From cells to laminate: probing and modeling residual stress evolution in thin silicon photovoltaic modules using synchrotron X-ray micro-diffraction experiments and finite element simulations

Fracture of silicon crystalline solar cells has recently been observed in increasing percentages especially in solar photovoltaic (PV) modules involving thinner silicon solar cells (<200 μm). Many failures due to fracture have been reported from the field because of environmental loading (snow, wind, etc.) as well as mishandling of the solar PV modules (during installation, maintenance, etc.). However, a significantly higher number of failures have also been reported during module encapsulation (lamination) indicating high residual stress in the modules and thus more prone to cell cracking. We report here, through the use of synchrotron X-ray submicron diffraction coupled with physics-based finite element modeling, the complete residual stress evolution in mono-crystalline silicon solar cells during PV module integration process. For the first time, we unravel the reason for the high stress and cracking of silicon cells near soldered inter-connects. Our experiments revealed a significant increase of residual stress in the silicon cell near the solder joint after lamination. Moreover, our finite element simulations show that this increase of stress during lamination is a result of highly localized bending of the cell near the soldered inter-connects. Further, the synchrotron X-ray submicron diffraction has proven to be a very effective way to quantitatively probe mechanical stress in encapsulated silicon solar cells. Thus, this technique has ultimately enabled these findings leading to the enlightening of the role of soldering and encapsulation processes on the cell residual stress. This model can be further used to suggest methodologies that could lead to lower stress in encapsulated silicon solar cells, which are the subjects of our continued investigations. Copyright © 2017 John Wiley & Sons, Ltd

Probing stress and fracture mechanism in encapsulated thin silicon solar cells by synchrotron X-ray microdiffraction

Thin (<150 µm) silicon solar cell technology is attractive due to the significant cost reduction associated with it. Consequently, fracture mechanisms in the thin silicon solar cells during soldering and lamination need to be fully understood quantitatively in order to enable photovoltaics (PV) systems implementation in both manufacturing and field operations. Synchrotron X-ray Microdiffraction (µSXRD) has proven to be a very effective means to quantitatively probe the mechanical stress which is the driving force of the fracture mechanisms (initiation, propagation, and propensity) in the thin silicon solar cells, especially when they are already encapsulated. In this article, we present the first ever stress examination in encapsulated thin silicon solar cells and show how nominally the same silicon solar cells encapsulated by different polymer encapsulants could have very different residual stresses after the lamination process. It is then not difficult to see how the earlier observation, as reported by Sander et al. (2013) [1], of very different fracture rates within the same silicon solar cells encapsulated by different Ethylene Vinyl Acetate (EVA) materials could come about. The complete second degree tensor components of the residual stress of the silicon solar cells after lamination process are also reported in this paper signifying the full and unique capabilities of the Synchrotron X-Ray Microdiffraction technique not only for measuring residual stress but also for measuring other potential mechanical damage within thin silicon solar cells

Microscale Initiation And Propagation Of Yielding In Duplex Stainless Steel Under Multiaxial Loading

A major challenge for modeling the mechanics of engineering alloys is linking phenomena across multiple length scales. In this work, the link between crystal plasticity, occurring at the microscale, and macroscopic yielding is explored for biaxial loading of dual phase alloys. In particular, dual phase austenitic-ferritic stainless steel LDX-2101 is studied. However, the analysis is generally applicable to a range of single and dual phase alloys. Combined neutron diffraction experiments and polycrystalline finite element simulations are used to investigate the elasto-plastic deformation of LDX-2101 under biaxial loading. A new formulation of strength-to-stiffness parameter is developed for generalized multiaxial loading. The strength-to-stiffness parameter exhibits strong correlation with the macroscopic stress at which subgrain regions of a polycrystal yield. In contrast, traditional Schmid and Taylor factors do not correlate to the macroscopic stress at which yielding occurs, for materials with high single-crystal elastic anisotropy. The strength-to-stiffness analysis is extended into a methodology for predicting the macroscopic stress at which subgrain regions yield. A new, physically-based yield criterion is also developed. The yield criterion is based on detecting the existence of a yield band, an interconnected region of yielded material that separates opposing domain surfaces. The new yield criterion is used in conjunction with simulated and predicted spatial distributions of yielding to evaluate the yield surface for LDX-2101 in biaxial stress space. The combination of the spatial yield distribution prediction algorithm and new yield criterion provides a fast, accurate methodology for yield surface evaluation