Electronic charge injection and transport in organic field-effect transistors

Electronic devices based on organic semiconductors, such as field-effect transistors (FETs) and light emitting diodes have attracted much interest as possible inexpensive and flexible alternatives to inorganic devices. Despite considerable improvement in device properties, a better understanding of the nature of charge transport in these devices and the physics of contacts is crucial to further development of optoelectronic organic devices. This work outlines our findings in understanding and characterizing the injection and transport mechanisms of charge carriers in solution processed poly (3-hexylthiophene) (P3HT) field-effect devices. We measured hole transport in P3HT FETs with Au electrodes at submicron channel lengths as a function of gate voltage and a wide range of temperatures. The strongly nonlinear and gate modulated transport is shown to be consistent with a model of Poole-Frenkel-like hopping mechanism in the space-charge limited current regime. Charge injection from different source/drain electrodes such as Au, Cu and Cr was examined over a broad temperature range, and the contact current-voltage characteristics were extracted from the dependence of conductance on channel length. The differences between linear vs. nonlinear charge injection were carefully studied and compared to recently developed models of charge injection. In addition, the effect of doping-dependent charge injection in devices with Au and Pt contacts was studied, revealing large contact resistances and marked non-Ohmic transport at low dopant concentrations. Ultraviolet photoemission spectroscopy (UPS) reveals that metal/P3HT band alignment is rearranged as samples are dedoped, leading to an increased injection barrier for holes, with a greater shift for Au/P3HT. We also performed a study using dipole-containing self-assembled monolayers on the Au source and drain electrodes to strongly manipulate the charge injection process across the metal/organic interface. We have shown that chemically increasing the injecting electrode work function significantly improves hole injection relative to untreated Au electrodes.