Otimização e comparação de processos para formação do campo retrodifusor com boro em células solares

AUTOR(ES)
FONTE

IBICT - Instituto Brasileiro de Informação em Ciência e Tecnologia

DATA DE PUBLICAÇÃO

27/03/2012

RESUMO

The aim of this thesis was to develop and compare methods for the formation of the back surface field and emitter with boron using n-type and p-type solar-grade Si-Cz wafers as well as analyze and optimize a manufacturing process of solar cells with structure n+pp+ and back surface field formed by boron. The highly doped p+ region was formed by spin-on deposition of the liquid dopants PBF15 and PBF20 and diffusion in standard quartz tube furnace. It was found that with diffusion at 900 C, it is possible to obtain the suitable sheet resistance to form the emitter. However, to form the back surface field, the boron diffusion should be implemented at 1000 C. For both temperatures, the sheet resistance obtained in n-type substrate is higher than that measured in p-type wafers, independent of diffusion time. Boron diffusion, in general, increases the minority carrier lifetime (τ) in p-type wafers and it decreases in n-type substrates. However, the average value after the boron diffusion is high in n-type samples, because the initial value of τ in this type of substrate is approximately 150 % higher than in p-type wafers. The results are similar for both dopants evaluated and they are slightly better for the dopant PBF20. The best bulk minority carrier lifetime after the boron diffusion occurred to the temperature of 1000 C and diffusion time of 20 minutes for both types of substrates. In the optimization of the fabrication process, it was found that the type of gas at the entrance of the wafers in the quartz tube to phosphorus diffusion slightly influences the efficiency of solar cells and the best results were found for the oxidation times of 30 minutes and 40 minutes.The firing temperature of the metallization pastes of 830 C gave the highest efficiency of 13,7 % for the belt speed of 200 cm/min, due to the increased fill factor. The better efficiency was obtained to phosphorus diffusion temperature of 920 C. The efficiency obtained with the dopant PBF25 is slightly higher than that obtained with dopant PBF20 due to the small difference in fill factor and short-circuit current density. The greater efficiency of 14.1% was obtained with this dopant. The reduction in the percentage area of the metal grid on the back side of 52.5 % to 9.4 %, slightly affects the performance of solar cells. With the increase of the solar cell area from 4 cm2 to 61.58 cm2, the efficiency decreased from 14.2 % to 13.0 % due to the reduction of the short-circuit current density. Solar cells with back surface field formed by aluminum efficiency reached 15.4 % and devices presented higher shortcircuit current density. The developed cells with back surface field formed by boron diffusion presented higher open circuit voltage, thus demonstrating that cells with p+ back surface field formed with boron was more effective.

ASSUNTO(S)

engenharia de materiais cÉlulas solares energia solar sistemas fotovoltaicos engenharias

ACESSO AO ARTIGO

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