Tuning the energy gap: A novel approach for organic semiconductors


Date:
June 10, 2021
Source:
Technische Universita"t Dresden
Summary:
What is already established for inorganic semiconductors stays a
challenge for their organic counterparts: Tuning the energy gap
by blending different semiconducting molecules to optimize device
performance. Now, scientists demonstrated how to reach this goal.



FULL STORY ========================================================================== Organic semiconductors have earned a reputation as energy efficient
materials in organic light emitting diodes (OLEDs) that are employed in
large area displays. In these and in other applications, such as solar
cells, a key parameter is the energy gap between electronic states. It determines the wavelength of the light that is emitted or absorbed. The continuous adjustability of this energy gap is desirable. Indeed, for
inorganic materials an appropriate method already exists -- the so-called blending. It is based on engineering the band gap by substituting atoms
in the material. This allows for a continuous tunability as, for example
in aluminum gallium arsenide semiconductors. Unfortunately, this is
not transferable to organic semiconductors because of their different
physical characteristics and their molecule-based construction paradigm,
thus making continuous band gap tuning much more difficult.


========================================================================== However, with their latest publication scientists at the Center for
Advancing Electronics Dresden (cfaed, TU Dresden) and at the Cluster
of Excellence "e- conversion" at TU Munich together with partners from University of Wu"rzburg, HU Berlin, and Ulm University for the first time realized energy-gap engineering for organic semiconductors by blending.

For inorganic semiconductors, the energy levels can be shifted towards one another by atomic substitutions, thus reducing the band gap ("band-gap engineering"). In contrast, band structure modifications by blending
organic materials can only shift the energy levels concertedly either up
or down. This is due to the strong Coulomb effects that can be exploited
in organic materials, but this has no effect on the gap. "It would be
very interesting to also change the gap of organic materials by blending,
to avoid the lengthy synthesis of new molecules," says Prof. Karl Leo
from TU Dresden.

The researchers now found an unconventional way by blending the material
with mixtures of similar molecules that are different in size. "The
key finding is that all molecules arrange in specific patterns that
are allowed by their molecular shape and size," explains Frank Ortmann,
a professor at TU Munich and group leader at the Center for Advancing Electronics Dresden (cfaed, TU Dresden). "This induces the desired change
in the material's dielectric constant and gap energy." The group of Frank Ortmann was able to clarify the mechanism by simulating the structures
of the blended films and their electronic and dielectric properties.

A corresponding change in the molecular packing depending on the shape
of the blended molecules was confirmed by X-ray scattering measurements, performed by the Organic Devices Group of Prof. Stefan Mannsfeld at
cfaed. The core experimental and device work was done by Katrin Ortstein
and her colleagues at the group of Prof. Karl Leo, TU Dresden.

The results of this study have just been published in the journal Nature Materials. While this proves the feasibility of this type of energy-level engineering strategy, its employment will be explored for optoelectronic devices in the future.

========================================================================== Story Source: Materials provided by Technische_Universita"t_Dresden. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Katrin Ortstein, Sebastian Hutsch, Mike Hambsch, Kristofer
Tvingstedt,
Berthold Wegner, Johannes Benduhn, Jonas Kublitski, Martin Schwarze,
Sebastian Schellhammer, Felix Talnack, Astrid Vogt, Peter Ba"uerle,
Norbert Koch, Stefan C. B. Mannsfeld, Hans Kleemann, Frank Ortmann,
Karl Leo. Band gap engineering in blended organic semiconductor
films based on dielectric interactions. Nature Materials, 2021;
DOI: 10.1038/s41563-021- 01025-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/06/210610135553.htm

--- up 4 weeks, 6 days, 22 hours, 45 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1337:3/111)