Rodrigo Martins

In the present work, the spectroscopic ellipsometry (1.5 - 5.5 eV) was used to investigate the effects of current density induced microstructural variations and their influence on the electronic states of as-prepared and a-Si:H coated porous silicon (PS). The pseudodielectric responses of the low and high current densities (5 and 40 mA/cm2) were analyzed using a multilayer model within the effective medium approximation. The FTIR investigation reveals me enhancement of surface oxide (Si-Ox) layer with current density and the improvement of the Si-Hx band after a-Si:H coating.

In the present work, we investigate the influence of hydrocarbon (CHx) thin film coating on porous silicon (PS) by plasma enhanced chemical vapor deposition (PECVD) technique to detect CO2 gas. The fabricated CHx/PS heterojunction device shows up to one and two orders of magnitude enhancement in current under CO2 gas exposure. FTIR spectroscopy measurements reveal a remarkable structural modification of the CHx/PS device during CO2 gas exposure. Further, the enhancement of CHx related absorbance bands by a factor 6.2 for the CHx/PS specimen in comparison with PS confirm the good quality of the deposited CHx thin films. © 2007 Elsevier B.V. All rights reserved.

This work refers to the fabrication of hybrid solar cells based on porous silicon (PS) filled with copper phthalocyanine (CuPc) to form GZO/CuPc-PS/Si/Al structure. We used n-type Si since photoinduced charge transfer from CuPc to Si is observed only for n-type Si. The characteristic 100% peak at 6.8°2θ from XRD confirms that the CuPc coating on PS exists in α-CuPc polymorph. The systematic increase in FWHM of XRD peak at 69.1°2θ and red-shift of LO phonon Raman spectra indicate a progressive reduction of Si nanocrystallites size with increasing etching time and the estimated size agrees well with atomic force microscopic images. The FTIR results confirm the formation of α-CuPc polymorph and it is also corroborated well with XRD and Raman results. © 2008.

In this work Laponite was combined with a modified abundant natural polymer, (caboxymethyl cellulose), acrylic sodium salt polymer and lithium perchlorate aiming to produce inexpensive and sustainable nanocomposite electrolytes for functional electrochemical devices. Optical, electrochemical, structural, morphological and rheological characterization was performed in order to evaluate their properties and potential advantages as electrolyte. It was verified that the addition of Laponite led to an ionic conductivity at room temperature (25 C) in the range of 6 to 9 × 10- 5 Scm - 1 this value being then determined by the composition of the nanocomposite. These electrolytes were applied to electrochromic devices using evaporated nickel oxide thin film as the electrochromic layer. The devices exhibited a significant transmittance modulation that exceeds 45 % at 600 nm. © 2013 Elsevier B.V.

The aim of this paper is to present results concerning the role of the buffer layer on pin devices, deposited in a single chamber for plasma enhanced chemical vapor deposition, using high hydrogen dilution and pressures at 27.12 MHz. By doing so, we allow the incorporation of nanoparticles into the i-layer, during plasma process. The results show solar cells with 8.8% efficiency with a collection efficiency of 95% in the blue region of the spectra. Apart from that, the results from impedance spectroscopy, imaginary impedance vs. real impedance, show difference of a semicircle radius as function of sample temperatures, which could be explained by total device series resistance variation. © 2005 Elsevier B.V. All rights reserved.

This paper aims to characterize the growth process of polymorphous/ nanocrystalline silicon (pm-Si:H) films produced by PECVD at 13.56 MHz. The emphasis is in determining the plasma parameters that allow to control the conditions where pm/nc-Si:H can be obtained under high hydrogen dilution, where the only varied parameter is the silane gas flow, fixing rf power, deposition pressure and substrate temperature. The data achieved show that good pm/nc-Si:H films are produced at 240 Pa using a silane gas flow of 5sccm (dilution 1:70) to which it corresponds films with photosensitivities exceeding 106, optical gaps close to 1.80 eV and 18 at% hydrogen contents. The data also show that under certain deposition conditions the pm-Si:H films peel-off.

Nanostructured silicon films were deposited on the amorphous to microcrystalline transition region by plasma enhanced chemical vapor deposition, using an rf frequency of 27.12 MHz. Micro-Raman spectroscopy data show that in the transition region the peaks typically associated with amorphous silicon are slightly shifted towards higher wavenumber and become narrow, which could be explained by the short range order improvement or by the incorporation of very small Si nanocrystallites. The hydrogen evolution spectra from samples deposited in this region show two peaks, one at low temperature (LT) and the other at high temperature (HT), around 698 K and 840 K, respectively. These peaks represent activation energies of 87 (LT) and 135 (HT) kJ/mol, respectively, as deduced from the so-called Kissinger's method. The solar cells fabricated using i-layers produced in this transition region show good performances, with current density = 14.96 mA/cm2, short circuit voltage = 0.95 V, and fill factor = 0.67, which leads to efficiencies of 9.52%. © 2006 Elsevier B.V. All rights reserved.

This work deals with the study of the role of the buffer layers thickness on the TCO/p-a-SiC:H/buffer1/buffer2/i(nc-Si/a-Si:H)/n-a- Si:H/Al solar cell I-V and impedance performances. The aim was to improve the p/i interface region, which has a large influence on the solar cell characteristics and stability. In order to match the difference between the p and i layers optical gaps, the buffer layers were deposited using, for each layer, different methane to silane mixtures, aiming to obtain a gradual match of the corresponding optical gaps. The intrinsic layer was deposited at high hydrogen dilution rates at 27.12 MHz in conditions that allowed the incorporation of nanoparticles/nanoclusters. Solar cells with fill factor of 0.63; open circuit voltage of 0.93 Volts; short circuit current density of 16.13 mA/cm2 and an efficiency of 9.4% were produced with buffer layers around 1.3 nm thick. When comparing these solar cells with conventional amorphous silicon solar cells we notice that the quantum efficiency from ultraviolet to green regions is improved up to 13%, in average. Concerning solar cell capacitance, the data show that the best solar cells exhibit the highest capacitance, meaning that the films are compact and dense, in-line with the other electrical characteristics obtained. ©2005 IEEE.

Nanostructured silicon thin films were produced in a single PECVD (Plasma Enhanced Chemical Vapour Deposition) reactor using an excitation frequency of 27.12 MHz. The process parameters were selected to allow the films' production to be performed at the transition region (from amorphous to microcrystalline), aiming their use in solar cells. The real and imaginary parts of pseudo-dielectric function of these nanostructured films show a shift to higher energies and the order factor reveals an improvement on the short atomic range order of the films produced. The solar cells with a structure of ZGO/p-a-SiC:H/buffer1/buffer2/i-(nc/a-Si:H)/n-a-Si:H/Ag/Al were deposited with nanostructured intrinsic layer, showing a good performances, with current densities of about 14.48 mA/cm2, open circuit voltage of 0.94 V, and fill factor of 0.67, which lead to efficiencies of 9.12%. The solar cell degradation study performed under AM1.5 spectrum conditions up to 100 hours revealed a decrease on the solar cell efficiency of about 8.11%, mainly related to the decreasing of current density. Despite that, the open circuit voltage increases slightly after the degradation.

In this work we studied the influence of the power density of hydrogen plasma on electrical and optical properties (Hall mobility, free carrier concentration, sheet resistance, optical transmittance and a.c. impedance) of indium zinc oxide films, aiming to determine their chemical stability. This is an important factor for the optimization of amorphous/nanocrystalline p-i-n hydrogenated silicon (a/nc-Si:H) solar cells, since they should remain chemically highly stable during the p layer deposition. To perform this work the transparent conductive oxide was exposed to hydrogen plasma at substrate temperature of 473 K, 87 Pa of pressure and 20 seem of hydrogen flow. The results achieved show that IZO films were reduced for all plasma conditions used, which leads mainly to a decrease on films transmittance. For the lowest power density used in the first minute of plasma exposition the transmittance of the IZO films decreases about 29%.

This work deals with the study of nanostructured/amorphous silicon solar cell deposited by plasma enhanced chemical vapor deposition at 27.12 MHz by impedance spectroscopy. The solar cell studied present fill factor of 0.67, open circuit voltage of 0.94 V and short-circuit current density of 14.48 mA/cm2, which leads to the efficiency of 9.12%. The impedance spectroscopy analysis was performed under dark and illumination conditions. The data obtained were used to define an electrical equivalent circuit model able to explain the role of the different solar cell components, including the interfaces, on the solar cell performance. © 2006 Elsevier B.V. All rights reserved.

In this work we report the study of spectral response on large area amorphous silicon solar cells (30×40 cm2), deposited through plasma enhanced chemical vapour deposition technique (PECVD) at excitation frequencies of 27.12MHz. To perform this work, the solar cells were split in units of area of 0.126 cm2, which allows determining the device homogeneity over all the entire solar cell. Emphasis of this work is put the role of thickness and optical band gap of p-doped layer on the collection efficiency, spectral response, current density-voltage curves under standard condition and spectroscopy impedance. The results show that high transparent p-doped layer can be deposited at 42mW/cm2, which allows increasing the collection efficiency in 45%, at the blue region. The spectroscopy impedance performed showed to be efficient in analyzing the device shunt resistance, interfaces role on the device performances and the behaviour of the device depletion region, for the range of frequencies analysed.

In this paper we study the electro-optical behaviour and the structure of different TCOs, namely the ZGO, ITO and IZO films before and after being submitted to different hydrogen plasma power densities, for times up to 60 s, aiming their use in a/nc-Si:H solar cells. The results achieved show that ZGO films do not reduce for all plasma conditions used and so, the solar cells produced evidence high current density, about 17% larger that the one recorded in the other TCOs. Besides that, by combining the electrical and optical characteristics of the films through a figure of merit, the data reveal that for the ITO and IZO films even when exposed to very low hydrogen power plasma, the figure of merit is reduced up to 50%. © 2005 Elsevier B.V. All rights reserved.

This work reports a technique to obtain electronic grade intrinsic amorphous silicon using the plasma enhanced chemical vapour deposition technique at 13.56 MHz. The batch processing method consists of igniting the plasma process through a neutral gas such as hydrogen or helium and only feeding the carrier gas containing the species to be decomposed into the reactor when the plasma is stabilized. By doing so, no surface damage is induced in the first deposited layers and so a more compacted and stable film is produced, compared to amorphous films grown by conventional methods. The best deposition conditions to produce films with good transport properties for optoelectronic applications are: temperature ≈ 473 K, 60 < pressure 87 Pa, power density of 32 mW/cm2 and flow of silane ≈ 10 sccm. The growth rate and the microstructure factor are 1.5 Å/s and 3.3×10-2, respectively, while the activation energy ≈ 0.8 eV; dark conductivity at room temperature ≈ 4.37×10-10 (ωcm)-1; photosensiti-vity ≈ 5.02×l06; density of states ≈ 6.6×1015 cm-3; bonded hydrogen concentration ≈ 20 at% and optical band gap ≈ 1.75 eV.

The aim of this paper is to compare the density of bulk states (DOS) of polymorphous silicon (pm-Si:H) films produced by plasma enhanced chemical vapor deposition at 13.56 and 27.12 MHz using the constant photocurrent method and the space charge limited current (SCLC) technique. The data achieved revealed that the set of films produced present similar DOS. Apart from that, data concerning the correlation of the deposition conditions that lead to the production of pm-Si:H as well as their characteristics, such as the hydrogen content and how hydrogen is bonded, will be discussed, giving special emphasis to the set of mechanical stresses developed. By doing so we could get a better understanding of the nature of hydrogen bonding in pm-Si:H films as well as to determine the role of the excitation frequency on the film's performances, where films with amounts of hydrogen around 20 at.% can have DOS as low as 8 × 10 14 cm-3 with Urbach energies in the range of 41-50 meV. © 2004 Elsevier B.V. All rights reserved.

In this work we study the optical and electrical behavior of ZnO:Ga, ITO and IZO films deposited on glass after sustaining different hydrogen plasma conditions and exposure times. This work was complemented by analyzing the surface morphology of the set of films, which allow us to determine the role of hydrogen plasma on the film's properties such as Hall mobility, free carrier concentration, sheet resistance, optical transmittance, figure of merit and state of the surface. Apart from that, the performances of solar cells using an intrinsic layer constituted by nanocrystalline silicon will be also presented. The data show that the electrical properties of solar cells were improved by using ZnO:Ga as front contact, allowing a high current density collection and single pin solar cells with efficiencies exceeding 11%. © 2005 Materials Research Society.

The aim of this work is to present data concerning the optimization of performances of a large area amorphous silicon p-i-n solar cell (30×40 cm2) deposited by plasma enhanced chemical vapour deposition (PECVD) at 27.12 MHz. In this work the solar cell was split into small areas of 0.126 cm2, aiming to study the device performance uniformity, where emphasis was put on the role of the n-layer thickness. The solar cells were studied through the spectral response behaviour in the 400-750 nm range as well as by the behaviour of the AC impedance. Solar cells with fill factor of 0.58, open circuit voltage of 0.83 V, short circuit current density of 17.14 mA/cm2 and an efficiency of 8% were obtained at growth rates higher than 0.3 nm/s. © 2004 Elsevier B.V. All rights reserved.

ZnO/CNT composites were prepared using ZnO nanoparticles and tetrapods synthesized by the Laser Assisted Flow Deposition method. The co-operative behaviour between these two materials may give rise to the production of advanced functional materials with a wide range of applications in electronics and optoelectronics. Despite some degree of aggregation in the case of the nanoparticles, scanning electron microscopy images evidence that the produced ZnO structures are well dispersed in the CNT buckypapers. Independent of the ZnO morphology the samples resistivity was shown to be of the order of ∼10-1 Ω cm while in the case of the electron mobility, the composite with tetrapods reveals a lower value than the ones obtained for the remaining samples. Well-structured ZnO luminescence was observed mainly in ultraviolet highlighting the high optical quality of the produced structures. The temperature dependence of the luminescence reveals a distinct trend for the composites with ZnO tetrapods and ZnO nanoparticles. © 2015 Elsevier B.V.

In this paper ageing effects of the solution used to prepare fluorine-doped ZnO films by the spray pyrolysis technique were investigated, concerning its role on the structure, the electrical and optical properties of films produced. The data reveal that the sheet resistance of the ZnO:F thin film decreases with the age of the solution used, reaching a minimum of 24 Ω/□, after 15 days. On the other hand the optical transmittance increases for films deposited using 6 days aging solution, decreasing afterwards as the aging time increases, being the optical transmittance in the visible range below 55%, for films deposited from solutions 36 days in age. The X-ray diffraction spectra show that the aged films are polycrystalline in nature with a [100] predominant orientation. The data also show that the intensity of (100) peak increases as the time of solution age increases, which is related to an improvement of the film crystallinity. © 2009 Elsevier B.V. All rights reserved.

A double-chamber system was used to deposit large-area hydrogenated amorphous silicon films for photovoltaic applications. The electro-optical characterisation of films of area 400 cm2 deposited on glass substrates is described in this paper. The deposition rate of the films is dependent on the r.f. power delivered, the substrate bias and the partial pressure of the reactive gas. The film thickness was observed to have a uniformity of better than 0.5%. The best film quality was obtained for a deposition rate of about 1.5 Å s-1. The optical gap, activation energy, photosensitivity, density of gap states and hydrogen content were determined. © 1985.

Solution-processed field-effect transistors are strategic building blocks when considering low-cost sustainable flexible electronics. Nevertheless, some challenges (e.g., processing temperature, reliability, reproducibility in large areas, and cost effectiveness) are requirements that must be surpassed in order to achieve high-performance transistors. The present work reports electrolyte-gated transistors using as channel layer gallium-indium-zinc-oxide nanoparticles produced by solvothermal synthesis combined with a solid-state electrolyte based on aqueous dispersions of vinyl acetate stabilized with cellulose derivatives, acrylic acid ester in styrene and lithium perchlorate. The devices fabricated using this approach display a ION/IOFF up to 1 × 106, threshold voltage (VTh) of 0.3-1.9 V, and mobility up to 1 cm2/(V s), as a function of gallium-indium-zinc-oxide ink formulation and two different annealing temperatures. These results validates the usage of electrolyte-gated transistors as a viable and promising alternative for nanoparticle based semiconductor devices as the electrolyte improves the interface and promotes a more efficient step coverage of the channel layer, reducing the operating voltage when compared with conventional dielectrics gating. Moreover, it is shown that by controlling the applied gate potential, the operation mechanism of the electrolyte-gated transistors can be modified from electric double layer to electrochemical doping. © 2014 American Chemical Society.

Total hip replacement is a common practice in every day clinical work. Artificial hip implants consist of a femoral component and an acetabular component. Nowadays the acetabular component is composed of a polymeric cup and a metallic shell. This study focuses the development of an innovative acetabular component substituting the metallic shell by a multilayer coating on the acetabular cup. A titanium coating was deposited onto ultra-high molecular weight polyethylene (UHMWPE) samples by physical vapour deposition (PVD), having an in situ pre-treatment with argon ion bombardment in order to optimize the adhesive strength by surface modification, followed by the deposition of a thin film of hydroxyapatite (HA) using rf magnetron sputtering technique, at room temperature. Results obtained seem to indicate that these multilayer coatings can be a viable alternative to the metallic shell, leading to the substitution of a two part for a one part acetabular component.

The optimization of tungsten trioxide (WO3) nanoparticles produced via hydrothermal synthesis for application in electrochromic (EC) devices is reported. The structure and morphology of the nanoparticles are controlled by changing the acidity of the aqueous solvent added to the sol-gel precursor (peroxopolytungstic acid) during synthesis. Orthorhombic hydrated WO3 nanorods or monoclinic WO3 nanoslabs are obtained when HCl is added, while synthesis only in aqueous medium results in a mixture of both types of polymorphs. Dual-phase thin films are processed by inkjet printing deposition of the nanoparticles in flexible polyethylene terephthalate substrate with indium tin oxide coating (ITO PET) followed by the deposition of the precursor solution. When compared with purely amorphous tungsten oxide films, the dual phase ones present higher optical densities and improved capacity, and cyclability stability. The best results, obtained for orthorhombic hydrated nanoparticles (ortho -WO3 ·0.33H2 O), are due to its high surface area and improved conductivity. Additionally, the ex situ X-ray diffraction (XRD) lithiation studies show evidence of a higher distortion of the monoclinic when compared with the orthorhombic crystallographic structure, which contribute to the inferior EC performance. These results validate the use of inkjet printing deposition with low processing temperatures for EC dual-phase thin films containing optimized nanoparticles which are compatible with low-cost substrates. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This work reports the structural, optical, electrical and thermoelectric properties of vanadium pentoxide (V2O5) thin films deposited at room temperature by thermal evaporation on Corning glass substrates. A post-deposition thermal treatment up to 973 K under atmospheric conditions induces the crystallization of the as-deposited amorphous films with an orthorhombic V2O5 phase with grain sizes around 26 nm. As the annealing temperature rises up to 773 K the electrical conductivity increases. The films exhibit thermoelectric properties with a maximum Seebeck coefficient of -218 μV/K and electrical conductivity of 5.5 (Ω m) -1. All the films show NIR-Vis optical transmittance above 60% and optical band gap of 2.8 eV. © 2013 Elsevier B.V. All rights reserved.

The aim of this paper is to present a set of electric data concerning the performances before and after ageing of Cu-Sn-Cu joins used to solder power diodes and to compare the results achieved with the ones obtained in diodes soldered using the conventional technology. The set of results achieved show that the Cu-Sn-Cu joins present even better performances than the ones exhibited by diodes soldered using the conventional technology, without requiring the use of Mo discs to be inserted between the silicon crystal and the metal contacts (stud or finger) to compensate thermal mismatches.