Minimizing surface reflection loss is critical when designing high efficiency solar cells. In recent years, biomimetic
antireflection nanostructures (such as moth-eye structures), with their extraordinary broadband and omnidirectional
antireflection properties, have caught much attention. Single side biomimetic antireflection (AR) coatings
show good performance in suppressing broadband reflection between air and glass interface. However, reflection
from the interface between absorption layer and transparent window layer still remains. In this study,
we proposed a double-side gradient-index nanostructure, and examined its reflection spectrum in comparison
with different biomimetic nanostructures using a finite-difference time-domain (FDTD) simulation and effective
medium theory (EMT). In order to minimize surface reflection, all abrupt interfaces were replaced by gradientindex
biomimetic nanostructures, including air/glass interface and absorber/glass interface. Monolayer of silica
spheres serve as double-side gradient-index nanostructures, partially immersed into photoabsorbing material.
Spheres with diameter smaller than incoming light wavelength show excellent antireflection properties. From
simulation results, in normal incidence, average reflection rate of optimized AR coating structure was lower to
around 5% compared to originally above 25% within visible spectrum region (350nm – 850nm). Details of how
to apply such biomimetic nanostructures in thin film solar cells were also discussed.