On the Growth of Evaporated NaF on Phenyl-C61-butyric Acid Methyl Ester and Poly(3-hexylthiophene)

NaF has been considered as a material for the electron extraction layer in organic photovoltaic devices. Thin layers of NaF were evaporated onto phenyl-C61-butyric acid methyl ester and poly(3-hexylthiophene) thin films. The chemical state of NaF was observed for various layer thicknesses via X-ray photoelectron spectroscopy (XPS) for a very low deposition (similar to 1 angstrom) layer, a thick 30 angstrom layer, and also for sequential depositions of the salt. The chemical states were also mapped at different surface depths via angle-resolved XPS measurements. NaF was not found to dissociate; however, an extra chemical state in fluorine was observed on all samples and it is suspected that incidental nonstoichiometric clusters were deposited along with the expected NaF monomer units. The electronic properties of the valence band were determined via UV-photoelectron spectroscopy and metastable-induced electron spectroscopy for a range of thicknesses with the sequential depositions up to a nominal value of 30 angstrom NaF. The interfacial dipole was investigated and was seen to be affected by the deposition method of the salt and also by the salt layer thickness. Concentration depth profiles were obtained via neutral impact collision ion scattering spectroscopy, and it is believed that the salt layer forms islands with some diffusion.