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Letter
Light-Independent Ionic Transport in Inorganic Perovskite and Ultra-Stable Cs-Based Perovskite Solar Cells Wenke Zhou, Yicheng Zhao, Xu Zhou, Rui Fu, Qi Li, Yao Zhao, Kaihui Liu, Dapeng Yu, and Qing Zhao J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01851 • Publication Date (Web): 17 Aug 2017 Downloaded from http://pubs.acs.org on August 17, 2017
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Light-Independent Ionic Transport in Inorganic Perovskite and Ultra-Stable Cs-Based Perovskite Solar Cells
Wenke Zhou,1 Yicheng Zhao,1 Xu Zhou,1,3 Rui Fu,1 Qi Li,1 Yao Zhao,1 Kaihui Liu,1,2 Dapeng Yu1,4 and Qing Zhao1,2* 1
State Key Laboratory for Mesoscopic Physics and Electron Microscopy Laboratory, School of Physics, Peking University, Beijing 100871, China.
2
Collaborative Innovation Center of Quantum Matter, Beijing 100084, China.
3
Academy for Advanced Interdisciplinary Studies, Peking University, Beijing 100871, China.
4
Institute for Quantum Science and Technology and Department of Physics, South
University of Science and Technology of China (SUSTech), Shenzhen 518055, China.
# These authors contribute equally to this work.
*Correspondence to:
[email protected] 1
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Abstract Due to light-induced effects in CH3NH3-based perovskites, such as ion migration,
defects
formation
and
halide
segregation,
the
degradation
of
CH3NH3-based perovskite solar cells under maximum power point is generally concerned. Here we demonstrated that the effect of light-enhanced ion migration in CH3NH3PbI3 can be eliminated by inorganic Cs substitution, leading to an ultra-stable perovskite solar cell. Quantitatively, the ion migration barrier for CH3NH3PbI3 is 0.62 eV under dark, larger than CsPbI2Br (0.45 eV); however, it reduces to 0.07 eV for CH3NH3PbI3 under illumination, smaller than CsPbI2Br (0.43 eV). Meanwhile, photo-induced halide segregation is also suppressed in Cs-based perovskites. Cs-based perovskite solar cells retained >99% of the initial efficiency (10.3%) after 1500 hours’ maximum power point tracking under AM1.5G illumination, while CH3NH3PbI3 solar cells degraded severely after 50 hours operation. Our work reveals an uncovered mechanism on the stability improvement by inorganic cation substitution in perovskite-based optoelectronic devices.
TOC graphic
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Organic-inorganic hybrid perovskite solar cells (PSCs) exhibits a 22.1% record certified efficiency recently1, while the long-term stability issue still remains as achilles heel for their commercialization. Among the high-performance perovskite solar cells2-8, [methylammonium (MA)/formamidinium (FA)]+ and [PbI3]- constitute the matrix to fabricate high-quality crystal film; however, the corresponding devices still degrade at maximum power point (MPP) under illumination even with UV filter. This issue is supposed to relate with material decomposition and ion migration in the perovskites9-19. Although the material itself passes damp heat stability test20, a fast degradation behavior is still observed at the early stage, the so-called ‘burn-in’ degradation, implying an ion migration-related destruction in devices. Recently, the Cs-enhanced crystal structure of perovskites is manifested by thermal stability, humidity resistance and photo-stability3,21-25; however, a detailed study is still lacking on whether the ion migration can be tailored by Cs incorporation in perovskite. It is, therefore, valuable to compare the ionic transport for MA- and Cs-based perovskite, under dark and illumination. We mainly focused on MAPbI3 and CsPbI2Br perovskite films by taking thermal/ambient phase stability and bandgap (1.92 eV) into consideration22,23,26,27. The corresponding ion migration barrier in both MAPbI3 and CsPbI2Br thin films are extracted by analyzing cryogenic Galvanostatic28-30 and current-voltage experiments in Au/Perovskite (MAPbI3 or CsPbI2Br)/Au lateral structure over various temperature (100-295 K) and light intensity (0.1-25 mW/cm2). For MAPbI3, ion migration energy barrier (Ea) is reduced by approximately one order of magnitude (from 0.62 eV to 3
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0.07 eV) when light illumination changes from 0.1 to 25 mW/cm2. However, the Ea CsPbI2Br is almost light-independent with 0.45 eV. Moreover, the related light-induced halide segregation in MA-based mixed-halide perovskites30,31 is not detected in Cs-based perovskites. The halide stabilization is consistent with the light-independent nature in inorganic mixed-halide perovskites under strong illumination. For the solar cell operation under AM1.5G illumination, CsPbI2Br device presents 250 K) for CsPbI2Br (Fig. 2e) and MAPbI3 films (Fig. 2f), respectively. Ea (T>250K) 7
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under different light intensity for CsPbI2Br and MAPbI3 are summarized in Table 1.
Table 1: Activation energy for ion migration under various light intensities, for CsPbI2Br and MAPbI3 films.
Ion migration barrier (Ea) in MAPbI3 films presents a striking decrease from 0.62 eV to 0.07 eV when light intensity changes from 0.1 to 25 mW/cm2. The much lower barrier for ion migration under stronger light will cause more severe ion migration in MAPbI3 film, leading to a fast burn-in degradation of solar cells10,19,30,34,35. In contrast, Ea in CsPbI2Br remains constant under different light intensity (0.45-0.43 eV), revealing the light-independent Ea of CsPbI2Br perovskite. The elimination of light-enhanced ion migration by Cs substitution highlights the key role organic cation plays in the complicated interplay between photon excitation and ionic transport. Gottesman et al. demonstrates a softer lattice of MAPbI3 perovskites under illumination, arising from a reduced binding between the MA+ ions and the inorganic frames, which might be a contributing factor for the light-enhanced ion migration36. 8
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Recently, polaron formation with deformed lattice surrounding the photocarriers had been demonstrated in MA/FAPbBr3 while not in CsPbBr337. The organic cation-dependent polaron formation matches well with cation-dependent ionic transport property, implying another potential mechanism for light-enhanced ion migration38. We further move on to examine the halide segregation in Cs-based perovskite film under laser excitation. Light-induced halide segregation has been widely demonstrated in MAPb(IxBr1-x)3 (0.2
