Carbazole–pyrene derivatives for undoped organic light-emitting devices S.L. Lai a , Q.X. Tong b,c,⇑ , M.Y. Chan d , T.W. Ng a , M.F. Lo a , C.C. Ko e , S.T. Lee a , C.S. Lee a,⇑ a Center of Super-Diamond and Advanced Films (COSDAF) and Department of Physics and Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region b Department of Chemistry, Shantou University, Guangdong 515063, China c Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, China d Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong Special Administrative Region e Department of Biology and Chemistry, City University of Hong Kong, Hong Kong Special Administrative Region article info Article history: Received 29 October 2010 Received in revised form 29 December 2010 Accepted 31 December 2010 Available online 13 January 2011 Keywords: Small-molecule Organic light-emitting diode Electroluminescence Undoped Carbazole Pyrene abstract Two carbazole–pyrene derivatives, namely 3,6-dipyrenyl-9-(4 0 -tert-butylphenyl) carbazole (BPyC) and 3,6-dipyrenyl-9-(4 0 -pyrenylphenyl) carbazole (TPyC), have been designed and synthesized for application in organic light-emitting devices (OLEDs). While the two com- pounds have similar chemical structures and photoluminescent properties, OLEDs based on them show distinct electroluminescence (EL) spectra. The BPyC-based devices show a single peak saturated blue emission with CIE coordinates of (0.15, 0.18); whereas the TPyC-based devices exhibit two emission peaks at blue and yellow hues with CIE coordi- nates of (0.22, 0.29). The difference in their EL spectra is attributed to the substitution of the t-butyl unit of BPyC with a pyrenyl group to form TPyC, which effectively increases the electron-donating property and results in exciplex formation at its interface with the electron-accepting TPBI. A high external quantum efficiency of 3.11% is achieved in the TPyC-based devices. Influences of chemical structure and fluorescent quantum yield on the efficiency of exciplex emission are discussed. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Organic light-emitting device (OLED) is emerging as a leading next generation technology for full-color flat-panel displays and ambient illumination sources because of its huge market potential and excellent performance in terms of brightness, operating voltage and color gamut. Incorpo- ration of extrinsic dyes into conductive hosts is an effective way to customize emitting color of OLEDs that is particu- larly useful for fabricating white OLEDs with two comple- mentary or three primary colors [1,2]. Electroluminescence (EL) in terms of Commission Internationale de L’Eclairage (CIE) coordinates can be fine-tuned and optimized through precise control of dopant concentration and thickness for each light-emitting layer (EML). However, this approach would complicate device configuration as well as fabrica- tion process, and inevitably increases the total manufac- turing cost. In addition, the shortened device lifetime due to morphological phase separation caused by operation in- duced heating is another challenge for doped OLEDs. Recent studies [3–9] have demonstrated a simple way to tailor the emitting color via interfacial excited-state charge-transfer complex (termed exciplex) formation to control emission peak location and/or intensity by using different binary combinations of electron-donor/hole- transporting materials (HTMs) and electron-acceptor/elec- tron-transporting materials (ETMs), i.e., hm exciplex = E LUMO (ETM)  E HOMO (HTM), ignoring the band bending at the organic/organic interface [10]. In the equation, E HOMO and E LUMO are the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbi- tal (LUMO), respectively. However, only certain combina- tions of materials were reported to be capable of giving 1566-1199/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2010.12.026 ⇑ Corresponding authors. Address: Department of Chemistry, Shantou University, Guangdong 515063, China (Q.X. Tong). E-mail addresses: qxtong@gmail.com (Q.X. Tong), apcslee@cityu. edu.hk (C.S. Lee). Organic Electronics 12 (2011) 541–546 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel