Photocatalytic solar water splitting is a promising technology for producing green hydrogen that has the potential to compete economically with other commercially utilized carbon-based fuels. While this field has existed for decades, there has yet to be a demonstration of a system that can produce hydrogen in a cost-competitive manner. Recently, photocatalyst particles employing bulk heterojunctions (BHJs) of organic semiconductors (OSs) have emerged as an alternative approach to traditional photocatalytic water splitting using inorganic and/or single photoabsorber systems. OS materials are uniquely positioned to shift the paradigm in photocatalytic water splitting as result of their unmatched tunability, cheap and abundant constituent atoms, solution processability, and optoelectronic properties. This work aims to expand on the initial demonstrations of these OS-based BHJ photocatalysts, and, more importantly, develop a deeper knowledge of the particle formation mechanism from BHJ formation to co-catalyst deposition. The first chapter provides a brief summary of the work that has already been done in this field and the main challenges that must be addressed. The second chapter explores all-polymer BHJ nanoparticle formation made via a nanoprecipitation technique. We observe that photocatalyst particles made in this way exhibit exceptional stability, yet their hydrogen evolution performance remains modest. Importantly, we tune nanoparticle size by varying the amount of organic solvent in the surfactant solution which offers a launching point for further exploration of how to control this property. In the third chapter we shift our focus to understanding particle formation using a miniemulsion method. Under common synthetic conditions we show that the purported mechanism behind particle formation does not occur using a PTB7-Th:ITIC BHJ system. We further demonstrate this using the well-studied donor polymer P3HT as a model system with both SDS and TEBS surfactants. This brings a critical perspective to the field as the miniemulsion method is widely used; thus, understanding it fundamentally is necessary to better design the photocatalyst particles. Finally, we shift our attention to the photodeposition of a Pt co-catalyst on PTB7-Th:ITIC BHJ photocatalyst particles. We develop a halted photodeposition method which allows for better control over co-catalyst loading and morphology resulting in dramatically improved hydrogen evolution performances (on a Pt basis). Together, these chapters address some of the outstanding challenges in this field by providing important insights on OS BHJ nanoparticle photocatalyst formation and offering tools for influencing critical system properties.