Fink group - Theoretical Chemistry, Computational Physical Chemistry

Applicable Theory

The Fink group develops and applies electronic structure methods in order to address scientific issues in the realm of Physical Chemistry. Our work focuses on

• Theory of organic solar cells
• Calculation of core electron spectra
• Development of quantum chemical approaches

Theory of organic solar cells

Conductive organic dyes are key-materials of a new group of low-cost, high-performance electrical components. Important applications are field-effect transistors, organic light-emitting diodes (OLEDs) and organic solar cells. Although such materials are already used in technical applications their functionality is only partially understood. This is partly due to the complex mostly amorphous, semicrystalline and / or polymorphic structure of these materials, which cannot be treated as periodic symmetric crystal structures. For the optoelectronic properties the solid-state structure plays a crucial role. However, even for purely crystalline substances the important transport properties are not well understood.

Using the example of 3,4:9,10 Perylentetracarboxy-bisimide, we have shown recently that quantum chemical calculations can provide insights into the optoelectronic properties of organic materials. The structures of aggregates of these molecules can be correlated with crystal structures of derivatives of this class of molecules and we have established a quantitative relationship between the crystal structures and absorption properties. Furthermore, we are modeling transport properties of organic materials and we are identifying potential transport-limiting processes. Further work will deal with the structure of such materials at interfaces and simulate their importance for opto-electronic properties.

The work in the Fink group aims

1. to gain an understanding and quantitative prediction of the energetic interactions that are responsible for the structure of solids,
2. to predict absorption and transport properties of interesting materials,
3. to explain the often very limited mobility of charge carriers and excitons, and to find possibilities to avoid these in organic materials.

Calculation of core electron spectra

The electronic structure is the major basis of all chemical properties: core electron spectroscopy is a method to obtain detailed knowledge of electronic structure. Electrons are excited from core orbitals and the decay of the resulting highly excited states is observed.

In the Fink group the wave functions of the states occurring during the core electron spectroscopy are calculated. Thus, their energies and transition probabilities for the excitation and decay processes can be obtained. This gives rise to, a purely theoretical equivalent of core electron spectra, which can be used to aid in their interpretation. A fundamental understanding of this spectroscopy is also obtained.

Development of quantum chemical methods

The key question for quantum chemical calculations is often whether a method is available that allows to answer a given chemical question with the available computational resources and with sufficient accuracy.

In the Fink group quantum chemical methods are extensively used. At the same time new wave-function-based ab initio methods like the RE-PT-S2 and MP methods are developed. Benchmark calculations on small molecules suggest that these methods require comparably small computational demands for a rather high accuracy. As these methods provide electronic wave functions, a comparison with very accurate counterparts from highly accurate coupled cluster and full CI calculations is feasible. This is going to be used to derive new accurate quantum chemical methods with low computational complexity for a wide application range.