Radical Cation-Anion Coupling-Induced Work Function Tunability in Anionic Conjugated Polyelectrolytes

By coating conjugated polyelectrolytes (CPEs) on metals, the work function (WF) of metals can be tuned by electric dipoles formed through the self-assembly of cation-anion pairs on the side chains of CPEs. Recently, it has been reported that a pertinent oxidative doping of anionic CPEs results in a reversed direction in the net electric dipoles, thereby yielding opposite WF tunability compared with pristine CPEs. However, the fundamental mechanism of this reversed WF tunability is not clearly understood. Here, through a systematic study of the relationship between the chemical structure of anionic CPEs (n-type) and the corresponding oxidation-processed CPEs (p-type), it is clearly demonstrated that radical cations are generated on the π-conjugated backbone of the n-type CPEs and strongly paired with anions on the side chains of the n-type CPEs, thereby reversing the direction of the total net electric dipoles. Furthermore, it is found that the degree of radical cation-anion coupling in the CPEs is a key factor in determining their WF tunability. It is envisioned that the results will provide a fundamental understanding of CPEs for versatile interface engineering in organic electronics. The work function tunability of p-type conjugated polyelectrolytes (CPEs) is found to be strongly dependent on the degree of coupling between the radical cation on the conjugated backbone and the anion on the side chain, which can be controlled by the ionization energy of pristine CPEs. These results promise versatile interface engineering in organic electronics with substantially enhanced device efficiencies and lifetimes.