Benzo[1,2-b:4,5-b′]dithiophene-Based Conjugated Polymers for Highly Efficient Organic Photovoltaics

ConspectusIn the past several years, power conversion efficiencies (PCEs) of bulk heterojunction (BHJ) single-junction organic photovoltaic (OPV) cells have increased rapidly because of the innovation of photovoltaic materials, including polymer donors and nonfullerene acceptors (NFAs), device engineering, and morphology optimization. The development of photovoltaic materials has been deemed as the core to imporve the PCEs of OPV cells. Regarding the diversification of NFAs, the rational design of polymer donors is becoming more and more challenging, because an ideal polymer donor is required not only to meet matched molecular energy levels and complementary absoprtion spectra with NFAs but also to improve BHJ morphology and electroluminescence quantum efficiency of OPV cells. Among tens of thousands of polymer donors, benzo[1,2-b:4,5-b′]dithiophene (BDT)-containing conjugated polymers have become dominant donor materials because they are closely related to the frequent breakthroughs in the PCEs of OPV cells. A comprehensive understanding of the correlation among their chemical structures, optical properties, aggregation behaviors, and photovoltaic efficiencies is urgently required to develop next-generation outstanding polymer donors.In this Account, we focus on the molecular design strategies of BDT-containing polymer donors with the goal of developing new photovoltaic materials for fabricating the state-of-the-art OPV cells. First, we summarize our recent achievements in developing high-efficiency BDT-containing polymer donors. In the meantime, the role of surface electrostatic potentials of active layer materials on exciton dissociation of BHJ layer is briefly discussed; the influence of fluorine and chlorine atoms in polymer donors on molecular energy levels, molecular torsion, and photovoltaic efficiencies is analyzed in detail. Second, to pursue higher PCEs of OPV cells, we highlight the following three aspects for in-depth discussion of the BDT-based polymer donor design. (i) The BHJ morphologies are regulated by changing the steric hindrance of the flexible chains, intermolecular interactions, and side chain orientations of polymers to modulate their aggregation in solution. (ii) The optical gaps of polymer donors are fine-controlled by combining the theories of the frontier orbitals hybrid and electron delocalization to develop the ideal polymer materials for versatile applications of OPV cells. (iii) The reduction of nonradiative recombination energy losses of OPV cells is discussed in detail by adding a third component, the reduction of energetic offsets of active layer materials, and the development of polymer donor with strong electroluminescence quantum efficiency. On the basis of the aforementioned molecular design considerations, we achieved PCEs up to 19.0%, 20.2%, and 28.4% for single-junction, double-junction, and indoor light OPV cells, respectively. Lastly, we briefly discuss the opportunities and challenges to further improve the PCEs of OPV cells.