Study On White Organic Light-emitting Devices Using Complementary Colors And Their Color Stability

In the background of the global energy shortage, organic light-emitting diodes (OLEDs) hasbeen recognized as the most promising high-tech field in21th century because of their excellentproperties, such as wide material sources, high efficiency, low power consumption, rich colors,flexible display in large area and environmental friendliness etc. Over the rapid development inlast two decades, the activities on OLEDs turn to focus on the development of the commercialproducts, derived from the monotonously fundamental research. In order to satisfy therequirements of next generation of flat panel display and solid-state lighting, much more effortshould be made on the further improvements of OLEDs’ comprehensive performance.Currently, the performance of the white organic light-emitting diodes (WOLEDs), such asefficiency and lifetime has basically reached the commercial standards. In practical applications,both in the fields of lighting and display, a superior stability of WOLEDs is also required.However, in the process of fabricating devices, we found that most WOLEDs have the problemof color shift, regardless of a single emitting layer or multi emitting layer is applied. The varietyof the spectrum is the main factor of the color shift. In this paper, some mechanisms of theinfluences of color stability are revealed and WOLEDs based on complementary colors withhigh efficiency and color stability are fabricated via the structure of a single emitting layer ormulti emitting layer. The main contents involved in this dissertation are listed as follows.1. We firstly analyzed the influence of color shift, e.g., carrier trapping, exciton quenching andexciton recombination zone alternation. After that, some methods are put forward to improvethe color stability, such as (1) Rational utilizing of energy transfer mechanism, and limitingcarrier trapping mechanism. Low dopant concentration if there exist carrier trapping.(2)Employ the methods such as extending recombination zone, balancing the carrier injection andtransport, or reducing interface barrier potential of organic layer to decrease the color shiftcaused by exciton annihilation.(3) Select organic materials with carrier mobility independentelectric field to stable exciton recombination zone.(4) Regulate carriers and exciton through theinterface layer to balance various dye excitons. Red and yellow color are the basic colors of WOLEDs, whereas carriers trapping mechanism widely exists in red and yellow color withnarrow band gap. So we clearly analyzed that limit carriers trapping mechanism can stablizespectrum in WOLEDs through specific experiments.2. Highly efficient all fluorescent single emitting layer WOLEDs have been fabricated by themethod of co-doping and non-doping. WOLEDs fabricated in these two methods both have acurrent efficiency (CE) over11cd/A and superior color stability over a wide range ofluminance. The main reasons are as follows:(1) In co-doping system, the shift ofelectroluminescent (EL) spectra often occurs due to concentration quenching and carriertrapping of yellow or red dye. While in our WOLED based on the method of co-doping, theconcentration quenching and carrier trapping are almost non-exist because of the lowdoping concentration of PT-01(~0.5wt.%).(2) In non-doping system, the reason of colorstability not only due to the emission of ultrathin PT-01and PT-86from energy transfer notcarrier trapping, but also due to the carrier mobility of PT-05independent of electric field.From the analyses of the singlet exciton distribution and the influences of the thickness of thehost layer, PT-86and PT-01are evaporated at the proper location in the emitting layer andgetting a high efficiency and stable all fluorescent WOLEDs. The devices based on the methodof non-doping not only have similar performance as the co-doping one, but also have higherrepeatability, which makes them appropriate for a mass-production process.3. For the WOLEDs with multi emitting layer, the charge carriers and exciton were regulated byinserting the interlayer Ir(ppz)3between the two emissive zones based on two complementaryblue (DBFDPOPhCz:Firpic) and yellow (CBP:(Fbi)2Ir(acac) or PO-01) emitters. Theefficiencies and spectra of WOLEDs can be easily tuned by the thickness of the interlayer andthe yellow-EML (Y-EML). The device with1nm thick interlayer and3nm thick yellow emittinglayer obtains very high efficiencies of42.4cd/A and47.6lm/W and a luminance of1056cd/m2was realized at a low voltage of3.5V. In addition, another device with2nm thick interlayer and5nm thick Y-EML exhibited nearly voltage-independent EL spectra. Commission Internationalde L’Eclairage (CIE) coordinates of this device only changes from (0.333,0.436) at a luminanceof100cd/m2to (0.330,0.434) at that of10000cd/m2, nearly independent of the driving voltage.4. In order to simplify the device structure and reduce the production costs, high efficiency andcolor stability all phosphorescent WOLEDs were fabricated by the single host with multi emitting layer. External quantum efficiency (EQE) of the PO-01based yellow OLED issignificantly increased by co-doping FIrpic and PO-01into the common host of mCP. Detailedinvestigation indicates the enhancement is ascribed to effective triplet exciton gathering byFIrpic, then followed to PO-01by efficient energy transfer. A noticeable EQE enhancement isrealized in the WOLED according to this principle. At the same time, the spectra of WOLEDsare rather stability due to the emission of PO-01mostly from energy transfer not carrier trapping.This work makes it easier for a single host WOLED to simultaneously harvest high efficiencyand color stability. In order to test this mechanism whether suitable for other broadband gaphosts, we choose TCTA as host and find that the energy transfer from FIrpic to PO-01also exsit.The device with TCTA as host and hole transport layer shows high efficiency and color stability.A maximum EQE, PE and CE of16.1%,41.0lm/W and47.4cd/A at1mA/cm2, and a luminanceof1000cd/m2was realized at a low voltage of3.9V. In addition, The CIE coordinates only showa slightly shift from (0.401,0.473) to (0.404,0.474) as the luminance varied from67cd/m2to5094cd/m2.