Strong Electron-Phonon Coupling and Self-Trapped Excitons in the Defect Halide Perovskites A₃M₂I₉ (A = Cs, Rb; M = Bi, Sb)

The optical and electronic properties of Bridgman grown single crystals of the wide-bandgap semiconducting defect halide perovskites A₃M₂I₉ (A = Cs, Rb; M = Bi, Sb) have been investigated. Intense Raman scattering was observed at room temperature for each compound, indicating high polarizability and strong electron-phonon coupling. Both low-temperature and room-temperature photoluminescence (PL) were measured for each compound. Cs₃Sb₂I₉ and Rb₃Sb₂I₉ have broad PL emission bands between 1.75 and 2.05 eV with peaks at 1.96 and 1.92 eV, respectively. The Cs₃Bi₂I₉ PL spectra showed broad emission consisting of several overlapping bands in the 1.65-2.2 eV range. Evidence of strong electron-phonon coupling comparable to that of the alkali halides was observed in phonon broadening of the PL emission. Effective phonon energies obtained from temperature-dependent PL measurements were in agreement with the Raman peak energies. A model is proposed whereby electron-phonon interactions in Cs₃Sb₂I₉, Rb₃Sb₂I₉, and Cs₃Bi₂I₉ induce small polarons, resulting in trapping of excitons by the lattice. The recombination of these self-trapped excitons is responsible for the broad PL emission. Rb₃Bi₂I₉, Rb₃Sb₂I₉, and Cs₃Bi₂I₉ exhibit high resistivity and photoconductivity response under laser photoexcitation, indicating that these compounds possess potential as semiconductor hard radiation detector materials.