When the PV solar cell is used in a multi layer form, the goal is to benefit of a large interval of solar radiations. I think that the gap and the contact of different layers must have certain properties. How to choose the different materials for this application: (gap and type of contact) ?
In order to optimise the best contact and Eg for solar cell, you should manage the energy level alignments for your materials.
1- Lower band gap means higher absorption in the visible and NIR region. but you could manage different materials to cover more regions in the spectrum.
2- More layer could increase the device resistance.
3- The contacts will be employed to collect a specific charge carrier (e or h). i.e. Al usually used to collect electrons while gold used for holes.
I'm not sure what you mean by "contact type". Different interfaces between the layers of a multijunction solar cell exist:
-pn-junctions in each sub-cell, tunneljunctions between sub-cells
-interface between two different semiconductor layers
-metal-semiconductor junctions
...
To reach maximum efficiency you have to choose proper energy gaps for a given sun spectrum.
For a tandem solar cell (two sub-cells) at 1 sun conditions a combination of 0.98 eV for bottom cell and 1.87 eV for top cell is ideal. For 3-J cell 0.82 eV, 1.44 eV, 2.26 eV is the ideal combination.[1]
You have to consider that your sub-cells are series connected and therefore the ideal gap combinations slightly change [2].
Another constraint is the following:
To produce the cell, you have to grow the sub-cells on a substrate in a MOCVD reactor. To obtain a good film quality, it is necessary to choose materials that are matched in lattice constant and of course have the same lattice structure. Also other possibilities are developed [3].
To design a perfect MJ solar cell you have to choose materials with the same lattice constant and nearly the band gap combinations given above.
For example a lattice matched GaInP/GaInAs/Ge cell (state-of-the-art for space and concentrator application) with a gap combination of 1.8eV/1.4eV/0.7eV was realized with an efficiency of 41.6% (AM1.5d, 364 suns) [4].
Further informations about MJ cell design and different approaches can be found in ref. [4].
1. Martí, A. & Araújo, G. L. Limiting efficiencies for photovoltaic energy conversion in multigap systems. Sol. Energy Mater. Sol. Cells 43, 203–222 (1996).
2. Brown, A. S. & Green, M. A. Detailed balance limit for the series constrained two terminal tandem solar cell. Phys. E Low-dimensional Syst. Nanostructures 14, 96–100 (2002).
3.Dimroth, F.; Roesener, T.; Essig, S.; Weuffen, C.; Wekkeli, A.; Oliva, E.; Siefer, G.; Volz, K.; Hannappel, T.; Haussler, D.; Jager, W. & Bett, A. W. Comparison of Direct Growth and Wafer Bonding for the Fabrication of GaInP/GaAs Dual-Junction Solar Cells on Silicon IEEE Journal of Photovoltaics, 2014, 4, 620-625
4. Philipps, S. P., Dimroth, F. & Bett, A. W. Solar Cells. Sol. Cells 353–381 (Elsevier, 2013). doi:10.1016/B978-0-12-386964-7.00012-3
Adding to Michael, the multijunction solar call is a tandem vertical solar cells connected in series. The solar spectrum is distributed among the tandem cells such that their optimum maximum power point current are equal . This is the matching condition for cells connected in series. since every cell would absorb specific band of wavelengths, the cells acts a band absorption filter with two cut off wavelengths. The larger cut off wavelength of the cell determines its bandgap such that Eg= h C/ Lambda cut off larger. It is the availability of the matched and compatible material combinations that determines the number of possible tandem cells and consequently the energy gaps and the photocurrent of the multijunction structure.
Dear Kebbab
Band engineering of nanoparticles such as QDs can be useful for fabricating multilayer structures.
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