Chalcogenide thin films for solar cells: growth and properties

Thin film solar cells have in recent years gained market quota against traditional silicon photovoltaic panels. These developments were in a large part due to CdTe solar panels on whose development started earlier than their competitors. Panels based on Cu(In,Ga)Se2 (CIGS), despite being more efficient in a laboratory and industrial scale than the CdTe ones, still need a growth technology cheaper and easier to apply in industry. Although usually presented as a good candidate to make cheap panels, CIGS uses rare and expensive materials as In and Ga. The price evolution of these materials might jeopardize CIGS future. This thesis presents three different studies. The first is the study of different processes for the incorporation of Ga in a hybrid CIGS growth system. This system is based on sputtering and thermal evaporation. This technology is, in principle, easier to be applied in the industry and solar cells with efficiencies around to 7% were fully made in Aveiro. In the second part of this thesis, a new material to replace CIGS in thin film solar cells is studied. The growth conditions and fundamental properties of Cu2ZnSnSe4 (CZTSe) were studied in depth. Suitable conditions of temperature and pressure for the growth of this material are reported. Its band gap energy was estimated at 1.05 eV and the Raman scattering peaks were identified. Solar cells made with this material showed efficiencies lower than 0.1%. Finally, preliminary work regarding the incorporation of selenium in Cu2ZnSnS4 (CZTS) thin films was carried out. The structural and morphological properties of thin films of Cu2ZnSn(S,Se)4 have been studied and the results show that the incorporation of selenium is higher in films with precursors rather with already formed Cu2SnS3 or Cu2ZnSnS4 thin films. A solar cell with 0.9 % of efficiency was prepared.

Understanding and modelling hydrological and soil erosion processes in burnt forest catchments

Forest fires implications in overland flow and soil erosion have been researched for several years. Therefore, is widely known that fires enhance hydrological and geomorphological activity worldwide as also in Mediterranean areas. Soil burn severity has been widely used to describe the impacts of fire on soils, and has being recognized as a decisive factor controlling post-fire erosion rates. However, there is no unique definition of the term and the relationship between soil burn severity and post-fire hydrological and erosion response has not yet been fully established. Few studies have assessed post-fire erosion over multiple years, and the authors are aware of none which assess runoff. Small amount of studies concerning pre-fire management practices were also found. In the case of soil erosion models, the Revised Universal Soil Loss Equation (RUSLE) and the revised Morgan–Morgan–Finney (MMF) are well-known models, but not much information is available as regards their suitability in predicting post-fire soil erosion in forest soils. The lack of information is even more pronounced as regards post-fire rehabilitation treatments. The aim of the thesis was to perform an extensive research under the post fire hydrologic and erosive response subject. By understanding the effect of burn severity in ecosystems and its implications regarding post fire hydrological and erosive responses worldwide. Test the effect of different pre-fire land management practices (unplowed, downslope plowed and contour plowed) and time-since-fire, in the post fire hydrological and erosive response, between the two most common land uses in Portugal (pine and eucalypt). Assess the performance of two widely-known erosion models (RUSLE and Revised MMF), to predict soil erosion rates during first year following two wildfires of distinctive burn severity. Furthermore, to apply these two models considering different post-fire rehabilitation treatments in an area severely affected by fire. Improve model estimations of post-fire runoff and erosion rates in two different land uses (pine and eucalypt) using the revised MMF. To assess these improvements by comparing estimations and measurements of runoff and erosion, in two recently burned sites, as also with their post fire rehabilitation treatments. Model modifications involved: (1) focusing on intra-annual changes in parameters to incorporate seasonal differences in runoff and erosion; and (2) inclusion of soil water repellency in runoff predictions. Additionally, validate these improvements with the application of the model to other pine and eucalypt sites in Central Portugal. The review and meta-analysis showed that fire occurrence had a significant effect on the hydrological and erosive response. However, this effect was only significantly higher with increasing soil burn severity for inter-rill erosion, and not for runoff. This study furthermore highlighted the incoherencies between existing burn severity classifications, and proposed an unambiguous classification. In the case of the erosion plots with natural rainfall, land use factor affected annual runoff while land management affected both annual runoff and erosion amounts significantly. Time-since-fire had an important effect in erosion amounts among unplowed sites, while for eucalypt sites time affected both annual runoff and erosion amounts. At all studied sites runoff coefficients increase over the four years of monitoring. In the other hand, sediment concentration in the runoff, recorded a decrease during the same period. Reasons for divergence from the classic post-fire recovery model were also explored. Short fire recurrence intervals and forest management practices are viewed as the main reasons for the observed severe and continuing soil degradation. The revised MMF model presented reasonable accuracy in the predictions while the RUSLE clearly overestimated the observed erosion rates. After improvements: the revised model was able to predict first-year post-fire plot-scale runoff and erosion rates for both forest types, these predictions were improved both by the seasonal changes in the model parameters; and by considering the effect of soil water repellency on the runoff, individual seasonal predictions were considered accurate, and the inclusion of the soil water repellency in the model also improved the model at this base. The revised MMF model proved capable of providing a simple set of criteria for management decisions about runoff and erosion mitigation measures in burned areas. The erosion predictions at the validation sites attested both to the robustness of the model and of the calibration parameters, suggesting a potential wider application.

Participação ativa no desenvolvimento de comunidades online

Esta tese tem como objetivos identificar e caraterizar os modos de Comunicação Mediada por Computador (CMC) utilizados nas Comunidades Online (CO), relevantes para a prática de criação visual, contextualizando-os na produção de Type Design (TD). E concetualizar um modelo de CO de prática, desenvolvida segundo uma metodologia participativa, avaliando as estratégias de dinamização e viabilização no contexto nacional. Atualmente, com a disseminação das tecnologias de informação e comunicação a prática de TD, uma atividade isolada e individual, está sujeita a um paradigma de mudança—tornar-se uma atividade de partilha cada vez maior. A investigação dividiu-se em três fases. Na primeira fase procura perceber os modos relevantes de CMC para a prática de TD. Para isso, observa-se as motivações e comportamentos dos utilizadores nestas plataformas, apoiado pela recolha de dados sobre o uso destas plataformas no contexto académico. Juntamente com o desenvolvimento e avaliação de uma experiência preliminar numa plataforma online existente. Na segunda fase, apresenta-se o modelo concetual de uma CO para a produção de TD apoiado na identificação e caraterização das Virtual Community of Practice de TD existentes. E o respetivo desenvolvimento e implementação de um protótipo funcional. Na terceira fase o protótipo foi avaliado recorrendo à participação dos utilizadores—uma amostra de Type Designers profissionais portugueses—no desenvolvimento das estratégias comunitárias segundo uma abordagem participativa de Community-Centered Development. A utilização de um Focus Group de dimensões reduzidas, e as limitações temporais da investigação prática não permitem generalizar os resultados encontrados. No entanto, os resultados revelam que quanto mais específica for a atividade da prática partilhada na CO, mais especializados e restritos devem ser os modos de CMC implementados para a comunicação e interação dos membros online. A integração dos modos de CMC sobre a comunicação assíncrona através de artefactos visuais tem um potencial de comunicação mais eficaz e satisfatório para estes utilizadores. E as barreiras à participação, apesar de poderem ser minimizadas através da inovação nos modos implementados, são um desafio de sociabilização que devem ser geridos, equilibrando a comunicação explícita, os usos, perceções e expectativas dos diferentes papéis dos membros da comunidade.