TORONTO, July 16, 2015 — Aligning the crystalline structures of quantum dots (QDs) and perovskites could yield high-efficiency LEDs.
Emitting in the IR region, these hybrid materials could have applications in night-vision and gesture-recognition technology, biomedical imaging and telecommunications. The concept could also be applied to LEDs that emit in the visible spectrum for lighting and displays, said the developers at the University of Toronto.
An artist's rendering of a quantum dot seamlessly integrated into a perovskite crystal matrix. Courtesy of the Sargent Group/University of Toronto, Engineering.
"It's a pretty novel idea to blend together these two optoelectronic materials, both of which are gaining a lot of traction," said doctoral candidate Xiwen Gong. "We wanted to take advantage of the benefits of both by combining them seamlessly in a solid-state matrix."
The team designed a way to embed colloidal QDs into organohalide perovskites, a family of materials that is easily manufactured from solution and allows high charge-carrier mobility and minimal loss or capture due to defects.
The result is a black crystal that relies on the perovskite matrix to "funnel" electrons into the QDs with 80 percent efficiency. The researchers observed combined structures as large as 60 nm and containing at least 20 mutually aligned QDs.
"When you try to jam two different crystals together, they often form separate phases without blending smoothly into each other," said postdoctoral fellow Riccardo Comin. "We had to design a new strategy to convince these two components to forget about their differences and to rather intermix into forming a unique crystalline entity."
The process of lining up two different crystal structures at the atomic level, called heteroepitaxy, is the basis of electrically driven lasers, multijunction solar cells, and blue LEDs. Here the Toronto researchers applied the approach to align the QDs and perovskites smoothly, without defects forming at the seams.
Researchers Riccardo Comin, left, and Xiwen Gong examine a hybrid crystal incorporated into an early prototype device. Courtesy of Marit Mitchell/University of Toronto, Engineering.
"We started by building a nanoscale scaffolding 'shell' around the quantum dots in solution, then grew the perovskite crystal around that shell so the two faces aligned," said Zhijun Ning, a former Toronto postdoctoral fellow and current faculty member at ShanghaiTech University.
Combining the two materials in this way also solved the problem of self-absorption, which occurs when a substance partly reabsorbs the same spectrum of energy that it emits, with a net efficiency loss.
"These dots in perovskite don't suffer reabsorption because the emission of the dots doesn't overlap with the absorption spectrum of the perovskite," Comin said.
The researchers designed their material to be compatible with solution processing so it could be readily integrated with conventional thin film manufacturing processes.
Next the team plans to use the process to build a working LED device and test its power efficiency against existing LEDs.
Funding came from the Ontario Research Fund: Research Excellence Program, the Natural Sciences and Engineering Research Council of Canada, and King Abdullah University of Science & Technology.
The research was published in Nature (doi: 10.1038/nature14563).
For more information, visit www.engineering.utoronto.ca.