Chapter 3 Development of Di-Chromatic White Organic Light-Emitting
3.5 Development of p-i-n di-chromatic WOLED device
In this section, we demonstrate a dual emission layer (DEML) system for
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p-i-n WOLEDs in which the first emission layer is the co-dopant emitting layer
with MADN: 5% NPB: 3% DSA-Ph: 0.2% Rb and the second one is a blue emitting layer of MADN: 5% NPB: 3% DSA-Ph. Three types of p-i-n WOLED devices were fabricated as depicted in Figure 3-5. The total thickness of DEML in these p-i-n devices is only 15 nm. Device I is the p-i-n WOLED without co-deposited NPB in the DEML system while device II is the p-i-n WOLED with the DEML system. The pure NPB layer of device II was replaced with 1,1-bis[N,N-di(p-tolyl)aminophenyl]cyclohexane (TAPC) [21] as electron-blocker in device III. In our p-i-n architecture, Cs2CO3 is co-evaporated with BPhen [22], which has a high electron mobility of 2.4 × 10-4 cm2/Vs, as then-doped electron transport layer (n-ETL). BPhen also possesses a deep highest
occupied molecular orbital (HOMO) of 6.4 eV, which can effectively block hole carriers in emission layer. On the other hand, NPB doped with WO3 is used as the p-doped hole transport layer (p-HTL) [23]. In the DEML system, MADN, NPB, DSA-Ph and Rb were used as host material, assistant dopant, blue and yellow fluorescent dopants, respectively.Figure 3-5 Schematic device architecture of p-i-n WOLEDs.
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It is known that Rb with LUMO/HOMO of 3.2/5.4 eV can be a carrier trap for electrons, especially at low electric field [24], which will cause the problematic white emission color change with various drive currents in thin emission layer of p-i-n di-chromatic WOLEDs. In order to alleviate the unstable color issue associated with the carrier-trapping property of Rb, we have purposely co-evaporated Rb with low doping concentration of 0.2% and 3%
DSA-Ph in MADN, which would cause the yellow emission generated by the energy-transfer process from blue to yellow emitter.
The energy-transfer process can be demonstrated by the solid-state emission spectra depicted in Figure 3-6, the thin film composed of MADN: 3% DSA-Ph:
0.2% Rb (90 nm) emits intense yellow emission and relatively weak blue-greenish emission. It is evident that the emission of MADN around 430 nm essentially quenched and there is an effective energy-transfer characteristic from DSA-Ph to Rb, which is primarily due to the favourable spectral overlap between the emission peak of DSA-Ph and the absorption peak of Rb at 495 nm [25].
Figure 3-6 Absorption spectrum of Rb and solid PL spectra of DSA-Ph and composite thin film.
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(a) (b)
Therefore, we designed and developed a DEML system by inserting a blue emitting layer of MADN: 3% DSA-Ph after the co-evaporated EML into the structure of device I to enhance the blue emission intensity. From Figure 3-7(a), the EL spectrum of device I indeed shows the enhancement of the blue emission intensity with respect to the solid PL of co-evaporated thin film. However, the yellow emission intensity is still much higher than the blue emission, leading to undesirable white emission with CIEx,y
coordinates of (0.37, 0.47). Furthermore,
a significant EL color shift of device I is observed with respect to various drive currents as the CIEx,ycoordinates is shifted from (0.410, 0.496) at 1 mA/cm
2 to (0.321, 0.419) at 100 mA/cm2 with ΔCIEx,y = ±(0.089, 0.077) as shown in Figure 3-7(b), in which the yellow emission intensity decreases with the increasing current density. We inferred the unstable EL color is due to the RZ shifts towards the blue emitting layer under high current stress.Figure 3-7 (a) Solid PL spectrum of composite film and EL spectra of device I at 20 mA/cm2. (b) CIEx,y coordinates vs current density characteristics of device I.
Therefore, we co-deposited 5% NPB in the DEML system of device II as the assistant dopant with the purpose of shifting the RZ towards the blue
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emitting layer to balance the blue and yellow emission intensity under low current density. Figure 3-8(a) reveals that device II also has a better J-V characteristic than device I which indicates that NPB molecules play an important role in enhancing the hole-transport in the DEML system. In addition, the EL spectrum of device II depicted in Figure 3-8(b) shows a more balanced white emission with CIEx,y of (0.33, 0.43) at 20 mA/cm2 and the relative intensity of blue light has been increased as compared with device I. This phenomenon exhibits the co-deposited NPB molecules can indeed shift the RZ to the blue emitting layer and improve white CIEx,y coordinates. Moreover, the EL color shift with respect to varying drive currents has also been improved to ΔCIEx,y = ±(0.05, 0.04) from 1 mA/cm2 to 100 mA/cm2 as shown in Figure 3-8(c). However, the reduced relative intensity of yellow emission leads to a lower EL efficiency of 7.4 cd/A at 20 mA/cm2.
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Figure 3-8 (a) J-V characteristics of devices I and II. (b) Solid PL spectrum of composite thin film and EL spectra of device I and II at 20 mA/cm2. (c) CIEx,y
coordinates vs current density characteristics of device I and II.
To further enhance the efficiency and improve the color stability of these
p-i-n white devices, we turn to refine the exciton confinement in device III, in
which the NPB layer was replaced by TAPC with a high LUMO energy level (2.0 eV) and high hole mobility (10-4–10-3 cm2/Vs) [21], which can be an effective electron-blocking as well as hole transport material. Indeed, both blue and yellow intensity of device III have been enhanced as shown in Figure 3-9(a) and its emission achieved a white CIEx,y of (0.32, 0.43). Furthermore, the EL performance can be boosted to 9.9 cd/A and 8.2 lm/W at 20 mA/cm2. It is also observed that the white emissive color becomes more stable with respect to(a) (b)
(c)
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drive current density as the EL color shift is only of ΔCIEx,y = ±(0.013, 0.009) from 1 mA/cm2 to 100 mA/cm2 as shown in Figure 3-9(b).
Figure 3-9 (a) EL spectra at 20 mA/cm2. (b) CIEx,y coordinates vs current density characteristics of devices I, II, and III.
Detailed EL performances of these devices are summarized in Table 3-2.
The voltage, current efficiency, power efficiency, external quantum efficiency (E.
Q. E.), and color coordinates were measured at 20 mA/cm2. All p-i-n white devices show a much lower drive voltage and a dramatic gain in power efficiency as compared with those of conventional white device. Device III can achieve 10 cd/A and 9.3 lm/W at 1000 cd/m2 which are considerably better than those of device II with 7.3 cd/A and 6.8 lm/W. It is noteworthy that both L-V and
J-V curves of device III are steeper than those of conventional white device as
depicted in Figure 3-10. The threshold voltage of device III is around 2.9 V.When driven at 6 V, device III can reach 10000 cd/m2 which is nearly 8 times brighter than the conventional device. These results prove that the efficient exciton confinement is one of the most important factors in controlling the RZ shift under various drive currents and it is also indispensable for the
(a) (b)
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development of a highly efficient and color stable p-i-n WOLED.
Table 3-2 EL performances of p-i-n white OLED devices at 20 mA/cm2.
Figure 3-10 L-J-V characteristics of conventional white device and device III.