Excited state properties of novel solution processed inorganic and hybrid photovoltaic materials
Authors:   , Butkus Justinas 
Published: November 2021

<p>Novel solution processed inorganic and hybrid photovoltaic materials have shown great promise in low-cost energy market and advanced optoelectronic device applications. Materials like colloidal nanocrystals could potentially transform device manufacturing with printing or spraying based processing, however often the excited state properties of these emerging materials are not well understood. The lack of fundamental knowledge of charge carrier formation, trapping and relaxation processes obstructs the development of the materials and devices. Therefore we employ time-resolved femtosecond spectroscopy methods such as differential reflectance, diffusion, interferometry and primarily transient absorption spectroscopy to study the characteristics of a range of novel photovoltaic materials. Different materials and their aspects are analysed in the six data chapters of this thesis with first three chapters concerning inorganic colloidal nanocrystals and final three concentrating on the analysis of metal halide perovskite family in bulk and nanocrystal forms.  Iron pyrite nanocrystals, which tend to show poor performance in devices due to active surface states, are examined. In particular, the effects of novel surface passivation approach of coating iron pyrite nanoparticles with shells is analysed using transient absorption and long wavelength range steady state absorption spectroscopy techniques. The coated nanocrystals were found to display significant modulation of the surface state behaviour which was governed by the shell properties while the iron pyrite core remained photoactive. The analysis revealed the effectiveness of the new iron pyrite nanocrystal surface defect modulation approach which could potentially solve the issues associated with iron pyrite performance in devices.  Cu₂ZnSnS₄ (CZTS) is another novel semiconductor which suffers from trapping effects due to its tendency to form defects. We investigated CZTS nanocrystal properties using transient absorption along with standard steady state and integrating sphere absorption measurements. Rapid relaxation of band edge carriers to a broad defect distribution was found which limited CZTS carrier lifetimes. In addition, plasmon influence induced by CZTS nanocrystal defects was also resolved. The prominent sub-state state influence on the CZTS excited state dynamics revealed that the defects must be efficiently passivated or their formation must be suppressed for effective material performance.  Transient absorption spectroscopy along with steady state property analysis was also used to help characterize the properties of a range of silicon nanocrystals doped with different transition metals. Doping of nanoparticles can open new ways to control their properties but the field is relatively new and doped silicon nanoparticles have not been widely explored. By comparing undoped particle excited stated dynamics to doped nanocrystals we discovered rapid exciton transfer to new dopant introduced states which must be taken into account for the consideration of doped silicon nanocrystal applications.  Recently metal halide perovskites emerged as the leading novel photovoltaic material family and while a lot of research efforts have been put into bulk material, perovskite nanocrystals have been less widely researched. We analysed a range of CsPbBr₃ nanocrystals spanning different quantum confinement regimes in relation to the bulk material using transient absorption spectroscopy. Comprehensive analysis of different aspects of quantum confinement influence was carried out considering state focusing, bandgap renormalization, degeneracy and carrier cooling. The results highlighted that quantum confinement effects were mostly significant for the smallest CsPbBr₃ nanocrystals that are currently available (~4 nm) while the larger nanocrystal photophysics was better described as a small perturbation on the free carrier photophysics established for the bulk material.  The final two chapters concentrated on the photorefractive metal halide perovskite effects. Transient absorption and a range of differential reflectance measurements were employed to study anomalous metal halide perovskite TA response which obstructed the excited state resolution in previous literature. Here the anomalous features were isolated and attributed to the photorefractive effect and were further analysed using frequency domain interferometry. Photoinduced changes in the refractive index of the metal halide perovskite films and nanocrystals were measured with a femtosecond time resolution. The findings allowed for better understanding of metal halide perovskite excited state dynamics and accurate hot carrier temperature and effective mass determination as well as photoinduced refractive index characterization which is crucial for the design of such applications as optical switching, modulating and recording devices, probing techniques, lasers, single photon emitters and hot carrier solar cells.  Collectively, this thesis provides an insight into fundamental properties of a range of novel solution processed inorganic and hybrid photovoltaic materials.</p>