Ultrafast exciton-polaron transformations and nonlinear terahertz photocurrents in inorganic Cs2SnI6 double perovskite

Cs 2 SnI 6 , a lead-free vacancy-ordered double perovskite, is promising for optoelectronics due to its suitable band gap, high absorption coefficient, and ambient stability. However, its underwhelming photovoltaic performance and nonlinear optoelectronic responses remain unclear. We employed optical pump-terahertz (THz) probe (OPTP) and THz emission spectroscopy (TES) to study ultrafast carrier dynamics and nonlinear photocurrent generation in Cs 2 SnI 6 thin films. OPTP shows that photoexcitation generates hot carriers within 0.6 ps, which relax via electron-phonon coupling to form excitons by 1.2 ps. These excitons dissociate into free carriers that couple with phonons to form polarons within 2.53 ps, followed by polaron relaxation (∼27 ps). This exciton-polaron transformation limits carrier mobility and photovoltaic efficiency. TES reveals THz emission from both in-plane and out-of-plane nonlinear photocurrents, dominated by the photogalvanic effect (PGE) and photo-Dember effect, respectively. Polarization-dependent TES shows a helicity-sensitive circular PGE contribution up to 28.7% at 45° incidence, 7.8 times higher than in MAPbI 3 , evidencing Rashba spin splitting in this centrosymmetric perovskite. These results clarify the mechanisms of ultrafast exciton-polaron dynamics and nonlinear photocurrents in Cs 2 SnI 6 , explaining its photovoltaic limitations and highlighting its potential as a tunable, polarization-controllable THz emitter.