Transient space-charge-limited current pulse shapes in molecularly doped polymers

The transient current response of molecularly doped polymers have been numerically modelled under space-charge-limited (SCL) conditions for the situation in which a step potential is applied to an ideal injecting contact. Under trap-free conditions, the transient SCL current pulse shape is found to be sensitive not only to the underlying field dependence of the injected carrier mobilities and diffusivities, but also to the magnitude of the applied step potential. A progressive reduction in the ratio of the peak current density j p to the final steady-state magnitude j ss is obtained by increasing either the field strength of the mobility or the relative amount of diffusion. It is demonstrated, however, that for times preceding the location t p of the current peak, the rate of current increase displays a gradual transition from a super-linear to linear time dependence upon the introduction of diffusion. The diminishing observability of j p / j ss is accompanied by a shift in the position of t p relative to the space-charge-free carrier transit time t 0 . The classical fixed-mobility value t p / t 0 = 0.786 is modestly reduced as the field strength of the mobility or amount of carrier diffusion is enhanced. The numerical predictions are compared with experimental SCL current transients obtained from hydrazone doped polyester samples fitted with gold contacts.