Charge transport in disordered organic host–guest systems: effects of carrier density and electric field

Abstract

We investigate charge transport in disordered organic host-guest systems with a bimodal Gaussian density of states (DOS). The energy difference between the two Gaussians defines the trap depth. By solving the Pauli master equation for the hopping of charge carriers on a regular lattice with site energies randomly drawn from the DOS, we obtain the dependence of the charge-carrier mobility on the relative guest concentration, the trap depth, the energetic disorder, the charge-carrier density and the electric field. At small and high guest concentrations, our work provides support for recent semi-analytical model results on the dependence of the mobility on the charge-carrier density at zero field. However, at the cross-over between the trap-limited and trap-to-trap hopping regimes, where the mobility attains a minimum, our results can almost be one order of magnitude larger than predicted semi-analytically. Furthermore, it is shown that field-induced detrapping can contribute strongly to the electric-field dependence of the mobility. A simple analytical expression is provided which describes the effect. This result can be used in continuum drift-diffusion models for charge transport in devices such as organic light-emitting diodes.

Field-induced detrapping in disordered organic semiconducting host-guest systems

“…The approach is based on the results of transport modeling using a master-equation ͑ME͒ approach. In an earlier study, 27 it was already found that the effect of FID on the mobility as obtained from ME modeling is significantly different from the effect as predicted by a semianalytical effective-medium theory. Using the ME modeling results, we show that the HL model can be extended to finite values of the electric field by using a generalized FD distribution function that depends only on the shape of the host DOS and the electric field, and not on the guest DOS.…”

Electron transport in polyfluorene-based sandwich-type devices: Quantitative analysis of the effects of disorder and electron traps

“…From an estimate of the effect based on an analysis given in Ref. 27 for the case of a bimodal Gaussian DOS, we have found that the effect is very small for the systems and experimental conditions considered in this Brief Report. In our analysis, FID was therefore neglected.…”

“…It was used successfully for treating the mobility in a bimodal Gaussian DOS, 26 as confirmed by numerically exact masterequation calculations. 27 It is known that in the presence of a field the effective mobility in a system containing trap states can be larger than as obtained from the MTR model. 28 For materials with the shape of the DOS assumed in this Brief Report ͑Fig.…”

Quantitative analysis of the guest-concentration dependence of the mobility in a disordered fluorene-arylamine host-guest system in the guest-to-guest regime

“…In this regime, a more complicated model is required. 26 We remark that for higher concentrations the quality of the fit was less sensitive to the value of N t , leading to a large error margin in N t .…”