The electronic structure of doped epitaxial organic films

Although thin films of conjugated organic molecules are entering the marketplace as the active elements in various optoelectronic devices, the basic understanding of their electronic structure, crucial to their function, is lacking. The electronic band structure, electron energy versus momentum E(k), of the conjugated p system defines both the electronic properties and the optical properties of the so-called organic semiconductors. The aim of this project is to measure and understand the full electronic structure of conjugated molecules and to study the effects of functional groups and doping on both their intra-molecular electronic structure and their inter-molecular crystal band structure. By choosing either alkali metals or halides as dopants both n- and p-doping can be achieved. In this project the occupied valence band together with the unoccupied conduction band structure will be determined by using a combination of angle resolved ultraviolet photoemission spectroscopy and angle resolved inverse photoelectron spectroscopy. The molecules chosen are not only directly device relevant, but also good experimental model systems accessible to ab-initio calculations. The project can be partitioned into four major topics: I) the preparation of doped single crystalline organic films, II) the electronic band structure of bulk doped organic semiconductors, III) the unoccupied electronic bands and IV) the modification of the electronic band structure arising from functional groups. We will be producing and characterizing crystalline films of sexiphenyl, sexithiophene, pentacene, pentaphenyl and phthalocyanines from very low up to very high doping (approx. 20%) concentrations, i.e. creating films not only with semiconducting but also with conducting properties. These doped crystalline films will be used to measure both, the intra-chain dispersion of polaron and bipolaron bands, which appear with increasing doping level and the inter-molecular band structure. After commissioning the inverse photoemission spectrometer we plan to measure the unoccupied conduction band states above the Fermi level, which together with the valence band will directly yield the transport gap of the organic semiconductor. In particular, we aim at measuring the band dispersion E(k) of the unoccupied states to achieve the first experimental determination of the full electronic band structure. In the final part of this project it is foreseen to employ functionalized molecules, e.g. functionalized n-phenyls and phthalocyanines. Here, the influence of the attached functional group or heteroatom, which might be regarded as internal doping, on the valence and conduction band structure, will be explored.

Although thin films of conjugated organic molecules are entering the marketplace as the active elements in various optoelectronic devices, the basic understanding of their electronic structure, crucial to their function, is lacking. The charge transport properties of films of such organic semiconductors are strongly changed via alkali metal doping. This is expressed by the appearance of gap states in the molecular electronic structure. The aim of this project was to understand these changes in the molecular electronic structure upon alkali metal and halide doping. As a prerequisite ordered/oriented molecular films and monolayers had to be prepared, which did not disorient during the doping process.Angle resolved ultraviolet photoelectron spectroscopy is a standard method to investigate the electronic structure. Within this project we have further developed this method into orbital tomography together with partners from Prof. S. Tautzs group (FZ Jülich, Germany) and from solid state theory (assoz. Prof. P. Puschnig. KF University Graz, Inst. of Physics) (Proc. Nat. Acad. Sci. (PNAS) 111 (2014) 605). Using the angular distribution of the photoelectrons we can now assign/reconstruct the molecular orbital these originate from. It was thus possible to identify the emerging gap states upon doping, to clearly assign the molecular orbitals and even to infer the orientation of the (doped) molecules. The method requires a special analyzer at the synchrotron radiation facility BESSY II/ Helmholtz Zentrum Berlin. In the course of the project approximately 4-6 weeks/year have been awarded.Major results are illustrated by the Cs doping of a sexiphenyl monolayer on Cu(110). First the gap states appearing upon doping, termed bipolarons and interacting bipolarons, could be identified as the filled lowest unoccupied molecular orbital (LUMO) and LUMO+1, using orbital tomography. Second, the doping proceeds not continuous but in three distinct stages: decoupling of the molecules from the substrate (1), filling of LUMO (2) and filling of LUMO+1(3). In some cases, like sexiphenyl or pentacene on Ag(110) the decoupling stage causes a reorientation of the first molecular layer by 90. Investigations of the maximum number of electrons that can be transferred to a n-phenyl (n = 1 to 6) chain-molecule has shown that up to n = 3 two electrons can be transferred, while for n = 4 to 6 four electrons can be accommodated. In cooperation with the group of Prof. Karl-Heinz Ernst, EMPA, Switzerland, the doping of corannulene, a buckybowl molecule, has been investigated. The corannulene molecule could host 4 additional electrons, which is significant compared to the three times larger C60 molecule, where a 6 electron transfer was reported. This suggests that corannulene might be an interesting acceptor material for organic semiconducting devices.