The idea of using quantum dots as a path to high efficiency was first noted by Barnham 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.
DSSC efforts Another modern cell design is the
dye-sensitized solar cell, or DSSC. DSSCs use a sponge-like layer of titanium dioxide| as the semiconductor valve as well as a mechanical support structure. During construction, the sponge is filled with an organic dye, typically
ruthenium-polypyridine, which injects electrons into the titanium dioxide upon photoexcitation. 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.
Cadmium telluride (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
conductive polymer. 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
cadmium and
telluride are rare and highly toxic metals subject to price swings.
The Sargent Group used
lead sulfide as an
infrared-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
gold substrate as an electrode, although
nickel works just as well.
Hot-carrier capture 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 as heat.
Amorphous thin-film silicon was tried as an alternative, but the defects inherent to these materials overwhelmed their potential advantage. Modern thin-film cells remain generally less efficient than traditional silicon. Nanostructured donors can be cast as uniform films that avoid the problems with defects. These would be subject to other issues inherent to quantum dots, notably resistivity issues and heat retention.
Multiple excitons The Shockley-Queisser limit, which sets the maximum efficiency of a single-layer photovoltaic cell to be 33.7%, assumes that only one electron-hole pair (exciton) can be generated per incoming photon. Multiple exciton generation (MEG) is an exciton relaxation pathway which allows two or more excitons to be generated per incoming high energy photon. In traditional photovoltaics, this excess energy is lost to the bulk material as lattice vibrations (electron-phonon coupling). MEG occurs when this excess energy is transferred to excite additional electrons across the band gap, where they can contribute to the short-circuit current density. Within quantum dots, quantum confinement increases coulombic interactions which drives the MEG process. This phenomenon also decreases the rate of electron-phonon coupling, which is the dominant method of exciton relaxation in bulk semiconductors. The phonon bottleneck slows the rate of hot carrier cooling, which allows excitons to pursue other pathways of relaxation; this allows MEG to dominate in quantum dot solar cells. The rate of MEG can be optimized by tailoring quantum dot ligand chemistry, as well as by changing the quantum dot material and geometry. In 2004,
Los Alamos National Laboratory reported spectroscopic evidence that several excitons could be efficiently generated upon absorption of a single, energetic photon in a quantum dot. Capturing them would catch more of the energy in sunlight. In this approach, known as "carrier multiplication" (CM) or "
multiple exciton generation" (MEG), the quantum dot is tuned to release multiple electron-hole pairs at a lower energy instead of one pair at high energy. This increases efficiency through increased photocurrent. LANL's dots were made from
lead selenide. In 2010, the
University of Wyoming demonstrated similar performance using DCCS cells. Lead-sulfur (PbS) dots demonstrated two-electron ejection when the incoming photons had about three times the bandgap energy. In 2005,
NREL demonstrated MEG in quantum dots, producing three electrons per photon and a theoretical efficiency of 65%. In 2007, they achieved a similar result in silicon.
Non-oxidizing In 2014 a University of Toronto group manufactured and demonstrated a type of CQD n-type cell using PbS with special treatment so that it doesn't bind with oxygen. The cell achieved 8% efficiency, just shy of the current QD efficiency record. Such cells create the possibility of uncoated "spray-on" cells. However, these air-stable n-type CQD were actually fabricated in an oxygen-free environment. Also in 2014, another research group at MIT demonstrated air-stable ZnO/PbS solar cells that were fabricated in air and achieved a certified 8.55% record efficiency (9.2% in lab) because they absorbed light well, while also transporting charge to collectors at the cell's edge. These cells show unprecedented air-stability for quantum dot solar cells that the performance remained unchanged for more than 150 days of storage in air. ==Quantum Dot Solar Cells Market==