Abstract
Solar photovoltaic (PV) modules generate pollution-free energy via the direct conversion of solar energy into electricity. In the current context of fossil fuel depletion and increased energy demand, PV modules provide a promising alternative to conventional energy sources. Currently, crystalline silicon (c-Si)-based PV modules dominate the industry, commanding the largest market share, as they are the most matured technology with well-established manufacturing processes. However, recent reports show that some PV modules fail due to mechanical stress and fractures of c-Si cells during fabrication and operation. While cell fractures during manufacturing reduce the production yield and increase costs, premature field failures can incur warranty costs for the manufacturer. In extreme cases, they may also cause catastrophic failures, leading to accidents and life-threatening hazards. Therefore, it is essential to understand the origins of stress and fractures in c-Si cells and develop techniques to characterize them in a quantitative manner. Current PV stress characterization techniques are of limited capability and most of them are unable to probe stress in encapsulated cells. Moreover, the highly localized nature of the residual stress near the soldered interconnects in the cells makes stress characterization even more difficult. In this dissertation, we aim to address this shortcoming by employing a curvature-based stress evaluation methodology, developed using the unique synchrotron scanning X-ray microdiffraction (µSXRD) technique. This method exploits the high penetration and high-resolution capabilities of the synchrotron X-ray beam to directly probe stress in the encapsulated c-Si cells. Thus, this method allows for the systematic quantitative characterization of residual stress in c-Si cells at different stages of the PV module manufacturing process, which is a novel achievement. Further, we develop a physics-based finite element model to sequentially simulate the thermomechanical deformations and stress during the soldering and lamination processes. With this model, the physical mechanism driving the high residual stress is elucidated. This unique methodology combining experiment and simulation allows for the systematic evaluation of the effects of various parameters (different materials, geometries and processes on cell stress. This in turn enables us to draw meaningful conclusions and recommendations, which are useful for the design of highly reliable, next-generation thin silicon PV modules.