Fabrication And Optical Performance Of Structured Ultrathin Crystalline Silicon Over Large Areas

Since silicon absorber layer of ultrathin silicon-based photovoltaic device is too thin to absorb enough solar photon, photoelectric conversion efficiency of the device is very low. Therefore structuring silicon absorber layer is adopted to extend the optical path within the layer, increasing its light trapping. In this paper, ultraviolet (UV) nanoimprint lithograph, wet etching and dry etching are applied to realize the size control of structure on silicon with large area. The light absorption is enhanced in structured silicon within a wide wavelength range.Firstly, the method to fabricate ultrathin single crystalline silicon on large area is studied in this paper. Alkaline corrosion is applied to fabricate the ultrathin silicon from thick silicon. We explore a method to immerse the thick silicon completely into the corrosive liquid, ensuring that the thickness of silicon is uniform. The obtained silicon has an area of 3 ×3 cm2 and a thickness of 20 μm.Secondly, we study the ultraviolet nanoimprint lithograph that is applied to ultrathin silicon. The etching mask layer is made by ultraviolet nanoimprinting method. We explore the spin-coating regularity of the resist on the ultrathin silicon and the appropriate process parameters of fabricating different patterns by UV nanoimprint (such as grating,001 resist, optimum rotational speed:5500 rpm).Thirdly, we respectively apply wet etching and dry etching after UV nanoimprint to fabricate structures on one side of ultrathin silicon, transfering the pattern on resist to silicon substrate. On one hand, it is difficult to obtain periodic structures after wet etching. On the other hand, periodic structures can be obtained when combining the UV nanoimprint and dry etching. we study the influence of etching gas and etching time on the same imprinted structure. The vertical structure is obtained when the gas flow ratio is CF4:Ar=50:12 sccm; the cone shape is obtained when the gas flow ratio is CF4:O2=50:6 sccm; the hyperbolic shape is obtained when the gas flow ratio is SF6:O2=15:5 sccm. The three mentioned shape become obvious as the etching time increases, while the increased power mainly improve the etching rate.Lastly, on the basis of one-side structured ultrathin silicon, the method to fabricate double-side structured ultrathin silicon is studied and its optical properties were measured. The result indicates that the fabrication process of one-side structured ultrathin silicon is also suitable for double-side structured ultrathin silicon. The absorptivity of double-side structured ultrathin silicon is about 15% higher than one-side structured silicon. Meanwhile, finite difference time domain (FDTD) simulation is applied and it also proves that double-side structured ultrathin silicon has better light absorption.