Efficient Polymeric Donor for Both Visible and Near-Infrared-Absorbing Organic Solar Cells

A new ternary polymer, TP, incorporating 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT), 5-octyl-2-(2-octyldodecyl)-4,6-di(thieno[3,2-b]thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,3(2H,5H)-dione (PPD), and 2-ethylhexyl 3-fluorothieno[3,4-b]thiophene-2-carboxylate (TT) units at a 2:1:1 ratio, was prepared to develop a potential candidate for visible and near-infrared (NIR)-absorbing organic solar cells (OSCs). TP displayed an optical bandgap (E_g) of 1.65 eV with a maximum absorption peak at 558 nm. TP indicates good complementary absorption with visible light, absorbing most typically used fullerene-based acceptors (PC₇₀BM, intense absorption range: 300-450 nm with E_g of 1.80 eV) and nonfullerene acceptors (NFAs), namely, ITIC (intense absorption range: 500-800 nm with E_g of 1.59 eV). In addition, TP exhibits good complementary absorption with NIR-absorbing NFAs, namely, IEICO-4F (intense absorption range: 600-1000 nm with E_g of 1.24 eV). The determined energy levels (-5.31 eV and -3.66 eV) of TP were found to be suitable as an electron donor for both fullerene and NFA OSCs. The TP:PC₇₀BM blend provided a maximum power conversion efficiency (PCE) of 6.51% with an open-circuit voltage (V_oc) of 0.79 V, short-circuit current (J_sc) of 13.50 mA/cm², and fill factor (FF) of 0.61. The PCE was remarkably improved to 8.18% (V_oc ≈ 0.89 V, J_sc ≈ 15.31 mA/cm², and FF ≈ 0.60) and 8.73% (V_oc ≈ 0.74 V, J_sc ≈ 21.44 mA/cm², and FF ≈ 0.55), for the OSC device prepared using TP and NFAs such as ITIC and IEICO-4F. Surprisingly, the OSC device created using a ternary blend, TP:ITIC:IEICO-4F, provided an outstanding PCE of 9.80% (V_oc ≈ 0.87 V, J_sc ≈ 19.09 mA/cm², and FF ≈ 0.59).