The Molybdenum Oxide Interface Limits the High-Temperature Operational Stability of Unencapsulated Perovskite Solar Cells

We report on the improved operational stability of unencapsulated perovskite solar cells (PSCs) aged in an ambient atmosphere at elevated temperatures (70 °C) for >1000 h under constant illumination and bias at 30-50% relative humidity. We identify a previously unseen interfacial degradation mechanism concerning the use of a MoOx interlayer, which was originally added to increase operational stability. Specifically, the hole-transport layer/MoOx interface buckles under illumination at 70 °C, which leads to delamination and rapid losses of short-circuit current density corresponding to an average t80 of ∼55 h. By judiciously evaluating various hole-transport layers, interlayers, and contacts, we find that replacing the MoOx with a VOx interlayer, regardless of the other components in the solar cell, alleviates this buckling issue due to its higher activation barrier toward crystallization, leading to significant gains in PSC operational stability. Unencapsulated devices aged in an ambient atmosphere with a VOx interlayer retain 71% of their initial PCE on average after constant illumination and bias at 70 °C for 1100 h (t80 ∼645 h). Currently, this is the highest temperature reported for the operational stability of unencapsulated n-i-p PSCs aged in air. Identification of a new facet of the complex degradation mechanisms in PSCs will allow for targeted acceleration testing to speed the deployment of low-cost, long-lasting electricity generation under realistic operating temperatures.