Parasitic absorption in the rear reflector of a silicon solar cell: Simulation and measurement of the sub-bandgap reflectance for common dielectric/metal reflectors

The rear side of a silicon solar cell is often designed to minimize surface recombination, series resistance, and cost, but not necessarily parasitic absorption. We present a comprehensive study of parasitic absorption in the metal layer of solar cells with dielectric/metal rear reflectors. The sub-bandgap reflectance of a solar cell or test structure is proposed as an experimentally accessible probe of parasitic absorption, and it is correlated with short-circuit current density. The influence of surface texture, dielectric refractive index and thickness, and metal refractive index on sub-bandgap reflectance—and thus current—is then both calculated and measured. From the results, we formulate design rules that promote optimum infrared response in a wide variety of silicon solar cells. •Comprehensive analysis of the reflectance of dielectric/metal reflectors at the rear of silicon solar cells.•Sub-bandgap reflectance is a readily measureable metric of parasitic absorption and is correlated with short-circuit current density.•Calculation and measurement of sub-bandgap reflectance for common textures, dielectrics, and metals.•General design rules for optimizing rear reflectance: low-refractive-index dielectrics at least 100nm thick suppress absorption in even relatively poor metal reflectors.