Kenji Okumoto

Green fluorescent organic light-emitting device with external quantum efficiency of nearly 10%

Green fluorescent organic light-emitting device (OLED) exhibiting a high external quantum efficiency of nearly 10% has been developed. The OLED consists of simple three organic layers, using NPB, 0.8% C545T doped TPBA, and DBzA as a hole-transporting layer, an emitting layer, and an electron-transporting layer, respectively, [fluorocarbon coated indium tin oxide/NPB (60 nm)/08% C545T doped TPBA (40 nm)/DBzA (20 nm)/LiF (1 nm/Al], where NPB is 4,4′-bis (N-phenyl-1-naphthylamino)biphenyl, C545T is 10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-benzo[l]pyrano[6 7 8-ij]quinolizin-11-one, TPBA is 9,9′,10,10′-tetraphenyl-2,2′-bianthracene, and DBzA is 9,10-bis[4-(6-methylbenzothiazol-2-yl)phenyl]anthracene. The high external quantum efficiency is maintained in the wide range of current density of 2–100 mA∕cm2. The current efficiency and power efficiency of the OLED are also very high, 29.8 cd/A and 26.2 lm/W, respectively, at a current density of 20 mA/cm2. The OLED is promising for prac...

High efficiency red organic light-emitting devices using tetraphenyldibenzoperiflanthene-doped rubrene as an emitting layer

Red organic light-emitting devices (OLEDs) have been developed employing a novel fluorescent emitting layer, tetraphenyldibenzoperiflanthene-doped rubrene. The devices are characterized by low driving voltage below 4V at a current density of 20mA∕cm2 and high color purity with Commission Internationale de l’Eclairage coordinates of (0.66, 0.34). The OLED using the novel emitting layer in combination with the electron-transporting layer consisting of 9,10-bis[4-(6-methylbenzothiazol-2-yl)phenyl]anthracene exhibits a high power efficiency of 5.3lm∕W at a current density of 20mA∕cm2. The half-luminance lifetime of the red OLED is 223h at a current density of 80mA∕cm2 (initial luminance of 3570cd∕m2). Both the driving voltage and current efficiency of the device are significantly improved compared to a device using tris(8-quinolinolato)aluminum as an electron-transporting layer. The studies on charge transport for the host materials indicate that the high efficiency is attributed to the improved charge inject...

High Efficiency Stacked Organic Light-Emitting Diodes Employing Li2O as a Connecting Layer

We demonstrate the high-efficiency stacked organic light-emitting diodes (OLEDs) introducing new connecting layers. In the green stacked OLEDs, the external efficiencies increase proportionally to the number of the stacked units without suffering the decrease in power efficiency. The current, power and external efficiencies at 0.5 mA/cm2 of the stacked OLED with six stacked units (6-stacked OLED) have reached 235 cd/A, 46.6 lm/W, and 65.8%, respectively. Furthermore, we have applied the connecting layers to a white stacked OLED and fabricated an active-matrix full-color display with a low temperature polysilicon thin film transistor backplane. In the device, the current efficiency of the white 2-stacked OLED is enhanced by a factor of 2.2. The initial luminance drop is significantly suppressed for the white 2-stacked OLED compared to 1-stacked OLED. The proposed white stacked OLED technology can be applied to a full-color display for a practical use.

Organic light-emitting devices using polyacene derivatives as a hole-transporting layer

Three polyacene derivatives, 2-tert-butyl-9,10-di(2-naphthyl)anthracene (TBADN), 9,9′,10,10′- tetraphenyl-2,2′-bianthracene, and 5,6,11,12-tetraphenylnaphthacene (rubrene), are found to successfully function as hole-transporting layers (HTLs) in red organic light-emitting devices (OLEDs) using tetraphenyldibenzoperiflanthene as an emitter. Compared with an OLED with the most widely used HTL material, the arylamine derivative, 4,4′-bis(N-phenyl-1-naphthylamino)biphenyl (NPB), the OLEDs with the polyacene derivatives exhibit advantageous performance such as lower driving voltage, higher electroluminescence efficiency, or longer luminance lifetime, depending on the employed HTL material. Current efficiency of the red OLED with a TBADN HTL is 5.5cd∕A with Commission Internationale de L’Eclairage chromaticity coordinates of (x=0.66, y=0.34), which is much higher than the value of 2.1cd∕A with the same chromaticity for the OLED with a NPB HTL. The driving voltage and luminance lifetime of the red OLED with rubr...

Reduction in Power Consumption for Full-Color Active Matrix Organic Light-Emitting Devices

The active matrix organic light-emitting diode (AMOLED) is expected to serve as next generation flat panels display with the outstanding features of wide viewing angle, vivid images, and quick response. For practical use of full-color AMOLEDs in mobile devices, it is essential to reduce the power consumption, which is generally higher than that of liquid crystal displays (LCDs). For this aim, a red, green, blue, and white (RGBW) pixel format combined with an RGB color filter array (RGBW format) with a common white emission layer (EML) has been developed. We find that the RGBW format can successfully reduce the power consumption of a full-color AMOLED by nearly half that of a conventionally filtered RGB pixel format. This improved power consumption is almost equal to the power consumption of a same-sized LCD. The RGBW format is a promising technique for the further reduction of the power consumption of a full-color AMOLED.