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

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.