To study the emission characteristics of the OLED shown schematically in Fig.1, we employed the finitedifference time-domain (FDTD) method. For OLED devices in which the electron-transporting layer (ETL) is only a few tens of nanometers distant from a metallic reflector and in which the layer structure is complex, the FDTD method has been shown to be very effective8, 9.
The radiation profiles of both the horizontal dipole source (dx, y) and the vertical dipole source (dz) were investigated by varying the distance Da between the active layer and the metallic cathode. Figures 2(a) and 2(b) show the radiation profiles of the dx and dz dipoles in the y – z plane for several values of the ETL layer thickness (60, 80, and 100 nm). It was found that most of the radiation from the dz dipoles is emitted below the critical angle and thus cannot escape from the glass into the air. Therefore, we concentrate on the in-plane dipoles dx and dy. Note that since the image dipole of dx, y induced by the metallic cathode is out of phase with the original dx, y, constructive interference is expected when the ETL thickness Da is ~λ/ (4nETL). Clear enhancement of the vertical radiation is seen in Fig. 2(a), when the value of Da (≈80 nm) satisfies the condition for constructive interference. We then spatially integrated the outcoupled radiation power over several values of the viewing angle 90°±40°, and confirmed that the optimized ETL layer thickness is about 80 nm. The thickness of the ETL layer and the position of the active layer are critical to the design of the OLED structure.
The extraction efficiency enhancement produced by the photonic crystal pattern is related to three factors: the lattice constant(Λ), the depth of the pattern(d), and the size of the rod. In this study, a 2-D square lattice pattern was used because this pattern is easily fabricated by two-beam holographic lithography. Figure 3 shows the relative extraction efficiency as a function of the lattice constant for several pattern depths. Here, the SiNx layer thickness is 600 nm and the radius of the rod is 0.3Λ. In the calculation of the relative extraction efficiency, the finite size of the pixels (200×50μm2) of the real OLED display must be considered. However, this size is too large for direct FDTD calculation. Instead, we placed four perfect reflectors at the domain boundaries of the computation and temporally integrated the energy extracted into the air up to the average time needed for photons to reach the boundary of the pixel. To make the light emission isotropic, we distributed the dx, y, z dipoles evenly in the active layer.
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