VU Physicists Develop Technologies That Can be Applied in Foldable TVs to Energy-Generating Clothes

Organic optoelectronic technologies are some of the most rapidly developing technologies – last year, their global market was valued at about $60 bln and is now forecast to hit $300 bln in 2027. The biggest segment of this market is claimed by organic light emitting diode (OLED) screens. The scientists of the Organic Optoelectronics Group at the Institute of Photonics and Nanotechnology, Vilnius University (VU), have been actively working in this field for more than 15 years, engaged in all stages of research, from creating new molecules to demonstrating device prototypes. Dr Karolis Kazlauskas, the head of the Organic Optoelectronics Group, discusses scientific research being conducted in VU in this field and a wide range of applications of this technology.

Multidisciplinary approach is a must in research

According to Dr Kazlauskas, organic optoelectronics is inseparable from organic semiconducting materials, of which various optoelectronic devices are made, e.g. organic light emitting diodes and lasers, organic solar cells, transistors and sensors. VU Organic Optoelectronics Research Group works on creating and adapting efficient materials for the production of these devices.

Materials in organic optoelectronics often combine characteristics found both in common non-organic and organic semiconductors, therefore they can boast not only electric conductivity, light absorption and emission, but also mechanic stiffness or flexibility, lightness, solubility. “It is exactly the latter qualities of organic materials that open the door to the use of cheap solution-based casting or printing technologies as an alternative to quite expensive vacuum evaporation technology in production of energy-saving optoelectronic devices, such as OLED,” the scientist claims.

Dr Kazlauskas explains that, as opposed to standard LED technology, which is based on forming epitaxial layers in a vacuum and in high temperatures from non-organic semiconductors, OLED technology is associated with material- and energy-saving printing technology. It basically works as an inkjet printer, where micron-size droplets of organic materials are dripped with great accuracy onto the necessary spots on a tray.

“Research in OLED is interdisciplinary and requires competences in the fields of chemical engineering, physics and technology. A wide competence spectrum is what enables the building of some of the most efficient OLED devices that glow in blue spectral range with cheap casting technology. In these devices, all electrically injected charge is converted into light without losses. Having the necessary scientific instruments at our disposal: modern organic optoelectronics technological equipment and an inert gas glovebox system that we devised together with a German company MBRAUN, also helps us perform these tasks well,” says Dr Kazlauskas.

About to become attractive investment for business

Scientists at VU are now actively developing the technology for 3-generation emitters, also called thermally activated delayed fluorescence (TADF) emitters. The best TADF-OLED devices constructed by the research group of Dr Kazlauskas demonstrate internal quantum efficiencies of up to 100 per cent.

“Commercial OLED TVs currently use inefficient first-generation fluorescent emitters to give off blue colour and second-generation phosphorescent emitters containing atoms of noble metals for green and red. Replacing them with the newest generation TADF emitters might reduce energy losses and production costs,” the scientist believes.

Dr Kazlauskas remarks that with the casting method, their group managed to create TADF-OLED devices that do not lose their high efficiency in the blue spectral range even under high luminance, making these emitters adaptable not only to OLED screens but also to lighting panels.

“Although TADF-OLED devices are not commercialized yet, hopes are that due to their high quantum efficiency and potential in OLED screen production and lighting, they will become very attractive for business in the future,” says Dr Kazlauskas.

Organic electronic devices can be applied in medicine

OLED devices, being compact sources of light, can be easily transferred to the medical field, to measure human physiological parameters (pulse, blood oxygen levels) or even to treat skin cancer on the principle of photodynamic therapy. “In the latter case, a flexible plaster made of OLED of a particular wavelength can be put anywhere you need on the body and the therapy is then performed without constraining the patientʼs movements,” says the scientist, citing the capabilities of OLED devices in medicine.

Dr Kazlauskas points out that organic electronic devices are more compatible with biological systems than their counterparts made of non-organic materials. This gives us tremendous opportunities to use them in biomedicine, where it is important to ensure electrical connection with neurons. Organic transistors can operate as sensors of electric impulses that receive signals from neurons, or even perform the stimulating function.

“Organic semiconductors can be used in medicine to restore vision affected by retinal degenerations. These retina-related vision problems can be both inborn and caused by aging. In 2020, Italian scientists did experiments with rats and demonstrated that when you inject some nanoparticles of a polymer poly(3-hexylthiophene) (P3HT) into their eyes, the particles distribute themselves in the retina and perform the function of photoreceptors. These photosensitive receptors form a close connection with neurons in the retina, ensuring transmission of information via electric impulses,” the scientist explains.

“Fashionable” solar cells and foldable OLED screens

Today the world is particularly interested in long-diagonal OLED screens, large-area OLED lighting panels and organic solar cells. OLED and organic solar cells are very thin (can be just a few microns thick) and flexible, so they can be integrated even into clothing. One collection of such futuristic clothes is proposed by a Dutch designer Pauline van Dongen. Solar cells in these clothes do not only perform their direct function, i.e., generate energy for devices the person has on them, but also set up new fashion trends.

“Transparent OLED screens are already in the market, and the rollable OLED TV by LG, which saves living space, is currently gaining a lot of attention in Japan. Another attractive innovation is semi-transparent large-area organic solar cells, which can be installed right on the windows with transparent OLED lighting panels. These solar cells not only reduce the amount of sunlight entering the room during the day functioning as a kind of semi-transparent curtains, but also convert sunlight into energy, which the OLED panels can use to light the room at night,” presents the scientist.

Dr Kazlauskas shares that his research group aims to increase the efficiency of organic solar cells with the help of light conversion shortening the wavelength. The light conversion is based on the phenomenon of triplet-triplet annihilation (TTA) – in the materials of a particular molecular structure, two lower-energy triplet excitons are converted into one singlet with higher energy. This type of conversion opens possibilities for new-generation organic solar cells to generate energy using a big part of infrared solar radiation, which is nowadays wasted.

“Our group, in cooperation with a team of chemists, led by VU professor Edvinas Orentas, solves these efficiency problems by creating more effective TTA materials, increasing triplet exciton diffusion and decreasing harmful non-radiative exciton relaxation processes. These pieces of research are also important in that they mark the way for further development of TTA materials and their application in promising fields such as photocatalysis, bioimaging or contactless drug photoactivation through the skin,” says Dr Kazlauskas.