A multi-functional halogen-free cesium salt bulk-doping treatment toward performance-enhancement of perovskite solar cells

Dopant engineering involving molecular/ionic additives is a kind of usual technique to achieve state-of-the-art perovskite solar cells (PSCs) with favorable stability. It is critical to maximize the functions of doping agents for perovskite due to its unstable structure, inherent defect, weak water resistance, and so on. To ameliorate these problems, herein, we introduce a multi-functional halogen-free cesium trifluoroacetate (CsCF3COO, CsTFA) into organic amine salt precursor via a conventional two-step sequential deposition approach. The carboxyl group (-COO-) of CsTFA can passivate inherent traps that exist in perovskite crystal lattice through interacting with the Pb–I framework, which effectively suppresses non-radiative recombination and further reduces open-circuit voltage deficit. Cesium ion (Cs+) supplies an A-site cation to tune tolerance factor while the trifluoromethyl section (-CF3) improves hydrophobicity of devices. These three synergistic effects finally drive the power conversion efficiency of the champion device from 21.1% to 22.3% with improved stability. This work paves a new way to enhance the performance and the stability of photovoltaic device using halogen-free doping materials.