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 SnF2 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 approximate to 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 approximate to T-1.2, suggesting that it is limited mostly by intrinsic interactions with lattice vibrations. For the lowest doping concentration of 7.2 x 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.