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ET04.02.08 : Trapped-Charges as the Core Origin for Degradation of Perovskite Crystals

4:30 PM–4:45 PM Nov 26, 2018 (US - Eastern)

Hynes, Level 3, Room Ballroom C

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Namyoung Ahn1 Min-cheol Kim1 Kwisung Kwak1 Mansoo Choi1

1, Seoul National University, Seoul, , Korea (the Republic of)

At the current state of perovskite photovoltaics field, it is no exaggeration to say that the most important issue is their stability upon real operation. To find out engineering ways on long-term stability with commercially-viable level, it is more urgent to seek the root origin of decomposition of perovskite crystals than any other. Therefore, we have explored what decisively destroys these hybrid perovskite crystals from experiments and theoretical approaches. From all our studies, we conclude that trapped-charges induce irreversible degradation of perovskite crystals as the core origin. First, a novel experimental set-up utilizing the deposition of ions with different polarity via a corona discharger revealed that perovskite materials irreversibly degrade along grain boundaries where ion charges trapped in the presence of either water or oxygen. In the case of light-illuminated perovskite films, such degradation was observed mainly along grain boundaries where provide a lot of trap states for photo-generated electrons and holes, which was also evidenced by Kelvin probe force microscopy (KPFM) measurements. Additionally, ab initio molecular dynamics (AIMD) simulation based on density functional theory (DFT) strongly supports trapped-charge driven degradation of perovskite crystals from the theoretical viewpoint, by showing that CH3NH3PbI3 crystals surrounded by either water or oxygen molecules break down with trapped-charges (localized at one crystal), whereas the decomposition never takes place without trapped-charge. These simulations not only uncover atomistic mechanisms for trapped-charge driven degradation but also bring out the observation of the deprotonation of organic cation (CH3NH3+) for the first time. Finally, we obtain more powerful evidence to support trapped-charge driven degradation through light-induced degradation test of CH3NH3PbI3 perovskite films coated on different charge-selective layers. The test results showed that the polarity of localized charges inside CH3NH3PbI3 perovskite films determines degradation speed and pathways depending on the kind of surrounding gaseous molecules. Based on these experimental results and additional calculations, we discovered the effect of polarity-dependent localized charges on the degradation of CH3NH3PbI3 crystals through close observations on real operation of CH3NH3PbI3-based photovoltaics

[1] N. Ahn, K. Kwak, M. S. Jang, H. Yoon, B. Y. Lee, J.-K. Lee, P. V. Pikhitsa, J. Byun, and M. Choi, “Trapped-charge driven degradation of perovskite solar cells”, Nature Communications, (2016)
[2] K. Kwak, E. Im, N. Ahn, J. Heo, K. Bang, M. Kim, S. Kim, and M. Choi, “Atomistic mechanism of trapped-charge driven degradation of perovskite solar cells”, arXiv:1709.04130 (2017)

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