Redox Chemistry of the Subphases of α-CsPbI(2)Br and β-CsPbI(2)Br: Theory Reveals New Potential for Photostability

The logic in the design of a halide-mixed APb(I(1−x)Br(x))(3) perovskite is quite straightforward: to combine the superior photovoltaic qualities of iodine-based perovskites with the increased stability of bromine-based perovskites. However, even small amounts of Br doped into the iodine-based materials leads to some instability. In the present report, using first-principles computations, we analyzed a wide variety of α-CsPbI(2)Br and β-CsPbI(2)Br phases, compared their mixing enthalpies, explored their oxidative properties, and calculated their hole-coupled and hole-free charged Frenkel defect (CFD) formations by considering all possible channels of oxidation. Nanoinclusions of bromine-rich phases in α-CsPbI(2)Br were shown to destabilize the material by inducing lattice strain, making it more susceptible to oxidation. The uniformly mixed phase of α-CsPbI(2)Br was shown to be highly susceptible towards a phase transformation into β-CsPbI(2)Br when halide interstitial or halide vacancy defects were introduced into the lattice. The rotation of PbI(4)Br(2) octahedra in α-CsPbI(2)Br allows it either to transform into a highly unstable apical β-CsPbI(2)Br, which may phase-segregate and is susceptible to CFD, or to phase-transform into equatorial β-CsPbI(2)Br, which is resilient against the deleterious effects of hole oxidation (energies of oxidation >0 eV) and demixing (energy of mixing <0 eV). Thus, the selective preparation of equatorial β-CsPbI(2)Br offers an opportunity to obtain a mixed perovskite material with enhanced photostability and an intermediate bandgap between its constituent perovskites.