Comparative Study on Hole Transport in N,N'-bis(naphthalen-1-yl)-N,N'- bis(pheny) Benzidine and 4,4',4''-tri(N-carbazolyl)triphenylamine

Hole transport characteristics in N,N'-bis(naphthalen-1-yl)-N,N'-bis(pheny) benzidine (NPB) and 4,4',4''-tri(N-carbazolyl)triphenylamine (TCTA) are comparatively investigated. The current density-voltage (J-V) characteristics of hole-only devices based on NPB and TCTA at different temperatures and thicknesses show that the hole current is dominated by the bulk conduction with an exponential trap distribution. Detailed analyses of the J-V characteristics give the trap densities N_t of (6.3±0.3)×10¹⁸ and (1.9±0.02)×10¹⁸ cm³, characteristic trap depths of 135±6 and 117±5 meV, hole mobilities of (8.1±0.5)×10^-5 and (1.9±0.1)×10^-4 cm²V^-1s^-1 for NPB and TCTA, respectively. It is found that TCTA exhibits higher hole mobility. Obviously, this is directly related to the lower trap density and shallow trap depth in TCTA films, leading to good charge carrier transport.