Reversible wavelength-dependent photo-bleaching in free-standing polycrystalline films of MAPbI3 monitored under the intense visible light flux
Andrzej Sienkiewicz a b, Konstantins Mantulnikovs b, Márton Kollár b, Endre Horváth b, László Forró b
a ADSresonances SARL, Route de Genève 60B, CH-1028, Prèverenges, Switzerland
b Laboratory of Physics of Complex Matter, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of International Conference on Perovskite and Organic Photovoltaics and Optoelectronics (IPEROP19)
Kyōto-shi, Japan, 2019 January 27th - 29th
Organizers: Hideo Ohkita, Atsushi Wakamiya and Mohammad Nazeeruddin
Oral, Andrzej Sienkiewicz, presentation 044
Publication date: 23rd October 2018

We report on the recurrent photo-bleaching and photo-recovery cycles of photoluminescence (PL) in free-standing polycrystalline films of methyl-ammonium lead iodide, CH3NH3PbI3 (MAPbI3), which were monitored under intermittent illumination with strong fluxes of the visible light. To this end, the PL responses of MAPbI3 thin films, coated onto glass substrates and protected with a thin layer of Poly(methyl-methacrylate) (PMMA), were acquired under repeatable illuminations at two different wavelengths of 470 nm and 546 nm. Specifically, the same spot of the MAPbI3 deposit was exposed for several times to multiple cycles of ‘light (470 nm) – dark - light (546 nm)’. The emission PL spectra were acquired using a custom-modified epi-fluorescent microscope. During the subsequent illumination cycles, the exposure times were of ~2 min and ~10 min, for the excitation wavelengths of 470 nm and 546 nm, respectively. Each short exposure to the wavelength of 470 nm repeatedly resulted in a rapid diminishment of the PL intensity (by ~50%). This marked PL decrease could also be followed as a function of time under the continuous excitation at 470 nm. Interestingly, after switching the excitation wavelength to 546 nm, the PL intensity could be partially recovered on a time scale of ~10 min. Overall, the recurrent exposures to 470 nm light resulted in a relatively rapid photo-bleaching, whereas exposures to 546 nm light lead to a somewhat slower recovery of the PL intensity. Moreover, since the photo-recovery of the PL intensity was not entirely complete, after the recurrent photo-bleaching events, a net decrease in the PL intensities was also observed as a function of illumination time. 

In summary, our results point to marked wavelength-dependent mechanisms of light-induced changes in the PL intensity of free-standing polycrystalline films of MAPbI3. Taking into account the time scales of these partially reproducible variations of the PL emission, we infer the occurrence of two types of light-induced processes: (i) rapid ‘photo-bleaching’ at excitation at 470 nm due to the interband transitions within the split valence and conduction bands [1,2], and (ii) partially reversible structural transformation and degradation of polycrystalline deposits of MAPbI3 under excitation at wavelengths from 460 to 480 nm [3].

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