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|GML (Gradient Multi-Layer) nano-film|
Recently, a new type of nanostructure - GML (Gradient Multi-Layer) nano-film - attracted attention of the researchers in the solar cell industry. The properties of this structure can be used in design of more efficient and less expensive solar cells, as well as other nano-structured devices.
GML Nano films
Quantum Dots are spherical nano-particles (or nano-crystals) typically made of semiconductor or metal.
Nano-structured Materials (or Devices) typically contain two or more interpenetrating nano-scale networks (Bulk Hetero Junctions or BHJs) including Organic (Polymer Blends), Inorganic (Quantum Dots only), and Hybrid (Polymer with Embedded Quantum Dots).
Gradient Multi-Layer (GML) Nano-structure (or nano-film) is a stack of Quantum Dot layers arranged to form a size gradient, composition gradient, density gradient or composition/size modulation with the strict control of each layer thickness and composition. The GML nano-film may include two or more types of Quantum Dot material to form Bulk Hetero Junctions or it can be embedded in the organic material (polymer).
GML Nano films have been described so far in only a few publications i.e. "Microchemical Nanofactories", US Patent Publ. 20080108122 (chemical method of building GML Nano); "Nanophotovoltaic Device with Improved Quantum Efficiency", US Patent Publ. 2008142075 2008.(mine); "Energy transfer between quantum dots of different sizes for quantum dot solar cells", 34th PV Spec.Conf., 2009 (Stanford research).
GML Nano films applications for PhotoVoltaics
Assembly of Quantum Dot layers can be designed to efficiently absorb the most of the Sun spectrum (0.3-2.0+ eV) by size and composition tuning. Specifically, one type of Quantum Dots can be selected from the low band-gap material (i.e. PbSe, InAs, Ge, others) to be able to absorb InfraRead part of the spectrum, and by tuning the QDs' sizes Quantum Dots made from the same material will absorb "green/yellow" part. The other Quantum Dots may have a wider band gap to absorb the "blue/UV" part of Spectrum. Due to the Quantum Confinement light absorption in Quantum Dot layers is very strong so the entire Sun spectrum can be absorbed within several tens to a hundred of nanometers.
Quantum Dots generally exhibit Multi Exciton Generation (MEG) phenomena, i.e. generation of more than one electron-hole pair by a single high-energy photon. GML Nano-films, containing low band gap QD's are expected to exhibit and utilize this phenomena, at least in some portion of the film thus enhancing Power Conversion Efficiency of GML Nano-film solar cell.
Size gradient in the GML Nano film creates corresponding gradient of the electro-chemical potential, which is equivalent to generation of high built-in Electric field in the film, which enhances transport of electrons and holes thus improving internal quantum efficiency (IQE) and photo current.
GML Nano films can exhibit phenomena of light trapping and photon re-emission, which additionally enhances IQE.
In case the efficient method of building GML Nano film is available the efficient PV structure can be formed that will provide the highest possible Power Conversion Efficiency.
PV expectations and main challenges
Theoretically, assuming a perfect transport (IQE), Multi Exciton Generation and perfect light trapping/photon re-emission and max possible Voc, the PCE can reach 65+% (still below the thermodynamic limit of about 86%). It is, however, a very challenging goal (see e.g. "Prospects of Nanostructure-Based Solar Cells...", Int. Journ. Of Photoenergy, 2009, id 154059).
One challenge here is to design a GML Nano structure in such precise and well-controlled way that allows for successful utilizing most or all of the above advantages of QD systems. But, even if the "smart" design of an efficient GML Nano structure had been available it would be unclear as to how to build such a structure in a well controlled, reliable, inexpensive and production-worthy way.
This represent another big challenge as most of the known methods of building nanostructured material are either unable to form a GML Nano structure (spin-coating, generally all solution coating methods) or are very expensive with low throughput and difficulties to be transferred to a full-scale production environment (Atomic Layer Deposition (ALD), Langmuir-Blodget or Microchemical method of US 20080108122).
For more information, please click here
Dr. Boris Gilman
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