Nov 7 2018
A new efficiency record has been set by scientists for LEDs based on perovskite semiconductors, which surpasses that of the best organic LEDs (OLEDs).
Credit: University of Cambridge
Compared to OLEDs, which are extensively used in high-end consumer electronics, the perovskite-based LEDs, created by researchers at the University of Cambridge, can be produced at much lower costs and can be adjusted to radiate light across the near-infrared and visible spectra with high color purity.
The scientists have designed the perovskite layer in the LEDs to display close to 100% internal luminescence efficiency, opening up future applications in lighting and communications, display, as well as next-generation solar cells.
These perovskite materials are of the same type as those employed in making very efficient solar cells that could in the future substitute commercial silicon solar cells. While perovskite-based LEDs have already been established, they have not been quite as efficient as conventional OLEDs at changing electricity into light.
The earlier hybrid perovskite LEDs, first created by Professor Sir Richard Friend’s group at the University’s Cavendish Laboratory four years ago, showed potential, but losses from the perovskite layer due to minute defects in the crystal structure, restricted their light-emission efficiency.
Currently, Cambridge scientists from the same group and their collaborators have demonstrated that by designing a composite layer of the perovskites along with a polymer, it is possible to accomplish much higher light-emission efficiencies, not far off the theoretical efficiency limit of thin-film OLEDs. Their results have been illustrated in the journal Nature Photonics.
“This perovskite-polymer structure effectively eliminates non-emissive losses, the first time this has been achieved in a perovskite-based device,” said Dr Dawei Di from Cambridge’s Cavendish Laboratory, one of the corresponding authors of the paper. “By blending the two, we can basically prevent the electrons and positive charges from recombining via the defects in the perovskite structure.”
The perovskite-polymer blend employed in the LED devices, called a bulk heterostructure, is composed of two-dimensional and three-dimensional perovskite components and an insulating polymer. Pairs of electric charges that transport energy move from the 2D regions to the 3D regions in a trillionth of a second when an ultra-fast laser is shone on the structures, which is a lot faster than previous layered perovskite structures used in LEDs. Separated charges in the 3D regions then recombine and discharge light very efficiently.
Since the energy migration from 2D regions to 3D regions happens so quickly, and the charges in the 3D regions are isolated from the defects by the polymer, these mechanisms prevent the defects from getting involved, thereby preventing energy loss.
Dr Dawei Di, Cambridge’s Cavendish Laboratory.
“The best external quantum efficiencies of these devices are higher than 20% at current densities relevant to display applications, setting a new record for perovskite LEDs, which is a similar efficiency value to the best OLEDs on the market today,” said Baodan Zhao, the paper’s first author.
While perovskite-based LEDs are starting to compete with OLEDs with regards to efficiency; they will still require improved stability if they are to be accepted in consumer electronics. When perovskite-based LEDs were first designed, they had a service life of just a few seconds. The LEDs created in the present research have a half-life close to 50 hours, which is a massive improvement in just four years, but still not close to the lifetimes necessary for commercial applications, which will require a wide-ranging industrial development program. “Understanding the degradation mechanisms of the LEDs is a key to future improvements,” said Di.
The study was sponsored by the Engineering and Physical Sciences Research Council (EPSRC) and the European Research Council (ERC).