Organic Photovoltaics of Regiorandom Copolymers Composed of Unsymmetric Diketopyrrolopyrrole with Different Alkyl Chain Lengths

Diketopyrrolopyrrole (DPP) is an authentic π-conjugated building unit that has been utilized as the electron acceptor of narrow-bandgap copolymers for organic photovoltaics (OPVs). The conventional DPP unit is symmetrically flanked with identical aromatic rings, whereas unsymmetric DPP with different aromatic units has remained mostly unexplored. Here, we report the synthesis and characterization of DPP copolymers composed of thiophene (Th) and pyridine (Py) as the unsymmetric configuration of side aromatics. The unsymmetric DPP unit is appended with three different lengths of alkyl chain and coupled with a Th donor to simplify the polymer structure, which is in contrast to the benchmark, high-cost benzodithiophene donor. With the elongation of the alkyl chain, a significant deepening of the highest occupied molecular orbital (HOMO) level by ∼0.3 eV is observed. Furthermore, the crystallinity as well as the HOMO levels are also subject to alteration by the processing solvents, yielding a maximum power conversion efficiency of 5.59% (blended with Y6 non-fullerene acceptor), much higher than those of IT4F-blended OPVs (∼1%). We provide a comprehensive evaluation of the bulk heterojunction morphology, polymer crystallinity, fluorescence quenching, and space-charge-limited current mobility, highlighting the effect of the alkyl chain length of unsymmetric DPP copolymers on the OPV performances.