Sabine Ludwigs

Electrochemistry of conducting polymers--persistent models and new concepts.

2.2. Cathodic Electropolymerization 4732 2.2.1. Electropolymerization of PPXs and PPVs 4732 3. Charging-Discharging of Conducting Polymers 4733 3.1. Redox Properties of Oligomers and Polymers 4733 3.2. Specific Phenomena of n-Doping 4739 3.3. Conductivity in Charged Systems 4740 4. Controlling the Electropolymerization Process 4742 4.1. Influence of the Polymerization Technique 4742 4.2. Influence of Experimental Conditions 4743 4.3. Electropolymerization in Novel Electrolytic Media 4745

A Bicontinuous Double Gyroid Hybrid Solar Cell

We report the first successful application of an ordered bicontinuous gyroid semiconducting network in a hybrid bulk heterojunction solar cell. The freestanding gyroid network is fabricated by electrochemical deposition into the 10 nm wide voided channels of a self-assembled, selectively degradable block copolymer film. The highly ordered pore structure is ideal for uniform infiltration of an organic hole transporting material, and solid-state dye-sensitized solar cells only 400 nm thick exhibit up to 1.7% power conversion efficiency. This patterning technique can be readily extended to other promising heterojunction systems and is a major step toward realizing the full potential of self-assembly in the next generation of device technologies.

Anisotropic Charge Transport in Spherulitic Poly(3-hexylthiophene) Films

Poly(3-hexylthiophene) (P3HT) is one of the most important semiconducting polymers for organic photovoltaics and optoelectronics.[1] Yet, because of the structural complexity of semicrystalline polymers, comparatively little is known conclusively about the relationship between charge transport and microstructure in P3HT-containing semiconductors.[2–9] This lack of information arises mainly from an inability to manufacture samples with spontaneous long-range crystalline order approaching macroscopic dimensions, that allow characterisation in a similar fashion to inorganic single crystals. We have overcome this limitation and grown well-ordered P3HT spherulites, sufficiently large to measure charge transport within single oriented crystal domains, enabling measurements of the charge mobility both parallel and perpendicular to the characteristic π-stacked nanocrystalline lamellae and across spherulite boundaries. The importance of local order and molecular orientation in determining the electronic properties of polymer semiconductor films is now well established.[3] In the case of the poly(alkylthiophenes) (PAT)s an edge-on orientation of the molecules, such that the alkyl sidechains (a-axis) stand vertically and both the chain backbone (c-axis) and π-stacking direction (b-axis) lie in the plane of the film, is critical in achieving high in-plane

On the Efficiency of Charge Transfer State Splitting in Polymer:Fullerene Solar Cells

The field dependence and yield of free charge carrier generation in polymer:fullerene blends with varying energetic offsets is not affected when the excitation energy is varied from above band-gap to direct CT state excitation. Instead, the ability of the CT state to split is dictated by the energetic offset between the relaxed CT state and the charge separated (CS) state.

Morphology of P3HT in Thin Films in Relation to Optical and Electrical Properties

The search for renewable and environmentally friendly energy sources has made organic electronics an interesting field of research. Semiconducting polymers stand out because they offer cheap and easy processability at a large- scale from solution, combined with impressive optoelectronic properties. Polythiophenes, in particular poly(3-hexylthiophene) (P3HT), are the most prom- inent and investigated representatives of semiconducting polymers and have been applied in various devices such as solar cells and field-effect transistors. For this class of polymers, it has been well established that the morphology of the functional layer has a significant impact on the device performance. However, transport bottlenecks are hard to determine due to the complex semicrystalline microstruc- ture, which is composed of a mixture of crystalline and amorphous domains. In order to gain a deeper understanding of the correlation between microstructure and functional properties, precise control of nucleation and growth of semicrystalline polymers such as P3HT is crucial. This article gives an overview of recent publi- cations addressing the morphology and crystallization of regioregular P3HT, both in solution and thin film, and attempts to correlate these structural features to the functional (i.e. optical and electrical) properties of the polymer.

Electrochemically Induced Reversible and Irreversible Coupling of Triarylamines

The electrochemical coupling and dimerization behavior of the low molecular compounds triphenylamine (TPA) and 9-phenylcarbazole (PHC) in comparison to tri-p-tolylamine (p-TTA) with para-blocked methyl groups has been investigated in detail. In contrast to the unsubstituted radical cations of TPA and PHC, the radical cations of p-TTA are stable in the radical cation state and do not undergo any further coupling reactions. However, we found that the dicationic state of p-TTA does undergo two different competitive reaction pathways: (1) an irreversible intramolecular coupling reaction which leads to phenylcarbazole moieties and (2) a reversible intermolecular dimerization leading to charged σ-dimers. The σ-dimers become decomposed upon discharging at low potentials (E(pc) = -0.97 V vs Fc/Fc(+)) so that the starting monomer p-TTA is partially regenerated. In particular, the reversible dimerization reaction has not been described in literature so far. Polymeric systems containing para-methyl blocked triarylamines in the side chain exhibit similar coupling behavior upon electrochemical doping.

Room temperature vacuum-induced ligand removal and patterning of ZnO nanoparticles: from semiconducting films towards printed electronics

In this manuscript we present a new approach for the fabrication of ZnO nanoparticle based semiconducting thin films. The films are obtained by spin coating of stable nanoparticle dispersions with low boiling point ligands. As the ligands might hinder efficient charge transport between the particles in electronic devices, we present a method to remove them by a vacuum-induced “sintering” process of the particles at room temperature. Amine stabilized ZnO nanoparticles were obtained by the decomposition of diethylzinc (Et2Zn) in the presence of amines. If butylamine is used as the ligand, NMR and XPS measurements show that complete removal of butylamine can be achieved by storing the nanoparticles in vacuum overnight. Ligand removal leads to electronic interparticle contact as measured with field effect transistors. The ability to process at room temperature makes this approach highly interesting for temperature-sensitive substrates. The potential of our approach for printed electronics is further shown by patterning nanoparticle dispersions via micro-injection moulding in capillaries as a soft lithographic method.

Optoelectronic properties of hyperbranched polythiophenes.

Branched conjugated architectures should possess the advantage of isotropic charge transport compared to conventional linear conjugated polymers, as for example poly(3-hexylthiophene) (P3HT) which is commonly used in organic solar cells. This contribution investigates the optoelectronic properties of branched poly(thiophene)s p3T and p4T synthesized in a straightforward one-pot procedure by oxidative coupling of branched trithiophene and tetrathiophene monomers with FeCl(3). These polymers can be regarded as model systems for ideal amorphous conjugated materials. Optical characterization in solution and in thin films together with cyclic voltammetry data suggests the applicability of these materials for the use in organic solar cell devices. In particular, the HOMO and LUMO levels of the branched polythiophenes are shifted to lower energy values as compared to linear P3HT. Field effect mobilities are in the order of 10(-4) cm(2)/(V s). A first optimization of solar cell devices based on the branched polythiophene materials in combination with PCBM as acceptor resulted in efficiencies of 0.6% with open-circuit voltages being about 30% higher (up to 714 mV) than normally found with P3HT.

Control of gyroid forming block copolymer templates: effects of an electric field and surface topography

The control over 10 nm scale porosity derived from self-assembly of copolymers is an extremely promising method for the synthesis of organic–inorganic hybrid materials applied, for example, in solar cells. Here, we report the thin film behaviour of a poly(4-fluorostyrene)-b-poly(D,L-lactide) PFS-b-PLA block copolymer which adopts the bicontinuous gyroid phase in the bulk and may be used to form a porous template suitable for patterning functional materials by selective degradation of the minority PLA domains. The response of the copolymer morphology to DC electric fields is probed at temperatures where the bulk copolymer adopts either the gyroid or the cylindrical phase. At 150 °C electric field alignment results in vertical arrays of cylinders, lamellae, and perforated lamellae while at 180 °C the gyroid phase coexists with a standing perforated lamellar phase. We show that both polymer–substrate interactions and substrate topography are critical factors determining substrate reconstruction of the gyroid phase. Spontaneous cross-film percolation of the minority network phase on a given substrate, a prerequisite for electrochemical replication, is dependent on surface topology at the scale of the gyroid unit cell. Importantly, under suitable processing conditions all these complex copolymer morphologies can be electrochemically replicated to produce highly ordered freestanding nanostructured arrays over large areas.

P3HT Revisited – From Molecular Scale to Solar Cell Devices

Progress in the synthesis of poly(3-hexylthiophene).- Morphology of P3HT in Thin Films in Relation to Optical and Electrical Properties.- Epitaxy: a versatile method to orient and nanostructure P3HT.- P3HT and other polythiophene field-effect transistors.- Morphology and charge transport in poly(3-hexylthiophene): A theorists Perspective.- P3HT-based solar cells: Structural properties and photovoltaic performance.