On the field and temperature dependence of hole mobility in molecularly doped polymer

The charge transport properties of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) doped polycarbonate (PC) have been measured as a function of field and temperature. Hole mobility is observed to follow log μ ∝ E1/2 (i.e. Poole-Frenkel behavior) at relatively high field strengths while at low field strengths mobility initially decreased with increase in field strength. Mobility value undergoes minima at a particular field value for a given temperature. Minima in mobility are observed to occur at higher field strength as the temperature is lowered. Mobility data and transport parameters are analyzed using the formalism of Gaussian disorder model (GDM). The mobility behavior at low field and shift of mobility minima with temperature suggests large disorder in the film. Investigation of film morphology using photoluminescence, X-ray diffraction and scanning electron microscopy suggests that large positional disorder in the film is probably due to film morphology and TPD-PC interaction.