High stress and fracture of silicon crystalline solar cells has recently been observed in increasing percentages especially in solar photovoltaics (PV) modules involving thinner silicon solar cells (<200 um). Many failures due to fracture of cells have been reported from the field and handling. However, a significantly higher number of failures have also been reported during module integration (soldering/ stringing and lamination) indicating a PV laminate/module with significantly high residual stresses and hence more prone to cell fractures. We characterize the residual stress evolution in crystalline silicon solar cells during module integration process, which is the current knowledge gap. The residual stress characterization was achieved through a systematic research using synchrotron X-ray submicron diffraction experiments coupled with physics-based Finite Element modeling of the PV module integration process. Thought this work we also demonstrate the unique capability of Synchrotron X-ray submicron diffraction to quantitatively probe residual stress in encapsulated silicon solar cells that has ultimately enabled these findings leading to the enlightening of the role of soldering and encapsulation processes. While our experiments quantify the stress at different process states including encapsulated cells, our FEA simulations, for the first time unravel the physical reasoning for the stress evolution and expected to bridge the knowledge gap. 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.

A B S T R A C TThin ( < 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.

Silicon is considered as a promising anode material for the next-generation lithium-ion battery (LIB) due to its high capacity at nanoscale. However, silicon expands up to 300% during lithiation, which induces high stresses and leads to fractures. To design silicon nanostructures that could minimize fracture, it is important to understand and characterize stress states in the silicon nanostructures during lithiation. Synchrotron X-ray microdiffraction has proven to be effective in revealing insights of mechanical stress and other mechanics considerations in small-scale crystalline structures used in many important technological applications, such as microelectronics, nanotechnology, and energy systems. In the present study, an in situ synchrotron X-ray microdiffraction experiment was conducted to elucidate the mechanical stress states during the first electrochemical cycle of lithiation in single-crystalline silicon nanowires (SiNWs) in an LIB test cell. Morphological changes in the SiNWs at different levels of lithiation were also studied using scanning electron microscope (SEM). It was found from SEM observation that lithiation commenced predominantly at the top surface of SiNWs followed by further progression toward the bottom of the SiNWs gradually. The hydrostatic stress of the crystalline core of the SiNWs at different levels of electrochemical lithiation was determined using the in situ synchrotron X-ray microdiffraction technique. We found that the crystalline core of the SiNWs became highly compressive (up to -325.5 MPa) once lithiation started. This finding helps unravel insights about mechanical stress states in the SiNWs during the electrochemical lithiation, which could potentially pave the path toward the fracture-free design of silicon nanostructure anode materials in the next-generation LIB.

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