Crystalline Silicon |
Thin-Film |
Multi-Junction |
Mono-crystalline: The atoms form a regular lattice. Due to the regular structure, mono-crystalline has a better response rate. Purity is > 99.99999% (7N) |
Thin film modules use compounds with very stron light absorption characteristics, requiring only a thin layer. The absorption materials are deposited on glass or foil. They include Amorphous Silicon (a-Si): Atoms do not form diamonds, but are randomly put together. Not surprisingly, the response rate is much lower than in mono-crystalline structure. However, it captures more of the highly-intensive light than crystalline silicon and can be changed by alloying it with, among others, Germanium or Carbon. CdTe: Cadmium-Telluroid: Inexpensive technology with medium efficiency. CIS/CIGS (cadmium-indium-gallium-selenide and copper indium gallium diselenide): Have efficiencies as high as crystalline silicon. |
Differences in spectral sensitivities between materials can be exploited by adding multiple absorption layers on top of each other, resulting in multiple p-n junctions.
The material with the largest band gap is positioned closest to the incoming light, absorbing all the high- energy photons. For low- energy (long wavelength) photons, this first layer will be transparent until they hit the second layer with a lower band gap. This way, overall efficiency can be increased to 40%, compared to 20% for monocrystalline silicon. |
References:- Hyperphysics: Semiconductor Band Gaps |
The Principles of Photovoltaics |
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The layers of a solar module
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The Photo-effectIn simple terms, the photo effect describes the conversion of light into an electric current. To describe this mechanism more formally, it is best to think of light in terms of a stream of photons where each photon carries one quantum of energy. Each photon is associated with just one wavelength or frequency. High-frequency photons have more energy than the ones with low frequency. Intrinsic SemiconductorIn a pure semi-conductor the outermost electron of the underlying molecule is not heavily bound. An incoming photon with enough energy can promote the electron from the valence band to become a free electron in the conduction band. See figure above. This in turn leaves a positive hole in the valence band. The minimum energy that is necessary for this to happen is called the band gap. The band gap varies from material to material and also varies with temperature, which is why performance of solar modules deteriorates with higher temperatures. However, in an intrinsic semiconductor, no resulting electric current is observed, since the promoted electrons re-combine again with the holes.Doped SemiconductorsDoping means the addition of a small percentage of foreign atoms in regular crystal lattice of the semiconductor.
Semiconductor with p-n JunctionWhere p-type and n-type layers join at the p-n junction, electrons and holes diffuse to create the charge-free depletion zone. Moreover, the junction creates a slope in the resulting energy bands. Now, when a photon promotes an electron to the conduction band, it can subsequently "roll down" through the depletion zone into a lower energy band rather than instantly re-combine with a hole. This is what generates the photo current.
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Band Gaps
To convert electron-Volts into wavelenghts of incoming light, we use the forumla:
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Spectral SensitivityIn reality, the photo effect is not like an on- off- switch depending on the energy of the photon. At longer wavelengths, electrons may still flow due to energy from ambient temperature. On the other hand, short wave photons may not be able to be absorbed, as they have too much energy. The Response Rate measures a material's ability to convert light into an electric current: Due to the spectral sensitivities, this rate is highly dependant on the wavelength of the incoming light. Most of the energy of the sun light is in the visible spectrum.
Although silicon captures a wider spectrum than the human eye, it is more sensitive to infrared than to visible light, thus less efficient in converting the all-important visible spectrum.. Other materials (III-V compounds) such as GaAs, GaInP or GaAsAl cover wider ranges where the spectral sensitivity is better matched to the incident sun light, thus increasing efficiency of the cell. By applying two materials in a tandem or multi-junction cell, an even wider spectrum can be captured.
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