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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 vapour phase epitaxy (MOVPE) and directly link the density of antiphase boundaries to optical emission modulation using correlative electron microscopy and cathodoluminescence (CL). Moreover, it is observed through CL that grain boundaries act as non-radiative sinks for excited charge carriers. The effect extends several micrometres into the grains, making grain boundaries detrimental to the applicability of Zn3P2 in real devices. 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. In addition, these results directly link extended defects to recombination pathways in Zn3P2 and highlight grain-size control as a key strategy for improving earth-abundant photovoltaic absorbers.