Photovoltaic (PV) module reliability issues, due to silicon cell cracking, are gaining more and more attention due to increasing demand for solar power and reduction of cell thickness to reduce cost. ...Recent reports show significant effect of encapsulation polymer material on cell cracks leading to the idea of tailoring encapsulation materials for more reliable PV modules. This paper investigates the effect of encapsulation modulus on the cell residual stress using Synchrotron scanning X-ray microdiffraction (µSXRD), which has been proven to be an effective technique to probe the stress in silicon solar cells, especially once they are encapsulated. The post lamination residual stress in the encapsulated multi-crystalline silicon (mc-Si) solar cells was reported for the first time using µSXRD in this manuscript and provide quantitative evaluation of the effect of encapsulation modulus on the cell residual stress. Further, simple approximate finite element (FE) model was also developed to evaluate the effect of the encapsulation polymer on the cell stress. The FE simulations predict the trend of the stress variation with encapsulation polymer modulus very well. Dynamic mechanical analysis and rheological testing of the encapsulation polymers was also performed to correlate the polymer behaviour with the experimental and simulated stresses. Both experimental and simulation results show a similar trend of significant cell stress variation with encapsulation polymer modulus. In the case of external loading, the temperature of load application is observed to be very significant as it dictates the elastic state of the encapsulant, leading to critical conclusion that the encapsulant needs to be selected based on elastic behaviour over the temperature history of the encapsulant during module fabrication and operation. The results and discussion presented are expected to be very useful for development of more reliable PV modules.
•Residual stress in the encapsulated mc-Si cells measured using μSXRD was reported in this work for the first time. The effect of encapsulation polymers on the cell residual stress was evaluated using μSXRD experiments, polymer dynamic mechanical and rheological analyses and simplified FE simulations.•The experimental results show that the elastic modulus of different encapsulants vary significantly with temperature and hence affects the cell residual stress after lamination, as shown by the stress maps obtained from μSXRD. The post-lamination residual stress in cell is directionally proportional to the encapsulant modulus at room temperature.•The stress maps obtained from μSXRD experiments also show that a thicker encapsulant (EVA) reduces the cell residual stress and vice-versa.•It was further shown with FE analysis that the front encapsulant has significant effect on cell stress whereas the back encapsulant effect is negligible.
Variation of the max. cell residual stress (from μSXRD) with the front encapsulant storage modulus at RT. Display omitted
In this study, the evolution of dislocation densities during compressive deformation of nanoscale Cu/Nb single crystal multilayers with individual layer thickness of 20nm is investigated using ...Synchrotron X-ray micro-diffraction. The samples were subjected to successive compression straining up to a final cumulative strain of 35%. The nanolayer composite exhibited a maximum flow strength of ~1.6GPa at approximately 24% compressive strain. Synchrotron X-ray micro-diffraction experiments, using a monochromatic beam of 10keV energy were performed after each compression strain increment. We observed a significant increase in X-ray ring width peak broadening in both Cu and Nb layers up to strains of ~3.5% followed by saturation broadening at higher strains. This observation indicates that the interfaces of the Cu/Nb nanolayers are very effective in trapping and annihilating dislocation content during mechanical deformation.
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Through-silicon via (TSV) has been used for 3-dimentional integrated circuits. Mechanical stresses in Cu and Si around the TSV were measured using synchrotron X-ray microdiffraction. ...The hydrostatic stress in Cu TSV went from high tensile of 234MPa in the as-fabricated state, to −196MPa (compressive) during thermal annealing (in situ measurement), to 167MPa in the post-annealed state. Due to this stress, the keep-away distance in Si was determined to be about 17μm. Our results suggest that Cu stress may lead to reliability as well as integration issues, while Si stress may lead to device performance concerns.
Recently, there has been a strong commercial push toward thinner silicon in the solar photovoltaic (PV) technologies due to the significant cost reduction associated with it. Tensile stress (normal, ...in-plane) and fracture of the silicon cells are increasingly observed and reported for products of crystalline solar cell technologies. In an effort to shed light on these topics, stress measurements and mapping of the solar cells in the vicinity of the most typically observed crack initiation locations using synchrotron X-ray microdiffraction technique was conducted and are reported in this paper. The technique is unique as it has the capabilities to quantitatively determine stresses in silicon and to map these stresses with a micron resolution, all while the silicon cells are already encapsulated.
With this technique, we aim to gain fundamental understanding of the stress magnitudes as well as characteristics that could lead to crack initiation and propagation. We have thus far found evidences of both extrinsic (device related) as well as intrinsic (crystallographic) nature of silicon cracking, which further confirm that the control of mechanical stress is the key to enable thin silicon solar cell technologies in the coming years. This study represents an ongoing high impact technology research that addresses real and important fundamental materials issue facing the crystalline silicon solar PV industry and contributes directly to the industry drive to reduce cost of PV systems to grid parity.
•We measure stress in the silicon cell in its package.•The stress measurement is enabled by using the synchrotron X-ray microdiffraction.•Investigation of stress evolution and fracture mechanisms of PV system in the field.•Examine the microstructure of the solder joint as the source of the stress.
Fracture in silicon crystalline solar cells has been a long-standing challenge encountered in the photovoltaic (PV) industry. This occurs as result of stresses developing in the cells due to ...thermo-mechanical stresses that arise when there is a mismatch in the coefficient of thermal expansion (CTE) between the different constituent materials during the manufacturing processes as well as mechanical stresses from hail, snow and wind during field operations. Through finite element simulations, this paper investigates the effect of interconnect cross-sectional geometry on stresses developed in silicon cells during the entire PV manufacturing cycle and provides physical reasoning to its stress evolution. The simulations are performed in a sequential manner whereby the residual stresses developed at the end of one step is brought forward to the beginning of the next step. It is found that the highest cell stresses occur at the back surfaces of the cells at the end of the pressure-ramping step during lamination. In addition, increasing the thickness of the front interconnects significantly increases the stresses developed in the cells during lamination. This predicted trend is verified experimentally.
•The effect of interconnect geometry on stresses in silicon solar cells in a PV laminate is analysed.•An increase in interconnect thickness leads to higher stress, and the physics behind it is proposed.•Thicker interconnect trend in industry with 5BB (vs. 3BB) could lead to lower PV reliability.
•The effect of the encapsulants on the IBC silicon cell stresses in a PV module were studied using finite element analysis.•The encapsulant modulus and thickness affect the cell stress significantly ...and the effect of CTE of the encapsulant is negligible.•A constrained cell curvature model is proposed to explain the effect of the encapsulant.•The cell stress is directly proportional to encapsulant modulus and inversely proportional to encapsulant thickness.•The increased cell stresses due to thinning of the silicon cells can be negated by adopting to a softer and thicker encapsulant.
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.
•Unique and aesthetic Polycarbonate-sandwiched PV module design.•PC-EVA interface adherence is fully investigated using WTCB method.•New proposed method for enhancement of the PC-EVA interface ...adhesion strength.
Light weight photovoltaic (PV) modules have advantages both to reduce costs of PV installations as well as to enhance their further integration with building and other urban structures such as roofs of public parking areas and bus stations. Polycarbonate (PC) materials have been studied before as glass or front sheet replacement, but recently the PC-PC module design (where both front and back sheets are made of PC) has gained attractions both in industry as well as in research. It offers further additional benefits, such as improved reliability (in terms of silicon cell cracks) and enhanced ease of integration into not only flat surfaces, but also curved ones such as integrated PV in cars, trucks, and boats. These benefits may enable more aesthetic design that can increase the appeal of the PV technology. However, a few technological issues stand between the current forms of PC-PC PV module design and the increasing market demands for more aesthetically pleasing and contour-confirming PV integrations. Two of the most important ones are the silicon sensitivity against fracture (especially in bending mode) and interface delamination (especially between PC and the encapsulants). Both are driven by mechanical stresses in the silicon cells as well as in other materials in the PV module/package design. Having published widely our studies in stresses in silicon solar cells in PV module design, both experimentally as well as computationally as shown in the literature review of this manuscript, our research group has obtained unique insights in the primary driving forces of failures in the typical PV module design (glass-back sheet). In the present manuscript, we use the Width Tapered Cantilever Beam (WTCB) method to investigate the interfacial delamination issues systematically. We found that the energy release rate for delamination or fracture at the interfaces between PC and EVA (Ethylene Vinyl Acetate) was fairly low (i.e. low adhesion strength), but could be enhanced with process parameters during the PV lamination step or separate thermal treatment. Hence, this could be taken advantage for enabling unique and highly innovative PV modules based on PC, and for aesthetic purposes for the wider applications of PV technology.
Plastic deformation mechanisms in metal–metal nanolayer composites (nanolaminates) have been studied extensively during the last decade. It has been observed that, for the case of metal–metal ...nanolaminates with a semicoherent interface, such as Cu/Nb, low interface shear strength increases the interface barrier to dislocation crossing, which improves nanolaminate plasticity. In this study, we use Cu (63 nm)/Nb(63 nm) accumulative roll-bonded nanolaminates, which have a large anisotropy of the interface shear strength between rolling and transverse directions (RD and TD, respectively), to study the effect of interface shear strength on the failure in metal–metal nanolaminates with a semicoherent interface during in situ clamped beam bending. Further, finite element analysis is used to understand the observed behavior. The results show a substantial difference between the fracture behaviors along the RD and TD owing to differences in the interface shear strength and grain size. For the RD beams, the slip bands originate from the Nb layers at the notch/crack tip followed by crack propagation along these bands. For the TD beams, the crack propagation is inhibited by interface shear. We suggest that shear bands form subsequently through the beam and lead to the final beam failure. However, under the assumption of the presence of the grain boundaries near the stress concentration zone, the interface shear in the TD beams could be inhibited. In this case, the crack growth can be attributed to the formation of microcracks at grain boundaries beside the main crack.
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Synchrotron x-ray microdiffraction (
μ
XRD
) allows characterization of a crystalline material in small, localized volumes. Phase composition, crystal orientation and strain can all be probed in ...few-second time scales. Crystalline changes over a large areas can be also probed in a reasonable amount of time with submicron spatial resolution. However, despite all the listed capabilities,
μ
XRD
is mostly used to study pure materials but its application in actual device characterization is rather limited. This article will explore the recent developments of the
μ
XRD
technique illustrated with its advanced applications in microelectronic devices and solar photovoltaic systems. Application of
μ
XRD
in microelectronics will be illustrated by studying stress and microstructure evolution in Cu TSV (through silicon via) during and after annealing. The approach allowing study of the microstructural evolution in the solder joint of crystalline Si solar cells due to thermal cycling will be also demonstrated.
When crystalline materials are mechanically deformed in small volumes, higher stresses are needed for plastic flow. This has been called the “smaller is stronger” phenomenon and has been widely ...observed. Various size-dependent strengthening mechanisms have been proposed to account for such effects, often involving strain gradients. Here we report on a search for strain gradients as a possible source of strength for single-crystal submicron pillars of gold subjected to uniform compression, using a submicron white-beam (Laue) X-ray diffraction technique. We have found, both before and after uniaxial compression, no evidence of either significant lattice curvature or subgrain structure. This is true even after 35% strain and a high flow stress of 300
MPa were achieved during deformation. These observations suggest that plasticity here is not controlled by strain gradients or substructure hardening, but rather by dislocation source starvation, wherein smaller volumes are stronger because fewer sources of dislocations are available.
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