Deciphering the Metal-C60 Interface in Optoelectronic Devices: Evidence for C60 Reduction by Vapor Deposited Al

The formation of interfacial midgap states due to the reduction of bucluninsterfullerene (C-60) to amorphous carbon upon subsequent vapor deposition of Al is confirmed using Raman spectroscopy and X-ray, ultraviolet, and inverse photoemission spectroscopies. We demonstrate that vapor deposition of Al results in n-type doping of C-60 due to an electron transfer from Al to the LUMO of C-60, resulting in the formation of midgap states near the C-60 Fermi level. Raman spectroscopy in ultrahigh vacuum clearly identifies the presence of the C-60 anion radical (C-60(center dot-)) as well as amorphous carbon created by further degradation of C-60(center dot-). In contrast, the interface formed by vapor deposition of Ag shows only a slight Ag/C-60 interfacial charge displacement with no evidence for complete metal-to-C-60 electron transfer to form the anion radical or its further degradation products. These results confirm previous speculations of metal-induced chemical damage of C-60 films after Al deposition, which is widely suspected of decreasing charge collection efficiency in OPVs, and provide key insight into charge collection at metal/organic interfaces in such devices.