Organic light-emitting diode (OLED) technology spans a thirty-year history. Over the years, the development and improvement of OLEDs has involved using specialized equipment like glove boxes and spin coaters in research and development setups. Today, OLEDs touch our everyday lives through our reliance on devices like smartphones but, despite being first conceived over three and a half decades ago, OLEDs are still considered an emerging technology.
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Timeline of important OLED developmentsIn the lead up to the invention of OLED technology, a ripple of scientific breakthroughs occurred. As early on as the s, scientists first observed the phenomenon of organic electroluminescence.
In , researchers working at the Admiralty Materials Laboratory, Dorset, discovered thermally activated delayed fluorescence (TADF) in the compound eosin (a fluorescent dye commonly used in microscopy). The mechanism involved was not fully understood at the time. A couple of years later, researchers at New York University found another dye, known as anthracene, that could emit electroluminescence when a sufficiently high voltage was applied.
Besides eosin and anthracene, a number of fluorescent organic compounds were known at the time. However, there were limiting factors in realising the fabrication of practical electroluminescent devices. These were later articulated by Tang and Slyke, the co-discoverers of OLED technology:
Operating voltage and efficiency were both problematic factors. In the meantime, organic materials were discovered to have extremely high fluorescence quantum efficiencies in the visible spectrum, including the blue region, with some approaching 100%. Even so, it would take almost three decades before light emission at lower voltages could be fully realised.
In at the Eastman Kodak Company, two scientists &#; Ching Tang and Steven Van Slyke &#; built the first OLED device. It was operational at a sufficiently low voltage and marked the discovery of OLED technology. For the first time scientists had combined modern thin film deposition techniques with suitable materials and structure to build a double layer OLED device.
Since then, the structural architecture of the OLED has evolved over time from a simple structure, to a more complex multi-layered design that has considerably enhanced efficiency.
Multi-layer structures have increased OLED efficiencyFirst generation OLEDs used fluorescence emitters. Then second-generation emitters were doped with heavy metals, like iridium, giving rise to phosphorescence. This vastly improved upon efficiency. The evolution of three successive generations of OLEDs is given below:
Generation
Emission-type
First
Fluorescence
Second
Phosphorescence
Third
Thermally activated delayed fluorescence (TADF)
Both academia and industry have contributed equally to advances in OLED technology. It was close to ten years after the work at Eastman Kodak that the first commercial OLED device was produced by Pioneer and used in car audio systems.
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More environmentally friendly than traditional LEDs, they are proving ever popular on the consumer market. OLED displays are commonly found in everyday consumer devices, and, unlike traditional liquid crystal displays, they do not require a backlight system.
OLED research and development has come a long way since . Current research in academia and industry is now progressing toward the fourth generation of OLEDs comprising foldable and stretchable displays. We have already seen curved OLED TV screens and foldable smartphone displays. OLEDs based on soluble materials can be printed on flexible substrates &#;&#;think wearable devices and clothing. Meanwhile, microLED technologies are being used in the field of virtual reality.
Written by
Dr. Nicola Williams
Professional Science Writer
Organic light emitting diodes (devices) or OLEDs are monolithic, solid-state devices that typically consist of a series of organic thin films sandwiched between two thin-film conductive electrodes. When electricity is applied to an OLED, under the influence of an electrical field, charge carriers (holes and electrons) migrate from the electrodes into the organic thin films until they recombine in the emissive zone forming excitons. Once formed, these excitons, or excited states, relax to a lower energy level by giving off light (electroluminescence) and/or unwanted heat.
The basic OLED cell structure consists of a stack of thin organic layers sandwiched between a conducting anode and a conducting cathode. Breakdown of an OLED structure:
Emi
ssive
Layer
&#; The heart of the device and where light is made, the emissive layer consists of a color defining emitter doped into a host. This is the layer where the electrical energy is directly converted into light.To generate red, green and blue light to render full-color images, there are two main approaches currently being used. The first is to pattern red, green and blue OLED sub-pixels in each pixel of the display, as shown below. This is generally the preferred approach for high-resolution mobile displays.
RGB OLED side-by-side
The second approach is to produce white light in every pixel, and then use a color filter to make red, green and blue sub-pixels.
White OLED with Color Filter (CF)
OLED displays including TVs, smartphones, wearables, IT and VR
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