Nanosecond transients in the electroluminescence from multilayer blue organic light-emitting devices based on 4,4′-bis(2,2′diphenyl vinyl)-1,1′-biphenyl

Abstract

Nanosecond electroluminescence (EL) overshoots observed when multilayer blue 4,4′-bis(2,2′-diphenyl vinyl)-1,1′-biphenyl (DPVBi)-based organic light-emitting devices (OLEDs) are excited by rectangular voltage pulses are described. The overshoots occur at the voltage turn-off and exceed the cw brightness by up to an order of magnitude. Time-resolved images of the OLEDs demonstrate that the emission from most of the sample surface decays with a single time constant τ1=13±3 ns. This decay is attributed to recombination of charges which accumulate at the interface of the electron and hole transporting layers, possibly at intrinsic trapping sites. In areas of increased electron injection and EL, such as cathode edges and morphological defects, a second slower decay time 20 ns<τ2<1 μs is observed, apparently due to release of carriers from localized trap states in the organic/cathode interface. Only marginal variations in τ1 are found between bright and dim areas of the devices. At a bias of 10 V, the amplitude of the overshoot is found to peak at a pulse duration of ∼20 μs. Its behavior is believed to result from increased quenching of singlet excitons by the accumulated charges.