A synergistic Cs2CO3 ETL treatment to incorporate Cs cation into perovskite solar cells via two-step scalable fabrication

Abstract

Triple cation CsFAMA perovskite films fabricated via a one-step method have recently gained attention as an outstanding light-harvesting layer for photovoltaic devices. However, questions remain over the suitability of one-step processes for the production of large-area films, owing to difficulties in controlling the crystallinity, in particular, scaling of the frequently used anti-solvent washing step. This can be mitigated through the use of the two-step method which has recently been used to produce large-area films via techniques such as slot dye coating, spray coating or printing techniques. Nevertheless, the poor solubility of Cs containing salts in IPA solutions has posed a challenge for forming triple cation perovskite films using the two-step method. In this study, we tackle this challenge through fabricating perovskite films on a caesium carbonate (Cs₂CO₃) precursor layer, enabling Cs incorporation within the film. Synergistically, we find that Cs₂CO₃ passivates the SnO₂ electron transport layer (ETL) through interactions with Sn 3d orbitals, thereby promoting a reduction in trap states. Devices prepared with Cs₂CO₃ treatment also exhibited an improvement in the power conversion efficiency (PCE) from 19.73% in a control device to 20.96% (AM 1.5G, 100 mW cm⁻²) in the champion device. The Cs₂CO₃ treated devices (CsFAMA) showed improved stability, with un-encapsulated devices retaining nearly 80% efficiency after 20 days in ambient air.