Luminescent solar concentrators (LSC) are used in photovoltaic applications to concentrate direct and diffuse sunlight without tracking. We employed 2D FDTD simulations to investigate the concept of a photonic LSC (PLSC), where the luminescent material is embedded in a photonic crystal to mitigate the primary losses in LSCs: the escape cone and reabsorption. We obtain suppressed emission inside the photonic band gap, which can be utilized to reduce reabsorption. Furthermore, the efficiency of light guiding is strongly enhanced in a broad spectral range, reaching up to 99.7%. Our optimization of design parameters suggests emitting layers of sub-wavelength thickness.

In solar applications microstructured polymer surfaces can be used as optically functional devices. Examples are antireflective surfaces, daylighting, sun protection systems, concentrator photovoltaic modules and light trapping structures in organic solar cells. The examples and the principles of function of the respective microstructures are described in detail. The suitability of different manufacturing methods is discussed. Two of them, ultraprecision machining and interference lithography are described. For the latter experimental results are shown. Finally, the opportunities and the risks of the shown approaches are discussed.

In this paper we report on research activities at the Fraunhofer Institute for Solar Energy Systems (ISE) and Concentrix Solar in the area of secondary optics for FLATCON® modules. This concentrator photovoltaic (CPV) technology is based on Fresnel-lenses as primary optics, passive heat spreaders and triple-junction III–V solar cells. In the first part of the paper, a field performance analysis is presented for Concentrix CPV-systems recently installed in Spain. Subsequently, the performance of the first FLATCON® modules with reflective and refractive secondaries are evaluated (FLATCON® II) in indoor and outdoor measurements. As a result of this development, the first module with automated assembly process of the secondary optics could be manufactured. The highest outdoor efficiency measured for this kind of module is 29.1 %, which is the highest module efficiency achieved at the Fraunhofer ISE so far.

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 recombination losses during the charge carrier extraction. The large versatility of organic semiconductors and compositions requires specific optimization of each system. Due to the small thickness of the functional layers in the order of several ten nanometres, coherent optics has to be considered and therefore interference effects play a dominant role. Here we present and discuss concepts for light trapping in organic solar cells. These are wide gap layers in planar solar cells, folded solar cell architectures benefiting from the illumination under inclined incident angles and multiple reflections and absorptions as well as diffraction gratings embossed into the photoactive layer. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The series interconnection of organic photovoltaic cells on the scale of several hundred nanometres results in 1390 interconnected elementary cells per mm. This organic photovoltaic nanomodule generates the highest reported voltage per length for an organic photovoltaic device. As a first application we demonstrate the switching of an organic field effect transistor with a 590 mu m wide photovoltaic nanomodule providing a sufficiently high gate voltage.

In this study we present a theoretical and experimental analysis of the application of photonic band stop filters on top of photovoltaic fluorescent concentrators in order to increase the photon collection efficiency. The light guiding effect of the fluorescent concentrator relies on total internal reflection. The escape cone of total internal reflection is their major loss mechanism. Our ray tracing simulation allows to calculate the beneficial effect of photonic band stop reflection filters, which reduce these losses, and to simulate the angular distribution of the light trapped in the concentrator. We present simulations of the optical properties of 1D and 3D photonic structures and how 3D structures are realized with colloidal opals. We also show that the application of a 1D photonic structure increases the efficiency of a real system by 20% relative. (C) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The Yablonovitch limit for light trapping in solar cells with Lambertian surfaces can be increased using angle selective absorbers thereby exploiting the limited incidence angle of solar radiation. We simulate the efficiency gain or loss caused by an angular and energy selective filter on top of the absorber, compared to a Lambertian and a flat absorber. Additionally, we introduce two possible implementations of such a filter, a Rugate stack and inverted opal layers.

thermally annealed polymer photovoltaics” †Appl. Phys. Lett. 90, 163511 „2007...‡ Moritz K. Riede, Toni Mueller, Bert Maennig, Karl Leo, Kristian O. Sylvester-Hvid, Birger Zimmermann, Michael Niggemann, and Andreas Gombert Technische Universität Dresden, Institut für Angewandte Photophysik, D-01062 Dresden, Germany Freiburger Materialforschungszentrum, University of Freiburg, Stefan-Meier-Strasse 21, D-79104 Freiburg i. Br., Germany Fraunhofer Institute for Solar Energy Systems ISE, Heidenhofstrasse. 2, D-79110 Freiburg i. Br., Germany

In this paper we present a high throughput testing setup for organic solar cells that is necessary for an efficient analysis of their behaviour. The setup comprises process parameter logging, automated measurement data acquisition and subsequent data management and analysis. Utilising this setup the reproducibility of solar cells and the effect of production parameter variations has been tested with a set of 360 solar cells based on the poly-3-hexylthiophene:1-(3-methoxycarbonyl)-propyl-1-1-phenyl-(6,6)C-61 bulk heterojunction. Variations in power conversion efficiency between 1 and 3% were observed on varying production parameters hardly mentioned in literature. The conditions during the vacuum deposition of the aluminium cathode turned out to have a significant effect. The key solar cell parameter affecting the performance was the fill factor (FF). As such the work exemplifies the necessity for a combined approach to analyse the complex behaviour of organic solar cells. The developed high throughput testing setup provides a basis for an efficient testing of production parameter variations and materials and additionally opens the door for statistical analysis. Copyright (C) 2008 John Wiley & Sons, Ltd.

A nondestructive method for assessing the thickness of the photoactive layer in poly(3-hexyl-thiophene):1-(3-methoxy-carbonyl)propyl-1-phenyl-(6,6)C-61 (P3HT:PCBM) solar cells is reported. In the approach the absorption spectrum of the solar cell as derived by optical simulations is fitted to the corresponding measured spectrum, varying only the P3HT:PCBM layer thickness. Within the 50-250 nm thickness range, a linear correlation between the position of a certain spectral minimum and the P3HT:PCBM layer thickness is shown, based on simulated absorption spectra. As an initial application, absorption spectra for 240 P3HT:PCBM solar cells prepared at four different spin-coating speeds were recorded, and the average P3HT:PCBM layer thickness estimated for each spin-coating speed. The simulated fraction of light absorbed in the P3HT:PCBM layer of the solar cells is compared with the P3HT:PCBM absorption spectra measured for films spin coated on simpler substrate types. The latter spectra cannot account for the light harvested in the photoactive layer of P3HT:PCBM solar cells because of substantial optical interference in the solar cells. The measured short circuit current densities J(sc) for the solar cells vary with the spin-coating speed in a manner confirmed by optical simulations of the maximal short circuit current densities. The measured efficiencies follow the same pattern. On average the measured J(sc) is 1-2 mA/cm(2) below the simulated maximal short circuit current densities. Based on the resemblance of the measured and simulated absorption spectra such difference can be attributed to recombination exclusively. (c) 2007 American Institute of Physics.

The production process of organic solar cells (OSCs) is investigated and the effects of parameter variations on experimental results are analysed with the Principal Component Analysis (PCA). This statistical method is applied to an exemplar data set, in which the materials' concentration in the absorber solution and the spincoating speed of the absorber solution were varied intentionally. In addition to the remaining production parameters, the time intervals between the steps were included in the analysis. A large part of the variance in the experimental results can be explained with the evaporation conditions, the spincoating speed and the concentrations in the absorber solution. The PCA also confirms that the OSC is a complex and interdependent system, where one has to analyse the influence of several parameters at the same time in order to understand their effects on the OSC properties. The PCA results will be used to focus further experiments on the identified key parameters.

In this paper we present detailed optical simulations of organic bulk-heterojunction solar cells built with inverted layer sequence as compared to the commonly used setup which is based on indium tin oxide (ITO) covered glass or plastic substrates and where the metal electrode is evaporated on top of the active absorber blend. The inverted setup may have production related advantages over the conventional setup, as the metal electrode is first evaporated onto the substrate and afterwards only wet chemical processes are needed. Additionally ITO can be replaced with a suited module concept. The effects of light trapping with an optical spacer, namely a transparent conductive layer between the absorber and the metallic electrode are investigated for the inverted setup. The results show that the insertion of an optical spacer does not increase the maximal obtainable short circuit current density and is only beneficial if a decrease of film thickness of the active absorber results in a higher internal quantum efficiency, open circuit voltage or fill factor. In the experimental section we show that the inversion of the layer sequence can be realised without any loss in device efficiency as compared to devices with the conventional layer sequence.

We report on experimental characterization of a microstructured optical retarder fabricated using interference lithography technique. The microstructured element is designed to be a quarter-wave plate in visible range. The measurements of retardation distribution were carried out against wavelength in incoherent light using a rotating analyzer technique. We also investigated the dependence of retardation upon angle of incidence for two orientations of the rotation axis. The measurements showed that the birefringent microstructured element is relatively uniform and introduces 70deg retardation at lambda=510 nm

In solar control devices based on total internal reflection and microstructured surfaces, the Goos-Hanchen shift can lead to a significant decrease in the geometrical optical solar shading effect. The knowledge of the maximal size of the GoosHanchen shift for a specific geometry is an important information to estimate its effect on the desired function of the system. Quantitative measurements of the shift for optical wavelengths seems not feasible and analytical approaches are not suited to identify the maximal shift. By using newly developed numerical techniques, namely the rigorous coupled wave analysis (RCWA), the maximal Goos-Hanchen shift for given parameters can be determined.