**5. Morphological instabilities on ITO**

In general, instability at the ITO/active layer interface can be related to morphological, chemical and electronic changes over the lifetime of the device. There are four major criteria which lead to an unstable interface: surface energy mismatch, low glass transition tempera‐ ture (Tg) materials, surface reactivity with organics, and work function instability. The sur‐ face energy mismatch and low transition temperatures are the driving characteristics for the morphological instability discussed in this chapter.

degree oforder [98]; however, they are metastable at room temperature, showing very strong dewetting during storage at room temperature under vacuum for one month [99]. Though sometimes observed under high temperature treatments [3, 100], dewetting is not as significant a problem for polymer systems, due to the generally higher Tg of polymer mate‐ rials [101]. However, device performance is heavily influenced by the morphology, especial‐ ly in polymer-based bulk heterojunction solar cells [102]. The optimal morphologies require the spontaneous phase segregation of the donor and acceptor polymers during co-deposi‐ tion. As such, the interpenetrating morphology required for high device performance, is also

Dewetting Stability of ITO Surfaces in Organic Optoelectronic Devices

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235

Beyond the limitations in the shelf life, metastability of the active layers can be a signifi‐ cant driving force for degradation during operation. The organic layers are subjected to thermal stress in a variety of ways at a number of points in the device lifetime. During fabrication, the metal cathode is vacuum deposited directly on the organic films, which can lead to localized heating of the organic film. During normal device operation, low mobility in the organic films can lead to high electric fields and local Joule heating [106-110]. Zhou et al. observed that the surface temperature for TPD based OLEDs can

show poor natural heat dissipation [110, 112-113], such large temperature variations can‐ not be handled by the limited heat sink at the glass surface. Choi et al. [114] were able to find an inverse correlation between the OLED device lifetime and the internal device

temperature, as measured by a scanning thermal microscope.

(a) (b)

800nm 800nm

after deposition (b) after one month in a low vacuum, low humidity environment.

**Figure 3.** AFM micrograph of diindenoperylene (DIP) deposited on ITO surface at room temperature (a) immediately

OPVs have the additional burden of illumination induced heating and cooling cycles, which can cause the internal temperatures to reach well beyond the Tg when coupled with Joule heating. Sullivan et al. [115] observed that UV induced heating (through UV absorption by the glass substrate) lead to a structural reorganization of pentacene, decou‐ pling it from the ITO surface, causing kinks to form in the current density-voltage (J-V)

C, suggesting that the temperature inside the actual devices could be

C [110]. Tessler et al. [111] saw temperature variation during operation

C in the recombination zone. As semiconducting organic molecules tend to

highly metastable, and driven by the substrate surface energy mismatch [103-105].

reach as high as 86o

higher than 200o

as high as 60o

As the ITO surface consists of many dangling O, surface treatments tend to saturate the sur‐ face with hydroxides, making it hydrophilic [80]. Advancing aqueous contact angles range from ~0-30o on treated surfaces [80-86]. By contrast, many electron donating organic materi‐ als are hydrophobic. Two widely used HTLs for small molecule OLEDs, TPD and NPB, have advancing contact angles of 80°, and 90° respectively [80]. This large mismatch in surface en‐ ergy makes it difficult to grow continuous films necessary for devices. Thermally evaporat‐ ed oligomers, such as NPB [87-88] and TPD [89] as well as many others, show a strong tendency to island (Volmer-Weber) growth (see figure 2a), with highly active surface diffu‐ sion to step edges and defects. Often, the initially formed islands can ripen laterally with continued deposition to form what appear to be continuous films [87], which are highly metastable. For molecules deposited from solution, the surface energy mismatch with ITO can also lead to inhomogenous deposition, as seen in figure 2b for PEDOT:PSS.

Even when continuous films are able to form upon deposition, the relatively low Tg for many oligomer hole transporting materials (NPB 96o C, TPD 65o C [53]), can lead to dewet‐ ting under mild thermal treatments or even with storage over time at ambient temperatures [42, 90]. Diindenoperylene, a novel material of interest due to its well defined ordering [91], interesting growth behaviour [92-93], promising electron transport properties [94-95], fa‐ vourable electronic structure [96], long exciton diffusion lengths [97], has recently been shown by us to have tuneable behaviour in solar cells based on its morphology [98]. As seen in figure 3, upon initial deposition, films of DIP form large flat islands on ITO with a high

**Figure 2.** (a) SEM image of (EtCz)2 films deposited at Ts = 90oC on a bare-ITO substrate. Reprinted with permission from [68]. Copyright 2002 John Wiley and Sons (b) Incomplete coverage of ITO surface with PEDOT:PSS deposited from solution.

degree oforder [98]; however, they are metastable at room temperature, showing very strong dewetting during storage at room temperature under vacuum for one month [99].

**5. Morphological instabilities on ITO**

234 Optoelectronics - Advanced Materials and Devices

morphological instability discussed in this chapter.

many oligomer hole transporting materials (NPB 96o

from ~0-30o

from solution.

In general, instability at the ITO/active layer interface can be related to morphological, chemical and electronic changes over the lifetime of the device. There are four major criteria which lead to an unstable interface: surface energy mismatch, low glass transition tempera‐ ture (Tg) materials, surface reactivity with organics, and work function instability. The sur‐ face energy mismatch and low transition temperatures are the driving characteristics for the

As the ITO surface consists of many dangling O, surface treatments tend to saturate the sur‐ face with hydroxides, making it hydrophilic [80]. Advancing aqueous contact angles range

als are hydrophobic. Two widely used HTLs for small molecule OLEDs, TPD and NPB, have advancing contact angles of 80°, and 90° respectively [80]. This large mismatch in surface en‐ ergy makes it difficult to grow continuous films necessary for devices. Thermally evaporat‐ ed oligomers, such as NPB [87-88] and TPD [89] as well as many others, show a strong tendency to island (Volmer-Weber) growth (see figure 2a), with highly active surface diffu‐ sion to step edges and defects. Often, the initially formed islands can ripen laterally with continued deposition to form what appear to be continuous films [87], which are highly metastable. For molecules deposited from solution, the surface energy mismatch with ITO

Even when continuous films are able to form upon deposition, the relatively low Tg for

ting under mild thermal treatments or even with storage over time at ambient temperatures [42, 90]. Diindenoperylene, a novel material of interest due to its well defined ordering [91], interesting growth behaviour [92-93], promising electron transport properties [94-95], fa‐ vourable electronic structure [96], long exciton diffusion lengths [97], has recently been shown by us to have tuneable behaviour in solar cells based on its morphology [98]. As seen in figure 3, upon initial deposition, films of DIP form large flat islands on ITO with a high

**Figure 2.** (a) SEM image of (EtCz)2 films deposited at Ts = 90oC on a bare-ITO substrate. Reprinted with permission from [68]. Copyright 2002 John Wiley and Sons (b) Incomplete coverage of ITO surface with PEDOT:PSS deposited

C, TPD 65o

C [53]), can lead to dewet‐

can also lead to inhomogenous deposition, as seen in figure 2b for PEDOT:PSS.

(a) (b)

on treated surfaces [80-86]. By contrast, many electron donating organic materi‐

Though sometimes observed under high temperature treatments [3, 100], dewetting is not as significant a problem for polymer systems, due to the generally higher Tg of polymer mate‐ rials [101]. However, device performance is heavily influenced by the morphology, especial‐ ly in polymer-based bulk heterojunction solar cells [102]. The optimal morphologies require the spontaneous phase segregation of the donor and acceptor polymers during co-deposi‐ tion. As such, the interpenetrating morphology required for high device performance, is also highly metastable, and driven by the substrate surface energy mismatch [103-105].

Beyond the limitations in the shelf life, metastability of the active layers can be a signifi‐ cant driving force for degradation during operation. The organic layers are subjected to thermal stress in a variety of ways at a number of points in the device lifetime. During fabrication, the metal cathode is vacuum deposited directly on the organic films, which can lead to localized heating of the organic film. During normal device operation, low mobility in the organic films can lead to high electric fields and local Joule heating [106-110]. Zhou et al. observed that the surface temperature for TPD based OLEDs can reach as high as 86o C, suggesting that the temperature inside the actual devices could be higher than 200o C [110]. Tessler et al. [111] saw temperature variation during operation as high as 60o C in the recombination zone. As semiconducting organic molecules tend to show poor natural heat dissipation [110, 112-113], such large temperature variations can‐ not be handled by the limited heat sink at the glass surface. Choi et al. [114] were able to find an inverse correlation between the OLED device lifetime and the internal device temperature, as measured by a scanning thermal microscope.

**Figure 3.** AFM micrograph of diindenoperylene (DIP) deposited on ITO surface at room temperature (a) immediately after deposition (b) after one month in a low vacuum, low humidity environment.

OPVs have the additional burden of illumination induced heating and cooling cycles, which can cause the internal temperatures to reach well beyond the Tg when coupled with Joule heating. Sullivan et al. [115] observed that UV induced heating (through UV absorption by the glass substrate) lead to a structural reorganization of pentacene, decou‐ pling it from the ITO surface, causing kinks to form in the current density-voltage (J-V) curves for PEN:C60 solar cells. Paci et al. [116] observed that the metastable morphology of P3HT:PCBM solar cells was modified in a similar way under illumination as by delib‐ erate thermal annealing.

**6. Preventing interfacial morphological instability**

**6.1. Increasing the glass transition temperature**

**6.2. Interfacial structure stabilizing interlayers**

Tg (~96o

There has been much research into methods of counter-acting dewetting from the ITO sur‐ face, as the integrity of the active layers is of paramount importance in the device perform‐ ance and stability. Incorporation of the unstable film into a device, with a mutli-layer film stack, already significantly suppresses the dewetting of single films [40, 125, 134]. Deliberate use of a stable inorganic capping layer, such as Al2O3 [135] or an electrode metal such as Ag [136] or Au [137], within the device can also greatly improve the stability of the underlying organic phases. Utilizing a rough substrate can also encourage wetting and stability [32, 46, 138-140], but that can lead to undesirable morphologies in the original deposition [99]. In or‐ der to further improve the stability at the ITO surface, two general approaches have been adopted: increasing the Tg of the active layer itself, or introducing stabilizing interlayers.

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There has been much work over the last 30 years focused on finding new hole transporting/ electron donating molecules that have high morphological stability, as recently reviewed by Shirota [56]. General strategies for designing morphologically stable molecules were devel‐ oped by Shirota [55-56], Wirth [141] and Naito and Muira [53] -- the underlying philosophy was to decrease the degrees of freedom and increase the rigidity of the molecules, through replacement/augmentation of the core; linear or branch linkages; or long substituents lead‐ ing to starbust type molecules. The first and most widely used replacement of the original triphenyldiamine used in the Tang OLED (TPD) [12], was a benzedine derivative, NPB. It had excellent hole transporting and film forming properties in addition to a slightly higher

C) [133, 142-143]. Though NPB was widely adopted in OLEDs, unfortunately it too

had a propensity to crystallization and dewetting albeit at higher temperatures [118, 144]. The original family of triphenyldiamines were successful in most other aspects as a hole transporting layer leading to the development of various derivatives, including triphenyl amines [120, 123], biphenyl amines [142], binaphlathene diamines [145], asymmetric triaryl‐ diamines (TPD derivatives) [146], triphenylamine-based starburst molecules [147-149], and recently star-shaped oligotriarylamines [150]. Other approaches include using a fluorene core to increase the rigidity of biphenyl HTLs [121], carbazole derivatives [151], vinyl-type polynorborenes with ethyl ester linked triarylamines [152], thermoclevable densified poly‐ mers [153-154], defect reduced polymers [130, 155-156], among many others. Though new materials are synthesized regularly with significantly higher glass transition temperatures than the classically utilized molecules, the correct combination of high hole mobility, good energetic compatibility with electron-accepting materials, and good optical absorbance has proven elusive, and many of these materials currently do not see widespread use in devices.

Due to the difficulties in finding complete replacements to traditional hole transport/elec‐ tron donating layers, much research has gone into introducing thin adhesion-promoting buffer layers between the active material and the ITO anode. The ideal interlayer should ex‐

Irreversible device failure has also been observed in operating OLEDs [38] and OPVs [117] heated above Tg of one of the molecular organic components. For many devices, the weak‐ est link in the device lifetime is the low Tg of the HTL immediately adjacent to the ITO sur‐ face [117-121], as many electron accepting and transporting materials have relatively high Tg [53]. Fenter et al. proposed that device failure was a result of significant expansion of the least thermally stable material, TPD, leading to delamination from the ITO surface [38]. Do et al. for MEH-PPV [3] and Choi et al [114] for PFO, observed buckling behaviour, where the polymer layer completely detached from the ITO in a number of areas on the electrode sur‐ face. This inhomogenous delamination results in significant disruption of the multilayer structure, which can cause non-uniform contact between the electrode and organic layers. TPD undergoes signifcant structural changes even well below its glass transition tempera‐ ture: at ~60o C, changes have been observed in its thickness and density by XRR [38] and seri‐ ous dewetting has been observed by AFM [122]. Tokito et al. [123] observed a direct correlation between the glass transition temperature of the HTL and device degradation as a function of temperature; however, this was disputed by Adachi et al. [51], who saw no cor‐ relation specifically with the Tg, relating the stability more to the interfacial barrier to injec‐ tion (and hence to Joule heating [124]). It is conceivable that dewetting, which compromises the integrity of the hole transport layer, would lead to inefficient hole injection by non-uni‐ form coverage, and a modification of the other layers deposited atop the dewetted layer [125].

Any local failures due to morphological inhomogeneities on the ITO surface (In spikes, thin‐ ner organic regions, etc) [90], where the high local electric field would already encourage Joule heating, would also tend to both accelerate and be accelerated by dewetting. As many materials are also subject to annealing heat treatments for improved performance, film met‐ astability can have catastrophic consequences for the device in operation. Do et al. observed complete separation of the active MEH-PPV layer during operation in a number of areas on the electrode surface, resulting in dead areas that eventually covered the whole surface [3]. Even without dewetting induced failures, over-annealing or elevated temperature storage over long time can lead to significant phase-segregation beyond the exciton diffusion length for bulk heterojunction devices [126-128], which ultimately decreases the performance over time [101, 127, 129-130].

Both dewetting and work function instability at ITO surfaces are accelerated by humidity [86, 90] or light irradiation [115-116]. The same mechanism is at work in both cases – the modification of the hydroxyl termination on the ITO surface [47], which leads to a change in the surface energy. This also helps to explain the strong correlation observed between the interfacial barrier to hole injection and OLED device degradation [51, 131-133], as both are related to this hydroxyl terminated surface.
