The role of sodium in photovoltaic devices under high voltage stress: A holistic approach to understand unsolved aspects

The paper reviews the role of sodium in the degradation of PV devices under high voltage stress with the purpose to identify issues still not solved and to introduce a comprehensive method of investigation. If a limited amount of Na diffusion into the CIGS cell structure is beneficial for the cell performance, on the other side when moving in the module structure it could create unpleasant effects, like corrosion and shunts. Soda-lime glass with a concentration of sodium around 13-15% is widely used both as cell substrate and as front layer in PV modules. Glass is not a static material and Na movement is easily activated by different triggering causes (stress, voltage bias, environmental variables). This paper is considered a prelude to further research. For this reason, a methodological approach is outlined, with emphasis on the consideration that PV devices, such as modules, are complex systems of systems. Recalling methodological discussions both in system engineering and in risk analysis, the proposed approach emphasizes the importance to study such complex systems adopting a holistic approach, thus taking into consideration interactions between system components and the surrounding environment. Probabilistic risk analysis (PRA) is suggested as a valid method for representing and understanding the interactions between different system components and environmental factors, while studying system reliability and risks for electricity supply.