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Host Engineering for High Quantum Efficiency Blue and White Fluorescent Organic Light-Emitting Diodes Wook Song, Inho Lee, and Jun Yeob Lee* Full color organic light-emitting diodes (OLEDs) possess red, green, and blue emitting pixels and the light-emitting performances of each pixel affect the efficiency, power consumption, and lifetime of the full color OLED panel. Therefore, the device performances of each red, green, and blue devices need to be maximized to develop high performances full color OLED panel. The most well-known approach to maximize the device performances of OLEDs was to use phosphorescent emitting materials instead of traditional fluorescent emitting materials for high quantum efficiency.[1,2] Theoretically, the quantum efficiency of the phosphorescent OLEDs can be higher than that of fluorescent OLEDs by four times and high quantum efficiency above 20% has already been reported in the red, green, and blue phosphorescent OLEDs.[3–8] Additionally, the lifetime of the red and green phosphorescent OLEDs can meet the requirement of the commercial OLED products. Therefore, the red and green phosphorescent OLEDs replaced old fluorescent OLEDs and upgraded the device performances of OLED panel. However, blue phosphorescent OLEDs could not be commercialized because of their short lifetime, in spite of their high quantum efficiency above 20%.[6–8] There are several reasons for the short lifetime of the blue phosphorescent OLEDs, but one of critical factors is intrinsic weak chemical bond between Ir and organic ligand of blue triplet emitters which are exposed to high energy blue emission.[9–11] This problem can be avoided by using organic-based fluorescent blue-light-emitting materials instead of Ir based blue triplet emitters. However, it was difficult to improve the quantum efficiency of the blue fluorescent OLEDs because triplet excitons of the fluorescent blue emitters were wasted. Although there was a literature reporting improved quantum efficiency by partially harvesting triplet excitons by triplet–triplet fusion process,[12–14] the degree of improvement was limited because theoretical maximum internal quantum efficiency by the triplet–triplet fusion process is 62.5%. One solution for the limited quantum efficiency of the typical blue fluorescent OLEDs can be to generate only singlet excitons
W. Song, Prof. J. Y. Lee School of Chemical Engineering Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi 440-746, South Korea E-mail: [email protected] Dr. I. Lee Department of Polymer Science and Engineering Dankook University 126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448-701, South Korea
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of the blue fluorescent emitter without generation of triplet excitons. The singlet exciton generation without any triplet exciton generation can be possible by photoluminescence (PL) process induced by Förster energy transfer from host to dopant material.[15,16] In this case, the host material should harvest both singlet and triplet excitons for high quantum efficiency. Therefore, the host materials harvesting both singlet and triplet excitons for Förster energy transfer may enhance the quantum efficiency of the fluorescent OLEDs. In an early work, TADF sensitized fluorescent emitting system was reported,[15] but three materials should be coevaporated and the instability of wide gap host is a serious problem. It would be better to develop a single host material which does not need any wide gap host material. In this work, a blue thermally activated delayed fluorescent (TADF) emitter, 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF), was developed as the host material of a common 2,5,8,11-tetra-tert-butylperylene (TBPe) emitter to improve the quantum efficiency of the TBPe blue fluorescent OLEDs. Maximum quantum efficiency of 15.4% and a quantum efficiency of 11.5% at 1000 cd m−2 were demonstrated in the TBPe device using the CzAcSF host. The quantum efficiency of the TBPe device was improved by more than three times compared with that of the conventional TBPe fluorescent device by engineering the host material. Additionally, white fluorescent OLEDs with TBPe and 2,8-ditertbutyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (TBRb) doped in the CzAcSF host exhibited 14.0% external quantum efficiency, which was better than any other data reported in all fluorescent white OLEDs. In the conventional blue OLEDs with an emitting layer of 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) and TBPe, only singlet–singlet energy transfer by Förster energy transfer process can give rise to light emission and triplet excitons of the MADN host decay non-radiatively. Therefore, only 25% of total excitons can be converted into photons and maximum internal quantum efficiency by energy transfer process is 25%. The 25% internal quantum efficiency of the MADN:TBPe device can be further enhanced if the triplet excitons can be converted into singlet excitons by reverse intersystem crossing process.[17] Assuming that all triplet excitons are harvested by the reverse intersystem crossing process, close to 100% internal quantum efficiency can be realized in the blue fluorescent OLEDs by Förster energy transfer process although exactly 100% internal quantum efficiency cannot be achieved due to charge trapping and Dexter energy transfer by the fluorescent dopant. Therefore, the use of the TADF material which can effectively transform triplet excitons into singlet excitons by reverse intersystem crossing process can enhance the quantum efficiency of the blue fluorescent OLEDs. The emission process
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of the fluorescent device with the TADF material as a host is schematically described in Figure 1. The host material of the TBPe fluorescent emitter should have higher singlet energy than TBPe for efficient energy transfer from the host to TBPe dopant. Therefore, a TADF emitter, CzAcSF, was developed to exhibit deep blue emission to transfer emission energy to the TBPe fluorescent emitter. The CzAcSF molecule was designed as a TADF emitter to emit at short wavelength by connecting a weak diphenylsulfone acceptor with an 9,9-dimethyl-9,10-dihydroacridine and a carbazole donor. As the TADF emitters give singlet emission from charge transfer (CT) excited state, the emission wavelength depends on the donor and acceptor strength of the emitters. Molecules with strong donors and acceptors give emission at long wavelength, while molecules with weak donors and acceptors emit at short wavelength. Therefore, the combination of weak diphenylsulfone acceptor with moderate donors such as carbazole and acridine may induce blue emission in the CzAcSF by TADF process. Compared with bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS),[18] CzAcSF may emit at short wavelength because one acridine unit of DMAC-DPS was replaced with weak electron donating carbazole. Additionally, asymmetric molecular structure of CzAcSF may be helpful to suppress selfluminous quenching by preventing molecular stacking.
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Figure 1. Schematic diagram showing the emission process of blue fluorescent device with a blue fluorescent emitter doped in a TADF host.
Scheme 1 shows synthetic process of CzAcSF. The preparation of the CzAcSF TADF emitter was simply carried out by stepwise direct amination of bis(4-fluorophenyl) sulfone with carbazole and acridine. The 9-(4-((4-fluorophenyl)sulfonyl)phenyl)carbazole intermediate with only one carbazole was not isolated and only the final CzAcSF compound was purified by column chromatography and vacuum train sublimation. The final synthetic yield was 40.8%. Basic material characteristics of CzAcSF were analyzed by ultraviolet–visible (UV–vis), PL, absolute PL and cyclic voltammetry (CV) measurements. The normalized UV–vis absorption spectrum in toluene, the PL spectrum in solid polystyrene (1 wt%), and the low temperature PL spectrum in toluene are presented in Figure 2a. Combined UV–vis absorption of carbazole and acridine was observed in the CzAcSF emitter and the absorption edge could reach 412 nm by the n–π* absorption of the acridine moiety. The PL emission peak position in polystyrene was 443 nm and the low temperature PL peak position was 466 nm. The singlet and triplet emission energies of CzAcSF were 3.11 and 3.04 eV, respectively, from the onset of singlet and triplet emission spectra. The singlet energy of CzAcSF was slightly higher than that of DMAC-DPS (3.07 eV) due to weak CT character by carbazole substitution instead of acridine. Photophysical properties of CzAcSF were compared with those of DMAC-DPS in Table S1, Supporting Information. The PL quantum yield of CzAcSF in tetrahydrofuran was 0.71. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of CzAcSF were estimated from oxidation and reduction potentials of oxidation and reduction curves in Figure S1, Supporting Information. The HOMO and LUMO of CzAcSF were −5.89 and −3.00 eV, respectively, by oxidation of the acridine moiety of CzAcSF and reduction of the diphenylsulfone moiety of CzAcSF as can be predicted from the simulated molecular orbital in Figure 2b. B3LYP 6–31G* of Gaussian program was a basis set of the electronic state calculation. Localization of the HOMO on the acridine moiety and of the LUMO on the diphenylsulfone moiety was noted in the molecular simulation and can explain the HOMO and LUMO levels of CzAcSF. The HOMO and LUMO were separated with weak overlap of the HOMO and LUMO in the phenyl unit of diphenylsulfone. The separated HOMO and LUMO of CzAcSF donated charge transfer character to the CzAcSF molecule as can be proved by the solvent dependent PL spectra in Figure S2, Supporting Information. Gradual shift of the PL emission from short wavelength to long wavelength by increasing the polarity of the solvent was observed. Additionally, there was small overlap of the HOMO and LUMO in the phenyl unit of diphenylsulfone, which is beneficial for the light emission of CzAcSF. The CzAcSF compound was also thermally stable and high glass transition temperature of 117 °C and high decomposition temperature of 371 °C were detected. Thermal analysis data of CzAcSF are summarized in Figure S3, Supporting Information.
Scheme 1. Synthetic scheme of CzAcSF.
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PL spectra of CzAcSF and TBPe doped CzAcSF films were measured to investigate the energy transfer from CzAcSF to TBPe. Figure 3a represents the PL spectra of pure CzAcSF and TBPe doped CzAcSF films. Comparing the PL spectra of CzAcSF and CzAcSF:TBPe films, PL emission of CzAcSF was not observed in the TBPe doped CzAcSF films, indicating efficient energy transfer from CzAcSF to TBPe. The energy transfer from the CzAcSF to TBPe was studied in more detail by transient PL measurement. Transient PL decay curves of CzAcSF and CzAcSF:TBPe are shown in Figure 3b. Pure CzAcSF film showed fast decay component by normal fluorescence and delayed decay component by delayed fluorescence process, indicating that CzAcSF pure film also possesses TADF character by up-conversion of triplet excitons into singlet excitons. The TBPe doped CzAcSF film also showed similar decay curve although the delayed PL emission was relatively weak. This result suggests that the emission of TBPe is predominantly dominated by energy transfer from CzAcSF to TBPe. The fast decay emission includes energy transfer from singlet excitons of CzAcSF to TBPe and direct emission of TBPe by excitation, while the delayed emission includes energy transfer from triplet excitons of CzAcSF to TBPe via up-conversion process of the triplet excitons into singlet excitons. The rather reduced delayed emission in the TBPe doped CzAcSF is due to direct excitation of TBPe which does not have any delayed PL component. Blue fluorescent OLEDs of TBPe were developed by fabricating TBPe devices with a device structure of ITO (120 nm)/ PEDOT:PSS (60 nm)/mCP (30 nm)/CzAcSF or CzAcSF:TBPe (25 nm)/TSPO1 (5 nm)/TPBI (30 nm)/LiF (1 nm)/Al (200 nm),
Figure 2. Solution UV–vis, solid PL in polystyrene and low temperature PL spectra in toluene of a) CzAcSF, b) HOMO and LUMO distribution of CzAcSF calculated using Gaussian 09 program, and c) temperature dependent PL spectra of CzAcSF at different temperatures.
Delayed fluorescent behavior of CzAcSF was analyzed using transient PL measurements. Figure 2c shows temperature dependent transient PL decay curves of CzAcSF from 100 to 298 K. Delayed emission of CzAcSF was weak at 100 and 150 K, but it was intensified at 200 and 298 K as observed in other TADF molecules.[17] The numerical fitting of the transient PL decay curve gave excited state lifetime for TADF emission of 5.6 µs.
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Figure 3. a) PL spectra of pure CzAcSF and TBPe doped CzAcSF films. b) Transient PL decay curves of pure CzAcSF and TBPe doped CzAcSF films.
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Figure 4. a) Current-density–voltage–luminance curves, b) quantum efficiency–luminance curves, and c) EL spectra of pure CzAcSF and CzAcSF:TBPe devices.
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level of −5.89 eV and LUMO level of −3.00 eV of the CzAcSF host resulted in the low turn-on and driving voltage. Quantum efficiency of the blue devices was plotted against luminance in Figure 4b. Maximum quantum efficiency of the TBPe device with the MADN host was 4.3% and the maximum quantum efficiency of the CzAcSF:TBPe device was 15.4% at a doping concentration of 0.3%. There was more than three times improvement of the quantum efficiency by using CzAcSF instead of MADN as the host material of TBPe. Although there was efficiency roll-off in the CzAcSF:TBPe device, the quantum efficiency at 1000 cd m−2 was still 11.7%. The quantum efficiency obtained in this work is better than any other data reported in blue fluorescent device. It was even better than the quantum efficiency of the TADF sensitized blue fluorescent device.[15] In particular, the quantum efficiency of the CzAcSF and CzAcSF:TBPe devices was higher than that of corresponding device with DMAC-DPS instead of CzAcSF as summarized in Figure S4, Supporting Information. It seems that asymmetric molecular structure of CzAcSF contributed to the high quantum efficiency of the CzAcSF and CzAcSF:TBPe devices by suppressing self-quenching of light-emission compared to symmetric molecular structure of DMAC-DPS. The high quantum efficiency of the CzAcSF:TBPe device can be explained by the efficient up-conversion of triplet excitons into singlet excitons in the pure CzAcSF film, efficient Förster energy transfer from CzAcSF to TBPe, little charge trapping by TBPe, and little Dexter energy transfer from CzAcSF to TBPe. The triplet to singlet reverse intersystem crossing of pure CzAcSF film confirmed by the delayed PL emission and a high maximum efficiency of 15.7% of the pure CzAcSF device support the efficient up-conversion of the triplet excitons into singlet excitons. The efficient energy transfer proved by the PL emission in Figure 3 also contributed to the high quantum efficiency because the Förster energy transfer process induces only singlet exciton formation without triplet excitons wasted. The similar quantum efficiency of the pure CzAcSF and TBPe doped CzAcSF devices hints that singlet excitons of CzAcSF were effectively converted into singlet excitons of TBPe by efficient Förster energy transfer process. Little charge trapping and little Dexter energy transfer by low TBPe doping concentration of 0.3% are another important factors for the high quantum efficiency of the CzAcSF:TBPe device. In general, both singlet and triplet excitons are formed by charge trapping process by TBPe, but triplet excitons of TBPe are lost due to forbidden transition from triplet excited state to singlet ground state. Therefore, maximum internal quantum efficiency of the TBPe device by charge trapping process is only 25% compared to 100% by Förster energy transfer process. The charge trapping can be avoided by minimizing the doping concentration of TBPe and low TBPe doping concentration of 0.3% assisted reducing the charge trapping and increasing the quantum efficiency. Similarly, Dexter energy transfer from CzAcSF to TBPe was also suppressed by low TBPe doping concentration and played a role of enhancing the quantum efficiency of the TBPe device. Device performances of the CzAcSF:TBPe system were optimized at 0.3% doping concentration because of incomplete energy transfer below 0.3% doping and charge trapping effect above 0.3% doping. The quantum efficiency could be further improved up to 18.0% by adding wide gap
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where ITO is indium tin oxide, PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), mCP is 1,3-bis(Ncarbazolyl)benzene, TSPO1 is diphenylphosphine oxide-4(triphenylsilyl)phenyl and TPBI is 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene. The doping concentration of TBPe was 0.3%. Figure 4a displays current-density–voltage and luminance– voltage characteristics of the CzAcSF and CzAcSF:TBPe devices. The current density of the blue devices was similar in the CzAcSF and CzAcSF:TBPe devices and the doping of TBPe in the CzAcSF host material did not greatly affect the current density. The luminance of the blue devices was slightly high in the pure CzAcSF device, which is due to large portion of long wavelength component of CzAcSF as shown in the PL spectra in Figure 3a. Turn on voltage at 1 cd m−2 was below 3.0 V and the driving voltage at 1000 cd m−2 was around 5.0 V in the blue devices. The HOMO
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bis[2-(diphenylphosphino)phenyl]ether oxide host to take advantage of TADF sensitized fluorescent emitting system (Figure S5, Supporting Information). Figure 4c represents electroluminescence (EL) spectra of the CzAcSF and CzAcSF:TBPe devices. Broad EL emission of the CzAcSF device disappeared in the CzAcSF:TBPe device because of the energy transfer from CzAcSF to TBPe and sharp TBPe emission with vibrational peaks was observed. The color coordinate of the CzAcSF was (0.17, 0.28), while the color coordinate of the CzAcSF:TBPe devices were (0.15, 0.23) irrespective of the doping concentration. The EL spectra of CzAcSF:TBPe device were not changed according to the driving voltage of the device because the EL emission is mainly from TBPe (Figure S6, Supporting Information). In addition to the high quantum efficiency and stable EL spectra, the lifetime of the CzAcSF:TBPe device was much longer than that of the CzAcSF device (Figure S7, Supporting Information) due to stable emission from TBPe although the absolute lifetime of the device was short because of instability of charge transport materials used in the device structure. White fluorescent OLEDs were further developed by additional doping of TBRb in the CzAcSF:TBPe emitting layer. Current-density–voltage–luminance plot, quantum efficiency– luminance plot and EL spectra of the white OLEDs are shown in Figure 5a–c. Doping concentration of TBRb was 0.2% and 0.4%. In the current-density–voltage–luminance curves, the current density and luminance were slightly reduced at 0.4% TBRb doping concentration by charge trapping effect of TBRb. Maximum quantum efficiencies of the fluorescent OLEDs at 0.2% and 0.4% doping concentrations were 15.2% and 14.0%, respectively, and the quantum efficiency of the white fluorescent OLEDs was comparable to that of CzAcSF:TBPe blue fluorescent device. In spite of additional TBRb doping, the quantum efficiency of the blue fluorescent device could be maintained without large non-radiative loss of excitons by low doping of TBRb. Extra doping of TBRb did not have great influence on the quantum efficiency although slight reduction of the quantum efficiency at 0.4% TBRb doping concentration was observed. Color coordinates of the 0.2% and 0.4% TBRb doped devices were (0.23, 0.30) and (0.31, 0.37), respectively. Pure white emission was obtained at 0.4% doping concentration. Device performances of all devices fabricated in this work are summarized in Table 1. In conclusion, high quantum efficiency blue and white fluorescent OLEDs with external quantum efficiencies of
Figure 5. a) Current-density–voltage–luminance curves, b) quantum efficiency–luminance curves, and c) EL spectra of CzAcSF:TBPe:TBRb devices.
Table 1. Device performances of the CzAcSF-based TADF and TADF sensitized fluorescent OLEDs. Emitting layer
a)
QE [%]
PEb)
CEc)
QEa)
[cd A−1]
[%]
PEb) [lm W−1]
CEc) [cd A−1]
Color coordinate
[lm W−1]
CzAcSF
15.7
26.3
28.6
10.3
11.4
18.6
(0.17,0.28)
CzAcSF:TBPe
15.4
23.4
23.7
10.7
9.7
16.5
(0.15,0.23)
CzAcSF:TBPe:TBRb (0.2%)
15.2
33.4
32.6
12.2
16.4
24.8
(0.23,0.30)
CzAcSF:TBPe:TBRb (0.4%)
14.0
36.2
35.1
11.8
18.9
28.4
(0.31,0.37)
a)QE:
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quantum efficiency; b)PE: power efficiency; c)CE: current efficiency.
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Experimental Section The device structure of CzAcSF and CzAcSF:TBPe devices was ITO (120 nm)/PEDOT:PSS (60 nm)/mCP (30 nm)/CzAcSF or CzAcSF:TBPe (25 nm)/TSPO1 (5 nm)/TPBI (30 nm)/LiF (1 nm)/Al (200 nm). TBPe was doped at a doping concentration of 0.3% to avoid charge trapping. White fluorescent OLEDs were constructed to contain CzAcSF:TBPe:TBRb emitting layer with TBRb doping concentrations of 0.2% and 0.4%. mCP was a hole transport type exciton blocking layer and TSPO1 was an electron transport type exciton blocking layer to prevent triplet exciton quenching of CzAcSF. PL and device performance measurements are described in our previous work.[19]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2013R1A1A2007991) and Ministry of Science, ICT, and future Planning (2013R1A2A2A01067447).
Adv. Mater. 2015, 27, 4358–4363
Received: March 1, 2015 Revised: May 17, 2015 Published online: June 15, 2015
[1] M. A. Baldo, D. F. O. Brian, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson, S. R. Forrest, Nature 1998, 395, 151. [2] C. Adachi, M. A. Baldo, Mark E. Thompson, Stephen R. Forrest, J. Appl. Phys. 2001, 90, 5048. [3] S. Su, H. Sasabe, Y. Pu, K. Nakayama, J. Kido, Adv. Mater. 2010, 22, 3311. [4] S. O. Jeon, S. E. Jang, H. S. Son, J. Y. Lee, Adv. Mater. 2011, 23, 1436. [5] Q. Wang, I. W. H. Oswald, X. Yang, G. Zhou, H. Jia, Q. Qiao, Y. Chen, J. Hoshikawa-Halbert, B. E. Gnade, Adv. Mater. 2014, 26, 8107. [6] K. Udagawa, H. Sasabe, C. Cai, J. Kido, Adv. Mater. 2014, 26, 5062. [7] H. Shin, S. Lee, K. Kim, C.-K. Moon, S.-J. Yoo, J.-H. Lee, J.-J. Kim, Adv. Mater. 2014, 26, 4730. [8] M. Kim, J. Y. Lee, Adv. Funct. Mater. 2014, 24, 4164. [9] S. Schmidbauer, A. Hohenleutner, B. König, Adv. Mater. 2013, 25, 2114. [10] L. Yang, F. Okuda, K. Kobayashi, K. Nozaki, Y. Tanabe, Y. Ishii, M.-A. Haga, Inorg. Chem. 2008, 47, 7154. [11] E. Baranoff, B. F. E. Curchod, J. Frey, R. Scopelliti, F. Kessler, I. Tavernelli, U. Rothlisberger, M. Grätzel, M. K. Nazeeruddin, Inorg. Chem. 2011, 51, 215. [12] C.-J. Chiang, A. Kimyonok, M. K. Etherington, G. C. Griffiths, V. Jankus, F. Turksoy, A. P. Monkman, Adv. Funct. Mater. 2013, 23, 739. [13] Y. Kondakov, T. D. Pawlik, T. K. Hatwar, J. P. Spindler, J. Appl. Phys. 2009, 106, 124510. [14] S. Sinha, A. P. Monkman, Appl. Phys. Lett. 2003, 82, 4651. [15] H. Nakanotani, T. Higuchi, T. Furukawa, K. Masui, K. Morimoto, M. Numata, H. Tanaka, Y. Sagara, T. Yasuda, C. Adachi, Nat. Commun. 2014, 5, 4016. [16] D. Zhang, L. Duan, C. Li, Y. Li, H. Li, D. Zhang, Y. Qiu, Adv. Mater. 2014, 26, 5050. [17] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature 2012, 492, 236. [18] Q. Zhang, B. Li, S. Huang, H. Nomura, H. Tanaka, C. Adachi, Nat. Photonics 2014, 8, 326. [19] Y. J. Cho, K. S. Yook, J. Y. Lee, Adv. Mater. 2014, 26, 6642.
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15.4% and 14.0% were developed by doping fluorescent emitters in the TADF type CzAcSF host material. The CzAcSF host material efficiently up-converted triplet excitons into singlet excitons by TADF process and harvested singlet excitons of fluorescent emitters by Förster energy transfer process, which resulted in more than three times increase of the quantum efficiency of the conventional TBPe device. Moreover, the CzAcSF host enhanced the quantum efficiency of TBPe a TBRb codoped fluorescent white OLEDs by efficient energy transfer process. Therefore, the development of high efficiency single layer TADF device and the use of the TADF material as the host of a fluorescent dopant can boost the quantum efficiency of the blue and white fluorescent devices above 20%.
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Host Engineering for High Quantum Efficiency Blue and White Fluorescent Organic Light-Emitting Diodes Wook Song, Inho Lee, and Jun Yeob Lee* Full color organic light-emitting diodes (OLEDs) possess red, green, and blue emitting pixels and the light-emitting performances of each pixel affect the efficiency, power consumption, and lifetime of the full color OLED panel. Therefore, the device performances of each red, green, and blue devices need to be maximized to develop high performances full color OLED panel. The most well-known approach to maximize the device performances of OLEDs was to use phosphorescent emitting materials instead of traditional fluorescent emitting materials for high quantum efficiency.[1,2] Theoretically, the quantum efficiency of the phosphorescent OLEDs can be higher than that of fluorescent OLEDs by four times and high quantum efficiency above 20% has already been reported in the red, green, and blue phosphorescent OLEDs.[3–8] Additionally, the lifetime of the red and green phosphorescent OLEDs can meet the requirement of the commercial OLED products. Therefore, the red and green phosphorescent OLEDs replaced old fluorescent OLEDs and upgraded the device performances of OLED panel. However, blue phosphorescent OLEDs could not be commercialized because of their short lifetime, in spite of their high quantum efficiency above 20%.[6–8] There are several reasons for the short lifetime of the blue phosphorescent OLEDs, but one of critical factors is intrinsic weak chemical bond between Ir and organic ligand of blue triplet emitters which are exposed to high energy blue emission.[9–11] This problem can be avoided by using organic-based fluorescent blue-light-emitting materials instead of Ir based blue triplet emitters. However, it was difficult to improve the quantum efficiency of the blue fluorescent OLEDs because triplet excitons of the fluorescent blue emitters were wasted. Although there was a literature reporting improved quantum efficiency by partially harvesting triplet excitons by triplet–triplet fusion process,[12–14] the degree of improvement was limited because theoretical maximum internal quantum efficiency by the triplet–triplet fusion process is 62.5%. One solution for the limited quantum efficiency of the typical blue fluorescent OLEDs can be to generate only singlet excitons
W. Song, Prof. J. Y. Lee School of Chemical Engineering Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi 440-746, South Korea E-mail: [email protected] Dr. I. Lee Department of Polymer Science and Engineering Dankook University 126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448-701, South Korea
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of the blue fluorescent emitter without generation of triplet excitons. The singlet exciton generation without any triplet exciton generation can be possible by photoluminescence (PL) process induced by Förster energy transfer from host to dopant material.[15,16] In this case, the host material should harvest both singlet and triplet excitons for high quantum efficiency. Therefore, the host materials harvesting both singlet and triplet excitons for Förster energy transfer may enhance the quantum efficiency of the fluorescent OLEDs. In an early work, TADF sensitized fluorescent emitting system was reported,[15] but three materials should be coevaporated and the instability of wide gap host is a serious problem. It would be better to develop a single host material which does not need any wide gap host material. In this work, a blue thermally activated delayed fluorescent (TADF) emitter, 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF), was developed as the host material of a common 2,5,8,11-tetra-tert-butylperylene (TBPe) emitter to improve the quantum efficiency of the TBPe blue fluorescent OLEDs. Maximum quantum efficiency of 15.4% and a quantum efficiency of 11.5% at 1000 cd m−2 were demonstrated in the TBPe device using the CzAcSF host. The quantum efficiency of the TBPe device was improved by more than three times compared with that of the conventional TBPe fluorescent device by engineering the host material. Additionally, white fluorescent OLEDs with TBPe and 2,8-ditertbutyl-5,11-bis(4-tert-butylphenyl)-6,12-diphenyltetracene (TBRb) doped in the CzAcSF host exhibited 14.0% external quantum efficiency, which was better than any other data reported in all fluorescent white OLEDs. In the conventional blue OLEDs with an emitting layer of 2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) and TBPe, only singlet–singlet energy transfer by Förster energy transfer process can give rise to light emission and triplet excitons of the MADN host decay non-radiatively. Therefore, only 25% of total excitons can be converted into photons and maximum internal quantum efficiency by energy transfer process is 25%. The 25% internal quantum efficiency of the MADN:TBPe device can be further enhanced if the triplet excitons can be converted into singlet excitons by reverse intersystem crossing process.[17] Assuming that all triplet excitons are harvested by the reverse intersystem crossing process, close to 100% internal quantum efficiency can be realized in the blue fluorescent OLEDs by Förster energy transfer process although exactly 100% internal quantum efficiency cannot be achieved due to charge trapping and Dexter energy transfer by the fluorescent dopant. Therefore, the use of the TADF material which can effectively transform triplet excitons into singlet excitons by reverse intersystem crossing process can enhance the quantum efficiency of the blue fluorescent OLEDs. The emission process
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of the fluorescent device with the TADF material as a host is schematically described in Figure 1. The host material of the TBPe fluorescent emitter should have higher singlet energy than TBPe for efficient energy transfer from the host to TBPe dopant. Therefore, a TADF emitter, CzAcSF, was developed to exhibit deep blue emission to transfer emission energy to the TBPe fluorescent emitter. The CzAcSF molecule was designed as a TADF emitter to emit at short wavelength by connecting a weak diphenylsulfone acceptor with an 9,9-dimethyl-9,10-dihydroacridine and a carbazole donor. As the TADF emitters give singlet emission from charge transfer (CT) excited state, the emission wavelength depends on the donor and acceptor strength of the emitters. Molecules with strong donors and acceptors give emission at long wavelength, while molecules with weak donors and acceptors emit at short wavelength. Therefore, the combination of weak diphenylsulfone acceptor with moderate donors such as carbazole and acridine may induce blue emission in the CzAcSF by TADF process. Compared with bis[4-(9,9-dimethyl-9,10-dihydroacridine)phenyl]sulfone (DMAC-DPS),[18] CzAcSF may emit at short wavelength because one acridine unit of DMAC-DPS was replaced with weak electron donating carbazole. Additionally, asymmetric molecular structure of CzAcSF may be helpful to suppress selfluminous quenching by preventing molecular stacking.
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Figure 1. Schematic diagram showing the emission process of blue fluorescent device with a blue fluorescent emitter doped in a TADF host.
Scheme 1 shows synthetic process of CzAcSF. The preparation of the CzAcSF TADF emitter was simply carried out by stepwise direct amination of bis(4-fluorophenyl) sulfone with carbazole and acridine. The 9-(4-((4-fluorophenyl)sulfonyl)phenyl)carbazole intermediate with only one carbazole was not isolated and only the final CzAcSF compound was purified by column chromatography and vacuum train sublimation. The final synthetic yield was 40.8%. Basic material characteristics of CzAcSF were analyzed by ultraviolet–visible (UV–vis), PL, absolute PL and cyclic voltammetry (CV) measurements. The normalized UV–vis absorption spectrum in toluene, the PL spectrum in solid polystyrene (1 wt%), and the low temperature PL spectrum in toluene are presented in Figure 2a. Combined UV–vis absorption of carbazole and acridine was observed in the CzAcSF emitter and the absorption edge could reach 412 nm by the n–π* absorption of the acridine moiety. The PL emission peak position in polystyrene was 443 nm and the low temperature PL peak position was 466 nm. The singlet and triplet emission energies of CzAcSF were 3.11 and 3.04 eV, respectively, from the onset of singlet and triplet emission spectra. The singlet energy of CzAcSF was slightly higher than that of DMAC-DPS (3.07 eV) due to weak CT character by carbazole substitution instead of acridine. Photophysical properties of CzAcSF were compared with those of DMAC-DPS in Table S1, Supporting Information. The PL quantum yield of CzAcSF in tetrahydrofuran was 0.71. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of CzAcSF were estimated from oxidation and reduction potentials of oxidation and reduction curves in Figure S1, Supporting Information. The HOMO and LUMO of CzAcSF were −5.89 and −3.00 eV, respectively, by oxidation of the acridine moiety of CzAcSF and reduction of the diphenylsulfone moiety of CzAcSF as can be predicted from the simulated molecular orbital in Figure 2b. B3LYP 6–31G* of Gaussian program was a basis set of the electronic state calculation. Localization of the HOMO on the acridine moiety and of the LUMO on the diphenylsulfone moiety was noted in the molecular simulation and can explain the HOMO and LUMO levels of CzAcSF. The HOMO and LUMO were separated with weak overlap of the HOMO and LUMO in the phenyl unit of diphenylsulfone. The separated HOMO and LUMO of CzAcSF donated charge transfer character to the CzAcSF molecule as can be proved by the solvent dependent PL spectra in Figure S2, Supporting Information. Gradual shift of the PL emission from short wavelength to long wavelength by increasing the polarity of the solvent was observed. Additionally, there was small overlap of the HOMO and LUMO in the phenyl unit of diphenylsulfone, which is beneficial for the light emission of CzAcSF. The CzAcSF compound was also thermally stable and high glass transition temperature of 117 °C and high decomposition temperature of 371 °C were detected. Thermal analysis data of CzAcSF are summarized in Figure S3, Supporting Information.
Scheme 1. Synthetic scheme of CzAcSF.
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PL spectra of CzAcSF and TBPe doped CzAcSF films were measured to investigate the energy transfer from CzAcSF to TBPe. Figure 3a represents the PL spectra of pure CzAcSF and TBPe doped CzAcSF films. Comparing the PL spectra of CzAcSF and CzAcSF:TBPe films, PL emission of CzAcSF was not observed in the TBPe doped CzAcSF films, indicating efficient energy transfer from CzAcSF to TBPe. The energy transfer from the CzAcSF to TBPe was studied in more detail by transient PL measurement. Transient PL decay curves of CzAcSF and CzAcSF:TBPe are shown in Figure 3b. Pure CzAcSF film showed fast decay component by normal fluorescence and delayed decay component by delayed fluorescence process, indicating that CzAcSF pure film also possesses TADF character by up-conversion of triplet excitons into singlet excitons. The TBPe doped CzAcSF film also showed similar decay curve although the delayed PL emission was relatively weak. This result suggests that the emission of TBPe is predominantly dominated by energy transfer from CzAcSF to TBPe. The fast decay emission includes energy transfer from singlet excitons of CzAcSF to TBPe and direct emission of TBPe by excitation, while the delayed emission includes energy transfer from triplet excitons of CzAcSF to TBPe via up-conversion process of the triplet excitons into singlet excitons. The rather reduced delayed emission in the TBPe doped CzAcSF is due to direct excitation of TBPe which does not have any delayed PL component. Blue fluorescent OLEDs of TBPe were developed by fabricating TBPe devices with a device structure of ITO (120 nm)/ PEDOT:PSS (60 nm)/mCP (30 nm)/CzAcSF or CzAcSF:TBPe (25 nm)/TSPO1 (5 nm)/TPBI (30 nm)/LiF (1 nm)/Al (200 nm),
Figure 2. Solution UV–vis, solid PL in polystyrene and low temperature PL spectra in toluene of a) CzAcSF, b) HOMO and LUMO distribution of CzAcSF calculated using Gaussian 09 program, and c) temperature dependent PL spectra of CzAcSF at different temperatures.
Delayed fluorescent behavior of CzAcSF was analyzed using transient PL measurements. Figure 2c shows temperature dependent transient PL decay curves of CzAcSF from 100 to 298 K. Delayed emission of CzAcSF was weak at 100 and 150 K, but it was intensified at 200 and 298 K as observed in other TADF molecules.[17] The numerical fitting of the transient PL decay curve gave excited state lifetime for TADF emission of 5.6 µs.
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Figure 3. a) PL spectra of pure CzAcSF and TBPe doped CzAcSF films. b) Transient PL decay curves of pure CzAcSF and TBPe doped CzAcSF films.
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Figure 4. a) Current-density–voltage–luminance curves, b) quantum efficiency–luminance curves, and c) EL spectra of pure CzAcSF and CzAcSF:TBPe devices.
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level of −5.89 eV and LUMO level of −3.00 eV of the CzAcSF host resulted in the low turn-on and driving voltage. Quantum efficiency of the blue devices was plotted against luminance in Figure 4b. Maximum quantum efficiency of the TBPe device with the MADN host was 4.3% and the maximum quantum efficiency of the CzAcSF:TBPe device was 15.4% at a doping concentration of 0.3%. There was more than three times improvement of the quantum efficiency by using CzAcSF instead of MADN as the host material of TBPe. Although there was efficiency roll-off in the CzAcSF:TBPe device, the quantum efficiency at 1000 cd m−2 was still 11.7%. The quantum efficiency obtained in this work is better than any other data reported in blue fluorescent device. It was even better than the quantum efficiency of the TADF sensitized blue fluorescent device.[15] In particular, the quantum efficiency of the CzAcSF and CzAcSF:TBPe devices was higher than that of corresponding device with DMAC-DPS instead of CzAcSF as summarized in Figure S4, Supporting Information. It seems that asymmetric molecular structure of CzAcSF contributed to the high quantum efficiency of the CzAcSF and CzAcSF:TBPe devices by suppressing self-quenching of light-emission compared to symmetric molecular structure of DMAC-DPS. The high quantum efficiency of the CzAcSF:TBPe device can be explained by the efficient up-conversion of triplet excitons into singlet excitons in the pure CzAcSF film, efficient Förster energy transfer from CzAcSF to TBPe, little charge trapping by TBPe, and little Dexter energy transfer from CzAcSF to TBPe. The triplet to singlet reverse intersystem crossing of pure CzAcSF film confirmed by the delayed PL emission and a high maximum efficiency of 15.7% of the pure CzAcSF device support the efficient up-conversion of the triplet excitons into singlet excitons. The efficient energy transfer proved by the PL emission in Figure 3 also contributed to the high quantum efficiency because the Förster energy transfer process induces only singlet exciton formation without triplet excitons wasted. The similar quantum efficiency of the pure CzAcSF and TBPe doped CzAcSF devices hints that singlet excitons of CzAcSF were effectively converted into singlet excitons of TBPe by efficient Förster energy transfer process. Little charge trapping and little Dexter energy transfer by low TBPe doping concentration of 0.3% are another important factors for the high quantum efficiency of the CzAcSF:TBPe device. In general, both singlet and triplet excitons are formed by charge trapping process by TBPe, but triplet excitons of TBPe are lost due to forbidden transition from triplet excited state to singlet ground state. Therefore, maximum internal quantum efficiency of the TBPe device by charge trapping process is only 25% compared to 100% by Förster energy transfer process. The charge trapping can be avoided by minimizing the doping concentration of TBPe and low TBPe doping concentration of 0.3% assisted reducing the charge trapping and increasing the quantum efficiency. Similarly, Dexter energy transfer from CzAcSF to TBPe was also suppressed by low TBPe doping concentration and played a role of enhancing the quantum efficiency of the TBPe device. Device performances of the CzAcSF:TBPe system were optimized at 0.3% doping concentration because of incomplete energy transfer below 0.3% doping and charge trapping effect above 0.3% doping. The quantum efficiency could be further improved up to 18.0% by adding wide gap
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where ITO is indium tin oxide, PEDOT:PSS is poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), mCP is 1,3-bis(Ncarbazolyl)benzene, TSPO1 is diphenylphosphine oxide-4(triphenylsilyl)phenyl and TPBI is 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene. The doping concentration of TBPe was 0.3%. Figure 4a displays current-density–voltage and luminance– voltage characteristics of the CzAcSF and CzAcSF:TBPe devices. The current density of the blue devices was similar in the CzAcSF and CzAcSF:TBPe devices and the doping of TBPe in the CzAcSF host material did not greatly affect the current density. The luminance of the blue devices was slightly high in the pure CzAcSF device, which is due to large portion of long wavelength component of CzAcSF as shown in the PL spectra in Figure 3a. Turn on voltage at 1 cd m−2 was below 3.0 V and the driving voltage at 1000 cd m−2 was around 5.0 V in the blue devices. The HOMO
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bis[2-(diphenylphosphino)phenyl]ether oxide host to take advantage of TADF sensitized fluorescent emitting system (Figure S5, Supporting Information). Figure 4c represents electroluminescence (EL) spectra of the CzAcSF and CzAcSF:TBPe devices. Broad EL emission of the CzAcSF device disappeared in the CzAcSF:TBPe device because of the energy transfer from CzAcSF to TBPe and sharp TBPe emission with vibrational peaks was observed. The color coordinate of the CzAcSF was (0.17, 0.28), while the color coordinate of the CzAcSF:TBPe devices were (0.15, 0.23) irrespective of the doping concentration. The EL spectra of CzAcSF:TBPe device were not changed according to the driving voltage of the device because the EL emission is mainly from TBPe (Figure S6, Supporting Information). In addition to the high quantum efficiency and stable EL spectra, the lifetime of the CzAcSF:TBPe device was much longer than that of the CzAcSF device (Figure S7, Supporting Information) due to stable emission from TBPe although the absolute lifetime of the device was short because of instability of charge transport materials used in the device structure. White fluorescent OLEDs were further developed by additional doping of TBRb in the CzAcSF:TBPe emitting layer. Current-density–voltage–luminance plot, quantum efficiency– luminance plot and EL spectra of the white OLEDs are shown in Figure 5a–c. Doping concentration of TBRb was 0.2% and 0.4%. In the current-density–voltage–luminance curves, the current density and luminance were slightly reduced at 0.4% TBRb doping concentration by charge trapping effect of TBRb. Maximum quantum efficiencies of the fluorescent OLEDs at 0.2% and 0.4% doping concentrations were 15.2% and 14.0%, respectively, and the quantum efficiency of the white fluorescent OLEDs was comparable to that of CzAcSF:TBPe blue fluorescent device. In spite of additional TBRb doping, the quantum efficiency of the blue fluorescent device could be maintained without large non-radiative loss of excitons by low doping of TBRb. Extra doping of TBRb did not have great influence on the quantum efficiency although slight reduction of the quantum efficiency at 0.4% TBRb doping concentration was observed. Color coordinates of the 0.2% and 0.4% TBRb doped devices were (0.23, 0.30) and (0.31, 0.37), respectively. Pure white emission was obtained at 0.4% doping concentration. Device performances of all devices fabricated in this work are summarized in Table 1. In conclusion, high quantum efficiency blue and white fluorescent OLEDs with external quantum efficiencies of
Figure 5. a) Current-density–voltage–luminance curves, b) quantum efficiency–luminance curves, and c) EL spectra of CzAcSF:TBPe:TBRb devices.
Table 1. Device performances of the CzAcSF-based TADF and TADF sensitized fluorescent OLEDs. Emitting layer
a)
QE [%]
PEb)
CEc)
QEa)
[cd A−1]
[%]
PEb) [lm W−1]
CEc) [cd A−1]
Color coordinate
[lm W−1]
CzAcSF
15.7
26.3
28.6
10.3
11.4
18.6
(0.17,0.28)
CzAcSF:TBPe
15.4
23.4
23.7
10.7
9.7
16.5
(0.15,0.23)
CzAcSF:TBPe:TBRb (0.2%)
15.2
33.4
32.6
12.2
16.4
24.8
(0.23,0.30)
CzAcSF:TBPe:TBRb (0.4%)
14.0
36.2
35.1
11.8
18.9
28.4
(0.31,0.37)
a)QE:
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quantum efficiency; b)PE: power efficiency; c)CE: current efficiency.
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Experimental Section The device structure of CzAcSF and CzAcSF:TBPe devices was ITO (120 nm)/PEDOT:PSS (60 nm)/mCP (30 nm)/CzAcSF or CzAcSF:TBPe (25 nm)/TSPO1 (5 nm)/TPBI (30 nm)/LiF (1 nm)/Al (200 nm). TBPe was doped at a doping concentration of 0.3% to avoid charge trapping. White fluorescent OLEDs were constructed to contain CzAcSF:TBPe:TBRb emitting layer with TBRb doping concentrations of 0.2% and 0.4%. mCP was a hole transport type exciton blocking layer and TSPO1 was an electron transport type exciton blocking layer to prevent triplet exciton quenching of CzAcSF. PL and device performance measurements are described in our previous work.[19]
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology (2013R1A1A2007991) and Ministry of Science, ICT, and future Planning (2013R1A2A2A01067447).
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Received: March 1, 2015 Revised: May 17, 2015 Published online: June 15, 2015
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15.4% and 14.0% were developed by doping fluorescent emitters in the TADF type CzAcSF host material. The CzAcSF host material efficiently up-converted triplet excitons into singlet excitons by TADF process and harvested singlet excitons of fluorescent emitters by Förster energy transfer process, which resulted in more than three times increase of the quantum efficiency of the conventional TBPe device. Moreover, the CzAcSF host enhanced the quantum efficiency of TBPe a TBRb codoped fluorescent white OLEDs by efficient energy transfer process. Therefore, the development of high efficiency single layer TADF device and the use of the TADF material as the host of a fluorescent dopant can boost the quantum efficiency of the blue and white fluorescent devices above 20%.
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