Degradation of polymer encapsulants for photovoltaic application [electronic resource]
Keywords: Materials Science and Engineering.

Polymeric and thin film materials are ubiquitous in modern photovoltaic modules. To be economically competitive, photovoltaic modules need to maintain an operational lifetime of greater than 25 years. The reliability of solar modules poses a unique materials challenge as the module experiences weathering from solar radiation, temperature and active environmental species. Under these conditions, the polymeric and thin film materials are susceptible to degradation. Defects can from and evolve in the polymeric material and its adjacent interfaces, which can lead to delamination and mechanical failure of the joints and subsequent failure of the module. Understanding the underlying mechanisms governing the degradation of these materials and interfaces under synergistic stress conditions is critical for commercialization of photovoltaic technology. In this dissertation, I present studies of photo-degradation of polymeric and thin film materials in photovoltaic application. To study the mechanisms of photo-degradation, we examined the encapsulation used in the optical elements of concentrator photovoltaic (CPV) modules. As the modules operate in extreme conditions of enhanced solar flux, elevated operating temperature, and frequent thermal cycling, they can provide unique insights into degradation modes of solar materials. A survey of the CPV industry was conducted to determine the most critical interface for CPV reliability. Fracture mechanics based metrologies were developed to characterize the adhesion of the silicone encapsulant and its adjacent interfaces. Specimens were weathered in outdoor concentrator to simulate operating conditions and under UV to determine the effects of photo-degradation. Significant changes in the adhesion energy of the encapsulant and its adjacent interfaces were observed in both outdoor concentrator and UV weathering. Oxidation of the silicone encapsulant was observed during weathering. A physics based model was developed to explain and predict the changes in adhesion energy as a result of the changes in chemical structure with the silicone encapsulant. To study the degradation of the interface of polymer and oxide thing films, we examined the barrier films used in flexible photovoltaic applications. The flexible diffusion barrier films utilized alternating layers of polymeric and oxide thin films to protect CIGS/OLED solar cells from atmospheric moisture. Accelerated aging was conducted under UV in an environmental chamber. We observed a significant decrease in adhesion between the polymer and oxide layers following UV exposure. To strengthen the interface, we developed a lithographical process to pattern the interface between a polymer substrate and an oxide thin film. By leveraging mechanical adhesion, the interface is better able to resist delamination from photo-degradation. The patterned interface increased the adhesion energy between the film and the substrate by an order of magnitude compared to the planar interface. Further, the patterned interface maintained a 3 to 5 fold increase in adhesion energy after UV exposure. Using this technique, we can increase the reliability and durability of barrier films and ensure a greater than 25 year operational lifetime in photovoltaic modules.