Formamidine disulfide oxidant as a localised electron scavenger for >20% perovskite solar cell modules

Abstract

Formamidinium lead iodide (FAPbI₃)-based perovskites possess high light absorption and long diffusion lengths, making them strong candidates for highly efficient solar cells. However, despite these properties, primary intrinsic defects in FAPbI₃ (i.e. iodine vacancy) induce strong electron localisation and become deep traps and recombination centres upon photoexcitation. Consequently, the carrier lifetime is significantly reduced and the superior properties are not fully utilised. Therefore, the manipulation of intrinsic defects has become a critical issue for realising highly efficient solar cells. Herein, formamidine disulfide dihydrochloride (FASCl) is used because the FAS²⁺ ion is a strong oxidant or electron scavenger. Substitution of the FAS²⁺ ion for the FA⁺ ion makes the iodine vacancy lose the strongly localised electrons and removes the deep traps. The incorporation of FASCl induces the formation of intermediate phases with a perovskite precursor, which can effectively stabilise the black α-phase FAPbI₃ and retard the crystallisation rate, leading to compact full-coverage perovskite layers with high crystallinity and a large grain size. As a result, the optimal unit device (0.14 cm²) exhibits a remarkable power conversion efficiency (PCE) of 23.11%, a stabilised power output (SPO) of 22.83%, a low voltage deficit of 0.343 V, and a notable fill factor of 83.4%. Without encapsulation, the device retains ∼92.5% and ∼91.7% of its initial efficiency after 1000 h of either heating at 85 °C (thermal) or 50% relative humidity atmospheric testing, respectively. Moreover, the perovskite solar modules (PSMs) achieve PCE values of 20.75% (with a notable fill factor of 78.5%) and 17.44% for the active areas of 23.27 and 59.33 cm², respectively.