Rational design of a novel isoindigo-based conjugated terpolymer with panchromatic absorption and its application to polymer solar cells

In this study, a panchromatic absorptive conjugated terpolymer, BDTID-BDT3MT, is synthesized, which consists of electron-donating benzodithiophene (BDT), isoindigo (ID) as a strong electron-accepting unit, and methyl-3-thiophenecarboxylate (3 MT) as a weak electron-accepting unit. By combining these three monomers into the structure of a conjugated terpolymer, the absorption spectrum of BDTID-BDT3MT is induced to exhibit an unusually broad, strong, and uniform band in the wavelength interval from 300 to 750 nm, which helps achieving highly efficient light harvesting under solar illumination. The intriguing panchromatic absorption behavior of BDTID-BDT3MT was explained on the basis of theoretical calculations using simplified repeating units. Polymer solar cells (PSCs) based on BDTID-BDT3MT as a donating polymer and non-fullerene acceptors (e.g., ITIC-4F) exhibited a high power conversion efficiency (PCE) of 5.38%, high open circuit voltage (V_oc) of 0.88 V, and short circuit current density (J_sc) of 13.74 mA/cm², while PSCs based on ternary blend systems consisting of BDTID, BDT3MT, and ITIC-4F exhibited lower PCE and J_sc of 3.74% and 11.15 mA/cm², respectively. The superior performance of PSCs based on BDTID-BDT3MT can be attributed to their high light harvesting efficiencies and relatively more favorable nano-phase film morphologies. Our results establish that BDT, ID, and 3 MT units serve as useful building blocks in the structure of conjugated terpolymers due to their remarkably broad panchromatic absorption band.