A two-dimensional halogenated thiophene side-chain strategy for balancing Voc and Jsc and improving efficiency of non-fullerene small molecule acceptor-based organic solar cells

Abstract

A two-dimensional (2D) halogenated thiophene side chain strategy was proposed and two new 2D halogenated electron-rich fused central core units, FBDT and ClBDT, based on linear benzo[1,2-b:4,5-b′]di(cyclopenta[2,1-b:3,4-b′]dithiophene) with fluorinated and chlorinated thiophene side chains have been developed for the first time, respectively. Then, a family of multihalogenated A–D–A structured non-fullerene small molecule acceptors (NF-SMAs), namely HBDT-4Cl, FBDT-4Cl and ClBDT-4Cl, based on the same dichlorinated 1,1-dicyanomethylene-3-indanone (IC) as the terminal group and three different electron-rich central donor cores (a simple thiophene side chain functionalized central donor core HBDT, its fluorinated analogue FBDT, and its chlorinated analogue ClBDT), respectively, were synthesized for high performance organic solar cells. Compared to nonhalogenated analogues, HBDT-4Cl, FBDT-4Cl and ClBDT-4Cl exhibit slightly wider bandgaps and enhanced molar absorption coefficients, slightly down-shifted energy levels, and obviously improved crystallinity in neat thin films. After blending with the polymer donor PBDB-T-2F, the as-cast FBDT-4Cl/PBDB-T-2F blend films show the highest and most balanced hole/electron mobilities, the highest charge dissociation and collection efficiency, and the most proper phase separation feature among these blend films. Thus, the as-cast FBDT-4Cl/PBDB-T-2F based device exhibits the best power conversion efficiency (PCE) of 12.36%, which is higher than that of ClBDT-4Cl (11.65%) and HBDT-4Cl (10.35%) based devices owing to the higher J_sc and FF. The PCE of 12.36% is the highest value for fluorinated central core functionalized NF-SMA-based binary organic solar cells. Our results revealed that introduction of halogenated thiophene side chains, especially fluorinated thiophene side chains, in an electron-rich fused central core is an effective strategy to achieve high-performance two-dimensional aromatic side-chain functionalized NF-SMAs.