Band Structure Engineering in Organic Semiconductors

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Band structure engineering, i.e., the possibility to "tune" the band energies of semiconductors by mixing different elements, is a key technology for modern electronics and optoelectronics. In organic semiconductors, this approach seemed to be impossible since it requires delocalized electronic states - a situation which usually is not being observed in polycrystalline or amorphous organic thin films which are relevant for device applications. However, we have recently made the surprising observation that it is nevertheless possible to tune electronic energies in organic semiconductors by mixing molecular substituents: Long-range Coulomb interactions due to quadrupole moments of the molecules average the energies. This opens promising new perspectives for applications, such as the fine-tuning of open-circuit voltages in organic solar cells or emission wave lengths in organic light emitting diodes (OLED), or controlling the injection properties at contacts. In this project, we want to go far beyond the initial observations: We want to address key open questions, like the detailed interplay between local morphology and energy structure, the impact of molecular dipole moments on electronic energies, the effect of long-range interactions on energy level beyond the highest occupied orbitals, and many more. In parallel, the project also addresses first applications.