The hydrogen/deuterium isotope effect of the host material on the lifetime of organic light-emitting diodes

Abstract

The hydrogen/deuterium primary kinetic isotope effect provides useful information about the degradation mechanism of OLED host materials. Thus, replacement of labile C–H bonds in the host with C–D bonds increases the device lifetime by a factor of five without loss of efficiency, and replacement with C–C bonds by a factor of 22.5.

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Organic light-emitting diodes (OLEDs) being innovative devices for display and illumination,^1,2 their device lifetime and hence the mechanism of device degradation remain the most pressing subjects of research.^3–5 Many of the studies reported thus far have focused on the analysis of the degradation products,⁶ while a few have reported on the kinetics of the molecular processes of the degradation.⁷ We focused on the hydrogen/deuterium (H/D) primary kinetic isotope effect (KIE) as a mechanistic probe, where the rate of a chemical reaction slows down by a factor of 6.5 (a theoretical value at 298 K)⁸ when the cleavage of the C–H(D) bond in question is the rate determining step in the overall reaction sequence.^9–14 We report here a finding that deuteration of the benzylic methyl groups in a green phosphorescent host material (i.e., CH₃CZBDF vs. CD₃CZBDF in Fig. 1a) increases the device lifetime by a factor of five (from 0.2 h to 1.0 h) without affecting the other properties of the device much. This rate retardation factor of five is consistent with the KIE value reported for the heterolysis of a benzylic C–H bond.¹⁵ Taking together the isotope effect and the resonance-stabilizing effect of a furan ring (Fig. 1b), we consider the degradation of the methyl group in CH₃CZBDF to be involved as a critical step of the degradation process; that is, the molecule is oxidized to a radical cation (i.e., hole formation) during operation of the OLED device and suffers from the loss of either a proton or a hydrogen radical. Guided by this analysis, we replaced the heterolytically labile C–H bonds in CH₃CZBDF with more stable C–CH₃ bonds (t-BuCZBDF) and found the lifetime to increase by a factor of 22.5 under the same device configuration. The lifetime enhancement by a factor of five through H to D change in the OLED performance makes an interesting contrast to a reported decrease of solar cell performance by 50% where no C–H(D) cleavage is involved.¹⁶ These data indicate that isotope effects are of considerable practical and mechanistic values in organic electronic research.^17,18

Fig. 1 Host materials discussed in this study. (a) Chemical structures of CZBDF derivatives and (b) a plausible degradation path of CH₃CZBDF.

The host material CH₃CZBDF, which we focus on in this work, was designed on the basis of our previous host material PhCZBDF (Fig. 1a),^19–21 whose triplet energy (T₁) level of PhCZBDF, ca. 2.0 eV, was too low to be suitable as a host for green and blue phosphorescent OLEDs (PHOLEDs). We thus designed a less conjugated host CH₃CZBDF in order to raise the T₁ level (Fig. 1a; synthesis in ESI†), having a concern about the device durability due to the intrinsically labile benzylic methyl groups (cf.Fig. 1b). Thus, we also synthesized the deuterated (CD₃CZBDF) and the tert-butyl analogues (t-BuCZBDF) for comparison.

We first found that CH₃CZBDF and CD₃CZBDF have essentially the same photophysical properties. This is reasonable because the C–H(D) bonds are not directly involved in the photoexcitation processes (see ESI,† Table S1). The ionization potentials (IPs) of CH₃CZBDF and CD₃CZBDF were the same (6.04 and 6.03 eV, respectively), as determined using photoemission yield spectroscopy (PYS).²² The IP of t-BuCZBDF was slightly lower (6.00 eV). The optical band gap (ΔE_g) was the same for all compounds (3.49 eV). Using the IP and the ΔE_g values, the electron affinities (EAs) were calculated to be 2.55, 2.54 and 2.51 eV for CH₃CZBDF, CD₃CZBDF and t-BuCZBDF, respectively, which are large enough to confine carriers within green phosphorescent Ir(ppy)₃²³ (Fig. 2b). As determined by phosphorescence measurements at 77 K, the triplet energy levels (E_T) are also high enough to suppress energy transfer from Ir(ppy)₃ (E_T = 2.4 eV) to the host materials: 2.64, 2.65 and 2.67 eV for CH₃CZBDF, CD₃CZBDF and t-BuCZBDF, respectively.

Fig. 2 Materials and device structure of the CZBDF-based PHOLEDs. (a) Chemical structure of Ir(ppy)₃ and device structure. (b) Energy diagram of the materials.

PHOLED devices were fabricated using alkyl-CZBDF as a host material in the emission layer doped with green phosphorescent Ir(ppy)₃ with a standard heterojunction configuration as follows: ITO/PEDOT:PSS/α-NPD/alkyl-CZBDF:Ir(ppy)₃/BCP/Alq₃/Liq/Al, where PEDOT:PSS and α-NPD serve as hole injection and transport layers, BCP as a hole-blocking layer, and Alq₃ and Liq as electron transport and injection layers, respectively (Fig. 2a). The PEDOT:PSS layer was spin-coated, and the other layers were deposited under vacuum. Electroluminescence from Ir(ppy)₃ was observed from all devices (Fig. 3a), indicating that the carriers are appropriately confined within Ir(ppy)₃.

Fig. 3 Device performance of the CZBDF-based PHOLEDs. (a) EL spectra. (b) L–V characteristics. (c) EQE–V characteristics. (d) Normalized L–t characteristics.

A comparison of the performances of the CH₃CZBDF- and CD₃CZBDF-based devices indicates that deuteration of the host material significantly enhanced the device stability without affecting the electroluminescence spectra (Fig. 3a) and the device efficiency (Fig. 3b and c). Thus, the driving voltage and luminance efficiency at 1000 cd m⁻² (V₁₀₀₀ and η₁₀₀₀, respectively) and external quantum efficiency (EQE) barely changed (V₁₀₀₀ = 7.0 and 6.9 V; η₁₀₀₀ = 8.9 and 8.4 lm W⁻¹; EQE = 5.5 and 5.5% for devices A and B, respectively; Table 1). The maximum luminance of the CD₃CZBDF device was higher than that of the CH₃CZBDF device, 22 270 and 13 800 cd m⁻², respectively, because the former device degraded much less during the measurement. In consonance with this data, the half-life of the former was five times longer than the latter (1.0 h and 0.2 h, respectively, Fig. 3d and Table 1). This difference of stability is consistent with the reported KIE values for the heterolysis of a benzylic C–H bond,^11,15 and hence suggests that the degradation of the α-methyl groups in CH₃CZBDF (Fig. 1b) is a rate-determining step in this rapidly degrading device.

Table 1 Summary of the OLED performance data^a

Host	V 1000 ^b [V]	η 1000 ^c [lm W^−1]	EQEmax ^d [%]	L max ^e [cd A^−1]	τ 1/2 ^f [h]
a These data were collected at a luminance of 1000 cd m^−2. b Driving voltage. c Luminance efficiency. d Maximum external quantum efficiency. e Maximum luminance. f Device half-lifetime measured at a constant current density of 12.5 mA cm^−2.
CH3CZBDF	7.0	8.9	5.5	13 800	0.2
CD3CZBDF	6.9	8.4	5.5	22 270	1.0
t-BuCZBDF	7.4	6.1	4.5	20 770	4.5

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We next considered the possibility that the conversion of the α-C–H bonds into heterolytically more stable C–C bonds will further elongate the device lifetime (Fig. 1a). Consistent with this expectation, the t-BuCZBDF device achieved a 22.5 times longer lifetime (Fig. 3d, Table 1) and higher maximum luminance (20 770 cd m⁻²) than the CH₃CZBDF-based device. There were slight increases in the driving voltage (7.4 V) and lower efficiencies (η₁₀₀₀ = 6.1 lm W⁻¹, EQE = 4.5%), which was possibly caused by the difference of molecular packing.²⁴ In a similar vein, a similar device using PhCZBDF (having a heterolytically stable phenyl–furyl bond) has a very long lifetime of >4000 h.¹⁹

In conclusion, we have shown that the hydrogen/deuterium exchange of a heterolytically labile C–H bond in an OLED host material increases the lifetime of the device. The deuterium atoms in CD₃CZBDF did not change the device performance much except for the lifetime (and the maximum luminance which is related to the lifetime), because the photophysical and electrical processes as well as morphological changes do not include C–H(D) bond cleavage (cf. secondary kinetic isotope effects). Identification of the heterolytically labile C–H bonds in CH₃CZBDF then led us to replace these bonds with C–C bonds to achieve a further increase in the lifetime. We expect that some of the data reported in the literature and patents on the favourable effects of deuterated materials may be rationalized in terms of the kinetic and equilibrium isotope effects.

We thank MEXT for financial support (KAKENHI for H.T., No. 20685005). E.N. thanks the Strategic Promotion of Innovation Research and Development from the Japan Science and Technology Agency (JST).

Notes and references

1. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.
2. (a) K. Müllen and U. Scherf, Organic Light-Emitting Devices: Synthesis, Properties and Applications, Wiley-VCH, 2006; (b) J. Shinar, Organic Light-Emitting Devices: A Survey, Springer, 2004; (c) Z. Li and H. Meng, Organic Light-Emitting Materials and Devices, CRC Press, 2007; (d) Y. Sato, in Electroluminescence I, ed. G. Mueller, Academic Press, San Diego, 2000, vol. 64, pp. 209–254; (e) Y. Sato, S. Ichinosawa and H. Kanai, IEEE J. Sel. Top. Quantum Electron., 1998, 4, 40–48.
3. For reviews: (a) H. Aziz and Z. D. Popovic, Chem. Mater., 2004, 16, 4522; (b) F. So and D. Kondakov, Adv. Mater., 2010, 22, 3762.
4. (a) H. Aziz, Z. D. Popovic, N.-X. Hu, A.-M. Hor and G. Xu, Science, 1999, 283, 1900; (b) Y. Luo, H. Aziz, G. Xu and Z. D. Popovic, Chem. Mater., 2007, 19, 2079.
5. For a recent review: A. Chaskar, H.-F. Chen and K.-T. Wong, Adv. Mater., 2011, 23, 3876.
6. (a) D. Y. Kondakov, W. C. Lenhart and W. F. Nichols, J. Appl. Phys., 2007, 101, 024512; (b) S. Scholz, K. Walzer and K. Leo, Adv. Funct. Mater., 2008, 18, 2541; (c) V. Sivasubramaniam, F. Brodkorb, S. Hanning, O. Buttler, H. P. Loebl, V. van Elsbergen, H. Boerner, U. Scherf and M. Kreyenschmidt, Solid State Sci., 2009, 11, 1933; (d) I. R. de Moraes, S. Scholz, B. Lüssem and K. Leo, Org. Electron., 2011, 12, 341; (e) I. R. de Moraes, S. Scholz, B. Lüssem and K. Leo, Org. Electron., 2012, 13, 1900.
7. (a) N. C. Giebink, B. W. D'Andrade, M. S. Weaver, P. B. Mackenzie, J. J. Brown, M. E. Thompson and S. R. Forrest, J. Appl. Phys., 2008, 103, 044509; (b) N. C. Giebink, B. W. D'Andrade, M. S. Weaver, J. J. Brown and S. R. Forrest, J. Appl. Phys., 2009, 105, 124514.
8. Estimated using a well-established equation for kinetic isotope effect: k_H/k_D = exp[hc(ν_H − ν_D)/2kT], where ν_H and ν_D stands for stretching frequencies of C–H (∼3000 cm⁻¹) and C–D (∼2100 cm⁻¹), respectively.
9. N. Yoshikai, H. Matsuda and E. Nakamura, J. Am. Chem. Soc., 2008, 130, 15258.
10. (a) K. B. Wiberg, Chem. Rev., 1955, 55, 713; (b) F. H. Webtheimer, Chem. Rev., 1955, 55, 713.
11. For recent reviews on chemical reactions: (a) E. M. Simmons and J. F. Hartwig, Angew. Chem., Int. Ed., 2012, 51, 3066; (b) T. Giagou and M. P. Meyer, Chem. – Eur. J., 2010, 16, 10616; (c) W. D. Jones, Acc. Chem. Res., 2003, 36, 140; (d) M. Gómez-Gallego and M. A. Sierra, Chem. Rev., 2011, 111, 4857.
12. For recent reviews on energy conversion: (a) S. Fukuzumi, Y. Yamada, T. Suenobu, K. Ohkubo and H. Kotani, Energy Environ. Sci., 2011, 4, 2754; (b) S. Hammes-Schiffer, Acc. Chem. Res., 2009, 42, 1881.
13. For recent reviews on biological process: (a) W. W. Cleland, Arch. Biochem. Biophys., 2005, 433, 2; (b) D. Antoniou, S. Caratzoulas, C. Kalyanaraman, J. S. Mincer and S. D. Schwartz, Eur. J. Biochem., 2002, 269, 3103.
14. T. D. Nguyen, G. Hukic-Markosian, F. Wang, L. Wojcik, X.-G. Li, E. Ehrenfreund and Z. V. Vardeny, Nat. Mater., 2010, 9, 345.
15. (a) K. B. Wiberg and L. H. Slaugh, J. Am. Chem. Soc., 1958, 80, 3033; (b) I. Howe and F. W. McLaffert, J. Am. Chem. Soc., 1971, 93, 99.
16. M. Shao, J. Keum, J. Chen, Y. He, W. Chen, J. F. Browning, J. Jakowski, B. G. Sumpter, I. N. Ivanov, Y.-Z. Ma, C. M. Rouleau, S. C. Smith, D. B. Geohegan, K. Hong and K. Xiao, Nat. Commun., 2014, 5, 3180.
17. For deuterium effect on functional materials and enhancement of OLED efficiency; (a) C. C. Tong and K. C. Hwang, J. Phys. Chem. C, 2007, 111, 3490; (b) J.-L. Zuo, J.-X. Yang, F.-Z. Wang, X.-N. Dang, J.-L. Sun, D.-C. Zoub, Y.-P. Tian, N. Lin, X.-T. Tao and M.-H. Jiang, J. Photochem. Photobiol., A, 2008, 199, 322; (c) T. D. Nguyen, G. Hukic-Markosian, F. Wang, L. Wojcik, X.-G. Li, E. Ehrenfreund and Z. V. Vardeny, Nat. Mater., 2010, 9, 345.
18. For deuterium effect on iridium complexes: (a) T. Abe, A. Miyazawa, H. Konno and Y. Kawanishi, Chem. Phys. Lett., 2010, 491, 199; (b) P. Wang, F.-F. Wang, Y. Chen, Q. Niu, L. Lu, H.-M. Wang, X.-C. Gao, B. Wei, H.-W. Wu, X. Caic and D.-C. Zou, J. Mater. Chem. C, 2013, 1, 4821.
19. (a) H. Tsuji, C. Mitsui, L. Ilies, Y. Sato and E. Nakamura, J. Am. Chem. Soc., 2007, 129, 11902; (b) H. Tsuji, C. Mitsui, Y. Sato and E. Nakamura, Adv. Mater., 2009, 21, 3776; (c) C. Mitsui, H. Tanaka, H. Tsuji and E. Nakamura, Chem. – Asian J., 2011, 6, 2296; (d) C. Mitsui, H. Tsuji, Y. Sato and E. Nakamura, Chem. – Asian J., 2012, 7, 1443.
20. Fused furan materials reported by ourselves: (a) H. Tsuji, C. Mitsui, Y. Sato and E. Nakamura, Heteroat. Chem., 2011, 22, 316; (b) C. Mitsui, J. Soeda, K. Miwa, H. Tsuji, J. Takeya and E. Nakamura, J. Am. Chem. Soc., 2012, 134, 5448; (c) H. Tsuji, L. Ilies and E. Nakamura, Synlett, 2014, 25, 2099.
21. Furan-containing functional materials: (a) R. Shukla, S. H. Wadumethrige, S. V. Lindeman and R. Rathore, Org. Lett., 2008, 10, 3587; (b) N. Hayashi, Y. Saito, H. Higuchi and K. Suzuki, J. Phys. Chem. A, 2009, 113, 5342; (c) U. Caruso, B. Panunzi, G. N. Roviello, G. Roviello, M. Tingoli and A. Tuzi, C. R. Chim., 2009, 12, 622; (d) H. Li, P. Jiang, C. Yi, C. Li, S.-X. Liu, S. Tan, B. Zhao, J. Braun, W. Meier, T. Wandlowski and S. Decurtins, Macromolecules, 2010, 43, 8058; (e) J. Santos-Pérez, C. E. Crespo-Hernández, C. Reichardt, C. R. Cabrera, I. Feliciano-Ramos, L. Arroyo-Ramírez and M. A. Meador, J. Phys. Chem. A, 2011, 115, 4157; (f) J. Xiao, B. Yang, J. I. Wong, Y. Liu, F. Wei, K. J. Tan, X. Teng, Y. Wu, L. Huang, C. Kloc, F. Boey, J. Ma, H. Zhang, H. Y. Yang and Q. Zhang, Org. Lett., 2011, 13, 3004; (g) J. C. Bijleveld, B. P. Karsten, S. G. J. Mathijssen, M. M. Wienk, D. M. de Leeuw and R. A. J. Janssen, J. Mater. Chem., 2011, 21, 1600; (h) C. Moussallem, F. Gohier, C. Mallet, M. Allain and P. Frère, Tetrahedron, 2012, 68, 8617; (i) K. Niimi, H. Mori, E. Miyazaki, I. Osaka, H. Kakizoe, K. Takimiya and C. Adachi, Chem. Commun., 2012, 48, 5892; (j) B. M. Kobilka, A. V. Dubrovskiy, M. D. Ewan, A. L. Tomlinson, R. C. Larock, S. Chaudhary and M. Jeffries-El, Chem. Commun., 2012, 48, 8919; (k) H. Li, P. Tang, Y. Zhao, S.-X. Liu, Y. Aeschi, L. Deng, J. Braun, B. Zhao, Y. Liu, S. Tan, W. Meier and S. Decurtins, J. Polym. Sci., Part A: Polym. Chem., 2012, 50, 2935; (l) C. Mitsui, T. Okamoto, H. Matsui, M. Yamagishi, T. Matsushita, J. Soeda, K. Miwa, H. Sato, A. Yamano, T. Uemura and J. Takeya, Chem. Mater., 2013, 25, 3952; (m) K. Nakahara, C. Mitsui, T. Okamoto, M. Yamagishi, K. Miwa, H. Sato, A. Yamano, T. Uemura and J. Takeya, Chem. Lett., 2013, 42, 654; (n) H. Li, J. Ding, S. Chen, C. Beyer, S.-X. Liu, H.-A. Wagenknecht, A. Hauser and S. Decurtins, Chem. – Eur. J., 2013, 19, 6459; (o) S. Li, J. Yuan, P. Deng, W. Ma and Q. Zhang, Sol. Energy Mater. Sol. Cells, 2013, 118, 22; (p) M. J. Bosiak, M. Rakowiecki, K. J. Orlowska, D. Kedziera and J. Adams, Dyes Pigm., 2013, 99, 803; (q) O. Gidron, A. Dadvand, E. W.-H. Sun, I. Chung, L. J. W. Shimon, M. Bendikov and D. F. Perepichka, J. Mater. Chem. C, 2013, 1, 4358; (r) C. Mallet, Y. Didane, T. Watanabe, N. Yoshimoto, M. Allain, C. Videlot-Ackermann and P. Frère, ChemPlusChem, 2013, 78, 459; (s) S. Shi, X. Xie, C. Gao, K. Shi, S. Chen, G. Yu, L. Guo, X. Li and H. Wang, Macromolecules, 2014, 47, 616; (t) C. Moussalem, O. Segut, F. Gohier, M. Allain and P. Frére, ACS Sustainable Chem. Eng., 2014, 2, 1043; (u) W. Huang, B. Yang, J. Sun, B. Liu, J. Yang, Y. Zou, J. Xiong, C. Zhou and Y. Gao, Org. Electron., 2014, 15, 1050; (v) K. Nakahara, C. Mitsui, T. Okamoto, M. Yamagishi, H. Matsui, T. Ueno, Y. Tanaka, M. Yano, T. Matsushita, J. Soeda, Y. Hirose, H. Sato, A. Yamano and J. Takeya, Chem. Commun., 2014, 50, 5342; (w) Z. Li, H. Li, S. Chen, T. Froehlich, C. Yi, C. Schönenberger, M. Calame, S. Decurtins, S.-X. Liu and E. Borguet, J. Am. Chem. Soc., 1014, 136, 8867; (x) C. G. Péterfalvi, I. Grace, D. Z. Manrique and C. J. Lambert, J. Chem. Phys., 2014, 140, 174711; (y) T. Kojima, R. Yokota, C. Kitamura, H. Kurata, M. Tanaka, H. Ikeda and T. Kawase, Chem. Lett., 2014, 43, 696.
22. M. Honda, K. Kanai, K. Komatsu, Y. Ouchi, H. Ishii and K. Seki, Mol. Cryst. Liq. Cryst., 2006, 455, 219.
23. (a) C. Adachi, M. A. Baldo and S. R. Forrest, Appl. Phys. Lett., 2000, 77, 904; (b) T. Hofbeck and H. Yersin, Inorg. Chem., 2010, 49, 9290.
24. H. Tsuji, G. Cantagrel, Y. Ueda, T. Chen, L.-J. Wan and E. Nakamura, Chem. – Asian J., 2013, 8, 2377.

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Footnote

† Electronic supplementary information (ESI) available: Synthetic details, summary of physical properties of materials and device fabrication. See DOI: 10.1039/c4cc05108d

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