Photoluminescence (PL) of organometal halide perovskite has been broadly investigated as a fundamental signal to understand the photophysics of these materials. Complicated PL behaviors have been reported reflecting complex mechanisms including effects from crystal defects/traps whose nature still remains unclear. Here in this work we observed, besides the PL enhancement, a surprising PL decline phenomenon in methylammonium lead triiodide (CH3NH3PbI3) perovskite showing a high initial PL intensity followed by a fast decline in time scale of milliseconds to seconds. The similarity between the PL enhancement and PL decline suggests both processes are due to PL quenching traps in the material. Combining experimental and theoretical results, two interstitial defects of iodide and lead were identified to be responsible for the PL enhancement and PL decline, respectively. Both traps can be switched between active and inactive states, leading to a reversible process of PL enhancement and PL decline. The identification of  the chemical nature of the PL quenching traps is an important step toward fully understanding the crystals defects in these materials.

Photoluminescence (PL) of CH3NH3PbI3 perovskites depends strongly on sample preparation, atmosphere, crystal size, and so forth. However, the origin of these dependencies is always misunderstood because of the co-works of many different factors. Herein, we prepared hexagonal-shaped single crystals with tens of micrometers in size and observed a red-shifted PL emission (800–830 nm) mainly from the crystal edges besides the usual band-to-band transition (760 nm) from the central regions. Also, significantly different time-resolved dynamics and excitation power dependencies were observed. To summarize, we conclude that the red-shifted component comes from the depth of the crystal, where monomolecular recombination occurs involving photogenerated charges and unintentional doped charges, while the normal PL is emitted by bimolecular recombination from the surface layers. These results showed the significance of pure optical effects in perovskite crystals and would promote detailed understanding of the charge dynamics and recombination in perovskite crystalline materials of different geometries and sizes.

Summary The emergence of all-inorganic halide perovskites has shown great potential in photovoltaic and optoelectronic devices. However, the photo-induced phase segregation in lead mixed-halide perovskites has severely limited their application. Herein, by real-time monitoring the photoluminescence (PL) spectra of metal mixed-halide perovskites under light irradiation, we found that the photo-induced phase transition can be significantly inhibited by B-site doping. For pristine mixed-halide perovskites, an intermediate phase of CsPbBr x I 3-x can only be stabilized under low excitation power. After introducing Sn 2+ ions, such intermediate phase can be stabilized in nitrogen atmosphere under high excitation power and phase segregation can be started after the exposure in oxygen due to oxidization of Sn 2+ . Replacing Sn 2+ by Mn 2+ can further improve the intermediate phase's tolerance to oxygen proving that B-site doping in perovskites structure by Sn 2+ or Mn 2+ could effectively minimize the light-induced phase segregation and promote them to serve as promising candidates in photovoltaic and light-emitting devices.

Two-dimensional (2D) perovskites are attracting broad attention for their stability and wavelength tunability. However, random crystallization of sample preparation makes it difficult to obtain 2D perovskites with pure structure, especially when the number of layers is large. Herein, we prepared 2D perovskite (C8H17NH3)2(MA) n−1Pb n I3n+1 with different layers (n = 1–10). For the first time, we experimentally identified the band gap energy E g of 2D perovskite (C8H17NH3)2(MA) n−1Pb n I3n+1 with layers up to 10 by investigating specific pieces of crystal with pure emission spectra using fluorescence microscopy. Intriguingly, the relationship between E g and n perfectly fits an exponential function rather than the pure quantum confinement effect in good agreement with the theoretical calculation based on first principles. Our results suggest that the band gap of the 2D perovskite is determined not only by quantum confinement effect, but other factors including chemical components also give significant contribution.