Controlling charge injection properties in polymer field-effect transistors by incorporation of solution processed molybdenum trioxide

Abstract

A simply and facilely synthesized MoO₃ solution was developed to fabricate charge injection layers for improving the charge-injection properties in p-type organic field-effect transistors (OFETs). By dissolving MoO₃ powder in ammonium (NH₃) solvent under an air atmosphere, an intermediate ammonium molybdate ((NH₄)₂MoO₄) precursor is made stable, transparent and spin-coated to form the MoO₃ interfacial layers, the thickness and morphology of which can be well-controlled. When the MoO₃ layer was applied to OFETs with a cost-effective molybdenum (Mo) electrode, the field-effect mobility (μ_FET) was significantly improved to 0.17 or 1.85 cm² V⁻¹s⁻¹ for polymer semiconductors, regioregular poly(3-hexylthiophene) (P3HT) or 3,6-bis-(5bromo-thiophen-2-yl)-N,N′-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole (DPPT-TT), respectively. Device analysis indicates that the MoO₃-deposited Mo contact exhibits a contact resistance R_C of 1.2 MΩ cm comparable to that in a device with the noble Au electrode. Kelvin-probe measurements show that the work function of the Mo electrode did not exhibit a dependence on the thickness of MoO₃ film. Instead, ultraviolet photoemission spectroscopy results show that a doping effect is probably induced by casting the MoO₃ layer on the P3HT semiconductor, which leads to the improved hole injection.