Achieving graded refractive index by use of ZnO nanorods/TiO₂ layer to enhance omnidirectional photovoltaic performances of InGaP/GaAs/Ge triple-junction solar cells

In this study, we theoretically and experimentally demonstrate high performance of antireflection (AR) coating composed of the ZnO nanorods (NRs) and TiO₂ layers applied on InGaP/GaAs/Ge triple-junction solar cells. The high performance of this AR coating is due to the realization of a smooth gradient profile of refractive index fabricated by only two physical layers. First, due to the inherent inhomogeneous-nanoporous geometry along the surface normal, the ZnO NRs are reasonable to be considered as discrete multiple optical layers with low scattering loss, leading to the index smoothly increasing from air ambient toward down to the solar cells. Second, to compensate the gap of index between ZnO and solar cells, an additional TiO₂ layer with index in between is hence necessary to insert under ZnO NRs, significantly further reducing the Fresnel reflection loss of the entire device. The ZnO NRs/ TiO₂ layer shows a low wavelength-averaged (solar spectrum weighted) reflectance of 6.00% (4.78%) over a wide spectral range of λ=380-1800 nm, and exhibits a hydrophobic surface with a water contact angle of 128.2°. At device level, we compared the photovoltaic performance of solar cells with and without AR coating, the short-circuit density (J_SC) is enhanced by 31.8% and 23.8% for solar cells integrated with the ZnO NRs/TiO₂ layer and conventional SiO₂/TiO₂ double-layer AR (DLAR) coating, respectively. Under a very large incident angle of solar illumination (θ=80°), the ZnO NRs/TiO₂ layer remains static with J_SC enhancement of 35.2%, whereas the J_SC enhancement of conventional DLAR coating drops down to 9.4%. In addition, the ZnO NRs/TiO₂ layer barely affects the open-circuit voltage and fill factor of the solar device. Therefore, the proposed ZnO NRs/TiO₂ layer with a smoother graded refractive index change is highly promising for the AR coating applications for the next-generation solar cells.