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

Abstract

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-C60 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.

The effect of excitation wavelength and metallic nanostructure on SERS spectra of C60

“…Although, the excitation wavelength dependence is a major factor affecting the enhancement, as demonstrated by Ye et al [25] and by Alvarez-Puebla, [26] greater understanding can be gained by combining the excitation wavelength dependence with an investigation of the SERS substrate morphology. In our study, we thermally evaporating tapered thin films of Au using a shadow mask that causes a shadowing effect across the Au film's borders and generates a range of Au nanostructure morphologies at this region, as similarly published by Oates et al [27] Fullerene C 60 is a molecule with closed-cage structure that has been widely studied and used in technological applications such as biological sensing, [28] drug delivery, [28] organic light emitting diodes, [29] organic photovoltaics, [30,31] and organic field effect transistors. [32,33] We chose Fullerene C 60 as our SERS probe molecule because it is a highly symmetric molecule that produces uniform films via thermal evaporation, it is optically stable, and it does not have strong fluorescence that would have limited our excitation range.…”

“…Fullerene C 60 is a molecule with closed‐cage structure that has been widely studied and used in technological applications such as biological sensing, drug delivery, organic light emitting diodes, organic photovoltaics, and organic field effect transistors . We chose Fullerene C 60 as our SERS probe molecule because it is a highly symmetric molecule that produces uniform films via thermal evaporation, it is optically stable, and it does not have strong fluorescence that would have limited our excitation range.…”

Growth, thermal desorption and low dose ion bombardment damage of C60 films deposited on Cu(111)

“…The literature on C 60 films growth on metal substrates is abundant [8,[13][14][15][16][17][18][19][20][21][22][23][24]. Particularly on Cu(111) substrates, STM experiments combined with theoretical calculations of bands structure [16], and photoemission [19] were used to demonstrate that electron transfer from the Cu(111) substrate to C 60 varies between 1 and 2 electrons per molecule.…”

Spontaneous generation of interlayers in OPVs with silver cathodes: enhancing Voc and lifetime

“…2 In the direct architecture, the top metal contact should attain a low work function for electron extraction, and hence, aluminum and calcium are the common preferences (F = 4.11 eV and 2.8 eV respectively). 3,4 However, their low work function inherently imposes low environmental stability due to oxidation when exposed to ambient conditions 5,6 and interaction with the underlying organic film. 7 Indeed, the limited lifetime associated with direct architecture devices is often attributed to the oxidation of the metal electrode and not necessarily the degradation of the organic materials.…”