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Multicrystalline Silicon Ribbons Grown Over a Dust Substrate
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O Silício sobre Substrato de Pó (SDS da sigla em inglês) é um processo desenvolvido para fabricar fitas de silício multicristalino directamente a partir de uma fonte gasosa (silano), evitando as etapas industriais de deposição de poli-silício, crescimento de cristal e corte em bolachas. Este processo tem por objectivo alcançar um material com boa qualidade cristalográfica para o fabrico de células solares, aliado a uma expressiva redução do custo global dos sistemas fotovoltaicos.
O foco do trabalho apresentado nesta tese é o aperfeiçoamento de toda a técnica SDS, a qual consiste em três passos principais: (i) produção de pó de silício; (ii) deposição química em fase de vapor (CVD da sigla em inglês) de silício sobre um substrato de pó de silício; e (iii) recristalização por zona fundida flutuante (ZMR da sigla em inglês) da fita microcristalina obtida no passo de CVD. Adicionalmente, foram identificadas as melhores práticas e parâmetros experimentais ideais para os três passos, que possibilitam obter fitas de silício multicristalino de melhor qualidade.
Um novo sistema experimental para a produção de pó de silício com granulometria micrométrica a partir de bolachas de silício multicristalino foi testado, caracterizado e usado na produção de seis pós de silício com intervalos bem definidos de dimensão de partículas, variando entre ≤25 e ]180; 250] μm.
A dimensão das partículas, massa por unidade de área e porosidade são propriedades do substrato de pó que têm uma importante influência no sucesso do processo de CVD e nas propriedades físicas da pré-fita de silício microcristalino crescida sobre o substrato de pó, tais como rácio de pó, taxa de crescimento e porosidade. Foi demonstrado que à medida que a dimensão das partículas do substrato de pó diminui, a taxa de crescimento por CVD aumenta (até 52.8 μm/min) e ambos os valores de porosidade e rácio de pó da pré-fita diminuem (até 52.7 ± 7.3% e 0.60 ± 0.01, respectivamente).
Consequentemente, o êxito do processo ZMR é fortemente afectado pelas características da pré-fita, de tal modo que o material cristalizado de melhor qualidade foi obtido a partir de pré-fitas crescidas sobre substratos de pó com partículas de menor dimensão (≤75 μm), as quais também têm menor porosidade e incorporação de pó do substrato. Foram produzidas fitas de silício multicristalino com sucesso, tendo-se obtido largas áreas cristalinas, medindo aproximadamente 2×4 cm2, com crescimento cristalino colunar e com uma dimensão média do cristal no intervalo de 1 a 10 mm. O valor de resistividade obtido foi 0.70 ± 0.05 Ω.cm, equivalente a uma concentração de dopante de 2.1×1016 cm-3 e o valor obtido para o tempo de vida de portadores minoritários foi de 0.3 ± 0.1 μs.
Foi demonstrada a capacidade de produção de fitas de silício multicristalino, por CVD sobre um substrato de pó, previamente obtido a partir da moagem de pedaços silício, seguido de um passo de recristalização por zona fundida.
The Silicon on Dust Substrate (SDS) is a gas-to-wafer process, developed to manufacture multicrystalline silicon ribbons directly from gaseous feedstock (silane), avoiding the standard industry stages of polysilicon deposition, crystal growth and wafering. It aims to achieve good quality material for solar cell manufacturing with a significant reduction of the overall photovoltaic systems cost. The focus of the work presented in this thesis is the improvement of the entire SDS technique, which consists of three main steps: (i) production of silicon powder; (ii) chemical vapour deposition (CVD) of silicon over a silicon powder substrate; and (iii) zone melting recrystallization (ZMR) of the microcrystalline pre-ribbon obtained in the CVD step. Additionally, the best practices and optimal experimental parameters across the three steps were identified. A new experimental setup to produce micrometric sized silicon powders from multicrystalline silicon wafers was tested, characterized and used to manufacture six silicon powders of well-defined particle size intervals, ranging from ≤25 to ]180; 250] μm. The powder substrate properties, such as particle size, mass per unit of area and porosity, have a preponderant influence on the success of the CVD process and the physical characteristics, like powder ratio, growth rate and porosity, of the microcrystalline pre-ribbon grown over the powder substrate. It was demonstrated that as the powder substrate particle size decreases, the CVD growth rate increases (up to 52.8 μm/min) and both pre-ribbon porosity and powder ratio decreases (down to 52.7 ± 7.3% and 0.60 ± 0.01, respectively). The ZMR process performance is substantially impacted by the pre-ribbon physical characteristics, as the best crystallized material was obtained from pre-ribbons grown over powder substrates with smaller particle size (≤75 μm), which also have a lower porosity and powder incorporation from the substrate. Multicrystalline silicon ribbons were successfully produced, having large crystalline areas measuring approximately 2×4 cm2, with visible columnar crystal growth and an average crystal size in the 1 to 10 mm range. The measured resistivity was 0.70 ± 0.05 Ω.cm, equivalent to a dopant concentration of 2.1×1016 cm-3 and a measured minority carrier lifetime of 0.3 ± 0.1 μs. The ability to produce multicrystalline silicon ribbons by CVD over a powder substrate, previously obtained from grinding small silicon chunks, followed by a recrystallization step with a linear molten zone was demonstrated.
The Silicon on Dust Substrate (SDS) is a gas-to-wafer process, developed to manufacture multicrystalline silicon ribbons directly from gaseous feedstock (silane), avoiding the standard industry stages of polysilicon deposition, crystal growth and wafering. It aims to achieve good quality material for solar cell manufacturing with a significant reduction of the overall photovoltaic systems cost. The focus of the work presented in this thesis is the improvement of the entire SDS technique, which consists of three main steps: (i) production of silicon powder; (ii) chemical vapour deposition (CVD) of silicon over a silicon powder substrate; and (iii) zone melting recrystallization (ZMR) of the microcrystalline pre-ribbon obtained in the CVD step. Additionally, the best practices and optimal experimental parameters across the three steps were identified. A new experimental setup to produce micrometric sized silicon powders from multicrystalline silicon wafers was tested, characterized and used to manufacture six silicon powders of well-defined particle size intervals, ranging from ≤25 to ]180; 250] μm. The powder substrate properties, such as particle size, mass per unit of area and porosity, have a preponderant influence on the success of the CVD process and the physical characteristics, like powder ratio, growth rate and porosity, of the microcrystalline pre-ribbon grown over the powder substrate. It was demonstrated that as the powder substrate particle size decreases, the CVD growth rate increases (up to 52.8 μm/min) and both pre-ribbon porosity and powder ratio decreases (down to 52.7 ± 7.3% and 0.60 ± 0.01, respectively). The ZMR process performance is substantially impacted by the pre-ribbon physical characteristics, as the best crystallized material was obtained from pre-ribbons grown over powder substrates with smaller particle size (≤75 μm), which also have a lower porosity and powder incorporation from the substrate. Multicrystalline silicon ribbons were successfully produced, having large crystalline areas measuring approximately 2×4 cm2, with visible columnar crystal growth and an average crystal size in the 1 to 10 mm range. The measured resistivity was 0.70 ± 0.05 Ω.cm, equivalent to a dopant concentration of 2.1×1016 cm-3 and a measured minority carrier lifetime of 0.3 ± 0.1 μs. The ability to produce multicrystalline silicon ribbons by CVD over a powder substrate, previously obtained from grinding small silicon chunks, followed by a recrystallization step with a linear molten zone was demonstrated.
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Fotovoltaico Pó de Silício Matéria-prima Fitas de Silício Deposição Química em Fase de Vapor Recristalização por Zona Fundida Photovoltaic Silicon Powder Silicon Feedstock Silicon Ribbons Chemical Vapour Deposition Zone Melting Recrystallization