The emission of the new inorganic EL device using perovskite-type oxides
Japan's National Institute of Advanced Industrial Science and Technology (AIST) developed a red inorganic EL device that enables surface emission at a low AC voltage of approximately 10V.
The emission voltage of the device is 1/10 or less that of the existing inorganic EL devices, according to AIST. With the EL device that enables surface emission of red light at a low voltage, it is possible to achieve a wider view angle with the surface emission method, while reducing the size of the power supply.
AIST expects to apply the device to lighting equipment, light sources, displays, etc in the future by improving the luminance and increasing the number of emission colors.
The new inorganic EL device uses a perovskite-type oxide for the material of each constituent layer. According to AIST, perovskite-type oxides are chemically stable, and their properties are less likely to be deteriorated by oxidation or heat. Thus, sealing and other processes can be shortened, simplifying the production process, AIST said.
Thus far, AIST has discovered that many perovskite-type oxides emit strong fluorescent light by UV excitation and succeeded in reducing the thickness of emission layer. This time, it made efforts to develop a highly stable inorganic EL device by stacking those perovskite-type oxides with insulator thin films.
The newly developed inorganic EL device was produced by forming an insulating layer, an emission layer and another insulating layer on an electrode substrate in this order by pulsed laser deposition (PLD). An ArF excimer laser (wavelength: 193nm) was used as the light source.
These layers were formed at a substrate temperature of 700°C and a growth atmosphere of 10Pa oxygen. Then, the layers were heat-treated in the air, and a transparent electrode was formed by PLD to produce the inorganic EL device, AIST said.
The electrode substrate, emission layer and insulating layer are all made of relatively abundant materials, according to AIST. Specifically, the electrode substrate is made of strontium titanate doped with 1% niobium (Nb) (1%Nb-SrTiO3). The emission layer is made of calcium strontium titanate ((Ca0.6Sr0.4)TiO3), which is a perovskite-type oxide (ABO3), doped with a small amount of praseodymium (Pr) as a luminescent center in the A-site. Strontium titanate (SrTiO3), another perovskite-type oxide, is used for the insulating layer.
These thin films were grown successively by PLD to form a "thin-film EL device with a double insulating layer structure," which is a device composed of an emission layer sandwiched between two insulating layers. An indium tin oxide (ITO) or SnO2 film is used for the transparent electrode on the device.
At first, AIST fabricated an inorganic EL device with a single emission layer. When a 14V, 1kHz AC voltage was applied, the entire surface of the transparent electrode emitted red light. The device had an emission spectrum with a sharp peak at the center wavelength of 612nm.
This is considered to be due to the energy transition of Pr3+ ions from the 1D2 to 3H4 level. The voltage at the beginning of emission was about 10V.
Furthermore, AIST made a device with two emission layers and achieved an intensive surface emission of red light at 24V, which is approximately double the voltage applied to EL devices with a single emission layer.
The research results will be published in German science journal "Advanced Materials."
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