Two optoelectronic materials getting a lot of press these days are perovskite and quantum dots. Both have been individually utilized by researchers to boost sunlight conversion to electrical current in solar cells, and to increase the efficacy of electrically-generated light. Now engineers at the University of Toronto (U of T) have combined both of these materials to create an ultra-efficient, super-luminescent hybrid crystal that they say will enable new records in power-to-light conversion efficiencies.

To create the crystal, researchers in The Edward S. Rogers Sr. Department of Electrical & Computer Engineering had to come up with a way to incorporate highly luminescent colloidal quantum dot nanoparticles into perovskite. They achieved this by using a technique known as heteroepitaxy, where an atomically aligned crystalline film is "grown" on top of a different crystalline substrate.

In order to achieve heteroepitaxy, the team created a method to join the atoms on the ends of the two crystalline materials so that they lined up accurately and without any faults at the seams.

"We started by building a nano-scale scaffolding 'shell' around the quantum dots in solution, then grew the perovskite crystal around that shell so the two faces aligned," said Dr. Zhijun Ning, who was a post-doctoral fellow at U of T at the time of the research.

"When you try to jam two different crystals together, they often form separate phases without blending smoothly into each other," adds Dr. Riccardo Comin, a post-doctoral fellow at U of T. "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 resultant form is a black-colored crystal whose light production depends on the perovskite matrix's ability to guide electrons into the quantum dots, which then super-efficiently convert electricity to light. Merging these two materials has also solved the problem of self-absorption that occurs when a physical medium partially re-absorbs the same spectrum of energy that it emits, resulting in 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," said Dr. Comin.

In explaining the remarkable optoelectronic properties of these so-called "heterocrystals", the team claims that this is due to the fact that photoelectrons and holes generated in the larger bandgap of the perovskite are transferred with 80 percent efficiency to become excitons in the quantum dot nanocrystals. This, then, leverages the superior photocarrier diffusion of the perovskite to produce bright-light emission.

Producing light at the near-infrared, the researchers have also specifically designed their new crystal material to be suitable for use in solution-processing (that is the use of chemical deposition in a solution), so the material may be easily incorporated with inexpensive commercial methods of manufacturing solar film and other PV devices.

The material could also form the basis of the basis for a new family of highly energy-efficient near-infrared LEDs, which could be used to improve night-vision technology, biomedical imaging, and high-speed telecommunications. The team next plan to build and test hardware to prove the practicality of the technology.

"We’re going to build the LED device and try to beat the record power efficiency reported in the literature," said Gong.

The work was recently published in the journal Nature.

Source: University of Toronto

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