Cation influence on carrier dynamics in perovskite solar cells

Rubidium and Cesium cations (Rb+ and Cs+) incorporation recently emerged as a viable strategy to enhance perovskite solar cells (PSCs) efficiency. However, a clear understanding of the impact of these cations on the structure-function relationship in relation to the device performance is severely lacking. Here, we systematically investigate the influence of Rb+ and Cs+ on the carrier dynamics using transient optical spectroscopy and correlate with solar cell performance. Unlike Rb+, Cs+ integrates well with methylammonium (MA+) and formamidinium (FA+) yielding increased perovskite grain size, longer charge carrier lifetimes and improved power conversion efficiency (PCE). Concomitant incorporation of Cs+/Rb+ cooperatively retards radiative recombination by ~60% in the quaternary-cation based perovskite system (RbCsMAFA) compared to the dual-cation MAFA samples. By suppressing the defect density, PCEs around 20% are obtained along with more balanced charge carrier diffusion length and comparable photoluminescence quantum yield in quaternary-cation perovskites. While the synergistic addition of Rb+ and Cs+ is attractive for controlling defects and recombination losses in efficient solar cells development, sole incorporation of Rb+ is still an engineering challenge. Importantly, our study explicates the underlying mechanisms behind the synergistic combination of cations to minimize the charge carrier losses and achieve high efficiency perovskite solar cells.