Impact of contact integrity during thermal stress testing on degradation analysis of copper-plated silicon solar cells

•Non-ohmic behaviour is observed in plated contacts after extended thermal exposure.•Diffusion of copper through defects in the capping metal results in void formation.•These voids are shown to result in non-ohmic behaviour in Suns-Voc measurements.•Severity of non-ohmic behaviour dependent on capping and sintering conditions.•Errors in fitted diode parameters as a result impact cell degradation analysis.

In this study, silicon solar cells with copper-plated front side metallisation were exposed to long-term reliability thermal stress conditions and the material integrity of the plated contacts after stress testing was investigated using imaging and electrical measurements. Significant voltage 'bend-back' was observed in Suns-VOC measurements at high illumination intensities (> 1 Sun) following thermal stress testing at 200 °C for 500 h of laser-ablated cells with a nickel/copper/silver plated front metal grid. Using a combination of focussed ion beam milling, high resolution imaging and energy dispersive X-ray spectroscopy, it was shown that large voids can form between the silver capping layer and the main copper stack during thermal annealing. However, even more revealing was the detection of a new metal layer comprising largely of diffused copper overlying the silver capping. The cause of the Schottky 'bend-back' behaviour was theorised to be due to increased contact resistance arising from the voids which are presumed to form as a result of grain boundary diffusion of copper through the silver capping layer. Errors of 5-10% in the determination of pFF from Suns-VOC occur as a result, with the scale of the error dependent on the capping method and sintering conditions. Collapsing the voids was subsequently shown to remove the Schottky behaviour and improve reliability of the fitted diode parameters extracted from Suns-VOC measurements.

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