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July 29, 2012

Colloidal Quantum dot film solar cells reach 7% efficiency

Researchers from the University of Toronto (U of T) and King Abdullah University of Science & Technology (KAUST) have made a breakthrough in the development of colloidal quantum dot (CQD) films, leading to the most efficient CQD solar cell ever

The researchers, led by U of T Engineering Professor Ted Sargent, created a solar cell out of inexpensive materials that was certified at a world-record 7.0% efficiency.

"Previously, quantum dot solar cells have been limited by the large internal surface areas of the nanoparticles in the film, which made extracting electricity difficult," said Dr. Susanna Thon, a lead co-author of the paper. "Our breakthrough was to use a combination of organic and inorganic chemistry to completely cover all of the exposed surfaces."

A 3 page article from Nature Photonics explains what is needed to get colloidal quantum dots to a commercial breakthrough

Given the practical limitations associated with reducing balance-of-systems costs, it is widely believed that a long-term viable solar technology — even one with an unprecedentedly low module cost — must offer a clear roadmap to achieving power- conversion efficiencies of more than 20%.

Half of the Sun’s energy the Earth lies in the visible band, while the other half is in the infrared range. If a single light-absorbing semiconductor is employed in a solar module, its bandgap must lie in the near-infrared (around 1.1–1.4 to offer a theoretical limit of 31% under unconcentrated illumination conditions.)

When a single-junction power-conversion efficiency of 10% is eventually reached, the
multijunction strategy can be deployed to engineer these low-cost, flexible materials
into devices with efficiencies of 15%.



The Sun’s visible and infrared radiation can be more efficiently harnessed if a number of different light-absorbing materials are employed in series. In such a multijunction solar cell, a stack of semiconductors, chirped through the visible to the infrared range, capture not only the abundant energy available in each visible photon, but also the considerable photon current available in the short-wavelength infrared photons, which ordinarily would not be absorbed in a single-junction device. This strategy enables theoretical efficiencies in the range of 40–60%, depending on the number of junctions employed. Colloidal quantum dots have tunable absorption properties so making multijunctions would be simpler.

The electronic properties of colloidal quantum dot films currently limit device performance. To exceed power-conversion efficiencies of 10% in a single-junction planar cell, a material’s electron and hole mobility should exceed 10–1 cm2 V–1 s–1 and its bandgap should be as trap-free as possible (less than 10^14 cm–3 deep traps). Much progress has been made towards this objective through advances in the packing and passivation of colloidal quantum dots in thin solid films.

Quantum dots are semiconductors only a few nanometres in size and can be used to harvest electricity from the entire solar spectrum – including both visible and invisible wavelengths. Unlike current slow and expensive semiconductor growth techniques, CQD films can be created quickly and at low cost, similar to paint or ink. This research paves the way for solar cells that can be fabricated on flexible substrates in the same way newspapers are rapidly printed in mass quantities.

The U of T cell represents a 37% increase in efficiency over the previous certified record. In order to improve efficiency, the researchers needed a way to both reduce the number of "traps" for electrons associated with poor surface quality while simultaneously ensuring their films were very dense to absorb as much light as possible. The solution was a so-called "hybrid passivation" scheme.

"By introducing small chlorine atoms immediately after synthesizing the dots, we're able to patch the previously unreachable nooks and crannies that lead to electron traps," explained doctoral student and lead co-author Alex Ip. "We follow that by using short organic linkers to bind quantum dots in the film closer together."

Work led by Professor Aram Amassian of KAUST showed that the organic ligand exchange was necessary to achieve the densest film.



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