Organic Photodiodes for Use as Infrared Sensors

Researchers from Siemens Corporate Technology (CT) and the Universities of Linz and Karlsruhe (TH) have produced organic photodiodes that are very sensitive to infrared radiation with wavelengths of more than one micrometer (µm). These detectors are important for many measuring processes, including gas sensors, night vision systems, and cancer diagnostics. When used in these technologies, organic photodiodes with semiconductor nanocrystals could result in substantial cost savings. An article concerning the research activities recently appeared in the renowned specialist publication Nature Photonics.

To produce their diodes, the Siemens researchers at Organic Electronics in Erlangen used quantum dots, which are semiconductor crystals with a diameter of just a few nanometers. Conventional photodiodes made of silicon are not sensitive to radiation with wavelengths of more than 1.1 m m. For such radiation, III/V semiconductors such as gallium arsenide, which are much more costly, are used today.

The new organic photodiodes, on the other hand, are not based on crystalline semiconductors, but rather on plastics. The advantage is in their production, which is very inexpensive for diodes with large surface areas: A solution containing the plastics is applied to a substrate. The cost of the process is nearly unaffected by the area of the diode, but prices of crystalline diodes are directly determined by their size. The solution contains semiconducting polymers and fullerenes, which take on the roles of the electron donors and acceptors of the semiconductor.

Organic photodiodes primarily detect visible light, and they are efficient detectors of infrared radiation with a wavelength of not more than about 1 µm. With quantum dots, which consist of about 50 atoms, the researchers can expand this range to as much as 2 µm. The sensitivity range is determined by the characteristic band gap energy of the semiconductors. The band gap depends directly on the number of atoms and the diameter of the semiconductor nanocrystals. The researchers use this quantum mechanics effect to set custom-tailored sensitivity ranges.

Using their method of imbedding quantum dots in organic diodes, they have achieved service life periods of at least one year. The measurements also indicate extremely high quantum efficiencies, which means the diodes convert the incident light into electric current with a high degree of efficiency. With a 256x256 pixel image sensor, the researchers show that the technology can even be used on an active-matrix circuit, demonstrating for the first time that solution-based semiconductor nanocrystals are suitable for industrial imaging technology.