Tuesday, 20 Mar 2018

Gauss Centre for Supercomputing and TU Dresden use simulation and experiment to study next-generation organic semiconductors

Theoretical physicists and experimentalists collaborate to identify dopants capable of enabling new designs of organic semiconducting materials

Gauss Centre/TU Dresden -  Illustration of a doped organic semiconductor based on fullerene

8 Mar 2018 | Editor

A team of scientists at TU Dresden used the SuperMUC supercomputer at the Leibniz Supercomputing Centre to refine its method for studying organic semiconductors.

Specifically, the team uses an approach called organic semiconductor doping, a process in which impurities are intentionally introduced into a material to give it specific semiconducting properties. It recently published its results in Nature Materials.

When someone changes a material's physical properties, he or she also changes its electronic properties and, therefore, the role it can play in electronic devices. Small changes in material makeup can lead to big changes in a material's characteristics--in certain cases one slight atomic alteration can lead to a 1000-fold change in electrical conductivity.

While changes in material properties may be big, the underlying forces--exerting themselves on atoms and molecules and governing their interactions--are generally weak and short-range (meaning the molecules and the atoms of which they are composed must be close together). To understand changes in properties, therefore, researchers have to accurately compute atomic and molecular interactions as well as the densities of electrons and how they are transferred among molecules.

Introducing specific atoms or molecules to a material can change its conducting properties on a hyperlocal level. This allows a transistor made from doped material to serve a variety of roles in electronics, including routing currents to perform operations based on complex circuits or amplifying current to help produce sound in a guitar amplifier or radio.

Quantum laws govern interatomic and intermolecular interactions, in essence holding material together, and, in turn, structuring the world as we know it. In the team's work, these complex interactions need to be calculated for individual atomic interactions, including interactions among semiconductor "host" molecules and dopant molecules on a larger scale.

The team uses density functional theory (DFT)--a computational method that can model electronic densities and properties during a chemical interaction--to efficiently predict the variety of complex interactions. It then collaborates with experimentalists from TU Dresden and the Institute for Molecular Science in Okazaki, Japan to compare its simulations to spectroscopy experiments.

To test its computational approach, the team simulated materials that already had good experimental datasets as well as industrial applications. The researchers first focused on C60, also known as Buckminsterfullerene.

Buckminsterfullerene is used in several applications, including solar cells. The molecule's structure is very similar to that of a soccer ball--a spherical arrangement of carbon atoms arranged in pentagonal and hexagonal patterns the size of less than one nanometer. In addition, the researches simulated zinc phthalocyanine (ZnPc), another molecule that is used in photovoltaics, but unlike C60, has a flat shape and contains a metallic atom (zinc).

As its dopant the team first used a well-studied molecule called 2-Cyc-DMBI (2-cyclohexyl-dimethylbenzimidazoline). 2-Cyc-DMBI is considered an n-dopant, meaning that it can provide its surplus electrons to the semiconductor to increase its conductivity. N-dopants are relatively rare, as few molecules are "willing" to give away an electron. In most cases, molecules that do so become unstable and degrade during chemical reactions, which in this context can lead to an electronic device failure. 2-Cyc-DMBI dopants are the exception, because they can be sufficiently weakly attractive for electrons--allowing them to move over long distances--while also remaining stable after donating them.

The team got good agreement between its simulations and experimental observations of the same molecule-dopant interactions. This indicates that they can rely on simulation to guide predictions as they relate to the doping process of semiconductors. They are now working on more complex molecules and dopants using the same methods.

Despite these advances, the team recognises that next-generation supercomputers such as SuperMUC-NG--announced in December 2017 and set to be installed in 2018--will help the researchers expand the scope of their simulations, leading to ever bigger efficiency gains in a variety of electronic applications.

"New kinds of semiconductors, organic semiconductors, are starting to get used in new device concepts."

"Some of these are already on the market, but some are still limited by their inefficiency. We are researching doping mechanisms--a key technology for tuning semiconductors' properties--to understand these semiconductors' limitations and respective efficiencies."

"Electrical conductivity can come from many dopants and is a property that emerges on a much larger length scale than just interatomic forces."

"Simulating this process needs more sophisticated transport models, which can only be implemented on high-performance computing (HPC) architectures."

"We need to push the accuracy of our simulations to the maximum."

"This would help us extend the range of applicability and allow us to more precisely simulate a broader set of materials or larger systems of more atoms."

"We are often limited by system memory or CPU power."

"The system size and simulation's accuracy are essentially competing for computing power, which is why it is important to have access to better supercomputers. Supercomputers are perfectly suited to deliver answers to these problems in a realistic amount of time."

Dr. Frank Ortmann, team leader

Insight into doping efficiency of organic semiconductors from the analysis of the density of states in n-doped C60 and ZnPc

Christopher Gaul, Sebastian Hutsch, Martin Schwarze, Karl Sebastian Schellhammer, Fabio Bussolotti, Satoshi Kera, Gianaurelio Cuniberti, Karl Leo & Frank Ortmann

Nature Materials (2018) : doi:10.1038/s41563-018-0030-8

Received: 08 March 2017 | Accepted: 23 January 2018 | Published online: 26 February 2018


Doping plays a crucial role in semiconductor physics, with n-doping being controlled by the ionization energy of the impurity relative to the conduction band edge. In organic semiconductors, efficient doping is dominated by various effects that are currently not well understood. Here, we simulate and experimentally measure, with direct and inverse photoemission spectroscopy, the density of states and the Fermi level position of the prototypical materials C60 and zinc phthalocyanine n-doped with highly efficient benzimidazoline radicals (2-Cyc-DMBI). We study the role of doping-induced gap states, and, in particular, of the difference Δ1 between the electron affinity of the undoped material and the ionization potential of its doped counterpart. We show that this parameter is critical for the generation of free carriers and influences the conductivity of the doped films. Tuning of Δ1 may provide alternative strategies to optimize the electronic properties of organic semiconductors.

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