2-D closely packed SiO2 nanosphere arrays serving as the photonic structure for light absorption enhancement on top of ultra-thin Cu(In1-xGax)Se-2 solar cells are investigated both theoretically and experimentally. It is theoretically demonstrated that whispering gallery modes and high order Mie resonances contribute to the light absorption enhancement for the large spheres and an anti-reflection effect is prominent for small ones. The ultra-thin ClGSe solar cells achieve the optimum absorption enhancement for the small sphere array with a diameter of 110 nm, contrary to the larger spheres used in Si solar cells. The reason is attributed to the strong parasitic absorption in the AZO/ZnO/CdS front layers. They absorb mainly in the short wavelength range where the Mie resonances occur. Additionally, it is shown that the 110-nm-diameter sphere array exhibits a better angular tolerance than a conventional planar anti-reflection layer, which shows the potential as a promising anti-reflection structure. (C) 2016 Elsevier B.V. All rights reserved.