Amorphous silicon carbide/crystalline silicon heterojunction solar cells: A comprehensive study of the photocarrier collection

We have studied the I-V characteristics of p+ a-SiC:H/ n c-Si heterojunction solar cells at different conditions. Under standard test conditions (300K, 100mW/cm2, AM1.5) these cells show normal I-V characteristics with a high fill factor (FF=0.73) and a high efficiency (??13%). However, below room temperature and at illumination levels above 10mW/cm2 they exhibit an S-shaped I-V curve and a low fill factor. Simulation studies revealed that this effect is caused by the valence band discontinuity at the amorphous/crystalline interface which hinders at low temperatures the collection of photogenerated holes at the front contact. At low temperatures a high hole accumulation at the interface combined with extra trapping of holes inside the p+ a-SiC:H layer causes a shift of the depletion region, from the c-Si into the p+ a-SiC:H. This leads to an enhanced recombination inside the c-Si depletion region causing a significant loss in photocurrent (S-shape). Tunnelling through the valence band spike can reduce these effects. For lower doped p a-SiC:H layers (Eact >0.4eV) this S-shape can even occur at room temperature.