Organic electronics: a new semiconductor in the carbon-nitride family

The illustration is alluding to the laser experiment in the background and shows the structure of TGCN.

The illustration is alluding to the laser experiment in the background and shows the structure of TGCN. © C.Merschjann/HZB

Teams from Humboldt-Universität and the Helmholtz-Zentrum Berlin have explored a new material in the carbon-nitride family. Triazine-based graphitic carbon nitride (TGCN) is a semiconductor that should be highly suitable for applications in optoelectronics. Its structure is two-dimensional and reminiscent of graphene. Unlike graphene, however, the conductivity in the direction perpendicular to its 2D planes is 65 times higher than along the planes themselves.

Some organic materials might be able to be utilised similarly to silicon semiconductors in optoelectronics. Whether in solar cells, light-emitting diodes, or in transistors – what is important is the band gap, 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. Band gaps of one to two electron volts are ideal.

Talent for optoelectronics

A team headed by chemist Dr. Michael J. Bojdys at Humboldt University Berlin recently synthesised a new organic semiconductor material in the carbon-nitride family. Triazine-based graphitic carbon nitride (or 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 form 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 exactly the opposite: the perpendicular conductivity is about 65 times greater than the planar conductivity. With a band gap of 1.7 electron volts, TGCN is a good candidate for applications in optoelectronics.

Detailed analysis of transport properties

HZB physicist Dr. Christoph Merschjann subsequently investigated the charge transport properties in TGCN samples using time-resolved absorption measurements in the femto- to nanosecond range at the JULiq laser laboratory, a JointLab between HZB and Freie Universität Berlin. These kinds of laser experiments make it possible to connect macroscopic electrical conductivity with theoretical models and simulations of microscopic charge transport. From this  approach he was able to deduce 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 neighbouring 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, this is probably not sufficient to explain the actual measured factor of 65. “We do not yet fully understand the charge transport properties in this material and want to investigate them further”, adds Merschjann. At ULLAS / HZB in Wannsee, the analysis lab used subsequent to JULiq, the setup is being prepared for new experiments to accomplish this.

 “TGCN is therefore the best candidate so far for replacing common inorganic semiconductors like silicon and their crucial dopants, some of which are rare elements”, says Bojdys. “The fabrication process we developed in my group at Humboldt-Universität, produces flat layers of semiconducting TGCN on an insulating quartz substrate. This facilitates upscaling and simple fabrication of electronic devices.”

To the Publication:

Angewandte Chemie: "Directional Charge Transport in Layered Two‐Dimensional Triazine‐Based Graphitic Carbon Nitride" Yu Noda, Christoph Merschjann, Ján Tarábek, Patrick Amsalem, Norbert Koch, Michael J. Bojdys

DOI: 10.1002/anie.201902314

 

arö

You might also be interested in

  • Stability of perovskite solar cells reaches next milestone
    Science Highlight
    27.01.2023
    Stability of perovskite solar cells reaches next milestone
    Perovskite semiconductors promise highly efficient and low-cost solar cells. However, the semi-organic material is very sensitive to temperature differences, which can quickly lead to fatigue damage in normal outdoor use. Adding a dipolar polymer compound to the precursor perovskite solution helps to counteract this. This has now been shown in a study published in the journal Science by an international team led by Antonio Abate, HZB. The solar cells produced in this way achieve efficiencies of well above 24 %, which hardly drop under rapid temperature fluctuations between -60 and +80 Celsius over one hundred cycles. That corresponds to about one year of outdoor use.
  • Scientists Develop New Technique to Image Fluctuations in Materials
    Science Highlight
    18.01.2023
    Scientists Develop New Technique to Image Fluctuations in Materials
    A team of scientists, led by researchers from the Max Born Institute in Berlin and Helmholtz-Zentrum Berlin in Germany and from Brookhaven National Laboratory and the Massachusetts Institute of Technology in the United States has developed a revolutionary new method for capturing high-resolution images of fluctuations in materials at the nanoscale using powerful X-ray sources. The technique, which they call Coherent Correlation Imaging (CCI), allows for the creation of sharp, detailed movies without damaging the sample by excessive radiation. By using an algorithm to detect patterns in underexposed images, CCI opens paths to previously inaccessible information. The team demonstrated CCI on samples made of thin magnetic layers, and their results have been published in Nature.
  • Recommended reading: Bunsen magazine with focus on molecular water research
    News
    13.01.2023
    Recommended reading: Bunsen magazine with focus on molecular water research
    Water not only has some well-known anomalies, but is still full of surprises. The first issue 2023 of the Bunsen Magazine is dedicated to molecular water research, from the ocean to processes in electrolysis. The issue presents contributions from researchers cooperating within the framework of a European research initiative in the "Centre for Molecular Water Science" (CMWS). A team at HZB presents results from the synchrotron spectroscopy of water. Modern X-ray sources can be used to study molecular and electronic processes in water in detail.