Investigation of the forming process and characteristics of Al-BSF in silicon solar cells

This paper is intended to give insights into the forming process and characteristics of aluminum back surface field (Al-BSF) made by thermal evaporation and conventional thermal processing. It is indicated that Al-BSF formation is the result of the recrystallization of silicon and the dissolved silicon is responsible for the BSF thickness. A sufficient oxygen supply at the early stage of firing process is essential to thicken and strengthen oxide layer to protect the liquid aluminum and silicon during firing. It will be formed Al-BSF/Al-Si eutectic/Al₂O₃ multilayer structure at the back side of Si solar cells and the X-ray measurement demonstrates that the eutectic layer mainly contains Al_3.21Si_0.47. The Al-BSF has good electrical properties compared with boron-doped p⁺-Si by Hall tests. Solar cell with Al-BSF significantly increases long-wavelength response compared with the cells with diffused or ion implanted boron-BSF, and the back surface recombination velocity can be reduced as low as to 200 cm/s for Al-BSF passivation.