Alloy-induced phase transition and enhanced photovoltaic performance: the case of Cs3Bi2I9−xBrx perovskite solar cells

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Vol. 7; no. 15; pp. 8818 - 8825
• Main Authors: Bin-Bin, Yu, Liao, Min, Yang, Jingxiu, Chen, Wei, Zhu, Yudong, Zhang, Xusheng, Duan, Tao, Yao, Weitang, Su-Huai Wei, He, Zhubing
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 2019
• Subjects: Alloying elements; alloys; bismuth; bromides; bromine; cesium; crystal structure; Density functional theory; Energy conversion efficiency; Energy gap; engineering; iodides; Lead free; Optoelectronic devices; Perovskites; phase transition; Phase transitions; Photovoltaic cells; Photovoltaics; Solar cells; solar energy
• ISSN: 2050-7488; 2050-7496; 2050-7496
• DOI: 10.1039/c9ta01978b

Cs3Bi2I9 is a promising inorganic, nontoxic, and stable perovskite material for solar cell applications, but the power conversion efficiency of Cs3Bi2I9-based solar cells is relatively low because its band gap is too large and indirect. Here we show that the band gap and optoelectronic properties of Cs3Bi2I9 can be improved by alloying Br into Cs3Bi2I9, despite the common expectation that the band gap of the Cs3Bi2I9−xBrx alloy will increase with Br concentration. By systematically growing and characterizing Cs3Bi2I9−xBrx perovskite materials, we discover that at x = 3 Cs3Bi2I6Br3 shows the lowest bandgap of 2.03 eV, while that of Cs3Bi2I9 is 2.20 eV. Our photophysical characterization and density functional theory calculations show that this unusual band gap reduction and better optoelectronic properties at x ∼ 3 are mainly caused by a phase transition from the P63/mmc phase to the P3m phase. Moreover, we found that the composition engineering also leads to compact Cs3Bi2I6Br3 films with high crystallinity and orientation, which results in a champion device with a PCE of 1.15%. This novel discovery will definitely deepen our understanding of Cs3Bi2I9−xBrx materials and afford more promising strategies for highly stable lead-free perovskite optoelectronic devices.