The application of polyethyleneimine (PEI) on the cathode has been shown to improve the stability of organic thin-film devices. We performed molecular dynamics (MD) calculations to model linear and branched PEIs (lPEI, bPEI) and simulate the polymer/ZnO interfaces. The results confirm the formation of an electric double layer at the PEI/vacuum interface, while at the ZnO/PEI interface, N-H adsorption leads to charge density dispersion and a vacuum level shift of about 0.3 V. This reveals a mechanism for improved device stability consistent with the experimental results.