Photoanode is a key component in the development of third-generation solar cells. The performance of the photoanode is critically determined by the morphology and size of oxide semiconductors. In this research, we investigate the size effect of spherical ZnO nanoparticles in N719 dye-sensitized solar cell (DSSC) and CdSe quantum dot-sensitized solar cell (QDSSC). The light harvesting and photovoltaic performance of these cells are highly dependant on the Mie scattering effect. The larger particle shows better scattering and photovoltaic performance. The role of the ZnS buffer layer on QDSSCs is also investigated. After the ZnS treatment, an enhancement in power conversion efficiency, light harvesting ability and inhibition of charge recombination have been observed. Thus, the ZnS act as an excellent buffer layer for reducing the backscattering of carriers. The work experimentally verified the role of synthesized ZnO spherical nanoparticles as an excellent photoanode material for DSSCs and QDSSCs.