Dependence of Phosphorescent Emitter Orientation on Deposition

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Dependence of phosphorescent emitter orientation on deposition technique in doped organic films Thomas Lampe, Tobias D. Schmidt, Matthew J Jurow, Peter I. Djurovich, Mark E Thompson, and Wolfgang Brutting Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04607 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on January 24, 2016

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Chemistry of Materials

Dependence of phosphorescent emitter orientation on deposition technique in doped organic films Thomas Lampe,∗,† Tobias D. Schmidt,† Matthew J. Jurow,‡ Peter I. Djurovich,‡ Mark E. Thompson,‡ and Wolfgang Br¨utting∗,† †Institute of Physics, University of Augsburg, 86135 Augsburg, Germany ‡Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA E-mail: [email protected]; [email protected] Abstract Introducing an overall horizontal orientation of transition dipole moments of heteroleptic Ir-complexes is a promising concept to improve efficiencies of organic lightemitting diodes. To investigate the impact of deposition technique on molecular orientation, we prepared doped films of four different phosphorescent iridium complexes in various organic host materials both by thermal evaporation and by solution processing and compared the observed emitter orientation. All heteroleptic Ir-complexes show comparable horizontal alignment if fabricated from the gas phase, while isotropic orientation or even a slightly vertical trend is observed in the solution processed samples. These findings can be explained by the creation of an interface between vacuum and the aromatic host material during evaporation and the lack of this feature when processed from solution. The underlying mechanism of molecular orientation can then be explained by the interaction of aliphatic and aromatic parts of the Ir-complexes

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with this interface during deposition from the gas phase. The absence of horizontal orientation of the phosphors in layers deposited from liquid preparation techniques has important implications for solution processed OLEDs.

Keywords organic light-emitting diode, emitter orientation

Introduction The control of molecular orientation in emissive guest-host systems is a promising concept for efficiency enhancement in state-of-the-art organic light-emitting diodes (OLEDs). 1–4 Horizontal alignment of the emitting transition dipole moments increases the external quantum efficiency (EQE) of OLEDs due to reduced coupling to lossy optical modes. Hence, the outcoupling factor can be significantly increased from ca. 20% (for the isotropic case) up to 46% (when perfect horizontal alignment is achieved) 3 without using complex outcoupling enhancements such as nanostructured electrodes 5 or scattering particles. 6 Here, we compare two different preparation techniques for emissive guest-host layers containing phosphorescent metal-organic iridium-complexes doped in an organic matrix material. Thermal evaporation in high vacuum is the most common fabrication process used in modern OLED materials. During this deposition process, the dye molecules assemble at an interface formed between the so far deposited material (mainly the matrix due to the low doping concentrations of approx. 5%) and vacuum. The emitting species interacts with both media at this interface until it is covered with subsequently deposited molecules, which occurs in the range of seconds for the typically used evaporation rates of 0.1 nm/s. 1,3,7 In contrast, during the solution processing, the deposition takes place in one step with the absence of such cumulative interfacial effects. Thus, if the process generating the horizontal alignment of common heteroleptic Ir-complexes is based on an interface interaction during deposition, 2 ACS Paragon Plus Environment

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the two preparation techniques should show significant differences in the orientation of the dye molecules and the resulting alignment of the emissive transition dipole moments. Because the direct measurement of molecular orientation is not possible in these guesthost systems due to low doping concentrations, we followed the indirect investigation route to measure dipole alignment presented by Frischeisen et. al. in 2011 8 using the angular dependent emission pattern to determine the net orientation of the transition dipole moment vectors (TDVs) relative to the substrate. TDVs have a fixed orientation to the moleular frame of each dopant, so the TDV orientation is directly tied to the molecular orientation. The orientation parameter Θ is defined as the ratio of energy radiated by vertically aligned transition dipole moments to the total radiated power. 2 Based on this definition an isotropic distribution of the TDVs results in a Θ value of 0.33, a perfect horizontal alignment is identified by Θ = 0.0 and a completely vertical orientation yields Θ = 1.0. In general the connection between Θ and an ensemble of molecules, each having different possible orientations of the TDVs is: P

ai

i

Θ= P

P j

ai

P

i

bj p2z,ij

bj |~pij |2

(1)

j

where ai denotes the relative contribution of each dye molecule, bi describes the relative contribution of the j-th transition dipole moment p~ij on the i-th molecule and pz,ij denotes the vertical component of the corresponding transition dipole moment vector. 9

Experimental The samples were prepared as 15 − 30 nm thick layers on glass substrates containing a doping concentration of 6 − 8 % wt. of the investigated iridium complex. For the phosphorescent dye molecules Bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) (Ir(mdq)2 (acac)) and



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Bis(1-phenylisoquinoline)(acetylacetonate)iridium(III) (Ir(piq)2 (acac)) the matrix material N,N-Di(1-naphthyl)-N,N-diphenyl-(1,1-biphenyl)-4,4-diamine (NPB) was used in both evaporated and solution processed samples. For Bis[2-(2-pyridinyl-N)phenyl-C](acetylacetonate)iridium(III) (Ir(ppy)2 (acac)) and tris[2-phenylpyridinatoC2,N]iridium(III) (Ir(ppy)3 ) as dyes, two different materials, 4,4-Bis(9-carbazolyl)-1,1-biphenyl, 4,4-N,N-Dicarbazole-1,1-biphenyl (CBP) and 2,7-Bis(carbazol-9-yl)-9,9-spirobifluorene (Spiro2CBP) have been used as hosts. Doped films of CBP were prepared by both solution processing and thermal evaporation. Spiro2-CBP was only investigated in solution processed layers. Additionally Poly(methyl methacrylate) (PMMA) was evaluated as a matrix material for spin cast films of Ir(ppy)2 (acac). The solution processed emission layers were prepared by spin coating at 5000 RPM in either a chloroform solution (5 mg/ml) for the NPB, CBP and Spiro2-CBP hosts or from a toluene solution (1.25 mg/ml) for the PMMA based layers. After deposition the samples were dried under cleanroom conditions for at least 1.5h without further annealing treatment. The samples based on evaporated host materials were deposited at an average rate of 1.0 ˚ A/s in a vacuum chamber with a base pressure of 5 × 10−7 mbar. The experimental setup contains a macroscopic outcoupling prism (fused silica), which is index matched to the glass substrate of the sample under investigation. Both are mounted on a rotary stage and excited by a laserbeam with a wavelength of 375 nm (Oxxius OXV375). The emission passes a polarization filter and is afterwards collected by a collimator lens attached via an optical fiber to a spectrometer. The outcoupling through the fused silica prism is necessary to extract the substrate modes of the sample, which actually contain the required information about the orientation of the TDVs of the emissive species in the guesthost system. Figure 1 shows an exemplary measurement of two samples prepared by each technique. To extract the orientation parameter Θ from the measured angular dependent photoluminescence spectra, a cross-section at the peak wavelength was analyzed by optical simulation. 8 To improve the quality, measured values were averaged over a small wavelength range (±3 nm


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to the peak intensity) and 1 − 3 different samples. The resulting angular dependent emission pattern was used for fitting, the standard deviation of these values was used as measurement error. Figure 1c shows the simulated angular dependent emission spectra for selected orientations of the TDVs together with measurements and the corresponding fits. Note, that the simulation of isotropic host materials is based on the model by Barnes et al., 10 while the calculation taking into account the presence of anisotropic media was performed following the approach by Penninck et al.. 11 A detailed description of the optical models behind the numerical simulation tool used for the following investigations is given in Refs. 12,13 The calculation of the emitted light intensity depends on the outcoupling angle and therefore uses the layer thickness, optical constants, wavelength and dipole orientation of the emitting molecules as input parameters.

Results and discussion Four different emissive Ir-complexes were doped into a variety of host materials and were deposited by evaporation and from solution. The materials NPB and CBP were chosen as common host materials for the investigated phosphors. PMMA and Spiro2-CBP were selected due to their higher glass transition temperatures to suppress crystallization in the films deposited by spin coating. All investigated heteroleptic emissive dopants (Ir(mdq)2 (acac), Ir(piq)2 (acac) and Ir(ppy)2 (acac)) contain one acetylacetonate (acac) ligand and show net horizontal alignment of the TDVs in evaporated films. 1,3,7,9 (Ir(ppy)3 ) shows no preferred alignment in evaporated systems. 7,14 The results for all analyzed guest-host systems are summarized in figure 2. Full orientation fits can be found in the supporting information. 22 All investigated Ir-complexes show the orientation expected from literature in samples fabricated by thermal evaporation. 1,3,7,14 The emissive molecules containing an acetylacetonate (acac) group exhibit predominantly horizontal alignment, while the homoleptic complex Ir(ppy)3 shows no preferred orientation. The comparison between the different host



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a)

b)

c)

Figure 1: a), b) Plots of emission intensity versus detection angle for films of NPB doped with Ir(mdq)2 (acac), fabricated from evaporation (a) and from chloroform solution (b). c) Investigation of the anisotropy factor Θ using a cross section of the angular dependent spectra at the peak intensity. The lines indicate the simulated values for different anisotropy factors Θ using a layer thickness of 20 nm. Fitting the measured data points results in Θ values of 0.36 and 0.26 for the solution processed (circles) and evaporated samples (diamonds), respectively.


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CBP

NPB

PMMA Spiro2-CBP

N N

O Ir

vertical

O 2

Ir(mdq)2(acac) O Ir O

N

2

Ir(piq)2(acac) N

O Ir O

horizontal

2

Ir(ppy)2(acac) N Ir 3

Ir(ppy)3

Figure 2: Summary of all observed anisotropy values. The filled symbols indicate measurements on samples processed from solution, the open symbols show the data from thermally evaporated films. The four different dyes are discriminated by different colors. The data for evaporated layers containing Ir(ppy)2 (acac) and Ir(ppy)3 in a Spiro2-CBP host are taken from Ref. 15



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materials indicates no dependency of the anisotropy factor Θ on the higher glass transition temperature of Spiro2-CBP. Earlier reported mechanisms for the orientation process of guest molecules propose increased alignment of the TDVs for high glass transition temperatures of the matrix material, if the guest and host molecules have comparable masses. This would lead to more horizontal orientation in films using Spiro2-CBP as host material. 15 However, in the investigated films the mass of the Ir-complex is much higher compared to the matrix material and this model can not be applied. The horizontal alignment of the heteroleptic complexes is governed by the deposition process, as none of the samples fabricated from solution developed this kind of preferred orientation. Comparison of the equivalent values measured for solution processed films of the normally horizontally oriented Ir(ppy)2 (acac) and the always isotropic Ir(ppy)3 species demonstrates that orientation in solution processed films is not influenced by molecular symmetry of the phosphorescent dopant. A similar effect has been oberved for the orientation of differently shaped fluorescent molecules in neat layers. 21 Furthermore, the orientation for solution processed samples is slightly influenced by the host material. Films of CBP as matrix material doped with either Ir(ppy)3 or Ir(ppy)2 (acac) show slightly vertical alignment. This kind of orientation is not favorable, as it would decrease the outcoupling efficiency in OLEDs. Due to the absence of this effect in PMMA and Spiro2-CBP, it can be connected to crystallization processes. Such effects have been reported for CBP at room temperature and for NPB based layers at 50 ◦ C. 17,18 Furthermore, the crystallization could be enhanced by solvent residues in the layer after the spin coating process, which could cause ”solvent vapor annealing”. This effect is known to activate or increase the crystallinity in other organic thin films. 20 While the influence of these effects on the orientation of TDVs is not yet clearified, the investigated samples show a trend to vertical alignment in the presence of crystallization processes. This could possibly be enhanced by post annealing treatments below the glass transition temperature of the host material, which were not investigated in this work.


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The significant dependency of the horizontal alignment of the heteroleptic phosphors on the fabrication process can be explained by the formation of an interface between the aromatic matrix and vacuum during the sample preparation by evaporation. 9 This interface is not present during the spin coating process, thus the alignment of the emissive Ir-complexes is disabled in these samples. Figure 3 illustrates this process.

Figure 3: Sketch of the model, which explains the different orientation in the evaporated and solution processed layers, using CBP (a) and Ir(ppy)2 (acac) (b) as an exemplary guesthost system. c) Orientation process during thermal evaporation, where the molecules are deposited ”step-by-step”. The aliphatic acetylacetonate group is highlighted in red together with the symmetry axis (C2) pointing from the Iridium central atom to this group. The C2 axis orients perpendicular to the aromatic surface in the direction of the vacuum. d) Sample preparation from solution by spin coating. The molecules (i.e. the C2 symmetry axis) do not align in a predominant direction in the solvent. This random behavior is conserved during the film drying process and thereby the deposited film shows isotropic orientation.

Conclusion In summary, the orientation of four different iridium complexes doped into a range of host materials was compared in solution processed and evaporated layers. All of the evaporated heteroleptic iridium complexes develop predominantly horizontal orientation in the matrix 9 ACS Paragon Plus Environment


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material, while the solution processed layers show nearly isotropic or slightly vertical orientation. The isotropic orientation of the TDVs is explained due to the missing interface between the aromatic matrix material and vacuum during the liquid-phase fabrication process. However the slightly vertical orientation in some emission layers still needs further investigation, but could be tentatively assumed to originate from a matrix effect connected to the glass transition temperature such as crystallization. The observed effects have important consequences for OLEDs as the horizontal alignment of the emissive TDVs of common phosphorescent Ir-complexes can not be achieved by solution based fabrication techniques.

Acknowledgement The research described here was carried out with the support of the Deutsche Forschungsgemeinschaft (DFG Br 1728/13-1) and Bavaria California Technology Center (BaCaTeC) for the Augsburg part as well as the Humboldt Foundation and Universal Display Corporation for the University of Southern California.

Supporting Information Available Figs. S1–S3, showing measured angular dependent emission patterns for all phosphorescent guest-host systems together with fits and error margins of the extracted orientation parameters. This material is available free of charge via the Internet at http://pubs.acs.org/.

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¨ mmich, J. Frischeisen, D. Michaelis, (2) T. D. Schmidt, D. S. Setz, M. Fla ¨ tting: Evidence for non-isotropic emitB. C. Krummacher, N. Danz and W. Bru ter orientation in a red phosphorescent organic light-emitting diode and its implications for determining the emitters radiative quantum efficiency. Appl. Phys. Lett. 99 (2011), 163302 (3) S.-Y. Kim, W.-I. Jeong, C. Mayr, Y.-S. Park,K.-H. Kim, J.-H. Lee,C.¨ tting and J.-J. Kim: Organic Light-Emitting Diodes with 30% K. Moon, W. Bru External Quantum Efficiency Based on a Horizontally Oriented Emitter. Adv. Funct. Materials 23 (2013), 3896-3900 ¨ tting: In(4) J. Frischeisen, D. Yokoyama, A. Endo, C. Adachi and W. Bru creased light outcoupling efficiency in dye-doped small molecule organic light-emitting diodes with horizontally oriented emitters. Org. Electron. 12 (2011), 809-817 (5) Y. Qu, M. Slootsky and S. R. Forrest: Enhanced light extration from organic light-emitting devices using a sub-anode grid. Nature Phot. 9 (2015), 758763 ¨ ller-Meskamp, (6) H.-W. Chang, J. Lee, S. Hofmann, Y. H. Kim, L. Mu B. Lssem, C.-C. Wu, K. Leo and M. C. Gather: Nano-particle based scattering layers for optical efficiency enhancement of organic light-emitting diodes and organic solar cells. J. Appl. Phys. 113 (2013), 204502 ¨ ssem, K. Leo and M. C. Gather: (7) P. Liehm, C. Murawski, M. Furno, B. Lu Comparing the emissive dipole orientation of two similar phosphorescent green emitter molecules in highly efficient organic light-emitting diodes. Appl. Phys. Lett. 101 (2012), 253304 ¨ tting: Determination of (8) J. Frischeisen, D. Yokoyama, C. Adachi and W. Bru molecular dipole orientation in doped fluorescent organic thin films by photoluminescence measurements. Appl. Phys Lett. 96 (2010), 073302 11 ACS Paragon Plus Environment


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¨ tting (9) M. Jurow, C. Mayr, T. D. Schmidt, T. Lampe, P. I. Djurovich, W. Bru and M. E. Thompson: Understanding and predicting the orientation of heteroleptic phosphors in organic light-emitting materials. Nature Mat. 15 (2015), 85-91 (10) W. L. Barnes: Fluorescence near interfaces: The role of photonic mode density. J. Mod. Opt. 45 (1998), 661 (11) L. Penninck, P. De Visschere, J. Beeckman and K. Neyts: Dipole radiation within one-dimensional anisotropic microcavities: a simulation method. Opt. Express 19 (2011), 18558-18576 ¨ tting, J. Frischeisen, T. D. Schmidt, B. J. Scholz and C. Mayr: De(12) W. Bru vice efficiency of organic light-emitting diodes: Progress by improved light outcoupling. physica status solidi (a) 210 (2013), 44-65 ¨ tting: Efficiency Analysis of (13) T. D. Schmidt, B. J. Scholz, C. Mayr and W. Bru Organic Light-Emitting Diodes Based on Optical Simulations. IEEE Sel. Top. Quant. Elect. 19 (2013), 1-12 (14) K.-H. Kim, C.-K. Moon, J.-H. Lee, S.-Y. Kim and J.-J. Kim: Highly Efficient Organic Light-Emitting Diodes with Phosphorescent Emitters Having High Quantum Yield and Horizontal Orientation of Transition Dipole Moments. Advanced Materials 26 (2014), 3844-3847 ¨ tting: Influence of molecular orientation in organic light(15) C. Mayr and W. Bru emitting diodes Chem. Mat. 27 (2015), 2759-2762 (16) C-K Moon, K-H Kim, J. W. Lee and J-J Kim: Influence of Host Molecules on Emitting Dipole Orientation of Phosphorescent Iridium Complexes Chem. Mat. 27 (2015), 2767-2769


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Dependence of Phosphorescent Emitter Orientation on Deposition

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