The kinetics controlling the electrical transport inside the c-Si tunnel-recombination junction ͑TRJ͒ of a-Si:H/a-Si:H tandem solar cells was studied in detail with computer simulations. Trap assisted recombination tunneling and Poole-Frenkel mechanisms were included in our analysis. Three different c-Si tunnel junctions were investigated: ͑a͒ n-p, ͑b͒ n-oxide-p and ͑c͒ n-i-p. The highest theoretical efficiencies in a-Si:H/a-Si:H tandem cells were achieved with the n-i-p tunnel junction structure. The impact of the c-Si effective masses, mobility gap, and mobilities in the tandem solar cell efficiency is also studied in this article. Several a-Si:H/a-Si:H tandem solar cells were made with the c-Si tunnel configurations of types ͑b͒ and ͑c͒. In all of these samples one extra oxide layer was needed at the i-a-Si:H/n-c-Si interface. Both tunnel junctions lead us to comparable experimental tandem solar cell efficiencies. When the n-i-p structure is implemented as TRJ in the a-Si:H/a-Si:H tandem solar cell, efficiencies sensitively depend upon the tunnel junction intrinsic layer thickness. The optimization of this thickness provides a more controlled way of maximizing the tandem solar cell efficiency. Illuminated J-V and QE characteristics were successfully fitted using computer modeling.
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 (E
act>0.4 eV) this S-shape can even occur at room temperature.
Amorphous silicon pin solar cell with a twolayer back electrode Performance and analysis of amorphous silicon pin solar cells made by chemicalvapor deposition from disilane
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