History of OLEDs
Organic light-emitting diode (OLED) technology spans over a thirty-year history. It touches our everyday lives through our reliance on devices like smartphones. OLEDs are still considered an emerging technology, even though they were first conceived of over three and a half decades ago.
In the lead up to the invention of OLED technology, a ripple of scientific breakthroughs occurred. As early on as the 1960s, scientists first observed the phenomenon of organic electroluminescence.
In 1961, 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:
- High voltage is generally required to inject charges into organic crystals
- The long-term stability of organic EL diodes was unknown
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.
The First OLED Device
In 1987 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.
Development of OLEDs
First 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:
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. The key innovations after 1987 are as follows:
A Bright Future in OLED Technology
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 1987. 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.
Adachi, C. et al. (2012) Highly efficient organic light-emitting diodes from delayed fluorescence. Nature, 492, 234–238.
Hong, G. et al. (2021) A Brief History of OLEDs—Emitter Development and Industry Milestones. Advanced Materials, 33, (9). Doi: 10.1002/adma.202005630.
Hung, L. et al. (2002) Recent progress of molecular organic electroluminescent materials and devices. Mat. Sci Eng. Rep., 39, 143-222.
Tang, C. W. and S. A. Van Slyke (1987) Organic electroluminescent diodes. Appl. Phys. Lett., 51, 913.
Professional Science Writer