Screens are ubiquitous in modern life — from phones to computers, ATMs to watches, eBook readers to TVs—we're surrounded by visual display technology that’s constantly improving.
Organic light-emitting diodes (OLEDs) have revolutionized the display industry with their vivid colors, exceptional contrast ratios, deep blacks, and high resolutions. OLEDs are also very energy efficient, making them ideal for small portable devices like a phones or tablets, as well as huge outdoor billboards.
OLED technology continues to evolve, and the latest innovations have made bendable and foldable screens possible. Fully transparent displays for augmented reality (AR) applications are also in development, as are many other uses for this technology.
We’ve seen extensive growth in publications relating to OLEDs in the CAS Content CollectionTM, the largest human-curated repository of scientific information, over the last decade (see Figure 1). Notably, the patent-to-journal ratio is about 3:1, which shows that manufacturers are bringing new products to market constantly.
How OLEDs work
Display technology began to improve with liquid crystal displays (LCDs) and the first LEDs. LCDs function by using liquid crystals to selectively transmit or block light which passes through it. In contrast, LEDs are tiny light-emitting diodes that can be individually controlled to produce a wide spectrum of colors. Both technologies operate by blocking the transmission of light from a backlight to generate visual images.
OLEDs produce colored light by using an organic material that can emit red, green, and blue light in the emissive layer or by adding a colored filter into one of the transparent layers.
OLEDs, however, do not use a backlight. They utilize an organic (carbon-based) semiconductor material that emits light in response to an electrical current. This emissive layer and a conductive layer are sandwiched between a cathode and anode (see Figure 2a). When electricity is applied, electrons generated at the cathode move to the emissive layer, and holes generated at the anode move to the conductive layer. As holes from the conductive layer recombine with electrons in the emissive layer, they release energy in the form of a photon.
By placing an array of red, green, and blue OLEDs in proximity and activating them independently, they function as individual pixels allowing us to generate complex and high-resolution color images ref, ref (Figure 2b).
This system is contained in protective layers of glass or plastic. The top layer is known as the seal and the bottom layer as the substrate. Typically, at least one of the top or bottom layers is transparent so that light can escape from the device. Because OLEDs use self-emissive pixels and have no need for a backlight, displays can be much larger and thinner than those using LCD or LED technology.
Foldable screens, transparent displays, and more
OLEDs can be categorized into several types based on three key factors: the organic material used, the driving method, and the light emission pathway (see Figure 3).
Some of the most exciting OLED innovations include:
- Flexible OLEDs (FOLEDs): FOLED technology utilizes flexible materials, enabling screens to bend, fold, or roll without damage. This innovation paves the way for thinner, lighter, and more energy-efficient displays. Applications of this technology include foldable phones, rollable TVs, and even AR/VR glasses and wearable devices.
- Transparent OLEDs (TOLEDs): TOLED displays have both transparent or semi-transparent cathodes and anodes. This dual-sided transparency enables light emission from both the top and bottom surfaces. The high transparency allows for clearer viewing even in an off state. TOLEDs also offer sharp contrast ratio, vivid colors, flexibility, and wide viewing angles, delivering an unparalleled visual experience. Since they can show content and remain transparent even when they’re not in use, TOLED displays are ideal for TVs, AR/VR glasses, and interactive retail displays.
- Small Molecule OLEDs (SMOLEDs): Most displays, including TVs, tablets, laptops, and smartphones, use SMOLEDs today. These OLEDs have small organic molecules as electroluminescent materials in the emissive layer. They offer several benefits, including structural flexibility due to the wide range of available organic emitters, easy purification, and simpler chemical modifications in the chromophores. This technology is further categorized into Fluorescence OLEDs and Phosphorescence OLEDs (PHOLEDs) based on the light emission mechanism followed by their emitters. PHOLEDs are more energy efficient compared to fluorescent technology, which loses most of its energy as heat.
- Active-Matrix OLEDs (AMOLEDs): AMOLED displays employ a thin-film transistor (TFT) backplane integrated with a storage capacitor to directly control and activate each pixel independently, resulting in superior resolution, high-contrast, vibrant colors, wide-viewing angles, and larger display sizes. They also feature fast refresh rates, energy efficiency, and thin and flexible display potential. AMOLED displays are commonly used in smartphones, smart watches, laptops, and TVs.
- White OLEDs (WOLEDs): OLED panels that can emit white light are known as WOLEDs. This technology is highly energy efficient and can be used in solid-state lighting, TVs, monitors, and other displays.
Further improvements to OLED technology
OLEDs haven’t fully replaced typical LEDs and other display technologies yet, despite their superior consumer experience. Why is this the case?
Until recently, OLEDs have had issues with short lifespan, specifically with blue emitters degrading over time. Elevated temperatures, humid conditions, and increased brightness can also accelerate the degradation process, thereby reducing the overall operational lifespan of OLED displays.
Static images displayed for an extended period on an OLED screen can cause a “burn-in” effect, which appears as a permanent shadow left on the screen. If TVs, for example, are left on one channel for hours, this can become an issue.
Addressing these challenges will be crucial to expanding OLED applications. Researchers are exploring innovations such as blue PHOLEDs, plasmonic PHOLEDs, and different types of emitters such as thermally activated delayed florescence (TADF) and hyperflorescence (HF) emitters to overcome persistent issues and improve the visual aspects of this technology. These improvements can also help bring down the cost, which remains expensive compared to other display technologies. If more OLED devices can be produced, economies of scale can be achieved to bring those costs down.
As the dynamic landscape of OLED technology evolves, one can imagine rollable newspaper screens, bendable smartphones, and transparent interactive displays in stores. Wearable devices, biomedical imaging, and commercial lighting could all be changed by cutting-edge OLEDs as well.
