Study On Large Area And High Efficiency Tandem Organic Light-Emitting Devices

Over three decades of research development, organic light emitting diodes(OLEDs) technology has been successfully used in the display and lighting products due to its superior advantages of high efficiency, healthy light, flat emission, and flexible feasibility. OLED studies, including the development of OLED devices and synthesis of new materials, are still the research hot topics in the field of organic electronics. In this thesis, we mainly focus on the investigation of device structure including novel n-type dopant in tandem OLEDs, intermediate connector structure and its physical mechanism, synthesized bipolar host material, and inverted OLEDs with low driving voltage. In industrial application, we successfully fabricated the largest area of OLED lighting panels in China using our self-designed large-area organic thin film evaporator. Detailed research contents are listed as follow.(1) In chapter 1 and 2, we introduce the technical characteristics, the industrial development status, as well as the issues in OLED lighting area, describe the principle of tandem OLEDs and corresponding internal carrier injection, transportation and recombination processes.(2) In chapter 3,Lithium hydride(LiH) is employed as a novel n-dopant in the intermediate connector for tandem OLEDs because of its easy co-evaporation with other electron transporting materials. The tandem OLEDs with two and three electroluminescent(EL) units connected by a combination of LiH doped 8-hydroxyquinoline aluminum(Alq3) and 1,4,5,8,9,11–hexaazatriphenylene- hexacarbonitrile(HAT-CN) demonstrate approximately twofold and threefold enhancement in current efficiency, respectively. No extra voltage resulted from the intermediate connector is observed. Particularly, the lifetime(T75%) in the tandem OLED with two and three EL units is substantially improved by 3.8 times and 7.4 times, respectively. The doping effect of LiH into Alq3, the charge injection and transport characteristics of LiH-doped Alq3 are further investigated by ultraviolet photoelectron spectroscopy(UPS) and X-ray photoemission spectroscopy(XPS).(3) In chapter 4, an intermediate connector(IC) consisting of lithium(Li) doped 4,7-diphenyl-1,10- phenanthroline(BPhen)/Al/tetra?uoro-tetracyanoquinodimethane(F4-TCNQ)/1,4,5,8,9,11–hexaazatriphenylene-hexacarbonitrile(HAT-CN) is developed for fabricating tandem white organic light-emitting diodes(WOLEDs). The investigation of charge generation and separation process in Bphen: Li/Al/ F4-TCNQ/HAT-CN, which is carried out by the analysis of current-voltage and capacitance-voltage characteristics, shows that the proposed IC structure is suitable as a connecting unit in tandem OLEDs. The tandem WOLED based on a silicon compound host material of 10-phenyl-2’-(triphenylsilyl)-10H-spiro [acridine-9,9’-fluorene](SSTF) with proposed IC structure exhibits a maximum current efficiency of 159.2 cd/A and a maximum power efficiency of 69.4 lm/W, respectively. For application in large-area OLEDs, a 150×150 mm2 tandem lighting panel with maximum efficiencies of 231.8 cd/A and 52.9 lm/W, correlated color temperature of 3000 K and Commission International de I’Eclairage(CIE) coordinates of(0.34, 0.45) is also demonstrated.(4) In chapter 5, high efficiency blue phosphorescence devices with quantum efficiencies above 25% are developed by using a new bipolar host material, diphenyl(10-phenyl-10H-spiro[acridine-9,9’-fluoren]-2’-yl)phosphine oxide(POSTF). The separation of bipolarity from effective SAF structure is elucidated and the versatility is evaluated by two kinds of blue phosphors. Noticeably, large-size white light-emitting panel(150×150 mm2) based on this new host is fabricated with a maximum power efficiency of 75.9 lm/W.(5) In chapter 6, green phosphorescent inverted OLEDs(IOLEDs) with 1,4,5,8,9,11–hexaazatriphenylene-hexacarbonitrile(HAT-CN)/Aluminium/n-doped 4,7-diphenyl-1, 10-phenanthroline(Bphen) used as interlayer are demonstrated. The IOLED shows the lowest driving voltage of 4.5 V at 10000 cd/m2 to date. For application in large-size OLEDs, a 120×120 mm2 flexible IOLED with same device structure is successfully fabricated.(6) In chapter 7, we fabricate the 300 × 300 mm2 OLED lighting panels using self-designed large area thermal evaporator with a good uniformity(3%) for the linear source. We also systemly study the luminous uniformity and the operating temperature of the OLED lighting panels. The luminous uniformity of the lighting panels can be improved largely by using a proper outcoupling film. At the same time, we have successfully developed a large-size(150×150 mm2) flexible OLED lighting panel.The thesis demonstates a fundamental study on n-type dopant used in tandem OLEDs, intermediate connector, novel bipolar host materials, and low operating voltage IOLEDs, which shows the significance in realization of large area OLEDs for practical applications.