Bipolar device model results

From the results for the bipolar single layer device, we know that as the voltage increases, the recombination rate profile shifts from the cathode to the anode. In order to let the multilayer LED emit various colors, three different kinds of emitting layer with different electroluminescent spectrum must be included in the device. There will be energy barrier between het-erojunction interface. Since, for most of the organic materials, the electric field dependence factor of the electron mobility is larger than that of hole, the recombination rate profile will be dominated by the profile of the elec-tron density. In order to clarify how the interface elecelec-tron barrier affect the electron density distribution, then the recombination rate profile, as voltage increase, we calculate the electric field, carrier density, and recombination rate for the bilayer device with electron barrier in the heterojunction inter-face. Since unipolar LED can not emit light, we examine bipolar case for bilayer device directly.

Figure 4.20 presents the structures of two double carrier bilayer devices.

Each layer has the thickness of 50nm, the difference for these two layer is that the LUMO for the material of the right side is 2.8eV and that for the material of the left side is 3.0eV. Such LUMO deviation introduce a electron barrier between these two layers. Fig. 4.20(a) shows the case for the symmetric carrier mobility, Fig. 4.20(b) shows that for the asymmetric carrier mobility. The zero field carrier mobility, µ0, and the field dependence factor, E₀, for the symmetric device (Fig. 4.20(a)) are 10⁻¹⁰m²/Vs and 4.3 × 10⁶V/m respectively. Fig. 4.20(b) presents the case for asymmetric carrier mobility. The zero field hole (electron) mobility, µ0h (µ0e), and the field dependence factor, E_0h(E_0e), for the asymmetric device are µ₀ (0.1×µ₀) and 4.3 × 10⁶V/m (1.6 × 10⁶V/m) respectively.

Figure 4.21 demonstrates the electric field profiles for the two devices in fig. 4.20. Fig. 4.21(a) is for the case of symmetric carrier mobilities, Fig.

4.21(b) is for the case of asymmetric carrier mobilities. Fig. 4.21(b) presents the electric field profile variation for voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line). Because of the discontinuities for the carrier density (electron) across the interface, the electric field around the interface shows a sudden jump. Since the electron density on the right side material is much lower than that of the left side material, higher electric field is needed to provide steady current for the right side material. In Fig. 4.21(b), the electric field jump beside the heterojunction for low voltage is smaller than that for high voltage.

Figure 4.22 shows the electron and hole density profiles for the two devices in Fig. 4.20. Fig. 4.22(a) presents the electron and hole density for the case

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Figure 4.20: The structures of the double carrier bilayer devices with (a) symmetric and (b) asymmetric carrier mobility.

of symmetric carrier mobility. Figs. 4.22(b) and (c) show the electron and hole density for the case of asymmetric carrier mobility respectively. The carrier density profiles variation for varying voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line) are shown in Figs. 4.22(b) (electron) and (c) (hole). In Fig. 4.22(a), electron density has a sudden jump in the interface due to the electron barrier introduced by the heterojunction. The hole density, which has no energy barrier beside the junction, is not smooth across the heterojunction, because the hole density must deviate from its smooth value to match the electron density to satisfy the requirement of steady state current density. In Fig. 4.22(b), the electron density for low voltage (3V) decreases rapidly on the material of the right side. As the voltage being 6V, more electrons can reach the right contact. As the voltage being even higher to 10V, the electron density is raised to about one order magnitude larger than that just across the junction, owing to the attraction from the high density of holes injected from the contact of the right side with a small injection barrier. In Fig. 4.22(c), the density of hole also demonstrate the deviation behavior to match the sudden jump of the electron density around the interface. As the voltage increases, the deviation gets larger, and more hole can reach the contact of the left side.

Figure 4.23 demonstrates the recombination rate profiles for the two de-vices in Fig. 4.20. Fig. 4.23(a) is for the case of symmetric carrier electron mobility, Fig. 4.22(b) is for that of asymmetric carrier electron. Fig. 4.21(b) presents the recombination rate profiles variation for voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line). In Fig. 4.23(a), for the case of the symmetric carrier mobility, the recombination rate on the right side

100 CHAPTER 4. COLOR TUNABLE LIGHT-EMITTING DIODES

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Figure 4.21: The electric fields profiles for the two double carrier bilayer devices in fig. 4.20: (a) is for symmetric device in fig. 4.20(a); (b) is for asymmetric device in fig. 4.20(b). The profiles variation for voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line) are presented.

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Figure 4.22: (a) The electron (dashed line) and hole (solid line) density profiles for the symmetric double carrier bilayer device in fig. 4.20(a); (b) the electron and (c) hole density profile for the asymmetric double carrier bilayer device in fig. 4.20(b). The profile variation for voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line) are presented in (b) and (c).

102 CHAPTER 4. COLOR TUNABLE LIGHT-EMITTING DIODES beside the junction keeps a large fall with that on the left side beside the junction. In Fig. 4.23(b), for the case of the asymmetric carrier mobility, as the bias being low voltage (3V), the rate profile jump around the junc-tion is broad and the rate profile on the material of the right side decays rapidly near the the contact of the right side. As the bias increases, the rate profile jump around the junction is getting more narrow and higher and the rate profile on the material of the right side is lifted to a level being able to compete with that of the left side. Hence, the recombination rate profile for the multilayer organic LED, which has asymmetric carrier mobility, can shift from cathode to anode as voltage increases.

In summary, results of device model with electron barrier heterojunction and asymmetric carrier mobility are presented. Recombination rate profiles for symmetric and asymmetric carrier mobility shows that as the device volt-age increases, the jump of the rate profile is getting more narrow and higher.

The higher jump of the rate profile near the junction lifts the the rate profile near the hole injection contact to a level being able to compete with that near the electron injection contact. The profile variation as increasing voltage for the case of asymmetric carrier mobility shows the exciton recombination zone shifting toward the hole injecting side. This profile shifting due to volt-age increasing is the basic principle for the color-tuning mechanism of the multilayer LED.

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Figure 4.23: The recombination rate profiles for the double carrier bilayer devices in fig. 4.20: (a) is for symmetric device in fig. 4.20(a); (b) is for asymmetric device in fig. 4.20(b). The profile variation for voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line) are presented in (b).

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