High-performance Exciplex-type Host for Multicolor Phosphorescent

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High-performance Exciplex-type Host for Multicolor Phosphorescent Organic Light Emitting Diodes with Low Turn-on Voltages Linlin Jia, Lu Jin, Kai Yuan, Lingfeng Chen, Jie Yuan, Shen Xu, Wenzhen Lv, and Runfeng Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01155 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 19, 2018

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ACS Sustainable Chemistry & Engineering

High-performance Exciplex-type Host for Multicolor Phosphorescent Organic Light Emitting Diodes with Low Turn-on Voltages Linlin Jia, Lu Jin, Kai Yuan, Lingfeng Chen, Jie Yuan, Shen Xu,

Wenzhen Lv, Runfeng Chen*

Key Laboratory for Organic Electronics and Information Displays & Institute of Advanced Materials, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing 210023, China.

E-mail: [email protected]

KEYWORDS: Exciplex emission, Host material, Exciplex formation mechanism, PhOLEDs, TADF

ABSTRACT: Rational design and selection of suitable donor and acceptor components for optimal thermally activated delayed fluorescence (TADF) exciplex-type emitters or hosts is presently challenging. Here, we constructed successfully a blue-emitting bulk exciplex system with efficient TADF emission and high

triplet

energy

(ET)

based

[3-methylphenyl(phenyl)amino]triphenylamine

on

a and

donor an

of

acceptor

4,4’,4’’-tris of

1,3,5-tri

(m-pyrid-3-yl-phenyl)benzene. Systematic experimental and theoretical studies show

ACS Paragon Plus Environment

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that the matched frontier orbital energy levels, high ET, facile intersystem crossing, high oscillator strength of the exciplex, and efficient energy transfer channels should be the main considerations during the design of high-performance exciplex-type TADF emitters and bipolar host materials. Therefore, this bulk exciplex system can behaviour not only as blue emitters for organic light emitting diodes (OLEDs), but also as universal hosts for the green, yellow, and red phosphorescent OLEDs (PhOLEDs). Impressively, even under a very low guest doping level of 2 wt%, the PhOLEDs exhibit very low turn-on voltages (~2.2 V) and high maximum external quantum efficiencies up to 18.5%. These promising device results, along with the theoretical understandings, could shed important light on the rational design of exciplex systems and their applications as either TADF emitters or bipolar host materials for high-performance and low-cost OLEDs.

Introduction Phosphorescent organic light emitting diodes (PhOLEDs) have attracted massive attention due to their broad application prospect in the solid-state lighting and flat-panel display market with theoretically 100% internal quantum efficiency by harvesting both singlet and triplet excitons for electroluminescence.1-3 However, host materials are generally required in PhOLEDs to disperse the long-lived phosphorescence emitters to alleviate the concentration and triplet state-involved quenching effects.4-6 To electrically drive the host-guest PhOLED system, the host


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materials should be capable of supporting efficient and balanced hole and electron injection and transportation during the operation of the devices, confining triplet excitons on phosphor guests for high luminescence efficiencies and possessing high thermal stability for long-term device operation at elevated temperature.7, 8 Various polymer,9 dendrimer,10 bipolar,11 self-adaptive,12 thermally activated delayed fluorescence (TADF),13 and high singlet exciton formation14 host materials with appropriate frontier orbital energy levels, high carrier mobility, high triplet state energy (ET), and stable molecular structures have been elaborately designed and investigated to develop high-performance host materials for PhOLEDs.

Recently, it has been demonstrated that host materials with small singlet-triplet energy splitting (∆EST) can significantly reduce the operation voltage and improve the external quantum efficiency (EQE) of the PhOLEDs.15-18 For instance, using single-molecule TADF host materials with small ∆EST to change the energy transfer model from the short-range Dexter transfer to the long-range Förster transfer, the TADF-hosted PhOLEDs have shown high efficiency, low efficiency roll-off and long lifetime at low dopant concentration and low driving voltage.19-22 Nevertheless, the design of TADF host with both high ET and small ∆EST is nontrivial, which requires careful selection and combination of the donor and acceptor moieties for ingenious separation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) within one framework to an appropriate extent to maintain simultaneously the high ET and suitable HOMO and LUMO energy levels. Alternatively, exciplex systems formed intermolecularly between donor and acceptor



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molecules were also found to give TADF emission with intrinsically smaller ∆EST values and more efficient reverse intersystem crossing (RISC) process compared to the single-molecule TADF compounds.23-25 Moreover, the exciplex-type TADF materials containing both hole-transportable donor and electron-transportable acceptor components possess an inherent bipolar characteristic, which would facilitate the charge transport balance in the emission layer and minimize the efficiency roll-off at high luminance.16 Therefore, PhOLEDs hosted by either bulk exciplexes formed by blending donor and acceptor molecules or interface exciplexes formed at the interface of donor and acceptor layers, have shown high device performance with EQEs above 25%.26, 27 And, compared to the TADF host molecules, the multi-component TADF exciplexes can benefit from a widely available donors and acceptors already developed to support also the efficient long-range Förster energy transfer for a significant concentration reduction of the noble metal-based phosphors, resulting low cost PhOLEDs in both host and guest materials.28 However, despite the spectacular progress of exciplex-type materials for optoelectronic applications, it is still a challenge to precisely predict the likelihood of a particular pair of molecules giving rise to exciplex emission as well as high ET for either TADF OLEDs or host materials of PhOLEDs.

In

this

work,

4,4’,4’’-tris[3-methylphenyl

widely

used

hole-transporting

(phenyl)amino]triphenylamine

material

of

(m-MTDATA)

and

electron-transporting material of 1,3,5-tri [(3-pyridyl)-phen-3-yl]benzene (TmPyPB) were selected as donor and accept molecules respectively to construct a bulk exciplex


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system. Blue exciplex emission from m-MTDATA/ TmPyPB was observed in both photoluminescence (PL) and electroluminescence (EL), when their weight proportion were controlled to be 70:30 in solid film. High ET of the novel m-MTDATA/TmPyPB bulk exciplex system was also identified, which qualifies them as host materials of PhOLEDs. The yellow device shows an ultralow turn-on voltage of 2.5 V and a high EQE of 18.5%, which is among the best results of bis(2-phenylbenzothiazolato) (acetylacetonate)iridium(III) (Ir(bt)2(acac)) based PhOLEDs. Also, the green, red, and white PhOLEDs hosted by the exciplex all exhibit low turn-on voltages (10%), demonstrating clearly the high performance of this pair of molecules for exciplex-type host materials (Fig. S1). Theoretical investigation was further performed to understand the exciplex formation mechanism based on this efficient exciplex system. It was found that the matched HOMO of m-MTDATA and LUMO of TmPyPB, high ET of both m-MTDATA and TmPyPB, high oscillator strength of their exciplex for emission, and suitable energy transfer channels to efficiently excite the doped Ir(III) complexes should be the main reasons for their high device performance. This work would offer valuable clues on the rational design of exciplex systems and their applications as host materials of PhOLEDs with high efficiency, low driving voltage, low roll-off, and low cost as well.

Results and discussion

Molecular structures of the donor (m-MTDATA) and acceptor (TmPyPB) used in constructing the exciplex system are shown in Fig. 1a. From the frontier orbital energy levels (Fig. 1b), the energy difference between the HOMOs of m-MTDATA ACS Paragon Plus Environment


and TmPyPB is as high as 1.6 eV, which can effectively block the hole transport from m-MTDATA to TmPyPB; meanwhile, the large difference between their LUMO energy levels hinders significantly the electron transport from TmPyPB to m-MTDATA.12, 29 Therefore, the excitons can be generated efficiently at the interface of the m-MTDATA (donor) and TmPyPB (acceptor) to form exciplexes. To experimentally verify the formation of exciplex, photoluminescence (PL) spectrum of the m-MTDATA/TmPyPB blended (50:50, wt/wt) film was measured and found to be significantly red-shifted and broadened with slightly higher PL quantum efficiency (PLQY) in comparison with that of pure m-MTDATA and TmPyPB films (Fig. 2a and Table S1). Moreover, the time-resolved PL measurements also show apparently elongated lifetime (73.4 ns) of the m-MTDATA/TmPyPB blended film (Fig. S2). The red-shifted broad PL spectrum of the blend with increased lifetime is coincident with the typical features of exciplex emission, confirming the successful establishment of exciplex system using m-MTDATA and TmPyPB.30


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Fig. 1. (a) Molecular structures and (b) LUMO and HOMO energy levels of m-MTDATA and TmPyPB functioned as donor and acceptor molecules in exciplex-type host materials for yellow, green, and red PhOLEDs using Ir(ppy)3, Ir(bt)2(acac) and Ir(piq)2(acac), respectively.

To probe the exciplex formation mechanism, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations of the dimer of m-MTDATA and TmPyPB were performed (Fig. 2b, c). As a preliminary attempt, these calculations are based on the DFT optimized geometry of the dimer; although the practical molecular packing structures in the amorphous emitting layer of PhOLEDs is very complicated, this optimal dimer structure could also give valuable clues on the basic understandings of exciplex formations and functions. A short inter-atom distance about 0.3 nm was observed in the dimer, suggesting a strong intermolecular



interaction between m-MTDATA and TmPyPB.31 This strong interaction maybe an important reason for the strong exciplex emission, which is also found in the TD-DFT simulated PL spectra with red-shifted PL and strong oscillator strength of the m-MTDATA/TmPyPB dimer (Fig. 2b). Due to the electron-donating nature of m-MTDATA, HOMO of the dimer is mainly distributed on the moiety of m-MTDATA, while the electron-accepting TmPyPB dominates LUMO of the dimer, leading to separated HOMO and LUMO (Fig. 2c) for the formation of charge-transfer singlet (1CT) and triplet (3CT) excited states.32 From the fluorescent and phosphorescent PL (Fig. S3) peaks of the exciplex, the energy levels of 1CT and 3CT were identified to be 2.58 and 2.54 eV at a weight ratio 70:30 of m-MTDATA and TmPyPB, respectively. Therefore, the singlet-triplet splitting (∆EST) of the exciplex is only 0.04 eV, confirming the formation of the intermolecular TADF system using m-MTDATA and TmPyPB.33 As to the possible intersystem crossing channels, electron density differences (EDD) between the ground state (S0) and the singlet and triplet states were computed by TD-DFT calculations with the aid of Multiwfn (Fig. S4).34 The similar EDD isosurfaces of S1 and T3, T4 and T5 (Fig. 2d) suggests enhanced spin-orbit coupling through these channels for facilitated intersystem crossing processes. Moreover, S1, S2, and S3 have similar EDD isosurface and close energy levels to support also facile intersystem crossing through Sm (m=1, 2, and 3) →Tn (n=3, 4, and 5), and T1 to T5 are also close in energy, which means that these exciplex-type excitons can change from T1 to Tn (n=2, 3, 4, and 5) easily with the aid of environmental thermal energy. Such energy and spin orbital features of the singlet


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and triplet excited states of the m-MTDATA/TmPyPB exciplex should be important to provide massive intersystem crossing (ISC) and RISC channels for this intermolecular TADF system.33, 35

Fig. 2. (a) Normalized experimental photoluminescence (PL) spectra of m-MTDATA, TmPyPB and m-MTDATA/TmPyPB (50:50, wt/wt) bulk exciplex films excited at 320 nm, (b) theoretical emission spectra of m-MTDATA, TmPyPB and their dimer, (c) theoretical

HOMO

and

LUMO

energy

levels

and

their

isosurfaces

of

m-MTDATA/TmPyPB dimer, (d) TD-DFT calculated excited state energies and their electron density differences (EDD) upon excitation transitions from the ground state (S0), and (e) energy diagram showing the energy transfer processes in the m-MTDATA/TmPyPB (70:30) exciplex-type host.



It has been proposed that high-T1 constituting molecules are preferable for designing exciplex emitters with high PLQY, because the high T1 can prevent energy leakage and enable efficient RISC.36 From Fig. 2e, both m-MTDATA and TmPyPB show high T1 with ET of 2.61 and 2.78 eV, respectively; their exciplex has lower T1 at 2.54 eV. Therefore, internal conversion (IC) and ISC can populate facilely the low-lying 1CT and 3CT, while efficient RISC from 3CT to 1CT supports the efficient formation of TADF exciton for emission in TADF OLEDs or Förster energy transfer (FRET) to the guest in PhOLEDs. PLQYs of m-MTDATA/TmPyPB exciplexes with different component weight ratios were found to be over 10%, as measured by an integrating sphere under the excitation of 300 nm. Such a high ET of the exciplex with TADF properties would be an attractive host material for the green and yellow phosphors in PhOLEDs.

The exciplex-type TADF OLEDs were then fabricated and investigated with the device structure of ITO/MoO3 (2 nm)/15 wt% MoO3: m-MTDATA (30 nm)/m-MTDATA (10 nm)/x wt% m-MTDATA: TmPyPB (30 nm)/TmPyPB (30 nm)/LiF (1 nm)/AL (100 nm), where x=70, 50, 30 (m-MTDATA: TmPyPB=70:30, 50:50 or 30:70), ITO is the anode, MoO3 is the hole injection layer, 15 wt% MoO3: m-MTDATA and m-MTDATA is the hole transport layer, x wt% m-MTDATA: TmPyPB is the emitting layer, TmPyPB is the electron transport layer, and LiF/AL acts as the cathode. (Fig. S5).29 An ultralow turn-on voltage (2.7 V) was observed for the blue TADF OLEDs, but the electroluminescence (EL) brightness and efficiencies of current (CE), power (PE), and external quantum (EQE) are quite low, due to the


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mainly the low PLQY of this exciplex system. It is interesting that the EL peaks can shift from 504 to 464 nm by changing the weight ratios of m-MTDATA and TmPyPB from 30:70 to 70:30 in the bulk films, while the interface exciplex exhibits much broader

EL

spectra

covering

almost

all

the

EL

peaks

under

varied

m-MTDATA/TmPyPB ratios (Fig. S6). Therefore, besides the HOMO of m-MTDATA and LUMO of TmPyPB, the interactions between the donor and acceptor can also influence the optoelectronic properties of the exciplex significantly. Further, it was found that the exciplex-type excitons were formed majorly at m-MTDATA by selectively doping Ir(bt)2(acac), which can harvest the host exciton by energy transfer, at different layers of the interface exciplex OLEDs; much better device performance was observed when Ir(bt)2(acac) was doped in the m-MTDATA side that close to TmPyPB (Fig. S7). Therefore, larger amount of m-MTDATA in the exciplex would be helpful in generating sufficient excitons for emission.



Fig. 3. (a) Normalized PL spectra of m-MTDATA/TmPyPB (70:30) bulk exciplex as well as m-MTDATA, TmPyPB and their exciplex films doped by 2 wt% yellow guest (G) of Ir(bt)2(acac), (b) EL spectra of the yellow PhOLED (2 wt% Ir(bt)2(acac)) hosted by m-MTDATA/TmPyPB (70:30) bulk exciplex at various driving voltages from 4 to 12 V.

In light of the moderate PLQY of the exciplex and low EQEs of its TADF OLEDs, the m-MTDATA/TmPyPB (70:30) bulk exciplex was then tested as a host material of phosphors. From Fig. 3a, PL spectra indicate efficient energy transfer from the host to the guest (Ir(bt)2(acac)) at a doping lever as low as 2 wt%, due to the high absorbance of the yellow phosphor (Fig. S8) at the PL peaks of the hosts to support efficient energy transfer.34 Therefore, complete energy transfer from the bulk


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exciplex-type host to the guest of Ir(bt)2(acac) leads to complete and stable guest emission peaks at 560 and 602 nm under the different driving voltages of PhOLEDs (Fig. 3b). The device configuration of the PhOLEDs is optimized to be ITO/MoO3 (2 nm)/15 wt% MoO3: m-MTDATA (30 nm)/m-MTDATA (10 nm)/2 wt% Ir(bt)2(acac): bulk exciplex (30 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al (100 nm) (Fig. 4a). To our delight, extraordinary high EL brightness up to 37,300 cd/cm2 and maximum CE, PE, and EQE up to 50.7 cd/A, 57.6 1m/W and 18.5% respectively were achieved under a very low turn-on voltage of 2.5 V (Fig. 4b, c). These results are among the best of the Ir(bt)2(acac)-based PhOLEDs.38-40 The high EQE up to 18.5% suggests clearly a mostly 100% internal quantum efficiency (IQE) and highly efficient energy transfer from the exciplex to Ir(bt)2(acac) in the PhOLEDs with a normal out-coupling efficiency of 20%, indicating obviously the TADF nature of the bulk exciplex.



Fig. 4. (a) Device configuration, (b) luminance–current density–voltage (L-J-V) characteristics, and (c) efficiency–luminance characteristics of the yellow PhOLEDs hosted by m-MTDATA/TmPyPB (70:30, wt/wt) exciplex with 2 wt% doped Ir(bt)2(acac). Inset of (b): the schematic ET diagram of the energy transfer process from the exciplex to the yellow dopant.


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Single carrier devices were further fabricated to probe the high performance of the yellow PhOLEDs. As illustrated in Fig. S9, the m-MTDATA/TmPyPB (70:30) bulk exciplex shows a highly balanced carrier transportation with similar hole and electron mobilities of 5.21×10-6 cm2 v-1 s-1 and 4.97×10-6 cm2 v-1 s-1, respectively, according to the space charge limited current (SCLC) model.13 Such a super-balanced charge transportation of the host should be the main reason for the low driving voltages and high device efficiencies of the PhOLEDs. Encouraged by the high efficiencies of the yellow PhOLEDs, we also prepared the green and red PhOLEDs based on the exciplex-type host under the same device configuration using green and red phosphorescent guests of tris(2-phenylpyridine) iridium(III) (Ir(ppy)3) and bis(1-phenylisoquinoline) (acetylacetonate)iridium(III) (Ir(piq)2(acac)), respectively (Fig. S10).40-42 From the luminance-current density-voltage (L-J-V) characteristics (Fig. 5a) and the EL spectra (Fig. 5b) of the green and red PhOLEDs hosed by the m-MTDATA/TmPyPB exciplex, stable EL from the metal complexes was observed at varied driving voltages, indicating a complete energy transfer from the exciplex host to the guests even when their concentration is as low as 2 wt%. High efficiencies of the green and red PhOLEDs were also achieved; both of them show EQEs over 10% (Fig. S11). Impressively, the very low driving voltages were observed in these devices and the turn-on voltage of the green PhOLEDs is even as low as 2.2 V. Moreover, the stability of the green PhOLEDs is very high: the roll-off of these devices is only 0.26% at 10000 cd m-2 and 3.84% at 20000 cd m-2, respectively.



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Fig. 5. (a) Luminance–current density–voltage (L-J-V) characteristics and (b) normalized

EL

spectra

of

the

green

and

red

PhOLEDs

hosted

by

m-MTDATA/TmPyPB (70:30, wt/wt) exciplex with 2 wt% doped Ir(ppy)3 and Ir(piq)2(acac), respectively. Considering the excellent device performance of the yellow PhOLEDs, we also fabricated the complementary white PhOLED using the yellow dopant of Ir(bt)2(acac) and the blue one of bis[(4,6-difluorophenyl)-pyridinato-N,C2](picolinate) iridium(III) (FIrpic). The device structure is designed as ITO/MoO3 (2 nm)/15 wt% MoO3: m-MTDATA (30 nm)/m-MTDATA (10 nm)/2 wt% Ir(bt)2(acac): bluk exciplex (30 nm)/10 w% FIrpic: TCTA (10 nm)/TmPyPB (30 nm)/LiF (1 nm)/Al (100 nm) (Fig. 6a), where Tris(4-carbazoyl-9-ylphenyl)amine (TCTA) acts as both an electron transporting material for the exciplex-hosted emitting layer and a host for FIrpic. The


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yellow EL from Ir(bt)2(acac) and blue EL from FIrpic leads to complementary white EL (Fig. S12). Similar to the yellow PhOLEDs, the white device shows strong brightness, high device efficiencies, and low turn-on voltage (2.3 V). The maximum luminance, CE, PE and EQE are up to 21800 cd/m2, 32.2 cd/A, 37.0 lm/W and 11.9%, respectively (Fig. 6b, c).

Fig. 6. (a) Device structure, (b) luminance–current density–voltage (L-J-V), and (c) efficiency–luminance characteristics of the complementary white PhOLEDs. Inset of



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(b): the schematic ET diagram of the energy transfer processes from the exciplex to the yellow dopant (Ir(bt)2(acac)) and from TCTA to the blue dopant (FIrpic).

Table 1. Device performance of the blue exciplex OLED and multicolor exciplex-hosted PhOLEDs using m-MTDATA/TmPyPB (70:30, wt/wt)

Device

Driving Voltage [V]

a)

Efficiencyb)

Lmax -2

-1

-1

CIE

[cd cm ]

CE [cd A ]

PE [lm W ]

EQE [%] 0.9, 0.6, 0.3

Blue

2.7, 4.3, 8.7

1400

1.9, 1.3, 0.5

2.2, 1.0, 0.2

[x, y] (0.06, 0.25)

Green

2.2, 3.0, 3.5

99500

39.1, 19.6, 30.4

27.9, 20.0, 26.8

11.5, 5.7, 8.9

(0.32, 0.61)

Yellow

2.5, 4.1, 6.3

37300

50.7, 49.8, 44.9

57.6, 37.8, 22.4

18.5, 18.2, 16.4

(0.51, 0.49)

Red

2.7, 3.8, 5.2

12900

9.4, 9.2, 8.1

9.6, 7.5, 4.9

10.8, 10.6, 9.2

(0.67, 0.33)

White

2.3, 3.4, 4.3

21800

32.2, 29.4, 24.5

37.0, 27.6, 17.9

11.9, 10.9, 9.1

(0.36, 0.41)

a

-2

In the order of onset, 100 and 1000 cd m ;

b

In the order of max, 100 and 1000 cd cm-2

Conclusions

Based on widely used hole and electron transport materials of m-MTDATA and TmPyPB, we established efficient blue TADF exciplex emission (PLQY=12.3% and ∆EST =0.04 eV) by varying their component proportions. Owing to the high ET of both m-MTDATA and TmPyPB, energy leakage from the exciplex to its components was completely eliminated, while the close-lying CT and 3LED in energy supports the dominated TADF mechanism to populate the CT states of the exciplex, as revealed by a combined experimental and theoretical studies of this exciplex system. With the high ET (2.54 eV) and suitable frontier orbital energy levels inherited from the


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constituting molecules, m-MTDATA/TmPyPB exciplex was found to be a universal host for the green, yellow, and red PhOLEDs using Ir(ppy)3, Ir(bt)2(acac), and Ir(piq)2(acac), respectively. Efficient and complete energy transfer from the bulk exciplex host to the guests was realized even under a very low doping level of 2 wt%, showing high device performance of the exciplex-hosted multicolor PhOLEDs. Remarkably, very low turn-on voltages (~2.2 V) were observed and the yellow PhOLEDs show maximum CE, PE and EQE up to 50.7 cd/A, 57.6 m/W and 18.5%, respectively. These findings could provide important guidance on the rational design of exciplex systems based on TADF mechanism and their applications as bipolar host materials for PhOLEDs to achieve high device performance using readily available electron donor and acceptor molecules.

Experiments methods OLED devices were prepared in the following procedures. The patterned ITO glass substrates were ultrasonically cleaned with detergent, alcohol, acetone, and deionized water for 30 min respectively, and then dried at 120°C in a vacuum oven for more than one hour. After ultraviolet (UV)-ozone treating for 15 min, the samples were transferred to a thermal evaporator chamber. Then, the MoO3 (2 nm), MoO3: m-MTDATA (30 nm), m-MTDATA (10 nm), emissive layers (EMLs), TmPyPB (30 nm), LiF (1 nm), and Al (100 nm) were deposited subsequently by thermal evaporation under a pressure of 5×10−4 Pa. Organic materials were deposited at a rate



of 1–2 Å s

−1

and the deposition rates were 0.1 and 10 Å s

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−1

for LiF and Al,

respectively. The active area of the device was controlled to be 3×3 mm2. Devices without encapsulation were measured at room temperature under ambient atmosphere conditions. The luminance-current-voltage (L-J-V) characteristics of the devices were recorded by a Keithley source-meter (model 2602) and a calibrated luminance meter. Electroluminescence (EL) spectra were obtained using a spectra-scan PR655 spectrophotometer. Ultraviolet-visible (UV-Vis) spectra were recorded on an UV-3600

SHIMADZU

UV-VIS-NIR

spectrophotometer.

Steady

state

photoluminescent spectra of the exciplex emitters and their pristine materials in solid films were recorded on an RF-5301PC spectrofluorophotometer with a Xenon lamp as light source. Phosphorescence spectra of the compounds in thin film were collected by a time-resolved Edinburgh LFS920 fluorescence spectrophotometer at 77 K with a 10 ms delay time after the excitation (λ=300 nm) using a microsecond flash lamp. Photoluminescent decay characters were measured at a time-resolved Edinburgh LFS920 fluorescence spectrophotometer at room temperature with the excitation of a 315 nm diode laser. DFT calculations were performed on Gaussian 09 revision D.01 package. Molecular structures were fully optimized by the Becke’s three-parameter exchange functional along with the Lee-Yang-Parr correlation functional (B3LYP) for neutral states at the standard split valence plus polarization function 6-31G(d) basis set. Molecular structures at the lowest singlet excited states (S1) were optimized via time-dependent DFT (TD-DFT) of TD-B3LYP/6-31G(d).


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:xxxx.

Detailed photophysical property measurements, DFT calculations, device fabrication and performance measurements (Supplementary Figures S1-S12 and Tables S1).

AUTHOR INFORMATION Corresponding Author *Runfeng Chen. E-mail: [email protected]

ACKNOWLEDGMENT This study was supported in part by the National Natural Science Foundation of China (21674049, 21001065, 21274065, 21601091 and 61136003); Qing Lan project of Jiangsu province; Science Fund for Distinguished Young Scholars of Jiangsu Province of China (BK20150041); Natural Science Foundation of Jiangsu Province of



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China (BK20160891); 1311 Talents Program of Nanjing University of Posts and Telecommunications (Dingshan).

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TOC Figure

Synopsis Rationally constructed m-MTDATA/TmPyPB bulk exciplex shows blue TADF emission, high triplet energy, and bipolar charge transport property for high-performance host materials of multicolor PhOLEDs.


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High-performance Exciplex-type Host for Multicolor Phosphorescent

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