A Brief History of OLEDs-Emitter Development and Industry Milestones.

Organic light emitting diodes (OLEDs) have gained great interest in the last years due to their potential for future flat panel display and solid state lighting applications. OLEDs are a novel and very attractive class of solid-state light sources, which generate a diffuse, nonglaring illumination with high color rendering. Compared to the other major lighting technologies in the market – incandescent, fluorescent, high intensity discharge (HID) lamps, LED and electroluminescent, OLED technology has the potential of achieving substantial energy and CO2 savings, without compromising color rendering or switching speed. The unique features of OLED lighting are inspired the imagination of designers who are exploring various OLED applications: windows, curtains, automotive light, decorative lighting and wall papers. The OLED technology is now being commercialized as a multibillion dollar market. OLEDs are already used in small displays in cellular phones, car stereos, digital cameras, etc. The rapidly growing market for OLED displays and lighting is driving research in both advanced materials and improved manufacturing processes. In spite of the spectacular results achieved, there are still many problems concerning the efficiency, stability and lifetime of OLEDs, materials selection and optimization, encapsulation, uniformity over large areas, manufacturing cost, colour saturation, etc. to be solved. OLED represents a quite complicated system of many very thin layers of various materials situated between electrode layers (one of which is transparent); this system emits light when placed under electric potential. The type of material used as the light emitter determines the specific characteristics of such devices. Two types of OLEDs are developed – on the bases of “small” molecules (SM-OLED) (Tang & VanSlyke, 1987) and conjugated polymers (PLED) (Burroughes et al., 1990), oligomers, etc. Potential emitters for SM-OLED are metal complexes from the lanthanide and platinum groups as well as complexes of Al, Zn, Cd, Cu, Be, B with carefully selected ligands from the group of heterocyclic compounds like as hydroxyquinoline, benzoxazole, benzothiazole, triarylamines, etc. (Petrova & Tomova, 2009). The first generation of efficient devices, pioneered by Tang and Van Slyke from Eastman Kodak (1987), was based on fluorescent

A pure-blue light-emitting material is one of the key components in the preparation of organic light-emitting diode (OLED) displays. Although high-efficiency blue OLEDs have been realized in therma...

Since Tang and Van Slyke's breakthrough with the first organic light-emitting diode (OLED), these devices have been thoroughly researched for real-world displays.(Tang and VanSlyke, 1987) Their appeal lies in their lightweight build, wide viewing angles, rapid response times, easy chemical customization of emitting molecules, low energy usage, compatibility with flexible plastic substrates, and adaptability to diverse display forms. Moreover, pure organic materials are vital in OLEDs, significantly influencing their luminescence. Recent advancements in OLEDs have shown significant improvements in efficiency and lifespan.(Chan et al., 2021;Vasilopoulou et al., 2021;Naveen et al., 2022) Still, the broad emission spectra of materials pose challenges for achieving the high color purity required for future high-end displays like UHDTV. The BT 2020 color gamut standard mandates narrowband emissions for red, green, and blue, which is essential for enhancing color purity.(Vasilopoulou et al., 2021) Recently, boron-based multi-resonant (MR) narrowband emitters have exhibited high color purity and significant potential in OLED applications, offering promising alternatives for improved OLED performance. (Madayanad Suresh et al., 2020;Kim and Yasuda, 2022;Mamada et al., 2024)This Research Topic comprises four submissions, each focusing on distinct aspects. Three submissions explore boron-based materials for OLED applications, likely investigating synthesis, characterization, and performance evaluation to enhance color purity and overall efficiency. Conversely, one article in the collection focuses on designing and synthesizing a new [1 + 4+2] framework of multilayer targets with detailed photophysical characterizations. Xiaofeng et al. delved into the ‘Sterically-wrapped MR fluorophores’ concept for quench-resistant and narrowband OLED applications. Their exploration included a thorough analysis of reported materials and their device performances. Additionally, they guided various approaches for achieving stable ultra-wide color gamut OLED displays. Yogesh et al. elaborated into hyperfluorescence-based stable OLEDs, focusing primarily on material design, a deeper understanding of energy transfer mechanisms, and examining previously reported hyperfluorescence devices. Their exploration provides insights into the prospects for the display industry, highlighting the importance of innovative material design and a comprehensive understanding of energy transfer processes for advancing OLED technology. Sudhir et al. introduced a novel Se-doped emitter, SeBSe, characterized by a helically distorted structure and a high reverse intersystem crossing (RISC) rate surpassing 108 s-1, resulting in a narrowband sky blue emission. Remarkably, OLEDs fabricated using the SeBSe material achieved a maximum external quantum efficiency of 9.3%. Zhang et al. successfully synthesized a new [1 + 4+2] multilayer framework employing modified dual Suzuki–Miyaura cross-couplings and group-assisted purification (GAP) chemistry. Comprehensive characterization via spectroscopic analysis and X-ray crystallography was conducted, along with detailed investigations into physical properties, including aggregation-induced emission (AIE).This Research Topic is truly a testament to the collective dedication of researchers in unraveling the complexities of luminescent materials, particularly boron-based materials. The dynamic nature of this field underscores the relentless pursuit of knowledge and the collaborative spirit that propels scientific advancement forward in OLEDs. By laying the groundwork for future investigations, this work not only expands our understanding of chemistry but also holds promise for addressing pressing societal challenges in the display industries. It exemplifies the synergy that arises from the exchange of ideas and the shared commitment to pushing the boundaries of science for the betterment of humanity.The gratitude extended to the contributors of this Research Topic is heartfelt and well-deserved. Their unwavering dedication and groundbreaking contributions have significantly enriched our collective understanding of high color purity materials, opening doors to future innovations, particularly in OLEDs. Their diverse insights and findings illuminate the multifaceted nature of chemistry's pursuit to harness the potential of these compounds, offering exciting new avenues for discovery and application. It's through their collaborative efforts that we continue to push the boundaries of scientific exploration and pave the way for transformative advancements in boron-based narrowband materials

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

New organic light emitting diodes (OLEDs) with dual peak emission spectra were fabricated and their electrical and optical characteristics were investigated. In the experiments, the blue emission materials GDI602 and GDI602 were doped with GDI691(2%)[GDI602∶GDI691(2%)] and the yellow emission material Alq3 was doped with Rubrene(10%)[Alq3∶Rubrene(10%)]. Under an applied voltage of 10 V, the OLED with the GDI602/Alq3∶Rubrene(10%) emission layer had a luminance of 325 Cd/m2 and a power efficiency of 0.7 lm/W, whereas the OLED with the GDI602∶GDI691(2%)/Alq3∶Rubrene(10%) emission layer had a luminance of 500 Cd/m2 and a power efficiency of 0.94 lm/W. The dual peak wavelengths of the OLEDs were fixed but the relative intensities of the peak wavelengths varied according to the applied voltage. The OLED with the GDI602/Alq3∶Rubrene(10%) emission layer showed a somewhat reddish-white emission with a Commission Internationale de L'Eclairage(CIE) coordinate of (0.33, 0.32) at 9 V. In contrast, the OLED with the GDI602: GDI691(2%)/Alq3∶Rubrene(10%) emission layer showed an almost pure white emission with a CIE coordinate of (0.32, 0.33) at 6 V; furthermore, the color changed to blue as the applied voltage was increased.

Organic light-emitting diodes (OLEDs) have been successfully applied as displays and also recognized as a next-generation lighting technology because their several advantages such as self-emission, high luminous efficiency, full-color capability, wide viewing angle, high contrast, low power consumption, low weight, large area manufacture, transparence and flexibility. Now, the red and green phosphorescent OLEDs are qualified for the commercial products. The bottleneck still is the blue OLED. Currently, the high efficiency (100% internal quantum efficiency, and over 20% external quantum efficiency, EQE) blue OLED can be achieved using phosphorescent and thermal activated delay fluorescent (TADF) emitters, which were the crucial factor to determine color purity, efficiency, and lifetime of device. Beside the emitters, the other important factor is the host material. In this talk, we synthesized several novel carbazole-based, benzimidazole-based, and their combined derivers to be the universal hosts with wide energy bandgap for red, green, blue phosphorescent emitters and also applied for green and blue TADF emitters. These novel host materials could be classified as electron transport, hole transport, and bipolar. The carrier dynamics in OLEDs with these host materials were investigated by probing the position of main recombination zone and optimizing the efficiency performance. With increasing electron mobility of host material, the main carrier recombination zone in emitting layer was moved from the ETL side to the HTL side. Finally, the high EQE of over 30% was achieved in green and blue phosphorescent and TADF OLEDs with our host system. In addition, the operational lifetime was also improved by using our host system, comparing that of OLED with commercial mCP host.

Open AccessCCS ChemistryMINI REVIEW1 Aug 2020The Leap from Organic Light-Emitting Diodes to Organic Semiconductor Laser Diodes Chihaya Adachi and Atula S. D. Sandanayaka Chihaya Adachi *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka 819-0395 Google Scholar More articles by this author and Atula S. D. Sandanayaka *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Center for Organic Photonics and Electronics Research, Kyushu University, Fukuoka 819-0395 Department of Physical Sciences and Technologies, Faculty of Applied Sciences, Sabaragamuwa University of Sri Lanka, Belihuloya 70140 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000327 SectionsAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail In recent years, organic light-emitting device technology has expanded from organic light-emitting diodes (OLEDs) to organic semiconductor laser diodes (OSLDs) with the progress of sophisticated molecular and device architectural designs. In OLEDs, the development of thermally activated delayed fluorescence molecules has been intensively investigated recently. As a result, the internal quantum efficiency of OLEDs containing relatively simple aromatic compounds without precious metals has reached almost 100%. Furthermore, incorporating a distributed feedback resonator structure into the OLED architecture has yielded OSLDs that exhibit the features of current-pumped lasing. In this short review, the authors describe the recent paradigm shift from OLEDs to OSLDs, mainly from the perspective of materials innovation. Download figure Download PowerPoint Progress of Emitter Materials in Organic Light-Emitting Diodes In an organic light-emitting diode (OLED), electrons and holes are injected from the cathode and anode, respectively, into multiple organic layers with thicknesses of ∼100 nm and transported in these layers. The recombination of electrons and holes in the light-emitting layer generates excitons, which might deactivate radiatively, leading to light emission from the OLED. At the time of exciton generation, four eigenstates are formed from a combination of electrons and holes according to spin statistics (Figure 1).1 In this event, the excited singlet state with spin s = 0 is generated with a probability of 25% and the excited triplet states with s = 1 are generated with a probability of 75%. Singlet excitons are usually generated with a probability of almost 100% in the photoexcitation process, whereas triplet excitons are generated with a 75% probability in the electrical excitation process (Figure 2). Thus, achieving radiative deactivation of triplet excitons generated by electrical excitation is the key to realizing highly efficient OLEDs. However, most organic molecules are fluorescent materials that emit light from singlet excitons; emission from their triplet excited states is not usually observed at room temperature because of the competition of nonradiative deactivation. Therefore, triplet-state emission from fluorescent materials is typically limited to low temperatures, such as that of liquid nitrogen. OLED research started in the 1950s with single crystals of anthracene, which is a typical fluorescent molecule (first generation).2,3 Until around 1997, only fluorescent materials were used as light-emitting materials (Figure 3). It was then discovered that the excited triplet energy level of anthracene derivatives could be controlled systematically by introducing a wide variety of substituents, which led to the use of triplet–triplet upconversion (TTU) to raise their electroluminescence (EL) efficiency to higher than that of typical fluorescent molecules. At present, the external quantum efficiency (EQE) of TTU-based OLEDs is >10%, which exceeds the theoretical limit of fluorescence-based OLEDs, that is, the EQE = 5%. In addition, TTU-based emitters with durable molecular structures have been developed, resulted in their practical application in blue OLEDs. Figure 1 | Four eigenstates generated under current excitation. Statistically, the recombination of electrons and holes produces 25% excited singlets and 75% excited triplets. (a) Conceptual diagram of the four spin states. (b) Spin function. Download figure Download PowerPoint Since the early 1950s, it has been widely recognized from theoretical considerations that high EL efficiency can be obtained in OLEDs by using phosphorescence, which is direct luminescence from a triplet excited state. In the early 1990s, some phosphorescent materials such as keto-coumarin derivatives,4,5 Eu derivatives,6,7 and Tb derivatives7 were examined. However, the EQE of OLEDs containing these phosphorescent materials was much lower than that of fluorescent OLEDs. Then in the latter half of the 1990s, some organometallic complexes containing heavy metals such as Os, Au, Pt, and Ir were examined, aimed for OLED application. In fact, Ma and Che first demonstrated the feasibility of metal complexes to obtain high-efficiency OLEDs using Os(CN)2(PPh)3X,8 although their very first device showed a rather low EQE of <0.1%. This study initiated the examination of various luminescent materials, which revealed that PtOEP9 and Ir(ppy)310,11,12 showed great promise for use in OLEDs. Indeed, an internal quantum efficiency (IQE) of almost 100% was realized for OLEDs with Ir(ppy)3 derivatives and sophisticated device architectures,12 giving rise to second-generation luminescent materials. Then the molecular structure of Ir complexes was optimized considering device durability, resulting in current practical devices that operate in the green and red regions. However, Ir is inherently scarce and expensive. Furthermore, even after 15 years of research and development, it is still difficult to achieve highly stable blue phosphorescent OLEDs.13 Figure 3 | Progress of OLED light-emitting molecules: first generation (fluorescent molecules), second generation (phosphorescent molecules), and third generation (TADF). TTA is an extension of first-generation technology. Download figure Download PowerPoint In 2012, our research group reported a current-to-photon conversion efficiency of nearly 100% using advanced thermally activated delayed fluorescence (TADF) materials as third-generation luminescent materials,14 following our lead studies.15–17 To achieve efficient TADF, a small energy difference between the lowest singlet and triplet excited states (ΔEST) is needed to facilitate reverse intersystem crossing (RISC). In TADF, the RISC process is used as an emission light path (Figure 2). Moreover, the phenomenon of TADF itself was first confirmed in the 1930s,18 but the efficiency of upconversion was rather low, masking it as a possible OLED mechanism.19–21 However, focusing on precise molecular design with the aim of minimizing ΔEST has led to pure aromatic compounds with ΔEST as small as several hundreds of millielectronvolts with almost 100% upconversion efficiency. As a result, OLEDs with an IQE of 100% were realized.14 Figure 2 | Mechanisms of exciton generation under current excitation. (a) Conventional fluorescence and phosphorescence emission mechanisms under optical and electrical excitations. In case of fluorescence molecules, only 25% of electrically generated excitons contributes for light emission, while phosphorescence molecules can harvest 100% excitons for light emission via direct triplet exciton formation and indirect triplet formation through ISC. (b) TADF mechanism. In case of thermally activated delayed fluorescence (TADF) mechanism, both electrically generated singlet and triplet excitons contribute for prompt and delayed emissions, leading to 100% emission from the singlet state. ISC, intersystem crossing; RISC, reverse intersystem crossing; TADF, thermally activated delayed fluorescence; NRD, nonradiative decay process. Download figure Download PowerPoint So far, many reported TADF molecules comprise donor–acceptor (D–A) structures in which the electronic configurations of the ground and excited states are orthogonal to each other, like the n–π* transition but not π–π*. Thus, it is vital to understand the mechanism of effective spin upconversion in the TADF system. In the case of D–A-type TADF molecules, it has been well recognized that there are two major electronic states, such as locally excited (LE) and charge-transfer (CT) states, that form multiple energy levels depending on the molecular structures.14 A recent study clarified that LE and CT states could mix partially to form ψ(LE + CT) states. Upconversion from an excited triplet to an excited singlet state is a transition between different spin states, and according to the El-Sayed rule,22 a transition between triplet CT and singlet CT states or triplet LE and singlet LE states is a forbidden process when the wavefunctions of these states are composed of pure components. Thus, the transition between the same types of pure electronic states does not occur, but instead, as a mechanism to promote the triplet-to-singlet RISC transition, and a model was proposed in which the transition between the singlet CT and triplet CT states goes through an intermediate triplet LE transition state (Figure 4). Quantum chemical calculations have also revealed that in actual molecules pure CT and LE states do not exist, and in many cases, the electronic level is a mixture of CT and LE states.23–25 Furthermore, it has been pointed out that the presence of different CT levels, such as through-space and through-bond levels, plays an essential role in upconversion.26,27 Figure 4 | A possible mechanism of the electronic transition from the lowest triplet excited state to the lowest singlet excited state. Spin conversion from 3CT to 1CT occurs via 3LE. The CT state is based on the electronic transition from the donor site to the acceptor site in a molecule, and the LE state is the electronic state localized at the donor site. In practical devices, the mixing of CT and LE states occurs, promoting the RISC process. Download figure Download PowerPoint D–A compounds are considered the fundamental TADF structure for designing high-performance TADF molecules, and many such molecules have now been developed. It has also been clarified that high-performance TADF properties could be achieved using other novel molecular skeletons. In 2014, it was reported that an n–π*-type heptazine derivative without a D–A skeleton exhibited TADF properties.28 Although the photoluminescence quantum yield of guest–host thin films with the heptazine derivative was about 30%, its TADF lifetime was extremely short (about 250 ns). Furthermore, Hatakeyama et al.29–33 proposed a separation mechanism of the highest occupied and lowest unoccupied molecular orbitals using the charge-resonance effect, which yielded a high-performance TADF molecule. Since this molecule had a rigid molecular skeleton, it showed a very narrow emission spectrum with a full width at half maximum (FWHM) of 27 nm, making it an excellent candidate for display applications. Currently, the molecular skeletons of TADF materials include D–A type, charge-resonance type, multiple heterocycles utilizing the n–π* excited state, and proton transfer molecules.32 Therefore, a wide variety of molecular skeletons could be used to realize TADF, and it is expected that further molecular designs would be developed in the future. In this way, OLED research started with fluorescent molecules, progressed to the development of room-temperature phosphorescent molecules, and then rapidly evolved to focus on TADF molecules. Besides, very recent studies have demonstrated some novel conceptual light-emitting materials based on organic radical and organic–inorganic perovskite materials, which use triplet-to-triplet,33 doublet-to-doublet,34 and band-to-band transitions,35,36 respectively. Indeed, various developments are being made because of the high degree of freedom in the molecular design of organic molecules. Active Molecules for Organic Lasers Another attractive feature of organic light-emitting molecules is their ability to amplify light; that is, laser action. Since the first reports of lasing from organic materials using Eu complexes by Sorokin, Lankard, and Schafer more than 50 years ago,37–41 various molecular skeletons have been developed for this purpose. Research has centered on styrylamine-, coumarin-, and cyanine-based materials, keeping their application to liquid dye lasers in mind, and the number of such lasing materials exceeds tens of thousands.42 Especially since 1995, the development of materials for solid-state waveguide thin-film lasers has progressed along with that of OLED light-emitting molecules, and various molecular skeletons exhibiting low lasing thresholds have been reported.43–58 Figure 5 summarizes the lasing/amplified spontaneous emission (ASE) threshold of representative laser materials in solid films. It has been recognized that stilbene and fluorene units in both small molecules and polymers provide excellent lasing behaviors, indicating all possessing rigid backbones with high photoluminescent quantum yield (PLQY) and radiative decay rates. Actually, some reports have aimed to develop current injection lasers using organic materials.59,60 Because organic molecules exhibit strong concentration quenching, a thin solid film consisting of a few mol % of the laser molecules dispersed in a host matrix, that is, guest–host system, is used in such current injection lasers. Figure 5 | Correlation between the molecular structures of organic laser molecules and thresholds of ASE and lasing. Download figure Download PowerPoint Of these various molecular skeletons, it has been reported that laser molecules with a stilbene skeleton exhibit a low threshold value for ASE and lasing.61,62 In particular, 4,4′-bis[(N-carbazole)styryl]biphenyl (BSB-Cz) showed an ASE oscillation wavelength (λASE) of 461 nm in a thin-film waveguide structure with 6 wt % BSB-Cz: 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl CBP as the active layer and an ASE threshold (Eth) of 0.32 ± 0.1 μJ/cm2, which is extremely low (Figure 6).61 The fluorescence lifetime (τf) of this thin film was short (∼1.0 ns), its fluorescence quantum yield (Φf) reached almost 100%, and its radiative deactivation rate constant (kr) was large (1 × 109 s−1). Because Φf and τf of this film did not show temperature dependence from 5 to 300 K, nonradiative deactivation was suppressed entirely even at room temperature. λASE of BSB-Cz occurs near the 0–1 transition in its fluorescence emission spectrum, which suggests the slight self-absorption of the 0–0 transition. Here, λASE is discussed based on kr, the stimulated emission cross section (σem), and the absorption cross section (σABS). kr (kr = ΦPL/τf) is calculated from τf and the emission quantum efficiency (ΦPL) of each codeposited thin film. σem is calculated using the following formula,63,64 σ em ( λ ) = λ 4 E f ( λ ) 8 π n 2 ( λ ) c τ f (1) n f = ∫ E f ( λ ) d λ (2) Figure 6 | Laser oscillation characteristics and optical properties of a 6 wt % BSB-Cz:CBP thin film as an active layer. (a) Chemical structures of BSB-Cz and CBP as an active emitter and host, respectively. (b) Temperature dependence of the emission quantum efficiency and emission lifetime of the thin film. (c) Lasing oscillation spectrum. (d) Excitation power dependence of emission intensity. The threshold is around 0.32 μJ/cm2. Download figure Download PowerPoint In Eq. (1), Ef(λ) is the quantum yield distribution, and n is the refractive index. σABS54 was calculated using Eq. (3), in which n = 1.8. σ ABS , Sol ( λ ) = 1000 ɛ ( λ ) ln 10 N A (3)where ɛ(λ) is the molar extinction coefficient, and NA is Avogadro’s number. In the 6 wt % BSB-Cz:CBP thin film, a high value of σem = 2.7 × 10−16 cm2 was obtained. Furthermore, the effective stimulated emission cross section (σemeff) is the difference between σem and the cross section related to a loss (σABS and the singlet and triplet excited-state absorption cross sections, σSS and σTT, respectively), and is given by Eq. (4). σ emeff = σ em − ( σ ABS + σ SS + σ TT ) (4) Figure 7 shows the spectra of σeff and σABS and the excited-state absorption spectrum of a 6 wt % BSB-Cz:CBP coevaporated thin film. In BSB-Cz, the singlet excited-state absorption, triplet excited-state absorption, and ground-state absorption spectra do not have a large overlap with λASE. Thus, the 6 wt % BSB-Cz:CBP codeposited thin film has a high kr (i.e., a large σem), σABS as small as <10−19 cm2 at λASE, and an excited-state absorption. The absence of these absorptions provides a very large σemeff, leading to a very low Eth. Figure 7 | (a) Ground-state absorption spectrum (solid red line), fluorescence spectrum, laser oscillation spectrum (solid blue line), S–S absorption spectrum (blue circles), and T–T absorption spectrum (orange circles) of BSB-Cz. (b) Energy-level diagram of BSB-Cz. Download figure Download PowerPoint Laser Oscillation Characteristics Under Optical Excitation As described earlier, BSB-Cz is suitable for optical amplification because of its high Φf, low probability of intersystem crossing, and the absence of overlapping excited-state absorption in the λASE region.66 For laser oscillation, it is necessary to introduce an optical resonator structure; however, in an amorphous organic thin film with a thickness of several hundred nanometers, it is difficult to form an end face like in the case of an inorganic semiconductor crystal with a distributed Bragg reflector structure. Thus, for organic thin-film lasers, it is ideal for forming a distributed feedback (DFB) resonator structure, which could outcouple the emission perpendicular to the longitudinal direction of the device. Among DFB resonator structures, the mixed-order DFB structure, which has a primary feedback region that produces strong optical feedback and a secondary Bragg scattering region that allows light extraction, is suitable for organic thin-film lasers. In a DFB resonator structure, the Bragg condition is given by Eq. (5), m λ Bragg = 2 n eff Λ (5)where m is the diffraction order, λBragg is the Bragg wavelength, neff is the effective refractive index of the gain medium, and Λ is the grating period. Laser oscillation occurs when this condition is satisfied.25 Using the reported values of neff and λBragg for BSB-Cz, the optimum Λ for m = 1 and 2 in DFB laser devices are 140 and 280 nm, respectively. Figure 8 shows a DFB grating observed by scanning electron microscopy (SEM). The DFB grating was designed to possess a depth of 65 ± 5 nm and Λ of 140 ± 5 and 280 ± 5 nm. The primary and secondary DFB grating lengths were approximately 15.12 and 10.08 µm, respectively. By forming a 200 nm-thick BSB-Cz film on the grating by vacuum deposition, the surface morphology of the organic layer possessed a lattice structure with a surface modulation depth of 20–30 nm. Figure 9 shows the oscillation characteristics of a mixed-order DFB device under optical excitation. With increasing excitation intensity, the FWHM decreased remarkably, and at Eth = ∼0.2 μJ/cm2, laser oscillation occurred from the vicinity of the stopband at the central oscillation wavelength of 481 nm. In this mixed-order-type DFB structure, compared with those of devices with ASE and second-order DFB structures, Eth was decreased by about 1/3 and 1/2, respectively, demonstrating the superior performance of the mixed-order-type DFB structure. These results confirmed the light confinement effect of the mixed-order DFB structure with BSB-Cz. Figure 8 | (a) Schematic of a mixed-order DFB structure with first- and second-order gratings. (b, c) SEM images of the DFB structure with a 140-nm primary structure and 280-nm secondary structure. Download figure Download PowerPoint The limited overlap of the excited-state absorption, ground-state absorption, and emission spectra of BSB-Cz suggest the possibility to realize quasi-continuous-wave (qCW) laser oscillation. Figure 10 shows the qCW laser oscillation characteristics of a device with BSB-Cz. Continuous laser action was obtained even at a high frequency of 80 MHz. Besides, continuous laser action was observed even with a long pulse excitation of 800 µs to 30 ms.55 The optical gain and loss coefficient estimated from the ASE characteristics of the doped film (optical waveguide structure with a thickness of 200 nm) using the variable stripe method were 40 and 3 cm−1, respectively. These results confirmed that BSB-Cz is an attractive candidate for qCW lasers able to operate even under long-pulsed light excitation. Figure 10 | Quasi-CW lasing characteristics of mixed-order DFB structures. Streak images of the oscillation state when (a) the excitation frequency was changed from 0.01 to 80 MHz, and (b) the pulse width was 30 ms and 800 (c) Excitation dependence of the lasing threshold The doped film exhibited a lower Eth than that of the film. continuous Download figure Download PowerPoint of Organic Semiconductor Laser With the of fluorescent molecules, phosphorescent molecules, and TADF molecules, OLEDs, of conversion with the IQE of 100%, are now At the same the of an organic semiconductor laser diode has long been a in organic semiconductor proposed an using an Eu in the active layer in but 30 years have since organic semiconductor laser that by current excitation is expected to have great because of its low and wavelength from the to the Furthermore, such organic semiconductor lasers are attractive for use at the of organic such as in optical on Figure 9 | (a) Laser oscillation characteristics of the mixed-order DFB structure. (b) of oscillation and FWHM on excitation intensity. (c) of emission around the with the calculated Download figure Download PowerPoint In our research group reported of lasing by current The device structure was based on that of a OLED. To electrical the primary and secondary DFB structures in the optical resonator and a fluorescent thin film of BSB-Cz as the organic semiconductor active layer were between an thin and cathode (Figure In this organic amorphous thin-film a thin-film structure is to a high of for effective current and the thickness of the organic active layer was limited to nm. Furthermore, to form with the the cathode of the organic active layer was with and a layer was on the of the organic active layer to achieve Figure | Schematic diagram of the structure of a was obtained using BSB-Cz on the cathode and layer on the cathode Download figure Download PowerPoint Conventional OLEDs are based on a to and electrons and holes and the generated excitons in the light-emitting layer. This structure to achieve high and the EL emission efficiency of OLEDs did not to current of about 1 However, an injection of 1 is needed for laser oscillation. At such a high current and various exciton deactivation by the Therefore, it is necessary to use a structure containing to both and deactivate excitons in the of the light-emitting layer. current revealed that the of both electrons and holes in the BSB-Cz layer were about Thus, that there are at the between the and the organic the recombination site be near the of the BSB-Cz emission layer. In fact, the characteristics of the showed a constant EQE to a high current of without A organic molecules under current excitation is the presence of and radical absorption. Because many organic molecules have a absorption spectrum in the radical state, the absorption by overlap with the oscillation wavelength laser oscillation. BSB-Cz shows strong absorption around to 1000 nm, which does not overlap with the emission spectrum near nm. Therefore, BSB-Cz the overlap of the absorption from the ground state, excited singlet absorption, excited triplet absorption, and absorption, the to achieve excellent performance as a laser molecule for current excitation. A laser was using a 6 wt % BSB-Cz:CBP codeposited thin film as an active and as an optical resonator structure suitable for an first- and second-order DFB structures were into an OLED device to Figure shows the laser oscillation characteristics of the device under the current excitation. a current of about a narrow and emission were obtained. A in FWHM was observed with a and a width of nm or was obtained. The current threshold was almost to the threshold value estimated under The efficiency under current excitation was which was the same as that under optical excitation In the efficiency of the device without a metal was which suggests that the loss by the metal was Figure | Lasing characteristics of a current-pumped (a) of the oscillation spectrum near the threshold on current (b) of oscillation and FWHM on current Download figure Download PowerPoint the of laser oscillation, the current a very short device of OLEDs has been by by and and chemical of light-emitting molecules using device However, a very high current compared with that used for OLEDs. the yield of OSLDs was because of the of device DFB In the along with the of the laser the mechanism of by the of the excited triplet states be BSB-Cz is to because of its which is relatively to it has been confirmed that the of triplet into organic semiconductor laser structures the of lasing In the in to the triplet exciton deactivation mechanism, it is necessary to the

Among the three primary colors, blue emission in organic light-emitting diodes (OLEDs) are highly important but very difficult to develop. OLEDs have already been commercialized; however, blue OLEDs have the problem of requiring a high applied voltage due to the high-energy of blue emission. Herein, an ultralow voltage turn-on at 1.47 V for blue emission with a peak wavelength at 462 nm (2.68 eV) is demonstrated in an OLED device. This OLED reaches 100 cd/m2, which is equivalent to the luminance of a typical commercial display, at 1.97 V. Blue emission from the OLED is achieved by the selective excitation of the low-energy triplet states at a low applied voltage by using the charge transfer (CT) state as a precursor and the triplet-triplet annihilation, which forms one emissive singlet from two triplet excitons. We found that the essential component for efficient blue emission is a smaller energy difference between the CT state and triplet exciton, accelerating the energy transfer between the two states and achieving the optimal performance by avoiding direct decay from the CT state to the ground state. Our study demonstrates that the developed OLED allows for a much longer operation lifetime than that from a typical blue phosphorescent OLED because the blue emission originates from a stable low-energy triplet exciton that avoids degrading the constituent materials.

Organic Light Emitting Diodes (OLEDs) are predicted to revolutionize next generation consumer electronics by offering many advantageous device characteristics, including low power consumption, low heat dissipation, a tunable and wider color gamut, high resolution and contrast, light weight, flexibility, and semi-transparency. However, a major limiting factor for OLEDs to reach their full potential is that only a few known blue OLED emitters with substantial spectral purity and longevity are available to date. Therefore, focus of this research is on understanding and addressing limitations of OLED emitters, with an emphasis on improving the characteristics of blue emitters. The work presented in this dissertation includes understanding structure-property relationships of OLED blue emitters using four structurally related pyrenylpyridines as model compounds (chapter 2), applying these structure-property relationship concepts to synthesize three novel blue emitters derived from pyrene-benzimidazole conjugates with substantially improved spectral properties (chapter 3), as well as synthesizing and characterizing propidium luminophore (3,8-diamino-5-[3-(diethylmethylammonio)propyl]-6-phenylphenanthridinium dication) based GUMBOS (group of uniform materials based on organic salts) to evaluate how these GUMBOS materials can be applied to address the aforementioned limitations of OLED emitters. A number of analytical tools were applied to study the characteristics of these compounds, including morphology, spectroscopy, photothermal stability, and electrochemistry. Also, OLED prototypes were fabricated and characterized with selected compounds to understand the luminance, current, and power relationships of these materials.

Much effort on the development of new host materials has been devoted to increase the emission efficiency of organic light-emitting diodes (OLEDs). In particular, the use of phosphorescent molecules as guest emitters, which harvests both singlet and triplet excitons, leads to internal quantum efficiency (ηint) approaching 100%. 2 Energy transfer from a host to a dopant in OLEDs can result in high external quantum efficiency (ηext). Therefore, the development of suitable hosts for blue, green and red phosphorescent emitters has been one of the major issues in the field of OLEDs. In general, host materials should have a higher triplet energy than that of the dopant molecules due to the energy transfer from the host to the guest while prohibits the energy transfer of triplet excitons from the guest to the host material. From the point of this consideration, carbazole derivatives are often use as host materials, because they have high triplet energies (2.9 eV or less). Polyaromatic compounds bearing naphthyl groups have formed an important class of highly efficient and stable bluelight emitting materials. It has been suggested that nonplanar derivatives of naphthalene due to steric factors may hinder close packing and improve the device performance; hence, the electroluminescence (EL) polyaromatic compounds bearing naphthyl groups have been designed on the basis of this principle. During our ongoing efforts on the development of emitting materials and host for phosphorescence OLEDs, we have observed that naphthyl derivatives linked by 9-ethylcarbazole unit as a core, such as 3,6-di(1naphthyl)-9-ethylcarbazole (1-DNEC) and 3,6-di(2-naphthyl)-9-ethylcarbazole (2-DNEC), have high thermal stability, reversible electrochemical behaviors and larger band gap compared to those of other carbazole derivatives. These facts have prompted us to investigate electroluminescent (EL) characteristics of both compounds. Herein, we discuss electroluminescent characteristics and device data for OLEDs employing both 1-DNEC and 2-DNEC as host. We fabricated multilayer devices with the configuration of ITO/NPB (30 nm)/(device 1) 1-DNEC and 2-DNEC (device 2): Ir(ppy)3 (45 nm: 3%)/Alq3 (25 nm)/LiF (2 nm)/Al. The layers of the device consist of ITO (indium-tin-oxide) as the anode, NPB (N,N'-di-1-naphythyl-N,N'-diphenyl-byphenyl4,4'-diamine) as the hole transporting layer, 1-DNEC or 2DNEC as host, 3%-Ir(ppy)3 (ppy = 2-phenylpyridine) as the emitter, Alq3 (q = 8-hydroxyquinolate) as the electron transporting layer, LiF as the electron injection layer, and Al as the cathode, respectively (Figure 1). In this study, we chose Ir(ppy)3 as the emissive material, which is a prototypical dopant for green OLEDs. The electroluminescence spectra of all devices consist only of green emission (λmax = 458 nm) from Ir(ppy)3 without any emissions from the host and Alq3,

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