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

• We theoretically and experimentally demonstrate a high performance of antireflection coating.
• ZnO nanorod (NR) is reasonable to be considered as discrete multiple optical layers with low scattering loss.
• An additional TiO2 layer inserted under ZnO NRs can significantly reduce the Fresnel reflection loss of the solar device.
• The ZnO NRs/TiO2 layer shows a good hydrophobicity characteristic.

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 (JSC) 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 JSC enhancement of 35.2%, whereas the JSC 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.