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International audience; The vapor-liquid-solid (VLS) and vapor-solid-solid (VSS) growth mechanisms are widely used to obtain silicon nanowires. In this paper, we report on a hybrid method based on the use of a dual-phase catalyst made of liquid Sn and solid Cu 3 Si, which results in a liquid-assisted VSS (LA-VSS) mechanism. The silicon atoms are brought by atomic hydrogen-assisted dissociation of silane molecules. We observe the growth in situ, in the transmission electron microscope, at

International audience; Optoelectronic devices based on high aspect ratio nanowires bring new challenges for transparent electrodes, which can be well addressed by using hybrid structures. Here we demonstrate that a composite contact to radial junction nanowire solar cells made of a thin indium-tin oxide (ITO) layer and silver nanowires greatly improves the collection of charge carriers as compared to a single thick ITO layer by reducing the series resistance losses while improving the transparency. The optimization is performed on p-in solar cells comprising of dense non-vertical nanowires with a p-doped c-Si core and an ultra-thin a-Si:H absorption layer grown by plasma-enhanced chemical vapor deposition on glass substrates. The optimal hybrid contact developed in this work is demonstrated to increase the solar cell conversion efficiency from 4.3% to 6.6%.

International audience; In the quest for the replacement of indium tin oxide (ITO), Ti-doped zinc oxide (TZO) films have been synthesized by atomic layer deposition (ALD) and applied as an n-type transparent conductive oxide (TCO). TZO thin films were obtained from titanium (IV) i-propoxide (TTIP), diethyl zinc, and water by introducing TiO 2 growth cycle in a ZnO matrix. Process parameters such as the order of precursor introduction, the cycle ratio, and the film thickness were optimized. The as-deposited films were analyzed for their surface morphology, elemental stoichiometry, optoelectronic properties, and crystallinity using a variety of characterization techniques. The growth mechanism was investigated for the first time by in situ quartz crystal microbalance measurements. It evidenced different insertion modes of titanium depending on the precursor introduction, as well as the etching of Zn−Et surface groups by TTIP. Resistivity as low as 1.2 × 10 −3 Ω cm and transmittance >80% in the visible range were obtained for 72-nm-thick films. Finally, the first application of ALD-TZO as TCO was reported. TZO films were successfully implemented as top electrodes in silicon nanowire solar cells. The unique properties of TZO combined with conformal coverage realized by the ALD technique make it possible for the cell to show almost flat external quantum efficiency (EQE) response, surpassing the bell-like EQE curve seen in devices with a sputtered ITO top electrode.

The radicals and atoms generated by a plasma have the effect, among others, of changing the surface energies of materials, which allows one to prepare nano-objects that would not stabilise in other conditions. This is the case of the Sn catalysed silicon nanowires (NWs) we present in this paper: without plasma, the liquid Sn at the top of NWs is unstable (because Sn naturally wets the Si) so that no growth is allowed, while in presence of the H atoms generated by the plasma, the balance of surface energies is drastically changed; the Sn droplet stabilises and can be used efficiently by the vapour-liquid-solid (VLS) mechanism of growth. Thus, if one wants to study the growth mechanisms of such NWs in situ in the transmission electron microscope (TEM), one has to adapt a plasma system on the TEM. This is precisely what was done at École polytechnique on the NanoMAX environmental TEM. The paper reports on the plasma effects, on the catalyst and on NW growth, recorded in situ in real time, at atomic resolution. The results are discussed in the light of density functional calculations of bare and hydrogenated Si surface energies.

The optimized design of a photonic platform based on a nanowire light emitting diode (LED) and a nanowire photodetector connected with a waveguide is proposed. The light coupling efficiency from the LED to the detector is optimized as a function of the geometrical parameters of the system using the finite difference time domain simulation tool Lumerical. Starting from a design reported in the literature with a coupling efficiency of only 8.7%, we propose an optimized photonic platform with efficiency reaching 65.5%.

We study light trapping in hydrogenated amorphous silicon thin film solar cells fabricated by plasma-enhanced chemical vapor deposition on various nanostructured back reflectors. The back reflectors are patterned using polystyrene assisted lithography. We have investigated the correlation between the back reflector optical properties and the corresponding solar cell performance. We have introduced double size polystyrene sphere patterned back reflectors and have provided experimental evidence for improved light trapping performance compared to single size polystyrene sphere patterned back reflectors. We have achieved high performing nanostructured amorphous silicon solar cells with an initial power conversion efficiency of 7.53% and over 20% enhancement of the short-circuit current compared with the reference flat solar cell.

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International audience

Conversion of carbon dioxide into hydrocarbons using solar energy is an attractive strategy for storing such a renewable source of energy into the form of chemical energy (a fuel). This can be achieved in a system coupling a photovoltaic (PV) cell to an electrochemical cell (EC) for CO

Conversion of carbon dioxide into hydrocarbons using solar energy is an attractive strategy for storing such a renewable source of energy into the form of chemical energy (a fuel). This can be achieved in a system coupling a photovoltaic (PV) cell to an electrochemical cell (EC) for CO2 reduction. To be beneficial and applicable, such a system should use low-cost and easily processable photovoltaic cells and display minimal energy losses associated with the catalysts at the anode and cathode and with the electrolyzer device. In this work, we have considered all of these parameters altogether to set up a reference PV–EC system for CO2 reduction to hydrocarbons. By using the same original and efficient Cu-based catalysts at both electrodes of the electrolyzer, and by minimizing all possible energy losses associated with the electrolyzer device, we have achieved CO2 reduction to ethylene and ethane with a 21% energy efficiency. Coupled with a state-of-the-art, low-cost perovskite photovoltaic minimodule, this system reaches a 2.3% solar-to-hydrocarbon efficiency, setting a benchmark for an inexpensive all–earth-abundant PV–EC system. ISSN:0027-8424 ISSN:1091-6490

International audience; A flexible nitride p-n photodiode is demonstrated. The device consists of a composite nanowire/polymer membrane trans- ferred onto a flexible substrate. The active element for light sensing is a vertical array of core/shell p−n junction nanowires containing InGaN/ GaN quantum wells grown by MOVPE. Electron/hole generation and transport in core/shell nanowires are modeled within nonequilibrium Green function formalism showing a good agreement with experimental results. Fully flexible transparent contacts based on a silver nanowire network are used for device fabrication, which allows bending the detector to a few millimeter curvature radius without damage. The detector shows a photoresponse at wavelengths shorter than 430 nm with a peak responsivity of 0.096 A/W at 370 nm under zero bias. The operation speed for a 0.3 × 0.3 cm2 detector patch was tested between 4 Hz and 2 kHz. The −3 dB cutoff was found to be ∼35 Hz, which is faster than the operation speed for typical photoconductive detectors and which is compatible with UV monitoring applications.

International audience; A new strategy for the development of III-V/Si tandem solar cells has recently been proposed consisting in low temperature PECVD epitaxy of silicon or silicon-germanium on gallium-arsenide. This paper thus gives first insights about theoretical but realistic maximum performance of such tandem cells by means of full numerical simulations considering perfect layers and interfaces. The consequences of using a thin epi-Si bottom cell instead of a thick silicon substrate are investigated. In case no light trapping scheme is considered, a minimum epi-layer thickness of 20 μm is mandatory for the tandem to exhibit higher conversion efficiencies than a single GaAs solar cell. The epi-Si can yet be advantageously replaced by an epitaxial silicon-germanium alloy to increase the bottom cell optical absorption and thus decrease the minimum required thickness by a factor of ∼4 (∼5 μm). Finally, simulations show that over 33% efficiency can be obtained for AlxGa1-xAs/epi-Si0.63Ge0.27, which confirms that this is a promising new concept.

The fabrication of arrays of silicon nanowires (Si NWs) with well-defined surface coverage using the vapor-liquid-solid process requires a good control of the density and size distribution for the metal catalyst. We report on a cost-effective bottom-up approach to produce Si NWs by a low-temperature deposition technology using plasma-enhanced chemical vapor deposition and tin dioxide (SnO2) nanoparticles as the source of tin catalyst. This strategy offers a straightforward method to select specific particle sizes by conventional colloidal techniques, and to tune the surface coverage using a polyelectrolyte layer to efficiently immobilize the particles on the substrate by electrostatic grafting. After a further step of reduction into tin metal droplets using hydrogen plasma treatment, the catalyst particles are used for the growth of Si NWs. This approach allows the prodcution of controlled Si NWs arrays which can be used as a template for radial junction thin film solar cells.

We study light trapping and parasitic losses in hydrogenated amorphous silicon thin film solar cells fabricated by plasma-enhanced chemical vapor deposition on nanostructured back reflectors. The back reflectors are patterned using polystyrene assisted lithography. By using O2 plasma etching of the polystyrene spheres, we managed to fabricate hexagonal nanostructured back reflectors. With the help of rigorous modeling, we study the parasitic losses in different back reflectors, non-active layers, and last but not least the light enhancement effect in the silicon absorber layer. Moreover, simulation results have been checked against experimental data. We have demonstrated hexagonal nanostructured amorphous silicon thin film solar cells with a power conversion efficiency of 7.7% and around 34.7% enhancement of the short-circuit current density, compared with planar amorphous silicon thin film solar cells. Published version

International audience

International audience; Silicon nanowires have been grown by a plasma-assisted vapour–liquid–solid method using tin as the catalyst. Transmission electron microscopy in the [1-210] zone axis shows that the diamond hexagonal (P63/mmc) crystal structure is present in several nanowires. This is the first unambiguous proof of the natural occurrence of this metastable phase to our knowledge.

The appropriate choice of nanoparticles is proved to be essential in tuning the properties of F:SnO2 (FTO) nanocomposites. With the use of more conductive sulphur-doped TiO2 (S:TiO2) nanoparticles, the sheet resistance of S:TiO2–FTO nanocomposites is successfully reduced down to 38% as compared to the standard flat FTO (11.7 Ω sq-1), while the haze factor of the S:TiO2–FTO nanocomposites can be varied from almost zero (reference flat FTO) up to 60%; moreover the majority of 〈110〉 oriented S:TiO2 nanoparticles leads to a strong (110) texture in the resulting S:TiO2–FTO nanocomposites by local epitaxy. Careful morphology analyses and angle-resolved measurements reveal that the haze factor is proportional to the total surface coverage of the S:TiO2 nanoparticle agglomerates, while the feature size of the agglomerates determines the angular distribution of the scattered light – this is confirmed by an angle-resolved Mueller matrix polarimeter which allows obtaining the optical microscopic and angleresolved images of the exact same textured region. Our work establishes the guidelines to fabricate FTO and other transparent conductive oxide (TCO) nanocomposites as promising electrodes in solar cells with tunable structural, electrical, and optical properties. published

International audience; Recent developments have unlocked the main issues arising from the combination of III-V and silicon and have opened a new way to fabricate tandem solar cells. In this study we provide a detailed analysis of III-V/epi-SiGe tandem devices performance using opto-electrical models and parameters acquired from previous experimental realizations of single junction devices. At first, we present the validation of our top and bottom cells models by comparison with previously published solar cells. The analysis of the current matching and the impact of the Al content in AlGaAs absorber on the open circuit voltage is performed on a very wide range of thickness and Al content. The optimal configurations for tandems with thin film absorbers are found with an empirical expression. This expression relates the required bottom absorber thickness to the Al content for current matching in a flat tandem device. Low-temperature epitaxial SiGe growth on III-V materials is an inverted growth technique, meaning that the last material grown is the Si(Ge) bottom cell. We can thus easily texture the back of the bottom cell for higher photon absorption. The proposed nanostructurization of the back reflector shows that, to reach the same efficiency, only half of the thickness is required if a 2D grating is combined with a silver reflector. The detailed influence of the bulk and interface electrical quality in the epi-SiGe bottom cell is also assessed. Finally, the prediction of the tandem device performance according to different realistic scenarios is presented.

A comprehensive study of the silicon nanowire growth process has been carried out. Silicon nanowires were grown by plasma-assisted-vapor-solid method using tin as a catalyst. We have focused on the evolution of the silicon nanowire density, morphology, and crystallinity. For the first time, the initial growth stage, which determines the nanowire (NW) density and growth direction, has been observed step by step. We provide direct evidence of the merging of Sn catalyst droplets and the formation of Si nanowires during the first 10 s of growth. We found that the density of Sn droplets decreases from ~9000 Sn droplets/μm2 to 2000 droplets/μm2 after just 10 s of growth. Moreover, the long and straight nanowire density decreases from 170/μm2 after 2 min of growth to less than 10/μm2 after 90 min. This strong reduction in nanowire density is accompanied by an evolution of their morphology from cylindrical to conical, then to bend conical, and finally, to a bend inverted conical shape. Moreover, the changes in the crystalline structure of nanowires are from (i) monocrystalline to (ii) monocrystalline core/defective crystalline shell and then to (iii) monocrystalline core/defective crystalline shell/amorphous shell. The evolutions of NW properties have been explained in detail.

International audience; Ultrathin c-Si solar cells have the potential to drastically reduce costs by saving raw material while maintaining good efficiencies thanks to the excellent quality of monocrystalline silicon. However, efficient light trapping strategies must be implemented to achieve high short-circuit currents. We report on the fabrication of both planar and patterned ultrathin c-Si solar cells on glass using low temperature (T < 275 °C), low-cost, and scalable techniques. Epitaxial c-Si layers are grown by PECVD at 160 °C and transferred on a glass substrate by anodic bonding and mechanical cleavage. A silver back mirror is combined with a front texturation based on an inverted nanopyramid array fabricated by nanoimprint lithography and wet etching. We demonstrate a short-circuit current density of 25.3 mA/cm2 for an equivalent thickness of only 2.75 μm. External quantum efficiency (EQE) measurements are in very good agreement with FDTD simulations. We infer an optical path enhancement of 10 in the long wavelength range. A simple propagation model reveals that the low photon escape probability of 25% is the key factor in the light trapping mechanism. The main limitations of our current technology and the potential efficiencies achievable with contact optimization are discussed.

International audience; Both surface photovoltage and photocurrent enable to assess the effect of visible light illumination on the electrical behavior of a solar cell. We report on photovoltage and photocurrent measurements with nanometer scale resolution performed on the cross section of an epitaxial crystalline silicon solar cell, using respectively Kelvin probe force microscopy and conducting probe atomic force microscopy. Even though two different setups are used, the scans were performed on locations within 100-μm distance in order to compare data from the same area and provide a consistent interpretation. In both measurements, modifications under illumination are observed in accordance with the theory of PIN junctions. Moreover, an unintentional doping during the deposition of the epitaxial silicon intrinsic layer in the solar cell is suggested from the comparison between photovoltage and photocurrent measurements.

International audience; Spectral Mueller matrices measured at multiple angles of incidence as well as Mueller matrix images are recorded on the exoskeletons (cuticles) of the scarab beetles Cetonia aurata and Chrysina argenteola. Cetonia aurata is green whereas Chrysina argenteola is gold-colored. When illuminated with natural (unpolarized) light, both species reflect left-handed and near-circularly polarized light originating from helicoidal structures in their cuticles. These structures are referred to as circular Bragg reflectors. For both species the Mueller matrices are found to be nondiagonal depolarizers. The matrices are Cloude decomposed to a sum of non-depolarizing matrices and it is found that the cuticle optical response, in a first approximation can be described as a sum of Mueller matrices from an ideal mirror and an ideal circular polarizer with relative weights determined by the eigenvalues of the covariance matrices of the measured Mueller matrices. The spectral and image decompositions are consistent with each other. A regression-based decomposition of the spectral and image Mueller matrices is also presented whereby the basic optical components are assumed to be a mirror and a circular polarizer as suggested by the Cloude decomposition. The advantage with a regression decomposition compared to a Cloude decomposition is its better stability as the matrices in the decomposition are determined a priori. The origin of the depolarizing features are discussed but from present data it is not possible to conclude whether the two major components, the mirror and the circular polarizer are laterally separated in domains in the cuticle or if the depolarization originates from the intrinsic properties of the helicoidal structure.-matrix characterization of bee-tle cuticle: polarized and unpolarized reflections from representative architectures," Appl. Opt. 49, 4558–4567 (2010).-induced polarization effects in the cuticle of scarab beetles: 100 years after Michelson," Phil. Mag. 92, 1583–1599 (2012). 4. H. Arwin, T. Berlind, B. Johs, and K. Järrendahl, "Cuticle structure of the scarab beetle Cetonia aurata analyzed by regression analysis of Mueller-matrix ellipsometric data," Opt. Express 21, 22645–22656 (2013). 5. matrices: how to decompose them?," Phys. Status Solidi A 205, 720–727 (2008). 6. S. R. Cloude, "Group theory and polarization algebra," Optik (Stuttgart) 75, 26–36 (1986). 7. S. R. Cloude and E. Pottier, "A review of target decomposition theorems in radar polarimetry," IEEE Trans.

Using a plasma to generate a surface texture with feature sizes on the order of tens to hundreds of nanometers (“nanotexturing”) is a promising technique being considered to improve efficiency in thin, high-efficiency crystalline silicon solar cells. This study investigates the evolution of the optical properties of silicon samples with various initial surface finishes (from mirror polish to various states of micron-scale roughness) during a plasma nanotexturing process. It is shown that during said process, the appearance and growth of nanocone-like structures are essentially independent of the initial surface finish, as quantified by the auto-correlation function of the surface morphology. During the first stage of the process (2 min to 15 min etching), the reflectance and light-trapping abilities of the nanotextured surfaces are strongly influenced by the initial surface roughness; however, the differences tend to diminish as the nanostructures become larger. For the longest etching times (15 min or more), the effective reflectance is less than 5 % and a strong anisotropic scattering behavior is also observed for all samples, leading to very elevated levels of light-trapping.

The radial junction (RJ) architecture has proven beneficial for the design of a new generation of high performance thin film photovoltaics. We herein carry out a comprehensive modeling of the light in-coupling, propagation and absorption profile within RJ thin film cells based on an accurate set of material properties extracted from spectroscopic ellipsometry measurements. This has enabled us to understand and evaluate the impact of varying several key parameters on the light harvesting in radially formed thin film solar cells. We found that the resonance mode absorption and antenna-like light in-coupling behavior in the RJ cell cavity can lead to a unique absorption distribution in the absorber that is very different from the situation expected in a planar thin film cell and that has to be taken into account in the design of high performance RJ thin film solar cells. When compared to the experimental EQE response of real RJ solar cells, this modeling also provides an insightful and powerful tool to resolve the wavelength-dependent contributions arising from individual RJ units and/or from strong light trapping due to the presence of the RJ cell array.

Optical and magneto-optical (MO) properties of the MO nanometer-size gratings (nanogratings) are modeled using improved rigorous couple wave analysis (RCWA). The improvements include S-matrix propagation algorithm and Fourier factorization rules and lead to necessary numerical precision for the modeled quantities. The rigorous model is compared with effective medium approximation (EMA), used for simple description of the grating parameters, and the areas of differences are discussed. Dependence of optical and MO elements of permittivity tensor on the fill factor is presented together with comparison to the EMA. Basic polar, longitudinal, and transverse MO configuration are studied.

International audience; We used azimuthally-resolved spectroscopic Mueller matrix ellipsometry to study a periodic silicon line structure with and without artificially-generated line edge roughness (LER). The unperturbed, reference grating profile was determined from multiple azimuthal configurations using a generalized ellipsometer, focusing the incident beam into a 60 μm spot. We used rigorous numerical modeling, taking into account the finite numerical aperture, introducing significant depolarization effects, and determining the profile shape using a four trapezoid model for the line profile. Data obtained from the artificially perturbed grating were then fit using the same model, and the resulting root-mean-square error (RMSE) values for both targets were compared. The comparison shows an increase in RMSE values for the perturbed grating that can be attributed to the effects of LER.

The formation of GaSb nanopillars by low energy ion sputtering is studied in real-time by spectroscopic Mueller matrix ellipsometry, from the initial formation in the smooth substrate until nanopillars with a height of 200 – 300 nm are formed. As the nanopillar height increased above 100 nm, coupling between orthogonal polarization modes was observed. Ex situ angle resolved Mueller polarimetry measurements revealed a 180° azimuth rotation symmetry in the off-diagonal Mueller elements, which can be explained by a biaxial material with different dielectric functions εx and εy in a plane parallel to the substrate. This polarization coupling can be caused by a tendency for local direction dependent alignment of the pillars, and such a tendency is confirmed by scanning electron microscopy. Such observations have not been made for GaSb nanopillars shorter than 100 nm, which have optical properties that can be modeled as a uniaxial effective medium. Open Access

Development of the characterization tools and techniques used by semi-conductor industry directs not only towards increasing of the instrumental precision, to push limits of the optical characterization methods to the smallest lines, but also towards a decreased dependency of the used tools on the tool-to-tool calibration procedures. The challenge approached in this work is to use multiple independent optical configurations to determine overall accuracy of the results with minimal or no assistance of the other non-optical methods. The approach presented here is based on a well-known change of sensitivity of the optical model parameters at the different azimuthal measurement configurations. The full potential of this method can only be unlocked using complete Mueller matrix measurements providing complete information on the anisotropic nature of the gratings. The measurements at multiple azimuth configurations, used in this work, illustrate the potential of the method on the experimental data provided by the angle-resolved and spectrally resolved Mueller matrix polarimetric tools. The results are consistent with the single-line AFM measurements used as an independent reference.

The measurement of the Mueller matrix when the probing beam is placed on the boundary between two (or more) regions of the sample with different optical properties may lead to a depolarization in the Mueller matrix. The depolarization is due to the incoherent superposition of the optical responses of different sample regions in the probe beam. Despite of the depolarization, the measured Mueller matrix has information enough to subtract a Mueller matrix corresponding to one of the regions of sample provided that this subtracted matrix is non-depolarizing. For clarity, we will call these non-depolarizing Mueller matrices of one individual region of the sample simply as the non-depolarizing components. In the framework of the theory of Mueller matrix algebra, we have implemented a procedure allowing the retrieval of a non-depolarizing component from a depolarizing Mueller matrix constituted by the sum of several non-depolarizing components. In order to apply the procedure, the Mueller matrices of the rest of the non-depolarizing components have to be known. Here we present a numerical and algebraic approaches to implement the subtraction method. To illustrate our method as well as the performance of the two approaches, we present two practical examples. In both cases we have measured depolarizing Mueller matrices by positioning an illumination beam on the boundary between two and three different regions of a sample, respectively. The goal was to retrieve the non-depolarizing Mueller matrix of one of those regions from the measured depolarizing Mueller matrix. In order to evaluate the performance of the method we compared the subtracted matrix with the Mueller matrix of the selected region measured separately.

Anisotropic lamellar sub-wavelength gratings (nanogratings) are described by Effective Medium Approximation (EMA). Analytical formulas for effective medium optical parameters of nanogratings from arbitrary anisotropic materials are derived using approximation of zero-order diffraction mode. The method is based on Rigorous Coupled Wave Analysis (RCWA) combined with proper Fourier factorization method. Good agreement between EMA and the rigorous model is observed, where slight differences are explained by the influence of evanescent higher Fourier harmonics in the nanograting.

International audience; We characterized two samples consisting of photoresist layers on silicon with square arrays of square holes by spectroscopic ellipsometry (SE) and Mueller matrix polarimetry (MMP). Hole lateral dimensions and depths were determined by fitting either SE data taken in conventional planar geometry or MMP data in general conical diffraction configurations. A method for objective determination of the optimal measurement conditions based on sensitivity and parameter correlations is presented.When applied to MMP, this approach showed that for one of the samples the optimal incidence angle was 45◦, much below the widely used 70 degrees value. The robustness of the dimensional characterisation based on MMP is demonstrated by the high stability of the results provided by separated fits of the data taken at different azimuthal angles.

Spectral properties of lamellar sub-wavelength gratings (nanogratings) are described by effective medium approximation (EMA). Analytical spectral formulae for ordinary and extraordinary effective optical functions are derived for nanogratings consisting of material described by Sellmeier, damped harmonic oscillator, and Drude formulae. Spectral origin for birefringence of dielectric nanogratings and linear dichroism for absorbing ones is discussed for model cases and gratings consisting of real natural materials. Simple approximation by zero-order diffraction is compared with rigorous modeling based on Rigorous Coupled Wave Analysis (RCWA). Limits of applicability of effective medium approximation is discussed in the spectral domain. Web of Science 1 art. no. 06015

Color in living organisms is primarily generated by two mechanisms: selective absorption by pigments and structural coloration, or a combination of both. In this study, we investigated the coloration of cuticle from the wings (elytra) of the two ground beetle species Carabus auronitens and Carabus auratus. The greenish iridescent color of both species is created by a multilayer structure consisting of periodically alternating layers with different thicknesses and composition which is located in the 1-2 µm thick outermost layer of the cuticle (epicuticle). Illuminated with white light, reflectance spectra in both linear polarisation show an angle-dependent characteristic peak in the blue/green region of the spectrum. Furthermore, the reflected light is polarised linearly. Scattering experiments with laser illumination at 532 nm show diffuse scattering over a larger angular range. The polarisation dependence of the scattered light is consistent with the interpretation of small inhomogeneities as scattering centres in the elytra.

Background: Treatment with high dose chemotherapy and autologous stem cell rescue in pacients with poor risk germ cell tumors is still controversial. Results of multicentric randomized EBMT study IT 94 presented at ASCO 2002 show benefit in 1-year EFS in high dose arm (52% versus 48%), 3-year EFS was the same in both arms (53%) in salvage treatment. Individual treatment with stem cell rescue as upfront treatment offers a survival benefit. Methods:Autologous stem cell rescue was provided in our center, from September 1997 to May 2006 to 52 patients. High dose chemotherapy was indicated to 32 patients in salvage setting after 2nd line of treatment (VeIP) and to 20 patients as upfront treatment after 1st line treatment (BEP). Median age was 29 years and tumor markers were elevated: HCG in 9 pts, AFP in 13 pts. Stem cell mobilization was performed after the 3rd cycle of VeIP or BEP in combination with G-CSF. The amount of CD34+ cell/kg b.w. was between 2,0 – 13.4×106. High - dose conditioning regimen CARBOPEC (carboplatin 1600 – 2 200 mg/m2, etoposide 1 800mg/m2, cyclophosphamide 6 400 mg/m2) was used. The treatment was well tolerated without transplant - related mortality. Results: WHO criteria non - hematological toxicity was predominantly grade 2 to 3. Engraftment was rapid, recovery of hematopoiesis in neutrofils over 1.0×109/l and platelets over 50×109/l was reached an average on days +10 and +13 respectively. Additional post-transplant treatment for persistence, progression or relaps had 20 patients (8pts had 2nd line treatment VEIP, 12pts had 3nd line treatment with paclitaxel+gemcitabine and 5 pts had retroperitoneal lymfadenectomy). The follow - up period ranges from 3 to 99 months, at present 38 (73 %) patients are alive, 14 (27 %) pts died. Median TTP of all pts is 10 months, median OS of all pts is 39 months. Median DFS of surviving pts is 38 months. Conclusion: high-dose chemotherapy with autologous stem cell rescue in patients with poor risk germ cell tumors is feasible and beneficial method of the individual treatment. High-dose chemotherapy as upfront treatment for poor prognosis germ cell tumors and as salvage treatment in good risk pts seems to be good possibility of the individual treatment.

We demonstrate flexible nanowire white light-emitting-diodes (LEDs) with an optimized colour quality. The devices consist of flexible InGaN/GaN nanowire LEDs acting as pumps, capped with removable phosphor-doped polydimethylsiloxane membranes. Five different phosphors with tens of microns in grain size emitting from green to orange are investigated using both violet-blue and a blue-green nanowire-based LED pumps. In addition, a flexible nanowire white LED with a warm white emission is demonstrated using two layers of different phosphors. Compared to the previous realizations of flexible nanowire white LEDs, these novel LEDs improve the colour rendering index from 54 to 86 and show a colour tuneable from a bluish cool white colour to natural white and finally to warm white. The flexibility tests show that the LEDs can be bent down to 1.5 cm curvature radius without significant degradation. Therefore, the replacement of the nano-phosphors used in the previous realization by relatively inexpensive micro-phosphors does not degrade the good mechanical flexibility of the white nanowire LEDs.

Silicon nanowires (SiNWs) are attracting an increasing attention due to their unique one-dimensional geometry with properties suitable for different optoelectronic applications. Recently, small diameter NWs (below 10 nm) have become the target due to predicted quantum size effects. Moreover, small diameter SiNWs with hexagonal diamond crystalline structure, which are predicted to have a direct bandgap that can be tuned by the diameter control, have been synthesized directly in a PECVD reactor. This thesis is dedicated to fabricating small diameter SiNWs with hexagonal phase using PECVD. The analysis on the previous work at LPICM has shown that hexagonal SiNWs were not obtained with a single Sn catalyst. We have demonstrated SiNW growth using evaporated SnCu mixed catalysts with a controlled composition for the first time, which leads to a core-shell structure on the top of SiNW with a crystalline Cu3Si core and a Sn-rich shell most probably liquid during the growth. The SiNWs have been grown through a combined VLS and VSS growth process. We have investigated the influence of the catalyst composition and PECVD conditions on the SiNW growth systematically and achieved ultra-dense (5.7 × 1010 /cm2) and ultra-thin silicon quantum nanowire arrays with average crystalline diameter of around 4 to 5 nm and with a narrow distribution. TEM observation showed that hexagonal phase can be obtained with a substrate temperature between 385 and 447 °C, and within a SiH4 partial pressure range from 0.026 to 0.068 mbar, on a Cu TEM grid, and on (100) Si wafer substrates using different SnCu compositions. The hexagonal phase appeared statistically rarely (usually with a yield of around 5 – 6%), with no particular growth condition except for the small diameter (with crystalline diameters from 4.0 to 7.2 nm). By replacing Sn with In element with similar properties in a mixed catalyst with Cu, dense SiNW arrays with an average diameter of 10.5 nm have been obtained, among which we found a hexagonal SiNW with crystalline diameter of 5 nm. In addition, the use of AuCu mixed catalysts has led to faster SiNW growth rate when compared to pure Au catalyst. Therefore, using mixed catalysts allows for a control of SiNW diameter (narrower distribution), length (growth rate), and density that cannot be obtained by using pure metal catalysts. Finally, PIN radial junction solar cells based on ultra-dense and thin SiNWs synthesized with SnCu and InCu co-catalysts have been fabricated successfully., Les nanofils de silicium bénéficient d’une attention accrue du fait de propriétés uniques liées à leur géométrie unidimensionnelle, propriétés qui intéressent différentes applications en opto-électronique. Récemment, des nanofils de très petit diamètre (inférieur à 10 nm) ont été l’objet de plusieurs études, car leurs propriétés prédites témoignent d’intéressants effets de confinement quantique. De plus, la structure hexagonale-diamant a été observée dans ce type de nanofils, et les calculs prédisent qu’ils auraient alors un gap direct, ajustable par contrôle du diamètre. Ces nanofils de silicium hexagonal ont été synthétisés lors d’une précédente étude au LPICM, dans un réacteur de dépôt chimique en phase vapeur assisté par plasma (PECVD). Le travail présenté ici a pour but la fabrication par PECVD de nanofils de très petit diamètre, avec la phase hexagonale diamant. Notre analyse des précédents travaux menés au LPICM montre que le catalyseur utilisé alors – nominalement de l’étain – comprenait en réalité un part de cuivre venant du substrat – une grille en cuivre de microscopie électronique en transmission (TEM). Nous avons donc démarré cette thèse par l’étude de catalyseurs mixtes SnCu de composition contrôlée. Ce type de catalyseur acquiert lors des traitements préliminaires une structure de type « cœur-coquille ». Lors de la croissance, le cœur devient du Cu3Si cristallin tandis que la coquille demeure un eutectique riche en étain, très certainement liquide à la température de croissance. Ce mode de croissance combine donc VLS (vapeur-liquide-solide) et VSS (Vapeur-solide-solide). Nous avons étudié de manière systématique l’influence de la composition du catalyseur et des conditions PECVD sur la croissance. Nous avons ainsi obtenu des tapis ultra-denses (5.7 × 1010 /cm2) de nanofils ultrafins (4-5 nm de diamètre cristallin). Nos observations TEM montrent que la phase hexagonale est présente, dans cette gamme de diamètres, pour toutes les conditions de croissance testées (température de 385 à 447 °C, pression de SiH4 de 0,026 à 0,068 mbar, sur grille TEM en Cu comme sur Si (100) et avec des catalyseurs de compositions en Sn et Cu variables). La phase hexagonale apparaît cependant avec une relativement faible probabilité (typiquement de 5 à 6%), sans condition de croissance particulière si ce n’est la faible taille (les valeurs des diamètres cristallins des nanofils hexagonaux sont entre 4.0 et 7.2 nm). En remplaçant l’étain par l’élément In, qui a des propriétés similaires, dans le catalyseur mixte avec Cu, nous obtenons également des réseaux denses de nanofils de Si, avec toutefois des diamètres plus grands pour une épaisseur de catalyseur égale. Dans un réseau de diamètre moyen de 10.5 nm, nous avons trouvé un nanofil de Si de 5 nm de diamètre avec une structure partiellement hexagonale. Nous avons également essayé le catalyseur mixte AuCu qui a donné des vitesses de croissance accrues, par rapport à l’Au pur. À l’issue de l’ensemble de ces expériences, nous pouvons affirmer qu’utiliser des catalyseurs mixtes, plutôt que des catalyseurs de métaux purs, permet un meilleur contrôle : du diamètre des nanofils (en autorisant des diamètres plus petits dans des distributions étroites) ; de leur longueur (par la vitesse de croissance), et de leur densité. Pour finir, nous avons fabriqué des cellules solaires à jonction radiale en silicium amorphe hydrogéné sur ce type de petits nanofils très denses, dopés p cette fois, synthétisés à partir des catalyseurs SnCu and InCu. Ces premières cellules apparaissent fonctionnelles.

In this thesis, we have used a low-temperature plasma-enhanced chemical vapor deposition (PECVD) reactor to fabricate Si nanowire radial junction solar mini-modules on 5x5 cm2 glass substrates with the assistance of the laser scribing technique for the series connection of the cells.We have used fluorine-doped tin oxide (FTO) deposited on soda-lime glass substrates (SLG) as a back contact as well as the source of the Sn catalyst which was formed by a direct reduction of FTO using a H2 plasma. Subsequently, p-type SiNWs were grown using plasma-assisted vapor liquid solid (VLS) process, followed by the deposition of intrinsic a-Si:H and n-type µc-SiOx:H layers to achieve pin radial junction solar cells. We have obtained an energy conversion efficiency of 6.3 % with an active area of solar cells of 0.126 cm2, which is to our knowledge, the highest efficiency obtained based on FTO layers as a source of Sn catalyst.Laser scribing was used to perform a selective removal of thin-film materials in order to fabricate minimodules. With laser scribing, a monolithic series connection between adjacent RJ SiNW solar cells on the same glass substrate was achieved. In particular, the laser scribing system has been used to perform selective removal of FTO thin-film and RJ SiNWs, which are commonly known as step P1 and P2, respectively, and to perform a final scribe to isolate the active region from the rest of the substrate. The transparent top ITO contact was sputtered and cell stripes were defined using the lift-off technique (step P3).We have carried out a detailed study of the P2 laser scribe obtained with either green (532 nm) or IR (1064 nm) laser setups. The power of the laser has to be controlled as it has a direct impact on the removal of SiNW RJs and it can damage the underneath FTO contact. We have found that the scribing using a green laser produces a partial melting outside the scribed spots, unlike the IR laser which provides a cleaner scribing and less crystallized material at the edges of scribed spots. Mapping of the scribed spots using Raman spectroscopy allowed analyzing the material composition within the scanned area inside the craters left by the laser pulses. We have demonstrated that the use of the IR laser is preferable for P2 scribing because it can provide a high-quality series connection between cells.Finally, the optimized 10 cm2 SiNW RJ mini-module has reached an energy conversion efficiency of 4.37 % with power generation of 44 mW, thanks to the improved P2 laser scribing and the dense Ag grid printed using the ink-jet method. This performance represents, to the best of our knowledge, the highest reported power generation for silicon nanowire-based solar modules on glass substrates., Dans cette thèse, nous avons utilisé un réacteur de dépôt chimique en phase vapeur assisté par plasma (PECVD) à basse température afin de fabriquer des minimodules à base de cellules solaires à jonction radiale (RJ SiNWs) sur des substrats de verre de 5x5 cm² en employant la gravure laser pour la mise en série des cellules.Nous avons utilisé une couche de 600 nm d’épaisseur de dioxyde d’étain dopée au fluor (FTO) déposée sur du verre sodocalcique (SLG). La couche de FTO sert à la fois de contact arrière pour le module et de source de catalyseur Sn une fois la couche de FTO réduite par un plasma de H2. Ensuite, on fait croître des SiNW dopés p par le procédé Vapor-Liquid-Solid (VLS) assisté par plasma, suivi d’un dépôt d’une couche de Si intrinsèque a-Si:H et d’une couche de Si dopée n µc-SiOx:H, afin d’obtenir une cellule solaire à jonction radiale PIN. Nous avons obtenu une efficacité énergétique de 6.3 % avec une surface active de 0.126 cm². C’est à notre connaissance l’efficacité la plus élevée obtenue en utilisant une couche de FTO comme source de catalyseur Sn.La gravure laser a été utilisée pour retirer localement des couches minces dans l’objectif de fabriquer des minimodules solaires. Grâce à la gravure laser, une connexion monolithique en série entre les cellules solaires à jonction radiale adjacentes a pu être accomplie. Dans cette thèse, la gravure laser a servi à retirer localement la couche de FTO ainsi que les RJ SiNWs, étapes appelées respectivement P1 et P2. On dépose ensuite une couche transparente d’oxyde d’indium-étain (ITO), servant de contact avant, par pulvérisation cathodique (étape P3), et on procède à la séparation en bandes par la technique « lift-off ». Nous avons mené une étude détaillée de l’étape P2 de gravure obtenue par un laser vert (532 nm) et IR (1064nm). La puissance du laser a un impact direct sur l’ablation des RJ SiNWs, et peut aussi endommager le contact arrière de la cellule. Nous avons déterminé que le laser vert entraîne une fonte partielle de matériau sur les bords de la zone gravée, contrairement au laser IR qui produit des gravures de meilleure qualité. La cartographie Raman des zones gravées permet une analyse des matériaux dans la zone étudiée, et a donné des indications sur la composition des résidus laissés par les impulsions laser. Nous avons démontré que l’utilisation du laser IR pour l’étape P2 de gravure est préférable. Elle permet d’avoir des connexions en série de haute qualité entre les cellules.Enfin, le mini-module optimisé de 10 cm² à base de RJ SiNWs a atteint un rendement de conversion énergétique de 4.37 % avec une puissance générée de 44 mW, grâce à l’amélioration de l’étape P2 et de l’impression par jet d’encre d’une grille dense d’Ag. À notre connaissance, cette puissance générée est la plus élevée rapportée pour des modules solaires à base de cellules à jonction radiale.

Research in photovoltaics aims at lowering the price per watt of generated electrical power. Substantial efforts aim at searching for new materials and designs which can push the limits of existing solar cells. The recent development of complex materials and nanostructures for solar cells requires more effort to be put into their characterization and modeling. This thesis focuses on optical characterization, modeling, and design optimization of advanced solar cell architectures.Optical measurements are used for fast and non-destructive characterization of textured samples for photovoltaic applications. Surface textures enhance light-trapping and are thus desired to improve the solar cell performance. On the other hand, these textures make optical characterization more challenging and more effort is required for both, the optical measurement itself and subsequent modeling and interpretation of obtained data. In this work, we demonstrate that we are able to use optical methods to study the widely used pyramidal textures as well as very challenging randomly oriented silicon nanowire arrays.At first, we focused on the optical study of various pyramidal surfaces and their impact on the silicon heterojunction solar cell performance. We have found that vertex angles of pyramids prepared using various texturing conditions vary from the theoretical value of 70.52° expected from crystalline silicon. This change of the vertex angle is explained by regular monoatomic terraces, which are present on pyramid facets and are observed by atomic resolution transmission electron microscopy. The impact of a vertex angle variation on the thicknesses of deposited thin films is studied and the consequences for resulting solar cell efficiency are discussed. A developed optical model for calculation of the reflectance and absorptance of thin film multi-layers on pyramidal surfaces enabled a solar cell design optimization, with respect to a given pyramid vertex angle.In-situ Mueller matrix ellipsometry has been applied for monitoring the silicon nanowire growth process by plasma-enhanced vapor-liquid-solid method. We have developed an easy-to-use optical model, which is to our knowledge a first model fitting the experimental ellipsometric data for process control of plasma-assisted vapor-liquid-solid grown nanowires. The observed linear dependence of the silicon material deposition on the deposition time enables us to trace the fabrication process in-situ and to control material quality., La recherche sur le photovoltaïque vise à réduire le prix par watt de puissance électrique générée. Des efforts considérables sont menés pour rechercher de nouveaux matériaux et des conceptions qui repoussent les limites des cellules solaires existantes. Le développement récent de matériaux et nanostructures complexes pour les cellules solaires nécessite des efforts plus importants pour mener à bien leur caractérisation et leur modélisation. Cette thèse porte sur la caractérisation optique, la modélisation et l'optimisation de la conception d'architectures de cellules solaires de pointe.Les mesures optiques sont utilisées pour la caractérisation rapide et non destructive des échantillons texturés pour les applications photovoltaïques. Les textures de surface améliorent le piégeage de la lumière et sont donc souhaitées pour améliorer les performances des cellules solaires. D'autre part, ces textures rendent la caractérisation optique plus difficile et des efforts plus importants sont nécessaires non seulement pour la mesure optique elle-même mais également pour la modélisation et l'interprétation ultérieure des données obtenues. Dans ce travail, nous démontrons que nous sommes en mesure d'utiliser des méthodes optiques pour étudier les textures pyramidales très répandues ainsi que les réseaux de nanofils de silicium à orientation aléatoire dont l'analyse est très difficile.Premièrement, nous nous sommes concentrés sur l'étude optique de diverses surfaces pyramidales et de leur impact sur les performances des cellules silicium à hétérojonction. Nous avons constaté que les angles au sommet des pyramides, préparées à l'aide de différentes conditions de texturation, diffèrent de la valeur théorique de 70.52° attendue pour le silicium cristallin. Cette modification de l'angle au sommet est expliquée par la présence, sur les facettes pyramidales, de terrasses monoatomiques régulières, observées par microscopie électronique à transmission de résolution atomique. L'impact d'une variation de l’angle au sommet sur les épaisseurs des couches minces déposées est étudié et les conséquences sur l'efficacité des cellules solaires résultantes sont discutées. Un modèle optique développé pour le calcul de la réflectance et de l'absorption des couches minces en multicouches sur surfaces pyramidales a permis l’optimisation de la conception de la cellule solaire pour un angle au sommet pyramidal donné.L'ellipsométrie matricielle Mueller a été utilisée in-situ pour caractéiser le processus de croissance - par méthode vapeur-liquide-solide activée par plasma - des nanofils de silicium. Nous avons développé un modèle optique facile à utiliser, qui, à notre connaissance, est le premier modèle utilisant des données ellipsométriques expérimentales pour contrôler le procédé de croissance, en phase vapeur-liquide-solide assisté par plasma, des nanofils. La dépendance linéaire observée du dépôt de matériau de silicium avec le temps de dépôt nous permet de suivre le processus de fabrication in situ et de contrôler la qualité du matériau.