Philipp Lellig

Transparent, Thermally Stable and Mechanically Robust Superhydrophobic Surfaces Made from Porous Silica Capsules

Superhydrophobic surfaces are advantageous for a cost-effective maintenance of a variety of surfaces. The combination of micro and nano-sized roughness increases the contact angle of water such that water droplets cannot adhere but roll off. Therefore, superhydrophobic coatings are self-cleaning and anticorrosive. If the superhydrophobic surface were even transparent, the range of possible applications could be expanded to glass-based substrates such as goggles or windshields and, equally important, prevent an efficiency degradation of solar cells by pollution accumulation. Moreover mechanical robustness is also particularly critical because the dual scale roughness can easily be destroyed irreversibly leading to a rapid decrease of the contact angle and an increase of contact angle hysteresis. We use porous silica capsules as key components to build lotus leaf-like superhydrophobic surfaces. The latter are highly transparent as well as mechanically and thermally stable, see Fig. 1, left. When used as transparent coatings for organic solar cells they leave their performance unaffected, see Fig. 1, right.

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.

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.

Infiltration of Polymer Hole-Conductor into Mesoporous Titania Structures for Solid-State Dye-Sensitized Solar Cells

The degree of filling of titania nanostructures with a solid hole-conducting material is important for the performance of solid-state dye-sensitized solar cells (ssDSSCs). Different ways to infiltrate the hole-conducting polymer poly(3-hexylthiophene) (P3HT) into titania structures, both granular structures as they are already applied commercially and tailored sponge nanostructures, are investigated. The solar cell performance is compared to the morphology determined with scanning electron microscopy (SEM) and time-of-flight grazing incidence small-angle neutron scattering (TOF-GISANS). The granular titania structure, commonly used for ssDSSCs, shows a large distribution of particle and pore sizes, with porosities in the range from 41 to 67%, including even dense parts without pores. In contrast, the tailored sponge nanostructure has well-defined pore sizes of 25 nm with an all-over porosity of 54%. Filling of the titania structures with P3HT by solution casting results in a mesoscopic P3HT overlayer and consequently a bad solar cell performance, even though a filling ratio of 67% is observed. For the infiltration by repeated spin coating, only 57% pore filling is achieved, whereas filling by soaking in the solvent with subsequent spin coating yields filling as high as 84% in the case of the tailored titania sponge structures. The granular titania structure is filled less completely than the well-defined porous structures. The solar cell performance is increased with an increasing filling ratio for these two ways of infiltration. Therefore, filling by soaking in the solvent with subsequent spin coating is proposed.

Colloidal aggregates tested via nanoindentation and quasi-simultaneous 3D imaging

The mechanical properties of aggregated colloids depend on the mutual interplay of inter-particle potentials, contact forces, aggregate structure and material properties of the bare particles. Owing to this variety of influences, experimental results from macroscopic mechanical testings were mostly compared to time-consuming, microscopic simulations rather than to analytical theories. The aim of the present paper was to relate both macroscopic and microscopic mechanical data with each other and simple analytical models. We investigated dense amorphous aggregates made from monodisperse poly-methyl methacrylate (PMMA) particles (diameter: 1.6

Low-Temperature Sol-Gel Synthesis of Nanostructured Polymer/Titania Hybrid Films based on Custom-Made Poly(3-Alkoxy Thiophene)

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Micromechanical properties of colloidal structures

m via nanoindentation in combination with confocal microscopy. The resulting macroscopic information was complemented by the three-dimensional aggregate structure as well as the microscopic strain field and strain tensor. The measured strain field and tensor were in reasonable agreement with the predictions from analytical continuum theories. Consequently, the measured force-depth curves could be analyzed within a theoretical framework that had been frequently used for nanoindentation of atomic matter such as metals, ceramics and polymers. The extracted values for hardness and effective Young’s modulus represented average values characteristic of the aggregate. On the basis of of these parameters we discuss the influence of the strength of particle bonds by introducing polystyrene (PS) between the particles.Graphical abstract

Application of hybrid blocking layers in solid-state dye-sensitized solar cells.

A low-temperature route to directly obtain polymer/titania hybrid films is presented. For this, a custom-made poly(3-alkoxy thiophene) was synthesized and used in a sol-gel process together with an ethylene-glycol-modified titanate (EGMT) as a suitable titania precursor. The poly(3-alkoxy thiophene) was designed to act as the structure-directing agent for titanium dioxide through selective incorporation of the titania precursor. The nanostructured titania network, embedded in the polymer matrix, is examined with atomic force microscopy (AFM) and scanning electron microscopy (SEM) measurements. By means of the scattering technique grazing incidence wide-angle X-ray scattering (GIWAXS), a high degree of crystallinity of the polymer as well as successful transformation of the precursor into the rutile phase of titania is verified. UV/Vis measurements reveal an absorption behavior around 500 nm which is similar to poly(3-hexyl thiophene), a commonly used polymer for photoelectronic applications, and in addition, the typical UV absorption behavior of rutile titania is observed.

Comparative study of conventional and hybrid blocking layers for solid-state dye-sensitized solar cells

The production, further processing as well as the product properties of nanostructured aggregates and colloidal films are specified by characteristics of the particles and particle structures. Especially, the effect of the colloidal structure on the micromechanical properties is of particular interest. The investigation of this structure-micromechanical property relationship was the main objective of this study. Therefore, the colloidal structure and micromechanical properties of silica model aggregates and colloidal PMMA films were analyzed and compared to theoretical considerations and simulations of the deformation and fracture behavior using the discrete elements method.

Stress-Structure Correlation in PS-PMMA Mixed Polymer Brushes

A hybrid blocking layer consisting of a conducting TiO2 network embedded in a ceramic matrix is implemented in a solid-state dye-sensitized solar cell. This novel type of blocking layer is thinner than the classical blocking layer films as shown with SEM and XRR measurements, and thereby the conductivity of the hybrid film is increased by 110%. A percolating TiO2 network, proven by TEM/ESI and GISAXS measurements, allows for the charge transport. Although being thinner, the hybrid film completely separates the rough electrode material from the hole-transport medium in solar cells to avoid the recombination of charge carriers at this interface. In total, the power conversion efficiency of solar cells is improved: the application in photovoltaics shows that the efficiency of devices with the hybrid blocking layer is increased by 6% compared to identical solar cells employing the conventional blocking layer.