Christian Melzer

Supramolecular self-assembly and opto-electronic properties of semiconducting block copolymers

Abstract With continuous and nanometre-scale interpenetrating phases of electron donor and acceptor components, a novel diblock copolymer, in which one block is poly(p-phenylene vinylene) (PPV) and the other is a C60-functionalized polystyrene, is designed to be an efficient photovoltaic material. The synthesis involves the polymerization of a styrene derivative from a PPV-based macroinitiator via living free radical polymerization, and its subsequent functionalization with C60 via atom transfer radical addition. In selective solvents for the polystyrene block, aggregation is detected by means of optical spectroscopy and small-angle neutron scattering. Solid films exhibit honeycomb-structuring at the micrometre level when cast from CS2. As active layer in a device, the donor–acceptor diblock copolymer shows enhanced photovoltaic response relative to a blend of its constituent polymers.

Photovoltaic devices from fullerene oligophenyleneethynylene conjugates

Exploitation of the particular electronic properties of fullerenes for solar energy conversion has become a field of intensive investigation. The modification of semiconductor or metal electrodes with fullerene thin films has revealed promising photoelectrochemical properties. Good solar energy conversion efficiencies have also been obtained with sandwich-type conjugated polymer/fullerene heterojunctions. As part of this research, we have recently shown that fullerene derivatives, in which an oligophenylenevinylene (OPV) group is attached to C60, can be incorporated into photovoltaic cells. This molecular approach for solar energy conversion appears to be particularly interesting, since the bicontinuous network obtained by chemically linking the hole-conducting OPV moiety to the electron-conducting fullerene subunit prevents any problems arising from bad contacts at the junction, as observed for polymer/C60 blends. Furthermore, this new synthetic approach also offers great versatility for designed tuning of the photovoltaic system. We now report the preparation of plastic solar cells from fullerene ±oligophenyleneethynylene (OPE) derivatives 1 ±4. For compounds 1 and 2, the efficiencies of the resulting photovoltaic devices are similar to those prepared from corresponding fullerene ±OPV conjugates. Interestingly, by increasing the donating ability of the

Self-consistent analytical solution of a problem of charge-carrier injection at a conductor/insulator interface

We present a closed description of the charge-carrier injection process from a conductor into an insulator. Common injection models are based on single electron descriptions, being problematic especially once the amount of charge-carriers injected is large. Accordingly, we developed a model, which incorporates space-charge effects in the description of the injection process. The challenge of this task is the problem of self-consistency. The amount of charge carriers injected per unit time strongly depends on the energy barrier emerging at the contact, while at the same time the electrostatic potential generated by the injected charge carriers modifies the height of this injection barrier itself. In our model, self-consistency is obtained by assuming continuity of the electric displacement and the electrochemical potential all over the conductor/insulator system. The conductor and the insulator are properly taken into account by means of their respective density of state distributions. The electric-field distributions are obtained in a closed analytical form and the resulting current-voltage characteristics show that the theory embraces injection-limited as well as bulk-limited charge-carrier transports. Analytical approximations of these limits are given, revealing physical mechanisms responsible for the particular current-voltage behavior. In addition, the model exhibits the crossover between the two limiting cases and determines the validity of respective approximations. The consequences resulting from our exactly solvable model are discussed on the basis of a simplified indium tin oxide/organic semiconductor system.

High net gain at 514 nm in a photorefractive polymer doped with a chalcone derivative

We report on the photorefractive properties of a low Tg composite consisting of functionalized polysiloxane doped with a chalcone derivative. The high transparency of this doping molecule enabled the observation of high net gain at 514 nm. Orientational and Pockels contributions to the total refractive index variations were measured by frequency-dependent ellipsometry experiments. Finally, the field dependence of the gain coefficient is described using Kukhtarev’s model for the space charge field with an effective trap density as a single fitting parameter.

Complementary organic field effect transistors by ultraviolet dielectric interface modification

The realization of p- and n-type pentacene organic field effect transistors and an organic inverter stage is reported based on selective ultraviolet (UV) modification of the polymer dielectric in air. Apart from the UV radiation treatment, the device structures are identical. The achieved field effect carrier mobilities for both transistor types are ≈0.1cm2∕Vs. Similar performance data for both transistor types as well as an observed low current hysteresis qualify the UV treatment for organic complementary metal oxide semiconductor (O-CMOS) technology. The realized O-CMOS inverter exhibits stable operation below its supply voltage, as well as a gain of 17.

High mobility indium zinc oxide thin film field-effect transistors by semiconductor layer engineering.

Indium zinc oxide thin-film transistors are fabricated via a precursor in solution route on silicon substrates with silicon dioxide gate dielectric. It is found that the extracted mobility rises, peaks, and then decreases with increasing precursor concentration instead of rising and saturating. Investigation with scanning probe techniques reveals full thickness variations within the film which are assumed to adversely affect charge transport. Additional layers are coated, and the extracted mobility is observed to increase up to 19.7 cm(2) V(-1) s(-1). The reasons for this are examined in detail by direct imaging with scanning tunneling microscopy and extracting electron density profiles from X-ray reflection measurements. It is found that the optimal concentration for single layer films is suboptimal when coating multiple layers and in fact using many layers of very low concentrations of precursor in the solution, leading to a dense, defect and void free film, affording the highest mobilities. A consistent qualitative model of layer formation is developed explaining how the morphology of the film develops as the concentration of precursor in the initial solution is varied.

A color-tuneable organic light-emitting transistor.

An organic light-emitting field-effect transistor whose emission color can be changed by the applied voltage is presented. The transistor is based on a parallel layer stack of acenes serving as organic transport and emission layers. During operation, the position of the recombination zone can be moved by a proper change in the drain and gate bias from one organic semiconductor of the stack to the other, inducing a change in the emission color from green to red.

Self-consistent theory of unipolar charge-carrier injection in metal∕insulator∕metal systems

A consistent device model to describe current-voltage characteristics of metal∕insulator∕metal systems is developed. In this model the insulator and the metal electrodes are described within the same theoretical framework using density of states distributions. This approach leads to differential equations for the electric field which have to be solved in a self-consistent manner by considering the continuity of the electric displacement and the electrochemical potential in the complete system. The model is capable of describing the current-voltage characteristics of the metal∕insulator∕metal system in forward and reverse biases for arbitrary values of the metal∕insulator injection barriers. In the case of high injection barriers, approximations are provided offering a tool for comparison with experiments. Numerical calculations are performed exemplarily using a simplified model of an organic semiconductor.

Electroluminescence from a pentacene based ambipolar organic field-effect transistor

In this letter light emission from a pentacene-based ambipolar organic field-effect transistor is reported. Electroluminescence appears from a narrow region extending over the entire transistor channel width. The position of the light-emitting recombination zone is spatially controllable by the applied voltages. The current/voltage characteristics of the ambipolar device exhibit a hysteresis caused by trapped electrons in the transistor channel, which is also mirrored in the voltage dependent position of the light-emitting region. From the voltage dependence of the light intensity it can be concluded that Ohmic contacts are formed between pentacene and the utilized metal contacts calcium and gold.

Electron-hole pair mechanism for the magnetic field effect in organic light emitting diodes based on poly(paraphenylene vinylene)

We investigated the magnetic field effect (MFE) on current and electroluminescence in organic light emitting diodes based on poly(paraphenylene vinylene). The MFE was strictly positive in the full range of device operation and showed nonmonotonic dependencies on applied voltage and temperature. Furthermore, the MFE on current obtained in bipolar devices was significantly larger than in hole-dominated devices. We discuss our results in the framework of an electron-hole pair model and show that the model can explain all functional dependencies observed in our devices.