Texturization processes of monocrystalline silicon with Na2CO3/NaHCO3 solutions for solar cells

Abstract

A novel Na2CO3/NaHCO3 texturization process for the surface structuring of the monocrystalline silicon substrate, aimed to improve the silicon optical characteristics to be used in solar cells, has been proposed in this thesis. The proposed process consists in the wet chemical anisotropic texturization of (100) oriented silicon with a new solution which has been designated as ultra-low concentration Na2CO3/NaHCO3 solution. The alkaline solution forms micrometric pyramids on the (100) silicon surface. This process presents several operation advantages: (i) no additives are required, contrarily to NaOH or KOH based processes; (ii) the 20 times lower concentration of chemicals compared to previously established high concentration Na2CO3/NaHCO3 processes allows saving costs and simplifies a possible implementation in the industry; (iii) the process temperature at semi-industrial scale is lower than the temperature of Na2CO3/NaHCO3 texturization processes found in the literature; (iv) the texturing solution can be reused several times; and (v) it can be applied to (100) silicon substrates with different characteristics. The resulting morphology obtained with the ultra-low concentration Na2CO3/NaHCO3 solution presents low values of standard weighted reflectance and smaller and more sharpened pyramids than the high concentration texturization process. Furthermore, it produces smoother facets of the pyramids, which is advantageous regarding to the mitigation of the surface recombination velocity. Different influencing parameters of the process, as the process temperature or the initial roughness of the substrate, have been studied. The ultra-low concentration Na2CO3/NaHCO3 solution has been used to study the mechanism of pyramid formation on (100) silicon by means of AFM and SEM. The proposed ultra-low concentration Na2CO3/NaHCO3 solution has been applied to a solar cell process as texturization step. The influence of the Na2CO3/NaHCO3 texturing on solar cell processing steps, as formation of phosphorous doped emitter layer, passivation, and deposition of thin layers, has been studied. The comparison of p-Cz phosphorous diffused and screen printed silicon solar cells produced with the high concentration and the new ultra-low concentration Na2CO3/NaHCO3 solutions demonstrate enhanced solar cell electrical parameters for the ultra-low Na2CO3/NaHCO3 concentration process. This is related to a lower front reflection and enhanced light trapping, and to the smaller size of the pyramids, which benefit cell processing, and probably to the lower facets nanoroughness, compared to the high concentration Na2CO3/NaHCO3 textured surface. The here proposed texturization method is a promising candidate to be implemented in the industry.