Achieving High-Performance Solution-Processed Orange OLEDs with

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Achieving High-Performance Solution-Processed Orange OLEDs with the Phosphorescent Cyclometallated Trinuclear Pt(II) Complex Xiaolong Yang, Bo Jiao, Jing-Shuang Dang, Yuanhui Sun, Yong Wu, Guijiang Zhou, and Wai-Yeung Wong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18330 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 6, 2018

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ACS Applied Materials & Interfaces

Achieving High-Performance Solution-Processed Orange OLEDs with the Phosphorescent Cyclometallated Trinuclear Pt(II) Complex Xiaolong Yang,† Bo Jiao,‡ Jing-Shuang Dang,† Yuanhui Sun,† Yong Wu,† Guijiang Zhou,*,† Wai-Yeung Wong*,¶

†

MOE Key Laboratory for Nonequilibrium Synthesis and Modulation of Condensed

Matter, Institute of Chemistry for New Energy Material, Department of Chemistry, School of Science, State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P. R. China. E-mail: [email protected] ‡

Key Laboratory for Physical Electronics and Devices of the Ministry of Education,

School of Electronic and Information Engineering, Xi’an Jiaotong University, Xi’an 710049, P. R. China. ¶

Department of Applied Biology and Chemical Technology, The Hong Kong

Polytechnic University, Hung Hom, Hong Kong, P. R. China. E-mail: [email protected]

Keywords Organic light-emitting diodes; Phosphorescence; Trinuclear Pt(II) complex; Triphenylamine; Efficiency Abstract Cyclometallated Pt(II) complexes can show intense phosphorescence at room 1

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temperature. Their emission properties are determined by both the organic ligand and the metal center. However, most of the related studies focus on tuning the properties by designing different types of organic ligands, only several reports investigate the key role played by the metal center. To address this issue, phosphorescent Pt(II) complexes with one, two and three Pt(II) centers are designed and synthesized. With more Pt(II) centers, the cyclometallated multinuclear Pt(II) complexes display red-shifted emissions with increased photoluminescence quantum yields (PLQYs). Most importantly, solution-processed organic light-emitting diodes (OLEDs) with the conventional device structure using the multinuclear Pt(II) complexes as emitters show excellent performance. The controlled device based on the conventional mononuclear Pt(II) complex shows the peak external quantum efficiency (EQE), current efficiency (CE) and power efficiency (PE) of 6.4%, 14.4 cd A−1 and 12.1 lm W−1, respectively. The efficiencies are dramatically improved to 10.5%, 21.4 cd A−1 and 12.9 lm W−1 for the OLED based on the dinuclear Pt(II) complex, and 17.0%, 35.4 cd A−1 and 27.2 lm W−1 for the OLED based on the trinuclear Pt(II) complex. To the best of the authors’ knowledge, these efficiencies are among the highest ever reported for the multinuclear Pt(II) complex-based OLEDs.

1. Introduction Organic light-emitting diodes (OLEDs) have drawn increasing attention from both scientific and industrial communities due to their great potential application in 2


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next-generation displays and solid-lightings.1, 2 As the key active materials in OLEDs, emitters take the unique position of deciding the emission colors as well as the OLED performance. Since the Pt(II) and Os(II) complexes were successfully used as emitters for OLEDs,3, 4 phosphorescent organic complexes containing heavy-metal Ir(III) or Pt(II) centers become a research hotspot because they can fully utilize electrogenerated singlet (25%) and triplet (75%) excitons to reach 100% internal quantum yield.5-11 Generally, phosphorescence emissions of organic complexes originate from triplet metal-to-ligand charge transfer states (3MLCT) or a mixture of 3

MLCT with ligand-based triplet excited states.12, 13 Therefore, the metal center has

significant effect on the emission properties of related organic complex. However, to date, numerous efforts have been paid to design and synthesize various organic ligands to manipulate the properties of organic complexes.14, 15 In comparison, little attention is paid to the question that what if one more metal center is incorporated into the organic complexes.16-18 At present, only a handful of studies investigated the properties of multinuclear Pt(II) complexes. Kozhevnikov and co-workers synthesized some cyclometallated dinuclear Pt(II) complexes (Scheme 1) and reported that the dinuclear Pt(II) complexes could show red-shifted phosphorescent emissions with enhanced emission efficiencies compared with their corresponding mononuclear Pt(II) complexes.19, 20 Zhu and co-workers reported some cyclometallated dinuclear Pt(II) complexes and fabricated OLEDs using these dinuclear Pt(II) complexes as emitters.21-24 The solution-processed OLED based on Pt2L3 could show a maximum luminance of 205 cd m−2 and a maximum current efficiency (CE) of 0.1 cd A−1.22 The 3



device based on Pt2L4 showed a peak external quantum efficiency (EQE) of 8.86% at the current density of 0.76 mA cm−2.23 Tronnier and Strassner synthesized a series of cyclometalated dinuclear N-heterocyclic biscarbene Pt(II) complexes.25 At a doping level of 30 wt%, the thermal-deposited OLED based on Pt2L5 could show the highest EQE, CE and power efficiency (PE) of 8.5%, 24.9 cd A−1 and 10 lm W−1, respectively. Although trinuclear Pt(II) complexes were also reported,26-30 only two trinuclear Pt(II) complexes (Scheme 1) were used for fabricating OLEDs.30 The OLED based on Pt3L1 displayed the best performance with a peak luminance of 375 cd m−2 and PE of 0.1 cd A−1. The abovementioned studies indicate that multinuclear Pt(II) complexes can show very different properties compared with conventional mononuclear Pt(II) complexes. With rigid molecular structure, the PLQYs of Pt(II) complexes can be increased by incorporating one more Pt(II) center (e.g., Pt2L1 and Pt2L2). Therefore, phosphorescent multinuclear Pt(II) complexes have great potential application in fabricating high performance OLEDs. However, the efficiencies of OLEDs based on multinuclear Pt(II) complexes are still very low. This may be result from their low PLQYs, e.g., 0.009 for Pt2L3, or the lack of the functional group to facilitate the charge injection/transport process. To solve these problems, in this study, three triphenylamine-based Pt(II) complexes have been designed and synthesized. The triphenylamine core is chosen for two reasons. 1) The electron-rich triphenylamine core has the advantage to improve the hole injection/transport ability of the resultant complexes.31 2) The star-shaped triphenylamine core is very suitable for synthesizing 4


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multinuclear Pt(II) complexes with relative rigid molecular structure to improve their PLQYs.32 The phosphorescent emission peaks are red-shifted from ca. 577 nm for the mononuclear Pt(II) complex to ca. 590 nm for the multinuclear Pt(II) complexes. Furthermore, the PLQYs are increased from 0.24 for the mononuclear Pt(II) complex to 0.74 for the trinuclear Pt(II) complex. Most importantly, compared with the reference OLED based on the mononuclear Pt(II) complex, the solution-processed orange OLEDs based on the multinuclear Pt(II) complexes exhibit dramatically improved performance, i.e., the electroluminescence efficiencies are greatly increased, representing the highest values for OLEDs based on multinuclear Pt(II) complexes so far. Compared with the studies that incorporating two metal centers into one molecule to enhance the emission efficiencies,16-20, 33, 34 this work further demonstrates that it is promising to improve the emission efficiencies by incorporating one more metal center to form the trinuclear complex emitter, which implies that Pt(II) complexes with suitable organic ligands containing four or five Pt(II) centers may display stronger emission. However, even though this study provides a new strategy to synthesize the multinuclear Pt(II) complexes with much higher yields compared with convectional ways, the yield significantly drops with increasing metal centers. Therefore, new efficient synthetic method should be developed to obtain complexes with metal centers more than three. There is another point that is worth to be mentioned. Maybe some simple and efficient synthetic routes will be discovered in the future, and then complexes bearing four or even more metal centers will be popular. However, even the PLQY would increased up to 1.0 for complexes with four 5



or even more metal centers, it is diseconomy to fabricate OLEDs based on these multinuclear complexes for their high cost. Therefore, there exits a trade-off between the efficiency and the cost. Hence, it seems that dinuclear and trinuclear Pt(II) complexes may be a wise choice for this trade-off. Nevertheless, this study provides a new way to develop efficient phosphorescent emitters for high performance OLEDs.

6


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Scheme 1. Chemical structures of some di- and trinuclear Pt(II) complexes.19, 20, 22, 23, 25, 30

N

N

Pt

O

Pt O

O

N

Pt

N

O

O

O

Pt

O O

Pt2L2 (ref. 20)

Pt2L1 (ref. 19) F

F CH2O

F N

O

OH2C

O

F N

Pt O

Pt O

O

Pt2L3 (ref. 22)

O

O

C8H17O

O

S

O

N N Pt

N

Pt

N

N

Pt

N

N

N N S

OC8H17

O

N

Pt

O O

Pt2L4 (ref. 23)

Pt2L5 (ref. 25) F

F N Pt O

N

O O O N

N Pt

O

O

O

6

N

O

O

O

F

6

6

O

O

Pt3L1 (ref. 30)

N Pt F

F

F

N

N

O

Pt

F

O

6

O

O

F

N

N Pt

O

F

O O

6

O

O Pt3L2 (ref. 30)

6

N

O O

F

N Pt

O

F

F

7



2. Results and Discussion 2.1. Synthesis The synthetic route to the mono-, di- and trinuclear Pt(II) complexes has been illustrated in Scheme 2. The detailed synthetic procedures are described in the Supporting Information (SI). At first, three formyl-substituted triphenylamine compounds were obtained through Vilsmeier–Haack reaction.35 Then, the organic ligands (L1−L3) were prepared by treating 2-aminobenzenethiol with the corresponding formyl-substituted triphenylamine compounds in the presence of H2O2 and HCl. The mononuclear Pt(II) complex MPt was synthesized according to the reported procedure by reacting K2PtCl4 with the ligand L1 at molar ratio of ca. 1.0.36 However, if the feeding ratio between K2PtCl4 and organic ligand was 2.0 for DPt or 3.0 for TPt in order to achieve high yield according to the conventional method, the coordination sites in the organic ligand should be fully utilized. In this case, nearly insoluble large chloride-bridged precursor Pt(II) complexes with the configuration of linear for DPt or bulky for TPt should be formed (see Figure S1a in SI) to make the synthesis of DPt and TPt very difficult. Therefore, DPt and TPt were hardly to be prepared using the conventional method (the yield of TPt < 1%). To solve this problem, we adopted a new strategy to facilitate their synthesis by adding the terminating ligand of 2-phenylpyridine (Hppy). After adding Hppy (1/2 amount to K2PtCl4), the much smaller chloride-bridged precursor Pt(II) complex (Figure S1b) would be formed more easily, which could be beneficial to improve the yield. Although the addition of Hppy was an economically ineffective way due to the 8


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formation of (2-phenylpyridinato-N,C2')Pt(2,4-pentanedionato-O,O) [(ppy)Pt(acac)], DPt and TPt had been successfully synthesized in much higher yields (9.2% and 5.6%, respectively). At last, all the desired Pt(II) complexes were obtained as orange solids, and their well-defined chemical structures had been confirmed by NMR spectroscopy (Figure S2−S4).

Scheme 2. Synthetic route to the mono-, di- and trinuclear Pt(II) complexes.

2.2. Physical Characterization We studied the thermal properties of MPt, DPt and TPt by thermogravimetric analysis (TGA). The TGA results showed that the decomposition temperature (Td) of these complexes were higher than 260 °C (Table 1, Figure S5), indicating their good thermal stability. UV−vis absorption spectra of MPt, DPt and TPt in THF at room temperature are 9



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shown in Figure 1a. Strong absorption bands peaking at ca. 260 nm observed for all of these complexes can be assigned to π-π* ligand-centered (LC) transitions. Due to their high molar extinction coefficient (ε > 24000 M−1 cm−1) and broad profile, the arresting absorption band located in the range of 425-490 nm can be ascribed to the intraligand charge transfer (ILCT) (from triphenylamine group to benzothiazole unit). In detail, the more electron-acceptor moiety (benzothiazolyl) exists in the Pt(II) complex, the stronger absorption in the range of 425-490 nm observed (log ε = 4.38, 4.73, 5.12 for MPt, DPt and TPt, respectively). Besides, the onset electronic transition absorptions have been significantly red-shifted from 495 nm for MPt to 520 nm for both DPt and TPt (Figure 1a). These results further imply the ILCT character of the intense absorptions in the range of 425-490 nm. It is interesting that the absorption peak of TPt (479 nm) is slightly blue-shifted compared with that of DPt (485 nm). In longer wavelength region, the typical low-energy metal-to-ligand charge-transfer (MLCT) absorption bands cannot be detected clearly, which indicates the low participation of Pt(II) center in frontier molecular orbitals of these Pt(II) complexes.

Table 1. Photophysical, thermal data and HOMO/LUMO levels for these Pt(II) complexesa

MPt

DPt

TPt

λem (nm) at 77 K

PLQY

Td

HOMO

/in film

(in THF/film)

(°C)

/ LUMO (eV)

494 (3.2 ns),

570 (1.83 µs)

0.24/0.28

266

–5.24/–2.66

375 (4.21), 451 (4.38)

577 (0.19 µs)

/577 (1.86µs)

254 (4.75), 325 (4.30),

513 (1.5 ns),

583 (1.33 µs)

0.53/0.61

345

–5.27/–2.68

386 (4.39), 485 (4.73)

591 (0.19 µs)

/592 (1.33 µs)

255 (5.16), 310 (4.92),

512 (2.3 ns),

579 (1.97 µs)

0.74/0.81

315

–5.28/–2.69

392 (4.92), 479 (5.12)

588 (0.24 µs)

/590 (1.0 µs)

λabs (nm)

λem (nm) at r.t.

264 (4.42), 303 (4.25),

10


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a

The λabs was measured in THF at a concentration of 10–5 M, log ε values are shown in parentheses; the λem and

corresponding lifetime shown in parentheses were measured in THF solutions at room temperature (r. t.) and 77 K

as well as in doped films; the PLQY was measured in degassed THF solutions relative to fac-[Ir(ppy)3] (PLQY = 0.96 )37 at r.t.; the PLQY of doped film was measured using an integrating sphere; the Td was tested under N2 atmosphere at a heat rate of 20 K min–1; the HOMO and LUMO energy levels were calculated using the first

anodic peak potential (Epa) and cathodic peak potential (Epc) according to the equations EHOMO = – (Epa + 4.8) eV and ELUMO = – (Epc + 4.8) eV.

Figure 1. Absorption and photoluminescence of these Pt(II) complexes: (a) absorption spectra at r.t., (b) PL spectra at r.t., (c) PL spectra in doped PMMA films at ca. 10.0 wt%, (d) PL spectra at 77 K, and (e) PL spectra of MPt in toluene, chloromethane (DCM), THF and DMF at r.t. (numbers stand for the peak wavelengths). 11



Upon UV light excitation, all complexes display dual-band emission spectra in THF solutions (Figure 1b). As listed in Table 1, the short lifetimes (3.2 ns for MPt, 1.5 ns for DPt and 2.3 ns for TPt) of these high-energy emissions peaking around 500 nm imply their fluorescent character which origins from singlet states. The low-energy emissions in the range from 577 nm to 591 nm are phosphorescence from triplet excited states indicated by their much longer lifetimes in the microsecond regime (0.19 µs for MPt, 0.19 µs for DPt and 0.24 µs for TPt). When doped in polymethyl methacrylate (PMMA) films, these Pt(II) complexes show similar emissions in low-energy region to those in the solutions. However, the emission lifetimes are greatly increased to the range of 1.0 − 1.86 µs, indicating their triplet character. Besides, the intensities of low-energy emissions have been significantly increased due to the absence of oxygen in films and the effective restriction of intra-molecular motion by the rigid PMMA matrix to promote radiative decay of the triplet states. This result further evidences the phosphorescent character of the low-energy emissions. As shown in Figure 1d, compared with the emissions recorded at room temperature, the intensity of the low-energy emissions are also greatly enhanced, and the maximum emission wavelengths are blue-shifted at 77 K, which have been widely observed for other phosphorescent organometallic emitters.38-41 According to the previous studies,19, 20 phosphorescent emissions should be notably red-shifted by introducing more Pt(II) centers, which is the case for MPt and DPt. However, the emission maximum for TPt shows a slight hypochromic effect with respect to that of DPt. This interesting phenomenon has not been reported previously, 12


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but it is reasonable since the lowest-energy absorption band of TPt is also slightly blue-shifted compared with that of DPt. Another interesting observation is that with increasing the number of Pt(II) centers, the intensities of the fluorescence are decreased while the phosphorescence are enhanced. Accordingly, the PLQYs are increased from 0.24 for MPt to 0.53 for DPt and 0.74 for TPt in solutions, which demonstrates the success of our design strategy. The solvatochromism behaviors of these Pt(II) complexes were investigated in different solvents (toluene, chloromethane, THF and DMF). As shown in Figure 1e and Figure S6, the fluorescent emission peaks of these Pt(II) complexes are red-shifted with increasing the polarity of the solvent. For example, the fluorescent emission of MPt displays a vibronic structure with the peak located at 485 nm in toluene, while this emission becomes broad and structureless with the peak red-shifted to 515 nm in DMF. The positive solvatochromism indicates the strong intramolecular charge transfer from the electron donor (arylamine) to the electron acceptor (benzothiazole) moieties, implying that the fluorescences of these Pt(II) complexes could be dominated by 1ILCT states.42 However, the phosphorescent emissions of these Pt(II) complexes show very weak solvatochromic effect, implying that the phosphorescent emissions mainly result from the 3π-π* states.43 2.3. Theoretical Calculation

To provide insight understanding about the nature of the excited states of these complexes with different numbers of metal centers, theoretical calculations based on 13



density functional theory (DFT) and time-dependent density functional theory (TD-DFT) were performed. We firstly investigated the properties of highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of these Pt(II) complexes (Figure 2 and Table 2). Both HOMOs and LUMOs of these Pt(II) complexes are mainly located on the organic ligands, with minor contributions from the Pt(II) centers, and the HOMO→LUMO transitions make over 80% contribution to both T1 and S1 of these Pt(II) complexes, indicating that the HOMO→LUMO transitions show dominated ligand-centered characters. Therefore, it is reasonable that no obvious MLCT absorption bands could be detected for MPt, DPt and TPt (Figure 1a). As indicated by the more dispersed patterns of the frontier molecular orbitals of DPt and TPt, the introduction of more Pt(II) centers as well as electron-acceptor benzo[d]thiazole groups to form the dinuclear and trinuclear Pt(II) complexes will effectively extend the conjugation length as well as enhance the donor-acceptor charge-transfer process. Besides, although the contribution from Pt(II) centers to the HOMOs is minor, it increases with the introduction of more Pt(II) centers into complexes, which may also help to enhance the charge-transfer process to some extent. Therefore, the effective extension of the conjugation length and the enhancement of the donor-acceptor charge-transfer process should be responsible for the red-shift of absorption from the mononuclear complex to the multinuclear complex. Compared with DPt, TPt possesses one more electron-acceptor benzo[d]thiazole moiety and Pt(II) center. However, the third electron-acceptor moiety and Pt(II) center make more contribution to the HOMO than to the LUMO of 14


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TPt, meaning that the HOMO is more dispersive than the LUMO in TPt, which may stabilize the energy level of the HOMO more than that of the LUMO, leading to an enlarged energy gap. Hence, TPt shows slightly blue-shifted absorption band compared with DPt. This calculation result is in good accord with the absorption experimental result. Table 2. Theoretical calculation results for these Pt(II) complexes based on their optimized S0 a

geometries

MPt

DPt

TPt a

Contribution from

Contribution from

Pt to HOMO

Pt to LUMO

4.08%

4.14%

5.30%

6.04%

4.71%

4.82%

State

λcal (nm)

S1

444

T1

569

S1

491

T1

597

S1

495

T1

591

f 0.7606

Percentage contribution of H→L to S1 or T1 H→L (95.91%) H→L (90.53%)

1.0317

H→L (97.70%) H→L (88.34%)

0.7873

H→L (97.55%) H→L (81.30%)

H→L: HOMO to LUMO transition; λcal: calculated wavelength; f : oscillator strength.

Figure 2. HOMOs and LUMOs distributions for these Pt(II) complexes.

15



In order to further understand the nature of the electronic transitions, natural transition orbitals (NTOs) for the S0→T1 excitations were evaluated based on the geometry of T1 state for these Pt(II) complexes. The calculated results suggest that the phosphorescent emissions of these Pt(II) complexes mainly arise from the 3π‒π* excited states, which has been experimentally supported by the solvatochromic behavior in solvents with different polarities. As shown in Figure 3, the weights of the hole-particle contribution to the excitations of these Pt(II) complexes are over 97%, and both hole and particle orbitals are obviously localized on the cyclometalating ligand (> 90%, Table S1 in SI), which reveals the predominated ligand-centered (LC) excitation character of T1 states. Although the contributions from Pt(II) centers to both hole and particle orbitals are very similar for these Pt(II) complexes, the distribution of both hole and particle orbitals for DPt and TPt are more dispersive on the cyclometalating ligand as well as the Pt(II) centers with increasing the number of Pt(II) centers, which will stabilize the excited states, and thereby lead to red-shifted emissions. Due to the very similar hole and particle orbitals distribution of TPt to those of DPt, which indicates the very similar excited states, TPt can show almost identical phosphorescent emission to DPt. This theoretical calculation result is consistent with the experimental observations.

16


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Figure 3. NTOs for S0→T1 excitation for these Pt(II) complexes at optimized T1 geometries.

2.4. Electrochemical Properties

The redox potentials of these Pt(II) complexes were measured by cyclic voltammetry (CV) in MeCN solutions containing n-Bu4NPF6 as the supporting electrolyte. The ferrocene/ferrocenium couple (Fc/Fc+) was used as the reference. (Figure S7). Two irreversible oxidation processes were observed for MPt with the anodic peak potentials located at 0.44 and 0.99 V, which are assigned to the oxidation process of the electron-rich triphenylamine core and the Pt(II) center, respectively.44, 45 Complexes DPt and TPt also showed two irreversible oxidation processes similar to MPt. However, the first anodic peak potentials of DPt and TPt were 0.47 and 0.48 V, respectively. Their slightly increased anodic peak potentials may caused by the fact that more electron-acceptors in DPt and TPt will lower the electron density of the 17



triphenylamine core. Therefore, the HOMO levels of DPt and TPt (–5.27 and –5.28 eV, respectively) are slightly deeper than that of MPt (–5.24 eV), but they are still higher than that of the (ppy)Pt(acac) (–5.32 eV) without triphenylamine group,46 indicating the influence of triphenylamine core on the HOMO levels. During the cathodic scan process, MPt, DPt and TPt showed similar reduction behaviors with the cathodic peak potentials located at –2.14, –2.12 and –2.11 V, respectively, which may result from the reduction of benzo[d]thiazole moieties. Anyway, with more benzo[d]thiazole moieties, trinuclear complex TPt possesses the deeper LUMO level of –2.69 eV, which may be beneficial to the charge injection process in OLEDs.

2.5. Electroluminescence Characterization To evaluate the electroluminescence (EL) properties of these Pt(II) complexes, solution-processed OLEDs were fabricated with conventional device structure of indium tin oxide (ITO)/PEDOT:PSS (45 nm)/x wt% Pt emitter:TCTA (30 nm)/TPBI (45 nm)/LiF (1 nm)/Al (100 nm), in which the poly(ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) layer was the hole-injection/transport layer. The compound 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA) was used as the host, and these Pt(II) complexes were doped at the levels of 7, 9 and 11 wt% (Figure 4). The compounds 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) and LiF were used as the electron-transport and the electron-injection materials, respectively.

18


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DPt MPt

−2.66 −2.68 −2.69 −2.7

-3

ITO −4.8

Pt:TCTA

−5.24 −5.27 −5.28

TPBI (45 nm)

−3.4

-4

-5

TPt −2.4

PEDOT:PSS (45 nm)

-2

E (eV)

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


−5.2

-6

−5.7

−2.9

LiF/Al Pt emitter

Device (Doping level)

MPt :

A1 (7 wt%), A2 (9 wt%), A3 (11 wt%)

DPt :

B1 (7 wt%), B2 (9 wt%), B3 (11 wt%)

TPt :

C1 (7 wt%), C2 (9 wt%), C3 (11 wt%)

−6.2

x wt% Pt : TCTA (30 nm)

Figure 4. Device configuration and molecular structures of the materials for the OLEDs.

19



350 (b) A2 B2 C2

0.8 0.6 0.4 0.2 0.0 450

500

550

600

650

700

Current density (mA cm− 2)

(a)

250

104

200

103

150

102

100

101

50 100

0

750

105

A2 B2 C2

300

−

EL Intensity (a.u.)

1.0

Luminance (cd m−2)

4

6

8

10

12

14

Voltage (V)

Wavelength (nm) 102 (d)

(c)

10

2

EQE (%) 10

101

101

−

CE (cd A− 1)

101

CE

0

100

A2 B2 C2

100

101

102

103

104

105

PE (lm W−1)

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

Page 20 of 32

10

PE

10

0

A2 B2 C2 0

10

1

10

2

10

3

10

4

10

5



Figure 5. EL characteristics of OLEDs based on MPt, DPt and TPt: (a) EL spectra, (b) J – V – L properties, (c) curves of EQE vs luminance, and (d) curves of CE and PE vs luminance. The EL characteristics of OLEDs based on MPt, DPt and TPt are shown in Figure 5, Figure S8 and Figure S9. The key EL data are summarized in Table 3. Although the EL spectra show feeble emissions located around 510 nm, all of the OLEDs can emit intense orange light with EL emission peaks similar to the phosphorescence peaks in PL spectra obtained from the doped films (Figure 1c), indicating that the EL emissions predominately origin from the triplet excited states of these Pt(II) complexes. The turn-on voltages of these OLEDs are in the range of 3.0–4.0 V, which decrease slightly as increase the number of Pt(II) centers in the Pt(II) complexes. The devices based on the mononuclear Pt(II) complex MPt show common efficiencies with the peak EQEs, CEs and PEs over 4.7%, 10.5 cd A−1 and 20


Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


7.5 lm W−1, respectively. However, the devices based on the dinuclear Pt(II) complex DPt display much better performance. The peak efficiencies are significantly increased up to 10.5%, 21.4 cd A−1 and 12.9 lm W−1, and can still remain as high as 10.1%, 20.5 cd A−1 and 9.4 lm W−1 at a high luminescence of 1000 cd m−2. To the best of our knowledge, this is the best performance ever reported for OLEDs using dinuclear Pt(II) complexes as emitters.21-25, 47-49 The most exciting result is that the device based on the trinuclear Pt(II) complex TPt achieves unexpected high peak efficiencies of 17.0%, 35.4 cd A−1 and 27.2 lm W−1 with very low efficency roll-off at a high luminescence of 1000 cd m−2. This is a benchmark for OLEDs based on trinuclear Pt(II) complexes.30 The remarkably improved device performance can be attributed to the significantly increased PLQYs from the mononuclear to the dinuclear and to the trinuclear Pt(II) complex, and better charge balance with more Pt(II) centers,

demonstrating the

great potential of developing highly efficient

phosphorescent emitters with multimetal centers. Table 3. EL data for these solution-processed devices

a

Device

Emitter

A1

MPt (7 wt%)

λELmax

V

Lmax a

(nm)

(V)

580

3.8/7.8/10.7

EQE −2

CE

b

(cd m )

(%)

21204

5.7/4.8

PE −1 b

(cd A )

(lm W−1)b

12.8/10.7

11.4/4.3

A2

MPt (9 wt%)

580

3.3/7.6/10.6

23784

6.4/5.6

14.4/12.5

12.1/5.2

A3

MPt (11 wt%)

580

3.6/8.9/12.5

18934

4.7/4.4

10.5/9.7

7.5/3.4

B1

DPt (7 wt%)

592

3.4/7.8/11.7

23555

8.1/6.7

16.5/13.7

13.9/5.5

B2

DPt (9 wt%)

592

3.3/6.8/10.2

22895

10.5/10.1

21.4/20.5

12.9/9.4

B3

DPt (11 wt%)

592

3.5/8.2/13.2

17542

6.9/6.2

14.0/12.6

8.0/4.9

C1

TPt (7 wt%)

592

3.0/6.5/10.2

30355

14.0/12.9

29.1/27.0

17.7/13.1

C2

TPt (9 wt%)

592

3.0/7.1/10.0

45475

17.0/15.9

35.4/33.2

27.2/14.6

C3

TPt (11 wt%)

592

3.7/8.5/11.8

33812

11.2/10.3

24.0/21.2

13.2/7.7

−2

b

Driving voltages (V) in the order of at 1, 1000 and 10000 cd m , respectively. EQE, CE and PE in the order of

the maximum value and at 1000 cd m−2.

21



3. Conclusions In conclusion, phosphorescent Pt(II) complexes with one, two and three Pt(II) centers have been designed and synthesized to investigate the effects from the heavy-metal center on the properties of the resultant emitters. Compared with the mononuclear Pt(II) complexes, the multinuclear Pt(II) complexes exhibit red-shifted absorption and emission bands. Moreover, by incorporating more Pt(II) centers, the phosphorescent emission can be greatly enhanced with the PLQYs increasing from 0.24 to 0.74 in solutions. Accordingly, the device performance of solution-processed orange OLEDs using these Pt(II) complexes is remarkably improved. The peak EQE, CE and PE of the device based on the trinuclear Pt(II) complex are 17.0%, 35.4 cd A−1 and 27.2 lm W−1, respectively, which are over two-fold higher than those of the device based on the mononuclear Pt(II) complex. These results demonstrate that it is promising to increase the PLQYs as well as the related EL efficiencies by incorporating more hevay-metal centers into the organometallic emitters, providing a new way to develop high performance phosphorescent materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Detailed synthesis of these Pt(II) complexes. NMR, TGA, CV tests, EL efficiencies of other Devices.

AUTHOR INFORMATION Corresponding Author 22


Page 22 of 32

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*E-mail: [email protected] *E-mail: [email protected]

Notes The authors declare no competing financial interest. Acknowledgements The authors acknowledge the financial support received from the National Natural Science Foundation of China (Nos. 21602170 and 21572176), the Natural Science Foundation of Shaanxi Province (No. 2016JQ2011), the China Postdoctoral Science Foundation (No. 2015M580831), the Fundamental Research Funds for the Central Universities (Nos. cxtd2015003 and xjj2016061) and Creative Scientific Research Team in Yulin City.

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31


photo-physical

and


TOC Graphic

S

O

N Pt

O N EQE: 6.4% CE: 14.4 cd A−1 PE: 12.1 lm W−1

EQE: 17.0% CE: 35.4 cd A−1 PE: 27.2 lm W−1

32


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Achieving High-Performance Solution-Processed Orange OLEDs with

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