Low bandgap isoindigo-based copolymers: design, synthesis and photovoltaic applications

Abstract

Three new low bandgap isoindigo-based conjugated polymers were synthesized, namely poly {(N-octyl)-carbazole-2,7-diyl-alt-[N,N′-(2-ethylhexyl)-isoindigo]-6′,6′′-diyl} (PCzID), poly{(9, 9-dioctylfluorene)-2,7-diyl-alt-[N,N′-(2-ethylhexyl)-isoindigo]-6′,6-diyl} (PFID), and poly{4, 8-bis(2-ethylhexoxy)-benzo[1, 2-b,3,4-b]-dithiophene-2,6-diyl-alt-[N,N′-(2-ethylhexyl)-isoindigo]-6′,6′′-diyl} (PBDTID), by Stille or Suzuki coupling polymerization reaction. All of the polymers were soluble in common organic solvents, such as chloroform, tetrahydrofuran, and chlorobenzene with good film forming properties. The polymer films exhibit broad absorption bands in the wavelength region from 300 nm to 810 nm. Especially, PBDTID possesses the smallest bandgap of 1.54 eV calculated from its absorption cut-off at 806 nm. Preliminary photovoltaic cells based on the device structure of ITO/PEDOT:PSS/PBDTID:PC₆₀BM (1 : 1 w/w)/Ca/Al showed an open-circuit voltage of 0.56 V, a power conversion efficiency of 0.9% and a short circuit current of 3.81 mA cm⁻².