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 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-Tₓ (x=1–3), are designed and synthesized. These Y5-IC-Tₓ are blended with small molecule acceptor L8-BO, respectively, for forming alloy acceptor layers that are subsequently deposited onto PM6 donor to form PM6/L8-BO:Y5-IC-Tₓ 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, respectively. These enhancements result from additional light absorption of Y5-IC-TT with its optimum molecular packing with L8-BO that improves charge transport in the device. The density functional theory calculation results reveals that Y5-IC-TT has the smallest dihedral angle of 14.87° and largest binding energy values 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 L8-BO film with wide-angle X-ray scattering. These types of binary alloy blend acceptors that involve molecular engineering of conjugated end-group as the acceptor layer strategy effectively improves the PCE of planar junction organic photovoltaics, providing pathways for the development of high-performance organic photovoltaic devices.