WOLEDs have a great application prospect in daily lighting owing to their merits of low energy consumption, eye protection, and additional flexibility potential. Compared with stacked WOLEDs or multiple-emitting-layer WOLEDs, SEL-WOLEDs are favored for commercialization due to the drastically simplified device structure and reduced production costs. However, a formidable challenge lies in the rational control of both singlet and triplet excitons between host materials and different color emitters in the single-emitting-layer, making simultaneous efficiency improvement and color control a fundamental issue in SEL-WOLEDs.
The observed white electroluminescence in typical SEL-WOLEDs usually involves complicated energy transfer processes from host material to emitters and between different color emitters, which induces the device design complexity. Furthermore, efficient energy transfer between different color emitters makes the emission spectra correlate greatly to the doping concentration. Actually, rather low doping concentration (
In order to simplify device design and fabrication, reducing energy transfer channels between host material and emitters in the multicomponent single-emitting-layer is an intuitive method. Recently, scientists found that host materials are hardly involved in energy transfer process when luminescent lanthanide d-f transition complexes were used as the emitters, which may be a solution for simple energy transfer regulation. In addition, d-f transition complexes have many advantages as emitters in OLEDs, such as theoretical high efficiency, short excited state lifetime, tunable emission color, and low cost due to the abundance of cerium in Earth's crust is even slightly higher than that of copper. Therefore, the exploration of d-f transition emitters in SEL-WOLEDs is promising.
In a new paper published in Light Science & Application, a team of scientists, led by Professor Zhiwei Liu from Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, China, have synthesized a sky-blue emitting cerium(III) complex Ce-TBO2Et and an orange-red emitting europium(II) complex Eu(Tp2Et)2 with d-f transition characteristic, and fabricated their SEL-WOLEDs with a simple three-layered device structure. It is found that the energy transfer from host material to emitters is eliminated and that between two emitters is hindered, hence they obtained efficient and color-stable white electroluminescence with a controllable weight doping ratio of 10% Ce-TBO2Et and 5% Eu(Tp2Et)2.
Prior to constructing SEL-WOLEDs, the electroluminescence properties of Ce-TBO2Et as a sky-blue emitter and Eu(Tp2Et)2 as an orange-red emitter were separately investigated. The Ce-TBO2Et based device B1 showed a maximum luminance of 18200 cd m-2 and a maximum EQE of 22.3%, while the Eu(Tp2Et)2 based device O1 showed a maximum luminance of 15800 cd m-2 and a maximum EQE of 11.1%. Noticeably, these performance surpass the best reported blue and red OLEDs with d-f transition lanthanide complexes.
The differences between electroluminescence and photoluminescence spectra of Ce-TBO2Et or Eu(Tp2Et)2 in mCP suggest that hole and electron recombination dominantly occurs on d-f transition complexes rather than the host material, which avoids energy transfer from host molecule to the doped complex, so the emission of host material is no longer observed in the electroluminescence spectra.
Encouraged by the results that both efficient sky-blue electroluminescence and orange-red electroluminescence were realized with the same simple three-layered device structure, a SEL-WOLED was designed by only changing the single emitting-layer with Ce-TBO2Et and Eu(Tp2Et)2 co-doped in mCP, and the fabricated device W1 exhibts good color stability and a maximum EQE of 15.9%.
The weight doping concentration of Eu(Tp2Et)2 (5 wt%) in W1 is controllable, avoiding the low concentration (2acac) was used as the orange-red emitter and the white emission was only obtained with a doping concentration of 0.1 wt%. Such difference results from the different energy transfer efficiencies from Ce-TBO2Et to Eu(Tp2Et)2 (20%) or Ir(bt)2acac (100%). The hindered energy transfer between the d-f transition emitters provides an advantage on doping concentration optimization in SEL-WOLEDs.
