Effects of Mg composition on open circuit voltage of Cu2O–MgxZn1−xO heterojunction solar cells

MgxZn1−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 Ag–Cu2O–MgxZn1−xO–fluorine-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 (VOC) with the increase of x, from 251 mV at x=0 to 570 mV at x=10%, was observed. In order to understand how VOC increases with Mg%, the band alignment between Cu2O and MgxZn1−xO was demonstrated using X-ray photoelectron spectroscopy (XPS) measurements. The result indicates that the conduction band of MgxZn1−xO moves closer to the vacuum level with increasing of x, leading to a decrease of the conduction band offset between MgxZn1−xO and Cu2O and hence an enhancement of theoretical VOC. Another improvement with the increase of Mg% was realized on the shunt resistance (Rsh) of devices. With the improved VOC and Rsh, a relatively high solar power conversion efficiency (ηAM1.5=0.71%) was obtained on the MgxZn1−xO (x=10%) based solar cell.