Improved bulk and emitter quality by back-side aluminum doping and annealing of polycrystalline silicon solar cells

The influence on solar-cell properties of thermal annealing polycrystalline silicon wafers is presented. Electrical performance and quantum efficiency measurements revealed that cells produced from wafers which received a 1-h anneal at 700 degrees C with an aluminum-doped back side exhibit a bulk diffusion length enhancement of 15% over nonannealed cells. A 5% bulk diffusion length improvement was found when Al was absent during the anneal. The blue response was improved compared to unannealed cells and was independent of emitter sheet resistance for cells with an Al-doped back side during the anneal. In standard cells and cells annealed without Al, this blue response decreased with increasing sheet resistance. These data suggest that in wafers with low emitter phosphorus concentration, aluminum takes over the passivating and/or gettering action of phosphorus. Aluminum presumably reaches the front side of the cell by grain-boundary diffusion through the entire wafer thickness, thereby passivating grain and subgrain boundaries.