Sun is the largest carbon-neutral energy source that has not been fully utilized. Although there are solar cell devices based on inorganic semiconductor to efficiently harvest solar energy, the cost of these conventional devices is too high to be economically viable. This is the major motivation for the development of organic photovoltaic (OPV) materials and devices, which are envisioned to exhibit advantages such as low cost, flexibility, and abundant availability.[1] The past success in organic light-emitting diodes provides scientists with confidence that organic photovoltaic devices will be a vital alternate to the inorganic counterpart.At the heart of the OPV technology advantage is the easiness of the fabrication, which holds the promise of very low-cost manufacturing process. A simple, yet successful technique is the solution-processed bulk heterojunction (BHJ) solar cell composed of electron-donating semiconducting polymers and electron-withdrawing fullerides as active layers.[2] The composite active layer can be prepared as a large area in a single step by using techniques such as spin-coating, inkjet-printing, spraycoating, gravure-coating, roller-casting etc.[3] In the last fifteen years, a significant progress has been made on the improvement of the power-conversion efficiency (PCE) of polymer BHJ solar cells, and the achieved efficiencies have evolved from less than 1% in the poly(phenylene vinylene) (PPV) system in 1995,[2] to 4-5% in the poly(3-hexylthiphene) (P3HT) system in 2005, [4] to around 6%, as reported recently.[5] However, the efficiency of polymer solar cells is still significantly lower than their inorganic counterparts, such as silicon, CdTe and CIGS, which prevents practical applications in large scale.There are many factors limiting the performance of the BHJ solar cells.[6] Among them, the properties of materials of the active layer are the most determining factor in the overall performances of polymer solar cells. [7] Ideally, the polymers should have a broad absorption in the solar spectrum to ensure effective harvesting of the solar photons and a high chargecarriers mobility for charge transport. Further, suitable energy levels of the polymer are required that match those of the fullerides. The polymer should have a low-lying highest occupied molecular orbital (HOMO) energy level to provide a large open-circuit voltage (V oc ) and a suitable lowest unoccupied molecular orbital (LUMO) energy level to provide enough offset for charge separation. In addition, morphology of the active composite layer plays a very important role. It is imperative that a bicontinuous network with a domain width approximately twice that of the exciton diffusion length and a high donor/acceptor interfaces is formed, which favors the exciton dissociation and transport of the separated charges to the respective electrode. [8] Most of the polymers reported to date are far from ideal to fulfill all these requirements. [9] We have developed a series of novel semiconducting polymers based on alternating e...

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This paper describes synthesis and photovoltaic studies of a series of new semiconducting polymers with alternating thieno[3,4-b]thiophene and benzodithiophene units. The physical properties of these polymers were finely tuned to optimize their photovoltaic effect. The substitution of alkoxy side chains to the less electron-donating alkyl chains or introduction of electron-withdrawing fluorine into the polymer backbone reduced the HOMO energy levels of polymers. The structural modifications optimized polymers' spectral coverage of absorption and their hole mobility, as well as miscibility with fulleride, and enhanced polymer solar cell performances. The open circuit voltage, V(oc), for polymer solar cells was increased by adjusting polymer energy levels. It was found that films with finely distributed polymer/fulleride interpenetrating network exhibited improved solar cell conversion efficiency. Efficiency over 6% has been achieved in simple solar cells based on fluorinated PTB4/PC(61)BM films prepared from mixed solvents. The results proved that polymer solar cells have a bright future.