Laurent Coolen

Core-crown CdSe/CdS nanoplatelets combine improved quantum yield with promising optoelectronic performance, yet their emission dynamics remain governed by reversible charge trapping. By correlating single-particle luminescence decays with intensity fluctuations (blinking/flickering), we uncover a distinct emission regime at intermediate brightness levels where delayed luminescence arises. This behavior reflects a reversible trapping process that reduces light emission and introduces a secondary, slow decay channel. Time-resolved analysis reveals stochastic switching between active and inactive reversible trapping within an individual emitter. Under higher excitation powers, these reversible traps are supplanted by stronger nonradiative quenching pathways.

Obtaining high‐performance films of organo‐metallic halide perovskites is still a challenging task, with tremendous potential outcomes for devices involving light absorption such as next‐generation photovoltaics or photodetectors. In many experimental reports, particularly on perovskite solar cells, the addition of metallic nanoparticles (gold, silver…) has demonstrated promising performance improvements. However, while light management strategies based on plasmonic resonances are the initial motivation for these experiments, various other explanations have been proposed and the plasmonic nature of the performance boost is not always clear. In this article, optical simulation analysis is combined with a general review of the experimental reports to elucidate the role of nanoparticles in perovskite devices from a multifaceted perspective. Performance improvements were recently reported for various devices (solar cells, photodetectors) of different perovskite materials where gold nanoparticles were introduced either by spin coating or evaporation. Alongside a comprehensive examination of conventional optical effects, a novel function is identified of nanoparticles in regulating the crystallization rate of perovskite films, leading to enhanced film quality and ultimately boosting light absorption and device optical performance. This analysis enriches the mechanistic understanding for future studies on the use of nanoparticles in perovskite‐based devices, offering a novel approach for optimizing perovskite films.

Numerous experiments have demonstrated improvements on the efficiency of perovskite solar cells by introducing plasmonic nanoparticles, however, the underlying mechanisms are still not clear: the particles may enhance light absorption and scattering, as well as charge separation and transfer, or the perovskite's crystalline quality. Eventually, it can still be debated whether unambiguous plasmonic increase of light absorption has indeed been achieved. Here, various optical models are employed to provide a physical understanding of the relevant parameters in plasmonic perovskite cells and the conditions under which light absorption may be enhanced by plasmonic mechanisms. By applying the recent generalized Mie theory to gold nanospheres in perovskite, it is shown that their plasmon resonance is conveniently located in the 650–800 nm wavelength range, where absorption enhancement is most needed. It is evaluated for which active layer thickness and nanoparticle concentration a significant enhancement can be expected. Finally, the experimental literature on plasmonic perovskite solar cells is analyzed in light of this theoretical description. It is estimated that only a tiny portion of these reports can be associated with light absorption and point out the importance of reporting the perovskite thickness and nanoparticle concentration in order to assess the presence of plasmonic effects.

An extensive analysis of biexciton luminescence in high-quality, large perovskite CsPbBr₃ nanocrystals shows how the biexciton Auger decay rate deviates from the “universal” volume scaling as the exciton confinement becomes weaker.

Self-assembled linear chains of CdSe nanoplatelets areknown toexhibit highly efficient Fo''rster resonant energy transfer (FRET)leading to fast exciton diffusion between platelets. Here, we comparethe luminescence decay dynamics of single nanoplatelets, clustersof a few platelets, and self-assembled chains. As the number of stackedplatelets is increased, we show that the luminescence decay becomesfaster, which can be interpreted as the FRET-mediated effect of quenchers:excitons may diffuse to nearby quenchers so that their decay rateis increased. On the other hand, a minor slow decay component is alsoobserved for single platelets, corresponding to trapping-detrappingmechanisms in nearby trap states. The contribution of the slow componentis enhanced for the platelet chains. This is consistent with a FRET-mediatedtrapping mechanism where the excitons would diffuse from plateletto platelet until they reach a trap state. Finally, we develop toymodels for the FRET-mediated quenching and trapping effects on thedecay curves and analyze the relevant parameters.

Probing the transition dipoles responsible for the luminescence of a nanoemitter is essential to understanding its physical properties, its interactions with its environment, and its potential applications. Various methods in photoluminescence microscopy, based on polarimetry or Fourier imaging, have been developed to measure an emitter’s dipole properties: the number of radiating dipoles, the oscillator strength ratio between them, and their orientation. In this article, we model the most used of these protocols and show that their sensitivity depends crucially on the experimental conditions: substrate material, presence of another lower or upper layer, and objective numerical aperture. We develop guidelines to optimize the measurement sensitivity by tailoring the experimental conditions, depending on the type of protocol used and the dipole property to be measured.

The 3D orientation of a single gold nanoparticle is probed experimentally by light scattering polarimetry. We choose high-quality gold bipyramids (AuBPs) that support around 700 nm a well-defined narrow longitudinal localized surface plasmonic resonance (LSPR) which can be considered as a linear radiating dipole. A specific spectroscopic dark-field technique was used to control the collection angles of the scattered light. The in-plane as well as the out-of-plane angles are determined by analyzing the polarization of the scattered radiation. The data are compared with a previously developed model where the environment and the angular collection both play crucial roles. We show that most of the single AuBPs present an out-of-plane orientation consistent with their geometry. Finally, the fundamental role of the collection angles on the determination of the orientation is investigated for the first time. Several features are then deduced: we validate the choice of the analytical 1D model, an accurate 3D orientation is obtained, and the critical contribution of the evanescent waves is highlighted.