On the Growth of Evaporated LiF on P3HT and PCBM

The electronic properties of thin layers of LiF evaporated onto phenyl-C₆₁-butyric acid methyl ester and poly(3-hexylthiophene) thin films have been examined to better understand how alkali halide salt layers at the interface between the metal electrode and the organic conducting material improve device performance. The chemical state of LiF was investigated for various layer thicknesses via X-ray photoelectron spectroscopy (XPS) and angle-resolved XPS. LiF was found to remain nondissociated up to a layer thickness of 30 Å layer and also for sequential depositions of the salt. The electronic properties of the valence band were determined for a range of thicknesses with ultraviolet-photoelectron spectroscopy. Concentration depth profiles were obtained via ion scattering spectroscopy. An interfacial dipole forms on the organic surfaces, and a closed layer of salt was not formed with 16 Å of salt deposition as shown via metastable induced electron spectroscopy. The combined results indicate that the salt layer forms as nanoscale islands with some diffusion into the polymer layer.