All-Solution-Processed Thermally and Chemically Stable Copper

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All-Solution-Processed Thermally and Chemically Stable Copper-Nickel Core-Shell Nanowire based Composite Window Electrodes for Perovskite Solar Cells Kyungmi Kim, Hyeok-Chan Kwon, Sunihl Ma, Eunsong Lee, SeongCheol Yoon, Gyumin Jang, Hyunha Yang, and Jooho Moon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b09266 • Publication Date (Web): 17 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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ACS Applied Materials & Interfaces

All-Solution-Processed Thermally and Chemically Stable Copper-Nickel Core-Shell Nanowire based Composite Window Electrodes for Perovskite Solar Cells Kyungmi Kim, Hyeok-Chan Kwon, Sunihl Ma, Eunsong Lee, Seong-Cheol Yoon, Gyumin Jang, Hyunha Yang, and Jooho Moon* Department of Materials Science and Engineering, Yonsei University 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea

KEYWORDS: perovskite solar cells, transparent bottom electrodes, copper nanowires, allsolution-processing, core-shell nanowires

*Corresponding author, e-mail: [email protected]

tel.: +82-2-2123-2855, fax: +82-2-312-5375.

ACS Paragon Plus Environment

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ABSTRACT Organic-inorganic hybrid perovskite solar cells (PSCs) have recently attracted tremendous attention due to their excellent efficiency and the advantage of a low-cost fabrication process. As a transparent electrode for PSCs, the application of copper nanowire (CuNW)-network was limited due to its thermal/chemical instability, despite its advantages in terms of high optical/electrical properties and low-cost production. Here, the copper-nickel core-shell nanowire (Cu@Ni NW)-based composite electrode is proposed as a bottom window electrode for PSCs, without the involvement of a high-cost precious metal and vacuum process. The dense and uniform Ni protective shell for CuNWs is attainable by simple electroless plating, and the resulting Cu@Ni NWs exhibit outstanding chemical stability as well as thermal stability compared with bare CuNWs. When the Ni layer with the optimal thickness is introduced, the Cu@Ni NW electrode show a high transmittance of 80.5% AVT at 400–800 nm, and a sheet resistance of 49.3 ± 5 Ω sq-1. Using the highly stable Cu@Ni NWs, the composite electrode structure is fabricated with sol-gel-derived Al-doped zinc oxide (AZO) over-layer for better charge collection and additional protection against iodine ions from the perovskite. The PSCs fabricated with AZO/Cu@Ni NW-based composite electrode demonstrate a power conversion efficiency (PCE) of 12.2% and excellent long-term stability maintaining 91% of initial PCE after being stored for 500 h at room temperature. Experimental results demonstrate the potential of highly stable Cu@Ni NW-based electrodes as the cost-effective alternative transparent electrode, which can facilitate the commercialization of PSCs.


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Organometal halide perovskite has emerged as a promising candidate as a light absorber due to its excellent properties for photovoltaic devices, achieving power conversion efficiency (PCE) of over 22% since it was first introduced in 2009 with PCE of 3.8%.1-5 The high-quality perovskite films are obtainable through a solution process, enabling relatively simple and cost-effective solar cell fabrication.6-8 However, the high energy consuming vacuum processes are still involved in the fabrication process of perovskite solar cells (PSCs), which possibly raises the production cost and hinders its commercialization. The most commonly used vacuum-processed components in PSC are metallic top electrodes of Au, Ag, Al, etc. and indium tin oxide (ITO) or fluorine-doped tin oxide (FTO) for transparent electrodes.9-10 These energy-intensive processes should be excluded to fully exploit the benefit of cost-effective solution-processable perovskite absorber. Recently, metal nanowire (MeNW)-based transparent conductive electrodes (TCEs) have gained tremendous interest as a promising alternative to vacuum processed metal or ceramicbased counterparts due to its low sheet resistance, flexibility, controllable figure of merit, and solution processability.11-13 MeNW-based TCEs have been successfully employed for both the top and bottom electrodes for PSC alternatives to vacuum-processed electrodes. For example, the MeNW-based TCEs top electrode can be applied to achieve the semitransparent or tandem solar cells.14–15 In such a case, care must be taken not to decompose the underlying perovskite layer by the solvent suspending MeNW during the electrode deposition. Less solvent involved or dry approaches such as spray coating and mechanical transfer methods have been developed to deposit MeNW-network electrodes while minimizing the damage to the perovskite.16-19 In contrast to the top electrode, applying the MeNWs as a bottom window electrode for PSCs is more challenging. The MeNWs need to be thermally stable to withstand repeated heat treatments


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during the successive deposition of the upper functional layers. Without thermal stability, the MeNWs are likely oxidized or collapsed when heated over 300 °C.20-21 Upon exposure to a perovskite precursor solution containing iodide ions, in particular, the MeNW should have high chemical stability against chemical reaction. Otherwise, the MeNWs decompose, losing their electrical conductivity.22 To address these stability issues, a protective layer has been introduced to silver nanowire (AgNW) electrode-based PSCs. For instance, solution-processed graphene oxide flakes were utilized as the anti-corrosive barrier, revealing a PCE of 7.92%.23 Our group proposed the AgNW composite electrode sandwiched sol-gel derived ITO and zinc oxide (ZnO) to prevent physical contact with the perovskite layer, while to facilitate the carrier transport, resulting in a PCE of 8.44%.24 Recently, Lee et al. successfully demonstrated a protected AgNWs composite electrode by pin-hole free amorphous Al-doped zinc oxide (AZO), and the resulting PSCs exhibited a high PCE of 13.93%.25 Although these promising results clearly demonstrate the feasibility of MeNW based electrodes as an alternative to the ceramic-based TCEs, the use of expensive precious metal such as Ag poses a barrier to realize low-cost PSCs. Copper nanowires (CuNWs) have attracted much attention because of high conductivity comparable to Ag as well as abundancy.26-27 However, utilizing CuNWs as a bottom window electrode in the PSCs is very challenging because Cu is susceptible not only to copper iodide (CuI) formation when in contact with the perovskite phase,28-29 but also to oxidation due to contact with ambient air.30 It was demonstrated a highly thermal/chemical stable CuNW composite electrode with the aid of the sputtered ITO layer. Despite high PCE of 12.95%, their approach still involved a high-cost vacuum process like sputtering and usage of rare indium element, all of which make it difficult to apply to large-scale cost-effective manufacturing.28


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Recently, we reported outstanding performance of CuNW window electrode based PSCs without any usage of precious metals, such as In or Ag, in which the sandwich structure of a sputtered AZO layer and spin-coated poly(ethylenimine) film was utilized as a protective layer to shield the CuNWs from oxidation and CuI formation.29 For the ultimate realization of low-cost PSCs, the sputtering process should be avoidable in TCE fabrication. In this regard, solutionprocessable

and

earth-abundant

CuNW

based

window

electrodes

with

improved

thermal/chemical stabilities are highly demanded. Herein, all-solution-processed copper-nickel core-shell NW (Cu@Ni NW) based composite electrodes (i.e., AZO/Cu@Ni NW) are presented for PSCs. Specifically, the low-cost nickel shell plays a role as a protective layer from oxidation and halide formation of CuNWs; while the sol-gel derived AZO is introduced as an upper layer of Cu@Ni NWs to further improve charge collection ability. The Ni shelling is obtainable by a simple electroless plating onto the as-prepared CuNWs network film, which preserves the direct junction between CuNWs to facilitate electron transport avoiding electrical passage through the Ni layer. The optimized Cu@Ni NWs electrodes show a sheet resistance of approximately 49 ± 4 Ω sq-1 and average visible light transmittance (AVT) of 80.1%. The electroless plated Cu@Ni NW-based electrodes are successfully employed to PSCs, leading to a PCE of 12.17%, which is comparable to the sputtered FTO based PSCs. The Cu@Ni NW based composite electrodes shed light on the possibility to achieve ultimate cost-effective, all-solution processed PSCs.

RESULTS AND DISCUSSION To address the problematic issues of chemical and thermal instabilities of the copper film, CuNW and Cu particle coated with various metals (e.g. Ag, Au, Pt, Ni ...) (i.e., Cu@metals) have


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previously been suggested.31-35 As a bottom electrode for the PSC, however, Ag shell is an inferior candidate because of its incompatibility toward the perovskite phase in which the Ag electrode is decomposed upon the reaction with iodine ions.36 Even the commonly used Au is vulnerable to the reaction with the perovskite phase, leading to significant PSC degradation.37-39 Meanwhile, Pt could be suitable as a protective layer due to its high stability, but it is a precious metal, attenuating the advantage of cost-effective Cu based TCE. Conversely, Ni is an earth abundant material and has been reported as a chemically stable top electrode for PSCs.40-42 In addition, Ni could be electrolessly plated on the surface of the CuNW network as a protective layer. In this regard, Ni was selected as a protection layer for CuNWs preventing the surface oxidation and decomposition. A schematic illustration for preparing the Cu@Ni NW by the electroless plating around the CuNWs as well as its protective effect is shown in Figure 1a. First, the CuNWs were synthesized according to a previously developed method43 from which the resulting nanowires have an average diameter of 32.9 ± 3.2 nm with a few hundred micrometers in length. The CuNW random network films were fabricated by the vacuum filtering of the CuNW dispersion containing lactic acid to eliminate the surface residues such as copper oxide and hydroxide (see Experimental Section for details).20 The resulting films were annealed at 150 °C under N2 atmosphere to make a fused junction between the CuNWs. The fabricated CuNW transparent electrodes exhibited good optical/electrical properties, which are comparable to representative results reported in the literature for CuNW electrodes (Figure S1 in Supporting Information). To initiate Ni plating, lactic-acid treated oxide-layer-free bare CuNWs network films were immersed in an activation solution containing Pd ions, from which Pd seeds formed onto the outer surface of CuNWs (Figure S2). To minimize the oxidation of CuNW surface, the Pd activation process was conducted immediately after the lactic-acid treatment. After Pd


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activation, the CuNW films were transferred to electroless Ni plating solution and both processes were performed at atmospheric condition. During the electroless plating, Ni ions began to be reduced around the Pd seeds and gradually covered the surface of CuNWs.44 Figure 1b and c show clear differences in thickness and morphology before and after Ni plating on the CuNWs. The thickness of NWs was uniformly increased by approximately 10–11 nm after Ni plating at 50 °C for 70 s (Figure 1c). Furthermore, the dense and pin-hole free Ni layers were uniformly deposited along the CuNWs as well as around the slightly fused junctions between CuNWs (Figure S3). Transmission electron microscope (TEM) and energy-dispersive spectroscopy (EDS) mapping analyses were conducted to characterize the distribution of each element for Cu@Ni NWs (Ni plating at 50 °C for 70 s), as shown in Figure 2. The EDS mapping images revealed that the continuous and thin Ni layer was formed around the surface of CuNWs, indicating the successful formation of a Cu@Ni core-shell structure. Furthermore, in order to identify the phases of Cu-core and Ni-shell, the edge structure of Cu@Ni NW was investigated with highresolution TEM (HR-TEM). In Figure S4, the lattice spacing of the Cu core was clearly observed while there were no such features in the area of Ni shell. This observation suggests that the electroless plated Ni layer exists as an amorphous phase. To confirm this hypothesis, the crystallinity of Ni shell layer was investigated by the X-ray diffraction (XRD) measurement. Since a very thin Ni shell layer is likely undetectable by XRD, thick plated Ni film (~ 40 nm) was prepared on the thermally evaporated Cu film with a thickness of 50 nm on the glass substrate using the same electroless plating method. As shown in Figure S5, the XRD patterns exhibited two distinguishable diffraction peaks at 2θ = 43.5° and 50.7°, corresponding to the (111) and (200) planes of crystalline Cu (JCPDS 03-1018), respectively. Meanwhile, there was


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no noticeable diffraction peak after the thick Ni was electrolessly plated on the Cu film, suggesting the amorphous Ni film. The electrolessly deposited Ni layer can be crystallized after post-annealing at above 330 °C.45 The crystalline Ni has better electrical conductivity compared to the amorphous counterpart. However, the CuNW network is likely disconnected due to local melting when annealed at above 300 °C, making it difficult to crystallize the Ni protection layer. Although amorphous Ni might deteriorate to some extent, the electrical conductivity of the metal nanowire-based electrode, Ni shell is ultrathin and the CuNWs have slightly fused junctions prior to Ni plating, so that its electrical detrimental effect would be insignificant. The stability issues against the oxidation and chemical degradation pose a hurdle to application to the photovoltaic device as a bottom electrode. To evaluate thermal and chemical stabilities of Cu@Ni NWs and determine the optimum thickness of the Ni shell, the resistance change (R/R0) of Cu@Ni NW networks with various thicknesses of Ni layer was monitored. Ni plating was performed at the same temperature of 50 °C for all the Cu@Ni NWs in which the Ni thicknesses were controlled by varying the plating times (30 s, 50 s, 70 s, and 90 s). Prior to the stability test, the average initial sheet resistance for the Ni plated samples under various plating conditions was summarized in Table S1. When comparing the sheet resistance for the samples performed after various Ni plating times, there was only a small difference in sheet resistance. This is because the electrons are mainly transported through the CuNWs with low resistivity even though the Ni shell conformally covers the CuNWs. From this point of view, it is reasonable to select the optimized thickness of Ni shell based on the consideration of the minimum Ni shell enough to protect the CuNW from the oxidation/degradation. The Cu@Ni NW networks were exposed to three different specific conditions, i.e., i) 75 °C and 75% relative humidity (RH) oxidative atmosphere, ii) perovskite precursor solution, and iii) AZO sol-gel


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precursor solution. Relative resistance with respect to the initial value (R/R0) for bare CuNWs degraded abruptly as shown in Figure S6, and the resistance could not be detected even after 10 min exposure, indicative of a complete oxidation of CuNWs. Conversely, the oxidation of Cu@Ni NWs was strikingly suppressed with the increasing Ni layer thickness. After 60 min exposure, R/R0 values for the Cu@Ni NWs, samples with different plating times of 30 s, 50 s, 70 s, and 90 s were dramatically reduced from 6.2×102, 4.5, 1.75 to 1.3, respectively. When the plating time exceeded 70 s, the R/R0 values remained below 2, demonstrating that a conformal Ni shell layer can completely protect the CuNWs from oxidation. Oxidation stability as expressed by low R/R0 value is comparable to previously reported CuNWs protected by sputtered AZO and chemical vapor deposited graphene.29,

46

The electroless plating-based core-shell structured

Cu@Ni NW exhibited excellent thermal stability without the involvement of any high-cost vacuum processes. During the fabrication of PSCs, the MeNW based bottom electrode might be damaged and lose its conductivity after exposure to the perovskite precursor solution. As schematically depicted in Figure 3a, the perovskite precursor solution was spin-coated on top of the CuNWs, and the resistance variation of the electrodes was monitored as a function of the aging time under ambient conditions. When measured immediately after perovskite phase deposition, the bare CuNW showed a sharp increase in resistance (Figure 3b). This is presumably due to the formation of an insulating CuI phase as a result of chemical reaction between Cu and iodine ions contained in the precursor solution, leading to electrical conductivity loss.29 In contrast, Cu@Ni NW exhibited remarkably reduced resistance increase. In particular, the Cu@Ni NW with the increasing thickness of Ni shell (i.e., plating time change from 30 s to 90 s) showed better stability. Even after stored for 143 h in ambient air under 10–20% RH, the samples plated more


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than 70 s showed a slight increase in resistance (R/R0 = 5.19–1.75). This observation clearly proves that the Cu@Ni NW network retains superior stability against chemical attack by halogen ions. To apply MeNW-based electrode as a bottom electrode of PSCs, additional issues should be resolved; i) rough surface finish of nanowires may hinder the uniform deposition of upper layers and ii) the limited contact area between adjacent layers and nanowires may make charge carrier collection ineffective, both of which likely degrade the PSC performance. To address these issues, sol-gel solution-processed indium-free transparent conducting oxide, e.g., AZO, was utilized with which the Cu@Ni NW was made into the layered composite electrode. As schematically depicted in Figure 3c, the AZO sol-gel solution was spin-coated on top of the Cu@Ni NW and annealed at 190 °C for 40 min, followed by the perovskite precursor deposition and annealing at 100 °C for 10 min. The resistance variation of the electrodes was monitored in the course of the composite electrode fabrication. The Cu@Ni NWs successfully maintained their conductivity when exposed to the AZO sol-gel solution, and even after 190 °C annealing process for 40 min, while bare CuNW was easily oxidized during the annealing process of AZO (Figure 3d). As similar to the results of Figure 3b and S6, the Cu@Ni NW with the increasing thickness of Ni shell (i.e., plating time varied from 30 s to 90 s) showed more stability. After 80 h-long aging in ambient air under 10–20% RH condition followed by the perovskite layer deposition/annealing, the resistance of Cu@Ni NWs with Ni plated for 70 and 90 s was almost unchanged (R/R0 = 1.31–1.15). This observation demonstrates that the Cu@Ni NW endures the AZO sol-gel layer formation and the composite structure of AZO/Cu@Ni NW reveals even more stable behavior against various thermal and chemical attacks. X-ray photoelectron spectroscopy (XPS) depth profile analysis was performed to verify the thermal stability of the AZO layer coated on either CuNWs (i.e., AZO/CuNW) or Cu@Ni


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NWs (i.e., AZO/Cu@Ni NW) with Ni plating for 70 s at 50 °C. In Figure S7, the x-axis of the XPS depth profile corresponds to the etching time associated with the depth of the composite structure. As shown in Figure S7a and b, it is shown that Cu or Ni exists at the bottom side in the AZO layer as represented by Zn and O in both samples. However, the XPS spectral analysis in the Cu 2p region revealed different features in both samples, as shown in Figure S7c. The AZO/Cu@Ni NW showed the binding energy peaks at 930.8 eV and 951.3 eV in Cu 2p region, whereas AZO/CuNW had the corresponding peaks at 931.8 eV and 952.3 eV. The peak positions for Cu 2p of AZO/CuNW were shifted to higher binding energies with respect to those of AZO/Cu@Ni NW. This confirms that all states of Cu metal in bare CuNW were converted to the Cu2+ oxidation state in case of AZO/CuNW structure during the AZO overlayer formation in ambient air.20, 47-48 In addition, although Ni tends not to be oxidized in low temperature, there is a risk to Ni oxidation during the formation of AZO composite structure annealed at 190 °C. However, as shown in Figure S7d, the Cu@Ni NWs shows only two main peaks of Ni even after AZO coating process without peak splitting or shift, which are usually observed for the formation of NiO.49-50 This result suggests that the Cu@Ni NWs are stable against the oxidation during the composite electrode formation. To further confirm the chemical stability of Cu@Ni NWs against iodine compound formation, the perovskite film was deposited on bare CuNW (i.e., perovskite/CuNW) and AZO/Cu@Ni NW (i.e., perovskite/AZO/Cu@Ni NW) composite structure. The depth profile elemental analysis of perovskite/CuNW sample is shown in Figure 4a. Cu is unexpectedly detected in the regions of Pb and I where the perovskite layer is present. This implies that the Cu migrates towards the perovskite layer and forms a CuI phase upon the decomposition of perovskite phase. Previous studies on the degradation mechanism of metal electrodes used in


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PSCs suggest that the metal electrode and iodine ion are spontaneously inter-diffused, which accelerates the decomposition of the metal electrode and perovskite, resulting in severely degraded performance.25, 37-39 This is consistent with the result of bare CuNW in which Cu and iodine ions are simultaneously diffused when the CuNW and perovskite layers are in direct contact. Moreover, XRD measurement detects the peaks corresponding to CuI, demonstrating that the Cu ions are reacted with iodine in perovskite phase when the perovskite is coated on the bare CuNW electrode (Figure S8). This observation clearly supports the detection of Cu in the perovskite layer (Figure 4a) and explains a sharp increase in sheet resistance because of the CuI formation between CuNW and perovskite (Figure 3b). Meanwhile, the elemental depth profile for perovskite/AZO/Cu@Ni NW is present in Figure 4b. The Cu and Ni remain inside the AZO regime, implying that they are immobilized at the bottom side without undesirable reaction with the perovskite owing to dual protection by Ni and AZO. Consequently, the structural integrity and thermal/chemical stabilities of the Cu@Ni NWs based composite electrode are clearly demonstrated, suggesting the possibility of application as a bottom electrode for PSCs. The optical/electrical performance of the Cu@Ni NW electrode should be carefully evaluated for application as a window electrode of PSCs. The Cu@Ni NW films with various transmittances were fabricated by varying the amount of CuNWs dispersion through vacuum filtration, followed by Ni plating (Figure S9). The plot of measured optical transmittance (T, at 550 nm wavelength) versus sheet resistance (Rsh) for the bare CuNW and Cu@Ni NW films with the plating time at 50 s, 70 s, 90 s is shown in Figure 5. Bare CuNW exhibited optical/electrical properties with a 37 Ω sq-1 and a 90.3% at 550 nm, whereas the Cu@Ni NWs showed a slight increase in the sheet resistance about 44–49 Ω sq-1 and a decrease in transmittance. As the plating time of Ni increased from 50 s to 90 s, the T (@ 550 nm) of Cu@Ni NWs decreased from


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82 to 76%. Longer plating time allowed Ni deposition around the nanowires, making them thicker NWs (Figure S10) and in turn diminishing the transparency of Cu@Ni NW. Taking into consideration in term of both transparency and stabilities of Cu@Ni NW, the plating time of 70 s for Cu@Ni NW electrodes is considered an optimized condition for perovskite solar cell. The optical transmittance and the sheet resistance of optimized Cu@Ni core-shell nanowires are averaged visible transmittance (AVT, in the wavelength of 400–800 nm) 80.5 ± 2% and 49.3 ± 5 Ω sq-1. A cross-sectional SEM image revealed rough surface finish of the Cu@Ni NW electrode (Figure 6a). However, the composite electrode after AZO over-coating showed a smooth surface, implying that the AZO layer effectively reduced the roughness of the Cu@Ni NWs (Figure 6b). Moreover, the Cu@Ni NW networks were fully covered by the conductive AZO layer, enlarging the contact area of Cu@Ni NWs with the adjacent layer to facilitate charge transfer. As confirmed by Figure 3b and d, the AZO layer also plays a role as an additional protective layer to endow better thermal/chemical stabilities. This is likely due to dense and pin-hole-free surface structure of the AZO over-layer as shown in Figure S11. The thickness of the AZO over-layer was optimized by varying the number of coating repetitions using a fixed sol-gel solution in order to make it as thin as possible, while reducing the surface roughness (Figure S12). The thickness of the fabricated AZO/Cu@Ni NW composite electrode with the optimized AZO layer is about 140–160 nm, and the transmittance spectra of bare CuNW, Cu@Ni NW electrode, AZO/Cu@Ni NW composite electrode, and commercial FTO electrode were compared in Figure 6c. Comparing with AVT 91% of the bare CuNW electrode, the transmittance of Cu@Ni NW electrode showed about AVT 80.5% due to the presence of Ni. There was slightly further loss in


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the transmittance in the wavelength 350–700 nm after the formation of the composite structure of AZO/Cu@Ni NW, exhibiting the AVT around 77.4%.25 Finally, all-solution-processed Cu@Ni NW based composite electrode was prepared as the bottom electrode for PSCs. The AZO/Cu@Ni NW composite electrode-based PSCs were fabricated by a successive spin-coating method, except for the gold electrode (Figure 6d). The combustion sol-gel derived ZnO (~ 65 nm) and mesoporous-Al2O3 (m-Al2O3) film (~ 160 nm) were spin-coated on the composite electrode as an electron transport layer (ETL) and scaffold layer, respectively, followed by depositing a CH3NH3PbI3 layer and a 2,2',7,7'-tetrakis(N,N-dipp-methoxyphenyl-amine)-9,9-spirobifluorene (spiro-OMeTAD) layer as the light absorber and the hole transport layer (HTM), respectively. Finally, the gold electrode was deposited by the thermal evaporation to form the back contact. The fabricated PSCs with the structure Au/spiroOMeTAD/CH3NH3PbI3/m-Al2O3/ZnO/AZO/Cu@Ni NW was observed by cross-sectional SEM and back-scattered electron (BSE) images as depicted in Figure 7a. The combustion sol-gel derived ZnO ETL layer was chosen to obtain high crystallinity at a relatively low temperature (~200 °C) annealing process. The combustion sol-gel derived ZnO involves less unwanted hydroxyl surface group, which is known to decompose the perovskite phase when in direct contact.51-52 However, the basic properties of ZnO could not be completely eliminated, so that mAl2O3 was introduced as an additional blocking layer. The current density-voltage (J-V) curves of PSCs with the AZO/Cu@Ni NW based composite electrode is shown in Figure 7b, and the performance parameters and its deviation of each parameter are shown in Figure 7c and summarized in Table 1. The performance of PSCs was obtained under 1 sun illumination (AM 1.5G, 100 mW cm-2) irradiated onto the bottom electrode side. The champion cell using the AZO/Cu@Ni NWs electrode showed an open circuit


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voltage (VOC) of 1.06 V, short-circuit current (JSC) of 16.63 mA cm-2, fill factor (FF) of 69.11%, and PCE of 12.17%. Figure 7d exhibited the external quantum efficiency (EQE) of the device. The calculated JSC from the EQE spectrum is 16.4 mA cm-2, which is consistent with the value of 16.63 mA cm-2 from the J-V curve. This PCE is a high or comparable value when compared to other CuNW bottom electrode based PSCs involving high-cost vacuum processes.28-29 Meanwhile, these PCE values of the AZO/Cu@Ni NW based PSCs are relatively lower than the FTO reference cells, which showed a maximum PCE of 15.37%. The drop of JSC can explain lower PCEs, which presumably result from relatively low performance of the composite electrodes compared to the FTO electrode. Figure S13 shows a plot to obtain the series resistance (RS) value of the devices based on both electrodes.53 The resolved RS value for FTO based cell (2.23 Ω·cm2) was three times lower than that of AZO/Cu@Ni cell (6.51 Ω·cm2). Since all layers except for the transparent electrodes are identical, this observation can be attributed to three times higher sheet resistance of AZO/Cu@Ni NW based electrode (45–55 Ω sq-1) as compared to FTO (15 Ω sq-1), which becomes one of the reasons for explaining low JSC in AZO/Cu@Ni NW based solar cell. The difference in the transmittance between FTO (AVT 93%) and AZO/Cu@Ni NW (AVT 77.4%) could be another cause of the lower JSC of AZO/Cu@Ni NW based devices. To quantitatively evaluate how much the difference of the photon energy transmitting through each electrode that actually reaches the perovskite layer, the cumulative irradiance was calculated. Figure S14 exhibits cumulative irradiance value in the wavelength of 350–750 nm under 1 sun illumination, resulting in a 16.49% difference in irradiance between FTO (479.68 W m-2) and AZO/Cu@Ni NW (400.59 W m-2). The cumulative irradiation value of bare CuNW is not significantly different from that of FTO, indicating that the formation of Ni shell is the main reason of decrease in transmittance and cumulative irradiance in the


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AZO/Cu@Ni NW based electrode. To further improve the performance of CuNW based PSCs, it is required to develop highly conductive and transparent protection material for CuNW without reliance on precious elements and a vacuum process. In addition, the synthesis of thinner and longer CuNWs will become an important strategy for developing better NW based transparent electrodes, allowing for the enhanced optical/electrical property. Finally, the J-V hysteresis behavior for the AZO/Cu@Ni NW based PSCs was investigated. A considerable hysteresis between forward and reverse J-V scan was observed in both FTO and AZO/Cu@Ni NW based cells (Figure S15). Even when the dwell time was varied, similar hysteresis was observed for both cells. Since all layers of the PSCs except for the bottom electrode were fabricated under the same conditions, the hysteresis can be attributed to the other layers in PSCs such as the perovskite absorber or ETL layers rather than the AZO/Cu@Ni NWbased electrodes. The stabilized current density and PCE at the maximum power point of the AZO/Cu@Ni NW based PSCs measured at 1 sun illumination for 100 s were shown to be slightly lower (13.1 mA cm-2 and 10.5%) than the top performance as shown in Figure S16. The hysteresis issues are generally associated with multiple factors such as ferroelectric polarization, ion migration, charge accumulation, and recombination at interfaces with the perovskite absorber layer, and charge transport imbalance.54-56 Many studies have reported that the hysteresis is strongly governed by the quality of interfaces between adjacent layer and perovskite, rather than the bulk defects of perovskite.57 In the cell configuration, the combustion sol-gel derived ZnO layer annealed at 190 °C might give rise to a hysteresis.58 To evaluate long-term stability of the AZO/Cu@Ni NW composite electrode in full cell structure, the samples were stored in dry condition (storage in box with silica gel, 10–30% RH) and monitored the PCE values. As shown in Figure 8, both Cu@Ni NW and FTO based PSC


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showed considerable stability over 500 h and maintained about 91% of initial efficiency. The JSC values which are mainly attributed to the performance of the electrodes remained almost at initial values for both PSCs, suggesting that the Cu@Ni NW electrode remains as stable as the FTO electrode even when stored for extended periods of time. Slight drops in both VOC and FF were observed, which may be the main cause of PCE deterioration. It is likely that repeated exposure to external bias, light, and air during the J-V measurements can induce the degradation in perovskite layer, rather than the electrode. This result implies that the solution-processable electroless-plated Ni layer and AZO composite layer effectively protect CuNWs, demonstrating promising long-term stability.

CONCLUSIONS In summary, we fabricated vacuum-free, silver and indium-free composite window electrodes using CuNWs with Ni protecting layer. From the morphological analysis, the solution processed, electrolessly plated Ni layer was densely and uniformly formed along the surface of CuNWs. The Cu@Ni NW showed improved chemical and thermal stabilities compared to the bare CuNW, preventing both oxidation and iodine compound formation. With the optimized Ni layer, the Cu@Ni NW electrode exhibited a transparency of 80.5% AVT at 400–800 nm and a sheet resistance of 49.3 ± 5 Ω sq-1. The Cu@Ni NW electrode maintained its conductivity during the AZO formation process and showed high stability even after introducing the perovskite precursor solution and annealing the film in ambient air. The smooth and pin-hole-free AZO layer played a role in not only promoting charge carrier efficiency as a composite electrode, but also providing an additional protective layer against halogen ion attack. The PSCs based on AZO/Cu@Ni NW electrode exhibited outstanding performance with the best PCE of 12.17%. Furthermore, the


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AZO/Cu@Ni NW based PSCs showed excellent long-term stability, maintaining 91% of the initial PCE when stored at room temperature with 10–30% RH condition for 500 h. These results clearly demonstrate the great potential of our all-solution-processed AZO/Cu@Ni NW composite electrode for application as a highly stable and cost-effective window electrode for PSCs.

EXPERIMENTAL METHODS Synthesis of copper nanowires (CuNWs): In order to synthesize CuNWs, the precursor solution was prepared, and 0.013 M of anhydrous copper chloride (97%, Sigma-Aldrich), 0.011 M of anhydrous dextrose (Sigma-Aldrich), and 0.056 M of 1-hexadecylamine (98%, Sigma-Aldrich) were dissolved in deionized water (450 mL) at room temperature. A well dispersed Cu(II)alkylamine complex was obtained after stirring t for 12 h. The emulsion was then kept in a hydrothermal reactor at 120 °C for 15 h. After cooling to room temperature, the solution was centrifuged for 3 min at 12000 rpm and then washed several times with deionized water (DI water) followed by a mixture of isopropyl alcohol (IPA, Duksan Pure Chemicals, Korea) and hexane (Duksan Pure Chemicals) to remove any unreacted impurities and excess alkylamine. To obtain well-dispersed CuNWs in IPA, the ligands were exchanged with polyvinylpyrrolidone (PVP) (10000 g/mol, Sigma-Aldrich) by adding the prepared CuNWs into the solution of PVP in IPA (2 mM). The PVP-capped CuNWs were finally obtained for the production of CuNW transparent electrodes. Preparation of Cu@Ni NWs: To prepare transparent CuNW electrodes, the 22 mm-thick soda-lime glass substrates were cleaned with acetone, DI water, and ethanol for 15 min each


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under ultra-sonication. For removing the oxide and hydroxide layers of CuNW surfaces, 7 wt% of lactic acid was added to PVP-capped CuNWs dispersion, followed by a vacuum filtration using a mixed cellulose ester membrane with a pore size of 1 µm, followed by transferring to a soda-lime glass. To remove a residual membrane filter, the CuNW network films were immersed in acetone for 90 s and blown with dry N2 gas. The annealing process was performed under N2 atmosphere at 150 °C for 20 min to completely eliminate the residual organics and to form a slightly fused junction between nanowires. To construct Cu@Ni core-shell structure, the Pd2+ activation solution was prepared by dissolving sodium tetrachloropalladate (99.99%, SigmaAldrich, 4 mg) in ethanol (50.7 mL) and hydrochloric acid (Duksan Pure Chemicals, 3 µL). The commercial electroless Ni plating solution (ENF-M and ENF-A solutions, Yong-In Plachem, Korea) was prepared by mixing ENF-M (8.15 mL) and ENF-A (4.89 mL) with 140 mL of DI water. The prepared bare CuNW electrode film was dipped into the Pd2+ activation solution for 20 s at 23 °C, followed by Ni plating for the controlled duration at 50 °C. Characterization of Optical, structural, electrical property: The surface and crosssectional structure of the samples were obtained by field emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL Ltd, Japan) and TEM (Talos F200X, FEI, USA). The total optical transmittance was measured with a UV-visible spectrophotometer (V-670, Jasco, Japan) equipped with an integrating sphere (ARMN-735, Jasco). The analysis of elemental depth profiling was obtained using an XPS (K-alpha, Thermo Fisher Scientific inc., Waltham, MA, USA). The phase evolution of CuNW electrode and perovskite/CuNW sample were determined by an X-ray diffractometer (Rigaku Miniflex 600, The Woodlands, USA). The sheet resistance of the electrodes was measured using a four-point probe meter (Rs8, BEGA Technologies, Seoul, Korea).


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Fabrication of AZO/Cu@Ni NWs composite electrode: The AZO/Cu@Ni NWs composite electrodes were fabricated by a spin-coating process in an ambient atmosphere. The 0.5 M of zinc precursor solution was prepared by dissolving zinc acetate dihydrate (≥ 98%, Sigma-Aldrich) in 2-methoxyethanol (anhydrous, 99.8%, Sigma-Aldrich). To further prepare the 0.5 M of AZO precursor solution, aluminum nitrate nonahydrate (99.997%, Sigma-Aldrich) was dissolved in zinc precursor solution to obtain 2 at% of Al/(Zn + Al) ratio. The solution was stirred for 30 min at 60 °C, the ethanolamine (≥ 98%, Sigma-Aldrich) was added as a stabilizing agent and stirred for 1 h at 60 °C. Finally, the solution was overnight aged at room temperature. To fabricate the metal oxide overlayered composite layer, the precursor solution was deposited onto the Cu@Ni NW film by spin-coating at 2000 rpm for 40 s and drying at 150 °C for 5 min, followed by annealing at 190 °C for 10 min. The AZO layers were obtained by coating three times. Fabrication of perovskite solar cells: To prepare ETL layer on top of the composite electrode, the 0.2 M of ZnO combustion sol-gel solution in which zinc nitrate hexahydrate (99%, Alfa-Aesar, USA) and zinc acetylacetonate (99.9995%, Sigma-Aldrich) were dissolved in 2methoxyethanol (anhydrous, 99.8%, Sigma-Aldrich) were utilized at a 1:1 of molar ratio. On the composite or commercial electrode substrate, the ZnO combustion solution was coated three times by spin-coating followed by annealing at 190 °C for 40 min. The 3 wt% m-Al2O3 nanoparticle dispersion (Sigma-Aldrich) was spin-coated on the ZnO ETL layer at 3000 rpm 60 s and annealed at 150 °C for 30 min. For the fabrication of perovskite absorber layer, a 53 wt% mixture of lead iodide (Alfa-Aesar), methylammonium iodide (Dyesol, Australia), and dimethylsulfoxide (Sigma-Aldrich) with the mole ratio of 1:1:1 was dissolved in dimethylformamide (Sigma-Aldrich). This solution was spin-coated on the m-Al2O3 coated


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substrates at 4000 rpm for 25 s, followed by annealing at 100 °C for 10 min. The precursor solution for the HTM was prepared by dissolving spiro-OMeTAD (99.8%, Borun Molecular, China, 72 mg) in chlorobenzene (1 mL) with 4-tert-butylpyridine (28.8 µL) and lithium salt solution (520 mg mL-1 lithium bis-(trifluoromethylsulfonyl)imide in acetonitrile, 17.6 µL) as additives. The HTM precursor solution was spin-coated on the perovskite layer at 3000 rpm for 30 s. Finally, 70 nm thick Au electrode was deposited onto the spiro-OMeTAD film by thermal evaporation. Characterizations of photovoltaic properties: The photovoltaic performances were determined using a solar simulator (Sol3A Class AAA, Oriel Instruments, Stratford, CT, USA) under air mass (AM) 1.5 and 1 sun (100 mW cm-2) condition with a Keithley 2400 source measurement unit (Keithley Instruments Inc., Cleveland, OH, USA). The selected area (0.06 cm2) of PSCs covered by the aperture mask was exposed to the light. The current was determined while the voltage was scanned over -0.1–1.2 V range at a rate of 0.52 V s-1 with a delay time of 50 ms. The EQE was also measured by a quantum efficiency measurement system (QEX10, PV Measurements, Inc., Colorado, USA) in the wavelength range of 300–900 nm.

ACKNOWLEDGMENT This work was supported by a grant from the National Research Foundation of Korea funded by the Korean government (MISP) (No. 2012R1A3A2026417).

SUPPORTING INFORMATION


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Performances of bare CuNWs, top and cross view SEM images and TEM images of Cu@Ni NWs, XRD patterns, thermal stability data, XPS depth profiling, and scheme of fabrication procedure, plot of VOC versus JSC, stabilized current density and PCE of full cell, and cumulative irradiance value of each electrode. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1. (a) Schematic representation showing the synthesis procedure of lactic-acid treated CuNW network film followed by annealing at 150 °C and the CuNW networks protected with electrolessly plated Ni (i.e., core-shell Cu@Ni NW). Inset photographs represent the corresponding CuNW and Cu@Ni NW network films. SEM images exhibiting the surface structures of (b) bare CuNW and (c) Cu@Ni NW.


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Figure 2. (a) TEM image of as-synthesized Cu@Ni NW. Elemental mapping images including (b) all elements, (c) elemental Ni, and (d) elemental Cu.


29


Page 30 of 37

Figure 3. (a) Schematic diagram of preparing the samples for stability test about iodination resistance. (b) Sheet resistance variation of Cu@Ni NWs with various thickness of nickel layer as a function of time after perovskite deposition. The as-prepared Cu@Ni NWs were exposed to perovskite precursor solution and annealed at 100 °C for 10 min then aged for 80 h and 143 h in dry condition (at room temperature and under 10% RH.) (c) Schematic illustration showing chemical stability test during the composite electrode fabrication. (d) Corresponding resistance variation as the as-prepared Cu@Ni NWs are coated with AZO sol-gel solution and annealed at 190 °C for 40 min, then aged for 80 h at room temperature, followed by the perovskite precursor deposition and annealing at 100 °C for 10 min.


30

Page 31 of 37

(a)

Top

(b)

Bottom

Top

Bottom

70 40

60

CH3NH3PbI3

CH3NH3PbI3

50 40

I 3d Pb 4f Cu 2p

30 20

Atomic ratio (%)

Atomic raito (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


AZO

Cu@Ni NW

30

I 3d Pb 4f Cu 2p Zn 2p Ni 2p O 1s

20

10

10 0

0

500

1000

1500

2000

2500

3000

0

0

500

Etching time (s)

1000

1500

2000

2500

3000

3500

Etching time (s)

CH3NH3PbI3 AZO

CH3NH3PbI3 Cu Cu Cu Cu

Ni

Ni

Cu Cu glass

glass

Ni

Ni

Cu Cu

Figure 4. XPS elemental depth profiling of (a) perovskite/CuNW structure and (b) perovskite/AZO/Cu@Ni NW. The zero time corresponds to the interface of air and perovskite. The corresponding analyzed sample structures are also shown.


31


100

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

90

80

70

Bare CuNW o

Cu@Ni NW_50 C 50 s o

Cu@Ni NW_50 C 70 s

60

o

Cu@Ni NW_50 C 90 s

25

50

75

100

800

1200

Sheet resistance (Ω /sq) Figure 5. Measured optical transmission (at the wavelength of 550 nm) versus sheet resistance of Cu@Ni NW electrodes with varying Ni thicknesses by plating time. Optical/electrical performance of bare CuNWs is also presented for comparison purpose.


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Figure 6. (a) Cross-sectional SEM morphologies of Cu@Ni NW and (b) AZO/Cu@Ni NW composite electrode. (c) Transmission spectra of bare CuNW, Cu@Ni NW, AZO/Cu@Ni NW electrode, and an FTO electrode as a function of wavelength. (d) Schematic diagram of successive solution processing used to fabricate the AZO/Cu@Ni NW composite electrodebased PSCs.


33


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Figure 7. (a) Cross-sectional SEM images of a full device employing AZO/Cu@Ni NWs as a bottom electrode. (b) J-V curve of the best device under standard 1 sun AM 1.5G illumination. (c) Device performance statistics of the VOC, JSC, FF, and PCE for the PSCs with AZO/Cu@Ni NW composite electrode and FTO electrode. (d) EQE of the best devices with AZO/Cu@Ni NW based electrode and FTO electrode.


34

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Table 1. Photovoltaic performances of top and average values for the PSCs based either AZO/Cu@Ni NW composite electrode or FTO electrode.

VOC [V]

JSC [mA cm–2]

FF [%]

PCE [%]

Top

1.06

16.63

69.11

12.17

Average

1.04±0.02

16.12±0.67

65.62±2.83

11.01±0.56

Top

1.03

20.55

72.35

15.37

Average

1.00±0.02

19.66±0.53

69.98±1.9

13.77±0.67

Electrode type

AZO/Cu@Ni NW

FTO


35

Normalized JSC Normalized FF

Normalized PCE

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Normalized VOC


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1.2 0.8 0.4

AZO/Cu@Ni NW FTO

0.0 1.2

0.8 0.4

AZO/Cu@Ni NW FTO

0.0 1.2 0.8 0.4

AZO/Cu@Ni NW FTO

0.0 1.2

0.8 0.4 0.0

AZO/Cu@Ni NW FTO

0

100

200 300 Time (h)

400

500

Figure 8. Normalized open-circuit voltage, short-circuit current density, fill factor, and power conversion efficiency of PSCs based on AZO/Cu@Ni NW composite electrode as a function of storage time in dry-air condition (box with silica-gel, about 10–15% RH).


36

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Graphical Abstract


37

All-Solution-Processed Thermally and Chemically Stable Copper

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