Electron Trapping in Acceptor Doped Polymers

Abstract

Eletron mobilities have been measured in N,N′-bis(1,2-dimethylpropyl)-1,4,5,8-naphthalenetetracarboxylic diimide (NTDI)-doped poly(styrene) containing a series of acceptor traps with depths between 0.11 and 0.41 eV. The results are explained within the framework of a formalims based on disorder. The formalism is premised on the assumption that charge transport occurs by hopping through a manifold of localized states that are distributed in energy. The key parameter of the formalism is σ, the energy width of the hopping site manifold. The results are compared to recent simulations of Wolf et al. and Borsenberger et al., and show that the presence of a distribution of traps, off-set from the intrinsic hopping manifold σ by an energy Et, does not change the basic phenomenology of hopping transport, as revealed by the field and temperature dependencies of the mobility or by the temporal features of the photocurrent transients. The effect of trapping can be quantitatively accounted for by the replacement of σ with an effective width σeff whose square increases linearly with the trap depth.