Estimate of hole mobilities of some organic photoconducting materials using the time-of-flight method

Abstract

1,1-bis-(4,4'-diethylaminophenyl)-4,4-diphenyl-1,3-butadiene (DEPB) and N,N'-diphenyl-N,N'-bis-(3-methylphenylene)-1,1'-diphenyl-4, 4'-diamine (TPD) are used as organic photoconductors (OPC) and as hole transport materials for organic light-emitting diodes (LED). The hole mobilities of cast or vapour-deposited DEPB and TPD films are measured using the time-of-flight method. The hole mobility of a cast DEPB/polycarbonate film increases with the concentration of DEPB, but the cast pure DEPB film could not be prepared because of crystallization. Since crystallization in a vapour-deposited DEPB film proceeded very slowly, it was possible to measure the hole mobility, which is 5-8*10-5 cm2 V-1 s-1. The hole mobilities of TPD films were consistent with those reported by Abkowitz et al. (1986). The hole mobility of vapour-deposited TPD film was several times 10-4 cm2 V-1 s-1 and the electric field dependence of its hole mobility was consistent with that for a cast pure TPD film. In other words, it is possible to estimate the mobility of vapour-deposited OPC material by measuring that of a cast pure OPC film, for cases when a relatively thick, vapour-deposited OPC film is difficult to prepare.