Zn3P2 is a promising earth-abundant absorber for thin-film photovoltaics, yet its development is hindered by the lack of lattice-matched substrates, its incompatible thermal expansion coefficient, and a complex defect landscape. Here, we demonstrate the quasi-van der Waals epitaxy of Zn3P2 on graphene by metal–organic vapor phase epitaxy (MOVPE) and demonstrate a clear spatial correlation between antiphase boundary density and local variations in the optical emission using correlative electron microscopy and cathodoluminescence (CL). Moreover, CL measurements show that grain boundaries act as efficient non-radiative recombination centers, with an effective recovery length extending several micrometres into the grains, indicating that grain boundaries can significantly influence carrier transport and recombination in Zn3P2. Further comparison with molecular beam epitaxy grown films reveals the suppression of strain-related sub-bandgap emission in MOVPE-grown material. Overall, quasi-van der Waals epitaxy of Zn3P2 by MOVPE resulted in larger grains and improved material quality. These results establish clear relationships between extended defects and the local optical response in Zn3P2, while highlighting grain-size control as a key strategy for improving earth-abundant photovoltaic absorbers.