Drift mobilities in amorphous charge-transfer complexes of trinitrofluorenone and poly-n-vinylcarbazole

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Abstract
Hole and electron drift mobilities have been measured in thin films of charge-transfer complexes of 2,4,7-trinitro-9- fluorenone (TNF) with poly-n-vinylcarbazole (PVK). The film compositions ranged from pure PVK to mixtures of TNF : PVK in a 1.1 : 1 molar ratio with respect to the monomer unit. Measurements have also been made on amorphous films of pure TNF. The drift mobilities are extremely low (< 10−6 cm2/V sec) and are strongly field and temperature dependent. The field dependence is due to the measured activation energies decreasing as the square root of the applied field from zero-field values of about 0.7 eV for both carriers. The field dependence of the mobilities is similar to that observed for conductivity modified by the Poole-Frenkel mechanism; however, several objections make this model unacceptable for this case. At this time, no satisfactory explanation of the observed field dependence can be given. As the TNF concentration is increased, the hole mobility decreases rapidly, while the electron mobility increases. The results suggest that charge carriers are highly localized with transport taking place by a thermally assisted intermolecular hopping process as for the case of small ppolaron motion. Hole transport is apparently due to hopping via uncomplexed carbazole states, while electron transport is due to hopping between TNF sites which may be complexed or uncomplexed. Estimates of the charge-carrier localization obtained from the exponential dependence of mobility on the site separation give characteristic distances of 1.1 × 10−8 cm for holes and 1.8 × 10−8 cm for electrons.