[15p-2N-1~17] 12.2 Characterization and Materials Physics

[15p-2N-10] [Young Scientist Presentation Award Speech] Anion Accumulation at the Surface of Polar Molecular Films Studied by Negative-Ion Photoemission Spectroscopy

〇Hiroumi Kinjo¹, Hyunn Lim¹, Jyunki Yamazaki¹, Tomoya Sato¹, Yusuke Ozawa¹, Shigehito Yakamitsu¹, Nguyen Thanh Luan¹, Yuki Uragami¹, Yutaka Noguchi², Yasuo Nakayama³, Hisao Ishii^1,4 (1.Chiba Univ. AIS, 2.Meiji Univ., 3.Toukyou Univ. of Sci., 4.Chiba Univ. CFS)

Keywords:Organic Semiconductor,Surface Potential,Electron Affiity

Spontaneous orientation polarization has often been observed for organic light-emitting diode (OLED)-related materials. For example, 500nm thick Alq3 film evaporated in dark condition gives about 30V potential across the film. This is called giant surface potential (GSP). When this polarized film was illuminated by visible light, this potential was decayed. Negative ions created by photocarrier generation are drifted toward the surface and finally accumulated as anions. Then the GSP is cancelled. Thus formed Alq3 anion has excess electron in their LUMO level. In this study, we observed the ionization energy of anion which corresponding to the electron affinity of neutral molecule by using high-sensitivity photoelectron spectroscopy and photoelectron yield spectroscopy. In comparison with IPES, the electron affinity investigated by NI-PES is slightly larger than that by IPES. Anions, which is negative carrier, in organic films are relaxed by polarizations of surrounding molecules. In IPES, anion in final state is relaxed by mainly electron polarization which is fast process. On the other hand, anion in NI-PES is in initial state and other slow relaxation proses can contribute to NI-PES. Moreover, polarization charges originated from spontaneous orientation also relax anions by Coulomb interaction. The energetic relaxation of anions by the polarization charge can be utilized to control the device performance such as injection nature. The method to investigate the anion state by photoemission can be developed to extend it as a tool to probe the unoccupied state of organic materials without sample damage. The obtained electron affinity reflects not only several types of relaxation processes but also the effect of polarization charge.