A novel design of hybrid solar cell combining traditional GaAs-based solar cell with colloidal CdS quantum dots (QDs) is demonstrated. By applying CdS QDs on the surface of GaAs device, we can obtain an effective anti-reflection coating at long wavelength. Moreover, due to its strong absorption at ultraviolet range and efficient light emission at blue photon, CdS quantum dots can down-convert the high-energy UV photons into visible ones, which are more easily to be collected in GaAs solar cell. The short circuit current and power conversion efficiency of this new device increases by 21% and 18.9%, respectively, when we compare them to the uncoated GaAs solar cell. An increase of the surface photoconductivity was also detected due to UV presence, and the fill factor of the CdS QDs solar cell can be improved accordingly.