Improving charge injection in high-mobility rubrene crystals: From contact-limited to channel-dominated transistors

摘要

With progressively improving charge carrier mobility in organic semiconductors and miniaturization of organic field-effect transistors (OFETs), low contact resistances become ever more important. To study the capabilities of metal electrodes in OFETs and to explore the transition from contact-limited to channel-dominated transistor operation, we used flip-crystal FETs with gold electrodes having different contact resistances R_c to high-quality rubrene crystals. 4-terminal transfer and output measurements reveal that R_c decreases from 10~5-10~6 Ω cm for 15 min air exposure to 3 × 10~3 Ωcm for at least 5 h air exposure of the gold electrodes before the flip-crystal FET is assembled. We conclude the reduction of R_c to be caused by a growing contamination layer on the gold electrodes that weakens the electrostatic coupling between rubrene crystal and gold electrode, and lowers the Schottky contact diode parameter V_0. In channel-dominated (low R_c) FETs, the mobility is in the range of 10-17 cm~2/(Vs); in contrast, in contact-limited (high R_c) FETs, the apparent mobility decreases significantly with increasing contact resistance. The apparent μ - R_c dependence is not intrinsic, but rather the result of incorrect assumptions of the potential and the charge carrier density in the channel region. Thus, the development of high-mobility organic semiconductors requires further efforts to improve contacts bevond traditional metal electrodes.