Increased conjugated backbone twisting to improve carbonylated-functionalized polymer photovoltaic performance

Two conjugated polymers (PBTCO-TT and PBTCO-T), differing only in their conjugated backbones with thiophene (T) units in the latter and thieno[3,2-b]thiophene (TT) units in the former, were synthesized and applied in non-fullerene (NF) polymer solar cells (PSCs). The two polymers displayed similar absorption profiles in solutions and films at room temperature, but PBTCO-TT in solution showed a stronger temperature-dependent aggregation than PBTCO-T. Cyclic voltammetry and density functional calculations confirmed similar molecular energy levels for the two polymers (HOMO/LUMO of −5.41/−3.56 eV for PBTCO-TT and −5.43/−3.55 eV for PBTCO-T). After blending each of these polymers with a NF ITIC acceptor, the resulting PBTCO-TT:ITIC film showed a significantly greater surface roughness and stronger aggregation than the resulting PBTCO-T:ITIC film. In NF PSCs, the PBTCO-TT:ITIC-based PSC only gave a PCE of 5.31%, with a VOC of 0.881 V, JSC of 13.01 mA cm−2 and FF of 46.31%, while the PBTCO-T:ITIC-based PSC showed a significantly higher PCE of 10.40% with imultaneous improvement of the VOC to 0.959 V, of the JSC to 16.21 mA cm−2 and of the FF to 66.89%.