Thiophene and thieno[3,2-b]thiophene π-bridged pyrrolo[3,4-c]pyrrole-1,3-dione-based wide band-gap polymers for fullerene and non-fullerene organic solar cells

The conventional polymerization of 2,5-bis(trimethylstannyl)thiophene (T) or 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (Tt) with 4,6-bis(5-bromothiophen-2-yl)-5-octyl-2-(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (TPPDT) afforded new alternating polymers, namely P(T-TPPDT) or P(Tt-TPPDT), respectively. Both P(T-TPPDT) and P(Tt-TPPDT) exhibited similar thermal stabilities (5% weight loss at around 420 °C) and absorption maxima (λmax ≈ 515 nm) as films. The absorption bands of P(T-TPPDT) and P(Tt-TPPDT) extend between 300 and 620 nm, with optical band gaps (Egs) of 2.02 eV and 2.00 eV, respectively. The highest-occupied/lowest-unoccupied molecular-orbital energies of P(Tt-TPPDT) (HOMO/LUMO: −5.35 eV/–3.33 eV) were found to be somewhat higher than those of P(T-TPPDT) (HOMO/LUMO: −5.31 eV/–3.31 eV). Organic solar cells (OSCs) prepared using 1:1.5 (w/w) P(T-TPPDT):PC70BM or P(Tt-TPPDT):PC70BM (PC70BM = [6,6]-phenyl C70 butyric acid methyl ester) blends provided power-conversion efficiencies (PCEs) of 3.50% or 4.71%, respectively. On the other hand, OSCs prepared with 1:1 (w/w) P(T-TPPDT):ITIC or P(Tt-TPPDT):ITIC (ITIC = 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)indanone))-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2′,3′-d’]-s-indaceno[1,2-b:5,6-b’]dithiophene) blends exhibited improved PCEs of 5.32% or 6.35%, respectively.