Ž . Synthetic Metals 107 1999 203–207 www.elsevier.comrlocatersynmet Letter Improved efficiencies of light-emitting diodes through incorporation of charge transporting components in tri-block polymers J.P. Chen, D. Markiewicz, V.Y. Lee, G. Klaerner, R.D. Miller, J.C. Scott ) IBM Research DiÕision, Almaden Research Center, 650 Harry Road, San Jose, CA 95120-6099, USA Received 29 July 1999; accepted 11 August 1999 Abstract Ž . Ž . Ž . 9,9-Di-n-hexylfluorene-co-anthracene DHF-ANT random copolymer and triphenylamine TPA - and oxadiazole OXA -containing Ž . tri-block copolymers along with a crosslinked TPA polymer are used to fabricate single- and double-layer light emitting diodes LEDs . In both single-layer and double-layer devices, TPA hole transporting components improve hole injection and transport, leading to Ž . improved device performance when low work function calcium Ca is used as the cathode, whereas OXA electron transporting Ž . components improve electron injection when high work function aluminum Al is used. This suggests that DHF-ANT copolymer and DHF-ANT blocks in the two tri-block polymers are hole-limited in devices with Ca cathodes, and electron-limited with Al cathodes. Furthermore, double-layer devices including a separate hole transporting crosslinked TPA layer increase device efficiencies by at least one order in magnitude over the corresponding single-layer devices, due to the improved charge injection, charge confinement and charge recombination. q 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Blue luminescent polymer; Hole transport; Electron transport; Polymer light-emitting diode 1. Introduction Polymer LEDs have been a subject of intensive study because of both academic interest and industrial potential wx Ž for applications in flat panel displays 1 . Poly phenylen- . w x Ž . wx Ž . evinylene 2,3 , poly p-phenylene 4 and poly thiophene wx 5 derivatives are among the most used emitting polymers in constructing LEDs. A typical polymer LED is con- structed with at least one polymer emitting layer sand- wiched between an anode such as transparent indium–tin Ž . oxide ITO and a cathode such as Al or Ca. Under a Ž . forward bias, the charges holes and electrons are injected into the emitting layer. Some of these holes and electrons combine within the emissive layer, generating singlet exci- ton states, which may decay radiatively. Polyfluorenes represent another class of conjugated polymeric emitting materials. They emit blue light because of the larger energy gap between the highest occupied molecular orbital and the lowest unoccupied molecular ) Corresponding author. Tel.: q1-408-927-1150; fax: q1-408-927- 2100; e-mail: jcscott@almaden.ibm.com wx orbital 6 . In a previous publication, we have demon- strated that a variety of high molecular weight, thermally Ž. stable polyfluorenes can be synthesized by nickel 0 wx mediated arylene condensation polymerization 7 . Further- more, excimers that often appear as broad red-shifted bands in the emission spectra can be suppressed by ran- domly introducing a small amount of an anthracene chro- mophore via the 9,10-positions into a polyfluorene main chain. This effect may be due, in part, to the strongly wx twisted anthracene structure 8 . In order to further understand the relationship between the electronic structure of the emissive polymer and LED performance, it is useful to introduce hole or electron transporting components into the emitting layer. Tripheny- Ž . wx lamine derivatives TPAs 9 are well-known hole trans- porting materials which, because of their lower ionization potential, lead to reduced hole injection barriers with the Ž . w x anode, whereas oxadiazoles OXAs 10 are electron transporters which facilitate electron injection from the cathode. Both TPA- and OXA-based chromophores have been blended as well as covalently incorporated in random w x copolymers for LEDs 9,10 . Here, we report the fabrica- tion and characterization of single-layer LEDs from three Ž . emissive copolymers random and block all derived from 0379-6779r99r$ - see front matter q 1999 Published by Elsevier Science S.A. All rights reserved. Ž . PII: S0379-6779 99 00168-X