Overcome The Limits of Conductivity in Doped Organic Semiconductors

We study the conductivity of doped organic semiconductors and vary (i) the electrostatic environment with anion exchange of the dopant and (ii) the dielectric constant with oligo ethylene glycol side chains. Since organic semiconductors are commonly disordered materials such as polythiophenes, it is crucial to look at the conductivity on different length scales. The comparison between electrical four point probe and optical THz time domain spectroscopy is the experimental core of this study resulting in the conductivity over the long-range (millimeters) and short-range (nanometers) respectively.<br/>Anion exchange from F₄TCNQ^— to PF₆^— doubles the short-range conductivity of P(g₃2T-T) up to 610 S/cm, which is related to an increased density of contributing charges. Similarily for P(g₃BTTT) upon anion exchange the conductivity doubles to 1280 S/cm. Since PF₆^— is smaller than F₄TCNQ^— we suggest the improvement stems from a better insertion of the anion rather than an electrostatic effect. Hereby, we see an increased density of contributing charges while their mobility is limited. This is verified by fitting the short-range conductivity with the Drude-Smith model for disordered systems. Furthermore, we track the changes in absorbance in-situ during chemical doping and electrochemical doping and find a substantial relation between polaron/bipolaron ratio and the best conductivity.<br/>Oligo ethylene glycol side chains increase the short-range conductivity by increasing the number of contributing charges and their mobility in comparison to P3HT with alkyl side chains. We find that glycolated side chains help the absolute conductivity as well as its restoration (&gt;90%) from the short-range (nanometers) to the long-range (millimeters) transport. The improvement hereby can be assigned to (i) the higher dielectric constant as well as (ii) lower energetic disorder. Understanding the conductivity on the short- and long-range is key for designing efficient devices based on doped organic semiconductors.