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-Si C:H/n c-Si heterojunction solar cells at different conditions. Under s tandard test conditions (300 K, 100 mW/cm(2), AM1.5) these cells show normal I-V characteristics with a high fill factor (FF = 0.73) and a r elatively high efficiency for their simple structure (eta approximate to 13%). However, below room temperature and at illumination levels ab ove 10 mW/cm(2) they exhibit an S-shaped I-V curve and a low fill fact or. Simulation studies revealed that this effect is caused by the vale nce band discontinuity at the amorphous/crystalline interface which hi nders at low temperatures the collection of photogenerated holes at th e front contact, At low temperatures a high hole accumulation at the i nterface 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 de pletion region causing a significant current loss (S-shape). Tunnellin g through the valence band spike can reduce these effects. For lower d oped p a-SiC:Ii layers (E-act > 0.4 eV) this S-shape can even occur at room temperature.