Light-trapping by wave interference in intermediate-thickness silicon solar cells

The power conversion efficiency of crystalline silicon (c - Si) solar cells have witnessed a 2.1% increase over the last 25 years due to improved carrier transport. Recently, the conversion efficiency of c - Si cell has reached 27.1% but falls well below the Shockley-Queisser limit as well as the statistical ray-optics based 29.43% limit. Further improvement of conversion efficiency requires reconsideration of traditional ray-trapping strategies for sunlight absorption. Wave-interference based light-trapping in photonic crystals (PhC) provides the opportunity to break the ray-optics based 4n(2) limit and offers the possibility of conversion efficiencies beyond 29.43% in c - Si cells. Using finite difference time domain simulations of Maxwell's equations, we demonstrate photo-current densities above the 4n(2) limit in 50 - 300 mu m-thick inverted pyramid silicon PhCs, with lattice constant 3.1 mu m. Our 150 mu m-thick PhC design yields a maximum achievable photo-current density (MAPD) of 45.22mA/cm(2). We consider anti-reflection coatings and surface passivation consisting of SiO2-SiNx-Al2O3 stacks. Our design optimization shows that a 80 - 120 - 150nm stack leads to slightly better solar light trapping in photonic crystal cells with thicknesses <50<mu>m, whereas the 80 - 40 - 20nm stack performs better for cells with thicknesses >100 mu m. We show that replacing SiNx with SiC may improve the MAPD for PhC cells thinner than 100 mu m. For a fixed lattice constant of 3.1 mu m, we find no significant improvement in the solar absorption for 50 and 100 mu m-thick cells relative to a 15 mu m cell. A substantial improvement in the MAPD is observed for the 150 mu m cell, but there is practically no improvement in the solar light absorption beyond 150 mu m thickness. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement