The current density versus voltage [J (V)] curves of hole-only sandwich-type devices containing a blue-emitting polyfluorene-based copolymer were measured for a wide range of temperatures and for several thicknesses of the active organic layer. We show that the J (V) curves cannot be accurately described using a commonly used model within which the mobility depends only on the electric field, but that a consistent and quantitatively precise description of all curves can be obtained using the recently introduced extended Gaussian disorder model (EGDM). Within the EGDM, the mobility depends on the electric field and on the carrier concentration. Two physically interpretable parameters, viz. the width of the density of states, ?, and the density of transport sites, Nt, determine the shape of the curves. For the semiconductor studied, we find ?=0.13Ý0.01 eV and Nt = (6Ý1) × 1026 m-3. Consistent with the EGDM, the logarithm of the mobility in the low carrier concentration and low-field limit is found to show a 1/ T2 temperature dependence. It is shown that analyses which neglect the carrier-concentration dependence of the mobility yield an apparent 1/T temperature dependence, as reported for many different materials, and that the incorrectness of such an approach would readily follow from a study of the layer thickness dependence of the mobility.