Quantifying Doping Efficiency to Probe the Effects of Nanoscale Morphology and Solvent Swelling in Molecular Doping of Conjugated Polymers

We study the doping of conjugated polymers from droplets of molecular dopant solutions, as might be used in additive manufacturing approaches. We compare the doping efficiency of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) solutions between two model conjugated polymers, regioregular poly(3-hexylthiophene) (P3HT) and poly(bithiophene-thienothiophene) copolymer with a triethylene glycol side chain (P(g(3)2T-TT)). We find that F4TCNQ dopes P(g(3)2T-TT) more efficiently from solution, producing films with >10(3) times higher conductivity. Using spectroelectrochemistry to calibrate polaron spectra to known hole injection levels, we quantify the doping efficiency (polarons created/dopant molecule added) to be higher than 170% for P(g(3)2T-TT) but only 47.2% for P3HT. We further explore the differences in molecular doping using a combination of scanning Kelvin probe microscopy (SKPM) and conductive atomic force microscopy (cAFM). We explore doping efficiency and aggregation as a function of the solvent of the dopant solution, side chain, and regioregularity of conjugated polymers; we show that the doping efficiency and dopant aggregation are both correlated with the ability of the dopant/solvent solution to swell the conjugated polymer, with combinations that swell, resulting in more efficient doping and smoother films with less aggregation.