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Abstract
The modification of the work function of Sn-doped In2O3 (ITO) by vacuum adsorption of 4-(Dimethylamino)benzoic acid (4-DMABA) has been studied using in situ photoelectron spectroscopy. Adsorption of 4-DMABA is self-limited with an approximate thickness of a single monolayer. The lowest work function obtained is 2.82 ± 0.1 eV, enabling electron injection into many organic materials. In order to identify a potential influence of the ITO substrate surface on the final work function, different ITO surface orientations and treatments have been applied. Despite the expected differences in substrate work function and chemical bonding of 4-DMABA to the substrate, no influence of substrate surface orientation is identified. The resulting work function of ITO/4-DMABA substrates can be described by a constant ionization potential of the adsorbed 4-DMABA of 5.00 ± 0.08 eV, a constant band alignment between ITO and 4-DMABA and a varying Fermi energy in the ITO substrate. This corresponds to the behaviour of a conventional semiconductor heterostructure and deviates from the vacuum level alignment of interfaces between organic compounds. The difference is likely related to a stronger chemical bonding at the ITO/4-DMABA interface compared to the van der Waals bonding at interfaces between organic compounds.
Highlights
Tin-doped indium oxide (ITO) is often used as transparent anode material in organic optoelectronic applications like organic light emitting diodes or photovoltaics [1,2,3,4,5,6,7]
In order to reduce uncertainties of the surface potentials originating from the combination of two different excitation sources, the values for the work function discussed in this work are extracted from a combination of XP and UP spectra according to φXPS = I P − EVB,XPS, where the ionization potential I P = Evac − EVB is derived from the UPS measurement I P = 21.2 eV − ( ESEC − EVB,UPS )
Films were deposited at a substrate temperature of 400 ◦ C and 5 Pa either in Ar atmosphere or in a 10/90 O2 /Ar atmosphere
Summary
Tin-doped indium oxide (ITO) is often used as transparent anode material in organic optoelectronic applications like organic light emitting diodes or photovoltaics [1,2,3,4,5,6,7]. The modification of the work function of transparent ITO electrodes has been widely investigated [6,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. Another approach for modifying the work function is the formation of self-assembling monolayers (SAM), which utilize the self-limited adsorption of organic molecules [23,24]. X-ray and ultra violet photoelectron spectroscopy (XPS and UPS) were applied to monitor the surface potentials
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