Electric-field dependence of the hole drift mobility in molecularly doped polymers: Importance of the disorder of hopping sites

The electric-field dependence of the drift mobility of holes in a polymer matrix doped with various different molecules was studied by means of time-of-flight (TOF) photoconductivity measurements. It was first found that the degree of the positive field dependence of the drift mobility increases with decreasing magnitude of the drift mobility at zero field. The tail of the Gaussian TOF current signal becomes broad in molecularly doped polymers having a large disorder of hopping site energies. It was pointed out that the field dependence of the mobility and the energy barrier for charge transport are largely related to energetic disorder of hopping sites.