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.

A series of donor−acceptor isoindigo (iI)-based copolymers synthesized with increasing numbers of thiophene rings in the repeat unit (from zero to three thiophene rings, including silole and germole-bridged fused bithiophene units) is applied toward solutionprocessed OFET devices. Differential pulse voltammetry on thin films of the polymers recorded LUMO energy levels confined within a 0.1 eV range around 3.9 eV, while their bandgaps are estimated at 1.5 to 1.7 eV. The interchain π-stacking distance of each sample was evaluated from the 2D-WAXS diffraction patterns of annealed extruded filaments and the GIWAXS patterns of thin films, and were found to be all in the same range, between 3.65 and 3.75 Å for the thin films. Both p-type and n-type charge transport in thin film bottom gate, bottom contact transistor devices were recorded. In particular, the copolymer P(T-iI) containing one thiophene ring afforded wellbalanced ambipolar p-type and n-type mobilities of 0.04 cm 2 /(V s) and 0.1 cm 2 /(V s), respectively. Under our processing conditions, the charge transport properties evolved from exclusively n-type to solely p-type as the number of thiophene rings within the repeat unit is increased to three rings in the case of P(T3-iI). This was observed despite all polymers displaying similar LUMO energy levels, interchain π-stacking distances, and microscopic thin film morphology (all face-on arrangement on the dielectric surface). This prompted a molecular-scale morphological analysis of P(T-iI) and P(T3-iI) in particular, using solid-state NMR spectroscopy in order to further investigate the stark difference in n-type mobilities between these two polymers. Using the complete assignment of solution 2D-NMR spectra of a thiophene-iI-thiophene model compound as guideline, the analysis of proton−carbon correlations in the solid-state 2D 13 C{ 1 H} FSLG-HETCOR NMR spectra of P(T-iI) and P(T3-iI) revealed differences in the molecular environment surrounding each iI unit. The latter suggests a stronger correlation of neighboring iI units in P(T-iI), whereas a stronger intermixing of iI and thiophenes prevails in P(T3-iI). We conclude that, in this study, the choice of the donor unit length within the primary structure of the D−A polymer can be responsible for hindering its n-type character.

The conjugated copolymer of isoindigo and dithienosilole was synthesized and studied in bulk heterojunction solar cells with fullerene derivatives.