Blue electroluminescence and dynamics of charge carrier recombination in a vacuum-deposited poly(p-phenylene) thin film

We have studied the electroluminescence (EL) and the dynamics of charge carrier recombination in the vacuum-deposited poly(p-phenylene) (PPP) thin film that shows a bright blue EL emission at about 450 nm. The current-voltage-luminescence (I-V-L) characteristics are systematically studied in the temperature range between 12 and 325 K in the light-emitting devices of ITO/PPP/Al and the bilayer devices with hole transporting layers of aromatic diamine (TPD) and poly(9-vinylcarbazole) (PVK). The EL quantum efficiency (QE) of ITO/PPP/Al and ITO/PVK/PPP/AI is almost temperature-independent, indicating carrier injection via tunnelling mechanism. However, ITO/TPD/PPP/Alshows the increasing QE with decreasing temperature and the power-law I-V-L dependence, characteristics of a space-charge-limited current in a trap-filled insulator. From the time delay between the onset of the voltage pulse and that of the EL we can estimate the charge carrier mobility of mu approximate to 4 x 10(-6) cm(2)/N s in ITO/PPP/Al and mu approximate to 5 x 10(-5) cm(2)/V s in ITO/PVK/PPP/Al. (C) 2000 Elsevier Science S.A. All rights reserved.