Effects of process and post-process treatments on the electrical conductivity of the PEDOT:PSS films

Poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrene sulfonate) (PSS) complex is of considerable interest for a variety of applications such as the hole-collecting electrode in solar cells. The conductivity of PEDOT:PSS is low, therefore it is doped or treated with solvents having high boiling points and high dielectric constants resulting in enhancement of conductivity and improvement of efficiency of devises fabricated from this polymer. In this work, to enhance the conductivity of PEDOT:PSS films, N-methylpyrrolidone (NMP) was added into aqueous dispersion of PEDOT:PSS and the conductivity of PEDOT:PSS films prepared on glass was investigated as a function of annealing temperature. The electrochemical properties of untreated and solvent-treated PEDOT:PSS films were studied through cyclic voltammetry. To study the impact of preparation conditions, the films were prepared at different coating speeds and coating times. To investigate the effect of annealing conditions on the conductivity, the films were annealed in an inert environment (N-2) and air. Surface topography images obtained by atomic force microscopy showed that addition of NMP into PEDOT:PSS resulted in a uniform distribution of PEDOT domains and smoother phase edges, which could enhance the charge transfer. The conductivity of films increased with lowering the coating speed, which resulted in a higher thickness. Slower coating speeds providing enough time for PEDOT to crystallize led to a uniform distribution of PEDOT regions as compared to faster speeds. Conductivity of the treated films decreased with thermal annealing in both air and N-2 indicating that the O-2 molecules had no negative effect on the conductivity of PEDOT:PSS films. Hence, the decreased conductivity could be ascribed to interruption of the chains, which changed the morphology and conductivity. The encapsulated films stored in a normal laboratory atmosphere for 30 days exhibited a similar to 50-60 % decrease in conductivity. However, with applying a second thermal annealing step, the original value of the conductivity was recovered.