Adjusting molecular weight optimizes electronic transport of extrinsically N-type doped conjugated polymer incorporating glycolated side chains

Abstract

Conjugated polymers incorporating glycolated side chains have been widely used for those organic electronic devices that use the properties of the doped state, such as organic thermoelectrics (OTEs) and organic electrochemical transistors (OECTs). This work pioneers the study of adjusting the molecular weight of such conjugated polymers for optimizing the electronic transport of both OTE and OECT devices. The example conjugated polymer P-3O has a naphthalene-alt-bithiazole-based backbone bearing polar side chains. The molecular weight of P-3O was tuned from M_w = 37 kDa to M_w = 81 kDa by controlling the polymerization conditions. The molecular weight strongly influenced the crystalline domain size and π-stacking paracrystallinity, while the interaction between polar side chains and the dopant molecules largely dictates the doping. This finding seems simple but is significant as it enables us to individually control free charge density and mobility in the doped state. As such, an appropriate molecular weight (M_w = 68 kDa) promotes the formation of broad charge transport pathways, leading to the best thermoelectric performance (σ ≈ 7 S cm⁻¹ and PF ≈ 25 μW m⁻¹ K⁻²) and the highest μC* ≈ 27 F cm⁻¹ V⁻¹ s⁻¹ for OECT devices. This study proves the strength of regulating molecular weight to enhance electronic transport in extrinsically-doped conjugated polymers.