ZnO:Al/i-ZnO bi-layers deposited on large substrates by pulsed D.C. magnetron sputtering for chalcogenide photovoltaics

In this paper crucial factors for the deposition of AZO/i-ZnO bi-layers to be used as front contact in chalcogenide photovoltaics are considered, with emphasis on effect of the deposition parameters on the electrical and optical properties. Thereby, a systematic study focused on the substrate temperature, substrate to target distance and i-ZnO and AZO thickness has been performed with the aim of scaling up the deposition processes on large areas. The transmittance of the AZO/i-ZnO/glass thin film bi-layer structure resulted between 85% and 92%. Moreover, while the increase of the i-ZnO film thickness hinders the bi-layer electrical properties, they were found to be improved with increasing AZO thickness. This phenomenon can be assigned to the increase of the grain size in the thickest films that reduces the grain boundary scattering. A figure of merit, expressed as ratio of the logarithm of the transmittance Tr (average value in the 400–800 nm wavelength) to the resistivity ρ of the films, as high as 22700 Ω−1 cm−1 was obtained for the deposition performed at a substrate temperature of 150 °C. This figure of merit was obtained on the basis of a resistivity value of 4.84×10−4 Ω cm and a transmittance of 91% in the visible range. Therefore the results obtained in this work on large substrates match the requirements for high quality optical and photovoltaic applications.