Efficient Fluorescent/phosphorescent Hybrid White Olymer Light-emitting Diodes

White polymer light-emitting devices (WPLEDs) have attracted intense attentionin both scientific and industrial communities due to their potential applications infull-color flat-panel displays and solid-state lighting sources, given their uniqueadvantages, such as ease of fabrication, low-cost manufacturing usingsolution-processed technology, and tunable molecule structure and emission color.Therefore, WPLEDs have become a hot research topic in the field of organicelectronics, and represent promising candidate for the next-generation of solid-statelighting sources.On the other side, the performance of WPLEDs is much lower than that of theorganic light-emitting devices (OLEDs) based on vapor-deposition technology so far,mainly in terms of efficiencies. Therefore, it is an urgent task to improve theperformance of WPLEDs in order to meet practical applications. By harvesting bothsinglet and triplet excitons, it can allow an internal quantum efficiency approach100%.Currently, due to their high efficiency, phosphorescent emitters based on smallmolecule have received intense research interest, while the most efficient WPLEDscritically rely on these phosphorescent materials. For PLEDs based on phosphorescentemitters, host-guest doping system is usually employed, rendering the deviceperformance not only depends on the development of the host and the phosphorescentguest materials, but is also related to their device structure. For the host material, itstriplet energy level should be higher than that of guest in order to avoid triplet energyback transfer from the phosphor to the polymer host, and possess efficient andbalanced carriers transporting properties. Therefore, many researches have beenfocused on the development of host materials with wide band gap and bipolartransporting properties. However, the synthesis of materials with wide band gap is notso easy and the materials are very unstable. Herein, the purpose of this study is toinvestigate and realize efficient WPLEDs from different host materials and noveldevice structures.The host materials used in this study including non-conjugated polymerpoly(N-vinylcarbazole)(PVK), polyfluorene and its derivatives (PFs). PVK is widelyemployed as host material due to its higher triplet energy states, good film-formingproperties, higher glass transition temperature and good hole transporting property. At present, the luminous efficiency (LE) of WPLEDs based on PVK host had exceed60cd/A, and power efficiency (PE) reached40lm/W. Polyfluorene is also an attractiveconjugated material due to its efficient, good color purity of blue emission as well asgood film-forming properties. However, its lower-lying triplet energy states canseverely quench the phosphorescent emission from the triplet guest. Here, wedemonstrate that the quench effect can be effectively suppressed by incorporating ananode buffer layer PVK between PEDOT: PSS and emitting layer. On the basis of this,a series of all-phosphorescent, hybrid (with both fluorescent and phosphorescentemission) WPLEDs based on PFs host were realized. These results broaden the rangeof options of host materials for WPLEDs and have vital significance tophosphorescent PLEDs. In particularly, when using deep blue emitting emitter as host,it can greatly broaden the white emission spectra, thus, WPLEDs with high efficiencyand good color quality can be readily achieved. When incorporating water/alcohol-soluble polyfluorene copolymers or derivatives with amino group (PFN) as electroninjection layer, devices efficiency can be further enhanced. The LE and PE of theoptimized WPLEDs reached21.4cd/A and15.2lm/W, respectively. Moreover, thecolor rendering index (CRI) was as high as90. In addition, when this type ofcopolymer is used as host, air-stable metal such as Al, Ag can be used as contactcathode, despite its higher turn-on voltage (Von).Finally, we also investigate the realization of stacked PLED by using a newcharge generation layer (CGL) which comprising water/alcohol-soluble conjugatedpolymer, metal Al and metal oxide MoO3. By using PFN/Al/MoO3CGL and emittingmaterial P-PPV, solution-processed stacked monochromatic PLEDs with emissionfrom two sub-units are fabricated, and the peak LE reached16.65cd/A. By using red,green, blue emitting materials PFO-DBT15, Alq3and PFSO, stacked whitelight-emitting devices were also fabricated with a PF-EP/Al/MoO3CGL. Themaximum LE reaches7.16cd/A, with a maximal PE of1.95lm/W. The Vonis foundto be8.75V, which is approximately the sum of that of sub-units. Moreover, the CIEcoordinates of white emission is (0.340,0.373), with a CRI of97, very close to that ofincandescent lamp of100.