Narrowing the Band Gap: The Key to High-Performance Organic Photovoltaics

ConspectusOrganic photovoltaics (OPVs) have attracted considerable attention in the last two decades to overcome the terawatt energy challenge and serious environmental problems. During their early development, only wide-band-gap organic semiconductors were synthesized and employed as the active layer, mainly utilizing photons in the UV-visible region and yielding power conversion efficiencies (PCEs) lower than 5%. Afterward, considerable efforts were made to narrow the polymer donor band gap in order to utilize the infrared photons, which led to the enhancement of the PCE from 5% to 12% in about a decade. Since 2017, the study of narrow-band-gap non-fullerene acceptors helped usher in a new era in OPV research and boosted the achievable the PCE to 17% in only 3 years. In essence, the history of OPV development in the last 15 years can be summarized as an attempt to narrow the band gap of organic semiconductors and better position the energy levels. There are multiple benefits of a narrower band gap: (1) considerable infrared photons can be utilized, and as a result, the short-circuit current density can increase significantly; (2) the energy offset of the lowest unoccupied molecular orbital energy levels or highest occupied molecular orbital energy levels between the donor and acceptor can be reduced, which will reduce the open-circuit voltage loss by minimizing the loss caused by the donor/acceptor charge transfer state; (3) because of the unique molecular orbitals of organic semiconductors, the red-shifted absorption will induce high transmittance in the visible region, which is ideal for the rear subcells in tandem-junction OPVs and transparent OPVs.In this Account, we first summarize our work beginning in 2008 on the design and synthesis of narrow-band-gap polymer donors/non-fullerene acceptors. Several strategies for constructing these materials, including enhancing the intramolecular charge transfer effect and steric hindrance/energy level engineering are discussed. In this part, in addition to systematic analyses of the design of narrow-band-gap polymer donors based on BDT/TT or BDT/DPP, donors/acceptors based on the new donor moieties DTP or BZPT are discussed as well. Especially, we highlight our work on the first report on the narrow-band-gap acceptor Y1 (based on the new donor moiety BZPT), which pioneered the future development and usage of acceptors belonging to the Y1 family (or series). Subsequently, we analyze several reported certified world record single-junction or tandem-junction OPVs that use these narrow-band-gap donors or acceptors. We share our experiences and insights from a device perspective in terms of donor/acceptor selection, energy level alignment management, film morphology control, current matching of subcells, interconnecting layer construction, interface engineering, and device geometry selection. In this part, the construction of high-performance ternary-blend OPVs and transparent OPVs based on these narrow-band-gap donors/acceptors is also discussed. Finally, in order to push the field into the 20-25% high-efficiency era in the next few years, some suggestions to further develop narrow-band-gap donors/acceptors and related device technologies are proposed.