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

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  • 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.

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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