We investigate the local changes in photophysics at different microregions of a methylammonium lead bromide (MAPbBr 3 ) perovskite crystal under illumination. Our results show that the emission from the structurally homogeneous region is blue-shifted compared to the emission from the inhomogeneous regions. The yield and spectrum of the emission from the structurally homogeneous region do not vary with the illumination time, whereas distinct light-induced changes are seen in the spectra from the inhomogeneous region. The changes in the spectra at long illumination time suggest that ionmigration inhibits the emission from the inhomogeneous regions. The measurements of the emission lifetime suggest that the emission from the inhomogeneous regions is dominated by the defect-related emission at short illumination times and the band-to-band emission at the longer illumination times. Our work provides direct evidence for the light-induced healing of the defect centers, which is important in the design of photoactive devices of MAPbBr 3 .

Photoluminescence (PL) spectra from thin films (TFs) and bulk crystals (BCs) of hybrid organo-halide perovskites are significantly different, the origin of which and their impact on the efficiency of the perovskite-based photoactive devices have been debated. We have used two-photon PL to study the temperature-dependent changes in the spectra of the TFs and the BCs of methylammonium lead bromide (MAPbBr3) perovskites in order to clarify the origin of the differences. Our results show that the differences in the spectra are due to the variation in the phase composition. At room temperature, the tetragonal (TE) phase is dominant in the BCs, while the orthorhombic (OR) phase is dominant in the TFs. The PL spectra of the TFs also show discernible contributions from the TE and the cubic phases. At lower temperatures, the increase in excitonic recombination causes a red shift of the PL spectra from the TFs, while a phase transition from the TE to the OR phase results in a blue shift of the PL from the BCs. The temperature-dependent narrowing of the PL linewidths shows a stronger coupling between the longitudinal optical phonons and the free carriers in the OR phase as compared to the TE phase, implying a reduced carrier mobility. However, as the OR phase is metastable at the room temperature, the slow phase transition to the TE phase should improve the photocurrent yield in the TFs, provided that the sample is shielded from other types of degradation.

Heterostructures of zero-dimensional/two-dimensional (0D/2D) materials, especially quantum dots (QDs)/nanosheets (NSs), have attracted significant attention for extracting photogenerated electrons and holes. Herein, we report the dissociation of excitons at the heterojunction of CdSe (cadmium selenide) QDs and MoS 2 (molybdenum disulfide) nanosheet utilizing steady-state and time-resolved spectroscopic techniques. Quasi type II semiconductor-like band energy alignment of the 0D/2D heterojunction facilitates exciton breaking via hole transfer from the QD to MoS 2 . Furthermore, we demonstrate the extraction of two holes from doubly excited QDs (created via high-power excitation) following the dissociation of a biexciton at the 0D/2D interface. This work is expected to provide a new approach of exploiting multiple exciton generation in quantum dot-sensitized solar cells by harvesting multiple carriers.

Two-dimensional (2D) perovskites with alternating cations in the interlayer space (ACI) represent a new type of structure with different physical properties compared to the more common Ruddlesden-Popper (RP) counterparts. Still, there is a lack of understanding of photophysical and optoelectronic properties. In our work, we have used transient absorption spectroscopy to get better insight into the nature of photoexcitations in ACI perovskites with varying layers. We observed that multiple exciton recombination predominates, compared to self-trapping of excitons, with increasing the number of layers (expressed by n). With decreasing n, the ACI perovskite shows less many-body exciton recombination due to a very fast self-trapping of the excitons. Furthermore, we observed that the optical anisotropy increases with increasing number of the inorganic layers as the polarization orientation increases.Comparing the photophysical properties of the three different systems, we suggest that ACI3 is most promising for photovoltaic and optical polarization devices.

Quasi-two-dimensional (2D) perovskites have attracted extraordinary attention for next-generation lighting and displays because of their high color purity and performance. Here, we present the results of an investigation of the distribution of nphases and their impact on photophysical properties of quasi-2D methylammonium lead bromide perovskite film (q-MPB) where n represents the number of PbBr 4 octahedral layers. We find that the emission from the low-n-phase region is blue-shifted compared to the emission from the medium-n-phase and high-n-phase regions. The yield and lifetime of the emission are higher for the medium-n-phase region due to the higher quasi-2D nature of the particles. Temperature-dependent two-photon photoluminescence measurements show that exciton−phonon interaction is stronger for the low-nphase region, and it decreases when the n-phases increase. Our work demonstrates the impact of spatial heterogeneity of n-phases on light-emission properties, which is of considerable importance for the development of highly efficient next-generation q-MPBbased LEDs.