Effects of the PbBr(2):PbI(2) Molar Ratio on the Formation of Lead Halide Thin Films, and the Ratio’s Application for High Performance and Wide Bandgap Solar Cells

We investigate the effects of the molar ratio (x) of PbBr(2) on the phases, microstructure, surface morphology, optical properties, and structural defects of mixed lead halides PbI(2(1−x))Br(2x) for use in solar cell devices. Results indicate that as x increased to 0.3, the surface morphology continued to improve, accompanied by the growth of PbI(2) grains. This resulted in lead halide films with a very smooth and continuous morphology, including large grains when the film was formed at x = 0.3. In addition, the microstructure changed from (001)-oriented pure PbI(2) to a highly (001)-oriented β (PbI(2)-rich) phase. The plausible mechanism for the enhanced morphology of the lead halide films by the addition of PbBr(2) is proposed based on the growth of a Br-saturated lead iodide solid solution. Furthermore, iodine vacancies, identified by X-ray photoelectron spectroscopy, decreased as the ratio of PbBr(2) increased. Finally, an electrical analysis of the solar cells was performed by using a PN heterojunction model, revealing that structural defects, such as iodine vacancies and grain boundaries, are the main contributors to the degradation of the performance of pure PbI(2)-based solar cells (including high leakage, low stability, and high hysteresis), which was significantly improved by the addition of PbBr(2). The solar cell fabricated at x = 0.3 in air showed excellent stability and performance. The device lost merely 20% of the initial efficiency of 4.11% after 1500 h without encapsulation. This may be due to the dense microstructure and the reduced structural defects of lead halides formed at x = 0.3.