Interdigitated back contact silicon hetero-junction solar cells: The effect of doped layer defect levels and rear surface i-layer band gap on fill factor using two-dimensional simulations

Interdigitated back contact silicon hetero-junction (IBC-SHJ) solar cells using a-Si emitter and contact layers show significant potential advantages over standard hetero-junction devices: higher short-circuit current (J_sc) since there is no grid shading and higher open-circuit voltage (V_oc) due to better surface passivation. However, they often suffer from low fill factor (FF). Using two-dimensional simulations to model IBC-SHJ devices on FZ n-Si, we found that the FF was nearly independent of the defect concentrations in contact and passivating i-layers but strongly dependent on the defects in emitter and the band gap in the rear i-layer. V_oc and J_sc were nearly independent of defects in either doped layer. In a-Si doped layers it is well known that the number of defects increase with doping. We find that the FF is sensitive to either mid-gap or band tail states and that S-shaped JV curves responsible for low FF can be eliminated by a decrease in p-layer mid-gap or band tail defect levels, or by decreasing the rear i-layer´s band gap. The insensitivity of FF to defects in the n-layer or in the i-layer suggests the FF is dominated by minority carrier injection/collection from the p-type emitter layer. The dependence of FF on the rear i-layer band gap suggests that increasing the offset in the valence band impedes minority carrier collection.