Symposium ES3 : Perovskite Solar Cell Research from Material Properties to Photovoltaic Function

Organo-metal trihalide perovskites have revolutionized the field of thin film solar cells due to their meteoric rise of power conversion efficiency (PCE) of a record value over 22%.¹ The advantages of perovskite material are low cost precursors, and capable of being processed a variety of scalable methods.^2-3 These materials exhibit salient properties such as strong light absorption from the visible into the near-infrared spectral region, long carrier diffusion length, and tailorable optoelectronic properties through compositional engineering of halides and cations. These properties are subservient to the formation and nature of the crystals, morphology, and growth. Despite the high efficiency, and excellent opt-electronic properties the biggest problem of organo-metal trihalide perovskite is stability under heat and light soaking conditions. In addition, an anomalous hysteresis behavior in the current-voltage characteristics is often observed in perovskite solar cells possibly related to ion migration and imbalanced extraction of charges.⁴ In this talk we address the hysteresis, and stability issues by using molecularly engineered novel hole transporting materials.
References
(1). www.nrel.gov/ncpv/images/efficiency_chart.jpg.
(2). Lee, M. M., Teuscher, J., Miyasaka, T., Murakami, T. N. & Snaith, H. J. Science 338, 643−647 (2012).
(3). Burschka, J., Pellet, N., Moon, S-J., Humphry-Baker, R., Gao, P., Nazeeruddin & Grätzel, M, Nature 499, 316−319 (2013).
(4). A molecularly engineered hole-transporting material for efficient perovskite solar cells, Michael Saliba & Mohammad Khaja Nazeeruddin, Nature Energy 1, Article number: 15017 (2016), doi:10.1038/nenergy.2015.17

Organic–inorganic hybrid perovskites, in particular CH₃NH₃PbX₃ have taken up the center stage in the solar cell field, with power conversion efficiencies (PCE) values exceeding 22%. Along with the development of photovoltaic technologies, the exquisite optoelectronic properties render perovskite materials other application possibility, such as photodetector, light-emitting diodes (LED) and laser. It has always been an essential issue to achieve a delicate control of perovskite nanostructure, and thus the resulted device performance. Here, we demonstrated a novel technique to thoroughly study the additive coordination effect on hybrids perovskite crystallization. With this method, we successfully control the nucleation/growth rate by the judicious selection of additives during film formation. The underlying mechanism regarding the perovskite film growth is also studied. The corresponding perovskite film shows extraordinary large grain size, crystallinity, and carrier lifetime, leading to an enhanced efficiency of 19.7%. In addition, a comprehensive investigation designed to explore the materials, the materials growth, materials properties, and device operation that governs high performance optoelectronic devices are discussed here.

Closed Space Sublimation (CSS) is a vacuum process well established for industrial large scale thin film deposition e.g. of CdTe thin film solar cells. We apply CSS in a laboratory system) using a two step deposition process of organic-inorganic perovskites, as CH₃NH₃PbI₃ (MAPI) and CH₃NH₃SnI₃ (MASI) to produce solar cells in planar architecture. In an integrated deposition and XPS/UPS analysis ultra high vacuum system a metal halide layer is deposited via physical vapor deposition and transferred to the methyl-ammonium (MAI) CSS chamber for the perovskite transformation. The UHV system offers in addition the in-situ possibility to deposit in separate chambers electron transport front contact layers (ETL) and hole transport back contact layers (HTL) without breaking vacuum. The aim of the project is to demonstrate vacuum processing using CSS as a possible route to up scaling and to characterize the chemical and electronic interface structures of the different layers of functional devices under well controlled conditions free of contaminations due to ex situ treatment.
In contast to a liquid deposition line to produce standard cells with mesoscopic architecture with regular efficiency of 13% for MAPI the CSS cells still suffer from inhomogeneous film morphology despite the expected principal advantage of CSS of higher processing temperatures which may lead to advantageous morphologies according to electron scanning microscopy. In our first experiments we reach above 3% using an Au hole contact sputtered in situ directly onto the absorber layer. The observed discrepancy between the expected advantages of CSS and the actually observed efficiencies can be either related to incomplete transformation of the PbI₂ or SnI₂ precursor layers in the MAI CSS process or to the incomplete coverage os the substrate by the absorber layer.
In summary, we were able to show the poof of principle of CSS as a vacuum based deposition technique for halogenide perovskites. Further improvement of cell efficiency of fully vacuum deposited solar cells using variations of perovskite absorbers and contact materials as well as the characterization of involved interfaces by XPS/UPS will be continued in next future.

Metal organic halide perovskites are promising materials for solar cells with a maximum certified efficiency of 22.1%. However, there are only a handful of reports on larger area modules, where efficiencies drop with increased use of active area. Chemical vapor deposition (CVD) is a technology used in many industrial applications demonstrating potential for scale up. We used a CVD process to fabricate solar cells and larger multi cell modules, and investigated scaling issues. In addition we fabricated modules using an established MAPbI₃ solution process and compared them to CVD grown modules. The solution processed cells performed better than CVD cells when comparing PCEs determined from J-V measurements, but the steady state power of solution processed solar cells decreased quickly with increased area. In contrast, FAPbI₃ CVD grown solar modules maintained much of their PCEs transitioning from J-V measurements to the steady state operating conditions, suggesting that the FAI based CVD process may outperform MAI based solution processed modules when scaled up to practical sizes.

A rare advantage in future manufacture of perovskite solar cells is vacuum-free low temperature process that significantly contributes to minimising the process cost. This merit is possibly larger than low material cost of the solar cell. Low temperature printing process (<120^oC) can be applied to hybrid perovskite cells by using organic materials (PEDOT, PCBM, etc.) and/or nanocrystalline SnO₂, ZnO, and their composites as carrier transporting layers. With high voltage >1.0-1.1V, power conversion efficiency (PCE) of these cells can compete with sintered metal oxide (TiO₂, etc.)-based perovskite cells on glass substrates, reaching PCE >16%. The printing process also meets fabrication of flexible cells on plastic substrates. Recently, SnO₂ emerged as a promising collector in terms of highly durable and hysteresis-less photovoltaic performance, possibly due to suppressed recombination and good interfacial connects at the junction with perovskite crystals. We showed that low temperature composite layer of Al₂O₃/ZnO/SnO₂ works as stable collector of MAPbI₃ with PCE>15% [1]. Further, FAPbI₃ prepared on ZnO is chemically more stable than MAPbI₃ on ZnO, exhibiting PCE>16% [2]. In chemical stability, TiO₂ is basically higher than ZnO and SnO₂ although preparation of good quality mesoscopic layer needs high temperature sintering. As an exception, brookite nanocrystal is capable of strong interparticle necking at temperature <120^oC through dehydration condensation reaction that enables formation of dense uniform layer [3]. We fabricated brookite-based plastic (ITO-PEN) film perovskite cell by using amorphous SnO_x as a compact layer. It shows non-hysteretic performance with PCE 14% and mechanical stability against bending over 100 times [4].
Lead-free perovskite solar cells are also made on low-temperature prepared metal oxide collectors. We have fabricated Bi-based perovskite device on various metal oxide underlayers, which exhibited strong influence on the morphology of (CH₃NH₃)₃Bi₂I₉ [5,6]. By interfacial engineering using organic additives, dendritic absorber layer was converted to flat uniform layer with large crystalline grains, leading to improvement in photovoltaic performance. All above experiments corroborate importance of the design of metal oxide collector by using chemical processes that modify the interfaces involved in hetero-junctions.
References
[1] J. Song, E. Zheng, X.-F. Wang, W. Tian, T. Miyasaka, Solar Ener. Mat. Solar Cells, 2016, 144, 623-630.
[2] J. Song, W. Hu, X.-F. Wang, G. Chen, W. Tian, and T. Miyasaka, J. Mater. Chem. A, 2016,4, 8435-8443.
[3] T. Miyasaka, M. Ikegami, and Y. Kijitori, J. Electrochem. Soc., 2007, 154, A455-A461.
[4] A. Kogo, M. Ikegami, and T. Miyasaka, Chem. Commun., 2016, DOI: 10.1039/C6CC02589G.
[5] T. Singh, A. Kulkarni, M. Ikegami, and T. Miyasaka, ACS Appl. Mater. Interfaces, 2016, 8, 14542-14547.
[6] A. Kulkarni, T. Singh, B. Chaudhary, M. Ikegami, and T. Miyasaka, submitted.

W.C. H. Choy, H. Zhang
Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong.
E-mail: [email protected]

Recently, researchers have focused more to design highly efficient flexible perovskite solar cells (PVSCs), which enables the implementation of portable and roll-to-roll fabrication in large scale. We will report new room temperature approaches for forming perovskite solar cells. Regarding hole transport layer (HTL), NiO_x is a promising material for candidate for fabricating efficient PVSCs on rigid substrate, the reported NiO_x HTLs are formed using different multi-step treatment (such as 300-500 degC annealing O₂-plasma, UVO etc.), which hindering the development of flexible PVSCs based on NiO_x. Meanwhile, the features of nanostructured morphology and flawless film quality are very important for the film to function as highly effective HTL of PVSCs. However, it is difficult to have the two features co-exist natively particularly in solution process that flawless film will usually come with smooth morphology. Here, we demonstrate the flawless and surface-nanostructured NiO_x film from a simple and controllable room-temperature solution process [1].
Meanwhile, we will propose a new room temperature scheme formation of perovskite films with the features of PbI₂ residue-free, large grain-sizes, and highly crystalline. We further layout the design rules for the broad, rational extension of our scheme to form high-quality perovskite films. Using our approach, a room-temperature processed PVSC with no hysteresis, high power conversion efficiency of 17.10% which is the best of the perovskite solar cells fabricated by low-temperature techniques to date. Additionally, the device is very stable with performance maintance of 95% after 1000 hours. This work contributes to the large-sale and low-cost production of perovskite solar cells with high device performances.


H. Zhang, J. Cheng, F. Lin, H. He, J. Mao, K.S. Wong, A.K.Y. Jen, W.C.H. Choy, “Pinhole-free and Surface-Nanostructured NiOx Film by Room-Temperature Solution Process for High-Performance Flexible Perovskite Solar Cells with Good Stability and Reproducibility”, ACS Nano, vol. 10, pp 1503-1511, 2016.

Organic-inorganic halide perovskite materials have emerged as attractive alternatives to
conventional solar cell building blocks. Their high light absorption coefficients and long
diffusion lengths suggest high power conversion efficiencies (PCE),1-5 and indeed
perovskite-based single band gap and tandem solar cell designs have yielded impressive
performances.1-16 One approach to further enhance solar spectrum utilization is the graded
band gap, but this has not been previously achieved for perovskites. In this study, we
demonstrate graded band gap perovskite solar cells with steady-state conversion
efficiencies averaging 18.4%, with a best of 21.7%, all without reflective coatings. An
analysis of the experimental data yields high fill factors of ~75% and high short circuit
current densities up to 42.1 mA/cm2. These cells, which are based on a novel architecture of
two perovskite layers (MASnI3 and MAPbI3-xBrx), incorporating GaN, monolayer
hexagonal boron nitride, and graphene aerogel, display the highest efficiency ever reported
for perovskite solar cells.

Lead halide perovskite solar cells have attracted substantial academic and commercial interest due to their ability to achieve relatively high power conversion efficiencies (~20%) using readily scalable solution-based fabrication protocols. The precursor solution chemistry (precursor molar ratio, solvent, additives, etc.) has been shown to strongly affect the final thin film morphology (roughness, coverage, grain size). Specifically, it has been demonstrated that a molar excess of the organic precursor, methylammonium iodide (MAI), is required to produce a continuous, compact, dense film whereas a solution containing stoichiometric amounts of PbI₂ and MAI produces a highly discontinuous morphology consisting of needle-like one dimensional features. As the final film morphology dramatically affects photovoltaic performance, it is imperative to understand the relationship between solution chemistry and film morphology.

In this work, we use x-ray absorption spectroscopy to characterize the lead coordination environment in solution as a function of precursor stoichiometry as well as using the processing additive hydriodic acid (HI). By analyzing the extended x-ray absorption fine structure we are able to determine that in a stoichiometric precursor solution lead ions are coordinated by both solvent molecules and iodide ions but as a molar excess of MAI is added solvent molecules are displaced by additional iodide ions, thus increasing the iodide coordination number. Similarly, the addition of HI to a stoichiometric solution of PbI₂ and MAI results in the displacement of solvent ions and an increase in the iodide coordination number, albeit at slightly larger Pb-I bond distances than when using excess MAI. This data is presented in the context of how precursor chemistry and the lead species present in solution affect the final film morphology. This work therefore provides mechanistic insight into how precursor solution chemistry can be tuned in order to readily produce smooth, continuous, dense lead halide perovskite films for use in high efficiency solar cells.

Organometallic lead halide Perovskite material have become a popular semiconductor for an eclectic spectrum of applications including solar cells, LED, transistors etc. Its processing holds a history of wide variety of routes which includes, blade coating, spin coating, spray and vacuum. In the route of spin coating, one branch goes towards the use of anti-solvent (like Chlorobenzene, toluene, dichloromethane, ethyl ether etc.) during the deposition stage followed by thermal treatment of thin film. Multiple reports have shown brilliance of this process resulting in smooth films with pin hole free morphologies. High PV device performances have been reported through this process complimenting the need to study the details about the processing conditions. It triggers an important query of the actual role played by anti-solvent during the processing. It is generally considered that dropping of anti-solvent nucleates perovskite and thermal annealing facilitates the crystal growth of perovskite. In my talk, I will reveal surprising results on the actual role of anti-solvent, with the impact of different solvents and ratios of them. Through in-situ spin coating and annealing under GIWAXS, we have captured this well-used process with the time scale of 0.4 seconds to make sure all the phases either stable or unstable can be sketched out. Results of GIWAXS were further confirmed through FTIR of as cast films and annealed films. This study has a potential to completely alter the way solution processing of perovskite, through anti-solvent, is understood today

Next-generation solar cells based on new concepts and/or novel materials are currently attracting wide interests. In this study, several types of emerging hybrid photovoltaics have been investigated. In order to improve the energy conversion efficiency of the emerging photovoltaics, the extension of absorption range of the sensitizer to near-infrared region is an important issue. In our study, panchromatic photoelectric conversion up to around 1100 nm has been accomplished by DX-sensitized Ssolar cells. On the other hand, we investigated planar or meso-porous heterojuction perovskite solar cells with high quality perovskite films through a simple solution process. Analysis of content of chloride and iodide in the final annealed perovskite films revealed that the low chloride content of the perovskite formed through chloride-assisted growth. The effects of condition of air on the perovskite film quality and cell performance have been investigated. We observed that the quality of the film depends significantly on the level of humidity in the atmosphere. For efficient solar cells based on organometal halide perovskites, the real origin of the I-V hysteresis became a big issue and has been discussed widely. In this study, simulated I-V curves of different equivalent circuit models were validated with experimental I-V curves of a planar perovskite solar cell. We found that an equivalent circuit model with a series of double diodes and capacitors produces simulated I-V curves with large hysteresis matching with the experimentally observed curves. The electrical capacitances generated by defects due to the lattice mismatch at the TiO₂/CH₃NH₃PbI₃ and CH₃NH₃PbI₃/spiro-OMeTAD interface are responsible for the hysteresis in perovskite solar cells. We prepared the various types of tandem solar cells using dye-sensitized and perovskite solar cells. The panchromatic DSSC are useful for a series-connected tandem solar cell. The panchromatic DSSC are also useful for spectral splitting organic photovoltaics using perovskite solar cell, where a high overall power conversion efficiency (η) of about 21.5% has been accomplished.

Non-TiO₂ Inorganic Semiconductors as Electron Transfer Layer for Perovskite Solar Cells
Y. T. Shi*¹, K. Wang¹, Q. S. Dong¹, T. L. Ma*^2
1State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian 116024, P. R. China
²Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Fukuoka, 808-0196, Japan
*[email protected]n; [email protected];
Perovskite solar cells (PSCs) is at the frontier of photovoltaics today and one key component in PSCs is the electron transfer layer (ETL) which is responsible for extraction of photo-generated electrons and simultaneously acts as hole-blocking. Inorganic semiconductors with wide band gaps, e.g. TiO₂ and ZnO, are usually used as ETL materials since they have correct alignment in band structures with perovskite photoabsorbers. Low conductivity and high-temperature sintering of TiO₂-ETL, as well as the poor chemical stability of ZnO-ETL have become obstacles to the further development of PSCs.
We for the first time developed amorphous WO_x into ESL via a simple solution route at extremely low temperatures of 70 °C or even lower. Together with chemical modifications by Ti⁴⁺ or Nb⁵⁺, the WO_x-based amorphous ESLs exhibited excellent optoelectronic properties in charge dissociation, conductivity, light transmittance, as well as passivation of charge recombination. Planar PSCs using WO_x-based ESLs fabricated at 150 °C and room temperature illustrated high power conversion efficiencies (PCEs) of 13.45% and 11.56%, respectively. In addition, we found that Nb⁵⁺ modification of WO_x helps to enhance the donor density, modify the energy band structure and reduce the depletion layer at the interface of ITO/ESL. As a result, the photovoltaic performance of flexible PSCs was improved remarkably and a high PCE of 15.65% was obtained.
We also conducted systematical investigations on the optoelectronic advantages of SnO₂ as ETL. Benefitting from the cooperation with TiO₂ mesoporous scaffold, we reported one rational strategy using SnO₂ as underlayer to reconstruct ETL for highly efficient M-PSCs. The concept “interfacial synergism of SnO₂/TiO₂ nanocomposite as ETL underlayer” was proposed for the first time which well explained the facilitation of charge transfer kinetic process and the passivation of strong charge recombination. Based on this new strategy, the strong charge recombination observed in previous SnO₂-based ETLs was remarkably suppressed. In the meantime, the internal series resistance of entire M-PSC was decreased a lot. The steady-state and champion PCEs of the M-PSCs using the new ETL were 16.31% and 17.58%, respectively, higher than that of M-PSCs with traditional ETLs.
References:
[1] K. Wang, Y. Shi, Q. Dong, Y. Li, S. Wang, X. Yu, M. Wu, T. Ma, J. Phys. Chem. Lett., 2015, 6, 755.
[2] K. Wang, Y. Shi, B. Li, L. Zhao, W. Wang, X. Wang, X. Bai, S. Wang, C. Hao, and T. Ma, Adv. Mater., 2016, 28, 1891.

Although the performance of hybrid perovskite solar cells has rapidly improved in recent years, there is still a pressing need to understand and control the light and moist induced chemical reactions that currently cause material degradation. In order to assess device stability, macroscopic light I-V’s are now presented as a function of time, which often show power conversion efficiency changes within ~50 seconds after illumination. Here, we implement a novel 4D microscopy based on Kelvin probe force microscopy to spatially and temporally resolve the local variations of the photo-voltage in hybrid perovskites upon different illumination treatments. We implement high speed (16 frames/sec) while maintaining spatial sensitivity [1] (in a low humidity environment), to image, in real-time, the changes that take place in the open-circuit voltage (Voc) of the perovskite devices with spatial resolution <100 nm [2]. Surprisingly, we observed local and reversible changes in the open-circuit voltage of the devices upon post-illumination treatments. Further, we quantified the voltage variations using spectrally dependent microscopy measurements. Voc local variations > 300 mV were consistently detected, not revealed by conventional microscopy tools [3].

[1] J. Garrett et al. Nanotechnology, 27, 245705 (2016). Front COVER
[2] E. M. Tennyson et al. Adv. Energy Materials 5, 1501142 (2015). Front COVER
[3] J. Garrett et al. Submitted.

Metal halide perovskites such as CH₃NH₃PbI₃ are highly promising materials for a variety of optoelectronic applications including solar cells, light-emitting diodes, lasers and photodetectors. A key enabling property of the perovskites for these applications is their high photoluminescence quantum efficiency, suggesting that these materials could in principle approach the thermodynamic device efficiency limits in which all recombination is radiative. However, recent reports have demonstrated the presence of non-radiative recombination sites which vary heterogeneously from grain to grain and limit performance. The origin of this heterogeneity and non-radiative decay still remains unclear.

Here, we seek to elucidate the origin of the emission heterogeneity by assessing the impact of local variations in chemistry and structure on the local photophysical properties of high quality neat CH₃NH₃PbI₃ perovskite films. We first use time-resolved confocal photoluminescence (PL) measurements to map the local variations in emission intensities across the grains and grain boundaries. We then correlate these variations in emission with local morphology and chemistry through use of energy-dispersive X-ray (EDX) measurements coupled with a scanning electron microscope. Furthermore, we also correlate the emission with local variations in crystallinity and stress by utilizing the X-Ray micro-diffraction beamline at the Advanced Light Source facility. To enable the correlations of the same regions between measurement setups, we use uniquely-shaped gold nanoparticles as fiducial markers.

Our results reveal substantial variations in local chemistry, morphology and structure which correlate with the variations in local PL emission and lifetime. Our work suggests that controlling local stoichiometry is crucial for minimizing non-radiative decay and ultimately reaching device efficiency limits. Moreover, it reveals the intimate relationship between structure and optoelectronic behavior. These findings explain the heterogeneity in high quality films and, in turn, will guide the controlled growth of better performing perovskite materials and devices.

During past few years we were witnessing an unprecedented boost of perovskite device efficiency. We were also gaining a lot of scientific insight into the operational mechanism of the perovskite solar cells. Not less significant events took place on the business side of the enterprise.
In my lecture I would like to address practical aspects of bringing perovskite solar cells from a lab to the market. We will discuss viability and maturity of the technology from business perspective.

Organometal trihalide perovskites (CH₃NH₃PbX₃) are promising candidates for optoelectronic devices. Fundamental understanding on carrier dynamics of these materials under environmental perturbations such as electrodes and doping, becomes substantial. In this work, by using symmetric electrode contacts and optical doping, the carrier dynamics in bulk single crystalline CH₃NH₃PbX₃ perovskites were analyzed by optical spectroscopy and electrical transport measurement. Doping concentration-dependent bulk and surface recombination lifetimes were clearly identified using time-resolved photoluminescence. One order of increase in magnitude of bulk lifetime was induced with low optical doping concentration. To avoid interference from asymmetric electrodes configuration on carrier dynamics, we introduced and configured symmetric Zn electrodes on a single-crystal-based vertical device. With such configuration, under one-side illumination, the device showed asymmetric photocurrent. Combined with our Hall effect measurement, this suggests that hole mobility is about one order higher than electron’s in the intrinsic crystal. By optical doping, we have achieved reduced carrier recombination lifetime leading to tripled response speed. We predicted that over GHz photo response speed can be realized in micron scale single crystalline CH₃NH₃PbX₃ films, suggesting their tantalizing applications in optoelectronic devices and systems.

Perovskite solar cells (PSCs) based on thin films of organolead trihalide perovskites (OTPs) provide unprecedented promise for future high-efficiency, low cost photovoltaics. Typical PV performance parameters, including short circuit current, open circuit voltage, and maximum power, are usually benchmarked at the macroscopic scale. But for insight into the still unanswered fundamental PV mechanisms in PSC’s, and their localized dependence on the OTP thin-film microstructure, it is crucial to probe such photoresponses at the nanoscale. Accordingly, photoconductive AFM spectroscopy is presented to analyze spatial variations of PV performance metrics for polycrystalline methylammonium lead triiodide (MAPbI₃) thin films that are hole-transport-layer free. The photoresponse is shown to vary substantially with thin-film microstructural features such as grains, grain boundaries, apparently planar defects, and even grain-aggregates. Coupled with evidence of piezoelectric domains from previous efforts, this work suggests routes for microstructural tailoring of PSC’s for further improving PV performance.

The performance and lifetime of organic and perovskite solar cells are strongly affected by the formation of defects during the fabrication process. Understanding and managing these defects is critically important if printed solar cells are to become a commercially viable technology. This work develops transient photovoltage/photocurrent (TPV/TPC) measurements as a spatially-resolved mapping technique which provides unique insights for the characterisation of defects in terms of the charge carrier dynamics.
TPV and TPC measurements considering a whole device are routinely used for analysing charge carrier dynamics in order to understand the operation of organic and hybrid solar cells. Adapting this technique for mapping (~ 50 μm resolution) introduces a number of challenges, particularly related to the interpretation of results. We address this by developing a 2D finite element model of the sample to simulate the measurement and comparing the simulated results to experimental data in order to validate our interpretation. In this way we are able to identify local variation in charge recombination and extraction dynamics correlated with various types of defects in the printed solar cell. We apply this technique to organic-inorganic hybrid perovskite devices. This novel approach enables us to probe the mechanisms by which different defects affect the performance of photovoltaic devices, as well as the magnitude of their impact. Furthermore we perform these transient measurements in-situ, while samples are exposed to controlled atmospheric conditions, in order to understand the effect of defects on the degradation behaviour of perovskite solar cells. Information of this kind provides a basis for the defect management strategies that must be developed for the successful scale-up of printed solar cells.

Emerging solar cells based on a group of solution-processed organometallic halide perovskite materials have reached a NREL-certified 22% power conversion efficiency in several years. Recent research has focused interest on understanding and further controlling the already low-cost and simple processing approaches toward even higher efficiencies, in order to approach the Shockley-Queisser limit of ~ 30% through improved material design, film processing control, device design, and advanced optical management. In particular, the optimization of controllable film growth processes for high-quality perovskite films with large grains and low defect density requires understanding of the underlying mechanisms that govern crystal nucleation and growth during the complex chemical reactions responsible for the synthesis of perovskite films. In addition, during the chemical synthesis and crystallization of perovskite thin films, an interesting question is the competition between multiple halide species (e.g. I^-, Cl^-, Br^-) in the formation of the mixed halide perovskite crystals.
In this talk, an introduction to the kinetics of crystal formation and evolution of crystalline order during the growth of organometallic halide perovskites, as revealed by time-resolved in-situ x-ray diffraction measurements, will be presented. And then, I will introduce in situ X-ray diffraction measurements of crystallization dynamics, which are combined with ex situ time-of-flight secondary ion mass spectrometry (TOF-SIMS) chemical analysis, to reveal that Br^- or Cl^- ions can promote crystal growth, yet reactive I^- ions prevent them from incorporating into the lattice of the final perovskite crystal structure. These findings significantly advance our understanding of the role of halogens during synthesis of hybrid perovskites, and provide an insightful guidance to the engineering of high-quality perovskite films, essential for exploring superior-performance and cost-effective photovoltaic devices.
Acknowledgement: This research was conducted at the Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.
References:
1. Yang, B. et al. J. Am. Chem. Soc. 138, 5028–5035 (2016)
2. Yang, B. et al. J. Am. Chem. Soc. 137, 9210-9213 (2015)

High photovoltaic (PV) conversion efficiencies in lead halide perovskites have been primarily attributed to factors that enable long charge-carrier lifetimes, most notably the shallow energy levels and/or high formation energies of intrinsic point defects that might otherwise enhance electron-hole recombination. However, the effect of impurities on solar cell performance in lead halide perovskites remains largely unexplored, despite the increased risk of impurity incorporation when employing solution-based synthesis and the known detrimental effect of impurities in other PV absorber materials, most notably 3d transition metals in crystalline silicon (c-Si).

In this work, we experimentally determine the threshold at which photovoltaic performance in methylammonium lead halide begins to decrease due to the incorporation of iron by intentionally dissolving iron(II) iodide into our solution feedstock, which contains a 3:1 molar ratio of methylammonium iodide to lead(II) chloride dissolved in dimethyl formamide at 38 wt%. We then fabricate solar cells at contamination levels ranging from 1 ppm to 1% of iron and perform current-voltage sweeps to evaluate performance. Our results indicate that the threshold for performance reduction lies between 10–100 ppm of iron, which is 2–3 orders of magnitude higher than in p-type c-Si devices. We also find that efficiency losses in the most highly contaminated devices (100 ppm and above) can be temporarily and partially recovered by applying a static voltage bias before taking a reverse sweep, which exhibits time-dependent behavior even in perovskite cell architectures that typically exhibit low hysteresis.

To correlate device performance losses with charge-carrier lifetime, we perform time-resolved photoluminescence measurements. The trend in our data generally agrees with the monotonic decrease in open-circuit voltage with increasing iron incorporation, suggesting the presence of iron enhances non-radiative recombination. To investigate the distribution of any precipitated iron, we perform synchrotron-based X-ray fluorescence microscopy on samples with the highest iron incorporation. We compare these results to estimates of total iron incorporation using inductively-coupled plasma optical emission spectroscopy, which allows us to estimate the relative proportion of iron precipitates to point defects that are suspected to enhance recombination in methylammonium lead halide. Our work demonstrates the high tolerance that methylammonium lead halide has to iron impurities in addition to intrinsic point defects, which helps explain its ability to achieve high efficiencies despite the use low-cost fabrication techniques where purity is less controlled.

In recent years, organic-inorganic halide perovskites have attracted significant attention due to their outstanding performance as absorber layers in solar cells with power conversion efficiencies (PCE) exceeding 20%. Due to their wide band-gap > 1.6 eV they are promising candidates for the integration as top-cells in tandem devices.

Mixed-halide compounds of CH₃NH₃Pb(I_1-xBr_x)₃ enable band-gap tuning in a wide range between 1.6 and 2.3 eV. Substituting the commonly used methyl ammonium cation with formamidinium and caesium enhances stability and can suppress phase transitions.

Despite the strong progress made, many fundamental material properties such as the exciton binding energy (relevant for the efficiency of charge carrier separation in the devices) are still under debate and not sufficiently well determined. We used temperature-dependent absorption measurements from T = 5 K up to room-temperature in order to investigate the band-gap behavior and exciton binding energy of various mixed-cation lead mixed-halide absorber materials. By fitting an Elliott-type formula to the absorption spectra we could obtain the T-dependent band-gap and exciton binding energy of the different perovskites.

First we studied CH₃NH₃Pb(I_1-xBr_x)₃ and investigated the composition-dependent band-gap variation and phase transition from the low-temperature orthorhombic phase to the tetragonal phase above T = 165 K. The exciton binding energies evaluated from the spectra at different temperatures are consistent over the whole range investigated. We obtained values around 25 meV for methyl ammonium lead iodide. For increasing bromine content, excitonic features become more pronounced and the binding energy increases up to 35–45 meV.

By substituting the methyl ammonium cation partially by formamidinium and caesium, a perovskite modification with enhanced stability could be investigated. The temperature-dependence of the band-gap shows a similar behavior as the low-temperature phase of methyl ammonium lead iodide but without the characteristic phase-transition related jump of 110 meV to lower energies at around 160 K. This could indicate a suppression of the phase transition. The exciton binding energy of these compounds is slightly below the value of 25 meV for pure methyl ammonium lead iodide and should be beneficial for solar cell operation due to improved charge carrier separation.

Solar cell devices fabricated from these mixed-cation lead mixed-halide perovskites show a PCE up to 15% and an open-circuit voltage up to 1.16 V. Combined with an increased stability and suppression of phase transitions they seem to be promising candidates for the integration in tandem solar cells.

Perovskite solar cells have attracted much attention recently due to their high power conversion efficiencies, convenient fabrication process and potentially low cost. Semitransparent solar cells that can absorb light from both sides are promising for some special applications, such as building-integrated photovoltaics, tandem cells and wearable electronics. However, the performance of semitransparent perovskite solar cells has been limited by the quality of transparent top electrodes. Here, we report the semitransparent perovskite solar cells prepared by laminating graphene transparent electrodes on the top for the first time. The device performance is optimized by improving the conductivity of the graphene electrodes and the contact between graphene and the perovskite active layers during the lamination process. The devices prepared at the optimum conditions show high power conversion efficiencies when they are illuminated from both sides due to the high transparency and conductivity of the graphene electrodes. Moreover, the graphene electrodes are flexible and can be transferred on various conformal surfaces, being compatible with roll-to-roll process. Therefore, graphene is an ideal transparent-electrode material that can be used in various types of perovskite solar cells.

Recently, organometal halide perovskite solar cells have passed the threshold of 20 % power conversion efficiency (PCE). This was primarily achieved by optimizing the employed materials and processing conditions. While the PCE of perovskite solar cells is already competitive to other photovoltaic technologies, processing of large-area devices is still a challenge.
Most of the devices reported in literature are prepared by small-scale solution-based processing techniques (e.g. spin-coating). Furthermore, Perovskite solar cells processed by vacuum thermal evaporation (VTE) have also been presented, which show uniform layers and achieve higher PCE and better reproducibility. Regarding the co-evaporation of the perovskite constituents, this technology suffers from differences of the thermodynamic characteristics of the two species. While the organic components evaporate instantaneously, higher temperatures are needed for reasonable deposition rates of the metal halide compound. In addition, hybrid vapor phase deposition techniques have been developed employing a carrier gas to deposit the organic compound while the metal halide compound has been solution-processed in advance. Generally, vapor phase processes have been proven to be a desirable choice for industrial large-area production.
In this work, we present a setup for the direct chemical vapor phase deposition (CVD) of methylammonium lead iodide (MAPbI₃) employing nitrogen as carrier gas. For the volatilization of lead iodide, an evaporation source for temperatures up to 500 °C was developed. The temperature of the methylammonium iodide source can be independently chosen (typically 120 - 150 °C). Both deposition rates are separately controlled by carrier gas flows. In the pressure regime of 10 - 20 mbar, a perovskite film is formed on 2.5 cm x 2.5 cm large either fluorine-doped tin oxide on glass or silicon substrates by a reaction of both co-deposited components. To control film formation, the substrate can either be heated or cooled. We show that the ratio of the individual deposition rates and the substrate temperature have great influence on the structural properties of the perovskite layer. X-Ray diffraction (XRD) and scanning electron microscopy (SEM) measurements are carried out to investigate the crystal quality and structural properties.
By optimizing the deposition parameters, we produced perovskite films at a deposition rate of 30 nm/h which are comparable to those fabricated by solution processing. Furthermore, the developed CVD process can be easily scaled up to larger substrates, thus rendering this technique a promising candidate for manufacturing large-area devices. Moreover, CVD of perovskite solar cells can overcome most of the limitations of liquid processing, e.g. the need for appropriate and orthogonal solvents.

Perovskite morphology and crystallization are two issues of crucial importance for high efficiency of perovskite solar cells (PSC). Among the common factors such as chemical stoichiometry, composition and processing conditions and so on, the direction for formation of full coordinated perovskite framework is a determinant factor for high quality perovskite with minimum defects. Here we develop a novel method for the fabrication of perovskite MAPbI3/FAPbI3 through anion exchange process by the incorporation of Chloride. The intermediate complex HPbI2Cl, HPbICl2, HPbCl3 were introduced here that can vibrantly react with MAI or FAI to form highly crystallized perovskite. This perovskite provides a durable phase and humidity stability up to around 2 months compared to the traditional one step method. Accordingly, solar cell devices with perovskite produced in this method exhibited an efficiency as high as 18.2%.

Organic-inorganic perovskite solar cells have recently experienced an incredible rise in power conversion efficiency reaching performance levels similar to the leading commercial photovoltaic materials. However, one of the largest barriers to the commercialization of the perovskite materials is their instability, which can be related to internal processes such as ion migration throughout the perovskite film. We first study the effects of ion migration on the nanoscale optical properties of CH₃NH₃PbI₃ films using cathodoluminescence in scanning transmission electron microscopy (CL-STEM) and show that strongly luminescent nanoscale areas correspond to iodide-enriched regions. We then use electron beam induced current (EBIC) to study the effect of voltage biasing on the ion migration and resulting changes in current extraction. By biasing devices in-situ, we compare nanoscale inhomogeneities in current extraction before and after forward biasing and relate this to variations in chemical composition. These studies allow us to directly understand the role of voltage biasing and ion migration in both performance improvements and long term degradation of perovskite solar cell devices and ultimately in the improvement of the device performance and stability.

High power conversion efficiency of perovskite-based solar cells offers promise for low-cost and scalable production of renewable energy. The need to harness solar energy has recently motivated the search to alternate ETL/HTL hybrid materials, specifically graphene/perovskite films. Hybrid organic-inorganic methylammonium lead halides, MAPbX₃ (X=I, Br, Cl)/mixed-halides (I_3-xCl_x, I_3-xBr_x) have been reported as light harvesting layers with their superior optoelectronic properties: tunable bandgap, long electron-hole diffusion lengths and high electron/hole mobility. Nevertheless, halide-based perovskites require in situ investigation for film growth, degradation and perovskite formation mechanisms to overcome detrimental effects of incomplete lead precursor conversion, inconsistent crystallite formation/film uniformity, and weak cation-anion-solvent coordination. Graphene-derived hybrids have recently emerged as an ETL/HTL replacement in these devices, however understanding the origin of interfacial chemical reactions between deposited perovskite films over graphene-derived materials is still lacking. Graphene/perovskite structure-property relationships are, however, not well understood due to unclear chemistry at the ETL/perovskite/HTL interfaces. Moreover, effect of film thickness, lead content, stoichiometry control, overlayer/underlayer morphology/composition, stability issues and cation-anion electrostatic interactions ought to be examined for better charge transport at the graphene/perovskite interfaces. Stability factors also need to be studied for charge mechanisms to unravel device performance challenges. Indeed, underlayer ETLs (TiO₂/Al₂O₃) and overlayer HTLs (spiro-OMeTAD) were rarely studied with graphene. To address scalability and stability issues, we examined degradation, nucleation and growth mechanisms in reduced graphene/graphite oxide (RGO) upon halide-based (I, Cl, Br) perovskite deposition. Chemical interactions were interpreted at perovskite/RGO interfaces for the grain size, orientation, boundaries, and surface/bulk effects using variable-temperature (≤600°C, Ar(g)) in situ spectroscopy (infrared absorption, micro-Raman, UV-vis-NIR, luminescence). Controlled perovskite formation was achieved at room temperature for bromide/chloride-based perovskites resulting in improved chemical stability with heat (vs. iodide derivative). Overall, perovskite decomposition was observed at ~150°C. Oxygen-induced chemical reactions occurred at ≤150°C, eliminated hydroxyls/carboxyls in RGO, and maintained ethers/epoxides upon perovskite decomposition. Poor perovskite formation was observed on RGO due to varying electron affinity and reactivity of precursor halides, resulting in film degradation in air (O₂, H₂O). Film morphology was explored by characterization techniques such as SEM, XRD, XPS, TEM, and AFM. 1. Acik et al. J. Mater. Chem. A, 4, 6185, 2016. 2. Acik et al. Nature Mater. 9, 840, 2010.

Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The abstract has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Office of Science User Facility under Contract No. DE-AC02-06CH11357. M.A. also acknowledges support from the Joseph Katz Named Fellowship at Argonne National Laboratory.

Perovskite solar cells have had rapid success in optimization of device efficiency; however, the operational durability of these devices remains a critical concern. To address this key issue, we will discuss a reliability-testing approach using consensus stability testing protocols (ISOS-L-1) for benchmarking perovskite solar cell durability in air and in different humidity levels. We investigate and characterize the stability of the power output from methylammonium lead iodide perovskite photovoltaic devices produced with various hole-collecting anode configurations. We demonstrate an equivalent efficiency, low cost contact consisting of molybdenum oxide (MoO_x) and aluminum (Al), which is far more commercializable than gold (Au) and silver (Ag). While we find that the MoO_x/Al contact is equivalent in terms of the efficiency, it far outperforms Au and Ag contacts in terms of device stability. The unencapsulated devices were operated under constant illumination and constant load conditions in laboratory ambient with periodic current-voltage testing. Although the initial efficiencies of devices were comparable across these configurations, the stability of these devices varied significantly due to subtle differences in the electrode structure. By careful selection of the anode configuration, the T80 – the operating time at which the device efficiency to reduce to 80% of its initial efficiency – quadrupled from 20 hours to 80 hours. We can further bolster the device stability with a thin encapsulant to increase the T80 further to approximately 250 hours. While the open-circuit voltage and fill factor of these devices declined slightly, the significant reduction in the short-circuit current is primarily responsible for the decrease in efficiency. We demonstrate that a thin MoO_x layer inhibits decomposition of the perovskite films under illumination in ambient laboratory conditions, but that greater improvements in device stability are achieved specifically with MoO_x/Al electrodes, which prevent losses in the short-circuit current density. We investigated the role of the MoO_x interlayer in the MoO_x/Al electrodes by exploring the effect of relative humidity and the MoO_x interlayer thickness on the perovskite solar cell stability and conduct further studies in stability enhancement through alternative perovskite chemistries. Furthermore, we will discuss results using modified cation compositions and the role that the A-site cation can have on the operational, compositional and phase stability of the material.

Recently developed organic-inorganic hybrid perovskite solar cells combine low-cost fabrication and high power conversion efficiency. Advances in perovskite film optimization have led to an outstanding power conversion efficiency of more than 20%. Looking forward, shifting the focus towards new device architectures holds the great potential to induce the next leap in device performance. Here, we report the demonstration of a perovskite/perovskite heterojunction solar cell. We developed a facile solution-based cation infiltration process to deposit layered perovskite (LPK) structures onto methylammonium lead iodide (MAPI) films. Grazing-incidence wide angle X-ray scattering experiments were performed to gain insights into the crystallite orientation and the formation process of the perovskite bilayer. Our results show that the self-assembly of the LPK layer on top of an intact MAPI layer is accompanied by a reorganization of the perovskite interface. This leads to an enhancement of the open-circuit voltage and power conversion efficiency due to reduced recombination losses, as well as improved moisture stability in the resulting photovoltaic devices. Our work paves the way for the implementation of all-perovskite heterojunction architectures in highly efficient perovskite solar cells.

Sound understanding of the structural behaviour of hybrid perovskite photovoltaic materials underpins comprehension of their fundamental physical properties. However, this entails surmounting challenges that stem both from the structural complexity inherent in these materials and the limitations of conventional X-ray diffraction (XRD) techniques. We have demonstrated neutron powder diffraction (NPD) to be a powerful method for investigating the complete atomic structure of hybrid materials, distinguishing carbon from nitrogen due to dissimilar neutron scattering lengths and obtaining hydrogen atom positions. From NPD data collected at D20, ILL, we have been able for the first time to refine the inorganic and organic components of fully hydrogenous MAPbI₃, including hydrogen positions.(1) The perovskite α-phase of formamidinium lead iodide is shown to be cubic (a = 6.3620(8) Å) at room temperature, rather than trigonal or tetragonal as previously described in the literature, using high resolution NPD at HRPD, ISIS.(2)

Novel hybrid organic-inorganic lead halides for optoelectronic applications (3) and the A-site solid solution series MA_xFA_1-xPbI₃ have been synthesised and characterised for structure and composition by XRD and allied spectroscopic techniques. MA_xFA_1-xPbI₃ compounds are cubic across the whole composition range, except for tetragonal MAPbI₃, with no observed non-perovskite δ-phase formation, as for FAPbI₃. The phase behaviour of MA_xFA_1-xPbI₃ has been investigated using variable-temperature single crystal XRD. The implications of these results for understanding the behaviour of these materials within photovoltaic cells will be discussed.

1) M. T. Weller, O. J. Weber, P. F. Henry, A. M. Di Pumpo, and T. C. Hansen, Chem. Commun., 2015, 51, 4180–4183.
2) M. T. Weller, O. J. Weber, J. M. Frost, and A. Walsh, J. Phys. Chem. Lett., 2015, 6, 3209–3212.
3) O. J. Weber, K. L. Marshall, L. M. Dyson, and M. T. Weller, Acta Crystallogr. B., 2015, 71, 668–78.

Surface morphology of perovskite film is critical for highly efficient perovskite-based solar cells. The solution processed perovskite films tend to have voids and pin-holes between the crystals, which has been cited as very detrimental to device performance. The voids and pin-holes not only cause electrical shorting but also deleteriously impact charge dissociation/ transport/ recombination because of the defects existed on the surface and grain boundaries of the perovskite films. Here we demonstrate passivation of the perovskite surface and grain boundaries by small molecules through infiltration of subphthalocyanines (SubPc) into the perovskite film. We found that filling up the voids and pin-holes is easier due to its lower viscosity of SubPc compared to that of the polymer. Moreover, compared to the polymer passivation, thermal annealing is avoided for the spin-coated SubPc solution, which eliminated the possible damage to the perovskite by the further thermal annealing. Free charge carrier lifetime of perovskite film was increased, which indicated effective passivation of the surface defect trap-states. As a result, the power conversion efficiency for the solution-processed planar heterojunction solar cells has been enhanced from 9.96% for the contrastive perovskite solar cells to 13.6% for the SubPc passivated perovskite solar cells. Different from the methods of improving the quality of perovskite crystal in the process of crystallization, the passivation method provide a simple and low-cost avenue to improve the quality of the perovskite film and further increase its efficiency.

Semiconducting lead triiodide perovskites (APbI₃) have shown remarkable performance in applications including photovoltaics and luminescence^1-3. Despite many theoretical possibilities⁴ for A⁺ in APbI₃, our current experimental knowledge is largely limited to two of these materials: methylammonium (MA⁺) and formamidinium (FA⁺) lead triiodides, neither of which is stable under the ambient environment. By hypothesizing that an ideally cubic structure may improve the semiconducting performance and structural stability of a perovskite material, we identified empirical criteria to stabilize cubic APbI₃ at room temperature, and synthesized a family of MA_1−xEA_xPbI₃ perovskites (x = 0.09–0.24, EA⁺ = ethylammonium) accordingly. Powder and single-crystal X-ray diffraction (XRD) reveals that as a solid solution, MA_1−xEA_xPbI₃ possesses the cubic perovskite structure unlike the room-temperature structure of any known single-component APbI₃. The steady-state absorption and photoluminescence spectra of MA_1−xEA_xPbI₃ are similar to those of MAPbI₃ but red-shifted by ~30 nm. A photodetecting device fabricated from a 1.8-mm-thick MA_0.85EA_0.15PbI₃ single crystal features a very high external quantum efficiency (EQE) of 1.5% at 830 nm, nearly one order of magnitude greater than the value of a MAPbI₃-based device. The performance highlight of MA_0.85EA_0.15PbI₃ is also corroborated by its high carrier mobility of 1.6×10² cm² V⁻¹ s⁻¹ from space-charge-limited current (SCLC) measurements. Furthermore, no change could be observed by the naked eye when cubic MA_1−xEA_xPbI₃ perovskites were stored in hydriodic acid for 60 days, or under 80% relative humidity for 3 days at room temperature. By comparison, MAPbI₃ would have turned from black to yellow within several hours under the same conditions. The significantly enhanced performance and stability of cubic MA_1−xEA_xPbI₃ indicate that this new family of perovskites can be a top candidate for semiconducting applications, and the effectiveness of the design process illustrates our updated understanding of the perovskite structures beyond the conventional Goldschmidt tolerance factors.

References
[1] Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J., Science 2015, 347 , 967-970.
[2] Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I., Science 2015, 348, 1234-1237.
[3] Chen, W.; Wu, Y.; Yue, Y.; Liu, J.; Zhang, W.; Yang, X.; Chen, H.; Bi, E.; Ashraful, I.; Gratzel, M., Science 2015, 350, 944-948.
[4] Filip, M. R.; Eperon, G. E.; Snaith, H. J.; Giustino, F., Nat. Commun. 2014, 5, 5757.

Hybrid metal halide perovskites have been paid enormous attentions in photophysics research, whose excellent performances were attributed to their intriguing charge carriers proprieties.^[1]-[2] However, it still remains far from satisfaction in the comprehensive understanding of perovskite charge-transport properities, especially about trap-assisted recombination process. In the present work, through time-resolved transient absorption (TA) and photoluminescence (PL) measurements, we provided a relative comprehensive investigation on the charge carriers recombination dynamics of CH₃NH₃PbBr₃ (MAPbBr₃) perovskite films and quantum dots (QDs), especially about trap-assisted recombination.^[3] It was found that the integral recombination mode of MAPbBr₃ films was highly sensitive to the density distribution of generated charge carriers and trap states. Additional, Trap effects would be gradually weakened with elevated carrier densities. Furthermore, the trap-assisted recombination can be removed from MAPbBr₃ QDs through its own surface passivation mechanism and this specialty may render the QDs as a new material in illuminating research. This work provides deeper physical insights into the dynamics processes of MAPbBr₃ materials and paves a way toward more light-harvesting applications in future.
References
[1] Z-Y Zhang, et al. J Phys Chem C (Revision).
[2] Z-Y Zhang, et al. Phys Chem Chem Phys 17, 44, 30084-30089; DOI: 10.1039/c5cp04333f (2015).
[3] Z-Y Zhang, et al. Sci. Rep. 6, 27286; DOI: 10.1038/srep27286 (2016).

Smooth, compact and defect free morphology of perovskite film is highly desired for enhanced device performance. Several routes such as thermal annealing, use of solvent mixtures, growth under controlled humidity has been adopted to obtain crystalline, smooth and defect free perovskite film. Herein, we have studied the crystallization of perovskite film by using humidity from ambient atmosphere and by direct use of water as co-solvent in perovskite precursor solution. In the first case, perovskite crystallization was achieved by keeping the film in ambient atmosphere having 40% RH at room temperature for several hours. Perovskite films were completely crystallized in 5 hours in ambient air leading to rod shape morphology. In the second case the optimization of amount of water as co-solvent in precursor solution was done in order to achieve smooth and dense film. Varying concentration of water was used in precursor solution of CH₃NH₃I and PbI₂ mixed in γ-butyrolactone (GBL) and dimethylsulfoxide (DMSO). Perovskite films were crystallized using toluene assisted solvent engineering method using GBL:DMSO:H₂O as solvent mixture. The amount of water was varied from 1% to 25%, which resulted in change in film morphology and perovskite crystallinity. It was concluded that an appropriate amount of water is required to assist the crystallization process to obtain smooth pin-hole free morphology. The room temperature, ambient air crystallized perovskite film gave an highest efficiency of 16.83%, whereas low temperature crystallized perovskite film using water as co-solvent gave highest efficiency ~14%, which was significantly higher than devices made from perovskite film without adding water. We also showed that addition of up to 25% by volume of water does not significantly change the device performance.

Organic-inorganic lead halide perovskites have attracted much attention as emerging photoactive materials. Their unique merits, including excellent optical absorption, long-range balanced carrier-diffusion length and low-cost processing methods, allow them to be extremely promising for the next-generation photovoltaics.¹ Lead halides are the most commonly used lead sources for perovskite active layers.² Recently, lead acetate (Pb(Ac)₂) has shown its superiority as a potential candidate to replace the traditional lead halides due to its simple fabrication process.^3-5 However, to date, the perovskite solar cells based on Pb(Ac)₂ precursors have shown relatively lower performances in comparison to those reported planar heterojunction perovskite solar cells based on lead halide precursors.

Here, we demonstrate a strategy to improve the device performance for the perovskite solar cells based on Pb(Ac)₂ precursors. We used trace amounts of a CH₃NH₃Br (MABr) additive in the Pb(Ac)₂ and CH₃NH₃I precursor solution, resulting in uniform, compact and pinhole-free perovskite films. Further studies show that MABr could be beneficial to improve the film morphology and crystallinity, tune the optical and electrical properties of the perovskite photoactive layer and enhance the charge carrier extraction at the interfaces between the perovskite layer and charge collection layers, leading to improved device performance.

By adopting state-of-the-art fabrication process, a champion power conversion efficiency of 18.32% was achieved in the inverted planar heterojunction perovskite solar cells by simultaneously improving three factors, a J_sc of 22.34 mA/cm², a V_oc of 1.00 V and an FF of 0.82. Meanwhile, this device also delivered a stabilized power output of 17.60% at the maximum power point. The optimized devices exhibited negligible current density-voltage (J-V) hysteresis under different scanning conditions. MABr additive is also shown to improve the reproducibility of device performance. These results hold tremendous promise for Pb(Ac)₂ as a lead source for perovskite solar cells.

References
1 S. D. Stranks et al. Science 2013, 342, 341.
2 N.J. Jeon et al. Nature 2015, 517, 476.
3 W. Zhang et al. Nat. Commun. 2015, 6, 6142.
4 J. Qing et al. ACS Appl. Mater. Interfaces 2015, 7, 23110.
5 W. Zhang et al. Nat. Commun. 2015, 6, 10030.
6. L. Zhao, D. Luo, R. Zhu, et al. Adv. Funct. Mater. 2016, 26, 3508.

Organic-inorganic trihalide perovskites have been widely studied as light absorbing materials for potential applications in high-performance solar cells. This group of materials possess a variety of unique properties. For example, in MAPb(BrxI1-x)3 , one attractive behavior is the ability to continuously tune its bandgap by changing the Br/I ratio. However, despite their larger bandgap, mixed halide perovskite solar cells do not generate a higher open circuit voltage than triiodide perovskite solar cells [1]. This poor voltage performance has been attributed to ion migration in mixed halide perovskites [2]. Recent studies have explained ion migration phenomena in mixed halide and pure iodide systems in terms of thermodynamic instabilities [3] and light induced local electric field effects [4]. However, the origin of reversible iodide-rich domain formation is still unclear. Here, the photoluminescent properties of MAPb(BrxI1-x)3 thin films are studied to elucidate the ion migration behavior as well as the resulting shift in peak wavelength in MAPb(BrxI1-x)3 under structured laser and white light illumination. While one spot on sample is excited, we study the PL spectrum and PL lifetime of the surrounding regions. Local bromide concentration is inferred from the PL peak wavelength and Br-concentration-bandgap relation [5]. We will show how these spatial, spectral, and temporal measurements can provide a detailed map of iodide-bromide redistribution to help study the origin of ion migration and photostability issues.

[1] Sneha A. Kulkarni, et al. "Band-gap tuning of lead halide perovskites using a sequential deposition process." J. Mater. Chem. A 2 (2014): 9221-9225.
[2] Eric T. Hoke, et al. "Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics." Chem. Sci. 6 (2015): 613-617.
[3] Federico Brivio, Clovis Caetano, and Aron Walsh. "Thermodynamic Origin of Photoinstability in the CH3NH3Pb (I1–xBrx)3 Hybrid Halide Perovskite Alloy." J. Phys. Chem. Lett. 7 (2016): 1083.
[4] Wei Zhang et al. "Photo-induced halide redistribution in organic-inorganic perovskite films." Nat. Commun. 7, 11683 (2016).
[5] Jun Hong Noh et al. “Chemical management for colorful, efficient, and stable inorganic–organic hybrid nanostructured solar cells.” Nano Lett. 13 (2013): 1764-1769.

Cadmium sulfide has recently attracted interest as a possible electron transport material for perovskite solar cells, and is considered an attractive material for this application due to its simple, rapid, and low-temperature deposition process; furthermore, perovskite solar cells using CdS as electron transport layer have displayed enhanced stability when compared with those using TiO₂. [1,2] However, the chemistry of the CdS/perovskite interface remains largely unexplored. In this study, we have conducted X-ray photoemission spectroscopy measurements of CH₃NH₃PbI₃ perovskite films spin-coated on top of CdS, which indicate that Cd can diffuse into the perovskite absorber. To better understand the effects of Cd incorporation, small amounts of CdI₂ were intentionally added into the perovskite precursor. Cd doping increased the grain size of the perovskite films substantially, but significantly harmed device performance. Time-resolved photoluminescence spectroscopy measurements indicate that Cd impurity in the perovskite does not appear to cause bulk trap states (i.e., there is no impact on minority carrier lifetime), but regions of secondary phase are visible in SEM images of the perovskite films when Cd concentration in the precursor exceeds 1 mol%. We have identified this phase to be (CH₃NH₃)₂PbI₄, and have determined that it is a wide band gap material and therefore likely to act as a barrier in the devices, accounting for the loss in device performance. Formation of this phase is especially vigorous in perovskite films grown on CdS using an alternative to spin coating, the matrix-assisted pulsed laser evaporation (MAPLE) technique, even when Cd is not intentionally added to the precursor. The presence of large amounts of (CH₃NH₃)₂PbI₄ in the MAPLE-grown films emphasizes that reaction between the substrate (i.e., CdS) and perovskite precursors can be quite severe, and devices made using these films are strongly afflicted by the concomitant barrier formation. We propose a reaction mechanism for the formation of (CH₃NH₃)₂PbI₄ in this manner, report strategies by which it may be avoided during device fabrication, and demonstrate spin-coated devices with stabilized power conversion efficiency exceeding 15%, which we believe to be the first time this milestone has been achieved for perovskite solar cells using CdS as the electron transport layer. Our findings demonstrate the importance of understanding interfacial effects between perovskite and charge transport layers, and that, as other transport materials are considered, a thorough investigation of interfacial reactivity is critical to ensuring both high efficiency and stability of devices.

1. Hwang, I. and Yong, K. ACS Appl. Mater. Interfaces 2016, 8, 4226-4232.
2. Peng, H. et al. J. Photon. Energy 2016, 6, 022002.

Perovskite solar cells, based on state-of-the-art light absorbing inorganic-organic halide perovskite materials, have shown remarkable progress on account of achieving power conversion efficiencies greater than 22%. Due to their poor humidity-, photo-, and thermal-stability, mixed halide perovskite materials have been briskly researched. Especially, halide gradient perovskite materials have been expected to enhance the hole extraction rate because the valence band level of perovskite materials is modified in a gradational manner. However, as far as we know, the halide concentration gradient perovskite materials have not been realized due to the difficulty in developing synthetic techniques. Mixed halide CH3NH3PbI_3-xBr_x perovskite solar cells are traditionally obtained from solution process using pre-mixed solution. Such wise only generate entirely mixed perovskite films, not partially substituted. Here we describe novel and facile halide conversion method using bromine fume produced by vaporized of hydrobromic. This method enables spatial halide substitution of the upper part of pristine iodine based perovskite film, thereby attain halide ion gradient inside the post-treated perovskite film. Accelerated hole extraction and enhanced lifetime due to Br gradient was verified by observing photoluminescence properties. We also examine the behavior and location of bromide ions that constitute halide gradient via the combination of secondary ion mass spectroscopy and transmission electron microscopy with energy-dispersive X-ray spectroscopy analysis. Mixed halide perovskite solar cells produced via this facile conversion method exhibits a power conversion efficiency (PCE) of 18.94 % due to increased open circuit voltage (V_oc) from 1.08 to 1.11 V and fill-factor (FF) from 0.71 to 0.74. Long-term stability is also dramatically enhanced after conversion process, the PCE of the post-treated device remains over 97% of the initial value under high humid condition (40-90%) without any encapsulation for 4 weeks.

The optical characteristics of organo-lead halide perovskite single crystals (MAPbX₃; MA=CH₃NH₃⁺, X=Cl^-, Br^-, or I^-) have been investigated by using steady-state photoluminescence (PL) and time-resolved PL (TRPL) spectroscopies. Perovskite single crystals (MAPbCl_3-xBr_x, MAPbBr_3-xI_x) were synthesized using the inverse temperature crystallization (ITC) with proper solvents. Then, the PL spectra were performed using confocal micro-spectroscopy (NTEGRA spectra, NT-MDT) equipped with a solid state laser with 405 nm and an objective lens with numerical aperture of 0.7, yielding a high spatial resolution of ~ 380 nm. For an analysis of carrier dynamics for the single crystals, TRPL, multifunctional confocal microscopy including a time-correlated single photon counting (TCSPC) system was employed (NTEGRA, NT-MDT). Figure 1 shows an evolution of PL spectra of the MAPbBr₃ and MAPbBr_2.5I_0.5 as functions of laser irradiation times. As shown, the sharp contrast between MAPbBr₃ and MAPbBr_2.5I_0.5 PL spectra was observed. The PL spectra of MAPbBr₃ remained with a band edge peak of ~ 530 nm. However, the MAPbBr_2.5I_0.5 developed the additional satellite peaks at lower energy bandgaps while the bandgap peak of 540 nm gradually disappeared. Interestingly, when the single crystals are left for 15 minutes in the dark, the PL spectra reverted to the initial PL states, indicating these photo induced changes are completely reversible. To further understand the nature of lower energy peaks, power and temperature dependent PL measurements were investigated and will be further discussed.

Organic-inorganic hybrid perovskite has been focused by many scientists due to its excellent properties in photovoltaic and optoelectronics. Especially, perovskite solar cell recorded 20.1% power conversion efficiency despite of short research period. However, high quality of film in devices is crucial for fabricating high performance device. For improving film quality, some researchers fabricated devices based on organic lead halide perovskite single crystal and demonstrated high performance. But, they synthesized only three representative organic lead halide perovskite single crystals(CH₃NH₃PbX₃,CH₃NH₃ MA, X=Cl, Br, I). For the expansion of applicability, bandgap tuning is essential. Therefore, we prepared bandgap modulated organic lead halide perovskite single crystals (MAPbCl_3-xBr_x, MAPbBr_3-xI_x) by changing halogen ratio within 24h using retro-grade solubility property and anti solvent diffusion. Synthesized samples were characterized with XRD, UV-Vis, FT-IR, photoluminescence, TGA, SEM and etc.

Recently, organo-metal halide perovskite appears as a superb solution processed active layer material in solar cell application. Besides, the band gap and electrical transportation properties can be easily tuned by mixing different organic molecules and halides. By adding small amount of MA and Br ions into FAPbI3 system could help to increase the power convert efficiency of perovskite solar cell notably. However, it is found that perovskite with different organic molecules and halides tend to form different crystal phase and have different optimal annealing temperature. The directly mixing of different perovskite components will bring phase competition thus unsatisfactory crystallinity. Here, we developed ion exchange method through slight solvent to replace traditional directly mixing method and synthesis uniform mixed perovskite thin film with high reproducibility. By further investigation on the ion exchange two-step process, a rapid interfacial process followed by a fine dissolution and recrystallization process, we proved that the phase and crystallinity could be retained during ion exchange process. As the result of high crystallinity, the illumination and moisture stability of mixed perovskite thin film could also be strikingly improved through this method. At the end, we applied the phase remaining strategy for mixed perovskite on thin film solar application and get a high PCE of 17.8%.

Although perovskite has been intensively studied as a promising material for solar cell technology due to its high power conversion efficiency, its stability should be improved for real applications. In this study, we have developed a protocol for discovery/design of new materials in perovskite solar cells. In this protocol, first, we explore the crystal ionic radii for all elements and select possible stable perovskites possessing cubic structure. A Goldschmidt tolerance factor of 0.95 is used to filter only cubic perovskites that are stable at room temperature. After removing rare or radioactively unstable elements, quantum mechanical density functional theory (DFT) calculations are performed on remaining perovskites to assess whether their electronic properties are suitable for solar cell applications. Similar calculations are performed on Ruddlesden-Popper phase of remaining perovskites with suitable properties. These calculation results are compared to results obtained from a Methylammonium Lead Iodide Perovskite reference. The stability is then assessed experimentally by formation of thin films of Ruddlesden-Popper and Perovskite phases of Methylammonium Lead Iodide. Thin films are then exposed to 373 K for 5 hours. Further instrumental analysis is implemented on remaining films to assess change in crystal structure and evolution of lead iodide

Recently, researchers have focused more to design mechanically flexible perovskite solar cells (PVSCs), which enables the implementation of portable and roll-to-roll fabrication in large scale. While NiO_x is a promising material for hole transport layer (HTL) candidate for fabricating efficient PVSCs on rigid substrate, the reported NiO_x HTLs are formed using different multi-step treatment (such as 300-500 degC annealing O₂-plasma, UVO etc.), which hindering the development of flexible PVSCs based on NiO_x. Meanwhile, the features of nanostructured morphology and flawless film quality are very important for the film to function as highly effective HTL of PVSCs. However, it is difficult to have the two features co-exist natively particularly in solution process that flawless film will usually come with smooth morphology. Here, we demonstrate the pinhole-free and surface-nanostructured NiO_x film from a simple and controllable room-temperature solution process for achieving high-performance flexible PVSCs with good stability and reproducibility.[1] The power conversion efficiency (PCE) can reaches a promising value of 14.53% with no obvious hysteresis (and a high PCE of 17.60% for PVSC on ITO glass). Furthermore, the NiO_x based PVSCs show markedly improved air stability. Regarding the performance improvement, the pinhole-free and surface-nanostructured NiO_x film can make the interfacial recombination and monomolecular Shockley-Read-Hall recombination of PVSC reduce. In addition, the formation of an intimate junction of large interfacial area at NiO_x film/the perovskite layer improve the hole extraction and thus PVSC performances. This work contributes to the evolution of flexible PVSCs with simple fabrication process and high device performances.
[1] H. Zhang, J. Cheng, F. Lin, H. He, J. Mao, K. S. Wong, A. K. Y. Jen, W. C. H. Choy, ACS Nano 2016, 10, 1503.

Developing planar structure is the trend of perovskite solar cells research due its low temperature and simple device fabrication processing. Unfortunately, planar structure, especially n-i-p planar devices usually showed serious I-V hysteresis and lower stable device efficiency compared with mesoporous structure. Here, a new electron transport layer was replacing traditional TiO2 condensed layer. The devices with this new electron transport layer are almost free of hysteresis, and a 19.9±0.6% certificated efficiency was obtained. The devices fabrication can be easily processed under low temperature (150oC), providing a new and efficient method for large scale production of perovskite solar cells.

Microscopic understanding of interaction between H₂O and MAPbI₃ is essential to further improvement of efficiency and stability of perovskite solar cells. A complete picture of perovskite from initial physical uptake of water molecules to final chemical transition to its monohydrate MAPbI₃●H₂O is obtained with in-situ infrared spectroscopy, mass monitoring and X-ray diffraction. Despite strong affinity of MA to water, MAPbI₃ appears hydrophobic and absorbs almost no water from ambient air. Water molecules penetrate the perovskite lattice and share the space with MA up to one H₂O per MA at high humidity levels. However, the interaction between MA and H₂O through hydrogen bonding is not established until the phase transition to monohydrate where H₂O and MA are locked to each other. This lack of interaction in water infiltrated perovskite is a result of dynamic orientational disorder imposed by tetragonal lattice symmetry. The apparent inertness of H₂O along with high stability of perovskite in ambient provides a solid foundation for its long-term application in solar cells and opto-electronic devices.

As the fabrication process for organometal halide perovskite (OHP) film, varieties of deposition techniques have been reposted. Among the techniques, it has been expected that vacuum deposition techniques have advantages for fabricating smooth and uniform OHP films on large-area substrates. We investigate the effect of vacuum deposition conditions on the growth of methylammonium lead triiodide (MAPbI₃) and formamidinium lead triiodide (FAPbI₃) films.
The MAPbI₃ films were grown on a substrate by the sequential deposition of PbCl₂ and CH₃NH₃I (MAI). The influences on the growth of MAPbI₃ film of the MAI deposition rate and the pressure inside vacuum chamber during the evaporation were studied. Despite the thickness of PbCl₂ layer was kept constant, the low deposition rate (~ 0.9 Å/s) of MAI caused the MAPbI₃ film to be thicker compared with that grown by the high deposition rate. The x-ray diffraction patterns and the x-ray photoemission spectra of the samples suggested that the fast deposition rate of MAI induced the presence of the unreacted (or insufficiently reacted) PbCl₂. In addition, it was observed that the pressure during the MAI evaporation also affected the growth of the MAPbI₃ films; the thickness of the MAPbI₃ film fabricated under 1 × 10⁻¹ Pa was thicker than that fabricated under 1 × 10⁻³ Pa.
The FAPbI₃ films were grown by the co-deposition of PbCl₂ (or PbI₂) and HC(NH₂)₂I (FAI). The effect of the deposition rate of PbCl₂:FAI and the substrate temperature on the quality of FAPbI₃ film was studied. At room temperature substrate, FAPbI₃ was formed with the deposition rate of PbCl₂:FAI = 1:10. However, the quality of FAPbI₃ film was poor. The FAPbI₃ film fabricated with the deposition rate of PbCl₂:FAI = 1:20 and the substrate temperature of 80 °C showed good uniformity and crystallinity.
Based on these results, the appropriate conditions of the vacuum deposition for the fabrication of the MAPbI₃ and the FAPbI₃ films will be discussed.

We present a study of the energy levels in a FTO/TiO₂/CH₃NH₃PbI₃/Spiro solar cell device. The measurements are performed using a novel ambient pressure photoemission (APS) technique alongside Contact Potential Difference data from a Kelvin Probe. The Perovskite Solar Cell energy band diagram is demonstrated for the device in dark conditions and under illumination from a 150W Quartz Tungsten Halogen lamp. This approach provides useful information on the interaction between the different materials in this solar cell device. Additionally, non-destructive macroscopic DC and AC Surface Photovoltage Spectroscopy (SPS) studies are demonstrated of different MAPBI₃ device structures to give an indication of overall device performance. AC-SPS measurements, previously used on traditional semiconductors to study the mobility, are used in this case to characterise the ability of a perovskite solar cell device to respond rapidly to chopped light. Two different device structures studied showed very different characteristics: Sample A (without TiO₂): (ITO/PEDOT:PSS/polyTPD/CH₃NH₃PbI₃/PCBM) had ~4 times the magnitude of AC-SPS response compared to Sample B (including TiO₂): (ITO/TiO₂/ CH₃NH₃PbI₃/Spiro). This demonstrates that the carrier speed characteristics of device architecture A is superior to device architecture B. The TiO₂ mesoporous layer has been associated with carrier trapping which is illustrated in this example. However, the DC-SPV performance of sample B is ~5 times greater than that of sample A. The band gap of the MAPBI₃ layer was determined through DC-SPS (1.57 ± 0.07 eV), Voc of the devices measured and qualitative observations made of interface trapping by DC light pulsing. The combination of these (APS, KP, AC/DC-SPV/SPS) techniques offers a more general method for measuring the energy level alignments and performance of Organic and Hybrid Solar Cell Devices.

Perovskite based solar cells is the center for many researchers due to number of their properties such as high mobility, high light absorption coefficient, large hole-electron diffusion length and better charge transportation behavior. Different methods have been adopted for preparing better Perovskite layer while on the other side structural engineering like conventional or inverted is also under evaluation by many groups for achieving hysteresis free devices. In this work we followed solvent engineering approach for preparing all solution processed planar inverted structure Perovskite solar cell (PSCs) with induction of silver nanoparticles and solution processed BPhen (sBPhen) as anode and cathode side modifications respectively. Effects of nanoparticles on Perovskite layer growth, charge collection and the overall power conversion efficiency (PCE) of device was examined for better understanding of device Physics. Our results reveal that PSCs with PEDOT:PSS+AgNPs for HTL and PC₆₀BM/sBphen bilayers ETL possess favorable charge collection efficiency, better Perovskite crystallinity leading the improvement in photocurrent density thereby an improved PCE. Whole fabrication was done at a low temperature ≤90°C for making it more acceptable for large scale production at low cost.

Today, the verified power conversion efficiency of perovskite solar cells (PSCs) have reached by 21.02 %, which are usually fabricated by spin-coating process with toluene dropping process. However, the spin-coating process is hardly to scale-up because the uniformity of perovskite layer is required. Furthermore, the toxic solvents, such as DMF, toluene, etc., are involved in the spin-coating process, which is unhealthy for humans. For these reasons, we believed that enlarged area with non-toxic process is required instead of the spin-coating process. Electrodeposition process is a low-cost and mature industrial technique for preparing large-scale coatings via electrochemical reduction-oxidation. The PbO₂ thin film prepared via electrodeposition process is easily to control including film thickness and uniformity. In this study, we try to optimize the electrodeposition process conditions by controlling the deposition current to fabricate a large-area and uniform PbO₂ film. In addition, the two-step process to fabricate CH₃NH₃PbI₃ is introduced in this study. The absorbance septum clearly show the PbO₂ can indeed convert into perovskite film in a short time. The conversion efficiency of 9.38% can be achieved with active area in 4 cm² under 1 sun.

Since the adopt of methylammonium lead triiodide (CH₃NH₃PbI_3, MAPbI₃) perovskite as light harvester in the solar cells with power conversion efficiency (PCE) of 3.8% in 2009, and the PEC has boosted to the current world record of 21.0% in 2016. The rapid advances in perovskite solar cells are attributed to its excellent optoelectronic properties such as remarkably high absorption coefficient, low exciton binding energy about 0.03 eV, carrier diffusion length in the range of micrometer owing to the recombination occurring on a timescale of hundreds of nanoseconds, and tunable energy bandgap. However, the surface coverage and the morphology of MAPbI3 are hardly to control, because the nucleation rate and crystal growth rate is inconsistent. To overcome this issue, the antisolvents usually were introduced during the preparation of MAPbI₃ thin film. There is many kind of anti-solvents, it should be not dissolving the perovskite materials and is miscible with DMSO and γ-butyrolactone (GBL). Therefore, selecting antisolvent becomes an important factor to fabricate high performance perovskite solar cells. Here, we have a systematic studied different types of anti-solvent including toluene (TL), chloroform (CF), chlorobenzene (CB), dichlorobenzene (DCB), isopropyl alcohol (IPA), and some common organic solvents. We find that low dielectric constant (<5), low dipole moment (<1) and low donor number (<5) is more suitable to perovskite especially toluene (TL). Because it has the largest supersaturated driving force. We also use TL as the anti-solvent to demonstrate a sub-module (larger-area) perovskite solar cell with a PCE as high as 11.60%.

Methylammonium lead iodide (MAPbI₃) perovskite based solar cells have recently emerged as a serious competitor for large scale and low-cost photovoltaic technologies. However, since these solar cells contain toxic lead, a sustainable procedure for handling the cells after their operational lifetime is required to prevent exposure of the environment to lead and to comply with international electronic waste disposal regulations.

In this presentation we will demonstrate an environmentally responsible and cost-efficient recycling process for solar cells based on MAPbI₃. Our results show that perovskite solar cells can be stripped down in a layer-by-layer approach, and that the collected materials can be reused without significant losses to device performance. In particular, we show that the toxic PbI₂ can be recycled and, after recrystallization, can be employed to prepare highly efficient devices. With this approach, the risk of lead contaminating the environment can be decreased, while still retaining PbI₂ as the starting material for the production of highly efficient solar cells. In addition, we were able to recycle the most expensive part of the solar cell, the FTO/glass substrates, several times without any loss of device performance. With our simple recycling procedure, we address both the risk of contamination and the waste disposal of perovskite based solar cells, while further reducing the cost of the system. This brings perovskite solar cells one step closer to their introduction into commercial systems.


A. Binek, M. L. Petrus, N. Huber, H. Bristow, Y. Hu, T. Bein, P. Docampo, ACS Appl. Mater. Interfaces, 2016, 8 (20), 12881-12886.

The degradation of organometallic perovskite (CH₃NH₃PbI₃) by the humidity and temperature under ambient conditions is the fundamental concern for successful commercialization, and therefore secure stability is required with the satisfaction of high power conversion efficiency (PCE). The CH₃NH₃PbI₃ perovskite phase is known to be vulnerable by the hygroscopic nature of methylamine molecule (CH₃NH₃⁺), and accordingly it necessitates the microstructural rigidity to stand the environmental changes. From the fact that material characteristic of instability influences the device performance, we have controlled the solvent and precursors to boost the grain morphology and crystallinity of organometallic perovskite. In addition, the chemistry of CH₃NH₃PbI₃ is tuned to withstand against the phase transformations. As a result, remarkably stabilized perovskite is synthesized with the improved PCE.

[1] J. Kim, T. Hwang, S. Lee, B. Lee, J. Kim, G. S. Jang, S. Nam, and B. Park, Sci. Rep. 6, 25648 (2016).
[2] T. J. Jacobsson, J. Correa-Baena, M. Pazoki, M. Saliba, K. Schenk, M. Gratzel, and A. Hagfeldt, Energy Environ. Sci. 9, 1706 (2016).

Corresponding Author: Byungwoo Park: [email protected]

The remarkable performance of hybrid perovskite-based solar cells has launched a new paradigm in the area of photovoltaic research.¹ Facilitated by the processing flexibility of hybrid lead halide perovskites, e.g. MAPbI₃ and FAPbI₃ (MA = methylammonium, FA = formamidinium), the turn-over efficiencies have soared to over 20% within a short period of time.^2–4 However, the toxicity of lead may limit wider commercialization of such materials. In the search for lead-free alternatives, Bi³⁺ based compounds have recently attracted attention. In such materials, the perovskite formula is essentially doubled and the divalent metal cations, e.g. Pb²⁺ cations, are replaced by a monovalent (B^I) and a trivalent cation (B^III), e.g. Na⁺ and Bi³⁺.

Based on previously synthesized (MA)₂KBiCl₆⁵, density functional theory (DFT) screening of the hybrid double perovskites (MA)₂B^IBiX₆ (B^I = K, Cu, Ag, Tl; X = Cl, Br, I) shows that systems with band gaps similar to those of the MAPbX₃ lead compounds can be expected for B^I = Cu, Ag, Tl. Motivated by these findings, (MA)₂TlBiBr₆, isoelectronic with MAPbBr₃, was synthesized and found to have a band gap of ~2.0eV.⁶



1 M. D. McGehee, Nat. Mater., 2014, 13, 845–846.

2 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050–6051.

3 M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith, Science, 2012, 338, 643–647.

4 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin and M. Grätzel, Nature, 2013, 499, 316–319.

5 F. Wei, Z. Deng, S. Sun, F. Xie, G. Kieslich, D. M. Evans, M. A. Carpenter, P. D. Bristowe and A. K. Cheetham, Mater. Horiz., 2016, 3, 328–332.

6 Z. Deng, F. Wei, S. Sun, G. Kieslich, A. K. Cheetham and P. D. Bristowe, 2016, J. Mater. Chem. A, 2016, 4, 12025–12029.

Perovskite solar cells are the subject of increasing attention, with a large increase in their power conversion efficiency (PCE) from 3.81% to 22.1% in just 6 years. However, they still suffer from stability issues, degrading when exposed to moisture, UV light, heat, and voltage. In terms of UV light, many studies have been carried out under the full solar spectrum in air (i.e., in the presence of moisture and oxygen), thus making it difficult to determine the specific effects of UV light alone.
We herein examined the degradation of perovskite solar cells in the presence of UV light alone, excluding moisture, oxygen, and other wavelengths of light. Perovskite solar cells were exposed to 365 nm UV light (7 mW/cm² intensity, at open circuit condition, ~25 °C) over 1,000 h under argon atmosphere at <0.5 ppm humidity without encapsulation. The long term UV light stability and degradation of perovskite solar cells were examined by UV-visible spectroscopy (absorbance and transmittance), X-ray diffraction (XRD), and light current-voltage (LI-V), external quantum efficiency (EQE), μ-photoluminescence spectroscopy (μ-PLS), and μ-light beam induced current (μ-LBIC) measurements.
Interestingly. 1-sun light illumination after UV degradation makes solar cell parameters recovery, mainly Fill factor (FF) and PCE. Over 60% of the initial PCE degradation recovery was observed for the 540 h UV exposed device. This degradation/recovery phenomenon of perovskite solar cells also were observed in other devices in this experiment over a range of UV degradation times. Also during consecutive UV exposure, decreased J_sc and QE were bounced back. 1-sun light soaking induced recovery considered to be caused by the resolving of stacked charges and neutralizing of meta-stable trap states by photo-generated carrieres. J_sc and QE bounce back phenomenon attributed to beneficial effects of the degradation by-product PbI₂

Organic–inorganic perovskite solar cells have recently emerged as high-performance solar cells and moved to the forefront of photovoltaics research. Here we compare electrical, chemical and optical properties of CH₃NH₃PbI₃ and CH₃NH₃PbI_3-xCl_x in inverted structure perovskite solar cells. The device structure embeds the absorber perovskite layer consisting of CH₃NH₃PbI₃ or CH₃NH₃PbI_3-xCl_x between respective hole and electron transport layers of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) and Fullerene (C₆₀). Apart from the ITO and Ag electrode deposition, the solar cell process is fully solution-based. To optimize the perovskite thin film morphology, a solvent engineering approach was used. For this purpose, a mix of gamma butyrolactone (GBL) and dimethylsulphoxide (DMSO) was used to dissolve the perovskite precursor, followed by a toluene drop-casting step during the thin film deposition. The later step facilitates the formation of a highly smooth and closed perovskite layer, which is required for preventing leakage currents. The presented results provide essential information regarding the role of the precursor solution in the solar cell optimization process.

Perovskite solar cells, with the latest record of 22.1% efficiency¹, are a promising candidate for next-generation photovoltaic technology, owing to their high efficiency and low fabrication cost. Despite the high efficiency achieved over a short period of time, the poor stability of perovskite cells in ambient moisture is still a problem. Few studies have focused on improving the stability by encapsulating the cell or incorporating an inert/hydrophobic material into or above the perovskite layer^2,3. Most of the previous studies are performed in an inert atmosphere, which restricts the large-scale production and application of perovskite solar cells.
In this work, an aerosol technique, electro-hydrodynamic atomization (electrospray) is used to deposit the perovskite layer, with the goal to enhance the stability of the cells without any additives. Electrospray is an established technique for generating aerosols by liquid atomization using an electric field, which has been used for thin film fabrication previously⁴. The cell design is FTO/dense-TiO₂/mesoporous-TiO₂/perovskite (CH₃NH₃PbI₃)/spiro-MeOTAD/Au. A two-step deposition technique is used to coat the perovskite layer, where PbI₂ is spin coated and CH₃NH₃I (MAI) is electrosprayed on the PbI₂ layer. Compared to the spin coating technique, the electrospray deposition results in solid, charged MAI particles, gradually supplied into the grounded PbI₂ layer. The entire cell fabrication and testing is performed in ambient condition (30-50% relative humidity).
The cells are tested under AM1.5 solar simulator and an efficiency of 11.35% has been achieved. Electrospray provides a uniform and smooth film compared to the spin coating technique, resulting in high cell efficiency. The effect of various electrospray parameters such as, substrate-to-nozzle distance, MAI concentration and flow rate (droplet size), on the performance of the solar cells is studied. The results showed that there is an optimum value for each of these parameters to achieve high efficiency.
The perovskite solar cells fabricated by electrospray are more stable than cells fabricated by traditional spin coating method, without any additive, retaining an efficiency of >80% of the initial efficiency after 1000 hours and >60% after 1500 hours. The reasons for the higher stability of the cells are: 1) smoother film produced by electrospray is resistant to moisture which is confirmed by the contact angle measurement (66.09° for electrospray deposited sample and 33.12° for spin coated sample) and 2) higher interaction between the precursors (MAI and PbI₂) due to the increased force of ballistic transport of MAI into PbI_2, which is observed by the reversible perovskite phase formation after an intentional moisture exposure, characterized by X-ray diffraction.

1. NREL Best Research-cell Efficiency Chart, 2016
2. Li et al., Nature Chemistry, 2015, 7, 703-711
3. Barbara et al., submitted
4. Kavadiya et al., Nanoscale, 2016, 8, 1868-1872

Lead halide perovskite solar cells have recently attracted huge attention because of their dramatic rise in power conversion efficiency (PCE) within few years. Their PCE could be potentially boosted over than 30 % when combined with Si or CIGS solar cells as tandem structure. In order to apply perovskite solar cells to the tandem device, the structure compatibility requires perovskite solar cells usually be an inverted structure (p-i-n type) as well as enhanced thermal and photo stability of the cell. Charge transport layers are also should have high transmittance and efficient charge transporting properties. Here, we report highly efficient perovskite solar cells having a long-term stability that adapts dense and transparent inorganic charge transport layer (NiO and Ti-S compounds) uniformly grown by atomic layer deposition (ALD) at relatively lower temperature. ALD-grown NiO has extremely low optical loss (>90 % of visible transmittance) and efficient hole transport property, which exhibited a highest PCE of 16.40 % with high open circuit voltage of 1.04 V and fill factor of 0.72 with a negligible current-voltage hysteresis in ITO/NiO/perovskite/PCBM/Ag. Furthermore, highly dense and transparent inorganic electron transport layer (TiO_x and Ti-S compounds) has been directly deposited onto perovskite using non-aqueous ALD process at room temperature, which ensures no degradation of perovskite during the process. These inorganic layers demonstrated efficient electron transporting as well as highly passivating performance. Finally, perovskite solar cells that using inorganic charge transport layers sandwiched by TCO electrodes (ITO/NiO/perovskite/TiO_xS_y/ITO) were fabricated and their performance as well as long-term stability under light soaking were characterized in order to apply to monolithic tandem device with Si or CIGS solar cells.

Solar cells based on organic-inorganic perovskite materials have dramatically developed in just a short period of time because of efficient light absorption and their unique semiconducting nature. The best power conversion efficiency is continuously recorded via various approaches; devices from sensitized mesoporous structures to planar structures, modified composition of perovskite, and fabrication methods. Despite the amazing improvement, many groups have encountered low reproducibility and lower device efficiency than reported results because of the perovskite’s own instability and sensitivity under ambient conditions; thus, we need a realistic and experimental investigation. In this study, we compare solution processing representative fabrication methods (1-step: additive (CHP), anti-solvent dripping (CBdrp), 2-step: interdiffusion (IFF), dipping (DPP)), which influence the growth, crystal orientation, and the electronic properties of CH₃NH₃PbI₃, device reproducibility, and efficiency in planar perovskite solar. This comparative study gives a realistic overview of the fabrication methods for planar perovskite solar cells.

Organohalide perovskite is promising for applications in photodetectors due to its high charge mobility, large absorption coefficient, and solution-based processes which are beneficial for quick response time, high responsivity and ease of fabrication. In this work, we focused on developing fast response and high detectivity perovskite-based photodiodes by incorporating a periodic nano-structured PEDOT:PSS hole transporting layer (HTL). The periodic nano-structures in the PEDOT:PSS HTL were created by nano-imprinting method. The improved contact area between the periodic nano-structured HTL and perovskite layer favors the light absorption and charge extraction, resulting in an improvement in the responsivity of the photodiodes. Our results reveal that the perovskite photodiodes with periodic nano-structured PEDOT:PSS HTL possess a fast response time, a very promising detector technology for high frequency light detection.

Organic–inorganic hybrid perovskite solar cells are fabricated using a water-soluble, self-doped conducting polyaniline graft copolymer based on poly(4-styrenesulfonate)-g-polyaniline (PSS-g-PANI) as an efficient hole-extraction layer (HEL) because of its advantages, including low-temperature solution processability, high transmittance, and a low energy barrier with perovskite photoactive layers. Compared with conventional poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) dispersed in water solution, PSS-g-PANI molecules are dissolved in water because of the polymeric dopant covalently bonded with PANI, and can steadily remain as an initial solution during long-term storage and over a wide pH range to fabricate a HEL with fewer surface defects. The built-in potential and device characteristics are substantially improved because of the surface energy state of PSS-g-PANI below Fermi-energy level. Moreover, the PSS-g-PANI mixed with electron-withdrawing perfluorinated ionomer (PFI) exhibits a higher work function (5.49 eV) and deeper surface energy state below the Fermi level; thus, an ohmic contact at the HEL/methylammonium lead iodide perovskite interface is obtained. Finally, the power conversion efficiency was increased from 7.8% in the perovskite solar cells with PEDOT:PSS to 12.4% in those with the PSS-g-PANI:PFI.

Perovskite-halide semiconductors are an emerging class of solution-processed materials that have exhibited very high performance in solar cells and have demonstrated good potential for light-emitting devices and photodetectors.^[1,2,3]
Here we focus on a fully inorganic perovskite, CsPbBr₃, which exhibits a wide band-gap of around 2.33 eV. Wide band-gap perovskite-halides that can achieve high open-circuit voltages are of particular interest to multijunction solar cells, visible light-emitting devices, and solar water splitting applications. However, due to the relative insolubility of the CsBr precursor, it is difficult to achieve a high level control over the thin-film morphology and optoelectronic properties using conventional approaches. Thus, thin-film devices based on CsPbBr₃ have remained relatively unexplored.
In this work, we study optoelectonic devices based on CsPbBr₃ in which the photoactive layer has been fabricated following different approaches: i) conventional crystallization of the thin film on the substrate starting from precursor solutions^[4] ii) solution-processing of a colloidal dispersion of pre-synthesized, high-quality nanocrystals. The latter approach allows the formation of thin-films with lower defect densities. The passivating nature of the nanocrystal’s ligands gives rise to extremely good photoluminescence properties (quantum yield > 85%), without significantly hindering charge-carrier conduction. Thanks to this formulation, it is possible to obtain solar cells with an open circuit voltage higher than 1.5 V, with power conversion efficiencies exceeding 5%. This is the first example of a ink-based devices, and its efficiency is among the highest achieved for wide band-gap perovskite-halides.
This strategy enables to highlight the role of structural and chemical defects, from point defects to interfaces, on the primary properties of the same material by combining structural and morphological characterizations, to the photophysical ones. Finally, the possibility to chemically tune the composition of the nanocrystals, that has already been demonstrated,^[5] represent a promise for future developments.

[1] Research Cell Efficiency Records http://www.nrel.gov/ncpv (April 2016).
[2] Sutherland, B. R. and Sargent, E. H. (2016). Perovskite photonic sources. Nature Photonics, 10, 391–402.
[3] Dou, Letian, et al. (2014). Solution-processed hybrid perovskite photodetectors with high detectivity, Nature communications 5.
[4] Kulbak M. et al. (2015). Cesium Enhances Long-Term Stability of Lead Bromide Perovskite-Based Solar Cells, J. Phys. Chem. Lett., 7, 167−172.
[5] Akkerman, Quinten A., et al (2015). Tuning the optical properties of cesium lead halide perovskite nanocrystals by anion exchange reactions, Journal of the American Chemical Society 137.32, 10276-10281.

Here, we report a novel way to form highly luminescent nanocrystalline CH₃NH₃PbBr₃ (MAPbBr₃) in CH₃NH₃Br (MABr) scaffold using adduct approach. Highly uniform MAPbBr₃ film was formed by nonstoichiometric precursor solution. PL intensity was enhanced by more than one order of magnitude as scaffold concentration was increased. The enhancement of PL intensity was found to be due to the pronounced polycrystallinity of MAPbBr₃, and the reduced excitonic feature in absorption spectra. Scanning electron microscopic image with elemental distribution mapping revealed that nonstoichiometric precursor regulated nanocrystalline size of MAPbBr₃. Furthermore, transmission electron microscopic images showed that nanometer scale spherical MAPbBr₃ probably confined the photo-generated electron and hole pair to accelerate the geminated recombination as confirmed from time-resolved PL measurement. Finally, highly efficient green light emitting diode was fabricated using the self-formed nanocrystalline MAPbBr₃ film.

Organic-inorganic hybrid perovskite solar cells have recently reached power conversion efficiencies (PCE) of more than 21%, yet only when a mesoporous structure was used, which involves a high temperature (≥ 500 °C) annealing step during device fabrication and is thus impractical for large scale applications. Instead, planar perovskite solar cells (PSCs) can be prepared at lower temperatures, but their efficiency still lacks behind that of mesoporous devices. Hence, improving the performance of PSCs is one of the key objectives for their application. In this study we investigated the effect of different glycol ether additives, namely 2-methoxyethanol, 2-ethoxyethanol and 2-propoxyethanol, on device performance using a planar ITO/ZnO/perovskite/spiro-OMeTAD/MoO_X+Ag device structure. The photoactive methylammonium lead iodide (CH₃NH₃PbI₃) absorber layer was prepared by a conventional two-step interdiffusion method, modified by addition of a small amount of a glycol ether to the methylammonium iodide (MAI) solution in the second step of the preparation protocol. An enhancement in lead iodide (PbI₂) to CH₃NH₃PbI₃ conversion was observed when the glycol ether/MAI solution mixture was used instead of an MAI solution. We found that adding glycol ethers to the precursor solution affected the morphology of the perovskite layers resulting in enlarged grain sizes of up to 1 µm. This in turn led to a considerable enhancement of the photovoltaic performance from 14.9% to 17.3% after addition of 2-methoxyethanol, largely due to an improvement in fill factor (FF) from 62% to 68%, while maintaining the already high short circuit current (J_SC). This enhancement of FF implies a more efficient conversion of PbI₂ to CH₃NH₃PbI₃ and the formation of a uniform and continuous perovskite film. Finally, to understand the effect of glycol ethers on the optical properties and charge carrier dynamics in CH₃NH₃PbI₃ PSC, ultrafast transient absorption (TA), time-resolved photoluminescence (TR-PL), and photo-thermal deflection spectroscopy (PDS) were performed.

Organohalide lead-perovskites have revolutionized the hybrid/organic photovoltaics landscape. Despite the fast efficiency increase, some of the materials properties related to their extraordinary photovoltaic performance remain largely not understood. Further advances in the perovskite solar cells (PSCs) field may be boosted by computational design and screening of new materials, with researchers examining material characteristics that can improve device performance and/or stability. Suitable modeling strategies may allow researchers to observe the otherwise inaccessible but crucial hetero-interfaces that control the operation of PSCs, allowing researchers the opportunity to develop new and more efficient materials and to further optimize the solar cells photophysics.
We illustrate the results of ab initio molecular dynamics simulations coupled to first principles electronic structure calculations on the effect of light absorption on the electronic and dynamical properties of organohalide lead perovskites. The role of the organic cation dynamics and of ion/defect migration is analyzed in relation to photoinduced structural transformations and solar cell operation. It is found that Frenkel defects, relatively abundant in MAPbI3 and related perovskites, undergo a light-induced dynamical transformation which may account for the observed enhanced photoluminescence quantum yield following sample irradiation. We also show how the fluctuations of the organic cations may locally (in space and time) impart the otherwise centrosymmetric perovskite lattice with spin-orbit coupling dependent properties which are typical of ferroelectric crystals characterized by lack of inversion symmetry. A possible polaronic mechanism, triggered by a photoinduced structural deformation, is finally presented which may be responsible for the reduced electron/hole recombination observed in organohalide perovskites.
References:
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The most extensively studied perovskite for photovoltaic applications has been CH₃NH₃PbI₃ (or its analogous but using chlorine precursor: CH₃NH₃PbI_3-xCl_x) as absorber material, in combination with electron (TiO₂) and hole (spiro-OMeTAD) selective contacts. The J-V curves of perovskite solar cells have been found to present a hysteresis-like distortion. Hysteresis has raised many concerns about the feasibility and long term stability. Recent experimental work shows a connection between capacitive current and hysteresis behavior in hybrid lead halide perovskites.¹ The microscopic phenomena responsible for capacitive currents in the dark is the ionic electrode polarization, similar to double layer capacitive effects, while the perovskite absorber layer behaves as a mixed conductor able to interact with the contacting transport layers. Here we show how contact phenomena distort the current-voltage curve by using different cell structures. We identify the different types of interactions of the standard electrode contacts. They display qualitatively diverse sources of reactivity at the interface between MAPbI₃ and the transporting layers. At TiO₂ contact, reversible capacitive currents are observed. Irreversible ionic reaction occurs between mobile ions in MAPbI₃ and spiro-OMeTAD organic hole extracting layer. Only the latter irreversible behavior may cause significant long term aging by reduction of spiro-OMeTAD conductivity, and it is therefore a key point for engineering of the solar cell towards long time robust operation.² We distinguish thus between capacitive and noncapacitive currents giving rise to specific hysteretic responses in the J-V curves. It is reported that capacitive current causing hysteresis dominate in regular structures with TiO₂ as bottom electron selective layer. This is mainly caused by the charge, both ionic and electronic, accumulation ability of the TiO₂/perovskite interface, but has no influence on the steady-state operation. Noncapacitive hysteresis is observable at slow enough scan rates in all kind of architectures. Inverted structures, including organic compounds as bottom hole selective layers and fullerene materials as top contact, exhibit larger noncapacitive distortions because of the inherent reactivity of contact materials and absorber perovskites.³

1. Almora, O.; Zarazua, I.; Mas-Marza, E.; Mora-Sero, I.; Bisquert, J.; Garcia-Belmonte, G., Capacitive Dark Currents, Hysteresis, and Electrode Polarization in Lead Halide Perovskite Solar Cells. J. Phys. Chem. Lett. 2015, 6, 1645−1652.

2. Carrillo, J.; Guerrero, G.; Rahimnejad, S.; Almora, O.; Zarazua, I.; Mas-Marza, E.; Bisquert, J.; Garcia-Belmonte, G., Ionic Reactivity at Contacts and Aging of Methylammonium Lead Triiodide Perovskite Solar Cells. Adv. Energy Mater. 2016, 6, 1502246.

3. Almora, O.; Aranda, C.; Zarazua, I.; Guerrero, G.; Garcia-Belmonte, G., Noncapacitive Hysteresis in Perovskite Solar Cells at Room Temperature. ACS Energy Lett. 2016, 1, 209−215

Perovskite solar cells have shown a tremendous rise in power conversion efficiency with record efficiencies report of over 20%. This makes this new technology very promising as low cost alternatives to conventional inorganic solar cells. However, currently due to a ‘hysteretic’ behaviour when recording a current-density – voltage curve for device, which strongly depends on the scan rate, device and measurement history, preparation method, device architecture, etc. the standard measurement protocol used for solar cells does not give reproducible results. However, for commercialization aspect and also for the possibility to compare devices measured in different laboratories, it is necessary to have a measurement method which gives the same results. Here we describe a reliable measurement protocol based on power point tracking. By comparing perovskite solar cells with different architectures, that behave very differently, we demonstrate the effects of "measurement tricks" and show how our protocol gives for all tested cells reliable measurements.

Perovskite solar cells [1] are emerging as a leading technology in the landscape of thin film solar cells. They have numerous advantages compared to other technologies as they have an excellent light absorption, low recombination rate and high open circuit voltage. Moreover, the possibility to grow the film of perovskite using different processes, from evaporation to solution process, opens interesting perspectives on the possible application of this technology.
Despite this great success many questions remain to be fully addressed. One of the most important, concerning the physics of the device, is the presence of an hysteresis in the J-V characteristics. Several physical mechanisms to explain that have been proposed: giant permittivity [2], trapping/detrapping at interfaces [3], ion migration [4] and ferroelectric effects [5]. However, no conclusive solution has been reached so far.
In order to investigate this problem we have developed a simulation tool [6], based on drift-diffusion, that allows to compute the complete J-V characteristics of the device including the real 3D mesoporous morphology. This will cast light on the effects of interface and mesoporous morphology on the overall performance of the device.

[1] H. S. Jung et al., Small, 2015, 11, 10–25.
[2] R. Sanchez et al., J. Phys. Chem. Lett., 2014, 5, 2357-2363.
[3] G. Xing et al., Small, 2015, 11, 3606–3613.
[4] H. Sanith et al., J. Phys. Chem. Lett., 2014, 5, 1511-1515.
[5] B. Chen et al., Nano Energy, 2015, 13, 582-591.
[6] A. Gagliardi et al., Nanoscale, 2015, 7, 1136-1144.

Starting at 2011, organic-inorganic halide perovskites have emerged as one of the most promising competitors to silicon-based PVs. This is enabled by the unique potential of this family of materials to satisfy the stringent requirements of coupling low energy and low-cost production methods utilizing earth-abundant raw materials with high power conversion efficiency (>21%). Despite a significant research effort which is evident in the constant increase of cell efficiency, the operation mechanism of this PV family is not yet resolved. Aiming at insight to perovskite solar cells mechanisms we performed photoconductivity, Raman, photoluminescence, photovoltage dependence on illumination intensity and open-circuit voltage decay (OCVD) measurements on both free standing perovskite films and full solar cells. All measurements show significant effect of light soaking and return to the original characteristics after dark treatment. In all cases the changes occur over many minutes with slower return (hours) in the dark. The extremely slow changes observed in all measurements cannot be regarded as electronic processes which are much faster but rather as photo-induced structural changes both in the bulk of the perovskite film and at its interface with the electron selective contact. A comprehensive approach to the interpretation of the five measuring techniques, three different perovskite materials and both free standing films and operating cells, provide new insight to the operation mechanism of perovskite solar cells.

Solar cells using organic-inorganic light absorbing materials based on methylammonium lead halides perovskite (MAPbX₃, X= Cl, Br, I) has demonstrated as the most promising candidate for next generation cost-effect PV technologies. A typical perovskite solar cell (PSC) consists of n-type semiconductor such as TiO₂ for transporting electron, MAPbX3 based light absorbing material and a p-type semiconductor for transporting hole. To date, organic semiconductors like Spiro-OMeTAD or P3HT are the most widely used p-type material in PSC. Nevertheless, the issues of high cost involved in synthesis of high purity organic semiconductor and the lack of stability of these organic materials in air may restrict their application in practice. Therefore, it is important and necessary to develop new stable and sustainable p-type semiconductor material or material composites for PSC.
we have found that carbon materials including carbon nanotubes and graphene exert significant influence on the charge transport, stability and performance of PSC. The results based on theoretical calculation show that the charge transfer at the interface between graphene and MAPbI₃ perovskite compound is very efficient. Hole from MAPbI3 can easily transfer to graphene, leading to well separated electron-hole pair which contributes to reduced recombination at the interface. Similar charge transport process is also expected at the interface between MAPbI₃ and other carbon material such as carbon nanotubes. Our experimental results have shown that the effective electron lifetime in the perovskite solar cell was increased nearly one order of magnitude when carbon nanotubes was employed as additives in P3HT based hole transport material for PSC.² As a result, the device performance was improved over two-fold.

We investigated the origin of hysteresis in I-V curves of a planar perovskite cell using different equivalent circuit models. A planar cells showing huge hysteresis with PCE 18.0% on reverse scan and 8.8% on forward scan was used for validating the equivalent circuits. We found that the conventional equivalent circuit with a diode, a series resistance and a shunt resistance does not reproduce the hysteresis of I-V curves. Even for the incorporation of a capacitive component in the circuit, the model also did not match the experimental I-V curves. However, an equivalent circuit model composed of two series connected diodes, two capacitors, two shunt resistances and a series resistance clearly reproduced the hysteretic I-V curves. For this model, fitting parameters (Rs, Rsh1, Rsh2,) were chosen by using calculated series and shunt resistance values from experimental I-V data. Initial C1 and C2 were randomly selected. Then parameters were optimized (by trial and error method) to minimize deviation between simulated and experimental curves. According to this equivalent circuit model, the computationally simulated curves matched closely with the experimental one. This suggests that perovskite cell has two active interfaces; TiO₂/CH₃NH₃PbI₃ and CH₃NH₃PbI₃/spiro-OMeTAD. Hysteresis is essentially caused by carrier accumulation at these active interfaces. The electrical capacitances generated by defects due to the lattice mismatch at the TiO₂/CH₃NH₃PbI₃ and CH₃NH₃PbI₃/spiro-OMeTAD interface are truly responsible for the hysteresis in the perovskite solar cells.

Based on above experience and knowledge, we also examined to evaluate the cell performance at low light intensity condition. Very surprisingly, due to the charge / discharge property with internal capacitance, we found the limitation to define the cell performance from the I-V curve because of the fake current. To solve this issue, we newly propose the Maximum Power Point Tracking (MPPT) technique to define the most accurate cell performance of the hysteric device.

References
[1] L. Cojocaru, S. Uchida, Y. Sanehira, J. Nakazaki, T. Kubo and H. Segawa, Chemistry Letters, (2015), 44, 5, 674.
[2] L. Cojocaru, S. Uchida, Y. Sanehira, V. Gonzales-Pedro, J. Bisquert, J. Nakazaki, T. Kubo and H. Segawa, Chemistry Letters, (2015), 44, 11, 1557.
[3] L. Cojocaru, S. Uchida, A. K. Jena, T. Miyasaka, J. Nakazaki, T. Kubo and H. Segawa, Chemistry Letters, (2015), 44, 8, 1089.
[4] L. Cojocaru, S. Uchida, V. V. J. Piyankarage, S. Kaneko, J. Nakazaki, T. Kubo and H. Segawa, Chemistry Letters, (2015), 44, 12, 1750.
[5] L. Cojocaru, S. Uchida, D. Matsubara, H. Matsumoto, K. Ito, Y. Otsu P. Chapon, J. Nakazaki, T. Kubo and H. Segawa, Chemistry Letters, (2016) in press.

Hybrid halide perovskites have rich solid state physics. A unique characteristic is their soft nature, with response processes over timescales on many orders of magnitude. The key question to understand is how a solution processed (and thus inevitably defective) can have such long recombination times, and thereby long minority charge carrier diffusion lengths and high photovoltaic performance.

In this work we present a multi-scale approach to understanding this problem. We combine solid state models for Frohlich polaron location, with quantitative lattice dynamic calculations. The multi time scales of response[1] requires an extension of standard solid-state models, developed for more simple tetrahedral semiconductors.

We propose that the uniquely low energy optical modes[2], and soft zone boundary acoustic modes are responsible for carrier scattering and the modest mobility. We build a model for the formation of the polaron, and its migration through the material, based on our prior monte-carlo simulation method of the disordered material[3]. We quantify the beneficial decrease in recombination rate due to segregation of electrons and holes in the 'ferroelectric highways', versus the detrimental decrease in mobility due to disorder. Our new model quantifies the contribution of short-range ferroelectric order on carrier stability and electron-hole recombination in this unique class of materials.

Reduced recombination can occur due to the spin-split indirect-gap. Local ferroelectric distortions[4] generating a crystal field interacts with the high spin-orbit coupling of the lead and iodide atoms. We have directly calculated the reduction in recombination due to this band-structure effect[5].

We implement mode-following to recover a potential energy landscape from our lattice dynamic calculations[6], inputting this to a numeric quantum oscillator solution, which is then used within the frozen-phonon approximation to calculate the electron-phonon coupling for the phonon modes.

This work has benefited from funding by the EPSRC and close collaboration with the groups of Mark van Schilfgaarde (King's College London), Piers Barnes (Imperial College London), and Simon Billinge (Columbia, New York).

[1] J.M. Frost, A Walsh, Acc. Chem. Res., 2016, 49 (3), pp 528–535 (2016).
[2] P Azarhoosh, et al., ArXiv: 1604.04500 (2016)
[3] J. M. Frost, K. T. Butler and A. Walsh, APL Mater. 2, 081506 (2014).
[4] J.M. Frost et al, Nano letters 14 (5), 2584-2590 (2014).
[5] A.M.A. Leguy et al, ArXiv: 1606.01841 (2016)
[6] F. Brivio, J. M. Frost et al., Phys. Rev. B 92, 144308 (2015).

Organo lead halide perovskites have emerged as one of the most promising materials in thin-film photovoltaics due to an extraordinary increase in performance in only few years. While power conversion efficiencies as high as 22% have already been reported, the fundamental working principles of solar cells made out of this material still remain to be completely understood. Here, we apply thermally stimulated current (TSC) analysis on solution processed methylammonium lead halide perovskite solar cells in order to identify electronic trap states and their possible effect on device performance. In this method the sample is cooled down to 10K initially, where possible trap states are subsequently filled via illumination. Afterwards, the sample is heated back up again to 300K at a constant rate, which causes trapped charge carriers to be thermally released from the semiconductor. Any weak current flow caused by this gradual release is precisely monitored and therefore facilitates conclusions about the energy distribution, depth and density of the corresponding trap sites. We recently successfully applied this method and identified energetically deep trap states in the methylammonium lead iodide (MAPI) perovskite layer itself [1]. By further studying perovskite solar cells in normal as well as inverted device configuration, we investigate the impact of various electron and hole transport materials on interfacial trap states and their distribution. This includes the widely-used electron selective layer TiO₂, where we report a substantial contribution to the energetic trap landscape. Additionally, we altered the crystal structure of the perovskite layer itself by changing the halide content, incorporating bromide as well as chloride to partly replace the iodide. Using scanning electron microscopy and X-ray diffraction, as complementary characterization techniques, we study the effects of this alteration of the chemical composition as well as the morphology of the perovskite on the electrically active trap state distribution.

References
[1] A. Baumann, S. Väth, P. Rieder, M.C. Heiber, K. Tvingstedt and V. Dyakonov, J. Phys. Chem. Lett. (2015), 6 (12), pp 2350-2354.

Surfaces and grain boundaries play key roles in determining the performance of polycrystalline solar cells. In this talk, I will present the recent progress in understanding the influence of surface and grain boundaries on ion migration and grain stability in perovskite based solar cells. The ion migration have been speculated to be the origin or an important contributing factor for many observed unusual phenomenon in OTP materials and devices, such as current hysteresis, switchable photovoltaic effect, giant dielectric constant, diminished transistor behavior at room temperature, photoinduced phase separation, photoinduced self-poling effect, self-healing effect, and electrical-field driven reversible conversion between lead iodide (PbI₂) and methylammonium lead triiodide (MAPbI₃). Thorough insight into the ion-migration mechanism is highly desired for the development of perovskite based devices to improve intrinsic stability in the dark and under illumination. I will also present the progress in suppress surface and grain boundaries related ion migration and grain degradation which result in enhanced the efficiency and stability of perovskite solar cells.

Rather than what was and is often stated, halide perovskites resemble, with notable exception of their elasto-mechanics and defect chemistry & physics, known inorganic semiconductors and care should be taken in comparing them with organic semiconductors in general, and their PV or LED behaviour with concepts from OPV, OLEDs and dye-sensitized solar cells. Still, there are the above-stated exceptions that allow excellent quality optoelectronic material to be made fast, at low temperatures, from solution, which is amazing for electronic materials research. I will discuss progress in understanding this behavior by scrutinizing apparent inconsistencies in the experimental data on these materials, using published data, as well new ones from our (collaborative) research.

* Work done with GARY HODES, T. M. Brenner, D. Egger, S. Gupta, H. Kaslasi, N. Kedem, M. Kulbak, I. Levine, Y. Rakita, A. Zohar; WIS collaborations with D. Ehre, E. Meirzadeh, M. Petoukhova and I. Lubomirski; O. Bar-Eli and D. Oron, S. Cohen; D. Egger and L. Kronik; other collaborations are with the Kahn (Princeton U), Snaith (Oxford UK), Bolink (Valencia), Seok (UNIST), Balberg-Milo (HUJ) and other groups.

Support: Weizmann Institute of Science via its Alternative sustainable Energy Research Initiative (AERI); Israel Ministry of Science, Israel National Nano-initiative

Organic-inorganic semiconductors with perovskite crystal structure have perturbed the landscape of contemporary photovoltaics research, due to their relatively low cost and record high light-to-electricity conversion efficiency. While a vast amount of theoretical and experimental investigations have been dedicated to the structural stability, electrical, and optical properties of hybrid halide perovskite materials in relation to their applications in solar cells, the thermal transport property, another critical parameter to the design and optimization of relevant solar cell modules, receives less attention. In this paper, we evaluate the lattice thermal conductivity of a representative methylammonium lead triiodide perovskite (CH₃NH₃PbI₃) with direct non-equilibrium ab initio molecular dynamics simulation (NEAIMD). We illustrate the details of the mysterious vibration of the methylammonium cluster (CH₃NH₃⁺) and present an unambiguous picture of how the organic cluster interacting with the inorganic cage and how the collective motions of the organic cluster drags the thermal transport, which provide fundamental understanding of the ultra-low thermal conductivity of CH₃NH₃PbI₃. We also reveal the strongly localized phonons associated with the internal motions of the CH₃NH₃⁺ cluster, which contribute little to the total thermal conductivity. Our study highlights the physical origin of the ultralow thermal conductivity of the hybrid organic-inorganic halide perovskite structures, and also provides new insights into the phonon transport from the view of organic-inorganic coupling (rotational vs. translational), which would be of great significance to the design and discovery of novel perovskite materials for better energy conversion performance.

Metal-halide perovskites are promising materials for opto-electronic applications. Their mechanical and electronic properties are directly connected to the nature of their lattice vibrations. Whereas the mid infrared (IR) range contains mainly information on the internal vibrations of the methylammonium cation,^1–3 the lead-halide lattice vibrations are located in the far-IR. Herein we report far-IR spectroscopy measurements of CH₃NH₃Pb(I/Br/Cl)₃ thin films and single crystals at room temperature and a detailed quantitative analysis of the spectra. We find strong broadening and anharmonicity of the lattice vibrations for all three halide perovskites, which indicates dynamic disorder of the lead-halide cage at room temperature. We determine for the first time the frequencies of both the transversal and longitudinal optical phonons, and use them to calculate the static dielectric constants, polaron masses, and upper limits for the phonon-scattering limited charge carrier mobilities. Our findings have important implications for our basic understanding of charge transport processes in metal halide perovskites.

(1) Glaser, T.; Müller, C.; Sendner, M.; Krekeler, C.; Semonin, O. E.; Hull, T. D.; Yaffe, O.; Owen, J. S.; Kowalsky, W.; Pucci, A.; Lovrinčić, R. J. Phys. Chem. Lett. 2015, 6 (15), 2913–2918.
(2) Bakulin, A. A.; Selig, O.; Bakker, H. J.; Rezus, Y. L. A.; Müller, C.; Glaser, T.; Lovrincic, R.; Sun, Z.; Chen, Z.; Walsh, A.; Frost, J. M.; Jansen, T. L. C. J. Phys. Chem. Lett. 2015, 6 (18), 3663–3669.
(3) Müller, C.; Glaser, T.; Plogmeyer, M.; Sendner, M.; Döring, S.; Bakulin, A. A.; Brzuska, C.; Scheer, R.; Pshenichnikov, M. S.; Kowalsky, W.; Pucci, A.; Lovrinčić, R. Chem. Mater. 2015, 27 (22), 7835–7841.

The demand for understanding the photophysical processes in perovskite solar cells has become more evident in view of the rapid rise in device efficiency. The lifetime and mobility of light induced-carriers in photoactive layers are about the most important parameters governing the efficiency of a solar cell. However, there is limited knowledge about the relationship between e.g. preparation route, morphology and precursors and these photo-physical properties. Information on these parameters is obtained by complementary microwave photo-conductance (TRMC) and photo luminescence (PL) measurements. We developed a kinetic model to describe both TRMC and PL experiments with one set of global kinetic parameters. In this kinetic model a short laser pulse leads to photoexcitation of electrons to the conduction band, from where they can recombine with holes in the VB. In competition with second order recombination, electrons can be immobilized in intra-band gap trap states. Finally, in case of a non-intrinsic semiconductor, there are additional holes on top of the photo-generated holes. Solving the coupled differential equations yield the time dependent concentrations of holes (n_p), electrons (n_e) and trapped electrons (n_t) from which the TRMC and PL traces can be calculated.
We could successfully describe the dynamics of different systems including planar and mesoprous layers of MAPI¹ and of MAPI single crystals² using this kinetic model. From the found rate constants and mobilities the corresponding diffusion lengths for minority and majority carriers were deduced. In addition a relation could be made between the synthetic route and number of trap states. Interesting findings include: i) The trap density in polycrystalline films is three orders of magnitude higher than in single crystals. ii) The minority carrier diffusion length in MAPI perovskites could exceed 10 μm if it is not limited by the crystal domain size. More recently we extended these studies by introducing a specific electron or hole transport layer allowing us to derive the charge injection rates.³ In short, our methodology gives apart from fundamental insight into the dynamics in perovskites a versatile way to optimize the electronic properties of photoactive layers and their charge transport layers.

References
¹Hutter, E. M.; et al JPCL 2015, 6, 3082-3090. ²Bi, Y.; et al JPCL 2016, 923-928. ³Ponseca, C. S.; et al J. Am. Chem. Soc. 2015, 137, 16043-16048.

Organic-inorganic hybrid perovskites, such as the prototypical methylammonium lead iodide (MAPbI₃), have been recognized as a promising solar photovoltaic, light-emitting diode, and thermoelectric material. Energy-related functionality and performance of MAPbI₃ highly depend on its thermal transport behavior. Using equilibrium molecular dynamics simulations, we discovered that the thermal conductivities of MAPbI₃ under all three phases (cubic, tetragonal, and orthorhombic) are less than 1 W/m/K, and is as low as 0.31 W/m/K at the room temperature. Such ultralow thermal conductivity can be attributed to the small phonon group velocities due to their low elastic stiffness as reflected in our observed soft phonon modes, in addition to their short phonon lifetimes (< 100 ps) and mean-free-paths (< 10 nm) due to the enhanced phonon-phonon scattering from highly-overlapped phonon branches. The contribution from optical phonon branches in the low-frequency domain plays a considerable role in the total phonon transport. The anisotropy in thermal conductivity at lower temperatures is found to associate with preferential orientations of organic MA⁺ cations. Among all atomistic interactions, electrostatic interactions dominate thermal conductivities in ionic MAPbI₃ crystals. Furthermore, we qualitatively estimated thermal conductivities of general hybrid perovskites MABX₃ (B = Pb, Sn; X = I, Br), and found that Sn- or Br-based perovskites possess higher thermal conductivities than Pb- or I-based ones due to their much higher elastic stiffness. Finally, we provided atomistic insights into optimal selections and rational designs of the ionic components for hybrid perovskites with desired thermal conductivity for thermally-stable photovoltaic energy harvesting and highly-efficient thermoelectric energy conversion applications.

After years of exploration of the photophysics of methylammonium lead iodide perovskites, recent studies have pointed towards the critical role of dynamic structural disorder in the observed low defect scattering rates and long carrier lifetimes. Theoretical calculations indicate the importance of structural fluctuations in the charge generation and transport processes and recent experiments demonstrate dynamic reorientation of the organic cations upon photoexcitation. Here we present femtosecond electron scattering experiments which directly probe the first atomic-scale steps following absorption of a photon in methylammonium lead iodide perovskites. Following above-gap photo-excitation, we resolve the energy transfer from hot carriers to the lattice by following the increase in room mean square (RMS) atomic displacements, occurring on a time-scale of ~10 ps. The extracted RMS displacements (~ 0.1 Å) agree well with first-principle calculation. For resonant excitation at the band-edge lattice heating is negligible, consistent with minimal carrier-defect scattering. Analysis of the time-dependent change in diffraction pattern allows for extraction of a time-resolved pair distribution function and identification of the relative contribution of correlated atom pairs to the lattice distortion. We observe a giant broadening effect in the iodine-iodine correlation function with no resolvable change in Pb-I distance, indicative of a light-induced rotationally-disordered octahedral structure developing on picosecond time-scales. This dynamic structural response of the inorganic framework, itself coupled to the reorientation of methylammonium ions, may underlie the long carrier lifetime exhibited by the hybrid perovskites.

Perovskite solar cells are currently one of the most promising photovoltaic technologies for highly efficient and cost-effective energy production. In only several years, an unprecedented progression of preparation procedures and material compositions delivered a prototype technology that exploits most of the potential for perovskites as photovoltaic materials. However, there still remains a huge scope to demonstrate that perovskite solar cells are stable under working conditions
Migration of ions within the perovskite crystal lattice has been widely investigated to explain the “hysteresis” of current-voltage (J-V) characteristic of perovskite solar cells. The results of these studies indicated that, regardless of the particular device architecture and materials composition, halides (and their vacancies) migrate within the perovskite layer and accumulate at the interface with charge selective contacts. Depending on particular voltage and light bias conditioning, accumulation of ions (and their vacancies) reduces the charge collection efficiency. This mechanism has been suggested as the most likely cause of J-V “hysteresis”, but it may also have a significant impact on the long-term stability of devices under working conditions. Understanding the impact of ion migration on device long-term performance is of paramount importance because it will answer the question whether or not there is an intrinsic instability that may ultimately prevent from using perovskites for photovoltaics.
In this talk, I will demonstrate ion migration in perovskite solar cells working under different voltage bias conditions and I will discuss the impact of ion migration on the initial device power conversion efficiency and long-term stability. Thus, I will demonstrate perovskite solar cells stabilised for several hundred hours close to the initial efficiency.

Emerging photovoltaic solar cells based on organometallic trihalide perovskites (OTP) are among next-generation economically efficient thin film photovoltaic technologies. The superior optoelectronic properties of intriguing OTPs are ascribed to their unique electronic structure and the chemistry of lead (Pb) and halogens (e.g. I, Cl, Br). The perovskites display a number of interesting physical properties, such as ion migration. However, the role of ionic migration on the photovoltaic performance, or alternatively whether the ionic migration plays a positive or negative role in determining superior photovoltaic performance, is still unclear. Here, we investigate mixed halide perovskite containing both I, Cl using time-of-flight secondary ion mass spectrometry (ToF-SIMS) and band excitation contact kelvin probe force microscopy (BE-cKPFM) to understand the role of the Cl ion on the system. ToF-SIMS results show that the Cl- segregates to the grain boundaries while the cKPFM point to the increased ionic mobility at the grain boundaries. The cKPFM measurement shows that the ionic mobility in Cl- segregated grain boundaries is higher than that of pure CH3NH3PbI3 perovskites, leading us to speculate that the improved ionic mobility in the mixed halide perovskite containing both I, Cl is due to the presence of migrated Cl ions in the grain boundaries.
This work was conducted at the Center for Nanophase Materials Sciences, which is a Department of Energy (DOE) Office of Science User Facility

Inorganic and hybrid halide perovskites, important members of the perovskite family, have shown exceptional photovoltaic performance with efficiencies exceeding 20%. Instability of these materials is a major issue for real world applications including both chemical breakdown and current-voltage hysteresis. Most perovskites adopt cubic crystal structures (Pm-3m type) at high temperature, and undergo various phase transitions and adopt lower symmetry space groups at lower temperature due to structural instability.[1] This phenomenon is due to a combination of octahedral tilting, molecular rotation and cation disorder within the perovskite unit cell. Depending on the specific transition path, large changes in the electronic and optical properties can take place that can affect photovoltaic performance.
Here we present a comprehensive analysis of the vibrational (phonon) structure for CH₃NH₃PbI₃ and a series of 24 inorganic metal halides: AMX₃ (A = Cs, Rb; M = Ge, Sn, Pb; X = F, Cl, Br, I). Calculations have been performed using lattice dynamics with forces based on density functional theory building on recent work [2]. Starting with the cubic perovskite phases we identify a range of lattice instabilities that are correlated the chemical composition and size of the ions. We find that a number of crystal structures reported at room temperature exhibit dynamic instabilities, which suggests the existence of symmetry-broken local domains that give rise to an observable higher symmetry structure. The implications on the electronic structure, due to electron-phonon coupling, and photovoltaic performance, through carrier separation and transport, will be discussed.
[1] Howard, C. J.; Stokes, H. T. Acta Crystallogr. Sect. B 1998, 54, 782–789.
[2] Brivio, F.; Frost, J. M.; Skelton, J. M.; Jackson, A. J.; Weber, O. J.; Weller, M. T.; Goni, A. R.; Leguy, A. M. A.; Barnes, P. R. F.; Walsh, A. Phys. Rev. B 2015, 92, 144308.

In the course of the experimental developments in perovskite absorber layers, a plethora of strategies has arisen to tune them for the purpose of high efficiency devices. A number of deposition methods have been proposed, and much effort has been invested in altering the precursor composition to induce better film quality.[1] Despite the wealth of deposition approaches, the community experiences a great deal of irreproducibility from laboratory to laboratory and between different preparation methods.[2] Another remarkable point is that the viability of most –if not all– film enhancement strategies through deposition and precursor tweaking has been seminally established by demonstrating the successful operation of one particular type of perovskite solar cells (both in terms of composition and device architecture), but they typically require rigorous optimization when applied for different perovskite compositions and device architecture. These generally encountered observations in perovskite research illustrate the need for a universal and uncomplicated deposition strategy that is applicable for many compositions of perovskite, and insensitive to minor changes in deposition conditions. Aiming to contribute in this aspect, we developed a robust and expedient method for the solution deposition of hybrid perovskite thin films. We present a range of different perovskite compositions with single, double and triple cations, all delivering state-of-the-art efficiencies. With this, our method offers a reliable standard practice for the fabrication of a non-exhaustive variety of perovskites exhibiting excellent film morphology and corresponding high performance in both regular and inverted structured solar cells.

References
1. A. Sjarenko and M.F. Toney, J. Am. Chem. Soc. (2016), 138, 463-470.
2. J. Berry et al., Adv. Mater. (2015), 27, 5102-5112.

Organometal halide perovskites have received considerable attention in recent years, and substantial research efforts have been made to increase the overall power conversion efficiency and long term stability in various environments of the perovskite films. It is of paramount importance that these perovskite films are stable under light irradiation if they are intended to be used for consumer solar cell devices. In this study we have investigated the light degradation effects on in-situ co-evaporated CH₃NH₃PbI₃ thin films by using X-ray Photoelectron Spectroscopy (XPS) and Scanning electron microscopy (SEM). A blue laser (408 nm) was used to simulate five solar intensities to ensure a reasonable timed experiment. An illuminated and non-illuminated control position were monitored for core atomic level position and intensity changes. In as little as 120 minutes, substantial degradation became apparent for the illuminated position which only increased with further exposure. After the full exposure there were significant losses in iodine and nitrogen concentrations, and the production of new metallic lead in the surface of the illuminated position. SEM images further confirmed the enhanced degradation and decrease in film uniformity of the illuminated position. Substantially less degradation was seen in the non-illuminated position and were comparable to other unexposed in-situ co-evaporated samples. This work strongly indicates that light can cause or possibly accelerate the decomposition of CH₃NH₃PbI₃ perovskite thin films, and that much further work is needed before the perovskite solar cells can reach the market.

Methylammonium Lead Iodide (MALI) is one of the most actively investigated materials for use in photovoltaic devices. MALI is a promising material as it displays critical properties, notably a band gap that makes it usable as a photoabsorber, and a significant light extinction coefficient that makes the material ideal for the fabrication of solid state thin film solar cells [1-3]. Futhermore, MALI is an excellent charge carrier, and so unlike most organic dyes used in photovoltaics, it does not require a mesoporous charge conducting substrate. While a great deal of computational effort has been directed at modelling the electronic structure of MALI [4-6], and determining how the band gap may be tuned to create an optimal photoabsorber [7], or to understand the influence of surface defects on the material and how that in turn influences the manner in which water and solvents penetrate the material [8], no effort has been directed towards understanding the mechanism of formation of MALI.

MALI is a perovskite, MAPbI₃, where the A site methylammonium cation is surrounded by corner sharing PbI₆ octahedra. MALI is readily made through simple grinding, of PbI₂ and MAI in solvent [9]. This means that the transition from the edge sharing PbI₆ octahedra to corner sharing is facile. We present simulations that simulate the initial conditions that lead to the formation of MALI, using both Density Functional Theory and a newly parameterized force field. Our simulations demonstrate how the methylammonium ion is able to penetrate the MALI crystal and how this leads to a rapid conversion to the perovskite lattice.

1. Park, N.-G. Mater. Today 18, 65–72 (2015).
2. Snaith, H. J. J. Phys. Chem. Lett. 4, 3623–3630 (2013).
3. Brittman, S., Adhyaksa, G. W. P. & Garnett, E. C. MRS Commun. 5, 7–26 (2015).
4. Walsh, A. J. Phys. Chem. C 119, 5755–5760 (2015).
5. Brivio, F., Walker, A. B. & Walsh, A. APL Mater. 1, 042111 (2013).
6. Frost, J. M. et al. Nano Lett. 14, 2584–2590 (2014).
7. Filip, M. R., Eperon, G. E., Snaith, H. J. & Giustino, F. Nat. Commun. 5, (2014).
8. Geng, W. et al. J. Materiomics 1, 213–220 (2015).
9. Prochowicz, D. et al. J. Mater. Chem. A 3, 20772–20777 (2015).

Methylammonium lead halide perovskites are highly promising materials to fabricate efficient solar cells. However, the stability issue in various environment has prevented the material from being a competitive candidate in the long term. In this study, we have investigated the degradation of freshly cleaved CH₃NH₃PbBr₃ single crystals using X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM). The single crystals were exposed to nitrogen, oxygen, ambient air and water, respectively. The results indicate that CH₃NH₃PbBr₃ single crystal is not sensitive to nitrogen or oxygen. The XPS results of H₂O exposure experiment show that the single crystal started to degrade at ~10⁸ L (1 L= 10^-6 Torr●sec), and degraded completely at ~5×10¹¹ L. The AFM measurements reveal that the morphology of the film changed drastically from smooth to rough by water exposure, but only changed very insignificantly when exposing to ambient environment. The experiments illustrate that the CH₃NH₃PbBr₃ single crystal is stable in dry air, but will degrade when exposed to H₂O. The degradation can be characterized by the new peaks of C 1s, N 1s and Pb 4f_7/2, and the changing of band structure.

The simple processability of lead halide perovskite semiconductors, typically from solutions at temperatures close to room temperature, exposes this class of materials to a non-negligible level of unintentional structural and chemical defects. Ascertained that their primary optoelectronic properties meet the requirement for high efficiency optoelectronic technologies, a lack of knowledge of the nature of the defects and their role in the device operation currently represents a major challenge, limiting the further enhancement of the device efficiency and importantly their stability and reliability. Here, we use novel excitation correlation photoluminescence (ECPL) spectroscopy [1,2] to investigate the recombination dynamics of the photo-generated carriers in lead bromide perovskites and quantitatively describe the carrier trapping dynamics within a generalization of Shockley-Read-Hall formalism. The superior sensitivity of our spectroscopic tool to the non-linear carrier interactions enables us to identify the nature and energetics of the defects unambiguously. In the case of polycrystalline films, depending on the synthetic route [3], we demonstrate the presence of both deep and shallow carrier traps. The shallow defects which are situated at about 20meV below the conduction band dope the semiconductor leading to a substantial enhancement of the photoluminescence quantum yield in spite of carrier trapping. On the other hand, at high excitation densities relevant for lasing, we observe a highly correlated regime of photo-carriers that leads to the suppression of non-radiative Auger recombination, thus aiding the lasing action. Furthermore, we demonstrate that colloidal nanocrystals [4] represent the possibility of achieving a defect-free system, suffering from non-radiative quenching only due to sub-picosecond Auger like interactions at high excitation density. By correlating the fabrication conditions to the non-radiative loss channels, this work provides essential guidelines for material engineering towards better optoelectronic device performances.
[1] Srimath Kandada, A.R. et al. Submitted Manuscript.
[2] von der Linde, D., Kuhl, J. & Rosengart, E. J. Lumin. 24/25, 675–678 (1981).
[3] Cho, H. et al. Science 350, 1222–1225 (2015).
[4] Akkerman, Q. A. et al. J. Am. Chem. Soc. 137, 10276–10281 (2015).

Perovskite solar cells based on CH₃NH₃PbI₃ have attracted enormous attention in the last few years due to their rapid improvement and high-certified efficiencies over 22%. The architectures of the devices have been categorized to mesoscopic structure or simple planar heterojunction and the devices exhibit large hysteresis in J-V characteristics especially in the planar structure. The standard type planar perovskite solar cell presents a big mismatch in the power conversion efficiency (PCE) from forward (short circuit to open circuit) and reverse scan (open circuit to short circuit). The origin of hysteresis has been discussed on the intrinsic proprieties like ferroelectric polarization and/or ionic migration of the perovskite to date. On the other hand, PCBM in the inverted cell could extract the carriers (electrons) more efficiently than TiO₂ without accumulation at the interface, and the hysteresis can be eliminated. Lattice mismatch of the interfaces containing organic compounds could be ignored and consequently the hysteresis can be also reduced. The importance of lattice mismatch is widely known and discussed for inorganic thin film solar cells. Due to the higher expansion coefficient of CH₃NH₃PbI₃ than TiO₂, an interfacial stress is created at the interface of TiO₂ /CH₃NH₃PbI₃ which changes with temperature.^1-3 The above reports strongly suggest that defects and/or traps at the interface between compact TiO₂ and CH₃NH₃PbI₃ play an important role in causing the hysteresis. The charge trapping / detrapping and/or charge accumulation caused by the lattice mismatch or voids at this interface act as capacitors.⁴ In the present study, we will discuss about the interfaces on standart and inverted structure planar perovskite solar cells using as support the equivalent circuit model to simulate the hysteretic I-V curves including the interfacial capacitive components.

[1] Cojocaru, L., Uchida, S., Sanehira, Y., K., Nakazaki, J., Kubo, T., Segawa, H. Chemistry Letters, 44, 5, 674, 2015.
[2] Cojocaru, L., Uchida, S., Y. Sanehira, Gonzales-Pedro, V., Bisquert, J., Nakazaki, J., Kubo, T., Segawa, H. Chemistry Letters, 44, 11, 1557, 2015.
[3] Cojocaru, L., Uchida, S., Jena A. K., Miyasaka T., Nakazaki, J., Kubo, T., Segawa, H. Chemistry Letters, 44, 8, 1089, 2015.
[4] Cojocaru, L., Uchida, S., Piyankarage V. V. J., Kaneko S., Nakazaki, J., Kubo, T., Segawa, H. Chemistry Letters, 44, 12, 1750, 2015.

Amorphous WOx as Electron Selective Layer for Highly Efficient Flexible Perovskite Solar Cells
K. Wang¹, Y. T. Shi*¹, T. L. Ma*^{2, 3
1}State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian 116024, P. R. China
²School Petroleum and Chemical Engineering, Dalian University of Technology, Panjin Campus, Panjin 124221, P. R. China
³Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Fukuoka, 808-0196, Japan
* [email protected]; [email protected]
Perovskite solar cell (PSC) has been a rising star in the third generation solar cells for their considerable potential applications in the field of photovoltaic. While the flexible and lightweight thin film PSC is one direction for further development that enjoys favorable prospect on account of its versatile functionality and convenience of integration. In PSCs, the electron select layer (ESL) is responsible for extraction of photo-generated electrons and simultaneously serves as a blocking layer to prohibit the direct contact of the conductive substrate with hole transport material. However, high temperature treatment is commonly required in the fabrication of ESLs, which precludes the fabrication of flexible PSCs based on plastic substrate. Although presently several solution routes have been proposed for low temperature fabrication of ESLs, it can be found that spin-coating highly dispersed pre-synthesized nanoparticles on flexible substrates as ESLs is a general route for the preparation of flexible PSCs.
Here, we present a simple route by which inorganic ESLs based on Nb doped amorphous WOx for flexible PSCs can be fabricated at low temperature or even room temperature for the first time. It is revealed that Nb doping could improve the charge injection and transmission as it could notably improve the donor density, modify the energy band structure and reduce the depletion layer at the interface of ITO/ESL. As a result, the photovoltaic performance for planar flexible PSCs is notably improved and a high power conversion efficiencies (PCE) of 15.65% has been obtained based on doped ESLs fabricated at very low temperature. Meanwhile, PSCs with ESLs fabricated at room temperature are also very efficient, with a maximum PCE of 13.14% obtained. Our work presents a novel strategy to develop amorphous, inorganic, and composite functional materials as ESLs to fabricate highly efficient flexible PSCs.
References:
[1] J. T.-W. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H. J. Snaith, R. J. Nicholas, Nano Lett. 2014, 14, 724.
[2] K. Wang, Y. Shi, Q. Dong, Y. Li, S. Wang, X. Yu, M. Wu, T. Ma, J. Phys. Chem. Lett. 2015, 6, 755.
[3] K. Wang, Y. Shi, B. Li, L. Zhao, W. Wang, X. Wang, X. Bai, S. Wang, C. Hao, and T. Ma, Adv. Mater. 2016, 28, 1891.

Depletion of fossil fuel and the following environmental impact has increased the importance of developing new energy source. In particular, there is a pressing need to develop new energy sources that are efficient, conservative and carbon-neutral to ensure minimal environmental impact. Solar, water and wind-based energy generation are widely considered to be some of the most promising technologies that can meet these requirements. Among these, solar cell has received wide attention from many scientists and engineers since it has several advantages such as environmentally friendly and non-depleting and long-term durability. Although the silicon-based solar cell devices show good energy efficiency (>20%), complexity of manufacturing system and high production cost have limited its wide range applications. Recently, hybrid perovskite solar cell, which uses (CH₃NH₃)PbI₃ as a photoactive layer, is receiving extensive attention because high energy efficiency has been achieved without complex manufacturing process. However, the inclusion of toxic lead has increased the necessity of developing pb-free photoactive materials. In this study, thus we have searched alternative perovskite materials which can replace (CH₃NH₃)PbI₃. Especially, we have employed First-Principles-based novel materials design technique to reduce time and cost required for synthesis and characterization of traditional experimental approaches. We have extensively studied on ABX₄, A₂BX₄, A₃B₂X₉ backbone structures to find alternative materials for perovsktie solar cell. Our study has identified a couples of candidate materials which are suitable for photoactive layer based on bandgap and energy level alignment. In addition, analysis on electronic structure has led the understanding of the relationship between constituent elements and bandgap.

Metal halide perovskite solar cells (PSCs) are thought to be promising energy suppliers because of their feasibility for high power conversion efficiency (PCE), light weight, and flexible architecture. The preparation of charge transporting layers at low temperature has been essential for such devices. Recently, low-temperature-processed metal oxides have been a desirable material for charge transport and air stability for PSCs, instead of organic semiconductors. However, pristine metal oxides fabricated at low temperature have still precluded high performance of the device because of their low conductivity and large deviation in energy levels from the conduction band or valance band of the perovskite. Therefore, doping metals in the metal oxides has been considered as an effective method to endow suitable electrical properties. Here, we developed a highly efficient electron transporting layer (ETL) consisting of Li-doped SnO₂ (Li:SnO₂) prepared at low temperature (185 ^oC). The doping effect produced the enhanced conductivity as well as induced a downward shift of the conduction band minimum of SnO₂, which facilitated injection and transfer of electrons from the conduction band of the perovskite. The PCE was measured to be 18.2% and 14.78% for the rigid and flexible substrates, respectively. The high-performance and flexible PSCs could be potentially used as a wearable energy supplier.

Organometal halide perovskites exhibit great promise for lightweight, low-cost and high efficiency solar cells due to their tunable optical band gaps, strong light absorption and especially excellent carrier transport properties. Ideally, growth of single-crystalline perovskites with a completely removed grain boundary and defects would enable the perovskite solar cells (PVSCs) to approach the Shockley–Queisser maxima power conversion efficiency (PCE) of ~30%. However, these single-crystalline perovskites with a large size are difficult to be used for solar cells mostly due to the challenge of crystal engineering to accommodate the device architecture. In this work, we have successfully grown single-crystalline lead halide perovskite arrays on a poly(3,4-ethylenedioxythiophene):polystyrenesulfonic acid (PEDOT:PSS) coated ITO substrate by the droplet-pinned crystallization (DPC) method and, for the first time, single-crystalline perovskite solar cells have been fabricated with a power conversion efficiency of 1.73%.

In the past few years, metal halide perovskite semiconductors have attracted much attention because of their potential utilization in solar cells, photodetectors, light-emitting diodes, and lasers. All-inorganic cesium lead halide perovskite nanocrystals (NCs) have recently shown to exhibit remarkable optical properties resulting from unique size-effects of NCs combined with the perovskite structure [1]. For further understanding of these novel materials, accurate determination of their basic optical quantities is necessary. Especially, the absorption cross-section is an important quantity for application purposes. However, there is a variation within the reported values of the absorption cross-sections [2,3]. We consider that this uncertainty is caused by differences in the experimental methods and analysis. In this study, we report the absorption cross-sections of cesium lead bromide (CsPbBr₃) all-inorganic perovskite NCs determined from different experimental techniques.
The samples used in this work were CsPbBr₃ NCs dispersed in hexane solution. Using femtosecond transient absorption spectroscopy, we could verify the Auger recombination dynamics under high excitation conditions. After the biexciton recombination, the bleaching of the remaining excitons was observed. The exciton bleaching showed a saturation behavior for increasing excitation density. From this saturation behavior, we determined the absorption cross-section of the perovskite NCs. This saturation behavior is expected to be observed also in the photoluminescence dynamics. By comparing the transient absorption and time-resolved photoluminescence data, we analyzed the photoabsorption processes and calculated the cross-sections. The above two experiments revealed ensemble properties, and furthermore we tried to reveal the single NCs properties with an additional experiment. The cross-sections determined from the different techniques are compared, and we discuss the optical properties of CsPbBr₃ perovskite NCs.
Part of this work was supported by JST-CREST.

[1] L. Protesescu et al., Nano Lett. 15, 3692 (2015).
[2] N. S. Makarov et al., Nano Lett. 16, 2349 (2016).
[3] F. Hu et al., ACS Nano 9, 12410 (2015).

Medium band gap solar cell absorber materials acquire immense interest when designing devices for building integrated PV and tandem applications. Earlier reports have employed a two-step deposition technique fabricating meso-porous FAPbBr₃ devices close to 7%. Here we report a simple planar p-i-n structure FAPbBr₃ devices via modified single step spin coating using HBr as an additive resulting in highly efficient devices (~8.3%). This was achieved through a comparative study on FAPbBr₃ devices making a transition from all organic transport layers to all inorganic transport layers was performed in detail. In th device structure, PEDOT:PSS on FTO substrate acts as the organic hole transport layer to start with. Though PCBM has served as an effective ETL for MAPbI₃ devices in previous studies, when coupled with FAPbBr₃ absorber they are ineffective in blocking holes resulting in major V_OC loss. Employing sputtered ZnO instead, serves as a better hole blocking layer, improving the open circuit potential.
The importance of the work comes from the ability to sputter inorganic metal oxide layers such as ZnO directly on top of planar perovskite layer. However certain amount of plasma damage on FAPbBr₃ is observed under direct exposure and is expected to cause interfacial recombination. . A detailed numerical simulation based on drift diffusion model was performed which helped to validate the underlying loss mechanisms associated with each of the device configurations.However, better performing devices were obtained by employing PCBM/ZnO bi-layer as ETL resulting in an open circuit potential of 1.35V. The PCBM interfacial layer reduced plasma related damage while depositing ZnO and the use of ZnO helps in effective hole blocking, thus device performance was greatly enhanced. The devices employing sputtered ZnO exhibited better stability when compared to PCBM based devices under continuous illumination. The bi-layer devices offered efficiencies as high as 8.3%, resulting in the best performing devices using FAPbBr₃.
Replacing organic PEDOT:PSS HTL with inorganic NiO (Atomic Layer Deposition) with ZnO as ETL, further improves the open circuit potential due to better band alignment. The transition from organic transport layers to wide band inorganic transport layers has greatly increased the performance and stability of the FAPbBr₃ devices. Direct deposition of sputter coated ZnO on FAPbBr₃ is used as an avenue to produce devices with moderate to high efficiencies but possibly limited by the interfacial damages during the plasma process. However, upon optimization, such process can open up an opportunity to use many inorganic charge transport materials resulting in a considerable cost reduction by eliminating the costly organic polymer based charge transport layers.

Since low cost and high performance organometal halide perovskite solar cells were developed in 2009, the stability of perovskite solar cells becomes the main barrier for its practical applications. The hybrid vapor chemical deposition (HCVD) is a promising technique for growing high quality perovskite films. Carefully adjusted cooling rate of HCVD process can eliminate the pin-holes formation in perovskite film, and therefore reduces the density of defect states at the grain boundaries. Mixing 15% O₂ carrier gas during the HCVD growth can also effectively passivate the trap states of the perovskite film. MesoscopicTiO₂ (mp-TiO₂) layer is also found to be useful for reducing defect states in the devices. The low-frequency current noise (LFCN) and low-frequency voltage noise (LFVN) of HCVD-grown CH₃NH₃PbI₃ (MAPI) solar cells is measured in dark to study the impact of the growth ambient, cooling rate and device configuration on the density of defect states of devices. Furthermore, the devices were characterized using X-ray diffraction (XRD), time-resolved photoluminescence (TRPL) and capacity-voltage (CV) measurements which were performed to investigate the underlying mechanism of enhanced performance of oxygen-annealed devices. The degradation of power conversion efficiency (PCE) and the stability of HCVD-grown MAPI devices with different ambient were monitored as a function of time. Our results indicated that the degradation is accompanied by the increase in the defect states. The PCE of O₂ passivated devices can maintain above 60% of its initial value for ~ 4000 hours and above 70% of its initial PCE if mp-TiO₂ is employed while the devices without O₂ passivation drop to 10% in 1250 hours.

Perovskite solar cells based on organometal trihalide have reached over 22% power conversion efficiency (PCE) in few years due to their extraordinary properties such as high absorption coefficient, long exciton diffusion length and low exciton binding energy. The device performance of perovskite is highly related to film quality of perovskite. Therefore, many research groups have tried to make high quality film, however most devices showing high performance have been fabricated by spin coating process. The various methods used in spin coating process such as gas quenching, introduction of additives and solvent engineering are difficult to keep optimized processing condition of spin coating in printing process due to different mechanism of film formation between them. When translating spin coating process to other scalable printing methods, the most different factor is drying mechanism of wet film, which is closely related to formation of nucleation and crystal growth. In this study, we introduced the gas quenching system for low coating temperature and processing additives, CHP (N-cyclohexyl-2pyrrolidone) and DMSO (dimethylsulphoxide) for reproducible wide processing window in slot-die coating process. CHP inhibits the rapid crystal formation due to high boiling point and extremely low vapor pressure and remaining CHP molecules produce homogeneous nucleation during solidification. Also, DMSO strongly coordinates with PbI₂ and forms PbI₂-DMSO-MAI intermediate states. These properties induce the controlled crystal growth of perovskite and the device fabricated by slot-die coating showed power conversion efficiency over 10% at coating temperature of 50 ^○C.

Solution processed hybrid inorganic-organic perovskite materials have been generating huge interest for application in a range of optoelectronic and electronic devices such as solar cells, light emitting diodes and transistors. In recent years, spectacular advances have been made in the power conversion efficiency of perovskite solar cells with recent reports of devices exhibiting PCE’s over 20%. However, these materials have some shortcomings, namely their toxicity due to their heavy metal component and their inherent instability. I will discuss recent investigations into this instability; I will unearth some of the fundamental issues that cause the degradation of methylammonium lead iodide to lead iodide under the action of light and oxygen [1-3]. I will begin by explaining how the perovskite mediates the generation of a reactive oxygen species, superoxide, which is key to its sensitivity to oxygen and light. The probe measurements used to identify this species will be explained to indicate how the species was identified, whilst the effects it has on the material will be shown via absorption properties. Following on from this, I will elucidate the nature of the perovskite crystal that facilitates the mechanistic generation of superoxide, which was achieved via a combination of experimental and computational data. Included in this will be an exploration into the effects of thin-film microstructure on stability, which has been shown to affect device performance [4]. Additionally, experiments that highlight why the material is so highly sensitive to oxygen will also be discussed. Towards the end of the presentation I will begin to examine the effects of introducing water into the oxygen and light induced degradation and how the role of water alters the reactivity of superoxide enhancing the rate of degradation. The talk will conclude with some of the lessons that can be learnt from the data collected and how it can be used to design more efficient and stable hybrid perovskite materials.


References:
[1] O’Mahony et al. J. Mater. Chem. A. 2015, 3,7219 –7223.
[2] Aristidou et al. Angew. Chemie. Int. Ed. 2015, 54,8208 –8212
[3] Bryant et. al. Energy Environ. Sci., 2016, 9, 1655-1660
[4] Im et. al. Nat. Nanotech., 2014, 9, 927–932

Perovskite solar cell has garnered a lot of attention from the research community as a highly promising source of energy that can easily be fabricated using wet-synthesis method, thereby lowering the cost of production. Simultaneously, research on inkjet printing technology has been growing over the decade. Its use has been moving away from the conventional home/office use to more advanced application such as printed electronics. This technology has a huge potential in being used for roll-to-roll fabrication of perovskite solar cells.

The study presented here focuses on inkjet printing of the different metal oxide components of a perovskite solar cell; mainly the transparent conducting electrode (Antimony doped tin oxide), the electron-transport layer (mesoporous Anatase titanium dioxide), and the insulating layer (mesoporous zirconium oxide). The different layers have been printed separately using aqueous-based nanoparticle-loaded inks. All metal oxide nanoparticles started out as nanoparticle powders that have been dispersed in water ultrasonically; Antimony tin oxide (Sigma Aldrich) is sized at 50 nm or less, the Titanium dioxide (US Research Nanomaterials, Inc.) sized at 18 nm, and Zirconium oxide (US Research Nanomaterials, Inc.) sized at 20 nm. The inks were then formulated with water as the main solvent, metal oxide nanoparticles as the deposited material, glycerol as viscosity enhancing co-solvent and humectant, non-ionic surfactants (Pluronic F-127 and Triton-X100) as dispersant and surface tension contrasting agent, and acetic acid and ammonium hydroxide as pH-altering reagent. The inks were formulated to have a pH that is far away from the iso-electric point (pI) to prevent rapid precipitation of the metal oxide nanoparticles via same charge repulsion. Adding a non-ionic surfactant also helped in the stabilization of the nanoparticle in the ink, but the amount of surfactant has been regulated to allow the ink to be jettable (within the inverse Ohnesorge number range of Z=4-14) and to control the wetting/spread of ink on the substrate. The printing experiments were done using a system built with a glass tipped 60 μm inkjet nozzle (Microfab Technologies), jetting driver (Microfab Technologies Jet drive III controller), 28 mm linear stages (Zaber technologies), droplet imaging camera (Sentech CCD USB Camera with Navitar optical zoom lens), and fiducial top-viewing camera (Hitachi CCD camera with Navitar optical zoom lens). The study has demonstrated that by careful manipulation of the different composition of the ink, we get stable and jettable inks that produce uniform films of metal oxide layers for use in perovskite solar cells.

Solar cells composed of the hybrid perovskite methylammonium lead iodide (MAPI) are notorious for their sensitivity to moisture, which limits the device lifetime. Ensuring the long-term stability of perovskite solar cells under operational conditions is crucial for their implementation into large-scale applications. Therefore, the degradation mechanisms occurring within the perovskite material upon exposure to moisture need to be elucidated.
Here, we show that MAPI undergoes a stepwise transformation into two species of transparent hydrated MAPI crystal phases upon exposure to moist air (75% relative humidity) at room temperature, and that these phase changes can be fully reversed when the material is subsequently dried. We have modeled the structural and optical data on thin films and single crystals to determine the time-dependent changes to MAPI films exposed to moisture. Our results indicate that the conversion into the monohydrate phase occurs isotropically within the granular perovskite film, suggesting rapid transport of water molecules along grain boundaries. The reverse reaction was observed in the successive dehydration of hydrated MAPI single crystals under low air humidity. In contrast to water vapor, the presence of liquid water led to the irreversible decomposition of MAPI to form PbI₂. Finally, we show that the same process occurs in perovskite solar cells. Vapor phase hydration of a device without encapsulation quickly resulted in a more than 90% drop in short circuit photocurrent and a 20% loss in open circuit voltage. However, the device performance was fully recovered after the cell was dried under a nitrogen flow at room temperature. Based on our observations, we suggest that perovskite solar cells which exhibit a dramatic decrease in performance due to the formation of a hydrated layer at the MAPI grain boundaries may be recovered by a simple drying step.

Emerging perovskite solar cells and their advanced flexible technologies get toward commercialization of renewable clean electrical power source. A delicate control of carrier dynamics in the perovskite device, including a balance of charge extraction rate and a transport/extraction efficiency, has decisive effect on hysteresis phenomenon and their power conversion efficiency. Perovskite solar cell embedding organic charge transport layers as an alternative to inorganic transport layer, which has slow carrier dynamics, have the various advantages of low-temperature process, fast carrier mobility and they do not show hysteresis problem. However, organic transport layer based perovskite device is very unstable in the atmosphere.
In our group, we have fabricated stable and efficient perovskite solar cell, which are attributed to the excellent performance of bi-structured inorganic-organic. A ~20 nm of thin organic layer formed on the several nanometers of plasma enhanced atomic layer deposited-TiO_x afford enhancing electron extraction property than only TiO_x layer. In addition, the perovskite solar cells, incorporating organic layer between inorganic TiO_x and perovskite layer demonstrate high stability compare to fully organic based perovskite solar cells. Moreover, all fabrication processes for perovskite solar cell are performed on the low-temperature, we realized flexible perovskite solar cells. These flexible devices shown high efficient that boost a world-class level without hysteresis due to the complementary effect of bi-structured transport layer.

The use of hybrid organic-inorganic perovskites (HOIPs) in perovskite solar cells (PSCs) is revolutionizing the field of photovoltaics, which is being led by advances in the processing of HOIP thin films. Here, we look at fundamental phenomena pertaining to nucleation/growth, coarsening, and microstructural evolution involved in the solution-processing of HOIP thin films for PSCs from a materials-science perspective.¹ Established scientific principles that govern some of these phenomena are invoked in the methodologic evolution in the HOIP solution processing. In this context, to induce the crystallization (or conversion) of HOIPs via methylamine (or formamidine) gas treatment is found to be favorable for the formation of HOIP thin films with ultra-smoothness and beneficial grain structures.^2-4 A series of in-situ or ex-situ characterization (optical/photoluminescence microscopy, TEM, XRD, etc.) experiments are conducted to reveal the crystallization mechanism at multiple scales, which has indeed led to the discovery and understanding of some unusual materials science or chemistry involved in these amazing HOIP materials. Notably, based on these fundamentals, we have established a set of scalable perovskite processing strategies utilizing the transformative or morphology-preserving natures of the precursor reactions, which leads towards high-performance, large-area perovskite solar cells with high heat and/or moisture tolerance.

REFERENCE:
1. Y. Zhou, O.S. Game, S. Pang, N.P. Padture. Journal of Physical Chemistry Letters, 2015, 6, 4827-4839.
2. Y. Zhou, M. Yang, S. Pang, K. Zhu, N.P. Padture. Journal of American Chemical Society, 2016, 138, 5535-5538
3. S. Pang, Y. Zhou, Z. Wang, M. Yang, A.R. Krause, K. Zhu, N.P. Padture, G. Cui. Journal of American Chemical Society, 2016, 138, 750-753
4. Z. Zhou, Z. Wang, Y. Zhou, S. Pang, H. Xu, Z. Liu, N.P. Padture, G. Cui. Angewandte Chemie, 2015, 54, 9705-9709.

Perovskite solar cells (PSCs), due to simple fabrication method and high power conversion efficiency, have been attracting more and more attention. In traditional vertical structures of PSCs, if the perovskite layer sandwiched by charge-selecting layers and electrodes degrades, the whole device should be discarded. Meanwhile, choices for sealing materials and processes for PSCs are limited, since the effect of the sealing material and process on both perovskite and charge-selecting layers should also be taken into consideration. These factors lead to high running cost and limited stability of PSCs.
The integrated-back-contacted (IBC) structure has been widely used in highly efficient inorganic single-crystal solar cells. If PSCs are fabricated on IBC substrates, the sealing and perovskite layers can be removed by solution process, and the IBC substrates can be recycled and reused. Thus the cost of PSCs could be largely reduce. Moreover, the IBC PSCs can be sealed with simple methods, such as spin-coating resins on perovskite layers.
Some groups have attempted to fabricate PSCs with IBC structure.^[1,2] However, due to the small crystal size of the perovskite layer, the charges have to travel through tens of or even hundreds of grain boundaries to reach electrodes. High recombination possibility has limited the performance of the IBC PSCs. Here, we propose a novel method to form pinhole-free perovskite layer on IBC substrates. It is demonstrated that the perovskite layers fabricated by the proposed method is composed of large single-crystal grains (up to 50 µm).^[3] We confirmed that large single crystals can be formed on patterned substrates. And carriers travel between two electrodes without passing through any grain boundary. By fabricating photo-detectors using the proposed method, we showed that the charge transportation significantly improved in perovskite layers with large crystal size. Our results pave the way for realization of highly efficient IBC PSCs.

[1] A. N. Jumabekov et al, J. Mater. Chem. C, 2016, 4, 3125.
[2] L. M. Pazos-Outón et al, Science, 2016, 351, 1430.
[3] T. Ma et al, submitted.

In 2009, the CH₃NH₃PbI₃ (MAPI) hybrid organic-inorganic perovskite structure was plucked from obscurity and propelled into the forefront of solar cell research. In just seven years, perovskite solar cells (PSCs) have reached a record efficiency of 22.1% which is attributed to strong absorption, small band-gap, high carrier mobility and ever-more sophisticated electron and hole transport materials (ETMs and HTMs). While the efficiency of PSCs is similar to the well-established silicon solar cell, the rapid expansion of this field has left a great deal of fundamental questions about their performance unanswered.
While some progress has been made with regard to understanding, and controlling, the properties of the perovskite material itself, little attempt has been made to discern the underlying processes behind hole transfer to the HTM. With the Shockley-Queisser limited efficiency of MAPI at around 31%, optimising hole transfer to, and subsequent transport through, the HTM may well represent the final frontier for PSC research.
In this study, we employ transient absorption spectroscopy (TAS) and time correlated single photon counting (TCSPC) to unravel the fundamental parameters that govern hole transfer dynamics from perovskites into a wide range of HTMs. The choice of HTM has a large effect on the yield of hole transfer and a two-order of magnitude bearing on the recombination time.
While many studies have focused on the ‘energetic driving force’ at the perovskite/HTM interface, we find that the overall picture of hole transfer dynamics is more complex. The size of perovskite crystal, along with the mobility, morphology and dopant concentration of the HTM are also found to be of high importance. By deconvoluting these factors we present the most sophisticated study of solar cell HTMs to date, demonstrating that device performance is inextricably linked to the perovskite/HTM interaction.

Thin film tandem solar cells, comprising of a perovskite top device and a radiation hard CIGS bottom solar cell would be attractive for space applications since they can be lightweight, flexible, and efficient. In this work, the ability to withstand the harsh radiation environment in space, consisting mainly of high-energy protons is demonstrated. In-situ measurements of the J-V characteristics during proton irradiation with a proton energy of 68 MeV show that organic-inorganic perovskites are radiation hard. The investigated CH₃NH₃PbI₃ based perovskite solar cells possess a negligible degradation for proton doses of up to 10¹² p cm^-2. ^[1] This exceeds the proton dose at which c-Si degrades by almost 3 orders of magnitude. Even more surprising is the observation that the solar cell performance improves with time after the proton irradiation is terminated. Hence, the localized defects caused by proton irradiation are metastable and vanish with time. In addition, proton irradiation partially compensated the photo-degradation effect of the perovskite solar cells. Compared to untreated devices the open circuit voltage and the fill factor improved. This is mainly attributed to smaller recombination losses due to a decrease of the density of localized defects. Simultaneously, proton irradiation caused the formation of shallow defects, which act as doping centers. The observed radiation hardness and self-healing capabilities render inorganic-organic perovskite absorbers highly attractive for space applications.

[1] F. Lang, N. H Nickel, J. Bundesmann, S. Seidel, A. Denker, S. Albrecht, V. Brus, J. Rappich, B. Rech, G. Landi, H. Neitzert, 2016, under review.

Organo metal halide perovskite solar cells exhibit exceptionally good power conversion efficiency values exceeding already 20% under laboratory conditions. To allow further improvements it is important to understand the limiting processes e.g. charge carrier recombination in this new photovoltaic technology. The charge carrier lifetime is an important parameter in solar cells as it defines together with the mobility the diffusion length of the charge carriers. Most of the recombination studies presented so far have been focused on perovskite films or crystals but only a few have studied the charge carrier lifetime in complete devices. We report on charge carrier lifetime values in bromine containing planar perovskite solar cells with varying the bromine content by using different precursor materials. The lifetime and corresponding charge carrier density has been derived from transient photovoltage (TPV) and charge carrier extraction (CE) experiments. We found increased lifetime values in solar cells with bromine (MAPbI_3-xBr_x) compared to pure methylammonium lead iodine (MAPbI₃) [1]. Our results demonstrate that the chemical composition of the perovskite layer is an important factor defining the defect density of the crystal and thus the lifetime of the charge carriers. Furthermore, the CE signals from the bromine containing solar cells obey two different extraction parts on short and long time scale which leads to anomalously high extracted charge values. In contrast, the MAPbI₃ solar cell reveals a CE signal which is saturated after 0.1 ms leading to a recombination current which is comparable to the respective short circuit current of the solar cell. To examine the origin of both parts in the Br containing device, we performed additional measurements in the dark. We discuss the experimental findings in regard of possible different contributions to the CE signal.

[1] D. Kiermasch, P. Rieder ,K. Tvingstedt, A. Baumann, V. Dyakonov, submitted, 2016

The photovoltaics landscape changed drastically over the past few years with the rapid evolution of organometallic perovskite solar cells. Their high carrier mobility, tunable band gap and strong absorption in the visible make it an excellent candidate for the production of low cost and high efficiency solar panels. However their stability is precarious and their current lifetime (~ 2000 solar peak hours) is still pretty low. In order to take this new material to the market, a better understanding of the photophysics and the degradation mechanisms is necessary. With this in mind, vacuum evaporated perovskite solar cells and perovskite single crystals were characterized using hyperspectral imaging. Spectrally resolved photoluminescence (PL), electroluminescence (EL) and transmittance maps were acquired on different samples over a million points with a spatial resolution close to one µm. PL and transmittance maps of a perovskite crystal aged in air showed strong inhomogeneities. Furthermore, PL and EL maps of perovskite devices were coupled with absolute calibration method in order to obtain the exact number of photons emitted from every point of the device, at every wavelength. Using the generalized Planck’s law, it is possible to obtain maps of the depth-averaged quasi-Fermi level splitting (Δµ), which dictate the maximum achievable open circuit voltage (Voc) of solar cells. Also, using the generalized reciprocity relations the charge transfer efficiency of the cells can be analyzed from the hyperspectral data. Once again, considerable spatial inhomogeneities, both in quasi-Fermi level splitting (Δµ) and in charge transfer efficiency, are found in these perovskite devices. Hence, hyperspectral imaging provides a useful insight of the local composition and electronic properties of perovskite solar

Hybrid organic-inorganic lead halide perovskite solar cells have emerged as one of the most attractive photovoltaic technologies, showing an unprecedentedly fast increase in power conversion efficiency. However, the high toxicity of lead stands as an unresolved issue for future large-scale applications. Tin-based perovskite counterparts have been reported as a potential alternative in devices, but with very poor stability due to spontaneous Sn²⁺ oxidation to Sn⁴⁺ in ambient air.^1,2 Even in trace amounts, presence of Sn⁴⁺ in the perovskite crystal structure introduces p-type self-doping in the material, making it adopt a metal-like behaviour and hindering its photovoltaic performance.^3,4 Promisingly, self-assembled layered tin halide perovskites ((C₄H₉NH₃)₂(CH₃NH₃)_n-1Sn_nI_3n+1), where n is the number of perovskite layers) show a progressive trend from metallic to semiconducting behaviour when decreasing n. ⁵
In the study herein shown, layered tin-based perovskites are presented as pontential light harvesters in lead-free solar cells. Photoinduced interfacial charge carrier transfer is demonstrated via time-resolved techniques. This, along with the great tunability of their optoelectronic properties via stoichiometric dimensionality control, makes layered tin-based perovskites an interesting candidate to achieve semiconducting metal oxide sensitisation for photovoltaic applications. The stability of layered tin-based absorbers as a function of their dimensionality will be investigated as an effort to overcome the inherent constraints of the conventional 3D tin-based perovskite.

1 Energy Environ. Sci., 2014, 7, 3061–3068.
2 Nat. Photonics, 2014, 8, 489–494.
3 Dalton Trans., 2011, 40, 5563–5568.
4 J. Solid State Chem., 2013, 205, 39–43.
5 Nature, 1994, 369, 467–469.

Organic-inorganic halide perovskites, especially methylammonium (MA) lead halides, have recently led to a remarkable breakthrough in photovoltaic devices as the efficiency of perovskite based solar cells rapidly improved in the past a few years.¹ Although most studies focused on MAPbI₃, there is an increasing interest in alternative candidates to improve device performance, for example with mixed cations on A-site², and to reduce the environmental concerns, for example by replacing Pb with Sn and Ge on the B-Site.³ In this study, we explored the possibilities of tuning the chemical compositions to achieve tailored materials properties, which leads to a range of perovskite-related materials spaning from zero dimensional (0D) isolated polyhedra to three dimensional frameworks.
By introducing a relatively large cation, guanidinium, in the A-site,⁴ guanidnium lead iodides in the form of (CN₃H₆)PbI₃ (1D), (CN₃H₆)₂PbI₄ (2D) and (CN₃H₆)₃PbI₅ (1D) were synthesized. Structural analysis was carried out by X-ray diffractions, and DFT calculations were employed to assess the stability and band structures. The connectivity of the Pb polyhedra has an important influence on the optical properties.
Another example of B-site substitution is to replace the toxic lead with bismuth. For example, a family of perovskite-related phases have been studied using ammonium as the cation. Inorganic connectivities, including Bi₂I₉^3-, BiI₄-, BiI₅^2- and BiI₆^3-, were synthesised, structurally characterized and studied by DFT calculations. The conductivity was measured for (NH₄)₃Bi₂I₉, which has a 2D perovskite-like architecture, in order to examine the potential application of low dimensional bismuth based perovskite and related phases as alternatives to the lead containing perovskites in solar cells.⁵

1 M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501, 395–398.
2 N. Pellet, P. Gao, G. Gregori, T. Y. Yang, M. K. Nazeeruddin, J. Maier and M. Gratzel, Angew. Chem., Int. Ed., 2014, 53, 3151–3157
3 C. C. Stoumpos, L. Frazer, D. J. Clark, Y. S. Kim, S. H. Rhim, A. J. Freeman, J. B. Ketterson, J. I. Jang and M. G. Kanatzidis, J. Am. Chem. Soc., 2015, 137, 6804–6819
4 G. Kieslich, S. Sun and A. K. Cheetham, Chem. Sci., 2014, 5, 4712–4715
5 S. Sun, S. Tominaka, J.-H. Lee, F. Xie, P. D. Bristowe and A. K. Cheetham, APL Mater., 2016, 3, 031101

The performances of lab-scale, fully spin-coated, methyl ammonium lead trihalide (MALT) perovskite-based solar cell devices are subject to a near infinite level of variation based on individuals’ expertise and the reported strategies to achieve such high performances are not sufficient enough for successful commercialization. The photovoltaic properties of perovskite solar cells are highly dependent on the active layer film morphology and crystallization, and as such controlled deposition of defect-free perovskite films is of significant interest towards wide adoption of this inorganic-organic hybrid technology. Mitigating defects during an all-low temperature, solution processed perovskite thin-film growth through automation is highly desirable to facilitate the lab-to-fab process transfer of this emerging solar technology. Critical to this goal is the implementation of novel fabrication protocols which are robust and fully compatible with commercial manufacturing processes. As a step forward, we have introduced aerosol-jet (AJ) deposition techniques as a means of precisely controlling the thin-film perovskite growth in a planar heterojunction p-i-n solar cell device structure. Here the roles of some of the user defined AJ deposition parameters (e.g. flow rates, stepping) and ink formulation are studied for the fabrication of the pure MALT thin films under near ambient conditions, and preliminary power conversion efficiencies upto 11.5% are achieved when such films are incorporated in a PEDOT:PSS-perovskite-PCBM type device format. We further show that depositing the materials as an aerosol inherently facilitates the formation of a highly uniform and PbI₂ residue-free MALT film and offers advantages over the conventional two-step solution-processed approach by avoiding the detrimental solid-liquid interface induced perovskite crystallization. It is found that new chemistries and engineering are pivotal for such high quality MALT film growth. This work suggests that the automated AJ deposition process, once fully optimized, could form a universal platform for the lab-to-fab process transfer of solution based perovskite photovoltaics.

In the last 5 years, hybrid halide perovskites have emerged as a highly efficient class of solar absorbers, with efficiencies reaching 22.4 %, quickly surpassing other 3rd generation devices. The highest performing hybrid perovskite is the cubic CH₃NH₃PbI₃ (MAPI), which is made from earth-abundant elements and can be easily solution processed, dramatically reducing manufacturing costs. Unfortunately, chemical stability is a major concern for these materials and much effort has been devoted to increasing the stability of MAPI based devices.

Recently, partial substitution of iodine with the pseudohalide ion, SCN-, to form the layered (CH₃NH₃)₂Pb(SCN)₂I₂ (MAPSI), has been suggested as novel route to increase stability whilst retaining high efficiencies. Here we present density functional theory results which explain why MAPSI can still possess an ideal electronic structure for light absorption, despite the loss in connectivity when moving from a cubic to a layered structure. We also explain, for the first time, why MAPSI is more stable than MAPI. Lastly we explore the possibility of a MAPSI related family of materials whose optoelectronic properties can be fine-tuned for use in photovoltaic applications.

Publications

A. M. Ganose, C. N. Savory and D. O. Scanlon, (CH₃NH₃)₂Pb(SCN)₂I₂: A More Stable Structural Motif for Hybrid Halide Photovoltaics?, J. Phys. Chem. Lett., 2015, 6 (22), pp 4594–4598

The conventional unstable and expensive hole transporting materials (HTM) has been replaced by cost effective modified carbon hole extraction layer. Herein, we demonstrated a new recipe toward air stable and waterproof modified carbon hole extraction layer for efficient perovskite solar cells (PSCs). The commercial available carbon ink modified with MAI was used as hole extraction layer for ambipolar perovskite solar cells. The fabricated optimized perovskite solar cell having Glass/FTO/mp-TiO₂/MAPbI_3-Cl_x/carbon+MAI/Carbon devices exhibited h=13.87% power conversion efficiency (PCE) with open circuit voltage (V_OC) 0.997V, current density (J_SC)=21.41 mAcm^-2 and fill factor (FF) 0.65. Furthermore, the air stability were tested at room temperature in open atmosphere. The water proof stability was tested under water flushing. Our results revealed that, although our carbon based devices show lower PCE (η=13.87%) compared to spiro-MeOTAD HTM (η=15%), the fabricated PSCs could even retain >90% after water exposure >20 times and ambient air stability more than 160 days. Further the large area device (>1cm²) device shows 13.04 % PCE with Jsc=21.47 mAcm^-2, V_OC=0.996 V and FF=0.61. We have also demonstrated >13 % efficiency for large area device (>1.1 cm²), demonstrating that the developed method is simple, cost effective and promising towards large area device fabrication. The developed methodology based on low cost carbon hole extraction layer will be helpful towards waterproof and air stable perovskite solar cells for large-area devices.

Metal halide perovskites such as methylammonium lead iodide (CH₃NH₃PbI₃) are generating great excitement due to their outstanding optoelectronic properties, which lend them to application in high efficiency solar cells and light-emission devices. However, there is currently debate over why these materials have such unexpectedly slow second order electron-hole recombination. Here, we explain this anomalous behavior by proposing that the band gap in CH₃NH₃PbI₃ has a direct-indirect character. Time-resolved photo-conductance measurements show that generation of free mobile charges is maximized for excitation energies just above the indirect band-gap. Furthermore, we find that second-order electron-hole recombination of photo-excited charges is retarded at lower temperature and thermally enhanced with an activation energy of 47 meV. These observations are consistent with a slow phonon-assisted recombination pathway via the indirect band-gap. Interestingly, this is not observed in the low-temperature orthorhombic phase, where fast electron-hole recombination occurs independent of the temperature and photon excitation energy. Our work elucidates the origin of a number of previously unexplained spectroscopic observations. Moreover, it provides a new framework to understand the optoelectronic properties of metal halide perovskites, rationalize their unique suitability for devices, and analyze spectroscopic data.

The hybrid perovskite CH₃NH₃PbI₃ (MAPI) exhibits long minority-carrier lifetimes and diffusion lengths[1]. We show that slow recombination originates from a slightly indirect gap as a consequence of relativistic effects. Large internal electric fields act on spin-orbit-coupled band extrema, shifting band-edges to inequivalent wavevectors, making the fundamental gap indirect[2]. From a description of photoluminescence within the quasiparticle self-consistent GW approximation for MAPI, CdTe and GaAs, we predict carrier lifetime as a function of light intensity and temperature. At operating conditions we find radiative recombination in MAPI is reduced by a factor of more than 350 compared to direct gap behavior.
The numerical prediction for minority-carrier lifetime agrees well with reported measurements on single-crystal MAPI. We also show that the indirect gap persists under a realistic description of disorder, by examining snapshots of the band edges from molecular dynamics simulations. Remarkably, the position of the band edge changes in time, as does the bandgap.
This study explains why hybrid halide perovskites have such long lifetimes, a prerequisite for efficient solar cells.

[1] Dong et al. Science 347,967 (2015)
[2] Brivio et al. Phys. Rev. B 89,155204 (2014)

The facile solution-processability of methylammonium lead iodide (CH3NH3PbI3) perovskites has catalyzed the development of inexpensive, hybrid perovskite based optoelectronics. It is apparent, though, that solution processed CH3NH3PbI3 films possess local emission heterogeneities, stemming from electronic disorder in the material. Herein we investigate the localized optical response of CH3NH3PbI3 films using spatially resolved and correlated emission/ absorption measurements. Variations in emission recombination dynamics are deduced from measurements of local emission intensity versus excitation intensity dependencies wherein such power-dependent measurements provide insight into local carrier kinetics, the existence of carrier trap states and their corresponding densities. What results is evidence for large spatial heterogeneities in carrier recombination dynamics under one sun excitation intensity conditions, wherein some regions of CH3NH3PbI3 films show predominantly bimolecular radiative recombination while others exhibit optical responses strongly influenced by carrier trapping. Of importance is that subsequent analyses enable quantitative estimates of nonuniform trap state distributions in CH3NH3PbI3 films as well as corresponding variations in emission quantum yields. Extracted local trap densities shows large spatial variations with values in the range of 10¹⁶ - 10¹⁸ cm^-3. Emission quantum yield (QY) at one sun excitation intensity are in the range of 4-8% for different locations in the films. The lower QY values are observed in the areas with higher trap densities. Of additional note is that the observed spatial extent of the optical disorder extends over length scales greater than that of underlying crystalline domains, suggesting the existence of other factors, beyond grain boundary-related nonradiative recombination channels, which lead to significant intrafilm optical heterogeneities. Also, we report the effect of the trap densities on the local electrical and emissive properties of the CH3NH3PbI3 solar cell devices.

Organolead trihalide perovskite solar cells have been attracting tremendous attention in both scientific and industrial communities in the past few years owing to the soaring efficiency achieved. However, the instability of hybrid perovskite materials to water and moisture arises as one of the major challenges to be addressed before any practical application.
Here we report a facile strategy that can simultaneously enhance the stability and efficiency of p-i-n planar heterojunction-structure perovskite devices ^[1]. Crosslinkable molecules with hydrophobic function groups are bonded onto fullerene to turn the fullerene layer into water-resistant. Methylammonia iodide is introduced in the crosslinked fullerene layer for n-doping via anion-induced electron transfer, resulting in dramatically increased conductivity by >100 times. With crosslinkable silane functionalized and doped fullerene electron transport layer, the perovskite devices deliver an efficiency of 19.5% with a high fill factor of 80.6%. Crosslinked silane modified fullerene layer also enhances the water and moisture stability of the non-sealed perovskite devices by retaining nearly 90% of their original efficiencies after 30 days exposure in ambient environment. This work has paved a new way towards addressing the main hindrance to the practical application of perovskite devices under ambient conditions.
[1] Bai, Y. et al., Enhancing Stability and Efficiency of Perovskite Solar Cells with Crosslinkable Silane Functionalized and Doped Fullerene. Nat. Commun., under review.

Lead-based halide perovskites have emerged as remarkable photovoltaic absorber materials in recent years. However, despite the extremely good performance in solar cells with efficiencies approaching 20%, fundamental issues remain to be addressed, including the toxic element lead (Pb) and instability under ambient moist air. Thus, here we present the synthesis and characterization of bismuth-based halide perovskites with the general formula Cs₃Bi₂I_9-xCl_x and study their optoelectronic properties to explore their potential in photovoltaic applications, providing a new family of Pb-free inorganic halide perovskites. The bismuth-based halide perovskites have advantages of low-toxicity, ambient stability, and low-temperature solution-processability, promising to solve the toxicity and stability challenges in organic-inorganic lead perovskites. The presence of Cl^- is essential to improve the material stability and enhance the perovskite crystallization. By changing the ratio of I/Cl, the optical absorption can be fine-tuned and with Cl^- a longer photocharge diffusion length has been observed, which is very important for photovoltaic applications. To investigate its photovoltaic performance, thin films were fabricated and deposition parameters were also studied to optimize integration of these materials into a photovoltaic device structures. The solar cell performance and currently limiting efficiency will be discussed to provide guidelines for further optimization and inspire more research work on developing lead-free inorganic peroviskites.

Lead halide perovskites such as methylammonium lead triiodide (CH₃NH₃PbI₃) have outstanding optical and electronic properties for photovoltaic applications, yet a full understanding of how this solution processable material works so well is currently missing. The material exhibits significant nanocrystallinity, defects and dynamic disorder; characteristics not normally associated with high efficiency photovoltaic devices. Using high energy resolution inelastic X-ray (HERIX) scattering, we measure phonon dispersions in CH₃NH₃PbI₃ and find an overlooked form of disorder in single crystals: large amplitude anharmonic zone-edge rotational instabilities of the PbI₆ octahedra that persist to room temperature and above, left over from structural phase transitions that take place tens to hundreds of degrees below. Phonon calculations show that the orientations of the methylammonium couple strongly and cooperatively to these modes. The result is a non-centrosymmetric, instantaneous local structure, which we observe in atomic pair distribution function (PDF) measurements. This local symmetry breaking is unobservable by Bragg diffraction, but can explain key material properties such as the structural phase sequence, ultra low thermal transport, and large minority charge carrier lifetimes despite moderate carrier mobility.

Perovskite on silicon tandem solar cells offer a path to high conversion efficiencies at low fabrication costs [1]. Research so far has focused on tandems with an n-type silicon bottom cell [2]. However, most industrially fabricated silicon solar cells today use p-type material. Enabling perovskite tandem solar cells with p-type silicon faces a number of challenges.
Because of the changed polarity of the silicon bottom cell, the perovskite top cell needs to be inverted; i.e. the hole transport layer and the electron transport layer need to switch places. Moving the electron transport layer to the top requires making this layer as transparent as possible for the entire wavelength range that silicon can use (300nm to 1200nm). Additionally, the tunnel junction needs to be redesigned. For an n-type silicon solar cell, we have demonstrated that a p++/n++ tunnel junction can be realized by recrystallizing amorphous silicon [3]. Inverting the polarity in this process is not possible, as phosphorous (used for p-doping in silicon) diffuses to the solar cell front. Therefore, a substitute for this tunnel junction needs to be found.
In this contribution we demonstrate a perovskite on p-type silicon tandem solar cell. The most critical component we developed was the tunnel junction. We compared n++/p++ Si, ITO/PEDOT:PSS and ITO/NiO_x tunnel junctions. We find that the diffuse tunnel junction is severely limited by the formation of an insulating SiO₂ layer during the oxygen plasma/UV-ozone treatment needed to make the surface hydrophilic for deposition of the solution-processed top cell. Here, we show how we overcame this limitation through a protective ITO layer. We compare p-type PEDOT:PSS vs. NiO_x for forming the tunnel junction. We find that NiO_x is highly advantageous by enabling a higher V_OC, and higher V_OC stability under illumination in ambient air.
[1] H. Snaith, Perovskites: the emergence of a new era for low-cost, high-efficiency solar cells, The Journal of Physical Chemistry Letters, 4 (2013), 3623-3630.
[2] C. Bailie et al., Semi-transparent perovskite solar cells for tandems with silicon and CIGS, EES 8 (3) (2015), 956-963.
[3] J. P. Mailoa et al., A 2-terminal perovskite/silicon multijunction solar cell enabled by a silicon tunnel junction, Applied Physics Letters 106 (2015), 121105.

Methylammonium lead halide perovskite solar cells have recently attracted great attention because of their excellent photovoltaic efficiencies and economic processing. In order to further improve planar heterojunction perovskite solar cells, two common strategies, interfacial engineering and chemical additives, have been proposed. Alternatively, Förster resonance energy transfer (FRET) via non-radiative dipole-dipole interaction had been successfully demonstrated in organic polymer solar cells and dye-sensitized solar cells to harvest excitons. Perovskite’s strong transition dipole moments imply the advantage of utilizing FRET. Here, we demonstrate FRET-based bulk-heterojunction perovskite solar cells that incorporate squaraine molecules. The high absorbance of squaraine in the near-infrared region overlaps with strong emission profile of the perovskite and gives a large Förster radius of 10.1 nm. Femtosecond spectroscopy studies reveal highly efficient (over 90%) excitation energy transfer from perovskite to squaraine occurring on a sub-picosecond timescale. We demonstrate a 20% increase in averaged power conversion efficiency to reach 13.7±0.3%. This architecture allows multiple donor materials to work synergistically by taking advantage of FRET, thereby enabling an improvement in light absorption and conversion.

Organic/inorganic hybrid perovskites are particularly promising materials for photovoltaic applications with an efficiency of more than 20%. While methylamonium-containing perovskites, e.g., CH₃NH₃PbI₃ and CH₃NH₃SnI₃, are of primary interest, the hybrid perovskite family is significantly diverse in terms of both the possible organic cations [1] and the structural motifs [2]. To accelerate the exploration of this family by emerging computational methods, we develop a dataset of more than 1,000 low-energy structures of organic/inorganic halide perovskites, containing 16 possible organic cations [3]. The structures in this dataset are determined by combining density functional theory computations with the minima-hopping structure predictions method. The band gaps of these hybrid perovskites are calculated at the HSE06 level [4], enabling a direct comparison with the experimental data. We will discuss the preparation of the dataset, our analysis and some insights that emerged from it.

References
[1] B. Saparov and D. Mitzi, Chem. Rev. 116, 4558 (2016).
[2] T. D. Huan, V. N. Tuoc, and N. V. Minh, Phys. Rev. B 93, 094105 (2016).
[3] C. Kim et al., in preparation (2016).
[4] J. Heyd, G. E. Scuseria, and M. M. Ernzerhof, J. Chem. Phys. 124, 219906 (2006).

Recently, methylammonium lead iodide (MAPbI₃) perovskite has attracted worldwide attention owing to its very high potential for solar cells, light-emitting diodes, and lasing applications. Comprehensive understanding of fundamental photoelectrical properties of MAPbI₃ is important to realize practical applications. Therefore, extensive studies have been devoted to investigating the photocarrier recombination and relaxation processes in MAPbI₃. Consequently, the photocarrier dynamics in the high-temperature tetragonal phase of MAPbI₃ is now well understood [1]. In contrast, there are only a few works examining the optical properties of the low-temperature orthorhombic-phase MAPbI₃ [2]. The exciton binding energy of orthorhombic-phase MAPbI₃ is about several tens of meV [3,4]. However, a dominant contribution of excitons to the recombination dynamics in orthorhombic-phase MAPbI₃ films has not been observed. The physical reason behind the lack of exciton features is still unclear and studies on single crystals are essential to solve this issue.
In this work, we revealed the photocarrier recombination dynamics in MAPbI₃ polycrystalline films and single crystals at low temperatures using photocurrent (PC), time-resolved photoluminescence (PL), optical transient absorption (TA), and THz TA spectroscopy. The data for thin films indicate that the high-temperature tetragonal phase remains even at low temperatures in addition to the major orthorhombic phase. A fast charge transfer from the orthorhombic phase to the tetragonal phase prevents the formation of excitons in the orthorhombic phase. Consequently, the recombination dynamics of photocarriers in both orthorhombic and tetragonal phases of MAPbI₃ thin films at low temperatures follow a free-carrier model rather than an excitonic model [5]. On the other hand, the PL and PC data of single crystals clearly show a dominant, sharp exciton peak at low temperatures. We discuss the exciton-phonon coupling and the exciton binding energy in single crystals.
Part of this work was supported by JST-CREST, JSPS KAKENHI (16F16017), the Collaborative Research Program of Institute for Chemical Research, Kyoto University (2016-18), and the MEXT Project of Integrated Research on Chemical Synthesis.

[1] Y. Yamada et al., J. Am. Chem. Soc. 136, 11610 (2014).
[2] R. Milto et al., Adv. Funct. Mater. 25, 2378 (2015).
[3] Y. Yamada et al., IEEE J. Photovolt. 5, 401 (2015).
[4] A. Miyata et al., Nat. Phys. 11, 582 (2015).
[5] L. Q. Phuong et al., J. Phys. Chem. Lett. 7, 2316 (2016).

Organic-inorganic hybrid solar cells based on organometal halide perovskite have recently emerged as one of the transformative photovoltaic technologies. With various interface engineering technologies, the power conversion efficiency has now exceeded over 20%. However, the potential toxicology issue of lead, which is a key component in the perovskite structure, is faced with the environment problem. And this problem hinders their market acceptance in future commercialization. To overcome this problem, finding suitable elements for replacing lead is needed.
In this work, by substituting lead to germanium, which is also the elements of group 14, fulfill the lead-free organic–inorganic germanium halide perovskite solar cells. Since the bandgap of perovskite structure is correlated with A-site cations, cesium was doped to A-site where methylammonium is located. Further, chloride was doped to X-site where halogens located to obtain the beneficial effect of highly conductive light harvesting material by combining two different halogens inside the perovskite crystalline structure. As a result, organic–inorganic germanium halide perovskite showed good semiconducting behavior and cesium and chloride co-doped methylammonium germanium iodide perovskite device showed clear photovoltaic effects.

Low-temperature processed perovskite solar cells are under immense interest due to their ease of fabrication and potential for mass production on flexible substrates. However, the stability issues remain one of the most vulnerable aspect in the perovskite based photovoltaic technologies. Our earlier studies revealed long-term stability over 6 months if stored in the ambient conditions. Some degree of degradation was obvious if they were kept under continuous illumination of 1 SUN. The degradation was so rapid for one the “p-i-n” structure of ITO/PEDOT:PSS/Perovskite/ PCBM/Ag. In one study, the power conversion efficiency (PCE) decreased from ~ 13% to less than 5% in 24 hours even with the glass encapsulations. Here, we report the improved stability of the “p-i-n” type perovskite solar cells using inorganic interface layers. For example, the replacement of one of the organic interface layer, PEDOT:PSS, in the “p-i-n” configuration with spattered NiO layer resulted in the continuous operation over 1000 hours at the maximum power point tracking (MPPT) under 1 SUN. Furthermore, the open circuit voltage can be improved by ~ 0.1V, and it is possible to achieve over 1 V with the structure of ITO/NiO/Perovskite/PCBM/Ag.

Recently, immense interests of organic-inorganic hybrid perovskite solar cells have been concentrated because of their high power conversion efficiency (PCE) which is higher than 21 %. However, for commercialization of perovskite solar cells, it is inappropriate to apply conventional spin-coating methods because of their critical limitations about continuous and large scale fabrication. Therefore, it is obvious that scalable and continuous fabrication method should be developed, such as, roll-to-roll printable blade-coating and spray-coating methods.
To relieve these problems, we have developed a novel spray-coating system using electrospray method under the ambient condition for realizing continuous and multi-scale fabrication of perovskite solar cells. Using high electro-potential, the electrospray-coating method can generate micro-sized monodisperse droplets as well as disject the droplets uniformly in a given area. Therefore, without any post process, the system can fabricate dense and uniform perovskite film. Furthermore, the system consumes very low perovskite solution for fabricating perovskite solar cells comparing to conventional spin-coating methods. Finally, high PCE (>12 %) device could be achieved after optimizing the electrospray-coating system.

Within the last few years metal halide perovskites have risen to become a very promising PV material, captivating the research community. In the most efficient devices, which now exceed 22% solar to electrical power conversion efficiency, the perovskite is present as a solid absorber layer sandwiched between n and p-type charge collection contacts. Increasing importance of improving solar cell operation is reliant upon understanding and controlling thin-film crystallisation and controlling the nature of the p and n-type contacts. In addition, understanding and enhancing long term stability of the materials and devices if a key driver. Despite the competitive efficiency, and assuming that stability challenges will be surmountable, for perovskites to feasibly enter the PV market, the commercial modules need to deliver something which other technologies cannot: Their unique selling point is ease of tenability of band gap which can deliver both hybrid and all-perovskite multifunction solar cells, with a feasibility of much higher efficiency than current commercial flat plate PV technologies. Here I will present our work on developing thin film perovskite solar cells, and specifically highlight recent advances in understanding the thin film crystallisation and enhancing the long term operational stability through compositional design of the perovskite, in addition to appropriate choice and adaptation of charge selective contacts. I will demonstrate efficient perovskite solar cells with band gaps ranging from 1.2 to 1.8 eV and show these materials integrated into hybrid tandem solar cells with silicon, in addition to all perovskite monolithic 2 terminal tandem modules. Finally I will discuss the further challenges to overcome before this technology is ready for production.

Organic-inorganic hybrid perovskites are a special class of low cost semiconductors that have revolutionized the prospects for photovoltaic and optoelectronics technologies. The inorganic chemistry of this class of materials is fascinating. These compounds adopt the three-dimensional ABX₃ perovskite structure, which consists of a network of corner-sharing BX₆ octahedra, where the B atom is a divalent metal cation (typically Ge²⁺, Sn²⁺ or Pb²⁺) and X is a monovalent anion (typically Cl⁻, Br⁻, I⁻); the A cation is selected to balance the total charge and it can be a Cs⁺ or a small molecular species. Such perovskites afford several important features including excellent optical properties that are tunable by controlling the chemical compositions, they exhibit ambipolar charge transport with high mobilities. Some members exhibit long electron and hole diffusion lengths. The fundamental similarities and differences between MeNH₃PbI₃, MeNH₃SnI₃ and MeNH₃GeI₃ perovskites as well as other low dimensional materials will be discussed. Another class of materials gaining significance are the two-dimensional (2D) perovskites -a blend of perovskites with layered crystal structure- (Ruddlesden-Popper type) offer a greater synthetic versatility and allow for more specialized device implementation due to the directional nature of the crystal structure. A remarkable advantage of the 2D perovskites is that their functionality can be easily tuned by incorporating a wide array of organic cations into the 2D framework, in contrast to the 3D analogues which have limited scope for structural engineering.

The discovery and development of the inorganic-organic halide perovskites (IOHP) as photovoltaic (PV) materials have been phenomenal not only in the PV research but also in the whole field of materials science. In less than five years, the power conversion efficiency (PCE) jumped from the initial 3.8% to 15%. In another two years, the PCE has exceeded 20%. Two of the remaining challenges for the deployment of IOHP-based PV technology are: 1) the stability of the material and 2) the environmental concern due to the use of lead. Intensive researches are currently undergoing to solve these issues.
Perovskite represents the first high-PCE (>20%) material that is not derived from the silicon structure, where all atoms are four-fold coordinated. In this sense, its success should not be viewed as just the success of one particular material. Instead, it could open the opportunity for a new class of materials to be discovered. In this work, we explore the chalcogenide-based perovskite materials in quest of suitable ones for PV applications. We considered the ABX₃ compounds, where A and B represent 2+ and 4+ cations, respectively, and X represents either S or Se. By using first-principles calculations with hybrid density functional, as well as quasi-particle GW calculations, we identified several compounds, namely, CaTiS₃, BaZrS₃, CaZrSe₃, and CaHfSe₃, which could possess a suitable direct band gap (1.0 – 1.7 eV) for making solar cells if they can be stabilized in the distorted perovskite phase [1]. Our recent experiment has successfully synthesized BaZrS₃. The band gap is found to be about 1.7 eV based on optical absorption and photoluminescence measurements [2], in agreement with our theoretical prediction. The defect properties of BaZrS₃, which affect the carrier lifetime and diffusion length, will be discussed and compared with those found in IOHP [3].
This work was supported by NSF DMR-1104994 and CBET-1510121.
[1] Y. Y. Sun, et al., Nano Lett. 15, 581 (2015).
[2] S. Perera, et al., Nano Energy 22, 129 (2016).
[3] M. L. Agiorgousis, et al., J. Am. Chem. Soc. 136, 14570 (2014).

Suitable optoelectronic properties and low-cost solution processability of hybrid organic-inorganic halide perovskite based semiconductors make them attractive materials for use in a wide range of optoelectronic devices. Here we present a two-step low pressure vapor-assisted solution process to grow high quality and phase pure CH₃NH₃PbI_3-xBr_x perovskite films over the full band gap range of 1.6 eV to 2.3 eV. Moreover, the CH₃NH₃PbI_3-xBr_x films show improved photo-stability compared to previously reported solution processed films. New insights into the optoelectronic properties of Br-containing hybrid organic-inorganic perovskites as a function of optical carrier injection are provided by using photoluminescence light-in versus light-out characterization techniques employing pump-powers over a six order of magnitude dynamic range. The internal luminescence quantum yield of wide band gap perovskites reaches impressive values up to 30% which translates into substantial quasi-Fermi level splitting, and high “optically implied” open-circuit voltage. Most importantly, both attributes, high internal quantum yield and high optically implied open-circuit voltage, are demonstrated over the entire band gap range (1.6 eV ≤ E_g ≤ 2.3 eV). These results demonstrate the versatility of Br-containing perovskite semiconductors for a variety of applications and especially for the use as high quality top cell in tandem photovoltaic devices in combination with industry dominant Si bottom cells.

Photon recycling, where recombination of electrons and holes generates photons that are absorbed elsewhere, plays an important role in highly efficient GaAs cells. Recently, the same effect has been demonstrated in perovskites, allowing for long charge extraction lengths, however its practical applications have not been assessed.
Here, we develop strategies to include photon recycling effects in drift-diffusion models of perovskite solar cell operation. One-dimensional and two-dimensional models are developed, using a photon gas approximation. These models are implemented for different cell geometries, including those with interdigitated back-contact electrodes and other back-contact designs. This allows the importance of photon recycling to be quantitatively assessed, and compared with experimental data from localised excitation of samples. Due to the bimolecular nature of radiative recombination in perovskites, photon recycling is found to play an important role at high illumination intensities.

Perovskite based solar cells, mostly employ solution processed perovskite layers. Evaporated methylammonium lead iodide perovskite layers have also been reported and been employed in solar cells. Our group has developed several perovskite based solar cells, using vacuum based perovskite preparation methods. These metal oxide free p-i-n type perovskite cells exhibit high power-conversion efficiencies. We have extended this work to fully evaporated perovskite devices reaching power conversion efficiencies as high as 20 % in a planar device. We directly compare n-i-p and p-i-n type structures using exactly the same materials. Additionally, using a multi-layer stack of different perovskite layers we generate tandem structures with high efficiency and open circuit voltage above 2.3 V.

Lead-halide perovskites have emerged as high-performance photovoltaic materials. In this work we demonstrate photon recycling in lead iodide perovskite solar cells [1]. We mapped the propagation of photogenerated luminescence and charges from a local photoexcitation spot in thin films of lead tri-iodide perovskites using a confocal microscopy setup with independent positioning of excitation and collection objectives. We observed regenerated PL emission at distances as far as 50 micrometers away from photoexcitation.

The internal photon distribution in the film was measured by monitoring the emission in a scratch in the film to increase out-scattering. We found that with increasing excitation transport distance the peak of the internal photon spectrum red-shifts from 765 to ≥800 nanometers. This is caused by the sharp decay of the absorption coefficient at the band tail, which allows longer wavelength photons to travel further between emission and absorption events, originating charges far from excitation. Thus, energy transport is not limited by diffusive charge transport but can occur over long distances through multiple absorption-diffusion emission events. We fabricated a lateral-contact solar cell with selective electron- and hole-collecting contacts, using a combination of photolitography and electrodeposition. We used these devices as a platform to study photocurrent propagation and found that charge extraction can be achieved well beyond 50 micrometers away from the excitation.

We connect these two observations by comparing the decay in intensity of the recycled component of the PL (which is around 765 nm) with the decay in photocurrent, taking into account that PL is proportional to the square of charge density, whilst photocurrent is proportional to charge density. Photon recycling leads to an increase in internal photon densities, which leads to a build-up of excited charges. This increases the split of quasi-Fermi levels and enhances the achievable open circuit voltage in a solar cell. Photon recycling allows for long charge extraction lengths through multiple absorption-emission events and contributes to the exceptionally high reported open-circuit voltages in perovskite solar cells.

1. Pazos-Outon, L. M., Szumilo, M., Lamboll, R., Richter, J. M., Crespo-Quesada, M., Abdi-Jalebi, M., Beeson, H. J., Vrucinic, M., Alsari, M., Snaith, H. J., Ehrler, B., Friend, R. H. & Deschler, F. ‘Photon recycling in lead iodide perovskite solar cells’. Science, 351, 1430–1433 (2016)

Lead-halide perovskite semiconductors CH₃NH₃PbX₃ (X=I, Br, Cl) have been attracting much attention as a new class of materials for optoelectronic devices. In case of thin-film solar cells based on perovskite CH₃NH₃PbI₃, the power conversion efficiencies have shown a rapid increase, now exceeding 22%. The excellent material properties of CH₃NH₃PbI₃, such as large absorption coefficients, long-lived free-carriers, and large carrier diffusion lengths, are cited as reasons for the high solar-cell power-conversion efficiencies [1,2]. In addition, intensive studies are performed also for other optoelectronic devices, such as photo-detectors, light emitting diodes, and lasers, because CH₃NH₃PbX₃ perovskites have good luminescence properties. A thorough understanding of the fundamental properties of CH₃NH₃PbX₃ is needed to achieve further improvements in optoelectronic device performance. So far, the optical properties of CH₃NH₃PbX₃ perovskites have been studied using thin-film samples, which usually exhibit grain structures. However, the diffusion and recombination processes of photocarriers should be affected by the polycrystalline grain structure in thin films. Therefore, optical experiments on high-quality single-crystal samples without grain structures are needed to clarify the essential characteristics of CH₃NH₃PbX₃ perovskites.

In this study, we investigated the optical spectra and photocarrier recombination dynamics of CH₃NH₃PbBr₃ single crystals to reveal their intrinsic properties using time-resolved two-photon excited photoluminescence (PL) spectroscopy [3]. We found that the PL decay dynamics in the near-surface region under one-photon excitation are different from those in the interior bulk region under two-photon excitation. Under one-photon excitation, the PL peak redshifted with time. On the other hand, the PL peak exhibited no red-shift with time under two-photon excitation. By changing the focal point of the excitation laser, we clarified the influence of re-absorption on the PL emitted from the interior region. As the distance between the focal point and the surface increases, the PL peak redshifts and the PL lifetime becomes longer. This behavior can be explained by considering carrier diffusion and photon recycling. The impact of photon recycling on the PL lifetime and carrier diffusion in CH₃NH₃PbBr₃ single crystals is discussed.

Part of this work was supported by JST-CREST.

[1] Y. Yamada, et al., J. Am. Chem. Soc. 136, 11610–11613 (2014).
[2] Y. Yamada, et al., J. Am. Chem. Soc. 137, 10456–10459 (2015).
[3] T. Yamada, et al., Adv. Electron. Mater. 2, 1500290 (2016).

Lead halide perovskite semiconductors are widely investigated for realizing highly efficient thin-film solar cells. Their power-conversion efficiencies increased steadily, now reaching up to 22%. In thin film solar cells, localized states such as defects and impurities cause carrier trapping, which leads to photocurrent (PC) losses. To understand how to improve the conversion efficiency, detailed understanding of localized states is important. Radiative localized states can be investigated by photoluminescence (PL) measurements, but non-radiative localized states are not understood sufficiently. In this study, we present a new high-sensitive detection technique and clarify the existence of non-radiative localized states near the band edge of lead halide perovskite semiconductor by comparing results from PL, absorption, and PC beat spectroscopies. PC beat spectroscopy is a method to determine the energy position of localized states. We have developed this spectroscopic technique to detect even a small number of localized states. The high sensitivity of this spectroscopy has been confirmed with the observation of radiative and non-radiative PC generation sites in semiconductors [1,2]. In the PC beat spectroscopy, a femtosecond phase-locked pulse pair is used as an excitation. The PC intensity is measured as a function of the time separation between the two pulses. It was shown that the time-resolved PC can show beating signals corresponding to the energy position of localized states [1,2]. The comparison between PL, absorption, and PC beat spectroscopies provides the details of the localized states. The investigated polycrystalline CH₃NH₃PbI₃ thin films showed a coexistence of two phases at low temperatures: the orthorhombic and tetragonal phases, where the orthorhombic phase was dominant [3,4]. An absorption edge was observed at the band edge of the orthorhombic phase. However, a PL peak was observed at the tetragonal phase. These spectra were not able to clarify the existence of localized states near the band edge. Using the PC beat spectroscopy, we successfully observed beating PC signals, which are evidence of localized states [5]. We found that the energy difference between the localized states and the band edge is a few tens of meV. We consider that the small number of localized states is caused by a low density of defects and impurities. Even a small number of defects can determine device performance. The removal of the small number of localized states can lead to the suppression of non-radiative trapping and improvement of the solar cell efficiency. Part of this work was supported by JST-CREST.

[1] H. Tahara and Y. Kanemitsu, Phys. Rev. B 90, 245203 (2014).
[2] H. Tahara and Y. Kanemitsu, Appl. Phys. Express 9, 032403 (2016).
[3] Y. Yamada et al., Appl. Phys. Express 7, 032302 (2014).
[4] L. Q. Phuong et al., J. Phys. Chem. Lett. 7, 2316 (2016).
[5] H. Tahara et al., J. Phys. Chem. C 120, 5347 (2016).

Organic-Inorganic perovskite absorber materials have drawn attention exponentially in the photovoltaic research due to the impressive improvements in the power conversion efficiencies of up to 20 % in just few years.⁽¹⁾ Numerous advanced properties are pointed out, among which materials are earth abundant, solution-processed active layers, and low-cost production. Different device configurations including mesoporous and planar (regular or inverted structures in both cases) succeed in achieving good device performances, being different interfaces between materials in such architectures. In particular, we discuss the important role of the interface between perovskite layer and n-type or p-type transport layer in a mesoporous-TiO₂ solar cells. To that end, a new current peak at forward bias in the dark current–voltage curves has been identified, which peak appears only under some specific experimental conditions.⁽²⁾ This behaviour was found in a standard perovskite solar cells with absorber layer methylammonium lead iodide CH₃NH₃PbI₃ and hole transport material (HTM) spiro-OMeTAD, and other materials were analyzed, achieving similar trends. When the solar cells were kept for several seconds under short-circuit conditions before starting the reverse measurement, it is considered that shallow and/or deep trap states located at the interface are dynamically filled during the reverse voltage scan. This uncommon diode shape disappears when the above experimental conditions are not applied. Therefore, in order to reduce these traps, the perovskite layer was passivated and consequently enhanced the overall cell performance from 14.5 % to 17.6 %. The passivation effect may prevent recombination processes at perovskite/HTM interface, being the charge recombination time longer from 0.3 μs for non-passivated perovskite solar cells to 60 μs for perovskite-passivated solar cells. It was expected that the most serious charge recombination occurs at interface between perovskite/n-type, where light is absorbed intensively and electron/hole densities are high. However, these studies corroborate that the interface between perovskite and p-type material should be paid particular attention.
References
⁽¹⁾A. M. Green, et al., Prog. Photovolt: Res. Appl., 2015, 23, 1-9.
⁽²⁾T. S. Ripolles, et al., Phys.Chem.Chem.Phys., 2016, 18, 14970.

In a search for lead-free materials that could be used as alternatives to the hybrid perovskites, (MA)PbX₃, in photovoltaic applications, we have discovered hybrid double perovskites, (MA)₂B^IBiX₆ (B^I = K, Tl, Ag and X = Cl, Br), which shows strong similarities to the lead analogues.
Solvent evaporation and hydrothermal methods were used for the synthesis. By optimization of synthesis conditions, we are able to make different hybrid double perovskites. Our first success was (MA)₂KBiCl₆,¹ but due to the highly ionic nature of K⁺ and Cl^-, it has a relatively large bandgap of 3.04eV. Materials with suitable bandgaps for photovoltaic applications have been sought through replacing K⁺ and Cl^- by Pearson softer ions such as Tl⁺, Ag⁺ and Br^-, I^-. Another hybrid double perovskite, (MA)₂TlBiBr₆, isoelectronic with MAPbBr₃, shows a direct bandgap of 2.16eV measured by UV vis spectroscopy,² and more importantly, the environmentally benign (MA)₂AgBiBr₆, gives an indirect bandgap of 2.0eV, comparable with MAPbBr₃ (2.2-2.35eV).^3,4 Hybrid double perovskites adopt 3D perovskite architectures and exhibit interesting phase transitions from cubic to rhombohedral, indicated by resonant ultrasound spectroscopy, single crystal X-ray diffraction and variable temperature synchrotron powder diffraction. Mechanical properties are very important for film formation. Nanoidentations on single crystals indicate that double perovskites show greater compliance than their corresponding lead analogues. The Young’s modulus of (MA)₂KBiCl₆ is smaller than that of MAPbCl₃ along the equivalent packing directions, and (MA)₂TlBiBr₆ is softer than MAPbBr₃, too. Similar mechanical behavior has been observed in ZIF compounds.⁵
These successes confirm the possibility of making new bromide and iodide double perovskites as potential light absorbers. Our findings provide a new and viable route towards lead-free, hybrid semiconductors.

1. F. Wei, Z. Deng, S. Sun, F. Xie, G. Kieslich, D. M. Evans, M. A. Carpenter, P. D. Bristowe and A. K. Cheetham, Mater. Horiz., 2016, DOI: 10.1039/c6mh00053c.
2. Z. Deng, F. Wei, S. Sun, G. Kieslich, A. K. Cheetham and P. D. Bristowe, 2016, arXiv:1606.02916.
3. T. Baikie, N. S. Barrow, Y. Fang, P. J. Keenan, P. R. Slater, R. O. Piltz, M. Gutmann, S. G. Mhaisalkar and T. J. White, J. Mater. Chem. A, 2015, 3, 9298–9307.
4. H. Bin Kim, I. Im, Y. Yoon, S. Do Sung, E. Kim, J. Kim and W. I. Lee, J. Mater. Chem. A, 2015, 3, 9264–9270.
5. T. D. Bennett, J.-C. Tan, S. A. Moggach, R. Galvelis, C. Mellot-Draznieks, B. A. Reisner, A. Thirumurugan, D. R. Allan and A. K. Cheetham, Chem. Eur. J., 2010, 16, 10684-10690.

Despite of the rapid improvement on the device efficiency of organic-inorganic lead (Pb) halide perovskite solar cells (PVSCs) in the past years,^[1-4] the use of of toxic Pb may severely limit their broad applications and commercialization. The development of Pb-free PVSCs is highly desirable and has drawn extensitive attention. Methylammonium tin iodide (MASnI₃) based Pb-free PVSCs with PCEs of around 6% have been reported in 2014, which have stimulated considerable follow-up studies on Pb-free Sn-based PVSCs.^[5-8] However, since 2014, no further improvement of the PCEs of Pb-free Sn-based PVSCs was reported.
We report on fabrication of efficient lead (Pb)-free planar tin (Sn)-based PVSCs, which consist of formamidinium tin triiodide (FASnI₃) as light absorbers. Along with solvent engineering, the use of tin fluoride additives is critical for synthesizing highly uniform and pinhole-free compact FASnI₃ perovskites with suitable carrier density. Our Pb-free FASnI₃ PVSCs have achieved average PCEs of 5.41±0.46% under forward voltage scan, exhibiting small J-V hysteresis and high reproducibility. The encapsulated cells stored in dark and glove box exhibited a good stability for 30 days, maintaining 85% of its initial efficiency when measured in ambient. Our work suggests that the planar architecture and solvent engineering are promising approaches to further enhance the performance of Pb-free PVSCs.

References
[1] D. P. McMeekin, G. Sadoughi, W. Rehman, G. E. Eperon, M. Saliba, M. T. Hörantner, A. Haghighirad, N. Sakai, L. Korte, B. Rech, M. B. Johnston, L. M. Herz, H. J. Snaith, Science 2016, 351, 151.
[2] W. S. Yang, J. H. Noh, N. J. Jeon, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Science 2015, 348, 1234.
[3] D. Zhao, W. Ke, C. R. Grice, A. J. Cimaroli, X. Tan, M. Yang, R. W. Collins, H. Zhang, K. Zhu, Y. Yan, Nano Energy 2016, 19, 88.
[4] D. Zhao, M. Sexton, H.-Y. Park, G. Baure, J. C. Nino, F. So, Adv. Energy Mater. 2015, 5, 1401855.
[5] F. Hao, C. C. Stoumpos, D. H. Cao, R. P. H. Chang, M. G. Kanatzidis, Nat. Photon. 2014, 8, 489.
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[7] N. K. Noel, S. D. Stranks, A. Abate, C. Wehrenfennig, S. Guarnera, A. Haghighirad, A. Sadhanala, G. E. Eperon, S. K. Pathak, M. B. Johnston, A. Petrozza, L. Herz, H. Snaith, Energy Environ. Sci. 2014, 7, 3061.
[8] S. J. Lee, S. S. Shin, Y. C. Kim, D. Kim, T. K. Ahn, J. H. Noh, J. Seo, S. I. Seok, J. Am. Chem. Soc. 2016, 138, 3974.

The cubic phase of CsPbI₃ has a band gap of 1.73 eV, absorbing most of the visible region of the solar spectrum. However, this structure only forms at high temperatures (310 °C) but transforms into the wide bandgap (3.01 eV) orthorhombic non-perovskite phase shortly after annealing. We have now developed a low temperature approach to form cubic CsPbI₃ through a halide exchange reaction using films of sintered CsPbBr₃ nanocrystals. The temperature dependent halide exchange process followed an Arrhenius relationship. The method also allows to obtain partially exchanged films featuring CsPbBr_xI_3-x compositional gradient that is customizable by adjusting film thickness and exchange time.. The charge carriers in these graded films are rapidly transferred to iodide rich regions near the film surface within the first several picoseconds after excitation. This ultrafast vectorial charge transfer process illustrates the potential of utilizing compositional gradients to direct charge flow in perovskite based photovoltaics.

The hybrid lead halide perovskites have risen to become some of the highest efficiency single-junction solar absorber materials, with the record cells now matching those of CdTe and CIGS above 20% efficiency.^1,2 Despite their success, the inclusion of lead remains a problem towards manufacture and commercialisation, with encapsulation required to limit environmental damage. An avenue for lead-free materials that retain the successful perovskite framework is to move from the APbX₃ (A = MA, Cs) formula to the double perovskite A₂MBiX6 (A = Cs, M = Ag, Cu, Au), replacing lead with the much less toxic bismuth. So far this year, multiple groups have already synthesised and examined members of the Cs₂AgBiX₆ family: McClure et al. synthesised both the chloride and bromide,³ Slavney et al. have shown that Cs₂AgBiX₆ has a long photoluminescence lifetime comparable to MAPI,⁴ and Volonakis et al. calculated the band gaps of hypothetical members of the Cs₂MBX₆ family (M = Cu, Ag, Au; B = Bi, Sb; X = Cl, Br, I) using hybrid density functional theory (DFT).⁵ Ultimately, however, Cs₂AgBiCl₆ and Cs₂AgBiBr₆ both possess indirect band gaps above 2eV, too high to see use in single-junction solar cells.

In this study, we examine the Cs₂MBiX₆ family using hybrid DFT and fundamentally evaluate their electronic structure, with particular attention to their comparison with the APbX₃ perovskites, and how inherent orbital interactions might significantly affect their optical behaviour and suitability as photoabsorbers.⁶ We also consider the effects of thermodynamic stability and cationic disorder, and their consequences on the applicability of these systems. As such, we hope to provide an outlook for the future of the double perovskites in photovoltaics and potential directions for experimental research in the area.

(1) Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; Dunlop, E. D. Prog. Photovoltaics Res. Appl. 2016, 24, 3.
(2) Brenner, T. M.; Egger, D. A.; Kronik, L.; Hodes, G.; Cahen, D. Nat. Rev. Mater. 2016, 1 (1), 15007.
(3) McClure, E. T.; Ball, M. R.; Windl, W.; Woodward, P. M. Chem. Mater. 2016, 28 (8), 1348.
(4) Slavney, A. H.; Hu, T.; Lindenberg, A. M.; Karunadasa, H. I. J. Am. Chem. Soc. 2016, 138 (7), 2138.
(5) Volonakis, G.; Filip, M. R.; Haghighirad, A. A.; Sakai, N.; Wenger, B.; Snaith, H. J.; Giustino, F. J. Phys. Chem. Lett. 2016, 4, 1254.
(6) Savory, C. N.; Walsh, A.; Scanlon, D. O. 2016, In submission.

Tandem solar cells offer a promising avenue for increasing the efficiency of existing solar cell technologies with a minimal cost increase. A top cell material with a bandgap around 1.8 eV that can be deposited inexpensively at a large scale could enable the manufacture of highly-efficient tandems with a silicon bottom cell, and since processing could be integrated into the silicon manufacturing infrastructure that is already in place, such technologies are very commercially attractive. Solution-processable organic-inorganic halide perovskites have recently generated considerable excitement as absorbers in single-junction solar cells due to their excellent electronic properties including low exciton binding energies, long range hole and electron diffusion lengths, and high carrier mobilities, but materials engineering is needed to optimize other key properties before they can be employed in high-efficiency, commercially viable tandem technologies. While it is possible to tune the bandgap of (CH₃NH₃)Pb(Br_xI_1-x)₃ between 2.3 and 1.6 eV by controlling the halide concentration, optical instability due to photo-induced phase segregation limits the voltage that can be extracted from compositions with appropriate bandgaps for tandem applications. Moreover, these materials have been shown to suffer from thermal degradation at temperatures within the processing and operational window. By replacing the volatile methylammonium cation with cesium, we synthesize a mixed halide absorber material with appreciably improved thermal stability, a stabilized photoconversion efficiency of 6.5%, and a bandgap of 1.9 eV. We demonstrate that the addition of Br structurally stabilizes the CsPbBr_xI_1-x perovskite phase, which had previously been shown to be unstable at room temperature, and that the range of mixed halide compositions over which the material is stable to photo-induced phase segregation is wider than in (CH₃NH₃)Pb(Br_xI_1-x)₃. Thus with material and processing optimization, further bandgap tuning and higher device performance are within reach. We note that mixed cation phases which include small amounts of Cs have demonstrated the highest single-junction cell efficiencies to date, so further work towards understanding and tuning the influence of the inorganic cation on the photophysical properties of the perovskite could be the key to developing a thermally and optically stable top cell material that paves the way for the future of low-cost photovoltaics.

Metal halide perovskite solar cells have rapidly reached performances rivaling those of commercial silicon and thin film photovoltaic technologies. The best devices have bandgaps of around 1.6 eV, but the bandgap of the lead-based perovskites can be readily tuned from 1.5 – 2.2 eV by substituting halides, enabling the use of wide gap perovskites in tandem solar cells with silicon or CIGS rear cells. Such high efficiency tandem approaches offer extremely promising routes to supplying photovoltaic energy at a large scale. Still, they will suffer from the same manufacturing and deployment limitations as those faced by the Silicon or CIGS rear cells. The ultimate solution would be to make efficient perovskite – perovskite tandems that rival the performance of perovskite – silicon tandem solar cells but benefit from the processing advantages of perovskite solar cells. These should be able to reach up to ~35-40% PCE in theory, with correctly chosen bandgaps. While high bandgap (1.5 - 2.2 eV) perovskite materials are already used in high performance solar cells, good photovoltaic performance has not yet been achieved when using small gap (< 1.3 eV) perovskites, such as would be suitable for a rear cell in a tandem configuration.
Here, we explore the use of small gap perovskite materials based on binary mixtures of lead and tin. By tuning the A and B site composition of the ABX₃ perovskite structure and developing a new processing route to form smooth continuous films over large areas, we are able to make highly efficient (13.6 %) planar heterojunction solar cells with small (~ 1.2 eV) bandgaps, suitable for all-perovskite tandem solar cells. Previous reports of tin based perovskite solar cells have demonstrated these to be extremely unstable under operation and in air. We however find that our mixed perovskite is remarkably stable, showing no signs of degradation under operation in air for over an hour. We will discuss preliminary results of monolithic and mechanically stacked tandems with wide gap (1.8 eV) perovskite top cells, which demonstrate that our small gap solar cells should be ideal for use in all perovskite tandem solar cells.

Since the first report in 1991 of the highly efficient dye-sensitized solar cells (DSC), mesoscopic solar cells have been attracting great attention for the high energy conversion efficiency and the low cost of materials and fabrication process. A recent surging example is the perovskite-mesoscopic solar cell (PSC), which had been first studied by applying organolead halide perovskite as inorganic sensitizers in DSC and later as a thin-film light absorber and/or charge transport material.
In this presentation, starting from the fundamental understanding of the device equivalent circuit, I will demonstrate that chemically decorating pervoskite grain surface or the mesoporous scaffold surface by bifunctional ammonium surfactants allows controlling the crystal growth and substantial enhancement in device performance and durability. Then I will emphasize a universally useful idea for film morphology control to prepare high-quality perovskite films to improve the reproducibility of high efficiency with scalability. Finally, I will introduce a strategy of using triple-layered architecture to improve the device stability of PSC.

Metal halide perovskite have drawn immense attention specifically due to their continuously remarkably increasing power conversion efficiencies in photovoltaics reported in recent years. Owing to their relative low processing cost, numerous of activities with these materials have been conducted, although still fundamental thin film properties are not fully controllable.
Despite a massive and a broad range of exploitation the complexity of controlling the properties of these particular materials has emerged to controversies. Furthermore, the nature of various interactions and structural disorder has led to an intricate challenge to be resolved by the vital spectroscopic techniques. Consequently, our objective is to bridge the device performance on presence of PbI₂, hence, selected techniques were adopted for the quantification of elementary photoexcitation processes.
In current study, we have examined the charge carrier dynamics by means of ultrafast transient absorption spectroscopy as the lead-iodide concentration in the perovskite is varied on CH₃NH₃PbI₃ material. More specifically, the associated recombination dynamics have been thoroughly investigated. We found a correlation when varying the concentration of the lead-iodide as the carrier dynamics is significantly reduced on the perovskite/TiO₂ surface and additionally the device performance is improved. However, an increased lead-iodide amount of concentration does not contribute to a significant reduced carrier recombination. Moreover, our spectroscopic and characterization results confirm structural modifications, morphology alterations and crystallization changes. Our accomplishments may lead to an enhanced knowledge and control of the chemical constituents and the photo physics of perovskites potentially leading to an enhanced understanding of improved device architecture and performance.

Transparent conductive oxides, FTO and ITO, have been variously used for solar cells, which are continuing to request the supply of quality FTO and ITO for their performance improvement.
A spray pyrolysis deposition (SPD) is one of chemical wet processes for thin film formation or surface coating on a substrate. SPD is mostly carried out using double fluids of raw solution and compressed air at room temperature under an atmosphere, where gives tailored thin films from atomized raw material solutions using simple apparatus with easy operation. The spraying operation in our technique is done not consecutively but intermittently for suppressing remarkable reduction of a prescribed substrate temperature between 200 and 600 °C, depending on the component and composition of thin film deposited [1-3].
The coating of 150 mm square and 800 nm thick FTO with a visible light transmittance 81% and a sheet resistance 7.4 ohm/square on 1.1 mm thick glass substrate was achieved by the above-mentioned SPD from a mixed ethanol solution including di-butyl tin (IV) di-acetate and ammonium fluoride after optimizing deposition conditions such as substrate temperature, intermittent time, rotation of substrate, nozzle moving velocity, and distance between nozzle and substrate. High quality of the FTO was assured also from small numerical deviations of transmittance and sheet resistance data measured at 36 different locations. Furthermore, we could obtain almost the same specifications of transmittance 81%, sheet resistance 7.1 ohm/square and small deviation each in 300 mm square FTO (Film thickness 0.86 mm) after discussed on enlargement. It has been proved that our spray pyrolysis deposition is suitable for the preparation of quality transparent conductive FTO. The surface roughness and electronic properties of FTO obtained in this study will be discussed from AFM observation and Hall measurement, respectively.

[1] I. Yagi and S. Kaneko, Chem. Lett., 1991, 156-9.
[2] K. Murakami et al., J. Am. Ceram. Soc., 79 (1996) 2557-62.
[3] E. V. A. Premalal et al., Thin Solid Films, 520 (2012) 6813-17.

Composite Interfacial Layer for High Performance Flexible and Semi-transparent Printed Perovskite Solar Cells
Jie Wang, Hui Lu, Lianping Zhang, Jian Lin, Na Wu, Yiling Wang, Xiaorui Jia, Qun Luo,^* Chang-Qi Ma
[email protected]
Printable Electronics Research Center, Suzhou Institute of Nano-Tech and Nano-Bionics,
Chinese Academy of Sciences (CAS), Suzhou, 215123, P. R. China.
In the organic or organic-inorganic hybrid perovskite solar cells, interfacial layer plays an important role with the functions of interfacial work function modification, substrates smoothening, carrier selectively extraction, and etc. Previously, we have developed a series of composite interfacial layer for application in organic solar cells, ^[1-4]. The results show the attraction of composite as potential interfacial materials for high efficiency and stable organic optoelectric devices.
Recently, we found that the composite interfacial layers also work well in the flexible or semi-transparent printed perovskite solar cells. ^[5-7] With a work function tunable composite interfacial layer, flexible perovskite solar cells with high conversion above 14% was obtained using the Ag grid as the bottom electrode.^[6] In the meanwhile, we also fabricated semi-transparent printed perovskite solar cells of 12% with Ag nanowire top electrode though inkjet printing. ^[7] In this device, this composite interfacial layer can effectively prevent solvent corrosion of perovskite films upon printing the Ag nanowire on the top of perovskite film, as well as effectively improve charge extraction efficiency through work function regulation. In this report, we will introduce our work on the fabrication of high performance flexible and semi-transparent printed perovskite solar cells.

References
Na Wu, Qun Luo, Chang-Qi Ma, et al. Sol. Energy Mat. Sol. Cells, 2015, 141, 248-259.
Yiling Wang, Qun Luo, Chang-Qi Ma, et.al. ACS Appl. Mater. Interface 2015, 7, 7170-7179.
Hui Lu, et. al. Qun Luo, Appl. Phys. Lett. 2015, 106.
Na Wu, Qun Luo, Zhongmin Bao, Yan-Qing Li, Chang-Qi Ma, Under revised.
Xiaorui Jia, Lianping Zhang, Qun Luo, et. al. ACS Appl. Mater. Interface Accepted.
Jie Wang, Qun Luo, et. al. Manuscript under preparation.
Hui Lu, Qun Luo, et. al. Manuscript under preparation.

In this study, we analyzed halide ion substitution reaction in CH₃NH₃PbX₃(MAPbX₃, X = I or Br) films, which was performed by dipping the films in MAX in 2-propanol solution. To determine the reaction rate constant for the halide exchange reaction, reaction time and concentration of MAX solution was varied while the composition of resulting film was determined by UV-vis absorption and photoluminescence spectra. From the determined composition depending on reaction time, order and rate constant of the reaction was investigated. Both of the reactions to form MAPbBr₃ from MAPbI₃ (reaction 1 : MAPbBr₃→ MAPbI₃) and MAPbI₃ from MAPbBr₃ (reaction 2 : MAPbI₃→ MAPbBr₃) are found to be first order reaction, in which rate constants at 0.126M MAX solution are calculated to be 3.24 x 10^-2 s^-1 and 4.62 x 10^-5 s^-1 respectively. Much faster rate constant for reaction 2 might be due to higher solubility of MAPbI₃ in 2-propanol compared to MAPbBr₃. Morphology dependent reaction kinetics is investigated by comparing reaction rate constant of reactant film with and without scaffolding oxide film. The reaction occur faster with scaffolding oxide, which imply the halide exchange reaction occur from grain boundary to interior. Finally, based on established reaction model, a novel perovskite film with locally controlled halide composition is demonstrated.

A dramatic increase in the development of a new class of solar cell based on mixed organic-inorganic halide perovskite over the past few years has been observed. Commencing from mid-2012 up to 2015, a range of 3.8% to 20% efficiency for perovskite solar cells has been reported in literature. This substantial increase of efficiency has captured the attention of the photovoltaic research community.
One of the key components of perovskite solar cell fabrication is the counter electrode. Carbon materials have been exploited in addressing cost-effectiveness of the fabrication process to replace expensive Au-based counter electrode. In this study, the electrical performance of the carbon counter electrode (CCE) was probed by electrochemical impedance spectroscopy (EIS) with varying carbon compositions and different deposition methods: spin-coating and spray deposition. EIS was performed on symmetric and asymmetric sandwich type cells with and without perovskite in order to examine the contribution of perovskite in the performance of CCE. Results show that the carbon black-biomass composite has the highest resistance in the symmetric sandwich design (R=160.6 Ω cm^-2) as compared to the commercially sold carbon (R=84.25 Ω cm^-2) with and without perovskite. Factors such as inefficient packing and low surface area may cause the increase of resistance. Additional characterizations were done by SEM and XRD.

Perovskite solar cells (PSCs) have recently emerged as promising cost-effective and high-efficiency solar cells. Unfortunately, power conversion efficiencies (PCE) of PSCs is caused to decrease by degradation in the air. It is thought the degradation of perovskite layer is their weakness to moisture and oxygen included in the air. Therefore, we introduce the system is no air exposure from fabrication to measurement, which feature is globe box connected into evaporation chamber. In this work, we try to detect the influence of air exposure during fabrication steps.
Device structure is ITO / compact-TiOx / CH₃NH₃PbI₃ (MAPbI₃) / spiro-OMeTAD / Au. MAPbI₃ was deposited by sequential vacuum deposition of PbI₂ and CH₃NH₃I (MAI) layer, not co-evaporation method. This films were rinsed with 2-propanol, which due to remove MAI remains on MAPbI₃. We clarify the influence of perovskite solar cells which exposed to the air for 30 min after MAI deposited, isopropanol washed or spiro-OMeTAD deposited, respectively. We obtained PCE of 4.09% by no air exposure, whereas PCE of air exposure is 0.45% after MAI deposited, 3.62% after 2-propanol rinsed and 2.48% after spiro-OMeTAD deposited. These result indicated that all solar cell performance under air exposure is reduced compared with reference of no air exposure. Especially, PSC exposed after MAI deposition is damaged deeply. This result suggests that the MAI remains on perovskite surface are trigger of degradation.
To clarify the influence of air exposure on MAPbI₃ films, we tend to introduce the O₂ or (H₂O+N₂) gases in cylindrical holder within which deposited film once MAI deposition is placed. Analysis of the film revealed that the UV-vis spectrum of O₂ atmosphere condition was almost same as that of reference while not air exposure. In contrary, the UV-vis spectrum of (H₂O+N₂) atmosphere condition was significantly decreased at light absorption region from 400 to 700 nm. The AFM images show that 300 nm uniform particle shaped grains were observed in MAPbI₃ films while not air exposure and with O₂ atmosphere conditions. In contrary, the film exposed in (H₂O+N₂) atmosphere has morphology with non-uniform 1 µm large grains. Hence, these results indicate that MAI remains on MAPbI₃ surface became rough by H₂O molecules intercalation into MAPbI₃ film. The XRD pattern shows the main diffraction peak at around 14 degrees assigned to the (110) crystal plane, while other diffraction peaks at around 10 and 12 degrees assigned to the (001) planes of MAI and PbI₂, respectively. The MAI crystalline plane (001) was observed in reference without air exposure and with O₂ atmosphere conditions. On the other hand, the MAI diffraction peak (001) disappeared during exposure in (H₂O+N₂) atmosphere condition. We assume that the reacted MAI molecules with H₂O molecule act as a new organic molecule with another chemical and physical property. Therefore, MAI diffraction peak (001) was disappeared and band gap of film was changed.

Hybrid lead-halide perovskites have recently led to a step-change in the power conversion efficiency of solution-processed photovoltaics and hold great promise for optoelectronic devices in general. Here, we address the discrepancy between reports on low external photoluminescence quantum yields (PLQE) in thin films of ~20% [1], and recent reports on laser cooling [2], which require high internal quantum yields, well above 50%.

Contrary to previous reports, which attributed low PLQEs mostly to trap assisted recombination, we find that these low values mainly originate from light trapping due to the high refractive index of lead halide perovskites: only 7% of generated light directly leaves the perovskite film. The effect of trap assisted recombination on internal PLQEs is therefore much smaller than previously reported.

We measure external PLQEs of ~60% in textured films, which optimize light out-coupling, compared to values of ~20% in planar samples. We discuss in detail how external and internal PLQEs are affected by the process of photon recycling, which we recently demonstrated in lead-halide perovskites [3]. For this, we study the photoexcited carrier dynamics and use a rate equation to relate radiative and non-radiative recombination events to measured PLQEs.

Obtaining high external quantum yields has been reported as one of the crucial parameters to boost the performance of highly efficient photovoltaic devices [4]. We discuss how similar approaches of surface texturing can lead to performance boosts of both perovskite-based solar cells and LEDs.

References
[1] S. D. Stranks et al., Phys. Rev. Appl. (2014), 2, 034007.
[2] S.-T. Ha et al., Nat. Photonics (2016), 10, 115–121.
[3] L. M. Pazos-Outon et al., Science (2016), 351, 1430-1433.
[4] O. D. Miller et al., IEEE J. Photovoltaics (2012), 2, 303-311.

Perovskite-based materials have become one of the most promising and rapidly developing materials for optoelectronic applications during the last few years. They combine almost perfect optoelectronic properties with the ease of fabrication by a variety of methods, whereby both solution- and evaporation-based processes have already been reported. These properties allow for large scale production and market maturity in the foreseeable future. The current focus of perovskite research is in the field of photovoltaic applications. However, other fields of application like light emitting diodes, lasers, and photodetectors are currently being investigated as well.

At the moment most research groups prepare perovskite-based thin-films by solution-processes. Even though these processes promise a cheap way of fabrication with readily available equipment, the strong interaction between solids, solvents, and the environment leads to crucial complications. Therefore, the processing of devices with reproducible material and layer properties requires an inert gas environment increasing thereby the equipment costs and production expenses. In our group we recently started to investigate the deposition of perovskite-based layers by thermal evaporation whose main benefits compared to the solution-based processes are the decoupling of the raw material from the environment, the elimination of solvents, and a strong reduction of the number of process parameters.

The focus here will be on the most common metal-organic compound CH₃NH₃PbI₃. Preparation is planned to be carried out either as a single vacuum process based on the co-evaporation of the organic and the inorganic halide or as a mixed two-step process starting with the evaporation of the inorganic halide followed by the conversion to perovskite in a solution containing the organic halide. The different processes will then be investigated in regards of their optical, electrical, morphological, and compositional properties and compared with the layers prepared by a single-step solution-process. Of particular interest are the compositional differences of these films which will be investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS), to get both lateral and depth information of the samples. In the end the suitability of thermal evaporation for the deposition of solar cell absorbers will be examined by preparing and analyzing perovskite-based solar cells in terms of their performance, stability and hysteretic behavior.

Evaporation is less sensitive with regards to variations in the structure and composition of the substrate. Therefore, we will also investigate the evaporation on structured materials which are important for optically pumped distributed feedback lasers (DFB). DFB-lasers for optical emission in the visible spectral range will also be prepared, investigated, and finally compared with the characteristics of the corresponding solar cell absorbers.

While the power conversion efficiency (PCE) of solar cells based on organo-lead halide perovskites (PSCs) has skyrocketed to levels of >20%,^[1] concerns about intrinsic and extrinsic stability are still intimately linked to perovskite photovoltaic technology.^[2] For example, the most commonly used perovskite methyl-ammonium lead iodide (CH₃NH₃PbI₃) shows stability issues in the presence of humidity and at elevated temperatures. Secondary effects of perovskite decomposition such as the degradation of functional building blocks in the solar cell due to decomposition products, like HI or CH₃NH₃I, which lead to the corrosion of metal electrodes like Ag or Al has been identified as a critical issue.^[3] Overall, without proper concepts to overcome these reliability issues, the prospects of wide-spread application and commercialization of organo-lead halide perovskite technology may be significantly compromised.
We propose inverted PSCs with a bi-layered electron extraction layer (EEL) of Al:ZnO and SnO_x, which substantially suppresses the decomposition of the perovskite both in ambient air and under elevated temperatures.^[4] The Al:ZnO is deposited from a NP dispersion and serves as electron extraction layer and as a nucleation layer for the SnO_x. As we have shown, the SnO_x layers which were grown by atomic layer deposition (ALD) are unique in a sense that they are optically highly transparent with a band gap of 3.8 eV, electrically conductive and provide outstanding gas permeation barrier properties.^[5] As evidenced by X-ray diffraction and X-ray photo-electron spectroscopy, our EELs effectively hinder the ingress of moisture towards the perovskite and - more importantly - they prevent the egress of decomposition products, like e.g. CH₃NH₃I, HI out of the perovskite.
While the efficiency of cells based on LiF/Al or AZO degraded to less than 50% of the initial value within less than a day in ambient air, devices based on AZO/SnO_x showed superior stability even after more than 350 h. Under inert atmosphere, AZO based devices degraded to roughly 50% of its initial efficiency within 100 h due to thermally induced decomposition of the perovskite. On the contrary, the suppressed out-diffusion of decomposition products due to the diffusion-barrier properties of the AZO/SnO_x EEL significantly slows down the decomposition of the perovskite even after hundreds of hours. The concept of impermeable EELs is generally applicable and therefore is expected to provide an avenue to achieve a substantially improved device lifetime of solar cells based on organo-metal halide perovskites even beyond MAPbI₃.

[1] W. S. Yang et al., Science 2015, 348, 1234.
[2] T. Leijtens et al., Adv. Energy Mater. 2015, 5, 1500963.
[3] Y. Kato et al., Adv. Mater. Interf. 2015, 2, 1500195.
[4] K. Brinkmann et al., Nat. Comms. (submitted).
[5] A. Behrendt et al., Adv. Mater. 2015, 27, 5961

Organic-inorganic hybrid perovskite solar cells (PSCs) have rapidly become candidates for large scale solar deployment, due to their demonstrated lab based power conversion efficiencies of >20%, ease and diversity of processing methods and their assumed low cost. One of the promising applications for PSCs is in tandem devices, combining silicon and perovskite absorbers.
Recently we have shown that PSC combined balance of module costs can range from $1-0.5/Wp with the current state of art efficiencies and materials sets proven in literature. We have further shown that theoretically a rigid glass-glass PSC module could reach as low as $0.28/Wp using demonstrated lab efficiencies and processing improvements.
However so far little work has been done on analysing the embedded cost associated with incorporating a PSC device in tandem structure with a silicon module. Current targets aim for between a 3-7.5% PSC device to coated atop of a silicon module at an added cost of ~$6. Overall this would see the cost of a silicon module drop to ~$0.35/Wp, representing a significant drop in price compared with the marginal decrease predicted before 2020 for silicon modules alone. However while the cost contribution of the perovskite photoactive layer is relatively small the materials cost for of the additional layers combined with the added cost of deposition and additional encapsulation required raises significant questions of if and how the PSC-silicon tandem cost-efficiency targets could be met.
Here we present an optical-cost model to analyse a range of different materials, architecture and processing parameters. With this model materials can be selected for each of the layers and a deposition method, speed and yield chosen as well as additional treatments and associated module costs added. We consider a range of different transparent conducting layers in the PSC module, as well as chemically differing perovskite photoactive layers and hole & electron transport materials. Each of these are in turn modelled for transmission through the layers based on materials absorption, thickness and aperture. From this the efficiency of the tandem device can be modelled using optical and electrical constants, backed up with real world data.
From this model we are able to highlight the key cost contributing areas such as evaporation steps and materials such as Indium, which are combined with the predicted power output to give an overall balance of tandem module cost for a range of scenarios. This then allows for the suggestion of how the cost may be saved through accurate layer design such as decreased thickness or materials selection and how this will affect the efficiency, and ultimately the $/Wp.
Finally we consider all potential scenarios to establish whether there are combinations of materials sets and deposition techniques already known that can achieve a target costs and analyse the viability and inception into the market of a PSC-silicon tandem module.

Organometal trihalide perovskites (OTPs), such as CH₃NH₃PbX₃ (MAPbX₃), HC(NH₂)₂PbX₃ (FAPbX₃) (X=I, Br, and Cl), and mixed perovskites, are considered to be a central photovoltaic material owing to their peculiar properties and remarkable increase of device efficiency in past several years. Their device efficiency is already exceeded 20%, comparable to those of currently commercialized inorganic solar cells. However, the OTP solar cells are still far from being commercial viability because they have experienced critical problems, such as stability, toxicity, photocurrent hysteresis, interfacial degradation, etc. Thus, current works have mainly focused on finding solutions to address such issues. Along with these efforts, many researchers have attempted to develop a simple, versatile, and generic solution approach for achieving reliable and reproducible OTP solar cells. Here, we introduce a simple and versatile approach based on OTP crystalline powders for high-performance OTP solar cells. The powders were simply synthesized by dispersing OTP precursor solution in anti-solvents. Then they were used as starting chemicals for one-step solutions methods. Their structural phase and compositional ratio were controlled by tuning variable parameters, such as input ratio, polar solvents, and anti-solvents. Through such powder engineering, we could not only fabricate various OTPs, such as MAPbX₃, FAPbX₃, and (MA,FA)PbX₃, crystalline powders with different phases and composition, but also achieve high-device performance with exceeding 16% from various OTPs solar cells. In this talk, we will discuss the formation behaviors of powders and their correlation with the device performance.

Halide perovskites (mainly methylammonium lead iodide (MAPbI₃) and its bromide analog, MAPbBr₃) are the next wave of light-harvesting materials for solar cell applications, with the best cells showing certified solar to electrical energy conversion efficiencies over 22 %. The origin for such outstanding performance intrigues the scientific community for the last couple of years. Ferroelectricity has repeatedly been suggested as a possible reason for some of the outstanding properties, especially the low carrier recombination rate and high voltage efficiency. Classical measurements to (dis)prove ferroelectricity require high electric fields, which may give rise to experimental artifacts for these materials, because of possible ion migration and the materials’ low formation energies.² Since a necessary condition for a material to be ferroelectric is that it will have a non-centrosymmetric and polar nature (point group), we examine these two prerequisite conditions via careful Second-Harmonic-Generation (SHG) polar mapping and pyroelectricity characterization (using the Chynoweth method¹). These two experimental methods are more accurate than X-ray- or neutron- diffraction methods, which are limited in explicitly determining small deviations in (non-)centrosymmetric structures.
After reporting³ a clear conclusion on the non-ferroelectric nature of MAPbBr₃ (which will be presented as well), we continue our investigation on MAPbI₃ and will report our most recent results on it (which, so far, does show some differences with MAPbBr₃ ).


1.Fan, Z. et al.; J. Phys. Chem. Lett. 6, 1155–1161 (2015).
2.Lubomirsky, I. & Stafsudd, O. ; Rev. Sci. Instrum. 83, 051101 (2012).
3.Rakita, Y. et al. ;APL Mat. 4, 051101 (2016)

Metal halide perovskite based solar cells have made tremendous progress, with record efficiencies beyond 20%. Beside leading to very efficient devices, hybrid perovskites are also extremely versatile, for example their bandgap can be easily modified by controlling their composition. Most efforts have been directed towards the developments of small bandgap absorber for efficient single junction solar cells, while wide bandgaps systems have been studied only on a limited basis. Realizing efficient solar cells with bandgap of around 2 eV would enable the fabrication of perovskite-perovskite tandem solar cells with efficiencies exceeding 23%, exceeding the efficiency of state of the art single junction devices. In this work, we present efficient perovskite-perovskite tandem structures prepared through a combination of solution and vacuum deposition methods. The resulting cells are more efficient than each of the perovskite subcells used, with high fill factor and photovoltage. This work demonstrates how perovskite-perovskite tandems technology is a potential candidate for the next generation, high efficiency, perovskite solar cells.

Hybrid (organic-inorganic) perovskites have become widely studied semiconducting materials, mostly due to their impressive photovoltaic performances. These are consequence of their unique properties, such as high absorption coefficient, high carrier mobility and long range carrier diffusion. On the other hand, the optical and electronic properties of hybrid perovskites vary largely with the material composition and morphology. Recently, hybrid perovskites such as methylammonium lead bromide (MAPbBr₃) have emerged as potential candidate for future electroluminescent devices. Generally, in LEDs, a high photoluminescence quantum yield (PLQY) of the active layer is desirable, since it is thought to determine the final efficiency of the device. Unfortunately, when the pure MAPbBr₃ is prepared by simple solution methods (single step spin-coating of the precursor solution), its PLQY is rather limited, at least if measured at low excitation intensity. In analogy to inorganic semiconductors, the most promising strategy to promote radiative recombination is the spatial confinement of the charge carriers. In this work we present strategies to improve the PLQY of hybrid perovskites by limiting their growth using a template material (i.e. polymer, small molecule or a porous inorganic scaffold) which is compatible with the solution processing of highly luminescent thin-films. The structural and optical properties, together with the application of such materials in LEDs, will be discussed.

Solar cells based on organic-inorganic metal halide perovskites such as methyl ammonium lead iodide (MAPbI₃) have demonstrated power conversion efficiencies (PCEs) greater than 22.1 %. The large absorption coefficient, long carrier diffusion lengths, desirable optical band gap, and fast carrier collection rates make this material very attractive for use in photovoltaic devices. However, non-radiative recombination of carriers is a primary energy loss mechanism, so methods are sought to reduce the density of charge carrier traps (N_t). Although the intrinsic defects in bulk MAPbI₃ do not create deep level trap states, grain boundaries and surfaces in the polycrystalline films can introduce defect levels that promote recombination. The density, N_t, and effect of these traps can be reduced by fabricating large grains with a high degree of crystallinity.
Here, we show that the nucleation and growth of MAPbI₃ films can be significantly altered to enhance grain size and crystallinity by using specific additives to the two-step sequential deposition process. The additive-assisted two-step deposition process resulted in grains as large as 1.5 μm, a factor of three improvement relative to control experiments. Furthermore, the additive-assisted approach resulted in smoother and more compact films. Charge carrier lifetimes of MAPbI₃ films were determined using time-resolved photoluminescence (TRPL) decay measurements. The lifetimes for materials prepared from the standard and additive-assisted two-step deposition process were ~ 100 and ~260 ns, respectively. The increase in the lifetime will be discussed in the context of a reduction in N_t, resulting in an improvement of the (PCE) from 7.1 % to 13.8 %.

Thermal annealing is a crucial step for the formation of crystalline perovskite films from precursor solution. However, the evolution of the structural properties and the bulk composition of perovskite films upon annealing has not been satisfactorily characterized to date. Such characterization is essential for accurately describing the mechanisms leading to the complete conversion of the precursor to functional perovskite, and, thus, for rationalizing the origin of the high photovoltaic (PV) performance achievable with perovskite films. By using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), supported by Grazing-Incidence X-Ray Diffraction (GIXRD), we succeeded in spatially resolving changes in both the molecular composition and structure of CH₃NH₃PbI_3-xCl_x mixed-halide perovskite films at different annealing stages, that is, in particular, before and after complete perovskite crystallization. At an early stage of annealing, our results indicate phase separation throughout the entire film depth between domains where perovskite is already formed and domains where a lead chloride-rich phase still dominates. After sufficiently long annealing, we then find a single perovskite phase of homogeneous composition. Importantly, parallel to this composition evolution, chloride diffuses from the perovskite layer towards the substrate. This remaining chloride in the final perovskite film corroborates the suggested implication of chloride in the optoelectronic properties of mixed-halide perovskite-based solar cells by modifying the interfacial electronic structure [1]. GIXRD performed at various annealing stages demonstrates growth of the films in the (pseudo-)cubic polymorph in pronounced fiber texture with respect to the substrate. Overall, our results highlight the importance of chloride for the formation of highly oriented films, which is crucial for enhancing the PV performance of perovskite solar cells [2].
Finally, via photoemission spectroscopy (PES) we monitored changes in the surface electronic structure with the annealing time. In particular, for longer annealing time we identify the perovskite film to become more n-type, which correlates with a lower relative methyl ammonium content. Such evolution of the electronic structure at the surface dictates the energy-level alignment at the interface between perovskite and the organic overlayer, and, therefore, impacts on the performance of the solar cells.

[1] S. Colella, et. al., J. Phys. Chem. Lett. 5, 3532-3538 (2014)
[2] P. Docampo, et. al., APL Mater. 2, 081508 (2014)

Due to their unprecedented development in the performance, perovsktie solar cells (PSCs) attract wildly attention during recently years. However, the stability of both the perovskite material and the charge transport layers are major issues that hamper stability of device because of their intrinsic structural and thermal instability.^1-3 We investigate the interfaces variation caused by the ion diffusion of perovskite, variety of temperatures during the aging test in both inverted planar structure and normal mesoporous structure in the size of 1cm². The XRD, SEM, AFM and EIS results showed that the variety at the interfaces play important role rather than decomposition of materials during the aging test. The different compounds of perovskite such as MAPbI₃, FAPbI₃ and mixed cation perovskite based PSCs are compared to further studied the interfacial variation in the aging test. Finally, the preferred perovskite material based on inverted structure is proposed under 1000hs aging test.

In this work, we report the synthesis of Nb₂O₅ transparent nanotube (NT) arrays shorter than 100 nm via a simple galvanostatic anodization process for the first time. One-dimensional, transparent nanotubular structures allow for superior carrier transport properties advantageous for electron transporting/hole blocking in photovoltaic devices. XRD, SEM, TEM, and XPS measurements are performed to verify the crystallinity and the chemical characteristics of the fabricated NTs. Moreover, UPS measurements are performed to estimate the band edge positions for the Nb₂O₅ structures with respect to the vacuum energy level. We demonstrate the potential for these NT arrays to be useful in solar cell devices by employing them as the electron transport layer in halide perovskite solar cells. The perovskite film formation was found to be greatly affected by the topography of the underlying NT array, yielding films with larger grain sizes and fewer pinholes and enabling the fabrication of solar cells with enhanced performance relative to devices utilizing only planar Nb₂O₅. The concept of ultra short Nb₂O₅ NTs both as an electron transport layer and as a means of guiding perovskite film growth could prove useful for the future development of perovskite and other thin film optoelectronic devices.

Inverted planar perovskite solar cells are of considerable interest since the discovery of bipolar transport properties in perovskites and the report on the perovskite/fullerene planar heterojunction structure in 2013. Inverted perovskite solar cells are more compatible with facile solution processing techniques used for producing organic solar cells. Particularly, inverted perovskite/fullerene planar heterojunction solar cells effectively eliminate or suppress photocurrent hysteresis associated with perovskites.
In an inverted architecture, the perovskite film is deposited on top of a hole extraction layer (HEL), typically PEDOT:PSS. However, the acidity of PEDOT:PSS is often detrimental to device performance and stability. Several solution processable inorganic substitutes to PEDOT:PSS have been examined as HELs; unfortunately, their processing methods are more complicated and less environmentally friendly than PEDOT:PSS cast from aqueous solutions. Here, we shown an anionic poly(phenylene vinylene) polyelectrolyte (PVBT-SO₃) works as an efficient hole extraction layer (HEL) cast from aqueous solution that substitutes for PEDOT:PSS, and does not require thermal annealing to achieve high performance. A maximum solar cell efficiency of 15.9% was achieved, ~26% higher than devices with PEDOT:PSS. Electrochemical impedance spectroscopy (EIS) measurements suggest that PVBT-SO3 containing devices have larger recombination resistance and more efficient charge extraction relative to those with PEDOT:PSS. Kelvin probe force microscopy (KPFM) characterization confirms that PVBT-SO3 films show higher work-function than that of PEDOT:PSS, which provides a larger build-in potential in devices containing PVBT-SO3 and thus a better performance, while time-resolved photoluminescence measurements further confirmed an efficient charge extraction of PVBT-SO3 interlayers from perovskite. The information presented here for PVBT-SO3 as a new hole extraction material to improve solution processed perovskite solar cells indicates conjugated polyelectrolytes may possess superior charge extraction properties, which will encourage the design and utility of new polyelectrolytes to advance photovoltaic technology.

Achieving high power conversion efficiency (PCE) has been the most interesting issue in the organolead halide perovskite solar cells since the CH₃NH₃PbI₃ (MAPbI₃) perovskite solar cell demonstrated 9.7% of PCE by employing spiro-MeOTAD as a hole transport material in 2012. While the PCE has dramatically increased, its unstable long-term performance hinders a commercialization of the perovskite solar cells. Even though considerable efforts have been made to improve its long-term stability, limited studies focusing on the perovskite material itself prevents us from understanding overall degradation mechanism of the devices. In this study, we found that the long-term stability of the device was significantly dependent on its selective contacts where ‘NiO/MAPbI₃/PCBM’ structure demonstrated better long-term stability compared to ‘TiO₂/MAPbI₃/spiro-MeOTAD’ structure. The interfacial reaction of selective contact with migrated ions from MAPbI₃ is strongly regarded as an origin of the long-term degradation which can be readily improved by adopting inert selective contacts. The deteriorated charge transport characteristics with increasing exposure to one sun illumination were systematically investigated by the aid of absorbance, photo luminescent and impedance spectroscopy.

The efficiency of organometal trihalide perovskites solar cells have reached parity with single crystal silicon and its nature-abundant raw material and solution-process capability promise a bright future for commercialization. The choice of the transparent electrode in perovskite-based solar cells affects not only the cost by also the performance and the long-term stability of the device, through chemical processes taking place at the electrode/absorber interface. It is therefore of great interest to investigate different cost-efficient, solution-processed electrodes in direct comparison to the state-of-the art, sputtered tin-doped indium oxide (ITO).
Solution-processed doped ZnO films deposited by spray coating/spray pyrolysis, as well as highly conductive PEDOT:PSS layers, also in combination with metallic nanowires and carbon nanotubes offer a wide range of choice to replace the costly ITO electrode on rigid and flexible substrates. In this work we provide a direct comparison between ITO electrodes and the aforementioned ITO-free materials, employing a solar cell architecture based on the lead-methylammonium iodide/PCBM junction.

Methylammonium lead iodide (MAPbI₃) is a promising new photovoltaic material with high power conversion efficiency. We have used first-principles density functional theory calculations to study the stability and optoelectronic properties of different polymorphs of MAPbI₃ under volumetric and epitaxial strain. We predict a novel hexagonal (H) phase of MAPbI₃ with P6₃ / mmc space group to be thermodynamically the most stable phase among all observed phases of MAPbI₃. The H-phase consists of alternating layers of face-shared PbI₆ octahedra and organic MA cations, which is in contrast to the corner connectivity of the octahedra observed in the experimentally reported orthorhombic, tetragonal (T), and cubic phases. The change in octahedral connectivity leads to large changes in the band structure and optoelectronic properties. The H-phase has an indirect band gap of 2.6 eV, which is ~1.2 eV larger than the direct band gap of T-phase. The optoelectronic properties of the new H-phase in contrast to the experimentally reported phases will be discussed. Finally, a route to obtain massive band gap changes in MAPbI₃ by tuning between the different polymorphs using strain will be presented.

The emerging organic-inorganic halide perovskites solar cells (PSCs) have rapidly progressed in the past five years. With >22% power conversion efficiencies, simple fabrication processes, and low manufacturing costs, there is a great potential to proceed towards commercialization. However, PSCs are currently limited by the instabilities in the materials and devices, especially due to reactions with water. To address this issue, it is important to know why the perovskite materials are unable to retain the excellent optoelectronic properties after exposure to humid air.
Here, we propose a quasi-ternary CH₃NH₃I-PbI₂-H₂O phase system to explain the relationship between the chemical composition changes of the materials, the optoelectronic response of the devices, and the device stability. Several critical phases with different chemical compositions are identified in the phase diagram, and investigations of the phase transformation, structural, optical, and electrical properties will be presented. By combining spatially resolved photocurrent measurements with material characterization, low-current generation can be correlated to the occurrence of low-dimensional perovskites (LDPs) that are formed when excess CH₃NH₃I [1] or H₂O [2] is present in the system. Perovskites in equilibrium withCH₃NH₃I-rich or H₂O rich environments exhibit weak photovoltaic responses and limit device performance. In contrast, high performance perovskite devices typically need to be prepared in slightly PbI₂-rich and H₂O-free environments
Water induced device degradation is also studied by mapping the spatial and temporal evolution of photocurrent [2]. We show that the hydration of the perovskite phase leads to a significant drop in the external quantum efficiency. However, this process is reversible: drying the device can convert the hydrated phases back to an unhydrated perovskite and recover the desired optoelectronic properties. Understanding of the phase stability and device performance of PSCs gives insight toward improving the long-term stability.

[1] Song et al., Chem Mater, 27, 4612, 2015.
[2] Song et al., Adv Energy Mater, in press, 2016.

Recent progress has shown that low-temperature processed tin oxide (SnO₂) is an excellent electron selective layer (ESL) material for fabricating highly efficient organic-inorganic metal-halide perovskite solar cells with the planar cell structure. Here, we report that plasma-enhanced atomic layer deposition (PEALD) is able to lower the deposition temperature of SnO₂ ESLs to below 100 ^oC and still achieve high device performance. With C₆₀-self-assembled monolayer passivation, our PEALD SnO₂ ESLs deposited at ~100 ^oC led to average power conversion efficiencies of 18.21% (maximum of 19.03%) and 15.57% (maximum of 16.80%) under reverse voltage scan for solar cells fabricated on glass and flexible polymer substrates, respectively. Our results demonstrate the potential of low–temperature PEALD process of SnO₂ ESLs for large-scale manufacturing of efficient perovskite solar cells.

Metal-halide perovskites have been actively researched in the past few years as an efficient light harvester material, that can be solution processed at low temperatures. The highest certified power conversion efficiency of these materials has reached 20%. This class of materials can also be incorporated into other devices such as light emitting diodes and lasers. In this presentation we will present results of computational efforts, supported by experimental synthesis and characterization studies, aimed at identifying lead-free double perovskite halides with desirable properties for replacing the lead-based halide perovskites that have been a main focus of research in this field. We present results for band gaps, thermodynamic stability and effective masses derived by a computational screening study based on density functional theory methods. Double perovskite compounds with composition A₂BB’X₆ (A = K, Rb, Cs; B = monovalent cation; B’ = trivalent cation; X = F, Cl, Br, I) were considered. As initial selection, compositions with unfavorable tolerance factors and octahedral factors were removed from the compounds dataset. Their thermodynamic stability was evaluated through the Materials Project database with calculated energies to determine whether they would decompose to simpler binary phases. Several energetically stable compounds containing Bi, Sb, or In with desirable band gaps and effective masses were identified as promising light harvesters.

Acknowledgement: This work was supported by the Singapore Berkeley Research Initiative for Sustainable Energy(SinBeRISE) Program.

Nanostructured carbon (NC) are very attractive nanomaterials for numerous applications because of their unique mechanical and optoelectronic properties. So far, few groups have explored the potential of NC when combined with perovskites materials. In this combination, the perovskites is the charge generator while the NC is expected to act both as charge collector and transporter. In this work, we fabricate NC/perovskites hybrid based photodetectors via a simple method to control amount of NC followed by spin coating the perovskite material. Here, we will present the structural, optical and unprecedented photoconductive properties of the hybrid material and their dependence on the NC amount.

Perovskite-based materials exhibit a number of functional properties from ferroelectricity, piezoelectricity, thermoelectricity, superconductivity to excellent light conversion and light emission. Hybrid organic-inorganic perovskites have shown great promise in an already exciting functional system. We explore potential new compositions for these hybrid structures by beginning with methyl-ammonium lead iodide (MAPbI) and building substitutional compounds that utilize different organics, lead-free compositions, and alternatives to halide anions. We explore the origins of why a relatively non-interacting organic molecule can cause drastic changes in performance and response using ab initio calculations on the elastic, piezoelectric, and dielectric response. Finally, we use this insight to suggest potential new hybrid organic inorganic perovskites for study.

Lead-halide perovskites have emerged quickly as high-performance photovoltaic materials. For future application in commercial thin film photovoltaics, it could be beneficial to implement these materials in efficient interdigitated back contact architectures, which are currently the performance-leading technology in silicon-based photovoltaics. Here, we developed a technique that allows for the simple fabrication of a back-contact device architecture for perovskite solar cells. We studied recombination at perovskite/electrode interfaces in a well operating back-contacted solar cell and quantified the effect of diffusion length on photovoltaic performance. By spatially mapping the external quantum efficiency and photoluminescence we found that uniform photocurrent generation can be achieved. Furthermore, by varying the pitch distance of the back-contacts we studied the transport characteristics of charges. We found that after injection of photo-generated electrons into the electron transporting electrode, holes left in the perovskite layer contribute to substantial photocurrent over distances far exceeding their reported thin-film diffusion lengths (1, 2). The development of electrodes capable of fast electron extraction opens a pathway to well-performing perovskite back-contact devices for low-cost, large-scale fabrication.
(1) S. D. Stranks, G. E. Eperon, G. Grancini, C. Menelaou, M. J. P. Alcocer, T. Leijtens, L. M. Herz, A. Petrozza, H. J. Snaith, Science 2013, 342, 341.
(2) G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, T. C. Sum, Science 2013, 342, 344.

Organic–inorganic hybrid perovskites such as methylammonium lead halide (MAPbX₃, MA= CH₃NH₃⁺, X=I^-, Br^-, Cl^-) attract more and more attention due to excellent optoelectronic properties, such as high carrier mobility, long-range balanced electron and hole diffusion lengths, long carrier lifetime, strong absorption in the whole visible range. In particular, optical and electrical properties can be considerably enhanced in single crystalline perovskites, compared to their polycrystalline thin film. However, perovskite single crystals with small size can generally be observed in a single perovskite growth process as previously reported by solution-based methods. Single crystals with large size should be grown by several repeated processes upon exposure to environment.
In this work, we reported a new approach to synthesize MAPbX₃ single crystals with size larger than 10cm. This approach was based on continuous-mass transport process (CMTP). Firstly, a perovskite seed obtained by inverse temperature crystallization was transferred to a specially designed equipment with circulating solution in two flasks, one for crystal growth and the other for the raw materials. The continuous growth of single crystals with good crystallinity was controlled by the temperatures of two flasks. This growth method of single crystals can uninterruptedly provide raw materials through mass transport, so that single crystal with large size can be achieved continuously. The size of crystal is only limited by the diameter size of the growth flask. Notably, both the size and shape of the crystals can be controlled by manipulating different crystallization parameters through this approach.

Perovskite-phase cesium lead iodide is a promising material for photovoltaics, however, under ambient conditions it rapidly transforms to a non-functional, so-called yellow phase. We recently reported a new approach to stabilizing thin films in the functional perovskite phase by co-assembling cesium lead iodide nanocrystals with cesium lead chloride dopant nanocrystals.[1] The resulting doped nanocrystal solid is subsequently fused into a polycrystalline thin film by chemically-induced, room-temperature sintering. This process ensures nanometer-scale mixing even at compositions that exceed the bulk miscibility of the two constituents. Spectroscopy and X-ray diffraction indicate that a portion of the chloride is further dispersed during sintering and a polycrystalline CsPbI_3-xCl_x phase is formed. By comparison to DFT-calculated values, the relative change in band gap and the lattice contraction are shown to be consistent with a Cl:I ratio of a few percent in the mixed majority phase. Additionally we find evidence of a portion of unmixed CsPbCl₃ phase inclusions within the film and we explore the possible role these structures play in the film properties. With this film structure and composition, the half-life of the functional perovskite phase in a humid atmosphere increases by more than an order of magnitude.
[1] S. Dastidar, D.A. Egger, L.Z. Tan, S.B. Cromer, A.D. Dillon, S. Liu, L. Kronik, A.M. Rappe, A.T. Fafarman, High Chloride Doping Levels Stabilize the Perovskite Phase of Cesium Lead Iodide, Nano Lett. (2016). doi:10.1021/acs.nanolett.6b00635.

Perovskite solar cells can outperform third generation photovoltaics cells, reaching the same level of cadmium tellurium (CdTe) and copper indium gallium selenide (CIGS) solar cells efficiency in only few years [1]. The state of the art is over 20% of power conversion efficiency, although its stability remains an issue [2]. A common technique used to prepare methylammonium lead iodide perovskite solar cells is named two-step deposition technique. It consists on the deposition of lead iodide (PbI₂) dissolved in dimethylformamide (DMF) on titanium oxide (TiO) substrate. The morphological and structural properties of the PbI₂ layer are detrimental to the reaction with methlylammonium iodide (CH₃NH₃I) in the second step and the formation of methylammonium lead iodide (CH₃NH₃PbI₃). In this work we have investigated, combining theoretical and experimental approaches, the effect of adding dimethylsulfoxide (DMSO) on the preparation of PbI₂ layer in solar cells with configuration FTO/Ti₂O/CH₃NH₃PbI₃ /SPIRO-MeOTAD/Ag. Our results show that the addition of DMSO leads to formation of PbI₂ layers with different superficial morphology and crystallinity which may be attributed to the strong coordination of Pb²⁺ with DMSO. In order to gain further insights on these features we have also carried out a classical molecular dynamics (MD) using the universal force field [3] as implemented in the well-known LAMMPS code [4]. Overall the solar cells have reached power conversion efficiency around 10%, with their structural stability significantly enhanced by the addition of DMSO.

[1] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami and H. J. Snaith. Science, 338, 643 (2012).
[2] Best Research-Cell Efficiencies. National Renewable Energy Laboratory (NREL) (2015).
[3] A. K. Rappé, C. J. Casewit, K. S. Colwell, W. A. Goddard III and W. M. Skiff. J. Am. Chem. Soc., 114, 10024 (1992).
[4] S. N. Laboratories, LAMMPS Users Manual, 209 (2014).

Hysteresis phenomena observable in current-voltage measurements of lead halide perovskite solar cells are currently at the center of interest to the solar cells community. While there is promise to utilize this class of materials for next generation photovoltaics, the discrepancy in measured performance depending on electrical history of these devices seems to potentially compromise the effectiveness of their use for solar energy conversion. On the other hand, the 'memory' effect observed in perovskite solar cell has raised interest for other type of electronic devices where electrically induced changes in material properties can be used to store energy, store information, or even construct complex hardware-based network systems.^1,2

In this work, we present the experimental observation of quasi steady state bistability in perovskite solar cells. We find that the stabilized value of the photocurrent in methyl ammonium lead iodide (MAPI) devices varies depending on the preconditioning of the device. In particular, opposite dependence of the photocurrent at long timescale to the preconditioning is found to the corresponding dependence at early timescale. We compare the magnitude of this observation for bottom cathode or top cathode architectures using lead halide perovskite containing either iodide, iodide-chloride or bromide as anion.

Deeper understanding of the role of ion migration on the working principle of these devices has recently outlined that the evolution of the internal field contributes to the degree of hysteresis.³ Here we discuss whether analogous processes may underlie this long timescale effect, raising the hypothesis of surface trap passivation induced by electrical preconditioning. We support our hypothesis with drift diffusion simulations of the quasi steady state photocurrent in these devices as a function of prebias.

Finally, we outline considerations for the correct characterization of the efficiency of perovskite solar cells given the presence of multi-stable photocurrent behavior.

1 Xiao, Z. et al. Nat Mater 14, 193-198, 2015
2 Xiao, Z. & Huang, J. Adv. Electron. Mat., doi:10.1002/aelm.201600100, 2016
3 Calado, P. et al. submitted to Nat Commun

Ternary bismuth iodides (A₃Bi₂I₉, A = K, Rb, Cs, NH₄⁺, MA, FA) have recently been proposed as less-toxic alternatives to lead iodide perovskites (APbI₃) for photovoltaic applications.^1,2 The compositional and structural flexibilty of A₃Bi₂I₉ makes them especially attractive for optimizing their optoelectronic properties. We have used first principles density functional theory calculations to understand the role of A-cation size on the optoelectronic properties of A₃Bi₂I₉. In contrast to APbI₃ perovksites, where their band gap is significantly reduced from 2.1 eV in KPbI₃ to 1.48 eV in FAPbI₃, we predict that increasing the size of the A-cations in A₃Bi₂I₉ leads to only a small variation of ~0.1 eV in the band gap. All the calculated compounds have a theoretical band gap of 2.23±0.04 eV. We further show that the band gap in A₃Bi₂I₉, which arises due to the overlap of the Bi 6p and I 5p states, is controlled by a complex interplay between three specific factors: the chemical pressure due to the size of the A-cation; spin-orbit coupling effects; and the strength of H-bonds between the organic cations and the iodide ions. Our theoretical predictions are confirmed by experiments, which show a band gap of 1.92 eV for K₃Bi₂I₉ and 1.95 eV for FA₃Bi₂I₉. We provide strategies to optimize the properties of A₃Bi₂I₉ compounds for photovoltaic applications by selectively tuning the abovementioned factors.


References:
1 Park, B. W. et al. Bismuth Based Hybrid Perovskites A₃Bi₂I₉ (A: Methylammonium or Cesium) for Solar Cell Application. Adv Mater 27, 6806-6813, (2015).
2 Lehner, A. J. et al. Crystal and Electronic Structures of Complex Bismuth Iodides A₃Bi₂I₉(A= K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics. Chemistry of Materials 27, 7137-7148, (2015).

Hybrid halide perovskite (for example, CH₃NH₃PbI₃) solar cells now display solar power conversion efficiencies exceeding 20% [1]. In these materials, excitonic and free-carrier regimes of primary photoexcitations are possible depending on crystalline microstructure of the active layer [2], with device microstructures typically featuring a combination of these two regimes. Recent literature suggests that photocarriers in these materials may be large polarons [3], with this notion motivated by observation that charge transport is limited by acoustic phonon scattering, and not by impurities and crystalline defects present ubiquitously in these polycrystalline microstructures. In order to explore the nature of photocarriers in these materials, we implement two-dimensional coherent photocurrent excitation (2D-PCE) spectroscopy as described elsewhere [4] on an optimized solar cell based on CH₃NH₃PbI₃ [5]. Via the time-resolved total correlation spectrum, we identify both excitonic and continuum resonances. By following the temperature dependence of the rephasing zero-time spectrum, we explore the possible polaronic character of the exciton and continuum resonances and address directly whether this measurement reflects such phonon coupling.

[1] NREL Solar Cell Efficiency Chart.
[2] Grancini et al, Nat. Photonics 9, 695−701 (2015).
[3] X.-Y. Zhu & V. Podzorov, J.Chem. Phys. Lett. 6, 4758−4761 (2015).
[4] arXiv:1602.04205 [cond-mat.mtrl-sci]
[5] Tao et al, Energy Environ. Sci. 8, 2365 (2015).

PSC (Perovskite Solar Cell) is regarded as a promising technology in energy devices with its fast development in efficiency, low manufacturing cost. In PSCs, MALI (Methyl-ammonium lead iodide) is the most generally used material as photo sensitizing layer. For well operation of PSC in relatively harsh condition under sunlight, the thermal stability of MALI is considered as one of the most important issue as main component of solar cell. However, there are limited methods for investigate the thermal stability of MALI. The property of MALI such as optical property changes with handling condition. As exposure temperature changes, stress applied to MALI and optical reflectivity change a lot, which will influence operation of PSCs. It is important to analyze thermal stability of MALI for more reliable PSCs.
In this study, we analyzed thermal induced stress and optical property changes as temperature increased to 200^oC with reflected laser beam pattern analysis. By simultaneously measuring deflection of few arrays of laser beam from MALI thin film, we measured stress applied to MALI film by measuring curvature change and optical property changes by measuring reflected intensity of laser. The experiment with in-situ deflection analysis reflects real time changes of MALI film in changing temperature, containing more specific and reliable results.
With in-situ data from laser beam pattern, we observed the tendency of applied stress on MALI and intensity of reflected laser, which corresponds to the efficiency of PSCs. We found that the intensity changes are closely related to the phase change and morphology change of MALI and efficiency of MALI PSCs. Up to 90^oC, MALI remains stable with small grain growth and intensity also remain unchanged. After 90^oC, MALI starts to decompose to PbI2 and surface become porous. Following this, intensity start to decrease after slight increase. The efficiency, corresponding to deflection beam data, become large until 90^oC and decrease in higher temperature. With these research in-situ, we figured out the thermal stability of MALI and the thermal induced changes of MALI.

Organic – inorganic hybrid perovskites have rapidly reached a power conversion efficiency above 22%. Recently, cesium lead iodide (CsPbI₃) has emerged as an all-inorganic alternative to the hybrid perovskites that alleviates the limitations to long term stability posed by the volatility and reactivity of the organic cation. However, the perovskite phase of CsPbI₃, also called the ‘black’ phase, is thermodynamically favored only at elevated temperature (~320 °C). At room temperature, especially in humid atmosphere, it rapidly transforms into a nonfunctional ‘yellow’ phase. In this work, we have employed thermal analysis to evaluate the thermodynamics and kinetics of the phase transformation of CsPbI₃. Thermogravimetric (TGA) analysis reveals that the thermal decomposition of CsPbI₃ starts at much higher temperature (more than 400 °C) compared to the hybrid perovskite (e.g. methylammonium lead iodide). As a consequence, it is possible to directly measure the enthalpies of phase changes using differential scanning calorimetry (DSC). The DSC results show a narrow endothermic peak around ~320 °C and a much wider exothermic plateau around ~270 °C, signifying the yellow to black phase conversion and reappearance of yellow phase, respectively. By analyzing the onset temperature and peak width, it is possible to study the kinetics of the phase transition in temperature regimes wherein the perovskite phase is metastable. We examine the effects of atmosphere, rate of heat flow and annealing on the metastability. These DSC studies inform the interpretation of spectroscopic studies of thin films of CsPbI₃ as a function of temperature and point to ways to improve the stability of these materials.

The carrier transport in organic-inorganic hybrid perovskites (such as CH₃NH₃PbI₃) has been extensively studied due to their superior performance in photovoltaic applications recently. We have studied the carrier transport in 3D CH3NH3PbI3 structure and its 2D counterparts using ultrafast spectroscopy and microscopy techniques. For 3D perovskite structure, initial above-band gap excitation creates hot carriers, and it is observed that hot carriers' cooling spans two time scales: one occurs in sub-picosecond and the other one occurs in tens of picoseconds. Transient absorption microscopy was able to directly image the band gap renormalization effect and Burstein-Mossing band filling effect from the hot carriers. It is demonstrated that ~10% of the hot carriers have a much longer life time, up to hundreds of picoseconds, than those previously estimated from ensemble transient absorption or photoluminescence (PL) measurements. Even though those hot carriers have been identified with unusually long life time, their migration appears to be diffusive, and the diffusion coefficient is much similar to the normal carriers. This observation implies that the hundreds-of-picosecond relaxation time might be related to some slow phonon-phonon scattering process. Using temperature dependent, time resolved micro-PL measurements, we also found that the exciton scattering rate in 2D perovskites follows ~T^-1.5 relationship for both the intra-band relaxation processes and the inter-band relaxation processes. This observation points to a dominated carrier scattering source by very low energy phonons (either acoustic phonons or low energy optical phonons) in the 2D perovskite structure.

Organic-inorganic hybrid perovskite solar cells based on lead halide compounds have attracted much attention from their high photoelectric conversion efficiency. In the perovskite solar cells, the perovskite crystal layer as well as the titania layer, which play an important role in the career generation and transport properties. On the one hand, the advantages of the perovskite solar cells is not only exhibit high photoelectric conversion efficiency but also it is possible to fabricate all construction layer, such as the crystal and a hole transport layer except electrode, under atmospheric condition.
Mist-vapor deposition is advantageous fabrication method for organic semiconductor devices. This film-formation technique will become a one of candidates for the roll-to-roll processing. A fine droplet generated by ultrasonic modulator enables to form uniform thin films under atmospheric pressure. In this research, we discuss one-step crystallization processing of perovskite layer based on methyl ammonium lead- halide organic-inorganic hybrid materials. Moreover, the other functional layers of the solar cell such as electron and hole transport layer were also formed by the same fabrication method.
For sample preparation, titania powder dispersed water and methyl ammonium lead-iodide solution were prepared. These two kinds of solution were atomized and mist particles were generated, which were transferred by nitrogen carrier gas with the flow rate of 4.0 L/min to the substrate area. Titania thin film and perovskite crystal layer were deposited on the heat controlled substrate. In order to clarify the film condition, XRD analysis and AFM measurement were performed, and moreover photoelectric conversion properties were discussed.
As a result, only controlling the substrate temperature while spraying within a range of up to 140°C from 110°C, the organic-inorganic hybrid perovskite crystal was formed directly on the substrate. Furthermore, the thickness, the surface morphology and even the device performance were also possible controlling with deposition temperature.

Perovskite solar cells have been demonstrating potential for a promising alternative to conventional silicon solar cells as they require low fabrication costs and have reported power conversion efficiencies exceeding 20%. In this study, we present results on plasmonically enhanced thin film perovskite solar cells where a plasmonic concentrator embedded in the top metal contact succeeds in increasing the cell’s efficiency through enhancing the active layer’s absorption properties. To fabricate the plasmonic concentrator, nanoimprint lithography is performed on the hole conducting layer of the thin film perovskite solar cell before the top contact deposition step so as to define a hexagonal periodical pattern of nanoholes. The stamps comprise 150 nm high nanopillar arrays with pitch ranging 200 – 400 nm and diameters ranging 100 – 200 nm on 1 cm² silicon substrates. After the deposition of the top contact, the nano-corrugated metal is able to support the generation of surface plasmon polaritons when illuminated, therefore acting as a plasmonic concentrator structure leading to enhancement of the thin film’s absorbance. This increase in absorbance results in an elevated open circuit voltage and in turn a higher efficiency compared to a cell comprising the same thickness but without plasmonic absorption enhancement. In addition, we perform optical characterization of the perovskite material, which serves as the active absorbing layer, in order to determine the absorption coefficient and extract the optical bandgap as a function of thickness. The film thicknesses are measured via SEM, ellipsometry, and profilometry in order to obtain an accurate result. Following these measurements, reflectance and transmittance measurements are performed over the 400 – 1100 nm optical wavelength range. Based on these experimental data, the multiple interference model, originating from the Fresnel equations for a four layer system is solved in order to calculate the film’s absorption coefficient. Having knowledge of the absorption coefficient’s spectral behavior, the Tauc and Cody models are implemented in order to extract the film’s optical bandgap. To conclude, in this study we have shown that plasmonic concentrator structures fabricated through nanoimprint lithography suggest a promising way of boosting thin film perovskite solar cell efficiencies.

Stability of hybrid perovskite based photovoltaic devices against humidity and temperature remains a significant challenge to its commercialization. Numerous attempts have been made to improve the stability including two-dimensional (2D) perovskites, macroscopic encapsulation, and modification of chemistry, the effect is, however, often modest, transient, and adds device complexity. Furthermore, while ultrathin oxide overlayers are widely utilized for passivation, the sensitivity of hybrid perovskites against majority of the conventional deposition processes significantly limits the application of such layers.
Here, we utilize a hybrid organic-inorganic electron selective layer in an inverted hybrid perovskite photovoltaic device to passivate the photoactive perovskite layer against ambient while allowing for efficient transport of electrons. The cells remained stable in ambient atmosphere for > 300 hours without additional macroscopic encapsulation. Without masking, the stabilized cells exhibited relatively high short-circuit current density (J_sc) and open circuit voltage (V_oc) of 19.6 mAcm^-2 and 0.934 V, respectively. However, owing to a modest fill factor (0.477), the devices ultimately showed 8.8 % efficiency measured in ambient under simulated one sun illumination. We expect that with further encapsulation, these stabilized devices have the potential to fulfill ambient stability requirements that are required for commercialization.

Hybrid perovskite (HPs) based solar cells have made substantial progress in past few years. Especially, the formamidinium lead halide FAPbI₃ (FAPI) has emerged as a promising replacement for methylamnoinum lead iodide MAPbI₃ (MAPI) due to broad-band absorption and higher thermal stability of the former. However, there is a deal of obscurity in the causal relation between the point defect chemistry of HPs and several transport related phenomena such as anomalous J-V hysteresis, ion migration, trap-assisted recombination/transport and dielectric/ferroelectric polarization. Importantly, the point defects in perovskites show strong temperature dependence. Therefore, the temperature dependent measurements can help to understand and establish the relation between defects and transport in FAPI based HPs. Here, we investigate the temperature-dependent (80-400K) transport in FAPI (in comparison with MAPI) using techniques including (i) pulsed current vs time measurement to separate semiconducting, ionic and dielectric properties; (ii) transient photoconductivity measurements to understand ion-migration effects; and (iii) ferroelectric polarization to investigate the presence of ferroelectricity. These measurements are complemented by experimental techniques for identification/quantification of defects in FAPI and temperature dependent x-ray diffraction.

In the highly active research field of organolead halide perovskite solar cells, fabrication control is an important issue. Vacuum deposition was considered to be useful for the controlled fabrication of perovskite. However, there is a great difficulty in the deposition control of amine halide because of the vaporization during the deposition. We have been developing laser deposition method specially designed for the fabrication of perovskite in order to achieve precise deposition control.
808-nm continuous-wave semiconductor laser was used for the evaporation of CH₃NH₃I and PbI₂. Based on the locally irradiated laser power, the vaporization issue of CH₃NH₃I is significantly reduced, where the vacuum under deposition is 10^-4 Pa. The controllability was improved, where the deposition rate was adjusted by tuning the duty ratio of squarely modulated laser. Deposition rate was stabilized for several-hour co-deposition period.
Organic photovoltaic (OPV) type perovskite solar cells were constructed. After the trials of the various kinds of hole-transport materials, the device structure of ITO/NiO_x/PCDTBT/CH₃NH₃PbI₃/PCBM/BCP/Al resulted in the high power conversion efficiency. The power conversion efficiency of 15.7 % and 16.0 % were obtained for forward scan and backward scan, respectively. The reduced hysteresis is the typical feature for OPV-type architecture. In this study, the precise control of the deposition rate resulted in the much reduced hysteresis. In addition, we found that the co-deposition condition to obtain tuned stoichiometry of perovskite was different among the substrate materials and surface conditions. Fundamentals of the growth mechanism of perovskite should be elucidated for the further control of the film quality. The laser deposition method will be helpful for the fundamental analysis because of its high controllability.

Acknowledgments: This work was financially supported by the new energy and industrial technology development organization (NEDO) of Japan.

Thin film photovoltaic cells based on hybrid organic/inorganic perovskite absorbers, such as methylammonium lead halide CH₃NH₃PbX₃ (X=Cl, Br, or I) have recently attracted great interests because of their advanced photovoltaic properties since they has led to the improvement in the power conversion efficiency up to 20 %. This high performance is attributed to the superior electrical transport properties including the long diffusion length of photogenerated electron and holes and fast charge transport at the interfaces with semiconductors[1]. Recently, several groups have investigated the role of the intrinsic defects of CH₃NH₃PbX₃ on its outstanding electrical and photovoltaic properties[2-3]. The comprehension of defect properties in actual organometal halide perovskite films is a critical requirement to design and process these films with higher photovoltaic performance. However, the fundamental understanding concerning the defect properties remains unknown. In addition, perovskite solar cells are known to degrade at moderate temperatures and upon moisture ingress. Understanding how to extend the long term stability of these films is another important challenge. Both defects and degradation in CH₃NH₃PbX₃ films are considered to be significantly affected by the evolution of phase transitions and crystal morphology during synthesis and environmental exposure. Solid-state NMR spectroscopy offers several techniques for the investigation of the morphological, structural and dynamic properties of solid materials. In this study, we investigated the defect structures and properties of molecular motions of methylammonium cation which related with phase transition behavior in CH₃NH₃PbX₃ films with different deposition process by means of solid-state NMR spectroscopy, Powder XRD and DSC. We also investigated the effects of degradiation of these films by moisture exposure. Detailed results will be presented in the conference.

References
1)G. Xing, N. Mathews, S. Sun, S. S. Lim, Y. M. Lam, M. Grätzel, S. Mhaisalkar, T. C. Sum,
Science 2013, 342, 344-347
2) W.-J. Yin, T. Shi, Y. Yan, Appl. Phys. Lett. 2014, 104, 063903
3) Kim, Jongseob, Lee, Sung-Hoon, Lee, Jung Hoon, Hong, Ki-Ha, Journal of Physical Chemistry Letters, 5,8,(2014) 1312-1317.
Corresponding Author: H. Ogata Tel: +81-42-387-6229, Fax: +81-42-387-6229, E-mail: [email protected]

Organometal halide perovskite-based solar cells have recently been reported to be highly efficient, giving an overall power conversion efficiency of up to 20.1 %. To take full advantage of the characteristics of perovskite materials in solar cell, it is essential to improve the scaffold layer as well as the highly crystalline and planar perovskite film. However, detailed relationship between the scaffold layer and the crystallinity of perovskite film and their photovoltaic performance has remained fully unknown. Here, we have employed the several kinds of scaffold films made of some metal oxide nanoparticles, mesoporous materials and investigated the relationship between their crystallinity and transport properties of perovskite films and their cell performances systematically. We also investigated the diameter dependence of metal oxides nanoparticles and pore size dependence of mesoporous materials on the crystallinity of perovskite film and their photovoltaic performances. Layered lead halide perovskite (CH₃NH₃PbX₃(X=Br or I)) films were spin-coated on TiO₂ single crystal, TiO2 nanoparticles, the other metal oxides or several kinds of mesoporous materials on FTO substrates. Spiro-OMeTAD were spin-coated on theses films as hole transport layer. It was found that some devices using metal oxides nanoparticles or mesoporous materials as scaffold layer exhibit higher crystallinity and higher photoluminescence quenching ratio than those using TiO₂.
In this presentation, detailed results of crystallinity of perovskite layers, surface morphologies and photovoltaic properties will be presented.

References
1.Meng Zhang et. al. Chem. Commun. 50 (2014) 11727.
2.Zhao et al., J. Phys. Chem. 10 (2014) 1021.
Corresponding Author: H. Ogata Tel: +81-42-387-6229, Fax: +81-42-387-6229, E-mail: [email protected]

By enabling the probing of physical and chemical properties at the functionally relevant length scales of most materials and devices, near-field optical imaging and spectroscopy accesses information that is unobtainable with other methods. Over the past decade, major advances in nanofabrication and probe development have resulted in rapid adoption of near-field approaches for studying novel nanoscale materials and phenomena. Current applications span basic science studies to cutting-edge industrial characterization, with e.g. semiconductor companies recognizing near-field scanning optical microscopy (NSOM) as the only technique capable of characterizing circuit failure in their next generation node (i.e. far-field super-resolution techniques cannot provide the needed information).
At the Molecular Foundry, we have been developing the Campanile probe design, which provides high near-field coupling efficiency over a broad spectral range for both the optical excitation and signal collection. Recently, we have applied these probes for nanoscale hyperspectral characterization of technologically-important nanomaterials, including light-harvesting nanowires [1] and 2D semiconducting materials [2]. Unfortunately, the complexity of fabrication and characterization of these nanostructured probes has greatly limited campanile tip use, with it typically taking Foundry scientists > 1 week to fabricate a tip. Here, we describe our collaborative work on characterizing batch-fabricated campanile probes, which are produced via a novel nanoimprint lithography (NIL) process. Initial tests are quite promising, enabling the hyperspectral imaging of luminescent beads, the mapping of light transmission through a nanogap grating structure (Fig. 1), as well as hyperspectral imaging of perovskite thin films.

There is currently a great deal of justified excitement about the hybrid perovskite (ABX₃) based solar cells. This paper discusses intrinsic point defect concentrations in these materials, and their role in determining properties. These intrinsic point defects and the defect-property correlations are relatively well studied in more "classic" oxide perovskites. In those materials it has been important to understand the differences between the perovskites that have a volatile molecule containing a cation species (ie PbO in Pb(Zr,Ti)O₃) in equilibrium at the processing temperature, and those which do not (such as BaTiO₃). In the case of the former, the stoichiometry tends to be self-correcting during processing. Thus one can begin with an A-site excess, PbO volatilizes during processing, the activity decreases until reaching a pseudo-equilibrium, leaving the concentration of A-site and O-site vacancies around 2-4 at.%. These defects can impact diffusion rates, charge carrier concentrations, electronic transport, and when associated, can act as defect dipoles. In all the materials (with volatile A-site components or without), the oxygen vacancy concentrations are mobile under applied field even at room temperature, and tend to be critical in determining the lifetimes of the materials used in electronic devices (e.g. capacitors).
In contrast, the defect chemistries of the hybrid perovskites have barely been studied, although there has been speculation about the role of mobile halide ion vacancies. This work focuses on MAPbI₃, where we show that volatile MAI also controls the concentration of the vacancies in the organic and halide sites. The phase behavior, single phase compositional range, and point defect concentrations were measured by a combination of thermogravimetric, chemical analysis, and X-ray methods. Samples with varied and controlled defect concentrations were prepared, and the effect on charge transport characterized over a range of temperatures, thus allowing the electronic and ionic contributions to be separated.

Organo-lead halide perovskite has attracted much attention for photovoltaic devices. Although the performance of the solar cell is outstanding, devices have shown many problems of long-term stability, simply because organic materials, e.g., Spiro-OMeTAD and PEDOT:PSS, as selective contacted layers, induce instability of the devices. As alternatively, a few inorganic transparent materials were reported including NiO, CuSCN and MoO₂, which have p-type property, wide bandgap, valence band edge matching to perovskite. However, usually conductivity of p-type TCO is limited by low hole mobility, because effective hole mass is large. Here, we present perovskite solar cell based on highly conductive and stable Copper(I) Chromium Oxide (CuCrO₂) nano-crystalline layers as an inorganic HTL synthesized by hydrothermal method. Optical band gap of CuCrO₂ is as large as 2.95-3.30 eV, appropriate valence band level (-5.3eV) with perovskite and high electrical conductivity up to 1.0 S cm^-1[1-4]. Pure crystalline phase of delafossite CuCrO₂, which defect phases (CuCr₂O₄, CuO) are not included, are confirmed by X-ray Diffraction (XRD) analysis. Scanning Electron Microscope (SEM) and Transmission electron microscopy (TEM) are performed to characterize morphology and microstructure. Electrical conductivity is conducted by Hall effect measurement. Inverted-structure solar cells of FTO/p-CuCrO₂/perovskite (CH3NH3PbI3)/PCBM/Ag were fabricated and resulting enhanced stability and efficiency of organo-lead perovskite solar cells. In addition current density (J) was measured with respective to the applied voltage under 1.5 AM illumination.

KEYWORDS
Inorganic hole transport material, Copper(I) Chromium Oxide (CuCrO2), perovskite solar cells

[1] D. Li, X. Fang, Z. Deng, S. Zhou, R. Tao, W. Dong, T. Wang, Y. Zhao, G. Meng and X. Zhu, J. Phys. D: Appl. Phys., 2007, 40, 4910.
[2] R. Gillen and J. Robertson, Phys. Rev. B: Condens. Matter, 2011, 84, 035125.
[3] J. Wang, P. Zheng, D. Li, Z. Deng, W. Dong, R. Tao and X. Fang, J. Alloys Compd., 2011, 509, 5715.
[4] R. Nagarajan, A. D. Draeseke, A. W. Sleight and J. Tate, J. Appl. Phys., 2001, 89, 8022.

Metal halide perovskites have come to the forefront of the research community due to their excellent performance in optoelectronic devices such as solar cells and light-emission applications. The ionic nature of these materials increases the versatility in physical properties and distinguishes them from other commercial semiconductor technologies, yet the extent of the ionic mobility influence on the optoelectronic performance is still unclear.
Here, we use temperature-dependent X-Ray Diffraction (XRD) measurements to study the low temperature tetragonal-orthorhombic phase transition in thin films and powders of CH₃NH₃PbI₃. We find there is a strong hysteresis in the phase transition temperature upon cooling compared to heating. Intriguingly, we find that the phase transition hysteresis effects manifest themselves in other important physical properties including photoluminescence, absorption and photo-conductance, suggesting an intimate relationship between structural characteristics of the films and their optoelectronic performance. We use micro-photoluminescence measurements to reveal that the origin of the hysteretic effect is small microscopic tetragonal inclusions in the orthorhombic phase and vice versa, even above and below the nominal phase transitions. Furthermore, the cooling/heating cycle through the tetragonal-orthorhombic phase transition induces a change in the texture of the films and manifests in variances of photoemission character, revealing strong links between crystal orientation and non-radiative decay losses. Our work will help guide controlling perovskite fabrication towards further improved optoelectronic properties and devices.

Though lead halide perovskite solar cells has achieved a remarkable power conversion efficiency exceeding 20%, the toxicity issue of lead remains as an obstacle to realize the practical manufacturing production. Tin based perovskite solar cell, e.g. formamidinium tin iodide (FASnI3) perovskite solar cell is a good candidate to solve the environmental issue, however Sn2+ is easily oxidized to Sn4+ in the ambient air, so that such device is not air stable. Here we incorporated Antioxidant additive during perovskite film formation, to achieve homogeneous dispersion of SnI2 film. Besides we utilized carbon counter electrode to further isolate the device with ambient air, achieving 4% efficiency and significantly improved device stability.

Organic-inorganic hybrid perovskite solar cells (PSCs) have attracted tremendous attention due to the dramatic improvement in efficiency from 3.8% to 22% in about seven years. The technology, however, faces several challenges such as strong effect of humidity, hysteresis in the current-voltage characteristics, degradation and lead toxicity that adversely affect its commercial viability. We recently demonstrated that the stability of these cells could be improved significantly using titania nanotubes as encapsulations for protecting the perovskite absorber from ambient effects. Nonetheless, understanding the root cause of ambient dependent inconsistent performance of PSCs is of paramount importance for ultimately solving this issue, particularly in cells having a planar geometry. We made a comprehensive study on the effect of fabrication as well as cell characterization ambient on the performance of the cells in this geometry. Our results showed that although high humidity levels during the fabrication reduce the open circuit voltage and deteriorate the cell characteristics, a certain level of humidity is beneficial for the high and stable performance of mixed halide cells. The influence of dry and humid air as well as nitrogen on the absorber material and other cell components was studied. We will present these results as well as the mechanism of interaction as revealed by in situ photoimpedance spectroscopy.

Organic trihalide perovskites have attracted intensive research interests in recent years due to its high power conversion efficiency and long photo-carrier diffusion lengths. Integrate them with two dimensional (2D) transition metal dichalcogenide such as MoS₂, can siginificantly broaden their absorption spectra. In addition, the 2D material can also be used as transparent conductors with high flexibility, which opens up the opportunity to build flexible optoelectronics applications. Here we report the fabrication and the study of the photo-response of MoS₂-CH₃NH₃PbI₃(MAPbI₃) hybrid devices. MoS₂ films were deposited on sapphire substrates via rapid thermal process growth. We then deposited gold electrodes via DC sputtering and made them into two point devices with channel length of 50 μm. About 200 nm thick trihalide perovskite MAPbI₃ was spin coated on to the MoS₂ devices. We studied the switching dynamics of the photocurrent and observed a photoresponsivity of ~ 10-100 mA/W. All devices exhibit power law dependence on the incident power, which makes them very promising for optoelectronic applications. We observed a three orders of magnitude enhancement in photo-current and photo-responsivity compared to the MAPbI₃ single channel devices. The photo-response enhancement in the hybrid device is attributed to the charge transfer induced by the band alignment between these two materials, which facilities the charge separation at the interface.

Organolead-halide perovskite solar cells (PSCs) have rapidly emerged as one of the most promising candidates for next generation photovoltaics due to their high power conversion efficiency (PCE) and low fabrication cost. However the potential of this technology has been severely limited by the poor device stability under ambient atmosphere. Recent studies have shown that the stability of PSCs can be significantly improved by substituting the organic charge transport layers with more stable metal oxide nanoparticle transport layers.^[1] In this study, we report efficient charge transport layers fabricated from metal oxide nanoparticles synthesized by a flame assisted spray pyrolysis method, which is a well-established industrial method for large scale production of nanoparticles.^[2] By controlling the growth parameters, such as the precursor, solvent, fuel, residence time and temperature, the size, composition and physicochemical characteristics of nanoparticle were fine tuned to match the requirements as charge transport layers in perovskite solar cells. As a result, we were able to fabricate perovskite solar cells with PCE > 14% under 50% relative humidity using modified NiO_x as the hole transport layer and ZnO as the electron transport layer. The cells displayed significantly improved long term stability in air and better photo-stability over the reference cells using conventional organic transport layers.


[1] J. You, L. Meng, T.-B. Song, T.-F. Guo, Y. (Michael) Yang, W.-H. Chang, Z. Hong, H. Chen, H. Zhou, Q. Chen, Y. Liu, N. De Marco, Y. Yang, Nat. Nanotechnol. 2015, 11, 1.
[2] H. K. Kammler, L. Mädler, S. E. Pratsinis, Chem. Eng. Technol. 2001, 24, 583.

Methylammonium lead iodide is an emerging photovoltaic material that has shown record increases in solar cell efficiency over the last five years. In spite of this phenomenal increase in device efficiencies, there remain many open questions about several intrinsic properties of this material such as its piezoelectric and ferroelectric response. The growth of large single crystals allows for the characterization of the symmetry specific properties, such as piezoelectricity and ferroelectricity. This work investigates the electrical, electromechanical, optical and electro-optical response of methylammonium lead iodide, MAPbI₃, and methylammonium lead bromide, MAPbBr₃ single crystals. As multiple, previous reports differ in both their techniques and their findings, we have investigated multiple characterization techniques including Rayleigh measurements, piezoresponse force microscopy, second harmonic generation, x-ray diffraciton, and DFT theory. Rayleigh analysis, in conjunction with piezoresponse force microscopy and second harmonic generation, suggest that these materials have polar regions at room temperature, and dielectric characteristics that show frequency dispersion below 60 °C, and elimination of the frequency dispersion at higher temperatures. Two-photon photoluminescence and electrical force microscopy were taken before and after polling and show measurable changes in the physical and optical properties of the crystals. These results point towards the influence ferroelectric domains on the optoelectronic properties.

Solar cells employed in spacecraft power systems require light-harvesting materials that are highly efficient, radiation-resistant, lightweight, and low cost. Organometallic halide perovskite materials having high absorption coefficient allows the cell to be very thin and lightweight, and the solution-processed method enables large and flexible device at low fabrication cost. To further qualify the candidacy of perovskites in space power applications, they must demonstrate durability and robustness in the space environment, specifically: electron radiation, proton radiation, UV light radiation, and thermal cycling. In this work, we characterized perovskite solar cells irradiated with 1-MeV energy electrons with fluences ranging from 10¹² to 10¹⁶ cm^-2, using the Dynamitron electron irradiation facility at the Jet Propulsion Laboratory. We found that there were no changes in the in the perovskite thin film morphology crystallinity, and importantly the perovskite solar cells showed no degradation in photovoltaic open-circuit voltage, short-circuit current density or fill factor performance and no significant changes in spectral response. Monte Carlo simulations of high energy electron irration indicate that most of the electrons completely penetrate through all of the solar cell layers with little scattering or energy loss. Under similar irradiation conditions, GaAs solar cells undergo significant radiation damage. Broadly, these results suggest the perovskites have superior electron radiation tolerance under space environmental conditions.

The success of CH₃NH₃PbI₃ in photovoltaics and other optoelectronic applications has spurred research toward understanding perovskite crystallization from precursor and intermediate phases. The in situ monitoring of crystal structure, chemical composition, and component desorption has enabled a more complete picture of the formation and degradation of solution-processed lead halide perovskites and the precursor composition-induced variations in mechanism, kinetics, and thermodynamics. We find that the addition of Cl^- to lead to widely different precursor evolution and perovskite formation kinetics. In addition, Cl^- significantly modifies the thermal degradation of the resulting perovskite, changing the mechanism and thermodynamics of perovskite decomposition. This work highlights the important role that precursor chemistry plays in both the formation and degradation of halide perovskites.

In this study, we propose high-performance lead-free perovskite infrared LEDs based on low-temperature solution-processed CsSnI₃ perovskite. Two methods, one-pot solution synthesis method and toluene dripping method were employed to prepare the CsSnI₃ films. The CsSnI₃ films synthesized via the one-pot solution synthesis method exhibit a sponge-like morphology. The infrared LEDs fabricated using such sponge-like CsSnI₃ films exhibit poor performance. Contrarily, the CsSnI₃ films prepared via the toluene dripping method exhibit compact micrometer-sized CsSnI₃ grains with very few pinholes and cracks at the grain boundaries. Based on time-resolved photoluminescence measurement, the PL lifetime of the CsSnI₃ film is in femtosecond region. Infrared LEDs fabricated with such high-quality CsSnI₃ films exhibit high device performance. Electroluminescence at 950 nm was obtained with maximum radiance of 40 W sr⁻¹ m⁻² at a current density of 364.3 mA/cm² and maximum EQE of 3.8% at 4.5 V, respectively. Distributions of the peak EQEs of devices indicate the reproducibility of our devices. Based on impedance spectroscopy analysis, the recombination resistance of the device fabricated by one-pot solution synthesis method is lower than the one of the device fabricated by the toluene dripping method for about one order of magnitude, which implies an increase of the recombination rate in the device fabricated by one-pot solution synthesis method. Since the radiance of the device fabricated by one-pot solution synthesis method is inferior to the one of the device fabricated by the toluene dripping method, it can be concluded that the superior performance of the device fabricated by the toluene dripping method is attributed to its lower non-radiative recombination. Furthermore, the trap density of state (tDOS) in CsSnI3 films prepared by both methods can be derived from the angular frequency dependent capacitance. The tDOS in the device fabricated by one-pot solution synthesis method is higher than the one of the device fabricated by the toluene dripping method, which explains the inferior performance of the device fabricated by one-pot solution synthesis method. These high-performance and lead-free infrared LEDs are suitable for infrared lighting, optical communications, and noninvasive biomedical imaging.

One Effective Strategy for Room Temperature Fabrication of Inorganic Electron Elective Layer for Perovskite Solar Cells
K. Wang¹, Y. T. Shi*¹, T. L. Ma*^{2, 3
1}State Key Laboratory of Fine Chemicals, School of Chemistry, Dalian University of Technology, Dalian 116024, P. R. China
²School Petroleum and Chemical Engineering, Dalian University of Technology, Panjin Campus, Panjin 124221, P. R. China
³Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Fukuoka, 808-0196, Japan
*[email protected]; [email protected]n;
As the key component of perovskite solar cells (PSCs), the electron select layer (ESL) is responsible for extraction of photo-generated electrons and simultaneously acts as blocking layer for prohibition of the direct contact of the conductive substrate with hole transport material. Presently, it has been widely observed and accepted that some inorganic materials (TiO₂, ZnO, and SnO₂, etc) are capable of serving as ESLs only when crystallized state is formed (due to the need of electronic properties like energy band structure and charge mobility) by high-temperature annealing, which is unsuitable for the fabrication of flexible PSCs based on plastic substrate at low temperature. Although presently several solution routes have been proposed for low temperature fabrication of ESLs, it can be found that crystallization was still a prerequisite.
Here, we present a simple solution route by which inorganic ESLs can be fabricated at extremely low temperatures of 70 °C or even room temperature. This solution route is significant because the feasibility of fabricating inorganic ESL without any thermal treatment is confirmed in the field of PSCs for the first time. Unlike conventional ESLs, the WO_x-based amorphous and composite ESLs exhibit favorable optical and electric properties, as reflected by light transmittance, electrical conductivity, charge dissociation, and the suppression of charge recombination. As a result, the photovoltaic performance for planar PSCs is notably improved and a high PCE of 14.47% was obtained based on composite ESLs fabricated at 150 °C. Meanwhile, PSCs with ESLs fabricated at 70 °C and room temperature are also very efficient given that maximum PCEs of 13.45% and 11.56% are obtained, respectively. Our work presents a new strategy to develop amorphous, inorganic, and composite functional materials for the fabrication of ESLs in efficient PSCs to reduce production cost and energy payback time further.
References:
[1] J. T.-W. Wang, J. M. Ball, E. M. Barea, A. Abate, J. A. Alexander-Webber, J. Huang, M. Saliba, I. Mora-Sero, J. Bisquert, H. J. Snaith, R. J. Nicholas, Nano Lett. 2014, 14, 724.
[2] K. Wang, Y. Shi, Q. Dong, Y. Li, S. Wang, X. Yu, M. Wu, T. Ma, J. Phys. Chem. Lett. 2015, 6, 755.
[3] K. Wang, Y. Shi, B. Li, L. Zhao, W. Wang, X. Wang, X. Bai, S. Wang, C. Hao, and T. Ma, Adv. Mater. 2016, 28, 1891.

The solid-state perovskite solar cell with power conversion efficiency (PCE) of 9.7% was first reported in 2012 by our group. Its PCE now reaches 22%. It is believed that perovskite solar cell is promising next-generation photovoltaics (PVs) due to superb performance and very low cost. In this talk, grain boundary healing process will be described for PCE of more than 20%. Nonstoichiometric precursor induces self-formed grain boundary layers. Grain boundary is found to play important role in charge transporting, where charge conduction is pronounced for the perovskite film prepared from grain boundary healing process. Stability and I-V hysteresis are now critical issues in perovskite solar cell. Light-soaking and temperature-dependent test results will be discussed to understand photo- and thermal stability of perovskite solar cells. In addition, beyond PV, halide perovskite possesses multifunctionality. Results on ultrasensitive X-ray imaging system, resistive memory device and light emitting display based on halide perovskites will be discussed.

Hybrid organic-inorganic perovskite solar cells have been demonstrated as an attractive alternative to Si cells in photovoltaic applications. Beside CH₃NH₃PbI₃ (the most widely studied perovskite material), to date a number of other perovskite materials as well as various device architectures have been proposed and laboratory-scale solar cells can routinely achieve power conversion efficiencies exceeding 10%. On the other hand, there are still a few key challenges facing the field. Among these challenges the most significant one is the much shorter lifetime of perovskite solar cells under operation conditions as compared to Si cells. In OIST, a team of researchers in the Energy Materials and Surface Sciences Unit has been making concerted efforts to study perovskite solar cell lifetime and possible degradation mechanisms under operation conditions (i.e. continuous cell operation at maximum power point under 1 Sun illumination), based on which we develop strategies to fabricate perovskite solar cells with longer lifetime [1-5].

[1] Z. Hawash, L.K. Ono, Y.B. Qi*, Adv. Mater. Interfaces (2016) 1600117.
[2] Z. Hawash, L.K. Ono, S.R. Raga, M.V. Lee, Y.B. Qi*, Chem. Mater. 27 (2015) 562.
[3] L.K. Ono⁺, S.R. Raga⁺, M. Remeika, A.J. Winchester, A. Gabe, Y.B. Qi*, J. Mater. Chem. A 3 (2015) 15451 (⁺These authors contributed equally)
[4] Y. Kato, L.K. Ono, M.V. Lee, S. Wang, S.R. Raga, Y.B. Qi*, Adv. Mater. Interfaces 2 (2015) 1500195.
[5] M.C. Jung, S.R. Raga, L.K. Ono, Y.B. Qi*, Sci. Rep. 5 (2015) 9863.

Organic-inorganic hybrid perovskites are promising novel materials for emerging photovoltaic applications. The conversion efficiencies of perovskite-based solar cells have been increasing rapidly, now exceeding 20% [1]. Various studies on perovskite thin-films and single crystals revealed their superior optoelectronic properties, explaining the high conversion efficiencies achieved so far [2]. However, there are still problems to solve for practical implementation, such as device stability. For further improvement of the conversion efficiency and the device durability, in-depth understanding of the photocarrier dynamics in the device structure during operation is indispensable. In particular, the carrier dynamics and the device durability under electric field should be clarified. The perovskite photovoltaic device is usually composed of a polycrystalline perovskite layer, charge transport layers, and electrodes. To investigate the complicated optoelectronic properties of such a device, a combination of various spectroscopic methods is useful. Electroluminescence (EL) is a powerful technique for studying the combined optical and electrical properties of the whole device, while photoluminescence (PL) selectively probes the carrier dynamics within the perovskite layer. With PL and EL measurements, the physics of the device operation and performance degradation can be discussed in detail.
In this study, we investigated the performance degradation of the perovskite CH₃NH₃PbI₃ solar cells under bias voltage in air and nitrogen atmospheres, and revealed the degradation mechanism utilizing current-voltage, time-resolved PL, and EL measurements. We fabricated a perovskite solar cell consisting of a CH₃NH₃PbI₃ absorber layer, compact and mesoporous TiO₂, and Spiro-OMeTAD transport layers. It was found that the application of forward bias voltage in dark caused a decrease in the conversion efficiency of the solar cell in air, while no decrease was observed in nitrogen atmosphere. The time-resolved PL measurements under bias voltage clearly showed that the bias application in air led to an increase of nonradiative recombination centers in the perovskite layer [3]. We consider that the deterioration of the perovskite layer is the main reason for the performance degradation. Furthermore, we found that under bias voltage in air, the EL intensity decreased with time, and simultaneously the current through the device increased. From these PL and EL results, we conclude that the device degradation is initiated by chemical modification of the perovskite grain surface. We will discuss the degradation of the perovskite solar cells under bias voltage, with respect to the carrier transport in the perovskite layer and carrier injection into the charge transport layers.
Part of this work was supported by JST-CREST.

[1] W. S. Yang et al., Science 348, 1234 (2015).
[2] Y. Yamada et al., J. Am. Chem. Soc. 137, 10456 (2015).
[3] T. Handa et al., Opt. Express 24, A917 (2016).

Organolead halide perovskites have recently emerged as a fascinating light harvesting material benefiting from its simple fabrication process and excellent electronic properties. The power conversion efficiency (PCE) of perovskite solar cells (PSCs) has been rapidly improved from 3.8% to 22.1% within the past several years. However, the instability of PSCs is its Achilles’ heel, and the issue originates not only from the perovskite itself, but also from the use of doped organic charge transport materials, such as spiro-MeOTAD. In this work, we report new strategies to fabricate stable PSCs with metal-oxides as charge transport layers. Firstly we developed a two-step method to synthesize PbI₂ precursor thin films with nano-pores and tunable crystal sizes. We found that the unique porous structure allows formation of an organic salt-rich, e.g., MAI-rich, environment during the reaction of the organic salt and PbI₂, thus creating a condition that is favorable for the growth of large perovskite crystalline domains. We then incorporated the perovskite layer in a FTO/TiO₂/MA_xFA_1-xPbI_yBr_3-y/NiO/metal structure. Through modification of the NiO/metal interfaces, we realized high-efficiency PSCs with excellent air stability.

Perovskite solar cells (PSCs) have captivated researchers due to high efficiencies and easy fabrication. Hole transport layer (HTL) is key to attaining higher efficiencies in PSCs. Currently the most widely used HTL is spiro-MeOTAD. The addition of LiTFSI dopants to spiro-MeOTAD HTL is necessary to achieve desirable hole transport properties. It has been observed that several hours of ambient air exposure is needed to reach highest solar cell performance. Using photoemission spectroscopy and mercury drop electrode I–V measurements on hole-only devices, we investigated the influences of H₂O vapor (RH 90%, RT), dry O₂, and ambient air (RH 50%, RT) on charge transport properties of LiTFSI-doped spiro-MeOTAD HTL. Our results reveal that H₂O vapor exposure resulted in an irreversible enhancement of LiTFSI-doped HTL conductivity. XPS showed that this enhancement was mainly due to LiTFSI redistribution across the HTL. Such an effect is believed to be major cause for increased cell performance when perovskite solar cells are exposed to ambient air for a period of time (typically several hours).

Reference:
Zafer Hawash, Luis K. Ono, Yabing Qi*, Adv. Mater. Interfaces (2016) DOI: 10.1002/admi.201600117

Organolead trihalide perovskite (OTP) materials have drawn tremendous attention in the past years because of its great promise to fabricate next generation of low-cost and highly efficient solar cells. In conventional solar cells, recombination of photogenerated carriers plays the major limiting role in the cell efficiency. Benefitting from the unique defects physics, charge recombination in the grain interior of OTP materials can be negligible due to the absence of deep traps in OPT grains.^{1, 2} The carrier diffusion lengths can be much longer than the light penetration length, thanks to the enlarging grains and improving crystallinity of the perovskite films with recent rapid improvement in material morphology controlling.^{3, 4} The enlargement of grains also significantly reduces carrier recombination at perovskite grain boundaries (GBs), which is facilitated by the advance of passivation techniques to further reduce recombination at GBs and film surface.⁵ Now that the photo-generated carriers can flow through perovskite films with negligible charge recombination, minimizing the charge recombination at the contacts becomes increasingly important to achieve highly efficient perovskite solar cells.
In this presentation, I will report a simple technique to suppress the surface charge recombination of perovskite solar cells to achieve a high power conversion efficiency of 20.3 % under one-sun illumination. Our idea was initially inspired by the hallmark technique applied in the commercial silicon solar cells, which resulted in the most efficient silicon solar panels in the world. However, we found that strictly copy the method used in silicon solar cells to perovskite solar cells was not feasible because OPT materials are more vulnerable to high temperature fabrication process. Encouragingly, we modified the original method by using low temperature solution-coated polymers, which potentially makes this technique low-cost and simple to be applied. What’s more, I will report this technique could significantly increase the perovskite device’s resistance to water-caused damage. The solar cells without further encapsulation could still operate well even when they were immersed in water.

1. Yin W-J, Shi T, Yan Y. Applied Physics Letters 2014, 104(6): 063903.
2. Kim J, Lee S-H, Lee JH, Hong K-H. The journal of physical chemistry letters 2014, 5(8): 1312-1317.
3. Bi C, Wang Q, Shao Y, Yuan Y, Xiao Z, Huang J. Nature communications 2015, 6.7747
4. Xiao Z, Dong Q, Bi C, Shao Y, Yuan Y, Huang J. Advanced Materials 2014, 26(37): 6503-6509.
5. Shao Y, Xiao Z, Bi C, Yuan Y, Huang J. Nature communications 2014, 5.5784

We studied the photoelectrochemical behavior of single crystal wafer and thin film of CH₃NH₃PbI₃ perovskite (MAPbI₃) in contact with a series of redox couples in dichloromethane (DCM). Experiments quantified the open-circuit photovoltage (V_oc), resistivity, and impedance for single crystal wafers of synthesized material as well as spin-coated, thin-layer electrodes with Ag back contact and systematically compared to results from carrier density and carrier lifetime measurements. Photoelectrochemical measurements were collected under 100 mWcm^-2 of ELH-simulated Air Mass 1.5 illumination for both bulk crystal and thin film electrodes utilizing the band edge positions and barrier heights. Impedance spectroscopy and photoelectrochemistry results showed n-type behavior for most of the MAPbI₃ electrodes in contact with the Ferrocence/Ferrocenium redox couple family in the potential range of -0.2~0.3 (Vvs. SCE) and Ohmic behavior in contact with the Cobaltocene/Cobaltocenium redox couple family in the potential range of -1.0~-1.2 (Vvs. SCE). A linear trend was observed in V_oc values under illumination as a function of the Nernstian solution potential, E(A/A^-), for both single crystal wafer and thin film perovskite electrodes. However, the results under illumination and dark condition indicate higher resistivity for thicker MAPbI₃ wafers with longer diffusion length.

The past five years have witnessed rapid progress of metal halide perovskite solar cells. However, device stability remains a primary concern for this technology to succeed. In order to enable improvements in device stability, a firm understanding of degradation pathways needs to be gained.
Here, we report an in-situ degradation study on solar cells using in-situ SEM and in-situ XRD under controlled humidity and vacuum pressure. Both in-situ studies show that the degradation and ultimate decomposition of devices must be considered in view of the overall device stack. In contrast to the belief that device degradation is due to the release of HI and methylamine by the decomposition of perovskite after water vapor exposure, our results show that it is actually due to redox reactions between the metallic Al electrode and MAPbI₃, in which Al⁰ strongly reduces Pb²⁺ to Pb⁰. In particular, the in-situ SEM measurement shows that moisture serves as a chemical diffusion agent to enable the continuous reaction of Al electrode and perovskite, well before serving as a decomposition agent for the perovskite film. In-situ XRD measurements under controlled moisture in the dark provide more detail about chemical degradation, showing that Al will reduce Pb²⁺ to Pb⁰, converting MAPbI₃ to MA₄PbI₆●2H₂O, and then to MAI. Notably, this decomposition occurs without producing PbI₂, an often-invoked decomposition product of perovskite absorbers. Surprisingly, when samples are exposed to high humidity (~90% relative humidity) for 4-5 h, well beyond the point of device degradation (device power conversion efficiency is reduced from 12.5% to less than 10% of its original value within 1 min), the full device stacks including the Al electrodes become transparent, with an overall transmittance of ~80%, due to redox reactions between Al and perovskite.
Our work brings important insight regarding electrode and buffer layer choices in perovskite solar cells to allow for enhanced stability. Further, our results stress that, because instability may be triggered from any of the layers in a device, degradation studies need to consider the device as a whole, rather than only considering the stability of perovskite films.

Metal halide perovskite solar cells are now considered not only as a low-cost alternative to commercialized solar cells, but also as a new functional cell with flexible applications. However, the perovskite solar cells have shown fast deterioration under actual operation even with encapsulation and its reason has been elusive. Here, we show that trapped charges play a decisive role in the degradation of perovskite materials regardless of the polarity[1]. A novel experimental set-up utilizing the deposition of ions with different polarity via a corona discharger reveals that perovskite materials degrade irreversibly through grain boundaries only when moisture is introduced on charge trapped perovskite surface, which indicates that the moisture induced irreversible dissociation of perovskite materials is triggered by trapped charges. This not only explains why the degradation of perovskite solar cells begins from different side of interface contacting hole or electron extracting layer depending on the use of different charge extraction layer, but also why the light soaking on the perovskite materials consistently induces irreversible degradation in the presence of moisture. The Kelvin Probe Force Microscopy measurements confirm that charges are trapped preferentially on the grain boundaries even under uniform deposition of ions or uniform illumination of light supporting the role of trapped charges in the degradation through grain boundaries. We also identified the synergetic effect of oxygen on the process of moisture induced degradation. Our study suggests that the prevention of charge accumulation at interfaces and grain boundaries is very important in addition to proper encapsulation for developing commercially viable perovskite solar cells.
[1] N. Ahn, K. Kwak, M. S. Jang, H. Yoon, B. Y. Lee, J.-K. Lee, P. V. Pikhitsa, J. Byun, and M. Choi, “Trapped charge driven degradation of perovskite solar cells”, arXiv:1604.07912, (2016)

In the last few years, the advent of metal halide perovskite solar cells has revolutionized the prospects of next-generation photovoltaics [1]. As this technology is maturing at an exceptional rate, research on its environmental impact is becoming increasingly relevant. With arising consciousness regarding the heavy metal content of this technology, and the possible strain this imposes on the public perception and acceptance, we provide an overall and well-balanced summary of the toxicity associated with perovskite solar cells from cradle to grave. We highlight for each phase of the device life-cycle different routes of intoxication and environmental burden – both on an occupational and non-occupational level –, stressing the relevance of the exact chemical nature of precursor materials (organic or inorganic metal compounds), solvents and their relation to various deposition methods [2].We also provide insight into the kinetics of Lead or Tin entering the human body with respective threshold values, symptoms and diagnostics [3]. Above all, we discuss some less obvious hazards in the device lifecycle and elaborate on challenges that still have to be overcome (cfr. robust encapsulations and recycling strategies). We also revisit Cadmium Telluride solar cells, a commercial solar technology also employing notorious heavy metals, and point out differences, similarities, and key facets that could allow perovskite to thrive commercially as well [4].

References
[1]. S. D. Stranks, H.J. Snaith, Nature Nanotech. (2015), 10, 391- 402.
[2]. World Health Organization, Environmental Health Criteria 3: Lead, 1977.
[3]. World Health Organization, Exposure to lead: a major public health concern, 2010.
[4]. J. Zayed, S. Philippe, Int. J. Toxicol. (2009), 28, 259-265.

In the past several years, organolead halide perovskites have emerged as a revolutionary photovoltaic (PV) material that can be low-temperature processed while delivering high solar cell efficiency. An important advantage of this class of perovskites is their synthetic simplicity and versatility: the materials can be made from a variety of precursor combinations and deposited through different physical processes, such as one-step spin-coating, sequential deposition, vapor-assisted deposition, etc. This property provides a great room for tuning the electronic properties of the perovskite devices through material engineering. In this talk I will describe our recent endeavors toward realizing high-efficiency and stable perovskite solar cells. Particular topics to be addressed include development of new precursors, passivation of perovskite films and substitutional doping. I will also share our findings on how these engineering processes affect the photophysical processes in perovskite solar cells and demonstrate that high efficiency and high air stability can be simultaneously achieved with a proper material passivation process.

In this presentation, I will demonstrate our latest achievement on invert PSCs that a certified efficiency of 18.2% was obtained from devices with working area over 1 cm². To realize this achievement, we put our focus on two aspects: one is improving the reproducibility of device performance through controlling the morphology and uniformity of perovskite layer and charge extraction layers in the solar cells; the other is increasing the long-term stability of device by developing new inorganic charge extraction materials with high conductivity. For example, we developed a new method for fabricating uniform CH₃NH₃PbI₃ layer with using uncrystallized PbI₂ precursor films, which overcame the problem of incomplete conversion and uncontrolled particle sizes of perovskite, thus greatly increasing the film uniformity and reproducibility. Inorganic semiconductors such as NiO and TiO2 were used as charge extraction materials. In order to reduce pinhole, the thick charge extraction layers were used. In order to increase conductivity, these layers were heavily doped with Mg²⁺, Li⁺ and Nb⁵⁺. As a result, we achieved energy conversion efficiency of 18.2% with cell area of cm². Finally, we also develop novel method to fabricate large area of perovskite solar cells.

Perovskite solar cells (PSCs) represent high power conversion efficiencies at potentially low material costs. This makes them a promising technology for new light harvesting applications, e.g. building integrated photovoltaics. However, the rather low light stability of PSCs remains a key challenge for their economical breakthrough. Recent reports explain light degradation of PSCs with the formation of metastable recombination centers in the perovskite. In this work, we provide new insights about the formation of these recombination centers, and propose strategies to improve device stability.

First, we study the metastability and lifetime of light-induced recombination centers in a commonly used PSC stack with 14% efficiency. We observe lifetimes exceeding hundreds of hours, thus defect healing during outdoor day-night cycles will be negligible, and the collected photocurrent will decrease over time. Therefore, understanding the conditions for the generation of these recombination centers is of key importance to increase the reliability of PSCs. In the next phase, we show that the light-induced generation of recombination centers depends on the device stack and can be controlled through appropriate architecture selection. We report that 4-tert-Butylpyridine (tBP), an additive employed in the hole transport layers of PSCs with record efficiencies, enhances the generation of recombination centers. These results demonstrate the need to replace tBP by other additives, or to use dopant-free hole transport layers, to improve device stability. For this purpose we track the maximum power point and measure the current-voltage characteristics of devices with promising transport layers under continuous illumination.

Perovskite solar cells have been recognized as a newly emerging solar cell with the potential of achieving relatively high efficiency with a low cost fabrication process. In particular, facile solution processed cell fabrication facilitated rapid development of optimum cell structure and composition. Over the last few years, the cell efficiency has rapidly been improved and now exceeds 20%. However, despite this development, fundamental cell operation mechanism such as a charge separation or recombination process at the perovskite interface has not been well understood.

A typical perovskite solar cell employs a nanocrystalline TiO₂ film, CH₃NH₃PbI₃ perovskite and spiro-OMeTAD hole conductor. Following light absorption, an electron and a hole are separated at the perovskite film interface, and are collected at the FTO and Au electrodes, respectively. We have recently clarified the role of the TiO₂ nanocrystalline film, and concluded that this type of solar cell is particularly suitable to be operated under 1 sun condition [1]. In this presentation, we will demonstrate charge separation and recombination dynamics at the perovskite interfaces employing a series of transient absorption and emission spectroscopies. Nanosecond transient emission spectroscopy clarifies charge separation processes, while Vis-NIR sub-microsecond to millisecond transient absorption spectroscopies identify charge separation efficiency and charge recombination rates. The influence of the cell conditions on the charge transfer dynamics will be discussed.

This work is financially supported by JST PRESTO program (Photoenergy Conversion Systems and Materials for the Next Generation Solar Cells), Japan. The author also acknowledges Australian Research Council (ARC) LIEF grant (LE140100104) and the Office for University-Industry Collaboration, Osaka University, for the financial supports.

[1] S. Makuta, M. Liu, M. Endo, H. Nishimura, A. Wakamiya, Y. Tachibana, Chem. Commun., 52, 673 - 676 (2016).

Recent development of hybrid perovskites (HYPE) is steering the progress made in current solar cell research. Their success is predominantly based on properties like high absorption coefficients and simultaneous effective separation of photogenerated charges. Though well knowing that material properties are influenced by crystal quality and morphology, as in conventional semiconductors used in solar cell devices. Therefore controlled nanostructures are a mighty tool to improve the properties of almost every semiconductor.^[1]

Allocation of unique HYPE nanostructures lag behind the evolution of optimizations in device processing. Especially, as nucleation and growth are difficult to control for the common used precipitation procedures from supersaturated solution, the synthesis of nanostructures bears major difficulties as HYPE materials are delicate to various synthetic parameters like humidity, temperature or oxygen atmosphere. Defined single-source precursors for the synthesis of HYPE like CH₃NH₃PbI₃ (MAPI) are rarely developed, but show enormous potential for controlled material conversion.^[2] Here, we introduce a systematic precursor system based on oligo-glycols with single-source characteristics, organizing MAPI on the molecular level, which allows the growth of defined nanostructures by different approaches.

The presented new oligo-glycol based single-source precursors can be designed from solid to liquid compounds, dependent on chain length and coordination chemistry. The liquid precursor system opens up the unique access to a microwave assisted crystallization of MAPI scaffolds, revealing a local photoluminescence enhancement at the outer crystal surface.^[3] With a solid precursor, nanoporous MAPI single crystals are obtained by a crystal-to-crystal transformation which is based on a spinodal demixing mechanism of the unique precursor structure.^[4] Further, the particle shape and properties of MAPI nanocrystals can be controlled by selective binding of capping agents on the surface, which leads to materials with various aspect ratios and functionalities. Thus, the attachment of designed molecules (e.g. conductive nature) can be envisioned to passivate charged surface defects leading to less recombination sites while retaining good conductivity in optoelectronic devices.

[1] Alivisatos, A. P. Science 1996, 271, 933–937. 10.1126/science.271.5251.933
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[3] Kollek, T.; Fischer, C.; Göttker-Schnetmann, I.; Polarz, S., Chem. Mater. 2016, 10.1021/acs.chemmater.6b01263
[4] Kollek, T.; Gruber, D.; Gehring, J.; Zimmermann, E.; Schmidt-Mende, L.; Polarz, S. Angew. Chemie Int. Ed. 2015, 54, 1341–1346. 10.1002/anie.201408713

The fundamental nature of charge carrier transport and recombination in heavy metal halide perovskites is of great interest because of their exceptional performance in photovoltaic devices. This talk will focus the optical properties of the vacancy-ordered triple perovskite Cs₃Bi₂Br₉. Using DFT calculations, we find evidence that the top of the valence band and bottom of the conduction band minimum are dominated by Bi-s and Bi-p states respectively. Experiments show that this produces a sharp exciton peak in the absorption spectra even at room temperature and produces a highly structured emission as a result of exciton-phonon interactions. We also demonstrate the band gap can be tuned by almost 1 eV through substitution of iodine for bromine, while maintaining the layered topology of the structure. These results will be discussed in the context of developing new lead free alternatives to CH₃NH₃PbI₃ and the importance of the structure in determining materials performance.

Metal halide perovskites have rapidly advanced thin film photovoltaic performance; as a result, the materials’ observed instabilities urgently require a solution. Using density functional theory (DFT), we show that a low energy of formation, exacerbated in the presence of humidity, explains the propensity of perovskites to decompose back into their precursors. We find, also using DFT, that intercalation of phenylethylammonium between perovskite layers introduces quantitatively appreciable van der Waals interactions; and these drive an increased formation energy and should therefore improve material stability.
In this work, we introduce reduced-dimensionality (quasi-2D) perovskite films that exhibit improved stability while retaining the high performance of conventional three-dimensional perovskites. Continuous tuning of the dimensionality, as assessed using photophysical studies, is achieved by the stoichiometry chosen via materials synthesis. We achieve hysteresis-free solar power conversion in a planar perovskite solar cell and observe greatly improved performance longevity.
Organometal halide perovskites are also of intense interest in light emission applications. In LEDs, which rely on the forward injection of electrons and holes, perovskites’ excellent mobilities contribute to the efficient capture of nonequilibrium charge carriers to rare nonradiative centres; and the lack of bound excitons weakens the competition of desired radiative over undesired nonradiative recombination.
We therefore explore the design and deployment of a perovskite mixed material, one comprised of a series of differently quantum-size-tuned grains, that funnels photoexcitations to the lowest-bandgap light-emitter in the mixture. The materials function as charge carrier concentrators, ensuring that radiative recombination successfully outcompetes trapping and hence nonradiative recombination. We use the new material to build devices that exhibit an external quantum efficiency (EQE) of 8.8% and a radiance of 80 Wsr^-1m^-2. These represent the brightest and most efficient solution-processed near-infrared LEDs to date.