Thin-film silazane/alumina high emissivity double layer coatings for flexible Cu(In,Ga)Se2 solar cells

In order to increase the efficiency of thin film Cu(In,Ga)Se2 (CIGSe) solar cells in space applications, it is necessary to reduce the working temperature of the cells under these special conditions. The most promising approach to achieve this is to increase the thermal emissivity. In this work, a new alumina/silica double layer coating system with a thickness of less than 1 µm that provides high values of emissivity on flexible CIGSe solar cells is presented. The coatings were applied by dip coating using a polysilazane, poly(alkyliminoalane) and an alumina sol as precursors. As the temperature for the thermal treatment of the coatings is limited to about 280 °C, due to thermal stability of the CIGSe device, the applied precursor based coatings must show good adhesion to the substrate and with each other as well as a high reactivity at low temperatures to form oxides. Conversion behaviour, coating thickness, morphology, mechanical properties, optical and thermal properties are investigated. Results of the optical and thermal measurements show that the coating system is transparent and can increase the emissivity remarkably from about 0.38 (uncoated CIGSe) to more than 0.7. Thus the developed double layer system is a promising candidate as high-ε coating on solar cells for space applications.