Symposium EN08 : Halide Perovskites for Photovoltaic Applications—Devices, Stability and Upscaling

Perovskite solar cells promise to yield efficiencies beyond 30% by further improving the quality of the materials and devices. Electronic defect passivation, and suppression of detrimental charge-carrier recombination at the different device interfaces has been used as a strategy to achieve high performance perovskite solar cells.
In this presentation, I will discuss the role of electronic defects and how these can be passivated to improve charge-carrier lifetimes and to achieve high open-circuit voltages. I will discuss the characterization of 2D and 3D defects, such as grain boundaries, crystal surface defects, and precipitate formation within the films, by synchrotron-based techniques. The importance of interfaces and their contribution to detrimental recombination will also be discussed. As a result of these contributions to better understanding 2D and 3D defects, the perovskite solar cell field has been able to improve device performance. Albeit the rapid improvements in performance, there is still a need to improve these defects to push these solar cells beyond the current state-of-the-art.

Since the first report on the 9.7% efficiency, 500 h-stable solid-state perovskite solar cell (PSC) in 2012 by our group, following two seed works on perovskite-sensitized liquid junction solar cells in 2009 and 2011, a power conversion efficiency (PCE) of 24.2% was recorded in 2019. According to Web of Science, publications on PSC increase exponentially since 2012 and total number of publications reaches over 12,000 as of May 2019, which indicates that PSC is considered as promising photovoltaics. Although high photovoltaic performance was achieved from small area cell, scalable technologies are required for commercialization of PSC. In order to shift from small-area device to large-area module, the cheap materials and an effective coating procedure are highly required. We developed cost-effective materials based on delta FAPbI₃ powder for high efficiency PSC. The best PCE of 22.6% was achieved using the synthesized perovskite powder. For large-area uniform perovskite coating, a precursor solution containing perovskite cluster was developed. Homogeneous MAPbI₃ perovskite film (>100 cm²) was D-bar coated within 20 s, which demonstrated the PCE approaching 18%. Bifacial stamping technique was developed, which led to both high quality FAPbI₃ and MAPbI₃ on a large scale at milder condition.

An effective surface engineering is developed to prepare effective SnO₂-based electron transport layer (ETL). Together with an additive engineering strategy, solar cell efficiency is improved to 22% when rigid glass is used as the substrate. When PET substrate is used, flexible solar cells are designed and fabricated with the cell efficiency improved to as high as 20.9%, the highest efficiency for this category of flexible solar cells. It is found that by using an additive to react with Pb²⁺ to form an intermediate complex. Crystallization process is slowed down during the perovskite formation, leading to enlarged grain size and improved crystalline quality. In fact, using the SnO₂ ETL and the additive engineering, the trap state density of the resultant perovskite thin films is effectively reduced comparing to the films without it, demonstrating that the additive effectively retards transformation kinetics during the thin film formation process. Meanwhile, the surface modification is found critical to form a nucleation layer and a good interface for the perovskite thin film development. Furthermore, the environmental stability of the flexible solar cells is significantly enhanced comparing to the devices without them. Large area solar cells are fabricated to demonstrate its scalability.

Hybrid organic-inorganic perovskite solar cells (PSCs) are promising for applications due to unparalleled processing-performance characteristics. Due to improvements in both perovskite active layer composition and the use of newly-developed interfacial materials, PSC PCEs have increased steadily from ~4% to >23%. High-performance PSCs relies on: (1) Appropriate interfacial energy level alignment for reducing energy losses. (2) Minimize defects and ion vacancies at the perovskite interface and bulk to maximize charge transfer. Here, we report two complementary strategies for fabricating/modifying the device electron transporting layer (ETL) based on inorganic and organic materials. In the first strategy, we demonstrate high-quality and dense ZnO films with negligible organic impurities, high crystallinity, and a self-passivated surface by combustion synthesis enabling cells with a PCE of 19.69 %. In the second strategy, we introduce two non-conjugated multi zwitterionic organic small-molecule electrolytes (NSEs) as effective bottom-up modifier to passivate not only interfacial but the bulk defects of perovskite for solution-processed PSCs. These NSEs enable to achieve an ideal WF for the ETL of 3.78 eV, thus resulting in a barrier free electron transfer, as well as to passivate interfacial defects between perovskite and ETL, preventing back electron transfer and suppressing charge recombination. Cells based on SnO2/NSE deliver PCEs surpassing 21.18% with an ultra-high VOC of 1.19 V. Both approaches strongly suppressed hysteresis and enhance environmental stability.

The physical properties of semiconducting solids depend on the imperfections they contain [1]. Defects come in a few flavours: conductivity-promoting defects create free carriers that enable electronics; killer defects (deep, charged centres) trigger recombination; and charge scattering defects reduce mobility.

Our understanding of the defect chemistry and physics of halide perovskites is limited in comparison to inorganic semiconductors. I will discuss recent progress, from theory and experiment, to identify, characterise and control point defects and defect processes in this family of compounds. I will cover charge compensation mechanisms [2], carrier trapping phenomena [3], the effect of grain boundaries [4], and how this understanding can be applied to engineer defect populations and distributions. The use of `defect tolerance` as a metric to develop and screen post-perovskite materials will be critically addressed.

[1] "Instilling defect tolerance in new compounds" Nature Mater. 16, 964 (2017); https://www.nature.com/articles/nmat4973

[2] "Self-regulation mechanism for charged point defects in hybrid halide perovskites" Angew. Chem. 54, 1791 (2015); https://doi.org/10.1002/anie.201409740

[3] "H-center and V-center defects in hybrid halide perovskites" ACS Energy Lett. 2, 2713 (2017); https://doi.org/10.1021/acsenergylett.7b00995

[4] "Accumulation of deep traps at grain boundaries in halide perovskites" ACS Energy Lett. 4, 1321 (2019); https://doi.org/10.1021/acsenergylett.9b00840

Molecular doping of inorganic semiconductor is a rising topic in the scope of organic/inorganic hybrid electronics. In this talk, we present two typical molecular doping for NiO and SnO₂, both of which play a crucial role in carrier transport in perovskite solar cells. First, NiO hole-transport-layer was realized p-doping successfully by 2,2’-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6TCNNQ). Determined by XPS and UPS, the E_F of NiO_x HTLs is increased from -4.63 eV to -5.07 eV and VBM-E_F declines from 0.58 eV to 0.29 eV after F6TCNNQ doping. The energy level offset between the VBMs of NiO_x and perovskites declined from 0.18 eV to 0.04 eV. Combining with first-principles calculations, electrostatic-force-microscopy was the first time applied to verify directly electron transfer from NiO_x to F6TCNNQ. The average PCE of CsFAMA mixed cations PSCs were boosted by ca. 8% depending on F6TCNNQ doped NiOx HTLs. Strikingly, the champion cell conversion-efficiency of CsFAMA mixed-cations and MAPbI₃ based devices got 20.86% and 19.75%, respectively.
However, it’s difficult for n-doping, especially for oxides, because rare molecules own both strong reducibility and stability simultaneously in ambient atmosphere. In the second work, a simple, air-robust and cost-effective triphenylphosphine oxide molecule successfully realizes n-doping of SnO₂, Strikingly, we discovered that electrons were transferred from the R₃P⁺-O^- σ-bond to the peripheral tin atoms other than the directly interacted ones at the surface. That means those electrons are delocalized. The course was verified by multi physical characterizations. This doping effect accounts for the enhancement of conductivity and the decline of work function of SnO₂, which enlarges the built-in field from 0.01 eV to 0.07 eV and decreases the energy barrier from 0.55 eV to 0.39 eV at the SnO₂/Perovskite interface enabling an increase in the conversion efficiency of perovskite solar cells from 19.01% to 20.69%.
Our discoveries offer an extremely promising molecular doping method for inorganic carrier transport layers in PSCs. This methodology definitely paves a novel way to modulate the doping in hybrid electronics more than perovskite and organic solar cells.

Metal-halide perovskites are promising solution-processable semiconductors for efficient solar cells and show unexpectedly high diffusion ranges of photogenerated charges. In this contribution, we report on charge extraction and recombination in metal-halide perovskite back-contact devices. These structures provide a powerful platform to spatially-resolve electron and hole transport dynamics separately, to gain insights not accessible in vertical devices. For this, we deposit thin films of metal-halide perovskite over laterally-separated electron (hole) selective materials of SnO2 (NiOx). Upon illumination, electrons (holes) rapidly transfer to the back-contact SnO2 (NiOx) collection electrode, leaving holes (electrons) to diffuse laterally as majority carriers in the perovskite layer. Under these conditions, we find that charge carrier recombination is strongly suppressed. Resulting recombination velocities are below 2 cm s-1, an order of magnitude lower than in the presence of both carrier types, and approaching values of high-quality silicon. Using frequency-modulated photo-current detection, we extract spatially-resolved time-scales of charge extraction and trapping, from which we directly extract the charge trapping length in hybrid perovskite semiconductors. We report diffusion lengths of electrons and holes to exceed 12 µm in our full device architectures, an order of magnitude higher than reported to date from luminescence quenching experiments in stacked device architectures. We fabricate back-contact solar cells with short-circuit currents as high as 18.4 mA cm-2, reaching 70% external quantum efficiency. Our results highlight the currently limiting aspects of electrode interfaces for back-contact, and vertical, perovskite solar cell architectures, and gives directions for future opportunities to improve efficiencies.

Organic-inorganic hybrid halide perovskites have recently attracted great attention due to high potential in photo- and electroluminescence and photovoltaic applications. However there is still a lack of understanding of phase transformation during solution-processing perovskite crystals and its influences on optoelectronic properties and photovoltaic performance. Here I will discuss our recent studies toward a better understanding of phase transformation mechanism and effective manipulation for the complex phase transformation from disordered precursors to intermediate phases and perovskite crystal during solution-casting, and the equilibrium between nucleation and growth during single-crystal fabrication.^[1-8] Our findings suggest an important role of the phase transformation control for dimensional tailoring, scalable fabrication, and flexible single-crystal of perovskite. In addition, I will briefly discuss the relationship of phase transformation-crystal quality-device performance in terms of solar cells and optoelectronics.

Conventionally, 3D perovskite materials are used for fabricating high-efficiency perovsite solar cells (PSCs), but they demonstrate poor device stability due to their high moisture sensitivity. Conversely, 2D Ruddlesden-Popper perovskite materials exhibit high moisture stability, but render lower device performance. Therefore, mixed dimensional 2D/3D PSCs have gained substantial research interest recently due to their high efficiency and ambient stability. Contemporary studies on mixed 2D/3D PSCs are based on either a thin 2D perovskite surface layer on top of 3D perovsite at perovskite/hole transport layer (HTL) interface or 2D perovsite incorporated in the bulk of 3D perovskite. This work demonstrates that the application of 2D surface material with appropriate energy band alignment can be as effective for perovskite/electron transport layer (ETL) interface, as reported for perovskite/HTL interface. Using such double-sided 2D surface passivation of n-butylammonium iodide (C₄H₁₂IN) sandwiching the bulk 3D quadruple cation Cs_0.07Rb_0.03FA_0.765MA_0.135PbI_2.55Br_0.45 [CH₃NH₃ (MA), CH(NH₂)₂ (FA)] ] perovskite enabled over 22% stabilized power conversione efficiency (PCE) with mixed dimensional 2D/3D PSCs in n-i-p device configuration. Compared to the control 3D PSCs, mixed 2D/3D PSCs demonstrate an average open circuit voltage (V_OC) enhancement of 20-40 mV and enhanced fill factor (FF) by 4% absolute value. The 2D surface layer contributes to the suppression of nonradiative recombination at perovskite/ETL and perovskite/HTL interfaces, which leads to enhanced photovoltaic performance with mixed 2D/3D PSCs. XRD spectral analysis also suggests reduced surface strain of perovskite film with 2D passivation layer that contributes to the superior device performance of 2D/3D device.

Hybrid halide perovskites combine top-notch optoelectronic properties with solution-deposition. This unprecedented combination has led to the development of solar cells that outperform established industry staples such as poly-Si. The road towards these achievements has been marked by a constant improvement of perovskite deposition techniques fuelled by our increasing understanding of the crystallization processes. However, stability still remains a key challenge. Here, I will focus on the effects of moisture-induced degradation on state-of-the-art perovskite materials and put forward several strategies to minimize the effect. I will show that the composition of the perovskite critically affects its stability while popular stabilization routes, such as introducing bulky hydrophobic cations - forming 2D/3D hybrid materials - in fact do not prevent moisture ingress into the structure. Our results show that the perovskite exhibits a surprisingly effective self-healing effect when combined with a thin 2D/3D hybrid perovskite bi-layer configuration. Furthermore, the effect can be minimized by the choice of hole transporter used where our developed amide-based hole transporting materials enhance both the performance and stability of the system.

Using carbon materials as the back electrode for perovskite solar cells (C-PSCs) has attracted significant attention due to their low cost and excellent stability. In general, the device structure of based C-PSCs has two types, one is hole transport layer (HTL) free and another one is with HTM. For these two cases, the carbon paste electrodes (CPEs) need directly collect the photo-generated holes or/and transport the holes. Therefore, interfacial engineering between the perovskite and CPE plays a crucial role in charge collection and affects the performance of C-PSCs.
Our group have been carried out the interfacial engineering between carbon layer and perovskite layer using several methods to improve the performance of the C-PSCs. Herein, we will report our recent results, including development a simple process and an effective sandwich structure-based on coal carbon materials and 2D titanium carbide for improving the efficiency of PSCs. We also review the current progress in the studies on the carbon-based PSCs.
References:
1. Meng F., Gao L., Yan Y., Cao J., Wang N., Wang T., Ma T. Ultra-low-cost coal-based carbon electrodes with seamless interfacial contact for effective sandwich-structured perovskite solar cells, Carbon, 145, 290 –296, 2019
2. Meng F., Gao L., Yan Y., Cao J., Wang N., Wang T., Ma T. Current progress in interfacial engineering of carbon-based perovskite solar cells, J. Mater. Chem. A, 2019,7, 8690-8699

Metal halide perovskite solar cells are now effectively competing with their inorganic counterparts in terms of power conversion efficiencies. However, state-of-the-art perovskite solar cells still suffer from limited fill factor (FF), open circuit voltage (V_oc) and long-term stability. Charge transport layers (CTLs) are key components of diffusion-controlled perovskite solar cells, however, we found that the CTL/perovskite interfaces induce additional non-radiative recombination pathways, which limit the V_oc of the cell. In order to harvest the full thermodynamic potential of the perovskite absorber, the interfaces of both the electron and hole transport layers (ETL/HTL) must be properly addressed and improved. Given the knowledge obtained from our previous studies, we developed a novel polymeric surface treatment based on poly(ionic-liquid) (PIL) materials. Notably, the application of this surface modification to a triple cation Cs₅(MA_0.17FA_0.83)₉₅)₉₅Pb(I_0.83 Br_0.17)₃ perovskite in pin-type solar cell enables a concomitant improvement of the FF and the V_oc. The resulting solar cell devices show outstanding FF values of up to 83% and V_oc of 1.17V, which lead to extraordinarily high PCEs up to 21.5%. Through combined photoluminescence and cathodoluminescence studies we find that the PIL treatment helps to reduce the non-radiative recombination at the perovskite surface by acting as a defect passivating agent in the bare material. Additionally, we find that the recombination of charges across the perovskite/C₆₀ interface is strongly reduced, increasing the V_oc in the actual device. Moreover, photoemission spectroscopy and conductive atomic force microscopy highlighted a specific charge redistribution at the perovskite surface upon PIL treatment, which conceivably leads to increased conductivity and charge extraction, consistently with the enhanced FF. Ultimately, the hydrophobic nature of the PIL provides a shielding coverage of the perovskite which helps to prevent degradation of the active material by moisture and air. The PIL modified devices show exceptionally long dark storage stability and enhanced lifetimes under maximum power point tracking (MPP). We show that by addressing and carefully modifying the perovskite surface/interface with this novel material, it is possible to efficiently suppress the non-radiative recombination of charges, promote the charge extraction and improve the stability at the same time. Additionally, given the simplicity of this post-treatment, our results can be representative of a more general methodology for device modification, therefore, potentially applicable to other compositions and cell architecture, including tandem solar cells. Finally, our results could open the doors to e new class of materials to be implemented in perovskite solar cells for future development.

State-of-the-art halide perovskite solar cells employ semiconductor oxides as electron transport materials. Defects in these oxides, such as oxygen vacancies (O_vac), act as recombination centres and, in air and UV light, reduce the stability of the solar cell. Under the same conditions, the PbZrTiO₃ ferroelectric oxide employs O_vac for the creation of defect-dipoles responsible for photo-carrier separation and current transport, evading device degradation. Here, we report the application of PbZrTiO₃ as the electron extraction material in triple cation halide perovskite solar cells. The application of a bias voltage (poling) up to 2 V, under UV light, is a critical step to induce charge transport in the ferroelectric oxide. Champion cells result in power conversion efficiencies of ∼11% after poling. Stability analysis, carried out at 1-sun AM 1.5 G, including UV light in air for unencapsulated devices, shows negligible degradation for hours. These devices show also swithching properties and thus, are also applied as solar transistors. Our results demonstrate, the application of ferroelectric/ferroelectric interfaces for efficient and stable PSCs and Solar transistors. These findings are also a step forward in the development of next generation ferroelectric oxide-based electronic and optoelectronic devices.

[1] Pérez-Tomás, A.; Lima, A.; Bilion, Q.; Shirley, I.; Catalan, G.; Lira-Cantú, M., A Solar Transistor and Photoferroelectric Memory. Adv. Func. Mater. 2018, 28 (17), 1707099.
[2] Pérez-Tomas, et.al., M., PbZrTiO3 Ferroelectric Oxide as electron extraction material in Halide Perovskite Solar Cells. Sustainable Energy & Fuels 2019, DOI: 10.1039/c8se00451j.
[3] A. Hagfeldt, M. Lira-Cantu, Recent concepts and future opportunities for oxides in solar cells, Applied Surface Science, (2018) in press.
[4] A. Perez-Tomas, A. Mingorance, Y. Reyna, M. Lira-Cantu, Metal Oxides in Photovoltaics: All-Oxide, Ferroic, and Perovskite Solar Cells, in: M. Lira-Cantu (Ed.) The Future of Semiconductor Oxides in Next Generation Solar Cells, Elsevier, 2017, pp. 566.
[5] M. Lira-Cantú, Perovskite solar cells: Stability lies at interfaces, Nature Energy, 2 (2017) nenergy2017115.

Organic/inorganic lead halide perovskite solar cells (PSCs), reaching over 24% power conversion efficiency demonstrates that they are the most promising class of materials for next-generation thin film photovoltaics. The unprecedented increase in the device performance in less than ten years is because of improved processing protocols, and compositional engineering of cations, and anions of the perovskite material. Nevertheless, the remarkable power conversion efficiency (PCE) progression of perovskite solar cells, confronts with serious stability concerns, due to an intrinsic decomposition of the materials compromising their potentiality as a future market technology. In this talk, we discuss how to mitigate the stability of perovskite solar cells by compositional and interface engineering.

Lead halide perovskite solar cells are attractive because they can be processed from solution at low temperature. Nevertheless, the perovskite community adheres to relatively strict processing protocols to form high-performance perovskite layers with the frequently used “anti-solvent” procedure.^[1] Thus, to establish a perovskite process in a lab is closely linked with cumbersome fine-tuning, processing practice and ambient control (temperature, atmosphere). As a result, reproducibility issues from lab to lab are frequently encountered. Some of the issues associated with the anti-solvent approach can be overcome by the so called “complex assisted gas quenching” (CAGQ) technique, that replaces the anti solvent exposure by a nitrogen flow driven drying process.^[2]
In any case, the perovskite precursor inks usually consist of highly polar solvents, which gives rise to de-wetting issues, especially if hydrophobic organic hole transporting layers, like poly triarylamine (PTAA), are considered.^[3]
Here, we will show, that for the most frequently used solvent system of DMSO:DMF precursor de-wetting is of particular importance as the wetting behavior deteriorates during the gradual formation of supersaturation. We unambiguously identify DMSO to be the main reason for de-wetting of the precursor solution and the consequent pinhole formation on hydrophobic hole transport layers. In striking contrast, we will show that n-methyl-2-pyrrolidon (NMP), which has a lower hydrophilic-lipophilic-balance, can be favorably used instead of DMSO to strongly mitigate these de-wetting issues. The resulting high-quality perovskite layers are extremely tolerant with respect to the mixing ratio (NMP:DMF), process handling and timing. Thus, our findings afford an outstandingly robust, easy to use and failsafe deposition technique yielding single (MAPbI₃) and double (FA_0.94Cs_0.06PbI₃) cation perovskites as well as mixed halide, wide bandgap (FA_0.6Cs_0.4PbI₂Br) perovskites for solar cells with high efficiencies without the need to adjust process parameters when switching between material systems. Most notably, the statistical variation of the devices with NMP:DMF solvent system is significantly reduced, even if the deposition process is performed by different persons with no significant lab experience whatsoever.^[4]
We foresee, that our results will further the reliable preparation of perovskite thin films and mitigate process-to-process variations that still hinder the prospects of upscaling perovskite solar technology.

[1] M. Saliba et al. Chem. Mater. 2018, 30, 4193
[2] B. Conings et al. Adv. Mater. 2016, 28, 10701
[3] X. Xu et al. J. Power Sources 2017, 360, 157
[4] K. Brinkmann et al. ACS Appl. Mater. Interfaces (submitted)

Solar energy can lead a “paradigm shift” in the energy sector with a new low-cost, efficient, and stable technology. Nowadays, three-dimensional (3D) methylammonium lead iodide perovskite solar cells are undoubtedly leading the photovoltaic scene with their power conversion efficiency (PCE) >23%, holding the promise to be the near future solution to harness solar energy [1]. Tuning the material composition, i.e. by cations and anions substitution, and functionalization of the device interfaces have been the successful routes for a real breakthrough in the device performances [2]. However, poor device stability and still lack of knowledge on device physics substantially hamper their take-off. Here, I will show a new concept by using a different class of perovskites, arranging into a two-dimensional (2D) structure, i.e. resembling natural quantum wells. 2D perovskites have demonstrated high stability, far above their 3D counterparts [3]. However, their narrow band gap limits their light-harvesting ability, compromising their photovoltaic action. Combining 2D and 3D into a new hybrid 2D/3D heterostructure will be here presented as a new way to boost device efficiency and stability, together. The 2D/3D composite self-assembles into an exceptional gradually organized interface with tunable structure and physics. To exploit new synergistic function, interface physics, which ultimately dictate the device performances, is explored, with a special focus on energy and charge transfer dynamics, as well as charge recombination and trapping processes happening over a time scale from fs to ms. As shown in Fig.1, when 2D perovskite is used on top of the 3D, charge transfer happens, while electron hole recombination at the perovskite/hole transporter interface is prevented. This results in improved device efficiency. In concomitance, the stable 2D perovskite is used as a sheath to physically protect the 3D underneath, with the aim to enhance the device stability. The joint effect leads to PCE=20% which is kept stable for 1000 h [3,4]. Incorporating the hybrid interfaces into working solar cells is here demonstrated as an interesting route to advance in the solar cell technology bringing a new fundamental understanding of the interface physics at multi-dimensional perovskite junction. The knowledge derived is essential for a deeper understanding of the material properties and for guiding a rational device design, even beyond photovoltaics.

Various strategies have been explored to improve the long-term stability of metal halide perovskite solar cells, including the use of barrier and passivation layers and device encapsulation. Atomic layer deposition (ALD) has emerged as an effective tool for all of these applications, due to the precise control it affords over film thickness and composition and the excellent barrier properties of the resultant films. In practice, ALD metal oxides are often grown on top of organic carrier transport layers in devices. For example, ALD SnO₂ grown on fullerenes is widely used for high-efficiency all-perovskite, perovskite-Si and perovskite-CIGS tandems.^1-3 The physical barrier and electronic properties of an ALD film grown on an organic substrate, such as a polymer, small molecule, or self-assembled monolayer largely depend on the structure of the organic material. To ensure the formation of an abrupt organic/oxide interface and avoid subsurface growth, a high density of functional sites (e.g. hydroxyls, amines) at the surface are desired to facilitate ligand-exchange reactions with the ALD metal-organic precursors.

In this work, we apply these insights to improve the nucleation, and consequently the barrier properties, of ALD metal oxides as selective contacts, with a focus on improving operational device stability. We demonstrate the application of a thin (~1 nm), nucleophilic polymer to promote the growth of ALD SnO₂ on C₆₀ as electron contacts in n-i-p devices. The nucleation-enhanced SnO₂ effectively protects against the degradation of Cs_0.25FA_0.75Pb(Br_0.20I_0.80)₃ films held at 200°C in air for up to 2 hours and upon direct exposure to water for at least 10 minutes. Ultimately, we show the integration of nucleation layers into single-junction, opaque devices with a champion power conversion efficiency (PCE) of 18.5%. On average, devices with nucleation-enhanced ALD SnO₂ and no further encapsulation maintain 82% of their initial PCE after 250 hours of continuous, illuminated operation at maximum power in air at 60°C, demonstrating a 150% improvement in stability at these harsh conditions over devices with ALD SnO₂ grown directly on C₆₀. These results highlight the impact that a high-quality contact/barrier layer can have on protecting the perovskite from various degradation pathways, such as moisture or oxygen ingress, halide egress, or metal-halide reactions. This study also presents a framework upon which to further improve the quality of barrier/encapsulation layers for various configurations and architectures of perovskite optoelectronic devices.

1. Palmstrom, A. F. et al. Enabling Flexible All-Perovskite Tandem Solar Cells. Joule (2019). doi:10.1016/j.joule.2019.05.009
2. Sahli, F. et al. Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency. Nat. Mater. 17, 820–826 (2018).
3. Kim, D. H. et al. Bimolecular Additives Improve Wide-Band-Gap Perovskites for Efficient Tandem Solar Cells with CIGS. Joule (2019). doi:10.1016/j.joule.2019.04.012

A novel naphthalene diimide (NDI)–based polymer (P(NDI2DT–TTCN)) which has strong electron withdrawing dicyanothiophene group is employed as the electron transport layer (ETL) in place of the conventional fullerene-based ETL in inverted–structure perovskite solar cells (Pero–SCs). A variety of characterization techniques are used to observe and analyze the interface between perovskite and P(NDI2DT–TTCN) or [6,6]–phenyl–C61–butyric acid methyl ester (PCBM). It is confirmed that P(NDI2DT–TTCN) helps not only to facilitate the electron extraction but also to prevent ambient atmosphere interference by forming a hydrophobic ETL surface. Furthermore, P(NDI2DT–TTCN) shows excellent mechanical stability in flexible Pero–SCs. With these improved properties, the device performance based on P(NDI2DT–TTCN) is superior to that of based on PCBM from 14.3% to 17% with negligible hysteresis.

For commercializing perovskite solar cells (PSCs), moisture-tolerant materials are required for each component of PSCs, such as electron transport layer (ETL), perovskite layer, and hole transport layers (HTL), because low relative humidity (RH) under 10~20% cannot be controlled in actual production line. Most of conversional PSCs with a high power conversion efficiency (PCE) over 20% have widely implemented Li doped mesoporous titanium oxide (mp-TiO₂:Li) as an n-type semiconductor for ETL. This Li treatment could facilitate an electron extraction of ETL, but could affect the device stability during the fabrication of PSCs under humid atmospheric environment due to its hydroscopic nature.
In this work, we employ mesoporous BaSnO₃ (mp-BSO), which is moisture tolerant ETL, and fabricate stable PSCs without scarifying the performance. The mp-BSO based PSCs show certified PCE of 21.3% and steady-state PCE of 21.4%, which is compare to that of mp-TiO₂:Li based PSCs. However, with respect to the storage stability, wach device exhibits different behavior according to the humidity in air. Both devices are stable at RH-10% atmosphere. At high humid environment (RH 40%), mp-TiO₂:Li based device shows severe degradation even after 24 hours, whereas mp-BSO based device shows good stability similar to the result at RH 10%. This is mainly attributed to the formation of Li_xO on TiO₂ surface analysis of mp-TiO₂:Li (i.e. IR, XPS, and TEM). For resultant device with mp-TiO₂:Li as ETL, electron transfer/transport could be impeded at the interface between ETL and perovskite layer at a high humid condition. On the other hand, the device with mp-BSO exhibits a superior moisture stability. It is also supported by transient photocurrent/photovoltage and time resolved photoluminescence studies. Along with outstanding performance, mp-BSO based PSCs show excellent storage and light stability during 500 hours. We believe that this strategy of introducing mp-BSO ETL into PSCs will accelerate the commercialization of PSCs¹

1) J.Chung et.al., Joule (2019) article in press, DOI: 10.1016/j.joule.2019.05.018

Perovskite solar cells are considered the photovoltaics technology of the future due to their low manufacturing costs and potential for high performance. Perovskite solar cells often use oxides to perform multiple functions; oxides such as TiO₂, SnO₂, and ZnO are used as electron transport layers (ETL) and NiO, WO₃ as hole transport layers (HTL). The ETL material must simultaneously have optimal conduction band alignment, facilitate carrier extraction, prevent recombination, and provide a chemically stable interface with the volatile perovskite semiconductor. The multiple requirements placed on this one layer dictate different material properties at different positions within the film, specifically bulk and interfacial properties. Given that the layer thickness ranges from 10 nm to 200 nm, the requirements demand positional control of the material composition at the nano-scale. By using the pulsed laser deposition (PLD) technique to control the chemical composition at nanometer scale to form material alloys of ZnO and MgZnO as ETL for the perovskite solar cell device, we demonstrate that digital alloy gradients can be tuned to significantly outperform either of the parent materials in perovskite solar cells.

SnO2-based oxide is very promising electron transport layer (ETL) for high efficient and stable perovskite solar cell. However, electron affinity of SnO2 is rather low, considering the otimal conduction band offset (CBO) between ETL and absorber layer. We studied device simulation to get optimal CBO by tuning the electron affinity of SnO2-based ETL and perovskite absorber. SCAPS simulation showed that power conversion efficiency (PCE) is increased from cliff-type CBO to spike-type CBO, and decreased with larger spike-type CBO. With the obtained simulation result, we fabricated SnO2 and doped-SnO2 ETLs with up-shifted conduction band edge and compared performance of fabricated device with simualtion result. This research indicates that CBO should be optimized for higher efficiency SnO2-based perovskite solar cell.

We have investigated perovskite solar cells (PSCs) employing novel polymer semiconductor as hole transporting material (HTM). Dithiophene-benzene (DTB) copolymer-based PSCs have been reported higher short circuit current density than other polymer-based PSCs. In addition, it was expected that DTB was low cost and high stability compared to typical HTM of Spiro-OMeTAD. However, the influence of oxygen and dopant on DTB have not been examined in detail yet.
In this research, we fabricated mesoporous-type PSCs employing DTB copolymer as HTM without and with dopant under nitrogen atmosphere and measured the device performance in air. We used a lewis acid tris(pentafluorophenyl)boran (BCF) as a dopant.
In case of the device without dopant, initial power conversion efficiency (PCE) showed 2.98% after exposure to air for 5 hours. However, the device performance gradually improved by keeping in dry air. After keeping in dry air for 1 week, PCE greatly improved to 16.15%. These results indicate that increase of carrier density in DTB occurred due to oxygen-doping. In case of the device with BCF dopant, initial PCE showed 13-14% after exposure to air for 5 hours. The PCE improved to 15-16% by keeping in dry air. The cause of this improvement of the initial device performance was due to increase of carrier density in DTB by BCF dopant.

Tandem-junction perovskite/silicon devices present the potential for highly efficient solar energy conversion devices. We investigate low-temperature organic monolayer functionalization of silicon substrates to chemically and electrically tether perovskite thin-films while maintaining carrier selectivity and a soft, flexible interface. We have covalently grafted vertically oriented perylene dianhydride overlayers on oxide-free Si (111) through and imide linkage to a surface-bound aniline. Secondary functionalization of the terminal anhydride with phenylenediamine yields A-type cation moieties that serve as a chemical hook for perovskite deposition. UPS characterized the band-energy levels of functionalized surfaces aligned to facilitate electron transport from MAPbI₃ to the silicon substrate. Transflection Infrared Spectroscopy and X-ray photoelectron spectroscopy revealed a high coverage of vertically oriented perylene diimides. Liquid-junction photoelectrochemical experiments of MAPbI₃/perylene diimide terminated, oxide-free silicon interfaces yield improved energy conversion relative to long-chain alkyl terminated silicon surfaces. We ascribe these improvements to enhanced carrier selective transport and reduced recombination. This work presents a bench-top functionalization of soft, robust, electron transport layers to further integrate low-cost perovskites with mature silicon PV.

Recent advances in flexible perovskite solar cells (PSCs) have attracted considerable attention owing to their great potential for bendable and wearable electronic devices. Particularly, developing high-quality low-temperature processed electron transport layers (ETLs) plays a pivotal role in realizing highly efficient flexible PSCs. Herein, we develop a facile strategy to fabricate graphene quantum dot/SnO₂ composites (GQD@SnO₂) as effective ETLs for PSCs. Through systematically optimizing GQDs' size and concentration, higher film conductivity, better film coverage uniformity, and raised fermi energy level matching with perovskite were reached, which is remarkably beneficial to facilitate electron transfer and suppress interfacial charge recombination, resulting in great enhancement of photovoltaic performance. As a result, PSCs based on SnO₂ blending with GQD of ca. 5 nm in diameter (G5@SnO₂) exhibit superior photovoltaic performance with a champion power conversion efficiency (PCE) of 19.6% and an average PCE of 19.0%. Significantly, flexible PSCs based on G5@SnO₂ obtain a best PCE and stabilized PCE of 17.7 % and 17.2%, respectively. Moreover, these flexible devices demonstrate outstanding durability, retaining 91% of their original PCE after 500 bending cycles at a radius of 7 mm. This work provides a facile route to develop effective ETLs for high-performance flexible PSCs and paves the way for further advances in flexible photovoltaic devices and optoelectronic applications.

Organic–inorganic hybrid perovskite solar cells have attracted great attentions in recent years, because they can be fabricated by solution processes and the power conversion efficiency (PCE) has been drastically improved up to more than 24%. In a normal type perovskite solar cells having mesoporous structure, a metal oxide layer such as titanium oxide is used. However, it is not suitable for the application to flexible solar cells because it requires high temperature treatment for film formation. Therefore, studies on the inverted type perovskite solar cells which can be formed a thin film at low temperature have attracted attention. Poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) widely used for the hole transport layer, is considered to have a negative influence on the solar cell characteristics due to strong acidity, hygroscopicity, etc. Therefore, the development of chemically inert hole transport layers is one of the important challenges.¹⁾²⁾
Interface engineering of heterojunction interfaces which reducing the surface recombination is also a crucial strategy to improve the performance of the perovskite solar cells.
In this study, inverted perovskite solar cells using metal oxide as the hole transport layer are prepared, and the effect of various acid treatments on the surface of the metal oxide layer such as NiO_x, WO₃, MoO₃ and V₂O₅ on its local structure, charge transport characteristics and photovoltaic characteristics were investigated systematically.
It was found that the perovskite layer prepared on the acid-treated metal oxide film exhibits higher PL quenching ratio than that on the non-acid-treated metal oxide film.
In this presentation, we will report the detailed experimental results on the effect of acid treatment to metal oxide film on solar cell characteristics under various conditions.

An alcohol based nickel oxide nanoparticle dispersion was prepared and used to deposit hole conductor medium for a polymeric substance free perovskite solar cell in mesoscopic n-i-p configuration. In contrast to conventional p-i-n configuration or inverted type perovskite solar cells, nickel oxide layer was spin coated directly on perovskite layer and the perovskite layer was sandwiched between two metal oxides, TiO₂ and NiO, resulting in n-i-p configuration. High surface area nickel oxide nanoparticles were synthesized by precipitation and succesfully dispersed in isopropanol with the aid of ball milling, which was confirmed to breakdown the aggregates and stabilize the dispersion without the assistance of a stabilizing agent. This strategy leads to deposition of nickel oxide nanoparticles on perovskite layer without damaging the underneath perovskite layer and inhibiting the charge transfer between individual nanoparticles, confirmed by scanning electron microscopy, photoluminescence quenching and J-V measurements. Ultraviolet photoelectron spectroscopy analysis showed excellent matching of the band alignment of nickel oxide layers with that of perovskite. An efficiency of 10.89% was achieved after optimizing the nickel oxide layer thickness and comparing with a hole conductor free device using J-V measurements and electrochemical impedance spectroscopy revealed that nickel oxide layer possesses excellent electron blocking ability and reduces the recombination rate, which also in turn stabilizes the power output and hysteresis of the cells. This strategy is believed to be applicable for other metal oxides employed in charge selective layers of perovskite and organic solar cells.

The power conversion efficiency (PCE) of inverted perovskite solar cells (i-PSCs) is lower than the normal structures. The low efficiency is mainly ascribed to the inferior properties of commonly-used [6,6]-phenyl C61 butyric acid methyl ester (PCBM) electron transport layers (ETLs) such as complexity in achieving high quality films, low electron mobility, imperfect energy level for electron extraction, and large electron capture region. Herein, we develop the bulk heterojunction (BHJ) ETLs, which compose of PCBM and conjugated polymer materials. The electron mobility of BHJ film is enhanced by more than 3 times compared to pristine PCBM, and its Fermi level is close to the conduction band of perovskite. The high electron mobility and suitable energy level result in efficient electron extraction. Further, the electron capture region of BHJ film significantly decreases to 1.20 × 10^-18 cm^-3 from 3.70 × 10^-17 cm^-3 for pristine PCBM due to increased relative permittivity (enhanced to 5.32 from 3.66 for PCBM), leading to reduced trap-assistant recombination at perovskite/ETL interface. As a result, the PCE of i-PSCs is up to 18.84%, increased by about 15% compared to pristine PCBM. Under ambient air after 30 days, the PCE of unsealed devices based on BHJ ETLs keeps about 85% of the initial efficiency, while the devices with PCBM retain only about 49% of the initial value under the same aging conditions. Meanwhile, the unsealed devices with BHJ ETLs show about 83% of the initial efficiency under continuous illumination for 60 hours, while the devices based on pristine PCBM give about 63% of the initial value. The excellent stability is attributed to the more hydrophobic BHJ films and full coverage of perovskite surface, which effectively prevent the moisture permeation into the perovskite devices. We believe this breakthrough will provide the path to improve the efficiency and stability of i-PSCs.

In the design of electron transport layers (ETLs) to enhance the performance of perovskite solar cells (PSCs), efficient electron extraction and transport are important aspects. Herein, we examine the effect of different titanium oxide (TiO₂) polymorphs such as anatase and brookite in the resultant PSCs performance. In this work, pure-phase, high-stable, high conductive, and single-crystalline brookite TiO₂ nanoparticles (NPs) with an average diameter of approximately 30 nm ~ 50 nm were synthesized by a facile, nontoxic, environmentally friendly hydrothermal method by using a water-soluble titanium complex as the titanium source that helps for green and scalable fabrication of the PSCs. We design and fabricate, single-crystalline and high-phase-purity brookite-based TiO₂ heterophase junctions on fluorine-doped tin oxide (FTO) as the substrate. We attempt to investigate and compare single phase anatase (A) and brookite (B) and heterophase anatase−brookite (AB) and brookite−anatase (BA) as ETLs in PSCs. The PCE of PSCs with single crystalline FTO-B only layer as the ETL was as high as 14.92% that is the highest reported efficiency of FTO-B-based single-layer PSC (Nano Lett., 2019, 19, 598). This result suggests that FTO-B acts as an active phase, which can be a potential candidate as an ETL scaffold in planar PSCs. Hetero junction FTO-AB ETLs based PSCs showed PCEs as high as 16.82%, which is higher to those of PSCs with single-phase anatase (FTO-A) and brookite (FTO-B) as the ETLs (13.86% and 14.92%, respectively). The large surface area of brookite TiO₂ NPs facilitates more electron injection and subsequent transport, which can balance the electron current density and hole current density in the resultant PSCs and, therefore, can promote high performance. This facile synthesis and employing of brookite TiO₂ NPs provide a clean and eco-friendly fabrication of PSCs with respect to an alternative approach to anatase TiO₂ phase as the ETL bilayer.

Perovskite solar cells (PSCs) has remarkably increased its power conversion efficiency (PCE) to 23.7% due to its high open-circuit voltage (V_OC) and photocurrent. To make it commercialized, the modification of electron extraction layer (EEL) for improving the photovoltaic performance became another important issue. For the EEL, TiO₂-based material is widely applied for the next-generation solar cells because of its non-toxicity, chemical stability, inexpensiveness, and high charge transportability. Doping Zn into TiO₂ could accelerate the photo-induced electron-hole separation and could tune the bandgap of TiO₂-based material. To further enhance the photovoltaic performance of PSC, we systematically study the effect of different Zn-doped TiO₂ (Zn:TiO₂) EEL on PSCs, including (1) planar Zn:TiO₂ EEL, (2) mesoscopic Zn:TiO₂ EEL, and (3) low-temperature Zn:TiO₂ EEL. For the planar Zn:TiO₂ EEL, it demonstrates serious hysteresis and it is unstable in the atmosphere. Although the mesoscopic Zn:TiO₂ EEL based PSCs shows higher PCE than planer Zn:TiO₂ EEL, the high-temperature calcination leads to cost increment and energy consumption and hinders the commercial potential. Therefore, the low-temperature process to prepare the nanostructure TiO₂ layer has been regarded as a breakthrough. By optimizing the parameters of PSCs and the energy band alignment between perovskite absorber layer and 3 types of Zn:TiO₂ EEL, the average PCE for PSCs based on (1) planerZn:TiO₂ EEL is ~14.0%, (2) mesoscopic Zn:TiO₂ EEL is ~16.8%, and (3) low-temperature Zn:TiO₂ EEL is 19.52%. The PCE of the champion device based on low-temperature Zn:TiO₂ EEL reached as high as 20.4%.

Organometallic halide perovskite solar cell (PSC) is the next generation thin film photovoltaic that harvests infinite solar energy. The strength of PSC lies in its potential price competitiveness from solution process availability and its ability to be made thin and be flexible due to high absorption coefficient of organometallic halide perovskite. However, use of brittle, indium-embedded, high-cost transparent conducting oxide (TCO) limits the commercialization of PSCs and their application to flexible devices.
Carbon nanotube (CNT) has received a lot of spotlight as a transparent electrode for electronic devices on grounds of its high conductivity, good transmittance, and abundance of elements. Single-walled carbon nanotube (SWNT) has been a subject of much research because of its low light density. However, its poor solubility restricts SWNT from cost-effective, large area available solution processes, preventing the commercialization of SWNT. Double-walled carbon nanotube (DWNT) is structurally midway between SWNT and multi-walled carbon nanotube (MWNT), with high transparency of SWNT and excellent dispersibility of MWNT. Taking advantage of these features, we demonstrate solution processed DWNT electrodes and apply them to the PSCs.
We treat nitric acid and trifluoromethanesulfonic acid (TFMS) on DWNT for optimization of sheet resistance and duration of doping effect. We systematically study the effect of the two dopants on the outer- and inner-wall through Raman spectroscopy and near-infrared (NIR) spectroscopy, and the results are consistent with the trend obtained from DFT calculation. We analyze PSCs based on DWNT with photoluminescence (PL) and dark current density-voltage (Dark J-V) measurements. Maximum power point tracking (MPPT) shows that optimized device exhibits stabilized power conversion efficiency (PCE) of 17.2% under 1 sun illumination.

Developing perovskite solar cells (PSCs) with a high performance and a low-temperature process has great potential for the scalable, economic renewable photovoltaic devices. Among critical elements, ZnO has many advantages as electron transporting layer (ETL) in PSCs, due to promising features including a suitable energy structure and a high electron mobility, which can potentially reduce the recombination loss as ETL. Also, ZnO can be easily solution-processed at low temperature. Despite these advantages, several obstacles must be resolved to utilize ZnO for viable perovskite-based devices; one of them is the thermal instability of CH₃NH₃PbI₃ photoactive layer deposited on ZnO ETL. The basic nature of the ZnO surface can induce the proton transfer reaction taking place between ZnO and CH₃NH₃PbI₃ layer, which can reduce the device performance.
In this work, we demonstrated modification of the ETL by self-assembled monolayers (SAMs) to suppress the decomposition of perovskite for low-temperature processed and high performance PSC. The SAM layer was deposited on ZnO ETLs by dip coating for several hours to induce the condensation reaction between the hydroxyl group of ZnO ETL and the carboxylic acid group of the SAM layer. It was found that utilization of SAMs, especially 4-methoxybenzoic acid (MBA), 3, 4-dimethoxybenzoic acid (DMBA) and 3, 4, 5-trimethoxybenzoic acid (TMBA) on ZnO ETL prevents the direct contact between the perovskite and ETLs, avoiding the proton transfer reaction between perovskite and ZnO ETL. Also, strong dipole effect induced by a methoxy functional group of SAM interlayer can enhance the LOMO energy level and the work function of ZnO ETL, resulting in a reduction of electron transport barrier at the ZnO/CH₃NH₃PbI₃ interface and enhancing the built-in voltage of the device.
We verified the decrease in the energy gap between LOMO level of ZnO ETL and that of perovskite by photoluminescence quenching. PL intensity of ZnO/CH₃NH₃PbI₃ was quenched after applying SAM interlayer, proving that charge transfer at ZnO/CH₃NH₃PbI₃ interfaces was significantly facilitated via insertion of SAM interlayer. The effect was maximized when the ZnO was modified with TMBA which has the highest dipole moment among the SAM materials employed. Also, microscopic studies revealed that the perovskite active layer on TMBA modified ZnO ETLs showed optimized morphology.The power conversion efficiency (PCE) of the device was enhanced from 0.86 to 12.65% after optimizing a SAM interlayer, which was mainly ascribed for the highly increased short circuit current (J_SC).

Recent advances in photovoltaic research have proved the great potential of copper(I) thiocyanate (CuSCN) as a hole transport layer (HTL) for organic inorganic hybrid perovskite solar cells. Here, we have investigated CuSCN as HTL instead of conventional 2,2’,7,7’-tetrakis-(N,N-di-4-methoxyphenylamini)-9,9’-(spirobifluorene) (Spiro-OMeTAD) and systematically observed the relationships with methyl ammonium lead iodide (MAPbI₃) perovskite absorber layer. The CuSCN was completely dissolved in diethyl sulfide solvent and then deposited by spin coating method on the MAPbI₃ absorber layer. For the creation of compact CuSCN layer the post-annealing process was performed with various temperatures and times using hot plate. However, we observed the undesirable decomposition from MAPbI₃ to PbI₂ because post-annealing process performs feature long time annealing steps at elevated temperature. To solve this problem we were investigated by the increasing of methyl-ammonium iodide (MAI) ratio in MAPbI₃ precursor solution. The device and material properties were analyzed by current-voltage curve, quantum efficiency, scanning electron microscopy, UV-visible spectroscopy, and x-ray diffraction for finding the optimum condition of MAPbI₃ solar cells with CuSCN HTL. When excess MAI ratio is present in MAPbI₃ precursor solution, it can be compensates for the loss of any MAI and suppresses or delays the formation of a PbI₂ phase during annealing.

Despite the high power conversion efficiency (PCE) of perovskite solar cells (PSCs), poor long-term stability is one of the main obstacles preventing their commercialisation. Several approaches to enhance the stability of PSCs have been proposed. However the accelerating stability test of PSCs at high temperature under the operating conditions in ambient air remains still to be demonstrated. Herein, we show interface engineered stable PSCs with inorganic charge transport layers (p-NiO and n-Al:ZnO grown via atomic layer deposition). First of all, NiO has been chosen as hole transporting layers due to its a wide band gap (~3.6 eV) and p-type semiconducting porperties. It also has good optical transparency and high chemical stability, and thus has the capability aligning the band edges to the perovskite (CH₃NH₃PbI₃) layers with efficient energy transfer. Ultra-thin and un-doped NiO films with much less absorption loss were prepared by atomic layer deposition with highly precise control over thickness without any pinholes. Thin enough (5–7.5 nm in thickness) NiO films with the thickness of few time the Debye length (L_D = 1–2 nm for NiO) show enough conductivities achieved by overlapping space charge regions. Second, the highly conductive Al doped ZnO films have been chosen as an efficient electron transporting layers while acting as dense passivation layers. This layer prevents underneath perovskite from moisture contact, evaporation of component, and reaction with a metal electrode. Finally the inverted-type PSCs with inorganic charge transport layers exhibited a PCE of 18.45 % and retained 86.7 % of the initial efficiency for 500 hours under continuous 1-sun illumination at 85 °C in ambient air with electrical biases (at maximum power point tracking).

The electrical and optical properties of the transport layers play an important role in achieving efficient and hysteresis-free perovskite solar cells. Nickel oxide (NiO_x) with its wide band gap and suitable valance energy band alignment with perovskite is a promising candidate for hole extraction. Although hysteresis-free and rather stable perovskite solar cells were developed by utilizing NiO_x hole transport layer, its relatively small mobility and carrier concentration prevent achieving perovskite solar cells over 20% efficiency. On this respect, there are numerous studies in the literature on doping of NiO_x with various elements (i.e. Cu, Y, Li, etc.) to increase its mobility and carrier concentration. In this study, the impact of manganese (Mn) incorporation into the NiO_x thin film (Mn:NiO_x) is investigated to enhance the charge extraction capability of NiO_x without compromising its optical properties. Nickel(II) acetate tetrahydrate and different amounts of manganese(II)acetate tetrahydrate salts are used to prepare the Mn:NiO_x precursor solutions and spin coated on TCO substrates. While transmission and reflection, and spectroscopic ellipsometry measurements are performed to determine the optical properties of uniform Mn:NiO_x films, ultraviolet spectrum is utilized to determine their work function and valance energy band position. Impedance spectroscopy is carried out to determine surface passivation property and measure the mobility of the layers. We demonstrate the Mn doping into NiO_x thin films provides to enhanced efficiency in hysteresis-free perovskite solar cells.

Among many strategies to develop high-performance perovskite solar cells, the interface engineering is considered as a promising approach for achieving high power conversion efficiency. Specifically, high optical transparency and excellent electrical properties are essential in optimized hole transport materials in the inverted-type planar perovskite solar cells. In this study, we demonstrate the molecular doping of copper thiocyanate (CuSCN) by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) significantly enhances the photovoltaic performance of perovskite solar cells. The incorporation of F4TCNQ in CuSCN leads to successful electron transfer from CuSCN to F4TCNQ, which affords more balanced energy level alignment at the interface of perovskite layer for hole conduction. The device analyses reveal faster charge transport and less carrier recombination in the F4TCNQ-doped CuSCN-based devices, contributing to not only the improved efficiency but also the hysteresis elimination. At the optimized doping concentration, the doped CuSCN exhibited ~35% increased efficiency as high as 14.19% in the inverted-type planar perovskite solar cells.

SnO₂ has received a great deal of attention in normal and planar type perovskite solar cells with its excellent optical and electrical properties; i.e., wide bandgap, good chemical stability and can be grown by low temperature processing as an electron transporting layer (ETL). Among various deposition methods, SnO₂ thin films used as ETL are deposited by Atomic Layer Deposition (ALD). ALD shows numerous advantages of being able to control film thicknesses with nanometer precision and to produce high quality thin films without any pinholes. SnO₂ has a deeper conduction band, compared to TiO₂ deposited by ALD which has energy level mismatch with MAPbI₃, thus energetic alignment of SnO₂ is more appropriate for extracting photogenerated electrons than TiO₂, resulting in high power conversion efficiency (PCE ; ~ 18.3 % with pure MAPbI₃ based PSCs). In addition, processing temperature of ALD-SnO₂ thin films (at ~ 180 ^oC and/or even R.T.) is much lower than that of ALD-TiO₂ thin films (~ 400 ^oC). Furthermore, thin films of Zn doped SnO₂ also prepared in this study and showed enhanced conductivity compared to undoped ones, as a result, photovoltaic performances are improved in the normal planar structure (FTO / Zn:SnO₂ / Perovskite / Spiro-OMeTAD / Ag) of the perovskite solar cells.

Keyword : perovskite solar cell, atomic layer deposition, SnO₂

For perovskite solar cells (PSCs) without hole transport layer (HTL), the hole transport energy barrier (ΔE_h) is defined as the energy difference between the Fermi level (E_F) of the electrode and the valence band maximum (VBM) of the perovskite. Highly efficient HTL-free PSCs require that ΔE_h should be reduced or eliminated to improve hole collection. In order to minimize ΔE_h, one strategy is to adjust the VBM of the perovskite closer to the E_F of the electrode. However, it is rather challenging because the electronic structure of the perovskite depends, to a large extent, on the specifics properties of the system, as well as the preparation and environmental conditions. Another strategy to minimize ΔE_h is to shift the E_F of ITO closer to the VBM of the perovskite. In this way, organic monolayer (ML) materials which have been widely used in organic electronics can adjust the electrode E_F. However, to date organic ML is rarely used to prepare efficient HTL-free PSCs. Besides, the energy level arrangement between electrode and HTL still needs optimization, and the feasibility of using organic ML to construct highly efficient PSC without HTL remains to be explored. We report the monomolecular layer strategy toward barrier-free contact of the energy level arrangement at the electrode/perovskite interface, facilitating efficient charge transfer and inhibiting non-radiative carrier recombination. The HTL-free PSCs based on ML-modified ITO produced an efficiency of 19.4%, much higher than the HTL-free PSCs (10.26%) on the original ITO, and it is even comparable to the PSCs with HTL. This study provides an in-depth understanding of the interface level layout mechanism and is helpful in designing advanced interface materials for simplified and efficient PSCs.

Efficiency of the halide perovskite solar cells (HPSCs) has recently reached over 24% by solution-based compositional engineering as well as surface passivation technique. Low stability of halide perovskite materials still remains many challenging problems. Inverted configuration of HPSCs with a hole transporting layer (HTL) of inorganic materials have shown great promise to improve the resulting stability. In this study, ultra-thin films of NiO as HTLs were deposited by thermal atomic layer deposition (ALD) ALD with H₂O₂ and O₃ as an oxygen source. were deposited by. With the analysis of XPS, it was able to identify the difference between Ni and O compositions in the deposited thin films either by H₂O₂ or O₃ as oxygen sources. Band diagrams of HPSCs with ultra-thin NiO layers were revisited on the basis of the optical property, and the analysis of valence band maximum and work function using ultraviolet photoelectron spectroscopy. The present study shows that a couple of nanometers-thick and pin-hole-free NiO films could be successfully prepared by ALD and served as HTLs by modifying interfacial characteristics to halide perovskite layers and by demonstrating improved stability of HPSCs.

Perovskite solar cell (PSC) has been considered one of the most promising candidates for the next generation renewable energy device due to its power conversion efficiency over 22% in a short time. Many fabrication methods for the perovskites have been reported, such as one-step deposition, sequential deposition, vapor assisted deposition. The sequential deposition involving the conversion of perovskite from lead halide is particularly attractive because of its controllability of perovskite property via crystallization kinetics. Though previous studies have correlated the microstructure of lead halide, solution concentration, and loading time with the resulting perovskite film, the effect of underlying layer on the sequential deposition seemingly remains few.
In this regard, a series of TiO₂ nanorods with the length from 200 nm to 800 nm on FTO are grown by the hydrothermal process, followed by the sequential deposition of CH₃NH₃PbI₃. In order to investigate the effect of underlying layer, the SEM and XRD are carried out to analyze the properties of the sequential deposited CH₃NH₃PbI₃, such as the morphology, crystallization and conversion level, on a series of TiO₂ nanorods. The UV-Vis and EIS are performed to detect the optical property and charge transport, respectively, of perovskite films deposited on different length of TiO₂ nanorod. J-V characteristics of the perovskite solar cells using different TiO₂ nanorods are tested under AM 1.5G illumination. Our results show that the different length of TiO₂ nanorod can potentially affect the level of PbI₂ residue and the perovskite morphology after the sequential deposition. An optimized length of TiO₂ nanorod is simultaneously beneficial for obtaining a high quality perovskite film along with the improved charge collection. More details of the performance of perovskite solar cells, influence by the underlying TiO₂ nanorods, will be discussed in the presentation.

Organic-inorganic halide perovskites have found increasing potential in a wide range of optoelectronic applications due to their excellent optoelectrical properties e.g., low energy bandgaps (E_g ~ 1.6 eV), low exciton binding energy (<10 meV), long diffusion length (up to 1 μm), and high absorption coefficients (1.5 × 10⁴ cm^-1 at λ = 550 nm). In this context, perovskites solar cells (PSCs) have reached a rapid development of best power conversion efficiency (PCE) with a certified 23.7% efficiency up to date. In this work, we have focused our attention on the design of efficient and stable hole transporting layer (HTL) materials in organic-inorganic PSCs, towards a facile hole extraction and enhanced photocurrent generation. To date, cost ineffectiveness, insufficient hole mobility (~10^-4 cm² V^-1 s^-1), and low electrical conductivity (~10^-5 S cm^-2) have hindered the commercialization of the reference 2,2’,7,7’-Tetrakis-(N,N’-di-4-methoxyphenylamino)-9,9’-spirobifluorene (Spiro-OMeTAD) HTL material. Similarly, conventional additives and dopants e.g., lithium bis (trifluoromethylsulfonyl)-imide (LiTFSI) and 4-tert-butylpyridine (TBP), have been shown to enhance moisture absorption, leading to inherently low stability and reproducibility, and to disturb the equilibrium between the CH₃NH₃PbI₃ and its degraded components, respectively.
In this work, we have developed for the first time a novel and efficient HTL material by optimized doping of a conjugated polymer, poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl)diphenylamine)] (TFB), with a non-hygroscopic p-type dopant, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). The F4-TCNQ-doped TFB PSC exhibited a PCE of 17.46%, which surpassed that of 16.64% and 11.01% for the reference LiTFSI+TBP-doped Spiro-OMeTAD and LiTFSI+TBP-doped TFB-based devices, respectively. The appropriate band alignment of the introduced F4-TCNQ-doped TFB HTL material with the highest occupied molecular orbital (HOMO) of the perovskite was proposed as primary factor to facilitate efficient charge extraction from the perovskite to the electrode as well as increased short-circuit current. Conducted steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) analyses have herein further corroborated a reduced charge recombination. Most importantly, the hydrophobic nature of F4-TCNQ-doped TFB was shown to dramatically improve the long-term stability of the device under an ambient condition with RH of 45%. In detail, the device herein reported maintained ca. 80% of its initial efficiency after 10 days, significantly superior to both LiTFSI+TBP-doped Spiro-OMeTAD (ca. 30%) and LiTFSI+TBP-doped TFB (ca. 10%) counterparts. This simple, yet novel strategy is believed to reflect a promising route for a wide range of highly efficient solar cells and other photovoltaic applications.

Environmental stability remains one of the most significant challenges impeding the commercialization of perovskite solar cells. The presence of defects has been linked to the stability of perovskite devices, for example, the photoinduced formation of superoxide upon oxygen exposure in the presence of iodine vacancy defects has been shown to play a key role in the degradation of methylammonium lead iodide (MAPbI₃) perovskite thin films [1]; however, the details of this process remain unclear. Recently, our group has demonstrated that the type and density of defects on the surface of MAPbI₃ perovskites can be tuned by incrementally adjusting the stoichiometry of the precursor solution [2]. Herein, we exploit this method to elucidate the interaction between specific defects in MAPbI₃ and the environment, and further expand it to understand these effects in bromide-based perovskites (MAPbBr₃).

Our methodology is based on a multi-technique approach. X-ray photoelectron spectroscopy is used to identify the density and type of surface defect in each type of perovskite film. To then isolate the interaction of the defect with the environment, films are exposed to environments of nitrogen mixed with varying (but controlled) oxygen and water content and completed with extraction layers and an electrode. In parallel, the changes in optoelectronic properties are monitored with photothermal deflection spectroscopy and photoluminescence spectroscopy, allowing us to correlate the changes in defect type and density with changes in photovoltaic performance. Finally, the PV performance and optoelectronic properties are monitored over time to assess the shelf-storage stability as a function of defect density and type.

We find that the photovoltaic performance of the MAPbI₃ samples deficient in iodine shows a tremendous drop in the short-circuit current (J_sc) upon exposure to oxygen and light, while samples with excess iodine show performance that is similar to pristine films. While the latter show low photoluminescence quantum efficiency (PLQE) and high Urbach energies, suggesting the presence of deep trap states, these states do not interact with oxygen under illumination. In contrast, the PLQE of iodine-deficient samples initially increases upon exposure to oxygen, indicative of trap healing; but this process is short lived, with significant degradation taking place after 2h, in agreement with the decline in PV performance of these samples. The bromide-based perovskite solar cells show little dependence of the initial photovoltaic performance on the trap density, but upon exposure to air and light, bromide-deficient solar cells undergo a large J_sc boost, resulting in a near tripling of their initial PCE. These results highlight the strong role that chemical composition plays in the stability of perovskite PVs and suggest that defect engineering is a potential strategy for enhanced stability, especially in the single-cation perovskite compositions.

[1] “Fast oxygen diffusion and iodide defects mediate oxygen-induced degradation of perovskite solar cells” N. Aristidou, et al, Nature Comm. 8, 15218 (2017).
[2] “Fractional Deviations in Precursor Stoichiometry Dictate the Properties, Performance and Stability of Perovskite Photovoltaic Devices" P. Fassl, et al, Energy Environ. Sci. 11, 3380 (2018).

Organic-inorganic halide perovskites (OIHP) remains as one of the hottest research topics in the past several years due to their exceptional optoelectronic properties in broad fields relating light emitting, photovoltaic, photo-detecting, gas sensing and even photosynthesis, etc. However, the exist of grain boundaries in OIHP thin films made using current preparation method creates large number of defects/traps, resulting in possible low photo conversion efficiency (PCE) of its corresponding devices. The presence of grain boundaries can also become a “starting point” for structural degradation to take place, which makes the film highly degradable when exposing to excessive environmental stress, such as moisture, heat and light.
In this study, we effectively passivated the grain boundaries in methylammonium lead iodide (MAPI) perovskite thin films using one type of the polyester called polycaprolactone (PCL). We observed the successful attachment of PCL on MAPI grains using FTIR, AFM and (HR)TEM. The blueshifts of C=O group were found both in PCL incorporated MAPI layer and the PCL mixed PbI₂ sample, which indicated the formation of coordination between electron donating O-C=O group on PCL and electron acceptor Pb²⁺ complex in MAPI via Lewis acid and base adduct. We also visualized the presence of PCL polymer on grain boundaries of MAPI by using friction mode AFM and (HR)TEM. The brighter boundaries and darker grains on friction AFM for MAPI with PCL indicated stronger interaction of AFM tip on the boundaries and implied the gathering of softer polymeric materials between the grains, while the pure MAPI barely showed the contrast difference using same AFM operation mode. The HRTEM images on both surface and the focused ion beam (FIB) prepared cross-sectional samples further proved the passivation of PCL at the boundaries with highly crystalline MAPI grain areas being separated by amorphous PCL polymer channels. The passivation has also been confirmed by showing increased photoluminescence (PL) emission intensity and decay lifetime due to reduce of boundary- borne defects/traps, which can act as centers for the non-radioactive recombination. Such optoelectronic improvement by PCL directly promoted the photovoltaic performance of the corresponding device, showing an increased open circuit voltage with the highest PCE exceeding 19%. More importantly, the direct boundary passivation by PCL greatly enhanced the stability of MAPI thin films against three major environmental stresses (moisture, heat and light). All samples with PCL incorporation largely maintained pure MAPI crystal structure with much suppressed growth of PbI₂, while the reference MAPI films became degraded with obvious PbI₂ appearing after the exposure. Moreover, we used the SIMS to reveal the suppression effect of ion migration (especially for iodide ion) due to the increase of the activation energy of MAPI film with PCL passivation. After heating the corresponding devices, the PCL passivated one presented only little, if any, migration of iodide towards the HTM and electrode layer, while significant migration with much stronger iodide ion signal has been detected at the HTM layer for the reference MAPI device.
We believe that the unique chain structure of PCL with the functional ester groups (O-C=O) on backbone offered the outstanding passivation by forming direct backbone attachment of PCL on the grain boundaries of MAPI. The repeating backbone segments composed of six methylene between two attaching ester groups can serve as a strong hydrophobic moisture repellent layer and can also help maintain high structural purity of MAPI when exposing to heat and light, as well as hinder the mobile of iodide ion by increasing the activation energy of MAPI after the direct polymer chain attachment. (This work was supported by the Morin Foundation Trust and the NSF, Inspire program #1344267)

Perovskite-based solar cells have overseen tremendous development reaching power conversion efficiencies exceeding 24% in the laboratory scale. Part of the development was made possible by utilizing passivated SnO₂ as an electron transporting layer. Chlorinated and colloidal precursors such as SnCl₄ and SnO₂ nanoparticles are usually used to fabricate efficient SnO₂ films. In this work, we explore the effect of halogen additives on the passivation properties yielded on SnO₂ films. This was done by employing three novel acetylacetonate-based precursors of which one is halide-free and two are halogenated with Cl and Br respectively. These precursors form stable, non-colloidal solutions that yield a unique SnO₂ film morphology as we show with TEM. Additionally, we demonstrate that the halide additives electrically passivate the SnO₂ film and alter the optimum annealing temperature of SnO₂ film formation. Our optimized SnO₂ films achieved high-performing planar perovskite solar cell devices with a power conversion efficiency of 22.19% (21.4% certified by Newport) with 0.16 cm² active area, and 16.7% for mini-modules with 15 cm² active area.

Organometallic halide perovskites attract much attention in the field of optoelectronics and in photovoltaic research thanks to its high performance. However, perovskite photovoltaics have yet a major obstacle to overcome - stability. The perovskite surface plays an important role in the cells' stability and efficiency. Surface states were previously found to induce recombination and accelerated degradation. Passivation of the surface and interfaces of the perovskite film and neutralizing surface states can be achieved by various techniques. In this research we focus on the effect of passivation using conjugated small molecules on the perovskite photo-stability. The perovskite surface is modified using porphyrin molecules resulting in the formation of new hybrid films. The molecules are added into the precursor solution and are aimed to be located at the perovskite grain boundaries. The additive porphyrins differ by their functional group while each group is designated to passivate specific surface dangling bonds towards increasing the photo-stability. We find that different functional groups differently affect the perovskite crystallization, hence the resulting solar cell efficiency and stability.

The performance of perovskite solar cells (PSCs) depends on the crystallization of the perovskite layer. Herein, we demonstrate an effective photoannealing (PA) process by a halogen lamp. During the PA process, on the one hand, the lower energy photon, that is, near IR up to ∼1015 nm photon, drives the crystallization of the perovskite film, similar to the conventional thermal annealing (TA). On the other hand, the higher energy photon of PA can excite the trapped carriers and release the space charges, thus leading to an ideal perovskite layer with better crystallinity and lower density of defect when compared to that of TA. A maximum power conversion efficiency (PCE) has been obtained to be 20.41% in the CH3NH3PbI3-based planar PSCs based on PA because of the increase of Jsc and Voc, much higher than the control device based on the conventional TA with a maximum PCE of 18.08%. Therefore, this result demonstrates that PA is an effective method to promote the device performances and reduce the fabrication cost, which provides a potential approach for the commercial application of perovskite devices.

Organic-inorganic hybrid lead halide perovskites have become the most sought-after materials for various opto-electronic applications. Their low-temperature (<150°C) and solution processability enable them to function on flexible platforms. The power conversion efficiencies of perovskite solar cells have reached 22%. Interdisciplinary efforts are going into realizing perovskite-silicon tandem cells for the better utilization of the solar bandwidth.
The major drawback of these devices is their inferior stability. The high diffusivity of iodide ions in the perovskite lead to chemical reactions with atmospheric oxygen and moisture. This in turn disintegrates the perovskite structure and hence, photon absorptivity is lost. Encapsulation of these devices does improve the stability but is nowhere near standards needed for commercialization.
The shallow and deep trap states play a major role in determining the performance and stability of perovskite solar cells. These states are created by ionic defects at the at the grain boundaries and surface respectively. The use of Lewis acids and bases have been demonstrated to pacify these defects. The Lewis base interacts with the cation and the acid with the anion. The interaction of a Lewis acid or base at a grain boundaries results in defect states. These states hinder inter-grain transport and act as parasitic resistances.
In this work, we propose to design a bifunctional organic molecule with HOMO and LUMO matching the band edges of the perovskite absorber. The bifunctionality (Lewis acid and base) enables passivation of both cations and anions simultaneously. The trap state passivation inhibits ion migration in the crystal and hence improves the stability of the device. It also helps in impeding trap assisted recombination to in turn enhance device performance. The band edge matching on the HOMO and LUMO levels empowers inter-grain transport of charge carriers. This will result in larger charge carrier diffusion lengths and lower bimolecular recombination. The overall performance and stability of the device is augmented. The use of organic molecule for passivation also increases the hydrophobicity of the perovskite layer. This results in better moisture resistance for the perovskite.
The stability of the passivated perovskite is monitored through JV measurements, X-ray diffraction and absorption measurements. The interaction of the passivation molecule with perovskite is estimated using FTIR spectroscopy and is mapped using an X-ray synchrotron source. The effect of passivation is estimated by calculating the trap density of states and also observing the long-term aging behavior.

With power conversion efficiencies (PCE) of more than 24%, halide perovskites (HAPs) now even rival silicon as photoactive semiconductor in solar cells.^[1] At the same time, serious concerns about material stability are still inevitably linked with HAPs, e.g. when in contact with water etc..^[2,3] The sensitivity of HAPs not only states a challenge with respect to long term stability, but likewise limits the choice of processes for the deposition of subsequent functional materials, such as charge transport layers and electrodes.
Processes involving polar solvents such as water or ethyl acetate are typically excluded as candidates for follow-up processing, as they have been shown to severely damage the HAP. Unfortunately, a wide variety of solvents that would be desirable for “green” upscaling classify as corrosive for HAPs.
In this work we will show that the use of an ultrathin, impermeable, yet conductive AZO/SnO_x hybrid layer that serves as electron transport material is able to mitigate most of the above issues. This is possible due to the unique nature of the ALD grown SnO_x, which is transparent, conductive and, most importantly, physically dense and impermeable to a wide variety of corrosive solvents.^[4] Based on the AZO/SnO_x hybrid we were able to realize perovskite solar cells with semitransparent top electrode based on silver nanowires (Ag NWs), that were spray coated from an aqueous dispersion. The resulting semi-transparent cells showed a PCE exceeding 15%. In a similar sense a carbon electrode could be doctor bladed from a paste based on ethyl acetate. This afforded the first p-i-n (inverted) perovskite solar cells with carbon top-electrode. Most importantly, if both processes were used without the AZO/SnO_x protection layer, the solar cells turn out to be non-functional.
The now functional devices comprising the Ag-NW top electrode reveal a further important issue related to the interface of Ag-NW and SnO_x, which shows rectifying behavior, due to a parasitic Schottky barrier between Ag-NW and SnO_x. We have conducted an in-depth study using Kelvin probe analysis, photoemission spectroscopy and specific unipolar test devices. We demonstrate that the issues associate with the interfacial barrier can either be eliminated by UV-light soaking or by the introduction of a dedicated interlayer with a high carrier density n > 10¹⁸ cm^-3 between Ag-NW and SnO_x.
Taken together, we will present the first perovskite solar cells with a water processed Ag NWs top electrode and as well as the first proof of concept p-i-n (inverted) perovskite solar cell with a carbon top electrode.

[1] NREL Efficiency Chart (https://www.nrel.gov/pv/cell-efficiency.html)
[2] H. Cho et al. Adv. Mater. 2018, 1704587
[3] L. Zhao et al. ACS Energy Lett. 2016, 1, 595
[4] A. Behrendt et al., Adv. Mater. 2015, 27, 5961.

After a decade of metal halide perovskite solar cell research and owing to the intensive research efforts across the world, there is no doubt that power conversion efficiencies (PCEs) comparable to several other photovoltaic technologies (e.g., multicrystalline Si, CIGS, CdTe) are achievable [1]. However, when moving perovskite solar cell technology from laboratory to industrial products, high-PCE, low-cost, and up-scaling are key metrics [2]. At OIST, a team of researchers in the Energy Materials and Surface Sciences Unit is making concerted efforts to develop processes (i) aiming at high PCE, (ii) high-throughput, (iii) minimum batch-to-batch variation, and (iv) compatible with large-area perovskite solar cells and modules. In this talk, we will present our recent progress on the use of chemical vapor deposition CVD [2-4] to fabricate perovskite solar cells and modules.

[1] L.K. Ono and Y.B. Qi*, Research Progress on Organic-Inorganic Halide Perovskite Materials and Solar Cells. J. Phys. D Appl. Phys. 51 (2018) 093001
[2] L. Qiu, L.K. Ono, Y.B. Qi*, Advances and Challenges to the Commercialization of Organic–Inorganic Halide Perovskite Solar Cell Technology. Mater. Today Energy 7 (2018) 169-189.
[3] L. Qiu, S. He, Y. Jiang, D.-Y. Son, L.K. Ono, Z. Liu, T. Kim, T. Bouloumis, S. Kazaoui, and Y.B. Qi*, Hybrid Chemical Vapor Deposition Enables Scalable and Stable Cs-FA Mixed Cation Perovskite Solar Modules With a Designated Area of 91.8 cm² Approaching 10% Efficiency. J. Mater. Chem. A 7 (2019) 6920-6929.
[4] Y. Jiang, M.R. Leyden, L. Qiu, S. Wang, L.K. Ono, Z. Wu, E.J. Juarez-Perez, and Y.B. Qi*, Combination of Hybrid CVD and Cation Exchange for Up-Scaling Cs-Substituted Mixed Cation Perovskite Solar Cells with High Efficiency and Stability. Adv. Funct. Mater. 27 (2018) 1703835.

We will report on the progress on vapor phase deposited perovskites, including low bandgap Pb-Sn and wider bandgap multiple cation versions. The performance of these materials in both single and double junction solar cells will be reported. The influence of the use of thin organic charge extraction layers, using strong dopants, ionic compounds and conjugated polymers will be described.

Developments in perovskite photovoltaics dominantly rely on thin films of polycrystalline morphology. The record efficiency for PSCs which is 24.2% [1] is still far from their Shockley-Queisser (SQ) limit which is ~30.5% power-conversion efficiency (PCE) for methylammonium lead iodide (MAPbI3).[2] Despite the marked efforts in improving polycrystalline perovskite films, they have significant parasitic non-radiative recombination due to their inherent grain size and surface defects.[3] In contrast to polycrystalline perovskite films, single-crystal perovskites are orders of magnitude superior in terms of defect density, charge carrier mobility, and diffusion length.[4] These advantages make the single-crystal perovskites a strong candidate to achieve their SQ limit. Unfortunately, there are a limited amount of studies on single-crystal perovskite solar cells (SC-PSC), most of which show inferior PCEs compared to polycrystalline counterpart. The highest PCE so far for SC-PSCs was reported in 2017 is 17.8% by Huang and co-workers.[5]

Here, we achieved highly efficient SC-PSCs with PCEs reaching 21.1% and FFs of up to 84.3% (under 1-sun illumination).[6] This is the first time for an SC-PSC to exceed the 20% PCE benchmark. These devices, based on a ~20 micrometers-thick MAPbI3 single-crystal active layer in an inverted p-i-n structure, set a new record for SC-PSC efficiency and a new potential benchmark for FFs that PSCs should aim for, which polycrystalline PSCs have struggled to achieve. We describe the crucial fabrication steps and measurement conditions required to achieve these high efficiencies. Additionally, our study is also answering an ongoing debate over the diffusion length of carriers in perovskite single-crystals. The different methods like 1D, SCLC, and 3D methods showed a diverse range from several microns to mm. Our work confirms the extraction length should be over 40 μm since the external quantum efficiency (EQE) is over 80% all over the absorbed spectrum. While there is still room for substantial interfacial optimization, our findings highlight the promise of single-crystals for advancing perovskite optoelectronic technology, which could be a parallel growing path to the one taken by their polycrystalline counterparts.


[1] NREL Best Research-Cell Efficiencies. 2019, https://www.nrel.gov/pv/cell-efficiency.html
[2] Pazos-Outón, L. M.; Xiao, T. P.; Yablonovitch, E., Fundamental efficiency limit of lead iodide perovskite solar cells. J. Phys. Chem. Lett. 2018, 9 (7), 1703-1711.
[3] Stranks, S. D., Nonradiative losses in metal halide perovskites. ACS Energy Lett. 2017, 2 (7), 1515-1525.
[4] Shi, D.; Adinolfi, V.; Comin, R.; Yuan, M.; Alarousu, E.; Buin, A.; Chen, Y.; Hoogland, S.; Rothenberger, A.; Katsiev, K., Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals. Science 2015, 347 (6221), 519-522.
[5] Chen, Z.; Dong, Q.; Liu, Y.; Bao, C.; Fang, Y.; Lin, Y.; Tang, S.; Wang, Q.; Xiao, X.; Bai, Y.; Deng, Y.; Huang, J., Thin single crystal perovskite solar cells to harvest below-bandgap light absorption. Nat. Commun. 2017, 8 (1), 1890.
[6] Chen, Z.; Turedi, B.; Alsalloum, A.; Yang, C.; Zheng, X.; Gereige, I.; AlSaggaf, A.; Mohammed, O. F.; Bakr, O. M., Single-Crystal MAPbI3 Perovskite Solar Cells Exceeding 21% Power Conversion Efficiency. ACS Energy Letters 2019, 4, 1258-1259.

In recent years, halide perovskites have upstaged decades of development in solar cells by reaching power conversion efficiencies that surpass polycrystalline silicon performance. The efficiency improvement in the perovskite cells is related to repeated recycling between photons and electron-hole pairs, reduced recombination losses and increased carrier lifetimes. Here, we demonstrate a novel approach towards enhancing the efficiency of perovskite solar cells by invoking the Forster Resonance Energy Transfer (FRET) mechanism for predicting the strength and range of exciton transport between separated molecules based on Born-Oppenheimer approximation. FRET occurs in the near-field region as photosensitive protein, bacteriorhodopsin (bR) and perovskite have similar optical gaps. Titanium dioxide functionalized with bR protein is shown to accelerate the electron injection from excitons produced in the perovskite layer. FRET predicts the strength and range of exciton transport between separated perovskite and bR layers. We show that the cells incorporating bR/ TiO2 layers exhibit much higher photovoltaic performance. These results open the opportunity to develop a new class of bio-perovskite solar cells with improved performance and stability.

Perovskites have emerged as low-cost, high efficiency photovoltaics with certified efficiencies of 24.0% approaching already established technologies. The perovskites used for solar cells have an ABX₃ structure where the cation A is methylammonium (MA), formamidinium (FA), or cesium (Cs); the metal B is Pb or Sn; and the halide X is Cl, Br or I. Unfortunately, single-cation perovskites often suffer from phase, temperature or humidity instabilities. This is particularly noteworthy for CsPbX₃ and FAPbX₃ which are stable at room temperature as a photoinactive “yellow phase” instead of the more desired photoactive “black phase” that is only stable at higher temperatures. Moreover, apart from phase stability, operating perovskite solar cells (PSCs) at elevated temperatures (of 85 °C) is required for passing industrial norms.
Recently, double-cation perovskites (using MA, FA or Cs, FA) were shown to have a stable “black phase” at room temperature.(1,2) These perovskites also exhibit unexpected, novel properties. For example, Cs/FA mixtures supress halide segregation enabling band gaps for perovskite/silicon or perovskite/perovskite tandems.(3) In general, adding more components increases entropy that can stabilize unstable materials (such as the “yellow phase” of FAPbI₃ that can be avoided using the also unstable CsPbI₃). Here, we take the mixing approach further to investigate triple cation (with Cs, MA, FA) perovskites resulting in significantly improved reproducibility and stability.(4) We then use multiple cation engineering as a strategy to integrate the seemingly too small rubidium (Rb) (that never shows a black phase as a single-cation perovskite) to study novel multication perovskites.(5)
One composition containing Rb, Cs, MA and FA resulted in a stabilized efficiency of 21.6% and an electroluminescence of 3.8%. The V_oc of 1.24 V at a band gap of 1.63 eV leads to a very small loss-in-potential of 0.39 V, one of the lowest measured on any PV material indicating the almost recombination-free nature of the novel compound. Polymer-coated cells maintained 95% of their initial performance at 85°C for 500 hours under full illumination and maximum power point tracking. This is a crucial step towards industrialisation of perovskite solar cells.

Lastly, to explore the theme of multicomponent perovskites further, molecular cations were revaluated using a globularity factor. With this, we calculated that ethylammonium (EA) has been misclassified as too large. Using the multication strategy, we studied an EA-containing compound that yielded an open-circuit voltage of 1.59 V, one of the highest to date. Moreover, using EA, we demonstrate a continuous fine-tuning for perovskites in the "green gap" which is highly relevant for lasers and display technology.
The last part elaborates on a roadmap on how to extend the multication to multicomponent engineering providing a series of new compounds that are highly relevant candidates for the coming years.(6,7)
(1) Jeon et al. Nature (2015)
(2) Lee et al. Advanced Energy Materials (2015)
(3) McMeekin et al. Science (2016)
(4) Saliba et al., Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency. Energy & Environmental Science (2016)
(5) Saliba et al., Incorporation of rubidium cations into perovskite solar cells improves photovoltaic performance. Science (2016).
(6) Turren-Cruz et al. Methylammonium-free, high-performance and stable perovskite solar cells on a planar architecture. Science (2018)
(7) Saliba. Polyelemental, Multicomponent Perovskite Semiconductor Libraries through Combinatorial Screening. Advanced Energy Materials (2019)

Halide-based perovskite semiconductors (e.g., CH₃NH₃PbI₃ and lower-dimensional analogs) have attracted substantial recent interest for photovoltaic and other optoelectronic application, due to large optical absorption coefficients, high carrier mobilities, long minority carrier lifetimes, and relatively benign defects and grain boundaries achieved within these materials. Coupled with the demonstrated high performance of associated films/devices, the ability to employ a diverse array of simple and low-temperature film deposition approaches provides an opportunity for very low processing costs and wide-ranging substrate form factors (e.g., flexible, light weight, curved surface). This talk will explore several recently described pathways for versatile thin-film deposition [1], including additive engineering [2], melt-processing [3,4], lamination and resonant-infrared matrix-assisted pulsed-laser evaporation (RIR-MAPLE) [5]. Considerations will include focus on grain growth, interfacial reactivity/passivation and deposition pathways for more complex 2-D hybrid perovskite films. Advances in film deposition and control offer promise of enhanced device performance, improved operational stability and greater device form factor flexibility.

[1] W. Dunlap-Shohl, Y. Zhou, N. P. Padture, D. B. Mitzi, Chem. Rev. 2019, 119, 3193.
[2] Q. Han et. al., Energy Environ. Sci. 2017, 10, 2365.
[3] T. Li et. al., Chem. Sci. 2019, 10, 1168.
[4] T. Li et. al., Chem. Mater. 2019, 31, 4267.
[5] W. A. Dunlap-Shohl et. al., Mater. Horiz. 2019, DOI: 10.1039/C9MH00366E (Adv. Article).

Organic-Inorganic Perovskite solar cells (PSCs) have attracted great interest over the past few years due to their outstanding power conversion efficiencies. However, operational stability still needs to be improved for broad commercial application. A commonly observed phenomenon for PSCs is hysteresis in their current–voltage characteristics, which has been suggested to be related to issues with long-term stability. Understanding of the processes involved in hysteresis may also guide the way to further improving the cells efficiencies. We fabricate planar pin organic–inorganic hybrid PSCs using three different organic transport layer architectures by vacuum deposition in a new multi-source tool and measure their hysteresis and long-term stability behavior. The organic transport layer architectures use controlled doping methods based on our pioneering work on organic doping, which is currently broadly commercially used in OLED displays.
Our results show that all-vacuum-deposited PSCs show much weaker hysteresis than in previous studies, where many types of PSCs were involved. For our devices, we also report an inverted hysteresis, where efficiency is somewhat higher for a voltage sweep from short circuit to forward-bias conditions than for the reverse sweep at lower scan rates, which has not commonly been observed in previous studies of PSCs.

High-throughput experimentation can be of great advantage for exploring the complex compositional phase space of halide perovksites. Here we demonstrate the application of such techniques for the investigation of CsPbI₃ thin-films and solar cells. Cesium lead iodide thin films were fabricated with a lateral compositional gradient by co-evaporation of CsI and PbI₂ onto a non-rotating substrate. The composition, crystal structure, grain size, charge carrier mobility, lifetime and photoluminescence external quantum yield (PLQY) were determined by mapping these properties on the 5x5cm samples. Correlation of these results provides complete structure-property relationships and shows that stable high quality γ-CsPbI₃ can be obtained by Cs-rich low temperature deposition, without need of a high temperature annealing step. 12% efficient solar cells are demonstrated based on these results.[1]
Furthermore, samples obtained at different substrate temperatures, containing both the γ-phase and the δ-phase, were annealed at 320 °C for a comparison of the properties of the as-deposited γ-phase with the γ-phase obtained after high temperature annealing. It is found that grain sizes increase from ~100 nm in the as-deposited films to ~500 nm in the annealed films, accompanied by an increase in the PLQY by about an order of magnitude (only). The degradation/conversion to the δ-phase is investigated systematically for various deposition temperatures and Cs/Pb composition ratios in a humidity-controllable environmental chamber. Overall, a significantly enhanced stability is observed for the Cs-rich deposited perovskite samples.

[1] P. Becker, J. A. Márquez, J. Just, A. Al-Ashouri, C. Hages, H. Hempel, M. Jošt, S. Albrecht, R. Frahm, and T. Unold, Adv. Energy Mater. 2019, 1900555, DOI: 10.1002/aenm.201900555.

This year, we are celebrating 10 years of perovskite solar cell (PSC). Since our first discovery of PSC in 2009, enormous efforts have been put into different aspects of PSCs and the progress has been incredibly fast on all fronts. While preparing a comprehensive review¹ on the background, on-going R&Ds, and future direction of PSC research recently, we realized that, although efficiency level has gone beyond 24%, PSCs face serious challenges of practical stability and durability required for industrialization. Although compositional engineering of perovskites by mixing different cations and anions, using modulator molecules and mixing 2D and 3D structures have doubtlessly improved the stability of perovskites against heat and moisture, use of organic moieties still remain a challenge to improve the stability further. Intrinsic stability of the perovskite crystal structure and robust properties of carrier transport materials are going to be the keys to the long term durability of the device. In this respect, use of all-inorganic perovskite materials and combination with dopant-free carrier transport materials are highly desired. We have conducted some work in this direction which includes stabilization of CsPbI₃ black phase ² and use of dopant-free hole transport materials (HTMs). Dopant-free HTMs combined with all-inorganic perovskites have yielded sufficiently high efficiency of 15%. Intrinsic thermal stability of the device was improved without using diffusible dopants. In our collaboration with JAXA, stability of perovskites was investigated in space environment for satellite applications. Here, organic cations in perovskite such as methyl ammonium are instable under exposure to vacuum and high temperatures >100^oC. Perovskite materials demonstrate its high stability against exposure to high energy particle radiations (proton and electron beams) up to dose of 10¹⁵ particles/cm² due to use of thin absorber films that can avoid accumulation of particles and also exhibit defect tolerant nature. ³ The lecture will also introduce our current efforts in making PSCs based on both lead and lead-free perovskites, and future perspectives of perovskite photovoltaics.
A. K. Jena, A.，Kulkarni, T. Miyasaka, Chem. Rev. 2019, 119, 3036–3103.
A. K. Jena, A. Kulkarni, Y. Sanehira, M. Ikegami, and T. Miyasaka, Chem. Mater. 2018, 30, 6668-6674.
Y. Miyazawa, M. Ikegami, H.-W. Chen, T. Ohshima, M. Imaizumi, K. Hirose, T. Miyasaka, iScience 2018, 2, 148-155.

Halide Perovskites (HaPs) present a remarkable case of just stable (against decompositon into binaries) compounds that can function as active component for PV, light-emission and radiation detection , all demanding functions, over time periods that seem incompatible with their free enrgy of formation (again, from the binaries). Actually, the Achilles heel of the materials is their surface, true to Pauli's famous dictum, because of the law of mass-action: as long as the system remains a hermetically close one, it can withstand the onslaught of electronic carriers and photons. Add to that that it are to a large extent the interfaces that make the devices function (again following a famous dictum, this time from Kroemer) and.. those are made with surfaces. Thus, what might appear esoteric materials chemistry and physics issues, are highly relevant for device design. In principle, apart from the remarkable self-healing ability (known to some exent also for CIGS) this is not new as nearly every (inorganic) semiconductor material that we use today went through the process of taming its surfaces to get control over interfaces made with them. In this talk I will, depending on developments in the half year between abstract writing and presentation, present data on the self-healing and (de)stabilization processes of HaPs and put their behaviour in perspective with respect to some other semiconductors, to arrive at insights that should be useful to device design and building.

Surfaces of metal halide perovskites (MHP) present an interesting set of questions and challenges that are only beginning to be addressed, namely the existence, origin, density and energy of electronic surface gap states. MHP surfaces are prone to ion diffusion and chemical reactions, and to degradation under various environmental conditions and probing tools, all of which likely induce some density of electronic gap states. In sufficient density, these states affect the electronic structure and optoelectronic performance of all MHP interfaces and devices. There is currently no consensus on potential profile and occurence of band bending in perovskite films, from substrate to surface. Available data point to a strong dependence of these profile on the nature and surface potential of the substrate on which the films are fabricated, and on the perovskite processing conditions. Limits in Fermi level excursion have been observed and also point to some low density of (surface or bulk) gap states.[1] To address some of these issues, several groups have started to use organic molecular dopants (reductants and oxidants), as “gentle” probes of the electronic occupation and density of surface states, and as modifier agents of these states. [2,3] This talk describes our recent investigations of the interaction between molecular reductants (mostly [RuCp*Mes]₂) and oxidants (Mo(tfd)₃ and derivatives) and surfaces of 3D MHPs CsPbBr₃ and FA_xMA_1-xPb(I_yBr_1-y)₃, and the 2D Ruddlesden-Popper phase BA₂PbI₄. Using a combination of electron spectroscopies and contact potential measurements, we determine the sign and magnitude of surface photovoltage occurring at these free surfaces, and the extent of surface energy level shifts resulting from charge exchange with the molecular dopants. We also show evidence obtained via photoemission spectroscopy of a density of (surface) filled states above the valence band maximum of FAPbBr₃ following electron bombardment, and of MAPbI₃, likely due to the presence of DMSO in the precursor solution. The role of surface doping in changing the occupation of these states will be discussed.

[1] Zohar et al., ACS Energy Lett. 4, 1 (2019)
[2] Zu et al., ACS Appl. Mat. & Interf., 9, 41456 (2017)
[3] Perry et al., Adv. Electron. Mater. 4, 1800087 (2018)

Semiconducting metal-halide perovskites present various types of chemical interactions which give them a characteristic fluctuating structure sensitive to the operating conditions of the device, to which they adjust. This makes the control of structure-properties relationship, especially at interfaces where the device realizes its function, the crucial step in order to control devices operation. In particular, given their simple processability at relatively low temperature, one can expect an intrinsic level of structural/chemical disorder of the semiconductor which results in the formation of defects.

Here, first I will summarize our understanding of the nature of defects and their photo-chemistry, which leverages on the cooperative action of density functional theory investigations and accurate experimental design. Then, I will show the correlation between the nature of defects and the observed semiconductor instabilities. Instabilities are manifested as light-induced ion migration and segregation, eventually leading to material degradation under prolonged exposure to light. Understanding, controlling and eventually blocking such material instabilities are fundamental steps towards large scale exploitation of perovskite in optoelectronic devices. By combining photoluminescence measurements under controlled conditions with ab initio simulations we identify photo-instabilities related to competing light-induced formation and annihilation of trap states, disclosing their characteristic length and time scales and the factors responsible for both processes. We show that short range/short time defect annihilation can prevail over defect formation, happening on longer scales, when effectively blocking undercoordinated surface sites, which act as a defect reservoir. Finally, based on such knowledge, I will discuss different synthetic and passivation strategies which are able to stabilize the perovskite layer towards such photo-induced instabilities, leading to improved optoelectronic material quality and enhanced photo-stability in a working solar cell.

The surface of hybrid perovskites plays a crucial role in the performance and stability of optoelectronic devices, as it strongly influences the recombination rate of excited charge carriers. Recently, it has been reported that molecular ligands such as benzylamine are capable of significantly reducing the surface trap state density in thin films. Here I will report on the mechanism that governs the surface passivation of hybrid perovskites by benzylamine. To this end, we developed a versatile approach to investigate the influence of benzylamine passivation on the well-defined crystal surface of freshly cleaved methylammonium lead tribromide single crystals. We show that benzylamine is capable of permanently passivating surface trap states in these single crystals, resulting in enhanced photoluminescence intensities and charge carrier lifetimes. Additionally, we show that exposure of the perovskite surface to benzylamine leads to replacement of the methylammonium cations by benzylammonium, thereby creating a thermodynamically more stable two-dimensional perovskite (BA)2PbBr4 on the surface of the 3D crystal. This conversion from a 3D to 2D perovskite drives an anisotropic etching of the crystal surface, with the {100} planes being most prone to etching.

Tungsten trioxide electron-selective transport layer (ETL) was formed by sol-gel method using acetylated peroxotungstic acid (APTA) solution as a precursor. The formed layer was in triclinic phase, as shown by the x-ray diffraction patterns. Both compact and mesoporous structure can be made using the same precursor solution and process, but with the Polyehtylene glycol as an additive for the mesoporous films. Electronic and optoelectronic properties of the synthesized WO₃ ETL was determined from electrochemical impedance spectroscopy, photoluminescence, and UPS measurement. Three types of WO₃ layer was used for the cell fabrication, which were in compact, mesoporous, and mesoporous/compact double layer. Also, the effect of TiO₂ interlayer for the cell performance was also studied.

Much research related to perovskite solar cells follows a familiar pattern. A group of postdocs and PhD students enter the lab where they with skill and determination meticulous handcraft a staggering number of solar cells, where after they carefully measure the device characteristics of the cells. This process triggers the writing of papers in which insights are summarised, and where a subset of the generated data is used for plotting figures and filling tables. Over the last ten years, this has been repeated about 10000 times. Sadly, much of the data presented in figures are non-trivial to extract, and much of the raw data newer leaves the hard drives of the students doing the work, and when they move on, the data is many times forever lost in the growing heap of unconsidered data. We firmly believe that much could be gained if more of this data could be stored in one place where it is easily accessible for the research community. So is also an increasing number of funding agencies. Such a collection of data could lead to new insights that are hard to get when the data is scattered over thousands of articles, and it could thus be a way to accelerate the pace of development. In this project, we are working on creating a platform for what could be described as a Wikipedia of perovskite device data. We will in this presentation describe what we have created, what can be conclude from the data so far accumulated, as well as the potential benefits for the perovskite community and beyond if this approach to data sharing reach full penetration in the field.

With the advent of the deployment of halide perovskites in optoelectronics, various combinations of precursor halide salts have been tried to tune the perovskite’s photophysical properties which include changing the stoichiometric ratio of different ions that make up the perovskite structure. Simple compositional engineering opens a new world to tune the bandgap from 1.1 to 3.0 eV making the fabrication of all perovskite tandem devices and IR-RGB LED’s possible. One way to achieving efficient and stable perovskite photovoltaic devices is through improving the crystallinity and morphology of the active absorber layer. Due to the different chemistry of various halide precursors in solution, it is difficult to develop a common processing condition for thin film deposition of all the compositionally engineered perovskite.

In this work, we demonstrate a common post-deposition vapor annealing technique that improves the quality of pure and binary MASn_xPb_1-xI₃ perovskite thin-films. MAPbI₃ perovskite forms an optically bleached frustrated intermediate complex (MAPbI₃.xMA) when exposed to methylamine (MA) vapor for 3 seconds at room temperature. Upon removal at 25°C, the complex decomposes and gives back MAPbI₃ parent phase in the next 2-3 seconds with degassing of MA gas. This increases the surface coverage from 90 to 100% with uniform grains of 200 nm size as compared to needle-like non-uniform crystals obtained in as-deposited control films. MA exposure improved the crystallinity and carrier lifetime of these films by two times. The grain sizes are further increased to 10-15 microns by prolonging the recrystallization time to 90 minutes in a MA filled pressure-controlled glass reactor heated at 140°C.

In the case of pure MASnI₃ films, the MA exposure results into optically bleached stable Lewis pair amorphous complex of SnI₂.xMA at room temperature, with MAI crystalizing out of the film as confirmed by XRD. We find that prolonged heating of MA exposed film at 140-150°C breaks the complex back to SnI₂ which simultaneously reacts with MAI resulting into MASnI₃ parent phase. Annealing below 150°C results into partial conversion to perovskite with remnant MAI primary XRD peak appearing at ~10°. Annealing beyond 150°C leads to degradation of films into SnI₂. A step-annealing technique on MA exposed films is employed whereby the temperature is progressively increased from 25°C to 150°C and maintained at 150°C for 60 minutes to control the nucleation and grain-growth kinetics. This technique increases the grain size from 100-150 nm in as-deposited films to 1-2 microns in step-annealed films along with 7 times improvement in film crystallinity. Due to lower grain boundary density, the oxygen and moisture permeation is checked leading towards 75 nm of Burstein-moss red-shift in the absorption onset. This reveals that step-annealed MASnI₃ films (1.2 eV) are fairly less doped as compared to degenerate p-type self-doped as-deposited MASnI₃ films (1.3 eV).

For the binary MASn_0.5Pb_0.5I₃ (1.2 eV) perovskite films, the MA exposure forms an intermediate stable complex, but it doesn’t show any characteristic XRD peak of MAI instead, an unknown peak at 11.3° is observed, which diminishes as the film is annealed. At 85°C, the parent MASn_0.5Pb_0.5I₃ perovskite phase is fully recovered. Annealing below 85°C results into partial conversion to perovskite, whereas annealing beyond 85°C results into the small appearance of PbI₂ peak at 12.6° suggestive of degradation onset. The recrystallization temperature increases with increase in Sn content in MASn_xPb_1-xI₃ films from 25°C, 85°C to 150°C for x = 0, 0.5 and 1.0 respectively suggestive of different affinity towards MA following different complex formation and recrystallization mechanism. The MA vapor annealing method provides a way to obtain high quality pure and binary MASn_xPb_1-xI₃ perovskite thin-films to be employed in tandem and singlet fission devices leading towards enhanced efficiency.

Hybrid organic-inorganic perovskite materials have taken the photovoltaic (PV) research world by storm, setting new power conversion efficiency (PCE) records at an unprecedented rate. However, significant irreproducibility issues continue to plague the field, even at world-leading laboratories. These issues are often due to small, unnoticed factors. For example, our group recently demonstrated that fractional variations in the precursor solution stoichiometric ratio significantly impact subsequent device performance.¹ These findings motivated us to look for other hidden variables affecting the performance and reproducibility of perovskite PVs. Herein, we seek to further our understanding of the root causes of this irreproducibility by closely examining the fabrication procedure of cutting edge triple cation perovskite solar cells. Through a variety of characterization techniques including electron microscopy, x-ray diffraction, and others, we reveal that subtle differences during the critical “antisolvent” fabrication step lead to significant microstructural changes, and thus dramatically affect the final PV performance². By deliberately introducing device-to-device variation in this step, we find that perovskite film formation is highly altered, and show that the resulting PCE can vary from over 21% to less than 15%, for devices fabricated in the same batch and from the same precursor solutions. Additionally, we performed these experiments with 15 different antisolvents, and find that they fall into four categories based on their physical and chemical properties, with each possessing significantly different optimal processing conditions. Furthermore, by careful control over the antisolvent fabrication step, high performance (~20% PCE) can be obtained using almost all studied antisolvents. These results challenge some of the prevailing beliefs currently held by the research community, regarding not only what the highest performing antisolvents are, but also in general the role of the antisolvent in fabricating high performance perovskite solar cells.

(1) Fassl, P.; Lami, V.; Bausch, A.; Zhiping, W.; Klug, M. T.; Snaith, H. J.; Vaynzof, Y. Fractional Deviations in Precursor Stoichiometry Dictate the Properties, Performance and Stability of Perovskite Photovoltaic Devices. Energy Environ. Sci. 2018, DOI: 10.1039/c8ee01136b.
(2) Taylor, A. D.; Sun, Q.; Paulus, F.; Vaynzof, Y. Understaing the Role of Antisolvent Quenching in Film Formation, Device Performance, and Reproducibility of Triple Cation Perovskite Solar Cells. Submitted, 2019.

Despite rapid advancements in power conversion efficiency in the last decade, perovskite solar cells still perform below their thermodynamic efficiency limits. Non-radiative recombination, in particular, has limited the external radiative efficiency and open circuit voltage in the highest performing devices. I will review the historical progress in enhancing perovskite external radiative efficiency and determine key strategies for reaching high optoelectronic quality. Specifically, I will focus on non-radiative recombination within the perovskite layer and highlight novel approaches to reduce energy losses at interfaces and through parasitic absorption. I will highlight a rapid photoluminescence method capable of quantifying non-radiative loss throughout the device stack. By strategically targeting defects, it is likely that the next set of record performing devices with ultra-low voltage losses will be achieved.

Metal halide perovskite solar cells (PSCs) have attracted much attention due to the rapid increase in their power-conversion efficiencies (PCEs), and their amenability to solution-processing and low-cost manufacturing. Typical lead (Pb)-based 3D perovskites show an onset of light absorption around 800 nm; extending light absorption beyond 800 nm into the near-infrared (NIR) of such photoactive layers should increase photocurrent generation and further improve photovoltaic efficiency. While efforts to tune and formulate new materials compositions have resulted in tin (Sn)-containing perovskites with narrower bandgaps than their Pb-counterparts, these new materials exhibit low chemical stability, and are often more prone to oxidation compared to Pb-based perovskites. Tandem structures composed of two or more sub-cells with complementary absorption profiles serially connected have also demonstrated enhanced photo-response compared to single-junction Pb-based perovskite solar cells. The use of a tandem structure, however, necessarily complicates processing and dramatically increases device complexity. There currently does not exist a simple and effective strategy to enhance the NIR photovoltaic response of PSCs. Here, we demonstrate a facile approach to incorporate a NIR-chromophore that is also a Lewis-base, namely IEICO-4F, into an active layer comprising CsFAMAPbI₃ to broaden its photo-response and increase its photovoltaic efficiency. Given the energy-level alignment between the organic chromophore and perovskite, efficient hole and electron transfer take place readily. Compared with control experiments conducted on PSCs without IEICO-4F, these solar cells generate photocurrent in the NIR, beyond the band edge of perovskite. Given the Lewis-basic nature of IEICO-4F, its addition to the photoactive layer also effectively passivates perovskite defects. These films thus exhibit significantly reduced trap densities, enhanced hole and electron mobilities, and suppressed illumination-induced ion migration. As a consequence, perovskite solar cells with IEICO-4F exhibit an enhanced PCE of 21.6% relative to those without IEICO-4F at 20.0%, and substantively improved operational stability under continuous one-sun illumination. Borrowing from the organic photovoltaic playbook, we have successfully demonstrated the effectiveness of incorporating a narrow bandgap chromophore to extend the photoresponse of perovskite absorber. The judicious selection of the organic chromophore simultaneously increases photovoltage and photocurrent generation to PSCs, and imbues enhanced stability to their operation.

Improving the quality of the perovskite active layer is crucial to obtaining high performance perovskite solar cells (PSCs). In this work, by introducing formic acid into the formamidinium lead iodide (FAPbI₃) precursor solution, we managed to achieve reduced colloidal size in the solution, leading to more uniform deposition of FAPbI₃ film with lower trap state density and higher carrier mobility. The solar cells based on the FAPbI₃ absorber layer modified with formic acid show significantly better photovoltaic performance than that on the reference FAPbI₃ film without formic acid. The device performance shows a close correlation with the colloidal size. Within the range studied from 6.7 to 1.0 nm, the smaller the colloidal size is, the higher the solar cell efficiency. More specifically, the cell efficiency is improved from 17.82% for the control cell without formic acid to 19.81% when 0.764 M formic acid was used. Formic acid has also been added into a CH₃NH₃PbI₃ (MAPbI₃) precursor solution, which exhibits a similar effect on the resulting MAPbI₃ films and solar cells, with efficiency improved from 16.07% to 17.00%.

A novel alcohol/water-soluble small molecule was obtained using a p-type planar backbone. The synthesized molecule was dissolved in organic solvents and highly polar solvents. The 3,3'-(((3,3'''-dimethyl-[2,2':5',2'':5'',2'''-quarterthiophene]-5,5'''-diyl)bis(4,1-phenylene))bis(oxy))bis(N,N-dimethylpropan-1-amine) (QTA) film exhibited a red-shifted spectrum compared with the solution spectrum owing to its many more planar molecular conformations in the solid state. According to X-ray diffraction (XRD) measurements, the QTA film showed sharp diffraction peaks near 3.6–11.0°, which indicates the formation of an interdigitated and ordered structure as an out-of-plane peak (100) due to the alkyl side chain of the quarter-thiophene backbone. A comprehensive analysis of the out-of-plane and in-plane XRD data suggests that a large fraction of the QTA derivatives was oriented edge-on relative to the substrate. A photovoltaic device containing QTA exhibited an open-circuit voltage of 0.85 V, current density of 15.5 mA/cm², fill factor of 62.9%, and power-conversion efficiency of 8.4%. The photovoltaic device containing the QTA derivative exhibited improved power conversion efficiency compared with those containing PFN (8.0%) due to the ordered orientation and compact molecule packing of QTA. Moreover, the PCE values of the device with QTA decreased by only approximately 91 ~ 92 % after 450 hours, which showed superior durability compared with that with PFN (68 ~ 69 % after 450 hours) because of the electrochemical stability of QTA.

Organic-inorganic hybrid halide perovskite has emerged as a light harvest material for solar energy. Perovskite has a lot of advantages such as low manufacturing cost, unique photoelectric properties and so on. Those properties are related to the morphology of perovskites such as grain size, pinhole, and thickness. Moreover, the most common method to synthesis perovskite is a solution process method. In that method, anti-solvent plays a critical role to get the optimal perovskite layer. The kind and volume of anti-solvent have been studied in recent years. Only a few people study influence about dropping speed and time of the anti-solvent. In our research, we use a fancy perovskite, triple cation perovskite (Cs_X(MA_0.17FA_0.83)_100-XPb(I_0.83Br_0.17)₃). Three anti-solvents (TL, TFT, and 1,2-DCB) were used to study which is the best one for the triple cation perovskite. We try the anti-solvent dropping speed from 0.6 ml/min to 4.8 ml/min under a constant time condition and dropping time from the 10s to 60s in certain dropping speed. The changes of morphology and phase have been observed. Grain size is decreased by increasing the speed and time. The density of pinhole and thickness also decreased. The amount of black α-FAPbI3 phase is increased when we increase speed and time. We can show that the performance of perovskite solar cell is increased to 7.66% in the normal environment by changing the anti-solvent dropping speed and time.

Organic-inorganic metal halide perovskite solar cells are an emerging photovoltaic technology that has the potential to compete with commercially available silicon solar cells. Over the last decade, perovskite solar cells have have undergone significant development; reaching a high power conversion efficiency of 24.2% with low material and processing costs.¹ However, most of these certified solar cells are processed from solution, depositing the perovskite layer from solvents such as dimethyl sulfoxide (DMSO) or dimethyl formamide (DMF). Solvent-free, vacuum-based methods are mature technologies widely adopted in the semiconductor industry but marginally explored for perovskite optoelectronics.²Its suitability for large-scale fabrication of high-purity systems merges with the ability of precise control over film thickness and composition, low-temperature processing and the possibility of preparing multilayer structures.³ All these advantages call for more in-depth investigation of this alternative device preparation method. Here we present a detailed study of the chances and challenges of the vacuum-deposition process of hybrid inorganic/organic lead halide perovskites, which once completed, will facilitate the transfer from lab scale to large-scale application.
¹Best Research-Cell Efficiency Chart. Available online: https://www.nrel.gov/pv/cell-efficiency.html (accessed on 5 June 2019).
²J. Ávila; C. Momblona; P. P. Boix; M. Sessolo; H. J. Bolink, Joule 2017, 1, 431-442.
³O. Malinkiewicz, A. Yella, Y. H. Lee, G. M. Espallargas, M. Graetzel, M. K. Nazeeruddin, H. J. Bolink, Nat. Photonics 2014, 8, 128–132.

Perovskite solar cells based on methylammonium lead iodide (CH₃NH₃PbI₃) and related materials have emerged as an exciting development for next generation photovoltaic technologies. Solar cells based on them have achieved impressive energy conversion efficiencies, but their stability is still limited. Understanding degradation mechanisms in such materials is key to developing strategies to increase their lifetime. The present work reports on the nanoscale characterization of perovskite photovoltaic films with respect to their stability and degradation mechanisms. We investigated the local conductance and surface potential variation of perovskite films at the nanoscale using conducting atomic force microscopy (CAFM) and Kelvin probe force microscopy (KPFM). CAFM measurements revealed that the current is larger at grain boundaries. The surface potential at grain boundaries and on top of the grain is almost similar which suggests a negligible energy barrier at grain boundaries. We investigated the effect of sunlight exposure on the nano-scale conductance and surface potential of perovskite thin films towards better understanding of photo-induced degradation mechanisms.

A high performance heterojunction ultraviolet (UV) photodetector was fabricated by growing vertical aligned ZnO nanowire arrays (ZnO NWAs) on ITO substrate followed by deposition of novel wide-bandgap inorganic perovskite Cs₂SnCl₆ nanoparticles (NPs) thin film on the top. The ZnO-Cs₂SnCl₆ heterojunction photodetector demonstrated a high external quantum efficiency of 86% at the wavelength of 378 nm and fast response speed with rise and fall time on the order of µs. In addition, the photodetector achieved a large linear dynamic range of 178dB. The excellent performance of this photodetector originates from enhanced separation and collection efficiency of the photo-generated carriers at the ZnO-Cs₂SnCl₆ hetero-interface, which will be discussed in details in terms of the energy band diagrams of the device and the carrier dynamics upon UV illumination. The results presented in this work may trigger further research interest in development of perovskite based heterojunction photodiodes.

The certified efficiency of Pb perovskite solar cells (PSCs) with more than 1 cm² cell is reported to be 20.9%. In the smaller cells, the cell efficiency with 23.3% has been reported, which is close to those of multi-crystalline inorganic solar cells such as 22.3% of multi Si solar cells. However, the use of Pb in electronic is limited according to the EU RoHS Directive. Thus, it is desirable to develop Pb-free PSCs. Among Pb-free perovskite candidates, Sn perovskite is one of the most promising candidates for the light harvesting layer for the Pb-free PSCs, because they have perovskite structure similar to Pb-perovskite. The efficiency with more than 9% have been reported for Sn-PSC consisting of 2D and 3D structures. In this report, the efficiency increases during the storage in N₂. However, no clear explanation is given regarding this observation. We have also seen similar phenomena with our SnGe PSCs where the initial efficiency is 4.48 % increased to 6.90 % after 3 days. Here, we will discuss the efficiency enhancement from the viewpoint of perovskite lattice relaxation during storage time. The inverted structure of ITO / PEDOT:PSS / FA_0.75MA_0.25SnI₃ + GeI₂ (5 mol%) / C₆₀ / BCP / Ag / Au showed an efficiency of 6.3 % immediately after fabrication and the value increased up to 7.6 % after 6^th day. The increased is mainly contributed by the improvement of V_OC and a slightly improved FF. Interestingly, the J_SC remains the same throughout the study. Photoluminescence peak shifted from 920nm (1^st day) to 910 nm (5^th day). The lattice strain determined from the Williamson-Hall plot showed a value of 1.56% on the 1^st day, and decreased gradually to 0.26% on the 5^th day. These results show that the efficiency enhancement during storage phenomena can be explained through lattice strain relaxation

Further breakthroughs in perovskite solar cells require advances in new compositionsand underpinning materials science. Indeed, a greater fundamental understanding ofperovskite materials requires atomic-scale characterization of their underlyingstructural, defect and transport behaviour. In this context, combined modelling-experimental work is now a powerful approach for investigating these properties at theatomistic level. In my talk I will describe such studies on hybrid perovskites [1-3] intwo related areas: (i) the beneficial impact of partial A-site substitution on the iondiffusion in methylammonium lead iodide (MAPI). Combining ab initio modelling and multiple spectroscopic techniques, we demonstrate for the first time that partial guanidinium substitution into MAPI strongly suppresses iodide ion transport; [1] (ii)
Application of strain for systematically modifying the photovoltaic performance of
halide perovskites. Charge carrier generation, transport and recombination in these materials can be largely modified by controlled lattice distortion. [2-4] These insights are very important for the future design of stable perovskite solar cells with enhanced performance.
[1] D. Ferdani, S. Pering, D. Ghosh et al. Energy Environ. Sci, 2019 (Just Accepted)
[2] D. Ghosh, A. R. Smith, A. B. Walker and M. S. Islam, Chem. Mater 2018, 30 , 5194
[3] D. Ghosh, A. Aziz, J. A. Dawson, A. B Walker and M. S. Islam, Chem. Mater 2019
(Just Accepted)
[4] D. Ghosh, D. Acharya, L. Zhou, A. J. Neukirch and S. Tretiak, (Submitted)

A key bottleneck on way to practical application of perovskite solar cells (PSC) is the development of non-expensive, sustainable and stable hole-transporting materials (HTM). Molecular HTMs hold much promise and already allowed obtaining power conversion efficiencies (PCE) in excess of 20% while often providing better stability than the reference Spiro-OMETAD. Further developments are still needed to improve stability, cost, and efficiency.
Ab initio - typically at the DFT level - modeling is useful to obtain and compare key electronic properties of such materials which helps rationalize their performance and helps design them rationally. While it is relatively easy to perform molecular-level DFT modeling, as is done in much of the literature, ultimately solid HTM layers are used in devices, and it is important to include effects due to molecular aggregation.
Specifically, at the single molecule level, there are no significant changes in the electronic properties such as frontier orbital energies or absorption spectra of molecules differing by the length of alkyl chains. Chain length is an important design variable; redox potentials and other properties of films are known to change with chain length; there can also be strong effect on charge transport which is not captured in molecular calculations.
We will present a combined Density Functional Theory (DFT) - Density Functional Tight Binding (DFTB) and Time-dependent (TD-) DFT/DFTB modeling of the effects of aggregate state on electronic properties of diketopyrrolopyrrole based molecular HTMs which showed PCEs of about 20%. Based on experimentally measured packing structures and computed electronic properties, we show that alkyl chain length can have effects on electronic properties which would be missed in molecular calculations and which are important to understand the differences in device performance with molecules featuring different chains.

Increases in the global energy demand necessitate the development of new approaches to energy conversion and storage. In particular, the utilisation of solar energy could provide a basis of evolving technologies capable of meeting modern and future demands.

By combining the photovoltaic and newfound electrochemical properties of organo-metal hybrid perovskite materials in a single device, a novel photobattery technology is proposed [1]. Utilising the photovoltaic performance of perovskite materials in combination with the intercalation and conversion mechanisms available to Lithium ion species, a device with the ability both to convert light to electrochemical energy and store it is demonstrated. The motivation for such a device will be discussed, with its inherent impact in areas such as off-grid energy solutions and the internet of things. The fabrication techniques are described and characterisation techniques common to both photovoltaic and electrochemical disciplines, with their recent results are discussed. Modifications are made to conventional electrochemical coin cells and pouch cells, in order to facilitate optical access to the electrode material. Using in-operando x-ray diffraction and optical probing, the fundamental mechanisms of charge storage and ion conversion are investigated both in the presence of light, to replicate a light-charging cycle and in the absence of light.

[1] Nano Lett. 2018, 18, 3, 1856-1862

The success of perovskite-perovskite tandem solar cells hinges crucially on the development of efficient wide-bandgap cells. However, when the Bromide content is increased to widen the bandgap, cells cease to deliver the expected increase in open-circuit voltage (V_oc). This loss is usually attributed to halide-segregation in the perovsite. Naturally the question arises: how much voltage loss does halide-segregation incur? Our work answers this question by quantifying the V_oc loss due to halide segregation using Fourier Transform Photocurrent Spectroscopy (FTPS) coupled with thermodynamic calculations. Our results show that the main voltage loss in contemporary Br rich perovskite cells is not due to halide segregation, but the relatively low initial luminescent radiative efficiency in the mixed halide thin films. We also present a model that quantitatively predicts expected voltage loss due to halide-segregation at different bandgaps and segregation severities. Our results suggest that V_oc of up to 1.33V is within reach, even if halide segregation cannot be supressed, and that focusing upon maximising the initial radiative efficiency of the mixed halide films is more important than attempting to supress halide segregation.

We studied ion migration in p-i-n MAPb(I_1-xBr_x)₃-based solar cells at varying AM1.5G light intensities with electrochemical impedance spectroscopy (EIS). We calculated the diffusion coefficients (D), ionic conductivities (σ_ion), and capacitive elements for each hybrid organic-inorganic perovskite (HOIP)-based device. We found that D values increase with increasing illumination intensity in MAPbI₃, MAPb(I_0.8Br_0.2)₃, and MAPb(I_0.2Br_0.8)₃, whereas in MAPbBr₃, MAPb(I_0.6Br_0.4)₃, and MAPb(I_0.4Br_0.6)₃ compositions, D values exhibit a weak dependence on light intensity. Furthermore, we compared the magnitude of ion diffusion in different compositions of MAPb(I_1-xBr_x)₃ (x= 0.2 to 0.8) and found that MAPb(I_0.8Br_0.2)₃ has the highest resistance to ion diffusion. Interestingly, the MAPb(I_0.8Br_0.2)₃ devices show a superior charge extraction capability to both MAPbI₃ and MAPbBr₃ devices. Based on our impedance analysis, we expect the MAPb(I_0.8Br_0.2)₃ composition to be more stable and efficient than pristine MAPbI₃.

Metal organic perovskite materials have shown promise as solution processable, high efficiency, optically active semiconductors for a variety of applications. This amazing property has led many to believe these materials are capable of high-speed roll-to-roll manufacturing for large solar cells and LED displays. However, the main solvents used to dissolve traditional precursor materials are undesirable from a toxicity, coating and manufacturing perspective. Solvents like DMF and Acetonitrile are readily absorbed through the skin and are highly toxic. DMF has high surface tension, viscosity and a high boiling point limiting upper manufacturing speeds. In addition, these solvents have legally regulated exposure limits in both the United States by the Occupational Safety and Health Administration (OSHA) or by the European Union Agency for Safety and Health at Work (EU-OSHA). These exposure limits have the overall effect of practically limiting coating speed under normal ventilation conditions or requiring specialized equipment for vapor management.

To solve this problem we turn to Lead-Alkylamine complex systems as they show promise for being able to extend the solubility of lead halides into additional organic solvents. We use the Hansen solubility model to dissolve solar cell perovskite precursors into a more benign and less limited mixed solvent system of Tetrahydrofuran and Methanol. Although the resulting perovskite film crystalizes in milliseconds, we show that high power conversion efficiency is only achieved when the solution is doped with multiple organic halides improving charge carrier diffusion length as well as external photoluminescence quantum yield. This work provides a path forward for a more benign and industry focused solvent system for perovskite devices.

Despite perovskite solar cells (PSCs) efficiency reaching 24.2%, PSCs stability is still well behind 25-year-stable silicon wafer solar cells. To push PSCs into commercialization, improving their environmental stability is a critical step. Recent studies by Zhou et al., and Dong et al. have suggested that depositing 2D perovskite capping layer, which is also known as buffer layer in PV community, between perovskite absorber and hole-transport layers in a device can improve both environmental stability under ambient temperature with 40%-50% relative humidity (RH) and efficiency by up to 20% relative improvement [1], [2]. Understanding how various 2D perovskite capping layers can be optimized will help improving the cells stability and device architecture design process in the future. We propose a detailed study exploring 7 different types of 2D perovskites for interface layer with methylammonium lead iodide (MAPI) as the photon-absorber layer. The samples are degraded in a well-controlled environmental chamber, under high humidity (85% RH), heat (85°C), and illumination (0.17 sun). In order to accelerate the exploration process of 7 different types of 2D perovskites and numerous deposition conditions, Bayesian optimization (BO) algorithm is used to rapidly optimize each capping layer’s stability based on their deposition conditions. The preliminary results show that optimized tetrabutylammonium iodide capping layer lasts 3.6 times longer than bare MAPI films. Furthermore, by comparing the stability of various types of 2D perovskites through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscope (SEM) data analysis, we can deduce a capping layer design guideline in terms of grain size and film thickness for more stable perovskite solar cells.

References
[1] Q. Zhou et al., “High-Performance Perovskite Solar Cells with Enhanced Environmental Stability Based on a ( p -FC ₆ H ₄ C ₂ H ₄ NH ₃ ) ₂ [PbI ₄ ] Capping Layer,” Adv. Energy Mater., vol. 9, no. 12, p. 1802595, Mar. 2019.
[2] H. Dong et al., “Conjugated Molecules ‘Bridge’: Functional Ligand toward Highly Efficient and Long-Term Stable Perovskite Solar Cell,” Adv. Funct. Mater., vol. 29, no. 17, p. 1808119, Apr. 2019.

Organic–inorganic lead halide perovskite materials have attracted great attention to realize low–cost and high–efficiency solar cells. The power conversion efficiency of Pb–based halide perovskite solar cells (PSCs) has increased from 3.8 % to above 20 % in the last decade[1,2]. Nevertheless, the toxicity and instability of Pb–based perovskite materials are preventing their commercial implementation. So far, Sn–based PSCs showed the most promising results in Pb–free perovskite solar cells with enough efficiencies. However, Sn–based PSCs showed a much smaller open–circuit voltage (V_OC) than Pb–based PSCs[3]. Many previous studies have focused on improve the quality of Sn-based perovskite layers, but the V_OC of Sn–based PSCs has still been much lower than Pb–based PSCs
To increase the V_OC of Sn–based PSCs, we focused on the interface between Sn–based perovskites and electron transport layers (ETLs). There is a large conduction band offset at the interface between Sn–based perovskites and TiO₂, which is often used as ETLs in both Pb–based and Sn-based devices. Such offset is essentially caused by the fact that Sn–based perovskites have the shallower electron affinity (EA) of -3.5 eV than Pb–based perovskites of -4.0 eV. Moreover, p–n junction is mainly formed at interface between ETLs and Sn-based perovskite layers because Sn–based perovskites show p–type characteristics. So we investigated the relationship of the V_OC and the conduction band offset between a perovskite layer and an ETL.
In this work, we use three oxides with different EA, SnO₂, TiO₂ and Nb₂O₅, as ETLs of formamidinium tin iodide (FASnI₃) PSCs. Each oxide was fabricated by the spin–coating or sputtering method. We measured that the EA of SnO₂, TiO₂ and Nb₂O₅ are -4.6 eV, -4.0 eV and -3.3 eV, respectively using the ultraviolet photoelectron spectroscopy and the UV–Vis spectroscopy. We demonstrated the performances of Sn-based PSCs in a planar n–i–p heterojunction device (ITO/ATO/ETL/FASnI₃/PTAA/Au) using each oxide as ETLs. The V_OC of 100 mV, 300 mV and 420 mV were obtained using SnO₂, TiO₂ and Nb₂O₅, respectively. The device with power conversion efficiency (PCE) reached 5.1 % using Nb₂O₅, which was higher than PCE of 2.5 % and 0.5 % using TiO₂ and SnO₂, respectively. As the conduction band offset between FASnI₃ and the ETL oxide become smaller, a V_OC and a PCE of the device increase. Consequently, for Sn–based PSCs a conduction band offset of interface between ETLs and perovskites layers is an important factor to increase V_OC, and Nb₂O₅ is a good ETL material for high performance of Sn–based device.
[1] A. Kojima, K. Teshima, Y. Shirai , T. Miyasaka, “Organometal halide perovskites as visible-light sensitizers for photovoltaic cells.” J. Am. Chem. Soc. (2009)
[2] Q. Jiang, Yang Zhao, X. Zhang, X. Yang, Y. Chen, Z. Chu, Q. Ye, X. Li, Z. Yin and J. You, “Surface passivation of perovskite film for efficient solar cells.” Nature Photonics (2019).
[3] W. Ke, C. C. Stoumpos and M. G. Kanatzidis, ““Unleaded” perovskites: status quo and future prospects of tin-based perovskite solar cells.” Adv. Mater. (2018).

Metal halide perovskites have properties including strong absorption coefficients and long charge diffusion lengths¹ (relative to film thickness) making them close to ideal for solar cells. For perovskite devices to achieve their efficiency limits, charge carrier recombination and the role of photon recycling within films must be better understood and controlled.
Current approaches to measure charge carrier recombination rates are typically based on time intensive and expensive transient spectroscopic measurements². Here we demonstrate that we can employ steady state approaches to extract these parameters, thereby allowing for faster screening of materials. We show the potential of this approach by a comparison with results from transient absorption spectroscopy.
To further understand photon recycling we use our methodology to quantify the photon escape probability. While there are some calculations of the escape probability in the literature³, this quantity has not been previously quantified in metal halide perovskites, despite its importance for solar cell operation including photon recycling. We present measured values and compare with those previously calculated.
Using results from our rapid screening process we calculate the limiting efficiency and number of photon recycling events per initially absorbed photon within devices. We carry out calculations for a range of materials including low bandgap and wider gap mixed-halide perovskites. Our results show that in addition to minimising the charge trapping rate it is necessary to maximise the escape probability for optimal efficiency, thereby reducing the number of photon recycling events. We extend our analysis to two and three bandgap tandem configurations, demonstrating that photon recycling between absorber layers has a relatively small effect on efficiency at the maximum power point. Our combined experimental and modelling approach allows us to target specific device parameters to reach limiting efficiencies.
References
1. M. A. Green, A. Ho-Baillie and H. J. Snaith, Nat. Photonics, 2014, 8, 506–514.
2. J. M. Richter, M. Abdi-Jalebi, A. Sadhanala, M. Tabachnyk, J. P. H. Rivett, L. M. Pazos-Outón, K. C. Gödel, M. Price, F. Deschler and R. H. Friend, Nat. Commun., 2016, 7, 13941.
3. T. Kirchartz, F. Staub and U. Rau, ACS Energy Lett., 2016, 1, 731–739.

Mixed organic-inorganic or fully inorganic halide perovskite solar cells (PSCs) have drawn appreciable interest of research community as highly efficient light harvesters reaching efficiency of about 24% in a past few years. Despite its promising potentials to replace the other commercialized technologies, the principal hurdle is the instability issue when exposed to ambient environment. On the other one cannot wish away from the fact that lead (Pb) which is highly toxic and carcinogenic is an inseparable part of PSCs. Researchers worldwide have tried hard to replace it with some other non-hazardous counterparts but failed to achieve similar optoelectronic properties. Disposal of hazardous waste is also a major issue because of its high expenses, so safe disposal becomes a major hurdle especially for the developing countries.¹ Cost analysis studies of similar architectures have shown that a major fraction (40-60%) of the total cost embedded is owned by fluorine doped tin oxide (FTO) substrate.² Other profound contributor to the cost of the technology is the expensive spiro-OMeTAD hole transporting layer, but a number of efficient alternatives to spiro-OMeTAD have been testified. Compact titanium dioxide (TiO₂) which serves as electron transporting layer though not much cost intensive but consumes a good amount of primary energy owing to its high sintering temperature requirement. In our work we have tried efficient recycling of the final decomposed product, which is PbI₂ back to perovskite. The reuse of same Pb and hence underneath TiO₂ and FTO layers will cut down the cost of the technology, decreasing the payback time even further. This work renders a comparative study of the feasibility of efficient recycling in three different fabrication routes i.e. single step mixed halide, single step acetate and sequential deposition route. We encountered that all the films cannot be recovered expeditiously, which was corroborated from the XRD spectra. XRD spectra elucidated that the degraded sequentially deposited films cannot be completely recovered back to methyl ammonium lead iodide perovskite on incorporation of methyl ammonium iodide. Degraded sequentially deposited films have shown drastically improved crystallinity, which if somehow can be converted back to perovskite completely will result in altogether superior optoelectronic properties. Films deposited via single step acetate and sequentially deposition route has shown efficient recovery of optoelectronic properties as compared to single step mixed halide route, as ascertained from the steady state photoluminescence. Absorption spectra recorded have shown the recovery of the characteristic wide absorbance range back upon recycling. Reusing the same Pb multiple times without compromising with the optoelectronic properties will move the technology a step ahead in the path of commercialization.

References
[1] Hailegnaw, B.; Kirmayer, S.; Edri, E.; Hodes, G.; Cahen, D. Rain on methylammonium lead iodide based perovskites: possible environmental effects of perovskite solar cells. The journal of physical chemistry letters 2015, 6(9), 1543-1547.
[2] Binek, A.; Petrus, M. L.; Huber, N.; Bristow, H.; Hu, Y.; Bein, T.; Docampo, P. Recycling Perovskite Solar Cells to Avoid Lead Waste. ACS Appl. Mater. Interfaces 2016, 8 (20), 12881–12886.

Perovskite solar cell shows great possibilities for the solar power field as exhibiting uprising power conversion efficiency from 3.81% to 23.7%. Recent study has been focused on how to get market competitiveness between other solar cells. Those attempts include deposition of perovskite on texture and large-area substrate such as textured silicon.
Perovskite layer has been mainly deposited on mesoporous or compact layer by spin-coating perovskite solution on substrate. Spin-coating method has its limitation in deposition on textured, large-area surface. It also uses a poisonous solvent such as dimethylformamide, dimethyl sulfoxide. Other deposition methods such as evaporation, electroplating and slot die coating have also been proposed as methods for depositing perovskite layer. To obtain conformal and uniform layer of perovskite material on a large-scale and textured substrate, radio-frequency(RF) magnetron sputtering can be used to deposit a uniform lead iodide film on the substrate first. Conformal perovskite layer can be prepared by sequentially interacting the lead iodide film with methylammonium iodide(MAI). Lead iodide deposited through sputtering shows uniformly packed and conformal films on planar or textured substrate. RF magnetron sputtering is competitive with other deposition methods because highly uniform and conformal lead iodide films can be deposited on textured and large-area substrate and mass production is available. Unlike evaporation method, it does not need high temperature environment hindering coating film on flexible substrate
In this study, lead iodide films were deposited by RF magnetron sputtering. Uniform and dense lead iodide films are obtained on a 1.5 cm x 1.5 cm FTO-glass substrate coated with c-SnO₂. Characteristics of film deposited under various sputtering conditions were analyzed and compared with conventional spin-coated lead iodide film. Lead iodide films deposited using same method on textured silicon were also investigated. Perovskite layer are prepared by loading certain amount of MAI solution on sputtered lead iodide films. The effect of loading time and concentration of MAI solution on formation of perovskite layer were studied. Several measurement methods were used including X-ray diffraction method, Raman spectroscopy, X-ray photoelectron spectroscopy, Ultraviolet-visible spectroscopy, Transmission electron microscopy and Atomic force microscope.

Polycrystalline metal-halide perovskite thin films share many optoelectronic properties of their high quality, inorganic semiconductor counterparts. However, local heterogeneity of the perovskite has been shown to limit device performance.[1] Fundamental semiconductor properties such as the quasi-Fermi level splitting (QFLS), absorptance, Urbach energy and photoluminescence quantum efficiency (PLQE) energy are important metrics that determine the upper bound efficiency of solar cells made with these materials, however complete microscale measurements of these properties in perovskites have not been performed. While photoluminescence mapping is common in the field, confocal illumination and collection geometries are often employed which necessitates much higher illumination intensity and real effects can be masked by carrier diffusion away from the illuminated area.[2, 3] Wide-field illumination microscopy, where the entire imaged area is illuminated simultaneously, enables microscopy at solar cell relevant carrier densities. This technique has been used to investigate the local QFLS in perovskites,[4] and in other thin film technologies such as CIGS.[5]

Here we use a wide-field illumination, spectrally and spatially calibrated microscope to study thin films of MAPbI₃ and triple cation perovskites (Cs_0.05FA_0.79MA_0.16Pb(I_0.83Br_0.17)₃) at 1 sun equivalent illumination intensities. We develop novel methods to measure absolute reflectance, transmittance and absorptance spectra of these films in addition to absolute-intensity calibrated photoluminescence spectra with diffraction limited resolution. Correlating these spectra allows the extraction of local QFLS, Urbach energy and PLQE. We examine local spatial correlations between these properties in order to test the assertions of detailed-balance at the scale of individual grains. The results reveal considerable microscale heterogeneity in many fundamental, device relevant parameters that, on average, match with macroscale observations. We see grain to grain heterogeneity in the emission, but also larger scale heterogeneity over clusters of tens of grains. Strikingly we find local PLQE variation by up to an order of magnitude. We also use the technique to identify the most effective defect passivation treatments to our films resulting in enhanced device performance. Finally, we note the potentially wide applicability of these methods to a broad range of other emerging semiconductor technologies.

1. Correa-Baena, J.-P., et al., Homogenized halides and alkali cation segregation in alloyed organic-inorganic perovskites. Science, 2019. 363(6427): p. 627.
2. de Quilettes, D.W., et al., Impact of microstructure on local carrier lifetime in perovskite solar cells. Science, 2015. 348(6235): p. 683.
3. deQuilettes, D.W., et al., Tracking Photoexcited Carriers in Hybrid Perovskite Semiconductors: Trap-Dominated Spatial Heterogeneity and Diffusion. ACS Nano, 2017. 11(11): p. 11488-11496.
4. El-Hajje, G., et al., Quantification of spatial inhomogeneity in perovskite solar cells by hyperspectral luminescence imaging. Energy & Environmental Science, 2016. 9(7): p. 2286-2294.
5. Delamarre, A., et al., Quantitative luminescence mapping of Cu(In, Ga)Se2 thin-film solar cells. Progress in Photovoltaics: Research and Applications, 2015. 23(10): p. 1305-1312.

Perovskite solar cells represent one of the most promising photovoltaic technologies. Over the past few years, the performance of perovskite solar cells was gradually improved up to >23%, which is close to the characteristics of crystalline silicon photovoltaics. However, high toxicity and low stability of complex lead halides used as absorber materials hamper commercialization of this technology. The compositional engineering of lead perovskites was actively pursued in order to improve their stability and/or performance. In particular, a partial or full replacement of Pb²⁺ in MAPbI₃ is highly desirable in terms of developing more environmentally friendly materials. Here we present a systematic study of lead substitution in MAPbI₃ with >20 different cations introduced in atomic concentrations ranging from 10^-5 to 20-30%. It was shown that replacing even minor fraction of lead could change significantly the perovskite film crystallinity and morphology. Importantly, the efficiency and stability of p-i-n and n-i-p perovskite solar cells were improved considerably by appropriate modification of MAPbI₃ films. Moreover, important relationships were established between the nature of the substituting ions (e.g. ionic radius or charge) and their effects on electronic properties and photovoltaic performance of the resulting hybrid perovskites. The obtained results should facilitate the rational design of more stable and less toxic absorber materials for advanced perovskite solar cells.

Organic-inorganic hybrid perovskites are a class of semiconducting materials with great potential for solar cell absorbers. This is due to their high absorption coefficients, tunable direct bandgap, demonstrated high power conversion efficiencies in devices, and disruptively low manufacturing costs. However, they are currently limited by their tendency to degrade under strong (UV-)illumination, and elevated temperatures, both critical issues for a photovoltaic material. While these degradation effects have been widely reported in their electrical behavior, there is comparatively little information on the effects on their optical constants, the refractive index and absorption coefficient.

In this work we investigate the changes in optical constants of a variety of device relevant multi-cation perovskite compositions under the effects of UV light soaking and high temperatures. The compositions investigated include the widely used triple cation perovskite (Cs_0.05(MA_0.17FA_0.83)_0.95Pb(Br_0.17I_0.83)₃), Rb-stabilized and methylammonium-free compositions (Rb_0.05Cs_0.05(MA_0.17FA_0.83)_0.90PbI₃, Rb_0.05Cs_0.10FA_0.85PbI₃), as well as MAPbI₃ for comparison. UV light soaking was performed with a Xe lamp in air, and heating was done in a high temperature stage with nitrogen purging at 100°C and 120°C. The optical measurements were carried out using Variable Angle Spectroscopic Ellipsometry in the wide wavelength range of 190 nm – 25 μm, from the UV to the Middle Infrared (MIR). We demonstrate that it is possible to observe degradation effects around the bandgap and high absorption regions, which are of interest for solar cell performance, while changes in chemical composition can be observed at the same time through the MIR absorption bands.

A variety of behaviors were observed with increasing degradation time under both UV and heat exposure. The most recurring was a rapid loss of methylammonium ions from the film. This was visible as a reduction of IR peaks associated to methylammonium and a decrease in film thickness, which was confirmed by SEM imaging. IR peaks related to formamidinium ions also showed a decrease, but the rates were much lower. The loss of methylammonium was coupled with phase segregation and the formation of PbI₂ in the film: at energies above 2 eV, a superposition of the absorption coefficients of PbI₂ and the base perovskite was observed, with the bandgap of PbI₂ clearly visible at 2.4 eV. Overall, the most UV stable composition was the standard triple cation perovskite, closely followed by the methylammonium-free Rb_0.05Cs_0.10FA_0.85PbI₃, which showed the best thermal stability.

Recently, the interest in low-cost hole-conduction-layer-free perovskites solar cells is growing rapidly due to the high-power conversion efficiencies (exceeding 17 %) recorded [1]. Previous studies have reported that halide hybrid perovskites thin films produced a more stable and efficient hole-conduction-layer-free solar cell when compared with its non-hybrid halide perovskites components, owing to their excellent blend of light absorption and hole-conduction properties [2]. However, far too little attention has been paid to their synthesis by physical vapor deposition, which may further increase stability. Herein, a hybrid methylammonium lead tri-iodide/methylammonium lead tri-bromide (MAPbI₃/MAPbBr₃) perovskites thin film was prepared by sequential physical vapor deposition of MAPbBr₃ on MAPbI₃ thin film. The structural, optical, morphological and electrical properties of the MAPbI₃, MAPbBr_3, and MAPbI₃/ MAPbBr₃ thin films were compared. X-ray diffraction patterns confirmed the tetragonal crystal structure of MAPbI₃, a cubical structure of MAPbBr₃ and the two clearly separate phases for MAPbI₃/MAPbBr₃ [3]. In addition, the crystallite sizes were observed to decrease from 32.74 to 22.06 nm while dislocation density increased from 9.33 x 10¹⁰ to 2.05 x 10¹¹ cm^-2 for MAPbI₃, MAPbI₃/MAPbBr_3, and MAPbBr₃ respectively. UV-Vis absorption spectra revealed onsets of absorption for MAPbI₃, MAPbI₃/MAPbBr_3, and MAPbBr₃ at 760, 628 and 550 nm respectively, which correspond to respective optical bandgaps of 1.63, 1.97 and 2.25 eV. Scanning electron microscopy micrographs depicted densely packed grains which fully covered the substrates, required to minimize leakage currents. Electrical properties showed that the trap densities increased from 2.56 x 10¹⁵ to 1.89 x 10¹⁶ cm^-3 whereas the carrier mobilities decreased from 2.34 x 10^-1 to 1.89 x 10^-2 cm² V⁻¹ s⁻¹ for MAPbI₃, MAPbI₃/MAPbBr₃ and MAPbBr₃ respectively. This study introduces a facile way of growing hybrid halide perovskites thin films for efficient hole-transport-layer-free solar cells.
Keywords: Hybrid methylammonium lead tri-iodide/methylammonium tri-bromide; sequential physical vapor deposition; hole-conduction-layer-free solar cell.
References
[1] Z.W. Xin Wu, L. Xie, K. Lin, J. Lu, K. Wang, W. Feng, B. Fan, P. Yin, J. Mater. Chem. A 7 (2019) 12236-12243.doi:10.1039/C9TA02014D.
[2] S. Aharon, B. El Cohen, L. Edgar, J. Phys. Chem. C. 118 (2014) 17160–17165. doi:10.1021/jp5023407.
[3] F. Lehmann, A. Franz, D.M. Többens, S. Levcenco, T. Unold, A. Taubert, S. Schorr, RSC Adv. 9 (2019) 11151–11159. doi:10.1039/c8ra09398a.

To date, the primary limitation of perovskite solar cell is still related to device instability. Because intrinsic and extrinsic factors such as light, temperature, bias, humidity, and oxygen can all contribute to material degradation, we propose the use of machine learning (ML) as a strategy to identify the individual and combined contribution of each aforementioned factor through a reap-rest-recovery cycle, which will be discussed in details. Moreover, we will present the dynamic electrical and optical behavior of MAPbI3, MAPbBr3, and multi-cation perovskites (including Cs and Rb ions) by implementing environmental atomic force microscopy (AFM) and micro-photoluminescence (micro-PL) microscopy. In the realm of the perovskite’s electrical response, we use Kelvin probe force microscopy (KPFM) and photoconductive (pc-) AFM to map changes in voltage and current that occur upon submitting the devices to different light treatments. Through fast-KPFM we quantify, in real-time, a dynamic open-circuit voltage (Voc) response as a function of chemical composition, resulting from ion motion. Optically, we resolve the correlation between the luminescence response of the perovskites and its chemical composition by exposing the perovskites to different environments, including loops in relative humidity ranging from < 5% to 55%. Combined, our microscopy probes enable us to define the contribution of each stressor to device instability, elucidating their effects on the physical behavior of the grains and boundaries composing the perovskite solar cells.

Understanding the degradation mechanisms and realizing high operational stability have become major issues in perovskite photovoltaics. Especially, recent researches point that stable perovskite solar cells can be enabled by controlling defects of conventional devices, as well as developing novel materials.^[1,2] Herein, based on our comprehension upon the defect characteristics and analyses,^[3,4] we introduce our approaches for efficient and stable perovskite solar cells via the development of materials and trap controls. Using copper thiocyanate (CuSCN) for the inorganic hole-transport layer, we suggest polydimethylsiloxane (PDMS) as a novel crosslinking material for the interlayer between perovskite and CuSCN, which brings the effect of interfacial modification and defect passivation. With its unique crosslinking behavior observed, PDMS mitigates the trap formation and enhances the charge extraction at the interface realizing thermally stable perovskite solar cells with high power-conversion efficiency over 19%. By extension, further modification on the hole-transport layer is applied in order to prevent degradation of electrode and ionic interdiffusion, the dominant factors deteriorating the device performance under light and applied bias. Embracing our recent research on the stability of perovskite solar cells, the correlation between the interfacial defects and device stability will be discussed, with future insights to realize highly stable and efficient perovskite solar cells.

[1] H. Zhou et al., Nat. Energy 4, 408 (2019).
[2] L. Li et al., Angew. Chem. Int. Ed. (2019) 10.1002/ange.201904945.
[3] B. Park et al., Adv. Mater. 30, 1704208 (2018).
[4] B. Park et al., ACS Appl. Mater. Interfaces 11, 6907 (2019).

Organic-inorganic hybrid perovskites solar cell (PVSC) has received worldwide attention in the past decade. Its power conversion efficiency (PCE) has struck to 24.2% from 3.8% since its first debut in 2009. Even though the perovskite materials possess exceptional semiconducting properties, such as ambipolar charge-transporting capability, low exciton binding energy, long carrier diffusion length, their generally light-harvesting range is limited in the visible-light region, from 300 nm to 800 nm, which engender great loss in spectral while illumination pass through the device. The enhancement in near-IR (NIR) region usually could be attained by composition engineering but the results were not significant, on the other hand, it could also be reached through device engineering but the researches mostly focused on conventional configuration. Herein, we describe a simple and accessible method to enhance the NIR photoresponse of inverted PVSC by rational design of electron-transporting layer.

In general, fullerene is the most commonly used ETL for inverted PVSCs. However, owing to its symmetric geometry, the light absorption of fullerene is quite weak. Recently, low bandgap non-fullerene acceptor (NFAs) with NIR light absorption has been vigorously develop in the field of organic photovoltaics. As inspired by this impressive progress, we herein conceive a study to enhance the NIR response of inverted PVSC by engineering the fullerene ETL with NFAs with NIR light absorption. Owing to the low bandgap, the NFA can form type II charge transfer with fullerene to dissociate the photoexction induced by NIR absorption. However, our result manifests that such binary blend is not sufficient to enhance the NIR photoresponse of derived inverted PVSCs. The ETL consisting of PCBM and NFA blend could not convert any near-IR photons into resulting photocurrent, as evidenced by the corresponding EQE spectrum. Intriguingly, we demonstrated a strategy to efficiently intercept the near-IR light and convert the photons into current through further modification of this hybrid ETL with NIR absorbing capability. As a result, the optimized near-IR ETL can enable its derived inverted PVSC to possess an extended photoresponse from visible region to 950 nm and present 40% EQE in the NIR region, contributing to 8.4% of the overall photocurrent (22.5 mA/cm²). Combined with the high Voc of 1.15 V and decent FF of 70%, an inverted PVSC with a high PCE of 18.1% was finally demonstrated. Our study unveils an effective approach to enhance the NIR photoresponse of inverted PVSCs through simple design of compatible charge-transporting layer.

Metal halide perovskites have witnessed a remarkable growth in efficiency that has yet to be recorded for any other photovoltaic technology to date (1). At the heart of these high performances lie exceptional material properties, which only in the recent years have started to be studied more profoundly alongside research efforts focusing on boosting the efficiency. An aspect attracting particular recent interest in this respect has been the significant impact of low fractional Cs substitution of the composition and the addition of sacrificial additives containing elements such as Rb, K and Na on the stability and/or performance of perovskite solar cells (2,3). Already clearly linked to effects such as halide homogenization in the light harvester, there are also strong indications that these compounds have an influence on crystallographic and phase intermediates of the soft and forgiving semiconductor (2,3,4). In an attempt to unravel the mechanism underlying the effect of these operations, this work studies the spatial photoluminescence of multi-cation and –anion perovskites by hyperspectral imaging. We demonstrate that sacrificial and substitutional compounds result in the elimination of distinct domains with varying intensities and emission maxima, which cannot be correlated to compositional inhomogeneities. Instead, we link this observation to effects inherent to the deposition method used to fabricate the absorber, and hypothese that the observed results are due to strain effects in the semiconductor grains (5). In addition, we also elaborate on the stability of these PL inhomogeneities in terms of exposure to circumambient atmosphere, and reflect on their potential effects in corresponding devices.

References:
(1) NREL efficiency chart, 13^thJune 2019.
(2) Correa-Baena, J. P., Luo, Y., Brenner, T. M., Snaider, J., Sun, S., Li, X., ... & Poindexter, J. R. (2019). Homogenized halides and alkali cation segregation in alloyed organic-inorganic perovskites. Science, 363(6427), 627-631.
(3) Gratia, P., Zimmermann, I., Schouwink, P., Yum, J. H., Audinot, J. N., Sivula, K., ... & Nazeeruddin, M. K. (2017). The many faces of mixed ion perovskites: unraveling and understanding the crystallization process. ACS Energy Letters, 2(12), 2686-2693.
(4) Dang, Hoang X., et al. "Multi-cation Synergy Suppresses Phase Segregation in Mixed-Halide Perovskites." Joule (2019).
(5) Jones, T. W., Osherov, A., Alsari, M., Sponseller, M., Duck, B. C., Jung, Y. K., ... & Li, Y. (2018). Local Strain Heterogeneity Influences the Optoelectronic Properties of Halide Perovskites. Condens. Matter, 1-13.

An open counter [1] is a unique detector that can operate in air at atmospheric pressure to detect and count a small number of low-energy photoelectrons. Therefore, photoemission yield spectroscopy in air (PYSA) can be performed by employing an open counter as the detector [2]. PYSA has the unique advantage of measuring high energy-resolution and low photo-excitation energies in a non-vacuum environment. After the commercial model was released, it was used to analyze industrial materials, e.g., semiconductors, organic photoconductors, toners, organic light-emitting diodes, and magnetic disk drives. Furthermore, the successive change on the practical surfaces, such as fresh Al surface exposed to air [3], has been successfully observed.
PYSA measurement was performed as follows. UV light emitted from a deuterium lamp was monochromatized using a grating monochromator, which was then focused on the sample surface. The number of photoelectrons emitted from the sample surface was counted using an open counter.
In recent years, PYSA has been applied to measure halide perovskites [4, 5]. Last year, we proposed a novel PYSA measurement technique and examined the change in the threshold energy of photoemission, which corresponds to the first-ionization potentials, at approximately the phase transition temperature of CH₃NH₃PbI₃ [6]. Here, we discuss the applicability of PYSA for evaluate the long-term stability of halide perovskites.

[1] H. Kirihata and M. Uda, Rev. Sci. Instrum. 52 (1981) 68
[2] M. Uda, Jpn. J.Appl. Phys. 24 (1985) 284
[3] M. Uda, Y. Nakagawa, T. Yamamoto, M. Kawasaki, A. Nakamura, T. Saito, K. Hirose, J. Electron Spectrosc. Relat. Phenom. 88-91 (1998) 767
[4] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131 (2009) 6050
[5] L. Cojocaru, S. Uchida, A. K. Jena, T. Miyasaka, J. Nakazaki, T. Kubo, H. Segawa 44 (2015) 1089
[6] D. Yamashita, Y. Nakajima, S. Uchida, H. Segawa, 2018 MRS Fall meeting

Organolead halide perovskites (OHP) have emerged as a promising material for applications in photovoltaics, LEDs and light detectors. This group of materials offers the simplicity of wet-chemistry methods while keeping competitive optoelectronic properties. However, photodegradation of OHPs especially under ambient conditions and high intensity illumination has been a significant bottleneck for its commercialization. Although the actual mechanism for this detrimental phenomenon is still under debate, the role of surface defects has been proposed. Recently, the presence of these surface defects has also been associated to the stochastic photoblinking of OHP nanocrystals. Employing super-resolution localization methods, we were able to estimate the spatial distribution of these surface defects along a single OHP nanocrystal. We then deliberately expose the particles at elevated light intensity and ambient conditions and observe the spatial evolution of its photodegradation and correlated it with the estimated localization of surface defects.

Perovskite solar cells have been recognized as a newly emerging solar cell with the potential of achieving high efficiency with a low cost fabrication process. In particular, facile solution processed cell fabrication facilitated rapid development of optimum cell structure and composition. Over the last few years, the cell efficiency has exceeded 23%.

A typical perovskite solar cell employs a perovskite layer sandwiched by p-type semiconductor (such as spiro-OMeTAD, PEDOT or NiO) and n-type semiconductor (such as TiO₂, ZnO or PCBM) layers. Following light absorption, an electron and a hole are separated at the perovskite film interface, and are collected at the back electrodes. Choice of the most suitable solar cell structure is crucial to improve the performance further. In this presentation, we will present parameters controlling charge separation and recombination dynamics at the perovskite interfaces employing a series of transient absorption and emission spectroscopies. Nanosecond transient emission spectroscopy (Vis-ns-TES) clarifies charge separation processes, while Vis-NIR submicrosecond-millisecond transient absorption spectroscopies (VisNIR-smm-TAS) identify charge separation efficiency and charge recombination rates. The mechanisms of interfacial charge transfer and recombination processes have been clarified. Correlation of the dynamics results with the solar cell performance will be discussed [1-4]. An optimum cell structure for methylammonium lead iodide (MAPbI₃) perovskite sandwiched by TiO₂ and spiro-OMeTAD layers, among planar heterojunction, mesoporous structure and extremely thin absorber structure will be identified. The method to determine the best structure will be applied to a different type of semiconductor.

This work was financially supported by the JST PRESTO program (Photoenergy Conversion Systems and Materials for the Next Generation Solar Cells) and partly by JSPS KAKENHI Grants (16K05885) and (19H02813) and the Collaborative Research Program of Institute for Chemical Research, Kyoto University (grant number 2019-50), Japan. The author also acknowledges Australian Research Council (ARC) LIEF grant (LE170100235) and the Office for Industry-University Co-Creation, Osaka University, for the financial supports.

References
[1] S. Makuta, M. Liu, M. Endo, H. Nishimura, A. Wakamiya, Y. Tachibana, Chem. Commun., 52, 673 - 676 (2016).
[2] Tachibana et al., J. Photopolym. Sci. Technol., 30(5) 577-582 (2017).
[3] Tachibana et al., ACS Appl. Energy Mater., 1(8) 3722-3732 (2018).
[4] Tachibana et al., J. Photopolym. Sci. Technol., in press (2019).

The efficiencies of small-pixel perovskite photovoltaics have increased to well above 20%, while the question is whether fabrication methods can be transferred to scalable manufacturing process. Here we report a method of fast blading large area perovskite films at an unprecedented speed of 99 millimeter-per-second or higher in ambient condition by tailoring solvent coordination capability. Combing volatile non-coordinating solvents to Pb²⁺ and low-volatile, coordinating solvents achieves both fast drying and large perovskite grains at room temperature. The reproducible fabrication yields a record certified module efficiency with aperture area of 63.7 cm². The perovskite modules also show a small temperature coefficient of -0.13%/°C and nearly fully recoverable efficiency after 58 cycles of shading, much better than commercial silicon and thin film solar modules. The application of the coating method to perovskite/silicon tandem cells and will also be presented. We will answer the question whether the perovskite layers can be fabricated at the speed of silicon cells are produced in the regular production lines.

Crystalline Si (c-Si) solar cells are driving the progression of renewable electricity generation technologies thanks to lowering costs and increasing efficiencies. One solution to maintain this cost-competitiveness on the long-term involves increasing efficiencies beyond the limit of c-Si by stacking a perovskite solar cell on a commercial c-Si one to form a photovoltaic tandem device. The tunable bandgap, soft processing conditions and high single-junction performance of perovskites indicate that this approach could upgrade c-Si solar cells to efficiencies >30% through a few extra process steps with low additional process costs.

For maximum photocurrent and compatibility with existing c-Si process flows, the perovskite solar cell should be deposited directly on the textured front side of the c-Si solar cell, a texture that improves light management in the c-Si. But this pyramidal texture imposes several microfabrication challenges as the perovskite absorber is typically deposited via solution processing, and is about one order of magnitude thinner than the height of the pyramids it needs to cover. Achieving a conformal deposition of all the layers of the top cell on this pyramidal texture and hence maximum performance requires a fine control over the layer formation to avoid pinhole formation.

In that regard, electron microscopy techniques, notably analytical transmission electron microscopy (TEM), can shed some light on the device nanostructure and its dependence on processing/operation conditions, guiding the development of devices. However, the fragile nature of perovskite solar cells complicates their preparation into thin cross-sections necessary for TEM observations and their analysis with high-energy electrons. This presentation will review artifacts that may occur during TEM sample preparation and observation, elaborate several strategies to identify and mitigate them, before discussing several topics correlating nanostructure and performance of perovskite single-junction and tandem solar cells. This contribution will present how electron microscopy data coupled with other techniques provide valuable inputs to guide the development of high-efficiency (>25%) tandems featuring textured n- and p-type c-Si solar cells^1,2, notably by i) identifying optimal bottom cell contact nanostructures, ii) isolating crystallographic and chemical features enabling the recombination junction to quench shunts,³ iii) guiding the removal of shunts running through the perovskite absorber on textured c-Si by adapting process conditions and iv) visualizing the dewetting of charge-selective layers during the crystallization of the perovskite solar cell on certain recombination junctions. In addition, degradation pathways triggered by reverse voltages (also investigated through TEM in situ biasing experiments)⁴, during long-term operation at maximum power point at various temperatures, or during damp heat tests (85°C/85% relative humidity) will be examined. These results highlight the dynamic nature of the perovskite nanostructure (ionic migration within the absorber and into the contacts, volatilization of species, crystallographic phase change/decomposition) depending on the external stimuli and its influence on the solar cell performance.

1 Sahli, F. et al. Nat. Mater. 17, 820–826 (2018)
2 Nogay, G. et al. ACS Energy Lett. 4, 844–845 (2019)
3 Sahli, F. et al. Adv. Energy Mater. 8, 1701609 (2018)
4 Jeangros, Q. et al. Nano Lett. 16, 7013–7018 (2016)

Integrating metal halide perovskite top cells with crystalline silicon or CIGS bottom cells into monolithic tandem devices has recently attracted increased attention due to the high efficiency potential of these cell architectures. To further increase the tandem device performance to a level well above the best single junctions, optical and electrical optimizations as well as a detailed device understanding of this advanced tandem architecture need to be developed. Here we present our recent results on monolithic tandem combinations of perovskite with crystalline silicon and CIGS, as well as tandem relevant aspects of perovskite single junction solar cells.
By selecting a front contact layer stack with less parasitic absorption and utilizing the p-i-n perovskite top cell polarity, a certified conversion efficiency of 25.0% for a monolithic perovskite/silicon tandem solar cells was enabled. Further fine-tuning of the stack optics as well as contact layers improved the efficiency to 26.0% (0.8 cm² area) and we present how especially the fill factor of highly efficient tandem solar cells behaves under current-mismatch conditions. In strong mismatch the FF of the tandem cell is enhanced which reduces the sensitivity of efficiency to spectral mismatch. ^[1] Additionally, the introduction of light trapping foils with textured surfaces is presented together with the influence on texture position on lab performance and outdoor energy yield.^[2]
The monolithic combination of perovskite and CIGS was highly challenging up to now as the CIGS surface is rather rough. By implementing a conformal hole transport layer, an 21.6% efficient monolithic perovskite/CIGS tandem (0.8 cm² area) was realised. Absolute photoluminescence of the perovskite and CIGS sub-cells gives insights into the contributions to the tandem open-circuit voltage (Voc). To further improve the tandem efficiency, the Voc of perovskite top cells needs to be enhanced via reduction of non-radiative recombination at the interface between perovskite and the charge selective layers. This can either be done via proper interlayers or via fine-tuned charge selective contacts.
Recently we have shown that self-assembled monolayers (SAM) could be implemented as appropriate hole selective contacts.^[4] The implementation of new generation SAM molecules enabled further reduction of non-radiative recombination losses with Voc’s up to 1.19 V and efficiency of 21.2% for p-i-n perovskite single junctions with band gaps of 1.63 eV and 1.55 eV, respectively.
References:
[1] Köhnen, Jost, Stannowski, Albrecht et al., Sustainable Energy and Fuels, doi: 10.1039/C9SE00120D
[2] Jost, Topic, Stannowski, Albrecht et al., Energy and Environmental Science 2018, 11, 3511
[3] Jost, Bertram, Koushik, Albrecht et al., ACS Energy Letters 2019, 4 , 583
[4] Magomedov, Al-Ashouri, Albrecht, Getautis et al., Advanced Energy Materials 2018, 8, 1801892

Wide-bandgap (~1.7-1.8 eV) perovskite solar cells have attracted substantial research interest in recent years due to their great potential to fabricate efficient tandem solar cells via combining with a lower bandgap (1.1-1.3 eV) absorber (e.g., Si, copper indium gallium diselenide, or low-bandgap perovskite). However, wide-bandgap perovskite solar cells usually suffer from large open circuit voltage (V_oc) deficits caused by small grain sizes and photoinduced phase segregation. We show that in addition to large grain sizes and passivated grain boundaries, controlling interface properties is critical for achieving high V_oc’s in the inverted wide-bandgap perovskite solar cells. We adopt guanidinium bromide solution to tune the effective doping and electronic properties of the surface layer of perovskite thin films, leading to the formation of a graded perovskite homojunction. The enhanced electric field at the perovskite homojunction is revealed by Kelvin probe force microscopy measurements. This advance enables an increase in the V_oc of the inverted perovskite solar cells from an initial 1.12 V to 1.24 V. With the optimization of the device fabrication process, the champion inverted wide-bandgap cell delivers a power conversion efficiency of ~19% and sustains more than 72% of its initial efficiency after continuous illumination for 70 h without encapsulation. The improvement on performance of wide-bandgap perovskite subcells enables us to fabricate efficient and stable perovskite tandem solar cells.

The record power conversion efficiency is 28 % for perovskite-silicon tandems and 23.2 % for perovskite-perovskite tandems. One of the challenges that must be overcome to achieve efficiency greater than 30% is to reduce the voltage loss in high bandgap perovskite cells and prevent light-induced phase separation. We have found that treating the surface of perovskites can dramatically reduce the extent of light -induced phase separation, which has interesting implications for how the process occurs. We have also developed new strategies for increasing the bandgap in perovskite compounds that have modest amounts of bromine. We have been able to make semitransparent high bandgap solar cells with greater than 20% power conversion efficiency that do not suffer from light-induced phase separation. These advances in combination with improvements in the atomic layer deposition of highly impermeable metal oxide contacts enable the fabrication of highly efficient and stable tandems.

Halide ion mobility in metal halide perovskites plays an important role in determining the performance of perovskite solar cells. For example, the high ionic conductivity in MAPbI₃ compared to the electronic conductivity along with high native ionic disorder leads to the trapping of the electronic carriers. The intrinsic ionic defects, specifically halide ion vacancies, often dictate the mobility of halide species within the perovskite film during the operation of solar cells. Whereas entropy of mixing explain the thermally activated mixing of halide ions to yield mixed halide perovskite, an opposite trend is observed when the mixed halide film is subjected to photoirradiation. This process, commonly referred as photoinduced phase segregation or halide segregation, yields Br-rich and I-rich perovskite domains. The interesting part is the total recovery when the film is left in the dark as thermal activation drives back to original composition. Formation of such Br-rich and I-rich domains severely affects the solar cell performance. The kinetic and thermodynamic arguments that explain the photoinduced halide segregation and dark recovery will be discussed.

The promising photovoltaic properties of organometallic halide perovskite based solar cells (PSCs) make it highly competitive with the existing solar technologies. Recently, the record power conversion efficiencies (PCEs) of 24.2 % for the single junction perovskite solar cell and 28 % for the monolithic perovskite/silicon tandem solar cell were reported. The rapid enhancement in device efficiency has attracted increasing attentions in the field of photovoltaics. Tremendous research efforts have been devoted to optimizing the performance of perovskite solar cells and increasing the device area to meet the criteria for future commercialization. Passivation engineering is an effective approach to enhance the stability and PCE of PSCs. In this work, we demonstrated effective passivation strategies for the bulk of mixed perovskite and the electron transport layer (ETL). Systematic experiments were conducted to investigate the electron transport materials (ETM) and their layer architecture. It is found that the SnO₂ based ETM in a multilayer structure is superior in suppressing the I-V hysteresis and enhancing the performance of PSCs due to the optimized morphology of the ETL and the passivation effects attributed to the presence of additional nanoparticles and thin films between the ETL/perovskite interface. The performance of PSCs can be further improved by incorporating a controlled amount of organic cross linker in the perovskite absorber. The experimental results obtained from photothermal thermal deflection spectroscopy, time resolved photoluminescence and scanning confocal microscopy show that an optimal amount of the organic cross linker can passivate the defect states in the bulk of mixed perovskite. However, excessive organic cross linkers will significantly lower the device performance of PSCs due to the generation of new defects and the insulating properties of the organic cross linker itself. A PCE of 19 % with negligible hysteresis can be achieved after careful optimizations of the perovskite absorber and material interfaces in PSCs. This work will provide useful information for the photovoltaic community to optimize the fabrication process of PSCs through the passivation engineering.

As the stability and efficiency of perovskite solar cells improve, methods for scaling laboratory performance to industrial production are increasingly sought after. Current efforts focus primarily on transferring the chemistry of spin-coating to high-throughput, continuous processes. However, perovskites are also compatible with vapor-processing, meaning that established thin film PV manufacturing methods could be used to address scale-up challenges. For instance, First Solar’s Vapor Transport Deposition (VTD) process has made CdTe the dominant thin film PV on the market; and VTD is particularly appealing for perovskites due to its high-throughput, dry processing and its highly efficient use of toxic material.

Here, an all-vapor process for CH₃NH₃PbI₃ perovskite deposition using close-space vapor transport (CSVT) is described, and its viability as a fabrication route is confirmed. The film is formed in a 2-step process with deposition of a PbI₂ precursor layer followed by its reaction in CH₃NH₃I vapor. A pilot CSVT reactor was designed and constructed with control of critical process variables including: source temperature, substrate temperature, system pressure, carrier gas flow rate, and the distance between the source and the substrate. As a result, a range of deposition rates, reaction rates, mass transport regimes, and morphologies are accessible which can be used to improve device performance.

Films of PbI₂ were deposited on CdS/ITO/SLG substrates in low vacuum at pressures ranging from 1 to 10 torr. CdS was chosen as the electron-selective contact due to its favorable band alignment with CH₃NH₃PbI₃ and its low-temperature, robust processing through chemical surface deposition. Temperatures, pressures, and deposition times were optimized for deposition of 200 nm films of PbI₂. At a source temperature of 260 °C, substrate temperature of 215 °C, and pressure of 1 torr, full coverage of the CdS substrates at the 200 nm target thickness was achieved after a 20 min deposition. Control of morphology and crystallinity in highly-oriented PbI₂ films was demonstrated, and their dependence on processing conditions will be described.

For the 2^nd processing step, PbI₂ films were reacted with CH₃NH₃I vapor by CSVT to form CH₃NH₃PbI₃. Temperature was found to have a significant effect on the rate of reaction and control of film morphology. At 160 °C, complete conversion from PbI₂ to CH₃NH₃PbI₃ is achieved within 10 min, but the CH₃NH₃PbI₃ grains coalesce at this temperature, leaving the CdS substrate exposed in the form of large pinholes. On the other hand, at 100 °C reacted films are pinhole-free with full surface coverage, but they require 150 min for complete conversion.

While isothermal reactions at 100 and 160 °C produced single-phase perovskites, reactions with a temperature gradient between the source and substrate promoted the formation of uncharacterized secondary phases. A brief rinse in isopropanol removes these phases, leaving behind only single-phase tetragonal CH₃NH₃PbI₃. The structure and composition of these secondary phases are under investigation, but it is possible that the temperature gradient promotes CH₃NH₃I deposition which drives an alternate chemical reaction.

Devices with CSVT CH₃NH₃PbI₃ films were completed with spin-coated, hole-selective spiro-OMeTAD and evaporated gold back contacts, and a 12.1% efficient cell with V_OC = 980 mV, J_SC = 21.9 mA/cm², and FF = 56.6% was measured. This result demonstrates the viability of CSVT as an all-vapor processing method for perovskite solar cells.

When there is no clear study of doped ternary metal oxide for efficient hole transport layers (HTL), we propose the ingenious and multifunctional synthesis approach of In doped CuCrO2 nanoparticles (NPs) HTL including simplifying the synthesis requirements, enabling doping and achievement treatment-free HTLs. To be more specific, we demonstrate an azeotropic promoted approach (APA) to synthesize In doped CuCrO2 nanoparticles (NPs) [1].
Remarkably, compared with the conventional method for synthesizing CuCrO2 NPs, the reaction time is dramatically shortened by 90% and the calcination temperature is lowered by one-third, which not only promote the high throughput production but also reduce power consumption and cost in synthesis. Equally important, we successfully dope Indium into CuCrO2, which is fundamentally difficult to low temperature process. The In doping offers less d-d transition of Cr3+ and p-type doping characteristics for improving the transmittance and conductivity of the hole transport layer (HTL) respectively. Interestingly, the In doped CuCrO2 HTL with these improvements can be achieved by simple ambient condition process and exhibits thermal stability up to 200 ^oC, which is beneficial for realizing highly performed perovskite solar cells (PSCs). Our results show that PSCs with the power conversion efficiency (PCE) of 20.54 % has been achieved with higher short-circuit current density (Jsc) and fill factor (FF) than that of the control PSCs with pristine CuCrO2 NPs. Meanwhile, the devices show good repeatability and photostability. Consequently, we demonstrate an effective approach for realizing efficient HTL from the synthesis of material all the way to film formation featuring with substantial simplification of in synthesis conditions of In doping CuCrO2 NPs to treatment-free high-quality film process for favoring the practical applications of highly performed PCSs.

[1] B. Yang, D. Ouyang, Z. Huang, X. Ren, H. Zhang, W. C. H. Choy*, "Multifunctional Synthesis Approach of In:CuCrO2 nanoparticles for Hole Transport Layer in High-Performance Perovskite Solar Cells", Adv. Function. Mater., in press.

With the developement of compositional engineering, the power conversion efficiencies (PCE) and air stabilities of organic-inorganic hybrid perovskite solar cells (OIHPs) have been greatly improved. Triple A cations of Cs, MA and FA are the most popular compositions used in OIHPs recording high PCEs over 22%. Different from the modulation of cations in A site, manipulation of cations in B site is relatively limited compare to the cations in A site due to the quite unstable properties of elements used in B site such as Ge and Sn. Recently, the use of GeI₂ in perovskite solar cells was reported to improve the PCEs and efficiencies of cells. However, Ge-Pn or Ge-Sn systems were mainly focused on inorganic based perovskite solar cells because of extremely poor stability of GeI₂ when organic cations in A site such as MA and FA are used, leading to poor electronic performances. We found out that the incorporation of MACl in the precursor significantly improves the solubility of GeI₂ in the precursors. Based on this fact, the triple B cations composed of Pb-Sn-Ge were used in OIHPs based on organic A cations composed of MA and FA. As a result, genernal optoelectronic properties were improved with high Jsc and Voc leading to superior PCEs.

Perovskite solar cells are the fastest developing solar cell technology till time. In a short span of time, power conversion efficiencies exceeding 23% in laboratory cells were reported by different research groups, hereby challenging existing dominant photovoltaic technologies. The focus of PeroPrint and the follow up project Upero-funded by the Swiss Federal Office for Energy is to develop perovskite solar cells with high efficiency, using industrially relevant printing and coating techniques.
The layered architecture of perovskite solar cells, as well as the ability of raw materials to disperse and dissolve into inks, opens the doors for solution processing of the cells. On the other hand, printing and coating technologies are large area deposition methods, which can drastically reduce production costs, especially when performed under ambient conditions. This is contrary to existing industrial photovoltaic products, which rather employ energy and waste intensive manufacturing techniques, such as high-purity ingot growth or vacuum deposition. The latter techniques also require huge capital expenditure to get production plants up and running at a profitable scale, a domain where simpler printing and coating techniques could lower the entry barrier to production.
In this project, we were able to develop novel inks to efficiently slot die coat the full stack of carbon based monolithic perovskite solar cells (Glass/FTO/compact TiO₂, mesoporous TiO₂, mesoporous ZrO₂, carbon with infiltrated MAPbI₃ into the porous structure) with power conversion efficiencies exceeding 12% for a large area cell. Here we also demonstrate an inventive co-firing method to fabricate monolithic perovskite solar cells where only one high temperature step is required in comparison to four reported till time for this architecture, thus reducing the fabrication time from 14 hours to 3.5 hours. We also demonstrate high shelf-life of the slot die coated cells exceeding 1 year when stored under ambient condition without any encapsulation.

Long-term stability has been a key challenge towards commercialization of organic-inorganic metal halide perovskite solar cells despite their high-power conversion efficiency and low-cost. To enable improvements in stability, it is of paramount importance to understand the chemical processes that could trigger degradation and the associated reaction pathways within the perovskite as well as its interaction with other materials. To this end, we employ temperature programmed desorption (TPD) and Fourier transform infrared (FTIR) spectroscopy techniques to study the thermal stability and decomposition of organic halide precursors (e.g. methylammonium (MAX), formamidinium (FAX), X = I, Cl, Br) and halide perovskites by themselves, as well as when they are in contact with metal oxide materials used as transport layers. In our experiments, the powder sample is heated with a controlled rate at atmospheric pressure. The gases evolved during heating from desorption, reaction, or decomposition are carried using an inert gas to a mass spectrometer and FTIR connected inline with the furnace and are simultaneously analyzed. The composition of these released gases as a function of temperature enables direct deduction of the thermal processes and helps elucidate the reaction steps involved in the degradation mechanism. Using this approach, we determine that the decomposition of MAI (CH₃NH₃I) proceeds via the formation of CH₃I and NH₃ above 250 °C. However, when MAI is mixed with NiO, the decomposition temperatures lowers to 175 °C with release of CH₃NH₂ and H₂O. Although, both NiO and SnO₂ reacts with MAI, NiO is found to be more reactive, consistent with n-i-p devices with SnO₂ are more stable than p-i-n devices with NiO. Studies will also be performed on other halide precursors, halide perovskites, and other commonly used metal oxide transport layer materials. These studies provide fundamental knowledge that will facilitate the choice of suitable perovskite and transport layer materials and processing conditions to further engineering of perovskite solar cells with enhanced stability.

Lightweight, plastic photovoltaic (PPV) devices are attractive renewable energy devices because they can be fabricated using high-throughput printing process and they enable the possibility to use solar power on curved surfaces. Generally, the power conversion efficiency (PCE) of a PPV is lower than that of its rigid counterpart, primarily owing to the limitations of the low thermal budget of plastic substrates. Apart from this problems, its flexibility is, ironically, another troublesome factor that causes PCE low due to insufficient light capture from its thin absorbing layer. Perovskite solar cells (PSCs) utilizing organometal halide perovskite absorber is considered a favorable candidate of PPV because of its unrivalled extinction coefficient, low exicton binding energy and low formation temperature. However, most published processes to deposit perovskite absorber involves the use of polar organic solvent such as DMF or DMSO, which is toxic and can not be vastly used in production scale. In the past few years, we devoted ourselves to develop a low-toxic process to fabricate high PCE PSC using only water as the primary solvent.
In this presentation, the journey of our efforts in this protocol and our targets in near future is shared. The conversion mechanism, morphology control and device engineering will be highlighted. Currently, we have achieved 16.5% plastic PSC using Pb(NO3)2/water as the starting material to prepare organometal halide perovskite layer.

Perovskite solar cells (PSCs) have been rapidly improved in their energy converging performances, currently exhibiting up to 24.2 % of power conversion efficiencies (PCEs). Despite their great performances, they still suffer from various issues such as stabilities and scaling-up problems. PSCs struggle to maintain their performance owing to fast chemical decomposition or performing instability caused by hysteretic behavior or ion migration. Large-area perovskite film fabrication and cost problems of electrodes should be resolved for practical photovoltaic application.
Among present problems, electrode selection is one of most important assignments in solving both hurdles for commercialization because the most popular metal electrodes, Au or Ag, are instable chemically and it is too much expensive to be used in mass production system, especially Au electrode. PSCs with Ag electrode performs low PCEs with low open circuit voltage and fill factor showing an s-shape kink in current density-voltage (J-V) curves at first. However, with aging time, the performance of PSCs recovers to the normal level and such abnormal performing instability can be a huge obstacle for commercial application of Ag electrode. The s-shape J-V curves are mainly attributed to inferior majority charge carrier extraction from the photo-absorbing area to the cathode or anode, however, the underlying reason for these phenomena, especially for the abnormal aging and recovery mechanism in PSCs with Ag electrode has not been elucidated at all. It is important to understand the origin of the s-shape J-V curves of PSCs with Ag electrode at early stage and the mechanism of aging and recovery phenomena with elimination of s-shape for securing the possible use of Ag electrode in commercialized PSCs.
Here in this work, we present the different time-dependent performance change trends of PSCs with regard to metal electrodes (Au and Ag). PSCs with Ag electrode exhibit a clear s-shape J-V curves in early stage, however they recover to their best performance with 12 h of aging time. On the other hands, PSCs with Au electrode exhibit their best performance from the beginning. In order to clarify this abnormal aging and recovery process in PSCs with Ag electrode, we also investigate the time-dependent carrier dynamics alteration by photo-luminescence behaviors which proves the performance changes with aging time. By the measurement and simulation results, we are able to verify that the work function difference between Ag electrode and hole transport materials, Spiro-MeOTAD, induces a significant injection barrier for holes, and it clearly cause the formation and alleviation of the s-shape J-V curves in PSCs with Ag electrode. We finally address the potential of Ag electrodes for commercial use by demonstrating highly stable PSCs with Ag electrode for durations exceeding 350 h under light-illumination, which is close to the results of PSCs with Au electrode.

Perovskite solar cells (PSCs) have attracted intensive attention from many researchers due to the excellent properties of perovskite materials and simple device fabrication process. Thanks to worldwide efforts, PSCs have achieved the most rapid development with certified efficiencies rising from 3.8% in 2009 to 24.2% in 2019. Different from mesoscopic structure or planar structure, the triple mesoscopic perovskite solar cells are based on a TiO₂/ZrO₂/carbon triple-layer mesoporous scaffold, acting as the electron transporting layer, spacer layer and counter electrode, respectively. All these functional layers are fabricated by screen printing along with subsequent sintering. Innovatively, the triple mesoscopic PSCs eliminate the use of noble metal electrodes and hole transport materials, which can simplify the fabrication procedures and reduce the fabrication cost. More impressively, the stability of PSCs with the triple mesoscopic structure is quite superior and different groups have confirmed it in recent years. Photostability of more than 1000 h in ambient air under full sunlight irradiation was first reported in 2014. Subsequently, stability of more than 10,000 h with zero loss in performances was obtained under controlled standard conditions in 2017. In addition, thermal stability of the triple mesoscopic PSCs is also excellent for they can maintain initial efficiency for over 1500 h during the 100°C thermal test in the dark. Based on these advantages, the triple mesoscopic PSCs have drawn lots of interest and become a competitive candidate for the new generation of photovoltaic technology. And a series of work have been performed to improve the performance of the triple mesoscopic PSCs, including perovskite modification, interface engineering, optimization of each functional layers, post-treatment and encapsulation. Up to now, the state-of-the-art power conversion efficiency of 15.77% has been obtained, exhibiting promising future of commercialization for the triple mesoscopic PSCs.

In an efficient perovskite solar cell (PSC), the quality of the charge transport layers should be significantly considered. In case of the electron transport layer (ETL) materials, TiO₂ is a most commonly used inorganic semiconductor as ETL. However, due to the limitation of the intrinsic properties of TiO₂, it always suffers from high processing temperature, low electron mobility, and strong photocatalytic activity, resulting into the undesirable carrier recombination for TiO based PSCs. Comparatively, zinc oxide (ZnO) is an alternative metal oxide ETL owing to its high electron mobility and comparable energy level as well as low temperature fabrication. In addition, ZnO films can be simply prepared through the various solution- or vacuum-based techniques, which is beneficial to the mass and low-cost production of photovoltaic devices. Although the solution processed ZnO-based PSCs have acquired the significant achievements, some problems still need to be settled for the future commercial applications. An important issue is the hydroxyl groups and/or residual acetate ligands remained on the solution-processed ZnO surface, which accelerates the chemical decomposition of the perovskite film during the fabrication processing. In comparison to various solution methods, magnetron sputtering technique is a traditional vacuum-deposition approach to form stoichiometry and morphology controlled metal oxide semiconductors at a desirable temperature. On the basis of sputtered ETLs-based normal or inverted PSCs, the thermochemical stability of the devices is rarely discussed. Most importantly, the sputtered samples contain few hydroxyl groups because the hydrogen-free ceramic target and working gases are used during the whole fabrication process, which effectively avoid the chemical decomposition of the perovskite layer and is beneficial to enhance the stability of the devices. Hitherto, the ETLs with various amounts of hydroxyl groups prepared by vacuum and solution methods are seldom considered.
In this study, the thermochemical stability of the perovskite layer and PSCs on ZnO substrates containing the different amount of hydroxyl groups is investigated. To evaluate the chemical decomposition of the perovskite layer caused by the amount of hydroxyl groups remained on the interface of ZnO/perovskite films, ZnO films with different amount of hydroxyl groups are prepared by magnetron sputtering and spin-coating methods. The sputtered ZnO film contains the negligible hydroxyl groups because the hydrogen-free working gases of argon and oxygen are introduced during the sputtering process. In case of the optimized deposition condition under Ar/O₂ ratio of 1:4, the sputtered ZnO film shows larger grain size, bigger water contact angle, higher energy-level matching, better optical transmission, and higher conductivity in comparison with the spin-coated ZnO film. Moreover, the planar perovskite solar cells (PSCs) based on the sputtered ZnO electron transport layers (ETLs) yields an optimal power conversion efficiency of 17.22%, sharply outperforming that based on the spin-coated ZnO ETL (14.24%). More notably, the thermochemical stability of the device based on the sputtered ZnO ETL was also significantly improved.

The performance of perovskite solar cells has been developing rapidly since 2009, the certified power efficiency has exceeded 24% in a few years. It has a very broad application prospect on basic research fields and new energy development, thus which has attracted great attentions. Ultraviolet radiation in solar spectrum is inevitable that can destroy the performance of perovskite solar cells in the same time. In order to solve this problem, we introduced photoluminescent glass in perovskite solar cells replacing traditional glass basement. The key point is SBOET photoluminescent glass, which can convert high-energy ultraviolet light to visible light for improving the stability and efficiency of perovskite solar cells. On the one hand, it can decrease the decomposition of perovskite material because less ultraviolet radiation can pass through SBOET glass. On the other hand, it can increase visible radiation by photoluminescence conversion. So that utilization of dual function SBOET glass can both improve stability and efficiency of the perovskite device.

Small cation (Cs⁺ and Rb⁺) doped formamidinium lead triiodide (FAPbI₃) is one of the most promising perovskite materials for solar cell applications because of the elimination of thermally unstable methylammonium (MA) cation and the reduced phase transition of FAPbI₃ from photoactive α-phase to photo-inactive δ-phase. Comprehensive studies have been performed on the stability of small cation doped FAPbI₃ perovskites with respective to oxygen, humidity, and temperature. However, the stability under the operational condition, such as applied electric bias and solar illumination, is equally important, yet has not been well studied.
In this work, we fabricated MA-free Rb_0.05Cs_0.10FA_0.85PbI₃ perovskite via hot casting and anti-solvent method. Diethyl ether and chlorobenzene were applied as the anti-solvent. Diethyl ether served as a better anti-solvent than chlorobenzene, yielding densely packed, uniform α-phase Rb_0.05Cs_0.10FA_0.85PbI₃ perovskite thin films confirmed by SEM and XRD. Perovskite solar cells with the structure of ITO/PEDOT:PSS/Rb_0.05Cs_0.10FA_0.85PbI₃/PC₆₀BM/BCP/Ag exhibited an average PCE of 12.14% and a steady Jsc at the maximum power for 60 min. The PCEs of the devices, stored in N₂ glove box, dropped to ~60% of the initial value over the first 15 days and retained the same level for the next 15 days with a testing interval of 3 days in these 30 days, and kept the same PCEs when tested in another 20 days later. No interfacial degradation was confirmed via light intensity- and effective voltage-dependent J_SC measurements. SEM images showed that the perovskite thin film still remained crystalline grains with clear grain boundaries over the entire cross-section. However, XRD patterns showed that small amount of photo-inactive δ-phase perovskite formed in the region right under the electrodes while the region in between electrodes remained photoactive α-phase, indicating the initial PCE drop is due to the δ-phase perovskite formation and electric-field plays an important role.
The impact of precursor preparation on the formation of phase pure Rb_0.05Cs_0.10FA_0.85PbI₃ perovskite was further investigated. Rb/CsPbI₃ phase segregation occurred in the formation of Rb_0.05Cs_0.10FA_0.85PbI₃ perovskite thin film if the precursor solution was not well mixed before spin coating. While a higher average PCE of 13.23% and a steady Jsc at the maximum power for 60 min were achieved from the devices prepared with the unwell-mixed precursor solution, the devices exhibited an accelerated degradation and strong dependence on testing frequency, which means the exposure to the applied electric field. With the testing intervals of 3, 15, and 30 days, the PCE reduced to 8, 20, and 44% of the initial values, respectively. The perovskite under electrodes turned to yellow while in between electrodes were still black. No interfacial degradation was confirmed. The perovskite material degradation was investigated via SEM, which showed small pin holes in the cross-sectional area right under the electrode, and XRD, which showed the significantly decreased α-phase peaks accompanied by the significantly increased δ-phase peaks. The perovskite in between electrodes, again, remained crystalline grains with clear grain boundaries and α-phase with trace δ-phase formation. We proposed that the electric field induced degradation via small cation migration between vacancies could be a possible mechanism. More experiments were conducted to verify this proposal. Furthermore, the possible reducing phase segregation methods were also proposed and experimentally tested.
This work provides an insight on the selection of anti-solvent for making mixed-cation perovskite thin films, demonstrates the importance of precursor preparation to the operational stability of devices, reveals a perovskite solar cell degradation mechanism via the electric field driven phase segregation, and offers the means to reduce such phase segregation and to enhance perovskite solar cell stability.

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted considerable attention due to their remarkable power conversion efficiency (PCE) that lately exceeds 22 %. However, stability issue remains regarding their use in real-life environments with the most urgent matter being their long-term stability under humid conditions. The hybrid perovskites are inherently vulnerable to water molecules, which can induce degradation of perovskite photoactive chemicals such as MAPbI₃, FAPbI₃. Therefore, to realize the commercial-level long-term PSC stability, the adsorption and infiltration of water into perovskite films must be minimized. Herein, it is demonstrated that polydimethylsiloxane (PDMS) introduced simultaneously during perovskite spin-coating is highly beneficial to passivate perovskite grains and adjacent grain boundaries (GBs). It promotes the formation of lead oxide (PbO) bonding that prevent a water-perovskite reaction and contributes to reducing a Pb defect density related to trap-assisted recombination. Owing to the reduced trap-assisted recombination, the best PDMS-passivated PSC exhibits a PCE of 16.16 %, FF of 70 % and J_sc of 21.10 mA/cm². The photovoltaic performance of the PDMS-passivated PSC is notably enhanced compared to a reference PSC (without PDMS). Furthermore, the PDMS with abundant methyl moieties is hydrophobic, effectively limiting water infiltration into the perovskite film and PbO bonding formed by PDMS passivation can prevent catalytic reaction between Pb atoms and water molecules. Thus surprisingly, more than 90 % of the initial PCE (~15 %) is sustained after laboratory storage of 5000 h under 70 % relative humidity, while only 20 % of original PCE of reference PSC is retained after 2000 h under the same conditions. These results will pave the way for developing commercial perovskite optoelectronic devices.

Many organic cations in halide perovskites have been studied for their application in perovskite solar cells (PSCs). Most organic cations in the PSCs are based on protic nitrogen cores, which are susceptible to deprotonation. Here, a new candidate of fully alkylated cation (target cation) was designed and successfully assembled into PSCs with the aim of increasing humidity stability. To ensure promising photovoltaic performance of three-dimensional perovskites, trace amount of target cation was assembled into perovskite solar cells by using typical solution spin-coating method. From the results of the structural and morphology studies combined with XPS analysis, it was evident that the target cation was introduced in the grain boundaries and/or surface of perovskites without change in 3-D perovskite lattice, which resulted in the comparable photovoltaic performance (η ~ 19 %) of 3-D MAPbI₃ under 100 mWm^-2 irradiation.
Regarding the stability of perovskite film on moist, it was found that the generic feature of A-site cations in the form of ABX₃ perovskites significantly affected humidity stability of perovskites, with ammonium-based perovskites giving a poor moisture-tolerance. However, the target cation-driven perovskites showed a much more pronounced effect on the increase in humidity stability, which emphasizes a generic electronic difference between protic versus aprotic cation. From the point of view on the humidity stability, it should be added that not only hydrophobic alkyl (or aromatic) chain but also basicity of cations should be considered when engineering large cations for their use in perovskites. Based on the prominent photovoltaic performance and stability of target additive-driven PSCs, current additive is considered as logical cations for further design of 2-D, 3-D, or 2D/3D hybrid perovskite.

In organic-inorganic hybrid solar cells using a lead halide-based perovskite compound as a light absorbing layer, improvement in durability is one of the major issues in practical use. Several studies have been reported on improving the durability of the perovskite layer. In particular, it have been reported that the stability of the compounds is enhanced by mixed crystal formation of lead halide perovskite compounds. In particular, it has been reported that stability is improved by mixing inorganic cations such as Cs and Rb with organic cations such as CH₃NH₃⁺ (MA⁺) and HC (NH₂)₂⁺(FA⁺) ^{(1) (2)}. In addition, recently, it has been reported that energy conversion efficiency and stability are improved by mixing CH₆N₃⁺ as an organic cation ⁽³⁾, but these deterioration suppressing mechanisms have not been sufficiently elucidated yet. We are conducting research focusing on the durability mechanism of the perovskite layer with mixed cations and halide anions.
In this study, the effects of temperature and humidity on the crystal structure, morphology, charge transport properties and photovoltaic properties of the mixed perovskite compound films based on FAPbI _3-y Br_y were systematically investigated. Furthermore, the results of evaluating the influence of the ratio of the inorganic cation (Cs or Rb) and the organic cations of the mixed perovskite compound thin film on the durability are also reported. Detailed results will be discussed on the conference.

Solution-processable perovskite materials have already been proven the potential for applications in optoelectronic devices, including solar cells, photodetectors, light-emitting diodes etc. Stability of the perovskite solar cells is particularly important for the eventual application. The formation of secondary phase in the complex perovskite active layer, typically with multiple ions, is one of the causes leading to the deterioration in the cell performance. In this work, we report our effort to understand the thermal stability of the solution-processed methylammonium- and bromine-free formamidinium-based perovskite (FA_0.85Cs_0.1Rb_0.05PbI₃) layers, and the relationship between the formation of secondary phase and long-term stability of the FA_0.85Cs_0.1Rb_0.05PbI₃-based perovskite solar cells. It shows that the emergence of secondary phase in the perovskite layer is closely associated with the storage time of the perovskite precursor solution. An obvious decrease in power conversion efficiency was observed when the secondary phase was formed in the perovskite active layer in the cells. The formation of the secondary phase in the perovskite active layer leads to an increase in charge recombination, due to the reduction in the built-in potential across the cell. The improved understanding of the formation mechanism of the secondary phase and the solutions to supress the formation of the secondary phase are the prerequisite for attaining high performing perovskite solar cells.

Here we present the preparation of solution processed mixed cation GA_x(Cs_0.05MA_0.15FA_0.8)_1-xPb(I_0.85Br_0.15)₃ (GA= guanidinium; MA= methylammonium; FA= formamidinium) organic-inorganic hybrid perovskite films and their incorporation into mesoporous perovskite solar cells by spin coating solution processing. We found that incorporation of GA into Cs, MA, and FA triple cation perovskite enables the formation of highly crystalline black phase perovskite which enhance charge carrier transfer and inhibits charge recombination at the perovskite/spiro-OMeTAD interfaces and therefore improves short-circuit current density of the device. Time-resolved photoluminescence (TRPL) data confirm a faster decay of the PL signal in the presence of the perovskite/spiro-OMeTAD with GA incorporation perovskite solar cell in comparison with triple cation perovskite solar cell. The effect of GA substitution was systemically investigated, and the 5% GAI was the optimum condition to achieve high performance perovskite solar cells. This study will provide a basic insight for the mixed-cation perovskite for the future energy harvesting applications.

High power conversion efficiency,¹ high specific power (W/g) and stowed packing efficiency (W/cm³),² low processing cost (US cents/kWh)³ and high tolerance against space environmental threats (high energy and charged particle radiation^4,5) makes perovskite solar cell (PSC) a promising candidate as space-based photovoltaics. However, vacuum causes outgassing of perovskite absorber,⁴ raising concerns on its long-term reliability. In this work, we investigated how different pressure levels affect the operational stability of non-encapsulated PSCs with the state-of-the-art (SOA) architecture. Stability of PSCs decreases upon reducing the operational pressure while they are kept at the maximum power point (MPP) condition under 1 sun continuous illumination. By performing XRD, UV-Vis, SEM and SIMS measurements, we reveal the vacuum induced efficiency loss mechanism. In addition, we developed two PSC architectures that effectively delay/mitigate these vacuum induced detrimental effects. PSCs with one of the new developed device architectures show a low PCE loss rate of 0.007%/h in 1037 h at MPP condition under 50 mbar, equaling to a projected T₈₀ lifetime of 4750 h. The findings present in this study lay the foundations towards highly stable PSC structures for applications in low-pressure environment, as encountered in outer space.

Reference:
Best Research Cell Efficiencies (NREL, 2019); https://www.nrel.gov/pv/cell-efficiency.html and https://www.nrel.gov/pv/module-efficiency.html
M. Kaltenbrunner et al., Flexible high power-per-weight perovskite solar cells with chromium oxide–metal contacts for improved stability in air Nat. Mater. 14, 1032–1039 (2015).
M. Cai, Y. Wu, H. Chen, X. Yang, Y. Qiang, Li. Han Cost-Performance Analysis of Perovskite Solar Modules, Adv. Sci. 4, 1600269 (2017).
Yu Miyazawa et al., Tolerance of Perovskite Solar Cell to High-Energy Particle Irradiations in Space Environment, iScience 2, 148–155 (2018).
Felix Lang et al., Radiation Hardness and Self-Healing of Perovskite Solar Cells, Adv. Mater. 28, 8726–8731 (2016).

In this work, two lead halide perovskite photodetector were fabricated by laser-assisted rapid fabrication method. A microchannel was engraved on an indium tin oxide (ITO) coated polyethylene terephthalate (PET) conductive flexible substrate using a CO2 laser source. The channels were filled by methylammonium lead halide perovskite (CH₃NH₃PbI₃) using the capillary motion of perovskite precursor. CYTOP and the low-cost commercially available FluroPel were used as a top protective coating layer to suppress the decomposition of the perovskite channel. X-ray diffraction pattern (XRD) was used to measure the stability of the perovskite. Strong humidity resistant and self-healing behavior were observed in both devices. The performance of the two photodetectors was compared by measuring electrical and optical characteristics over time. This study will help in the low-cost fabrication of perovskite-based devices.

Perovskite solar cells have exhibited impressive power conversion efficiency, but to be considered for large scale commercial applications these cells have to be stable under operating conditions including exposure to sunlight, heat and electric bias. Here we present stability studies using concentrated sunlight, which allows rapid screening of the degradation parameters in the cells. Specifically, accelerated degradation studies to determine factors affecting degradation at different bias conditions were performed.
Our experimental methodology allows independent control of sunlight intensity, the sample temperature and environment during the exposure. Stress testing of perovskite solar cells showed that faster degradation was found for cells held at SC under concentrated sunlight and on the initial stage of outdoor exposure. However, cells kept at short circuit (SC) showed better long-term stability compared to cells kept at open circuit (OC) upon real operational conditions. We also found that intensity was more important than dose for cells degradation at SC conditions, while dose was the determining factor at OC. This indicates that different degradation mechanisms are dominant at different degradation stages and under different bias conditions and that nano-scale understanding of degradation mechanisms is required to suggest ways to increase the device life-time.

Over the last few years, perovskites have emerged as promising photovoltaic materials because of their ease of fabrication and high power conversion efficiency. However, their long-term stability remains a major concern for their commercialization. Here we present stability studies using concentrated sunlight, which allows rapid screening of the degradation parameters in the cells.
Studies conducted on dual cation perovskite solar cells revealed that there are two different degradation mechanisms being dominant at different exposure intensities. At 10 to 30 suns exposure, the PCE degradation behaved entirely different from that of 40 and 50 suns exposed samples. We have studied the mechanisms behind this PCE degradation with the help of photoluminescence mapping and by electrical measurements including photo-induced charge carrier extraction by linearly increasing voltage (photo-CELIV), transient photo-current and impedance, using the PAIOS instrument. PL mapping shows non-homogeneous degradation only at intensities of 40-50 suns, which is non-reversible, in contrast with that at lower sunlight concentrations. Electrical measurements showed that cell degradation up to 30 suns can be due to the generation of traps. The different acting degradation mechanisms indicate that accelerated stability testing using concentrated sunlight is limited to smaller sunlight concentrations.

Hybrid halide perovskites (HOIHPs) have been extensively studied in recent years due to their potential use as light-harvester in photovoltaic devices. While the efficiencies of such devices pose no limitation to commercial applications, the severe lack of stability of the materials remains an important issue to be overcome. Indeed, HOIHPs are known to easily degrade under moderate thermal stress,^[1] or upon oxygen^[2] and/or light exposure.^[3] Notably, recent calorimetric studies even suggested that, in agreement with DFT calculations,^[5] some HOIHPs (MAPbI₃ in particular and MAPbBr₃ to a lesser extent) could be thermodynamically unstable ^[4] with respect to decomposition into their binary precursors. In contrast, other studies indicate the materials to be intrinsically stable.^[1,6] Obviously these questions need to be unambiguously answered. In this contribution, we discuss the underlying thermodynamics of HOIHPs. We treat the intrinsic (vs temperature) and the extrinsic (vs oxygen) stability, as well as decomposition processes induced by light and applied bias. Intrinsically, we find the materials to be stable under standard conditions (albeit, in the case of MAPbI₃, only marginally), and we also can assess the favourable degradation path upon heating. A strong tendency towards instability is expected under real conditions. Extrinsically, our considerations reveal a large tendency towards degradation of HOIHPs in the presence of oxygen, which is especially severe under real conditions. Notably, light itself can activate a relevant photodecomposition pathway.^[7] We discuss these issues on a quantitative level, in conjunction with experimental observations of the degradation phenomena.

References
[1] B. Brunetti et al., Sci. Rep. 2016, 6, 31896.
[2] N. Aristidou et al., Angew. Chemie 2015, 54, 8208.
[3] Y. Li et al., J. Phys. Chem. C. 2017, 121, 3904.
[4] G. P. Nagabhushana et al., Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 7717.
[5] E. Tenuta et al., Sci. Rep. 2016, 6, 37654.
[6] I. L. Ivanov et al., J. Chem. Thermodyn. 2018, 116, 253.
[7] G. Y. Kim et al., Nature Mater. 2018, 17, 445.

Organic-inorganic halide perovskite (OIHP) based solar cells have rapidly emerged as a promising photovoltaic technology because of fascinating optical and electronic properties of OIHPs. However, perovskite solar cells (PSCs) are extremely fragile and have cohesion energy values lower than that of even the organic solar cells. Also, the devices have multi-layered structures with functional layers possessing different thermal and mechanical properties. So, thermal cycles during device fabrication can induce macroscopic residual stresses in the OIHP film. The possibility of these stresses leading to different modes of film fracture (surface cracks, channelling cracks and interface cracks) is evaluated. The behaviour of the devices is also studied under externally applied stresses. The effect of interface modification on the cohesion energy of the devices is demonstrated, leading to more mechanically reliable PSCs. The OIHP film’s mechanical stability is evaluated by subjecting the films deposited on flexible substrates to high strains. The microstructural and the residual stress changes during a bending cycle are characterized to understand the mechanisms of failure. This brings insights to the fundamental aspects of the mechanical behaviour of OIHPs which is very likely to be critical inorder to emerge as a highly reliable solar cell technology.

Thin film solar cells based on organo-metal halide perovskite have reached efficiencies above 24%, promising to be part of next generation photovoltaics. However, one of the preconditions for its commercialization is performance stability. In recent years, extrinsic stability of perovskite solar cells (PSCs) has significantly improved due to better encapsulation. As a follow, increasing attention has been attracted towards the intrinsic stability of PSCs. In this regard, many PSCs have been reported to be stable for over 1000 hours under continuous illumination at constant temperature in inert environment. However, in real life outdoor operation solar cells undergo light-dark cycling and temperature variation due to day-night periods. Thus, it’s necessary to understand PSC performance changes due to these variable conditions.

Motivated by this cause, we have tracked the changes in the maximum power of PSCs in three scenarios progressively evolving from conventional laboratory settings to approximate diurnal outdoor settings, i.e. from continuous light at constant temperature to light-dark cycling with respective constant temperatures ending with light-dark cycling with temperature variation under light. Study shows that first and second scenarios do not significantly affect PSC performance, while the third scenario results in irreversible performance degradation. Based on analysis using numerical simulation and advanced electrical characterizations, we arrive at the understanding of this degradation with respect to defect dynamics.

To conclude, this work reveals the combination of day-night cycling and varying temperature causes PSC degradation, owing to the intrinsically unstable property of perovskite, i.e. defects generation and migration. Thus, this work further accents the necessity to control defects formation in polycrystalline perovskite films in order to achieve stable PSCs at outdoor operation settings.

Perovskite solar cells (PSCs), reported for the first time in 2009, are experiencing rapid increase in efficiency from 3.81% at the initial efficiency to 24.2% at present, attracting many researchers with the next generation solar cells. Basically, CH₃NH₃PbI₃ perovskite films (PFs) can be fabricated through the synthesis of the precursor (ex:PbI₂) and CH₃NH₃I. One of the key factor for producing a highly efficient PSCs is to produce a uniform thin films on substrates.
One of the major research topic in the future of PSCs is commercialization strategy, and it is important to deposit thin films uniformly over a large area. However, it is expected that the conventional solution process will have a limitation in uniformly depositing PSCs on a large area. In order to fabricate a large-area PSCs, a method of depositing PFs through the vacuum process is required.
In addition, the vacuum process is also needed from the viewpoint of silicon/perovskite tandem solar cells, deposition of conformal PFs over silicon pyramids is limited by solution processes on textured commercial silicon substrates. For this reason, the silicon/perovskite tandem solar cell has been fabricated to have a flat structure at the front face so that a lot of reflection loss was generated. Recently, there has been a report on fabricating silicon/perovskite tandem solar cell with double-sided texture through evaporation process to reduce reflection loss and increase current density.
In this study, the PFs were fabricated by sputtering and chemical vapor deposition process which are widely used in commercial applications. Perovskite precursor were deposited by sputtering, followed by chemical vapor deposition process for phase converted into the PFs. The films were analyzed by X-ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM). A PSCs with glass/FTO/compact-TiO₂/CH₃NH₃PbI₃/spiro-OMeTAD/Au structure were fabricated. Finally, it was confirmed that the PFs were conformally deposited on the 4-inch textured silicon substrate. The uniformity of reflectance and thickness was higher than 86% and 92%, respectively. This fabrication processes of the PFs are expected to be applicable to silicon/perovskite tandem solar cells.

Currently, various multi-junction solar cell structures are being studied to overcome the theoretical limitation of silicon solar cell. The III-V tandem structure can achieve high efficiency, but it can not be commercialized because of a high cost. In order to overcome this problem, research on perovskite / silicon tandem solar cell using a low cost perovskite solar cell which is rapidly emerging as a next generation solar cell is being conducted. However, most hybrid tandem cell research is based on n-type heterojunction silicon cells, which occupy only a small fraction of the total solar market. Therefore, for commercialization, research on perovskite / silicon tandem solar cell based on PERC structure emerging as the mainstream of next-generation silicon solar cell and Al-BSF structure which is mainstream of silicon solar cell is essential.

Here, we have fabricated perovskite / silicon tandem solar cells by introducing Al-BSF structure and PERC structure. Optimized tandem cells based on Al-BSF structure, fabricated with a 310-nm-thick perovskite layer of (FAPbI3)0.8(MAPbBr3)0.2 and a hole transport layer of PTAA, have significantly increased efficiency of 21.19% compared to stand-alone 13.4% perovskite and 12.8% Si cells. Our tandem cell represents the highest efficiency increment among all monolithic perovskite / silicon tandem cells and the highest efficiency among monolithic perovskite / silicon tandem cells based on p-type homojunction silicon cell with Al back-surface field at the same time. The design rules suggested in this study can be applicable to different types of perovskite / silicon tandem cells as well. We believe that this approach opens up a pathway for the commercialization of perovskite solar cells with industry-standard monocrystalline Si solar cells as highly efficient monolithic tandem devices.

In recent years the significant effort has been made to develop new photovoltaic materials configurations and architectures, among which the considerable attention was drawn to tandem cells. It was theoretically proven that tandem configuration involving two or more solar cells has a great potential to overcome the Shockley-Queisser single-junction efficiency limit. Theoretical calculations predict that the combination of large band gap energy (~1.5 eV) material as top cell and low band gap energy (~1.0 eV) material as bottom cell can allow the conversion efficiency of around 30%. Among solution-processed photovoltaic materials, metal halide perovskites with their broad light absorption spectrum, tunable band gaps, long charge carrier diffusion, and low fabrication cost together with lead sulfide (PbS) colloidal quantum dots (QDs) with highly tunable band gap and strong infrared absorption, shows great opportunity to create such highly efficient tandem cells. However, the biggest challenge here is to incorporate the sensitive and unstable perovskite film into such tandem structure.
Herein, I will demonstrate efficient solution-processed tandem solar cells using Methylammonium Lead Iodide (MAPbI₃) Perovskite in the front cell and PbS QDs in the rear cell and discuss the progress on developing and optimizing semitransparent middle interlayer in such tandem solar cell configuration. Also, further strategies for improving this interlayer together with short overview of other challenges on how to provide even more complementary absorption will be discussed.

The crystalline Si-based solar cells have been dominating the ever-expanding global PV market with about 95% market share. With its highest cell efficiency of 26.6% reaching its theoretical limit, there is little hope to raise its performance by just device optimization within the single-junction cell architecture. As the PV market is so huge with its annual revenue in the order of trillion US dollars, even 1% cost reduction would mean net profit of billions of US dollars per year. Since efficiency increase is proven to be the most reliable means to reduce cost, it makes sense for industries like Hanergy and Shanghai Institute of Microsystem and Information Technologyto support us to develop the tandem cells with potential of significantly increased cell efficiency. There are two basic designs for the tandem cells: (1) two-terminal configuration in which the perovskite top-cell is deposited on the silicon bottom-cell; and (2) four-terminal design in which both the perovskite top-cell and the silicon bottom-cell are completed independently. For the first, in case the top-cell is degraded for any reason, the integrated tandem cell is affected by the same proportion. For the latter, if the perovskite goes bad for any reason, it can be simply removed from the integration and the silicon cell will work just fine. Considering that the silicon solar modules are so robust with product warranty of 25-35 years, and the perovskite solar cells are still in the research stage without any reliability data from the market, the 4-terminal cell design is therefore selected in the present work to not only taking the advantages of higher cell efficiency for reduced cost, but also minimized influence of uncertain reliability of perovskite on the silicon solar modules.

Recently, semi-transparent perovskite soler cells (PSCs) have been investigated as as building integrated photovoltaics (BIPV) for next generation smart building. To realize semi-transparent PSCs, both anode and cathode electrodes have a high transparency and conductivity. However, unlike bottom anode electrodes prepared by typical sputtering, the preparation of top cathode electrode is very difficult because the semi-transparent PSC is severely damaged by irradiation of high energy plasma during the sputtering process. Here, we reported semi-transparent perovskite solar cells (PSCs) with InZnSnO (IZTO) top cathode prepared by two-step sputtering at room temperature. At first sputtering process, 30 nm thick IZTO top cathode was very slowly sputtered at a low DC power to minimize the direct plasma damage of PSCs. Then, thick IZTO top cathode was directly sputtered on the thin IZTO buffer layer to obtain high conductivity ^l and transparency of the IZTO cathode electrode. LFTS-grown IZTO top cathode sputtered at an optimized condition has a sheet resistance of 20.95 Ohm/square and optical transmittance of 85.97 % at wavelength 550 nm. In addition, we confirmed LFTS-grown IZTO top cathode has appropriate energy band structure as a top cathode cathode for semi-transprent PSC by using ultraviolet photoelectron spectroscopy (UPS). The semi-transparent PSCs with LFTS-grown IZTO top cathode has comparable power conversion efficiency (PCE) of 14.08% to PCE(16.46%) of reference opaque PSC with Ag top cathode. Similar PCE value of the semi-transparent PSC prepared by LFTS indicates that the LFTS is a promising thin film process to make IZTO top cathode on the PSCs without plasma damage which was easily found in typical DC or RF magnetron sputtering of TCO top cathode in semi-transparent PSCs.

The tandem solar cells based on perovskite top cells are attracting significant interest due to its high conversion efficiency and decent stability.[1,2] The theoretical calculation shows that the combination of a top cell with a large bandgap energy (1.5~1.7 eV) and a bottom cell with a low bandgap energy (1.0~1.1 eV) can lead to a conversion efficiency higher than 40%. Given that the bandgap energy of most commercial single junction solar cells is around 1.1 eV, the perovskite solar cell with a bandgap energy around 1.6 eV must be a very promising candidate for the top cell of tandem solar cells.
Although the efficiency of the perovskite-based tandem solar cells is rapidly increasing, the accurate characterization of their photovoltaic/electrical properties is quite tricky due to their monolithic device structure. We have recently demonstrated that the photovoltaic/electrical properties of the highly efficient (~24%) perovskite-Si tandem solar cells and each subcell can be accurately characterized using a 3-terminal characterization platform,[3] where we used subcell EQE spectra for precise current matching. In this presentation, the concept of 3-terminal characterization platform and how we utilized it for developing highly efficient perovskite-Si tandem solar cells will be introduced. In addition, some of our recent studies focusing on the unique photovoltaic/electrical properties of each subcell in the monolithic tandem solar cells such as the effect of the subcell illumination on the electron dynamics and the effect of the current mismatching on the photovoltaic properties will be discussed.

References
[1] Best Research-Cell Efficiency Chart (https://www.nrel.gov/pv/cell-efficiency.html)
[2] K. A. Bush et al., Nature Energy 2, 17009 (2017)
[3] I. J. Park et al., Joule 3, 807 (2019)

Using carbon to function as both hole transport layer (HTL) and back contact electrode in perovskite solar cells (PSC) can significantly lower the cost, improve the device stability and expand applications to flexible substrates [1]. The interface between HTL and the perovskite layer which is the only effective hole extraction interface in this device is expected to have a significant impact on the device performance [2]. Besides the effective HTL, a high open-circuit voltage (Voc) of carbon-based PSCs is also key to achieving high performance. One of the reasons responsible for the low Voc in carbon electrode-based PSC devices is the energy level mismatch between the carbon electrode (e.g., graphite, carbon black) and the perovskite film (e.g., CH₃NH₃PbI₃ (MAPbI₃), CH(NH₂)₂PbI₃, CH₃NH₃PbBr₃) [3]. In this work, we develop an elaborate process to engineer carbon paste to optimize the properties of carbon electrodes and the perovskite/carbon interface to fabricate all low-temperature processed carbon electrode-based lead halide perovskite solar cells. In addition, the low-temperature coating process is fully compatible with large scale and continuous fabrication. Carbon electrode-based flexible solar cells can be also fabricated on flexible conductive substrates thanks to the low-temperature process and their compatibility with printing processes.

[1] Z. Wu, Z. Liu, Z. Hu, H. Zafer, L. Qiu, Y. Jiang, L.K. Ono, Y.B. Qi*, Adv. Mater. 2019, 31, 1804284.
[2] L. Qiu, L.K. Ono, Y. Jiang, M.R. Leyden, S.R. Raga, S. Wang, Y.B. Qi*, J. Phys. Chem B 2018, 122, 511.
[3] H. Chen, S. Yang, Adv. Mater. 2017, 29, 1603994.

Hybrid metal halide perovskite solar technology poses exciting global potential for terawatt-level generation and penetration of solar energy into new markets such as transportation and lightweight wireless devices. However, large area scaling and operational reliability remain formidable challenges hindering commercialization. High-throughput open-air fabrication methods such as spray coating and printing could offer low capital-expenditure routes to continuous manufacturing of perovskites, but understanding and overcoming ambient factors such as relative humidity (RH) has been a persistent challenge for achieving high performance and device stability.

In this work, we present moisture-immune open-air spray-plasma processing of high-performance double cation perovskite solar cells that leverages solvent engineering and dual source annealing to precisely control film drying kinetics. Rapid plasma curing and air blade drying of perovskite films (< 100 ms) at elevated substrate temperatures > 100 °C and high processing speeds (>20 cm/s) afford immunity to ambient moisture ingress even under high humidity conditions (up to 50% RH). Our study of the influence of precursor composition, processing speed, and perovskite plasma curing kinetics on film morphology and crystallinity, as well as solar cell performance, allows a generalizable correlation of micron-scale morphologies with photovoltaic efficiency.

This detailed understanding of open-air perovskite processing allowed fabrication of highly reproducible planar inverted Cs_.17FA_.83Pb(Br_.17I_.83)₃ cells with stabilized PCE > 16 %, fill factors as high as 78 %, and standard deviation < 1.5% absolute PCE among independent batches fabricated from 25%–50% RH. Importantly, these devices are processed exclusively by scalable methods (no spin coating), utilizing spray deposited inorganic NiO hole transport layers to complete a highly photostable cell architecture that retains 98% of initial PCE over the course of 5●10⁴ s of maximum power point tracking under illumination. This combination of rapid open-air processing methods with an efficient and highly stable multiple cation perovskite composition can offer a viable path towards module upscaling.

Perovskite solar cells (PSCs) show great promise as an emerging photovoltaic technology (PV) because of their low cost and relative ease of manufacturing. In a previous study, we demonstrated the application of flexible solar cells for wireless systems, using them as external power sources for radio-frequency identification (RFID) sensors [1]. Traditionally, the performance of PSCs deposited on plastics is inferior to their on-glass counterparts, both in terms of efficiency and stability. We, therefore, performed a diagnostic study to determine the key performance limiting factors and understand the root-cause of the performance drop. Our results identified shunting effect as a major contributor for low efficiency in flexible cells. Hence, we performed process optimization through cleaning, etching, morphology control and chemical doping, and achieved an improvement in efficiency from 10% to over 13% in an ongoing process. We evaluated the device stability via an accelerated aging test in our in-lab stability chamber and performed a comparative study on the degradation mechanisms of PSCs deposited on glass and plastics, respectively. Our study shows significant improvement in the performance of flexible PSCs and contributes to the development of flexible PSCs for wearable technologies, such as solar-powered RFID.
[1] S. N. R. Kantareddy, I. Mathews, R. Bhattacharyya, I. M. Peters, T. Buonassisi, and S. E. Sarma, “Long Range Battery-Less PV-Powered RFID Tag Sensors,” IEEE Internet Things J., p. 1, 2019.

Copper (I) thiocyanate (CuSCN) is an optically transparent, wide bandgap (3.4-3.9 eV), p-type semiconductor used in organic and perovskite solar cells. In comparison to other hole transporting materials (HTMs), CuSCN is economically favourable and comparable in performance, with efficiencies for hybrid organic-inorganic metal halide perovskite solar cells exceeding 20% [1].

Current research involves several solution-processable methods such as spin coating, spray coating and doctor blading. This study reports the first ever deposition of CuSCN via aerosol assisted chemical vapour deposition (AACVD). AACVD is a scalable ambient-pressure technique with low processing costs and simple reactor design. An added advantage is the flexibility over material choice due to the process being dependent on the solubility of the precursor, rather than the volatility as in conventional chemical vapour deposition techniques [2].

The study involves deposition of CuSCN using three different solvents: dipropyl sulfide, diethyl sulfide and ammonium hydroxide. For each CuSCN/solvent combination, deposition temperature, concentration of solution and amount of precursor solution deposited were varied. CuSCN films produced by diethyl sulfide solvent were further optimised by studying the growth characteristics. CuSCN films deposited via AACVD were compared to films produced by conventional spin coating methods. Scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet-visible spectroscopy (UV-Vis) were used to determine morphological, structural and optical information.

Using the optimised conditions, AACVD CuSCN was incorporated into a working methylammonium lead iodide perovskite solar cell. Addition of interlayers, different electron transport layers and device architectures were explored to determine structure with highest performance. Studies into the solar cell performance allowed further understanding of the less well-known CuSCN HTM. Solar cells were tested under AM 1.5 and a champion efficiency of 10.44% was achieved, which represents the first ever working solar cell using AACVD CuSCN.

[1] N. Arora, M. I. Dar, A. Hinderhofer, N. Pellet, F. Schreiber, S. M. Zakeeruddin and M. Grätzel, Science (2017) 768-771
[2] P. Marchand, I. A. Hassan, I. P. Parkin and C. J. Carmalt, Dalton Trans. (2013) 42, 9406-9422

Organometal–halide perovskite solar cells (PSC) have attracted attention by its unprecedented rise in device efficiency. However, the PSCs were mostly tested for the unit cell of small area, Therefore, it is important to upscale to solar modules that provide desired voltage and power output. Laser scribing technology, proven for the conventional thin film solar cell fabrication process, can be effectively extended for PSC module fabrication. The P1 scribing to isolate the transparent bottom electrode layer is essentially identical to the conventional laser scribing task. However, the P2 laser scribing to expose the bottom electrode for serial interconnect with the minimal contact resistance has been recognized as a challenging process; it is important to develop reliable single-step P2 process removing necessary stack of films, e.g., including hole blocking layer, without damaging bottom electrode, depending on the PSC material architectures selected. Although the P3 laser scribing to isolate the top electrode layer has been assumed to be less difficult, close examination of possible electrical shunt and/or unwanted damage of underlying structures are required. On the other hand, the P4 laser scribing to fabricate see-through patterns should minimize electrical shunt and/or degradation of surrounding structures with high optical transmissivity or visuality. Current study will focus on addressing the aforementioned challenges by exploring the optimal PSC laser scribing parameters based on pico- and nano- second lasers of various wavelengths from ultraviolet to visible and near-infrared, illuminated both from film and substrate sides. Relevant mechanisms and further improvement plans will be also discussed based on the measured module performance in conjunction with morphological, compositional, optical and spectroscopic characterization results.

The cathode interface plays a critical role in achieving high-performance fullerene/perovskite planar solar cells, governing the charge extraction and transport behaviors between the perovskite layer and the electrode. However, there is a high energy mismatch between PCBM and the commonly used metal cathode electrode aluminum (Al) and silver (Ag), severely restricting the electron extraction at the contact and therefore leading to poor device performance. ^[1]

Herein, organic dyes, Isatin and Isatin-Cl, which are abundant, cheap, air stable, and easy to modify in terms of their chemical structure, are introduced at the back contact PCBM/Al as a cathode modification interlayer due.^[2] It is revealed that the Isatin interlayers facilitate electron transport/extraction and suppress electron recombination, attributed to the formation of negative dipole potential steps and the passivation of the interfacial trap density. The average power conversion efficiencies of the resulting devices are significantly improved by 11% from 17.68 % to 19.74%, with an enhancement in all device parameters including short-circuit current, open-circuit voltage and fill factor. The hysteresis index is found to disappear. In addition, such interlayer enhances device stability under ambient conditions compared to the control devices due to suppression of moisture-induced degradation of the perovskite films. Our findings provide a comprehensive understanding of the engineering of the back contact between PCBM and the metal electrode to improve efficiency and stability of perovskite solar cells.

[1] F. Tan, et al. Adv. Mater. 2019, 31, 1807435
[2] S. Xiong, et al. Adv. Optical Mater. 2019 (Accepted)

Surface ligand treatment provides a promising approach to passivate defect states and improve the photoluminescence quantum yields (PLQY), charge-carrier mobilities, material and device stability, and photovoltaic (PV) device performance of organic metal halide perovskites (OMHPs). Numerous surface treatments have been applied to OMHP thin films and shown to passivate defect states and improve the PLQY and PV performance of OMHPs, but it is not clear which surface modifiers bind to the surface and to what extent. As surface ligands have the potential to passivate defect states, alter interface energetics, and manipulate material and device stability, it is important to understand how different functional groups interact with the surfaces of OMHP thin films. In this study, we investigate a series of ligand binding groups and systematically probe the stability of the bound surface ligands, how they influence PL properties, film stability, and PV device performance. We observe phosphonic acids (PAs) form reasonably stable surface bonds, whereas carboxylic acids (CAs) and thiols do not. Trichlorosilane (SiCl₃) groups react rapidly and lead to thicker surface layers that restrict charge transfer at the perovskite/electron transport layer (ETL) interface. Ammonium salts react rapidly to displace the methylammonium cations, thus resulting in less control over formation of only a surface layer. Despite the fact that we do not detect thiol or CA binding with XPS measurements, both of these surface ligands lead to increased PL intensity and increased stability. Furthermore, toluic acid treatment of MAPbI₃ based inverted PV devices results in an improved power conversion efficiency from 16.5% to 19.3%.

Mechanical strength of crystalline silicon wafers sliced by diamond wire sawing process depends on the surface and subsurface damage (in form of microcracks) caused by material removal processes. The type and the extent of damage depends on material removal being ductile or brittle. In this work we investigated the chemo-mechanical effect of cutting fluid on material removal mechanisms in single crystal silicon by scribing experiments, which simulates the material removal mode of industrial diamond wire slicing process. We show experiments between of scribing single crystal silicon in dry and in presence of water-based cutting fluids. Our results show more ductile material removal of silicon in presence of the cutting fluid – such chemomechanical effect of fluid on silicon has not been reported in literature, and provide new ways of reducing surface and subsurface damage in silicon wafers.

Multi-crystalline silicon (mc-Si) is cheaper than single-crystalline silicon and hold promise of delivering affordable photovoltaic energy. However, manufacturing process of multi-crystalline silicon wafers by slicing with diamond wire sawing faces challenges of reduced efficiency with increased consumption of the diamond abrasives on the wire. We simulate the cutting process by fundamental scribing or scratch experiments with diamond on single and multi-crystalline silicon substrates. We used two tips of same geometry conical diamond indenters, and monitored the wear of diamond by microscopy and radius of curvature measurements, apart from contact forces for mc-Si versus single -crystal silicon. We found that the forces are higher for diamond contacting for mc-Si, and corresponding increase in tip radius being higher in mc-Si versus single-crystal silicon, due to higher stresses and wear of the tip. We found the fracture surfaces of diamond show micro-fracture and blunting from high resolution scanning electron microscopy and confocal microscopy. Stress induced phased transformation of diamond along with compressive residual stresses were measured using Raman spectroscopy for difference in silicon material structure. We explain the results using material inhomogeneity of multi-crystalline silicon for increased wear of diamond cutting the material, compared to single crystal silicon.

Perovskite is a promising light harvester for use in photovoltaic solar cells. In recent
years, the power conversion efficiency of perovskite solar cells has been dramatically
increased, making them a competitive source of renewable energy.
This work will discusses new directions related to organic inorganic perovskite and their applications in solar cells.
In low dimensional systems, stability of excitons in quantum wells is greatly enhanced due to the confined effect and the coulomb interaction. The exciton binding energy of the typical 2D organic-inorganic perovskites is up to 300 meV and their self-assembled films exhibit bright photoluminescence at room temperature.
* In this work we will show the dimensionality in the perovskite structure. The 2D perovskite structure should provide stable perovskite structure compare to the 3D structure. The additional long organic cation, which is added to the perovskite structure (in the 2D structure), is expected to provide hydrophobicity, which will enhance the resistivity of the perovskite to humidity. Moreover we will demonstrate the use of 2D perovskite in high efficiency solar cells.
* Organic-inorganic halide perovskite is used mainly in its “bulk” form in the solar cell. Confined perovskite nanostructures could be a promising candidate for efficient optoelectronic devices, taking advantage of the superior bulk properties of organo-metal halide perovskite, as well as the nanoscale properties. In this talk, I will present our recent progress related to the synthesis and characterization of perovskite NPs- i.e. Inorganic and hybrid organic-inorganic NPs. New nanostructures such us: NRs and NWs will be presented and the introduction of other cations such us Rb will be shown.

Combining halide perovskites with tailored dimensionality into two/three-dimensional (2D/3D) systems has revealed a powerful strategy to boost the performances of perovskites photovoltaics (PVs). Despite the recent advances, a clear understanding on the intimate link between interface structure and physics is still missing, leading so far to a blind optimization of the 2D/3D PVs. Here, we reveal the impact of 2D/3D crystal alignment in driving the interface charge-recombination dynamics. The 2D crystal growth and orientation is manipulated by specific fluorination of the phenethylammonium (PEA), used here as the organic cation backbone of the 2D component. By means of time-resolved optoelectronic analysis from femto- to microsecond, we demonstrate a static function of the 2D as electron barrier and homogeneous surface passivant, together with a dynamical role in retarding back charge recombination. Our results reveal a crucial dependence of such beneficial effect with the 2D crystal orientation, leading to enhanced open circuit voltage (V_OC) if 2D lies parallel on the 3D. Such findings provide a deep understanding and delineate precise guidelines for the smart design of multi-dimensional perovskite interfaces for advanced PV and beyond.

Two-dimensional (2D) hybrid halide perovskites are promising in optoelectronic applications, particularly solar cells and light emitting devices (LEDs), and for their increased stability compared to 3D perovskites. Here, we report a new series of structures using a propylammonium (PA⁺) cation which results in a series of Ruddlesden-Popper (RP) structures with the formula (PA)₂(MA)_n-1Pb_nI_3n+1 (n = 3, 4) and a new homologous series of “step-like” (SL) structures where the PbI₆ octahedra connect in both a corner- and face-sharing motif with the general formula (PA)_2m+4(MA)_m-2Pb_2m+1I_7m+4 (m = 2, 3, 4). The RP structures show a blue-shift in bandgap for decreasing n while the SL structures have an even greater blue-shift. DFT calculations show that, while the RP structures are electronically 2D quantum wells, the SL structures are electronically 1D quantum wires with the chains of corner-sharing octahedra “insulated” by blocks of face-sharing octahedra. These blocks form an additional lateral carrier confinement barrier that prevents any significant electronic coupling across the face-sharing octahedra and results in 1D electronic dimensionality. Dark measurements for RP crystals show high resistivity perpendicular to the layers but a lower resistivity parallel to them. The SL crystals have varying resistivity in all three directions, confirming both RP and SL crystals’ utility as anisotropic electronic materials. The RP structures show strong photoresponse and are utilized in solar cell devices. Solar cells were made with n = 3 using “hotcasting” and solvent engineering methods giving an efficiency of 7.04% (average 6.28 ± 0.65%) with negligible hysteresis.

Quasi-2D perovskites are inevitably formed with multi-layer nanoplates of different n values. Normally, the nanoplates are vertically arranged from small-n-value to large-n-value structures between bottom and top surfaces in quasi-2D perovskite films, forming a common approach to facilitate charge transfer for developing high-performance optoelectronic devices. Here, we report a different strategy of uniformly arranging different-n-value nanoplates (PEA₂MA_n-1Pb_nI_3n+1) between bottom and top surfaces in quasi-2D perovskite films to generate high-performance solar cells by introducing vacuum poling treatment right after spin-coating of precursor solution. By mechanically peeling off the monolayers of nanoplates with our delicate scotch-tape method while monitoring the photoluminescence (PL), we un-ambiguously show that our vacuum poling method can create the uniform dispersion of different-n-value nanoplates between bottom and top surfaces in quasi-2D perovskite films, as compared to the conventional ordered dispersion of nanoplates from n = 1 to n = 5 prepared without vacuum poling treatment. The transient absorption (TA) measured through bottom and top surfaces show that the efficient carrier transfer occurs within 10 ps from small-n-value nanoplates to large-n-value nanoplates under uniform dispersion, presenting an efficient carrier extraction mechanism. The PL dynamics indicate that uniformly arranging different-n-value nanoplates leads to small-n-value and large n-value nanoplates functioning as passivation agents and light-emitting centers with significantly enhanced PL intensity and lifetime. Moreover, the record-high fill factor (FF) of 82.4 % was realized with the power conversion efficiency (PCE) of 18.04% (Voc = 1.223V and Jsc = 17.91 mA/cm²), showing an extremely efficient carrier extraction occurring in the quasi-2D perovskite device (ITO/PTAA/quasi-2D perovskites/PC₆₁BM/PEI/Ag) based on this new strategy. Therefore, uniformly arranging different-n-value nanoplates offers a new materials engineering strategy to enhance carrier transfer and extraction for developing high-performance quasi-2D perovskite solar cells.

The all-inorganic lead halide perovskite without volatile component would be a promising alternative candidate for high efficiency photovoltaics. However, the all inorganic CsPbI₃ with most suitable band gap face the challenges of low room temperature phase stability and relative low efficiency. To enhance the performance and stability of all-inorganic CsPbI₃ perovskite, the 2D/3D configuration especially the (110) oriented 2D perovskite component was introduced to stabilize the α-CsPbI₃. The (110) oriented 2D based on EDAPbI₄ help stabilize α-CsPbI₃ to achieve up to >11% efficiency. Impressively, the Cs cation in CsPbI₃ would not ion exchanged with organic cation, which is significantly different from the hybrid and a signal for the promising cation stability of inorganic perovskite. Therefore, a facile organic cation surface termination approach was developed to significantly enhance the stability and performance of α-CsPbI₃ solar cell with >15% efficiency. The bifunctional stabilization of CsPbI₃ with gradient Br doping and organic cation termination finally improve the efficiency of CsPbI₃ perovskite solar cells to a record value of 17% with enhanced stabilities.

Recently, the all-inorganic CsPbI₂Br perovskite attracts increasing attention owing to its outstanding thermal stability and suitable bandgap for optoelectronic devices. However, the substandard power conversion efficiency (PCE) and large energy loss (E_loss) of CsPbI₂Br perovskite solar cells (PSCs) caused by the low quality and high trap density of perovskite film, still limit the application of devices. Herein, we utilize the post-treatment of evaporating cesium bromide (CsBr) on top of perovskite surface to passivate the CsPbI2Br/hole-transporting layer interface and reduce E_loss. The results of micro zone photoluminescence indicate that the evaporated CsBr gathered at grain boundaries of CsPbI₂Br layers and Br-enriched perovskites (CsPbI_xBr_3-x, x<2) are formed, which can provide protection for CsPbI₂Br. Therefore, the gaps between crystal grains are filled up and the recombination loss of the all-inorganic CsPbI₂Br PSCs is reduced accordingly. The champion device exhibits the high open-circuit voltage and PCE of 1.271 V and 16.37%, respectively. To the best of our knowledge, it’s the highest PCE among all-inorganic CsPbI₂Br PSCs reported so far. In addition, the stability of CsPbI₂Br PSCs is effectively improved by CsBr passivation and the device without encapsulation can retain 86% of its initial PCE after storage of 1368 hr, which is beneficial to the practical application.

The all-inorganic CsPbI₃ perovskite offers the prospect of a more stable alternative to hybrid organic-inorganic perovskites. Unfortunately, like many other polymorphic perovskite systems, the metastability of its high-temperature black phase (1) stands in the way of realizing room-temperature (RT) stable optoelectronic devices. It is of interest to find ways to shift the energetics of CsPbI₃, in order to secure RT black phase formation over its yellow non-perovskite phase (2). In this presentation, we outline the role of glass substrate clamping and biaxial strain (induced by the large thermal expansion mismatch) to realize RT stable black-phase CsPbI₃ thin films. Employing synchrotron-based grazing incidence wide angle X-ray scattering with extremely fast acquisition time (~0.1 s), we track the introduction of crystal distortions and texture formation within black CsPbI₃ thin films as they are cooled from annealing temperatures. The thermal stability of black CsPbI₃ thin films are vastly improved by the strained interface, a response verified by ab initio thermodynamic modelling. This contribution introduces substrate clamping as an important parameter in the rational design of stable RT black inorganic halide perovskite thin films.

1. R. J. Sutton et al., Cubic or Orthorhombic? Revealing the Crystal Structure of Metastable Black-Phase CsPbI₃ by Theory and Experiment. ACS Energy Letters. 3, 1787–1794 (2018).
2. Y. Yang, J. You, Make perovskite solar cells stable. Nature. 544, 155–156 (2017).

Hybrid organic-inorganic halide perovskite solar cells (PSCs) have attracted a great deal of attention as they show promises toward the development of next generation solar cells, with the highest power conversion efficiency (PCE) reaching 24%. However, these types of PSCs, such as PSCs based on methylammonium lead iodide (MAPbI₃) or formamidinium lead iodide (FAPbI₃), exhibited poor stability against moisture and heat. Moreover, hole transport materials (HTMs) in these traditional PSCs (Spiro or PTAA) are very expensive and need some additives to improve their mobility. In order to overcome these disadvantages, we develop all-inorganic PSCs based on CsMX3 (M = Pb, Sn, In; X = I, Br, Cl) and low-cost carbon films. First, we employed CsPbBr₃ as the absorb layer in the all-inorganic PSC, which showed excellent stability even under environmental stress and without any encapsulation. However, because the bandgap of CsPbBr₃ was 2.3 eV, the light absorption for CsPbBr₃ was only up to ~540 nm, which led to the low PCE. As CsPbI₃ possesses a smaller bandgap (1.7 eV) than CsPbBr₃, CsPbI₃-based all-inorganic PSCs are subsequently developed. However, they still exhibited lower PCEs because of the existence of defects and poor stability. Therefore, in our recent work, we engineered the defect densities in CsPbI₃, and a new all-inorganic perovskite material, CsPbI₃:Br:InI₃, was prepared. This new perovskite retained the same bandgap as CsPbI₃, while the intrinsic defect concentration has been largely suppressed. Moreover, it can be prepared under extremely high humidity atmosphere thus a glovebox was not required. By completely eliminating the labile and expensive components in traditional PSCs, the all-inorganic PSCs based on CsPbI₃:Br:InI₃ and carbon electrode exhibited the PCE and open-circuit voltage as high as 12.04% and 1.20 eV, respectively. More importantly, the encapsulated all-inorganic PSCs based on CsPbI₃:Br:InI₃ demonstrated excellent stability in air for more than two months, while those based on CsPbI₃ can just survived a few days in air. The progresses reported here open the door for all-inorganic PSCs with long-term stability under harsh conditions, making practical application of the PSC with high performance a real possibility.

The state-of-the-art organic-inorganic halide perovskite solar cells (PSCs) have scored tremendous achievements as reflected by the highest power conversion efficiency (PCE) skyrocketing to over 24.2% in just a few years.^[1] Nevertheless, the long-term stability of the hybrid perovskite inherited from its organic moiety remains a major hurdle preventing it from commercialization. For further development of PSCs, a more robust inorganic cesium halide perovskite (CsPbX₃, X = I or Br) without a fragile organic component and possessing a range of spectacular advantages, including superb thermal stability (> 300 °C) and high electron mobility, is considered to have more potential than the organic-inorganic hybrid material. As such, it is extremely crucial to design inorganic perovskite-based solar cells possessing both excellent photovoltaic performance and outstanding stability.
Among the shared inorganic perovskite employed in PSCs, CsPbIBr₂ is considered a feasible material for tandem solar cells after balancing the band gap and stability of the inorganic perovskite. Nevertheless, the performance of CsPbIBr₂ PSCs has lagged behind that of other analogs, far from its highest theoretical efficiency. In general, the inferior performance is attributed to substantial interface recombination and non-radiative recombination within the perovskite.
Accordingly, we report a facile interface modification strategy for inorganic PSCs, wherein a low-cost lanthanide halide, SmBr₃, is incorporated to modify the TiO₂/CsPbIBr₂ interface.^[2] Apart from its effects at the interface, SmBr₃ can diffuse into the perovskite layer and induce a blocking layer with a gradient energy band gap. Therefore, SmBr₃ interface modification can not only optimize the charge transfer process and restrain interface recombination, but also suppress the non-radiative recombination within the perovskite material. Finally, the devices with SmBr₃ interface modification yield a high PCE of 10.88%.
In addition, Ba²⁺ was doped in CsPbIBr₂ to suppress the inside non-radiative recombination.^[3] Ba²⁺ with large radius can result in an undesired tolerance factor less than 0.8, but strikingly, the Ba²⁺-doped CsPbIBr₂ has beneficial morphology and excellent crystallinity as well as suppressed carrier recombination. Compared with the control sample, Ba²⁺ doping boosts the device efficiency from 8% to 10.5%. This finding indicates that the perovskite films are tolerant to Goldschmidt-rule-deviating homovalent heteroatoms, extending the substitution engineering of inorganic perovskite.
References:
[1] https://www.nrel.gov/pv/cell-efficiency.html
[2] W. Subhani, K. Wang, M. Du, X. Wang, S. Liu, Adv. Energy Mater. 2019, 1803785.
[3] W. Subhania, K. Wang, M. Du, S. Liu, Nano Energy 2019, 61, 165.

Over a span of a decade, perovskite solar cells have evolved as a promising photovoltaic technology by displaying a meteoric rise in the power-conversion efficiencies. However, it is equally important that the performance of perovskite solar cells sustains under operating conditions over a long period of time. Therefore, their stability under different conditions such as illumination, humidity, and thermal stress needs to be thoroughly investigated. It is well known that the stability problems of perovskite solar cells arise from the degradation of the light-absorber and/or charge extraction layers and the deterioration of various interfaces across the solar cell. We have employed three main strategies to develop better perovskite-based devices that are not only efficient but also stable. We have investigated the interfaces on both sides of the hole transporting material (HTM) i.e., perovskite absorber/HTM and HTM/back contact interface in a solar cell device to mitigate the catastrophic stability issues associated with perovskite solar cells. We explored three main strategies including employing all-inorganic charge extraction layers [1]. This has led to the CuSCN hole extraction layer based devices with stabilized efficiencies exceeding 20%. Another strategy involves using highly water-repellent wide bandgap perovskite layer inserted between the 3D light-harvesting perovskite film and HTM. This approach combines the suitable properties of 2D layer (ultrahydrophobicity, improved interfacial hole extraction, enhanced operational stability under humid conditions) enabling perovskite solar cells with 3D/2D architecture with efficiencies exceeding 22% [2]. The third strategy is to introduce electrochemically stable or inert layers, which, respectively, facilitate or allow the transport of holes, either in between perovskite and hole transporting layer or in between hole conductor and metallic back contact [1, 3]. It is worth emphasizing that these approaches not only allowed us to realize high efficiencies but also outstanding stability under operational conditions for perovskite solar cells. In my presentation, important fundamental insights gained through various structural, morphological, and spectroscopic characterization techniques directly impacting the performance and stability of perovskite solar cells will be discussed.

References

1) N. Arora,† M. Ibrahim Dar,†* A. Hinderhofer, N. Pellet, F. Schreiber, S. M. Zakeeruddin, M. Grätzel Perovskite solar cells with CuSCN hole extraction layers yield stabilized efficiencies greater than 20%. Science 2017, 358, 768-771. DOI: 10.1126/science.aam5655.
2) Y. Liu, S.Akin, L. Pan, R. Uchida, N. Arora, J. V. Milic, A. Hinderhofer, F. Schreiber, A. R. Uhl, S. M. Zakeeruddin, A. Hagfeldt, M. Ibrahim Dar*, M. Grätzel Ultra-Hydrophobic 3D/2D Fluoroarene Bilayer-Based Water-Resistant Perovskite Solar Cells with Efficiencies Exceeding 22%. Science Advances, 2019, 5, eaaw2543, DOI: 10.1126/sciadv.aaw2543.
3) M. Abdi-Jalebi, M. Ibrahim Dar, S.P. Senanayak, A. Sadhanala, Z. Andaji-Garmaroudi, L.M. Pazos-Outón, J.M. Richter, A.J. Pearson, H. Sirringhaus, M. Grätzel, R.H. Friend. Charge extraction via graded doping of hole transport layers gives highly luminescent and stable metal halide perovskite devices. Science Advances, 2019, 5, eaav2012. DOI:10.1126/sciadv.aav2012.

The photovoltaic efficiency of perovskite solar cells (PSC)PSCs has remarkably developed, while poor stability is a huge barrier for the commercialization of PSCs. The strategies to enhance stability are portrayed in terms of moisture and illumination endurance of photoactive layer as well as defects at interfaces of different layers.
Here we effectively improve the stability of perovskite solar cell by molecular design and defects passivation. Firstly, the quasi 2D molecular with several ammonium salts are design for enhance the hydrophobic properties of photoactive layer. The halogen functional groups are further introduced to improve the poor charge transport with high-quality perovskite films with better crystallization. The optimal 2D PSCs exhibit high efficiency of 20.08% as well as retain 92% initial PCE after aging at 50 ± 5% relative humidity for 2400 h. Moreover, the defects of photoactive layer are reduced by controlling the growth and nucleation of perovskite film and adding passivation additives. The C60/TiO₂ bilayer were employed to gain intimate contact between the ETL and perovskite, which results in an enhanced perovskite crystallization and a reduced charge recombination at the interface. The bilayer-based PSC shows outstanding stability, retaining 83% and 90% of its initial performance after 312 h UV irradiation and 1000 h exposure to ambient air, respectively.

Developing multijunction perovskite solar cells (PSCs) is an attractive route to boost PSC efficiencies to above the single-junction Shockley-Queisser limit. However, commonly used tin-based narrow-bandgap perovskites have shorter carrier diffusion lengths and lower absorption coefficient than lead-based perovskites, limiting the efficiency of perovskite-perovskite tandem solar cells. We discover that the charge collection efficiency in tin-based PSCs is limited by a short diffusion length of electrons. Adding proper additive into tin-perovskite precursors reduce the background free hole concentration and electron trap density, yielding a long electron diffusion length of 2.72±0.15 µm. It increases the optimized thickness of narrow-bandgap perovskite films to 1000 nm, yielding exceptional stabilized efficiencies of 20.2% and 22.7% for single junction narrow-bandgap PSCs and monolithic perovskite-perovskite tandem cells, respectively. This work provides a promising method to enhance the optoelectronic properties of narrow-bandgap perovskites and unleash the potential of perovskite-perovskite tandem solar cells.

Multi-junction solar cells composed of complementary absorber layers with appropriate bandgaps offer avenues to improve the AM1.5 power conversion efficiency (PCE) beyond the Shockley-Queisser (SQ) limit of single-junction solar cells. In particular, PV technologies based on low-temperature solution processing have attracted interest in the pursuit of low-cost, flexible, and lightweight solar cells. Solution-processed perovskite/perovskite tandem solar cells have achieved significant progress in recent years. However, face a fundamental limitation on the path to reaching their full tandem potential due to the lack of efficient low-temperature processed IR back solar cell technology. Perovskite:colloidal quantum dot (CQD) hybrid tandems offer a route to overcoming this limitation. PbS CQDs are desirable for third-generation solar cell technology, owing to their superior photoelectric properties, facile bandgap tunability, and low-temperature solution-processability. The band gaps of CQDs can reach 0.5 eV providing a path to harness photons in the near to deep infrared region of the solar spectrum, beyond the capacity of crystalline silicon and perovskites. Here we report a solution-processed four terminal (4T) tandem solar cell composed of a perovskite front cell and a PbS CQD back cell, with a PCE exceeding 21% and a stabilized efficiency of 20%. This is the highest reported PCE of the perovskite/CQD tandem solar cell to date, and it surpasses the performance of both the perovskite and the CQD single junction double-pass solar cells.
We developed a dielectric-metal-dielectric (DMD) electrode to enable semi-transparent perovskite solar cells that combine enhanced transmittance and conductance for record-performing front cells. The DMD structure is designed to have high IR transmittance by optimizing the thickness of top dielectric oxide based on the zero-reflection condition in the optical admittance diagram. As a result, we achieved an increase of 25% in IR transmittance, and this enabled us to take advantage of CQDs as IR absorbers and boost the photon collection beyond the capacity of prior solar cells. The semi-transparent perovskite solar cell has a best PCE of 19.0% and a stabilized efficiency of 17.5%. The back cell is fabricated based on the IR PbS CQD absorber layers that is complementary to the IR transmittance characteristic of semi-transparent perovskite solar front cell for the optimized 4T tandem performance. The CQD:erovskite 4-terminal tandem is designed by mechanically stacking the 1.15 eV PbS-CQD solar cell as the back cell to the semi-transparent perovskite (1.63 eV) front cell. The EQE measurements of 1.15 eV and 0.95 eV CQD absorbers showed high EQE values at the exciton peaks; which indicates that the solar cells have an excellent IR charge collection efficiency from light harnessed beyond the perovskite absorption at ~750 nm. The short circuit current of the filtered CQD solar cell were ~6 mA/cm2 after filtering the perovskite front sub-cell. The Voc of the device is only marginally reduced, while a small increase in the fill factor is obtained. In the 4T measurement of the CQD solar cell, for the best performing cell, we were able to get a filtered PCE of 2.11%. Together with the 19.02% efficiency of the perovskite front cell, this gives 21.13 % efficiency for the tandem device.
Significant progress could be achieved in this perovskite:CQD tandem device in future, with concurrent advances in the light management in the perovskite sub-cell and IR charge collection techniques in the CQD sub-cell. Solution-processed hybrid tandem photovoltaics combining these two emergent PV technologies open the possibility to realize not only high-efficiency solar cells with substantial reductions in the cost of solar electricity but also next-generation flexible photovoltaic (PV) devices.

Combining wide-bandgap and narrow-bandgap perovskites to construct monolithic all-perovskite tandem solar cells offers avenues for the continued increase in photovoltaic power conversion efficiencies (PCEs). However, the actual efficiencies of all-perovskite tandem solar cells today are diminished by the subpar performance of narrow-bandgap subcells. Here we report a strategy to reduce Sn vacancies in mixed Pb-Sn narrow-bandgap perovskites. We increase thereby the charge carrier diffusion length in narrow-bandgap perovskites, and we obtained a PCE higher than 20% for 1.2-eV narrow-bandgap solar cells. We then developed a recombination junction that avoids sputtered transparent-conductive-oxide and takes advantage instead of robust and compact recombination layer processed via atomic layer deposition. We fabricated monolithic all-perovskite tandem solar cells with PCEs beyond 23%. The tandem solar cells retained 90% of their performance after more than 400 hours of operation at the maximum power point under full one-sun illumination.

All solid-state solar cells based on organometal trihalide perovskite absorbers have already achieved distinguished power conversion efficiency (PCE) to over 24% and further improvements are expected up to 25%. Now, the research on perovskite solar cells has been moving toward commercialization. To facilitate commercialization of these great solar cells, some technical issues such as long-term stability, large scale fabrication process, and Pb-related hazardous materials need to be solved. Also, exploitation of flexible solar cell is of great importance for commercialization in niche markets.
This talk is dealing with our recent efforts to facilitate commercialization of perovskite solar cells.For examples, we introduce a recycling technology of perovskite solar cells, which covers the regeneration process of Pb contained perovskite layer as well as recycling process of Au electrodes and transparent conducting oxide glass. Also, simple fabrication technologies for large scale perovskite module is going to be introduced. Finally, recent interesting results regarding ultra-flexible perovskite cells will be discussed in terms of stress analysis.

The past few years have witnessed a rapid evolution of hybrid organic-inorganic perovskite solar cells (PSCs) with both low cost and boosted high power conversion efficiency (PCE) over 22%. Despite the achievements, MAPbI₃ suffers from inherent instability over ambient operation conditions due to the low formation energy of the material itself and the high hydrophilicity of the organic cations. Efforts such as developing novel device architectures as well as exploring novel materials have been tried to improve the device stabilities. Among them, the two-dimensional (2D) perovskites that are crafted using bulkier alkylammonium cations in place of methylammonium exhibit appealing environmental stability. However, the insulating alkylammonium spacer cations hinder charge transport and limit the efficiencies of the devices based on such perovskites. In this scenario, an exploration of alternative yet effective organic spacer cations that increase the charge transfer is imperative to enhance the efficiency.
Herein, we design such an alternative bi-functional conjugated cation AB. We report the first time the incorporation of AB in 2D/3D perovskites and its implementation on solar cells. The use of bi-functional conjugated cations enhances significantly the performance of the cells, reaching a highest power conversion efficiency of 15.6% with improved stability. The efficiency remained around 90% of the initial value after 100 h continuous illumination, much more stable than MAPbI₃ perovskite. By comparing this cation with a mono-functional cation and a bi-functional non-conjugated cation with similar structure, we found that the bi-functional conjugated cation not only benefits the growth of perovskite crystals in the mesoporous network, but also facilitates the charge transport. Our approach helps explore new rational designs of cations in perovskites.

All-perovskite–based polycrystalline thin-film tandem solar cells have the potential to deliver efficiencies of >30%. However, the performance of all-perovskite–based tandem devices has been limited by the lack of high-efficiency, low–band gap tin-lead (Sn-Pb) mixed-perovskite solar cells (PSCs).We found that the addition of guanidinium thiocyanate (GuaSCN) resulted in marked improvements in the structural and optoelectronic properties of Sn-Pb mixed, low–band gap (~1.25 electron volt) perovskite films.The films have defect densities that are lower by a factor of 10, leading to carrier lifetimes of greater than 1 microsecond and diffusion lengths of 2.5 micrometers.These improved properties enable our demonstration of >20% efficient low–band gap PSCs. When combined with wider–band gap PSCs, we achieve 25% efficient four-terminal and 23.1% efficient two-terminal all-perovskite–based polycrystalline thin-film tandem solar cells.

All-perovskite tandem solar cells have the potential to reach high efficiency but still suffer from limited stability. While a large effort of the community has been focused on improving the efficiency and stability of wide-gap (Eg>1.5eV) perovskite solar cells, Tin-based low-gap (Eg~1.2eV) perovskites still have a large room for improvement, especially in terms of stability, hindered by potential problems of Tin oxidation and improper charge selective contacts.
We will present our latest development on designing the Tin-Lead perovskite absorber composition and deposition conditions to reach high efficiency and high stability in tandem configuration. The growth conditions of the absorber are adapted to yield smooth, compact and large grained films. Then, we will discuss the crucial role of the hole selective contact in the efficiency/stability compromise. Several hole transport materials are under investigation. In particular, we will show low gap cells that can maintain >95% of initial efficiency after 1000hrs at 85°C, in air with no encapsulation. This can be achieved by eliminating the commonly used hole transport layer PEDOT:PSS and use instead the band bending effect existing at the ITO/perovskite heterojunction [1]. Further adapting an ALD-deposited barrier and sputtered TCO layer for the Sn/Pb perovskites can improve the device environmental stability. Overall, Tin-Lead perovskite solar cells having a bandgap of ~1.25 eV will be presented with >16% efficiency and improved light, thermal and environmental stability.
We will also present our progress at integrating these low gap cells into efficient and stable all-perovskite tandem solar cells. In that respect, a new perovskite tandem structure in substrate orientation will be introduced and discussed at the conference, including its potential for easier industrial penetration and lower module fabrication costs.

[1] Rohit Prasanna, et al. in peer review

Hybrid organic-inorganic metal-halide perovskites stand out as obvious candidates for solar energy generation and photovoltaic applications owing to their much-heralded material property trio: remarkable absorption coefficients, long-lived photocarriers and tuneable bandgaps. After the first introduction of the solid-state device architectures in 2012,^1,2 tremendous efforts from both academia and industry have been devoted to research fundamental properties of metal-halide perovskites as well as to advance relevant photovoltaic technologies.
Power conversion efficiency (PCE) of perovskite solar cells has since continued to soar over the past years. This skyrocketing progress has reflected on a recent record breaking PCE over 24% certified for perovskite single-junction cells, fast-approaching the performance offered by incumbent crystalline-silicon (c-Si) technologies.³ Such rapid development of perovskite photovoltaics has directly led to their emergence in the hope of preventing natural resource depletion as well as mitigating global climate change. Among the applications of perovskite photovoltaics pioneered so far, perovskite-Si tandem solar cells that feature two sub-cells operating in their optimised absorption spectrum appear to be one of the most promising photovoltaic technologies to achieve remarkable energy conversion efficiency in a cost-effective manner. The latter is largely thanks to the simplicity and versatility of metal-halide perovskite material processing.⁴
In this contribution, we will present high-durability metal-halide perovskite compounds enabled by a series of additives. Our approach allows ultra-high-longevity perovskite solar cells to sustain an aggressive ageing condition that combines high-strength light and heat. With numerous material and spectroscopic characterisation techniques, we will shed light on the mechanism and factors that could eventually help perovskite solar cells to resist extreme environmental conditions. Integrating our high-resilience compounds into monolithic two-terminal perovskite-Si tandem solar cells has led to a 27% PCE, surpassing the highest recorded efficiencies for single-junction c-Si solar cells. Our demonstration represents a significant advancement that validates the approach of using perovskite-Si tandem configurations to ultimately enhance the absolute performance of market-dominating Si-based photovoltaics.^5,6

References
1. Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Efficient Hybrid Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 338, 643-647 (2012).
2. Kim, H. S. et al. Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2, 591 (2012).
3. NREL. Best Research-Cell Efficiency Chart, <https://www.nrel.gov/pv/cell-efficiency.html> (2019).
4. Nayak, P. K., Mahesh, S., Snaith, H. J. & Cahen, D. Photovoltaic solar cell technologies: analysing the state of the art. Nat. Rev. Mater. 4, 269-285 (2019).
5. Yoshikawa, K. et al. Silicon heterojunction solar cell with interdigitated back contacts for a photoconversion efficiency over 26%. Nat. Energy 2, 17032 (2017).
6. Green, M. A. et al. Solar cell efficiency tables (Version 53). Progress in Photovoltaics: Research and Applications 27, 3-12 (2019).

Perovskite solar cells (PSCs) have attracted much attention in the past several years due to the ever-increasing power conversion efficiency (PCE) and simple fabrication process.<span style="font-size:10.8333px"> </span>Particularly, we developed a printable triple mesoscopic architecture for PSCs, which uses carbon electrodes instead of traditional noble metals and can be fabricated with screen printing process. Through optimizing the material systems, modifying the interfaces, and diversing the device structures, the efficiency of such printable triple mesoscopic PSCs has increased from the initial ~6% in 2013 to ~18% in 2019. Besides the lower material cost and simpler fabrication process, printable triple mesoscopic PSCs also have shown promising stability under various conditions, such as high temperature (85 oC), continuous illumination (AM1.5G), and outdoor conditions.<span style="font-size:10.8333px"> In 2018, we enlarged the cell area to module level (3600 cm2) and installed a power system for outdoor exposure.</span>

Quasi-2D perovskite has higher stability and easier tunability compared with conventional 3D organic-inorganic hybrid perovskite. Tuning the phase distribution according to the device structure is necessary. We manipulate the structure of quasi-2D perovskite film and probe how the structure affects the solar cell device performance and stability. To better understand the structure-function relationship, we utilize femtosecond laser spectroscopy to study photophysics of different structures.

The addition of a small amount of hydrophobic cation of large size into 3D perovskite can improve both efficiency and stability. We find that the propylammonium is the most effective by comparing a family of different cations. The cations of large size preferentially segregate at the grain boundaries and surface, verified by transient absorption and reflection spectroscopy. Such passivation enhances device efficiency up to 20.1% and improves both device and precursor stabilities. The efficiency of devices with treatment only decreases by ~3% after 600 hours under low (10-15% RH) and high humidity (45–50% RH). (J. Mater. Chem. A, 2019, DOI:10.1039/C9TA01755K)

We also manipulate the phase purity and vertical distribution of quasi-2D perovskite. Ethyl acetate is utilized as antisolvent to tune the vertical phase distribution and the direction of carrier transfer is reversed accordingly. CH₃NH₃Cl and dimethyl sulfoxide are used to control the phase purity. These phenomena are proved by transient absorption spectroscopy. We find that solar cell performance is more sensitive to phase purity than vertical phase distribution. (Solar RRL, 2019, 3, 1800359)

In recent years, perovskite solar cells that can be easily manufactured at low cost by solution processing have achieved high energy conversion efficiency exceeding 24%. Halide perovskites of an ABX₃ structure [A=CH₃NH₃⁺(MA), HC(NH₂)₂⁺(FA), Cs⁺ ; B=Pb²⁺, Sn²⁺ ; X=I^-, Br^-, Cl^-] are used photovoltaic materials. In perovskite solar cells, a perovskite compound containing methylammonium or formamidinium, which is an organic cation is widely used as a photoactive layer because it exhibits high energy conversion efficiency, but its durability against heat and moisture is low.
It has been reported that thermal resistance is improved by using an inorganic cation such as an alkali metal instead of an organic cation. In addition, it has been reported that the CsPbX₃ thin film using cesium as an inorganic cation changes in band gap energy and durability due to mixed crystallization of halide ions.^(1)~(3) Furthermore, it has been reported that CsPbX₃ thin film causes a structural change δ phase which is non perovskite phase by moisture, which is a problem for practical use. ⁽⁴⁾
In this study, the effects of temperature and humidity on the crystal structure, morphology and charge transport properties and durability of several kinds of CsPbX_3-yX'_y thin films with different proportions of halide ions (X and X') were systematically investigated. Detailed experiment results will be presented.

The inorganic CsPbBrI₂ perovskite has attracted ever-increasing attention for its outstanding optoelectronic properties and ambient phase stability. However, to achieve high power conversion efficiency (PCE), it is imperative to minimize recombination at interface between the CsPbBrI₂ and the hole-extraction layer (HEL) for maximized carrier extraction efficiency. Herein, we developed the AI treatment to provide a general method for optimizing the interfacial properties. In our present work, quantum-dots (QDs) were firstly used to form dimension-graded heterojunction structure with the CsPbBrI₂ perovskite bulk film for optimized energy alignment in the solar cell (PSC) structure. Then, we ventured to conduct interface engineering by post treatmentment of the CsPbBrI₂ perovskite film with a series of A-site cation based iodine salts (AI, where A = formamidinium (FA⁺), methylammonium (MA⁺), ethylenediamine (EDA⁺), phenylethylammonium (PEA⁺) or n-butylammonium (BA⁺)) to achieve further improved device performance. In the applied QDs/film structure, an ultra-thin iodine-ion-enriched perovskite layer was formed on the top of CsPbBrI₂ film, and QDs surface were proved capped with AI after AI salt post-treated. We found such a phenomenon leads to proper band edge bending, decreased surface defects, and high-quality QDs modified layer. As a consequence, these changes proved effectively decreased recombination loss with improved hole extraction efficiency. More specifically, the FAI treated device yields an ultra-high PCE of 14.12% which positioning above the best reported PCE for CsPbBrI₂ PSCs to date. We believe such strategy should have significant potential for future applications in other optoelectronic devices.

Up to now, the best conversion efficiency of perovskite solar cells (PSCs) has reached over 23%, which is approaching to theoretical limit, and therefore, not only high efficiency, but also development of related technologies and new applications such as flexible device, module, lead-free material, high voltage, X-ray detector, and power source for IoT device are focused as next targets of researches. Recently, Cs-based all inorganic perovskite solar cells are energetically developed due to their high thermal stability. CsPbX₃ (X = I, Br and their mixture) perovskites possess wide band gap compared to the FA and MA-based perovskites, enabling to obtain high-voltage PSCs. Unfortunately, a-phase of iodide-rich CsPbX₃ perovskites such as CsPbI₃ and CsPbI₂Br is unstable under ambient condition, because of too small Cs ion size to form 3-D cubic perovskite structure at room temperature. Therefore, various effort to stabilization of the a-phase is devoted. On the other hand, reports of pure bromide, CsPbBr₃ is increasing because of high phase stability and high open-circuit voltage (V_OC) up to 1.6 V. We focused on CsPbBr₃ and developed a fully inorganic planar-type PSCs using TiO_X compact layer and a carbon-based electrode.
Because of very poor solubility of the CsPbBr₃ perovskite in any solvent (maximum less than 0.5 mM for DMF and DMSO), the CsPbBr₃ film is commonly prepared sequential deposition method; a precursor PbBr₂ film was treated with CsBr solution. The CsBr treatment is repeated four times to perfectly convert the PbBr₂ to CsPbBr₃ perovskite. However, because of no TiO₂ mesoporous scaffold, CsPbBr₃ film converted on the TiO_X CL became very rough and there are many pinholes due to low adhesion power between perovskite and TiO_X surface. To obtain pinhole-free and high-quality perovskite film, we modified a recipe of precursor solution and CsBr treatment procedure, and obtained pinhole-free CsPbBr₃ film on the TiO_X CL by CsBr treatment once. Furthermore, by passivation of surface of the TiO_X CL, V_OC of up to 1.5 V was obtained.

Understanding the phase transitions in inorganic halide perovskites is crucial to increase the stability of solar cells and to allow possible application of these materials in thermochromic solar cells and phase-change based optoelectronic devices [1, 2]. Here we present a combination of several in-situ techniques to reveal the phase transformation relations between the α, β and γ (black) perovskite phases and the δ (yellow) non-perovskite polymorph in thin films. Structural changes were tracked as a function of relative humidity (RH) and temperature by in-situ X-ray diffraction with a liquid-metal jet X-ray source. We find that at room temperature, the transition from the γ-black to the δ-yellow phase is exclusively triggered by the % of RH whereas O₂ is not involved in the mechanism. The transformation is preceded by an expansion of the unit cell volume of the perovskite phase Spectrally filtered in-situ optical microscopy reveals that this transformation does not affect the grain structure. An intermediate state is observed, where the optical transmission at 700 nm is significantly reduced, suggesting a pseudo-amorphous state during the atomic reordering.

For a detailed understanding of the atomic motion during the phase transitions we also studied the conversion from the yellow non-perovskite to the black-perovskite polymorph during in-situ heating using scanning transmission electron microscopy (STEM) revealing the transition from the δ- to the α-phase with atomic resolution where a layered structure is observed as intermediate.

[1] J. Lin, M. Lai, et al. Nature Materials (2018), 17, 261–267
[2] P. Becker, J.A. Márquez, et al. Advanced Energy Materials (2019), 1900555.

The huge performance enhancements of the organometal halide perovskite solar cells (OHPSCs) have appealed enormous attention within recent ten years. Although the rapid growth of the device power conversion efficiency (PCE) has attained over 24%, the contamination of health-hazardous components still holds back its sustainable applications. To reduce the lead usage, many groups have tried chemical lead reduction solutions: substituting the lead by other group 14 metal elements to realize the low-lead OHPSCs. Unfortunately, neither the PCE nor the stability, low-lead OHPSCs all lag far behind the state-of-the-art conventional lead-based OHPSCs. In this work, we present a physical lead reduction (PLR) concept by reducing the perovskite film thickness to restrict the perovskite hazard risk with minor scarification in device performances. Through the simulation of transfer matrix model, we theoretically demonstrated that by introducing the optical space layer, the device PCE could maintain 96% of the original maximum value while attenuating the perovskite film thickness to one-third. This means that the usage of lead can be reduced by ~70% with PLR concept, which could have broad appeal as a new lead reduction strategy towards high performance OHPSCs.

While lead-based perovskite and quantum dot (QD) solar panels are rapidly approaching the technological maturity for widespread commercialization, their environmental impacts and end-of-life disposal requirements have been largely uncharacterized. Here we present the United States Environmental Protection Agency Toxicity Characteristic Leaching Procedure (TCLP) as a means of rapidly characterizing the human and environmental health impacts of laboratory-scale solar cells and their resulting disposal requirements. We employ the TCLP to screen the toxicity and regulatory requirements of perovskite solar cells on various substrates, revealing increased lead leaching and potentially higher disposal costs for perovskite solar cells on lightweight, flexible substrates compared to heavier, rigid ones. We also use the TCLP to guide the development of a lower-toxicity manufacturing method for lead sulfide (PbS) QD solar cells. By developing a new solution-phase QD ligand exchange method that employs tetrabutyl ammounium idodide (TBAI) as the source of ligands, we maintain high device performance while reducing the total lead content in the synthesis byproducts by nearly 250 times and the total volume of solvents used by 80% compared to previous methods that employ lead halides. TCLP analysis reveals that QD solar cells made with this ligand exchange method leach less lead than the EPA limit, and as a result are unlikely to cause environmental harm during end-of-life disposal in municipal landfills. Further, the TBAI ligand exchange method offers significant cost savings compared to previous lead-halide-based methods, reducing synthesis costs by more than 70%. By using the TCLP to obtain architecture-specific toxicity information of lab-scale devices, we are able to design new solar technologies with low environmental risk.

Inorganic halide perovskite materials have attracted wide attention for optoelectronic applications due to its high mobility, long diffusion length and outstanding visible light harvesting performance. However, there are two major challenges for industrial applications, stability, and toxicity. By introducing environmental benign element Tin (Sn), toxicity problem could be addressed. Compared with divalent Sn-based perovskite which must be handled in an inert condition, tetravalent Sn-based perovskite materials are stable in ambient condition.

In this work, we demonstrate a novel solution processed all-inorganic perovskite Cs₂SnI₆ with excellent optoelectronic properties. At room temperature, the solution processed Cs₂SnI₆ photodetectors are sensitive to red-light(650nm), exhibiting a high detectivity. These Sn-based devices possess good linearity photo-response, and their optoelectronic performance is comparable to most of the all inorganic perovskite photodetectors. Our results indicate that with proper synthesis and device construction, lead-free inorganic perovskite materials are promising for stable, low-cost, environmentally-benign and high-performance photodetectors.

Perovskite solar cells (PSCs) have advanced intensively over the last few years and have reached in 24.2% power conversion efficiency (PCE). Because the achievement of high efficiency is enough for commercial applications, the stability and scalability issues have attracted attention for the potential commercialization of PSCs. However, when exposed to moisture, methylammonium lead iodide (MAPbI₃), which is used as the active layer of PSCs, decomposes into methylammonium iodide (MAI) and lead iodide (PbI₂), thereby degrading the device performance. For this reason, PSCs have to be fabricated under humidity-controlled environments.
Recently, it is reported that the humidity plays an important role in the crystallization of the MAPbI₃ and MAPbI₃ fabricated in ambient conditions has had a higher performance than that coated at low humidity. Thereafter, studies were conducted to fabricate the MAPbI₃ layer at various humidities, and it has been shown that when moisture is absorbed, the adjacent grain merges and the grain size increases. However, while the Spiro-OMeTAD material that is often used as a HTL has the property of absorbing moisture like MAPbI₃, few studies have reported on Spiro-OMeTAD coated at various humidities.
In this report, we investigate the tendency of characteristic changes when both Spiro-OMeTAD and MAPbI₃ were fabricated at relative humidity (RH) 20%, 40%, 60%, and 80%. As a result, PSCs fabricated at RH 60% showed PCE 15.5%, which is higher than the PCE 12.9% fabricated at RH 20%. We demonstrated that MAPbI₃ is more critical for the PCE of PSCs than Spiro-OMeTAD when we deposited MAPbI₃ and Spiro-OMeTAD separately under different humidity conditions. In this regard, the amount of PbI₂ produced by the humidity condition was characterized by employing an X-ray diffractometer and an ultraviolet–visible spectrophotometer. The surface morphology of the MAPbI₃ image were obtained by a scanning electron microscope and the charge lifetime of the MAPbI₃ surface was analyzed by steady-state photoluminescence (PL) and time-resolved PL. In this result, we can confirm that the efficiency increases when all experimental processes were performed in ambient condition at RH 60%. We expect this to be useful for commercialization because the fabrication of PSCs in ambient environment does not require expensive equipment to maintain low humidity.

The perovskite solar cells (PSCs) have been attracting great attention due to their promising high performance and low fabrication cost. Researchers have put a huge amount of effort to optimize the bulk and interfaces of perovskite layer in the PSCs to achieve high power conversion efficiency (PCE) and high stability at the same time. As a result, the PCE of PSCs has increased from 3.8% to 24.2% in only 10 years.[1] However, as the internal quantum efficiency of the PSCs approaching 100%, the space left for further optimization is getting smaller. Previously, by using numerical simulation methods, we have demonstrated that the performance of the PSCs could be improved by 11% by replacing the widely-used sandwich structure with an integrated back-contact (IBC) structure.[2] However, the reported PCEs of the IBC-PSC are still lower than that of their sandwich-PSC counterparts.[3]
Here, based on our simulation and experiment results, we propose that the grain boundary is one of the main factors limiting the performance of IBC-PSCs. In IBC structure, since the photo-generated charges need to travel laterally through a longer distance than that in a sandwich structure, if the distance of the electrodes is larger than the grain size, the charges might have to cross several grain boundaries to reach the electrode, resulting in a high recombination possibility. To reduce grain boundaries in the charge transporting path, we adopt a unique method to form a uniform perovskite layer with large single crystal grains as large as 50 µm.[4] In the fabricated IBC devices, most of the adjacent electrodes were connected by a single crystal of perovskite material. The performance of the IBC-PSCs was therefore significantly improved due to the reduced grain boundaries.

Reference
[1] https://www.nrel.gov/pv/cell-efficiency.html. Accessed on Jun. 13, 2019.
[2] T. Ma et al., ACS Appl. Energy Mater. 2018, 1, 970-975.
[3] M. Wong-Stringer et al., Energy Environ. Sci., 2019, 12, 1928-1937.
[4] T. Ma et al., J. Phys. Chem. Lett. 2017, 8, 720-726.

In the field of perovskite thin-film solar cells (PSC), the formation of the perovskite absorber via hybrid thermal evaporation / spin-coating processes represents an important step towards large-area deposition methods and potential industrial feasibility, while also enabling deposition on textured substrates¹. One caveat of this hybrid method is that it requires two physically separate layers of precursors to interdiffuse and crystalize, forming the perovskite absorber. This interdiffusion is complex, and many process parameters influence the resulting perovskite crystal quality and associated interfaces. In turn, these material qualities strongly affect the photovoltaic performance of the fabricated PSC.
Directly measuring the perovskite material quality would be an ideal shortcut to drive process development without the need to make large numbers of full devices In this work, photo-thermal deflection spectroscopy (PDS), atomic force microscopy (AFM), and X-Ray diffraction (XRD) measurements were used to investigate material quality in perovskite films formed via a hybrid evaporation / spin-coating process. Specifically, the temperature, dew point², and anneal time of the interdiffusion process were investigated. This led to insights about the crystal quality and stoichiometry of the perovskites, which was shown to directly link back to PSC performance, with efficiency gains of over 3% absolute. However, verification by PSC production revealed that these metrics (PDS, AFM, XRD) did not tell the whole story of the interdiffusion process. The remaining effects were attributed to modification of the perovskite layer interfaces. Further investigations into this phenomenon will be discussed at the conference.
The understanding of this hybrid fabrication method was then applied to PSC deposited on textured substrates. In theory, PSC employing a textured absorber interface could eschew the optical losses typical of thin films via improved light management. As our hybrid method allows for perovskite deposition on textured surfaces, textured PSC development focused more on optimization of the substrate texture. Along this route, we first performed optical ray-tracing simulations using AFM scans of textured surfaces formed by KOH-etched silicon (c-Si) wafers, as demonstrated in our previous publications^3,4, and boron-doped zinc oxide (BZO). Optimal BZO and c-Si textured surfaces were then used to fabricate PSC in the p-i-n architecture, with the perovskite layer deposited via the optimized hybrid method. Demonstrated PSC verified the trends predicted by simulation, with increased short-circuit current density (J_SC) and removed interference fringes from external quantum efficiency curves. However, improved optical performance and J_SC are not the only factors relevant to the overall performance of our PSC. Optimization of the full device and the impact of textured interfaces on other device parameters will be discussed at the conference.

References:
1. F. Sahli et al. Nature Materials. 17, 820–826 (2018)
2. Y. Ko et al. Journal of Materials Chemistry A. 6, 42, 20695-20701 (2018)
3. S. Altazin et al. Optics Express. 26 (10), A579 (2018)
4. T. Lanz et al. Optics Express. 23 (11), A539 (2015)

Two-dimensional (2D) organic–inorganic perovskites have recently emerged as one of the most important thin-film solar cell materials owing to their excellent environmental stability. The remaining major pitfall is relatively poor photovoltaic performance in contrast to 3D perovskites. In this work we demonstrate cesium cation (Cs+) doped 2D (BA)2(MA)3Pb4I13 perovskite solar cells giving power conversion efficiency (PCE) as high as 13.7%, the highest among reported 2D perovskite based devices. In addition, it has excellent humidity stability. The enhanced efficiency is attributed to perfectly controlled crystal orientation, increased grain size of 2D planes, superior surface quality, reduced trap-state density, enhanced charge-carrier mobility and charge-transfer kinetics. To our surprise, the Cs+ doping yields superior environmental stability and humidity tolerance. The device doped using 5% Cs+ degrades only ca. 10% after 1400 hours exposure in 30% relative humidity (RH), and exhibits significantly improved stability under heating and high moisture environment. Our results provide an important step toward air-stable and fully printable low dimensional perovskite as a next-generation renewable energy source.

Cesium titanium bromide, Cs₂TiBr₆, has attracted research interest as a semiconductor due to its ~1.8 eV band gap, which makes it desirable for forming tandem junctions with silicon. However, little is known about the surface of the material. To ascertain stability of the material in ambient air we conducted controlled air exposure experiments, and to determine the reactivity and basicity of interfacial halides, we reacted the surface with a series of Lewis acids. X-ray photoelectron spectroscopy (XPS) established correct synthesis, effects of atmospheric exposure, and surface coverage of Lewis adducts. Powder X-ray diffraction (pXRD) quantified the effects of atmospheric exposure to the crystal over time. Temperature programmed desorption (TPD) quantified bond strengths of resulting Lewis adducts. The presence of Lewis adducts at the surface of Cs₂TiBr₆ demonstrated consistent loss of cesium bromide on the surfaces and surface exposure to ambient air resulted in rapid surface oxidation. These results elucidate the reactivity of the chemical states on Cs₂TiBr₆ surfaces, and future routes through which interfacial reactivity may be used to improve atmospheric stability.

Lead-free double perovskites of Cs₂AgBiBr₆ wherein two divalent Pb²⁺ cations are substituted by one monovalent Ag⁺ cation and one trivalent Bi³⁺ cation, respectively, are considered as potential candidates for developing materials for perovskite solar cells. We have now prepared 8—10 nm diameter Cs₂AgBiBr₆ nanocrystals using a hot-injection method and investigated their excited state properties. In addition, we have also probed the excited state interactions of double perovskite quantum dots (QDs) with various metal oxides (TiO₂, ZnO, SnO, and ZrO₂) and established the electron transfer kinetics between the two. Spin-coated QD films were annealed (at 225 °C) to obtain bulk double perovskite films. UV-vis absorption, high resolution SEM images, and X-ray diffraction have been employed to track the transformation of QDs to bulk films. The annealed films can be directly employed in the fabrication of solar cell architecture: FTO/c-TiO₂/m-TiO₂/Cs₂AgBiBr₆/Spiro-oMeTAD/Au. The double perovskite solar cells show relatively low solar energy conversion efficiency (less than 1.0 %) with J_sc of 0.40 mA/cm² and V_oc of 0.65 V. Understanding of nanocrystal growth mechanism and correlation between the grain size and solar cell efficiency is important for improving the efficiency of double halide perovskite for solar cells.

Organic-inorganic hybrid perovskite solar cells (SCs) have emerged as one of the most promising contenders to traditional silicon solar cells, due to their active layers outstanding photoelectric properties, such as appropriate direct bandgap, balanced high carrier mobility and long carrier diffusion length, the identified power conversion efficiency (PCE) has reached to 24.2 %. But the toxic lead, a key component in the archetypical light harvesting material, is a large obstacle to commercialization.
Double perovskite Cs₂AgBiBr₆ has emerged as a promising optoelectronic material due to its excellent physics and photoelectric properties. We first reported double perovskite planar heterojunction SC with a high quality Cs₂AgBiBr₆ film, fabricated by a low-pressure assisted solution processing method under ambient conditions, which presents a highest PCE of 1.44 %. Moreover, we reported highly efficient and stable self-powered ultraviolet and deep-blue photodetector based on Cs₂AgBiBr₆/SnO₂ heterojunctionwith two responsivity peaks at 350 and 435 nm, which is suitable for ultraviolet-A (320–400 nm) and deep-blue light detecting. A high responsivity of 0.11 A/W at 350 nm and a quick response time of less than 3 ms are obtained, which is significantly higher than other semiconductoroxide heterojunction based UV-detectors. More importantly, the stability is significantly better than most of the hybrid perovskite photodetectors reported so far. Its photocurrent shows no obvious degradation after more than six months storagein ambient conditionswithout any encapsulation.Consequently, the utilization of Cs₂AgBiBr₆ film is a practical approach for high performance, large-area lead-free perovskite photodetector application. The low toxicity and high stability of this double perovskite active layer make it very promising for practical application. For mechanism, we found that photogenerated carriers in Cs₂AgBiBr₆ film are separated at Cs₂AgBiBr₆/SnO₂ heterojunction interface by its build-in field, which is very important for the future double perovskite based optoelectronic device structure design.
Silver bismuth iodide (Ag−Bi−I) as an environmentally friendly semiconductor with suitable band gap and high stability has been regarded as a potential photovoltaic material, while the reported mesoscopic devices all showed poor open circuit voltage (V_oc) of 0.5−0.6 V. We reported silver bismuth iodide-based solar cells with a planar heterojunction structure of “ITO/SnO₂/AgBiI₄/PTAA/Au” by a solution method. Li-TFSI additive (2 wt %) was added into the AgBiI₄ precursor and induced a fully covered pinhole-free morphology. The readily coordinated organic component, TFSI⁻, was proven to play a role of assisting the film growth. The optimal device could achieve a V_oc of 0.82 ± 0.01 V and PCE up to 2.50 ± 0.20 %, much higher than the one without Li-TFSI of 0.66 ± 0.02 V and 1.67 ± 0.09 %. Moreover, we fabricated AgBiI₄ SC with a thicker absorber film, which could even have a record V_oc of 0.88 V, but the depressed J_sc implied its limited carrier diffusion length. In addition, the double layer devices with a structure of “ITO/SnO₂/AgBiI₄/Au” or “ITO/AgBiI₄/PTAA/Au” showed the carrier separation occurred in the interface of SnO₂/AgBiI₄. Above results pointed out the significance of film quality improvement and interface charge extraction in further research.

Halide perovskites have generated tremendous interest as low-cost semiconductors for optoelectronic applications such as photovoltaics. The bulk of work surrounding halide perovskites has centered around Pb-based perovskite derivatives; however, recent years have seen a rise in Pb-free perovskites which are less harmful and more environmentally friendly. In this regime the leading alternative to Pb is Sn, which has successfully been used in photovoltaics to achieve over 9% power conversion efficiency. The further development of Sn based perovskite photovoltaics necessitates a firm understanding of the material properties and the ways in which tin derivatives differ from their lead counterparts. One key property is the energetic structure of Sn-based perovskites and the energetic alignment of their transport states with those in adjacent layers. Herein lies an issue where current literature regarding the valence and conduction band energies cannot reach consensus even for formamidinium tin iodide (FASnI3), the most popular tin-based composition, for which reported ionization energies (IEs) already span a range greater than 1 eV. Using low energy ultraviolet and inverse photoelectron spectroscopy, we find the IE and electron affinity (EA) to equal 5.3 and 3.9 eV for FASnI3. In addition, we observe that the amount of tin(II) fluoride, a nearly ubiquitous additive employed to reduce tin oxidation, shifts these energetics by hundreds of meV. Further, we find that short air exposures (<1 minute), synonymous with sample transfer, result in minimal changes to IE, EA, and PV performance, whereas longer exposure times (5-10 minutes), analogous of a sample measurement in air, shows changes of hundreds of meV in both IE and EA as well as degradation of device performance. At the interface between FASnI3 and C60 we detect migration of iodide from the perovskite into the fullerene and the formation of a surface dipole. Additional fullerene derivatives are examined to elucidate the prevalence of iodide migration and the impacts on energy level alignment with the perovskite and photovoltaic performance. Finally, surface modifiers are explored to prevent iodide diffusion out of the perovskite layer. These results are important for understanding the electronic structures of tin-based perovskites, how interface energetics influence charge extraction in perovskite photovoltaics, and minimizing iodide diffusion is critical to improving stability.

Tin-lead (Sn-Pb) perovskite exhibits an ideal band gap (1.2 eV-1.4 eV) to achieve higher power conversion efficiency (PCE) than pure Pb based PSCs [1]. However, being a low band gap material and prone to oxidation of Sn²⁺ to Sn⁴⁺, Sn-Pb solar cells exhibit low open circuit voltage (Voc) [2,3]. To increase the Voc, a spike structure strategy was reported by our group, Voc of 0.75 V and PCE of 17.6% were obtained [2]. To further improve Voc, in this work we will demonstrate that by decreasing the strain in the Sn-Pb perovskite lattice, Voc more than 0.80 V can be obtained. Also, we will show that with the change in conducting glass substrate, i.e by selecting a conducting glass with better IR transmittance, short circuit current density (Jsc) more than 30 mA/cm² can be obtained. The methodology discussed in the work can direct to PCE of more than 20%.
Cs⁺ added triple cation-based Sn-Pb perovskites, (Cs_xFA_1-xSnI₃)_0.5(MAPbI₃)_0.5, were prepared. Voc was improved from 0.76V to 0.81V with the addition of optimized amount of Cs⁺. This observation was well supported by the decrease of lattice strain at the optimized concentration of Cs⁺. In the work, we will discuss in detail about the characterization techniques such as X-ray diffraction (XRD), UV-Vis spectroscopy, Trap density measurement by Thermally stimulated current (TSC) measurements, etc, which led to the conclusive discussions regarding the optimized precursor for Sn-Pb PSCs.
Strain relaxation in Sn-Pb PSCs was reported which led to the increase in Voc more than 0.80 V. It was demonstrated that better transmittance of conducting glass in infrared region can increase Jsc of Sn-Pb PSCs more than 30 mA/cm². Furthermore, PCE of more than 20% can be achieved in the case of Sn-Pb PSCs despite of oxidation problem. Also, the results obtained in the work encourage implementation of Sn-Pb PSCs as a low band gap solar cell in tandem solar cells.
References
W. Shockley, H. J. Queisser, J. Appl. Phys. 1961, 32, 510.
G. Kapil and S. Hayase et al., Nanoletters, 2018, 18, 3600-3607.
J. Tong and K. Zhu et al. DOI: 10.1126/science.aav7911.

Organometal halide perovskites have received a lot of attention in recent years as an efficient light absorbing material for use in a solar cell, where the device efficiency has reached 24.2% in early 2019. Compared to traditional silicon solar cells, the perovskite thin films can readily be made in a lab environment with techniques such as spin coating, blade coating, and thermal vapor deposition. In particular, thermal deposition is well-known for its ability to grow highly uniform perovskite thin films, often times with fewer defects and a more ideal chemical stoichiometry than films grown by solution-based techniques. The interactions of the prototypical perovskite precursors, lead iodide (PbI₂) and methylammonium iodide (MAI), while in the vapor phase and their depositions are poorly understood, despite their role they play in perovskite film growth. Herein, we observed the partial pressure changes of the perovskite precursors in the vapor phase with a residual gas analyzer, and each precursor was individually evaporated in a vacuum chamber which was continually pumped by a turbomolecular pump. No increase in the MAI partial pressure was observed during the MAI evaporation and this suggested that it was not evaporating as a whole compound, but rather it was dissociating into several other species including CH₃NH, CH₃NH₂, and HI. Surprisingly, residual water vapor was also observed to behave differently in the two different individual evaporations. During the MAI evaporation, the water vapor pressure was reduced and this indicated that the water vapor was being consumed either by MAI or MAI dissociation products. This was not observed during the evaporation of PbI₂ or the heating of an empty tungsten boat used as a control investigation. This was surprising as the water vapors initial pressure was 5*10^-7 torr, and such a low amount of water vapor was not expected to influence the depositions. In literature, it had previously been thought that MAI depositions were problematic due to a low sticking coefficient. However, this work suggests that the MAI’s and its dissociation component’s vapor pressures in previous investigations may have unknowingly also been reduced due to residual water vapor in the vacuum growth chambers. This reduction effect of the MAI related vapor pressures may play a critical role in the initial deposition of MAI films, and subsequently, any deposition of perovskite thin films by vapor deposition.

Cs₂AgBiBr₆ has received great attention as an emerging double perovskite material. Some researches shown that the performance of the double perovskite material can be improved by doping other elements, but there is no report about solar cell with doping.
We demonstrate Rb⁺ doping in Cs₂AgBiBr₆ can effectively increase the current and PCE of the device. Doping Rb⁺ can effectively modify the perovskite to reduce the density of defect states, while the absorption of long-wavelength is strengthened. We also found the optimizing ratio of Cs⁺ and Rb⁺ is 9:1. The device with (Cs_0.9Rb_0.1)₂AgBiBr₆ achieves a maximum PCE of 1.52% and an ultra-high FF of 0.788 with the planar structure. And the average PCE is nearly 15% higher than the devices with Cs₂AgBiBr₆. However, excessive Rb⁺ would lead to formation of Rb₃Bi₂Br₉, that means the degradation of the double perovskites.

Research in perovskite photovoltaics (PV) has witnessed a tremendous development over the last decade, as leaps in efficiency and stability were achieved by improvements in the device architecture. Progress in the charge selective layers led to better charge collection and band alignment, whereas development of multiple cation mixed halide perovskite phases resulted in robustness beyond 500 h under harsh environmental conditions.^[1,2] Yet, the broad commercialization of perovskite PV panels has remained an elusive goal, as issues remain regarding the modules’ scalability and their long-term moisture- and air-sensitivity.
While typical stability tests employ high ambient humidity and elevated temperatures to accelerate device degradation, it must be pointed out that real-life module operation involves device soaking, mechanical stress and temperature changes, as experienced by a solar panel under say a heavy rain. Accordingly, the investigation of device robustness when submerged in aqueous, potentially corrosive electrolyte solutions may provide a more realistic benchmark for evaluating encapsulation strategies.
In this contribution, we introduce photoelectrochemical (PEC) device testing as a way to evaluate encapsulation methods, which are employed for perovskite PV devices of different sizes. Accordingly, we will focus on the recent advances in solar cell encapsulation achieved using a thin layer of Field’s metal (FM), a low melting InBiSn alloy.^[3,4,5] The metal layer has a dual function, as protective barrier and electrical contact, which can act as an electronic interface to a surface-bound catalyst for PEC applications. In particular, the FM encapsulation demonstrates an advantage for the J-V characterization of larger single pixel perovskite solar cells, which can be straightforwardly employed as hydrogen-evolving photocathodes after electroless deposition of a platinum nanoparticle catalyst.^[5] In this respect, we will also place special focus on the scalability of FM encapsulated solar cells, highlighting the interplay between device performance and serial resistance losses for single pixel perovskite PV devices of sizes of up to 10 cm².
In this context of photoelectrochemical benchmarking, we will also give a brief introduction to the basic principles of solar-fuel research and its advantages in terms of simultaneous energy production and storage,^[6] showcasing our recent progress in the development of perovskite-BiVO₄ PEC tandem devices for unassisted water splitting.^[5] Taking our current progress on 300 cm² large scale BiVO₄ panels into account,^[7] we envision rooftop PEC devices by combining the BiVO₄ photoanodes with perovskite photocathodes of matching sizes. In the context of commercial applications, we will further address the potential of lowering down the overall device costs by appropriate choice of components.

[1] M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo, A. Ummadisingu, S. M. Zakeeruddin, J.-P. Correa-Baena, W. R. Tress, A. Abate, A. Hagfeldt, M. Grätzel, Science 2016, 354, 206.
[2] T. J. Jacobsson, S. Svanström, V. Andrei, J. P. H. Rivett, N. Kornienko, B. Philippe, U. B. Cappel, H. Rensmo, F. Deschler, G. Boschloo, J. Phys. Chem. C 2018, 122, 13548.
[3] M. Crespo-Quesada, L. M. Pazos-Outón, J. Warnan, M. F. Kuehnel, R. H. Friend, E. Reisner, Nat. Commun. 2016, 7, 12555.
[4] S. Ahmad, A. Sadhanala, R. L. Z. Hoye, V. Andrei, M. H. Modarres, B. Zhao, J. Rongé, R. H. Friend, M. de Volder, ACS Appl. Mater. Interfaces 2019. DOI: 10.1021/acsami.9b04963.
[5] V. Andrei, R. L. Z. Hoye, M. Crespo-Quesada, M. Bajada, S. Ahmad, M. De Volder, R. Friend, E. Reisner, Adv. Energy Mater. 2018, 8, 1801403.
[6] V. Andrei, K. Bethke, K. Rademann, Energy Environ. Sci. 2016, 9, 1528–1532.
[7] H. Lu, V. Andrei, K. J. Jenkinson, A. Regoutz, N. Li, C. E. Creissen, A. E. H. Wheatley, H. Hao, E. Reisner, D. S. Wright, S. D. Pike, Adv. Mater. 2018, 30, 1804033.

Halide perovskite solar cells (PSCs) have leap-frogged many photovoltaic systems in the past decade due to their excellent optoelectrical properties and solution-processability. Meanwhile, metal oxide (MO) transport layers have been shown to exhibit better stability, lower cost, and more tolerant processing parameters than their organic counterpart, leading to a promising path towards PSC commercialization. However, the bottleneck to high-throughput manufacturing of PSCs is the thermal annealing step in both the perovskite active layer and the MO transport layer, especially for mixed-cation perovskite materials and sol-gel MO films, which both require long thermal annealing time and high temperature. Conventional thermal annealing methods are not conducive to high-throughput manufacturing of large area PSCs because they translate to hundreds of meters long annealing tools and tremendous energy consumption due to the heat loss. In this work, we explored photonic curing as a rapid thermal annealing technique to replace conventional thermal annealing for both the bottom MO transport layer and the perovskite layer. Photonic curing, which is often used to sinter conductive nanoparticles on flexible substrates, delivers intense pulsed light on a sample within ~1ms. The absorption of light by the precursor film can increase its temperature by several hundreds of degrees within a short time window while minimizing heating of the substrate due to overall low energy. Thus, it is suitable for plastic substrates that can only be processed at low temperatures, e.g. PET. In addition, many photonic curing parameters--pulse length, pulse shape, pulse rate, energy per pulse, and number of pulses—can be optimized to process different materials due to their different light absorption coefficients and thermal properties. Our p-i-n PSCs are fabricated on PET/ITO substrates using NiO_x as the hole transport layer. We spin coat and photonic cure NiO_x sol-gel precursor films and perovskite precursor films sequentially. The materials’ properties, including optical absorption, crystallinity, surface morphology and roughness, and work function, as well as device performance are compared for films processed by photonic curing vs. conventional thermal annealing. We also compare the processing time and cost between photonic curing and hot plate annealing to demonstrate photonic curing as a potential candidate in high-throughput manufacturing of halide PSCs.

Organic-inorganic halide perovskite solar cells have shown a rapid increase of performance with a maximum power conversion efficiency (PCE) up to 24.2 % in 2019, making these devices promising alternatives to those of established solar cell technologies. Besides the toxicity of the commonly used Pb-based compounds, the lack of deposition tools for large areas is one of the major challenges for the commercialization of perovskite photovoltaics. Chemical vapor deposition (CVD) is an appealing choice because it furthermore features high purity and superior process control.
In this work, we present a custom-developed showerhead-based perovskite CVD tool as well as novel low-vacuum (total pressure 10 hPa) CVD processes to fabricate dense perovskite films by simultaneous deposition of the precursors from the gas phase. We employ an organo-halide compound (CH₃NH₃I (MAI)) and a metal halide (either PbI₂ or BiI₃ as a non-toxic alternative) to obtain either MAPbI₃ or (MA)₃Bi₂I₉ (MBI) perovskite layers on a typical solar cell anode stack (glass / FTO / compact TiO₂ / mesoporous TiO₂, substrate size 2.5 cm x 2.5 cm). The thermally sublimated precursors are transported via N₂ carrier gas (500 sccm) through the heated showerhead. The carrier gas flow is spread homogeneously enabling deposition on large-area substrates. Condensation and reaction of the precursors occur on the temperature-controlled substrate. A quartz crystal microbalance (QCM) is used to determine the deposition rates, usually between 3 and 7 nm/min for perovskite growth. For morphological, structural and optical characterization of fabricated layers, scanning electron microscopy (SEM), X-ray diffraction (XRD) and absorption measurements are performed. To obtain stoichiometric perovskites, we analyze the impact of the deposited-volume ratio between MAI and PbI₂/BiI₃ (adjusted by the QCM rates). In case of the Pb-based perovskite, an excessive supply of MAI (volume ratio MAI/PbI₂ > 5) was found to be responsible for the formation of the dihydrate perovskite phase (CH₃NH₃)₄PbI₆^.2H₂O (XRD reflex at 11.3 °, measured in ambient air), leading to fast degradation of the perovskite layer. Providing the right amount of MAI during deposition (volume ratio between 2 and 4) results in stoichiometric perovskite films without any residues of the precursors (XRD spectrum perfectly matches the calculated pattern). We fabricated dense layers with thicknesses of 400 nm, exhibiting a cubic morphology with grain sizes of 200 nm.
For the MBI perovskite, a volume ratio of 5 was found to be sufficient for a complete conversion of BiI₃. Depositions with MAI/BiI₃ volume ratios exceeding 5 do not exhibit XRD peaks which can be assigned to either MAI or a perovskite hydrate. Obviously, the simultaneous deposition of BiI₃ and MAI has a self-limitation for MAI incorporation, still leading to the formation of stoichiometric MBI. Furthermore, we investigated the influence of the substrate temperature and layer thickness on film properties. Decreasing the substrate temperature from 88 to 50 °C leads to a more rounded grain morphology. The size of the crystallites scales to some extent with the layer thickness. Large grains up to 500 nm diameter are formed for a film with 225 nm layer thickness. The MBI films were implemented in non-inverted solar cell structures (Spiro-MeOTAD / Au contacts on top) with a maximum PCE of 0.016 %. Most likely, higher efficiencies can be achieved by optimizing the device architecture as the mesoporous TiO₂ matrix is not penetrated by the perovskite film. Finally, we deposited and characterized dense MBI films on large-area FTO substrates (12 cm x 9 cm), demonstrating the high degree of homogeneity. The formation of dense perovskite layers combined with the versatility and scalability of the process render this technology to be an auspicious candidate for commercial large-area production.

Perovskite solar cells (PSCs) are one of the most promising photovoltaic technology. However, lead perovskites face long-term instability issues under stimuli of heat, oxygen, moisture, electric field and light irradiation. Here we incorporate a series p-type organic conductors as additives in perovskite layer, increasing the device efficiency and markedly improving the long-time device stability. We close by positing organic conductor, reduce the defect density on the perovskite surface, by tying up both halide-rich antisites and undercoordinated lead center. After optimization, the carrier recombination lifetime was increased and the boost efficiency of 20.4% inverted perovskite solar cells was obtained. The thermal stability of the PSCs with passivation was evaluated, in which the devices were heated to 85°C inside a N₂-filled glove box. there is almost no PCE reduction for the perovskite-conductor device after being heated for 24 hours.

Highly efficient and low-cost halide perovskite solar cells (HPVKSCs) are regarded as one of the most promising photovoltaic technologies to realize commercialization in the near future.
However, the long-term stability of HPVKSCs under real working conditions, involving stresses from moisture, heat, light, and the electric field, is still a challenge that must be addressed. The irreversible degradation of HPVKSCs can be mainly attributed to three phenomena: (1)volatilization of the organic components in organic–inorganic hybrid perovskites, especially for the benchmark methylammonium lead triiodide (MAPbI₃) under thermal aging conditions above 85°C, (2)permeation of external H₂O/O₂ and its induced degradation of perovskites, and (3) reactions between the normally used metal electrodes and halogens from the perovskites during prolonged operation. For the last two issues, especially the third one, their resolutions are largely related to the essential properties of the interfacial layers (i.e., morphologic pin-hole free, chemical resistivity to H₂O/O₂ and halides), which could act as robust permeation barriers to block the undesired processes. If the interfacial layers are permeable or have pin-holes, sufficient separation between the perovskites and electrode metals cannot be achieved, and the corrosive reaction could be fast.
Here, we demonstrate a facile processing strategy for highly stable HPVKSCs with the structure FTO/Li⁺-doped Ni_xMg_1−XO (NiMgLiO)/Perovskite (PVK)/PCBM/BCP/Bi/Ag by introducing the compact Bi interlayer. The devices and films were characterized by photoluminescence (PL), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), time of flight secondary ion mass spectrometry (ToF-SIMS), etc., which demonstrate that the Bi interlayer acts as a robust penetration barrier that blocks the unfavorable external molecules from diffusing into the devices and suppresses the diffusion of ions between the metal electrode and the perovskites. As a result, our unencapsulated MA-HPVKSCs retain 88% of their initial efficiency after being maintained in the dark in ambient air without humidity control for 6000h. During long- term light soaking or thermal aging at 85°C in the dark, the devices with Bi interlayers are considerably more stable than devices without Bi. This Bi-interlayer strategy is easily scalable and highly repeatable and may pave the way to the future practical application of large-area and highly stable HPVKSCs.

Lead halide semiconductors with a perovskite crystal structure and APbX₃ stoichiometry [A = Cs+; X = I−, Br−, Cl− or mixtures thereof have recently turned into the most strongly considered category of inorganic optoelectronic materials. These materials are now widely utilized in light-emitting diodes, lasers, photo-detectors as well as in X-ray and gamma-ray (γ-ray) detectors. In these applications, large single crystals (SCs) grown by precursors solutions of perovskites are mostly utilized because of low trap density and less grain boundary. Therefore, the rapid emergence of halide perovskites based single crystalline materials for a wide variety of as mentioned applications has drawn much research attention in recent years. Many efforts have been made in optimizing their macroscale device performance by improving material processing and device fabrication and over the last few years. But, to fully obtain and understand the intrinsic characteristics of perovskites, a variety of high-quality single-crystalline materials are genuinely necessary.

In our present study, we have reported the 3mm size lead free perovskite single crystals grown by the slow cooling method. The synthesis, crystal growth, and detailed structural and optical characterization have been carried out by Optical microscopy, FESEM, EDXA, single crystal XRD, UV-Vis, PL, and TCSPC. The synthesized single crystals have orthorhombic structure and Pnma space group and the corresponding lattice parameters and reciprocal lattice parameters are calculated. Moreover, symmetry operators in the crystal, hkl values, linear absorption coefficient and reciprocal lattice pattern of the synthesized single crystals have been calculated. Transmission microscope and FESEM images have shown clearly the single crystalline morphology and EDXA have shown the stoichiometry matching by means of elemental analysis of the crystals. The I-V characteristics of this lead-free perovskite single crystals are also discussed.

Perovskite solar cells (PSCs) demonstrated fast- raising power conversion efficiency (PCE) in past couple years. However, one of the problems associated with PSCs is the formation of small sized grains with large density of grain boundaries (GB) and defects within the perovskite layer, which largely results from the conventional post annealing method used in perovskite layer preparation.
To promote crystalline quality and reduce GB density, we utilized a so-called “hot-casting” technique, in which the perovskite precursor solution and substrate are preheated at a relatively high temperature before spin-casting. This allows annealing and spin-casting to occur at the same time, prolongs the grain growth time, and as a result, enhances the crystal quality. Therefore. PSCs made by hot-casting can have promoted PCE by growing large crystal grains and decreasing defects/traps.
We compared the morphology and crystalline structure of perovskite layer and PV performance of corresponding devices prepared via conventional post annealing versus the hot-casting. We observed in scanning electron microscope (SEM) images that hot-casted samples produced an average grain size of 11.40 μm, nearly 60 times greater in size than the average grain size of 194 nm obtained for the post annealing method. Furthermore, we observed in atomic force microscopy (AFM) images that hot-casted perovskite demonstrated a canyon-like crystal growth due to such vigorous growth that neighboring grains collided at the edges, forming a uniform, fully covered, and highly crystalline film with micron sized grains; in contrast, perovskite made by post annealing produced a film with much smaller and nanosized grains. We also used x-ray diffraction (XRD) to confirm that hot-casting generated perovskites possessed the tetragonal crystal phase, same with the sample made by post annealing method; in fact, hot-casting enhanced the crystallinity of perovskite, which exhibited stronger XRD peak intensity than the conventional ones. Most notably, PV performance tests demonstrated a PCE increase of ≈15% when using hot-casting, from 12.2% prepared conventionally to 13.9% prepared via hot-casting. Thus, hot-casting has demonstrated to be an effective method to increase PCE of PSCs through the grain size management. (We acknowledge support from the Louis Morin Charitable Trust and NYS Department of Economic Development.)

Organic-inorganic hybrid perovskites, such as the prototypical methylammonium lead iodide (CH₃NH₃PbX₃ or MAPbX₃, X=I^-, Br^-, Cl^-), have emerged as promising light absorbers in photovoltaic (PV) cells or as emitters in light-emitting diodes (LEDs). However, they generally suffer from moisture instability, which limits the long-term use of perovskite-based devices in ambient environment and impedes the rapid commercialization. In this work, we have studied the ion diffusion and surface moisture-induced degradation behaviors for two types of perovskites: 3D MAPbI₃ and 2D MAPbBr₃. In order to greatly decrease the ion dissociation rate and improve the moisture stability, we have applied molecular dynamics (MD) simulations and first-order reaction kinetics theory to model the ion dissociation process and estimate the associated free energy barrier with and without ligand passivation. We design 3D MAPbI₃ and 2D MAPbBr₃ structures with ligand-passivated surfaces by replacing MA cations with ligands composed of long-chain alkyl-ammoniums. Ligands with different chain lengths, such as CH₃(CH₂)_nNH₃⁺ (n = 3, 7), and under different surface coverages (σ = 25%, 50%, 75%, 100%), are considered here. We discover that ligand passivation can greatly help 3D MAPbI₃ and 2D MAPbBr₃ perovskites protect MA cations on the surface due to the much higher dissociation energy barriers of these ligands compared to that of MA cations. For I anions, ligand passivation can also shield them from water contacts, except for long-chain ligands, such as CH₃(CH₂)_nNH₃⁺ (n = 7) at full surface coverages (σ = 100%), due to the reduced dissociation free energy barriers of long-chain ligands. As an interesting finding, the reduced dissociation free energy barriers for long-chain ligands at high surface coverages could be explained by their larger tendencies to micellize, which serves as additional driving force for their dissociation. Additionally, water contact angle simulations have also been performed for 3D MAPbI₃ perovskites to compare the hydrophobicity of different ligand-passivated surfaces and verify that the surface moisture stability of 3D MAPbI₃ surface has been improved by the ligand passivation technique. We also observe suppressed and anisotropic ion diffusion in 2D MAPbBr3 perovskites due to the layered ligand phase sandwiched by the PbI₂ crystals, which resembles an organic-inorganic superlattice. This work significantly motivates future experimental efforts in designing new surface ligands to improve the moisture stability of halide perovskites.

Within the last decade, perovskite (PVSK) has emerged as a solar cell material rivaling those of silicon and quantum dots due to their increasing power conversion efficiency (PCE) . PVSK serves as the active layer within a planar solar cell and have the common hybrid organic inorganic halide structure ABX3. The increasing efficiency of these cells can be attributed to several optoelectronic characteristics such as a high absorption coefficient, tunable bandgap and ambipolar carrier transport . However, PVSK is limited because the ABX3 classic structure is unstable. The most common ABX3 structure is methylammonium lead iodide (MAPbI₃). The organic cation of methylammonium (MA) is hygroscopic, causing the cell to degrade under conditions of moisture, heat, oxygen, and light [3]. To reduce degradation and enhance efficiency, Cs and formamidinium (FA) cations and Br halide are introduced to create a mixed cation/halide structure CsFAMAPbIxBr1-x. The combination of these cations and halides can allow for increased stability and optoelectronic performance by reducing transformation into a photo inactive delta phase, while maintaining the preferable bandgap produced by the FA structure.
Pervious research studies have used different pure PVSK compounds such as CsPbI₃ or FAPbI₃ to either prevent degradation or to optimize absorption. However, due to the existence of inactive phases, these materials suffer from structural instability. Furthermore, studies that have observed mixed structures that do not include FA (Cs_xMA_1-xPbI₃ for example) can exhibit lower efficiencies due to a bandgap increase . This study tries a combination of Cs, FA, MA, and Br with a spin-coating method to optimize both efficiency and stability of the cell.
One-step spin coating was used to prepare the PVSK film. PbI2, MAI, CsI, FAI, and PbBr2 at a molar ratio of 1:.7:0.15:0.15 were placed in a mixed solvent of DMF and DMSO (8:2). Titanium dioxide was spin coated onto FTO substrates and annealed to form the electron transport layer (ETL). The precursor PVSK solution was then deposited via spin coating and chlorobenzene was applied onto the surface as an anti-solvent for crystal generation. Spiro-OMeTAD was coated as a hole transport layer (HTL) and physical vapor deposition (PVD) was used to add the gold electrodes.
UV-Visible Spectroscopy indicated that the changing of the cation/halide component did not influence the absorption of the photoactive layer. Scanning electron microscopy (SEM) morphology results showed increased grain size for the mixed PVSK. Atomic force microscopy showed correspondent morphologies to SEM results, but with an increase in roughness, which is within acceptable range. Moreover, XRD results implicated that the mixed PVSK had two possible crystal phases (α and �� phases) compared to the single peak of (110) of MAPbI3 PVSK, while only the cubic α phase is photoactive. Therefore, the mixed PVSK layer was annealed at various conditions to optimize the cubic alpha photoactive phase. This is because the partial phase segregation can lead to increased recombination at Iodine rich centers, and would therefore hinder PCE. After the optimization, results revealed that at a temperature of 120 Celsius and a time of 10 minutes allowed for the preferable crystallization of the mixed PVSK with a strong alpha peak and negligible delta phase. Furthermore, the PCE measurement indicated the mixed PVSK has higher PCE, probably due to increased grain size. The moisture and heat stability tests (XRD) revealed enhanced structural stability against excessive heat, supporting that the mixed structure can successfully generate better performance and enhance the durability. (We acknowledge support from the Louis Morin Charitable Trust and NYS Department of Economic Development.)

The single crystals of methylammonium lead iodide (MAPbI₃) CH₃NH₃PbI₃, which recently draws much research interest as light-harvesters in dye-sensitized solar cells because of the high photovoltaic efficiency and the structural flexibility for all solid-state devices, were prepared by the solution growth method and characterized by ¹²⁷I NQR (nuclear quadrupole resonance) techniques. ¹²⁷I NQR spectrum, linewidth, spin-spin relaxation rate 1/T₂, and spin-lattice relaxation rate 1/T₁ were measured as a function of temperature down to 70 K to microscopically understand the iodine dynamics associated with the ionic conductivity. As temperature decreases, ¹²⁷I NQR frequency slightly increases from ~ 82 MHz at room temperature and then disappears around 180 K with the rapidly increasing linewidth. At lower temperatures, the NQR signal appears again with the fast increase of resonance frequency but the narrowed linewidth remains almost same. The spin-echo signal is observed below 120 K and 1/T₂ decreases quickly at low temperatures. 1/T₁ increases down to 180 K, below which it starts to decrease. Based on these NQR data, we conclude that the iodine dynamics change significantly around 180 K.

Methylammonium lead iodide (CH₃NH₃PbI₃, MAPbI₃) belongs to the group of organic-inorganic halide perovskite (OIHP) that exhibit exceptional electrical and optical behavior for photovoltaic applications. However, its structural and chemical stability and robustness remain a challenge in order to be considered a feasible active material in optoelectronic devices. Therefore, it is crucially important to understand mechanisms by which MAPbI₃ undergoes chemical degradation under operating conditions such as a relatively humid environment. We use dark pulse discharge measurements and current-voltage (I–V) variations of MAPbI₃ under different moisture levels to investigate the nature of structural degradation in (OIHPs). We show that while relatively low levels of humidity (RH<65%) have limited impact on structural stability, exposure to higher levels of moisture results in the formation of PbI₂ and CH₃NH₃I which fundamentally change the charge transport phenomenon in MAPbI₃. Our findings suggest an explanation of the ongoing debate on the presence of a critical level of humidity that triggers irreversible structural transformation that leads to full degradation of MAPbI₃.

The efficiencies of small-pixel perovskite photovoltaics have increased to well above 20%, while the question is whether fabrication methods can be transferred to scalable manufacturing process. Here we report a method of fast blading large area perovskite films at an unprecedented speed of 99 millimeter-per-second or higher in ambient condition by tailoring solvent coordination capability. Combing volatile non-coordinating solvents to Pb²⁺ and low-volatile, coordinating solvents achieves both fast drying and large perovskite grains at room temperature. The reproducible fabrication yields a record certified module efficiency with aperture area of 63.7 cm². The perovskite modules also show a small temperature coefficient of -0.13%/°C and nearly fully recoverable efficiency after 58 cycles of shading, much better than commercial silicon and thin film solar modules. The application of the coating method to perovskite/silicon tandem cells and will also be presented. We will answer the question whether the perovskite layers can be fabricated at the speed of silicon cells are produced in the regular production lines.

It is known that the V_OC of solar cells are strongly related to the defects recombination in the bulk or on the surface of absorber layer. Solution processed perovskite film is usually polycrystal, which contains substantial structural disorder such as grain boundary defects, crystallographic defects. Although it was theoretically predicted that most of the defects formed in perovskite layer could be shallow defects, several experimental results still showed that the suppression of defects in the solution-processed perovskite films are very critical for further enhancing the performance of PSCs toward their thermodynamic limits. Surface is the place where the defects could be easily formed, passivation of the surface defects is always the most important tasks in any type of solar cells. In PSCs, several efficient surface passivation methods have been adopted previously. Recently, we developed an organic halide salt phenethylammonium iodide (PEAI) for post-treatment of mixed perovskite to suppress the surface defects of perovskite polycrystal film for efficient solar cells. We find that the PEAI can form on the perovskite surface and results in higher efficiency cells by reducing the defects and suppressing non-radiative recombination. As a result, planar PSCs with a certificated efficiency of 23.32% (Quasi-steady-state) are obtained. More recently, we have pushed the efficiency of PSCs beyond 24%.

References
1. Q. Jiang, …, J. B. You*. Nat Photon., doi.org/10.1038/s41566-019-0398-2 (2019).

Organic-inorganic perovskite materials, due to their excellent optoelectronic properties and low-cost processing have attracted many research groups nowadays. These materials with ABX₃ crystal structure (A: cesium (Cs), methylammonium (MA) or formamidinium (FA), B: Pb or Sn, and X: Cl, Br or I) have been a good candidate for the fabrication of highly efficient perovskite solar cells (PSCs) with a certified power conversion efficiency (PCE) of 24.2%. Interfacial engineering is one of the effective strategies for improving the efficiency and stability of PSCs. In this regard, combination of two-dimensional (2D) materials with PSCs would be so effective to improve the photovoltaic properties of devices. Here, we study the applications of monolayer graphene synthesized by chemical vapor deposition (CVD), MoS₂, and BN in the perovskite architecture. We considered graphene as top and bottom electrodes in the device and found that this would be a great way to replace metal electrode such as gold and fabricate efficient semi-transparent PSCs. Based on graphene as a top electrode in CsPbI₃ QDs PSC, we achieved a PSC with power conversion efficiency (PCE) of 6.8%, which is slightly lower than the reference cell (9.6%). Additionally, by tuning the thickness of the active layer, a PSC with PCE of 4.95% and average visible transmittance (AVT) of 53% is demonstrated, indicating the potential of CsPbI₃ QDs for the fabrication of semi-transparent devices applicable in windows.
Moreover, we employed MoS₂ as an electron transporting layer (ETL) in the PSCs and a PSC with PCE of 18% was achieved with this architecture, indicating the potential of 2D materials in the PSCs.
In addition to the above study, we considered BN monolayer as an interface layer in inverted PSC device. Based on this modification, we improve the open circuit voltage (V_oc) and stability of the PSCs drastically. Based on this modification, a PSC with PCE of over 20% was achieved.

Hybrid metal halide perovskite materials have become a leading technology in the photovoltaic scene since its first application in solar cells in 2009. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been reached from 3.8 to 24.2% in the past ten years. Compositional tuning, i.e., by substituting the anions or cations, and interface functionalization have been the key to the breakthrough of the device performances. However, the state-of-art device performance is, so far, achieved by employing three-dimensional (3D) perovskite materials as the light absorber. The major drawback in using 3D perovskites is the poor stability against moisture, which hinders the widespread application of this technology. On the other hand, two-dimensional (2D) perovskite materials have demonstrated superior stability compared to the 3D counterparts. However, 2D perovskites have limited light-harvesting ability, resulting in low PCE. Combining 2D and 3D into a new hybrid 2D/3D has been a popular alternative to boost both device efficiency and stability. In this work, we design and employ a family of thiophene-based organic cations, as building blocks for layered two-dimensional (2D) perovskites. The 2D layer functionalizes the interface between the optimized triple-cation (formamidinium (FA), methylammonium (MA), and cesium) lead halide perovskite layer and the organic hole transporting material (HTM) employed in n-i-p solar cells configuration. As an immediate result, the functional 2D interlayer actively improves device performances by significantly improving the long-term device stability by retaining up to 90% of the initial efficiency after continuous illumination and delivering efficiency of >20%. In parallel, the thiophene-based 2D capping layer reduces the commonly observed current voltage hysteresis behavior to a great extent.

Lead halide perovskites have drawn extensive research attention due to their excellent optoelectronic properties and remarkable performance in photovoltaic devices. The power conversion efficiency of perovskite solar cells has already reached over 24 %, surpassing those of mainstream thin film photovoltaic technologies like CdTe and CIGS. Despite these encouraging progresses, perovskite solar cells still suffer from poor stability owing to their vulnerability to oxygen, moisture and light.
It has been demonstrated that the degradation of perovskite initializes from the defective surface and grain boundaries, due to the higher reactivity of defects site, making them vulnerable to be attacked by moisture and oxygen.^[1] Therefore, the passivation of these defects is imperative for the commercialization and large-scale production of this photovoltaic technology.
Here we present a simple strategy via the in-situ conversion of surface perovskite to water-insoluble lead salts, which exhibits dual effects: 1) passivate the surface defects and thus enhance the power conversion efficiency to over 21 %; 2) the formation of compact surface lead salt layer exhibits good resistance to hazardous stimuli under ambient atmosphere and light irradiation. Consequently, our encapsulated devices show excellent stability after operation at maximum power point under simulated AM 1.5G irradiation for over 1000 hours at 65 ^oC.

References:
[1] Wang, Q. et al. Scaling behavior of moisture-induced grain degradation in polycrystalline hybrid perovskite thin films. Energ. Environ. Sci. 10, 516-522

To reveal the device physics of solar cells, various photoconductivity approaches such as time-resolved microwave photoconductivity (TRMC), time-of-flight (TOF), and photo charge extraction by linearly increasing voltage (CELIV) are used in-situ. However, their time resolution is within the range of couple nanoseconds to microsecond due to the device structure, resulted in that the carrier dynamics study is dominated by trapping states, rather than trapping-free phenomena. Therefore, it remains a challenge to study in-situ solar cells in an ultrafast fashion.
In this report, we use a more than 20 % perovskite thin film solar cell as a test-bed, and integrated them into an ultrafast photocurrent spectroscopy with sub-40 ps time resolution to study the carrier dynamics from the sub-40 ps to couple microseconds. We address the evolution of carrier dynamics such as relaxation, recombination, transport, and trapping.

Nanostructures, interfaces and active layer crystallization are critical to the functioning of the new generation solar cells. The hybrid organic/inorganic halide perovskite solar cells have emerged as among the most competitive photovoltaic technologies of the future thanks to their superb and rapidly improving power conversion efficiencies (PCEs). For realistic deployment of the perovskite photovoltaic technology in large scale, however, device stability has become more and more important issue of the day. I will update our recent work on the development of high-efficiency and high-stability perovskite solar cells. First, we have developed a new hole transport material by embedding an ultra-low concentration of gold nanoparticles into a NiO film, significantly boosting the performance of the p-i-n perovskite solar cells. Second, by incorporating an ultrathin ferroelectric oxide PbTiO₃ layer between the electron transport material and the halide perovskite, we are able to significantly increase both PCE and stability of the perovskite solar cells. Finally, we discovered that a large excess of cesium iodide induces spinodal decomposition of CsPbI₂Br perovskite films, and these films could also be made into high performance perovskite solar cells. Implications of these results on the future development of perovskite solar cells will be discussed.

Compared with silicon-based solar cells, organic-inorganic hybrid perovskite solar cells (PSCs) possess a distinct advantage, i.e., its application in flexible field. However, the efficiency of flexible device is still lower than that of rigid one. Firstly, we found that dense perovskite film can be obtained with the help of N-methyl-2-pyrrolidone (NMP) by low pressure assisted method (LPAM). In addition, CH₃NH₃Cl (MACl) as the additive could preferentially form MAPbCl_3-xI_x perovskite seeds to induce perovskite phase transition and crystal growth. Finally, by using FAI-PbI₂-NMP+x%MACl as the precursor, i.e., ligand and additive synergetic process (LASP), FA-based perovskite film with large grain size, high crystallinity and low trap density was obtained on a flexible substrate in ambient due to the synergetic effect, e.g., MACl could enhance the crystallization of the intermediate phase of FAI-PbI₂-NMP. As a result, a record efficiency of 19.38% in flexible planar PSCs was achieved by using FAI-PbI₂-NMP+15%MACl as the precursor, and it could retain about 89% of its initial PCE after 230 days without encapsulation in ambient. The PCE retained 92% of the initial value after 500 bending cycles with a bending radii of 10 mm. Our results show a robust way to fabricate highly efficient flexible PSC.

Perovskite solar cells are approaching silicon photovoltaic counterparts in performance, but mostly on a small area. To commercialize this low-cost technology, scalable fabrication is one of the main challenges to be addressed, for both perovskite active layers and charge transport layers [1]. For large-area, uniform and high-quality perovskite deposition, we have developed industry compatible hybrid chemical vapor deposition (H-CVD) to deposit MAPbI₃, FAPbI₃, and Cs_xFA_1-xPbI₃ films with high quality for solar cells/modules [2-5]. In this talk, I will focus on fully scalable perovskite solar modules with high-geometric fill factor by a further optimized hybrid chemical vapor deposition process and scalable sputtering deposition of electron transport layer SnO₂ [6-7]. With optimized interface structure between SnO₂ and Cs_xFA_1-xPbI₃, 10 cm x 10 cm solar modules showed a designated area efficiency approaching 10% with a geometric fill factor greater than 90%. We show that H-CVD is capable of fabricating large-area and high performance perovskite solar modules.

[1] L. Qiu, L.K. Ono, Y.B. Qi*, Mater. Today Energy 2018, 7, 169.
[2] M.R. Leyden, L.K. Ono, S.R. Raga, Y. Kato, S. Wang, Y.B. Qi*, J. Mater. Chem. A 2014, 2, 18742.
[3] M.R. Leyden, M.V. Lee, S.R. Raga, Y.B. Qi*, J. Mater. Chem. A 2015, 3, 16097.
[4] M.R. Leyden, Y. Jiang, Y.B. Qi*, J. Mater. Chem. A 2016, 4, 13125.
[5] Y. Jiang, M.R. Leyden, L. Qiu, S. Wang, L.K. Ono, Z. Wu, E.J. Juarez-Perez, Y.B. Qi*, Adv. Funct. Mater. 2018, 28, 1703835.
[6] L. Qiu, Z. Liu, L.K. Ono, Y. Jiang, D.-Y. Son, Z. Hawash, S. He, Y.B. Qi*, Adv. Funct. Mater. 2019, 29, 1806779.
[7] L. Qiu, S. He, Y. Jiang, D.-Y. Son, L.K. Ono, Z. Liu, T. Kim, T. Bouloumis, S. Kazaoui, Y.B. Qi*, J. Mater. Chem. A 2019, 7, 6920.

The past ten years have witnessed the booming development of perovskite solar cell (PSC) with efficiency exceeding 24%, which is recognized as a superstar in photovoltaic research fields. However, the stability issue still limits their further development and commercialization. One of the main factors which lead to the instability issue is the ion diffusion between metal electrodes (Ag or Au) and perovskite absorbers. Carbon nanotubes have attracted tremendous attention with merits of low cost, outstanding electrical and mechanical properties, great chemical stability as well as large-area fabrication. Aiming at improving the long-term stability and reducing the fabrication cost of PSCs, we have devised several high-performance PSCs based on cross-stacked superaligned carbon nanotube electrodes (CSCNT). Via subtly optimizing the device architecture and chemically modifying the CNT electrodes, we have significantly enhanced the photovoltaic performance and stability of CNT based PSCs under multi-environment. We first modify CNT with iodine and polyethylenimine (PEI) for regular HTM-free PSCs and inverted planar PSCs, respectively. Devices show better photovoltaic performance than that of the pristine CNT based ones owing to the enhanced charge transfer. Then we successfully construct inverted PSCs with all inorganic charge transport layer based on SnO₂@CNT electrode and obtain a champion power conversion efficiency (PCE) of 14.3% and 10.5% on the rigid and flexible substrate, respectively. Meanwhile, devices retain 88% of their original PCE aging at light illumination in air or thermal treatment in N₂ for 550 h. Finally, we design all-carbon-electrode based flexible PSCs with graphene as anode and CNT as cathode with a best PCE of 11.9%. Our devices show outstanding stability and durability under continuous light soaking in air for 1000 hours or bending cycles over 2000 times, retaining ca. 90% of their original PCEs respectively. Considering the increasing efforts dedicated to developing low-cost and stable energy conversion devices, the promising efficiency, flexibility, and long-term stability of CNT based PSCs clearly reveal sufficient compatibility. In addition, we have successfully developed a perovskite-thermoelectric tandem cell, which promotes the device PCE up to 23.2%.

Hybrid perovskite materials have emerged as attractive alternatives for cost-effective solar cells. Impressive device performance has been realized through generic interface engineering. However, approaching hysteresis-free, stable and efficient solution-processed perovskites solar cells has remained a significant fundamental challenge. In this study, we report a strategy that utilizes polymer to anchor the counter ions in the perovskite lattices to suppress the formation of point defects, reduce the migration of ions/vacancy and to facilitate crystal growth in a more thermodynamically preferred orientation. Systematical investigations indicate that polymer indeed form hydrogen bonds at the crystal interface, which reduce the formation of kinetically-driven point defects, minimizes charge carrier recombination and maintains the sharp distribution of the density of states. As a result, un-encapsulated perovskite solar cells by novel hybrid perovskites thin film exhibit efficient, hysteresis-free characteristics and a decent device lifetime after being operated in air with 50% relative humidity in comparison to the reference perovskite solar cells by pristine hybrid perovskites thin film. Our studies demonstrate that development of hybrid perovskite materials crosslinked with polymers is one option to approach hysteresis-free, stable and efficient solution-processed perovskite solar cells.

Perovskite solar cells in which metal halide perovskite layers are used as light harvesting materials are gaining much attention because of the high efficiency although these solar cell can be prepared by low temperature printing technology. The efficiency of Pb-perovskite solar cells with more than 1 cm² is 20.9 % which became close to those of inorganic multi-crystalline solar cells such as MC-Si, CIGS, and CdTe. In small cells with less than 1cm², the efficiency of 24.2% has just been reported for the perovskite solar cells.
Conventional perovskite solar cells consisting of Pb have band gap of 1.5-1.6 eV and can harvest the light in the visible region up to 850nm. According to Shockley-Queisser limit, light harvesting layer with 1.2-1.4 eV band gap gives the highest efficiency. Mixed metal PbSn perovskite materials have a narrow band gap of around 1.2 eV. Therefore, mixed metal SnPb perovskite solar cell is expected to give higher efficiency than Pb-perovskite solar cells. In addition, the narrow band gap solar cell is useful for bottom cells for all perovskite tandem solar cells. When SnPb mixed metal perovskite solar cells was firstly reported by us, the efficiency was around 4%. However, the efficiency has been enhanced gradually and recently efficiency higher than 20% has been reported by several groups including our Lab. How the efficiency was enhanced will be discussed in the presentation^1-3.
The conventional perovskite layer consists of Pb ions. Therefore, the perovskite solar cells have to recover completely, for which a new business model is needed such as CdTe solar cells. In addition, the use of Pb ions is limited by the law such as RoHS directive in Europe. From this view point, there is a strong request for Pb-free perovskite solar cells solar cells. Bismuth halide compounds such as Cs₃Bi₂I₉, MA₃Bi₂I₉ Ag₃BiI₆, AgBi₂I₇, Cs₂AgBiBr₆, antimony halide compounds such as Rb₃Sb₂I₉, titanium halide compounds such as Cs₂TiBr₆, and copper halide compounds such as MA₂CuI₄ have been reported to replace lead. However, the solar cell efficiency based on these materials were less than 5% and was not satisfactory. Among all lead-free perovskite materials, Sn-based perovskite is one of the most promising candidates as the light harvesting layer for Pb-free PSCs, because they have perovskite structure similar to Pb perovskite. The efficiency has been enhanced to 10% by several research groups including us, however, the efficiency is not still satisfactory, when compared with Pb-perovskite. The cause of the low efficiency and how the efficiency will be enhanced is discussed^4-7. In addition, wide gap perovskite solar cells needed for all perovskite tandem cells will be reviewed including our results on Pb-perovskite consisting of Ge ion⁶.

References
1. Kapil G., Hayase, S., et.al., Nano Letters, 18, p.3600-3607, 2018., 2. Ogomi, Y., Hayase, S. et.al., J. Phys. Chem. Lett., 5, p.1004-1011, 2014., 3. Ripolles, Teresa, Hayase, Shuzi, et.al., J. Phys. Chem., Part C: DOI: 10.1021/acs.jpcc.8b09609, 2018. , 4. Ito, N. Hayase, S., et.al., J. Phys. Chem. Lett., 9, p.1682-1688, 2018., 5. Chi Huey Ng, Shuzi Hayase, et.al., Nano Energy, https://doi.org /10.1016/j.nanoen. 2019.01.026, 2019. 6. Yang, F., Hayase, S., et.al., Angew. Chem. In. Ed., 2018, 57, doi:10.1002/anie.201807270, 2018., 7. Shuzi Hayase, Review Article, Current Opinion in Electrochemistry, 11, p.146-150, 2018.

Tin perovskite solar cells (PSCs) show the most promise to replace the more toxic lead-based perovskite solar cells. However, the efficiency of tin-based PSCs falls short of that of lead-based PSCs by a large margin, as a result of low open-circuit voltage (Voc). This is due to the tendency of Sn²⁺ to oxidize into Sn⁴⁺ in the presence of air together with the formation of defects and traps caused by the fast crystallization of tin perovskite materials. Here, we performed post-treatment of tin perovskite layer with Lewis base to suppress the recombination reaction in tin halide PSCs resulting in efficiencies more than 10 %, which is the highest reported efficiency to date for pure tin-halide PSCs. The Voc increased by as much as 0.1 V with optimum concentration of the Lewis base. Upon analyzing the X-ray photoelectron spectroscopy and impedance spectroscopy data, we came to the conclusion that the amine-group bonded the under-coordinated perovskite, passivating the dangling bonds and defects resulting in suppressed charge carrier recombination. This also has the effect of prolonged lifetime and higher charge carrier mobility. Our findings will provide the groundwork for improving the efficiency of pure tin halide PSCs to compete with that of lead-based PSCs by simple post-treatment with organic molecules.

Inorganic-organic hybrid lead halide perovskite solar cells have attracted tremendous research attention due to rapid rise in power conversion efficiency (PCE) from merely 3.8% to 24.2% within 10 years. Such rise in PCE is attributed to exceptional optoelectronic properties such as long charge carrier diffusion length, low exciton binding energy, ambipolar charge mobility, tunable narrow bandgap. Despite of these advantages, one of the major problems encountered with this new technology, apart from structural and chemical stability, is the toxicity associated with heavy metal lead. Among various metals explored, bismuth-based perovskite materials provide promising optoelectronic properties including a high absorption coefficient and can be processed from solution using a variety of wet chemical deposition techniques and additives. In our research, we focused on different various bismuth-based perovskite materials having zero dimensional to higher dimensional networks and investigated their optical, morphological and photovoltaic properties. Interestingly, we could observe, that by increasing the bismuth halide network dimensionality, the power conversion efficiency could be improved. This increase in solar cell performance could be achieved by introducing monovalent noble metals. The incorporation of these monovalent cations lead to 3D networking of Bismuth halides, resulting in rudorfitte structure.

The state-of-the-art high efficiency perovskite solar cells (PSCs) contain lead and organic cations in the perovskite light-absorber. However, lead-toxicity and intrinsic instability of these PSCs are major hurdles in the path towards the commercialization of PSCs. While there has been effort towards replacing lead cation with less toxic cations, typical lead-free PSCs still suffer from the low power-conversion efficiency (PCE) and poor stability to ambient. Herein, we have utilized a defects passivation strategy in Sn-based perovskite materials, and realized PSCs with a promising PCE of up to 10%. Furthermore, the defects passivation via native oxides passivation layer or 2D phase passivation result in superior air-stability of Tin-based perovskite solars within 1000-hours continuous operation under one-sun illumination. Thus, this work provides a new avenue for the design and development of high performance and stable Sn-based PSCs.

Solution processing of organic-inorganic metal halide perovskites represents a major advantage for low-cost solar cell fabrication. However, scalable processes must be developed in place of spin-coating in order to successfully commercialize the technology. Slot die and ambient spray coating have emerged as alternatives, but like spin-coating, these methods require lengthy anneals that pose significant challenges for inline high-speed fabrication.

In this work, we fabricate all of the perovskite, charge transport, and barrier layers in open air with scalable processes. Using a p-i-n planar architecture, we first spray coat an aqueous NiO hole transport layer that demonstrates improved efficiency and fill factor relative to a spin coated organic NiO precursor. A double cation perovskite film consisting of Cs_.17FA_.83Pb(Br_.17I_.83)₃—a composition with excellent thermal and photostability—is then formed using the Rapid Spray Plasma Processing (RSPP) method, where films are sprayed and immediately exposed to a plasma at an ultrafast linear processing rate > 10 cm/s. The combination of reactive species (ions, radicals, metastables, and photons) and convective thermal energy produced by the plasma rapidly transfer energy to the perovskite-precursor solvate, enabling the growth of efficient and mechanically robust perovskite films in seconds with no post-annealing necessary.

A fiber laser (1064 nm) and CO₂ laser (10.6 μm) are used for the respective P1 and P2 scribes to create a monolithic, series-interconnected module. Module design is optimized for the RSPP process to achieve >70% Geometric Fill Factor. After completion of the module, an organosilicate barrier film is deposited with RSPP to improve thermal and environmental stability in addition to providing protection from thermal degradation of the perovskite during encapsulant lamination. A stabilized value of 12.4% PCE was achieved after 1000 s illumination in air for a 12.5 cm² series-connected module with six subcells and a module V_oc > 6.2 V. Additionally, encapsulated modules with in-situ barriers demonstrate validated reliability and retain >80% of their initial PCE after 500 hours of indoor (damp heat testing at 85 °C) and outdoor (full spectrum illumination) ageing tests.

The commercial application of perovskite based thin film photovoltaic technology requires highly efficient large area devices. A high drop in power conversion efficiency of 40%rel. is observed when comparing record solar cells of 0.1 cm² aperture area with the best 100 cm² modules. As Li et.al reported in Nature review in 2018, perovskite based TFPV still have higher upscaling loss than any other PV technology. Recently, with better understanding of perovskite crystallization dynamics, module efficiencies of 17 % on 17 cm² and 12.6 % on 355 cm² have been achieved.

Upscaling losses going from cell to module can be grouped into four types of losses: sheet resistance, interconnection resistance, inactive area and layer inhomogeneity loss. In order to achieve high performance upscaled devices, we process and analyze devices up to 156 cm², using techniques such as spin coating, blade and slot die coating. We employ integrated circuit-based simulation techniques, as well as electrical and optical characterization methods including J-V measurements, transmission line measurement, luminescence imaging, thermography and electron spectroscopy to analyze each of the loss mechanism separately.

As a result, we identify specific process optimization steps that should be undertaken in order to minimize each performance loss, including module design, laser interconnection patterning, optimizing deposition method used, improving processing conditions and solvent engineering. Following an ideal process implementation, the relative upscaling losses going from cells to modules could decrease to 5 %, making large area perovskite modules with 20 % power conversion efficiencies a reality.

Development of large-area high-throughput deposition techniques is necessary to successfully commercialize perovskite solar cell technology. Vapor transport deposition (VTD) is one such potential technique that uses a carrier gas to transfer sublimed salts from source to substrate, where they react to form perovskite films. Unlike vapor thermal evaporation, during VTD the material deposition rate is decoupled from material temperature allowing for high throughput deposition. During the VTD process there is also independent control of chamber pressure and deposition rate, parameters which can be tuned to change the film crystallization kinetics. Similar to thermal evaporation, VTD allows for precise thickness control and eliminates hazardous solvents from device fabrication, allowing for facile growth of complex multi-layer device structures such as tandem solar cells. The high throughput deposition coupled with low vacuum operation reduces capex requirement for VTD deposition tools, and has already led to commercialization of the technique for CdTe and organic semiconductor materials manufacturing.¹

In our work, through the use of a custom-built VTD setup² we study perovskite film formation via co-evaporation of lead iodide and methylammonium iodide (MAI). We find that control of MAI degradation during vapor transport and its deposition rate is a critical challenge that needs to be overcome. We describe design parameters and deposition conditions necessary to incorporate undegraded MAI into the methylammonium lead iodide film. We describe numerical simulation of material diffusion and gas flow necessary to narrow the design parameter space. We study the impact of substrate temperature and carrier gas flow rate on film formation kinetics by examining metrics such as photoluminescence, x-ray diffraction, morphology, as well as device efficiency. Through this systematic study we demonstrate VTD to be a viable new deposition tool for large-area high-throughput deposition of perovskite solar cells.

¹Powell, R. C. Research leading to high throughput manufacturing of thin-film CdTe PV modules. NREL Annual Subcontract Technical Report (2006). DOI:10.2172/881482
²Hoertner, M. T. et al. High-peed vapor transport deposition of perovskite thin-films. ACS Applied Materials & Interfaces (2018). In Review.