EFFECTS OF GEOMETRICAL DISORDER ON HOLE TRANSPORT IN MOLECULARLY DOPED POLYMERS

The mobility of holes is compared between two molecularly doped polymer systems. The charge-transporting material in one [TAPC, 1,1-bis(di-4-tolylaminophenyl)cyclohexane] is a dimeric analog of the other (TTA, tri-4-tolylamine). The charge-transport sites are essentially identical, but their packing is different because of their pairing in TAPC. The influence of packing and geometrical (off-diagonal) disorder is investigated, and current theories and notions of the effects of such disorder are tested. At equal, low site concentrations, TAPC probably has the greater degree of geometrical disorder. As expected on this basis, the mobility is a less rapidly increasing (or more rapidly decreasing) function of electric field strength in TAPC than in TTA. Contrary to predictions of the Gaussian Disorder Model [H. Bassler, Phys. Status Solidi B 175, 15 (1993)], there is no concomitant effect on the magnitude of the mobility as extrapolated to zero field strength. Other predictions of the model, concerning the dependence of the mobility on field and temperature, are also not verified. The manner in which the field dependence in TTA varies with varying concentration is qualitatively consistent with the results of recent simulations [B. Hartenstein ct al., Chem. Phys. 191, 321 (1995)], but the specific field dependence is not. A comparison of TTA at very low concentrations with a tetrameric analog leads to a proposal on the specific influence of site clustering (dimeric or tetrameric) on transport. The low-concentration data also exhibit a drift velocity that decreases with increasing field strength, nondispersive transport at low field strengths, and dispersive transport at high field strengths. The relative importance of molecular packing in general and the implications for the interpretation of transport data on other systems are discussed. (C) 1995 American Institute of Physics.