Chlorinated Thiophene End Groups for Highly Crystalline Alkylated Non-Fullerene Acceptors toward Efficient Organic Solar Cells

End group engineering is critical for developing A-D-A-type non-fullerene acceptors with suitable absorption properties and high crystallinity for organic solar cells. Here, we report the synthesis of a novel chlorinated thiophene end group and its application in A-D-A-type acceptors. When combined with a tetraoctyl-substituted indacenodithieno­[3,2-b]­thiophene core, the resulting C8-ITCC-Cl acceptor exhibited red-shifted absorption and downshifted energy levels relative to those of the nonchlorinated analogue, C8-ITCC, due to the strong electron-withdrawing ability of the chlorine atoms as revealed by theoretical calculations. Moreover, the noncovalent interactions induced by the chlorine atoms enabled improved lamellar and π–π stacking of C8-ITCC-Cl, leading to a higher electron mobility of 6.6 × 10^–4 cm² V^–1 s^–1 and a fill factor of 73% of the PBDB-TF:C8-ITCC-Cl devices. Consequently, a high power conversion efficiency of 12.7% was achieved by the C8-ITCC-Cl-based devices, which outperformed the C8-ITCC-based ones (10.8%). This work highlights the potentials of halogenated thiophene end groups for highly crystalline A-D-A-type non-fullerene acceptors.