Impact of Surfaces on Photoinduced Halide Segregation in Mixed

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Impact of Surfaces on Photo-induced Halidesegregation in Mixed-halide Perovskites Rebecca A. Belisle, Kevin A. Bush, Luca Bertoluzzi, Aryeh Gold-Parker, Michael F. Toney, and Michael D. McGehee ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01562 • Publication Date (Web): 05 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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Impact of Surfaces on Photo-Induced Halide-Segregation in Mixed-Halide Perovskites Rebecca A. Belisle1,2, Kevin A. Bush2, Luca Bertoluzzi2, Aryeh Gold-Parker3,4, Michael F. Toney4, Michael D. McGehee5* 1 Physics

Department, Wellesley College, Wellesley, MA 02481 Materials Science & Engineering, Department, Stanford University, Stanford, CA 94305 3 Chemistry Department, Stanford University, Stanford, CA 94305 4 Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025 5 Chemical and Biological Engineering, University of Colorado Boulder, Boulder, CO 80309 2

Abstract: Photo-induced halide segregation currently limits the perovskite chemistries available for use in high bandgap semiconductors needed for tandem solar cells. Here, we study the impact of post-deposition surface modifications on photo-induced halide segregation in methylammonium lead mixed-halide perovskites. By coating a perovskite surface with the electron-donating ligand trioctylphosphine oxide (TOPO), we are able to both reduce non-radiative recombination and dramatically slow the onset of halide segregation in CH3NH3PbI2Br films. This result, coupled with the observation that the rate of halide segregation can be tuned by varying the selective contact, provides a direct link between surface modifications and photo-induced trap formation. Based on these observations, we discuss possible mechanisms for photo-induced halide segregation supported by drift-diffusion simulations. This work suggests that improved understanding and control of perovskite surfaces provides a pathway towards stable and high-performance wide bandgap perovskites for the next generation of tandem solar cells. Table of Content’s Graphic: Average Photoluminescence [nm]

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Perovskite tandem solar cells have recently emerged as highly promising photovoltaic technologies. Unlike conventional solar cells, tandem cells use multiple semiconductors of varying bandgaps in a single photovoltaic device to absorb more of the solar spectrum and thereby increase power conversion efficiency1–4. For monolithic perovskite-silicon tandems, the ideal perovskite bandgap is ~1.7 eV on top of 1.1 eV silicon5. This optimized bandgap should allow for a theoretical power conversion efficiency (PCE) approaching 39%, well above the 25% PCE currently achieved by high efficiency perovskite tandems5,6. The ideal top-cell bandgap increases to ~1.8 eV if a second perovskite is used for the bottom subcell since the lowest bandgap currently achievable with perovskites is 1.19 eV7,8, enabling a theoretical PCE near 38% from a potentially lower-cost tandem solar cell that can be manufactured at a large scale5,9. However, while perovskites with ideal bandgaps are synthesizable via chemical substitution of bromide for iodide (i.e. CH3NH3Pb(IxBr1-x)3)10–12, this substitution makes them susceptible to halide segregation13,10,14. This process of photo-induced halide segregation – often referred to as photo-induced halide segregation15 – prevents the increase in bandgap from realizing a proportional increase in open circuit voltage, ultimately limiting the benefits of bandgap-tunable, mixed-halide perovskites as top cells in tandems16. Photo-induced halide segregation was first observed in CH3NH3Pb(IxBr1-x)3 compounds with bromine contents exceeding 20%. In these mixed-halide compounds the photoluminescence was seen to shift to lower energies, centered at the emission energy of that in CH3NH3Pb(I0.8Br0.2)3, under continuous illumination of the sample14. This shift in photoluminescence is understood to coincide with demixing of the perovskite into iodide-rich and bromide-rich domains, where photo-generated carriers are funneled into the lower band gap iodide-rich regions, which act as traps and increase the rate of recombination. This effect is also observed to be reversible if the samples rest in the dark14,17. Further work has found that the rate and behavior of photo-induced halide segregation depends upon A site cation chemistry for ABX3 perovskites11,18–20, can be expedited or suppressed by varying halogen to lead ratios21,22, is morphology dependent23,24, is driven by increasing injected carrier density in addition to photo-generated carriers23, and appears dependent upon carrier generation gradient21. To explain photo-induced halide segregation, specifically the reversibility and the 20% bromide threshold for methylammonium perovskites, several groups have developed thermodynamic models that predict the formation of a miscibility gap in the free energy curve of the perovskite solid solution under illumination25–27. These models point to the existence of either large polarons25 or a high density of free carriers under illumination26 as the thermodynamic driving force for demixing, and allow for the observation that, when returned to the dark, the samples could return to a mixed equilibrium state. These models would suggest that photo-induced halide segregation is an intrinsic property of certain perovskite chemistries, largely independent of film morphology, and that avoiding it would require new perovskite compositions with, for instance, reduced electron-phonon coupling25. However, previously mentioned experimental observations appear to challenge these thermodynamic models by showing that the rate of halide segregation is impacted by tuning precursor stoichiometry21,22, film morphology23,24, or generation profile21, suggesting that our understanding of photo-induced halide segregation is either incorrect or incomplete. Recently, Barker et al. have observed that photo-induced halide segregation can be suppressed not by varying chemistry, but by instead varying the illumination profile incident on the perovskite film – linking heterogeneity in carrier generation to the halide segregation process, and suggesting that photo-induced halide segregation arises from bromine preferentially moving away from illuminated areas in cubic perovskites21. However there remains insufficient data to link all of these observations with a complete model of photo-induced halide segregation. To intelligently select materials that are resistant to photo-induced halide segregation and therefore suitable for high-efficiency tandems, we must better understand the origin and nature of this photo-induced halide segregation. Here we present our observations that perovskite surfaces are an integral part of the progression of the halide-segregation process. We show that by modifying surfaces to control carrier trapping we can substantially slow photo-induced halide segregation, and in doing so we provide new insights into the mechanism of halide segregation in mixed-halide


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perovskites. Outside of the explanation set forth by Barker et al.21, prior models cannot straightforwardly explain how a surface modification would have such a large impact. We therefore present possible mechanisms of halide-segregation, supported by drift-diffusion modeling, which are consistent with our experimental observations in the hopes of catalyzing further modeling and a deeper understanding of photo-induced halide segregation. Controlling and understanding perovskite surfaces has proved essential for increasing the efficiency and stability of perovskite solar cells28–30. As evidence of the importance of controlling surfaces and surface defects, many researchers have focused on the development of surface treatments to reduce non-radiative recombination and increase photovoltaic performance of the pure iodide perovskite CH3NH3PbI331–36. Given the benefits of a subset of these treatments – namely trioctylphosphine oxide31,37, iodopentafluorbenzene36, and polystyrene38 – on the pure iodide perovskites, we were motivated to investigate the impacts of these post perovskite deposition chemical treatments on the photoluminescence behavior of the mixed halide perovskite CH3NH3PbI2Br. Of the surface treatments explored, the Lewis base and ligand trioctylphosphine oxide (TOPO) was observed to have the most significant impact on light-induced phase segregation in CH3NH3PbI2Br (results of other surface treatments in SI Figure 1). In addition to TOPO, polystyrene was observed to slow the photo-induced shift in photoluminescence, albeit not as radically as the TOPO treatment (SI Figure 1). Both the TOPO and polystyrene are deposited as solutions with chlorobenzene, yet their impact on photoluminescence far exceeds that of the solvent (chlorobenzene) on its own (SI Figure 2), suggesting that the impact is due to the additives themselves. Figure 1 shows the impacts of a TOPO surface treatment on halide segregation. All samples were illuminated with 0.1 suns equivalent of 488 nm illumination such that the evolution in photoluminescence could be resolved in a period of seconds. Additionally, all samples were tested in a nitrogen filled chamber with a fixed sample position and alignment, such that photoluminescence intensity can be qualitatively compared between samples.

Figure 1: Behavior of CH3NH3PbI2Br with and without Lewis base treatment trioctylphosphine oxide (TOPO) as monitored by photoluminescence under 0.1 suns equivalent of 488 nm illumination through top-surface of a perovskite film over a two minute time period (a) without and (b) with treatment, and (c) after 10 minutes of continuous light exposure. Evolution of photoluminescence highlighted with red arrows. With no surface treatment the standard photo-induced halide segregation process is observed (Figure 1a), with the formation of sub-bandgap emission over a period of seconds and a continued increase in the photoluminescence from smaller bandgap domains over a two-minute time period. After 10 minutes the signature of the wide band gap cannot be detected (Figure 1c). Note that the photo-induced halide effect is strongly dependent on the light intensity, as described elsewhere and shown in SI Figure 315. However, when


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the same perovskite material is capped with TOPO (Figure 1b), not only is the initial photoluminescence intensity substantially increased, in line with previous reports31, but the photoluminescence shift to lower energy appears halted within the same time window. After prolonged illumination, evidence of photo-induced halide segregation in the TOPO-treated samples is seen to emerge (Figure 1c), however the rate of halidesegregation as monitored by photoluminescence is substantially reduced with the TOPO treatment (SI Figure 4). This is additionally impressive given that the increased photoluminescence intensity of the TOPO sample suggests an increase in the steady-state perovskite carrier density, and increased carrier density has been previously linked to a faster rather than a slower shift in the photoluminescence23. This suppressed rate of halide-segregation with the use of TOPO is additionally observed at higher light intensities and with other perovskite compositions with higher bromide concentration (SI Figure 5). Finally, we confirm that beyond being an optical effect, this slowed photoluminescence-shift reflects a delay in halide-segregation by performing insitu X-ray diffraction (XRD) to monitor the cubic perovskite’s structure during illumination. XRD results for illuminated MAPbIBr2 samples coated with TOPO show reduced structural distortion as compared to untreated controls (SI Figure 6). This lattice distortion, although not completely understood, has been previously linked to the formation of a lower bandgap perovskite phase14,21. It should be noted that a higher bromine content perovskite was selected for this experiment to better resolve the change in lattice structure under illumination. Given the exceptional behavior of the TOPO surface treatment it is important to consider how it is impacting the perovskite. Previous reports on similar Lewis base ligands suggest that they are capable of donating electrons to the surface of the perovskite and leave the bulk of the perovskite largely unaffected31. In line with these previous reports, X-ray photoelectron spectroscopy (XPS) of perovskite films capped with TOPO shows a substantial presence of phosphorous at the surface 2of the perovskite, which is quickly reduced below 44 4 C rbon hosphorous 2 C aa rbon C arbon 11 ss1s PP hosphorous Phosphorous pp2p 11 00 10 5 .41.4 11 .4 000 50 000 instrument detection limits after 5removal of the film surface by sputtering (SI Figure 7). Additionally, XPS 8 00 40 80TOPO 0 provides insights into the impact of448the treatment on the energetics of the perovskite surface. Employing .21.2 11 .2 39 techniques previously described ,44the relative binding energies of elements unique to the perovskite with and 66 00 40 60 0 without the electron-donating TOPO surface treatment can be compared to explore variations in surface 444 00 40 400 11 1 vacuum level alignment. Figures 2a and 2b highlight the binding energy for lead and iodide peaks for perovskite 44 22 00 40 200 samples with and without TOPO normalized to a surface carbon peak at 285 eV (SI Figure 8). 130.7 X:X: 130.7 X: 130.7 4625 Y:Y: 4625 Y: 4625

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Figure 2: Impact of Lewis base treatment on the surface energetics of CH3NH3PbI2Br perovskite samples as monitored by shifts in core-level binding energies of (a) lead and (b) iodine in reference to surface carbon, and (c) summarized in a schematic energy band diagram with the film surfaces represented by vertical lines and the red-blue ellipses representing the dipole introduced by the TOPO treatment. In these results, a consistent shift to lower binding energy is observed for photoemitted electrons from the perovskite layer when coated in TOPO. This apparent lower binding energy implies an increase in kinetic energy for emitted electrons from TOPO samples with respect to the neat films, suggesting the TOPO induces a surface dipole that points away from the perovskite surface and accelerates electrons as they pass through this layer40. A similar effect is observed by monitoring photoemitted electrons from photoemission spectroscopy in air, where an increase in the kinetic energy of emitted electrons from TOPO-coated films shifts the observed


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ACS Energy Letters

takeoff in photoemitted electrons to lower energy (SI Figure 9). We attribute this observed surface dipole to the electron-donating nature of TOPO: the ligand may donate electrons to passivate some surface defect on the perovskite, thereby giving the TOPO a net positive charge, and in doing so will create a surface dipole that shifts the vacuum level to lower energy by as much as 500 meV (as estimated by the shift in peak binding energy observed in XPS). The result is a perovskite surface that will accumulate electrons and repel positive charge, as shown schematically in Figure 2c. Our data suggests that this variation in surface energetics with the TOPO treatment results in decreased non-radiative recombination and increased photostability, manifesting in increased photoluminescence intensity and slowed photo-induced halide segregation. To further understand how surface treatments modulate photo-induced halide segregation, we prepared heterojunction samples with varying selective contacts and measured the photoluminescence evolution. As can be seen in Figure 3a, with the introduction of either the hole-transport layer (HTL) poly(triaryl amine) (PTAA) or the electron-selective contact (ETL) C60, the initial photoluminescence is greatly quenched. However, this photoluminescence quenching and associated reduction in steady-state carrier density does not in and of itself prevent photo-induced halide segregation. The photoluminescence of the light-soaked heterojunction samples is shown in Figure 3b. After ten minutes of illumination, the photoluminescence of the HTL capped perovskite is largely unaffected, while the perovskite capped with the ETL has substantial photoluminescence from a lower bandgap phase.

Figure 3: Impact of hole- and electron-selective contacts on the photoluminescence of CH3NH3PbI2Br perovskite samples (a) immediately after light exposure and (b) after ten minutes of light soaking. The suppression of photo-induced halide segregation with the addition of an HTL shows a trend similar to the perovskite samples treated with TOPO. While the HTL can accept holes, it cannot efficiently accept electrons. As such, and given the highly quenched PL under illumination conditions, we expect holes to be efficiently transferred to the HTL, leaving electrons in the perovskite and preventing the accumulation of positive charge within the perovskite. Such an effect could result in an electronic landscape that is similar to the TOPO/perovskite interface. This further reinforces our observation that photo-induced halide segregation is greatly impacted by variations in perovskite surfaces, and provides guidance for how surface treatments may be developed to suppress photo-induced halide segregation in operational solar cells. Given the observed variation in PL evolution between the PTAA and C60 heterojunction samples observed in Figure 3, constructing perovskite solar cells in a p-i-n architecture, could allow for a post perovskite-deposition Lewis base treatment to suppress photo-induced halide segregation before the addition of an ETL. Overall, our experimental observations link changes in the perovskite surface and surface energetics to changes in photo-induced halide segregation, a process that up to now has been largely believed to be an intrinsic



property of some perovskite compositions, only addressable by changes in perovskite chemistry18,25. In light of our results, we now propose a general model to explain how surface treatments that result in reduced nonradiative recombination or charge accumulation may inhibit halide segregation. In short, we suggest that carrier trapping at surface states induces electric fields that impact the movement or accumulation of ionic defects, and that this movement could drive local variations in chemistry that result in photo-induced trapping. We should note that this model does not preclude a thermodynamic stabilization of an iodide-rich perovskite phase under illumination, but perhaps suggests a mechanism by which these stable domains begin to form.

Figure 4: Annotated band-diagrams produced from drift-diffusion simulations highlighting the hypothesized mechanism by which surfaces encourage halide segregation for mixed-halide perovskites on glass. (a) Photoinduced occupancy of surface defect states, (b) band-bending induced by trapping, (c) halide vacancy and hole redistribution to compensate the trap-induced field, and (d) the TOPO treatment partially passivating surface states and consequently less charges accumulate at the perovskite surface. Band diagrams are rendered with the substrate on the left and top surface on the right. Figure 4 presents this model, showing how surfaces impact halide segregation under illumination and how this effect may be suppressed by a surface treatment such as TOPO. Using a drift-diffusion model (full details presented in the SI) we investigate the impacts of carrier trapping on halide redistribution. Figure 4a shows the initial condition of the system, right after the light as been switched on (i.e. before any trapping process or ion migration occurs). We assume that mobile halide vacancies are uniformly distributed and are chargecompensated by immobile ions or other ionized point defects.41 In addition, we consider a density of surface traps with a capture cross-section that favors electron trapping at the top-surface of an intrinsic perovskite. These assumptions are consistent with previous reports of likely trap states in CH3NH3PbI3 and the predicted intrinsic nature of these perovskites42–45. Before trapping occurs, we consider the effective charge of these traps to be neutral, which could either be due to the charge of the defect state itself or due to the existence of a defect pair of compensating charge at the surface46–48.


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ACS Energy Letters

Trapping of electrons (Figure 4b) induces the formation of an electric field pointing towards the surface of the film. To shield the trap-induced field halide vacancies migrate towards and accumulate near the surface states, leaving a depletion region populated by immobile anions (Figure 4c)49. Given that for lead-halide perovskites the concentration of vacancies is predicted to far exceed that of free carriers within the tested illumination conditions49–52, halide vacancies (as opposed to holes) will preferentially accumulate at the perovskite surface. In the case of the Lewis base TOPO, the molecular dipole opposes this trap-induced field and less holes and vacancies accumulate at the surface as a result (Figure 4d). We suggest that it is the redistribution of ions in response to carrier trapping that drives photo-induced halide segregation, and that a treatment such as TOPO slows this process by reducing the driving electric field. In order for the redistribution of ions to precipitate halide-segregation, i.e. photo-induced halide segregation, this movement must impact bromide and iodide differently. While the details of ionic conductivity in mixed perovskites remains an open area of study, work on single-halide perovskites suggest that such an effect is possible. For one, defect calculations suggest a lower activation energy for the formation of bromide vacancies with respect to iodide vacancies53. As such, in a mixed-halide perovskite the drift of positively charged vacancies towards the perovskite surface is likely to be equivalent to a preferred drift of bromide away from the surface, and the accumulation of halogen vacancies should therefore result in the formation of an iodide rich phase. Secondly, Kim et al. have recently described the photo-induced formation of iodide vacancies, where free holes oxidize iodide otherwise in the perovskite lattice54. If iodide is easier to oxidize than bromide in the mixed-halide perovskite, illumination should preferentially increase the formation of iodide vacancies and consequently the conductivity of iodide, again resulting in a local change to the perovskite chemistry. In the model outlined above the driving force for halide segregation increases with the number of trapped electrons. Consequently any treatment or chemical variation that changes either the concentration or the energy of surface trap states would impact the evolution of photo-induced halide segregation, and could explain experimental observations linking morphology and chemical variation to the rate of photo-induced halide segregation21–24. This is consistent with our observation that, in order to slow photo-induced halide segregation, one can either reduce the number of surface defect states (as with TOPO) or prevent the accumulation of positive charges at the interface (as with TOPO and PTAA). Both strategies would negate an electric field that drives halogen vacancy accumulation towards the surface. Additionally, such a trapping mechanism could also be consistent with the observation that photo-induced halide movement in CH3NH3Pb(IxBr1-x)3 perovskites leads to a 20% bromide phase14. Changing the halogen ratio in mixed halide perovskites is experimentally observed to change the ionization potential and bandgap of the materials55,56, and is theoretically predicted to change the defect formation energies and trap depths48,53. If the band structure for CH3NH3Pb(IxBr1-x)3 perovskites is such that at ~20% bromide the prevailing surface trap is no longer energetically stable or is no longer a deep trap, we would predict the driving force for photo-induced halide segregation to be negated. Such a mechanism could also explain why Cs compounds, which have been observed to have a deeper valance band than their methylammonium counterparts56,57, appear more stable to photo-induced halide segregation up to higher bromine contents11,18. Our results definitively show that perovskite surfaces are an integral part of the halide segregation process and that any model which seeks to fully explain photo-induced halide segregation must take surfaces into consideration. Through post-deposition treatments of perovskites, we were able to slow down the formation of lower bandgap iodide-rich domains in mixed-halide perovskite. These results suggest that carrier trapping and charge accumulation at perovskite surfaces are principle drivers of photo-induced halide segregation, and efficiently passivating and treating surfaces is a pathway towards stabilized wide-bandgap perovskites. While the surface treatments presented here only slowed photo-induced halide segregation, it is possible that deeper understanding will allow for a complete suppression of photo-induced halide segregation by surface



passivation, and materials previously thought unusable for photovoltaic applications might be candidates for high-efficiency perovskite tandems. Supporting Information: Experimental and modelling methods; and additional characterization of the impacts of surface treatment on mixed-halide perovskites including: additional photoluminescence data of mixed-halide perovskites with alternative light intensities and surface treatments, in-situ X-ray diffraction data, additional X-ray photoelectron spectroscopy data, and photoemission spectroscopy data. Author Information: *Corresponding Author E-mail: [email protected] Notes: The authors declare no competing financial interest Acknowledgments This research was supported by NSF EAGER Award Number 1664669, the Swiss National Science Foundation “Postdoc Mobility” Fellowship Number P400P2_180780, the NSF GRFP Award Number DGE-1147470, and the Hybrid Perovskite Solar Cell program of the National Center for Photovoltaics, funded by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)

Bush, K. A.; Palmstrom, A. F.; Yu, Z. J.; Boccard, M.; Cheacharoen, R.; Mailoa, J. P.; McMeekin, D. P.; Hoye, R. L. Z.; Bailie, C. D.; Leijtens, T.; et al. 23.6%-Efficient Monolithic Perovskite/Silicon Tandem Solar Cells with Improved Stability. Nat. Energy 2017, 2, 17009. Eperon, G. E.; Leijtens, T.; Bush, K. A.; Prasanna, R.; Green, T.; Wang, J. T.-W.; McMeekin, D. P.; Volonakis, G.; Milot, R. L.; May, R.; et al. Perovskite-Perovskite Tandem Photovoltaics with Optimized Band Gaps. Science 2016, 354, 861–865. Duong, T.; Wu, Y.; Shen, H.; Peng, J.; Fu, X.; Jacobs, D.; Wang, E.-C.; Kho, T. C.; Fong, K. C.; Stocks, M.; et al. Rubidium Multication Perovskite with Optimized Bandgap for Perovskite-Silicon Tandem with over 26% Efficiency. Adv. Energy Mater. 2017, 7, 1700228. Sahli, F.; Kamino, B. A.; Werner, J.; Bräuninger, M.; Paviet-Salomon, B.; Barraud, L.; Monnard, R.; Seif, J. P.; Tomasi, A.; Jeangros, Q.; et al. Improved Optics in Monolithic Perovskite/Silicon Tandem Solar Cells with a Nanocrystalline Silicon Recombination Junction. Adv. Energy Mater. 2017, 1701609. Kurtz, S. R.; Faine, P.; Olson, J. M. Modeling of Two-Junction, Series-Connected Tandem Solar Cells Using Top-Cell Thickness as an Adjustable Parameter. J. Appl. Phys. 1990, 68, 1890–1895. Bush, K. A.; Manzoor, S.; Frohna, K.; Yu, Z. J.; Raiford, J. A.; Palmstrom, A. F.; Wang, H.-P.; Prasanna, R.; Bent, S. F.; Holman, Z. C.; et al. Minimizing Current and Voltage Losses to Reach 25% Efficient Monolithic Two-Terminal Perovskite–Silicon Tandem Solar Cells. ACS Energy Lett. 2018, 3, 2173–2180. Hörantner, M. T.; Leijtens, T.; Ziffer, M. E.; Eperon, G. E.; Christoforo, M. G.; McGehee, M. D.; Snaith, H. J. The Potential of Multijunction Perovskite Solar Cells. ACS Energy Lett. 2017, 2, 2506–2513. Zhao, B.; Abdi-Jalebi, M.; Tabachnyk, M.; Glass, H.; Kamboj, V. S.; Nie, W.; Pearson, A. J.; Puttisong, Y.; Gödel, K. C.; Beere, H. E.; et al. High Open-Circuit Voltages in Tin-Rich Low-Bandgap Perovskite-Based Planar Heterojunction Photovoltaics. Adv. Mater. 2017, 29, 1604744. Eperon, G. E.; Leijtens, T.; Bush, K. A.; Prasanna, R.; Green, T.; Wang, J. T.-W.; McMeekin, D. P.; Volonakis, G.; Milot, R. L.; May, R.; et al. Perovskite-Perovskite Tandem Photovoltaics with Optimized Band Gaps. Science (80-. ). 2016, 354, 861–865. Noh, J. H.; Im, S. H.; Heo, J. H.; Mandal, T. N.; Seok, S. Il. Chemical Management for Colorful, Efficient, and


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ACS Energy Letters

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Impact of Surfaces on Photoinduced Halide Segregation in Mixed

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