High-performing random terpolymer-based nonfullerene polymer solar cells fabricated using solvent additive-free as-cast blend films

Two new random terpolymers containing 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT) as a donating unit, methyl-3-thiophenecarboxylate (3MT) as a weak accepting unit, and the more electron-deficient benzo[c][1,2,5]thiadiazole (BTz) or 5,5′-bis(2-ethylhexyl)-4H,4′H-1,1′-bithieno[3,4-c]pyrrole-4,4′,6,6′(5H,5′H)-tetraone (BiTPD) unit as a third monomer (respectively yielding 3MTB and 3MTT) were synthesized in order to achieve improved physical and optoelectronic properties relative to the 3MT-Th copolymer bearing only BDT and 3MT. The UV–vis absorption spectra and optical bandgap energies of these terpolymers were broader and smaller than those of the 3MT-Th copolymer. In thin films, the terpolymers displayed the face-on polymer chain orientation, indicating that the BTz or BiTPD unit in the terpolymer backbones did not affect the molecular arrangement on the substrate. In comparison with 3MT-Th-based polymer solar cells (PSCs) as control devices, the PSCs employing blend films of these terpolymers as the donor and 2,2′-[[6,6,12,12-Tetrakis(4-hexylphenyl)-6,12-dihydrodithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6-b′]dithiophene-2,8-diyl]bis[methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]]bis[propanedinitrile] (ITIC) as the acceptor exhibited higher power conversion efficiencies (>8.0%) for the same device configuration. In addition, the 3MTB- and 3MTT-based PSC devices displayed excellent shelf-life even after aging for over 1000 h.