Available online at www.sciencedirect.com Synthetic Metals 158 (2008) 213–218 Cross-linkable aromatic amines as materials for insoluble hole-transporting layers in light-emitting devices S. Lengvinaite a , J.V. Grazulevicius a , S. Grigalevicius a,∗ , B. Zhang b , J. Yang b , Z. Xie b , Lixiang Wang b a Department of Organic Technology, Kaunas University of Technology, Radvilenu plentas 19, LT50254 Kaunas, Lithuania b State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China Received 28 September 2007; received in revised form 7 December 2007; accepted 10 January 2008 Available online 4 March 2008 Abstract A series of cross-linkable aromatic amines has been synthesized by the multi-step synthetic rout. Full characterization of their structure by 1 H NMR-, IR- and mass spectrometry is presented. The synthesized materials were examined by various techniques including differential scanning calorimetry, thermogravimetry, UV and electron photoemission spectrometry. The electron photoemission spectra of the layers showed their ionisation potentials of 5.1–5.2eV. One of the derivatives was used for the preparation of insoluble hole-transporting layers by photoinduced polymerization. The layers obtained were tested in multilayer light-emitting diodes. The device-containing hole-transporting layer of PEDOT/cross- linked network exhibited the best overall performance with a turn-on voltage of 5 V, a maximum photometric efficiency of 2.8 cd/A and a maximum brightness of ca. 5600 cd/m 2 . © 2008 Elsevier B.V. All rights reserved. Keywords: Aromatic amine; Cross-linkable material; Ionisation potential; Light-emitting diode 1. Introduction Light-emitting diodes (LEDs) based on organic small molecules and polymers have recently attracted much attention because of their potential use in flat panel displays [1]. The efficient organic LEDs can be obtained by building multilayer structures [2]. Two approaches are normally used for their fabri- cation. The organic layers can be fabricated by vapor deposition or by spin-coating from solution. The latter rout has some advan- tages, e.g. the production costs are reduced, large areas can be coated, and the molar mass of the materials is not limited. A severe difficulty in the preparation of such devices is the solu- bility of the material which forms the bottom layer onto which the top layer has to be cast, because most organic semiconduc- tors are soluble in the same organic solvents. One approach that has been employed to circumvent this problem is to render the material insoluble by using cross-linking reactions. ∗ Corresponding author. Tel.: +370 37 456 525; fax: +370 37 456 525. E-mail address: saulius.grigalevicius@ktu.lt (S. Grigalevicius). Recently, several series of photo-cross-linkable polymers [3] and monomers [4] for the fabrication of LEDs have been reported. Among the photo-cross-linkable monomers earlier studied, triphenyldiamine (TPD) core having derivatives were mostly used. For the purpose of developing of organic layers for LEDs, we have been studying the synthesis and applications of new photo-cross-linkable monomers with carbazolyl fragments. Films of the compounds should demonstrate suitable ionisation potentials and could be used as hole-transporting layers in mul- tilayer devices [5]. On the other hand, we have used short photo- cross-linkable oxetanyl moieties which neither form additions after polymerization nor decrease considerably the concentra- tion of the charge transporting aromatic chromophores. 2. Experimental 2.1. Instrumentation 1 H NMR spectra were recorded using Varian Unity Inova (300 MHz) apparatus. Mass spectra were obtained on a Waters ZQ 2000 spectrometer. FTIR spectra were recorded using 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.01.004