The bulk heterojunction (BHJ) concept, which has been successfully developed for solution-processed organic semiconductor-based photovoltaic (OPV) devices, offers a promising route to high-performance, scalable, and inexpensive photocatalyst nanoparticles (NPs) for solar-driven hydrogen production. However, state of the art BHJ NPs exhibit quantum yields far below OPVs, thus understanding how NPs characteristics affect their performance is a major goal. Herein we present insights to NP formation and co-catalyst deposition that demonstrate the effect of NP size and co-catalyst morphology on the performance of PTB7-Th:ITIC BHJ NPs. Specifically, using the emulsion NP preparation route, we report how BHJ NP size influences photocatalytic H2 evolution, with a drastic increase in H2 production when reducing NP diameter from 230 nm to 25 nm. Moreover, we introduce a halted photodeposition-dialysis method that affords unprecedented control over platinum (Pt) co-catalyst loading and morphology. Applying this method with typical Pt deposition conditions gave a max H2 evolution rate of 140 mmol h−1 g−1 (based on semiconductor mass) with only 15.2 wt % Pt and suggested an optimum loading of < 20 wt % Pt, above which parasitic light absorption decreases the H2 evolution rate.