2. Optical simulation model
silicon thin film solar cell with a substrate configuration was used in this study, as shown inFig. 1. The cell consists of a 80 nm thick Aluminium doped zinc oxide (ZnO:Al) front contact layer, followed by a 300 nm silicon active layer, a 40 nm thick (ZnO:Al) layer, a 200 nm thick Ag layer, and a thick Aluminium foil with periodic hemispherical nanopits on it, which can be realized and controlled by the experiments [13], [14] and [15]. All of these layers are textured by the periodic nanopit array of the Aluminium foil, which resulted in the same size and spacing of the nanopits for each layer. The effect of Aluminium periodic nanopits on the light absorption in silicon thin film solar cells were studied by calculating the optical wave propagation and absorption in the silicon thin film cells with different nanopits sizes (d) and spacing (s). Three dimensional optical wave propagation and absorption in the silicon cells was investigated by rigorously solving Maxwell's equations using finite difference time domain (FDTD) algorithm (FDTD Solutions 8.7.1, Lumerical Inc.).
The silicon thin film cells were illuminated by a TM polarized (with the electric field parallel to the incident plane), normally incident plane wave with wavelength range from 300 to 800 nm, covering most of the useful solar spectrum for silicon thin film solar cells. Perfectly matched layer boundary conditions were used in the light incident direction to prevent interference effect, and periodic boundary conditions were used in the lateral direction to simulate ordered array of nanopits. Optical constants of Aluminium, Copper and silicon were taken from Palik [16] and ZnO:Al was taken from Ref. [17]. For simplicity, effect of doping on the absorption of silicon layer was neglected.