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Thickness influence on structural, optical and electrical properties of sputtered indium tin oxide (ITO) with thickness ranging from 60 up to 430 nm films has been studied. At the increase of the film thickness crystallinity degree and grain size increased, whereas tensile structural distortion as well as resistivity decreased. It was observed that a microstructure evolution takes place: the initial amorphous layer evolved in polycrystalline phase, with a grain–subgrain surface morphology. Carrier concentration increased at the increase of the film thickness and a general relationship between electrical characteristics and structural distortion has been found. In thinner films larger tensile distortion allowed to include larger amount of interstitial O and/or Sn atoms in the lattice. An appreciable impact of the thickness was also observed on electro-optical properties in terms of changes in energy gap, resistivity and optical absorption. Silicon heterojunction solar cells have been produced and Jsc as high as 33.0 mA/cm2 has been obtained.

Low-emissive (low-E) coatings, based on alternating thin layers of aluminium nitride (AlN) and ultra-thin layers of silver (Ag), have been deposited by sputtering for use on glass surface outside the inert gas filled gap of insulating multiple-pane windows. Such low-E coatings being optical filters containing thin metallic layers, undergo degradation phenomena when exposed to air with the appearance of a bluish colour and delamination. An in-depth study has been conducted to identify the degradation mechanism in moist atmosphere by means of several characterization techniques before and after accelerated aging tests. Photoluminescence measurements have evidenced formation of the complex defect VAl-3ON at the AlN/Ag interfaces as responsible for the hydrolytic degradation process. Relationship between degradation process and chemical-physical nature of AlN lattice defects has been established. Modifications in the sputtering process of AlN films were implemented and the antireflective layer was replaced with less hydrophilic material. The coating deposited with modified structure has been found to be highly stable and with excellent thermo-optical performances (SHGC 0.54, visible transmittance 80%, emissivity 4.8%). The modified low-E coatings have been also deposited on polycarbonate substrate with similar results of high stability in the air. This stabilized coating on polymer can be considered an excellent candidate for low-cost energy-saving retrofitting.

Dry and maskless texturing process of mono-crystalline silicon wafers using CF4/O2 plasma in a reactive ion etching (RIE) system has been developed with the aim of obtaining optimized surface texture characterized by low optical reflection loss for photovoltaic applications. In this study c-Si has been successfully subjected to plasma etching at unusual process conditions, specifically high value of substrate temperature (15 °C) coupled with high value of process pressure (40 Pa). Different c-Si surface textures are produced at different etch durations. Specifically, pyramid-like morphology appears at very short etch time and crater-like nanostructures form at longer etch time, whereas the former gives the lowest average reflectance (

The development of a simple, economic and low-temperature synthesis procedure for titanium oxide thin films is an important challenge, due to their diffusion and to the limitations imposed by high temperature annealing. The knowledge on the properties of amorphous TiO2 films, as well as on the effect of their functionalization by organic compounds, is still inadequate. Here we describe a hydrolytic sol-gel procedure to prepare TiO2-based hybrid (inorganic-organic) thin films containing four different complexing agents in their structure (acetylacetone, dibenzoylmethane, citric acid and diethanolamine). The process variables were explored in order to attain stable sols and to tune the features of the amorphous films resulting by drying at 80 °C. Their structural, morphological, optical and electrical properties were studied by SEM, AFM, XRD, ellipsometry, FT-IR and UV–vis-NIR spectroscopy and resistivity measurements. The effect of the organic component, precursors concentration, deposition parameters and annealing temperature was considered. Moreover, the modifications induced by the exposure of the hybrid films to UV light were deeply investigated. A short UV treatment has a strong impact on the microstructure and wettability of the films and promotes a striking increase in their electrical conductivity.

ZnO:Al films, obtained by RF-sputtering and textured by wet etching, have been subjected to argon plasma etching treatment in order to stabilize their electrical properties over time, being well known the electrical degradation of these transparent conductive films when exposed to air. Specifically focused on photovoltaic application as front electrode, the effect of argon plasma treatment on long-term electrical stability of ZnO:Al thin film was monitored by performing damp heat degradation cycles in environmental test chamber. There was evidence for strong slowdown of electrical degradation when ZnO surface had been subjected to Ar plasma treatment. Different etching times have been used and it has been observed that surface modification induced by plasma treatment gave its more pronounced beneficial effect after the first 10 min of the process. In particular, for no-plasma treated samples the carrier mobility worsening is about three times larger than that related to Ar-treated samples after long-lasting aging test. The stabilization mechanism of the electrical properties was investigated by studying the main modifications of the plasma etched materials in terms of optical, chemical, morphological and structural properties. Structural analysis revealed that crystalline rearrangement occurs down to 30 nm depth below the plasma treated film surface, where a thin nanocrystalline layer forms. PL analysis revealed a different distribution of deep defect centres for plasma treated film surface compared to untreated one. In particular, donor-like lattice defects (Vo and Zni) decreased whereas Vzn defects became predominant. Crystalline rearrangement and change of type and/or amount of deep defect centres inside the plasma modified top layer have been hypothesized as responsible for the enhancement of the electrical stability. © 2016 Elsevier B.V.

Hydrogen-doped indium oxide (IO:H) layers with very high carrier mobility have been developed by two-step fabrication procedure. In the first step IO:H films were deposited by radio frequency (RF) magnetron sputtering from In2O3 target in Ar/water vapour gas mixtures. Different sputtering powers and partial pressures of H2O (pH2O) have been explored and effects induced by changes of these process parameters on final film structure and on electro-optical properties have been investigated. In the second step a post-deposition annealing under vacuum were performed. During this treatment a complete and complex amorphous-crystalline transition occurred. Growth of large crystalline domains give rise to IO:H films with excellent carrier mobility values, up to 138 cm2/Vs. For each sputtering power used, the pH2O range has been identified which allows to obtain the highest carrier mobility value for IOH post-annealed films. Growth of large crystallites occurred in IO:H annealed films and, at this purpose, a selective chemical etching method has been developed to give better evidence for the polycrystalline grains. IO:H films appeared formed by fairly large crystalline domains with presence of sub-grains. Silicon heterojunction cells fabricated by using IO:H layers as front electrode, compared with those obtained from a conventional ITO showed improved short-circuit current density and the resulting conversion efficiency.

ZnO:Al (AZO) thin films have been deposited by radio frequency (RF) magnetron sputtering and RF power applied to the target has been varied in the range 600–1200 W. RF power effect on structural, electrical and optical properties was investigated and the relationship existing between these properties and the film lattice defect distribution was discussed. At the increasing of the RF power it was found that AZO films, having a preferential growth orientation along (002) direction, showed a decrease of the lattice distance indicating a less defected structure. Furthermore, at the increase of the RF power a higher optical absorption by free carriers, coupled with an increase of the band gap value, was observed. Resistivity varied from 1.1 × 10 − 3 Ω cm at 600 W down to a minimum value of 5.6 × 10 − 4 Ω cm at 1200 W, whereas the carrier density increased up to 1 × 10 21 cm − 3 . Lattice defect variation of AZO films was analyzed by photoluminescence (PL) measurements. Presence and amount of different lattice defects were evaluated for AZO films deposited at different RF powers. At 600 W the film structure was dominated by zinc vacancies (V Zn ), whereas for higher RF power the PL band associated with V Zn decreased and interstitial oxygen (O i ) band remarkably increased. PL analysis revealed that extrinsic Al doping is the dominant effect on the conductivity enhancement. It was hypothesized that at higher RF power a more effective diffusion phenomenon can give more effective Al doping and less amount of zinc vacancies. As a consequence, Al atoms are more effectively trapped into the structure.

In this paper an automated procedure for the analysis of Transparent Conductive Oxides (TCO) layers exhibiting rough surfaces is proposed. The method is based on the interaction between MATLand the Sentaurus TCAD and is aimed to the reduction of computational efforts needed for full three dimensional analyses. Experiments performed on CVD deposited ZnO layer, showing the reliability of the method for describing their optical properties, are reported. A semi-empirical technique for the extraction of the TCO refractive index is shown as well.

Silicon nano-crystallites dispersed in an insulating matrix of zinc silicates have been obtained by a two-step process, consisting of sputtering deposition of several alternated amorphous silicon and zinc oxide thin layers, followed by a chemical reaction carried out under vacuum at comparatively low temperature (from 450 °C to 560 °C) between stratified reagents. This low cost, low temperature and scalable process has to be considered promising in quantum dot fabrication technology. Several characterization techniques have been employed and large evidence has been found for the presence of silicon and zinc oxide nano-crystallites showing quantum confinement effect and related band gap enlargement. Photoluminescence phenomenon has been observed in the range 550-850 nm and a consistent explanation has been given in terms of exciton radiative recombination on trap states at the interface between Si dot and silicate matrix. Electrical measurements have given evidence for an expected material anisotropy where large difference between lateral and perpendicular resistivity values has been found. A technological strategy for transport property improvement has been prospected. © 2014 Elsevier B.V.

Double-textured ZnO:B layers with enhanced optical scattering in both short and long wavelength regions have been successfully fabricated using MOCVD technique through a three step process. Growth of double-textured structures has been induced by wet etching on polycrystalline ZnO surface.Our double-layer structure consists of a first ZnO:B layer wet etched and subsequently used as substrate for a second ZnO:B layer deposition. Polycrystalline ZnO:B layers were etched by utilizing diluted solutions of fluoridic acid (HF), chloridric acid (HCl) and phosphoric acid (H3PO4) and their effect on surface morphology modification was systematically investigated. The morphology of the second deposited ZnO layer strongly depended on the surface properties of the etched ZnO first layer. Growth of cauliflower-like texture was induced by protrusions presence on the HCl etched surface. Optimized double-layer structure shows a cauliflower-like double texture with higher RMS roughness and increased spectral haze values in both short and long wavelength regions, compared to conventional pyramidal-like single texture. Furthermore, this highly scattering structure preserves excellent optical and electrical properties. © 2013 Elsevier B.V. All rights reserved.

Electrical properties of transparent conductive ZnO:B films, obtained by the low-pressure metal organic chemical vapor deposition (LP-MOCVD) technique, were monitored during the given time, and a heavy worsening of these properties was observed. At the same time, other ZnO:B samples subjected to a post-deposition treatment by argon plasma etching showed an appreciable enhancement of the electrical stability in the given time. A degradation mechanism, involving formation of O- adsorbate and its diffusion inside the grain boundary, is proposed as responsible of the carrier mobility worsening. Furthermore, the beneficial effect of the plasma treatment is explained in terms of a rearrangement of the grain boundary with a decrease of positively charged defect density and, consequently, a slowing down of O - formation rate. Finally, argon plasma post-deposition treatment is proposed as an effective process for obtaining ZnO:B films with an appreciable electrical stability in the given time. © 2013 American Chemical Society.

This paper presents a method for determining the complex refractive index of a Transparent and Conductive Oxide that is embedded in a multilayer structure. The procedure assumes that the optical properties of others materials forming the structure are known and only the real and the imaginary part of the refractive index are unknowns. The refractive index is determined by exploiting the matrix model of light propagation along with experimental measurements of the light transmitted through the complete multilayer device. A simple multilayer structure, formed by a ZnO thin film deposited on a thick glass substrate, is analyzed to evidence the reliability of the method. Results show that the by adopting for the ZnO the refractive index extracted with the proposed procedure, a reliable description of light propagation through multiple layers can be obtained. © 2017 IEEE.

The main aspects of the sol–gel synthesis of ZnO thin films were critically examined with particular reference to the optimization of the process parameters that influence specific properties of the films, such as the transparency, the crystallization behavior and the electrical resistivity. Starting from zinc acetate dihydrate, anhydrous ethanol and triethanolamine, a valuable sol–gel route was set. This route allowed the preparation of transparent ZnO thin films by dip-coating. Triethanolamine, thanks to its high chelating efficiency towards Zn2+, permitted to prepare stable zinc acetate dihydrate solutions wholly at room temperature. The structural, optical and morphological characteristics of the films were examined by glancing incidence X-ray diffraction, UV–vis spectroscopy and atomic force microscopy. Single layer films, with a thickness of about 120 nm, characterized by a high adhesion force (about 140 Kg cm−2), were obtained. The crystallization behavior of dip-coated amorphous films was explored in the 500–600 °C range; polycrystalline films, formed by randomly oriented ZnO nanocrystals (about 20 nm), with an average transmittance higher than 90% in the visible region, were obtained. Finally, a strong influence of annealing environment on electrical resistivity was observed.

Starting from stratification of alternating thin films of Si and ZnO produced by sputtering technique, followed by an annealing process at a sufficiently low temperature (560 °C) under vacuum (10–4 mbar), we were able to form crystalline nano-aggregates of Si and ZnO in a dielectric matrix of zinc silicate and silicon oxide. The grain size of the Si crystallites depended on the Si/ZnO ratio in the starting material and on the annealing duration. Materials with tuned band gaps have been produced (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An optimized solar multilayer coating based on W-Al 2 O 3 graded cermet layer was fabricated in our laboratories by magnetron sputtering technique. To use this coating for the receiver tube of solar parabolic trough plants it is necessary for it to have very high structural, physical and chemical stability at high temperature (580 °C) under vacuum, maintaining good performances in terms of high solar absorption and low thermal emissivity. An investigation on the stability of our coating was carried out by means of several material characterizations before and after annealing processes at high temperature. The salient characteristics emerging in this study were the partial oxidation of the tungsten nano-particles inside the cermet forming shells of WO x , the nucleation of a nano-crystalline porous alumina γ phase in the matrix of the prevailing amorphous one and the phase transition from β to α of the tungsten back-reflector. All these phenomena were responsible for the coating reflectance modification promoted by the increased temperature. However we verified that such light optical modification was not progressive but at an annealing time of 2 days a definitive adjustment of the coating optical properties was reached, attesting to a remarkable thermal stability of this solar coating under vacuum and making it very suitable to be used at high temperature.

LP-MOCVD deposited ZnO:B thin films, post-etched by argon plasma processes, were investigated in this study in order to optimise the ZnO:B/p-layer interface when the ZnO:B is used as front electrode of p–i–n a-Si:H solar cells. At varying etching time different surface roughness was obtained and the evolution of the surface morphology was correlated with the texture characteristic and its scattering properties. Atomic force microscopy data were analysed and discussed together with the scattering properties, which are haze parameter and angular resolved scattering (ARS) distribution. The presence of several preferential scattering angles was hypothesized and a deconvolution approach was applied to each angular scattering curve. For each fixed preferential scattering angle θ i we associated a Gaussian distribution of the scattered light amount related to a well-defined scattering surface. The different preferential scattering angles were correlated to different scattering phenomena, the modifications of the angular scattering curves well agreed with SEM and AFM images. It is well known that a:Si–H solar cells fabricated on MOCVD deposited ZnO:B substrates show poor FF and V oc values with good J sc value. We demonstrated that only an effective sharp edge rounding off produced by an appropriately long plasma etching treatment is able to make MOCVD deposited ZnO:B perfectly suitable for high quality a-Si:H based devices.

Thin film photovoltaics is a particularly attractive technology for building integration. In this paper, we present our analysis on architectural issues and technological developments of thin film silicon photovoltaics. In particular, we focus on our activities related to transparent and conductive oxide (TCO) and thin film amorphous and microcrystalline silicon solar cells. The research on TCO films is mainly dedicated to large-area deposition of zinc oxide (ZnO) by low pressure-metallorganic chemical vapor deposition. ZnO material, with a low sheet resistance ( 82%) in the whole wavelength range of photovoltaic interest, has been obtained. “Micromorph” tandem devices, consisting of an amorphous silicon top cell and a microcrystalline silicon bottom cell, are fabricated by using the very high frequency plasma enhanced chemical vapor deposition technique. An initial efficiency of 11.1% (>10% stabilized) has been obtained.

In terms of both high photo-thermal efficiency and high stability, multi-layer structures based on metal-dielectric composites (cermet) can be considered the most attractive selective solar absorbers for receiver tubes operating at medium-high temperatures in the field of solar thermodynamic plants. The double cermet layer approach represents a very simple fabrication method and can give high performances in terms of high solar absorptance and low hemispherical emittance. Optimised solar coatings based on cermet layers were fabricated in our laboratories by sputtering technique following the double layer approach. The joined employment of ellipsometric measurements and optical simulation is proposed as an effective method to optimise and fabricate coatings showing the best performances at a fixed operating temperature of the receiver tube. Interesting results concerning an optimised spectrally selective coating are shown. Solar absorptance higher than 0.94 and hemispherical emittance at 580 °C lower than 0.13 were obtained.

Textured boron-doped zinc oxide (ZnO:B) films, suitable as transparent and conductive layers in thin film silicon-based solar cells, have been obtained by low-pressure metalorganic chemical vapour deposition (LP-MOCVD) technique. The complex role of the boron doping in ZnO layers was examined and its influence on the morphological and structural properties was analysed. Furthermore, a correlation between such properties, intrinsic stress and carrier mobility in the film has been analysed. ZnO:B films have a polycrystalline structure with a columnar texture shape, they show a rough surface with pyramidal large grains. At the increase of the boron doping the pyramidal shape of the grains deteriorates and the average grain size reduces. Furthermore the a-lattice parameter decreases indicating that the boron incorporation introduces a deformation in the crystal. In addition to these large structural modifications, the decrease of the carrier mobility at the increase of the doping content is observed. At the same time, the boron content also plays a meaningful role on the intrinsic stress inside the film. Stress behaviour in ZnO films has been investigated by X-ray diffraction measurements using the sin2 ψ method. Tensile stresses have been observed and the thermal and intrinsic components have been calculated. Respect to undoped ZnO films, boron incorporation on substitutional or interstitial sites increases the tensile stress by means of a lattice strain mechanism that reduces the d-spacing value.

A patented solar coating for evacuated receiver, based on innovative graded WN-AlN cermet layer, has been optically designed and optimized to operate at high temperature with high performance and high thermal stability. This solar coating, being designed to operate in solar field with molten salt as heat transfer fluid, has to be thermally stable up to the maximum temperature of 550 °C. With the aim of determining degradation behaviour and lifetime prediction of the solar coating, we chose to monitor the variation of the solar absorptance αs after each thermal annealing cycle carried out at accelerated temperatures under vacuum. This prediction method was coupled with a preliminary Differential Thermal Analysis (DTA) in order to give evidence for any chemical-physical coating modification in the temperature range of interest before performing accelerated aging tests. In the accelerated aging tests we assumed that the temperature dependence of the degradation processes could be described by Arrhenius behaviour and we hypothesized that a linear correlation occurs between optical parameter variation rate (specifically, Δαs/Δt) and degradation process rate. Starting from Δαs/Δt values evaluated at 650 and 690 °C, Arrhenius plot gave an activation energy of 325 kJ mol-1 for the degradation phenomenon, where the prediction on the coating degradation gave a solar absorptance decrease of only 1.65 % after 25 years at 550 °C. This very low αs decrease gave evidence for an excellent stability of our solar coating, also when employed at the maximum temperature (550 °C) of a solar field operating with molten salt as heat transfer fluid. © 2016 Author(s).

In “parabolic trough” technology for concentrating solar plants, optimization and fabrication of effective and thermally stable solar coatings represent the most challenging technological goal in order to maximize the photo-thermal conversion efficiency of a solar field operating at high temperature. Starting from the typical cermet-based thin film stratification constituting high-performance solar coating, the optical properties of the metallic layer deposited as “back reflector” strongly determines the thermal loss amount, where solar coating emissivity should have to be as low as possible. Thanks to its very high reflectance in the infrared region, silver is more promising respect to transition metals such as molybdenum and tungsten. An adaptive tungsten layer deposited on stainless steel substrate has been employed with the aim of promoting improvements both on adhesion and on thermal stability of the Ag infrared reflector. Optimization procedure has been in detail described to design and to realize effective cermet-based solar coatings. Six different solar coatings with Ag as back reflector have been optically designed by means of a semi-empirical procedure where a layer-on-layer ellipsometric characterization was utilized. These optimized coatings were fabricated at increasing solar absorptance values coupled with emissivity values, evaluated at 580 °C, lower than 14%. Improved performances have been obtained respect to similar optimized solar coatings with W as back reflector and the highest photo-thermal conversion efficiency has been reached (>92% at 400 °C and >85% at 550 °C) among the solar coatings at the present state of the art. Excellent thermal and optical stability has been proved by means of annealing cycles at 580 °C under vacuum for a total duration of 85 days. © 2016 Elsevier B.V.

Aluminium induced texture (AIT) method has been used for obtaining highly textured glass substrate suitable for silicon based thin film solar cell technology. Wet etch step parameters of AIT process have been varied and effect of different etchants and different etching times on morphological and optical properties has been analyzed. The resulting morphology features (shape, size distribution, inclination angle) have been optimized in order to obtain the best scattering properties. ZnO:Ga (GZO) films have been deposited by sputtering technique on AIT-processed glass. Two different ZnO surface morphologies have been obtained, strongly depending on the underlying glass substrate morphology induced by different etching times. Very rough and porous texture (σrms ∼ 150 nm) was obtained on glass etched 2 min showing cauliflower-like structure, whereas a softer texture (σrms ∼ 78 nm) was obtained on glass etched 7 min giving wider and smoother U-shaped craters. The effect of different glass textures on optical confinement has been tested in amorphous silicon based p-i-n devices. Devices fabricated on GZO/high textured glass showed a quantum efficiency enhancement due to both an effective light trapping phenomenon and an effective anti-reflective optical behaviour. Short etching time produce smaller cavities (

With the aim of enhancing light trapping phenomenon in PV thin film silicon devices, a method has been developed to produce highly textured TCO surfaces by the growth of ZnO:B on a buffer layer consisting of UV-cured highly transparent acrylic polymer (IBA) whose surface had been modified by RIE process. Different RIE etching conditions, particularly RF-power and process duration, have been used to obtain different polymer surface modifications. The effect of different treatment conditions on polymer surface has been systematically investigated and subsequently correlated to the properties of ZnO:B films deposited by MOCVD on these modified polymer surfaces. The surface morphology of zinc oxide film, growth on IBA buffer layer, changed from conventional pyramid-like single-texture to cauliflower-like double-texture. The morphology of double-textured ZnO strongly depended on the surface properties of the etched polymer. Excellent scattering properties were obtained and at the wavelength of 800 nm the haze value increased from 7.5% (pyramidal-like single-texture ZnO layer on glass) to 68% (double textured ZnO film obtained on etched IBA, treated for 20 min at RF-power of 80 W). The RMS roughness showed similar behaviour: it increased at the increasing of the etching time and the highest value of 160 nm was obtained for cauliflower-like ZnO:B film. Good electrical properties were maintained for double-textured ZnO:B films with excellent scattering properties. © 2014 Elsevier B.V.

Improvement of structural and morphological stability of Ag layers has to be pursued for effective employment in solar coating technology for “parabolic trough” thermal plants. Starting from the consideration that stainless steel surface of a receiver tube does not behave as an excellent substrate for fabricating appropriate Ag layers in terms of adhesion strength and thermal stability, both substrate pre-treatments and adaptive layer deposition have been taken into account. Beneficial effect of substrate ionic etching pre-treatment on adhesion coupled with utilization of “α phase” tungsten film as adaptive layer gave evidence for large improvement both on adhesion strength and on morphological stability of Ag layers subjected to thermal cycles at 400 °C under vacuum. Using Ag as infrared reflector and W α as adaptive layer, an optimized cermet-based selective coating suitable for receiver tube operating at 400 °C has been fabricated. This coating, subjected to annealing processes at 400 °C under vacuum up to 29 days, showed excellent thermal and optical stability.

Amorphous WO3 films have been deposited by E-beam evaporation technique at a substrate temperature of 80°C using different base pressure values. The best results have been obtained starting from a mild base pressure (1×10−5 Torr) probably due to the presence of water and oxidative atmosphere in the evaporation chamber. TEM investigations have shown that thermally evaporated WO3 films have an amorphous metastable structure, which subjected to energetic solicitations changes towards a nanocrystalline phase. The amorphous material subsequently annealed at a temperature of 500°C exhibits a fully crystalline structure. Also, in this case good electrochromic properties have been observed. Afterwards a WO3 double layer structure has been realised depositing an amorphous film upon a crystalline layer. The amorphous/crystalline structure properties have been compared with the amorphous and crystalline single layer ones. The colouration response and the transmittance asymptotic value during the bleaching phase have been better for the double layer.

Polysilicon thin films have been obtained by Laser Induced Crystallization utilizing a Q-switched diode pumped, frequency-doubled Nd:YLF laser at 523-nm wavelength. Intrinsic and n-doped amorphous materials, of different thickness, have been deposited on Corning 1737 by LPCVD technique. The irradiation conditions have been varied in order to study their influence on crystallized material properties. Electrical and optical properties of as-deposited and crystallized films have been determined. Structural characterization has been performed to evaluate average grain size and distribution. Larger grain size has been observed in intrinsic materials compared to n-doped materials and the largest grain size (≈1 μm) has been obtained on materials having thickness of 50 nm. Critical role of doping in the crystallization process has been pointed out.

Amorphous and crystalline WO3 films show electrochromic properties by means of double injection of cations and electrons. At low substrate temperature thermal evaporation technique produces an amorphous material mixed to a nanocrystalline phase. When this material is annealed at temperatures higher than 400°C a fully crystalline structure is obtained. Different absorption mechanisms are responsible for colored amorphous and crystalline WO3 layers, small polaron transitions and Drude-like free electron scattering, respectively. Furthermore, both O/W ratio and crystalline structure affect electrochromic efficiency of a WO3 film. It has been confirmed that a hydrogen tungsten bronze structure appears when the material is colored. Finally, a self-bleaching process occurs when a colored WO3 film is exposed to an oxidative environment.

Aluminium-doped ZnO thin films, with a doping level in the range 2–2.8 at.%, were deposited by RF magnetron sputtering. Sputtering processes with pure Ar and Ar/H 2 gas mixtures have been explored. Electrical conductivity and Hall mobility of ZnO:Al films were measured in a wide temperature range. It has been found that the addition of hydrogen to the sputtering gas is an effective method to modify the morphological, structural and electrical properties of the ZnO:Al films. A low hydrogen dilution is able to produce a noticeable improvement of the conductivity by means of a better effectiveness of the Al doping. ZnO:Al films deposited at low hydrogen dilution showed a columnar structure whereas at high hydrogen dilution spherical shaped domains were present, formed by many stacked crystallites. Carrier mobility for the former structure was limited by bulk mechanisms, particularly by acoustical phonon and ionized impurity scattering. Carrier mobility for the latter structure was limited by grain boundary mechanisms, particularly by tunnelling effect between neighbouring spherical macroaggregates.

The morphological and structural properties of copper-indium alloys selenized in a box by selenium vapours have been studied by varying the thermal cycle of the selenization process. In the first series of selenizations, equal metallic precursors were exposed for 30 min to the selenium vapours at different temperatures in the range 175–405°C The chalcopyrite CuInSe2 phase was found at temperatures as low as 250°C and as a single phase at T > 375°C. At low temperatures different copper selenides were found, like CuSe, Cu2 − xSe and CuSe2, which affected in a different way the composition and morphology of the film. Indium loss was detected in the 250–300°C range and was attributed to the evolution in the gas phase of the In2Se species. Short selenization cycles were also used with the purpose of identifying the chemical precursors of CuInSe2. The determination of the selenium content in the alloys at different temperatures allowed us to determine a 7 kcal/mol activation energy for the kinetics of selenium incorporation. Long selenizations at 450°C allowed us to obtain a large-grained, compact layer of chalcopyrite CuInSe2 which could be used for the fabrication of photovoltaic devices.

The operation of micro-optoelectronic devices relies on optical properties of materials, first of all the wavelength dependent refractive index. Standard measurement techniques are often prone to significant ambiguities caused by the periodic nature of the optical response. In this paper we propose a procedure which exploits transmittance measurements, performed at variable light incidence angle, to reconstruct the complex refractive index as a function of the wavelength. Experiments performed on a thin TCO layer prove the reliability of the method. © 2014 IEEE.

The sol-gel synthesis of undoped and B- or Al-doped ZnO thin films were critically examined with particular reference to the influence of the pH of the reaction medium on some of their specific characteristics, such as thickness, morphology, doping level and optical properties, in view of their application in the photovoltaic field. Using triethanolamine (TEA) as chelating agent, a range of basic pH from 7.66 to 8.76 was explored starting from a very concentrated zinc acetate dehydrate (ZAD) solution in ethanol, [Zn2+] = 1.0 M, and keeping the ZAD/TEA = 1. A more basic environment gives more porous films whose thickness and crystallinity are higher than those achieved at lower pH. It was found that the morphology, as well as the sheet resistance (Rs) of films, depends on both pH and doping. Increasing the pH the Rs decreases for both undoped and doped films. At a certain pH undoped films exhibit a granular microstructure and lower Rs than B- or Al-doped films which exhibit a finer texture, characterized by a lower porosity. Optical properties strongly depend on the pH as well. Increasing the pH, a noticeable blue shift effect was observed, that was attributed mainly to structural changes and to a lesser extent to the Burnstein-Moss effect. © 2014 Elsevier B.V.

TCO (Transparent Conductive Oxides) layers often exhibit very rough surfaces and their optical properties, can not be described by one dimensional analytical models and numerical approaches must be adopted. However, computational time becomes, often, too large. In this paper we propose an automated procedure, based on the interaction between MATLAB and the Sentaurus TCAD environment which finds, in a generic rough surface, the smallest area characterized by the same statistical features of the whole surface so that full 3D analysis can be performed. © 2014 IEEE.

Thin films of copper–indium alloy electroplated on molybdenum substrates show different morphologies related to the substrate treatment. An ammonia treatment of the molybdenum electrode can remove a thin oxide layer from the electrode surface, changing the morphological structure of the deposit. The nucleation kinetics of copper were investigated by potentiostatic current–time transients. The deposition process involves instantaneous nucleation for untreated substrates and progressive nucleation for ammonia treated substrates, with diffusion-controlled growth of the three-dimensional nuclei. A higher nucleation density is observed for ammonia-treated molybdenum substrates. The film morphology of the copper–indium alloys can be closely related to the copper nucleation mechanism. On the untreated substrates a low nucleation density together with high adatom mobility lead to the formation of a globular structure. However, when the nucleation density is high and the nuclei are formed continuously during crystal growth, the film evolves towards a porous structure. After the selenization step the film morphology appears related to the precursor structure. © 1997 Elsevier Science S.A. All rights reserved.

The aim of this work was to study a method of electrodeposition of stoichiometric copper-indium alloy to be used as a precursor for obtaining CuInSe2 by selenization. This compound represents one of the most promising materials in terms of low cost and high efficiency for the realization of large-area thin film solar cells for terrestrial applications. However, to avoid the growth of secondary phases such as CuSe and In2Se3, the stoichiometry and morphology of the alloy have to be carefully controlled. We investigated the relationship between the ratio of Cu2+ to In3+ in the solution and the metal ratio in the deposited film. In order to avoid the use of complexing agents we chose to control the stoichiometry of the copper-indium alloys, electrodepositing them under diffusion-limiting current. Since the diffusion currents are, as a first approximation, proportional to the concentrations, there is a simple relationship between the individual diffusion currents of the ions and the stoichiometry of the deposited alloy. Thus we can change the CuIn content in the deposited film by controlling the ratio between the ions in the deposition bath.

Thin CuInSe 2 films have been prepared in a two stage process by sputtering on Mo/glass substrates alternate Cu, In layers followed by selenization by vacuum evaporation. The compostional and the structural analysis of the compounds has been carried out by EDS and XRD, respectively. Selenium incorporated in the ternary compound assumes a constant value around 48% (at/at) while the Cu/In ratio ranges between 0.8 and 1.2. As the copper content is further lowered, selenium decisively increases up to almost 51% when Cu/In is 0.6. The inversion point in the Se-Cu/In diagram has been correlated to the formation of a secondary phase identified as the chalcopyrite vacancy-variant compound CuIn 3 Se 5 (OVC). The results are independent on the type of metallic precursors and on their preparation conditions.

The influence of molecular structure and composition of ethylene-co-vinyl acetate (EVA) copolymers, differing in molecular mass and, for a given molecular mass, in vinyl acetate content (wt/wt), on morphological and structural changes undergone by injection-moulded samples of Nylon 6 (PA6)/EVA blends during uniaxial tensile straining at room temperature up to sample rupture and on properties of such materials was studied. Direct correlations between molecular structure and composition of EVA copolymers and the shape and size of domains of dispersed phase and ultimate properties of PA6/EVA materials have been drawn. For the dumb-bell-shaped specimens strained just beyond the yielding point, it was found that the original layered structure was strongly modified by straining. Three layers were, in fact, generated: a skin surface, where no domains of EVA dispersed phase were observed, an outer layer, where the EVA domains were elongated along the draw direction assuming mainly cylindrical shape, and a core where the EVA phase was segregated in ellipsoidal-shaped domains having their major axis oriented along the draw direction. For the samples strained to break it was found that the PA6/EVA fibres showed a single glass transition temperature, whose value is lower than that found for the fibre of plain PA6, but higher than that shown by PA6 phase in the unstrained samples; such a value, moreover, tends to be decreased on increasing the vinyl acetate content along the EVA chain, irrespective of copolymer molecular mass. The calorimetric measurements performed revealed, moreover, a noticeable increase of the observed melting temperature values of EVA phase in PA6/EVA fibres. In the PA6/EVA samples strained to break, only two different layers were seen: an outer skin, free of EVA domains, and a core where the EVA dispersed phase segregated in ellipsoidal-shaped domains. The values of stress at break, σB, and of the elongation at break, ɛB, exhibited by PA6/EVA fibres were found to be lower than those shown by the plain PA6. The trends of σB and ɛB values against the EVA vinyl acetate content showed, moreover, that σB and ɛB values decreased with increasing vinyl acetate content along the copolymer chain.

In this investigation the surface properties optimisation of a flexible PEN foil to use as substrate for thin film silicon solar cells is presented. The polymer surface, usually hydrophobic and inactive to chemical reactions, can give poor adhesion for films deposited on it. Furthermore, gas desorption from the polymer sometimes causes serious problems to the quality of the devices. To overcome these problems a thin film of silica-like functional material has been developed on polymer foil. Silica-like films were produced by sol-gel process starting from an organic silanes compound (APTMS) as precursor and the solution was deposited by spin-coating. Amorphous silica-like films were obtained with a hydrophilic surface. They were smooth, dense, homogeneous, transparent and exhibited an excellent adhesion to the polymer substrate due to the chemical bond between amine groups of the APTMS with carbonyl bonds in PEN. Physical properties such as elastic modulus and hardness and the UV irradiation effect on structure and surface hydrophilicity of the silica-like coatings have been analysed. A water contact angle of 34° was obtained after UV irradiation. Nanoindentation analysis showed that the silica-like coating have an hardness and an elastic modulus up to 2.0 GPa and 13.2 GPa respectively much higher than that of pure PEN. Oxygen permeability measured on silica-like coated PEN gave a value of 5.7 x 10-9 cc m/m2 s atm showing larger barrier properties respect to pure PEN. Strong adhesion, improved mechanical properties and barrier effect of our silica-like coating make the modified PEN substrate suitable to be used in thin film solar cell technology.

A study of the mode and state of dispersion of ethylene-vinyl acetate (EVA) copolymers, differing in molecular mass and, for a given molecular mass, in vinyl-acetate content (wt/wt), in injection-moulded samples of polyamide-6 based blends is reported. An attempt has been made to correlate molecular structure and composition of EVA copolymers to shape and size of domains of dispersed phase and elastic and tensile yielding behaviour of blended materials.

The development of a hybrid heterojunction fabricated by growing ultrathin amorphous silicon by Plasma Enhanced Chemical Vapor Deposition using temperatures below 250°C offers the potential of obtaining high efficiency solar cells deposited on glassy substrates. The surface preparation represents one of the most critical steps. The first aim of etching is to remove the native oxide layer from the surface of the crystalline wafer, before amorphous layer deposition. The possibility of obtaining this goal with a dry procedure that reduces the exposure of the sample to the environment is not trivial.We performed several dry etching processes but the best results were obtained using an etching process involving CF4/O2 gases. We have found evidence that plasma etching acts by removing the native oxide and the damaged surface of textured silicon and by leaving an active layer on silicon surface suitable for the emitter deposition. SEM analysis has confirmed that it is possible to find plasma process conditions where no appreciable damage and changes in surface morphology are induced. Detailed investigation was performed to find compatibility and optimization of amorphous layer deposition both on flat and textured cast silicon by changing the plasma process parameters. By using this process we achieved on cast silicon for solar applications photovoltaic conversion efficiencies of 12.9% on 51 cm2 and 9.2% on 45 cm2 active areas for amorphous crystalline heterostructure devices realized on monocrystalline and polycrystalline silicon respectively. We also investigated the compatibility of the process with industrial production of large area devices.

Low resistivity (5×10−4Ω cm) and high transmittance (T > 82 %) ZnO films have been obtained by RF magnetron sputtering deposition of ZnO/Al2O3 (2.5 % wt.) target. The doped oxide films have been investigated by optical and electrical measurements and by scanning electron microscopy. The optimized recipes have been applied to the realisation of amorphous p-i-n solar cells back reflector in the structure: TCO/pin/ZnO/Ag.The application of ZnO/Ag leads to an increase of the Jsc current of about 13% but a poorer fill factor steadily affects the characteristics of the devices. We demonstrate that a thicker n+ layer of about three times the standard one is effective in driving the FF toward the normal values.The cells' behaviour has been ascribed to a thinning of the n+ layer resulting from a sputtering action of energetic oxygen atoms during the ZnO deposition process. This effect has been evidenced by measuring the absorbance of SnO2/a-Si (20 nm) /ZnO in comparison with SnO2/a-Si (20 nm) structure.

Tungsten nitride (WNx) thin films were produced by reactive dc magnetron sputtering of tungsten in an Ar–N2 gas mixture. The influence of the deposition power on the properties of tungsten nitride has been analyzed and compared with that induced by nitrogen content variation in the sputtering gas. A combined analysis of structural, electrical and optical properties on thin WNx films obtained at different deposition conditions has been performed. It was found that at an N2 content of 14% a single phase structure of W2N films was formed with the highest crystalline content. This sputtering gas composition was subsequently used for fabricating films at different deposition powers. Optical analysis showed that increasing the deposition power created tungsten nitride films with a more metallic character, which is confirmed with resistivity measurements. At low sputtering powers the resulting films were crystalline whereas, with an increase of power, an amorphous phase was also present. The incorporation of an ...

Thin films of phosphorus doped μc-Si have been prepared in a glow discharge reactor starting from hydrogen diluted silane using less than 5 watt RF power. Conductivities as high as 41 S/cm have been measured on samples deposited at 210°C. The discharge parameters and reactor geometry will be examined in order to correlate the really dissipated RF power to the structural and electrical properties of the material. XRD, SEM, Raman spectroscopy and electrical characterization techniques have been used. All the samples show clusters that increase with the film thickness, while at the same time the crystallite size changes in the range 30–75 A.

Thin CuInSe2 films were prepared by selenization of sputtered metal precursors. The surface morphology, phase composition and adhesion at the Mo/glass substrate of the ternary compound have been correlated to the structure of the precursor. Single phase, homogeneous, small grained (1 µm) CuInSe2 arises from precursors showing stable phases (Cu11In9 and In). An ordered vacancy compound (CuIn3Se5) is likely present in In-rich (In>27 at.%) films. Precursors with a high content of the metastable CuIn2 phase show a very poor adhesion to the substrate.