Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States.
ACS Appl Mater Interfaces. 2013 Jul 10;5(13):6001-8. doi: 10.1021/am400640x. Epub 2013 Jun 26.
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.
通过拉曼光谱和 X 射线、紫外光和反向光发射光谱证实，富勒烯（C60）在随后蒸发沉积铝时还原为非晶碳，导致形成界面中间隙态。我们证明，由于电子从 Al 转移到 C60 的 LUMO，导致 C60 费米能级附近形成中间隙态，因此蒸发沉积 Al 会导致 C60 的 n 型掺杂。超高真空中的拉曼光谱清楚地表明了 C60 阴离子自由基（C60(•-))以及由 C60(•-)进一步降解产生的非晶碳的存在。相比之下，由蒸发沉积 Ag 形成的界面仅显示出轻微的 Ag/C60 界面电荷位移，没有证据表明完全的金属到 C60 的电子转移以形成阴离子自由基或其进一步的降解产物。这些结果证实了先前关于 Al 沉积后金属诱导的 C60 薄膜化学损伤的推测，这种化学损伤广泛被怀疑降低了有机光伏器件中的电荷收集效率，并为这些器件中金属/有机界面的电荷收集提供了关键的见解。
