**1. Introduction**

Heterojunctions are inherent in and essential to all molecular optoelectronic devices. In or‐ ganic light emitting diodes (OLEDs), the interfacial region between the active organic layers and the inorganic contacts plays a primary role in device performance, through the control of effective carrier injection and long term device reliability. In organic solar cells (OPVs), heterojunctions play a defining role in all of the major processes: charge separation relies on effective organic/organic interfaces; charge transport is critically determined by the structure of the thin film, controlled by the organic/inorganic interfaces with substrates; and charge extraction can only occur at high quality inorganic/organic interfaces at the electrodes. Stud‐ ies of various organic/inorganic interfaces have indicated that a wide range of interfacial types are possible in organic optoelectronic devices. To foster the next generation of devices, it is critical to understand the connections between heterojunction structure and morpholo‐ gy, and device performance. This connection is especially important with regard to the inter‐ facial stability and lifetime in organic optoelectronic devices. Control of the complex interactions and the microstructure at the electrode-organic interfaces would allow the opti‐ mization of performance and lifetime.

In this chapter, we aim to review the current state of the art with regards to interfacial stabil‐ ity and control of the anode (indium tin oxide) electrode/active layer interfaces to under‐ stand the performance of organic optoelectronic devices. From examples of our own research and others relating to interfacial morphological changes, a comprehensive picture of the role of the interface in device stability can be formed. This chapter begins with a brief overview of degradation in organic devices, including definitions. Following that, the main focus of the chapter is on the morphological instability at the ITO surface as a main mecha‐ nisms of device degradation. Various approaches to overcoming device instability are given,

© 2013 Turak; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Turak; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

with special attention paid to the various interlayers that have been introduced into devices. This also includes examples where dewetting is used advantageously to produce novel de‐ vice architectures and surprising solutions to device degradation.

OLEDs [18] and OPVs [19-20], in general it is more instructive to look at the relative im‐

Known degradation mechanisms include diffusion of molecular oxygen and water into the device, crystallization or oxidation of organic layers, degradation of interfaces, inter‐ layer and electrode diffusion, electrode reaction with the organic materials, electrode oxi‐ dation, phase segregation or intermixing, dewetting from the substrate, delamination of any layer, and the formation of particles, bubbles, and cracks. There are four major de‐ cay mechanisms related to the bulk active layers: organic layer oxidation, crystallization, charge carrier/exciton damage, and photobleaching. There are also four decay mecha‐ nisms directly associated with degradation at the top contact: electrode oxidation, dark spot formation, electrode bubbling and delamination, and metal diffusion. As this chap‐ ter is focussed on the morphological stability on the anode surface, interested readers are directed to recent topical reviews specifically focussed on polymer photovoltaics [21-22], on OLEDs [23], and on interfaces [24], for a comprehensive look at degradation and deg‐ radation mechanisms. As many of the issues related to anodic degradation at interfaces are common for both OPV and OLEDS, and for polymer and small molecule active lay‐

It is the interplay between molecule-molecule self-interaction and substrate-molecule in‐ teractions that determines the stability on a given surface [25-26]. For thin films (<100nm) coated onto non-wetting substrates, van der Waals forces play the dominant role in de‐ termining film stability [27-28]. The Hamaker model [29] allows quantification of the in‐ stabilities that arise in thin films when VdW forces induce an attractive potential

> <sup>2</sup> ( ) 12 *AH E h* p*h*

(*A*H*=A*F*-A*FS) and *h* is the film thickness on an infinite substrate.

2

( )

derivative of the energy. For a single film, this is given by

where AH is the effective Hamaker constant for the film and film-substrate interactions

The thermodynamic instability is given by the "disjoining pressure" [26, 30], or the second

2 4

2 *d Eh A A F FS dh h*

p

As the disjoining pressure is inversely proportional to the film thickness to the fourth pow‐ er, producing stable and defect free films is particularly difficult as the thickness decreases.


Dewetting Stability of ITO Surfaces in Organic Optoelectronic Devices

http://dx.doi.org/10.5772/52417

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provement in the device lifetime, which is how it will be discussed in this chapter.

ers, all types will be discussed within this chapter.

**3. Dewetting theory**

between two interfaces
