Jochen S. Gutmann

Template-Assisted Fabrication of Free-Standing Nanorod Arrays of a Hole-Conducting Cross-Linked Triphenylamine Derivative: Toward Ordered Bulk-Heterojunction Solar Cells

Free-standing nanorod arrays of a thermally cross-linked semiconducting triphenylamine were fabricated on conductive ITO/glass substrates via an anodic aluminum oxide (AAO) template-assisted approach. By using a solution wetting method combined with a subsequent thermal imprinting step to fill the nanoporous structure of the template with a cross-linkable triphenylamine derivative, a polymeric replication of the AAO was obtained after thermal curing and selective removal of the template. To obtain well-aligned and free-standing nanorod arrays, aggregation and collapse of the nanorods were prevented by optimizing their aspect ratio and applying a freeze-drying technique to remove the aqueous medium after the etching step. Because of their electrochemical properties and their resistance against organic solvents after curing, these high density nanorod arrays have potential application in organic photovoltaics.

Fabrication and characterization of nanostructured titania films with integrated function from inorganic–organic hybrid materials

Nanostructured titania films are of growing interest due to their application in future photovoltaic technologies. Therefore, a lot of effort has been put into the controlled fabrication and tailoring of titania nanostructures. The controlled sol-gel synthesis of titania, in particular in combination with block copolymer templates, is very promising because of its high control on the nanostructure, easy application and cheap processing possibilities. This tutorial review gives a short overview of the structural control of titania films gained by using templated sol-gel chemistry and shows how this approach is extended by the addition of further functionality to the films. Different expansions of the sol-gel templating are possible by the fabrication of gradient samples, by the addition of a homopolymer, by the combination with micro-fluidics and also by the application of novel precursors for low-temperature processing. Moreover, hierarchically structured titania films can be fabricated via the subsequent application of several sol-gel steps or via the inclusion of colloidal templates in a one-step process. Integrated function in the block copolymer used in the sol-gel synthesis allows for the fabrication of an integrated blocking layer or an integrated hole-conductor. Both approaches grant a one-step fabrication of two components of a working solar cell, which make them very promising towards a cheap solar cell production route. Looking to the complete solar cell, the top contact is also of great importance as it influences the function of the whole solar cell. Thus, the mechanisms acting in the top contact formation are also reviewed. For all these aspects, characterization techniques that allow for a structural investigation of nanostructures inside the active layers are important. Therefore, the characterization techniques that are used in real space as well as in reciprocal space are explained shortly as well.

Templated Organic and Hybrid Materials for Optoelectronic Applications

The review highlights different approaches to template organic materials as well as hybrid materials that find or are expected to find application in optoelectronic devices. The first templating approach focuses on the use of preformed nanoporous membranes as templates for organic materials and polymeric materials. Such nanoporous templates can be track-etched membranes, anodic aluminum oxide membranes and other variants thereof, or block copolymer templates. Further, opals have been described as templates. In the second part, we have summarized developments that take advantage of self-assembly processes to pattern hybrid materials. Examples are sol-gel templating techniques using amphiphiles, evaporation-induced self-assembly, lyotropic templating as well as templating from block copolymers. Both routes are very promising templating approaches for optoelectronic materials and represent complementary rather than competing techniques.

Hierarchically Structured Titania Films Prepared by Polymer/Colloidal Templating

Hierarchically structured titania films for application in hybrid solar cells are prepared by combining microsphere templating and sol-gel chemistry with an amphiphilic diblock copolymer as a structure-directing agent. The films have a functional structure on three size scales: (1) on the micrometer scale a holelike structure for reduction of light reflection, (2) on an intermediate scale macropores for surface roughening and improved infiltration of a hole transport material, and (3) on a nanometer scale a mesoporous structure for charge generation. Poly(dimethyl siloxane)-block-methyl methacrylate poly(ethylene oxide) (PDMS-b-MA(PEO)) is used as a structure-directing agent for the preparation of the mesopore structure, and poly(methyl methacrylate) (PMMA) microspheres act as a template for the micrometer-scale structure. The structure on all levels is modified by the method of polymer extraction as well as by the addition of PMMA particles to the sol-gel solution. Calcination results in structures with increased size and a higher degree of order than extraction with acetic acid. With addition of PMMA a microstructure is created and the size of the mesopores is reduced. Already moderate microstructuring results in a strong decrease in film reflectivity; a minimum reflectivity value of less than 0.1 is obtained by acetic acid treatment and subsequent calcination.

Phosphonated Hexaphenylbenzene: A Crystalline Proton Conductor

Proton conductivity has been widely studied because of its importance in biological and chemical processes. A fuel cell (FC) is a promising device that can provide electrical energy with high efficiency and low environmental impact. A critical issue that severely hampers FC performance is the synthesis of proton-exchange membranes (PEMs) that simultaneously provide high proton conductivity that is constant over temperature. In the case of automotive applications, a guideline of close to 1 10 1 Scm 1 for the proton conductivity of the membrane at 120 8C and 50 % relative humidity (RH) was established by the U.S. Department of Energy as target operating conditions. State-of-the-art polymeric electrolytes are sulfonic acid based perfluorinated polymers such as Nafion. These electrolytes present high but temperaturedependent proton conductivity, since proton transport is governed by the vehicle mechanism that is based on the diffusion of proton-containing groups. Alternatively, phosphonic acid has been suggested as a protogenic group for intrinsically conducting separator materials because of its amphoteric properties. It has been demonstrated that a high concentration of these acidic groups, which are able to aggregate, is required for a high intrinsic proton conductivity. Inorganic crystals (solid acid proton conductors) have been proposed as alternative materials to polymer electrolytes. However, despite their high intrinsic conductivities (10 –10 3 S cm ; Grotthuss-type mechanism), these crystals have certain disadvantages, such as poor mechanical properties, water solubility, and high-temperature operating conditions (above 230 8C under atmospheric pressure for CsH2PO4). [6] Although research has focused to date on increasing the flexibility of the protogenic groups, for example, by introducing spacers or by adding small molecules, we have followed a different approach, in which we proposed to increase proton mobility by using a self-assembly and preorganization concept. Herein, organic crystals of small molecules are suggested as an alternative to common polymeric electrolytes and inorganic crystals employed as PEM in FC systems. Although a lot of effort has been made to investigate inorganic crystals, very little information on entirely organic crystals has been reported to date. The crystal structure of hexaphenylbenzene (HPB) and its derivatives have been known for a long time. Most recently, crystallographic studies on acidic derivatives have been carried out. It was found that almost all the molecules present multiple hydrogen bonds in the molecular plane and additionally form hydrogen bonds between adjacent sheets in such a way that columnar supramolecular networks are formed. Inspired by the properties of phosphonic acids, as well as by the supramolecular self-assembly of HPB derivatives, we introduced phosphonic acid groups into the nonplanar structure of HPB. Hexakis(p-phosphonatophenyl)benzene (p-6PA-HPB) was synthesized in a three-step reaction (Scheme 1). Powder X-ray measurements at different temperatures and RH values were performed (see Figure 1 and Figures S4 and S5 in the Supporting Information). It was found that p6PA-HPB is crystalline and that the local order is only slightly affected by changing these parameters. These changes are attributed to small local packing variations probably caused by the evaporation of water (see Figure 1).

Durable press finishing of cotton fabrics: An overview

Durable press (DP) or easy care finishing is almost always used for cotton fabrics or textiles with a high content of cellulosic fibers. This finish provides resistance against shrinkage and improved wet and dry wrinkle recovery to cellulosic textiles. Inhibition of easy movement of the cellulose chains by crosslinking with resins/polymers is the mechanism of a DP finish. Initially, derivatives of urea such as urea-formaldehyde and melamine-formaldehyde resins were used. Environmental concerns and the potential danger of formaldehyde led to the introduction of formaldehyde-free finishes. Among them, polycarboxylic acids such as 1,2,3,4-butanetetracarboxylic acids and citric acids are the most promising chemicals. To enhance the flexibility, tensile strength and whiteness of the easy care finished textiles, novel finishing agents have been recently considered; for example, ionic crosslinking, polyamino carboxylic acids and non-ionic polyurethane, as well as employing nano-materials as the catalyst or co-catalyst. The possible application of the easy care treatment with other functional finishes, mainly antimicrobial, flame retardancy and water–oil repellency, has been also been focused upon.

Sponge-like structures for application in photovoltaics

Large surface areas at an interface between two different materials are desired in many research fields where the interaction between these materials significantly affects the performance of the physical system. This behaviour is illustrated on sponge-like structures, which assign for such a high surface area, and demonstrate the development from bulk material to thin films and a variety of applications. The focus is on sponge-like nanostructures consisting of a network of aggregated titania nanoparticles applied in hybrid structures for photovoltaics. Examples based on a sol–gel process for the preparation of titania nanostructures in thin films, mimicking the sponge morphology, are shown. In general, titania films are widely used in photovoltaics, contributing to a large surface area available for interfacial reactions, e.g. charge carrier transfer routes. Interpenetrating networks with dimensions matching exciton diffusion lengths in the polymer component of a hybrid organic–inorganic photovoltaic structure are highly desirable. To characterize the fabricated morphology, atomic force microscopy and field-emission scanning electron microscopy are employed in real space. The advanced scattering technique of grazing-incidence small-angle X-ray scattering complements the characterization in reciprocal space. From the obtained results, the sponge-like morphology is verified, a physical description of the morphology with statistical relevance is constructed and the successful complete filling of the network is shown. According to this description, the presented sponge-like titania nanostructures are well suited for use in hybrid organic–inorganic solar cells.

Permanent flame retardant finishing of textiles by allyl-functionalized polyphosphazenes.

Despite their excellent flame retardant properties, polyphosphazenes are currently not used as flame retardant agents for textile finishing, because a permanent fixation on the substrate surface has failed so far. Here, we present the successful synthesis and characterization of a noncombustible and foam-forming polyphosphazene derivative, that can be immobilized durably on cotton and different cotton/polyester blended fabrics using photoinduced grafting reactions. The flame retardant properties are improved, a higher limiting oxygen index is found, and the modified textiles pass several standardized flammability tests. As flame retardant mechanism a synergistic effect between the immobilized polyphosphazene and the textile substrate was observed. The polyphosphazene finishing induces an earlier decomposition of the material with a reduced mass loss in thermogravimetric analysis. The decomposition of cotton and polyester leads to the formation of phosphorus oxynitride, which forms a protecting barrier layer on the fiber surface. In addition, the permanence of the flame retardant finishing was proven by laundry and abrasion tests.

Dewetting of Thin Polymer-Blend Films Examined with GISAS

Abstract The morphology of dewetted thin polymer-blend films of deuterated polystyrene (dPS) and polyparamethylstyrene (PpMS) on top of silicon surfaces is investigated. The film thickness of the originally homogeneous films is varied between 19 and 104 A. Compared to the radius of gyration of the unperturbed molecule, R g =106 A , the as-prepared films are confined in the direction perpendicular to the sample surface. The dewetting results from the storage of the samples under toluene vapor atmosphere. Atomic force microscopy (AFM) and grazing incidence small-angle scattering (GISAS) are used. From the differences in the GISAS data measured with X-rays compared to data measured with neutrons a random distribution of the molecules inside the individual droplets is determined. Thus from dewetting under toluene atmosphere no periodicity in the internal structure exists. The, within all methods derived, most prominent in-plane length corresponds to the mean droplet distance. Its function of film thickness is explainable by the spinodal dewetting model.

Nanostructuring of Titania Thin Films by a Combination of Microfluidics and Block‐Copolymer‐Based Sol–Gel Templating

Sol-gel templating of titania thin films with the amphiphilic diblock copolymer poly(dimethyl siloxane)-block-methyl methacrylate poly(ethylene oxide) is combined with microfluidic technology to control the structure formation. Due to the laminar flow conditions in the microfluidic cell, a better control of the local composition of the reactive fluid is achieved. The resulting titania films exhibit mesopores and macropores, as determined with scanning electron microscopy, X-ray reflectivity, and grazing incidence small angle X-ray scattering. The titania morphology has three features that are beneficial for application in photovoltaics: 1) a large surface-to-volume ratio important for charge generation with disordered hexagonally arranged mesopores of 25 nm size and a film porosity of up to 0.79, 2) enhanced light scattering that enables the absorption of more light, and 3) a dense titania layer with a thickness of about 6 nm at the substrate (bottom electrode) to prevent short circuits. An optical characterization complements the structural investigation.