Thermal evaporation of small molecules-A study of interfacial, bulk and device properties for molecular electronics

Doctoral thesis, 2008

Electronic devices based on organic materials have recently become an emerging technology for many applications. Promising aspects are the compatibility with almost any substrate and low cost processing methods. The more or less infinite number of organic molecules as well as the means to tailor the molecular properties through different chemical reactions further extends the possibilities for devices. The organic device is a complex structure. In order to fully understand and improve its properties, both fundamental as well as device issues must be treated in parallel. A first, major part of this thesis is a characterization of the chemical composition and surface morphology of the transparent electrode material indium tin oxide (ITO) and its effect on the initial growth of molecules and devices. One particular system is the copper phtalocyanine (CuPc)/ITO interface. The Fermi level of ITO is highly sensitive to surface treatments and have a clear effect on the electronic levels in the CuPc layer with two distinct pinning levels formed based on the work function of the ITO. Further, we have investigated the molecular order in thin films of CuPc grown under a magnetic field on ITO, and found that the orientation and order of the molecules in the films can be changed. We have also studied the initial interaction of CuPc and 3,4,9,10-Perylene tetracarboxylic dianhydride (PTCDA) with Cu(100). Both molecules adsorb very strongly in the first monolayer and this is reflected in the formation of interface states observable with UV photoemission spectroscopy. The mechanisms behind these states are different for the two molecules. The PTCDA molecules undergoes a chemical reaction with the Cu surface with the loss of oxygen atoms as a consequence, whereas the interface state in the case of CuPc is interpreted as a quantum well state formed in the first monolayer which is quenched as the film thickness is increasing. The second part of the thesis concerns device studies. In order to increase the efficiency of n-type organic field effect transistors, we have studied the influence of grafting self assembly monolayers (SAM’s) on the contacts and substrate surface. We found that the SAM’s decreased the contact resistance with 1-2 orders of magnitude and increased the device mobility ~10 times. We believe the reason for this was two-fold. First, the SAM’s changed the work function of the gold contacts, secondly, they also affected the hydrophobic properties of the substrate which allowed the solution of the organic molecules to more easily flow down into the contact channels, thus improving the film quality and the efficiency. Another important device is the OLED, for which we have investigated the effect of different ITO preparations on the efficiency. It was found that an oxidative treatment with UV-light followed by nitric acid, decreased the device turn-on voltage from ~19 to ~2V. This was contributed to a reduced hole-injection barrier, due to an increased ITO work function. We have also studied the influence of fluorescent dopants. The doping molecules were porphyrin derivatives with different central atoms, zinc and hydrogen. Both porphyrins affected the light emission with strong new peaks (red shifted) in the spectra. Further, the position of the spectral peaks of the porphyrins are such that the inclusion of different porphyrin dopants in the same device may allow for a white emission OLED, which is important for solid state lighting applications. Finally the thesis also includes studies of encapsulation of OLED’s. We have shown that it is possible with a simple technique to substantially prolong the lifetime of the device. Encapsulated devices were twice as efficient after two weeks compared to non-encapsulated ones.