Non-doped phosphorescent organic light-emitting devices with an exciplex forming planar structure for efficiency enhancement

Non-doped phosphorescent organic light-emitting devices with an exciplex forming planar structure for efficiency enhancement

Accepted Manuscript Non-doped phosphorescent organic light-emitting devices with an exciplex forming planar structure for efficiency enhancement Mengg...

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Accepted Manuscript Non-doped phosphorescent organic light-emitting devices with an exciplex forming planar structure for efficiency enhancement Mengge Wu, Zijun Wang, Yufan Liu, Yige Qi, Junsheng Yu PII:

S0143-7208(18)32531-2

DOI:

https://doi.org/10.1016/j.dyepig.2019.01.020

Reference:

DYPI 7293

To appear in:

Dyes and Pigments

Received Date: 19 November 2018 Revised Date:

13 December 2018

Accepted Date: 14 January 2019

Please cite this article as: Wu M, Wang Z, Liu Y, Qi Y, Yu J, Non-doped phosphorescent organic light-emitting devices with an exciplex forming planar structure for efficiency enhancement, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.01.020. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Non-doped phosphorescent organic light-emitting devices with an exciplex forming planar structure for efficiency enhancement Mengge Wu, Zijun Wang, Yufan Liu, Yige Qi, Junsheng Yu*

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State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and

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Email: [email protected] (Mengge Wu)

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Technology of China (UESTC), Chengdu 610054, PR China.

[email protected] (Zijun Wang) [email protected] (Yufan Liu) [email protected] (Yige Qi)

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[email protected] (Junsheng Yu)

*

Corresponding author. E-mail: [email protected] Tel: 86-28-83207157. 1

ACCEPTED MANUSCRIPT Abstract

Organic light-emitting devices (OLEDs) consisting of a non-doped phosphorescent dye that inserted in an exciplex forming planar structure have been fabricated, and the

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thickness of ultrathin phosphor layer is optimized to achieve high efficiency. The results showed that OLEDs based on the exciplex interface and a 0.5 nm thick phosphorescent dye have a power efficiency, a current efficiency, and an external quantum efficiency of

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37.4 lm/W, 40.5 cd/A, and 14.3%, respectively, which are almost two-folds higher than those with non-exciplex interface devices. Meanwhile, the efficiency roll-off is

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significantly suppressed. These improved device performances are attributed to the elimination of triplet energy leakage from exciplexes to constituting molecules, efficient energy up conversion of triplet exciplexes and complete host-guest Förster energy transfer. These results will provide an easily-fabricated and time-saving approach for

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high-performance OLEDs.

Keywords: Organic light-emitting device (OLED); planar structure; phosphorescent dye;

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non-doped; exciplex

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ACCEPTED MANUSCRIPT 1. Introduction Organic light-emitting devices (OLEDs) have been studied intensively to accelerate the industrialization of full-color flat-panel displays and solid-state lighting, attributed to

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their advantages such as ultrathin structure, wide viewing angle, large area emission and compatibility with flexible substrates [1-3]. Fluorescent OLEDs have been paid extensive attention in terms of high reliability and low cost. However, the upper limit of internal

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quantum efficiency (IQE) is only 25%, which is an inevitable obstacle to achieving high

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efficiency [4]. Afterwards, phosphorescent OLEDs (PHOLEDs) become research hotspots, where 100% IQE can be theoretically obtained. The strong spin-orbit coupling of electronic states induced by the heavy metal-atom effect can promote the probability of inter-system crossing (ISC) [5]. Consequently, phosphorescent dye can harvest both

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singlet and triplet energies for emission via the radiative recombination of triplet excitons [6]. Generally, phosphorescent dyes are used as the dopants and dispersed into the host matrix, alleviating the concentration quenching effect caused by the intermolecular

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interactions of phosphors [7]. Meanwhile, the dopant concentrations are usually in the

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range of 5-10 wt%, aiming at realizing the efficient host-guest energy transfer of triplet excitons via short-radius Dexter type. The distance between host and dopant molecules cannot exceed the radius of Dexter energy transfer, otherwise the triplet excitons on host will diffuse to the host molecules near dopants rather than dopants, causing the triplet excitons energy loss via non-radiative recombination way [1]. In this host-guest luminous system, host molecules are the major charge transport channels and recombination centers, therefore, the properties of host materials are critical for the performance of 3

ACCEPTED MANUSCRIPT PHOLEDs [8]. Thermally activated delayed fluorescence (TADF) emitters and exciplexes are well known for the soar of IQE from 25% to 100% [9-11], and they forming co-hosts have

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been attempted as promising alternatives to replace conventional fluorescence dyes as host [12-15]. As well known, exciplexes are intermolecular charge-transfer excited-state complexes between electron donors (D) and acceptors (A), which are generally

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corresponding to hole and electron transport materials, respectively [16]. The

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intrinsically small split between singlet and triplet enable triplet excitons can up-convert to singlet effectively via reverse inter-system crossing (RISC) process [9]. Consequently, except for Dexter energy transfer, the efficient up conversion can provide another way to alleviate triplet energy loss significantly [17]. Although D:A mixed co-hosts have been

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widely employed in PHOLEDs [18,19], the accurate control and rapid modulation in physical vapor co-deposition rate of D:A co-hosts and phosphors are challenging [20,21]. At the same time, the electroluminescent (EL) performances of PHOLEDs are very

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sensitive to dopant concentration, which requires enough precision and time-saving of

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deposition equipment for maintaining high repeatability in large-scale industrial production.

In order to shorten the deposition time, the complicated host-guest luminous

systems with dopants dispersed in D:A co-hosts should be simplified. According to the atomic force microscopy images, ultrathin non-doped phosphorescent dye layer occupies the concave and convex sites of bottom layer surface [22]. When the discontinuous phosphorescent layer is embedded in D/A planar structure, the D/A interfacial 4

ACCEPTED MANUSCRIPT intermolecular interactions still exist and they can form interface excitons [23]. Consequently, this system with phosphorescent dye layer sandwiched between D/A co-hosts avoid doping processes, which results in not only easily fabricated and

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cost-effective, but also high-efficiency potentially. Furthermore, fundamental researches about the above host-guest systems have not been intensively carried out, such as the EL mechanisms, or their effects on the efficiency, efficiency roll-off of PHOLEDs and so on.

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In this work, easily-fabricated PHOLEDs with non-doped phosphorescent dye layer

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have been obtained. The donor of 4,4′,4′′-tris(N-carbazolyl)-triphenylamine (TCTA) and the acceptor of 1,3,5-tris(N-phenylbenzimidazole-2-yl) benzene (TPBi) are introduced to constitute the exciplex forming planar structure in this PHOLEDs system, where Bis(4-tert-butyl-2-phenylbenzothiozolato-N,C2′)iridium(III)

(acetylacetonate)

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((tbt)2Ir(acac)) is inserted in them as phosphorescent dye. And PHOLEDs with non-exciplex forming structures are fabricated for comparison. The obtained PHOLEDs can achieve 2-folds higher efficiencies and lower efficiency roll-off than those of

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comparison devices. Meanwhile, the EL mechanisms are systematically investigated to

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reveal the origins of the improved performance and the hindered efficiency roll-off.

2. Experimental 2.1 Methods

Indium tin oxide (ITO) anode coated glass substrates with a sheet resistance of 15 Ω/sq were cleaned in detergent, acetone, deionized water and isopropyl alcohol for 15 min at each ultrasonic step. They were subsequently dried with nitrogen gas flow and treated by oxygen plasma to further clean the ITO surface. The organic functional layers 5

ACCEPTED MANUSCRIPT and metallic cathode were thermally evaporated in separate vacuum chambers under pressures of 3×10-4 Pa and 3×10-3 Pa, respectively. Meanwhile, deposition rate and layer thickness were in situ monitored by a quartz crystal oscillator. The whole devices have an

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active area of 0.3 cm2, and the device configurations are as follows: ITO/ molybdenum trioxide (MoO3) (5 nm)/ 4,4'-bis(N-(1-naphthyl)-N-phenylamino) biphenyl (NPB) (50 nm)/(tbt)2Ir(acac) (X nm)/TPBi (10 nm)/ 4,7-diphenyl-1, 10-phenanthroline (Bphen) (40

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nm)/Mg: Ag (100 nm) for A-series devices and ITO/MoO3 (5 nm)/NPB (40 nm)/TCTA

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(10 nm)/(tbt)2Ir(acac) (X nm)/TPBi (10 nm)/Bphen (40 nm)/Mg: Ag (100 nm) for

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B-series devices.

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Fig. 1. Energy level diagram of exciplex-based PHOLEDs.

The energy level diagrams of PHOLEDs are depicted in Fig. 1. Therein, MoO3 was

adopted as a hole injection layer, while NPB and Bphen were used as hole and electron transport layers, respectively [24,25]. The hole mobility of NPB is equal to the electron mobility of Bphen on the order of 10-4 cm2/Vs, which is beneficial to charge transport balance [26,27]. The differences in structures between A- and B-series devices were that, the thickness of NPB layer was decreased from 50 to 40 nm, meanwhile, a 10 6

ACCEPTED MANUSCRIPT nm-thickness TCTA layer was deposited on the NPB layer in B-series devices. TCTA was introduced as an electron and exciton blocking layer, with the shallow lowest unoccupied molecular orbital (LUMO) level of 2.3 eV and the triplet state of the first

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excited state (T1) of 2.8 eV [28]. TPBi was employed as a hole and exciton blocking layer, with the deep highest occupied molecular orbital (HOMO) level of 6.2 eV and the high T1 of 2.7 eV. Moreover, TCTA and TPBi were used as the electron donor and

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acceptor to form exciplex, respectively. The phosphor (tbt)2Ir(acac) was adopted as

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emitters, where X represented 0.1, 0.5 and 1 nm for devices A1-A3 and B1-B3. It is noteworthy that, the T1 of (tbt)2Ir(acac) is 2.2 eV, which is lower than those of TCTA and TPBi to forbid triplet energy leakage from phosphorescent layer to neighboring layers [29,30].

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2.2 Device Characterization

The ultraviolet-visible (UV-Vis) absorption spectra were collected with a SHIMATZU UV-1700 spectrophotometer. The photoluminescence (PL) spectra were

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characterized with a PerkinElmer LS55 spectrometer at an excitation wavelength of 325

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nm. The EL spectra were recorded with an OPT-2000 spectrometer. The PL transient decay characteristics were measured with a HORIBA Scientific Single Photon Counting Controller FluoroHub-B, integrating a NanoLED-370 excitation light source and a TBX photon detector. The current density-voltage-luminance (J-V-L) characteristics were tested with a Keithley 4200 source and a ST-86LA luminance meter. All the measurements were performed in air at room temperature without encapsulation, except for PL transient decay curves recorded under nitrogen atmosphere. 7

ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Optical properties of the films The UV-Vis absorption and PL spectra of organic functional materials are depicted

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in Fig. 2. The molar ratios of both NPB: TPBi and TCTA: TPBi blend films are 1:1. It can be seen clearly in Figs. 2(a) and 2(b) that, the absorption spectra of NPB: TPBi and TCTA: TPBi are nearly a simple superposition of the absorption bands of individual

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component, indicating that there is no change in the ground states of NPB: TPBi and

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TCTA: TPBi blends. As shown in Fig. 2(c), the emission band of NPB: TPBi blend film peaked at approximately 440 nm are almost overlapped with that of NPB neat film. Therefore, the PL spectrum of NPB: TPBi only exhibits the intrinsic emission of NPB, indicating that the intermolecular interactions between NPB and TPBi are not effective to

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form exciplexes. Fig. 2(c) also exhibits the emission band of TCTA: TPBi blend film, which is red-shift and broad compared to the intrinsic ones of TCTA and TPBi, and should be ascribed to TCTA: TPBi interface exciplex [31]. As well known, the energy of

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exciplex emission peak (Eexciplex) is linearly correlated to the energy difference (ID-AA)

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between the ionization potential of electron donor (ID) and the electron affinity of electron acceptor (AA), which can be empirically described as follows: Eexciplex = (I D - AA ) + Constant

(1)

where the absolute value of constant is reported to be ≤0.4 eV [32,33]. Since the emission peak of the TCTA: TPBi at 436 nm, suggesting that the energy of their exciplex is 2.8 eV, which is 0.2 eV lower than the energy difference value of 3 eV between the ID of TCTA and the AA of TPBi. According to Eq. (1), it is apparent that the emission band 8

ACCEPTED MANUSCRIPT peaked at 436 nm originates from TCTA: TPBi interfacial exciplex. The UV-Vis absorption spectrum of (tbt)2Ir(acac) film is depicted in Fig. 2(c), in which the absorption peaks from 300 to 350 nm are assigned to ligand-centered π-π*

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transitions, meanwhile, the low-energy absorption peaks located at 450-490 nm originate from singlet and triplet metal to ligand charge transfer (1MLCT and 3MLCT) transitions, respectively. It is well known that, the strength of host-guest energy transfer closely

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depends on the spectral overlap between the emission band of host and the absorption

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band of guest [34]. Besides, the PL spectrum of (tbt)2Ir(acac) is also described in Fig. 2(c), in which a maximum at 573 nm and a shoulder at 600 nm originate from the vibronic levels 0-0 and 0-1 electronic transitions between T1 and singlet ground

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state, respectively [35,36].

Fig. 2. UV-Vis absorption and PL spectra of (a) NPB and TPBi pristine and blend films along with (b) TCTA and TPBi pristine and blend films. (c) UV-Vis absorption and PL spectra of (tbt)2Ir(acac) film, 9

ACCEPTED MANUSCRIPT along with PL spectra of blend films.

3.2 Electrical properties of devices There are two primary EL mechanisms in the PHOLEDs based on phosphorescent

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dyes, which are host-guest energy transfer and direct exciton formation on phosphors by charge trapping. As for direct exciton formation, phosphorescent dyes act as the charge trapping centers attributed to the deeper LUMO and/or shallower HOMO levels than

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those of neighboring hosts. The amount of trapped charge carriers increases along with

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the thickness of phosphorescent layer, leading to the decrease of current density at a constant voltage. Therefore, there exists an apparent dependence of J-V curves on the thickness of phosphorescent layer [37]. In a host-guest luminous system dominated by energy transfer, hosts become the major charge transport channels and recombination

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centers, and the charge trapping effect on phosphorescent dyes is negligible. As a result, the J-V curves are almost independent on the thickness of phosphorescent layer [38]. Fig. 3(a) exhibits the J-V characteristics of devices A1-A3. It can be seen that, the current

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density at a constant voltage decreases with increasing the thickness of phosphorescent

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layer, revealing that direct exciton formation by charge trapping is dominant in A-series devices. However, as shown in Fig. 3(b), the J-V curves of devices B1-B3 exhibit the independence on the thickness of phosphorescent layer, indicating that the host-guest energy transfer dominates the EL mechanism. It is interesting that, the power efficiency-current density-current efficiency (PE-J-CE) characteristics of both A- and B-series devices exhibit a similar variation tendency with increasing the thickness of phosphorescent layer in Figs. 3(c) and 3(d), 10

ACCEPTED MANUSCRIPT respectively. The efficiencies of PHOLEDs based on a 0.5 nm-thickness phosphorescent layer are the highest at a constant current density. Since the EL mechanism is probably dominated by direct exciton formation in A-series devices, the highly efficient device A2

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with a moderately thick phosphorescent layer are attributed to the trade-off between sufficient emitting centers and alleviated concentration quenching [39-41]. Meanwhile, since host-guest energy transfer s likely dominant in B-series devices, there also exists a

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trade-off between efficient exciplex formation and complete energy transfer from

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exciplexes to phosphors, so that device B2 is highly efficient. Specifically speaking, on one hand, there is efficient exciplex formation at the TCTA/TPBi interface of device B1 with a 0.1 nm-thickness phosphorescent layer, but the energy transfer is incomplete from exciplexes to (tbt)2Ir(acac) molecules, which will be verified by the EL spectra of device

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B1 later. As well known, TADF emitters are generally not efficient due to low radiative decay rates [9]. Hence, the emission of TADF exciplexes results in the low efficiencies of device B1. On the other hand, there is complete host-guest energy transfer in device B3.

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However, a 1 nm-thickness phosphorescent layer becomes an obstacle to TCTA/TPBi

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interfacial molecular interactions for exciplex formation, which leads to the low efficiency of device B3.

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Fig. 3. J-V-L characteristics of (a) A- and (b) B-series devices. PE-J-CE characteristics of (c) A- and (d) B-series devices.

3.3 Electrical properties of single carrier devices

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To confirm the dominant EL mechanism in A- and B-series devices, the J-V characteristics of single charge carrier devices and PL transient decay characteristics are

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measured and analyzed. The configurations of hole- and electron-only devices in terms of device A2 are ITO/NPB (50 nm)/(tbt)2Ir(acac) (0 or 0.5 nm)/TPBi (10 nm)/NPB (40

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nm)/Ag and Mg: Ag/Bphen (40 nm)/NPB (10 nm)/(tbt)2Ir(acac) (0 or 0.5 nm)/TPBi (10 nm)/Bphen (40 nm)/Mg: Ag, respectively. It can be seen in Fig. 4(a) that, the current density of hole-only devices substantially decreases at a constant voltage, as a (tbt)2Ir(acac) layer is embedded between NPB and TPBi. Meanwhile, the J-V curves of electron-only devices with and without (tbt)2Ir(acac) are almost overlapped. Consequently, (tbt)2Ir(acac) molecules play the role of hole trapping centers in A-series devices, due to the shallower HOMO level than that of neighboring NPB. The EL 12

ACCEPTED MANUSCRIPT mechanism is dominated by direct exciton formation. Fig. 4(b) shows the PL transient decay characteristics of TCTA: TPBi blend films with the increasing concentration of (tbt)2Ir(acac), in which the transient PL intensities are observed at 440 nm to characterize

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the delayed emission of TCTA: TPBi exciplex. As can be seen, the lifetime of the delayed component of TCTA: TPBi exciplex reduces successively, as (tbt)2Ir(acac) is doped in TCTA: TPBi co-hosts with the gradually increased doping concentration. Since

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triplet excitons are converted from photo-excited singlet ones via ISC. Meanwhile, the

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rate constant of Förster energy transfer is approximately 1010 s-1, which is two orders of magnitude faster than that of ISC (<108 s-1) [42]. The gradually shortened lifetime of delayed component is attributed to the more efficient host-guest Förster energy transfer, which suppresses ISC and decreases the population of triplet exciplexes [16,22].

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Therefore, there is efficient Förster energy transfer from TCTA: TPBi exciplexes to (tbt)2Ir(acac) molecules, and the dominant EL mechanism in B-series devices is host-guest energy transfer.

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According to the above analyses, the EL process of A- and B-series devices can be

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schematically described in Figs. 4(c) and 4(d). In A-series devices dominated by direct exciton formation, holes are transported on the HOMO of NPB and subsequently trapped by (tbt)2Ir(acac) molecules. As electrons are injected from TPBi into (tbt)2Ir(acac), excitons are formed by charge recombination, and undergo ISC process followed by radiative decay to generate light. In B-series devices dominated by host-guest energy transfer, holes and electrons mainly accumulate at the interface of TCTA/TPBi, due to the large charge injection barriers. Afterwards, interface exciplexes are effectively formed by 13

ACCEPTED MANUSCRIPT interfacial molecular interactions between TCTA and TPBi, i.e., columbic attraction between ions. Finally, singlet and triplet energy of exciplexes are transferred to

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(tbt)2Ir(acac) molecules through Förster and Dexter types, respectively [43,44].

Fig. 4. (a) J-V curves of hole- and electron-only devices with and without a (tbt)2Ir(acac) layer in

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device A2. (b) Experimental and simulated PL transient decay characteristics of TCTA: TPBi exciplex with and without (tbt)2Ir(acac) in the blend film. (c) and (d) are Schematic diagram of emission

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processes in A- and B-series devices.

3.4 EL properties of PHOLEDs The EL parameters are summarized in Table 1. It can be seen that, the efficiencies of

B-series devices are much higher than those of A-series devices. EQE is calculated by Eq. (2) as following:

EQE =

N photon N electron

=

I (λ ) ⋅ λ dλ 380 683 ⋅ hc Ie e

π ⋅ A⋅ ∫

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780

(2)

ACCEPTED MANUSCRIPT where Nphoton and Nelectron are the number of photons emitted externally generated and electrons injected, respectively. λ (nm) is the wavelength, and I(λ) is the relative EL intensity at each wavelength. h is the Planck constant, and c is the velocity of light. Ie is

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the current and e is the electron charge. Specifically, a power efficiency (PE) of 37.4 lm/W, a current efficiency (CE) of 40.5 cd/A and an external quantum efficiency (EQE) of 14.3% have been obtained for device B2 with a 0.5 nm-thickness phosphorescent layer.

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However, the highest PE, CE and EQE of A-series devices observably drop to 17.7 lm/W,

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21.3 cd/A and 7.9%, respectively, which are nearly half of those of device B2. Apparently, there is still some room for boosting EQE to 20%. For example, optimizing the interface morphology of TCTA and TPBi to obtain more excitons, constructing more efficient exciplex forming planar structure and selecting more suitable luminous dyes to improve

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the exciton utilization rate and radiative recombination efficiency. Table 1 EL characteristics of A- and B-series devices. Von (V)

Lmax (cd/m2)

PEmax (lm/W)

CEmax (cd/A)

[email protected] cd/m2 (cd/A)

EQEmax (%)

[email protected] cd/m2 (%)

A1

2.9

15700

18.1

17.9

8.0

6.7

3.0

A2

3.0

19400

17.7

21.3

14.4

7.9

5.4

A3

2.9

14400

17.6

17.7

8.1

6.6

3.1

B1

3.4

19300

30.9

31.4

20.0

10.3

6.7

B2

3.6

21000

37.4

40.5

33.1

14.3

11.5

B3

3.7

22300

26.5

29.5

21.8

11.0

8.2

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The EQE-Luminance curves along with EQE roll-off of devices A2 and B2 are

compared in Fig. 5 and its Inset, respectively. It is noteworthy that, except for the much higher EQE of device B2 at a constant luminance, the EQE roll-off of device B2 is also alleviated significantly in comparison with that of device A2 at a practical luminance of

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ACCEPTED MANUSCRIPT 1000 cd/m2. Meanwhile, device B2 exhibits a lower efficiency roll-off even at a high luminance of 10000 cd/m2. B-series devices with enhanced efficiency and decreased efficiency roll-off are beneficial from the following critical factors: (I) Triplet energy

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leakage from exciplexes to constituting molecules is effectively eliminated by TCTA and TPBi with high-lying T1. Meanwhile, triplet excitons are efficiently up-converted to singlet ones by RISC, which substantially reduces the triplet energy loss through the

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non-radiative triplet states of exciplex. (II) It is reported that, exciton aggregation in a

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phosphorescent layer can be alleviated by host-guest energy transfer [45]. Moreover, except for Dexter energy transfer, efficient Förster energy transfer suppresses ISC to decrease triplet population and density, which dramatically alleviates Triplet-Triplet

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Annihilation on exciplexes.

Fig. 5. EQE-Luminance characteristics of devices A2 and B2. Inset: EQE roll-off of devices A2 and B2.

Since B-series devices achieve relatively superior performance, we further characterize the EL spectra versus luminance in Fig. 6(a). It can be seen that, the EL peaks of B1 devices are 417, 559, and 595 nm, respectively. Among them, 417 nm originates from the emission of TCTA/TPBi exciplex, while 559 and 595 nm are from 16

ACCEPTED MANUSCRIPT (tbt)2Ir(acac). There are more exciplexes formed at the TCTA/TPBi interface with increasing the amount of injected charges, meanwhile, the exciton accommodation sites provided by a 0.1 nm-thickness (tbt)2Ir(acac) phosphorescent dye easily become

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saturated. Both of these factors result in incomplete host-guest energy transfer and the increasing amount of exciplexes released from the TCTA/TPBi interface. However, (tbt)2Ir(acac) molecules provide sufficient exciton accommodation sites in devices B2

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and B3 with the thickness of phosphorescent layer above 0.5 nm, which are shown in

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Figs. 6(b) and 6(c). Consequently, there exists complete energy transfer, and the emission composition proportion of exciplex becomes negligible. According to the EL spectra of B-series devices, the schematic diagram of EL mechanism is described in Fig. 4(d). There are two paths, i.e., Dexter energy transfer and up conversion followed by Förster

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energy transfer to harvest the triplet energy of exciplexes for phosphorescence. In device B1, except for the phosphorescence of (tbt)2Ir(acac), there still exist the prompt and delayed fluorescence of TCTA/TPBi exciplex. In devices B2 and B3, there is only the

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emission of (tbt)2Ir(acac) phosphorescent dye due to efficient host-guest energy transfer.

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Fig. 6. (a), (b) and (c) EL spectra versus luminance of B-series devices with the increasing thickness of (tbt)2Ir(acac).

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4. Conclusions

In conclusion, we have demonstrated easily-fabricated PHOLEDs with a (tbt)2Ir(acac) phosphorescent dye embedded in an TCTA/TPBi exciplex forming interface.

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The best PHOLEDs have a PE of 37.4 lm/W, a CE of 40.5 cd/A and an EQE of 14.3%.

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The EL mechanism is dominated by direct exciton formation via host-guest energy transfer. Furthermore, the EQE roll-off is also substantially reduced in comparison with those of non-exciplex devices. The excellent performance of exciplex based PHOLEDs are beneficial from the high-lying T1 of TCTA and TPBi, efficient up conversion of triplet excitons and complete host-guest Förster energy transfer. This work gains an insight into the EL mechanisms of OLEDs based on the non-doped phosphorescent dye, and paves a way for easily fabricated, time-saving, cost-effective and high-performance 18

ACCEPTED MANUSCRIPT devices. Funding

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This work was financially supported by the Foundation of the National Natural Science Foundation of China (Nos. 61675041, 61421002, and 51703019) and the National Key R&D Program of China (Grant No. 2018YFB0407100-02).

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Competing Interests

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The authors declare that they have no competing interests.

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ACCEPTED MANUSCRIPT Acknowledgements Mengge Wu designed and carried out the experiments. Mengge Wu and Zijun Wang participated in the work to analyze the data and prepared the manuscript initially. Yufan

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All authors read and approved the final manuscript.

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Liu and Yige Qi fabricated devices and characterized. Junsheng Yu supervised this work.

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