In contrast to conventional electronics, where inorganic conductors and semi-conductors like copper and silicon are used, the circuits of organic electronics consist of conducting polymers and small organic compounds. This difference allows the realization of electronic devices with completely new material properties. For example devices with transparent an flexible components are possible. This extends the applicability of organic electronics to new fields, which were out of scope for conventional, silicon based, electronics.
Furthermore, many compounds are soluble and posses self-assembling abilities, which enables a facile and inexpensive processing at moderate conditions.
However, there is one issue that inhibits the advent of organic electronics: most materials suffer from a low electron mobility. This deficiency affects strongly the performance of field effect transistors and the efficiency of solar cells. In order to optimize this property, a better understanding of the charge transport mechanism in these materials is required.
There are three characteristics which strongly influence the charge transport:
- First of all the charge transport is dependent of the average magnitude of the electronic coupling between orbitals of neighboring molecules and their relative energies.
- Equally important are the fluctuation of these couplings and energies.
- Another factor is the response of the environment to the moving charge. In nonpolar environments this is achieved mainly by electronic polarization and in polar environments mainly by reorientation of the surroundings.
In our approach we address these issues by performing a molecular dynamics simulation using a QM/MM formalism. The charge carrier is propagated using quantum mechanics, while the dynamic of the system and its response to the moving charge is modeled using molecular mechanics. With this approach we are capable of simulating charge transfer in the range from hopping between localized states to partially delocalized transport.