White‐Light Emission from a Single‐Emitting‐Component Organic Electroluminescent Device

Abstract

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...

White-light-emitting diode based on a single-layer polymer

“…[10][11][12][13] In case of PFB, the electromer could be regarded as an excited state of paired sites with opposite charging (hole and electron, respectively) on the excited PFB molecules (PFB + /PFB -)* under electric excitation. We can also consider that one of the paired PFB molecules carries an excessive hole while the other carries an electron.…”

“…[6][7][8][9] Alternatively, WLEDs can be achieved on the basis of electroplex or electromer emissions, where new specimens form in the active layer between different or identical molecules to produce the required spectrum for filling the gaps in the white-light spectrum. [10][11][12][13] Electromer is referred to as a pair of chemically identical molecules charged by electrons and holes, respectively, which are statistically independent of each other. 12 The corresponding device generally produces red-shifted EL spectrum as compared to the intrinsic molecular emission.…”

“…[10][11][12][13] Electromer is referred to as a pair of chemically identical molecules charged by electrons and holes, respectively, which are statistically independent of each other. 12 The corresponding device generally produces red-shifted EL spectrum as compared to the intrinsic molecular emission. Kalinowski et al reported electromer emission in the red using diodes based on 1,1-bis((di-4-tolylamino)phenyl)cyclohexane (TAPC), where the electromer forms between two tri(p-tolyl)amine (TTA) molecules chemically bridged by a cyclohexane ring.…”

High‐Efficiency White‐Light Emission from a Single Copolymer: Fluorescent Blue, Green, and Red Chromophores on a Conjugated Polymer Backbone

“…[14] Despite these efforts, the external quantum efficiency of WPLEDs remains much lower than that of a smallmolecule WOLED fabricated by thermal deposition. It is noted that some efforts have been made to synthesize a single white-light-emitting small-molecule device [15] and to synthesize a white phosphorescent polymer that shows a slightly higher device efficiency of 4.7 cd A -1 with CIE coordinates of (0.38,0.35). …”

High‐Efficiency White‐Light‐Emitting Devices from a Single Polymer by Mixing Singlet and Triplet Emission

“…In the past few years, studies in this field have attracted growing interest and encouraging progress has been made. Li et al [9] fabricated a single-component white organic light-emitting diode (OLED) by using 1,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB) as the emitting species. The green-and red-light emission of the electroluminescence (EL) device from TECEB is produced by electric excitation.…”