Synergistic regulation of ester side chains and thermal annealing to enhance electrical conductivity in polydiketopyrrolopyrrole-based conjugated polymers for thermoelectric generation

Abstract

Ester side chains have been recognized for enhancing the stacking and performance of conjugated polymers in applications such as solar cells and organic field-effect transistors. However, their application in thermoelectric materials has received limited attention. In this study, we synthesized diketopyrrolopyrrole (DPP)-based polymers incorporating alkyl and alkoxy ester tert-butyl (t-Boc) side chains, designated as PDPPC6B-10 and PDPPO4B-10 (where “10” represents the molar content of ester side chains). A control polymer, PDPPB-0, lacking ester side chains, was also synthesized for comparative analysis. Grazing incidence wide-angle X-ray scattering (GIWAXS) analysis revealed that the incorporation of functional ester side chains significantly improved polymer stacking and doping efficiency. Consequently, the resulting polymers exhibited substantially higher power factors (PF) than the control PDPPB-0. Under the same doping conditions, PDPPO4B-10 and PDPPC6B-10 achieved the highest power factors, with values of 48.60±1.74 μW m-1 K-2 and 39.72±3.13 μW m-1 K-2, respectively. These values correspond to 3.95-fold and 3.23-fold increases relative to PDPPB-0 (PF = 12.30±1.24 μW m-1 K-2). Moreover, thermal cleavage of the t-Boc groups further enhanced polymer stacking and electrical conductivity. The PDPPO4B-10-A (after thermo-removal of side chains of PDPPO4B-10) reached 144.46 S cm-1, which is 21 times higher than that of PDPPB-0 under similar doping conditions. These findings suggest that introducing ester side chains represents a viable and effective strategy for developing high-performance thermoelectric conjugated polymers.