Ultrathin polyethylenimine (PEI) interlayers are frequently used to reduce the work function of electrode materials; however, to exploit the full potential of this material for application in optoelectronic devices, further fundamental understanding is required. Here, it is demonstrated that intermixing of PEI deposited from water solution onto the conductive polymer poly(3,4-ethylene-dioxythiophene) (PEDOT):polystyrene sulfonate can be avoided by using anhydrous butanol as solvent. At the same time, this choice of solvent prevents de-doping of PEDOT and preserves the electrode's transparency. For inorganic electrode materials, i.e., ZnO and graphite, it is demonstrated that residual solvent in PEI films, present even after annealing in inert gas atmosphere, leads to significant surface dewetting, and thus inhomogeneity in terms of the morphology and work function. However, annealing in ultrahigh vacuum removes residual solvent, establishes homogenous surface coverage with PEI, and allows one to achieve the maximum work function reduction. For the transparent electron transport material ZnO, a record low work function value of 2.3 eV is achieved, which can enable the formation of ohmic contacts to most semiconductors.