Study On Materials Of OLED Via Photoelectron Spectroscopy

Study on materials of OLED via Photoelectron SpectroscopyDespite great efforts have put on the development of organic light-emitting devices, there were still some major problems need to be solved. There is no satisfactory theory based on these problems, such as luminescence properties, carrier transport properties, the relationship between the molecular structure of materials and luminescence behavior. Compared to inorganic semiconductor, organic semiconductors have not yet formed a complete theoretical framework. The main reason impeding its application is the accelerated failure of these devices at elevated temperature. This may be caused by reactions with impurities or degradation at the electrode/organic interface. Also the high production cost of OLED displays is far more expensive than the commercial LCD dispalys. Intensive studies on some important basic problems of organic light-emitting devices are needed in order to enhance the performance of the devices and meet the requirements of commercial production.Organic light-emitting devices usually use more multi-layered sandwich structure, which involves the anode/anode barrier layer, anode barrier layer/organic emitting layer, organic emitting layer/cathode. We can get the qualitative elements of valence, surface energy and surface elements with the content of information using the current most popular surface analysis equipment: x-ray and ultraviolet photoelectron spectroscopy. We can intuitively understand the behavior of various interfaces, and the doping mechanism by analysising the photoelectron spectroscopy. The results provide a convincing theoretical support to the production of highly efficient and stable OLED device.Based on these points, this paper studied on the ITO anode OLED by means of photoelectron spectroscopy and optimized the performance of the device, using ErF3 thin layer as an electrode buffer layer for the first time.In this paper, two issues were studied by XPS as following: 1,The influence to device performance of OLED from different concentrations of sulfuric acid treatment on the ITO anode and the washing of ITO substrate on the residual sulfuric acid affects on test results.Treating ITO cleaned with conventional cleaning process with 60℃water bath for 10 minutes ultrasound using the difference of 0%, 30%, 60%, 98% sulfuric acid solution. We removed the residual acid with organic solvents. The ITO will be treated as an anode of OLED, producing a device structure: ITO/NPB (40nm)/AlQ (40nm)/Al (120nm). Device test results showed the best device performance after treatment in the sulfuric acid of 60%. XPS test results are very different using different methods to washing residual sulfuric acid of the surface when we study on photoelectron spectroscopy of ITO which on the treatment carried out for the production of the device. New organic pollution, mainly C contamination will introduced when organic solvents (acetone, ethanol) are used for cleaning residual sulfuric acid .It may result of errors in subsequent analysis. Cleaning the residual sulfuric acid with deionized water, it will leave a residual S element resulting in acid pollution, and the effect on NPB film and the ITO work function and carrier is unclear. Meanwhile, the study found: with increasing of the concentration of sulfuric acid solution treating the surface of ITO, the oxygen vacancy concentration on ITO surface is greatly reduced; the pollution of c has decreased significantly. It is most obvious when the ITO surface is treated with 98% sulfuric acid in the experiment. Reduction of oxygen vacancies and decrease of the c pollution make the work function of ITO increase, so the Fermi level of ITO and organic layers HOMO more matches, more conducive to the hole injection. But in tests of Luminance, the 60% sulfuric acid-treated ITO device made of the highest brightness, not the 98%. The surface of ITO film heated 60-degree water bath for 10 minutes of concentrated sulfuric acid in the ultrasonic overreacting leads to the destruction of surface, so that the quality of NPB film in the above deteriorates. Device of 4 # low-voltage existing in a larger current from the device's current - voltage characteristic curve also prove uneven NPB's film. These factors result in the luminance level of device made of 60% sulfuric acid treatment is higher than concentrated sulfuric acid. UPS test results showed the surface work function of ITO treated by 60% sulfuric acid solution increase about 0.22ev comparing with conventional cleaning. The test demonstrates the surface work function of ITO treated by 60% sulfuric acid increases indeed.2,For the first time,ErF3 was used as the cathode modification material.we studied the influence to the efficiency of OLED when ErF3 thin layer was used as a cathode modification layer on the OLED. And also, we studied the feasibility when ErF3 was used as the anode modification material.We used the ErF3 ultra thin layer as the cathode modification layer for the first time.And we produced a modified device, the structure is: ITO / NPB (40nm) / AlQ (40nm)/ErF3 (xnm) / Al (100nm), the thickness x were 0, 0.5, 1, 1.5 nm. Device brightness test showed: a thickness of 0.5nm was the best; the maximum brightness is 4 times the brightness without modification. We analyzed the following reasons:(1) With XPS test tools,we found, although between AlQ and ErF3 had reactted, and in the XPS spectra is mainly reflected in the F 1s and Al 2p binding energy of the high end of the emergence of new peaks. AlQ reacted with ErF3, in which F atom released electronics to the AlQand Al 2p emerged of new peaks due to Al-F formation. ErF3 thin layer as a cathode with the introduction of modified layer was the same principle as LiF thin layer in References [48], effective optimization of the device.(2) ErF3 is an insulator, which resistivity is more than the Organic layer's. ErF3 thin layer, only 0.5nm, as the cathode modification layer, electric of the ErF3 layer is far outclass organic layers. According to I.D.Parker tunneling model, the Fermi energy of cathode and organic film LUMO can be differential, thereby increasing the risk of electronic injected.(3) ErF3 thin layer can stop the Al atoms spreeding to AlQ.if the Al atoms had spreaded to the AlQ, the grid of AlQ might distortion and even cause AlQ recrystallizate.And they will affect the life and brightness of OLED. In addition, in the environment of water, the hydrolysis of AlQ will happen, the hydrolysis product will reaction with adsorption of oxygen that AlQ had.The product may affect the efficiency of OLED.In conclusion, ErF3 ultra thin film as a Cathode modified layer can not only However, as the thickness of increases, the electric field in film will be greatly reduced, the electron tunneling probability greatly reduced, and the device's turn-on voltage increased. Therefore its thickness should be moderate.As the anode layer of OLED, ErF3 thin layer affectted device shine performance has the following respects:(1)Basing on XPS test tools, we found that,among NPB,ITO and ErF3 did not produce chemical reaction, but the ErF3 thin layer can be conducive to the growth of NPB.(2)As a insulator, ErF3 thin layer make the hole injection only by tunneling, blocking the part of the hole, to a certain extent, electronic and hole can be more balance,and the efficiency of OLED was enhanced.(3)ErF3 thin layer can stop the Indium and Tin of ITO to NPB,it can reduce the luminescence quenching centers and light traps.In conclusion, ErF3 thin layer used as an anode modification can improve the performance of OLED. In the visible range, ErF3 is not transparent, if make OLED as the bottom emission structure, ErF3 thin layer used as an anode modification would dramatically reduce the Luminous intensity of OLED. In order to solve this problem, OLED can be made as the top emission structure.But limited the conditions of our team, we have not made the OLED as the top emission structure.