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The molecular origin of anisotropic emission in an organic light-emitting diode Thomas Lee, Bertrand Caron, Martin Stroet, David Mark Huang, Paul L. Burn, and Alan Edward Mark Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b03528 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 14, 2017
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δ=0°
δ=49°
δ=0°
δ=16°
1.4 1.2 probability density
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Ir(ppy)3
1.0 0.8 0.6 0.4
Ir(ppy)2(acac)
0.2 0.0 0.0
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1.0 0.0 sinψ
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Figure 5. Distribution of the transition dipole orientation |sin ψ| for Ir(ppy)3 (red) and Ir(ppy)2 (acac) (blue) for two choices of TDV angular offset from the Ir–N bond (illustrated above the plot): δ = 0◦ for both emitters (left) and δ = 49◦ and 16◦ for Ir(ppy)3 and Ir(ppy)2 (acac), respectively (right). Light emitted along the vertical axis originates from the horizontal components of the TDVs. The dashed line represents an isotropic distribution. Error bars represent the standard errors of the results of 20 independent simulations.
density approximately equal to 1.2 and 0.6 when |sin ψ| ≈ 0 and |sin ψ| ≈ 1, respectively). Based on these distributions, the fractions of light emitted from the horizontal component Ir(ppy)3 of the TDVs were calculated to be ΘH = 0.68±0.01 and Ir(ppy)2 (acac) ΘH = 0.74 ± 0.01. These are in close agreement Ir(ppy)3 with the experimental measurements ΘH = 0.69 ± 0.02 Ir(ppy)2 (acac) 2 and ΘH = 0.77 ± 0.02. Despite the clear preference for the principle symmetry axis of Ir(ppy)3 to align vertically, the orientation of the TDVs is essentially isotropic assuming δ = 0◦ . This is because the three Ir–N bonds in Ir(ppy)3 are nearly orthogonal, as illustrated in Figure 5 (top-left). Assuming δ = 49◦ , the TDVs are no longer orthogonal. This results in a small but significant net horizonIr(ppy)3 tal alignment of Ir(ppy)3 TDVs, with ΘH = 0.71 ± 0.01 (Figure 5, right). This result is still within the experimental uncertainty. Conversely, the net horizontal alignment of Ir(ppy)2 (acac) TDVs is reduced by using a larger δ. UsIr(ppy)2 (acac) ing δ = 16◦ , ΘH = 0.72 ± 0.01. This comparison between the simulations and published experimental results suggests that the TDVs of Ir(ppy)3 and Ir(ppy)2 (acac) lie closer to the Ir–N bond than indicated by TDDFT calculations on isolated emitters in vacuum. To understand the effect of the deposition process on the alignment of the emitter molecules, the layers were annealed at 400 K for 10 ns, well above the glass transition temperature of the CBP host (335 K). 18 Neither emitter displayed a net alignment after annealing. This indicates that the alignment of these emitters results from the asymmetry in the en-
vironment at the solid–vapor interface as they are deposited. The alignment induced at the interface is maintained as the emitters are buried providing that the temperature is below the glass transition temperature. The simulations suggest a net vertical alignment of the principal symmetry axes of Ir(ppy)3 and Ir(ppy)2 (acac) within blended emission layers. The TDVs of Ir(ppy)3 orient isotropically, despite the net molecular alignment, due to the relative directions of the TDVs within the emitter. In contrast, the net molecular alignment of Ir(ppy)2 (acac) leads to a net alignment of the TDVs. Thus, the simulations provide a clear explanation for the higher outcoupling efficiency of OLEDs containing Ir(ppy)2 (acac). The work demonstrates the importance of understanding the relative directions of the individual TDVs within the emitter molecules in order to optimize the efficiency of the device. The results suggest that the TDVs of Ir(ppy)3 and Ir(ppy)2 (acac) lie closer to the Ir–N bond than indicated by calculations in vacuum. This highlights the need to better understand the influence of the solid-state environment on emission properties. Overall, the work shows that atomistic molecular simulations can be used to mimick the vapor deposition of blended phosphorescent emissive layers containing either homoleptic or heteroleptic complexes. Such deposition simulations also have the potential to provide insight into the impact on OLED efficiency of other aspects of the emission layer morphology. For example, energy losses due to triplet-triplet annihilation depend strongly on emitter aggregation. 13,19 Beyond OLEDs, similar simulations could be used to understand the morphology and properties of wide variety of organic thin films, such as those used for photovoltaics, biosensors, and organic electronics. Acknowledgement The authors acknowledge funding from the Australian Research Council (ARC) grant DP150101097. A.E.M. is a University of Queensland ViceChancellor’s Research Focused Fellow. P.L.B. is an ARC Laureate Fellow (FL160100067). This work was supported by computational resources provided by the Australian Government through the National Computational Infrastructure under the National Computational Merit Allocation Scheme. References (1) Barnes, W. L. J. Mod. Optic. 1998, 45, 661–699. (2) Liehm, P.; Murawski, C.; Furno, M.; Lüssem, B.; Leo, K.; Gather, M. C. Appl. Phys. Lett. 2012, 101, 253304. (3) Graf, A.; Liehm, P.; Murawski, C.; Hofmann, S.; Leo, K.; Gather, M. C. J. Mater. Chem. C 2014, 2, 10298–10304. (4) Kim, K.-H.; Lee, S.; Moon, C.-K.; Kim, S.-Y.; Park, Y.-S.; Lee, J.-H.; Woo Lee, J.; Huh, J.; You, Y.; Kim, J.-J. Nat. Commun. 2014, 5, 4769. (5) Jurow, M. J.; Mayr, C.; Schmidt, T. D.; Lampe, T.; Djurovich, P. I.; Brutting, W.; Thompson, M. E. Nat. Mater. 2015, 15, 85–91. (6) Flämmich, M.; Frischeisen, J.; Setz, D. S.; Michaelis, D.; Krummacher, B. C.; Schmidt, T. D.; Brutting, W.; Danz, N. Org. Electron. 2011, 12, 1663–1668. (7) Moon, C. K.; Kim, K. H.; Kim, J. J. arXiv:1706.00172. (8) Flämmich, M.; Gather, M. C.; Danz, N.; Michaelis, D.; Bräuer, A. H.; Meerholz, K.; Tünnermann, A. Org. Electron. 2010, 11, 1039–1046. (9) Heil, A.; Gollnisch, K.; Kleinschmidt, M.; Marian, C. M. Mol. Phys. 2016, 114, 407–422. (10) Gonzalez-Vazquez, J. P.; Burn, P. L.; Powell, B. J. Inorg. Chem. 2015, 54, 10457–10461. (11) Vanhelmont, F. W. M.; Strouse, G. F.; Güdel, H. U.; Stückl, A. C.; Schmalle, H. W. J. Phys. Chem. A 1997, 101, 2946–2952. (12) Hofbeck, T.; Yersin, H. Inorg. Chem. 2010, 49, 9290–9299. (13) Tonnelé, C.; Stroet, M.; Caron, B.; Clulow, A. J.; Nagiri, R. C. R.; Malde, A. K.; Burn, P. L.; Gentle, I. R.; Mark, A. E.; Powell, B. J. Angew. Chem. Int. Edit. 2017, 56, 8402–8406. (14) Kutzner, C.; van der Spoel, D.; Lindahl, E. J. Chem. Theory Comput. 2008, 4, 435–447.
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(15) Malde, A. K.; Zuo, L.; Breeze, M.; Stroet, M.; Poger, D.; Nair, P. C.; Oostenbrink, C.; Mark, A. E. J. Chem. Theory Comput. 2011, 7, 4026–4037. (16) Schmid, N.; Eichenberger, A. P.; Choutko, A.; Riniker, S.; Winger, M.; Mark, A. E.; van Gunsteren, W. F. Eur. Biophys. J. 2011, 40, 843–856. (17) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.; Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem. 2001, 40, 1704–1711. (18) Tsai, M. H.; Hong, Y. H.; Chang, C. H.; Su, H. C.; Wu, C. C.; Matoliukstyte, A.; Simokaitiene, J.; Grigalevicius, S.; Grazulevicius, J. V.; Hsu, C. P. Adv. Mater. 2007, 19, 862–866. (19) Reineke, S.; Baldo, M. A. Phys. Stat. Solidi A 2012, 209, 2341– 2353.
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Graphical TOC Entry Light emission
Transition dipole moment
Ir(ppy)2(acac)
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