Organic Thin-Film Sensors for Security

Spectroscopy, the branch of science that measures and interprets the electromagnetic spectra resulting from the interaction between radiation and the sample, has found many uses in differentiating between real and fake banknotes, secure documents and pharmaceuticals.

The technique’s main advantages are that it is non-destructive, fast, requires no sample preparation, and provides a fingerprint of the physical and chemical composition of the item under inspection.

These attributes often come at a high price. But in a recent publication in the journal ‘Advanced Materials’1, a team of physicists and chemists from TU Dresden present an affordable organic thin-film sensor that works in a completely new way to identify the wavelength of light and achieves a spectral resolution better than one nanometre.

As integrated components, the thin-film sensors could eliminate the need for external spectrometers in the future. A patent application has already been Spectroscopy comprises a group of experimental methods that separate radiation according to a specific property, such as wavelength or mass. It is considered one of the most important analytical methods in research and industry. Spectrometers can determine colours (wavelengths) of light sources and are used as sensors in various applications, such as medicine, engineering, the food industry and anti-counterfeiting.

Commercially available instruments are usually relatively large and very expensive. They are mostly based on the principle of the prism or grating: light is refracted, and the wavelength is assigned according to the angle of refraction.

At the Institute for Applied Physics (IAP) and the Dresden Integrated Center for Applied Physics and Photonic Materials (IAPP) of the TU Dresden (Technische Universität Dresden), such sensor components based on organic semiconductors have been researched for years. With the spin-offs Senorics and PRUUVE, two technologies have already been developed towards market maturity.

Now, researchers at the IAP and IAPP, in cooperation with the Institute of Physical Chemistry, have developed a thin-film sensor that describes a completely new way of identifying the wavelength of light and, due to its small size and cost, has advantages over the current commercially available spectrometers.

The sensor works like this. Light of unknown wavelength(s) excites luminescent materials in a hair-thin film. The film consists of a mixture of long-glowing (phosphorescent) and short-glowing (fluorescent) entities, which absorb the light under investigation in different ways. The intensity of the afterglow can be used to derive the wavelength(s) of the unknown input light.

‘We exploit the fundamental physics of excited states in luminescent materials,’ explains Anton Kirch, doctoral student at the IAP. ‘Light of different wavelengths excites in such a system, when properly composed, certain proportions of long- lived triplet and short-lived singlet spin states. And we reverse that dependence. By identifying the spin fractions using a photodetector, we can identify light wavelengths.’

Using this strategy, the scientists have achieved sub-nanometre spectral resolution and have successfully tracked minor wavelength changes of light sources.

One of the applications under consideration for the novel sensors is in counterfeit protection. ‘The small and inexpensive sensors could be used, for example, to quickly and reliably check banknotes or documents for certain security features and thus determine their authenticity, without any need for expensive laboratory technology,’ explains Anton Kirch.

1 - https://onlinelibrary.wiley.com/doi/10.1002/adma.202205015.