Current collecting channels in RGS silicon solar cells : are they useful?

Abstract

Ribbon growth on substrate (RGS) silicon could be the crystalline silicon material for PV of the future. The extremely fast production technique avoiding any material losses due to sawing drastically reduces the wafering costs. On the other hand, one has to deal with more crystal defects (grain boundaries, dislocations, impurities), which especially limit the diffusion length and normally result in small short-circuit current densities Jsc. The charge carrier collection probability can be increased by a macroscopic V-texture of the surface, but even more effective would be a 3-dimensional emitter structure within the whole bulk cell volume. This was observed in some RGS solar cells showing minority carrier lifetimes of only around 0.4 μs after cell processing, but Jsc of above 34 mA/cm2. In these cells, the whole bulk volume collects current despite the small diffusion lengths. This behaviour was investigated using spatially resolved lifetime and internal quantum efficiency mappings, capacitance measurements and a special EBIC technique, where the electron beam hits the backside of the wedge-shaped solar cell. From our results, we conclude that the collecting structures may be caused by inversion in combination with a high O content. Cells with large areas of collecting channels exhibit lower fill factors, but nearly no loss in open-circuit voltage as compared to the standard RGS cells. For both types of cells, confirmed record efficiencies of 12.5% have been obtained.