Transparent conducting oxide layers for thin film silicon solar cells

Abstract

Texture etching of ZnO:1%Al layers using diluted HCl solution provides excellent TCOs with crater type surface features for the front contact of superstrate type of thin film silicon solar cells. The texture etched ZnO:Al definitely gives superior performance than Asahi SnO2:F TCO in case of nanocrystalline silicon (nc-Si) type of solar cells. The stress of the ZnO:Al film changes from tensile to compressive with the increase in substrate temperature of sputter deposition and the rms roughness and the haze of the film seem to have a correlation with the stress of the film prior to etching; the sample made at 150 °C is most tensile and the etching rate and the evolved roughness is least at this condition whereas the sample made at 350 °C with a compressive stress character gives a high roughness. At present the ZnO:Al made at room temperature provides the best combination of the electrical property and the scattering property of the texture etched layer. A current density of 24 mA/cm2 has been obtained for a nc-Si cell of 2200 nm thick. To apply such a texturing technique to make rough ZnO:Al TCO layers on PET and PEN substrates for solar cells on plastics, an additional step of embossing the plastics prior to the sputter deposition of the ZnO:Al layers was employed to release the undue stress. The texture etching of such layers on plastics showed excellent scattering properties in addition to the good electrical properties. As far as ZnO:Al as back reflector is concerned, use of a thick, low doped ZnO:Al in combination with white reflectors, instead of metals, will be a possible solution to avoid surface plasmon absorption loss. We have successfully applied this concept using 0.5% Al doped ZnO to a superstrate type a-Si solar cell using upconversion material at the back of the solar cell. In case of substrate type solar cells on plastics, the ZnO:Al layers that are used as the Ag/ZnO:Al back reflector as well as barrier layers, have to be thin and made at a low stress condition. Such a process resulted in 6% efficiency of n-i-p a-Si solar cells on PET and PEN substrates.