Effects of Mg composition on open circuit voltage of Cu₂OMg _xZn_1-xO heterojunction solar cells

Mg_xZn_1-xO (0≤x≤0.13) films grown by metal-organic chemical vapor deposition (MOCVD) were chosen as the n-type semiconductor layer forming a heterojunction with electrodeposited p-type cuprous oxide (Cu₂O) for photovoltaic applications in this study. We investigated the effects of Mg contents (x) on the performance of AgCu ₂OMg_xZn_1-xOfluorine-doped tin oxide (FTO)-glass heterojunction solar cells, where Ag and FTO are used as top and bottom electrodes, respectively. An enhancement of the open-circuit voltage (V _OC) with the increase of x, from 251 mV at x=0 to 570 mV at x=10%, was observed. In order to understand how V_OC increases with Mg%, the band alignment between Cu₂O and Mg_xZn_1-xO was demonstrated using X-ray photoelectron spectroscopy (XPS) measurements. The result indicates that the conduction band of Mg_xZn_1-xO moves closer to the vacuum level with increasing of x, leading to a decrease of the conduction band offset between Mg_xZn_1-xO and Cu ₂O and hence an enhancement of theoretical V_OC. Another improvement with the increase of Mg% was realized on the shunt resistance (R_sh) of devices. With the improved V_OC and R _sh, a relatively high solar power conversion efficiency (η_AM1.5=0.71%) was obtained on the Mg_xZn _1-xO (x=10%) based solar cell.