A Two-Dimensional Numerical Simulation of Organic Light-Emitting Devices
Y. Li, B-S Lee, P. Chen, J-W Lee, C-S Lu and W-H Chen
National Chiao Tung University, TW
2D simulation, OLED, electrical characteristics, recombination rate, structure optimization
Organic light-emitting devices (OLEDs) add a completely new dimension to current display technology [1-3]. OLED displays are thinner, lighter, brighter, and cheaper to manufacture; which also consume less power than the current thin displays do. In having an OLED with better electroluminescence (EL) efficiency, an optimization of device structure is strongly needed. However, the traditional used empirical based method is very inefficient [1-3]. In this paper, we employ a two-dimensional numerical simulation to explore device structure for both the voltagecurrent (IV) and radioactive recombination. Moreover, the spatial distributions of charge and electric field are also investigated for structure optimization of OLEDs. The two-dimensional numerical simulation of the continuity and Poisson equations has been carefully extended to treat the interfaces of multi-layer organic structure. In handing the interface property of the realistic OLED structure, introducing of traps is necessary in this simulation. The formation of traps is expressed as: ::::::::::::::::::::::::::: where ntj is the density of trapped electrons for the jth trap, the Ntj is total trap density which equal to Nt0exp[- Etj/kBT], and the Etj is the trap energy relative to the conduction band edge. The experimental structure is shown in Fig. 1 which implies that a two-dimensional device simulation is necessary. The simulated electrostatic potential, electronic field and current density distribution are shown in Fig. 2. It is found that the distributions of electrostatic potential, electric field, and current density are not uniform spread over the organic thin films; therefore, the traditional used one-dimensional methods [1-3] can be improved. Two parameters have been considered in the device structure for the optimization study; the Schttoky barrier between contacts and organic semiconductor, the trap density at the multi-layer organic semiconductor. Figs. 3 and 4 exhibit IV characteristics for various Schottkybarrier- height and mid-band trap density. Additionally, the radioactive recombination rate for different Schottkybarrier- height and mid-band trap density is also presented in the Figs. 5 and 6. It can be summarized from those four figures that a lower Schottky-barrier-height will increase both the current density and radioactive recombination rate. That is, a lowering of the Schottky-barrier-height is very helpful for the improvement of EL efficiency. On the other hand, the effect of the trap density will saturate at about 1*1012; therefore, a trap density higher than 1*1012 is not preferred in OLED fabrications. This work is supported in part by the National Science Council of TAIWAN under contracts NSC-93-2215- E-429-008 and NSC 93-2752-E-009-002-PAE, and the grant of the Ministry of Economic Affairs, Taiwan under contract No. 92-EC-17-A-07-S1-0011.
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Nanotech 2005 Conference Program Abstract