Role of Polymer-Dopant Distance on Charge Transport in Amorphous and Crystalline Domains in Doped Conjugated Polymers

Conjugated polymers lack free carriers and thus need to be chemically doped to increase conductivity. This process, however, introduces energetic disorder and charge traps. Therefore, there is a compelling need to identify factors that affect dopant-polymer interactions and modify them suitably for improved charge transport. The effect of Coulomb interaction of charged polymer and dopant counterion is one such factor. In literature, some studies ascribe reduced Coulomb interactions to increased counterion size while others to increased molecular ordering. We resolve this apparent conflict by evaluating the impact of doping the crystalline and amorphous domains on the density of states (DOS) and how each domain type contributes to charge transport. We measured Seebeck coefficient (α) and electrical conductivity (σ) of gradually dedoping of regioregular and regiorandom P3HT films and their blends. We used wide angle X-ray scattering and UV-Vis-near IR spectroscopy to characterize the doped P3HT films and fitted data to a phonon-assisted hopping model of charge transport to extract the dopant-induced disorder from the calculated DOS. We show that the regiorandom and regioregular domains in the doped polymer simultaneously contribute to the charge transport with their individual contribution depending on their doping level. We also show that any observable impact of counterion size or paracrystallinity on charge transport can be correlated to polymer-dopant distance, <i>R_s</i>. Our two-DOS model of crystalline and amorphous regions connects well to the observed trends in the α-σ curves of regiorandom and regioregular P3HT blends. Our work shows the amorphous domain encounters a larger dopant-induced disorder and also contributes to charge transport significantly. Thus, it is important to take into account the effect of dopant-induced disorder on particular structural regions while designing polymers and dopants for improved charge transport.