Optical Simulation of Organic Light Emitting Diode by Transfer Matrix Method with a Green’s Function Approach and 2D FDTD

 

Introduction

Organic light emitting diode (OLED) has been getting much attention over the past decades in the field of displays and lighting applications for its excellent efficiency, color quality and color tunability. Optical modeling of OLEDs is one important development issue to create high performance devices [1]. In this article, several optical simulations of OLEDs are presented by using recently implemented features in Atlas. First, we present the multiple dipole sources emission from a stacked layers structure, including the interference effect between the emitted light and reflected light using Transfer Matrix Method (TMM) with a Green’s Function Approach [2-4]. Then, we present the 2D FDTD analysis on the device with the grating structure using the finite difference time domain (FDTD) [5].

 

1. Multiple Dipole Sources Emission

White OLEDs are emerging and expected as the next generation light sources in the field of AM-OLED TVs and lighting devices. Various device structures have been proposed to realize white light emission [1]. We have selected a multiple stacked layers structure with/without mixtures of coherent and incoherent layers, and performed analysis of multiple dipole sources emission using TMM with Green’s function.

1.1 Coherent case
A stacked layers structure for the analysis of the multiple dipole emission shown Figure 1 is taken from the reference paper [6].

Figure 1. Multilayer structure device with 3 point dipoles inside.

 

The multilayer structure of OLED device composed of 8 coherent layers with 3 emissive dipoles is created using Radiant [7], which is an interactive tool to simulate LED and OLED devices. Three point dipoles are set in the center of each emitting layer (EML_Dipole1, EML_Dipole2 and EML_Dipole3) and the spontaneous emission rate for each dipole is set to 1. Photo Luminescence data corresponding to each layer is selected from the Silvaco Spectrum Library, Alq3.spc, DCM2Alq3.spc and Alq3_PtOEP.spc are set to EML_Dipole1, EML_Dipole2 and EML_Dipole3, respectively.

In the syntax of Atlas, these settings are defined by the “MATERIAL” statement as follows.

material region=EML_Dipole3 emit.num=1 emit.rate=1 emit.file=Alq3_PtOEP.spc out.uspec=PL_red.spc

The TMM Green’s Function method is enabled by the “TMM.GF” parameter on the “SAVE” statement, and solves the 3D field of a point dipole inside the stack. The dipole is assumed to be the Hertzian and randomly oriented for the coherent simulation. Output spectral power density at viewing angle 0 degree is calculated.

In the syntax of Atlas, these settings are defined by “SAVE” statement as follows.

save tmm.gf ^emit.bot bot.hs numrays=1 view.min=0 view.max=0 emin=1.5 emax=3.5 nsamp=501 angpower=as0.log out.spect=pspect0.log spect.angle=spect_angle0.log ^calc.emission.fac dipole polar=45 theta=45 horizontal=0.666

Simulated output spectral power density for the multiple dipole sources is shown in Figure 2. Total output spectral power density is calculated by summing up the power density emitted by each dipole source. The profile can be controlled and calibrated by changing the spontaneous emission rate of the dipoles, and the contribution of each dipole source can be estimated.

Figure 2. Simulated output spectral power densities at viewing angle 0 degree for the multiple dipole sources.

 

1.2 Incoherent case
The multilayer structure of OLED device for the incoherent case is shown in Figure 3.

Figure 3. The Multilayer structure device with thicker glass.

A thicker oxide layer of 10 microns is placed on top of the structure shown in Figure 1 as to investigate the effect of the interference by the emitted light and reflected light within the device. Three point dipoles are set in the center of the emitting layer and the emitting rate for each dipole is the same as the coherent case in the previous section. Output spectral power density at viewing angle 0 degree is calculated.

The incoherent mode is enabled by the “INCOHERENT” parameter on the “SAVE” statement. If this parameter is activated, the interference within the specified layers by the “MATERIAL” statement is not considered.

In the syntax of Atlas, these settings are defined by “MATERIAL” and “SAVE” statement as follows.

material material=oxide real.index=1.5 imag.index=0.0 incoherent
save tmm.gf ^emit.bot bot.hs numrays=1 view.min=0 view.max=0 emin=1.5 emax=3.5 nsamp=501 angpower=as0.log out.spect=pspect0_adj.log spect.angle=spect_angle0.log ^calc.emission.fac dipole polar=45 theta=45 horizontal=0.666 normalize.pl normalize.spec incoherent

Simulated output spectral power densities with and without considering the effect of the interference are shown in Figure 4. Fine fringe patterns appeared on the spectrum if the interference within the layers is not considered.

Figure 4. Simulated output spectral power densities at viewing angle 0 degree with / without considering the effect of the interference.

 

2. 2D FDTD

The typical OLED device has a periodic layout. Recently, the layout size of the OLED or driving TFT is scaled to smaller sizes. If the layout size is closer to the wavelength of the incident light from outside of the device, the interference effect between the incident light and reflected light can not be neglected. The interference affects the characteristics on the viewing angle of the device. We have performed 2D FDTD analysis for sine wave and Gaussian pulse incidence.

 

2.1 Optical simulation for the grating structure by sine wave incidence
The OLED device with the grating structure is shown in Figure 5(a). The settings of Perfect Matching Layer (PML) and Total Field and Scattering Field (TF/SF) for optical simulation are shown in Figure 5(b). The steady state field response for the sine wave incidence at 75 degree is calculated.

Figure 5. Device structure and settings for optical simulation.
(a) Device structure (b) Optical settings

 

In the syntax of Atlas, the scattering region in FDTD in the TF/SF setup is defined by “BEAM” statement as follows.

beam num=1 x.origin=0 y.origin=-0.5 angle=75 \
fdtd fd.auto tm td.srate=4 sine \
prop.length=10 big.index wavelength=0.45 \
td.every=250 td.many=250 td.err=0.0025 dt=2.5e-18 \
td.log=fdtdlog_test td.file=fdtd_test fdtd.lum \
FACET=nearfar_test scat.top=0.5 scat.bottom=0.2
x.periodic angl.res=0.8

Simulated intensity of the electric field (Ez) is shown in Figure 6.

Figure 6. Steady state simulation of electric field intensity (Ez).

The incident light is reflected on the surface of gratings and the bottom electrode. Above the scattering region (y<-1.5), reflected light is separated from the incident light.

 

2.2 Optical simulation for the grating structure by Gaussian pulse
The device structure and the optical settings are the same as the previous section 2.1. The transient field response for the Gaussian pulse incidence at 75 degree is calculated. Using Gaussian pulse, the details of light propagation, reflection and scattering can be evaluated.

In the syntax of Atlas, Gaussian pulse setup is defined by “BEAM” statement as follows.

beam num=1 x.origin=0 y.origin=-0.5 angle=75 \
fdtd fd.auto tm td.srate=4 pulse td.width=5e-16 \
prop.length=10 big.index wavelength=0.45 \
td.every=250 td.many=250 td.err=0.0025 \
td.log=fdtdlog_test td.file=fdtd_test fdtd.lum \
FACET=nearfar_test scat.top=0.5 scat.bottom=0.2
x.periodic angl.res=0.8

Simulated intensities of the electric field (Ez) are shown in Figure 7.

Figure 7. Transient simulation of electric field intensity (Ez).

 

The incident light is propagated from the source on the top of the device, then reflected on the surface of the gratings and the bottom electrode. The details of the light propagation within the device can be precisely evaluated.

 

Conclusions

The newly implemented TMM Green’s function in Atlas has successfully simulated the output spectral power density from multiple dipole sources within the multilayer OLED device for both coherent and incoherent conditions.
2D FDTD in Atlas simulated steady state and transient state field responses for oblique sine wave and Gaussian pulse incidence to the structure with periodic boundary condition. These newly implemented features are very useful for the optical modeling of OLEDs.

 

References

  1. Reineke Sebastian, Michael Thomschke, Björn Lüssem, and Karl Leo. “White organic light-emitting diodes: Status and perspective.” Reviews of Modern Physics 85, no. 3 (July 2013): 1245-1293.
  2. N. Danz, R. Waldhäusl, A. Bräuer, and R. Kowarschik, “Dipole lifetime in stratified media”, J. Opt. Soc. Am. B 19, 412 (2002).
  3. K. Celebi, T. D. Heidel and M. A Baldo, “Simplified calculation of dipole energy transport in a multilayer stack using dyadic Green’s functions”, Optics Express, Vol. 15, No.4, 1762 (2007).
  4. S. Jung, K. Kim, Y. Lee, J. Youn, H. Moon, J. Jang, and J. Kim, “Optical Modeling and Analysis of Organic Solar Cells with Coherent Multilayers and Incoherent Glass Substrate Using Generalized Transfer Matrix Method”, Jpn. J. Appl. Phys. 50, 122301 (2011).
  5. C. Oh and M. J. Escuti, “Time-domain analysis of periodic anisotropic media at oblique incidence: an efficient FDTD implementation”, Optics Express Vol. 14, No. 24, 11870 (2006).
  6. M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M. E. Thompson and S. R. Forrest, “Highly efficient phosphorescent emission from organic electoluminescent devices”, Nature vol.395 No.6698 p.151.
  7. “Radiant: GUI-based Design Software for Performing Simulations of Optoelectronic Thin Film Devices Such as LED and OLED”, simulation Standard, Volume 25, Number 2, April-May-June 2015.