Role of Thick-Lithium Fluoride Layer in Energy Level Alignment at Organic/Metal Interface: Unifying Effect on High Metallic Work Functions

We have investigated the function of ~3 nm thick lithium fluoride (LiF) buffer layers in combination with high work function metal contacts such as coinage metals and ferromagnetic metals for use in organic electronics and spintronics. The energy level alignment at organic/LiF/metal interfaces is systematically studied using photoelectron spectroscopy and the integer charge transfer model. The thick-LiF buffer layer is found to pin the Fermi level to ~3.8 eV regardless of the work function of the initial metal due do energy level bending in the LiF layer caused by depletion of defect states. At 3 nm thickness, the LiF buffer layer provides full coverage, and the organic semiconductor adlayers are found to physisorb with the consequence that the energy level alignment at the organic/LiF interface follows the integer charge transfer model’s predictions.