Study On Microcavity Top-emitting Organic Light-emitting Devices With Periodic And Gradual Change Of Cavity Length

Organic light-emitting device,(OLED) due to small size, light weight, high brightness, low cost, wide viewing angle and flexible has become a promising new type flat panel display technology.OLED devices are generally divided into the top-emitting OLED (TOLED) and the bottom-emitting OLED (BOLED). TOLED has a lot of advantages such as high brightness, large aperture ratio, but the TOLED make only certain wavelengths of light with resonant microcavity mode can emit in a particular point, the Full Wave at Half Maximum (FWHM) of light waves will be narrow. The intensity and wavelength are also will be different at different angles. This paper interests in solving the problem of TOLED of the light wavelength changing at different angles. We will fabricate TOLED with grating. At this condition, the micro-resonant cavity length can change from single to manifold. Then grating can weaken or eliminate the luminous intensity and color changing at different angles.Based on the above solution is proposed, microcavity effect of TOLED can be solved. We completed the following work:First of all, we use MATLAB software simulating the transmission spectra of TOLED with grating and TOLED without grating. And we obtained wider spectrum of TOLED with grating. Than we compare different grating height effect on the spectrum, grating height are 0nm, 10nm, 20nm, 30nm, 40nm, 50nm, 60nm, the results showed than optimal grating height are 50-60nm; after that we optimize the distance from light-emitting layer (EML) to the top-interface. We choose distance which are 10nm, 20nm, 30nm, 40nm, finally get the optimal distance are 20-30nm; finally, make the minimum distance fine-tuning between light-emitting layer to the bottom-interface. We obtain the best conditions: grating height is 60nm, the distance from EML to the top-interface is 30nm, the minimum distance from EML to the bottom-interface is 45nm, The largest distance from EML to the bottom-interface is 105nm.Second, we fabricate grating micro-structure by means of double-beam interference. Spin-coating speed, solubility of photoresist, laser-power have been determined. They are 0.025g/ml, 8000rpm and 0.25mW. We change exposure time of the nanosecond laser to get the best conditions for grating to meet the requirements of TOLED. Selected exposure times are 5s, 8s, 10s, 12s, 15s, 20s. Eventually determine the best time is 15s. And we need evaporate anode Ag after fabricated grating, we use the atomic force microscope to test their surface morphology and then observe deposition whether affect the surface of grating. The results showed that surface topography of grating and surface topography of evaporated silver Ag just changed a little.Third, in order to improve the performance of the device, we need to select an anode buffer layer. We choose a method which is exposed Ag UV in a specific UV light box, and we can obtain a thin layer of Ag2O at the anode surface as an anode buffer layer. The reason is that Ag2O's the top of valence band and exactly match the HOMO energy level of NPB, making the hole injection barrier of Ag/Ag2O/NPB less than the Ag/NPB of hole injection barrier. However, the thickness of the silver oxide can not be too thick, otherwise the electrode reflectivity and conductivity will be worse. We get the optimal condition by controlling the exposure time. At beginning, Ag exposure time are 20s, 40s, 60s, 80s and we measure the devices, found that efficiency just has a little difference under the different conditions, but charge injection are better when Ag exposure time are 60s, 80s, and then we fabricated devices whose Ag exposure time are 60s, 70s, 80s, 90s respectively. The results showed that optimal UV exposure time is 70s.In order to fill the grating to make the device have several resonant cavity length, we also need spin-coating a hole injection or transport layer after get the anode buffer layer by UV exposure to reduce the height of the original grating. Three different solutions are attempted to spin-coating: 1.PEDOT, 2.PVK: m-MTDATA (1:1, 10 mg/ml, solvent: THF), 3.PVK: m-MTDATA (1:1, 10 mg/ml, solvent: chloromethane). And we measured and observed the grating's surface morphology to determine which kind of solution is the best. We finally selected PVK: m-MTDATA (1:1, 10 mg/ml, solvent THF) as the hole injection and transport materials for the fabrication of the devices.Finally, we have obtained many optimal conditions before fabricating the devices. Then we fabricated the devices and obtained the results that peak of wavelength moving with the different are much more weakenedIn summary, we fabricated TOLED with grating and used the fluctuation of grating in TOLED to change the disadvantage that they have only one micro-resonant cavity length and make them have multiple micro-resonant cavity. This can solved the problems that only certain wavelengths of light with resonant cavity mode has been able to emit at a particular point. The phenomenon of the intensity and wavelength changing at different angles phenomenon no longer exist. This work has an important meaning on the solution of microcavity effect of TOLED, develop a new way on studying the wide-spectrum TOLED, propose a new solution for the intensity and color of light changing at different viewing angle in display applications.