Triplet Harvesting by a Fluorescent Emitter Using a Phosphorescent

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Triplet Harvesting by a Fluorescent Emitter using a Phosphorescent Sensitizer for Blue Organic Light-Emitting Diodes Hyun-Gu Kim, Hyun Shin, Yeon Hee Ha, Ran Kim, Soon-Ki Kwon, Yun-Hi Kim, and Jang-Joo Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17957 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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ACS Applied Materials & Interfaces

Triplet Harvesting by a Fluorescent Emitter using a Phosphorescent Sensitizer for Blue Organic Light-Emitting Diodes Hyun-Gu Kim1, Hyun Shin1, Yeon Hee Ha3, Ran Kim3, Soon-Ki Kwon4*, Yun-Hi Kim3* and Jang-Joo Kim1,2*

1Department

of Materials Science and Engineering, Seoul National University, Seoul, 08826,

South Korea 2Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul, 08826, South Korea 3Department of Chemistry and RIGET, Gyeongsang National University, Jinju, 52828, South Korea 4Department of Materials Engineering and Convergence Technology and ERI, Gyeongsang National University, Jinju, 52828, South Korea

Keywords blue fluorescent organic light-emitting diodes (OLEDs), phosphorescent sensitizer, spinmixing process, Fӧrster energy transfer, conventional fluorescent emitter

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Abstract Development of highly efficient blue organic light-emitting diodes (OLEDs) with good stability is currently the most important issue in OLED displays and lighting. This paper reports an efficient blue fluorescent OLED based on a deep-blue emitting phosphorescent sensitizer ((dfpysipy)2Ir(mpic)) and a conventional fluorescent emitter (TBPe). Efficient triplet harvesting by the fluorescent emitter occurs in the OLED due to sensitization even though the difference in emission energy between the phosphorescent and fluorescent emission was only 0.05 eV. These results clearly demonstrate the potential for realizing highly efficient blue fluorescent OLEDs using phosphorescent sensitizers without requiring ultraviolet-emitting phosphorescent dye.


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The development of efficient blue organic light-emitting diodes (OLEDs) with long operational lifetimes has been a challenging issue for practical applications such as flat-panel displays and solid-state lighting. Heavy metal complexes and thermally activated delayed fluorescence (TADF) materials have yielded highly efficient blue OLEDs via harvesting of triplet excitons.1-10 However, the blue OLEDs still suffer from limited operational lifetime. Rather than developing stable phosphorescent or TADF molecules, conventional fluorescent emitters with either phosphorescent or TADF sensitizers have been proposed as a way to realize a long operational lifetime and high efficiency at the same time, where excited states are spin-mixed in phosphorescent or TADF sensitizers followed by energy transfer (ET) to fluorescent emitters.11–18 The use of TADF sensitizers has proven highly successful in recent years, demonstrating efficient blue fluorescent OLEDs with external quantum efficiencies (EQEs) up to 18.8% (achieved by reducing triplet ET from the sensitizer to the fluorescent emitter),13,19–22 as well as a highly efficient deep-blue fluorescent OLED with an EQE of 19.0% and Commission Internationale de L’Eclairage (CIE) coordinates of (0.14, 0.15) (achieved by using a fluorescent emitter with a small Stokes shift).23 However, spin-mixing in the TADF sensitizers is not always efficient because reverse intersystem crossing (RISC) of the TADF materials depends on the charge transfer (CT) and locally excited states, which can be influenced by the host polarity.24–26 Additionally, if incomplete ET from the sensitizer to the fluorescent emitter occurs, blue fluorescent OLEDs with high color purity would not be expected due to additional broad spectral emission from the TADF materials, caused by the CT characteristics. In terms of spin-mixing process, phosphorescent dyes as the sensitizer would be more efficient than TADF dyes because of the large spin-orbit coupling. Using phosphorescent dyes as the sensitizer was proposed many years ago,11,12 and highly efficient conventional fluorescent OLEDs with long lifetimes have been achieved using phosphorescent dyes as the sensitizers.17 However, efficient blue fluorescent OLEDs with long lifetimes would be much 3 ACS Paragon Plus Environment


more difficult to achieve than those of other colors, and have not yet been reported to our best knowledge. The realization of phosphorescent dye-sensitized blue fluorescent OLEDs is difficult because of the challenges in finding the ultraviolet (UV) emitting phosphorescent dyes required for efficient ET from the phosphorescent dye to the blue emitting fluorescent emitter. In this paper, we first report an efficient blue fluorescent OLED using a phosphorescent sensitizer and a conventional fluorescent emitter. Although the energy difference between the triplet of the phosphorescent sensitizer and the singlet of the fluorescent emitter was small, we fabricated a fluorescent OLED with an EQE of 15.3% by efficient triplet harvesting via sensitization. Combined with the recent development of deep blue phosphorescent OLEDs, this work demonstrates the potential for realizing highly efficient blue fluorescent OLEDs using phosphorescent sensitizers. Absorption of organic materials in solution or films were measured using a spectrophotometer (Cary 5000; Agilent). A continuous-wave He-Cd laser (325 nm) was used as the excitation source for photoluminescence (PL) measurements. PL spectra and angle-dependent PL were measured using a fiber optics spectrometer (Maya2000; Ocean Optics) as the optical detection system. The PL quantum yields (PLQYs) of the films were measured using an integrating sphere and a photomultiplier tube. Details of the experimental procedures are described elsewhere.27,28 The transient PL was measured using a pulsed nitrogen laser (337 nm; Usho Optical Systems) as the excitation light source and a streak camera (C10627; Hamamatsu Photonics) as the optical detection system. OLEDs were fabricated by the deposition of organic layers and the Al electrode on pre-cleaned 70-nm-thick ITO-coated glass substrates. All the layers were deposited by thermal evaporation under a pressure of 5 × 10–7 Torr. Current density, luminescence, and electroluminescence (EL) spectra were measured using a programmable source meter (Keithley 2400) and a spectrometer (Spectrascan PR650; Photo Research). The operational lifetimes of the OLEDs were measured until the luminance 4 ACS Paragon Plus Environment

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decreases to 50% of an initial luminance (LT50) under constant current density using a lifetime measurement system (M6000T; Polaronix). Figure 1a shows the molecular structures of the compounds used in this study. 3,3-di(9Hcarbazol-9-yl)biphenyl (mCBP) and diphenylphosphineoxide-4-(triphenylsilyl)phenyl (TSPO1) were combined into a mixed co-host with a molar ratio of 1:1. 2,5,8,11-tetra-tertbutylperylene (TBPe) was used as a conventional fluorescent emitter, and bis(2′,6′-difluoro-4(trimethylsilyl)-2,3′-bipyridine)Ir(III)(3-methylpyridinepicolinate) ((dfpysipy)2Ir(mpic)) as a phosphorescent sensitizer. Figure 1b compares the absorption and PL spectra of (dfpysipy)2Ir(mpic) and TBPe in methylene chloride with the PL spectra of mCBP and TSPO1 films. The PL spectra of mCBP and TSPO1 overlapped with the absorption of (dfpysipy)2Ir(mpic) and TBPe, and the absorption spectrum of TBPe also overlapped with the PL spectrum of (dfpysipy)2Ir(mpic). The Fӧrster radius from (dfpysipy)2Ir(mpic) to TBPe was calculated to be 3.17 nm, indicating that Fӧrster ET from the sensitizer to the fluorescent emitter can be efficient even at low doping concentrations. The triplet energies of mCBP (2.90 eV)29 and TSPO1 (3.36 eV)1 are higher than that of (dfpysipy)2Ir(mpic) (2.76 eV; estimated from the peak wavelength of PL emission), so that triplet exciton quenching from the phosphorescent dye to the host triplets is expected to be small. Additionally, it is possible to prevent the singlet excited state of TBPe from quenching to the triplet of (dfpysipy)2Ir(mpic), since the triplet energy of (dfpysipy)2Ir(mpic) is higher than the singlet energy of TBPe. The PL spectra of the films codoped with TBPe (1 wt%) and (dfpysipy)2Ir(mpic) in the mCBP:TSPO1 mixed-host showed almost same spectra of TBPe-doped films, indicating that the PL emissions of the co-doped film originated from the fluorescence of TBPe. (Figure 1c) In the PL spectra of films, slight host emission was also observed at a wavelength shorter than about 420 nm. The PLQY of doped TBPe (1 wt%) in the mCBP:TSPO1 mixed-host was 73±2 and 60±2%, respectively, when 1 and 5 wt% of (dfpysipy)2Ir(mpic) was additionally doped. 5 ACS Paragon Plus Environment


To understand the emission process, we measured the transient PL of the films with and without the phosphorescent sensitizer at the excitation wavelength of 337 nm where the absorption by mCBP and (dfpysipy)2Ir(mpic) is dominant. Figure 2a shows the transient PL of the films integrated from 435 to 620 nm. The doping concentration of TBPe and (dfpysipy)2Ir(mpic) was 1, and 5 wt%, respectively. TBPe doped in the mCBP:TSPO1 mixedhost without the (dfpysipy)2Ir(mpic) showed mono-exponential decay with a lifetime of 5.65 ns (inset of Figure 2a). The transient PL of (dfpysipy)2Ir(mpic) doped in the co-host without TBPe also showed mono-exponential decay with an exciton lifetime of 2.75 μs, characteristic of phosphorescent emission. When the phosphorescent and fluorescent dyes were co-doped, however, the transient decay curve showed multi-exponential decay combined with prompt emission in the range of nanoseconds, and delayed emission with a lifetime of 1.21 μs. The decay rate of the delayed emission was faster than the phosphorescent emission. Notably, this emission originated from TBPe as manifested in the time-resolved PL spectra (Figure 2b). The prompt and delayed emission spectra of the film doped with (dfpysipy)2Ir(mpic) and TBPe matched the emission spectrum of the TBPe-doped film in terms of the positions and intensities of the emission peaks. The prompt and delayed emission from TPBe emission in the co-doped film indicates that various excitonic processes were operating within the system; direct ET from the host molecules (mostly mCBP, considering the excitation wavelength and absorption spectra of the host molecules displayed in Figure S1, Supporting Information) to TBPe, direct ET from (dfpysipy)2Ir(mpic) to TBPe, and cascade ET from the host to (dfpysipy)2Ir(mpic) to TBPe. The increase of the delayed portion in the transient PL with an increase in the concentration of (dfpysipy)2Ir(mpic) resulted from the increased cascade ET from the host to the TBPe via (dfpysipy)2Ir(mpic), or direct ET from (dfpysipy)2Ir(mpic) to TBPe. (Figure S2, Supporting Information). Taken together, these results imply that the spinmixing process in the sensitizing system can be efficient with very little efficiency loss in OLEDs. 6 ACS Paragon Plus Environment

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The fabrication of blue fluorescent OLEDs based on our sensitizing system proved that triplet harvesting by spin-mixing in the sensitizer was indeed efficient. Figure 3a illustrates the device structures of the fluorescent OLEDs; indium tin oxide (ITO) (70 nm)/mCBP:6 wt% Re2O7 (40 nm)/mCBP (10 nm)/emitting layer (EML) (30 nm)/TSPO1 (55 nm)/Rb2CO3 (1 nm)/Al (100 nm). mCBP and TSPO1 were used as the hole transporting layer (HTL) and electron transporting layer (ETL), respectively, and as the mixed co-host to improve charge balance in the EML. p-doped mCBP and Rb2CO3 layers were used for efficient charge injection from the electrodes. Three different fluorescent OLEDs with different concentration of the (dfpysipy)2Ir(mpic) sensitizer (0, 1, and 5 wt%) in the EML were fabricated. Figure 3b shows the current density–voltage–luminance (J–V–L) curves of the devices. The devices showed the same turn-on voltage of 3.6 V, but the current density was reduced as the concentration of the sensitizer was increased. This indicates that charge trapping to the sensitizer must contribute the reduced current density in the device. The EL spectra of the devices are shown in Figure 3c. The emission peaks of EL spectra were the same and the CIE coordinates of the devices were slightly changed from (0.13, 0.19) to (0.14, 0.19) when the sensitizer was co-doped regardless of the concentration of sensitizer. The EQEs of the devices are compared in Figure 3d. Even though the PLQY of the doped TBPe was reduced as as the concentration of sensitizer was increased, the EL efficiency of device was increased. This demonstrates that charge trapping to the sensitizer resulted in efficient triplet harvesting via sensitization in the devices. The maximum EQEs of the fluorescent OLEDs was increased from 7.0% to 15.3% when 5 wt% of (dfpysipy)2Ir(mpic) was co-doped, which correspond to the maximum current efficiency and power efficiency of 18.0 cd A-1 and 15.7 lm W-1 for the fluorescent OLED with the sensitizer, respectively. (Figure S3, Supporting Information) According to the classical dipole model, with a PLQY of 60±2% and a horizontal dipole ratio of 76(±1)% for TBPe (Figure S4, Supporting Information),28 69% of the total excitons are converted to light in the device with the sensitizer under the assumption of an exciton 7 ACS Paragon Plus Environment


formation efficiency of 100% without any electrical loss. The singlet ratio in the sensitized device can be higher if electrical loss without forming excitons is considered because the device structure cannot confine the electrons in EML due to the low electron barrier between the EML and HTL. The fluorescent OLED with 5 wt% of sensitizer also showed higher EQE (5.6%) at 1,000 cd m-2 and lower efficiency roll-off compared to the fluorescent OLED without the sensitizer (1.7%). (Table S1, Supporting Information) The efficiency roll-off characteristics of the fluorescent OLEDs at high current density might also originate from the electrical loss caused by field-dependent mobility differences or charge trapping.30 If we can use a deep blue exciplex co-host having balanced hole and electron mobilities, and a lower triplet energy level than those of the constituent materials for efficient spin mixing, the fluorescent OLEDs would have further increased efficiency and yielded better roll-off characteristics. We also measured operational lifetimes of three devices to compare the device stability. (Figure S5, Supporting Information) The LT50 of the fluorescent OLED with 5 wt% of the sensitizer at the initial luminance of 400 cd m-2 was 0.89 hr, which was about 10 times longer than that of the fluorescent OLED without the sensitizer. However, the LT50 of three devices in this study are still short compared to those in industry. TSPO1 used as EML and ETL in the devices must contribute to the short operational lifetime due to low bond dissociation energy of 1.84 eV in the anion state. (Table S2, Supporting Information). Further studies about the development of stable host materials should be accompanied to realize improved device stability with high efficiency. It is also beneficial for phosphorescent dye-sensitized fluorescent OLEDs that the difference in emission wavelength between the phosphorescent sensitizer and fluorescent emitter used in this study was only 10 nm, with emission energies of 2.76 eV for phosphorescent emission and 2.71 eV for fluorescent emission. This demonstrates that sensitization is possible with blue emitting phosphorescent dyes, with no requirement for a UV emitting phosphorescent 8 ACS Paragon Plus Environment

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dyes, contrary to common belief. This work, together with recent reports of high-efficiency deep blue OLEDs, suggests a bright future for phosphorescent dye-sensitized blue OLEDs with comparable efficiency to phosphorescent OLEDs but with greatly improved device lifetime comparable to that of fluorescent OLEDs. In conclusion, an efficient blue fluorescent OLED was developed using a deep blue emitting phosphorescent sensitizer (dfpysipy)2Ir(mpic) and a blue emitting conventional fluorescent emitter (TBPe). The emission energy difference between the phosphorescent and fluorescent emission was only 0.05 eV, but the emission from the fluorescence dopant was dominant in the device. The EQE of the sensitized OLED was 15.3%, which is 2.2 times that of the fluorescent OLED without the sensitizer (7.0%). These results clearly demonstrate that sensitization by a phosphorescent dye to harvest triplets by a fluorescent emitter does not require a large energy difference between the phosphorescent sensitizer and fluorescent receiver. Using the recently developed deep blue phosphorescent dyes as a sensitizer in the phosphorescent dye-sensitized OLEDs developed in this work is expected to yield fluorescent OLEDs with comparable efficiency to phosphorescent OLEDs but with greatly improved operational lifetime comparable to that of fluorescent OLEDs.

Associated Content Supporting Information Absorption spectra of host materials (Figure S1), transient PL of films according to the concentration of sensitizer (Figure S2), angle-dependent PL intensity of TBPe (Figure S3), efficiencies of OLEDs (Figure S4, Table S1), operational lifetime (Figure S5), and bond dissociation energies of host materials (Table S2).

Author Information 9 ACS Paragon Plus Environment


Corresponding Authors *J.-J.K: [email protected]. *Y.-H.K: [email protected]. *S.-K.K: [email protected]. ORCID Jang-Joo Kim: 0000-0003-4926-2346 Yun-Hi Kim: 0000-0001-8856-4414 Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by a grant (2018R1A2A05018455) from National Research Foundation (NRF) of Korea.


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Figures

Figure 1. (a) Molecular structures of the host (mCBP, TSPO1), iridium (Ir) complex sensitizer ((dfpysipy)2Ir(mpic)), and fluorescent emitter (TBPe) (b) Absorption and photoluminescence (PL) spectra of (dfpysipy)2Ir(mpic) and TBPe in methylene chloride with the PL spectra of mCBP and TSPO1 films. (c) The PL spectra of films in the mCBP:TSPO1 mixed host doped with 1 wt% TBPe (black square), 1, 5 wt% (dfpysipy)2Ir(mpic) (red circle, blue triangle), and 1, 5 wt% (dfpysipy)2Ir(mpic) with 1 wt% TBPe (dark cyan down triangle, magenta diamond). 11 ACS Paragon Plus Environment


Figure 2. (a) Transient PL of three films in mCBP:TSPO1 mixed host doped with 5 wt% (dfpysipy)2Ir(mpic) (red), and 5 wt% (dfpysipy)2Ir(mpic) + 1 wt% TBPe (blue). The inset shows the transient PL of 1 wt% TBPe in mCBP:TSPO1 mixed film. (b) Time-resolved PL spectra of the three films.


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Figure 3. (a) Schematic diagram of fluorescent organic light-emitting diodes (OLEDs) (b) Current density–voltage–luminance (J–V–L) characteristics, (c) Electroluminescence (EL) spectra at 1,000 cd m-2, and (d) External quantum efficiencies of OLEDs as a function of luminance



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Sim, B.; Moon, C.-K.; Kim, K.-H.; Kim, J.-J. Quantitative Analysis of the Efficiency of OLEDs. ACS Appl. Mater. Interfaces 2016, 8 (48), 33010–33018.



ToC figure


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Figure 1. (a) Molecular structures of the host (mCBP, TSPO1), iridium (Ir) complex sensitizer ((dfpysipy)2Ir(mpic)), and fluorescent emitter (TBPe) (b) Absorption and photoluminescence (PL) spectra of (dfpysipy)2Ir(mpic) and TBPe in methylene chloride with the PL spectra of mCBP and TSPO1 films. (c) The PL spectra of films in the mCBP:TSPO1 mixed host doped with 1 wt% TBPe (black square), 1, 5 wt% (dfpysipy)2Ir(mpic) (red circle, blue triangle), and 1, 5 wt% (dfpysipy)2Ir(mpic) with 1 wt% TBPe (dark cyan down triangle, magenta diamond). 170x399mm (300 x 300 DPI)

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Figure 2. (a) Transient PL of three films in mCBP:TSPO1 mixed host doped with 5 wt% (dfpysipy)2Ir(mpic) (red), and 5 wt% (dfpysipy)2Ir(mpic) + 1 wt% TBPe (blue). The inset shows the transient PL of 1 wt% TBPe in mCBP:TSPO1 mixed film. (b) Time-resolved PL spectra of the three films. 292x114mm (300 x 300 DPI)


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Figure 3. (a) Schematic diagram of fluorescent organic light-emitting diodes (OLEDs) (b) Current density– voltage–luminance (J–V–L) characteristics, (c) Electroluminescence (EL) spectra at 1,000 cd m-2, and (d) External quantum efficiencies of OLEDs as a function of luminance 299x250mm (300 x 300 DPI)


Triplet Harvesting by a Fluorescent Emitter Using a Phosphorescent

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