Isoindigo, as a new electron acceptor unit for organic electronic materials, was integrated into two low-energy gap oligothiophenes. Optical and electrochemical studies of the newly synthesized oligomers demonstrate broad absorption through the visible spectrum, along with appropriate energy levels, as desired for light harvesting donors for organic solar cells when blended with [6,6]-phenyl-C(61)-butyric acid methyl ester (PC(60)BM). Molecular heterojunction solar cells were fabricated using these oligomers and exhibit a power conversion efficiency up to 1.76% with a V(oc) of 0.74 V, I(sc) of 6.3 mA/cm(2) and fill factor of 0.38.

Isoindigo (iI) has proven successful as an electron-accepting building block for the preparation of electroactive materials for organic electronics. Its high yielding and scalable synthesis has enabled the rapid development of a large number of molecular and polymeric iI-based materials with remarkable physical properties. This perspective provides an overview of the fundamental properties of isoindigo and summarizes the progress in the development of new materials for varied electronic applications during the last 3 years, focusing in particular on organic photovoltaics (OPVs) and organic field effect transistors (OFETs). The fundamental electronic properties of isoindigo are discussed in the context of the substitution pattern effect (5,5′ vs 6,6′) on the frontier orbitals energies and optical properties. The development of molecular systems in the 6,6′-iI configuration for OPVs is examined with an emphasis on molecular design for improved electronic properties thanks to fine-tuning of the active layer morphology via crystallization control. Numerous copolymers of iI have been reported, with both electron-rich and electron-poor comonomers. The homopolymer of isoindigo displays electron-accepting and electrochromic properties and serves as a polymeric surrogate for fullerenes in all-polymer solar cells. The copolymers’ absorption profiles span the entire visible spectrum into the near-infrared, up to 900 nm. Bulk-heterojunction solar cells based on iI copolymers have reached up to 6.3% efficiency. While the effect of processing additives and cell architecture are important, the unique electronic properties of iI polymers also provide useful insight on energetic losses within blends with fullerenes. Selected copolymers also perform highly in air-stable field effect transistors, with p-type mobilities exceeding 3 cm2/(V s). New concepts concerning the effect of backbone curvature and side-chain branching or polarity have been investigated using iI copolymers. Additionally, some all-acceptor copolymers display n-type mobility. As the design of iI materials evolves, structural modifications of the iI core emerge, targeting ambipolar charge transport and enhanced backbone planarity. Overall, isoindigo provides the field of organic electronics with impressive performance as well as a valuable platform for structure–property relationship investigation.

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The conjugated electron acceptor isoindigo was used to synthesize two conjugated polymers with backbones composed exclusively of electron-deficient units. Suzuki polycondensation afforded the homopolymer of isoindigo and a copolymer with 2,1,3-benzothiadiazole as repeat unit. The materials are thermally stable up to 380 °C, along with being soluble in and processable from common organic solvents. The polymers absorb light broadly throughout the visible spectrum, with optical bandgaps of 1.70 and 1.77 eV, respectively. Both polymers reduce reversibly with LUMO energy levels at −3.84 and −3.90 eV for the homopolymer and the copolymer, respectively, close to the value of −4.10 eV found for fullerenes such as PC60BM when measured under identical conditions. The polymers HOMO levels were calculated at −5.54 and −5.67 eV, respectively, based on their optical band gaps. Spectroelectrochemical measurements on thin films of the homopolymer showed the generation of stable negative charge carriers, accompanied by colored-to-transmissive electrochromism in the films upon reduction. The n-type character of these polymers motivated the fabrication of all-polymer solar cells using blends of poly(3-hexylthiophene) and the homopolymer of isoindigo, yielding efficiencies approaching 0.5%, with room for optimization based on the observed surface morphology of the blend films.

Solvent additives provide an effective means to alter the morphology and thereby improve the performance of organic bulk‐heterojunction photovoltaics, although guidelines for selecting an appropriate solvent additive remain relatively unclear. Here, a family of solvent additives spanning a wide range of Hansen solubility parameters is applied to a molecular bulk‐heterojunction system consisting of an isoindigo and thiophene containing oligomer as the electron donor and [6,6]‐phenyl‐C61‐butyric acid methyl ester (PC61BM) as the electron acceptor. Hansen solubility parameters are calculated using the group contribution method and compared with the measured solubilities for use as a screening method in solvent additive selection. The additives are shown to alter the morphologies in a semipredictable manner, with the poorer solvents generally resulting in decreased domain sizes, increased hole mobilities, and improved photovoltaic performance. The additives with larger hydrogen bonding parameters, namely triethylene glycol (TEG) and N‐methyl‐2‐pyrrolidone (NMP), are demonstrated to increase the open circuit voltage by ~0.2 V. Combining a solvent additive observed to increase short circuit current, poly(dimethylsiloxane), with TEG results in an increase in power conversion efficiency from 1.4 to 3.3%.