Bipolar results and discussion

In order to emit light, there should be significant excitons recombined through injected electrons and holes. In unipolar LED cited in last section, the den-sity for one type of the carriers is so low that the recombined excitons can not provide enough light to be radiated. Bipolar LED, which has the contacts with small injection barrier for electrons and holes, can provide significant recombined excitons to radiate light. Figure 4.14 shows structures for bipo-lar single layer devices with symmetric and asymmetric carrier mobilities.

Figure 4.14(a) presents the case for symmetric carrier mobility. The zero field carrier mobility, µ₀, and the field dependence factor, E₀, for the sym-metric device are 10⁻¹⁰m²/Vs and 4.3 × 10⁶V/m respectively. Figure 4.14(b) presents the case for asymmetric carrier mobility. The zero field hole (elec-tron) mobility, µ_0h (µ_0e), 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) re-spectively. The difference between the electron mobility of these two types of devices will cause vary important effects of the recombination rate profile.

The recombination rate profile for the asymmetric device will shift from cath-ode to ancath-ode. This is the fundamental principle for color-tuning mechanism concerned mostly in this work.

Figure 4.15 demonstrates the electric field profiles for the two devices in fig. 4.14. Fig. 4.15(a) is for the symmetric case in fig. 4.14(a), fig. 4.15(b) is

4.2. SINGLE LAYER DEVICES 89

0 20 40 60 80 100

10⁰ 10¹ 10² 10³

0 20 40 60 80 100

10¹⁸ 10¹⁹ 10²⁰ 10²¹

X (nm) X (nm)

(a)

(b)

Figure 4.13: The recombination rate profiles for the two hole-only devices in fig. 4.10: (a) is for standard device in fig. 4.10(a); (b) is for standard device in fig. 4.10(b).

90 CHAPTER 4. COLOR TUNABLE LIGHT-EMITTING DIODES

Figure 4.14: The structures of double carrier device with (a) symmetric carrier mobility and (b) Asymmetric carrier mobility.

for the asymmetric case in fig. 4.14(b). The profile variation for voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line) are presented in fig.

4.15(b). The electric field for the symmetric device has a symmetric shape, that for the asymmetric device has nearly symmetric shapes for various bias voltage. In both of the cases, the electric fields near the two injected contacts are all lower than the field among the middle of the devices. The injected carrier densities from the contacts are higher than that among the middle of the device because the electric field needed to maintain the device current density steady for the parts near the contacts is lower than that for the parts among the middle of the device. As the voltage increases, the electric field profile lifts their averaged value, and the profile shape keep familiar. Fig.

4.15(c) shows the potential for the symmetric case in fig. 4.14(a). Compared with fig. 4.11(c), there is no maximum in fig. 4.15(c), it means that large diffusion current is not needed to stabilize the drift current for the bipolar case here.

Figure 4.16 presents the electron and hole density profiles for the two double carrier devices in fig. 4.14. Fig. 4.16(a) shows the electron (dashed line) and hole (solid) density for the symmetric case. Fig. 4.16(b) and (c) shows the electron and hole density variation for voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line) respectively. The electron density for low voltage (3V) in the asymmetric case concentrate near the electron injecting contact. As the voltage increases, the electron mobility grows grad-ually higher than hole mobility that the electron density distribution tends to extend to the hole injecting contact. At the case for high voltage, the electron density on the side for hole injecting contact is even higher than that on the side for electron injecting contact. As the voltage increases, hole

4.2. SINGLE LAYER DEVICES 91

Figure 4.15: The electric fields profiles for the two double carrier devices in fig. 4.14: (a) is for symmetric device in fig. 4.14(a); (b) is for asymmetric device in fig. 4.14(b). The profiles variation for voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line) are presented; (c) is the potential for the symmetric case.

92 CHAPTER 4. COLOR TUNABLE LIGHT-EMITTING DIODES mobility also increases to let the distribution extend to the electron injection side. However, the field dependence for the hole mobility is weaker than that for electron, the variation of the hole density for voltage increasing is not so obvious as that of the electron.

Figure 4.17 presents the recombination rate profiles for the two double carrier devices in Fig. 4.14. Fig. 4.17(a) is for the symmetric case in Fig.

4.14(a), Fig. 4.17(b) is for the asymmetric case in Fig. 4.14(b). Fig. 4.16(b) shows the recombination rate profile variation for voltage as 3V (dotted line), 6V (dashed line), and 10V (solid line) respectively. For the case of the sym-metric device in Fig. 4.17(a), the recombination rate profile has symsym-metric shape and distributes rather smoothly. For the case of the asymmetric device in Fig. 4.17(b), as the voltage increases, the recombination rate profile tends to extend to the hole injecting side. At high voltage, the recombination rate at the hole injecting side is even higher than that at the electron injecting side. The recombination rate profile shifting due to voltage increasing in the case for asymmetric device is the basic principle for color-tunable multilayer organic LED, which will be introduced in details at the later sections.

In summary, bipolar devices with both electrons and holes were described with a device model using carrier mobilities and Schottky energy barriers to injection determined in single carrier devices. Results of a device model for single layer organic LEDs which includes charge injection, transport, recombi-nation, and space charge effects in the organic material have been presented.

The role of transport was investigated by considering cases with field depen-dent mobilities those whose prefactors are both large (symmetric device), and cases where electron has a prefactor 10 times lower than hole, but the electric field dependence factor for electron is larger than that for hole (asym-metric device). Bimolecular carrier recombination was used with a Langevin form for the recombination coefficient. The results for the symmetric device are all symmetric and have no voltage variation on profile. However the results for the asymmetric device demonstrate obvious variation as voltage increases. The electron density distribution and therefore the recombination rate profile extends to the hole injection contact as the voltage increases.

This distribution shifting is the basis for the color-tuning multilayer organic LEDs.