Title: Unified thermodynamic theory for light-induced phase separation in mixed halide perovskites
Abstract | Under illumination, mixed halide perovskites that are thermodynamically stable in the dark can demix due to a free energy lowering of photocarriers that funnel to a nucleated phase with different halide composition and lower band gap than the parent phase. This is problematic for applying mixed halide perovskites in solar cells. I will present in this seminar a unified thermodynamic theory for this light-induced phase separation and apply it to five mixed iodine-bromine perovskites. The spinodals separating metastable and unstable regions in the composition-temperature phase diagrams shift upward in temperature under illumination, while new light-induced binodals separating stable and metastable regions appear that signal nucleation of a low-band gap iodine-rich phase. The threshold photocarrier density for phase separation is mainly governed by the band gap difference of the parent and iodine-rich phase. Partial replacement of organic cations by cesium reduces this difference and therefore has a stabilizing effect.