Cation influence on carrier dynamics in perovskite solar cells

Abstract

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.