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Estudo de técnicas de preparação de contactos seletivos para células solares de muito alta eficiência usando filmes de SiO2/TiO2
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Resumo(s)
As principais fontes de produção de energia estão, nos dias de hoje, associadas a problemas de impacto ambiental, devido à queima de combustíveis fósseis como o petróleo e o carvão. Como forma de mitigar esses problemas, a exploração de fontes alternativas de energia, como a renovável, aumentou. Entre as várias fontes de energia renovável destaca-se a energia solar devido ao seu fácil acesso, tornando-se assim um meio energético com grande interesse. E, por isso, de modo a obter um aproveitamento máximo da energia solar, têm surgido vários métodos de conversão desta em energia elétrica, através de dispositivos fotovoltaicos. A passivação das superfícies das células de silício é importante para se obter elevada conversão de energia, permitindo reduzir a recombinação dos portadores de carga minoritários. A técnica mais utilizada para passivação de células solares é o crescimento de dióxido de silício (𝑆𝑖𝑂2) através de vários tipos de oxidação: térmica e química. Passivaram-se amostras através destes dois tipos de oxidação variando-se a temperatura (𝑇𝑜𝑥𝑖) e o tempo (𝑡𝑜𝑥𝑖) de oxidação de crescimento das camadas de 𝑆𝑖𝑂2 de forma a determinar o impacto nos tempos de vida, e por sua vez na recombinação. Variou-se a 𝑇𝑜𝑥𝑖, para o processo térmico de 800ºC a 900ºC e o 𝑡𝑜𝑥𝑖 de 60min a 120min. Para o processo de oxidação química variou-se a 𝑇𝑜𝑥𝑖 entre 25ºC (temperatura ambiente) e 80ºC e o 𝑡𝑜𝑥𝑖 de 30min a 180min. Estas variações permitem traçar um perfil aperfeiçoado da qualidade de passivação, através das medidas resultantes dos tempos de vida. Para ambos os processos de oxidação, observa-se que com o aumento da temperatura e do tempo de oxidação atingem-se melhores qualidades de passivação. De modo a acrescer o desempenho da célula, utilizam-se contactos seletivos, optando-se pelo dióxido de titânio (𝑇𝑖𝑂2), com base em estudos anteriores, devido aos desvios otimizados das suas bandas de condução e valência. Os melhores resultados antes e após a deposição da camada seletiva em ambas as faces da amostra foram os da passivação por oxidação térmica, obtendo-se valores de tempo de vida de 101,75μs.
The main sources of energy production are, today, associated with problems of environmental impact, due to the burning of fossil fuels such as oil and coal. As a way of mitigating these problems, the exploration of alternative energy sources, such as renewable, has increased. Among the various sources of renewable energy, solar energy stands out due to its easy access, thus becoming an mean of energy with great interest. And, therefore, in order to obtain a maximum use of solar energy, several methods of converting it into electrical energy, through photovoltaic devices have emerged. The passivation of the surfaces of the silicon cells is important to obtain high energy conversion, allowing to reduce the recombination of minority charge carriers. The most used technique for passivating solar cells is the growth of silicon dioxide (𝑆𝑖𝑂2) through various types of oxidation: thermal and chemical. Samples were passivated through these two types of oxidation by varying the temperature (𝑇𝑜𝑥𝑖) and the time (𝑡𝑜𝑥𝑖) of oxidation for the growth of the 𝑆𝑖𝑂2 layers in order to determine the impact on the lifetime and, thus, recombination. The 𝑇𝑜𝑥𝑖 was varied for the thermal process from 800ºC to 900ºC and the 𝑡𝑜𝑥𝑖 from 60min to 120min. For the chemical oxidation process, the 𝑇𝑜𝑥𝑖 varied between 25ºC (room temperature) and 80ºC and the 𝑡𝑜𝑥𝑖 from 30min to 180min. These variations make it possible to draw an improved profile of the passivation quality, through the measures resulting from the lifetime. For both oxidation processes, it is observed that with the increase in temperature and oxidation time, better passivation qualities are achieved. In order to increase the performance of the cell, selective contacts are used, opting for titanium dioxide (𝑇𝑖𝑂2), based on previous studies, due to the optimized deviations of its conduction and valence bands. The best results before and after deposition of the selective layer on both sides of the sample were those of passivation by thermal oxidation, obtaining lifetime values of 101.75μs.
The main sources of energy production are, today, associated with problems of environmental impact, due to the burning of fossil fuels such as oil and coal. As a way of mitigating these problems, the exploration of alternative energy sources, such as renewable, has increased. Among the various sources of renewable energy, solar energy stands out due to its easy access, thus becoming an mean of energy with great interest. And, therefore, in order to obtain a maximum use of solar energy, several methods of converting it into electrical energy, through photovoltaic devices have emerged. The passivation of the surfaces of the silicon cells is important to obtain high energy conversion, allowing to reduce the recombination of minority charge carriers. The most used technique for passivating solar cells is the growth of silicon dioxide (𝑆𝑖𝑂2) through various types of oxidation: thermal and chemical. Samples were passivated through these two types of oxidation by varying the temperature (𝑇𝑜𝑥𝑖) and the time (𝑡𝑜𝑥𝑖) of oxidation for the growth of the 𝑆𝑖𝑂2 layers in order to determine the impact on the lifetime and, thus, recombination. The 𝑇𝑜𝑥𝑖 was varied for the thermal process from 800ºC to 900ºC and the 𝑡𝑜𝑥𝑖 from 60min to 120min. For the chemical oxidation process, the 𝑇𝑜𝑥𝑖 varied between 25ºC (room temperature) and 80ºC and the 𝑡𝑜𝑥𝑖 from 30min to 180min. These variations make it possible to draw an improved profile of the passivation quality, through the measures resulting from the lifetime. For both oxidation processes, it is observed that with the increase in temperature and oxidation time, better passivation qualities are achieved. In order to increase the performance of the cell, selective contacts are used, opting for titanium dioxide (𝑇𝑖𝑂2), based on previous studies, due to the optimized deviations of its conduction and valence bands. The best results before and after deposition of the selective layer on both sides of the sample were those of passivation by thermal oxidation, obtaining lifetime values of 101.75μs.
Descrição
Tese de mestrado integrado, Engenharia da Energia e Ambiente, Universidade de Lisboa, Faculdade de Ciências, 2020
Palavras-chave
Célula Solar Tempo de vida μW-PCD QSSPC Silício Dióxido de titânio Contactos seletivos Teses de mestrado - 2020