**2. Degradation**

Unlike the field of inorganic electronics, organic electronics encompasses highly diverse technologies with devices that can be prepared with different architectures, using many dif‐ ferent materials, processed by many different methods. Unlike their inorganic counterparts, all organic devices are to some extent unstable and their performance degrades over time [1]. After efficiency, lifetime is the second most important parameter for organic devices [2]. While inorganic semiconductors are for the most part intrinsically chemically stable, and in‐ sensitive to the ambient environment, for organic devices, the polymer or small molecule ac‐ tive layers themselves, the inorganic electrodes, and the interfaces between them are all potential locations for degradation. Degradation for organic devices is, therefore, highly complex and typically cannot be described by a single mechanism.

OLEDs [3] and OPVs [4-6] are known to degrade during both operation and storage (called shelf life or dark stability). From the moment the metal electrode is applied, the device is subject to degradation: in vacuum, in the dark and under operation [1]. The three physical mechanisms that degradation can take are the loss of conjugation and irreversible deteriora‐ tion of the active organic layers; degradation of the interface conductive properties; and me‐ chanical disintegration of device (dewetting, phase segregation, crystallization), all of which manifest themselves as a change in the electrical properties. The basic requirement of an emissive or absorptive technology with regards to lifetime is to provide adequate device performance over the intended time of use for the application. Stability for short term dis‐ plays, such as cell phone monitors, requires different criteria than long term high perform‐ ance solid state lighting. For solar cells, the requirements are different still with exposure to external environments that are not even a consideration for OLEDs, aggravated by the fact that many organic molecules undergo serious degradation in electrical properties upon ex‐ posure to light [7-11]. The desire for flexible substrates for both technologies brings yet an‐ other set of challenges. These requirements have stimulated much research in the thirty years since the first OLEDs and OPVs were produced at Kodak [12-13].

Though efforts are underway to establish standardized protocols for OLED and OPV char‐ acterization [14-16], official qualification procedures have not yet been established for life‐ time testing. Stability testing protocols were proposed for OPVs by consensus among 21 international research groups in May 2011 to improve the reliability of reported values [17], and are still in the process of being adopted by other researchers. A comparison of reported lifetime values between different groups is difficult, as device lifetime is greatly affected by the driving voltage, number of duty cycles, length of rest cycles, initial luminance or power conversion efficiency, deposition conditions, and exposed environment. Though it is possi‐ ble to estimate values for standardized test conditions using acceleration factors for both OLEDs [18] and OPVs [19-20], in general it is more instructive to look at the relative im‐ provement in the device lifetime, which is how it will be discussed in this chapter.

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‐ ers, all types will be discussed within this chapter.

### **3. Dewetting theory**

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‐

Unlike the field of inorganic electronics, organic electronics encompasses highly diverse technologies with devices that can be prepared with different architectures, using many dif‐ ferent materials, processed by many different methods. Unlike their inorganic counterparts, all organic devices are to some extent unstable and their performance degrades over time [1]. After efficiency, lifetime is the second most important parameter for organic devices [2]. While inorganic semiconductors are for the most part intrinsically chemically stable, and in‐ sensitive to the ambient environment, for organic devices, the polymer or small molecule ac‐ tive layers themselves, the inorganic electrodes, and the interfaces between them are all potential locations for degradation. Degradation for organic devices is, therefore, highly

OLEDs [3] and OPVs [4-6] are known to degrade during both operation and storage (called shelf life or dark stability). From the moment the metal electrode is applied, the device is subject to degradation: in vacuum, in the dark and under operation [1]. The three physical mechanisms that degradation can take are the loss of conjugation and irreversible deteriora‐ tion of the active organic layers; degradation of the interface conductive properties; and me‐ chanical disintegration of device (dewetting, phase segregation, crystallization), all of which manifest themselves as a change in the electrical properties. The basic requirement of an emissive or absorptive technology with regards to lifetime is to provide adequate device performance over the intended time of use for the application. Stability for short term dis‐ plays, such as cell phone monitors, requires different criteria than long term high perform‐ ance solid state lighting. For solar cells, the requirements are different still with exposure to external environments that are not even a consideration for OLEDs, aggravated by the fact that many organic molecules undergo serious degradation in electrical properties upon ex‐ posure to light [7-11]. The desire for flexible substrates for both technologies brings yet an‐ other set of challenges. These requirements have stimulated much research in the thirty

Though efforts are underway to establish standardized protocols for OLED and OPV char‐ acterization [14-16], official qualification procedures have not yet been established for life‐ time testing. Stability testing protocols were proposed for OPVs by consensus among 21 international research groups in May 2011 to improve the reliability of reported values [17], and are still in the process of being adopted by other researchers. A comparison of reported lifetime values between different groups is difficult, as device lifetime is greatly affected by the driving voltage, number of duty cycles, length of rest cycles, initial luminance or power conversion efficiency, deposition conditions, and exposed environment. Though it is possi‐ ble to estimate values for standardized test conditions using acceleration factors for both

vice architectures and surprising solutions to device degradation.

complex and typically cannot be described by a single mechanism.

years since the first OLEDs and OPVs were produced at Kodak [12-13].

**2. Degradation**

230 Optoelectronics - Advanced Materials and Devices

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 between two interfaces

$$E(h) = \frac{-A\_H}{12\pi h^2} \tag{1}$$

where AH is the effective Hamaker constant for the film and film-substrate interactions (*A*H*=A*F*-A*FS) and *h* is the film thickness on an infinite substrate.

The thermodynamic instability is given by the "disjoining pressure" [26, 30], or the second derivative of the energy. For a single film, this is given by

$$\frac{d^2E(h)}{dh^2} = \frac{-A\_F + A\_{FS}}{2\pi h^4} \tag{2}$$

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. At the lowest limits, thermally or mechanically induced fluctuations (capillary waves) tend to cause film rupture [27, 31-32], a process known as spinodal dewetting. The surface undu‐ lations give rise to a pressure gradient which drives film instability if the effective Hamaker constant is negative (i.e. non-wetting) [31]. As the film thickness decreases, there is a tradeoff between destabilizing vdW forces and stabilizing surface tension that leads to an amplifi‐ cation of capillary waves [33] and can therefore cause spontaneous rupture if the film is thin enough (typically <200nm, greatly enhanced at <10nm [32-33]). Additionally, the glass tran‐ sition temperature (Tg) is lower for thin films due to confinement effects [34], further aggra‐ vating dewetting effects. Most OLED and OPV layers are less than 200nm thickness, making them highly susceptible to spinodal dewetting.

properties, high optical transparency to visible light, and a smooth, often amorphous mor‐ phology with good film forming properties [12, 51-59]. In small molecule OLEDs, excellent examples, and the first HTL materials [12], are triphenylamines such as TPD and NPB; in polymer OLEDS and OPVs, this function is often fulfilled by PEDOT:PSS (Poly(3,4-ethylene‐

Dewetting Stability of ITO Surfaces in Organic Optoelectronic Devices

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

233

An indium tin oxide (ITO) thin film on a glass substrate could be considered an archetypical anode for both OLEDs and OPVs due to its high transparency over the visible region, high electrical conductivity, and high work function [61-64]. Although ITO use is almost ubiqui‐ tous as the transparent conducting electrode in organic optoelectronic devices, it has a num‐ ber of drawbacks, including variations in the surface properties depending on preparation method [47, 62, 64-67], poor energetic compatibility with active organics [68-69], and insta‐ bility with a wide variety of hole transporting materials that directly impacts on the device stability. ITO has a bixbyite crystal structure [70-71] and the surface of polycrystalline thin films are dominated by the oxygen terminated (111) plane with many dangling O bonds [72] (see figure 1). Due to this rich oxygen landscape, the electrode surface has a highly variable electronic structure [73] that can be modified by a wide variety of surface treatments, and is very susceptible to moisture [74] and light irradiation [63, 75]. As the difference between the active layer highest occupied molecular orbital (HOMO) and the electrode surface work function plays a limiting role in device performance [76], ITO surface modification is typi‐ cally focussed on increasing work function, by passivation with surface-active species [77]

1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

(222)

q~=2.14

(411)

q~=2.62

**q// (**Å**-1)**

**Figure 1.** Grazing incidence x-ray diffraction of polycrystalline indium tin oxide surface with major planes identified showing the predominance of (111) planes, with a schematic of the ITO (111) surface with hydroxyl and oxo-termina‐

dioxythiophene):poly(styrenesulfonate)) [60].

**Intensity (arb. units)**

tions as an inset.

(200) q~= 1.24

or by over 50 kinds of chemical and physical treatments [76, 78-79].

(211)

q~=1.52

Aside from capillary wave destabilization, dewetting can be driven by nucleation and hole growth from defects (i.e. airborne particles) [27, 35], by the release of residual stress [36], by density variations [37] or by thermal expansion mismatch between substrate and organic film [27, 38], which can be highly anisotropic for organic molecules [39].

Dewetting effects are strongly related to the crystalline structure of organic thin films [40], and are thus quite different from the wetting – dewetting problems of an isotropic liquid. The situation is further complicated by the fact that many organic molecular crystals exhibit several distinct crystal structures, which are energetically very similar and may coexist [41-42]. For crystalline films, pseudo-epitaxy with the substrate can simultaneously drive both film stabilization and dewetting [42]. The predominance of physisorption, combined with the relatively large size of the molecule compared to the inorganic substrate allows or‐ ganic films to accommodate much larger strains than those observed in inorganic epitaxy [42]. As having lateral organization in the thin film can stabilize against dewetting [40], the amorphous films often used in devices are even more susceptible to extreme morphological instabilities.

Regardless of the mechanism, dewetting begins with a nucleation event leading to the for‐ mation of a hollow which proceeds to grow by the transport of material away from the nu‐ cleation site to a retreating rim surrounding the hole. These holes eventually intersect, leading to the formation of ribbons of material along the contact line [27].
