Synthesis and optoelectronic properties of a luminescent fluorene derivative containing hole-transporting triphenylamine terminals

Abstract

Solution-processable 4,4′,4″-{[9,9-bis(hexyl)-9H-fluorene-2,4,7-triyl]tri-2,1-ethenediyl}tris(N,N-diphenyl)benzeneamine (TF), consisting of a fluorene core and terminal triphenylamine groups, was synthesized by the Wittig reaction. The characteristics of TF were analyzed by ¹H-NMR, elemental analysis, differential scanning calorimetry, optical spectra, cyclic voltammetry, and atomic force microscopy. The triphenylamine terminals were intentionally incorporated to increase its hole-transporting ability that reduces the barrier height of hole-injection. A multilayer light-emitting diode using TF as a hole-transporting layer [ITO/PEDOT:PSS/TF/Alq₃(70 nm)/LiF(0.5 nm)/Al(100 nm)] was readily fabricated by the spin-coating process. Its maximum brightness (9390 cd m⁻²) and current efficiency (2.90 cd A⁻¹) were superior to those using conventional NPB as a hole-transporting layer. In addition, the TF could also be blended with polyfluorene (PF) to be applied as an emitting layer; the devices [ITO/PEDOT:PSS/blend (110 nm)/LiF(0.5 nm)/Ca(50 nm)/Al(100 nm)] exhibited high maximum brightness (4040 cd m⁻²) and current efficiency (1.62 cd A⁻¹). Our results indicate that the TF is a potential optoelectronic material which is readily processed by wet processes.