The cells are connected in series via a tunnel junction as intermediate layers. How does the intermediate layer work? Say we have a system of anode/n1/p1/tunnel junction/n2/p2/cathode. 1 high energy photon causes EH generation at the n1/p1 region. 1 low energy photon causes 1 n2/p2 region to have EH generation. A majority carrier hole is collected from the p1 by the tunnel junction, a majority carrier electron is collected from n2 by the tunnel junction. They both then meet at the tunnel and recombine, while the majority carrier electron collected by the anode from n1 and majority carrier hole is collected by the cathode from p2. Does this mean 2 photons can only generate 1 electron flow through the outer circuit? Is my understanding correct? If yes isn't that less efficient?
Dear Christopher,
In electronics/spintronics, a tunnel junction is a barrier, such as a thin insulating layer or electric potential, between two electrically conducting materials. Electrons (or quasiparticles) pass through the barrier by the process of quantum tunnelling.
The tunneling effect is a quantum phenomenon that allows carriers to cross a potential barrier without jumping over it. If the barrier is thin enough and empty sites are available in the right range of energies, carriers from one side can tunnel to these empty sites. It should be noted that although the word “tunnel” carries the image of going through, it is rather the case that carriers disappear and then reappear on the other side of the barrier.
Tunnel Junction in multi-junction solar cells is conductive, optically transparent semiconductor layer used to join different semiconductor materials in order to increase overall device efficiency.
Good luck.
Dear Dr Tan: The following is my input as germane to your question:
Multiple-Junction Solar Cells. In such a tandem solar cell, the larger-band gap material, of which the top cell is made, collects the high-energy photons, while the lower-energy photons are collected in the lower cell.
Article Too Many Junctions? A Case Study of Multijunction Thin-Film ...
Poster Investigation of Metal and Metal Oxide Intermediate Layers i...
Article PbS quantum dot tandem solar cell based on solution-processe...
I trust this will help you in your way forward. Kind regards--
Ijaz Durrani Idris Bouchama
Thank you for your comments. Perhaps showing an image here might explain my question better. As you can see, in a tandem organic solar cell. The high energy photon generates an EHP at the high bandgap layer, and another low energy photon generates an EHP at the low bandgap layer. However, at the end of the day only 1 hole is collected by the anode, and 1 electron is collected by the cathode. The other electron and hole from the high and low bandgap layer, respectively recombines in the middle.
So for this case does it mean only 1 EHP can be collected per 2 photons? Then how does tandem layers increase the efficiency? I understand the Voc can be increased due to the spectral matching that reduces thermalization, but how does that compromise the huge loss in EHP collected?
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