Phase segregation affects electron–phonon coupling in perovskite solar cell materials

Abstract

Phase segregation has been described as a significant factor that limits solar cell efficiency and long-term stability in mixed organic–inorganic halide perovskite materials. It is often microscopically linked to the electron–phonon coupling when the device is under operation. Through computational calculations, Migdal-Eliashberg theory and the Fröhlich large polaron model, we examined the control of phase segregation, in bulk I/Br and FA/Cs mixtures, over the electron–phonon coupling strength. We revealed that either A-site or X-site phase segregation destabilizes the material but reduces the electron–phonon coupling and increases the charge carrier mobility. Segregation promotes higher frequency vibrations and phonon instability is generally caused by [PbI₆]⁴⁻ octahedral torsions and liberations in FA⁺. Phonon dispersion has stronger control over the electron–phonon coupling than electronic bands. We expect that our theoretical findings will influence future discussions regarding the interplay of phase segregation and electron–phonon interactions in perovskite solar cells.