The Doping Effect Simulation on the OLED Devices Using ATLAS

Introduction

Organic light emitting diodes (OLEDs) have attracted great attention for full-color flat-panel displays since the demonstration of efficient electroluminescent devices [1]. Due to electrical doping of the electron and hole transport layer with an intrinsic emission layer sandwiched in bewteen OLED, the devices have reached high performance and high luminance at low voltage.

In this article, we show the doping effect of the 2 layer NPB and Alq3 with C545T as the dopant using the 2 Dimensional Device Simulator ATLAS [2]. The electrical characteristics as the I-V curve  and the local free carrier densities and ionized trapped charge densities were considered in this article using ATLAS. The Cole-Cole impedance [3] plot of the powerful method on characterizing many of the electrical properties of materials show the effective to doping effects.

 

Device Structure and Simulation Model

The simulated devices for the multilayer structure consist of ITO, NPB(65nm), Alq3(60nm), and Lif/Al for the without-doping device; and ITO, NPB(65nm), Alq3/C545T(30nm), Alq3(30nm) and Lif/Al for the with-doping device.

Figure 1 shows the simulated organic electronic device thickness and the energy diagram with LUMO and HOMO. The work function of ITO was chosen 5.05eV and LiF/Al was chosen the same energy with LUMO of the Alq3 as 3.0eV from the vacuum energy level. The hole trapped level of dopant, C545T, is 0.19eV from LUMO of Alq3 and the electron trapped level of dopant is 0.13eV from HOMO of Alq3.

Figure 1. Schemetic energy diagram and thickness of the simulated device.

 

 

Hopping Mobility for the Doping

The intrinsic of organic layer has the constant value as the zero-field mobility. But when the dopant was introduced to into organic layer, the mobility should be consider the double peak gaussian distribution along the energy as like the Figure 2.

 

Figure 2. The effective transport hopping energy model and defines the defects using a double peak Gaussian distribution [4].

 

Figure 3. shows that the double peak Gaussian distribution should be considered mobility due to the doping level.
For example, when the doping level is below 1e-2 %, which is the intrinsic organic layer, the mobility is 2.5e-4 [cm2/Vs] at the Gamma=3.0e7 [6], but when the doping level is 10%, then the mobility increased to 4e-2 [cm2/Vs]. The doping is very effective for mobility in this case.

 

Figure 3. Charge carrier mobility in doped semiconducting polymers [5].

 

 

Dopant Effect on Transport Process

The recombination process for free carriers and charged dopant can be treated as the attractive Coulombic interaction between the free carrier and trapped charges plus the dopant’s dipole because most dopant molecules are polar molecules.
The exciton generation rates on host and dopant for the singlet materials arising from the electron-hole recombination[6].

 

Simulation Results and Discussion

In the simulation of electrical process, the energy levels (HOMO ad LUMO) are shown in Figure 1. The mobility depend on the doping level was considered as Figure 3. These are the key input data for the OLED electrical process simulation.

At first, we simulated the without dopant device using the structure as in Figure 1.

The free carrier concentration distribution at 9V on the anode is shown in Figure 4. At the 65nm of the interface NPB/Alq3 has the maximum carrier concentration.

 

Figure 4. Free Carrier concentration profile without dopant device.

 

For the device with dopants in the emitting layer, Alq3, as like Figure 5, there are electrons and holes trapped by dopants, and the trapped charge modifies the carrier transport process and their distribution in the device. With ATLAS, the organic device with dopant was simulated the free carrier concentration and trapped carrier concentration very clearly and simutaneousely.

 

Figure 5. Free Carrier and trapped electron and hole concentration of 1.0% dopant device.

 

The comparison of the recombination rate at 9V with and without dopant is shown in Figure 6. The free carrier recombination rate for the device without dopant is confined to a very narrow region nearby the NPB/Alq3 interface. But for the device with dopant in the emitting layer, it could be observed that the free carrier recombination profile has been affected by the existing dopant and became broader. So the total recombination rate with dopant is greatly enhanced compared the without dopant device.

 

Figure 6. Langevin Recombinaiton rate profiles.

 

Finally the I-V characteristics for the multilayer devices with 4 different doping level are shown in Figure 7. The dopant effect can be simulated using ATLAS from getting the IV curve and it could be compared to measured data which is in good agreement.

 

Figure 7. I-V characteristics with different doping ratios.

 

 

Impedance Spectroscopy Method

The measurement of the bias and frequency-dependent device capacitance is a well-established technique for the investigation of conductivity, doping and trap states in organic semiconductors. In ATLAS, with ac analysis, the Cole-Cole plot could get the Z parameters or Y parameters.

Typical data of the complex impedence Z=Re[Z]-Im[Z] are shown in Figure 8 for the organic device with and without dopant.

Using this Impedance Spectroscopy, the Cole-Cole plot could converted as shown in Figure 9 and 10.

Figure 9 shows this without the dopant and Figure 10 shows the 0.5% dopant effect.

 

Figure 8. Impedance Spectroscopy method of Z Parameter.

 

Figure 9. Cole-Cole plot without dopant.

 

Figure 10. Cole-Cole plot with 0.5% dopant.

 

Conclusion

We have presented efficiency calculation of the electrical characteristics for the OLED with dopant in the host material based due to the dopant level.

We have also shown the Cole-Cole plot with the Impedance Spectroscopy method for the investigation of the conductance using ATLAS Device simulator.

 

Reference

  1. C. W. Tang, S. A. Vanslyke, Appl. Phys. Lett. 51 (1987) 913.
  2. ATLAS User’s Manual, Baseline 2010 Version
  3. H. Naito, “Impedence Spectroscopy of the Organic Electronic Devices”, JSAP. 76 No. 11, pp.1252-1258 (2007) Japanese
  4. Arkhipov, V.I., P. Heremans, E.V. Emelianova, G.J. Adriaenssens, H. Bassler, “Charge carrier mobility in doping disordered organic semiconductors”, Journal of Non-Crystalline Solids, Vol. 338-340 (2004): 603-606.
  5. V.I. Arkhipov, P. Heremans, E.V. Emelianova, G.J. Adriaenssens, H. Bassler, Applied Physics Letters, Vol. 82, No. 19, pp 3245-3247, 2003
  6. ATLAS User’s Manual, Chapter 15, Baseline 2010 Version

 

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