Imidazolium chloride salt was demonstrated as a novel water-processable, strong base-free electron dopant for single-walled carbon nanotubes (SWCNTs). By thermoelectric evaluation and various photoelectron spectroscopy analyses associated with the comparison of the redox potentials, the valence change of the anion (Cl-/Cl3-) was presented as a possible mechanism for electron injection into the nanotube conduction band. The obtained n-type SWCNT films presented good thermoelectric power factor exceeding 100 mu W m(-1) K-2, nearly identical to that of the original p-type film.