FIELD-EFFECT TRANSISTORS MADE FROM SOLUTION-PROCESSED ORGANIC SEMICONDUCTORS

We present results on metal-insulator-semiconductor field-effect trans istors using conjugated organic semiconductors which can be processed from solution. The polymer odecyloxy(-alpha,alpha'-alpha,alpha''-)tert hienyl) is processed directly from solution whilst the polymer poly(th ienylene vinylene) and the molecule pentacene are processed via solubl e precursors. The operation mechanism of the transistors has been expl ained and expressions to extract mobility values and to interpret on/o ff 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 b e simulated using as input the transistor sheet conductivity as a func tion of the gate bias. Within a simple model the bulk conductivities a nd field-effect mobilities along with their temperature dependences ar e explained using variable-range hopping for heavily doped systems and polaronic thermally activated transport for lightly doped systems. Th is is a consequence of the density of stares of conjugated systems whi ch dynamically changes upon introduction of charge either by a held ef fect or through doping. The observed relationships for the various org anic semiconductors are rationalized in a schematic mobility-conductiv ity plot. Transistors constructed from semiconductors processed from p recursors display characteristics that allow for the construction of s imple logic gates. Results from inverter, NOR and NAND gates and a sim ple ring oscillator circuit are shown. Switching frequencies of a few kHz have been achieved. The shelf-lives of devices are encouraging. Un der stress operation, however, it is found that relaxation processes g ive rise to reversible current loss. This relaxation effect is intrins ic to the semiconductor. The origin may be related to the density of s tates for conjugated molecules being dynamic upon doping.