Improving the thermoelectric performances of polymer via synchronously realizing of chemical doping and side-chain cleavage

Side-chain cleavage is an effective strategy to reconstruct the arrangement of polymer main chains, thus improving their electrical properties and corrosion stability. However, simultaneously realizing side-chain cleavage and chemical doping of polymer semiconductors have not been reported. Herein, a series of p-type polymer semiconductors with the incorporation of ester-cleavage groups (PBDTTT-TETx, X is the molar ratio of the cleavage ester groups and nondegradable alkyl groups) were designed and synthesized. The introduced functional ester side-chains on PBDTTT-TET0.5 can be degraded by a commonly used p-type dopant FeCl3 readily, resulting in the maximum 120-fold enhancement in electrical conductivity and 15-fold increase in power factor at the FeCl3 concentration of 30 mM, compared to the reference one (PBDTTT-PET0.5 with the nondegradable ester side-chains). Scanning electron microscopy, atomic force microscopy measurements and X-ray diffraction reveal that the increase of crystalline domains along with the side-chain cleavage contributes to the formation of (bi)polaron network. The improved doping efficiency can be reflected by their ultraviolet photoelectron spectroscopy and UV–vis-NIR spectroscopy results. Moreover, a further optimized power factor can be achieved for PBDTTT-TET0.05, which is 5.8 times higher than that of PBDTTT-TET. Therefore, molecule design strategy by incorporating FeCl3-cleavage ester groups into the π-conjugated backbones provided an effective approach to establish advanced TE materials.