This PDF file contains the front matter associated with SPIE Proceedings Volume 7002, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

Artificial photonic antenna systems have been realised by incorporating organic dyes in a nanoporous material. We have been using zeolite L in most of our experiments as it has proven to be a very versatile host. Its crystals are cylindrically shaped porous aluminosilicates featuring hexagonal symmetry. The size and aspect ratio of the crystallites can be tuned over a wide range. A nanometre sized crystal consists of many thousand one-dimensional channels oriented parallel to the cylinder axis. These can be filled with suitable organic guests. Geometrical constrains of the host structure lead to supramolecular organisation of the guests in the channels. Thus very high concentrations of non- or only very weakly interacting dye molecules can be realised. A special twist is added to these systems by plugging the channel openings with a second type of fluorescent dye, which we call stopcock molecule. The two types of molecules are precisely tuned to each other; the stopcocks are able to accept excitation energy from the dyes inside the channel, but cannot pass it back. The supramolecular organisation of dyes inside the zeolite channels is what we call the first stage of organization. It allows light harvesting within the volume of a dye-loaded zeolite L crystal and also radiationless energy transport to either the cylinder ends or centre. The second stage of organisation represents the coupling to an external acceptor or donor stopcock fluorophore at the ends of the zeolite L channels, which can then trap or inject electronic excitation energy. The third stage of organization is realised by interfacing the material to an external device via a stopcock intermediate. We observed that electronic excitation energy transfer in dye-zeolite L materials occurs mainly along the channel axis. This important finding means that macroscopically organised uni-directional materials can be prepared. In order to achieve this, we prepared oriented zeolite L monolayers, filled them with luminescent dyes, and finally added a stopcock. The new materials offer unique possibilities as building blocks for optical, electro-optical and sensing devices.

The nowadays quite indispensable enhancement of PV conversion efficiency cannot be obtained without new mechanisms. The most useful of these mechanisms have to appear in the front face of the device, i.e. in the emitter, so as to allow exploitation of the energetic photons of the solar spectrum. Such an improvement can be realized through a multistage PV conversion starting by primary generation (photon absorption) followed by secondary generations (hot carrier collisions with low-energy generation centers). This cascade-like process is possible, for example, in multiinterface devices containing several emitter strata. Some of these strata assume the primary free-carrier generation while others do the secondary free-carrier generation. In this work we report investigations of new mechanisms based on I(V) curves measured on test samples with different multiinterface architectures, electronic passivations, front grids, collecting electrodes and so on. The measurements have been performed under a variable intensity incident light beam conserving always its spectral (solar) composition, except for analogous measurement cycle without a UV component. The same beam intensities with a filtered UV component complemented these investigations. The measurements have been compared with those of a weak excitation from a typical halogen lamp (relatively stable flux without a UV component). The test structures show a clear improvement of the PV conversion in the UV range induced by impact ionization within the superficial nanostratum.

Thermal processing of as-made fluorozirconate glasses which were additionally doped with neodymium and chlorine ions leads to enhanced up-conversion fluorescence intensities in these glass ceramics. The samples were annealed between 240°C and 290°C while the optimum value was found for the 270°C sample. We investigated the power dependence of the infrared fluorescence, the 2-photon up-conversion, and the 3-photon up-conversion fluorescence intensities as well as the corresponding radiative lifetimes. In analogy to the up-conversion intensity, the radiative lifetime of the Nd³⁺ fluorescence at about 880 nm depends significantly on the annealing temperature: the longest lifetime was observed for the 270°C sample.

A new concept of ultra-thin film photovoltaic solar cell including a planar photonic crystal is proposed. The goal is to couple the incident light into broad resonances guided in the absorbing layer. To achieve this, a periodic lattice is patterned within the active layer, for example made of holes in amorphous silicon. By adjusting the pattern dimensions, the spectral position and quality factor of these resonances can be controlled so as to optimise the global absorption. Design details will be discussed in this communication.

The concept of a 3D photonic crystal structure as diffractive and spectrally selective intermediate filter within 'micromorphous' (a-Si/μc-Si) tandem solar cells has been investigated numerically and experimentally. Our device aims for the enhancement of the optical pathway of incident light within the amorphous silicon top cell in its spectral region of low absorption. From our previous simulations, we expect a significant improvement of the tandem cell efficiency of about absolutely 1.3%. This increases the efficiency for a typical a-Si / μc-Si tandem cell from 11.1% to 12.4%, as a result of the optical current-matching of the two junctions. We suggest as wavelength-selective optical element a 3D-structured optical thin-film, prepared by self-organized artificial opal templates and replicated with atomic layer deposition. The resulting samples are highly periodic thin-film inverted opals made of conducting and transparent zinc-oxide. We describe the fabrication processes and compare experimental data on the optical properties in reflection and transmission with our simulations and photonic band structure calculations.

The Luminescent Solar Concentrator (LSC) consists of a transparent plate with solar cells connected to one or more sides. The plate contains luminescent species, like e.g. organic dyes or quantum dots. Part of the light emitted by the luminescent species is guided towards the solar cells by total internal reflection, the plate functioning as a waveguide. We have developed a ray-tracing model that describes the experimental results and can be used to determine the different loss mechanisms in the LSC. We will present a parameter study using this model which indicates some of the important aspects of the LSC.

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

We present the optimization of coatings that can be applied on top of solar cells. The purpose of these coatings is to reduce the light acceptance cone only in the long wavelength range. This allows for high transmission of light into the solar cell independent on the angle of incidence in the short wavelength range where the solar cell material shows high absorption. Furthermore, it allows for enhanced absorption of the direct sunlight in the long wavelength range by trapping such light using total internal reflection. The coating for this purpose is based on a combination of various design strategies with well defined impacts on the reflection spectra. It provides a set of free parameters which describe the index profile of the filter. Realistic numerical procedures for calculating efficiencies for crystalline silicon solar cells are used to optimize the free parameters of the coating. We show that these filters can lift the efficiency of 10μm thick solar cells up to 30.1%. This is a factor of 1.05 above the ≈ 28% limit found for unconcentrated illumination by Kerr et al.

For thin-film silicon solar cells, light trapping schemes are of uppermost importance to harvest all available sunlight. Typically, randomly textured TCO front layers are used to scatter the light diffusively in p-i-n cells on glass. Here, we investigate methods to texture the back contact with both random and periodic textures, for use in n-i-p cells on opaque foil. We applied an electrically insulating SiOx-polymer coating on a stainless steel substrate, and textured this barrier layer by nanoimprint. On this barrier layer the back contact is deposited for further use in the solar cell stack. Replication of masters with various random and periodic patterns was tested, and, using scanning electron microscopy, replicas were found to compare well with the originals. Masters with U-grooves of various sub micrometer widths have been used to investigate the optimal dimensions of regular patterns for light trapping in the silicon layers. Angular reflection distributions were measured to evaluate the light scattering properties of both periodic and random patterns. Diffraction gratings show promising results in scattering the light to specific angles, enhancing the total internal reflection in the solar cell.

Randomly textured zinc oxide surfaces with and without amorphous silicon deposited on top are studied by near-field scanning optical microscopy. By virtue of a three dimensions it allows to access the local light intensity in the entire spatial domain above the structures. Measurements are compared with large scale finite-difference time-domain simulations. This study provides new insight into light trapping in thin-film silicon solar cells on a nanoscopic scale. Light localization on the surface of the textured interface and a focusing of light by the structure further away are observed as the key features characteristic for such surfaces.

We report on a series of organic solar cells based on heterojunctions of oligothiophene derivatives with varying chain length and C60 fullerenes. Devices are based on either p-i-n or p-i-i structure. In the first the intrinsic photovoltaic active layer is sandwiched between a p-type and n-type doped organic wide-gap layer for hole and electron transport respectively. In the latter the electron transport layer is replaced by a thin layer of wide-gap material as exciton blocker. Through optimization of transport and absorber layers we are able to reach in devices with single heterojunctions an open circuit voltage V_oc of about 1V, a short circuit current density J_sc of about 5.6mA/cm² and a fill factor FF above 50% under an AM1.5 illumination with 1000W/m². However, still only a small part of the available solar spectrum is used. Thus, based on these materials stacked solar cells have been made to further improve the light absorption. The thickness of each layer is optimized using optical simulations to match the currents delivered by each of the solar cells in the stack. Through the incorporation of a very efficient recombination zone between the stacked solar cells the resulting V_oc nearly reaches the sum of the V_oc of the two serially connected solar cells.

Organic photovoltaic represents an emerging technology thanks to its ability to give flexible, light weight and large-area devices, with low production cost by simple solution process or printing technologies. But these devices are known to exhibit low resistance to the combined action of sunlight, oxygen and water. This paper is focused on the behaviour of the active layer of the devices under illumination in the presence and absence of oxygen. The monitoring of the evolution of the chemical structure of MDMO-PPV submitted to accelerated artificial ageing permitted the elucidation of the mechanisms by which the polymer degrades. Extrapolation of the data to natural ageing suggested that, if well protected from oxygen (encapsulation), MDMO-PPV:PCBM based active layer is photochemically stable for several years in use conditions. In addition the charge transfer between the two materials was observed to remain efficient under exposure. The study of P3HT:PCBM blends allowed to point out the Achilles heel of P3HT towards the impact of light. In addition, P3HT:PCBM blends were shown to be much more stable under illumination than MDMO:PCBM blends. Preliminary results devoted to the AFM monitoring of the morphological modifications of P3HT:PCBM blends under the impact of light are also reported.

One material system of interest for photovoltaic cells is the combination of the p-conducting copper-phthalo-cyanine (CuPc) and the n-conducting fullerene (C₆₀) as donor and acceptor materials, respectively. Therefore the transport properties for diodes containing neat and blended organic films are analysed in the space charge limited current regime. The charge carrier mobilities are found to decrease upon dilution of the respective conducting phase by the other species. Photovoltaic cells can be realised with bilayered or blended organic donor/acceptor films. The influence of both photo-active layer types on the electronic structure and the open circuit voltage is investigated. From photoelectron spectroscopy a higher open circuit voltage is predicted for bilayered solar cells. Due to mixing of the organic materials the intermolecular gap between the highest occupied molecular orbital of the donor and the lowest unoccupied molecular orbital of the acceptor is reduced. This prediction is proven true by photocurrent measurements.

Using a dense organic monolayer, self-assembled and directly bound to n-Si, as high quality insulator with a thickness that can be varied from 1.5-2.5 nm, we construct a Metal-Organic Insulator-Semiconductor (MOIS) structure, which, if fabricated with semi-transparent top electrode, performs as a hybrid organic-inorganic photovoltaic device. The feasibility of the concept and the electrical properties of the insulating layer were first shown with a Hg top electrode, allowing use of prior know-how from electron transport through molecular monolayers, but with photon collection only from around the electrode. We then used another bottom-up fabrication technique, in addition to molecular self-assembly, electro-less metal deposition, to implement an all-covalently bound solid state device. Electro-less Au deposition yields an electrically continuous, porous and semi-transparent top electrode, improving photon harvesting. Aside from being a nearly ideal insulator, the monolayer acts to passivate and protect the interfacial Si layer from defects and to decrease the surface state density. In addition the cell, like any MIS solar cell, benefits from that the light needs only to cross a few thin transparent layers (anti-reflective coating, organic insulator) to reach the photovoltaically active cell part. This helps to generate carriers close to the junction area, even by short wavelength photons, and, thus, to increase light collection, compared to p-n junction solar cells. Due to low temperature cell fabrication without high vacuum steps, the MOIS approach might be interesting for low cost solar cells.

Our work deals with the improvement of "light harvesting" in organic photovoltaic cells by using photonic nanostructures. We have theoretically studied a periodically nanostructured poly (3-hexylthiophene)(P3HT)/6,6-phenyl C61-butyric acid methyl ester (PCBM) thin film in order to increase its absorption in the near infrared spectral range. We have used a software, based on the FDTD (Finite-Difference Time-Domain) method, to calculate the absorption of light in organics solar cells. We have also considered the nanostructured photoactive layer of solar cells as a photonic crystal and we have computed band diagrams to study the dispersion curves of this structure. We have first studied a blend (bulk heterojunction) with the same proportions of P3HT and PCBM. This material provide at this time the best results in terms of photovoltaic efficiency. Nevertheless, in order to improve the transport of charges to the electrodes, a model with P3HT and PCBM independently nanostructured (ordered heterostructure) was also used. Moreover, this periodic nanostructuration allows "slow Bloch modes" to be coupled inside the device with a low group velocity of electromagnetic waves. Thus, the interaction duration between light and organics materials is improved. The P3HT/PCBM photonic crystal parameters have been adjusted to maximize the density of Bloch modes and to obtain flat dispersion curves. We have found that the light matter interaction was strongly enhanced which resulted in a 35.6% increase of absorption in the 600 nm to 700 nm spectral range. In order to realize nanostructured organic solar cells, we are also developing an experimental prototype, based on a patented process, which allows to nanostructure several kinds of polymers.

β-FeSi₂ has many attracting properties as a semiconductor not consisting of toxic chemical elements and is an ideal semiconductor as a thin film solar cell owing to its extremely high optical absorption coefficient. To evaluate β-FeSi₂ as a solar cell, photo-response measurement is critically important and useful. Since β-FeSi₂ thin films are normally deposited on Si substrates, intrinsic photo-response of β-FeSi₂ is usually difficult to be collected due to the strong contribution from Si substrates. We here present the photo-response from bulk β-FeSi₂ crystals, expecting that we can eliminate the contributions coming from the Si substrates and the crystallographic defects existing at the β-FeSi₂/Si interfaces when we use β-FeSi₂ thin films. We prepared bulk specimens by chemical vapor transport method (CVT) in which needle-like and plate-like β-FeSi₂ crystals were obtained. We chose the former specimens for the formation of Al/n-β-FeSi₂ Schottky contacts to measure their photo-responses. These contacts were found to form Schottky diodes even though there are large series resistances and leakage currents. Under laser light illumination of 1.31 μm through optical fiber, the positive voltage was observed between the Al contact and the In solder glued to the back-surface of β-FeSi₂ bulk specimen. Two-dimensional distribution of photo-responses were measured by scanning the above optical fiber with the spot size of 50 μm. The highest photo-response was obtained in the vicinity of Al wire, and was 7.7 mA/W for the as-grown sample, and 31 mA/W for the annealing one, respectively. These observations state that β-FeSi₂ holds appropriate optical features to be used as a solar cell.

Completing one-step PV conversion by additional new low-energy mechanisms is one of the most important challenges of modern photovoltaics. Si is a basic PV material which is not efficient enough to convert the light into electricity in its bulk or thin-film form because of its indirect bandgap. Progress in conversion efficiency requires breakthroughs. One way has been indicated by low-dimensional or nanostructured Si materials such as, for example, nanoscale Si-layered systems combined with an active interface with its crystalline defects. We have demonstrated low-energy carrier multiplication experimentally under attenuated solar excitation in nanostructured Si in both the optical and electronic approaches. This paper gives results, among others, concerning: 1) a simulation based on previously determined parameters and 2) experiments using a reference cell. In the simulation, the nanolayer thicknesses are 5 nm for a-Si (estimation based on TEM images) and 20 nm for (estimation based on EELS data). To be simple and directly useful in the future development, the project has been limited to comparative measurements of the short-circuit currents of our test cells relative to the new generation photovoltaics. The ratio of short-circuit currents shows steps which allow estimating a characteristic energy E_δ = 0.274 eV, previously determined by us from spectral response and modeling. The effect is particularly visible under weak incident beams. Thanks to these investigations, the fabrication of a very highly efficient Si solar cell becomes more realistic. The results suggest scientific and technological prospectives.

The optoelectronics and photonics properties of silicon are fundamentally influenced by the density of carriers present near the sample surface. One way of generating very large densities of such carriers is to confined them in a Si monolayer made by implantation in an amorphous Si (Si-a) nanolayer followed by a suitable thermal treatment. In that way, one can photo-generate a two dimensional (2D) plasma which modifies the complex refractive index of the nanolayer. Our work describes the modification of the reflection and absorption induced by incident light in the same devices when varying their electronic passivation. The different studied samples contain a buried amorphous substructure and are distinguishable exclusively by the thickness of the SiO₂ covering layer. The measurements of the different samples show large differences in their absorption coefficients. The absorption coefficient α measured at 450 nm for a Si sample with a thin passivation layer is α~4.0x10⁴ cm^-1 that is nearly 5 times lower than a sample which is processed with a complementary passivation α~1.8x10⁵ cm^-1. In both cases average flux of photons (10¹⁵ s^-1 cm^-2) is the same. This result confirms the role of the free-carrier population, induced by the incident light, which is confined near the surface. In the conventional modeling of the absorption only the surface recombination rate governed by the thickness of the SiO₂ covering layers has to be taken into account. In the present work, we show that the carrier confinement also plays an important role. Such results are very interesting in the context of optoelectronics and photonics silicon nano-structured devices.

Solar photon energy can be better used when totally transformed on collectable free-carriers. The conversion of one energetic photon could result in more than one free-carrier pair if a low-energy mechanism is involved. Such PV conversion represents a multistage nonlinear process and requires especially dedicated low-energy centers. A cascade-like progression is induced by the primary/fundamental/interband absorption. As shown by us previously, the corresponding structure can be realized, for example, with nanostructured Si. The experimental devices convert 400 nm photons into collectable primary and secondary free-carriers. The excess carriers can be drawn out into the external electrical circuit even in a multiinterface architecture containing a carrier collection limit. The superficial effect seems to be totally independent of the presence or not of a buried amorphized layer. This is the first simple experimental evidence for low-energy generation. The performance is inversely proportional to the incident light intensity. The thermodynamic limit of conventional photovoltaic conversion is lower than 30%, while in the case of the mechanism reported here, it can be propelled above 60%. An optimization of the effect by a suitable conditioning and annealing should be possible, opening the way to different applications, especially in the areas of nanophotovoltaics and very high efficiency solar cells.

Intermediate Band (IB) solar cell is a concept belonging to the third generation of photovoltaic converters potentially having an efficiency limit exceeding the one of single gap solar cells. Its performance is based on the existence of an intermediate band within the conventional gap of a host semiconductor which facilitates two step absorptions of photons with energy below the band gap. An extension of intermediary band solar cells is the multiband solar cell achieved by increasing the number of IBs in the host gap. When the number of IBs increases to infinite, the device becomes equivalent to an infinite serial tandem, which exhibits a conversion efficiency close to the thermodynamic limit. One proposes a simple but accurate model for a multiband solar cell having an arbitrary number of IBs that may be implemented via Quantum Dot (QD) technology. The novelty of this approach lies in the computation procedure of the energy band diagram using the transfer matrix method and the derivation of an effective absorption coefficient for the IB system. The presented model is suitable as a tool for investigating electro-optical properties of QD multiband cells. Results of numerical simulations performed for finding maximum conversion efficiency for given architectures are presented.

Fluorescent photovoltaic collectors are examined theoretically with the help of Monte-Carlo simulations. The classical construction of mounting solar cells on each side surface of the collector is compared to alternative set-ups: solar cells only partly cover the collector side surfaces or they are mounted on the collector back surface, respectively. We find that the collection probability of photons in most situations is higher for systems with solar cells mounted on the sides of the collector compared to systems with solar cells at the bottom. In all cases collection probabilities in excess of 90 % are only achieved if a photonic band stop filter is applied to the top surface of the collector. This filter acts as an omni-directional spectral band stop and prevents the light emitted by the dye from leaving the collector. The application of such a filter allows a solar cell area reduction of 99 %. However, inclusion of non-radiative losses in the collector deteriorates the photon collection probability in all cases. This effect is especially important in systems with band stop filter.

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

We suggest three-dimensional photonic crystals as a direction selective filter for ultra-light trapping in solar cells. 3D photonic crystals allow tailoring of the photonic stop gap in space and energy. We analyzed different photonic crystal structures concerning their spectral and direction selective properties and defined two figures of merit for our application: a quality factor and a transmission coefficient. By analyzing different experimentally feasible 3D photonic crystals, we found that the inverted opal has the best properties. We verified the direction selective properties of the inverted opal in the microwave spectral range and found a very good agreement between experiment and simulation.

In this study, we demonstrated the textured structure on silicon surface by metal assisted etching method, using Au nanoparticles as catalysts in HF and H₂O₂ solution. The size and density of the nanoparticles could be tuned easily. The porous layers filled with cylinder- or cone-shaped were uniformly formed by immersing the gold deposited silicon wafers in a mixed solution containing HF and H₂O₂ under different etching conditions. The optimized textured structure was close-packed pyramids-like surface in subwavelength scale and showed the lowest reflectance less than 0.5% over whole visible and near IR wavelengths. The large reduction of reflectance was attributed from the gradient refractive index of the silicon surface with the depth along the light propagation.

Computer modeling has become a necessity in the solar cells design. A computer model allows the study of the physical behavior of the device offering valuable information on the effects of each parameter on device performance. Dye-sensitized solar cells (DSSC) have attracted a lot of interest in recent years, in research as well as in industry. In present, the development has reached a stage where detailed physical models may contribute considerably to the optimization of these devices. Up to now, there is not a comprehensive model which links material parameters of a DSSC based on TiO₂ nanocrystals DSSC to the electrical performance of the whole cell, such as I-V characteristic and spectral response. Typically, a DSSC consists of two layers, a TiO₂ porous structure coated with a suitable light-absorbing charge-transfer dye wetted with an iodide/triiodide redox electrolyte and a bulk electrolyte layer, sandwiched between two glass substrates which are coated with transparent conductive oxide (TCO) layers. In this paper we present a model for the transport processes inside the DSSC based on the classical transport equations in one dimension. The equations are solved using the monodomain approach, which consists of using a single set of equations, with different values for the transport coefficients inside the two regions of the computational domain. The transport coefficients for the porous medium are calculated using homogenization techniques. The model permits the computation of the dye-sensitized solar cell I-V curves and efficiency. As model application, the influence of the most important material parameters on the cell performances investigated by numerical simulation is reported.

Alternatives for replacing the expensive ITO are explored and Poly(ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) is introduced as one possibility. We present the first small-molecule organic solar cells employing only PEDOT:PSS as transparent electrode. Solar cells on glass and on flexible plastic foil were prepared, using a p-doped hole transporting material, zinc phthalocyanine (ZnPc) and C₆₀ as donor-acceptor heterojunction, and an exciton blocking layer. Different methods to structure the PEDOT:PSS electrodes were investigated and are presented. As proof of principle, non-optimized prototype cells with efficiencies of over 0.7% on glass and 0.9% on flexible plastic foil substrate were obtained.