Mobility with negative coefficient in Poole–Frenkel field dependence in conjugated polymers: Role of injected hot electrons

In this work we study the electric field dependence of mobility extracted from Electroluminescence Transient (ELT) measurements in both single carrier and double carrier Polymer Light emitting Diode (PLED) configurations by varying the cathode. We have chosen two different families of conjugated light emitting polymers (structurally different hole transporting materials). The Poole–Frenkel (P–F) plot of mobility μ as a function electric field F   (logμ∝F) shows negative slope (β < 0) in the case of double carrier injection, while positive slope is observed as expected in the case of dominantly single carrier injection. The mobility values and their corresponding field dependent parameters for both the carriers are derived. It is shown that electrons injected to higher lying transport states can have high mobilities behaving as hot electrons. We propose a phenomenological model based on alignment of Gaussian local density of states (DOS) to explain the occurrence of reduction of mobility with increasing electric field for electrons, which do not fully relax into equilibrium distribution within the DOS in the timescales of transit time.