Some organic materials cannot be utilized similarly to silicon semiconductors in optoelectronics. Whether in solar cells, light-emitting diodes, or in transistors, what is important is the bandgap, i.e. the difference in energy level between electrons in the valence band (bound state) and the conduction band (mobile state). Charge carriers can be raised from the valence band into the conduction band by means of light or an electrical voltage. This is the principle behind how all electronic components operate. Bandgaps of one to two electron volts are ideal.
A new organic semiconductor material in the carbon-nitride family was synthesized. Triazine-based graphitic carbon nitride (TGCN) consists of only carbon and nitrogen atoms and can be grown as a brown film on a quartz substrate. The combination of C and N atoms forms hexagonal honeycombs similar to graphene, which consists of pure carbon. Just as with graphene, the crystalline structure of TGCN is two-dimensional. With graphene, however, the planar conductivity is excellent while its perpendicular conductivity is very poor. In TGCN, it is the opposite: the perpendicular conductivity is about 65 times greater than the planar conductivity. With a bandgap of 1.7 electron volts, TGCN is a good candidate for applications in optoelectronics.
The charge transport properties in TGCN samples were investigated using time-resolved absorption measurements in the femto- to nano-second range. These laser experiments make it possible to connect macroscopic electrical conductivity with theoretical models and simulations of microscopic charge transport. From this approach, researchers deduced how the charge carriers travel through the material. They do not exit the hexagonal honeycombs of triazine horizontally, but instead move diagonally to the next hexagon of triazine in the neighboring plane. They move along tubular channels through the crystal structure.
This mechanism might explain why the electrical conductivity perpendicular to the planes is considerably higher than that along the planes; however, it is probably not sufficient to explain the actual measured factor of 65.
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