Full Paper

Optimizing OLED Efficacy of 2,7-Diconjugated 9,9-Dialkylfluorenes by Variation of Periphery Substitution and Conjugation Length†

  1. H. P. Rathnayake1,
  2. A. Cirpan2,
  3. Z. Delen1,
  4. P. M. Lahti1,
  5. F. E. Karasz2

Article first published online: 5 DEC 2006

DOI: 10.1002/adfm.200600089

Issue

Advanced Functional Materials

Advanced Functional Materials

Volume 17, Issue 1, pages 115–122, January, 2007

Additional Information

How to Cite

Rathnayake, H. P., Cirpan, A., Delen, Z., Lahti, P. M. and Karasz, F. E. (2007), Optimizing OLED Efficacy of 2,7-Diconjugated 9,9-Dialkylfluorenes by Variation of Periphery Substitution and Conjugation Length. Adv. Funct. Mater., 17: 115–122. doi: 10.1002/adfm.200600089

Author Information

  1. 1

    Department of Chemistry, University of Massachusetts, Amherst, MA 01003, USA

  2. 2

    Department of Polymer Science and Engineering, University of Massachusetts, Amherst, MA 01003, USA

  1. †

    This work was supported by the U.S. Department of Energy (H.P.R., P.M.L.; DE-FG02-05ER15695), the National Science Foundation (Z.D., P.M.L.; CHE 0415716), and the Air Force Office of Scientific Research (A.C., F.E.K). Supporting Information is available online from Wiley InterScience or from the author.

Publication History

  1. Issue published online: 4 JAN 2007
  2. Article first published online: 5 DEC 2006
  3. Manuscript Revised: 19 JUN 2006
  4. Manuscript Received: 25 JAN 2006

Funded by

  • U.S. Department of Energy. Grant Number: DE-FG02-05ER15695
  • National Science Foundation. Grant Number: CHE 0415716
  • Air Force Office of Scientific Research

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Keywords:

  • Electroluminescence;
  • Fluorenes;
  • Light-emitting diodes, organic (OLEDs);
  • Luminescence

Abstract

The photoluminescence (PL) and electroluminescence (EL) of four 2,7-bis(phenylethenyl)fluorenes (OFPVs) and two 2,7-diphenylfluorenes (OFPhs) are compared to evaluate effects of nonconjugating peripheral substitution and conjugation length on their EL emissions. The OFPVs exhibit very similar PL spectra with 460–480 nm emission maxima but show large variation in the organic light-emitting diode (OLED) efficacy: from a material that does not give persistent emission in test OLEDs (9,9-diheptyl substitution on the fluorene ring) to materials with luminance efficiencies of 0.5 cd A–1 and greater (9,9-diethyl substitution on the fluorene ring, methoxy and methoxy/heptyloxy substituents on the phenylethenyl rings). The best OFPV in an ITO/PEDOT:PSS/(emitter)/Ca–Al (ITO: indium tin oxide; PEDOT: poly(ethylenedioxythiophene); PSS: poly(styrene sulfonate)) OLED configuration has 9,9-diethyl substitution and terminal heptyloxy substitution (maximum luminance of 1500 cd m–2 at 12 V). Unlike the OFPVs, the neat OFPhs show not only EL at the desired blue output of ca. 400–410 nm emission maxima but also an undesired green emission component at 500–550 nm. Blending the OFPhs with poly(methyl methacrylate) eliminates the long-wavelength component when the emitter load is 10–25 %, but the OFPh luminance efficiencies, turn-on voltages, and maximum luminance tend to be poorer than those of the OFPVs. The deficiencies of the OFPhs appear to be attributable to thermal degradation and oxidative reactivity, although solid-state annealing and a nonoptimal bandgap match to the OLED device configuration may also contribute.

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