A Victim of Halide Ion Segregation. How Light Soaking Affects Solar

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A Victim of Halide Ion Segregation. How Light Soaking Affects Solar Cell Performance of Mixed Halide Lead Perovskites. Gergely Ferenc Samu, Csaba Janáky, and Prashant V. Kamat ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00589 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 25, 2017

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ACS Energy Letters

A Victim of Halide Ion Segregation. How Light Soaking Affects Solar Cell Performance of Mixed Halide Lead Perovskites. 1,3,4

Gergely F. Samu,

3,4

Csaba Janáky,

and Prashant V. Kamat

1,2*

Radiation Laboratory and Department of Chemistry and Biochemistry University of Notre Dame, Notre Dame Indiana, 46556, United States and Department of Physical Chemistry and Materials Science University of Szeged, Rerrich Square 1. Hungary

*Corresponding author: [email protected] 1

Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana, 46556, United States

2

Department of Chemistry and Biochemistry, University of Notre Dame

3

Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich Square 1, Szeged, H-6720, Hungary 4

MTA-SZTE “Lendület” Photoelectrochemistry Research Group, Rerrich Square 1, Szeged, H-6720, Hungary

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2

Abstract Photoinduced segregation in mixed halide perovskite has a direct influence on decreasing the solar cell efficiency as segregated I-rich domains serve as charge recombination centers. The changes in the external quantum efficiency mirrors the spectral loss in the absorption, however the timescale of the IPCE recovery in the dark is slower than the absorption recovery, showing the intricate nature of the photoinduced halide segregation and charge collection in solar cell device.

TOC Graphics

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3 The ability to tune the bandgap of mixed halide perovskites through halide composition is attractive to design solar cells with tailored response.1-3 Several research groups have evaluated solar cell performance under short term irradiation and they consider the photovoltaic performance to be stable (See for example, Table 1 in reference4). Halide ion migration in perovskite solar cells plays an important role in determining the overall photovoltaic performance.5-7 Light soaking, for example, leads to migration of ions and vacancies toward the oxide interface influencing the observed photovoltage.8 Another property of the mixed halide lead perovskite is its ability to undergo phase segregation under continuous illumination.4, 9-13 For example, when methyl ammonium lead halide (Br:I, 1:1 ratio) is subjected to longer term (>20 min) visible irradiation it undergoes phase segregation to yield iodide rich and bromide rich domains. (Reaction 1) nMAPbBrxI3-x+ hν → (n – 2m)MAPbBrxI3-x + mMAPbBrx-yI3-x+y+ mMAPbBrx+yI3-x-y .

(1)

Upon stopping the illumination, the segregated phase is restored to the original mixed phase. In previous studies, we have tracked the kinetics of halide ion segregation as well as dark recovery.12-13 The origin of the segregation has been explained using different models. These include entropy of mixing,9 halide ion defect driven movement of ions,13 and polaron induced lattice strain.14 It was also found that the excitation intensity and duration of irradiation determines the dark recovery.12-13 These intriguing behaviors of mixed halide perovskite have led us to probe the halide ion segregation on the solar cell performance. The obvious question is, What is the impact of longer term light irradiation (or light soaking) on the performance of mixed halide perovskite cells? We prepared two sets of cells, one with a partial assembly excluding top gold contact and the other complete solar cell assembly using the same mixed halide perovskite deposition on a mesoporous TiO2 film (see Figure S1). The procedure used for cell fabrication as well as the methodology used for spectral and photovoltaic characterization are given in the Supporting Information. The methylammonium lead halide films were cast using the stoichiometric composition with Br:I ratio of 1:1. The partially assembled cell was used for absorption measurements. The absorption spectra recorded before and after 30 min irradiation with visible light (100 mW/cm2) are shown in Figure 1A. The decrease in absorbance at 620 nm suggests the diminution of the mixed halide phase. The concurrent increase in the absorption at shorter and longer wavelength represents formation of bromide rich and iodide rich phases. To better visualize this process the difference absorption spectra were recorded during the dark recovery (Figure 1B). In about 3 hours we see almost complete recovery of the mixed halide phase. The details on the absorption changes and recovery can be found in our earlier studies.12-13 The next step was to evaluate the photovoltaic performance of fully assembled solar cells. The J-V characteristics are shown in Figure S1 (Supporting Information). The champion cell we tested for these measurements yields a power conversion efficiency of 4.4 %, an open-circuit voltage of 1.04 V and short circuit current of 6.4 mA/cm2, and fill factor of 66%. The cell was then subjected to visible light irradiation (100 mW/cm2) for 30 minutes under open circuit conditions. This allowed us to induce halide ion segregation through visible light excitation (identical to the conditions in Figure 1A). Upon evaluation of J-V curves (Figure S2 in Supporting Information) we found significant decrease in the solar cell performance with Voc of 0.91 V and Jsc of 4.2 mA/cm2 resulting in a net photoconversion efficiency of 1.4

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4 %. This phenomenon was also observable for different cells irradiated for 15 and 30 min (as seen in

Figure 1. (A) Absorption spectra of mixed halide perovskite film (a) before and (b) after irradiation for 30 minutes. Spectrum (c) shows dark recovery after 3 hours. (B) Difference absorption spectra after 30 min light soaking shows dark recovery. (C) External quantum efficiency (IPCE) of mixed halide perovskite solar cell (a) before light soaking and (b) – (f) during dark recovery following 30 min light soaking. (A 420 nm cut-off filter was used for IPCE measurements and residual dark currents were corrected for IPCE measurements). (D) Difference IPCE spectra showing the recovery of the solar cell response in the 600-700 nm region. (The spectra were normalized at 470 nm and the IPCE recorded before the continuous irradiation was used as the reference.) Figure S2). We also recorded IPCE spectra at different times as the cell recovered in the dark (Figure 1C). The IPCE spectrum recorded immediately after 30 min irradiation shows a nearly 50% drop in IPCE. The IPCE spectra were seen to recover slowly as we left the cell in the dark. Almost 90% of the recovery was seen in 36 hours. This recovery time is notably longer than the recovery timescale we see in the absorption spectra (see Figure S3). The quick absorption recovery is indicative of the fact that the majority of the segregated phases have reversed back. However, the slower recovery in IPCE indicates that a few remaining charge carrier traps which are not captured through absorption changes are responsible poor

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5 charge carrier extraction. This difference in the recovery time thus suggests the necessity of longer time to alleviate residual traps following light induced halide ion. Upon closer examination, we see a change in the shape of the IPCE spectra of 30 min irradiated cell as compared to the spectrum recoded prior to irradiation. The drop in the IPCE response at longer wavelengths appears to be greater than that at shorter wavelengths. In order to obtain a closer look at this phenomenon we recorded difference IPCE spectra at different recovery times following normalization (Figure 1D). The difference IPCE spectra recorded immediately after the 30 min irradiation of the solar cell shows depletion in the 550-680 nm region with a maximum around 620 nm. This depletion in IPCE matches closely to the decreased absorbance seen in the absorption spectrum after irradiation. This result confirms that the loss of contribution in the IPCE at longer wavelengths arises from the fraction of mixed halide that is lost in the segregation process. As we continue the recovery process in the dark, this depleted IPCE (500-670 nm) region recovers. This recovery of IPCE performance parallels the reversibility seen in the photoinduced segregation (see for example Figure 1A & B). It is important to note that the irradiated MAPbI3 solar cells do not show such loss of IPCE response in the red region (see Figure S4). The observed decrease in IPCE following the 30 min of light soaking of mixed halide film arises from the halide ion segregation and not from the film degradation (both the absorption properties and IPCE recover almost completely when left in dark). As noted in our previous study, low lying I-rich phase serves as a sink for photogenerated electrons and holes.12 As a result of this accumulation we see prominent emission arising from the I-rich phase in the photoirradiated films. The gain in the IPCE at wavelengths greater than 700 nm is rather small and cannot account for the loss we see in the 500-670 nm region. We attribute this discrepancy to the halide ion segregated domains within the film which block the flow of charge carrier. While the conduction bands of mixed halide and segregated phases are isoenergetic, the valence bands show an energy gradient causing I-rich region to favor the hole accumulation (See Scheme in TOC). The I-rich domains thus serve as recombination centers resulting in eventual decrease in the IPCE and overall photoconversion efficiency.10, 12, 15 As the mixed films recover in dark, these recombination centers are removed and we see recovery of photovoltaic performance. To summarize, the photoinduced segregation seen in mixed halide films makes the photovoltaic performance of perovskite solar cells less efficient as iodide rich domains block the flow of charge carriers and promote charge carrier recombination. The dark reversibility is a silver lining here and one should focus on speeding up the dark recovery time. Indeed, increased halide concentration is one way to speed up the dark recovery and minimize segregation effects. Efforts are underway to further explore the influence of halide composition on the photovoltaic performance of mixed halide films under long term irradiation. ASSOCIATED CONTENT Supporting Information. Experimental methods including film preparation, solar cell fabrication and related spectroscopic and photovoltaic measurements are presented in the Supporting Information. AUTHOR INFORMATION

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6 Corresponding Author *Address correspondence to this author: [email protected] twitter: @kamatlabND Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We would like to thank Seog Jun Yoon and Anselme Mucunguzi for their assistance in film preparation. P.V.K. acknowledges the support of King Abdullah University of Science and Technology (KAUST) through Award OCRF-2014-CRG3-2268. This collaborative research was partially supported by the GINOP-2.3.215-2016-00013 project (GFS and CJ): “Intelligent materials based on functional surfaces−from syntheses to applications”. This is contribution number NDRL No. 5179 from the Notre Dame Radiation Laboratory which is supported by the by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy through award DE-FC02-04ER15533. REFERENCES (1) McMeekin, D. P.; Sadoughi, G.; Rehman, W.; Eperon, G. E.; Saliba, M.; Hörantner, M. T.; Haghighirad, A.; Sakai, N.; Korte, L.; Rech, B., et al. A Mixed Cation Lead Mixed Halide Perovskite Absorber for Tandem Solar Cells. Science 2016, 351, 151-155. (2) Jesper Jacobsson, T.; Correa-Baena, J.-P.; Pazoki, M.; Saliba, M.; Schenk, K.; Gratzel, M.; Hagfeldt, A. Exploration of the Compositional Space for Mixed Lead Halogen Perovskites for High Efficiency Solar Cells. Energy & Environ. Sci. 2016, 9, 1706-1724. (3) Klein-Kedem, N.; Cahen, D.; Hodes, G. Effects of Light and Electron Beam Irradiation on Halide Perovskites and Their Solar Cells. Acc. Chem. Res. 2016, 49, 347-354. (4) Slotcavage, D. J.; Karunadasa, H. I.; McGehee, M. D. Light-Induced Phase Segregation in HalidePerovskite Absorbers. ACS Energy Lett. 2016, 1199-1205. (5) Mosconi, E.; De Angelis, F. Mobile Ions in Organohalide Perovskites: Interplay of Electronic Structure and Dynamics. ACS Energy Lett. 2016, 182-188. (6) Eames, C.; Frost, J. M.; Barnes, P. R. F.; O'Regan, B. C.; Walsh, A.; Islam, M. S. Ionic Transport in Hybrid Lead Iodide Perovskite Solar Cells. Nat Commun 2015, 6, Article No. 8497 (7) Barker, A. J.; Sadhanala, A.; Deschler, F.; Gandini, M.; Senanayak, S. P.; Pearce, P. M.; Mosconi, E.; Pearson, A.; Wu, Y.; Srimath Kandada, A. R., et al. Defect-Assisted Photoinduced Halide Segregation in Mixed-Halide Perovskite Thin Films. ACS Energy Lett. 2017, 2, 1416–1424. (8) Hu, J.; Gottesman, R.; Gouda, L.; Kama, A.; Priel, M.; Tirosh, S.; Bisquert, J.; Zaban, A. Photovoltage Behavior in Perovskite Solar Cells under Light-Soaking Showing Photoinduced Interfacial Changes. ACS Energy Lett. 2017, 2, 950-956. (9) Brivio, F.; Caetano, C.; Walsh, A. Thermodynamic Origin of Photoinstability in the CH3NH3Pb(I1– Br ) Hybrid Halide Perovskite Alloy. J. Phys. Chem. Lett. 2016, 7, 1083-1087. x x 3 (10) Hoke, E. T.; Slotcavage, D. J.; Dohner, E. R.; Bowring, A. R.; Karunadasa, H. I.; McGehee, M. D. Reversible Photoinduced Trap Formation in Mixed-Halide Hybrid Perovskites for Photovoltaics. Chem. Sci. 2015, 6, 613-617. (11) Hentz, O.; Zhao, Z.; Gradečak, S. Impacts of Ion Segregation on Local Optical Properties in Mixed Halide Perovskite Films. Nano Lett. 2016, 16, 1485-1490.

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7 (12) Yoon, S. J.; Draguta, S.; Manser, J. S.; Sharia, O.; Schneider, W. F.; Kuno, M.; Kamat, P. V. Tracking Iodide and Bromide Ion Segregation in Mixed Halide Lead Perovskites during Photoirradiation. ACS Energy Lett. 2016, 290-296. (13) Yoon, S. J.; Kuno, M.; Kamat, P. V. Shift Happens. How Halide Ion Defects Influence Photoinduced Segregation in Mixed Halide Perovskites. ACS Energy Lett. 2017, 1507-1514. (14) Bischak, C. G.; Hetherington, C. L.; Wu, H.; Aloni, S.; Ogletree, D. F.; Limmer, D. T.; Ginsberg, N. S. Origin of Reversible Photoinduced Phase Separation in Hybrid Perovskites. Nano Lett. 2017, 17, 10281033. (15) Hoffman, J. B.; Schleper, A. L.; Kamat, P. V. Transformation of Sintered CsPbBr3 Nanocrystals to Cubic CsPbI3 and Gradient CsPbBrxI3–x through Halide Exchange. J. Am. Chem. Soc. 2016, 138, 8603–8611.

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