Metallization of rear contact in crystalline silicon solar cell using infrared pulsed laser

Abstract

The main problem of silicon-based solar cells is the high fabrication cost due to the high cost of thick silicon (Si) wafers. As a result, the thickness of the Si wafer is being reduced in an effort to lower fabrication costs. Thinning Si wafers, on the other hand, results in increased mechanical stress and caused wafer bowing. The thermal expansion mismatch between Si and aluminium (Al) during the high-temperature metallization process causes this mechanical stress. As a result, a room temperature laser-based metallization technique is one of the options for overcoming this problem because of its capacity to anneal localised regions. The type of laser used in this thesis for the annealing process is an infrared pulse laser with a wavelength of 1064 nm. The annealing process also uses an Al metal type paste on the rear surface of Si without a passivation layer. The metallization optimization process is based on laser power parameters and contact geometry patterns. The laser power range starts from 3.3 – 47.0 W with a circular contact geometry pattern with diameters of 0.05, 0.10 and 0.15 mm and a line-shaped contact geometry pattern with a width of 0.05 mm. The entire laser-based metallization process was performed on p-type czochralski (CZ) Si wafers with a wafer thickness of 200 μm. For laser-based metallization characterization, five Al/Si rear surface contact configurations were selected to measure the performance of light current-voltage (I-V) solar cells: (a) rear surface contact of annealed Al paste on the back surface field (BSF) layer with circle geometry pattern, (b) rear surface contact of Al paste on BSF layer with circle geometry pattern, (c) rear surface contact of Al paste on Si wafer without BSF layer with circle geometry pattern, (d) rear surface contact of Al paste on Si wafer without BSF layer with one-dimensional line geometry pattern, and (e) rear surface contact of Al paste on Si wafer without BSF layer with two-dimensional line geometry pattern. Solar cells for the five configurations obtained efficiencies of: (a) 8.52%, (b) 4.29%, (c) 2.99%, (d) 9.31%, and (e) 10.06%. The solar cell with optimum efficiency of 10.06% is from a twodimensional line geometry pattern with a laser power of 8.3 W. It produces an opencircuit voltage (Voc) of 573 mV relative to 603 mV, short circuit current density (Jsc) of 29.94 mA/cm² relative to 31.20 mA/cm², and fill factor (FF) of 58.29% relative to 68.00% of commercial solar cells with an efficiency of 12.80% respectively. In conclusion, the infrared pulse laser-based metallization technique is capable of annealing paste-type metal without the presence of a passivation layer for the rear contact of Si solar cells.