Optimization of ITO TCO for thin-film silicon and HIT solar cells

Abstract

Within thin-film silicon solar cells, the so-called Transparent Conductive Oxide (TCO) plays an important role. The optimization of TCOs is a very meticulous process since these thin layers not only need to be both highly transparent and conductive, but are also used for light management; that is manipulating the path of the light through the active layer by means of a textured TCO surface which scatters the light. Besides, the TCOs are also used as back reflector in combination with metallic films. By using these methods both the short circuit current and the overall conversion efficiency of the thin-film silicon solar cells is improved. This report focuses on the actual optimization of the RF magnetron sputtering deposition process of tin-doped indium (ITO) TCO’s for application in thin-film silicon and HIT solar cells. By varying deposition parameters such as power, pressure and temperature an optimal recipe for each specific type of thin-film solar cell could be established. The optimized ITO TCO recipes are utilized as both front and back TCO in a-Si:H, ?c-Si:H and HIT thin-film solar cells. Thanks to these optimizations we were able to achieve efficiencies which were comparable, and in some cases even better, to similar devices made with the already optimized aluminum-doped zinc oxide (AZO) TCO’s. A highest initial efficiency of 10.7% was achieved for a p-i-n solar cell with front Asahi U-type TCO and back ITO TCO. Besides the actual optimization process the consistency and repeatability of the ITO depositions with time was investigated. Since the optimization process is so elaborate, repeatability of depositions is very important. Regularly scheduled depositions were made to check the variations of thin-film quality with time. The results show that the quality of the ITO thin-films is quite variable. The ITO quality is known to drop significantly after a deposition chamber ventilation. The number of hours of sputtering needed to return to original and acceptable quality was investigated by means of a post-ventilation recovery series.