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 TiO 2 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 TiO 2 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/ TiO 2 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/TiO 2 layer and conventional SiO 2 /TiO 2 double-layer AR (DLAR) coating, respectively. Under a very large incident angle of solar illumination ( θ =80°), the ZnO NRs/TiO 2 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/TiO 2 layer barely affects the open-circuit voltage and fill factor of the solar device. Therefore, the proposed ZnO NRs/TiO 2 layer with a smoother graded refractive index change is highly promising for the AR coating applications for the next-generation solar cells. [ABSTRACT FROM AUTHOR]