Cesium Oleate Passivation for Stable Perovskite Photovoltaics

Despite their emergence as promising materials for low-cost and efficient energy power generation technology, the instability of hybrid organic-inorganic lead-halide perovskites toward moisture and heat stress remains a serious obstacle that needs to be tackled for commercialization. Here, we show improved moisture and thermal stability through the use of cesium oleate to modify the perovskite/hole transporting material interface. Passivation using cesium oleate does not induce the formation of any low-dimensional perovskites, suggesting that the organic species only passivate the perovskite's surface and grain boundaries. As a result, enhanced hydrophobic character of the perovskite film is realized upon passivation, evidenced by a large water contact angle of 107.4° and improved stability at ambient conditions (a relative humidity of ∼70%, room temperature). Concomitantly, the proposed passivation strategy leads to an increased amount of cesium concentration within the films, resulting in beneficial enhanced thermal stability of the film at 85 °C. By maintaining the three-dimensional (3D) structure of the solar absorber while concurrently passivating the interfacial defects and vacancies, improved open-circuit voltage (V_oc) and unsacrificed short-circuit current density (J_sc) were obtained from the treated devices, leading to power conversion efficiencies of more than 18%. When stored in a humid environment (a relative humidity of ∼55%), devices with cesium oleate passivation maintain 88% of their initial power conversion efficiency after 720 h, degrading two times slower than those of the control. This work offers a strategy of coating 3D perovskites with a unique combination of inorganic cations and long-chain organics to provide hydrophobicity and moisture stability to the solar absorber layer while maintaining good device performances.