Optimised solder interconnections in crystalline silicon (c-Si) photovoltaic modules for improved performance in elevated temperature climate

A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of Philosophy.The operations of c-Si PV modules in elevated temperature climates like Africa and the Middle East are plagued with poor thermo-mechanical reliability and short fatigue lives. There is the need to improve the performance of the system operating in such regions to solve the grave energy poverty and power shortages. Solder interconnection failure due to accelerated thermo-mechanical degradation is identified as the most dominant degradation mode and responsible for over 40% of c-Si PV module failures. Hence the optimisation of c-Si PV module solder interconnections for improved performance in elevated temperature climate is the focus of this research. The effects of relevant reliability influencing factors (RIFs) on the performance (thermo-mechanical degradation and fatigue life) of c-Si PV module solder interconnections are investigated utilising a combination of ANSYS finite element modelling (FEM), Taguchi L25 orthogonal array and analytical techniques. The investigated RIFs are operating temperature, material combination and interconnection geometry. Garofalo creep relations and temperature dependent Young’s Modulus of Elasticity are used to model solder properties, EVA layer is modelled as viscoelastic while the other component layers are modelled using appropriate constitutive material models. Results show that fatigue life decays with increases in ambient temperature loads. A power function model =721.48−1.343, was derived to predict the fatigue life (years) of c-Si PV modules in any climatic region. Of the various ribbon-contact material combination models investigated, Silver-Silver, Aluminium-Aluminium, Silver-Aluminium and Aluminium-Silver are the top four best performing solder interconnection models with low deformation ratios, , normalised degradation values, 1. Further findings indicate that only the solder layer demonstrates good miniaturisation properties while the standard dimensions for ribbon and contact layers remain the best performing geometry settings. Additionally, from the Taguchi robust optimisation, the Aluminium-Silver ribbon-contact material combination model (ribbon = 180μm, solder = 56μm, contact = 50μm) demonstrated the best performance in elevated temperature climate, reduced solder degradation by 95.1% and is the most suitable substitute to the conventional c-Si PV module solder interconnection in elevated temperature climate conditions – in terms of thermo-mechanical degradation. These findings presented provide more insight into the design and development of c-Si PV modules operating in elevated temperature climates by providing a fatigue life prediction model in various ambient conditions, identifying material combinations and geometry which demonstrate improved thermo-mechanical reliability and elongated fatigue life.Schlumberger Faculty for the Future Foundation (FFTF

Thermomechanical degradation mechanisms of silicon photovoltaic modules

The durability and lifetime of photovoltaic (PV) modules is one of the chief concerns for an industry which is rapidly approaching maturity. Guaranteeing the economic viability of potential PV installations is paramount to fostering growth of the industry. Whilst certification standards have helped to improve the reliability of modules, with a significant reduction in early failures, long-term performance degradation and overall lifetimes are yet to be addressed in a meaningful way. For this, it is necessary to quantify the effects of use-environment and module design. Long-term degradation of the solder bonds in PV modules causes steady power loss and leads to the generation of more devastating, secondary mechanisms such as hot-spots. Whilst solder bond degradation is well-recognised and even tested for in certification protocols, the potential rate of degradation is not well understood, particularly with respect to different environmental conditions and material selection. The complex nature of a standard silicon PV module construction makes it difficult to observe the stresses experienced by the various components during normal operation. This thesis presents the development of a finite-element model which is used to observe the stresses and strains experienced by module components during normal operating conditions and quantifies the degradation of solder bonds under different environmental conditions. First, module operating temperatures are examined across a range of climates and locations to evaluate the thermal profiles experienced by modules. Using finite-element techniques, the thermomechanical behaviour of modules is then simulated using the same thermal profiles and a quantification of solder bond degradation potential in each location is achieved. It is shown that hot climates are responsible for the highest degradation potential, but further to this, hot environments with many ii clear sky days, allowing for large swings in module temperature, are significantly more damaging. A comparison is drawn between indoor accelerated stress procedures and outdoor exposure, such that an equivalence between test duration and location-dependent outdoor exposure can be determined. It is shown that for the most damaging climate studied, 86 standard thermal cycles is appropriate for one-year of outdoor exposure whereas the least damaging environment would require 11 standard thermal cycles. However, these conclusions may only be applicable to the specific module design which was modelled as the material selection and interaction within a device plays a major role in the thermomechanical behaviour and degradation potential. In addition to a study on the influence of use-environment, a study on the influence of the encapsulating material is conducted with a particular focus on the effects of the viscoelastic properties of the materials. It is shown that the degradation of solder bonds can vary depending on the encapsulating material. Furthermore, the intended use-environment could inform the selection of the encapsulating material. The temperature-dependency of the material properties means that some materials will mitigate thermomechanical degradation mechanisms more than others under certain conditions i.e. hotter or colder climates

Effect of operating temperature on degradation of solder joints in crystalline silicon photovoltaic modules for improved reliability in hot climates

Accelerated degradation of solder joint interconnections in crystalline silicon photovoltaic (c-Si PV) modules drives the high failure rate of the system operating in elevated temperatures. The phenomenon challenges the thermo-mechanical reliability of the system for hot climatic operations. This study investigates the degradation of solder interconnections in c-Si PV modules for cell temperature rise from 25 °C STC in steps of 1 °C to 120 °C. The degradation is measured using accumulated creep strain energy density (Wacc). Generated Wacc magnitudes are utilised to predict the fatigue life of the module for ambient temperatures ranging from European to hot climates. The ANSYS mechanical package coupled with the IEC 61,215 standard accelerated thermal cycle (ATC) profile is employed in the simulation. The Garofalo creep model is used to model the degradation response of solder while other module component materials are simulated with appropriate material models. Solder degradation is found to increase with every 1 °C cell temperature rise from the STC. Three distinct degradation rates in Pa/°C are observed. Region 1, 25 to 42 °C, is characterised by degradation rate increasing quadratically from 1.53 to 10.03 Pa/°C. The degradation rate in region 2, 43 to 63 °C, is critical with highest constant magnitude of 12.06 Pa/°C. Region 3, 64 to 120 °C, demonstrates lowest degradation rate of logarithmic nature with magnitude 5.47 at the beginning of the region and 2.25 Pa/°C at the end of the region. The module fatigue life, L (in years) is found to decay according to the power function L=721.48T-1.343. The model predicts module life in London and hot climate to be 18.5 and 9 years, respectively. The findings inform on the degradation of c-Si PV module solder interconnections in different operating ambient temperatures and advise on its operational reliability for improved thermo-mechanical design for hot climatic operations

Thermo-mechanical deformation degradation of crystalline silicon photovoltaic (c-Si PV) module in operation

Reliability and mean-time-to-failure (MTTF) of crystalline silicon photovoltaic (c-Si PV) module operating at elevated temperature can be increased through in-depth understanding of the mechanics of thermo-mechanical deformation and degradation of the laminates bonded together in the system. The knowledge is critical to developing the next generation of robust c-Si PV modules. Deployment in elevated ambient temperature reduces the 25-year design life by inducing excessive deformation that results in significant laminate degradation. The research investigates the thermo-mechanical deformation of c-Si PV module. Analytical and simulation methods are employed in the investigation. The IEC 61215 test qualification is used. Ethylene vinyl acetate (EVA) and solder materials responses are modelled as temperature dependent with appropriate material models. Analytical technique for validating simulation results of the response of c-Si PV module to temperature load is presented. The laminate’s stiffness is found to be governed by the stiffness ratio magnitude of silicon which is the most stressed component. The deformation ratio of EVA is highest and significantly determines the degree of variation of gap between solar cells. The EVA exhibits the highest susceptibility to thermo-mechanical deformation followed by the solder which is found to accumulate the highest magnitude of strain energy density. The research presents an analytical method that can be used to validate the output of computer-simulation of the magnitude of strain energy density of solder in c-Si PV modules

Thermo-visco-elastic modelling of photovoltaic laminates: Advanced shear-lag theory and model order reduction techniques

During lamination, residual thermo-mechanical stresses are induced in the encapsulated solar cells composing photovoltaic (PV) modules. Depending on the material and geometrical configuration of the layers of the laminate, this residual stress field can be beneficial since it may lead to a compressive stress state in Silicon and therefore crack closure effects in the presence of cracks, with a recovery of electrical conductivity in cracked solar cells. It is therefore important to investigate the distribution of thermo-mechanical stresses within the PV laminate with a view to optimizing the coupling between the electrical response and elastic deformation in the operation of PV modules. A promising approach proposed in the present thesis regards the prediction of residual stresses in composite laminates by using a shear-lag theory to model the epoxy-vinil-acetate polymeric layers, accounting for their thermo-visco-elastic response. Moreover, it will be shown that thermomechanical formulations for stress analysis of a PV laminate lead to a system of higher order ordinary differential equations or partial differential equations in which the exact solutions may be impossible to be determined in closed form and hence numerical schemes become desirable. However, the computational cost associated with the implementation of the numerical scheme may be significantly expensive. Therefore, a method to reduce the computational complexity is expected to be very important. To this aim, Model Order Reduction (MOR) techniques are applied hierarchically, first to the thermal system of a PV module in service, and then extended to coupled thermo-mechanical problems. A combination of proper orthogonal decomposition (POD) and discrete empirical interpolation method (DEIM) with a modified formulation is proposed for the first-order thermal equations of photovoltaic system during service and a new coupled second-order Krylov based formulation is developed for model order reduction of the coupled thermo-mechanical model of the photovoltaic module. The results of these reduction schemes show a huge computational gain in the reduced system solutions and a high accuracy of the reduced system outputs

Optimization of thermo-mechanical reliability of solder joints in crystalline silicon solar cell assembly

This is an accepted manuscript of an article published by Elsevier in Microelectronics Reliability on 28/12/2015, available online: https://doi.org/10.1016/j.microrel.2015.12.031 The accepted version of the publication may differ from the final published version.© 2015 Elsevier Ltd All rights reserved. A robust solder joint in crystalline silicon solar cell assembly is necessary to ensure its thermo-mechanical reliability. The solder joint formed using optimal parameter setting accumulates minimal creep strain energy density which leads to longer fatigue life. In this study, thermo-mechanical reliability of solder joint in crystalline silicon solar cell assembly is evaluated using finite element modelling (FEM) and Taguchi method. Geometric models of the crystalline silicon solar cell assembly are built and subjected to accelerated thermal cycling utilizing IEC 61215 standard for photovoltaic panels. In order to obtain the model with minimum accumulated creep strain energy density, the L9 (33) orthogonal array was applied to Taguchi design of experiments (DOE) to investigate the effects of IMC thickness (IMCT), solder joint width (SJW) and solder joint thickness (SJT) on the thermo-mechanical reliability of solder joints. The solder material used in this study is Sn3.8Ag0.7Cu and its non-linear creep deformation is simulated using Garofalo-Arrhenius creep model. The results obtained indicate that solder joint thickness has the most significant effect on the thermo-mechanical reliability of solder joints. Analysis of results selected towards thermo-mechanical reliability improvement shows the design with optimal parameter setting to be: solder joint thickness - 20 μm, solder joint width - 1000 μm, and IMC thickness - 2.5 μm. Furthermore, the optimized model has the least damage in the solder joint and shows a reduction of 47.96% in accumulated creep strain energy density per cycle compared to the worst case original model. Moreover, the optimized model has 16,264 cycles to failure compared with the expected 13,688 cycles to failure of a PV module designed to last for 25 years.The authors acknowledge funding provided by the Petroleum Technology Development Fund (PTDF, PTDF/E/OSS/PHD/ZMT/623/12), Nigeria used in carrying out this study.Published versio