Charge density modulation on asymmetric fused-ring acceptors for high-efficiency photovoltaic solar cells

Charge density modulation on a thieno[2 '',3 '':5 ',6 ']-s-indaceno[2 ',1 ':4,5]dithieno[3,2-b:2 ',3 '-d]pyrrole (IPT) core has been conducted for a systematic study of its impact on the electronic structure, molecular packing and photovoltaic performance of asymmetric fused-ring acceptors (FRAs). Herein, a series of IPT-based FRAs (i.e.IN-4F, INO-4F, IPT-4F and IPCl-4F) are designed by incorporating 2-ethylhexyl, 2-ethylhexyloxy, hydrogen and chloro side chains, respectively, onto the IPT core. Enhancing the electron-withdrawing ability of the side-chains decreases the optical bandgap but lowers the lowest unoccupied molecular orbital (LUMO) level, which yields a trade-off between J(SC) and V-OC in organic solar cells (OSCs). Furthermore, the FRAs exhibit tunable miscibility and crystallinity, reflected in the FF and J(SC) values of the OSCs. By pairing with polymer donor PM6, IPT-4F based devices achieve the highest PCE of 14.62% with a balanced V-OC of 0.88 V and J(SC) of 22.15 mA cm(-2) and a high FF of 75.01%. Our research demonstrates that electronic density modulation on asymmetric FRAs is an effective way to systematically optimize the device parameters in pursuit of high performance OSCs.