Halide Perovskite Photovoltaics: Background, Status, and Future

Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX

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Halide Perovskite Photovoltaics: Background, Status, and Future Prospects Ajay Kumar Jena,† Ashish Kulkarni,† and Tsutomu Miyasaka*,†,‡ Graduate School of Engineering and ‡Faculty of Medical Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba, Yokohama, Kanagawa 225-8503, Japan

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†

ABSTRACT: The photovoltaics of organic−inorganic lead halide perovskite materials have shown rapid improvements in solar cell performance, surpassing the top efficiency of semiconductor compounds such as CdTe and CIGS (copper indium gallium selenide) used in solar cells in just about a decade. Perovskite preparation via simple and inexpensive solution processes demonstrates the immense potential of this thin-film solar cell technology to become a low-cost alternative to the presently commercially available photovoltaic technologies. Significant developments in almost all aspects of perovskite solar cells and discoveries of some fascinating properties of such hybrid perovskites have been made recently. This Review describes the fundamentals, recent research progress, present status, and our views on future prospects of perovskite-based photovoltaics, with discussions focused on strategies to improve both intrinsic and extrinsic (environmental) stabilities of high-efficiency devices. Strategies and challenges regarding compositional engineering of the hybrid perovskite structure are discussed, including potentials for developing all-inorganic and lead-free perovskite materials. Looking at the latest cutting-edge research, the prospects for perovskite-based photovoltaic and optoelectronic devices, including non-photovoltaic applications such as X-ray detectors and image sensing devices in industrialization, are described. In addition to the aforementioned major topics, we also review, as a background, our encounter with perovskite materials for the first solar cell application, which should inspire young researchers in chemistry and physics to identify and work on challenging interdisciplinary research problems through exchanges between academia and industry.

CONTENTS 1. Discovery and Background of Perovskite Photovoltaics 1.1. Photovoltaics of Halide Perovskites 1.2. Discovery and History of Perovskite Photovoltaics 1.2.1. From Oxide to Halide Perovskites 1.2.2. Discovery of Halide Perovskite Solar Cells 2. Fundamental Structure, Working Mechanism, and Major Milestones of Progress 2.1. Semiconductor Properties of Organic Lead Halide Perovskites 2.2. Working Principle 2.3. Major Milestones of Progress 3. Metal Oxide-Based Electron Transport Layers in Perovskite Solar Cells 4. Compositional Engineering of Perovskites 4.1. Mixed Compositions 4.1.1. A-Site Cations Mixture 4.1.2. X-Site Anions Mixture 4.1.3. Both Cations and Anions Mixture 4.2. Mixed Dimensions 5. Stability of Perovskite Solar Cells 5.1. Stability Issues with Perovskites 5.1.1. Structural/Intrinsic Stability 5.1.2. External/Environmental Stability © XXXX American Chemical Society

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B B

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B B C E E G H

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I K K L O O S T T U Y

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5.2. Stability Issues with Hole Transport Materials and Contacts All-Inorganic Perovskites 6.1. CsPbI3: Stabilization of Black Phase 6.2. Cesium−Lead Mixed Halide Perovskites Lead-Free and Low-Lead Perovskites 7.1. Lead-Free Perovskite Materials 7.1.1. Tin (Sn)-Based Perovskites 7.1.2. Germanium (Ge)-Based Perovskites 7.1.3. Lead-Free Binary Metal Halide Perovskites 7.1.4. Group 15 Metal-Based Perovskite/Nonperovskite Materials 7.2. Low-Lead Perovskite Materials: Reducing Toxicity and Enhancing Efficiency Toward Commercialization 8.1. Scaling Up and Reproducibility Challenges 8.2. Perovskite Tandem Cells 8.3. Low-Temperature Process and Flexible Device 8.4. Potential Applications in Optoelectronic Devices Conclusions and Future Prospects

AA AD AE AH AJ AJ AJ AL AM AN AQ AR AR AT AV AW AZ

Special Issue: Perovskites Received: August 30, 2018

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DOI: 10.1021/acs.chemrev.8b00539 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 9.1. Further Improvement in PCE 9.2. How To Increase Intrinsic Stability Further? 9.3. How To Increase Environmental Stability Further? 9.4. Potential of Lead-Free Perovskites 9.5. Steps toward Commercialization Author Information Corresponding Author ORCID Notes Acknowledgments References

Review

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1. DISCOVERY AND BACKGROUND OF PEROVSKITE PHOTOVOLTAICS 1.1. Photovoltaics of Halide Perovskites Figure 1. Year-wise citation history of the first paper1 on perovskite solar cells based on data obtained from Clarivate Analytics.

The evolution of organic−inorganic lead halide perovskite solar cells (PSCs) has accomplished the most notable progress in the field of photovoltaics (PV). In the decade since the publication of our peer-reviewed paper in 2009,1 the first on the topic, which was based on our experiments that had started as early as 2005, the power conversion efficiency (PCE) of PSCs has rapidly increased to reach the latest record of 23.7% (reported by the U.S. National Renewable Energy Laboratory (NREL), https://www.nrel.gov/pv/assets/pdfs/pv-efficiencychart.201812171.pdf), approaching the top values achieved with single-crystalline silicon solar cells. Not only the high performance but also the low-cost solution-based processes used for device fabrication attest to the immense potential of PSCs to be a PV technology of the future. While solutionbased syntheses of the perovskite materials involve a lot of chemistry, crystallization engineering and the optical and electrical characterization of solid-state crystals (semiconductors) have their backgrounds in physics. Because of the interdisciplinary nature of perovskite PV, necessitating expertise in chemistry, physics, and optoelectronics, the research field has gained interest from a large community of researchers around the world. As a result, research progress in perovskite PV has been tremendous. Interestingly, with the discoveries of its rare properties, perovskites have found many applications beyond PV, expanding into the areas of lightemitting diodes, photodetectors, X-ray detectors, memory devices, and so on. Such multiple functions of halide perovskites have promoted considerable interdisciplinary research. On the basis of information from Clarivate Analytics in 2018, we estimate that researchers at more than 1000 institutes worldwide are presently working on halide perovskite-related photovoltaics and optoelectronics, and this has produced more than 8000 scientific papers in the field. Figure 1 shows, by year (from 2009 to 2018), the history of citations of our first paper,1 obtained from Clarivate Analytics, representing the explosive escalation of interest in perovskites. Being a magic box of many mysterious properties, perovskites have also triggered fundamental studies on ion migration, defect tolerance, carrier dynamics in this soft semiconductor. However, while some of the mysterious properties of perovskite are very valuable, others are creating issues that impair practical applications of the new technology. For instance, while the defect tolerant nature of perovskite contributes to high efficiency the ion-migration phenomenon stands as a potential threat against stability. As efficiency and

stability are two crucial factors for device implementation, in this Review, in addition to high efficiency aspects of PSCs, we focus on the stability issues and strategies to overcome them. Although there are challenges in industrialization of PSCs, results coming from both research laboratories and industries show prominent potential. The tremendous success of perovskite PV was quite unexpected by our group.1 With a long silence (almost no citations) for about 3 years after our first publication and a series of presentations at international conferences, we had not anticipated PSCs becoming the subject of such a huge amount of research activity. In fact, it was only this success that made us realize the importance of the beginning. Hence, we take this opportunity to share the background story of our discovery of perovskites as PV materials first, followed by reviews and discussions on their performance, their stability, industrialization, present challenges, and future prospects. 1.2. Discovery and History of Perovskite Photovoltaics

1.2.1. From Oxide to Halide Perovskites. The term “perovskite solar cell” might sound familiar to most of us today, but it was alien in 2005, when the journey to explore PV applications of halide perovskites began in the Miyasaka laboratory at Toin University in Yokohma, Japan. At that time, “perovskite” generally meant metal oxides having perovskite structures, most of which are classified as either ferroelectric or piezoelectric materials. Perovskite generally represents a kind of crystal structure with chemical formula ABX3, in which A and B are cations and X is an anion. In an ideal cubic structure, the B cation has a 6-fold coordination, surrounded by an octahedron of anions, and the A cation has a 12-fold cuboctahedral coordination. The cubic unit cell of such compounds is composed of A cations at cube corner positions, B sitting at the body-center position, and X anion occupying the face-centered positions (see Figure 3a). In the history of minerals, perovskite was first discovered in a piece of chloriterich skarn by the Prussian mineralogist Gustav Rose in 1839.2 The mineral was composed of CaTiO3 and was named after the renowned Russian mineralogist Count Lev A. Perovskiy (1792−1856) upon the request of a notable Russian mineral collector, August Alexander Kämmerer. Later, many inorganic metal oxides, such as BaTiO3, PbTiO3, SrTiO3, BiFeO3, etc., B


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Figure 2. Solid-state mixing routes for synthesis of (a) CH3NH3PbBr3 and (b) CH3NH3PbI3. Photographs of CH3NH3PbBr3 powder spread on paper and exposed to (c) room light and (d) UV lamp (black light), showing strong emission under the UV light. Solutions of (e) CH3NH3PbBr3 and (f) CH3NH3PbI3. Photographs of (g) TiO2/CH3NH3PbBr3 and (h) TiO2/CH3NH3PbI3 photoanodes. Scanning electron micrographs of (i) bare TiO2 and (j) TiO2 loaded with CH3NH3PbBr3. Photographs of (k) MAPbBr3 single crystals, (l) precursor solution (dated Aug 4, 2009), and (m) carbon-based perovskite photovoltaic devices used as the first solid-state perovskite solar cell, all made by Kojima.

researchers to use other cations in place of Cs. Weber6,7 found that the organic cation methylammonium (CH3NH3+) replaces Cs+ to form CH3NH3MX3 (M = Pb,6 Sn,7 X = I, Br) and reported the first crystallographic study on organic lead halide perovskites. Toward the end of the 20th century, a large variety of halide perovskites were synthesized by David Mitzi using small and large organic cations.10−12 Mitzi had focused his studies on the physical properties of twodimensional (2D) perovskite materials with a large organic group.11 Based on Mitzi’s studies, in the late 1990s, Prof. Kohei Sanui was conducting a project through a Japanese national research program (JST-CREST). This project dealt with selforganized quantum confinement structures using the above perovskites; optical properties of 2D13,14 and 3D crystals were investigated.15,16 Although the research opened applications of these materials to nonlinear optics and electroluminescence (i.e., light-emitting diodes, LEDs) by utilizing sharp monochromatic optical absorption and luminescence,17,18 there was no idea that these materials could utilize solar energy, because 2D perovskites are not suitable for harvesting light over the wide spectral range of sunlight. 1.2.2. Discovery of Halide Perovskite Solar Cells. Firstand second-generation solar cells comprising silicon waferbased and thin-film solar cells, respectively, have done well in terms of efficiency and stability. However, ultra-high-pure metallic silicon (>99.9999%), which can be obtained by crystallization of melted Si in a furnace at more than 1400 °C, is required for the solar cells. Thus, the high cost of materials and processing of the wafers has prevented the popular use of such solar cells as alternatives to fossil-fuel-based energy sources such as thermal power generation (present cost is

were found to have the perovskite structure, so therefore, perovskite compounds are more commonly known as metal oxides, with formula ABO3. Although David Cahen et al. mentioned in their review3 that a perovskite might have been first synthesized in 1882 by the Danish chemist and crystallographer Haldor Topsøe (1842−1935), it seems that the first synthesis had been attempted in 1851 by French researcher Jacques-Joseph Ebelmen, who synthesized CaTiO3 by a flux growth method.2 Oxide perovskites are in use in various ferroelectric, piezoelectric, dielectric, and pyroelectric applications, etc., But except for some limited compositions like LiNbO3, PbTiO3, and BiFeO3, which show some PV effect due to ferroelectric polarization (known as ferroelectric photovoltaics),4 these metal oxide perovskites do not exhibit good semiconducting properties that would make them suitable for PV applications. However, a class of halide perovskites which differ from oxide perovskites in having halide anions in place of oxide anions (ABX3; A = cation, B = divalent metal cation, X = halogen anion) shows the semiconducting properties that are desired for PV applications. The discovery of such halide perovskites dates back to the 1890s. In 1893, Wells et al. performed a comprehensive study on the synthesis of lead halide compounds from solutions including lead halide and cesium, CsPbX3 (X = Cl, Br, I),5 ammonium (NH4),6 or rubidium, RbPbX3.7 Much later, in 1957, the Danish researcher C. K. Møller found that CsPbCl3 and CsPbBr3 have the perovskite structure,8,9 existing as a tetragonally distorted structure which undergoes a transition to a pure cubic phase at high temperature.9 The simple solution process for synthesis of these cesium lead halide ionic crystals might have inspired C


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Figure 3. (a) Crystal structure of organo-lead halide perovskite compounds. (b) SEM image (scale bar = 10 nm) of particles (shown by an arrow) of nanocrystalline CH3NH3PbBr3 deposited on the TiO2 surface. (c) Incident photon-to-current conversion efficiency (IPCE) action spectra for photoelectrochemical cells using CH3NH3PbBr3/TiO2 (solid line) and CH3NH3PbI3/TiO2 (dashed line) photoanodes with a liquid electrolyte, 0.4 M LiBr and 0.04 M Br2 dissolved in acetonitrile for the former and 0.15 M LiI and 0.075 M I2 dissolved in methoxyacetonitrile for the latter photoanode. (d) Photocurrent−voltage characteristics for cells using CH3NH3PbBr3/TiO2 (solid line, PCE = 3.13%) and CH3NH3PbI3/TiO2 (dashed line, PCE = 3.81%) under 100 mW cm−2 AM 1.5 irradiation. Reproduced with permission from ref 1. Copyright 2009 American Chemical Society.

studies from our team have been presented as a collaboration between three universities (TPU, TUY, and UT) and Peccell. Our perovskite-based PV cell first employed CH3NH3PbX3 (X = I, Br) as the sensitizer on a TiO2 mesoporous electrode used in conjunction with a lithium halide-containing electrolyte solution. Some of Kojima’s initial works, carried out before 2009, are presented in Figure 2. On the assumption that the perovskite would function as a quantum dot-like sensitizer, deposition of the perovskite was done by spin-coating of the precursor solution, in which the loading amount of the perovskite was adjusted so as to obtain the thinnest layer of nanocrystalline perovskite, to cover a large surface area of a thick TiO2 layer (∼10 μm), similar to DSSCs. This architecture was different from the present perovskite solar cell that uses a thin TiO2 film (1 μm) in polycrystalline perovskite films,31,93 which generally work as absorbers with thicknesses of more or less than 0.4 μm, efficient electron extraction can also be achieved with use of a nonporous flat ETM film, a dense TiO2 film, or even organic materials such as PCBM. In fact, the efficiency obtained with PCBM-based inverted structure devices94,95 has been close to those observed with TiO2-based devices. Nevertheless, many high-performance devices with PCE exceeding 20% have been fabricated on mesoporous TiO2 layers with varying compositions of perovskites. Although there is no comprehensive study on structural and compositional difference at the interface of perovskite with mesoporous TiO2 and compact TiO2, crystallization of perovskite on the two ETLs must be very different, which can certainly alter the interface. In a regular device structure, in which ETM is coated on transparent conductive substrates such as FTO and ITO, metal oxide ETMs are convenient to use in terms of their chemical stability (insolubility) against perovskite precursor solutions that are deposited on the ETM. Metal oxide ETMs are not necessarily employed in the form of mesoporous layers and can be used as nonporous compact layers (CLs) in the planar structure devices. The existence of a CL is essential to ensure selective electron extraction and hole-blocking function at the surface of the negative electrode. It is also a prerequisite even when a mesoporous metal oxide ETM layer is used as the scaffold for the perovskite layer. Interestingly, when we used a mesoporous Al2O3 layer on the TiO2 CL in a solid-state perovskite device,29 we found higher efficiency and VOC compared to those observed with a meso-TiO2-based cell used as reference. In this architecture, because of its nonconductive (insulator) property, Al2O3 does not conduct I


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Figure 10. (a) Cross-sectional scanning electron micrograph of ALD-SnO2-based perovskite ((FAPbI3)0.85(MAPbBr3)0.15) solar cell. (b) J−V characteristics of both ALD-TiO2- and ALD-SnO2-based planar heterojunction PSCs. (c) Schematic illustration of the existence (in the case of TiO2) or absence (in the case of SnO2) of an energy barrier between the electron transport material (ALD-TiO2 or ALD-SnO2) and the mixed perovskite. Reproduced with permission from ref 102. Copyright 2015 Royal Society of Chemistry, under Creative Commons Attribution NonCommercial 3.0 Unported License.

Figure 11. SnO2- and ZnO-based planar structure perovskite solar cells made by a low-temperature process. (a) Illustration of device structure and (b) energy level diagram showing the electron transport mechanism for a SnO2-based perovskite cell using MAPbI3 as absorber, in which PbI2 was used as an intermediate layer to suppress recombination. (c) J−V characteristics of the SnO2 perovskite cell showing VOC = 1.08 V. Reproduced with permission from ref 103. Copyright 2015 Royal Society of Chemistry. (d) J−V characteristics of the ZnO-based perovskite solar cell, using Csx(MA0.17FA0.83)1−xPb(I0.83Br0.17)3 as absorber, showing VOC = 1.11 V. Reproduced with permission from ref 104. Copyright 2017 Royal Society of Chemistry.

the case for PSCs. Perovskite solar cells using SnO2 and ZnO exhibit VOC and JSC comparable with those of TiO2-based cells. Indeed, one of the record efficiencies has been obtained with SnO2.102 The differences between DSSCs and perovskite cells must be due to differences in their device structure and working mechanism. The thicknesses and volumes of metal oxide semiconductors employed in both cells are very different. It is reasonable that a thin film (compact layer, 20 mA/cm2 obtained

the electrons, but its role is to accommodate perovskite into its porous network so that carriers are conveyed to the substrate (electrode) through the infiltrated perovskite. This is, in fact, enabled by the long distance diffusivity of carriers in perovskite. In this aspect, a meso-Al2O3-based perovskite cell works basically the same as a planar heterojunction structure cell without a mesoporous ETM layer. As examples of further developments in such Al2O3-based devices, we accomplished a PCE of 17% (Figure 9),24 while Snaith et al.97 also obtained a PCE up to 16.7% in a meso-Al2O3-based MAPbI3 solar cell. However, after those reports, much less work was done on the use of Al2O3 because devices with Al2O3 scaffolds often showed low fill factors (FFs), attributed to high impedance of the Al2O3 network, which limited the cell’s performance. However, the ability to use Al2O3 provided the first evidence that perovskite is capable of transporting carriers over long distances without significant recombination.24 A number of metal oxide semiconductors other than TiO2, which had been already employed in DSSCs, have also been used as ETMs in PSCs. SnO298,99 and ZnO100,101 have been frequently employed as alternatives to TiO2 in PSCs. In DSSCs, the energy level of the CB significantly influences the VOC and short-circuit photocurrent (JSC) of the cells. In particular, as VOC in DSSCs is determined by the energy gap between the CB level of the semiconductor and the redox potential of the iodide-based redox agent in the electrolyte (HTM), SnO2, having a deep CB level (large work function), always results in low VOC. However, interestingly, this is not J


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Figure 12. Energy levels of lead halide perovskite absorbers and various metal oxide electron transport materials and hole transport materials (HTMs) employed in solar cell devices. In the perovskites, MA and FA stand for methylammonium and formamidinium, respectively. PCBM denotes [6,6]-phenyl-C 61-butyric acid methyl ester. In HTMs, PTAA, P3HT, and PEDOT:PSS denote poly[bis(4-phenyl)(2,4,6trimethylphenyl)amine, poly(3-hexylthiophene-2,5-diyl), and poly(3,4-ethylenedioxythiophene)−polystyrene sulfonate, respectively.

analyzed in high-vacuum conditions, such as by ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS), do not necessarily indicate the actual energy levels of thin films exposed to ambient air and or of those in chemical contact with other charge transport or absorber materials in the device structure. In this regard, the summary in Figure 12 is only a reference for approximate comparison of energy levels to estimate the possibility of charge-transfer processes. All of these ETMs are capable of sufficiently high VOC. VOC of the device tends to be more influenced by the quality of the perovskite film and its heterojunction interfaces than by the bulk properties of the ETM. The kinds of defects and density of defects that strongly affect the carrier collection and recombination at the ETM/perovskite interface are also important to the overall performance of the cells. One of our studies, where we explored the use of a TiO2−MgO bilayer as the ETM layer (ETL), indicated that defects (trap states) present in TiO2, which can be largely influenced by preparation methods, can cause losses in the open-circuit voltage of the cells.116 Defects/traps at the ETM/perovskite interface have been found to directly or indirectly influence the cell’s performance, especially J−V hysteresis. Different surface modification techniques have been adapted to change the properties of the ETM so as to improve its interface with perovskite. The roles of metal oxide ETMs and of the interface between perovskite and ETM in device performance are comprehensively reviewed by Mahmood et al.117 and Fakharuddin et al.,77,118 respectively. In general, the physical and optoelectronic characteristics of ETMs influence the performance of PSCs remarkably and need to be optimized specifically according to the device structure for high and stable performance.

with ALD-SnO2. As explained by the authors, the poor photocurrent in the case of TiO2 was due to band misalignment between perovskite and TiO2, forming an energy barrier against electron transfer. It must be noted that the low photocurrent in that case is not even comparable with those of solution-based TiO2 films, which often produce current >20 mA/cm2 at even not-so-perfectly optimized conditions. This result asserts the greater importance of thin-film properties over bulk properties of metal oxide ETMs in PSCs. As different metal oxides (TiO2, ZnO, SnO2, etc.) have different work functions and conductivity, the rate of electron transfer from the perovskite varies with the different metal oxides used in the cell. However, TiO2, ZnO, and SnO2 ETMs have all shown VOC > 1 V, indicating insignificance of the energy level of these layers in determining the VOC of the cell. Figure 11 displays examples of planar structure PSCs using thin, compact films of SnO2103 and ZnO104 as ETMs, prepared by a low-temperature-based (non-sintering) solution-coating process.103 Although the device efficiency changes depending on the metal oxide and perovskite is used, which affects the JSC and FF, the VOC values of both devices are almost same (1.08 and 1.12 V), comparable with those obtained with TiO2-based cells. In addition to TiO2, SnO2, and ZnO, a variety of other metal oxides, such as Nb2O5,105−107 WO3,108,109 Nb-doped TiO2,110 Mg-doped ZnO,111 MgO/TiO2,112 etc., have been investigated for use in PSCs. Figure 12 shows a summary of the energy levels (work function) of some typical metal oxide electron extraction materials in relation to those of perovskite absorber materials and HTMs. Energy level data listed here are based on our measurements103,105,113−115 and also published studies. For metal oxide materials, the band gap energy and CB level do not necessarily show good agreement with the open-circuit voltage observed in PSCs, probably due to its dependence on the form of material subjected to measurement. Such mismatches of values, if not large, can originate in the differences between single bulk crystal and polycrystalline films, film samples in vacuum and in air, and pure and defectrich samples. Furthermore, it is considered that energy levels

4. COMPOSITIONAL ENGINEERING OF PEROVSKITES 4.1. Mixed Compositions

In the past few years, while the efficiency of PSCs continued increasing, the long-term stability of the cells also went up K


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methylammonium (MA), formamidinium (FA), Cs, and Rb and anions like I, Br, and Cl, and their combinations, have been explored in the past few years. Table 1 provides a list of A-, B-, and X-site ions and their sizes that are or can be used in certain combinations to form perovskite structures. Out of all possible combinations,124 the mixed perovskite that has become most popular in the recent years is (MA/FA/ Cs)Pb(I/Br)3, which is commonly known as a triple-cationbased perovskite. The quadruple-cation-based perovskite, including Rb (i.e., (MA/FA/Cs/Rb)Pb(I/Br)3) as the fourth cation, has also gained interest due to its high cell efficiency and stability. Although it was believed earlier that Rb occupies the A-site, a recent study has revealed that it likely does not sit in any lattice site and is instead expelled out to grain boundaries.125,126 4.1.1. A-Site Cations Mixture. According to DFT calculations, in lead halide perovskites, Pb 6s 6p−I 5p interactions lead to generation of the two bands: the valence band maximum (VBM) is formed by antibonding (σ*) Pb 6s− I 5p interactions, while the conduction band minimum (CBM) is formed by empty Pb 6p orbitals128 and/or by Pb 6p−I 5p interactions.129,130 Therefore, the cations in the A-site are considered not to contribute directly toward the band structure, but they play a significant role in providing structural stability by charge compensation within the PbI6 octahedra, largely based on their electrostatic (van der Waals) interactions131 with the inorganic cage. Nevertheless, any change in the size of cations in the A-site can either contract or expand the crystal lattice, thereby altering the optical properties of the perovskite. Smaller cations like Cs and Rb are expected to contract the lattice and thus increase the band gap, while larger cations like formamidinium (FA+) are supposed to expand the lattice and decrease the Eg. FA+ was the first cation that was used instead of methylammonium (MA+). FA+, having a larger ionic radius (r = 0.253 nm) than MA+ (r = 0.217 nm), expands the crystal a bit, resulting in decreased Pb−I bond distance, which eventually lowers the band gap. As measured, pure FAPbI3 shows a band gap (Eg) of 1.47 eV,36 while that for pure MAPbI3 is 1.55 eV. Although,

remarkably. One major development that has stood out among others and contributed substantially to enhanced stability is incorporation of different cations in the A-site and different halides in the B-site. This compositional play with the perovskite has not only exerted a strong influence on efficiency but also raised the stability substantially. Although such compositional mixing does not always succeed in forming a homogeneous solid solution, certain combinations of cations in the A-site and halides in the B-site of the perovskites have demonstrated superiority over single-cation/halide perovskites in terms of both efficiency and stability. Indeed, all the cells published in the NREL chart of certified efficiency, except for the first one, comprise a mixture of cations or anions or both (Figure 13).

Figure 13. Perovskite solar cell performances certified by NREL (14.1%,119 16.2%,63 17.9%,120 20.1%,121 21.02%,122 and 22.1%123). The compositions of perovskites used in three recent high-efficiency cells (22.7%, 23.3%, and 23.7%) are not in the public domain yet.

Based on ionic size and geometrical tolerance factor (τ = which is an empirical index widely used for

rA + rX ), 2 (rB + rX )

predicting perovskite crystal structure, different cations such as

Table 1. Ionic Radii of Some of Common A-Site Cations, B-Site Cations, and X-Site Anions Used in Hybrid Perovskitesa cation (A-site) NH4+ methylammonium [CH3NH3]+, (MA) formamidinium, [CH(NH2)2]+, (FA) hydrazinium, [NH3NH2]+ azetidinium, [(CH2)3NH2]+ hydroxylammonium, [NH3OH]+ imidazolium, [C3N2H5]+ ethylammonium, [(CH3CH2)NH3]+ dimethylammonium, [(CH3)2NH2]+ guanidinium, [(NH2)3C]+ tetramethylammonium, [(CH3)4N]+ thiazolium, [C3H4NS]+ 3-pyrrolinium, [NC4H8]+ tropylium, [C7H7]+ K+ Rb+ Cs+

effective ionic radius (pm)

metal ion (B-site)


anion (X-site)


146 217

Pb2+ Sn2+

119 110

F− Cl−

129 181

253 217 250 216 258 274 272 278 292 320 272 333 164 172 188

Ge2+ Mg2+ Ca2+ Sr2+ Ba2+ Cu2+ Fe2+ Pd2+ Eu2+ Bi3+ Sb3+

73 72 100 118 135 73 78 86 117 103 76

Br− I−

196 220

a

Adapted from ref 127. Copyright 2015 Springer Vienna. L


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Figure 14. (a) Schematic illustration of the importance of the size of the A-site cation in determining the tolerance factor and structural properties of perovskite. Substitution of small amounts of FA with Cs on the A-site results in more structurally stable compounds. Changes in UV−vis spectra of (b) FAPbI3 and (c) FA0.85Cs0.15PbI3 over 18 days. (d) XRD patterns of original FAPbI3 and FA0.85Cs0.15PbI3 films and those films after 30 days of storage. (e) Change in XRD patterns of FAPbI3 thin film after its exposure to high humidity. (f) Photos of FAPbI3 and FA0.85Cs0.15PbI3 films under high-humidity conditions. (g) J−V curves of FA0.85Cs0.15PbI3 solar cells at 0−15 days of storage under 15% RH. (h) Normalized PCE of FAPbI3 and FA0.85Cs0.15PbI3 solar cells at different storage times. Reproduced with permission from ref 141. Copyright 2016 American Chemical Society.

degradation) as easily as MA.138 Therefore, partial replacement of MA with FA in MA-based perovskites also improves the thermal stability of the perovskites (FAxMA1‑xPbI3) remarkably. Improved structural stability, associated with an increased tolerance factor and a stronger interaction of FA with iodide ions, results in greater thermal stability of the FAxMA1‑xPbI3 perovskites. However, the FA inclusion strategy falls short of long-term durability because FA is, unfortunately, more hygroscopic than MA and, thus, shows greater vulnerability to humidity.138 Therefore, incorporation of the inorganic cation Cs became an alternate choice. Indeed, it has been found that substitution of MA with Cs simultaneously improves efficiency and thermal stability by a certain amount. As reported by Niu et al.,139 about 9 mol% of Cs in CsxMA1‑xPbI3 (x = 0.09) shows better performance (18.1%) and thermal stability (no color change when heated at 120 °C for 3 h) than pristine MAPbI3 (15.8%), while a higher concentration of Cs shows surprisingly worse stability. The unencapsulated Cs0.09MA0.91PbI3 cell retains above 80% of its initial performance, whereas the performance of the pure MAPbI3 cell deteriorates to less than 40% of its initial performance after heat treatment at 85 °C for 60 min. From temperature-dependent reflectance measurements, Gong et al.140 also found that regular MAPbI3 degrades completely when heated at 200 °C for 10 min, changing its absorption edge to that of PbI2, while the absorption spectrum of Cs0.05MA0.95PbI3 remains almost unchanged under the same treatment conditions. To our knowledge, there is no study that directly compares the moisture stability of MAPbI3 with that of (Cs/MA)PbI3. However, there are several reports on comparisons of the moisture resistance of FAPbI3 with that of (Cs/FA)PbI3, showing results that strongly endorse the fact that moisture stability is improved by Cs inclusion in the perovskite. Small and large tolerance factors respectively for CsPbI3 (τ = 0.85) and FAPbI3 (τ = 0.98) stabilize the

based on the Eg value (1.47 eV), FAPbI3 is supposed to perform better than MAPbI3 due to its extended absorption edge (800 nm), the best efficiency achieved so far with FAPbI3 is ∼18%,132 while that for pure MAPbI3 is >20%.133−135 With an extended absorption edge, FAPbI3 cells show higher photocurrent (JSC), but their efficiency is mainly limited by poor FF, which is possibly related to phase instability of FAPbI3. FAPbI3 readily crystallizes into a photo-inactive phase (δ-FAPbI3) at room temperature (RT), and this phase is transformed to a photo-active black phase (α-FAPbI3) at temperatures between 125 and 165 °C.120 This α-FAPbI3, formed at high temperatures, slowly transforms into δ-FAPbI3 when kept at RT. However, partial substitution of FA with MA increases the stability of α-FAPbI3 at RT. Specifically, a trigonal α-FAPbI3 phase is stabilized at RT with a composition of 20 mol% of MA in MAxFA1‑xPbI3 (x = 0.2), although the best efficiency is reported (∼18.3%) for FA-based perovskites having a composition of MA0.4FA0.6PbI3. For 0.2 ≤ x ≤ 1, MAxFA1‑xPbI3 exists in the tetragonal phase, indicating the dominance of MAPbI3, which is stabilized in a tetragonal phase at RT. It has been proposed that, when a MA+, which has almost 10 times greater dipole moment than FA+, is incorporated, it exhibits stronger interactions with the PbI64− octahedra and thus stabilizes the 3D arrangement of α-FAPbI3 with little lattice shrinkage or changes in the optical properties.136 Although the efficiency of FAPbI3 cells falls behind that of the MAPbI3 cells, FAPbI3 has a significantly greater thermal stability compared to MAPbI3.137 FAPbI3, when heated at 150 °C for hours, does not change color, whereas MAPbI3 becomes yellow (formation of PbI2) after heating at 150 °C (or even lower) for just 30 min.36 It is proposed that FA has a stronger interaction with iodide and, therefore, does not allow easy breaking of the network. It is also explained that FA, being less acidic than MA, does not undergo deprotonation to furnish HI (the first step of M


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Figure 15. Changes in UV−vis absorbance spectra of (a) FAPbI3, (b) Cs0.05FA0.95PbI3, and (c) Rb0.05FA0.95PbI3 perovskite films stored at 85% RH, 25 °C, and in the dark for different durations. As highlighted by this experiment, the stability of Rb0.05FA0.95PbI3 films was significantly superior even to that of Cs0.05FA0.95PbI3. Reprinted with permission from ref 145. Copyright 2015 John Wiley and Sons.

Figure 16. UV−vis absorption spectra and photographs of MAPbI3‑xBrx. (a) UV−vis absorption spectra of FTO/c-TiO2/mp-TiO2/MAPbI3‑xBrx/ Au cells. (b) Photographs of TiO2/MAPbI3‑xBrx bilayer nanocomposites on FTO glass substrates. (c) Quadratic relationship of the band gaps of MAPbI3‑xBrx as a function of Br concentration (x). (d) Power conversion efficiencies of the heterojunction MAPbI3‑xBrx solar cells as a function of Br composition (x). (e) J−V characteristics of the MAPbI3‑xBrx cells (x = 0, 0.06, 0.13, 0.20, 0.29, 0.58, 1.0). Reprinted with permission from ref 158. Copyright 2013 American Chemical Society.

remain stable (Figure 14f), and correspondingly, the performance of the FA0.85Cs0.15PbI3 devices are found to be more stable than that of the pure FAPbI3 cells (Figure 14g,h).141 In a similar study, Lee et al.143 observed that 10% of Cs in FAPbI3 (i.e., Cs0.1FA0.9PbI3) improved the cell performance from 16.3%, measured from pure FAPbI3 cells, to 17.1% in Cs0.1FA0.9PbI3. Interestingly, the absorbance of the Cs0.1FA0.9PbI3 film and performance of the cells made with Cs0.1FA0.9PbI3 deteriorated much less in comparison to those of FAPbI3 when the films and cells were aged under light/dark in a humid atmosphere. Similar results of higher efficiency and better stability were also reported for Cs0.2FA0.8PbI3-based devices by Yi et al.144 In addition to Cs, Rb and K have also attracted attention, as FAPbI3 perovskites including either Rb or K have demonstrated improvements in the PV performance of the cells. Although its location in the crystals is still a matter of controversy, it is believed that a small amount (x ≤ 0.05) of Rb+ can be included in FAPbI3, and higher concentrations lead

perovskites in orthorhombic (yellow) and hexagonal structures (Figure 14a), respectively. Due to the difficulty in determining the ionic radii of organic cations accurately, slight discrepancies may occur between the structures predicted from the tolerance factor values and the structures observed experimentally. For instance, while some reports141 agree on formation of the hexagonal structure of FAPbI3 at RT, some other studies report a cubic structure142 of the same. Nonetheless, tuning of the tolerance factor by mixing ions of different sizes has been a successful strategy to stabilize the perovskites in cubic structures. For example, the effective tolerance factor can be tuned to form a cubic structure (0.9 ≤ τ ≥ 1) by alloying CsPbI3 with FAPbI 3 (FA xCs 1‑x PbI3 ) over a range of proportions (Figure 14a). In comparison to pure FAPbI3, the structural/phase stability and therefore the cell performance are improved for a mixed FA/Cs composition with 15% of Cs (i.e., FA0.85Cs0.15PbI3). It has been also observed that FAPbI3 perovskite thin films, when exposed to a humid environment for 18 days, degrade fast, while FA0.85Cs0.15PbI3 thin films N


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Figure 17. (a) Dependence of powder XRD peaks on the value of x in (FAPbI3)1‑x(MAPbBr3)x single crystals. The hollow circles represent data points, and the solid lines are Gaussian fits of the data. The shift of all the peaks to higher 2θ implies contraction of the lattice due to alloying of FAPbI3 with MAPbBr3. Reproduced with permission from ref 163. Copyright 2017 American Chemical Society. (b) Schematic illustration of strain relaxation after MAPBr3 alloying into FAPbI3 (side view). Reproduced with permission from ref 164. Copyright 2016 American Chemical Society.

to phase segregation.125 Devices based on Rb-mixed FAPbI3 (i.e., Rb0.05FA0.95PbI3) outperform those based on FAPbI3, and more importantly, the stability of this Rb0.05FA0.95PbI3 film, as shown in Figure 15, against humid conditions is superior to that of Cs0.05FA0.95PbI3.145 K, being even smaller than Rb, has been also used in mixedcation compositions, and its effect on the performance of the cells has been found to be significant, which is discussed with regard to mixed cation−mixed halide in the following section. 4.1.2. X-Site Anions Mixture. Like different cations in the A-site, different halides, like Cl, Br, and even non-halide/ pseudohalide ions like SCN−, have been incorporated into pure MAPbI3, FAPbI3, or mixed-cation (FA/MA, FA/MA/Cs, FA/Cs, FA/MA/Cs/Rb, etc.) lead iodide perovskites. But, unlike cations, different halides mixed in the X-site impart a dramatic effect on optical and electronic properties, absorption and emission spectra (band gap), and carrier lifetime and diffusion length. Although the ambiguity of inclusion of Cl in pure iodide perovskites like MAPbI3 remains unresolved, a majority of reports claim that Cl easily sublimes out of the perovskite film during preparation and, therefore, does not exist in the final film,146−148 despite the starting solution containing Cl-based precursors (PbCl2 or MACl). In contrary, a good number of studies also show evidence of the existence of a trace amount of Cl in MAPbI3,149−152 even though the amount measured was always substantially lower than that used in the starting materials. It is presumed that MAPbI3‑xClx is either a metastable phase or has a high formation energy153 and, therefore, is not formed in the final perovskite film despite the starting materials containing Cl. Nevertheless, distinct effects of Cl present in the precursor solution on the quality, morphology, and crystallinity of the perovskite films have been observed in all the studies. PbCl2 as the source of Pb- or Clbased additives, such as HCl,154,155 NH4Cl,156 and MACl,157 improves the film quality by slowing down the crystallization, thus resulting in more uniform and pinhole-free films, which consequently improves the performance as well as the stability of the devices. Besides, the longer diffusion length of electrons in MAPbI3‑xClx than in MAPbI3 films157 has been credited with improving the performance of the cells. But, as the carrier lifetime or diffusion length is dependent on the morphology (grain size and grain boundaries) of polycrystalline films, it is not easy to separate the effect of Cl from effects of morphology on the electronic properties. Hence, it seems that Cl− in the precursor solution has an indirect but positive effect on the overall performance of the cells. Unlike Cl, the presence and effect of Br in the X-site in mixed Br/I perovskites have been

observed clearly and directly. The influence of Br on the optical and electronic properties has been remarkable and recorded well. Incorporation of smaller Br − ions in MAPbI3‑xBrx increases the band gap of the mixed-halide perovskite, and this increment in Eg follows a quadratic relation with the concentration of Br, as show in Figure 16c. Through such band gap tuning, Noh et al.158 reported an initial best efficiency above 12% with Br content 20%) were lower, the cells displayed better resistance against high humidity (RH = 55%), which was correlated with a tetragonalto-pseudocubic structural transition (at x = 0.13). Following this first report, a number of reports came out showing continuous improvement in cell efficiency, which was accomplished through optimization of the methods of preparation. A certified PCE of 16.2% was reported by Jeon et al.,63 which they achieved by using a solvent engineering method to prepare a uniform and dense MAPbI3‑xBrx film. In a similar manner, FA-based mixed I/Br (FAPbI3‑xBrx) perovskites with a range of Br concentrations (x = 0−1)36 were studied, and a similar effect of increasing band gap with Br incorporation was observed. But the interesting and surprising fact about FAPbI3‑xBrx is that it does not form any crystalline phase (amorphous) for 2.3 ≤ x ≥ 2.5.36 The origin of this amorphous regime is not yet understood. However, like in the case of MAPbI3‑xBrx,159 photoinduced phase segregation was also observed in FAPbI3‑xBrx.160 A detailed discussion about such photoinduced phase segregation is presented in section 5.1.2. A few studies have been undertaken on systems like MAPb(Br/Cl) and MAPb(I/Br/Cl). In contrast to MAPb(I/ Cl), Cl has been found to coexist in MAPb(Br/Cl). The smaller difference in ionic radii (RI− = 2.07 Å, RBr− = 1.84 Å, RCl− = 1.67 Å) and the higher degree of ionic character between Br and Cl are responsible for the easier miscibility of Br/Cl than I/Cl.161 To our knowledge, application of MAPbBr3‑xClx in PV devices has not been explored yet, but devices based on triple-halide compositions (MAPbI3‑x‑yBrxCly) have been reported to show PCE > 16%.162 4.1.3. Both Cations and Anions Mixture. A number of “mixed cations and mixed halides” perovskites have been synthesized and employed in PV devices. In fact, such simultaneous mixing of cations and anions has led to further improvements in cell efficiency and stability. Based on results of enhanced VOC by Br inclusion and increased structural stability in the FA-MA mix, (FA/MA)Pb(I/Br) perovskite O


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Figure 18. (a) XRD patterns and (b) corresponding UV−vis spectra (dashed lines) and photoluminescence (PL) spectra (solid lines) of perovskite upon addition of Cs to the series Csx(MA0.17FA0.83)(1‑x)Pb(I0.83Br0.17)3, abbreviated as CsxM, where M stands for “mixed perovskite” and x = 0, 5, 10, and 15%. Cross-sectional scanning electron microscopy (SEM) images of (c) Cs0M- and (d) Cs5M-based perovskite solar cells. (e) Current− voltage scans for the best-performing Cs5M device, showing PCE exceeding 21%. The inset shows the power output under maximum power point tracking for 60 s, starting from forward bias and resulting in a stabilized power output of 21.1% (at 960 mV). (f) Aging for 250 h of highperformance Cs5M and Cs0M devices in a nitrogen atmosphere held at RT under constant illumination and maximum power point tracking. The maximum power point was updated every 60 s by measuring the current response to a small perturbation in potential. A J−V scan was taken periodically to extract the device parameters. The device efficiency of Cs5M drops by about 20% (red curve, circles), and then it stays relatively stable for at least 250 h. This is not the case for Cs0M (black curve, squares). Reproduced with permission from ref 167. Copyright 2016 Royal Society of Chemistry, under Creative Commons Attribution-NonCommercial 3.0 Unported License.

Figure 19. (a) Cross-sectional scanning electron micrograph, (b) J−V curves, and (c) PCE histogram plot of triple-cation perovskite solar cells fabricated in ambient air under controlled humidity (15−25%). Reproduced with permission from ref 38. Copyright 2018 Chemical Society of Japan.

of FAPbI3, a slight amount of the photo-inactive yellow phase always remained in the resultant perovskite, which was considered to be detrimental for long-term stability. In order to avoid this yellow phase completely, Saliba et al. explored the inclusion of Cs+, which is significantly smaller than MA+, as a third cation in mixed FA-MA and I-Br perovskites.167 In their study, Cs prevented formation of the yellow phase completely and the improved morphology of the perovskite film through further grain growth. A triple-cation composition with 5% of Cs (i.e., Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3) produced excellent results, with a stabilized PCE above 21%, which dropped to about 18% in a few hours and then remained stable for up to 250 h of continuous operation at maximum power point (Figure 18). Further, Cs (20%) in the triple-cation mixed perovskites (Cs/FA/MA/Pb/I/Br) demonstrated an incredibly long life (1000 h); the perform-

became a popular mixed perovskite for study. A good amount of work has been done on this mixed perovskite to find the optimum composition for maximizing device performance and to understand more about the correlation between its composition and optoelectronic properties. Incorporation of cations with smaller effective radius (MA+) into FAPbI3 can adjust the Goldschmidt tolerance factor close to 1163 by contracting the lattice (Figure 17a) or relaxing the crystal strain of FA-based perovskites (Figure 17b) to stabilize the cubic phase of the perovskite.164 As a result, simultaneous mixing of FA-MA cations and Br-I anions improved the performance and stability of the devices. Devices based on MA0.17FA0.83Pb(I0.83Br0.17)3 have been reported to work at PCE > 20%, which has been accomplished either by combining it with a newly designed HTM165 or by adding a slight excess of PbI2 in the solution.166 Although MA, with a smaller effective size than FA, worked here as a crystallizer for the black phase P


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Figure 20. (a) Schematic illustration of the device architecture of perovskite solar cells, (b) maximum power point tracking of encapsulated PSCs under constant 1 sun AM1.5G illumination measured in air, and (c−f) box chart presentation of photovoltaic parameters of PSCs. The current− voltage (J−V) curves of 17 cells of each type were recorded at a scan rate of 0.1 V s−1. Reproduced with permission from ref 171. Copyright 2018 John Wiley and Sons.

Figure 21. Schematic illustration of (a) black single-cation α-FAPbI3, (b) black double- (CsFA, RbFA), triple- (CsMAFA), or quadruple-cation (RbCsMAFA) compositions (X = I, Br), and (c) yellow non-perovskite δ-FAPbI3. The table shows the incorporation capacity of Rb+ and Cs+ into the FAPbI3 lattice. Reproduced with permission from ref 125. Copyright 2017 American Chemical Society.

ance of cells made with Cs0.2FA0.8Pb2.84Br0.16 film almost did not change, even after being stored for 1000 h in the dark. The two identifiable attributes of these triple-cation-based perovskite films are phase purity (no δ-phase formation) and uniform grains, which are apparently responsible for the enhanced PV performance of the cells. Inclusion of Cs into the FA-MA-based perovskite completely prevents formation of the δ-phase, improving the phase purity of the resultant perovskite film. Recently, Zhou et al. tried to understand the underlying mechanism by which Cs helps in preventing formation of the δ-phase.168 As found in their study, a triple-coordinated intermediate phase consisting of Pb2+, DMSO, and Cs+ retards crystallization of PbI2 in the precursor films, suppressing formation of the yellow δ-phase. The greater structural stability of triple-cation mixed perovskites and the presence of the intermediates/colloids (not well investigated yet) in the precursor solution are probably the keys to formation of

good-quality perovskite films with high reproducibility and lower sensitivity to the environment. Triple-cation perovskites, either processed in a N2 environment or in ambient conditions (humidity −15−25%), result in high efficiency. Figure 19 shows one example of a high-efficiency (>21%) triple-cation perovskite solar cell fabricated in ambient conditions in our laboratory.169 The triple-cation recipe was then followed by a quadruplecation mixed perovskite that included Rb as the fourth cation.170 With this quadruple-cation mixed perovskite with 5% of Rb, Saliba et al. achieved stabilized efficiencies up to 21.6% (average value: 20.2%) and open-circuit voltage of 1.24 V for a band gap of 1.63 eV (0.39 V loss in potential), and the polymer-coated cells maintained 95% of their initial performance until 500 h of operation at the maximum power point at 85 °C.170 In comparison to Cs/FA/MA triple-cation devices, Rb-based quadruple-cation perovskite cells show slightly Q


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Figure 22. Effect of K inclusion on J−V hysteresis of different perovskites. J−V curves of perovskite solar cells employing different perovskite materials FA0.85MA0.15PbI2.55Br0.45, FA0.85MA0.1Cs0.05PbI2.7Br0.3, MAPbI3, and FAPbI3 doped with (B, D, F, H) and without (A, C, E, G) 10 μmol of KI, measured on reverse (filled circles) and forward (empty circles) scans at the scan rate of 130 mV/s (= voltage settling time of 200 ms) under AM 1.5G 1 sun illumination (100 mW/cm2). Aperture mask area was 0.125 cm2. Reproduced with permission from ref 177. Copyright 2018 American Chemical Society.

Table 2. Summary of Key Effects of Cs, Rb, and K Inclusion into (FA/MA)Pb(I/Br)3 Mixed Perovskites role and effects of Cs •gets incorporated into lattice at A-site •helps in formation of cubic perovskite phase (complete removal of yellow phase) •enhanced stability •reduces trap states •resists phase segregation in mixed I/Br perovskites (improves photostability)

role and effects of Rb •most likely not incorporated into lattice at A-site •segregation preferably near to the ETL and at grain boundaries •enhanced stability •enhancement in charge mobility •no effect on trap landscape

Role and effects of K •most likely not incorporated into lattice A-site •shift of XRD peaks/lattice expansion (contrary to what expected if it was incorporated) •reduction/elimination of hysteresis •hinders formation of δ-FAPbI3 •grain boundary passivation

Like Rb+ (172 pm), K+ (164 pm), having a similar ionic size, is also considered not to occupy any crystal site in FAPbI3 or mixed perovskites, but it enhances the cell performance by passivating the defects sites in perovskite. Although K+ was assumed to occupy the A-site, as reported in earlier studies, several independent and recent studies support its existence in interstitial sites172−174(evidenced from shift in XRD peaks and absorption edges), while a few other studies claim that it does not occupy any crystal site175 and is expelled out of the crystal, segregating at grain boundaries or on surface.176 Hence, although its site of location in perovskite film and its active role are not entirely clear, its positive impact on cell performance has been commonly observed in all the studies. K inclusion in a variety of perovskite compositions has been found to eliminate hysteresis in the J−V curves of the cells (Figure 22) and, therefore, has been proposed to be a universal method to eliminate hysteresis.177 Among all alkali metal ions (Li+, Na+, K+, Rb+, Cs+), K+ works best for reducing/eliminating hysteresis. It is proposed that K+ prevents formation of Frenkel defects, which are responsible for hysteresis, but direct evidence supporting the active mechanism involved in elimination of hysteresis is yet to come. It has been also found that K+ incorporation hinders formation of photoinactive δ-FAPbI3 almost completely, resulting in enhanced lifetime of charge carriers, reduced recombination, and shift of conduction band edge toward better energy alignment with SnO2 ETL.178 As a result, K0.03(MA0.17FA0.83)0.97PbI2.5Br0.5 perovskite solar cell based on tin oxide (SnO2) as ETL yields hysteresis-free PCE over 17%. Based on the reports so far, a summary of major roles and effects of Cs, Rb, and K in mixed FA-MA perovskite is given in

higher FF, reduced hysteresis, and greater photostability (Figure 20), which were essentially attributed to reduced recombination and less defects.171 Hu et al.171 found that Rb addition leads to increased charge carrier mobility but has only a marginal effect on the trap landscape of the perovskite layer. In contrast, Cs incorporation significantly reduces the number and the depth of trap states in the perovskite crystals, but it has barely any effect on the charge carrier mobility. The observed reduction in trap density is in excellent agreement with the enhancement in VOC and FF for Cs-containing devices compared to FA-MA (Figure 20). Upon combining Cs and Rb in quadruple-cation (Rb-Cs-FAMA) perovskite mixtures, the highest mobility and the lowest trap density were observed, which subsequently resulted in solar cells with the highest stabilized power output. This difference between the effects of Cs and Rb essentially arises from incorporation or segregation of these cations in the crystal lattice. As evident from solid-state NMR studies,125 Cs can get incorporated into the crystal of (FA/MA)Pb(I/Br)3 up to 15 mol%, while Rb does not occupy any crystal sites.125,126 Instead, it segregates as rubidium-rich phases RbPbI3 mixed cesium−rubidium lead iodides, mixture of rubidium halides, various rubidium lead bromides, depending on the exact composition (Figure 21). A ToF-SIMS depth profile also shows a preferable accumulation of Rb+ species at the TiO2 interface, while Cs+ is very homogeneously distributed in the film.171 Hence, it is believed that enrichment of Rb cations near the electron transport layer interface is possibly related to a reduction of surface recombination in the vicinity of the electron transporting layer, which in turn affects device hysteresis and power output stability. R


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Figure 23. (a) UV−vis absorption and (b) steady-state PL emission spectra of FAMA mixed perovskite doped with Cs, Rb, K, Na, and combinations of (Cs, Rb), (Cs, K), and (Cs, Rb, K). (c) Table listing corresponding band gap values. Reproduced with permission from ref 126. Copyright 2018 American Chemical Society.

Figure 24. Schematic illustration summarizing the main difference between the perovskite materials containing two (MA/FA), three (Rb/MA/FA and Cs/MA/FA), and four (Rb/Cs/MA/FA) cations, observed by HAXPES (hard X-ray photoelectron spectroscopy). Quantification of different ions in bulk and on surface displays that unreacted FAI increases on surface with increasing addition of Cs or Rb in the precursor. Reproduced with permission from ref 179. Copyright 2017 American Chemical Society.

all the studies which show improvement in performance by slight modification of the surface of perovskite also support this fact that compositional modification at the surface of perovskite, which can come from different methods, can have a strong effect on performance, especially on VOC. It is expected that deeper understanding related to roles of different cations or anions in the mixed perovskites will come through more studies in future, but one thing that has been commonly noticed in most of these mixed perovskite studies is the enhanced structural/intrinsic stability, which is doubtlessly an important development.

Table 2. As the studies have conceded, Cs helps in formation of the black phase, preventing formation of the yellow phase completely. Furthermore, it also imparts a significant effect of grain growth. As a result, thermal stability as well as moisture stability of Cs-incorporated mixed perovskites has been witnessed to be remarkable. However, it is not known how exactly a small amount of Cs prevents the reaction of the perovskites with water. It is unconvincing that this small amount of Cs protects the perovskite from humidity just because Cs is stable against moisture. Instead, it seems that the enhanced structural stability improves the moisture resistance of Cs/Rb-based mixed perovskites. Hence, structural stability can be a major factor involved in moisture instability. In all three cases, Cs, Rb, and K, the improvement in PCE is basically due to higher VOC and better FF, which is a result of traps-passivation. It has been found (Figure 23) that the optical band gap remains unaltered when (FA/MA)Pb(I/Br)3 is doped with either of Cs, Rb, K, or Na.126 This result complements the fact that improved performance in all three cases is related to defects passivation. However, the questions that remain to be answered are, “What kind of defects exist in the films, and how do these cations help in passivating them? Why do Na+ and Li+ not exhibit such traps-passivation effects? Is it that the distributions of these ions are different and therefore they demonstrate different effects?” Not just distribution of these ions, it is rather very much possible that these ions strongly impact distribution of other ions like Pb2+, I−, Br−, MA+, and FA+ in the films. As a matter fact, it has been found that inclusion of Cs and/or Rb in (FA/MA)Pb(I/Br)3 films increases the amount of unreacted FAI on the surface of the films (Figure 24), which is indeed responsible for an increase in VOC in the devices with Cs, or Rb doped FAMA perovskite.179 And, this understanding that slight excess of FA on the surface improves the VOC is also consistent with the results of increased VOC in the cases where slight excess of organic cations were used in the precursor solution.180 In fact,

4.2. Mixed Dimensions

Another recent strategy that has succeeded in improving intrinsic stability or essentially the moisture stability of perovskites is by mixing the 2D structures with 3D structures. It is well known that 2D perovskites are more stable to heat and humidity181,182 but they lag behind the 3D perovskites in terms of performance because of their narrow absorption band in addition to poor electron transport properties.183 However, mixing a small amount of 2D perovskite to 3D perovskite structures have been found to work with higher efficiency and improved long-term stability. Several 2D/3D mixed perovskite compositions and their cell efficiency and stability are listed in Table 3. For instance, incorporation of 0.8 mol% of ethylenediammonium iodide (EDAI) (which forms a 2D perovskite structure when mixed alone with PbI2) into 3D MAPbI3 structure improves the PCE by reducing recombination184, and the MAPbI3-EDAI cell retains about 75% of its initial performance (18%) after 72 h of continuous operation under illumination while the regular MAPI3 loses 90% of its initial performance (17%) in just 15 h of operation (Figure 25). Similarly, a combination of properties of enhanced stability from 2D perovskite and excellent optoelectronic properties from 3D perovskite has been observed in a mixeddimensionality and mixed-compositional (MDMC) lead iodide perovskite based on [CF3CH2NH2]2(FA0.825MA0.15Cs0.025)n‑1S


189

188

stability measured at maximum power point up to 2 weeks; better stability than 3D structure stability not measured 8.5

17.7

187 better thermal stability than 3D perovskite >18

185

17.6

Figure 25. Long-term evolution of normalized (a) PCE, (b) VOC, (c) JSC, and (d) FF for the planar PSCs based on pristine MAPbI3 and MA1−2xEDAxPbI3 (x = 0.008) tested under 1 sun irradiation, at 50 °C and RH 50%. Absolute starting parameter values were PCE = 16.9% and 17.6%, VOC = 1.03 and 1.04 V, JSC = 22.2 and 22.3 mA cm−2, FF = 0.74 and 0.76 for MAPbI3 and MA1−2xEDAxPbI3 (x = 0.008), respectively. Reproduced with permission from ref 184. Copyright 2017 John Wiley and Sons.

(IC2H4NH3)2(CH3NH3)n‑1PbnI3n+1

BA0.09(FA0.83Cs0.17)0.91Pb (I0.6Br0.4)3

MA1‑2xEDAxPbI3 (x = 0.008)

[CF3CH2NH2]2(FA0.825MA0.15Cs0.025)n‑1Pbn(I0.85Br0.15)3n+1

(BA)2(MA)n‑1PbnI3n+1

FAxPEA1‑xPbI3

butylamine (BA)

ethylenediamine (EDA) CF3CH2NH2

butylamine (BA)

phenylethylamine (PEA) PEA

FTO/c-TiO2/m-TiO2/perovskite/ spiro-OMeTAD/Au FTO/SnO2/C60/perovskite/spiroOMeTAD FTO/c-TiO2/perovskite/spiroOMeTAD/Au FTO/c-TiO2/m-TiO2/perovskite/ spiro-OMeTAD/Au ITO/PTAA/perovskite/PCBM/ C60/BCP/Cu ITO/NiOxperovskite/PCBM/C60/ Ag FTO/TiO2/perovskite/spiroOMeTAD/Au

18

187

retains 80% of initial performance up to 1000 h (for non-encapsulated cells) or above 3000 h (for encapsulated cells) retains 75% of initial performance after 72 h under AM 1.5G irradiation, 50% RH, 50 °C device temperature better stability compared to 3D under 65% RH for up to 4 weeks 17.2

184

186 stability not checked 9.3

Review

IC2H4NH3

Pbn(I0.85Br0.15)3n+1 series (n = 1−∞, CF3CH2NH2-TFEA).185 The hydrophobic nature of the trifluoroethylamine chain and the multilayered structure of the mixed perovskite (Figure 26) result in enhanced moisture resistance; MDMC PSCs (n = 30) without encapsulation maintain over 90% of the initial PCE under relative humidity of 65% at RT for up to approximately 28 days (Figure 26b). A more promising result has been obtained with a 2D/3D mixed perovskite with about 3 mol% of aminovaleric acid iodide (AVAI) in 3D MAPbI3 perovskite. This mixed perovskite employed in a HTM-free carbon-based device has exhibited stability more than 10 000 h under 1 Sun illumination.

5. STABILITY OF PEROVSKITE SOLAR CELLS 5.1. Stability Issues with Perovskites

At present, while PCE of above 20% in a lab-scale device is being achieved by most of the leading laboratories, long-term stability and toxicity of Pb stand as two formidable obstacles for commercialization of PSCs. For outdoor installation like Si PV panels, PSCs must guarantee production of stable power at operating conditions of real sun radiation, raised temperature due to heating, and under atmospheric moisture and oxygen for a period of ∼25 years. Thus, these conditions are considered as the requisite for commercialization. Long-term stability/instability therefore include both intrinsic and extrinsic stability issues with perovskite. Both structural/ intrinsic stability and stability under different external environmental stresses such as heat, light, humidity, and oxygen are critically important. Performance deterioration and/or material degradation issues under continuous operation of the cells needs serious attention and should be solved in the coming days.

PEA2MAn‑1PbnBr3n+1

PCE (%) device structure mixed perovskite spacer cation

Table 3. Compositions of 2D/3D Mixed Perovskites, Corresponding Solar Cell Performance, and Stability

stability

ref

Chemical Reviews

T


Chemical Reviews

Review

Figure 26. (a) Schematic representation of the stacking structures of multidimension multicomposition (MDMC) perovskites based on trifluoroethylamine ammonium cations. (b) Power conversion efficiency as a function of storage time (under a relative humidity level of 65% at room conditions) for conventional 3D and MDMC (n = 30) perovskite solar cell, tested with an interval of 1 day. (c) XRD patterns of fresh and aged perovskite film (4 weeks). Reproduced with permission from ref 185. Copyright 2017 John Wiley and Sons.

When τ is in the rage of 0.8 < τ < 1, ideal cubic perovskite structures or distorted perovskite structures with tilted octahedra are favored. Specifically, 0.9 < τ < 1 favors a cubic perovskite structure while for 0.8 < τ < 0. 9, a distorted perovskite structure is formed. Values of τ < 0.8 and τ > 1 diminish the possibility of formation of perovskite structures. Therefore, it can be expected that τ close to the middle of the range from 0.8 and 1, away from both the non-perovskite zones (Figure 27), would form a stable perovskite.190 For FAPbI3, τ is close to 1, which is the near the upper boundary for perovskite structure, and therefore, FAPbI3 is prone to formation of a hexagonal δ-phase which is photoinactive. CsPbI3 with τ ≈ 0.8, is at the edge of lower boundary for perovskite structures, and it normally crystallizes into a δphase. MAPbI3 with τ ≈ 0.9 is close to the middle of the perovskite zone and forms a black photoactive perovskite phase. As discussed in section 4.1, compositional engineering of perovskite; mixing of different cations and anions improves structural stability of perovskites, essentially by adjusting the value of τ close to middle of perovskite zone. Addition of Cs or MA to FAPbI3 moves τ value down from 1 to stabilize the cubic phase of FAPbI3. Although precise calculation of resultant/effective tolerance factor (τ) for mixed perovskites is not easy, a simple mixture rule can be applied to obtain an approximate value. According to the mixture rule, for (AxA′1‑x)B(XyX′1‑)3, rA(eff), and rX(eff) can be calculated by using the following equations:

5.1.1. Structural/Intrinsic Stability. Structural stability of perovskite compounds can be primarily judged by the Goldschmidt tolerance factor (τ), which is an empirical index widely used to predict formation of different crystal structures of ABX3. The value of τ (relation is given in section 4.1) varies with the size of the ions in ABX3. Since ionic radii of organic cations (A) cannot be determined accurately, a certain amount of uncertainty lies in the calculated tolerance factors. Nevertheless, the values can be used to compare cations of reasonably different ionic radii. Figure 27 shows the calculated tolerance factors of APbI3 systems where A = Na, K, NH4, Rb, Cs, MA, FA, EA (ethylamine), and EDA (ethylenediamine).

Figure 27. Calculated tolerance factors (τ) for different cations (A) in APbI3 perovskite system. Commonly used cations like Cs, MA, and FA give rise to a τ-value in the range of 0.8−1.0, indicating formation of the cubic perovskite phase structure. Ionic size of A cations used in calculation are values referring to XII coordination, not VI coordination. Ethylammonium (EA) and ethylenediamine (EDA) cations are too big, giving rise to a tolerance factor >1.0 and thus fall into the “upper forbidden zone” and cannot form perovskite alone. The group I alkali metal cations (Na, K, Rb) and NH4 have a τ-value

Halide Perovskite Photovoltaics: Background, Status, and Future

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