Designing tandem organic light-emitting diodes (OLEDs) is an effective approach to increasing efficiency, extending operation lifetime and tuning the emission spectrum to achieve desired performance by doping different emitters into individual stacks. In this paper, a series of three and four-color tandem white OLEDs with high color-rendering index (CRI) are created by connecting multiple electroluminescence (EL) units using an optimized charge generating unit. 9,10-bis[4-(6-methylbenzothiazol-2-yl) phenyl]anthracene (DBzA), 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN), N-(4-Tert-butylphenyl)-1,8-naphthalimide-9,9-diphenyl-9,10-dihydroacridine (NAI-DPAC) and 7,10-Bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino-phenanthrene (TPA-DCPP) serve as blue, green, orange and red emitters, respectively. All of the resulting tandem OLEDs exhibit excellent white emission with extremely high spectral stability and maximum CRI beyond 75 and 95 for devices with two and three EL units, respectively. The proposed WOLED with three EL units achieved current efficiency of 110.9 cd/A, correlated color temperatures of 4311 K, and an operation lifetime exceeding 104 hours at 1000 cd/m2. This device design strategy provides a new avenue for achieving simple-structured WOLEDs with ultra-high CRI.

White organic light-emitting diodes (WOLEDs) incorporating a blend of blue, green and red phosphorescent small molecular materials are presented in this article. 4,4′,4″-Tris(carbazol-9-yl)triphenylamine (TcTa) and 9-(4-tert-Butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) with different transmission characteristics were selected as hosts for different emitting layers aim to promote holes transport, which will reinforce carriers’ balance and broaden carrier composite. On account of adaptive energy levels of the utilized dopants and hosts, secured phosphorescent WOLED displayed high efficiencies, low operating voltage and slow efficiency roll-off. In addition, distribution of carriers’ recombination zone and spectral of change were studied in detail to further understand the light-emitting mechanisms of obtained WOLEDs. Finally, by majorizing the dosage concentration of (fbi)2Ir(acac) (bis(2-(9,9-diethyl-9H-fluoren-2-yl)-1-phenyl-1H-benzoimidazol-N,C3)iridium(acetylacetonate)) and the architectures of WOLEDs, the optimal device exhibited the maximum efficiencies of 44.92 cd A−1, 42.85 lm W−1, 16.8%, respectively, turn on voltage of 2.6 V and Commission International de l’Eclairage coordinates of (0.337, 0.458) at the brightness level of 3000 cd m−2.

The selection of host materials has a great impact on the performance of doping devices, especially near-infrared (NIR) emitters. In this investigation, four diverse host materials serve as a potential barrier layer (PBL) to confine and balance holes and electrons within the potential well layer (PWL), TPA-DCPP, for fabricating nondoped NIR thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) (NNT-OLEDs) with double quantum wells' (DQWs) structure. The hole-type host (mCP) forms an optimum interface energy barrier (IEB) and disperses carriers and excitons in each well, which helps to widen the recombination interval of carriers and restrain the quenching of excitons. Finally, OLEDs with pure red emission and a maximum external quantum efficiency (EQE(max)) of nearly 15% (with 0.5 nm well width), deep-red emission with an EQE(max) of 12% (with 1.0 nm well width), and NIR emission with an EQE(max) of 3.3% (with 5.5 nm well width) are achieved with tunable emission peaks within the range of 641-700 nm by adjusting well width, which are significantly superior to those of doped devices based on the same emitter. This investigation demonstrates a simple, feasible, and effective design strategy for achieving efficient fluorescent OLEDs with controllable wavelength, which solves the blue shift problem in traditional NIR devices of host-guest structure.

Broad-spectrum white organic light-emitting diodes (WOLEDs) based on all-fluorescent materials with excellent color stability were realized by precisely optimizing the doping concentrations of guests and the thickness of each functional layer. High-efficiency blue fluorescent emissive material 9,10-bis[4-(6-methylbenzothiazol-2-yl) phenyl]anthracene (DBzA) was selected as blue emitter and doped into first light emitting layer (EML), while red-emitting dopant 4-(dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidn-4-yl-vinyl)- 4H pyran (DCJTB) was doped into green-emitting host material tris(8-hydroxyquinoline) aluminum (Alq(3)) as the second EML. Thin hole limit layer (HLL) was inserted to balance carriers' distribution within the two EMLs and to modulate the luminous intensity ratio of different emissions. Finally, the optimal WOLED exhibited the maximum current efficiency of 9.34 cd/A, power efficiency of 10.06 lm/W, brightness up to 29,364 cd/m(2) and turn-on voltage of only 2.7 V. In addition, this device displayed stable Commission International de I'Eclairage coordinates from (0.339, 0.382) to (0.324, 0.354) with increasing current density. The highest color rendering index and corresponding correlated color temperature reach 85 and 5492 K, respectively, and the T50 lifetime reaches 7912 h. The achievement of these results fully exhibits the effectiveness of the HLL in improving spectral stability and operation lifetime.

In this work, we have experimentally demonstrated the efficacy of micro-cavity effect in realizing high-performance top-emitting organic light-emitting diodes (TEOLEDs). By optimizing the thickness of top Yb/Ag electrode and cavity length, highly efficient green TEOLED with external quantum efficiency as high as 38% was achieved. A strong dependence of electroluminescent (EL) performances and spectrum on cavity length was observed, and there was also a significant angle dependence of EL spectrum. Ultimately, ultra-high current efficiency up to 161.17 cd A −1 (3.2 V) was obtained by the device with emission peak at 552 nm, which is 35 nm longer than the intrinsic emission peak (517 nm) of utilized green emitter. Interestingly, this device displayed narrow emission with full-width at half-maximum of less than 20 nm, which was obtained by increasing the Ag layer thickness.

Top-emitting organic light-emitting diodes (TEOLEDs) achieved an ultra-high power efficiency of up to 186.38 lm W −1 , an impressive EQE as high as 47.4%, and a low operation voltage.

In this work, we have reported full phosphorescent warm white organic light-emitting diodes (WOLEDs) consisting of three light-emitting layers not only achieving high external quantum efficiency (EQE) but also demonstrating significant color stability and low efficiency roll-off. The WOLED exhibits slight shift in Commission Internationale de l'Eclairage coordinates (CIEx,y) from (0.409, 0.405) at 1000 cd m(-2) to (0.372, 0.398) at 10000 cd m(-2). Moreover, the maximum external quantum efficiency is as high as 34.5% and remains >28% over wide brightness range of 100 cd m(-2) to 5000 cd m(-2). Our work not only provides effective design strategy for physiologically friendly warm WOLEDs but also paves the way for achieving high EQE and low roll-off all phosphorescent WOLEDs.

In this work, we report the design and optimization of yellow organic light-emitting diodes (OLEDs) with tris [2-(p-tolyl)pyridine]iridium (III) (Ir (mppy)(3)) doped 4,4',4'-tris(N-carbazolyl)triphenylamine (TCTA) and bis(2-phenylquinoline) (2,2,6,6-tetramethylheptane-3,5-dionate)iridium (III) (PQ(2)Ir (dpm)) doped 2,6-bis(3-(9H-carbazol-9-yl)phenyl)phyridine (26DCzPPy) films as green and red light-emitting layers (EMLs). Experimental results demonstrated that the designed double EMLs system is beneficial in carriers' distribution, exciton confinement and reducing triplet exciton quenching because of the matched energy levels and triplet energies of hosts and dopants. Finally, the optimal device showed a very maximal current efficiency of 65.51 cd/A, slow roll off of efficiency with a very high current efficiency of 63.54 cd/A at 5000 cd/m(2), and excellent yellow emission with CIE coordinates of (0.468, 0.493) at 10 mA/cm(2).

A high performance deep-blue electroluminescent device was prepared by employing bis(2-(2-hydroxyphenyl)pyridine)beryllium (Bepp2) as a fluorescent guest material with an energy level matching host material. Device structure and thickness of individual layer were also optimized to reduce accumulation holes, balance carriers' distribution on emitter molecules, and eliminate the penetration of electrons via hole dominant light-emitting layer. Consequently, exciplex emission was efficiently eliminated, which is helpful in improving color purity and enhancing luminescent efficiency. Finally, deep-blue device with Commission Internationale de L'Eclairage (CIE) coordinate of (0.150, 0.096) was obtained, and the optimal device realized the highest brightness of 11780 cd m- 2, current efficiency (eta c) of 2.40 cd A-1, power efficiency (eta p) of 2.35 lm W-1, and external quantum efficiency (EQE) of 3.0%.

We design and fabricate highly efficient red organic electroluminescent (EL) devices by utilizing iridium(III) bis(2-phenylquinoly-N,C2’)dipiva1oy1methane $(\mathrm{P}\mathrm{Q}_{2}\mathrm{I}\mathrm{r}(\mathrm{d}\mathrm{p}\mathrm{m}))$ and tris(phenylpyrazole) iridium $(\mathrm{I}\mathrm{r}(\mathrm{p}\mathrm{p}\mathrm{z})_{3})$ as emitter and hole-type sensitizer, respectively. Experimental results demonstrated that sensitizer molecules within hole-dominant EML function as hole trappers, thus delaying the transport of holes. Finally, the optimized co-doped device obtained the external quantum efficiency, maximum brightness and current efficiency up to 18.9%, 87370 cd/m2 and 51.09 cd/A, respectively.

为了提高黄光有机电致发光器件的效率,以二(2-苯基喹啉)(2,2,6,6-四甲基庚烷)-二酸合铱[PQ2 Ir(dpm)]和三(2-(4-甲苯基)苯基吡啶)合铱[Ir(mppy)3]为发光材料,选择4,4',4"-三(9-咔唑基)三苯胺(TcTa)和2,6-二(3-(9H-咔唑基-9-基)苯基)吡啶(26DczPPy)分别作为主体材料,设计了阶梯式能级结构的器件,制备出一系列双发光层有机电致发光器件并考察了红光材料掺杂量对电致发光器件性能和电致发光光谱的影响.结果 表明:制备的双发光层有机电致发光器件的启亮电压为3.5V、最大亮度为147900cd/m2、最大电流效率为88.91cd/A、最大功率效率为61.05lm/W、最大外量子效率为33.2％;亮度为1000cd/m2时,该器件的电流效率和外量子效率分别为84.9cd/A和32.1％.

The OLEDs doped of two red iridium(iii) complexes containing the four-membered Ir–S–C–S backbone exhibit an EQEmax of 26.66% with low efficiency roll-off.

The electroluminescent (EL) performances of a green iridium complex tris(2-(4-tolyl)phenylpyridine)iridium Ir(mppy)3 were significantly improved by employing another hole-type iridium complex as a sensitizer.