Asymmetry is key to creating more stable blue perovskite LEDs

Date:
January 25, 2022
Source:
Okinawa Institute of Science and Technology (OIST) Graduate
University
Summary:
For the first time, researchers have created blue LEDs using
layers of metal halide perovskite linked with asymmetrical bridges,
solving a critical instability problem.



FULL STORY ==========================================================================
From street and household lighting, to television and mobile displays,
light emitting diodes (LEDs) play an essential role in modern life. Now, researchers from the Okinawa Institute of Science and Technology Graduate University (OIST) have developed blue LEDs based on a material called
metal halide perovskite, that, for the first time, uses asymmetrical
bridges to hold the layers of perovskite together, creating a more
stable structure.


==========================================================================
The study, published recently in theJournal of the American Chemical
Societyon November 18th 2021, could bring perovskite LEDs one step closer
to commercialization.

"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." Aside from efficiency levels, perovskite LEDs also have numerous advantages over
current LED technologies on the market, as they have the potential to
produce brighter, purer colors 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, in particular, have lagged behind red and green-colored LEDs, with a lifetime of less than 2 hours and around half
the level of efficiency.

But without blue LEDs, practical applications using perovskites in color displays or as light sources are limited, as red, green and blue light
need to be mixed to produce the full array of colors, including white, explained Professor Yabing Qi, senior author of the paper and head of
the OIST Energy Materials and Surface Sciences Unit.



========================================================================== "Historically, blue emission has always been much more difficult to
achieve," Prof. Qi continued. "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. So, there
is very much a need for research into new blue-emitting materials, like perovskites." In the study, the scientists looked at one of the major
issues seen in blue perovskite LEDs -- the halide segregation problem.

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.

However, when a voltage is applied across a perovskite LED, which is
required for the LED to emit light, 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 color to shift to a greener hue.

To try and combat the halide segregation problem, the researchers created
blue LEDs with a type of perovskite structure called a Dion-Jacobson
phase structure, where two-dimensional (2-D) 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 that were created were
symmetrical, which means that both ends of the molecule looked the same.

Now, for the first time, the researchers explored whether using an
asymmetrical bridge, where each end was different, 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,
therefore 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, therefore 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.

As well as solving the problem of halide segregation in perovskite LEDs,
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 concluded.

This study received support from the OIST Technology Development and
Innovation Center's Proof-of-Concept Program.

========================================================================== Story Source: Materials provided by Okinawa_Institute_of_Science_and_Technology_(OIST)
Graduate_University. Original written by Dani Ellenby. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Yuqiang Liu, Luis K. Ono, Guoqing Tong, Tongle Bu, Hui Zhang,
Chenfeng
Ding, Wei Zhang, Yabing Qi. Spectral Stable Blue-Light-Emitting
Diodes via Asymmetric Organic Diamine Based Dion-Jacobson
Perovskites. Journal of the American Chemical Society, 2021; 143
(47): 19711 DOI: 10.1021/ jacs.1c07757 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220125124021.htm

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