Balance the Carrier Mobility To Achieve High Performance Exciplex

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Balance the Carrier Mobility to Achieve High Performance Exciplex OLED Using a New Triazine-Based Acceptor Wen-Yi Hung, Pin-Yi Chiang, Shih-Wei Lin, Wei-Chieh Tang, Yi-Ting Chen, Shih-Hung Liu, Pi-Tai Chou, Yi-Tzu Hung, and Ken-Tsung Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11895 • Publication Date (Web): 28 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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ACS Applied Materials & Interfaces

Balance the Carrier Mobility to Achieve High Performance Exciplex OLED Using a New Triazine-Based Acceptor

Wen-Yi Hunga*, Pin-Yi Chianga, Shih-Wei Linb, Wei-Chieh Tangb, Yi-Ting Chenb, Shih-Hung Liub, Pi-Tai Choub*, Yi-Tzu Hungb, and Ken-Tsung Wongb,c*

a

Institute of Optoelectronic Sciences, National Taiwan Ocean University, Keelung 202, Taiwan.

b c

Department of Chemistry, National Taiwan University, Taipei 106, Taiwan.

Institute of Atomic and Molecular Science, Taipei 106, Taiwan.

†Corresponding

Authors

W.-Y.

Hung

([email protected]),

P.-T.

([email protected]), K.-T. Wong ([email protected]).

Keywords: thermally activated delayed fluorescence (TADF), exciplex, electron-transporting material, 1,3,5-triazine, balance mobility 1

ACS Paragon Plus Environment

Chou


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Abstract: A star-shaped 1,3,5-triazine/cyano hybrid molecule CN-T2T was designed and synthesized as a new electron acceptor for efficient exciplex-based OLED emitter by mixing with a suitable electron donor (Tris-PCz). The CN-T2T/Tris-PCz exciplex emission shows a high ΦPL of 0.53 and a small ∆ET-S= ‒0.59 kcal/mol, affording intrinsically efficient fluorescence and highly efficient exciton up-conversion. The large energy level offsets between Tris-PCz and CN-T2T and the balanced hole and electron mobility of Tris-PCz and CN-T2T, respectively, ensuring sufficient carrier density accumulated in the interface for efficient generation of exciplex excitons. Employing a facile device structure composed as ITO/ 4% ReO3:Tris-PCz (60 nm)/ Tris-PCz (15 nm) / Tris-PCz:CNT2T(1:1) (25 nm)/ CN-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm), in which the electron-hole capture is efficient without additional carrier injection barrier from donor (or acceptor) molecule and carriers mobilities are balanced in the emitting layer, leads to a highly efficient green exciplex OLED with external quantum efficiency (EQE) of 11.9%. The obtained EQE is 18% higher than that of a comparison device using an exciplex exhibiting a comparable ΦPL (0.50), in which TCTA shows similar energy levels but higher hole mobility as compared with Tris-PCz. Our results clearly indicate the significance of mobility balance in governing the efficiency of exciplex-based OLED. Exploiting the Tris-PCz:CN-T2T exciplex as host, we further demonstrated highly efficient yellow and red fluorescent OLEDs by doping 1 wt% Rubrene and DCJTB as emitter, achieving high EQE of 6.9 and 9.7%, respectively.

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Introduction Materials with thermally activated delayed fluorescence (TADF) have recently attracted great attention as emitters for next generation organic light emitting diodes (OLEDs) owing to their harvesting almost 100% internal quantum efficiency.1-11 In general, TADF materials with small singlet–triplet energy splitting, ∆ET-S (defined as T1 minus S1) allow highly efficient up-conversion from radiative forbidden triplet states (T1) to radiative singlet states (S1) by thermally activated reverse intersystem crossing (RISC). The small ∆ET-S can be theoretically obtained via tailor-made molecules with weakly-coupled electron donor (D) and acceptor (A) components, inducing an intramolecular charge transfer (ICT) behaviour such that the electron exchange energy that contributes to the energy difference between singlet and triplet states, can be minimized. To maximize the TADF efficiency and device operational stability, it is necessary to introduce host and carrier transport molecules with higher triplet energies than that of a guest TADF molecule, similar to conventional phosphorescence-based OLEDs. Because the exchange energy decreases as the HOMOLUMO (electron-hole) separation distance increases, alternatively, small ∆ET-S can be readily realized via intermolecular charge transfer by spatially separating the LUMO and HOMO on two dissimilar donor and acceptor molecules.12,13 Fluorescent OLEDs based on pure TADF emitters have been reported with > 20% external quantum efficiencies (EQE). However, it is still a challenge to access highly efficient exciplex systems. This is mainly because exciplex formation is commonly accompanied with a large red shift of emission spectra and long radiative lifetime, which tend to diminish photoluminescence quantum yield (PLQY) as well as electroluminescence (EL) performance. Until now, exciplex-based OLEDs with EQE above 10% are still scarce.14-18 In a previous study, we reported a strategy to realize a high efficiency bilayer-type exciplexbased OLED (EQE~7.7%) by using a triazine-centered electron-transporting (ET) molecule 3PT2T as the acceptor and a carbazole-based hole-transporting (HT) material TCTA as the donor.19 3



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We also developed a triazine-based ET compound PO-T2T possessing very low LUMO/HOMO. As a result, the exciplex emission from blue to red can be attained via systematically tuning HOMO of the HTL material. Subsequently, this leads to the successful construction of a tandem, allexciplex-based white light OLED (WOLED). 20 These previous works, though representing a remarkable advance, however still suffer relatively low luminous efficiency and fast roll-off of device efficiency. It is noted that the physical properties of triazine-based ET materials are strongly governed by the structural features of the peripherals (e.g. pyrazole vs. diphenyl phosphine oxide), resulting in the formation of exciplexes with different characteristics. Therefore, for searching higher efficiency exciplex, further exploration of new triazine-based ET-type molecule is demanding. In this study, we report on a new exciplex system by mixing a new electron acceptor 3′,3′″,3′″″-(1,3,5-triazine-2,4,6-triyl)tris(([1,1′-biphenyl]-3-carbonitrile)) (CN-T2T) with an electron donor, 9,9′,9″-triphenyl-9H,9′H,9″H-3,3′:6′,3″-tercarbazole (Tris-PCz)21 (Scheme 1). Tris-PCz is a hole-transporting material possessing three electron donating carbazole moieties with high triplet state energy. CN-T2T is modified from 1,3,5- triazine-based ET-type molecules that reported in our group 22 by incorporating a peripheral polar group benzonitrile onto the meta-positions of C3symmetry 2,4,6-triphenyl-1,3,5-triazine. Both Tris-PCz and CN-T2T exhibit high triplet energy and the same order of magnitude of hole and electron mobility. To probe the influence of mobility balance on exciplex formation, Tris-PCz was replaced by TCTA which exhibits similar energy levels but higher hole mobility. The promising physical properties of Tris-PCz and CN-T2T, together with the barrier free hole injection from ITO electrode to the EML, realizes highly efficient green exciplex OLED with an extremely low driving voltage of 2.6 V at 100 cd m−2 and EQE up to 11.9% (37 cd A−1, and 46.5 lm W−1). This EQE is 18% higher than that of CNT2T/TCTA-based exciplex OLED, verifying the importance of charge balance in exciplex formation regardless of the energy levels and photoluminescence quantum yield (PLQY). In addition, the utilization of CN-T2T/Tris-PCz exciplex as host system for conventional fluorescent

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emitters such as Rubrene and DCJTB was also explored to give high EQE of 6.9 and 9.7%, respectively.

Results and Discussions: CN-T2T was synthesized in two steps with a high yield from a triazine tribromo compound (1), which was firstly transformed into triboronic ester (2) and subsequently reacted with readily available 3-bromobenzonitrile under Suzuki coupling condition (Scheme 1). CN-T2T was characterized with satisfactory spectroscopic data and elemental analysis.

Scheme 1. Synthesis of CN-T2T and molecular structures of CN-T2T and Tris-PCz.

CN-T2T exhibits excellent thermal stability with a high decomposition temperature (Td) of 418 °C (refer to 5% weight loss) measured by thermogravimetric analysis (TGA). No evident glass transition temperature but a melting temperature was observed at 286 oC for CN-T2T by differential scanning calorimetry (DSC) analysis. In addition, one quasi-reversible reduction potential (-2.02 V vs. Fc/Fc+) of CN-T2T was detected by cyclic voltammetry analysis (Figure 1). This result was used to calculate the LUMO energy level to be -2.78 eV as referred to the redox couple of ferrocene and the HOMO (-6.70 eV) was estimated by subtracting the energy gap from the LUMO level. The data are summarized in Table 1. 5



Table 1 Physical properties of CN-T2T

CN-T2T a

Tg /Tma (oC)

Tdb (oC)

n.d. h /286

418

λabs (nm)c sol. 270

film 268

λPL (nm)c sol. 420

film 422

Egd (eV)

E Te (eV)

E1/2Red f (V)

3.92

2.82

-2.02

HOMO/LUMOg (eV) -6.7/-2.78

Determined by DSC measurement. b Calculated from TGA result (5% weight loss). c The solution spectra were probed

in CH2Cl2.

d

Calculated from the absorption onset of the solution spectrum.

e

Probed in 2-MeTHF. f The reduction

potential was probed in THF solution containing 0.1 M TBAP and was referred to the Fc/FC+ redox couple. g LUMO level was calculated from reduction potential as referred to the HOMO of ferrocene. HOMO = LUMO - Eg.

h

Not

detected.

1.0

I(a.u.)

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

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0.5

CN-T2T

0.0 -1.0

-1.5 -2.0 + Potential(V vs Fc/Fc )

Figure 1. Cyclic voltammogram of CN-T2T in THF containing 0.1 M TBAPF6 as supporting electrolyte. A glassy carbon, Pt wire and Ag/AgCl were used as working, counter and reference electrodes, respectively.

Exciplex formation requires an efficient charge transfer between donor and acceptor, and this requires considerable energy level offset (> 0.4 eV)13 between the HOMO/LUMO levels of the donor and acceptor, respectively. We find that energy level offsets of TrisPCz/CN-T2T are rather large, which is also beneficial for carriers’ accumulation at the TrisPCz/CN-T2T interface (Scheme 2). The ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectra of CN-T2T in CH2Cl2 and thin films show an absorption maximum centred at 270 and 268 nm as well as an emission peak at 420 and 422 nm, respectively, ascribed to the π−π* transitions of the 1,3,5-triphenyltriazine core (Figure 2a). The triplet energy (ET) of CN-T2T is estimated to be 2.82 eV from the highest-energy peak

6


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of phosphorescence spectrum at 77 K. The UV-vis absorption and PL spectra of Tris-PCz, CN-T2T, and Tris-PCz:CN-T2T (1:1) mixed films are depicted in Figure 2b. The normalized absorption spectra of Tris-PCz:CN-T2T blend exactly originate from their individual components. No new absorption band appeared at longer wavelengths, indicating that electronic interactions on the ground state of Tris-PCz:CN-T2T are rather weak and no ground-state charge transfer complex exists. This makes the description of Tris-PCz:CNT2T structure very difficult. In a primitive attempt (in top-to-top displacement) where the geometry optimization of Tris-PCz:CN-T2T in ground state was simulated with density functional theory (DFT) at the B3LYP/6-31g(d) levels, a weak interaction between cyanobiphenyl and carbazole moieties is resolved (Figure 3) with a complexation energy of -1.04 kcal/mol. Nevertheless, it should be noted that this configuration may be at a local minimum, while in reality there exist a number of other interactions in the solid film. The HOMO and LUMO of Tris-PCz:CN-T2T are located only at Tris-PCz and CN-T2T, respectively. The S0-S1 energy gap for the complex is calculated to be 485.9 nm assigned to HOMO → LUMO (96%), resulting in a spatially separated charge transfer process.

Scheme 2. Device structure and energy levels of the exciplex OLED.

7



0.8

CN-T2T

1.0

solu. / film / Abs. / PL / Phos.

0.8

(b) Abs. (a.u.)

ET= 2.82 eV

1.0

0.8

0.6

0.6

0.4

0.4 0.2

0.6

0.4

0.4

0.2

0.2

0.2

0.0

0.0 200

300

400 500 Wavelength (nm)

600

1.0

0.8

0.6

0.0

Tris-PCz CN-T2T Tris-PCz:CN-T2T

300

400 500 600 Wavelength (nm)

PL (a. u.)

Abs. (a.u.)

(a) 1.0

PL (a. u.)

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

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0.0 700

Figure 2. (a) Room-temperature UV-vis absorption and photoluminescence (PL) spectra of CN-T2T in CH2Cl2 solution and neat film as well as corresponding phosphorescence (Phos) spectra recorded in EtOH solution at 77 K. (b) UV-vis absorption and PL spectra of TrisPCz, CN-T2T and Tris-PCz:CN-T2T (1:1) neat films.

Top view

HOMO

LUMO

Side view

HOMO

LUMO

Figure 3. The computation optimized structure of the Tris-PCz:CN-T2T complex in dichloromethane. The distance between Tris-PCz and CN-T2T is calculated to be around 4.50 Å. Also shown are the calculated HOMO and LUMO of the Tris-PCz:CN-T2T complex.

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The PL of Tris-PCz:CN-T2T film exhibits typical exciplex characteristics with a peak at 530 nm and a narrow half width at half maximum (HWHM = 90 nm), which is significantly red-shifted by 90 nm in comparison to those of Tris-PCz and CN-T2T. The photon energy of the exciplex emission can be directly correlated to the energy difference (2.82 eV) between the HOMO (-5.60 eV) of Tris-PCz and the LUMO (-2.78 eV) of CN-T2T, which is consistent with the onset of the PL spectrum (450 nm). The PLQY (ΦPL) for Tris-PCz:CNT2T film was measured using an integrating sphere to be 52.9% (Figure S1 in supporting information (SI)). More importantly, both of the triplet energy of Tris-PCz (2.76 eV)19 and CN-T2T (2.82 eV) are higher than the singlet energy of the formed exciplex, enabling efficient confinement of the Tris-PCz:CN-T2T exciplex triplet state in the emitting matrix. Figure 4a shows the time resolved PL spectra of the Tris-PCz:CN-T2T film with different delay times at 300 K. The prompt component (delay = 0, gate width = 10 ns) is assigned to the exciplex fluorescence, and the delayed component (delay = 10 µs, gate width = 1 µs) can be assigned to exciplex emission via the reverse ISC process. A slight red shift (∆λ ≤ 10 nm) was resolved between the prompt and delayed components. This small shift has been reported in several other exciplex systems and was interpreted by the stabilization of charge transfer state due to the slow (microseconds to milliseconds) dipole reorientation of surrounding solid matrix.12, 23 Figure 4b reveals the relaxation dynamics of the 550 nm exciplex emission. The transient PL decay curve of Tris-PCz:CN-T2T film was resolved into prompt and delayed decay components, consisting of a fast decay component of 28.6 ns and a slower decay with a lifetime of 2.85 µs. Applying the TADF kinetics under the assumption of pseudo-equilibrium between S1 and T1 states, the time-resolved fluorescence intensity, [S1]f, can be expressed as follows (see Eqs. S1 in ESI for detailed derivation):

 krisc  kisc [S1 ]f = I 0 ⋅  e−t / τ1 + e−t / τ 2  kisc + krisc  kisc + krisc 

(1)

9



, where I0 is a proportional constant incorporating both radiative decay rate constant of the exciplex emission and instrument factor, τ1 and τ2 are the observed lifetime of the fast and slow decay components. As a result, the equilibrium constant Keq = kisc/krisc can be obtained by the ratio of the pre-exponential factor (extrapolation at t = 0) in eq. (1), (see Figure 4b), which is deduced to be 8.1. According to the ∆ET-S-Keq relationship expressed as ∆ET-S = RTln(Keq/3) where a factor of 3 stands for the triplet degenerate states, ∆ET-S is then deduced to be ‒0.59 kcal/mol.

1.0 0.8

529 539

delay=0 gate width =10 ns delay=10 µs gate width =1000 ns

0.6 0.4 0.2

(b) PL intensity (Count)

(a)

PL (a.u.)

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

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ex355_em500 Fitting IRF

Delay 3

10

I(t)=A1exp(-t/τ1)+A2exp(-t/τ2) A1=0.89; τ1=28.6 (ns) 2

A2=0.11; τ2=2.85 (µs)

10

1

0.0 400

10 500 600 Wavelength (nm)

0

700

5

10 Time (µs)

15

20

Figure 4. Transient PL decay properties of Tris-PCz:CN-T2T blend film at 300K. (a) Prompt (delay = 0, gate width = 10 ns) and delayed (delay = 10 µs, gate width = 1 µs) components of PL spectra. (b) The decay dynamics monitored at 550 nm.

The intensity of exciplex emission relies on the charge carrier density accumulated at donor/acceptor interface, which in turn determines the device efficiency. Hence, charge transport balance of donor and acceptor plays an important role in highly efficient exciplexbased OLEDs.8,24 To gain further insight into the charge-carrier transport properties, we used the time-of-flight (TOF) technique25 to evaluate the carrier mobility. Representative TOF transients for holes of Tris-PCz and electrons of CN-T2T are shown in Figure 5a-b. Figure 5c depicts the carrier mobility as a function of the square root of the electric field. The hole mobility of Tris-PCz varied within the range from 1.9 x 10-5 to 3.2 x 10-4 cm2 V-1s10


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1

as electric fields increase from 3.1x105 to 1.5 x 106 V cm-1, while the electron mobility of

CN-T2T varied within the range from 2.8 x 10-5 to 1.6 x 10-4 cm2 V-1s-1 for fields increasing from 4.6 x 105 to 1.1 x 106 V cm-1. Therefore, we observed almost identical values for the electron and hole mobility of CN-T2T and Tris-PCz, respectively, over a wide range of electric field, leading to high electron–hole capture probability that benefits the device performance.

100 50

tT

100 10 1

0.1

1 Time (µ sec)

10

0

CN-T2T (electron) Photocurrent (µA)

150

Photocurrent ( µA)

(b)1000

Tris-PCz (hole) Photocurrent (µA)

Photocurrent ( µA)

(a) 200

800 600 400

tT

1000 100 10

200

0.1

1 10 Time (µsec)

0 0

2

4

6

8

10

0

Time ( µsec)

3

6

9

12

15

Time ( µsec)

(c) µ (cm / V s)

.

1E-4

2

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


Tris-PCz (h) TCTA (h) CN-T2T (e) 1E-5 400

600

800

E

1/2

1000

(V/cm)

1200

1/2

Figure 5. Representative TOF transients for (a) Tris-PCz (1.62 µm thick) at E = 1.1 × 106 V/cm, (b) CN-T2T (1.54 µm thick) at E = 8.5 × 105 V/cm and insets are the double logarithmic plots. (c) Carrier mobility of compounds plotted with respect to E1/2.

To investigate the capability of using Tris-PCz:CN-T2T as exciplex emitters, we used a very simple device structure composed of Tris-PCz and CN-T2T as functional materials: ITO/ 4% ReO3:Tris-PCz (60 nm)/ Tris-PCz (15 nm) / Tris-PCz:CN-T2T(1:1) (25 nm)/ CN-

11



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T2T (50 nm)/Liq (0.5 nm)/Al (100 nm). Tris-PCz, CN-T2T, 8-Hydroxyquinolinolatolithium (Liq), and Al are used as the hole-transporting layer (HTL), electron-transporting layer (ETL), electron injection layer (EIL), and cathode, respectively. Scheme 2 shows the device structure and the schematic diagram of the energy levels of materials used in the device. To lower the hole injection barrier from ITO to Tris-PCz, we used rhenium oxides (ReO3) as a dopant material in Tris-PCz to produce ohmic contact.26,27 Both electron mobility of CNT2T and hole mobility of Tris-PCz have the same order of magnitude as 10-4 cm2 V-1s-1 at E = 106 V cm-1. The molar ratio of Tris-PCz:CN-T2T in the co-deposited emission layer is 1:1 to balance the carrier in EML, attaining high electron–hole capture probability. To gain further insight into the influence of carrier mobility, we also selected another holetransporting material 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA)17 as donor, which shows similar energy levels (HOMO/LUMO = -5.62/-2.2 eV) and higher hole mobility (Figure 5c). We then compare the device performance with that of using as Tris-PCz as donor under the device structure: ITO/ 4% ReO3:TCTA (60 nm)/ TCTA (15 nm) / TCTA:CN-T2T(1:1) (25 nm)/ CN-T2T (50 nm)/Liq (0.5 nm)/Al (100 nm). Figure 6 displays the EL characteristics of the Tris-PCz:CN-T2T and TCTA:CN-T2T green exciplex devices, and pertinent data are summarized in Table 2. The Tris-PCz:CNT2T device gives a maximum brightness (Lmax) of 73800 cd m−2 at 11.0 V (2070 mA cm−2) with CIE coordinates of (0.33, 0.57). The maximum external quantum (EQE), current (CE), and power efficiencies (PE) were 11.9%, 37 cd A−1, and 46.5 lm W−1, respectively, which are significantly higher than those for TCTA:CN-T2T device (9.7%, 31.1 cd A−1, and 39.6 lm W−1). Consider the comparable energy levels bewteen TCTA and Tris-PCz, and the similar PLQY (0.50) of TCTA:CN-T2T mixed film (Figure S1 in SI), the ~18% increase in EQE of the Tris-PCz:CN-T2T device (cf. the TCTA:CN-T2T device) clearly manifests the key role of charge balance in EML, which renders high electron–hole capture probability beneficial for the device performance. 12


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For a highly efficient OLED, ideally, the operating voltage should approximately match to the photon energy (hv) of the emitted photons. However, the operating voltages in most of ever-reported exciplex-based OLEDs were far above the photon energies since the charge injections at the interfacial contacts are characterized with energy barriers. In this study, the Tris-PCz:CN-T2T green exciplex device exhibits a turn-on voltage of as low as 2.0 V, and the operating voltage at 1000 cd m−2 is only 3.6 V. It is also worthy to note that the brightness of 100 cd m−2 is achieved at 2.6 V, which almost equals to the emitting photon energy (2.64 eV, calculated from the EL spectrum onset (470 nm)). The high power efficiency is ascribed to the efficient electron-hole capture without additional carrier injection barrier from donor (or acceptor) molecules and carrier balance in EML, which resulted in low operation voltage.28

10

2 1

10

1 -1

2

0

2

4 6 8 Voltage(V)

Quantum Efficiency (%)

2

Brightness (cd/m )

3

3

10

50 12 10

40

8

30

6

10

2 0 0 10

10

20

4

1

10

10

2

3

10

0

10

4

2

Brightness (cd/m )

EML CIE Tris-PCz:PO-T2T (0.33,0.57) TCTA:PO-T2T (0.36,0.56) 1% Rubrene (0.49,0.50) 1% DCJTB (0.59,0.40)

(c) 1.0 EL. (a.u.)

(b)

Tris-PCz:CN-T2T TCTA:CN-T2T 1% Rubrene 1% DCJTB

5

10

Power Efficiency (lm/W)

(a)

Current Density (A/cm )

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


0.5

0.0

500


800

Figure 6. (a) Current density–voltage–luminance (J–V–L) characteristics. (b) external quantum (EQE) and power efficiencies (PE) as a function of brightness. (c) EL spectra for device.

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Table 2. EL performances of devices EML

Vonb [V]

TrisPCz:CNT2T 2.0 TCTA:CNT2T 2.0 1wt% Rubrene a 2.0 1wt% DCJTB a 2.0 a

Lmax [cd m‒2]

PE EQE c at 103 cd m‒2 EQE CE ‒1 ‒1 [%] [cd A ] [lm W ] [%, V]

Imax [mA cm‒2]

CIE [x,y]

73800 (11.0V)

2070

11.9

37.0

46.5

11.1, 3.6

0.33,0.57

30400 (10.2V)

1850

9.7

31.1

39.6

9.0, 4.4

0.36,0.56

166000 (7.8 V)

3300

6.9

21.4

28.1

6.6, 3.0

0.49,0.50

140500 (8.0 V)

3250

9.7

19.3

23.3

9.1, 3.1

0.59,0.40

The doped 1wt% Rubrene and DCJTB indicate the devices fabricated with co-host TrisPCz:CNT2T, respectively. b

Turn-on voltage at which emission became detectable. c The values of driving voltage and EQE of device at 1000 cd m– 2

are depicted in parentheses.

We then made a further attempt by using the Tris-PCz:CN-T2T as co-host for conventional fluorescent dopants [1 wt% 5,6,11,12-tetraphenylnaphthacene (Rubrene) and 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB)29] with the same device structure. The absorbance of the Rubrene and DCJTB in CH2Cl2 and the emission spectrum of the exciplex are displayed in Figure 7, in which the spectral overlaps between them allow efficient energy transfer from the exciplex to the dopants. The yellow emission device doped 1 wt% Rubrene reveals a maximum Lmax of 166000 cd m−2 at 7.8 V (3300 mA cm−2) with CIE coordinates of (0.49, 0.50), which exhibited the EQE of 6.9%, CE of 21.4 cd A−1, and PE of 28.1 lm W−1. The red emission device doped 1 wt% DCJTB reveals a maximum Lmax of 140500 cd m− 2 at 8.0 V (3250 mA cm− 2) with CIE coordinates of (0.59, 0.40), which exhibited the EQE of 9.7%, CE of 19.3 cd A−1, and PE of 23.3 lm W−1, the data are also summarized in Table 2. To our knowledge, EQE of the present fluorescent emitter doped devices are higher than the conventional fluorescent OLEDs. The DCJTB-based device using Tris-PCz:CN-T2T as host is comparable to those of state-of-the-art exciplex system as host.30,31

14


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Rubrene (Abs,solu.) DCJTB (Abs, solu.) Tris-PCz:CN-T2T (PL, film)

Abs. (a.u.)

1.0 0.8

1.0 0.8

0.6

0.6

0.4

0.4

0.2

0.2

0.0

400


700

PL (a. u.)

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0.0

Figure 7. The absorbance of Rubrene and DCJTB in CH2Cl2 solution as well as the normalized PL spectrum of Tris-PCz:CN-T2T film, exhibiting spectral overlap between them to facilitate the energy transfer.

Conclusion: In conclusion, taking the advantage of the 1,3,5-triazine-centered molecules, we report that a new but rather simple molecule CN-T2T that can serve as an electron acceptor for efficient exciplex by mixing with a suitable electron donor (Tris-PCz). The exciplex in the Tris-PCz:CN-T2T mixed layer shows a high ΦPL of 0.53 and a small ∆ET-S= ‒0.59 kcal/mol (-25.6 meV), affording intrinsically efficient fluorescence and highly efficient exciton upconversion. We employed a simple device structure composed of only Tris-PCz and CNT2T as charge-transporting and emitting materials. The combination of several advantageous factors, namely the same order of magnitude of the hole and electron mobility of Tris-PCz and CN-T2T, respectively, and the barrier free hole injection from the ITO to the EML and high triplet energies, leads to the green exciplex device with exceptionally low driving voltage of 2.6 V at 100 cd m−2 and EQE up to 11.9% (37 cd A−1, and 46.5 lm W−1). A comparison exciplex with TCTA exhibiting similar energy levels but higher hole mobility as donor gave a comparable PLQY but inferior device characteristics (9.7%, 31.1 cd A−1, and 39.6 lm W−1). This result clearly infers that the charge balance in EML is a crucial factor governing the efficiency of exciplex-based OLED. The Tris-PCz:CN-T2T exciplex 15



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system also serves as an excellent host for fluorescent dopants. By doping 1 wt% Rubrene and DCJTB as emitter in Tris-PCz:CN-T2T that acts as the co-host, the devices attained high EQE of 6.9 in yellow and 9.7% in red, respectively. Experimental Section: Synthesis. The bromo-substituted triazine intermediate 1 was obtained according to our previous report.32 A 100 mL two-neck flask was filled with bis(pinacolato)diboron (1.67 g, 6.57 mmol), 2,4,6-tri(3-bromophenyl)-1,3,5-triazine (1.0 g, 1.83 mmol), potassium acetate (1.3 g, 13.2 mmol), PdCl2(dppf) (0.08 g, 0.11 mmol) and stir bar. Using degassed THF (35 mL) as solvent, the mixture was refluxed for 12 hours. The reaction was worked up by adding water and extracting the mixture with THF, and washing the organic phase with brine, and followed by drying over MgSO4. Purification was performed with silica-gel column chrogatography, eluting with 1:2 EtOAc/hexanes to afford the white solid compound 2 (1.04 g, 1.51 mmol, 83%). mp 309~310 oC；IR (KBr) ν 2979, 1604, 1524, 1483, 1422, 1343, 1262, 1211, 1142, 1077, 964, 854, 798, 702, 678, 509 cm-1, 1H NMR (CDCl3, 400 MHz) δ 9.19 (s, 3H), 8.91 (d, J = 8.0 Hz, 3H), 8.05 (d, J = 7.2 Hz, 3H), 7.61 (t, J = 7.6 Hz, 3H), 1.43(s, 27H)；13C NMR (CDCl3, 400 MHz) δ 171.1, 138.3, 135.2, 134.9, 131.6, 127.6, 121.6, 83.9, 25.3；HRMS (m/z, FAB+) Cacld. for C39H48N3O6B3 687.3822, found 687.3826.

CN-T2T. A mixture of triboronic ester 2 (3.0 g, 4.37 mmol), 3-bromobenzonitrile (2.64 g, 14.5 mmol), Pd(PPh3)4 (0.78 g, 0.67 mmol) was stirred in degassed toluene (100 mL) solution under argon atmosphere. After tri(tert-butyl)phosphine (26.2 mL, 0.05 M in toluene, 1.34 mmol) and Na2CO3(aq) (19.8 mL, 2.0 M in water, 39 mmol) were added into the solution, the mixture was refluxed for 72 h. The reaction mixture was cooled to room temperature and extracted with CH2Cl2. The solvent was removed by rotary evaporation, and the crude was purified on a silica gel column chromatography by using gradient eluents of hexanes/CH2Cl2 (3:1 to 1:1) to provide CN-T2T as a white solid (2.5 g, 4.1 mmol, 93%). mp: 290 oC (DSC) IR (KBr) ν 3062, 2230, 1587, 1527, 1488, 1452, 1392, 1347, 1262, 1166, 1136, 1088, 900, 784, 689, 648, 492, 438 cm-1, 1H NMR (CDCl3, 16


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400 MHz) δ 8.97 (s, 3H), 8.85 (d, J = 7.6 Hz, 3H), 8.03 (s, 3H), 7.99 (d, J = 7.8 Hz, 3H), 7.85 (d, J = 7.6 Hz, 3H), 7.77-7.72 (m, 6H), 7.67 (t, J = 7.6 Hz, 3H), 13C NMR (CDCl3, 400 MHz) δ 171.6, 142.0, 139.5, 136.9, 131.6, 131.4, 131.2, 130.8, 129.9, 129.7, 129.0, 127.5, 118.9, 113.2, HRMS (m/z, FAB+) Cacld. for C42H24N6 612.2026, found 612.2060, Anal. Calcd. C, 82.33; H, 3.95; N, 13.72, found C, 82.37; H, 4.14; N, 13.56.

Computational Method. All the calculations were performed with the Gaussian 09 program package. The geometry optimization of ground states of CN-T2T and Tris-PCz monomer was simulated first with density functional theory (DFT) at the B3LYP/6-31g(d) levels using the dichloromethane as the solvent. The initial structure of Tris-PCz:CN-T2T complexation was done by inserting CN-T2T to Tris-PCz from various angles and spaces then progressing the all atoms freely geometry optimization procedure until a local minimum was obtained.

Supporting Information Details of the general experimental procedures, photophysical and time-of-flight mobility measuremants, OLED fabrication, and 1H,

13

C NMR spectra of CN-T2T. This materialis

available free of charge via the Internet athttp://pubs.acs.org.

Acknowledgement

Hung W.-Y., Wong K.-T. and P.-T. Chou thank the Ministry of Science and Technology, Taiwan (MOST 103-2112-M-019-004-MY3, 104-2113-M-002-006- MY3 and MOST 102-2119-M-002003) for the financial support.

References

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Graphic for manuscript 20


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Balance the Carrier Mobility To Achieve High Performance Exciplex

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