Solar cells convert light into electricity based on photoelectric effect and are used as for providing clean energy without pollution. Nowadays, solar cells have been widely researched and manufactured and are gradually adopted to replace traditional electric power.
Generally, solar cells are formed with a semiconductor device, such as monocrystalline silicon, multicrystalline silicon, amorphous silicon or other thin film semiconductor devices which include a p-n junction. The method of forming the p-n junction includes doping n-type or p-type ions into semiconductor device by diffusion or ion implantation. When the semiconductor device is illuminated by light, electron hole pairs are generated in semiconductors, such as silicon, and electrons and holes are separated drifted to form a current by an internal field formed by the p-n junction. In order to collect the current generated by the solar cells, metal electrodes are formed on surfaces of the solar cells. As shown in FIG. 1, the metal electrodes formed on the solar cells 10, 20 include back electrodes 12, 22, finger electrodes 13, 23 which are formed on the front side 11, 21, and busbar (or buss bar) electrodes 14, 24 which are formed on the two terminals of the finger electrodes 13,23.
The finger electrodes 13, 23 and the busbar electrodes 14, 24 are usually formed by coating silver paste on surfaces of the solar cells 10, 20 by screen printing. Besides, the finger electrodes 13, 23 are usually made with smaller widths and arranged in parallel with a predetermined interval to minimize shading effect. The back electrodes 12, 22 also can be formed by screen printing and is usually made of aluminum. However, the back electrodes 12, 22 are different from the finger electrodes 13, 23 that they can be formed by coating aluminum on a whole surface of the solar cells 10, 20.
A single solar cell usually provides an output voltage of about 0.5 volt to about 0.7 volt, and therefore a plurality of solar cells are connected in series to constitute a solar module to provide a required operating voltage. As shown in FIG. 1, neighboring solar cells are usually series connected with their front and back electrodes via ribbons 30. For example, a terminal of the ribbon 30 is connected to the busbar electrode 14 formed on the front side of the solar cell 10 while the other terminal of the ribbon 30 is connected to the back electrode 22 of the solar cell 20 and therefore a solar module 1 is formed with serially-connected solar cells, hereinafter a “series-connected type solar module.”
The above process requires steps of forming metal electrodes and soldering ribbons. Since traditional soldering process is usually performed at a higher temperature, micro cracks may be generated on the surface of the crystalline silicon solar cells because of the thermal expansion/shrinkage coefficient difference between metal electrodes and crystalline silicon substrate.