Ribbon growth on substrate : a roadmap to higher efficiencies

Abstract

Ribbon Growth on Substrate (RGS) is a very promising low cost silicon ribbon material for photovoltaics. The RGS production principle allows the direct growth of silicon wafers ready for solar cell processing without any aftertreatment such as wiresawing with a very high production speed of one wafer per second. The material quality of the wafers was continuously improved during the last years in terms of a lowered oxygen content. The lowered oxygen content allows a faster hydrogen diffusion which enables an enhanced passivation of bulk defects. As a result the lifetime of the RGS material strongly improved and shows average values of up to 5 μs on present material within an industial type screen-printing firing through silicon nitride solar cell process. Until a new, totally rebuilt production facility is finalized, which is currently built up, the efficiencies of RGS solar cells can be improved by different process related approaches such as gettering of impurities, hydrogenation of bulk defects, avoidance of material and process induced shunts as well as surface texturisation. A further basic approach is to utilize the occuring drift field of gallium doped RGS wafers in order to enhance Jsc. Due to the fine crystal structure of RGS material with crystal sizes in the submillimeter range different surface texturisation methods had to be analysed. Reflectivity and quantum efficiency measurements demonstrate the achievable gain in Jsc. Because of the limited output of the lab-scale R&D machine solar cell processing was performed on 5x5 cm2 wafers in the past and resulted in cell efficiencies close to 13%. 12% efficient 10x10 cm2 RGS solar cells are presented and demonstrate the scalability of both the material homogeneity and the cell process.