Fabircation And Study Of New Doping Structure White Organic Light-Emitting Diodes

In1987, C. W. Tang and VanSlyke firstly used aromatic diamine derivative (NPB) as holetransporting material,8-hydroxyquinoline aluminum (Alq3) as electron transporting and emittingmaterial to fabricate organic light-emitting diodes (OLEDs) with vacuum evaporation method,and achieved a low operating voltage (<10V), high brightness (>1000the cd m-2) and highefficiency (1.5lm W-1) device. C. W. Tang and coworkers’ pioneering work was a milestone inthe development of the organic electronics. Since then, the OLED research carried out in theworldwide. Subsequently in1993, J. Kido and his coworkers fabricated a white organiclight-emitting diodes(WOLED) using a doping system, which is another leap forward of theOLED, and greatly expanded the scope of application. After rapid development of nearly20years, white organic light-emitting diodes has been recognized as a new generation of solid statelighting, full-color display and LCD backlight, and since the advantages and potentialapplications, OLED has become the most promising flat panel display technology. Theadvantages of OLED displays are that it can provide more realistic colors, more outstandingdynamic pictures. At present, the small size OLED display products have been published, suchas the Samsung GALAXY AMOLED mobile phones of South Korea; Samsung7.7-inchAMOLED Tablet PC; Sony head-mounted3D OLED displays HMZ-T1and so on. But there arealso many deficiencies of OLED products, such as the high price, and limit for large-size displayapplications.Nowadays, the fabrication process and preparation method are not perfect for WOLED,which can not meet the large-scale industrial production needs. One of the main factors affectingthe application of WOLED is the white color stability. For developing low-cost, highperformance, high-quality, color-stable white emission, we designed two new doping structures of all-fluorescent white organic electroluminescent devices. We tested the EL properties and thenanalysed and discussed the improvement of the white color stability in detail.In Chapter II, we use DCJTB as the dopant material, and doped it into the hole-transportingmaterial NPB and the electron-transporting material Bepp2, simultaneously, to fabricate adual-doping emitting layer white device. When DCJTB was doped into Bepp2separately,through comparing the EL spectrum and analysing the electroluminescent characteristics, wefound that when the doping concentration was changed from0.2wt%to0.1wt%, the efficiencyincreased1.5times, but the light became blue. This dual-doping structure can achieve highefficiency WOLED as well as reduce the doping concentration. We test the EL properties of thedevice, the turn-on voltage is3.2V, the maximum luminance is19670cd m-2(903mA cm-2) at12V, and the maximum current efficiency and luminous efficiency is4.29cd A-1and4.21lmW-1, respectively.Generally, the hole mobility of the organic photoelectric small molecule is larger than theelectron mobility. The mismatching carrier mobility often leads to a too large current density,low efficiency, and poor voltage tolerance. So we introduced TCTA as a hole buffer layer tobalance the number of hole and electron in the recombination zone. As a result, the currentdensity has declined, the maximum current efficiency is5.97cd A-1, the maximum luminousefficiency is4.49lm W-1, the maximum brightness is23910cd m-2, and the CIE coordinates is(0.327±0.006，0.340±0.007) which is very close to the ideal white equal-energy point (0.33,0.33). From the point of view of brightness, CIE coordinates, spectra stability, it is an ideal whiteemission device. The feature of this device is fully aware of the equivalent carrier transmissioncapacity of NPB and Bepp2, which results in the recombination and excitons on both sides of theinterface, and takes full advantage of the excitation state energy with the dual-doping structure,thus achieving efficient and stable white emission.In Chapter III, firstly, based on the same device structure, a dual-doping WOLED ofRubrene is fabricated. The turn-on voltage is2.8V, maximum brightness is10060cd m-2, themaximum current efficiency and luminous efficiency is up to5.47the cd A-1and4.37lm W-1.But its chromaticity stability is poorer than the device based on DCJTB which is owing to the fact that the energy transfer from Bepp2to Rubrene is bad. So we dope Rubrene into NPB andBepp2,separately, to fabricate two single-doping white devices. NPB single-doping device’sperformance is much better than that of Bepp2, and similar to that of the dual-doping device. itsturn-on voltage is2.9V, the maximum brightness is6717cd m-2, the maximum currentefficiency and luminous efficiency is6.06cd A-1and4.13lm W-1, respectively. In addition, wefabricate the doping film of DCJTB:Bepp2and Rubrene:Bepp2with a concentration of0.1wt%. The photoluminescence spectra show that DCJTB has a much larger emission intensity inthe same excitation conditions. These results suggest that the defects of the energy transfer fromBepp2to Rubrene is the main reason of the morderate device performance and the chromaticitystability.In Chapter IV, we choose DCM as a red dopant. Through analysing the photophysicalproperties of DCM, Bepp2and their doping film, we design a new doping structure with gradientconcentration of0.2wt%and0.5wt%DCM in Bepp2. We obtained an ideal white lightemission CIE coordinates (0.334±0.002,0.337±0.007), and almost in all the driving voltagethe chroma stability is very high, which is much better than that of Rubrene and DCJTB. Inaddition, the device has a maximum current efficiency of14.0±0.35cd A-1, maximum luminousefficiency of9.2±0.25lm W-1, maximum external quantum efficiency of5.6±0.15%, and thecolor rendering index is up to79-81, the correlated color temperature is5400K-5600K. It hasmet requirements of high-quality white emission. This device’s most important feature is throughusing the means of different doping concentration in different regions of the emitting layer todeal with the moving or widening of the exciton recombination zone as well as the differentexcitation degree of the host-guest materials under different driving voltage, in order toachieving more stable white emission in the entire drive voltage.In summary, we have designed and fabricated two new type of all-fluorescent dopingstructure WOLED, which improve the chromaticity stability, and achieved high-efficiency,high-quality white emission. The low-cost fluorescent materials and the excellentelectroluminescent properties show the potential application value of these two newstructures in OLED development.