Effect of the back-surface field on the open-circuit voltages of p +-n-n + and n +-p-p + silicon solar cells

Abstract

The effect of the back-surface field (BSF) on the open-circuit voltages V oc of front-illuminated p +-n-n + (p/n BSF) and n +-p-p + (n/p BSF) silicon solar cells was investigated. A theory based on the assumption that the front emitter has a unit injection efficiency was formulated and this theory can be applied to both low and high level conditions in the bulk regions of p/n and n/p BSF cells. The effect of perfect and imperfect minority carrier blocking at the low-high (L-H) junction end of the bulk region was analysed. The theory gives some insight into the physics of the BSF cells. It shows that the minority carrier blocking at the L-H back junction both by itself and by causing a back reflection of minority carriers towards the front-junction end of the bulk region leads to an improvement in the open-circuit voltage of the cells. The contribution of back reflection is independent of the intensity of illumination and the resistivity ϱ B of the bulk region, whereas the contribution of minority carrier blocking increases with both the intensity of illumination and ϱ B . For low level conditions the improvement in V oc is mainly due to the contribution of back reflection. However, for high level conditions the contribution of minority carrier blocking can be quite large. For high level conditions the increase in recombination at the front surface and in the front region is more harmful to the V oc value of a p/n BSF cell. However, a decrease in the minority carrier blocking at the L-H junction is more harmful to the V oc value of an n/p BSF cell. The fact that V oc for BSF cells is independent of ϱ B is attributed to the high level conditions in their bulk regions.