Role of GeI2 and SnF2 additives for SnGe perovskite solar cells

Highlights: • The trap densities of a new type SnGe perovskite was first revealed through thermally stimulated current. • GeI₂ dopant substantially reduced trap densities and improved the carrier dynamics of SnGe device. • The SnF₂ taking a role to suppress the undesirable Sn²⁺ oxidation process. • This SnGe-based device exhibited the highest efficiency to date (PCE=7.9%). -- Abstract: The distribution of trap states within perovskite vicinity or hetero-interfaces is the prime attribution towards carrier dynamics inferiority and low photovoltaic performances. In this work, thermally stimulated current (TSC) is performed to unravel the impact of germanium (Ge) addition in passivating and reducing trap states; consequently, to improve its carrier dynamics. The addition of 5 mol% germanium into the FA_0.75MA_0.25Sn_1-xGe_xI₃ (FMSGI(5)) framework strikingly suppresses the trap density from 10¹⁵–10¹⁷ cm⁻³ (without Ge) to 10⁸–10¹⁴ cm⁻³, thereby renders longer charge diffusion length (~1 µm) and lifetime; coupled with excellent charge mobility and efficiency of 7.9%. Interestingly, the FMSGI(5) perovskite exhibits indistinguishable trap densities profile from that of MAPbI₃ perovskite and exhibits a long charge diffusion length of 1 µm. Another important information to be highlighted in this paper is the advantage of SnF₂ to subside the vacancy of Sn²⁺. This study provides deep intuitive on trap landscape, which unlocks opportunities for the designation of high performance lead-free perovskite solar cells.