Thermomechanical Stress in Solar Cells and Modules: A Finite Element Modelling and Experimental Investigation

With the swift progress in Si Photovoltaic (PV) cell and module technologies, significant changes in geometry, materials, and technological processes are taking place. The adoption of emerging technologies raises concerns about potential durability issues like solder joint failure and cell cracking, which can be attributed to thermomechanical stress induced during the fabrication process. This thesis aims to deepen the understanding of the evolution of thermomechanical stress in Si cells and modules by utilising finite element modelling (FEM) complemented by experimental validations. Models were developed to analyse the stress evolution in Multi-Busbar (MBB) tiled modules after soldering and lamination. The impact of cell overlapping in modules and the selection of solders on maximum stress and stress distribution of MBB modules was investigated. Simulation results predicted that a maximum 1st principal stress of up to ~230 MPa occurred in the Si wafer at the edge of outmost Ag pads. In addition, highly localised stress areas occurred at the overlapped area when tiled which was verified by Raman spectroscopy. The application of SnBi solder can effectively reduce the maximum 1st principal stress in the tiled module to ~160 MPa, although its material properties tend to result in higher stress in the interconnection with a cell gap than SnPb solder. The evolution of thermomechanical stress in screen printed Ag and Cu plated Silicon Heterojunction (SHJ) cells throughout annealing and soldering was explored. It was found that Ag electrodes induced only negligible stress after annealing, with a comparatively low stress of ~160 MPa in Si cells. In contrast, Cu pads underwent both elastic and plastic deformation, with an average stress of ~90 MPa observed along the edges of the Cu pads post-soldering, and a significant stress concentration of ~220 MPa at the edges of the outermost Cu pads. Raman mapping confirmed the FEM's prediction of stress distribution. The FEM also suggested that increasing the width of Cu pads in the direction of the Cu wires effectively reduces soldering induced stress. Finally, a detailed methodology was presented for determining the constitutive model of plated Cu on SHJ cells by integrating nanoindentation and reverse FEM. SHJ cells with Cu pads were mounted in epoxy resin and mechanically polished for nanoindentation. The power law constitutive model of the plated Cu was obtained by aligning the FEM and experimental load-displacement curves. This model enables the precise prediction of thermomechanical stress evolution in Cu plated SHJ cells and deepens the understanding of how cell metallisation can potentially affect the durability of modules.

Grantor dc:publisher

UNSW, Sydney

Year dc:date

2025

Author dc:creator

• Wang, Zhimeng ; https://orcid.org/0000-0003-2767-3216

• Finite element modelling
• Thermal stress
• Silicon
• Interconnection
• Raman spectroscopy
• Nanoindentation
• anzsrc-for: 400910 Photovoltaic devices (solar cells)

Statement dc:rights

• open access
• CC BY 4.0
• free_to_read

Licence

• https://purl.org/coar/access_right/c_abf2
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

en

Identifier

https://doi.org/10.26190/unsworks/31896

OAI identifier oai:identifier

oai:unsworks.library.unsw.edu.au:1959.4/106639