Sep
19
Quantum Dots for Photovoltaic Cells Get Better
September 19, 2011 | Leave a Comment
Researchers from the University of Toronto (U of T), the King Abdullah University of Science & Technology (KAUST) and Pennsylvania State University (Penn State) are reporting a the most efficient quantum dot assembly technique of solar cells in the latest issue of Nature Materials. In the paper entitled “Collodial-quantum-dot photovoltaics using atomic-ligand passivation,” the researchers demonstrate how the wrappers that encapsulate the quantum dots can be shrunk to a mere layer of atoms.
Professor Ted Sargent, corresponding author on the work and holder of the Canada Research Chair in Nanotechnology at U of T said, “We figured out how to shrink the passivating materials to the smallest imaginable size.”
Collodial Quantum Dots (CQD) are nanoscale semiconductors that capture light and convert it into electrical energy. 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 common silicon-based version.
A crucial challenge in quantum dot cell assembly 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. But the quantum dots are usually capped with organic molecules that add a nanometer or two. When working on a nanoscale, that is big and bulky. Yet the organic molecules have been an important ingredient in creating a colloid, which is a substance that is dispersed in another substance like pigment in paint. This allows the quantum dots to be painted on to other surfaces.
To solve the problem, the researcher team 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.
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 explains, “We wrapped a single layer of atoms around each particle. As a result, they packed the quantum dots into a very dense solid.”
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.
Professor John Asbury of Penn State, a co-author of the work takes the explanation further, “The team proved that we were able to remove charge traps – locations where electrons get stuck – while still packing the quantum dots closely together.” The combination of close packing and charge trap elimination enabled electrons to move rapidly and smoothly through the solar cells, thus providing record efficiency.
Professor Dmitri Talapin of The University of Chicago, who is a research leader in the field, considers the results saying, “This finding proves the power of inorganic ligands in building practical devices. 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.”
Professor Aram Amassian of KAUST, a co-author on the work said, “At KAUST we were able to visualize, with incredible resolution on the sub-nanometer lengthscale, the structure and composition of this remarkable new class of materials. 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.”
Solar cells fabricated following the team’s technique show up to 6% solar AM1.5G power-conversion efficiency. The CQD films are deposited at room temperature and under ambient atmosphere, rendering the process amenable to low-cost, roll-by-roll fabrication.
When it comes to the roll to roll or printing processes the CQD material is the current top efficiency choice. Producers and consumers still have to wait to get an ideas on the pricing. One isn’t expecting that CQD is a one time use disposable product – the nirvana of photoelectric solar. But its getting closer.
If the economy could pick up perhaps this level of sophistication would get better legs into the market. For now though the flow to industry and products is going to be slowed up. We don’t need any more massive bankruptcy news from photoelectric.
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