**4. Conclusion**

In this chapter, main effort has been directed to disclose the structure-property relationship for solar cell polymers. The requirements and criteria for an efficient polymer donor in BHJ solar cell have been discussed with representative examples. Key factors are: absorption efficiency, solubility, stability (thermal-, photo-), low band gap, HOMO/LUMO energy level, charge carrier mobility and morphology. In order to achieve high power conversion efficiency, a good balance among these factors should be met. On the other hand, choice of acceptor counterpart and device engineering for the BHJ device also play important roles for power conversion efficiency improvement. Nowadays choice of donor/acceptor combination and device fabrication is still 'a state of art' but more and more rules of thumb have been pointed out. Provided that if BHJ concept still prevails for the next 10 years or longer, newer device design is also urgently required. Tandem solar cell device reported is one example to address the efficiency issue from this point of view. But no matter what kind of new changes will be brought out, the photon flux capture material, which is conjugated polymer in PSC, will still be the core of the device.
