Bulk electrical properties of rubrene single crystals: Measurements and analysis

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±0.02 eV above the valence band.