High Light Extraction on OLED with Microcavity Effects

 

Introduction

OLEDs have been researched for application in display devices due to their considerable advantages. In order to further improve and optimize devices for practical applications, the electrical and optical parts have to be considered.
In this article, we show that the optical properties include the excitation distribution of the microcavity device consisting of two organic layers with varying thickness of NPB and Alq3. As the optimal thickness of the organic layers is in the order of a wavelength, A standing wave pattern is observed. To investigate the effects of organic layer thickness, it is necessary to combine electrical and optical models together for the most critical parameters for the light generation.

 

Optical Model

As in [1], the electrical part was fully considered including carrier density and electric field distribution during the biasing. The Cole-Cole impedance plot was considered as a powerful analysis method for device characterization of the electrical properties on organic materials.

The impedance plot was dependant on the biasing and doping rate. ATLAS can simulate with high accuracy not only the steady state but also AC analysis to obtain the Impedance Spectroscopy

In ATLAS, the continuity equations and the Poisson equation are solved to obtain the carrier concentrations, electric field distributions and recombination rate. The thickness of recombinarion region can be determined from the recombination rate, which can be used to estimate the width of emission region(taking into account of exciton diffusion) to be cincluded into the optical model.

The devices with structure Bottom-Mirror/NPB/Alq3/Ag were investigated for different combination of NPB/Alq3 thickness(153 nm/51 nm and 51 nm/153nm) and for two different bottom mirror materials : Cu (25nm) and Ag(80nm). Using the Transfer Matrix Method, the standing wave pattern was considered. The refractive indices for all the thin film layers in the structure were determined by spectrum data.

 

Parameters Used in Simulation

The material parameters used for modelling of carrier transport are obtained from literature. The devices were simulated with forward bias of 3 volts.

Parameters
NPB
Alq3

Permittivity
mun
E0
Nc
Nv
LUMO
HOMO

3.0
6.1e-6
6.1e-4
4.44e5
1e21
1e21
2.4
5.4

3.0
1.9e-6
1.9e-8
7.1e4
1e21
1e21
3.0
5.7

 
Cu
Ag
Workfuncion
5.1
3.3
Table 1. Electrical simulated parameters.

 

Results and Discussion

Figure 1 shows the simulated optical field intensity in these two devices. It is well known that the strongest EL enhancement is obtained when the emission layer is aligned with the position of nt mode of the cavity.


(a) Excition Distribution


(b) Optical Intensity

Figure 1. Excition distribution and optical field distribution with different NPB/Alq thickness.

 

In 153/51 of NPB and Alq3 device, the peak of the optical field coincides with the peak of the excition region in the vicinity of the NPB/Alq3 interface. On the other hand, the emission region of 51/153 of NPB/Alq3 device is not aligned with the antimode of the optical field: in fact the emission region is positioned near the node of the optical field. This phenomenon has a major a effect on EL intensity as shown in Figure 2.

Figure 2. Simulated Output Coupling spectrum for devices with 23nm of emission layer thickness.

Red plot is NPB/Alq3 = 153/51, Blue plot is NPB/Alq3 = 51/153

 

As the bottom metal layer has high reflectivity, the top emission patterns can be obtained as shown in Figure 3 displayed as angular power distribution. ATLAS can also produce a CIE chart as shown in Figure 4.

Figure 3. Simulated far field pattern, angular power distribution.

 

Figure 4. Simulated CIE chart, peak wavelength has 450 nm.

 

 

 

Conclusion

We have simulated both electrical and optical behavior of the bilayer OLEDs using ATLAS.

The electrical behavior was investigated in [1]. In this article, the optical behavior was investigated with the Transfer Matrix Method to get the standing wave intensity along the thickness. In order to achieve better recombination, exciton distribution and efficiency, balanced carrier transport to the emissive layer is essential. By aligning the position of the antimode of optical field intensity within the device with the electrical recombination region, high excition distribution, enhancement in the overall light output of the device was achieved.

 

Reference

  1. �The Doping Effect Simulation on the OLED Devices using ATLAS�, Simulation Standard, Volume 21, No. 1, Jan-Feb-Mar. 2011, 7-9
  2. Kristiaan Neyts, “Microcavity effects and the out coupling of light in displays and lighting applications based on thin emitting films”, Applied Surface Science 244(2005) 517-523
  3. H.K. Kim, et al, “Deep blue, efficient, moderate microcavity organic light-emitting diodes”, Organic Electronics 11(2010) 137-145
  4. C.J. Lee et al, “Microcavity Effect of Top-emission Organic Light-emitting Diodes Using Aluminum Cathode and Anode”, Bull. Korean Chem. Soc. 2005, Vol.26, No.9
  5. Michael Thomschke, et al, “Optimized efficiency and angular emission characteristics of white top-emitting organic electroluminescent diodes”, Applied physics Letters 94, 083303 (2009).

 

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