Some organic materials could be used in optoelectronics similar to silicon semiconductors. Whether as solar cells, light-emitting diodes or transistors – the so-called band gap, i.e. the energy difference between electrons in the valence band and the conduction band, is important. By means of light or an electrical voltage, charge carriers can be lifted from the valence band into the conduction band – in principle, this is how all electronic components work. Band gaps between 1-2 electron volts are ideal.
A team led by the chemist Dr. Michael J. Bojdys at Humboldt University Berlin has now synthesized a new organic semiconductor material from the carbon nitride family. The triazine-based graphitic carbon nitride or TGCN consists only of carbon and nitrogen atoms and can be grown as a film on a quartz substrate. The C and N atoms together form hexagonal honeycombs, similar to graphene, which consists of pure carbon. As with graphene, the crystalline structure of TGCN is two-dimensional. With graphene, however, the conductivity in the plane is excellent, perpendicular to it very poor. With TGCN it is exactly the other way round: the conductivity perpendicular to the plane is about 65 times greater than in the plane itself. With a band gap of 1.7 electron volts, TGCN is a good candidate for applications in optoelectronics.
The HZB physicist Dr. Christoph Merschjann then investigated the transport properties in samples of TGCN with time-resolved absorption measurements in the femto- to nanosecond range in the JULiq laser laboratory, a joint lab between HZB and Freie Universität Berlin. Such laser experiments make it possible to combine macroscopic conductivity with microscopic transport models. From the measured data he was able to deduce how the charge carriers diffuse through the material: “They do not leave the hexagonal honeycombs of triazine units horizontally, but move obliquely to the next triazine unit in the neighboring plane. They move along tubular channels through the crystal structure.” This mechanism could explain that the conductivity perpendicular to the planes is significantly higher than in the planes. However, it is probably not sufficient to explain the actual measured factor of 65. “We have not yet fully understood the transport properties in this material and want to investigate them further,” announces Merschjann.
“TGCN is therefore so far the best candidate to replace common inorganic semiconductors such as silicon with their partially critical “dotants” of rare elements,” says Bojdys. The developed manufacturing process leads to flat layers of semiconducting TGCN on insulating quartz glass. According to the scientists, this should enable upscaling and simple device production.
More information: https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.201902314