Beyond Integer Charge Transfer: A New Design Rule Linking Dopant Strength and Counterion Separation to Efficient Carrier Transport in Conjugated Polymers

Organic semiconductors, particularly conjugated polymers, are critical for advancing flexible electronics. Their electrical conductivity is enhanced by increasing carrier concentration through doping by blending small atoms or molecules, which can introduce Coulombic traps and disrupt packing. Here, we elucidate a molecular design principle that leverages dopant energy depth quantified by ΔE, which is the difference between appropriate frontier orbital energies of the doped polymer and its counterion, to mitigate disorder and simultaneously improve conductivity and the Seebeck coefficient. Using a modified Gaussian disorder model that incorporates carrier screening, we simulate the density of states and validate our findings experimentally with iodine- and Magic Blue-doped poly(3-hexylthiophene) (P3HT). Our results reveal that deeper dopant frontier energy levels significantly reduce energetic disorder and enhance transport properties when the carrier-counterion separation is small. We identify a saddle-point behavior in conductivity as a function of ΔE and show that ΔE can significantly boost conductivity when the dopant is located close to the backbone. This work introduces ΔE as a powerful molecular lever for designing high-performance organic semiconductors, offering a new design rule to overcome the long-standing trade-off between conductivity and Seebeck coefficient, thereby increasing thermoelectric efficiency in doped conjugated polymers.