A Long-Term View on Perovskite Optoelectronics - Accounts of

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A Long-Term View on Perovskite Optoelectronics Published as part of the Accounts of Chemical Research special issue “Lead Halide Perovskites for Solar Energy Conversion”. Pablo Docampo* and Thomas Bein* Department of Chemistry and Center for NanoScience (CeNS), University of Munich (LMU), Butenandtstrasse 5-13, 81377 Munich, Germany CONSPECTUS: Recently, metal halide perovskite materials have become an exciting topic of research for scientists of a wide variety of backgrounds. Perovskites have found application in many fields, starting from photovoltaics and now also making an impact in light-emitting applications. This new class of materials has proven so interesting since it can be easily solution processed while exhibiting materials properties approaching the best inorganic optoelectronic materials such as GaAs and Si. In photovoltaics, in only 3 years, efficiencies have rapidly increased from an initial value of 3.8% to over 20% in recent reports for the commonly employed methylammonium lead iodide (MAPI) perovskite. The first light emitting diodes and light-emitting electrochemical cells have been developed already exhibiting internal quantum efficiencies exceeding 15% for the former and tunable light emission spectra. Despite their processing advantages, perovskite optoelectronic materials suffer from several drawbacks that need to be overcome before the technology becomes industrially relevant and hence achieve long-term application. Chief among these are the sensitivity of the structure toward moisture and crystal phase transitions in the device operation regime, unreliable device performance dictated by the operation history of the device, that is, hysteresis, the inherent toxicity of the structure, and the high cost of the employed charge selective contacts. In this Account, we highlight recent advances toward the long-term viability of perovskite photovoltaics. We identify material decomposition routes and suggest strategies to prevent damage to the structure. In particular, we focus on the effect of moisture upon the structure and stabilization of the material to avoid phase transitions in the solar cell operating range. Furthermore, we show strategies to achieve low-cost chemistries for the development of hole transporters for perovskite solar cells, necessary to be able to compete with other established technologies. Additionally, we explore the application of perovskite materials in optoelectronic applications. We show that perovskite materials can function efficiently both as a film in light-emitting diodes and also in the form of nanoparticles in light-emitting electrochemical cells. Perovskite materials have indeed a very bright future.

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INTRODUCTION Recently, metal halide perovskite materials have become an exciting topic of research for scientists of a wide variety of backgrounds. Perovskites have found application in many fields, starting from photovoltaics and now also making an impact in lighting. This new class of materials has proven so interesting since it can be easily solution-processed while exhibiting materials properties approaching the quality of industry staples such as GaAs and Si. In photovoltaics, in only 3 years, efficiencies have rapidly increased from an initial value of 3.8% to over 20% in recent reports for the commonly employed methylammonium lead iodide (MAPI) perovskite.1−3 The first light emitting diodes4−6 and light-emitting electrochemical cells7 have been developed already, exhibiting internal quantum efficiencies exceeding 15% for the former as well as tunable light emission spectra. Despite their processing advantages, perovskite optoelectronic materials suffer from several drawbacks that need to be overcome before the technology becomes industrially relevant and hence achieve long-term application. Chief among these are the sensitivity of the structure toward moisture8,9 and crystal phase transitions in the device operation regime,10,11 © XXXX American Chemical Society

unreliable device performance dictated by the operation history of the device, that is, hysteresis,12 the inherent toxicity of the structure13,14 and the high cost of the employed charge selective contacts.15 In this Chemical Research account we highlight recent results in these areas and other advances toward the long-term viability of perovskite optoelectronics.

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DISCUSSION

Background

Perovskite optoelectronic devices are generally based on an n-ip junction, where the intrinsic perovskite material sits between a hole and an electron extraction or injection layer, depending on its intended function, as schematically depicted in Figure 1. Perovskite solar cells can be built in a “regular” configuration, with a typical device structure of FTO/TiO2/Perovskite/SpiroOMeTAD/Au16 or in an “inverted” configuration, with a typical device structure of FTO/PEDOT:PSS/Perovskite/PCBM/ TiOx/Al.17 Both configurations lead to highly efficient devices, Received: October 14, 2015

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DOI: 10.1021/acs.accounts.5b00465 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

Our results showed that unfavorable interactions between the substrate and the perovskite precursor mixture can lead to the formation of nonuniform films with low substrate area coverage.16,17 In fact, maximizing surface coverage proved to be a successful strategy to maximize device performance.18,19 In the case of perovskite LEDs, the requirement for thin perovskite films of under 50 nm to maximize the device internal quantum efficiency leads to a very unfavorable perovskite morphology and thus a high leakage current.4 Every aspect of the preparation process, such as the addition of certain chemicals that aid the film formation,21 the thermal annealing step,22 the precursor composition,23 the used solvent,24−26 the precursor concentration,27 and the substrate conditions, has a critical influence on the perovskite crystallization rate and therefore the morphology of the resulting perovskite layer. This makes perovskite film optimization challenging for one-step deposition techniques. Deposition Method

Figure 1. (a) Generalized crystal structure of ABX3 perovskites.19 (b) Energy level alignment schematic for typical perovskite solar cell materials.20 (c) Photograph of a flexible perovskite solar cell.17 (d) Solar cell layout for planar heterojunction solar cells.20 Figure 1a is reproduced with permission from ref 19. Copyright 2015, John Wiley and Sons. Figure 1b and d is reproduced with permission from ref 20. Copyright 2014 Royal Society of Chemistry. Figure 1c is reproduced with permission from ref 17. Copyright 2013 Macmillan Publishers Ltd.: Nature Communications.

In order to reliably achieve 100% surface coverage, we have adapted a two-step deposition approach28−30 based on the conversion of an initial lead halide layer into the perovskite phase via immersion in an alcoholic solution containing the organic cation for planar heterojunction architectures.10,31 A schematic of the process is shown in Figure 2. As the conversion into the perovskite phase is dictated by the diffusion speed of the ions and crystallization rate of the perovskite, both dependent on the temperature and concentration of the solution, fine-tuning of these parameters is crucial to obtain complete conversion and full surface coverage.32 The addition of small amounts of methylammonium chloride (MACl) to the methylammonium iodide (MAI) containing immersion solution (up to 5%) proved to be critical to improve the solar cell performance from ∼10% to 15%.31 The assembled films consisted of MAPI cuboids of a few hundred nanometers in size, as shown in Figure 2. The orientation of the resulting perovskite cuboids can be guided by tuning the temperature of the conversion solution, with higher temperatures leading to a higher degree of orientation of the (110) planes perpendicular to the substrate.32 Highly ordered perovskite crystals lead to higher short circuit currents on average, and more reproducible devices.32 Additionally, the rough surface created by the

but inverted architectures do not require high temperature sintering steps and can therefore be deposited on flexible substrates, as shown in Figure 1c.17 The optimization of the deposition methods has been a key factor behind the outstanding increases in perovskite device performance. Initially, perovskite films were simply deposited via a one-step deposition method with a subsequent annealing step, where the precursor materials were simply dissolved in an appropriate solvent (e.g., dimethylformamide or γ-butyrolactone) and then spin-coated onto the appropriate substrate. This leads to the formation of a film composed of a solventperovskite complex, for example MAI−PbI2−DMSO,18 which is then annealed and finally crystallized into the perovskite phase.

Figure 2. (a) Experimental protocol for two-step deposition. (b) SEM top view of the prepared perovskite film. (c, d) SEM cross sections of state-ofthe-art perovskite solar cells. (e) Schematic representation of the crystallization process of perovskite films prepared via sequential deposition. Figure 2a-d adapted with permission from ref 31. Copyright 2014 Wiley & Sons. Figure 2e reproduced with permission from ref 34. Copyright 2015 American Chemical Society. B


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Figure 3. (a) Infrared perovskite LED device architecture and light emitting properties, (b) yellow-emitting perovskite LEC device architecture and light emission properties, and (c) blue-green perovskite LED device architecture and light emitting properties. (d) Photographs of perovskite LEDs and UV-illuminated perovskite films with a range of chloride to bromide concentrations. Figure 3a,d adapted with permission from ref 4. Copyright 2014 Macmillan Publishers Ltd.: Nature Nanotechnology. Figure 3b adapted with permission from ref 7. Copyright 2015 American Chemical Society. Figure 3c adapted with permission from ref 6. Copyright 2015 American Chemical Society.

is only a small fraction of the total cost, with other manufacturing costs such as installation, housing, assembly, etc. taking the biggest shares.35 In fact, recent reductions in commercial solar cell costs arise from improvements to the device efficiency, rather than further material or processing cost reduction. Therefore, a straightforward solution to bring down the costs further is to enhance the performance of already established technologies with perovskite solar cells by fabricating tandem architectures.35 For this type of applications, band gap control is crucial to maximize the performance of the system. This control is also important in order to introduce perovskite materials into lighting consumer products such as displays or ambient lighting. The exchange of iodide bromide allows for band gap tuning from 1.6 to 2.3 eV, and the exchange of bromide for chloride allows further tuning to 3.1 eV.6,36 Fine tuning can be achieved by exchanging methylammonium with formamidinium, which leads to a subtle narrowing of the band gap, for both bromide and iodide-based perovskites.10,37 We have recently fabricated perovskite-based LEDs covering the whole visible spectrum, from blue to red and the near-infrared, and LECs emitting yellow light, as shown in Figure 3, demonstrating the wide gamut available with perovskite materials. When employing mixed halide perovskites in solar cells, although exhibiting single crystalline phases with clearly defined band gaps and clean band edges,38 the resulting devices exhibit very poor performance.39 This has been ascribed to lightinduced phase separation of the halide ions in the structure, which induces the formation of deep traps in the iodide-rich areas and thus causes a voltage loss as a result.39 We have demonstrated that solar cells based on methylammonium lead bromide samples achieve 1.45 V open circuit voltage, although the short circuit current is low at approximately 1 mA cm−2.37 When methylammonium (MA) is substituted for formamidinium (FA), however, high currents approaching the theoretical maximum for the band gap of the material and correspondingly

cuboids further enhanced light collection in the active layer without modifying its thickness.31,33 We have recently studied the crystallization process with 2-D X-ray scattering for the MAPI perovskite for two-step deposition processes, as shown in Figure 2.34 Starting from a film of crystalline PbI2, where the crystals are arranged in a pancake-like stack, the MAI diffuses from the top down through the stack32 and begins the conversion into the perovskite phase. As the structure expands upon inclusion of MAI, growth is constrained in the lateral direction, leading to predominant growth perpendicular to the substrate. Lead iodide sheets below the topmost layer experience stronger confinement effects and begin to crack leaving smaller grains of perovskite, while the crystals closer to the top surface can expand in an Ostwald-type ripening and form large cuboid crystals.34 Metal halide precursors and organic cations exhibit very different chemical properties and, as such, often show markedly different solubilities in the same solvent. This greatly complicates their deposition using one-step approaches as all ingredients must be dissolved in the same solution. Therefore, any change to the precursor mixture affects the whole film formation and crystallization dynamics. In two-step deposition approaches, as the lead precursor deposition and the inclusion of the organic cation is done in two separate steps, appropriate solvents can be targeted for both, and thus require almost no additional optimization beyond tuning of temperature and reaction times. This deposition technique therefore gives access to a large number of highly crystalline perovskite structures with little effort. Band Gap Tuning

For perovskite solar cells to compete in the current photovoltaic sectors, a compelling functional advantage is required. While perovskite films can be prepared from earthabundant materials which are low in cost and readily available, beating current competing photovoltaic technologies in price is unlikely to occur. The module price in mainstream applications C


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Figure 4. (a) Hydration reaction scheme and schematic of the resulting structures. (b) XRD patterns of MAPI films exposed to moisture for a range of times under constant cold air flow. (c) XRD patterns of MAPI films exposed to moist warm air for 20 min. Adapted with permission from ref 8. Copyright 2015 American Chemical Society.

results in the decomposition into HI (soluble in water), solid PbI2, and CH3NH2 either released as gas or dissolved in water as observed in previous reports.8,43 In this instance, the process is irreversible and losses of device performance are incurred.42 When the films are exposed to cool humid air, and thus no water condensation occurs, then water is slowly incorporated into the crystal to form a new crystal structure with isolated [PbI6]4− octahedra.8,44 The crystal structure formed depends on the humidity content, temperature and exposure time, with the initial formation of a monohydrated phase followed by the formation of a dihydrated crystal phase. The dihydrated phase already appears for films exposed for around 2 h at room temperature and 80% humidity. The water incorporation process appears to be isotropic, meaning that it occurs more or less homogeneously throughout the sample, rather than crystallization beginning at the outside and moving inward, suggesting that water is highly mobile within the lattice. In this case, we have shown that the process is reversible in films and in solar cells, where the photovoltaic performance of “degraded” devices was recovered with a simple drying step under a nitrogen stream for a few hours.8 Not only moisture, but simple heating of the material also leads to structural changes in the commonly employed MAPI perovskite. This material undergoes a reversible crystal phase transition between tetragonal and cubic symmetry in the temperature range between 54 and 57 °C, which corresponds to common solar cell operating temperatures during summer.45 High efficiency solar cells employ fine-tuned charge extraction layers, and thus any change to the electronic band structure can potentially reduce the photovoltaic performance considerably.45 Additionally, cycling between the two crystal phases during the day and night cycles is likely to lead to material fatigue and shorter device lifetimes. Several perovskite structures, such as methylammonium lead bromide or cesium lead iodide, are available in order to avoid phase transitions at solar cell operating temperatures. However,

high device performance is achieved. The main difference found between the two materials is that FA-derived perovskite structures exhibit orders of magnitude longer lifetime of the photoexcited species, which leads to significantly higher charge collection efficiencies. Stability

Ensuring the stability of MAPI photovoltaics under operational conditions is one of the biggest challenges to be addressed before commercializing the technology. Recent studies show that perovskite solar cells are relatively stable up to 1000 h in accelerated degradation tests,40 dropping only to approximately 80% of the initial performance. This, however, falls short of the required 10 000 h for commercial applications. At present, little is understood about the failure mechanisms of devices. Fully understanding the degradation pathways is crucial in order to develop effective strategies to improve stability and thus achieve market standards. Two main degradation pathways for perovskite materials have been identified: humidity-induced crystal changes and decomposition of the structure via heat and light. Water interacts strongly with the state-of-the-art methylammonium lead iodide (MAPI) perovskite. As the structure is soluble in water, the presence of humidity during film processing can significantly influence the thin film morphology16 and can lead to improved solar cell performance.3,41 However, when fully formed perovskite solar cells are exposed to air with a water vapor content above 50% at room temperature, a loss in performance is incurred.8,42 We have recently studied the effect of moisture on perovskite thin films, solar cells and single crystals, and our results are summarized in Figure 4.8 We have identified two distinct degradation pathways depending on whether water condensation occurs on the surface of the film or not. If the films are exposed to warm humid air with a water vapor content above 50% at room temperature, and thus water condensation occurs, then this D


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Figure 5. XRD pattern for a range of temperatures for neat FAPI (left) and FAMAPI (right), where 13% of the FA cations are exchanged with MA. The two FAPI crystal structures are depicted above the XRD patterns and the characteristic reflection positions are marked with an arrow. Adapted with permission from ref 10. Copyright 2015 American Chemical Society.

cation. The hydrogen bonding strength depends on both the number of hydrogen atoms and the dipole moment of the molecule.48,49 The dipole moment of MA is 10 times higher than that of FA,43 with a comparable number of hydrogen atoms, and thus, it is more likely that stronger hydrogen bonds are formed when MA is introduced into the α-FAPbI3 structure. In the second case, the structure can be made more stable through the increase of the Madelung energy. This value is a result of the Coulombic interaction between the charged species in the crystal. In the case of hybrid halide perovskites, the Coulombic interaction between charges and dipoles is proportional to the strength of the dipole. This, combined with a stronger dipole arising from MA, likely results in an overall Madelung energy increase and thus a more stable structure can be expected.

exchanging the halide for bromide leads to a suboptimal band gap of 2.3 eV, and a correspondingly lower power conversion efficiency, whereas perovskites based on cesium are less chemically stable overall. Currently, the best option is to exchange the organic cation from methylammonium (MA) to formamidinium (FA) in iodide-based perovskites. This leads to a slight narrowing of the band gap to 1.53 eV for the trigonal αphase,10,46 which is near the optimum for single junction solar cells, and a phase transition at 125 °C, well beyond cell operating temperature.47 Unfortunately, the more thermodynamically stable phase at room temperature for formamidinium lead iodide (FAPI) is not a “3D structure”. Here, the material crystallizes in a hexagonal structure, similar to the PbI2 lattice, with the lead halide octahedra sharing edges, in the so-called δ-phase. This results in disrupted charge transport and a wide band gap of approximately 2.2 eV, clearly unfavorable for solar cells. However, depending on the synthesis temperature, FAPI can be crystallized in the “α-phase” which is a pseudocubic structure, that is, the lead halide octahedra share the corner atoms, with trigonal symmetry. Thus, the challenge is to find a synthesis route which leads to stable FAPI in the dark phase. We have recently shown that the exchange of approximately 15% of the formamidinium cations with methylammonium leads to the crystallization of a material with the same crystal structure as FAPI. However, no trigonal to hexagonal phase transitions were observed in the temperature range between room temperature and 220 °C, as shown in Figure 5. The resulting material exhibits the same lattice parameters, photoluminescence emission peak wavelength, and band gap as those found for neat FAPI, meaning that no significant lattice contraction occurred after the inclusion of the smaller MA cation.10 State-of-the-art solar cells based on this approach now reach a power conversion efficiency above 20%.2 The stabilization of the α-phase of FAPI after the organic cation exchange can arise from an increase in the overall hydrogen bonding strength between iodide and the organic cation, an increase in the Madelung energy or a combination of the two. In the first case, the structure can be made more stable by increasing the number of hydrogen bonds between the iodide ions and the hydrogen of the ammonium moiety in the

Low-Cost HTMs

Photovoltaic technologies currently on the market, that is, polySi or CdTe, reach a remarkable raw device cost around $0.30 per peak Watt. Current record-breaking perovskite solar cells employ expensive organic layers for optimal charge extraction, leading to a projected cost just from the hole transporter, assuming 20% power conversion efficiencies, of ∼$0.30 per peak Watt.15 Clearly, for perovskite photovoltaics to enter the market, new low-cost charge extraction layers are necessary. Most commonly employed hole and electron transporters are synthesized via expensive cross-coupling reactions that require stringent reaction conditions and extensive product purification. This generally leads to material costs exceeding $200 g−1 and thus reaching the module target costs is challenging.15 The development of new chemistries with simple to purify products is thus an important area of research for perovskite photovoltaics. We have recently studied Schiff base condensation reactions under ambient conditions based on azomethines, where the only byproduct of the reaction is water.15,50−52 In particular, we have designed a new small molecule, termed “EDOTOMeTPA”,15 based on previous azomethine work. Here, the solubility and energy level alignment with the material was optimized via the inclusion of a cyclic ether and methoxy groups. This material, when employed in perovskite solar cells, achieved device efficiencies comparable to those based on the E


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Figure 6. Synthetic schemes for low cost hole transporting materials EDOT-OMeTPA and PEDOT.

range of typical solar cell panels in summer. The second challenge can be solved by employing a mixture of formamidinium and methylammonium in the crystal lattice, but the sensitivity to moisture remains. We have identified the degradation routes in this latter instance, which can be irreversible if water is condensed on the surface, with the immediate formation of lead iodide and thus severe losses in device performance. Barring that scenario, water molecules are easily incorporated into the perovskite crystal lattice, forming mono- and dihydrated crystals. However, the reaction is reversible and the performance of solar cells can be recovered upon a simple drying step under a nitrogen stream for a few hours. In either case, the exposure to moisture is undesirable, and the development of effective encapsulation techniques and moisture barriers is paramount. Further challenges to be resolved include the unreliable device performance dictated by the operation history of the device, that is, hysteresis, and the inherent toxicity of the lead in the structure. In any case, the great structural and compositional tunability of this family of materials and its excellent optoelectronic properties give researchers hope that these challenges are surmountable and the future of the technology is still as bright as ever.

state-of-the-art hole transporter Spiro-OMeTAD. The reaction scheme and molecule are shown in Figure 6. We have performed a cost analysis for the developed EDOTOMeTPA molecule, and estimate a total material cost of only $10 per gram, which results in a negligible material cost contribution of ∼$0.004 Wp−1 in the final device. The costs are comparable with recently developed highly conductive polymer PEDOT (not to be confused with PEDOT:PSS) as the hole transport material on top of the perovskite, with reported efficiencies similar to Spiro-OMeTAD.53 With these materials in hand, commercial applications of perovskite photovoltaics can be targeted. However, the developed low-cost hole transporters currently exhibit a rather narrow band gap, and thus cannot be employed in tandem applications with an inorganic bottom cell.

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OUTLOOK AND CONCLUSIONS In this Account, we have reviewed current advances toward the long-term viability of perovskites in commercial applications. Perovskites are particularly attractive for a variety of applications because they can be easily solution-processed, they are compatible with established scale-up techniques, they are composed of cheap earth-abundant materials, and they already exhibit device performance competitive with established commercial technologies. The ease of structural control via simple tuning of the synthesis route, such as two-step deposition, gives access to a wide variety of films of highly crystalline perovskite structures. This leads to precise control of the optoelectronic properties of the materials, such as the band gap. This in turn makes the materials very attractive in the lighting sector, where access to a wide color range is essential, and for photovoltaic tandem applications with an inorganic bottom cell, where the light absorption of the subcells must be matched well. In this latter instance, however, the commonly employed organic extraction layers must be replaced with lower cost materials in order to meet market standards, with target module costs well below $0.50 per peak Watt. Further developing new low-cost chemistries resulting in easily purified compounds, such as those based on azomethine bonds, will be an essential part of future molecular design for perovskite solar cells. However, no technology is free of challenges, and perovskites are no different. Regardless of the advances toward device performance, if structural stability over thousands of hours is not demonstrated, the devices will probably not be attractive for commercial applications. The performance of the devices can be severely reduced via exposure to moisture, and the typical perovskite structure, methylammonium lead iodide (MAPI), undergoes crystal phase transitions in the operating

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Funding

The authors acknowledge funding from the Bavarian Ministry of the Environment and Consumer Protection, the Bavarian Network “Solar Technologies Go Hybrid”, and the DFG Excellence Cluster Nanosystems Initiative Munich (NIM). P.D. acknowledges support from the European Union through the award of a Marie Curie Intra-European Fellowship. Notes

The authors declare no competing financial interest. Biographies Pablo Docampo received his DPhil in 2012 from the University of Oxford and subsequently performed postdoctoral research with Thomas Bein at the LMU Munich. He was appointed assistant professor at the LMU Munich in 2013 and is now a Marie Curie Fellow at the same institution since 2014. He now leads a research group investigating the long-term applications of hybrid halide perovskite materials, from the development of new materials and solar cell device architectures to new lighting applications such as lasers, LEDs, and LECs. F


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Thomas Bein graduated in Chemistry from the University of Hamburg in 1981 and obtained his PhD in Chemistry in 1984. He continued his studies as Visiting Scientist at the DuPont Central Research and Development Department in Wilmington, DE. From 1986 until 1991, he was Assistant Professor of Chemistry at the University of New Mexico in Albuquerque. In 1991, he joined Purdue University in West Lafayette, Indiana as Associate Professor, and was promoted to Full Professor of Chemistry in 1995. In 1999, he became Chair of Physical Chemistry at the University of Munich (LMU). His current research interests lie in the synthesis and physical properties of functional nanostructures.

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