Current Challenges and Prospective Research for Upscaling Hybrid

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Current Challenges and Prospective Research for Upscaling Hybrid Perovskite Photovoltaics Spencer T Williams, Adharsh Rajagopal, Chu-Chen Chueh, and Alex K. -Y. Jen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b02651 • Publication Date (Web): 11 Feb 2016 Downloaded from http://pubs.acs.org on February 12, 2016

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The Journal of Physical Chemistry Letters

Current Challenges and Prospective Research for Upscaling Hybrid Perovskite Photovoltaics Spencer T. Williams, Adharsh Rajagopal, Chu-Chen Chueh, Alex K.-Y. Jen* Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195, United States *Address correspondence to: [email protected]

ABSTRACT Organic-inorganic hybrid perovskite photovoltaics (PSCs) are poised to push toward technology translation, but significant challenges complicating commercialization remain. While J-V hysteresis and ecotoxicity are uniquely imposing issues at scale, CH3NH3PbI3 degradation is by far the sharpest limitation to the technology’s potential market contribution. Herein, we offer a perspective on the practical market potential of PSCs, the nature of fundamental PSC challenges at scale, and an outline of prospective solutions for achieving module scale PSC production tailored to intrinsic advantages of CH3NH3PbI3. While integrating PSCs into the energy grid is complicated by CH3NH3PbI3 degradation, the ability of PSCs to contribute to consumer electronics and other niche markets like those organic photovoltaics have sought footing in rests primarily upon the technology’s price point. Thus slot die, roll-to-roll processing has the greatest potential to enable PSC scale-up and herein we present perspective on the

research

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Organic-inorganic hybrid perovskite photovoltaics (PSCs) have transfixed researchers in the solar energy field since the first exploration of CH3NH3PbI3 as a light harvesting material.1 While soaring efficiencies2 and increasingly flexible processability make this technology potentially transformative, there remains measured skepticism3 about ultimate impact because of intrinsic toxicity and instability.4 The implementation of an energy generation technology necessitates a pragmatic balance between energy investment, energy return, and overall impact of implementation. As a technology and societies using it change, this balance shifts. This is partly manifested in the gradually changing experience curve of a PV technology: production quality as a function of cost. Currently, the experience curve of silicon PV is daunting to contend with,3 but it is difficult to fairly compare silicon and perovskite experience curves because of the very different stages of development. Moreover, even with an established technology, accurately analyzing true economic and energetic investment/return is difficult. Changes in commodities, government subsidies, regulations, implementation, environmental factors like insolation, and continued developments at all levels of production complicate the generation of unambiguous price points. To understand cumulative economic and environmental costs of a technology over its entire service life it is common to use lifecycle analysis (LCA), a framework regulated by the International Organization of Standardization. This framework provides structure for such a wide


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array of systems and scales that some subjectivity is entailed in approaching specific sets of questions, and resources are available to guide analysis.5 As has been noted by the National Renewable Energy Lab, market and policy complexities make prices defining a PV technology’s threshold of profitability highly dependent upon the economic performance metric chosen.6 One of the most holistically meaningful metrics is the levelized cost of electricity (LCOE) which accounts for subsidized and direct capital investment as well as continued investment in facility overhead. LCOE is essentially the cost at which electricity must be generated to completely recoup initial and continued investment averaged over a technology’s lifetime. The US Energy Information Administration estimates that the LCOE for photovoltaics in general will be roughly $125 per megawatt-hour (MW-hr) in 2020 compared to ~$100 per MW-hr for coal.7 For perspective on what is becoming realistically achievable, SunEdison, an industry leader in monocrystalline silicon photovoltaics, is building to meet a $46 per MW-hr power purchasing agreement (PPA) by 2016. More recently, a Nevada utility signed a PPA for ~$39 per MW-hr power from a thin film solar farm being developed by First Solar. These price points reflect sharp changes from as recently as 2014 when the same Nevada utility paid an average of ~$140 per MW-hr for renewable energy.8 Champion PSCs have reached over 20% power conversion efficiency (PCE) in just a few years compared to the ~40 years of research behind champion 25% PCE monocrystalline silicon devices and 21.5% PCE CIGS and CdTe thin film devices.2 This combined with relatively low material and fabrication costs9 makes PSCs appear viable as on-grid competitors for existing technologies; however, this remains completely dependent on addressing ecotoxicity, J-V hysteresis, and degradation. Both LCOE and levelized avoided cost of electricity (LACE) are necessary metrics for fairly


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comparing viability of different generation strategies in real settings, but the sensitivity to economic and regional realities make these mechanics cumbersome for comparing various PV technologies on equal footing. Energy payback time (EPBT) is the service time required for a solar cell to generate net positive energy after accounting for all energy invested in its creation. It has recently been shown that PSC EPBT can be as low as 0.22 years compared to ~2.4 and ~0.7 years for crystalline silicon and thin film PV technologies respectively.9 Deleterious impacts on freshwater and human health identified by recent LCAs have been linked to Pb in CH3NH3PbI3 and other environmentally expensive materials in PSCs like FTO/ITO glass and noble metal electrodes.9 While encapsulation10 makes Pb escape during PSC service negligible, disposal11 is a serious issue. The remarkably low PSC EPBT may allow greater investment in end-of-life infrastructure for this technology, as well as aggressive device encapsulation to address the most direct impediment to PSC implementation: CH3NH3PbI3 degradation.12 Not only is CH3NH3PbI3 vulnerable to degradation through hydration, exposure to illumination, bias, and heat accelerate the process in unique ways.12 Moreover, CH3NH3PbI3 is thermodynamically unstable relative to PbI2 at operating temperatures making eventual degradation inevitable regardless of encapsulation.13 While this issue makes PSC commercialization potentially prohibitive, compositional engineering13,14 has shown promise in addressing both thermodynamic instability of CH3NH3PbI3 and J-V hysteresis in PSCs. Currently, most markets necessitate 15 to 20 year warrantees for competitive residential and commercial PV systems. Depending on how successfully the research community addresses CH3NH3PbI3 degradation this service life may not be possible for some time. While this limits immediate PSC on-grid potential, the remarkably low EPBT, wide and vivid color tunability,15 and device flexibility readily enable contributions to the same niche markets organic


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photovoltaics (OPV) have sought footing in.16,17 Semitransparent, neutral density18 and colored PSCs15 have demonstrated the technology’s potential for building integrated photovoltaics (BIPV) as well.19 Impediments to PSC energy grid integration make market contribution more likely via gradual percolation through these niche markets. Thus, scaling up large area, flexible device fabrication allows greater access to the value this technology offers. For the research community, these current market realities highlight fully printable PSCs as a driving goal for innovation. In terms of fundamental chemical research, this implicates thermodynamic control of CH3NH3PbI3 growth and degradation as important directions to fuel continued growth of the field. This short piece offers a perspective on challenges in up-scaling PSC fabrication as well as potential solutions. Scaling-up fundamental complexities of PSCs Each challenge inherent to CH3NH3PbI3 is tied to a key aspect of the material’s utility. Here, we discuss how the challenges of ecotoxicity, J-V hysteresis, and instability take on new and more imposing character in any attempt to scale up PSC production. The ensuing discussion is summarized in Figure 1. Ecotoxicity Pb possesses electronic properties that make it a very unique part of the periodic table. While also partly the source of its cytotoxicity,20 the size of Pb makes the 6s electrons that stabilize the 2+ oxidation state inert. While in the 2+ oxidation state, Sn easily oxidizes to 4+ creating instability in CH3NH3SnI3 even more troublesome than in CH3NH3PbI3.4 This makes Pb elimination more a matter of new material design rather than simple atomic replacement.21 The challenge thus shifts to CH3NH3PbI3 device encapsulation,12 end-of-life measures like recycling, and integration into other Pb based technology lifecycles.11 Recent LCAs indicate that device encapsulation can effectively limit Pb escape during PSC service10 and end-of-life techniques like incineration and Pb recovery can further minimize exposure.11 Ultimately, the degree of Pb


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exposure indicated by current LCAs is moderate and comparable to existing technologies especially when PSCs are used for Pb sequestration from already societally prevalent sources like Pb-acid batteries.11

Figure 1. Schematic showing how each challenge intrinsic to PSCs takes on a new weight at scale. Ecotoxicity and J-V hysteresis are serious issues, but environmental stability and limited service life are the sharpest determinants of potential markets for PSCs. Specific environmental regulations controlling the implementation of photovoltaics vary throughout the world, but in terms of Pb toxicity this technology will face considerations similar to those relevant to the application of cadmium based thin film PV. High throughput production


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has its own environmental implications regardless of Pb toxicity, and at scale the many other materials in PSCs present their own environmental challenges. While solution processing allows use of facile, large area deposition techniques, it also means a great deal of solvent waste and the need to manage solvent vapor during fabrication. The FTO/ITO coated substrates often used in PSCs require energetically costly vacuum deposition, and noble metal availability creates a resource scarcity issue with PSC electrodes.9 In terms of end-of-life considerations at scale, emissions from incinerating polymer based flexible substrates required for processes like roll coating are a non-trivial issue.11 In defining solutions to these ecological issues, PSC device design plays as significant a role as addressing Pb toxicity. J-V hysteresis In photovoltaics, hysteresis commonly manifests as a change in the shape of J-V characteristics. Early in the evolution of PSCs, J-V hysteresis anomalously exacerbated by slow voltage scan rates was observed.22 Through fierce investigation it has been unveiled that the changing electric field profile within CH3NH3PbI3 responsible for hysteresis is largely caused by motion of ions within the CH3NH3PbI3 lattice. 23-24 In CH3NH3PbI3, the delicate balance between Pb, iodide, and methylammonium binding enables the low temperature self-assembly responsible for this material’s potentially transformative impact in the PV sector.13 Pb can exist in many different coordination states with halogens which is a key issue in the chemistry behind CH3NH3PbI3 growth,25 but this flexibility also likely adds lability to the Pb-I bond in the solid state. By virtue of calculated activation energies, methylammonium, iodide, and their respective charged vacancies in CH3NH3PbI3 are primarily implicated in ion motion behind hysteresis.23 While this understanding simplifies the matter, exposing devices to illumination, bias, heat, and sources of degradation like water modifies how hysteresis manifests in J-V behavior, and thus the issue may be physically quite complex.26 Whether or not ion motion can fully be halted


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through CH3NH3PbI3 material design as has been suggested by recent work,14 unstable PSC performance can be addressed through PSC interface and device design as interfacial charge extraction efficiency plays a key role in hysteresis.27,28 At the module level, electrical consequences of unstable device function caused by transient processes in CH3NH3PbI3 may be magnified by the series and parallel connections knitting together an overall module. On the laboratory scale, accounting for hysteresis is important to preserve accuracy of PSC performance data.22 On the module scale, failing to account for hysteresis may make entire blocks (individual devices in parallel) perform unstably and introduce mismatch effects along entire strings (blocks connected in series). This can be addressed through module design including use of bypass and blocking diodes, but failing to address hysteresis at the material level means inevitable performance loss through module J-V instability. Degradation Although oxidation is not a primary concern for CH3NH3PbI3, spontaneous hydration and/or transformation to PbI2 are accelerated by device service conditions and constitute the sharpest limiting factors facing PSC implementation. Metastable monohydrate (recoverable) and stable dihydrate (unrecoverable) formation is driven by hydrogen bonding between water molecules and CH3NH3+ in the CH3NH3PbI3 lattice. Transformation to PbI2 from any of these states can occur through proton transfer yielding highly volatile CH3NH2 or gradual CH3NH3I sublimation.12 While CsPbI3 perovskites eliminate this hydrogen bonding and transfer, elimination of CH3NH3+ complicates processing and reduces performance.4 Great strides have been made with PSC encapsulation, but this doesn’t address the fundamental thermodynamic instability of CH3NH3PbI3 with respect to PbI2.13 That intrinsic limitation means compositional changes of the CH3NH3PbI3 lattice are necessary to ensure thermodynamic stability regardless of device level accommodations. Still, at the device level there are unique challenges to stability


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including the acid base chemistry that accelerates degradation at a ZnO/CH3NH3PbI3 interface.29 Degradation is uniquely troublesome on the module level for the same reasons J-V hysteresis is problematic, but with degradation individual cells may eventually be completely removed from the circuit. Complete water exclusion can be energetically costly in many high throughput device fabrication techniques, and water interacts with CH3NH3PbI3 growth even at small concentrations.12 Environmental CH3NH3PbI3 exposure can accelerate Pb liberation10 and exacerbate J-V hysteresis26 tying together all fundamental challenges in PSC design. Although toxicity and hysteresis affect PSC price point, degradation establishes sharp ceilings for device lifetime and thus largely determines what markets the technology can contribute to. Although PSC stability over 1000 hours has been demonstrated,30-31 in the short term this means that the most significant contribution from PSCs may be to niche markets like portable electronic devices, flexible PV coatings, and BIPV. Integration of PSCs into the energy grid may become possible, but only through opportunities in material and device design. Scaling-up fabrication of PSCs Spin-coating provides an expedient route for thin-film deposition from solution because at the high spin rates used, viscosity and interaction with the substrate largely determine film dimensions. This process produces a uniform film of solution, and in ideal cases optimization is geared toward controlling solvent removal. CH3NH3PbI3 growth however proceeds through many solution and solid states making even spin-coating difficult to control.32 While large area CH3NH3PbI3 deposition techniques are already under investigation, fully printable module level PSC production remains a significant challenge. Practically realizing this goal entails device level challenges similar to those the OPV field has faced33 and material level challenges unique to CH3NH3PbI3 growth.13,23 Using the context of the previous section, below we attempt to illuminate potential solutions from material to device


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levels to address fundamental PSC challenges at scale. Technique

Electrodeposition

Spray Coating

Inkjet Printing

Screen Printing

Blade Coating

Slot-die R2R coating

Key characteristics Established industrial technique already being utilized for functional coatings and inorganic thin film solar cells Promising for fabrication of inorganic mesoporous scaffold and epitaxial films, which can be coupled with liquid or vapor based treatments for conversion to perovskite Large amount of liquid bath used in the process is concerning in terms of solution wastage and disposal Utilization of low solid concentration in solution compared to spin coating on laboratory scales Impractical on large scale due to low resolution patterning and excessive restrictions on the boiling points of solvent used Programmable fabrication enables precise control of patterns and interfaces. Multiple channels can ease compositional engineering of perovskite via in situ mixing of ink from separate cartridges Speed limitations and restrictions on ink properties limits the utility of printing all layers in R2R compatible device fabrication Effective in formation of nanostructured scaffolds and very thick layers (10-500 µm) for use as interlayers and electrodes Expensive when transferred to flexible substrates for high throughput processing via rotary screen printing Convection facilitated drying leading to slower film formation favoring realization of large crystalline domains and photonic nanostructures for efficient and colorful perovskite solar cells Efficient materials usage and high throughput deposition for a wide variety of perovskite chemistries Difficulty in fabrication of thin interface layers Unique crystallization kinetics controllable via solution feed, drum speed and temperature eases efficient printing of well-defined stripes for solar module fabrication Desired thickness and properties of perovskite and interlayers can be achieved via ink, surface and interface modifications Economically viable because of its effective material usage for printing few meters per minute with minimal dead space

PCE (Area) Reference 4.5% (4 cm2) 34

7.3% (0.07 cm2) 35 11.6% (0.15 cm2) 36 15% (0.16 cm2) 37 10.4% (0.1 cm2) 15, 38 Semi-9.2% (0.1 cm2) Full-3.4% (0.5 cm2) 39-41

Table 1. Summary of current, roll-to-roll compatible techniques for CH3NH3PbI3 deposition. Performance data is reported from work most indicative of real large area device performance; and efficiency and device area is reported along with the key characteristics of each technique. Scalable processing techniques and device level challenges The low temperature, solution processability of CH3NH3PbI3 allows the use of flexible substrates like polyethylene terephthalate (PET) thus enabling more cost effective production, transportation, and deployment strategies relative to silicon PV. Thus, a wide range of techniques including but not limited to electrodeposition,34 spray coating,35 inkjet printing,36 screen printing,37 blade coating,15,38 and slot-die roll to roll (R2R) coating39–41 are being employed to address challenges associated with


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PSC processing. A brief summary of the characteristics, highest device performance, and merits and demerits associated with each technique being used is provided in Table 1. As in all photovoltaics,33 increasing PSC size and flexibility increases series resistance and magnifies instability so PCEs much lower than lab scale devices are inevitable. Performance data reported in Table 1 is the work most faithfully indicative of real large area device potential, and device area for each PCE is included in the table. The only fully printed, R2R PSC demonstrated to date currently has a PCE below 5% demonstrating significant room for innovation.39 Many methodologies in Table 1 focus only on scaling-up CH3NH3PbI3 deposition rather than full device preparation. This is partially because of the wealth of work that has been devoted to encapsulation, printable electrodes, and low temperature charge transporting interlayer (CTL) processing through the effort to scale up OPVs.33 Taking full advantage of PSC market potential requires access to the same niche markets OPVs have sought footing in,16,17,19 which makes realizing fully printable, flexible devices pivotal. Among the many options, slot die R2R coating is the most attractive route for PSC scale-up because of the technique’s amenability to large throughput device fabrication.33 Combining low EPBT of PSCs9 with high power to weight ratio of flexible devices is likely necessary to make PSCs market contributors. Interfaces play key roles in CH3NH3PbI3 stability29 and hysteresis27-28,42 which necessitates rational CTL design. Fullerene based self-assembled monolayers (SAM) initially employed in OPVs have shown promise in addressing both of these issues,42 despite the intrinsic difficulty in achieving ultra-thin CTLs and SAMs with scalable techniques like blade and slot die R2R coating.40 Addressing the challenge of SAM deposition would allow access to a wider variety of inorganic CTLs to take advantage of their good ambient and thermal stability in PSC design. Inorganic materials can be made amenable to printable device fabrication through low


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temperature processing techniques like combustion synthesis and nanoparticle deposition, and some inorganic surfaces have proven effective at templating CH3NH3PbI3 growth.43 Fullerene based CTLs have proven effective at addressing PSC hysteresis while preserving high PCE, and many of the other CTLs employed in PSCs are organic and readily solution processable. Proper doping of organic CTLs can widen the organic material toolbox in PSC deisgn.44 Design of the top electrode remains a challenge no matter what processing technique is used. In OPV, the most common choice of top electrode is screen printed and flexographic printed silver.33 In PSCs however, silver electrodes have been found to facilitate CH3NH3PbI3 degradation to PbI212 and the printing itself requires an orthogonal electrode ink to avoid damaging the underlying layers. Alternatives like carbon nanotube deposition45 and Ni-mesh lamination46 are currently being explored. Even with these challenges in mind, the successful application of many CTLs, electrodes, and encapsulation techniques already employed in OPV means that experience accumulated through OPV scale-up supports efforts to scale up PSCs.44 Challenges inherent to scaling-up perovskite growth The primary issue that separates PSC scale-up from other classes of solution processable photovoltaics is the complex growth behavior of CH3NH3PbI3 and the equally complex structure-property relationships that result.32 hybrid perovskite colloidal and coordination chemistry creates an intimate relationship between solution chemistry and final film quality.25 The nature of the just-deposited film of solution thus determines a great deal about how CH3NH3PbI3 growth proceeds. Slot die and blade coated films initially contain much more material overall as well as a much greater fraction of solvent than comparable spin-coated films. This means that much of the work devoted to optimizing CH3NH3PbI3 growth32 cannot be directly applied to scaled-up production without re-optimization and in some cases fundamental restructuring. The greater solvent content in films deposited with


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R2R coating likely shifts CH3NH3PbI3 and intermediate growth kinetics. Many relationships established between initial conditions, temperature, time, atmosphere, solution composition, growth, and resulting device performance must thus be remapped in conditions relevant to particular scalable methods. The thermodynamically limited CH3NH3PbI3 growth in these high solvent content films makes controlling growth thermodynamics through solution25 and interfacial chemistry43 key to scaling-up production. The solvent washing deposition method serves as an excellent example of the reformulation necessary to adapt lab-scale strategies to the large scale.14 This approach produces remarkably high quality CH3NH3PbI3 films on the laboratory scale, but it does so by capitalizing on the ability to almost instantaneously add and remove an anti-solvent through spin-coating. Since extreme substrate motion is impossible for large devices, this strategy must be fundamentally reformulated for module scale production. The physical balance responsible for the success of this technique on the lab scale is chemically intrinsic to the choice of solvent, anti-solvent, and species constituting CH3NH3PbI3. Thus it should still be possible to use an anti-solvent to control CH3NH3PbI3 formation in large devices. That said, for printed films a different physical means of anti-solvent exposure like vapor treatment becomes necessary, which entails another regime of exploration and optimization. Further complication stems from inevitable non-idealities intrinsic to large area processing like spatial variations in temperature, vapor composition, and solution concentration. While possible to address these issues with optimization, as has been attempted with regard to water exposure during fabrication,38 complex interrelated processes like hydration, J-V hysteresis, and degradation12 complicate optimization. Because complexity in CH3NH3PbI3 growth is not limited to the solid state,25 regulation of solution feed stock will become a practical consideration for industrial production. Time


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dependent solution equilibria like aggregation47 and complex ion formation25 will complicate consistency, an issue that illustrates the possible utility of two-step CH3NH3PbI3 growth for scalable deposition.39 Entirely vapor based deposition techniques avoid the complexity of CH3NH3PbI3 growth from solution at the expense of increased cost and the difficulty inherent in controlling vapor concentration and flow across large areas. Although one-step CH3NH3PbI3 growth from vapor circumvents CH3NH3PbI3 · Solvent intermediates,32 other intermediate phases implicit in CH3NH3PbI3 growth like CH3NH3PbCl3 still persist.48 This means that the requirement to carefully control growth through processing conditions remains relevant. CH3NH3PbI3 growth is linked to almost every identifiable processing parameter but our understanding of the structure-property relationships defining electronic conduction, absorption, J-V hysteresis, and degradation in CH3NH3PbI3 is still developing. Both CH3NH3PbI3 carrier concentration and J-V hysteresis26 are linked to halide stoichiometry which changes throughout growth to a degree dependent on growth route and conditions. The importance of microstructure in carrier recombination has been demonstrated,49 but the exact origins for the behavior remain unclear. The convolution of all relevant properties with illumination, bias, and temperature50 makes CH3NH3PbI3 a uniquely complex system to fully grasp at the design level, and thus work towards fundamental understanding still plays a role in the development of this technology. The material (A), chemical (B), and process (C) engineering necessary to achieve scaled up PSC production is outlined in Figure 2 and discussed below.


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Figure 2. Summary of research directions necessary to fuel PSC technological translation via slot die R2R coating. (A) material engineering can address intrinsic ion diffusivity and degradation of CH3NH3PbI3, (B) optimization of perovskite growth under R2R conditions is necessary to reach performance requirements, and (C) device engineering is required to extend device life and expand the material toolbox for PSC design. Scalable solutions from material to device levels

Fundamental thermodynamic instability,13

susceptibility to hydration,12 and high ionic mobility23 are properties intrinsic to CH3NH3PbI3.


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Thus, truly scalable PSC solutions addressing these key challenges must incorporate compositional control of the perovskite lattice itself (Figure 2a). Bromide and fomamidinium (FA) have been successfully incorporated into the CH3NH3PbI3 lattice to mitigate J-V hysteresis14 and eliminate fundamental thermodynamic instability.13 The increased bond polarity of Pb-Br relative to Pb-I plays a beneficial role in crystallization. Bromine inclusion shifts the band gap4 which allows color tuning for BIPV19 and tandem device architectures.4 Additionally, many have found that a small amount of Br and FA lead to a maximum in overall device PCE.14 The precise mechanism behind these enhancements in fundamental material properties remains unclear and under illumination the otherwise complete solid solubility of Br in CH3NH3PbI3 shifts limiting effective substitution to roughly 20 atom percent.51 Regardless of these complexities, compositional optimization of CH3NH3PbI3 is an inevitably necessary tool for effective scale-up. Taking CH3NH3PbI3 compositional engineering and unique conditions of R2R coating into account in the optimization of CH3NH3PbI3 growth, microstructure, and composition is an important step in realizing module scale PSCs (Figure 2b). Although the sensitivity of CH3NH3PbI3 growth toward almost all processing parameters makes production a challenge,32 it also creates unique opportunities for exerting control. The shift from spin-coating to R2R coating represents a shift from kinetically limited CH3NH3PbI3 growth to thermodynamically limited growth. The most effective way to control growth thus becomes tuning the thermodynamics driving growth through precursor solution composition,25 the chemistry of the surface on which growth occurs,43 annealing conditions, and atmosphere during growth.4 Remapping thermal annealing behavior under conditions directly relevant to large area deposition is readily done. Solution additives, solvent engineering, and spectator ions like Cl- can


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be exploited to fundamentally alter CH3NH3PbI3 growth route and control kinetics of competing processes.32 The retrograde solubility of CH3NH3PbI3 in certain solvent systems creates unique opportunities for tuning growth thermodynamics with temperature,52 both before deposition by controlling aggregation in solution and after deposition by controlling solubility. While an inevitable complication of large area deposition, the influence of atmosphere during CH3NH3PbI3 growth allows control of morphology and film quality through tuning the concentration of solvent, methylamine, and water vapor present.4 Interface modification through SAM deposition, solvent treatment, and a variety of other methods exploit relationships between substrate surface chemistry and CH3NH3PbI3 growth to control crystallization without external intervention. This makes interface modification an important consideration in scale-up despite the challenge of ultra-thin film deposition with R2R coating. Vapor based post deposition modification with primary amines and moderately sterically hindered Lewis bases,53 processes readily adaptable to R2R coating, have shown a dramatic influence on CH3NH3PbI3 interfacial microstructure and chemistry respectively. Because thermal and ambient instability are the sharpest limitations to PSC market potential, the EPBT and potential price point of fully printable, flexible PSCs are central to the technology’s current capacity for commercialization (Figure 2c). Limitations in SAM and ultrathin film deposition through slot die R2R coating constitute an opportunity for innovation in realizing effective PSC modules. Tuning solution feed remains the most expedient route to controlling thickness, and creative processing techniques realizing SAM modification with R2R coating40 will enable a larger material tool box for PSC fabrication at scale. Replacement of the often employed silver electrode is necessary to avoid expediting CH3NH3PbI3 degradation, and electrode deposition itself is an area ripe for innovation. This is especially true for transparent


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electrodes to enable module scale, semi-transparent PSCs. Inevitable contributions from J-V hysteresis must be addressed by pragmatic module design to avoid unstable performance. Device encapsulation and end-of-life techniques like recycling are practically necessary to implement PSCs at any scale; but ultimately the color tunability, EPBT, and flexibility offered by fully printable PSCs enable contributions to consumer electronics and other niche markets like BIPV. Concluding remarks on technology translation As the recent inclination within the field toward exploring new systems demonstrates, development of CH3NH3PbI3 based PSCs has reached the plateau appropriate to support pursuit of technological translation. Challenges facing the technology remain substantial, but through focused fundamental research addressing CH3NH3PbI3 degradation as well as pragmatic engineering addressing Pb ecotoxicity and J-V hysteresis this technology may eventually become viable for energy grid integration. In the immediate future, techniques like slot die R2R processing are poised to enable the price point necessary for PSCs to contribute to the same markets sought after by OPV. PSC scale-up, however, requires sensitivity to the complex growth of CH3NH3PbI3 as well as the challenges intrinsic to R2R coating. Necessary solutions to these issues range from material to device levels. CH3NH3PbI3 compositional engineering is necessary to address fundamental material issues like ion mobility and stability. Optimization of CH3NH3PbI3 growth specific to the thermodynamically limited conditions inherent to R2R coating along with the exploration of post deposition treatments is necessary for ensuring quality material preparation at scale. The sensitivity that makes CH3NH3PbI3 growth depend on virtually all processing parameters creates an equivalently wide variety of strategies for controlling growth. Intrinsic limitations of R2R coating in extremely thin film and SAM deposition along with electrode design and deposition complicate fully printable PSC fabrication, but if addressed the resulting price point of the


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technology may enable PSC market permeation via percolation through niche applications like BIPV and consumer electronics. While herein we have focused on the market potential of PSCs alone, CH3NH3PbI3 has great potential for application in tandem devices with silicon or CIGS based devices as well as application in light emitting and detecting technologies. Despite the challenges, CH3NH3PbI3 has proven to have potential limited only by our ingenuity.

AUTHOR INFORMATION NOTES The authors declare no competing financial interests. BIOGRAPHIES Spencer T. Williams received a B.A. in Physics and Chemistry from Colorado College in 2010. He is currently a Ph.D. candidate in the Department of Materials Science and Engineering at the University of Washington. His research interests include material physics; nanomaterial, pnictogen, chalcogen, and hybrid metal halide chemistry; and electron microscopy & spectroscopy. Adharsh Rajagopal received a B.Tech in Materials Science and Metallurgical Engineering from National Institute of Technology-Bhopal in 2012 and M.S. in Materials Science and Engineering from University of Florida in 2014. He is currently a Ph.D. candidate in the Department of Materials Science and Engineering at the University of Washington. His research interests include optical engineering and device physics of thin-film photovoltaics. Chu-Chen Chueh received a B.S. and Ph.D. in Chemical Engineering at National Taiwan University. He is currently the leader of the organic-inorganic perovskite solar cells thrust in Prof. Jen’s group in the Department of Materials Science at the University of Washington. His


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research interests include organic/hybrid semiconductors and their applications in optoelectronic devices. Alex K.-Y. Jen is the Boeing-Johnson Chair Professor of Materials Science and Engineering at the University of Washington. He is also serving as Chief Scientist for the Clean Energy Institute endowed by the Washington State Governor. He earned his Ph.D. in 1984 from the University of Pennsylvania. He has co-authored more than 560 papers with >27,000 citations and an H-index of 84.

QUOTES Because thermal and ambient instability are the sharpest limitations to PSC market potential, the EPBT and potential price point of fully printable, flexible PSCs are central to the technology’s current capacity for commercialization Fundamental thermodynamic instability, susceptibility to hydration, and high ionic mobility are properties intrinsic to CH3NH3PbI3. Thus, truly scalable PSC solutions addressing these key challenges must incorporate compositional control of the perovskite lattice itself The thermodynamically limited CH3NH3PbI3 growth in these high solvent content films makes controlling growth thermodynamics through solution and interfacial chemistry key to scaling-up production. Although the sensitivity of CH3NH3PbI3 growth toward almost all processing parameters makes production a challenge, it also creates unique opportunities for exerting control.

ACKNOWLEDGMENT The authors thank the support from the Asian Office of Aerospace R&D (No. FA2386-11-14072), the Office of Naval Research (No. N00014-11-1-0246), and the Department of Energy SunShot (DE-EE0006710). This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-0718124 & DGE-


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1256082. A. K.-Y. Jen thanks the Boeing-Johnson Foundation for their financial support.

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Current Challenges and Prospective Research for Upscaling Hybrid

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