|Budget Amount *help
¥4,060,000 (Direct Cost: ¥3,700,000、Indirect Cost: ¥360,000)
Fiscal Year 2007: ¥1,560,000 (Direct Cost: ¥1,200,000、Indirect Cost: ¥360,000)
Fiscal Year 2006: ¥2,500,000 (Direct Cost: ¥2,500,000)
This project was conducted toward clarifying mechanism of electrophosphorescence in the organic-inorganic nano-hybrids and application of the nano-hybrids to light emitting diode. The results of this project are summarized as follows.
1. Preparation of organic-inorganic nano-hybrids and electrophosphorescence in the nano-hybrid
1) Nano-hybrid films embedded with rare earth halides (CeI_3, CeBr_3, CeF_3, TbI_3) nano-particles were prepared by simultaneous vacuum-deposition of an organic semiconductor, 4,4'-N,N'-dicarbazole-biphenyl (CBP), and rare earth halides at a base pressure of 10^<-4> Pa. The obtained films were transparent and uniform. TEM observation demonstrated that spherical nano-particles of rare earth halides were dispersed in CBP medium. The diameter of the nano-particles varied within the range of approximately 10 to 15 nm.
2) By photo-excitation, the all nano-hybrid films exhibited fluorescence from CBP. Phosphorescence from CBP and emission of rare earth halides were not o
bserved. 3) Using the nano-hybrid films, one fabricated electroluminescent (EL) devices combined with organic semiconductors, ITO/hole transport layer/nano-hybrid film/electron-transport layer/Al-Li. All devices exhibited phosphorescence (electrophosphorescence) from CBP.
2. Mechanism of electrophosphorescence in nano-hybrids
Using hole-transporting organic semiconductors and electron-transporting organic semiconductors, 6 kinds of nano-hybrid films embedded with CeF_3 nano-particles.
1) Only fluorescence from the organic semiconductors was observed in the all nano-hybrid films.
2) When hole-transporting organic semiconductors employed as emissive layer in EL devices, electrophosphorescence from the organic semiconductors was observed. On the other hand, emission from CeF_3 was observed in EL devices having the nanohybrids with electron-transporting organic semiconductor and CeF_3.
From above-mentioned results, the following mechanism of electrophosphorescence was suggested. Firstly, Ce^(3+), which has one electron in the f-orbital, easily ionizes Ce^<4+> to form closed shell. Then, the Ce^<4+> captures electron, and forms C^(3+*). In the hole-transporting organic semiconductors with low ionization potential, Dexter-type energy transfer from C^(3+*) is carried out, and forms excited triplet state of the organic semiconductors. As a result, electrophosphorescence is observed. In the electron-transporting organic semiconductor with high ionization potential, on the contrary, the energy transfer does not occur. Accordingly, emission form CeF_3 is observed. Less