To improve two-step metallization processes for silicon solar cells, the influence of different seed layers and surface textures on the light-induced plating (LIP) deposition behaviour at different plating rates has been studied. Typical seed layers of screen-and aerosol printed silver paste have been tested on alkaline (random pyramid) and acidic texture. Silver deposition at high rate has been found to be problematic on very thin aerosol printed layers, especially for alkaline texture with high roughness. In an attempt to obtain high process speed while maintaining high conductivity, aerosol seed layers were subjected to different multi-step plating processes. Morphological features (aerosol particles, alkaline texture) were leveled by a slow pre-plating step. In a second plating step, the deposition rate was varied. The results were compared to similar experiments with untreated aerosol seed layers. Both pre-plating steps were found to effectively improve the deposition quality of the second plating step with high plating rate, and to improve the process robustness. On acidic texture (multicrystalline solar cells), high plating rates were found to give better qualities compared to random pyramids, even without pre-treatment. Both behaviours are attributed to differences in local field and current density between sharply and smoothly shaped plating bases. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3571266] All rights reserved.

We modified the optical properties of organic semiconductor distributed feedback lasers by introducing a high refractive index layer consisting of tantalum pentoxide between the substrate and the active material layer. A thin film of tris-(8-hydroxyquinoline) aluminium doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4H-pyran was used as the active layer. By varying the intermediate layer thickness we could change the effective refractive index of the guided laser mode and thus the laser wavelength. With this technique we were able to tune the laser emission range between 613 nm and 667 nm. For high index layer thicknesses higher than 40 nm the laser operated on the TE(1)-mode rather than the fundamental TE(0)-mode.

It is possible to increase the efficiency of fluorescent concentrator systems with photonic structures. This is achieved by reducing the losses caused by the loss cone of total internal reflection. Examples of fluorescent concentrators we are currently working with are given and different photonic structures designed for the application on these fluorescent concentrators are presented. We discuss the optical characteristics of the photonic structures and their effects on the light guiding efficiency of the fluorescent concentrators. An analytical model is established to analyze and quantify the effects of these filters on the light guiding efficiency theoretically. This model is used to analyze the given photonic structures in detail. We show that with a real photonic structure the loss cone losses can be reduced by more than 75%.

We modified the photonic band structure of organic distributed feedback lasers by introducing a patterned high index intermediate layer of tantalum pentoxide. This layer was oblique angle evaporated onto one dimensional surface gratings with a periodicity of 400 nm. The dielectric broadened the stopband due to its high refractive index compared to both the substrate and the active layer. By tuning the layer thickness we could increase the stopband from 3 to 16 nm.

Photonic structures can be used to eliminate the main loss mechanism in fluorescent concentrators. Simulation routines have been established to investigate the optical characteristic of different photonic crystals. Especially two kinds of structures with an appropriate characteristic have been examined closely. The first is the rugate filter, a one-dimensional photonic structure. In the rugate filter the refractive index is varied sinusoidally over the thickness of the filter. The second is the opal, a three-dimensional photonic crystals made of spheres that are arranged in a self organization process. Filters from these structures have been designed and optimized for the application and fluorescent concentrators and have been optimized. Additional aspects of the structures like angular effects have been examined.

We report on the development of two types of organic solar cell modules one for energy autonomous systems and one for large area energy harvesting. The first requires a specific tailoring of the solar cell geometry and cell interconnection in order to power an energy autonomous system under its specific operating conditions. We present an organic solar cell module with 22 interconnected solar cells. A power conversion efficiency of 2% under solar illumination has been reached on the active area of 46.2 cm(2). A voltage of 4 V at the maximum power point has been obtained under indoor illumination conditions. Micro contact printing of a self assembling monolayer was employed for the patterning of the polymer anode.Large area photovoltaic modules have to meet the requirements on efficiency, lifetime and costs simultaneously. To minimize the production costs, a suitable concept for efficient reel-to-reel production of large area modules is needed. A major contribution to reduce the costs is the substitution of the commonly used indium tin oxide electrode by a cheap material. We present the state of the art of the anode wrap through concept as a reel-to-reel suited module concept and show comparative calculations of the module interconnection of the wrap through concept and the standard ITO-based cell architecture. As a result, the calculated overall module efficiency of the anode wrap through module exceeds the overall efficiency of modules based on ITO on glass (sheet resistance 15 Omega/square) and on foils (sheet resistance 60 Omega/square). (c) 2007 Elsevier B.V. All rights reserved.

Organic solar cells have the potential to make cheap photovoltaic devices feasible. In order to achieve this, material and production costs have to be minimised by using device architectures, which are suited to tap the full potential of reel-to-reel production. The inversion of the layer sequence in organic bulk-heterojunction solar cells is motivated by the possibility to omit the commonly used expensive indium tin oxide electrode utilising the so-called wrap through concept. In this concept, the hole contact is formed by a highly conductive formulation of poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate), which is led through via holes in the solar cell to the backside of the substrate in a regular pattern, where it is contacted with a metal layer with low sheet resistance. In this way, a scalable parallel connexion is realised. If higher voltage is desired, one can also connect several such cell segments in series monolithically. We will show that the inversion of the layer sequence is possible without loss of device performance. Using the results of small area inverted devices, we calculate the optimal dimensions of the wrap through solar cell module. First devices with active areas of 2-4cm(2) with parallel and serial wrap through connexion will be shown as proof of concept. (c) 2006 Elsevier B.V. All rights reserved.

The current–voltage characteristic and the performance of organic bulk-heterojunction solar cells are very sensitive to small variations in the production steps or environmental influences. In our experiments, we found a large variation of the short-circuit current, which does not correspond to the device thickness as one might expect. The fill factor of some devices is below 25% under illumination, while the best devices have a fill factor of about 70%. Electrical impedance spectroscopy can provide information about the conductivity of different regions within the device. In earlier measurements, it was observed that devices with a thick absorber layer might consist of a conductive bulk region and a very poorly conductive depletion region at the metal contact. Using a standard semiconductor device model, it is shown in this paper that this reduces the charge collection efficiency under short-circuit conditions, as there is no electrical field in the bulk region, supporting the charge separation. For devices with the low fill factor, a thin-current limiting layer under forward bias can be identified by electrical impedance spectroscopy and is suggestive of a corroded metal contact.

We present a detailed experimental and theoretical investigation of the lasing characteristics of organic photonic crystal lasers. These lasers are based on strongly modulated two-dimensional polymer surface relief structures on which thin films of optically active organic materials have been deposited. We determine the in-plane photonic band structure of the corresponding quasiguided modes within an effective two-dimensional model. In addition, we calculate the total (three-dimensional) losses associated with these modes. This allows us to identify the lasing thresholds for square lattice geometries and to understand the emission pattern.

One key problem in optimizing organic solar cells is to maximize the absorption of incident light and to keep the charge carrier transport paths as short as possible in order to minimize transport losses. The large versatility of organic semiconductors and compositions requires specific optimization of each system. We investigate two model systems, the MDMO-PPV:PCBM blend and the P3HT:PCBM blend. Due to the small thickness of the functional layers in the order of several ten nanometers, coherent optics has to be considered and therefore interference effects play a dominant role. The influence of the thickness of the photoactive layer on the light absorption is investigated and compared with experimental data. The potential of an optical spacer which is introduced between the aluminium electrode and the photoactive layer to enhance the light harvesting is evaluated by optical modelling. Optical modelling becomes more complex for novel solar cell architectures based on nanostructured substrates. Exemplary optical simulations are presented for a nanoelectrode solar cell architecture.

A novel cell architecture for organic solar cells is presented which is based on a functional microprism substrate. In contrast to the most widely used planar cell architecture with a transparent indium tin oxide (ITO) electrode, the microstructure results in a folded solar cell. Of benefit are the light trapping effect and the substitution of the ITO-electrode by a highly conductive polymer layer with a supporting metal grid. Optical simulations were performed and reveal a gain in absorption in the photoactive layer due to the inclined incidence of radiation and due to the second reflection. Optimal dimensions of the microstructure were calculated by taking the sheet resistivity of the polymer anode and the shading effect of the metal grid into account. The metal grid with a low effective sheet resistance below 1 Ω/□ was realised by evaporation techniques using the microstructure as a self aligning mask. Investigations on the thin film formation are presented. First microprism solar cells with solar efficiencies comparable to planar ITO-reference solar cells were realised.

We investigate fluorescence and lasing action from strongly modulated two-dimensional surface relief structures with hexagonal symmetry onto which thin films of optically active organic material have been deposited. As compared with second-order laser structures with square symmetry, these organic photonic crystal lasers exhibit unusual feedback mechanisms. As a result, we observe surface-emitting lasing action with a central beam normal to the surface and a hexagonal emission pattern of side-beams whose direction slightly deviates from the normal. A corresponding theoretical analysis allows us to determine the photonic bandstructure and the low-threshold laser modes in this system. These results agree very well with fluorescence data and confirm the hexagonal lasing pattern and the corresponding emission angles.

New developments in large-scale micro-structuring of surfaces offer a wide range of applications in glazings with solar-control and light-redirecting properties. In many cases, e.g. when using prism or compound parabolic concentrator (CPC) arrays, the geometry of these structures allows or requires single facets of the structure to be selectively coated with optical coatings. This facet-selective coating technique offers a way to further improve the optical performance. On one hand it is capable of reducing unwanted refractive and diffractive effects of the structure, on the other hand it offers the advantage of combining the static light-guiding abilities of structured surfaces with the possibility to control the properties of a glazing actively by using switchable coatings.

This paper is to report on the development and investigation of a specific type of growth mode and surface morphology of PVD-coatings, namely a columnar growth mode where the individual columns are terminated by pyramid-shaped tips. TiAlN was chosen as a representative of the cubic crystal system, TiO2 as a representative of the tetragonal crystal system. The coatings were sputter-deposited on silicon and steel substrates by a reactive RF-magnetron process under variation of total pressure, reactive gas flow and RF-power. Investigations of microstructure and surface morphology were carried out with electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction techniques (XRD) and are described in more detail for TiO2-coatings. The individual columns with diameters between 100 and 500 nm were terminated by pyramid-shaped tips where the geometric forms of the pyramids were determined by the crystal system of the coating material. A power law was found for the dependance of the mean column diameter on coating thickness in qualitative agreement with phenomenological models for thin film growth. Both coating materials were deposited on steel tools and hot embossed into the surfaces of PMMA-clear-view screens. The resulting screen's surface morphologies proved to be comparable to the anti-reflective surfaces of moth-eyes and also exhibited a pronounced decrease in reflectivity.

We measured the electrical impedance spectra of organic bulk-heterojunction solar cells based on an absorber blend of poly(3-hexylthiophene) and [6,6]-phenyl C-61-butyric acid methyl ester. Comparing the spectra of the non-treated device and after two consecutive treatments with applied forward bias voltage at 110 degrees C, we observed a region in the semiconductor with a low conductivity, which was expanding after the treatments. We concluded that this region is a depletion region at the aluminium contact. This was confirmed by the bias dependence of the impedance spectra.

This paper reports on the development of nanostructured, multifunctional TiAlN- and TiO2-coatings where the multifunctionality stems from their nanoscale surface topography. The coatings were sputter-deposited on silicon and steel substrates by a reactive RF-magnetron-process. For both types of coatings deposition parameters which led to a columnar growth with mean column diameters between 100 nm and 500 nm were elaborated. The columns itself were terminated by pyramid-shaped tips where the geometric forms of the pyramids were determined by the crystal system of the coating material. Investigations of microstructure and surface morphology were carried out with scanning electron microscopy, atomic force microscopy and X-ray diffraction techniques. A power law was found for the dependence of the mean column diameter on coating thickness in qualitative agreement with phenomenological models for thin film growth. In order to test the coating's suitability for industrial applications, both coating materials were deposited on steel tools and replicated into the surfaces of PMMA sheets. The resulting sheet's surface morphologies proved to be comparable to the antireflective surfaces of motheyes and led to a pronounced decrease in reflectivity. A second possible application, namely the modification of wetting properties of surfaces due to their structuring, was tested by fluorination of the nanostructured TiO2-surface which led to an ultrahydrophobic surface with water contact angles up to 150°.

Interference lithography is a manufacturing technique which allows the origination of various types of microstructures on large areas. Micro-structured surfaces with optical functions are useful for radiation power management applications as well as light management applications in displays and in solar energy systems. This paper presents the interference lithography process, different types of possible structures, replication technologies, some applications and new approaches with high aspect ratio photoresists. Both conventional positive-tone diazonaphthoquinone (DNQ)-photoresists like AZ9260 and chemically amplified negative-tone photoresists like NANOTM SU-8 have been investigated. For the structure origination a large scale interferometer setup with an argon ion laser is used. To this end the laser beam is divided into two beams in a first step. Then the two beams are directed by mirrors, expanded and finally superposed. The exposure intensity profile resulting of two superposed coherent light waves is sinusoidal. Therefore in general continuous microstructure profiles will result after development. In this paper thick film photoresists and their usability for interference lithography are demonstrated. Therefore one-dimensional microstructures as well as prismatic microstructures for daylighting applications have been fabricated in AZ9260. Two-dimensional photoresist structures in NANOTM SU-8 for display applications are also introduced. With these new approaches, microstructures with high aspect ratios and structure depths up to 100μm and more have been realised due to the low UV-absorptance of such photoresists. Additional replication processes such as electroforming and soft-embossing as well as replicas in polymers are presented. Thus a complete process chain for a cost-effective fabrication of micro-structured optical components is given.

Optical losses in waveguides comprising metallic contacts are thought to be a major hurdle to the realization of organic laser diodes. We demonstrate here that careful tuning of the waveguide mode in flexible distributed feedback lasers can allow lasing action to occur in organic thin films in the presence of contacting electrodes with virtually no difference when compared to metal free devices. A metallic electrode is most suited as the bottom contact between the polymer and the substrate as it reduces mode leakage into the substrate and enhances modal gain. In contrast, a thin transparent electrode such as a metal oxide is preferable for the top electrode, where confinement is not a problem.

With surface-relief structures, optical functions that are required for radiation power management such as antireflection, light trapping, or light distribution and redirection can be obtained for new applications in solar energy systems and in displays. There, structures with submicrometer features must be distributed over large areas homogeneously. We address the design and the whole experimental process chain from the microstructure origination on large areas to the replication and the system integration in the specific application. Topics are antireflective surfaces for solar systems and displays, light trapping in polymer solar cells, sun protection systems for facades, and diffusers for projection displays and in glazing. For the microstructure origination we investigate the suitability of holographic recording in photoresist using a large-scale interferometer. We use an argon ion laser as a coherent light source at a wavelength of 364 nm. Periodic and stochastic interference patterns are recorded in positive photoresist with the interferometer setup. In the case of periodic structures, grating periods between 200 nm and 20 mum are realized. By carefully modeling the resulting resist profiles it is possible to originate even prismatic surface-relief profiles. Structures with good homogeneity are originated on areas of up to 4800 cm(2) by optimizing the interferometer setup and the photoresist processing. (C) 2004 Society of Photo-Optical Instrumentation Engineers.

Structuring surfaces on a microscopic scale allows to modify their optical properties. The exact tailoring of these properties requires very precise manufacturing techniques. On large areas, mainly replication techniques allow competitive production cost. This paper addresses the challenge of originating and replicating microstructures with optical functions with dimensions between 200nm and 50μm on areas of up to half a square meter. The whole experimental process chain is described and discussed. For the microstructure origination, interference lithography was used. An argon ion laser was chosen as a coherent light source at a wavelength of 364nm. Periodic and stochastic interference patterns were recorded in positive photoresist by using large interferometer set-ups. Structures with good homogeneity were originated on areas of up to 4800 cm2 by optimizing the set-up and the photoresist processing. By carefully modeling resulting resist profiles it was possible to originate a wide variety of surface-relief profiles including prismatic ones. Different replication techniques like hot compression molding and UV casting are discussed. Some applications of large-area micro-structured films and sheets are presented.