Orientation of Thiocyanate Ions Tuning the Electron-Phonon Interactions in Pseudohalide Perovskites

Although the importance of electron-phonon interactions on the optoelectronic properties of perovskites has been well documented, the structural origin of electron-phonon interactions remains largely unexplored. In this study, using pseudohalide perovskites Cs₂Pb(SCN)₂I_2(1-Br₂ as a model, we have revealed how the orientation of SCN^- anions tunes the electron-phonon interactions and the effective charge-carrier mobility by utilizing femtosecond sum frequency generation vibrational spectroscopy, supplemented by photoluminescence spectroscopy and femtosecond optical-pump terahertz-probe spectroscopy. The coupling between neighboring SCN^- anions decreases as the Br content (x) increases but does not have a significant effect on the electron-phonon interactions. In contrast, the orientation angle of SCN^- anions has a strong correlation with the electron-phonon interaction and effective charge-carrier mobility, that is, a more parallel orientation of SCN^- anions leads to a higher electron-phonon interaction and lower effective charge-carrier mobility. This finding provides a molecule-level understanding of the inorganic lattice structure in tuning electron-phonon interactions and may offer valuable guidance for optimizing the optoelectronic properties of perovskites.