Analysis of recombination losses in screen-printed aluminum-alloyed back surface fields of silicon solar cells by numerical device simulation

The Al-alloyed back surface field (Al-BSF) is a current topic in research and development of crystalline Si solar cells for mass production. We simulate the solar cells in full-size with numerical device modeling, based on incomplete ionization and updated parameters of the Al-O complex in the Al-BSF region, combined with SPICE circuit models. The simulations show that for full-area rear contact, the full-area Al-BSFs should optimally be about 6 μm deep; in shallower Al-BSFs, recombination at the metal contact dominates; and in deeper Al-BSFs, recombination via Al-O defects dominates. The recombination in the Al-BSF dominates the cell's total recombination in well optimized cells. The optimally attainable cell efficiency then is about 18.8% after degradation of the B-O complex in the boron-doped wafer, or 19.1% after deactivating the B-O complexes as completely as currently possible. In PERC cells, having screen-printed Al-BSF fingers and a rear Al2O3 passivation layer, cell efficiency reaches about 19.6% or 20.3% after degradation or deacivation, respectively, with an improved homogeneous emitter that is feasible for mass production, and with screen-printing front contacts. Ideally, the design strategies for PERC structure with various wafer resistivities and rear finger widths are shown. The reduction in cell efficiency due to three different types of voids (cavities) in local Al-BSFs is also quantified in detail.