The Effects of Doping Density and Temperature on the Optoelectronic Properties of Formamidinium Tin Triiodide Thin Films

Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI 3 ) thin films, whose background hole doping density is varied through SnF 2 addition during film fabrication. Monomolecular charge‐carrier recombination exhibits both a dopant‐mediated part that grows linearly with hole doping density and remnant contributions that remain under tin‐enriched processing conditions. At hole densities near 10 20 cm −3 , a strong Burstein–Moss effect increases absorption onset energies by ≈300 meV beyond the bandgap energy of undoped FASnI 3 (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (10 20 cm −3 ), temperature‐dependent measurements indicate that the effective charge‐carrier mobility is suppressed through scattering with ionized dopants. Once the background hole concentration is nearer 10 19 cm −3 and below, the charge‐carrier mobility increases with decreasing temperature according to ≈ T −1.2 , suggesting that it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2 × 10 18 cm −3 , charge‐carrier mobilities reach a value of 67 cm 2 V −1 s −1 at room temperature and 470 cm 2 V −1 s −1 at 50 K. Intraexcitonic transitions observed in the THz‐frequency photoconductivity spectra at 5 K reveal an exciton binding energy of only 3.1 meV for FASnI 3 , in agreement with the low bandgap energy exhibited by this perovskite.