Surface and interface state analysis of the TPD/Alq_3 using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM)

摘要

The surface and interface state of N, N'-diphenyl-N, N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD)/tris-(8-hydro-quinoline)aluminum (Alq_3) thin film was investigated using X-ray photoelectron spectroscopy (XPS). The morphology TPD/ Alq_3 has also been investigated using atomic force microscopy (AFM). XPS results at raw surface of the sample indicate that the O atoms originated from the absorbed O_2. The appearance of Al 2p peaks indicates the formation of interdiffusion system of TPD and Alq_3 during vacuum deposition and corresponds to the binding energy of Al~(3+). There are three components at 285.3, 286.4 and 287.3 eV, respectively. With the continuous increasing of the sputtering time, the increases of the relative intensity of the three components are visible. Additionally, all components undergo ～0.3 eV chemical shifts to lower binding. A band bending in the interface of the TPD/Alq_3 caused the observed energetic shifts. The XPS interface analysis of the TPD/Alq_3 shows that the binding energy of Al 2p, O 1s and N 1s spectra also undergo chemical shifts during sputtering. The Al 2p core level undergoes a 0.62 eV chemical shift to higher binding energy, reflecting the energy band bending at the TPD/Alq_3 interface. The chemical shift of N 1s suggests that more net charge is expected to transfer from N atoms to the Al ions for argon ion sputtering. TPD molecules have influence on Al-N bond. With the increase of sputtering time, three new components of O 1s appear at 531.9, 531.0 and 530.3 eV corresponding to C-O, C=O and Al-O bond, respectively.