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Non-fullerene acceptors (NFAs) are highly promising materials for organic photovoltaics (OPVs). Exciton diffusion in NFAs is crucial to their photovoltaic performance, but is not yet well understood. Here we systematically examine exciton diffusion in a fused-ring electron acceptor (IDIC) based on a first-principles framework. We discover that low-energy excitons in disordered IDIC are charge-separated with electrons and holes residing on neighboring molecules, yielding long exciton lifetimes. With low energetic disorder, high exciton density of states (DOS) and long lifetimes, the disordered IDIC is predicted to exhibit large exciton diffusion lengths and high quantum efficiency. The temperature and energy dependences of exciton diffusion are explored and the manner in which various materials properties (exciton energy, DOS, energetic disorder, and phonon frequency) conspire to influence exciton diffusion is elucidated. Finally, we show that dilation could be an effective strategy to increase the exciton diffusion length in IDIC. 

Three fused-ring electron acceptors ( SIDIC , DIDIC and TIDIC ) were designed and synthesized using single bond, vinylene and acetylene units linked indaceno[3,2- b ]dithiophene dimers as electron-rich cores and 3-(1,1-dicyanomethylene)-5,6-difluoro-1-indanone as electron-deficient termini. These molecules exhibit strong absorption from 550 to 900 nm with large attenuation coefficients of 1.8–2.0 × 10 5 M −1 cm −1 and high electron mobilities of 2.2–4.9 × 10 −3 cm 2 V −1 s −1 . In combination with wide-bandgap polymer FTAZ as a donor, organic solar cells exhibit efficiencies of 9.3–13.1%. Effects of the linking units on optical, electronic, morphologic, and photovoltaic properties were revealed. Relative to SIDIC , vinylene-bridged DIDIC shows red-shifted absorption, while acetylene-bridged TIDIC shows blue-shifted absorption. Compared with SIDIC and DIDIC , TIDIC has a lower HOMO, higher electron mobility, and higher device efficiency. 

Two polymer donors, FTAZ and J71 , and two fused-ring electron acceptors, ITIC1 and ITIC2 , are used to investigate the effects of conjugation dimension on the performance of organic solar cells (OSCs). FTAZ and J71 , and ITIC1 and ITIC2 share the same molecular backbone, respectively, while J71 and ITIC2 possess conjugated thienyl side chains. The addition of conjugated side chains slightly red-shifts the absorption spectra and lowers the bandgap due to the extended 2D conjugation. Conjugated side chains on the acceptor induce the self-aggregation of the acceptors, while conjugated side chains on the donor increase the miscibility of the donors and acceptors, thus optimizing the morphology of the active layers. The blends based on mixed combinations, namely 1D donor/2D acceptor and 2D donor/1D acceptor, show better performance relative to 1D donor/1D acceptor and 2D donor/2D acceptor. 

We compared an indacenodithiophene(IDT)-based fused-ring electron acceptor IDIC1 with its counterpart IHIC1 in which the central benzene unit is replaced by a naphthalene unit, and investigated the effects of the benzene/naphthalene core on the optical and electronic properties as well as on the performance of organic solar cells (OSCs). Compared with benzene-cored IDIC1, naphthalene-cored IHIC1 shows a larger π-conjugation with stronger intermolecular π–π stacking. Relative to benzene-cored IDIC1, naphthalene-cored IHIC1 shows a higher lowest unoccupied molecular orbital energy level (IHIC1: −3.75 eV, IDIC1: −3.81 eV) and a higher electron mobility (IHIC1: 3.0 × 10 −4 cm 2 V −1 s −1 , IDIC1: 1.5 × 10 −4 cm 2 V −1 s −1 ). When paired with the polymer donor FTAZ that has matched energy levels and a complementary absorption spectrum, IHIC1-based OSCs show higher values of open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency relative to those of the IDIC1-based control devices. These results demonstrate that extending benzene in IDT to naphthalene is a promising approach to upshift energy levels, enhance electron mobility, and finally achieve higher efficiency in nonfullerene acceptor-based OSCs.