Inhomogeneous Active Area

ITO/organic interface properties and effects are crucial for charge injection into OLED (compare chapter 3.3). Therefore, stable interface properties versus operating time are important for OLED lifetime and homogenous luminance over the active area. It is found that by operation time or if devices are stored at elevated temperatures, inhomogeneities in luminance appear on the active area (Fig. 5.1).

Fig. 5.1: Photo of inhomogeneous active area (on-state) of OLED (3.8 cm x 3 cm) after storage at elevated temperatures. 4.25 h at 70°C (left). 21 h at 80°C (right). In Fig. 5.2, the organic layer stack of the investigated OLED of Fig. 5.1 is shown.

LiF/Al

20 nm ETM001

30 nm SMB013:SEB115 (5%)

150 nm NPB

ITO

Fig. 5.2: Organic layer stack of investigated OLED.

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A 150 nm HTL consisting of NPB is followed by a 30 nm fluorescent blue EL and a 20 nm ETL consisting of ETM001. The blue EL consists of the host material SMB013 doped with 5% vol. of the fluorescent blue emitter SEB115 (for organic materials compare Tab. 4.1). At 1000 cd/m2_{, a current efficacy of 7.7 cd/A at a drive voltage of}

3.7 V can be measured. The appearance of inhomogeneities over the active area is a result of unstable hole injection at the ITO to HTL interface. If the hole injection is enhanced for example by a dipole layer, the partial degradation of this dipole layer causes inhomogeneous emission (Fig. 5.3).

Fig. 5.3: Closed dipole layer and homogeneous hole injection and emission over active area (left) and degraded dipole layer followed by inhomogeneous emission (right). To operate OLED at higher luminance (and thereby higher temperatures if not cooled) or for storage in rough environments with large temperature variations like cars, a more stable interface has to be developed. Different processes are commonly applied to activate the ITO surface after particle cleaning and removal of gross contaminants. While the usage of oxygen and an O2 plasma or UV/Ozone treatment is an often

utilized method to create a surface dipole on the ITO surface, different alternative concepts exist as well. As explained in chapter 3.3, the injection can be enhanced by three different methods, which are:

 dipole layers

 band-bending by doping  charge extraction layers

All of these approaches are investigated in the following. Compared to an O2 plasma,

the creation of a more stable dipole layer is realized by connecting functionalized molecules to the ITO surface via wet chemistry (chapter 5.2). Stabilization by band- bending is investigated by p-doping the HTL at the interface (chapter 5.3) and the stable extraction of charge carriers of the first organic layer is investigated employing thin oxide buffer layers (chapter 5.4).

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To evaluate the different approaches in terms of interface stability between ITO and first organic layer, large-area (e.g. 3.8 cm x 3 cm) OLED (organic layer stack shown in Fig. 5.2) are stored at elevated temperatures of 70°C in the off-state, which simulates the storage in a rough environment. However, 70°C guarantees to be lower than the glass transition temperature of NPB of 95°C [47] (lowest glass transition temperature of investigated OLED stack). An energetically long-term stable interface should not be affected by the storage. To measure the quality of the interface, the active area is checked periodically for inhomogeneities in the on-state. Although the optical inspection of inhomogeneities is to some extent depending on how good the person who inspects the OLED is trained, it is the only feasible method to acquire information about the interface. In this way, macroscopic result of microscopic effects at the ITO/organic interface is observed. It is probably not possible to investigate the effect on a microscopic scale. Necessary would be a microscopic method which could detect electronic changes at the interface. In principle, electron loss spectroscopy (EELS), which belongs to the group of transmission electron microscopy (TEM), would be suitable. But with regard to the electron exposure density necessary for high resolution (0.1 to 0.2 C/cm2_{s), a critical electron dose for organic materials of 10}-3_{-1 C/cm}2 _[68]

would be reached within seconds. If electrons are excited in an organic molecule, the relaxation can easily destroy bonds (radiolysis), which was observed already in very early TEM investigations [69-70]. Apart from the visual description of inhomogeneities, a further quantitative qualification of the interface is only possible in some special cases. The lifetime e.g. is sensitive to many different factors (compare chapter 3.5 and chapter 7), so that a direct connection between the stability of the interface and the lifetime is mostly not given. Furthermore, if Fermi energy pinning occurs at the ITO/organic interface, IV curves do not necessarily change if different methods of activation are applied.

As described in chapter 4.3, O2 plasma cleaning is the standard activation procedure in

this work. A comprehensive overview of O2 plasma cleaning of ITO surfaces is given in

[71]. An increase in the work function after O2 plasma treatment can be traced back to

the creation of a surface dipole. It was found that the work function increase has a positive influence on the injection of holes from the ITO into the HOMO of the adjacent organic layer. But even if O2 plasma activation guarantees excellent short-

term injection properties, it does not seem to be the ideal method to guarantee a long- term stable ITO/organic interface. At elevated storage temperatures of 70°C, inhomogeneities in the active area of OLED appear after 240 h. The excellent short- term injection properties of O2 plasma activated substrates are not influenced if the

substrates are stored in N2 atmosphere (at 25°C and up to 24 h) before the deposition

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