Effect of Methanol Addition on the Resistivity and Morphology of PEDOT:PSS Layers on Top of Carbon Nanotubes for Use as Flexible Electrodes.

Overcoating carbon nanotube (CNT) films on flexible poly(ethylene terephthalate) (PET) foils with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) ( PEDOT: PSS) layers reduces the surface roughness, which is interesting for use in organic electronics. Adding methanol to the PEDOT: PSS aqueous solution used for spin coating of the PEDOT: PSS layer improves the wetting behavior of the CNT/PET surface. Samples with different volume fractions of methanol (0, 33, 50, 67, and 75 vol %) are compared with respect to the transmission, horizontal, and vertical resistivity. With grazing-incidence small-angle X-ray scattering, the film morphologies are probed, which is challenging because of the substrate flexibility. At 50 vol %, methanol optimum conditions are achieved with the resistivity close to that of the bare CNT/PET substrates because of the best contact between the PEDOT: PSS film and CNT surface. At lower methanol ratios, the PEDOT: PSS films cannot adapt the CNT morphology, and at higher methanol ratios, they rupture into domains and no continuous PEDOT: PSS layers are formed.

Monolayer properties of asymmetrically substituted sexithiophene.

We present a structural comparison of monolayers on a SiO2 substrate of two asymmetrically substituted sexithiophenes (6T). Molecule 1 consists of 6T with a branched alkyl chain at one end only and shows a crystalline structure. In molecule 2, the bifunctional 6T has in addition at the other end a linear alkyl chain. It displays thermotropic liquid crystalline (LC) behavior. Both compounds form readily single molecular layers from solution. Remarkably, full monolayer coverage can be achieved before multilayer growth starts. LC properties promote preordering near the interface as well as exchange of molecules between the growing domains, thus regulating the domain sizes. As a result, the LC compound 2 forms single-molecule islands with larger domain sizes compared to compound 1. Surface X-ray investigations indicate that the 6T cores are tilted relative to the layer normal. The tilt angle is as large as 54° for compound 2 compared to 28° for compound 1. For molecule 2, interfacial constraints and packing requirements because of the asymmetric substitution cause a rather loosely organized core structure.

Evolution of lateral structures during the functional stack build-up of P3HT:PCBM-based bulk heterojunction solar cells.

Bulk heterojunction (BHJ) solar cells from 1,2-dichlorobenzene solution processed regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT): phenyl-C61-butyric acid methyl ester (PCBM) are prepared and investigated at different steps of the multilayer stack build-up of the device. The inner structure is probed from the molecular to the mesoscale with grazing incidence small/wide-angle X-ray scattering (GISAXS/GIWAXS) and X-ray reflectivity (XRR). The surface morphology is detected with atomic force microscopy (AFM). Therefore, an in-depth knowledge of the three-dimensional morphology of the bulk heterojunction solar cell, starting from the cleaned ITO substrate up to the final post-treated solar cell, is generated. The active layer structure is influenced by the annealing as well as by the top contact deposition. Structures coarsen during the evaporation of the metal contacts. The P3HT crystal structure strongly depends on the device processing as well. These morphological changes together with the diffusion of aluminum atoms to the active layer are of importance for the device efficiency.

Influence of nanoparticle surface functionalization on the thermal stability of colloidal polystyrene films.

The installation of large scale colloidal nanoparticle thin films is of great interest in sensor technology or data storage. Often, such devices are operated at elevated temperatures. In the present study, we investigate the effect of heat treatment on the structure of colloidal thin films of polystyrene (PS) nanoparticles in situ by using the combination of grazing incidence small-angle X-ray scattering (GISAXS) and optical ellipsometry. In addition, the samples are investigated with optical microscopy, atomic force microscopy (AFM), and field emission scanning electron microscopy (FESEM). To install large scale coatings on silicon wafers, spin-coating of colloidal pure PS nanoparticles and carboxylated PS nanoparticles is used. Our results indicate that thermal annealing in the vicinity of the glass transition temperature T(g) of pure PS leads to a rapid loss in the ordering of the nanoparticles in spin-coated films. For carboxylated particles, this loss of order is shifted to a higher temperature, which can be useful for applications at elevated temperatures. Our model assumes a softening of the boundaries between the individual colloidal spheres, leading to strong changes in the nanostructure morphology. While the nanostructure changes drastically, the macroscopic morphology remains unaffected by annealing near T(g).

Phase Separation and Molecular Intermixing in Polymer-Fullerene Bulk Heterojunction Thin Films.

The phase separation and molecular intermixing in poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunction thin films are investigated as a function of the overall PCBM content. The structural length scales, phase sizes, and molecular miscibility ratio in bulk heterojunction films are probed with grazing incidence small-angle neutron scattering (GISANS). The PCBM content is varied between 9 and 67 wt %. For the symmetric P3HT/PCBM ratio, which is typically highly efficient in photovoltaic devices, a structure size of 20 nm, the largest PCBM phases, and 18 vol % dispersed PCBM in the amorphous P3HT phase are found. The molecularly dispersed PCBM content is found to increase with the overall PCBM content. Absorption measurements complement the GISANS investigation.

Influence of film thickness on the phase separation mechanism in ultrathin conducting polymer blend films.

The film morphology of thin polymer blend films based on poly[(1-methoxy)-4-(2-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly(N-vinylcarbazole) (PVK) is probed as a function of film thickness. Blend films are prepared with spin-coating of polymer solutions with different concentrations on top of solid supports. The blending ratio of both conducting polymers is kept constant. The film and surface morphology is probed with grazing incidence ultrasmall-angle X-ray scattering (GIUSAXS) and atomic force microscopy (AFM). A linear dependence between the film thickness and the averaged phase separation is found. In addition, X-ray reflectivity measurements show an enrichment of PVK at the substrate interface. UV/vis spectroscopy measurements indicate a linearly increasing amount of both homopolymers in the blend films for increasing film thicknesses. The generalized knowledge about the influence of the film thickness on the phase separation behavior in conducting polymer blend films is finally used to describe the phase separation formation during the spin-coating process, and the results are discussed in the framework of an adapted Flory-Huggins theory for rodlike polymers.

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.

Influence of annealing and blending of photoactive polymers on their crystalline structure.

Thin photoactive polymer films of poly(3-octylthiophene-2,5-diyl) (P3OT) and poly(2,5-di(hexyloxy)cyanoterephthalylidene) (CN-PPV) are investigated. With X-ray reflectivity measurements, a linear concentration-thickness dependence is found for both polymers and the molecular weight of CN-PPV is determined from this concentration-thickness dependence. Based on the molecular weights, the critical blending ratio is determined. Grazing incidence wide-angle X-ray scattering (GIWAXS) is used to probe the crystallinity of thin films and to determine characteristic length scales of the crystalline structure. Moreover, the orientation of the crystalline parts regarding the substrate of both the homopolymer and the blended films is probed with GIWAXS. Temperature annealing is found to improve the crystallization for both homopolymers. In addition, reorientation of the predominant crystalline structures takes place. Blending both polymers reduces or even suppresses the crystallization during spin coating as well as temperature annealing. Absorption measurements complement the structural investigations.

Photoactive nanostructures of polypyrrole.

The creation of nanostructures of the photoactive polymer polypyrrole (PPy) on glass substrates with the spin-coating method is described. No additional post-production treatment is necessary to obtain uniformly distributed photoactive nanostructures on macroscopically scaled substrates. Based on X-ray reflectivity measurements, the critical solution concentration of PPy below which these nanostructures develop is determined. The PPy nanostructures are displayed with atomic force microscopy (AFM) measurements, which prove that the nanostructures form directly on the substrate. With UV/Vis spectroscopy the absorption behavior of the nanostructures is probed in comparison to PPy films and PPy solutions. A linear dependence of the absorption of the nanostructure on the surface coverage measured with AFM is detected. The influence of confinement on the conjugation length results in a modified absorption behavior of the nanostructures.

Thin films of photoactive polymer blends.

The morphology inside photoactive blended films of two conjugated homopolymers poly [(1-methoxy)-4-(2-ethylhexyloxy)-p-phenylene-vinylene] (MEH-PPV) and poly(3-hexylthiophene-2,5-diyl) (P3HT) is investigated. For both homopolymers a linear dependence of the installed film thickness from the concentration of the polymer solution used in spin coating is probed. This dependence allows preparation of an efficient series of blended films with constant thickness and different blending ratios. Information about the lateral structure inside the films is gained from grazing incidence small angle X-ray scattering. At the calculated critical blending ratio the smallest lateral separation between adjacent domains is found representing the highest surface contact between both homopolymers in the films. The presence of wetting layers at both interfaces as detected with X-ray reflectivity and atomic force microscopy is promising for photovoltaic applications. UV/Vis spectroscopy complements the structural investigation.

In situ GISAXS study of gold film growth on conducting polymer films.

The growth of a thin gold film on a conducting polymer surface from nucleation to formation of a continuous layer with a thickness of several nanometers is investigated in situ with grazing incidence small-angle X-ray scattering (GISAXS). Time resolution is achieved by performing the experiment in cycles of gold deposition on poly(N-vinylcarbazole) (PVK) and subsequently recording the GISAXS data. The 2D GISAXS patterns are simulated, and morphological parameters of the gold film on PVK such as the cluster size, shape, and correlation distance are extracted. For the quantitative description of the cluster size evolution, scaling laws are applied. The time evolution of the cluster morphology is explained with a growth model, suggesting a cluster growth proceeding in four steps, each dominated by a characteristic kinetic process: nucleation, lateral growth, coarsening, and vertical growth. A very limited amount of 6.5 wt % gold is observed to be incorporated inside a 1.2-nm-thick enrichment layer in the PVK film.

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.