HIGH FILL-FACTORS OF BACK-JUNCTION SOLAR CELLS WITHOUT FRONT SURFACE FIELD DIFFUSION

We investigate by numerical simulations the influence of the front surface field (FSF) in high- efficiency interdigitated back contact (IBC) solar cells . It is often found that IBC cells with FSF show higher fill factors (FF) as compared to cells without FSF. By analyzing the local recombination currents we show that the major benefit of the FSF for the fill factor of IBC cells is not based on supplying lateral conductivity in the FSF. More important is that a diffused FSF ensures low level injection conditions at the surface. Recombination in low-level injection results in a lower ideality factor and therefore in higher FF as compared to recombination in high-level injection. Furthermore we show that the diffused FSF can be replaced by a sufficient amount of fixed charges at the surface without causing a decrease in FF. Fixed negative charges of front-side Al2O3 passivation create in low-injection an inversion layer at the surface of n-type solar cells and maintain the surface in low-injection even when the bulk enters high-injection. In addition the absence of the doped region at the front produces less Auger recombination compared to front sides with FSF. Consequently, Al2O3 front side passivation without FSF enables higher efficiencies compared to front surfaces with FSF.