Marco Matters

Field-effect transistors made from solution-processed organic semiconductors

Abstract 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.

Bias-stress induced instability of organic thin film transistors

We have investigated the stability of polythienylene vinylene field-effect transistors under gate bias stress. On time scales up to 1000 s and temperatures up to 140 °C, we only observe reversible charge relaxation effects and no degradation. We show the time dependence of the threshold voltage shift at different temperatures. Furthermore, we discuss the influence of water and oxygen on the relaxation process.

The Meyer–Neldel rule in organic thin-film transistors

We have measured and analyzed the temperature and gate voltage dependencies of the field-effect mobility in organic thin-film transistors. We find that the mobility prefactor increases exponentially with the activation energy in agreement with the Meyer–Neldel rule. This behavior is demonstrated in the mobility data of solution-processed pentacene, poly(2,5-thienylene vinylene) and in mobility data reported in literature. Surprisingly, the characteristic Meyer–Neldel energy for all analyzed materials is close to 40 meV. Possible implications for the charge transport mechanism in these materials are discussed.

Organic field-effect transistors and all-polymer integrated circuits

Electrical properties of field-effect transistors made of different solution processable organic semiconductors are described. The temperature and gate-voltage dependence of the mobility is shown and theoretically described using a model based on the variable-range hopping of charge carriers in an exponential density of states. Furthermore, a technology has been developed to make all-polymer integrated circuits. It involves reproducible fabrication of field-effect transistors on flexible substrates, where the semiconducting, conducting and insulating parts are all made of polymers. Integrated circuits consisting of more than 300 field-effect transistors are demonstrated.

Polymeric integrated circuits and light-emitting diodes

All-polymer flexible integrated circuits have been fabricated using a potentially low-cost technology. Combination with commodity anti-theft stickers has led to functional radio frequent identification transponders. Polymeric light-emitting diodes (PLEDs) opened the way to cost efficient, large area display-applications. Directions for further enhancement of their performance are presented.

Low-cost all-polymer integrated circuits

A technology has been developed to make all-polymer integrated circuits. It involves reproducible fabrication of field-effect transistors in which the semiconducting, conducting and insulating parts are all made of polymers. The fabrication on flexible substrates uses spin-coating of electrically active precursors and patternwise exposure of the deposited films. In the whole process stack-integrity is maintained. Vertical interconnects are made mechanically. As a demonstrator for the technology functional 15-bit programmable code generators are fabricated. These circuits still operate when the foils are sharply bent. Due to the limited number of process steps the technology is potentially inexpensive.

The isokinetic temperature in disordered organic semiconductors

We have investigated the field dependence of the in-plane conductivity in poly(2,5-thienylene vinylene) thin films. The conductivity is found to have a square root dependence on the lateral electric field with values of the activation energy, Δ=0.46 eV, B=2.3.10 -5 eV(m/V) 1/2 and the characteristic temperature T 0 =5.2.10 2 K. A similar value (T 0 =4.9.10 2 K) is found for the isokinetic temperature in Meyer-Neldel experiments on poly(2,5-thienylene vinylene) field-effect transistors. Based on these results, we argue that entropy changes due to hopping of charge carriers should be incorporated in theoretical descriptions of the field dependent mobility in disordered organic semiconductors.