The Influence of Backbone Correlation Length and Dopant Strength on the Thermoelectric Power Factor of Semiconducting Polymers

The performance of doped semiconducting polymers is strongly governed by processing methods and underlying thin-film microstructure. We report on the influence of different doping methods (solution vs vapor) on the thermoelectric power factor (PF) of PBTTT molecularly doped with F$_{n}$TCNQ ($n=$ 2 or 4). The vapor doped films have over two-orders of magnitude higher electronic conductivity ($\sigma )$ relative to solution-doped films. On the basis of resonant soft x-ray scattering, vapor-doped samples are shown to have a large orientational correlation length (OCL) ($e.g. $length-scale of aligned backbones) that correlate to a high apparent charge carrier mobility ($\mu )$. Interestingly, the Seebeck coefficient ($\alpha )$ is largely independent of OCL. This reveals that, unlike \quad electronic conductivity, leveraging strategies to improve $\mu $ does not have a dramatic impact on $\alpha $. Overall, our work introduces important general processing guidelines for the continued development of doped semiconducting polymers for thermoelectrics.