Intermediate phase-enhanced Ostwald ripening for the elimination of phase segregation in efficient inorganic CsPbIBr₂ perovskite solar cells

Mixed halide perovskites with the ability to tune bandgaps exhibit attractive applications in tandem solar cells, building integrated photovoltaic and wavelength-tunable light-emitting devices. However, halide demixing under illumination or in the dark with a charge-carrier injection in both hybrid and inorganic perovskites results in bandgap instability and current-density-voltage (J-V) hysteresis, which can significantly hamper their application. Here, we demonstrate that halide segregation and J-V hysteresis in mixed halide inorganic CsPbIBr₂ solar cells can be effectively mitigated by introducing an intermediate phase-enhanced Ostwald ripening through the control of the chemical composition in the CsPbIBr₂ precursor solution. Excess amounts of either PbBr₂ or CsI are incorporated into originally even molar amounts of PbBr₂ and CsI precursor solutions. With the PbBr₂-excess, we observed an enlarged perovskite grain size, no detectable halide phase segregation at the grain boundaries nor the perovskite/TiO₂ interface, an increased minority carrier lifetime, a reduced J-V hysteresis, and an improved solar-cell performance. However, different CsI:PbBr₂ stoichiometric ratios were found to have different effects on the performance of the perovskite solar cell. The excessive lead phase is reactive with the dimethyl sulfoxide (DMSO) in the precursor solution to form the Pb(I, Br)₂-DMSO complex and the quasi-two-dimensional (2D) CsPb₂(I, Br)₅, which are conducive to Ostwald maturation and defect extinction. Finally, the CsPbIBr₂ solar cell with a PbBr₂-excess precursor composition reaches a power conversion efficiency (PCE) of 9.37% (stabilized PCE of 8.48%) and a maximum external quantum efficiency of over 90%.