Effects of sulphurization time on Cu2ZnSnS4 absorbers and thin films solar cells obtained from metallic precursors

We report the results of a study of the sulphurization time effects on Cu2ZnSnS4 absorbers and thin film solar cells prepared from dc-sputtered tackedmetallic precursors. Three different time intervals, 10 min, 30min and 60 min, at maximum sulphurization temperature were considered. The effects of this parameter' change were studied both on the absorber layer properties and on the final solar cell performance. The composition, structure, morphology and thicknesses of the CZTS layers were analyzed. The electrical characterization of the absorber layer was carried out by measuring the transversal electrical resistance of the samples as a function of temperature. This study shows an increase of the conductivity activation energy from 10 meV to 54meV for increasing sulphurization time from 10min to 60min. The solar cells were built with the following structure: SLG/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Ni:Al grid. Several ac response equivalent circuit models were tested to fit impedance measurements. The best results were used to extract the device series and shunt resistances and capacitances. Absorber layer's electronic properties were also determined using the Mott–Schottky method. The results show a decrease of the average acceptor doping density and built-in voltage, from 2.0 1017 cm−3 to 6.5 1015 cm−3 and from 0.71 V to 0.51 V, respectively, with increasing sulphurization time. These results also show an increase of the depletion region width from approximately 90 nm–250 nm.

Cu2ZnSnS4 absorber layers obtained through sulphurization of metallic precursors: Graphite box versus sulphur flux

In this work we employed a hybrid method, combining RF-magnetron sputtering with evaporation, for the deposition of tailor made metallic precursors, with varying number of Zn/Sn/Cu (ZTC) periods and compared two approaches to sulphurization. Two series of samples with 1×, 2× and 4× ZTC periods have been prepared. One series of precursors was sulphurized in a tubular furnace directly exposed to a sulphur vapour and N2+5% H2 flux at a pressure of 5.0×10+4 Pa. A second series of identical precursors was sulphurized in the same furnace but inside a graphite box where sulphur pellets have been evaporated again in the presence of N2+5% H2 and at the same pressure as for the sulphur flux experiments. The morphological and chemical analyses revealed a small grain structure but good average composition for all three films sulphurized in the graphite box. As for the three films sulphurized in sulphur flux grain growth was seen with the increase of the number of ZTC periods whilst, in terms of composition, they were slightly Zn poor. The films' crystal structure showed that Cu2ZnSnS4 is the dominant phase. However, in the case of the sulphur flux films SnS2 was also detected. Photoluminescence spectroscopy studies showed an asymmetric broad band emission whichoccurs in the range of 1–1.5 eV. Clearly the radiative recombination efficiency is higher in the series of samples sulphurized in sulphur flux. We have found that sulphurization in sulphur flux leads to better film morphology than when the process is carried out in a graphite box in similar thermodynamic conditions. Solar cells have been prepared and characterized showing a correlation between improved film morphology and cell performance. The best cells achieved an efficiency of 2.4%.

Hopping conduction and persistent photoconductivity in Cu2ZnSnS4 thin films

The temperature dependence of electrical conductivity and the photoconductivity of polycrystalline Cu2ZnSnS4 were investigated. It was found that at high temperatures the electrical conductivity was dominated by band conduction and nearest-neighbour hopping. However, at lower temperatures, both Mott variable-range hopping (VRH) and Efros–Shklovskii VRH were observed. The analysis of electrical transport showed high doping levels and a large compensation ratio, demonstrating large degree of disorder in Cu2ZnSnS4. Photoconductivity studies showed the presence of a persistent photoconductivity effect with decay time increasing with temperature, due to the presence of random local potential fluctuations in the Cu2ZnSnS4 thin film. These random local potential fluctuations cannot be attributed to grain boundaries but to the large disorder in Cu2ZnSnS4.

CuxSnSx+1 (x = 2, 3) thin films grown by sulfurization of metallic precursors deposited by dc magnetron sputtering

We report the results of the growth of Cu-Sn-S ternary chalcogenide compounds by sulfurization of dc magnetron sputtered metallic precursors. Tetragonal Cu2SnS3 forms for a maximum sulfurization temperature of 350 ºC. Cubic Cu2SnS3 is obtained at sulfurization temperatures above 400 ºC. These results are supported by XRD analysis and Raman spectroscopy measurements. The latter analysis shows peaks at 336 cm-1, 351 cm-1 for tetragonal Cu2SnS3, and 303 cm-1, 355 cm-1 for cubic Cu2SnS3. Optical analysis shows that this phase change lowers the band gap from 1.35 eV to 0.98 eV. At higher sulfurization temperatures increased loss of Sn is expected in the sulphide form. As a consequence, higher Cu content ternary compounds like Cu3SnS4 grow. In these conditions, XRD and Raman analysis only detected orthorhombic (Pmn21) phase (petrukite). This compound has Raman peaks at 318 cm-1, 348 cm-1 and 295 cm-1. For a sulfurization temperature of 450 ºC the samples present a multi-phase structure mainly composed by cubic Cu2SnS3 and orthorhombic (Pmn21) Cu3SnS4. For higher temperatures, the samples are single phase and constituted by orthorhombic (Pmn21) Cu3SnS4. Transmittance and reflectance measurements were used to estimate a band gap of 1.60 eV. For comparison we also include the results for Cu2ZnSnS4 obtained using similar growth conditions.

Influence of selenization pressure on the growth of Cu2ZnSnSe4 films from stacked metallic layers

Cu2ZnSnSe4 (CZTSe) is a p-type semiconductor with a high absorption coefficient, 104 to 105 cm-1, and is being seen as a possible replacement for Cu(In,Ga)Se2 in thin film solar cells. Yet, there are some fundamental properties of CZTSe that are not well known, one of them is its band gap. In order to resolve its correct value it is necessary to improve the growth conditions to ensure that single phase crystalline thin films are obtained. One of the problems encountered when growing CZTSe is the loss of Sn through evaporation of SnSe. Stoichiometric films are then difficult to obtain and usually there are other phases present. One possible way to overcome this problem is to increase the pressure of growth of CZTSe. This can be done by introducing an atmosphere of an inert gas like Ar or N2. In this work we report the results of morphological, structural and optical studies of the properties of CZTSe thin films grown by selenization of DC magnetron sputtered metallic layers under different Ar pressures. The films are analysed by SEM/EDS, Raman scattering and XRD.

Growth and characterization of Cu2ZnSn(S,Se)4 thin films for solar cells

Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) with their band gap energies around 1.45 eV and 1.0 eV, respectively, can be used as the absorber layer in thin film solar cells. By using a mixture of both compounds, Cu2ZnSn(S,Se)4 (CZTSSe), a band gap tuning may be possible. The latter material has already shown promising results such as solar cell efficiencies up to 10.1%. In this work, CZTSSe thin films were grown in order to study its structure and to establish the best growth precursors. SEM micrographs reveal an open columnar structure for most samples and EDS composition profiling of the cross sections show different selenium gradients. X-ray diffractograms show different shifts of the kesterite/stannite (1 1 2) peak, which indicate the presence of CZTSSe. From Raman scattering analysis, it was concluded that all samples had traces of CZTS and CZTSSe. The composition of the CZTSSe layer was estimated using X-ray diffraction and Raman scattering and both results were compared. It was concluded that Se diffused more easily in precursors with ternary Cu–Sn–S phases and metallic Zn than in precursors with ZnS and/or CZTS already formed. It was also showed that a combination of X-ray diffraction and Raman scattering can be used to estimate the ratio of S per Se in CZTSSe samples.

Iron Overload Favors the Elimination of Leishmania

Iron plays a central role in host-parasite interactions, since both intervenients need iron for survival and growth, but are sensitive to iron-mediated toxicity. The host’s iron overload is often associated with susceptibility to infection. However, it has been previously reported that iron overload prevented the growth of Leishmania major, an agent of cutaneous leishmaniasis, in BALB/c mice. In order to further clarify the impact of iron modulation on the growth of Leishmania in vivo, we studied the effects of iron supplementation or deprivation on the growth of L. infantum, the causative agent of Mediterranean visceral leishmaniasis, in the mouse model. We found that dietary iron deficiency did not affect the protozoan growth, whereas iron overload decreased its replication in the liver and spleen of a susceptible mouse strain. The fact that the iron-induced inhibitory effect could not be seen in mice deficient in NADPH dependent oxidase or nitric oxide synthase 2 suggests that iron eliminates L. infantum in vivo through the interaction with reactive oxygen and nitrogen species. Iron overload did not significantly alter the mouse adaptive immune response against L. infantum. Furthermore, the inhibitory action of iron towards L. infantum was also observed, in a dose dependent manner, in axenic cultures of promastigotes and amastigotes. Importantly, high iron concentrations were needed to achieve such effects. In conclusion, externally added iron synergizes with the host’s oxidative mechanisms of defense in eliminating L. infantum from mouse tissues. Additionally, the direct toxicity of iron against Leishmania suggests a potential use of this metal as a therapeutic tool or the further exploration of iron anti-parasitic mechanisms for the design of new drugs.

O impacto da comunicação interna na criação da cultura organizacional

A importância da comunicação interna recebe cada vez mais atenção do seio das organizações. Ela é vista, atualmente, como um elemento fundamental da criação da cultura das empresas, e estas dedicam-lhe cada vez mais tempo e recursos. Este estudo visa avaliar o impacto da comunicação interna na transmissão dos valores e da cultura corporativa. Para tal, foi realizado um estudo empírico, através da realização de inquéritos a 52 colaboradores do BNP Paribas CIB Portugal. As hipóteses analisadas no estudo variavam no conhecimento (ou falta dele) da cultura da instituição e na importância dada (ou não) à comunicação interna como transmissora da cultura organizacional. Os resultados dos questionários comprovam a hipótese inicial, provando que a amostra conhece a instituição e avalia como positivas quer as ferramentas de comunicação interna quer a atitude geral da empresa no que diz respeito à comunicação.

A (In)visibilidade do Conselho Geral

Este estudo teve como principal objetivo determinar as causas que levam à invisibilidade do Conselho Geral(CG). Este, conjuntamente com o Diretor, o Conselho Pedagógico e o Conselho Administrativo, constituem os órgãos de gestão e administração dos Agrupamentos de Escolas ou escolas não Agrupadas (AE/E). O CG representa o órgão de maior poder na hierarquia formalmente definida para a gestão e administração dos AE/E, pelo que devia ser conhecido por toda a comunidade escolar. Foi desenvolvido em três AE/E do concelho de Matosinhos, tendo sido inquiridos professores, alunos e assistentes operacionais/técnicos na tentativa de averiguar o (re)conhecimento do órgão em estudo e das suas competências. Foram ainda entrevistados os Diretores e os presidentes dos Conselhos Gerais de cada escola de forma aferir o tipo de relação que se estabelece entre eles. Fizeram ainda parte da nossa amostra Conselheiros, grupo privilegiado quer a nível de conhecimento das competências do Conselho Geral quer no reconhecimento do tipo de relação que se estabelece entre Diretor e Conselho Geral. Os resultados por nós obtidos mostram que o Conselho Geral é um órgão pouco conhecido da comunidade escolar. No cômputo geral, a comunidade escolar mais restrita desconhece o seu funcionamento bem como as suas competências. Face aos resultados foi por nós elaborado um Plano de Ação que visa melhorar o conhecimento que a comunidade tem relativamente ao Conselho Geral, e com ele melhorar a gestão estratégica dos AE/E.

Alterações da sinalização purinérgica gastrointestinal na Diabetes

A dismotilidade entérica é uma complicação a longo prazo da Diabetes mellitus (DM) que causa desconforto significativo em 76% dos pacientes diabéticos. Sabendo que as purinas estão envolvidas na neuromodulação colinérgica e que no SNC de animais diabéticos foram encontradas alterações na expressão de recetores purinérgicos, decidimos investigar se na dismotilidade diabética a neuromodulação purinérgica se encontra preservada. O modelo animal escolhido de diabetes tipo I resultou da administração de estreptozotocina (STZ, 55 mg/kg, IP) a ratazanas (Rattus norvegicus, Wistar). Este modelo STZ provou ser adequado para o estudo, apresentando 2 semanas após a indução polidipsia, poliúria, polifagia, hiperglicemia e um atraso da motilidade gastrointestinal. A caraterização morfológica macroscópica dos animais STZ revelou um aumento significativo do cego e do intestino. Funcionalmente, estudos preliminares indicam que as contrações espontâneas do íleo dos animais STZ perdem ritmicidade e apresentam maior amplitude que as dos animais controlo de uma forma insensível à TTX, sugerindo o comprometimento das ICC. Paralelamente, estudos imagiológicos revelaram uma perda neuronal mioentérica, principalmente de neurónios nitrérgicos, sendo os colinérgicos preservados. Contudo, a resposta muscular do íleo de animais diabéticos à acetilcolina (ACh) foi inferior à dos controlos, estando a libertação de ACh modulada pela adenosina modificada. Verificou-se que a inibição promovida pelos recetores A1 se mantinha, mas que se perdia a facilitação mediada pela ativação de recetores A2A, cuja imunorreatividade também se encontrava diminuída. Curiosamente, apesar do catabolismo do ATP e dos seus metabolitos estar aumentado nos animais STZ, não se verificou um aumento dos níveis extracelulares de adenosina. Nos animais diabéticos a adenosina é rapidamente desaminada e recaptada por transportadores de nucleósidos, com principal relevância para os concentrativos, sendo os equilibrativos responsáveis pelo transporte da adenosina em animais controlo. Os resultados apresentados nesta tese sugerem que a dismotilidade diabética pode dever-se à perda da atividade nitrérgica, das ICC e da neuromodulação purinérgica mediada por recetores A2A, comprometendo assim a libertação de ACh e consequentemente a motilidade GI.

A importância do estúdio de ópera na formação do aluno de canto

Relatório de Estágio para a obtenção do grau de Mestre em Ensino da Música, Ramo - Canto