Chapter 2 Literature Review
2.1 Introduction of OLED
The next great emerging technology in the display industry was expected to come from organic light-emitting-diode (OLED). [12] A typical OLED device structure was shown in Fig. 2.1, in which organic material such as Alq3 was presented in the form of thin film with a thickness not exceeding the depth of the ink on the paper. OLED have gained momentum in the past few years because it was an emissive system creating its own light rather than relying on modulating a backlight.
In addition, OLED possessed fast response time (<10 ms), wide view angle (>170°), true colors, excellent contrast ratio, brightness, low operating voltage (3-10 V) and potentially less power consumption compared to those of LCD or LED. [1] As a result, OLED had the opportunity to replace LED or LCD in display industry. [1] However, its broad adoption has been hindered by several issues; namely (1) poor production yields which makes the product not cost attractive and deter investments and (2) device lifetime and reliability due to its sensitive to moisture and oxygen, which can quickly degrade the device performance and produce the black spots if unprotected. [2, 3]
Figure 2.1 The film stack and structure of OLED device.
2.1.1 Mechanisms of Degradation
Although organic light-emitting diodes (OLEDs) had many above-mentioned advantages, the short lifetime was still a critical issue to be addressed. Many mechanisms have been proposed and reported for the decay in luminance, ranging from crystallization of the organic molecules,[13, 14] decomposition of Alq3
molecules, [15] delamination at the cathode/Alq3 interface, [13, 16, 17] and electrochemical reactions at the organic/electrode interfaces. [17, 18]
H. Aziz et al. [13] proposed the Alq3 molecules became crystalline clusters by exposing to humidity and delamination occurred between cathode/Alq3 interface because the crystalline Alq3 clusters was thicker than the surrounding amorphous regions and lead the cathode/Alq3 contact isolated. F. Papadimitrakopoulos et al. [15]
suggested a series of probable reaction for the failure of Alq3-based OLEDs as shown in Fig.2.2.Reaction (1) showed water acts as a catalyst to hydrolyze Alq3 to become freed 8-hydroxyquinoline (8-Hq) and then (2) an oxidative condensation reacted with yielding a nonemissive polymeric byproduct.
Figure 2.2 Scheme of (1) Trans-Complexation of 8-Hq with Water in the Alq3
Complex and (2) Oxidative Polymerization of 8-Hq. [15]
M. Schaer et al. [17] presented two different degradation mechanisms under moisture and oxygen environments as shown in Figs.2.3 (a) and (b), respectively.
Fig.2.3 (a) showed water vapor entered from the defects, such as cracks, pinholes etc.
and diffused to the cathode/organic interface. When the device was working, water was then reduced at the cathode and yielded hydrogen gas. The electrochemical reduction was:
-2 2
2H O+2e →H +2OH- (2.1) Then, the bubbles were formed by the evolving hydrogen gas and the organic/electrode interfaces were separated. In contrast, Fig. 2.3 (b) showed oxygen enters from the defects and diffuses to the cathode/organic interface. As the metal layer was oxidized at the cathode/organic interface, the molecular volume which increased of the oxide led to interface delamination.
In summary, organic layers and metal cathodes of OLEDs were sensitive to moisture and oxygen, no matter what mechanism was involved. Therefore, preventing moisture and oxygen permeation in OLED was an immediate solution to extend the life time of devices.
Figure 2.3(a) Mechanism of dark spots formation during operation under water vapor environment. (b) Mechanism of dark spots formation for an unbiased device under a pure oxygen atmosphere. [17]
2.1.2 Requirements of WVTR and OTR
Fig. 2.4 showed the water vapor transmission rate (WVTR) and oxygen transmission rate (OTR) required for various organic electronic devices. [19] Among these devices ranging from LCD, solar cell to OLED, the requirements for OLED application was most stringent. In general, water vapor transmission rate (WVTR) should be lower than 10-6 g/m2-day and oxygen transmission rate (OTR) should be lower than 10-5 g/m2-day for OLED’s applications. The WVTR requirements for TFTs
or LCD were not as critical as those for OLEDs. The sensitivity to moisture and oxygen in OLED was much more than 10000 times in LCD. Apparently, oxygen and moisture permeation was a serious issue for developing OLED applications.
Figure 2.4 Requirements of WVTR and OTR for different applications. [19]
2.1.3 Methods for Measuring WVTR
There were many methods for measuring WVTR, which have been developed in the past few years. These methods and their detection principles were listed in Table 2.1, along with their merits and disadvantage. Among these methods, optical calcium degradation test was the most commonly used for flexible OLED’s applications, which was often named as the “Ca test” or “Ca button test”. This method was based on the corrosion of calcium films and subjected at a fixed temperature and humility. At the start, the calcium film was a reflecting metallic layer as shown in Fig. 2.5(a). As water and oxygen permeated into metallic calcium, calcium would convert to a transparent oxide or hydroxide salt as shown in Fig. 2.5(b).
The related chemical reactions of calcium with H2O and O2 were assumed to be the following:
2Ca+O2→2CaO, (2.2) Ca+H2O→CaO+H2, (2.3) CaO+H2O→Ca(OH)2 . (2.4) Then, the reacted sites can be observed as spots on the Ca film. Although it did not distinguish between oxygen and water permeation and it was more qualitative comparison, it was still a simple and direct method to compare the permeability of different materials.
Table 2.1 Summary of WVTR measuring methods, their detection principles and advantages and disadvantages.
Method Detection principle Advantage Disadvantage Reference Gravimetric loss of water or gain of
water on P2O5
optical density Simple、easy、
cheap
change in resistance easy data-point acquisition
Ca storage [21]
Radioactive tritiated water (HTO) or CO
direct
high sensitivity (~10-8g/ m2day)
expensive [22]
glass Ca barrier
(a)
glass Ca barrier
H2O
H2O H2O
(b)
Figure 2.5(a) Illustration of the structure for Ca test before testing, (b) the structure changed after H2O permeating.