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Metal halide perovskite semiconductors often exhibit photoluminescence blinking and flickering when luminescence of individual small nano- or even microcrystals is monitored. The nature of these fluctuations is not well understood but must be related to the presence of metastable non-radiative recombination channels and efficient charge migration in these materials. Here we report on the excitation wavelength dependence of photoluminescence flickering effect in degraded methylammonium lead iodide (MAPbI3) thin films. While the luminescence intensity is temporary stable when excited in the blue region with wavelength shorter than 530 nm, excitation with red light (wavelength longer than 530 nm) results in luminescence flickering. It is hypothesised that the wavelength dependence reflects the excitation energy dependence of the photochemical mechanism that switches non-radiative recombination channels on and off. The effect can also be related to hindered charge carrier diffusion due to their localization in the interfacial layer between MAPbI3 and PbI2 which is formed in the course of degradation.

Under illumination, the photoluminescence of a mixed-halide perovskite such as MAPb(I1-xBrx)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(I1-xBrx)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. (Less)

Whether optoelectronic devices based on metal-halide perovskite semiconductors will become a commercially viable technology will be determined by their intrinsic and operational stability. Recent results indicate there is some reversibility of perovskite degradation in thin films and devices, although mechanistic insight into the processes driving degradation and recovery are still scarce. We here present a comparative spectroscopic study of methylammonium lead iodide (MAPbI3) films having undergone either photo- or thermal degradation under controlled conditions. We confirm that the degradation mechanism pertaining to each type of stress is inherently different. Our results from photoluminescence microscopy measurements paint a spatially, spectrally and temporarily resolved picture showing that, unlike thermally degraded samples, photodegraded samples are in a state that intermittently recovers to luminescent MAPbI3 upon laser excitation. This indicates that rather than irreversibly decomposing, photoinduced degradation leaves MAPbI3 structurally or compositionally intact but induces defects causing non-radiative recombination losses.