Field-effect transistors made from solution-processed organic semiconductors

We present results on metal-insulator-semiconductor field-effect transistors using conjugated organic semiconductors which can be processed from solution. The polymer poly(β′-dodecyloxy(-α,α′-α,α″-)terthienyl) is processed directly from solution whilst the polymer poly(thienylene vinylene) and the molecule pentacene are processed via soluble precursors. The operation mechanism of the transistors has been explained and expressions to extract mobility values and to interpret on/off ratios have been presented. The obtained transistor characteristics are explained in relation to the measured dopant concentrations, bulk conductivities and field-effect mobilities. All characteristics can be simulated using as input the transistor sheet conductivity as a function of the gate bias. Within a simple model the bulk conductivities and field-effect mobilities along with their temperature dependences are explained using variable-range hopping for heavily doped systems and polaronic thermally activated transport for lightly doped systems. This is a consequence of the density of states of conjugated systems which dynamically changes upon introduction of charge either by a field effect or through doping. The observed relationships for the various organic semiconductors are rationalized in a schematic mobility-conductivity plot. Transistors constructed from semiconductors processed from precursors display characteristics that allow for the construction of simple logic gates. Results from inverter, NOR and NAND gates and a simple ring oscillator circuit are shown. Switching frequencies of a few kHz have been achieved. The shelf-lives of devices are encouraging. Under stress operation, however, it is found that relaxation processes give rise to reversible current loss. This relaxation effect is intrinsic to the semiconductor. The origin may be related to the density of states for conjugated molecules being dynamic upon doping.