Instability of tin iodide perovskites: Bulk p-doping versus surface tin oxidation

Tin halide perovskites represent the only realistic route toward lead-free perovskite optoelectronics. Despite significant progress, however, the device efficiency and stability of solar cells are still limited by the perovskite self-p-doping and by Sn(II) oxidation to Sn(IV). By employing state-of-the-art density functional theory simulations, we unveil the mechanistic features and energetics of Sn(II) → Sn(IV) oxidation in pristine and defective models. Surprisingly, tin oxidation is predicted to be considerably unfavorable in bulk MASnI3 while it is energetically favored at unpassivated perovskite surfaces. As a consequence, bulk Sn(IV) spontaneously transforms into Sn(II), releasing two holes to the valence band and p-doping the perovskite, while surface Sn(IV) acts as a deep electron trap and contributes to nonradiative carrier recombination. The stoichiometry and the valence band surface pinning are found to largely influence the formation of Sn(IV), pointing to surface passivation as the main strategy to obtain efficient and stable tin halide solar cells.