Highly Efficient Long-Wavelength Thermally Activated Delayed

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Highly Efficient Long-Wavelength Thermally Activated Delayed Fluorescence OLEDs Based on Dicyanopyrazino Phenanthrene Derivatives Shipan Wang, Zong Cheng, Xiaoxian Song, Xianju Yan, Kaiqi Ye, Yu Liu, Guochun Yang, and Yue Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14796 • Publication Date (Web): 01 Mar 2017 Downloaded from http://pubs.acs.org on March 2, 2017

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ACS Applied Materials & Interfaces

Highly Efficient Long-Wavelength Thermally Activated Delayed Fluorescence OLEDs Based on Dicyanopyrazino Phenanthrene Derivatives Shipan Wang,† Zong Cheng,† Xiaoxian Song, † Xianju Yan,† Kaiqi Ye,† Yu Liu*,† Guochun Yang,*,‡ and Yue Wang*,† †

State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, P. R. China.

‡

Centre for Advanced Optoelectronic Functional Materials Research and Key Laboratory for UV Light-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, Changchun 130024, P. R. China

ACS Paragon Plus Environment

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Abstract

Highly efficient long-wavelength thermally activated delayed fluorescence (TADF) materials are developed using 2,3-dicyanopyrazino phenanthrene (DCPP) as the electron acceptor (A), carbazole (Cz), diphenylamine (DPA) or 9,9-dimethyl-9,10-dihydroacridine (DMAC) as the electron donor (D). Due to the large, rigid π-conjugated structure and strong electronwithdrawing capability of DCPP, TADF molecules with emitting colors ranging from yellow to deep-red are realized with different electron-donating groups and π-conjugation length. The connecting modes between donor and acceptor, i.e, with or without the phenyl ring as π-bridge are also investigated to study the π-bridge effect on the thermal, photophysical, electrochemical and electroluminescent properties. Yellow, orange, red and deep-red organic light-emitting diodes (OLEDs) based on DCPP derivatives exhibit high efficiencies of 47.6 cd A−1 (14.8 %), 34.5 cd A−1 (16.9 %), 12.8 cd A−1 (10.1 %) and 13.2 cd A−1 (15.1 %), with Commission Internationale de L’Eclairage (CIE) coordinates of (0.44, 0.54), (0.53, 0.46), (0.60, 0.40) and (0.64, 0.36), respectively, which are among the best values for long-wavelength TADF OLEDs.

Keywords: thermally activated delayed fluorescence, dicyanopyrazino phenanthrene derivatives, organic light-emitting diodes, long-wavelength emitters, intramolecular charge transfer excited states

1. Introduction Organic light emitting diodes (OLEDs) based on noble metal-free thermally activated delayed fluorescence (TADF) are raising increasing interest for their potential to achieve 100% internal


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quantum efficiencies through efficient up-conversion of non-radiative triplets to radiative singlets.1−3 Though the performance of most TADF OLEDs are still not as good as that of phosphorescent OLEDs, TADF OLEDs show the great potential as the next-generation OLEDs due to high costs and limited resources of phosphorescent materials.4−5 Recently, significant progress has been made in green and blue TADF OLEDs, with the external quantum efficiency (EQE) exceeding 20%.6−19 Nevertheless, as one of the three primary colors, the electroluminescent (EL) performance of red TADF devices, which meet the standard of emission maximum (λmaxEL) beyond 600 nm and the Commission Internationale de L’Eclairage (CIEx,y) coordinates of (x ≥ 0.60, y ≤ 0.40), are still far from satisfactory due to their low EQE (450 °C) than those of DMAC-based compounds (Td10%) far beyond the 5% theoretical EQE limit of traditional fluorescent OLEDs, which was evidence of effective utilization of electrically generated 75% triplet excitons. As confirmed by the PL measurements, the Cz-DCPP and Cz-Ph-DCPP based OLEDs emitted yellow light with emission maxima at 560 and 564 nm and Commission Internationale de L’Eclairage (CIE) coordinates of (0.44, 0.54) and (0.46, 0.52), respectively. Due to higher ΦPL of Cz-DCPP film, the Cz-DCPP based device


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showed a higher peak current efficiency (CE) of 47.6 cd A−1, peak power efficiency (PE) of 44.0 lm W−1, and maximum EQE of 14.8% than those of Cz-Ph-DCPP based device (35.5 cd A−1, 32.8 lm W−1 and 11.6%). The DPA-DCPP based OLED gave red EL centered at 616 nm with CIE coordinate of (0.61, 0.38), while the DPA-Ph-DCPP based OLED emitted deep-red light

Figure 5. The energy-level diagram of the multilayer devices and chemical structures of the used materials. with an emission maximum at 644 nm and CIE coordinate of (0.64, 0.36). Notably, compared with our previous work using a unipolar electron-transport host TPBi (EQE~10%), the performance of DPA-Ph-DCPP based device was greatly enhanced using the bipolar host mCPPy2PO. The DPA-Ph-DCPP based device showed a turn-on voltage of 3.2 V and maximum CE of 13.2 cd A−1, PE of 12.9 lm W−1 and EQE of 15.1%. The EL performance were much better than that of D-A type compound DPA-DCPP (EQE~10.4%), which was the reason that the larger kTADF rate of DPA-Ph-DCPP induced more triplet excitons harvested. The EL performance


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of DPA-Ph-DCPP with various doping concentrations was shown in Figure S19. DPA-Ph-DCPP showed concentration-dependent spectral red-shift with increasing doping concentration from 5 wt% to 30 wt%. Besides, in our previous work, its non-doped device showed an emission peak at 710 nm.25 One reason for this significant spectral red-shift with increasing concentration could be ascribed to the strong polarization effect as shown in other reported red ICT compounds.32-35 On the other hand, its relatively planar structure predicted by the theoretical calculations probably induces strong dipole–dipole interactions or intermolecular π-π stacking in neat solid state films.36-38 The devices showed similar maximum EQE values of nearly 15% at low doping level of 5-10 wt%. With further increasing doping concentrations, the maximum EQE value decreased rapidly to 5.9% for 20 wt% doping level and 3.3% for 30 wt% doping level. In addition to the aggregation-induced quenching effect, we further prepared the single-carrier devices to study the influence of charge balance. As shown in Figure S20, with increasing doping concentration, the hole-current gradually increased while the electron-current decreased, indicating that the more unbalanced hole and electron transport at high doping concentrations, which may be another reason for the reduced EQE value. The OLED containing DMAC-DCPP exhibited red emission peaking at 624 nm with CIE coordinate of (0.60, 0.40), while the DMACPh-DCPP device emitted orange light with an emission peak at 596 nm and CIE coordinate of (0.53, 0.46). Due to the much higher ΦPL and kF of DMAC-Ph-DCPP, its device also realized higher efficiencies of 34.5 cd A−1, 32.8 lm W−1 and 16.9% than those of the DMAC-DCPP device (12.8 cd A−1, 12.2 lm W−1 and 10.1%), which were among the best values of orange-red TADF OLEDs (see Table S3). With increasing luminance, the EQEs of the Cz and DPA based devices were dramatically decreased. Take DPA-DCPP with a long τTADF of 579 µs for example, its maximum EQE was


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10.1% at 1 cd m−2, and rapidly decreased to 0.9% at a practical brightness at 500 cd m−2. Because the bipolar host possessed good charge balance, the severe efficiency roll-offs should be mainly attributed to the long τTADF of Cz and DPA based compounds endowing increased triplet exciton density, which could easily induce triplet–triplet annihilation (TTA) or singlet–triplet annihilation (STA) processes.18,22 Because of the much smaller ∆EST and relatively shorter τTADF of the DMAC-based compound, their devices showed reduced EQE roll-offs and their EQE remained at 4.2% for DMAC-DCPP and 6.3% for DMAC-Ph-DCPP at a practical brightness at 500 cd m−2. Therefore, in addition to high ΦPL, TADF materials with high kTADF rate to lower τTADF and triplet density should be synthesized to enhance the EQE and improve the efficiency roll-offs in long-wavelength TADF OLEDs. Due to the lack of related encapsulation equipment, we conducted the operational stability experiment of the OLEDs in air. The lifetime of DCPP derivatives based devices were compared with that of a well-known phosphorescent material factris(2-phenylpyridine) iridium(III) (Ir(ppy)3) based device.39 Figure S21 presented the relative luminance of the OLEDs as a function of time under a constant current. The devices were operated at an initial luminance of 200 cd m-2. The operational lifetimes of Cz-DCPP, DPA-PhDCPP, DMAC-Ph-DCPP and Ir(ppy)3 devices were 0.37, 1.00, 0.16 and 1.10 h, respectively, when the luminance drops to 50% of the initial value. The lifetime of DPA-Ph-DCPP device was comparable to that of Ir(ppy)3 device, suggesting the molecular structure of DPA-Ph-DCPP was electrically stable. The lifetime of DMAC-Ph-DCPP and Cz-DCPP device were much shorter than that of DPA-Ph-DCPP, which may be due to the electrochemical instability of the DMAC and Cz donors for their irreversible oxidation reactions.


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Figure 6. a) Current density–voltage–luminance (J–V–L), b) EQE–luminance characteristics, c) Power efficiency–luminance characteristics, d) EL spectra recorded at 10 mA cm-2 of the devices based on the DCPP-based TADF compounds. Table 3. Electroluminescence characteristics of the OLEDs. Device a)

Von

Lmax

EQEmax

CEmax

PEmax

EQE500

CIE100

(V)

(cd m−2)

(%)

(cd A−1)

(lm W−1)

(%)

(x, y)

Cz-DCPP

3.4

13130

14.8

47.6

44.0

3.1

0.44,0.54

Cz-Ph-DCPP

3.4

8978

11.6

35.5

32.8

1.9

0.46,0.52

DPA-DCPP

3.0

4219

10.4

14.4

15.1

0.9

0.61,0.38

DPA-Ph-DCPP

3.2

5333

15.1

13.2

12.9

1.6

0.64,0.36

DMAC-DCPP

3.3

7917

10.1

12.8

12.2

4.2

0.60,0.40

DMAC-Ph-DCPP

3.3

11540

16.9

34.5

32.8

6.3

0.53,0.46

a)

Von: turn-on voltage at 1 cd m−2, Lmax: maximum luminance, EQEmax: maximum external EL quantum efficiency, CEmax: maximum current efficiency, PEmax: maximum power efficiency, EQE500: obtained at 500 cd m−2, CIE100: CIE coordinates measured at 100 cd m−2.


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4. Conclusions In conclusion, we have developed a series of long-wavelength TADF emitters using 2,3dicyanopyrazino phenanthrene (DCPP) as the electron acceptor. With enhanced electrondonating groups and elongated conjugation length, these TADF molecules successfully show yellow (560 nm) to deep-red (650 nm) emission. To illustrate the molecular structure and property

relationship,

we

systematically

investigated

their

thermal,

electrochemical,

photophysical and electroluminescent properties. Compared with the Cz and DPA donor based molecules, the DMAC donor based molecules showed extremely smaller ∆EST values due to the large steric hindrance and dihedral angles. The introduction of π-bridge in the D-A-D-type molecules has a different influence on the kF and kTADF of diverse donors. Enlarged undesirable rotational relaxation between the donor and acceptor may cause a lower ΦF for the Cz and DPA donor based molecules. Using a new bipolar host mCPPy2PO, the OLED devices based on these emitters exhibited high efficiencies of 47.6 cd A−1 (14.8%) for Cz-DCPP, 34.5 cd A−1 (16.9%) for DMAC-Ph-DCPP, 12.8 cd A−1 (10.1 %) for DMAC-DCPP and 13.2 cd A−1 (15.1%) for DPA-Ph-DCPP with CIE coordinates of (0.44,0.54), (0.53,0.46), (0.60, 0.40) and (0.64,0.36), respectively. However, the devices of Cz and DPA donors based compounds suffered from more severe efficiency roll-offs compared with that of DMAC donor based molecules due to that the larger ∆EST and longer delayed fluorescence lifetimes induce serious excitons annihilation. Thus, the molecular structures should be carefully designed for fabricating low efficiency roll-off OLEDs. We believe that these results give a platform for developing novel high performance long-wavelength TADF materials.


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ASSOCIATED CONTENT Supporting Information. Equations of the rate constants; TGA and DSC; cyclic voltammogram; the absorption and PL spectra with different solvents, phosphorescence spectra, temperature dependence of the transient PL spectra, and single carrier device performance. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (Y.L.) *E-mail: [email protected]. (G.Y.) *E-mail: [email protected]. (Y.W.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (91333201), the National Basic Research Program of China (2015CB655003) and Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT101713018).


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Highly Efficient Long-Wavelength Thermally Activated Delayed

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