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

Abstract

Optoelectronic properties are unraveled for formamidinium tin triiodide (FASnI₃ ) 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 10²⁰ cm^-3 , a strong Burstein-Moss effect increases absorption onset energies by ≈300 meV beyond the bandgap energy of undoped FASnI₃ (shown to be 1.2 eV at 5 K and 1.35 eV at room temperature). At very high doping densities (10²⁰ 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 10¹⁹ 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 × 10¹⁸ cm^-3 , charge-carrier mobilities reach a value of 67 cm² V^-1 s^-1 at room temperature and 470 cm² 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 FASnI₃ , in agreement with the low bandgap energy exhibited by this perovskite.