Solar energy conversion offers a carbon free and renewable alternative to fossil fuel consumption. Current materials still fall short of the target solar-to-hydrogen efficiency needed to make the technology economically viable. In order to increase the efficiencies, a deeper understanding of charge generation, transfer and recombination, to further improve their photocatalytic activities. Within this dissertation n-type gallium phosphide is investigated for photocatalytic hydrogen production. P-type gallium phosphide and carbon nitride are investigated for photoelectrochemical solar energy conversion. Characterization of these materials consists of surface photovoltage spectroscopy, optical spectroscopy, electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.Chapter 1 provides an introduction to the topic and important measurements. Chapter 2 investigates n-type gallium phosphide as a photocatalyst for hydrogen production, looking at defect states, space charge layer effects on the charge generation, cocatalyst effect on charge transfer and recombination processes to optimize activity for solar water splitting. A quantum efficiency of 14.8 % was achieved by 4 % (w/w) dinickel phosphide nanoparticles loaded onto gallium phosphide microparticles in an aqueous solution of 0.3 M sodium sulfide and 0.3 M sodium sulfite under 525 nm LED illumination. Chapter 3 reveals how the photovoltage and photocurrent of p-type gallium phosphide photoelectrodes is affected by a cadmium sulfide passivation layer, added platinum cocatalyst, altered electrolyte composition, and added hydrogen or oxygen. The champion photocathode drives hydrogen evolution with a quantum efficiency of 62 % at 0.0 V RHE and an open circuit photovoltage of 0.43 V at 250 mW/cm2 (400 nm). Finally, Chapter 4 explores the photovoltage of carbon nitride films prepared using different fabrication techniques and in different electrolytes. The best performance was achieved by a photoanode prepared via doctor blading in 0.1 M KOH with O2 purging. This film reached 200 µA/cm2 photocurrent at 1.23 V vs RHE and 1 sun illumination and a photovoltage of 1.1 V under 79 mW/cm2 illumination (405 nm).

Solar water splitting has gained increasing attention as a sustainable approach for producing clean hydrogen fuel. However, achieving the theoretical energy conversion efficiency of photocatalyst materials remains challenging due to limiting control over carrier separation in semiconductor photocatalysts. This dissertation explores doping, nanostructuring, and surface states as key factors affecting carrier separation, providing insights into the relationship between electronic property and performance. Semiconducting photocatalysts were prepared via solid state reaction, sol-gel reaction, electrodeposition, or sourced commercially. Their structural and optoelectronic properties were characterized using electron microscopies, X-ray diffraction (XRD). UV-Vis spectroscopy, surface photovoltage spectroscopy (SPS), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (EPR) spectroscopy. Photocatalytic performance was evaluated by photocatalytic oxygen evolution and photoelectrochemical (PEC) measurements with different redox active species.In Chapter 2, we explored p-type aliovalent doping strategies in BiVO4 hoping to optimize carrier separation and enhance photocurrent without compromising photovoltage, using microcrystal BiVO4 and nanoporous BiVO4 films as model systems. In the microcrystal system, monoclinic BiVO4 microparticles were synthesized through a room-temperature method with in situ Sr doping using Sr(NO3)2. The synergistic effect of 5 % Sr doping and Ar annealing led to a photocurrent density of 260 μA/cm2 at 1.23 V vs. RHE for sulfite oxidation under intensified Xe irradiation, representing 5.7-fold enhancement over pristine BiVO4. However, this enhancement diminished under 1-Sun illumination. Ar-annealed BiVO4 microparticles exhibited a visible-light-driven oxygen evolution rate of 202 μmol/h under 550 mW/cm2 irradiation, but Sr doping provided no further improvement. Sr was not detected in BiVO4:Sr by EDX or XPS, likely due to inefficient dopant incorporation at low temperature. In the nanoporous system, p-type aliovalent doping with Ca, Sr, or Ti was attempted at 500°C using electrodeposited BiVO4 films. Ca doping led to limited photocurrent enhancement from 1.56 to 1.73 mA/cm2 for sulfite oxidation under 1-Sun illumination. While Sr doping and Ti doping consistently reduced photocurrent. EPR analysis confirmed the absence of detectable V4+ in pristine BiVO4. N,p co-doping with 0.5 % Mo and Ca/Ti was also explored, achieving a photocurrent density of 2.94 mA/cm2 with Mo doping alone, but no additional enhancement with Ca or Ti. In Chapter 3, we demonstrate the synthesis, characterization, and the corresponding photocatalytic application of single crystal BiVO4 nanowires. In this study, single-crystal BiVO4 nanowires were synthesized for the first time by recrystallization of BiVO4 microparticles from NaVO3 flux at 700°C – 550°C, exhibiting an average thickness of 433.4 ± 110.6 nm with lengths exceeding 20 μm. XRD and electron microscopy confirmed their single crystallinity with a monoclinic Scheelite structure, growing along the [010] direction. With a band gap of 2.41 eV, the nanowires demonstrate photocatalytic activity under visible light (390 mW/cm²), achieving a water oxidation rate of 28.75 µmol/h and an apparent quantum efficiency of 0.44 % at 405 nm. Photolabeling experiments with silver and manganese 2+ ions demonstrate that both photoholes and electrons are extracted along the cylindrical nanowire surface. The absence of a facet-induced charge separation mechanism explains the relatively low photocatalytic activity of the nanowires. Despite this, the unique morphology of nanowire BiVO4 holds significant potential for applications in sustainable energy conversion and photocatalysis. In Chapter 4, we demonstrate the application of X-ray photoelectron spectroscopy (XPS) on electronic property characterization in semiconductor photocatalysts and their devices in three systems. In Chapter 4.1, the defect chemistry of single crystal SrTiO3 (111) annealed in H2 at 1050 ℃ with electric polarization was investigated using XPS. After H2 treatment, a Fermi level shift of 0.58 eV towards a more reducing level was observed, indicated by consistent shifts in the Sr, Ti, and O elemental peaks in XPS. The increased abundance of surface -OH signals in O 1s spectra confirms the successful incorporation of oxygen vacancies. And the negligible change in Sr, Ti, O signals after both reverse and forward polarizations rules out electron segregation as the cause of local polarization. In Chapter 4.2, XPS was employed with SPS on as-purchased n-GaP to investigate the chemical origin of an unknown surface defect state located 1.3 eV below its conduction band. Two separate surface treatments, air oxidation at 600°C and surface coating with a metallic Ga layer, were conducted on the bare n-GaP surface for XPS analysis to decouple the contributions of surface oxides and metallic Ga. A metallic Ga(0) shoulder peak, with an 1.98 eV energy difference from the major peak, was detected on the as-received n-GaP surface and identified as the source of the 1.3 eV defect level. These findings provide critical insights for surface treatments in GaP-based devices. In Chapter 4.3, detailed XPS analysis was performed on various substrate/Bi2S3 photoanodes to obtain surface chemistry information and the valence band edge position. From the measured 0.92 eV valence band onset and other optoelectronic data, a Schottky-junction model was derived to successfully explains the substrate-dependent photocurrent trends observed in the photoanodes when different conductive substrates were employed.

Transition metal oxide (TMO) semiconductors are abundant and inexpensive materials with remarkable electrical and chemical properties suitable for solar energy conversion. Solar power has the potential to deliver energy that is clean, renewable, and sustainable provided that it can be harvested efficiently. Semiconducting TMOs are extensively developed and utilized for this purpose, particularly in the construction of energy-harvesting solar devices. This research focuses on the thin film fabrication and utilization of TMO semiconductors namely, Cu2O, BiVO4, ZnO, TiO2, SnO2, and NiOx for photoelectrochemical (PEC) water splitting and photovoltaic (PV) devices. Chapter I provides a brief introduction to the chemical and electrical properties of semiconducting TMOs and their most common methods of preparation, as well as their broad applications in solar energy conversion.

In Chapter II, the fabrication of a direct tandem Cu2O/Mo:BiVO4 PEC solar device is discussed. Here, we present a fabrication scheme that is facile and scalable, using electrochemical deposition for Cu2O and electrophoretic deposition for Mo:BiVO4. We prepared nanoparticles of BiVO4 and successfully doped the material with Mo to improve its electrical conductivity. Thermal annealing converts the Mo:BiVO4 into a semi-transparent material, allowing light to reach the Cu2O layer and partially protect it from corrosion. We show that Cu2O/Mo:BiVO4 on fluorine-doped tin oxide (FTO) substrate works as a photoanode for water oxidation, capable of evolving oxygen at a Faraday efficiency of 92% and achieving a photocurrent of 1.45 mA cm^-2 at 1.23 V vs. RHE under simulated sunlight conditions. The FTO/Cu2O/Mo:BiVO4 exhibited about nine times higher incident photon-to-light conversion efficiency (IPCE) value at 400 nm than the FTO/Mo:BiVO4 film. The FTO/Cu2O/Mo:BiVO4 device is still functional after 15 hours of continuous operation. Using surface photovoltage spectroscopy (SPS) and open circuit potential (OCP) measurements, we show that the Cu2O and Mo:BiVO4 form a p-n heterojunction and work as tandem under applied bias conditions. Chapter III emphasizes the use of SPS for the construction of a Cu2O-based PV cell. Here, we fabricated Cu2O by electrochemical deposition onto FTO, Mo and Ni conductive substrates. We show that Ni gives the highest surface photovoltage for Cu2O among all the substrates, providing up to 1.7 V at 285.75 mW-cm^-2 light intensity using a 405 nm light emitting diode (LED ) lamp. We also electrochemically deposited a ZnO overlayer to extract the electrons from Cu2O, which resulted in about 30% increase in the surface photovoltage. ZnO likely formed a deep depletion layer and removed the defect states in Cu2O as previously seen in the literature, causing the increase in photovoltage. Using SPS, we determined that the 1500-s deposition time for ZnO is optimal. Pre-made silver nanowires and conductive Ag paint were added to complete the final PV device structure. The completed device achieved an OCP value of ~16 mV under simulated 1 sun illumination, however, it still suffers from low conductivity issues. Improving the lateral conductivity in the ZnO electron transport layer (ETL ) layer and selecting metal front contact will be key to achieving a better-performing Cu2O-based PV device.

Chapter IV focuses on the thin film fabrication of TiO2, SnO2, and NiOx selective transport layers and their application in the FTO/Cu2O/Mo:BiVO4 PEC device. Using combined spin coating and solution combustion synthesis methods, we optimized the thin film fabrication of these selective contacts at 250 oC. We monitored the reaction progress using infrared spectroscopy, and the conductivity and rectifying behavior of the films were evaluated by PEC measurements. Using SPS, we show that NiOx promotes hole extraction from Cu2O to the surface and remains the same after adding SnO2. Although all elements were detected, we could not confirm the presence of each selective transport layer in the sandwich FTO/Cu2O/NiOx/SnO2/Mo:BiVO4 FTO/TiO2/Cu2O/NiOx/SnO2/Mo:BiVO4 structures due to the thickness and low concentration of these materials in the device. Here, we report 25% and 50% photocurrent enhancements in the FTO/Cu2O/NiOx/SnO2/Mo:BiVO4 and FTO/TiO2/Cu2O/NiOx/SnO2/Mo:BiVO4 sandwich film structures, respectively at 1.23 V vs. reversible hydrogen electrode (RHE ) versus the PEC device structure without any charge selective contact.

Solar energy conversion devices capture the energy in sunlight to produce electricity or fuels and include photovoltaics, photoelectrochemical systems, and photocatalysts. Such devices rely on semiconductor photoabsorbers that are able to produce mobile charge carriers under solar illumination and charge carrier-selective contacts that are able to selectively direct charge carriers to separate locations in the device. Understanding how each interface contributes to charge carrier separation is key to understanding how well a device functions.This dissertation presents three studies examining charge carrier separation in solar energy conversion devices, with relevance to photovoltaic, photoelectrochemical, and photocatalytic systems. Surface photovoltage, electrical, and photoelectrochemical measurements are employed to probe charge carrier separation in devices and identify interfaces that improve or hinder device operation. The first work investigates the contribution of hole transport layers to the photovoltage of GaP heterojunction solar cells. Then, a novel ternary nitride, ZnTiN2, is demonstrated as an electron-selective contact with additional protective benefits in silicon photocathodes. Lastly, charge carrier separation at the GaP/BiVO4 interface is evaluated to assess the possibility of constructing a tandem photocatalyst particle for overall water splitting.