Rodrigo Martins

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 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.

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.

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 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.

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%.

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 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 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.

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 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.

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.

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.

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.