Transmission electron microscope observation of organic–inorganic hybrid thin active layers of light-emitting diodes
© Jitsui and Ohtani; licensee Springer. 2012
Received: 16 July 2012
Accepted: 12 October 2012
Published: 25 October 2012
We performed transmission electron microscope (TEM) observation of organic–inorganic hybrid thin films fabricated by the sol–gel reaction and used as the active layers of organic light-emitting diodes. The cross-sectional TEM images show that the films consist of a triple-layer structure. To evaluate the composition of these layers, the distribution of atoms in them was measured by energy-dispersive X-ray fluorescence spectroscopy. As a result, most of the organic emissive material, poly(9,9-dioctyl-fluorene-co-N-4-butylphenyl-diphenylamine (TFB), was found to be distributed in the middle layer sandwiched by SiO and SiO2 layers. The surface SiO layer was fabricated due to the lack of oxygen. This means that the best sol–gel condition was changed due to the TFB doping; thus, the novel best condition should be found.
Organic light-emitting diodes (OLEDs) have been energetically investigated for application to flat-panel displays and illumination light sources[1–6]. However, the operation lifetime of OLEDs is shorter than that of inorganic LEDs because OLEDs are strictly affected by the oxidant effect. Very recently, we fabricated organic–inorganic hybrid LEDs in which the active layers consisted of organic emissive materials dispersed in SiO2[7, 8]. These novel LEDs exhibit very long operation lifetime because the organic emissive materials in SiO2 are protected against the oxidant effect. However, the structures of the fabricated organic–inorganic hybrid active layers are still unknown. In this research, we observe them by transmission electron microscope (TEM).
The organic–inorganic hybrid thin films were fabricated by sol–gel method. Poly(9,9-dioctyl-fluorene-co-N-4-butylphenyl-diphenylamine) (TFB) was used as an organic emissive material, while perhydropolysilazane (PHPS) was used as a sol–gel reaction accelerator. The ratio of TFB to PHPS was changed from 1 to 50 wt.%. They were dissolved together in xylene at a density of 3.5 wt.%. Then, a thin film of the TFB-PHPS solution was fabricated on a sufficiently cleaned SiO2 substrate by spin-coating method. Next, the samples were annealed to remove the xylene. Finally, the thin film was turned into the organic–inorganic hybrid material by humidity treatment of 90% RH at 50°C for 180 min. This temperature and humidity constituted the best condition for the sol–gel reaction of PHPS recommended by a chemical company, Sanwa Kagaku Corp. (Nagoya, Japan).
TEM observation was performed using JEM2100F (JEOL, Tokyo, Japan). To evaluate the distributions of atoms in the organic–inorganic films, energy-dispersive X-ray fluorescence spectroscopy (EDS) observation was additionally performed using JEM2100F. The samples were irradiated using an N2 laser for the photoluminescence (PL) measurement. PL spectra were measured using a multi-channel spectroscope (USB-2000, Ocean Optics, Dunedin, FL, USA). PL measurements were performed in atmosphere and at room temperature.
Results and discussion
Mass distributions of the three atoms, carbon, oxygen, and silicon in the three layers
Organic–inorganic hybrid thin films fabricated by sol–gel reaction were investigated in detail by TEM and EDS observations. They consisted of triple-layer structures; most of the organic emissive material TFB was distributed in the middle layer sandwiched by SiO and SiO2 layers. The surface SiO layer was formed due to the lack of oxygen. This means that the best sol–gel condition was changed due to the TFB doping; thus, the novel best condition should be found.
The authors would like to thank Miwako Toda and Junko Morita for their cooperation in the TEM observation.
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