Colloidal quantum dots: Performance boost next-generation solar cell technology

ScienceDaily (Sep. 19, 2011) — Researchers from the University of Toronto (U of T), the King Abdullah University of Science & Technology (KAUST) and Pennsylvania State University (Penn State) have created the most efficient solar cell ever made based on colloidal quantum dots (CQD).

The discovery is reported in the latest issue of Nature Materials.

Quantum dots are nanoscale semiconductors that capture light and convert it into an energy source. Because of their small scale, the dots can be sprayed on to flexible surfaces, including plastics. This enables the production of solar cells that are less expensive to produce and more durable than the more widely-known silicon-based version. In the work highlighted by the Nature Materials paper, the researchers demonstrate how the wrappers that encapsulate the quantum dots can be shrunk to a mere layer of atoms.

"We figured out how to shrink the passivating materials to the smallest imaginable size," states Professor Ted Sargent, corresponding author on the work and holder of the Canada Research Chair in Nanotechnology at U of T.

A crucial challenge for the field has been striking a balance between convenience and performance. The ideal design is one that tightly packs the quantum dots together. The greater the distance between quantum dots, the lower the efficiency.

However the quantum dots are usually capped with organic molecules that add a nanometer or two. When working on a nanoscale, that is bulky. Yet the organic molecules have been an important ingredient in creating a colloid, which is a substance that is dispersed in another substance. This allows the quantum dots to be painted on to other surfaces.

To solve the problem, the researchers have turned to inorganic ligands, which bind the quantum dots together while using less space. The result is the same colloid characteristics but without the bulky organic molecules.

"We wrapped a single layer of atoms around each particle. As a result, they packed the quantum dots into a very dense solid," explains Dr. Jiang Tang, the first author of the paper who conducted the research while a post-doctoral fellow in The Edward S. Rogers Department of Electrical & Computer Engineering at U of T.

The team showed the highest electrical currents, and the highest overall power conversion efficiency, ever seen in CQD solar cells. The performance results were certified by an external laboratory, Newport, that is accredited by the US National Renewable Energy Laboratory.

"The team proved that we were able to remove charge traps -- locations where electrons get stuck -- while still packing the quantum dots closely together," says Professor John Asbury of Penn State, a co-author of the work.

The combination of close packing and charge trap elimination enabled electrons to move rapidly and smoothly through the solar cells, thus providing record efficiency.

"This finding proves the power of inorganic ligands in building practical devices," states Professor Dmitri Talapin of The University of Chicago, who is a research leader in the field. "This new surface chemistry provides the path toward both efficient and stable quantum dot solar cells. It should also impact other electronic and optoelectronic devices that utilize colloidal nanocrystals. Advantages of the all-inorganic approach include vastly improved electronic transport and a path to long-term stability."

"At KAUST we were able to visualize, with incredible resolution on the sub-nanometer length scale, the structure and composition of this remarkable new class of materials," states Professor Aram Amassian of KAUST, a co-author on the work.

"We proved that the inorganic passivants were tightly correlated with the location of the quantum dots; and that it was this new approach to chemical passivation, rather than nanocrystal ordering, that led to this record-breaking colloidal quantum dot solar cell performance," he adds.

As a result of the potential of this research discovery, a technology licensing agreement has been signed by U of T and KAUST, brokered by MaRS Innovations (MI), which will will enable the global commercialization of this new technology.

"The world -- and the marketplace -- need solar innovations that break the existing compromise between performance and cost. Through U of T's, MI's, and KAUST's partnership, we are poised to translate exciting research into tangible innovations that can be commercialized," said Sargent.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Toronto, via EurekAlert!, a service of AAAS.

Journal Reference:

Jiang Tang, Kyle W. Kemp, Sjoerd Hoogland, Kwang S. Jeong, Huan Liu, Larissa Levina, Melissa Furukawa, Xihua Wang, Ratan Debnath, Dongkyu Cha, Kang Wei Chou, Armin Fischer, Aram Amassian, John B. Asbury, Edward H. Sargent. Colloidal-quantum-dot photovoltaics using atomic-ligand passivation. Nature Materials, 2011; DOI: 10.1038/nmat3118

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Understanding next-generation electronic devices: Smallest atomic displacements ever

ScienceDaily (Sep. 3, 2011) — An international team of scientists has developed a novel X-ray technique for imaging atomic displacements in materials with unprecedented accuracy. They have applied their technique to determine how a recently discovered class of exotic materials -- multiferroics -- can be simultaneously both magnetically and electrically ordered. Multiferroics are also candidate materials for new classes of electronic devices.

The discovery, a major breakthrough in understanding multiferroics, is published in Science dated 2 September 2011.

The authors comprise scientists from the European Synchrotron Radiation Facility (ESRF) in Grenoble (France), the University of Oxford and the University College London (both UK). Helen Walker from the ESRF is the main author of the publication.

Everybody is familiar with the idea that magnets are polarized with a north and a south pole, which is understood to arise from the alignment of magnet moments carried by atoms in magnetic materials. Certain other materials, known as ferroelectrics, exhibit a similar effect for electrical polarisation. The exotic "multiferroic" materials combine both magnetic and ferroelectric polarizations, and can exhibit a strong coupling between the two phenomena.

This leads to the strange effect that a magnetic field can electrically polarise the material, and an electric field magnetise it. A class of strong multiferroics was discovered ten years ago and has since led to a new, rapidly growing field of research, also motivated by the promise of their exotic properties for new electronic devices. One example is a new type of electronic memory, in which an electric field writes data into the memory and a magnetic detector is used to read it. This process is faster, and uses less energy than today's hard disk drives.

However, the origin of the electric polarisation in multiferroics remained mostly elusive to date. The team's work unambiguously shows that the polarization in the multiferroic studied proceeds from the relative displacement of charges of different signs, rather than the transfer of charge from one atom to another.

As the displacement involves a high number of electrons, even small distances can lead to significant polarisation. The actual distance of the displacement still came as a surprise: about 20 femtometres, or about 1/100,000th of the distance between the atoms in the material. Measuring such small displacements was actually believed to be impossible.

"I think that everyone involved was surprised, if not staggered, by the result that we can now image the position of atoms with such accuracy. The work is testament to the fantastic facilities available in Grenoble to the UK science community," says Prof. Des McMorrow, Deputy Director of the London Centre for Nanotechnology, leader of the UCL part of the project.

Walker and her colleagues developed a smart new experimental technique exploiting the interference between two competing processes: charge and magnetic scattering of a powerful, polarized X-ray beam. They studied a single crystal of TbMnO3 which shows a strong multiferroic coupling at temperatures below 30K, and were able to measure the displacements of specific atoms within it with an accuracy approaching one femtometre (10-15m). The atoms themselves are spaced apart 100,000 times this distance.

The new interference scattering technique has set a world record for accuracy in absolute measurements of atomic displacements. (It is also the first measurement of magnetostriction in antiferromagnets.) Most significantly the identification of the origin of ferroelectricty in a multiferroic material is a major step forward in the design of multiferroics for practical applications.

"By revealing the driving mechanism behind multiferroics, which offer so many potential applications, it underlines how experiments designed to understand the fundamental physics of materials can have an impact on the wider world," concludes Dr. Helen Walker who led the work at the ESRF.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by European Synchrotron Radiation Facility, via EurekAlert!, a service of AAAS.

Journal Reference:

H. C. Walker, F. Fabrizi, L. Paolasini, F. De Bergevin, J. Herrero-Martin, A. T. Boothroyd, D. Prabhakaran, D. F. Mcmorrow. Femtoscale Magnetically Induced Lattice Distortions in Multiferroic TbMnO3. Science, 2 September 2011: Vol. 333 no. 6047 pp. 1273-1276 DOI: 10.1126/science.1208085

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