How luminous animals could change the structure of OLEDs

Contents

How can a firefly light up an OLED display? A research team led by Stefan Schramm, Professor of Applied Organic Chemistry at the Dresden University of Applied Sciences, is looking into this question. He aims to replace environmentally harmful heavy metal compounds in displays with biodegradable substances. The idea is to use molecules with which luminous animals switch on their light.

The problem: OLEDs (organic light-emitting diodes) contain heavy metal compounds that can be highly toxic to the environment, explains Schramm. If the technical devices in which they are installed – such as smartphones – are not recycled correctly, harmful substances like these could be washed out and released into the environment. Nature cannot break down some substances by itself, explains Schramm. Instead, they accumulate. "In Asia, researchers have already found molecules that are associated with OLEDs and LCDs in fish and even in human milk," says Schramm – including various fluorenes, anthracenes and triazines. In addition, the production of metals such as iridium or platinum releases a lot of CO₂ during manufacture and a lot of water is needed to process them.

More than 3500 species can generate light themselves

"Our goal is to develop so-called BiOLEMs, i.e. 'biologically inspired organic light-emitting molecules'," explains Schramm. These molecules are based on systems used by animals and plants that are capable of emitting light themselves. Bioluminescent organisms would not need an external impulse for this, but would light up through a chemical reaction. "These include luminous algae, fireflies and jellyfish," explained the chemist.

More than 3500 species can produce light themselves. Interestingly, nature has come up with this ingenuity in different places: "This is called convergent evolution," says Schramm. In other words, different starting points lead to the same result without any connection to each other. The ability to produce bioluminescence has evolved over 50 times independently of each other. But how does the ability of fireflies, jellyfish or snails help to make a display glow?

Schramm and his team are concentrating primarily on fireflies. The advantage: there is already a wealth of knowledge about these fireflies. "Fireflies were one of the first systems to be studied in the 1950s and 1960s," says Schramm. There have been numerous international studies on the glow of molecules in fireflies. As a result, it has long been possible to chemically modify the molecules so that, for example, the color changes and organic applications are possible. "Unfortunately, this has not yet made it to market application," explains the chemist.

Interaction of the molecules is crucial

In addition to the chemical molecules, physical aspects are also important for use in OLED displays. For many applications, the molecules must be able to be vaporized or printed onto a surface. "In the future, we also want to work more closely with physicists and engineers to build organic sensors and organic field-effect transistors by making targeted use of various quantum mechanical effects," says Schramm. "Only by interacting with many other molecules in the OLED will we ultimately be able to build a light-emitting diode that is efficient, long-lasting and environmentally friendly."

While in nature a chemical reaction triggers the lighting effect, in the case of displays, this is done by an electric current. The process is called electroluminescence. The molecules do not decompose due to the lack of a reaction, explains Schramm. "The OLED needs a substance that lights up," says the chemist. This is the luminescent molecule. Electricity excites the molecule and puts it into an excited state. "It has a lot of energy in this state," explains Schramm. To return to a relaxed ground state, the molecule has to dissipate the excess energy. This is the point at which it converts the energy into light. "It depends on the energy difference between the excited and the relaxed ground state," says the 33-year-old.