A new finding to develop more energy efficient OLEDs

Organic light-emitting diodes (OLEDs) are currently used in high end displays, where they offer more vibrant colours, deeper blacks, and superior energy efficiency compared to older technologies, such as liquid crystal displays (LCDs). To make an OLED, a light emitting molecule, known as a ‘dopant’, is mixed into in a very thin layer of inert molecules, known as the ‘host’. This ‘emissive layer’ is then sandwiched between two metal layers through which electricity flows into the device, where it is converted into light by the dopant molecule. Dispersing the dopant in a host is important to avoid undesirable interactions between the dopant molecules that result in the wasteful conversion of electricity into heat instead of light. However, up until now, the host material was thought to make no direct contribution to the ability of the isolated dopant molecule to convert electricity into light.

The results provide a really exciting advance in our understanding of how OLEDs work. It is a great example of how something that appears to be unassuming at first glance, like an inert host material, can actually be playing a vital role in OLED operation beyond what was previously thought.  Dr Alex Gillett

Working with the group of Professor David Beljonne at the University of Mons in Belgium, researchers at the Cavendish Laboratory, University of Cambridge, have recently discovered how the host material plays a much more important role in how OLEDs work than previously thought. Published in Nature Materials, the team’s findings reveal that far from being an inert component of an OLED device, the host material is actually enabling the dopant to convert electricity into light.  Using very short laser pulses about a millionth of a billionth of a second long, the team in Cambridge were able to detect the signatures of the dopant molecule vibrating in a host material. These characteristic vibrations can be used to reveal the structure of the dopant, that is, how the atoms that make up the molecule are arranged. With this technique, the team was able to show that the vibrational signature, and therefore the structure of the dopant, changed rapidly after it absorbed light from a laser pulse. Importantly, absorbing laser light is comparable to how the dopants interact with electricity, meaning the processes investigated by the researchers should be the same as in an OLED.

To understand where this effect came from, the research group at the University of Mons ran a computational simulation of the dopant molecule surrounded by hundreds of individual host molecules. This simulation recreates the emissive layer of an OLED and allows the interactions between the host and dopant to be explored in detail. Using this model, they were able to accurately recreate the experimental results, which suggested that change in the structure of the dopant was directly caused by the host molecules rotating and vibrating themselves. Further calculations revealed that without the vibrations and rotations of the host molecules, the dopant would not be able to convert electricity to light as efficiently.

These findings open up a new way to improve OLEDs through altering the host material, including changing its rigidity to enhance the beneficial vibrations, or chemically modifying the host molecules to interact more strongly with the dopant. “The results provide a really exciting advance in our understanding of how OLEDs work. It is a great example of how something that appears to be unassuming at first glance, like an inert host material, can actually be playing a vital role in OLED operation beyond what was previously thought”, shares the first author of the paper Alex Gillett, Leverhulme Early Career Fellow at the Cavendish Laboratory.

If the full potential of the host material can be tapped into, this will allow for the development of more energy efficient OLEDs. This can benefit mobile phones that can go longer between being charged to more energy efficient household appliances, such as TVs and lighting.

Reference: Gillett, A.J., Pershin, A., Pandya, R. et al. Dielectric control of reverse intersystem crossing in thermally activated delayed fluorescence emitters. Nat. Mater. 21, 1150–1157 (2022). https://doi.org/10.1038/s41563-022-01321-2

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