A narrow bandgap molecular acceptor, IPIC-4Cl, featuring an indacenobis(dithieno[3,2-b:2ʹ,3ʹ-d]pyrrol) (INP) core with 2-butyl-1-octyl sidechains and chlorinated (dicyanomethylidene)-indan-1-one (IC) as electron-accepting endgroup, has been rationally designed as non-fullerene acceptors (NFAs) for organic solar cells (OSCs). The impact of chlorination on the acceptor unit is revealed by a comparison study with two counterpart NFAs bearing fluorinated or non-halogenated IC unit. The synergetic photophysical and morphological analyses reveal that PBDB-T:IPIC-4Cl blend possesses efficient exciton dissociation and charge collection integrated with higher crystallinity and optimized phase separation. Consequently, the OSCs constructed by PBDB-T:IPIC-4Cl obtain a champion power conversion efficiency (PCE) of 13.4% with an extremely low energy loss of 0.51 eV. More encouragingly, we achieve a higher photovoltaic performance of 14.3% for ternary solar cells by using a combination of optimal amount of PC71BM with PBDB-T:IPIC-4Cl blend. 3 TOC Organic solar cells (OSCs) have emerged as one of the promising photovoltaic technology, owing to their superior performance and capacity for low-cost and scalable solution-processing fabrication. 1-6 In comparison with traditional fullerene-based electron acceptor, the non-fullerene acceptors (NFAs) with acceptor-donor-acceptor (A-D-A) structure exhibited significant advantages, such as variability of energy levels and 4 absorption range, strong absorption strength and adjustable chemical structure. Among the families of NFA small molecule, the fused-ring electron-acceptors (FREAs) have achieved dramatic progress in realizing high-performance bulk-heterojunction (BHJ) OSCs. 7-14 The power conversion efficiencies (PCEs) of the single-junction OSCs based on FREAs were boosted to 15.7 % as a result of optimization of the central core units, electron-withdrawing terminal groups, and the sidechain in core units. 15-18 Moreover, in combination with tandem cell architecture, the photovoltaic performance containing FREAs can further be improved to over 17%. 19 Rational design and synthesis of novel FREA as well as relationship of their structure-to-performance (especially the energetic loss (Eloss, Eloss = Eg opt -eVoc, Eg opt : opticl bandgap, Voc: open-circuited voltage) vs the short-circuited current densities (Jsc)) are indispensable to develop highly efficient OSCs for real-world application. 20-22 Summarizing the literature about FREA, benefitting from the well-balanced feature between open-circuited voltage (Voc) and Jsc, FREA material with bandgap between

Molecular orientation and π-π stacking of nonfullerene acceptors (NFAs) determine its domain size and purity in bulk-heterojunction blends with a polymer donor. Two novel NFAs featuring an indacenobis(dithieno[3,2-b:2′,3′-d] pyrrol) core with meta-or para-alkoxyphenyl sidechains are designed and denoted as m-INPOIC or p-INPOIC, respectively. The impact of the alkoxyl group positioning on molecular orientation and photovoltaic performance of NFAs is revealed through a comparison study with the counterpart (INPIC-4F) bearing para-alkylphenyl sidechains. With inward constriction toward the conjugated backbone, m-INPOIC presents predominant face-on orientation to promote charge transport. The as-cast organic solar cells (OSCs) by blending m-INPOIC and PBDB-T as active layers exhibit a power conversion efficiency (PCE) of 12.1%. By introducing PC 71 BM as the solid processing-aid, the ternary OSCs are further optimized to deliver an impressive PCE of 14.0%, which is among the highest PCEs for as-cast single-junction OSCs reported in literature to date. More attractively, PBDB-T:m-INPOIC:PC 71 BM based OSCs exhibit over 11% PCEs even with an active layer thickness over 300 nm. And the devices can retain over 95% of PCE after storage for 20 days. The outstanding tolerance to film thickness and outstanding stability of the as-cast devices make m-INPOIC a promising candidate NFA for large-scale solutionprocessable OSCs.

Two novel dithieno[3,2-b:2,3-d]pyrrol fused electron acceptors (FREAs) with branched alkyl side-chains have been developed.