p-ZnTe for Back Contacts to CdTe Thin Film Solar Cells

Abstract

AbstractCdTe thin film solar cells still suffer from problems related to back contacts which provide stable ohmic contacts without electrical losses. In previous studies metal/CdTe contacts have shown significant limitations. A promising option for the realization of ohmic back contacts is the use of a highly p-doped ZnTe interlayer on top of the CdTe absorber, in particular since metal/ZnTe contacts have shown very good electrical properties. In this work we studied the electronic und chemical properties of nitrogen doped p-ZnTe films. p-ZnTe:N films were prepared by using reactive RF magnetron sputtering with N2/Ar gas mixtures or by thermal evaporation with an additional nitrogen plasma source. Samples and their contacts have been prepared in DAISY-SOL (DArmstadt Integrated SYstem for SOLar energy research) which combines a full vacuum production with an in-situ photoelectron spectroscopy (XPS/UPS) analysis. The results of XPS/UPS investigations and electrical measurements will be discussed in comparison to previous results on metal/CdTe contacts. As ZnTe forms an interlayer in the CdTe thin film solar cell, the ZnTe/CdTe interface properties must be also taken into consideration. Our experiments have shown in agreement to previous studies that the valence band offset is nearly ideal for hole transport across the interface. Also electrical measurements have been carried out to investigate the metal/ZnTe/CdTe layer sequences in their contact properties.