Vol. 27, No. 16 | 5 Aug 2019 | OPTICS EXPRESS A1014nom.mse.hust.edu.cn/pdf/2019_OPT_EXPRESS.pdf · 2019. 7. 15. · Vol. 27, No. 16 | 5 Aug 2019 | OPTICS EXPRESS A1016 . where k0

Simulation method for study on outcoupling characteristics of stratified anisotropic OLEDs

XIANHUA KE,1 HONGGANG GU,1, 2 XUENAN ZHAO,1 XIUGUO CHEN,1 YATING SHI,1 CHUANWEI ZHANG,1 HAO JIANG,1 AND SHIYUAN LIU

1, 3 1State Key Laboratory of Digital Manufacturing Equipment and Technology, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China [email protected] [email protected]

Abstract: We derive explicit power dissipation functions for stratified anisotropic OLEDs based on a radiation model of dipole antennas inside anisotropic microcavity. The dipole field expressed by vector potential is expanded into plane waves whose coefficients are determined by scattering matrix method, and then an explicit expression is derived to calculate the energy flux through arbitrary interfaces. Taking advantage of the formulation, we can easily perform quantitative analysis on outcoupling characteristics of stratified anisotropic OLEDs, including outcoupling efficiency, normalized decay rate and angular emission profile. Simulations are carried out on a prototypic stratified OLED structure to verify the validity and capability of the proposed model. The dependencies of the outcoupling characteristics on various emission feature parameters, including dipole position, dipole orientation, and the intrinsic radiative quantum efficiency, are comprehensively evaluated and discussed. Results demonstrate that the optical anisotropy in different organic layers has nonnegligible influences on the far-field angular emission profile as well as outcoupling efficiency, and thereby highlight the necessity of our method. The proposed model can be expected to guide the optimal design of stratified anisotropic OLED devices, and help to solve the inverse outcoupling problem for determining the emission feature parameters.

© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction

Organic light emitting diodes (OLEDs) have been the subject of intensive investigation in recent years due to their applications in displays and lighting [1–4]. The high-performance demands of OLED outcoupling characteristics call for comprehensive and quantitative optical analysis of the OLED layered system, especially the effect of various emission features [5–10]. The rapid design cycles of OLED devices prefer to resort to computer models to accelerate the optimal design. Besides, during recent years, abundant organic thin films have been reported to possess a preferred orientation which leads to significant optical anisotropy in materials [11–14]. Therefore, it is necessary to develop a proper model with optical anisotropy considered to accurately analyze and evaluate the dependencies of stratified anisotropic OLEDs outcoupling characteristics on various emission feature parameters.

In most of early researches, under the implicit assumption that all layers are isotropic, various approaches have been developed to investigate the optical behavior of OLED stacks. Broadly, these methods can be roughly divided into two typical models in the OLED optical simulation: (1) one is based on the microcavity model [15–20]; (2) the other one is based on the dipole model [21–28]. The former describes the OLED structure as a simplified Fabry-Perot cavity, which can serve as a simple and intuitive quick guide to predict the irradiance spectra and angular emission. For example, by using the microcavity model, Hoang and Lee et al. [16,17] presented theoretical studies on the performance of the red/blue phosphorescent OLEDs constructed by epsilon negative electrodes, and they acquired the optimal luminous efficiency by varying the layer thickness of the structure. While the latter method treats the

Vol. 27, No. 16 | 5 Aug 2019 | OPTICS EXPRESS A1014

#367540 https://doi.org/10.1364/OE.27.0A1014 Journal © 2019 Received 14 May 2019; revised 11 Jun 2019; accepted 11 Jun 2019; published 27 Jun 2019

https://doi.org/10.1364/OA_License_v1

https://crossmark.crossref.org/dialog/?doi=10.1364/OE.27.0A1014&domain=pdf&date_stamp=2019-06-27

exciton emission in OLEDs as radiation from a bounded harmonically oscillating dipole antenna, and can be employed to rigorously calculate the electromagnetic field inside/outside the layer structure and provide complete information of the optical characteristics in the OLED stacks. Based on the dipole model, Meerheim et al. [25] analyzed the distribution of the different energy loss mechanisms in bottom and top emission OLED stacks, and obtained remarkably high external quantum efficiencies by optimizing the stack structure. Zhu et al. [26] analyzed the light outcoupling and angular performance of quantum dot light emitting diode, resulting in an enhancement in the outcoupling efficiency by combining optimal structure and a high refractive index substrate. However, all these mentioned approaches assumed that the materials in the OLED device are isotropic, which leads to unsuitable problems for the structure with anisotropic layers.

In recent years, as anisotropic materials being widely applied in high quality OLEDs, the dipole model has been remarkably extended to address the calculation of dipole radiation in stratified anisotropic mediums [29–35]. Chance et al. [29] firstly used the Hertzian vector to describe the field of the emissive dipole and applied the plane wave expansion method to determine the emissive electromagnetic field. Wasey et al. [30] extended the plane wave expansion method to outline a classical model for spontaneous emission decay rate within birefringent materials. Moon et al. [32] detailed a robust algorithm for the computation of electromagnetic fields radiated by a dipole source in the cylindrically stratified anisotropic mediums. Kim et al. [34] presented an optical dipole model to calculate the luminescence from emissive dipoles in a birefringent medium to describe the outcoupling efficiency. These approaches mentioned can rigorously calculate the electromagnetism in the uniaxial mediums, but they usually suffer from lack of a clear and simple way to calculate the emission from the multilayer structure with arbitrary anisotropic layers of an OLED structure. Considering that all the layers in a stack of multi-layer films are anisotropic, Penninck et al. [35] presented an explicit and general expression for electric field of the dipole radiation in the anisotropic medium with arbitrary optics axis orientation. However, due to the complex two-dimensional integral expression of the electric field, only an implicit representation of the Poynting vector was given in the form of a four-dimensional integral, which may lead to inconvenience in the process of numerical integration. Since the optics axis of the OLED material is generally normal to the substrate plane, the theory and method can be significant simplified to deal with this situation. Furthermore, they focused on the dipole energy of layered anisotropic systems and did not give formulations about the outcoupling characteristics of the cavity structures, such as the outcoupling efficiency, the normalized decay rate, and the far-field angular emission profile. Therefore, when dealing with more common cases in the stratified anisotropic OLED stacks, it would be more convenient if simplified and explicit formulations are derived for these outcoupling characteristics of the stratified anisotropic OLED stacks.

In this paper, we derive explicit power dissipation functions for the general stratified anisotropic OLED structure based on a radiation model of dipole antennas inside anisotropic microcavity. This proposed formulation brings us much convenience to quantitively analyze the dependencies of the outcoupling characteristics of stratified anisotropic OLEDs on various emissive properties. Simulations are performed on a prototypic OLED structure to investigate the influence of emission features on outcoupling characteristics of the OLED device to verify the effectiveness and necessity of the proposed method.

2. Theory and methods

A multilayer OLED made of various functional layers is superior in terms of efficiency and lifetime. A typical stack layout of such an OLED is displayed in Fig. 1(a), and from top to bottom are encapsulation, cathode, electron injection layer (EIL), electron transport layer (ETL), emission layer (EML), hole transport layer (HTL), hole injection layer (HIL), anode and substrate. According to the investigation of Chance, Prock and Silbey (CPS) [29], the


exciton formation in OLEDs can be treated as a bounded harmonically oscillating dipole. Therefore, the formulation model is simplified as the dipole source embedded in stratified anisotropic layers illustrated in Fig. 1(b). Then the problem becomes to calculate the energy flux through any interface z = zi with a dipole source embedded in the s-th layer with each layer characterized by dielectric tensor ε

v h v( ) .ε ε ε= + −ε I zz (1)

Here the bold I is the unit dyad, εh and εv are corresponding to the dielectric eigenvalues in the planes which parallel and perpendicular to the z axis, respectively.

Fig. 1. (a) Schematic of a typical OLED structure. (b) Simplified formulation model for the stratified anisotropic OLED structure with dipole in the s-th layer.

2.1 Power dissipation functions for a stratified anisotropic OLED

Considering a dipole located at R′ = (x′, y′, z′) and assuming a harmonic time component ejωt common to all field terms, the magnetic vector potential Ax by x orientated dipole and Az by z orientated dipole can be written by [36]

jh00

v

jˆ( , ) ( ) d ,

4πzvk z zz

zzv

kz e J k e k

kρ

ρ ρε μρ ρε

∞ ′− −= − A (2)

( )

j00

j j00

jˆ( , ) ( ) d

4π

1ˆ sgn( ) ( ) d ,

4π

zh

zh zv

k z zxx

zh

k z z k z zz

kz e J k e k

k

e z z J k e e kx k

ρρ ρ

ρ ρρ

μρ ρ

μ ρ

∞ ′− −

∞ ′ ′− − − −

= −

∂′+ − −∂

A (3)

where ˆxe and ˆ

ze denote the unit vectors in x and z directions respectively, j is the imaginary

unit, μ is the magnetic permeability, kρ is the surface-parallel wavevector, and J0 is the zero-order Bessel function. The sign (sgn) function denotes the correspondence of the positive (negative) sign to the wave propagating in the + z (−z) direction, and

2 2( ) ( ) ,x x y yρ ′ ′= − + − (4a)

2 20 h ,zhk k kρε= − (4b)

2 20 h h v/ ,zvk k kρε ε ε= − (4c)


where k0 is the wavevector in vacuum, and for the convenience, the position coordinates (ρ, z) are omitted in the following.

Applying the relationship between magnetic vector potential and electromagnetism filed 2j ( 1/ )E A k Aω= + ∇∇ ⋅ and 01/ ( )H Aμ= ∇× , we can obtain the z components of the source

electric and magnetic fields as

3

jh00

v v

1( ) d ,

4πzvk zz

zzv

kE J k e k

kρ

ρ ρε ρ

ωε ε∞ −= − (5)

0,zzH = (6)

j00

v

jsgn( )( ) d ,

4πzvk zx

z

zE k J k e k

x ρ ρ ρρωε

∞ −∂=∂ (7)

j00

j( ) d .

4πzhk zx

zzh

kH J k e k

y kρ

ρ ρρ∞ −∂=

∂ (8)

Here the dipole is set as the origin of the coordinates for simplicity, and the z component of the electric filed corresponds to TM waves while those of the magnetic field corresponds to TE waves.

Using the previous expression as a source term within the microcavity, and considering additional terms due to reflection and transmission at the interfaces, the integral expression of the previous equations can be reconstructed as the following forms

, , ,

3j j j,h

, 00,v ,v ,

1( ) ( ) ( )d ,

4πl zv l zv l zvk z k z k zlz

l z l ll l l zv

kE z A e A e l m e J k k

kρ

ρ ρ

εδ ρ

ωε ε∞ − −− ′= + + − (9)

, , ,j j j, 00

,v

j ( )( ) ( )d ,

4π sgn( )l zv l zv l zvk z k z k zx

l z l ll

l mE z k B e B e e J k k

x zρ ρ ρδ ρ

ωε∞ − − ∂ −′= + + ∂ (10)

, , ,j j j, 00

,

j( ) ( ) ( ) d .

4πl zh l zh l zhk z k z k zx

l z l ll zh

kH z C e C e l m e J k k

y kρ

ρ ρδ ρ∞ − −∂ ′= + + − ∂ (11)

Here the coefficients A′l, B′l, C′l and Al, Bl, Cl correspond to the forward and backward traveling waves in the l-th layer respectively. The source terms with Dirac delta function δ are omitted hereinafter to achieve a simplified formulation. The boundary conditions between l-th layer and (l + 1)-th layer are [37]

,v , 1,v 1, ,v , 1,v 1,,h 1,h

1 1, ,l l z l l z l l z l l z

l l

E E E Ez z

ε ε ε εε ε+ + + +

+

∂ ∂= =∂ ∂

(12a)

, 1, , 1,, .l z l z l z l zH H H Hz z+ +

∂ ∂= =∂ ∂

(12b)

Based on these interface equations and by setting A′1 = B′1 = C′1 = 0 and AN = BN = CN = 0, we can calculate the ratios of the coefficients starting at the outer layers by the scattering matrix method [38,39]. Then the source term is added into the field to calculate coefficients of the emission layer from the calculated ratios. Once again recursively utilizing the interface equations, the coefficients for each layer can be determined, then the z components of electric and magnetic fields can be obtained. Finally, the x and y components can be calculated by the following equations directly derived from Maxwell equations


v2 2

h

j,z z

x

H EE

y z xk kρ ρ

εωμε

∂ ∂∂= − +∂ ∂ ∂

(13a)

v2 2

h

j,z z

y

H EE

x z yk kρ ρ

εωμε

∂ ∂∂= +∂ ∂ ∂

(13b)

v2 2

j 1,z z

x

E HH

y z xk kρ ρ

ωε ∂ ∂∂= +∂ ∂ ∂

(13c)

v2 2

j 1.z z

y

E HH

x z yk kρ ρ

ωε ∂ ∂∂= − +∂ ∂ ∂

(13d)

The energy flux through the any infinite plane σ then can be calculated as

*Re ( ) d ,Sσ

σ= × ⋅ E H n (14)

where n is the unit normal vector of the plane, the superscript symbol * denotes the complex conjugation, and Re indicates taking the real part. After applying Bessel closure equation [40,41] into the integrand and divided by the total energy of the dipole in unbounded emission materials, the normalized energy flux through any z plane in the l-th layer of the z orientated dipole can be calculated by

( ), , , ,

*2 3 4, ,hTM,

***2 * 2,, , ,h ,v , ,

*j j j j

| |3Re Re

4 1 /

d ,l zv l zv l zv l zv

s v lz sl

l vs v s h l s l v l v

k z k z k z k zl l l l

u nqS

u

A e A e A e A e u

ε εεε ε ε ε ε ε

− −

=

−

′ ′− +

(15)

where the normalized wavevector u is defined as 0 ,vRe sk ukρ ε= , q is the intrinsic radiative

quantum efficiency, and the subscript s denotes the emission layer index. Similarly, the normalized energy flux through any z plane in the l-th layer of the x orientated dipole is

, , , ,*, j j j jTE,

2 2, , ,

31Re d ,

2 3 /

l zh l zh l zh l zhs h k z k z k z k zx sl l l l l

s h s v l h s

q unS u C e C e C e C e

n u

εε ε ε

− −

′ ′= + − + −

(16)

, , , ,

2 2 2*, l, j j j jTM,

, , ,

3 1 /1Re d .

2 3l zv l zv l zv l zvs h s s v k z k z k z k zx

l l l l ls h s v l h

q un u nS u B e B e B e B e

ε εε ε ε

− − −

′ ′= − + +

(17)

The Eqs. (15)–(17) are expressions of the power dissipation functions we derived for general stratified anisotropic OLEDs. Thereby, the dipole energy coupling to each individual layer can be obtained by taking the difference of the flux magnitude at both boundaries of the layer.

2.2 Outcoupling characteristics of OLED with a dipole in anisotropic microcavity

Outcoupling characteristics, including the outcoupling efficiency, the normalized decay rate, as well as the angular emission profile, have a pronounced influence on the quantum efficiency, and further can be applied to extract the parameters that cannot be measured directly. Upon the proposed model we derived, the outcoupling characteristics could be easily calculated as following.


Based on Kuhn model [29] and by using the reflected filed at the emitter’s position in Eqs. (9) and (10), we obtain an expression for the normalized decay rate in the cavity system as

( )3

0 20

31 1 Re ,

2 1

zs s

b uq q du A A

b u

∞ ′= − + + + −

(18)

( ), 2

0 2, , , ,

31 1 Re 1 .

3 1 /

s vx s ss s

s h s v s v s h

ub C Cq q du u B B

b u

εε ε ε ε

∞ ′+ ′= − + + − − − + −

(19)

Therefore, the outcoupling efficiency of a top emitting OLED can be easily derived as

( ). , ,outcoupling 0/ / ,x z x z x z

NS b bη = (20)

where N is the outmost layer index of the OLED structure, and S is the normalized energy flux which can be calculated through Eqs. (15)–(17). The x and z in the superscript denote the x oriented and z oriented dipole respectively.

In the case of plane waves propagating through multilayers, the emitting angle θair in the air region is associated with u and wavevector k0 = 2πnair/λ, therefore, the angular emission profile can be easily calculated by Eqs. (15)–(17) just by replacing u with θair in the outmost layer

, , ,

airair air

( , ) ( , ) ( , )( , ) (1 ) ,

2π tan( ) 2π tan( )

TM z TE x TM xN N NS u S u S u

Iλ λ λθ λ α α

θ θ+

= + − (21)

where S is the power emitted in Eqs. (15)–(17) as a function of u, and α denotes the dipole orientation: α = 1 denotes that the dipoles are orientated perpendicular to the substrate plane, α = 0 denotes that the dipoles are orientated parallel to the substrate plane, and α = 1/3 means a random distribution of the dipole orientation.

3. Simulation and discussion

Based on the proposed model, comprehensive analysis can be performed for the OLED stack outcoupling characteristic, including the outcoupling efficiency, the normalized decay rate and the far-field angular emission profile. Here we focus on the effect of the emission features, such as the relative emissive dipole position, the dipole orientation and the intrinsic radiative quantum efficiency q. Besides, in order to highlight the necessity and effectiveness of the proposed method, effects of the organic layer birefringence on the far-field angular emission profile as well as outcoupling efficiency are meanwhile quantitatively predicted and analyzed in this section. The optical simulations are performed by custom-made MATLAB codes.


(a) Ag (20nm)

Mg:Ag (100nm)

ETL (50nm)

EML (30nm)

HTL (50nm)

PEDOT:PSS (30nm)

ITO (160nm)

Glass

(b)

Fig. 2. (a) A detailed structure of a prototypic OLED device used for simulation. (b) Intrinsic emission spectrum of the emitting materials with a peak wavelength corresponding to 539 nm.

The state-of-the-art OLED stack under investigation and the intrinsic emitter spectrum of the emitting materials are shown in Fig. 2. This structure can be described as: the substrate is glass precoated with 160 nm thick layer of indium tin oxide (ITO). Next, a 30 nm thick layer of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulphonate) (PEDOT:PSS) is spun on. Then 50 nm of N,N`-diphenyl-N,N`-bis(3-methylphenyl)-[1,1`-biphenyl]4,4`-diamine (TPD) is evaporated as HTL layer, 30 nm of tris(8-hydroxyquinoline) aluminum (Alq3) is as EML layer and 50 nm of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) is as ETL layer. The cathode is a 100nm thick Mg:Ag layer with 60:1 Mg:Ag ratio with a 20 nm thick Ag cap on top. For more information about the OLED device structure or fabrication, we refer to [24]. Figure 2(b) shows the intrinsic luminance spectrum with a peak emission of about 539 nm. Applied Eqs. (18)–(21) for this structure stack, we can obtain the outcoupling efficiency, the normalized decay rate as well as the far-field angular emission profile.

3.1 Verification of the validity of the proposed model

Since the calculated outcoupling efficiency and normalized decay rate of Celebi et al. [24] have been accurately verified by the measurement in the experiment using a reverse bias technique, here we compare the simulation results using our methods with those using Celebi’s methods to verify the validity of the model we proposed. In simulations, the dipole is located at the middle of the EML layer, the intrinsic radiative quantum efficiency is assumed as q = 1, and wavelength is set as λ = 539 nm corresponding to the peak in intrinsic emission spectrum. The simulated outcoupling efficiency and normalized decay rate as a function of the ETL layer are shown in the top subfigure of Fig. 3. For simplicity, dipoles orientated perpendicular to the substrate plane are denoted as VED and those orientated parallel to the substrate plane are denoted as HED. The phrase “My” and “Re” in the legend respectively denote the simulation results from our model and the referent model of Celebi et al. [24]. It can be easily observed that the curves are completely consistent not only for the outcoupling efficiency but also the normalized decay rate. In order to further quantitatively analyze the results of the two models, the differences between these results are presented in the bottom subfigures of Fig. 3, where “δVED” and “δHED” in the legend denote the difference between their model and our proposed model for VED and HED respectively. It can be seen from Fig. 3 that the difference between two models is of the level of 10−3, and therefore the model we proposed can be verified with the simulation results of stratified isotropic OLED structure.


Fig. 3. Simulated (a) outcoupling efficiency and (b) normalized decay rate as a function of the ETL layer thickness and the difference between their model and our proposed model. Dipoles orientated perpendicular to the substrate plane are denoted as VED and those orientated parallel to the substrate plane are denoted as HED. The phrase “δVED” and “δHED” in legend denote the difference between their model and our model for VED and HED respectively.

In order to further verify the validity of the proposed model, we also compare the simulation results using our methods with those using the commercial software Fluxim Setfos [42]. The forward normalized emission spectrums under different ETL layer thickness is shown in Fig. 4. The configuration in this simulation is the same with previous one, except that the dipole orientation is assumed as HED here, since the radiation contribution of VED is neglectable in the direction normal to the substrate. The thickness of the ETL layer ranges from 40 nm to 160 nm with a step of 40 nm, and the results using our model are denoted by solid lines while those using Setfos are denoted by open circles as shown in Fig. 4. It can be easily observed that these results by our model and Fluxim Setfos are completely consistent with each other, which further highlights the validity of the model we proposed.

Fig. 4. Simulated normalized emission in the direction normal to the substrate under various ETL layer thickness by our proposed method and the commercial software Fluxim Setfos The results using our model are denoted by lines while those using Setfos are denoted by circles.


Fig. 5. Simulated outcoupling efficiency of the OLED stack under investigation as a function of the dipole position and wavelength for (a) VED and (b) HED.

3.2 Influence of the emission features on the outcoupling characteristics

Based on the proposed model, we can easily predict and analyze the influence of the emission features, including the dipole position, the dipole orientation, and intrinsic radiative quantum efficiency, on the outcoupling characteristics of the OLED stacks. The simulated outcoupling efficiency for the VED and HED as a function of the ETL layer thickness and wavelength are shown in Figs. 5(a) and 5(b), respectively. The dipole is located at the middle of the EML layer, and the intrinsic radiative quantum efficiency is assumed as q = 1. The ETL layer thickness ranges from 5 to 200 nm, and as the parameter to tune the distance from the dipole position to the metal, it is the main determining factor for the weak microcavity interference effects in this OLED stack. As we can obtain from the figure, the outcoupling efficiency for VED can reach the peak value 10% at the wavelength λ = 539 nm and 140 nm thick ETL layer. While the outcoupling efficiency for HED can reach its extremum as high as 34.3% at the wavelength λ = 541 nm combined with 30 nm thick ETL layer, and 36.4% at the wavelength λ = 535 nm combined with 200 nm thick ETL layer. From the simulation results we found that, compared with the VED, the HED significantly increases the maximum of the outcoupling efficiency, and therefore, the control of the dipole orientation has been identified as a particularly powerful handle for outcoupling efficiency enhancement.

In order to analyze of the influence of various emission features on the outcoupling efficiency more straightforward, without loss of generality, the simulation are carried on mono-chromatic wave and the wavelength is assumed as λ = 539 nm which corresponds to the maximum value of the intrinsic emission spectrum. The optical constants of OLED layers within this wavelength are shown in the Table 1. The dielectric eigenvalues in the Eq. (1) under the isotropic assumption then can be related to the refractive indices n and the extinction coefficient k as εh = εv = (n + jk)2.

Table 1. The optical constants of OLED layers at wavelength λ = 539 nm.

Ag Mg:Ag ETL EML HTL PEDOT:PSS ITO

n 0.102 0.305 1.727 1.716 1.727 1.518 1.811

k 3.904 4.926 3.19e-5 0 3.19e-5 0.011 0.010


Fig. 6. Simulated outcoupling efficiency of the OLED stack as a function of the ETL thickness under various radiative quantum efficiency q for (a) VED and (b) HED.

Fig. 7. Simulated normalized decay rate of the OLED as a function of the ETL layer thickness under various radiative quantum efficiency q for (a) VED and (b) HED.

Under the wavelength λ = 539 nm, the outcoupling efficiency of the OLED stack as a function of the ETL layer thickness under various intrinsic radiative quantum efficiency q are shown in Figs. 6(a) and 6(b) for VED and HED, respectively. And Fig. 7 demonstrates the behavior of the normalized decay rate as a function of the ETL layer thickness under various q for VED and HED. Both curves exhibit a qualitatively similar trend with two maxima separated by a pronounced minimum, and the pace of the oscillations between the outcoupling efficiency and the normalized decay rate curves is almost the same, which means that the higher outcoupling efficiency is produced mainly due to the stronger normalized decay rate. Furthermore, it can be clearly seen from the figures that the oscillations of the outcoupling efficiency and the normalized decay rate are both damped for decreasing radiative quantum efficiency q. And the first order optical cavity is more efficient owing to a higher outcoupling and a stronger normalized decay rate when compared to the second order in the OLED structure under investigation. Therefore, for simulation-based optimization of OLEDs in this section, the emissive dipole should be placed in the first cavity maximum for all the radiative quantum efficiency q ranged from 0.2 to 1.

The knowledge of the intrinsic radiative quantum efficiency is paramount important for OLED performance, and this information is usually not available in the directly measurement. Based on the relationship of the normalized decay rate and the lifetime τ:


( )

0 0

,b q

b

ττ

= (22)

herein, the two free parameters q and lifetime τ0 in the absence of the cavity can be easily determined by comparing measured lifetime data with simulation results. Due to that the normalized decay rate is critically depends on q and the difference significantly increases at the cavity maximum, the thickness of ETL layer should be the peak maximum if one wants to determinate the q by fitting the measured excited states’ lifetimes. As shown in Figs. 6 and 7, the optimal ETL thickness is 145 nm for VED, while 48 nm or 220 nm for HED.

3.3 Influence of optical anisotropy on the angular emission profile

These simulation results demonstrate that the proposed model can be utilized simply to predict and evaluate the dependencies of the stratified OLEDs outcoupling characteristics on various emission feature parameters. Nevertheless, in order to highlight the necessity and effectiveness of the proposed method, we also need to quantitatively study the impact of anisotropic organic layers on these outcoupling characteristics. Note that the angular emission profile not only allows us to calibrate the layer thicknesses but also to predict or determine the emission zone profile as well as the dipole orientation, and the accuracy of the investigations on the angular emission profile could be improved with the anisotropy considered in the organic layers. In this section we focus on analyzing influence of the anisotropy on the far-field angular emission profile.

The radiative quantum efficiency hereinafter is assumed to be q = 1, and the dipole is located at the middle of the EML layer. The extraordinary refractive indices ne of the functional layers are set as ne = n + Δn, where n denotes the ordinary refractive index shown in the Table 1 and the Δn is the birefringence. Then the dielectric eigenvalues in the Eq. (1) becomes

( )2j .v n n kε = + Δ + (23)

Substituting this dielectric eigenvalue into Eqs. (15)–(17), then based on the Eq. (21), we can calculate the far-field angular emission profile with anisotropy considered.

Figures 8 and 9 present the normalized far-field angular emission intensity versus the emission angle under various ETL and HTL birefringence respectively. From the Figs. 8(a) and 9(a), we found that for VED, the birefringence of the ETL layer leads to a peak shift while the influence of the HTL birefringence on the far-field angular emission profile can be neglectable. As the birefringence Δn increasing from −0.4 to 0.4, the intensity maximum peak shifts from 43.6° to 49.2° under anisotropic ETL layer. The influence of the anisotropy in organic layer on HED is quite different compared with VED. As shown in Figs. 8(b) and 9(b), the low angle range emission intensity rises obviously with the birefringence Δn increasing from −0.4 to 0.4 for both the ETL layer and the HTL layer. The simulation results therefore demonstrate that the anisotropy in ETL and HTL layer of OLED structure has non-neglectable influence on the far-field angular emission profile. Moreover, when the angular emission profile is applied to fit the emitter properties, including dipole position and dipole orientation, the proposed formulation with the capability of considering the optical anisotropy in arbitrary layer can be expected to provide more accurate analysis in the fitting process.


Fig. 8. Simulated far-field normalized angular emission profile with ETL layer birefringence for (a) VED and (b) HED under the wavelength λ = 539 nm.

Fig. 9. Simulated far-field normalized angular emission profile with HTL layer birefringence for (a) VED and (b) HED under the wavelength λ = 539 nm.

3.4 Influence of optical anisotropy on the outcoupling efficiency

To further perform analysis about the effect of optical anisotropy in organic functional layers on the outcoupling efficiency of the OLED, power dissipations in different channels, including absorption in top contact, waveguiding in organic layers, waveguiding in ITO layer, waveguiding in glass substrate, and emission into air, are studied by using the proposed optical simulation model. The proportion of power emission into the air to the total power emission is usually defined as the outcoupling efficiency. Substituting the organic function layer birefringence which has been defined in Eq. (23) into Eqs. (15)–(17), we can calculate the dipole energy coupling into l-th layer by (Sl − Sl−1) b0/b, and then the power dissipations of different optical channels can be obtained.


Fig. 10. Amount of power coupled to different optical channels for the OLED in dependence of the layer birefringence and dipole orientation. The radiative quantum efficiency is assumed as q = 1, and the simulation was performed under monochromatic wave with wavelength λ = 539 nm. Figures 10(a) and 10(b) denote the corresponding influence of the ETL and HTL layer birefringence on power dissipation for VED, while Figs. 10(c) and 10(d) denote that for HED.

Figure 10 shows power dissipations in different channels versus the birefringence in the ETL and HTL. In the simulations, a monochromatic wave with the wavelength λ = 539 nm is used. The radiative quantum efficiency here is assumed as q = 1, the dipole is in the middle of the EML layer, and the birefringence ranges from −0.4 to 0.4. The simulation results for VED independently with birefringence in the ETL and HTL layer are shown in Figs. 10(a) and 10(b), respectively. It can be obviously observed from Fig. 10(a) that the power dissipation into glass substrate waveguiding increases with negative birefringence in ETL layer. Therefore, if the enhancement tools are applied to extract light from the glass substrate of the OLED under investigation, ETL layer with negative birefringence could be beneficial to improve the outcoupling efficiency when the dipole orientation are VED domain in the OLED device. The trend of the power dissipation for HED is completely different compared with that for VED. As shown in Fig. 10(c), the outcoupling efficiency could benefit from negative birefringence in the ETL, and it can be improved from 26.2% to 37.2% when the birefringence in ETL layer reduces from 0.4 to −0.4. By comparison as shown in Fig. 10(d), negative birefringence HTL layer leads to a decrease in outcoupling efficiency, which denotes that for HED, the HTL and the ETL have opposing birefringence requirements. Important to note here is that the effect of the HTL layer is notably smaller than the effect of the ETL layer, since the outcoupling efficiency increases only from 29.5% to 32.9% as birefringence in HTL layer raises from −0.4 to 0.4.

Results in Fig. 10 demonstrate that the optical anisotropy in organic layers of the OLED structure has non-neglectable influence on the power dissipation into different optical channels and the final outcoupling efficiency. Therefore, the proposed optical formulation model with the capability of considering the optical anisotropy in arbitrary layer can be expected to provide accurate analysis and prediction on the detailed power dissipations and to guide the optimal design for stratified anisotropic OLEDs with high efficiency.

Since the OLED layers are almost thin films in panel display such as smartphones and tablets, the proposed work focuses on the simulation and quantitative analysis on this kind of OLEDs straightforwardly. Whereas, in applications of general lighting, periodical structures or random patterns are usually employed to improve the light extraction and enhance the final outcoupling efficiency of OLEDs [43–48]. Therefore, how to simulate structured or patterned OLEDs is currently a key issue in this field. It should be pointed out that although the formulations in this work cannot deal with structures or patterns in current version because the optical refractive index of each layer is a tensor with constants in the formulation, the


proposed model could be well extended to structured or patterned OLEDs with periodicity structures by simply introducing the mature algorithms, such as the rigorous coupled wave analysis (RCWA) method [49–51].

4. Conclusion

In this paper, explicit power dissipation functions are derived for stratified anisotropic OLED. In construction of the model, the dipole field expressed by the vector potential is expanded into a superposition of plane waves, whose coefficients are determined by the scattering matrix method. Then explicit power dissipation functions are derived to calculate the Poynting vector through an arbitrary interface in the anisotropic multi-layer stack. Based on the proposed methods, we can define the calculation of the outcoupling characteristics of a stratified anisotropic OLED, including the outcoupling efficiency, the normalized decay rate and the far-field angular emission profile. Simulations are carried out on a prototypic stratified OLED structure to verify the proposed method and to evaluate the dependencies of the outcoupling characteristics on various emission feature parameters. Results demonstrate the capability and validity of the proposed model, and the necessity of our method is also highlighted due to the non-neglect influence of the layer anisotropy on the far-field angular emission profile and the outcoupling efficiency. Therefore, the proposed formulation model with the capability of anisotropy considered in arbitrary layer can be expected to guide the structure optimization by providing more accurate analysis on the outcoupling characteristics and even prediction on emission features of stratified anisotropic OLEDs.

It is worth noting that, some existing commercial softwares, such as FDTD solution and Fluxim Setfos have been applied to perform the simulation and optimization of stratified anisotropic OLEDs. However, to the best of our knowledge, the FDTD solution is based on the OLED layer discretization which works at the expense of more computation resources, and the Setfos complies with the assumption that the emitting layer must be transparent. In this paper, the proposed model without such constraints is universal and therefore can be flexibly employed to quantitatively analyze or even optimize the performance of stratified anisotropic OLED structures. Moreover, the formulation proposed could be well extended to the thin patterned OLEDs with periodicity structures if the rigorous coupled wave analysis methods are combined.

Funding

National Natural Science Foundation of China (51727809, 51805193, 51525502, and 51775217), China Postdoctoral Science Foundation (2016M602288 and 2017T100546), National Science and Technology Major Project of China (2017ZX02101006-004), and the Natural Science Foundation of Hubei Province of China (2018CFB559 and 2018CFA057).

References

1. M. C. Gather, A. Köhnen, and K. Meerholz, “White organic light-emitting diodes,” Adv. Mater. 23(2), 233–248 (2011).

2. A. Salehi, X. Y. Fu, D. H. Shin, and F. So, “Recent advances in OLED optical design,” Adv. Funct. Mater. 29(15), 1808803 (2019).

3. H. W. Chen, J. H. Lee, B. Y. Lin, S. Chen, and S. T. Wu, “Liquid crystal display and organic light-emitting diode display: present status and future perspectives,” Light Sci. Appl. 7(3), 17168 (2018).

4. S. Reineke, M. Thomschke, B. Lussem, and K. Leo, “White organic light-emitting diodes: status and perspective,” Rev. Mod. Phys. 85(3), 1245–1293 (2013).

5. Z. Wang and S. J. Su, “Molecular and device design strategies for ideal performance white organic light-emitting diodes,” Chem. Rec. 18, 1–14 (2018).

6. K. Saxena, V. K. Jain, and D. S. Mehta, “A review on the light extraction techniques in organic electroluminescent devices,” Opt. Mater. 32(1), 221–233 (2009).

7. M. C. Gather and S. Reineke, “Recent advances in light outcoupling from white organic light-emitting diodes,” J. Photon. Energy 5(1), 057607 (2015).


8. W. Brutting, J. Frischeisen, T. D. Schmidt, B. J. Scholz, and C. Mayr, “Device efficiency of organic light- emitting diodes: progress by improved light outcoupling,” Phys. Status Solidi., A Appl. Mater. Sci. 210(1), 44–65 (2013).

9. S. Nowy, B. C. Krummacher, J. Frischeisen, N. A. Reinke, and W. Brutting, “Light extraction and optical loss mechanisms in organic light-emitting diodes: influence of the emitter quantum efficiency,” J. Appl. Phys. 104(12), 123109 (2008).

10. M. Chakaroun, A. T. Diallo, S. Hamdad, S. Khadir, A. P. A. Fischer, and A. Boudrioua, ““Experimental and theoretical study of the optical properties optimization of an OLED in a microcavity,” IEEE T,” Electron Dev. 65, 4897–4904 (2018).

11. T. D. Schmidt, T. Lampe, D. Sylvinson, M. R. P. I. Djurovich, M. E. Thompsom, and W. Brutting, “Emitter orientation as a key parameter in organic light-emitting diodes,” Phys. Rev. Appl. 8, 037001 (2017).

12. D. Yokoyama, “Molecular orientation in small-molecule organic light-emitting diodes,” J. Mater. Chem. 21(48), 19187–19202 (2011).

13. T. Lee, B. Caron, M. Stroet, D. M. Huang, P. L. Burn, and A. E. Mark, “The Molecular origin of anisotropic emission in an organic light-emitting diode,” Nano Lett. 17(10), 6464–6468 (2017).

14. K. H. Kim and J. J. Kim, “Origin and control of orientation of phosphorescent and TADF dyes for high efficiency OLEDs,” Adv. Mater. 30(42), e1705600 (2018).

15. H. Cho, J. Chung, J. Song, J. Lee, H. Lee, J. Lee, J. Moon, S. Yoo, and N. S. Cho, “Importance of Purcell factor for optimizing structure of organic light-emitting diodes,” Opt. Express 27(8), 11057–11068 (2019).

16. T. V. Hoang, S. E. Lee, J. G. Lee, Y. K. Kim, and J. H. Lee, “Optimum thickness of epsilon negative tri-metal layer electrodes for maximizing OLED outcoupling efficiency,” Opt. Express 25(25), 31006–31016 (2017).

17. S. E. Lee, T. V. Hoang, J. H. Lee, and Y. K. Kim, “Investigation of light out-coupling efficiency of blue OLED using microcavity effects,” Physica B 550, 122–126 (2018).

18. K. Kang, Y. Lee, J. Kim, H. Lee, and B. Yang, “A generalized Fabry-Perot formulation for optical modeling of organic light-emitting diodes considering the dipole orientation and light polarization,” IEEE Photonics J. 8(2), 1–19 (2016).

19. D. Poitras, C. C. Kuo, and C. Py, “Design of high-contrast OLEDs with microcavity effect,” Opt. Express 16(11), 8003–8015 (2008).

20. H. Peng, J. Sun, X. Zhu, X. Yu, M. Wong, and H.-S. Kwok, “High-efficiency microcavity top-emitting organic light-emitting diodes using silver anode,” Appl. Phys. Lett. 88(7), 073517 (2006).

21. W. Lukosz, “Theory of optical-environment-dependent spontaneous-emission rates for emitters in thin layers,” Phys. Rev. B 22(6), 3030–3038 (1980).

22. K. G. Sullivan and D. G. Hall, “Enhancement and inhibition of electromagnetic radiation in plane-layered media. I. Plane-wave spectrum approach to modeling classical effects,” J. Opt. Soc. Am. B 14(5), 1149–1159 (1997).

23. K. A. Neyts, “Simulation of light emission from thin-film microcavities,” J. Opt. Soc. Am. A 15(4), 962–971 (1998).

24. K. Celebi, T. D. Heidel, and M. A. Baldo, “Simplified calculation of dipole energy transport in a multilayer stack using dyadic Green’s functions,” Opt. Express 15(4), 1762–1772 (2007).

25. R. Meerheim, M. Furno, S. Hofmann, B. Lussem, and K. Leo, “Quantification of energy loss mechanisms in organic light-emitting diodes,” Appl. Phys. Lett. 97(25), 253305 (2010).

26. R. Zhu, Z. Luo, and S. T. Wu, “Light extraction analysis and enhancement in a quantum dot light emitting diode,” Opt. Express 22(S7 Suppl 7), A1783–A1798 (2014).

27. A. A. Shcherbakov, A. V. Tishchenko, D. S. Setz, and B. C. Krummacher, “Rigorous S-matrix approach to the modeling of the optical properties of OLEDs,” Org. Electron. 12(4), 654–659 (2011).

28. M. Qian, X. B. Shi, Y. Liu, Z. M. Jin, X. L. Wang, Z. K. Wang, and L. S. Liao, “Theoretical model for the external quantum efficiency of organic light-emitting diodes and its experimental validation,” Org. Electron. 25, 200–205 (2015).

29. R. R. Chance, A. Prock, and R. Silbey, “Molecular fluorescence and energy transfer near interfaces,” Adv. Chem. Phys. 37, 1–65 (2007).

30. J. A. E. Wasey, A. Safonov, I. D. W. Samuel, and W. L. Barnes, “Effects of dipole orientation and birefringence on the optical emission from thin films,” Opt. Commun. 183(1-4), 109–121 (2000).

31. K. Sainath, F. L. Teixeira, and B. Donderici, “Robust computation of dipole electromagnetic fields in arbitrarily anisotropic, planar-stratified environments,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 89(1), 013312 (2014).

32. H. Moon, B. Donderici, and F. L. Teixeira, “Stable evaluation of Green’s functions in cylindrically stratified regions with uniaxial anisotropic layers,” J. Comput. Phys. 325, 174–200 (2016).

33. M. K. Callens, D. Yokoyama, and K. Neyts, “Anisotropic materials in OLEDs for high outcoupling efficiency,” Opt. Express 23(16), 21128–21148 (2015).

34. C. K. Moon, S. Y. Kim, J. H. Lee, and J. J. Kim, “Luminescence from oriented emitting dipoles in a birefringent medium,” Opt. Express 23(7), A279–A291 (2015).

35. L. Penninck, P. De Visschere, J. Beeckman, and K. Neyts, “Dipole radiation within one-dimensional anisotropic microcavities: a simulation method,” Opt. Express 19(19), 18558–18576 (2011).

36. Z. H. Xiong, “Electromagnetic fields of electric dipoles embedded in a stratified anisotropic earth,” Geophys. 54(12), 1643–1646 (1989).

37. W. C. Chew, Waves and Fields in Inhomogeneous Media (IEEE, 1995), Chap. 2.


38. D. Y. K. Ko and J. R. Sambles, “Scattering matrix method for propagation of radiation in stratified media: attenuated total reflection studies of liquid crystals,” J. Opt. Soc. Am. A 5(11), 1863–1866 (1988).

39. H. Kim, I. M. Lee, and B. Lee, “Extended scattering-matrix method for efficient full parallel implementation of rigorous coupled-wave analysis,” J. Opt. Soc. Am. A 24(8), 2313–2327 (2007).

40. I. S. Gradshteyn and I. M. Ryzhik, Table of Integrals, Series and Products (Academic, 2007), Chap.6. 41. G. B. Arfken and H. J. Weber, Mathematical Methods for Physicists (Academic, 2005), Chap. 11. 42. A. G. Fluxim, “Semiconducting emissive thin film optics simulator SETFOS,” http://www.fluxim.com. 43. H. Liang, H. C. Hsu, J. Wu, X. He, M. K. Wei, T. L. Chiu, C. F. Lin, J. H. Lee, and J. Wang, “Corrugated

organic light-emitting diodes to effectively extract internal modes,” Opt. Express 27(8), A372–A384 (2019). 44. M. Kovacic, P. A. Will, B. Lipvsek, M. Topic, S. Lenk, S. Reineke, and J. Krc, “Coupled optical modeling for

optimization of organic light-emitting diodes with external outcoupling structures,” ACS Photonics 5(2), 422–430 (2018).

45. H. Liang, Z. Luo, R. Zhu, Y. Dong, J. H. Lee, J. Zhou, and S. T. Wu, “High efficiency quantum dot and organic LEDs with a bach-cavity and a high index substrate,” J. Phys. D Appl. Phys. 49(14), 145103 (2016).

46. H. Liang, R. Zhu, Y. Dong, S. T. Wu, J. Li, J. Wang, and J. Zhou, “Enhancing the outcoupling efficiency of quantum dot LEDs with internal nano-scattering pattern,” Opt. Express 23(10), 12910–12922 (2015).

47. J. W. Kim, J. H. Jang, M. C. Oh, J. W. Shin, D. H. Cho, J. H. Moon, and J. I. Lee, “FDTD analysis of the light extraction efficiency of OLEDs with a random scattering layer,” Opt. Express 22(1), 498–507 (2014).

48. W. H. Koo, S. M. Jeong, F. Araoka, K. Ishikawa, S. Nishimura, T. Toyooka, and H. Takezoe, “Light extraction from organic light-emitting diodes enhanced by spontaneously formed buckles,” Nat. Photonics 4(4), 222–226 (2010).

49. M. G. Moharam, E. B. Grann, D. A. Pommet, and T. K. Gaylord, “Formulation of stable and efficient implementation of the rigorous coupled wave analysis of binary gratings,” J. Opt. Soc. Am. A 12(5), 1068–1076 (1995).

50. L. Li, “Use of Fourier series in the analysis of discontinuous periodic structures,” J. Opt. Soc. Am. A 13(9), 1870–1876 (1996).

51. S. Liu, Y. Ma, X. Chen, and C. Zhang, “Estimation of the convergence order of rigorous coupled-wave analysis for binary gratings in optical critical dimension metrology,” Opt. Eng. 51(8), 081504 (2012).


Vol. 27, No. 16 | 5 Aug 2019 | OPTICS EXPRESS A1014nom.mse.hust.edu.cn/pdf/2019_OPT_EXPRESS.pdf · 2019. 7. 15. · Vol. 27, No. 16 | 5 Aug 2019 | OPTICS EXPRESS A1016 . where k0

Documents