Molecular Insight into Efficient Charge Generation in Low-Driving-Force Nonfullerene Organic Solar Cells

ConspectusFor organic solar cells (OSCs), charge generation at the donor/acceptor interfaces is regarded as a two-step process: driven by the interfacial energy offsets, the excitons produced by light absorption are first dissociated into the charge-transfer (CT) states, and then the CT states are further separated into free charge carriers of holes and electrons by overcoming their Coulomb attraction. Meanwhile, the CT states can recombine through radiative and nonradiative decay. Owing to the emergence of narrow-band-gap A-D-A small-molecule acceptors, nonfullerene (NF) OSCs have developed rapidly in recent years and the power conversion efficiencies (PCEs) surpass 18% now. The great achievement can be attributed to the high-yield charge generation under low exciton dissociation (ED) driving forces, which ensures both high photocurrent and small voltage loss. However, it is traditionally believed that a considerable driving force (e.g., at least 0.3 eV in fullerene-based OSCs) is essential to provide excess energy for the CT states to achieve efficient charge separation (CS). Therefore, a fundamental question open to the community is how the excitons split into free charge carriers so efficiently under low driving forces in the state-of-the-art NF OSCs.In this Account, we summarize our recent theoretical advances on the charge generation mechanisms in the low-driving-force NF OSCs. First, the A-D-A acceptors are found to dock with the D-A copolymer or A-D-A small-molecule donors mainly via local π-π interaction between their electron-withdrawing units, and such interfacial geometries can provide sufficient electronic couplings, thus ensuring fast ED. Second, the polarization energies of holes and electrons are enhanced during CS, which is beneficial to reduce the CS energy barrier and even leads to barrierless CS in the OSCs based on fluorinated A-D-A acceptors. Moreover, the exciton binding energies (E_b) are substantially decreased by the strong polarization of charge carriers for the A-D-A acceptors; especially for the Y6 system with three-dimensional molecular packing structures, the remarkable small E_b can enable direct photogeneration of free charge carriers. Accordingly, the excess energy becomes unnecessary for CS in the state-of-the-art NF OSCs. Third, to simultaneously decrease the driving force and suppress charge recombination via the triplet channel, it is imperative to reduce the singlet-triplet energy difference (ΔE_ST) of the narrow-band-gap A-D-A acceptors. Importantly, the intermolecular end-group π-π stacking is demonstrated to effectively decrease the ΔE_ST while keeping strong light absorption. Finally, hybridization of the CT states with local excitation can be induced by small interfacial energy offset. Such hybridization will result in direct population of thermalized CT states upon light absorption and a significant increase of luminescence quantum efficiency, which is beneficial to concurrently promote CS and reduce nonradiative voltage loss. We hope this Account contributes to the molecular understanding of the mechanisms of efficient charge generation with low driving forces and would be helpful for further improving the performance of organic photovoltaics in the future.