Charge Transfer in Organic Semiconductor Systems Probed by Photoemission Spectroscopy

In the present work, charge transfer in organic semiconductors is investigated by means of photoemission spectroscopy. Organic charge transfer systems consist of electron donors and acceptors and in some cases exhibit new electronic properties that are not observed in the individual constituents. Examples are metallic conductivity and changed optical or transport gaps. The main focus were interfaces between donor and acceptor molecules that were prepared as thin films by thermal evaporation in ultra-high vacuum. In particular, the strong electron acceptor F6TCNNQ was combined with several scientifically relevant donors, with the aim of achieving a large charge transfer. As reference systems, potassium doped F6TCNNQ and the interface between F6TCNNQ and gold were studied. In both systems, a large electron transfer to F6TCNNQ with similar spectroscopic signatures was observed. The investigated organic interfaces all showed charge transfer that manifested itself in form of changes in the core levels of F6TCNNQ that were similar for each system. Also, new occupied states in the former gaps of the molecules were found. For every investigated interface the Fermi energy was pinned above the respective highest occupied molecular orbitals which entails semiconducting behaviour and no metal-like delocalised charge carriers. For the combination of F6TCNNQ and dibenzopentacene, a blended film was prepared by co-deposition and compared with the corresponding interface. It was found that the electronic properties of the blend are initially determined by electrostatic interactions, whereas annealing leads to a large charge transfer due to a temperature induced change of molecular orientation. Moreover, the acceptors F2TCNQ and F16CoPc were used in order to compare systems with the same donor and different acceptors. Differences in the degree of charge transfer and interface morphology were observed. The last part of this work addresses the electronic properties of an organic rectifier that was fabricated by collaboration partners. It is built up of an organic heterojunction of two phtalocyanines (CuPc and F16CoPc) between two gold contacts. The energy level alignment across the device and the charge transfer reactions at the different interfaces are discussed with regard to the functionality of the device