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Solar Engineering is based on the use of bulk semiconductive materials for solar-energy conversion in photovoltaic devices. In these materials, a single photon can only produce one exciton, having a limited efficiency in electrical current production. The production of multiple excitons by a single photon in quantum-confined systems may improve the efficiency of optoelectronic devices, solar cells or panels, carbon nanotube antennas, low-threshold lasers, etc.
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What is an Exciton?
When a high energy photon hits the surface of a semiconductive material such as Silicon, an electron leaves the valence band and enters the conduction band. This is possible when the energy of the photon is equal to the material's energy gap. The electron will leave an empty space behind, a hole. Exciton is the term that describes the electron-hole pair created by this process.
In order for a solar cell to be efficient, the incident photon's energy should match the bandgap energy of the cell's material. Silicon's band gap energy for example, corresponds to the infrared region of the electromagnetic spectrum. An infrared photon will create an exciton and loose all of its energy. Each photon can only transfer its energy to one electron, consequently producing only one exciton. In the case of higher-energy photons (visible or ultraviolet), the excessive energy is converted into heat through crystal lattice vibrations (or phonons) and therefore, it can not contribute any further to the production of electrical current.
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The Multiple Exciton Generation (MEG) Effect
It has been experimentally demonstrated that it is possible to utilize the lost photon energy in order to produce multiple excitons and consequently an amplified electrical current signal from a single photon. The multiple-exciton energy can never exceed the amount of energy carried by the incident photon, so that the whole process is in accordance with the conservation of energy principle.
A similar process is even possible when bulk materials are used and it is called impact ionization. However, the enhancement of solar-energy conversion is only possible in quantum-confined systems, such as quantum dots, where the Multiple Exciton Generation Effect – MEG is observed.
The observation of the MEG can only be explained through the complicated quantum dynamics of nanostructured semiconductors. More specifically, these quantum confined systems have a small Exciton Bohr Radius which is a measure of the maximum distance between an electron and the hole. In a bulk semiconductor the Exciton Bohr Radius is very small compared to the dimensions of the crystal, so the exciton can wander throughout the crystal. In the case of quantum dots, the exciton can not move freely throughout the crystal due to the small size of the dot.
The use of quantum dots for the increase in conversion efficiency, still has a certain weakness. The exciton's lifetime is only a few hundred femtoseconds (order of 10-15sec). This means that an electron can only stay in the conductive band for a limited period of time before it reunites with a hole and the exciton disappears. The exciton's brief lifetime is due to the small Bohr radius of the quantum dot.
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Can Multiple Excitons Really Increase the Conversion Efficiency of Solar Cells?
Researchers have already accomplished the production of 7 excitons per photon in a PbSe quantum dot. The small lifetime of these multiple excitons, introduces certain difficulties in the manufacturing process of photovoltaic devices. The design of new material systems could enhance the performance of these devices and achieve the ambitious 50% percentage of conversion efficiency for solar cells.
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- “Seven Excitons at a Cost of One: Redefining the Limits for Conversion Efficiency of Photons into Charge Carriers”, R.D.Schaller, M.Sykora, J.M.Pietryga, V.I.Klimov, 2006
- “High-efficiency carrier multiplication through direct photogeneration of multi-excitons via virtual single-exciton states”, R.D.Schaller, V.M.Agranovich, V.I.Klimov, 2005
- “Multiple excitons could improve solar cells”, nanotechweb.org