Symposium ES16—Perovskite Photovoltaics and Optoelectronics

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.

Non-radiative recombination loss specially at the interfaces is a big challenge in the perovskite solar cells (PSCs) and affect the performance, stability, reproducibility of devices. Engineering the interfacial regions of the PSCs using an interface layer or additives is an effective strategy to address this issue. Our new findings report on a major breakthrough in the PSCs research. We discovered amazingly effective ways to mitigate the radiationless recombination of charge carriers at the interfaces of the perovskite with either electron transporting layer (ETL) or the hole transporting layer (HTL). In terms of ETL modification, we treat the surface of ETLs in the PSCs using SnO₂ and graphene, results in drastically improved the charge transfer properties, retarded the recombination rate and reduced the unwanted interfacial reactions at the interfaces with perovskite film. Moreover, we found that incorporation of the molecular modulators such as adamantanes either at the interface of perovskite with HTL or into the HTL solution prevents non-radiative recombination drastically, resulting in a highly efficient and stable PSC. Based on these techniques, we achieved PSCs (for both planar and mesoporous structures) with electric to power conversion efficiency (PCE) of ~22%, an open circuit voltage of up to 1245 mV, external electroluminescence yield of 2.5% and a great operational stability, which are the records for triple A-cation PSCs with respect to the band gap (1.61 eV). Our proposed approaches open up a promising route for fabrication of cost-effective solar cell and pave the way for the development and commercialization of the PSCs.

Understanding defects in perovskite films is key to improving device performance and stability. It has been hypothesized that vacancy defects enable ion migration¹, which has been implicated to cause current-voltage hysteresis and long-term material degradation. Additionally, defects at the surface of the perovskite film may affect the interfaces in a device, and interface engineering is seen as an important avenue for improving device performance. Scanning tunneling microscopy (STM) offers the ability to probe the surface of OHPs with atomic resolution, including resolving individual vacancy defects.^2-4 Scanning the same area multiple times allows for observation of dynamic events. Here, multiple types of defects were resolved and dynamic ion migration to and from the surface was imaged at the atomic scale. DFT calculations indicate vacancy defects are MABr vacancies and that vacancy defects at the surface of the film change the local work function, which has important implications for energy level alignment and charge transfer between layers in a photovoltaic device.

References
¹ J. Azpiroz, E. Mosconi, J. Bisquert and F. De Angelis, Eng. Environ. Sci. 8, 2118 (2015).
² L. She, M. Liu, D. Zhong, ACS Nano. 10, 1126 (2016).
² R. Ohmann, L.K. Ono, H.S. Kim, H. Lin, M.V. Lee, Y. Li, N.G. Park, Y.B. Qi, J. Am. Chem. Soc. 137, 16049 (2015).
⁴ Y. Liu, K. Palotas, X. Yuan, T. Hou, H. Lin, Y. Li and S.T. Lee, ACS Nano. 11 2060 (2017).

Abstract. Organic and inorganic cation alloying on the A-site in mixed halide perovskites solar cells has enabled remarkable improvements in efficiency and environmental stability. However, the added compositional complexity may lead to undesired phase segregation that detracts from optoelectronic performance. To clarify this balancing act, we assess the nanoscale chemical and electronic impacts of alkali-metal cation addition by means of synchrotron-based nanoprobe X-ray fluorescence and induced current. We find that the halide distribution homogenizes upon the addition of CsI and RbI precursors for films prepared with stoichiometric or excess lead halide. The halide homogenization coincides with long-lived charge carrier decays and spatially homogenous carrier dynamics visualized by ultrafast microscopy. At the same time, we observe Rb-rich clusters that phase separate within the film. We identify these Rb aggregates as recombination active sites using X-ray and E-beam induced current microscopy. Based on our microscopy investigations upon careful tuning of precursor stoichiometry, we will share quantitative precursor design strategies for preparing high-performance alloyed perovskite films that take advantage of the beneficial effects of alkali cations on homogenizing the lead-halide electronic backbone while minimizing secondary phase formation. These insights provide a more comprehensive understanding of the role of alkali cations in alloyed perovskite and indicate directions to further improve the performance and stability of perovskite optoelectronic devices.

Grain boundaries (GBs) are the most prominent microstructural features that play significant roles in determining the physical properties and photovoltaic functions of halide perovskite (HP) thin films. While enormous effort has been devoted to modifying the HP GBs and making them benign, the microstructures in these modified/functionalized HP thin films have been somehow random and/or uncertain. Herein, we demonstrate several unique chemical approaches to functionalize the HP GBs in a continuous, precisely-controlled manner. The key to the unprecedent success of the confocal functionalization is the strong molecular interaction between HPs grains and functionalizing agents. Microscopic characterization methods including analytical transmission electron microcopy have been employed to confirm the microstructures in our HP thin films. Combined experimental and theoretical studies have showed that the confocal functionalization of HP GBs not only leads to electronic passivation of defects, but also prevents HP grains from moisture/oxygen ingression and unfavorable phase transformation. As a result, highly efficient and stable perovskite solar cells are demonstrated. The concept of continously functionalization of the HP GBs is paving the way for developing higher-performance perovskite solar cells of the future.

We are developing effectively transparent superstrates for perovskite solar cells. These superstrates incorporate triangular-shaped grid fingers with thin transparent conductors, which mitigate shading losses associated with conventional grid fingers and/or thick TCO layers. We have investigated various types of polymers for use as superstrates in both rigid (glass-backed) and flexible applications. We have developed an electroplating process to fabricate the triangular gridlines, which improves their conductivity vs. prior silver ink printing approaches. We will present a summary of perovskite solar cells that were fabricated on these effectively transparent superstrates, including a discussion of their photovoltaic efficiency, spectral response, and LBIC mapping. The results demonstrate that effectively transparent superstrates enable an improvement in the short-circuit current density and fill factor for large-area perovskite solar cells.
Perovskite solar cells are of great interest due to their potential for low cost and high performance. One of the challenges to attaining high photovoltaic conversion efficiency, particularly for large-area cells, is the tradeoff between the optical and electrical performance of the top contact. Because perovskite absorbers and selective electrode materials provide very little lateral conductivity for current collection, a transparent conductive oxide (TCO) such as indium tin oxide (ITO) must be used for the front contact. However, TCOs offer a tradeoff between transparency and conductivity, resulting in a compromise between short-circuit current density (due to optical losses) and fill factor (due to resistive losses) for solar cells. A solution is to increase the density of the grid fingers such that thinner TCOs can be used; however, this increases the shading losses.
Recently, a method to produce effectively transparent front contact grids has been described (Adv. Optical Mater. 4 (10), 1470-1474 (2016); Photovoltaic Specialists Conference (PVSC) IEEE 43rd, 3612-3615, (2016); Sustainable Energy and Fuels, 1 (3), 593-598, (2017)). This approach yields a relatively dense array of high-aspect-ratio, triangular-shaped front contact fingers, in which light striking the metal is reflected towards the cell. Our current work pertains to the application of this technology to perovskite solar cells.

Solution processable hybrid organic-inorganic perovskites such as CH₃NH₃PbI_3-zCl_z have attracted enormous fundamental and applied interest because of their outstanding optoelectronic properties. There is considerable interest in establishing methods to control perovskite film morphology, for example, using micropatterning. Here, hydrophilic poly(N-vinylformamide)-based swellable colloidal sponge-like particles were dispersed in perovskite precursor solution which was then spin coated to deposit CH₃NH₃PbI_3-zCl_z films for the first time. Remarkably, the CH₃NH₃PbI_3-zCl_z films formed disordered inverse opal (DIO) films. Our unique approach enables micropatterning of the perovskite capping layer in a single step. The CH₃NH₃PbI_3-zCl_z pore wall thickness is shown to be controlled by the concentration of sponge-like colloid particles used. The particles not only caused CH₃NH₃PbI_3-zCl_z to be more efficiently deposited but also increased light absorption and photoluminescence intensity. Results from demonstration solar cells constructed containing the DIO CH₃NH₃PbI_3-zCl_z films are discussed. A mechanism for DIO film formation is also presented[1]. We also present recent results obtained by extending this approach to prepare semi-transparent perovskite solar cells.

[1] Dokhan et al, Phys. Chem. Chem. Phys., DOI: 10.1039/c8cp05148h.

The efficiency of perovskite solar cells has skyrocketed from 3.8% to 23.3% in the past few years. For the best MAPbI₃ perovskites, the open-circuit voltage deficit, defined as the difference between bandgap and open-circuit voltage (V_oc), is only 0.37 V, approaching other best technologies such as GaAs [1]. However, similar to III-V materials, higher bandgap perovskites tend to have larger V_oc deficits. For example, for 1.64-eV and 1.7-eV perovskites, the best V_oc deficits are 0.49 V and 0.51 V, respectively [2]. This voltage loss limits the performance of wide-bandgap perovskites, and their applications on, for instance, tandem solar cells.
The V_oc of photovoltaic devices is governed, hierarchically, by recombination in the absorber and the carrier-selective contacts of the devices. Grain engineering has been proven to be an effective way to reduce defect density and, thus, suppress recombination to enhance the V_oc [2][3]. In our work, we focus on exploring different carrier-selective contact materials for perovskites.
With inverted architecture, we have replaced PTAA hole contact with boron-doped a-Si:H material. Our first device shows a V_oc of 1.02 V on a 1.6-eV CsMAFABr perovskite, and 0.92 V on a 1.67-eV perovskite. These numbers are very close to the PTAA control devices with V_oc of ~1.1 V. Although the FF is rather poor due to shunts and possible large series resistance due to band misalignment. We are in the process of making new devices without shunts, and characterizing the band alignment between our a-Si:H hole contact and perovskite absorber. By the time we present, we believe we’ll have such information. In addition to the current-voltage measurement, we will use surface photovoltage and XPS/UPS etc technique to obtain the band information, and also use a Suns-V_oc tool that we developed to probe the series-resistance-free fill factor of the device with a-Si:H contacts. We’ll also further employ phosphorous-doped a-Si:H as an electron contact to perovskite, and perform a through analysis on that as well.

To maximize the scalable deployment of perovskite PV, we investigate low-temperature organic monolayer-based morphologies to chemically and electrically glue perovskites to their contacting phases while maintaining a “soft”, flexible interface and carrier selectivity. We have covalently grafted vertically oriented perylene dianhydride overlayers on TiO₂ through and imide linkage to a surface-bound silane. Secondary functionalization with a diamine to the terminal anhydride yields A-type cation moieties that serve as a chemical hook for perovskite deposition. UPS-quantified HOMO and LUMO levels of perylene diimide molecules aligned to facilitate electron transport to the TiO₂ substrate while blocking holes. Reflection-Absorption Infrared Spectroscopy (RAIRS) and X-ray photoelectron spectroscopy revealed a high coverage of vertically oriented perylene diimides. Perylene interfacial monolayers yield improved energy conversion relative to TiO₂/perovskite interfaces that we ascribe to enhanced interfacial carrier selectivity and transport, and reduced recombination. We discuss the present results in the context of other morphologies relevant to tandem photovoltaics.

Perovskite solar cells are surprising the photovoltaic community as unconventional behaviors have been reported. In most of the cases the origin of these behaviors is not completely understood and also, very important for the final application, how them influence the final performance of the device. On the other hand Impedance spectroscopy is a non-destructive characterization technique that can help in the understanding of these devices. Impedance spectroscopy is a characterization method in the frequency domain that allows to decouple physical processes with different characteristic times at the working conditions i.e. under illumination and applied bias. Despite the huge potentiality of this technique for the characterization of perovskite solar cell a complete model of impedance for this kind of cells applicable in all the conditions and configuration has been elusive for the moment. Undoubtedly, the combined action of electron and holes and ions in perovskite solar cell is at the base of the complex behavior observed in this kind of devices. In this talk we compare the well know dye sensitized solar cells with the perovskite solar cells highlighting similarities and differences. In addition the interest of impedance characterization of different types of perovskite solar cells is discussed.

Mobile ions in hybrid perovskite semiconductors introduce a new degree of freedom to electronic devices suggesting applications beyond photovoltaics. An intuitive device model describing the interplay between ionic and electronic charge transfer is needed to unlock the full potential of the technology. We describe the perovskite-contact interfaces as transistors which couple ionic charge redistribution to energetic barriers controlling electronic injection and recombination. This reveals an amplification factor between the out of phase electronic current and the ionic current. Our findings suggest a strategy to design thin film electronic components with large, tuneable, capacitor-like and inductor-like characteristics. The resulting simple equivalent circuit model, which we verified with time-dependent drift-diffusion simulations of measured impedance spectra, allows a general description and interpretation of perovskite solar cell behaviour.

arXiv:1805.06446

Photo-induced phase separation has been reported for a range of mixed-halide perovskites, which limits the available band-gap energies for photovoltaic applications [1,2]. An enhanced understanding of the phase separation mechanism is essential to rationalize limitations and design stable perovskite semiconductors. Up to now, phase separation and segregation has been detected by means of X-ray diffraction (XRD), photoluminescence (PL) and cathodoluminescence experiments [3,4].
During electron microscope experiments, the electron beam may cause changes in halide perovskites [4, 5] and we here show that phase separation can be induced by electron beam irradiation of the sample. Inorganic CsPb(Br_xI_1-x)₃ thin films were deposited by spin-coating and the phase separation investigated in-situ by using transmission electron microscopy (TEM). By this approach, it was possible to directly monitor the phase separation of CsPb(Br_0.5I_0.5)₃ thin films on the nanoscale into CsPbI₃ and probably PbBr₂ domains. We discuss the comparability of the interactions of electrons and photons with the halide-perovskite thin film and correlate the in-situ TEM with XRD and PL measurements of the same samples. Furthermore, we present different approaches of sample preparation for this in-situ TEM investigation.


[1] E. T. Hoke et al. “Reversible photo-induced trap formation in mixed- halide hybrid perovskites for photovoltaics”, Chem. Sci.,2015, 6, 613
[2] E. L. Unger et al. “Roadmap and roadblocks for the band gap tunability of metal halide perovskites,” J. Mater. Chem. A,2017, 5, 11401
[3] Barker et. al “Defect-Assisted Photoinduced Halide Segregation in Mixed-Halide Perovskite Thin Films”, ACS Energy Lett. 2017, 2, 1416−1424
[4] W. Li, et al. “Phase Segregation Enhanced Ion Movement in Efficient Inorganic CsPbIBr2 Solar Cells”, Adv. Energy Mater. 2017, 7, 1700946
[5] Z. Dang et al. “In Situ Transmission Electron Microscopy Study of Electron Beam-Induced Transformations in Colloidal Cesium Lead Halide Perovskite Nanocrystals”, ACS Nano 2017, 11, 2124−2132

In recent years, the organic-inorganic hybrid perovskite has gained an increasing research interest in academia for applications in thin film solar cells, due to rapidly increased efficiency (from 3.8 to 23.3% within a decade) [1], high absorption coefficient [2], low-cost fabrication process, and material availability [3]. Among the hybrid perovskites, MAPbI₃ (CH₃NH₃PbI₃) based solar cells have shown high power conversion efficiencies but with several obstacles such as thermal instability, hysteresis loss at room temperature. Therefore, the commercialization of these solar cells is still a challenge. Understanding and resolving these issues necessitate the investigation of the sample at the atomic scale to determine the underlying fundamental processes.
Here, we present the growth and experimental characterization of thin MAPbI₃ films on Au (111) under ultra-high vacuum conditions (UHV=1x10^-10 Torr). The thin films were prepared by vacuum evaporation of the precursor molecules MAI and PbI₂ with a thickness of a few monolayers (approx. 4 nm). We characterize the sample with scanning tunneling microscopy (STM), and X-ray photoelectron spectroscopy (XPS), obtaining information about the atomic structure, and chemical composition. For the electronic properties analysis, we used ultraviolet photoemission spectroscopy (UPS), and inverse photoemission spectroscopy (IPES). Our study will provide the basis for further understanding ion incorporation and stability at the atomic scale.

Mixed halide lead perovskites offer a useful strategy for continuous tuning of semiconductor bandgap. Based on their tunable optical properties these mixed halides are being considered as attractive candidates to develop single junction and multijunction tandem solar cells. The ease of halide ion exchange property poses a problem to create a tandem structure with layers of metal halide perovskites of different compositions. In order to keep the lead halide perovskite nanocrystals intact without undergoing exchange of halide ions and retain the original band structure one needs to suppress the halide ion migration across the nanocrystals. We have now successfully achieved this task by capping CsPbBr₃ and CsPbI₃ nanocrystals with PbSO₄-Oleate. Absorption measurements show that the nanocrystal assemblies maintain their identity as either CsPbBr₃ or CsPbI₃, for several days. Furthermore, we have electrophoretically deposited these assemblies as hierarchical structures on electrode surfaces and employ them in light emitting devices. The effectiveness of PbSO₄-Oleate capping of lead halide perovskite nanocrystals offers new opportunities to overcome the challenges of halide ion exchange and aid towards the tandem design of perovskite light harvesting assemblies.

Metal halide perovskites have sparked tremendous enthusiasm in the photovoltaic community with their promising performances. Currently holding a record power conversion efficiency of 23.3%, this exciting material class is approaching the mighty Si-powerhouse, concurrently opening encouraging avenues in (all-) perovskite multi-junction applications while approaching commercial exploitation.¹At the origin of these highly performing devices lie superior absorber properties that so far have been investigated by a plethora of characterisation techniques. Amongst all, electron microscopy (EM) remains one of the most extensively employed tools for studying the absorber in terms of crystallography, morphology, interfaces, lattice defects, compositions, and charge carrier collection and recombination properties. However, a crucial challenge upon using EM remains the beam sensitivity of the absorber, especially upon imaging with an increased electron dose. Resulting in structural and chemical changes in the light harvester in function of the total electron dose, it becomes extremely challenging to validate obtained results and those reported in literature.^3,4 Therefore, a quantitative characterisation of the beam damage is highly necessary to avoid, or at least mitigate, changes to the sample upon imaging in future analysis. Therefore, in this work, we investigate the current degeneration in multi-cation and –anion perovskite solar cells in function of the total electron dose by means of electron beam induced current (EBIC) measurements on rough cross-section of corresponding devices—providing an indirect yet quantitative route to characterise the electron beam induced damage to the absorber. Allowing the extraction of current decay profiles, various compositional perovskites are compared in terms of beam sensitivity, and operational parameters are indicated to avert (or at least mitigate) sample damage during imaging.

Halide perovskite solar cells (HPSCs) present a cutting-edge nanotechnology which has demonstrated remarkable power conversion efficiency increases to a recently reported value of 23% [1]. The high efficiency combined with low-cost production make HPSCs one of the most promising solar cell technologies but significant challenges related to their stability are yet to be solved. Exposure to oxygen, humidity, temperature, atmospheric pressure and UV light stimulates a complex set of degradation mechanisms such as diffusion of molecular oxygen and water, interfaces and the active material degradation, electrode reaction with the organic material to morphological and macroscopic changes[2]. For this reason, devices are encapsulated which prolongs their lifetime and also increases mechanical stability[3]. As the natures and timescales for each of these chemical reactions differ, it is challenging to elucidate the degradation processes. Secondary ion mass spectrometry (SIMS) is one of the few techniques that can offer valuable insights into potential degradation mechanisms by obtaining spatially resolved chemical analysis of organic and inorganic layers and interfaces. In this work, we present the high mass-resolving power depth profiling of HPSCs using the 3D OrbiSIMS[4].
The 3D OrbiSIMS comprises dual beam and dual analyser configuration by integrating a Q Exactive™ HF Hybrid Quadrupole-Orbitrap™ mass spectrometer with a high-resolution imaging ToF-SIMS platform. Advantages of this instrument include a significantly higher duty cycle compared with time-of-flight instruments (using pulsed ion beams), high-mass resolving power (>240,000) and high-mass accuracy (~ 1 ppm), up to 4 orders of magnitude dynamic range for organic materials and the ability to do MS/MS.
Here, we apply single beam depth profiling combined with high-mass resolving power to study the interface and interlayer chemistry in HPSCs and the degradation mechanisms that occur when exposed to air. Critical to achieving this is air-free transfer between a sample preparation glovebox and the 3D OrbiSIMS instrument. The air-free transfer protocol will be presented in this work.
[1] National Renewable Energy Laboratory, “Best Research-Cell Efficiencies”, 2018. https://www.nrel.gov/pv/assets/pdfs/pv-efficiencies-07-17-2018.pdf
[2] V. I. Madogni, B. Kounouhéwa, A. Akpo, M. Agbomahéna, S. A. Hounkpatin, and C. N. Awanou, “Comparison of degradation mechanisms in organic photovoltaic devices upon exposure to a temperate and a subequatorial climate,” Chem. Phys. Lett., vol. 640, pp. 201–214, Nov. 2015.
[3] A. Distler et al., “Effect of PCBM on the Photodegradation Kinetics of Polymers for Organic Photovoltaics,” Chem. Mater., vol. 24, no. 22, pp. 4397–4405, Nov. 2012.
[4] M. K. Passarelli et al., “The 3D OrbiSIMS—label-free metabolic imaging with subcellular lateral resolution and high mass-resolving power,” Nat. Methods, vol. 14, no. 12, pp. 1175–1183, Nov. 2017.

Organolead halide perovskites are a new class of photovoltaic materials for potential use in thin film solar cells, but suffer from rapid degradation under light and humidity, limiting their viability. Recently developed quasi-2D Ruddlesden-Popper phase perovskites show improved stability but, at present, the mechanism behind the stability is poorly understood. Here, we have used time-of-flight secondary ion mass spectrometry (ToF-SIMS), photoluminescence spectroscopy (PL), electrochemistry, and other techniques to study the chemistry and performance of 2D and 3D phases of methylammonium lead triiodide (MAPI) solar cells as a function of humidity exposure. ToF-SIMS depth profiles of both 2D and 3D MAPI planar solar devices (n-i-p, C₆₀/perovskite/PEDOT:PSS) show the formation of a significant hydrolysis-based degradation layer at the surface of only 3D MAPI devices after humidity exposure. Isotopic D₂O studies confirm the layer is caused by ambient humidity, while electrochemistry confirms it is directly related to loss of cell performance. The growth of this hydrolysis layer is found to be inhibited in 2D MAPI devices by the formation of a thin protective layer composed of more thermodynamically-stable 2D phases, clearly observed in PL spectra, which protects the less stable 2D bulk. To confirm that this newly formed 2D layer improves stability, we created solar cells consisting of 3D MAPI films coated with a thin layer of the 2D perovskite, and show that these cells are more stable that un-treated 3D MAPI devices. As a result, we conclude that using 2D perovskite as a protective interface may be a simple way to improve device stability.

Power conversion efficiency (PCE) of lead halide perovskite solar cell (over 23%) has surpassed those of CIGS and CdTe, approaching the top value of crystalline Si cell. Our group has been able to achieve PCE over 21% by low cost ambient fabrication. However, high PCE of single-cell enabled by lead halide-based perovskite absorbers are now being saturated, taking the Shockley Queisser (SQ) limit of open-circuit voltage (V_OC) (ca.1.32V) into account. Tandem cell making, which can further increases PCE up to 28% or more, leads to higher material and process cost and will raise a question if performance/cost ratio can be accepted in industry. Therefore, a smart way is to create a single cell which has high PCE comparable with that of GaAs (>28%) by reducing bandgap energy to <1.4 eV without accompaniment of increase in V_OC loss. This possibility will be in a family of metal halide perovskite out of those depending on use of lead. In addition to such efficiency issue, high performance of organo lead halide materials is not compatible with robust high stability required for practical use. Ensuring the intrinsic thermal stability (desirably >200^oC) of the perovskites is a key issue before industrialization. In addition, toxicity of lead-based perovskites are going to become the most formidable challenges for real use (commercialization), in particular, for applications to IoT society, which is one of the most promising field of perovskite photovoltaic device in terms of high voltage output even under weak illumination. These thoughts urge us to concentrate our next research of perovskite photovoltaics (PV) more on development of non-lead high efficiency absorbers. Sn perovskite is still a strong candidate because Sn(II) has been found to be stabilized against ambient air by metal doping method (such as Ge). Regarding Bi-based perovskites, we found AgBi₂I₇ as a promising all-inorganic absorber having high thermal and moisture stability. Stability also highly depends on the property of charge transport materials (CTMs), especially, the kind of hole transporter. Spiro-OMeTAD does not work at high temperature while P3HT, for example, is thermally stable. In our collaboration with JAXA, P3HT-based perovskite devices showed robust stability by exposure to high (100^oC) and low (-80^oC) temperatures and also to high energy particle radiations (iScience, 2018, 2, 148). Selection of CTMs is another important key in combination with non-lead perovskite materials. In conclusion, next direction of perovskite PV should be to enhance PV performance of non-lead all-inorganic semiconductor materials by extended compositional engineering, in parallel with developing thermally stable CTMs. Our on-going studies on non-lead perovskite materials in our group will be introduced in the talk.

Since the first report on the high efficiency, 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, PSC demonstrated its power conversion efficiency (PCE) of 23.3% in 2018. According to Web of Science, publications on PSC increase exponentially since 2012 and total number of publications reaches already over 10,000 as of October 2018, which is indicative of a paradigm shift in photovoltaics. Although small area cell exhibited superb efficiency surpassing the performance of thin film technologies, scale-up technology is required toward commercialization. In addition, further higher efficiency toward Shockley–Queisser limit is required in parallel. In this talk, Large-area coating technology is introduced using perovskite cluster embedded coating solution, followed by brief introduction on history of perovskite solar cell. Bi-facial stamping method was developed for not only scale-up technique but also interface modification and low-temperature phase stabilization. For higher efficiency, managing recombination is critical. Methodology reducing recombination is developed via interface and bulk engineering. Current-voltage hysteresis is also discussed because hysteresis is related to the stability of perovskite solar cell. Ion migration is now visualized and confirmed to correlate with hysteresis.

Organic-inorganic perovskites enable a combination of useful organic and inorganic properties within a single molecular-scale composite and have attracted substantial interest for use within organic-inorganic electronic devices [1], in part due to the high carrier mobilities, long minority carrier lifetimes, tunable band gaps and relatively benign defects and grain boundaries for systems based on Group 14 metals (e.g., Ge, Sn and Pb) [2]. Indeed, these materials have enabled unprecedented rapid improvement in perovskite photovoltaic performance to levels above 20% power conversion efficiency and with open circuit voltages above 1V for a single junction photovoltaic (PV) device [3]. This talk will provide an historical perspective on foundational work related to the organic-inorganic perovskite semiconductors, including discussion of crystal structure flexibility [4,5], semiconducting properties, film deposition approaches and electronic device applications of the three-dimensional and lower-dimensional perovskite structures. Recent trends in the field, as they relate to application in photovoltaics and related devices, will also be coupled into this discussion.

[1] D. B. Mitzi, K. Chondroudis, C. Kagan, IBM J. Res. Develop. 45, 29 (2001).
[2] W.-J. Yin, T. Shi, Y. Yan, Adv. Mater. 26, 4653 (2014).
[3] W. S. Yang et. al., Science 356, 1376 (2017).
[4] B. Saparov and D. B. Mitzi, Chemical Reviews 116, 4558 (2016).
[5] D. B. Mitzi, Prog. Inorg. Chem. 48, 1 (1999).

In our work on perovskite solar cells (PSC) we have achieved efficiencies above 22% with a mixed composition of iodide/bromide and organic and inorganic cations. With the use of SnO₂ compact underlayers as electron acceptor contacts we have constructed planar perovskite solar cells with a hysteresis free efficiency above 20%. Through the compositional engieneering larger preovskite grains grown in a monolithic manner are observed and reproducibility and device stability are improved. With regards to lifetime testing, we have shown a promising stability at 85 ^oC for 500 h under full solar illumination and maximum power point tracking (95% of the initial performance was retained). Recently, we have also commented on the standardization of PSC aging tests.

Perovskite solar cell research continues to progress rapidly on various fronts. My group at OIST is making efforts to use surface science and advanced material characterization to obtain in-depth understanding about perovskite materials and solar cells [1]. In this talk, I will present our research progress on understanding the degradation mechanism of perovskite materials [2,3] as well as developing strategies to improve stability of perovskite solar cells [4-6].
[1] L. K. Ono, Y. B. Qi*, J. Phys. Chem. Lett. 7, 4764 (2016).
[2] S. Wang, Y. B. Qi* et al., Nature Energy 2, 16195 (2016).
[3] E. J. Juarez-Perez, Y. B. Qi* et al., J. Mater. Chem. A 6, 9604 (2018).
[4] Z. Wu, Y. B. Qi* et al., Adv. Mater. 30, 1703670 (2018).
[5] J. Liang, Y. B. Qi* et al., Adv. Energy Mater. 8, 1800504 (2018).
[6] Z. Liu, Y. B. Qi* et al., Nat. Commun. 9, 3880 (2018).

Tin oxide (SnO₂) is a promising material for the electron transport layer in planar perovskite solar cells (P-PSCs) due to its suitable energy level and high electron mobility. SnO2-based P-PSCs show the highest power conversion efficiency among planar structure devices, but the PCE remains still low compared to the mesoporous TiO₂-based PSCs and there is a lack of thermal stability study. In this study, we develop a simple interface engineering to improve optoelectronic properties and the thermal stability of the P-PSCs. The modified SnO2 shows high conductivity, effective charge extraction ability and high recombination resistance. Empirically, efficiencies of 21.43% and 20.5% were reached for the device with doped Spiro-OMeTAD and with dopant-free asy-PBTBDT, respectively, in the present study. The devices with modified SnO₂ show excellent stability under mild (humidity of 25%) and harsh conditions (humidity of 85%; temperature of 85°C). Thus, our newly developed method guarantees highly efficient and thermal stable P-PSCs.

Organic-inorganic hybrid perovskite solar cells show the promises as the next-generation photovoltaic technology. The efficiency has quickly increased from 3.8% [1] to 23.2% since 2009. [2] However, instability of perovskite solar cell is a big issue hindering its commercialization and practical applications. By combining thermal-insensitive organic-inorganic perovskite and moisture-resistant electrode, we report successful fabrication of stable organic-inorganic hybrid PSCs which shows enhances stability under tough aging condition. Surface contact is another critical factor affecting solar cell efficiency. After suitable surface modification, solar cell efficiency is significantly improved from 12.2% to 14.9%.
¹ A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131, 6050 (2009).
² https://www.nrel.gov/pv/assets/images/efficiency-chart.png

Metal halide perovskites hold tremendous promise for next-generation solar cells, more than any other recently developed low-cost active PV material. To fulfill this promise, perovskites must first overcome the chemical and thermomechanical instability that has long been observed in them. While some promising results have recently been achieved in scalability with power conversion efficiencies (PCE) of 12% for 100 cm² perovskite devices, the thermomechanical instability of perovskites remains a significant challenge to producing module-scale perovskite solar cells with operational lifetimes comparable to c-Si and CdTe.

In particular, stresses are generated in perovskite films during processing and magnified in service by environmental effects such as thermal cycling, resulting in the formation of defects and propagation of fracture and delamination. Additionally, cracks that develop in the film are a source of accelerated degradation for the transport of gases, moisture, and other environmental species. Unfortunately, perovskite layers are exceptionally fragile and susceptible to delamination as measured by their fracture energy—less robust than organic photovoltaics (OPVs) by an order of magnitude and c-Si or copper indium gallium diselenide (CIGS) solar cells by two orders of magnitude. Despite the significance of film stresses for device stability, the origin and magnitude of stresses in perovskite films have been largely overlooked. In addition to causing fracture, stress accelerates the rate of photochemical degradation in many materials—such as fuel cell membranes, laser diodes, and encapsulants—and a recent report shows that perovskite are no exception.

Perovskites can accumulate residual stresses during processing through several pathways. For example, in solar cell devices, perovskites have much higher coefficients of thermal expansion (CTE) than the other device layers and the glass substrate. When perovskites are annealed after deposition and subsequently cooled back to room temperature, a lower-CTE substrate constrains the perovskite from contracting, an effect that generates lattice strain in the film. Given the extreme mechanical fragility of perovskite films, and the possibility for further stresses developing during fabrication and operation (e.g., exposure to environmental stressors such as thermal cycling), these residual stresses could contribute significantly to light, heat, and moisture-based chemical degradation as well as fracture in devices.

In this work, we report on an overlooked factor affecting stability: the residual stresses in perovskite films, which are tensile and can exceed 50 MPa in magnitude, a value high enough to deform copper. These stresses provide a significant driving force for fracture. Films are shown to be more unstable under tensile stress—and conversely more stable under compressive stress—when exposed to heat or humidity. Increasing the formation temperature of perovskite films directly correlates with larger residual stresses, a result of the high thermal expansion coefficient of perovskites. Specifically, this tensile stress forms upon cooling to room temperature, as the substrate constrains the perovskite from shrinking. No evidence of stress relaxation is observed, with the purely elastic film stress attributed to the thermal expansion mismatch between the perovskite and substrate. Additionally, the authors demonstrate that using a bath conversion method to form the perovskite film at room temperature leads to low stress values that are unaffected by further annealing, indicating complete perovskite formation prior to annealing. It is concluded that reducing the film stress is a novel method for improving perovskite stability, which can be accomplished by lower formation temperatures, flexible substrates with high thermal expansion coefficients, and externally applied compressive stress after fabrication.

Integrated circuits fabricated in the microelectronic industry heavily rely on complex interfaces which drive switching of the electrical components; i.e., transistors at the front end; capacitors & resistors at the back end. A combination of low resistive metal (Cu) and a low dielectric constant material (not limiting to SiO₂) are required to form a multi-level metallization scheme with electrical wiring to reduce the time constants (RC) at the back end. Such schemes facilitate faster and efficient signal and data processing to the front end. However, adhesion at these intricate interfaces and structures remain an issue under operational conditions resulting in a wide range of failure mechanisms at the back end of line. Mechanical probing using Cross-Sectional Nanoindentation (CSN) is a well-established method used to measure the fracture toughness and reliability of these interconnects. Subsequently electron microscopy is performed to identify the modes of failure mechanisms. On similar grounds, the perovskite and two-terminal silicon/perovskite tandem solar cells involve a variety of low resistive metals (Au, Ag, Al, carbon) and organic molecules/polymeric dielectric materials (Spiro-OMeTAD, PTAA etc), hence making it feasible to apply CSN method to characterize the mechanical reliability of these solar cells. Semi-quantitative assessment of the interfacial adhesion or cohesion in both single and multi-junction solar cells will be estimated from load-displacement curves and measuring the crack lengths using electron microscopy techniques (SEM, TEM). Modelling using non-classical finite element modelling will be used to generate load-displacement curves involved to predict the various modes of failure in such sub-micrometre single-junction perovskite solar cells and will be extended to multi-junction solar cells/modules. Understanding the failure mechanisms will facilitate better design and fabrication of reliable perovskite single-junction and multi-junction solar cells/modules.

In the current work, CSN testing method is extended to measure the interfacial delamination in perovskite and silicon/perovskite tandem solar cells (SPTS), which could lead to lowering of the fill factor (FF) under operating conditions of solar cells. In this test configuration, nanoindentation is performed on the substrate which is typically ~5-20 μm away from the electrode to air interface. The load-displacement curve obtained from the nanoindentation testing will be used to determine the fracture energy with the aid of a semi-quantitative analytical model to compute and compare adhesion and mechanical properties of samples with different interfaces.[2] At the conference, details pertaining to the CSN test and subsequent analytical analysis to get the fracture energy will be presented for various configurations of perovskite solar cells.

References
1. Sánchez, J.M., et al., Cross-sectional nanoindentation: a new technique for thin film interfacial adhesion characterization. Acta Materialia, 1999. 47(17): p. 4405-4413.

2. Mulmudi, H.K., Application and Further Development of Cross-Sectional Nanoindentation for Mechanical Stability Assessment of Complex BEoL Stacks with Cu/low-k, (Unpublished, Master Thesis, performed at the Center for Center for Complex Analysis, GLOBALFOUNDRIES, Dresden, 2009).

The issue of long-term stability in perovskite solar cells under atmospheric conditions remains at the forefront of making them viable technologies. To facilitate a firm understanding of the processes at stake and their influence, it is important to employ a systematic investigation of degradation [1]. The interactions between various degradation phenomena (e.g. the effect of load, atmosphere, etc.) can be substantial, warranting the use of multifactorial methodologies to extract dominant effects and their interactions. As such, we use a design of experiments (DOE) methodology, which allows us to identify salient confluent factors and their interactions as they affect the operation of the solar cells. Specifically, we observe the influence of the combination of temperature, humidity, perovskite type, and loading condition on electrical (I-V), crystallographic (XRD), and optical (UV-Vis) characteristics. We also track their impact on the concentration and activation energy of deep level traps in Cs_xFA_1-xPbI₃ and MAPbI₃ solar cells as a function of degradation via photo-induced conductivity transient spectroscopy. This study thus constitutes a framework upon which we can further our mechanistic understanding of degradation in perovskite solar cells.

Reference
[1] K. Domanski, E. A. Alharbi, A. Hagfeldt, M. Gratzel, and W. Tress, Nature Energy (2018), Vol. 3, pp. 61–67.

The wide and tunable bandgap of perovskites makes them attractive for use in tandem solar cells either as a top cell in perovskite/silicon and perovskite/thin film solar cells or as both top and bottom cells in all-perovskite tandem solar cells. For a tandem device, a transparent conducting electrode must be deposited on top of the perovskite solar cell to allow light transmission into the device. The conventional technique to deposit transparent conducting oxide (TCO) materials is sputtering. However, the high kinetic energy of the sputtered atoms and the UV radiation damages underlying layers of the cells (mainly the perovskite and hole contact layers), resulting in an S-shaped or shunted I-V curve with poor cell performance¹. Previously, a thin protective buffer layer of MoO_x² or SnO₂³ has been used before sputtering TCO, which places several processing constraints such as low-temperature deposition with mild precursors, high transparency of the buffer layer, and appropriate energy level alignment.

In this work, an aerosol-based technique is developed to deposit a uniform TCO thin film on perovskite solar cells without any damage to the other layers of the cell. The technique involves a two-step spraying process. In the first step, a colloidal solution of indium tin oxide (ITO) nanoparticles is sprayed through an ultrasonic atomizer nozzle generating droplets containing nanoparticles. Once the solvent from the droplet evaporates, in the second step, the dry nanoparticles are passed through a fine orifice nozzle along with a carrier gas, creating a spray of nanoparticles which then deposit on the substrate. The pressure downstream the orifice nozzle is almost 10-fold lower than the upstream pressure, which reduces the drag force on the fine nanoparticles and enables them to deposit on the substrate by impaction rather than flowing away along with the carrier gas. The absence of energetic atoms and radiation makes this technique suitable to deposit any kind of nanoparticle layer on top of perovskites without damage. The presentation will discuss this novel technique and its application in depositing a uniform layer of ITO nanoparticles as a top electrode in perovskite-based tandem solar cells to further boost their efficiency.

References:
1 Kanda, H. et al. Analysis of Sputtering Damage on I–V Curves for Perovskite Solar Cells and Simulation with Reversed Diode Model. The Journal of Physical Chemistry C 120, 28441-28447 (2016).
2 Duong, T. et al. Semitransparent perovskite solar cell with sputtered front and rear electrodes for a four-terminal tandem. IEEE Journal of Photovoltaics 6, 679-687 (2016).
3 Bush, K. A. et al. 23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability. Nature Energy 2, 17009 (2017).

Currently, perovskite solar cells (PSCs) with high performances greater than 20% contain bromine (Br), causing a suboptimal bandgap, and the thermally unstable methylammonium (MA) molecule. Avoiding Br and especially MA can therefore result in more optimal bandgaps and stable perovskites. We show that inorganic cation tuning, using rubidium and cesium, enables highly crystalline formamidinium-based perovskites without Br or MA. On a conventional, planar device architecture, using polymeric interlayers at the electron- and hole-transporting interface, we demonstrate an efficiency of 20.35% (stabilized), one of the highest for MA-free perovskites, with a drastically improved stability reached without the stabilizing influence of mesoporous interlayers. The perovskite is not heated beyond 100°C. Going MA-free is a new direction for perovskites that are inherently stable and compatible with tandems or flexible substrates, which are the main routes commercializing PSCs.

The major obstacles delaying the large scale commercialization of thin film solar cells based on hybrid organic halide perovskite are the complicated multi-step fabrication process, expensive materials, and environment and ion migration induced instability in the devices. In this research, we present a simple film-cover strategy to deposit light absorption layers (LALs) and hole transporting layers (HTLs) on a large area (4 x 4 cm²) through a single step process which is fundamentally based on polar and nonpolar interfacial induced self vertical phase separation between perovskite and HTLs. This process is highly saving material which is 3% Spiro-OMeTAD and 7.5% perovskite used compared to the traditional spin-coating process, it is dozens of times for the cost saving. Moreover, The specific gradient phase separation construction based on the doped-free Spiro-OMeTAD shows specific energy level and device construction, leading to efficient carrier transportation and higher power conversion efficiency (PCE). We further systematic studied the origin of stability in this phase separated film compared to the traditional layer by layer spin-coating film, it suggested that the gradient HTLs embed in perovskite crystal boundary which can effectively restrain ion migrate and moisture at the crystal boundary, providing an excellent environmental stability and retains 95% of the initial efficiency even after 68 days being stored in ambient condition without encapsulation. We believe that our strategy will pave a new way for simple, economical, efficient, stable, large-area and commercialized perovskite solar cells (PSCs) preparation, and provide more understanding on the nature of ion migrate in the PSCs devices.

Organic-inorganic hybrid perovskites are emerging as highly attractive materials for optoelectronic applications due to their high color purity, broadly tunable optical bandgap, facile solution process, and efficient narrow band emission. Due to these figure of merits, perovskite materials are highly utilized in the field of light-emitting diodes (LEDs) use as an active layer and have been successfully demonstrated to overcome the disadvantages of organic LEDs such as complex synthesis procedure, poor color purity, and high-cost processing. Because of these efforts, perovskite LEDs have recently reached external quantum efficiencies (EQEs) about 10 % for green and near-infrared emissions through reduce the grain size of the perovskite active layer. However, it still has the problem that increase the number of grain boundaries causing exciton quenching and nonradiative recombination, while reduce the grain size of the perovskites.
In this study, we investigated single-step solution-processed methylammonium lead-bromide (CH₃NH₃PbBr₃, MAPbBr₃) perovskite active layer for enhanced efficiency of LEDs. The single-step process contributed to strong exciton confinements inside the perovskite grain for improves the radiative recombination of the LEDs by achieving small grain size through a nanocrystal pinning method. In addition, the single-stepped perovskites can effectively reduced exciton quenching to the metallic Pb atoms by passivating the perovskite grain boundaries due to the MABr-rich molar proportion. As a result, the LED fabricated with single-step-processed MAPbBr₃ perovskite active layer (SSP LED) exhibited 33.8 % stronger maximum luminance (202 cdm^-2) and 17.6 % higher EQE (4.0 %) compared to LED fabricated with double-stepped perovskites (DSP LED). The SSP LED also showed a 11.5 % reduction in series resistance (1.31 Ωcm²) and a 7.3 % improvement in current efficiency (9.53 cdA^-1) compared to DSP LED, which was attributed to reduced Joule-heating than the double-stepped perovskite layer. The both LEDs consisting of ITO/PEDOT:PSS/MAPbBr₃/ PCBM/Ag structures with a peak emission wavelength of 538 nm.

In a perovskite solar cell (PSC), charge generation, transport and collection layers are stacked together, producing several interfaces, which have significant influences on the charge processes. At present, the interfaces in PSCs have been carefully manipulated by controlling the compositions, or introducing interfacial buffers and passivation layers. Here, unlike the widely-reported electron transport layers (ETLs) consisting of monolayer or multilayer metal oxide (TiO₂, ZnO, SnO₂, etc.), group-III nitride GaN work as ETL is explored to improve the electron transport and interfacial property. Benefiting from the self-limiting reactions of plasma enhanced atomic layer deposition (PEALD), the GaN thin films are conformal and pin-hole free with an accurate thickness control (<10 nm)，which thus in favor of the deposition of high quality perovskite film on GaN and suppression of charge recombination between GaN and perovskite layers. The results indicate that GaN thin-film thickness have significate effects on the electron transport and collection of the photovoltaic cells, and the devices based on a 5 nm thick GaN ETL can achieve the highest cell efficiency of 15.18% with J_sc of 22.56 mA/cm², V_oc of 0.977 V and FF of 0.689.

Perovskite solar cells (PSCs) based on lead-based halide perovskites (MAPbI₃, FAPbI₃, etc.) have achieved surprisingly high power conversion efficiencies (PCEs). However, lead-toxicity and intrinsic instability of these PSCs are major hurdles in the path towards the commercialization of PSCs. Typically, lead-free PSCs suffer from the low PCE (<10%) and poor stability. Herein, we introduce a series of Dion-Jacobson type 2D tin-based perovskite materials, and realized a promising PCE of up over 4% in hole-transport-layer-free PSCs. Theses PSCs shows high stability, with <10% PCE decay after 200 hours under continuous one-sun illumination. Thus, this work provides a new direction in the design and development of stable Sn-based perovskite solar cells.

The effect of water on the performance of perovskite solar cells has been intensively studied in recent years. However, the conflicting conclusion derived from different studies makes it impossible to fully understand the mechanism involved. Besides, all studies on water effect thus far have concentrated on single methylammonium cation perovskite materials. As a consequence, the effects of water on formamidinium and cesium perovskites are still unclear. Herein, we introduce water during the fabrication of triple-cation formamidinium and cesium based hybrid perovskites (abbreviated as Cs/FA/MA). By carefully controlling the water content, we demonstrate that water additive plays a significant role on the crystallization of the hybrid Cs/FA/MA perovskite film. More interestingly, the studies of the optical properties of the films indicate that water content determines the phase of the perovskites. Further, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS) characterizations are applied to analyze the components and the chemical environments of the films. Finally, By finely tuning the concentration of water, a more stable device with a significant enhancement in PCE could be fabricated. The findings in this work contribute to the understanding of the effect of water on hybrid perovskites. It is suggested that further theoretical and experimental studies should be carried out to gain full understanding of the mechanism of water on hybrid perovskites.

Formamidinium lead iodide (FAPbI₃) perovskites have great potential for their used in perovskite solar cells (PSCs) due to their near-ideal bandgaps. However, FAPbI₃ perovskite (α-phase) tends to crystalize in a hexagonal non-perovskite structure (δ-phase) at room temperature which is thermodynamically more stable yet photo-inactive. Herein, we report a novel method to stabilize the α-phase FAPbI₃ via chemical functionalization of grain boundaries. By protecting the α-phase FAPbI₃ from the ingression of moisture from the atmosphere, the resulting PSCs show significantly enhanced stability. This method also boosts the power conversion efficiency by affording higher phase-purity of as-crystallized α-phase FAPbI₃ and defects passivation. This work points to a new route for designing high-performance PSCs with long-term durability.

In this work, we synthesize ultra-small ternary oxide nanoparticles (NPs), NiCo₂O₄, by a novel strategy of controllable deamination of Co-NH₃ complexes in a system containing Ni(OH)₂. Through this approach, 5 nm NiCo₂O₄ NPs without exotic ligands are obtained and could be dispersed well in aqueous, which enables the formation of uniform and pin-hole free films. We apply the tightly covered NiCo₂O₄ film as hole transport layer in perovskite solar cells, and the corresponding devices reach a high power conversion efficiency of 18.23% and a promising stability (maintained ~90% PCE after 500 h light soaking) due to the formation of large perovskite grains and the reduction of film defects [1]. Besides improving device performances and stability, the newly synthesized NiCo₂O₄ with the interesting features of low-cost, low-temperature, simple and green solution process expose the opportunities of ternary oxides for the applications of emerging photovoltaic devices. To the best of our knowledge, it is the first time that spinel NiCo₂O₄ NPs are applied as hole transport layer in perovskite solar cells successfully. In summary, this work not only demonstrates the potential applications of ternary oxide NiCo₂O₄ as HTLs in PVSCs but also provides an insight into the design and synthesis of ultra-small and ligand-free NPs HTLs to enable cost-effective photovoltaic devices.

During the last years, inorganic-organic hybrid perovskite materials have been received attention as an absorber for next-generation photovoltaics because of its excellent photovoltaic properties for solar cells. Currently, the power conversion efficiency (PCE) of single-junction perovskite solar cells has raised to 23.3% within a short period. In addition, double-junction tandem solar cells with perovskite materials have been developed such as perovskite/Si, perovskite/thin-film and perovskite/perovskite. The monolithic perovskite/Si tandem cells with double-side textured silicon substrate exhibit the PCE of 25.2%. However, realizing an efficient and stable perovskite-based triple-junction tandem solar cell remains challenging despite their very high theoretical PCE (~49%). The critical issue is the poor stability of perovskite solar cell against the light because large bandgap perovskite (over 1.9 eV) with mixed halide could be easily segregated to I-rich and Br-rich material under the continuous light. For a highly efficient and stable triple-junction tandem solar cells, therefore, the perovskite absorber that exhibits the highly stable under the operating condition is essential.
Here, we report a highly durable large bandgap (1.9 eV) perovskite solar cell under continuous light. Perovskite absorber containing triple cation (FA/MA/Cs) was prepared by two-step sequential deposition method. We added trimethylolpropane triacrylate (TMPTA) as an additive into perovskite layer and cross-linked it by annealing process. By photoluminescence and X-ray diffraction analyses, we confirmed that a TMPTA additive retarded the phase segregation of mixed halide. We assumed that the additive prevents the ion migration at the grain boundaries, which improves the light stability. As a result, the devices with the additive-added perovskite exhibited outstanding stability under continuous light with negligible PCE drop during 1 hour in ambient condition. Finally, we successfully fabricated solution process-based perovskite/perovskite/Silicon triple-junction tandem solar cells on one-side textured Si substrates.

Organic-inorganic halide perovskites have shown exceptional semiconducting properties and microstructural versatility for inexpensive, solution-processable photovoltaic and other optoelectronic devices. The emerging solution-processed perovskite photodetector technologies show promising optoelectronic properties^[1–4] and generate a growing interest for large-area and flexible optoelectronic device production. In this work, we present a simple route for synthesizing efficient and reproducible halide perovskites under ambient conditions (>40%RH) by tailoring their physical and microstructural properties via mixed-anions and solvent additives. We fabricate an all-solution-based photodetector using high crystal quality perovskite nanowires grown on FTO and flexible Kapton substrates. The optimized photodetectors exhibit a high responsivity, a maximum specific detectivity of 10¹² cmHz^1/2W^-1 and a rise time as low as 55 µs for devices on the rigid substrate and 320 µs for devices on the flexible substrate. It also shows remarkable photocurrent stability after multiple bending cycles. Moreover, due to the hygroscopic nature of halide perovskites, a deposition of poly(methyl methacrylate) (PMMA) as a protective layer on the perovskite yields significantly better stability under ambient air operation: the PMMA-protected devices are stable for over one month. This work demonstrates a cost-effective fabrication technique for high-performance flexible photodetector and will open opportunities for research advancements on broadband and large-scale flexible perovskite-based optoelectronic devices.


[1] I. Ka et al., Sci. Rep. 2017, 7, 10.1038/srep45543.
[2] F. P. García De Arquer et al., Nat. Rev. Mater. 2017, 2, 1.
[3] I. Ka et al., Nanoscale 2018, 10, 9044.
[4] I. M. Asuo et al., Nano Energy 2018, 51, 324.

Low cost, solution processable perovskite solar cell has potential for large-scale fabrication. The slot-die coating is one of the solution process that uses less amount of material as compared with spin coating. Currently, the power conversion efficiency (PCE) of perovskite solar cell has reached to 18% by slot-die coating. However, the uniformity of perovskite film is determined by the rate of solvent evaporation that increase with gas quenching, vacuum quenching and thermal heating in the slot-die coating. In this work, we successfully fabricate the uniform perovskite film using near-infrared heating in 6 seconds. The highest PCE of 14.26% is reached for perovskite device without posting annealing in ambient.

Inorganic-organic perovskite is a promising material with unique advantages of high absorption coefficient, long carrier diffusion length, tunable bandgap, and simple preparation techniques for future photovoltaic development. The power conversion efficiency (PCE) of perovskite single-junction solar cells increases rapidly from 3.8% since 2009 to 22.7% in 2018. However, the PCE is ultimately limited by the Shockley-Queisser limit. All-perovskite tandem solar cell has been considered as the potential candidate for bring the PCE beyond the Shockley-Queisser limit of single-junction device and yet retaining the advantages of earth-abundant materials and solution processability. It is still a challenge to fabricate such solution-processed perovskite tandem device owing to the lack of robust interconnecting layers (ICL). The ICL should simultaneously achieve good electrical property for carrier transport and recombination, and high optical transmission. Meanwhile, the ICL should protect the bottom perovskite film against the damage of the solvent from the upper perovskite film because the solution process usually involves the same type of polar solvents. Here, a new thermionic emission-based ICL with enhanced solvent-resistance feature is demonstrated for monolithic tandem cells with solution-processed perovskite films. Fundamentally, the thermionic emission assisted electron transport mechanism is confirmed through both experimental and theoretical studies. Meanwhile, the new ICL protects the underlying perovskite film from the solvents of the upper perovskite by introducing a fluoride silane-incorporated polyethylenimine ethoxylated (FSIP) hybrid system. Besides the solvent resistance function, this novel FSIP hybrid system passivates perovskite defects to reduce nonradiative recombination leading to considerable enhanced performance. With optimized ICL, the tandem cells demonstrate high PCE up to 17.9%, which is the highest among solution-processed all-perovskite tandem cells with indium free ICL. Consequently, this work contributes to not only understanding the fundamental mechanism of ICL but also to promoting robust and low-cost photovoltaics.

Due to the soft nature of components in the metal halide perovskite absorber, it inevitably generates Pb⁰ and I⁰ defects during device fabrication and operation. These defects serve as not only recombination centers to deteriorate device efficiency, but also degradation starting point to hamper the device lifetime eventually. Herein, we demonstrate a sustainable method to eliminate Pb⁰ and I⁰ defects in perovskite simultaneously and solve the intrinsic degradation issue of perovskite materials. The resultant planar devices via low temperature process achieved the promoted efficiency of 21.98% (20.9%, certified by Newport, USA; 21.2%, certified by National Institute of Metrology, China) without hysteresis. Moreover, it leads to substantially improved long-term light and heat durability, and remaining more than 90% of the peak efficiency under AM 1.5 illumination or 85 ^○C for 1500 h, and 91% of its original stable PCE after maximum power point (MPP) tracking for 500 h, respectively. Additionally, we developed a robust hole transport composite, which prevent the inter-diffusion of perovskite components and Au electrode with the perseverance of efficient charge extraction and transportation. Moreover, we developed a reliable encapsulation method of PSCs, and the encapsulated devices can even withstand boiling water.

Key words: stability, perovskite solar cells, defects, encapsulation, hole transport material
(1) Zhou, H.; Chen, Q.; Yang, Y.* et al, Science 2014, 345, 542-546.
(2) Wang, L., Zhou, H.*; Sun, L. D.*; Yan, C. H. et al, ACS Appl. Mater. Interfaces 2017, 9 (31), 25985-25994.
(3) Zheng, G.; Zhu, C.; Hong, J.* Chen, Q.* Zhou, H.* et al, Nat. Commun. 2018, 9, 2793.

Remarkable recent advances in perovskite solar cells with efficiencies of over 23% have drawn world-wide as high-efficiency and low-cost solar cells. However, there are some problems to be solved in perovskite solar cells in order to develop high-efficiency, highly reliable and large-area solar cells and modules. Therefore, understanding and reducing non-radiative recombination losses in those materials and solar cells and resistance losses are necessary.
In this paper, efficiency potential of perovskite solar cells and perovskite/Si tandem solar cells is discussed by using external radiative efficiency (ERE), open-circuit voltage loss and fill factor loss and non-radiative recombination losses based on our previous analytical results for high-efficiency solar cells [1] and Si tandem solar cells [2].
One of problems to attain the higher efficiency perovskite solar cells is to reduce non-radiative recombination loss. The open-circuit voltage drop compared to bandgap energy (Eg/q-Voc) is dependent upon non-radiative voltage loss (Voc, nrad) that is expressed by external radiative efficiency (ERE). Open-circuit voltage is expressed by

where the second term shows non-radiative voltage loss, and is radiative open-circuit voltage and 0.28V was used as the value for perovskite solar cells in this study. Correlation between Voc values for perovskite solar cells in the references estimated by eqs. (1) suggests that perovskite solar cells have still non-radiative loss and further improvements in efficiency are thought to be possible by improving minority-carrier lifetime. In addition, reduction in resistance loss is suggested to be decreased.
The perovskite solar cells have efficiency potential of 26% by improving ERE from around 0.3-1% to 20%. As one of non-radiative recombination loss issues in perovskite solar cells, effects of gran size upon minority-carrier diffusion length in perovskite solar cells are discussed. In order to improve ERE in perovskite solar cells, effects of photon recycling are also discussed.
Recently, Si tandem solar cells including perovskite/Si tandem solar cells have become as one of hot topics. Most recently, 25.24% and 27.3% efficiency with perovskite/Si 2-junction tandem solar cells have been demonstrated by EPFL and Oxford Univ., respectively. Based on our analytical procedure [1,2], efficiency potential of perovskite/Si tandem solar cells is also analyzed in this paper. From the point of view of material quality, the ERE of perovskite/Si tandem solar cells are less than 10-5, which is several order-of-magnitude lower than that of III-V/Si tandem solar cells, This suggests that the material quality of perovskite/Si tandem cells still have large room to improve. By improving ERE from about 10^-5 to 0.01, high efficiency of more than 33% is expected to be realized for perovskite/Si tandem solar cells. For this purpose, improvements in material quality, optimization of layer structures to reduce optical and resistance losses, and effective usage of light-trapping are suggested.

References
[1] M. Yamaguchi et al., J. Mater. Res. 32, 3445 (2017).
[2] M. Yamaguchi et al., J. Phys. D: Appl. Phys. 51, 133002 (2018).

In recent few years, the efficiency of organic-inorganic metal halide perovskite-based solar cells (PSCs) has been improved rapidly, because of the significant efforts in materials development and device fabrications. Hole transporting materials (HTMs) play an important role to the PSCs in charge extraction and interface modification.^[1] Currently, the most extensively studied and applied HTM for perovskite devices is 2,2’,7,7’-tetrakis(N,N’-di-p-methoxyphenylamine)-9,9’- spirobifluorene (Spiro-OMeTAD).^[2,3] However, the spiro-OMeTAD possesses relatively low intrinsic conductivities, which need dopants like lithium bis(trifluoromethanesulfonyl)-imide (Li-TFSI), cobalt complexes, or 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquino-dimethane (F4-TCNQ) to enhance the material conductivity. Thus, for simplifying the device fabrication, reducing the cost, and solving some trade-off between photovoltaic properties and long-term stability and to circumvent the tight control of doping level and oxidation duration in the fabrication procedure, numerous dopant-free HTMs including small molecules and conjugated polymers have been vigorously explored. Side-chain polymers play important roles in the polymer and materials science with abundance structures, flexibility, and moisture protection ability. However, only a few side-chain polymers like the poly(9-vinylcarbazole) (PVK) derivatives were studied in the PSC fields with moderate efficiency of around 16%^[4,5].

Herein, we designed new side-chain polymer structures by integrating a methoxytriphenylamineconjugated-thiophene moiety as the HTM repeating unit on a polystyrene side-chain. The polymer (Poly-HTM) was synthesized by the radical polymerization of its relevant monomer (Mono-HTM). The results showed that with similar device preparation and measurement conditions, the hole mobility of polymer was about 3 times of that of its small molecular monomer. PSCs based on Poly-HTM as the dopant-free HTM afford an impressive highest power conversion efficiency of 17.2%, which is much higher than those obtained from Mono-HTM (9.7%). Moreover, the devices based on the Poly-HTM presented significantly higher stability than the device based on its monomer. Hence, this study demonstrates that the side-chain polymer strategy is an effective approach to achieve high efficiency and highly stable PSCs using dopant-free HTMs.

Reference
1. Calió, L.; Kazim, S.; Grätzel, M.; Ahmad, S., Angew. Chem. Int. Ed. 2016, 55, 14522-14545.
2. Ding, I. K.; Tétreault, N.; Brillet, J.; Hardin, B. E.; Smith, E. H.; Rosenthal, S. J.; Sauvage, F.; Grätzel, M.; McGehee, M. D., Adv. Funct. Mater. 2009, 19, 2431-2436.
3. Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I., J. Am. Chem. Soc. 2014, 136, 7837-40.
4. Xu, Y.; Bu, T.; Li, M.; Qin, T.; Yin, C.; Wang, N.; Li, R.; Zhong, J.; Li, H.; Peng, Y.; Wang, J.; Xie, L.; Huang, W., ChemSusChem, 2017, 10, 2578–2584.
5.Yang, L., Yan, Y.; Cai, F.; Li, L.; Wang, T., Sol. Energy Mater. Sol. Cells, 2017, 163, 210–217.

Metal halide perovskite films are endowed with the nature of ions and polycrystalline.^[1] Formamidinium iodide (FAI) -based perovskite films, which include large cations (FA) incorporating into the crystal lattice, are most likely to induce local defects due to the presence of the unreacted FAI species.^[2,3] Here, we demonstrate a diboron assisted strategy to control the defects induced by the unreacted FAI both inside the grain boundaries and at the surface regions. The diboron compound (C₁₂H₁₀B₂O₄) can selectively react with unreacted FAI leading to reduced defect densities. Nonradiative recombination between a perovskite film and a hole-extraction layer was mitigated considerably after the introduction of the proposed approach, and charge-carrier extraction was improved as well. We therefore obtained a champion power conversion efficiency of 21.11 % with a stabilized power output of 20.83 % at maximum power point for planar structure perovskite solar cells.^[4] The optimized device also delivered negligible hysteresis effect under various scanning conditions. This approach paves a new way for mitigating defects and improving device performance.
Reference:
[1] M. Saliba, T. Matsui, J. Y. Seo, K. Domanski, J. P. Correa-Baena, M. K. Nazeeruddin, S. M. Zakeeruddin, W. Tress, A. Abate, A. Hagfeldt, M. Gratzel, Energy Environ. Sci. 2016, 9, 1989.
[2] Q. Hu, L. Zhao, J. Wu, K. Gao, D. Luo, Y. Jiang, Z. Zhang, C. Zhu, E. Schaible, A. Hexemer, C. Wang, Y. Liu, W. Zhang, M. Gratzel, F. Liu, T. P. Russell, R. Zhu, Q. Gong, Nat. Comm. 2017, 8, 15688.
[3] J. A. Christians, P. Schulz, J. S. Tinkham, T. H. Schloemer, S. P. Harvey, B. J. Tremolet de Villers, A. Sellinger, J. J. Berry, J. M. Luther, Nat. Energy 2018, 3, 68.
[4] Y. Tu, X. Yang, R. Su, D. Luo, Y. Cao, L. Zhao, T. Liu, W. Yang, Y. Zhang, Z. Xu, Q. Liu, J. Wu, Q. Gong, F. Mo, R. Zhu, Adv. Mater. 2018, 1805085.

While lead halide-based perovskites have received an incredible amount of attention over the past several years as an emerging photovoltaic material, one aspect that has received minimal attention is their spectroelectrochemical response, due in part to their relative instability under applied potentials while immersed in an electrolyte solution. We identify new spectrally-distinct sub-gap NIR absorption features when we subject model CH₃NH₃PbI₃ perovskite solar cell materials to AC anodic and cathodic electrochemical potentials. First, we resolve a new differential absorption band with an onset near 850 nm under cathodic conditions, which is absent under anodic conditions. We find the electronic transition energy for this absorption to be commensurate with the energy separation that has been predicted and determined experimentally for the E_1/2u and F_3/2u conduction band minima in CH₃NH₃PbI₃. Additionally, our AC spectroelectrochemical signals appear to be spectrally coincident with NIR transient absorption features that we observe on picosecond to nanosecond timescales. Deconvoluting these spectra by target analysis shows the correlation in temporal evolution between the photogenerated conduction band electron and trap state bleaching. The kinetics of the species associated spectra we model based on charge extraction, bimolecular recombination, charge trapping, and Auger recombination processes. The AC nature of our spectroelectrochemical detection also allows us to virtually eliminate the film degradation that has plagued DC electrochemical studies of halide perovskite materials. These results are relevant to device performance metrics as we see through kinetic modeling simulations that an extended electron lifetime observed in films made with a 1-step fabrication procedure is due to a decreased trap state density compared to those made with a 2-step procedure, which also correlates to improved device performance due to increased fill factor.

For commercialization of perovskite solar cell, suppression of ion migration and reduction of trap states is highly needed. Recent works has proposed that 2D perovskites with layered structures can be mixed with 3D perovskites due to its low defect density and superior ambient stability. Despite diversity of long organic cation, correlation of molecular structure and structural features and finally electrical properties have been neglected. Here, we systematically investigated the effect of the size of the phenylalkylammonium ions focusing on charge transport and defect passivation. The results show that defect density was reduced regardless of the type of phenylalkylammonium ion, however carrier accumulation and recombination loss in the perovskite crystals were increased with larger ion. Organocation with stereic hindrance induced tilting of lead-halide octahedra which affected charge collection efficiency. Finally, we could obtain highly efficient and stable (PCE=20.5%) low band gap (1.48eV) formamidinium perovskite solar cell.

With the shortage of energy, developing the alternative energy sources becomes an important issue. Solar energy is considered as one of the most important alternative energy source because it is inexhaustible. Among many types of solar cells, the metal halide perovskite solar cells (PSCs) is the most promising solar cells due to its high power conversion efficiency (PCE). The PCE of PSCs has rapidly increased from 3.8% to 23.3% since 2009. However, the high-efficiency PSCs typically required high-temperature-sintered mesoporous (over than 450^oC) TiO₂ (HT-TiO₂) as electron transport layer (ETL) and the high-temperature to process often limits continues fabrication process. Furthermore, it can’t be used on the flexible substrate and it will spend more energy on fabrication process. Base on above reasons, high-temperature to process is not suitable for commercialization. In this study, all the fabrication processes were under low temperature. The TiO₂ nanocrystals (LT-TiO₂) were synthesized by non-hydrolytic sol-gel method under 150^oC and the spray-coating method was employed for the deposition of LT-TiO₂ ETL. Moreover, we used spray-coating method and blade coating for large area (over 25 cm²) module. In order to go into commercialization, the perovskite active layer should be fabricated in the air. Therefore, we controlled different ratio of perovskite precursor solution (dimethyl sulfoxide:-ButyrolactoneL) to obtain large grain size, high crystalline, pinhole-free and uniform perovskite active layer with blade coating method under various relative humidity. We used scanning electron microscope and atomic force microscope to investigate the correlation between perovskite crystallization and various relative humidity. We also used the photo-assisted Kelvin probe force microscopy (KPFM) to explore the charge transport efficiency in active layer which were prepared under various relative humidity. Toward future commercial application, a large size module is essential. Hence, the 5x5 cm² PSCs module was fabricated based on the low-temperature process. Finally, we successfully developed PSCs without the need of high-temperature sintering process and obtained higher PCE than the mesoporous PSCs. The large area PSCs module also demonstrated the high PCE with the continuous process.

The demand for renewable energy is escalating in recent years due to a dire need to address the global warming. Solar energy is one of the reliable alternative energy resources. Direct conversion of solar energy to electrical energy is a process known as photovoltaic conversion. Si based p-n junction solar cells with efficiency around 22% have captured commercial market despite high cost. Other solar cells based on semiconductor thin films, dye sensitizers, quantum dots, and organic inorganic perovskites are evolving to reduce the solar cell production cost while keeping up the efficiency similar to Si solar cells. Most of these devices are hampered by their long-term stability. In this front, all oxide photovoltaic devices are seems to be promising. Recently, ferroelectric oxides have captured significant attention in solar cell community due to demonstration of existence of unique anomalous photovoltaic effect, in which photovoltage (V_OC) is found to be larger than the bandgap of the materials [1]. Most of the ferroelectric materials have larger bandgap ~ 3 eV which inhibits the material to absorb entire solar spectrum. Short circuit current density (J_SC) observed in ferroelectric oxide based solar cells is in the range of nA to µA which severely limits efficiency of the devices [2].
BiFeO₃ (BFO) is a room temperature ferroelectric oxide which exhibits lower bandgap ~ 2.5 eV in bulk. Bandgap of BFO can be tailored over a wide range from 2.32 eV to 1.5 eV [3], which is promising for solar cell application. In this report we show the effect of oxygen vacancies (O_V) on ferroelectric, photoconductivity and photovoltaic properties of solar cell devices made using Bi_1-xCa_xFe_1-yTi_yO_3-d (BCFTO) [x = y = 0, (BFO); x = 0.1 and y = 0, (BC10FO); x = 0.2 and y = 0, (BC20FO); x = y = 0.1 (BC10FT10O); x = y = 0.2 (BC20FT20O)] thin films. All thin films are coated on FTO-glass substrates by simple spin coating. A mask having circular hole of 2 mm diameter is used to coat Au on top of the films. FTO/BCFTO/Au devices in capacitor configuration are used for photoconductivity and photovoltaic measurements. We observed an enhancement in photoconductivity of 3 orders in BC10FO and 5 orders in BC20FO compared to pristine BFO devices when illuminated with 1 Sun (~100 mW/cm²) power. Enhancement of photoconductivity in BC10FO and BC20FO is attributed to the O_V present in the system. It is observed that J_SC is increased but V_OC is reduced with increasing Ca doping concentration in pure BFO. FTO/BC20FO/Au and FTO/BC10FO/Au devices show J_SC ~ 3 mA/cm² and 30 µA/cm², respectively which are four and two orders larger than the J_SC observed in FTO/BFO/Au devices (~ 0.3 µA/cm²). V_OC observed in pure BFO devices are ranging from 0.35 V to 3 V. In case of BC10FO devices, this lies in between 0.2 V to 0.8 V and it is found to decrease further for BC20FO devices (~ 0.1 V to 0.3 V). A reduction in ferroelectric domain size is observed with increasing Ca content. Ferroelectric switching gets diminished in BC10FO and BC20FO. In contrary, titanium cation co-doping at Fe site strengthens the ferroelectric polarization and ferroelectric switching in BCFTO films. Enhanced photoconductivity and photovoltaic property are evidenced in BCFTO devices. High solar cell efficiency 0.5% are achieved with devices exhibiting V_OC (~ 0.8 V) and J_SC (~ 2 mA/cm²). BCFTO devices with high V_OC and J_SC are easily fabricable, stable and low fabrication cost. Therefore, ferroelectric oxide solar cells are very promising as a commercially viable devices.

References:
[1] S. Y. Yang et al. ‘Above-bandgap voltages from ferroelectric photovoltaic devices’, Nat. Nano 5, 143 (2010).
[2] Narimichi Makino et al. ‘Photocurrent properties of BiFeO₃ thin films prepared by chemical solution deposition’, Ferroelectrics, 453:20-25 (2013)
[3] Pavana S. V. Mocherla et al. ‘Tunable bandgap in BiFeO₃ nanoparticles: The role of microstrains and oxygen defects,’ Appl. Phys. Lett., 103, 022910–022915 (2013).

The efficiency of Perovskite solar cells (PSC) has increased from 3% to 22.1% in less than eight years. Because of its rapid increase in efficiencies and low cost fabrication process, the perovskite solar cell technology has been studied intensively and was named as one of the biggest scientific breakthroughs in the year of 2013 by Science. In the PSC structure, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is usually used as the hole transport layer (HTL) on top of ITO. It was found that interface engineering at electrodes is very important for increasing the performance of PSCs.

In this work, we show our study on how the efficiency of the Perovskite solar cells depend on the conductivity and work function of PEDOT:PSS. A series of PEDOT:PSS solutions with different conductivity and work function were used to make the Pervoskite solar cells. We will also report the interface processing we did in the p-i-n structure PSCs, which optimize device properties and charge transfer. In this work, all materials were prepared at low temperatures, showing potential for low-cost, scalable production of large-scale perovskite solar cells on flexible substrates.

Perovskites materials have been used in the fabrication of solar cell with efficiency exceeding 22%. Some of the proposed perovskite structures uses solution process of lead iodide (PbI₂) as precursor. Here we propose the production of perovskite by using sputtered PbI₂. The films were deposited by sputtering a PbI₂ target prepared by compression of PbI₂ powder and deposited at room temperature. The use of the sputtering technique allows the deposition of uniform films in large area. The films were converted in perovskite by using two routes: 1) spin coating the film with a solution of methylammonium iodide (MAI) and 2) dipping the film in the same solution. We also investigate the effect of using formamidinium iodide (FAI) and cesium iodide (CsI) solutions. X-ray measurements revealed that the PbI₂ films contain a small concentration of segregated lead (Pb) and that the perovskite films have (112) preferential orientation and some residual PbI₂. However, after conversion of the PbI₂ in perovskite we do not observe presence of segregated lead. Optical and structural properties were investigated through UV-Vis spectroscopy, SEM and AFM. Preliminary study of fabrication of perovskite solar cells supplied devices with efficiency of 2.5%. Optimization of the fabrication process is currently under way.

Perovskite solar cells (PSCs) are based on organo-metallic halides (perovskites) that act as a light-sensitive compound which produces excitons when placed under light. The general formula for a perovskite compound used in a PSC is expressed as ABX₃, where A is an organic/inorganic cation, B is a divalent metal cation and X₃ are halide ions. The most common compound used in a PSC is methyl-ammonium lead halide, CH₃NH₃PbI₃ (A = CH₃NH₃⁺, B = Pb²⁺, X = I^-) deposited on the mesoporous TIO₂ layer. These materials possess optoelectronic properties that are required for high efficient photovoltaic devices. However, PSCs have a limited operating lifetime due to the degradation of the perovskite layer. There are many factors that lead to the degradation of the perovskite layer.
This study aims to investigate the environmental factors that affect the performance and long-term stability of the PSCs. The structure of the PSC is used in this study is the regular structure containing the TCO, TiO₂, perovskite absorber, hole transport layer and the carbon layer. The PSCs were fabricated using two different methods, the doctor blading method, and the spin coating method. These solar cells were investigated under various environmental conditions like humidity (0 to 70%), with oxygen, without oxygen (nitrogen), normal environmental conditions, and longtime light soaking at elevated temperatures. The properties under investigations are the optical absorption of the perovskite absorber, the open circuit voltage (V_oc), short circuit current (I_sc), fill factor (FF), and efficiency of the PSCs. It was found that the PSCs do not degrade with the presence of oxygen in a dry and dark environment. The complete study of the effects of these environmental factors will be reported in this paper.

This investigation has applied simulation work to elucidate the photovoltaic performance of non-Pb based perovskite devices, utilizing Sn. In an effort to address additional key criteria that impede the commercial realization of this technology, we have investigated Cu2O (instead of the traditional spiro-OMeTAD) as an HTL that is cheap and easily synthesized, deposited and fabricated to form a fully functional device. Also, in an effort to reduce cost, in place of Ag as the back electrode, we have incorporated Cu. By optimizing each layer thickness, this work showcases not only a maximal efficiency device, but by critical analyses of Jsc, Voc and FF trends that manifest due to each component layer thickness modulation, we present key charge mobility and transfer kinetics within the structure of the cell. These optimal numbers and trends are critical when fabricating a physical device in the lab, and even more so once these components are investigated as viable ingredients in an industrial setting.

In this research, we have utilized simulation studies to probe non-toxic Sn-based perovskites with a goal to explore optimal fabrication methodologies. To that end, we have investigated a device structure with and without an ETL and assessed the impact on critical photovoltaic parameters. Our ETL of choice has been the traditional TiO2 matrix, which has a wide bandgap, and good transport properties. We have also in-situ performed optimization of individual layer thicknesses, thereby ultimately fabricating a device with the highest possible theoretical efficiency. The analyses provide critical insight into charge mobility and charge transfer kinetics within this non-Pb based device; by observing the effects on Jsc, Vcc, and FF as individual layer thicknesses are modulated, and doing comparisons of devices with and without an ETL, we present results of a structure that is not only optimal from an efficiency standpoint, but also from a technology viability and sustainability standpoint.

Recently, SnO₂ has received a great deal of attention in normal perovskite solar cells with its excellent optical and electrical properties, 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 has the advantage of being able to control the film thickness with nanometer precision and to produce high quality thin film without any pinholes. Also, SnO₂ has a deeper conduction band, compared to TiO₂ which has energy level mismatch with MAPbI₃, thus energetic alignment of SnO₂ is more appropriate for extracting photogenerated electrons than TiO₂. Thin films of Zn doped SnO₂ also prepared in this study and showed enhanced conductivity compared to undoped ones, as a result, photovoltaic performances were improved in the normal planar structure (FTO / Zn:SnO_x / Perovskite / Spiro-OMeTAD / Ag) perovskite solar cells.

Keyword : perovskite solar cell, atomic layer deposition, zinc doped SnOx

In this research, we have utilized simulation studies to probe non-toxic Sb-based perovskites with a goal to explore optimal fabrication methodologies. To that end, we have investigated a device structure with and without an ETL and assessed the impact on critical photovoltaic parameters. Our ETL of choice has been ZnO – which has a wide bandgap, and good transport properties. We have also in-situ performed optimization of individual layer thicknesses, thereby ultimately fabricating a device with the highest possible theoretical efficiency. The analyses provide critical insight into charge mobility and charge transfer kinetics within this non-Pb based device; by observing the effects on Jsc, Vcc, and FF as individual layer thicknesses are modulated, and doing comparisons of devices with and without an ETL, we present results of a structure that is not only optimal from an efficiency standpoint, but also from a technology viability and sustainability standpoint.

Solar cell technology is evolving into solar cells that increase efficiency, reduce cost, or have special functions such as transparent or flexible solar cells. Silicon solar cell technology, which has a market share of more than 90% of the entire solar cell market, has a wide spectrum of BSF, PERC, HIT and IBC in terms of efficiency and price. However, it is difficult to meet the efficiency and price of silicon solar cell at the same time, and breakthrough technology is needed to overcome it. Among these breakthrough technologies, the most promising technique is a tandem cell strategy in which two or more solar cells with different band gap energies are stacked. In particular, perovskite solar cell is an ideal material for tandemization with silicon solar cells because it is easy to control the bandgap by adjusting the chemical composition of perovskite. Thanks to these advantages, perovskite/silicon tandem solar cells have been actively studied and some results with high efficiency above 20% already have been reported. However, Most of the reported perovskite/silicon tandem solar cell are based on the high-cost SHJ planar silicon solar cell due to the problem of uniform perovskite top layers.
Here, we demonstrate a simple, low-cost, 2-terminal, silicon/perovskite mechanical tandem solar cell, which was fabricated by bonding a p-type silicon cell upside down on an n-i-p perovskite cell using a transparent conductive adhesive (TCA) layer. The TCA layer consists of Ag-coated poly(methyl 2-methylpropenoate) (PMMA) particles embedded in a polymer adhesive. The Ag-coated PMMA particles have the role of electrical current path and the polymer adhesive bond two sub-cells mechanically. The specific contact resistance and transmittance of the TCA layer was determined to be 5.46 x 10^-2 ohm cm² and > 97.0%, respectively. Because this TCA is flexible and soft, our tandem structure can use a conventional textured silicon solar cell compared to the other structures. Using optical simulations and experiments, the optimal current matching was achieved at a perovskite layer thickness of 150 nm for the CH₃NH₃PbI₃ solar cell in n-i-p structure and Al BSF p-type Si cells. The tandem cell fabricated under the optimal current matching condition exhibited a current density of 15.43 mA/cm², an open-circuit voltage of 1.59 V, and a fill factor of 79%, resulting in a steady-state PCE of 19.4%. To the best of our knowledge, the tandem cell efficiency of this study corresponds to the highest efficiency among all tandem cells based on Al-BSF silicon cells. The unique structure of this tandem cell showed excellent long term stability without encapsulation in a humid environment (100 h in RH 85%).

The power conversion efficiency (PCE) of perovskite solar cells (PSC) continuously break new records; however, there are still big challenges for large perovskite fabrication. Two of these being the solar cell’s grain growth and stability. In this investigation, Urea additions to the perovskite precursor solution prepared using lead acetate precursors show improved the device performance. The results show that the best content of Urea is 0.5 wt. %, when compared with the samples with and without Urea. The presence of Urea in the PSC increases the PCE of from 15% to 17%. X-ray diffraction analysis indicates that the addition of Urea promotes grain growth and also improves the grain’s orientation. This study reveals that the PCE is also closely related to the anneal time of the perovskite layer. Our findings show that the addition of Urea and proper anneal times enhance the crystallization process of the perovskite films and results in the resulting in larger and more orientated grains. This is the reason for the device’s improved carrier transport and conversion efficiency.

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 (~ 85 °C) 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, n-Al:ZnO, and SnO₂ grown via atomic layer deposition (ALD)). 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 ALD 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).

Perovskite solar cells (PSCs) have attracted attention due to tremendous rise in the power conversion efficiencies (PCEs) over the past decade and their possibility to replace traditional silicon solar cells has become realistic [1]. Although high life time is key requirement, the research has been mainly focused on improving the efficiency. A few studies have been performed to improve the stability of PSCs by synthesizing new hole or electron transport layer [2]. In the inverted PSCs, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) is the widely used hole transport layer (HTL), however, its acidic nature has been continuously raised as a concern for the long-term stability of PSCs. Therefore, there is an urgent need for other HTLs. Inorganic transport layers have been used in organic photovoltaics e.g. nickel oxide (NiO_x), copper iodide (CuI), copper thiocyanate (CuSCN) which have found successful in improving the stability. These HTLs can potentially improve the stability of PSCs having an impact on bulk transport and interfacial charge collection properties.
In this study, we have tested six HTLs, PEDOT:PSS, NiO_x, CuI, CuSCN, molybdenum oxide (MoO₃) and vanadium pentaoxide (V₂O₅) in the inverted perovskite solar cell structure. The used device structure was ITO/HTL/perovskite/PC₆₀BM/LiF/Ag, where FA_0.7MA_0.2Cs_0.1Pb(I_5/6Br_1/6)₃ (FMC) was the used as perovskite absorber layer. Stability tests were performed under continuous light exposure using an AM1.5G spectrum as well as using dark storage tests. Both the Power Conversion Efficiency (PCE) and lifetime were found dependent on the HTL material which was applied for the tests. PSCs with PEDOT:PSS showed the efficiency of 10.1 % which degraded to 60 % in 180 hours. PSCs with NiO_x showed the highest efficiency (>12 %) and lifetime. NiO_x based solar cell has maintained the efficiency for more than 180 hours. Additionally, the PSC with NiO_x also showed hysteresis free photovoltaic characteristics. CuSCN, MoO₃ and V₂O₅ based devices were also found to be more stable than PEDOT:PSS based device but with lower PCEs.
To understand the degradation in more detail, glow-discharge mass spectroscopy was used to analyse the films before and after degradation. The results will be presented in this talk. Additionally, atomic force microscope and X-ray diffraction was used to support the results and compare between different HTLs. Morphology of perovskite was found to be extremely homogenous on inorganic HTLs supporting the solar cell performance which yielded the best performance. Overall, the results indicate that the crystallinity of perovskite is found to be the crucial factor affecting the stability of solar cell.
In conclusion, the inorganic transport layers shows improved stability for perovskite solar cells. NiO_x was found to be the most efficient and stable hole transport layer.
References
[1] Saliba, M. et al. Energy Environ. Sci. 2016, 9, 1989-1997.
[2] J. Chen, N-G. Park J. Phys. Chem. C, 2018, 122, 14039-14063.

The self-limiting reactions of atomic layer deposition (ALD) enable conformal coatings/ interface layers with molecular-level control of thickness and composition, which has been found great benefit in tuning the interfacial structure and properties in solar cells. So far, various metal oxides (TiO₂, Al₂O₃, ZrO₂, SiO₂, etc) deposited by ALD have been used in some new generation solar cells, however, few group-III nitrides (AlN, GaN, InN, etc) are reported to explore their potential. Here, we study the application of GaN thin-films in perovskite solar cells (PSCs). The nitrides thin films are deposited via plasma enhanced atomic layer deposition (PEALD) by reaping the benefits of the lower deposition temperature and precise thickness control. The GaN thin film here serves electron transport layer (ETL) for PSCs. The results indicate that GaN thin-film thickness has a significate effect on the electron transport and collection of the photovoltaic cells. It is found that the devices based on the 50-cycle GaN thin-film (~5 nm) show the best cell performance with efficiency of 15.2%. The introduction of group-III nitrides has shown promise for improving solar cell performance and further enhancement of cell efficiency via improving the nitrides quality.

Hole transporting layer (HTL), where photogenerated holes are extracted from perovskite and transported to the back-metal contact, plays an important role in enhancing the power conversion efficiency (PCE) as well as improving the stability, which is a major bottleneck for commercialization of perovskite solar cell (PSC). Currently, the state-of-the-art PSCs use the small molecule 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (Spiro-OMeTAD) or poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine](PTAA) doped with common additives, such as Li-TFSI and tBP, which usually cause a serious degradation of device performance. Therefore, the development of dopant-free HTL becomes an essential requirement in the field of PSC. In this work, we designed and synthesized small-molecule-based dopant-free hole transporting material (HTM) called α, β-COTh-OMeTAD by fusing two strong electron donating moieties; saddle-shaped cyclooctatetrathiophene (COTh) and triphenylamine, to obtain sufficiently high hole transporting properties without additional dopants. The α, β-COTh-OMeTAD exhibits a deep highest occupied molecular orbital energy level (HOMO) of -5.3 eV and a high hole mobility of 2.88 × 10^-4 cm²V^-1s^-1. The thermal decomposition temperature of this small molecule is 308 °C and the phase transition temperature is 121.5 °C, showing a good thermal stability. It also exhibits a good solubility in several common organic solvents, which is favourable for essay processability and device fabrication. The solar cell fabricated with our small molecule HTM in the structure of FTO/compact TiO₂/mesoporous TiO₂/perovskite/HTL/Au gives a PCE of 15.66% with J_SC = 21.05 mA cm^-2, V_OC =1.04 V and FF=71.79%, which is comparable to the device based on doped spiro-OMeTAD as HTL, whose PCE is 16.69% (J_SC=20.24 mA cm^-2, V_OC=1.10 V, FF=74.62%).
In addition to functioning as HTM, α, β-COTh-OMeTAD also serves as an interfacial layer to passivate the grain boundaries of perovskite crystals. During the crystallization of perovskite film, solvent such as chlorobenzene (CB) or toluene is usually poured into the perovskite precursor solution under spinning to produce a uniform and high-quality perovskite film. Herein, we observed that incorporating a small amount of α, β-COTh-OMeTAD (10 mg/mL) in CB anti-solvent further improves the efficiency of solar cell. Using α, β-COTh-OMeTAD as both interfacial layer and HTL improves the PCE to 17.22% with J_SC = 21.51 mA cm^-2, V_OC = 1.05 V and FF=76.37%. SEM image of perovskite film treated with α, β-COTh-OMeTAD exhibits less apparent grain boundaries and denser film because of the passivation effect. Time resolved photoluminescence (TRPL) measurement also reveals that the faster decay lifetime τ₁ which corresponds to the quenching of the photogenerated free carriers transporting from the perovskite to HTL becomes shorter but the slower decay lifetime τ₂ which represents the radiative recombination of free carriers becomes longer, suggesting the efficient charge transfer between the perovskite and HTL and good passivation effect of the α, β-COTh-OMeTAD. Besides the hole transporting and passivating properties, α, β-COTh-OMeTAD also improves the stability of the device due to the hydrophobic nature of the material which prevents moisture from penetrating into the perovskite film, and deep HOMO level which prevents from reaction with iodine in the perovskite compound.
We also employed the commercially available conjugated polymer called PDTBDT-FBT in a separate study. Different from α, β-COTh-OMeTAD, the PCE of device is very low when PDTBDT-FBT is used as HTL but it is functioning well as interfacial layer to passivate the perovskite film. The PCE of 18.03% is obtained when PDTBDT-FBT dissolved in CB is used as anit-solvent and spiro-OMeTAD as HTL.

Since Prof. Miyasaka’s group first reported the organometal halide perovskite-sensitized photoelectrochemical cells in 2009,¹ perovskite-based light harvesters have launched the photovoltaics revolution due to their key advantages of low cost and solution processing. Moreover, some superior material properties, such as the high absorption extinction coefficient in the visible-near infrared wavelength region and excellent photocarrier separation/transport characteristics,^2,3 encourage the progressive development of organic-inorganic hybrid perovskite solar cells (PSCs).

After years of development, the power conversion efficiencies of PSCs have been dramatically increased to more than 20%, however, the instability issues are remain the obstacle to commercialization. In this seminar, I will discuss the relationship between interface engineering and the reliability of efficient PSC devices. In addition, I will also illustrate the strategies of designing the device interface and controlling the perovskite grain interface to achieve bendable and durable PSCs.

References
(1) A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050.
(2) Y.-C. Wang, J. Chang, L. Zhu, X. Li, C. Song, J. Fang, Adv. Funct. Mater. 2018, 28, 1706317.
(3) Y.-C. Wang, X. Li, L. Zhu, X. Liu, W. Zhang, J. Fang, Adv. Energy Mater. 2017, 7, 1701144.

Although methyl ammonium lead halide (MAPbX₃) perovskites exhibit unprecedented photovoltaic potential with excellent optical and carrier transport properties; but the toxicity of lead and intrinsic instability are the main issue hampering commercialization of MAPbX₃ solar cells. Therefore, photovoltaic community giving great attention to bismuth based halide perovskite material which has advantages of non-toxicity, ambient stability along with low-temperature solution-processability. Here, we prepared thin films of cesium bismuth iodide (Cs₃Bi₂I₉) materials using novel one-step spray coating method. Spray deposition technique is a versatile coating strategy that can be used to deposit utilitarian films of different materials over large areas at speed. An electric field generated by employing high DC voltage supply while spray deposition method. Applied voltages built accessory electric pressure along with carrier gas throughout atomization of droplets which facilitates further improvement in atomization process from the Coulomb fission process. Spray deposition parameters like the height of the nozzle over the substrate, flow rate of Cs₃Bi₂I₉ solution, substrate temperature and pressure of nitrogen cylinder were optimized. The utilization of electric field during the spray deposition plays an influential role for improving uniformity, morphology, crystallinity, optical and electronic properties of the Cs₃Bi₂I₉ perovskite film and hence enhancement in the photovoltaic parameters of Cs₃Bi₂I₉ solar cell expected.

Perovskite solar cells are especially promising when considering the technology lifespan. The advantages of using perovskite for the active layer include but are not limited to broad a light absorption spectrum, tunable band gaps, long charge carrier diffusion, and low fabrication cost. The current work involves the design, fabrication, and simulation of different configurations of Tin Iodide active layer based lead-free perovskite solar cells. The goal of the project is to develop a configuration that is non-toxic as well as stable in natural open air environments with a specific resistance to moisture. This design incorporates the use of organic and inorganic materials, as well as the efficient use of the perovskite crystalline structure that is produced by using the properties of the active layer. Different economical hole transport layers has also been explored to evaluate the impacts on opto-electronics, and photon-to-current conversion efficiencies. The stacking structure is further modulated within the cell by progressively including a recombination-inhibiting thin TiO2 or a ZnO layer with the FTO, followed by an electron-transport layer (various oxides) and a combination of CuO/Cu as the hole transport layer and back contact electrode. This work is intended to build up to a broader project of selecting the most optimal low-toxicity perovskite cell to stack onto a Si or FTO bottom cell for tandem configurations.

While hybrid organic-inorganic perovskite materials are promising for the next generation of solar cells, their successful commercialization requires development of high throughput manufacturing methods. Vapor transport deposition is a technique already successfully developed and commercialized for CdTe and organic semiconductor materials. It makes use of a carrier gas to transport sublimed precursors to a substrate where they react to form perovskite films. This technique eliminates the need for hazardous solvents in manufacturing while maintaining high deposition rates. In this work, we study the transport of lead iodide and methylammonium in nitrogen carrier gas in a quartz tube furnace as a first step toward developing vapor transport deposition of perovskites. We find that due to the vastly different molecular weight and vapor pressures of the two materials, they require uniquely different control mechanisms for achieving target deposition rate on the substrate and to minimize parasitic deposition. We characterize the co-deposited films using angle dependent x-ray diffraction and scanning electron microscopy to find perovskite formation ratio as a function of depth. We find that due to incorporation of excess available methylammonium, post-deposition thermal annealing is critical for achieving high-quality semiconducting films.

A silicon etched in the form of nanorods or nanoporous is called black silicon. Black silicon shows lower reflectivity over the entire wavelength range of light than pyramidal solar cells. Thus, black silicon can absorb more light than a pyramid texturing solar cell without an antireflective coating. Black silicon has the advantage of significantly reducing production time compared to pyramid texturing. This means that the production cost of the solar cell can be effectively reduced while a high quality silicon solar cell can be manufactured. With a simple structure solar cell process using black silicon, a single solar cell efficiency over 18% is reported.

Passivated Emitter and Rear Contact (PERC) solar cells, which are expected to account for the largest share in silicon solar cells in the future. It includes a dielectric passivation layer on the rear surface of the solar cell. Due to this, the electron recombination of the solar cell is reduced. In addition, the rear passivation layer improves the ability to capture light by reflecting light in the long wavelength region that silicon can not absorb. As a result, the overall solar cell efficiency can be increased.

Perovskite solar cell can adjust the bandgap by controlling the chemical composition of the material, making it ideal for tandemization with silicon solar cells. With the addition of tandemization technology, it is expected to develop solar cells that approach 30% efficiency. For this reason, recent studies on the tandemization of silicon solar cells and perovskite solar cells have been actively conducted.

In this work, we introduce a perovskite/black silicon solar cell. The bottom cell has a black-PERC solar cell structure, which increases the overall Jsc. The PERC rear structure was fabricated using photolithography methods to make electrode back contact opening more accurate, rather than a process that used a laser ablation. This will help reduce surface damage by the laser. The top cell used a perovskite solar cell. High Voc was obtained through the connection of the upper and lower solar cells. It is expected to develop high efficiency solar cells with relatively simple process.

Recently, the development of solar cells using perovskite has been studied for past 10 years. In perovskite solar cells(PSCs), the electron transport layer(ETL) is important component, which extract and convey electrons from perovskites to the cathode. This property has a direct influence on the efficiency of the cell. Since TiO₂ has superior charge extraction and injection characteristics, most of high-efficiency PSCs use TiO₂ and mesoporous TiO₂ as ETL materials. However, TiO₂ has several disadvantages such as vulnerable light stability against ultraviolet (UV) and low electron mobility which requires mesoporous TiO₂ with high-temperature process (~450°C) so that there is limit of using in flexible substrates. As a result, studies on metal oxide as ETL that can be synthesized at low temperature and capable of planar structure have been actively researched. Among them, SnO₂ has high mobility so that it enables fast transport of electrons and hysteresis-free can be obtained even in a planar structure. Also, SnO₂ has a wide bandgap which leads stability against UV, and less photocurrent so that reduces current loss. For these reasons, SnO₂ has been reported to be as efficient as a cell using TiO₂. Moreover, since SnO₂ is easily doped with various materials unlike TiO₂, open circuit voltage(Voc) can be enhanced through bandgap alignment. Not only high efficiency, but also easy doping process could make SnO₂ one of the most promising electron transport materials for planar perovskite solar cells.
Herein, we demonstrated highly efficient PSCs with planar structure with Nb:SnO₂ synthesized at low temperature. Compared with TiO₂-based cells, SnO₂ synthesized at temperature of 210 °C has higher current value but lower Voc. Therefore, Nb doping at various concentration was tested for bandgap alignment which made Voc and fill factor(FF) enhanced. By doping with Nb, the electron traps are passivated, resulting in improved conductivity and mobility. As a result, we achieved the efficiency of 18% at the optimum concentration. In summary, SnO₂ can be applied to flexible solar cells, and it is also advantageous in the production of tandem solar cells because it has been carried out at low temperature process dislike TiO₂. In addition, since it is hysteresis-free in the planar structure, it is excellent in terms of low-cost commercialization.

For commercialization of perovskite solar cell, suppression of ion migration and reduction of trap states is highly needed. Recent works has proposed that 2D perovskites with layered structures can be mixed with 3D perovskites due to its low defect density and superior ambient stability. Despite diversity of long organic cation, correlation of molecular structure and structural features and finally electrical properties have been neglected. Here, we systematically investigated the effect of the size of the phenylalkylammonium ions focusing on charge transport and defect passivation. The results show that defect density was reduced regardless of the type of phenylalkylammonium ion, however carrier accumulation and recombination loss in the perovskite crystals were increased with larger ion. Organocation with stereic hindrance induced tilting of lead-halide octahedra which affected charge collection efficiency. Finally, we could obtain highly efficient and stable (PCE=20.5%) low band gap (1.48eV) formamidinium perovskite solar cell.

With the shortage of energy, developing the alternative energy sources becomes an important issue. Solar energy is considered as one of the most important alternative energy source because it is inexhaustible. Among many types of solar cells, the metal halide perovskite solar cells (PSCs) is the most promising solar cells due to its high power conversion efficiency (PCE). The PCE of PSCs has rapidly increased from 3.8% to 23.3% since 2009. However, the high-efficiency PSCs typically required high-temperature-sintered mesoporous (over than 450^oC) TiO₂ (HT-TiO₂) as electron transport layer (ETL) and the high-temperature to process often limits continues fabrication process. Furthermore, it can’t be used on the flexible substrate and it will spend more energy on fabrication process. Base on above reasons, high-temperature to process is not suitable for commercialization. In this study, all the fabrication processes were under low temperature. The TiO₂ nanocrystals (LT-TiO₂) were synthesized by non-hydrolytic sol-gel method under 150^oC and the spray-coating method was employed for the deposition of LT-TiO₂ ETL. Moreover, we used spray-coating method and blade coating for large area (over 25 cm²) module. In order to go into commercialization, the perovskite active layer should be fabricated in the air. Therefore, we controlled different ratio of perovskite precursor solution (dimethyl sulfoxide:-ButyrolactoneL) to obtain large grain size, high crystalline, pinhole-free and uniform perovskite active layer with blade coating method under various relative humidity. We used scanning electron microscope and atomic force microscope to investigate the correlation between perovskite crystallization and various relative humidity. We also used the photo-assisted Kelvin probe force microscopy (KPFM) to explore the charge transport efficiency in active layer which were prepared under various relative humidity. Toward future commercial application, a large size module is essential. Hence, the 5x5 cm² PSCs module was fabricated based on the low-temperature process. Finally, we successfully developed PSCs without the need of high-temperature sintering process and obtained higher PCE than the mesoporous PSCs. The large area PSCs module also demonstrated the high PCE with the continuous process.

The demand for renewable energy is escalating in recent years due to a dire need to address the global warming. Solar energy is one of the reliable alternative energy resources. Direct conversion of solar energy to electrical energy is a process known as photovoltaic conversion. Si based p-n junction solar cells with efficiency around 22% have captured commercial market despite high cost. Other solar cells based on semiconductor thin films, dye sensitizers, quantum dots, and organic inorganic perovskites are evolving to reduce the solar cell production cost while keeping up the efficiency similar to Si solar cells. Most of these devices are hampered by their long-term stability. In this front, all oxide photovoltaic devices are seems to be promising. Recently, ferroelectric oxides have captured significant attention in solar cell community due to demonstration of existence of unique anomalous photovoltaic effect, in which photovoltage (V_OC) is found to be larger than the bandgap of the materials [1]. Most of the ferroelectric materials have larger bandgap ~ 3 eV which inhibits the material to absorb entire solar spectrum. Short circuit current density (J_SC) observed in ferroelectric oxide based solar cells is in the range of nA to µA which severely limits efficiency of the devices [2].
BiFeO₃ (BFO) is a room temperature ferroelectric oxide which exhibits lower bandgap ~ 2.5 eV in bulk. Bandgap of BFO can be tailored over a wide range from 2.32 eV to 1.5 eV [3], which is promising for solar cell application. In this report we show the effect of oxygen vacancies (O_V) on ferroelectric, photoconductivity and photovoltaic properties of solar cell devices made using Bi_1-xCa_xFe_1-yTi_yO_3-d (BCFTO) [x = y = 0, (BFO); x = 0.1 and y = 0, (BC10FO); x = 0.2 and y = 0, (BC20FO); x = y = 0.1 (BC10FT10O); x = y = 0.2 (BC20FT20O)] thin films. All thin films are coated on FTO-glass substrates by simple spin coating. A mask having circular hole of 2 mm diameter is used to coat Au on top of the films. FTO/BCFTO/Au devices in capacitor configuration are used for photoconductivity and photovoltaic measurements. We observed an enhancement in photoconductivity of 3 orders in BC10FO and 5 orders in BC20FO compared to pristine BFO devices when illuminated with 1 Sun (~100 mW/cm²) power. Enhancement of photoconductivity in BC10FO and BC20FO is attributed to the O_V present in the system. It is observed that J_SC is increased but V_OC is reduced with increasing Ca doping concentration in pure BFO. FTO/BC20FO/Au and FTO/BC10FO/Au devices show J_SC ~ 3 mA/cm² and 30 µA/cm², respectively which are four and two orders larger than the J_SC observed in FTO/BFO/Au devices (~ 0.3 µA/cm²). V_OC observed in pure BFO devices are ranging from 0.35 V to 3 V. In case of BC10FO devices, this lies in between 0.2 V to 0.8 V and it is found to decrease further for BC20FO devices (~ 0.1 V to 0.3 V). A reduction in ferroelectric domain size is observed with increasing Ca content. Ferroelectric switching gets diminished in BC10FO and BC20FO. In contrary, titanium cation co-doping at Fe site strengthens the ferroelectric polarization and ferroelectric switching in BCFTO films. Enhanced photoconductivity and photovoltaic property are evidenced in BCFTO devices. High solar cell efficiency 0.5% are achieved with devices exhibiting V_OC (~ 0.8 V) and J_SC (~ 2 mA/cm²). BCFTO devices with high V_OC and J_SC are easily fabricable, stable and low fabrication cost. Therefore, ferroelectric oxide solar cells are very promising as a commercially viable devices.

References:
[1] S. Y. Yang et al. ‘Above-bandgap voltages from ferroelectric photovoltaic devices’, Nat. Nano 5, 143 (2010).
[2] Narimichi Makino et al. ‘Photocurrent properties of BiFeO₃ thin films prepared by chemical solution deposition’, Ferroelectrics, 453:20-25 (2013)
[3] Pavana S. V. Mocherla et al. ‘Tunable bandgap in BiFeO₃ nanoparticles: The role of microstrains and oxygen defects,’ Appl. Phys. Lett., 103, 022910–022915 (2013).

The efficiency of Perovskite solar cells (PSC) has increased from 3% to 22.1% in less than eight years. Because of its rapid increase in efficiencies and low cost fabrication process, the perovskite solar cell technology has been studied intensively and was named as one of the biggest scientific breakthroughs in the year of 2013 by Science. In the PSC structure, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) is usually used as the hole transport layer (HTL) on top of ITO. It was found that interface engineering at electrodes is very important for increasing the performance of PSCs.

In this work, we show our study on how the efficiency of the Perovskite solar cells depend on the conductivity and work function of PEDOT:PSS. A series of PEDOT:PSS solutions with different conductivity and work function were used to make the Pervoskite solar cells. We will also report the interface processing we did in the p-i-n structure PSCs, which optimize device properties and charge transfer. In this work, all materials were prepared at low temperatures, showing potential for low-cost, scalable production of large-scale perovskite solar cells on flexible substrates.

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 such as flexible device, module, lead-free material, and high voltage are focused as next targets of researches. Among the various targets, we focused on a high voltage perovskite solar cell. To realize “High voltage”, wide band gap perovskite consisting of lead-bromide perovskite is often employed. The lead-bromide have approximately 2.2 eV band gap and is expected to output over 1.5 V of open-circuit voltage (V_OC). However, V_OC is highly sensitive to defects, grain boundary, and interface problems. Therefore, preparation of high quality perovskite crystalline film is required.
We have developed high quality FAPbBr₃ perovskite films by means of sulfoxide additives and applied to mesoscopic PSCs. We prepared the FAPbBr₃ film by sequential deposition method from a PbBr₂ precursor film. We added the additive into the precursor PbBr₂ solution and the additive significantly changed morphology of the resulting PbBr₂ film. The PbBr₂–additive film gave high quality perovskite films. Thickness of the perovskite film became thicker compared to the reference perovskite film prepared from pristine PbBr₂ film. By optimization of the preparation condition, the highest V_OC value reached to 1.5V, and after three week ageing treatment, the V_OC increased to 1.55 V, resulting the best conversion efficiency of 7.8% was obtained.

This investigation has applied simulation work to elucidate the photovoltaic performance of non-Pb based perovskite devices, utilizing Sn. In an effort to address additional key criteria that impede the commercial realization of this technology, we have investigated Cu2O (instead of the traditional spiro-OMeTAD) as an HTL that is cheap and easily synthesized, deposited and fabricated to form a fully functional device. Also, in an effort to reduce cost, in place of Ag as the back electrode, we have incorporated Cu. By optimizing each layer thickness, this work showcases not only a maximal efficiency device, but by critical analyses of Jsc, Voc and FF trends that manifest due to each component layer thickness modulation, we present key charge mobility and transfer kinetics within the structure of the cell. These optimal numbers and trends are critical when fabricating a physical device in the lab, and even more so once these components are investigated as viable ingredients in an industrial setting.

In this research, we have utilized simulation studies to probe non-toxic Sn-based perovskites with a goal to explore optimal fabrication methodologies. To that end, we have investigated a device structure with and without an ETL and assessed the impact on critical photovoltaic parameters. Our ETL of choice has been the traditional TiO2 matrix, which has a wide bandgap, and good transport properties. We have also in-situ performed optimization of individual layer thicknesses, thereby ultimately fabricating a device with the highest possible theoretical efficiency. The analyses provide critical insight into charge mobility and charge transfer kinetics within this non-Pb based device; by observing the effects on Jsc, Vcc, and FF as individual layer thicknesses are modulated, and doing comparisons of devices with and without an ETL, we present results of a structure that is not only optimal from an efficiency standpoint, but also from a technology viability and sustainability standpoint.

In this research, we have utilized simulation studies to probe non-toxic Sb-based perovskites with a goal to explore optimal fabrication methodologies. To that end, we have investigated a device structure with and without an ETL and assessed the impact on critical photovoltaic parameters. Our ETL of choice has been ZnO – which has a wide bandgap, and good transport properties. We have also in-situ performed optimization of individual layer thicknesses, thereby ultimately fabricating a device with the highest possible theoretical efficiency. The analyses provide critical insight into charge mobility and charge transfer kinetics within this non-Pb based device; by observing the effects on Jsc, Vcc, and FF as individual layer thicknesses are modulated, and doing comparisons of devices with and without an ETL, we present results of a structure that is not only optimal from an efficiency standpoint, but also from a technology viability and sustainability standpoint.

The power conversion efficiency (PCE) of perovskite solar cells (PSC) continuously break new records; however, there are still big challenges for large perovskite fabrication. Two of these being the solar cell’s grain growth and stability. In this investigation, Urea additions to the perovskite precursor solution prepared using lead acetate precursors show improved the device performance. The results show that the best content of Urea is 0.5 wt. %, when compared with the samples with and without Urea. The presence of Urea in the PSC increases the PCE of from 15% to 17%. X-ray diffraction analysis indicates that the addition of Urea promotes grain growth and also improves the grain’s orientation. This study reveals that the PCE is also closely related to the anneal time of the perovskite layer. Our findings show that the addition of Urea and proper anneal times enhance the crystallization process of the perovskite films and results in the resulting in larger and more orientated grains. This is the reason for the device’s improved carrier transport and conversion efficiency.

Hole transporting layer (HTL), where photogenerated holes are extracted from perovskite and transported to the back-metal contact, plays an important role in enhancing the power conversion efficiency (PCE) as well as improving the stability, which is a major bottleneck for commercialization of perovskite solar cell (PSC). Currently, the state-of-the-art PSCs use the small molecule 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-spirobifluorene (Spiro-OMeTAD) or poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine](PTAA) doped with common additives, such as Li-TFSI and tBP, which usually cause a serious degradation of device performance. Therefore, the development of dopant-free HTL becomes an essential requirement in the field of PSC. In this work, we designed and synthesized small-molecule-based dopant-free hole transporting material (HTM) called α, β-COTh-OMeTAD by fusing two strong electron donating moieties; saddle-shaped cyclooctatetrathiophene (COTh) and triphenylamine, to obtain sufficiently high hole transporting properties without additional dopants. The α, β-COTh-OMeTAD exhibits a deep highest occupied molecular orbital energy level (HOMO) of -5.3 eV and a high hole mobility of 2.88 × 10^-4 cm²V^-1s^-1. The thermal decomposition temperature of this small molecule is 308 °C and the phase transition temperature is 121.5 °C, showing a good thermal stability. It also exhibits a good solubility in several common organic solvents, which is favourable for essay processability and device fabrication. The solar cell fabricated with our small molecule HTM in the structure of FTO/compact TiO₂/mesoporous TiO₂/perovskite/HTL/Au gives a PCE of 15.66% with J_SC = 21.05 mA cm^-2, V_OC =1.04 V and FF=71.79%, which is comparable to the device based on doped spiro-OMeTAD as HTL, whose PCE is 16.69% (J_SC=20.24 mA cm^-2, V_OC=1.10 V, FF=74.62%).
In addition to functioning as HTM, α, β-COTh-OMeTAD also serves as an interfacial layer to passivate the grain boundaries of perovskite crystals. During the crystallization of perovskite film, solvent such as chlorobenzene (CB) or toluene is usually poured into the perovskite precursor solution under spinning to produce a uniform and high-quality perovskite film. Herein, we observed that incorporating a small amount of α, β-COTh-OMeTAD (10 mg/mL) in CB anti-solvent further improves the efficiency of solar cell. Using α, β-COTh-OMeTAD as both interfacial layer and HTL improves the PCE to 17.22% with J_SC = 21.51 mA cm^-2, V_OC = 1.05 V and FF=76.37%. SEM image of perovskite film treated with α, β-COTh-OMeTAD exhibits less apparent grain boundaries and denser film because of the passivation effect. Time resolved photoluminescence (TRPL) measurement also reveals that the faster decay lifetime τ₁ which corresponds to the quenching of the photogenerated free carriers transporting from the perovskite to HTL becomes shorter but the slower decay lifetime τ₂ which represents the radiative recombination of free carriers becomes longer, suggesting the efficient charge transfer between the perovskite and HTL and good passivation effect of the α, β-COTh-OMeTAD. Besides the hole transporting and passivating properties, α, β-COTh-OMeTAD also improves the stability of the device due to the hydrophobic nature of the material which prevents moisture from penetrating into the perovskite film, and deep HOMO level which prevents from reaction with iodine in the perovskite compound.
We also employed the commercially available conjugated polymer called PDTBDT-FBT in a separate study. Different from α, β-COTh-OMeTAD, the PCE of device is very low when PDTBDT-FBT is used as HTL but it is functioning well as interfacial layer to passivate the perovskite film. The PCE of 18.03% is obtained when PDTBDT-FBT dissolved in CB is used as anit-solvent and spiro-OMeTAD as HTL.

Perovskite solar cells are especially promising when considering the technology lifespan. The advantages of using perovskite for the active layer include but are not limited to broad a light absorption spectrum, tunable band gaps, long charge carrier diffusion, and low fabrication cost. The current work involves the design, fabrication, and simulation of different configurations of Tin Iodide active layer based lead-free perovskite solar cells. The goal of the project is to develop a configuration that is non-toxic as well as stable in natural open air environments with a specific resistance to moisture. This design incorporates the use of organic and inorganic materials, as well as the efficient use of the perovskite crystalline structure that is produced by using the properties of the active layer. Different economical hole transport layers has also been explored to evaluate the impacts on opto-electronics, and photon-to-current conversion efficiencies. The stacking structure is further modulated within the cell by progressively including a recombination-inhibiting thin TiO2 or a ZnO layer with the FTO, followed by an electron-transport layer (various oxides) and a combination of CuO/Cu as the hole transport layer and back contact electrode. This work is intended to build up to a broader project of selecting the most optimal low-toxicity perovskite cell to stack onto a Si or FTO bottom cell for tandem configurations.

While hybrid organic-inorganic perovskite materials are promising for the next generation of solar cells, their successful commercialization requires development of high throughput manufacturing methods. Vapor transport deposition is a technique already successfully developed and commercialized for CdTe and organic semiconductor materials. It makes use of a carrier gas to transport sublimed precursors to a substrate where they react to form perovskite films. This technique eliminates the need for hazardous solvents in manufacturing while maintaining high deposition rates. In this work, we study the transport of lead iodide and methylammonium in nitrogen carrier gas in a quartz tube furnace as a first step toward developing vapor transport deposition of perovskites. We find that due to the vastly different molecular weight and vapor pressures of the two materials, they require uniquely different control mechanisms for achieving target deposition rate on the substrate and to minimize parasitic deposition. We characterize the co-deposited films using angle dependent x-ray diffraction and scanning electron microscopy to find perovskite formation ratio as a function of depth. We find that due to incorporation of excess available methylammonium, post-deposition thermal annealing is critical for achieving high-quality semiconducting films.

Metal halide perovskites have attracted tremendous attentions owing to their remarkable optoelectronic properties and shown great potential in photodetector applications. However, the large dark current and environmental instability have significantly hindered the reliable device operation and their potential application in photodetection. In this work, we fabricated lead-halide perovskite and two-dimensional (2D) phosphorus doped graphitic carbon nitride (P-C₃N₄) hybrid thin films for high-performance visible light photodetectors. First, 2D P-C₃N₄ was synthesized by a facile thermal polymerization process using low-cost precursors and the band gap was reduced from 2.91 eV for the pristine graphitic carbon nitride to 2.77 eV, which is feasible to form a type-II junction with perovskites. Then 2D P-C₃N₄ was hybridized with lead-halide perovskites through a solution-processing method and the photodetection performance of hybrid thin films was examined. Most importantly, the dark current was significantly decreased in the hybrid films from ~10⁻⁹ A for perovskite-based devices to ~10⁻¹¹ A due to the bulk heterojunction between P-C₃N₄ and perovskites. As a result, the on/off ration of the hybrid based devices increased up to 10⁵ from 10³ and the photodetectivity enhanced by more than one order of magnitude, up to 10¹³ Jones. Meanwhile, the responsivity reached an elevated value of 14 A/W. Moreover, the addition of 2D P-C₃N₄ significantly modified the hydrophilicity and morphology of the perovskite films, which dramatically increased the stability of the devices. The hybrid photodetectors described in the present study present a promising direction toward the future low-cost and high-performance optoelectronic thin film device applications with solution-processed perovskites.

Over the past decade, perovskite solar cells (PSCs) have received great interest in solar cell research field because of their high efficiency and low cost. Nowadays, PSC has reached up to 23% of efficiency which makes it a next-generation photovoltaic material. Perovskite solar cells can be attractive owing to the synergetic advantages of the long diffusion length, long minority carrier lifetimes and high absorption coefficient. However, despite the simple process advantages of solution-based fabrication, there is a growing need for new ways to increase the size of solar cells for mass-production and commercialization. Even though there have been many attempts to up-scale the perovskite solar cells (such as spin-coating, flash evaporation, gas-induced method, doctor blading, etc.), still vacuum deposition method is achieving the highest efficiency of over 20%, while other methods having around ~10%. By controlling the partial pressure and deposition rate of metal halides and organic halides during co-evaporation, high quality perovskite layer with the optimum optical bandgap and stoichiometry could be achieved. Not only for upscaling, but also for high efficiency multi-junction solar cell application through conformal coating on micro pyramid Si surface is possible with vacuum-process which leads to higher current density.
In this work, we suggest PSCs with high quality MAPbI3 perovskite thin film made by co-evaporation method. The optimization was fulfilled in many ways; controlling the stoichiometry by amount of each precursor, deposition rate, substrate heating during deposition, and etc. For uniform MAPbI3 thin film layers on ETL, we increased substrate temperature up to 70°C which showed the highest fill factor (0.70) and open circuit voltage (1.02V). The vacuum-processed MAPbI3 with substrate annealing had very uniform thickness on 10 x 10 substrate and also reproducible thickness of ~330nm which showed the highest power conversion efficiency. Also, we used e-beam processed inorganic Nb2O5 ETL layer to enhance the total stability of the solar cell. By controlling the oxygen vacancy of the Nb2O5, we achieved the optimum efficiency of 16%. The vacuum-processed perovskite and carrier transport layer is thus very suitable for highly efficient and stable multi-junction solar cells.

Recently, inorganic lead halide perovskite nanocrystals show great potential in infrared, visible and multispectral photodetectors (PD) for sensing, security and electronics applications. Considerable progress has been obtained through solution-processed fabrication technique which combines ease of processing, tailorable optoelectronic properties, facile compatibility with flexible structures. However, the stability of the nanocrystals in ambient condition has always been a great challenge in as-prepared PDs. In this work, an air stable organic/ inorganic hybrid lead halide perovskite nanocrystalline PD has been obtained from full solution processes, using CsPbBr₃ quantum dots (QDs) as the photoactive layer, Poly(N,N'-bis-4-butylphenyl-N,N'-bisphenyl)benzidine (poly-TPD) and phenyl-C61-butyric acid methyl ester (PCBM) as the electron transport/hole blocking layer (ETL/HBL) . The resulting device has significantly suppressed dark current as low as 10 nA/cm². The responsivity of greater than 1 A●W^-1 was obtained for this unique hybrid structure, and it has been approved up to one order of magnitude enhancement when compared with the device without the organic combination layers of poly-TPD and PCBM. Especially, these as-prepared devices remain stable for more than one week even without encapsulation. In this solution process, hydrofluoroethers (M7100) was employed as an effective protective layer for QD nanocrystals. Further, we systematically studied the influence of different types of organic carrier transport layers on device performance.

This talk will focus on the dynamic optical and electrical behavior of perovskites, from the macro- to the nanoscale. We investigate a series of hybrid perovskites, including MAPbI₃, MAPbBr₃, CsxFA1−xPb(IyBr1−y)3 , and triple cation Cs-mixed. To elucidate the contribution of humidity, oxygen, temperature, light, and bias on materials’ properties and devices’ performance, we combine advanced scanning probe methods based on photoluminesce (PL) microscopy and atomic force microscopy (AFM). Through environmental PL microscopy we identify a humidity-induced PL hysteresis that strongly depends on the Cs/Br ratio [1]. Using fast Kelvin-probe force microscopy we quantify a dynamic open-circuit voltage (Voc) response as a function of perovskite chemical composition and illumination treatment, as will be discussed in details during the presentation [2,3]. The individual and combined effects of each intrinsic and extrinsic parameter is analyzed in terms of a machine learning approach (ML), key to elucidate the device conditions that can prevent degradation, and enable rest and recovery. Our functional imaging platform [4-6], combined with ML [7], can be leveraged to identify the recovery conditions of emerging perovskites, including Pb-free options.

References:
[1] J. M. Howard et al. J. Phys Chem Letters 9, 3463 (2018)
[2] J. L. Garrett et al. Nano Letters 17, 2554 (2017).
[3] E. M. Tennyson et al. ACS Energy Letters, 2, 2761 (2017). Invited Review
[4] E. M. Tennyson et al. ACS Energy Letters 2, 1825 (2017). Invited Perspective
[5] E. M. Tennyson et al. Advanced Energy Materials 5, 1501142 (2015).
[6] J. L. Garrett et al. ACS Appl. Materials & Interfaces 10, 28850 (2018)
[7] J. M. Howard et al., in review (2018)

Perovskite LEDs have now exceeded external quantum efficiencies of 20% in multiple spectral regimes. I will discuss progress in the synthesis, materials processing, interface engineering, and device design that has enabled these advances; and will review remaining challenges and paths forward for the field.

Charge transport layers (CTLs) are key components of diffusion controlled perovskite solar cells, however, they can induce additional non-radiative recombination pathways which limit the open circuit voltage (V_OC) of the cell. In order to realize the full thermodynamic potential of the perovskite absorber, both the electron and hole transport layer (ETL/HTL) need to be as selective as possible. By measuring the quasi-Fermi level splitting (QFLS) of perovskite/CTL heterojunctions [1], we quantify the non-radiative interfacial recombination current for a wide range of commonly used CTLs, including various hole-transporting polymers, spiro-OMeTAD, metal oxides and fullerenes [2]. We find that all studied CTLs limit the V_OC by inducing an additional non-radiative recombination current that is significantly larger than the loss in the neat perovskite and that the least-selective interface sets the upper limit for the V_OC of the device. The results also show that the V_OC equals the internal QFLS in the absorber layer of pin and nip-type cells with selective CTLs and power conversion efficiencies of up to 21.4% (in pin). However, in case of less selective CTLs, the V_OC is substantially lower than the QFLS which indicates additional losses at the contacts and/or interfaces. The findings are corroborated by rigorous device simulations which outline several important considerations to maximize the V_OC. This work shows that the real challenge to supress non-radiative recombination losses in perovskite cells on their way to the radiative limit lies in the suppression of carrier recombination at the perovskite/CTL interfaces.
[1] Stolterfoht et al. Visualization and suppression of interfacial recombination for high-efficiency large-area pin perovskite solar cells. Nat. Energy 3, 847–854 (2018).
[2] Stolterfoht et al. submitted for publication. Preprint at http://arxiv.org/abs/1810.01333.

Perovskite solar cells employing an ‘inverted’ structure (iPSC), where a p-type charge selective layer is typically deposited first, have many advantages over ‘normal’ structured devices. The use of low-cost and dopant-free charge selective layers such as PEDOT:PSS, NiOx, C₆₀ and PCBM in iPSCs make them an attractive option for commercial scale-up, while still surpassing the psychological power-conversion efficiency (PCE) barrier of 20%. Almost all reported devices in this configuration use fullerene-derivatives such as C60 or PCBM as their electron transporting layer. While such materials boast excellent electron mobilities and are solution processable, the Schottky barrier formed between organic semiconductor and metal electrode poses a severe barrier to high performance. This barrier is often reduced by the addition of thin tunnel junctions comprising of organic or inorganic materials. However, the addition of these layers can often increase the series resistance within the solar cell and, in many cases, do not remove the energy barrier completely leading to a limitation of the device’s potential performance. This barrier issue is present not only for iPSCs but also organic solar cells, organic light emitting diodes and field-effect transistors.

Here, we present a new interfacial bilayer which dramatically reduces the Shottky barrier between semiconductor and metal in photovoltaic devices. In perovskite solar cells, PCBM electron-transporter is coated with a Nb-doped TiOx nanoparticle layer followed by a layer of bathocuproene (BCP) before metal electrode deposition. We find that Nb doping of TiOx passivates trapped states commonly found in pristine TiOx, while BCP chelates with both the oxide nanoparticles and metal electrode to form a near perfect ohmic contact between PCBM and metal cathode. The resulting solar cells exhibits fill factors in excess of 84% - among the highest reported in any photovoltaic technology, with accompanying PCEs over 20%. We characterize this bilayer both in terms of its crystallographic and morphological nature, as well as electronically in solar cells, electron-only devices and thin-film transistors.

In addition, the universality of this method is exemplified in organic photovoltaics (OPVs) where we present fullerene-free, p-i-n ‘normal’ structure devices with fill factors of 71% and PCEs over 13%. This interfacial bilayer replaces a highly reactive and unstable Ca/Al electrode with a more stable Ag one, further improving device stability and yielding the highest performing OPV using such a structure.

This development of an approach to entirely remove electronic barriers between materials paves the way for ultra-high efficiency solar cells thanks to unprecedented fill factor improvements nearing the theoretical limit, while having implications in fields beyond that of photovoltaics.

Within the past few years, metal halide perovskites have been attracting significant interest due to their and their versatile use in a wide range of applications. These materials have been used in lasers, photodetectors, and most commonly, in photovoltaic devices and light emitting diodes. Despite the cheap and simple fabrication methods by which these materials are deposited, the resulting perovskite films are effectively high-quality semiconductors, and the power conversion efficiencies of lead halide perovskite solar cells are now exceeding certified values of 23%. However, perovskite-based devices are yet to achieve their full potential. One of the major hindrances to achieving this is an incomplete understanding of perovskite surfaces and interfaces. Deficiencies at these interfaces may be responsible for the largest losses in perovskite-based optoelectronic devices; limiting charge extraction, increasing non-radiative recombination rates and leading to hysteresis, and significantly increasing the voltage loss in perovskite photovoltaics. Herein, we will present interface modification strategies to mitigate these deficiencies. We utilise charge-transfer dopants to dope the perovskite at the interface, resulting in the formation of narrow homojunctions. These homojunctions result in reduced interfacial recombination, suppressed hysteresis and improved device performance, yielding steady-state device efficiencies of over 21%. We also explore the use fluoride-containing ionic liquids at the metal-oxide perovskite interface and show that not only do they affect the work function of the metal oxide, but also interact strongly with the perovskite, significantly improving the quality of the perovskite film. The utility of these interface modification strategies extends beyond perovskite solar cells and can also be used to further improve the performance of perovskite-based light-emitting diodes.

Perovskites solar cells have irrupted in the photovoltaic field with no precedent. Rising the power conversion efficiencies (PCE) from ~4% to 23.2%, can be consider as one possible substitute to silicon solar cells. However, long term stability together with the lack of a fully understanding of the intrinsic processes are still issues which need to be undertaken. To overcome those problems, different strategies such as compositional engineering of perovskite material as well as new synthesized hole transport materials (HTMs) were accomplished. Furthermore, in order to reduce internal losses, an optimized interface between the different layers of the perovskite device is paramount. In this regard, an optimized surface passivation layer is seen as an ideal approach to protect the surface from extrinsic factors, without altering the electro-optical properties.
In this work, we present the utilization of a passivation layer, which protect the perovskite layer from atmospheric degradation as well as compensate the possible iodine segregation on the perovskite layer. As result, an improved PCE (20%) was also obtained compare with reference devices (18%), as the passivation layer will diminish trap-states towards the HTM layers, reducing interfaces recombination processes and therefore improving the open circuit voltage.

Metal halide perovskite materials have been the focus of intense investigation due to their low-cost and superior photovoltaic performance, with record power conversion efficiencies exceeding 23.3%[1]. Despite numerous advantages, their poor stability hinders commercialization of perovskite-based devices. To increase perovskite stability various strategies have been envisaged[2]. Mixing different halides (I, Br, Cl) has been shown to have a strong impact on the device performance and stability[3-6]. However, the stabilizing effect of the halides critically depends on their distribution in the mixed compound, a topic that is currently under intense debate [7-8]. A fundamental understanding remains largely elusive regarding the correlation between the structure of the mixed-perovskites and their electronic properties and stability.
In this work, combining scanning tunneling microscopy (STM), density functional theory (DFT) and UV/X-ray photoelectron spectroscopy (UPS/XPS), we reveal the exact location of I and Cl anions in the mixed CH₃NH₃PbBr_3-yI_y and CH₃NH₃PbBr_3-zCl_z perovskite lattices. Additionally, we demonstrate the impact of halide-incorporation on the material electronic properties and stability. Furthermore, we determine the ideal Cl-incorporation ratio for stability increase without detrimental bandgap modification. The increased material stability induced by chlorine incorporation is verified by performing photoelectron spectroscopy on a device architecture. Our findings provide an important direction for the fabrication of stable perovskite devices.
References:
[1] National Renewable Energy Laboratory (NREL). http://www.nrel.gov/ncpv/images/efficiency_chart.jpg.
[2] Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. I., Chemical Management for Colorful, Efficient, and Stable Inorganic–Organic Hybrid Nanostructured Solar Cells. Nano Letters 2013, 13 (4), 1764-1769.
[3] Liu, J.; Prezhdo, O.V. J. Phys. Chem. Lett. 2015, 6 (22), 4463.
[4] Quarti, C.; Mosconi, E.; Umari, P.; De Angelis, F. Inorg. Chem. 2017, 56, 74.
[5] Colella, S.; Mosconi, E.; Pellegrino, G.; Alberti, A.; Guerra, V.L.P.; Masi, S.; Listorti, A.; Rizzo, A.; Condorelli, G.G.; De Angelis, F.; Gigli, G. J. Phys. Chem. Lett. 2014, 5 (20), 3532.
[6] Yu, H.; Wang, F.; Xie, F.; Li, W.; Chen, J.; Zhao, N. Adv. Funct. Mater. 2014, 24, 7102.
[7] Ye, M.; Hong, X.; Zhang, F.; Liu, X. J. Mater. Chem. A 2016, 4, 6755.
[8] Zhang, T.; Yang, M.; Benson, E.E.; Li, Z.; Lagemaat, J.; Luther, J.M.; Yan, Y; Zhu, K; Zhao, Y. Chem. Commun. 2015, 51, 7820.

In less than 10 years, hybrid organic-inorganic perovskites have emerged as a new generation of absorber materials for high-efficiency and low-cost solar cells [1], [2]. More recently, fully inorganic perovskite quantum dots (QD) also led to promising efficiencies [3], [4] and then become a serious alternative to hybrid organic-inorganic perovskites. Currently, the record efficiency for QD solar cells is obtained with CsPbI₃. High resolution in-situ synchrotron XRD measurements have been performed on CsPbI₃ as a function of the temperature and revealed a highly anisotropic variation of the lattice parameters. Moreover, CsPbI₃ can be temporarily maintained in a perovskite-like structure down to room temperature, stabilizing a metastable perovskite polytype (black-phase) crucial for photovoltaic applications. Structural, vibrational and electronic properties of the three experimentally observed black phases are further scrutinized using theoretical approaches revealing the importance of the anharmonicity of the lattice [5], [6]. A symmetry-based tight-binding model, calibrated with self-consistent GW calculations including spin-orbit coupling, affords further insight into their electronic properties. A Rashba effect is thus predicted for both cubic and tetragonal phases when using the symmetry breaking structures obtained through frozen phonon calculations.

The ab initio simulations have been performed on HPC resources of CINES under the allocation 2017-[x2017096724] made by GENCI (Grand Equipement National de Calcul Intensif).

[1] A. Kojima, et al., Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J.
Am. Chem. Soc. 2009, 131, 6050−6051.
[2] Best research-cell efficiencies; https://www.nrel.gov/pv/assets/images/efficiency-chart.png (accessed Nov 7, 2017).
[3] H. Bian et al., Graded Bandgap CsPbI_2+xBr_1-x Perovskite Solar Cells with a Stabilized Efficiency of 14.4%, Joule, 2018, 2, 1500-1510
[4] E. M. Sanehira, et al., Enhanced Mobility CsPbI₃ Quantum Dot Arrays for Record-efficiency, High-voltage Photovoltaic Cells. Sci. Adv. 2017, 3, eaao4204.
[5] A. Marronnier et al., Structural Instabilities Related to Highly Anharmonic Phonons in Halide Perovskites. . J. Phys. Chem. Lett. 2017, 8, 2659−2665
[6] A. Marronnier et al., Anharmonicity and Disorder in the Black Phases of Cesium Lead Iodide Used for Stable Inorganic Perovskite Solar Cells. ACS Nano 2018, 12, 3477−3486

The so-called one-step and two-step sequential deposition methods are the most used in the fabrication of perovskite films suitable for deployment within solar cell structures. The one-step deposition approach relies on mixed metal and halide precursor solutions being processed under stringent conditions to control the extent of precipitation and, thus, to ensure high quality perovskite thin films. Whilst useful for small-scale devices, this approach is challenging to scale, which has prompted us to further explore the two-step method. In two-step sequential deposition, a high-quality perovskite film can be achieved by exposing a neat lead halide film to a solution of an appropriate halide salt. In this work, we explore the role of solvent engineering to achieve microstructural control of the lead iodide (PbI₂) layer such that it is suitable for two-step sequential deposition. Through this modification, the diffusion of methylammonium iodide (MAI) into the structure to form methylammonium lead iodide perovskite during the second step is accelerated. Moreover, it enables for the formation of ultra-smooth thin perovskite films with large grains and photo-conversion efficiencies (PCE) of > 15% under planar configurations. The versatility of the approach is demonstrated also for all-inorganic CsPbBr_xI_3-x, with PCEs of > 10%.

Widespread adoption of renewable solar power requires the production of solar cells of improved power conversion efficiency at low cost. As conventional single-junction silicon solar cells approach their fundamental limits in efficiency, the use of tandem solar cells based on multiple semiconductors to absorb complementary parts of the solar spectrum offers a proven way to increase efficiency past 30%. Metal halide perovskite semiconductors, with their strong absorption, tunable band gaps, and defect tolerance under a variety of low-temperature processing methods offer the promise of fabricating such tandem solar cells at low cost. Specifically, all-perovskite tandems, which use perovskites for all active absorbers in the stack, promise to attain high efficiencies while keeping these benefits for all subcells.

Over 20% efficient monolithic tandem solar cells have recently been demonstrated using a lead iodide-bromide perovskite for the wide-gap absorber and a tin-lead iodide perovskite for the low-gap absorber. While the use of tin at the B-site of the ABX3 perovskite is crucial to achieving low band gaps required of the rear subcell, tin-containing perovskite solar cells have conventionally been thought to be unstable due to the propensity of tin to oxidize upon exposure to air. We show that tin can be drastically stabilized by alloying with lead, which blocks the facile oxidation pathway that pure tin perovskites undergo. We identify two different reaction pathways by which tin-containing perovskites can be oxidized. The pathway that mixed tin-lead perovskites (with 50% tin or lower at the B-site) are forced to take involves breaking more Sn-I bonds, which makes it unfavorable, therefore stabilizing these perovskites toward oxidative breakdown.

We show that PEDOT:PSS, the hole transporter most commonly used so far for tin-containing perovskites, adversely reacts with them upon aging at elevated temperature (85 C). This results in poor thermal stability, with charge extraction being severely reduced upon 100 - 200 hours of aging at 85 C. By designing a heterojunction with upward band bending in the perovskite, we achieve selective extraction of holes without using the unstable PEDOT:PSS.

We proceed to study the effect of morphology on the rate of oxidation, and find that increasing compactness and size of nanoscale morphological domains significantly improves stability in air. Combining these results, we fabricate a mixed tin-lead perovskite using a hole-selective heterojunction, processed in a way that yields a compact and large-grained film, and show that this device maintains 95% of its initial efficiency after 1000 hours of aging at 85 C in ambient air without any additional encapsulation, and maintains 95% efficiency after 1000 hours of aging in damp heat (85 C with 85% relative humidity) with encapsulation. Further, the solar cell stays above its initial efficiency when operated at maximum power point under illumination for over 1000 hours. These results represent the first demonstration of the benchmark 1000-hour stability tests being passed by low band gap tin-lead perovskites. Our work removes the main perceived hurdle to the implementation of single junction and tandem solar cells based on tin-lead perovskites.

Perovskite solar cells (PSCs) have become a competitive photovoltaic (PV) technology with rapid progress of efficiencies reaching above 23%. Uniquely, PSCs have the highest efficiencies when they are solution processed, so one can envision solar cells printed in a similar manner and scale as newspapers. Bandgap tunability through perovskite composition engineering is attractive for developing high-efficiency multijunction devices, including perovskite/perovskite, perovskite/silicon, or perovskite/thin-film absorber (e.g., CIGS). In this talk, I will discuss our recent progress in the development of perovskite-based tandem solar cells. For wide-bandgap (~1.7–1.8 eV) PSCs, the challenge is to reduce the voltage deficit. Developing strategies to reduce defect density is critical. For low-bandgap (~1.2–1.3 eV) PSCs based on Sn-Pb mixed perovskite films, key challenges lie at both the material and device levels. At the material level, the high defect density associated with oxidation of Sn²⁺ to Sn⁴⁺ and formation of Sn vacancies significantly limits carrier lifetime and charge collection. At the device level, it is difficult to synthesize uniform, pinhole-free, Sn-based thin films because of the rapid crystallization of Sn-based perovskite films. I will show our recent progress on solution chemistry engineering to reduce defect density in perovskites with a wide composition range covering both low- and wide-bandgap perovskite films. The precursor chemistry and growth conditions affect significantly the physical and optoelectronic properties of perovskites. Finally, I will also discuss some of our recent efforts on upscaling PSCs. These results demonstrate a promising path towards commercialization of the perovskite photovoltaic technology.

Thin film tandem photovoltaic (PV) devices fabricated with commercially viable Cu(In,Ga)Se₂ (CIGS) device as narrow bandgap bottom cell and emerging perovskite technology devices as wide bandgap devices for the top cell can offer low-cost and environmentally friendly solution processed route towards high efficiency devices. Another advantage in perovskite/CIGS tandems lies in variability of both top and bottom cell bandgaps. While multiple experimental and computational studies have optimized the perovskite/silicon tandem solar cells, the option of variability in the bottom cell have been overlooked. In this study we have simulated performances of the perovskite/CIGS tandem solar cells in two-terminal and four-terminal configuration while varying both top and bottom cell bandgaps and perovskite layer thickness. To this end we use actual measured refractive indices for all the layers including the absorber layers within transfer matrix method calculations to determine the optimal bandgap combinations for the top and bottom cells. These devices are also examined for their energy yields in the context of real-world conditions. For this part of the study we generate annual hourly solar irradiance spectra using updated National Renewable Energy Laboratory’s typical meteorological year data while accounting for sunlight’s angles of incident, PV device temperature and anisotropy in diffused horizontal irradiance for multiple locations in the USA. Our results show that certain combination of bandgaps for top and bottom cells do not perform as well under real-world conditions as they do for laboratory conditions. We explore the monthly spectral shifts due to seasonal climatic changes and demonstrate correspondence between the shifts in the solar irradiance spectra and functions of tandem devices.

Vacuum-deposition of halide perovskites and charge-selective materials enables a wide range of variations in the device architecture [1]. This technique permits an accurate control on the layer thickness and stacking sequence, which is particularly suitable for applications such as tandem devices. Here, we use the intrinsically additive nature of vapor-based processes to carry out a detailed study of the solar cell interfaces and perovskite optoelectronic properties. We analyze the main processes determining the performance of both nip [2] and pin configurations [3]. The use of optical and electrochemical spectroscopies allows distinguishing the role of band alignment and recombination, and how these affect the open circuit voltage and overall performance of the devices. As a result, CH3NH3PbI3-based solar cells with open circuit potential close to the thermodynamical limit and power conversion efficiency >20 % are achieved.
This work presents an outlook of the main processes determining the performance of vacuum-deposited solar cells, which can be crucial to developing the ideal system based on this industrially mature technique.
References
[1] Jorge Avila, Cristina Momblona, Pablo P. Boix, Michele Sessolo, and Henk J. Bolink, “Vapor-Deposited Perovskites: The Route to High-Performance Solar Cell Production?”, Joule. (2017), 1(3), pp. 431–442.
[2] Daniel Perez-del-Rey, Pablo P. Boix, Michele Sessolo, Afshin Hadipour, and Henk J Bolink, “Interfacial Modification for High-Efficiency Vapor-Phase-Deposited Perovskite Solar Cells Based on a Metal Oxide Buffer Layer” The Journal of Physical Chemistry Letters. (2018), 5, 1041–46.
[3] Benedikt Dänekamp, Nikolaos Droseros, Demetra Tsokkou, Verena Brehm, Pablo P. Boix, Michele Sessolo, Natalie Banerji and Henk J. Bolink, “Influence of hole transport material ionization energy on the performance of perovskite solar cells”, Journal of Materials Chemistry C. (2019), DOI: 10.1039/C8TC05372C

Hybrid organic-inorganic perovskites (HOIPs) have garnered significant interest as absorber layers in tandem photovoltaic devices (PVs) due to their low-temperature solution processability, high performance, and widely tunable band gap. Perovskites have been coupled in tandem devices with silicon, CIGS, and other perovskites, to boost the efficiency of the single-junction devices. However, they have yet to be employed in tandem with CdTe. One of the reasons for this is the need for a perovskite with either a very low or very high band gap to couple efficiently with the 1.5 eV band gap (E_g) of CdTe. Here, we show that tandem PVs can be made by coupling CdTe with wide band gap CH₃NH₃PbBr₃ (MAPBr) in a four-terminal device architecture to improve efficiency. We find that MAPBr films tend to exhibit a significant amount of haze, depending on the processing conditions of the film. This haze is a result of “wrinkling” in the film, which is observed using polarized light microscopy and scanning electron microscopy. By simulating device performance with a MAPBr top cell and a CdTe bottom cell using the Semiconductor Capacitance Simulator (SCAPS) software package, we identify the amount of haze that can be tolerated by the tandem device to still improve the performance relative to the CdTe device layer. We did this by applying optical filters to a simulated 15.5% power conversion efficiency (PCE) CdTe device and comparing simulated CdTe device PCE before and after the filter was applied. We see for a perfect MAPBr top cell (100% transmittance E_g<2.3 eV, 0% transmittance E_g>2.3 eV), a MAPBr device efficiency of 4.2% is required for the PCE of 4-terminal MAPBr-CdTe tandem to exceed 15.5%. Simulating instead a film with high haze, we show a MAPBr top cell with 50% sub-band gap transmittance would require a PCE of at least 9.8% for break even. Finally, optimizing the MAPBr top cell morphology, optical quality, and performance via solvent-antisolvent engineering and additive engineering, we construct and characterize HOIP-CdTe tandem devices.

Perovskite solar cells have shown tremendous improvement in their power conversion efficiency (PCE) in a short period of time reaching up to 23.3% [1]. Owing to excellent optical properties (sharp band-edge, high IR transmittance) and ability to tune their bandgap to wide energy range, make perovskites attractive candidates for top-cells in tandem applications [2].

We have previously demonstrated a 23.6% efficient monolithic, two-terminal perovskite/silicon tandem using 1.63 eV Cs_0.17FA_0.83Pb(Br_0.17I_0.83)₃ (CsFA) perovskite top-cell [3]. This PCE value is still below the 30% that is achievable with two junctions in two-terminal devices with silicon as a bottom sub-cell [2]. Reasons for low PCE include high reflectance loss due to planar front surface and parasitic absorption in layers other than perovskite and silicon. Additional way to improve PCE is to reduce the thermalization losses in tandem by using a perovskite top-cell of higher bandgap. Perovskite/silicon tandems are multi-layered devices, thus a comprehensive optical modeling is required to tune the thickness of perovskite top-cell to current-match it with silicon bottom-cell. To perform this analysis optical constants of wide-bandgap perovskites are required. So we first determined the optical constants using multi-angle spectroscopic ellipsometry and spectrophotometry of two CsFA perovskites having different bandgaps of 1.63 and 1.68 eV, and also of a common absorber, CH₃NH₃PbI₃. Then we show that the complex refractive index of CH₃NH₃PbI₃, can be used to generate approximate optical constants for an arbitrary wide-bandgap perovskite (in the range of 1.56 eV to 1.68 eV) by translating the data along the wavelength axis. Lastly, we used these optical constants to map the short-circuit current density of perovskite/silicon tandems with a range of different bandgap perovskites and textures to determine the top-cell thicknesses necessary to achieve current matching for varying top-cell bandgap. Our results show that ~28% PCE tandem is achievable with 1.68 eV perovskite, and even higher efficiency is possible by minimizing parasitic losses.

In the end we fabricated a two-terminal perovskite/silicon tandem by employing a 1.68 eV Cs_0.25FA_0.75Pb(Br_0.20I_0.80)₃ perovskite top-cell, rear textured silicon bottom-cell and textured polydimethylsiloxane (PDMS) anti-reflection foil at the front achieving 25% PCE [4].

[1] Available: NREL Efficiency Chart (https://www.nrel.gov/pv/assets/images/efficiency-chart.png).
[2] Z. J. Yu, M. Leilaeioun, and Z. Holman, "Selecting tandem partners for silicon solar cells," Nature Energy, vol. 1, p. 16137, 2016.
[3] K. A. Bush et al., "23.6%-efficient monolithic perovskite/silicon tandem solar cells with improved stability," Nature Energy, vol. 2, p. 17009, 2017.
[4] K. A. Bush et al., "Minimizing Current and Voltage Losses to Reach 25% Efficient Monolithic Two-Terminal Perovskite–Silicon Tandem Solar Cells," ACS Energy Letters, vol. 3, no. 9, pp. 2173-2180, 2018.

Two-terminal (2-T) monolithically integrated all-perovskite tandem solar cells are a promising candidate for the next generation solar cells with ultrahigh efficiency and low fabrication cost. Here, we report on the fabrication of 2-T all-perovskite tandem solar cells consisting of a 1.75-eV wide bandgap top cell and a 1.25-eV low bandgap bottom cell connected by a simple interconnection layer. We employed a bulk-passivation strategy to enlarge the grain size, increase carrier mobilities, reduce electronic disorder, and suppress trap-assisted recombination in the low bandgap mixed Sn-Pb perovskite films, which leads to improved performance of low bandgap perovskite solar cells with an absorber thickness over 750 nm. This advance in high-quality low-bandgap perovskite layers enables the fabrication of efficient 2-T all-perovskite solar cells with power conversion efficiencies of more than 21% and a decent operational stability [1]. Our approach provides a suitable pathway to realize ultra-high efficiency 2-T all-perovskite tandem solar cells.

[1] D. Zhao et al., Nature Energy 2018, DOI: 10.1038/s41560-018-0278-x.

In this work, aiming to develop the potential hole transporting materials (HTMs) with multifunctional features such as hole extraction, trap passivation and hydrophobic protection for perovskite solar cells (PrSCs) applications, three phenothiazine (PTZ) based A-π-D-π-D-π-A architectured small molecules containing various terminal acceptor units N-methyl rhodanine (SGT-420), dicyanovinylene (SGT-421) and indanedione (SGT-422) have been synthesized via an efficient and step-economical direct C–H arylation strategy. Two PTZ units in this architecture have been introduced to downshift the HOMO level closer to the valence band of the perovskite and the central thiophene is used to enhance the planarity in terms of reducing the torsional alleviation between the PTZ units. Energy levels of the synthesized HTMs are sufficiently lower than Spiro-OMeTAD but closer to the valence band of the perovskites and distinctly varied with respect to the end-capped acceptor units. Correlation of Optical-electrochemical and DFT studies reveals that dicyanovinylene unit in SGT-421 synergistically lowers the LUMO and HOMO (-5.41 eV) levels very closer to valance band (-5.43 eV) of the perovskites. Whereas N-methyl rhodanine in SGT-420 and indanedione in SGT-422 upshift the HOMO levels from the valence band of the perovskites and the increased energy level offset leads to energy loss which is deemed to be detrimental for efficient hole transport. To reinforce our envisaged design concept, SGT-421 exhibited an intrinsic superior hole mobility of 1.09 × 10^–4 cm² V^–1 s^–1 without any dopants as compared to spiro-OMeTAD (8.32 × 10^–5 cm² V^–1 s^–1). Using the structure of FTO/compact TiO₂/mesoporous TiO₂/CH₃NH₃PbI_3-xCl_x/HTM/Au, since excellent energy levels matching between perovskites and SGT-421 and high intrinsic hole mobility of SGT-421, PrSCs device based on SGT-421 exhibited an impressive photovoltaic performance of 17.27% as compared to the standard spiro-OMeTAD-based device (18.01%). In addition, the SGT-421-based device exhibits improved long-term stability which is originated from the combination of various contributions such as larger water contact angle, good film-forming ability and trap passivation effect toward the perovskite films via Lewis acid-base interactions. Eventually, we believe in that the structure-property relationship study of the terminal acceptor units offer detailed understanding about their behaviour obtained from optical-electrochemical profiles with respect to the valence band of the perovskites. The apparent demonstration of multifunctional features together with high intrinsic hole mobility and excellent PCE by SGT-421 from the A-π-D-π-A based HTMs shows that they can be a cost-effective and promising alternative to the expensive spiro-OMeTAD for PrSCs applications. The facile synthesis of the HTMs from commercially available cheap starting materials opens up a new avenue for the commercialization of perovskite solar cells through the development of efficient and low-cost HTMs with multifunctional features.

Moisture degradation is a serious hurdle for the commercialization of perovskite solar cells. Recently, mixed cation perovskites (MCPs) have shown promise in achieving relatively longer stability and consistently higher power conversion efficiency than single cation counterparts. Increased stability is due in part to the large grains created through the use of the mixed cation material, which leads to fewer pathways for the penetration of moisture. Furthermore, it has previously been shown that trapped charges are likely the core origin for degradation of perovskite material. Using MAPbI/Br as control and CsFAMAPbI/Br as the mixed cation perovskite, we measured trap density of states (tDoS) along the grain boundaries through KPFM and capacitance sweeps. These measurements were performed at 0%, 25%, 50%, and 75% relative humidity conditions for comparison. The mixed cation samples showed lower tDoS under moisture exposure, pointing to the reason for prolonged stability. Through this work, we can further understand the degradation pathway of mixed cation perovskite and the effect MA may play in the moisture stability of the device, moving one step closer to the commercialization of perovskite solar cells.

The rise of the efficiency of perovskite solar cells from 3.8% to 23.3% in less than 10 years has shaken up the scientific photovoltaic community. A comprehensive understanding of fundamental properties of these intriguing materials is paramount for further progress in device applications. In this work we investigate how the charge carrier cooling dynamics in MAPbI₃ thin films is affected by hydrostatic pressure up to 400MPa. In particular, charge cooling is probed by pump-probe transient absorption spectroscopy. We find a dramatic increase of carrier temperature at the picosecond timescale when the pressure is increased. We also investigate the effect of pressure on cooling timescales, as recently the slow cooling in those material has sparkled significant interest. Furthermore, we present a first insight on how the cooling is influenced by the phase transition, which occurs at 325 MPa for this perovskite. Our findings will contribute to the understanding of ultrafast carrier dynamics of metal halide perovskites.

Since the emergence of photovoltaic devices based on metal halide perovskite material, there has been extensive studies on replacing Pb for environmentally friendly technology. CsSnI₃, a representative Pb-free halide perovskite material, suffers from critical problems. When exposed to air, it is immediately oxidized from Sn²⁺ to Sn⁴⁺, prompting rapid spontaneous phase transition from the perovskite (black phase) to the non-perovskite (yellow phase) structure. This air-instability prohibits its practical applications. Herein, we demonstrate a new strategy to improve the air stability of the black perovskite phase of CsSnI₃ by chemical doping. We investigate structural changes of the undoped and doped black phase under various conditions, and find that moisture mainly causes the phase transition of the black phase. We reveal that specific dopants suppress the formation of competitive yellow phase by changing formation energy of CsSnI₃, and oxidation of Sn²⁺ in doped samples occurs at a significantly lower rate in air. As a result, doped CsSnI₃ maintains the black phase over a week under air. These dopants induce an unusual shift in optical band gaps in Sn-based perovskite materials, in contrast to Pb-based materials. Ab initio density functional theory calculations show that band gap tuning effect is attributed to the p orbitals of the dopants.

Organic-inorganic lead halide perovskite solar cells (PSCs) have attracted great interest due to its rapid progress in power conversion efficiency (PCE) within a few years, which is from 3.8% to the present record of 23.3%. However, this type of solar cell still exists some challenges to overcome. First, the toxicity issue of lead drags the PSC from commercialization. To advance the technology of PSC, the development of lead-free or lead-reduced perovskite is imperative. Second, the current density-voltage hysteresis (J-V hysteresis) is an important issue for PSCs. Especially, hysteresis effect often occurs in planar n-i-p structured PSCs. The hysteresis effect can lead to misjudgment of photovoltaic performance because the forward and reverse scans show different results. In order to solve these problems, we developed mesoscopic zinc-doped TiO₂ (meso-Zn:TiO₂) electron transport layer (ETL) and barium-doped perovskite (Ba-perovskite) active layer. The mesoscopic zinc-doped TiO₂ (meso-Zn:TiO₂) as ETL of PSC is synthesized by combining sol-gel and hydrothermal method. Surface morphology, electron mobility, charge carrier dynamic and electrical conductivity of various meso-Zn:TiO₂ ETL are systematically studied. To understand the electron extraction behavior between perovskite active layer and meso-Zn:TiO₂ ETL, the band levels and surface potentials of perovskite active layers are analyzed by ultraviolet photoelectron spectroscopy (UPS) and photo-assisted Kelvin probe force microscopy (KPFM), respectively. To achieve high PCE lead-reduced PSC, the stability of perovskite structure should be considered. By introducing formamidinium (FA) to Ba-perovskite, the crystallinity can be enhanced, which allows long-term stability for PSCs. The Ba-perovskite films are stable which can be fabricated in the atmosphere. In this study, we combined meso-Zn:TiO₂ with Ba-perovskite to fabricate high-performance lead-reduced PSCs. By combining the benefits of both MAFA mixed-cation perovskite and Ba-perovskite, the acceptable doping amount of Ba²⁺ can be increased to >10.0 mol%, and the device performance is better than the MAPbI₃ PSCs, indicating that mixed-cation perovskite is able to enhance the performance of lead-reduced perovskite and reduce the lead pollution to the environment. Obviously, when Ba-perovskite combines with meso-Zn:TiO₂ ETL, the J-V hysteresis effect can be completely eliminated. These results show that the ETL/perovskite interface optimization could enhance PCE and eliminate J-V hysteresis.

Low power conversion efficiencies and material instability caused by moisture effects under ambient conditions are the pertinent issues for perovskite solar cells. Herein, an optimal small amount of water additive into methyl ammonium lead iodide (CH₃NH₃PbI₃) solution helps to improve perovskite crystallization and stability of perovskite films. Addition of optimal fraction of water into N, N-dimethylformamide (DMF) results in controlled perovskite growth due to the higher vapour pressure and lower boiling point of water compared to DMF. The optimal water concentration of 1% by volume was found to yield preferential crystallization in the (110) plane, together with improved grain size,smaller amount of pin holes and defects compared to non-water added film. The device performance improved from 10.1% to 12.3% in 1% water added perovskite films, but decreases upon further inclusion of water content. Pump fluence dependent photoluminescence shows the reduced trap density from 4.8x10¹⁷ cm^-3 to 3.2x10¹⁷ cm^-3 on inclusion of 1% water to the perovskite film. The presence of the optimal amount of water enhances the charge carrier lifetimes and diffusion lengths in blend film confirmed by the transient measurements and hence improved device performance. We conclude that these detects in perovskite are dominated by surface trap states and inclusion of an optimal amount of water as an additive help to passivate these traps. This study correlates the device performance, thin film morphology with transient dynamics and provide a route to control the growth of crystal perovskites and new insights for improving the stability of organic–inorganic halide perovskites.

Organic-inorganic hybrid halide perovskite solar cells (PeSCs) have risen as promising candidates for photovotaics due to their superior properties such as outstanding performance, low cost and facile processability from solution state. In order to obtain higher power conversion efficiency of PeSCs, the suitable engineering of charge transport layer as well as that of active layer is needed. Herein, we first reported the application of zirconium-based metal-organic frameworks (MOFs) as an interlayer for PeSCs which are porous coordination polymers exhibiting long-range ordered metal clusters connected with organic linkers. There are some of reports expecting that MOFs could be utilized to optoelectronics, but a few reports regarding to MOF-included optoelectronics have also appeared. By using the rapid self-assembly process between metal source and organic linker to build MOFs structures, brushing method from precursor solution under hot-substrate condition was employed to prepare the thin MOFs films, incorporated into p-i-n structured PeSCs. By this straightforward strategy, the transparent and thin MOF films were deposited, film development and properties were analyzed by various methods and found to be appropriate for charge injection and transfer between perovskite and MOF layer. This result indicates MOFs could be novel and good hole transport materials for PeSCs.

Perovskite solar cells have been actively studied due to their great light absorption ability and a long excition diffusion length (~ 1.0 μm). The cell efficiency has rapidly grown up to ~23% using mesoporous TiO₂ layer which required a high temperature sintering processes. Such device structure with high temperature treatment is not compatible with flexible solar cells due to degradation of flexible substrate above 120 °C. Therefore, low temperature process below the T_g of flexible substrate (PEN 120 °C, PET 80 °C) is necessary for the application to the flexible substrate. So we adopt planar type device structure of substrate/ITO/polymer hole transport layer/amphiphilic conjugated polyelectrolyte/MAPbI₃/PCBM/BCP/Ag with low temperature solution process (except BCP/Ag electrode). Also, we investigate the effect of various additives on crystal growth and grain size of perovskite films at different thermal annealing temperatures. The reasonably high efficiency of 18% and 17% fabricated at low temperature were obtained on glass and flexible substrates, respectively.

Recently, metal halide perovskite appears to be the most promising material in the photovoltaic communities. Despite the dramatic increase in the certified device efficiency, the lack of a comprehensive understanding of the fundamental properties, such as the real elemental composition and distribution in the perovskite film, greatly limits the technological progress. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is a powerful analytical tool which can provide one-dimension (depth profile), two-dimension (image mapping) and three-dimension (3D tomography) compositional information and it was used to probe the perovskite film or the entire perovskite solar cell. However, we found out that, during the sputtering period, the organic group locates at the A site in the perovskite lattice was removed at a much higher rate than the inorganic species when using the commonly utilized O₂⁺ sputter source. The unequal sputter rate of the components, which is also known as preferential sputtering, can misguide researchers to a false conclusion and stall the scientific advance. In this study, we demonstrated that by substituting C₆₀⁺ for O₂⁺ as the sputter source, the preferential sputtering of organic species can be eliminated, yielding a depth profile without anomalous concentration gradient. We also investigated the 3D tomography of the perovskite film by using ToF-SIMS with C₆₀⁺ sputtering. By comparing our results with those reported in the previous studies, the importance of using C₆₀⁺ sputter source was highlighted for obtaining artifact-free information of metal halide perovskite via ToF-SIMS analysis.

Power conversion efficiencies of perovskite (PK) solar cells have rapidly improved in recent years obtaining efficiencies in excess of 20% [1]. Good absorption properties in the visible and infrared spectrum, high efficiencies and low fabrication costs have brought PK solar cells to the forefront. With single junction PK solar cells soon reaching theoretical efficiency limits a requirement for tandem cells, where materials of differing absorption characteristics convert a wider spectral range, is in demand. However, such tandem solar cells introduce fabrication complexities due to the thermal budget restrictions imposed by the complex layer structure [2].

Indium-tin oxide (ITO) is widely used as a transparent contact in PK cells. However, due to the high deposition temperatures or energetic process used in production ITO is not viable for tandem solar cells. Here we discuss the replacement of this transparent contact employing atomic layer deposited (ALD) Ti-doped ZnO as a candidate material. ALD has the ability to be scaled and offers excellent uniformity even at low temperatures. We focus on the optimum deposition conditions of Ti-doped ZnO its properties and compare those to the commonly used ITO. Solar cells with Ti-doped ZnO transparent contacts were successfully fabricated and their performance, with respect to power conversion efficiency, evaluated and compared to state of the art PK solar cells.

References:
[1] National Renewable Energy Laboratory: Efficiency Chart (2018). https://www.nrel.gov/pv/assets/pdfs/pv-efficiencies-07-17-2018.pdf
(accessed on 24^th October 2018)
[2] Sahli, F., Fully textured monolithic perovskite/silicon tandem solar cells with 25.2% power conversion efficiency, Nature Materials, 2018. 17(9): p. 820-826

Halide perovskites have been dominating solution-processable photovoltaic research with remarkable power conversion efficiency of over 23%. The presence of toxic lead in state-of-the-art halide perovskite, however, shackles the marketability of this technology. Although by completely replacing lead with elements such as tin, germanium, etc. lead-free halide perovskite solar cells can be achieved, they show much inferior efficiency and stability. Alternatively, partially replacing lead with other metals helps to reduce the lead content in perovskite solar cells while without significant efficiency loss. But, we identify that trap-assisted recombination at grain boundaries and on the perovskite film surface associated with Sn vacancies limit the efficiency of three-dimensional (3D) Sn-Pb alloyed perovskite devices. By designing two-dimensional/3D (2D/3D) alloyed perovskites, we passivated the non-radiative recombination channels and thus improved the efficiency of alloyed perovskite solar cells. Here, 2D/3D alloyed perovskites are synthesized by substituting small organic cations in 3D alloyed perovskites with bulkier phenylethylammonium cations. We optimized the content of lead and phenylethylammonium cations for achieving the highest photovoltaic performance in resultant 2D/3D alloyed perovskite solar cells. Consequently, an enhancement of 39% was achieved in tin-rich alloyed perovskites (7.66 % versus 4.78 %) and a 15% enhancement in lead-rich alloyed perovskites (13.60 % versus 11.49 %) was achieved as compared to 3D alloyed perovskites in p-i-n planar solar cells. Also, the current-voltage hysteresis originating from the ion migration in perovskite almost fully vanishes in 2D/3D alloyed perovskite devices, once again attributed to the passivation of trap states, which is in clear contrast to the large hysteresis displayed in 3D alloyed perovskites. Last but not the least, 2D/3D alloyed perovskite devices showed much better moisture stability and photostability. The work demonstrates that a 2D/3D engineering strategy is an effective approach towards realizing Sn-based high-performance perovskite solar cells.

Expensive hole transport materials (HTMs) such as Spiro-OMeTAD and PTAA hinder the commercial deployment of high-efficiency perovskite solar cells (PSCs). We report an iterative approach to developing highly efficient HTMs from inexpensive organic scaffolds that give rise to high-efficiency PSCs without sophisticated device engineering. Candidate molecules with small structural variations were made within three synthetic steps. The structure-property relationship of these compounds was revealed by interrogating their physical (e.g., crystallinity and glass transition temperature) and electrical properties (e.g., work function and charge extraction from perovskite layer). This knowledge informs molecular modifications to achieve optimal parameters in thin-film devices. As the result, device PCE above 20 % have been achieved in both direct and inverted cell configurations.

Metal oxide carrier transporting layers have been investigated widely in organic/inorganic lead halide perovskite solar cells (PSCs). Tin oxide (SnO₂) is a promising alternative to the titanium dioxide commonly used in the electron transporting layer (ETL), due to its tunable carrier concentration, high electron mobility, amenability to low-temperature annealing processing, and large band gap energy. In this study, a facile method was developed for the preparation of a room-temperature-processed SnO₂ electron transporting material that provided a high-quality ETL, leading to PSCs displaying high power conversion efficiency (PCE) and stability. A novel physical ball milling method was first employed to prepare ground SnO₂ nanoparticles (G-SnO₂), and then a sol–gel process was used to prepare a compact SnO₂ (C-SnO₂) layer. The effects of various types of ETLs (C-SnO₂, G-SnO₂, G-SnO₂/C-SnO₂) on the performance of the PSCs are described. The composite SnO₂ nanostructure formed a robust ETL having efficient carrier transport properties; accordingly, carrier recombination between the ETL and mixed perovskite was inhibited. PSCs incorporating C-SnO₂, G-SnO₂, and G-SnO₂/C-SnO₂ as ETLs provided PCEs of 16.40, 17.92, and 20.12%, respectively. In addition to their high efficiency, the devices featuring the composite SnO₂ (G-SnO₂/C-SnO₂) nanostructures possessed excellent long-term stability—they maintained 89% (with encapsulation) and 83% (without encapsulation) of their initial PCEs after 105 days (>2500 h) and 60 days (>1400 h), respectively, when stored under dry ambient air (20 ± 5 RH %).

Solar cells based on organolead halide perovskites have recently seen rapid progress as one of the most promising photovoltaic technologies, owing to their solution-processability, low-cost and high efficiencies [1-5]. However, serious concerns on the use of toxic lead as well as the instability of these perovskites in humid conditions have triggered efforts towards developing non-toxic and stable organic-inorganic alternatives, which is critically important for their market acceptance in future commercialization [6-8].
To address the toxicity and stability issue of the tin/lead based perovskites in this research field, we presented the first demonstration of applying organic-inorganic methylammonium bismuth (III) iodide (MBI) in solution-processable solar cells [10]. The organobismuth based material shows the advantages of non-toxicity, ambient stability and low-temperature solution-processibility, which may provide promising solution to address the toxicity and stability challenges in perovskite solar cells. Systematic study have been performed to understand the electrical, optical and optoelectronic properties of the organic-inorganic methylammonium bismuth (III) iodide. Crystallographic data as well as the crystal structure of the MBI single-crystal was achieved using single-crystal X-ray diffraction. First-principles electronic band structure calculations based on the density functional theory was carried out as well to determine the theoretical energy band structure and density of states in this semiconductor. According to the both the theoretical and experimental results, the energy band levels of the MBI are well-aligned to be used in a typical perovskite solar cells in replacing of tin/lead perovskite layers. Therefore, solar cells based on the MBI films were assembled and tested. The devices yielded a preliminary efficiency of ~0.190% with an open-circuit voltage of ~ 0.510 V in its planar structure (Fig. 1 (a-c)). The MBI film shows good long-term stability in the ambient air, which has been confirmed by monitoring the time-dependent XRD results (Fig 1 (d)). Reducing the background carrier concentration and its optical band-gap may be two effective strategies for further improving the device performance. Also, this is one of the earliest demonstrations of trivalent metal cation based, solution-processable organic-inorganic hybrid light-absorber for solar cell application, which may inspire more research work to develop and apply organic-inorganic hybrid materials beyond divalent metal cations (Pb (II) and Sn (II)) for solar energy utility.

An upscaled chemical vapor deposition process (CVD) for methylammonium lead iodide (MAPI, CH₃NH₃PbI₃) and formamidinium lead iodide (FAPI, CH₅N₂PbI₃) used in thin film perovskite solar cells has been built-up and optimized. The procedure uses exclusively gaseous precursors to transform the inorganic salt, i.e. lead iodide (PbI₂) to the respective perovskite films. The mechanism of perovskite formation has been confirmed using XPS and XRD. Further characterization techniques include SEM and solar simulator measurements.

Lead halide perovskite materials are particularly suited to solution processing and have been used extensively in coating processes combined with electron and hole transporting materials to produce multilayer solar cell devices with photovoltaic conversion efficiencies over 23%. The versatility of perovskite materials means that they can be applied to a range of substrates from glass to plastic and metal using a range of deposition processes from screen printing to spin and slot-die coating. This paper will consider the trials and tribulations of manufacturing perovskite solar cells at scale on glass and flexible PET substrates, in particular delivering an assessment on the matching of device architecture to deposition method to substrate with the view of unlocking large-scale production. The results will be discussed in the context of two recent pilot scale manufacturing trials on FTO coated glass and ITO coated PET.

In the first instance, the screen printing of glass mounted carbon perovskite solar cells (C-PSC), based on a triple mesoporous stack (titania, insulator, carbon) will be discussed. This structure represents likely the most promising architecture for successfully upscaling perovskite solar cells to modules and represents an exciting leap forward in the pursuit of industrial deployment with recent reports demonstrating 10,000 hours of continuous light soaking datausing inexpensive manufacturing processes. Alongside process improvements related to increasing throughput and mechanising manufacture, the performance and final cost of large area modules also depends on the ratio between the module’s active and total areas, the so-called geometrical fill factor, which has to be optimised to properly exploit the substrate available area and maximise the energy production. At present, series interconnected C-PSC modules are achieved through accurate registration of each of the patterned layers, using precision screen printing, however new developments will be presented using mechanical and laser scribing to increase the active area towards 90% for all printed modules.

Roll to roll processing of perovskite on PET requires a different approach primarily due to the temperature limitations of the substrate combined with the continuous non-batch deposition processes required. Here meniscus guide slot-die coating is used. Slot-die coating is a versatile technique able to coat a range of ink rheologies with little ink wastage. The paper will discuss the application of a meniscus guide to improving stripe definition, coating quality and reducing coating defects such as ribbing and discontinuous films. The lead iodide ink used in the perovskite layer is formulated in a non-toxic solvent and results in a film readily converted to perovskite through slot-die coating of methylammonium iodide without any other pre-treatments. The choice of solvent for the MAI ink is optimised by considering how the rheology and volatility of the formulation impacts on the conversion to perovskite. These developments allow us to demonstrate roll-to-roll coated perovskite solar cells.

Finally the paper will present the complex preparation required for the delivery of a pilot scale perovskite module trial. In order to transition from lab to module scale particular considerations for solvent and material handling are required. Extensive environmental monitoring was undertaken to understand and mitigate the potential for operator exposure and to ultimately ensure the safe delivery of these trials.

Perovskite solar cells have attracted tremendous attentions in recent years due to the high device performance and the superior optoelectronic properties of perovskite materials. In this contribution, I will show our efforts on improving the device performance of inverted perovskite solar cells.^[1-5] Research efforts, including the charge-carrier balance, composition optimization and in-situ dynamic observation of perovskite microstructure evolution, will be discussed.

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Organometal halide perovskites have powerful intrinsic potential to drive next generation solar technology, but their insufficient thermomechanical reliability and unproven large-area manufacturability limit competition with incumbent silicon PV. This work addresses these limitations by leveraging large-area processing and robust inorganic hole transport layers (HTLs). We present inverted p-i-n perovskite solar cells utilizing NiO HTLs deposited by rapid aqueous spray-coating that outperform spin coated NiO and lead to a 5X improvement in the fracture energy (G_c ), a primary metric of thermomechanical stability. The morphology, chemical composition, and optoelectronic properties of the NiO films were characterized to understand and optimize compatibility with an archetypal double cation perovskite, Cs_.17FA_.83Pb(Br_.17I_.83)₃. Perovskite solar cells with sprayed NiO show higher photovoltaic performance, exhibiting up to 82 % fill factor (FF) and 17.7 % power conversion efficiency (PCE)—the highest PCE reported for inverted cells with scalably-processed charge transport layers—as well as excellent stability under full illumination and 4,000-hour aging. By utilizing open-air techniques and aqueous precursors, this combination of robust materials and low-cost processing provides a platform for scaling perovskite modules with long-term reliability.

The recent discovery of hybrid organic-inorganic metal halide perovskites led to a renaissance of thin film photovoltaics.¹ The great diversity of hybrid perovskite compositions and preparation pathways makes them an excellent candidate for novel photovoltaic materials with unique combination of properties, the potential for low cost and easy processing along with relatively high power conversion efficiencies.² Crystallinity, density of defects and impurities are key factors for optoelectronic properties, and are also highly dependent on the materials formation processes for most inorganic semiconductors. Understanding this behaviour and the structure/property relationship is crucial for fundamental understanding of perovskite materials, and for extending their properties to other process-tolerant systems.In that context, the synthetic approach induced by mechanical forces has appeared as a new emerging methodology in materials science.³ The mechanochemical reactions in solid state offer a significant advance by avoid the use of solvent, dramatically shortening synthesis times and simultaneously increasing the purity and amount of product.
Herein, we present a facile mechanochemical route for the preparation of various hybrid perovskite particles for high-efficiency thin-film photovoltaics.⁴ We also demonstrate that such approach applied for preparation of perovskite materials has advantage over a solution-based synthetic routes in terms of hysteresis and device performance.⁵ In addition, mechanochemistry allows the facile synthesis of large quantities of polycrystalline materials that is particularly well-suited for solid-state NMR studies, which can provide direct information about cation dynamics and atomic level phase compositions.⁶ These studies highlight the essential need for atomic-level characterization of photovoltaic perovskite materials and provide fundamental understanding of photovoltaic parameters in these systems and their superior stability.

References
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[6] (a) D. J. Kubicki, D. Prochowicz, A. Hofstetter, S. M. Zakeeruddin, M. Grätzel, L. Emsley. J. Am. Chem. Soc. 2017, 139, 14173. (b) D. J. Kubicki, D. Prochowicz, A. Hofstetter, M. Saski, P. Yadav, D. Bi, N. Pellet, J. Lewinski, S. M. Zakeeruddin, M. Grätzel, L. Emsley. J. Am. Chem. Soc. 2018,140, 3345. (c) D. J. Kubicki, D. Prochowicz, A. Hofstetter, S. M. Zakeeruddin, M. Grätzel, L. Emsley. J. Am. Chem. Soc. 2018,140, 7232.

Organometallic halide perovskite solar cells have been attracting increasing attention in recent years due to their excellent photovoltaic efficiencies, and that they possess the advantages of suitable optoelectronic attributes and simultaneously low-cost solution processibility. Recently, perovskite solar cells (PSCs) with inverted planar heterojunction structures, wherein a polycrystalline perovskite film is sandwiched between a hole and an electron-extraction layer, have gained attention due to its easy fabrication, compatibility with flexible substrates, versatility of energy-band engineering, and the possibility of fabricating multijunction cells. Moreover, they have achieved power conversion efficiencies (PCE) exceeding 21%. Further improving their PCE is now mainly hindered by the relatively low open-circuit voltage (V_oc), in comparison with reported regular PSCs using similar bandgap perovskites materials. The lower V_oc of inverted PSCs is attributed to the nonradiative recombination losses both inside the perovskite bulk material and at the interfacial contacts because of the presence of a considerable density of defects or recombination centers. However, there are several reported approaches to reduce the nonradiative recombination, which includes increasing the grain size, surface passivation, ion compensation and heterojunction engineering.
On the other hand, selection of the charge extraction material is also critical to the device performance, not only for energy level matching and charge extraction, but also in order to protect the perovskite photoactive layer. Different materials have been used as the hole and electron extraction layers in planar PSCs, including both inorganic and organic materials. Organic transport layers, such as poly(3,4-ethyenedioxythiophene) polystyrene sulphonate (PEDOT:PSS), 2,2’,7,7’-tetrakis(N,N-p-dimethoxyphenylamino)-9,9’-spirobifluorene (spiro-OMeTAD), poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) have been used as p-type hole transport layers, and [6,6]-phenyl C₆₁ butyric acid methyl ester (PCBM), C₆₀ and their derivatives have been used as n-type electron transport layers. Inorganic transport layers, such as CuSCN, CuI, CuO and NiO_x have been used as the hole extraction layers and ZnO, SnO₂ and TiO₂ have been used as the electron extraction layers. It is known that metal oxide demonstrate much higher carrier mobility and superior stability than organic materials. Here, we report a vacuum-deposition-processed p-i-n type planar perovskite solar cell that uses metal oxide as hole and electron transport layers, respectively. This device structure shows a high V_oc of 1.16 volts and 1.18 volts, corresponding to a voltage deficit of 0.45 volts at a bandgap of 1.61 electron volts and a voltage deficit of 0.47 volts at a bandgap of 1.65 electron volts, respectively, which derives from reducing nonradiative recombination.

Fully inorganic cesium lead halide perovskites are of great interest in the field of optoelectronics due to their optical and electronic properties and thermal stability.^1–3 The poor solubility of Cs-based perovskites, which is encountered in conventional solution processing, can be bypassed by the use of vacuum deposition.^4–6 This technique allows the formation of high-quality perovskite thin films in a dry process without the need of extra chemicals (e.g., solvents) other than the salt precursors that form the perovskite compounds. In this study, single-source vacuum deposition (SSVD) is used as an easy, fast and dry process for the preparation of fully inorganic cesium lead halide perovskite thin films.⁷ We investigated the structural and optical characteristics of the so-formed films as a function of chemical composition (chloride, bromide and iodide films were formed), post-deposition thermal annealing, as well as previous mechanosynthesis of perovskite powders. We found that the CsPbX₃ perovskite was preferentially formed for the smaller halides and favored by previous ball-milling of CsX and PbX₂ precursors. When bigger halides were used and/or CsX and PbX₂ precursors were simply mixed without previous mechanosynthesis, PbX₂-rich compounds such as CsPb₂X₅ were preferentially formed in the thin films. We demonstrate that single-source vacuum deposition of ball-milled perovskites represents an easy, fast and dry process to form high-quality thin films of fully inorganic perovskites. We will also present the preliminary implementation of these films into different optoelectronic devices such as light-emitting diodes or solar cells.

1. Chen, C. Y. et al. All-Vacuum-Deposited Stoichiometrically Balanced Inorganic Cesium Lead Halide Perovskite Solar Cells with Stabilized Efficiency Exceeding 11%. Adv. Mater. 29, 1–7 (2017).
2. Kulbak, M. et al. Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells. J. Phys. Chem. Lett. 7, 167–172 (2016).
3. Eperon, G. E. et al. Inorganic caesium lead iodide perovskite solar cells. J. Mater. Chem. A 3, 19688–19695 (2015).
4. Protesescu, L. et al. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 15, 3692–3696 (2015).
5. Dirin, D. N., Cherniukh, I., Yakunin, S., Shynkarenko, Y. & Kovalenko, M. V. Solution-Grown CsPbBr ₃ Perovskite Single Crystals for Photon Detection. Chem. Mater. 28, 8470–8474 (2016).
6. Forgács, D. et al. Efficient wide band gap double cation-double halide perovskite solar cells. J. Mater. Chem. A 5, 3203–3207 (2017).
7. El Ajjouri, Y., Palazon, F., Sessolo, M. & Bolink, H. J. Single-Source Vacuum Deposition of Mechanosynthesized Inorganic Halide Perovskites. Chem. Mater. acs.chemmater.8b03352 (2018). doi:10.1021/acs.chemmater.8b03352

Perovskite photovoltaics has emerged as a new thriving technology for solar conversion due to the its rapidly increasing power conversion efficiency (PCE). Although great progress has been made to push the PCEs of perovskite solar cells (PSCs)PSCs up to over 23% at laboratory scale, the practical conversion of solar energy to electricity via PSCs requires the development of large-area, low-cost, and stable high-efficiency PSCs. This presentation will introduce our recent efforts on the exploration of: 1) air blading as a new fully spin-coating-free and anti-solvent-free processing for scalable high-efficiency PSCs;^[1] 2) high-efficiency electron-transport-layer-free PSCs with simpler device configurations for low-cost and convenient device processing;^[2] 3) new low-cost and readily-available organic hole-transporting material for PSCs with efficiency over 20%;^[3] and 4) new insights into the stability of all-inorganic CsPbI3 perovskite photovoltaics.^[4,5] These results may contribute to develop practical perovskite photovoltaics.
References
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Schockley and Queisser knew the absolute efficiency limit of single junction silicon long before technology caught up to within 90% of their 30% power conversion efficiency glass ceiling. The multijunction concept offers a roadmap for mass deployment of photovoltaic technology beyond 30% - raising the fundamental limit to 45% for silicon tandem and 49% for triple junction. In a few short years the perovskite technology has reached 23.3% in single junction and in tandem with silicon achieved 27.3% - exceeding for the first time the very best silicon single junction. Theoretical combination of existing best-in-class single junctions predict obtainable tandem and triple junction efficiencies of 34% and 39% respectively. So what is between us and the 34% efficient perovskite-silicon tandem? We discuss the specific challenges to overcome and our progress on the tandem roadmap beyond 30%, addressing fundamental loss mechanisms in the perovskite absorber and its contact heterojunctions as well as aspects of optical design and deployment in mono or bifacial format. We will address how these solutions can be delivered at the full wafer-scale in a production environment at rates of tens of thousands of wafers/hr.

Organic-inorganic metal halide perovskites have emerged as the most promising candidate for low-cost solar cells based on their compatibility with solution processing and rapid progress in device efficiency. Despite this remarkable potential, perovskites suffer from well-characterized chemical and thermal instability, as well as mechanical fragility; all of which are obstacles to achieve the required service lifetimes for commercialization. We previously introduced a new design concept of scaffold-reinforced compound solar cells (CSCs), in which planar perovskite devices are partitioned into many smaller microcells by a mechanically reinforcing scaffold. However, the introduction of scaffolds reduced device efficiency due to the parasitic absorption of the scaffold material, where the short-circuit current scaled inversely with the scaffold area (i.e., larger scaffolds resulted in lower current and efficiency).

In this work, we demonstrate a new low-cost lens array architecture which directs light away from the scaffolds and into the perovskite microcells, resulting in over 90% efficiency and current recovery. The microlens array is fabricated on the backside of the device substrate and uses light concentration to directly fabricate the scaffolds, thus making the scaffold self-aligned and removing the need for a photomask. In addition, both optical ray tracing simulations and light transmission experiments show that these lenses have passive light tracking capabilities up to angles of incidence greater than 30^o, making them strong candidates for integration into low-cost, high-performance perovskite devices. Finally, we demonstrate the thermomechanical reliability of these lenses under accelerated aging conditions which is imperative for longer service lifetimes so that perovskite solar cells can ultimately compete with CIGS, CdTe, and c-Si solar technology.

Lately, tremendous effort has been made, particularly in laboratory-scale spin-coating technique, to achieve unprecedented increase in power conversion efficiencies of perovskite solar cells (23.7%). However, the reported efficiency are mainly based on small-area perovskite solar cells (≤0.1 cm²). Scalable processing methods are still lagging behind the state-of-the-art spin-coated devices, principally due to the complexity in perovskite casting, drying and crystallization from its precursor solution. It can be foreseen that the upcoming challenge for commercializing perovskite PV technology will be translation of high quality perovskite film from laboratory-scale spin coating technique to large-area coating or printing methods. Since the perovskite ink chemistry for scalable deposition was underexplored, we demonstrated new perovskite ink formulation for single step anti-solvent-free deposition for high efficiency perovskite solar cells and its potential of fabricating large-area devices.

Organo-lead halide perovskites are excellent candidates for applications in light emitting diodes (LEDs). Methylammonium lead bromide (MAPbBr₃) is the most investigated perovskite for visible perovskite LEDs, but its use is still limited by a low photoluminescence quantum yield (PLQY). Efforts to increase the PLQY of this material mainly consist in fine-tuning the morphology and reducing the grain size of the polycrystalline perovskite to the micro- and nanoscale. However, the origins of the increased PLQY with reduced crystal size remain unclear. Here we elucidate the physical processes underlying the light emission of MAPbBr₃ thin films using time-resolved spectroscopy.¹ A systematic correlation between the enhanced PL properties and the reduction of the crystal size, achieved by using different additives during solution-processing, is observed. The co-existence of free carriers and excitons at low excitation densities is shown in as-cast polycrystalline MAPbBr₃ (crystal size of the order of few μm), while only excitons are present at high excitation densities. Using the Burstein-Moss and Saha models, important quantities such as the exciton binding energy, the reduced exciton effective mass and the trap density are estimated. We then explain the increased PLQY upon crystal size reduction by the presence of a bright exclusively exitonic population even at low excitation densities, together with reduced surface trapping thanks to passivation by the additives. ¹N. Droseros , G. Longo, J. C. Brauer, M. Sessolo , H. J. Bolink , N. Banerji. ACS Energy Lett. (2018), 3, 1458

In this research, we have utilized simulation studies to probe non-toxic Sn and Sb-based perovskites with a goal to explore optimal fabrication methodologies. To that end, we have investigated a device structure with and without an ETL (both TiO2 and ZnO have been explored) and assessed the impact on critical photovoltaic parameters. We have also explored Cu2O as a viable hole transport layer (HTL) alternative to the traditional Spiro OMeTAD structure owing to ease of fabrication. Lastly, we have incorporated Cu as a viable back-contact alternative to the expensive Ag. We have performed optimization of individual layer thicknesses, thereby ultimately fabricating a device with the highest possible theoretical efficiency. The analyses provide critical insight into charge mobility and charge transfer kinetics within this non-Pb based device with cheap and easily fabricated materials; by observing the effects on Jsc, Vcc, and FF as individual layer thicknesses are modulated, and doing comparisons of structures with and without the various ETL layers, we present results of a structure that is not only optimal from an efficiency standpoint, but also from a technology viability and sustainability standpoint.

While energy conversion efficiencies of metal halide perovskite solar cells continue to improve (from 3.8 % to 23.7 % in merely 9 years), durability becomes an even more pressing issue especially for practical applications. In particular, solar cells need to endure environmental stresses such as moisture, heat and light. I will talk about some of the simple encapsulation schemes developed in my research group for moisture and heat stabilities. It is found that high quality barrier film when applied appropriately is sufficient for perovskite solar cells to pass IEC61215:201615 thermal cycling, damp heat and humidity freeze tests. The pressing pressure when applying barrier-film-based encapsulation and the design of the solar cell structures play a key role in module durability. Preliminary results of gas chromatography and mass spectrometry will also be presented to investigate trends of outgassing by perovskite solar cells which are useful information for designing appropriate encapsulation schemes.

Emulation of brain-like signal processing is the foundation for development of efficient learning circuitry, but few devices offer the tunable conductance range necessary for mimicking spatiotemporal plasticity in biological synapses. An ionic semiconductor which couples electronic transitions with drift-diffusive ionic kinetics would enable energy-efficient analog-like switching of metastable conductance-states. Here, we utilize ionic-electronic coupling in halide perovskite semiconductors to create memristive synapses with a dynamic continuous transition of conductance-states. Co-existence of carrier injection barriers and ion migration in the perovskite films defines the degree of synaptic plasticity, more notable for the larger organic ammonium and formamidinium cations than the inorganic cesium counterpart. Optimized pulsing schemes facilitates a balanced interplay of short and long-term plasticity rules like paired-pulse facilitation and spike-time dependent plasticity, cardinal for learning and computing. Trained as a memory array, halide perovskite synapses demonstrate reconfigurability, learning, forgetting and fault tolerance analogous to the human brain. Network-level simulations of unsupervised learning of handwritten digit images utilizing experimentally derived device parameters, validates the utility of these memristors for energy-efficient neuromorphic computation, paving way for novel ionotronic neuromorphic architectures with halide perovskites as the active material.

Despite the significant progress in fabricating hybrid organic-inorganic lead halide perovskite solar cells, their toxicity and low stability remain as major drawbacks, thereby hindering large-scale commercialization. Given the isoelectronic nature of lead(II) and bismuth(III) ions, potentially stable and non-toxic alternatives for efficient light absorption in thin-film photovoltaic (PV) devices may be found among bismuth-based halide semiconductors. Recently, solution-based film fabrication approaches for several bismuth-based iodide semiconductors have been investigated. The current work reports on a facile and reproducible two-step coevaporation/annealing approach to fabricate A₃Bi₂I₉ (A = Cs, Rb), AgBi₂I₇, AgBiI₄ and Ag₂BiI₅ polycrystalline films with compact and pinhole-free morphology (Chem. Mater., 2018, 30, 3538–3544). The structural, compositional, optical and electrical characteristics of the resulting thin films are studied by X-ray diffraction, scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDX), optical spectroscopy and X-ray/UV photoelectron spectroscopy. A crystallographic phase transition from rhombohedral to cubic structure silver bismuth iodide is investigated via temperature-dependent X-ray diffraction (TD-XRD). Planar-junction photovoltaic (PV) devices are prepared based on the coevaporated rhombohedral AgBiI₄ films, with titanium dioxide (TiO₂) and poly(3-hexylthiophene) (P3HT) as electron- and hole-transport layers, respectively. The best-performing device exhibited a power conversion efficiency (PCE) of as high as 0.9% with open-circuit voltage (V_OC) > 0.8 V in the reverse scan direction.

Combining perovskites with well-established photovoltaic materials such as silicon or CIGS is an attractive option for producing cheap, high efficiency and high voltage solar cells. Perovskite-based tandem solar cells can potentially achieve over 30% tandem efficiency. We demonstrate a 4-terminal tandem perovskite/silicon configuration in which the efficiency is as high as 26%, with 23.9% for a perovskite/CIGS tandem. We also demonstrate a two-terminal monolithic tandem device with a perovskite top cell and a polysilicon passivated c-Si bottom cell, with an efficiency of over 24%, using a novel approach that avoids the need for a dedicated interconnection layer. Much of the success of these approaches has come from the reduction of optical losses, and we discuss the potential for further progress in this direction. In addition we have found that a combination of PMMA and PCBM can provide effective interface passivation for perovskite solar cells, resulting in a Voc of over 1.2V for a 1.6eV bandgap perovskite cell. Finally we show an efficiency of over 20% for a perovskite cell using a CuPc hole transport layer, which demonstrates excellent thermal stability. These results show the clear potential of perovskites in practical high efficiency devices.

All-perovskite tandem cell has been considered as the potential candidate for bringing the power conversion efficiency (PCE) beyond the Shockley-Queisser limit of single-junction device and yet retaining the advantages of earth-abundant materials and solution processability. However, a challenging issue of realizing such solution-processed devices is the fulfillment of complex and coupled requirements of the interconnecting layer (ICL) including solvent-resistance to protect underlying perovskite film, high electrical properties for carrier transport and recombination, and high optical transmission. In this work, a new thermionic emission-based ICL with enhanced solvent-resistance feature is demonstrated. Fundamentally, the thermionic emission plays a critical role in the electron transport process in the ICL, which is confirmed through both experimental and theoretical studies. Besides achieving high optical transmission and electrical properties, the new ICL chemically protects the underlying perovskite film by introducing a fluoride silane incorporated polyethylenimine ethoxylated (FSIP) hybrid system which also passivates the surface defects to reduce electrical loss. Our monolithic all-perovskite tandem cells demonstrate highest PCE of 17.9% [1]. Consequently, this work contributes to not only understanding the fundamental mechanism of ICL but also to promoting robust and low-cost photovoltaics.

[1] C. Li, Z. Wang, H.L. Zhu, D. Zhang, J. Cheng, H. Lin, D. Ouyang, W.C.H. Choy*, "Thermionic emission-based interconnecting layer featuring solvent resistance for monolithic tandem solar cells with solution-processed perovskites", Adv. Energy Mater., in press.

The energy demand is constantly growing worldwide and Paris Agreement requirements of a substantial de-carbonization of electricity generation by 2020 are very stringent. Renewable energy production efficiencies need to be maximized to match these urgent needs. Since solar irradiance is the largest resource of globally available energy, thus photovoltaics (PV) is in a prime position to deliver energy at costs similar to the fossil generated. Developing solar cells able to deliver high efficiencies, keeping their production cost and environmental impact low, would therefore address these issues. Nowadays, PV market is dominated by silicon solar cells delivering high efficiency and long-term stability. However, Si solar cells are approaching their practical efficiency limits and new methodologies are required to enhance SC power conversion efficiency. Tandem solar cells are the most promising and viable option for drastically improving the efficiency of existing PV technologies keeping the production cost low and versatile. ^1,7 Indeed, combining Si solar cells with a low-cost wide-band-gap absorber material, as perovskite, it is possible to create tandem solar cells that efficiently absorb light in complementary regions of the solar spectrum and overcame the transmission and thermalization losses of a single junction solar cells. Research on perovskite on silicon tandem SCs is picking up very fast and in just three years very promising results have been reported on both four-terminal (4T), mechanically stacked, and two-terminals (2T), monolithically integrated tandem, silicon–perovskite tandem SCs. The development of high efficiency semi-transparent perovskite solar cells is critical for the implementation of high performing tandem solar cells.
In this work, we present tandem structures including semi-transparent perovskite with different band-gaps as top-cells and both hetero-junctions and passivated emitter and rear cell (PERC) silicon solar cells as bottom cells. Semi-transparent perovskite solar cells with different architectures have been developed and efficiencies above 16.0% with average transparency >80% in near-infrared have been demonstrated. Semi-transparent electrode has been realize using sputtered indium tin oxide (ITO) as semi-transparent electrode and a thin Ag/MoOx buffer layer to protect the organic layers below and to guarantee the contact to be resistant to delamination but still highly transparency.⁸ We will also present different solar cells realized both by spin-coating and thermal evaporation and strategies to improve their specific efficiencies. We will also discuss the effects of different perovskite bandgap solar cells and the limiting factors of tandem solar cells efficiency, as parasitic absorption, reflection losses.
References
[1] J Werner, et al., Adv. Mater. Interfaces 2018, 5, 1700731
[2] Y Wu et al., Energy Environ. Sci., 2017,10, 2472.
[3] K.Bush, et al.Nature Energy 2017, 2, Article number: 17009
[4] T. Duong, et al. Adv. Energy Mater. 2017, 7, 1700228.
[5] F. Sahli, Nature Materials, 2018, 17, 820
[6] S.Albrecht, Energy Environ. Sci. 2016, 9 (1), 81-88
[7] J. Peng, .Adv.Energy Materials 2017, 7 (4), 1601768
[8] A. Guchhait et al ACS Energy Lett. 2, 4, 807-812

Wide-bandgap (~1.7 to 1.9 eV) perovskite solar cells have attracted substantial research intertest in recent years due to their great potential to fabricate efficient tandem solar cells via combining with a lower bandgap (1.1 to 1.3 eV) absorber (e.g., Si, CIGS, and 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. Here, we introduce a small amount of Pb(SCN)₂ as an additive into wide-bandgap FA_0.8Cs_0.2Pb(I_0.7Br_0.3)₃ perovskite precursor solution and simultaneously employing the solvent annealing method to fabricate high-quality wide-bandgap perovskite thin films. The synergetic effects of combining Pb(SCN)₂ additive and solvent annealing strategies lead to enhanced grain size from ~200 nm to > 2 μm and creation of excess PbI₂ to passivate defect states at grain boundaries. This advance enables an increase in the V_oc of the p-i-n type perovskite solar cells from 1.12 to 1.21 V. Further, we adopt guanidinium bromide (GuaBr) solution to tune the effective doping and electronic properties at the back surface of perovskite thin films, which further enhance V_oc to 1.24 V. The enhancement is attributed to the formation of a stronger electric field, revealed by Kelvin probe force microscope (KPFM) measurements. With the optimization of the device fabrication process, we demonstrate a champion power conversion efficiency of 18.19% with a decent stability under continuous illumination.

The emergence of metal halide perovskites as high efficiency, low-cost photovoltaic materials with a tunable band-gap has led to significant interest for perovskite-based tandems. Of existing perovskite-based tandem technologies, pairing wide-gap and low-gap perovskites in a monolithic all-perovskite tandem arguably offers the greatest potential by enabling efficiencies beyond the single-junction Shockley-Queisser limit with low-cost solution processing on a flexible, lightweight substrate. As of yet, efficiency of all-perovskite monolithic tandems has lagged behind perovskites paired with silicon or CIGS despite significant improvement to low-gap perovskite materials. The state-of-the-art all-perovskite 2T tandems have been limited by several main factors: 1) relatively low fill factors, 2) shunting due to the use of a thick (100nm) and conductive recombination layer of indium tin oxide allowing lateral connectivity of shunt pathways in either subcell, and 3) large voltage losses in the wide gap subcell due to photoinduced halide segregation, resulting in lower than ideal voltages for the tandem overall. Here, we develop strategies to overcome all of these issues and attain record-efficiency all-perovskite tandem devices.

We have substantially improved the recombination layer with an ultra-thin nucleophilic surface modification to C₆₀for improved atomic layer deposition (ALD), enhancing the solvent/sputter barrier properties and mechanical stability of ALD-grown recombination layers. This strategy enables the fabrication of recombination layers that are thinner and less laterally conductive (reduced shunting) than currently reported strategies. Secondly, we improve the voltage of our wide-gap cell through A-site cation bandgap tuning. A combination of large and small A-site cations are used to tilt the metal halide octahedral. This increases the bandgap and therefore reduces the amount of bromine needed to attain a certain bandgap. We achieved a band gap of 1.7eV with a solution bromine concentration of only 20%. We find that this strategy results in higher voltages and efficiencies that are stable under operation. Combining our robust recombination layer and high-Voc wide-gap perovskites, we demonstrate two-terminal all-perovskite tandems with efficiencies over 23% for rigid devices and 21% for flexible devices.

Sn perovskite solar cell is one of the most expected Pb-free perovskite solar cells with high efficiency. We have already reported the efficiency of SnGe-perovskite with 7.9 %. It has been also reported that Sn perovskite solar cell efficiency was enhanced to over 9% by inserting 2D structure partially. It is well known that the stability of perovskite structure is expected by Tolerance Factor. In the research on enhancing the Sn-perovskite solar cells, we focused on the relationship between the lattice strain of the Sn-perovskite calculated from XRD data and the solar cell efficiency. Sn perovskite solar cell is expressed as ASnI3, where A stand for cation. In this research, we employed (B0.1)(FA0.75MA0.25)0.9SnI3, where A cation is the mixture of (B0.1)(FA0.75MA0.25)0.9. In this report, B was varied was varied from small Na cation to large Butyl ammonium cations (Na cation, K cation, Et ammonium cation and Butyl ammonium cation). The lattice strain decreased with an increase in the ion radius from Na, K, Cs, Et ammonium, and Butyl ammonium. Replacement of FAMA with Et ammonium cation gave smallest lattice strain. By introducing Bu ammomium cation, the lattice strain increased with keeping the 3D structure at 5 mol %. By the addition of 10, and 20% addition, the strain decreased with the formation of 2D structure as it is well-known in the literature. Therefore, estimation of lattice strain by XRD peaks was proved to be useful. The solar cell efficiency had good correlation with the lattice strain. Low lattice strain gave high efficiency when MAFA was partially replaced by from Na ion to Ethyl ammonium cation. Introduction of Et ammonium ion with the smallest lattice strain gave the best results. The relative lattice strain of FA0.75MA0.25SnI3 was 0.057 (Tolerance factor: 0.979). When 10 mol % of Et ammonium cation was introduced, the relative lattice strain decreased to 0.035 (tolerance factor:0.985), the efficiency of the solar cells was enhanced from 2.34% without adding Et ammonium cation to 3.72% with 10 mon% addition. The efficiency was saturated at 20mol % concentration (3.45%). XRD signal showed that 3D structure was held and the increase in the efficiency was not explained by the insertion of 2D structure which has been reported before. All of the FF, Jsc, and Voc increased with a decrease in the lattice strain. By introducing 10% of EA ion, the efficiency of solar cells consisting of ITO/PEDOT-PSS/EA0.1(FA0.75MA0.25)0.9SnI3/PCBM/C60 was enhanced to 5.41 %. Sn perovskite solar cells with 7.9% is reported.

Organic-inorganic hybrid halide perovskite solar cells have attracted immense attention because of excellent optoelectronic properties and record power conversion efficiency (PCE) has surpassed 23.3%. within a few years. Despite the very high efficiency already attained by HPSCs (ABX3; A = MA, FA, Cs; B = Pb,Sn; X=I, Br, Cl) resulting from high absorption coefficient and electron-hole diffusion lengths, toxicity of Pb and stability of these materials are veritable issues. Composition space for Pb-free perovskite ABX₃, A₂BX₆ (on the order of > 10⁴ combinations possible) and so on is very large and a fast-growing field, however, there is a lack of systematic understanding of effect of defects, materials processing, crystal structures on bandgap and other opto-electronic properties. Replacement of Pb²⁺ with Sn²⁺ seems to be a logical solution for addressing the toxicity issues (e.g. CsSnI3). However, a major challenge for Pb-free perovskites is poor power conversion efficiencies as divalent Sn2+ and Ge2+ tend to oxidize into 4+ oxidation state, generating excessive defects (Sn and Ge vacancies), and result in short carrier diffusion lengths. Therefore, here we choose a model system based on Pt as the B-cation.

Cs₂PtI₆ has been successfully fabricated using solution processing using gamma-butyrolactone (GBL) with a bandgap of 1.45 eV in good agreement with first-principle calculations. However, replacing GBL with dimethyl-sulfoxide (DMSO) leads to wider bandgap material of 1.95 eV with incorporation of oxygen. This bandgap can be further widened to 2.2-2.4 eV by controlling the amount of oxygen in the films. Both materials resulted in preliminary surface photovoltage measurements of greater than 200 mV, high hole mobility, and absorption coefficients above or around 10^5 cm-1, making them interesting for further study as options for wide bandgap top cells.

Experimental results of effect of solvent engineering on the film morphology, composition, and optoelectronic properties will be discussed and supported by first-principle calculations. Solution processed Cs₂PtI₆ and CsPtI₃ based superstrate n-i-p device results will be presented for two different buffer layers CdS and Zn(O,S) processed by chemical bath deposition. To understand the effect of contact materials on degradation of perovskite film, two different contacts based on Au and carbon will be used. The oxygen-containing films are also observed to be more stable compared to pure Cs₂PtI₆, thereby addressing the issues of both stability and Pb-toxicity.

The state-of-the-art high efficiency perovskite solar cells (PSCs) contain lead and organic cations in the perovskite light-absorber. However, the toxicity associated with the lead and the volatility of the organic cations are hurdles in the path towards the future 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. Herein, we have utilized an alloying strategy in all-inorganic Sn-based perovskite materials, and realized PSCs with a promising PCE of up to 7.11%. Furthermore, the formation of a stable, robust native-oxide layer passivates the surface resulting in superior air-stability, with <10% PCE decay after 500 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.

Hybrid organic-inorganic perovskite (HOIP) solar cells – particularly those using APbX₃ [A = CH₃NH₃⁺, HC(NH₂)₂⁺, Cs⁺ and X = I^–, Cl^–, Br^–] as the absorber layer – have already exceeded the 23% conversion efficiency milestone, surpassing the record efficiencies of well-established photovoltaic technologies based on multicrystalline silicon, chalcopyrite and cadmium telluride absorbers [1]. However, a major obstacle in the commercialization of HOIP-based solar devices continues to be their short operational lifetimes, with significant performance decreases observed within the first hours in service [2]. Because this deterioration is linked to the intrinsic instability of the HOIP [3], reducing their defect concentrations seems a key strategy towards improving the long-term stability of this type of absorber and, therefore, of solar devices produced from them. Recently, it has been shown that the addition of divalent cation halide salts (e.g., SrI₂ and MgI₂) to the HOIP precursor mixture yields thin-films with lower defect concentrations, leading to boosts in resulting solar device performance [4,5]. The additives are expected to increase halide content, thus effectively filling halide vacancies, however an understanding of the doping role of the divalent cation and the mechanism inducing the observed device improvements is, at best, incomplete.
Using hard x-ray photoelectron spectroscopy (HAXPES), we have investigated the chemical and electronic structure of a Sr-doped MAPbI₃ thin-film sample series, prepared by varying the concentration of SrI₂ additive within a 0 to 2 mol% range. Sr is detected in all Sr-doped samples, and depth-dependent measurements reveal a pronounced surface accumulation of Sr in samples with higher Sr-doping concentrations, suggesting a Sr lattice incorporation saturation. Moreover, significant reductions in defect states (i.e., Pb⁰) are detected for all Sr-doped samples compared to (undoped) MAPbI₃. The effect of the Sr-doping concentration and detected Pb⁰ defect changes on the electronic structure is discussed in this contribution, as well as a comparison of the chemical and electronic properties with the device performance (enhancements) of corresponding solar cells.

[1] NREL Best Research-Cell Efficiencies. https://www.nrel.gov/pv/assets/images/efficiency-chart.png.
[2] R. G. Wilks and M. Bär, Nat. Energy 2017, 2, 16204; and references therein.
[3] B. Conings, et al., Adv. Energy Mater. 2015, 5, 1500477.
[4] D. Pérez-del-Rey, et al., Adv. Mater. 2016, 28, 9839.
[5] P. Caprioglio, et al., Poster session presented at the PSCO (2017).

Lead-free double-perovskites (A₂M⁺M³⁺X₆) have been recently proposed as a stable and environmental friendly alternative to lead-based perovskites. Among the vast number of possible double-perovskite compositions, Cs₂AgBiBr₆ has been the most investigated material, both from a theoretical and experimental point of view. To date, the majority of fundamental studies on this material have been conducted on single crystals or powders, which is due to the poor solubility of the material precursors and to difficulties in obtaining smooth and uniform films. Vapour deposition offers to be a promising alternative for the formation of Cs₂AgBiBr₆ films, overcoming the drawbacks of traditional solution chemistry methods and time-consuming growth of single crystals. In particular, physical vapour deposition can produce high-quality films with various thicknesses and smooth surfaces. In this way, numerous spectroscopic techniques which require non-scattering surfaces (like TRPL or THz spectroscopy), as well as thickness- and temperature-dependent transient photoconductivity measurements can be conducted achieving an insight in the optoelectronic properties of the material. Here, we disclose our recent findings on vapour-deposited Cs₂AgBiBr₆, demonstrating not only efficient double-perovskite solar cells, but also presenting a fundamental understanding of the carriers dynamics in the double-perovskite thin films, not achievable otherwise.

The low toxicity and a near-ideal bandgap make tin (Sn) perovskite an attractive alternative to lead perovskite for next generation solar cells. However, the development of Sn perovskite solar cells has been impeded by their extremely poor stability when exposed to oxygen. We report low dimensional Sn perovskites that exhibit markedly-enhanced air stability in comparison with their 3D counterparts. The reduced degradation under air exposure is attributed to the improved thermodynamic stability after dimensional reduction, the encapsulating organic ligands, and the compact perovskite film preventing oxygen ingress. We then explore these highly-oriented lowdimensional Sn perovskite films in solar cells. The perpendicular growth of the perovskite domains between electrodes allows efficient charge carrier transport, leading to power conversion efficiencies of 5.94% without the requirement of further device structure engineering. We tracked the performance of unencapsulated devices over 100 hours and found no appreciable decay in efficiency. Based on low dimensional structure, we prepared a 2D-quasi 2D-3D hiererachy structure by using NH₄SCN as an regulator to adjust the perovksite growth. In this structure, the 3D structure inside bring efficient carriers transport, while the surface 2D structure can further prevent the approach of oxygen to perovskite surface. In this way, we are able to get the highest efficiency of 9.4% for lead free perovskite solar cells. This work raise the prospects for preparing highe efficiency lead free perovskites for solar cells.
References
1. Y. Liao, H. Liu, W. Zhou, D. Yang, Y. Shang, Z. Shi, B. Li, X. Jiang, L. Zhang, L. N. Quan, R. Quintero-Bermudez, B. R. Sutherland, Q. Mi, E. H. Sargent, and Z. Ning*, J. Am. Chem. Soc., 2017, 139, 6693–6699.
2. F. Wang, X. Jiang, H. Chen, Y. Shang, H. Liu, J. Wei, W. Zhou, H. He, W. Liu, and Z. Ning*, Joule., 2018, 2, 1–12.

Lead (Pb) based perovskite solar cells have come a long way since their inception. However, toxicity issues due to the presence of lead have adversely affected their commercial potential. Strontium (Sr) a relatively non-toxic element is a viable candidate to replace Pb, given the similar ionic radius (Sr²⁺= 132 pm, Pb²⁺= 133 pm). We have prepared samples using acetate-based precursors to make devices with different substitution percentages of Pb with Sr, of 10% and 20%. We have obtained a reasonable device efficiency of 8.9% and 5.7% for 10% and 20% substitution devices, respectively. Charge carrier lifetimes gave 117.91ns for 10% and 109.36ns for 20% samples which are higher compared to the 103.79ns for a complete Pb based control device. Higher order substitution of Pb with more than 20% Sr has however resulted in a poor performance which can be due to the formation of SrO, Sr(C₂H₃O₂)₂ and SrCO₃ compounds which act as barriers for smooth charge transport.

The unsatisfactory performance of low-bandgap mixed tin (Sn)-lead (Pb) halide perovskite subcells has been one of the major obstacles hindering the progress of the power conversion efficiencies (PCEs) of all-perovskite tandem solar cells. By analyzing dark-current density and distribution, we identified that charge recombination at grain boundaries is a key factor limiting the performance of low-bandgap mixed Sn-Pb halide perovskite subcells. We further found that bromine (Br) incorporation can effectively passivate grain boundaries and lower the dark current density by two-three orders of magnitude. By optimizing the Br concentration, we successfully fabricated low-bandgap (1.272 eV) mixed Sn-Pb halide perovskite solar cells with open-circuit voltage deficits as low as 0.384 V and fill factors as high as 78%. The best-performing device showed a PCE of >19%. Our results suggest an important direction for improving the performance of low-bandgap mixed Sn-Pb halide perovskite solar cells.

In realizing new materials for low cost, high performing, solution processed photovoltaic (PV) devices, metal halide perovskites continue to show increasing potential. To enable solution processed tandem PVs by tuning the bandgap, as well as reducing the toxicity of conventional Pb-based perovskites solar cells (PSCs), Pb-Sn mixed perovskites have recently shown excellent capability (Eperon et al. Science 354, 861). However, one of the prevailing issues in these PSCs is the fast oxidation of Sn²⁺ to Sn⁴⁺ giving rise to a “self-doping” process (Noel et al. Energy Environ. Sci. 7, 3061). Sn⁴⁺ dopants play a major role in non-radiative recombination in Sn-based perovskites by acting as trap sites for mobile photo-generated charge carriers. This has made it difficult for these PCSs to have comparable efficiencies with Pb-only perovskites. Although there have been efforts to inhibit the oxidation process; most commonly with the addition of SnF₂ (Leijtens et al. ACS Energy Lett. 2, 2159), there are no known studies reported on the complete removal of Sn⁴⁺ defects.

Interestingly, it has been reported that the efficient extraction of Sn⁴⁺ from ionic mixtures in ores, alloys and food is dependent on the solvent system used. Based on this premise, we investigate a solvent extraction method to achieve the complete removal of Sn⁴⁺ through anti-solvent engineering for Sn-based perovskites, on a novel triple cation Pb-Sn mixed perovskite system with the formula Cs_0.05(FA_0.83MA_0.17)_0.95Pb_0.5Sn_0.5I₃ (bandgap 1.26 eV). The THz photoconductivity studies show high charge carrier mobilities for solvents which enable the highest Sn⁴⁺ extraction resulting in a maximum of 23.52 cm² V^-1 s^-1, the highest observed for Sn-based PSCs. This trend continues in XPS surface analysis which indicates a clear dependence on the anti-solvent system used. The most efficient Sn⁴⁺ extracting solvent also enables the highest grain sizes, lowest Urbach energies and highest PSC performance leading to champion PCE 11.62%. This is an 80% improved compared to other anti-solvents that extract Sn⁴⁺ less efficiently. Significant improvements were observed for the J_sc, where the highest value observed is 28.05 mA cm^-2 for the best performing devices compared to the highest values for the other two solvents tested in this work, 22.10 mA cm^-2 and 21.48 mA cm^-2, while dark leakage current is minimal with when Sn⁴⁺ is more efficiently extracted. Sn⁴⁺ is seen to act as shallow traps, as evident from the dark current analysis. Further strengthening the argument that solvent engineering aids in Sn⁴⁺ removal, the Pb-only analogue of the PSCs did not show a performance dependence on the anti-solvent used.

Next, we introduce a mechanism by which Sn⁴⁺ is extracted through complex formation with the optimized solvent extraction medium used. Building on this foundation, the complete removal of Sn⁴⁺ was achieved by tuning the solvent engineering process and annealing temperature. Finally, the performance is further enhanced by defect passivation by fabricating a wide bandgap perovskite structure at the grain surfaces, leading to efficiencies towards 15%. Here, we achieve 83% fill factor, which is the highest achieved for a Sn-based perovskite and strongly competes with the Pb-only PSCs.

To date, while solar cells (SCs) have been developed for years with striking advancement especially in silicon SCs and perovskite SCs, a new issue has been emerging in such progress: how to manufacture SCs in an environmental-friendly way with the help of non-toxic and durable materials. Taking such environmental consequences into consideration, recently, double perovskites A₂BCX₆ (A=Cs, B=Ag, Cu, In, C=In, Bi, Sb, X=Cl, Br, I) have provoked substantial interests among researchers, especially those who are eager to solve the toxic and unstable nature of lead halide perovskites. Since the capacity of the perovskite SCs for reaching high power conversion efficiencies has been widely revealed, it is of great potential that the A₂BCX₆ double perovskites would substitute the traditional perovskites for SCs in the future.
Continuous researches have revealed that Cs₂AgBiBr₆, though the processability and stability of which are favorable, has a relatively wide and intrinsically indirect bandgap which seems unsuitable for photovoltaics. To solve such problem, we pick up Cs₂AgBiBr₆ double perovskite as the matrix and, herein, report a feasible method for the synthesis of Cs₂AgSb_xBi_1-xBr₆ through the solution process, where antimony serves as the alloying element for narrowing the bandgap. Apart from former synthetic method through a time- and energy consuming process at over 300°C, in this work, we successfully dissolved the precursors in the dimethylsulfoxide in a short time then fabricated double perovskites thin films directly and quickly on the prepared substrates. Meanwhile, we also tested the thermostability and reliability of the thin films under different conditions, as well as the photoelectric properties and the band structure. Further, following a similar methodology of traditional perovskite SCs researches, we conducted an advanced study on the performance of the Cs₂AgSb_xBi_1-xBr₆ SCs. Conclusively, our results suggest that feasibility of controlled synthesis of Cs₂AgSb_xBi_1-xBr₆ thin films and indicate that Cs₂AgSb_xBi_1-xBr₆ double perovskites, benefited from its narrowed bandgap, would be more applicable as the photosensitive layer in SCs or photodetectors.

Lead halide perovskites have shown rapid advancements in power conversion efficiencies over a decade, leading to an increased focus in these materials for solar cells and other optoelectronics. However, stability issues and the unavoidable toxicity of lead prove potential barriers to large-scale applications and implementations of the lead halide perovskites. Bismuth has long been a replacement for lead in various applications, and some of the similarities in electronic properties make bismuth an interesting candidate to replace lead for optoelectronic materials. In this work, we describe bismuth halide perovskite and perovskite-inspired materials which demonstrate a wide range of properties. We show photoluminescene of bismuth halide precursor seeds in the synthesis of cesium bismuth halide nanocrystals, absorbance tunability of cesium bismuth halide nanocrystals based on halide composition, thermochromism of a layered double perovskite and the apparent sonocrystallization of nanocrystals from bulk crystals of layered double perovskites of various compositions. We identify promising properties of the bismuth materials and discuss major areas that require investigation in order to make bismuth halide perovskites viable materials for a variety of applications.

Within the PSC device stack, charge-selective contact selection has been shown to considerably impact PSC stability. Recently, a triarylamine-based hole-transport layer (HTL) doped with its oxidized organic salt analogue (EH44/EH44-ox) has been shown to bypass the need for hygroscopic lithium salt dopants, enabling high PSC stability in ambient conditions and room temperature. However, the improved stability came with a power conversion efficiency (PCE) penalty, and stability suffered at elevated temperatures. Broadening the applicability and understanding of this under-utilized HTL dopant system, we report design criteria for stable, synthetically simple triarylamine-based organic hole transport materials (HTM) and their corresponding oxidized salts as HTL dopants for improved PSC efficiency and stability at elevated temperature. The triarylamine-based dopants must contain at least two para-electron donating groups for radical cation stabilization to prevent impurity formation (e.g., dopant reduction) which significantly reduces PSC fill factors (FF). Furthermore, these dopants can be interchanged with respect to the HTL matrix for improved PSC FF and PCE. The stability of devices (unencapsulated in ambient atmosphere under constant load and illumination) prepared with these three new HTLs considerably outperform both EH44/EH44-ox and Li⁺-doped spiro-OMeTAD controls at 50 and 70 C. The ability to mix and match a stable dopant with a non-identical small-molecule-based HTL matrix for improved charge-transport properties has broadened the design scope for highly stable and cost-effective PSC without sacrificing performance. This is collaborative work with the National Renewable Energy Lab (NREL).

The further development of organic metal halide perovskites (OMHPs), and the optoelectronic devices in which they are used, requires an improved understanding of the factors influencing efficiency and stability. Application of surface ligands provides a route to passivate defect states, improve photoluminescence quantum yields (PLQYs), increase charge-carrier mobilities, increase material stability, and improve photovoltaic (PV) device performance. A number of surface ligands are reported in the literature to improve photoluminescence properties, stability, and PV performance, but a myriad of fundamental questions remain. For example, do these ligands remain on the surface? What sites are they binding to? Are they primarily passivating surface states or grain boundaries? Is the ligand binding dynamic or stable? We investigate a series of ligand binding groups and systematically probe the stability of the bound surface ligands, how they affect the PLQY, film stability, PV device performance, and PV device stability. We observe carboxylates, carboxylic acids, phosphonic acids, and zwitterionic molecules form reasonably stable surface bonds, whereas thiols do not. Thiols do not bind to the surface at a level that we can detect, however they do result in improved PLQYs and improved PV device performance. Ammonium salts react rapidly to displace the methylammonium cations, thus resulting in less control over formation of only a surface layer. Trichlorosilane also reacts rapidly and leads to thick surface layers that restrict charge transfer at the perovskite/electron transport layer (ETL) interface. Overall, our results indicate that thiol treatment provides a route to remove non-radiative trap states; however, when a surface monolayer on the perovskite is desired, such as to protect the perovskite film or manipulate the interfacial energetic, phosphonic or carboxylic acids are better suited.

We developed design principles for morphologically stable spiro-based hole transport materials (HTMs) and show the relevance of these findings to high-performance perovskite solar cells (PSCs). Chemical modification of an asymmetric spiro[fluorene-9,9′-xanthene] core is amenable to selective placement of redox active triphenylamine (TPA) units. We therefore leveraged computational and experimental techniques to investigate five HTMs bearing TPA groups judiciously positioned about this asymmetric spiro core. It was determined that TPA groups positioned about the conjugated fluorene moiety increase the free energy change for hole-extraction from the perovskite layer, while TPAs about the xanthene unit govern the Tg values. The synergistic effects of these characteristics resulted in an HTM characterized by both a low reduction potential (approximately 0.7 V vs. NHE) and a high Tg value (>125 °C) to yield a device power conversion efficiency (PCE) of 20.8 % in a PSC.

Perovskite solar cells have the potential to be utilized in space solar arrays due to potential cost and weight savings, and the considerable radiation hardness these systems have demonstrated [1]. Here, the potential of using high stability mixed cation perovskite solar cells for deep space high radiation missions is investigated. The suitability of these systems for space applications is evidenced by the lack of any phase transition between 4.2 K and 300 K, which demonstrates the excellent stability of these mixed cation perovskites. Temperature and intensity dependent current density-voltage (J-V) measurements are used to probe the solar cells under conditions consistent with the environments around the outer planetary systems of Mars, Jupiter, and Saturn. At low temperature, a barrier impeding current flow is observed in the structure and results in a largely decreased fill factor under 1-Sun AM0 conditions. However, under low intensity and low temperature (LILT) conditions, the fill factor and the power conversion efficiency are recovered. At low temperature, thermionic emission is the limiting factor for carrier extraction over the barrier and as such the small photocurrent generated at LILT conditions provide the conditions for efficient carrier extraction, even in situations where sample degradation may have occurred in transit.

[1] Y. Miyazawa et al., "Tolerance of Perovskite Solar Cell to High-Energy Particle Irradiations in Space Environment," iScience, vol. 2, pp. 148-155, 2018/04/27/ 2018.

Improving the open-circuit voltage (Voc) of photovoltaic devices (PVs) made with wide-bandgap (Eg>1.8 eV) perovskites is a major goal in the development of low-cost, high-efficiency tandem solar cells that use these materials as a top cell. There is significant interest in using perovskite quantum dots (PQDs) for this application. PVs made with CsPbI₃ PQDs with Eg=1.75 eV now hold the record power conversion efficiency (13.4%) for all quantum dot materials. PQD PVs show higher Vocs than PVs made with similar bulk perovskite composition, and furthermore, it is easy to manipulate the bandgap of the PQDs by synthetically tuning their size and composition. To date, however, PQD solar cells with Eg>1.8 eV have exhibited poor device performance, due especially to high voltage loss. To alleviate these problems, we have sought to develop PQD ligand chemistry that enables high efficiency PQD device fabrication. We have found that the performance of wide bandgap PQD PVs correlates with the photoluminescence (PL) lifetime and carrier diffusion length of the PQD layer, and that the performance of wide bandgap PQD PVs can be significantly improved with appropriate modifications of the PQD composition and surface chemistry. For instance, the exchange of native oleate and oleylammonium ligands with acetic acid and formamidinium significantly increases the carrier mobility, enabling devices with high Voc. We found that PQD PVs made from CsPb(I_1-xBr_x)₃ and FAPb(I_1-xBr_x)₃ with sufficiently wide bandgap for tandem devices exhibit decreased Vocs, short PL lifetimes, and decreased carrier diffusion lengths due to the high Br incorporation required for bandgap tuning. PL lifetime and carrier diffusion length of wider bandgap PQDs can be improved by mixing FA and Cs at the A-site, i.e., (FA_1-xCs_x)Pb(I_1-xBr_x)₃. Alloying of FA and Cs at the A-site balances Br incorporation at the X-site to yield wide bandgap PQD PVs with improved Voc. This work highlights potential strategies for producing the high-voltage, wide bandgap perovskite photovoltaic devices needed for a next generation of high efficiency tandem devices.

Design of new crystalline materials requires understanding the relationship between physical inputs of a system, such as chemicals, conditions, and molecular features, and the corresponding results. The chemical processes underlying crystallization in relatively simple systems can be confounded through small changes in solution concentrations, temperatures, or stochastic effects. Statistical models, such as machine learning, can aid in quantifying relationships between variations in chemical environment and reaction outcome. For success, statistical models targeting real-time data collection generally require data pipelines that assemble normalized datasets. Toward this end, we have developed a platform which brings together data storage, hardware-software interface, and simple experimental user interfaces into a single data pipeline. The key components of this pipeline include: generation of initial search parameters and robotic input files, accessible interfaces for data input by experimentalists, and in-line molecular feature calculations. Discussion topics will include the process for navigating the large initial experimental search space including automated physical constraints as well as user specifications, development of the technology stack to aid in data acquisition, and the resulting chemical ontology used to describe the underlying relationships between the chemistry and the data will be discussed. Particular attention will be given to what has emerged as ‘best practices’ for data curation, acquisition, and the development of a scalable workflow. An overview on the development and application to organohalide perovskite crystal formation will be discussed, and the work needed to implement similar processes for disparate chemical systems with moderate adjustment will be highlighted. This work provides an overview of the tools necessary for further development of real-time data capture in materials chemistry and outlines the first steps to construct larger databases for use in materials discovery.

Excellent flexibility with high mechanically stable of photonic devices are essential for next-generation integrated optoelectronic systems, wearable sensors and flexible consumer electronics. Here we describe hybrid perovskite based flexible photodetector for wearable sensor devices. The hybrid perovskites are excellent new materials with high carrier mobility, high photo conversion efficiency, tunable bandgap and excellent optoelectronic properties. These materials have advantage of low temperature and easy processing, and facile scale-up.
In this study, we discuss the design and experimental demonstration of hybrid perovskite based photodetector integrated on flexible sensor platform. We have fabricated perovskite-based flexible photodetector consisting of ITO/CdS/perovskite/V₂O₅/Au, focusing on the conformal coating of perovskite on the planar CdS by addition of PbBr₂ and PbCl₂ to PbI₂, which significantly increases the nucleation rate of perovskite-leading to highly smooth surface morphology of perovskite. This enhances the performance of photodetectors. The devices are fabricated on flexible PEN substrate which are withstand many bending cycles at a submillimeter radius without degradation of optoelectronic properties. Our findings provide a hybrid perovskite sensor platform for photonic devices to improve their role in the area of flexible and mechanically stable optoelectronic systems.