Influência de parâmetros de crescimento nas propriedades de Cu2ZnSnS4

Mestrado em Engenharia FísicaNeste trabalho estudam-se filmes finos de Cu2ZnSnS4 (CZTS) no sentido de avaliar a influência dos parâmetros de crescimento na morfologia e nas propriedades estruturais e óticas destes filmes de forma a otimizar a sua utilização como camada absorvente em células solares. O número de períodos de precursores metálicos foi variado (1, 2, 4) e a sulfurização foi realizada em caixa de grafite ou em fluxo de enxofre. Os estudos realizados consistiram em análises morfológica, estrutural e ótica com base nas técnicas de SEM, EDS, XRD, espetroscopia de Raman e fotoluminescência. Verificou-se que as amostras sulfurizadas em fluxo de enxofre apresentavam um tamanho de grão médio superior ao observado para as amostras sulfurizadas em caixa de grafite. Adicionalmente, para este último conjunto de amostras, a intensidade da luminescência medida é claramente inferior à obtida para as amostras sulfurizadas em fluxo de enxofre. Por outro lado, o incremento do número de períodos de precursores revelou-se vantajoso tanto do ponto de vista do tamanho de grão como do incremento da razão sinal/ruído da luminescência. A análise estrutural permitiu verificar que a fase de CZTS é dominante em todas as amostras estudadas. Para a amostra com quatro períodos e sulfurizada em fluxo de enxofre, as dependências na potência de excitação e na temperatura permitiram estabelecer um modelo de transições radiativas entre um eletrão na banda de condução e um buraco ligado a um nível aceitador sob a influência de flutuações de potencial na banda de valência. A profundidade das flutuações de potencial na banda de valência foi avaliada, obtendo-se o valor de 104,7 0,4 meV. Foi estimada uma energia de ionização do nível aceitador de 78 3 meV e um valor para a energia de hiato do CZTS a 17 K na gama 1,467-1,507 eV. Os mecanismos de desexcitação não radiativa foram investigados tendo-se estabelecido dois canais envolvendo, um nível discreto ou uma banda. Os resultados deste trabalho revelaram-se importantes no processo de otimização das técnicas de crescimento em filmes finos de CZTS.In this work we study Cu2ZnSnS4 (CZTS) thin films in order to evaluate the influence of the growth parameters on their morphology and structural and optical properties to optimize the application as absorbent layer in solar cells. The number of periods of metallic precursors was changed (1, 2, 4) and the sulphurization was done in a graphite box or under sulphur flux. The studies consisted of morphological, structural and optical analysis based on SEM, EDS, XRD, Raman spectroscopy and photoluminescence. It was found that the samples sulphurized in sulphur flux had an average grain size higher than that observed for the samples sulphurized in the graphite box. Additionally, the luminescence intensity for the last set of samples is clearly lower than the observed for the samples sulphurized in sulphur flux. Moreover, the increment in the number of periods of metallic precursors proved advantageous both from the viewpoint of grain size as the increase of signal/noise ratio of the luminescence. Structural analysis showed that the CZTS phase is dominant in all studied samples. For the sample with four periods and sulphurized on sulphur flux, the dependences of the emission on the excitation power and temperature allowed to establish a model of radiative recombination between an electron in the conduction band and a hole bound to an acceptor level under the influence of potential fluctuations of the valence band. The depth of the potential fluctuations in the valence band was evaluated, obtaining the value of 104,7 0,4 meV. An ionization energy for the acceptor level of 78 3 meV and a band gap at 17 K in the range 1,467-1,507 eV, were estimated. Two nonradiative channels involving, a discrete level or a band, were established. The results of this study have proved relevant in the optimization process of the growth of CZTS thin films

Estudo optoelectrónico de células solares baseadas em filmes finos de calcogenetos

In this thesis, we studied the role of the defects on the optoelectronic properties of two semiconductors with high potential for photovoltaic application, Cu2ZnSnS4 (CZTS) and Cu(In, Ga)Se2 (CIGS). Both materials are highly doped and strongly compensated, and their optoelectronic properties are governed by a highly complex electronic energy levels structure. In order to further understand the impact of these complex structures on the performance of the devices, several studies were carried out using mainly the photoluminescence technique, and complemented with other optical, morphological, structural, and electrical analyses. For CZTS based solar cells, three series of samples that include studies on the impact of: i) the time of maximum temperature of the sulphurization; ii) the sulphurization method; and iii) the postdeposition annealing on the optoelectronic properties of the CZTS layer, were studied. For CIGS based solar cells, three major topics were addressed: i) solar cells with conventional architecture, ii) solar cells that explore new architectures; and iii) theoretical and experimental study of the influence of defects on the devices performance. The influence of fluctuating potential was shown, and fully explain CZTS and CIGS characteristic luminescence. The optical studies carried in CZTS based solar cells, reveal several issues related to nonradiative recombination or recombination involving deep defects that were closely linked to a poor performance of the studied devices. For different series of CIGS based solar cells, an overall correlation of the influence of the fluctuating potentials with the performance of the devices were obtained. The CIGS related optical results revealed two main recombination deexcitation channels compatible with a shallow donors cluster and the VCu acceptor defect. From theoretical and experimental analyses of CIGS based solar cells, we obtained a higher degree of correlation of electrostatic fluctuating potentials with the open circuit voltage losses of the devices, in comparison with bandgap fluctuations. Finally, we demonstrate the influence of fluctuating potentials in CIGS technology at room temperature. In this thesis, we showed that the optoelectronic properties of CZTS and CIGS are consistent with the existence of the fluctuating potentials, being its impact as a limitative factor on the studied solar cells performance significantly different in the two studied technologies. CZTS based solar cells present serious recombination issues that are significantly more critical for the performance of the device, then the existence of fluctuating potentials. For CIGS based solar cells, the influence of the fluctuating potentials and the performance of the devices are remarkably correlated.Nesta tese, foi estudado o papel dos defeitos nas propriedades optoelectrónicas de dois materiais com grande potencial para aplicações fotovoltaicas, Cu2ZnSnS4 (CZTS) e Cu(In, Ga)Se2 (CIGS). Ambos os materiais são fortemente dopados e compensados, sendo que as suas propriedades optoelectrónicas são governadas pelas suas estruturas de níveis eletrónicos complexas. No sentido de melhor compreender o impacto da estrutura eletrónica no desempenho das células solares, diferentes estudos foram realizados utilizando principalmente a técnica de fotoluminescência, complementada com análise morfológica, estrutural e elétrica. Para as células solares baseadas em CZTS, foram estudadas três séries de amostras para as quais o impacto i) do tempo da temperatura máxima de sulfurização, ii) do método de sulfurização, iii) do tratamento térmico após a deposição, foi avaliado nas propriedades optoelectrónicas da camada CZTS. Para o CIGS, três tópicos principais foram abordados, i) células solares com arquitetura convencional, ii) células solares para as quais se exploram novas arquiteturas, iii) influência dos defeitos no desempenho das células solares a partir da comparação de modelos teóricos com resultados experimentais. A influência das flutuações de potencial foi evidenciada, sendo que a luminescência obtida, tanto envolvendo o CZTS como o CIGS, foi completamente explicada a partir de modelos de recombinação que envolvem a presença destas flutuações. Os estudos óticos desenvolvidos no âmbito das células solares de CZTS revelaram um grande impacto de mecanismos não radiativos e de recombinação envolvendo defeitos profundos que se relacionam com um fraco desempenho dos dispositivos estudados. Para diferentes séries de amostras de células solares baseadas em CIGS foi obtida uma correlação entre a influência das flutuações de potencial e desempenho dos dispositivos estudados. Os resultados óticos obtidos para CIGS revelaram dois mecanismos principais de desexcitação dos canais radiativos envolvendo aglomerados de dadores pouco profundos e o defeito aceitador VCu. A partir das análises teórica e experimental de células solares de CIGS, obtevese uma maior correlação entre a influência das flutuações de potencial electroestáticas com as perdas de tensão de circuito aberto, do que aquela observada para as flutuações de hiato. Finalmente, foi demonstrada a influência das flutuações de potencial na tecnologia CIGS à temperatura ambiente. Nesta tese, foi mostrado que as propriedades optoelectrónicas do CZTS e CIGS são consistentes com a existência de flutuações de potencial, sendo que o seu impacto no desempenho das células solares é significativamente diferente em cada uma das tecnologias. Enquanto no CZTS os mecanismos de recombinação aparecem como um problema com um impacto no desempenho das células solares mais significativo que as flutuações de potencial, no CIGS uma correlação entre a influência das flutuações de potencial e o desempenho das células solares é notória.The author acknowledge the financial support of the project UID/CTM/50025/2019, and IF/00133/2015/CP1325/CT0001 from the FCT.Programa Doutoral em Físic

Comparison of the healthcare system of Chile and Brazil: strengths, inefficiencies, and expenditures

Background: Governments in Latin America are constantly facing the problem of managing scarce resources to satisfy alternative needs, such as housing, education, food, and healthcare security. Those needs, combined with increasing crime levels, require financial resources to be solved. Objective: The objective of this review was to characterizar the health system and health expenditure of a large country (Brazil) and a small country (Chile) and identify some of the challenges these two countries face in improving the health services of their population. Methods: A literature review was conducted by searching journals, databases, and other electronic resources to identify articles and research publications describing health systems in Brazil and Chile. Results: The review showed that the economic restriction and the economic cycle have an impact on the funding of the public health system. This result was true for the Brazilian health system after 2016, despite the change to a unique health system one decade earlier. In the case of Chile, there are different positions about which one is the best health system: a dual public and private or just public one. As a result, a referendum on September 4, 2022, of a new constitution, which incorporated a unique health system, was rejected. At the same time, the Government ended the copayment in the public health system in September 2022, excluding illnesses referred to the private sector. Another issue detected was the fragility of the public and private sector coverage due to the lack of funding. Conclusions: The health care system in Chile and Brazil has improved in the last decades. However, the public healthcare systems still need additional funding and efficiency improvement to respond to the growing health requirements needed from the population. © 2022, The Author(s).info:eu-repo/semantics/publishedVersio

Exploiting the Optical Limits of Thin‐Film Solar Cells: A Review on Light Management Strategies in Cu(In,Ga)Se 2

Light management strategies are of utmost importance to allow Cu(In,Ga)Se2 (CIGS) technology market expansion, as it would enable a conversion efficiency boost as well as thinner absorber layers, increasing sustainability and reducing production costs. However, fabrication and architecture constraints hamper the direct transfer of light management architectures from other photovoltaic technologies. The demand for light management in thin and ultrathin CIGS cells is analyzed by a critical description of the optical loss mechanisms in these devices. Three main pathways to tackle the optical losses are identified: front light management architectures that assist for an omnidirectional low reflection; rear architectures that enable an enhanced optical path length; and unconventional spectral conversion strategies for full spectral harvesting. An outlook over the challenges and developments of light management architectures is performed, establishing a research roadmap for future works in light management for CIGS technology. Following the extensive review, it is expected that combining antireflection, light trapping, and conversion mechanisms, a 27% CIGS solar cell can be achieved.Fundação para a Ciência e a Tecnologia (FCT) and Fundo Social Europeu (FSE) are acknowledged through the projects IF/00133/2015, UIDB/ 50025/2020, UIDP/50025/2020, UIDB/04730/2020, UIDP/04730/2020, and DFA/BD/4564/2020. This research was also supported by NovaCell—Development of novel Ultrathin Solar Cell Architectures for low-light, low-cost, and flexible optoelectronic device project (028075) cofunded by FCT and ERDF through COMPETE2020. This research was supported by InovSolarCells–Development of innovative nanostructured dielectric materials for interface passivation in thin-film solar cells project (029696) cofunded by FCT and ERDF through COMPETE2020. The authors acknowledge the financial support of the project Baterias 2030, with the reference POCI-01-0247-FEDER-046109, cofunded by Operational Programme for Competitiveness and Internationalization (COMPETE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF)info:eu-repo/semantics/publishedVersio

Mitigation of ED Patient Boarding: Transferring Admissions from the Center City ED to Methodist

Objectives: Objectively analyze transferred patient transfers cases as far as LOS (length of stay), final diagnosis, and transfer failure. Assess patient satisfaction with the transfer process as means to identify areas for improvement as well as potential patient safety issues