  We developed a generalized model that considers light as energy contributed
through the thermalization of excited carriers to generate
excitation-intensity- and temperature-dependent phase diagrams. We find that
the model replicates the light-induced phase segregation behavior of the
MAPb(I,Br)3 system. From there, we sought to study how best to design new,
stable, mixed-halide alloys. The resultant first-principles predictions show
that the pseudo halide anion BF4- suppresses phase segregation in
FA0.83Cs0.17Pb(I0.6Br0.4)3, resulting in enhanced operating stability. The
findings reveal that photostability is linked with the structure and electronic
properties of materials and may be overcome using informed alloying strategies.
