Hot-wall epitaxy and molecular-beam epitaxy have been employed for growing quaterthiophene thin films on the (010) cleavage face of potassium hydrogen phthalate, and the results are compared in terms of film properties and growth mode. Even if there is no geometrical match between substrate and overlayer lattices, these films are epitaxially oriented. To investigate the physical rationale for this strong orientation effect, optical microscopy, atomic force microscopy, and X-ray diffraction are employed. A clear correlation between the morphology of the thin films and the crystallographic orientation is found. The results are also validated by surface potential calculations, which demonstrate the primary role played by the corrugation of the substrate surface.
White organic light-emitting devices (WOLEDs) have been attracting much interest because they offer low-cost and large-area alternatives as backlights for flat-panel displays and have good potential for lighting applications. Various strategies have been utilized to fabricate WOLEDs. These include the manufacture of multilayer OLEDs [1±5] by consecutive evaporation, where each layer emits a primary color, and that of the single-layer polymer blend devices, [6±8] where all the emitting components are mixed in one layer. In the case of multilayer white devices, charge blockers are usually used to confine the charges and excitons within the desired regions for improved emission. The usage of blockers often causes high driving voltages and consequently low efficiencies. Spincoating of blended polymer solutions to fabricate single-layer polymer devices offers a promising low-cost technique for large-area display applications. However, problems such as color dependence on the driving voltage and undesired För-ster-type energy transfer between chromophores do exist in such devices.[6±8] Furthermore, in both the multilayer organic device and the single-layer polymer device, the emission color is usually sensitive to the device structure parameters such as active layer thickness and doping concentration. In addition, white-light emission can also be obtained from excimers [9] and/or exciplexes [10,11] of organic or polymer chromophores.However, they have not produced satisfactory white-light emission in terms of color and efficiency. Most of the above problems seem avoidable if a single-component material can be used as the emitting species. Early in 1997, Yang [12] reported a single-emitting-component whitelight-emitting device in which the polyfluorene derivative, poly[9,9-bis(3,6-dioxaheptyl)fluorene-2,7-diyl] (BDOH-PF), was used as the emitter. POPPPVC3 copolymer [13] was also used to fabricate a single-emitting-component WOLED, in which the blue±green light of the white electroluminescence (EL) comes from the individual lumophore and excimer, whereas the red light originates from a new emitting species called an electromer. Recently, another single-emitting-component polymer white EL was obtained with red emission coming from aggregates and molecular blue fluorescence.[14]Although a few single-emitting-component white polymer devices have been described in the literature, no single-emittingcomponent WOLED has yet been reported. In this paper we report, for the first time, efficient single-emitting-component WOLEDs with a single-layer and three-layer structure fabricated from 1,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB). The luminance and efficiency of the three-layer device are the highest ever reported for any single-emittingcomponent polymer or organic white devices. The chemical structure and the synthetic procedure of the new compound TECEB are illustrated in Scheme 1. It can be prepared by the traditional Wittig reaction via the following procedure: the reaction of the brominated product of mesit...
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