1970s–1990s: Can organic semiconductors shine, too?
André Bernanose and his team at Nancy Université in France were the first to observe electroluminescence in organic materials in the early 1950s.
Largely due to the difficulties of efficiently injecting charge into organic materials, it was not until 1974 that the first polymer LED patent was filed by Roger Partridge at the National Physical Laboratory in the United Kingdom. One surprising feature of this patent is the title: with such a groundbreaking discovery of a very specific new type of electronic device, it seems strange that the inventor chose the very broad title “radiation sources” for his patent.
The first practical OLED devices were developed even some time later, in 1987, by chemists Ching Wan Tang and Steven van Slyke at Eastman Kodak. Tang and Slyke developed a two-layer structure with separate hole transporting and electron transporting layers, thus mitigating the problems of generally lower charge mobility in organic semiconductors. With this novel structure of organic thin films, prepared by vapor deposition, recombination and light emission could occur in a small interface region of the organic structure.
In the late 1990s and early 2000s, the first full-colour OLED displays were produced by Kodak and Sanyo.
2000s–Present: From indicators to infrastructure
With the spectrum conquered, LEDs began to displace traditional incandescent and fluorescent lighting on a global scale. Dramatic improvements in luminous efficacy, thermal management, and chip architecture made LEDs the lighting solution of choice in consumer electronics (backlighting, displays), automotive lighting (headlamps, brake lights), general illumination (homes, offices, streets), medical and scientific imaging, agriculture and aquaculture (specialised grow lights).
Even the topic of my experimental work nearly 25 years ago has come a long way since: flexible OLED displays are now widely used in foldable displays for smartphones and electronic paper applications, a far cry from the short-lived and somewhat unwieldy spin-coated polymer OLEDs that I worked on in a Munich physics lab in the early 2000s.
Samsung Galaxy foldable smartphones with OLED displays, image source: https://commons.wikimedia.org/wiki/File:Foldable_Smartphones.jpg
Searching for a suitable topic for an article for HLK’s 175th anniversary, I decided to go back into my own past: I used to be an experimental physicist working on organic semiconductor LEDs and lasers. It turns out that LEDs are actually quite a fitting topic for a 175th anniversary piece, since the theoretical foundations for LEDs date back to the 1850s with the birth of electroluminescence theory, and the first patents related to LEDs are nearly 100 years old.
