This PDF file contains the front matter associated with SPIE Proceedings Volume 7725, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Due to their electrical conductivity and transparency in the visible spectral range, transparent conductive oxides (TCOs) are suitable as transparent front electrodes for multiple cell concepts. One promising device is a semiconductor-insulator-semiconductor (SIS) solar cell, in which the TCO induces the pn juntion and acts as a transparent electrode at the same time. Due to its work function, the thin TCO layer leads to the inversion of the subsurface region. The high refractive index of transparent conductive oxides enables antireflection coating in a limited spectral range. One approach to achieve broadband antireflection properties with effective light coupling into the absorber is a nanostructured silicon interface. For large area modifications of silicon, the inductive coupled plasma (ICP) etching technology is a possible technique. The combination of the nanostructured surface and the SIS system leads to a novel solar cell concept with promising properties and a simple production process. In our study, we used pulsed dc magnetron sputtering for the deposition of thin ITO films on p-doped unstructured and ICP-structured silicon substrates. Optical and structural properties have been analyzed. Furthermore, the solar cell performance of the first devices under AM1.5G illumination will be shown and discussed.

Using thin films of crystalline silicon to make solar cells reduces the cost by reducing the amount of material needed and allowing poorer quality material with shorter carrier diffusion lengths to be used. However, the indirect band gap of silicon requires that a light trapping approach be used to maximize optical absorption. Here, a photonic crystal (PC) based approach is used to maximize solar light harvesting in a 400 nm-thick silicon layer by tuning the coupling strength of incident radiation to quasiguided modes over a broad spectral range. The structure consists of a double layer PC. We show an enhancement of maximum achievable photocurrent density from 7.1 mA/cm² for an unstructured film to 21.8 mA/cm² for a structured film for normal incidence. This photocurrent density value approaches the limit of 26.5 mA/cm², obtained using the Yablonovitch light trapping limit for the same volume of active material.

An effective light trapping system is required in silicon solar cells in order to collect a large amount of photons. That is why we focus our investigation on the fabrication and evaluation of two types of optical systems introduced on the back side of solar cells. The aim of these structures is to enhance the light trapping of the long wavelength photons (above 1000 nm). On the one hand, we evaluated a Si/SiO₂ linear nanograting; on the other hand, hexagonal nanostructures fabricated with SiO2 nanoparticles and a filling matrix are under investigation. In this paper, we describe the fabrication processes developed for both approaches and we present the solar cell results and characterisation. For the first approach, we show a reflectance reduction on test structures, which occurs at the same wavelength as the increase of absorption induced by the simulated gratings. Moreover, we demonstrate the feasibility of the fabrication of silicon solar cells with the hexagonal nanostructures as a diffractive back reflector. Although no short circuit current increase has been observed due to a poor rear side passivation, a current gain up to 0.3 mA/cm² is possible in the wavelength range of 1050-1150 nm due to these nanostructures. Finally, we also comment on the advantages and drawbacks of each approach and on the feasibility to introduce these systems in the solar cell process flow.

We present results of rigorous optical modeling of reactive ion etched crystalline silicon surfaces, so called Black Silicon, for different etching parameters and compare them to experimental data. Reactive ion etching of crystalline silicon with SF6 and O2 can produce a surface consisting of sharp randomly distributed needle like features with a characteristic lateral spacing of about a few hundreds of nanometers and a wide range of aspect ratios depending on the process parameters. Due to the very low reflectance over a broad spectral range such surface textures can be beneficial for photon management in photovoltaic applications. To gain a detailed understanding of the optical properties of Black Silicon surfaces we recovered the full three dimensional geometry of differently etched samples. With these data we calculated the optical response using the finite differences time domain method. From the calculations we will give insight into the magnitude of resonant phenomena within the Black Silicon and the resulting near field enhancement. Furthermore we will present carrier generation profiles which quantify the effect of absorption enhancement due to the nanostructured surface. We also investigate the angular forward scattering distribution into the silicon substrate and the resulting path length enhancement which is crucial for the near band edge absorption especially in thin solar cells.

In this paper the applicability and efficiency of different intermediate layer (IL) stacks for the implementation in recrystallized wafer equivalent (RexWE) solar cells are investigated. The requirements for the IL in the RexWE concept are short term stability at temperatures above 1400 °C, high reflectivity for wavelengths exceeding 600 nm, electrical conductivity and acting as a diffusion barrier against metallic impurities. Various combinations of stoichiometric SiC layers, silicon rich SiC layers and SiO₂ layers were tested as IL stacks regarding their performance after a zone melting recrystallization (ZMR) process. For the first time, samples with an IL consisting of a SiC multilayer were recrystallized and successfully processed to solar cells.

In this paper, a review of the recent progress in concentrator photovoltaics (CPV) is given. In the first part, an introduction to CPV includes the concepts of solar concentration and the specific advantages of CPV. Then, the various optical designs are presented and discussed. In the second part, the recent success in bringing this technology to market ready products is described. Exemplarily, the FLATCON® CPV technology is described in detail and data of the field performance are presented. The design of the FLATCON CPV module is based on Fresnel lenses and III-V multijunction solar cells (MJC). With these modules Concentrix installed a demo tracker and two power plants in Spain in 2008. Field data of these systems with a maximum AC efficiency of 23% are presented and discussed in detail. In 2009, the first systems were installed with the module CX-75 which is produced on the fully automated production line of Concentrix in Freiburg. This module has a DC efficiency of 27% flashed. The field data which are presented demonstrate an outstanding AC system efficiency of 25%.

Scaling down the dimensions of concentrating photovoltaic systems based on plane Fresnel lenses has several promising advantages. By conserving a designed concentration ratio and reducing the aperture size of the lens, the working distance decreases as well. This provides thinner modules and the dimensions of the used solar cells can be scaled down to the millimeter range. An important benefit of this miniaturization process is the avoidance of technically demanding cooling. In this work the design of a plane Fresnel lens is introduced and the basic limitations concerning the achievable concentration ratio are investigated based on geometrical optics. However, accompanied by the down scaling of the prism dimensions, pure ray tracing based on the geometrical optics approximation may no longer be valid for the determination of the concentration ratio. In terms of micro-structured Fresnel lenses for solar concentration, only a qualitative description of this limit - typically a rule of thumb - is provided in the literature. For this reason a quantitative investigation of the influence of the prisms' down scaling and thus the appearing wave optical effects on the obtained concentration ratio is presented. In a final step the introduced monochromatic investigations are extended to a polychromatic analysis. This allows for the prediction of the influence of miniaturization on the effective concentration ratio for a given spectrum and thus the adequate size of the receiver. A better quantitative understanding of the impact of diffraction in micro-structured Fresnel lenses might help to optimize the design of several applications in nonimaging optics.

Fresnel lenses are often used as primary optical components in concentrating photovoltaics (CPV). When applied in the field, varying conditions during operation lead to variations in lens temperature which has a strong impact on the optical efficiency of the lenses. A setup for indoor characterization with the ability to heat lens plates allows for the assessment of the quality of Fresnel lenses by means of their irradiance profiles in the focal plane. To analyze the measured temperature dependency we simulate thermal deformations of the lens geometry with finite element method (FEM) tools and use the resulting lens geometry as an input to ray tracing simulations. A close match between computer simulations and measurements of the irradiance profile in the focal plane is achieved, validating our simulation approach. This allows us to judge and optimize the temperature dependence of new lens designs before building and testing prototypes. The simulation enables us to analyze and understand all superimposed effects in detail. The developed tools in combination with detailed solar resource data and knowledge of the CPV system will be the basis for future assessment of overall performance and further optimization of optics for CPV applications.

To achieve higher efficiencies in organic solar cells, ideally the open circuit voltage (V_OC), fill factor (FF) as well as the short current density (J_SC) have to be further improved. However, only a few suitable acceptor molecules, e.g. C₆₀, are currently available for the photoactive layer. Despite a good electron mobility on the order of 1×10^-3 cm²/Vs the absorption of C60 in the visible sun spectrum is low. From polymer based solar cells it is known that the fullerene derivative [70]PCBM used in the photoactive layer shows a significant enhancement in JSC compared to [60]PCBM. This work investigates the application of fullerene C₇₀ as acceptor in comparison to the well known C₆₀ in vacuum processed small molecule solar cells. C₇₀ shows a broadened and red shifted absorption (abs. maximum around 500 nm) compared to C60. By fabricating p-i-i solar cells we show that the stronger absorption of C₇₀ leads to enhanced photon harvesting and increased external quantum efficiency. The bulk heterojunction p-i-i solar cell containing C₇₀ as acceptor and ZnPc as donor, co-evaporated with an optimized ratio of 2:1, and a layer thickness of 30 nm shows improved solar cell parameters: a 30% larger photocurrent of 10.1 mA/cm² is obtained. The V_OC of 0.56 V and FF of 55% remain comparable to C60-containing p-i-i solar cells. Therefore, the solar cell performance is mainly improved by JSC and leads to a mismatch corrected power conversion efficiency of 3.12%. Thus, we show that C70 is an alternative fullerene to C60 for solar cell applications.

The optimization of the conversion efficiency of dye sensitized solar cells (DSC) has become a key issue nowadays due to the search for alternative energy resources. Different approaches based on the optical absorption enhancement can be realized in this type of solar devices by modifying the optical design of the cell. In this respect, novel porous and highly reflecting one-dimensional photonic crystals (1DPC) have been recently implemented in DSC due to their great potential for the manipulation of light propagation. The periodic structure is built by alternating layers made of different types of nanoparticles that allow us to obtain a wide and intense Bragg reflection peak. The photonic crystal, with a thickness of just half-micron, is able to efficiently localize incident light within the nc-dyed TiO₂ electrode in a targeted wavelength range. So, significant optical absorption amplification in a wide spectral range occurs in these structures that combine the presence of a highly reflecting photonic crystal and a layer of absorbing material, being therefore enhanced the photogenerated current. Average power conversion efficiencies are improved between 15% and 30% with respect to the reference value attained for standard electrodes with no photonic crystal coupled.

Thin films with low-refractive index are of great interest to adjust the optical properties of optical components, e.g. reflectivity. In this respect, sol-gel coatings are very efficient for flat glass functionalisation because of ease of application, low cost and versatility. Mesoporous silica films prepared by surfactant self-assembly have been extensively studied but show poor refractive index stability as capillary condensation of atmospheric water can occur in the pores. One way to tackle this issue is to prepare films with larger pores for which capillary condensation is impossible at ambient humidity. Starting from custom made latex nanoparticles, we successfully prepared sol-gel porous silica film with pore size above 30nm and no microporosity in the silica walls. We are then able to independently and accurately tailor pore size, pore volume fraction and pore surface chemistry, leading to a stable refractive index that can be tuned in a large range (from 1.15 to 1.40 at 600 nm). Pore accessibility as a function of pore size and porous fraction was investigated through ellipsometry-porosimetry for ethanol adsorption, and a transition between open and closed pore structure at decreasing volume fraction was shown. Below the threshold, the films showed a closed porosity structure with a low stable refractive index (down to 1.29 at 600 nm), opening the way to their use for antireflective applications.

The subject of this work is the development of an electrically conductive Rugate filter for photovoltaic applications. We think that the optical as well as the electrical performance of the filter can be adapted especially to the requirements of crystalline Si thin-film and amorphous/crystalline silicon tandem solar cells. We have deposited amorphous hydrogenated Silicon Carbide layers (a-Si_xC_1-x:H) with the precursor gases methane (CH4), silane (SiH4) and diborane (B₂H₆) applying Plasma Enhanced Chemical Vapour Deposition (PECVD). Through changing just the precursor flows a floating refractive index n from 1.9 to 3.5 (at 633 nm) could be achieved quite accurately. Different complex layer stacks (up to 200 layers) with a sinusoidal refractive index variation normal to the incident light were deposited in just 80 min on 100x100 mm². Transmission measurements show good agreement between simulation and experiment which proofs our ability to control the deposition process, the good knowledge of the optical behaviour of the different SiC single layers and the advanced stage of our simulation model. The doped single layers show lateral conductivities which were extremely dependent on the Si/C ratio.

There are several ways to nanostructure Si. Some of them, e.g. nanoscale Si-layerd systems buried within the n+ layer of a crystalline Si can provide an initial material with unpredicted optoelectronic behavior. Such a transformation leads to a PV Si metamaterial, whose optoelectronic properties arise from qualitatively new response functions that are (i) not observed in the constituent materials and (ii) result from the inclusion of artificially fabricated, intrinsic and extrinsic, low-dimensional components. We show that an extremely strong c-Si:P absorptance, determined by the free-carrier population, is larger than can result from conventional conversion. The density of the population confined within the surface layer delimited by the nanoscale Si-layerd system increases by injection of additional carriers from a nanostratum (transformed up to a Si-metamaterial) lying just behind the top c-Si:P-layer.

A 3D photonic intermediate reflector for textured micromorph silicon tandem solar cells has been investigated. In thin-film silicon tandem solar cells consisting of amorphous and microcrystalline silicon with two junctions of a-Si/c-Si, efficiency enhancements can be achieved by increasing the current density in the a-Si top cell providing an optimized current matching at high current densities. For an ideal photon-management between top and bottom cell, a spectrally-selective intermediate reflective layer (IRL) is necessary. We present the first fully-integrated 3D photonic thin-film IRL device incorporated on a planar substrate. Using a ZnO inverted opal structure the external quantum efficiency of the top cell in the spectral region of interest could be enhanced. As an outlook we present the design and the preparation of a 3D self organized photonic crystal structure in a textured micromorph tandem solar cell.

Recent scientific publications have highlighted the possibility of enhancing solar conversion efficiency in thin film solar cells using surface plasmon (SP) waves and resonances. One main strategy is to deposit layers of metal nanoparticles on the top of a thin film silicon solar cell which can increase light absorption and consequently the energy conversion in the frequency range where the silicon intrinsic absorptance is low. In this paper, we investigate the effects produced on the light absorption and scattering by silver nanoparticles, arranged in a periodic pattern, placed on the top of amorphous silicon (α-Si) thin layer. We propose different geometry of metal objects, quantifying the scattering (back and forward) determined by the nanoparticles in dependence of their shapes and Si thickness. The analysis reveals that the thickness of the substrate has huge influence on the scattering, in particular on the back one, when the nanoparticles have corners, whereas it seems less dramatic when rounded profiles are considered (nanospheres).

We present the integration of an absorbing planar photonic crystal within a thin film photovoltaic cell. The devices are based on a stack including a hydrogenated amorphous silicon P-i-N junction surrounded by TCO layers, with a back metallic contact. Optical simulations exhibit a significant increase of the integrated absorption in the 300-720nm wavelength range. The global electro-optical characteristics of such a new solar cell, and the impact of surface passivation, are also discussed. Carrier generation rate maps calculated by optical simulations are introduced as input data in a commercial electrical simulation software. The fabrication of such a device is finally addressed, with a specific focus on the use of low cost nanopatterning processes compatible with large areas.

We introduce a model which allows for the description of scattering properties of randomly textured ZnO films by evaluating a Fourier surface analysis. The interface is developed into a series of periodic gratings with well defined diffraction angles. The scattering efficiency is assumed to be the Fourier transform of the surface profile. This model is applied on different kinds of textures and compared with experimentally obtained angularly resolved scattering. This Fourier model is extended to obtain the scattering properties with both spatial and angular resolution which allows the study of the light scattering of individual surface elements. The identification of structures which scatter light into larger angles is possible. The calculated scattering properties show a good agreement to the experimentally obtained data. The results are essential for the further improvement of surface texture to optimize light trapping in thin-film solar cells.

Along the road towards ubiquitous and low-cost solar cells, solutions to the seemingly mutually exclusive targets of reducing material consumption while increasing the efficiency has to be found. One potential solution seems to lie in thin film tandem solar cells. It offers the promise of moderate efficiencies combined with the advantage of relying on a well-established thin film fabrication technology at reasonable low costs. To finally make them serious competitors, various structures to be exploited for photon management may be incorporated into these solar cells with the aim of increasing their efficiency. Besides reducing reflection losses at the entrance facet via textured surfaces and eliminating the dissipation in the metallic backside reflector, the efficiencies of tandem cells can be significantly boosted by a wavelength-dependent steering of the spatial domain where light gets absorbed, i.e. either the top or the bottom cell. This is mainly possible by placing a spectrally selective intermediate reflector in between both cells. In the present contribution we apply well-adapted numerical routines, which solve Maxwell's equations rigorously, to quantitatively explore various intermediate reflector concepts for thin film solar cells from an optical point of view. The solar cells we focus on are silicon based, where the top layer is made of amorphous and the bottom layer of microcrystalline silicon, respectively. We explore state-of-the-art concepts for the intermediate reflector, such as homogenous layers based on dielectrics characterized by a lower permittivity as well as new photonic (such as, e.g., photonic crystals) and plasmonic concepts. Most notably we will address the issue how randomly textured interfaces, present in thin film solar cells, affect the performance of each intermediate reflector and how the randomness may contribute to the absorption enhancement. Guidelines for designing optimized systems will be given.

Optical absorption losses limit the efficiency of thin-film solar cells. We demonstrate how to increase the absorption in hydrogenated amorphous silicon solar cells by using a directionally selective optical multilayer filter covering the front glass. The filter transmits perpendicularly incident photons in the wavelength range 350 nm - 770 nm. In the regime of low absorptance, i.e. large optical absorption lengths, however, it blocks those photons impinging under oblique angles. Thus, the incoming radiation is transmitted with almost no loss while the emitted radiation is mostly blocked due to its wider angle distribution. We determine the enhancement in the optical path length from reflectivity measurements. In the weakly absorbing high wavelength range (650 nm - 770 nm) we observe a peak optical path length enhancement of κ ~ 3.5. The effective path length enhancement κ ~ calculated from the external quantum efficiency of the solar cell with filter, however, peaks at a lower value of only κ ~ 1.5 in the same wavelength range. Parasitic absorption in the layers adjacent to the photovoltaic absorber limit the increase in the effective light path enhancement. Nonetheless we determine an increase of 0.2 mAcm^-2 in the total short circuit current density.

In a Luminescent Solar Concentrator, short-wavelength light is converted by a luminescent material into longwavelength light, which is light guided towards a photovoltaic cell. In principle, a Luminescent Solar Concentrator allows for high concentration, since the heat generated by the conversion process can be used to lower the entropy of light. However, less controlled loss mechanisms prevent high concentration factors in practice. One important loss mechanism is the escape of luminescent radiation into directions that do not stay inside the light guide. To reduce this amount, wavelength-selective filters can be applied that reflect the luminescent radiation back into the light guide while transmitting the incident sunlight. However, a filter optimized for reflecting as much as possible luminescent radiation will reflect part of the incident sunlight at high angles. Depending on the luminophore properties, it may be possible to design a suitable filter. In this paper, the interdependence of the luminophore and filter properties will be clarified and quantified using simulations. Optimal luminophore-filter combinations will be discussed, as well as the feasibility to realize them in practice.

We present a photovoltaic tandem system made of two stacked fluorescent collector plates and gallium-indium-arsenide (GaInP), gallium-arsenide (GaAs) and silicon (Si) solar cells, utilizing the spectral selectivity of fluorescent conversion. Fluorescent collectors use fluorescent dye molecules embedded in a dielectric material to collect solar radiation. Incoming radiation is converted into radiation of lower energy, reduced by the Stokes shift energy ΔE. Total internal reflection keeps part of the converted radiation inside the collectors and guides it to the edges of the collector plates, where GaInP and GaAs solar cells are mounted. In order to make use of the spectral selectivity of each collector, the band gap energies Eg of the solar cells at the edges match the energy of dye emission. Optical transmission, reflection and photoluminescence measurements analyze the fluorescent collectors. A spectral transfer matrix formalism allows us to calculate the emitted photon flux of each collector as a function of the absorption/emission properties of the dye and the spectrum of incident radiation. By multiplying the transfer matrices tailored on each collector with the quantum efficiencies of the solar cells, we obtain the particular quantum efficiencies of each collector-cell sub-system and the overall quantum efficiency of the tandem system. The results show very good agreement in the shape of predicted and measured quantum efficiency curves of the tandem system.

Fluorescent concentrators concentrate both diffuse and direct radiation without requiring tracking of the sun. In fluorescent concentrators, luminescent materials embedded in a transparent matrix absorb sunlight and emit radiation with a different wavelength. Total internal reflection traps the emitted light and guides it to solar cells attached to the concentrator's edges. The escape cone of total internal reflection, however, limits the light collection efficiency. Spectrally selective photonic structures, which transmit light in the absorption range of the luminescent material and reflect the emitted light, reduce these losses. In this paper, we review different realizations of such structures and show that they increase collection efficiency by 20%. However, light emitted into steep angles in respect to the front surface, which would be lost without the photonic structures, has a very long effective path inside the concentrator until it reaches a solar cell. Therefore it suffers from path length dependent losses. We discuss how emission into the unfavorable directions can be suppressed by integrating the luminescent material into photonic structures, thus reducing these losses. We present possible realizations both for the concentrator design and for the solar cells used in such systems.

The efficiency of thin film solar cells can be improved with the addition of a photon down-conversion top layer. This layer converts incident ultraviolet light of the solar spectrum to visible light, which transmits through the glass and is efficiently absorbed by the active layer of the solar cell. The results of our investigations of thin dielectric films and fluorozirconate glass, both doped with Tb³⁺ ions, are presented. Tb3+ has absorption bands between 250 and 380 nm; the corresponding emission bands are in the spectral range between 400 and 630 nm. Thin SiO₂ and Al₂O₃ films with 0.04 - 10.18 at.% Tb were prepared by co-sputtering. For both as-deposited film systems, the highest fluorescence intensity is found for a Tb³⁺ doping level of approximately 1 at.%; the fluorescence intensity of Tb3+ in SiO₂ is higher than that in Al₂O₃. Thermal treatment leads to an enhancement of the fluorescence intensity by more than one order of magnitude and the highest fluorescence intensity is found for 2 at.% Tb for annealed thin SiO2 films containing Tb³⁺. For comparison, the absorption and emission properties of Tb³⁺-doped fluorozirconate glass are investigated for a doping level of 0.3 at.% Tb.

A stable and narrowly distributed dispersion of Mn-doped ZnS (sphalerite) nanoparticles with an average diameter of 1-2 nm, has been synthesized via chemical precipitation without using any surfactant. The surface of the particles has been functionalized with acrylic acid for compatibilization with polymethylmetacrylate (PMMA). A thorough morphological and optical characterization is proposed. A high nonlinearity in the response of the material is evidenced, with the maximum of luminescence obtained with 5%mol Mn doping.

Europium doped La2O3 nanocrystalline powders with sizes in the range of 50-200 nm have been obtained by the modified sol-gel Pechini method. These nanocrystals have been deagglomerated using sonication for 3 h and have been dispersed into a semiconductor P3HT polymeric matrix. X-ray diffraction (XRD) was used to study the evolution of the desired crystalline phase, and scanning electronic microscopy (SEM) to determine the size and the morphology of the nanoparticles. Transmission electronic microscopy (TEM) was used to analyse the sonication effect, and Raman technique to observe the dispersion of the nanocrystals into P3HT. Finally, we studied and analysed the luminescence properties of the trivalent europium in the hexagonal La₂O₃ nanocrystals by excitation and emission properties. The luminescence spectrum of Eu³⁺ in these nanocrystals is dominated by the 5D0 → 7F2 transition with a maximum intensity peak located at 626 nm. On the other hand, we observed that P3HT is a suitable polymer with an absorption band between 350-650 nm including the wavelength range in which the nanoparticles emit light. These properties will allow us to use this material as a down-converter material.

One of the ways in which the cell efficiency of solar cells may be improved by better exploitation of the solar spectrum makes use of the down-conversion mechanism, where one high energy photon is cut into two low energy photons. When energy transfer between rare earth ions is used to activate this process, high emission and absorption cross sections as well as low cutoff phonon energy are mandatory. Glass-ceramics can be a viable system to fulfill these requirements. The main advantage of the glass-ceramic is to combine the mechanical and optical properties of the glass with a crystallike environment for the rare-earth ions, where higher cross-sections of the rare earth ion can be exploited. In the case of silica-hafnia system the glass ceramic is constituted by nanocrystals of HfO₂, containing the rare earth ion, imbedded in the silica-hafnia host. Hafnia nanocrystals are characterized by a cutoff frequency of about 700 cm-1, so that nonradiative transition rates are strongly reduced, thus increasing the luminescent quantum yield of the rare-earth ions. In this work we investigated the Tb³⁺/Yb³⁺ energy transfer efficiency in a 70SiO₂-30HfO₂ glass-ceramic waveguide in order to convert absorbed photons at 488 nm in photons at 980 nm. The energy transfer efficiency was estimated as a function of the Tb³⁺/Yb³⁺ molar ratio as well as of the total amount of rare earth ions. A transfer efficiency of 38% was obtained for Tb³⁺/Yb³⁺ = 0.25 mol and a rare earth content [Tb+Yb]/[Si+Hf] = 5% mol.

Transparent glasses as up- or down-converters are attractive systems to increase the efficiency of solar cells. Er-doped fluorozirconate (FZ) glasses show an intense up-conversion upon excitation at 1540 nm. Transmission spectra show that the absorbance at 1540 nm grows linearly with the Er-doping level. In Eu-doped FZ glasses, which were additionally doped with chlorine ions, the growth of BaCl₂ nanocrystals can be observed upon thermal annealing. For high annealing temperatures a phase change from hexagonal to orthorhombic phase BaCl2 can be seen. Upon excitation in the ultraviolet (UV) spectral range these glass ceramics emit an intense blue emission. A combination of a silicon solar cell and an Eu-doped FZ glass ceramic as a down-converting top layer shows an increase in the short circuit current in the UV spectral range compared to a solar cell without a down-converting top layer.

In conventional silicon solar cells, photons with energies lower than the silicon band gap (1.12 eV) are not absorbed in the silicon layer. However, the near-infrared portion of the solar spectrum may still be able to contribute to photocurrent generation if use can be made of up-conversion processes that transform two or more infrared photons into a photon of sufficient energy to be absorbed in silicon. One possible material in which up-conversion processes occur are rare-earth ions such as Er³⁺. It has recently been shown that up-conversion in such ions could be enhanced by optical near-field coupling to metal nanoparticles in a highly controlled geometry. However, potential photovoltaic applications of the upconversion enhancement will certainly be characterized by different geometric arrangements, with random distances between ions and nanoparticles. Whether or not an overall enhancement of the up-conversion efficiency may be expected under such realistic conditions is an open question. In this work, we address an important aspect of this question, namely the particle-induced enhancement of the optical excitation rate in the rare-earth ions. Our model calculations show that the excitation rate in Er³⁺ ions can be enhanced using spherical gold nanoparticles. The model includes random distances between ions and nanoparticles, as well as random polarizations of the exciting light. The enhancement of the rate of excitation of the fundamental transition results in increases of the up-conversion rate by up to 20% for an excitation wavelength of 1523 nm, provided that photoluminescence-quenching effects due to nonradiative relaxation in the metal can be neglected.

The preferred surface spectral response for sub-ambient sky cooling varies according to the amount of water vapor in the atmosphere and the operating difference (T_a-T_s) between ambient and emitter surface temperatures. While all good candidates average high emittance from 7.9 μm to 13 μm, where the atmosphere is most transparent (the IR "sky window"), the preferred spectral response in the remainder of the Planck spectrum depends on a number of factors. Emittances E in studies to date have been near the two extremes of a high E ~ 0.85 to 0.95, and an E value between 0.3 to 0.4 for surfaces which emit strongly only in the sky window. Cooling rates and ideal spectral properties vary with operating conditions. The reasons behind this will be explained for select different coatings, using spectral densities for emitted outgoing radiation, which is T_s dependent, and the incoming radiation that is absorbed, which is fixed unless the atmosphere changes. Higher E surfaces always work best above and just below ambient but external factors that reduce incoming radiation from the atmosphere, including very low humidity or heat mirror apertures, extend this preference down to lower surface temperatures. Sky window spectrally selective coatings do not benefit as much because they already absorb little incoming radiation, but always have the potential to achieve very much colder temperatures if non-radiative heat gains are kept low.

The development of advanced materials for facades aims to achieve higher energy efficiency of buildings. Successful application of these materials depends on the availability of reliable characterization data. While data derived from integrated measurements of transmission and reflection is widely available, it does not allow to characterize the angular dependence of the performance of such materials. The Bidirectional Reflection-Transmission Distribution (BRTD) can be measured by commercially available Gonio-Photometers and, complimenting integrated transmittance and reflectance, allows the assessment of facade materials and thus supports both their development and application. Validation of the obtained data is crucial to back these measurements. Integration of validation procedures into the operation of a characterization laboratory allowing a well-defined approach to quality control is presented for a range of typical material and sample types: * consistency checks of measurement data * cross-checking of integrated material properties derived from BRTD data with integrating sphere measurements * round-robin comparison between laboratories using comparable devices The results of of these first measurements are discussed. Potential to further improve the availability of reliable angular resolved characterization data for the building sector is identified.

One possibility to enhance the solar cell efficiency above the Shockley Queisser limit is trapping the sunlight inside the absorber with angular selective filters like a 3d photonic structure to increase the optical path length of the radiation. In this study we analyze a 3d opal structure for light trapping in crystalline silicon solar cells. Therefore, we performed reflection and transmission measurements on the opal and luminescence measurements on a crystalline silicon wafer in conjunction with an opal. Results from measurements were compared with numerical simulations to identify the effects of the photonic stop gap which departs from ideal ones by limited number of layers (below 10 layers) and by lateral departure from translational symmetry due to ordered opal regimes of limited sizes (around 100 μm²). For the discussion of experimental observations we include as well the influence on luminescence yield of a hypothetic non-stop-gap over layer with similar refractive index.

Diffractive effects have the potential to greatly increase light trapping in solar cells. The simulation of solar cells with diffractive elements is, however, difficult. The reason for this difficulty is that wave optical considerations are required. Typically, for solar cell simulations a ray tracing approach complemented by a transfer matrix algorithm is sufficient to simulate the optical properties. In this paper we present a more fundamental method to consider wave optical effects for solar cells. The optical characteristics of a solar cell are externally simulated using a wave optical approach and are used as input parameters for a simulation of the electrical characteristics. This coupled method is tested on an exemplary system; a solar cell with a backside diffractive grating. In a first step we show that substituting the optical parameters by externally simulated ones is expedient. Furthermore we show that diffractive effects, which hitherto could not be considered, can be made accessible for solar cell simulations. Additionally, the potential for a grating within a solar cell was investigated, resulting in an increase of 1% efficiency absolute for a crystalline silicon solar cell.

A sophisticated light-management is indispensable for silicon thin-film silicon solar cells based on amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon. The optical properties of thin-film solar cells have a significant influence on the conversion efficiency. The topology of the nano-textured interfaces affects the optical path and absorption. A rough transparent conductive oxide (TCO) film leads to a high quantum efficiency and shortcircuit current density. Simulations of various geometries indicate the optimal texture. Therefore, we simulate 3-dimensional tandem thin-film solar cells with different interfaces. The roughness can be identified by atomic force microscope (AFM) scans. In order to accurately analyze all aspects of the light propagation in solar cells, numerical simulations of Maxwell's equations are needed. By standard simulation programs for solving Maxwell's equations, it is difficult to simulate realistic textures of the solar cell layers. Therefore, a simulation tool based on the finite difference time domain (FDTD) method and the finite integration technique (FIT) is developed, which is able to integrate AFM scan data. To incorporate the nanostructure of a relevant section in the AFM scans, high computational domains are needed. This leads to a large number of grid points in the resulting discretization. Parallel computations on high performance computers are needed to meet the large computational requirements. The simulations show that the light propagation in the investigated thin-film device is a complex phenomenon depending on the wavelength and phase of the incident light.

Investigations of optical losses induced by localized plasmons in protrusions on silver back contacts of thin-film silicon solar cells are presented. The interaction of electromagnetic waves with nanoprotrusions on flat silver layers is simulated with a three-dimensional numerical solver of Maxwell's equations. Spatial absorption profiles and spatial electric field profiles as well as the absorption inside the protrusions are calculated. The results presented here show that the absorption of irradiated light at nanorough silver layers can be strongly enhanced by localized plasmonic resonances in Ag nanoprotrusions. Especially, localized plasmons in protrusions with a radius below 60 nm induce strong absorption, which can be several times the energy irradiated on the protrusion's cross section. The localized plasmonic resonances in single protrusions on Ag layers are observed to shift to longer wavelengths with increasing refractive index of the surrounding material. At wavelengths above 500 nm localized plasmonic resonances will increase the absorption of nanorough μc-Si:H/Ag interfaces. The localized plasmon induced absorption at nanorough ZnO/Ag interfaces lies at shorter wavelengths due to the lower refractive index of ZnO. For wavelengths above 500 nm, a high reflectivity of the silver back contacts is essential for the light-trapping of thin-film silicon solar cells. Localized-plasmon induced losses at silver back contacts can explain the experimentally observed increase of the solar cell performance when applying a ZnO/Ag back contact in comparison to a μc-Si:H/Ag back contact.

We investigated a three dimensional inverted opal having the potential to notably increase light-trapping in solar cells. The 3D photonic crystal top layer is an angle- and direction-selective filter, which decreases the acceptance cone of the solar cell. Numerical optimisation methods are used to verify the optical and electrical properties for a large angluar and energy spectrum for a system consisting of an inverted opal on top of a thin crystalline silicon solar cell. It is numerically shown that an inverted opal grown in the Τ - Xdirection might fulfill the requirement for such a filter. An estimate for the theoretically achievable efficiency for nonconcentrated light is presented that do show an enchanced efficiency near the electronic band edge of the absorber. The fabrication of first opals grown in Τ - Xdirection is presented and discussed with respect to the quality and large scale fabrication.

At present there is considerable global concern in relation to environmental issues and future energy supplies, for instance climate change (global warming) and the rapid depletion of fossil fuel resources. This trepidation has initiated a more critical investigation into alternative and renewable sources of power such as geothermal, biomass, hydropower, wind and solar energy. The immense dependence on electrical power in today's society has prompted the manufacturing of devices such as photovoltaic (PV) cells to help alleviate and replace current electrical demands of the power grid. The most popular and commercially available PV cells are silicon solar cells which have to date the greatest efficiencies for PV cells. The drawback however is that the manufacturing of these cells is complex and costly due to the expense and difficulty of producing and processing pure silicon. One relatively inexpensive alternative to silicon PV cells that we are currently studying are dye-sensitised solar cells (DSSC or Grätzel Cells). DSSC are biomimetic solar cells which are based on the process of photosynthesis. The SFI Strategic Research Centre for Solar Energy Conversion is a research cluster based in Ireland formed with the express intention of bringing together industry and academia to produce renewable energy solutions. Our specific research area is in DSSC and their electrical properties. We are currently developing testing equipment for arrays of DSSC and developing optoelectronic models which todescribe the performance and behaviour of DSSCs.

Investigation of some light-matter interactions in Multi-Interface Novel Devices (MIND) containing a nanoscale Si-layered system have led to a method for predicting free-carrier density dependent nonlinear optical properties as a function of doping, light excitation intensity and carrier injection. The approach is based on the well-known t-matrix approximation. A simplified a few-layer optical model has been constructed that will reproduce the main features/parameters of real systems. The degree of model and simulation self-consistency is discussed using basic physical functions and published experimental data. Near perfect agreement between the simulated model and the corresponding experimental results has been obtained. In this way, the simulation allowed us to determine the main origins/components of the strong optical nonlinearity characteristic of one of the most specific MIND behaviours.

Borate glasses and borate glass ceramics are good candidates as a matrix material for fluorescent ions like samarium. The chosen network modifier influences the fluorescence efficiency of the incorporated rare earth ion. Sm³⁺-doped lithium, sodium, barium and lead borate glasses were examined with respect to their fluorescence properties and potential use as a down-converting top layer of a solar cell.

The surface grating technologies enable to control the thermal radiation spectrum. We are applying this technique to promote the chemical reaction to produce hydrogen in the methane steam reforming process by spectrally resonant thermal radiation. The thermal radiation spectrum is adjusted to the vibrational absorption bands of methane and water molecules near 3μm by making two-dimensional (2D) microcavities with the period Λ=2.6 μm on the radiative surface. By tuning the peak of thermal radiation to the absorption bands of these gases, it is clearly observed that the methane steam reforming is promoted by using spectrally selective emitter. Since the promotion of hydrogen production can be observed under resonant excitation of gases, it is suggested that the optical excitation of vibrational levels is contributed to this phenomenon. From the result, it is confirmed that the thermal radiation resonant with molecular absorption bands is effective to the high production rate of hydrogen in methane steam reforming process. To study the detail process of chemical reaction, under resonant excitation, the produced gas is analyzed by gas chromatograms.

We explore different approaches to achieve co-doping of glasses with rare earth ions and metallic nanoparticles, and to manipulate the spectral position of the particles' surface plasmon resonance. The final goal is to find a composite material with improved efficiency of frequency up-conversion of light for photovoltaic applications. The potential for improvement has been shown by theoretical calculations predicting that absorption and emission probabilities of the ions can be enhanced when the plasmon resonance of the nanoparticles is close to the respective transition frequency of the ions. In this work we demonstrate the sequential co-doping of glasses already containing rare-earth ions with Ag nanoparticles, as well as implantation of rare-earth ions in glasses which already contained metallic nanoparticles. It could also be demonstrated that the surface plasmon resonance of the created particles can be tuned by femtosecond laser induced shape transformation of the Ag clusters.

Nanostructured Si devices based on a nanoscale Si-layered system may constitute an interesting system for enlarging optical and electrical functions in Si optoelectronic technology. Strong enough physical interactions transform the initial material, without changing its chemical composition, leading to a Si metamaterial. We report here some specific electrical properties which illustrate the complexity of the electron transport in test structures. I-V measurements on samples differentiated exclusively by their surface features are given for PV and photodiode modes as well as time-resolved current collection under stabilized voltage. The measurements have been carried out over a large range of solar light excitation intensities.

A complete model for the heavily doped and/or highly excited Si:P dielectric function is presented in the limit of the free-carrier gas approach. New interesting features of Si:P dielectric functions are presented and discussed. The influence of dopants and free-carriers is taken into account independently and their common features usually assumed in the literature, are analysed. The influence of Drude damping time on the optical response of heavily doped Si:P is studied. All results are compared with experimental data.