The key role of methylenediammonium and tetrahydrotriazinium in the phase stability of FAPbI3

Formamidinium lead triiodide (FAPbI3) is the prime candidate for single-junction perovskite solar cells, despite the metastability of the phase. To improve its ambient-phase stability and produce world-record photoelectric conversion efficiencies, methylenediammonium (MDA) has been used as an additive in FAPbI3. However, the exact function and role of MDA are still uncertain. The MDA doping may exist in the perovskite lattice in either the original structure or the THTZ-H (tetrahydrotriazinium) structure. In this research, the effects of the MDA and THTZ-H doping FAPbI3 perovskite on its stability are explored by first-principles calculations. Both MDA and THTZ-H doping can improve the stability of FAPbI3 perovskite from a structural perspective due to lattice strain and stronger H–I bonds. However, the doping mechanisms differ significantly in terms of electronic properties. The MDA doping acts by the traditional passivation mechanism. It can eliminate the iodine interstitial defect states that trap charge carriers and inhibit iodine interstitial defect migration. The THTZ-H cation can directly contribute to the band edge construction in the FAPbI3 bulk. Electron delocalization in the π-conjugated ring structure lowered the frontier orbital separation of the THTZ-H organic molecule and enabled orbital overlap with the inorganic moiety. The in-depth understanding of the mechanism of improving stability in this study would facilitate the application of FAPbI3 perovskite optoelectronic devices.