  "Secondary doping" in
poly(3,4-ethylenedioxy-thiophene):poly(styrenesulfonate) (PEDOT:PSS) is quite
effective and a frequently used method for the conductivity enhancement. This
simple approach, the addition of co-solvents to the PEDOT:PSS solution before
film fabrication, is an essential way to derive the high electrical performance
of PEDOT:PSS, but the mechanism still remains unclear. In this study, nanoscale
structural changes synchronized with the conductivity enhancement via secondary
doping in PEDOT:PSS films were investigated. During secondary doping with
ethylene glycol near the critical dopant concentration, non-crystalized PEDOT
molecules uncoupled from PSS chains and then underwent nano-crystallization.
These structural changes might be the key driving force for conductivity
enhancement via secondary doping.
