n-Type Conjugated Polyisoindigos

Abstract

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 PC 60 BM 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.