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 buckminsterfullerene (C₆₀) 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 C₆₀ due to an electron transfer from Al to the LUMO of C₆₀, resulting in the formation of midgap states near the C₆₀ Fermi level. Raman spectroscopy in ultrahigh vacuum clearly identifies the presence of the C₆₀ anion radical (C₆₀^•-) as well as amorphous carbon created by further degradation of C₆₀^•-. In contrast, the interface formed by vapor deposition of Ag shows only a slight Ag/C₆₀ 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 C₆₀ 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.