Texturing approaches for the fabrication of nc-Si:H/c-Si tandem devices

Abstract

This thesis is dedicated to the fabrication of a novel silicon based tandem cell which combines hydrogenated nanocrystalline silicon (nc-Si:H) thin film(TF) photovoltaic (PV) technology and silicon heterojunction (SHJ) c-Si based PV technology. As a matter of fact, the growth of nc-Si:H on a flat c-Si substrate is not uniformas it irregularly peels off after deposition using the plasma enhanced chemical vapor deposition (PECVD). On the other hand, the growth of the nc-Si:H TF layers on the standard alkaline textured c-Si substrate with sharp pyramidal structures results in defective regions in the bulk of nc-Si:H grown material. A nc-Si:H layer with high defect density reduces the voltage and the fill factor (FF) of the tandem device. In order to minimize the defect density in the nc-Si:H absorber layer, different texturing approaches (TA) were developed for the c-Si
wafer to facilitate better growth of the nc-Si:H absorber. The surface morphology of the textured c-Si wafers for all TA’s at different etching time steps were investigated. Prior to TF layer deposition for the SHJ and the tandem devices, various cleaning approaches were investigated to improve the surface passivation of the textured c-Si wafers. The investigation of the grown nc-Si:H layer using different TA’s showed that smoothening of the sharp pyramidal structures significantly improved the nc-Si:H grown bulk as it helped in better growth of nc-Si:H with no significant defects. Finally, the current density-voltage (J-V) measurements were investigated for all TA’s for the SHJ and the tandem devices. The best performing tandem cell has an open circuit voltage (Voc ) of 1.02 V , short current density (Jsc ) of 13.34 mA/cm-2 and a FF of 0.44. It is expected that optimizing the tunnel recombination junction (TRJ) will further improve the electrical performance of the tandem devices for all TA’s.