From non-doped to dopable: The impact of methoxy functionalization on doping and thermoelectric properties of conjugated polymers

The introduction of alkoxy side chains into the backbone of conjugated polymers is an effective way to change their properties. While the impact on the structure and optoelectronic properties of polymer thin films was well-studied in organic solar cells and transistors, limited research has been conducted on their effects on doping and thermoelectric properties. In this study, the effects of methoxy functionalization of conjugated backbones on the doping and thermoelectric properties are investigated through a comparative study of diketopyrrolopyrrole-based conjugated polymers with and without methoxy groups (P29DPP-BTOM and P29DPP-BT, respectively). Methoxy-functionalization significantly enhances doping efficiency, converting undopable pairs to dopable ones. This dramatic change is attributed to the structural changes in the polymer film caused by the methoxy groups, which increases the lamellar spacing and facilitates the incorporation of dopants within the polymer crystals. Moreover, methoxy-functionalization is advantageous in improving the Seebeck coefficient and power factor of the doped polymers, because it induces a bimodal orientational distribution in the polymer, which contributes to the increased splitting of Fermi and charge transport levels. This study demonstrates the impact of methoxy-functionalization of a conjugated polymer on doping behavior and thermoelectric properties, providing a guideline for designing high-performance conjugated polymers for thermoelectric applications.