Small-molecule layers for devices -Evaporation growth and characterization of thin films

Licentiate thesis, 2005

Small-molecule layers for devices - Evaporation growth and characterization of thin films Måns Andreasson Applied Semiconductor Physics Department of Microtechnology and Nanoscience Chalmers University of Technology Abstract Organic semiconductors constitute a relatively recent group of materials available for the electronic device designer. Substantially cheaper production technologies and integration with plastic materials are anticipated. Many different devices have been realized such as organic light emitting devices (OLED's), photovoltaic cells and transistors. This thesis describes vacuum-evaporation growth of small-molecule, thin film layers for device applications. The work covers three main parts. The first describes an organic molecular beam deposition system and the initial growth made for the purpose to determine the operational parameters. The molecular flux as a function of evaporation source temperature is measured and the relation between the thickness monitor reading and the real thickness of the film are determined. For this purpose thin films of the well known molecule 3,4,9,10 perylene tetra carboxylic dianhydride is used. The film surfaces are characterized by reflective high energy electron diffraction and atomic force microscopy. They show amorphous and polycrystalline film with smooth to rough surface appearance depending on substrate and growth condition. The second part is an ultra violet photoemission study of thin films of copper phtalocyanine (CuPc) deposited on two differently treated indium tin oxide (ITO) substrates, wet treated or wet treated and subsequently heated respectively. Thin films of CuPc are commonly used in OLED's as a hole injection layer. Both the electronic properties of the substrate and the CuPc are found to depend on the substrate treatment. Specifically we find that the ITO workfunction is increased ~0.4-0.6 eV after heating, and that the highest occupied molecular orbital (HOMO) is shifted ~0.5 eV to a lower binding energy on the heated substrate. The shift in HOMO is interpreted as different Fermi level pinning at a spin split Cu derived orbital on the two surfaces. Finally three different fluorescent dopants are investigated in a standard OLED structure, 9,10-bis(phenyl-ethenyl)anthracene (BPEA), Zincporphyrin (ZnP), and Porhyrin (H2P). Current-voltage measurements show that BPEA doping increases the turn-on voltage whereas it remains the same for ZnP and H2P devices. Electroluminescence spectra show that BPEA has little effect on the emission spectra while ZnP and H2P doping result in a clear change with an almost complete energy transfer for the latter. Keywords: molecular semiconductors, OLED, evaporation deposition, CuPc, PTCDA