C. Tanase

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.

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.

Thickness scaling of the space-charge-limited current in poly(p-phenylene vinylene)

Charge transport in light-emitting diodes (LEDs) based on a poly(p-phenylene vinylene) (PPV) derivative is investigated as a function of sample thickness. Via the thickness dependence, the contributions from the electric field and charge carrier density to the mobility in space-charge-limited (SCL) diodes can be disentangled. It is demonstrated that a field-dependent mobility weakens the thickness dependence of the SCL current, whereas a carrier-density-dependent mobility gives rise to an enhanced thickness dependence. The enhanced thickness dependence of the experimental SCL current in PPV is in agreement with the predictions using a density-dependent mobility only. This observation confirms that in PPV-based LEDs, the hole transport is dominated by filling of the localized states.

Integrated complementary-like circuits based on organic ambipolar transistors

We report on an approach towards integrated complementary-like circuits based on organic ambipolar transistors. In particular, we show that ambipolar transport can be achieved within a single transistor channel using gold electrodes and a solution processable polymer-small molecule blend as the electroactive material. To demonstrate the suitability of these devices for practical utilisation in logic circuits we realise complementary-like voltage inverters comprised entirely of ambipolar transistors. Moreover, by integrating several such inverters we are able to demonstrate more complex circuits such as ring oscillators.

Charge transport in disordered organic field-effect transistors

In this thesis we study charge transport in organic semiconductors. We do this by focusing on the physical characterization of disordered organic field-effect transistors. It will be made clear that the disorder in the polymer films is crucial for the interpretation of the data. The field-effect transistor geometry allows variation of the charge carrier density in the semiconductor, without the presence of counter ions. Therefore, the transistor allows a rather clean study of the charge transport in organic semiconductors as a function of the charge carrier density and temperature. In the experiments we find that the organic transistors are in several respects not comparable to silicon MOSFETs. Therefore, in this thesis we redefine and re-evaluate basic transistor parameters, such as the threshold voltage, the field-effect mobility, the contact resistance and the dopant density. Subsequently, we study the charge transport as a function of charge density, temperature and electric field, giving insight into the charge transport mechanism. Based on our observations we propose as the main charge transport mechanism: multiphonon hopping of polaronic charge carriers in a Gaussian density of states. We investigate the electrical stability of the polymer layer in metal-insulator-semiconductor diodes, where we determine and analyse the dopant density changes as a function of oxygen and light exposure. The presence of contact resistances in the transistors is addressed by analysing the scaling behavior of the electrical characteristics as a function of the transistor channel length, and an empirical relation between the contact resistance and the charge carrier mobility in the polymer layer is observed. Finally, we discuss why typically only unipolar transistor behavior is observed experimentally, and we demonstrate ambipolar transistor behavior in organic field-effect transistors based on blends of organic semiconductors and on low band gap organic semiconductors.

Enhanced hole transport in poly(p-phenylene vinylene) planar metal-polymer-metal devices

In planar metal-poly(p-phenylene vinylene) (PPV)-metal devices the experimental current is five to six orders of magnitude larger as compared to the expected space-charge limited current. Comparing these measurements with field-effect transistors demonstrates that the enhanced current originates from a high surface charge carrier density at the polymer/substrate interface. This surface charge is found to be only weakly dependent on the substrate, device geometry, and chemical treatment of the substrate. The presence of such a conducting channel due to charging of the surface obscures the intrinsic in-plane conducting properties of PPV.

Hole transport in poly(p-phenylene vinylene) based light-emitting diodes revisited

Understanding of the charge transport properties is of great importance for the operation and the efficiency of polymer based light-emitting diodes (LEDs). We investigate the charge transport in hole-only diodes based on poly(p-phenylene vinylene) (PPV) as function of temperature T, charge carrier density p and electric field E. At low voltages the hole mobility is independent on the electric field and charge carrier density. At high voltages both the charge-carrier density and electric-field dependence of the mobility have to be taken into account to describe the hole transport in polymer LEDs.

Enhancement of the hole transport in poly(p-phenylene vinylene) based light-emitting diodes

The hole transport in various poly(p-phenylene vinylene) (PPV) derivatives has been investigated in hole-only diodes as function of temperature T and applied electric field E. A difference of three decades has been found in the hole mobility between a random copolymer with asymmetric sidechains and a PPV-derivative with symmetric sidechains. The temperature dependence of the hole mobility has been analysed within the correlated Gaussian disorder model. The large differences in the mobility values of these PPV derivatives are governed by a strong decrease of the energetic disorder. The high mobility PPV-based polymers are interesting candidates for being used as hole transport layers in heterojunction light-emitting diodes.

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.