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Organofunctional silane modification of aluminumdoped zinc oxide surfaces as a route to stabilization Rachael Matthews, Emily Glasser, Samuel Sprawls, Roger H. French, Tim Peshek, Emily Pentzer, and Ina Martin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 01 May 2017 Downloaded from http://pubs.acs.org on May 3, 2017

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ACS Applied Materials & Interfaces

Organofunctional silane modification of aluminumdoped zinc oxide surfaces as a route to stabilization Rachael Matthews,a Emily Glasser,a Samuel C. Sprawls,b Roger H. French,b,c,d,e Timothy J. Peshek,c Emily Pentzer,a,d,* Ina T. Martinb,c,* Case Western Reserve University 10900 Euclid Ave. Cleveland, OH 44106 a

Department of Chemistry b

c

d

Department of Physics

Department of Materials Science and Engineering

Department of Macromolecular Science and Engineering e

Department of Biomedical Engineering .

KEYWORDS: Surface modification, transparent conductive oxide, aluminum-doped zinc oxide, degradation, electron mobility.

ABSTRACT. Aluminum-doped zinc oxide (AZO) is a low-temperature processed transparent conductive oxide (TCO) made of earth abundant elements; its applications are currently limited by instability to heat, moisture, and acidic conditions. We demonstrate that the application of an

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organofunctional silane modifier mitigates AZO degradation, and explore the interplay between performance and material composition and morphology. Specifically, we evaluate degradation of bare AZO and APTES (3-aminopropyltriethoxysilane)-modified AZO in response to damp heat (DH, 85 °C, 85 % relative humidity) exposure over 1000 h, then demonstrate how surface modification impacts changes in electrical and optical properties, and chemical composition in one of the most thorough studies to date. Hall measurements show that the resistivity of AZO increases due to a decrease in electron mobility, with no commensurate change in carrier concentration. APTES decelerates this electrical degradation, without affecting AZO optical properties. Percent transmission and yellowness index of an ensemble of bare and modified AZO are stable upon DH exposure, but haze increases slightly for a discrete sample of modified AZO. Atomic force microscopy (AFM) and optical profilometer (OP) measurements do not show evidence of pitting or delamination after 1000 h DH exposure, but indicate a slight increase in surface roughness on both the nanometer and micron length scales. X-ray photoelectron spectroscopy data (XPS) reveal that the surface composition of bare and silanized AZO is stable over this time frame; oxygen vacancies, as measured by XPS, are also stable with DH exposure, which, together with transmission and Hall measurements, indicate stable carrier concentrations. However, after 1500 h of DH exposure, only bare AZO shows signs of catastrophic destruction. Comparison of the data presented herein to previous reports indicates that the initial AZO composition and microstructure dictate the degradation profile. This work demonstrates that surface modification slows the bulk degradation of AZO, and provides insight into how the material can be more widely used as a transparent electrode in the next generation of optoelectronic devices.


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Introduction. Transparent conductive oxides (TCOs), a subset of metal oxides, are of immense societal value due to their high transparency and conductivity. TCOs are critical components of advanced optoelectronic and photonic devices including photovoltaics (PV), light emitting diodes (LEDs), gas sensors, and smart windows.1-7 Whereas the most common TCOs used in these applications are indium-tin oxide (ITO)8 and fluorinated tin oxide (FTO), the former is unattractive due to the scarcity and expense of In, and the latter is limited by high processing temperature requirements (350 - 450 oC).2,9 An alternative TCO, aluminum-doped zinc oxide (AZO) consists of abundant materials that can be processed at low temperature. AZO deposition methods and parameters impact film thickness, composition and crystallinity, and therefore also the initial optical and electrical properties of the material.9,11-18 The surface of AZO is populated with hydroxyl groups (Fig. 1, left) and can adsorb and desorb atmospheric oxygen, carbon dioxide, and water.11-13 Currently, compared to ITO and FTO, AZO is relatively unstable and degrades in the presence of water, limiting its widespread use.10

Figure 1. Overview of the work presented herein: functionalization of AZO on glass with 3aminopropyltriethoxysilane (APTES) and exposure of both bare AZO and AZO/APTES to damp heat (DH), followed by characterization of the optical and electronic properties, composition, and morphology.


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Numerous studies have focused on optimizing AZO performance by varying annealing temperature,11,19 sputtering conditions,19,-21 and doping content.22,23 Although fewer studies have focused on the stability of AZO performance in response to external factors, damp heat (DH) is a stressor that has been shown to primarily affect AZO conductivity, but not its optical properties.24,25 Additionally, the thickness and microstructure of AZO films, as dictated by deposition method and parameters, influence the degradation pathways. Tohsophon et al. reported that compact films with large grain sizes (> 40 nm) and film thicknesses of >700 nm have more uniform electrical properties than thinner films (< 690 nm).24 Similar trends were observed by others, showing that thicker films with large grain sizes had subdued degradation of electrical properties.26,27 AZO films are expected to degrade from the surface, with chemisorption of water at grain boundaries and/or surface defects that introduce trap states to enhance scattering of free carriers and induce charge-carrier trapping, thus reducing carrier concentration and carrier mobility.8,24,28 Moreover, acidic conditions (pH < 6) substantially enhance water-induced degradation of AZO.29 As such, chemical reactions initiated on the surface and at grain boundaries can change the binding environment of component atoms and degrade the bulk properties of AZO thin films. Various small molecules and polymers have been used to modify TCO surfaces for specific applications, such as bridging the surface energy of an inorganic TCO with organic active layers in organic PV and LED devices.31-35 However, modifiers that optimize initial device performance can be detrimental to long term performance; for example, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), a polymer commonly used as a hole transport layer in organic electronics, is acidic and corrodes both ITO and AZO.36 Organofunctional silanes are one class of compounds commonly used as surface modifiers for TCOs; simple hydroxylation of


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the AZO surface via ozone cleaning renders the material ideal for silanization (Fig. 1, right). A scattering of reports suggests that silane modification is actually beneficial to TCO stability. For example, addition of an allyltriethoxysilane (ATES) layer between ITO and PEDOT:PSS prevented corrosion of ITO on a short time scale, without detriment to device performance.36 Further, APTES (3-aminopropyltriethoxysilane) modification of ZnO improved the uniformity of device performance in inverted architecture organic PV devices, giving higher average efficiency, and lower standard deviation.37 The effect of modifiers on initial device performance is well known, however a fundamental understanding of their influence on AZO stability and degradation has yet to be established. We propose that surface modification of AZO is an ideal and easily accessible route to mitigate bulk degradation, as such treatment would “cap” the moieties most sensitive towards disadvantageous reactions. Herein we evaluate the degradation of bare AZO and APTES modified AZO (AZO/APTES) using the standard IEC 61646 and 61215 protocol of 1000 h exposure to damp heat (DH, 85 °C and 85% relative humidity). Select samples were further exposed to DH for a total of 1500 h. The electrical (Hall mobility, carrier concentration, resistivity) and optical (percent transmittance, yellowness index, % haze) properties, and morphology/composition (surface roughness, chemical composition, binding environment) of the samples were analyzed. Significant findings include that the changes in resistivity and Hall mobility of AZO/APTES from 1000 h of DH exposure are mitigated compared to bare AZO, and that carrier concentration does not significantly change for either system. Further and more severe differences in the stability of the electrical properties and morphology are observed when comparing bare and modified AZO after 1500 h of DH exposure. Taken together, these data support that electrical degradation of AZO is slowed when surface functionalities attributed to degradation are


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covalently passivated, and help establish a foundation for characterizing, understanding, and ultimately preventing AZO degradation. Experimental Sample preparation. AZO on glass was purchased from Zhuhai Kaivo Electronic Components Co., Ltd and consisted of ultra-clear float glass sputtered with AZO (800-850 nm thickness, 3% wt Al). Specified values provided by the manufacturer are sheet resistance 10%, percent transmission >80% from 400-1000 nm, and haze 1000 nm) indicates that the carrier concentration is constant in the bulk of the film, as this region is sensitive to free carrier absorption associated with the electron concentration.58 These findings are consistent with the electrical property measurements described above (Fig. 2).44,45

Figure 3. Optical properties: A) Broad spectrum UV-Vis-NIR percent transmission spectra (%T) of bare AZO before and after DH; B) %T for AZO/APTES before silanization, and before and after DH; C) % haze (black squares) and yellowness index (YI, red triangles) of ensemble of samples; D) % haze (black squares) and yellowness index (YI, red triangles) of discrete samples. Bare AZO is represented by solid symbols and AZO/APTES is represented by open symbols. Two additional metrics of optical performance for TCOs are yellowness index (YI) and % haze. Increases in YI59 and % haze60 are generally undesirable as they reduce the amount of light


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reaching the absorber in a solar cell, or emitting from a display; moreover, YI has been reported as a surface sensitive measurement that can show degradation before changes in %T.37,46 The % haze and YI for bare AZO and AZO/APTES as a function of DH exposure time are shown in Fig. 3C for the ensemble of samples (different samples measured at each time point) and Fig. 3D (a discrete sample measured at each time point). Fig. 3C shows that the average % haze of bare AZO samples at time zero is 0.8 ± 0.4, slightly less than the % haze for AZO/APTES at time zero, 2.3 ± 0.8; YI does not significantly change upon silanization. From measurements of the ensemble of samples, DH exposure led to variation of % haze, but no clear trend. To exclude sample-to-sample variation, a discrete sample was tracked over the course of ~1000 h of DH exposure. Whereas YI remained consistent, % haze increased slightly for both bare AZO and AZO/APTES (Fig. 3D, black lines), from 0.8 to 2.6 and 2.1 to 5.1, respectively. Increase in % haze may be due to formation of aggregates or particles, and slight roughening of the surface (see OP data below), and could be related to reaction of surface bound APTES molecules. Morphology/Composition. To investigate the surface morphology and surface roughness of bare AZO and AZO/APTES, both AFM and OP were used. AFM measurements of 5 µm x 5 µm sample areas were collected to show changes in fine structure, which can signal initiation and propagation of physical degradation.49 In compliment, each OP image covers 180 µm x 130 µm, and thus can evaluate changes over a larger area. The measured root-mean-square (RRMS) and average roughness (Ra) values, as determined by AFM, were consistent with previous reports for bare AZO (Fig. 4A, raw data in Table S2).21,30,50,51 Addition of APTES resulted in a 25% increase in RRMS (compare Fig. S2), in agreement with the addition of a disorganized thin film.41 Fig. 4A and 4B show that bare AZO has a nominal change (4%) in RRMS after DH exposure for ~1000 h, and Fig. 4C and 4D show a similar trend for AZO/APTES (5% increase in RRMS). The


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OP measurements give RRMS values lower than those observed by AFM for all samples before DH exposure (see Table S2 and S3), due to the resolution limitations of this optical technique. By OP, RRMS of bare AZO was 1.2 ± 0.1 nm, and increased slightly after APTES deposition52 (Fig. S1 and 4G), consistent with AFM data. Exposure to DH for ~1000 h resulted in the appearance of elevated features for both bare AZO and AZO/APTES (Fig. 4F and 4H), leading to increased RRMS compared to the pre-exposed samples. These data reveal only minor changes in the surface and the RRMS of all samples remained less than 85 nm after ~1000 h DH exposure. The boxes drawn on the OP images of Fig. 4F and 4H contain areas of the film that have RRMS values similar to the non-DH treated films, highlighting the heterogeneity observed on a larger scale. These results are in contrast to previous reports illustrating catastrophic changes in morphology, including delamination and deep ditches in the material.52 The differences are likely due to contrasting thickness and grain size of AZO (previous studies used AZO films 100-200 nm in thickness compared to the AZO studied here, ~800 nm).


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Figure 4. AFM measurements of bare AZO A) before DH exposure; and B) after DH exposure for 980 h; and AZO/APTES C) before DH exposure; and D) after DH exposure for 910 h; OP measurements of bare AZO E) before DH exposure; and F) after DH exposure for 980 h; and AZO/APTES G) before DH exposure; and H) after DH exposure for 910 h. Changes in the elemental composition and chemical binding environment of the surface atoms of bare AZO and AZO/APTES were determined by XPS measurements. A 90° photoelectron takeoff angle (TOA) relative to the substrate surface was used for all measurements, which samples the top ~10 nm of the surface and therefore ensures evaluation of AZO under the APTES layer (< 5 nm thick).53 Survey spectra (0 to 1200 eV) were used to identify the component elements (C, O, Al, Zn, Si, N), and high-resolution spectra of these elements were collected for more accurate quantification of the elemental composition, and identification of their binding environments. XPS spectra of bare, ozone cleaned AZO showed the presence Zn (43 ± 3%), Al (0.5 ± 0.1%), and O (47 ± 1%) as expected for AZO (average of 9 high-resolution spectra). Adventitious C (8 ± 3%), and trace Si (~ 1%) and N (~ 0.6%) were also observed (Fig. S3). While C and N are likely to due to environmental contamination, the presence of Si in bare AZO can be attributed to contamination from preparation, and not the glass substrate, as AZO is 800 nm thick and XPS cannot evaluate to these depths. Compared to bare AZO, XPS spectra of AZO/APTES showed a decrease in Zn (13 ± 3%), Al (0.4 ± 0.1%), and O (37 ± 2%), and an increase in the elemental constituents of APTES: C (37 ± 6%), Si (6 ± 1%), and N (6 ± 1%) (average and standard deviation of six high-resolution spectra). Survey spectra showing qualitative comparisons are presented Fig. S3. Furthermore, high-resolution Si 2p spectra of AZO/APTES showed a single peak centered at 102.0 ± 0.2 eV (Fig. S4), consistent with the Si(O)3 binding environment of the APTES molecule.41 DH exposure did not affect the Al, Zn, and O


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elemental composition of bare AZO or AZO/APTES, up to ~1000 h (Fig. S5), suggesting that any adsorption of oxygen-containing small molecules such as O2, H2O, and CO2 is less than the variation between samples (~10 relative %). Furthermore, as grain boundaries comprise only a small portion of AZO volume, adsorption of small molecules and reaction of AZO in these spaces may not lead to detectable changes in the XPS spectra, given the sensitivity of the instrument (0.1 atomic %). Of note, for AZO/APTES, % Si remains constant yet % N slightly decreased during the first ~200 h of DH exposure, suggesting that the C-Si, C-C, or C-N bond may hydrolyze.

Figure 5. High-resolution O 1s spectra of A) bare AZO (black line) overlaid with AZO/APTES (pink line); O1s spectra for representative samples of B) bare AZO at 0 h; C) bare AZO after 980 h of DH exposure; D) AZO/APTES at 0 h; E) AZO/APTES after 880 h DH exposure. B-E are fit


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with the component peaks outlined in the table; the solid red line is the overall fit, and the dashed red line is the residual. Deconvolution of high-resolution XPS O 1s spectra was used to evaluate the binding environments of bare AZO and AZO/APTES, to provide further insight into the surface chemistry of the materials. Specifically, this analysis was used to evaluate changes in the features that are sensitive to oxygen vacancies, which have previously been related to the AZO conductivity.8,54 Fig. 5A shows an overlay of the high-resolution O 1s spectra of bare AZO and AZO/APTES and reveals that silanization leads to broadening of the O 1s peak, which can be attributed to the unique oxide character of the APTES bound to AZO. The O 1s spectrum of bare AZO was fit with three component peaks (Fig. 5B): OI is attributed to O2- anions in the wurtzite ZnO lattice (530.4 eV), OII is associated with O2- anions in the oxygen-deficient regions of the film (531.5 eV), and OIII is attributed to adsorbed oxygen-containing molecules (532.6 eV, O2, H2O).8,54,55,30 After 980 h of DH exposure, the O 1s spectrum of bare AZO did not significantly change (Fig. 5C), and the ratio of OI:OII:OIII remained constant. The presence of APTES complicates the O 1s spectral deconvolution, requiring fixing the position and FWHM values of the OI, OII, and OIII peaks, and the addition of two peaks: O* at 531.9 eV attributed to the O-Si bond from APTES,52 and O** at 529.0 eV. As O** comprises only ~3% of the total O 1s envelope and has a low binding energy (i.e., C=O), it may be attributed CO2 which has reacted with the amine of APTES.41,56,57 After 880 h of DH exposure, the O 1s spectrum of AZO/APTES does not significantly change (Fig. 5E), further supporting that the silane bond of APTES remains intact.


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Figure 6. Dependence of oxygen composition (as determined by fits of high-resolution O 1s spectra) on DH exposure time for A) bare AZO; and B) AZO/APTES. The percent composition of OI, OII, OIII, O*, and O** for the ensemble of samples of bare AZO and AZO/APTES are plotted as a function of DH exposure in Fig. 6. As OIII is due to surface adsorbed oxygen-containing species and can vary, the relationship between OI and OII can be used to evaluate changes in the binding environment of the oxygen atoms within the sample, calculated by the following equation: OII/(OI+OII). As can be seen in Fig. 6A and 6B, this ratio is independent of DH exposure time for both bare AZO and AZO/APTES, although sample-to-sample variations are apparent (0.32 ± 0.06 for all of the bare AZO samples, and 0.28 ± 0.03 for all of the AZO/APTES samples). Averages of the position and contribution of the O


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1s components for all bare AZO and AZO/APTES films exposed to up to 1000 h of DH are listed in Tables S4 and S5. Extended DH Exposure. Traditionally, the IEC 61646 protocol (1000 h in DH) is used to evaluate the lifetime performance of PV modules; as a result, it is also used to test degradation of constituent layers, including TCOs.27,42,52 To determine the limits of AZO stability and impact of APTES modification, we further evaluated the impact of 1500 h total DH exposure on select samples of bare AZO and AZO/APTES. Electrical data shown in Fig. S6 illustrate distinct differences between the silanized and bare samples from 1000 to 1500 h DH exposure; bare AZO has a sharp increase in resistivity and decrease in carrier concentration, far outside the trend line established in the first 1000 h. In contrast, AZO/APTES showed a decreased Hall mobility and increased resistivity within the same trend observed for the first 1000 h, and no change in carrier concentration over the same period of time. OP data show distinct differences in the surface characteristics of the two samples (Fig. S7). After 1500 h, bare AZO shows conventional signs of delamination including pits and holes in the surface, which support disruption of electrical pathways and changes in the properties discussed above. In contrast, after the extended DH exposure AZO/APTES only shows elevated features, which could be attributed to further reaction (i.e., hydrolysis) of APTES.52 XPS data shown in Fig. S8 illustrate a significant change in the shape of O1s envelope after 1500 h of DH exposure for bare AZO; the envelope is significantly broader and shifted to a higher binding energy. Moreover, this spectrum cannot be fit with the method established for AZO, and the decrease in intensity at lower binding energy values is consistent with loss of OI and OII components, suggesting significant degradation of the wurtzite structure. Additionally, XPS data show that after 1500 h DH exposure, bare AZO had a significant reduction in % Zn (6.2 ± 0.8) and increase in % Al (4.1 ± 1.0) and % Si (9.5 ± 1.1). In


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contrast to bare AZO, after 1500 h DH exposure, O1s spectra of AZO/APTES is very similar to the unexposed samples, and no significant change in the elemental composition is observed (Fig. S5 and S8). Taken together, these data suggest that the surface modification of the AZO impedes DH-induced surface reactions that change atomic composition and morphology, and disrupt electrical pathways in bare AZO films.49,55 Alternatively, APTES modification protects the sample from substantial degradation over the course of 1500 h, possibly by passivation of reactive species at grain boundaries. This is supported by both the lack of significant morphological change, and the constant carrier concentration.52,58 Clearly, investigation of the stability and degradation of functionalized AZO at extended times, and specifically beyond industry standards, is required to establish the full potential of surface modification to extend lifetime of the material. Collective View. A review of the reports on AZO degradation indicates that film thickness, composition, and grain size affect both the initial electrical properties, and the nature of the degradation of these properties upon DH exposure.8,24-26,52 AZO films with small grain sizes may be attractive due to processing conditions that require little energy; however, such films have lower mobility and carrier concentration, as the probability of charge carriers scattering increases with increased volume of grain boundaries. Moreover, these small grain films undergo faster degradation due to increased adsorption and reaction of atmospheric species at the grain boundaries;8,22-24,49 these reactions lead to increased recombination of carriers (i.e., decreased carrier concentration) and increased potential barriers (i.e., decreased µ).24-26,42 The AZO used in this study is relatively thick (800 nm) and has lateral grain sizes on the order of 100-200 nm (Fig. S9). Addition of APTES mitigates degradation of the AZO electrical properties upon DH exposure, without adversely affecting the initial electrical performance. The


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bulk performance metrics can be measured by UV-Vis spectroscopy and Hall measurements, but these characterization techniques do not give insight into changes at the surface, which are responsible for the onset of degradation. The surface characterization technique of XPS can characterize the binding environment of the constituent atoms, specifically oxygen, as described above; this can then give insight into subsequent changes in bulk properties. For AZO films < 500 nm thick, decreases in the OII peak associated with oxygen vacancies have been correlated with decreased carrier concentration, although such studies are not comprehensive. For example, Chen et al. showed a doubling of the relative intensity of the OII contribution between asdeposited and annealed AZO (400 °C), for ~440 nm thick films, but did not report electrical measurements (although annealing is reported to decrease resistivity).54 Likewise, Liu et. al showed that for 200 nm thick AZO, carrier concentration decreased ~4 x’s and was accompanied by a twofold reduction in the OII/(OI + OII) ratio; however, variations in the measured values were not reported and a standard protocol for DH exposure was not followed. In the study presented herein, the XPS data show that 800 nm thick AZO has a chemically stable surface, with a consistency in atomic composition and the OII component of the O 1s spectrum corresponding to stable carrier concentrations over 1000 h DH exposure, as consistent with the Hall measurements. Further, these XPS analyses demonstrate that careful fitting of the O 1s spectra allow differentiation between components related to carrier concentration and conductivity (OII), and components related to APTES attachment. As the chemical reactions of molecules adsorbed at AZO grains can be challenging to parse due to surface heterogeneity and concurrent reactions, XPS can be used to evaluate subtle changes and extended time studies can be used to demonstrate correlations to bulk measurements.


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Conclusion. In summary, we have demonstrated that APTES modification of ~800 nm thick AZO films significantly impedes the electrical degradation of the material caused by DH exposure, without significantly affecting the initial optical, electrical, or structural properties of the AZO films. Upon 1000 h of DH exposure, resistivity of both systems increased and can be attributed only to decreased mobility, as carrier concentration was consistent. We observed that APTES modification slowed the increase in AZO resistivity over 1000 h of DH exposure; however, the protective nature of APTES modification became critical after 1500 h (500 h beyond the standard protocol). At this extended exposure time, macroscopic degradation was observed only for bare AZO including pitting and delamination and was accompanied by a drastic increase in resistivity and decrease in carrier concentration. This study suggests that covalent passivation of AZO surface sites by silanization essentially “caps” reactive moieties, thereby improving the stability of the material. Further studies will give insight into how the type of functionalization (both binding group to AZO and pendant substitution), as well as the source of the AZO (i.e., film thickness and grain sizes) impact stability, which will give guiding principles for the incorporation of AZO as a transparent electrode in future device designs. ASSOCIATED CONTENT Supporting Information. The following files are available free of charge: contact angle, AFM, and OP measurements, XRD, XPS, and characterization of materials after 1500 h DH exposure. AUTHOR INFORMATION Corresponding Author *[email protected], *[email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The authors acknowledge the funding from DOE award DE-EE0007360 and CWRU College of Arts and Sciences. E. G. thanks the CWRU Summer Undergraduate Research in Energy and Sustainability (SURES) program. ACKNOWLEDGMENT The authors also thank the Materials for Opto/electronics Research and Education (MORE) Center (Ohio Third Frontier grant TECH 09-021), Solar Durability and Lifetime Extension (SDLE) Research Center (Ohio Third Frontier, Wright Project Program Award tech award 12004), the Swagelok Center for Surface Analysis of Materials (SCSAM) through the CWRU college of engineering, and Jon Mackey at the Glenn Research Center at Lewis Field, NASA for use of the Hall measurement system. ABBREVIATIONS AFM = Atomic Force Microscopy; XPS = X-ray photoelectron spectroscopy; OP = optical profilometry, AZO = aluminum doped zinc oxide, DH = damp heat, XRD = X-ray diffraction, RRMS = root mean square, RA = roughness average, TCO = Transparent conductive oxide, APTES = 3-aminopropyltriethoxysilane, YI = yellowness index, IEC = International Electrochemical Commission. REFERENCES


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