Morphological reasoning for the enhanced charge carrier mobility of a hole transport molecule in polystyrene
F. Khan, A.-M. Hor, and P. R. Sundararajan
Department of Chemistry, Carleton University, 1125
Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada; Xerox Research Centre
of Canada, 2660 Speakman Drive, Mississauga, Ontario L5K 2L1, Canada
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.
*Lecture presented at the symposium "Polymers in electronics and photonics: Synthesis, characterizations and device applications", as part of the 39th IUPAC Congress and 86th Conference of the Canadian Society for Chemistry: Chemistry at the Interfaces, Ottawa, Canada, 10-15 August 2003. Other Congress presentations are published in this issue, pp. 1295-1603.
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