ZnO nanowire-enabled light funneling effect for antireflection and light convergence applications

We present a new light trapping technique to reduce reflection loss, as well as for light, focusing at submicron scales for solar cell and image sensing applications. We have fabricated hexagonal arrays of ZnO funnel-like structures on Si substrate by the patterned growth of ZnO nanowires in a hydrothermal growth process. The funnels are optimized so that the effective refractive index along the vertical direction decreases gradually from the Si surface to the top of funnel to reduce Fresnel reflection at a device-air interface. Finite difference time domain simulation is used for optimization of the minimum reflectivity and to analyze optical properties such as angle dependency, polarization dependency, and funneling effect. The structures function similar to a GRIN lens in light trapping and convergence. An optimized structure reduces the average reflectivity close to 3% in the wavelength range of 300-1200 nm with the possibility of confining incident light to a few hundreds of nanometers.