Disordered nanophotonic surfaces for enhanced light collection in semiconductor solar cells

Abstract

We present a theoretical study of rough photonic surfaces and disordered photonic crystal wire and hole (slab) geometries as a scattering entrance layer to enhance the absorption in thin-film photovoltaic solar cells. One commonly accepted gold standard for scattering enhanced absorption in solar films is given by the Lambertian limit, which results in an optical path length enhancement of 4n2, where n is the refractive index. However, this limit is often an idealized upper limit and hard to realize in practice. Exploiting finite-difference time-domain simulations of realistic rough surfaces, we first explore the limitations and potential practical implementation of this ideal enhancement. A practical rough film, characterized by the RMS height (σRMS) and correlation length (Lx,y) of the random peaks, is numerically optimized for different substrate thicknesses over an optical wavelength range of 400–1100 nm. This optimized surface reflects one practical solution for Lambertian slab thicknesses above 500 nm and wavelengths in the near-UV, but performs worse in the near-IR (∼80%). The physical total height of the optimal rough surfaces is found to be of the order of 2 μm, which contradicts the common assumption that absorption losses within the rough film are negligible. We also investigate photonic crystal wire and slab holes structures with positional and radial disorder, for a fixed height of 1 μm with reflective Ag substrates, considering both GaAs and c-Si semiconductors. We show that the best performing GaAs nanowires give the largest total short-circuit current density (Jsc) of 30  mA/cm2. Relative to the unpatterned slabs of the same height, the greatest photonic crystal enhancements in absorption were seen for c-Si nanoholes and GaAs nanowires (enhancement factors of 1.65 and 1.33, respectively). The inclusion of lattice disorder is shown to increase the absorption enhancement in all photonic crystal structures by 4%–15%, except GaAs nanoholes, which remained within 1% of the ordered structure. Using an analysis of the computed local density of photon states, which is strongly dependent on the complex dielectric constant of the semiconductor, we demonstrate that, due to the higher inherent material losses in GaAs, the useful effects of disorder-induced broadening are minimized in GaAs, compared to c-Si.