Significance of the relationship between the alkyl side-chain lengths of diketopyrrolopyrrole-based polymers and their thermoelectric properties

• A series of DPP-based polymer with different lengths of alkyl side-chains is synthesized. • DPPSe-HD with the shortest alkyl side chains exhibits a wide and dense dendritic fibrillar morphology. • Shorter alkyl side-chains in DPP-based polymers lead to a higher proportion of edge-on stacking. • DPPSe-HD shows the highest electrical conductivity and power factor due to superior charge mobility. Side-chain engineering of conjugated polymers has emerged as a highly effective approach to improving the performance of organic thermoelectric devices by facilitating efficient doping and charge transport. In this study, a series of selenium‐substituted diketopyrrolopyrrole-based polymers was synthesized to investigate the influence of the alkyl side-chain length on the electrical and thermoelectric (TE) properties. The polymer with the shortest alkyl side chain demonstrated the highest electrical conductivity of 82 S cm −1 while retaining a comparable Seebeck coefficient of 110 µV K −1 , which resulted in a superior power factor of 70 µW m −1 K −2 . Charge transport and morphological analysis revealed that it had wider dendritic fibrillar structures with a higher electron density than the other polymers, which enhanced its charge carrier mobility. X-ray structural analysis indicated that it had a much higher proportion of edge-on stacking structures, which would be expected to enhance the charge carrier mobility and electrical conductivity and thus improve the thermoelectric performance. These findings offer key insights into molecular design strategies for the next generation of TE devices.