Asymmetry is key to stable blue perovskite LEDs

The study, conducted at Okinawa Institute of Science and Technology Graduate University (OIST) and published in the Journal of the American Chemical Society, could bring perovskite LEDs one step closer to commercialisation.

“Perovskites have the potential to be a real game-changer in the lighting industry,” said first author Dr. Yuqiang Liu, a former post-doctoral researcher in the OIST Energy Materials and Surface Sciences Unit and currently a professor at Qingdao University, China. “In only a few short years, the efficiency of perovskite LEDs – how well they can transfer electrical energy into light energy – has shot up to a level that rivals traditional LEDs, and soon will surpass them.”

Perovskite LEDs also have the potential to produce brighter, purer colours at a fraction of the production cost.

However, the stability of perovskite LEDs remains a huge barrier, with the operational lifetime of even the most stable LEDs lasting only a few hundred hours. Blue LEDs have lagged behind red and green-coloured LEDs with a lifetime of under two hours and around half the level of efficiency.

Without blue LEDs, practical applications using perovskites in colour displays or as light sources are limited, as red, green and blue light need to be mixed to produce the full array of colours, said Professor Yabing Qi, senior author of the paper and head of the OIST Energy Materials and Surface Sciences Unit.

“The Nobel prize-winning blue LEDs that were first made using gallium nitride took three decades longer to develop than red and green LEDs – and even now, creating large, high-quality crystals of gallium nitride remains challenging and expensive,” said Prof. Qi. “So, there is very much a need for research into new blue-emitting materials, like perovskites.”

In the study, the scientists addressed the issue of halide segregation. When metal halide perovskite crystals form, the halides bond in an octahedral shape around a metal atom. A positive ion is situated in between four of these octahedral shapes.

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When a voltage is applied across a perovskite LED it also causes the negative halide ions that form the octahedral structure to separate and migrate towards the positive end of the LED. The positive ions between the octahedral shapes also migrate to the negative end of the LED. This ion migration degrades the perovskite structure, causing the efficiency of the LED to plummet and the blue colour to shift to a greener hue.

To overcome this, the researchers created blue LEDs with a Dion-Jacobson phase structure, where 2D layers of perovskite crystal are stacked on top of each other. The perovskite layers are then linked together by molecular bridges, increasing the stability of the whole structure.

In previous research, the molecular bridges were symmetrical. For the first time the researchers explored whether using an asymmetrical bridge affected the overall properties of the perovskite LED.

The researchers found that when the bridging molecule was asymmetrical, it slowed down the migration of ions across the layers of perovskite, improving the stability of the perovskite structure.

The team proposed that the asymmetry causes changes in how the electrons are distributed across the bridging molecule, consequently creating a small dipole electric field in between the layers.

“We think this dipole electric field is what is interfering with the ion migration, and therefore maintaining stability,” said Prof. Qi.

The strategy of using asymmetrical bridges could also be applied to other perovskite-based devices, such as perovskite solar cells.

“It’s an exciting advancement towards creating all kinds of longer-lived perovskite devices,” Prof. Qi said.