Effect of polar molecules on the transport and localization of charge carriers in molecular materials

The influence of polar species on the transport of charge carriers in molec ular solids is reviewed. Guest molecules with permanent dipole moments intr oduced into molecular crystals can act as traps localizing carriers provide d certain energetic conditions are fulfilled. In this case their polar char acter may manifest itself in a held-dependent decrease of the trap depth. M oreover, irrespective of the positions of their energy levels, polar impuri ties may locally modify the polarization energy thus creating traps on neig hbouring molecules. The effect of polar dopants in disordered molecular sol ids can be described within the model due to Bassler: the presence of polar species broadens the density-of-states function thus decreasing the effect ive mobility of charge carriers.