Microespectroscopia fluorescente hiperespectral aplicado ao estudo das propriedades ópticas de filmes finos de nanocristais de perovskita CsPbBr(x)I(3-x)

Abstract

Perovskite nanocrystals (ABX3) has shown to be an extremely promising material for applications in solar cells, optoelectronic devices, photocatalysts, humidity and temperature sensors, memory devices, optical power limiters and low-cost high-efficiency photodetectors. Such characteristics are due to its high quantum efficiency of fluorescence, allied to the ionic conduction of the charge carriers. However, both the Ion Diffusion mechanisms triggered by photoexcitation and the energy transfer between nanocrystals (NCs) are poorly understood. To overcome these problems, halide perovskite NCs with different chemical compositions have been proposed both from a theoretical and experimental point of view. In this context, this dissertation aimed to study some properties triggered by photoluminescence, in Thin Films of CsPbBrxI(3-x) perovskite nanocrystals, through the technique of Frequency Resolved Hyperspectral Fluorescence Microspectroscopy. This technique was implemented at the Optical and Photonic Spectroscopy Laboratory of the Federal University of Alfenas, for immediate use in this project. The first part of this dissertation consisted of setting up the experimental apparatus and their respective data interpretation/analysis programs. Initial tests were carried out with letters printed on Chamex paper, with a weight of 90 g/m², in order to test and adjust the settings for probing the samples. Once the entire optical system was developed and tested, the optical processes presented by the Thin Films of Perovskite nanocrystals CsPbBr3, CsPbBr2I and CsPbBrI2 were investigated. For the CsPbBr3 samples, an increase in fluorescence intensity was identified in relation to the initial fluorescence (24 to 70%) and a slight increase in the maximum wavelength, towards the red, associated with the emission peak. In this case, potency accelerated the kinetics of the displayed effects. CsPbBr2I samples exhibited conversion of iodide to bromide phases as the main feature of photoluminescence. With the beginning of irradiation, the I phase suffered an abrupt drop until stabilization, while the bromide phase started a significant growth, surpassing the initial fluorescence intensity presented by the iodide phase. It was identified that increasing the power accelerates the photoconversion effect. It turns out that, with irradiation, the I atoms, which were initially organized together with the Br atoms along the length of the material, are expelled from the crystal lattice, causing rich regions to exist in each halide phase. Therefore, the Linear Unmixing method was used to determine the contribution of each halide phase to the total fluorescence presented, where a region rich in the Br phase was perceived (central region); mixing region between the halide phases, forming a halo in the regions adjacent to the central region; and an I-phase-rich region external to the halo. The central region that presented domains in the bromide phase was quantified and a trend towards an increase in the diffusion length of the iodide phase was observed, as the irradiation power increased. Finally, the CsPbBrI2 samples showed photodegradation effect. Even for the lowest radiated power, the photobleaching phenomenon was immediate. Recovery in the dark was also observed after 30 minutes, showing a total reversibility of the process, for the power of 50 µW. It can be concluded that the implemented technique is effective, as it allowed the study of several phenomena triggered by photoluminescence, whose results are in agreement with the results presented in the Literature.