Optimization of low temperature RF-magnetron sputtering of indium tin oxide films for solar cell applications

In this work we have studied the influence of argon working pressure, substrate temperature, low power plasma irradiation and partial pressure of hydrogen in the RF-magnetron sputtering of indium tin oxide (ITO) thin films on glass substrates. This work aims at identifying the best conditions to achieve good quality ITO films at low temperature for deposition on heat-sensitive substrates. Four sets of samples were prepared which were characterized by X-ray diffraction (XRD), Van der Pauw and transmittance measurements. It was found that structural, electrical and optical properties of the films depend strongly on the deposition parameters. ITO films with a thickness of ~300 nm, displaying a sheet resistance of 68 Ω/sq and average transmittance, in the visible range, of about 90% were produced performing the deposition at low pressure and at room temperature. However, further improvements in the sheet resistance up to a factor of 3 were obtained by decreasing a little more the argon working pressure or applying a low power plasma irradiation or adding a partial pressure of hydrogen to the working gas. Films produced at low working pressures are crystalline and have [222] preferential orientation. The conductivity and transmittance of these films are higher than those of films deposited at high pressures. The electrical resistivity of the ITO thin films decreased sharply either with low power plasma irradiation or the addition of a partial pressure of H2 to the working gas. All the films showed an average transmittance of over 80% in the visible range. Therefore, as a result of this work we established that the addition of a small partial pressure of H2 to the working gas during growth allowed us to achieve the main aim of depositing low resistivity ITO films at low substrate temperature suitable for the envisaged applications. At the same time, we concluded that this approach leads to a drastic reduction in the amount of target surface conditioning required every time a new target was installed because it turns the properties of the ITO films more independent of the target surface properties.publishe

Annealing of RF-magnetron sputtered SnS2 precursors as a new route for single phase SnS thin films

Tin sulphide thin films have been grown on soda-lime glass substrates through the annealing of RF-magnetron sputtered SnS2 precursors. Three different approaches to the annealing were compared and the resulting films thoroughly studied. One series of precursors was annealed in a tubular furnace directly exposed to a flux of sulphur vapour plus forming gas, N2 + 5%H2, and at a constant pressure of 500 mbar. The other two series of identical precursors were annealed in the same furnace but inside a graphite box with and without elemental sulphur evaporation again in the presence of N2 + 5%H2 and at the same pressure as for the sulphur flux experiments. Different maximum annealing temperatures for each set of samples, in the range of 300–570 C, were tested to study their effects on the properties of the final films. The resulting phases were structurally investigated by X-Ray Diffraction (XRD) and Raman spectroscopy. Annealing of SnS2 precursors in sulphur flux produced films where SnS2 was dominant for temperatures up to 480 C. Increasing the temperature to 530 C and 570 C led to films where the dominant phase became Sn2S3. Annealing of SnS2 precursors in a graphite box with sulphur vapour at temperatures in the range between 300 C and 480 C the films are multi-phase, containing Sn2S3, SnS2 and SnS. For high annealing temperatures of 530 C and 570 C the films have SnS as the dominant phase. Annealing of SnS2 precursors in a graphite box without sulphur vapour at 300 C and 360 C the films are essentially amorphous, at 420 C SnS2 is the dominant phase. For temperatures of 480 C and 530 C SnS is the dominant phase but also same residual SnS2 and Sn2S3 phases are observed. For annealing at 570 C, according to the XRD results the films appear to be single phase SnS. The composition was studied using energy dispersive spectroscopy being then correlated with the annealing temperature. Scanning electron microscopy studies revealed that the SnS films exhibit small grain structure and the film surface is rough. Optical measurements were performed, from which the band gap energies were estimated. These studies show that the direct absorption transitions of SnS are at 1.68 eV and 1.41 eV for annealing in graphite box with and without elemental sulphur evaporation, respectively. For the indirect transition the values varied from 1.49 eV to 1.37 eV. The results of this work show that the third approach is better suited to produce single phase SnS films. However, a finer tunning of the duration of the high temperature plateau of the annealing profile is required in order to eliminate the b-Sn top layer.info:eu-repo/semantics/publishedVersio

Effect of selenization conditions on the growth and properties of Cu2ZnSn(S,Se)4 thin films

The opto-electronic properties of copper zinc tin sulfide can be tuned to achieve better cell efficiencies by controlled incorporation of selenium. In this paper we report the growth of Cu2ZnSn(S,Se)4 (CZTSSe) using a hybrid process involving the sequential evaporation of Zn and sputtering of the sulfide precursors of Cu and Sn, followed by a selenization step. Two approaches for selenization were followed, one using a tubular furnace and the other using a rapid thermal processor. The effects of annealing conditions on the morphological and structural properties of the films were investigated. Scanning electron microscopy and energy dispersive spectroscopy were employed to investigate the morphology and composition of the films. Structural analyses were done using X-ray diffraction (XRD) and Raman spectroscopy. Structural analyses revealed the formation of CZTSSe. This study shows that regardless of the selenization method a temperature above 450 °C is required for conversion of precursors to a compact CZTSSe layer. XRD and Raman analysis suggests that the films selenized in the tubular furnace are selenium rich whereas the samples selenized in the rapid thermal processor have higher sulfur content

Artificial antibodies for troponin T by its imprinting on the surface of multiwalled carbon nanotubes: Its use as sensory surfaces

A novel artificial antibody for troponin T (TnT) was synthesized by molecular imprint (MI) on the surface of multiwalled carbon nanotubes (MWCNT). This was done by attaching TnT to the MWCNT surface, and filling the vacant spaces by polymerizing under mild conditions acrylamide (monomer) in N,N′-methylenebisacrylamide (cross-linker) and ammonium persulphate (initiator). After removing the template, the obtained biomaterial was able to rebind TnT and discriminate it among other interfering species. Stereochemical recognition of TnT was confirmed by the non-rebinding ability displayed by non-imprinted (NI) materials, obtained by imprinting without a template. SEM and FTIR analysis confirmed the surface modification of the MWCNT. The ability of this biomaterial to rebind TnT was confirmed by including it as electroactive compound in a PVC/plasticizer mixture coating a wire of silver, gold or titanium. Anionic slopes of 50 mV decade−1 were obtained for the gold wire coated with MI-based membranes dipped in HEPES buffer of pH 7. The limit of detection was 0.16 μg mL−1. Neither the NI-MWCNT nor the MWCNT showed the ability to recognize the template. Good selectivity was observed against creatinine, sucrose, fructose, myoglobin, sodium glutamate, thiamine and urea. The sensor was tested successfully on serum samples. It is expected that this work opens new horizons on the design of new artificial antibodies for complex protein structures

Influence of the sulphurization time on the morphological, chemical, structural and electrical properties of Cu2ZnSnS4 polycrystalline thin films

The effects of the sulphurization annealing time on the morphological, chemical, structural and electrical properties of CZTS thin films were investigated by scanning electron microscopy, X-ray energy dispersive spectroscopy, Hall effect and electrical conductivity measurements in samples annealed during different time intervals. The increase of the annealing time was found to improve the chemical composition of the samples and to, slightly, increase the crystallite size. Small amounts of Na were measured in the samples. However, the concentration of Na does not increase significantly with the annealing time and should not modify the characteristics of the CZTS thin films. It was also found that at high temperature the electrical conductivity is dominated by thermal emission of carriers over the inter-grain potential barriers. As the temperature decreases different hopping conduction mechanisms start to dominate. At first with nearest-neighbour hopping and successively changing to variable range hopping conduction with a crossover from Mott and Efros–Shklovskii behavior. The electrical conductivity, the concentration of free holes, acceptors and donors, traps0 density at the grain boundaries and the grain potential barriers height were found to increase with the annealing time. However, a significant drop in the compensation ratio from 0.8 to 0.5 was also detected.info:eu-repo/semantics/publishedVersio

Anomalous persistent photoconductivity in Cu2ZnSnS4 thin films and solar cells

A persistent photoconductivity effect (PPC) has been investigated in Cu2ZnSnS4 thin films and solar cells as a function of temperature. An anomalous increase of the PPC decay time with temperature was observed in all samples. The PPC decay time activation energy was found to increase when temperature rises above a crossover value, and also to grow with the increase of the sulfurization temperature and pressure. Both the anomalous behavior of the PPC decay time and the existence of two different activation energies are explained in terms of local potential fluctuations in the band edges of CZTS

Análise exploratória do modelo de simulação de rodovias de pista simples do VISSIM

A modelagem de rodovias de pista simples tem sido incorporada em simuladores de tráfego. Como a lógica de simulação em rodovias de pista simples teve que se adequar a outros submodelos comportamentais dos simuladores, como o de car-following e o de mudança de faixas, surge a necessidade de avaliar se a modelagem desse tipo de rodovia tem sido realizada adequadamente. Este artigo, portanto, teve como meta realizar essa análise para o VISSIM, devidamente calibrado por meio de dados de campo e com um Algoritmo Genético. Foram analisadas as variações de duas medidas de desempenho, velocidade média de viagem (ATSd) e número de ultrapassagens (NUd), de acordo com a variação das zonas de ultrapassagens proibidas, da porcentagem de veículos pesados e do fluxo de tráfego unidirecional. Não foram observadas grandes diferenças quando foram avaliados os valores de ATSd, ao contrário do que ocorreu com NUd.A modelagem de rodovias de pista simples tem sido incorporada em simuladores de tráfego. Como a lógica de simulação em rodovias de pista simples teve que se adequar a outros submodelos comportamentais dos simuladores, como o de car-following e o de mudança de faixas, surge a necessidade de avaliar se a modelagem desse tipo de rodovia tem sido realizada adequadamente. Este artigo, portanto, teve como meta realizar essa análise para o VISSIM, devidamente calibrado por meio de dados de campo e com um Algoritmo Genético. Foram analisadas as variações de duas medidas de desempenho, velocidade média de viagem (ATSd) e número de ultrapassagens (NUd), de acordo com a variação das zonas de ultrapassagens proibidas, da porcentagem de veículos pesados e do fluxo de tráfego unidirecional. Não foram observadas grandes diferenças quando foram avaliados os valores de ATSd, ao contrário do que ocorreu com NUd

Study of polycrystalline Cu2ZnSnS4 films by Raman scattering

Cu2ZnSnS4 (CZTS) is a p-type semiconductor that has been seen as a possible low-cost replacement for Cu(In,Ga)Se2 in thin film solar cells. So far compound has presented difficulties in its growth, mainly, because of the formation of secondary phases like ZnS, CuxSnSx+1, SnxSy, Cu2−xS and MoS2. X-ray diffraction analysis (XRD), which is mostly used for phase identification cannot resolve some of these phases from the kesterite/stannite CZTS and thus the use of a complementary technique is needed. Raman scattering analysis can help distinguishing these phases not only laterally but also in depth. Knowing the absorption coefficient and using different excitation wavelengths in Raman scattering analysis, one is capable of profiling the different phases present in multi-phase CZTS thin films. This work describes in a concise form the methods used to grow chalcogenide compounds, such as, CZTS, CuxSnSx+1, SnxSy and cubic ZnS based on the sulphurization of stacked metallic precursors. The results of the films’ characterization by XRD, electron backscatter diffraction and scanning electron microscopy/energy dispersive spectroscopy techniques are presented for the CZTS phase. The limitation of XRD to identify some of the possible phases that can remain after the sulphurization process are investigated. The results of the Raman analysis of the phases formed in this growth method and the advantage of using this technique in identifying them are presented. Using different excitation wavelengths it is also analysed the CZTS film in depth showing that this technique can be used as non destructive methods to detect secondary phases