Mg doping and alloying in Zn_3P_2 heterojunction solar cells

Abstract

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. We report the fabrication of Mg/Zn_3P_2 Schottky diodes with V_(oc) values reaching 550 mV, J_(sc) values up to 21.8 mA/cm^2, and photovoltaic efficiency reaching 4.5%. Previous authors have suggested that Mg impurities behave as n-type dopants in Zn_3P_2, but combined Hall effect measurements and Secondary Ion Mass Spectrometry (SIMS) show that 10^(17) to 10^(19) cm^(-3) Mg impurities compensate p-type doping to form highly resistive Zn_3P_2. Further device work with modified ITO/Mg/Zn_3P_2 heterojunctions suggests that the ITO capping layer improves a passivation reaction between Mg and Zn_3P_2 to yield high voltages > 500 mV without degradation in the blue response of the solar cell. These results indicate that at least 8-10% efficiency cell is realizable by the optimization of Mg treatment in Zn_3P_2 solar cells.