A semiconductor can absorb photons with the energy of its band gap in a solar cell. Photons with lower energy than the band gap of the devices are not absorbed. Solar radiation contains a broad range of wavelengths. Solar cell actually achieves nearly 100 % conversion at particular wavelengths of light, because electron hole pair can be produced from a single photon at a particular wavelength. However, an electron hole pair is also produced when a photon with a shorter wavelength (higher energy) is absorbed, in which case the excess energy is wasted as heat. This is a cause of low efficiency of the silicon solar cells for its small optical band gap. In multi-junction solar cells, each junction (subcell) absorbs sunlight from a specific region of the spectrum. The subcells can be stacked on top of one another so that sunlight first strikes the highest band gap subcell, which is tuned to light with the shorter wavelengths (highest energies). The longer wavelengths pass through the first subcell and strike the lower band gap subcells (second one), and so on. This arrangement offers a significant advantage over single-junction solar cells. In this way the efficiency of the solar cell is been enhanced. But problem is making a unique electric field from upper electrode to bottom electrode for moving the excited carriers to the outer circuit.
Yes Dr. Muhibbullah good question.
At first i would like to congratulate you with ramadan
The tandem solar cell structure is thought to be the highest efficiency solar cell structure. The subcells are vertically stacked such that electrical connection between them acts an ohmic contact with low resistance. This is achieved by using heavily doped p-n tunnel junction as an example the stack from the top to the bottom is n+/ p/ p+/ n+........../ p+. The bold designated junction between the two cells will be tunneling.
Tunneling junctions behaves nearly ohmic with very low resistance.
In this way the subcells are connected to each other as if they are separated cells connected with metallic wires.
Best wishes
To
Professor Dr. Abdelhalim Zekry
Department of Electronics and Electrical Communication
Ain Shams University
Cairo, Egypt
Dear Sir,
Thank you very much for your answer. It will be helpful in my research.
Sincerely yours,
Muhibbullah
Tandem solar cells promise very high efficiencies. The sub-cells may be connected in current-constrained or non current -constrained modes. Most experimental Silicon tandem solar cells are configured in current-constrained mode.
Much efforts are needed to achieve higher efficiencies compared with even single junction mono-crystalline silicon solar cells (~25%). This is because, the max current is limited by the current through the sub-cell with the lower current capacity. But, higher Voc are achieved in this series connection. In non current -constrained mode, better performance are possible with associated challenges in realization of such solar cells.
You should be interesting in the late work from R. J. Walters et al, "Detailed Characterization of the Radiation Response of Multijunction Solar Cells" presented at 2015 NSREC conference. This work presents the response of triple-junction solar cells to proton irradiation is analyzed using electroluminescence (EL). This analysis allows the dark current of each individual subcell to be determined providing insight into the radiation response mechanisms.
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