<title>Activation energies and hole transport in photochemically modified molecularly doped polymers</title>

Abstract

The activation energy for charge transport in molecularly doped polymer films containing p-diethylaminobenzaldehyde diphenylhydrazone (DEH) is measured before and after systematic UV irradiation. UV exposure has been shown to induce a photochemical reaction of the DEH molecule which dramatically reduces the molecule's capability to transport charge. Presumably, the reduction in mobility is associated with the substantial increase (above 1 eV) in the ionization potential of the DEH molecule after photocyclization. The increase in ionization potential effectively removes the photochemically modified molecule from the primary transport manifold. In previous work, we have demonstrated that systematic UV irradiation of molecularly doped polymer films containing DEH provides a novel approach for diluting the dopant concentration and effectively increasing the intersite separation between 'active' dopant molecules in situ. Furthermore, ab initio calculations suggest that the dipole moment of the photoproduct is significantly lower than that of the unconverted DEH molecule. We exploit this photochemical process to prove the hopping activation energy, (Delta) , in DEH doped polycarbonate films parametric in UV irradiation time. The principal observation of this work is that despite a systematic and substantial elimination of active hopping sites and despite the systematic increase in the population of lower dipole moment neighbors, the hopping activation energy for the DEH/polycarbonate system remains constant.