Positive effects of front surface field in high-efficiency back-contact back-junction n-type silicon solar cells

The role of the phosphorus-doped front surface field (FSF) in n-type back-contact back-junction silicon solar cells was analyzed. The FSF improves the quality of the front surface passivation and enables very high efficiencies even for the cells with higher front surface recombination velocity. The stability of the front surface passivation using the FSF with respect to UV-light was analyzed by measurements of lifetime samples. Application of the FSF significantly improves the UV-light stability. The surface saturation current density (J0e) of the textured lifetime samples without FSF increased form 30 fA/cm² to almost 450 fA/cm² after 55 hours of UV-light exposure. J0e of the samples with FSF showed only a marginal increase from 30 to 35 fA/cm². An additional positive effect of the FSF is the reduction of the lateral resistance losses. These losses are caused by a significant increase of the pitch on the rear cell side, when only low-cost structuring technologies (screen-printing and laser processing) are applied. An experimental study showed that the FSF strongly improved the fill factors of the cells with large pitches. Two-dimensional device simulations revealed that the FSF significantly contributes to the lateral transport of the majority carrier´s current. The best cell efficiency of 21.3 % was obtained for the solar cell with a 1 Ω cm specific base resistivity and a front surface field with a sheet resistance of 148 Ω/sq.