Strategies to Improve the Performance of n-Type Conjugated Polymers

The development of organic semiconductors (OSCs) has gained significant attention in the realm of organic electronics (OEs). Organic thermoelectrics (OTEs) are of interest owing to their potential for harnessing power from residual waste heat and powering autonomous wearable devices. Although P-type OTEs showed robust thermoelectric performance from an early stage, the development of n-type OTEs has been slower in comparison, owed to limited doping efficiency and rapid degradation of electronic properties. <u>Here, we present DPP-based n-type conjugated polymers (CPs) for OTEs with three different strategies to enhance their charge transport and TE performance. Our strategies are based on the Hansen solubility parameter (HSP) framework and provide insight to improve charge transport in doped OSCs from morphology control to solvent-host-dopant interaction.</u> First, we utilize HSP to screen thousands of solvents to refine our selection and succeed in controlling the mesoscale morphology of DPP-based n-type CPs films transitioning from nanofibrils into nanofibers. We found that the nanofiber morphology can simultaneously maintain their electrical properties and enhance its mechanical robustness. Second, we address the limitations of the electrical conductivity (σ) of n-type CP by employing HSP to assess the suitable solvent that improves miscibility and charge transport. We found that solvent having higher solubility of dopant, and conjugated backbone of the CP can increase σ without sacrificing Seebeck coefficient (S). This work emphasizes the effectiveness of HSP-guided solvent selection in hindering σ-S trade-off and limitations from solution mixing doping. Last, we propose the idea of self-induced anisotropy in n-type OTEs which enhance the thermoelectric power factor (PF) by mitigating the trade-off between σ and the S. Utilizing HSP theory, we try to understand the solvent-host-dopant intermolecular interactions and elucidate the origin of self-induced anisotropy. We found that a preferential edge-on orientation would increase the in-plane delocalization length and thus improved the electrical conductivity without hindering the S.