Effect of encapsulants on the thermomechanical residual stress in the back-contact silicon solar cells of photovoltaic modules - A constrained local curvature model

The fracture of the silicon cells and associated performance degradation is a major hindrance to the long-term viability of solar photovoltaics as a main-stream power source. The stress induced in the cells during the photovoltaic module integration (soldering and encapsulation) processes is significant as they may be high enough to cause cell fracture or propagate the pre-existing micro cracks. In a well-controlled cell soldering and encapsulation process in the industry, this stress may not cause significant cell cracking, but the stressed regions can still act as crack localization sites under the external operational loads. With the advent of thin silicon wafers to reduce material costs and increase productivity, the cells may become even more fragile and susceptible to fractures during the module fabrication processes. In this scenario, the PV industry is looking for methods and materials to reduce cell stresses. In this work we simulate the effect of the encapsulation polymers on cell stress and show that the encapsulant elastic modulus and thickness significantly affect cell stress, during the module fabrication and operation as well. The results suggest that the choice of the encapsulant can help to reduce the cell stress and improve the module reliability. The results further show that the effect of the encapsulant is much more significant for thinner cells, and that the effect of the front encapsulant is more significant compared to that of the back encapsulant. A physical model of constrained local curvature of the cell in the PV module is also proposed to explain the effects of the encapsulant modulus on the cell stress.