This PDF file contains the front matter associated with SPIE Proceedings Volume 8438, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Up-conversion (UC) and down-conversion (DC) of sunlight are two possible routes for improving energy harvesting over the whole solar spectrum. Via such processes it could be possible to exceed the Shockley-Queisser limit for a single-junction photovoltaic (PV) device. The effect of adding DC and UC layers to the front and rear of a solar cell, respectively, is to modify the incident solar spectrum. One of the materials more extensively studied for these propose have been the lanthanides or rare-earth systems, due to the suitability of their discrete energy levels for photon conversion inside a wide variety of host materials. While high quantum yields of 200% have been demonstrated with DC materials, there remain several barriers to realising such a layer that is applicable to a solar cell. These are, firstly, weak absorption of the lanthanide ions and, secondly, the competing loss mechanism of non-radiative recombination. For UC, these two barriers still exist, however an additional challenge is the non-linear nature of the UC process, thus favouring operation under concentrated sunlight. In this paper, we review the application of UC and DC to PV, discussing the material systems used and optical characterisation.

Sm³⁺-doped barium borate glasses are investigated for their potential as a superstrate for CdTe solar cells. The influence of the Sm³⁺ conversion efficiency and the Sm₂O₃ doping level on the short circuit current density of a CdTe solar cell is analyzed. CdTe solar cells with CdS layer thicknesses of 45 and 300 nm are evaluated. A 3.2 mm thick, 2 mol% Sm₂O₃- doped glass superstrate enables a relative increase in the short circuit current density of approximately 1.4% and 2.9% for a 45 and 300 nm CdS buffer layer, respectively, assuming 100% Sm³⁺ conversion efficiency.

The charge transfer state (CTS) band of the Eu³⁺:La₂O₃ nanocrystals were studied in detail in order to understand the shift of the maximum of this band. Eu³⁺:La₂O₃ nanoparticles present a broad CTS band. However, the maximum is peaking below 300 nm, far below the limit of the solar spectrum arriving to the surface of the Earth and it makes difficult the application of this material as down-shifting in solar cells. In order to shift the CTS band towards blue wavelengths, different synthesis methods such as hydrothermal and sol-gel modified Pechini methods were used to prepare these nanoparticles, and adding additional CTS bands with co-doping ions such as Bi³⁺ was explored as well. The crystalline structure of Eu³⁺:La(OH)₃, Eu³⁺:La₂O₃, Bi³⁺:Eu³⁺:La(OH)₃ and Bi³⁺:Eu³⁺:La₂O₃ and the cell parameters of Eu³⁺:La₂O₃ and Bi³⁺:Eu³⁺:La₂O₃ were analyzed by X-ray powder diffraction technique and their morphology was observed by transmission electron microscopy. Once we obtained the cell parameters refining the XRD patterns by Full-prof software, we were able to calculate the Eu³⁺-O distance by ATOMS software through the structure previously represented following the Pauling model. The results found suggest that we need to take into account another parameter apart from the Eu³⁺-O distance to explain the CTS band small shift.

Organic polymeric chiral nematic liquid crystalline (cholesteric) wavelength selective mirrors can increase the efficiency of luminescent solar concentrators (LSCs) when they are illuminated with direct sunlight normal to the device. However, due to the angular dependence of the reflection band, at larger incidence angles the cholesterics reflect away some incoming sunlight that could have been absorbed by the luminophore. As a result, the increase in LSC efficiency after application of a cholesteric reflector drops if the light incident to the device is at angles larger than 30 degrees. The cholesteric reflectors still have a positive impact on device performance for light incident up to 45-50 degrees but at larger angles efficiency decreases when a cholesteric reflector is added. This affects the performance of the LSC device when illuminated with indirect incident light, especially when the incident light has a large contribution of photons above 45 degrees.

Upconversion of otherwise lost sub-band-gap photons is a promising approach for more efficient solar cells. We investigate upconverter materials based on lanthanides, especially trivalent erbium. They are known for high upconversion efficiency of infrared photons under laser excitation at a wavelength around 1520 nm. However, the achieved upconversion efficiency is still not large enough and the absorption range of these materials is too narrow for an application in photovoltaics. Herein, we present an overview of different possibilities to enhance the efficiency of upconversion for silicon solar cells. The concepts discussed can be divided into two groups. The first group comprises internal concepts, e.g., the host material itself, size effects and dopant concentration. The second group consists of external methods, which change the physical environment around the upconverter to improve the absorption properties and enhance the upconversion quantum yield. By considering the different effects in a sophisticated rate equation model of the upconverting material β-NaY_0.8Er_0.2F₄, and comparing the results with experimental data, we show that there is a big potential to improve the upconversion properties for solar applications. Furthermore we show variety opportunities to increase the upconversion quantum yield are.

To achieve higher efficiencies in solar cells one possibility is to integrate angular selective filters, with the aim of decreasing losses caused by radiative recombination. In fact, thermodynamically, angular selectivity is equivalent to concentration. In both cases the Shockley-Queisser-Limit of solar cells is overcome by manipulating the ratio of incoming and outgoing radiation represented by the angles of incidence and emission. In concentrating systems the angle of incidence is increased, whereas in systems with an angular confinement the angle of emission can be decreased. Another possibility to achieve highest efficiencies is to combine both, concentration and angular confinement. Starting with a given concentrating system, photonic angularly selective filters such as thin film stacks are investigated and optimized for the use in this system. We present results of wave optical simulations of these filters and show some of their characteristics. The goal of this study is, however, not only to optimize optical filters but also to consider the whole system. One approach is to use results from optical simulations as input values for detailed balance simulations of the solar cell. So, the main advantage is, that in fact not the optical characteristics are optimized separately, but rather the whole system is taken into account, which allows predictions of theoretical efficiency enhancement.

In this work we theoretically investigate the light trapping properties of one- and two-dimensional periodic patterns etched in crystalline silicon solar cells with anti-reflection coating and back-reflector, in a wide range of active material thicknesses. The resulting short-circuit current (taken as the figure of merit for efficiency) and the optical spectra are compared with those of an unpatterned cell, and with the ultimate limits to light trapping in the case of a Lambertian (isotropic) scatterer. Photonic patterns are found to give a substantial absorption enhancement, especially for twodimensional patterns and for thinner cells, thanks to physical mechanisms like reduction of reflection losses, diffraction of light into the cell, and coupling into the resonant optical modes of the structure.

A sophisticated light management is important to construct thin film solar cells with optimal efficiency. This is based on suitable nanostructures of different layers and materials with optimized optical properties. To design thin film solar cells with high efficiency, simulation of light-trapping is a very helpful tool. Such a simulation has to take into account the underlying physical properties like plasmonic effects of silver or interferences. To this end, it is important to solve Maxwell's equations on a discretization grid. To obtain an accurate simulation, the roughness of the top transparent conductive oxide (TCO) layer is described by AFM-scan data. To meet the high computational amount in solving Maxwells equations on a finite difference discretization grid, high performance computers (HPC) are used.

Random structures are typically used for light trapping in thin-film silicon solar cells. However, theoretically periodic structures can outperform random structures in such applications. In this paper we compare random and periodic structures of similar shape. Both types of structure are based on atomic force microscopy (AFM) scans of a sputtered and etched ZnO layer. The absorption in a solar cell on both structures was calculated and compared to external quantum efficiency (EQE) measurements of samples fabricated on the random texture. Measured and simulated currents were found to be comparable. A scalar scattering approach was used to simulate random structures, the rigorous coupled wave analysis (RCWA) to simulate periodic structures. The length and height of random and periodic structures were scaled and changes in the photocurrent were investigated. A high height/length ratio seems beneficial for periodic and random structures. Very high currents were found for random structures with very high roughness. For periodic structures, current maxima were found for specific periods and heights. An optimized periodic structure had a period of Λ = 534 nm and a depth of d = 277 nm. The photocurrent of this structure was increased by 1.6 mA/cm² or 15% relative compared to the initial (random) structure in the spectral range between 600 nm and 900 nm.

The influence of the front texture of an etched transparent conductive oxide with crater-like structures of various sizes on the absorption of a thin amorphous silicon (a-Si:H) layer is investigated by rigorous optical simulations as part of two simplified systems: A simplified single junction device, using a perfect metal as back contact and a top cell of an amorphous/microcrystalline silicon tandem device, using a microcrystalline silicon halfspace adjacent to the amorphous layer. The texture is modified by stretching either in height or laterally and the average absorption in the a-Si:H layer is investigated relative to the original structure. We investigate the average absorption for each wavelength as well as the total absorption, weighted with an AM1.5g spectrum. Furthermore, the local absorption distribution inside the a-Si:H layer is examined to improve the understanding of local texture features and their influence on absorption and cell performance. For both modifications, an optimal point can be found to improve the absorption in the amorphous layer by up to 15% and 6% for a simplified single junction and tandem top cell, respectively. In case of the top cell of the simplified tandem device, it is found that additionally, the transmission into the microcrystalline silicon can be improved. Also, the local absorption distribution shows that there is an optimal size of the surface craters for all modifications, while steeper crater rims in general lead to higher absorption.

In order to improve the photoconversion efficiency, we consider the possibility of increasing the photocurrent in solar cells exploiting the electron photoemission from small metal nanoparticles into a semiconductor. The effect is caused by the absorption of photons and generation of local surface plasmons in the nanoparticles with optimized geometry. An electron photoemission from metal into semiconductor occurs if photon energy is larger than Schottky barrier at the metal-semiconductor interface. The photocurrent resulting from the absorption of photons with energy below the bandgap of the semiconductor added to the solar cell photocurrent can extend spectral response range of the device. We study the effect on a model system, which is a Schottky barrier n-GaAs solar cell, with an array of Au nanoparticles positioned at the interface between the semiconductor and the transparent top electrode. Based on the simulations, we chose to study disk-shaped Au nanoparticles with sizes ranging from 25nm to 50nm using electron beam lithography. Optical characterization of the fabricated devices shows the presence of LSP resonance around the wavelength of 1250nm, below the bandgap of GaAs.

The challenge of future solar cell technologies is the combination of highly efficient cell concepts and low cost fabrication processes. A promising concept for high efficiencies is the usage of nanostructured silicon, so-called black silicon. Due to its unique surface geometry the optical path of the incoming light through the silicon substrate is enhanced to nearly perfect light trapping. Combined with the semiconductor-insulator-semiconductor (SIS) solar cell concept it is possible to fabricate a low cost device by using conventional sputtering technologies. Therefore, a thin insulator is coated on the nanostructured silicon surface, followed by the deposition of a transparent conductive oxide (TCO), e.g. indium tin oxide (ITO) or aluminum doped zinc oxide (AZO). In such systems the TCO induces a heterojunction, hence, high temperature diffusion processes are not necessary. The optical and geometrical properties of different nanostructured silicon surfaces will be presented. Furthermore, the influence of the used TCO materials will be discussed and the solar cell performance under AM1.5G illumination of unstructured and structured SIS devices is shown.

In this work, microscopic three-dimensional (3D) simulations were performed on nanowire array solar cells to study the impact of surface recombination on the photovoltaic performance. Both axially and radially arranged p-n junction in III-V based structures were taken into consideration. From the cases with surface recombination velocity varying from 1e3cm/s to 1e6cm/s, the core-shell nanowire was found to provide better tolerance for surface recombination. The difference of surface recombination within the axial and core-shell structures is explained by analyzing the relevent minority carrier density, followed by a discussion on the impact of surface recombination on the performance of nanowires as photovoltaic devices.

The combination in hybrid heterojunction of nanocrystals and semiconductor polymers has great potential for light-toenergy conversion devices. For this reason, a great number of different quantum dots/polymer molecular solar cells have been investigated. However, less attention has been paid to the photo-induced charge transfer processes at the interface of these systems. Here we report a time resolved spectroscopic study of the electron injection and recombination transfer steps of CdSe/P3HT bulk heterojunction films. From the data obtained using Time Correlating Single Photon Counting (TCSPC) we have inferred that electron injection from P3HT excited state to CdSe nanocrystal conduction band occurs faster than 250 ps and the electron yield is higher than 85%, independently of the nanocrystal shape. On the other hand, the use of Laser Transient Absorption Spectroscopy allowed us to observe that all the studied interfacial charge transfer process can be fitted to dispersive stretched exponentials kinetics, independently of the QD's concentration and nanocrystal morphology, thereby offering evidence of multiple decay process in CdSe/P3HT bulk heterojunctions.

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. At first, these structure origination and replication technologies are presented in detail: With the interference pattern of two or more coherent waves, a wide variety of structures with feature sizes ranging from 100 nm to 100 μm can be generated in photoresist by interference lithography. Examples are linear gratings, crossed gratings, hexagonal structures, three dimensional photonic crystals or surface-relief diffusers. The strength of this technology is that homogeneous structures can be originated on areas of up to 1.2 x 1.2 m². The structures in photoresist, the so-called master structures, can serve as an etching mask for a pattern transfer, as a template for infiltration with different materials or they can be replicated via electroplating and subsequent replication processes. Especially in combination with replication steps, the industrially feasible production of elaborate structures is possible. As a particularly interesting process, nanoimprint lithography (NIL) is described in detail. As a way towards industrial production, a roller NIL tool is presented. After the description of the basic technologies, three application examples for solar cells are presented with details about the design of the structures, the structuring processes, sample characterization and evaluation: (1) honeycomb structures for the front side texturization of multicrystalline silicon wafer solar cells, (2) diffractive rear side gratings for absorption enhancement in the spectral region near the band gap of silicon, and (3) plasmonic metal nanoparticle arrays manufactured by combined imprint and lift off processes.

Two dimensional (2D) periodic photonic nanostructures, fabricated by nanoimprint lithography (NIL) and dry etching on the front surface of crystalline silicon (c-Si) layers, are investigated experimentally and theoretically in order to characterize their optical properties and demonstrate their relevance to photovoltaic (PV) applications. Nanoimprint lithography is performed on c-Si wafers and ultra-thin c-Si films with various thicknesses. A comparison with state-ofthe- art front side texturing with an antireflection coating is made. The 2D periodic photonic nanostructures result in an enhanced light absorption in the photoactive material. The results are validated through simulations based on Rigorous Coupled Wave Analysis (RCWA). The nanoimprinted substrates result in a similar absorption compared to the state-ofthe- art random pyramid texturing while consuming less than a micron of photoactive material. In contrast to the random pyramid texturing, the nanopatterning exhibits a robust performance for a wide range of incident angles up to 70°. The light trapping mechanism we propose is based on the combination of a graded index effect and the diffraction of light inside the photoactive layer at high angles.

We present a process chain to generate periodically arranged metallic nanostructures supporting plasmons for the use in solar cells. As proof-of-concept, platinum and silver nanoparticles with a period of 1 μm and a diameter of 600 nm were fabricated. For the platinum particles, an absorption enhancement for light with a wavelength of 3.6 μm was observed in silicon. By decreasing structure sizes the active spectral region can be shifted to wavelength relevant for solar cell applications. Therefore, in a second step a silver grating with a smaller period and also smaller diameter of approximately 200 nm was realized.

In this work, we studied the feasibility of surface texturing of thin molybdenum layers on a borosilicate glass substrate with Ultra-Short Laser Pulses (USLP). Large areas of regular diffraction gratings were produced consisting of Laserinduced periodic surface structures (LIPSS). A short pulsed laser source (230 fs-10 ps) was applied using a focused Gaussian beam profile (15-30 μm). Laser parameters such as fluence, overlap (OL) and Overscans (OS), repetition frequency (100-200 kHz), wavelength (1030 nm, 515 nm and 343 nm) and polarization were varied to study the effect on periodicity, height and especially regularity of LIPSS obtained in layers of different thicknesses (150-400 nm). The aim was to produce these structures without cracking the metal layer and with as little ablation as possible. It was found that USLP are suitable to reach high power densities at the surface of the thin layers, avoiding mechanical stresses, cracking and delamination. A possible photovoltaic (PV) application could be found in texturing of thin film cells to enhance light trapping mechanisms.

We present here some of the last results of the EUROPEAN project ALPINE. We present both the development of an adjustable fibre laser pulse source and scribing results on CdTe and CIGS solar cells. The scribing tests were performed at three different pulse durations: 400 fs, 8 ps and 250 ps. The results obtained with 250 ps are already very promising for P3 steps in both CdTe and CIGS solar cells. In both cases the results were validated electrically. In the case of P3 scribing for CIGS solar cells, shunt resistances as high as 125 kΩ.cm were obtained. Isolation resistances were higher than 1 MΩ.cm. The processing speed was 2 m/s.

Luminescent solar concentrators (LSCs) were developed over three decades ago as a simple route to obtain high concentration ratio for photovoltaic cells without tracking the sun. Despite their exciting theoretical potential, LSCs have thus far reached only modest concentration ratio in practice. Here, we introduce two new approaches to LSC optical design that enable significant increases in concentration ratio for any type of luminescent material. First, we discuss the conceptual basis for nonimaging optics in luminescent concentration and then present exact ray tracing results for the simplest implementation of compound parabolic concentrator-shaped edges that deliver ~15% increase in concentration ratio with negligible decrease in optical efficiency. Second, we extend the recently introduced concept of resonance-shifting to the case of shaped substrates and demonstrate that light can be channeled through an LSC with low loss and localized at specific points, opening up entirely new directions in LSC design.

TiO2 based sol-gel microstructuring layer is applied to increase efficiency of solar cells modules or the so-called PV planar concentrators, by reducing the amount of solar cells (silicon based solar cells) while increasing the amount of solar energy trapped into the modules. The proposed solution is based on linear grating whose role is to trap and diffract the incident beam to solar cells. The design of the module and the optimized gratings, to be angularly and spectrally tolerant, are presented. The paper also deals with the fabrication of large and long gratings (m² area), using the unique direct photopatterning sol-gel solution, based on the dynamic and continuous gratings writing using phase mask lithograghy.

Compact and inexpensive solar concentrators can be designed by using transmission gratings that diffract incident sunlight into a light guide. To this end a grating should have a small period and maintain a high diffraction efficiency over a wide range of incident angles. We numerically study the angular dependence of the diffraction efficiency of surface-relief gratings using Rigorous Coupled-Wave Analysis. It is shown how one can control the angular acceptance of gratings by tuning the refractive index or the grating topology. Gratings with a high refractive index maintain a high diffraction efficiency over a wide range of incident angles. By adjusting the topological symmetry one can design a grating with a high diffraction efficiency over a narrow range of incident angles, or a grating with a more homogenous distribution of the diffraction efficiency.

The use of upconverters (UC) to harvest light with photon energy below the bandgap of a photovoltaic cell is one possible route to overcome the Shockley-Queisser limit for single junction devices. The materials which have shown potential to enhance the performance of silicon (Si) cells are rare earths (RE) such as trivalent erbium (Er³⁺). Er³⁺ is limited by a low absorption cross section over a narrow bandwidth which requires high excitation powers to achieve good efficiencies due to its non-linear response. This material has predominantly been investigated under monochromatic excitation at 1523nm as this achieves strong resonance with the equidistant energy levels although, is not representative of its application under a spectrally broad solar irradiance. In this paper we show the importance of using broadband excitation (12nm and 38nm bandwidths) as a method to characterise these materials and understand their possible benefits. Using an oxyfluoride ceramic with active YF₃:Er³⁺ _10% nano-crystals (NC), and increasing the bandwidth by a factor of 3.17, lead to a 55 fold increase in emission for the same solar concentration. This is equivalent to achieving the same level of emission with a factor of 7.6 less Suns.

Neodymium-doped barium borate glasses are investigated for their potential as fluorescent concentrators for the near infrared spectral range. Additional doping of the glasses with silver oxide and subsequent heat treatment leads to a reduction of the doped silver ions and to the formation of metallic silver nanoparticles. The formation of the silver nanoparticles is indicated by a broad surface plasmon-related extinction band at approximately 410 nm. The influence of the silver nanoparticles on the fluorescence properties is investigated.

This paper proposes a sawtooth-shape planar lightguide solar concentrator. In this design, sunlight is collected by each lens in a two-dimensional lens array and is coupled into the planar lightguide using localized micro structure placed at each lens focus. Where the micro structure is the dimpled structure [2010 International Optical Design Conference, paper ITuE5P]. In this design, one side of the planar lightguide was shaped in a sawtooth-form shape. The usage of the sawtooth-shape planar lightguide prevents most leakage of guiding rays after multiple reflections in lightguide. Simulation results show that the sawtooth-shape planar lightguide solar concentrator can achieve 92% optical efficiency at 300× concentration.

Optical fibres as sunlight harvesting waveguides for use in concentrating photovoltaic (CPV) systems are proposed. Results of ray tracing simulations and experimental measurements in feasible optical fibre configurations are presented in this paper. The configurations incorporate spherical as well as aspheric lenses, a simpler precursor of the Fresnel lens. Step index fibres with SiO₂ as core material and preferably high numerical apertures and high incidence angles are utilised initially. Scenarios with sources of monochromatic and 1000 W/m² irradiance are considered on simulations. Although high concentrations can be achieved in practice, for CPV applications uniformity at the end receiver is also considered as a key factor to realise acceptable cell performance. We obtained more than 99% uniformity at the end receiver of the proposed configurations for flux concentration of 2000 suns.

We report an optimized inverted bulk-heterojunction (P3HT:PCBM) organic solar cell geometry in order to both efficiently trap incident light within in the cell (increasing light absorption) and at the same time provide efficient transport of the generated carriers to the electrodes (reducing the active layer thicknesses). To address these issues, we have used two approaches. The first one consists of including diffraction gratings that increase the light path length in the cell and thus enhance absorption in wavelength intervals matching the absorption peak of the organic active layer on the bottomelectrode, while the second approach includes Ag nanoparticles embedded on the solar cell topelectrode, which scatter the incident light into the solar cell active layer. The solar cells containing either gratings or nanoparticles exhibit a significant enhancement on the power conversion efficiency. Furthermore, the solar cells do not contain the rare metal indium, but employ a PEDOT:PSS based transparent electrode.

Adsorption geometry, nuclear vibrations, and molecular orientation of the dye with respect to the oxide surface affect significantly the performance of dye-sensitized solar cells. We compute the influence of these factors on injection and recombination conditions in organic amino-phenyl acid dyes differing by the donor group on the anatase (101) surface of titania. Nuclear motions affect significantly and differently between the dyes the driving force to injection Δ G. A temperature increase from 300 to 350 K does not have a noticeable effect on the distribution of injection rates in all studied system. Molecular dynamics simulations predict configurations in which dyes tend to lay flat on the oxide surface. The resulting proximity of the oxidation equivalent hole to the oxide is expected to promote recombination. Temporal evolution of the driving force to injection is found to be independent of dye orientation and uncorrelated to the oscillations of the O_dye Ti bonds through which the dye is attached to the surface. We conclude that the dynamics of Δ G(t) is explained by uncorrelated evolution of the energies of the dye excited state and of the conduction band minimum of the oxide due to their respective vibrations. This suggests that it must be possible to control independently conditions of recombination (e.g. by preventing the dye oxidation hole from approaching TiO₂ by using co-adsorbates) and of injection (e.g. by designing dyes where non-equilibrium geometries strongly destabilize dye's LUMO to increase Δ G).

We present experimental results and rigorous numerical simulations on the optical properties of Black Silicon surfaces and their implications for solar cell applications. The Black Silicon is fabricated by reactive ion etching of crystalline silicon with SF₆ and O₂. This produces 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 and a pronounced light trapping effect at the silicon absorption edge such Black Silicon surface textures are beneficial for photon management in photovoltaic applications. We demonstrate that those light trapping properties prevail upon functionalization of the Black Silicon with dielectric coatings, necessary to construct a photovoltaic system. The experimental investigations are accompanied by rigorous numerical simulations based on three dimensional models of the Black Silicon structures. Those simulations allow insights into the light trapping mechanism and the influence of the substrate thickness onto the optical performance of the Black Silicon. Finally we use an analytical solar cell model to relate the optical properties of Black Silicon to the maximum photo current and solar cell efficiency in dependence of the solar cell thickness. The results are compared to standard light trapping schemes and implications especially for thin solar cells are discussed.

Two different transparent conductive oxides (TCO) were deposited by magnetron sputtering on borate glasses. The influence of sputtering conditions on optical, electrical and microstructural properties was much higher for indium tin oxide (ITO) than for aluminium-doped zinc oxide (AZO) films. Specific resistivity values obtained from simulation of the optical spectra are in good agreement with values obtained from four-point probe measurements.

Lanthanide based dyes belong to one of the most promising fields of photovoltaic research, combining high quantum yields and large spectral shift. However, many challenges are faced when working with lanthanide dyes for spectral conversion: their thermal and chemical stability, which can greatly influence the shelf-life of the dyes; the absorption band position, which depends on the organic part of the dye, the so called "antenna"; self-quenching mechanisms, which lead to a photoluminescence emission loss. The chemical composition of the surrounding environment of the dyes has a fundamental role in their properties. In this paper, the optical and PLQY (photoluminescence quantum yield) properties of an europium-based dye embedded in a silica matrix are reported. The in-house synthesized dye consists of a bis(2- (diphenylphosphino)phenyl)ether oxide (DPEPO) ligand and three hexafluoroacetylacetonate (hfac) co-ligands coordinating a central europium ion. The dye has been included in porous core-shell particles, to study its optical properties once embedded in a solid dielectric matrix. The optical properties of the resulting samples have been characterized by photoluminescence emission and PLQY measurements. The results have been compared with data obtained from a commercially available dye (BASF Lumogen family) in similar conditions.

Photonic crystals modify the local density of photon states. These variations influence the emission properties of a dipole embedded within the photonic crystal. Furthermore, field enhancement can be observed within photonic crystals. In this paper, we investigate how these effects influence upconversion processes in β-NaYF₄:Er³⁺. For this purpose we use finite-difference time-domain (FDTD) simulations of a grating-waveguide-structure in combination with a rate equation model of the upconversion processes in β-NaYF₄:Er₃₊. The grating parameters are optimized to achieve large field enhancements within the structure for the combination of s- and p-polarized light. Furthermore, the variation of the spontaneous emission rates for dipole emitters within the structure is simulated. The varied transition rates, as well as the field enhancement, serve as input parameters for the rate equation model for upconversion. Using this approach, the influence of the structure on the upconversion quantum yield is calculated. For a simulated initial irradiance of 1000 W/m², we find enhancement factors of up to four for the field enhancement in the upconverter region and up to a factor of three for the upconversion quantum yield. In consequence, the incorporation of upconverting material in photonic structures in very promising to increase upconversion efficiencies.

Microcrystalline silicon (mc-Si) lms deposited using a Plasma Enhanced Chemical Vapour Deposition (PECVD) process constitute an important material for manufacturing low-cost, large-area thin-lm devices, such as solar cells or thin-lm transistors. Although the deposition of electronic-grade mc-Si using the PECVD process is now well established, the high substrate temperature required (~400°C) does not lend itself to electronic devices with exible form factors fabricated on low-cost plastic substrates. In this study, we rst investigated an intrinsic mc-Si layer deposited at plastic-compatible substrate temperatures (~150°C) by characterising the properties of the lm and then evaluated its applicability to p-i-n solar cells though device characterisation. When the performance of the solar cell was correlated with lm properties, it was found that, although it compared unfavourably with mc-Si deposited at higher temperatures, it remained a very promising option. Nonetheless, further development is required to increase the overall eciency of mc-Si exible solar cells.

We present a thermoreflectance-based metrology concept applied to compound semiconductor thin films off-line characterization in the solar cells scribing process. The presented thermoreflectance setup has been used to evaluate the thermal diffusivity of thin CdTe films and to measure eventual changes in the thermal properties of 5 μm CdTe films ablated by nano and picosecond laser pulses. The temperature response of the CdTe thin film to the nanosecond heating pulse has been numerically investigated using the finite-difference time-domain (FDTD) method. The computational and experimental results have been compared.

Long alkyl chain ligands such as oleic acid (OLA) which cover the as-prepared PbS nanodots act as an insulating layer that impedes efficient charge transfer in PbS nanodots:polymer hybrid solar cells. The replacement of OLA with tailored ligands of an appropriate chain length is needed to achieve a noticeable enhancement of photovoltaic performance. Several studies have centered on the ligand exchange prior to casting the PbS film1,2,3. However, this post synthesis approach requires careful consideration for the choice of a ligand as clustering of the nanodots has to be avoided. Recently, a new approach that allows direct chemical ligand replacement in a blended mixture of PbS:P3HT has been demonstrated 4,5,6. In this contribution, the latter approach (post-fabrication) was compared with the post-synthesis ligand exchange. We investigated the effect of the ligand exchange processes to the charge separation dynamics in the P3HT:PbS blends by steady-state and time-resolved photoluminescence (PL). Hexanoic acid and acetic acid were used as a short-length ligand for the post fabrication approach while decylamine, octylamine and butylamine were used for the post-synthesis approach. As expected, decreasing the chain length of the ligand led to an increase of the P3HT fluorescence quenching. The absence of enhancement of PbS luminescence due to energy transfer from P3HT and the dependence of the quenching efficiency on the bulkiness of the ligands coating the QDs suggest that the quenching of the P3HT fluorescence is dominated by electron transfer to PbS quantum dots (QDs). In addition, the fluorescence quenching is also less prominent in the P3HT with higher regioregularity (RR) suggesting an enhanced phase separation in the blend due to more densely packed nature of conjugated polymer with higher RR.

The scattering of light by the textured transparent conductive oxide (TCO) in thin-film silicon solar cells is frequently described by transmission haze and angular intensity distribution (AID) at the interface between the TCO and air. The scattering is expected to improve the light trapping and, therefore, the absorption of the solar cell. Using these scattering properties as input parameters for the electrical modeling of thin-film solar cells leads to significant deviations from the measurements for short circuit current densities. The major disadvantage of the AID measurement at the TCO/air interface is that in real thin-film silicon solar cells the TCO/Si interface is relevant. We use a model that is based on scalar scattering theory to calculate the scattering properties at the transition into air and into silicon. The model takes into account the measured surface topography and the optical constants of the adjacent media. For a series of μc-Si:H cells on ZnO:Al with different surface topographies, AID and the transmission haze into a μc-Si:H half space are calculated. From these results, a quantity is derived that describes the scattering efficiency. This quantity is compared to the short circuit current densities of μc-Si:H solar cells showing good agreement. It will be shown that for artificially modified textures an increase in the short-circuit current density and thus, the efficiency of thin-film silicon solar cells can be achieved.

This paper studies the effect of femtosecond laser treatment in air of hydrogenated amorphous silicon thin films (a-Si:H) on their structural, electrical and photoelectric properties. The possibility of laser-induced crystallization of a-Si:H films with controlled crystalline volume fraction was shown. A sufficient increase of dark conductivity was observed for laser treated a-Si:H films which crystallinity exceeds 7%. Such increase was attributed to change in conductivity mechanism. However, spectral dependences of absorption coefficient did not show any qualitative changes with the laser fluence increase. It was found that spallation and oxidation of the film took place when laser fluence became reasonably high.

Plasmonics has become a focus of recent research in photovoltaic applications primarily due to their effects in enhancing the absorption performance of solar cells. In this paper a review of different approaches that have been proposed to integrate plasmonics technologies into solar cells is presented. It has been observed that a range of metallic nanostructures that show plasmon resonance wavelength in the visible and near-infrared regime can be utilized to increase the coupling of light into the solar cell. This is widely used to increase the coupling of light that can be trapped in thin layers of active regions as in thin film technologies. In this review paper, more attention is given to the techniques of fabricating the metallic nanoparticles and the ways to control their plasmon resonance wavelengths. The role of the shape, size, dielectric permittivity of the host and the type of the metallic nanoparticles on tuning the resonance wavelength are analyzed. Furthermore, the cluster of nanoparticles gives different resonance wavelength from the individual nanoparticles due to dipolar coupling among the nanoparticles. In conclusion, we show how the plasmon resonance can be engineered to increase the absorption performance of conventional solar cells.

Silver nanowire films are a newly introduced choice for transparent electrodes in thin film solar cells. Simulation is an adequate and economic method to analyse and predict the optical properties of these films. We simulate the optical behavior of such films by solving Maxwell equations. The simulation technique is a finite integration technique (FIT) combined with a time harmonic inverse iteration method (THIIM) to handle the negative permittivity of silver. Parallel computation on high performance computers(HPC) is used to meet the large computational requirement of the problem. In agreement to preliminary experimental results, the simulation results show that transmission of light is larger than expected by a simple ray-tracing model.

In conventional luminescent solar concentrators (LSC) incident light is absorbed by luminophores and emitted isotropically. Most of the emitted light is trapped inside the LSC by total internal reflection and guided to solar cells at the edges. Light emitted towards the surfaces, however, is lost in the escape cone. Furthermore, when the luminophore emits light in its absorption range, light is lost due to reabsorption. To overcome these losses, we embed the luminescent material in photonic structures to influence the emission characteristics. Directional and spectral redistribution of emission is supposed to enhance the light guiding in LSCs and reduce reabsorption losses. For this purpose, we prepared opal films from PMMA colloids, in which Rhodamine B was embedded during the polymerization process. In direction-resolved luminescence measurements a strong dependence of the emitted spectrum on the detection direction was observed. Further, the light collection efficiency of the samples was determined with optical measurements and light guiding due to the intended absorption and emission process was observed. The overall performance, however, suffers from cracks and defects in the photonic crystal.

This work reports the formation process and the resulting optical properties of AlTiO selective transmitting layers. The AlTiO layers were deposited by reactive sputtering from Al and Ti targets in oxygen atmosphere. The compositions and thicknesses of the AlTiO layers were controlled by varying deposition parameters, and corresponding changes in transmittance and reflectance were measured. The reflectance decreased and then increased with increasing wavelength, and its minimum depending on the composition and thickness of AlTiO was found at a wavelength ranging from 380 to 750nm. The raised reflectance in the long wavelength regime suggests the possible use of the sputter-deposited AlTiO layers as a selective transmitting layer in high efficient and semitransparent Si thin film solar cells.

Light trapping due to rough interfaces is a common and industrially applied technique to enhance cell performance in silicon thin-film solar cells. The induced scattering enhances the absorption and consequently the conversion efficiency of the device. Periodic structures promise to further enhance the light trapping, allowing a beneficial reduction of the absorber layer thickness. In this work, solar cells with transparent front contacts with a two-dimensional (2D) grating structure produced by holographic lithography are investigated. The grating structures are characterized by various means and the results are used to calibrate finite-difference time-domain (FDTD) simulations. With the computational method, the influence of the grating height on the solar cell performance is investigated.