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High Efficiency in a Solution-Processed Thermally Activated Delayed-Fluorescence Device Using a DelayedFluorescence Emitting Material with Improved Solubility Yong Joo Cho, Kyoung Soo Yook, and Jun Yeob Lee* Solution-processed organic light-emitting diodes (OLEDs) have attracted great attention as a device for application in large-size OLEDs because of simple fabrication process and easy scalability of the solution process. However, the device performances of the solution-processed OLEDs are still inferior to those of vacuum-deposited OLEDs[1] and further improvement of the device performances of solution-processed OLEDs is required. There have been many papers reporting improved device performances in solution-processed OLEDs and most research has been directed to enhancing the quantum efficiency using soluble triplet host and phosphorescent emitting materials.[2–11] In most cases, a small-molecule host and dopant materials for vacuum-deposited phosphorescent organic light-emitting diodes were modified or used without modification for application in solution-processed devices, which improved the quantum efficiency of the solution-processed phosphorescent OLEDs. High quantum efficiency close to 20% was reported in red,[2–4] green[5–7] and blue phosphorescent OLEDs.[8–15] Although most studies on high-efficiency solution-processed OLEDs have focused on developing soluble phosphorescent OLEDs, thermally activated delayed-fluorescence (TADF) OLEDs can also be used to enhance the quantum efficiency of soluble OLEDs because of potentially high internal quantum efficiency of 100% by triplet harvesting for singlet emission via a reverse intersystem crossing mechanism. Several TADF emitting materials have been synthesized and were effective at improving the quantum efficiency of the OLEDs.[16–19] A maximum quantum efficiency close to 20% was achieved in the green TADF devices using a 2,4,5,6-tetra(carbazol-9-yl)-1,3-dicyanobenzene (4CzIPN) emitting material and it was comparable to that of phosphorescent OLEDs.[16] Therefore, the TADF emitting materials can also be useful to develop high-efficiency solution-processed OLEDs instead of phosphorescent emitting materials. However, there has been no study about solutionprocessed TADF devices. In this work, soluble TADF materials were synthesized to develop high-efficiency solution-processed TADF devices by modifying 4CzIPN with a methyl or tert-butyl group to improve
Y. J. Cho,[+] K. S. Yook,[+] Prof. J. Y. Lee Department of Polymer Science and Engineering Dankook University 126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448–701, Korea E-mail: [email protected] [+]These authors contributed equally to this work.
DOI: 10.1002/adma.201402188
Adv. Mater. 2014, DOI: 10.1002/adma.201402188
the solubility of the TADF dopant material in aromatic solvents. Two soluble TADF dopant materials, 2,4,5,6-tetra(3,6dimethylcarbazol-9-yl)-1,3-dicyanobenzene (m4CzIPN) and 2,4,5,6-tetra(3,6-di-tert-butylcarbazol-9-yl)-1,3-dicyanobenzene (t4CzIPN), were compared with 4CzIPN as the TADF material for the solution-processed TADF OLEDs. It was found that the t4CzIPN dopant was effective at improving the quantum efficiency of the solution-processed TADF OLEDs and a high quantum efficiency of 18.3% was demonstrated in the green TADF devices using the t4CzIPN dopant. The quantum efficiency of the soluble TADF OLEDs doped with the t4CzIPN TADF dopant was comparable to that of vacuum deposited TADF OLEDs with the same dopant. The m4CzIPN and t4CzIPN TADF materials were designed to allow the dopant materials to be soluble in aromatic solvents because the solubility of the well know 4CzIPN dopant is poor in aromatic solvents. The carbazole moiety of 4CzIPN was modified with methyl and tert-butyl units to improve the solubility of the TADF materials. The m4CzIPN and t4CzIPN were synthesized according to the synthetic procedure reported in other works except that 3,6-dimethylcarbazole and 3,6-di-tbutylcarbazole were used instead of carbazole.[16] Synthetic scheme of m4CzIPN and t4CzIPN is shown in Scheme 1. The synthetic yields of m4CzIPN and t4CzIPN were 89% and 49%, respectively, after purification by vacuum-train sublimation. The chemical structure of the dopant materials was confirmed with 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. The photophysical properties of m4CzIPN and t4CzIPN were analyzed by UV–vis and photoluminescence (PL) measurements. UV–vis, solution PL in toluene, and low temperature PL spectra in tetrahydrofuran of m4CzIPN and t4CzIPN are shown in Figure 1. The UV–vis spectra of m4CzIPN and t4CzIPN were similar, and the absorption edge of m4CzIPN and t4CzIPN was extended to 490 nm. The UV–vis absorption spectra of m4CzIPN and t4CzIPN were shifted to long wavelength by 20 nm, compared with that of 4CzIPN because of the electron-donating methyl and tert-butyl units. PL emission of m4CzIPN and t4CzIPN was also red-shifted by about 20 nm compared with the PL emission peak of 4CzIPN by the strong donor–acceptor character of m4CzIPN and t4CzIPN. The triplet energies of m4CzIPN and t4CzIPN were 2.39 eV and 2.38 eV, respectively, from low-temperature PL measurements. The TADF dopant materials were dispersed in diphenyldi(4(9-carbazolyl)phenyl)silane (SiCz), which was reported as a high-triplet-energy host material for phosphorescent dopant.[20] SiCz was chosen as the host material for the TADF dopant materials because the solid PL emission of SiCz was well overlapped
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m4CzIPN
t4CzIPN Scheme 1. Synthetic scheme of m4CzIPN and t4CzIPN.
with the PL emission of the TADF dopant materials for energy transfer (Figure S1, Supporting Information) and the triplet energy of SiCz (3.00 eV) was higher than that of the TADF dopant materials. Energy-transfer efficiency from SiCz to the TADF dopant materials was above 90% from PL measurement (Figure S2, Supporting Information). The PL quantum yields of the m4CzIPN and t4CzIPN were 0.67 and 0.78, respectively, and were lower than that of 4CzIPN (0.94). In addition, SiCz is compatible with the solution coating process due to amorphous nature of the molecular structure, suppressed crystallization and high glass transition temperature (109 °C). Transient PL measurements of 4CzIPN, m4CzIPN and t4CzIPN dispersed in SiCz host material were carried out to analyze the excited-state lifetime for the delayed PL emission. Figure 2 shows transient PL decay curves of vacuum-deposited
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and solution-coated 4CzIPN, m4CzIPN, and t4CzIPN. The deposition method did not greatly change the excited-state lifetime of the TADF emitting material, and a similar excited-state lifetime was observed in the vacuum-deposited and spin-coated TADF emitters. In addition, the methyl and tert-butyl substitution had little influence on the excited-state lifetime of the TADF emitters. The excited-state lifetimes of vacuum-deposited 4CzIPN, m4CzIPN, and t4CzIPN were 3.8, 2.6, and 3.2 µs, respectively, while those of spin-coated 4CzIPN, m4CzIPN, and t4CzIPN were 3.1, 2.6, and 2.9 µs, respectively. The SiCz host activated the delayed PL emission of 4CzIPN, m4CzIPN, and t4CzIPN. The TADF emission of the solution-processed SiCz:m4CzIPN and SiCz:t4CzIPN films was further studied by temperaturedependent transient PL measurements, which is shown in Figure S3 in the Supporting Information. The temperature was changed from 60 K to 297 K to check the delayed fluorescent emission of SiCz:m4CzIPN and SiCz:t4CzIPN films. The TADF emission was not clearly observed at 60 K, but it gradually intensified according to the temperature because the delayedfluorescence emission process is a thermally activated process by reverse intersystem crossing. This confirms that the delayed emission is caused by thermally activated delayed fluorescence. Delayed PL quantum efficiency of the delayed-fluorescence emission was calculated from the transient PL measurement and the delayed-fluorescence component was increased according to temperature by the thermally activated delayedfluorescence process (Figure S4 in the Supporting Information). The solubilities of the TADF dopant materials were compared to check the compatibility of the dopant materials in the solution process. Toluene was a solvent for the TADF dopant materials and the solubility was in the order of t4CzIPN (0.4%) > m4CzIPN ≈ 4CzIPN (0.1%). Although the methyl substitution could not solubilize the dopant further, tert-butyl substitution increased the solubility of the dopant material. Therefore, the tert-butyl substitution can make the TADF material compatible with the solution-coating process. The different solubilities of the dopant materials affected the film morphology of the spin-coated films and atomic force microscopy (AFM) analysis of the spin-coated SiCz:TADF dopant films revealed that the root-mean-square surface roughness was in the order of t4CzIPN (0.29 nm) < 4CzIPN (0.34 nm) < m4CzIPN (0.39 nm). AFM images are shown in Figure 3. In particular, the aggregation of the dopant materials was clearly observed in the 4CzIPN and m4CzIPN doped films. The tert-butyl moiety may prevent intermolecular interactions and hinder the aggregation of the dopant materials. Therefore, the tert-butyl moiety played a dual role of increasing the solubility of the dopant materials and stabilizing the morphology of the spin-coated films. High energy-transfer efficiency, uniform film morphology, and activated TADF emission of the solution-coated TADF dopant materials enabled the fabrication of solution-processed TADF devices. The device structure of the solution-processed TADF OLEDs was indium tin oxide (ITO) (120 nm)/poly(3,4-ethylenedioxythiophene);poly(styrenesulfonate) (PEDOT:PSS) (60 nm)/ poly(9-vinylcarbazole) (PVK) (15 nm)/SiCz:4CzIPN or m4CzIPN or t4CzIPN (25 nm, 1% doping)/diphenyl(4-(triphenylsilyl)phenyl)phosphine oxide (TSPO1) (5 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) (30 nm)/LiF (1 nm)/Al
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(200 nm). Current-density–voltage–luminance curves of the solution-processed TADF OLEDs are presented in Figure 4a. The order of the current density was 4CzIPN > m4CzIPN > t4CzIPN, but the difference of the current density was not significant. The current density of the TADF OLEDs depending on the dopant materials can be correlated with the charge-trapping effect by the TADF dopant materials. As the doping concentration of the TADF dopant materials was 1%, the dopant materials play the role of a charge trap rather than a charge-transport site. The narrow bandgap of t4CzIPN (2.51 eV) compared with that of m4CzIPN (2.53 eV) and 4CzIPN (2.61 eV) induced more-significant charge trapping by the dopant material and reduced the current density of the TADF devices. Therefore, the light-emission process of the t4CzIPN dopant is dictated by the charge trapping rather than energy transfer from the host to the t4CzIPN. In particular, electrons are trapped by the t4CzIPN due to strong electron-withdrawing CN units in the molecular structure, which induces dominant light emission by the charge-trapping process. The charge trapping was more significant at high doping concentration and the current density was decreased at 3% and 5% doping concentration. The external quantum efficiency of the solution-processed TADF OLEDs is plotted against current density in Figure 4b. The 4CzIPN TADF device exhibited a maximum quantum efficiency of 8.1%, whereas the m4CzIPN and t4CzIPN devices
showed maximum quantum efficiencies of 8.2% and 18.3%, respectively. Methyl substitution could not affect the quantum efficiency, but tert-butyl substitution greatly enhanced the quantum efficiency of the TADF devices. The quantum efficiency of the solution-processed TADF devices was more than doubled by using t4CzIPN instead of 4CzIPN and m4CzIPN. There are several key parameters for emitters that influence the quantum efficiency of the solution-processed TADF OLEDs. These are the quantum efficiency of the emitter, a uniform dispersion of the emitter in the spin-coated film, and charge balance in the emitting layer. The quantum efficiency of the emitter can be excluded as the origin of the high quantum efficiency of the TADF devices because the external quantum efficiency of the optimized 4CzIPN and m4CzIPN devices prepared by vacuum thermal evaporation is higher than that of the t4CzIPN device, as summarized in Table 1. Charge balance in the emitting layer is affected by charge trapping by the TADF emitters, but the charge balance by charge trapping may also have little effect on the quantum efficiency as the current density was slightly changed by the dopant materials. Therefore, the high quantum efficiency of the t4CzIPN devices can be correlated with the uniform dispersion of the dopant material, which can be inferred from the AFM image of the spin-coated emitting layer. Less aggregation of the dopant materials in the t4CzIPN film indirectly implies uniform dispersion of the
Figure 3. AFM images of solution-coated SiCz:4CzIPN, SiCz:m4CzIPN, and SiCz:t4CzIPN films.
Adv. Mater. 2014, DOI: 10.1002/adma.201402188
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40 35 30
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TADF dopants in the host materials, which suppresses exciton quenching and improves the quantum efficiency of the TADF OLEDs. However, the quantum efficiency of the solution-processed t4CzIPN TADF devices was degraded at high doping concentration. The quantum efficiency of the solution-processed TADF OLEDs was compared with that of vacuum-deposited TADF OLEDs having the same TADF dopants. The device performances are summarized in Table 1. The quantum efficiency of the 4CzIPN device dropped sharply from 26.0% to 8.1% and the quantum efficiency of the m4CzIPN device reduced from 19.6% to 8.2% by the solution process. However, the quantum efficiency of the t4CzIPN device was similar, irrespective of the coating process of the emitting layer. Only t4CzIPN could
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realize a similar quantum efficiency level, both in the vacuum and solution processes, which indicates that the t4CzIPN dopant is effective as the soluble TADF emitter. Electroluminescence (EL) spectra of the solution-processed TADF OLEDs are plotted in Figure 4c. The EL emission of m4CzIPN and t4CzIPN was red-shifted compared to the EL emission of 4CzIPN by the electron-donating methyl and tert-butyl substituents. The color coordinates of the 4CzIPN, m4CzIPN, and t4CzIPN devices were (0.20, 0.43), (0.29, 0.57), and (0.31, 0.59), respectively. No SiCz emission was observed in the solution-processed TADF OLEDs, confirming effective energy transfer from SiCz to TADF emitters. In conclusion, tert-buty modification of the carbazole moiety of 4CzIPN dopant improved the solubility of the TADF dopant and stabilized the film morphology of the solution-processed TADF emitting layer. Therefore, a high quantum efficiency of 18.3% was achieved in a solution-processed TADF device doped with the t4CzIPN TADF dopant and the quantum efficiency of the solution-processed TADF device was comparable to that of the vacuum evaporated TADF device.
Experimental Section Synthesis: Synthesis of m4CzIPN and t4CzIPN is described in the Supporting Information. Device Fabrication: TADF OLEDs were fabricated with a device structure of ITO (50 nm)/PEDOT:PSS (60 nm)/poly(9-vinylcarbazole) (15 nm)/ SiCz:TADF dopant (30 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm). The TADF dopant materials were 4CzIPN, m4CzIPN, and t4CzIPN and were doped at a doping concentration of 1%. PEDOT:PSS, PVK, and the emitting layer were coated by spin-coating and TSPO1, LiF, and Al were deposited by vacuum thermal evaporation. The PEDOT:PSS layer was coated from an aqueous dispersion of PEDOT:PSS (Clevios CH8000) diluted with isopropyl alcohol at a spin speed of 2000 rpm and was baked at 150 °C for 10 min in a glove box. The PVK layer was formed by spincoating of high-molecular-weight PVK (ca. 1 000 000 g mol−1, Aldrich Co. Ltd.) from 1 wt% dichlorobenzene solution at a spinning speed of 2000 rpm. Baking of the spin-coated PVK film was carried out at 120 °C for 10 min. The TADF emitting layer was deposited from 2 wt% toluene solution of SiCz and TADF dopant materials. The spinning speed was 2000 rpm and baking was performed at 100 °C for 10 min. TSPO1, LiF, and Al were vacuum-deposited at a base pressure of 1.0 × 10−6 Torr. The solution-processed devices were protected from moisture and oxygen by encapsulating the device using a glass cover and epoxy adhesive inside a glove box.
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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 the Ministry of Science, ICT and Future Planning (2013R1A2A2A010674). Received: May 15, 2014 Revised: June 30, 2014 Published online:
[1] L. Duan, L. Hou, T. Lee, J. Qiao, D. Zhang, G. Dong, L. Wang, Y. Qiu, J. Mater. Chem. 2010, 20, 6392. [2] H. Kim, Y. Byun, R. R. Das, B.-K. Choi, P.-S. Ahn, Appl. Phys. Lett. 2007, 91, 093512.
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Measurements: Basic chemical analysis and photophysical characterization methods are described in the Supporting Information. Transient PL measurements were performed using a pulsed Nd–YAG laser (355 nm) as the excitation source and a photomultiplier tube as an optical detector system. Surface morphology of the spin-coated film was characterized by AFM (Veeco Instrument MMAFM-2) in tapping mode. The doping concentration of the TADF-emitter-doped SiCz films was 3%. A CS2000 spectroradiometer was used for luminance measurements and a Keithley 2400 source measurement unit recorded the currentdensity–voltage data.
[3] J. Chen, C. Shi, Q. Fu, F. Zhao, Y. Hu, Y. Feng, D. Ma, J. Mater. Chem. 2012, 22, 5164. [4] L. Hou, L. Duan, J. Qiao, D. Zhang, G. Dong, L. Wang, Y. Qiu, Org. Electron. 2010, 11, 1344. [5] M. Zhu, T. Ye, X. He, X. Cao, C. Zhong, D. Ma, J. Qin, C. Yang, J. Mater. Chem. 2011, 21, 9326. [6] N. Rehmann, D. Hertel, K. Meerholz, H. Becker, S. Heun, Appl. Phys. Lett. 2007, 91, 103507. [7] M. Cai, T. Xiao, E. Hellerich, Y. Chen, R. Shinar, J. Shinar, Adv. Mater. 2011, 23, 3590. [8] J.-H. Jou, W.-B. Wang, S.-Z. Chen, J.-J. Shyue, M.-F. Hsu, C.-W. Lin, S.-M. Shen, C.-J. Wang, C.-P. Liu, C.-T. Chen, M.-F. Wud, S.-W. Liu, J. Mater. Chem. 2010, 20, 8411. [9] K. S. Yook, J. Y. Lee, Org. Electron. 2011, 12, 1711. [10] C. W. Lee, J. Y. Lee, Adv. Mater. 2013, 25, 596. [11] S. Gong, Q. Fu, Q. Wang, C. Yang, C. Zhong, J. Qin, D. Ma, Adv. Mater. 2011, 23, 4956. [12] S. Gong, Q. Fu, W. Zeng, C. Zhong, C. Yang, D. Ma, J. Qin, Chem. Mater. 2012, 24, 3120. [13] K. S. Yook, S. E. Jang, S. O. Jeon, J. Y. Lee, Adv. Mater. 2010, 22, 4479. [14] K. S. Yook, J. Y. Lee, J. Mater. Chem. 2012, 22, 14546. [15] H.-C. Yeh, H.-F. Meng, H.-W. Lin, T.-C. Chao, M.-R. Tseng, H.-W. Zan, Org. Electron. 2012, 13, 914. [16] H. Uoyama, K. Goushi, K. Shizu, H. Nomura, C. Adachi, Nature 2012, 492, 236. [17] F. B. Dias, K. N. Bourdakos, V. Jankus, K. C. Moss, K. T. Karntekar, V. Bhalla, J. Santos, M. R. Bryce, A. P. Monkman, Adv. Mater. 2013, 25, 3707. [18] Q. Zhang, J. Li, K. Shizu, S. Huang, S. Hirata, H. Miyazaki, C. Adachi, J. Am. Chem. Soc. 2012, 134, 14706. [19] H. Nakanotani, K. Masui, J. Nishide, T. Shibata, C. Adachi, Sci. Rep. 2013, 3, 2127. [20] S. H. Kim, J. Jang, S. J. Lee, J. Y. Lee, Thin Solid Films 2008, 517, 722.
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High Efficiency in a Solution-Processed Thermally Activated Delayed-Fluorescence Device Using a DelayedFluorescence Emitting Material with Improved Solubility Yong Joo Cho, Kyoung Soo Yook, and Jun Yeob Lee* Solution-processed organic light-emitting diodes (OLEDs) have attracted great attention as a device for application in large-size OLEDs because of simple fabrication process and easy scalability of the solution process. However, the device performances of the solution-processed OLEDs are still inferior to those of vacuum-deposited OLEDs[1] and further improvement of the device performances of solution-processed OLEDs is required. There have been many papers reporting improved device performances in solution-processed OLEDs and most research has been directed to enhancing the quantum efficiency using soluble triplet host and phosphorescent emitting materials.[2–11] In most cases, a small-molecule host and dopant materials for vacuum-deposited phosphorescent organic light-emitting diodes were modified or used without modification for application in solution-processed devices, which improved the quantum efficiency of the solution-processed phosphorescent OLEDs. High quantum efficiency close to 20% was reported in red,[2–4] green[5–7] and blue phosphorescent OLEDs.[8–15] Although most studies on high-efficiency solution-processed OLEDs have focused on developing soluble phosphorescent OLEDs, thermally activated delayed-fluorescence (TADF) OLEDs can also be used to enhance the quantum efficiency of soluble OLEDs because of potentially high internal quantum efficiency of 100% by triplet harvesting for singlet emission via a reverse intersystem crossing mechanism. Several TADF emitting materials have been synthesized and were effective at improving the quantum efficiency of the OLEDs.[16–19] A maximum quantum efficiency close to 20% was achieved in the green TADF devices using a 2,4,5,6-tetra(carbazol-9-yl)-1,3-dicyanobenzene (4CzIPN) emitting material and it was comparable to that of phosphorescent OLEDs.[16] Therefore, the TADF emitting materials can also be useful to develop high-efficiency solution-processed OLEDs instead of phosphorescent emitting materials. However, there has been no study about solutionprocessed TADF devices. In this work, soluble TADF materials were synthesized to develop high-efficiency solution-processed TADF devices by modifying 4CzIPN with a methyl or tert-butyl group to improve
Y. J. Cho,[+] K. S. Yook,[+] Prof. J. Y. Lee Department of Polymer Science and Engineering Dankook University 126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi 448–701, Korea E-mail: [email protected] [+]These authors contributed equally to this work.
DOI: 10.1002/adma.201402188
Adv. Mater. 2014, DOI: 10.1002/adma.201402188
the solubility of the TADF dopant material in aromatic solvents. Two soluble TADF dopant materials, 2,4,5,6-tetra(3,6dimethylcarbazol-9-yl)-1,3-dicyanobenzene (m4CzIPN) and 2,4,5,6-tetra(3,6-di-tert-butylcarbazol-9-yl)-1,3-dicyanobenzene (t4CzIPN), were compared with 4CzIPN as the TADF material for the solution-processed TADF OLEDs. It was found that the t4CzIPN dopant was effective at improving the quantum efficiency of the solution-processed TADF OLEDs and a high quantum efficiency of 18.3% was demonstrated in the green TADF devices using the t4CzIPN dopant. The quantum efficiency of the soluble TADF OLEDs doped with the t4CzIPN TADF dopant was comparable to that of vacuum deposited TADF OLEDs with the same dopant. The m4CzIPN and t4CzIPN TADF materials were designed to allow the dopant materials to be soluble in aromatic solvents because the solubility of the well know 4CzIPN dopant is poor in aromatic solvents. The carbazole moiety of 4CzIPN was modified with methyl and tert-butyl units to improve the solubility of the TADF materials. The m4CzIPN and t4CzIPN were synthesized according to the synthetic procedure reported in other works except that 3,6-dimethylcarbazole and 3,6-di-tbutylcarbazole were used instead of carbazole.[16] Synthetic scheme of m4CzIPN and t4CzIPN is shown in Scheme 1. The synthetic yields of m4CzIPN and t4CzIPN were 89% and 49%, respectively, after purification by vacuum-train sublimation. The chemical structure of the dopant materials was confirmed with 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. The photophysical properties of m4CzIPN and t4CzIPN were analyzed by UV–vis and photoluminescence (PL) measurements. UV–vis, solution PL in toluene, and low temperature PL spectra in tetrahydrofuran of m4CzIPN and t4CzIPN are shown in Figure 1. The UV–vis spectra of m4CzIPN and t4CzIPN were similar, and the absorption edge of m4CzIPN and t4CzIPN was extended to 490 nm. The UV–vis absorption spectra of m4CzIPN and t4CzIPN were shifted to long wavelength by 20 nm, compared with that of 4CzIPN because of the electron-donating methyl and tert-butyl units. PL emission of m4CzIPN and t4CzIPN was also red-shifted by about 20 nm compared with the PL emission peak of 4CzIPN by the strong donor–acceptor character of m4CzIPN and t4CzIPN. The triplet energies of m4CzIPN and t4CzIPN were 2.39 eV and 2.38 eV, respectively, from low-temperature PL measurements. The TADF dopant materials were dispersed in diphenyldi(4(9-carbazolyl)phenyl)silane (SiCz), which was reported as a high-triplet-energy host material for phosphorescent dopant.[20] SiCz was chosen as the host material for the TADF dopant materials because the solid PL emission of SiCz was well overlapped
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m4CzIPN
t4CzIPN Scheme 1. Synthetic scheme of m4CzIPN and t4CzIPN.
with the PL emission of the TADF dopant materials for energy transfer (Figure S1, Supporting Information) and the triplet energy of SiCz (3.00 eV) was higher than that of the TADF dopant materials. Energy-transfer efficiency from SiCz to the TADF dopant materials was above 90% from PL measurement (Figure S2, Supporting Information). The PL quantum yields of the m4CzIPN and t4CzIPN were 0.67 and 0.78, respectively, and were lower than that of 4CzIPN (0.94). In addition, SiCz is compatible with the solution coating process due to amorphous nature of the molecular structure, suppressed crystallization and high glass transition temperature (109 °C). Transient PL measurements of 4CzIPN, m4CzIPN and t4CzIPN dispersed in SiCz host material were carried out to analyze the excited-state lifetime for the delayed PL emission. Figure 2 shows transient PL decay curves of vacuum-deposited
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Wavelength (nm) Figure 1. UV–vis, solution PL in toluene, and low-temperature PL spectra of m4CzIPN and t4CzIPN.
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and solution-coated 4CzIPN, m4CzIPN, and t4CzIPN. The deposition method did not greatly change the excited-state lifetime of the TADF emitting material, and a similar excited-state lifetime was observed in the vacuum-deposited and spin-coated TADF emitters. In addition, the methyl and tert-butyl substitution had little influence on the excited-state lifetime of the TADF emitters. The excited-state lifetimes of vacuum-deposited 4CzIPN, m4CzIPN, and t4CzIPN were 3.8, 2.6, and 3.2 µs, respectively, while those of spin-coated 4CzIPN, m4CzIPN, and t4CzIPN were 3.1, 2.6, and 2.9 µs, respectively. The SiCz host activated the delayed PL emission of 4CzIPN, m4CzIPN, and t4CzIPN. The TADF emission of the solution-processed SiCz:m4CzIPN and SiCz:t4CzIPN films was further studied by temperaturedependent transient PL measurements, which is shown in Figure S3 in the Supporting Information. The temperature was changed from 60 K to 297 K to check the delayed fluorescent emission of SiCz:m4CzIPN and SiCz:t4CzIPN films. The TADF emission was not clearly observed at 60 K, but it gradually intensified according to the temperature because the delayedfluorescence emission process is a thermally activated process by reverse intersystem crossing. This confirms that the delayed emission is caused by thermally activated delayed fluorescence. Delayed PL quantum efficiency of the delayed-fluorescence emission was calculated from the transient PL measurement and the delayed-fluorescence component was increased according to temperature by the thermally activated delayedfluorescence process (Figure S4 in the Supporting Information). The solubilities of the TADF dopant materials were compared to check the compatibility of the dopant materials in the solution process. Toluene was a solvent for the TADF dopant materials and the solubility was in the order of t4CzIPN (0.4%) > m4CzIPN ≈ 4CzIPN (0.1%). Although the methyl substitution could not solubilize the dopant further, tert-butyl substitution increased the solubility of the dopant material. Therefore, the tert-butyl substitution can make the TADF material compatible with the solution-coating process. The different solubilities of the dopant materials affected the film morphology of the spin-coated films and atomic force microscopy (AFM) analysis of the spin-coated SiCz:TADF dopant films revealed that the root-mean-square surface roughness was in the order of t4CzIPN (0.29 nm) < 4CzIPN (0.34 nm) < m4CzIPN (0.39 nm). AFM images are shown in Figure 3. In particular, the aggregation of the dopant materials was clearly observed in the 4CzIPN and m4CzIPN doped films. The tert-butyl moiety may prevent intermolecular interactions and hinder the aggregation of the dopant materials. Therefore, the tert-butyl moiety played a dual role of increasing the solubility of the dopant materials and stabilizing the morphology of the spin-coated films. High energy-transfer efficiency, uniform film morphology, and activated TADF emission of the solution-coated TADF dopant materials enabled the fabrication of solution-processed TADF devices. The device structure of the solution-processed TADF OLEDs was indium tin oxide (ITO) (120 nm)/poly(3,4-ethylenedioxythiophene);poly(styrenesulfonate) (PEDOT:PSS) (60 nm)/ poly(9-vinylcarbazole) (PVK) (15 nm)/SiCz:4CzIPN or m4CzIPN or t4CzIPN (25 nm, 1% doping)/diphenyl(4-(triphenylsilyl)phenyl)phosphine oxide (TSPO1) (5 nm)/1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) (30 nm)/LiF (1 nm)/Al
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Figure 2. Transient PL curves of vacuum-deposited and solution-coated SiCz:4CzIPN (a), SiCz:m4CzIPN (b), and SiCz:t4CzIPN films (c).
(200 nm). Current-density–voltage–luminance curves of the solution-processed TADF OLEDs are presented in Figure 4a. The order of the current density was 4CzIPN > m4CzIPN > t4CzIPN, but the difference of the current density was not significant. The current density of the TADF OLEDs depending on the dopant materials can be correlated with the charge-trapping effect by the TADF dopant materials. As the doping concentration of the TADF dopant materials was 1%, the dopant materials play the role of a charge trap rather than a charge-transport site. The narrow bandgap of t4CzIPN (2.51 eV) compared with that of m4CzIPN (2.53 eV) and 4CzIPN (2.61 eV) induced more-significant charge trapping by the dopant material and reduced the current density of the TADF devices. Therefore, the light-emission process of the t4CzIPN dopant is dictated by the charge trapping rather than energy transfer from the host to the t4CzIPN. In particular, electrons are trapped by the t4CzIPN due to strong electron-withdrawing CN units in the molecular structure, which induces dominant light emission by the charge-trapping process. The charge trapping was more significant at high doping concentration and the current density was decreased at 3% and 5% doping concentration. The external quantum efficiency of the solution-processed TADF OLEDs is plotted against current density in Figure 4b. The 4CzIPN TADF device exhibited a maximum quantum efficiency of 8.1%, whereas the m4CzIPN and t4CzIPN devices
showed maximum quantum efficiencies of 8.2% and 18.3%, respectively. Methyl substitution could not affect the quantum efficiency, but tert-butyl substitution greatly enhanced the quantum efficiency of the TADF devices. The quantum efficiency of the solution-processed TADF devices was more than doubled by using t4CzIPN instead of 4CzIPN and m4CzIPN. There are several key parameters for emitters that influence the quantum efficiency of the solution-processed TADF OLEDs. These are the quantum efficiency of the emitter, a uniform dispersion of the emitter in the spin-coated film, and charge balance in the emitting layer. The quantum efficiency of the emitter can be excluded as the origin of the high quantum efficiency of the TADF devices because the external quantum efficiency of the optimized 4CzIPN and m4CzIPN devices prepared by vacuum thermal evaporation is higher than that of the t4CzIPN device, as summarized in Table 1. Charge balance in the emitting layer is affected by charge trapping by the TADF emitters, but the charge balance by charge trapping may also have little effect on the quantum efficiency as the current density was slightly changed by the dopant materials. Therefore, the high quantum efficiency of the t4CzIPN devices can be correlated with the uniform dispersion of the dopant material, which can be inferred from the AFM image of the spin-coated emitting layer. Less aggregation of the dopant materials in the t4CzIPN film indirectly implies uniform dispersion of the
Figure 3. AFM images of solution-coated SiCz:4CzIPN, SiCz:m4CzIPN, and SiCz:t4CzIPN films.
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40 35 30
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25 20
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Table 1. Device performances of vacuum and solution-processed TADF OLEDs.
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Wavelength (nm) Figure 4. Current-density–voltage–luminance curves (a), quantum-efficiency–current-density curves (b), and EL spectra of solution-processed SiCz:4CzIPN, SiCz:m4CzIPN, and SiCz:t4CzIPN devices.
TADF dopants in the host materials, which suppresses exciton quenching and improves the quantum efficiency of the TADF OLEDs. However, the quantum efficiency of the solution-processed t4CzIPN TADF devices was degraded at high doping concentration. The quantum efficiency of the solution-processed TADF OLEDs was compared with that of vacuum-deposited TADF OLEDs having the same TADF dopants. The device performances are summarized in Table 1. The quantum efficiency of the 4CzIPN device dropped sharply from 26.0% to 8.1% and the quantum efficiency of the m4CzIPN device reduced from 19.6% to 8.2% by the solution process. However, the quantum efficiency of the t4CzIPN device was similar, irrespective of the coating process of the emitting layer. Only t4CzIPN could
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Maxa)
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Maxa)
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realize a similar quantum efficiency level, both in the vacuum and solution processes, which indicates that the t4CzIPN dopant is effective as the soluble TADF emitter. Electroluminescence (EL) spectra of the solution-processed TADF OLEDs are plotted in Figure 4c. The EL emission of m4CzIPN and t4CzIPN was red-shifted compared to the EL emission of 4CzIPN by the electron-donating methyl and tert-butyl substituents. The color coordinates of the 4CzIPN, m4CzIPN, and t4CzIPN devices were (0.20, 0.43), (0.29, 0.57), and (0.31, 0.59), respectively. No SiCz emission was observed in the solution-processed TADF OLEDs, confirming effective energy transfer from SiCz to TADF emitters. In conclusion, tert-buty modification of the carbazole moiety of 4CzIPN dopant improved the solubility of the TADF dopant and stabilized the film morphology of the solution-processed TADF emitting layer. Therefore, a high quantum efficiency of 18.3% was achieved in a solution-processed TADF device doped with the t4CzIPN TADF dopant and the quantum efficiency of the solution-processed TADF device was comparable to that of the vacuum evaporated TADF device.
Experimental Section Synthesis: Synthesis of m4CzIPN and t4CzIPN is described in the Supporting Information. Device Fabrication: TADF OLEDs were fabricated with a device structure of ITO (50 nm)/PEDOT:PSS (60 nm)/poly(9-vinylcarbazole) (15 nm)/ SiCz:TADF dopant (30 nm)/TSPO1 (35 nm)/LiF (1 nm)/Al (200 nm). The TADF dopant materials were 4CzIPN, m4CzIPN, and t4CzIPN and were doped at a doping concentration of 1%. PEDOT:PSS, PVK, and the emitting layer were coated by spin-coating and TSPO1, LiF, and Al were deposited by vacuum thermal evaporation. The PEDOT:PSS layer was coated from an aqueous dispersion of PEDOT:PSS (Clevios CH8000) diluted with isopropyl alcohol at a spin speed of 2000 rpm and was baked at 150 °C for 10 min in a glove box. The PVK layer was formed by spincoating of high-molecular-weight PVK (ca. 1 000 000 g mol−1, Aldrich Co. Ltd.) from 1 wt% dichlorobenzene solution at a spinning speed of 2000 rpm. Baking of the spin-coated PVK film was carried out at 120 °C for 10 min. The TADF emitting layer was deposited from 2 wt% toluene solution of SiCz and TADF dopant materials. The spinning speed was 2000 rpm and baking was performed at 100 °C for 10 min. TSPO1, LiF, and Al were vacuum-deposited at a base pressure of 1.0 × 10−6 Torr. The solution-processed devices were protected from moisture and oxygen by encapsulating the device using a glass cover and epoxy adhesive inside a glove box.
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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 the Ministry of Science, ICT and Future Planning (2013R1A2A2A010674). Received: May 15, 2014 Revised: June 30, 2014 Published online:
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Adv. Mater. 2014, DOI: 10.1002/adma.201402188
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Measurements: Basic chemical analysis and photophysical characterization methods are described in the Supporting Information. Transient PL measurements were performed using a pulsed Nd–YAG laser (355 nm) as the excitation source and a photomultiplier tube as an optical detector system. Surface morphology of the spin-coated film was characterized by AFM (Veeco Instrument MMAFM-2) in tapping mode. The doping concentration of the TADF-emitter-doped SiCz films was 3%. A CS2000 spectroradiometer was used for luminance measurements and a Keithley 2400 source measurement unit recorded the currentdensity–voltage data.
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