Eduard Johannes Meijer

Dopant density determination in disordered organic field-effect transistors

We demonstrate that, by using a concentric device geometry, the dopant density and the bulk charge-carrier mobility can simultaneously be estimated from the transfer characteristics of a single disordered organic transistor. The technique has been applied to determine the relation between the mobility and the charge density in solution-processed poly(2,5-thienylene vinylene) and poly(3-hexyl thiophene) thin-film field-effect transistors. The observation that doping due to air exposure takes place already in the dark, demonstrates that photoinduced oxygen doping is not the complete picture.

Organic complementary-like inverters employing methanofullerene-based ambipolar field-effect transistors

We demonstrate a complementary-like inverter comprised of two identical ambipolar field-effect transistors based on the solution processable methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). The transistors are capable of operating in both the p- and n-channel regimes depending upon the bias conditions. However, in the p-channel regime transistor operation is severely contact limited. We attribute this to the presence of a large injection barrier for holes at the Au∕PCBM interface. Despite this barrier the inverter operates in both the first and third quadrant of the voltage output versus voltage input plot exhibiting a maximum gain in the order of 20. Since the inverter represents the basic building block of most logic circuits we anticipate that other complementary-like circuits can be realized by this approach.

Switch-on voltage in disordered organic field-effect transistors

The switch-on voltage for disordered organic field-effect transistors is defined as the flatband voltage, and is used as a characterization parameter. The transfer characteristics of the solution processed organic semiconductors pentacene, poly(2,5-thienylene vinylene) and poly(3-hexyl thiophene) are modeled as a function of temperature and gate voltage with a hopping model in an exponential density of states. The data can be described with reasonable values for the switch-on voltage, which is independent of temperature. This result also demonstrates that the large threshold voltage shifts as a function of temperature reported in the literature constitute a fit parameter without a clear physical basis.

Scaling behavior and parasitic series resistance in disordered organic field-effect transistors

The scaling behavior of the transfer characteristics of solution-processed disordered organic thin-film transistors with channel length is investigated. This is done for a variety of organic semiconductors in combination with gold injecting electrodes. From the channel-length dependence of the transistor resistance in the conducting ON-state, we determine the field-effect mobility and the parasitic series resistance. The extracted parasitic resistance, typically in the MΩ range, depends on the applied gate voltage, and we find experimentally that the parasitic resistance decreases with increasing field-effect mobility.

Local charge carrier mobility in disordered organic field-effect transistors

In conventional field-effect transistors, the extracted mobility does not take into account the distribution of charge carriers. However, in disordered organic field-effect transistors, the local charge carrier mobility decreases from the semiconductor/insulator interface into the bulk, due to its dependence on the charge carrier density. It is demonstrated that the conventional field-effect mobility is a good approximation for the local mobility of the charge carriers at the interface.

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.

Frequency behavior and the Mott–Schottky analysis in poly(3-hexyl thiophene) metal–insulator–semiconductor diodes

Metal–insulator–semiconductor diodes with poly(3-hexyl thiophene) as the semiconductor were characterized with impedance spectroscopy as a function of bias, frequency, and temperature. We show that the standard Mott–Schottky analysis gives unrealistic values for the dopant density in the semiconductor. From modeling of the data, we find that this is caused by the relaxation time of the semiconductor, which increases rapidly with decreasing temperature due to the thermally activated conductivity of the poly(3-hexyl thiophene).

Doping kinetics of organic semiconductors investigated by field-effect transistors

The kinetics of acid doping of the semiconductor regioregular poly-3-hexylthiophene with vaporized chlorosilane have been investigated using field-effect transistors. The dopant density has been derived as a function of temperature and exposure time from the shift in the pinch-off voltage, being the gate bias where current starts to flow. The doping kinetics are perfectly described by empirical stretched exponential time dependence with a saturation dopant density of 1±0.5× 10 26 m -3 and a thermally activated relaxation time. We show that a similar relationship holds for previously reported kinetics of poly-thienylene-vinylene doped with molecular oxygen.

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.

Hole transport in polymeric field-effect transistors and light-emitting diodes

The hole transport in the amorphous poly(2-methoxy-5-(3’,7’-dimethyloctyloxy)-p-phenylene vinylene) (OC1C10-PPV) and in the more ordered poly[2,5-bis(3’,7’-dimethyloctyloxy)-p-phenylene vinylene] (OC10C10-PPV) has been investigated both in field-effect transistors (FETs) and light-emitting diodes (LEDs) as function of temperature and applied voltage. From J-V measurements on LEDs a difference of 15x has been found in the hole mobility between OC1C10-PPV and OC10C10-PPV. In FETs the dependence of the field-effect mobility on the carrier density is much stronger in OC1C10-PPV than in OC10C10-PPV. These differences in the mobility in both FETs and LEDs are determined by the difference in microscopic transport parameters between the two materials, which results from a different ordering in the polymeric film of the PPV derivatives. Due to their specific chemical composition OC1C10-PPV is an amorphous polymer and the transport is the same in all directions, while OC10C10-PPV is more ordered and the transport shows anisotropy between sandwich and in-plane devices.