Synthesis and photovoltaic properties of conjugated copolymers bearing bis(9,9-di(2-ethylhexyl)-9H-fluoren-2-yl)quinoxaline subunit with deep HOMO level

Abstract

A series of alternating copolymers (PT-BDFQx, PC-BDFQx and PBDT-BDFQx) have been synthesized bearing novel planar bis(9,9-di(2-ethylhexyl)-9H-fluoren-2-yl)quinoxaline (BDFQx) as acceptor unit, using benzo[1,2-b:4,5-b′]dithiophene (BDT), thiophene (T) and carbazole (C) as donor units via Stille or Suzuki coupling reactions. XRD characterization indicated that the presence of planar BDFQx unit (monomer 8) is favorable for the promotion of crystallization in the solid state and GPC results illustrated that the import of multiple chains in planar BDFQx unit raises the polymers molecular weight. Electrochemical measurement results suggested that three copolymers possess deep HOMO energy level of -5.50-5.77 eV. The polymer solar cell with structure of ITO/PEDOT:PSS (30 nm)/polymer: PCBM (60 nm)/Bphen (10 nm)/Ag (100 nm) exhibited the highest Voc of 0.80 V with PBDT-BDFQx as p-type polymer, while the best power conversion efficiency (PCE) of 0.9% was obtained using a blend of PBDT-BDFQx and PCBM(1:4) as active layer.