Deciphering the Metal-C_(60) Interface in Optoelectronic Devices: Evidence for C_(60) Reduction by Vapor Deposited Al

摘要

The formation of interfacial midgap states due to the reduction of buckminsterfullerene (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)~(·-)) as well as amorphous carbon created by further degradation of C_(60)~(·-). 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 rnetal-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.