Research on fused‐ring small‐molecular‐acceptors (SMAs) has deeply advanced the development of organic solar cells (OSCs). Compared to fruitful studies of ladder‐type cores and end‐caps of SMAs, the exploration of side chains is monotonous. The widely utilized alkyl and aryl side chains usually produce a conflicting association between SMAs' crystallinity and miscibility. Herein, a fresh idea about the modification of side chains is reported to explore the subtle balance between the crystallinity and miscibility. Specifically, a phenyl is introduced to the tail of the alkyl side chain whereby a new acceptor IDIC‐C4Ph is reported. Moderately weakened crystallinity is observed, while maintaining preferred absorption profiles and face‐on orientations. Concurrently, remarkably improved heterojunction morphologies and stacking orientations are detected. PBDB‐T:IDIC‐C4Ph devices exhibit greater efficiency of 11.50% than devices from alky and aryl modified acceptors. Notably, the as‐cast OSCs of PBDB‐TF:IDIC‐C4Ph reveal outstanding FF over 76% with the best efficiency up to 13.23%. The annealed devices reveal further increased efficiency exceeding 14% with the state of the art FF of 78.32%. Overall, an effective but easily navigable approach is demonstrated to modulate the crystallinity of SMAs toward synergistically improved morphologies and molecular orientations of bulk heterojunction enabling highly efficient OSCs.

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energy source. [1][2][3][4] The past few years have witnessed the rapid progress of nonfullerene n-type acceptors and the PCE of nonfullerene-based PSCs devices have already surpassed 13% for single-junction devices. [5][6][7][8][9] To achieve efficient nonfullerene PSCs and mature fully from research into cost effective products, it is of critical importance not only to design high-performance small molecular acceptors (SMAs), but also to develop matching donor polymers, as well as to deeply understand the mechanism of the coordinated microstructure interactions between donor and acceptor components. [10][11][12][13][14][15] Key and fundamental strategies for designing high-effciency nonfullerene PSC devices are absorption spectrum and energy level. The donor and acceptor components should possess complementary absorptions and high extinction coefficient to enhance light harvesting to achieve high J SC . [16,17] The matching energy levels are crucial for donor and acceptor to ensure efficient driving force of exciton dissociation, to minimize voltage loss and thus to guarantee high V OC . However, the troublesome tradeoff between high V OC and J SC is inevitable which is the big challenge for further improving the PSCs performance. [18,19] In addition, the most difficult and complicated problem is how to precisely modulate the microstructure of the blending system. The donor should exhibit good morphology compatibility with the acceptor, including suitable crystallinity, appropriate aggregation, face-on orientation, and moderate domain size, and thus to achieve excellent charge transport and high fill factor (FF). [20][21][22] Therefore, establishing universal and general guideline of rational designing compatible D/A structure is extremely urgent and important for nonfullerene PSCs.Benzo[1,2-b:4,5-b′]dithiophene (BDT) and dithieno[2,3d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophene (DTBDT) are the dominant backbones for donor polymers because of their planar framework, showing the most high performance in PSCs. [23][24][25][26][27][28][29][30] Most researchers focus on the side chain engineering of BDT-and DTBDT-based polymers and demonstrate that optimized side-chain can easily control polymer packing In this work, a new asymmetrical backbone thienobenzodithiophene (TBD) containing four aromatic rings is designed, and then four polymers PTBD-BZ, PTBD-BDD, PTBD-FBT, and PTBD-Tz are synthesized. The planar and high degree of π-conjugation configuration can guarantee effective charge carrier transport and the distinct flanked dihedral angles between the TBD core and conjugated side chain can subtly regulate the molecular aggregation and crystallinity. The four polymer/3,9-bis(2-methylene-(3-(1,1dicyanomethylene)-indanone)-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]-dithiophene (ITIC) blending films exhibit predominantly face-on orientation. The photovoltaic devices based on wide bandgap polymers PTBD-BZ and PTBD-BDD achieve power conversion efficiencies (PCEs) as high as 12.02% and...