Extremely Simplified, High-Performance and Doping-Free White

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Extremely Simplified, High-Performance and DopingFree White Organic Light-Emitting Diodes Based on Single Thermally Activated Delayed Fluorescent Emitter Dongxiang Luo, Qizan Chen, Yuan Gao, Menglong Zhang, and Baiquan Liu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00711 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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ACS Energy Letters

Extremely Simplified, High-Performance and Doping-Free White Organic Light-Emitting Diodes Based on Single Thermally Activated Delayed Fluorescent Emitter Dongxiang Luo,1 Qizan Chen,1 Yuan Gao,2 Menglong Zhang,1 and Baiquan Liu*2,3 1

School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China 2

Luminous! Center of Excellence for Semiconductor Lighting and Displays, School of

Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore 3

Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

ABSTRACT: For the first time, extremely simplified yet high-performance thermally activated delayed fluorescent (TADF) WOLEDs have been demonstrated. Unlike previous concepts, only single molecular emitter is required for high-quality white emissions, where intrinsic TADF emitter is sandwiched between p-type and n-type layers, forming doping-free p-i-n WOLED. The WOLED exhibits a color rendering index (CRI) of 91, the first WOLED overtaking their counterparts (single-emitter white polymer/inorganic LEDs). The maximum total external quantum efficiency (28.4%) and power efficiency (68.5 lm W-1) are comparable to those of state-of-the-art doping TADF WOLEDs and doping-free phosphorescent WOLEDs, or higher than those of TADF WOLEDs with ultrahigh CRIs (≥90) and highquality single-emitter white LEDs. Significantly, this is the first TADF WOLED possessing ultrahigh CRIs at high luminance and 18796 cd m-2 is 370% higher than previous best one. Moreover, the proposed WOLED is the simplest TADF WOLED. 1 ACS Paragon Plus Environment

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Table of Contents

White organic light-emitting diodes (WOLEDs) have drawn research interest since they possess vast potential for the energy-saving lighting and display applications.1-14 To date, WOLEDs have been demonstrated to exhibit many merits (e.g., high efficiency, long lifetime and stable color).15-17 However, the cost still limits the commercial process of WOLEDs. This may be because: i) compared with monochromic OLEDs, WOLEDs intrinsically show more complicated device structures; ii) to realize the high-performance WOLEDs, noble metal complexes (e.g., Ir, Pt) which are rare and expensive are usually adopted; iii) multiple molecular emitters are generally selected.1-17 Thus, the key issue to accelerate the further development of WOLEDs becomes how to reduce the cost. To loosen the bottleneck, one of the most effective schemes is to exploit the simplified WOLEDs.18 However, the achievement of simplified structure and high performance trade-off is a huge challenge. A crucial reason for this phenomenon is that multiple molecular emitters are required for WOLEDs, leading to the fact that additional function layers are necessary to meet the requirement of these emitters.19 Hence, the reduction of the number of molecular emitters, as a result simplified WOLEDs, is desirable. For white polymer LEDs (WPLEDs), single polymer emitter has been demonstrated to emit high-efficiency and high-quality white emissions (e.g., a maximum external quantum efficiency (EQE) of 2.8%, luminance of 16610 cd m-2 and color rendering index (CRI) of 88 can be obtained).20,21 Even for white inorganic LEDs (WILEDs), single inorganic emitter has been recently reported to yield excellent white emission, exhibiting the maximum EQE of 1.9%, luminance of 1007 cd m-2 and CRI of 88.22 2 ACS Paragon Plus Environment

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ACS Energy Letters

However, no report has been documented that WOLED based on single molecular emitter can achieve such performance so far. Therefore, the gap between WOLEDs and their counterparts (e.g., WPLEDs and WILEDs) urgently needs to be bridged. Recently, thermally activated delayed fluorescent (TADF) materials have been considered as the third-generation OLED emitter after the breakthrough made by Adachi's group.23 By dint of the reverse intersystem crossing (RISC) procedure resulting from a small singlet-triplet energy gap, TADF materials can harvest triplets which are wasted in conventional fluorescent emitters, leading to a theoretical 100% internal quantum efficiency (IQE) in the absence of noble metal complexes.24,25 Thus, TADF materials have the potential to realize simplified yet high-performance WOLEDs. In fact, TADF WOLEDs have emerged as a new kind of WOLED.26-31 For example, Adachi et al. used blue TADF and complementary fluorescence materials to organize a WOLED, achieving a maximum EQE of 12.1% and power efficiency (PE) of 22.0 lm W-1.32 Lee et al. designed a single-emitting-layer (single-EML) WOLED comprising TADF host and fluorescence dopants, obtaining a maximum EQE of 14% (36.2 lm W-1).33 Su et al. utilized a chromaticity-adjustable yellow TADF emitter to furnish highefficiency and high-quality white light, yielding a maximum EQE of 15.6% (28.9 lm W-1) and CRI of 95 at 200- 2000 cd m-2.34 However, the structures of state-of-the-art TADF WOLEDs are still not simplified or even more complicated than the phosphorescent or hybrid WOLEDs (e.g., eight organic layers34). Besides, multiple evaporator sources are required to be simultaneously working due to the doping host-guest system (e.g., three sources32,33), complicating the fabrication processes. Moreover, the guest concentrations in single-EML TADF WOLEDs are too low (e.g., 0.3%33), which is difficult to control and reproduce in the coevaporation procedure.35-37 Therefore, compared with phosphorescent or hybrid WOLEDs, the unique advantages of TADF WOLEDs have not been fully unveiled. Currently, the prospect of doping-free OLEDs is flourishing, since the doping-free technique can greatly minimize the fabrication procedure, avert the host, simplify the device 3 ACS Paragon Plus Environment


engineering and lower the cost.38-41 For doping-free monochromatic TADF OLEDs, their performance can be comparable to those of doping counterparts.42,43 On the other hand, for doping-free WOLEDs (DF-WOLEDs), conventional fluorescent, phosphorescent and hybrid emitters have been extensively investigated to develop high-performance devices.44-48 Previously, we also developed a series of high-performance doping-free phosphorescent and hybrid WOLEDs.49-53 However, more than four layers and two molecular emitters are usually required for DF-WOLEDs, resulting in the fact that their structures are still not simplified enough. To address this issue, doping-free TADF WOLED may be effective. However, almost no attention has been paid on doping-free TADF WOLED. Therefore, many effects of doping-free TADF WOLEDs remain unknown. In this paper, for the first time, extremely simplified yet high-performance TADF WOLEDs have been demonstrated. Different from previous concepts which usually need at least two molecular emitters for high-quality white emissions, only single molecular emitter is required in the novel concept, in which intrinsic TADF emitter is sandwiched between p-type and n-type layers, forming doping-free p-i-n WOLED. The WOLED can exhibit a highquality white emission with a maximum CRI of 91, the first WOLED that outperforms their counterparts (e.g., WPLEDs and WILEDs). Besides, the WOLED possesses a maximum total EQE, current efficiency (CE) and PE of 28.4%, 65.4 cd A-1 and 68.5 lm W-1, respectively. To the best of our knowledge, these efficiencies are i) comparable to those of best doping TADF WOLEDs, ii) as efficient as those of highest doping-free phosphorescent WOLEDs, iii) new records for TADF WOLEDs with ultrahigh CRIs (≥90), and iv) much superior to previous records for single-emitter white LEDs with high-quality emissions (CRI≥88), including polymer and inorganic emitters. Remarkably, this is the first report that TADF WOLEDs can possess ultrahigh CRIs at high luminance (≥3000 cd m-2) and the maximum luminance of 18796 cd m-2 is 370% higher than previous best TADF WOLED with ultrahigh CRIs, paving

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ACS Energy Letters

their way toward practical applications. Moreover, the p-i-n WOLED represents the most simplified TADF WOLEDs. Figure 1 depicts the conceptual structure of the p-i-n WOLEDs, where an intrinsic TADF doping-free EML is sandwiched between the doping-free p-type and n-type layers. Firstly, we have selected 1-bis[4-[N,N-di(4-tolyl)amino]phenyl]-cyclohexane (TAPC) and 1,3,5-tri(mpyrid-3-yl-phenyl)benzene (TmPyPB) as the p-type and n-type layer, respectively, while DDCzTrz as the TADF EML, forming the three-layer structure of TAPC (40 nm)/ DDCzTrz (2 nm)/ TmPyPB (55 nm) for device W1. Besides, indium tin oxide (ITO)/MoO3 (5 nm) and LiF (1 nm)/Al (120 nm) have been utilized as the anode and cathode, respectively, guaranteeing that charges can be effectively injected.11-14 The detailed structures of fabricated devices are shown in Figure S1 (Supporting Information). The molecular structures of TAPC, TmPyPB and DDCzTrz are shown in Figure 1b. TAPC has been chosen as the p-type layer owing to i) its high hole mobility (10-2 cm2/V s) and suitable HOMO (highest occupied molecular orbital) of 5.43 eV (Figure 1c),54 which can effectively transport holes, lowering the voltage; ii) high LUMO (lowest unoccupied molecular orbital) of 2.0 eV,54-56 confining electrons. TmPyPB has been employed as the n-type layer because of i) its high electron mobility (10-3 cm2/V s) and appropriate LUMO (2.7 eV),57 leading to the efficient electron transport; ii) deep HOMO of 6.7 eV, blocking holes. DDCzTrz has been adopted as the TADF EML because i) it is one of the most efficient blue TADF emitters with high photoluminescence (PL) quantum yields (0.66);58 ii) the broad deep-blue emission is favorable to high-CRI WOLEDs; iii) the aggregation-caused quenching (ACQ) is not serious since high efficiency can be realized at high concentration,58 rendering its flexibility for doping-free WOLEDs.50 Furthermore, both the singlet energy (S1, 2.80 eV) and triplet energy (T1, 2.53 eV) of DDCzTrz are lower than those of TAPC and TmPyPB,54-58 indicating that excitons on EML can be efficiently confined. Therefore, although p-i-n WOLEDs are extremely simplified, they can form not only an effective charge transporting/confining 5 ACS Paragon Plus Environment


structure, but also an efficient exciton confining structure, which is a key feature for the high performance.1-5

Figure 1. (a) The conceptual structure of the p-i-n WOLEDs. (b) Molecular structures of TAPC, TmPyPB and DDCzTrz. (c) Schematic diagrams of the emission mechanisms of p-i-n WOLEDs. Red and black arrows represent hole and electron flow, respectively. The combination of exciplex and TADF emission ensures the white emissions. Based on the above factors, high-performance p-i-n WOLED has been developed (device W1), as shown in Figure 2 and Table 1. Since illumination sources are generally characterized by their total emitted power,59-61 W1 accordingly exhibits the maximum total EQE and PE of 28.4% and 68.5 lm W-1, respectively, demonstrating that around 100% IQE can be harvested by the extremely simplified p-i-n WOLED.61 Besides, a maximum total CE of 65.4 cd A-1 is obtained (Figure S2, Supporting Information). In fact, these efficiencies are i) comparable to those of best doping TADF WOLEDs, ii) as efficient as those of best dopingfree phosphorescent WOLEDs, and iii) new records for TADF WOLEDs with ultrahigh CRIs. 6 ACS Paragon Plus Environment

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ACS Energy Letters

Remarkably, our device is the most simplified TADF WOLEDs, which may render the novel concept more feasible to the commercialization. Similar to most of reported TADF OLEDs,58,62 the efficiency roll-off is somewhat serious (Table 1). Nevertheless, it is still better than that of previous state-of-the-art doping TADF WOLEDs.63 This problem may be originated from: i) the increased triplet-triplet annihilation (TTA), singlet-triplet annihilation and triplet-polaron quenching at high luminance usually happened in TADF OLEDs, due to the slow triplet exciton dynamics of TADF emitters;54,58 ii) relatively narrow exciton recombination zone,12 since the thickness of DDCzTrz EML is 2 nm; iii) the charge balance is not ideal, considering the charge mobility of TAPC and TmPyPB is inequal. To further improve the performance, the enhancement of materials design (e.g., developing TADF emitters with short triplet lifetimes using indolocarbazole-isomer derivatives to reduce tripletinvolved annihilation procedures,64 combining TADF emitter with aggregation-induced emission (AIE) characteristics to widen the exciton recombination zone12) and device engineering (e.g., exploring charge transport materials with same mobilities to optimize the charge balance6). Impressively, CRIs ≥90 can be achieved at ≥3000 cd m-2 (Figure 2a inset), which is the first TADF WOLED that can exhibit ultrahigh CRIs at such high luminance. Additionally, given the predicted per-m2 manufacturing cost of large-area illumination and the limited aperture ratio of high-resolution displays, the luminance should be ≥5000 cd m-2.64-66 Thus, it is easily seen that W1 can satisfy this practical requirement with CRIs ≥90 for general illuminations. Besides, considering that ultrahigh CRIs are usually achieved via more than four-layer structures and the doping technology, the presented results provide a new opportunity to realize ultrahigh CRIs by using simple structures. Moreover, it is well-known that most of ultrahigh CRIs are obtained via more than three molecular emitters and no WOLED based on single molecular emitter can possess ultrahigh CRIs. Herein, we have demonstrated the first WOLED based on single molecular emitter to show ultrahigh CRIs, 7 ACS Paragon Plus Environment


which fills the gap between WOLEDs and their counterparts (e.g., WPLEDs and WILEDs, the detailed comparisons are shown in Table S1, Supporting Information). For the ultrahigh CRIs, the utilization of DDCzTrz is crucial, since TADF materials usually exhibit broader full wavelength at half maximum (FWHM) than conventional fluorescent and phosphorescent emitters (e.g., 81 nm for DDCzTrz).23 Besides, the FWHM of the exciplex emission (will be demonstrated later) is also very wide. Hence, this blue-yellow strategy is effective to accomplish ultrahigh CRIs. For displays, pure-white colors are extremely desired. Significantly, W1 can exhibit verypure white emissions, where the Commission International de I’Eclairage (CIE) coordinates are very close to the white-equivalent point of (0.33, 0.33) (Figure S3, Supporting Information). For fluorescent WOLEDs, they can exhibit ultrahigh CRIs and pure-white emissions simultaneously. However, their efficiencies are very low due to the nonradiative triplets. For phosphorescent WOLEDs, they can show high efficiency and pure-white emissions simultaneously. However, it is difficult for them to exhibit ultrahigh CRIs owing to the difficult synthesis of deep-blue phosphors.38 For hybrid WOLEDs, they can yield high efficiency and ultrahigh CRIs simultaneously. However, they usually show warm-white or yellowish-white light since the triplets of blue fluorophor are harvested by phosphors.59-61 Herein, we have demonstrated that TADF WOLEDs can possess the high efficiency/ultrahigh CRI/pure-white emission trade-off, which has not been reported previously. Moreover, the maximum luminance of W1 is 18796 cd m-2 (Figure 1b), which is 370% higher than previous best TADF WOLED with ultrahigh CRIs.34


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ACS Energy Letters

Figure 2. (a) EQE and PE of W1. Inset: electroluminescent (EL) spectra at various luminances and a photograph of the WOLED. (b) The current density and luminance of W1.



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Table 1 Summary of performances of p-i-n WOLEDs. Vona

EQEmax/PEmax/CEmaxb

EQE1000/PE1000/CE1000c

Lmaxd

(V)

(%/lm W-1/ cd A-1)

(%/lm W-1/ cd A-1)

(cd m-2)

W1

3.0

28.4/68.5/65.4

3.9/5.0/8.7

18796

91

6355

(0.34, 0.35)

W6

3.0

5.5/12.1/11.5

1.6/1.8/3.3

6434

89

6120

(0.24, 0.25)

W7

3.0

13.9/32.9/31.4

2.3/2.7/5.2

10947

91

6521

(0.31, 0.33)

W8

3.2

21.1/46.3/47.2

3.5/4.1/7.9

17044

91

6056

(0.38, 0.37)

Device

a

d

CRIe

CCTf

CIEg

(K)

The turn-on voltage. bMaximum EQE, PE and CE. c EQE, PE and CE at 1000 cd m-2.

Maximum luminance. eMaximum CRI. f The color-correlated temperature (CCT) at the maximum CRI. gCIE coordinates at 1000 cd m-2.

It is an uncommon phenomenon that only single molecular emitter can be used to exhibit high-quality white emissions. To understand this fact, the emission mechanism of W1 has been unveiled. From the EL spectra of W1 (Figure 2a inset), it is noted that there are two emission peaks to guarantee the white emissions. In the case of the blue peak (462 nm), it is originated from the typical emission of blue TADF emitter DDCzTrz.58 To give more evidences for the emission of DDCzTrz, a doping OLED has been fabricated (device W2) with the structure of ITO/ MoO3 (5 nm)/ 1,3-bis(9H-carbazol-9-yl)-benzene (mCP, 30 nm)/ mCP: DDCzTrz (1: 10%, 10 nm)/ TmPyPB (55 nm)/ LiF (1 nm)/Al (120 nm), where the T1 of mCP (3.0 eV) is higher than that of DDCzTrz, indicating that excitons can be effectively consumed by DDCzTrz.7 As shown in Figure 3a, W2 exhibits a blue emission of 463 nm with the CIE coordinates of (0.18, 0.22), similar to the previous report.58 Additionally, the emission of W2 is almost overlapped with the blue region of W1, further conforming that the blue emission of W1 is directly originated from DDCzTrz and the orange intensity (572 nm) is not generated by DDCzTrz. Besides, the orange peak cannot be attributed to the emission of TAPC or TmPyPB, since i) the PL emission peak of TAPC and TmPyPB are 382 and 359 nm, respectively; ii) no 10 ACS Paragon Plus Environment

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ACS Energy Letters

obvious orange PL emission peak can be generated (Figure S4, Supporting Information). To further verify this effect, EL emissions of pure TAPC and TmPyPB have been examined via the classical structure of ITO/ MoO3 (5 nm)/ TAPC for device W3 or TmPyPB for device W4 (97 nm)/ LiF (1 nm)/ Al (120 nm).67 However, no visible light can be observed for W3 or W4 till they were broken down, indicating that the orange peak in W1 is not produced by TAPC or TmPyPB. Apart from the emitting molecular emission, the interface exciplex emission is another significant source for the efficient emission.68,69 Since the orange emission is not originated from TAPC, DDCzTrz or TmPyPB, it can only be generated from the interface exciplex emission.68,69 Although there are two interfaces (i.e., TAPC/DDCzTrz and DDCzTrz/TmPyPB interface), it is believed that the orange emission is generated from the TAPC/DDCzTrz interface,43 which can be understood as follows. Since the HOMO of TAPC and DDCzTrz are 5.43 and 6.01 eV, respectively, there is a large energy barrier (0.58 eV) for the hole transport, leading to the fact that holes are easily accumulated at the TAPC/DDCzTrz interface. On the other hand, the LUMO of DDCzTrz (2.9 eV) is lower than that of TmPyPB (2.7 eV), resulting in the fact that it is barrier-free when electrons are transported from TmPyPB to DDCzTrz. Hence, electrons are readily accumulated at the TAPC/DDCzTrz interface because of the LUMO barrier between TAPC and DDCzTrz (0.9 eV) together with the fact that TAPC is a ptype material. As a result, TAPC and DDCzTrz can be functioned as the electron-donating and electron-accepting molecules, respectively, generating the exciplex emission at the TAPC/DDCzTrz interface,68,69 as shown in Figure 1c. For the DDCzTrz/TmPyPB interface, electrons are not accumulated owing to the LUMO barrier-free characteristic, leading to the fact the exciplex emission is unfavorable to form at this interface. Besides, since the LUMO of DDCzTrz is lower than that of TmPyPB, holes on DDCzTrz would bound with electrons from DDCzTrz instead of TmPyPB to form excitons,2 recombining for the blue emission.



To provide further evidence that the exciplex emission is generated from the TAPC/DDCzTrz interface, 5 nm mCP has been inserted between TAPC and DDCzTrz to eliminate this interface to fabricate another device (device W5), where other layers of W5 remained the same as those of W1. As shown in Figure 3a, there is no orange emission and only blue emission can be observed in W5, further indicating that the exciplex emission is generated from the TAPC/DDCzTrz interface. This fact can be explained as follows. Although electrons can be accumulated at the mCP/DDCzTrz interface due to the LUMO barrier (0.5 eV), holes would not be accumulated at this interface since the HOMO of mCP (6.1 eV) is deeper than that of DDCzTrz,7 leading to no exciplex emission at the mCP/DDCzTrz interface.30 Besides, since there is a large LUMO barrier between mCP and DDCzTrz together with the fact that mCP is a p-type material,7 electrons are effectively confined in the DDCzTrz EML to furnish the blue emission, resulting in no exciplex emission at the TAPC/mCP interface. To give another direct evidence that the orange emission of W1 is generated from the TAPC/DDCzTrz interface exciplex emission, the PL spectra of TAPC, DDCzTrz, and TAPC: DDCzTrz mixed films have been examined. As shown in Figure 3b, compared with PL spectra of TAPC (382 nm) and DDCzTrz (466 nm), TAPC: DDCzTrz shows a new obviously red-shifted emission with a peak at 553 nm, indicating that TAPC and DDCzTrz form the exciplex excited state.53,68,69 Therefore, the orange emission of W1 is generated from the exciplex emission (553 nm). In a word, the emission nature of W1 is to simultaneously realize the TADF emission (originating from the TADF emitter DDCzTrz) and exciplex emission (generating from the ptype layer TAPC (electron donor) and DDCzTrz (electron acceptor). Besides, both S1 and T1 of the exciplex are lower than those of TAPC and DDCzTrz (e.g., the T1 of TAPC and DDCzTrz are 2.87 and 2.53 eV, respectively54-56,58), which indicates that the excitons of


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ACS Energy Letters

exciplex would not be quenched by the electron donor and acceptor, ensuring the efficient orange emission otherwise the exciton leakage can deteriorate the performance.43,68,69

Figure 3. (a) EL spectra of W1, W2 and W5 at 1000 cd m-2. (b) Photophysical properties of various samples. PL spectra of TAPC, DDCzTrz, and TAPC: DDCzTrz (1: 0.5) films, together with the EL spectrum of W1 at 1000 cd m-2. To provide more insights for the emission nature of p-i-n WOLEDs, DDCzTrz with various thicknesses have been used to develop devices (0.3, 0.8 and 5 nm for device W6, W7 and W8, respectively), where other layers of these devices are the same as those of W1 except for the EML. As shown in Figure S5 and Table 1, the maximum total EQE of W6, W7 and W8 are 13 ACS Paragon Plus Environment


5.5%, 13.9% and 21.1%, respectively, lower than that of W1, indicating that the thickness of DDCzTrz has a great influence on the performance of p-i-n WOLEDs. The CE of W6, W7 and W8 are shown in Figure S6 (Supporting Information). Particularly, W6 exhibits very low efficiency and no white emission at ≥1000 cd m-2. This is because i) when the EML is too thin, the emission zone is very narrow, which easily causes TTA due to a high triplet exciton density;14 and ii) too thin EML cannot effectively harvest excitons, leading to the low efficiency.14 Besides, due to the ultrathin DDCzTrz (electron acceptor), the exciplex emission is not efficient, resulting in the fact that almost only blue emission at high luminance.68,69 When the EML is thick (e.g., 5 nm), the ACQ of DDCzTrz deteriorates the efficiency, indicating that TADF emitter with AIE characteristics can further broaden the flexibility of pi-n WOLEDs.12 Furthermore, all devices (W1, W6, W7 and W8) show blue-shifted color with the increasing luminance, which can be explained as follows. At high electrical field, more holes can overcome the large barrier between TAPC and DDCzTrz to form excitons on DDCzTrz and then excitons will be harvested by DDCzTrz,2 increasing the blue emission. Besides, since fewer holes would be accumulated at the TAPC/DDCzTrz interface, the excitons harnessed by the exciplex are reduced, weakening the orange emission. Therefore, blue-shifted colors have been observed. To further understand the concept that single TADF emitter can exhibit high-quality white emissions, other kinds of single molecular emitters have been used to attempt p-i-n WOLEDs. Since 4,4’-bis(9-ethyl-3-carbazovinylene)-1,1’-biphenyl (BCzVBi) and iridium(III)bis[(4,6difluo-rophenyl)-pyridinato-N,C2] (FIrpic) are the most representative fluorescent and phosphorescent emitters, respectively,57,59 they have been used as the single molecular emitters to explore p-i-n WOLEDs (BCzVBi and FIrpic for device W9 and W10, respectively), where other layers of these devices are the same as those of W1 except for the EML. The energy levels of W9 and W10 are shown in Figure S7 (Supporting Information). 14 ACS Paragon Plus Environment

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ACS Energy Letters

As shown in Figure 4, W9 exhibits a blue emission (452 nm) with the CIE coordinates of (0.17, 0.18), the typical emission of BCzVBi.59 On the other hand, W10 shows a blue emission (474 nm) with the CIE coordinates of (0.18, 0.35), the own emission of FIrpic.57 Therefore, no white emission can be observed from W9 and W10. These results are also consistent with previous reports: OLEDs based on single fluorescent or phosphorescent emitters usually exhibit monochromatic emissions.38 Previously, TADF emitters have been demonstrated to develop high-efficiency OLEDs without noble metal complexes, which is a unique advantage compared with their counterparts (i.e., fluorescent, phosphorescent and hybrid emitters).23,32-34,62,64 Besides, Adachi et al. demonstrated another advantage that simple monochromatic undoped TADF emitters based OLEDs could be as efficient as the best doped OLEDs.42 Herein, we have demonstrated a new unique advantage of TADF emitters that single TADF emitter can be used to develop high-quality WOLEDs. By virtue of this unique advantage, simple yet high-performance TADF WOLEDs can be achieved, which may open a new avenue for the application of TADF materials.



Figure 4. EL spectra of W9 and W10 at 1000 cd m-2. Inset: chemical structures of BCzVBi and FIrpic. In summary, extremely simplified yet high-performance TADF WOLEDs, for the first time, have been developed. Unlike previous concepts, only single TADF emitter is needed for high-quality white emissions by forming doping-free p-i-n WOLED, which is the simplest TADF WOLED. The WOLED can exhibit a CRI of 91, the first WOLED based on single molecular emitter outperforming single-emitter WILEDs and WPLEDs. The maximum EQE, CE and PE are 28.4%, 65.4 cd A-1 and 68.5 lm W-1, respectively. Remarkably, this is the first report that TADF WOLED can possess ultrahigh CRIs at high luminance and 18796 cd m-2 is 370% higher than previous best one. Such results may i) present a novel concept to develop simplified yet high-performance WOLEDs, ii) bridge the gap between WOLEDs and their counterparts, iii) unlock a new unique advantage of TADF materials (i.e., single TADF emitter can be used to develop high-quality WOLEDs). Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX Experimental details, device architectures, current efficiency, CIE coordinates, PL spectra, energy levels, table of performance, and supplementary figures (PDF). AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (B. Liu)

Acknowledgements The authors are grateful to the National Natural Science Foundation of China (Grant No. 61704034 and 51602065), the Key Platforms and Research Projects of Department of 16 ACS Paragon Plus Environment

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Extremely Simplified, High-Performance and Doping-Free White

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