Multicrystalline Silicon Ribbons Grown Over a Dust Substrate

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

Temperature coefficients and crystal defects in multicrystalline silicon solar cells

The paper VIII is not published yet.The conversion efficiency of a photovoltaic device is strongly dependent on the operating temperature. For most devices, the efficiency, and hence the power production, decreases with increasing temperature due to fundamental, material, and process-related factors. Therefore, understanding the thermal behavior of photovoltaic devices is essential to accurately forecast the power production of photovoltaic installations and to optimize devices for different climatic conditions. The thermal behavior of crystalline silicon-based devices is of special interest because of the importance of the technology for industrial applications. This thesis expands the knowledge about temperature dependent performance by investigating how crystal defects influence the thermal behavior of multicrystalline silicon solar cells. Two parameters are given special attention: The temperature coefficient of the open-circuit voltage, which provides information about the temperature sensitivity of the device performance, and the so-called recombination parameter, containing information about the underlying physical mechanisms. In this thesis, temperature dependent performance is studied locally across multicrystalline silicon wafers and solar cells. The temperature sensitivity of grain boundaries, dislocations, and intra-grain regions is investigated at various processing steps, using a novel temperature dependent photoluminescence imaging tool developed during the PhD project. Significant variations in temperature sensitivity is observed for the various crystal defects. Dislocation clusters exhibit an especially interesting thermal behavior, which is discussed in detail. Brick position is found to significantly affect the average temperature sensitivity of wafers and cells, with reduced temperature sensitivity generally observed towards the top of the brick. This is found to arise mainly from the presence of dislocation clusters, because of associated low values, and a typically increasing density towards the top of a multicrystalline silicon brick. Finally, the influence of impurity atoms is investigated using a temperature and injection dependent numerical model, relating the recombination parameter to impurity recombination in crystalline silicon. The model is used to predict for various impurity atoms. Additionally, the temperature coefficient of the open-circuit voltage is predicted without a temperature dependent measurement, enabling more accurate temperature coefficient modeling.publishedVersio

Proceedings of the 25th Project Integration Meeting

Topics addressed include: silicon sheet growth and characterization, silicon material, process development, high-efficiency cells, environmental isolation, engineering sciences, and reliability physics

Gallium Phosphide Integrated with Silicon Heterojunction Solar Cells

abstract: It has been a long-standing goal to epitaxially integrate III-V alloys with Si substrates which can enable low-cost microelectronic and optoelectronic systems. Among the III-V alloys, gallium phosphide (GaP) is a strong candidate, especially for solar cells applications. Gallium phosphide with small lattice mismatch (~0.4%) to Si enables coherent/pseudomorphic epitaxial growth with little crystalline defect creation. The band offset between Si and GaP suggests that GaP can function as an electron-selective contact, and it has been theoretically shown that GaP/Si integrated solar cells have the potential to overcome the limitations of common a-Si based heterojunction (SHJ) solar cells. Despite the promising potential of GaP/Si heterojunction solar cells, there are two main obstacles to realize high performance photovoltaic devices from this structure. First, the growth of the polar material (GaP) on the non-polar material (Si) is a challenge in how to suppress the formation of structural defects, such as anti-phase domains (APD). Further, it is widely observed that the minority-carrier lifetime of the Si substrates is significantly decreased during epitaxially growth of GaP on Si. In this dissertation, two different GaP growth methods were compared and analyzed, including migration-enhanced epitaxy (MEE) and traditional molecular beam epitaxy (MBE). High quality GaP can be realized on precisely oriented (001) Si substrates by MBE growth, and the investigation of structural defect creation in the GaP/Si epitaxial structures was conducted using high resolution X-ray diffraction (HRXRD) and high resolution transmission electron microscopy (HRTEM). The mechanisms responsible for lifetime degradation were further investigated, and it was found that external fast diffusors are the origin for the degradation. Two practical approaches including the use of both a SiNx diffusion barrier layer and P-diffused layers, to suppress the Si minority-carrier lifetime degradation during GaP epitaxial growth on Si by MBE were proposed. To achieve high performance of GaP/Si solar cells, different GaP/Si structures were designed, fabricated and compared, including GaP as a hetero-emitter, GaP as a heterojunction on the rear side, inserting passivation membrane layers at the GaP/Si interface, and GaP/wet-oxide functioning as a passivation contact. A designed of a-Si free carrier-selective contact MoOx/Si/GaP solar cells demonstrated 14.1% power conversion efficiency.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201