Mwm. Vancleef et al., AMORPHOUS-SILICON CARBIDE CRYSTALLINE SILICON HETEROJUNCTION SOLAR-CELLS - A COMPREHENSIVE STUDY OF THE PHOTOCARRIER COLLECTION/, JPN J A P 1, 37(7), 1998, pp. 3926-3932
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.