New Electron Acceptor with End-Extended Conjugation for High-Performance Polymer Solar Cells

To develop high-efficiency polymer solar cells (PSCs), the acceptors in a bulk heterojunction (BHJ) blend are supposed to possess complementary absorption bands in the near-infrared region and a suitable energy level to be well-matched with the donors. In this work, a new small molecular acceptor (SMA) named IDTT8-N based on an indacenodithienothiophene (IDTT) core was designed and synthesized. In comparison to the counterpart molecule IDTN with an indacenodithiophene (IDT) core, IDTT8-N with the extended π-conjugation length of an IDT core not only exhibits a red shift of ca. 35 nm in optical absorption but also has little change on its lowest unoccupied molecular orbital (LUMO) energy level. Therefore, PSCs based on PM6:IDTT8-N exhibit a superior short-circuit current density (Jsc) and high open-circuit voltage (Voc). Moreover, apart from the strong face-on molecular stacking, distinct end-group π–π stacking of IDTT8-N can be observed in the blends, facilitating the charge transport. Therefore, the optimized PM6:IDTT8-N-based devices exhibit dramatically high and balanced electron mobility (μe) and hole mobility (μh), whose magnitudes are over 10–3 cm2 V–1 s–1. Consequently, an extraordinary PCE of 14.1% with a relatively high Jsc of 20.98 mA cm–2 and a Voc of 0.94 V was recorded. To our knowledge, it is the new record among PSCs with a SMA based on 2-(3-oxocyclopentylidene)malononitrile (INCN) as end groups. These results indicate that extending the π-conjugation length of the fused ring core of a SMA is an efficient method to both enhance the absorption and the molecular interaction of the acceptor as well as the photovoltaic performance of PSCs.