A two-step acidic texturization procedure for the manufacture of low-reflective multi-crystalline silicon solar wafer

摘要

Texturization of multi-crystalline silicon wafers for photovoltaic application comprises the removal of the saw damage and shaping the topography of the bulk surface to create a surface with a low reflectivity, the so-called texture. Etching of multi-crystalline silicon wafers is usually carried out with acid mixtures consisting of hydrofluoric acid (HF), nitric acid (HNO3) and hexafluorosilicic acid (H2SiF6). The present study reveals that such acid mixtures diluted by water or modified by the addition of ammonia solution, NH3 (added as ammonium hydroxide solution, NH4OH) can create textures with a significantly increased surface area exceeding that obtained by standard etching mixtures by a factor of 2.5-3. This yields a significantly reduced reflectivity of the etched wafer surface. However, the addition of water or NH3 causes a very low etching rate, which makes such mixtures inapplicable for industrial application. To overcome this disadvantage, a two-step etching regime was developed to produce surface-enlarged solar wafers within a timespan typical for industrial production lines. This procedure comprises a first step of slow etching with a NH3-modified etching mixture to pre-shape the as-cut wafer surface. The second etching step is performed with a typical HF/HNO3/H2SiF6 etching mixture that finalizes the texturization. Electrical measurements made on solar cells produced from such etched wafer confirm the improved surface quality of the two-step etched wafer compared to the reference wafer.