Plasmonic scattering back reflector for light trapping in flat nano-crystalline silicon solar cells

Most types of thin film solar cells require light management to achieve sufficient light absorptance. We demonstrate a novel process for fabricating a scattering back reflector for flat, thin film hydrogenated nanocrystalline silicon (nc-Si:H) solar cells. This scattering back reflector consists of an array of silica nanocylinders in a metal sheet. Typically, nc-Si:H solar cells are grown on textured substrates that scatter the incoming light. However, one needs to make compromises to the size and aspect ratio of the scattering features and the material growth process to prevent growth defects in the nc-Si:H layers on top of such (nano)-structured substrates. Here, we grow a high-quality nc-Si:H layer on a flat superstrate. On top of this, we apply a plasmonic scattering structure composed of a periodic array of dielectric nanocylinders, which is overcoated by silver. The use of a flat plasmonic scattering back reflector (PSBR) circumvents silicon growth defects and prevents the associated reduction of the open circuit voltage, while on the other hand, the scattering improves the short circuit current. The PSBR is fabricated over a large area using substrate conformal imprint lithography (SCIL). Optical modeling was performed using finite-difference time-domain (FDTD) simulations to determine the absorptance in the active layer of the cell. A particle swarm optimization algorithm is used to optimize the PSBR geometry, such that the photocurrent is maximized. We fabricated a 1 μm thick nc-Si:H cell and experimentally demonstrate a current improvement of 32% compared to a flat reference cell, without affecting the open circuit voltage. The PSBR structure is shown to be effective for nc-Si:H solar cells, and the concept of flat scattering back reflectors is compatible with a wide range of other thin film superstrate solar cells