Direct evidence of Mg-Zn-P alloy formation in Mg/Zn3P2 solar cells

Zinc phosphide (Zn₃P₂) 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 Zn₃P₂ cells of 6% (M. Bhushan et al., Appl. Phys. Lett., 1980) used a Mg/Zn₃P₂ 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/Zn₃P₂ 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/Zn₃P₂ devices. These results indicate that high efficiency should be realizable by optimization of Mg treatment in Mg/Zn₃P₂ solar cells.