A series of donor-acceptor oligomer OBTTh n (n＝1-7) and polymer PBTTh 1 and PBTTh 2 composed of alternative 2,1,3-benzothiadiazole and 3-hexylthiophene have been designed and synthesized for the purpose of investigation on the effect of chain length and side-chain regioregularity on their basic properties and photovoltaic performance. In the OBTTh n oligomers and PBTTh 1 polymer, all the hexyl side chains on thienyl units orient toward the same direction. Upon elongation of the chain length, the intramolecular charge transfer (ICT) absorption band in solution gradually redshifts from 398 nm for OBTTh 1 to 505 nm for OBTTh 7 , then to 512 nm for PBTTh 1 polymer. Meanwhile, the HOMO energy level increases from −5.45 eV (OBTTh 1 ) to −5.08 eV (OBTTh 7 ) and −5.09 eV (PBTTh 1 ), and the LUMO energy level decreases from −3.11 eV (OBTTh 1 ) to −3.30 eV (OBTTh 7 ) and −3.33 eV (PBTTh 1 ), thus giving a smaller and smaller energy bandgap for higher oligomers and polymers. Theoretical calculation suggests straight line-like backbone geometry for this series of oligomers and polymer. On the other hand, polymer PBTTh 2 possesses a different side-chain regioregularity, in which every two neighbor hexyl side chains are arranged in different orienting direction. It is theoretically suggested to have curved line-like backbone geometry. In solution, it shows similar photophysical and electrochemical properties as PBTTh 1 . However in film state, it displays a less redshift in the ICT band as refer to that in solution than PBTTh 1 . In combination with [6,6]-phenyl-C 61 -butyric acid methyl ester (PC 61 BM), these oligomers and polymers were used as donor material to fabricate organic bulk heterojunction solar cells. Again, chain length-dependent device photovoltaic performance was observed. The device based on OBTTh 4 showed a power conversion efficiency of 0.16%, while it increased to 0.36% and 0.49% for the devices based on OBTTh 6 and PBTTh 1 , respectively. However, the side-chain regioregularity has less influence on the device photovoltaic output since the device based on PBTTh 2 displayed an efficiency of 0.52%, comparable to that of PBTTh 1 .

Organic solar cells (OSCs) have attracted an extensive attention due to their light-weight, low cost and flexible features. Exploitation of novel polymeric donating materials is one of the mainstreams in the field. To date, a variety of conjugated polymers with excellent electron-donating properties have been developed. Based on these materials, power conversion efficiency of OSCs has reached to 9%. This review classifies these materials based on the chemical structures of the repeating units, and summarizes their progress accordingly. A number of representative materials, which have received much attention, are highlighted from the aspects of designing strategy, properties, device fabrication and performance. It is anticipated to penetrate more deeply the basic relationships among the chemical structure-packing structure-property, which could provide valuable information for the future materials exploitation.