Mechanochemical pretreatment of tin iodide perovskite precursors: effects of grinding temperature and time on solar cell performance

Abstract

Tin-based halide perovskite solar cells (Sn PSCs) have unacceptably low open-circuit voltages owing to the severe carrier traps induced by the facile oxidation of Sn²⁺. To prevent oxidation, Sn powder is often added to the precursor solutions; however, the optimal process conditions and the detailed underlying mechanism remain elusive. Here, the PSC precursors (Sn powder, SnF₂, SnI₂, and organic cation iodides) are subjected to mechanochemical pretreatment to enhance the power conversion efficiency (PCE) of PEA_0.1FA_0.9SnI₃ PSCs (PEA: phenylethylammonium and FA: formamidinium). Two systems are compared: the Sn powder–SnF₂ (Sn–SnF₂) system and all-chemical system, both ground using a vibrational ball-milling technique. The grinding temperature (5–28 °C) and duration (0.5–9 h) are found to significantly affect the PCEs. The processes with 4 h of grinding of Sn–SnF₂ at 13 °C followed by 1 h of thermal annealing (TA) at 170 °C and 2 h of grinding of the all-chemical sample without TA yield the highest PCEs of 7.76 and 7.60%, respectively, compared to 5.24% for the control (no grinding). The assumed mechanochemical reactions provide a rationale for the preferred low-temperature grinding process. An efficient and reliable method with accelerated reduction of SnI₄ in dimethyl sulfoxide and improved oxygen tolerance of Sn PSCs, both resulting from grinding, is proposed for the preparation of lead-free PSCs.