Regulating the Aggregation of Unfused Non‐Fullerene Acceptors via Molecular Engineering towards Efficient Polymer Solar Cells

Tuning molecular aggregation via structure design to manipulate the film morphology still remains as a challenge for polymer solar cells based on unfused non‐fullerene acceptors (UF‐NFAs). Herein, a strategy was developed to modulate the aggregation patterns of UF‐NFAs by systematically varying the π‐bridge (D) unit and central core (A’) unit in A−D‐A’−D−A framework (A and D refer to electron‐withdrawing and electron‐donating moieties, respectively). Specifically, the quantified contents of H ‐ or J ‐aggregation and crystallite disorder of three UF‐NFAs (BDIC2F, BCIC2F, and TCIC2F) were analyzed via UV/Vis spectrometry and grazing incidence X‐ray scattering. The results showed that the H ‐aggregate‐dominated BCIC2F with less crystallite disorder exhibited a more favorable blend morphology with polymer donor PBDB‐T (poly[(2,6‐(4,8‐bis(5‐(2‐ethylhexyl)thiophen‐2‐yl)benzo[1,2‐ b :4,5‐ b ']dithiophene)‐ co ‐(1,3‐di(5‐thiophene‐2‐yl)‐5,7‐bis(2‐ethylhexyl)benzo[1,2‐ c :4,5‐ c ']dithiophene‐4,8‐dione)]) relative the other two UF‐NFAs, resulting in improved exciton dissociation and charge tranport. Consequently, photovoltaic devices based on BCIC2F delivered a promising power conversion efficiency of 12.4 % with an exceptionally high short‐circuit current density of 22.1 mA cm −2 .