Perovskite LED luminescence efficiency boosted fourfold [Unboxing Lab]

[Financial News] A Kookmin University research team has made an efficient perovskite LED without an additional step to replace the molecules coating its surface. Its efficiency in converting electricity into light is 11.4%, more than four times the 2.7% achieved by nanocrystal LEDs made using conventional synthesis methods.

Perovskite is a material with a crystal structure that emits light well. Perovskite nanocrystals are particles of this material made so small they cannot be seen with the naked eye. They produce vivid colors and emit light efficiently. Because they can be made into a liquid and sprayed on, they are considered a key material for next-generation display LEDs.
The problem lies on the particles’ surfaces. They are covered with long-chain molecules called ligands. Ligands keep the particles from clumping together, but they also block the flow of electricity. As a result, replacing the long ligands with shorter molecules through a ligand-exchange process has been virtually essential when making LEDs.
This study produced nanocrystals that can be used without this exchange process. Kookmin University explained that “the process for making light-emitting devices will be simplified, and their applications will expand to a variety of devices that handle light and electricity.” In their paper, the research team said that “these nanocrystals could become a foundational material that can be used across a range of devices.”
The team said in its paper that these LEDs had the highest performance reported to date among those that use nanocrystals directly, without additional processing. However, it also noted that they still fall short of state-of-the-art devices.

The conventional synthesis method is hot-injection synthesis, in which a solution of other materials is rapidly added to a solution heated to 180°C to grow nanocrystals. In this method, two types of long-chain molecules—oleate and oleylammonium—cover the particles’ surfaces.
The research team used a heated synthesis method, putting all the materials in one container and heating them to 120°C for five minutes. A chemical reaction called an SN2 reaction occurs during this process: one molecule displaces bromine from another molecule and takes its place. Oleate is consumed as a reactant in this process, and bromide ions fill the vacated sites.
Oleylammonium remains. Even so, the density of ligands covering the surface was 34.6% lower than with the conventional method, according to the team’s calculations. The nanocrystals also grew to a more uniform size.
The team also confirmed that more bromine attached to the surface. Surface analysis found 4.42 bromine atoms for every lead atom, more than the 3.94 in the conventional method. Computer calculations also showed that bromine bound more strongly to the surface than oleate. As bromine filled vacant sites on the surface, defects decreased. Defects trap charge and prevent light from being emitted.
Before washing, the luminescence yield of the nanocrystal solution was 90.0% for heated synthesis and 84.4% for the conventional method. Luminescence yield is the proportion of light absorbed by nanocrystals that they emit again as light.

After synthesis, nanocrystals are washed with a solvent to remove impurities. After one wash, conventional nanocrystals retained oily residue. The analysis attributed this to the solvent used in the reaction remaining behind. Two or more washes were needed to produce a uniform film.
But the more the nanocrystals were washed, the lower their luminescence yield became. For conventional nanocrystals, it fell from 84.4% before washing to 55.1% after two washes. The paper attributed this to the washing solvent stripping molecules from the surface and creating defects.
Nanocrystals made by heated synthesis formed a uniform film after just one wash. Their luminescence yield after two washes was 77.5%, down from 90.0% before washing. The film surfaces also differed in roughness: 7.595 nanometers for heated-synthesis nanocrystals washed once, compared with 10.386 nanometers for conventional nanocrystals washed twice. A nanometer is one-billionth of a meter. The smaller the number, the smoother the surface.
The nanocrystals also contained less residual organic matter. The proportion by weight of organic matter driven off by heating was about 20% for conventional nanocrystals washed twice and 12% for heated-synthesis nanocrystals washed once.

The research team made thin films of the nanocrystals for comparison. The luminescence yield in film form was 65.8% for heated synthesis and 40.7% for the conventional method.
The flow of charge also improved as ligand density fell and the nanocrystals aligned more uniformly. A hole is a vacancy left by an electron; it moves like a positive charge and carries electricity. The mobility of these holes was more than three times higher than with the conventional method. Mobility indicates how easily something moves. The density of defects that trap charge was also about 21% lower.

The team used these nanocrystals directly in the LED’s light-emitting layer, without ligand exchange. The comparison was with an LED made in the same way using nanocrystals produced by the conventional synthesis method.
External quantum efficiency is the number of light particles emitted for every 100 electrons put into a device. The heated-synthesis LED reached a maximum of 11.4% at 3.0 volts—about 11 light particles for every 100 electrons. Under the same conditions, the conventional LED achieved 2.7%, or about three particles.
At peak brightness, luminance rose more than fivefold, from 59 to 314 candelas per square meter. The candela is a unit of luminous intensity. The voltage at which the LED began to emit light fell from 3.0 to 2.7 volts.
The width of the light’s wavelength range narrowed from 21 to 17 nanometers. This width indicates how much other colors are mixed in with a given color. The narrower it is, the more vivid the color. In other words, the green light became more vivid.
The LEDs also lasted longer. At an initial brightness of about 50 candelas, the time it took for the brightness to fall by half was about 2.3 times that of the conventional LED.

The research was conducted by Professor Kim Young-hoon’s team in the Department of Chemistry at Kookmin University, together with Professor Lee Bo-ram’s team at Sungkyunkwan University and Professor Lee Tae-gyeong’s team at Hanyang University. Master’s student Lee Yu-bin, who had completed coursework, participated as a co-first author alongside Kookmin University researcher Kim Ji-geon and Sungkyunkwan University researcher Seok Gyeong-eun. According to Kookmin University, Lee took part in key stages of the research, including analyzing the nanocrystals’ surface properties and applying them to light-emitting devices.
The paper was published in the international materials science journal Advanced Functional Materials.

[email protected] Kim Man-ki Reporter