Rubrene single crystals with well-developed (100) faces extending along the [010] direction were equipped with two planar gold electrodes deposited under low thermal load on the same crystal surface. The current-voltage curves measured on crystals with various thicknesses were analyzed by means of the space-charge-limited current model. In all cases, the curves present a clear trap-filled transition that allows extracting the trap-free mobility along the $b$ axis. Working in a gap-type geometry and by comparing the mobility calculated from the Geurst two-dimensional and Mott–Gurney three-dimensional models, a clear 2D to 3D transition is established for a thickness that roughly corresponds to half the distance between the two electrodes. Further quantitative analysis of the data with a differential method leads to the conclusion that charge-transport properties at room temperature are affected by a discrete trap level pointing at $0.48\pm 0.02\mathrm{eV}$ above the valence band.

• Received 30 July 2007

DOI:https://doi.org/10.1103/PhysRevB.77.115205