In a conventional solar cell light is absorbed by a, producing an electron-hole (e-h) pair; the pair may be bound and is referred to as an. This pair is separated by an internal electrochemical potential (present in p-n junctions or ) and the resulting flow of electrons and holes creates an electric current. The internal electrochemical potential is created by one part of the semiconductor interface with atoms tha. In a conventional solar cell light is absorbed by a, producing an electron-hole (e-h) pair; the pair may be bound and is referred to as an. This pair is separated by an internal electrochemical potential (present in p-n junctions or ) and the resulting flow of electrons and holes creates an electric current. The internal electrochemical potential is created by one part of the semiconductor interface with atoms that act as electron donors (n-type doping) and another with electron acceptors (p-type doping) that results in a. The generation of an e-h pair requires that the photons have energy exceeding the of the material. Effectively, photons with energies lower than the bandgap do not get absorbed, while those that are higher can quickly (within about 10 s) thermalize to the band edges, reducing output. The former limitation reduces, while the thermalization reduces the. As a result, semiconductor cells suffer a trade-off between voltage and current (which can be in part alleviated by using multiple junction implementations). The shows that this efficiency can not exceed 33% if one uses a single material with an ideal bandgap of 1.34 eV for a solar cell. The band gap (1.34 eV) of an ideal single-junction cell is close to that of silicon (1.1 eV), one of the many reasons that silicon dominates the market. However. A quantum dot solar cell (QDSC) is a design that uses as the captivating photovoltaic material. It attempts to replace bulk materials such as, () or (). Quantum dots have that are adjustable across a wide range of energy levels by changing their size. In bulk materials, the bandgap is fixed by the choice of material(s). This property makes quantum dots attractive for, where a variety of materials are used to improve efficiency by harvesting multiple portions of the. As of 2022, exceeds 18.1%. Quantum dot solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. Early examples used costly processes. However, the lattice mismatch results in accumulation of strain and thus generation of defects, restricting the number of stacked layers. Droplet epitaxy growth technique shows its advantages on the fabrication of strain-free QDs. Alternatively, less expensive fabrication methods were later developed. These use wet chemistry (for CQD) and subsequent solution processing. Concentrated nanoparticle solutions are stabilized by long that keep the nanocrystals suspended in solution. To create a solid, these solutions are cast down and the long stabilizing ligands are replaced with short-chain crosslinkers. Chemically engineering the nanocrystal surface can better passivate the nanocrystals and reduce detrimental trap states that would curtail device performance by means of carrier recombination. This approach produces an efficiency of 7.0%. A more recent study uses different ligands for different functions by tuning their relative band alignment to improve the performance to 8.6%. The cells were solution-processed in air at room-temperature and exhibited air-stability for more than 150 days without encapsulation. In 2014 the use of as a ligand that does not bond to oxygen was introduced. This maintains stable n- and p-type layers, boosting the absorption efficiency, which produced power conversion efficiency up to 8%. The idea of using quantum dots as a path to high efficiency was first noted by Burnham and Duggan in 1989. At the time, the science of quantum dots, or "wells" as they were known, was in its infancy and early examples were just becoming available. Another modern cell design is the, or DSSC. DSSCs use a sponge-like layer of as the semiconductor valve as well as a mechanical support structure. During construction, the sponge is filled with an organic dye, typically -polypyridine, which injects electrons into the titanium dioxide upon photoexcitation. This dye is relatively expensive, and ruthenium is a rare metal. Using quantum dots as an alternative to molecular dyes was considered from the earliest days of DSSC research. The ability to tune the bandgap allowed the designer to select a wider variety of materials for other portions of the cell. Collaborating groups from the and developed a design based on a rear electrode directly in contact with a film of quantum dots, eliminating the electrolyte and forming a depleted. These cells reached 7.0% efficiency, better than the best solid-state DSSC devices, but below those based on liquid electrolytes. Traditionally, multi-junction solar cells are made with a collection of multiple semiconductor materials. Because each material has a different band gap, each material's p-n junction will be optimized for a different incoming wavelength of light. Using multiple materials enables the absorbance of a broader range of wavelengths, which increases the cell's electrical conversion efficiency. However, the use of multiple materials makes multi-junction solar cells too expensive for many commercial uses. Because the band gap of quantum dots can be tuned by adjusting the particle radius, multi-junction cells can be manufactured by incorporating quantum dot semiconductors of different sizes (and therefore different band gaps). Using the same material lowers manufacturing costs, and the enhanced absorption spectrum of quantum dots can be used to increase the short-circuit current and overall cell efficiency. (CdTe) is used for cells that absorb multiple frequencies. A colloidal suspension of these crystals is spin-cast onto a substrate such as a thin glass slide, potted in a. These cells did not use quantum dots, but shared features with them, such as spin-casting and the use of a thin film conductor. At low production scales quantum dots are more expensive than mass-produced nanocrystals, but and are rare and highly toxic metals subject to price swings. The Sargent Group used as an -sensitive electron donor to produce then record-efficiency IR solar cells. Spin-casting may allow the construction of "tandem" cells at greatly reduced cost. The original cells used a substrate as an electrode, although works just as well. Another way to improve efficiency is to capture the extra energy in the electron when emitted from a single-bandgap material. In traditional materials like silicon, the distance from the emission site to the electrode where they are harvested is too far to allow this to occur; the electron will undergo many interactions with the crystal materials and lattice, giving up this extra energy a. Although quantum dot solar cells have yet to be commercially viable on the mass scale, several small commercial providers have begun marketing quantum dot photovoltaic products. Investors and financial analysts have identified quantum dot photovoltaics as a key future technology for the solar industry. • Quantum Materials Corp. (QMC) and subsidiary Solterra Renewable Technologies are developing and manufacturing quantum dots and nanomaterials for use in solar energy and lighting applications. With their patented continuous flow production process for perovskite quantum dots, QMC hopes to lower the cost of quantum dot solar cell production in addition to applying their nanomaterials to other emerging industries.• QD Solar takes advantage of the tunable band gap of quantum dots to create multi-junction solar cells. By combining efficient silicon solar cells with infrared solar cells made from quantum dots, QD Solar aims to harvest more of the solar spectrum. QD Solar's inorganic quantum dots are processed with high-throughput and cost-effective technologies and are more light- and air- stable than polymeric nanomaterials.• UbiQD is developing photovoltaic windows using quantum dots as fluorophores. They have designed a luminescent solar concentrator (LSC) using near-infrared quantum dots which are cheaper and less toxic than traditional alternatives. UbiQD hopes to provide semi-transparent windows that convert passive buildings into energy generation units, while simultaneously reducing the heat gain of the building.• ML System S.A., a producer listed on intends to start volume production of its QuantumGlass product between 2020 and 2021. Many heavy-metal quantum dot (lead/cadmium chalcogenides such as PbSe, CdSe) semiconductors can be cytotoxic and must be encapsulated in a stable polymer shell to prevent exposure. Non-toxic quantum dot materials such as AgBiS2 nanocrystals have been explored due to their safety and abundance; exploration with solar cells based with these materials have demonstrated comparable conversion efficiencies (> 9%) and short-circuit current densities (> 27 mA/cm ). UbiQD's CuInSe2−X quantum dot material is another example of a non-toxic semiconductor compound.
