Effects of Charge Transport and Heterogeneous Charge Transfer on the Operation of Inorganic Semiconductor Light-Harvesting Systems.

Abstract

This dissertation quantitatively details the operational features of five separate light-harvesting systems based on crystalline inorganic semiconductors. Specifically, charge transport and heterogeneous charge transfer are characterized in silicon (Si) and gallium phosphide (GaP) light-harvesting systems via both experiments and simulations. The goal of this work is to provide a quantitative framework to facilitate the design of efficient, scalable solar energy conversion systems.

Nanostructured, high-aspect-ratio semiconductors are attractive materials for light-harvesting systems due in part to their ability to decouple light absorption and carrier collection to minimize bulk recombination in low-purity materials. For n-type GaP photoelectrodes featuring short minority carrier diffusion lengths, high-aspect-ratio structuring leads to an order of magnitude increase in energy conversion efficiency for sufficiently thick macroporous films as compared to planar photoelectrodes. The design of most high-performance nanostructured devices has been elusive due to a lack of detailed information on their design and operation. Chapter III provides quantitative guidelines for the design of such systems via a finite-element simulation analysis that focuses on how charge transport and recombination affect the performance of nanowire photoelectrodes featuring various radii, dopant densities, defect densities, surface recombination velocities, nanowire tapering, and doping uniformity. Notably, a novel discrete-contact nanowire scheme featuring high open-circuit potentials is shown to significantly outperform analogous devices featuring conformal Schottky contacts over a broad range of these parameters. Charge transfer/transport is also investigated in two systems featuring inorganic semiconductors in contact with an organic polymer or chromophore. Specifically, the low electron collection velocity at n-Si/PEDOT:PSS interfaces is shown to mitigate carrier loss at heterojunctions, leading to increased open-circuit potentials and short-wavelength internal quantum yields as compared to n-Si/Au devices. Charge injection and collection are also characterized in planar dye-sensitized p-GaP photoelectrodes featuring strong electric fields within the depletion regions of the semiconductors. These fields effectively sweep holes away from interfacial recombination centers and facilitate large internal quantum yields even in systems with relatively poor kinetics for carrier collection. Taken together, the results in this dissertation provide clear design requirements for high-performance light-harvesting systems and highlight the importance of understanding and controlling charge transport/transfer in semiconductor devices.