Iodine and Chlorine Element Evolution in CH₃NH₃PbI_3–xCl_x Thin Films for Highly Efficient Planar Heterojunction Perovskite Solar Cells

Highly efficient planar heterojunction perovskite solar cells (PHJ–PSCs) with a structure of ITO/PEDOT:PSS/CH 3 NH 3 PbI 3– x Cl x /PCBM/C 60 /Ag was fabricated, in which the compact and pinhole-free CH 3 NH 3 PbI 3– x Cl x perovskite thin film was obtained using a mixture of precursors containing lead iodide (PbI 2 ), lead chloride (PbCl 2 ), and methylammonium iodide (CH 3 NH 3 I) at an optimized ratio of 1:1:4. The morphology and formation process of CH 3 NH 3 PbI 3– x Cl x thin film was closely related to the annealing temperature and time, which would result in the controllable performance for the PHJ–PSC devices. The morphology, crystallization process, and element analysis suggested that the chlorine gradually diffused and sublimated from the film surface while the iodine moved to the surface, together with the removal of the pinholes in the film. The PHJ–PSCs with the as-prepared CH 3 NH 3 PbI 3– x Cl x thin film showed good performance and excellent repeatability. The power conversion efficiency (PCE) up to 14.03% was achieved without obvious hysteresis under different scanning conditions. The understanding of the iodine and chlorine element evolving process during the thermal treatment is beneficial to develop a more efficient scalable one-step solution processing method for fabricating large-area, highly efficient CH 3 NH 3 PbI 3– x Cl x -based PSCs.