Solar cells based on functional copolymers were considered as promising devices and can be used to solve the intractable energy crisis for humankind. In this review, several key factors in molecular structures and morphologies which may depress the power conversion efficiency of devices are discussed first. Moreover, we concentrate on the molecular design strategies which can be applied in synthesizing functional polymers with appropriate band gap energy, prolonged exciton diffusion distance, good charge carrier transportations, as well as suitable self-assembly microphase morphologies in solid state. Once these design strategies are selectively combined, polymer solar cells with optimized performance can be approached.