The origin of dispersion of transiting charge carriers in molecular l y doped polymers

Abstract

The dispersion of transiting charge carriers in organic solids is analysed by examining the normalized time-of-flight current signals obtained over a wide range of electric fields, sample thicknesses and temperatures. Carriers injected into polycarbonate molecularly doped with N, N′-diphenyl-N, N-bis(3-methylphenyl)-[1, 1′-biphenyl]-4, 4′-diamine reach dynamic equilibrium with the environment in a small fraction of the overall transit time. The carrier dispersion then appears to follow the time-dependent Gaussian statistics with a broader Gaussian bandwidth observed at higher electric fields. The Gaussian dispersion arises from the off-diagonal disorder, in agreement with the prediction of Marshall's Monte Carlo simulation. The existence of a finite field-independent dispersion at very long transit times and the observed narrowing of the dispersion with increasing temperature suggest that diagonal disorder (a distribution of hopping site energies) also plays an important role in controlling the carrier dispersion.