Influence of Cl Incorporation in Perovskite Precursor on the Crystal Growth and Storage Stability of Perovskite Solar Cells

Solar cells based on organic-inorganic hybrid lead-halide perovskites are very promising because of their high performance and solution process feasibility. Elemental engineering on perovskite composition is a facile path to obtain high-quality crystals for efficient and stable solar cells. It was found that partially substituting I ^- with Cl ^- in the perovskite precursor promoted crystal growth, with the grain size larger than the layer thickness, and facilitated the generation of a self-passivation layer of PbI ₂ . Whereas the residual Cl ^- ions were suspected to diffuse to the hole-transport layer consisting of ubiquitously spiro-OMeTAD, the formation of highly bounded ionic pairing of Cl ^- with the oxidized state of spiro-OMeTAD led to insufficient charge extraction and severely reversible performance degradation. This issue was effectively alleviated upon Br ^- doping owing to the generation of Pb-Br bonds in the lattice that strengthened the phase stability by improving the binding energy between each unit. The binary halide (Br ^- /Cl ^- )-doped perovskites resulted in a champion power conversion efficiency of 20.2% with improved long-term storage stability.