Blue Electroluminescence and Dynamics of Charge Carrier Recombination in a Vacuum-Deposited poly(p-phenylene) Thin Film

Abstract

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/Al is almost temperature-independent, indicating carrier injection via tunnelling mechanism. However, ITO/TPD/PPP/Al shows 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 μ≈4×10−6 cm2/V s in ITO/PPP/Al and μ≈5×10−5 cm2/V s in ITO/PVK/PPP/Al.