An ultrafast spectroscopy study of excited state dynamics in perovskite semiconductors for optoelectronic applications
Authors: Wagner Isabella 
Published: June 2023

<p>Metal halide perovskites have shown remarkable performances in optoelectronic applications associated with their excellent properties such as a high optical absorption coefficient, long carrier diffusion length, high photoluminescence quantum yield and high defect tolerance. However, many photophysical mechanisms governing the function and device performance need to be studied in greater depth. This thesis investigates the photoexcitation dynamics in a range of metal halide perovskites using a novel ultrafast broadband photo-<br>
luminescence spectroscopy technique based on a transient grating method. With this novel approach, we conduct an in-depth study of several crucial topics.<br>
First, we examine the transport properties of lead-free inorganic double perovskites. We found that the sub-picosecond exciton self-localisation and a subsequent incoherent hopping process of these localised exciton-polarons dominate the carrier mobility of the material. The long lifetime in combination with polaron protected transport allows exciton diffusion exceeding 150 nm in this material. A doping strategy has been used to tune the double perovskite bandgap, making them not only more suitable for photovoltaic applications, but also increasing the mobility of the self-trapped excitons. However, an additional competing non-radiative recombination pathway has been identified to deteriorate the mobility boost, and a balance between enhanced absorption properties and non-radiative losses has yet to be maintained.<br>
The second study investigates the nature of the emissive species responsible for optical gain in strongly and weakly confined quasi-two-dimensional perovskite nanoparticles. For the highly confined case, attractive exciton-exciton interactions support optical gain from multi-exciton complexes, which are strongly coupled to lattice vibrations. This highly limits the biexciton stability, resulting in weak exciton binding energies and small optical gain coefficients. These results verify the absence of lasing reports from strongly confined perovskite nanoplatelets as described in the literature. It becomes apparent that strong electron-phonon coupling has not only beneficial effects.<br>
Next, we investigate the ultrafast lasing dynamics in mixed quasi-two-dimensional perovskite films by resolving their amplified spontaneous emission dynamics. A newly developed experimental scheme allows us to study singlet-triplet annihilation on an ultrafast timescale. We found that the triplet state accumulation highly depends on the dimensionality of the nanostructure and only occurs on strongly confined grains but has tremendous consequences for the bright singlet state carrier population, limiting the overall lasing performance. Based on these findings, knowledge about the triplet state properties is crucial to avoid their accumulation, and to optimise device performances. By using ultrafast photoluminescence spectroscopy to directly monitor the Anti-Stokes photoluminescence dynamics, the third and final study uses perovskite nanocrystals as a model system to identify the mechanism responsible for thermal photoluminescence up-conversion in perovskite semiconductors. The observations indicate that strong fluctuations of the perovskite crystal lattice dominated by optical phonons support the Urbach tail absorption process. These results provide a more profound understanding of the up-conversion mechanism in perovskite systems and demonstrate the capability of ultrafast broadband photoluminescence spectroscopy as a new approach to further investigate the influence of confinement effects or ligand properties on the up-conversion process.This thesis has given a more in-depth insight into the photophysics of perovskite semi-conductors, using ultrafast broadband photoluminescence spectroscopy as a valuable tool to selectively probe the emissive species. The results of this investigation provide new vital insights into the photoexcitation dynamics in perovskite semiconductors and guidelines for an efficient device operation and material design. </p>