| Due to its light weight, small size, low cost, high brightness, wide viewing angle and flexible display,Organic Light-Emitting Device,(OLED) has become a promising new type flat panel display technology. However there are still many problems in producing large size OLED display device, which severely limits its market process. Among all the problems, efficiency is the most serious one.There are Surface Plasmon Polaritons (SPP) on the upper and nether interfaces of the metal electrodes in TEOLED. In general devices, this mode propagate along the interface between organic/metal and metal/air. Its strength exponentially decays along the direction which is perpendicular to the interface of the metal, so this model can only propagate to a limited range, approximately micron or nanometer scale. Research shows that, in TEOLED there are about 40% of the energy dies away due to the existence of SPP when light transits the metal layers. Thus it can be seen, if we can stimulate the SPP of the OLED into visible light, the efficiency of the OLEDs will be significantly improved.Based on the above principles, we introduced periodic grating microstructure into OLED. Modulated the cross coupling of the SPP through the gratings, stimulated it into visible light emission, and finally to improve the efficiency of the devices. Specific work as follows:Firstly, we fabricated small molecule OLEDs using vacuum deposition method. The device structure is Glass/Alq3(100nm)/Ag(55nm)/NPB(60nm). Using Nd:YAG laser with the wavelength of 355nm as a illuminant for double-beam interference experiment. Calculated the interference light angle by the formula .Whereλis the wavelength, T is the grating period.We got grating with good uniformity by ablating NPB layer,. Using 405nm light as a pump, we stimulated the Alq3 to shine. In the photoluminescence device without grating structure, the intrinsic spectrum of Alq3 was observed. In the devices with grating structure, the photoluminescence peaks of the stimulated SPP was clearly observed. We measured the emission spectrum at different angles using fiber spectrometer. Compared the spectrums with the angle changing. We recorded the maximum wavelength and the viewing angle, and mapped the dispersion relation. The experimental results and theoretical simulation mainly agreed each other. We confirmed that in the photoluminescence device we made, the best condition of the grating period is 500nm.On that basis, we fabricated OLED with organic capping layer, the device structure is SiO2/ Ag(80nm)/ MoOx(5nm)/ NPB(50nm)/ Alq3(50nm)/ LiF(1nm)/ Al(1nm)/ Ag(20nm)/ NPB(60nm). Devices with grating structure were made by the ablation of NPB. We changed the grating period, and compared device performance with no grating devices. Both theoretical calculation and experimental results show that the device with 400nm grating can get the most significant increased performance. The maximum luminance is 121717 cd/m2, and the maximum efficiency is 7.02 cd/A, increased by 37%.In OLEDs with double corrugated electrodes that have been reported, because of the phase of the two SPP mode doesn't match each other, there are destructively interfere between them. The SPP mediated emission is weaken. In order to eliminate this destructively interfere, we increased the silver thickness to 60nm. Then we prepared TEOLED devices with one and two-dimensional grating structure of metal, with 400nm, 500nm and 600nm periodic grating. Among them, for 2D device, due to the structure of grating electrode material has been damaged by laser, its efficiency reduced. But the luminance and the efficiency of one-dimensional grating structure device are both increased. The device with 500nm grating can get the most significant increased performance. The maximum luminance is 128296 cd/m2, and the maximum efficiency is 6.67 cd/A, increased by 63%.Using the threshold effect of the ultraviolet absorption of silver electrode, we carried out interference photolithography for the whole device substrate without mask. Due to the low power laser can only ablate the organic part around the devices, we can create a device with grating structure around and the internal structure of the device has not been effected. Because of the loss of waveguide mode has been weaken, and there are feedback effect by the grating structure in organic layers. The performance of the device with grating around structure has increased by a certain degree. Among them, the one-dimensional 600nm period device, can get the maximum luminance of 84441cd/m2, the maximum efficiency is 6.16cd/A, increased by 46%. And for the 2D structure, 600nm device can get the maximum luminance 118822 cd/m2, the maximum efficiency is 7.51 cd/A, increased by 78%. So, by introducing grating microstructure around TEOLED, the device efficiency can be significantly improved. However, in the TEOLED we made, the mechanism of the feedback device structure and its relation to microstructure parameters is still unclear. We will continue to work on the theoretical analysis and experimental research, strive to promote TEOLED performance to a new level.To sum up, we introduced periodic grating microstructure into TEOLED. We fabricated TEOLED with grating structure in organic capping layer, TEOLED with grating of metal electrodes structure and TEOLED with grating around respectively. The three kinds of devices all have improved efficiency. Demonstrated the introduction of the periodic grating into OLED has important significance. |
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