Direct Evidence for the Role of the Madelung Potential in Determining the Work Function of Doped Organic Semiconductors

The work function of a model degenerately doped organic semiconductor $p$-doped poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonic acid) can be systematically tuned over an eV-wide range by exchanging excess matrix protons with spectator cations, without altering the organic semiconductor doping level or polaron density. Ultraviolet photoelectron spectroscopy reveals this to arise not from an interface dipole, but from a bulk effect due to a shift in the Madelung potential set up by the local counter- and spectator-ion structure at the polaron sites. Electrostatic modeling of this potential is in agreement with the observed shift in carrier energetics. The spectator cations also cause a systematic shift in electron-phonon coupling and carrier delocalization, as revealed by infrared and Raman phonon modes, and charge-modulated absorption, which can be related to disorder in this potential.