Photoluminescence and electroluminescence of vacuum-deposited poly(p-phenylene) thin film

We have studied the photoluminescence (PL) and electroluminescence (EL) properties of vacuum-deposited poly(p-phenylene) (PPP) thin film. The film shows the blue PL and EL emission at about 450 nm with well-resolved vibronic structures. The current–voltage–light (I–V–L) characteristics are systematically studied in the temperature range between 12 and 300 K in light-emitting devices of ITO/PPP/Al, and the devices with a hole transporting layer of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD), ITO/TPD/PPP/Al. The I–V dependence appears to follow the power law, I∝Vm+1, characteristics of a space-charge-limited current. The EL quantum efficiency increases with the decreasing temperature in ITO/TPD/PPP/Al. From the time delay between the EL emission and the onset of the bias voltage pulse in ITO/PPP/Al, we estimated the hole mobility of ∼4×10−6 cm2/V s in the vacuum-deposited PPP film.