Study On White Organic Light-Emitting Devices Based On Complementary Color Of Blue And Yellow

Organic Light-Emitting Devices (OLEDs) have the advantages of light weight, thin thickness, small volume, wide viewing-angle, highly efficiency, self-luminous and so on, which have made it become the most competitive display among the third-generation flat panel displays. Especially, the White Organic Light-Emitting Devices (WOLEDs) have very wide applications in the field of lighting and display. While the industrialization process of WOLEDs is hampered by the high cost of device production, instability in the performance and other factors. To solve these problems, this paper mainly focuses on studying the dual-emitting layer WOLEDs which were based on complementary color of blue and yellow, owing to the relatively simple production process. And we also try to improve the efficiency and stability of the device from the perspective of device structure.These devices of small molecules blue and yellow monochromatic OLEDs and WOLEDs in this paper were fabricated by vacuum evaporation. In fabricating the blue OLEDs, there are two kind of organic materials used as the light emitting layer. For the undoped device, the influences of light-emitting layer thickness on device performance were studied. While for the doped device, the impacts of doping concentration on the device were studied. Comparing the undoped and doped blue devices, a high efficiency device was obtained. For yellow devices, devices based on different host materials were fabricated and a novel yellow device structure was presented. At last, a WOLED with high efficiency and color stability was presented. The device was based on the complementary color and in combination with the obtained monochromatic blue and yellow emitting layers. And the mechanisms of high efficiency and color stability were analyzed in depth.The results from the blue devices with different emitting layer materials show that the performance of doped device was excellent. In the doped device, the emitting layer is formed by doping TBPe into MADN. The optimum doping concentration is5wt%, then the turn-on voltage and color coordinates are2.7V and (0.1347,0.1903), respectively. At the voltage of10V, the luminance and current density was62mA/cm and of12744cd/m, respectively. The maximum luminance of14000cd/m and luminous efficiency of7.3cd/A were obtained. The doping emitting layer can improve the luminous efficiency of the device.Yellow OLEDs can be fabricated by doping rubrene into different host materials. The novel structure of doping rubrene into MADN:TBPe to form an emitting layer can get a better performance than doping it into NPB or Alq3. For the novel yellow device, the turn-on voltage and color coordinates are3V and (0.4678,0.4970), respectively. At the voltage of10V, the luminance and current density was82mA/cm2and of13645cd/m2, respectively. The maximum luminance of21000cd/m2and luminous efficiency of10.6cd/A were achieved.Studies on the WOLEDs with dual-emitting layers have shown that the performances of these devices are strongly susceptible to the thickness of the emitting layer and the stack order of two emitting layers. When the thickness of blue-and yellow-emitting layer are20and10nm, separately, blue-light emitted from blue-emitting layer and yellow-light emitted from yellow-emitting layer are relatively well-balanced to give a white light. At the same time, the order of two emitting layers can also affect the device performance. It reveals that the emissive dopant rubrene acts as charge traps to improve electron-hole balance, provides sites for electron-hole recombination and thus makes carriers distribute more evenly in the dual-emitting layers which broaden the recombination zone and improve the stability of the CIE coordinates.Structure of ITO (160nm)/NPB (30nm)/MADN:5wt%TBPe:3wt%rubrene (10nm)/MADN:5wt%TBPe (20nm)/BCP (10nm)/Alq3(20nm)/Al (100nm) was determined to be the most favorable WOLED. The turn-on voltage is3.5V. And the color coordinates of (0.3263,0.3437) was obtained at applied current density of10mA/cm2, which is closer to the standard white light (CIE (0.33,0.33)) than the most recent reported WOLEDs. The maximum luminance of16000cd/cm2and luminance efficiency of8.6cd/A were obtained. Moreover, there is just slight variation of coordinates (△CIEx,y=0.0171,0.0167; corresponding Au’v’=0.0119) when the current density increases from10to100mA/cm2.