Deposition pathway-induced structural and energy-level variations at an organic–metal interface: C60 and PC61BM on Au(111)

Energy-level alignment at organic–metal interfaces governs charge injection and extraction processes in molecular electronics and organic optoelectronic devices. Frontier-level alignment at organic–metal interfaces is jointly governed by the intrinsic molecular electronic structure, the substrate work function, and molecule–substrate interactions, including charge transfer, interface-dipole formation, orbital hybridization, and polarization screening. Within this framework, the role of deposition pathway and growth kinetics in shaping interfacial electrostatics remains insufficiently clarified. In this work, we present a comprehensive comparative study of C60 and [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) thin films deposited on Au(111) by thermal evaporation and electrospray deposition. By combining x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, inverse photoemission spectroscopy, and low-energy electron diffraction (LEED), we correlate interfacial chemistry, structural ordering, and frontier electronic structure as a function of deposition method and film thickness. For C60, thermal evaporation and electrospray deposition produce markedly different long-range overlayer structures, as revealed by LEED, yet the HOMO- and LUMO-derived feature positions remain essentially invariant. In contrast, PC61BM exhibits deposition-dependent differences in XPS spectra and frontier-level alignment that become more evident at higher coverage, with thermally evaporated films showing an overall downward shift of the HOMO- and LUMO-derived feature positions relative to electrosprayed films. These results demonstrate that the relationship between deposition pathway, film structure, and frontier-level alignment is strongly molecule-dependent. The deposition-dependent structural differences observed for C60 are not accompanied by measurable frontier-level shifts, whereas the frontier-level alignment of PC61BM shows larger deposition-dependent differences at higher coverage.