Células solares bifaciais em lâminas finas de silício tipo N : otimização de processos de fabricação

Abstract

The challenges for the next generations of silicon solar cells are the reduction of the wafer thickness and the use of n-type wafers was well as the albedo with bifacial devices. The goal of this work was to develop and analyze 100 µm thick bifacial n-type solar cells with n+np+ structure. The firing temperature of the screen printing pastes, the contact coverage factor of the p+ face, the phosphorus diffusion and the thermal oxidation process were evaluated. Among the firing temperatures tested, the temperature of 840 °C allowed the fabrication of the devices with higher efficiency. The metallic coverage reduction of the p+ face combined with the use of an anti-reflection coating resulted in an absolute increase of 2 % in the efficiency of the devices when they were illuminated by the boron rear emitter. The SiO2 layer obtained by thermal process at 800 °C and during 45 min produced the most efficient devices. For phosphorus diffusion, it was found that at the temperature of 855 °C and diffusion times of 30 min and 50 min were those that produced the devices with higher efficiency: 16.0 % for illumination by n + region and 15.6 % for illumination by the p + region and 16.1 % in the n+ face and 15.4 % in the p+ face, for diffusion times of 30 min and 50 min, respectively. From the analyses of the internal quantum efficiency of the solar cells, it was found that the SiO2 layer passivated the n+ region more effectively than the p+ region and that the surface texture combined with SiO2 and TiO2 films provided an effective light trapping. The solar cells develop in this work presented a high bifaciality, reaching 0.96. Light-induced degradation in the solar cell after 5.5 hours of exposure to solar radiation was not observed. Concerning the silicon amount to produce each watt, the developed thin n+np+ devices presented a value of 1.5 g/W, about 40 % lower than the obtained in standard thick solar cells processed with the same fabrication sequence.