Module Layout for Reliable Aluminum-based Building-Integrated Photovoltaics

When manufacturing photovoltaic (PV) modules with aluminum rear covers, for instance for building integration purposes, one has to consider the large thermal expansion of the aluminum compared to silicon and glass. This holds for the manufacturing process as well as for the reliability of the resulting building-integrated PV (BIPV) modules. Concerning module reliability, the mismatch in thermal expansion coefficients of silicon solar cells and aluminum rear cover leads to mechanical stress in the solar cell strings under temperature changes. After several temperature shifts during operation, the copper wires interconnecting the silicon solar cells might suffer from fatigue breakage and thereby the module power output could be drastically reduced. This work investigates the influence of module layout in terms of solar cell width and solar cell string length on the electrical degradation of crystalline silicon PV modules with aluminum sheets as rear covers in thermal cycling tests. As a result, we find that aluminum-based modules with wide cells degrade faster and fail earlier in the thermal cycling test than modules with narrow cells. This can be explained by an increased cell gap change during thermal cycling for solar cell strings with wider cells. Furthermore, thermal cycling of modules with aluminum rear covers in lengths from 20 cm to 240 cm and accordingly long cell strings resulted in earlier degradation and failure of longer strings. In conclusion, from a thermomechanical point of view, a module layout with short strings of narrow cells is recommended.