Transformation of Lead Iodide (Cs0.05PbI2.05) during Annealing and Its Influence on Perovskite Formation and Solar Cells

This study explores the transformation of lead iodide (PbI₂) thin films including small amounts of cesium iodide (Cs_0.05PbI_2.05) during annealing. Furthermore, its impact on the formation of the perovskite thin film using a two-step method is investigated. A solution of PbI₂ and CsI is used for the spin-coating process, containing dimethylsulfoxide (DMSO) to benefit from retarded PbI₂ crystallization due to residual DMSO in the thin film, gradually evaporating during annealing. We show that the annealing conditions of the Cs_0.05PbI_2.05 thin film play a crucial role in determining the films’ crystallinity and density. A transition of the Cs_0.05PbI_2.05 films from an amorphous to an increasingly crystalline morphology is observed. The crystallization process starts with the formation of small, widely distributed crystallites at the surface. Upon further annealing, crystallinity increases, accompanied by a decrease in film thickness and weight due to effusion of DMSO molecules. The formed film structure has a strong impact on the surface morphology of the resulting perovskite thin film. It is shown that less crystalline Cs_0.05PbI_2.05 thin films allow for higher open-circuit voltages (V_oc) of the produced solar cells, while more crystalline Cs_0.05PbI_2.05 thin films lead to the smoothest perovskite surface but lower fill factor and V_oc.