Photophysics of Perovskite Nano- and Microcrystals

The demand and consumption of energy is increasing dramatically all around the world. Broad adoption of fossil fuels has triggered an enormous threat to the environment. To ensure sustainability of our species and habitat, new solutions to fulfill the energy demand have to be found. Innovative materials offer the possibility to generate scalable renewable energy, which is efficient and environmentally friendly. In recent years, it was demonstrated that lead halide perovskite (LHP) materials prepared with superior optoelectronic properties which are desirable for solar-cell applications. Moreover, the solution processing ensures cheap production of those materials. In order to optimize and widely adopt LHPs as a key component for solar energy generators, it is important to understand the fundamental photophysical processes governing their unique behavior.
In this thesis we studied the photophysics of LHP nano- (NC) and microcrystals (MC) using spectroscopic methods. We investigated the photostability of NCs, and found the light irradiation induced particle aggregation. Furthermore, we revealed large two-photon absorption cross sections of these NCs, and we conclude that the two-photon absorption process populates the exciton band through a virtual state. Moreover, we have demonstrated the crucial role of size distribution in explaining the difference between the one-photon excited and two-photon excited photoluminescence (PL).
We fabricated photodetectors using micrometer-sized LHP crystals as building blocks, which show high responsivity as well as fast response in both the visible (one-photon absorption process) and the near infrared (NIR) region (two-photon absorption process). We also elucidated the underlying mechanism for the enhanced photoresponse by efficient charge collection, low trap density and high charge carrier mobility in the MCs.
In order to develop environmentally friendly materials, we replaced the lead element with less toxic bismuth in LHP structures to form the bismuth-based perovskite NCs, which exhibit tunable PL but with low quantum yield. The photophysical studies in such NCs with time-resolved spectroscopy attributed the low PL emission intensity to a fast trapping process.