Dual Electric‐Double‐Layer Model for Work Function Reduction by Polyethyleneimine Coated on Zinc Oxide: All‐Atom Molecular Dynamics Simulations

Polyethyleneimine (PEI) can reduce work function when applied to cathode surface and to improve the drive efficiency of various organic electronic devices. Clarifying the mechanism of the work function reduction is important in developing alternative materials with higher stability. In this paper, a PEI thin film coated on a ZnO layer using all-atom molecular dynamics simulations is analyzed. The simulations show that the entire PEI thin film induces an electrostatic potential shift of 0.30 eV, whose magnitude and sign are in good agreement with experiments. Further analysis reveals that there are two electric double layers (EDLs) at the ZnO/PEI interface and PEI/vacuum surface and that the latter plays a major role. The coil-shell conformation of PEI at the PEI/vacuum surface forms the outer EDL and reduces the work function. At the ZnO/PEI interface, on the other hand, the coil-shell conformation is less dominant because of the Zn-N interaction and the inner EDL causes a small increase of the work function. This dual EDL model is different from the single EDL model for self-assembled monolayers in that polymer conformations are essential. The report clarifies the novel polymer-specific mechanism of work function reduction and opens up the possibility of new cathode modifiers.