Theoretical Studies of Atomic Transport in Ternary Semiconductor Quantum Dots and Charge Transport in Organic Photovoltaic Active Layers

Abstract

Ternary semiconductor quantum dots with thermodynamically stable structures are particularly important for achieving optimal performance in optoelectronic and photovoltaic applications. Ternary quantum dots (TQDs) are typically synthesized in the form of core/shell structures. However, misfit strain induced by the abrupt core/shell interface can change the nature of the TQDs dramatically, leading to unstable optoelectronic function. In this thesis, a transient species transport model is developed to predict species distributions in TQDs during their thermal annealing. Specifically, the interdiffusion kinetics is analyzed of group-VI species in ZnSe1-xSx and ZnSe1-xTex TQDs and of group-III species in InxGa1-xAs TQDs. The modeling results are used to interpret the evolution of near-surface species concentration during thermal annealing and predict the equilibrium species distribution as a function of TQD size and composition. A database of constituent species transport properties is generated for further design of post-growth processes that enables the development of thermodynamically stable TQD structures with optimal optoelectronic function grown through simple one-step colloidal synthesis techniques. Nanoparticle assemblies of organic semiconducting materials are particularly appealing for next-generation organic photovoltaic (OPV) devices because their low-cost aqueous synthesis reduces the usage of chlorinated solvents. Another class of novel semiconducting materials, organometallic halide perovskites, have emerged as promising materials for solar cells because of their high photo-absorption coefficient and high power conversion efficiency (PCE). Based on deterministic charge carrier transport models, this thesis presents a computational analysis of charge transport in photovoltaic devices with active layers of the above two types of materials and develops design protocols for improving photovoltaic device efficiency. Our results demonstrate that charge transport efficiencies in centrifuged organic nanoparticle assemblies are comparable with those in drop cast thin films. The effects on charge transport of excess stabilizing surfactant molecules and dispersion of insulating nanoparticles in the assemblies have been analyzed. The simulation results accurately reproduce experimental data and provide interpretations for the observed effects of the active layer nanostructure, i.e., nanoparticle size, ratio, and internal morphology, on charge transport and device PCE. Furthermore, the charge generation rate in the active layer is maximized and the device’s photovoltaic performance is optimized with respect to the OPV device parameters. For photovoltaic devices based on organometallic halide perovskites, the modeling results demonstrate quantitatively that incorporation of multi-walled carbon nanotubes (MWCNTs) into the perovksite layer reduces bimolecular recombination, thus increasing the device’s PCE. In addition, we find that electronic band offsets play an important role in determining the effects on device performance of the charge carrier mobilities and of majority doping in the electron and hole transporting layers (ETLs and HTLs). The modeling results provide guidelines for designing hybrid perovskite photovoltaic devices with enhanced photovoltaic performance.