The degradation of high-intensity BSF solar-cell fill factors due to a loss of base conductivity modulation

The loss of base conductivity modulation in high-intensity back-surface-field silicon solar cells is discussed. It is shown that the loss of conductivity modulation begins in a region immediately in front of the back-surface-field region and spreads toward the front of the device as the current through the cell increases. The effect of this loss of conductivity modulation, in cells which have high base resistivity, is to cause a significant degradation in the fill factor of the device. A computer simulation of the device is presented which shows that the degradation of the fill factor is more severe in n⁺-p-p⁺cells than in p⁺-n-n⁺cells of the equivalent geometry and base resistivity. The origin of this difference is found to lie in the mobility difference between electrons and holes in silicon. The effects of the loss of base conductivity modulation can be reduced either through an increase in the base doping or through a reduction in the base width.