Characterisation of Organic Semiconductor Growth using Real-time Electron Spectroscopy
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Characterisation of Organic Semiconductor Growth using Real-time Electron Spectroscopy. D. Andrew Evans1, Owain R. Roberts1, Gruffudd T. Williams1 and David. P. Langstaff1 1 Institute of Mathematics and Physics, Aberystwyth University, Aberystwyth SY23 3BZ, U.K.
ABSTRACT An organic molecular beam deposition system coupled to a soft x-ray excitation source has been developed to monitor the growth of organic semiconductor thin films in-situ and in real-time. Rapid collection of photoelectron spectra has been enabled using a multichannel array detector coupled to a hemispherical analyzer. The organic semiconductor tin phthalocyanine (SnPc) exhibits a Stranski-Krastanov growth mode on a polycrystalline gold substrate where the transition thickness between layered and clustered growth has been determined to be comparable to the thickness of a single molecular layer within which the molecules are standing on edge relative to the substrate plane. INTRODUCTION The electrical and optical properties of semiconducting organic materials in multilayer devices depend crucially on their physical and electronic structure and their interfaces with other materials within the device. For example, the conductivity of ordered small molecule films that have the highest mobility among organic semiconductors depends on the degree and direction of the molecular ordering within the film [1]. The molecular orientation and the larger-scale thin film morphology are sensitive to the substrate-molecule and molecule-molecule interaction at the interface and these are further influenced by thin film growth parameters such as temperature and rate. Metal phthalocyanines are widely applied in optoelectronic devices such OPV [2] and have other applications in fields as varied as cancer therapy and spintronics [3]. Few techniques can probe sufficiently broadly to provide direct correlations between these properties and the device performance and the usual approach is to apply different characterization methods sequentially on different device structures. One technique that can provide parallel information on energetics, chemical bonding and thin film morphology is photoelectron spectroscopy but it is not usually applied as a real-time, in-situ characterization method. However, with advances in electron detector and analyzer technology, photoelectron spectra can now be recorded sufficiently rapidly to enable data to be recorded during surface processing and thin film growth. The growth of SnPc, a non-planar metallo-organic dye, on a gold surface is chosen to illustrate the application of in-situ, real-time photoelectron spectroscopy to the formation of an inorganic-organic interface.
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EXPERIMENT Exposure of a polycrystalline gold surface to a flux of the organic semiconductor SnPc has been monitored in real-time using photoelectron spectroscopy. The substrate surface was prepared by ex-situ polishing and solvent cleaning followed by in-situ Ar sputtering and annealing. Photoelectrons were excited by soft x-rays of energies ~ 100 eV and collected u
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