Cesium-Incorporated Triple Cation Perovskites Deliver Fully Reversible and Stable Nanoscale Voltage Response

Perovskite solar cells that incorporate small concentrations of Cs in their A-site have shown increased lifetime and improved device performance. Yet, the development of fully stable devices operating near the theoretical limit requires understanding how Cs influences perovskites' electrical properties at the nanoscale. Here, we determine how the chemical composition of three perovskites (MAPbBr₃, MAPbI₃, and Cs-mixed) affects their short- and long-term voltage stabilities, with <50 nm spatial resolution. We map an anomalous irreversible electrical signature on MAPbBr₃ at the mesoscale, resulting in local V _oc variations of ∼400 mV, and in entire grains with negative contribution to the V _oc. These measurements prove the necessity of high spatial resolution mapping to elucidate the fundamental limitations of this emerging material. Conversely, we capture the fully reversible voltage response of Cs-mixed perovskites, composed by Cs_0.06(MA_0.17FA_0.83)_0.94Pb(I_0.83Br_0.17)₃, demonstrating that the desired electrical output persists even at the nanoscale. The Cs-mixed material presents no spatial variation in V _oc, as ion motion is restricted. Our results show that the nanoscale electrical behavior of the perovskites is intimately connected to their chemical composition and macroscopic response.