Light induced metastable modification of optical properties in CH3NH3PbI3−xBrx perovskite films: Two-step mechanism

•PL of MAPbI3−xBrx films red-shift excited by CW laser.•PL of MAPbI3−xBrx films don't shift excited by small repetition pulsed laser.•Ref-shift of PL is due to photoinduced metastable sub band states in film.•Photoinduced metastable states can be restored spontaneously.•Two-step mechanism is needed to explain the metastable modification.

We have used photoluminescence (PL) and photomodulation (PM) spectroscopy to investigate the reversible spectral changes of PL in CH3NH3PbI3−xBrx films, where x is 1.7. In an as-prepared film, the peak of PL spectra shifts from ∼640 nm near bandedge to ∼750 nm after excitation by a continuous wave (CW) or a pulsed laser with high repetition rate, but keeps at 640 nm excited by same pulsed laser with the repetition rate smaller than 500 Hz. The PM spectroscopy also shows the formation of sub bandgap states after illumination which is responsible for the red shift of PL. The light induced modification of optical properties is reversible after keeping the film out of illumination for several hours at room temperature. We analyze the photoinduced modification to be two-steps processes: the temporary sub bandgap states were first photogenerated in perovskite film, if those states interacting with more coming photons within their lifetimes, light induced metastable states responsible for red-shift of PL will be formed. This instability reduces the electronic bandgap and generates more traps which will degrade the performance of the related photovoltaic devices.

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