Symposium S.EN01—Next Steps for Perovskite Photovoltaics and Beyond

Formamidinium lead iodide (FAPbI₃) is one of the most extensively studied perovskite materials due to its narrow band gap and high absorption coefficient, which makes it highly suitable solar cells applications. Deposition from a solution containing lead iodide (PbI₂) and formamidinium iodide (FAI) or by sequential deposition of PbI₂ and FAI usually leads to the formation of films with poor morphology and unstable crystal structure that readily crystallizes into two different polymorphs: the yellow phase and the black phase. I will show that, 2D 2-phenylethylammonium lead iodide (PEA₂PbI₄) thin films deposited by a scalable doctor-blade coating technique can be used as a growth template for the high-quality 3D FAPbI₃ perovskite thin films, by organic cation exchange. I will discuss the structural, morphological and optical properties of these converted 3D FAPbI₃ perovskite films, comparing them to the properties of directly deposited 3D FAPbI₃ films. The converted FAPbI₃ thin films are compact, smooth, highly oriented and exhibit better structural stability in comparison to the directly deposited 3D films. These results not only underscore the importance of the employed deposition techniques in the formation of highly crystalline and stable perovskite thin films but also reveal a strategy to easily obtain very compact perovskite layers using doctor-blade coating.

All solid-state solar cells based on organometal trihalide perovskite absorbers have already achieved distinguished power conversion efficiency (PCE) to over 25% and further improvements are expected up to 27%. Now, the research on perovskite solar cells has been moving toward commercialization. To facilitate commercialization of these great solar cells, some technical issues such as long-term stability, large scale fabrication process, and Pb-related hazardous materials need to be solved. Also, flexible perovskite solar cell using plastic substrate can be used in niche applications such as portable electrical chargers, electronic textiles, and large-scale industrial roofing.
This talk is dealing with our recent efforts to facilitate commercialization of perovskite solar cells. For examples, we introduce a recycling technology of perovskite solar cells, which covers the regeneration process of Pb contained perovskite layer as well as recycling process of Au electrodes and transparent conducting oxide glass. Also, recent interesting results regarding ultra-flexible perovskite cells will be discussed in terms of stress analysis. Finally, simple fabrication technologies for realizing large scale perovskite module is going to be introduced.

Nitrides provide a rich, and generally underexplored, field of chemistries for solar energy conversion, with many of them displaying excellent optoelectronic, electronic, and mechanical properties, as well as chemical stability. Prediction of ternary nitrides via computational modeling can significantly streamline the materials discovery process. First principles calculations were used to facilitate materials discovery within the Mg-Sb-N phase space, providing crystal structures to compare to and a predicted chemical potential phase diagram to help tune parameters. Guided by these predictions of favorable phases and the required conditions to achieve them, radio-frequency (RF) magnetron co-sputtering was used to deposit the films. The computationally predicted Mg₂SbN₃phase, which has a wurtzite-derived structure, and a lattice constant similar to that of GaN, has been experimentally synthesized for the first time [1]. In addition, the antiperovskite Mg₃SbN phase, which has previously been seen only in bulk form, has been grown as a thin film. This Mg-rich phase has a variety of predicted properties ideal for photovoltaics, such as a large dielectric constant, low effective masses, and a measured near-gap PL at room temperature, at around 1.3 eV. Both of these phases show up within the same combinatorial sample across the spread of Mg to Sb.

[1] Heinselman et. al., Chemistry of Materials (2019), DOI: 10.1021/acs.chemmater.9b02380

Due to quantum confinement effects, nanoscale materials offer vastly different optoelectronic properties from their bulk counterpart. In the last decade, one promising nanomaterial that has emerged is the perovskite which has a crystal structure of ABX₃ where A and B are metallic cations and X is a halide anion. Perovskites have shown extremely large molar absorptivity, high quantum yield photoluminescence, wide visible spectral coverage, and large defect tolerance that together make them ideal for many optoelectronic applications including photovoltaics, light emitting diodes, and lasing. To maximize device applicability, monodispersed nanocrystals (NCs) must be synthesized and then assembled into superstructures over multiple length scales. Currently, most syntheses control the size of NCs by using long aliphatic surface passivating ligands which are inherently insulating molecules and create a barrier for interparticle electronic interactions. To overcome these challenges, we have synthesized nearly monodispersed (~3 nm) CsPbX₃ NCs (X = Cl, Br, and I) with poly(ethylene glycol)6 (PEG6–NH₂/COOH) surface passivating ligands. We selected PEG6 ligands because they offer several unique advantages including increased conductivity through electron dense oxygen atoms and their effectiveness to lead to hierarchical superstructure formation. Importantly, by varying the PEG concentrations in the reaction mixture, we control the mesoscale assembly of our NCs from lamellar to nanorice structures. Interestingly, these mesoscale assemblies result in a band gap variation of 240 meV with the same ~3 nm NCs due to the delocalization of the exciton wavefunction. We believe that the dipole-dipole interactions between NCs, along with van der Waals attractions between PEGs, induce the mesoscale assemblies. Furthermore, these mesoscale assemblies form micron length twisted ribbon superstructures. Thus, by using polymer assisted forces at the nano- and meso- scales, we synthesize various CsPbX₃ NCs assemblies, which interact collectively through exciton wavefunction delocalization providing a pathway to enhance future device applications.

The integration of ultrasonic monitoring techniques with silica sol-gel materials, for the purpose of tamper indication, presents a novel set of challenges and with it, various solutions through integration of both fields. Tamper indication plays a vital role in the dismantlement of nuclear equipment by ensuring that the intergrity of the stored equipment can be maintained.

Due to the responsive nature of sol-gel silica, it has the potential to be used as a visual indicator of tamper as sealants. In this research, the sol-gel precursor samples are synthesised using tetramethyl orthosilicate (TMOS) as a precursor and a base catalyst to amplify the effects of tamper. However, this technique can be subject to human error, therefore to alleviate such problems, ultrasonics will be explored in conjuction with the sol-gel chemistry. Ultrasonics has been used within the field of structural health monitoring (SHM) due to its high sensitivity and non-destructive nature. Piezoelectric materials are used to transmit the acoustic waves through a sample, as well as, receive any waves reflected back from the samples or any present defects. The combination of these techniques has not previously been fully explored, so the aim of this research is to develop a highly effective and accurate interdisciplinary method of tamper indication.

Metastable sol-gel formulations show shrinking and fracturing upon solvent evaporation but varying the catalyst concentration can tailor the material stability, so an appropriate concentration was decided upon. Initial experiments were then done to check compatibility of the piezoelectric transducer with the sol-gel formulations. After which, bulk wave transducers were used to test out the responsiveness of the system to sol-gel degradation. Results indicate that it is possible to use these piezoelectric transuders to monitor these sol-gel degradation points, as well as, determine the point of exposure.

The presentation will explore the capabilities of the ultrasonic components to monitor the degradation of sol-gel formulations during simulation of a tamper, as well as, the challenges overcome to ensure integration of both materials.

References:

1 D. Balageas, C.-P. Fritzen and A. Güemes, Structural Health Monitoring., John Wiley & Sons, 2010.
2 A. E. Danks, S. R. Hall and Z. Schnepp, Mater. Horiz., 2016, 3, 91–112.
3 C. H. Zhong, A. J. Croxford and P. D. Wilcox, Proc. R. Soc. London A Math. Phys. Eng. Sci.
4 G. Robin, F. Vander Meulen, N. Wilkie-Chancellier, L. Martinez, L. Haumesser, J. Fortineau, P. Griesmar, M. Lethiecq and G. Feuillard, Ultrasonics, 2012, 52, 622–627.
Ministry of Defence © Crown Copyright 2019/AWE

Rapid decline in fossil fuel energy necessitates the immediate need for renewable energy resources. Here, we report a previously unexplored class of nitrides AMN₂ (A=Na, K, Rb, Cs, M=V, Nb, Ta) keeping renewable energy applications in mind. Using a detailed structure and stability analysis using first principles simulation, we discovered twelve such compounds (few of which were already synthesized before), which are chemically, mechanically, and dynamically stable. These twelve compounds were then evaluated for their suitability for three renewable energy applications: (i) photovoltaics, (ii) water splitting, and (iii) thermoelectrics. Careful analysis of electronic structure reveals high optical transition strength resulting in a sharp rise in absorption. This in turn yields high short circuit current and hence excellent solar efficiency for a few compounds namely CsVN₂ and RbVN₂. Along with excellent absorption quality, some compounds show favorable band edge positions compared to water redox levels and hence are promising as photoelectrodes in photo(electro)chemical water splitting devices. Mixture of flat and dispersive bands in the band structure yields both high Seebeck and electrical conductivity, thus an excellent power factor for six compounds. Simulated lattice thermal conductivity shows moderate to ultralow values and thus the possibility of achieving a high thermoelectric figure of merit (ZT), even at lower temperatures. From the experimental perspective, we discuss the possible challenges that may arise while utilizing these compounds for the desired applications and suggest possible pathways to overcome them. We believe such theoretical prediction of promising materials is extremely useful for new materials discovery and anticipate rapid response from the experimental community.
Journal Reference : Phys. Rev. Materials 3, 105405 (2019).

Power conversion efficiencies (PCE) of perovskite solar cells (PSCs) has rising from the initial 3.8% to the state-of-the-art 25.2% within the past few years. Current PSCs intended for commercialization in the near future require both high efficiency and good long-term stability. Most highly efficient PSCs utilize an n-type layer of mesoporous titanium dioxide or tin oxide in an n-i-p device configuration, in which organic conductors are widely used to transport holes into an adjoined metal. Thus far, a variety of efforts have been devoted to achieve a defect-less perovskite film with high-quality morphologies to realize reduced loss-in-potential outcomes and enhanced efficiency levels. In this talk, we will discuss several challenges that need to be addressed in improving the photovoltaic performance and the stability of the perovskite solar cells, i.e. (1) preparation of high crystalline film of (FAPbI3)1-x(MAPbBr3)x with a large grain size and a preferred orientation, (2) incorporation of wide band gap halide into narrow band gap halide perovskites, (3) development of selective charge-transporting layers (CTL) with a superior stability, (4) interfacial control between the perovskite and the CTLs, and (5) the ion migration at the interface boosted by oxygen and light. Finally, our recent works on scale-up of PSCs will be discussed. Our strategy as presented in this work will offer new directions for those involved in the fabrication of highly efficient, stable and scalable PSCs.

Halide perovskites are currently one of the most heavily researched emerging photovoltaic materials. Despite achieving remarkable power conversion efficiencies, perovskite solar cells have not yet achieved their full potential, with the interfaces between the perovskite and the charge-selective layers being where most recombination losses occur. In this study we employ a fluorinated ionic liquid (IL) to modify the perovskite/SnO₂ interface. Using Kelvin probe and photoelectron spectroscopy measurements, we show that depositing the perovskite onto the IL results in the crystallisation of a perovskite film which has a more n-type character, evidenced by a decrease of the work function and a shift of the Fermi level towards the conduction band. We use photoluminescence spectroscopy and time-resolved microwave conductivity to investigate the optoelectronic properties of the perovskite grown on neat and IL-modified surfaces and find that the more n-type perovskite exhibits an order of magnitude lower trap density than the control film. When incorporated into solar cells, this interface modification results in a reduction in the current-voltage hysteresis and an improvement in device performance, with the best performing devices achieving steady-state PCEs exceeding 20%. Combining this modification with passivation treatments at the perovkite/HTM interfaces, allow us to achieve further enhanced PCEs (>22%) and improved long-term stability.

Vacuum deposition is a key technique in the field of perovskite photovoltaics because it presents significant advantages with respect to the classical solution deposition methods. In particular, avoiding the use of solvents and perovskite annealing steps makes it a good candidate for industrialization and upscaling, as it has already been demonstrated in the OLED field.
Despite these advantages, tuning the interfaces is essential to reach power conversion efficiencies as high as those achieved by solution processed perovskite solar cells. Metal oxides are an interesting choice for extraction layers due to their improved stability and reduced parasitic absorption. In this work we present our recent advances in using metal oxides as extraction layers optimising the interfaces in order to reach record open circuit voltages in CH₃NH₃PbI₃ perovskite and being able to use MoO₃ as extraction layer in both PiN and NiP structures through molecular passivation improving the stability and opening new possibilities for tandem applications. The developed pinhole-free interfaces show a perfect compatibility with the intrinsically large scalable vacuum deposition, making possible the implementation of those into large area devices and mini modules with minimal losses, opening an interesting door for industrial purposes.

After many decades of photovoltaic research we would expect that there should be a common scientific understanding on the necessary ingredients to make almost perfect solar cells. However, the unprecedented success of metal-halide perovskites (MHPs) stimulated discussions on whether there could be a certain, yet non-established secret that distinguishes this material from others. The present contribution compares the performance of MHPs to other photovoltaic materials in terms of a general top-down approach. This direct comparison allows one to judge the strengths and weaknesses of different technologies but also gives some insight on the specific achievements, e.g. that, on the one hand, the radiative yield of MHPs is second only to the best GaAs solar cells, which implies open circuit voltages that come close to the thermodynamic limits. On the other hand the relative contribution of fill factor losses to the overall efficiency losses in MHPs is relatively large compared to most other technologies. Fill factor issues can generally originate from recombination or voltage losses during charge carrier collection, both aspects playing a role for MHPs. This issue will be discussed in terms of new, general models for the charge carrier separation in solar cells. Finally, the question will be addressed whether specific bulk properties of MHPs, like the presence of an indirect band gap, the density of states, or small phonon energies could be of importance for their favorable photovoltaic behavior.

Metal halide perovskites (MHP) are the best solution-processed solar cell device technology to date. While record device have been achieved using spin-coating, larger area prototype realization requires the translation of processing strategies to scalable deposition methods. An in-depth understanding of how solvent molecules dictate the molecular structure of solution complexes as well as define thin film morphology by being incorporated into crystalline intermediate phases enables rational process optimization.
This talk will portray our team research on solution-processing of larger area MHP prototypes by inkjet printing and slot-die coating. We employ combinatorial inkjet printing for the systematic optimization of MHP composition for multi-junction solar cells as well as light-emitting devices. We discuss our approach to rapidly identify MHP compositions of suitable stability for large-scale deployment of MHP-based devices.
By investigating fundamental solution chemistry explaining spectral features of solution species through a combination of experiment and DFT calculations we rationally design inks for slot-die coating and inkjet printing. Spectral features of fundamental building blocks of MHP semiconductors identified in solution are then tracked to investigate the association of molecular species into extended poly-complexes, colloids and small perovskite seed crystals.
Through in-situ photoluminescence and absorption measurements during spin-coating we were able to identify the formation mechanism of several mixed cation/anion MHP semiconductors that enables us to translate film deposition strategies to scalable process techniques such as inkjet printing and slot-die coating. In-line process monitoring is used to optimize processing parameters to manufacture high-quality MHP semiconductor layers.
We will be presenting results on the fabrication of prototypes of MHP-based devices demonstrating inkjet-printed and slot-die coated small area solar cell devices with power conversion efficiencies > 15% and laser-scribed series interconnected mini-modules of > 12% on active areas > 10 cm². These results will be compared with the collected data of all MHP solar cell devices published that we are currently collecting in an open database through a crowd-sourced effort and that will be launched in spring 2020.

Manifested by high absorption coefficient, large carrier diffusion length, ambipolar charge transport and moderate mobility of charge carriers, organo-lead halide perovskites recently emerging as promising candidate for high efficiency and low cost solution processed solar cells, needs no introduction. Perovskite Solar Cells (PSCs) have already gathered considerable attention, showing a tremendous hike in efficiency from 3.8%1 by Kojima et al. (2009) to 22.1%2 by Yang et al. (2017). A lot of research has been done on the efficiency improvement of PSCs by optimization of the film morphology at the interfaces by various characterization techniques including electrochemical impedance spectroscopy (EIS) in a solid state active device geometry.3 Recently, the optimization of the film morphology at liquid electrolyte interface by EIS is trending as a more simplified approach. Li et. al. have measured the flat band potential, density and type of charge carrier at the perovskite-liquid interface from Mott-Schottky plot for spin coated and spray coated films of methylammonium lead tri-iodide (MAPbI3) perovskite.4 Srivastava et.al have optimized the morphology of MAPbI3 perovskite thin films at liquid interface by controlling nucleation and growth during film fabrication by spin coating the films on preheated substrates.5 However, there is a need of understanding the charge transfer at these interfaces in more detail.
Here, we have studied the kinetics of charge transfer and diffusion at the hybrid organic-inorganic perovskiteliquid electrolyte interface under the effect of applied bias. By applying the different dc bias from 0 to +1V and 0 to -1V, it was found that the ion diffusion at low frequency regime gets modulated due to charge accumulation. The charge transport resistance is initially increased to a maximum at around 0.4 V applied bias along with the decrease in the space charge capacitance. A transition state is observed at around 0.4 V to 0.6 V due to strong electronic-ion interaction where charge transport resistance decreases and capacitance increases. However at higher applied bias voltage charge transport resistance increases again and capacitance start to decrease due to excess ion accumulation. The perovskite films show similar trend of change in impedance under both positive and negative bias. The Mott-Schottky Plot for forward and reverse voltage scan shows n-type and p-type behavior which indicates the ambipolar nature of perovskite semiconductor. The significant difference in the impedance spectra can be seen in dark and light which is attributed to the high absorption coefficient of the perovskite material. We have proposed a model to explain the charge kinetics across the perovskite-liquid electrolyte interface.
References
(1) Akihiro Kojima Yasuo Shirai, and Tsutomu Miyasaka, K. T. J. Am. Chem. Soc. 2009, 131, 6050–6051.
(2) Yang, W. S.; Park, B. W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S.I. Science 2017, 356 (6345), 1376–1379.
(3) Bag, M.; Renna, L. A.; Adhikari, R. Y.; Karak, S.; Liu, F.; Lahti, P. M.; Russell, T. P.; Tuominen, M. T.; Venkataraman, D. J. Am. Chem. Soc. 2015, 137 (40), 13130–13137.
(4) Li, Z.; Mercado, C. C.; Yang, M.; Palay, E.; Zhu, K. Chem. Commun. 2017, 53 (16), 2467–2470.
(5) Srivastava, P.; Parhi, A. P.; Ranjan, R.; Satapathi, S.; Bag, M. ACS Appl. Energy Mater. 2018, 1, 4420–4425.

so far, an enormous progress has been made for perovskite photovoltaics reaching over 23% power conversion efficiencies. The compositional engineering as well as controlling the crystallization process were key factors in achieving such highly efficient devices. However, fundamental problems, such as the toxicity of hybrid lead halide perovskites, hysteresis and structural instability remain to be solved for perovskite solar cells. Indeed, the low-temperature solution-processing of perovskite films inevitably causes formation of a certain amount of defects on the surface and at the grain boundaries, which lead to serious trapping, charge accumulation, and recombination problems as well as stability issue. In this work, we will pursue the interface engineering and defects passivation by introducing organic ammonium salts on the perovskite active layer. We will discuss the origin of crystallization changes and performance improvement of treated devices based on the results achieved from optoelectrical and structural investigations.

A new type organic-inorganic hybrid perovskite has appeared to be a wonder material for its excellent optical absorption, long range charge-carrier diffusion and apparent tolerance to defects. In the last few years, it has been emerged as a primary candidate material for various photovoltaic, optoelectronic and photoelectronic applications. In just a few years, its solar cell efficiency has been improved from 3.8% to >25%. Moreover, the solar cell fabrication processes based on the planar architecture have been particularly enthusiastic thanks to their low temperature fabrication and compatibility with a range of substrates. Comparing solution deposition with vacuum deposition, the vacuum processes for thermal co-deposition and sequential deposition of PbCl₂ and CH₃NH₃I materials are recognized as efficient means to prepare perovskite film with good uniformity and high surface coverage. A vacuum deposition process has been developed to fabricate high efficiency perovskite solar cells with high stability using alternating layer-by-layer vacuum deposition. The new deposition process allows us to relax the strict deposition monitoring and control measures, while realizing superior uniformity in film morphology, surface coverage and smoothness, together with crystalline phase purity.
For the high efficiency perovskite solar cells, the power conversion efficiencies for the planar device is as high as 23.4%. More importantly, we have developed a superior low temperature modified SnO₂ material for ETL and transferred the cell fabrication process onto lightweight flexible polymeric substrate. The highest cell efficiency achieved was over 20%, it is also the highest efficiency among the flexible perovskite cells reported. Meanwhile, the devices show very good stability over long term exposure in ambient with very low degradation. After a representative cell was exposed in ambient lab condition for a year, its final cell efficiency is as high as over 95% of its initial efficiency with its degradation accounts for only smaller than 5%. Further analysis on the stability of the perovskite solar cells will be discussed. We have also developed a series of single-crystalline perovskites with superior stability and optoelectronic performance.

Reference
1. D. Yang, R. Yang, X. Ren, X. Zhu, Z. Yang, C. Li, S. Liu*, Advanced Materials, http://dx.doi.org/10.1002/adma.201600446.
2. D. Yang, R. Yang, J. Zhang, Z. Yang, S. Liu, C. Li, Energy Environ. Sci. 2015, 8, 3208.
3. D. Yang, Z. Yang, W. Qin, Y. Zhang, S. Liu, C. Li, J. Mater. Chem. A 2015, 3, 9401.

Microstructures of various interfaces (grain boundaries, heterophase interfaces, etc.) in halide perovskite materials/devices play very significant roles on the physical properties and optoelectronic functions. While enormous effort has been devoted to modifying the HP interfaces for better device performance, the insights into the interface microstructures and their exact role in devices have been obscured. In this talk, I will first present a fundamental discussion on HP interface microstructures from a materials-science perspective. Then, I will demonstrate several unique chemical approaches to tailor the HP interface microstructures in a precisely-controlled manner, which will not only lead to electronic passivation of defects, but also endow these interfaces with new functions. Finally, I will share my perspectives about future research in this promising direction.

In recent years, the interest in halide perovskites rose at a rapid pace due to their tremendous success in the area of photovoltaics; but other fields, like light emitting diodes, show great potential as well. In optoelectronic devices, the function and performance depend crucially on the proper alignment of the energy level landscape throughout the device, i.e. allowing for efficient charge transport across the various interfaces.
In perovskites it turned out that these interfaces can be rather complex. On the one hand, interface dipoles and band bending occur. But more importantly, the perovskite composition and formation can be significantly influenced by chemical reactions taking place at these interfaces.
In this talk I will show how UV as well as X-ray photoelectron spectroscopy measurements can help to probe and understand the processes going on at the various bottom contact materials. Intriguingly, clear differences are found between the formation of interfaces to organic materials and to metal oxide; furthermore, the perovskite processing (i.e. solution vs. evaporation) as well as the perovskite composition play significant roles as well.

The electrical properties of metal halide perovskite solar cells (PSCs) have been intensively studied by many researchers using capacitance-based techniques such as thermal admittance spectroscopy (TAS) and capacitance–voltage (C–V) measurements. The capacitance–frequency–temperature (C–f–T) spectra of PSCs reported in the literature [1-4] typically show two capacitance signatures, one at high frequency (> 10⁵ Hz) and the another at low frequency (> 10³ Hz). These capacitance signatures have been used to evaluate the properties of defects in PSCs [1-5]. Here, we show that the charge-transport layers (CTLs) in PSCs can have significantly influence on the capacitance signatures. We find that the capacitance signature at high-frequencies and low temperatures is due to the trap and detrap of charged carriers in the hole-transport layer (HTL). We further show that high-frequency capacitance measured from HTL-free PSCs represents the geometric capacitance, allowing a reliable calculation of the dielectric constant of perovskite layers. The presence of HTL in PSCs leads to apparent temperature dependent dielectric constant [3], due to the capacitance contribution of HTL. The low-frequency capacitance signature can be used to characterize the ionic transport properties of the perovskite layer. However, to ensure meaningful results, the capacitance coupling between perovskite layer and CTLs must be carefully avoid, by choosing the appropriate CTLs and device configuration. Our results explain well the large discrepancies of electrical properties of perovskites reported in the literature.


[1] H.-S. Duan, H. Zhou, Q. Chen, P. Sun, S. Luo, T.-B. Song, B. Bob, Y. Yang, The identification and characterization of defect states in hybrid organic–inorganic perovskite photovoltaics, PCCP, 17 (2015) 112-116.
[2] M. Samiee, S. Konduri, B. Ganapathy, R. Kottokkaran, H.A. Abbas, A. Kitahara, P. Joshi, L. Zhang, M. Noack, V. Dalal, Defect density and dielectric constant in perovskite solar cells, 105 (2014) 153502.
[3] O. Almora, I. Zarazua, E. Mas-Marza, I. Mora-Sero, J. Bisquert, G. Garcia-Belmonte, Capacitive Dark Currents, Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells, The Journal of Physical Chemistry Letters, 6 (2015) 1645-1652.
[4] W. Li, W. Zhang, S. Van Reenen, R.J. Sutton, J. Fan, A.A. Haghighirad, M.B. Johnston, L. Wang, H.J. Snaith, Enhanced UV-light stability of planar heterojunction perovskite solar cells with caesium bromide interface modification, Energy & Environmental Science, 9 (2016) 490-498.
[5] O. Almora, M. García-Batlle, G. Garcia-Belmonte, Utilization of Temperature-Sweeping Capacitive Techniques to Evaluate Band Gap Defect Densities in Photovoltaic Perovskites, The Journal of Physical Chemistry Letters, 10 (2019) 3661-3669.

This decade witnessed rapid progress in the development of organic-inorganic hybrid lead-halide perovskite-based solar cells. In particular, this class of solar cells showed high power conversion efficiency and significant potentiality to realize a low-cost terawatt-scale power generation. This is mainly due to its high optical absorption coefficient with sharp absorption onset, long charge-carrier lifetime, tunable band-gap and low-cost fabrication. However, despite advancement in the device performance, the underlying physics of charge-carrier dynamics needs important attention to develop insight into the performance bottlenecks and thereby better control and optimization over the device performances. Among the performance bottlenecks, the charge-carrier recombination is considered to be one of the dominant mechanisms that limit solar cell behavior. Therefore, in this work, the underlying interest is to propose a phenomenological model of recombination by simulating the temperature-dependent transient light ideality factor. We consider a standard sandwich device structure with CH3NH3PbI3 as an active layer. The Spiro and TiO2 are taken as hole- and electron-transporting layers, respectively. We utilized one-dimensional drift-diffusion equation to simulate transient open-circuit voltage Voc (t) with varying light-intensity, Φ between 0.2 to 2.0 Sun-equivalent and device temperature, T between 100 to 300 K. The transient light ideality factor, nid (t) is determined by evaluating the slope of the plots between Voc (t) vs ln(Φ). The plot of temperature-dependent transient light ideality factor shows a fast rise in its value between the temperatures 100 to 150 K and tends slowly towards 2.0 at room temperature and above. The observations clearly indicate that the recombination of charge-carriers are predominantly due to trap-states, interfacial or bulk, activated at higher temperatures. The work shed light on the recombination mechanisms in such class of solar cells and their alternative variants.

Halide perovskites offer exciting potential as photovoltaic materials and simply as semiconductors. Specifically, their structural tunability has become of greater interest as researchers begin to search for novel ways to tune the materials to achieve improved solar cell stability or to target new applications. One potential technology which halide perovskites could enable is dynamically switchable photovoltaic windows, windows which a user can transition between photovoltaically active (dark) and non-photovoltaic (transparent). We build toward this goal in this work by investigating the intercalation and deintercalation of methylamine gas into 2-dimensional Ruddelson-Popper phase halide perovskites. As has been shown with methylammonium lead iodide films, the intercalation of methylamine into the halide perovskite lattice results in a color change to a clear crystal phase. We find that in some 2-D perovskite systems, deintercalation of the methylamine gas is incomplete, resulting in the formation of some CH₃NH₃PbI₃; however, other 2-D perovskite phases show reversible intercalation/deintercalation with methylamine, indicating stronger binding between the long-chain ligand and the lead halide octahedra of the 2-D perovskite sheet. When integrated into a hybrid 2-D/3-D structure of the type A'₂(CH₃NH₃)_nPb_nI_3n+1, where A' is a strongly binding R-NH₃⁺ moiety such as phenethylammonium (PEA), these materials show promising reversibility for methylamine intercalation/deintercalation with little change in absorption or morphology. This work reveals the relative affinity of various R-NH₃⁺ molecules for the halide perovskite lattice, showing that many of these are not replaced by methylamine, and indicates that templating the 3-D CH₃NH₃PbI₃ structure with long-chain ammonium cations could lead to better reversibility in dynamic photovoltaic windows.

Multijunction all-perovskite solar cells offer a route towards efficiencies of III-V materials at low cost by combining the advantages of low thermalization loss in multijunction architectures with the beneficial properties of perovskites - namely low processing cost, high throughput fabrication, and compatibility with flexible substrates. One of the main limitations of all-perovskite tandems is the difficulty in achieving stable, high voltages with the wide bandgap subcell. Voltage losses in wide gap perovskites are generally high due to iodide-bromide segregation; mixing iodide and bromide on the X-site is the most common way to achieve the bandgaps of 1.7-1.85eV suitable for tandems.
Here, we demonstrate a bandgap tuning strategy based on incorporating A-site cations of mismatched size, namely dimethylammonium (DMA) and cesium cations. This increases the bandgap and therefore reduces the amount of bromine needed to attain a certain bandgap. We achieve a band gap of 1.7eV with a bromine concentration of only 20%, minimizing photoinduced halide segregation. We find that this strategy results in higher voltages and efficiencies that are stable under operation.
Surprisingly, we find that adding either DMA or cesium alone produce increases in bandgap. While cesium shrinks the lattice (as would be expected for an increase in bandgap), adding DMA causes the lattice to expand but still produces a bandgap increase. We will discuss the nature of this unusual mechanism and its use for further advances in high voltage wide bandgap devices. We will discuss the latest results demonstrating excellent stability of the DMA-containing materials under operation, thermal stress, and the combination of the two.
The robust and efficient nature of the DMA-containing wide bandgap materials makes them perfect for use in all-perovskite tandems. By combining these cells with advances in atomic layer deposition for robust and stable interlayers, we have fabricated two-terminal all-perovskite tandem solar cells with over 23% PCE; we will also discuss the latest developments in all-perovskite tandems.

Addressing the stability of organic-inorganic perovskite solar cells (PSCs) is needed to propel PSCs towards commercialization. Due to the inherent chemical reactivity of halide perovskites, most efforts have focused on improving their stability due to environmental agents. In contrast, intrinsic instability, including possible reaction between perovskite layer and contact materials in a device, has not been examined closely. Previously we showed that the stability is lowered and decomposition reaction is altered in methylammonium iodide (MAI) and methylammonium lead iodide (MAPbI₃) when they are in contact with NiO, but no change was observed when contacting TiO₂. In this talk, we will present a comprehensive study on thermal stability and degradation pathway in neat formamidine hydroiodide (FAI) versus when it is in contact with NiO or TiO₂. Using a combined temperature programmed desorption-mass spectrometry-Fourier transform infrared spectroscopy apparatus, we observe that the bulk decomposition of FAI powders proceeds with the formation of triazine (C₃H₃N₃) and hydrogen cyanide (HCN) as the major gas products at 265 °C while ammonia (NH₃) gas evolve above 300 °C. However, when FAI is mixed with NiO nanoparticles, the decomposition temperature lowers to 172 °C with NH₃ and water vapor releasing first, whereas C₃H₃N₃ and HCN are released at 243 °C. The releasing of NH₃ and water vapor at low temperature, which is absent when FAI is by itself, indicate interfacial reactions. Moreover, studies on FAI and FAI/NiO_x films show that the interfacial stability of films is further lowered to ≤ 150 °C, much lower than powder samples. These findings emphasize that interfacial reactions can be significant at typical processing conditions of FAI based perovskite devices and can induce intrinsic degradation in perovskite devices. We will explain the experimental results based on thermodynamic free energy calculations and propose mitigation strategies.

Wide bandgap metal halide perovskites are promising candidates to pair with low bandgap silicon, copper indium gallium diselenide, or perovskite photovoltaics for highly efficient next-generation tandems due to their excellent optoelectronic properties, low-cost manufacturability, and bandgap tunability.(1) However, wide bandgap perovskite solar cells have larger open circuit voltage deficits than their low or medium bandgap counterparts due to both their inferior charge carrier diffusion lengths and photoinduced halide phase segregation into high bandgap (bromine-rich) and low bandgap (iodine-rich) domains.(2) Here, we overcome these material challenges by alloying chlorine into the perovskite lattice to create a triple halide perovskite absorber, a phase space that has thus far been overlooked. Using x-ray diffraction and external quantum efficiency onsets, we show that chlorine is incorporated into the lattice in molar amounts up to 15% and increases the bandgap of the alloy to a greater extent than addition of bromine or cesium, which are commonly used in perovskite bandgap tuning. Importantly, the addition of chlorine drastically improves the optoelectronic properties of the material, resulting in doubled charge carrier mobilities and lifetimes. We show that a 1.67 eV wide bandgap triple halide perovskite does not experience photoinduced halide phase segregation as determined by photoluminescence at laser intensities up to 100 suns. Finally, we incorporate the triple halide perovskite absorber into solar cells, obtaining >20% efficient single junction devices with open-circuit voltages >1.2 V and 1 cm² monolithic perovskite/silicon tandem solar cells with voltages approaching 1.9 V and efficiencies >27%, exceeding the silicon single-junction record.(3) Solar cells with the triple halide perovskite absorber retain 95% of their initial efficiency under maximum power point tracking at 60°Cunder white light illumination for 1000 hours.

1. T. Leijtens, K. A. Bush, R. Prasanna, M. D. McGehee, Opportunities and challenges for tandem solar cells using metal halide perovskite semiconductors. Nat. Energy. 3, 828–838 (2018).
2. E. T. Hoke, D. J. Slotcavage, E. R. Dohner, A. R. Bowring, H. I. Karunadasa, M. D. McGehee, Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem. Sci.6, 613–617 (2015).
3. Best Research-Cell Efficiencies (2019), (available at https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20190923.pdf).

Our progress at developing both low and high band gap perovskites for tandem solar cells with be presented. We have made highly stable low band gap cells with greater than 19 % power conversion efficiency and triple halide high band gap cells with > 20 % stability that do not suffer from light-induced phase separation.

We will also show drift-diffusion modeling results that take into account how mobile halogen vacancies modify the electric field within perovskites. We have input reasonable ranges of charge carrier mobilities, bulk recombination rate constants, surface recombination velocities, mobile ion density and energy levels into the simulator. We will explain mysterious trends reported in the literature and make predictions of what needs to be done to improve efficiency.

Enormous efforts have been made in the last 8 years to up-grade the performance of lead halide hybrid perovskite solar cells (PSCs).¹ Although efficiency level has reached 25%, PSCs face serious challenges of practical stability and durability required for industrialization. Compositional engineering of lead halide perovskites by mixing different cations and anions, using modulator molecules and mixing 2D and 3D structures have improved the stability of perovskites against heat and moisture. However, organic cations in halide perovskites (methylammonium, etc.) and use of diffusible ionic dopants in hole transport materials (HTMs) are responsible for low stability of perovskites at high temperatures (>120^oC). In this respect, use of all-inorganic perovskite materials and dopant-free HTMs is highly desired.² We have conducted some work in this direction which includes stabilization of CsPbI₃ black phase³ and use of dopant-free HTMs. We could show that PSCs with all-inorganic perovskites and dopant-free HTMs are capable of efficiency up to 15%.² Further, Open-circuit voltage was found to be enhanced over 1.4V, amply high level as a visible light absorber. In preparation of all-inorganic perovskites, a big challenge should be directed to development of lead-free perovskite materials for environmental safety in practical applications.²
Among extensive applications of PSCs for outdoor and indoor power generation, use of PSCs in space environments is also promising because thin perovskite photovoltaic films demonstrate high stability and tolerance against exposure to severe space environment having high energy particle irradiations (proton and electron beams).⁴ Thin absorbers (<500 nm) avoid accumulation of particles and due to intrinsic defect tolerant nature of perovskites, radiation-induced collision damage is highly suppressed. Our current efforts in making PSCs based on both lead and lead-free perovskites, and future perspectives of perovskite photovoltaics will be introduced.

REFERENCES
1. A. K. Jena, A.，Kulkarni, T. Miyasaka, Chem. Rev. 2019, 119, 3036–3103.
2. T. Miyasaka, A. Kulkarni, Gyu Min Kim, Senol Öz, and A. K. Jena, Adv. Energy. Materials 2019, 1902500, 1-20.
3. A. K. Jena, A. Kulkarni, Y. Sanehira, M. Ikegami, and T. Miyasaka, Chem. Mater. 2018, 30,
6668-6674.
4. Y. Miyazawa, M. Ikegami, H.-W. Chen, T. Ohshima, M. Imaizumi, K. Hirose, T. Miyasaka, iScience 2018, 2, 148-155.

We tackle the individual and combined effects of extrinsic (humidity and oxygen) and intrinsic (light, bias, and temperature) stressors on halide perovskite materials by implementing photoluminescence (PL) under environmentally controlled conditions. These experiments are critical to assess the stability of the large variety of perovskite materials available for light-absorbing and -emitting applications. We clarify the influence of distinct humidity levels on the charge carrier recombination in Cs_xFA_1−xPb(I_yBr_1−y)₃ perovskites through in situ PL, where we temporally and spectrally measure light emission within loops of critical relative humidity (rH) levels. Our results demonstrate that the Cs/Br ratio strongly affects the spectral stability of light emission hysteresis, as well as its extent. The photo-emission dynamics in metal halide perovskites with both I and Br is also interrogated by environmental PL, where we find that the presence of Br suppresses hysteresis. The isolated effects of both temperature and photon excitation energy are also investigated. To pursue optimal ‘rest’ and ‘recovery’ conditions for device stable operation, we propose the implementation of a machine learning approach based on supervised learning combined with PL measurements upon exposing the samples to distinct values of humidity, oxygen, illumination, bias, and temperature, which will be discussed in details.

Eli Wolf¹ and Michael D. McGehee^2
1 Department of Applied Physics, Stanford University, Stanford, CA 94305, USA.
² Department of Chemical and Biological Engineering; University of Colorado; Boulder, CO 80309, USA
[email protected]
[email protected]

Commercialization of perovskite solar cells is dependent upon improving the stability of these devices. It has been shown that global average stress of a perovskite film is one significant contributing factor to its degradation rate. The global average stress of a perovskite film is mainly determined by two components:1) the thermal expansion coefficient mismatch between the perovskite film and the substrate and 2) the temperature at which the perovskite is formed (1). Most perovskite films are made on a glass substrate, whose thermal expansion coefficient is about an order of magnitude smaller than the perovskite film. They are commonly formed at temperatures around or above 100°C, resulting in residual tensile stresses of over 50 MPa in the film.

We show that due to the convolution of the effects of stress on degradation with classic thermal degradation, the resulting degradation rate in a perovskite film (Glass/FA_0.83Cs_0.17Pb(I_0.83Br_0.17)₃) is not Arrhenius. Stress is relieved at higher temperatures, reducing the degradation rate due to stress, which directly competes with the increasing degradation rate due to increased thermal energy for exceeding an energetic barrier associated with the degradation reaction. We use in-situ temperature dependent X-Ray Diffraction Spectroscopy to measure the degradation rate of perovskite films at multiple temperatures. The degradation rate of the perovskite film is measured by monitoring the growth of a PbI₂ peak and the regression of a perovskite peak.

Additionally, we directly measure the global average stress of a perovskite film as a function of temperature using in-situ temperature dependent profilometry to measure the changes in substrate curvature. The stress in a perovskite film changes with the temperature of the substrate even after the film is initially formed.

1. Rolston, N., Bush, K. A., Printz, A. D., Gold-Parker, A., Ding, Y., Toney, M. F., McGehee, M. D., Dauskardt, R. H., Adv. Energy Mater. 2018, 8, 1802139.

While the efficiency of metal halide perovskite (MHP) solar cells and light-emitting diodes continues to increase, their stability remains a critical challenge. Optical and electrical dynamics with varying degrees of reversibility arise in the presence of isolated or combined intrinsic (light, temperature, bias) and extrinsic (oxygen, water) operating factors [1]. My talk will discuss our investigations into the transient behavior of common MHP compositions, along with the ongoing implementation of statistical and machine learning techniques to predict these effects. First, we highlight our investigation into dynamics induced by environmental factors, such as light and moisture. For the archetypal MAPbI₃ and MAPbBr₃ (MA = CH₃NH₃⁺) perovskites we investigated the impact of illumination wavelength on surface photovoltage with Kelvin probe measurements [2]. For each composition, we record the transient voltage throughout an OFF-ON-OFF light cycle. Interestingly, after turning the light OFF, MAPbBr₃ displays a wavelength-dependent voltage decay 10x slower than the I-containing film, which exhibits no effects due to photon energy. Concerning moisture, we evaluated the in situ light-emission response of MAPbI₃ and MAPbBr₃ across multiple humidity cycles using micro-photoluminescence (micro-PL) [3, 4]. Our results establish that the intercalation of water molecules into the perovskite lattice depends on the lattice constants, with the I-based composition showing a 6× increase at 40% relative humidity (rH) while the Br-containing samples register a 15× enhancement at 65% rH. Subsequent dehydration of the MHP thin films returns the PL intensity to their original values. Our results, along with those from many other research groups, clearly establish that the issue of stability, or lack thereof, in perovskites calls for the ability to model and predict performance as a function of time. This includes times series forecasting of fundamental properties, such as photoluminescence and photoconductivity, as well as the overall macroscopic power output. With examples of their dynamics already introduced, I focus the second half of my talk on the implementation and evaluation of statistical and machine learning methods for predicting perovskite behavior. We explore and optimize standard methods, such as multiple linear regression, autoregressive integrated moving average (ARIMA), and echo state networks (ESN) on our rH-PL measurements, as well as J-V datasets from the literature. We provide a powerful analytical framework for evaluating any time-dependent MHP phenomena and, thus, tackling the open problem of stability. Our in situ measurements are a critical platform for informing the fabrication of devices with improved stability and the associated data analytics may serve as an essential tool for effectively predicting perovskite PV performance. We foresee further integration between combinatorial studies, in situ characterization, and predictive analytics, where all data streams can be collated as input for algorithms that optimize compositional tuning for sustained efficiency.

[1] J. M. Howard, et al., Joule, 3, 325– Invited Perspective, FRONT COVER
[2] E. M. Tennyson*, J. M. Howard*, et al., Chemistry of Materials, https://doi.org/10.1021/acs.chemmater.9b03089
[3] J. M. Howard, et al., in preparation.
[4] J. M. Howard, et al., Journal of Physical Chemistry Letter, 9, 3463 (2018)
.

Despite the rapid development in power conversion efficiency (PCE) from 3.8% to 25.2% within ten years, the long-term stability of perovskite solar cell (PVSC) severely hinders its commercial deployment. Herein, we have systematically studied the long-term stability of NiO_x based p-i-n structured PVSCs. After aging at maximum power point (MPP) under AM 1.5 G illumination, the PVSCs with organic interlayer (bathocuproine, BCP) and Ag electrode exhibit permanent degradation due to BCP failure and Ag diffusion. To address the above stability issue, an aluminum-doped zinc oxide (AZO) layer was employed as buffer layer to replace BCP and an indium tin oxide (ITO) layer was sputtered as top electrode to replace Ag. With this metal-oxide based structure, the long-term stability of the PVSCs has been significantly enhanced retaining 95% of its initial PCE after 30 days aging in glovebox. Furthermore, the PCE of metal-oxide based PVSCs exhibit active-rest-recovery (ARR) cycles under MPP tracking testing. During the active phase, the PVSCs produce output power under AM 1.5 G illumination at MPP and the device performance degrades gradually. Interestingly, the PVSC recovers to its initial PCE repeatedly after a rest phase under dark condition. Optoelectronic investigations such as electroabsorption (EA) and capacitance-frequency (CF) have illustrated that the ion migration within perovskite bulk is the origin of the ARR cycle. Finally, combining with an efficient Si bottom cell, a perovskite/Si 4T tandem solar cell with over 23% efficiency has been achieved. We anticipate the metal-oxide based PVSCs and the degradation mechanism demonstrated here would give credible impetus for developing long-term stable perovskite for both single junction and tandem applications.

In the last decade, hybrid perovskite photovoltaics have emerged as a promising new thin-film technology, capable of solution processing, with champion power conversion efficiencies (PCEs) exceeding 25% on a laboratory scale. The ability to solution process, coupled with an abundance of precursor materials, enables the fabrication of low-cost, large-area modules, either in a single junction configuration, or as a tandem solar cell utilizing established commercial photovoltaics, such as silicon and CIGS. However, PCEs of perovskite solar cells fabricated by scalable deposition techniques such as blade coating, spray coating or inkjet printing are consistently inferior to the record PCEs obtained from lab-scale spin coated devices. To overcome losses when transitioning to scalable deposition technologies, efficient optimization strategies are required.
As a novel strategy, we demonstrate simultaneous in situ thickness monitoring at multiple probing positions and exact drying control for a blade coated perovskite film. Control of the drying is established by a laminar air flow around a wing-shaped substrate holder in a flow channel, while in situ characterization is performed by five diode lasers, each with a respective photodiode probing the reflectivity of the film over time. This unique combination, simultaneously controlling and monitoring the drying process, ensures that the environment parameters of the fabrication process are exactly defined. Therefore, it becomes possible to derive an analytical drying model for perovskite solution films, using fundamental kinetic and thermodynamics equations. This model can accurately describe the measured, dynamic shrinkage of the film thickness during solvent evaporation in line with the inherent chemical properties of the perovskite solution. Because the mass transfer coefficients are precisely controlled, the model can be applied to arbitrary industrial-scale drying machines, for which these coefficients are mostly known.
In order to illustrate the relevance of the strategy presented here, we determine the optimal drying conditions for blade coated methylammonium lead iodide films incorporated into solar cells with the planar architecture ITO/SnO₂/perovskite/Spiro-MeOTAD/Au. Obtained PCEs and measured polycrystalline perovskite morphologies demonstrate a clear correlation with the drying parameters of substrate temperature and air flow velocity. Although both temperature and air flow velocity can control the drying speed, their effects on the crystal growth are fundamentally different. Obtaining a pinhole-free perovskite film and a high PCE requires restricting these variables to a distinct processing window. At optimal drying conditions, champion PCEs of 19.5% (as measured by the backward IV) and champion stabilized PCEs of about 17.3% are reached. These device PCEs do not differ significantly from those obtained from entirely spin coated devices fabricated with the same cell architecture from the same precursor solution. This equivalent device performance is evidently an example of an optimal process transfer from lab-scale spin coating to scalable blade coating fabrication method.
In summation, we provide a sophisticated upscaling strategy based on a fundamental understanding of the drying process in perovskite solution films. The derived drying model enables the prediction of the polycrystalline perovskite thin-film morphology for an arbitrary drying machine prior to experimental testing of the system. In future works, additional characterization methods will be explored to extend the model to apply beyond the onset of the crystallization process.
More detail on the described work can be found in the original, open access paper "Drying Dynamics of Solution-Processed Perovskite Thin-Film Photovoltaics: In Situ Characterization, Modeling, and Process Control" published in Advanced Energy Materials / Volume 9, Issue 39, October 17, 2019.

Perovskite solar cells are emerging candidates for next-generation thin-film photovoltaic technologies with the potential of extremely low fabrication cost and highest power conversion efficiency. Over the last decade, perovskite solar cells have gone through a rapid development in device efficiency from an initial 3.8% to a most recently certified 25.2%, though the challenges of long-term stability and lead toxicity still remain. Beyond the concerns in stability and lead toxicity, the cost of widely used materials such as ITO and Spiro-OMeTAD for high efficiency devices becomes another obstacle for the commercialization of perovskite solar cells in the near future. Carbon materials, ranging from zero-dimensional carbon quantum dots to three-dimensional carbon black materials, are promising candidates for reducing the material cost meanwhile boosting both efficiency and stability of perovskite solar cells, offering unique advantages for incorporation into various device architectures. In this talk, we will present a concise overview of important and exciting advancements of perovskite solar cells that incorporate different dimensions of carbon materials in their device architectures, followed by the research progress in our group utilising carbon materials (nanotubes and graphene) for a range of perovskite solar cell applications in an effort to simultaneously reduce the material cost and improve the long-term stability. We will discuss the major advantages and potential challenges of each technique that has been developed in our most recent work. Finally, we outline the future opportunities towards low-cost, efficient and stable perovskite solar cells utilising carbon materials.

Hybrid organolead-halide perovskite materials, with their remarkable optoelectronic properties, hold real promise for use as the active layer in low-cost next-generation solar cells. High-throughput open-air fabrication methods offer low capital-expenditure routes to continuous manufacturing of perovskites, but achieving reproducibility with ambient conditions and varying relative humidity (RH) has been a persistent challenge for achieving high performance and device stability. Our group has recently developed a Rapid Spray Plasma Processing (RSPP) method, where perovskite films are spray coated and immediately exposed to an open-air plasma at an ultrafast linear processing rate > 10 cm/s. The combination of reactive species (ions, radicals, metastables, and photons) and convective thermal energy produced by the plasma enables rapid (~100 ms) crystallization with high tolerance to RH.

In this work, we first control the plasma dose to vary curing kinetics and grain size in open-air spray coated Cs_.17FA_.83Pb(Br_.17I_.83)₃ perovskite films. We then correlate microstructure with carrier lifetime and photoluminescence measurements to show the relationship between curing source (room-temperature gas vs plasma), grain size, and film roughness on carrier dynamics and recombination. Improved steady-state photoluminescence is observed for plasma curing, an effect that is independent of microstructure and is attributed to the quenching of trap states during film deposition from the plasma. However, a strong correlation is observed between microstructure and carrier lifetimes: increased grain size in the plasma-cured perovskite films results in lifetimes > 1000 ns—exceeding spin-coated films fabricated in a glovebox by 5X—and suggesting that RSPP perovskite films have a lower defect density.

By optimizing plasma dose to produce perovskite films in 12.5 cm² series-connected perovskite modules, we demonstrate >14% stable power output fabricated entirely with scalable deposition processes of spray coating and evaporation. We identify smooth morphologies (R_q < 30 nm) as a key parameter to enable module stability under both maximum power point tracking and accelerated humidity (25°C, 70% RH) and heating (85°C, 25% RH) conditions for >100 hours without encapsulation. The control of microstructure using RSPP therefore improves optoelectronic properties, module performance, and stability compared to traditional spin coating.

Light and water are well known factors that can cause degradation to organic lead halide perovskites, which hinders their commercial applications. Here, we studied the degradation mechanism of FA_0.85Cs_0.15PbI₃ using peak force infrared microscopy, which enables subdiffractional spatial identification of the formamidinium (FA) cation and allows us to probe the degradation mechanism of individual grains. We found that upon exposure to light or water alone, no significant change in the spatial distribution of FA and no pure inorganic decomposition product was observed. However, upon exposure to both light and high humidity, we found that FA_0.85Cs_0.15PbI₃ degrades through a multistep pathway. First, α-FA_0.85Cs_0.15PbI₃ decays to δ_o-CsPbI₃, δ-FAPbI₃, and PbI₂ and the distribution of the FA cation became increasingly inhomogeneous. Second, the organic cations are lost and PbI₂ further photodecomposes.

Two-dimensional hybrid perovskites have attracted attention due to their superior ambient stability. In addition, due to local confinement, anisotropic dimensionality and versatility of organic chemistry synthesis, 2D perovskites have flexibility for the modulation of optoelectronic properties. Therefore, these materials are very promising alternative to 3D perovskites for solar cell, thin film transistors, light emitting diode and energy upconversion applications. In this work, using ultrafast pump probe spectroscopy, we studied the charge carrier dynamics in (MA)2Pb(SCN)2I2 perovskite. Carrier dynamics monitored at ~2.3 eV indicates that charges are depopulated with characteristics time constants of 3.5 ps and 57 ps. This indicates that the charge carrier recombination is finished within 100 ps. Also, transient absorption spectra suggest the less population of trap state in this structure. This work provides an improved understanding of fundamental photophysical processes in perovskite structures and provides the guideline for the design, synthesis and fabrication of optoelectronic devices including solar cells.

Optical metasurfaces have gained a considerable attention to the research community due to their application to planar optics with new functionality. So far most of the work has focused on implementing passive optical components like lenses and polarizers, where the optical bandwidth of the implemented structures is limited. Several optoelectronic applications like image sensors, infrared camera, and solar cells require broadband properties, which cannot be provided by most optical metasurfaces. In this study, optical metasurfaces are used to improve the light incoupling and light trapping of perovskite solar cells. Herein, a pyramid shaped metal oxide film is prepared through metal organic chemical vapor deposition (MOCVD), which later converted into optical metasurfaces. Optical metasurfaces are consist of an array of sub wavelength zinc oxide nanowires, where the incident light is controlled by the optical metasurfaces. Templated growth electrodeposition technique is used to grow sub wavelength nanowires. As a first step, metasurfaces are utilized on top of the flat perovskite single junction solar cells, which allows enhancing the short-circuit current density by >20% as compared to flat solar cell. As a second step, metasurfaces are applied to the implementation of perovskite/perovskite tandem solar cell, where materials with optimum bandgap combinations are selected to realize high energy conversion efficiency. The proposed perovskite/perovskite tandem solar cell integrated with optical metasurfaces exhibits a matched short-circuit current density beyond 18 mA/cm² and an estimated energy conversion efficiency exceeding 30%. Finite-difference Time-Domain simulations are used to investigate the optics of perovskite single junction and perovskite/perovskite tandem solar cells. Detailed guidelines for the realization of optical metasurfaces and solar cell structure will be provided.

References
[1] Mohammad I. Hossain et al., Optics of perovskite solar cell front contacts, ACS Applied Materials & Interfaces,11(2019). DOI: 10.1021/acsami.8b1658.
[2] Mohammad I. Hossain et al., Nanophotonic design of perovskite/silicon tandem solar cells, J. Mater. Chem. A. 6 (2018) 3625-3633. DOI: 10.1039/C8TA00628H.

An efficient hole transport material is crucial for realizing improved short-circuit current density and energy conversion efficiency of perovskite solar cells. In this study, Nickel-oxide (NiO_X) thin-film was used as a potential hole transport layer for efficient perovskite solar cells. The NiO_X films were prepared by electron-beam physical vapor deposition (EBPVD) at low temperatures. Necessary material characterizations were carried out to ensure high quality films and spectroscopic ellipsometry measurements allowed to determine the complex refractive indices of deposited films. Optics and optimization of perovskite solar cells were investigated by three-dimensional finite-difference time-domain simulations, where complex refractive indices were used as the input parameters. Optical simulations provided a very good agreement with the experiments in terms of quantum efficiencies and short-circuit current densities of planar perovskite solar cells. Hence, an optimized device design has been proposed, which can enhance short-circuit current density and energy conversion efficiency by 20% and over 25%, respectively in comparison to the planar perovskite solar cell. Such enhancement is due to improved light incoupling and light trapping, which allows realizing the estimated energy conversion efficiency beyond 22%. The optimized device design is further applied to the implementation of perovskite-based tandem solar cells reaching energy conversion efficiency over 30%. A detailed discussion of the proposed device structure will be provided to attain effective photon management.

A pin-hole free and uniformly distributed compact hole-blocking/electron-transport layer (ETL) is an essential requirement for reaching high power conversion efficiency (PCE) of planar perovskite solar cells (PSCs). In addition, the preparation of a hole-blocking layer has to be simple so that it reduces production costs and enables future commercialization of PSCs. Herein, high-performance planar PSCs were fabricated by employing spray pyrolysis deposition of TiO₂ compact ETL, where necessary material characterizations ensure obtaining high-quality films. The influence of TiO₂ layer thickness on the photovoltaic performance of the resulting devices was investigated by varying the precursor solution concentration from 0.15 M to 0.40 M. The photovoltaic performance is maximized for the 0.35 M TiO₂ precursor solution concentration, which allows realizing a PCE of 16.55% with short-circuit current density (J_SC) of 21.3 mA/cm², open-circuit voltage (V_OC) of 1.08 V, and fill factor (FF) of 72% from the champion device. Furthermore, three-dimensional (3D) finite-difference time-domain (FDTD) simulations were performed to investigate the mechanism of device operation from the optical point of view. Optical simulations were precisely used to model, design, and optimize the perovskite solar cells, where results from simulations provided a good agreement with experimental results which further reveal the correlation of the spray pyrolysis deposited TiO₂ thickness on the photovoltaic performance of PSCs. A detailed discussion of the proposed efficient planar PSC will be provided.

References
[1] Mohammad I. Hossain et al., Optics of perovskite solar cell front contacts, ACS Applied Materials & Interfaces,11(2019) 14693-14701. DOI: 10.1021/acsami.8b1658.
[2] Mohammad I. Hossain et al., Nanophotonic design of perovskite/silicon tandem solar cells, J. Mater. Chem. A. 6 (2018) 3625-3633. DOI: 10.1039/C8TA00628H.

Perovskite material systems have recently attracted substantial research attention as solar energy materials owing to their excellent inherent optoelectronic properties, while the manufacturing costs are assumed to be very low. In the past decade, the energy conversion efficiency of perovskite single-junction solar cell has reached over 24%. According to the detailed balance theory, the energy conversion efficiency of tandem solar cell can go beyond 45% if the optimum bandgap materials are selected. The energy conversion efficiency of perovskite/silicon tandem solar cells has already reached to 28%, however, details of fabrication methods and device structures are not yet available. In this study, all-perovskite planar tandem solar cells are investigated, where a wide bandgap (1.72 eV) perovskite is used as a top cell absorber and a low bandgap (1.16 eV) perovskite for the bottom cell. An optimized device design is investigated to realize a high efficiency planar all-perovskite tandem solar cell, where materials used for the contacts have a comparable refractive index to the perovskite absorbers. Herein, atomic layer deposited experimentally realized metal oxide is used to prepare the contacts. Three-dimensional finite-difference time-domain optical simulations are utilized to investigate the optics and optimize individual layers in the solar cell structure. The proposed planar all-perovskite tandem solar cell exhibits a short-circuit current density of 17.6 mA/cm² at matched condition, which allows estimating the energy conversion efficiency exceeding 29%. It is assumed that the proposed planar highly efficient all-perovskite tandem solar cell can be fabricated by solution-processed method at low cost. A detailed guideline on how to determine the solar cell design will be discussed.

Thin-film photovoltaic devices are emerging as a serious alternative to conventional silicon solar cells. Thin films enable for example utilizing resonant modes to decrease the amount of material required for the device, and to design the solar cell for special operational circumstances. On the other hand, thin films could enable designing solar cells from unconventional earth-abundant materials, many of which have favorable light absorption properties but insatisfactory electrical conductivity. However, to enable studying the prospects of new thin-film photovoltaic devices in detail, it would be extremely beneficial to have a modeling tool that enables studying their optical and electrical properties fully self-consistently. To this end, we have recently developed an interference radiative transfer-drift-diffusion (IRTDD) model for arbitrary thin-film device structures. The IRTDD model is especially well suited to study photon recycling and luminescent coupling in direct connection with electron-hole distributions affected by both illumination and bias voltage. It constructs a fully self-consistent solution between the fluctuational electrodynamics framework of photon dynamics and the drift-diffusion formalism of charge carrier dynamics. Therefore it also directly includes interference effects, such as Fabry-Perot resonances and emission enhancement. In this work, we use the IRTDD model to reproduce the device characteristics of selected single- and multi-junction thin-film solar cells and connect them with the underlying local photon transport and generation-recombination processes that are obtained as a direct result of solving the combined equation system. We focus on photon recycling and luminescent coupling and how they affect the overall device characteristics of different thin-film structures under illumination and in dark current conditions. We also investigate possible ways to incorporate diffractive elements into the model, such as gratings on the device surface.

The high efficiency organic-inorganic perovskite solar cells are generally fabricated as a multilayer structure in which the perovskite absorber layer is sandwiched in between an electron transport and a hole transport thin film creating an interface on either side. So far different hole transport materials have been investigated of which the organic hole transport layers have delivered the highest efficiencies. However, these organic hole transport layers based on octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine (Spiro-OMeTAD) and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) have displayed poor long term stability. Furthermore, their preparation involve expensive purification processes that hinders universal low-cost commercialization of perovskite solar cells. The alternative low-cost inorganic hole transport materials such as transition metal chalcogenides have been employed recently using the hot-injection method exhibiting superior device stability and high carrier mobility. However, hot-injection method has poor synthesis reproducibility and difficulty in scaling-up for large-scale production.

In this work we performed an ab initio study of facile inexpensive scalable syntheses of transition metal dichalcogenide (TiS₂) by electrodeposition from ionic liquids as a low-cost inorganic hole transport layer for perovskite solar cells. The TiS₂ thin films were electrodeposited from choline chloride–urea eutectic based ionic liquid electrolytes at 80°C with Na₂S₂O₃ as sulfur source. From the detailed cyclic voltammetry studies, the deposition potential of TiS₂ was optimized at -0.79 V vs Pt. The as-deposited TiS₂ hole transport layer exhibited polycrystalline structure with preferential growth along (001), (100), (002), (102), (110), (111) planes. The Raman spectroscopy investigations of these films showed peaks around 225 cm^-1 and 330 cm^-1 which are attribute to the E_g and A_1g Raman modes respectively. The synthesized thin films demonstrated sharp optical bandgap edge along with bandgap tunability as the bandgap (direct) decreased from 1.53eV to 1.49eV, 1.40eV, and 1.34 eV with gradual change in deposition potential from -0.79V to -0.9V, -1.0V, and -1.1V vs Pt respectively. This aspect may have potential for alignment of valance band edge in facilitating the hole transport at the perovskite-TiS₂ interface. The absorption coefficient of the as-deposited TiS₂ thin films likewise shown a dependence of synthesis potential for visible-light range which is highly conducive for application as a hole transport layer in multilayer solar cell structure. The TiS₂ thin films were observed to be p-type as shown from the Hall Effect studies with a carrier mobility up to 14.4 cm²V^-1s^-1. This paper will present a detailed study of the effect of the synthesis parameters on the optical and band-edge and its analysis for application as hole transport layer in perovskite solar cells.

Hybrid perovskite solar cells (PSCs) represent the hottest research topics due to its meteoric rise in its power conversion efficiency (PCE) to 25.2% since the first cell was fabricated in 2009 with PCE of ~ 3%. However, there is still a huge block in the form of stability and cost that hinders its path towards commercialization. The majority of the high-performance PSCs employ organic hole-transporting materials (HTMs) and precious metal as back contact (e.g. Au), which are expensive and operationally unstable. Through our work, we plan to tackle the stability and cost issues by employing cost-effective carbon as both the hole transport layer and back contact, thereby completely avoiding the organic hole HTMs. Carbon is a good replacement for the most commonly used precious metals such as gold and silver as a counter electrode as its work function (5.0 eV) is close to that of gold (5.1 eV). Our technique has the added advantage that it is suitable for the low-cost printing technology (e.g., roll-to-roll) as the material synthesis is done at low temperatures (<200 ^oC) and hence it gives rise to the possibility of fabricating flexible solar cell products. In this work, with SnO₂ quantum dots as an electron transport layer (ETL), and carbon as the counter electrode, we fabricated stable HTM-free planar heterojunction PSCs with the structure ITO/SnO₂/Cs_0.05FA_0.81MA_0.14PbI_2.55Br_0.45/Carbon. Our champion cell had an area of 0.08 cm_² and recorded a PCE of 12.12% aided by a very high open-circuit voltage (V_OC) of 1.07 V and a short-circuit current density (J_SC) of 20.5 mA/cm². This low-cost and highly efficient HTL free and carbon-based perovskite technology pave pathways to achieve affordable and stable perovskite thin-film solar cells.

Lead halide perovskite solar cell efficiencies have risen dramatically, reaching a certified 25.2%, promising as low-cost solar energy alternatives. However, perovskites are highly sensitive to temperature, moisture, oxygen, and UV light, presenting an obstacle to commercialization. Therefore, it is crucial to unravel the degradation mechanisms and the ways perovskites interact with water molecules. In the path to understand how water molecules are interacting with the material, a humid treatment was performed to perovskite solar cells with different precursor stoichiometries for hybrid organic-inorganic materials. By exposing the films for 48 hours in a high relative humidity environment, improvement in efficiency for the organic excess perovskite solar cells (highly efficient) was shown. Characterization was done from the material, to the device’s performance and using synchrotron advance X-ray methods to deepen in the properties of the perovskite. Synchrotron X-ray Fluorescence was realized in parallel to X-ray Beam Induced Current technique to exhibit a correlation between the elemental mapping and electrical behavior of the devices, in which heterogeneity upon moisture was observed.

The use of poly (3,4-ethylenedioxythiophene): poly(styrene sulfonate) sulfonic acid (PEDOT: PSS) as the hole transport layer (HTL) has been commonly reported in the field of perovskite solar cells during recent years. However, these devices suffer from stability issues due to the acidic and hygroscopic characteristics of PEDOT: PSS; therefore, rendering it as an inefficient option for large scale fabrication and commercialization. In this work, we have incorporated an additional layer of CuI along with PEDOT: PSS for efficient hole transport and stability. We have investigated different concentrations of CuI in acetonitrile and prepared devices with the inverted planar architecture of ITO/CuI/PEDOT: PSS/Perovskite/PCBM/Al. From our investigation, we found that 10mg/ml of CuI is the optimum concentration for better device performance as it showed an increase in efficiency by 28 % and current density by 31 % when compared to the devices with only PEDOT: PSS as the hole transport layer. In addition, the CuI devices also showed better stability under controlled atmospheres.

Solvent vapor annealing (SVA) is an effective post-treatment process to improve the quality of crystals and grains in metal halide perovskite films. Whereas SVA has been successfully adopted for the fabrication of perovskite films via the conventional two-step method, its adaptation to the simple single-step perovskite film deposition method has been limited because of the rapid transition of the precursor to the perovskite structure. In the present study, we demonstrate highly efficient and stable single-step-based inverted perovskite solar cells fabricated with perovskite films prepared using dimethyl sulfoxide (DMSO) and water as a combined solvent in an SVA treatment. The treatment with DMSO alone resulted in the growth of large grains (~900 nm) in the lateral direction at the surface region; however, voids and defects were observed in the vertical direction at the bottom interfacial region of the CH₃NH₃PbI₃ layer, which resulted in poor device properties. Interestingly, the combined DMSO–water induced vertical growth of almost single grains and the healing effect of water improved the grain quality. As a result, we fabricated co-SVA CH₃NH₃PbI₃ devices with a power conversion efficiency (PCE) of 19.52%. This PCE, which is much higher than that of pristine devices, was attributed to reduced nonradiative recombination. In addition, co-SVA solar cells displayed remarkable stability and their universality in various types of perovskite materials was demonstrated. We anticipate that our proposed process and mechanism will be widely used in the future development of perovskite-based devices.

Lead Perovskite oxide and halide based devices are especially promising when their applications in biomedical wearables. The advantages of using a perovskite oxide and halide layer for a coupled active layer include but are not limited to broad a light absorption spectrum, tunable band gaps, long charge carrier diffusion, enhanced electron mobility and low fabrication cost. The current work involves the design, fabrication, and simulation of a layered BaTiO₃-PMMA-Graphene perovskite oxide and a Tin Iodide coupled active layer structure for application as signal transducers and amplifiers in led based wearable and flexible biomedical health monitoring devices. 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, photon-to-current conversion efficiencies, variation in electron mobility and interfacial properties such as contact resistance and surface energy. The stacking structure is further modulated within the cell by progressively including a recombination-inhibiting thin BaTiO₃-PMMA-Graphene 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 coupled layer for biomedical devices.

Metal halide perovskites are a class of novel semiconductors, with tuneable band gap, high absorption coefficients, large charge diffusion length, and for these reasons are widely studied as absorbers in photovoltaics devices. One of the biggest challenge to achieve high-performing perovskite solar cells is the control over the perovskite film morphology. High crystallinity, low defect density, together with the formation of uniform films are important requirements in order to ensure efficient charge collection and minimize non-radiative recombination. The control over crystallization by solution techniques is challenging, due to the several parameters concurring in the film formation. For this reason, we explored solvent-free routes for the formation and post-treatment/recrystallization of perovskite thin films. Methylammonium lead iodide (MAPI) films were obtained by conversion of vacuum-deposited PbI₂ films with vapours of methylammonium iodide in a vacuum oven. With this technique, we deposited MAPI films, which are typically homogeneous and flat, composed by fine (< 100 nm) grains. Furthermore, we introduce the methylamine gas post-treatment to tune the film morphology of the sequentially deposited perovskite films. Upon controlled exposure to methylamine, the perovskite film is partially recrystallized, obtaining films with different and enhanced crystallinity. In particular, by tuning the temperature and gas concentration, we are able to substantially increase the average grain size. We will present extensive morphological and structural characterization, as well as the latest advances in solar cells obtained with solvent-free, vapour treated perovskite thin films.

Organic lead halide perovskite possesses desirable optical and electronic properties for solar cell application resulting in rapid progress in device performance for both single junction and multi-junction-tandem devices. I will be talking about some of the work in my research group related to optical properties, analysis of optical losses in single junction and tandem perovskite cells and strategies to overcome these losses. In addition, a simple and rapid method for measuring total hemispherical thermal emittance and its application in perovskite and silicon solar cells for thermal management will be also presented.

The expectional optical and electronic peropeties of the hybrid halide perovskites has motivated many researchers to explore structurally and compositionally similar materials. One aim is to try to replicate some of the peroerties of ABX₃ hybrid halide perovskites whilst resolving issues of toxcity (namely removal of lead) or stability.

The Fm-3m A₂BX₆ structure is formed by ordered removal of half of the B site cations from the cubic ABX₃ perovskite structure, and has been known for some time as the K₂PtCl₆ structure and more recently has become referred to as the defect perovskite or vacancy-ordered double perovskite. A₂BX₆ materials have close structural similarities with perovskites, but are in general much more stable with respect to oxidation or decomposition into the binary halides. Furthermore the doubling of the formal charge on the B site comapre with the perovskites allows a range of different compositions to be explored. In addition, the vacancies at half of the B site positions have consequences for the electronic structure, charge transport and optical properties and the tolerance of the structure to non ideal ion sizes. Understanding these issues, especially how they differ compared to the ABX₃ perovskites, will be necessary for identifying opportunites for A₂BX₆ materials in optoelectronic applications.

In this talk I will sumarise our findings on the basic crystal chemistry of A₂BX₆ materials. We have studied a range of compounds, with A = Rb⁺, Cs⁺, CH₃NH₃⁺, B = Sn⁴⁺, Te⁴⁺ and X = Cl^-, Br^-, I^-, and mixtures of ions on the A, B and X sites. We have identified distortion mechanisms analagous to the octahedral tilting well known in the perovskites, and also new distortions that cannot ouur in ABX₃ compounds. Local structure can be determined using Raman spectroscopy, which can identify the distribution of anions in the mixed halide materials: control of the anion distribution can lead to different optical properties in samples with the same overall chemical composition. We have studied the defect properties of these materials through divalent doping on the B site, as well as introduction of rare earth and post transition metal elements.

We present fundamental characterization, single-junction solar cells, and tandem solar cells based on the kesterite-inspired Cu₂BaSnS₄ (CBTS) and Cu₂SrSnS₄ (CSTS) absorbers. While CBTS has been investigated for a couple of years, CSTS material properties and solar cells are characterized here for the first time. Both absorbers are grown by thermal conversion of sputtered oxide films (Cu₂BaSnO₄ and Cu₂SrSnO₄) in an H₂S atmosphere at the same temperature normally used for sulfurization of kesterite [1]. Unlike the tetrahedrally-coordinated Cu₂ZnSnS₄, the CBTS and CSTS absorbers crystallize in a trigonal structure where Cu, Ba(Sr), and Sn have distinct coordination environments, making substitutional defect formation generally less favorable than in kesterite. In fact, both CBTS and CSTS are found to have sharper absorption and emission edges than kesterite, and their room-temperature photoluminescence peaks are well aligned to the band gap. These findings indicate that changing the cation in CXTS has dramatic consequences on band tailing in the material, as also confirmed by recent experiments on the Cu₂CdSnS₄ system [2]. Aided by temperature- and intensity-dependent photoluminescence experiments, will try to identify chemical trends in the deep and shallow defects of the CXTS material system and the X cation is changed.

In CBTS and CSTS solar cells, we modify the commonly used device architecture by using the high work-function metallic back compound TaS₂ as a back contact. The goal is to mitigate the losses associated with carrier transport across the semiconducting MoS₂ layer that invariably forms on a Mo back contact. Encouragingly, the TaS₂ back contact yields a 10% relative efficiency improvement over the Mo back contact. As CBTS and CSTS have optimal band gaps for a top absorber in tandem cells, a proof-of-concept monolithic CBTS/Si tandem cell is fabricated to evaluate possible process incompatibilities between the two subcells. Using a thin Ti(O,N) interlayer as a diffusion barrier and as a recombination layer between the two subcells, a working tandem cell is realized. Finally, we discuss the main hurdles in the development of more efficient single-junction and tandem cells based on CBTS and CSTS.

[1] A. Crovetto et al., ACS Appl. Energy Mater. 2, 7340 (2019).
[2] S. Hadke et al., Adv. Energy Mater. 1902509 (2019).

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

All-perovskite tandem solar cells, in which two perovskite solar cells with complementary bandgaps are monolithically integrated and electrically connected in series, offer a viable approach to suppress the Shockley-Queisser (SQ) radiative efficiency limits for single-junction solar cells. To date, all-perovskite solar cells have demonstrated power conversion efficiencies (PCEs) of up to ~25%.^1-4 To further improve the PCE of all-perovskite tandem solar cells, it is important to accurately assess photocurrent generation in individual subcells and rational design the tandem device architecture to maximize current output. However, accurate photocurrent assessment of individual perovskite subcells via external quantum efficiency (EQE) measurements presents some unique challenges due to the possible presence of shunting paths and unclear electric field distribution in both perovskite subcells. The accurate EQE experiments of tandem cells need to be carried out under representative illumination and electrical bias conditions to maintain accurate short-circuit condition for each subcell. Still, the detailed and accurate descriptions of EQE measurements of all-perovskite tandem solar cells have not been reported in the literature. Here, we report on a method to determine the required bias illumination and bias voltage for the EQE measurements, based on the behavior of individual subcells and optical modeling of tandem configuration. We show that the short-circuit current densities of the subcells in our all-perovskite tandem cells with PCEs of ~23%¹ obtained from the EQE integration are consistent with those from optical modeling and current-voltage measurements under AM1.5G illumination.

[1] J. Tong et al., Science 364, 475 (2019).
[2] A.F. Palmstrom et al., Joule 3, 2193 (2019).
[3] Z. Yang et al., Nature Communication 10, 4498 (2019).
[4] R. Lin et al., Nature Energy 4, 864 (2019).

Metal-halide perovskites as semiconductors with a direct band-gap are ideal candidates for optoelectronic on-chip devices such as lasers. These materials can be deposited onto existing chips and processed using back-end-of-line compatible techniques broadening their functionality. Recently, top-down lithographic patterning processes have been established in spite of the chemical sensitivity of perovskites. The first integrated devices show lasing thresholds that can compete with other technologies. Other recent works demonstrate continuous wave lasing operation and raising hopes for electrically pumped device concepts in the future. At the same time, monolithic integration onto different photonic substrates is feasible. This makes the integrated perovskite devices unique and, in the near future, more versatile compared to existing concepts such as hybrid-integrated III-V lasers.

In this talk we will demonstrate our latest achievements in integration of optoelectronic devices. Fabrication processes and treatment of the perovskite material has to be optimized differently in order to obtain efficient light emitting devices in comparison with photovoltaic applications. We work on a silicon nitride photonic platform and integrate perovskite processes seamlessly in our photonic fabrication line. The overall performance of the integrated devices is strongly dependent on the morphology of the used materials. First attempts of reducing the perovskite’s surface roughness and increasing the lateral dimensions of crystallites show the perspective of lowering laser threshold by order of magnitude. Scattering losses can be mitigated and thus polarization of the generated laser light can be stabilized. All devices work at room temperature and are currently optically pumped. Different types of perovskites (i.e. MAPbI₃ and CsPbBr₃) are integrated onto silicon nitride photonic chips. The generated laser light can directly be used by the photonic circuit underneath the perovskite laser. The emission is characterized after outcoupling through a silicon nitride waveguide. Furthermore, a detailed study about the optical properties of the perovskite after patterning by photolithography is carried out in order to define design optimizations needed for the future light emitting devices.

Our work focusses strongly on reproducible, mass manufacturable fabrication processes. Long term device stability is a central challenge for newly developed material and devices geometries. Possible industrial applications such as datacom and sensor devices are discussed.

The interest in perovskite solar cells is growing rapidly due to their versatile applicability for energy harvesting systems. In a short period of time, devices already reached efficiencies up to 25%, making them comparable to established thin-film solar cells like Cu(In,Ga)Se₂ or CdTe. The quantity of publications dealing with planar and rigid solar cells is growing tremendously; however, fibrous solar cells have not been in focus yet. Since the 1D structure provides a greater flexibility in comparison to planar systems, applications ranging from e-textiles/wearables to lightweight applications are feasible. Here, the single step fabrication of phase-pure organic-inorganic lead halide perovskite fibers by inert electrospinning technique is presented. Morphological, as well as optical/photonic properties have been studied and demonstrate first comprehensive data on electrospun organic-inorganic hybrid materials. Substitution of the absorbing layer in planar heterojunction solar cells with perovskite fibers resulted in a photoelectric response under simulated sunlight conditions. These flexible 1D hybrid perovskite fibers are potential elements for flexible optoelectronics and mark a starting point towards competitive fibrous solar cells.

Hybrid organic-inorganic perovskite semiconductors have shown potential to develop into a new generation of light-emitting diode (LED) technology with high light quality and energy efficiency. However, device operational stability is the biggest challenge for perovskite LEDs to become a commercial reality. Here we show that elevated device temperature caused by Joule heating is a major factor contributing to EQE roll-off and device degradation. Several thermal management strategies have been used to reduce Joule heating and improve thermal dissipation, such as adopting a thinner perovskite layer and doped charge transport layers, using substrates with higher thermal conductivity and optimizing device geometry. These strategies have allowed us to demonstrate perovskite LEDs with a peak EQE of 19.9%, a record high radiance of 1200 Wsr^-1m^-2, much reduced EQE roll-off (maintained an EQE of 5% at 4000 mA/cm²) and improved operational stability.

With only a few deep-level defect states having a high formation energy and dominance of shallow carrier non-trapping defects, the defect-tolerant electronic and optical properties of lead halide perovskites have made them appealing materials for high-efficiency, low-cost, solar cells and light-emitting devices. As such, recent observations of apparently deep-level and highly luminescent states in low-dimensional perovskites have attracted enormous attention as well as intensive debates. The observed green emission in 2D CsPb₂Br₅ and 0D Cs₄PbBr₆ poses an enigma over whether it is originated from intrinsic point defects or simply from highly luminescent CsPbBr₃ nanocrystals embedded in the otherwise transparent wide bandgap semiconductors. The nature of deep-level edge emission in 2D Ruddlesden-Popper perovskites is also not well understood. In this talk, I will analyze the experimental evidences that support the opposing interpretations and discuss challenges and root causes for the controversy. I will then present experimental approaches that can better correlate property with structure of a material and help resolve the controversies. Using combined Raman-photoluminescence under hydrostatic pressure in a diamond anvil cell, we prove that CsPbBr₃ nanocrystals are responsible for the bright green emissions in both 2D CsPb₂Br₅ and 0D Cs₄PbBr₆. Finally, I will show our preliminary understanding of the edge emission in 2D R-P perovskites.

References:
1. Chong Wang, Yanan Wang, Xinghua Su, Viktor G. Hadjiev, Shenyu Dai, Zhaojun Qin,Hector A Calderon Benavides, Yizhou Ni, Qiang Li, Jie Jian, Md Kamrul Alam, Haiyan Wang, Francisco C. Robles-Hernandez, Yan Yao,Shuo Chen, Qingkai Yu,Guoying Feng,Zhiming Wang, Jiming Bao, “Extrinsic Green Photoluminescence from the Edges of Two-dimensional Cesium Lead Halides”, Advanced Materials 1902492 (2019). DOI: 10.1002/adma.201902492.

2. Jiming Bao, Viktor G. Hadjiev, “Origin of Luminescent Centers and Edge States in Low Dimensional Lead Halide Perovskites: Controversies, Challenges and Instructive Approaches”, Nano-Micro Lett. 11, 26 (2019).

3. Zhaojun Qin, Shenyu Dai, Viktor G. Hadjiev, Chong Wang, Lixin Xie, Yizhou Ni, Chunzheng Wu, Guang Yang, Shuo Chen, Liangzi Deng, Qingkai Yu, Guoying Feng, Zhiming M. Wang, Jiming Bao, “Revealing the Origin of Luminescence Center in 0D Cs₄PbBr₆ Perovskite”, Chem. Mater. 2019, https://doi.org/10.1021/acs.chemmater.9b03426

Research on metal halide perovskites (MHP) has paved the way towards excellent optoelectronic materials, establishing them as a prime candidate for the forthcoming generation of large-area lighting and displays. Due to their high color purity, high radiative recombination rates, and bandgap tunability, MHP devices have been demonstrated to be in the lead for future LED applications. However, history repeats itself as for perovskite-based light-emitting diodes (PeLEDs) achieving an efficient performance for blue-emission remains a challenge. Compare to the green, red, and near-infrared, blue PeLEDs is lacking in terms of efficiency and luminance. Close attention should be paid towards the perovskite thin-film processing to ensure radiative recombination, especially in smaller grain-sized structure as in quasi-two-dimensional perovskites. Initially, these perovskites are tailored to an n number of inorganic slabs surrounded by spacer cations, but during the formation of thin-films, heterogeneous values of n coexist giving raised to different N-phase domains. It has been observed that by the addition of smaller organic molecules, one can suppress the formation of higher N values maintaining the excitation in the higher bandgap structures ensuring blue emission even as energy-funnelling is favored toward low bandgap domains. In this work, we report fully-solution-processed blue-light-emitting devices. Our devices have incorporated branched smaller spacer cations as additives to our PEA₂(MA_xCs_1-x)_n-1Pb_nBr_3n+1 perovskite films to hinder the formation of higher values of N, while prevailing an N-phase purity of 2 for n values of 1.5, 2, 2.5 in solution. This composition gave rise to devices with a dominant emission wavelength of 483 nm with color purity of 66 % at a maximum operating applied voltage, and a low threshold voltage as low as 3V.

Metal halide perovskites are of great promising candidates for optoelectronics research. Effective industrialization of perovskite technologies is impeded by the usage of high-boiling point, hazardous solvents, lab-scale processing techniques, and nitrogen environment processing, which significantly increases the cost. Here we propose a low- boiling point, less toxic solvent that enables to process the high-quality perovskite films at a high relative humidity (RH) and ambient atmospheric conditions. Our deposition route enables precursor dissolution and further on spin- or blade-coating, intermediate film phase is obtained. When the intermediate film is exposed to methylamine (MA) gas flow, the film immediately converts to MAPbI₃ perovskite. This method enabled deposition at moisture levels which span from 50 to 80 % RH. Incorporating the film in a low temperature processed inverted device structure, achieved stabilized PCEs up to 14%. Furthermore, the deposition is proved compatible with both spin- and blade-coating methods, to ensure scalability of the process. This method renders promising scalable, humidity robust, less hazardous, ambient atmosphere and low-temperature processing perovskite device fabrication.

Phenethylammonium (PEA) based 2D perovskite is an interesting example of 2D perovskites, serving as the gateway towards further introduction of functional conjugated organic cations into 2D perovskites for a variety of applications, for example, photovoltaics. However, the efficiency of photovoltaic devices based on PEA 2D perovskites only achieved ~ 7% for <n>=3, significantly lower than what have been achieved for other cations based 2D perovskites. Here we show that, by introducing propyl ammonium (C3A) into the PEA based 2D perovskites, the device efficiency can be improved to ~10% for 1:1 C3A:PEA based 2D perovskites (<n>=3). Further investigation discloses that, tuning the CH^…π interaction (between C3A and PEA or between two PEA) can have multiple beneficial impacts on such modified 2D perovskites, including (a) removal of undesirable n=1 phase, (b) lowering the density of trap states, and (c) achieving rather large crystalline grains. Additionally, when substituting with 50% C3A, other aromatic ammonium cations based 2D perovskites (<n>=3) also witness similar efficiency enhancement in their photovoltaic devices, boasting the general use of such method. Our results highlight that, the strategic tuning of non-covalent interactions (such as the CH^…π interaction) is a viable and important method in further developing 2D perovskites for photovoltaics.

The highest power conversion efficiency of perovskite solar cells has reached 25.2% which is approaching the Shockley-Queisser (S-Q) limit of single-junction devices. One solid strategy to break the S-Q limit is developing perovskite-perovskite tandem devices. The low-bandgap perovskite solar cells are ideal candidates as the rear subcell in the tandem device with the advantages of tunable bandgap, solution process and low cost. However, unlike Pb-based perovskites, it is still a challenge for realizing the targets of high performance and stability in mixed Pb-Sn perovskite solar cells owing to grain boundary traps and chemical changes in the perovskites. In this work, we propose the approach of in-situ tin(II) inorganic complex anti-solvent process for particularly tuning the perovskite nucleation and crystal growth process. Interestingly, by tuning the perovskite nucleation and crystal growth process through the tin(II) process, we uniquely form the quasi core-shell structure of Pb-Sn perovskite-tin(II) complex as well as heterojunction perovskite structure with energy offset at the same time for achieving the targets. The core-shell structure of Pb-Sn perovskite crystals covered by a tin(II) complex at the grain boundaries effectively passivates the trap states and suppresses the nonradiative recombination, leading to longer carrier life time. Equally important, the perovskite heterostructure with energy offset is intentionally formed at the perovskite top region for enhancing the carrier extraction and suppressing the bimolecular recombination. As a result, the mixed Pb-Sn low-bandgap perovskite device achieves a high power conversion efficiency up to 19.03% with fill factor over 0.8, which is among the highest fill factor in high-performance Pb-Sn perovskite solar cells. The perovskite device with the tin(II) process remains 92% of its initial PCE after stored in inert environment for 22 days. Remarkably, the device fail time under continuous light illumination is extended by over 18.5 folds from 30 h to 560 h beneficial from the protection of the quasi core-shell structure [1].
[1] C. Li, R. Ma, X. He, T. Yang, Z. Zhou, S. Yang, Y. Liang, X.W. Sun, J. Wang, Y. Yan, W.C. H. Choy**. Adv. Energy Mater. Under vision.

The promise of lead halide hybrid perovskites for optoelectronic applications makes finding less-toxic alternatives a priority. The double perovskite Cs₂AgBiBr₆ (1) represents one such alternative, offering long carrier lifetimes and greater stability under ambient conditions. However, the large and indirect 1.95 eV bandgap hinders its potential as a solar absorber. Here, I will show that alloying 1 with Sn allows for substantial modulation of the optical absorption to absorb more sunlight in a composition free from highly toxic elements. I will then present a detailed analysis, having employed a host of experimental techniques, of the speciation and substitution pathways of Sn alloyed into 1. Using this structural understanding, I will discuss the electronic consequences of Sn alloying in 1 as understood through band structure calculations. The understanding of heterovalent alloying in halide perovskites presented here opens the door to a wider breadth of potential alloying agents for manipulating their band structures in a predictable manner.

Halide perovskites provide a flexible platform for tuning optoelectronic properties through changes in composition, connectivity, and dimensionality. I will present our recent studies on increasing the accessible electronic structures of this versatile family of materials by synthesizing mixed-metal perovskites. Through both stoichiometric and substoichiometric substitutions we can synthesize a variety of halide perovskites, including those with unprecedented lattices with unusual electronic structures.

Two-dimensional (2D) hybrid halide perovskites offer high chemical versatility, strong potential in optoelectronic applications, and technological improvements in stability and performance of solar cells. The 2D perovskite family has a general formula of (A’)_m(A)_n-1B_nX_3n+1, where A’ is a large organic spacing cation (monovalent m=2 or divalent m=1), A is a small monovalent cation, B is a metal and X is halide. Here, we report the highest layer thickness (n = 7) that has ever been crystallographically characterized, for the Dion-Jacobson (DJ) phase. The seven-layered DJ phase (4AMP)(MA)₆Pb₇I₂₂ (4AMP = 4-aminomethylpyperidinium) shows less distortion in the crystal structure compared with the Ruddlesden-Popper (RP) phase (BA)₂(MA)₆Pb₇I₂₂ (BA = butylammonium). The comparison of the DJ and RP perovskites at the n = 7 level reveals that structural details still influence the optical properties, which the DJ phase has a smaller band gap than the RP due to smaller structural distortion. Another family derived from three-dimensional double perovskite, namely the layered double perovskite halides are reported here using propylammonium (PA) cation, with a general formula of A₄MM'X₈ (single-layered) and A₂A’MM'X₇ (two-layered) (A = PA, A’ = Cs, M = Ag, M' = Bi, In and X = Cl, Br). This system has demonstrated a high versatility, in which the layer thickness can be tuned up to two, and the organic spacer can be varied from singly protonated cations to a di-cation (1,4-butanediammonium, BDA). The band gap of the materials shows a wide tunability, varing from 2.41 eV (PA₄AgBiBr₈) to 3.96 eV (PA₄AgInCl₈). The diverse structural menu in 2D hybrid perovskite provides a wide selection in exploring the structure-property relations for better optoelectronic materials.

Organic-Inorganic halide perovskites have attracted much attention in the last few years. A certified efficiency of 23.7% has made them very attractive in the pursuit of cheaper alternatives to Si-based devices. However, the stability of the device has been a challenge given the very high moisture susceptibility in perovskites. The introduction of large cations is known to induce two-dimensional graded perovskite structures, which improved phase and ambient stability. Very few reports have emerged in this regard that looks at the influence of introducing large cations into a mixed triple cation perovskite, on the overall properties of the device. In this work, we investigated the impact of incorporating Phenyl ethyl ammonium (PEA) into the mixed triple cation composition of FAMACsPb(I_0.7Br_0.3)₃. For the 5% PEA in the quadruple composition, we observed improvements in the crystallinity and more uniform coverage with little to no pinholes through X-ray diffraction and electron microscopy studies. Device data showed increased V_OC from 0.8V to more than 1V for 5% PEA samples. Corresponding photoluminescence data also showed improvement in the intensity of absorption and charge carrier lifetimes.

Low dimensional perovskites are attracting interest for a variety of opto-electronic applications, owing to their tunable optical properties and their excellent ambient stability. In particular, metal halide perovskites of the Ruddlesden-Popper (RP) type – in which inorganic sheets are separated by relatively long organic cations - can be considered as strongly confined quantum well system. As these materials can be cast directly from solution, they hold the promise of procuring flexible and cost-effective films through large-scale fabrication techniques. However, such films typically possess a large degree of heterogeneity across different length scales, the origin and consequences of which are currently not well understood.

In this work, the influence of the stoichiometry of the precursor solution on the microscopic properties of films of the RP perovskite (PEA)₂PbI₄ are therefore investigated. It is found that the stoichiometry plays an important role in determining the crystallinity, morphology and optical properties of the resulting thin films. Pronounced differences in photoluminescence intensities are shown to exist even in films that are cast from stoichiometric solutions and manifest themselves on a sub-granular level. Using temperature-dependent photoluminescence mapping, an activation energy of 0.4 eV was extracted for the emergence of these local variations in non-radiative recombination rates. The spatial variation in the distribution of trap states is attributed to local fluctuations in the stoichiometry. In line with this, the surface can successfully be passivated by providing an excess of PEAI to an as-cast film, enhancing the photoluminescence intensity as much as 85% without significantly altering the film’s morphology.

Chiral-induced spin selectivity (CISS) occurs when the chirality of the transporting medium selects one of the two spin ½ states to transport through the media while blocking the other. Monolayers of chiral organic molecules demonstrate CISS but are limited in their efficiency and utility by the requirement of a monolayer in order to preserve the spin selectivity. Here we demonstrate CISS in a system that integrates an inorganic framework with a chiral organic sub-lattice inducing chirality to the hybrid system. Using magnetic conductive-probe atomic force microscopy we find that oriented chiral 2D layered Pb-iodide organic/inorganic hybrid perovskite systems exhibits CISS. The electron transport through the perovskite films depends on the magnetization of the probe tip and the handedness of the chiral molecule. The films achieve a highest spin-polarization transport of up to 86%. Magnetoresistance studies in modified spin-valve devices having only one ferromagnet electrode confirms the spin-dependent charge transport through the organic/inorganic layers.

The presence of lead in novel hybrid perovskite-based solar cells remains a significant issue regarding commercial applications due to toxicity and instability issues. Therefore, perovskite-inspired new lead-free compounds are sought-after as new candidates for photovoltaic applications. However, most of the suggested substitutions are either oxidation unsteady or suffer from indirect and wide bandgaps. Here, we demonstrate the first thin-film synthesis of MA₃Sb₂I₉ in the perovskite-like 2D polymorph with narrow and direct bandgaps and their application to solar cell devices. So far, MA₃Sb₂I₉ was reported to only crystallize in the zero-dimensional dimer structure with wide indirect bandgap properties. The formation of the 2D layered polymorph with its expected direct and narrow bandgap is more suitable for solar cell application. Using in situ XRD methods, we confirm the stability of the layered phase towards high temperature and moisture. We investigated the electronic band structure and confirmed experimentally the presence of a semi-direct bandgap at around 2.1 eV.
With the incorporation of a two-valent metal cation to the 2D antimony-based structure with the general structure formula A₃Sb₂X₉, the direct bandgap can be further narrowed down. The most known compound, Cs₄CuSb₂Cl_12, has a direct bandgap of around 1 eV with strong absorption and high stability towards temperature and polar solvents. However, the synthesis of highly crystalline material or thin-films is challenging. To overcome the limitation in the application of low-quality precipitants in optoelectronic devices, we introduce synthesis procedures requiring at least two-steps procedures. We synthesize single crystals larger than 500 µm, enabling their full characterization and application in optoelectronics.
Our work shows that careful control of nucleation via processing conditions can provide access to promising perovskite-like phases for photovoltaic applications.

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 the solar cells beyond the current state-of-the-art.

Recently organic–inorganic halide perovskite solar cell have great attention for its high performance together with easy production and wide variety of the process & flexibility of substrate materials. The power conversion efficiency has already reached over 25% in 2019 much beyond another solar cells such as CIGS or amorphous Si. The further performance still looks promising toward the Shockley–Queisser limit at around 30%. For that purpose, physical chemistry understanding based on the crystallography must be essential to design the good light harvesting, good charge separation and good charge transfer. Recently we reported the scientific revelation that the crystal phase of thin film CH₃NH₃PbI₃ consists of the mixture of tetragonal phase and cubic phase. They are about 15-20 nm well crystallized domain and randomly oriented in high resolution TEM analysis. Furthermore, multi stack sequence such as tetra-cubic-tetra resulted to form the superlattice with d-spacing 10.989Å (2θ=8.03°for CuKα). Such a superlattice may act beneficial for good charge separation & charge transfer that are well known property as a Type II superlattice. To make more efficient performance, the crystal phase control with some different approaches were examined as below.
(1) High pressure post treatment by cold isostatic pressing (CIP)
(2) Photo-flash rapid curing
(3) Crystal growth under the high voltage electrostatic field
Here in this research the results of advantage-disadvantage will be discussed.

References
[1] T.W. Kim, S.Uchida, T. Matsushita, L. Cojocaru, R. Jono, K. Kimura, D. Matsubara, M. Shirai, K. Ito, H. Matsumoto, T. Kondo and H. Segawa, Advanced Materials (2018), 1705230

Perovskite/silicon tandem solar cells have the potential to break the single-junction efficiency limit of silicon cells and further drive down the cost of silicon photovoltaics. To date, the reported monolithic perovskite/silicon tandem cells are fabricated primarily on silicon cells with front-side-polished surface, which not only cause large reflection loss but also require a costly chemical-mechanical polishing process. Thermal evaporation-involved fabrication of perovskite film has been reported for perovskite/silicon tandem cells on textured silicon to reduce the reflection loss, but it sacrifices the advantage of solution processability of perovskites. Here, we develop a scalable blade-coating technique to deposit perovskite film on double-side-textured silicon cells with sub-micrometer pyramids. This new tandem architecture combines the advances of textured silicon photovoltaics and solution processed perovskite photovoltaics to increase the efficiency while reducing the cost of monolithic perovskite/silicon tandem cells. The sub-micrometer pyramids are rough enough to scatter light within silicon nearly as efficiently as large pyramids but smooth enough to solution-process a perovskite film. Blade-coated perovskite film planarizes the textured silicon cell. With a textured light-scattering layer added to top of tandem to reduce front-surface reflectance, a monolithic perovskite/silicon tandem cell with an efficiency of 26% is achieved on textured silicon cells.

The rapid improvement in perovskite solar cells’ (PSCs) power conversion efficiency (PCE) has prompted the interest to adapt the perovskite technology in the production lines. Capitalizing on existing industrial processing methods will facilitate technology transfer from lab-scale research into mass production, thereby accelerating commercialization. In this work, we have proven the scalability of high quality thermally co-evaporation MAPbI₃ active layers in perovskite solar cells up to 4 cm² and mini-modules up to 50 cm², being able to guarantee minimal PCE losses. With a combined strategy of active layer engineering based on deposition optimization, effective surface, and interfacial treatments and light-dispersion minimization, we demonstrate PSCs and mini-modules (active area 21 cm²) achieving PCEs > 20% and ∼19% respectively. The un-encapsulated devices were able to retain ~ 90% of the initial PCE under continuous illumination at 1 sun (AM 1.5G) without active sample cooling. Looking towards both tandem applications and building-integrated photovoltaics (BIPV), we have also demonstrated thermally evaporated perovskite semi-transparent modules and semi-transparent PSCs. Colorful thermally evaporated semitransparent PSCs show a consistent PCE of ~16% for a set of colors ranging in the whole visible range. These results demonstrate the compatibility of perovskite technology with existing industrial processes and their potential for the next generation photovoltaics.

The Shockley-Queisser limit for solar cell efficiency of ~ 33% can be overcome if hot carriers can be harvested before they thermalize. Recently, carrier cooling time up to 100 picoseconds was observed in hybrid organic-inorganic lead halide perovskites, but it is unclear whether these long-lived hot carriers can migrate long distance for efficient collection. We report direct visualization of hot carrier migration in CH₃NH₃PbI₃ thin films by ultrafast transient absorption microscopy, demonstrating three distinct transport regimes. Quasi-ballistic transport was observed to correlate with excess kinetic energy; resulting in up to 230 nanometers transport distance in 300 fs that could overcome grain boundaries. The nonequilibrium transport persisted over tens of picoseconds and ~ 600 nanometers before reaching the diffusive transport limit. These results suggest potential applications of hot carrier devices based on hybrid perovskites.

The addition of large hydrophobic cations to lead halide perovskites has significantly enhanced the environmental stability of photovoltaic cells based on these materials. However, the associated formation of two-dimensional structures inside the material can lead to dielectric confinement, higher exciton binding energies, wider bandgaps and limited charge-carrier mobilities. In this work we investigate the impact of such effects to the charge transport properties of perovskite films comprising a 3D MAPbI₃ perovskite layer and a self-assembled quasi-2D perovskite region on the substrate side. We apply a combination of time-resolved photoluminescence and terahertz (THz) photoconductivity spectroscopy to reveal the charge-carrier recombination and transport through the film profile, when either the quasi-2D or the 3D layers are selectively excited.
Through modelling of the photoconductivity dynamics, we demonstrate that while the charge-carrier mobility is lower within the quasi-2D region, charge-carrier diffusion to the 3D phase leads to a rapid recovery in photoconductivity even when the quasi-2D region is initially photoexcited. In addition, the blue-shifted emission originating from quasi-2D regions overlaps significantly with the absorption spectrum of the 3D perovskite, allowing for highly effective heterogeneous photon recycling. We show that this combination compensates for the adverse effects of electronic confinement, yielding quasi-2D perovskite films with highly efficient charge transport properties.

Recent research in perovskite solar cells shows efficiencies beyond 20%. There are, however, several concerns that limit them from realizing the full potential. Defects on the surface of the perovskite are the significant reasons for non-radiative recombination, limiting the performance of the device. Several surface treatments show evidence as potential solutions to overcome these defects. Surface passivation of the perovskites is a viable option to minimize the defects and promote better charge carrier extraction. Also, the introduction of large cations into the perovskite induces a two-dimensional layered structure. In this work, we report the deposition of GUAI on mixed cation, FAMACsPb(I_0.7Br_0.3)_3, as a means to induce passivation and suppress non-radiative recombination. Results reveal improvements in structural properties and morphology. The open-circuit voltages (V_OC) with values >1V is indication of improvement with the addition of GUAI on the mixed cation perovskite.

To achieve maximal efficiency, a photovoltaic solar cell requires:
1. an excellent absorber, with high material quality and surface passivation to maximize its quasi-Fermi-level splitting under illumination (hereafter referred to as the implied or internal open-circuit voltage iV_oc), and
2. excellent electron and hole contacts, selective enough to maximize the extraction of the iV_oc and achieve a high external open-circuit voltage V_oc.
Consequently, deviations of the V_oc from the detailed-balance theoretical maximum is either due to limitations of the absorber itself—because of poor material quality or surface passivation, leading to non-radiative recombination—or of the contacts—not selective enough, with a drop of the majority-carrier quasi-Fermi level across the corresponding contact.

Based on this, we use a combination of characterization techniques to systematically investigate and quantify what limits the V_oc of wide-bandgap (1.68 eV) perovskite solar cells and screen contact schemes. First, we measure the external radiative efficiency (ERE)—also referred to as the photoluminescence quantum efficiency or yield (PLQE or PLQY)—of perovskite films and finished devices to determine their iV_oc. This voltage, which corresponds to the maximum voltage that could be extracted from the absorber if its contacts were perfect, is a metric of the material quality and the surface passivation of the absorber. Second, we measure the surface photo-voltage (SPV) of non-metallized devices. The SPV indicates the total band bending in the device; that is, the sum of the band bending at the absorber/ETL and at the absorber/HTL interfaces. Therefore, it is a metric of the selectivity of the contacts: the higher the SPV, the higher the band bending and, consequently, the lower the majority-carrier quasi-Fermi level drop across each contact. Finally, we compare the iV_oc with the V_oc. The ratio V_oc/iV_oc is also a metric of the selectivity of the contact and follows the same trend as the SPV. By comparing these three metrics (iV_oc, SPV, and V_oc/iV_oc) between cells with different contact architectures, we can guide the cell optimization process.

In particular, bare perovskite films on glass exhibit a very high luminescence (ERE>1%), corresponding to an iV_oc>1300 mV, more than 150 mV above the V_oc of the finished devices. Thus, the absorber material quality is not the limiting factor in the voltage of the final devices and contacts are responsible of this voltage deficit—either because of a poor surface passivation of the absorber, a poor selectivity, or a combination of both. Comparing NiO_x and PTAA as hole transport layers, we show that using NiO_x typically leads to a higher iV_oc but lower V_oc/iV_oc and SPV than PTAA. Consequently, NiO_x is more passivating but less selective than PTAA. Researchers working on NiO_x contacts should consequently focus on improving its selectivity, for example through modification of its work function. Conversely, for PTAA contacts, optimization of the surface passivation should be prioritized, for example by engineering the perovskite/PTAA interface to reduce its defect density.

The performance of perovskite solar cells continues to improve as progress is made in both the optimization of charge transport materials and the absorbers themselves. Mixed-halide perovskites (e.g. CH₃NH₃Pb(I_xBr_1-x)₃) are useful for their bandgap adjustment through change in chemical composition. The bandgap engineering of perovskites is of particular interest due to applications for tandem architectures in conjunction with lower-bandgap absorbers such as Si and CIGS. A notable observation in record-performance cells across the relevant bandgap range is the presence of a “bandgap deficit” for wider-bandgap perovskites. This has led to a number of studies that focus on reducing this deficit through modifying the energetics of perovskite bands at the interface with transport layers (TLs) or reducing the recombination at the perovskite/TL interfaces by incorporating passivation layers. The success of these approaches in reducing the deficit in performance with respect to thermodynamic limits points towards requirement of detailed analysis on the band alignment of perovskite/TL interfaces to realize the full potential of the device. Here we use 1-dimensional simulation of a perovskite solar cell with structure TiO₂/CH₃NH₃Pb(I_3-xBr_x)/Spiro-OMeTAD to observe the limiting factors of solar cell performance at wider bandgaps. We show that changing the bandgap of the absorber changes the band alignment at the absorber/TL interfaces, which results in increased band bending and recombination. By investigating the band positions of CH₃NH₃Pb(I_3-xBr_x) for intermediate values of x, we consistently find the optimal valence band offset to be +0.05 eV between the absorber and HTL and find differing optimal conduction band offsets for different absorber bandgaps. Most notably, the optimal CBO between the ETL and a 1.7 eV bandgap perovskite absorber is found to be -0.05 eV. A TiO₂ ETL and a Spiro-OMeTAD HTL form a -0.4 eV CBO and a -0.3 eV VBO, respectively, with the same absorber, significant deviations from their optimal alignments. We show that the downward trends in V_oc and PCE for widening bandgaps can be greatly improved through optimizing band alignment of the charge transport layers alone, leading to the recommendation of alternative transport layers that better suit wider-bandgap absorbers.

Small area (~0.1cm²) hybrid organolead-halide perovskite solar cells have made tremendous improvements in performance in recent years, approaching the efficiencies of single crystalline silicon cells. While scalable methods such as slot-die coating and spray coating offer the possibility of high quality large-area perovskite deposition, resistive losses across the transparent front electrode highlight the need for series interconnected module designs. Our group has developed laser scribing mechanisms for the ablation of the electrodes and active material utilizing a single wavelength (1064nm) high-frequency pulsed laser at ultrafast processing speeds (>20cm/s). This process coupled with improved module design that allows for the study of individual cell performance along with that of the complete module enables high throughput for both perovskite module development and characterization.
Our work first introduces the primary ablation mechanisms of our single-wavelength laser: direct ablation and lift-off. We demonstrate the unique characteristics of these scribing methods as they interact with the front electrode and highlight the unique capability of our single-wavelength laser system to perform each scribe step: patterning the front electrode (P1) with direction ablation, scribing the series interconnection (P2) with the lift-off mechanism, and patterning the back electrode (P3) with the lift-off mechanism. The unique lift-off characteristics for the P2 scribe result in improved efficiencies via increased short-circuit current and fill factor compared to modules utilizing direct ablation mechanisms from other laser sources.
The use of a single laser system for each scribe step also allows for high repeatability and accuracy, enabling a reduction in dead area to <600µm. We address performance losses due to the requisite dead area in conjunction with the resistive losses from the series interconnections and front electrode in order to motivate a module design with optimized cell and scribe geometries. The module design incorporates an adaptive architecture that enables complete characterization of both the entire module and its constituent cells. While laser-beam induced current (LBIC) measurements are valuable in evaluating the perovskite uniformity, particularly for large-area scalable deposition methods, previous module architectures force most of the module into reverse bias throughout the scan, resulting in inaccurate current outputs and the need to scan each cell individually. Our adaptative architecture allows for the complete characterization of large-area modules with a single LBIC scan, enabling high throughput single device and complete module characterization.

The uptake in expected future markets at TW-scale of photovoltaics demands stable, abundant, non-toxic materials. In addition to traditional silicon wafer-based solar cells, thin film offers a much wider flexibility in aesthetic designing, choice of substrates and module sizes, as well as unlocking new applications through the use of flexible and light-weight substrates. In these regards, Cu2ZnSnS4 (CZTS) solar cell has been regarded as a compelling alternative of the emerging photovoltaic materials. So far the certified highest power conversion efficiency of 11 % has been demonstrated for CZTS, where a layer of traditional CdS buffer material was applied. Considering the high toxicity issue of CdS, efforts have been put on partially or even fully removing the use of toxic Cd containing buffer materials. In this talk, our work on the Cd-less and Cd-free buffer based CZTS solar cells will be discussed. With SILAR-Zn_xCd_1-xS, we demonstrated >10% efficiency CZTS solar cells by finely tuning the chemistry process of ZnCdS and CZTS absorber composition, where the self-organised nanoscale Zn(O,S) at the initial growth process of Zn_xCd_1-xS was found playing a key role in the heterojunction interface passivation and band-alignment. For ALD-Cd-free Zn_xSn_1-xO buffer, we achieved >10% efficiency CZTS solar cells, which is also the first >10% efficiency fully environmentally-friendly kesterite solar cells and realised by adjusting the ratio of Zn and Sn for an optimised conduction band alignment with CZTS, and inserting ALD-Aluminum oxide (Al2O3) for heterojunction interface passivation. Our buffer materials engineering work showed that in addition to the heterojunction band alignment, combined effects from the composition of CZTS absorber (cation ratios of Cu/Zn/Sn), buffer synthesis process-induced sodium re-distribution and surface modification of absorber, as well as heterojunction interface passivation play major roles in determining the final device performance.

Lead halide perovskites have attracted much interest as an emerging photovoltaic technology due to high device efficiency and low-cost preparation methods. The highest efficiency record of Perovskite Solar Cells is 23% for device area of <0.1 cm² fabricated by spin-coating. Currently, perovskite solar cells are transitioning from small area devices to large area devices that are compatible with manufacturing. Blade-coating is a promising deposition technique because it is low cost, environmentally friendly, and easily transferable to roll-to-roll processing. Many studies demonstrating high efficiency devices using this method have been performed; however, these studies use a single step to deposit perovskite, which is challenging to scale because of the required addition of an antisolvent during deposition. In our previous study we demonstrated a compact perovskite film with large grain sizes deposited with the two-step blade-coating method in air, and we achieved solar cell with15% efficiency for small area ~ 0.08 cm². The two-step deposition method is widely used as a route for fabricating high-quality small area perovskite films using spin-coating because of its full coverage and excellent reproducibility. However, this method has not been investigated for perovskitesdeposited using a scalable blade coating method. In this work we investigate the possibility of a fully scalable deposition scheme for fabricating n−i−p perovskite modules of perovskites film with the two-step process. PbI₂ layer were blade coated on substrates with a transparent conducting oxide and SnO₂ electron transport layer (ETL) deposited by atomic layer deposit (ALD). The films were converted to perovskite by blade coating with a MAI solution. Small area devices and minimodules will be completed and characterized. For minimodule fabrication, the bottom conducting electrode layer will be laser scribed to form isolated substrates for the individual sub-cells; a second scribe will remove the active material (ETL, perovskite and HTL) to allow for interconnections between the sub-cells; and in the final scribe will remove the top electrode to isolate the individual sub-cells and define the outer boundaries of the module. Encapsulation the completed devices and minimodules with an inert SiO₂ layer prepared by RF-sputtering will also be investigated as a method to increase the stability.