Oligothiophenes and alkyl side-chain arrangement the structure-property study of their diketopyrrolopyrrole copolymers for organic photovoltaics

We have synthesized and characterized a series of diketopyrrolopyrrole (DPP)-oligothiophene copolymers, of which the number of regioregular oligothiophene ring (2T, 3T and 4T) and the arrangement of the alkyl side-chain on regioirregular quarterthiophene (4T0, 4T1 and 4T2) are variable. The side chains with regioregular lead to more planar copolymer backbones and higher short circuit current (JSC), but backbone torsion (due to regioirregular side chains) generates greater open-circuit voltages (VOC) for DPP-oligothiophene copolymers. The increasing thiophene ring progressively raises HOMO energy level of copolymers but marginally affects their band gaps. Additionally, the HOMO energy level was found declined significantly with side-chain regioirregularity, because of reducing length of π-conjugation. The HDDPP4T0 exhibits the strongest absorption, extensive network structure, and high hole mobility (μh = 6.04 × 10−4 cm2 V-1 s-1). These characteristics contribute to the exceptional high JSC of 18.96 mA/cm2 for OPV with PCE = 6.12%. However, the HDDPP4T1 having an optimal combination of π-conjugation and energy level affords the second highest VOC (0.73 V) and the third highest JSC (16.89 mA/cm2), resulting the best PCE of 7.51% among all. X-ray scattering, transmission electron microscopy, atomic force microscopy, and space-charge-limited-current (SCLC) measurements reveal that the solvent additive of diphenylether (DPE) enables PC71BM-blended copolymers thin film in crystallinic fibril with enhanced hole mobility. © 2018