Complex evolution of photoluminescence during phase segregation of MAPb(I_1-xBr_x)₃ mixed halide perovskite

Under illumination, the photoluminescence of a mixed-halide perovskite such as MAPb(I_1-xBr_x)₃ is known to undergo a significant intensity enhancement while spectrally shifting to lower energies. The evolution of low energy photoluminescence is attributed to the formation of iodide rich domains due to phase segregation. This process is detrimental for optoelectronic devices however the mechanism is not well-understood. Here we present a real-time study of the photoluminescence evolution in MAPb(I_1-xBr_x)₃ samples during light-induced phase segregation. We show that the evolution of photoluminescence proceeds via several intermediate stages making it more complex than previously suggested. Within the first few seconds of photo-excitation, we found a very rapid formation of a short-lived intense photoluminescence band with a peak energy even lower than the final emission of the fully segregated sample. We propose that this emission comes from small pure iodide nano-domains formed during the initial stage of photo-induced phase-segregation.