Ionic Reactivity at Contacts and Aging of Methylammonium Lead Triiodide Perovskite Solar Cells

Hybrid lead halide perovskites have reached very large solar to electricity power conversion efficiencies, in some cases exceeding 20%. The most extensively used perovskite-based solar cell configuration comprises CH3NH3PbI3 (MAPbI(3)) in combination with electron (TiO2) and hole 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spiro-bifluorene (spiro-OMeTAD) selective contacts. The recognition that the solar cell performance is heavily affected by time scale of the measurement and preconditioning procedures has raised many concerns about the stability of the device and reliability for long-time operation. Mechanisms at contacts originate observable current-voltage distortions. Two types of reactivity sources have been identified here: (i) weak Ti-I-Pb bonds that facilitate interfacial accommodation of moving iodine ions. This interaction produces a highly reversible capacitive current originated at the TiO2/MAPbI(3) interface, and it does not alter steady-state photovoltaic features. (ii) An irreversible redox peak only observable after positive poling at slow scan rates. It corresponds to the chemical reaction between spiro OMeTAD(+) and migrating I- which progressively reduces the hole transporting material conductivity and deteriorates solar cell performance.