Numerical modelling of Zinc Oxide nanowire anti reflective coatings

Zinc Oxide (ZnO) nanowire anti-reflective coating is modelled using wave optics methods, solving the Maxwell's equation numerically using commercially available finite element software. The model is used to investigate the effectiveness of the anti-reflective coating at air-glass interface. ZnO nanowires of both uniform and tapering cross sections are investigated, and reflection coefficient is calculated for incident angles of 0–90 degrees. Simulations are done for the wavelength range 300“800 nm considering the wavelength dependent optical properties of ZnO nanowires. With average diameter of 50 nm and height of 400 nm the simulated nanowires confirm to dimensions of low temperature hydrothermally grown nanowires. At normal incidence, tapered nanowires can reduce the reflections to 1.5% and uniform nanowires can reduce the reflections to 3%. Without an anti-reflection coating, reflections amount to about 4% at air-glass interface. Tapering nanowires which take in to account the graded refractive index effect due to density gradient of the nanowires are far more effective at higher incident angles. Tapered nanowires keep the reflections well below 12% for all the practical angles of incidences whereas without the anti-reflection coatings the reflections reach as high as 50%. The density gradient and packing density of the nanowires are varied to optimize the nanowire array for the particular wavelength range considered. The wavelength range considered for simulations includes the absorption bands of most commonly used absorbers employed in organic solar cells. Thus simulated properties of the anti-reflection coating is highly relevant for organic solar cells.