Light emission properties of inorganic lead-halide perovskites
We synthesize a variety of Cs-based perovskite nanocrystals (PNCs) and single crystal materials and study their excitonic/optical properties. Like their organic cousins (i.e. methylammonuim, MAPbI3 and the like), Cs-based perovskites have strong light absorption in the visible range. They, however, are more photo- and environmentally stable due to the presence of inorganic Cs cations. Cs-based PNCs possess alluring optical and electronic properties via compositional and structural versatility, tunable bandgap, high photoluminescence (PL) quantum yield (QY) and facile chemical synthesis. Perovskite structures can be conveniently depicted as comprised of lead halide octahedra [PbX6, X=Cl; Br or I] surrounded with Cs cations, residing in the voids between them and arranged with varying degrees of interconnections. Lower dimensionality polymorphs can be formed by manipulation of chemical synthesis conditions where Cs+ can stabilize 3D [PbX6] framework, resulting in 2D (nanosheet), 1D (nanowire) and 0D (nanodot) internal octahedra arrays within the bulk of the perovskite. The gamut of available experimental approaches is further expanded in colloidal perovskite NCs where both external size quantization and internal 0D structure may combine to achieve “multidimensional” electronic properties that are engineered both on atomic scale and nanoscale.
Laser and LED applications of CsPbX3 materials require detailed knowledge of the dynamics of mode propagation and spectral redistribution of the emission in the waveguiding structures. Repeated incoherent absorption and re-emission of photons that is called photon recycling in strongly absorbing media is expected to substantially affect carrier recombination dynamics and mode propagation, influencing quantum efficiency and light dynamics measurements. Using high quality single crystalline CsPbBr3 materials shaped in the form of microwires, we studied the effects of photon recycling and light waveguiding. Our results show propagation of the PL emission within individual microwires to distances in excess of a 100 µm from the excitation location. We found that PL spectral content and PL kinetics strongly depend on the propagation distance along the microwire and that the internal quantum efficiency of these materials is close to unity. (ACS Energy Letters 3, 1492-1498 (2018)) We are continuing work in this area to better understand photon recycling in other materials, including in the low-dimensional (1D and 2D) perovskite structures.
To fully exploit the potential of perovskites NCs and widely expand their light-conversion applications, one would need to obtain the precise knowledge of the origin and properties of their photoluminescence. Much higher density of point defects typically characterizes Cs-based perovskites materials with its structural lability and much lesser degree of chemical stability. Perhaps more intriguing, they show substantially smaller (as compared to chalcogenide QDs) PL spectral shift as a function of their size. Nevertheless, many perovskite NCs are bright emitters, with PL QY close to unity. We study emission statistics in 3D (CsPbBr3) and 0D (Cs4PbBr6) perovskite NCs by applying ultrasensitive single photon detection methods. We observed, contrary to conventional chalcogenide QDs, that an increase in excitation power induced the appearance of burst-like emission behavior with uniform distribution of PL lifetime values in 0D PNCs. Overall, the experimental data provide a compelling evidence that emission from both types of PNCs closely resembles that of the individual single molecules and is much less dependent on the external confinement factors. (Nat. Comm. 10, 2930 (2019))