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Highly efficient soluble blue delayed fluorescent and hyperfluorescent organic light-emitting diodes by host engineering Sang Kyu Jeon, Hee-Jun Park, and Jun Yeob Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17260 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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ACS Applied Materials & Interfaces

Highly efficient soluble blue delayed fluorescent and hyperfluorescent organic lightemitting diodes by host engineering Sang Kyu Jeon+, Hee-Jun Park+, Jun Yeob Lee* School of Chemical Engineering, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea E-mail: [email protected] + S. K. Jeon and H. -J. Park contributed equally.

Abstract Solution processed high efficiency fluorescent organic light-emitting diodes with an external quantum efficiency over 18% was developed by engineering a host material and device structure designed for solution process. A high triplet energy host material designed for solution process, (oxybis(3-(tert-butyl)-6,1-phenylene))bis(diphenylphosphine oxide) (DPOBBPE), worked efficiently as the host of blue fluorescent device due to good solubility, high photoluminescence quantum yield and good film properties. The DPOBBPE host enabled high external quantum efficiency of 18.8% in the fluorescent organic light-emitting diodes by solution process. Moreover, 25.8% external quantum efficiency in the soluble blue thermally activated delayed fluorescent devices was also realized. The 25.8% external quantum efficiency of the DPOBBPE delayed fluorescent device and 18.8% of the fluorescent device are the highest efficiency values achieved in the solution-processed blue fluorescent organic light-emitting diodes. Moreover, the solution-processed fluorescent device showed improved blue color coordinate of (0.14,0.20) compared to (0.17,0.31) of the delayed fluorescent device. Keywords: quantum efficiency⋅fluorescence⋅blue device⋅solution process⋅hyperfluorescence

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Introduction Organic light-emitting diodes (OLEDs) are generally produced by a vacuum thermal evaporation process, but solution-processed OLEDs are drawing a great deal of attention because large-scale color patterning of OLEDs using a fine metal mask is technically challenging. Solution-processed OLEDs use ink-jet printing technology for large-scale color patterning and can be a candidate for the fabrication of large OLEDs. However, there remain several challenging issues, such as the low efficiency and short lifetime of solution-processed OLEDs1. Although there has been much progress in terms of the device performance of solutionprocessed OLEDs, the efficiency and lifetime of these solution-processed OLEDs are still far lower than those of vacuum-processed OLEDs2,3. In particular, device performance of soluble blue OLEDs is much worse than that of soluble red and green OLEDs even though soluble blue phosphorescent OLEDs have been developed4-8. The best external quantum efficiency (EQE) achieved by soluble blue OLEDs was 20.0% using a blue phosphorescent bis[2-(4,6difluorophenyl)pyridinato-C2,N](picolinato)iridium(III) emitter9. An alternative to the highefficiency soluble blue OLED is the soluble blue thermally activated delayed fluorescent (TADF) OLED. It has been confirmed that soluble blue TADF OLEDs can perform as well as soluble blue phosphorescent OLEDs, achieving a high EQE of 19.7% using a 2,3,4,5,6penta(9H-carbazol-9-yl)benzonitrile emitter (5CzCN)10 and 20.0% using a 2,3,4,6tetra(9Hcarbazol-9-yl)-5-fluorobenzeonitrile emitter11. Therefore, soluble blue TADF OLEDs are potentially as efficient as soluble blue OLEDs. However, the soluble blue TADF OLEDs suffer from a lack of host materials satisfying both the high triplet energy and good solubility. In vacuum-deposited OLEDs, the well-known bis[2-(diphenylphosphino)phenyl]ether oxide (DPEPO) worked efficiently as the host12-17, but this performed poorly in the solutionprocessed OLEDs due to poor solubility in common aromatic solvents like toluene despite the high triplet energy of 3.1 eV. Moreover, carbazole-type host materials cannot fully harvest all 2 Environment ACS Paragon Plus

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excitons of the blue TADF OLEDs due to their relatively low triplet energy10, 18-20. Therefore, high triplet energy host materials showing good solubility are necessary in order to improve the efficiency of solution-processed blue TADF OLEDs. Moreover, the development of the soluble host materials for blue TADF emitters can realize soluble hyperfluorescent OLEDs which emit from fluorescent emitter sensitized by TADF emitters21-26. Until now, no blue hyperfluorescent OLED has been demonstrated by solution process due to lack of soluble host materials. Herein, we describe the synthesis of a soluble high triplet energy host material and device evaluation of the soluble host material in solution-processed blue TADF OLEDs and blue hyperfluorescent OLED. The host material developed in this work, (oxybis(3-(tert-butyl)-6,1phenylene))bis(diphenylphosphine oxide) (DPOBBPE), worked efficiently as the host of a blue-emitting 5CzCN TADF emitter10,23 and enabled 25.8% EQE in the soluble blue TADF OLEDs. Moreover, co-doping of 5CzCN and 2,5,8,11-tetra-tert-butylperylene (TBPe) in the DPOBBPE host allowed high EQE of 18.8% in the soluble blue hyperfluorescent OLEDs. The 25.8% EQE of the DPOBBPE device is the highest EQE achieved in any solutionprocessed blue OLEDs, including soluble blue phosphorescent OLEDs and the 18.8% EQE of the hyperfluorescent OLEDs is also the best EQE of soluble blue fluorescent OLEDs21. Moreover, the solution-processed fluorescent device showed improved blue color coordinate of (0.14,0.20) compared to (0.17,0.31) of the delayed fluorescent device. In fact, this is the first work reporting blue hyperfluorescent OLEDs by solution process.

Results and discussion In order to develop the hyperfluorescent OLEDs, a high triplet energy soluble host material is essential. Therefore, DPOBBPE was designed as a soluble host to have high triplet energy, limited intermolecular interaction, and good solubility in organic solvents. A diphenylphosphine oxide modified diphenylether backbone structure was the core structure to 3 Environment ACS Paragon Plus


obtain high triplet energy, and a t-butyl substituent was introduced for solubility as well as to suppress intermolecular interaction within emitters. Due to the diphenylphosphine oxide modified diphenylether backbone structure, the triplet energy of the newly designed host material is expected to be over 3.0 eV. This is sufficient for energy transfer to a blue emitting 5CzCN emitter, which is known to be one of the most efficient soluble TADF emitters10. The emission wavelength range of the 5CzCN emitter is from 400 nm to 580 nm, which is within the triplet energy of the DPEPO host material. Assuming that DPOBBPE has similar triplet energy to DPEPO, the host can fully harvest the triplet excitons of 5CzCN to achieve TADF emission without loss. Therefore, DPOBBPE can outperform the previous diphenyldi(4-(9carbazolyl)phenyl)silane (SiCz) host material and overcome the problem of poor solubility of DPEPO. For the synthesis of DPOBBPE, we adopted a similar procedure to that used in the preparation of DPEPO (Scheme 1). The phosphorylated intermediate, oxybis(3-(tert-butyl)6,1-phenylene))bis(diphenylphosphine) (DPBBPE), was obtained by a reaction between the starting material (4,4’-di-tert-butyldiphenyl ether) and chlorodiphenylphosphine with 50% yield. Subsequent oxidation of the intermediate with hydrogen peroxide produced DPOBBPE with a 90% yield. A detailed synthesis procedure is illustrated in the Supporting Information.

Scheme 1. Synthetic procedure of DPOBBPE.

Density functional theory (DFT) was employed to optimize the ground state of the host material. As shown in the calculation results by the B3LYP 6-31G basis set in Figure 1(a), 4 Environment ACS Paragon Plus

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the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of DPOBBPE were dispersed over the diphenylether and diphenylphosphine oxide, respectively. The electron-withdrawing character of the diphenylphosphine oxide unit dominated the HOMO and LUMO distribution. The dihedral angles of the diphenylether core in DPOBBPE (118.8o) was larger than that of the core in DPEPO (110.1o)22, suggesting a large distortion of the backbone structure and large steric hindrance. This may be advantageous to prevent intermolecular interaction in the emitting layer. Ultraviolet-visible (UV-Vis) absorption and room temperature and low temperature photoluminescence (PL) spectra of DPOBBPE in CH2Cl2 are shown in Figure 1(b). The DPOBBPE host exhibited high-energy absorption bands at around 230 nm, attributable to the π-π* transition of the diphenylether core structure. In addition, the weaker low-energy bands corresponding to n-π* transition appeared at around 295 nm. Their PL emission spectra, which ranged from 280 to 430 nm with peaks around 315 nm. To determine the triplet energy (T1), the phosphorescent emission peak was collected from a frozen solution of the compounds at 77 K after delay time of 1.5 µs. For DPOBBPE, the T1 energy was calculated to be 3.15 eV, indicating that this material is comparable to DPEPO in terms of T1. The T1 of DPOBBPE was high enough to cover the shortest emission wavelength of 400 nm of 5CzCN. Material characterization data of DPOBBPE are summarized in Table S1 in supporting information.



Figure 1. HOMO and LUMO calculation results of DPOBBPE (a), UV-vis, solution PL and low temperature PL spectra of DPOBBPE (b), solid PL spectra of DPOBBPE:5CzCN (c), and transient PL decay data of DPOBBPE:5CzCN (d).

PL characteristics of the DPOBBPE were further analyzed by doping the host with 5CzCN. Steady-state PL emission, transient PL decay and absolute PL quantum yield of the 5CzCNdoped DPOBBPE were examined to check the energy transfer and delayed fluorescence behavior. Steady-state PL data of the solution-coated DPOBBPE:5CzCN film in Figure 1(c) demonstrates complete energy transfer from the DPOBBPE host to the 5CzCN emitter. Solid PL data according to doping concentration are in Figure S1. As anticipated from the PL characterization results, the high T1 of the host allowed for efficient energy transfer, which was also confirmed by the transient PL data of the doped film. The energy transfer and delayed fluorescence of 5CzCN were found to be activated by the DPOBBPE host. One feature observed in the PL spectra is the blue-shift of the emission spectrum in the DPOBBPE 6 Environment ACS Paragon Plus

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host compared to that in the DPEPO host. This is due to intermolecular interaction blocking effect of t-butyl group in the DPOBBPE. Electronic orbital calculation results revealed that the dipole moment of DPOBBPE is similar to that of DPEPO, but the PL peak wavelength was shifted to short wavelength, indicating that solid state solvatochromic effect was insignificant in the DPOBBPE film. The t-butyl group spatially separated the highly polar phosphine oxide core from the 5CzCN emitter, which prevented red-shift of the PL emission spectra. Moreover, the PL spectra of DPOBBPE:5CzCN were not red-shifted at high doping concentration, suggesting that the DPOBBPE also hinders strong intermolecular interaction between 5CzCN emitter. The delayed fluorescence lifetime of the solution-coated DPOBBPE:5CzCN film based on the transient PL decay data shown in Figure 1(d) was 20.6 µs, which is comparable to that of DPEPO:5CzCN film10. Doping concentration dependent transient PL data of DPOBBPE:5CzCN films are also shown in Figure S2 in comparison with those of DPEPO:5CzCN films and the delayed fluorescent lifetimes are summarized in Table S2 . There was little change of the delayed fluorescent lifetime up to 15% in the DPOBBPE films, but there was gradual decrease of the delayed fluorescent lifetime in the DPEPO films, proving that the intermolecular interaction between 5CzCN emitters is suppressed by the DPOBBPE host. The PL quantum yield of the DPOBBPE:5CzCN film was 0.86 at 25% doping concentration, which was higher than that of DPEPO:5CzCN reported in previous work.[10] The increased PL quantum yield of 5CzCN in the DPOBBPE may be due to t-butyl functional group induced limited intermolecular quenching. PL quantum yields under nitrogen at 15% and 20% doping concentrations were 0.88 and 0.86, respectively, which was reduced under oxygen due to triplet exciton quenching. There was little change of the PL quantum yields according to 5CzCN doping concentration as shown in Table S3, demonstrating that intermolecular exciton quenching is suppressed by the DPOBBPE host. 7 Environment ACS Paragon Plus


The HOMO/LUMO levels of DPOBBPE were -6.69/-2.51 eV. Both HOMO and LUMO levels were similar to those of DPEPO host due to the two electron-withdrawing diphenylphosphine oxide units. The cyclic voltammetry data used to measure the HOMO/LUMO levels were added in Figure S3 in the supporting information. The solubility of DPOBBPE in toluene was analyzed because the main objective of designing the host was to guarantee high solubility for solution processing. The solubility value of DPOBBPE in toluene was 7.5 wt%, much higher than the 0.5 wt% of DPEPO or 3.3 wt% of previous SiCz hosts. Therefore, the DPOBBPE host fulfilled the requirement of high solubility for solution processing. Furthermore, the solution-coated DPOBBPE formed smooth film by spin coating. Atomic force microscopy analysis of the spin-coated DPOBBPE film provided root-mean-square surface roughness values of 0.18 nm (Figure S4 in supporting information), which was maintained even after doping of 5CzCN in the DPOBBPE host. Solution-processed blue TADF OLEDs were fabricated by doping the 5CzCN emitter in the DPOBBPE host at a doping concentration ranging from 15% to 25%. The doping concentration was adjusted high for application as a sensitizer in the hyperfluorescence OLEDs. Device characterization results, represented by current density–voltage, luminancevoltage, EQE-current density data, and electroluminescence (EL) spectra are presented in Figure 2. In the DPOBBPE host system, high doping concentration led to an increase in current density and luminance, as previously reported in other TADF devices fabricated using the DPEPO host13,15. As shown in the energy level diagram in Figure 2(d), there is a large HOMO gap between the host and 5CzCN, which induced hole trapping and transport by the 5CzCN emitter. Therefore, the current density was increased at high 5CzCN doping concentration due to improved hole transport. The EQE was optimized at 25% doping concentration in the DPOBBPE device. Because the DPOBBPE host is electron transport type host, the EQE was optimized at a rather high doping concentration by facilitated hole 8 Environment ACS Paragon Plus

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injection. The maximum EQE of the DPOBBPE device was 25.8%, and the EQE was 22.8% at 100 cd/m2. The 25.8% EQE of the DPOBBPE device is one of the best EQE values ever reported in blue OLEDs; the best EQE achieved by soluble blue phosphorescent OLEDs was 20%[9], while the best EQE achieved by soluble blue TADF OLEDs was also 20%11. The DPOBBPE:5CzCN device outperformed other soluble blue OLEDs in the literature. As explained by the material characterization data, the high T1 of 3.15 eV that can fully harvest singlet and triplet excitons of 5CzCN, good film-forming properties like the low surface roughness of 0.18 nm, the high solubility of 7.3 wt% in toluene, and the high PL quantum yield of 0.86 are major factors that contribute to the high EQE of the solution-processed DPOBBPE device. Electroluminescence (EL) spectrum at 100 cd/m2 in Figure 2(c) reflected only 5CzCN emission and the color coordinate of the DPOBBPE device was (0.17, 0.31). The EL emission spectra were not red-shifted at high doping concentration in the DPOBBPE devices because of restricted intermolecular interaction by the t-butyl unit. All device performances are listed in Table 1.



Figure 2. Current density-voltage-luminance data of DPOBBPE:5CzCN (a) and external quantum efficiency-current density data of DPOBBPE:5CzCN (b) and EL spectra of DPOBBPE:5CzCN devices according to doping concentration of 5CzCN at 100 cd/m2 (c). Energy level diagram of the DPOBBPE:5CzCN devices (d).

Figure 3. EQE-luminance data (a) and EL spectra (b) of the DPOBBPE:5CzCN:TBPe hyperfluorescent devices at 100 cd/m2.


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Table 1. External quantum efficiency, current efficiency and color coordinate of the DPOBBPE:5CzCN and DPOBBPE:5CzCN:TBPe hyperfluorescent devices.

Devices

Quantum efficiency [%]

Current efficiency [cd/A]

Power efficiency [lm/W]

Color coordinate

Max

100 cd/m2

Max

100 cd/m2

Max

100 cd/m2

DPOBBPE : 5CzCN 15%

20.0

16.0

37.6

30.2

18.5

9.4

(0.17, 0.28)

DPOBBPE : 5CzCN 20%

24.3

20.4

48.1

40.6

24.6

13.2

(0.17, 0.30)

DPOBBPE : 5CzCN 25%

25.8

22.8

52.3

46.8

27.1

17.3

(0.17, 0.31)

Device I

19.5

16.8

31.1

26.9

16.3

11.6

(0.15, 0.23)

Device II

17.2

12.9

25.3

19.2

17.2

11.2

(0.14, 0.20)

Device III

18.8

16.3

29.1

25.4

14.3

10.9

(0.14, 0.20)

Based on the high EQE of the DPOBBPE:5CzCN devices, soluble hyperfluorescence OLEDs were devised by doping a blue fluorescent TBPe emitter at a doping concentration range from of 0.5~1.0%. TBPe was chosen as the blue fluorescent emitter because of good solubility in toluene and emission wavelength for blue emission. The thickness of the emitting layer was also controlled to optimize the device performances. Three devices with different TBPe concentrations and emitting layer thicknesses were fabricated. Device I (25 nm, 0.5%) and II (23 nm, 1.0%) had different TBPe concentrations, and device III (40 nm, 1.0%) had different emitting layer thickness. EQE-luminance data and EL spectra of the hyperfluorescent OLEDs are displayed in Figure 3(a) and (b). Other device performances are in Figure S5 in supporting information. The current density and luminance were greatly dependent on the emitting layer thickness and device III showed the lowest current density and luminance. The maximum EQEs of the DPOBBPE:5CzCN:TBPe hyperfluorescent devices were reduced according to TBPe doping concentration as observed in other publications24. At doping



concentrations at 0.5%, 5CzCN emission dominates the device performances due to incomplete energy transfer, which resulted in the high EQE as shown in Figure 3(a). The TBPe emission was dominant at 1.0% doping concentration. The EQE of the device II was 17.2% and it was even further enhanced to 18.8% in the device III. Considering the EQE value lower than 5.0% of the conventional TBPe device, the EQE of the TBPe device was enhanced by more than three times due to sensitization by the TADF emitter.24-26 The EQE of the current soluble hyperfluorescent OLEDs is comparable to that of corresponding vacuum deposited device24. The DPOBBPE allowed for the fabrication of solution processed blue hyperfluorescent device assisted by TADF sensitizer and high EQE. The PL quantum yield of the DPOBBPEL:5CzCN:TBPe film was 0.80 under nitrogen, which contributed the high EQE of the hyperfluorescent OLEDs. This is the first demonstration of soluble hyperfluorescent OLEDs based on TADF sensitized blue fluorescent emission. The development of DPOBBPE realized the high EQE blue hyperfluorescent OLEDs by solution process. EL spectra of the hyperfluorescent OLEDs in Figure 3(b) reflected the EL emission of TBPe fluorescent emitter at 1.0% doping concentration. At doping concentrations below 1.0%, the TBPe emission was weak due to incomplete energy transfer from the 5CzCN TADF emitter to TBPe. The contributions of 5CzCN and TBPe emission were 69 and 31%, respectively, by peak separation of the EL spectra (Figure S6). Although the peak position of the 5CzCN is slightly red-shifted compared to that of TBPe, energy transfer from the 5CzCN to TBPe was observed. This behavior can be explained by different origin of the emission process. Charge transfer is the main emission process of 5CzCN and local emission is the emission process of TBPe. In the local emission of TBPe, vibrational peaks are observed and the highest energy peak is observed as the main peak in the spectrum. However, the peak wavelength is not the highest emission peak of the CT emission spectrum. There are several high energy emission peaks which have shorter wavelength than the peak wavelength. Therefore, energy transfer


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from the 5CzCN to TBPe is possible although the peak wavelength of the emission is slightly blue-shifted in the TBPe compared to that of 5CzCN

Conclusions In conclusion, high-efficiency solution-processed blue hyperfluorescent OLEDs with an EQE exceeding 18% and a blue color coordinate of (0.14,0.20) were firstly demonstrated using a soluble DPOBBPE host with a high triplet energy of 3.15 eV, a good solubility of 7.3 wt% in toluene and low surface roughness of 0.18 nm after spin coating. The new host material also demonstrated a state of the art EQE over 25% in solution-processed blue TADF OLEDs. Therefore, the DPOBBPE host can be applied in various blue solution-processed OLEDs involving triplet excitons in the light emission process.

Experimental Synthesis Detailed synthesis and analysis of the material is described in the supporting information.

Device Fabrication and Measurements: The basic structure of the device was indium tin oxide (ITO) (150 nm)/ MoO3 (15 nm)/poly(4-butylphenyl-diphenyl-amine) (Poly-TPD) (15 nm)/ poly(9-vinylcarbazole) (PVK) (70 nm)/DPOBBPE:5CzCN (40 nm)/ diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1) (5 nm)/ 1,3,5-tris( N -phenylbenzimidazole-2-yl) benzene (TPBi) (30 nm)/LiF (1 nm)/ Al (200 nm). The MoO3 solution was fabricated by applying Lin et al.'s method and used as a hole injection layer27. The dispersed MoO3 solution was spin coated at 5000 rpm followed by annealing at 150 °C for 30 min. Poly-TPD and PVK films were coated from 0.4 wt% and 1.2 wt% chlorobenzene solutions, respectively, and annealed at 120 °C for 30 min. The emitting layer films were prepared using 1.1 wt% toluene solutions by spin coating at a spin speed of 2000 rpm followed by annealing at 80 ºC for 30 13 Environment ACS Paragon Plus


min. MoO3, Poly-TPD, PVK and the emitting layer were coated by spin coating in a nitrogencirculated glovebox. TSPO1, TPBi, LiF and Al were vacuum deposited at a base pressure of 1.0 × 10-7 torr. In the case of the hyperfluorescence OLEDs, TPBe was additionally doped in the emitting layer at a doping concentration from 0.5 to 1.0 wt%. Transient decay time measured with a fluorescence lifetime spectrometer (C11367 Quantaurus-tau, Hamamatsu Photonics, Japan) using UV-LED (340 nm) source system. Electrical and optical analyses were performed using a Keithley 2400 source measurement unit and a CS2000 spectroradiometer.

Acknowledgements This work was supported by the Basic Science Research Program (2016R1A2B3008845), Nano Materials Research Program (2016M3A7B4909243) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning, and development of stretchable OLED by MOTIE.

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