Strategy for the Realization of Highly Efficient Solution-Processed All

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Organic Electronic Devices

Strategy for the realization of highly efficient solutionprocessed all-fluorescence white OLEDs – encapsulated thermally activated delayed fluorescent yellow emitters Xinxin Ban, Feng Chen, Yaqing Zhao, Aiyun Zhu, Zhiwei Tong, Wei Jiang, and Yueming Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b13101 • Publication Date (Web): 10 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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ACS Applied Materials & Interfaces

Strategy for the Realization of Highly Efficient Solution-Processed All-Fluorescence White OLEDs – Encapsulated Thermally Activated Delayed Fluorescent Yellow Emitters Xinxin Ban,*† Feng Chen,† Yaqing Zhao,† Aiyun Zhu,† Zhiwei Tong,† Wei Jiang*‡ and Yueming Sun‡ †

Jiangsu Key Laboratory of Function Control Technology for Advanced Materials, 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: Fabrication of highly efficient all thermally activated delayed fluorescence (TADF) white organic light-emitting diodes (WOLEDs) through solution-process still remains a big challenge. Here, two encapsulated TADF molecules with small singlet-triplet energy gap (∆EST) and high photoluminescence quantum yield (PLQY) were designed and synthesized as yellow emitters for solution-processed WOLEDs. The high current, power and external quantum efficiencies of 41.6 cd A-1, 30.4 lm W-1 and 17.3% were achieved for the solution-processed all-fluorescence WOLEDs with single emission layer. In contrast, even with the same ∆EST and PLQY, the corresponding unencapsulated parent emitters will account for near 50% loss of the potential device efficiency. This is the first time using small molecular TADF blue host and TADF yellow gust to construct solution-processed all-fluorescence WOLEDs, which exhibit a high efficiency comparable with most of the vacuum-deposited all-fluorescence white devices. These results not only demonstrate the great potential of TADF emitters in achieving highly efficient solution-processed WOLEDs, but also testify the key role of molecular encapsulation in reducing polar-exciton quenching and enhancing electroluminescence performance. Keywords: white emission, encapsulation, solution-process, TADF, organic light emitting diodes 1. Introduction White organic light-emitting diodes (WOLEDs) have been extensively studied due to their great promise for universal application in future solid-state lighting sources.1-3 In order to harvest 100% electrogenerated excitons for emission, organometallic emitters with strong spin-orbit coupling effect have been widely used in all-phosphorescence WOLEDs or fluorescence/phosphorescence hybrid WOLEDs.4-8 However, these precious-metal complexes are expense and rarity, which would limit the cost effectiveness and long-term mass production of WOLEDs. In addition to material selection, fabricating technique is the other main sources of cost. Comparing to the thermal evaporation, solution-processing is more promising by its advantages of large area, time saving and low energy consumption.9-12 Therefore, realizing solution-processed all-fluorescence devices with highly electroluminescent efficiency is pivotal for facilitating the commercialization of WOLEDs. To avoid the usage of precious-metal complexes, thermally activated delayed fluorescence (TADF) material has been explored as an alternative to harvest all of the excited excitons through reversed intersystem crossing from the triplet (T1) to singlet (S1).13-16 In fact, the TADF materials can not only be employed as emitters, but also as blue host of the complementary color to achieve white emission.17-20 By combining the blue and yellow TADF materials, the pure organic WOLEDs with external quantum efficiency above 20% have been reported, which fully demonstrate the superiority of TADF materials in facilitating the energy transfer and exciton utilization.21, 22 However, these high-performance vacuum-deposited WOLEDs usually need the

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complicated and multi-layered device structures, which are difficult to achieve for solution-processed devices due to the solvent erosion effect during the deposition of upper layers.23-26 In contrast, the simple device structure with a single-emission layer (EML), which has precious ratio of multiple emission components, is generally used to fabricate solution-processed WOLEDs.27-31 Since the solution-processable TADF emitters are scarcity, the common strategy is directly using the small molecular TADF materials designed for vacuum-deposited OLEDs to construct the corresponding solution-processed devices.32-35 However, even with the same emitters, the performance of these solution-processed OELDs are still much lower than their vacuum-deposited counterparts, which indicates the existence of other energy leakage pathway when transfer these small molecular TADF materials into solution-processed platform.36-38 As a result, no solution-processed all-fluorescence WOLEDs based on small molecular TADF emitters have been successfully fabricated up until now. Therefore, the exploration of effective method to qualify these classical small molecular TADF materials for solution-processed devices is essential for further improving the device efficiency and reducing the manufacturing cost of all-fluorescence WOLEDs. In this work, two encapsulated TADF materials, Cz-4CzPN and Cz-4CzTPN, were designed and synthesized as solution-processible yellow emitters. The molecular encapsulation by unconjugated alkyl chain keeps the TADF property of the parent molecules unchanged and makes the emitters more efficient for solution-processed devices. In view of the efficient triplet harvesting of encapsulated emitters, we have for the first time successfully fabricated high efficiency solution-processed all-fluorescence WOLEDs by using small molecular TADF materials. The current efficiency (CE), power efficiency (PE) and external quantum efficiency (EQE) as high as 41.6 cd A-1, 30.4 lm W-1 and 17.3% were achieved for the Cz-4CzPN based single-EML WOLEDs, which can be comparable with most of vacuum-deposited single- or multi-EML white devices based on pure organic emitters. However, despite the ∆EST and PLQY are similar, devices based on the unencapsulated parent emitters can only achieve half of the efficiency of the newly designed materials, which can be assigned to the unavoidable molecular collision and exciton-polaron quenching of the unencapsulated yellow emitters. In addition to manipulate the complicated molecular structures for the novel TADF emitters with small ∆EST and high PLQY, the results observed here indicate that the efficient molecular encapsulation is another important factor to reduce exciton annihilations and enhance device efficiency of the solution-processed WOLEDs. 2. Experimental Section All reagents were used as purchased without further purification. The anhydrous tetrahydrofuran (THF) were purified according to standard procedures and distilled under nitrogen. The intermediate 4-(6-(9H-carbazol-9-yl)hexyloxy)-9H-carbazole (Cz-Cz) was prepared according to the literature procedure.39 Synthesis of 3,4,5,6-tetrakis(4-(6-(9H-carbazol-9-yl)hexyloxy)-9H-carbazol-9-yl)benzene-1,2dinitrile (Cz-4CzPN): To the solution of Cz-Cz (1.8 g, 4 mmol) in anhydrous THF (40 ml), an anhydrous THF (20 ml) solution containing NaH (0.60 g, 5 mmol) was added dropwise for 15 min. After stirring for 3 h, 3,4,5,6-tetrafluorobenzene-1,2-dinitrile (0.20 g, 1 mmol) in anhydrous THF (20 ml) was added dropwise for 15 min. The solution was stirred for 24h at room temperature. Then, the reaction was quenched with the addition of 250 mL water. The mixture was extracted


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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 orange product (1.65 g, 91%). 1H NMR (300 MHz, CDCl3, δ): 8.05 (dd, J = 11.9, 7.9 Hz, 6H), 7.95 (t, J = 7.3 Hz, 2H), 7.50 (d, J = 7.3 Hz, 2H), 7.38 (td, J = 15.1, 7.6 Hz, 12H), 7.26 (d, J = 5.7 Hz, 6H), 7.23-7.10 (m, 8H), 7.08-6.89 (m, 10H), 6.80-6.68 (m, 4H), 6.50 (dd, J = 18.0, 7.7 Hz, 8H), 6.12 (d, J = 7.3 Hz, 2H), 4.27 (t, J = 6.8 Hz, 4H), 4.15 (t, J = 5.9 Hz, 4H), 3.96 (t, J = 6.2 Hz, 4H), 3.68 (t, J = 5.6 Hz, 4H), 1.93-1.83 (m, 8H), 1.80-1.63 (m, 8H), 1.51-1.40 (m, 16H). 13C NMR (75 MHz, CDCl3, δ): 155.92, 155.62, 140.85, 140.43, 140.10, 139.87, 138.68, 126.65, 125.61, 124.86, 124.66, 123.56, 122.83, 122.79, 120.34, 120.30, 119.60, 118.72, 113.47, 109.92, 108.65, 108.60, 103.32, 102.57, 101.08, 67.68, 42.96, 29.33, 29.00, 27.10, 26.15. MS (MALDI-TOF) [m/z]: calcd for C128H108N10O4, 1848.86; found, 1848.82. Anal. Calcd. for C128H108N10O4: C, 83.09; H, 5.88; N, 7.57. Found: C, 83.06; H, 5.90; N, 7.55. Synthesis of 2,3,5,6-tetrakis(4-(6-(9H-carbazol-9-yl)hexyloxy)-9H-carbazol-9-yl)benzene-1,4dinitrile (Cz-4CzTPN): This compound was prepared from intermediates Cz-Cz and 2,3,5,6tetrafluorobenzene-1,4-dinitrile following the same procedure as Cz-4CzPN. Orange red solid, yield: 93%. 1H NMR (300 MHz, CDCl3, δ): 8.07 (d, J = 7.7 Hz, 7H), 7.97 (d, J = 3.8 Hz, 4H), 7.46 – 7.29 (m, 17H), 7.23 (d, J = 7.4 Hz, 6H), 7.20 – 7.14 (m, 8H), 7.07 (dd, J = 16.6, 10.2 Hz, 10H), 7.00 – 6.79 (m, 4H), 6.51 (p, J = 8.2 Hz, 4H), 4.25 (t, J = 7.0 Hz, 8H), 3.95 (t, J = 6.5 Hz, 8H), 1.87 (m, 8H), 1.75 (m, 8H), 1.58 – 1.31 (m, 16H). 13C NMR (75 MHz, CDCl3, δ): 156.09, 155.41, 145.47, 140.27, 139.86, 139.10, 125.59, 124.00, 123.66, 122.81, 121.87, 120.31, 118.73, 116.83, 111.40, 109.30, 108.94, 108.60, 103.94, 103.09, 102.61, 101.94, 67.76, 42.88, 29.06, 28.89, 26.95, 25.95. MS (MALDI-TOF) [m/z]: calcd for C128H108N10O4, 1848.86; found, 1848.89. Anal. Calcd. for C128H108N10O4: C, 83.09; H, 5.88; N, 7.57. Found: C, 83.11; H, 5.85; N, 7.52. 3. Results and Discussions The synthetic scheme of Cz-4CzPN and Cz-4CzTPN was shown in Scheme 1. The Cz-4CzPN and Cz-4CzTPN were facilely prepared through aromatic nucleophilic substitution reactions by using Cz-Cz and the corresponding benzonitrile units. The two compounds are cost effective since no precious-metal are required during the synthetic process. The synthetic yields are both higher than 90% during the simple one-step reaction. 1H-NMR, 13C-NMR, mass spectrometry and elemental analysis were performed to characterize the compound structures. The thermal, photophysical, and electrochemical data of the two compounds are summarized in Table 1.

Scheme 1. Synthetic route of Cz-CzCN.



The geometrical and electronic properties of the compounds were studied using density functional theory (DFT) calculation to understand the structure-property relationship of the compounds. In view of the unconjugated alkyl chain, the encapsulated molecule can be divided into two independent functional fragments. The emissive core guarantees the electronic property of the molecule, while the peripheral carbazoles act as the steric shield of the emissive core to reduce the intermolecular interaction. Firstly, we optimized the molecule as a whole. As can be seen from Figure 1, the lowest unoccupied molecular orbitals (LUMOs) are mainly centered on the benzonitrile moieties, while the highest occupied molecular orbital (HOMO) are mainly delocalized over the peripheral carbazolyl moieties, which demonstrates that peripheral carbazoles not only act as steric shields, but also can used as hole transporting moieties. The calculated HOMO and LUMO levels of Cz-4CzPN and Cz-4CzTPN are -5.35/-2.54 and -5.35/-2.67 eV, respectively. Secondly, the parent cores of the two molecules were calculated to investigate the emission property. As shown in Figure S1, the HOMOs are localized on the carbazole unites, while the LUMOs mainly centered on the benzonitrile moieties. The highly twisted structures lead to spatially separated HOMO and LUMO of the emissive cores, which leads them to conclude the TADF behaviour. On the other hand, the certain degree of HOMO-LUMO overlap of the two molecules will facilitate to achieve high PLQY. The time-dependent DFT (TD-DFT) were also carried out to optimize the excited states. The theoretical calculated singlet state energies of Cz-4CzPN and Cz-4CzTPN are -2.33 and -2.15 eV, while the triplet state energies are -2.22 and -2.06 eV, respectively. Thus, the singlet-triplet energy splitting of the two design compounds are only 0.11 and 0.09 eV, which are sufficiently small for the reverse intersystem cross.

Figure 1. Molecular orbital amplitude plot calculated by B3LYP/6-31G (d).

Figure 2 shows the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) diagrams. Cz-4CzPN and Cz-4CzTPN exhibit the decomposition temperature (Td) of 410 and 400 oC according to 5% weight loss, while the glass-transition temperatures (Tg) are 120 and 113 oC, respectively. Both the high Td and Tg are important for the fabrication of OLEDs during solution-process. The electrochemical properties of the compounds were investigated by cyclic voltammetry (CV). Both Cz-4CzPN and Cz-4CzTPN show two overlapped quasi-reversible oxidations (Figure 3), which can be ascribed to hole delocalization over the carbazole units. To investigation the reason of the new peak in the reverse scan, the CV investigation for several circles were performed (Figure S2). In the first circle of front scan, only one peak around 1.3 V was achieved. However, a new peak around 0.8 V appeared in the first reverse scan. Moreover, the


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second front scan also appeared a new peak around 0.9 V, which was enhanced with the increased scan circles. Thus, the new appeared peaks in the CV scan can be attributed to the electrochemical polymerization of the carbazole units. The HOMO energy level should be determined by the onset of the first circle front scan. Therefore, the HOMO levels are estimated to be both -5.32 eV according to the values with regard to the energy level of ferrocene (4.8 eV below vacuum), while the LUMO levels are -2.84 and -3.05 eV for Cz-4CzPN and Cz-4CzTPN, respectively, which are calculated from the band gap and HOMO levels.

Figure 2. TGA curve of Cz-4CzPN and Cz-4CzTPN recorded at a heating rate of 10 °C min-1; Inset: DSC trace recorded at a heating rate of 10 °C min-1.

Figure 3. Cyclic voltammograms of Cz-4CzPN and Cz-4CzTPN at the first scan circle.

Figure 4(a) shows the UV-Vis absorption and photoluminescence (PL) features of Cz-4CzPN and Cz-4CzTPN in toluene. The absorption band around 285 nm can be attributed to π-π* transitions, while the absorption between 320 and 340 nm should be assigned to n-π* transitions of carbazoles. The longer wavelength around 400 nm for Cz-4CzPN and 480 nm for Cz-4CzTPN are charge transfer absorption from the electron-donating carbazole to the electron accepting benzonitrile moiety. The energy gaps of Cz-4CzPN and Cz-4CzTPN calculated from the on-set absorption bands are 2.48 and 2.27 eV, respectively. The photoluminescence (PL) spectra of Cz-4CzPN and Cz-4CzTPN in toluene show abroad yellow emission bands with the PL peaks at 560 and 572 nm, respectively. In pure film state, the PL emission of Cz-4CzPN is peaked at 567 nm (Figure S3), whereas the emission maxima of Cz-4CzTPN red shift to 588 nm. Figure S4 exhibits the phosphorescence spectra at 77 K in toluene. Since the triplet states are well resolved 3 ππ* states, the zero-zero energy (E0-0) can be identified from the highest energy peak of their



emission. In contrast, the PL emission without any vibronic structure at room temperature indicates their singlet states (E0-0s) are charge transfer states, which should be calculated from the onset of their broad emission band. The measured S1/T1 of Cz-4CzPN and Cz-4CzTPN are 2.52/2.42 and 2.41/2.33 eV, respectively. Thus, ∆EST were 0.10 and 0.08 eV for Cz-4CzPN and Cz-4CzTPN, respectively, which are very similar and in good agreement with the theoretical calculated values. To further investigate the photophysical properties, transient PL decay of the films were measured. As shown in Figure 4(b), Cz-4CzPN and Cz-4CzTPN exhibit the prompt component within several nanoseconds of 15.2 and 20.5 ns, and the delayed emission with lifetimes of 26.4 and 22.5 µs, respectively, which confirms the TADF character of the two compounds. Generally, the careful choice of blue host material is greatly important for the achieving of high device efficiency. PL emission of Cz-3CzCN was overlapped with UV-vis absorption of Cz-4CzPN and Cz-4CzTPN, suggesting the good energy transfer from blue host to the yellow emitters. The PL spectra of the doped films are also measured. As shown in Figure 5, both of the yellow emissions gradually enhanced with the increase of dopant concentration, which indicates the potential for realization of white emission by appropriate doping rate. Moreover, the photoluminescence quantum yield (PLQY) of Cz-4CzPN doped film is 88%, which is higher than that of Cz-4CzTPN doped one (62%). This trend is consistent with their parent emission cores, which exhibit the PLQYs of 85% and 59% for 4CzPN and 4CzTPN, respectively. Obviously, the structure optimization of the dicyanobenzene acceptor displays a strong influence on the luminous efficiency of the molecules.

Figure 4. (a) The UV-Vis absorption and PL features in toluene. (b) The transient PL decay curves of Cz-4CzPN and Cz-4CzTPN films.

Figure 5. The PL spectra of the doped films at different dopant concentration (a) Cz-3CzCN: Cz-4CzPN and (b) Cz-3CzCN: Cz-4CzTPN.


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Table 1. Physical properties of Cz-4CzPN and Cz-4CzTPN.

Td/Tg [°C]

λabs [nm]a

λems [nm]

Eg [eV]

S1/T1 [eV]

∆EST [eV]

HOMO [eV]

LUMO [eV]

Cz-4CzPN

410/120

325,346, 377,440

560a 567b

2.81c 2.48d

2.33/2.22c 2.52/2.42e

0.11c 0.10e

-5.35c -5.32f

-2.54c -2.84g

Cz-4CzTPN

400/113

325,346, 365,499

572a 588b

2.68c 2.27d

2.15/2.06c 2.41/2.33e

0.09c 0.08e

-5.35c -5.32f

-2.67c -3.05g

a)

Measured in toluene solution at 300 K.

Gauss simulation.

d)

b)

Measured in deposited films at 300 K.

Estimated from the absorption edges in CH2Cl2.

phosphorescent spectra.

f)

Determined by the CV measurement.

g)

e)

c)

Obtained from

Estimated from fluorescent and

Calculated from the energy gap and

HOMO level.

We proceed to investigate the influence of molecular modulation and molecular encapsulation on the performance of the solution-processed WOLEDs. The devices are fabricated with the general structure of ITO/PEDOT:PSS (40 nm)/Blue Host: x% Yellow Dopant (60 nm)/TPBi (40 nm)/ Cs2CO3 (2 nm)/Al (100 nm). The recently reported solution-processible TADF molecule Cz-3CzCN is used as Blue Host,39 while Cz-4CzPN and Cz-4CzTPN are used as Yellow Dopant. Firstly, the dopant concentration of 0%, 2% and 100% are investigated. As shown in Figure S5 and S6, when the dopant concentration is 0%, the device exhibits blue emission of Cz-3CzCN with the peak at 470 nm. When the dopant concentration is increased to 2%, the devices exhibit yellow emission with the EL peaks of 548 and 557 nm for Cz-4CzPN and Cz-4CzTPN, respectively. Therefore, the white mission can be anticipated by modifying the dopant concentration for both blue and yellow emissions. However, when the dopant concentration increases to 100%, the devices based on pure Cz-4CzPN and Cz-4CzTPN show red emission with EL peak of 590 and 600 nm, respectively, which are redshift about 20 nm comparing with their PL emission in film states. The redshift of EL emission can be attributed to the intermolecular interactions in the non-doped devices, which consists with PL emission trend of the doped films in Figure 5. Finally, the dopant concentration was optimized to be 0.6%. The Yellow Dopant is Cz-4CzPN for Device A and Cz-4CzTPN for Device B. The control devices with unencapsulated 4CzPN and 4CzTPN as yellow emitters are also fabricated for Device C and Device D, respectively. Figure 6 exhibits the energy level diagram of WOLEDs and molecular structures of the used emissive materials. Table 2 summarizes all the electro-luminescent data of the devices. Figure 7 shows the current density-voltage-luminance (J-V-L) characteristics and current efficiencies versus current density plots of Device A and B. As shown in Figure 7(b), Device A and B successfully present the dual-peak white emissions with the CIE coordinate of (0.34, 0.42) and (0.33, 0.39) at the luminance of 1000 cd m-2. The EL emission peaks of Cz-4CzPN and Cz-4CzTPN are well identified, which indicates the insufficient energy transfer from blue host to corresponding yellow guests. Moreover, the EL spectra of Device A and B exhibit small changes from 100 to 5000 cd m-2 (Figure 8). The CIE coordinates as a function of operating voltage were also investigated. As shown in Figure S7, the CIE coordinates are mainly located in the white range with a small variation, which demonstrate the high emission color purity of the WOLEDs.



Figure 6. Schematic energy-level diagram of the devices and the molecular structures of the emitters.

Figure 9a describes the dynamical process of excited states in these WOLEDs. Due to the low dopant concentration, the excitons are mainly formed on Cz-3CzCN with 25% on singlet states and 75% on triplet states. Partial singlet excitons decay immediately to generate blue fluorescence of Cz-3CzCN, while the others transfer to the singlet state of yellow dopant through the long range Förster energy transfer (FET), which can decay and produce yellow emission. On the other hand, the 75% triplet excitons of TADF blue host formed under electrical excitation can be thermally converted into singlet excitons through RISC process, resulting in delayed florescence of Cz-3CzCN or go through the same FET process mentioned above. Moreover, the TADF property of yellow emitters ensure the triplet excitons, which are partly derived from intersystem crossing (ISC) process or Dexter energy transfer (DET) from the adjacent blue host, can also up-converse to singlet states to achieve radiative decay. Thus, the white device with TADF blue host and TADF yellow emitter can harvest all the excitons to light emission.2, 21, 22.

Figure 7. (a) Current density-voltage-luminance (J-V-L) characteristics; (b) Current efficiencies versus current density plots; inset: the EL spectra of device A and B at 1000 cd m-2.


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Figure 8. The EL spectra of the WOLEDs at different voltages of Device A (a), Device B (b).

Figure 9. Energy transfer process (a) and molecular stacking diagram (b) in WOLEDs using TADF blue host and TADF yellow guest.

Intriguingly, the device with Cz-4CzPN as the yellow emitter realized the maximum CE, PE and EQE as high as 41.6 cd A-1, 30.4 lm W-1 and 17.3%, respectively. This is the first time using small molecular TADF emitters to construct highly efficient solution-processed all-fluorescence WOLEDs, which can be comparable with most of the reported all-fluorescence vacuum-deposited devices (Table 3). Contrarily, despite the efficient TADF property, Device B based on Cz-4CzTPN exhibited the maximum CE, PE and EQE of only 18.6 cd A-1, 12.9 lm W-1 and 7.8%, respectively (Figure S8). The obtained higher efficiency of Cz-4CzPN-based device as compared to that of Cz-4CzTPN can be attributed to the higher PLQY of Cz-4CzPN. Obviously, the suitable structure optimization for high PLQY will facilitate the achieving of high electroluminescence efficiency, which is accordance with the most reported vacuum-evaporated OLEDs. However, it does not always have a satisfactory effect when transfer the materials to solution-processed platform. As shown in Figure S9, although 4CzPN exhibit similar PLQYs comparing to the encapsulated Cz-4CzPN, Device C only achieves the peak EQE of 10.9% and CE of 27.3 cd A-1, which are much lower than that of Device A. Since the device concept with TADF blue host and TADF yellow guest will theoretically harvest all the electrogenerated excitons for light emission, the reduced EQE of 4CzPN-based device with the same PLQY can be assigned to the photoluminescence quenching, such as triplet-triplet annihilation, triplet-polaron annihilation, molecular collision and so on. In view of the insufficient energy transfer for achieving white emission, the device efficiencies are mainly determined by the exciton utilization of both blue host



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and yellow guest. Generally, the exciton utilization (߯) of the OLEDs can be estimated from the following Equation: ாொா

χ = ఊ∅

೛ ఎ೚ೠ೟

where, ߛ is recombination efficiency of the injected holes and electrons, ∅p is the photoluminescence quantum yields, ߟout is the optical out-coupling factor. By assuming ߛ = 1 and ߟout = 0.25, the calculated ߯ of Cz-4CzPN, 4CzPN, Cz-4CzTPN and 4CzTPN based devices are 78%, 51%, 50% and 29%, respectively. Obviously, the increased ߯ of encapsulated molecules fully demonstrated the effectiveness of steric shields in the suppression of molecular interaction induced exciton quenching. Table 2. Device Performances of the solution-processed OLEDs. Device

Vona

L maxb

CE/PE/EQEmaxc

CE/PE/EQE100d

CE/PE/EQE1000e

CRIf

CIE [x, y]g

18.3/9.2/7.6

68

(0.34, 0.42)

15.1/9.6/6.3

8.7/4.4/3.7

72

(0.33, 0.39)

27.3/20.0/10.9

19.0/12.9/7.6

9.6/5.4/3.8

65

(0.32, 0.43)

10.2/8.1/4.3

7.3/4.5/3.0

5.3/2.6/2.2

71

(0.33, 0.39)

[V]

[cd m ]

cd A / lm W / %

cd A / lm W / %

cd A-1/ lm W-1/ %

A

3.9

11000

41.6/30.4/17.3

35.4/24.3/14.6

B

3.9

9700

18.6/12.9/7.8

C

3.9

8500

D

3.9

7300

a)

-2

-1

-1

‒2 b)

Von = turn-on voltage at 1 cd m , -2 e)

PE and EQE at 100 cd m ,

-1

-1

Lmax = maximum luminance,

c)

maximum CE, PE and EQE, d) CE,

-2 f)

CE, PE and EQE at 1000 cd m , CRI = Color Rendering Index, g) CIE =

the Commission Internationale de L’Eclairage coordinates at the luminance of 1000 cd m-2.

Although the low doping rate of yellow emitter can reduce the concentration quenching of the excitons of emission guest, the unwanted molecular collision and exciton-polaron annihilation induced by the interaction between charged blue host and yellow emissive guest was also a potential energy leakage way.40-42 As shown in Figure 9b, the charged host surrounding the emission guest lead to a severe exciton-polaron interaction in the host-guest system, which cause the non-radiative transition of the guest. However, in view of the distance dependence of exciton-polaron annihilation process, the situation of encapsulated emission guest is different from the traditional emitter.43, 44 The peripheral bulky units will increase the host/guest distance and separate the exciton from the electric charges, which was an effective way to suppress the exciton-polaron interaction and stabilize the excited excitons of emission guest.45-47 Thus, the improved efficiency of Device A can be attributed to intermolecular separation through addition of steric bulk to the dopant molecule, which was validated again in Cz-4CzTPN based Device B comparing to 4CzTPN based Device D. Commonly, great efforts have been devoted for developing novel TADF emitters with small ∆EST and high PLQY by elaborately manipulating the molecular structures, which are rather complicate and difficult. In order to develop TADF materials compatible with the low-cost solution-processed WOLEDs, the results observed here may point out an alternative strategy to enhance the device efficiency by reducing the exciton quenching through efficient molecular encapsulation, which is simple and practicable.


Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Summary of EL performance of TADF based all-fluorescence WOLEDs EQEmax Blue Emitter S

M

Com. Color

PEmax -1

CEmax

CIE

-1

[%]

[lm W ]

[cd A ]

[x,y]

Sol.

Cz-3CzCN (T)

Cz-4CzPN (T)

17.1

30.4

41.6

(0.34,0.42)

This

Vac.

DMAC-DPS (T)

4CzTPN-Ph (T)

13.4

38.3

-/-

(0.29,0.39)

Ref. 48

Vac.

DMAC-DPS (T)

TBRb (F)

15.5

39.3

38.4

(0.28,0.35)

Ref. 49

Vac.

DMAC-DPS (T)

TBRb (F)

14.6

51.6

44.4

(0.37,0.47)

Ref. 50

Vac.

CzAcSF (T)

TBRb (F)

14.0

36.2

35.1

(0.31,0.37)

Ref. 51

Vac.

2CzPN (T)

AnbCz (T)

19.0

63.0

50.1

(0.34,0.44)

Ref. 52

Vac.

DMAC-DPS (E)

Rubrene (F)

7.5

15.9

20.2

(0.36,0.44)

Ref. 53

Vac.

2CzPN (T)

AnbTPA (T)

19.2

46.2

36.7

(0.33,0.38)

Ref. 54

Vac.

DDMA-TXO2 (T)

POZ-DBPHZ (T)

16.1

22.4

32.7

(0.31,0.40)

Ref. 55

Vac.

CC2BP (T)

p-Px2BBP (T)

6.7

16.4

-/-

(0.32,0.39)

Ref. 56

Vac.

3CzTRZ (T)

4CzTPN-Ph (T)

17.1

33.4

40.3

(0.30,0.38)

Ref. 20

Vac.

3,6-2TPA-TX (T)

3,6-2TPA-TXO (T)

20.4

48.6

49.5

(0.34,0.41)

Ref. 21

Vac.

DMAC-DPS (T)

DBP (F)

18.2

44.6

40.9

(0.31,0.39)

Ref. 57

Vac.

NI-1-PhTPA (F)

PXZDSO2 (T)

19.2

47.5

51.4

(0.34,0.45)

Ref. 2

Vac.

mCP:PO-T2T (E)

DTAF:PO-T2T (E)

11.6

15.8

27.7

(0.29,0.35)

Ref. 58

Vac.

DMAC-DPS (T)

DBP (F)

12.1

22.0

-/-

(0.25,0.31)

Ref. 59

Vac.

4CzPN (T)

TBRb (F)

15.1

47.4

48.9

(0.35,0.49)

Ref. 22

Vac.

pCNBCzoCF3 (T)

m-MTDATA (E)

18.8

19.3

53.8

(0.40,0.44)

Ref. 60

S= single emission layer, m= multi emission layers, Com. Color= complementary color, Sol.= solution-process, Vac.= vacuum-deposition, T= TADF, F= fluorescence, E= exciplex.

4. Conclusion In conclude, two encapsulated TADF materials, Cz-4CzPN and Cz-4CzTPN, were designed and synthesized as yellower emitter to construct two-color based all-fluorescence WOLEDs. The solution-processed single-EML WOLEDs contain TADF blue host and TADF yellow guest was successfully fabricated for the first time. Due to the efficient triplet harvesting of both blue and yellow emitters, the CE, PE and EQE as high as 41.6 cd A-1, 30.4 lm W-1 and 17.3% were obtained for the pure organic solution-processed WOLEDs, which can be comparable with most of the single- or multi-EML vacuum-deposited all-fluorescence WOLEDs. We also found that the EQE of the WOLEDs with unencapsulated parent yellow emitters are much lower than that of encapsulated counterparts even though the ∆EST and PLQY are similar. The enhanced EL efficiency of the developed materials fully demonstrated the key role of molecular encapsulation in suppressing of exciton quenching of solution-processible emitter. The current results not only demonstrate the great potential of TADF emitter for construct high performance solution-processed WOLEDs, but also highlights a viable strategy to qualify these classical small molecular TADF materials for solution-processed devices. Supporting Information PL spectra of thin films; phosphorescence spectra at 77K; EL spectra of WOLEDs at different voltages 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 National Natural Science Foundation of China (21805106), the Natural Science Foundation of Jiangsu Province (BK20181073), the Natural Science Fund for Colleges and Universities in Jiangsu Province (KK17150), the Postdoctoral Science Foundation of China (1107040175) and the Science Foundation of Huaihai Institute of Technology (KQ16025, KK17018). We also thank for the project supported by Science and Technology Bureau of Lianyungang (ZKK201702). Reference (1) Groves, C., Organic Light-Emitting Diodes: Bright Design. Nat. Mater. 2013, 12, 597-598. (2) Li, X. L.; Xie, G.; Liu, M.; Chen, D.; Cai, X.; Peng, J.; Cao, Y.; Su, S. J., High-Efficiency WOLEDs with High Color-Rendering Index based on a Chromaticity-Adjustable Yellow Thermally Activated Delayed Fluorescence Emitter. Adv. Mater. 2016, 28, 4614-4619. (3) Liang, J.; Zhao, S.; Jiang, X. F.; Guo, T.; Yip, H. L.; Ying, L.; Huang, F.; Yang, W.; Cao, Y., White Polymer Light-Emitting Diodes Based on Exciplex Electroluminescence from Polymer Blends and a Single Polymer. ACS Appl. Mater. Interfaces 2016, 8, 6164-6173. (4) Liu, B.; Nie, H.; Lin, G.; Hu, S.; Gao, D.; Zou, J.; Xu, M.; Wang, L.; Zhao, Z.; Ning, H.; Peng, J.; Cao, Y.; Tang, B. Z., High-Performance Doping-Free Hybrid White OLEDs Based on Blue Aggregation-Induced Emission Luminogens. ACS Appl. Mater. Interfaces 2017, 9, 34162-34171. (5) Wang, J.; Chen, J.; Qiao, X.; Alshehri, S. M.; Ahamad, T.; Ma, D., Simple-Structured Phosphorescent Warm White Organic Light-Emitting Diodes with High Power Efficiency and


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