Highly efficient OLEDs on ITO-free polymeric substrates

Green and low-cost approach to modify the indium tin oxide anodes in organic light-emitting diodes by electrochemical treatment in NaCl aqueous solution

Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer

Transparent conductive indium tin oxide (ITO) films with thin Ni-doped surface layers were prepared for organic light emitting diode (OLED) application. The top Ni-doped ITO surface layer were synthesized using Ni (RF) and ITO (DC) co-sputtering method at 120°C and annealed at 300°C for 10 minutes in vacuum to form a modulated work function layer in contact with the subsequently deposited light emitting organic layers. OLED devices with an Al/Alq3/NPB/Ni-doped ITO/ITO/glass structure were fabricated to investigate the effect of the Ni-doped ITO layer on the characteristics of the luminescence efficiency. The depositions of the Al/Alq3/NPB stacked films on top of the Ni-doped ITO/ITO/glass sample were conducted using thermal evaporation in a cluster tool without breaking the vacuum. Initial results show that the device turn-on voltage decreases from 10 volts to 6 volts and the luminescence efficiency was improved by 36% due to the existence of the Ni-doped ITO layer. It was also found that the optical transmittance of the ITO film decreased with the Ni concentration, resulting in external quantum efficiency deterioration by 3%. It was suspected that the presence of Ni (Φ~5.2eV compared to that of ITO ~4.2eV) on ITO surface decreases the heterojunction barrier height at the ITO/NPB interface, allowing more effective transportation of hole-carriers and hence an enhancement on the external quantum efficiency. However the optical impurity scattering of the Ni atoms in the ITO matrix caused the deterioration of the optical transparency and negative effect on the external quantum efficiency.

PSS). From the triple-stacked GO/MoO3/ PSS HSLs used as hole-injection layers (HILs) in the OLEDs, which outperform a conventional single HIL of PSS, it was found that OLEDs with triple-stacked HILs exhibited characteristic impedance properties, including low parallel resistance with trap-free space-charge-limited conductivity. Furthermore, it was shown that the relaxation frequency of a sample OLED with triple-stacked GO/MoO3/ PSS HILs was much higher than that of a reference device with a single PSS HIL. These impedance behaviors indicate that carrier (hole) injection in the sample OLED is more efficient than that in any of the other devices tested here. The results presented here clarify that the triple-stacked GO/MoO3/ PSS layers can act as efficient HILs on an ITO anode, representing a remarkable advance in relation to the mass production of high-performance solution-processable OLEDs.

The relatively high sheet resistance, low work function and poor compatibility with hole injection layers (HILs) seriously limit the applications of graphene as transparent conductive electrodes (TCEs) for organic light emitting diodes (OLEDs). Here, a graphene oxide/graphene (GO/G) vertical heterostructure is developed as TCEs for high-performance OLEDs, by directly oxidizing the top layer of three-layer graphene films with ozone treatment. Such GO/G heterostructure electrodes show greatly improved optical transmittance, a large work function, high stability, and good compatibility with HIL materials (MoO3 in this work). Moreover, the conductivity of the heterostructure is not sacrificed compared to the pristine three-layer graphene electrodes, but is significantly higher than that of pristine two-layer graphene films. In addition to high flexibility, OLEDs with different emission colors based on the GO/G heterostructure TCEs show much better performance than those based on indium tin oxide (ITO) anodes. Green OLEDs with GO/G heterostructure electrodes have the maximum current efficiency and power efficiency, as high as 82.0 cd A(-1) and 98.2 lm W(-1), respectively, which are 36.7% (14.8%) and 59.2% (15.0%) higher than those with pristine graphene (ITO) anodes. These findings open up the possibility of using graphene for next generation high-performance flexible and wearable optoelectronics with high stability.

Enhance external quantum efficiency of organic light-emitting devices using thin transparent electrodes

Open AccessCCS ChemistryCOMMUNICATIONS2 Sep 2022Towards Efficient Blue Delayed-Fluorescence Molecules by Modulating Torsion Angle Between Electron Donor and Acceptor Jinke Chen, Xing Wu, Hao Liu, Nuoling Qiu, Zhangshan Liu, Dezhi Yang, Dongge Ma, Ben Zhong Tang and Zujin Zhao Jinke Chen State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Xing Wu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Hao Liu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Nuoling Qiu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Zhangshan Liu State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Dezhi Yang State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Dongge Ma State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 , Ben Zhong Tang School of Science and Engineering, Shenzhen Institute of Aggregate Science and Technology, The Chinese University of Hong Kong, Shenzhen, Guangdong 518172 AIE Institute, Guangzhou Development District, Huangpu, Guangzhou 510530 and Zujin Zhao *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Luminescent Materials and Devices, Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates, South China University of Technology, Guangzhou 510640 https://doi.org/10.31635/ccschem.022.202202196 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Constructing blue thermally activated delayed-fluorescence materials for high-performance organic light-emitting diodes (OLEDs) remains challenging due to the intrinsically strong intramolecular charge transfer nature of the nearly orthogonal connection of electron donor (D) and acceptor (A), which results in long-wavelength emission. Herein, an effective delayed-fluorescence design strategy of modulating D–A torsion angles is proposed and efficient sky-blue, pure-blue, and deep-blue delayed-fluorescence molecules consisting of a xanthenone acceptor and carbazole-based donors are created by decreasing the torsion angles. They exhibit strong delayed fluorescence with high photoluminescence quantum yields of 85–94% in doped films, and their delayed-fluorescence lifetimes are elongated from 1.0 to 27.6 μs as the torsion angles decrease. These molecules can function as excellent emitters in OLEDs, providing efficient electroluminescence peaking at 442 nm (CIEx,y = 0.15, 0.08), 462 nm (CIEx,y = 0.15, 0.18), and 482 nm (CIEx,y = 0.17, 0.30) with state-of-the-art external quantum efficiencies of up to 22.2%, 33.7%, and 32.1%, respectively, demonstrating the proposed molecular design for efficient blue delayed-fluorescence molecules is successful and promising. Download figure Download PowerPoint Introduction Efficient blue organic luminescent materials are highly desired because they are one of the fundamental elements of the three primary colors in organic light-emitting diodes (OLEDs).1–7 Organic fluorescence molecules with blue emission, which are employed as the first-generation luminescent materials in OLEDs, can be readily designed, but only 25% of the electro-generated excitons under electrical excitation are used, leading to low external quantum efficiency with an upper limit of 5–7.5%.8–10 Several strategies, such as triplet–triplet fusion11–13 and hybridized local and charge-transfer excited states,14–17 have been proposed to enhance triplet exciton utilization of fluorescence molecules, but full exciton harvesting remains difficult. Second-generation noble-metal-containing phosphorescence materials have been developed, which can reach unity exciton utilization by converting singlet excitons to triplet excitons via intersystem crossing based on heavy-atom induced large spin–orbit coupling (SOC). But, because of the intrinsic metal-to-ligand charge-transfer (CT) characteristics, pure blue emissions are hardly achieved in most phosphorescence materials, and the long lifetimes of triplet excitons result in poor stability of these materials in OLEDs.18–21 After decades of continuous research, purely organic thermally activated delayed-fluorescence (TADF) molecules have been invented and are currently emerging as the third-generation luminescent materials for the fabrication of high-performance OLEDs, thanks to the advantages of easy molecular design, high exciton utilization, noble metal-free structures, and so on.1,22–28 The reported TADF molecules generally have a highly twisted conformation, consisting of an electron donor (D) and acceptor (A) connected in a nearly perpendicular manner, to minimize the exchange integral between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Thus, the energy split (ΔEST) between the lowest singlet excited (S1) state and the lowest triplet excited (T1) state can be reduced to allow fast reverse intersystem crossing (RISC), which results in the occurrence of delayed fluorescence.3,29–31 Although numerous efficient sky-blue to red TADF molecules have been successfully explored based on this design method,24,31–37 deep-blue to blue TADF molecules are still challenging because these nearly orthogonal D–A systems are inevitably accompanied by a strong intramolecular charge-transfer (ICT) effect that causes redshifted emissions.38–41 Weakening the ICT effect by choosing weak D and A groups3,42–44 or designing a through-space CT framework45,46 can to some extent shift the emission peaks to the short-wavelength region, but the corresponding electroluminescence (EL) efficiencies are often unsatisfactory. Therefore, modulating the torsion angles of proper D and A groups could be a promising strategy to explore high-efficiency deep-blue and pure-blue TADF molecules. As a proof of concept, we wish to report an effective design of blue luminescent molecules based on a xanthenone (XT) acceptor and two carbazole (Cz) donors (Figure 1a–c). XT is selected as electron acceptor because of its relatively weak electron-withdrawing nature, high structural rigidity, and ability to promote RISC by enlarging SOC stemming from the n−π* transition of the carbonyl group.40 A previous study demonstrated the ability to tune the color of blue TADF emitters by the introduction of methyl substituents.47 Here, the torsion angles between XT and Cz are tuned progressively by introducing methyl groups at the 1 and 8 positions of Cz, and the strength of the D–A interaction is further optimized by modification at the 3 and 6 positions of Cz with electron-donating tert-butyl groups. We found that the ICT effect is weakened sequentially as the torsion angles between XT and Cz diminishes, gradually blueshifting the emissions from 2MCz-XT to MCz-XT and then to Cz-XT. The introduction of tert-butyl groups can enhance the ICT effect, leading to moderately redshifted emission of 2TBCz-XT relative to Cz-XT. Meanwhile, all these molecules exhibit apparent delayed fluorescence, while the lifetimes of the delayed fluorescence are closely associated with the torsion angles and the strength of the ICT effect. By adopting these new molecules as emitters, highly efficient deep-blue, pure-blue, and sky-blue OLEDs with EL peaks at 442, 462, and 482 nm and outstanding maximum external quantum efficiencies (ηext,maxs) of 22.2%, 33.7%, and 32.1%, respectively, are obtained. These impressive EL performances demonstrate the significance of modulating torsion angles in the design of blue TADF emitters. Figure 1 | (a) Molecular design strategy. (b) Chemical structures of the new molecules with calculated torsion angles and (c) crystal structures of Cz-XT and 2MCz-XT with observed torsion angles. (d) Distributions of HOMOs and LUMOs and the calculated energy splits (ΔESTs) of the new molecules. Download figure Download PowerPoint Results and Discussion The target molecules Cz-XT, MCz-XT, 2MCz-XT, and 2TBCz-XT were facilely synthesized in good yields by palladium-catalyzed Buchwald–Hartwig C–N coupling reactions of 3,6-dibromoxanthen-9-one with Cz and Cz derivatives ( Supporting Information Scheme S1). The molecular structures were characterized by 1H NMR and 13C NMR ( Supporting Information Figures S1–S4) and high-resolution mass spectrometry with satisfactory results. They are thermally and morphologically stable with high decomposition temperatures of 397–459 °C and high glass-transition temperatures over 210 °C, as determined by thermogravimetry analysis and differential scanning calorimetry measurements, respectively ( Supporting Information Figure S5). Their electrochemical properties were measured by cyclic voltammetry using ferrocene as the calibration compound ( Supporting Information Figure S6). The experimental HOMO and LUMO energy levels of Cz-XT, MCz-XT, 2MCz-XT, and 2TBCz-XT are calculated to be −5.69 and −2.92; −5.66 and−2.93; −5.56 and −2.94; and −5.60 and −2.92 eV, respectively. Single crystals of Cz-XT and 2MCz-XT were obtained from a mixture of n-hexane and dichloromethane via slow solvent evaporation. Single-crystal X-ray crystallography analysis reveals that 2MCz-XT adopts a highly twisted D–A connection with large torsion angles of 83° and 91°, due to the severe steric hindrance imposed by the two methyl groups at the 1 and 8 positions of Cz. In contrast, Cz-XT shows a more planar molecular conformation, in which the torsion angles are decreased to 37° and 40°, indicating Cz-XT has a better π-conjugation between Cz and XT than 2MCz-XT. The optimized structures and molecular orbitals of these new molecules were calculated employing a density functional theory (DFT) method.48 As depicted in Figure 1d and Supporting Information Figure S7, the optimized geometry of 2MCz-XT has a similar highly twisted conformation to its crystal structure, with large torsion angles of 83°–84° between XT and Cz. However, MCz-XT and Cz-XT show gradually decreased torsion angles of 70° and 51°–52° due to the reduced steric hindrance. Similar molecular geometry is simulated for 2TBCz-XT compared with Cz-XT. The electron clouds of the HOMOs and LUMOs of these molecules are primarily distributed on Cz and XT, respectively. Due to the highly twisted molecular geometry, 2MCz-XT has the highest degree of separation between the HOMO and LUMO, which leads to the smallest ΔEST of 0.01 eV. The ΔEST of MCz-XT is increased to 0.12 eV due to decreased torsion angles. Cz-XT and 2TBCz-XT have overlapping HOMOs and LUMOs because of the relatively planar conformation. Thus, they have much larger ΔESTs of 0.23 and 0.21 eV than 2MCz-XT and MCz-XT. As displayed in Figure 2a, Cz-XT and 2TBCz-XT have strong absorption maxima at 365 and 384 nm in tetrahydrofuran (THF) solution, which are mainly comprised of the π–π* transitions. MCz-XT and 2MCz-XT have relatively weak absorption maxima at 362 and 368 nm, associated with the ICT states. 2MCz-XT exhibits a green photoluminescence (PL) peak located at 501 nm in THF solution, whereas the PL peaks are blueshifted progressively to 481 nm for MCz-XT and 459 nm for Cz-XT (Figure 2b) due to the weakened ICT effect. The PL peak of 2TBCz-XT is redshifted to 481 nm, which is ascribed to the strengthened ICT effect due to the presence of the tert-butyl groups. To evaluate the ICT effect, the PL spectra of the four molecules in various solvents are tested ( Supporting Information Figure S8). The spectral displacements gradually increase from Cz-XT (60 nm) to MCz-XT (63 nm) and then to 2MCz-XT (71 nm), in good agreement with the increased dihedral angles and strengthened ICT effect. 2TBCz-XT exhibits a larger spectral displacement of 70 nm than Cz-XT (60 nm) because of a stronger ICT effect. These solvation effects further validate that both enlarging D–A dihedral angles and introducing electron-donating groups strengthen the ICT effect of the molecules. When doped in (diphenylphosphoryl)-dibenzo[b,d]-furan (PPF) host at a concentration of 15 wt %, Cz-XT shows a PL peak at 459 nm, similar to that in THF solution, whereas MCz-XT and 2MCz-XT have redshifted PL peaks at 466 and 483 nm, respectively. The PL peak of 2TBCz-XT is located at 468 nm, which is redshifted by 9 nm compared with that of Cz-XT. The photoluminescence quantum yields (ΦPLs) of these molecules in doped films are in the range of 85–94%, higher than those in THF solution (Table 1). The molecular motions are active in solution, largely dissipating the excited-state energy and thus leading to low ΦPL values. But in the doped films, the intramolecular motions of the molecules are greatly suppressed so that the nonradiative dissipation pathways are blocked, accounting for the significantly improved ΦPL values.16,35 Figure 2 | (a) Absorption and (b) photoluminescence (PL) spectra of the new luminogens in THF solutions (10−5 M) and in doped films with a doping concentration of 15 wt % in PPF. Temperature-dependent transient PL decay spectra of (c) Cz-XT, (d) MCz-XT, (e) 2MCz-XT, and (f) 2TBCz-XT doped in PPF host with a doping concentration of 15 wt %, measured under nitrogen. Download figure Download PowerPoint Table 1 | Photophysical Properties of the New Molecules Solutiona Doped Filmb λabs (nm) λem (nm) ΦPLc (%) λem (nm) ΦPLc (%) τdelayedd (μs) Rdelayede (%) kFf (×107 s−1) kICg (×107 s−1) kRISCh (×105 s−1) ΔESTi (eV) Cz-XT 365 459 47 459 85 27.6 64 9.9 1.7 1.0 0.15 MCz-XT 362 481 52 466 86 11.3 51 7.0 1.1 1.8 0.04 2MCz-XT 368 501 47 483 91 1.0 60 1.1 0.1 25.0 0.01 2TBCz-XT 384 481 71 468 94 17.0 50 12.4 0.8 1.2 0.10 aMeasured in THF solution (10−5 M) at room temperature. bVacuum-deposited on a quartz substrate with a doping concentration of 15 wt % in PPF. cPhotoluminescence quantum yield (ΦPL) determined by a calibrated integrating sphere under nitrogen at room temperature. dDelayed fluorescence lifetime (τdelayed) evaluated at 300 K under nitrogen. eRatio of delayed component. fFluorescence decay rate. g Internal conversion decay rate from S1 to S0. hRate constant of RISC process. iEstimated from the high-energy onsets of fluorescence and phosphorescence spectra at 77 K. From the onsets of fluorescence and phosphorescence spectra of doped films ( Supporting Information Figure S9), the experimental ΔESTs of these molecules are calculated to be 0.01–0.15 eV, which are small enough to ensure the occurrence of RISC and thus delayed fluorescence (Figure 2c–f). By progressively reducing the torsion angles between Cz and XT, the ΔEST increases from 0.01 eV of 2MCz-XT to 0.04 eV of MCz-XT and to 0.15 eV of Cz-XT. The ΔEST of 2TBCz-XT is 0.10 eV, smaller than that of Cz-XT, although both molecules adopt nearly identical molecular conformations. These results demonstrate that enlarging the torsion angles and strengthening the ICT effect between D–A groups are conducive to achieving a small ΔEST. Because of the smaller ΔEST, 2MCz-XT exhibits a shorter delayed-fluorescence lifetime (τdelayed) of 1.0 μs and faster RISC, rate constant (kRISC) of 2.5 × 106 s−1, than Cz-XT (27.6 μs, 1.0 × 105 s−1) and MCz-XT (11.3 μs, 1.8 × 105 s−1). Compared with Cz-XT, 2TBCz-XT displays faster RISC, corresponding to a shorter τdelayed of 17.0 μs and a larger kRISC of 1.2 × 105 s−1 (Table 1). The temperature-dependent transient PL decay spectra indicate that Cz-XT and 2TBCz-XT have greatly promoted RISC with apparently enhanced delayed components (Rdelayeds) at high temperatures ( Supporting Information Table S1). However, the change in delayed fluorescence of 2MCz-XT by increasing temperature is obviously diminished, and its τdelayed and Rdelayed vary slightly from 77 to 300 K. These results manifest that the very small ΔEST allows 2MCz-XT to enjoy fast RISC even at low temperatures, while the large ΔESTs make Cz-XT and 2TBCz-XT more dependent on the thermal activation for sufficient RISC. The energy levels of Cz-based donors, XT acceptor, and the new molecules are measured from the phosphorescence spectra and shown in Supporting Information Figure S10. Generally, the locally excited triplet (3LE) energy levels of the donors are close to the 1CT states of MCz-XT, 2MCz-XT, and 2TBCz-XT, whereas the 3LE energy level of XT is close to the 1CT state of Cz-XT. For 2MCz-XT, the 3LE energy level of the donor is close to both 1CT and 3CT states, which may contribute to the fastest RISC and most efficient delayed fluorescence.49 Furthermore, the time-dependent DFT method is employed to gain insights into the RISC in these blue molecules. The natural transition orbital analysis reveals that the S1 and T1 states of the four molecules are dominated by CT transition, whereas the second triplet excited (T2) states are energetically close to the T1 states with LE transition characteristics ( Supporting Information Figures S11 and S12). The different transition natures of S1 and T2 are favorable for RISC.50,51 Furthermore, the calculated SOC matrix elements are also considerable between S1 and T2. These results suggest T2 is involved in RISC and facilitates the occurrence of delayed fluorescence in these molecules, which could be important for Cz-XT and 2TBCz-XT, who have small torsion angles and relatively large ΔESTs. To evaluate the EL performances of these blue molecules, doped OLEDs are fabricated with the configuration of indium tin oxide (ITO)/hexaazatriphenylenehexacabonitrile (HATCN) (5 nm)/1,10-bis(di-4-tolylaminophenyl)cyclohexane (TAPC) (50 nm)/tris[4-(carbazol-9-yl)phenyl]amine (TcTa) (5 nm)/1,3-di(carbazol-9-yl)benzene (mCP) (5 nm)/emitting layer (EML) (20 nm)/PPF or diphenyl-4-triphenylsilylphenyl-phosphine oxide (DPEPO) (5 nm)/3,3′-[5′-[3-(3-pyridinyl)phenyl][1,1′:3′,1″-terphenyl]-3,3″-diyl]bispyridine (TmPyPB) (30 nm)/lithium fluoride (LiF) (1 nm)/Al (Figure 3a–d), where the doped films of these molecules in PPF or DPEPO hosts with varied doping concentrations of 10, 15, and 20 wt % work as EMLs, HATCN and LiF serve as hole- and electron-injection layers, respectively, TAPC and TmPyPB perform as hole- and electron-transporting layers, respectively, TcTa serves as electron-blocking layer, PPF and DPEPO work as hole-blocking layers, and mCP functions as exciton-blocking layer. The key performance data of all the devices with corresponding configurations are summarized in Supporting Information Figures S13–S20 and Tables S2–S5. In general, these devices turn on at low voltages of 2.7–3.6 V and radiate strong light in deep-blue to sky-blue regions. In comparison with Cz-XT, MCz-XT and 2MCz-XT exhibit apparently redshifted EL emissions, and 2TBCz-XT shows redder EL emission than Cz-XT (Table 2). These EL behaviors are consistent with their PL behaviors. Cz-XT and MCz-XT have better EL efficiencies in the DPEPO host, whereas 2MCz-XT and 2TBCz-XT give better EL efficiencies in the PPF host. Whether in PPF or DPEPO, the EL spectra remain stable with minor redshifts less than 8 nm, but the maximum luminance (Lmax) is enhanced greatly by increasing doping concentrations from 10 to 20 wt %. Figure 3 | (a) Energy level diagram and chemical structures of the functional layers. (b) EL spectra at 4 V. (c) Plots of luminance–voltage–current density and (d) external quantum efficiency–luminance of the OLEDs based on the new luminogens. EML, doped films of the new molecules in PPF or DPEPO hosts. Download figure Download PowerPoint Table 2 | EL Performances of the Doped OLEDs Based on the New Molecules Emitter Von (V) ηC (cd A−1) ηP (lm W−1) ηext (%) Lmax (cd m−2) CIE (x, y) λEL (nm) Maximum Value/at 100/at 500 cd m−2 Cz-XT 3.5 15.9/10.3/5.8 13.9/7.5/3.5 22.2/14.4/8.0 1989 (0.15, 0.08) 442 MCz-XT 3.5 29.7/26.4/21.6 25.9/19.3/13.8 24.0/21.3/17.5 5751 (0.15, 0.15) 460 2MCz-XT 3.0 64.4/57.6/53.3 65.2/47.6/38.9 32.1/28.7/26.6 42250 (0.17, 0.30) 482 2TBCz-XT 2.9 47.9/38.6/31.9 50.2/33.7/24.5 33.7/27.1/22.4 20370 (0.15, 0.18) 462 Abbreviations:Von, turn-on voltage at 1 cd m−2; ηC, current efficiency; ηP, power efficiency; ηext, external quantum efficiency; Lmax, maximum luminance; CIE, Commission Internationale de I'Eclairage coordinates; λEL,EL peak. Cz-XT radiates deep blue-light with an EL peak at 442 nm, Commission Internationale de l'Eclairage (CIE) color coordinates of (0.15, 0.08), and an Lmax of 1989 cd m−2 in DPEPO host at a doping concentration of 10 wt %. The full width at half maxima value of the EL spectrum is 63 nm, similar to that of the PL spectrum (64 nm). The maximum current efficiency (ηC,max), maximum power efficiency (ηP,max), and ηext,max are 15.9 cd A−1, 13.9 lm W−1, and 22.2%, respectively. More importantly, 2TBCz-XT shows pure-blue light with an EL peak at 462 nm (CIEx,y = 0.15, 0.18) and a Lmax of 20370 cd m−2 in PPF host at a doping concentration of 20 wt %. The ηC,max, ηP,max, and ηext,max are 47.9 cd A−1, 50.2 lm W−1, and 33.7%, respectively. The device comprised of 2MCz-XT in PPF host at a doping concentration of 10 wt % displays sky-blue light with an EL peak at 482 nm (CIEx,y = 0.17, 0.30) and provides an ηext,max of 32.1% similar to that of 2TBCz-XT ( Supporting Information Tables S2–S5). In addition to the efficient RISC that ensures nearly full exciton utilization and excellent ΦPLs of 91% (2MCz-XT) and 94% (2TBCz-XT), the high horizontal orientation ratios of 78.0% and 84.0% of 2MCz-XT and 2TBCz-XT (Figure 4a,b), respectively, account for the outstanding ηext,maxs exceeding 30%.7,24,25,34,35,52,53 To the best of our knowledge, these impressive ηext,maxs demonstrate Cz-XT and 2TBCz-XT are among the currently reported state-of-the-art deep-blue and pure-blue TADF materials ( Supporting Information Table S6). Figure 4 | PL of (a) 2MCz-XT and (b) 2TBCz-XT in doped Download figure Download PowerPoint TADF molecules are in highly twisted D–A structures, which to efficient blue emissions because of a strong ICT effect. To this we a and effective strategy of modulating torsion angles of D–A groups for the of efficient blue delayed-fluorescence materials, based on a of deep-blue and pure-blue luminescent molecules consisting of Cz donor and XT By gradually decreasing the torsion the PL peak of Cz-XT is apparently blueshifted relative to those of MCz-XT and 2MCz-XT, accompanied by increased ΔEST and elongated By the electron-donating ability of Cz via the introduction of tert-butyl 2TBCz-XT shows a moderately redshifted PL and its ΔEST and τdelayed smaller and respectively, in comparison with Cz-XT. Although Cz-XT and 2TBCz-XT have more planar structures and larger they strong deep-blue and pure-blue delayed fluorescence in doped films with excellent ΦPLs of and respectively. The EL emissions of these molecules the as the PL emissions, thus achieving blue high-performance The device using Cz-XT as radiates deep-blue light with an EL peak at 442 nm (CIEx,y = 0.15, 0.08) and a high ηext,max of more efficient OLEDs are achieved by adopting 2TBCz-XT and 2MCz-XT as emitters, providing pure-blue and sky-blue light peaking at 462 nm (CIEx,y = 0.15, 0.18) and 482 nm (CIEx,y = 0.17, 0.30) with outstanding ηext,maxs of and 32.1%, respectively. These OLEDs are among the current state-of-the-art TADF OLEDs with similar which may the of efficient blue organic luminescent materials by of D–A torsion Supporting Information Supporting Information is and and fabrication and crystal data of Cz-XT and 2MCz-XT, analysis and differential scanning calorimetry cyclic transient PL decay fluorescence and phosphorescence and device performance of The of study is by the Science of China the Science of Guangdong and the State Key of Luminescent Materials and Devices, South China University of TADF Emitter and Efficient and Doped OLEDs with Wu Chen Ma Zhao Tang OLEDs with and by a of Efficient Materials for Blue Organic Efficient in Organic Yang Efficient by for Chen Wu of Blue and and Their OLEDs with Efficient Blue Based on and from and Their to Organic for Blue Organic Efficient Blue Based on Single and Organic The Key of Chen Wu for Efficient

An ultrathin layer of nickel oxide (NiO) was deposited on the indium tin oxide (ITO) anode to enhance the hole injections in organic light-emitting diode (OLED) devices. A very low turn-on voltage (3 V) was actually observed for the device with NiO on ITO. The enhancement of hole injections by depositing NiO on the ITO anode was further verified by the hole-only devices. The excellent hole-injection ability of NiO was also demonstrated by devising a device with patterned NiO on the ITO anode. Our results suggest that the NiO/ITO anode is an excellent choice to enhance hole injections of OLED devices.

We present highly power-efficient flexible organic light-emitting diodes by combining nanostructured metallic transparent conductor on plastic substrates, leading to a power efficiency over 160 lm/W with angular color stability. Flexible organic light-emitting diodes (OLEDs) hold great promise for future bendable display and curved lighting applications. One key challenge of high-performance flexible OLEDs is to develop new flexible transparent conductive electrodes (TCEs) with superior mechanical, electrical and optical properties. Indium-tin-oxide (ITO) electrode is the most widely used TCE in optoelectronic devices due to its excellent electrical conductivity and light transmission, but the high-temperature fabrication and the brittle nature under repeated bending condition hinder its application on flexible plastic substrates. An additional drawback of using ITO in flexible OLEDs on plastic substrates is the limited light outcoupling efficiency due to the severe trapping loss of the internally emitted photons. Various materials and structures have been proposed to function as flexible TCEs. Metal-dielectric composite electrode (MDCE) has been regarded as an effective TCE for flexible devices in terms of mechanical flexibility, electrical conductivity, optical transparency, and large-area film uniformity. Whilst MDCE may be an ideal candidate to replace ITO, particularly in flexible devices on plastic substrates, two key challenges should be overcome. First, forming an ultrathin metal film (≤ 10 nm) in a MDCE for improving the optical transmission is extremely difficult due to the dewetting problem with an isolated granular morphology, causing the adverse degradation in optical transmittance and electrical conductance with particle plasmon absorption and severe disconnection. Second, an optical microcavity effect is inevitable with the use of a planar MDCE structure, leading to the spectral and angular dependence of the emission characteristics. Herein, we demonstrate a new strategy to achieve a powerful transparent conductive electrode on plastic substrate that combines a quasi-random nanostructured optical coupling layer and an ultrathin metal alloy conduction layer [1,2]. The optimum electrical conductivity, optical manipulation capability, and high tolerance to mechanical bending are realized in this composite electrode, which is favorable for the fabrication of ITO-free flexible OLEDs with state-of-the-art performance on low-refractive-index plastic substrate. The angularly and spectrally independent boost in light outcoupling of white emission is obtained by minimizing the waveguide mode, metallic electrode-related microcavity effect and surface plasmonic loss due to the integrated quasi-random outcoupling structure in the composite electrode[3-5]. The resulting white flexible OLED exhibits the high enhancement in efficiency, e.g., external quantum efficiency ~70% and power efficiency over 160 lm/W. In addition, this composite electrode has a scalable manufacturing potential in large-area flexible electronic systems. Fig. 1. Device structure and performance of flexible white OLEDs. (a-c) Comparison of different transparent electrodes: (a) Optical properties. Inset is the schematic illustration of flexible OLED structure with NMDCE on plastic substrate. (b) Sheet resistance as a function of nominal film thickness. (c) Mechanical stability under continuous bending test. (d-f) Performance characteristics of white flexible OLEDs: (d) Current density and luminance as a function of driving voltage. (e) Current and power efficiencies as a function of luminance. (f) Normalized EL spectra. Inset is the photograph of white-emission flexible OLED. (g-i) Simulations of normalized cross-section intensity field distributions (at 520 nm) with propagation of OLEDs on (g) ITO, (h) MDCE and (i) NMDCE using FDTD method.

In this work, circular economy was investigated for commercial indium tin oxide (ITO) thin films deposited on glass substrates obtained from degraded organic light-emitting diodes (OLED). These devices were assembled and polarized at laboratory in a previous work. For each substrate, with geometry 2.5 × 2.5 cm, four OLEDs with active area of 3 × 3 mm were set up. These OLED devices were assembled with ITO as the electrode anode and successive depositions of other materials (layer-by-layer), to form the complete structure. To obtain the recovered ITO, all layers were removed from the samples containing the OLEDs previously mounted, remaining only the ITO thin films, that were cleaned with commercial product together with the received ITO/glass samples. Both samples were compared using some techniques, such as: colorimetry, electrical resistance, and Raman spectroscopy. A methodology with light-emitting diode (LED) device polarized emitting light crossing the ITO thin films was used, and the luminance with chromaticity coordinates was obtained, revealing the good transparency of the thin films. Electrical resistance of recovered ITO revealed five higher orders of magnitude in comparison to the one of received ITO. This fact can be tributed to a poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) layer, causing corrosion of the ITO thin films during the assembly of OLEDs or loss of the field lines created during the electrical measurements by probes of four-point probe. Raman spectroscopy did not show satisfactory results in the chemical composition analyses of the samples, but it indicated good cleaning process of the samples before the analyses.

We report the correlation between charge injection and charge balance using lithium fluoride/aluminum cathode and iridium oxide -coated indium tin oxide (ITO) anode in organic light emitting diodes (OLEDs). The turn-on voltage decreased from and luminous efficiency at increased from due to the presence of LiF and the high work function of . When was used in the anode region, the luminous efficiency increment rate decreased from 40% in Al cathode OLED to 7% in cathode OLED. The value of the carrier density suggested that the majority carrier changed from electrons in ITO- OLEDs to holes in ITO/ OLEDs, reducing the rate of increase. Therefore, it has been considered that charge injection should be increased and balanced in order to maximize the luminous efficiency.

Efficient single layer RGB phosphorescent organic light-emitting diodes

Hole injection enhancement has been reported for organic thin-film transistors and light-emitting diodes at the indium tin oxide (ITO) anode side by introducing a LiF layer, which is usually used as an electron injection layer at the cathode side to reduce the electron injection barrier. We report a revised mechanism for the hole injection enhancement by studying a prototype interface of pentacene/LiF/ITO anode. Upon deposition of LiF on ITO, the work function of ITO decreases, and energy level realignment occurs between the pentacene and ITO. The hole injection barrier from the ITO to the pentacene highest occupied molecular orbital increases significantly with LiF insertion. Thus, the reduction in the hole injection barrier is not a critical factor for the hole injection enhancement. We suggest that a LiF insulating buffer layer enhances both injection barriers and tunneling through the barrier when a bias is applied.

Deep blue, efficient, moderate microcavity organic light-emitting diodes

Improved performance of the single-layer and double-layer organic light emitting diodes by nickel oxide coated indium tin oxide anode