Hard-Cation-Soft-Anion Ionic Liquids for PEDOT:PSS Treatment.

A promising conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) experiences significant conductivity enhancement when treated with proper ionic liquids (ILs). Based on the hard-soft-acid-base principle, we propose a combination of a hydrophilic hard cation A + (instead of the commonly used 1-ethyl-3-methyl imidazolium, EMIM + ) and a hydrophobic soft anion X - (such as tetracyanoborate, TCB - ) as the best ILs for this purpose. Such ILs would decouple hydrophilic-but-insulating PSS - from conducting-but-hydrophobic PEDOT + most efficiently by strong interactions with hydrophilic A + and hydrophobic X - , respectively. Such a favorable ion exchange between PEDOT + :PSS - and A + :X - ILs would allow the growth of conducting PEDOT + domains decorated by X - , not disturbed by PSS - or A + . Using density functional theory calculations and molecular dynamics simulations, we demonstrate that a protic cation- (aliphatic N -alkyl pyrrolidinium, in particular) combined with the hydrophobic anion TCB - indeed outperforms EMIM + by promptly leaving hydrophobic TCB - and strongly binding to hydrophilic PSS - .