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

The transient current response of molecularly doped polymers have been nume rically modelled under space-charge-limited (SCL) conditions for the situat ion in which a step potential is applied to an ideal injecting contact. Und er trap-free conditions, the transient SCL current pulse shape is found to be sensitive not only to the underlying field dependence of the injected ca rrier mobilities and diffusivities, but also to the magnitude of the applie d step potential. A progressive reduction in the ratio of the peak current density j(p) to the final steady-stale magnitude j(ss) is obtained by incre asing either the field strength of the mobility or the relative amount of d iffusion. It is demonstrated, however, that for times preceding the locatio n 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 introduc tion of diffusion. The diminishing observability of j(p)/j(ss) is accompani ed by a shift in the position of t(p) relative to the space-charge-free car rier transit time t(0). The classical fixed-mobility value t(p)/t(0) = 0.78 6 is modestly reduced as the field strength of the mobility or amount of ca rrier diffusion is enhanced. The numerical predictions are compared with ex perimental SCL current transients obtained from hydrazone doped polyester s amples fitted with gold contacts.