Studies of The Electronic Structure of CuPc and PTCDA - Two Molecular Organic Semiconductors
Sammanfattning: Since the discovery of organic materials with conducting and semiconducting proper¬ties a few decades ago, the research in this field has intensified and has become in¬creasingly important. Expected advantages of both economical and technological nature have been powerful driving forces in this development, and today organic based devices are becoming a reality for commercial applications. It is predicted that the market for organic semiconducting devices and in particular organic light emitting devices (OLEDs) will expand dramatically in the near future and that OLEDs will replace LCDs in many applications. Thus, studies of organic molecular thin films with semi¬conducting properties and of organic/inorganic interfaces are highly motivated. The two substances investigated in this work, PTCDA and CuPc, are prototypes for this group of materials. They are both planar with a conjugated π-bonding system and they self-organize in well-ordered structures on a range of substrates. The studies aim at achieving better understanding for the electronic properties of thin molecular crys¬tal layers and their interaction with different inorganic substrates. The systems investi¬gated include three well characterized monocrystalline substrates: one extreme¬¬ly passive, Si(111):H, one that is expected to be suitable for quasiepitaxial growth, TiSe2, and one that is known to be quite aggressive in contact with molecular crystals, Cu(100). A fourth substrate, ITO, was also investigated in different states of prepa¬ration. Since ITO is a highly transparent conductor, it is used as electrode in optoelectronic devices. The studies of PTCDA/Si:H, revealed angularly dispersing valence band features, which were confirmed in more recent studies, and which show that the material forms a regular crystal perpendicular to the surface. No indication of in-plane crystal order was found, which was explained as an effect of very loose coupling to the substrate. The PTCDA/TiSe2 interface is more coherent, and supports quasiepitaxial growth of PTCDA. The local nature of charges in the molecular crystal was reflected via signi¬ficant substrate-related non-local screening of photoholes. Deposition of PTCDA on Cu(100) resulted in formation of new interfacial electronic states, clearly showing chemi¬¬sorptive interaction. For the interface between CuPc and ITO, we found that the electronic and morphological properties are highly dependent on the substrate surface preparation. Differently treated substrates alter the ITO work function and influence the Fermi level pinning in the CuPc thin films. Focusing on the properties of the central Cu atom in CuPc we found that the half-filled exchange split 3d-orbital states probably responsible for the two distinct Fermi level pinning situations reported by several other groups. Keywords: UPS, XPS, XAS, XES, Organic Semiconductor, PTCDA, CuPc.
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