Effect of direct current bias on MZO window layer in CdTe solar cells

Abstract

The band alignment between magnesium-doped zinc oxide (MZO) and cadmium telluride (CdTe) is critical for enhancing the efficiency of CdTe solar cells. In order to study the effect of direct current (DC) bias on the MZO film, various DC biases were applied during the radio frequency (RF) sputtering of the MZO films, with the target bias ranging from 79 V to 121 V. The compositional, structural, chemical, and electronic properties of the MZO films were investigated. The optical transmission and x-ray photoelectron spectroscopy results show that the DC bias could modify the position of the Fermi level while having minimal impact on the band gap of the MZO film. The MZO films with different DC biases were employed as the window layer in CdTe solar cells. The device without DC bias exhibited a low efficiency of 7.7%, corresponding to a cliff-like conduction band offset near −0.1 eV. The optimal DC bias for the MZO film was found to be 99 V, leading to an improvement in efficiency to 12.9%, with a spike-like conduction band offset near 0.2 eV. Further increases in the conduction band offset, however, decrease the photo-generated current, negatively impacting device performance. The results emphasize the significance of DC bias in the RF sputtering process of MZO films with regard to the functionality of CdTe solar cells.