Carrier transport assisted by dopants in doped poly(N-vinylcarbozole) light-emitting diodes

Abstract

We have investigated the transient electroluminescence (EL) onset of the double-layer light-emitting devices made from poly(N-vinylcarbozole) (PVK) doped with 4-(dicyanomethylene)-2-t-butyl-6(1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) and tris(8-hydroxy-quinoline) aluminium (Alq3). For the double-layered device in which PVK was doped with 0.1 wt% DCJTB, the EL onset of PVK lags that of DCJTB and Alq3, while the EL onset of DCJTB and Alq3 is simultaneous. However, the EL emission of the double-layered device of PVK/Alq3 originates only from Alq3. The results show that DCJTB dopants can not only help to tunnel electrons from Alq3 zone to PVK but can also assist electrons transfer in PVK under high electric field by hopping between DCJTB molecules or from DCJTB to PVK sites at a low doping concentration of 0.1 wt%. When the DCJTB doping concentration is 4.0 wt%, the EL onset of Alq3 lags that of DCJTB. The difference in the EL onsets of DCJTB, Alq3 and PVK is attributed to the slow build-up of the internal space charge in the vicinity of the interface between PVK and Alq3. The electron potential difference of the interface between Alq3 and PVK doped by DCJTB can be adjusted by changing the DCJTB doping concentration in double-layer devices.