Desenvolvimento de células solares bifaciais PERT Base n : análise da difusão com redução de etapas térmicas e da passivação com óxido de silício crescido em diferentes condições

Abstract

N-type solar cell technology is growing in the industry as well as the PERT bifacial structure, which enables the increase of power output. The objective of this work was to develop n-type bifacial solar cells with PERT structure, produced with boron and phosphorus diffusion in the same thermal step and with passivation using silicon oxide. The innovation lies on the diffusion of both dopants in the same thermal step, based on patent application BR1020180085760, and the analysis of the passivation with silicon oxide grown under different conditions. Si-Cz and Si-FZ wafers were used and the methodology can be summarized as: 1) to analyze the influence of boron diffusion temperature (from 940 °C to 980 °C) on the sheet resistance of the p+ emitter and n+ back surface field (BSF), the electrical parameters of bifacial solar cells, the bifaciality, the quantum efficiency and reflectance; and 2) to evaluate the thickness and passivation of the silicon oxide layer on the emitter and BSF, grown with reduced oxygen flow and in the presence of nitrogen. This involved estimating the layer thickness with ellipsometry method and measuring the minority carrier lifetime. We found that boron diffusion temperature (TB) affects the BSF sheet resistance, which increases with TB value. The temperatures of 960 °C and 950 °C for boron diffusion resulted in the highest power output in bifacial mode for solar cells produced on Si-Cz and Si-FZ wafers, respectively. The bifaciality reached the value of 0.99 for both substrate types. Due to the higher efficiency of Si-Cz solar cells, the power output in bifacial mode was 1.14 W, while for the Si-FZ device, it was 1.08 W. We also observed that the thickness of the silicon oxide layer was influenced by the boron diffusion temperature and affected the reflectance. In the back surface field, the reflectance at wavelengths near 350 nm decreased with increasing TB, regardless of the substrate type. The internal quantum efficiency in the boron emitter was lower than in the BSF for wavelengths below 400 nm due to increased recombination of minority carriers in this region. We concluded that the silicon oxide thickness was higher on the phosphorus-doped face, ranging from 38-40 nm, compared to the estimated value on the emitter side, which was 12.5-13.5 nm. It was also concluded that the addition of nitrogen tends to decrease the silicon oxide layer on both sides, and the reduction of O2 flow practically did not affect the thickness of the passivation layer. The growth of silicon oxide with the presence of nitrogen improved the substrate quality and provided passivation similar to what was found with the standard oxygen flow process. The results indicate that the boron and phosphorus diffusion process in the same thermal step is effective and enables the production of n-type bifacial solar cells with high bifaciality and minority carrier lifetime in the base