CONTRIBUTION OF DIPOLAR SPECIES TO THE FORMATION OF LOCAL STATES FOR CHARGE-CARRIERS IN MOLECULAR MATERIALS

Effect of polar impurities on transport of charge carriers in non-pola r molecular materials is reviewed. The presence of such species gives rise to structural traps. Calculations of local values of the polariza tion energy in the vicinity of a dipolar defect show that traps as dee p as several tenths of eV are created. If the polar impurity molecule can act as a trap itself, at high fields one may expect a dipole analo gue of the Poole-Frenkel effect, the lowering of the trap depth follow ing, however, a different dependence on the biasing electric field: De lta E-t alpha F-2/3. Moreover, the dipole-charge interactions contribu te to the broadening of local transport states density in disordered m aterials.