Photochemically-Induced Phase Segregation of Mixed Halide Perovskite Solar Cells

Abstract

The emerging low cost perovskite solar cell quickly reached power conversion efficiency (PCE) parity with crystalline Si solar cells [1]. Optional bandgap tuning of the perovskites by partial substitution of I- with Br- expand their application to tandem solar cells and LEDs. The only critical issue remaining to solve is their poor long term stability under operational conditions, specifically photochemical degradation by splitting into I- and Br- rich phases. Sufficient suppression of this process requires a thorough understanding of the underlying phenomena. Here we report the detailed investigation of the photoinduced phase transformation in stoichiometric and non-stoichiometric mixed halide CsPbI3-xBrx. Using in-situ Atomic Force Microscopy, we visualized real-time kinetics of halide phase segregation under illumination. The I-rich phase segregated predominantly along grain boundaries while the grain bulk remained enriched with Br. It was proposed that photogenerated Pb0 and I3 - species are selectively expelled from grain bulk into grain boundary resulting in the phase splitting. This work supports our focus in understanding degradation mechanisms in perovskite materials for next generation solar cells [2-3].[1] Best Research-Cell Efficiency Chart | Photovoltaic Research | NREL, (n.d.). https://www.nrel.gov/pv/cell-efficiency.html (accessed October 12, 2019).[2] Akbulatov, A.; Frolova, L.; Dremova, N.; Zhidkov, I.; Martynenko, V.; Tsarev, S.; Luchkin, S.; Kurmaev, E.; Aldoshin, S.; Stevenson, K. J.; Troshin, P. “Light or Heat: What Is Killing Lead Halide Perovskites Under Solar Cell Operation Conditions?," J. Phys. Chem. Lett. 2020, 11(1), 333-339.[3] Frolova, L. A.; Luchkin, S. Yu; Akbulatov, A. F.; Dremova, N. N.; Stevenson, K. J.; Troshin, P. A. “Efficient and Stable All-Inorganic Perovskite Solar Cells Based on Nonstoichiometric CsxPbI2Brx (x>1) Alloy,” J. Mater. Chem. A 2019 , 7(18), 5314-5323.