Modulares UHV Clustersystem für MBE und ARPES
Final Report Abstract
I. Functionalized Electrodes for high performance organic thin-film devices: The barrier to charge carrier injection across the semiconductor/electrode interface is a key parameter in the performance of organic transistors and optoelectronic devices, and the work function of the electrode material plays an important role in determining the size of this barrier. We have found a new, chemical route for making metal surfaces with low work functions, by functionalizing gold surfaces with self-assembled monolayers of n,n-dialkyl dithiocarbamates. Ultraviolet photoemission spectroscopy measurements show that work functions of 3.2 eV ± 0.1 eV can be achieved using this surface modification. Electronic structure calculations reveal that this low work function is a result of the packing-density, polarization along the N-C bond, and charge rearrangement associated with chemisorption. We demonstrate that electrodes functionalized with these monolayers significantly improve the performance of organic thin-film transistors and can potentially be employed in charge selective contacts for organic photovoltaics. II. Influence of surface energetics on organic thin-film evolution: The evolution of the growth of organic thin-films on substrates, which were energetically modified by the application of the polymeric dielectrics is investigated. Thin-films are produced by OMBD with film thickness varying from few monolayers to very thick films on the different dielectrics at two different evaporation rates. Thin-film evolution from a few monolayers to thick films in the case of the prototype organic molecule perylene is a three step process: Initial growth is dominated by substrate diffusion and the formation of high isolated islands. In the second step, with increasing amount of deposited material, the diffusion of perylene on perylene dominates the surface, resulting in the formation of molecular steps on top of the islands. In the final growth phase the islands start to coalesce. At the contact point of two islands dislocations can occur. Furthermore, the regime of coalescence can be accompanied by a pronounced spiral growth. Additionally some modified substrates show a significant dewetting effect in the first growth phase and high crystalline order, which was confirmed by X-Ray Diffraction (XRD) and Transmission Electron Microscopy (TEM) measurements. By combining measurements of the Surface Free Energy (SFE) of the differently modified substrates with the scaling relationship of nucleation, we were able to predict a profound influence of the dielectric on thin-film growth kinetics. In our study, we confirmed this prediction by showing a correlation between the surface energetics and the thin-film evolution. III. Organic heterojunctions for photovoltaic applications: Organic donor-acceptor heterojunctions are arguably the most promising system for the use as active layers in organic solar cells. Their key feature is a very efficient exciton dissociation at the interface between donor and acceptor which is crucial for device operation. Hence, one way towards high performance organic solar cells is the enlargement of the interface area, which can be achieved by simultaneous processing of donor and acceptor in order to deposit bulk heterojunctions. For certain donor-acceptor combinations significant performance improvements for bulk heterojunctions over simple bi-layer systems have already been demonstrated. For this study we are producing small molecule bi-layers and bulk heterojunctions based on Phtalocyanines as a donor and various acceptor materials. The structure of the resulting films can be tuned over a wide range by employing different substrate temperatures during evaporation in ultrahigh vacuum. The resultant films are investigated with regard to structural properties on absorption, charge transport and device performance which were investigated by means of x-ray diffraction, optical spectroscopy and electrical measurements. IV. Investigation of the electronic structure of phase change materials: We have recently added a sputter system to the cluster so that we can now also investigate occupied states of phase change materials. An inverse photoemission set-up is currently being added to enable the characterization of the entire electronic states in the vicinity of the Fermi energy.
Publications
- A New Route to Low Resistance Contacts for Performance-Enhanced Organic Electronic Devices. Advanced Materials Interfaces Volume 1, Issue 5, August, 2014
Philip Schulz, Tobias Schäfer, Christopher D. Zangmeister, Christian Effertz, Dominik Meyer, Daniel Mokros, Roger D. van Zee, Riccardo Mazzarello and Matthias Wuttig
(See online at https://doi.org/10.1002/admi.201300130)
