Stabilization of Black Photoactive Phase of CsPbI3 by Eu Inclusion for All-inorganic Perovskite Solar Cells
Ajay Jena a, Ashish Kulkarni a, Masashi Ikegami a, Tsutomu Miyaska a
a 1Toin Univeristy of Yokohama, Kanagawa, Japan
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference on Perovskite and Organic Photovoltaics and Optoelectronics (IPEROP19)
Kyōto-shi, Japan, 2019 January 27th - 29th
Organizers: Hideo Ohkita, Atsushi Wakamiya and Mohammad Nazeeruddin
Oral, Ajay Jena, presentation 088
Publication date: 23rd October 2018

Although organic-inorganic hybrid perovskites like MAPbI3 (MA = methylammonium), FAPbI3 (FA= formamidinium) and the mixed cation based perovskites (FA/MA, MA/Cs, FA/Cs, FA/MA/Cs, etc.) are leading in terms of efficiency, they have been facing a huge challenge of long-term stability. It is the organic cation part (MA or FA), which is believed to be responsible for poor thermal and environmental stability of these materials. For instance, MAPbI3 degrades easily at temperature as low as 120 oC, even at 80 oC if heated for long time. Therefore, replacement of organic cation with an inorganic cation is considered as a good strategy to improve stability. Cesium-based perovskite structures like CsPbX3 and CsSnX3 (X = Cl, Br, I) are indeed the first halide perovskites that were studied by Wells et al. in 1983. With their band gap controlled by the halide ions (CsPbI3; 1.77eV and CsPbBr3; 2.38eV), CsPbX3 perovskite have been considered as a model compound among all all-inorganic perovskites. However, the main challenge with such inorganic perovskites (CsPbI3) is stabilization/formation of the “black” photoactive phase (α-CsPbI3) at room temperature and ambient conditions because CsPbI3 usually crystallizes in a photo-inactive yellow phase (δ-CsPbI3) at room temperature. For application in solar cells, the black phase (absorbing visible light) is required. In the present work, we show how Eu (Eu2+ and Eu3+) inclusion could stabilize the black phase of CsPbI3  and the solar cells using these Eu-doped CsPbI3 showed a power conversion efficiency of ~ 7% in best cases. The study also reveals stability issues of the cells related to additives in the hole transport material.

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