Snaith group

Abstract:

Mixed lead-tin triiodide perovskites are promising absorber materials for low band-gap bottom cells in all-perovskite tandem photovoltaic devices. Key structural and electronic properties of the FAPb1-xSnxI3 perovskite are presented here as a function of lead:tin content across the alloy series. Temperature-dependent photoluminescence and optical absorption measurements are used to identify changes in the band-gap and phase transition temperature. The large band-gap bowing parameter, a crucial element for the attainment of low band-gaps in this system, is shown to depend on the structural phase, reaching a value of 0.84 eV in the low-temperature phase and 0.73 eV at room temperature. The parabolic nature of the bowing at all temperatures is compatible with a mechanism arising from bond bending to accommodate the random placement of unevenly sized lead and tin ions. Charge-carrier recombination dynamics are shown to fall into two regimes. Tin-rich compositions exhibit fast, mono-exponential recombination that is almost temperature independent, in accordance with high levels of electrical doping. Lead-rich compositions show slower, stretched-exponential charge-carrier recombination that is strongly temperature-dependent, in accordance with a multi-phonon assisted process. These results highlight the importance of structure and composition for control of band-gap bowing and charge-carrier recombination mechanisms in low band-gap absorbers for all-perovskite tandem solar cells.