Thin-Film Silicon PV Technology II

Consider a tandem (double junction) cell (see diagram above), the blue and green wavelengths of light are absorbed by the top cell, while red is absorbed in the bottom cell.  The hole generated in the a-Si top cell drifts to its p-layer and collects at the front contact, while the electron generated in the nc-Si bottom cell drifts to its n-layer and collects at the back contact.  The electron generated in the a-Si top cell drifts to its n-layer and recombines with the hole generated in the nc-Si bottom cell that drifts to its p-layer.  This happens because there is a recombination tunnel junction between the n-layer of a-Si and p-layer of nc-Si.  This is often thin and full of defects, which helps in the recombination.

The J-V curves of the tandem cell is shown in the diagram above on the right.  On the left are the a-Si and nc-Si curves shown separately.  The V_oc of the tandem cell is the sum of the V_oc of the top and bottom cells.  The J_sc of the tandem cell is, however, lower than the J_sc in either top or bottom cell.

The diagram above shows the various solar cells from single junction to triple junction, and their efficiencies.  However, for the last triple junction with an efficiency of 16.3%, the a-Si:H alloys suffer from light induced degradation (the Staebler-Wronski effect - SWE), and the stable efficiency reduces to below the 13.4% efficiency of the a/nc/nc triple junction.  SWE is caused by carrier recombination that generates some metastable defects in the absorber layers, and is one of the biggest challenges for thin film solar cells.  SWE can result in 10-15% reductions in efficiency of a-Si cells.

Finally, textured surfaces are used to scatter light and improve the absorption path length.  Intermediate reflector layers are also used.  This is done by separating the top and bottom cells of a tandem cell by a thin layer of low reflective index material (doped nanocrystalline silicon oxide), which causes more light to be reflected into the top cell.  Hence, the a-Si top cell can be thinner, and is less sensitive to SWE.  The intermediate layer also adds to the generation of the built in electric field over the absorber layers.



Reference:
5.2.2 Thin-Film Silicon PV Technology II, Delft University of Technology, https://www.youtube.com/watch?v=L-vIPE6uPck