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It’s being called the most efficient colloidal quantum dot (CQD) solar cell ever and a team of international researchers is being praised for the new development. Researchers from the University of Toronto (U of T), King Abdullah University of Science & Technology (KAUST) and Pennsylvania State University (Penn State) have created the new CQD solar cell based off the need for a design that compacts quantum dots together so that less distance between the quantum dots equals a higher efficiency.
"We figured out how to shrink the wrappers that encapsulate quantum dots down to the smallest imaginable size - a mere layer of atoms," states Professor Ted Sargent, corresponding author on the work and holder of the Canada Research Chair in Nanotechnology at U of T.
The team found that up until their discovery, quantum dots had been capped with organic molecules that separated the nanoparticles by a nanometer, which on a nanoscale, creates a large distance for electrons to travel. The researcher’s solution was to use inorganic ligands, sub-nanometer-sized atoms, which bind to the surfaces of the quantum dots and use less space.
"Our team at Penn State proved that we could remove charge traps - locations where electrons get stuck - while still packing the quantum dots closely together," says ProfessorJohn Asbury of Penn State, a co-author of the work.
The close quarters and charge trap elimination allowed for electrons to quickly and smoothly move through the solar cells and create the record efficiency. “We wrapped a single layer of atoms around each particle. This allowed us to pack well-passivated quantum dots into a 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 performance results were certified by Newport, an external laboratory, which is accredited by the US National Renewable Energy Laboratory. Newport found that the quantum dots had the highest electrical currents and the highest overall power conversion efficiency realized in CQD solar cells to date.
"It is very impressive that the team was able to make solar cells with power conversion efficiency up to 6% from quantum dots," states Professor Michael McGehee of Stanford University, a world-renowned expert in solution-processed organic solar cells. "There is a lot of surface area in these films that could have dangling bonds which would hinder the performance of solar cells by creating traps states.
"At KAUST, we used visualization methods with sub-nanometer resolution and accuracy to investigate the structure and composition of the passivated quantum dots," states co-author Professor Aram Amassian of KAUST in Saudi Arabia. "We proved that the inorganic passivants were tightly correlated with the location of the quantum dots and that it was the chemical passivation, rather than nanocrystal ordering, that led to the remarkable colloidal quantum dot solar cell performance," he adds.
The team’s research discovery has prompted U of T and KAUST to sign a technology licensing agreement as they foresee great potential for their research. The licensing is aimed at permitting the global commercialization of the new technology.
"This work proves the power of inorganic ligands in building practical devices," states Professor Dmitri Talapin of The University of Chicago, a pioneer in inorganic ligands and materials chemistry. "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."
