Tuning conjugated chain ends in small molecular acceptors for enhancing molecular packing in binary blend acceptors and inducing high efficiency in planar junction organic photovoltaics

Abstract

A series of AD'A'D'A type small-molecule acceptors based on the dithienothiophen-pyrrolo-benzothiadiazole core and dicyanomethylene-3-indanone (Y5-IC) with different types of conjugated chain end units such as thiophene (-T), thieno[3,2-b]thiophene (-TT), and dithieno[3,2-b:2′,3′-d]thiophene (-TTT), namely Y5-IC-T_x (x = 1–3), are designed and synthesized. These Y5-IC-T_x are blended with small molecule acceptor L8-BO, respectively, for forming acceptor layers that are subsequently deposited onto PM6 donor to form PM6/L8-BO : Y5-IC-T_x planar junction active layers. Among all devices, the champion planar junction PM6/L8-BO : Y5-IC-TT (wt ratio 8 : 2) device exhibits a power conversion efficiency (PCE) value of 19.3% and a high fill factor value of 79%, a substantial increase over the PCE value of 17.4% and fill factor value of 74% for the PM6/L8-BO control device. These enhancements result from additional light absorption of Y5-IC-TT with its optimal molecular packing with L8-BO that improves charge transport in the device. The density functional theory calculation results reveal that Y5-IC-TT has the smallest dihedral angle of 14.87° and the largest binding energy value of −1.5 eV with L8-BO in both anti-parallel and parallel configurations among all cases, being consistent with the enhanced crystalline coherence length value of 18.8 Å for L8-BO : Y5-IC-TT as compared to 17.1 Å for the L8-BO film from wide-angle X-ray scattering. These types of binary alloy blend acceptors that involve molecular engineering of conjugated end-groups as the acceptor layer strategy effectively improve the PCE of planar junction organic photovoltaics, providing pathways for the development of high-performance organic photovoltaic devices.