**4. Anode/active layer contacts**

The interface at the high work function electrode is especially influential in device stability as it often also forms the substrate upon which subsequent layers are deposited. The electri‐ cal, chemical, and morphological features of the electrode surface play a significant role for both OLEDs and OPVs as the quality of the interface and of the hole transporting (electron donating) (HTL) film deposited on it [38, 43] is often the limiting feature of the device, both for performance [38, 44-47] and stability [9, 38, 48-50].

For a high quality device, the HTL needs to fulfill a number of criteria including high hole mobility, good energy level matching with anodes and other active layers, good thermal 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‐ dioxythiophene):poly(styrenesulfonate)) [60].

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

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

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

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,

The interface at the high work function electrode is especially influential in device stability as it often also forms the substrate upon which subsequent layers are deposited. The electri‐ cal, chemical, and morphological features of the electrode surface play a significant role for both OLEDs and OPVs as the quality of the interface and of the hole transporting (electron donating) (HTL) film deposited on it [38, 43] is often the limiting feature of the device, both

For a high quality device, the HTL needs to fulfill a number of criteria including high hole mobility, good energy level matching with anodes and other active layers, good thermal

film [27, 38], which can be highly anisotropic for organic molecules [39].

leading to the formation of ribbons of material along the contact line [27].

them highly susceptible to spinodal dewetting.

232 Optoelectronics - Advanced Materials and Devices

instabilities.

**4. Anode/active layer contacts**

for performance [38, 44-47] and stability [9, 38, 48-50].

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

**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‐ tions as an inset.
