Highly Efficient All-Solution-Processed Fluorescent Organic Light

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Highly Efficient All-Solution-Processed Fluorescent Organic Light-Emitting Diodes Based on a Novel SelfHost Thermally Activated Delayed Fluorescence Emitter Xinxin Ban, Aiyun Zhu, Tianlin Zhang, Zhiwei Tong, Wei Jiang, and Yueming Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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ACS Applied Materials & Interfaces

Highly Efficient All-Solution-Processed Fluorescent Organic Light-Emitting Diodes Based on a Novel Self-Host Thermally Activated Delayed Fluorescence Emitter Xinxin Ban,*† Aiyun Zhu,† Tianlin Zhang,† Zhiwei Tong,† Wei Jiang*‡ and Yueming Sun‡ †

School of Chemical Engineering, Huaihai Institute of Technology, Lianyungang, Jiangsu, 222005, China

‡

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu, 211189, China

Abstract: Here, we conveniently designed and synthesized a self-host thermally activated delayed fluorescence (TADF) emitter, which can not only form a uniform thin film through wet-process, but also allow the subsequently deposition of electron transporting layer (ETL) by orthogonal solvent. By using this self-host material as emitter, the first all-solution-processed multilayer TADF organic light emitting diodes (OLEDs) was successfully fabricated. The maximum current, power and external quantum efficiencies of this nondoped device are 46.3 cd A-1 , 39.3 lm W-1 and 15.5%, respectively, which are much higher than the values of all-solution-processed OLEDs based on tranditional fluorescence and even comparable to the TADF devices with vacuum-deposited ETL. Moreover, the device maintains the high efficiency of 42.9 cd A-1 and 39.0 cd A-1 at the luminance of 100 cd m-2 for display and 1000 cd m-2 for practical lighting. The high efficiency and small efficiency roll-off of the all-solution-processed fluorescent OLEDs can be attributed to the superiority of the newly designed self-host TADF emitter, which possesses the perfect electroluminescent property and sufficient solvent resistance at the same time. Keywords: self-host, orthogonal solvent, solution-process, TADF, organic light emitting diodes 1. Introduction Organic light-emitting diodes (OLEDs) have become a commercial technology for display due to their unique advantage in flexibility, response time and picture quality.1 At present, the biggest challenge in the development of OLEDs is reduction of the manufacturing cost. Comparing with vacuum-deposition, solution-process techniques are more suitable for low cost and large area fabrication.2-3 However, the efficiencies of solution-processed OLEDs are generally low, especially for fluorescence devices.4-6 Recently, many efforts have been devoted to develop thermally activated delayed fluorescent (TADF) materials,7-10 which can achieve 100% internal quantum efficiency (IQE) through reverse intersystem crossing (RISC) from triplet state (T1) to singlet state (S1). Some important studies of TADF OLEDs fabricated by solution processing technique have been reported and the device efficiencies of them can even comparable to their vacuum-deposited counterparts.11-12 Unfortunately, although the emission layers (EML) of these devices are fabricated by spin-coating, the electron transporting layers (ETL) are still deposited by vacuum evaporation, which inevitably increases the manufacturing cost and complexity. To construct the fully solution-processed OLEDs from wet technology, it is important to prevent interface mixing during deposition of the subsequent layers.13-14 Normally, orthogonal solvent method is a well-established approach to overcome the redissolving problem. A series of fluorescent polymers and dendrimers have been successfully used as EML to make the device suitable to orthogonal processing, but the device efficiencies are generally below 5% because of the limited IQE.15-19 Up to now, no study of TADF material as emitter for all-solution-processed multilayer device has been reported due to many inevitable challenges. (i) Most of the TADF materials used for vacuum-deposited devices are not suitable in solution process due to the

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insufficient solubility and high crystallization. (ii) The conventional TADF emitters must be doped into suitable host materials to restrict the concentration quenching effect. (iii) Worst of all, the small molecular host and guest materials cannot resist to the solvent used for processing upper layers, which is the main reason for the reported solution-processed TADF OLEDs using vacuum-evaporation rather than solution-process to fabricate the ETL. Normally, the resistance to alcohols used for depositing upper layer remarkably increases with the molecular weight of the compounds. In view of the limited reproducibility and moderate device efficiency of polymer based OLEDs, Kido et al. have demonstrated that covalent dimerization can also afford the conventional small molecules sufficient resistance to alcohols used for processing upper layers.20 Nevertheless, there are few applicable large molecular TADF emitters can be used for multilayer solution-processing. Meanwhile, further extending the molecular weight with more complicated linkages and subunits will not only decrease the TADF feature, but also increase the cost of production. Recently, self-host TADF emitter comprised of host and guest into one molecule was demonstrated to be an efficient emitter for non-doped solution-processed OLEDs.21-23 Indeed, the self-host material will take over the doping technique to eliminate the phase separation phenomenon of the solution-processed emission layer. Furthermore, the high molecular weight and mechanical robustness of the self-host dendrimer can form an amorphous hydrophobic layer, which would make the EML to get over the threshold of compatibility with the subsequent solution process from orthogonal solvents. Although it is economically highly desired to produce multilayer TADF OLEDs by solution-processed fabrication technique, developing novel self-host TADF emitter with many characteristics such as easy of synthesis, high solubility, good film forming ability and excellent electroluminescent performance are essential to realize this goal. In this work, we conveniently constructed a self-host TADF emitter Cz-CzCN through aromatic nucleophilic substitution reactions by using commercial raw materials. The entire procedure was performed at mildly temperature without any palladium or rare-earth-metal catalysts, which completely accords with the low cost requirement of solution-process. The thermal, photophysical and electrochemical properties of Cz-CzCN were systematically investigated. The alkyl chain linked carbazoles sufficiently encapsulate the TADF unit and suppress the intermolecular interaction induced exciton quenching, which qualifies Cz-CzCN for a non-doped emitter of OLEDs. By using this self-host emitter, we successfully fabricated the all-solution-processed multilayer TADF OLEDs in which three functional layers including EML and ETL are fully spin-coated. The key point of the device is that the self-host emitter can not only form uniform thin film through spin-coating, but also afford the EML resistance to the orthogonal solvent uesd for subsequent depositing upper layer. As a result, the all-solution-processed TADF OLEDs achieves a turn-on voltage of 3.1 V, a maximum current efficiency of 46.3 cd A-1 and a maximum external quantum efficiency of 15.5%, which is much higher than the value of all-solution-processed OLEDs based on tranditional fluorescence. Moreover, the brightness of this all-solution-processed TADF OLEDs can even exceed 50000 cd m-2 at a high current denstiy of 400 mA cm-2, which fully proves the superiority of the self-host structure in suppressing the exciton quenching. To the best of our knowledge, this is the first time to open the way toward all-solution-processed multilayer TADF OLEDs with the efficiency comparable to the vacuum-deposited conterparts.


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2. Experimental Section The synthetic procedure for Cz-CzCN was illustrated in Scheme 1. All reagents were used as purchased without further purification. The intermediate Br-Cz was prepared according to the literature procedure.24-26 4-(6-(9H-carbazol-9-yl)hexyloxy)-9H-carbazole (Cz-Cz): A mixture of 9H-carbazol-4-ol (1.8 g, 10.0 mmol), 9-(6-bromohexyl)-9H-carbazole (3.3 g, 10.0 mmol) and K2CO3 (1.5 g, 12.0 mmol) was added to 50 mL DMF. The reaction was heated at 60 oC under nitrogen for 4 h. After cooling, the mixture was poured into 200 mL water. The crude product were filtered and purified by silica gel column chromatography. The target product was recrystallized by ethyl acetate to give the white product (3.6 g, 85%). 1H NMR (300 MHz, CDCl3, δ): 8.29 (d, J = 7.7 Hz, 1H), 8.10 (d, J = 7.8 Hz, 2H), 7.99 (s, 1H), 7.50-7.35 (m, 6H), 7.30 (t, J = 8.1 Hz, 1H), 7.23-7.16 (m, 2H), 7.01 (d, J = 8.0 Hz, 1H), 6.63 (d, J = 8.0 Hz, 1H), 4.32 (t, J = 7.1 Hz, 2H), 4.19 (t, J = 6.3 Hz, 2H), 2.04-1.88 (m, 4H), 1.74-1.60 (m, 2H), 1.56-1.48 (m, 2H). 13C NMR (75 MHz, CDCl3, δ): 140.97, 140.47, 138.73, 126.68, 125.63, 124.89, 122.99, 122.81, 120.35, 119.63, 118.76, 109.93, 108.67, 103.33, 101.14, 67.72, 42.78, 29.34, 28.99, 27.10, 26.15. MS (MALDI-TOF) [m/z]: calcd for C30H28N2O, 432.2; found, 432.1. Anal. Calcd. for C30H28N2O: C, 83.30; H, 6.52; N, 6.48. Found: C, 83.25; H, 6.45; N, 6.55. (2R,3R,4S,5S,6S)-2,3,4,5,6-pentakis(4-(6-(9H-carbazol-9-yl)hexyloxy)-9H-carbazol-9-yl)be nzonitrile (Cz-CzCN): Under nitrogen atmosphere, Cz-Cz (2.2 g, 5 mmol) in anhydrous DMF (40 ml) was added dropwise into a anhydrous DMF (20 ml) solution containing NaH (0.672g, 6 mmol) for 15 min and stirred for 3 h. Then, pentafluorobenzonitrile (0.2 g, 1 mmol) in anhydrous DMF (20 ml) was added dropwise for 15 min. Then the solution was stirred for 24h at 45 oC. After that, 250 mL water was added into the solution and the mixture was extracted with CH2Cl2 for three times. The combine organic layer was dried with anhydrous MgSO4 and the solvent was removed under vacuum. The precipitate was purified by column chromatography on silica gel, resulted in the bright green product (1.8 g, 80%). 1H NMR (500 MHz, CDCl3, δ): 8.11-8.00 (m, 10H), 7.97-7.90 (m, 2H), 7.56-7.46 (m, 4H), 7.45-7.31 (m, 15H), 7.27 (d, J = 8.4 Hz, 4H), 7.22-7.13 (m, 10H), 7.11 (d, J = 7.8 Hz, 4H), 7.02 (d, J = 7.9 Hz, 2H), 6.82 (d, J = 7.8 Hz, 4H), 6.75-6.63 (m, 6H), 6.62-6.50 (m, 6H), 6.47 (d, J = 8.0 Hz, 2H), 6.43 (d, J = 7.9 Hz, 2H), 6.14-6.08 (m, 2H), 6.05 (d, J = 7.8 Hz, 2H), 4.26 ((t, J = 6.9 Hz, 10H), 4.16 (t, J = 6.3 Hz, 10H), 1.86 (dt, J = 14.7, 7.4 Hz, 10H), 1.80-1.66 (m, 10H), 1.58-1.47 (m, 10H), 1.31-1.22 (m, 10H). 13C NMR (75 MHz, CDCl3, δ): 155.44, 155.36, 140.38, 140.23, 140.19, 140.14, 140.12, 138.07, 138.00, 137.95, 137.88, 126.88, 126.82, 126.77, 126.68, 125.57, 124.92, 124.81, 124.75, 124.66, 123.96, 123.89, 123.26, 123.17, 122.80, 121.79, 121.72, 120.28, 118.71, 113.77, 111.65, 108.94, 108.59, 103.85, 103.74, 102.27, 67.78, 67.74, 42.86, 29.07, 28.88, 26.94, 25.95. MS (MALDI-TOF) [m/z]: calcd for C157H135N11O5, 2254.1; found, 2254.0. Anal. Calcd. for C157H135N11O5: C, 83.59; H, 6.03; N, 6.83. Found: C, 83.54; H, 6.12; N, 6.81. 3. Results and Discussions Among the various electron-acceptors, cyano-group is widely applied in constructing donor-acceptor typed TADF molecule due to the strong electron-withdrawing property. Considering carbazolyl is a mild electron-donor unit with high triplet energy and excellent charge-transfer property, we chose this unit as donor part of the emissive core and peripheral wrapping group in the same time. The reported high efficiency TADF emitter, 2,3,4,5,6-penta (9H-carbazol-9-yl)benzonitrile (5CzCN), was used as the emissive core.27-28 To obtain the



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self-host TADF feature, it is necessary to introduce the peripheral host unit into the TADF core as well as remain the small single-triplet energy gap (∆EST) of the molecules through elaborate molecular design. Alkyl chains are non-conjugated linkers, which can not only enhance the solubility of the compound, but also render the electronic properties of the emissive core independent of the peripheral wrapping units. Thus, hexane was selected as the connection bridge to encapsulate the emissive core. Scheme 1 shows the synthetic route and chemical structure of Cz-CzCN. Carbazole was coupled with 1,6-dibromohexane by using sodium hydroxide in N,N-dimethylformamide (DMF) to generate Br-Cz, which was reacted with 4-hydroxycarbazole in DMF to provide the key intermediate Cz-Cz. The self-host emitter Cz-CzCN was finally synthesized through aromatic nucleophilic substitution reaction using pentafluoroben-zonitrile and Cz-Cz under the condition of alkali with 80% yield. The chemical structure of Cz-CzCN was characterized by 1H-NMR, 13C-NMR, mass spectrometry and elemental analysis.

Scheme 1. Synthetic route of Cz-CzCN. Before experiment test, we simulated the frontier orbital distribution of Cz-CzCN by density functional theory (DFT) calculation due to the internal connection between the molecular orbital and the physical properties. Figure 1 represents the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of the optimized molecular structure. The HOMO of Cz-CzCN was mainly distributed over the carbazolyl units of the emissive core, while the LUMO was delocalized on the benzonitrile moiety. The HOMO and LUMO of the molecule were entirely located on the emissive core, which indicates the alkyl chains make the electronic property of emissive core independent of the wrapping units. However, the HOMO distribution of Cz-CzCN shows a little different from that of 5CzCN, which can be attributed to the introduction of electron-donating methoxyl. The calculated HOMO and LUMO energy levels of Cz-CzCN are -5.26 eV and -2.03 eV, respectively. Due to the introducing of oxidation atom at 4-position of carbazole, the HOMO level of Cz-CzCN is shallower than its parent core 5CzCN (-5.54 eV), which would facilitate the hole injection for the self-host emitter.


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The extensively isolated HOMO/LUMO is an appropriate molecular orbital distribution of the TADF emitter, which ensures the small ∆EST of the emitter. According to the results estimated from time dependent-density functional theory (TD-DFT) calculation, the ∆EST of Cz-CzCN was only 0.09 eV, which is smaller than that of TADF core 5CzCN (0.19 eV) due to the enhanced electro-donating ability by oxidation atoms. The small ∆EST indicates the efficient reversed intersystem crossing and consequently efficient TADF emission. The alkyl chain connected carbazole moieties twisted around the emissive core like impellers around the axis, which endow the molecule a non-planar geometry to suppress the intermolecular interaction between the central TADF units. Thus, reducing the ubiquitous concentration quenching effect of the TADF molecule can be expected.

Figure 1. Optimized geometries and calculated HOMO and LUMO density maps for Cz-CzCN. The thermal properties of Cz-CzCN were investigated by the thermogravimetric analyzer (TGA) and different scanning calorimeter (DSC) under the nitrogen atmosphere. As shown in Figure 2, Cz-CzCN exhibits a glass transition temperature of 128 °C, and a decomposition temperature of 402 °C. Since the thermal stability of the material will facilitate the device stability, it can be assumed that Cz-CzCN would ensure the stable operation of the solution-processed device. The morphological property was characterized by atomic force microscopy (AFM). The spin-coated thin film of Cz-CzCN was quite smooth with a root-mean-square (RMS) value of 0.56 nm (Figure S1). No pinholes or cracks could be observed, which indicates that Cz-CzCN can form uniform amorphous film through solution process.

Figure 2. TGA curve of Cz-CzCN recorded at a heating rate of 10 °C min-1; Inset: DSC trace recorded at a heating rate of 10 °C min-1.



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The photophysical properties of Cz-CzCN were analyzed by the ultraviolet-visible (UV-vis) and photoluminescence (PL) spectra. As shown in Figure 3a, Cz-CzCN exhibits a broad absorption band at the low energy region around 400 nm, which should be attributed to the intramolecular charge transition (ICT). The energy bandgap (Eg) of the compound is calculated from the edge of the absorption, giving the value of 2.75 eV. The gradual red shit emission spectra in solvents with various polarities can further prove this donor-acceptor structure induced ICT character of Cz-CzCN (Figure S2). To determine the energy levels, we further measured the fluorescent and phosphorescent spectra of Cz-CzCN at room temperature and 77 K, respectively. As shown in Figure 3(c), Cz-CzCN exhibits a CT state broad fluorescence without any vibrational structure and the local state phosphorescence with characteristic vibrational structures. Therefore, the lowest energy of the S1 and T1 states of Cz-CzCN can be calculated from the onset of the fluorescent spectrum and the highest energy peak of the phosphorescence spectrum, respectively.28 Based on above principle, the S1 and T1 levels of Cz-CzCN are estimated to be 2.83 eV and 2.66 eV, respectively. Thus, the experimental ∆EST of Cz-CzCN is 0.17 eV, which is similar to the reported TADF core 5CzCN. The PL quantum yield (PLQY) of Cz-CzCN in toluene are measured to be 0.20 and 0.88 before and after bubbling nitrogen gas. The oxygen sensitive phenomenon indicates the TADF property of Cz-CzCN. The transient decay of Cz-CzCN in toluene were also measured to determine the oxygen sensitive property. As shown in Figure S3, after bubbling the nitrogen, Cz-CzCN shows double-exponential decay with the prompt and delayed lifetime of 13.5 ns and 2.8 us, respectively. Comparing to the sharp prompt component, the delayed emission can be attributed to the reverse intersystem crossing (RISC) from the non-radiative triplets to the radiative singlets induced by the sufficiently small ∆EST. For further exploring the emissive characteristics, the PLQY and transient decay of Cz-CzCN and 5CzCN in neat films were also measured (Figure 3b). Compared with the TADF core 5CzCN, the improved proportion of the delayed fluorescence of Cz-CzCN means that the peripheral wrapping units can effectively suppress the intermolecular interaction and excion quenching to facilitate the RISC process. The time-resolved photoluminescence spectra of Cz-CzCN in film state were measured. As shown in Figure 3(d), the similar photoluminescence spectra were obtained before and after applying delay time, which confirms that the delayed emission comes from the singlet states by reversed process. The PLQY of Cz-CzCN in film state was measured to be 0.52, while the PLQY of the TADF core 5CzCN in pure film state was only 0.21. The improved PL quantum yield of Cz-CzCN demonstrates that the alkyl chain linked carbazole units efficiently encapsulate the TADF unit and suppress the intermolecular interaction induced exciton quenching. Moreover, the temperature dependence of the delayed fluorescence was also explored. As shown in Figure S4, the delayed emission intensity increases with increasing temperature from 100 K to 300 K, which can be attributed to the enhanced up-conversion from T1 to S1 states based on thermal acceleration. These results fully confirm that the introduction of peripheral carbazole units by alkyl chain will keep the TADF property of the molecule unchanged. Since the main objective of designing Cz-CzCN was to develop all-solution-processed florescent OLEDs, the highly solvent resistance of Cz-CzCN is a prerequisite for multilayer device fabricated by sequential solution processes. Figure 4 shows the absorption spectral variations of 5CzCN and Cz-CzCN before and after spin-rinsing with isopropyl alcohol, which was used for processing the upper ETL. The absorption spectrum of pristine 5CzCN film almost disappeared after rinsing, while the remaining absorption intensity of Cz-CzCN film was above 95%, which demonstrates that the molecular encapsulation


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by functional unit is an effect way to afford the small TADF emitter to exceed the threshold molecular weight and compatible with the subsequent solution process through orthogonal solvent.

Figure 3. (a) UV and PL spectra of Cz-CzCN. (b) Fluorescence decays of the films at 300 K. (c) Fluorescence and phosphorescence of Cz-CzCN at room temperature and 77 K. (d) PL spectra of Cz-CzCN before and after applying delay time.

Figure 4. The absorption spectra of 5CzCN (a) and Cz-CzCN (b) films before and after rinsing with isopropyl alcohol. For developing solution-processible TADF molecule as efficient non-doped emitter, the frontier orbital energy level should be suitable to that of nearby layer to facilitate the charge injection and transporting. In order to get the experimental values of HOMO and LUMO energy levels, the electrochemical cyclic voltammetry analyses of the Cz-CzCN were performed (Figure S5). During the anodic sweeping, the Cz-CzCN exhibits a quasi-reversible oxidation curve. According to the onset potential, the HOMO energy level was calculated to be -5.32 eV, which was higher than that of 5CzCN (-5.55 eV). Similar trend was also observed in DFT calculation. The LUMO energy level was estimated from the HOMO and the optical energy band-gap, giving the value of -2.61 eV. The high-lying HOMO and appropriate LUMO of Cz-CzCN will ensure the effective injection



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and recombination of hole and electron in the EML, which is crucial to improve the device performance of non-doped OLEDs. In order to evaluate Cz-CzCN as an efficient EML of the solution-processed multilayer OLEDs, four kinds of devices were fabricated along with the following structures A, B, C and D (Figure 5). The schematic energy-level diagram of the device and the molecular structures of the organic materials are shown in Figure 6. In these sandwich geometries, PEDOT:PSS and Cs2CO3 were hole- and electron-injection layers, while tris(4-(diphenylphosphoryl)phenyl) benzene (PhPO) was an efficient phosphine oxide based alcohol-processable electron transporting material, which has been successfully used as ETL in the orthogonal sequential solution-processed multilayer phosphorescent OLEDs.29 The control device with a vacuum-deposited 1,3,5-tris(1-phenyl-1Hbenzimidazol-2-yl)benzene (TPBI) was fabricated (Device A). Moreover, the Device C with a vacuum-deposited PhPO and Device D without electron-transporting layer were also made as comparison. Figure 7 shows the current density-voltage-luminance (J-V-L) and current efficiency-luminance curves for the devices. The related device data are summarized in Table 1. For the electroluminescent performance, Device A shows a turn-on voltage of 3.5 V, a high current efficiency (CE) of 39.0 cd A-1, a maximum power efficiency (PE) of 32.2 lm W-1 and a maximum external quantum efficiency (EQE) of 13.4%, which are among the highest values of solution-processed TADF OLEDs based on vacuum- deposited ETLs. The high performance of Device A indicates that Cz-CzCN can be used as an efficient non-doped EML for solution-processed OLEDs.

Figure 5. The device configurations and fabrication processes of Device A, B, C and D. As the key objective of designing Cz-CzCN was to develop all-solution-processed fluorescent OLEDs, Device B with the spin-coated ETL was tested. The turn-on voltage of Device B (3.1 V) was smaller than Device A, which indicates that the PhPO layer facilitates the electron injection and transporting comparing to TPBI. The maximum CE, PE and EQE of Device B were 46.3 cd A-1, 39.3 lm W-1 and 15.5%, respectively, which were much higher than the devices with the tranditional fluorescent materials as emitters (Table 2). Obviously, the enhanced performance of Device B should be attributed to the efficient triplet havest by TADF emitter. Moreover, the device exhibited an extremely high luminenance of 54000 cd m-2 at the current density of 400 mA m-2, which verified the feasibility of self-host structure in suppressing the exciton quenching effect. To the best of our knowledge, this is the first time that using a TADF material as nondoped emitter to construct the all-solution-processed multilayer OLEDs. To further demonstrate the superiority of this self-host TADF emitter, Device C with a vacuum-deposited PhPO layer was tested. The maximum CE, PE and EQE of Device C were 49.7 cd A-1, 45.9 lm W-1 and 17.1%, respectively, which are among the highest efficiencies of the reported solution-processed devices with a vacuum-deposited ETL (Table 3). The similar turn-on voltages and comparable device efficiencies


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of Device B and Device C demonstrate that Cz-CzCN can afford the EML resistance to isopropyl alcohol used for processing upper ETL. The maximum luminescence of Device B and Device C are both above 50000 cd m-2, no matter how the ETL layer is prepared. Generally, the high luminescence is attributed to the high concentration of excitons, which indicates the self-host feature can effectively suppress the exciton quench of the TADF core. Device D without an electron-transporting layer exhibits a very poor device efficiency, which means the ETL is essentially required to offer efficient charge transporting and exciton recombination. As shown in Figure S6, these devices show very high color stability and the electroluminescence (EL) spectra of the devices are independent of the applied voltages from 5 V to 15 V, which manifested the complete exciton confinement on the emissive core of the self-host emitter. It is worth mentioning, the all-solution-processed Device B maintains the high efficiency of 42.9 cd A-1 and 39.0 cd A-1 at the luminance of 100 cd m-2 for display and 1000 cd m-2 for practical lighting (Figure S7). Even the luminance is up to 10000 cd m-2, the CE of Device B is still maintains as high as 33.0 cd A-1. The high efficiency and low efficiency roll-off of Device B can be attributed to the superiority of Cz-CzCN, which possesses a high PL quantum efficiency for light emitting, a small ∆EST for triplet harvesting and an efficient molecular encapsulation for the suppression of triplet-involved exciton quenching. To the best of our knowledge, this is the first report on all-solution-processed fluorescent OLEDs based on TADF emitter, which has a competitive performance comparing with the corresponding device with a vacuum-deposited ETL.39-40 Moreover, it can be expected that the optimization of peripheral wrapping units with bipolar feature and choosing a more effective TADF emissive core would further enhance the device efficiency and enrich the color emission of all-solution-processed fluorescent OLEDs.

Figure 6. The energy levels and molecular structures of the organic compounds used in the devices.

Figure 7. (a) Current density-voltage-luminance (J-V-L) characteristics; (b) current efficiencies



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versus current density plots. Table 1. Device Performances of the solution-processed OLEDs. Device

Vona [V]

CEmaxb [cd A-1]

PEmaxc [lm W-1]

EQEmaxd [%]

CEe [cd A-1]

L maxf [cd m-2]

CIE [x, y]g

A

3.5

39.0

32.2

13.4

29.4/23.3/16.7

31000

(0.26, 0.52)

B

3.1

46.3

39.3

15.5

42.9/39.0/33.0

54000

(0.25, 0.52)

C

3.1

49.7

45.9

17.1

49.2/46.8/38.7

52000

(0.26, 0.52)

D

3.2

6.5

4.2

2.1

6.3/4.6/--

6300

(0.29, 0.52)

a)

‒2 b)

Von = turn-on voltage at 1 cd m ,

efficiency,

d)

CEmax = maximum current efficiency,

EQEmax = maximum external quantum efficiency,

100 cd m-2, 1000 cd m-2 and 10000 cd m-2,

f)

e)

c)

PEmax = maximum power

Current efficiency at the luminance of

Lmax = maximum luminance,

g)

CIE = the Commission

Internationale de L’Eclairage coordinates.

Table 2. Comparison of highly efficient all-solution-processed fluorescent OLEDs EQEmax

PEmax

CEmax

[%]

[lm W-1]

[cd A-1]

Cz-CzCN a

15.5

39.3

46.3

52000

510

(0.25,0.52)

This work

GEP b SY b P-PPV b

3.2 6.6 7.8

9.1 16.8 -/-

11.6 21.2 23.8

20000 -/7923

530 550 540

(0.36,0.59) -/-/-

Ref. 30 Ref. 5 Ref. 31

EML

a)

Lmax [cd m-2]

ELpeak [nm]

CIE [x,y]

TADF, b) Traditional fluorescence.

Table 3. Comparison of solution-processed TADF OLEDs with vacuum-deposited ETL EQEmax

EML a

Cz-CzCN CDE1 a TZ-3Cz a PAPTC a LEP a P12 a CzDMAC-DPS a CBP:4CzIPN b SiCz:t4CzIPN b SiCz:TB-3PXZ b CBP:3ACR-TRZ b CBP:ACRDSO2 b CBP:PXZDSO2 b mCP:4CzCNPy b TCTA:pAcBP b TCTA:pCzBP b mCP:Copo1 b a)

PEmax

CEmax

-1

-1

[%]

[lm W ]

[cd A ]

Lmax [cd m-2]

17.1 13.8 10.1 12.6 10.0 4.3 12.2 18.5 18.7 13.9 18.6 17.5 15.2 11.3 9.3 8.1 20.1

45.9 -/-/37.1 -/11.2 24.0 -/42.7 32.6 -/-/-/14.8 20.3 9.0 40.1

49.7 -/30.5 41.8 -/10.7 30.6 -/-/41.5 -/53.3 45.1 38.9 31.8 24.9 61.3

54000 10000 22000 10251 -/-/-/-/-/10000 -/-/-/-/30800 5100 -/-

Non-doped device, b) Doped device.


CIE [x,y] (0.26,0.52) (0.40,0.54) (0.24,0.51) (0.30,0.59) (0.32,0.58) (0.24,0.43) (0.24,0.44) (0.32,0.56) (0.31,0.59) (0.23,0.54) -/(0.32,0.58) (0.42,0.55) (0.34,0.59) (0.38,0.57) (0.28,0.43) (0.36,0.55)

This work Ref. 22 Ref. 32 Ref. 33 Ref. 34 Ref. 35 Ref. 21 Ref. 36 Ref. 11 Ref. 7 Ref. 37 Ref. 12 Ref. 12 Ref. 10 Ref. 8 Ref. 8 Ref. 38

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4. Conclusion In summary, we have successfully fabricated all-solution-processed multilayered TADF OLEDs by sequential solution processing method. This breakthrough progress was enabled by the synthesis of a novel solution-processable self-host TADF emitter Cz-CzCN, which can not only form a uniform thin film through spin-coating, but also afford the EML resistance to isopropyl alcohol used for processing upper ETL. Importantly, this self-host TADF compound is easily synthesized with high yield and low cost, which matches the trends and requirements of the development of wet technology. The solution-processed multilayer device based on Cz-CzCN achieves a maximum current efficiency of 46.3 cd A-1 and a maximum external quantum efficiency of 15.5%, which are much higher than the values of all-solution-processed OLEDs based on tranditional fluorescence. This research indicates that the self-host emitters with TADF features are promising candidates for developing solution-processed multilayer OLEDs. Furthermore, the facile approach for constructing self-host TADF materials by linking the functional wrapping units and the emissive core with the alkyl chains can be readily adapted to obtain other well-defined TADF materials, which would further improve the efficiency and enrich the color emission of all-solution-processed fluorescent devices. Supporting Information Experimental details for DFT calculations; Physical measurements; Device fabrications; This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Authors * E-mail: [email protected] (X.X. Ban). * E-mail: [email protected] (W. Jiang). Notes The authors declares no competing financial interest. Acknowledgements We are grateful for the Grants from the Nature Science Fund of Jiangsu Province (BK20161294) and Science Foundation of Huaihai Institute of Technology (KQ16025, KK17018). We also thank for the project supported by Science and Technology Bureau of Lianyungang (ZK201702) and the project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. Reference (1) Zheng, H.; Zheng, Y. N.; Liu, N. L.; Ai, N.; Wang, Q.; Wu, S.; Zhou, J. H.; Hu, D. G.; Yu, S. F.; Han, S. H.; Xu, W.; Luo, C.; Meng, Y. H.; Jiang, Z. X.; Chen, Y. W.; Li, D. Y.; Huang, F.; Wang, J.; Peng, J. B.; Cao, Y., All-Solution Processed Polymer Light-Emitting Diode Displays. Nat.

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Highly Efficient All-Solution-Processed Fluorescent Organic Light

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