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Abstract: The interaction of nitrogen with liquid and solid silicon at high temperature is investigated in an electromagnetic levitation set-up. It is shown that the nucleation undercooling of Si decreases monotonically from 300 to 1K when the concentration of nitrogen in the solidified droplet increases from 0 to 600ppmw. Several α- and β-Si3N4 morphologies are observed and their growth conditions are linked to the various stages of the Si droplet cooling down. It follows that electromagnetic levitation is a valuable tool for investigation of the chemical behavior of highly reactive liquids at high temperature. [Copyright &y& Elsevier]

Abstract: The meso-porous silicon (PS) has become an interesting material owing to its potential applications in many fields, including optoelectronics and photovoltaics. PS layers were grown on the front surface of the n+ emitter of n+–p mono-crystalline Silicon junction. The thickness and the porosity of the PS layer were determined by an ellipsometer, as a function of time duration of anodization, and the variation law of the PS growth kinetics is established. Single layers PS antireflection coating (ARC) achieved around 9% of effective reflectivity in the wavelength range between 400 and 1000nm on junction n+–p solar cells. To reduce the reflectivity and improve the stability and passivation properties of PS ARC, silicon oxide layers were deposited by PECVD on PS ARC. SiO x layers of thickness of 105nm combined with PS layer led to 3.8% effective reflectivity. V oc measurements were carried out on all the samples by suns-V oc method and showed an improvement of the quality of the passivation brought by the oxide layer. Using the experimental reflectivity results and taking into account the passivation quality of the samples, the PC1D simulations predict an enhancement of the photogenerated current exceeding 44%. [ABSTRACT FROM AUTHOR]

Abstract: A simple and effective method for producing Ag nanoparticles at room temperature and on the time scale of several minutes on the surface of hydrogenated Si nitride (SiN x :H) films is presented. Depending on the stoichiometry of SiN x :H layers, Ag nanostructures with different sizes and densities can be produced. The structural properties of the deposited Ag nanoparticles are described in details with respect to the stoichiometries of SiN x :H films. [ABSTRACT FROM AUTHOR]

Abstract: We present an overview of the properties of silicon oxynitride material (SiON) deposited by plasma enhanced chemical vapor deposition (PECVD) for photovoltaic applications. SiON films were deposited using silane (SiH4), ammonia (NH3) and nitrogen protoxide (N2O) as precursor gases in a low frequency PECVD. Varying the gas flow mixture leads to a whole range of SiON layers starting from the silicon oxide to the silicon nitride with unique stoichiometries and properties. Thanks to spectroscopic ellipsometry measurements we have confirmed the suitability of SiON for antireflection coating layers due to the range of the refractive indexes attainable. SiON structure was analyzed by X-ray photo-electron spectroscopy. We have thus highlighted the critical role of oxygen behavior on the SiON network and the progressive replacement of nitrogen by oxygen atoms when the oxygen precursor increases. The type of chemical bonds present in SiON layers was also investigated by infrared spectroscopy. The SiON layers also contain a non-negligible amount of hydrogen which might be useful for passivation applications. The behavior of hydrogen content was thus analyzed by elastic recoil decay analysis and desorption characterization. A typical rapid thermal annealing was performed on the SiON samples in order to simulate the solar cells contact annealing and to investigate its impact on the dielectric film properties. It was found that hydrogen becomes weakly bonded to the films and strongly decreases in quantity with the annealing. The surface passivation effect is presented in the last part of this paper. The trend before and after a rapid thermal annealing showed opposite results which could be explained by the high porosity of the layers and the formation of Si–O bonds. [Copyright &y& Elsevier]

The rapid evolution of photovoltaic Si production induced a shortage of high purity silicon raw material. The use of lowest purity silicon has a strong effect on the casting conditions and ingot structure and properties. During solidification, solute rejection at the growth interface leads to an increase of the impurities concentration in the liquid phase and then to the precipitation of silicon nitride and silicon carbide. As a consequence, the grain structure of the ingot changes from columnar to small grains, also known as grits. A new electromagnetic levitation setup which has been developed in order to measure the undercooling versus impurity concentration is presented. The impurity concentration in the levitated Si drop is controlled by the partial pressure of nitrogen or hydrocarbon gas. As nucleation is a random phenomenon, statistical measurements are presented, from samples which showed numerous heating/melting and cooling/solidification phases. The effect of carbon impurities on the undercooling of silicon droplet is discussed. [ABSTRACT FROM AUTHOR]

Abstract: Thin crystalline silicon layers (50μm) were grown by vapour phase epitaxy on monocrystalline substrates. Minority carrier diffusion length and surface recombination velocity were evaluated by light beam induced current experiment. Although it appeared difficult to apply existing analytical models to thin and high quality layers, multi-dimensional simulator DESSIS was used successfully to extract diffusion length of the order of 300μm for p-type material and 80μm for n-type material with surface recombination velocity of the order of 100–1000cms−1 when the surface was passivated by a thin silicon nitrite coating. Results were compared with the diffusion length evaluated from internal quantum efficiency analysis in fabricated photovoltaic cells made of the same material, using spectral response and reflectivity measurements. [Copyright &y& Elsevier]

Abstract: The aim of the present work is to compare deposits made by electroless nickel–phosphorous (Ni–P) from two different baths and to see how it can be used for the fabrication of solar cell contacts: deposition on screen-printed contacts or directly on silicon as seed layer for subsequent electrolytic deposition of copper or silver. N-type silicon samples were plated, so as to study the feasibility of such deposits (homogeneity, adhesion). Scanning electron microscope (SEM) and contact resistivity measurements by transmission line model (TLM) were used to check the quality of the deposits. Dark I–V characteristics and external quantum efficiency have also been performed on standard silicon solar cells before and after Ni–P deposition on screen-printed contacts. [Copyright &y& Elsevier]

Abstract: This work is a contribution towards the understanding of the properties of hydrogenated silicon nitride (SiN x :H) that lead to efficient surface and bulk passivation of the silicon substrate. Considering the deposition system used (low-frequency plasma-enhanced chemical vapour deposition (PECVD)), we report very low values of surface recombination velocity S eff. As-deposited Si-rich SiN x :H leads to the best results (n-type Si: S eff=4cm/s – p-type Si: S eff=14cm/s). After annealing, the surface passivation quality is drastically deteriorated for Si-rich SiN x :H whereas it is lightly improved for low refractive index SiN x :H (n∼2–2.1). The chemical analysis of the layers highlighted a high hydrogen concentration, regardless the SiN x :H stoichiometry. However, the involved H-bond types as well as the hydrogen desorption kinetics are strongly dependent on the SiN x :H composition. Furthermore, “N-rich” SiN x :H appears to be denser and thermally more stable than Si-rich SiN x :H. When subjected to a high-temperature treatment, such a layer is believed to induce the release of hydrogen in its atomic form, which consequently leads to an efficient passivation of surface and bulk defects of the Si substrate. The results are discussed and compared with the literature data reported for the different configurations of PECVD reactors. [Copyright &y& Elsevier]

Abstract: The diffusion length of minority carriers is one of the most important electrical parameters to qualify silicon for photovoltaic applications. One way to evaluate this parameter is to analyse the decay of the current induced when a focused beam is scanned away from the collector using Light Beam Induced Current (LBIC) technique. The LBIC signal was numerically calculated with 2D-DESSIS software under different boundary conditions, as a function of material thickness and surface recombination velocity in order to verify the limitations of analytical models and to fit the LBIC signal measured in thin silicon samples. Samples with thickness ranging from 55 μm to 2500 μm were evaluated with diffusion length values ranging from 70 μm to 2.5 mm. Analytical expressions of the Internal Quantum Efficiency (IQE) were also used to extract the minority carrier bulk and effective diffusion lengths from surface averaged spectral response and reflectivity data in thick solar cells. [Copyright &y& Elsevier]

Abstract: Silicon nitride layers of 140nm thickness were deposited on silicon wafers by low pressure chemical vapour deposition (LPCVD) and irradiated at GANIL with Pb ions of 110MeV up to a maximum fluence of 4×1013 cm−2. As shown in a previous work these irradiation conditions, characterized by a predominant electronic slowing-down (S e =19.3keVnm−1), lead to damage creation and formation of etchable tracks in Si3N4. In the present study we investigated other radiation-induced effects like out of plane swelling and refractive index decrease. From profilometry, step heights as large as 50nm were measured for samples irradiated at the highest fluences (>1013 cm−2). From optical spectroscopy, the minimum reflectivity of the target is shifted towards the high wavelengths at increasing fluences. These results evidence a concomitant decrease of density and refractive index in irradiated Si3N4. Additional measurements, performed by ellipsometry, are in full agreement with this interpretation. [Copyright &y& Elsevier]

Abstract: The results presented in this paper concern the study of interdigitated back contact solar cells (IBC SC) emitter passivation by means of comparison of full and point contact solar cells and two type of passivation layers (SiN and SiO2) as well as different values of the emitter sheet resistance. The fabrication of contacts in the form of points does not influence significantly the I–V characteristic parameters in comparison to full contact cells for highly doped emitters, however, improvement of R sh is observed for a few point contact solar cells. A similar effect has been observed on higher sheet resistance emitters as well as a strong degradation of the dark current after the last annealing of the cell. We have attributed this effect to the short-circuiting of a too thin emitter. [Copyright &y& Elsevier]

Abstract: Hydrogenated silicon oxynitride (SiON) could be used in combination with silicon nitride (SiN) to create multi-layer antireflection coatings for silicon solar cells. It could also be used as a passivation layer, especially on the back side of the cell. This work deals with the passivation effect obtained on silicon surface by SiON layer deposited by Low Frequency Plasma Enhanced Chemical Vapour Deposition (LF-PECVD). SiON layers of different compositions have been deposited by varying the gas flow mixture (NH3, SiH4 and N2O) in the reactor. Infrared and X-ray photon–electron spectroscopy were made to determine the chemical structure of SiON layer. Minority carrier lifetimes were measured by the photoconductance decay method (PCD) before and after a rapid thermal anneal. Effective lifetime, measured on 5 Ω cm FZ-silicon wafers, can reach up to several hundreds microseconds, depending on the stoichiometry of the SiON layer. Low oxygen content samples (close to SiN layer) exhibit a good surface passivation of 250 µs but after annealing, this value is critically reduced to 6 µs. The opposite situation is observed for oxygen-rich layers: the effective lifetime increases from 10 µs to 150 µs. These behaviours could be partly explained by the composition of SiON, the evolution of the main peaks values of the FTIR spectra and the disappearance of Si–H bonds with anneal. [Copyright &y& Elsevier]

Abstract: Silicon nitride layers of 140nm thickness were deposited on silicon wafers by low pressure chemical vapour deposition (LPCVD). The samples were irradiated in the electronic slowing-down regime, with either Pb ions of 110MeV (S e =19.3keVnm−1) or Xe ions of 710MeV (S e =22.1keVnm−1). Using infrared absorption spectroscopy, the radiation-induced disorder in Si3N4 was analysed as a function of the ion fluence (up to 4×1013 cm−2). Some targets irradiated at low fluences (∼109 cm−2) were etched at room temperature in aqueous HF solution (10vol.%) for various durations. The processed surfaces were probed using atomic force microscopy (AFM) in order to evidence etched tracks and to measure their mean surface diameter. The non-simultaneous emergences of the nanopores at the Si3N4/Si interface and the limited etching efficiency allow to conclude that the tracks are probably discontinuous in the present irradiation conditions. [Copyright &y& Elsevier]

Abstract: High-efficiency silicon solar cells need a textured front surface to reduce reflectance and to improve light trapping. Texturing of monocrystalline silicon is usually done in alkaline solutions. These solutions are cheaper, but are pollutants of silicon technologies. In this paper, we investigate an alternative solution containing tetramethyl ammonium hydroxide ((CH3)4NOH, TMAH ). This study shows the influence of different parameters (concentration, agitation, duration and temperature), to obtain uniform and reliable pyramidal texturization on different silicon surfaces (as cut, etched and polished). Under optimized conditions, TMAH-textured surface led to an average weighted reflectance of 13%, without any antireflection coating independent of the initial silicon surface. Unlike potassium hydroxide (KOH) texturing solution, characterization of silicon oxide layer contamination after TMAH texturing process revealed no pollution, and passivation is less affected by TMAH than by KOH texturization. [Copyright &y& Elsevier]

Abstract: Hydrogenated silicon nitride SiN x :H films are largely used as antireflective coating as well as passivation layer for industrial crystalline and multicrystalline silicon solar cells. This work is focused on the optical and photoluminescence (PL) properties of SiN x :H deposited by either Plasma-Enhanced Chemical Vapour Deposition (PECVD) or UltraViolet photo-assisted CVD (UVCVD). Photoluminescence phenomena were investigated in SiN x :H having different stoechiometries. On the other hand, spectroscopic ellipsometry was carried out in order to obtain the optical properties of the films, from which the optical gap could be determined. The evolution of the photoluminescence with stoechiometry was correlated to the evolution of the optical properties, and especially the absorption within the SiN x :H layer. A good agreement was found considering the confinement of excitons in strongly absorbing silicon nanostructures (ns-Si) formed in the SiN x matrix, with different sizes according to the NH3 /SiH4 gas flow ratio and the deposition technique. The main PL peak showed an increase of the emission intensity along with a blueshift as silicon concentration decreases. These observations indicate a radiative recombination mechanism dominated by confined excitons within ns-Si rather than emission related to defects. Furthermore, these ns-Si are supposed to be responsible of the global higher absorption, and hence lower optical gap, of the near-stoechiometric SiN x :H films in comparison with the stoechiometric Si3N4 ones. These assumptions were confirmed thanks to transmission electron microscopy (TEM) images performed on one of the samples, showing crystalline silicon quantum dots (c-Si QDs) embedded in the SiN x matrix. [Copyright &y& Elsevier]

Thin film single crystal silicon on foreign substrate is an attractive way to realize cheap and efficient photovoltaic devices. In this paper we will compare epitaxial growth of silicon thin film on double porous sacrificial layers obtained by liquid or vapor phase epitaxy (LPE or VPE). Porous silicon is formed by electrochemical anodisation of monocrystalline silicon in a HF/ethanol solution. VPE is achieved in an atmospheric pressure chemical vapor deposition (APCVD) reactor under H2 atmosphere. Growth rate is in between 0.5–3 μm/min. LPE is realised in a graphite sliding boat using indium or tin as solvent. Growth rate is in the range 0.1–1 μm/min depending on the temperature, the cooling rate and the solvent. We discuss the substrate orientation, temperature, growth rate, layer homogeneity and electrical properties of the epilayers for both growing techniques. Diffusion length and mobility are measured respectively with LBIC and Hall effect technique. The values obtained for p-type (μ>100 V/cm2/s and Ln>100 μm) allows the realisation of solar cell using interdigitated technology on the top of this layer, which is detached and transferred onto mullite substrate. [Copyright &y& Elsevier]

This work intends to compare two different passivation methods for electromagnetically continuous pulling silicon (EMCP): remote plasma hydrogenation and remote plasma enhanced CVD of SiN followed by high-temperature sintering. All experiments are carried out on textured and non-textured EMCP samples from the same ingot. To check the effect of high-temperature diffusion on EMCP, a n+-emitter is formed on one group of the samples using POCl3 diffusion. Passivation capabilities of both techniques are checked using measurements of minority carrier lifetime by means of microwave photoconductance decay mapping. Solar cells are made to compare lifetime measurement with cell parameters. [Copyright &y& Elsevier]

Mesoporous germanium (Ge) is a new nanostructured material with a very high potential for numerous applications. Thermally induced structural evolution of this nanomaterial is a key point for many high-temperature processes, such as epitaxy, for example, in which it can be involved. In this work, we investigate these structural changes occurring during thermal annealing in vacuum in temperature range between 250 and 650 °C. The influences of the annealing temperature and time as well as of the initial porous layer thickness on the morphology reorganization of the mesoporous Ge layer are described in detail. The obtained results are discussed in terms of Lifshitz-Slyozov-Wagner theory. [ABSTRACT FROM AUTHOR]

A test method that measures spectrally resolved irradiance distribution for a concentrator photovoltaic (CPV) optical system is presented. In conjunction with electrical I-V curves, it is a means to visualize and characterize the effects of chromatic aberration and nonuniform flux profiles under controllable testing conditions. The indoor characterization test bench, METHOD (Measurement of Electrical, Thermal and Optical Devices), decouples the temperatures of the primary optical element (POE) and the cell allowing their respective effects on optical and electrical performance to be analysed. In varying the temperature of the POE, the effects on electrical efficiency, focal distance, spectral sensitivity, acceptance angle and multi-junction current matching profiles can be quantified. This work presents the calibration procedures to accurately image the spectral irradiance distribution of a CPV system and a study of system behavior over lens temperature.

An original plasmonic nano-Ag/SiN(x) substrate was elaborated to strongly enhance the nonlinear response of SiC NPs for the first time. A plasmon-induced two order of magnitude increase of second-harmonic generation and two-photon excited photoluminescence was experimentally achieved. The measured enhancement factors were correlated with local field intensities theoretically estimated by finite-difference time-domain calculations. The obtained plasmon-enhanced nonlinear response of the SiC nanostructures make them promising in nonlinear optics applications.

We report on the absorption of a 100nm thick hydrogenated amorphous silicon layer patterned as a planar photonic crystal (PPC), using laser holography and reactive ion etching. Compared to an unpatterned layer, electromagnetic simulation and optical measurements both show a 50% increase of the absorption over the 0.38-0.75micron spectral range, in the case of a one-dimensional PPC. Such absorbing photonic crystals, combined with transparent and conductive layers, may be at the basis of new photovoltaic solar cells.

We propose a design that increases significantly the absorption of a thin layer of absorbing material such as amorphous silicon. This is achieved by patterning a one-dimensional photonic crystal (1DPC) in this layer. Indeed, by coupling the incident light into slow Bloch modes of the 1DPC, we can control the photon lifetime and then, enhance the absorption integrated over the whole solar spectrum. Optimal parameters of the 1DPC maximize the integrated absorption in the wavelength range of interest, up to 45% in both S and P polarization states instead of 33% for the unpatterned, 100 nm thick amorphous silicon layer. Moreover, the absorption is tolerant with respect to fabrication errors, and remains relatively stable if the angle of incidence is changed.