CONTACT-INJECTED CURRENTS IN POLYVINYLCARBAZOLE

Steady-state measurements disclose different kinds of current-voltage characteristics in polyvinylcarbazole films depending on whether the electrodes are Au or Al. The current levels when Al is the anode are only two or three orders of magnitude smaller than when Au is the anode, which seems irreconcilable with the work-function difference of 0.8 eV. The discrepancies are resolved by assuming that the current is produced by hole-tunnelling from the anode and allowing for a normal distribution of the molecular state energies in polyvinylcarbazole. From Au, the holes tunnel directly into the transport band. From Al, they tunnel into surface states which are tentatively identified with the deep trapping states known to exist in the interior of polyvinylcarbazole. An estimated increase in the free-hole concentration by congruent-to 10(5), when the injection is from Au rather than Al, appears to correspond to a reduction in the mobility by congruent-to 10(3).