Limiting phosphonic acid interlayer–perovskite reactivity to stabilize perovskite solar modules

Phosphonic acid (PA)-based interlayers used in metal-halide perovskite solar cells (PSCs) can suffer from instability at elevated temperatures. We report that the acidic protons of PAs weakly bound to indium tin oxide (ITO) can accelerate the oxidation of iodide, decomposition of formamidinium, and reduction of lead ions and that these reactions accelerate at high temperature and on exposure to ultraviolet light. Also, some common PA molecules weakly bonded to ITO can desorb and react with perovskites. We synthesized a bis(diarylamino)biphenyl-based PA that binds more strongly to ITO and show that its use in PSCs led to an operational lifetime of nearly 3000 hours with 10% efficiency loss (T₉₀) at 85°C under a metal halide lamp (including ultraviolet light) with maximum power point tracking. Minimodules had power conversion efficiencies >22% with an aperture area of >20 square centimeters and exhibited a T₉₀ lifetime of ~2200 hours under similar testing conditions.