Elucidating Effect of Surface-Passivation on Charge-Carrier Relaxation Dynamics in Cs₄CuSb₂Cl₁₂ Microcrystals

In recent times, lead-free layered double perovskites have drawn considerable attention because of their nontoxic nature, structural stability in ambient conditions, and ability to alter optoelectronic properties by changing the stoichiometry of two metal ions. However, the existence of defect states still remains a major bottleneck for using this class of materials. In order to explore the potential applications of this intriguing family of materials as an active light-absorbing layer in solar cells, the nature of these defect states and their impact in trapping charge carriers must be understood in depth. In this work, we present a thorough investigation of the effects of capping ligands on surface passivation of microcrystals of Cs4CuSb2Cl12, a relatively new halide double-perovskite. We use different capping ligands with varying concentrations to explore such an effect. We further demonstrate how the surface trap states and their passivation influence ultrafast charge-carrier relaxation dynamics by using femtosecond transient absorption spectroscopy. While the nature of surface passivation is unlikely to affect the charge-carrier dynamics in microcrystals, owing to the remoteness of the surface from the core region where the majority of the charge carriers are photogenerated, our results show that even for microcrystals, the dynamics are significantly altered.