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In this work we present a method to fabricate Ge nanocrystals embedded in an oxide matrix by MBE deposition of amorphous Ge and Si layers combined with RTA and oxidation steps. A thin a-Ge layer was deposited on a SiO2 thermal oxide on p-Si substrate. The structures are then capped with a-Si. A dry oxidation is used to transform the a-Ge into a GeO2 layer. In order to precipitate the Ge in a SiO2 matrix, the sample was annealed under N2. The control of the formation of Ge traps in SiO2 has been optimized by the annealing time. For that purpose MOS capacitors have been characterized using and measurements. In addition, the electrical measurements allow the study of electron trapping in Ge. Those experiments have shown storage effects only for samples annealed for 30 s. Finally, we have optimized the process for the fabrication of Ge-based NVMs. [Copyright &y& Elsevier]

We report on the realization of flexible ion sensitive organic field effect transistors based on pentacene on which Parylene-C was deposited as top gate dielectric. In order to create proton sensitive sites at the insulator/electrolyte interface, Parylene-C surface has been covered with a thin layer of hydrogenated silicon nitride (SiN:H) deposited by photochemical vapor deposition at moderate temperature. The combination of Parylene and SiN:H enables the realization of highly reproducible and good performance transistors as well as ion sensitive sensors with an excellent pH response both in the acidic and alkaline pH range in a nearly all plastic technology. [ABSTRACT FROM AUTHOR]

Despite the widespread use of tunnel junctions in high-efficiency devices (e.g., multijunction solar cells, tunnel field effect transistors, and resonant tunneling diodes), simulating their behavior still remains a challenge. This paper presents a new model to complete that of Karlovsky and simulate an I (V) characteristic of an Esaki tunnel junction. A review of different analytical models of band-to-band tunneling models is first presented. As a complement to previous work on tunnel junction simulation, the transmission coefficient is precisely determined and incorporated, the valley current between the tunneling and drift regimes is included, and calculations of physical parameters are updated. It is found that the model works for a broad range of values of the forward bias. [ABSTRACT FROM AUTHOR]

Abstract: During solidification of low purity silicon for photovoltaic (PV) cells, solute rejection at the growth interface leads to an increase of the carbon concentration in the liquid phase and then to the precipitation of silicon carbide (SiC). When the precipitate radius becomes higher than the silicon critical nucleus radius, SiC can act as a refining agent for the Si and Si equiaxed grains appear in the liquid. The grain structure of the ingot changes from columnar to small grains, also known as grits. We developed a one-dimensional analytical model of this series of phenomena, including C segregation, SiC nucleation and growth, Si nucleation on the SiC precipitates and subsequent growth of the Si equiaxed grains. The equations are implemented under Matlab software in order to predict the columnar to equiaxed transition (CET) during the directional solidification of PV Si. We carried out calculations of the position and thickness of the equiaxed areas and of the number and size of Si grits as a function of the main process parameters: thermal gradient and growth velocity. Recommendations in order to adapt the growth process parameters to the initial carbon content are given. It is expected that coupling this model to global 3D numerical simulation codes could help improving the yield of ingot solidification. [Copyright &y& Elsevier]

Abstract: In this paper we analyse the passivation of the front surface of p-Si interdigitated rear contacts solar cell (IBC) by a thin porous silicon (PS) layer. Effectively, an efficiency improvement of 87% in relative was observed after porous silicon layer formation on the front surface of the IBC cell. The origin of surface passivation by the PS layer was studied by Laser Beam Induced Current (LBIC) method. The front surface of rear contacts cell with thin porous silicon layer was scanned by a modulated red laser beam in presence of a permanent light with different wavelengths and intensities. It was shown that without permanent illumination, the photocurrent of the cell with PS layer is very low, even lower than for a cell with unpassivated surface. However with short permanent wavelength illumination a strong increase of photocurrent was observed (8–10 times!). The light-dependent porous silicon passivation phenomenon is explained by a significant negative charge accumulation at the PS/p-Si interface traps under illumination. This leads to the formation of a hi-low (p+/p) junction at the front surface of the cell and to the reduction of the front surface recombination rate, like in Front Surface Field Solar Cell. [Copyright &y& Elsevier]

Abstract: Silicon nanostructures (ns-Si) show interesting optical and electrical properties as a result of the band gap widening caused by quantum confinement effects. Along with their potential utilization for silicon-based light emitters'' fabrication, they could also represent an appealing option for the improvement of energy conversion efficiency in silicon-based solar cells whether by using their luminescence properties (photon down-conversion) or the excess photocurrent produced by an improved high-energy photon''s absorption. In this work, we report on the morphological and optical studies of non-stoichiometric silica (SiO x ) and silicon nitride (SiN x ) layers containing silicon nanostructures (ns-Si) in view of their application for solar cell''s efficiency improvement. The morphological studies of the samples performed by transmission electron microscopy (TEM) unambiguously show the presence of ns-Si in a crystalline form for high temperature-annealed SiO x layers and for low temperature deposition of SiN x layers. The photoluminescence emission (PL) shows a rather high efficiency in both kind of layers with an intensity of only a factor ∼100 lower than that of porous silicon (pi-Si). The photocurrent spectroscopy (PC) shows a significant increase of absorption at high photon energy excitation most probably related to photon absorption within ns-Si quantized states. Moreover, the absorption characteristics obtained from PC spectra show a good agreement with the PL emission states unambiguously demonstrating a same origin, related to Q-confined excitons within ns-Si. Finally, the major asset of this material is the possibility to incorporate it to solar cells manufacturing processing for an insignificant cost. [Copyright &y& Elsevier]

Abstract: The photovoltaics industry is dominated by silicon solar cells technology mainly because of the low manufacturing cost. However, these solar cells show an important limiting factor on conversion efficiency due to the inefficient absorption of high energy photons as a consequence of the indirect bandgap structure of bulk silicon. In this article, we discuss about different possibilities to improve the efficiency of solar cells and we propose the use of silicon nanostructured layers to achieve this goal. We present the fabrication methods as well as the characterization results of two kinds of layers which can be used for solar cells'' efficiency improvement, namely non-stoichiometric silica layers (SiO x ) and non-stoichiometric silicon nitride layers (SiN x ). We demonstrate that the photoluminescence (PL) properties and/or the increased photocurrent (PC) signal at high-energy photon could be used to improve this efficiency. Finally, the major asset of these methods lies in their possibility to be incorporated to the solar cell processing for an insignificant cost. [Copyright &y& Elsevier]

Abstract: In this paper we present the results of the simulation of interdigitated back contacts solar cell on thin-film (∼50μm) silicon layer. The influence of several parameters (surface recombination rate, substrate thickness and type, diffusion length, device geometry, doping levels) on device characteristics are simulated using the accurate two-dimensional numerical simulator DESSIS that allows to optimise the cell design. [Copyright &y& Elsevier]

In this work, we present photoluminescence (PL) and electroluminescence studies on thermal SiO2 Si-implanted light-emitting devices (LEDs) containing crystallites of size 3 nm. The PL measurements reveal a bulk-type behavior of nc-Si-associated PL confirming that the PL obtained at ∼1.6 eV is closely related to Q-confined nc-Si. Analysis of electrical transport mechanisms confirms that the current is related to a tunneling process as expected and not to Fowler–Nordheim regime. Finally, threshold electric field for light emission obtained was as low as 5 MV/cm, confirming the real potential of a such Si-implanted material for LEDs based on Si. [Copyright &y& Elsevier]

This paper investigates the importance of incomplete ionization of dopants in compensated p-type Si and its impact on the majority-carrier density and mobility and thus on the resistivity. Both theoretical calculations and temperature-dependent Hall-effect measurements demonstrate that the carrier density is more strongly affected by incomplete ionization in compensated Si than in uncompensated Si with the same net doping. The previously suggested existence of a compensation-specific scattering mechanism to explain the reduction of mobility in compensated Si is shown not to be consistent with the T-dependence of the measured carrier mobility. The experiment also shows that, in the vicinity of 300 K, the resistivity of compensated Si has a much weaker dependence on temperature than that of uncompensated silicon. [ABSTRACT FROM AUTHOR]

Compensation effects are intensively studied on two highly doped ingots grown from solar-grade silicon feedstocks purified using metallurgical routes, through a comparison of the electrical properties at iso-carrier densities. Working at given carrier densities enables a clearer extraction of the compensation effects, at the wafer and solar cell levels. At the wafer level, the majority carrier mobility and the carrier lifetime are investigated. Regarding the mobilities, it was found that current models may underestimate the amount of incomplete ionization of boron leading to underestimated mobilities. In addition, the majority carrier mobility was found to be strongly affected at high compensation level. Regarding the carrier lifetimes, our results show that after a phosphorus diffusion step, dopants alone - and especially boron - can limit the lifetime in highly doped solar-grade silicon. At the cell level, I-V characteristics under standard illumination were studied. In particular, the observed reductions in short-circuit current on solar cells having a very high compensation level could be explained in terms of a compensation-induced reduction in the minority carrier mobility. We also report high conversion efficiencies of up to 15.9% on solar cells showing a boron content greater than two ppmw (2.6 × 1017 cm-3), which is generally considered unsuitable for solar cell manufacturing. [ABSTRACT FROM AUTHOR]

The Al/TiO[SUBx]/Si, Ti/TiO[SUBx]/Si, and Mo/TiO[SUBx]/Si interfaces are studied, before and after thermal treatment, by secondary ion mass spectrometry (SIMS), Rutherford backscattering spectrometry (RBS), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy. The metal layer is selected with regard to the formation of a titanium silicide layer. The reductive nature of the metal was found to be very significant in the interdiffusion of Si and Ti (from titanium oxide). This interdiffusion has the advantage to form a thin titanium silicide layer, which is known to have low contact resistivity. The SIMS, RBS, XPS, and XRD analyses show that after annealing for 10 min at 850°C under hydrogen ambient, titanium silicide interfacial layers such as Ti[SUB5]Si[SUB3] and TiSi[SUB2] C54 were formed in the case of Al/TiO[SUBx]/Si and Ti/TiO[SUBx]/Si structures, respectively. There is no significant reaction between Mo and TiO[SUBx] and no Ti and Si interfacial reaction in the Mo/TiO[SUBx]/Si system. With thermodynamic considerations, we confirm all the results found in this study. [ABSTRACT FROM AUTHOR]

Investigates the structural, electrical and piezoresistive properties of in situ boron-doped thin silicon layers deposited by plasma-enhanced chemical-vapor deposition on oxidized silicon substrates and subjected to a rapid thermal anneal. Sample preparation and experimental methods; Stress and structure of the films; Effect of doping on optical properties.

The improvement of optical confinement on the back crystalline silicon solar cell is one of the factors leading to its better performance. Porous silicon (PS) layer can be used as a back reflector (BR) in solar cells. In this work, single layers of porous silicon were grown by electrodeposition on a single crystalline silicon substrate. The measurement of the total reflectivity (RT) on Si/PS surface showed a significant improvement in optical confinement compared to that measured on Si/standard Al back surface field (BSF). The internal reflectivity (RB) extracted from total reflectivity measurements achieved 86 % for the optimized single PS layer (92 nm thick layer with 60 % porosity) in the wavelength range between 950 and 1200 nm. This improvement was estimated as more than 17 % compared to that measured on the surface of Si/BSF Al contact. To improve the stability and passivation properties of PS layer BR, silicon nitride layer (SiNx) was deposited by PECVD on a PS layer. The maximum measured total reflectivity for PS/SiNx achieved approximately 56 % corresponding to an improved RB of up to 83 %. The PS formation process in combination with the PECVD SiNx, can be applied in the photovoltaic cell technology and offer a promising technique to produce high-efficiency and low-cost c-Si solar cells. [ABSTRACT FROM AUTHOR]

We use photoluminescence to observe light-induced degradation in silicon in real time. Numerical simulations are used to match our results and lifetime decay data from the literature with theoretical models for the generation of the light-induced boron–oxygen defects. It is found that the existing model of the slowly generated defect SRC, where its saturated concentration is a function of the majority carrier concentration, does not explain certain results in both p- and n-type samples. A new model is proposed in which the saturated SRC concentration is controlled by the total hole concentration under illumination. [ABSTRACT FROM AUTHOR]

Abstract: Porous silicon Bragg mirrors at back-side of single crystalline and multicrystalline silicon solar cell were numerically simulated by transfer matrix method. It allows to choose the optimal parameters of porous stack of bi-layers (indexes of refraction, number of bi-layers) when the increase of photon absorption in 900–1050 nm spectral region is achieved. Application of Bragg mirrors at back-side of single crystalline solar cell can improve the efficiency on more than 0.8% in absolute for 200 μm both-side textured thickness wafer. The simulated results were compared with characteristics of Bragg mirrors fabricated by electrochemical etching of single- and multi-crystalline silicon. It is shown that despite the natural crystallites disorientation the efficient Bragg mirrors can be fabricated on multicrystalline silicon wafers in such way. Maximum measured reflectivity for Bragg mirrors on multicrystalline substrate achieves approximately 62%, whereas for single crystalline silicon the reflectivity in maximum is more than 90%. [Copyright &y& Elsevier]

Abstract: Selective laser ablation of silicon nitride layers on crystalline silicon wafers was investigated for solar cell fabrication. Laser processing was performed with a nanosecond UV laser at various energy densities ranging from 0.2 to 1.5Jcm−2. Optical microscopy was used as a simple mean to assess the ablation threshold that was correlated to the temperature at the interface between the silicon nitride coating and the silicon substrate. Minority carrier lifetime measurements were performed using a microwave photo-conductance decay technique. Band to band photoluminescence spectroscopy proved to be a sensitive technique to qualify the laser-induced damage to the silicon substrate. The crystalline structure of silicon seemed to be maintained after silicon nitride ablation as shown by UV reflectivity measurements. Laser parameters corresponding to fluences of around 0.4Jcm−2 were found to achieve selective ablation of SiN x without causing detrimental damage to the surrounding material. [Copyright &y& Elsevier]

An original plasmonic nano-Ag/SiNx 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. [ABSTRACT FROM AUTHOR]

Abstract: The formation of mesoporous Ge layers with different morphologies by bipolar electrochemical etching is reported for the first time. A detailed analysis of the effect of anodic/cathodic pulse duration on the mesoporous Ge formation is performed. It is shown that together with the applied current density and anodization time this parameter has the most important influence on porous Ge morphology. Six different types of porous morphologies were identified with the nanopore diameter being in the range from 5 to 15nm. Their crystalline nature has been confirmed by Raman spectroscopy. The diameters of the nanocrystallites of the porous Ge layers were extracted from Raman spectra by using the phonon-confinement model. Depending of the layer morphology their values varied between 4 and 10nm. A conceptual analysis of several aspects of the formation mechanisms of mesoporous Germanium based on the available information has allowed us to predict and create a new complex morphology. [Copyright &y& Elsevier]

Abstract: In this work, we study the influence of the amount of active phosphorus atoms in the emitter region of silicon N+/P solar cells. After emitter formation by thermal diffusion, a part of phosphorus atoms in the vicinity of the surface is electrically inactive. This results in a high emitter saturation current density and in a poor quantum efficiency in the blue wavelength range. This emitter region is usually known as dead layer. We first introduce a simple method to determine the percentage of electrically active phosphorus atoms in emitters obtained from a low pressure POCl3 diffusion furnace. Different phosphorus diffusions are performed using different parameters (temperature, time, POCl3/O2 ratio, and preoxidation time). We then investigate the effect of the fraction of active phosphorus on the doping level and on the emitter passivation. We show that diffusion parameters can greatly influence the active phosphorus percentage, particularly the POCl3/O2 ratio or the addition of a preoxidation step. Concerning the emitter passivation, a linear variation of the emitter saturation current density as a function of the active phosphorus percentage is observed. Using appropriate POCl3/O2 ratio or preoxidation time, we obtain an emitter saturation current density around 100fA/cm2. [Copyright &y& Elsevier]

Abstract: Electroless deposition of Ag nanoparticles on PECVD SiN x :H dielectric layers is a new straightforward chemical bath process well adapted to the realization of plasmonic solar cells. Depending on the stoichiometry, SiN x :H contains a certain number of Si nanoclusters embedded in the silicon nitride matrix, that act as reducing agents for Ag ions in solution and thus allows the deposition of Ag nanoparticles directly on the surface. We investigate the influence of the layer stoichiometry and deposition parameters on the nucleation and growth of Ag nanoparticles, and show how they can be adjusted to control the nanoparticle size and density. We also demonstrate that this facile metallization process can be extended to PECVD SiO x layers, another dielectric coating of interest in photovoltaics. [Copyright &y& Elsevier]

Abstract: Metallurgical grade silicon (MG-Si) has become a new source of raw material for the photovoltaic industry. The use of this material as an alternative feed stock has however introduced phenomena that are detrimental to both the yield of the manufacturing process and the performance of the photovoltaic cells produced. This is mainly related to the presence of carbon, which precipitates to silicon carbide (SiC) in the ingot. This article focuses on the effect of carbon on silicon nucleation. Statistical experimental results of silicon nucleation are obtained as a function of carbon concentration and are presented and compared to thermodynamic calculations. [Copyright &y& Elsevier]

We study the boron-oxygen defect in Si co-doped with gallium and boron with the hole density 10 times higher than the boron concentration. Instead of the linear dependence of the defect density on the hole density observed in boron and phosphorus compensated silicon, we find a proportionality to the boron concentration. This indicates the participation of substitutional, rather than interstitial, boron in the defect complex. The measured defect formation rate constant is proportional to the hole density squared, which gives credit to latent defect models against defect reactions limited by the diffusion and trapping of oxygen dimers by boron atoms. [ABSTRACT FROM AUTHOR]

Selective emitter solar cells can provide a significant increase in conversion efficiency. However current approaches need many technological steps and alignment procedures. This paper reports on a preliminary attempt to reduce the number of processing steps and therefore the cost of selective emitter cells. In the developed procedure, a phosphorous glass covered with silicon nitride acts as the doping source. A laser is used to open locally the antireflection coating and at the same time achieve local phosphorus diffusion. In this process the standard chemical etching of the phosphorous glass is avoided. Sheet resistance variation from 100 Ω/sq to 40 Ω/sq is demonstrated with a nanosecond UV laser. Numerical simulation of the laser-matter interaction is discussed to understand the dopant diffusion efficiency. Preliminary solar cells results show a 0.5% improvement compared with a homogeneous emitter structure. [ABSTRACT FROM AUTHOR]

Abstract: A study on the optimisation of front contacts of n-type multicrystalline silicon solar cells is presented. In this study the same cell processing was applied to two types of wafers: electronic grade (EG-Si) and metallurgic grade (MG-Si) silicon. The contact firing temperature was optimised, by measuring the contact resistivity of the front and back contacts for different firing temperatures. The front contacts were improved by deposing silver using an electrochemical process. The solar cells were characterised before and after the silver deposition. For all cells processed the line resistance was reduced by over 90% after the silver deposition. After the contact improvement, EG-Si cells showed an absolute efficiency improvement of 2.6%, but MG-Si cells suffered a reduction on the cell efficiency, an effect related to parasitic shunting existent in these cells. [Copyright &y& Elsevier]

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: Silicon solar cells still require cost reduction and improved efficiency to become more competitive. New architectures can provide a significant increase in efficiency, but today most of the approaches need additional fabrication steps. In this context, laser processing offers a unique way to replace technological steps like photolithography that is not compatible with the requirements of the photovoltaic industry. In particular laser induced thermal effects can be used to activate or re-organise dopants at the silicon surface to design new emitter geometries. In this paper dopant diffusion using a nanosecond UV laser on phosphorous-doped silicon emitters is studied. The presence of a phosphosilicate glass underneath a silicon nitride layer leads to a local decrease of the emitter sheet resistance from 100Ω/sq to 20Ω/sq. Laser induced damage, phosphorus diffusion profile and electrical shunt are assessed in the perspective of selective emitter silicon solar cells fabrication compatible with electrochemical metal contacts deposition. [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]

Abstract: This letter focuses on the variation of the Hall majority carrier mobility with the dopant compensation level in purely Boron-doped Czochralski grown silicon single crystals. Compensation was varied continuously at the sample scale via a step by step activation of the oxygen-based thermal donors. At room temperature, we show a strong drop in mobility for high compensation levels in both p- and n-type Si. Mobility models taking into account carrier scattering on ionized impurities and phonons could not reproduce this drop. We conclude that a specific effect of compensation must be taken into account to explain the observed behaviour. We qualitatively discuss physical mechanisms susceptible to reduce mobility in highly compensated Si. [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]

Bragg reflectors consisting of sequence of dielectric layers with a quarter wavelengths optical thickness are promising to create solar cells of third generation. SiOX /SiNX Bragg mirror (BM) at the backside of textured multicrystalline silicon solar cells was fabricated by PECVD method. BM with 9 bi-layers was optimized for the maximum reflectivity within the wavelength range Δλ = 820…1110 nm. The maximum measured reflectivity is approximately 82 %. Measured reflectivity values were compared with the simulated ones by using the transfer matrix. Effect of parameters for pyramids of several types on the total reflectivity of BM deposited on textured silicon surface was simulated. Enhancement of light absorption and external quantum efficiency in the longwave part of the spectrum (λ > 940 nm) was observed, and it was explained as increase of the photon absorption length. The influence of BM on passivation of SC rear surface was explored. The cell back contact was formed by Al diffusion through BM to the µc-Si wafer and promoted by a pulsed laser. For SC with BM, the efficiency 13.75 % is obtained comparatively with efficiency 13.58 % for SC without BM. [ABSTRACT FROM AUTHOR]

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]

In order to decrease reflectivity of silicon solar cells, NaOH or KOH texturisations are usually used leading to a pyramidal structure. However, these solutions are toxic and pollutant. Effectively, K+ or Na+ ions contaminate the passivation layer (SiO2 or SiN) deposited on the surface of the cell after texturisation. An alternative to KOH and NaOH texturisation is tetramethyl ammonium hydroxyde (CH3)4NOH) (TMAH). TMAH is not pollutant, not toxic and its use leads also to a pyramidal structure. Moreover, the etching rate and surface morphology can be controlled by the concentration of the solution, temperature and the addition of surfactant. Moreover, TMAH is selective with dielectrics and metals. It can therefore be used to produce self-aligned interdigitated back-contact solar cells (IBC). In this work, we have analysed the surface morphology and reflectivity after texturisation in TMAH in various experimental conditions. We have tested the possibility to use selective etching of the emitter of a back-contact solar cell by protecting the surface of the emitter with a metal grid. This new process permits to reduce the number of lithographic steps necessary to produce IBC solar cells. [ABSTRACT FROM AUTHOR]

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]

In this work, we describe how to fabricate good quality 3 nm nc-Si with low size distribution in thermal SiO2 oxides. Photoluminescence, excited photoluminescence, and photocurrent measurements are discussed on the basis of theoretical calculations of the quantified levels in nc-Si. The impact of shape and size in quantum dots on transition energies has been highlighted, thanks to 2D symmetrical self-consistent Poisson–Schro¨dinger simulations. Both direct and indirect gaps in silicon have been considered in order to carry out a better comparison between simulations and optical measurements. A good agreement is found between simulations and experimental data for the indirect gap of 3 nm dots which show a threshold energy around 2 eV. However, the optical recombinations seems to be related to lower energy states probably due to interfacial radiative defects around 1.58 eV. On the basis of highly luminescent nc-Si, we have fabricated an optimized light emitting device (LED) with a calculated design in order to favour both electron and hole injection. Stable red electroluminescence has been obtained at room temperature and the I–V measurements confirm that the current is related to a pure tunnelling process. A modelling of I–V curves confirms a Hopping mechanism with an average trap distance between 1.4 and 1.9 nm. The Fowler–Nordheim process is not observed during light emission for electric fields below 5 MV/cm. Finally, we have not hot carrier injection and thus it seems possible to develop Si-based LEDs with a good reliability. [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]

The size of silicon quantum dots (Si QDs) embedded in silicon nitride (SiNx) has been controlled by varying the total pressure in the plasma-enhanced chemical vapor deposition (PECVD) reactor. This is evidenced by transmission electron microscopy and results in a shift in the light emission peak of the quantum dots. We show that the luminescence in our structures is attributed to the quantum confinement effect. These findings give a strong indication that the quality (density and size distribution) of Si QDs can be improved by optimizing the deposition parameters which opens a route to the fabrication of an all-Si tandem solar cell. [ABSTRACT FROM AUTHOR]

Interconnected Si nanocrystals forming homogeneous thin films with controlled bandgap values from 1.2 to 2.9 eV were formed by pulsed plasma enhanced chemical vapor deposition technique under dusty plasma conditions. The chosen values of plasma duration time correspond to specific phases of the dust nanoparticle growth. Structural and optical properties of the deposited nanostructured films are described in details. These nanocrystalline Si thin films seem to be promising candidates for all-Si tandem solar cell applications. [ABSTRACT FROM AUTHOR]