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

Highlights: • Li ions enhance the formation of the non-desirable δ-FAPbI₃ phase. • Li ions have considerable impact one the photophysical properties of FAPbI₃ perovskite films. • Li ions reduce the non-radiative recombination. -- 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.