Nabij-infrarode luminescentie van diepe defecten in CsPbBr3 perovskiet

Metal halide perovskites (MHPs) have become immensely attractive semiconductor materials in the past years, with exciting applications in photovoltaics, light emitting diodes (LEDs) and photodetectors. Their popularity results from the combination of their easy, low-cost, solution-based processability with favorable optoelectronic properties, including high absorption coefficients, long carrier diffusion lengths and tunable bandgaps. The charge carrier dynamics of MHPs involve many different relaxation processes, both radiative and non-radiative. Achieving a complete picture of the photophysical pathways is a necessity to fully comprehend the behavior of these materials, and to optimize their performance in next-generation devices. In this context, this thesis studies the near-infrared (NIR) luminescence generated by all-inorganic CsPbBr3 perovskite microcrystals. The fundamental origin of this low-energy luminescence is elucidated in the first experimental chapter of this work. Results from several steady-state spectroscopy techniques demonstrate that a fraction of the excited states in CsPbBr3 and CsPb(Br/Cl)3 microcrystals decay through radiative transition between intrinsic deep electron and deep hole traps. Additionally, time-resolved luminescence spectroscopy experiments reveal that the picosecond-nanosecond excited state dynamics of CsPbBr3 is governed by a rise time component of 660 ps, corresponding to the population of the deep traps responsible for NIR emission. Subsequently, the effect of crystal size on the NIR emission is investigated. While MHP nanocrystals (NCs) are typically strongly fluorescent in the visible range, with quantum yields close to unity, they do not show NIR emission. On the other hand, larger (sub)-micrometer scale crystals show only weak visible-range emission, although NIR emission can be measured. This can be rationalized by considering that the formation energies of the deep defects are calculated to be very high and that the concentrations of these defects can be expected to be very low. When combining data on the excitation intensity dependence of the VIS and NIR emission with literature data on the SRH recombination in methylammonium lead iodide (MAPbI3) perovskite, densities in the neighborhood of 10^15 cm^-3 can be estimated for the donor-acceptor defect pairs involved in the NIR emission. The NIR emission of CsPbBr3 microcrystals extends over a broad wavelength range, between 700 nm and 1600 nm. This opens up possibilities to employ this material as a light source in commercially relevant applications. By means of proof-of-concept devices, the suitability of the CsPbBr3 NIR emission for chemical analysis, both qualitative and quantitative, as well as for sensitive biological imaging, is examined. Over time, the intensity of the NIR luminescence from individual CsPbBr3 microcrystals fluctuates, even under constant excitation intensity. This NIR photoluminescence intermittency effect is however much weaker and occurs on a longer time scale compared to the well-known blinking of the visible-range emission of MHP nanoparticles. Preliminary experimental results concerning this intriguing phenomenon are included in this work. To summarize, the overarching goal of this PhD thesis is to investigate the fundamental photophysical processes that take place after photoexcitation of CsPbBr3 perovskite microcrystals, which can contribute to developing and improving the efficiency of technologies such as LEDs, photovoltaics, and solid-state phosphors.