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

In a disordered organic semiconducting host-guest material, containing a relatively small concentration of guest molecules acting as traps, the charge transport may be viewed as resulting from carriers that are detrapped from the guest to the host. Commonly used theories include only detrapping due to thermal excitation, described by the Fermi-Dirac (FD) distribution function. In this paper, we develop a theory describing the effect of field-induced detrapping (FID), which provides an additional contribution at finite electric fields. It is found from three-dimensional simulations that the FID effect can be described by a field-dependent generalized FD distribution that depends only on the shape of the host density of states (DOS) and not on the guest DOS. For the specific case of a Gaussian host DOS, we give an accurate and easy-to-use analytical expression for this distribution. The application of our theory is demonstrated for sandwich-type devices under conditions typical of organic light-emitting diodes.