Interplay of structural and optoelectronic properties in formamidinium mixed tin-lead triiodide perovskites

Journal article

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.

Authors: Parrott, E; Green, T; Milot, R; Johnston, M; Snaith, H

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: Wiley
• Journal: Advanced Functional Materials
• Volume: 28
• Issue: 33
• Publication date: 2018-06-21
• Acceptance date: 2018-05-29
• DOI: 10.1002/adfm.201802803
• EISSN: 1616-3028
• ISSN: 1616-301X
• Keywords: recombination; perovskite solar cells; band-gap bowing; photoluminescence