Two-dimensional halogenated thiophene side-chain strategy for balancing Voc and Jsc and improving efficiency of non-fullerene small molecular acceptors-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 chain are firstly developed, respectively. Then, a family of multihalogenated A-D-A structural non-fullerene small molecular acceptors (NF-SMAs), namely HBDT-4Cl, FBDT-4Cl and ClBDT-4Cl, based on same dichlorinated 1,1-dicyanomehylene-3-indanone (IC) as terminal group and three different electron-rich central donor cores (simple thiophene side chain functionalized central donor core HBDT, fluorinated analogue FBDT, and chlorinated analogue ClBDT), respectively, were synthesized for high performance organic solar cells. Compared to nonhalogenated analogue HBDT-4Cl, FBDT-4Cl and ClBDT-4Cl exhibit slightly wider bandgap but enhanced molar absorption coefficient, slightly down-shifted energy levels, and obviously improved crystallinity in neat thin films. After blending with 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 best proper phase separation feature in among those 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 higher Jsc and FF. The PCE of 12.36% is the highest value for fluorinated central core functionalized NF-SMAs-based binary organic solar cells. Our results revealed that introduction of halogenated thiophene side chain, especially for fluorinated thiophene side chain, on electron-rich fused central core is an effective strategy to achieve high-performance two-dimensional aromatic side-chain functionalized NF-SMAs.