Fabrication, Mechanisms, and Properties of High-Performance

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Functional Nanostructured Materials (including low-D carbon)

Fabrication, Mechanisms, and Properties of HighPerformance Flexible Transparent Conductive Gas-Barrier Films Based on Ag Nanowires and Atomic Layer Deposition Dung-Yue Su, Che-Chen Hsu, Wen-Hsuan Lai, and Feng-Yu Tsai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09772 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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ACS Applied Materials & Interfaces

Fabrication, Mechanisms, and Properties of High-Performance Flexible Transparent Conductive Gas-Barrier Films Based on Ag Nanowires and Atomic Layer Deposition Dung-Yue Su, Che-Chen Hsu, Wen-Hsuan Lai and Feng-Yu Tsai* Department of Materials Science and Engineering, National Taiwan University, Taipei 10617, Taiwan * Corresponding author E-mail: [email protected]

Keywords: atomic layer deposition, silver nanowires, transparent conductive oxides, gas barriers, nanocomposites

Abstract

Thin films of Ag nanowires (NWs) offer many advantages as transparent electrodes for flexible electronics, but their applications are hindered by issues including poor stability/durability of Ag

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NWs, high processing temperatures, heterogeneity of surfaces, and lack of gas-barrier function. This study reports novel mechanisms through which a conductive Hf:ZnO (HZO) film by atomic layer deposition (ALD) can be integrated with a sprayed Ag NWs film to address the issues of Ag NWs. Firstly, the ALD surface reactions can induce fusing of the Ag NWs into a connected network without the need for a thermal-sintering process. Secondly, the ALD process can in situ functionalize the Ag NWs to yield defect-free (in terms of blocking gas permation) coverage of the ALD HZO over the entire nanowire surfaces, which also enhances the ALD-induced fusing of Ag NWs. The composite HZO/Ag NWs films exhibit low sheet resistance (15 Ω sq-1), low water vapor transmission rate (WVTR) (5.1 × 10-6 g m-2 day-1), high optical transmission (92%), excellent flexibility (12.5 mm bending radius), high stability/durability (against an extensive set of degradation modes and photolithographic patterning processes), and low processing temperature (90 C); and can be used in perovskite solar cells to obtain high power conversion efficiency (14.46%).

1.Introduction Flexible transparent conducting films (TCF) are an essential component of flexible and wearable electronics. The mainstream TCF material for rigid applications, indium tin oxide (ITO), offers little mechanical flexibility, which coupled with other concerns such as the scarcity of indium and high deposition temperatures have led to earnest efforts in developing alternative TCF with adequate flexibility. On this front, thin films consisting of randomly connected networks of silver nanowires (Ag NWs) have emerged as the most promising flexible TCF, which thanks to the high conductivity and ductility of Ag and the 1-D geometry of the NW have shown far superior


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conductivity, transparency, and flexibility to those of ITO and other alternatives.1–3 Additionally, Ag NWs are relatively inexpensive and compatible with large-area coating and roll-to-toll processes. Despite its advantages, the technology of Ag NWs TCF still faces several major obstacles that must be overcome for it to be practically viable. Firstly, Ag NWs have poor stability due to their large surface-to-volume ratio, which causes rapid corrosion in common usage environments—including the ambient air—and melting/coalescing under electrical loading.4,5 Ag NWs also lack mechanical durability as they can easily disintegrate or detach from their substrates upon abrasion or deformation.6,7 As a result of their poor stability/durability, Ag NWs are incompatible with photolithographic processes, making fine patterning of Ag NWs films a difficult challenge. Secondly, the area of an Ag NWs film is by design populated by open spaces, which sacrifice the surface homogeneity of an ideal electrode while depriving it of any gas-barrier function, an important attribute for shielding plastics-based flexible electronics from moisture and oxygen from the surroundings.

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Thirdly, Ag NWs are typically surface-functionalized with

surfactants—most commonly polyvinylpyrrolidone (PVP)—to aid their synthesis and dispersion in coating solutions. When Ag NWs are cast into a film, such surface-adsorbed surfactants can block the connection of individual Ag NWs, and therefore a thermal sintering process is often required to remove the surfactants to allow fusing of individual Ag NWs into a connected network. The thermal process can be incompatible with many sensitive materials such as plastic substrates. Various additives to Ag NWs have been shown to effectively improve upon some of these shortcomings, including carbon nanotube and graphene10,11, precursors for the sol-gel synthesis of metal oxides12,13, conducting and insulating polymers, and physical-vapor-deposited metal oxides14; however, compromises such as increased process complexity and deviations from Ag NWs’ optimal properties often accompany improvements, and a comprehensive solution has not


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been attainable. Consequently, a method that can simultaneously provide the functions of passivation, protection, surface homogeneity, gas-permeation barrier, and junction-fusing to the Ag NWs film while retaining its flexibility and processability will provide a major breakthrough to the development of the Ag NWs TCF technologies. This study attempts to devise such a comprehensive solution by utilizing an atomic-layerdeposited (ALD) transparent conductive Hf:ZnO (HZO) film to encapsulate ultrasonically sprayed Ag NWs into a composite HZO/Ag NWs film. ALD offers the unique capabilities of producing defect-free (in terms of blocking gas permeation) and conformal ultra-thin films which meet the exceptionally stringent encapsulation requirement—water vapor transmission rate (WVTR) ~10-6 g/m2 day—for organic electronics such as organic light-emitting diodes (OLED) and solar cells (OSC). 15,16 The high gas-barrier performance combined with ALD’s low processing temperatures and compatibility with non-vacuum, roll-to-roll processes have made ALD the method of choice for encapsulating organic electronics. 17,18 Likewise for Ag NWs films, encapsulation with an ALD insulating or conducting oxide film such as Al2O319, TiO220, ZnO21 and Al:ZnO22 has been shown to significantly improve the stability of Ag NWs films. There are, however, several key issues that remain unaddressed regarding this approach: (1) ALD’s conformal coating capability can block junctions of Ag NWs from fusing, and therefore high-temperature sintering of the Ag NWs before ALD is still required to establish a connected Ag NWs network20,22,23; 2) the Ag NWs surface— consisting of bare Ag and surfactants—lacks ALD-friendly functional groups, which can hinder the nucleation of the ALD film, making defect-free quality and adequate gas barrier performance difficult to obtain in the ALD/Ag NWs composite. In this study, through systematic analysis we identified a novel mechanism where ALD reactions on Ag NWs surface induced fusing of Ag NWs junctions without the need for a thermal driving force. We observed auxiliary mechanisms


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where the ALD HZO film contributed to the electrical conductivity of the HZO/Ag NWs composite film through bridging and connecting the Ag NWs. We discovered that the Ag NWs could be in-situ surface-functionalized during ALD with the use of a proper precursor to create ALD-friendly functional groups on Ag NWs, which was found to not only yield defect-free HZO coating on Ag NWs but also enhance the ALD-induced fusing of the Ag NWs. We engineered our previously reported ALD mixed-doping HZO process to minimize the deposition temperature and the required HZO thickness for adequate encapsulation of Ag NWs. Applications of our findings led to demonstration for the first time—to the best of our knowledge—of Ag NWs-based films with simultaneously low sheet resistance (15 Ω sq-1), low water vapor transmission rate (WVTR) (5.1 × 10-6 g m-2 day-1), high optical transmission (92%), excellent flexibility ( 12.5 mm bending radius for 100 cycles), high stability/durability (against an extensive set of degradation modes including photolithographic patterning processes), and low processing temperature (90 C). Lastly, we verified the utility of the HZO/Ag NWs film by fabricating highly efficient (up to 14.46%) perovskite solar cell (PSC) devices employing the HZO/Ag NWs film as a combination of electrode and electron-transporting layer (ETL).

2. Result and Discussion The content of this section is arranged as follows. Firstly, the individual properties of the ultrasonically sprayed Ag NWs film and the ALD HZO film are presented to serve as basis for the study of the HZO/Ag NWs composite film. Secondly, the in situ functionalization treatment is shown to be necessary for obtaining synergetic combination of the Ag NWs and the ALD HZO films. Thirdly, conductivity of ALD/Ag NWs composite films with various conductivity of the


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ALD layer—including an insulating Al2O3 one—is presented to elucidate the mechanisms governing the conductivity of the composite films. Lastly, the HZO/Ag NWs film’s stability/durability and its utility as an ETL/electrode for PSCs are examined.

2.1 Ultrasonically Sprayed Ag NWs Films The as-sprayed Ag NWs film had a high sheet resistance (Rs) of 10k Ω sq-1 due to the residual PVP surfactant adsorbed on the Ag NWs surface—a necessary ingredient in the synthesis and dispersion of Ag NWs—blocking the contact of the Ag NWs.12 Consistent with previous reports, the Ag NWs film required a high-temperature sintering step to overcome the blocking effect of the adsorbed PVP to fuse the Ag NWs into a connected network.4 We used 200 C and 30 min as sintering condition, which resulted in an Rs of 26 Ω sq-1, which served as a baseline for the nonsintered, ALD-coated Ag NWs film to compare to. Thanks to the high aspect ratio of the Ag NWs and the uniform spraying, as evidenced in the SEM image of the Ag NWs film (Figure S1, Supporting Information), the Ag NWs films showed excellent optical transparency of 94.4 % transmittance across the visible wavelengths (Figure S2a, Supporting Information ).

2.2 ALD HZO Films We obtained ALD HZO films that met the requirements of flexible Ag NW/HZO composite films—adequate gas-barrier capability (WVTR~10-6 g/m2 day) at minimal thickness (to minimize optical absorption and maximize mechanical flexibility of HZO)—by modifying our previously established mixed-doping ALD HZO technique 17 into a low-temperature (90 C) process, which


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also extended the compatibility of the process to more temperature-sensitive substrates such as poly(ethylene terephthalate) (PET), as shown in Figure 1a. Our previous process used a 150 C deposition temperature, which with the help of the defect-suppressing effect of the mixed-doping technique was capable of producing HZO films on PEN substrates with WVTR of ~10-6 g/m2 day at a 184 nm thickness, but at the small thickness (45 nm tested here) desired for the Ag NW/HZO composite, the WVTR became 310-3 g/m2 day. Moreover, the 150 C process temperature was not compatible with the PET substrate used here. Lowering the ALD temperature to 90 C addressed both issues, producing significantly lowered WVTR of ~10-6 g/m2 day at film thicknesses of 45, 33, and 30 nm on PET substrates (Figure 1a). The beneficial effect of lowering the ALD temperature on the WVTR was attributable to lowered ZnO crystallinity, which reduced defects associated with crystal grain boundaries, as shown in our previous works.15 Advantageously, the lower crystallinity also led to excellent mechanical flexibility as described in Section 2.5. The critical thickness, defined as the minimal thickness above which gas permeability becomes constant, 15,24 was determined to be about 33 nm for the 90 C process, which presented an optimum for the Ag NW/HZO composite and was thus used for fabricating TCBs in the following. The optical transmittance of the 33 nm HZO was 97.3% across the visible spectrum, as shown in Figure S2b of Supporting Information. Although the sheet resistance of the 90 C HZO film (54000 Ω sq-1) was much higher than that of the 150 C HZO (230 Ω sq-1), it was found to have minimal effect on the sheet resistance of the resultant Ag NWs/HZO TCB, as discussed in Section 2.4.

2.3 In situ functionalization of Ag NWs for fabricating HZO/Ag NWs Films


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We found that successful integration of the as-sprayed (not sintered) Ag NWs film with the ALD HZO film into a composite film required an in situ functionalization of the Ag NWs film to create ALD-friendly functional groups on its surface, for which we demonstrated that an addition of a first H2O2 half cycle into the ALD HZO process was effective. Figure 1a and 1b present the WVTR and sheet resistance (Rs), respectively, of the HZO/Ag NWs films fabricated with and without the in situ H2O2 functionalization. Direct combination of the ALD HZO and Ag NWs films without the functionalization treatment resulted in much higher WVTR than the ALD HZO on its own and higher Rs than the Ag NWs film on its own. Increasing the thickness of the HZO film from the above-determined 33 nm optimum brought down the WVTR and Rs of the HZO/Ag NWs film, but still fell far short of approaching the stand-alone values. SEM observation (Figure S3a, Supporting Information ), however, showed a defect-less conformal HZO coating on the Ag NWs, indicating that the WVTR and Rs of the HZO/Ag NWs were limited by subtle, molecularlevel defects. One probable source of such molecular-level defects was the lack of –OH as ALD chemisorption sites on the Ag surface as well as on the adsorbed PVP molecules, which hindered the nucleation of HZO. We first tested a gentle oxygen plasma pretreatment of the Ag NWs film as a means to create –OH groups on the Ag and PVP surfaces, but it proved to be too aggressive for the exposed Ag surface, making the Ag NWs film insultaing even after only 1 s of treatment at a low power of 6.8 W. Consequently, we devised the more gentle oxidative functionalization method involving a brief pulse of H2O2 vapor to the Ag NWs film in the beginning of the ALD process, which was able to keep the conductivity of the film intact. Water contact angle (WCA) measurements (Figure S4 of SI) showed markedly increased hydrophilicity of the Ag surface by the H2O2 functionalization treatements, with WCA reduced from 80º of the prinstine Ag surface to 67º of the H2O2-treated surface, indicating the creation of hydroxyl-associated functional


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groups.25,26 In addition to functionalizing the exposed Ag surface, the H2O2 treatment also created –OH groups on the adsorbed PVP molecules, as revealed by FTIR analysis (Figure 2), where a – OH stretching band centered at ~3422 cm-1 appeared and the carbonyl stretching peak of untreated PVP at 1691 cm-1 shifted to 1655 cm-1 due to hydrogen bonding between C=O and the created – OH 27 after the H2O2 functionalization. With both its Ag and PVP surfaces functionalized, the in situ H2O2-treated Ag NWs film indeed enhanced growth of the ALD HZO film, as the composite film started to exhibit WVTR and Rs equal to or better than the stand-alone values. Also, the critical thickness of the with-H2O2 HZO film on Ag NWs was the same as the 33 nm of the HZO film on PET, i.e. the WVTR and Rs reached steady-state values at HZO thicknesses  33 nm, indicating that the Ag NWs surface had been adequately functionalized into an ALD-friendly one. In terms of microstructure, the effects of the H2O2 functionalization treatment were not discernable under SEM (Figure S3, Supporting Information) which showed the same defect-less conformal HZO coating with or without the functionalization step, but they were observable in cross-sectional TEM of the HZO/Ag NWs film deposited on a Si wafer, as shown in Figure 3. Without the H2O2 step, the HZO showed a denser texture on the SiO2 surface of the Si substrate—which was populated with –OH groups—than on the Ag NWs; while the HZO texture became consistently dense on both SiO2 and Ag NWs with the H2O2 step. With the optimal 33 nm of HZO on H2O2-functionalized Ag NWs, the HZO/Ag NWs film obtained a lower Rs (15 Ω sq-1) than that of the sintered Ag NWs (26 Ω sq-1) (mechanisms to be detailed in Section 2.4), high optical transparency (92 % transmission across the visible wavelengths), and low WVTR (5.1 × 10-6 g m-2 day-1). These properties offer substantial


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improvements over those of commercial ITO films coated on PET substrates, as shown in Table S1 and Figure S5 of Supporting Information.

2.4 Mechanisms governing conductivity of HZO/Ag NWs Films By analyzing the Rs values from various combinations of as-sprayed or sintered Ag NWs films with different types of ALD films, as shown in Table 1, we identified the following three mechanisms through which the ALD coating imparted high electrical conductivity to the assprayed Ag NWs film without needing a high-temperature sintering step: (1) inducing fusing of the Ag NWs that were in contact with each other by ALD-reaction-generated contracting forces; (2) separating and bridging the Ag NWs with small gaps among them—which were narrower than 2 times the ALD film thickness—through infiltration of ALD precursors; (3) connecting the Ag NWs that were widely separated. It should be noted that heating the as-sprayed Ag NWs film at the ALD temperature tested (90 C) for more than the duration of the ALD process was verified to not cause any detectable change in the Rs of the film, and therefore whatever changes caused by the ALD coating to the as-sprayed Ag NWs could be attributed solely to the deposition and not to the thermal effect of the process. The first mechanism was observed by using an ALD Al2O3 film instead of HZO to overcoat the as-sprayed Ag NWs film, which despite the insulating nature of Al2O3 still exhibited a drastic reduction in Rs, from 10k to 49 Ω sq-1. Given that the as-sprayed Ag NWs film must have its individual Ag wires connected to achieve a low Rs, and that the insulating Al2O3 film would prevent such connections if all surfaces of the individual Ag wires were coated by it, it stands to reason that (a) there were some contacting surfaces among the Ag NWs not coated by Al2O3, and


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(b) the ALD process in some way—other than thermal sintering—induced those contacting surfaces to fuse. Evidence to the reasoning above was found in the TEM images in Figure 3, where two adjacent Ag NWs were fused together with the ALD layer conformally surrounding their exposed surface but not their shared interface. We speculate that such adjacent Ag NWs were in contact as sprayed, and when ALD was applied, the ALD precursor molecules could chemisorb and grow only onto their exposed surfaces but not their contact interface. And as the stress in the growing ALD film was coupled to the Ag NWs surfaces through the formation of -M-O-M- (M  metal in the ALD metal oxide films) linkages across the two Ag NWs, it induced fusing of the uncoated interface, as schematically illustrated in Figure 4a. Ag NWs have been shown to fuse at room temperatures under an applied pressure of 10 MPa, 28 and therefore the typically > 100 MPa stress of ALD ZnO and other oxide films29–31 would easily suffice as the driving force for the observed fusing of Ag NWs. This was consistent with the lower Rs of the Al2O3/Ag NWs and the HZO/Ag NWs made with the H2O2 functionalization step (49 and 15 Ω sq-1 respectively) than those without it (80 and 48 Ω sq-1, respectively). As discussed earlier, the H2O2 functionalization created ALD-friendly –OH groups over the Ag NWs surface, which served as anchors for the –MO-M- linkage to more readily form across adjacent Ag NWs, thereby enabling more thorough coupling of ALD film stress to the Ag NWs. The fact that the insulating Al2O3 coating brought the Rs down to near the HZO/Ag NWs value indicated that the ALD-driven fusing of Ag NWs was the primary mechanism over the other two. The required ALD coating thickness to fully fulfill this mechanism could be estimated from the Rs versus HZO thickness results shown in Figure 1b, where it required ~26 nm without and ~10 nm with the H2O2 functionalization for the Rs of the HZO/Ag NWs to complete its drastic reduction. The smaller required thickness with the H2O2


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functionalization was again attributable to its resultant –OH groups on the Ag NWs surface, which facilitated the formation of –M-O-M- linkages across the Ag NWs to enhance stress-coupling. The second mechanism, as illustrated in Figure 4b, was also observed from the Al2O3/Ag NWs samples. Although the Al2O3 coating significantly lowered the Rs of the as-sprayed Ag NWs film through the first mechanism, the Rs of the Al2O3/as-sprayed Ag NWs film—despite incrementally increased Al2O3 thickness—ultimately did not reach the 26 Ω sq-1 value of the bare (no ALD coating) thermally-sintered Ag NWs film (Figure S6, Supporting Information), whose Ag NW joints were all thermally fused. This indicated that in the Al2O3/as-sprayed Ag NWs film, there had been some thermally fusible Ag NWs that were prevented from fusing by the Al2O3 coating, as a result of those Ag NWs having small, molecular-level gaps among them which were not small enough to prevent the ALD precursor molecules from growing onto their surfaces. With the thermally-fusible gaps filled instead by Al2O3, such Ag NWs were insulated from one another, reducing the overall degree of connection of the Ag NWs film. In the case of the HZO/Ag NWs film, because HZO was conductive, such Ag NWs were able to maintain electrical connection among them, albeit with a higher contact resistance than that of a fused joint. Nevertheless, replacing a part of fused joints with HZO-connected ones caused only slightly increased overall Rs, as can be seen by comparing the Rs of the HZO/as-sprayed Ag NWs (15 Ω sq-1) and the HZO/sintered Ag NWs (14 Ω sq-1) samples. The HZO/sintered Ag NWs sample had all of the thermally fusible Ag NWs already fused by sintering before HZO was coated, and yet its Rs was only slightly lower than that of the HZO/as-sprayed Ag NWs, where some of the thermally fusible joints were joined by HZO instead. The third mechanism, as illustrated in Figure 4c, was intuitive and could be observed by comparing the Rs of the bare sintered Ag NWs (26 Ω sq-1), ALD Al2O3/sintered Ag NWs (26 Ω sq-


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and the HZO/sintered Ag NWs (14 Ω sq-1) samples. As all of the thermally fusible joints in the

three samples were fused by sintering, the differences in their Rs represented the contribution of the additional conduction routes among the unconnected Ag NWs provided by the ALD coatings. With the insulating Al2O3, the ALD coating did not contribute any conductive connection among the Ag NWs, and therefore Rs remained unchanged before and after Al2O3 coating. The conductive HZO coating, on the other hand, provided additional conduction paths to the Ag NWs to lower the overall Rs. To examine whether using a more conductive HZO coating could noticeably enhance this mechanism, we replaced the 90 C-deposited HZO film with a 150 C-deposited one on sintered Ag NWs. Despite the Rs of the 150 C HZO being ~1/235 that of the 90 C HZO, the Rs of the 150 C HZO/sintered Ag NWs only slightly improved to 13 Ω sq-1, as a result of the much higher conductivity of Ag than that of either the 90 or 150 C HZO film, which rendered the Ag NWs the dominant factor in determining Rs of the composite film. This also indicated that the Rs of the 90 C HZO at 33 nm thickness was already adequate for the minor role it played in the conductivity of the composite film. Knowledge of the above mechanisms and the relative magnitudes of their effects pointed to the following strategies for fabricating ALD/Ag NWs films that met the requirements of low Rs, low WVTR, high transparency, high flexibility, and low processing temperature: (a) as the first mechanism (ALD-induced fusing) was dominant over the other two (separating/bridging and connecting), the most important consideration was ensuring that the Ag NWs surface had sufficient –OH anchors for ALD film to bind to, which could be achieved without collateral damages to the Ag NWs by using the in situ H2O2 functionalization technique; (b) as the mechanisms of separating/bridging and connecting had only minor effects, the level of Rs at ~104 Ω sq-1 of the ALD film was shown to be adequate, which allowed prioritizing other important properties over


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Rs of the ALD film including WVTR, flexibility, transparency, and low processing temperature; (c) the prioritized properties were simultaneously optimized by using the 90 C ALD HZO process, which—besides being PET-compatible—enabled a low WVTR to be obtained at a small film thickness, which in turn led to high transparency and flexibility.

2.5 Flexibility, Stability/durability, and Photo-Patterning of HZO/Ag NWs films The defect-free, ultra-thin conformal ALD HZO coating allowed the HZO/Ag NWs film to retain its properties upon rigorous mechanical bending and to remain stable under an extensive set of harsh testing conditions, addressing the main concerns limiting practical applications of Ag NWs-based materials. In terms of flexibility, the HZO/Ag NWs film on PET withstood 100 cycles of bending to a 12.5 mm radius of curvature without a detectable change in WVTR or Rs, as shown in Figure 5. The Rs continued to remain constant even at 5 mm bending radius, as it was not as sensitive to the nano-/microscopic defects induced by over-bending as was WVTR. This presented vast improvements over a commercial ITO/PET sample used as a benchmark, which showed much higher WVTR and Rs throughout the bending radii tested. In terms of stability/durability, the HZO/Ag NWs film showed strong resistance against several known modes of Ag NWs degradation (with details of the test results provided in Supporting Information), including melting/coalescing under continuous current 4 (~ 176 mA cm-2 for > 500 h; Figure S7, Supporting Information), photo-degradation under UV illumination 5,32 (150 W, 320 – 450 nm wavelengths for 75 h; Figure S8, Supporting Information), oxidation in damp heat 32,33 (85 C and 85% R.H. for > 1000 h; Figure S9, Supporting Information), and damage or delamination by abrasion32 (peel test with an adhesive tape for 100 cycles; Figure S10, Supporting


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Information). As expected from the conformal coverage and defect-less quality of the ALD HZO film, the HZO/Ag NWs film retained its initial Rs in all of the tests (Table S2, Supporting Information), in sharp contrast to the rapid and drastic degradations of the bare Ag NWs film. The excellent stability of the HZO/Ag NWs film also manifested itself in the film’s compatibility with a typical photolithographic and dry-etching patterning process, as shown in Figure S11 of Supporting Information. The Rs of the HZO/Ag NWs remained unchanged after the series of process steps including spin-coating of photoresist, exposure to UV light, pattern development in an alkaline solution, dry-etching in an inductively-coupled Cl2–based plasma, and stripping of photoresist in an organic solution.

2.6 HZO/Ag NWs Film as Electrode/ETL for Perovskite Solar Cells We demonstrated utility of the HZO/Ag NWs film as a combination of transparent electrode and electron-transporting layer (ETL) in organic-inorganic hybrid perovskite solar cell (PSC) devices (architecture shown in Figure S12 of Supporting Information), which achieved higher power conversion efficiency (PCE) (12.78% average; 14.46% champion) than that of the control device with the common ITO/TiO2 electrode/ETL combination (12.17% average; 14.11% champion), as shown in Figure 6 and Table S3 of Supporting Information. Consistent with previous reports that Al-doping in ZnO resolved the problem of ZnO reacting with perovskites 34, the Hf-doped ZnO film of the HZO/Ag NWs film was also compatible with the CH3NH3PbI3 perovskite layer. What was remarkable about the HZO/Ag NWs-containing PSC device was its higher fill factor (FF) than that of the control, unlike typical PSC devices with Ag NWs-based electrodes, whose heterogeneous surface tends to result in lowered FF with increased series resistance and reduced


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shunt resistance.35 The HZO/Ag NWs film, with its surface being a conformal homogeneous HZO film with fair electrical conductivity, would conceivably avoid such drawbacks by providing uniform electron-transport paths throughout its surface. The homogeneity of electrical conductivity on the HZO/Ag NWs surface was verified with conductive atomic force microscopy (C-AFM), and the results are presented in Figure 7. The C-AFM scans indeed showed uniform current distribution on the HZO/Ag NWs surface, independent of the locations of the underlying Ag NWs shown in the topography image.

3. Conclusion The mechanisms of composite films composed of ultrasonically sprayed Ag NWs over-coated with a conductive ALD HZO film were analyzed to realize the critical properties required of transparent electrodes for flexible electronics, including low Rs (15 Ω sq-1), low WVTR (5.1 × 10-6 g m-2 day-1), high transparency (> 92% optical transmission), excellent flexibility ( 12.5 mm bending radius for 100 cycles), high stability (stable against an extensive set of degradation modes including photo-patterning processes), and low processing temperature (90 C). The ALD film contributed to the electrical conductivity of the HZO/Ag NWs film primarily through a novel mechanism where the ALD process induced fusing of the Ag NWs into a connected network with the contracting forces generated by its reaction at the Ag NWs surface, which was enhanced by in situ functionalization of the Ag NWs with a pulse of H2O2 vapor to create –OH groups for the ALD precursors to chemisorb to. Besides enabling the ALD-fusing mechanism to be completed at  10 nm of HZO thickness, the H2O2 functionalization also ensured defect-free coverage of the HZO film over the Ag NWs surface, preserving the low WVTR of the HZO film in the HZO/Ag


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NWs composite, which in turn resulted in the excellent stability/durability of the composite film. The intuitive mechanisms that the conductive HZO film added to the conductivity of the composite by bridging and connecting separate Ag NWs were found to be minor and adequately served by a HZO film with a mediocre Rs of ~104 Ω sq-1. Knowledge of the mechanisms allowed prioritizing other important properties over Rs of the ALD HZO film, including WVTR, optical transparency, mechanical flexibility, and low processing temperature. The prioritized properties were simultaneously optimized by using a 90 C ALD HZO process, which—besides being plasticscompatible—enabled the low WVTR to be obtained at a small film thickness (33 nm) thanks to the low resultant HZO crystallinity, which also led to the high transparency and flexibility. Finally, the practical utility of the HZO/Ag NWs film was demonstrated by incorporating it as a combination of transparent electrode and electron-transporting layer in organic-inorganic hybrid perovskite solar cell devices, whose power conversion efficiency (12.78% average; 14.46% champion) rivaled that of the control device with the common ITO/TiO2 electrode/ETL combination (12.17% average; 14.11% champion).

4. Experimental Methods Preparation of Ag NWs films. Ag NWs dispersed in isopropyl alcohol (20 mg ml-1) were purchased from ACS materials Co., Ltd (product number: Agnw-L30) and diluted with isopropyl alcohol to 0.1 mg ml-1 followed by sonication for 20 s before use. Ag NW films were fabricated by ultrasonic spraying of the diluted solution through a nozzle (Vortex nozzle, Sono-Tek Co., Ltd) directed at an automated substrate stage heated at 80 C for 21 passes. The substrates used included poly(ethelyene terephthalate) (PET) (DuPont , KEL10W, thickness = 125 µm), poly(ethylene


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naphthalate) (PEN) (Dupont, Teonex®-Q65HA, 125 µm), glass (Corning, Eagle-XG, thickness = 0.7 mm), and silicon wafer (Guv Team International Co.Ltd, 4” silicon wafer, thickness = 525 ± 25 µm). The as-sprayed Ag NWs films were either directly over-coated with the ALD HZO to form HZO/Ag NWs composite films or—as a baseline for the purpose of comparison—thermally sintered (200 C for 30 min in the ambient air)4. The oxygen-plasma treatment found to be detrimental to the Ag NWs was administered in a plasma cleaner (Harrick Scientific, model PDC32G) at 6.8 W for 1 sec. Atomic layer deposition of Hf:ZnO films. The as-sprayed Ag NWs films were over-coated with ALD HZO films in a commercial ALD system (Savannah 100, Cambridge NanoTech Co.) using diethylzinc (DEZn) (Tri Chemical, ≥ 99.9999% purity, used as received), tetrakis(dimethyl amido)hafnium (TDMAHf) (Sigma-Aldrich, ≥ 99.99% purity, used as received) and H2O as precursors. The ALD HZO process was based on our previously reported mixed-doping technique but with a lower deposition temperature of 90 C 17. The Ag NWs films were in situ functionalized before the main ALD HZO process with an ALD half cycle consisting of a 0.1 s pulse of H2O2 vapor followed by a 30 s purge. The ALD conditions were as follows: chamber pressure = 0.1 torr, nitrogen flow rate = 20 sccm, TDMAHf heating temperature = 75 C; DEZn, H2O, and H2O2 were not heated. The HZO films were deposited by alternating 1 mixed-deposition cycle with 4 ZnO deposition cycles for a total of 45, 180, and 230 cycles including 5 initial nucleation cycles of ZnO. The steps of the mixed-deposition cycle were as follows: simultaneous pulses of DeZn (0.05 s) and TDMAHf (0.05 s), 30 s purge, 0.02 s pulse of H2O, 30 s purge. The steps of the ZnO deposition cycle were as follows: 0.06 s pulse of DEZn, 30 s purge, 0.02 s pulse of H2O, 30 s purge. The steps of the ZnO nucleation cycle were as follows: 0.1 s pulse of DEZn, 30 s exposure, 30 s purge, 0.1 s pulse of H2O, 30 s exposure, 30 s purge. The thickness deposited per mixed-deposition/ZnO


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deposition dyad was 0.73 nm as determined by TEM (JEOL,JEM-2100F ) and elipsometry (Nanofilm Surface Analysis, EP3). For studying the mechanisms governing the electrical conductivity of the HZO/Ag NWs film, ALD Al2O3/Ag NWs films were prepared for comparison. The ALD Al2O3 process used trimethylaluminum (TMA) (Tri Chemical, ≥ 99.9999% purity, used as received ) and H2O as precursors with the following setting of a cycle: 0.03 pulse, 30 purge, 0.02 pulse, 30 purge. The number of cycles used was 330 (thickness = 33 nm) including 5 nucleation cycles which consisted of 0.1 s pulse of TMA, 30 s exposure, 30 s purge, 0.1 s pulse of H2O, 30 s exposure, 30 s purge. The deposition temperature was 90 C. Characterization of the Ag NWs, HZO, and HZO/Ag NWs films. The electrical properties were measured with a Hall-effect measurement system (ECOPIA HMS-3000) and a four-point probe (Loresta MCP T-410). Optical transmittance was measured with a UV/VIS/NIR spectrometer (Jasco V-570). Surface topography was analyzed with scanning electron microscopy (SEM) (JOEL JSM-6700F) and atomic force microscopy (AFM) (Bruker Dimension Icon), the latter of which in a conductive AFM mode was also used for mapping the aerial distribution of conductivity on the surface of the films. The water vapor transmission rates (WVTR) were measured in a MOCON Aquatran Model 1 permeation tester at 38 C and 100% relative humidity. For samples with WVTR below the measurement limit of the permeation tester at 5  10-4 g/m2 day, elevated-temperature measurements were carried out to allow determination of the 38 C WVTR values by extrapolation, as detailed in our previous report.36 X-ray diffraction (XRD) analysis was carried out in a Rigaku TTRAX 3 XRD system with a copper Kα line source. Water contact angles were measured on thermally evaporated Ag films with an automatic contact angle analyzer (SEO Phoenix 300), where a sessile drop of about 5 µl in volume was dispensed with a micro syringe and the contact angle was measured within 30 s after its formation. The FTIR


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analyses of the H2O2-functionalized PVP were conducted with a FTIR spectrometer (PerkinElmer, Spectrum Two) enclosed in an Ar-filled glove box. The PVP samples were spin-coated on KBr substrates from a 0.01 wt% PVP/anhydrous methanol solution in the glove box and treated with an ALD half cycle consisting of a 0.1 s pulse of H2O2 vapor followed by a 30 s purge at 90 C, and then baked at 120 C under vacuum for 1 hr before FTIR measurements. Stability/durability and bending tests. For the electrical loading test, a constant current density of 176 mA/cm2 was applied on the HZO/Ag NWs flms in the ambient air through a power supply (PROVA, PROVA 8000) with the corresponding voltage recorded with time. For the UV illumination test, the HZO/Ag NWs flms were illuminated with continuous UV light (Kinsten, KVB-30DT) under vacuum and measured for Rs periodically. For the damp heat test, the HZO/Ag NWs flms were placed in an environmental chamber (DICTEK, H-TH) set at 85 C and 85% relative humidity (RH) in the absence of light and measured for Rs periodically. For the peel test, a 3M Scotch tape with a width of 6 mm was used. For the photo-patterning test, the HZO/AgNWs films were spin-coated with a commercial photoresist (MicroChemicals, AZ® nLOF 2020), exposed to UV (365 nm) through a photomask, developed in an alkaline tetramethylammonium hydroxide (TMAH) solution, dry-etched in an inductively-coupled Cl2–based plasma, and stripped with N-methyl-2-pyrrolidone (NMP). For the bending test, a commercial bending tester (TQC, SP1820) was used. Fabrication and characterization of perovskite solar cells. The device architecture was glass/Ag NWs/HZO/perovskite/spiro-OMeTAD/Au with the HZO/Ag NWs film serving as both the electrode and the electron-transporting layer (ETL), as shown in Figure S12 of Supporting Information. The perovskite layer was spin-coated from a precursor solution of CH3NH3PbI3


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composed of 1:1 molar ratio of CH3NH3I (Sigma Aldrich, 98% purity) and PbI2 (Sigma Aldrich, 99% purity) in dimethylformamide (DMF) with a concentration of 1 M. The spin-coating steps were as follows: (1) the precursor solution was dispensed at 5000 rpm followed by 4 ~ 8 s of spinning before 200 µl of chlorobenzene was quickly dispensed onto the center of the substrate; (2) the coated film was dried at 100 °C for 10 min, at which point it turned black. The thickness of the perovskite layer was about 280 nm. The spiro-OMeTAD layer (thickness ~ 200 nm) was spin-coated from a solution composed of 217 mg of spiro-OMeTAD (Lumtec, > 99.5% purity), 53 μL of 520 mg ml-1 bis(trifluoromethane)-sulfonimide lithium salt (LiTFSI) (Sigma Aldrich, 99.95% purity) in acetonitrile (Sigma Aldrich, 99.8% purity), and 86.8 μL of 4-tertbutylpyridine (TBP) (Sigma Aldrich, 96% purity) in 3 mL of anhydrous chlorobenzene (Sigma Aldrich, 99.8% purity) at 2000 rpm for 30 s. A 100 nm gold top electrode was deposited by thermal evaporation through a shadow mask under high vacuum (~10-7 torr). The photocurrent density−voltage (J−V) characteristics of the devices were measured with a digital source meter (2400, Keithley Instruments, USA) under AM 1.5G illumination, which was calibrated with a solar simulator (Class AAA, Oriel, USA). The solar cells were masked with a metal aperture defining the active area (0.09 cm2) of the devices. The control device used ITO and TiO2 as the electrode and ETL, respectively. The TiO2 ETL (thickness = 3 nm) was deposited by ALD on ITO-coated glass substrates (Sigma-Aldrich,15 Ω/sq, used as received) using titanium isopropoxide (TTIP) (SigmaAldrich, purity ≥99.99%, used as received) heated to 80 C and H2O (unheated) as precursors at a chamber pressure of 0.1 torr with a 20 sccm N2 carrier gas flow. The settings of the ALD cycle (total = 200 cycles) were as follows: 0.1 s pulse of TTIP, 10 s exposure, 40 s purge, 0.02 s pulse of H2O, 10 s exposure, 40 s purge.


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Items provided in the Supporting Information are as follows: SEM images and UV-Vis transmission spectra of various ALD- and/or Ag NWs-based films, water contact angle (WCA) results, Rs of Al2O3/Ag NWs films as a function of Al2O3 thickness, comparisons of the HfO2/Ag NWs film with a commercial ITO film, stability/durability testing results of the bare Ag NWs and the HZO/Ag NWs films, and PSC device structures and performance metrics.

Acknowledgements This work was supported by funding provided to Feng-Yu Tsai by the Ministry of Science and Technology of Taiwan (Grant Nos. 106-2622-E-002-029-CC2, 106-2221-E-002-188-, 107-2221E-002-065-), National Taiwan University (NTUCC-108L892402), Industrial Technology Research Institute, and Bureau of Energy, Ministry of Economic Affairs of Taiwan.

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Figure 1. a)WVTR and b) sheet resistance of the 90 C ALD HZO and the HZO/Ag NWs films as a function of HZO thickness. The HZO/Ag NW films were fabricated either with or without the in situ H2O2 functionalization treatment.

Figure 2. FTIR spectra of polyvinylpyrrolidone (PVP) before and after the H2O2 treatment.


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Figure 3. Cross-sectional TEM images of the HZO/Ag NWs films fabricated (a) without and (b) with the in situ H2O2 functionalization treatment.

Table 1. Sheet resistances of the Ag NWs, ALD HZO, and HZO/Ag NWs films fabricated under various conditions. Film

Rs

Film

[Ω cm-1]

Rs [Ω cm-1]

Ag NWs, as-sprayed

1.03 ± 0.01 104

Al2O3/Ag NWs w/o H2O2

80 ± 5.5

Ag NWs, sintered

26 ± 0.5

Al2O3/Ag NWs w/ H2O2

49 ± 1.5

HZO (90 C)

5.4 ± 0.1104

HZO/Ag NWs w/o H2O2

48 ± 2.7

HZO (150 C)

230 ± 3.5

HZO/Ag NWs w/ H2O2

15 ± 0.5


27


Al2O3/sintered Ag NWs

26 ± 0.5

Page 28 of 31

HZO (150C)/Ag NWs w/ H2O2 13 ± 0.5

HZO/sintered Ag NWs

14 ± 0.5

Figure 4. Schematic illustrations of the mechanisms through which the ALD HZO coating contributed to the electrical conductivity of the HZO/Ag NWs films: a) ALD-induced fusing, b) separating and bridging, and c) connecting.


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Figure 5. Normalized a) sheet resistance and b) WVTR of the HZO/Ag NWs and commercial ITO films (both deposited on PET) as a function of bending radius after 100 cycles of bending. Ro of the HZO/Ag NWs and ITO was 15 and 18 Ω sq-1, respectively; WVTRo was 5.1 × 10-6 and 2.1 × 10-2 g/m2 day, respectively.

Figure 6. Current density-voltage characteristics of the champion PSC devices with the HZO/Ag NWs film or TiO2/ITO as the ETL/electrode. The average device metrics are provided in Table S3 of Supporting Information.


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Figure 7. (a) Topographic and (b) current mapping images of the HZO/Ag NWs film obtained with C-AFM.

Fabrication, Mechanisms, and Properties of High-Performance Flexible Transparent Conductive Gas-Barrier Films Based on Ag Nanowires and Atomic Layer Deposition

ToC Figure


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(8.25 × 4.45 cm)


31

Fabrication, Mechanisms, and Properties of High-Performance

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