Dual-sized CdS quantum dots were employed as light harvesters of solar cells to achieve high power conversion efficiency. Chemical bath deposition and annealing treatment were adopted to produce quantum dots with different sizes. The strategy of cosensitization by dual-sized CdS quantum dots was found effective to improve the performance of solar cells by forming a type-I band structure. Our work indicates that a reasonable configuration of quantum dots is crucial on boosting the performance of solar cells.