Metallization for high efficiency c-Si solar cells based on Cu-plating
Y.B. de Groot (TU Delft - Electrical Engineering, Mathematics and Computer Science)
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Abstract
To improve the performance of the high efficiency c-Si solar cells within the PVMD group several front metallization methods were investigated. The objective was to decrease the series resistance, which would lead to an increase in FF and thus efficiency. Three different cell structure are examined; i) a poly-poly cell with SiN¬x as an ARC, ii) a hybrid solar cell and iii) a silicon heterojunction (SHJ) device. Last two structures have a conductive TCO layer as a top layer.
For the poly-poly device four sorts of front metallization were examined; Al e-beam evaporation, DC Cu-plating, pulse Cu-plating and Al sputtering. Because Al evaporation was the most commonly used metallization method within the group, this will be the reference cell. The best reference cell had as external parameters a VOC of 689 mV, JSC of 38.3 mA/cm2 and a FF of 73.0%, resulting in a efficiency of 19.3%. With DC Cu-plating fingers with significantly higher aspect ratios (up to 0.28 compared to 0.02 of reference), but unwanted growth of Cu was also detected. Nevertheless, it gave an average increase of 2.3% absolute in FF, which resulted in a higher efficiency of 19.6%. In order to remove the unwanted Cu-growth, pulse Cu-plating was done. When pulse plating was done, no unwanted growth of Cu was detected. The efficiency of the pulse plating device was slightly lower than the other two devices. An efficiency of 18.7%. Sputtering of Al proves to have the same result as Al e-beam evaporation, when the same thickness is used.
Regarding the hybrid solar cells the best results was achieved with Al e-beam evaporation, with a VOC of 704 mV, JSC¬ of 40.7 mA/cm2 and FF of 73.8%. This results in a device with a efficiency of 21.0%. With Cu-plating technique and Ag-paste screen printing, these results were not reached. The Cu-plating technique did show an average increase in FF of 1.8% absolute, due to a lower series resistance. Cu contamination however lowered the VOC and JSC.
For SHJ solar cells the reference solar cell had a maximum efficiency of 17.7%, with an average FF of only 61.0%. Cu-plating on SHJ solar cell had de same contamination problems (low VOC and JSC), but because of the 10.9% absolute increase in FF, it showed higher efficiencies (with a maximum of 18.4%. Ag paste screen printing showed a smaller increase in FF (3.4% absolute).
Optical measurements for the rear reflector were done as well. This resulted in an advised rear contact of 100 nm thermally evaporated Ag and 6 μm of screen printed Ag paste.