Finite difference time domain simulations of absorption enhancement in thin GaAs solar cells with textured back surface reflectors

Three-dimensional finite difference time domain simulations were performed to evaluate the absorption enhancement of near-band edge light in 500 nm-thick GaAs solar cells with textured rear reflectors. Three types of the textures were examined: Triangular-prisms, pyramids, and a pseudo-random etch. The tested design parameters were the period and aspect ratios of the texture and then the thickness of the transparent rear scattering layer. It was found that a low-index layer between semiconductor and metal was required to achieve a high degree of absorption enhancement, as this layer greatly reduced parasitic absorption in the metal reflector. In terms of absorption enhancement, pyramids of nearly any period and aspect ratio were superior to triangular prisms by a factor between 2 and 3. The pseudo-random etch achieved performance generally between the two other texture types. The most promising texture in terms of feasibility and performance was pyramids with a period of 2µm and aspect ratio of 0.3, as this texture is accessible to wet-etching and granted especially high absorption enhancement. A cell thusly textured achieved 1-sun terrestrial short-circuit current of 27.4 mA/cm2 compared to 28.3 mA/cm2 for a conventional, 3µm-thick on-substrate cell. Current loss was due to parasitic absorption and light escaping through the front surface.