Simultaneous Enhancement of Solar Cell Efficiency and Photostability via Chemical Tuning of Electron Donating Units in Diketopyrrolopyrrole-Based Push–Pull Type Polymers

We synthesized a series of push–pull-type copolymers by copolymerizing an electron-deficient diketopyrrolopyrrole with three electron-donating benzodithiophene (BDT) moieties. PDPPDTT, which incorporated a dithienothiophene (DTT), showed a higher power conversion efficiency (PCE) of 6.11% compared to 3.31% for the BDT-based polymer (PDPPBDT). PDPPDTBDT, which incorporated a dithienobenzodithiophene (DTBDT), also exhibited superior performance, with a PCE of 4.75% although this value was lower than that obtained for PDPPDTT. The presence of the DTT unit in the polymer backbone lowered the energy bandgap of the polymer and induced an optimal morphology in the polymer:PC 71 BM blend film, resulting in higher charge carrier generation. Furthermore, the effectively delocalized frontier orbitals of PDPPDTT enhanced intermolecular interactions between the polymer chains by favoring effective π–…