Morphological reasoning for the enhanced charge carrier mobility of a hole transport molecule in polystyrene

Abstract

Pai and coworkers have reported that the charge carrier mobility of the hole transport molecule N,N'-diphenyl-N,N'-bis(3-methylphenyl)-[1,1'-biphenyl]-4,4'-diamine (TPD) is higher with polystyrene (PS) as the host polymer, in comparison to bisphenol A polycarbonate (PC) as the binder. It was proposed that the enhanced interaction of TPD with PC and the effect of the carbonyl dipole are responsible for such a phenomenon.We present a morphological study that lends support to the above proposal. The morphology and thermal behavior of TPD/polystyrene (TPD/PS) composites have been investigated as a binary solid solution and compared with that of the TPD/polycarbonate (TPD/PC) pair. The depression of the glass-transition temperature (Tg ) with the concentration of TPD is more pronounced with PC than with PS. On the other hand, the recovery of the Tg upon annealing is significantly higher in the case of PS. Fourier transform infrared (FTIR) study shows that the molecular- level interaction between TPD and PS is not as significant as in the case PC. Molecular modeling based on the MM+ molecular mechanics calculations also shows an electrostatic component to the total interaction energy in the case of PC. In addition, small crystals are present in the as-prepared films of TPD/PS that, upon chaining, could enhance the charge carrier mobility. Thus, it is suggested that the enhanced interaction between TPD and PC as well as the small crystals of TPD in PS are responsible for the higher mobility in the latter. PS serves as an inert host, while there are specific interactions between TPD and PC.