Effects of Chlorine Contents on Perovskite Solar Cell Structure Formed on CdS Electron Transport Layer Probed by Rutherford Backscattering

CdS synthesized by the chemical bath method at 70 °C, has been used as an electron transport layer in the planar structure of the perovskite solar cells. A two-step spin process produced a mixed halide perovskite of CH3NH3PbI3−xClx and a mixture of PbCl2 and PbI2 was deposited on CdS, followed by a sub-sequential reaction with MAI (CH3NH3I). The added PbCl2 to PbI2 in the first spin-step affected the structure, orientation, and shape of lead halides, which varied depending on the content of Cl. A small amount of Cl enhanced the surface morphology and the preferred orientation of PbI2, which led to large and uniform grains of perovskite thin films. In contrast, the high content of Cl produces a new phase PbICl in addition to PbI2, which leads to the small and highly uniform grains of perovskites. An improved surface coverage of perovskite films with the large and uniform grains maximized the performance of perovskite solar cells at 0.1 molar ratio of PbCl2 to PbI2. The depth profiling of elements in both lead halide films and mixed halide perovskite films were measured by Rutherford backscattering spectroscopy, revealing the distribution of chlorine along with the thickness, and providing the basis for the mechanism for enhanced preferred orientation of lead halide and the microstructure of perovskites.