Side-Chain Ionicity Governs Heavy Oxidative Doping and Electronic Structure Evolution in Conjugated Polyelectrolytes

Side chains influence not only solubility but also solid-state molecular organization and charge transport. Here, we establish side-chain ionicity as a decisive variable governing heavy oxidative doping. We compared poly(4H-cyclopenta[2,1-b:3,4-b′]dithiophene-alt-2,1,3-benzothiadiazole) (PCPDTBT) derivatives sharing an identical conjugated backbone but distinct side-chain functionalities: anionic CPE-Na, neutral CP, and cationic CPE-Br. Their solid-state organization spanned edge-on semicrystalline packing to an amorphous structure lacking detectable long-range order, yielding distinct doping behaviors. Only amorphous CPE-Br accommodated extensive dopant uptake through its loosely packed cationic network and preserved transport connectivity. This high local dopant density reorganized the density of states rather than rigidly shifting the Fermi level, reversing the Seebeck coefficient sign and yielding S = –34.8 μV K–1 at 20 mM FeCl3. To our knowledge, this is the first polarity inversion in a conjugated polyelectrolyte. Side-chain ionicity is therefore a molecular design parameter for oxidative doping, and long-range crystallinity is unnecessary for polarity inversion.