ELECTRON-TRANSPORT IN VAPOR-DEPOSITED MOLECULAR GLASSES - THE ROLE OF GROUP DIPOLE-MOMENTS

Electron mobilities are measured in vapor deposited layers of N,N'-bis(l,2-dimethyipropyl)-1,4,5,8-naphthalenetetracarboxylic diimide (NTDI). NTDI is a weakly polar compound with a molecular dipole moment of 0.3 D. The molecular moment is the vector sum of two group moments, each approximately 3.3 D. The results are described by a formalism based on disorder, due to Bassler and coworkers. The formalism is based on the assumption that charge transport occurs by hopping through a manifold of localized states with superimposed energetic and positional disorder. The key parameter of the formalism is the energy width of the hopping site manifold. For NTDI, the width is 0.093 eV. The width can be described by a model based on dipolar disorder. The model assumes that the width is comprised of a dipolar component and a van der Waals component. A comparison of the experimental results with predictions of the model leads to the conclusion that the dipolar component to the width is determined by group dipole moments of two carbonyl groups adjacent to the imide acceptor functionalities. The positional disorder parameter is 2.3 and attributed to packing constraints. The prefactor mobility is 1.0 x 10(-2) cm(2)/Vs.