Atomistic Insight into Molecular Orientation and

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Origin of High Efficiencies for Thermally Activated Delayed Fluorescence OLEDs: Atomistic Insight into Molecular Orientation and Torsional Disorder Hu Taiping, Guangchao Han, Zeyi Tu, Rui-Hong Duan, and Yuanping Yi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b08169 • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 10, 2018

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The Journal of Physical Chemistry

Origin of High Efficiencies for Thermally Activated Delayed Fluorescence OLEDs: Atomistic Insight into Molecular Orientation and Torsional Disorder Taiping Hu,†, ‡ Guangchao Han,†,‡ Zeyi Tu,†, ‡ Ruihong Duan,†, ‡ Yuanping Yi,*,†, ‡

†

CAS Key Laboratory of Organic Solids, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

*

University of Chinese Academy Sciences, Beijing, 100049, China

Corresponding authors. E-mail: [email protected].

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ABSTRACT Both the molecular orientation and conformation of thermally activated delayed fluorescence (TADF) emitter molecules that are doped in the host matrix are crucial to determine the performance of TADF-based organic light-emitting diodes (OLEDs). However, the amorphous molecular packing prohibits observation of the structural details at the atomic accuracy by experimental techniques. Here, using atomistic molecular dynamics simulations, we have uncovered the deposition process and molecular arrangements of a representative donor-acceptor (D-A) structured TADF emitter along with a host material on different model substrates. The simulated results point to that despite of distinct characters of the substrates, the emitter molecules in all the films exhibit preferential horizontal orientation due to the “rod-like” structure; thus the transition dipole moments (TDMs) of the lowest singlet excited state (S1) prefer to a horizontal distribution. This is beneficial to achieve a high out-coupling efficiency. In addition, the torsion angles between the D and A units of the emitter molecules show a broadened distribution around 90° due to thermal fluctuation and intermolecular interaction. Importantly, such torsional disorder can induce a drastic increase of both the S1 TDM and the spin-orbit coupling of S1 with the lowest triplet excited state (T1) while still keep a small energy difference between S1 and T1, which would facilitate the S1 radiative decay and the T1→S1 reverse intersystem crossing to obtain a high internal quantum efficiency. Our work provides an atomistic insight into the critical role of both molecular orientation and torsional disorder in achieving a high efficiency for an OLED based on the twisted D-A structured TADF emitter. 2


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I. INTRODUCTION Organic light-emitting diodes (OLEDs) have attracted considerable interest due to the application in flexible display and lighting.1-3 State-of-the-art OLEDs have a multilayer structure in which the emitting layer (EML) plays an important role. For an OLED device, the hole and electron carriers are electrically injected from the hole- and electron-transporting layer, respectively, to form singlet and triplet excitons with a ratio of 1:3 in the EML due to the spin statistics. Therefore, how to utilize the nonradiative triplet excitons is crucial to improve the internal quantum efficiency (IQE) for an OLED device. So far, heavy metal atoms (e.g. iridium and platinum) with strong spin-orbit couplings (SOCs) have been incorporated in organometallic compounds to improve the phosphorescence of the triplet exciton, thus increasing the exciton utilization efficiency and thus IQE of OLEDs.4-6 However, the use of high-cost rare metals and lack of stable efficient blue phosphors will limit the application and commercialization. Thermally activated delayed fluorescence (TADF) can harvest the nonradiative triplet excitons by thermally activated reverse (up-conversion) intersystem crossing (RISC) from the lowest triplet excited state (T1) to the lowest singlet excited state (S1) when the energy difference between S1 and T1 (ΔEST) is sufficiently small.7-14 Purely organic TADF emitters based on donor-acceptor (D-A) structures have been developed and successfully exploited in OLEDs. In such systems, the wave functions of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are essentially localized on the D and A units, respectively; the spatial separation between HOMO and LUMO will reduce the electron exchange energy and 3



result in small ΔEST, provided that both the S1 and T1 excitations are dominated by the HOMO→LUMO electronic transition.15-22 However, the same electronic transition nature will lead to vanishingly small SOC between S1 and T1 according to the El-Sayed rule.23-25 At the same time, owing to the strong charge transfer, the S1 state has very small transition dipole moment (TDM) and oscillator strength, which will limit the radiative decay process. Therefore, a comprehensive understanding of how to balance the TDM, SOC, and ΔEST by tuning molecular geometric structures is highly desirable to improve the IQE of the TADF OLEDs. Another key factor to determine the external quantum efficiency (EQE) of an OLED is the out-coupling efficiency, which can be improved by controlling the horizontal orientation and TDM (parallel to the substrate) of the emissive molecules.6, 26-27

Since the triplet exciton has a long life time, the TADF emitter usually needs to be

doped in a host matrix at a low concentration in the EML for the TADF OLED to suppress triplet-triplet annihilation.28 Therefore, the molecular orientation of the emitter can be influenced not only by the emitter itself but also by the substrate properties (e.g., surface stability and ordering or substrate temperature) as well as the host molecular structures.29-30 However, the amorphous nature of the EML inhibits experimental observation of the molecular packing details. Recently, TADF OLEDs with high EQEs have been prepared by many research groups. For instance, three TADF emitters were developed based on acridine-triazine twisted D-A structures (DMAC-TRZ31, DPAC-TRZ and SpriAC-TRZ32) and achieved high EQEs of 27.4%, 25.8% and 36.7% when doped into the 9-(3-(9H-carbazol-94


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yl)phenyl)-9H-carbazole-3-carbonitrile (mCPCN) host matrix, respectively. Wu et al prepared an OLED device with an EQE of 38% using a D-A-D structured TADF emitter.33 Such high EQEs indicate that not only the S1 radiative rate is very fast, but also the out-coupling efficiency breaks the traditional limit of 20%. Thus, it would be useful to probe the origin of high EQEs for the OLEDs based on these TADF emitters. In this work, taking the DMAC-TRZ:mCPCN system as representative, we have investigated the detailed molecular packing structures of the EML on different holetransporting substrates made of N,N-dicarbazolyl-3,5-benzene (mCP), by means of atomistic molecular dynamics (MD) simulations. The chemical structures of mCPCN, DMAC-TRZ and mCP are shown in Figure 1a. The results point to that the TADF emitter exhibits preferential horizontal molecular orientation and TDM due to the “rodlike” structure. Moreover, the torsion between the D and A units of the emitter molecules is properly broadened from the equilibrium perpendicular geometry due to the thermal fluctuation and intermolecular interaction. Such torsional disorder significantly enhances the TDM and SOC while keeps the ΔEST small enough. Our work demonstrates the importance of both favorable molecular orientation and proper torsional disorder to obtain a high EQE for the OLED based on a twisted D-A structured TADF emitter.

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Figure 1. (a) Chemical structures of mCPCN (host), DMAC-TRZ (emitter) and mCP (hole-transporting material). The D and A units of DMAC-TRZ are in red and blue, respectively; the related torsion angle is denoted as φ. (b) Three-dimensional representation of mCPCN and DMAC-TRZ. The carbon, nitrogen, and hydrogen atoms are in cyan, blue, and white, respectively. The arrows represent the principal axes of the molecules. (c) Illustration of the equilibrated mCP substrates with three different crystallographic planes.

II. METHODOLOGY Molecular dynamics simulations. All atomistic MD simulations were performed with the GROMACS 4.6.7 software package.34 The bonded and non-bonded interaction parameters of mCPCN, DMAC-TRZ, and mCP were built from the general Amber force field (GAFF)35 with the restrained electrostatic potential (RESP) charges36. Missing bonded parameters for mCPCN and mCP were taken from the work by Muccioli et al.37 The validations of force field parameters are presented in the

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supporting information (SI) (Figure S1 and Tables S1-S4). According to the previous works reported by Muccioli et al and ourselves, a quasi-equilibrium strategy was applied in the MD simulations of vacuum vapor deposition.38-39 The simulation details for vapor deposition and following high temperature annealing are described in the section of Simulation and Computational Details in SI. The final snapshots of the equilibrated EML film morphologies without substrate or on three different substrates were depicted in Figure 2a. There are about 150 emitter molecules and the emitter:host mole ratio is 1:9, corresponding to the weight concentration of 12% for the emitter. During the last 10 ns of the equilibrated trajectory, 1000 snapshots were sampled every 10 ps to analyze the geometric structures and 10 snapshots were chosen every 1 ns to calculate the excited-state properties considering the huge magnitude of first-principle calculations. Five independent simulations were performed for every kind of thin film. Final statistical results were obtained as the average over these five samples.

Electronic structure calculations. Since the S1 and T1 states of TADF emitters have strong charge-transfer (CT) character, the density functional could impact the calculated results. In addition, the torsion angle between the electron-donating and electronwithdrawing units will influence the nature of the excited states for the emitter. Therefore, we first performed model calculations to choose an appropriate functional. Rigid scan of the torsion angle was carried out starting from the geometry optimized by density functional theory (DFT) at the B3LYP/6-31G** level. It has been pointed out that long-range-corrected functionals with the optimally tuned range separation parameter  can well describe the CT excitations.40-41 Thus, the transition dipole 7



moment and ΔEST for each scan structure were then calculated by time-dependent DFT (TDDFT) with the B3LYP and B97Xd functionals in combination with the 6-31G** basis set. For B97Xd, the  value was optimized by a “gap-tuning” procedure (referred as *B97Xd hereafter);42 in addition, polarizable continuum model (PCM) was applied to take account of the polarization effect of solid state environment with the dielectric constant set to 3.0.43 Interestingly, B3LYP gives similar results of TDMs and ΔEST as those obtained with *B97Xd (Figures S3 and S4). Since the geometric structures can vary from different emitter molecules and different snapshots and samples, it would be impracticable to perform “gap-tuning” of the  value under every structure. Thus, the excited-state electronic-structure calculations for the emitter structures extracted from MD simulations were performed at the TDDFT-B3LYP/631G** level. All these calculations were performed with Gaussian16 A.03 package.44 The natural transition orbitals (NTOs) were used to describe the transition characters for the S0 → S1 and S0 → T1 transition.45 In order to quantitatively evaluate the CT component, we performed the orbital component analysis based on the highest occupied NTO (HONTO) and lowest unoccupied NTO (LUNTO) via the Hirshfeld method.46 The CT component is defined as the difference between the contributions of the electron-donating fragment to the HONTO and LUNTO, respectively. NTOs and orbital component analyses were done using Multiwfn program.47 In addition, the HONTO and LUNTO for the S0 → S1 and S0 → T1 transitions for the twist of 60º (or 120º) and 90º were depicted in Figure S5. The SOCs between S1 and T1 were calculated

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by using the single-particle BP operator with an effective charge approximation, as implemented in the PySOC program.48-49

(a)

long-axis

z

y

x

substrate

mixture

(b) Probability per 2 degrees

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.015

mCP(100)

mCP(010)

mCP(001)

mCPCN/DMAC-TRZ on mCP (010)

mCPCN/DMAC-TRZ on mCP(100)

0.010 0.005

x y z

0.000

mCPCN/DMAC-TRZ on mCP(001)

mCPCN/DMAC-TRZ mixture

0.010 0.005 0.000

0

30

60

90

120 150

0

30

60

90

120 150 180

θ (degree) Figure 2. (a) Illustration of the substrate and snapshots of the final deposition on the equilibrated substrates. The emitter molecules (red color) are shown in vdW representation. The host molecules are shown in licorice representation. The substrates (gray color) lie in the x-y plane and are shown in vdW representation. (b) Distributions of the angles between the molecular long-axis of the emitter and the x, y, and z directions for the (100), (010), and (001) substrate surfaces. The results for the blending mixture obtained by high temperature annealing method are also plotted.

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III. RESULTS AND DISCUSSION 3.1 Substrate properties. To obtain an in-depth understanding of the impact of holetransporting substrates on the molecular packing of EML, we have made a detailed analysis of the properties of the substrate surfaces. The structures of initial and equilibrated substrates as well as the substrates after deposition are plotted in Figures S6-S8, respectively. It can be found that after equilibration, the conformation changes are much greater for the (100) and (001) substrate surfaces with respect to the (010) surface, indicating that the (010) surface is more stable. As a result, the (100) and (001) surfaces become completely disordered while the (010) surface shows partial disorder after deposition of the EML; the interface between the substrate and EML undergoes substantial mixture for the (100) and (001) surfaces but relatively small mixture for the (010) surface. Similar interfacial mixtures were found between C60 and disordered pentacene or DTDCTB surfaces.39, 50 To quantitatively evaluate the stability of the substrate surfaces, the surface energies were estimated via energy minimization (EM) or NVT equilibration (see SI); the obtained results are shown in Figure S9. The two approaches give similar trends of the surface energies. Unexpectedly, the (001) surface has the smallest surface energy, even smaller than that of the most stable (010) surface. The reason is that the crosssectional area of the (010) surface is underestimated due to the existence of deep channels. To validate this hypothesis, the adhesion energies were calculated for the three surfaces. As expected, the (010) surface has the largest adhesion energy. The 10


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evolutions of molecular sites on the surfaces at the early deposition stage are depicted in Figures S10-S12. For the (010) substrate, the emitter or host molecules would be first situated in the deep and wide surface channels; analogous directional growths are not found for the other two surfaces.

3.2 Molecular orientation. The emitter molecular orientations in the EML have an important impact on the out-coupling efficiencies. The molecular long-axis of the emitter is from the electron-donating to electron-withdrawing unit (Figure 1b). The angles between the molecular long-axis and the x, y, and z directions of the substrates have been calculated, respectively; the distributions of these angles are shown in Figure 2b. It can be seen that the emitter molecules are randomly oriented if the substrate is not taken into account. When any of the substrates is considered, the emitter molecules will show preferential horizontal orientation (i.e., parallel to the substrates). In contrast, the host molecules exhibit relatively random orientation no matter whether the substrates are considered or not (Figure S13). Recently, Tonnelé et al also demonstrated that neither the Ir(ppy)3 emitter nor the CBP host adopts a preferred orientation in the film.51 It should be noted that random distribution of host molecules is beneficial to balance the hole and electron transport in OLEDs.26 The molecular orientations in the EML can be quantitatively evaluated by an orientation order parameter S,52 which is defined by the ordinary and extraordinary extinction coefficients κo and κe, 𝜅𝑒 − 𝜅𝑜 3〈cos 2 𝜃〉 − 1 𝑆= = 𝜅𝑒 + 2𝜅𝑜 2

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(1)


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Here, θ denotes the angle between the molecular long axis and the normal direction of the substrate surface (z-axis in Figure 2a) and 〈⋯ 〉 indicates the average of an ensemble molecules. Therefore, S = 1 when the molecules are completely perpendicular to the surface, S = -0.5 when they are completely parallel to the surface, and S = 0 means that the molecules are randomly oriented.53-55 The calculated S values are listed in Table 1. It can be found that neither the emitter nor the host exhibits a preferential orientation in the blending mixture produced by high temperature annealing (S ≈ 0), which is in full agreement with the random distribution of molecular orientations (Figures 2b and S13). Interestingly, the emitter molecules in the deposited films exhibit a slight priority for horizontal orientation (S = -0.05 ~ -0.07), while the host molecules still exhibit relatively random distribution (S = -0.02 ~ -0.03). The random distribution of the mCPCN host can be attributed to the relatively distorted (isotropic) geometric structure. On the contrary, the “linear-shaped” or “rod-like” structure of the DMAC-TRZ emitter would be beneficial for the molecules to be oriented horizontally.

Table 1. Calculated values of the orientation order parameters S and ΘH for the emitter and/or host in the films on different substrates and the blending mixture. (100) surface

(010) surface

(001) surface

mixture

-0.052±0.031 -0.031±0.010

-0.002±0.032 -0.002±0.012

S emitter hosta

-0.064±0.050 -0.020±0.010

-0.071±0.026 -0.026±0.014 ΘH

a

emitter -0.70±0.03 -0.71±0.02 -0.70±0.02 obtained by using the A2 axis for the host (see Figure 1b).

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-0.67±0.02

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Inevitably, the molecular orientation will affect the TDM direction that is directly related to the out-coupling efficiency in principle. Since the emitter molecules emit light perpendicular to the TDMs, the out-coupling efficiency can be maximized if the TDMs are completely parallel to the substrate surface. The degree of TDM alignment can be estimated by fitting the spectral radiant intensity to an optical simulation model taking the fraction of light emitted from horizontal components of the TDMs, ΘH, as a variable.56 Because of the good flexibility of the TADF emitter, the TDM magnitude of every emitter molecule in the film will be different from each other and the TDM direction could deviate from the molecular long axis. Therefore, for a film containing N emitter molecules, the fraction of light emitted from the horizontal components of TDMs can be calculated microscopically as the sum of the square of the horizontal component of each individual TDM,57 𝑁

1 𝜇𝑖,𝐻 2 𝛩𝐻 = ∑ ( ) 𝑁 𝜇𝑖

(2)

𝑖=1

where μi is the TDM magnitude of each individual emitter molecule and subscript H denotes the horizontal component. Fully horizontal and vertical alignment of the TDMs correspond to ΘH = 1 and 0, respectively; for the isotropic TDMs, ΘH = 0.67. Obviously, larger values of ΘH would result in a higher out-coupling efficiency.57 Here, our calculated values of ΘH were also listed in Table 1. It can be seen that the blending mixture obtained via high temperature annealing displays fully isotropic TDMs. However, the vapor deposition films on the substrates exhibit preferential horizontal TDMs and the calculated values of ΘH agree well with the experimental measurement 13



(0.72). This result confirms the importance of the substrates to obtain preferential horizontal alignment of molecular orientations and TDMs. Considering the high EQE measured for the OLED device, such preferential horizontal alignment of TDMs in the deposited EML can play a critical role in achievement of a high out-coupling efficiency.

3.3 Impact of torsional disorder. The torsion angle between the D and A units of the emitter has a significant impact on the electronic structure properties of the excited states. The distribution of the torsion angle for the emitter molecules in the deposited films and the blending mixture is shown in Figure 3. The equilibrium torsion angle for all the MD simulated samples corresponds well to the perpendicular configuration of the DFT-optimized geometry. Notably, the torsion angle of the emitter molecules in the all the films exhibits a similar broadened distribution from 50º to 130º due to the intermolecular interaction and thermal fluctuation. To illustrate the impact of torsional disorder on the electronic structure properties of the excited states, we have performed TDDFT calculations on a series of model twisted geometric structures (see the computational details in the METHODOLOGY section).

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0.030

Probability per 2 degrees

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0.025 0.020 0.015

(100) surface (010) surface (001) surface mixture

0.010 0.005 0.000 45

60

75

90

105

120

135

 (degree)

Figure 3. Distribution of the torsion angle between the D and A units of the emitter molecules in the deposited films on the (100), (010), and (001) surfaces and the blending mixture.

Figure 4a display the evolution of the TDM magnitude of S1 and ΔEST with the torsion angle for the emitter. Apparently, all the MD simulated samples show the same trend for the TDM magnitude (Figures 4b and S14). In addition, the results calculated using the model twisted structures can correspond well to the statistical results based on the MD simulated molecular geometries. When the D and A units are perpendicular to each other, both ΔEST and TDM are vanishingly small, implying that although the RISC process could be very fast, the radiative process will be prohibited. When the torsion angle deviates from 90º by ca. 40º as indicated from the MD simulations, △EST will be increased by 0.29 eV due to the ascending of the S1 energy and slight descending of the T1 energy (Figure 4c). Nonetheless, the △EST values are still small enough to ensure fast RISC (

Atomistic Insight into Molecular Orientation and

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