Anti-reflection properties of spherical top nanowire arrays for solar cell applications

Nanowires (NW)/nanopillars (NP) have unique optical and electrical properties that make them attractive for photovoltaic applications. Important factors such as diameter, length and array density of the nanowires have investigated in the recent years or their effect on light trapping. In this work, we study the effect of varying the NWs top tip morphology, and find significant differences in optical response, both via simulations and experiments. In the simulations, optical performance of NW with flat top and spherical top were investigated. The simulated 3D model is a CNT/Cr/a-Si/ITO coaxial structure with total diameter of 760nm. Our results show that as the spacing of the NW gets smaller, the influence of the top morphology on the nanowires' reflectance becomes more significant. For narrow spacing arrays (p<2d, where p is the period and d is the diameter of NWs) NW device with spherical top shows better antireflection performance than the one with flat top. This is due to the biomimetic antireflection (AR) effect introduced by the spherical top. For large spacing arrays (p>2d), AR effect introduced by spherical tops was almost negligible. It can be ascribed to the low volume concentration of the spherical top comparing to that of the planar surface. In addition, effect of structural defects were also investigated. Simulated result matched well with the experimental data by taking into account all these factors.