UC Davis

Photoelectrochemical (PEC) and photocatalytic (PC) water splitting are potentially promising ways to achieve solar energy conversion with renewable hydrogen fuel to meet the growing global energy demand. However, solar to hydrogen efficiencies achieved in current systems are still far behind the target value to make the technology economically viable. In order to increase the efficiencies, developing narrow bandgap photocatalyst and deeper understanding of the photophysics is of great importance. Therefore, this dissertation focuses on study of photochemical charge separation and transport in copper gallium selenide and metal oxides semiconductor materials as a function of light intensity, photon energy and chemical environment. We hope the results from this study can promote the development of PEC and PC water splitting systems.Chapter 2 employs liquid surface photovoltage (SPV) measurements combined with open circuit potential (OCP) measurements to investigate quasi-Fermi level splitting (QFLS) in CuGa3Se5 thin film photocathodes. Studies with different electrolyte contacts show that Fermi level pinning 0.5 eV above the valence band is the cause for the voltage loss during photoelectrochemical water and methyl viologen reduction. The effect of back contacts and CdS surface passivation layer on the QFLS of CuGa3Se5 thin film are also discussed. This work demonstrates a new approach to obtaining absolute minority carrier potentials in semiconductor/liquid junctions and identifying charge selective contacts and passivation layers. Chapter 3 examines copper gallium selenide (CGSe) as a particulate photocatalyst for proton reduction. P-type CGSe particles were synthesized via solid-state method. With cocatalyst and sacrificial donor, CuGa3Se5 showed only mild activity for hydrogen evolution. Surface photovoltage spectroscopy (SPS) is used to monitor photochemical charge separation and transport in these materials for the first time. The early photovoltage onset in SPS confirms interface/surface states in CuGa3Se5 particle films. SPS is also applied to study the effect of substrate/back contacts and electron transport layers on photochemical charge separation. A molybdenum back contact induced better charge carrier separation than fluorine-doped tin oxide (FTO). Nickel and TiO2 were identified as electron selective contacts. Chapter 4 employs surface photovoltage spectroscopy (SPS) and photoelectrochemical (PEC) methods to study the photochemistry of the metavanadates (CuV2O6, ZnV2O6 and Zn4V2O9). SPS verifies the existence of sub bandgap states of V4+ in the vanadates and their negative effect on the photovoltage and the PEC performance. Additionally, photochemical charge separation and the role of the space charge region (SCR) in CaFe2O4 particle films are investigated.