Effect of interconnect plasticity on soldering induced residual stress in thin crystalline silicon solar cells

Soldering of copper interconnects at ∼220 °C induces residual stresses in the crystalline silicon solar cells. These residual stresses may become detrimental in the context of thinner silicon cells (<180μm) causing cracking, performance loss and failures. In such a scenario a systematic evaluation of these residual stresses considering realistic material properties is required to assess the risk levels. In the present work, soldering induced residual stresses in the back contact c-Si solar cells were evaluated using finite element simulations. Effect of plasticity of the copper interconnects on the cell residual stress was studied. It was observed that the linear elastic material property of interconnect and solder over predicts the cell stress significantly and hence elastic-plastic material properties are needed for realistic predictions. Simulated stresses were also compared with those of synchrotron X-ray micro-diffraction experiments for cell thickness of 180μm to validate the finite element model. Further residual stress was evaluated as a function of silicon cell thickness in order to assess the risk of thinning cells for the current soldering process. These results are expected to enable optimization of soldering and interconnect parameters to mitigate high residual stress and thereby cell fractures

Thermomechanical residual stress evaluation in multi-crystalline silicon solar cells of photovoltaic modules with different encapsulation polymers using synchrotron X-ray microdiffraction

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

Thermopower characterization of InSb nanowires using thermoreflectance

\u3cp\u3eIncreasing energy demand and depleting fossil fuels make it imperative to shift to more efficient energy sources. Thermoelectric materials can convert waste heat into electrical energy by means of the Thermopower or Seebeck effect. Nanowires are promising candidates to increase the efficiency of thermo-electric conversion because their low dimensionality is predicted to increase the Seebeck effect due to confinement of charge carriers and decrease thermal conductivity due to boundary scattering of phonons. The Seebeck effect is characterized by the Seebeck coefficient which describes the output voltage per degree Kelvin in a thermoelectric material. Hence, in order to obtain the Seebeck coefficient, it is crucial to accurately measure the temperature difference along the axial direction of the nanowire which has a length of a few micrometers, at the locations where voltage measurements are performed under a temperature gradient. Here we demonstrate an application of thermoreflectance to measure the temperature difference along the nanowire allowing bi-directional measurements of Seebeck coefficient.\u3c/p\u3

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

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