Otimização das regiões altamente dopadas de células solares bifaciais base n e análise da passivação

Abstract

Bifacial silicon solar cells are designed to converter solar energy reaching on both faces. The n-type Si wafers have a higher tolerance to unwanted impurities, presenting a longer lifetime of the minority charge carriers, and do not present degradation when exposed to solar radiation due to the absence of boro-oxygen complexes. The objective of this work was to optimize experimentally the emitter and back surface field of n-type bifacial solar cells with p+nn+ structure, as well as to evaluate the passivation of the surfaces with SiO2. The study was based on previous work focused on monofacial p-type solar cells with a selective boron and aluminum back surface field and patent BR 10 2012 030 606 9, with boron diffusion and oxidation in the same thermal step. Solar grade Si-Cz wafers were used and the boron diffusion temperature (TB), which forms the emitter, was ranged from 960 ºC to 1,000 ºC. The phosphorus diffusion temperature (TP), which forms the back surface field (BSF), was ranged from 825 ºC to 855 ºC. The sheet resistance, the electrical characteristics, the reflectance and the internal quantum efficiency of the bifacial solar cells were evaluated. The passivation with SiO2, grown in dry oxidation on both faces and with different thicknesses was evaluated and compared. Lower values of fill factor were obtained in the emitter, independent of TP, from solar cells processed in TB of 960 ºC and 970 ºC. With the increase of TB at 980 ºC, TP = 845 ºC and TOXID = 860 ºC, the efficiency of 15.9 % and 16 % were obtained, respectively, in the emitter and in the BSF. The main result was the development of n-type bifacial solar cells with industrial process and efficiency of 15.5% and 16.6%, respectively, in the emitter and back surface field. This result was obtained with TB = 990 ºC, TP = 835 ºC and TOXID = 800 ºC. A thinner Si oxide did not result in quality passivation and passivation with a SiO2 layer in the range of 50-60 nm in the BSF resulted in an increase of 0.6% and 0.9% (absolute) in the efficiency when the device is illuminated on the back surface field and emitter, respectively.