Cluster Beam Deposition of Cu2-xS Nanoparticles into Organic Thin Films

Bulk-heterojunction films composed of semiconductor nanoparticles blended with organic oligomers are of interest for photovoltaic and other applications. Cu2-xS nanoparticles were cluster beam deposited into thermally evaporated pentacene or quaterthiophene to create bulk-heterojunction thin films. The nanoparticle stoichiometry, morphology, and chemistry within these all-gas phase deposited films were characterized by X-ray photoelectron spectroscopy (XPS) and electron microscopy. Cu2-xS nanoparticles were (at most) only slightly copper-deficient with respect to Cu2S; similar to 2.5 nm diameter, unoxidized Cu2-xS nanoparticles formed in both pentacene and quaterthiophene, as the matrix was not observed to impact the nanoparticle morphology or chemical structure. Cluster beam deposition allowed direct control of the nanoparticle stoichiometry and nanoparticle:organic ratio. Chemical states or Wagner plots were combined with other XPS data analysis strategies to determine the metal oxidation state, indicating that Cu(I) was predominant over Cu(II) in the Cu2-xS nanoparticles.