Optimization of highly porous ZnO thin film electrode structures is carried out for use in photoelectrochemical (PEC) cell. Spherical voids are formed in ZnO films using a sacrificial template of polystyrene spheres of similar to 400 nm sizes. The voids serve as light scattering centers for dramatically enhanced light absorption in the visible region of the solar spectrum, and as channels for electrolyte infiltration into the film. The UV-vis spectra reveal that light absorption at wavelength of 800 nm is enhanced to about 99%. The voids are further used as a route for the deposition of CdS nano-crystallites throughout the ZnO film. The CdS-decorated ZnO nanoparticulate structure shows a photon-to-electron conversion efficiency of 5.7% under a solar simulator. This efficiency value is the highest reported so far among ZnO-based electrodes, and is achieved by optimization of the nanostructure in the CdS-decorated ZnO films (C) 2013 The Electrochemical Society. All rights reserved.