What is an OLED?
The acronym ‘OLED’ stands for Organic Light-Emitting Diode - a technology that uses LEDs in which the light is produced by organic molecules. These organic LEDs are used to create what are considered to be the world’s best display panels.
OLED displays are made by placing a series of organic thin films between two conductors. When an electrical current is applied, a bright light is emitted. A simple design - which brings with it many advantages over other display technologies.
OLEDs enable emissive displays - which means that each pixel is controlled individually and emits its own light (unlike LCDs in which the light comes from a backlighting unit). OLED displays feature great image quality - bright colors, fast motion and most importantly - very high contrast. Most notably, “real” blacks (that cannot be achieved in LCDs due to the backlighting). The simple OLED design also means that it is relatively easy to produce flexible and transparent displays.
Where can OLEDs be found today?
OLEDs are mostly used today in mobile devices - many high-end smartphones use them. Over 500 million OLED panels are produced annually, by various display makers, and the market is growing as OLEDs offer better image quality, smaller form factors and flexibility - things that are hard to achieve with LCDs. It is expected that by 2022, over a billion OLED panels will be produced annually.
OLEDs can also be used to make TVs - some of the world’s top TV panels are produced based on innovative OLED technology. Various TV makers use OLED technologies to produce award-winning premium OLED TVs with outstanding image quality and ultra-thin form factors.
In the near future, other players are expected to join this market with new technologies - such as ink-jet printed OLEDs and quantum-dot hybrid OLEDs.
Next-Generation Foldable and Rollable Displays
OLEDs are a relatively new display technology - and its progress is still very fast. While many flexible OLED panels are already in use today, next-generation displays are under development and promise innovations like foldable devices, followed by rollable and stretchable displays.
The next wave of OLED displays is likely to be focused on foldable panels that will enable new mobile device form factors. Imagine phones that open into tablets, or smartwatches that can be opened to offer smartphone-sized displays. Several companies are gearing up to release the first such devices in 2019. Stay tuned!
OLED Device Structure
The basic OLED structure is simple - an organic emitter placed between two electrodes. But in order to create efficient and long-lasting devices, commercial OLEDs use several intermediate layers, like electron transport and blocking layers. The whole organic stack is placed between the electrodes, and this whole structure is deposited on the substrate (glass or plastic) and the display backplane (driver electronics). Some OLED displays on the market make use of dozens of different layers, one on top of the other.
OLED Production Processes
Currently, almost all OLED displays on the market are produced using an evaporation-based process, in which the OLED materials are deposited in a vacuum chamber. This has proven to be a great way to make OLEDs, but the process has its limitations - mainly material waste and high cost.
Companies are now developing next-generation deposition processes to enable more efficient production. One example is ink-jet printing, which makes use of soluble OLED inks that can be deposited using huge printers. This process is faster than the current evaporation process, and has almost no waste of materials. While there are still some challenges to overcome, it is expected that printed OLEDs will start entering the market soon - starting with TVs and monitor panels.
OLED Emitter Material Generations
The first OLED devices used so-called fluorescence emitters (1st-gen emitters). These are relatively stable and easy to produce compounds, but their internal efficiency is limited to around 25% - only a quarter of the energy is translated into light.
2nd-generation OLED emitters, called phosphorescence OLEDs, dope these emitters with heavy metals (usually iridium) which enables an internal efficiency of up to 100%. As of today, very efficient red and green phosphorescence OLEDs are available on the market and most OLED displays make use of these emitters to create highly efficient emitters.
An efficient and long-lasting blue emitter material has proven extremely hard to produce, and companies are still struggling to develop this material. Any much-anticipated success in doing so would surely have a dramatic effect on the power efficiency of OLED displays. While one option is to develop a blue phosphorescence OLED, there are also 3rd and even 4th-generation materials (TADF and Hyperfluorescence) OLED technologies which show great promise.
Testing an OLED Device
Testing an OLED device involves simultaneously measuring its electrical and optical properties to generate a current-voltage-luminance (IVL) curve. This allows the power efficiency and brightness to be characterised. Lifetime testing is also important to ascertain how quickly an OLED will degrade over extended use.
So, what’s next?
The future of OLEDs seems bright as their presence in the smartphone and TV markets is continuously growing, in addition to other markets (such as wearables, VR and more).
OLED technology is still an emerging technology, and many avenues are still open for new materials to be found and new processes to be developed that could further enhance OLED displays.
About the Author: This article was written by Ron Mertens, founder & CEO at Metalgrass LTD and OLED-info. For the past 14 years, Ron has been focused on the OLED market, gathering deep understanding of OLED technology, the industry and the market.