A ZnO composite film with an electron-collection center of ZnO nanorods (ZnNRs) embedded in a ZnO nanoparticle (ZnNP) film was fabricated on flexible photoanodes of back-illuminated dye-sensitized solar cells (DSSCs). Different lengths of the ZnNRs in ZnNP films were compared by considering their electron-collection and -scattering abilities. The morphologies and structures of the ZnO films were observed by examining their X-ray diffraction (XRD) patterns and scanning electron micrographs. The highest light-to-electricity conversion efficiency (eta) of 2.17% was obtained for DSSCs, the ZnO composite film of which had about the same thickness of ZnNPs as the length of ZnNRs on its photoanode. At a thickness of only 7 mu m of ZnNPs, the highest eta was achieved with the composite film, leading to a 66.9% enhancement in efficiency compared to that obtained with only ZnNPs (1.30%). When the ZnNR length was shorter than the thickness of the ZnNP film, electrons in the ZnNPs film were not effectively transferred. On the other hand, if the ZnNRs were too long and stuck out of the ZnNP film, the incident light was scattered, resulting in less light utilization. These explanations were substantiated by electrochemical impedance spectra, laser-induced photo-voltage transients, and incident photon-to-electron conversion efficiency curves. (c) 2012 Elsevier Ltd. All rights reserved.