Hexa-Alkoxytriphenylenes As Hole Transporting Materials Inc Stable Oleds Using ALQ 3 As Emitting Layer
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ACT Devices consisting of hexa-alkoxythriphenylene derivatives as hole transport materials and 8-hydroxyquinoline aluminium (Alq3) as emitting layer with ITO as anode and magnesium as cathode are presented. Moreover X-ray diffraction studies of evaporated triphenylene films were performed leading to a better understanding of crystal structure, morphology and the alignment of the triphenylene molecules on the surface of the substrate.
INTRODUCTION Typical hole injecting and transport materials in EL devices are low weight molecular triphenyldiamine (TPD) derivatives [1,2] with an ionization potential (5.1 eV) [3] approaching the work function of ITO (4,8 eV) [4]. Besides TPD, hexaalkoxytriphenylenes are known to be promising materials for HTL applications in OLEDs [10]. Hexa-alkoxytriphenylenes are columnar discotic mesogens which exhibit a highly ordered mesophase [5]. These molecules have the unique property of self organization into columnar stacks [6] providing a face to face orientation of the aromatic cores which leads to a large overlap of the π-bondings between the molecules. The photoinduced charge carrier mobility of hexa-alkoxytriphenylenes is ranging from 10-5 cm2V-1s-1 up to 10-2 cm2V-1s-1 [6,7] and HOMO values between 5.3 eV and 5.4 eV[8]. However, by now there is little work done on LEDs with a device setup of hexaalkoxytriphenylenes or unsymmetrical alkoxy-triphenylenes respectively as hole transporters, high emissive organic materials like Alq3 as emitting layer and low work function metals as cathode.
EXPERIMENTAL The materials used in this study were triphenylene derivatives (names and molecular structures see Fig.1) which were prepared according to known procedures [9,10,11,12]. NN´diphenyl-NN´-(3methylphenyl)-1,1´-biphenyl-4,4´diamine (TPD) with a purity of 99% was used as received by Aldrich corporation, whereas the 8-hydroxyquinoline aluminium (Alq3) was purified by sublimation techniques. Device preparation [13,14] was performed on substrates of ITO glass with 20Ω /sq., purchased from Donnely corporation, which were cleaned by ultrasonicating in baths of dry acetone, methyl alcohol and isopropyl alcohol 1 hour each and then plasma treated. The vapour
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a) TP6/1: OR = methoxy TP6/i5:OR = isopentyloxy
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b) TP4/1: OR =methoxy
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c) TP4/1-M: OR=methoxy TP4/6-M: OR=hexyloxy
Figure. 1: a) Molecular struture of 2,3,6,7,10,11-hexamethoxytriphenylene (TP6/1) and 2,3,6,7,10,11-hexaisopentyloxytriphenylen (TP6/i5); b) Molecular structure of 2,3,6,7tetramethoxytriphenylen (TP4/1); c) Molecular structure of 2,3,6,7-tetramethoxy-10metyltriphenylen (TP4/1-M) and 2,3,6,7-Tetrahexyloxy-10-metyltriphenylen (TP4/6-M) deposition of the organic compounds and the magnesium cathode was performed in a bell jar deposition chamber in a nitrogen glove box at a pressure of approximately 10-6 mbar. The deposition rates for the organic layers were about 2 – 3 Ås-1. The deposition rate for the cathode material was about 7 Ås-1. The cathode was patterned into six devices e
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