The synthesis of n-type organic semiconductors is challenging as reduced states are difficult to obtain due to their instability in air. Here, we report tailoring of semiconducting behavior through control of surfactant concentration during synthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles. Nanoparticles were synthesized by miniemulsion polymerization, where stable suspensions were used to produce polymer films by a simple casting technique on poly(ethylene terephthalate) (PET) substrates. The electrical conductivity and Seebeck coefficients were measured as a function of surfactant concentration. It was found that conductivity decreases 3 orders of magnitude as surfactant concentration increased, and remarkably the Seebeck coefficient switched from p-type to n-type. To further elucidate this finding, doping effects were studied by Raman, ultraviolet−visible (UV−vis), and electron spin resonance (EPR) spectroscopies. Finally, a thermoelectric module was developed by using the n-type PEDOT synthesized in this work and a standard p-type PEDOT:PSS.