Reflection of normally incident light from silicon solar cells with pyramidal texture

Abstract

A surface texture enhances the capacity of a solar cell to absorb incident radiation. In high efficiency and industry standard designs alike, pyramidal surface textures play the key role of reducing the reflectance of the cell surface. This reduction is achieved by ensuring that incident light rays suffer at least a double reflection in the various facets of the structure. In this work, we define a general expression for the reflectance of a pyramidal texture by identifying discrete paths of reflection and the fraction of reflected light that follows each of these paths. We apply the expression to analyse the reflection of normally incident light at textured surfaces. We examine three common morphologies, finding that a regular array of inverted pyramids just outperforms a random array of upright pyramids, with a regular array of upright pyramids showing poorer capacity to reduce front surface reflection. We extend the analysis to determine the transmittance of the various structures, thus permitting the calculation of a figure of merit that can be used to optimise the thickness of antireflection coatings (ARCs). Finally, by examining the angles at which light is reflected by the pyramidal textures, we find that an encapsulant of refractive index greater than 1.59 gives between 79 and 92% of the initially reflected light a second chance to enter the solar cell.