edioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS), [ 9 ] sol-gel-processed metal oxides, [ 10 ] self-assembled monolayers, [ 11 ] quantum dots, [ 12 ] 2D nanomaterials, [ 13,14 ] ionic compounds, [ 15,16 ] and oxygen-plasma treatment. [ 17 ] However, those methods suffer from various drawbacks, such as an acidic nature, a high processing temperature, poor stability in air, or processing that is incompatible with various printing fabrication techniques (i.e., the roll-to-roll printing method), which need to be overcome for their successful application to organic electronics.Conjugated polyelectrolytes (CPEs), π-conjugated polymers with ionic groups on the side chains, have been used to effectively reduce the energy barrier between the metal electrode and the organic active layer through the development of an electric dipole layer and a shift in the vacuum levels of the adjacent active layers. [ 5,8,[18][19][20][21] Furthermore, CPEs offer several advantages over other interlayer materials: neutrality (pH ≈ 7), which enhances the stability of the devices during operation; solution-processability, which provides their compatibility with roll-to-roll printing; a room-temperature process, which is necessary for plastic electronics; and property-controllability through a design of their chemical structure, such as π-conjugated backbones and ionic groups, [ 22,23 ] which ensures their boundless potential as interlayers for the fi ne modulation of the WFs of metal electrodes in various applications. [ 8 ] However, all CPEs developed thus far can only decrease the WFs of metals, and they have been utilized only as a cathode interlayer. [ 18 ] Recently, a new class of CPEs (henceforth referred to as p-type CPEs) for anodes was developed through a facile oxidative doping of pristine anionic CPEs (henceforth referred to as n-type CPEs) by Bazan's group in University of California, Santa Barbara (UCSB) and also our group in Gwangju Institute of Science and Technology (GIST). [ 24,25 ] The p-type CPEs show a broad WF tunability for various metal electrodes by forming self-aligned electric dipoles with a reversed direction to those of n-type CPEs, [ 25 ] and they exhibit superior hole-extracting ability in polymer solar cells (PSCs) or small molecule solar cells according to previous works. [25][26][27] However, in spite of the successful incorporation of p-type CPEs into various organic electronics as effi cient interlayers, the fundamental mechanism for how the direction and strength of electric dipoles are reversed By coating conjugated polyelectrolytes (CPEs) on metals, the work function (WF) of metals can be tuned by electric dipoles formed through the selfassembly 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, t...
