**6. Preventing interfacial morphological instability**

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‐

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‐

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

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

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

erate thermal annealing.

236 Optoelectronics - Advanced Materials and Devices

ture: at ~60o

[125].

time [101, 127, 129-130].

related to this hydroxyl terminated surface.

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.

#### **6.1. Increasing the glass transition temperature**

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 Tg (~96o 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.

#### **6.2. Interfacial structure stabilizing interlayers**

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‐ hibit strong adhesion to both the anode and the HTL, via physical adhesion or chemical bonds. Other desirable criteria for an effective buffer should include (1) high hole transport mobility; (2) easy deposition onto anode surfaces via straight-forward methods such as spincoating, vapor-deposition, or self-assembly; (3) good conformal matching to substrate; (4) substantial thickness control; and (5) well-defined microstructure free of pinhole defects.

ZnPc [63], for example, improved the stability of ZnPc:C60 bulk heterojunction solar cells by

The use of CuPc prevents the ambient dewetting observed for NPB directly deposited on ITO [88], providing a metastable equilibrium structure for devices at room temperature. CuPc was also seen to increase the crystallization temperature of NPB significantly, to above

figure 4), intermixing with NPB [118, 144, 213] and TPD [213], and dewetting. Ultimately, CuPc-buffered ITO does not prevent HTL crystallization and decohesion upon heating to temperatures near/above the HTL Tg. Additionally, CuPc is highly reactive with the ITO surface [118, 214], and its use leads to significant increases in the driving voltage for OLEDs [215]. To overcome some of these difficulties, researchers have used another buffer layer un‐

**Figure 4.** SEM micrographs for crystalline areas of: (a) CuPc type sample, and (b) NPB type sample. Reprinted with per‐

The second commonly used interlayer material is PEDOT:PSS [234]. Due to its high hole mobility [217, 234], PEDOT:PSS is often actually classified as a conductor and referred to as a polymeric anode [50], rather than a buffer layer. Considered as an essential compo‐ nent of most polymer based devices, both PLEDs and OPVs, it is used in almost all pol‐ ymer solar cells for its significant impact on the device performance rather than to improve stability. It was, however, initially introduced into PLEDs as a stabilizing layer [60, 166, 235] and many researchers saw substantial improvements in device lifetimes with its use [9, 60, 166, 235-237] (e.g. ~7x improvement in luminance t50 [217] table 1). The introduction of PEDOT:PSS into PLEDs allowed the device lifetime to go from a few days to hundreds [60, 166] or even thousands [235] of hours, effectively making early or‐ ganic devices into a viable technology. Figure 5 shows DIP deposited on PEDOT after storage in vacuum for one month [99]. Unlike on bare ITO, where severe dewetting was observed (figure 3), the film is completely stabilized with a PEDOT buffer layer. X-ray

der the CuPc layer, such as Pr2O3 [158] or LiF [161], with some success.

C [118, 144, 208]. This stabilization effect allows for longer lifetimes: Aziz et al. [169] ob‐ served more than a 5x increase in the luminance t50 using CuPc as a buffer (see table 1). Though the impact varies, many researchers have seen significant improvements in stability using CuPc under ambient conditions. However, poor device performance has been ob‐ served if the devices are used at even mildly elevated temperature [209-212]. It has been es‐

C leads to CuPc crystallization [80, 88] (see

Dewetting Stability of ITO Surfaces in Organic Optoelectronic Devices

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

239

~3.5x (see table 1).

tablished that moderate heating as low as 60o

mission from [88]. Copyright 2007 American Institute of Physics.

160o

In addition to promoting adhesion and stabilizing against dewetting, buffer layers often have a number of added benefits for the device, including enhancing initial device perform‐ ance [81, 86, 133, 157-167], encouraging better charge balance [45, 168-169] (often by prevent‐ ing hole injection [131]), preventing chemical reactions between the active layer and ITO [9, 162, 170-172], blocking In and Sn diffusion [170, 173-176], increasing mechanical strength [177], and smoothing the ITO surface (preventing electric field inhomogeneities that are po‐ tentially responsible for dark spot formation) [64, 161-162, 171, 175-176, 178-180].

There are two broad classes of interlayers that have been used to suppress dewetting specifically, or device degradation more broadly: modifications of HTLs (covalently bound or polymerized versions of traditional hole transport layers; HTL materials doped with or into a stabilizing material); and any organic, metal or oxide buffer that is not al‐ so used as the HTL.

#### *6.2.1. Generic buffer layers*

The most widely explored class is that of generic buffer layers. These run the gamut from vapor or solution deposited organic layers, self-assembled monolayers, dielectrics, conduct‐ ing oxides, insulating oxides, to metals. Virtually every element in the periodic table has been incorporated into the device with the aim of increasing the device lifetime. A wide va‐ riety of interlayers have also been introduced with no interest in their stabilizing properties, though film stability often also results as a side effect. These interlayers include metal doped ITO [181-183]; oxides including Y2O3 [184], Tb4O7 [184], TiO2 [184-185], ZnO [184], Nb2O3 [184], Ga2O3 [184], SnO2 [184], CuOx [186], Fe2O3 [187], SiO2 [188], VOx [189-190], RuOx [189], AZO [189], Al2O3 [191], NiO [192]; ultrathin metal layers such as Ni [184], Au [184], Sn [184], Pb [184]; F16CuPc [193]; conducting polymers [194]; and a wide variety of self-assembled monolayers [44-45, 195]. While this is not an exhaustive list, it demonstrates that a wide vari‐ ety of buffer layers have been attempted by researchers. The focus for the rest of this section will be on interlayers that are specifically shown to influence the device lifetime or stability of the organic layers deposited on the surface. The reported improvements in device lifetime for a variety of interlayers are summarized in table 1.

One of the first and still most widely used buffer layer for small molecule OLEDs is copper phthalocyanine (CuPc). Though an interesting semiconducting material in its own right [196-199] with current widespread use in solar cells as an electron donating material [200], it was first introduced as a stabilizing buffer layer for NPB [133]. It has been extensively em‐ ployed as an anode HTL buffer layer, mostly due to its reported ability to enhance OLED performance [44, 132-133, 164, 201-202], energetic level matching [203-205], high thermal sta‐ bility [204, 206], and low cost as a result of its use as a blue dye [207]. Other phthalocyanines have also been employed as buffer layers below the active layers [207]; the recent use of ZnPc [63], for example, improved the stability of ZnPc:C60 bulk heterojunction solar cells by ~3.5x (see table 1).

hibit strong adhesion to both the anode and the HTL, via physical adhesion or chemical bonds. Other desirable criteria for an effective buffer should include (1) high hole transport mobility; (2) easy deposition onto anode surfaces via straight-forward methods such as spincoating, vapor-deposition, or self-assembly; (3) good conformal matching to substrate; (4) substantial thickness control; and (5) well-defined microstructure free of pinhole defects.

In addition to promoting adhesion and stabilizing against dewetting, buffer layers often have a number of added benefits for the device, including enhancing initial device perform‐ ance [81, 86, 133, 157-167], encouraging better charge balance [45, 168-169] (often by prevent‐ ing hole injection [131]), preventing chemical reactions between the active layer and ITO [9, 162, 170-172], blocking In and Sn diffusion [170, 173-176], increasing mechanical strength [177], and smoothing the ITO surface (preventing electric field inhomogeneities that are po‐

There are two broad classes of interlayers that have been used to suppress dewetting specifically, or device degradation more broadly: modifications of HTLs (covalently bound or polymerized versions of traditional hole transport layers; HTL materials doped with or into a stabilizing material); and any organic, metal or oxide buffer that is not al‐

The most widely explored class is that of generic buffer layers. These run the gamut from vapor or solution deposited organic layers, self-assembled monolayers, dielectrics, conduct‐ ing oxides, insulating oxides, to metals. Virtually every element in the periodic table has been incorporated into the device with the aim of increasing the device lifetime. A wide va‐ riety of interlayers have also been introduced with no interest in their stabilizing properties, though film stability often also results as a side effect. These interlayers include metal doped ITO [181-183]; oxides including Y2O3 [184], Tb4O7 [184], TiO2 [184-185], ZnO [184], Nb2O3 [184], Ga2O3 [184], SnO2 [184], CuOx [186], Fe2O3 [187], SiO2 [188], VOx [189-190], RuOx [189], AZO [189], Al2O3 [191], NiO [192]; ultrathin metal layers such as Ni [184], Au [184], Sn [184], Pb [184]; F16CuPc [193]; conducting polymers [194]; and a wide variety of self-assembled monolayers [44-45, 195]. While this is not an exhaustive list, it demonstrates that a wide vari‐ ety of buffer layers have been attempted by researchers. The focus for the rest of this section will be on interlayers that are specifically shown to influence the device lifetime or stability of the organic layers deposited on the surface. The reported improvements in device lifetime

One of the first and still most widely used buffer layer for small molecule OLEDs is copper phthalocyanine (CuPc). Though an interesting semiconducting material in its own right [196-199] with current widespread use in solar cells as an electron donating material [200], it was first introduced as a stabilizing buffer layer for NPB [133]. It has been extensively em‐ ployed as an anode HTL buffer layer, mostly due to its reported ability to enhance OLED performance [44, 132-133, 164, 201-202], energetic level matching [203-205], high thermal sta‐ bility [204, 206], and low cost as a result of its use as a blue dye [207]. Other phthalocyanines have also been employed as buffer layers below the active layers [207]; the recent use of

tentially responsible for dark spot formation) [64, 161-162, 171, 175-176, 178-180].

so used as the HTL.

*6.2.1. Generic buffer layers*

238 Optoelectronics - Advanced Materials and Devices

for a variety of interlayers are summarized in table 1.

The use of CuPc prevents the ambient dewetting observed for NPB directly deposited on ITO [88], providing a metastable equilibrium structure for devices at room temperature. CuPc was also seen to increase the crystallization temperature of NPB significantly, to above 160o C [118, 144, 208]. This stabilization effect allows for longer lifetimes: Aziz et al. [169] ob‐ served more than a 5x increase in the luminance t50 using CuPc as a buffer (see table 1). Though the impact varies, many researchers have seen significant improvements in stability using CuPc under ambient conditions. However, poor device performance has been ob‐ served if the devices are used at even mildly elevated temperature [209-212]. It has been es‐ tablished that moderate heating as low as 60o C leads to CuPc crystallization [80, 88] (see figure 4), intermixing with NPB [118, 144, 213] and TPD [213], and dewetting. Ultimately, CuPc-buffered ITO does not prevent HTL crystallization and decohesion upon heating to temperatures near/above the HTL Tg. Additionally, CuPc is highly reactive with the ITO surface [118, 214], and its use leads to significant increases in the driving voltage for OLEDs [215]. To overcome some of these difficulties, researchers have used another buffer layer un‐ der the CuPc layer, such as Pr2O3 [158] or LiF [161], with some success.

**Figure 4.** SEM micrographs for crystalline areas of: (a) CuPc type sample, and (b) NPB type sample. Reprinted with per‐ mission from [88]. Copyright 2007 American Institute of Physics.

The second commonly used interlayer material is PEDOT:PSS [234]. Due to its high hole mobility [217, 234], PEDOT:PSS is often actually classified as a conductor and referred to as a polymeric anode [50], rather than a buffer layer. Considered as an essential compo‐ nent of most polymer based devices, both PLEDs and OPVs, it is used in almost all pol‐ ymer solar cells for its significant impact on the device performance rather than to improve stability. It was, however, initially introduced into PLEDs as a stabilizing layer [60, 166, 235] and many researchers saw substantial improvements in device lifetimes with its use [9, 60, 166, 235-237] (e.g. ~7x improvement in luminance t50 [217] table 1). The introduction of PEDOT:PSS into PLEDs allowed the device lifetime to go from a few days to hundreds [60, 166] or even thousands [235] of hours, effectively making early or‐ ganic devices into a viable technology. Figure 5 shows DIP deposited on PEDOT after storage in vacuum for one month [99]. Unlike on bare ITO, where severe dewetting was observed (figure 3), the film is completely stabilized with a PEDOT buffer layer. X-ray diffraction measurements (figure 5c) confirms that the crystal structure is also preserved during storage [238]. Although, PEDOT:PSS is widely used, much like CuPc, there are a number of drawbacks, most significantly its extremely high reactivity with ITO [59, 170, 239-241]. In some cases, this is a benefit, as the high solubility of In in PEDOT:PSS al‐ lows it to be used as a barrier again In migration into PPV [174, 242], or PCBM [242] im‐ proving the device stability. Again much like CuPc, one approach to overcoming these limitations is to introduce an underlying buffer layer, such as diamond-like carbon [170] or alkylsiloxane SAMs [173] to prevent In diffusion. PEDOT:PSS is also prone to oxida‐ tion [239], both from moisture in the ambient environment [243] and from the ITO sur‐ face, which decreases the device performance and stability [75, 230, 239, 244-245]. Unencapsulated P3HT:PCBM solar cell devices with PEDOT:PSS show rapid decay of the short circuit current, with t80 essentially equal with or without ITO [246]. Recently, the group of Karl Leo in Dresden introduced an ethylene glycol soaking treatment that im‐ proves solar cell lifetime by a factor of 1.3, by removing the insulating and hygroscopic PSS components in the layer [233]. As these PSS components can also react with other polymer layers [247], eliminating excess PSS leads to greatly increased lifetimes.

provements in t50 with plasma treatments. Not all plasma treatments lead to the same response, however. The study by Huang et al [67] showed that the wettability of NPB is in‐ creased by H2, CF4 and O2 plasmas, but decreases with Ar plasma. They suggested that de‐ contamination of the surface without hydroxylation leads to a dipole field on the surface that is stronger than the grain boundary and defect effects, promoting NPB nucleation. On the other hand, CF4 and O2 strongly encourage layer-by-layer growth, smoothing [219] and passivating the surface against defects that can lead to inhomogeneous electric fields. As an interesting aside, the results for UV-ozone treatment of the ITO surface by Fukushi et al [220] contradict the above. They saw ~5-6x improvement in t50 with a treatment that de‐

was possible, which contradicts thermodynamic wetting theory as the surface energy mis‐ match would be incredibly large. This underscores the likelihood that, as with many aspects of organic semiconductors, more than one mechanism is responsible for lifetime improve‐

Silane based SAMs, such as tetraaryldiamine (TAA) [80, 213, 250], epoxysilane functional‐ ized triphenylamine (TPA-silane) [81], dodecyltrichlorosilane (DDTS) [82], phenyltriethox‐ ysilane (PTES) [82], 3-aminopropyl-methyl-diethoxysilane (APMDS) [82], alkylsiloxane [173], and alkytrichlorosilanes (OTCS and FOTCS) [83] use a Si-O bond to anchor on the ITO surface, providing a robust interface with substantially higher surface energy. Chong et al [82] observed that the sticking and dewetting of NPB on various SAMs shows an inverse correlation with the measured aqueous contact angle, but Choi et al saw substan‐ tially longer lifetimes (x11 increase in t50 with 105o contact angle on FOTCS [83]). The im‐ proved adhesion of the active layer was attributed to interfacial reconstruction and interpenetration with the SAM [251]. Our recent study of quaterrylene molecules on SAM buffered surfaces showed a significant release of interfacial strain when the buffer was present [252]. As strain release is another mechanism that drives dewetting [36], bet‐ ter pseudo-epitaxial conformation with the surface for buffered films could be a mecha‐

A wide variety of other organic molecular interlayers have been used, with varying success in improving the lifetime, as summarized in table 1. These include oligomers such as 4,4',4''- (3-methylphenyl-phenylamino)triphenylamine (MTDATA) [147, 178, 253], parylene [162, 175, 254], cross-linked perylenediimide (PDMI) [255], N,N-bis(4-trifluoromethoxyben‐ zyl)-1,4,5,8-naphthalene-tetracarboxylic diimide (NTCDI-OCF3) [159], Alq3, or fullerene (C60). Most of these interlayers are thought to improve the active layer film formation on the surface, by smoothing [147, 162, 180, 255]. The high Tg of MTDATA [253] and NTCDI-OCF3 [159] are also thought to contribute to preventing crystallization of the HTL overlayer. Poly‐ mer approaches include polyanaline (PAni) [172], fluoropolymer (FC-227) [179], plasma polymerized fluorocarbons [74, 160], tetrahedral amorphous carbon [171], polyimide [176] or radical ion salt doped amine polymer [221], which are thought to prevent oxidation [74, 160, 172] or block hole injection [221], in addition to stabilizing the surface against dewetting

Metals such as Pt and Mg have also been used as interlayers, and though both show im‐ provements, the mechanisms are almost completely opposed. High work function Pt enhan‐

. They speculate that stronger adhesion of the HTL

Dewetting Stability of ITO Surfaces in Organic Optoelectronic Devices

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241

creased the aqueous contact angle to 4o

nism for the observed improvement in performance.

ments in devices.

**Figure 5.** DIP on PEDOT:PSS (a) AFM micrographs of the bilayer structure as deposited (b) AFM micrograph of the same surface after one month in a low vacuum, low humidity environment. (c) grazing incidence x-ray diffraction scan of characteristic DIP crystal planes with accelerated aging at 90oC with 60% humidity (13.5hr equivalent of 1 month ambient exposure).

As a third approach, plasma or ozone treatment of the ITO surface is widely used to modify the work function and surface energy prior to deposition of the active layers. Though not technically an interlayer, the modifications are limited to a small region immediately adja‐ cent to the active layer, resulting in effects similar to that of a buffer layer. The treatment of the ITO surface with plasma or UV ozone has a profound effect on the surface energy and hence the stability of the active layers deposited on the surface. The aqueous contact angle increases with plasma treatment [62, 85], correlated to the coverage of the surface with hy‐ droxyl terminated O [85, 248]. As this saturation also controls the surface work function, and wetting behavior has been recently shown to correlate to the surface work function [248-249], the hydroxylated surface should encourage improved adhesion of the active lay‐ ers and lead to better performance. Wu et al [78] saw almost two orders of magnitude im‐ provements in t50 with plasma treatments. Not all plasma treatments lead to the same response, however. The study by Huang et al [67] showed that the wettability of NPB is in‐ creased by H2, CF4 and O2 plasmas, but decreases with Ar plasma. They suggested that de‐ contamination of the surface without hydroxylation leads to a dipole field on the surface that is stronger than the grain boundary and defect effects, promoting NPB nucleation. On the other hand, CF4 and O2 strongly encourage layer-by-layer growth, smoothing [219] and passivating the surface against defects that can lead to inhomogeneous electric fields. As an interesting aside, the results for UV-ozone treatment of the ITO surface by Fukushi et al [220] contradict the above. They saw ~5-6x improvement in t50 with a treatment that de‐ creased the aqueous contact angle to 4o . They speculate that stronger adhesion of the HTL was possible, which contradicts thermodynamic wetting theory as the surface energy mis‐ match would be incredibly large. This underscores the likelihood that, as with many aspects of organic semiconductors, more than one mechanism is responsible for lifetime improve‐ ments in devices.

diffraction measurements (figure 5c) confirms that the crystal structure is also preserved during storage [238]. Although, PEDOT:PSS is widely used, much like CuPc, there are a number of drawbacks, most significantly its extremely high reactivity with ITO [59, 170, 239-241]. In some cases, this is a benefit, as the high solubility of In in PEDOT:PSS al‐ lows it to be used as a barrier again In migration into PPV [174, 242], or PCBM [242] im‐ proving the device stability. Again much like CuPc, one approach to overcoming these limitations is to introduce an underlying buffer layer, such as diamond-like carbon [170] or alkylsiloxane SAMs [173] to prevent In diffusion. PEDOT:PSS is also prone to oxida‐ tion [239], both from moisture in the ambient environment [243] and from the ITO sur‐ face, which decreases the device performance and stability [75, 230, 239, 244-245]. Unencapsulated P3HT:PCBM solar cell devices with PEDOT:PSS show rapid decay of the short circuit current, with t80 essentially equal with or without ITO [246]. Recently, the group of Karl Leo in Dresden introduced an ethylene glycol soaking treatment that im‐ proves solar cell lifetime by a factor of 1.3, by removing the insulating and hygroscopic PSS components in the layer [233]. As these PSS components can also react with other

polymer layers [247], eliminating excess PSS leads to greatly increased lifetimes.

**Figure 5.** DIP on PEDOT:PSS (a) AFM micrographs of the bilayer structure as deposited (b) AFM micrograph of the same surface after one month in a low vacuum, low humidity environment. (c) grazing incidence x-ray diffraction scan of characteristic DIP crystal planes with accelerated aging at 90oC with 60% humidity (13.5hr equivalent of 1 month

As a third approach, plasma or ozone treatment of the ITO surface is widely used to modify the work function and surface energy prior to deposition of the active layers. Though not technically an interlayer, the modifications are limited to a small region immediately adja‐ cent to the active layer, resulting in effects similar to that of a buffer layer. The treatment of the ITO surface with plasma or UV ozone has a profound effect on the surface energy and hence the stability of the active layers deposited on the surface. The aqueous contact angle increases with plasma treatment [62, 85], correlated to the coverage of the surface with hy‐ droxyl terminated O [85, 248]. As this saturation also controls the surface work function, and wetting behavior has been recently shown to correlate to the surface work function [248-249], the hydroxylated surface should encourage improved adhesion of the active lay‐ ers and lead to better performance. Wu et al [78] saw almost two orders of magnitude im‐

(a) (b)

240 Optoelectronics - Advanced Materials and Devices

ambient exposure).

(c)

Silane based SAMs, such as tetraaryldiamine (TAA) [80, 213, 250], epoxysilane functional‐ ized triphenylamine (TPA-silane) [81], dodecyltrichlorosilane (DDTS) [82], phenyltriethox‐ ysilane (PTES) [82], 3-aminopropyl-methyl-diethoxysilane (APMDS) [82], alkylsiloxane [173], and alkytrichlorosilanes (OTCS and FOTCS) [83] use a Si-O bond to anchor on the ITO surface, providing a robust interface with substantially higher surface energy. Chong et al [82] observed that the sticking and dewetting of NPB on various SAMs shows an inverse correlation with the measured aqueous contact angle, but Choi et al saw substan‐ tially longer lifetimes (x11 increase in t50 with 105o contact angle on FOTCS [83]). The im‐ proved adhesion of the active layer was attributed to interfacial reconstruction and interpenetration with the SAM [251]. Our recent study of quaterrylene molecules on SAM buffered surfaces showed a significant release of interfacial strain when the buffer was present [252]. As strain release is another mechanism that drives dewetting [36], bet‐ ter pseudo-epitaxial conformation with the surface for buffered films could be a mecha‐ nism for the observed improvement in performance.

A wide variety of other organic molecular interlayers have been used, with varying success in improving the lifetime, as summarized in table 1. These include oligomers such as 4,4',4''- (3-methylphenyl-phenylamino)triphenylamine (MTDATA) [147, 178, 253], parylene [162, 175, 254], cross-linked perylenediimide (PDMI) [255], N,N-bis(4-trifluoromethoxyben‐ zyl)-1,4,5,8-naphthalene-tetracarboxylic diimide (NTCDI-OCF3) [159], Alq3, or fullerene (C60). Most of these interlayers are thought to improve the active layer film formation on the surface, by smoothing [147, 162, 180, 255]. The high Tg of MTDATA [253] and NTCDI-OCF3 [159] are also thought to contribute to preventing crystallization of the HTL overlayer. Poly‐ mer approaches include polyanaline (PAni) [172], fluoropolymer (FC-227) [179], plasma polymerized fluorocarbons [74, 160], tetrahedral amorphous carbon [171], polyimide [176] or radical ion salt doped amine polymer [221], which are thought to prevent oxidation [74, 160, 172] or block hole injection [221], in addition to stabilizing the surface against dewetting

Metals such as Pt and Mg have also been used as interlayers, and though both show im‐ provements, the mechanisms are almost completely opposed. High work function Pt enhan‐ ces TPD wetting on ITO surfaces [46], while low work function materials such as Mg, Ca and Al give many orders of magnitude improvement in the luminance stability, correlated to the work function of the metal, by preventing the injection of holes into the Alq layer in OLEDs (intrinsic degradation) [169].

doped hole transport layers (MADN:NPB [226], F4TCNQ:NPB [131, 169], DSA\_Ph:NPB [225], Ir(piq)3:NPD [268]). As the focus of this section is on the stabilization at the ITO sur‐

Dewetting Stability of ITO Surfaces in Organic Optoelectronic Devices

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243

Doping with nanoparticles (LiF [88], C60 [32, 88, 269-271], NaCl [215], Au [33]) has also been very successful in stabilizing hole transport layers, though there is sometimes a trade-off be‐ tween stability and performance for such systems [88]. As particulates in the layer can act as nucleation centres for crystallization [272-273], care must be taken when selecting doping pa‐ rameters. The concentration and layer thickness must be chosen such that the electrical per‐ formance is not adversely affected by the presence of the doped layer. Interparticle or surface forces strongly influence suspension behaviour of nanoparticles; therefore, not every nanofil‐ ler works with every organic. We observed that LiF greatly enhances the stability of NPB at

C, while having no impact on the crystallization of CuPc [88]. There is a long history of nanoparticle inclusions for stabilization in non-conducting polymers [32, 274-275]. Luoet al. [275] suggests that a combination of factors are responsible for the stabilization effects, such as the mobility of nanofillers, their size, interaction with the organic, and additional pinning ef‐ fects at contact lines. Fillers work best if they are immobile; therefore, diffusion to and pinning at the substrate interface is one suggested mechanism for stabilization [32, 274]. Chu et al. [222] did in fact see similar improvement in stability with a C60 layer deposited at the interface below NPB as Yuan et al. [270] saw with doping C60 into NPB; Barnes et al. [32], however, saw greatly enhanced dewetting with C60 at the Si surface for polystyrene thin films. Additionally, there are a number of cases where diffusion to the substrate is unlikely, as no phase separation was observed [88, 120, 215] even though stability was improved. Mukherjee et al. [33] recently observed a concentration dependence on the stabilization, where dewetting droplets form a core-shell structure, rather than leaving behind nanoparticles as the polymer layer retreats as would be expected for substrate segregation. In such cases, strong electrostatic or chargetransfer interactions between the particle and the organic layer leading to a cross-linked net‐ work are the most likely route to highly stable films [88, 274]. Other possible mechanisms for stabilization include changing the Tg with high volume-surface area ratio (effectively modify‐ ing the film rheology), preventing heterogeneous nucleation, and relief of residual stress in

Another approach to stabilization, spearheaded by Tobin Marks' at Northwestern University [45, 80, 163-164, 213, 250, 276-278], focuses on functionalizing traditional hole transport mate‐ rials with siloxane groups. This allows the molecules to covalently bond to the ITO surface through the formation of Si-O bonds (see figure 6) in a manner similar to the SAMs discussed in section 5.2.1. Covalent bonds ensure strong adhesion and directly eliminate the surface en‐ ergy mismatch [84, 163]. As the interlayer is the same molecule as the HTL, deposition contin‐ ues in a self-epitaxial fashion, yielding uniform films as large as 25μm2 without cracks or pinholes [163]. Aqueous contact angles of 90° compared to 110° for the active layer ensures good wetting and physical cohesion in MDMO-PPV:PCBM bulk heterojunction solar cells

213, 276-277], NPB-Si2 [80, 163, 279], PABT-Si2 [278], penta(organo)fullerenes [84] (which use

C. Typical examples include TPD-Si2 [80, 163,

face, the summary in table 1 has been limited to doping in the HTL.

120o

the film through de-segregation [33].

[277-278] with dewetting prevention above 60o

*Functionalized HTL*

Most recently, MoOx has been successfully employed to improve lifetimes in both OLEDs [157, 218] and OPVs [75, 230-232], especially as a replacement for PEDOT:PSS, and though the mechanism is still controversial, the energy barrier at the active layer/ITO interface is thought to play a significant role. Other oxides, such as SiOxNy [86] or ZnO [256], which can act as oxygen and moisture getters, have been successfully used to stabilize the ITO surface against contamination.

#### *6.2.2. Modified HTLs*

There are three modifications that have been made to the hole transporting/electron donat‐ ing layer adjacent to the ITO surface to improve its wetting properties: doping, polymeriza‐ tion, and functionalization to allow for covalent bonding to the surface. Though many of the layers discussed in this section have also been tried as the HTL for a given device structure, they have found more success for high device performance coupled with stability when they are used as an ultra-thin buffer layer beneath the unmodified version of the molecule acting as the HTL.

#### *Doping*

The first approach to stabilization is through doping, either with nanoparticles or with other molecules. Doping with other molecules, often referred to as alloying or more recently as a bulk heterojunction, has been very successful in suppressing crystallization and dewetting in hole transporting molecules [131, 159, 169, 206, 225-227, 257-258]. Mori et al. used metalfree Pc to disrupt the crystallization of CuPc, showing ~2x improvement in lifetimes for op‐ eration above 85o C [206]. Chu et al. [159] and Lee et al. [227] used higher stability molecules NTCDI (diimide) and PFI (perfluorinatedionomer) to stabilize NPB and PEDOT:PSS, respec‐ tively. The incorporation of PCBM into various polymers, including P3HT [100, 259], PPV, MDMO:PPV [260], also stabilizes the morphology against heat treatment, and improves long-term stability [100, 259-260]. Addition of a diblock co-polymer of P3HT-C60 to a P3HT:PCBM composite has led to even greater stabilization against phase desegregation [261-262]. In some cases, the hole transport molecule was doped into a more stable matrix, such as TPD into high Tg polymers [263] or MgF2 [120], which significantly suppressed the crystallization. Ruberene:TPD [257], MADN:NPB [226], F4TCNQ:NPB [131, 169], DSA\_Ph:NPB [225]) combinations have all been employed with various levels of doping, leading in most cases to ~2x improvement in the 80% luminance lifetime in small molecule OLEDs (see table 2). One very successful method of improving device stability has been doping of the Alq3 layer in small molecule OLEDs with other molecules including TPD [258], NPB [210], NPD [218], quadricone [209], styrlamine [225], DMQA [264], rubrene [257, 265], DNP [266], Bphen [267], perylene [265-266], among many others [265]. This approach, however, is focused on combating the intrinsic degradation of Alq3 by holes [168]. This mechanism, hole blocking, was also suggested as an additional mechanism for a few of the doped hole transport layers (MADN:NPB [226], F4TCNQ:NPB [131, 169], DSA\_Ph:NPB [225], Ir(piq)3:NPD [268]). As the focus of this section is on the stabilization at the ITO sur‐ face, the summary in table 1 has been limited to doping in the HTL.

Doping with nanoparticles (LiF [88], C60 [32, 88, 269-271], NaCl [215], Au [33]) has also been very successful in stabilizing hole transport layers, though there is sometimes a trade-off be‐ tween stability and performance for such systems [88]. As particulates in the layer can act as nucleation centres for crystallization [272-273], care must be taken when selecting doping pa‐ rameters. The concentration and layer thickness must be chosen such that the electrical per‐ formance is not adversely affected by the presence of the doped layer. Interparticle or surface forces strongly influence suspension behaviour of nanoparticles; therefore, not every nanofil‐ ler works with every organic. We observed that LiF greatly enhances the stability of NPB at 120o C, while having no impact on the crystallization of CuPc [88]. There is a long history of nanoparticle inclusions for stabilization in non-conducting polymers [32, 274-275]. Luoet al. [275] suggests that a combination of factors are responsible for the stabilization effects, such as the mobility of nanofillers, their size, interaction with the organic, and additional pinning ef‐ fects at contact lines. Fillers work best if they are immobile; therefore, diffusion to and pinning at the substrate interface is one suggested mechanism for stabilization [32, 274]. Chu et al. [222] did in fact see similar improvement in stability with a C60 layer deposited at the interface below NPB as Yuan et al. [270] saw with doping C60 into NPB; Barnes et al. [32], however, saw greatly enhanced dewetting with C60 at the Si surface for polystyrene thin films. Additionally, there are a number of cases where diffusion to the substrate is unlikely, as no phase separation was observed [88, 120, 215] even though stability was improved. Mukherjee et al. [33] recently observed a concentration dependence on the stabilization, where dewetting droplets form a core-shell structure, rather than leaving behind nanoparticles as the polymer layer retreats as would be expected for substrate segregation. In such cases, strong electrostatic or chargetransfer interactions between the particle and the organic layer leading to a cross-linked net‐ work are the most likely route to highly stable films [88, 274]. Other possible mechanisms for stabilization include changing the Tg with high volume-surface area ratio (effectively modify‐ ing the film rheology), preventing heterogeneous nucleation, and relief of residual stress in the film through de-segregation [33].

#### *Functionalized HTL*

ces TPD wetting on ITO surfaces [46], while low work function materials such as Mg, Ca and Al give many orders of magnitude improvement in the luminance stability, correlated to the work function of the metal, by preventing the injection of holes into the Alq layer in

Most recently, MoOx has been successfully employed to improve lifetimes in both OLEDs [157, 218] and OPVs [75, 230-232], especially as a replacement for PEDOT:PSS, and though the mechanism is still controversial, the energy barrier at the active layer/ITO interface is thought to play a significant role. Other oxides, such as SiOxNy [86] or ZnO [256], which can act as oxygen and moisture getters, have been successfully used to stabilize the ITO surface

There are three modifications that have been made to the hole transporting/electron donat‐ ing layer adjacent to the ITO surface to improve its wetting properties: doping, polymeriza‐ tion, and functionalization to allow for covalent bonding to the surface. Though many of the layers discussed in this section have also been tried as the HTL for a given device structure, they have found more success for high device performance coupled with stability when they are used as an ultra-thin buffer layer beneath the unmodified version of the molecule acting

The first approach to stabilization is through doping, either with nanoparticles or with other molecules. Doping with other molecules, often referred to as alloying or more recently as a bulk heterojunction, has been very successful in suppressing crystallization and dewetting in hole transporting molecules [131, 159, 169, 206, 225-227, 257-258]. Mori et al. used metalfree Pc to disrupt the crystallization of CuPc, showing ~2x improvement in lifetimes for op‐

NTCDI (diimide) and PFI (perfluorinatedionomer) to stabilize NPB and PEDOT:PSS, respec‐ tively. The incorporation of PCBM into various polymers, including P3HT [100, 259], PPV, MDMO:PPV [260], also stabilizes the morphology against heat treatment, and improves long-term stability [100, 259-260]. Addition of a diblock co-polymer of P3HT-C60 to a P3HT:PCBM composite has led to even greater stabilization against phase desegregation [261-262]. In some cases, the hole transport molecule was doped into a more stable matrix, such as TPD into high Tg polymers [263] or MgF2 [120], which significantly suppressed the crystallization. Ruberene:TPD [257], MADN:NPB [226], F4TCNQ:NPB [131, 169], DSA\_Ph:NPB [225]) combinations have all been employed with various levels of doping, leading in most cases to ~2x improvement in the 80% luminance lifetime in small molecule OLEDs (see table 2). One very successful method of improving device stability has been doping of the Alq3 layer in small molecule OLEDs with other molecules including TPD [258], NPB [210], NPD [218], quadricone [209], styrlamine [225], DMQA [264], rubrene [257, 265], DNP [266], Bphen [267], perylene [265-266], among many others [265]. This approach, however, is focused on combating the intrinsic degradation of Alq3 by holes [168]. This mechanism, hole blocking, was also suggested as an additional mechanism for a few of the

C [206]. Chu et al. [159] and Lee et al. [227] used higher stability molecules

OLEDs (intrinsic degradation) [169].

242 Optoelectronics - Advanced Materials and Devices

against contamination.

*6.2.2. Modified HTLs*

as the HTL.

eration above 85o

*Doping*

Another approach to stabilization, spearheaded by Tobin Marks' at Northwestern University [45, 80, 163-164, 213, 250, 276-278], focuses on functionalizing traditional hole transport mate‐ rials with siloxane groups. This allows the molecules to covalently bond to the ITO surface through the formation of Si-O bonds (see figure 6) in a manner similar to the SAMs discussed in section 5.2.1. Covalent bonds ensure strong adhesion and directly eliminate the surface en‐ ergy mismatch [84, 163]. As the interlayer is the same molecule as the HTL, deposition contin‐ ues in a self-epitaxial fashion, yielding uniform films as large as 25μm2 without cracks or pinholes [163]. Aqueous contact angles of 90° compared to 110° for the active layer ensures good wetting and physical cohesion in MDMO-PPV:PCBM bulk heterojunction solar cells [277-278] with dewetting prevention above 60o C. Typical examples include TPD-Si2 [80, 163, 213, 276-277], NPB-Si2 [80, 163, 279], PABT-Si2 [278], penta(organo)fullerenes [84] (which use phosphonic acid linkages [72] rather than silane), and fluorinated triphenyldiamine (FTPD) [280]. An additional step of thermal curing leads to a cross-linked siloxane network, resulting in thin layer with HTL characteristics covalently anchored on surface [163, 280].

NTCDI-OCF3 1.2x NPB [159] Pr2O3 ~7x CuPc/TPD [158] Alq3 4.2x (t80) NPB [223] ta-C orders of mag PEDOT [171] Pani ~10x (t30) MEH:PPV [172] NaCl:NPB 2.3x NPB [215] metal free Pc:CuPc 3.4x TPTE [206]\* BHJ NPB:Alq 5.4x NPB [224]\* gradient HJ NPB:Alq 3.15x NPB [224]\* DSA\_Ph:NPB 1.3x NPB [225] F4TCNQ (5%-20%):NPB 40x NPB [169] F4TCNQ (30%):NPB 14x NPB [169] F4TCNQ (2%):NPB 2.5x NPB [169] MADN:NPB 2.7x NPB [226] PFI (3%):PEDOT:PSS 9.4x PFO [227]\*\*\* PFI (6%):PEDOT:PSS 8x PFO [227]\*\*\* heat treatment 35oc 3.5x MEH-PPV LB [228] heat treatment 65oc 5.5x MEH-PPV LB [228] heat treatment 110c 9.4x polyfluorene [229]\*\*\* heat treatment 150c 167.5x polyfluorene [229]\*\*\* **OPVs**

Dewetting Stability of ITO Surfaces in Organic Optoelectronic Devices

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

245

ZnPc ~3.5x ZnPc:C60 [63] MoO3 ~ 4x H2TPP [75] MoO3 ~24x α-NPD [75] MoO3 ~27x P3HT:PCBM [230]\*\*\* MoOx ~80x PCDTBT:PC70BM [231]\*\*\* sMoOx 39x P3HT:PCBM [232]\*\*\* PEDOT 8.7x (t70) FFTCNQ/ZnPc:C60 [233]

\* modified using data from [215] \*\* modified using data from [147] \*\*\* modified using data from [217]

stated.

the ITO surface1

1The most common description of lifetime for OLEDs is the illumination half-life (t50) – the time it takes for the lumi‐ nance to decrease to half of its initial value [216]. In OPVs, a similar standard has been used with t50 defined as the time for the power conversion efficiency (PCE) to decrease to half of its initial value [21]. More recently, it has become more common to report the t80, the time when the device has decayed to 80% of its initial performance [17]. In this chapter, t50 and t80 will be the commonly adopted lifetime values for OLEDs and OPVs respectively, unless otherwise

**Table 1.** Relative improvement in t50 (for OLEDs) & t80 (for OPVs) compared to bare ITO for various interlayers used at

### *Polymerized HTL*

The final widely used method of increasing the stability of the active layer on the ITO sur‐ face requires a crosslinked polymer or highly crystallized version of a traditional HTL as an interlayer [55, 143, 146, 280-283]. This approach has been most commonly applied to TPD [55, 146, 280-282], where significant stabilization was observed above 80o C with polymeriza‐ tion. Bellman et al also observed that the voltage increase with time for small molecule OLEDs was slower compared to those without the crosslinked interlayer [280], suggesting increased stability. In-situ polymerization/crystallization, by heat treatment or high temper‐ ature deposition (NPB [143, 284]) or by UV irradiation (TPD [282]), lead to significant in‐ creases in the shelf life (>2 months for NPB), and operational stability at high temperatures. This approach has also been applied to polymers, where the most common approach is to use a heat-treatment [228-229, 285](already widely used in solar cells to improve efficiency [102]), with as much as two orders of magnitude improvement in lifetime [229]. Some suc‐ cess has also be observed for doping and irradiation induced polymerization, mainly with PEDOT:PSS [286].


Dewetting Stability of ITO Surfaces in Organic Optoelectronic Devices http://dx.doi.org/10.5772/52417 245


phosphonic acid linkages [72] rather than silane), and fluorinated triphenyldiamine (FTPD) [280]. An additional step of thermal curing leads to a cross-linked siloxane network, resulting

The final widely used method of increasing the stability of the active layer on the ITO sur‐ face requires a crosslinked polymer or highly crystallized version of a traditional HTL as an interlayer [55, 143, 146, 280-283]. This approach has been most commonly applied to TPD

tion. Bellman et al also observed that the voltage increase with time for small molecule OLEDs was slower compared to those without the crosslinked interlayer [280], suggesting increased stability. In-situ polymerization/crystallization, by heat treatment or high temper‐ ature deposition (NPB [143, 284]) or by UV irradiation (TPD [282]), lead to significant in‐ creases in the shelf life (>2 months for NPB), and operational stability at high temperatures. This approach has also been applied to polymers, where the most common approach is to use a heat-treatment [228-229, 285](already widely used in solar cells to improve efficiency [102]), with as much as two orders of magnitude improvement in lifetime [229]. Some suc‐ cess has also be observed for doping and irradiation induced polymerization, mainly with

**OLEDs**

TMTPD+SbF-6:PC-TPB-DEGL 2700x NPB [221]

FOTCS 11x MEH:PPV [83] C60 8x NPB [222]

CuPc 1.8x NPB [215] CuPc 5.3x NPB [169] PEDOT:PSS 6.7x TDAPB [217] MoO3 5.9x (t90) α-NPD [218] MoOx 2.8x NPB [157]\* MoOx 3x NPB [157]\* sc-MTDATA 4.7x NPB [178]\*\* MTDATA 1.75x TPD [147] O2 plasma 20x PVK:Alq3:Nile red [78] O2 plasma 2x PEDOT [219] UV ozone 4.7x α-NPD [220] Mg ~60x (t90) NPB:F4TCNQ [169] CF3/CFx 2x NPB [74, 160] OTCS 4x MEHPPV [83]

C with polymeriza‐

in thin layer with HTL characteristics covalently anchored on surface [163, 280].

[55, 146, 280-282], where significant stabilization was observed above 80o

**Interlayer t50/t80 HTL**

*Polymerized HTL*

244 Optoelectronics - Advanced Materials and Devices

PEDOT:PSS [286].

\* modified using data from [215] \*\* modified using data from [147] \*\*\* modified using data from [217]

1The most common description of lifetime for OLEDs is the illumination half-life (t50) – the time it takes for the lumi‐ nance to decrease to half of its initial value [216]. In OPVs, a similar standard has been used with t50 defined as the time for the power conversion efficiency (PCE) to decrease to half of its initial value [21]. More recently, it has become more common to report the t80, the time when the device has decayed to 80% of its initial performance [17]. In this chapter, t50 and t80 will be the commonly adopted lifetime values for OLEDs and OPVs respectively, unless otherwise stated.

**Table 1.** Relative improvement in t50 (for OLEDs) & t80 (for OPVs) compared to bare ITO for various interlayers used at the ITO surface1

ic technologies mature, the metastability of interfaces becomes more and more significant in

Dewetting Stability of ITO Surfaces in Organic Optoelectronic Devices

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

247

The author would like to acknowledge collaborators at the Max-Planck-Institute for Metals Research, specifically Prof. Dr. H. Dosch and students supervised by the author (F. Maye, J.

[1] Krebs F C and Norrman K Analysis of the failure mechanism for a stable organic photovoltaic during 10000 h of testing Prog Photovoltaics 2007;15(8) 697-712.

[2] Brabec C J, Hauch J A, Schilinsky P and Waldauf C Production Aspects of Organic Photovoltaics and Commercialization of Devices MRS Bulletin 2005;30(Jan) 50-2. [3] Do L-m, Kim K, Zyung T and Kim J-j In situ investigation of degradation in polymer‐ ic electroluminescent devices using time-resolved confocal laser scanning microscope

[4] Reese M O, Morfa A J, White M S, Kopidakis N, Shaheen S E, Rumbles G and Ginley D S Pathways for the degradation of organic photovoltaic P3HT: PCBM based devi‐

[5] Paci B, Generosi A, Albertini V R, Perfetti P, Bettignies R D and Sentein C Time-re‐ solved morphological study of organic thin film solar cells based on calcium / alumi‐

[6] Jeranko T, Tributsch H, Sariciftci N S and Hummelen J C Patterns of efficiency and degradation of composite polymer solar cells Sol Energ Mat Sol C 2004;83(2-3)

[7] Kuwabara T, Nakayama T, Uozumi K, Yamaguchi T and Takahashi K Highly dura‐ ble inverted-type organic solar cell using amorphous titanium oxide as electron col‐ lection electrode inserted between ITO and organic layer Sol Energ Mat Sol C

Heidkamp) who have contributed some experimental results to this chapter.

Department of Engineering Physics, McMaster University, Hamilton, Canada

Appl Phys Lett 1997;70(25) 3470-2.

ces Sol Energ Mat Sol C 2008;92(7) 746-52.

nium cathode material Chem Phys Lett 2008;461(1-3) 77-81.

the quest for greater performance.

**Acknowledgements**

**Author details**

Ayse Turak

**References**

247-62.

2008;92(11) 1476-82.

**Figure 6.** Scheme for ITO surface modified by covalently bound HTL materials. Reprinted with permission from [163]. Copyright 2005 American Chemical Society.

### **7. Using dewetting as an advantage**

Though dewetting of the active layer is generally undesirable and implicated as a main mechanism in device failure, some groups have harnessed the effect to produce novel device architectures. As many of the films are metastable, they have a natural tendency to dewet into a stable equilibrium form, which can then be used as the starting point for device fabri‐ cation. Developing methods of tuning film morphology and rate of dewetting through total coverage, surface templating and temperature control are of significant interest in forming controlled organic nanostructures. Recently, we [98] used the strong island growth and dewetting tendency of DIP on ITO to produce columnar structures necessary for an interdi‐ gitated ideal bulk heterojunction solar cell, with four orders of magnitude improvement in the device efficiency. Ryu et al. [287] used the energy difference between PEDOT and PFO to form nanoscale dewetted islands of PEDOT at the internal interface in tandem polymer OLEDs. Wang et al. [288] produced sub-micrometer channel OFETs (field effect transistors) using SAM patterned SiO2 to force PEDOT:PSS dewetting. With the PEDOT:PSS acting as the source and drain electrodes, a submicrometer channel of F8T2 polymer was formed. Be‐ nor et al. [289] was able to produce resist patterns of PMMA or PEDOT using selective wet‐ ting on hydrophobic and hydrophilic SAM patterns. Deposition on these patterned substrates lead to the formation of mesoscale patterns for radio frequency ID tags or thin film transistor electrodes. Chen et al. [104] used a similar patterning motif with SAMs that were selectively wet by the two components to encourage phase separation of P3HT:PCBM into an interdigitated columnar structure. Most recently, Harirchian-Saei et al. [290] used the phase separation of PS and PMMA on OTS striped patterns to deliver a periodic array of CdS nanoparticles. By dissolution of the nanoparticle into only one component; then taking advantage of the selective wetting, a templated nanoparticle array was produced.

#### **8. Summary**

This chapter represents a comprehensive summary of the state of the art with regards to in‐ terfacial wetting stability in organic light emitting diodes and organic photovoltaics. Though the challenges are slightly different, both types of optoelectronic devices are heavily influ‐ enced by the stability of the interfaces with the bottom side contact. As organic optoelectron‐ ic technologies mature, the metastability of interfaces becomes more and more significant in the quest for greater performance.
