  Charge transport in disordered organic semiconductors occurs by hopping of
charge carriers between localized sites that are randomly distributed in a
strongly energy dependent density of states. Extracting disorder and hopping
parameters from experimental data like temperature dependent current-voltage
characteristics typically relies on parametrized mobility functionals that are
integrated in a drift-diffusion solver. Surprisingly, the functional based on
the extended Gaussian disorder model (eGDM) has been extremely successful at
this, despite it being based on the assumption of nearest neighbor hopping
(nnH) on a regular lattice. We here propose a variable range hopping (VRH)
model that has been integrated in a freeware drift-diffusion solver. The
mobility model has been calibrated using kinetic Monte Carlo calculations and
shows good agreement with the Monte Carlo calculations over the experimentally
relevant part of the parameter space. The model is applied to
temperature-dependent space charge limited current (SCLC) measurements of
different systems. In contrast to the eGDM, the VRH model provides a consistent
description of both p-type and n-type devices. We find a critical ratio of
aNN/$\alpha$ (mean inter-site distance / localization radius) of ~3 below which
hopping to non-nearest neighbors becomes important around room temperature and
the eGDM cannot be used for parameter extraction. Typical (Gaussian) disorder
values in the range 45-120 meV are found, without any clear correlation with
photovoltaic performance when the same active layer is used in an organic solar
cell.
