A New Class of Semiconducting Polymers for Bulk Heterojunction Solar Cells with Exceptionally High Performance

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Abstract
Solar cells based on the polymer−fullerene bulk heterojunction (BHJ) concept are an attractive class of low-cost solar energy harvesting devices. Because the power conversion efficiency (PCE) of these solar cells is still significantly lower than that of their inorganic counterparts, however, materials design and device engineering efforts are directed toward improving their output. A variety of factors limit the performance of BHJ solar cells, but the properties of the materials in the active layer are the primary determinant of their overall efficiency. The ideal polymer in a BHJ structure should exhibit the following set of physical properties: a broad absorption with high coefficient in the solar spectrum to efficiently harvest solar energy, a bicontinuous network with domain width within twice that of the exciton diffusion length, and high donor−acceptor interfacial area to favor exciton dissociation and efficient transport of separated charges to the respective electrodes. To facilitate exciton dissociation, the lowest unoccupied molecular orbital (LUMO) energy level of the donor must have a proper match with that of the acceptor to provide enough driving force for charge separation. The polymer should have a low-lying highest occupied molecular orbital (HOMO) energy level to provide a large open circuit voltage (Voc). All of these desired properties must be synergistically integrated to maximize solar cell performance. However, it is difficult to design a polymer to fulfill all these requirements. In this Account, we summarize our recent progress in developing a new class of semiconducting polymers, which represents the first polymeric system to generate solar PCE greater than 7%. The polymer system is composed of thieno[3,4-b]thiophene and benzodithiophene alternating units. These polymers have low bandgaps and exhibit efficient absorption throughout the region of greatest photon flux in the solar spectrum (around 700 nm). The stabilization of the quinoidal structure from thieno[3,4-b]thiophene is believed to be primarily responsible for these properties. Additionally, the rigid backbone enables the polymer to form an assembly with high hole mobility. Proper side chains on the polymer backbone ensure good solubility and miscibility with fullerene acceptors. The flexibility in structural tuning on the polymer backbone provides the polymers with relatively low-lying HOMO energy levels and enhanced Voc, short-circuit current density (Jsc), and fill factor (FF) and, thus, enhanced PCE. All of these features indicate that the polymer system exhibits a host of properties that are indeed synergistically combined, leading to the enhancement in solar cell output. Our preliminary results demonstrate why these polymers are excellent materials for solar energy conversion and represent prime candidates for further improvements through research and development.