Unveiling the Spectroscopic Signatures of Poly(3,4-ethylenedioxythiophene) by Controlling Polaron and Bipolaron Concentrations

The electronic and optical properties of conductive polymers (CPs) are related to the formation of polarons and bipolarons localized along the polymer backbone. Their concentration is affected by the conformational and morphological features of the CPs, making it crucial to understand how they are affected by chain conformation and π-electron delocalization. In this work, we studied the modulation of the concentration of polarons and bipolarons in PEDOT:PSS after post-treatment with hydrazine and H2SO4, which influence the optical and electronic properties. Through a combination of electron spin resonance spectroscopy, UV–vis–NIR spectroscopy, Raman spectroscopy, and conductivity measurements, we evaluate changes in the properties of PEDOT, which are related to the chain conformation and π-electron delocalization within the polymer backbone. We present experimental evidence supporting the recently revised interpretation of the UV–vis–NIR spectrum of PEDOT:PSS, and through a combination of theoretical and experimental analysis, we propose a new interpretation of the Raman spectrum of PEDOT:PSS. Furthermore, we provide the first experimental evidence of the presence of polaron pairs in the triplet state in PEDOT:PSS, which have a spin of S = 1, which were only predicted theoretically up to this point.