Novel process of grain boundary metallisation on mc Si solar cells

The electronic properties of multicrystalline silicon are heavily influenced by impurities concentrated along the grain boundaries that increase the recombination activity near the crystallite borders. Dopants can also diffuse preferentially down the grain boundaries, which leads to a low resistance path down the grain. These and other effects decrease the efficiency of multicrystalline silicon solar cells. Additionally, the efficiency is lowered by the shading of areas of silicon by metallisation lines due to the reduction of the active conversion area of the cell. We present a new way to combine the grain boundaries and the front contact grid with the aim to improve the efficiency of multicrystalline silicon solar cells. A first approach has been developed to produce multicrystalline silicon solar cells with a front contact metallisation following the grain boundaries: The different grain boundaries of a multicrystalline silicon wafer are detected by optical scanning of the wafer surface. Together with the emitter sheet resistivity this image serves as an input to calculate a net of finger lines that follow the grain boundaries wherever possible. Onto these detected grain boundaries the metallisation is performed by evaporative deposition of copper and photolithography. We report on the successful implementation of such a grid on 100×100 mm2 wafers.