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Open AccessCCS ChemistryRESEARCH ARTICLE30 May 2022Realizing External Quantum Efficiency over 25% with Low Efficiency Roll-Off in Polymer-Based Light-Emitting Diodes Synergistically Utilizing Intramolecular Sensitization and Bipolar Thermally Activated Delayed Fluorescence Monomer Yuchao Liu, Yanchao Xie, Lei Hua, Xingwen Tong, Shian Ying, Zhongjie Ren and Shouke Yan Yuchao Liu *Corresponding authors: E-mail Address: email protected E-mail Address: email protected E-mail Address: email protected Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science and Technology, Qingdao 266042 , Yanchao Xie Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science and Technology, Qingdao 266042 , Lei Hua State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029 , Xingwen Tong State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029 , Shian Ying Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science and Technology, Qingdao 266042 , Zhongjie Ren *Corresponding authors: E-mail Address: email protected E-mail Address: email protected E-mail Address: email protected State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029 and Shouke Yan *Corresponding authors: E-mail Address: email protected E-mail Address: email protected E-mail Address: email protected Key Laboratory of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science and Technology, Qingdao 266042 State Key Laboratory of Chemical Resource Engineering, College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029 https://doi.org/10.31635/ccschem.022.202201900 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Since polymer-based light-emitting diodes (PLEDs) are well-suited building blocks for large-area and low-cost flexible display equipment, state-of-the-art thermally activated delayed fluorescence (TADF) PLEDs are in high demand. To respond to this demand, light-emitting TADF units have initially been modified with electron-transporting units to balance the carrier transport of regiorandom TADF polymers, and simultaneously, an intramolecular sensitizing strategy has also been employed by covalently incorporating TADF sensitizers with light-emitting TADF units and hosts in conjugated polymers to accelerate the spin-flip of triplet excitons. Superior photophysical properties have been achieved by a rational regulation of the proportions of each component, achieving a photoluminescence quantum yield of 90%, an extremely high rate of reverse intersystem crossing of 3 × 106 s−1, and a relatively low nonradiative decay rate of around 105 s−1. As a result, the solution-processed PLEDs can attain an external quantum efficiency (EQE) value of 25.4% with emission peaks of around 550 nm, representing record-high performance for PLEDs. The efficiency roll-off can also be significantly suppressed, maintaining an EQE value of 24.2% at 1000 cd/m2 with ideal efficiency roll-off of lower than 5%. Encouragingly, this work provides a valid strategy to tackle the imperative need for PLEDs with high EQE and low efficiency roll-off. Download figure Download PowerPoint Introduction Since the thermally activated delayed fluorescence (TADF) mechanism was adopted to explore high-efficiency organic lighting emitting diodes (OLEDs) by Adachi et al. in 2012, newly designed TADF luminogens have made extremely impressive progress in enhancing the external quantum efficiencies (EQEs) of OLEDs.1–12 Up to now, the vacuum-deposited OLEDs employing TADF small molecules have reached high EQEs exceeding 30%,13–21 ranging from the deep-blue to the red region, even reaching 40% for the state-of-the-art green OLEDs, according to the latest reports.22–24 In contrast, the device performance of TADF polymer-based light-emitting diodes (PLEDs) still falls far behind that of the TADF small molecular emitters.25–33 The primary advantage of PLEDs lies in the fact that they are well-suited as building blocks for large-area and low-cost flexible display equipment through the printing or coating process by the virtue of their good solution processability.34–37 In this context, it is necessary to develop more efficient and stable polymeric emitters, especially TADF-conjugated polymers, which possess ideal electrical characteristics due to their intrinsic delocalized channels for rapid carrier delivery.38 The inferior electroluminescence efficiency of PLEDs is mainly associated with their low exciton utilization and unbalanced charge-carrier mobility. One of the most critical methods for enhancing exciton utilization is to promote the reverse intersystem crossing (RISC) process, which can restrain aggregation-induced quenching of triplet excitons and thus enable high luminescence efficiency and device stability.8,39–42 It has, however, been demonstrated that the traditional approach of reducing the energy splitting (ΔEST) between excited singlet (S1) and triplet states (T1) is deemed to be unsatisfactory in some cases, especially for the TADF polymeric emitters featuring typical charge transfer (CT) natures in the excited state.28,37 Utilizing the sophisticated hyperfluorescent system proposed in pioneering studies,43,44 the overwhelming majority of triplet excitons undergo rapid RISC processes with spin-flip transition before being transferred to singlet states in TADF-type sensitizers, and thus the accumulated singlet excitons are delivered into emissive states in emitters through favorable Förster resonance energy transfer (FRET), rather than the inefficient intersystem transition processes of emitters.45–53 Inspired by this strategy, we expected that covalently incorporating the TADF sensitizers into polymeric backbones, namely intramolecular sensitization, would simultaneously strengthen the device performance and avoid the potential phase segregation between the emitting materials and sensitizers. In PLEDs, hole transport usually presents trap-free and space-charge-limited characteristics whereas the electron transport is always hindered by trapping.54–56 Therefore, the recombination of trapped electrons with free holes should result in the reduction of nonradiative transition and subsequent efficiency. It has also been confirmed that the diffusion of electrons and holes toward each other under mutual Coulombic interaction is the crucial procedure for both trap-assisted and emissive bimolecular recombination, in which the rate coefficients are Langevin type.57,58 Nevertheless, for the major TADF emitters, conjugated polymer in particular, the diffusion rate of holes tends to be relatively higher than that of electrons, leading to exciton quenching and nonradiative trap-assisted recombination. Thus, in addition to boosting the efficiency of exciton utilization, another prerequisite for gaining high-efficiency PLEDs is the design TADF polymeric emitters with balanced carrier-transporting properties. Herein, we integrated the intramolecular sensitization strategy with the improvement of electron-transporting behavior of regiorandom TADF-conjugated polymers, in which the electron-transporting group with electronic inertness was incorporated into light-emitting TADF units. Then the TADF sensitizer was covalently connected with the TADF emitters, and the carbazole derivative hosts into polymer backbone through Yamamoto condensation. By rational regulation of the contents of the three components, the photoelectrical properties were further optimized, and thus the anticipated high-efficiency device was obtained herein. For the intramolecular sensitizing of TADF-conjugated polymers including P3–P5, the presupposed electroluminescent (EL) mechanism can be depicted in three steps as shown in Figure 1a. First, the holes and electrons are injected into host units, and most of them will combine into excitons on TADF-sensitizing units under electrostatic interactions as a result of their relatively high concentration in polymers. The excitons include 75% triplet excitons and 25% singlet excitons on account of spin statistics rule. Second, the triplet excitons can effectively convert to singlets via RISC processes because of the RISC high rate constant of sensitizers ( k RISC Sensitizer ). Then the accumulated singlet excitons can transfer to the emitting states of TADF emitters through the efficient FRET along conjugated backbones. Third, the residual carriers can also combine into excitons on the emitting units, which can undertake the effective spin-flip processes of triplet excitons via RISC as well. Thus, most of the excitons can be effectively utilized by radiation decay.40,43 Especially for P5 containing 14% sensitizers and 8% emitter units, the photoluminescence quantum yield (PLQY) can reach 90%, accompanied by an extremely high RISC rate (kRISC) of 3 × 106 s−1 and a relatively low nonradiative decay rate of triplet excitons ( k nr T ) of around 105 s−1. As a result, the solution-processed PLEDs have achieved a maximum EQE value of 25.4%, representing the record high efficiency for PLEDs. Additionally, thanks to the synergetic effect of the intramolecular sensitizing strategy and the balanced carrier transporting properties, efficiency roll-off has also been significantly suppressed, leading to the maintenance of the EQE value of 24.2% at 1000 cd/m2—the highest efficiency among TADF PLEDs under high luminance. Figure 1 | (a) Schematic diagram of presupposed EL mechanism in intermolecular sensitizing conjugated polymers, where FRET is the Förster resonance energy transfer process, and k RISC Sensitizer and k RISC Emitter are the rate constants of RISC process in sensitizer and emitter units, respectively. (b) Synthesis process of the intramolecular sensitizing regiorandom conjugated polymers. Download figure Download PowerPoint Experimental Methods Synthesis of polymeric emitters Under the protection of nitrogen atmosphere, a mixture of Ni(COD)2 (500 mg), bipyridine (2 mmol, 312 mg), prorata components of 4-triphenyl phosphine oxide-4′-(2,7-dibromo-9, 9-dimethylacridine)-diphenylketone (DMAC-BP-TPO-Br) as emitter units, 2,7-dibromo-10-4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl-9,10-dihydro-9,9-dimethyl-acridine (TRZ-DMAC-Br) as sensitizer units, and 3,6-dibromo-9-hexyl-carbazole (Cz-C6-Br) as host units were added into a reaction flask. After pumping and filling in with N2 three times, 1,5-cyclooctadiene (0.5 mL) and anhydrous tetrahydrofuran (THF) (10 mL) were successively injected into the reaction system. After stirring for 48 h at 85 °C, 0.5 mL of bromobenzene was added into the mixture to finish capping the polymer, and the mixture was stirred for another 12 h. When the system temperature was cooled down to room temperature, a sufficient amount of dichloromethane was poured into the system, and then the solution was fully washed by diluted hydrochloric acid to wipe off the remnant catalyst. After extraction by using dichloromethane, the organic layer was dried over anhydrous MgSO4. Then the organic layer was futher processed through filtration and evaporation. After that the crude product was dissolved in THF and dropped into cooled methyl alcohol to separate repeatedly. After careful settling separation, the product was further purified by Soxhlet using ethyl alcohol as the extractor. The synthesis routes of every intermediates have been depicted in Supporting Information Schemes S1–S4, and the 1H NMR and 13C NMR have been provided in Supporting Information Figures S1–S6. Additionally, the 1H NMR spectra of all five polymers are shown in Supporting Information Figure S7. Device fabrication and measurement Before device fabrication, the indium tin oxide (ITO) glass substrates were processed with Hellmanex III lotion by using ultrasonic treatment. Then the substrates were rinsed carefully using deionized water, followed by being dried under nitrogen and further dried under 100 °C for 15 min. After hydrophilic treatment by plasma for 10 min, the poly(styrene sulfonate) (PSS):poly(3,4-ethylenedioxythiophene) (PEDOT) aqueous solution was spin-coated on the ITO substrate. Then the substrates were heat-treated at 120 °C for 15 min to remove the water and transferred into the glove box. Then the layer was adequately solidified at 120 °C. The 10 wt % polymeric emitters mixed with 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host materials were dissolved in anhydrous chlorobenzene (10 mg/mL) at 60 °C for 1 h, and then spin-coated onto the PSS:PEDOT film. After heat treatment for another 30 min, all the substrates were transferred into the deposition system. The devices were fabricated under the pressure of below 1.0 × 10−5 Torr. The hole-blocking layer bis2-(diphenylphosphino)phenyl ether oxide (DPEPO) and electron-transferring layer 1,3,5-tri(3-pyridyl)-phen-3-ylbenzene (TmPyPB) were thermally evaporated in succession at a rate of 1.0 and 2.0 Å s−1, respectively. After that, the hole-injecting layer LiF was carefully deposited on the organic surface at a rate of 0.1 Å s−1, and aluminum (Al) electrode was thermally evaporated at a rate of 1.0 Å s−1, first for 10 nm and then at a rate of 3.0 Å s−1 for another 100 nm. The electroluminescence spectra and luminance of the devices were obtained on a PR670 spectrometer. First, the size of the luminous area was 0.25 cm × 0.25 cm, and each device substrate had four pixels. The device substrates were placed on the self-designed fixture. The electroluminescence characteristics of the devices were measured using a Keithley 2400 source meter at room temperature under an atmospheric environment without any encapsulation. Results and Discussion The triphenylphosphine oxide (TPO) as electron-transporting group and 9,9-dimethyl-9,10-dihydroacridine-2,4,6-triphenyl-1,3,5-triazine (DMAC-TRZ) as TADF sensitizer were integrated into regiorandom TADF-conjugated polymers. The TPO group with electronic inertness was used to modify the TADF emitter, 9,9-dimethyl-9,10-dihydroacridine-benzophenone (DMAC-BP), and DMAC-TRZ was covalently connected with the TADF emitters and the carbazole derivative hosts into polymer backbones through Yamamoto polycondensation under the catalysis of nickel metal, as shown in Figure 1b. To optimize the photoelectrical properties, we rationally regulated the proportions of three components, and eventually the proportions of emitters and sensitizers were accurately measured by elemental analysis to be 9% and 0% for P1, 17% and 0% for P2, 10% and 11% for P3, 11% and 7% for P4, and 8% and 14% for P5, respectively ( Supporting Information Table S1), in which the P1 and P2 without any TADF sensitizers were taken as control. The molecular weight and polydispersity index (PDI) were determined by gel permeation chromatography analysis, exhibiting typical weight-average molecular weights (Mws) of 16–25 kDa and PDI of 2.2–2.7, respectively ( Supporting Information Table S1). The thermal properties were evaluated by differential scanning calorimetry and thermogravimetric analysis. The glass transition temperatures (Tgs) of these polymers were in the range of 70–130 °C ( Supporting Information Figure S8), and the decomposition temperatures (Tds) were in the range of 350–450 °C ( Supporting Information Figure S9), indicating good thermal stability during device fabrication. Thanks to the alkyl chains of the hosts, all these polymers also exhibited excellent solubility in conventional organic solvents, such as dichloromethane, chloroform, toluene, xylene, and THF, ensuring formation of high-quality film for the emitting layer through solution processing. To reveal the effect of molecular structures on optoelectronic properties, density functional theory (DFT) simulations were performed. The foremost principle of choosing the simplified segments is to effectively separate emitting and sensitizing units by host units owing to quite high ratios of host (ca. 80%) existing in all copolymers. Therefore, we adopted two central carbazole units to separate emitting units from sensitizing units and one carbazole unit as end group. Additionally, we considered the chosen segment to be reasonable because the extended highest occupied molecular orbital (HOMO) distribution was no more than two carbazole units. As shown in Figure 2a of the optimized representative molecular fragment, the nearly orthogonal dihedral angles between electron donor and acceptor moieties for both TADF emitting and sensitizing units were accurately inherited from small molecules, potentially ensuring the predictable TADF characteristic in newly designed intramolecular sensitizing polymers. The computational electric dipole moment with direction nearly perpendicular to that of conjugated chains was 3.92 Debye. It is noteworthy that the dihedral angles between the different parts of conjugated polymeric skeletons were always lower than 40°. Correspondingly, it was also detected that relatively electron-rich regions were continuously distributed on conjugated backbones from total self-consistent field density performed by using the electrostatic potential method as shown in Figure 2b, enabling rapid migration and transformation of carriers and excitons. From the frontier molecular orbits (FMOs) shown in Figure 2c, both lowest unoccupied molecular orbital (LUMO) and LUMO+1 were almost completely distributed on electron acceptors of TADF emitter and sensitizer, respectively, whereas the HOMO and HOMO-1 were mainly distributed on electron donors of TADF emitter and sensitizer, respectively, partly delocalized on conjugated backbones. The relatively isolated FMO distributions reserved small ΔEST and thus effective triplet-to-singlet spin-flip processes, while the delocalized FMO distributions also suggested potential enhancement of the mobility of carriers for conjugated polymers. Additionally, it should be noted that there were few FMOs located in TPO groups, manifesting its electronic inertness in the photodynamic process.59,60 Figure 2 | (a) The optimized structure of representative molecular fragment for intramolecular sensitizing conjugated polymers. (b) The isosurface of molecular fragment from total SCF density mapped with ESP. (c) The FMOs of molecular fragment. Download figure Download PowerPoint The electrochemical and photophysical properties of these polymeric emitters were also evaluated in detail. From cyclic voltammetry curves shown in Supporting Information Figure S10, we deduced the HOMOs of these polymers to be 5.3–5.4 eV. From the fluorescence spectra (FL) at 300 K, the emission peak at around 510 nm for P1 and P2 was from TADF-emitting units, and these peaks exhibited a slight bathochromic-shift to 520–525 nm in intramolecualr sensitizing polymers, partly due to intramolecular interaction between sensitizing and emitting units in polymers. Additionally, the emission peaks of carbazole units located at 425 and 460 nm vanished when incorporating sensitizing units into polymers thus reflecting that the intramolecular sensitizing strategy can facilitate exchange of excitons between hosts and emitting units. From the phosphorescence spectra (Ph) at 77 K of polymers P1–P5 diluted in toluene (Figure 3a), the triplet excited states were calculated to be around 2.7 eV, which was mainly derived from carbazole-based main chains. The Ph of polymers exhibited fickle emission profiles because of intricate energy transfer processes. Especially for P5, the emission peaks around 455, 500, and 530 nm were mainly from excited triplet states of host, sensitizing units, and emitting units, respectively. Notably, the phosphorescence emission peak of TADF-emitting units for P2 completely disappeared, possibly caused by the severe triplet exciton quenching of the overmuch TADF units. Additionally, the inconspicuous emission peak of TADF units around 530 nm for P3 and P4 could also be ascribed to the undesirable quenching of triplet excitons, which might result in more inferior photophysical properties than those of P5. In Figure 3b, all these polymers diluted in toluene share quite similar UV–vis absorption spectra, containing π–π* absorption bands at 320 nm and characteristic CT bands of typical TADF molecules at 375 nm. Additionally, the signal intensity of CT bands gradually increased with the increasing percentage of the TADF emitters and sensitizers in polymers. From the PL spectra of 10 wt % emitters blended into CBP matrix, as shown in Figure 3b, the primary emission peaks of P1–P3 were located at 528 nm, originated from TADF emitter units with apparently residual emission peaks from host units. In contrast, the emissions of carefully modulated P4 and P5 exhibited a slightly bathochromic shift to 537 nm, and the extra emissions from host units could be effectively restrained, illustrating relatively superior energy transfer process in these two intramolecular sensitized polymers. According to the absorption and emission spectra of individual units for intramolecular sensitizing polymers ( Supporting Information Figure S11), there existed some spectral overlaps in the range of 400–450 nm, which benefited the acceleration of the FRET process from sensitizing units to emitting unis. To further probe the transient-state photophysical properties of these emitters in the solid state, the CBP blended films were detected by decay in the As shown in Figure all these decay spectra including fluorescence and delayed fluorescence components, respectively. the of and components were with regulation of the of these polymers, that the rate constants of decay or excitons were relatively to the of intramolecular sensitizing polymers. For P1 and P2 without any sensitizing units, the and of the were and and and respectively. After them with intramolecular sensitizing units, the the relatively spin-flip and high and for P3, and for P4, and and for P5, respectively In with the transient-state photophysical the of blended films for significantly those for P1 and P2 ( Supporting Information Figure Especially for P5, the value reached nearly 90%, that the triplet excitons can be effectively through an intramolecular sensitizing Additionally, it should also be noted that was detected from the PL spectra in and as shown in Figure which can be partly to the of the CBP with low or rapid of triplet excitons during FRET processes. Figure 3 | (a) The at 300 K and Ph at 77 K for polymers P1–P5 diluted in (b) The UV–vis absorption spectra of polymers diluted in toluene, and the PL spectra of 10 wt % emitters blended into CBP (c) The decay spectra of blended film in The PL spectra of P5 blended into CBP in and The PL decay spectra of the blended P5 films in The of the temperature of for all the blended Download figure Download PowerPoint Table 1 | The of Emitter ΔEST k T k T P1 1.0 P2 60 0.5 1.0 P3 P4 85 P5 3.0 splitting between and determined from of the temperature of in of of of rate constants of process calculated by of k ( PL ). rate constants of RISC process calculated by of k RISC PL ( 1 ) decay of triplet excitons calculated by of k T nonradiative decay of triplet excitons calculated by of k nr T ( 1 ) The rate constants of RISC ( k RISC ) and ( k ) processes have been calculated as in Table the effect of polymeric on excitons from the intramolecular sensitizing intersystem especially the RISC be significantly by the inefficient intersystem process on the emitter units. It is noteworthy that the intramolecular sensitized polymeric emitters exhibited higher k RISC especially for P5 with k RISC of 3.0 × 106 s−1, over 10 higher than P1 and P2 with k RISC of lower than 3.0 × 105 s−1. the decay rate constant ( k T ) of triplet excitons was × 105 s−1 for P5, significantly the nonradiative decay rate constants ( k T ) of × s−1, further that the modulated polymers can triplet excitons to the in intramolecular sensitizing with the relatively low PL P1 and P2 similar k T and k T of nearly 1.0 × 105 s−1 due to the unsatisfactory exciton utilization without sensitizers. In Figure and Supporting Information Figure the and proportions of delayed components for these polymers increased with the and the predictable TADF characteristic can be To probe into the exciton of the spin-flip process from excited triplet states to singlet states in we further the temperature of k RISC for the blended films by using the as shown in Figure and thus the ( a RISC ) were In this the a RISC value is to energy splitting between excited singlet and triplet states these emitters reveal a temperature due to the of the RISC Notably, similar different from P1 and P2, indicating that there channels to exciton for two of of the processes of spin-flip for intramolecular sensitizing emitters, the also higher k RISC to P1 and P2 over the temperature By the ΔEST of polymeric emitters can be for P1, for P2, for P3, for P4, and for P5, respectively, all of which are apparently small to the efficient RISC To the intrinsic between electronic structures and photophysical properties, we further the of excited states through using the
Liu et al. (Mon,) studied this question.