Process Practice and its Effects on Surface Defects on Flexible PV Barrier Films

To the present day, there is still an increasing interest in the development of Cu (In, Ga) Se2 (CIGS) thin film solar cells on flexible polyimide substrates. These cells offer advantages of low cost, light weight and excellent radiation hardness as well as facilitating building-integrated-photovoltaic (BIPV) applications. These solar cells typically consist of six different layers of thin film including; Ag / Transparent Conductive Oxide (TCO) of ZnO: Al /CdS/CIGS/Mo/Al2O3 [1]. The Al2O3 insulating layer is introduced into the structure of the solar cell in order to prevent diffusion of water vapour and oxygen to the CIGS layer. Hence, to optimize the photovoltaic (PV) module performance and lifespan, a study to understand the nature of surface defects in the Al2O3 barrier layer which allows water vapour and oxygen ingress to the active layer (CIGS) is required. Surface metrology techniques including; Optical Microscopy, White Light Scanning Interferometry (WLSI), Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were employed for collecting data on PV barrier layers defects down to nm scale. Feature segmentation analysis methods proved to be an effective tool for discrimination between insignificant and the most significant features which are directly responsible for PV module degradation.
Keywords: Photovoltaics, Defects, Surface topography, ALD and WVTR.