Strong Electron–Phonon Coupling Induced Self-Trapped Excitons in Double Halide Perovskites

Double halide perovskites exhibit impressive potential for the self-trapped exciton (STEs) luminescence. However, the detailed mechanism of the physical nature during the formation process of STEs in double perovskites is still ambiguous. Herein, theoretical research on a series of double halide perovskites Cs₂B¹B²Cl₆ (B¹ = Na⁺, K⁺; B² = Al³⁺, Ga³⁺, In³⁺) regarding their electronic structures, exciton characteristics, electron–phonon coupling performances, and geometrical configuration is conducted. These materials have flat valence band edges and thus possess localized heavy holes. They also show high exciton binding energies, and their short exciton Bohr radius indicates that the spatial size of their excitons is comparable to the dimension of their single lattice. Based on the Fröhlich coupling constant and Feynman polaron radius, the stronger electron–phonon coupling strength in Ga-series double halide perovskites is revealed. In particular, Cs₂NaGaCl₆ shows a high and effective Huang–Rhys factor of 36.21. The phonon characteristics and vibration modes of Cs₂NaGaCl₆ are further analyzed, and the Jahn–Teller distortion of the metal–halogen octahedron induced by hole-trapping after excitation is responsible for the existence of STEs. This study strengthens the physical understanding of STEs and provides effective guidance for the design of advanced solid-state phosphors.