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Laser ablation of the rear dielectric layer for PERC solar cells

The Passivated Emitter Rear Contact (PERC) solar cell architecture demands an efficient method to form localized uniform back surface field regions. Hence, the opening of the rear side thin dielectric layer without creating many damages to the silicon substrate underneath holds a vital role. Among the various methods such as photolithography, chemical etching, diamond scratching, laser ablation has the advantage as a fast, accurate, and cost-effective process. However, laser ablation is a thermal process; it is essential to choose optimal process parameters such that thermal damage of the silicon substrate is minimal without compromising on productivity. This has been achieved using a synergistic approach involving computational modelling, experiments, and process optimization (see figure below). We have developed a computational model of the laser heating process incorporating a double layer (passivation layer coated silicon substrate) to determine a ballpark estimate of the range of process parameters based on the temperature profiles as shown in the figure on the left below.
Further, experiments were carried out in the range of process parameters obtained from the computational model. Silicon oxynitride dielectric layer of thickness 85 nm was deposited using plasmaenhanced chemical vapour deposition system on single side polished silicon wafers. Green 532 nm laser developed at Sahajanand Laser Technology Ltd., Gujarat, India was used for carrying out the experiments. The repetition frequency of 30kHz, speed of 60 mm/sec, the pulse width of 80 ns and spot size of 17 μm were used in this study. The effectiveness of laser ablation was studied using surface imaging by field emission scanning electron microscope (FESEM) and energy dispersive spectroscopy (EDS). The reduction in the elemental percentage of nitrogen and oxygen at position 2 in the figure on the right below, validates the ablation of the silicon oxynitride layer.

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Methodology used in our study.

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Temperature variation with time at different values of fluence in the single pulse laser model.
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Top surface imaging and EDS analysis at the nonablated
(Position 1) and ablated (Position 2) regions.

Durga Prasad
Prof. Deepak
Prof. Anil