The Generic OLED
OLEDs (Organic LEDs) are devices that can produce light using electrical current and electro-phosphorescent organic material.
OLEDs have many advantages over other display technology such as LCD and LED displays. These advantages include the use of no backlight (which allows for true black images), higher efficiencies (as no backlight is needed), thinner displays, faster refresh rates, and curved displays.
These OLEDs are made with individual layers that are sandwiched together:
- Substrate – The outer layer that the OLED sits on (can be plastic, glass, etc.)
- Anode – This is the positive electrode of the stack
- Organic Layer – This sits on top of the anode and consists of organic molecules with either bonds or conductive polymers
- Conductive Layer – This sits above the organic layer and is used to transport “holes” from the anode to the emissive layer
- Emissive Layer – This layer contains the electro-phosphorescence organic molecules
- Cathode – This is the negative electrode of the stack
Construction of a typical OLED. Image courtesy of Newhaven Display.
OLEDs can offer brilliant images with no need for backlights but they do have inherent problems.
Firstly, OLEDs require a rare material for the anode, indium-tin oxide (ITO), because of its electrical conductivity and optical transparency.
Secondly, indium-tin oxide is rather brittle and does not work well with flexible displays, making it non-ideal for a flexible display. OLEDs also have low external quantum efficiencies (EQE) and, as the output luminescence increases, the efficiency becomes worse (efficiency roll-off).
Thirdly, the life-expectancy of OLEDs is much lower than that of LCD technology because of the use of ITO anodes. Indium-tin oxide molecules can atomise and diffuse into the luminescent layer, which prevents the emission of light (as ITO traps the charges that are needed to produce the light).
But these issues have been challenged by a South Korean team who have built an OLED stack that may be the key to the future of flexible high-efficiency displays.
The New OLED Stack
A South Korean team, led by Tae-Woo Lee from Pohang University of Science and Technology, have created an OLED stack that solves most of the current OLED problems. Their OLED stack is extremely flexible, not dependent on indium-tin oxide, and is very efficient as compared to current OLED technology.
Firstly, this OLED is exploiting the new craze material, graphene, to replace the indium-tin oxide anode. Graphene is not only very optically transparent but is also incredibly flexible, which makes it ideal for flexible displays. The graphene, unlike indium-tin oxide, does not readily atomise and diffuse into the luminescence layers, which increases the life of the OLED stack.
Graphene is strong, conductive, and transparent. Image courtesy of AlexanderAlUS [CC BY-SA 3.0].
The OLED stack also uses two electro-phosphorescence layers stacked on top of each other to improve the luminous current efficiency. However, depositing the extra layers needed for two stacked luminous layers requires high-temperature chemical deposition which can damage the existing substrate layer. To overcome this, the researchers developed a Charge Generation Layer (CGL).
A typical OLED stack (a) compared to the new OLED stack (b). Image courtesy of NPG Asia Materials.
This Charge Generation Layer is imperative in the OLED stack for two reasons:
- It is applied at low temperatures, unlike typical Chemical Vapor Deposition.
- It helps to carry and inject charges through the stack.
While there are other OLEDs using graphene anodes, this OLED stack really stands out with its high efficiency and efficiency roll-off. A typical single-stack, graphene-based OLED has an External Quantum Efficiency of 32.7%. By comparison, this OLED has an efficiency as high as 45.2% and 87.3% with a hemispherical lens.
Demonstration of the new flexible OLED. Image courtesy of NPG Asia Materials.
Future for OLEDs
So what does this mean for OLEDs in the future?
Recently, OLED displays have gotten cheaper, making them more appealing to consumer markets. The displays currently on the market are not very flexible at all which means there is still a large gap in the market for such flexible displays. Transparent displays are also a technology that companies have been trying to develop for the consumer market (for example, Samsung’s OLED transparent displays).
If this technology could be explored further and made more “mass production-friendly”, we could see the introduction of flexible displays with higher battery lives, better transparency, and lower costs.