Pure-Blue Fluorescence Molecule for Nondoped Electroluminescence with External Quantum Efficiency Approaching 13%

Open AccessCCS ChemistryRESEARCH ARTICLE1 Sep 2021Pure-Blue Fluorescence Molecule for Nondoped Electroluminescence with External Quantum Efficiency Approaching 13% Xianhao Lv†, Lei Xu†, Miao Cang, Runze Wang, Mizhen Sun, Huayi Zhou, Yuan Yu, Qikun Sun, Yuyu Pan, Yuwei Xu, Dehua Hu, Shanfeng Xue and Wenjun Yang Xianhao Lv† Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Lei Xu† Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Miao Cang Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Runze Wang Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Mizhen Sun Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Huayi Zhou Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Yuan Yu Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Qikun Sun Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Yuyu Pan School of Petrochemical Engineering, Shenyang University of Technology, Liaoyang 111003 , Yuwei Xu Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 , Dehua Hu Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640 , Shanfeng Xue *Corresponding author: E-mail Address: email protected Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 and Wenjun Yang Key Laboratory of Rubber-Plastics of the Ministry of Education/Shandong Province (QUST), School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 https://doi.org/10.31635/ccschem.020.202000392 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail 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 thermally activated delayed fluorescence (TADF) materials, they need to be dispersed into suitable host materials. Hence, exploring efficient nondoped, pure-blue luminous molecules is important. Herein, a novel "hot-exciton" material, 4-(2-(4-(10-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)anthracen-9-yl)phenyl)-1H-phenanthro9,10-dimidazol-1-yl)benzonitrile (tBuPCAPICN) is reported for the application of pure-blue fluorescent OLEDs. The nondoped tBuPCAPICN-based OLED exhibited excellent pure-blue electroluminescence (EL) performance with an emission peak at 452 nm and a full width at half maximum (FWHM) of only 53 nm, corresponding to the Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.11). Furthermore, the maximum external quantum efficiency (EQE) of the OLED reached 12.7%, and the exciton utilization efficiency (EUE) approached 80%, ranking it among the best performers in nondoped pure-blue OLEDs. The distinguished EL performance could be ascribed to the coordination of high molecular horizontal orientation (orientation factor Θ≈82%) and high-energy triplet exciton utilization. This work not only reveals the application potential of the tBuPCAPICN in pure-blue OLEDs, but also offers a useful approach for designing novel fluorescent materials with high-efficiency pure-blue performance. Download figure Download PowerPoint Introduction The third generation of organic light-emitting diode (OLED) materials, which incorporate advantages from the preceding generations, benefit from both the low cost and excellent stability of organic luminescence molecules and the high efficiency of phosphorescent materials (Ir and Pt complexes).1–10 Efficient pure-blue fluorescent luminescence molecules are urgently sought because they not only offer blue luminescence for organic displays but also could serve as host materials for generating other emissions via energy transfer.11–14 Usually, a high-efficiency OLED device is obtained via a doping method, which requires a proper host material with a large bandgap and precise regulation of the doping concentration.15–17 This doping process increases device fabrication costs and adversely affects commercial applications. In addition, color saturation is another key factor in full-color displays and lighting because it can reduce the power consumption in OLEDs.18–20 Therefore, developing efficient pure-blue emitters with outstanding color purity for nondoped OLED devices is important. Although noble metal (Ir, Pt, and Os)-based phosphorescent OLEDs can achieve efficient emission, they must be dispersed into wide bandgap host materials to alleviate quenching phenomena caused by the accumulation of triplet excitons.21–25 In addition, pure-blue phosphorescent OLEDs (POLEDs) with satisfactory color purity (a Commission Internationale de l'Eclairage (CIE) y value <0.10) are particularly rare. Recently, research on blue fluorescent emitters has made tremendous strides with excellent performance based on thermally activated delayed fluorescence (TADF),26–32 triplet–triplet annihilation (TTA),33–36 and "hot-exciton" fluorescence.37–39 Essentially, a high external quantum efficiency (EQE) of pure-blue OLEDs has been obtained using a slow process of reverse intersystem crossing (RISC) to transfer first-lowest triplet-excited state (T1) to first-lowest singlet-excited state (S1), which is known as the TADF mechanism, or converting two T1 excitons into one S1 exciton and one S0 exciton, which is known as the TTA mechanism.26,33 Unfortunately, like phosphorescent materials, the quenching effect caused by the long-lived T1 excitons still exists in most TADF and TTA materials, and they must be scattered into suitable host materials with high T1 energy levels to maintain high efficiency. Distinct from these two types of materials, "hot-exciton" materials undergo T → S conversion from a high-energy triplet excited state (T2) to S1 or a high-energy singlet excited state (S2) (usually T2 → S1 or T2 → S2). The fast RISC process starts at a high-energy triplet state of "hot excitons" thereby preventing the aggregation of T1 excitons at high brightness. Presently, highly efficient nondoped pure-blue electroluminescent devices with small y-value color-coordinates, a short electroluminescence (EL) peak (e.g., λ ≤ 450 nm), and a narrow half peak width remain scarce and need to be developed. The development of efficient nondoped pure-blue OLEDs is critical for practical applications in display and lighting, particularly for fluorescence materials with RISC of high-energy triplet-state excitons (T2 → S1). In this work, a pure-blue, "hot-exciton" fluorescent molecule, 4-(2-(4-(10-(4-(3,6-di-tert-butyl-9H-carbazol-9-yl)phenyl)anthracen-9-yl)phenyl)-1H-phenanthro9,10-dimidazol-1-yl)benzonitrile (tBuPCAPICN), which was constructed by attaching a tert-butyl-substituted phenylcarbazole (PCZ) group, anthracene, and cyano (CN)-substituted phenanthroimidazole (PI) was fabricated. In tBuPCAPICN, the carbazole group is a weak electron-donating unit with an excellent hole-transporting ability. The introduction of sterically hindered spacers like bulky tert-butyl groups effectively prevents the self-quenching effect originating from strong bimolecular interactions and controls the intermolecular distance to inhibit the aggregation-caused quenching (ACQ) effect.40 PI is an excellent near-UV fluorescent emitter. The incorporation of a CN group can efficiently adjust the electron-withdrawing ability of PI and further lower the charge injection barrier.41 The anthracene moiety is a conjugated core linking the donor (D) and acceptor (A). Recent studies of the energy level of anthracene derivatives indicate that they usually have stable T2 states,42 and their large energy gap (ΔET2−T1) and small energy difference (ΔET2−S1) inhibit the excitons' internal crossing (IC) process (T2 → S1) of excited states and facilitate the RISC process (T2 → S1) of excited states. Hence, incorporation of an anthracene moiety benefits the formation of "hot-exciton" channels and improves device efficiency. Thus, this twisting bipolar D–π–A molecule can effectively interrupt conjugation and inhibit the ACQ effect in a solid state, simultaneously improving color purity and photoluminescence quantum yield (PLQY). A nondoped OLED device was fabricated with tBuPCAPICN as emitter. It exhibited a pure-blue fluorescent emitting light with the maximum EL peak of 452 nm, associated with color coordinates of (0.15, 0.11). The maximum current efficiency (CE) and power efficiency (PE) were achieved at 12.3 cd A–1 and 7.4 lm W−1, respectively. The maximum EQE reached 12.7%, and the corresponding exciton utilization efficiency (EUE) was ∼80%. We confirmed that a "hot-exciton" channel with high-energy triplet-state excitons utilization and high molecular horizontal orientation is responsible for the excellent device performance. We believe that the "hot-exciton" channel and high molecular horizontal orientation are the key factors for the excellent device performance of this nondoped pure-blue device. Currently, these are the upmost results in a nondoped pure-blue OLED. Experimental Methods The synthetic pathway of tBuPCAPICN is shown in Supporting Information Scheme S1. First, the synthesis of the PPICN-Br (4-(2-(4-bromophenyl)-1H-phenanthro9,10-dimidazol-1-yl)benzonitrile) precursor was achieved by a simple and extremely efficient one-pot reaction method, and the carbazole derivative tBuPCZ-Br (9-(4-bromophenyl)-3,6-di-tert-butyl-9H-carbazole) was obtained by a C–N coupling reaction. Subsequently, the corresponding boron ester products, PPICNB (4-(2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-1H-phenanthro9,10-dimidazol-1-yl)benzonitrile) and tBuPCZB (3,6-di-tert-butyl-9-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole), were acquired by the Miyaura reaction. The intermediate compound PPICNA-Br was obtained by docking 9,10-dibromoanthracene with PPICNB via the Suzuki carbon–carbon coupling reaction. Under catalytic conditions, the Suzuki coupling reaction was repeated to couple PPICNA-Br (4-(2-(4-(10-bromoanthracen-9-yl)phenyl)-1H-phenanthro9,10-dimidazol-1-yl)benzonitrile) with tBuPCZB to obtain the final product tBuPCAPICN. The main intermediates and the high purity final product tBuPCAPICN were characterized by 1H NMR and 13C NMR ( Supporting Information Figures S1–S5). The molecular weight and elemental composition of the tBuPCAPICN were obtained by high-resolution mass spectrometry ( Supporting Information Figure S6) and elemental analysis. More detailed information on analysis methods is available in the Supporting Information section "Measurements". Results and Discussion Thermal properties We analyzed the performance of the tBuPCAPICN at high temperature by testing its decomposition temperature (Td) with thermal gravimetric analysis (TGA) under a nitrogen flow, and the glass-transition temperature (Tg) was obtained with differential scanning calorimetry (DSC) under nitrogen. The results are summarized in Table 1 and illustrated in Figure 1a. The Td was equal to 517 °C, which indicates excellent thermal stability and ensured that the deposition process did not occur during OLED device fabrication and operation. The Tg observed in the DSC curve was 230 °C, and this high Tg is indicative of excellent morphological stability during the heating process. Outstanding thermal and morphological properties are essential for OLED materials used in device applications. Table 1 | Basic Physical Properties of the Blue Emitter tBuPCAPICN Material λabs (s/f) (nm)a λPL (s/f) (nm)b Tg (°C)c Td (°C)d HOMO (eV)e LUMO (eV)e Eg (eV)e Eg (eV)f ηPL (s/f) (%)g tBuPCAPICN 395/409 438/454 230 517 −5.53 −2.62 2.91 2.85 82/49 aMaximal absorption spectrum in THF (10−5 M)/in thin film. bPL emission peak in dilute THF/in thin film. cGlass-transition temperature. dDecomposition temperature. eData of electrochemical measurement and energy gap calculations. fThe optical bandgap calculation is based on the onset of evaporated film absorption. gPLQY in THF (10−5 M)/in thin film. Figure 1 | (a) TGA and DSC curves of tBuPCAPICN, the insert shows the amplified DSC curve. (b) UV–vis absorption and PL emissive spectra of tBuPCAPICN in THF (10−5 M) and film. (c) PL emissive spectra of tBuPCAPICN in various solvents. (d) Transient PL decay spectrum of solid tBuPCAPICN. Download figure Download PowerPoint Photophysical properties To investigate the photoluminescence (PL) behavior and excited-state characteristics of tBuPCAPICN, the UV absorption and fluorescence emission of the material was examined. In Figure 1b, the PL spectrum of tBuPCAPICN in tetrahydrofuran (THF) emitted a deep-blue fluorescence at 438 nm. In the PL spectrum of the vacuum-evaporated film (454 nm), because of the aggregation behavior of molecules in the solid state, the PL spectra exhibited a 16 nm bathochromic shift compared with the spectrum of the sample in THF. The UV–vis absorption peaks of tBuPCAPICN in the range of 360–405 nm were caused by the π–π* electronic transition originating from the anthracene moiety,43 and the π–π* transition of the PPICN unit caused absorption ranging from 300 to 350 nm.44As illustrated in Figure 1c, when increasing the solvent polarity from hexane to acetonitrile, the PL spectra of tBuPCAPICN exhibited only a 7 nm red shift of its emission peak (λmax) from 432 to 439 nm, respectively. Such a faint solvatochromic effect suggests an almost complete localized excited character of the S1 state. As shown in Figure 1d, we can see the transient PL-decay curve of solid tBuPCAPICN, with a fluorescence lifetime of 2.40 ns, clearly exhibits a single index fluorescence attenuation process. No delayed component is observed on the curve, which suggests that the emission originated from the promoted decay in the S1 state. The PLQYs of tBuPCAPICN were equal to 0.82 in THF solution and 0.49 as solid; high PLQYs also indicate the locally emissive (LE) excited state of tBuPCAPICN. Electrochemical properties To facilitate device fabrication, a better comprehension of the material's energy levels is necessary. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated from the electrochemical cyclic voltammetry (CV) measurement. In Supporting Information Figure S7a, the oxidation potential onset of tBuPCAPICN was 0.94 V and reduction potential was −2.10 V as referenced to the ferricenium/ferrocene (Fc+/Fc and Fc−/Fc) redox couples. Calculated HOMO and LUMO energy levels were −5.53 and −2.62 eV, respectively, using Equation S1 in the Supporting Information. The electronic bandgap (Eg) equaled 2.91 eV. The energy alignment of the HOMO and LUMO and reversible oxidation–reduction curve of tBuPCAPICN illustrate the balanced charge injection ability of the EL devices. Theoretical calculations Via the Gaussian 09 B.01 package at the level of M062X/6-31 + G(d,p), both frontier molecular orbitals and molecular configuration calculations were conducted. The twisting-type molecular structure ( Supporting Information Figure S7b), linked with torsional angles of 52.3°, 82.2°, 78.8° and 70.7°, efficiently inhibited the π–π stacking effect caused by the molecular aggregation and interrupted the conjugation length, eventually stabilizing the novel blue emission. The twisting molecular configuration was responsible for the separation of the HOMO and LUMO. In Supporting Information Table S1 and Figure S7c, the corresponding singlet and triplet excited states' energy levels are listed. Illustrated in Figure 2, the natural transition orbitals (NTOs) were evaluated based on geometry structure of the S0 state. The NTO at the S1 state were almost positioned on the anthracene group, explaining the faint solvatochromic effect with solvent polarity change; the corresponding energy was 3.41 eV. The T1 of tBuPCAPICN was mainly populated on the anthracene part, with a calculated matching energy level of the T1 state of 2.09 eV. The NTO of the T2 state was distributed on the CN-substituted PI moiety, and the T2 state's energy level was 3.14 eV. The large energy difference between T1 and T2 (ΔET1–T2 = 1.32 eV) and the small one between S1 and T2 (ΔES1–T2 = 0.27 eV) indicate that a more competitive RISC behavior (T2 → S1) can be enhanced by the hot-exciton channel because of the narrower energy gap. The details of the NTOs of tBuPCAPICN are shown in Supporting Information Figure S8. Furthermore, we calculated the spin–orbital coupling (SOC) matrix constants of S1–T1, S1–T2 as well as S1–T3 excited states, the values are 0.11, 0.45, and 0.15 cm−1, respectively, and the SOC matrix constant of S1–T2 is the largest. In accordance with Fermi's golden rule, a more powerful SOC boosts the RISC rate constant.45 Figure 2 | The NTOs of tBuPCAPICN for S1/S0, T1/S0, and T2/S0. Percentage indicates possibility of transition. Download figure Download PowerPoint Electroluminescence properties For a better assessment of the potential use of tBuPCAPICN as the emitting material in devices, we fabricated a nondoped blue fluorescent OLED. The optimized device was comprised of the following: indium tin oxide (ITO) polymer poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) (40 nm), 4,4′,4″-tri(N-carbazolyl)-triphenylamine (TCTA) (30 nm), tBuPCAPICN (20 nm), 1,3,5-tri(phenyl-2-benzimidazolyl)benzene (TPBi) (30 nm), LiF (1 nm), Al (100 nm), where PEDOT:PSS was to function as the hole-injecting layer, TCTA was to function as the hole-transporting and electron-blocking layer, tBuPCAPICN was to function as the emitting layer, TPBi was to function as the electron-transporting and hole-blocking layer, and LiF was to function as the electron-injecting layer. ITO and Al were the anode and cathode, respectively. Table 2 displays the performance of the nondoped device. The device energy level structure is shown in Supporting Information Figure S9. The device emitted pure-blue fluorescence with maximum EL emission at 452 nm (Figure 3a) and maximum luminance of 5334 cd m–2 (Figure 3b). When the voltage was 6 V, the corresponding CIE coordinates were (0.15, 0.11). The relatively low CIE y value and narrow half peak width of 53 nm decreased the device's power consumption. As shown in Figure 3c, the maximum CE and PE were 12.3 cd A−1 and 7.4 lm W−1, respectively. As illustrated in Figure 3d and Table 3, the maximum EQE was 12.7%, currently the highest value with a CIE y ∼ 0.10 amongst the nondoped pure-blue OLEDs. The excellent EL performance of this device suggests that the hot-exciton mechanism plays an important role in creating highly efficient OLEDs. Another important reason for boosting the EQE of the OLED is that the horizontal orientation of tBuPCAPICN molecules improves the optical output efficiency (ηout).59–63 To understand the relationship between such a high EQE and the actual ηout of the nondoped device, the orientation factor (Θ, the fraction of horizontal emitting dipole) was obtained from the angle-dependent PL spectrum of the tBuPCAPICN-nondoped film ( Supporting Information Figure S10). The Θ value was calculated as 82%, which means that tBuPCAPICN in the emitting layer had many more horizontally orientated dipoles than vertically orientated dipoles (the corresponding Θ of isotropic dipole orientation is 66.7%). The large horizontal orientation fraction substantially contributes to enhancing the EQE. The ηout of a fully optimized pure-blue emissive OLED is ∼34% according to the previous reports by Kim et al.63 The EQE of OLEDs obeys this equation: ηEL = ηrec · ηs · ηPL · ηout, where ηEL is the EQE, ηs is the efficiency of the exciton utilization, ηPL is the PLQY of the solid film, and ηout is the efficiency of the light out-coupling ability (∼34%). The ηrec defines the efficiency of charge recombination, which equals 1 with the supposition of full electron–hole recombination. After calculation ηs was 76.2%, which implies that when the device works, most of the triplet excitons were effectively transformed to the singlet. The calculated energy difference value between the S1 and T1 states was equal to 1.32 eV ( Supporting Information Figure S7c), which was too large for the thermally activated RISC (T1 → S1) process to occur. As shown in the nanosecond-scale transient PL-decay life curve with a single-exponential decay (Figure 1d), no delayed fluorescence caused by the slow RISC (T1 → S1) process transpired. Therefore, we can exclude the possibility of TADF. Meanwhile, the transient EL decay spectra of the nondoped OLED in Supporting Information Figure S11a illustrate that the curves comprised two parts: (1) a fast EL attenuation caused by the fluorescent promotion of S1 and (2) a delay caused by the collisional recombination of electrons and holes retained in the device after the power shut off, which is a common phenomenon in the EL process.64,65 Furthermore, to clarify the possibility of the TTA mechanism in the EL process, we selected the delayed component of the device at 7 V and amplified it, and plotted the logarithm of EL intensity versus time ( Supporting Information Figure S11b). From previous reports,66 if the TTA mechanism dominates the EL process, the slope of the EL decay of the delayed component should be −1 at the start region of time and −2 in a longevity transient region. However, the actual corresponding slopes were −0.34 and −1.32, respectively. This result indicates that in this nondoped device, the TTA mechanism was not a main factor in the EL process. Table 2 | EL Performance of tBuPCAPICN-Based Device Device Vturn-on (V)a CEmax (cd A−1)b PEmax (lm W−1)c EQEmax (%)d Lmax (cd m−2)e CIE tBuPCAPICN 12.3 7.4 5334 452 53 voltage at 1 cd EQE. EL emissive peak of EL coordinates Figure | (a) EL spectra of tBuPCAPICN-based device (b) curves of tBuPCAPICN-based device. (c) curves of tBuPCAPICN-based device. (d) curve of tBuPCAPICN-based device, the is the tBuPCAPICN-based device configuration Download figure Download PowerPoint Table | Recent Nondoped OLEDs with CIE y value based on Materials Vturn-on (V)a CEmax (cd A−1)b PEmax (lm W−1)c EQEmax (%)d CIE tBuPCAPICN 12.3 7.4 452 This work 0.10 452 0.10 438 voltage at 1 cd EQE. emissive peak we compared transient EL decay of the tBuPCAPICN-based device with a device of the TTA mechanism ( Supporting Information Figure which can also exclude the of the TTA First, the EL intensity of the tBuPCAPICN-based device more compared with the device, that the fluorescence emission is substantially more delayed than the tBuPCAPICN-based fluorescence emission. with the voltage the in the fraction of the delayed of the device are caused by the triplet excitons quenching This phenomenon be to an quenching process among excitons and or because of with molecules in the device. However, the delayed of the tBuPCAPICN-based device and no voltage The two illustrate that the nondoped tBuPCAPICN-based device has a triplet excitons quenching process during the EL This result also suggests that the TTA mechanism is not the reason for the efficient EL process. triplet-state excitons utilization analysis To further understand the hot-exciton of the triplet and singlet excitons and the reason for the excellent EL the fluorescence and phosphorescent emission spectra of tBuPCAPICN with low temperature were (Figure A peak of fluorescence emission at 438 nm, and the energy of the excited state corresponding to this emission is eV. The phosphorescent emission of the tBuPCAPICN solution could not be observed from the delayed emission The large energy difference = 1.32 eV) between S1 and as calculated using ( Supporting Information Figure S7c), inhibited the intersystem crossing process from S1 to in a low of T1 no long-lived phosphorescent emission for a practical and energy transfer we used a phosphorescent material, with a low eV) to the T1 states of tBuPCAPICN. The emission was at nm on the delayed phosphorescent spectrum of the solution at Therefore, the energy