Tailoring Surfaces with Surface Modifications

  Depiction of a thin film transistor. I. Physikalisches Institut The performance of organic thin film transistors can be significantly improved by the application of surface modifications on the dielectric layer or on the electrodes (in this case gold).

Understanding morphology and electronic structure at an interface to organic materials is of crucial importance to improve the performance of organic semiconductor devices such as transistors or solar cells. Thereby surface modifications like self-assembling monolayer (SAM) and polymeric dielectrics can be applied to tailor surface/interfacial properties. For instance, SAM’s may improve charge carrier injection and extraction when applied to the noble metal contacts or may improve the overall device performance when applied onto the dielectric. Those surface modifications significantly impact the growth of the active, organic layer as well as the arrangement of the electronic energy levels at the interface. Eventually, cooperations with chemical institutes working on organic synthesis allows us to systematically tailor these surface modifications.

  Schematic structure of a self-assembeld-monolayer I. Physikalisches Institut Schematic structure of a self-assembeld-monolayer (SAM) to modify the electrical properties

Those surface modifications significantly impact the growth of the active, organic layer as well as the arrangement of the electronic energy levels at the interface. Eventually, cooperations with chemical institutes working on organic synthesis allows us to systematically tailor these surface modifications.

 
 

Transparent Electrodes

The basic principle of an organic solar cell is depicted. I. Physikalisches Institut The basic principle of organic solar cells.

In the above-mentioned organic devices, thin films of metals or transparent materials such as transparent conductive oxides (TCOs) are deposited as electrodes. These electrodes are either thermally evaporated or. The growth and the optical and electrical properties of the electrodes are investigated and optimized. Furthermore, the influence of the inorganic layers on the organic materials is investigated. By absorption of photons an exciton is generated, which is separated at the interface between the donor and acceptor into an electron and a hole. The charge carriers migrate to the electrodes thus enabling the conversion of light into electrical energy. Also in this process, an understanding of the operations at all interfaces is very important.

 

Our Tools

In our group we employ a large variety of different methods for sample preparation and analysis. We use different optical characterization methods like Fourier Transform Infrared Spectroscopy (FTIR), Photoluminescence Spectroscopy (PL), UV/VIS Absorption Spectroscopy and Variable Angle Spectrometical Ellipsometry (VASE). Furthermore we are equipped with setups for different conventional and advanced methods for microscopy (e.g. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy with optional Focussed Ion Beam (FIB-SEM)), X-ray based methods (X-ray diffraction (XRD), X-ray reflectrometry (XRR), Energy-dispersive X-ray spectroscopy (EDX)) as well as characterization tools for electrical transport. Last, but not least we operate a Photoemission Spectroscopy Setup with a X-ray as well as an UV-source (XPS/UPS) in our UHV-Cluster ORPHEUS (see next paragraph).

 

The ORPHEUS Cluster

Picture of the ORPHEUS-Cluster with connected glovebox. I. Physikalisches Institut The UHV-Cluster ORPHEUS contains of two deposition chambers, one analytic chamber for XPS/UPS and a directly attached inert gas glovebox.

With our state of the art UltraHigh Vacuum(UHV)-Cluster-Tool ORPHEUS we are able to perform a lot of different preparation steps and characterization in-situ, i.e. without exposure of our samples to ambient air which contains moisture and oxygen.

ORPHEUS contains of different UHV-chambers: One for deposition of organic materials via Organic Molecular Beam Deposition (OMBD), one metallization chamber via vacuum thermal evaporation (VTE) and one analytic chamber with our XPS/UPS spectrometer. In the near future our possibilities will be further enlarged by the addition of a sputter chamber for the deposition of transparent conductive oxide (TCO) electrodes as well as a second analytic chamber for Inverted PhotoEmission Spectroscopy (IPES). For the application of solution-based processes the whole UHV-Cluster is attached directly to an argon-atmosphere glovebox.