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Photovoltaic devices have helped us tackle and provide a promising solution to the world energy crisis. For example, photovoltaic devices like solar cells convert sunlight into electricity. These devices are reliable, cost-effective and have high efficiency. Some of the solar technologies being used are wafer, thin-film, and organic based solar cells.
Currently, about 90% of the photovoltaic markets are dominated by silicon wafer-based solar cells. These wafer-based solar cells have high efficiency but are costly. About 50% of the total module cost is due to the silicon wafer.
Thin film based solar cells are currently the most cost-effective option. They consist of semiconductor thin films, deposited on a glass substrate. Thin-film modules are mainly based on amorphous silicon (in single junction or multiple junction arrangements), chalcogenide compounds like CdTe or CdS, or polycrystalline silicon solar cells.
Advantages of Thin Film Solar Cells
Some of the advantages of thin film solar cells include low material consumption, shorter energy payback period, large area modules, monolithic integration, tuneable material properties, low-temperature processes, and transparent modules.
Shorter energy payback period is the time of cell operation in the field, during which it will produce the amount of energy equivalent to the energy required for its production.
Amorphous Silicon - Single Junction Solar Cells
Amorphous silicon (α-Si) is a direct bandgap material that can absorb a large fraction of sunlight from a few thin layers. However, the dangling bonds in α-Si result in short minority carrier diffusion lengths and abnormal electrical behavior.
Thus, to improve the minority carrier length, 10wt% of hydrogen is incorporated with α-Si called hydrogenated amorphous silicon (α-Si:H). Additionally, the incorporation of hydrogen lowers the manufacturing cost and causes a shorter energy payback time.
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These solar cells have a p-i-n junction structure. A transparent conductive indium-tin-oxide (ITO) layer is used between the silicon and the glass, as α-Si is not very conductive. The deposition of α-Si:H can be done by using PVD or CVD.
These α-Si solar cells have been used in calculators and digital watches.
Amorphous Silicon- Multiple Junction Solar Cells
Multi-junction solar cells and modules have multiple bandgaps that can be attuned by varied alloying. In these solar cells, the top junction has a higher bandgap than the bottom junction.
The solar spectrum has a range of photon energies, and photons with energies less than the semiconductor bandgap are not absorbed, e.g., in single junction solar cells. But this problem is overcome by multi-junction solar cells, as they have multiple bandgaps and can absorb multiple wavelength energies.
In multi-junction solar cells, two or more cells are arranged on top of one another. The top junction has a higher bandgap than the bottom junction, which improves the overall performance of the cell. By alloying it with germanium, the bandgap of hydrogenated amorphous silicon can be reduced.
Cadmium Tellurium Solar Cell
In 1972, the first solar cell based on Cadmium Tellurium/ Cadmium Sulphide (CdTe/CdS) was reported with an efficiency of 6%. After significant improvement, the efficiency was increased to 16.5%.
In this solar cell, the p-type semiconductor is CdTe and n-type is a CdS layer. A junction is formed between the p-type layer and the n-type layer. The CdS is a wide bandgap material (2.45eV) that absorbs photons with a wavelength less than 500nm. The absorption in CdS layer is low, and it mainly acts as a buffer layer. The CdTe layer is the main active and absorber layer.
A layer of CdS is deposited on the glass sheet coated with TCO followed by deposition of CdTe. CdTe is deposited by different techniques like close-space sublimation, chemical spraying, electroplating or vapor transport. Currently, two companies, i.e., First Solar in the United States and Antec in Germany are producing commercial CdTe based solar PV modules in large quantities
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