Organic semiconductors were discovered in the mid-1970s by Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa, who shared the Nobel Prize in Chemistry in 2000 for their work. The first effective OLED (Organic Light Emitting Diodes) was developed by Ching Tang and Steven VanSlyke at Eastman Kodak in 1987.
Through the years, OLED displays have gradually become more affordable, and so their adoption has increased. They are very popular for smartphones and other devices with small displays but are also used for TVs, PC monitors, and other large panels.
OLED structure
Before we get to the manufacturing process, it’s important to understand a little about how OLEDs work. OLEDs produce light using organic, i.e. carbon-based, materials. Their unique structure allows them to emit light when electricity is applied, making them a highly efficient source of illumination.
Unlike conventional LEDs, which are point light sources, OLEDs are made from sheets that emit diffuse-area light. This makes OLEDs suitable for larger, softer light sources that can be viewed directly, eliminating the need for shades, diffusers, or other light-manipulating tools.
The structure of an OLED is quite complex. It’s a solid-state device comprising a thin, carbon-based semiconductor layer that lights up in response to electricity applied by adjacent electrodes. For light to escape from the device, at least one of the electrodes must be transparent. The intensity of the light emitted is controlled by the amount of electric current applied by the electrodes, while the color of the light is determined by the type of emissive material used.
OLED structure, starting from the bottom:
- Substrate layer – The OLED foundation, which can be glass, metallic foil or plastic.
- Anode layer – This positively charged layer injects holes, meaning an absence of electrons, into the organic layers. It may be transparent depending on the type of OLED.
- Hole Injection Layer (HIL) – As the name implies, the HIL injects the anode holes further into the structure.
- Hole Transport Layer (HTL) – This layer transports the holes to the emissive layer.
- Emissive Layer – This layer is where light is brought into existence via the conversion of electrical energy. It is made up of a color-defining emitter doped into a host.
- Blocking layer (BL) – An adjustment layer that may be used to confine the charge-carrying electrons to the emissive layer.
- Electron Transport Layer (ETL) – Transports electrons across it so they can reach the emissive layer.
- Cathode – Being a cathode, it is negatively charged to inject electrons into the OLED’s organic layers. Depending on the type of OLED, it may be transparent.
When an electrical charge is applied to the OLED array, it results in a flow of electricity from the cathode to the anode. The anode draws electrons from the conductive layer while the emissive layer receives electrons from the cathode. This flow of displaced electrons results in electrofluorescence. The process makes conventional light-emitting diode (LED) backlights redundant, improving energy efficiency and allowing for the formation of thin, flexible, and/or transparent OLEDs.
The manufacturing process of OLED arrays
The manufacturing process of an OLED begins with the application of anodes to a substrate material of plastic or glass.
This can be done via:
- Vacuum deposition (or vacuum thermal evaporation) method: Organic molecules are heated in a vacuum chamber. They evaporate and are allowed to condense, forming thin films on cooled substrates.
- Organic vapor phase deposition method: This takes place in a low-pressure reactor chamber with hot walls. evaporated organic molecules are transported via a carrier gas transports onto cooled substrates, where they form thin films.
- Inkjet printing method: With this method, OLEDs are sprayed onto substrates much like inks on paper. This reduces the cost of manufacturing and allows for printing onto very large films to make large displays like big-screen TVs.
In the case of Inkjets, OLEDs are sprayed with the substrate material using a specialty dispersion, coating it with a uniform layer of conductive molecules. Subsequently, a second layer is added to the matrix through similar processes. This layer is composed of emissive molecules, and if necessary, additional layers can be applied.
These organic layers are then finished with the application of a cathode, effectively sandwiching the organic layers in a self-contained circuit, before a final seal of glass or plastic is applied to complete the OLED display.
The two types of OLEDs
There are two types of OLEDs: traditional OLEDs that use small organic molecules deposited on glass, and those that use large plastic molecules called polymers, also known as light-emitting polymers (LEPs) or Polymer LEDs (PLEDs).
LEPs are thinner and more flexible as they are printed onto plastic using a modified, high-precision version of an inkjet printer.
Large displays also differ in the way pixels are built up from individual OLED elements. Some designs arrange red, green, and blue pixels side by side, while others stack the pixels on top of one another. This provides higher resolution, though the display is correspondingly thicker.
Current and future applications
OLEDs are superior to the older LCD technology in many ways. They are thinner, lighter, and more flexible. They are also brighter and consume less energy, allowing for better battery life in portable devices. Rapid response time makes them suitable for fast-moving images such as sports on TV or computer games. Last but not least they also produce truer colors and offer wide viewing angles.
On the downside, their lifespan is shorter than that of conventional LEDs, and the organic molecules in OLEDs are sensitive to water, which might pose a challenge in portable products.
Nevertheless, OLEDs are now used in a variety of applications, including TVs, PC monitors, and of course mobile devices. Samsung started using OLED technology in its TVs back in 2013 and in its Galaxy smartphones the following year. Apple, initially lagging behind in OLED technology, released the Apple Watch with an OLED display in 2015, and the iPhone X became the first Apple smartphone with an OLED display in 2017.
What we might see more of in the future is, for example, animated billboards, electronic book readers, and clothing items with changing colors and patterns that are customizable by the wearer.
From the first OLED TVs showcasing the inherent flexibility of the technology to the unveiling of UHDTVs that can be rolled up safely when not in use, the innovative nature of OLED technology is evident. The desire to integrate OLEDs into car windows and windscreens for unobtrusive displays is pushing the boundaries of display panel design and use.