Coupling Photogeneration with Thermodynamic Modeling of Light-Induced Alloy Segregation Enables the Identification of Stabilizing Dopants

Halide segregation in perovskites for photovoltaics and light-emitting diodes is a topic of interest given its impact on long-term device reliability. We sought to develop phase diagrams of alloys that take account not only of temperature and composition but also include the effects of photon fluence: optical excitation that contributes, through the thermalization of excited carriers, to excitation-intensity-dependent phase diagrams. The model accurately replicates the experimentally observed light-induced phase segregation behavior of the MAPb(I,Br)₃ system. From there, we sought to study how best to design new, phase-stable, mixed-halide alloys. Using the model, we explored candidate dopants that could stabilize cubic (FA,Cs)-based mixed-halide perovskites. This leads to the prediction that the pseudohalide anion BF₄^- will suppress phase segregation. Experimentally, we find that BF₄^- incorporates into FA_0.83Cs_0.17Pb(I_0.6Br_0.4)₃; and that BF₄^- stabilized absorbers maintain >18% power conversion efficiency (PCE) over 800 h under 1-sun illumination at MPP with no performance loss. The model links photostability with the structure and electronic properties of materials and provides guidance on stabilizing via alloying.