High-Efficiency White Organic Light-Emitting Diodes Integrating

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High-Efficiency White Organic Light-Emitting Diodes Integrating Gradient Exciplex Allocation System and Novel D-Spiro-A Materials Xun Tang, Xiang-Yang Liu, Yi Yuan, Yongjie Wang, Hong-Cheng Li, Zuo-Quan Jiang, and Liang-Sheng Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09418 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 2018

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ACS Applied Materials & Interfaces

High-Efficiency

White

Organic

Light-Emitting

Diodes Integrating Gradient Exciplex Allocation System and Novel D-Spiro-A Materials Xun Tang,† Xiang-Yang Liu,† Yi Yuan,† Yong-Jie Wang,† Hong-Cheng Li,† Zuo-Quan Jiang,†* Liang-Sheng Liao †‡* †

Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of

Functional Nano & Soft Materials (FUNSOM), Soochow University, Suzhou 215123, China. ‡

Institute of Organic Optoelectronics, Jiangsu Industrial Technology Research Institute (JITRI),

Wujiang, Suzhou, Jiangsu 215211, China. Corresponding Author: [email protected] (Z. Q. J.); [email protected] (L. S. L.).

ACS Paragon Plus Environment

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ABSTRACT: How to maintain high power efficiency (PE) and color stability under operating brightness is critical for the white organic light-emitting diodes (WOLEDs). To this end, two novel spiro-type materials STPy3 and STPy4 were designed. These materials could act as a single host and achieved a remarkable external quantum efficiency (EQE) of 27.5% at 1000 cd m-2; to further optimize the PEs of OLEDs, STPy3/4 and PO-T2T were used as co-host induced exciplexes, which enhanced the PE of green OLED to over 148.0 lm W-1. Unfortunately, the lower triplet energy level of exciplexes than blue emitters, implied it is commonly unsuitable to fabricate WOLEDs. Herein, a new allocation of gradient exciplex (AGE) strategy was developed, in which the formed excitons could be rationally allocated in a consequently doped non-uniform profile. The AGE incorporated the advantages of the exciplex with an ultra-low turn-on voltage of 2.3 V, and efficiency stability of spiro materials. The PE at 1000 cd m-2 was enhanced to 72.7 lm W-1, representing the first exciplex WOLED with a performance exceeding that of conventional fluorescent tubes. KEYWORDS: white organic light-emitting diodes, gradient exciplex, power efficiency, spiro materials, low roll-off


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1. Introduction White organic light-emitting diodes (WOLEDs) have attracted attention worldwide as a promising technology for use in next-generation information displays, the lighting industry and even military or aerospace fields owing to their outstanding merits, such as flexibility, lightweight, power conservation and health.1-3 Improving the power efficiency (PE) of WOLEDs is undoubtedly a vitally important goal, and 70 lm W-1 is viewed as the benchmark at which a novel illumination technique can overtake the performance of a traditional fluorescent tube.4 In 2009, S. Reineke, K. Leo and co-workers first reported a WOLED that reached 90 lmW-1 at 1000 cd m-2 in phosphorescent structure, and it has drawn great attention from both academia and industry.5 However, this performance was accomplished using high-refractive-index substrates and a periodic out-coupling structure,6,

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and the original PE without the light extraction

techniques at 1000 cd m-2 requires further improvement.8 Spurred by the great potential of WOLEDs, enormous efforts have been made to design new materials or to develop device architectures that can achieve high PE.9,

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Unlike another

parameter, i.e., the external quantum efficiency (EQE) for OLED, the PE is distinctly sensitive to the driving voltage;11,

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thus, OLEDs with a high PE should satisfy the following strict

requirements: (i) efficient emitters with nearly 100% internal quantum efficiency (IQE);13 (ii) effective exciton harvesting from host to dopants; and (iii) low-energy barriers during carrier injecting and transporting process. Currently, new material systems for improving the PE of OLEDs are based on highly efficient phosphors and exciplex hosts because the former can achieve theoretical 100% IQE and the latter can improve carrier injection to decrease the driving voltage.14, 15 To date, several results from exciplex hosts implementing peak PE of approximately 100 lm W-1 have been reported under low brightness with different device structures, such as


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tandem exciplex WOLEDs,16 blue phosphor hosted by exciplex plus ultrathin yellow phosphor film,17 blue and yellow emitters doped in exciplex within one emissive layer,18 and blue fluorescent emitter hybridized with yellow phosphor in exciplex host.19 These reports revealed the promising and versatile applications of exciplex in designing high efficiency WOLEDs. Despite the high peak PEs in the above results, the PEs at 1000 cd m-2 can hardly exceed the 70 lm W-1 threshold without out-coupling techniques, which indicates there are must be large rolloffs at this practical brightness. Moreover, the releasing color stability is always very sensitive to the change of brightness,20, 21 which also requires further improvement. In this work, a new allocation of gradient exciplex (AGE) strategy is first identified in high quality WOLEDs, where the formed excitons could be rationally distributed in recombination zones. Hence, AGE could be considered as a “slope” between a single host and co-host formed exciplex by adjusting the D(onor)-A(cceptor) mixing ratio, leading to a consequently nonuniform dopant profile for the triplet energy level that ranges from high triplet energy to low triplet energy level. To assist this device strategy, two novel materials with D-spiro-A conformation were proposed and utilized. They possessed sufficiently high triplet energy (>2.8 eV) for blue emitters and could be mixed with another n-type material to form exciplex. In this D-spiro-A system, the D and A blocks are linked with unconjugated bonds, so the intramolecular interplay is weak, which facilitates the intermolecular interaction to form the exciplex. They can solely act as single host for traditional blue-yellow bicolor WOLEDs and achieve a peak EQE of 27.8%, however, the PE is only 50.0 lm W-1 due to high driving voltage at 1000 cd m-2. In contrast, by employing AGE, a remarkable enhancement of ultra-low driving voltage and roll-off at high brightness could be successfully obtained in WOLEDs as well as more improved color stability than other exciplex-based WOLEDs. In particular, the turn-on voltage was as low as 2.3


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V, which might be the lowest one to our knowledge; the maximum EQE of 28.2% indeed had a slight improvement but the PE could remain as high as 72.7 lm W-1 at 1000 cd m-2, which was a 45.4% improvement over the result obtained using a single host and was significantly higher than the PEs of other exciplex-based WOLEDs.

2. Experimental Details OLED Fabrication and Measurements: The OLEDs were fabricated through vacuum deposition under 2 × 10-6 Torr onto ITO-coated glass substrate with a sheet resistance of 15 Ω per square. The ITO substrates were ultra-sonic cleaned sequentially with acetone, ethanol, then dried in an oven at 110 °C for 1 h and finally subjected to UV-ozone treatment for 15 min. For each device, HAT-CN was deposited at 0.4 Å s-1, organic layers were deposited at a rate of 2-4 Å s-1, Liq was deposited at 0.4 Å s-1, and the metallic cathode was thermally deposited at 6 Å s-1 through a shadow mask subsequently. In the evaporating process of blue emissive layer, the doping ratio of blue emitter was 15 vol%, at the same time, the donor materials (STPy3/4) were thermally evaporated at the rate of 2 Å s-1 (including blue emitters and host materials), the evaporating rate of acceptor material was 0 Å s-1. Then, increasing the heating power of acceptor materials with a linear rising evaporating rate, at the same time, reducing the heating power of donor materials to decrease the evaporating rate of donors in order to maintain the total evaporating rate of 2 Å s-1. And at the terminal part of blue-emitting layer, the molar ratio of donor and acceptor materials is 1: 1, followed by yellow-emitting layer. The emitting areas of the devices were 0.09 cm2. Electroluminescence spectra, efficiencies, CIE coordinates, and current density-voltage-luminance characteristics were measured under a constant source of Keithley 2400s Source Meter, and a photometer of Photo Research PR 655 Spectrophotometer at a room temperature.


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3. Results and Discussion The materials STPy3 and STPy4 used in this study were newly designed and synthesized, which were shown in Figure 1(a) and (b). The m-linked fluorene part is effective to maintain high triplet energy level because of the poor conjugation. Unlike the previously reported derivatives, such as dibenzothiophene,22 diphenylamine, or carbazole,23 here the pyridine moiety was the electron-deficient group that was useful for electron transporting.24 The triphenylamine as D part and pyridine as A part were combined with an unconjugated spiro center,25, 26 thus offering independent channels for hole and electron transporting27. The two materials were also simulated by time-dependent density functional theory (TDDFT) at the B3LYP/6-31 g(d) level in Figure 1 (c) and (d). As expected, the HOMOs and LUMOs of STPy3 and STPy4 were separately distributed on each side of the spiro backbone. The HOMOs mainly located on the electron donating triphenylamine as expected, and the LUMOs were dispersed over the pyridine units with extension to the fluorene. These separated conformations were good for hole/electron transporting, and Figure S1 shows the hole and electron only characteristic curves for STPy3/4. The simulated energy levels (HOMO, LUMO & T1) had similar tendencies as did the tested ones. As the two materials were only position isomers, both materials had slight differences in photophysical properties in Figure 2. The STPy4, in which the nitrogen was embedded at the para-position to the fluorene core, showed bathochromic-shifts not only in UV-vis absorption but also in the photoluminescence spectra at room temperature and phosphorescence spectra at 77 K because the nitrogen in the pyridine of STPy4 could participate slightly more in the resonance to the neighboring fluorene. In particular, the maximum absorption for STPy3 and STPy4 were 263 and 269 nm, as shown in Figure 2 (a), and (b), respectively, corresponding to π-π* absorption


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from the backbone. The shoulder absorption at approximately 325 nm in the UV-vis spectra could be attributed to the intramolecular charge transfer (ICT) from the donor part to the acceptor part. The PL peak position shifted from 421 nm for STPy3 to 430 nm for STPy4, in accordance with the trend observed in the absorption spectra. The T1 energies were estimated to be 2.82 and 2.81 eV for STPy3 and STPy4, respectively, from the highest vibronic band in phosphorescence spectra, thus ensuring exothermic energy transfer from STPy3 or STPy4 to the guest emitters, such as blue phosphors and white phosphors consisting of blue/yellow components. The ultraviolet photoelectron spectroscopy (UPS) (Figure S2) and cyclic voltammogram. (Figure S3) were used to ascertain the HOMO energy levels. Moreover, STPy3/4 also exhibited very high thermal and morphological stabilities in thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements (Figure S4). All pertinent data and the physical characterization data are summarized in Table S1. In device tests, we first employed STPy3 and STPy4 as conventional single hosts for two corresponding bicolor warm WOLEDs (W1 for STPy3, and W2 for STPy4) according to the aforementioned photophysical characteristics. The single light-emitting layer (EML) is co-doped by a blue phosphor iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N, C2´] picolinate (FIrpic) (15 vol%) and a yellow phosphor iridium(III) bis(4-phenylthieno[3,2-c] pyridinato-N,C2′) acetylacetonate (PO-01) (1 vol%). Figure 3 (a) shows the energy alignment and the device structure, which was composed of indium tin oxide (ITO)/ 1, 4, 5, 8, 9, 11-hexaazatriphenylenehexacarbonitrile (HAT-CN) (10 nm)/ 1,1-bis[4- [N, N-di(p-tolyl) amino]phenyl]cyclohexane (TAPC) (40 nm)/ 4, 44’, 4’’-tris-(N-carbazolyl)-triphenylamine (TCTA) (10 nm)/ STPy3 or STPy4: blue dopant: yellow dopant (x and y vol%, 20 nm)/ 1,3,5-tri[(3-pyridyl)-phen-3yl]benzene (TmPyPB) (50 nm)/ quinolone lithium (Liq) (2 nm)/ aluminum (Al) (120 nm). In


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general, both host materials showed remarkable performance, such as high efficiency and low roll-off. In particular, the STPy3-based warm WOLED device performed the maximum EQE of 25.4%. At 1000 cd m-2, there was a negligible decrease for EQE of 25.2%, with a roll-off of just 0.7%, which was an inspiring performance for WOLEDs. Additionally, STPy4 exhibited a better performance than STPy3 did, with a maximum EQE of 27.8%. At 1000 cd m-2, there was a negligible decrease for EQE of 27.5%, with a roll-off of just 1.0%. The superiority of STPy4 to STPy3 could be partly attributed to the former’s deeper LUMO, which originated from the stronger π-electron-withdrawing effect of the para nitrogen in the pyridine moiety. Additionally, the color stability trade-off at different luminance and current densities is also significant in general lighting and display.28, 29 STPy3/4-based warm WOLEDs exhibited remarkable stability from 150 to 12000 cd m-2, which was shown in Figure S5. Despite achieving an ideal EQE, in fact, the maximum PE was just restricted to 64.2 lm W-1 and dropped to 50.0 lm W-1 at 1000 cd m-2, which was lower than that of a fluorescent tube. This, however, is not a unique case of a device with a very high EQE only providing moderate PE in WOLED.30-32 By analyzing these data and other relative works with “universal” hosts for WOLEDs,30, 33-35 we found that although the much higher triplet energy of a host could satisfy blue phosphors, severe energy loss could also occur during carrier injection by overcoming the wide bandgap of universal host materials, along with exciton transfer from host to yellow phosphors with low triplet energy level, which would lead to an increased driving voltage. (Corresponding device performances for STPy3- and STPy4-based RGB OLEDs are summarized in Figure S6, S7 and Table S2). To avoid the energy loss in hosting emitters with lower triplet energy than blue ones, it is useful to adopt the exciplex strategy in mono-chromatic OLEDs (MC-OLEDs).36-38 Figure 4 (a)


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shows the PL spectra of the selected host materials STPy3, STPy4, (1, 3, 5-triazine-2, 4, 6-triyl) tris(benzene-3, 1-diyl)) tris(diphenylphosphineoxide) (PO-T2T), co-deposited STPy3: PO-T2T, and co-deposited STPy4: PO-T2T (all of the samples were thin film state) at room temperature. Both STPy3: PO-T2T and STPy4: PO-T2T mixed films showed feature-less emission spectra. The corresponding wavelength peaks were located at 504 and 510 nm. Furthermore, the transient decay PL spectra (film, measured at 300 K) were measured to confirm the existence of the new exciplex by two hosts and PO-T2T. The curves in Figure 4 (b) were resolved into prompt and delayed decay components, including a fast decay component of 45 and 38 ns and a much slower decay with lifetime longer than 0.55 and 0.57 µs, respectively. The phenomenon indicated the successful formation of co-host exciplex, and the intramolecular D-spiro-A interaction could shift to the newly formed intermolecular D-A interaction. Therefore, the ultra-simplified device structures, doped with green phosphor bis(2phenylpyridine) iridium(III) (Ir(ppy)2(acac), G3 and G4) and yellow phosphor PO-01 (Y1 and Y2) were designed. The device architecture was well designed with little current leakage under low driving voltage (0~2 V), and the turn-on voltage of MC-OLEDs was 2.1 V, the maximum EQE was beyond 33.0%, and the corresponding PE was over 148.0 lm/W, which was very close to the highest record efficiency, to our best knowledge (details in Figure S8, S9, S10 and Table 1). However, the STPy3/STPy4: PO-T2T exciplexes had insufficient T1 energies to host blue emitters and thus appeared to be unsuitable for use in fabricating high efficiency WOLEDs, although they had an excellent impact on lowering driving voltages. In contrast, the STPy3/STPy4 themselves possessed high T1 energies for blue emitters but could not prevent the energy loss after the incorporation of yellow phosphors, or the wide bandgap for carrier injection, resulting in increased driving voltage. Thus, compared with the white OLEDs based on wide


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bandgap exciplex, which were summarized in Figure S11, the goal of high PE WOLEDs remained unaccomplished by directly combining the materials and co-host structure. To solve this problem, a novel strategy of allocation of gradient exciplex (AGE) was proposed to enhance the potential efficiencies of spiro-hosts and corresponding exciplexes. AGE was mainly utilized in the blue emissive layer to accomplish high efficiency blue light emitting, due to exciplex with low ET1. To be specific, in the conventional D-A co-host formed exciplex system, the fixed mixing ratio (such as 1: 1) of donors and acceptors is commonly utilized to obtain highest efficiency exciplex, however, in AGE system, the gradient mixing ratio leads to different exciplex distribution in the emissive region, besides the exciplex, the excess donors (STPy3/4) could also realized energy transfer to FIrpic. Moreover, at the terminal part of AGE emissive-layer, the mixing ratio is 1: 1, which is consistent with the mixing ratio in yellowemitting layer, thus there is non-barrier injection between two emissive layers. The device structure is established as ITO/ HAT-CN (10 nm)/ TAPC (40 nm)/ Gradient STPy3 or STPy4: PO-T2T (mixing ratio from 0 mol% - 50 mol%): FIrpic (15 vol%, 20 nm)/ STPy3 (W3) or STPy4 (W4): PO-T2T: PO-01 (6 vol%, 2 nm)/ PO-T2T (45 nm)/ Liq (2 nm)/ Al (120 nm). Where FIrpic and PO-01 were used as blue and yellow phosphors, respectively, and PO-T2T (acceptor) could be considered as dopants in D-A system, with the gradient mixing ratio from 0 mol% to 50 mol% in the blue emitting layer, and the D-A mixing ratio in yellow emitting layer was constantly 1: 1 for complete exciplex formation. In fact, the recombination zone deeply affects the exciton quenching, efficiency roll-off and device stability. Especially in the device composed of binary emitting-layers, the recombination interface is mainly located at the interface between yellow and blue emitting layers. In the conventional single host (blue-emission) + exciplex (yellow-emission) structure without AGE,


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due to the lower ET1 of exciplex than blue emitters, as well as the large energy barrier between single host and electron-transporting materials, the energy of excitons in exciplex cannot be effectively transferred to blue dopants, thus, leading to the accumulation at the interface. The aforementioned structure will severely narrow the recombination zone, then cause severe quenching and roll-off. Moreover, the restriction of exciton harvested by blue emitters will make it rather difficult to tune the white emission (in Table S3, S4, and Figure S12, S13). By contrast, it was flexible for OLED devices employing AGE to tune white-emission, even with stronger blue-emission (Figure S14 and Table S5). To better comprehend the recombination zone in AGE system, single-host + exciplex system and AGE system were selected to investigate the recombination zone, ultra-thin yellow dopant PO-01 (0.5 nm) was chosen as the quenching sensitizer, which was inserted into different regions of the light-emitting layer. As shown in the Table S6 and S7 and Figure S15, S16, the more reasonable exciton allocation in the emissive layer verified much wider recombination zone in AGE system. Furthermore, considering the single-host system, the triplet energy levels for STPy3 and STPy4 were 2.80 and 2.82 eV, respectively, which satisfied the requirement of energy transfer to FIrpic. However, the sharp change of LUMO severely hinders the electron injection. By contrast, the conventional exciplex system cannot avoid the energy loss due to the lower ET1, therefore, the mechanism of particular design in blue-emitting layer based on AGE is also significant. As shown in Figure S17 and Table S8, single-host, D-A indeuced exciplex with different mixing ratios (9:1, 7:3, 1:1), AGE-based blue-emitting layers were designed, the blue dopant was FIrpic. By analyzing the EL spectra, electrical properties, and efficiency, it was evident that AGE effectively decreased the driving voltage. In addition, through integrating the gradient mixing procedure, the excess donors could avoid the energy loss from FIrpic, thus, the


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blue-emitting device based on AGE exhibited higher efficiency. Obviously, the AGE system involves the incorporation the merits of universal hosts and an extremely low driving voltage, which could effectively reduce the strict requirements to achieve a higher PE in WOLEDs. Compared with traditional electron injecting and transporting in electron-transporting layer (ETL) and EML, the gradient co-host formed exciplex system provided a series of benefits: (1) a non-energy barrier to facilitate electron injection from ETL to EML, and reduced electron accumulation at ETL and EML interface; (2) blocking the exciton quenching via dispersed recombination density; and (3) a matched triplet energy level for corresponding color emitters with reduced energy loss. To verify the electro-properties of gradient co-host formed exciplex system, Figure 5 (b) shows the J-V-L curves of STPy3- and (c) STPy4-based devices. It was evident that the turn-on voltage for AGE-WOLEDs was only 2.3 V. At 1000 cd m-2, the operating voltage was as low as 3.4 V. The results perfectly confirmed our hypothesis for this device design. As shown in Table 2, the highest EQE for the STPy3-based device was 25.6%. At 1000 cd m-2, EQE could also be 24.9% with a low roll-off for the broadening of recombination zone. In addition, the highest power efficiency was 86.0 lm W-1, and even at 1000 cd m-2, PE was still over 65.0 lm W-1. The STPy4-based AGE-WOLED exhibited better performance. The highest EQE was over 28.1%, and the value remained at 28.0% at 1000 cd m-2, with an extremely low roll-off. The highest PE was over 91.0 lm W-1, even at 1000 cd m-2; the corresponding PE was 72.7 lm W-1, as well as good reproducibility (shown in Figure S18), which surpassed the 70 lm W-1 benchmark. As shown in Figure S19, for the STPy4-based AGEWOLED, there was a shift in the CIE coordinated ∆CIE = (0.019, 0.005), from (0.429, 0.485) at 300 cd m-2 to (0.410, 0.480) at 12000 cd m-2. The change was also plotted in CIELUV curves in Figure S19. Moreover, STPy3/4 exhibited very high thermal and morphological stabilities that


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were obviously superior to those of the commonly used small carbazole donors (such as mCP, CBP and mCBP), and the utilization of AGE could reduce the driving voltage and broaden the recombination zone, thus, spiro-materials and AGE based WOLED devices had better operational stability in Figure S20 and Table S9.

4. Conclusions In conclusion, this work started by exploring two new D-spiro-A hosts, STPy3 and STPy4, for use in high efficiency WOLEDs, and white OLED devices based on traditional structures exhibited high EQE that reached 27.8%. However, due to the restriction of structural design and the intrinsic problem of a single host, the PE showed a moderate performance of ~50.0 lm W-1 at 1000 cd m-2. By incorporating the new ambipolar materials as a donor for the co-host exciplex system, MC-OLEDs with remarkably high PEs could be achieved; however, the reduced triplet energies inhibited their use in WOLEDs. To promote the virtues and avoid the issues of using a single host and exciplex, the first identified allocation of gradient exciplex (AGE), in which formed excitons could be rationally distributed in recombination zones, was proposed to achieve an extremely low driving voltage and high PE. As a result, the ultra-low driving voltage was only 2.3 V, which is the lowest one achieved to date, to our best knowledge. At 1000 cd m-2, the EQE and PE remained as high as 28.0% and 72.7 lm W-1, which might be the highest result for exciplex-based WOLEDs (without any out-coupling techniques) under such practical brightness. In addition, the strategy of AGE contributed to extremely low efficiency roll-off for better exciplex and exciton distribution. This work also offered new options for material scientists to circumvent the strict selections of exciplex for WOLEDs


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because a wide bandgap D-A exciplex to host blue emitters is no longer indispensable for achieving highly efficient WOLEDs.

Figure 1. Molecular structures of (a) STPy3 and (b) STPy4. Frontier molecular orbitals (FMOs) distribution of (c) STPy3 and (d) STPy4.


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Figure 2. UV-vis absorption, fluorescence (298 K), and phosphorescent (77 K) spectra of (a) STPy3 and (b) STPy4.

Figure 3. Device performance of single-host WOLEDs based on STPy3 and STPy4. (a) The device structure and energy diagram of W1 and W2. (b) The current density to voltage curves of W1 and W2. (c) The EQE to luminance curves of W1 and W2. (d) The PE to luminance curves of W1 and W2.


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Figure 4. (a) PL spectra of STPy3, STPy4, PO-T2T, STPy3: PO-T2T, and STPy4: PO-T2T in deposited films at 300 K. (b) Transient PL decay curve of STPy3: PO-T2T and STPy4: PO-T2T co-doped films (measured at 300 K), respectively.


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Figure 5. (a) The schematic diagram of white OLED device structure based on the allocation of gradient exciplex process with shallow HOMO (-5.69 eV) donor materials in the blue emissive layer. Current density-voltage-luminance characteristics of “AGE” based white OLEDs (b) W3 and (c) W4. EQE-luminance-PE curves of “AGE” based white OLEDs (d) W3 and (e) W4.


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Table 1. Summary of device performances of exciplex based green, yellow and AGE based white OLEDs.

OLED

Power Efficiency (lm W-1)

EQE (%)

Voltage (V) Turn-on

1000 [cd m-2]

Maximum

500 [cd m-2]

1000 [cd m-2]

Maximum

500 [cd m-2]

1000 [cd m-2]

G3

2.1

3.3

29.2

29.1

28.6

129.5

107.2

102.3

G4

2.1

3.3

33.4

33.2

32.8

148.8

123.5

107.8

Y1

2.1

3.4

28.1

25.1

24.4

116.9

80.1

70.1

Y2

2.1

3.4

28.4

25.3

24.6

118.3

80.7

71.0

W1

-

4.6

25.4

25.3

25.2

57.0

50.1

46.0

W2

-

4.5

27.8

27.7

27.5

64.2

56.3

51.3

W3

2.3

3.4

25.6

25.4

24.9

86.0

72.6

65.7

W4

2.3

3.4

28.2

28.1

28.0

92.0

80.0

72.7

Table 2. Summary and comparison of EL performances of representative WOLEDs. Voltage [V]

PE [lm W-1]

EQE [%]

Turnon

1000 [cd m-2]

Max

1000 [cd m-2]

Max

1000 [cd m-2]

Roll -off [%]

W1

-

4.5

25.4

25.2

57.0

45.8

0.8

(0.37, 0.45)

W2

-

4.5

27.8

27.5

64.2

51.3

1.1

(0.40, 0.46)

W3

2.3

3.4

25.6

24.9

86.0

65.7

2.7

(0.42, 0.48)

W4

2.3

3.4

28.2

28.1

92.0

72.7

0.7

(0.42, 0.48)

Ref. 39

3.2

-

15.7

-

29.6

-

-

(0.46, 0.43)

40

2.5

-

25.5

14.8

84.1

24.2

42.0

(0.40, 0.43)

Ref. 41

3.1

-

20.8

19.6

51.2

38.7

5.8

(0.40, 0.45)

Ref. 42

OLEDs

Ref.

CIEa) [x, y]

-

-

19.6

11.4

50.2

-

-

(0.42, 0.48)

17

2.4

-

20.0

19.5

75.3

63.1

2.5

(0.42, 0.51)

Ref. 18

2.5

3.6

28.1

21.5

105

59.5

23.5

(0.40, 0.48)

19

2.5

4.1

28.3

25.8

102.9

63.5

8.8

(0.45, 0.48)

Ref.

Ref.

-2

a) CIEs were measured at the brightness of 1000 cd m .


18

Page 19 of 26 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


ASSOCIATED CONTENT Supporting Information. The synthetic routes of STPy3/4, the material characterization of UPS, CV, DSC traces and TGA curves. Device optimization of monochromatic OLEDs based on single host and co-host induced exciplex. The summary of physical properties of STPy3/4, and device performance of single host RGBW-OLEDs. The determination of recombination zone in AGE-based devices, and histogram of power efficiency for AGE-based WOLEDs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author [email protected] (Z. Q. Jiang) [email protected] (L. S. Liao) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes Any additional relevant notes should be placed here. ACKNOWLEDGMENT The authors acknowledge the financial support from the National Key R&D Program of China (Grant No. 2016YFB0400700). This work was also funded by the Collaborative Innovation Centre of Suzhou Nano Science and Technology (Nano-CIC), by the Priority Academic Program


19


Page 20 of 26

Development of Jiangsu Higher Education Institutions (PAPD), by the “111” Project of The State Administration of Foreign Experts Affairs of China, and by Yunnan Provincial Research Funds on College-Enterprise Collaboration.

REFERENCES 1.

D'Andrade, B. W.; Forrest, S. R. White Organic Light-Emitting Devices for Solid-State Lighting. Adv. Mater. 2004, 16, 1585-1595.

2.

Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Highly Efficient Organic Light-Emitting Diodes from Delayed Fluorescence. Nature 2012, 492, 234-238.

3.

Sun, Y.; Giebink, N. C.; Kanno, H.; Ma, B.; Thompson, M. E.; Forrest, S. R. Management of Singlet and Triplet Excitons for Efficient White Organic Light-Emitting Devices. Nature 2006, 440, 908-912.

4.

D'Andrade, B. W.; Holmes, R. J.; Forrest, S. R. Efficient Organic Electrophosphorescent White-Light-Emitting Device with A Triple Doped Emissive Layer. Adv. Mater. 2004, 16, 624-628.

5.

Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K. White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459, 234238.

6.

Nakamura, T.; Tsutsumi, N.; Juni, N.; Fujii, H. Thin-Film Waveguiding Mode Light Extraction in Organic Electroluminescent Device Using High Refractive Index Substrate. J. Appl. Phys. 2005, 97, 054505.


20

Page 21 of 26 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


7.

Heremans, P. L.; Gaertner, G.; Muccini, M.; Greiner, H.; Meulenkamp, E. A. Light Extraction from OLEDs with (High) Index Matched Glass Substrates. Proc. SPIE 2008, 6999, 69992T.

8.

Su, S. J.; Gonmori, E.; Sasabe, H.; Kido, J. Highly Efficient Organic Blue- and WhiteLight-Emitting Devices Having A Carrier- and Exciton-Confining Structure for Reduced Efficiency Roll-Off. Adv. Mater. 2008, 20, 4189-4194.

9.

Ding, L.; Dong, S.-C.; Jiang, Z.-Q.; Chen, H.; Liao, L.-S. Orthogonal Molecular Structure for Better Host Material in Blue Phosphorescence and Larger OLED White Lighting Panel. Adv. Funct. Mater. 2015, 25, 645-650.

10. Hung, W. Y.; Fang, G. C.; Lin, S. W.; Cheng, S. H.; Wong, K. T.; Kuo, T. Y.; Chou, P. T. The First Tandem, All-Exciplex-Based WOLED. Sci. Rep. 2014, 4, 5161. 11. Park, Y.-S.; Lee, S.; Kim, K.-H.; Kim, S.-Y.; Lee, J.-H.; Kim, J.-J. Exciplex-Forming CoHost for Organic Light-Emitting Diodes with Ultimate Efficiency. Adv. Funct. Mater. 2013, 23, 4914-4920. 12. Sasabe, H.; Nakanishi, H.; Watanabe, Y.; Yano, S.; Hirasawa, M.; Pu, Y.-J.; Kido, J. Extremely Low Operating Voltage Green Phosphorescent Organic Light-Emitting Devices. Adv. Funct. Mater. 2013, 23, 5550-5555. 13. Zhang, Q.; Li, B.; Huang, S.; Nomura, H.; Tanaka, H.; Adachi, C. Efficient Blue Organic Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence. Nat. Photon. 2014, 8, 326-332. 14. Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices. Nature 1998, 395, 151-154.


21


Page 22 of 26

15. Kim, K. H.; Lee, S.; Moon, C. K.; Kim, S. Y.; Park, Y. S.; Lee, J. H.; Woo Lee, J.; Huh, J.; You, Y.; Kim, J. J. Phosphorescent Dye-Based Supramolecules for High-Efficiency Organic Light-Emitting Diodes. Nat. Commun. 2014, 5, 4769. 16. Lee, S.; Shin, H.; Kim, J. J. High-Efficiency Orange and Tandem White Organic LightEmitting Diodes Using Phosphorescent Dyes with Horizontally Oriented Emitting Dipoles. Adv. Mater. 2014, 26, 5864-5868. 17. 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 Low Efficiency Roll-Off. ACS Appl. Mater. Interfaces 2016, 8, 10093-10097. 18. Wu, S.-F.; Li, S.-H.; Wang, Y.-K.; Huang, C.-C.; Sun, Q.; Liang, J.-J.; Liao, L.-S.; Fung, M.-K. White Organic LED with A Luminous Efficacy Exceeding 100 lm W−1 without Light Out-Coupling Enhancement Techniques. Adv. Funct. Mater. 2017, 27, 1701314. 19. Wu, Z.; Yu, L.; Zhao, F.; Qiao, X.; Chen, J.; Ni, F.; Yang, C.; Ahamad, T.; Alshehri, S. M.; Ma, D. Precise Exciton Allocation for Highly Efficient White Organic Light-Emitting Diodes with Low Efficiency Roll-Off Based on Blue Thermally Activated Delayed Fluorescent Exciplex Emission. Adv. Opt. Mater 2017, 5, 1700415. 20. Ho, C.-L.; Wong, W.-Y.; Wang, Q.; Ma, D.; Wang, L.; Lin, Z. A Multifunctional IridiumCarbazolyl Orange Phosphor for High-Performance Two-Element WOLED Exploiting Exciton-Managed Fluorescence/Phosphorescence. Adv. Funct. Mater. 2008, 18, 928-937. 21. Park, Y.-S.; Kang, J.-W.; Kang, D. M.; Park, J.-W.; Kim, Y.-H.; Kwon, S.-K.; Kim, J.-J. Efficient, Color Stable White Organic Light-Emitting Diode Based on High Energy Level Yellowish-Green Dopants. Adv. Mater. 2008, 20, 1957-1961.


22

Page 23 of 26 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


22. Dong, S.-C.; Liu, Y.; Li, Q.; Cui, L.-S.; Chen, H.; Jiang, Z.-Q.; Liao, L.-S. Spiro-Annulated Triarylamine-Based Hosts Incorporating Dibenzothiophene for Highly Efficient SingleEmitting Layer White Phosphorescent Organic Light-Emitting Diodes. J. Mater. Chem. C 2013, 1, 6575-6584. 23. Zhang, Y. X.; Zhang, L.; Cui, L. S.; Gao, C. H.; Chen, H.; Li, Q.; Jiang, Z. Q.; Liao, L. S. Control of Conjugation Degree via Position Engineering to Highly Efficient Phosphorescent Host Materials. Org. Lett. 2014, 16, 3748-3751. 24. Xiao, L.; Su, S.-J.; Agata, Y.; Lan, H.; Kido, J. Nearly 100% Internal Quantum Efficiency in An Organic Blue-Light Electrophosphorescent Device Using A Weak Electron Transporting Material with A Wide Energy Gap. Adv. Mater. 2009, 21, 1271-1274. 25. Romain, M.; Tondelier, D.; Geffroy, B.; Shirinskaya, A.; Jeannin, O.; Rault-Berthelot, J.; Poriel, C. Spiro-Configured Phenyl Acridine Thioxanthene Dioxide as A Host for Efficient PhOLEDs. Chem. Commun. 2015, 51, 1313-1315. 26. Romain, M.; Thiery, S.; Shirinskaya, A.; Declairieux, C.; Tondelier, D.; Geffroy, B.; Jeannin, O.; Rault-Berthelot, J.; Metivier, R.; Poriel, C. Ortho-, Meta-, And ParaDihydroindenofluorene Derivatives as Host Materials for Phosphorescent OLEDs. Angew. Chem. Int. Ed. 2015, 54, 1176-1180. 27. Wang, Y.-K.; Sun, Q.; Wu, S.-F.; Yuan, Y.; Li, Q.; Jiang, Z.-Q.; Fung, M.-K.; Liao, L.-S. Thermally Activated Delayed Fluorescence Material as Host with Novel Spiro-Based Skeleton for High Power Efficiency and Low Roll-Off Blue and White Phosphorescent Devices. Adv. Funct. Mater. 2016, 26, 7929-7936.


23


Page 24 of 26

28. Reineke, S.; Schwartz, G.; Walzer, K.; Leo, K. Reduced Efficiency Roll-Off in Phosphorescent Organic Light Emitting Diodes by Suppression of Triplet-Triplet Annihilation. Appl. Phys. Lett. 2007, 91, 123508. 29. Weichsel, C.; Reineke, S.; Furno, M.; Lüssem, B.; Leo, K. Organic Light-Emitting Diodes for Lighting: High Color Quality by Controlling Energy Transfer Processes in Host-GuestSystems. J. Appl. Phys. 2012, 111, 033102. 30. Mondal, E.; Hung, W.-Y.; Dai, H.-C.; Wong, K.-T. Fluorene-Based Asymmetric Bipolar Universal Hosts for White Organic Light Emitting Devices. Adv. Funct. Mater. 2013, 23, 3096-3105. 31. Lin, M.-S.; Chi, L.-C.; Chang, H.-W.; Huang, Y.-H.; Tien, K.-C.; Chen, C.-C.; Chang, C.H.; Wu, C.-C.; Chaskar, A.; Chou, S.-H.; Ting, H.-C.; Wong, K.-T.; Liu, Y.-H.; Chi, Y. A Diarylborane-Substituted Carbazole as A Universal Bipolar Host Material for Highly Efficient Electrophosphorescence Devices. J. Mater. Chem. 2012, 22, 870-876. 32. Gong, S.; Chen, Y.; Luo, J.; Yang, C.; Zhong, C.; Qin, J.; Ma, D. Bipolar Tetraarylsilanes as Universal Hosts for Blue, Green, Orange, and White Electrophosphorescence with High Efficiency and Low Efficiency Roll-Off. Adv. Funct. Mater. 2011, 21, 1168-1178. 33. Chaskar, A.; Chen, H. F.; Wong, K. T. Bipolar Host Materials: A Chemical Approach for Highly Efficient Electrophosphorescent Devices. Adv. Mater. 2011, 23, 3876-3895. 34. Hung, W.-Y.; Chi, L.-C.; Chen, W.-J.; Mondal, E.; Chou, S.-H.; Wong, K.-T.; Chi, Y. A Carbazole–Phenylbenzimidazole Hybrid Bipolar Universal Host for High Efficiency RGB and White PhOLEDs with High Chromatic Stability. J. Mater. Chem. 2011, 21, 1924919256.


24

Page 25 of 26 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


35. Li, J.; Ding, D.; Tao, Y.; Wei, Y.; Chen, R.; Xie, L.; Huang, W.; Xu, H. A Significantly Twisted Spirocyclic Phosphine Oxide as A Universal Host for High-Efficiency Full-Color Thermally Activated Delayed Fluorescence Diodes. Adv. Mater. 2016, 28, 3122-3130. 36. Kim, K. H.; Moon, C. K.; Lee, J. H.; Kim, S. Y.; Kim, J. J. Highly Efficient Organic LightEmitting Diodes with Phosphorescent Emitters Having High Quantum Yield and Horizontal Orientation of Transition Dipole Moments. Adv. Mater. 2014, 26, 3844-3847. 37. Lee, J. H.; Shin, H.; Kim, J. M.; Kim, K. H.; Kim, J. J. Exciplex-Forming Co-Host-Based Red Phosphorescent Organic Light-Emitting Diodes with Long Operational Stability and High Efficiency. ACS Appl. Mater. Interfaces 2017, 9, 3277-3281. 38. Lee, S.; Kim, K.-H.; Limbach, D.; Park, Y.-S.; Kim, J.-J. Low Roll-Off and High Efficiency Organic Light Emitting Diodes with Controlled Co-Doping of Green and Red Phosphorescent Dopants in An Exciplex Forming Co-Host. Adv. Funct. Mater. 2013, 23, 4105-4110. 39. Chen, Z.; Liu, X.-K.; Zheng, C.-J.; Ye, J.; Liu, C.-L.; Li, F.; Ou, X.-M.; Lee, C.-S.; Zhang, X.-H. High Performance Exciplex-Based Fluorescence–Phosphorescence White Organic Light-Emitting Device with Highly Simplified Structure. Chem. Mater. 2015, 27, 52065211. 40. Liu, X. K.; Chen, Z.; Qing, J.; Zhang, W. J.; Wu, B.; Tam, H. L.; Zhu, F.; Zhang, X. H.; Lee, C. S. Remanagement of Singlet and Triplet Excitons in Single-Emissive-Layer Hybrid White Organic Light-Emitting Devices Using Thermally Activated Delayed Fluorescent Blue Exciplex. Adv. Mater. 2015, 27, 7079-7085.


25


Page 26 of 26

41. Zhang, D.; Cai, M.; Zhang, Y.; Zhang, D.; Duan, L. Highly Efficient Simplified SingleEmitting-Layer Hybrid WOLEDs with Low Roll-Off and Good Color Stability through Enhanced Forster Energy Transfer. ACS Appl. Mater. Interfaces 2015, 7, 28693-28700. 42. Zhang, D.; Duan, L.; Zhang, Y.; Cai, M.; Zhang, D.; Qiu, Y. Highly Efficient Hybrid Warm White Organic Light-Emitting Diodes Using A Blue Thermally Activated Delayed Fluorescence Emitter: Exploiting the External Heavy-Atom Effect. Light: Science & Applications 2015, 4, e232.

TOC:


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High-Efficiency White Organic Light-Emitting Diodes Integrating

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