Tuning the Band Gap of Magnesium Zinc Oxide to Enhance Band Alignment with CdTe Based Photovoltaic Devices

Cadmium telluride (CdTe) based photovoitaic devices are usually coupled with a cadmium sulfide (CdS) emitter layer. Replacing the CdS with a material that has more transparency and a better band alignment with CdTe based devices could improve efficiency. The band gap of magnesium zinc oxide (MZO) has been seen to increase with increasing Mg content and sputtering at different temperatures can vary the Mg content. MZO is more transparent than CdS and the tunable band gap could help to improve band alignment between emitter and absorber layers. MZO films were deposited by radio frequency magnetron sputtering at $250^{\circ}\mathrm{C}, 300^{\circ}\mathrm{C}$ and $350^{\circ}\mathrm{C}$ and the optical properties were investigated. Cadmium selenide was then deposited by chemical bath on top of the MZO as a pre-cursor to forming a cadmium selenide telluride (CST) layer. CdSe was also deposited on Tec10 glass for comparison. Increasing the temperature of deposition of MZO from $250^{\circ}\mathrm{C}$ to $300^{\circ}\mathrm{C}$ and then $350{\circ}\mathrm{C}$ saw an increase in band gap from 3.85 eV to 3.87 eV to 3.97 eV respectively. The transparency and thickness were not affected. The CdSe on MZO films had a variable reduction in transmission with the CdSe on MZO sputtered at $300^{\circ}\mathrm{C}$ having the largest decrease in transparency between the wavelengths 350 - 650 nm and the CdSe on MZO sputtered at $250^{\circ}\mathrm{C}$ the smallest decrease when compared to CdSe alone. It was determined that interactions at the interface between CdSe and MZO were responsible for the transmission shift with parasitic absorption occurring.