Impact of Li doping on the photophysical properties of perovskite absorber layer FAPbI3

Abstract It has been demonstrated that ions used in the contact layers in a perovskite based solar device can migrate through the perovskite absorber and have a significant impact on the device performance. In this work, we investigate the direct effect of Li on the structural and optical properties of formamidinium lead triiodide perovskites thin films (FAPbI3). The ratio between the desirable α-phase black perovskite and the undesirable δ yellow phase is studied as a function of Li content in the film. Our results show that the presence of a small amount of Li ions can have a huge impact on the structural, optical and photophysical properties of the FAPbI3 layer by enhancing the formation of δ-FAPbI3 instead of the black α-FAPbI3 phase. The structural analyses show that the FAPbI3 doped by Li atoms leads to the distortion of the mean α-FAPbI3 perovskite structure due the appearance of non-perovskite structure based δ-FAPbI3. The optical measurements indicate a clear decrease of the absorbance with increasing the Li content. The photoluminescence (PL) intensity of the Li doped FAPbI3 decreases considerably indicating a significant quenching and enhanced carrier extraction after Li doping. Li ions have also been shown to reduce non-radiative recombination due to free electrons produced by n-type doping. This work provides insights into the effect of the Li ions on the photophysical properties of the FAPbI3 absorber layer and the recombination mechanism, and helps to unravel new ways for development of optoelectronic devices, more particularly thin film perovskite solar cells with high-stability and high-performance.