Efficiency Record for Perovskite LEDs

Researchers have set a new effi­ciency record for LEDs based on perovskite semi­conductors, rivalling that of the best organic LEDs. Compared to OLEDs, which are widely used in high-end consumer electronics, the perov­skite-based LEDs, developed by researchers at the Univer­sity of Cambridge, can be made at much lower costs, and can be tuned to emit light across the visible and near-infrared spectra with high colour purity.

This is an artistic impression of the perovskite-polymer heterostructure used in LEDs. (Source: U. Cambridge)

The researchers have engineered the perovskite layer in the LEDs to show close to 100% internal lumines­cence effi­ciency, opening up future appli­cations in display, lighting and communi­cations, as well as next-generation solar cells. These perovskite materials are of the same type as those found to make highly efficient solar cells that could one day replace commer­cial silicon solar cells. While perovskite-based LEDs have already been developed, they have not been nearly as effi­cient as conven­tional OLEDs at converting elec­tricity into light.

Earlier hybrid perovskite LEDs, first developed by Sir Richard Friend’s group at the University’s Caven­dish Labora­tory four years ago, were promising, but losses from the perov­skite layer, caused by tiny defects in the crystal structure, limited their light-emission efficiency. Now, Cambridge researchers from the same group and their colla­borators have shown that by forming a composite layer of the perovskites together with a polymer, it is possible to achieve much higher light-emission effi­ciencies, close to the theo­retical effi­ciency limit of thin-film OLEDs.

“This perovskite-polymer structure effec­tively elimi­nates non-emissive losses, the first time this has been achieved in a perovskite-based device,” said Dawei Di. “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 used in the LED devices, a bulk hetero­structure, is made of two-dimen­sional and three-dimensional perovskite compo­nents and an insu­lating polymer. When an ultra-fast laser is shone on the structures, pairs of electric charges that carry energy move from the 2D regions to the 3D regions in a trillionth of a second: much faster than earlier layered perovskite structures used in LEDs. Separated charges in the 3D regions then recombine and emit light extremely 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,” said Di. “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.

While perovskite-based LEDs are beginning to rival OLEDs in terms of efficiency, they still need better stabi­lity if they are to be adopted in consumer elec­tronics. When perovskite-based LEDs were first developed, they had a lifetime of just a few seconds. The LEDs developed in the current research have a half-life close to 50 hours, which is a huge improvement in just four years, but still nowhere near the lifetimes required for commer­cial appli­cations, which will require an extensive industrial develop­ment programme. “Understand the degra­dation mechanisms of the LEDs is a key to future improve­ments,” said Di. (Source: Cambridge Univ.)

Reference: B. Zhao et al.: High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes, Nat. Phot., online 5 November 2018; DOI: 10.1038/s41566-018-0283-4

Link: Cavendish Laboratory, University of Cambridge, Cambridge, UK

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