Novel approaches for tri-crystalline silicon surface texturing

Tri-crystalline silicon (Tri-Si) is a promising candidate to reduce the cost of solar cells fabrication because it can be made by a low-cost, fast process with a better mechanical strength, and needs a thinner wafer. One of the key parameters in improving the efficiency of the Tri-Si solar cells is the reflectance, which can be lowered by etching methods. However, Tri-Si is a crystal compound consisting of three mutually tilted monocrystalline silicon grains. In all grains boundaries the surface is (1 1 0)-oriented. A standard surface texture of etched random pyramids using an anisotropic etchant, such as NaOH, is not achievable here. In this paper, for the first time, a novel texturing method has been attempted, which consisted of two steps-HF:HNO₃:DI (2.5:2.5:5) etching was followed by exposure to the vapors to generate fine holes and an etching depth of 2.5 μm had been reached. A best result of 12.3% has been achieved for surface reflectance, which is about 10% lower than that using normal acidic texturing. Nanoporous structures were formed and the size of the porous structure varied from 5 to 10 nm. An antireflection coating of SiN_x SLAR was used to optimize the reflectance. A fill factor of 0.78 has been reached with an efficiency of 16.2% in 12.5 cm×12.5 cm. This high efficiency is mainly due to an increased short-circuit current density of 34 mA/cm².