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During the manufacturing of crystalline silicon solar cells, several wet chemical etching and cleaning steps play an important role for the resulting solar cell efficiency. One important step especially for monocrystalline silicon is texturing by anisotropic alkaline etching which takes place in an aqueous solution of potassium hydroxide and isopropanol at a temperature between 70 and 80 °C. The evaporation of isopropanol and water, the consumption of potassium hydroxide and the accumulation of alkaline silicates change constantly the composition of the process bath. A replenishment of the converted chemicals is necessary to keep the texturisation results at a constant high level. To this day, dosing parameters are often based on operator's experience, but not on analytical data. In this paper, we investigated the suitability of near-infrared spectroscopy as a fast and cost-efficient method for the analysis of alkaline texturisation solutions. The effects of the chemical composition of the alkaline texturisation solution on the near-infrared spectra were investigated, as well as the influence of temperature variations and process induced gas bubbles in the circulation line. A partial least squares (PLS) model, which includes all the disturbing effects was created and validated in a semi-industrial scale texturing system. The development of the chemical concentrations was compared with reference measurements like titration and HPLC (high performance liquid chromatography) as well as those, calculated from the silicon weight loss of the etched silicon wafers.

Light trapping is of very high importance for silicon photovoltaics (PV) and especially for thin-film silicon solar cells. In this paper we investigate and compare theoretically the light trapping properties of periodic and stochastic structures having similar geometrical features. The theoretical investigations are based on the actual surface geometry of a scattering structure, characterized by an atomic force microscope. This structure is used for light trapping in thin-film microcrystalline silicon solar cells. Very good agreement is found in a first comparison between simulation and experimental results. The geometrical parameters of the stochastic structure are varied and it is found that the light trapping mainly depends on the aspect ratio (length/height). Furthermore, the maximum possible light trapping with this kind of stochastic structure geometry is investigated. In a second step, the stochastic structure is analysed and typical geometrical features are extracted, which are then arranged in a periodic structure. Investigating the light trapping properties of the periodic structure, we find that it performs very similar to the stochastic structure, in agreement with reports in literature. From the obtained results we conclude that a potential advantage of periodic structures for PV applications will very likely not be found in the absorption enhancement in the solar cell material. However, uniformity and higher definition in production of these structures can lead to potential improvements concerning electrical characteristics and parasitic absorption, e.g. in a back reflector. (C) 2012 Optical Society of America

Light trapping is becoming of increasing importance in crystalline silicon solar cells as thinner wafers are used to reduce costs. In this work, we report on light trapping by rear-side diffraction gratings produced by nano-imprint lithography using interference lithography as the mastering technology. Gratings fabricated on crystalline silicon wafers are shown to provide significant absorption enhancements. Through a combination of optical measurement and simulation, it is shown that the crossed grating provides better absorption enhancement than the linear grating, and that the parasitic reflector absorption is reduced by planarizing the rear reflector, leading to an increase in the useful absorption in the silicon. Finally, electrooptical simulations are performed of solar cells employing the fabricated grating structures to estimate efficiency enhancement potential.

Many markets require large area surface relief micro- and nanostructures. Important examples are light management structures for display applications or the radiation power management in solar systems. Structuring techniques with both up-scaling and mass production potential are an essential precondition for their realization. Interference lithography is a promising technique to originate fine-tailored structures on areas of up to 1.2 × 1.2 m2. Subsequent replication techniques have the potential for enabling an industrial fabrication. After a description of the basic technologies, we present application examples of large area structures used as template for polarization optical films. Furthermore, light trapping structures for crystalline silicon solar cells are shown. For this application, etching masks were fabricated by nanoimprint lithography. A subsequent etching step was applied to transfer the structures into the silicon. Finally, 3-dimensional photonic structures with distinguished optical properties are presented.

A novel texturing method to realize a honeycomb texture on multicrystalline silicon solar cells is presented in this paper. The demonstrated process chain is based on nanoimprint lithography (NIL), where an ultra-violet (UV)-curable polymer layer is structured by mechanical embossing in a high-throughput process. This patterned polymer layer can then be used as an etching mask for plasma or wet chemical etching processes to transfer the defined pattern into the silicon substrates. Within this study, the whole process chain of this novel texturing scheme, using interference lithography, cast moulding, NIL, and plasma etching, is described with a focus on the NIL process. The textured substrates are characterized by reflection measurements, which are compared with standard solar cell textures. Besides optical measurements, first results of honeycomb textured solar cells are presented. Short-circuit current densities above 40 mA/cm2 were achieved on high-quality float zone material. To increase the feasibility of an industrial realization of this process chain, we are developing a roller-NIL tool to structure an etching mask in a continuous in-line process. First results of this novel tool are also shown in this study.

Recent investigations of Fresnel lens concentrator optics have shown that optical efficiency and module performance depend on lens temperature. In former works we have succeeded in modeling these effects, achieving very good agreement with measurements of silicone on glass (SOG) Fresnel lenses. In the work presented here, we combine this lens model with irradiation spectra and ambient temperature data at typical sites for concentrator photovoltaic power plants for a whole year of operation. The solar cell is simulated with a model based upon the detailed balance limit introduced by Shockley and Queisser. This concentrator photovoltaic system consisting of a triple junction solar cell and a Fresnel lens concentrator is simulated including changes in temperature and spectrum over daytime and during all seasons. Comparing two simulations, one with the Fresnel lens kept at constant temperature and one with a realistic temperature variation of the lens, allows us to analyze the impact of Fresnel lens temperature on system efficiency under realistic environmental conditions. In our analysis poly(methylmethacrylate) (PMMA) Fresnel lenses and SOG Fresnel lenses show comparable energy harvesting efficiency at close to market locations.

Restricting the angular range in which a photovoltaic system emits light, is a promising but rather unexplored approach to enhance conversion efficiency. In this paper we analyze and discuss the effect of a directionally selective filter on the absorption of light and the generation of charge carriers in a germanium solar cell. A directionally selective filter transmits photons of perpendicular incidence and reflects photons under oblique incidence in a given spectral range. To investigate its effect on light trapping, we perform reflection and quantum efficiency measurements. The reflection measurements show that a wavelength dependent absorption enhancement is induced by the application of the directionally selective filter. We calculate a maximum absorption enhancement of 45% at lambda approximate to 1900 nm. We show that the absorption enhancement is caused by light trapping of non-absorbed and scattered light and is not due to a suppression of radiative processes. A trapping of photons generated by radiative recombination could not be detected. Measurements of the quantum efficiency confirm the results of the reflection measurements. The generation of charge carriers is increased by up to 33% at lambda approximate to 1900 nm. A comparison of path length enhancement factors calculated from reflection and quantum efficiency measurements indicates a low parasitic absorption in the solar cell device.

Since micro- and nanostructures for photon management are of increasing importance in novel high-efficiency solar cell concepts, structuring techniques with up-scaling potential play a key role in their realization. Interference lithography and Nanoimprint processes are presented as technologies for origination and replication of fine-tailored photonic structures on large areas. The combination of these processes is presented as feasible route to generate high efficiency honeycomb textures on multicrystalline silicon.

In this work capacitance–voltage measurements of three different dielectric layers, thermal silicon oxide, plasma enhanced chemical vapor deposited (PECVD) silicon oxide, and PECVD silicon nitride, on p-type silicon have been performed in order to obtain characteristics as the energy distribution of the interface trap density and the density of fixed charges. Spatially resolved capacitance–voltage, ellipsometry and lifetime measurements revealed the homogeneity of layer and passivation properties and their interrelation. Additionally lifetime measurements were used to evaluate x-radiation induced defects emerged during electron beam evaporation for sample metallization.

This paper deals with the design of a single glazed flat plate Photovoltaic–Thermal (PV–T) solar collector. First, the thermal and electrical performances of several single glazed flat plate PV–T concepts based on water circulation are investigated, using a simple 2D thermal model, then different ways of improvement are presented. It mainly consists in focusing on the heat transfer between PV cells and fluid, and also on the optical properties of materials. Thus, the most appropriate concept configuration has been identified and suitable material properties have been selected. A prototype collector has been designed, built and tested. A high thermal efficiency was reached at zero reduced temperature. For this level of thermal efficiency, the corresponding electrical efficiency has is lower than efficiency of a standard PV panel using the same technology. However, this solar PV–T collector is reaching, in these standard conditions, the highest efficiency level reported in the literature.

The influence of phosphorus gettering is studied in this work applied to an acidic textured multicrystalline silicon substrate. The texturization was achieved with an HF/HNO3 solution leading to nanostructures on the silicon surface. It has been demonstrated in previous works that this textured surface decreases the reflectance on the solar cell and increases the surface area improving the photon collection and enhancing the short circuit current. The present study investigates the effect on the minority carrier lifetime of the phosphorous diffusion when it is carried out on this textured surface. The lifetime is measured by means microwave photoconductance decay and quasi steady state phototoconductance devices. The diffused textured wafers are used to fabricate solar cells and their electrical parameters are analyzed. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Restricting the angular emission characteristics of a solar cell results in a reduction of entropy generation and allows for higher solar cell efficiencies. In this paper, we present the concept of a photonic solar cell which exhibits the combination of a solar cell and a photonic crystal. The basic idea is to structure a solar cell in such a way that the applied structure is a photonic crystal consisting of the solar cell material. The aim of having a solar cell in the form of a photonic crystal is to restrict the emission of the solar cell into certain angles This reduction reduces the radiative losses and results in a higher VOC and, consequently, a higher efficiency of the cell. To calculate the efficiency of a photonic solar cell we introduce an adaptation of the detailed balance approach. This adaptation takes into account the reduced radiative losses due to the presence of a photonic band structure. In the second part of the paper we use this approach to calculat e the efficiency for an exemplary system. The exemplary system is an idealized edge filter, similar to a Bragg stack. The idealization concerns the reflection characteristic of this filter and its dependence on the polarization. For this idealized system we calculate an efficiency that exceeds that of a conventional solar cell by up to 2.5% absolute.

Dünne Schichten aus Silber werden bereits großflächig in Wärmeschutzverglasungen eingesetzt und zeigen gute Leitfähigkeiten bei hohen Transmissionen vor allem im sichtbaren Spektralbereich. Im nahen Infraroten fällt die Transmission ab, was jedoch durch eine Ausnutzung von Interferenzen wieder ausgeglichen werden kann. Dünne Schichten aus Silber können kostengünstig hergestellt und insbesondere auch gut auf Foliensubstraten abgeschieden werden. Durch zusätzliche dielektrische Schichten lässt sich das Schichtsystem auf weitere Anforderungen anpassen, ohne die Leitfähigkeit zu beeinträchtigen. Beispiele sind die Vermeidung von Barrieren, die Anpassung der Austrittsarbeit oder die Stabilisierung gegen folgende Prozessschritte. Dadurch sind Silberelektroden für organische Leuchtdioden oder Solarzellen von besonderem Interesse.

In this paper we give a mathematical derivation of how luminescence images of silicon solar cells can be calibrated to local junction voltage. We compare two different models to extract spatially resolved physical cell parameters from voltage images. The first model is the terminal connected diode model, where each pixel is regarded as a diode with a certain dark saturation current, which is connected via a series resistance with the terminal. This model is frequently used to evaluate measurement data of several measurement techniques with respect to local series resistance. The second model is the interconnected diode model, where the diode on one pixel is connected with the neighbor diodes via a sheet resistance. For each model parameter at least one image is required for a coupled determination of the parameters. We elaborate how also the voltage calibration can be added as an unknown parameter into the models, and how the resulting system of equations can be solved analytically. Finally the application of the models and the different ways of voltage calibration are compared experimentally.

A directionally selective multilayer filter is applied to a hydrogenated amorphous silicon solar cell to improve the light trapping. The filter prevents non-absorbed long-wavelength photons from leaving the cell under oblique angles leading to an enhancement of the total optical path length for weakly absorbed light within the device by a factor of kappa(r) = 3.5. Parasitic absorption in the contact layers limits the effective path length improvement for the photovoltaic quantum efficiency to a factor of kappa(EQE) = 1.5. The total short-circuit current density increases by DeltaJ(sc) = 0.2 mAcm(-2) due to the directional selectivity of the Bragg-like filter.

For decreasing thicknesses of wafer based silicon solar cells, photon management structures to maintain high quantum efficiencies will gain importance. Diffractive gratings on the wafer back side can be designed to achieve very high path length enhancements, especially for weakly absorbed infrared radiation. This technologically demanding concept has to be realised using processes with upscaling potential. Therefore, we present a fabrication process for producing photonic structures in silicon based on interference lithography and nanoimprint lithography (NIL). We realised linear as well as crossed gratings of different depths, which were etched into the wafer back side. Polarisation dependent reflection measurements were made to get information about potential absorption enhancement as well as the occurrence of parasitic absorption in the metal reflector. This is conducted for a PECVD silicon oxide buffer layer between grating and reflector as well as a spin coated silicon oxide layer. Besides these optical characterisations, we further investigated the electrical properties of the back surface, where we applied a concept in which electrical and optical properties are decoupled. This is realised by a layer stack on the wafer back side, consisting of a thin Al2O3 passivation and a doped amorphous silicon layer.