| Crystalline silicon(c-Si)solar cell is the most commonly used solar cell in recent years due to the dramatical advantages of high conversion efficiency,stable cell performance and mature production technologies.Texturization,which means to prepare specific anti-reflection morphologies on the surface of Si wafers,has been considered as one of the most important methods to reduce the optical losses of c-Si solar cells.Compared with the conventional texturization methods,such as acid etching,alkaline etching and reactive ion etching(RIE),metal-assisted chemical etching(MACE)can be utilized to fabricate various surface structures through flexible ways with low costs,making it a promising method for texturization of c-Si solar cells.In order to make full use of MACE method in the preparation processes of c-Si solar cells,the copper/silver-assisted chemical etching(Cu/Ag-ACE)method and copper-assisted chemical etching(Cu-ACE)method have been studied in this paper.In this research,a series of etching experiments was performed,and diverse surface structures with different characteristics were obtained.The reaction processes and etching mechanisms of these metal-catalyzed etching methods were analyzed as well.Relevant anisotropic etching theories were improved in this research.Based on the etching experiments,the textured surface morphologies on Si surface were effectively regulated,and the superiorities of these two texturing methods were highlighted.Furthermore,the outstanding omnidirectional anti-reflection advantages of the V-groove structure prepared through Cu-ACE was verified,which would contribute to the application of this metal catalyzed etching process.The research contents and results in this paper can be summarized as follows:Firstly,the Cu/Ag-ACE method was studied in this paper,and a submicron helical porous structure with excellent anti-reflection property on monocrystalline Si(mc-Si)wafer was fabricated through this etching process.The formation mechanism of this novel structure was investigated.According to the analyses,the main formation cause for the submicron helical porous structure was proved to be the rotation of the deposited metal nanoparticles(NPs)during etching.The rotation of the metal NPs can be attributed to the different etching rates at the interface between metal NPs and Si,which resulted from the irregular agglomerations of metal NPs.To effectively regulate the sizes and distributions of the helical pores through Cu/Ag-ACE,extensive etching experiments with different component concentrations in etchant solutions were performed,and thus the influences of the changing component concentrations on the experimental results were studied.In addition,a new texturization process was put forward by combining the Cu/Ag-ACE with a subsequent alkaline treatment,and a submicron inverted pyramid(IP)structure with an average reflectance of 9.10%in the wavelength range of 300-1000 nm was fabricated.By comparing the output performances of the quasi-monocrystalline Si(qc-Si)solar cells with IP structure and random upright pyramid(UP)structure,the superiorities of the texturization process consisted of Cu/Ag-ACE and alkaline post-treatment were confirmed,indicating great potentials for the Cu/Ag-ACE method to be applied in practical.To further reduce the costs of texturization process,we have also studied the Cu-ACE method,and the nanoporous structure,micro IP structure,V-groove structure,UP structure and hybrid structures were fabricated on the diamond wire sawn(DWS)mc-Si wafers.The etching mechanisms,the relationship between different textured surface morphologies,and the deposition behaviors of the Cu-NPs on Si surface were discussed.In this research,the Cu-NPs were observed to be selectively deposited in the micro cracks and deep grooves which were formed on Si surface in the wafer sawing processes,since the defects were densely distributed in these regions.Considering that the micro cracks and deep grooves were all parallel to the wafer slicing direction,the Cu-NPs gradually deposited and agglomerated in lines along the wafer slicing direction,resulting in the different etching rates on surface of DWS mc-Si wafers and the ultimate formation of V-groove structure.Meanwhile,the amorphous Si(a-Si)distributed on the surface of DWS mc-Si wafers was demonstrated to aggravate the differences in etching rate,thereby accelerating the formation of the V-groove structure.Additionally,the anisotropic etching theory of Cu-ACE was further improved as the textured structures terminated with Si {111} and {110} crystalline planes were fabricated using this Cu-assisted anisotropic etching method.Eventually,the transformation of different textured structures was studied,and the texturization processes as well as the anti-reflection performances of c-Si solar cells were effectively regulated through Cu-ACE.The omnidirectional anti-reflection property of the V-groove structure prepared through Cu-ACE has been investigated through the simulations and experiments.Based on the ray tracing simulation results,the deflection angle of incident light was proved as an important factor for the omnidirectionality of the V-groove structure;When the deflection angle was 0°,the reflectance of the V-groove structure declined at first and then rose with the increasing incident angle,but the reflectance didn't exceed 15%in the angle of incidence(AOI)ranging from 0° to 80°.We measured the reflectance of the V-groove structure under different incident angles in the wavelength range of 300-1000 nm.The experimental results showed that the average reflectance of the V-groove structure was slightly higher than the random UP structure when the AOI was in the range of 0-20°.But when the AOI was greater than 20°,the V-groove structure showed a significantly lower reflectance than the random UP structure.As a consequence,the V-groove textured structure fabricated through the Cu-ACE exhibited an excellent broadband omnidirectional anti-reflection property,which indicated great potentials for the fabrication of omnidirectional solar cells and photovoltaic modules.Moreover,a new quantitative calculation method was proposed in this chapter to intuitively evaluate the omnidirectional anti-reflection performances of samples and structures in specific ranges of wavelength and AOI.In summary,we have effectively regulated the etching processes of Cu/Ag-ACE and Cu-ACE,and the prominent omnidirectional anti-reflection advantages of V-groove structure prepared through the Cu-ACE have been confirmed based on the simulations and experiments represented in this paper,providing practical references and theoretical basis for these two texturization methods to be applied in fabrication of c-Si solar cells and photovoltaic modules with high conversion efficiencies or outstanding omnidirectional anti-reflection properties. |
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