Photophysical Study of Organic-Inorganic Lead Perovskites

Photophysical study of perovskite is of high importance as it can facilitate the understanding of the fundamental aspects of stability issues in the operation of perovskite solar cells. Photo-instability is one of the most complicated problems that contains several phenomenon such as photo-activated ion induced hysteresis and photo-induced passivation effect (defect self-healing) under the light soaking. These photo-induced abnormal behaviours are also dependent on the material properties of perovskites The aim of this thesis is to investigate the effect of abnormal carrier dynamics (e.g., electron/ hole recombination) observed in perovskite materials under illumination via the use of steady-state photoluminescence (PL) and time-resolved PL microscopy measurements. These PL techniques were used to perform quantitative analysis (e.g., carrier lifetime) unravelling and visualising the photo-physical related mechanism such as photo-activated ion motion and defect self-healing effect. In this thesis, the carrier dynamics study of methylammonium lead bromide (MAPbBr3) shows that the PL intensity and carrier recombination lifetime exhibit an excitation (laser) intensity dependent reduction in the time scale of seconds to minutes. Such slow carrier dynamics suggests that photo-activated ions exist in the perovskite and their accumulation at localized site can result in non-radiative carrier recombination, which relates to the ion-induced hysteresis. In addition, methylammonium lead iodide (MAPbI3) thin films were prepared by different deposition methods showed various PL enhancement/ quenching behaviours when the samples exposed to illumination. The results suggest that the carrier dynamics of perovskites are highly dependent on the illumination and material properties. This thesis also verifies experimentally the PbI2 induced passivation effect through comparing the carrier lifetimes between the PbI2-rich grains and the perovskite grains. This explains why PbI2-addition leads to power conversion efficiency (PCE) improvement. Overall, this thesis examines photophysical mechanisms of the hybrid perovskite material via photoluminescence analysis. The findings are helpful for a better understanding of the photo-stability issue associated with perovskites for improving the stability for solar cell operation in the future