Stable Dipole Layer by Phosphonic acid

Alternatively, a dipole layer can be created via the wet-chemical attachment of functionalized molecules to the ITO surface. Suitable molecules have to be able to form a chemical bond to the ITO surface while at the same time have to have a negatively polarized part pointing away from the surface (Fig. 3.10 (A) and Fig. 5.4).

Fig. 5.4: Unmodified ITO/organic interface (left).

ITO surface dipole shifts Evac and lowers the hole injection barrier Ebarrier (right).

The ITO surface dipole which is shown in Fig. 5.4 (right) raises the vacuum level Evac

and enhances the injection of holes into the HOMO of the organic material. The potential energy of holes crossing the surface dipole is lowered by the amount of the dipole energy Edipole.

Possible molecules to realize a surface dipole belong to the group of silanes, carboxylic acids and phosphonic acids [34], while phosphonic acids show the strongest bond to ITO surface. The attachment of different phosphonic acids with functionalized side groups to the ITO surface is described in [72]. Compared to air plasma activated OLED, a comparable efficiency could only be found if functionalized groups creating an ITO surface dipole which increases the work function of the ITO were chosen. In [73], additional functionalized phosphonic acids were investigated in terms of altering the ITO work function and OLED efficiency. Depending on the functional side group, the work function of ITO could be modified between 4.4 eV and 5.4 eV. In [74], the hole injection barrier of phosphonic acid treated ITO surfaces (work function between 4.9 eV and 5.4 eV) to NPB is investigated. Due to Fermi level pinning at the interface (work function of ITO pinned to HOMO energy of NPB), no influence on the hole injection barrier was found (injection barrier in all cases 1 eV). If the work function of the substrate is lower than the HOMO energy of the organic material, the injection barrier generally decreases with increasing work function of the substrate (e.g. unmodified to phosphonic acid treated ITO). However, as soon as the work function

5. Study of Interface Stabilization 46464646

comes close (in the range of 1 eV) to the HOMO energy of the organic material, the injection barrier does not seem to decrease any longer but remains constant [74]. Consequently, the work function of unmodified ITO has to be increased to guarantee a low hole injection barrier, but it is not necessary to match exactly the HOMO energy of the following organic material. Additionally to the work function, the surface energy of ITO is altered due to the attachment of phosphonic acids. A hydrophilic surface can be found after an O2 or air plasma activation, while a phosphonic acid activation creates a

hydrophobic surface [73].

Commercially available, pentafluoroethylphosphonic acid (here: FEPA) (Fig. 5.5) is chosen in this work to activate the ITO surface. Due to the difference in electronegativity FEPA molecules have a dipole moment.

Fig. 5.5: Molecular structure of pentafluoroethylphosphonic acid (FEPA).

The activation of ITO for OLED by this kind of functionalized phosphonic acid was never reported before. The complete activation procedure is shown in Fig. 5.6 schematically.

Fig. 5.6: Substrate activation procedure based on FEPA.

For ITO activation, a 1 mM solution of FEPA in chloroform and ethanol (2:1 ratio) is prepared and the ITO substrates are stored for 24 h in the solution. To complete the bonding between the phosphonic acid and the ITO substrate, the substrate is stored for 1 h on a heat plate at 140°C. In a next step, the substrate is cleaned from unbound molecules in an ultrasonic ethanol bath. After the activation procedure, a blue OLED according to the organic layer stack of Fig. 5.2 is deposited on the substrate. At 1000 cd/m2_{, a current efficacy of 7.8 cd/A and 3.9 V drive voltage can be measured.}

This is comparable to the O2 plasma activated reference device. In terms of drive

voltage, it is further investigated if the activation by an O2 plasma or FEPA results in

different IV characteristics. In Fig. 5.7 (left), the IV curves of OLED with both activation methods are plotted. No significant difference in the IV curves can be found.

+

5. Study of Interface Stabilization 47474747

Fig. 5.7: IV characteristics of blue OLED which are either O2 plasma or phosphonic

acid activated (left). FEPA molecule bound to ITO surface (right).

To investigate the long-term stability of the phosphonic acid activation method, the processed large-area (and FEPA activated) OLED are stored for 641 h at 70°C. No inhomogeneity can be found in the active area, so that the activation of the ITO surface by FEPA is a possibility to realize a long-term stable interface. In contrast, the commonly utilized oxygen-based dipole layer is not long-term stable and causes inhomogeneities (Fig. 5.1). It can be concluded, that a stable FEPA dipole layer was created and is present on the ITO surface (Fig. 5.7 (right)).

However, one major problem of the activation procedure described before seems to be the left-over of residuals of the solvents on the substrate. This can result in some artifacts in the active area of the OLED. Even though the activation by FEPA is suitable to create a long-term stable dipole layer on the ITO surface, an ITO stabilization procedure which could be directly integrated into a HV or OVPD process would be favorable compared to a wet-chemical approach. This would also circumvent the problem of inhomogeneities caused by the evaporation residua of the solvents.