Keywords: cooperations (143) materials research (69)

Science Highlight    18.06.2014

Electrostatics do the trick

When inserting an ultrathin dielectric between metal electrode and organic semiconductor, charge carriers (shown here for a positively charged holes in red) are, counter intuitively, more efficiently extracted from their transport level (blue) in the organic to the Fermi level (black) in the metal than without the interlayer.
Copyright: M. Oehzelt/HZB

A simple model describes what happens between organic semiconductors and metals

Organic semiconductors allow for flexible displays (OLEDs), solar cells (OPVCs), and other interesting applications. One common problem in these devices, however, is the interface between the metallic contacts and the organic semiconductor material, where undesirable losses occur. Now Dr. Martin Oehzelt has shown what these losses between the metal and the organic semiconductors depend upon and how to minimize them. In particular, his model also explains why a thin, electrically insulating layer between the two materials can even facilitate the transition of charge carriers. His results have recently been published in Nature Communications.

Currently, there are many different approaches describing the interface between organic semiconductor materials and metallic contacts. These somewhat contradictory theories, none of which is universally valid for all cases, have now been unified by Oehzelt and developed into a single coherent model based on the electrostatic potential caused by the charge carriers in the metal and the organic semiconductor. “I calculated the impact of the charge carrier distribution on the electronic states at the interface and how these changes feed back onto the charge carrier distribution”, he explains. Oehzelt is presently conducting research with Dr. Georg Heimel as a postdoc for Prof. Norbert Koch, who works at the Humboldt-Universität zu Berlin and the Helmholtz-Zentrum Berlin.

Such calculations have never been so comprehensively carried out before. Performing them, Oehzelt states: “it was surprising to me that the quantum physical level was not that important. The electrostatic effects predominated! The agreement between our model and the experimental data were astonishing.” On the example of pentacene, a common organic semiconductor, Oehzelt has quantitatively checked the model’s predictions for interface losses.

The energy distribution of the electronic states in organic semiconductors determines the minimum energy barrier the charge carriers have to overcome in transitioning from or into the metal. The calculation demonstrates that the shape of this energy barrier can vary, from a step-function to slow, continuously rising curves that lead to considerably lower losses. The latter can be achieved by introducing an extremely thin insulating layer between the organic semiconductor and the metal. Contrary to the general expectation, the introduction of an insulator thus improves the electrical contact.

The results of this work could notably simplify optimization of interfaces and contacts and, thereby, the development of more efficient organic electronic devices.

The work has recently been published in  Nature Communications:

doi 10.1038/ncomms5174
 
 

arö


           



You might also be interested in
  • NEWS      05.06.2019

    Photovoltaics are growing faster than expected in the global energy system

    Dramatic cost reductions and the rapid expansion of production capacities make photovoltaics one of the most attractive technologies for a global energy turnaround. Not only the electricity sector, but also transport, heating, industry and chemical processes will in future be supplied primarily by solar power, because it is already the cheapest form of electricity generation in large parts of the world. This is where opportunities and challenges lie - at the level of the energy system as well as for research and industry. Leading international photovoltaic researchers from the Global Alliance for Solar Energy Research Institutes describe the cornerstones of future developments in an article published in the journal "Science" on 31 May. [...]


  • <p>The illustration is alluding to the laser experiment in the background and shows the structure of TGCN.</p>SCIENCE HIGHLIGHT      05.06.2019

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

    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. [...]


  • NEWS      04.06.2019

    Federal Ministry of Education and Research supports the development of a miniaturised EPR spectrometer

    Several research institutions are developing a miniaturized electron paramagnetic resonance (EPR) device with industrial partner Bruker to investigate semiconductor materials, solar cells, catalysts and electrodes for fuel cells and batteries. The Federal Ministry of Education and Research (BMBF) is funding the "EPR-on-a-Chip" or EPRoC project with 6.7 million euros. On June 3, 2019, the kick-off meeting took place at the Helmholtz-Zentrum Berlin. [...]




Newsletter