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-Mg Zn1-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 (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 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 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 (eta(AM1.5) = 0.71%) was obtained on the MgxZn1-xO (x=10%) based solar cell. (C) 2011 Elsevier B.V. All rights reserved.