Study Of Efficient White Polymer Light-emitting Devices From Blends

Organic light-emitting diodes (OLEDs) include organic small molecule light-emitting diode (OLED) and polymer light-emitting diode (PLED), they have drawn great attentions by academic and industrial sectors because of the attractive prospect of application in the display, lighting, backlighting and other fields. After two decades of research, organic light-emitting materials and devices have come from the laboratory to the market, and begun to enter industrialization. However, all the commercial products are currently based on small molecule material by vacuum deposition process, which requires expensive equipment, and complicated the production of full color displays using traditional masking technologies. PLED base on solution-processes such as spin-coating, ink jet, and screen printing can overcome the disadvantagies and access full color and larger size display sizes at much lower costs, which attract more and more research specialist staff.In the area of display, the PLED technology include passive matrix PLED (PM-PLED) and active matrix PLED (AMPLED). In the future, at the PLED industrial development, the global trend is based on development of small-size PM-PLED and then to large-sized AM-PLED product. And the thin-film transistor (TFT) is essential in an active matrix display. Compared with inorganic TFT, organic thin film transistor (OTFT) have certain advantage in many respects, such as the diversity of material, to achieve flexible display on the flexisble substrate, easy to realize large area, and low-cost, simply processing ( such as spin coating, printing, evaporation and other means) and low temperature, so based on organic thin-film transistor driven organic electroluminescent (OTFT-PLED), namely so called "all organic display", pay more widespread attention. Therefore, in the previous chapter of this article, we proceed from prototype device of active matrix PLED, combined with the advantages of polymer light-emitting diodes and using screen-printing technology, investigated the technology and related physical problems for integrating organic thin-film transistor (OTFT) and polymer light-emitting diode (PLED).In addition, the white polymer light-emitting diodes (WPLED) as lighting and backlighting are important applications for PLED technology in the future. However, white emission PLEDs are less efficient with respect to power efficiency (PE) and luminous efficiency(LE), when compared with devices fabricated using vacuumdeposition technologies, and are still far away from practical applications for solid-state lighting. In order to improve the performance of white PLED, which related to the development of new materials, matching options of material systems, optimization of device structure to balance charge injection. The other part content of this article focuses on these issues and in order to obtain high-performance white polymer light-emitting device.We report a single emission layer white PLEDs by triple doping of RGB iridium metal complexes or double doping of red and blue Ir complexes with appropriate ratio into poly(N-vinylcarbazole) (PVK) host in presence of electron transport material 1,3-bis[(4-tert-butylphenyl)-1,3,4-oxadiazolyl] phenylene (OXD-7). After proper heat treatment, the triple-doped polymer WOLEDs have a peak PE of 9.53 lm W-1 (Device E) /9.95 lm W-1 (Device F) for forward viewing (corresponding to a total PE of 19/20 lm W-1) at 7.2/6.9 V, and a peak LE of 24.3 cd A-1 for forward viewing (corresponding to a total peak LE of 48 cd A-1), at 20.8 mA cm-2. Then, we use the sky-blue phosphor-based complexes emission FIrpic (~ 470 nm) and the yellow light emitting material with EL peak at 560-570 nm to compose complementary colors to obtain white light-emitting devices, and the current efficiency of the device achieved 42.9 cd / A for forward viewing (corresponding to the total current efficiency up to 80 cd / A), the power efficiency of 20 lm / W (corresponding to the total power efficiency up to 40 lm / W).An outstanding advantage of these polymer WOLEDs lies in very simple single emissive layer structure, only solution-processed technology is involved, that is no additive hole-blocking or electron transport layer was incorporated through extra vacuum-deposited technology, which ensures fully exploiting the potential of low-cost fabrication of polymer optoelectronic device.In most of the reported efficient WPLEDs, triplet emitter with sky-blue emission was used as a key component to fabricate phosphor-based device despite their relative poor color quality. Besides, unstable PVK was used as host material in these devices, which would degrade significantly during operation, thus limit their practiacal applications. In order to overcome this problem and obtain efficient WPLEDs with high color quality and long-term stability, an alternative approach is to use efficient deep-blue polymer as both host material and blue emitter to fabricate multiple dopants all-polymer WPLEDs. The polymers used here include, a newly synthesized efficient deep-blue emitting polyfluorene derivative named poly[(9,9-bis(4-(2-ethylhexyloxy)phenyl)fluorene)-co-(3,7-dibenziothiene-S,S-dioxide10)] (PPF-3,7SO10), a green light-emitting poly [2-(4-(3',7'-dimethyloctyloxy) -phenyl) -p- phenylenevinylene] (P-PPV) and an orange-red light-emitting 2-methoxy-5- (2'-ethyl-hexyloxy)-1, 4-phenylenevinylene (MEH-PPV), respectively. Optimized device shows a peak luminous efficiency of 14.0 cd A?1 and a peak power efficiency of 7.6 lm W-1, with a CIE of (0.33, 0.35) at a current density of 10 mA cm-2.Finally, the balanced carrier is key to improve the efficiency of white device. For the modification of cathode, white polymer light-emitting diodes (WPLEDs) with bilayer structure were fabricated by spin coating method using different solubility of polymers. By inserting a layer of water-soluble electronic transporting material of PFN approaching to the cathode, the maximal luminance efficiencies of 5.3 cd/A is achieved with CIE coordinates of (0.34, 0.36). By modified the cathode, the LE of WPLED was enhanced by 100 %. And for modification of anode, influence of three types of poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS), whose nominal conductivity varied in 5 orders in magnitude (~1 S cm-1, ~ 10?3 S cm-1, and 10?5 S cm-1, respectively), on the device performance of polymer-based phosphorescent organic light emitting diodes (PhOLEDs) were investigated. It was found that PEDOT:PSS (Baytron 8000) with the lowest conductivity resulted in superior device performance, in term of peak luminous efficiency (LE) and peak external quantum efficiency (EQE). When compared to that of devices with the routine PEDOT: PSS (Baytron P 4083) and the one with the highest conductivity (Baytron P), the device performance with PEDOT:PSS (Baytron 8000) as anode buffer layer was enhanced by 59 % and 91 %, respectively, in term of peak LE and peak EQE in PhOLEDs. It was found that improved manipulation of leakage current at small bias region, and more balanced charge carrier are responsible for the enhancement. Furthermore, novel glycerol modified PEDOT 8000 anode buffer layer whose conductivity increase as many as two orders of magnitude was developed to enhance overall device performance. These discoveries can potentially enable further improvement of the present efficiency of polymer light-emitting devices, indicating that polymer light-emitting devices (PLEDs) can achieve comparable device performance with vacuum-deposited small molecular devices.