We design and synthesize a series of regioisomeric n-type small molecules, which have an identical diketopyrrolopyrrole (DPP) core and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (INCN) terminal groups with octyl substituents at different positions. The isomeric structures are confirmed by two-dimensional NMR spectroscopy based on the heteronuclear multiple-bond coupling method. Incorporation of the electron-deficient DPP and strongly electron-withdrawing INCN groups yields deep frontier molecular orbitals with n-type charge-transport properties in solution-processed organic field-effect transistors (OFETs). Interestingly, a minor change in the substitution position of the octyl side chains significantly influences the optoelectronic and morphological properties of the thin film. The polycrystalline morphology of the as-cast films is reorganized differently with thermal annealing depending on the octyl topology, significantly affecting the OFET performance. With thermal treatment at 200 °C, the kinked DPP(EH)-INCNO1 (EH = 2-ethylhexyl) structures transform into single crystalline-like structures, exhibiting a remarkably improved electron mobility up to ∼0.6 cm2V−1 s−1 compared with DPP(EH)-INCNO2 isomers. The more linear DPP(EH or HD)-INCNO2 (HD = 2-hexyldecyl) molecules become more crystalline with thermal treatments, but their polycrystalline packing structures with large grain boundaries are the main reason for their lower electron mobility. When the solubilizing alkyl substituents are selected, careful molecular design is needed, with consideration of both the solubility and intermolecular packing, for optimizing the optoelectronic properties.
Two kinds of dumbbell-shaped acceptor–donor–acceptor (A–D–A)-type triad single-component (SC) photovoltaic molecules based on a benzodithiophene-rhodanine (BDTRh) core and [6,6]-phenyl-C61 butyric acid (PC61BA) termini, BDTRh–C2–PC61BA and BDTRh–C10–PC61BA, were synthesized by modulating the alkyl (C2 and C10) spacer lengths. Both SC photovoltaic structures had similar UV–vis spectra in solution, but BDTRh–C10–PC61BA showed a significantly higher absorption coefficient as a thin film. In films, a more facile intermolecular photo-induced charge transfer was observed for BDTRh–C10–PC61BA in the broad-band transient absorption measurements. BDTRh–C10–PC61BA also exhibited a higher hole mobility (by 25 times) and less bimolecular recombination than BDTRh–C2–PC61BA. By plotting the normalized external quantum efficiency data, a higher charge-transfer state was measured for BDTRh–C10–PC61BA, reducing its voltage loss. A higher power conversion efficiency of ∼2% was obtained for BDTRh–C10–PC61BA, showing higher open-circuit voltage, short-circuit current density, and fill factor than those of BDTRh–C2–PC61BA devices. The different carrier dynamics, voltage loss, and optical and photoelectrical characteristics depending on the spacer length were interpreted in terms of the film morphology. The longer decyl spacer in BDTRh–C10–PC61BA afforded a significantly enhanced intermolecular ordering of the p-type core compared to BDTRh–C2–PC61BA, suggesting that the alkyl spacer length plays a critical role in controlling the intermolecular packing interaction.
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