Two optoelectronic materials getting alot of press these days are perovskite and quantum dots. Both have been individuallyutilized by researchers to boost sunlight conversion to electrical current insolar cells, and to increase the efficacy of electrically-generated light. Nowengineers at the University of Toronto (U of T) have combined both of thesematerials to create an ultra-efficient, super-luminescent hybrid crystal that they say will enable new records in power-to-light conversion efficiencies.
Tocreate the crystal, researchers in The Edward S. Rogers Sr. Department ofElectrical & Computer Engineering had to come up with a way to incorporatehighly luminescent colloidal quantum dot nanoparticles into perovskite. Theyachieved this by using a technique known as heteroepitaxy, where an atomically aligned crystalline film is "grown" on top of adifferent crystalline substrate.
Inorder to achieve heteroepitaxy, the team created a method to join the atoms onthe ends of the two crystalline materials so that they lined up accurately andwithout any faults at the seams.
"Westarted by building a nano-scale scaffolding 'shell' around the quantum dots insolution, then grew the perovskite crystal around that shell so the two facesaligned," said Dr. Zhijun Ning, who was a post-doctoral fellow at U of Tat the time of the research.
"When you try tojam two different crystals together, they often form separate phases withoutblending smoothly into each other," adds Dr. Riccardo Comin, apost-doctoral fellow at U of T. "We had to design a new strategy toconvince these two components to forget about their differences and to ratherintermix into forming a unique crystalline entity."
The resultant form is ablack-colored crystal whose light production depends on the perovskite matrix'sability to guide electrons into the quantum dots, which then super-efficientlyconvert electricity to light. Merging these two materials has also solved theproblem of self-absorption that occurs when a physical medium partiallyre-absorbs the same spectrum of energy that it emits, resulting in a netefficiency loss.
"Thesedots in perovskite don’t suffer reabsorption, because the emission of the dotsdoesn’t overlap with the absorption spectrum of the perovskite," said Dr.Comin.
Inexplaining the remarkable optoelectronic properties of these so-called"heterocrystals", the team claims that this is due to the fact thatphotoelectrons and holes generated in the larger bandgap of the perovskite aretransferred with 80 percent efficiency to become excitons inthe quantum dot nanocrystals. This, then, leverages the superior photocarrierdiffusion of the perovskite to produce bright-light emission.
Producinglight at the near-infrared, the researchers have also specifically designed their newcrystal material to be suitable for use in solution-processing (that is the useof chemical deposition in a solution), so the material may be easilyincorporated with inexpensive commercial methods of manufacturing solar filmand other PV devices.
Thematerial could also form the basis of the basis for a new family of highlyenergy-efficient near-infrared LEDs, which could be used to improve night-visiontechnology, biomedical imaging, and high-speed telecommunications. The teamnext plan to build and test hardware to prove the practicality of thetechnology.
"We’regoing to build the LED device and try to beat the record power efficiencyreported in the literature," said Gong.
Thework was recently published in the journal Nature.
Source: University of Toronto
