Zinc phosphide (Zn3P2) has been lauded as a promising solar absorber material due to its functional properties and the abundance of zinc and phosphorous. In the last 4 decades, there has not been any significant improvement in the efficiencies of Zn3P2-based solar cells. This is vastly due to the limited understanding of how to tune its optoelectronic properties. Recently, significant progress has been made in the growth and charac-terization of the material, which has shed light on its potential. In this study, we report an energy conversion efficiency as high as 4.4% for a solar cell based on a polycrystalline Zn3P2/InP heterojunction. This device presents an open circuit voltage of 528.8 mV, 7.5% higher than the Zn3P2 homojunction record. We investigate the dominant recombination mechanisms within the film using different techniques to identify the key factors underpinning our device efficiency. In particular, we pinpoint that reduced carrier collection at the front of Zn3P2 is responsible for the reduced collection of high-energy photons. This allows us to suggest the design rules for next-generation Zn3P2-based heterojunction solar cells, which should allow us to go beyond the current con-version values.