Positional Disorder-Induced Mobility Enhancement in Rapidly Cooled Organic Semiconductor Melts

The study of the charge carrier transport mechanism in organic semiconducting films is a great challenge because of the weak interaction between organic molecules. Generally, high crystallinity is regarded as being conducive to carrier transport in organic films. However, the organic semiconducting films have been found to exhibit less crystallinity but improved mobility after being heated to melt and then rapidly cooled. In this paper, hole mobilities of the as-vacuum deposited and rapidly cooled films of two organic semiconductors, 4,4',4 ''-tris(N-3-methylphenyl-N-phenyl-amino)triphenylamine (mTDATA) and N,N'-diphenyl-N,N'-bis(3-methyl)-benzidine (TPD), were measured by the time-of-flight (TOF) method at different temperatures, ranging from 260 to 320 K. The Gaussian disorder model (GDM) has been employed to investigate the carrier transport properties of the organic films, and the energetic and positional disorder parameters were calculated quantitatively. It is found that the higher positional disorder Sigma in the rapidly cooled film gives rise to its high mobility. The positional disorder-induced improvement in hole mobility was explained in terms of the detour pathways for carriers through the film which were opened by the irregular intermolecular arrangement. Additionally, the effect of the chemical structure upon the molecular packing and the carrier transport was also discussed.