Zinc phosphide (Zn 3P 2) is a promising and earth-abundant alternative to traditional materials (e.g. CdTe, CIGS, a-Si) for thin film photovoltaics. The record solar energy conversion efficiency for Zn 3P 2 cells of 6% (M. Bhushan et al., Appl. Phys. Lett., 1980) used a Mg/Zn 3P 2 device geometry that required annealing to reach peak performance. Here we report photovoltaic device results and junction composition profiles as a function of annealing treatment for ITO/Mg/Zn 3P 2 devices. Mild annealing at 100°C in air dramatically increases V oc values from ∼150 mV to 550 mV, exceeding those of the record cell (V oc, record = 490 mV). In devices with thinner Mg films we achieved J sc values reaching 26 mA cm -2, significantly greater than those of the record cell (J sc, record = 14.9 mA cm -2). Junction profiling by secondary ion mass spectrometry (SIMS) and x-ray photoelectron spectroscopy (XPS) both show evidence of MgO and Mg-Zn-P alloy formation at the active photovoltaic junction in annealed ITO/Mg/Zn 3P 2 devices. These results indicate that high efficiency should be realizable by optimization of Mg treatment in Mg/Zn 3P 2 solar cells.