| The key to improving the photovoltaic(PV)conversion efficiency of crystalline silicon cells lies in more efficient absorption of sunlight and enhanced PV effect.The ways to increase the absorbance and the PV effect,and at the same time reduce the production costs have been the focus in the field of crystalline silicon cell.The light absorbing structure has a significant influence on the photoelectric characteristics of solar cells,making the research on the surface of the optical functional texture a hotspot worldwide.Although the light trapping texture etched directly on the surface of crystalline silicon can help increase the absorbance of the cell,it causes new surface defects and then reduces the PV effect.Therefore,the design and preparation of the optical functional textured surface have been one of the difficulties and attracted great attention in the research of high-efficiency crystalline silicon cells.The bottleneck problems existed in the conventional texturing process are analyzed and a light trapping film-smooth surface crystalline silicon laminated light-absorbing structure was proposed in this thesis.Firstly,the mathematical model for the outer-surface reflection loss and the internal-surface refraction loss of light on the triangular pyramidal optical functional textured film was developed,and the base angle of the triangular pyramid of the film was optimized.Taking the processing process into consideration,the structural parameters of the triangular pyramidal textured film were optimized.Secondly,the ultra-precision cutting of the original mold for the regular triangular pyramid of the textured film was studied,the working mold for the inverted triangular pyramid texture was prepared using the precision electroforming technique,and the triangular pyramidal textured film was prepared using the UV imprinting technique.The influence of theprocessing accuracy on the optical properties of the film was analyzed.In addition,by comparing the photoelectric characteristics of the cells we made with those of the textured-surface and the smooth-surface crystalline silicon cells,the performance of the light trapping film-smooth crystalline silicon laminated light-absorbing structure was comprehensively evaluated.The results indicate that most of the light irradiating on the outer surface of the triangular pyramidal textured film enters the film after being refracted multiple times by the sides of the triangular pyramids,and most of the light escaping from the inside of the film returns to the film after being retroreflected by the internal surfaces of the triangular pyramids.The triangular pyramidal texture film was laminated on a smooth-surface crystalline silicon cell,and the lowest reflection loss of this composite structure was 4.6% when the base angle of the triangular pyramid was taken as the optimized value of 56.5o.Considering the preparation process of large-area textured films,the length of the bottom side of the triangular pyramid was determined to be 115?m.The single-point diamond precision planing technique was used to make the original mold(size: 20mm×20mm)of the regular triangular pyramid texture where the roughness Ra of the side surface of the pyramid reached 9.2nm,and the micro-burrs on the bottom of the pyramid were removed by ultrasonic cavitation.The precision electroforming technique was used to reversely replicate the sub-mold of the inverted triangular pyramid texture,where the roughness Ra of the side surface of the pyramid reached 14.5nm.The small-area sub-molds were joined into a working mold(size: 200×200mm)using the seaming method.The inverted triangular pyramid textured film was prepared by UV imprinting technique,where the side-surface roughness Ra of the pyramid was 18.9nm,meeting the requirements of geometric optical specular reflection.After considering the defects of the prepared triangular pyramidal textured film,the optical loss of the light trapping film-smooth crystalline silicon laminatedstructure was calculated to be 5.75%.A uniformity coefficient was proposed for the quantitative characterization of the uniformity of the pyramidal texture on the surface of monocrystalline silicon cell,and was used to optimize the texturing process parameters of the monocrystalline silicon wafer.The maximal PV conversion efficiency of the textured monocrystalline silicon cell was 19.63%.The chemical polishing and the chemical-mechanical polishing(CMP)methods were used to prepare the smooth-surface crystalline silicon cells.The experimental results show that the smooth-surface cells prepared using the CMP method have the best PV effect,and laminating the light trapping film on them leads to a PV conversion efficiency as high as 20.68%,increasing by 1.05% compared with the best textured-surface crystalline silicon cell.The theory of light absorbing structure,which combines the light trapping film with the smooth-surface crystalline silicon cell,and its characteristics presented in this thesis provides a theoretical basis for clarifying the transfer and conversion of solar radiation energy in a laminated structure cell.It can help solve the problems of high reflection loss and low PV effect in the textured-surface crystalline silicon cell,and thus provides important theoretical basis and engineering application guidance for the research and development of the laminated-structure solar cell with high photovoltaic conversion efficiency. |
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