A multitude of basic chemical compounds can be detected due to their ability to absorb infrared light. It is therefore no surprise that infrared sensors are versatilely applied – e.g. to monitor the health conditions such as sugar concentration or oxygen saturation in the blood, to control the product quality for industrial processing or to predict the ideal harvesting date in agriculture. Even tough inorganic semiconductors handle these tasks reliably; spectrometers based on this material class are expensive in production, heavy and bulky.
As demonstrated by the success of organic light emitting diodes (OLEDs), hydrocarbon based semiconductors offer a serious alternative for opto-electronic applications. In contrast to their inorganic counterparts, such devices can be realised as light-weight, flexible, highly integrated and especially low-cost foils. Previous studies carried out at the Dresden Integrated Center for Applied Physics and Photonic Materials under direction of Prof. Karl Leo already underlined these advantages for OLEDs, organic photovoltaics and transistors. In the following, the research group headed by Prof. Koen Vandewal stated the question, whether organic semiconductors would also allow for (near) infrared sensing.
Initially, the scientists encountered the hurdle of identifying organic layers which can efficiently detect near infrared (NIR) signals, since organic compounds are transparent in this region. To solve this several decades old problem, the scientists of the TU Dresden considered utilizing an effect already known in organic solar cells: When blending two organic semiconductors on a molecular level, a very weak NIR absorption at the interface between both molecule species can be measured.
Finally, Bernhard Siegmund et al. achieved the breakthrough for organic NIR detectors, when they sandwiched such blend layers between ultra-thin mirror layers. By a careful choice of the distance between both metal mirrors, they succeeded in establishing a resonance, at which the incoming light transits the blend layer dozens of times via repeated reflections at the mirrors. The resulting device only detects light with a specific wavelength in the NIR. Utilizing a variation of the organic layer thickness, the sensor can scan several wavelengths simultaneously and hence, reconstruct spectral fingerprints of, for example, food or blood samples.
The authors see with this invention a novel research domain opening, giving rise to a multitude of exciting questions of both, applied and fundamental nature. For the coming years, Siegmund, Vandewal, and coauthors expect further progress for their sensors, making them appear almost entirely transparent to the human eye.
Source : Technische Universität Dresden