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–x)Br_2x 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.