Analysis of magnesium zinc oxide layers for high efficiency CdTe devices

CdTe thin film solar cells are the most successful thin film photovoltaic devices in terms of production yield, as attested by the remarkable market performance. Typically the junction is made with n-type CdS that, however, absorbs some of the light spectrum due to its band-gap of 2.4 eV. Therefore, in order to exceed 20% efficiency it is crucial to increase the wavelength range convertible by the absorber. One possible alternative buffer that, at the same time, allows a larger transparency and a proper band alignment with CdTe is Magnesium Zinc Oxide (MZO). In this paper, we present low temperature CdTe solar cells in superstrate configuration without CdS, by the introduction of MZO as buffer layer. Efficiencies above 12% have been obtained, with remarkable open circuit voltage and superior values of current density. The transmission and band gap of the different MZO layers have been analysed and presented. Moreover, the optimum type of MZO layer, made with higher oxygen content and with a band gap of 3.72 eV, has been characterized by X-ray diffraction, and its morphology has been studied by atomic force microscopy, whereas the finished cells have been characterized by current-voltage and by external quantum efficiency measurements.