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Amorphous Silicon Carbide/Crystalline Silicon Heterojunction Solar Cells: A Comprehensive Study of the Photocarrier Collection

We have studied the current–voltage (I–V) characteristics of p+ a-SiC:H/n c-Si heterojunction solar cells at different conditions. Under standard test conditions (300 K, 100 mW/cm2, AM1.5) these cells show normal I–V characteristics with a high fill factor (FF = 0.73) and a relatively high efficiency for their simple structure (η≈13%). However, below room temperature and at illumination levels above 10 mW/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 current loss (S-shape). Tunnelling through the valence band spike can reduce these effects. For lower doped p a-SiC:H layers (Eact>0.4 eV) this S-shape can even occur at room temperature.