ABSTRACT
Strong coupling between lead halide perovskite materials and optical resonators enables both polaritonic control of the photophysical properties of these emerging semiconductors and the observation of fundamental physical phenomena. However, the difficulty in achieving optical-quality perovskite quantum dot (PQD) films showing well-defined excitonic transitions has prevented the study of strong light-matter coupling in these materials, central to the field of optoelectronics. Herein we demonstrate the formation at room temperature of multiple cavity exciton-polaritons in metallic resonators embedding highly transparent Cesium Lead Bromide quantum dot (CsPbBr3-QD) solids, revealed by a significant reconfiguration of the absorption and emission properties of the system. Our results indicate that the effects of biexciton interaction or large polaron formation, frequently invoked to explain the properties of PQDs, are seemingly absent or compensated by other more conspicuous effects in the CsPbBr3-QD optical cavity. We observe that strong coupling enables a significant reduction of the photoemission line width, as well as the ultrafast modulation of the optical absorption, controllable by means of the excitation fluence. We find that the interplay of the polariton states with the large dark state reservoir plays a decisive role in determining the dynamics of the emission and transient absorption properties of the hybridized light-quantum dot solid system. Our results should serve as the basis for future investigations of PQD solids as polaritonic materials.
ABSTRACT
Quantum-dot (QD) solids are being widely exploited as a solution-processable technology to develop photovoltaic, light-emission, and photodetection devices. Charge transport in these materials is the result of a compromise between confinement at the individual QD level and electronic coupling among the different nanocrystals in the ensemble. While this is commonly achieved by ligand engineering in colloidal-based systems, ligand-free QD assemblies have recently emerged as an exciting alternative where nanostructures can be directly grown into porous matrices with optical quality as well as control over their connectivity and, hence, charge transport properties. In this context, we present a complete photophysical study comprising fluence- and temperature-dependent time-resolved spectroscopy to study carrier dynamics in ligand-free QD networks with gradually varying degrees of interconnectivity, which we achieve by changing the average distance between the QDs. Analysis of the photoluminescence and absorption properties of the QD assemblies, involving both static and time-resolved measurements, allows us to identify the weight of the different recombination mechanisms, both radiative and nonradiative, as a function of QD connectivity. We propose a picture where carrier diffusion, which is needed for any optoelectronic application and implies interparticle transport, gives rise to the exposure of carriers to a larger defect landscape than in the case of isolated QDs. The use of a broad range of fluences permits extracting valuable information for applications demanding either low- or high-carrier-injection levels and highlighting the relevance of a judicious design to balance recombination and diffusion.
ABSTRACT
Mixed-halide perovskites represent a particularly relevant case within the family of lead-halide perovskites. Beyond their technological relevance for a variety of optoelectronic devices, photoinduced structural changes characteristic of this type of material lead to extreme photophysical changes that are currently the subject of intense debate. Herein we show that the conspicuous photoinduced phase segregation characteristic of these materials is primarily the result of the local and metastable rearrangement of the iodide sublattice. A local photophysical study comprising spectrally resolved laser scanning confocal microscopy is employed to find a correlation between the defect density and the dynamics of photoinduced changes, which extend far from the illuminated region. We observe that iodide-rich regions evolve much faster from highly defective regions. Also, by altering the material composition, we find evidence for the interplay between the iodide-related defect distribution and the intra- and interdomain migration dynamics giving rise to the complexity of this process.