Effect of dipolar arrays on the localization of charge carriers in molecular materials

The paper presents results of calculations of the depth and distribution of local states for charge carriers created by polar impurities in a model mo lecular material. Equations resulting from the electrostatic model due to L yons have been employed in the calculations. The presence of polar species in a non-polar molecular material locally modifies the polarization energy, thus creating local states (traps) on neighboring molecules. Calculations performed for an isolated dipolar defect show that traps as deep as similar to 0.4 eV to similar to 0.5 eV can result in such a way, their depths and cross-sections depending on the dipole moment of the guest molecule. Result s of similar calculations carried out for arrays of spatially connected dip oles indicate that local states of a considerable density may be created, m odifying the density-of-states function, and hence influencing the effectiv e mobility of charge carriers.