Chapter IB-3 - Low-Cost Industrial Technologies for Crystalline Silicon Solar Cells

Abstract

Silicon substrates used in commercial solar cell processes contain a near-surface saw-damaged layer, which has to be removed at the beginning of the process. Thickness of the damage depends on the technique used in wafering of the ingot. A layer with thickness of 20 to 30 μm has to be etched from both sides of wafers cut by an inner-diameter blade saw, while only 10 to 200 μ m is enough when a wire saw is used. The etching process has to be slightly modified when applied to multicrystalline substrates. Too fast or prolonged etching can produce steps at grain boundaries. This can lead to problems with interruptions of metal contacts. This problem can be avoided by an isotropic etching based on a mixture of nitric, acetic, and hydrofluoric acids. However, a strong exothermic reaction makes this etching process difficult to control and toxicity of the solution creates safety and waste disposal problems. The silicon surface after saw damage etching is shiny and reflects more than 35% of incident light. The reflection losses in commercial solar cells are reduced mainly by random chemical texturing. Surface texturing reduces the optical reflection from the single crystalline silicon surface to less than 10% by allowing the reflected ray to be recoupled into the cell.