Ultrathin Silver Film Electrodes with Ultralow ... - ACS Publications

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Ultrathin silver film electrodes with ultralow optical and electrical losses for flexible organic photovoltaics Guoqing Zhao, Wenfei Shen, Eunwook Jeong, Sang-Geul Lee, Seung Min Yu, Tae Sung Bae, Gun-Hwan Lee, Seung Zeon Han, Jianguo Tang, Eun-Ae Choi, and Jungheum Yun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08578 • Publication Date (Web): 20 Jul 2018 Downloaded from http://pubs.acs.org on July 22, 2018

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ACS Applied Materials & Interfaces

Ultrathin silver film electrodes with ultralow optical and electrical losses for flexible organic photovoltaics Guoqing Zhao,† Wenfei Shen,ǁ Eunwook Jeong,† Sang-Geul Lee,‡ Seung Min Yu,± TaeSung Bae,± Gun-Hwan Lee,† Seung Zeon Han,§ Jianguo Tang, * ,ǁ Eun-Ae Choi, *,§ and Jungheum Yun*,† †

Surface Technology Division and §Materials Processing Innovation Research Division, Korea

Institute of Materials Science, Changwon, Gyeongnam 51508, Republic of Korea ǁ

Academy of Hybrid Materials, National Base of International Science & Technology

Cooperation on Hybrid Materials, Qingdao University, Qingdao 266071, People’s Republic of China ‡

Daegu Center, Korea Basic Science Institute, Daegu, 41566, Republic of Korea

±

Jeonju Center, Korea Basic Science Institute, Jeonju, Jeonbuk 54907, Republic of Korea

ABSTRACT: Improving the wetting ability of Ag on chemically heterogeneous oxides is technically important to fabricate ultrathin, continuous films that would facilitate the minimization of optical and electrical losses to develop qualified transparent Ag film electrodes in the state-of-the-art optoelectronic devices. This goal has yet to be attained, however, because conventional techniques to improve wetting of Ag based on heterogeneous metallic wetting layers are restricted by serious optical losses from wetting layers. Herein, we report on a simple and effective technique based on the partial oxidation of Ag nanoclusters in the early stages of Ag growth. This promotes the rapid evolution of the subsequently deposited pure Ag into

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completely continuous layer on the ZnO substrate, as verified by experimental and numerical evidence. The improvement in the Ag wetting ability allows the development of a highly transparent, ultrathin (6 nm) Ag continuous film exhibiting an average optical transmittance of 94% in the spectral range 400−800 nm and a sheet resistance of 12.5 Ω sq−1, which would be well suited for application to an efficient front window electrode for flexible solar cell devices fabricated on polymer substrates.

KEYWORDS: ultrathin Ag film; wetting layer; ultralow optical loss; flexible transparent electrode; organic photovoltaics


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1. INTRODUCTION Recent advances in the fabrication of highly conductive metals with two-dimensional (2D), ultrathin film-type geometries on chemically heterogeneous non-metallic substrates have led to their being considered for applications beyond conventional electrical connectors. Such applications include highly efficient transparent electrodes (TEs) that could be applied to stateof-the-art

optoelectronic

devices

including

photovoltaics,

light-emitting

diodes,

and

electrochromics, all of which would benefit from the superior optoelectrical performance of these TEs. Their performance clearly exceeds those of conventional transparent conductive oxides.1 The performance of metal-film TEs, typically designed as a metal−oxide multilayered structure with antireflection and anticorrosion features, is characterized by the optimum level of electrical conductivity and optical transparency, which would require an optimum combination of material choice and structural design.2−4 Given that Ag exhibits the highest electrical conductivity and the lowest optical loss of any metal over the visible spectral range (400−800 nm), the use of Ag as a metal-film candidate for TEs would be an excellent choice for improving the optoelectrical performance of TEs when only an ultrathin, continuous Ag film can be attained. This would overcome the detrimental trade-off between conductivity and transparency.1 However, this task remains highly challenging due to the poor wetting ability of pure Ag on an oxide substrate, giving rise to a need to alleviate the three-dimensional (3D) island growth of Ag while maintaining its chemical integrity and thus not sacrificing its inherent optoelectrical properties. Among the various techniques that are applied to improve the wetting ability of Ag on an oxide, the use of an atomically thin, metallic wetting layer (WL) that is deposited on the oxide prior to Ag growth has been recognized as the most effective strategy for facilitating the


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fabrication of ultrathin 2D-Ag films.5−22 Regarding the maintenance of the chemical integrity of any such Ag film, the technique exhibits a clear advantage over its counterparts that use metallic additives, predominantly Al

23−25

and Cu

26, 27

, in the Ag film as impurities with relatively high

concentrations in a range of a few to tens of atomic percent. Any WL material must be capable of realizing a strong bond both at the interface between the interlayer and the oxide substrate, as well as that between the interlayer and subsequently deposited Ag. Many metallic materials have proven to be capable of doing so, including Al,6−8, 11 Au,9−12 Cr,28 Cu,18 Ge,13−17, 19, 21, 22, 28, 29 Ni,16, 21, 22

Sn,5 and Ti 21. If, however, 2D ultrathin Ag films are to be used to further improve the

performance of Ag-based TEs for which electrical and optical losses must be minimized, the contribution of the WLs to the morphological improvement of the Ag films could be adversely offset by the strong optical loss incurred as a result of the inferior optical transparency of the metallic WLs relative to pure Ag films. This would greatly degrade the performance of a TE. There is currently no established technical strategy for improving the wetting ability of Ag that would equal the merits of the aforementioned heterogeneous metallic wetting layers (HMWLs) while simultaneously avoiding the optical losses incurred by HMWLs. Meanwhile, the ability of the Ag to wet the oxide substrate during the very early growth stages, prior to the development of a continuous layer, is dictated by the inherent 3D growth that can be represented by the highly granular evolution and coalescence of Ag nanoclusters. This growth mode is a thermodynamically favorable consequence forming a film of a high-surfaceenergy noble metal on a low-surface-energy oxide substrate, which is accompanied by poor adhesion and thus a high free energy at the interface of the noble metal with the substrate.30, 31 The ability of a metallic WL to suppress the 3D growth behavior of Ag on an oxide substrate can be regarded as being a direct consequence of the strong adhesion and thus good wetting of the


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interlayer with the oxides, probably due to either a redox reaction 32, 33 or charge transfer 34, 35 between the interlayer and oxides. In this respect, recently proposed techniques based on the gasadditive-mediated growth of Ag and Cu have attracted considerable attention because of their ability to suppress the 3D growth mode and thus improve the wetting ability of the target metals in the presence of trace gas additives including oxygen and nitrogen.36−40 Furthermore, a notable reduction has been observed in the surface and interface energies of ultrathin Cu layers grown on ZnO substrates, induced by the partial oxidation in the lattice of Cu.40 If the effects of the gas addition are comparable with those of the HMWLs as they relate to improving the Ag wetting of the oxide substrates, it is not unreasonable to suppose that there would be an excellent chance of resolving the technical challenges related to the optical losses in Ag TEs (as incurred by the use of HMWLs). The currently available, but optically inferior, HMWLs could be replaced with an optically transparent WL formed via gas-additive-induced growth. In the present study, we examined the evolution of an ultralow-optical-loss Ag film on a WL utilizing partially oxidized, but predominantly metallic, Ag (denoted Ag(O)) on a zinc oxide (ZnO) substrate, via the simple oxygen-mediated reactive sputtering of Ag at room temperature. The unique homogenous multilayer structure, Ag/Ag(O), maintains the chemical integrity of the Ag film and so successfully emulates the virtues of HMWLs to improve the wetting ability of the Ag. At the same time, it successfully prevents the optical losses caused by the WL. The improvement in the wetting ability of Ag/Ag(O) on ZnO substrates, which is comparable to the effects of HMWLs, indicates that a non-continuous Ag(O) geometry, consisting of relatively discrete, sparse, irregular clusters, is sufficient as a WL to enhance the wetting with the subsequently grown Ag. This is quite distinct from the common understanding that an ultrathin metallic layer, consisting of highly dense clusters, strongly bonded to its substrate, is an essential


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prerequisite for a candidate WL. To clarify the cause of this discord, computational investigations were undertaken using first-principles calculations based on density functional theory. These provided solid evidence for the Ag(O)-induced reduction in thermodynamic energies that affect the growth of Ag on a ZnO substrate. The Ag/Ag(O) is an efficient TE type which exhibits strong broadband transparency over the entire visible spectrum while maintaining the electrical superiority of the Ag. The simple reactive sputtering process of Ag/Ag(O) at room temperature facilitates the fabrication of an Ag/Ag(O) TE on flexible, heat-sensitive polymers, which is important for the realization of mass production of flexible solar cells.41 The potential of an Ag/Ag(O) TE, which exhibits a strong broadband transparency over the entire visible spectrum while maintaining the electrical superiority of Ag on a flexible polymer substrate, was demonstrated by fabricating a flexible organic solar cell. An excellent improvement in the photon-to-electron conversion efficiency was attained by designing the Ag/Ag(O) TE with an oxide/metal/oxide (OMO) configuration.

2. RESULTS AND DISCUSSION 2.1 Morphological evolution of Ag on WLs. The oxygen-additive-induced manipulation of Ag wetting was aimed at verifying the possibility of fabricating atomically thin Ag(O) to act as a WL to replace the HMWLs that are currently used to grow ultrathin, ultralow-optical-loss Agfilm electrodes. To achieve this goal, the optoelectrical performance of an Ag/Ag(O) film was quantitatively compared with that of Ag films formed on HMWLs using Ge and Cu. Among the metals applied to the HMWLs used for Ag growth, Ge has come to be recognized as the best candidate, as its use results in superior wetting by the Ag. It does, however, incur a considerable optical loss. Cu has also attracted attention because of its relatively low electrical loss, although


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its performance as a wetting agent appears to be inferior to that of Ge. The schematic diagrams (Figure 1a) and corresponding photographs (Figure 1b) of ZnO/Ag/Ag(O)/ZnO and ZnO/Ag/Ge/ZnO (denoted ZAOZ and ZAGZ, respectively) TEs, grown on highly flexible polyethylene terephthalate (PET) substrates with an area of 100 cm2, clearly demonstrate the superior optical transparency of the ZAOZ TE, relative to that of the ZAGZ TE, in that it successfully resolves the optical loss attributed to the Ge HMWL. More importantly, the achievement of this level of optical transparency did not incur losses in terms of the electrical conductivity of the ZAOZ TE, while a non-negligible reduction in the electrical conductivity was clearly observed for the ZAGZ TE due to the inferior conductivity of Ge relative to Ag. The optoelectrical performances of ZAOZ TEs clearly surpass those of the film-type TEs currently available for PET substrates, including ZnO/Ag/ZnO (denoted ZAZ), ZAGZ, ZnO/Ag/Cu/ZnO (denoted ZACZ), and amorphous ITO single-film TEs. This was confirmed by quantitatively comparing the change in the total transmittance, averaged for the visible spectral range (400−800 nm), with the sheet resistance (Figure 1c). The ZAOZ TE exhibited a total transmittance of 95.2% and 93.8% for sheet resistances of 23 Ω/sq and 12 Ω/sq, respectively, and maintained a total transmittance > 92% even for a sheet resistance < 10 Ω/sq. This was greatly superior not only to the total transmittances of the ZAGZ and ZACZ TEs but also to those of the ZAZ and ITO TEs. Morphological observations of the very early stages of Ag growth on a ZnO substrate provided evidence for the successful substitution of Ge- and Cu-based HMWLs with the Ag(O) WL. Ultrahigh-resolution field-emission scanning electron microscopy (UHR FE-SEM) was used to capture highly magnified morphological images of different WLs deposited on ZnO substrates, as well as the Ag that was subsequently deposited both with and without the WL


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(Figure 2a, b). In the present study, a ZnO film was selected to act as the substrate for the subsequently deposited metals because of its excellent optical transparency and relatively good wettability with Ag.1 Highly crystalline features with a preferred (0002) orientation were observed for the 5-nm ZnO substrate that was deposited at room temperature. There was no clear difference in the density and shape of the 0.7-nm Ag(O) layer, consisting of sparse, polygonal nanoclusters, relative to pure Ag of the same thickness. However, the Ge and Cu exhibited distinctly different morphologies, but which were assumed to be ideal for a WL. That is, they could be either a completely continuous 2D layer in the case of Ge or at least an array of highly dense, small nanoclusters in the case of Cu, with excellent wetting of the ZnO substrate (Figure 2a). A significant contribution of the Ag(O) WL to improve the wetting ability of the Ag was not attained until the thickness of the Ag(O) increased to a threshold value, in this case around 1.5 nm, which led to the formation of a random distribution of larger, but still sparse, irregular-shaped Ag(O) nanoclusters (Figure 2b). However, the nanoclusters of pure Ag continued to exhibit polygonal geometries as a result of the complete coalescence of neighboring nanoclusters into larger, but still polygonal geometries. This morphological feature of the Ag(O) nanoclusters could be attributed to their incomplete coalescence as a result of the oxygen-additive-induced mitigation of the volatile 3D clustering of the noble metal that is weakly bound to the ZnO substrate (Figure S1, Supporting Information). Assuming the coalescence between neighboring nanoclusters as being a consequence of the surface dynamics, leading to a reduction in the thermodynamic free energies of the nanoclusters by increasing the size of these nanoclusters, the incomplete coalescence of the Ag(O) nanoclusters could be attributed to a significant reduction in the free energies of the nanoclusters, of relatively small sizes, in the presence of oxygen.1, 37 Details of the relevant dynamics are discussed, together with the computational results, below.


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The performance of the Ag(O) as a WL was found to be very sensitive to changes in its thickness and oxygen level because of the influence of these parameters on the morphological feature of the Ag(O) nanoclusters. The minimum thickness of the Ag(O) as an efficient WL was observed at around 1.5 nm for an oxygen inclusion level generated by applying reactive sputtering under a plasma of Ar and O2 (an inlet Ar:O2 flow rate of 45:8 sccm) at room temperature. The chemical state of Ag(O) was identified as being predominantly the metallic Ag (Ag0) phase, including a small portion of an oxidized Ag (Ag+) phase with an oxygen concentration of roughly 7%, as determined by X-ray photoelectron spectroscopy (XPS) (Figure S2, Supporting Information). The Ag(O) was verified from the X-ray diffraction (XRD) patterns as being a highly metallic Ag(111) structure without any notable difference in the crystallographic features, relative to those of pure Ag (Figure S3, Supporting Information). Any reduction in the oxygen level and thickness of the Ag(O), which was examined by changing the Ar:O2 flow rate to 45:4 sccm and the Ag(O) thickness to 0.7 nm, had an adverse effect on the wetting ability of the Ag subsequently deposited on the Ag(O) WL (Figure S4, Supporting Information). The Ag(O) WL, with the significantly reduced surface density and coverage of Ag(O) nanoclusters, relative to the HMWL counterparts, still realized an improvement in the wetting ability of the subsequently deposited Ag. The Ag/Ag(O) exhibited a continuous film morphology with a thickness of as little as 6 nm due to the improvement in the wetting of the ZnO substrate, relative to an Ag film being directly deposited onto the substrate. Moreover, the increase in the wetting ability of the Ag/Ag(O) on the ZnO substrate as a function of the Ag thickness was found to be comparable to that of Ag/Ge and even better than that of Ag/Cu (Figure 2c) when the Ag wetting ability was approximated from plane FE-SEM images by determining the surface area of


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the ZnO substrates covered with Ag nanoclusters (Figure 2d). Morphological investigations using atomic force microscopy (AFM) were performed to further confirm the positive role of the Ag(O) WL in improving the morphology of the objective Ag film (Figure 3 and Table S1, Supporting Information). A comparison of the plane views and corresponding line profiles of the Ag surfaces clearly demonstrated large discrepancies in the morphological features of the Ag on different WL types (Figure 3a, b). The surface roughness of the Ag/Ag(O) was estimated to be around 0.6 nm in the case of the minimum thickness (around 6 nm) required to develop a continuous Ag/Ag(O) film. Although the surface roughness of the Ag/Ag(O) was estimated to be greater than that of the Ag/Ge that exhibited an atomically flat morphology with a surface roughness of less than 0.2 nm for the same film thickness, the surface roughness of the Ag/Ag(O) was always lower than that of the Ag deposited directly onto the ZnO substrate without a WL. Furthermore, an increase in the surface roughness that was observed with an increase in the thickness of the pure Ag was substantially alleviated in the case of the Ag/Ag(O). 2.2 Numerical predictions of wetting improvement. Numerical first-principles calculations based on density functional theory (DFT) were conducted to elucidate the working principles attributed to the improvement of the wetting ability of Ag on the Ag(O) WL. The thermodynamic stability of the Ag(O) WL on a ZnO substrate was compared with that of pure Ag and a Ge WL on a ZnO substrate. The thermodynamic cohesive energy (Ecoh) was employed as a criterion for determining the stability of the WLs in a layered geometry on the surface of a ZnO substrate. This was calculated using the following formula: =

− ( + ∆ + ∆ ) , ∆ + ∆

where EW is the total energy of the WL on ZnO, ES is the total energy of the oxygen-terminated ZnO substrate without any WL, ∆ and ∆ represent the amount of metal (either Ag or Ge)


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and oxygen in the WL, respectively, and and represent the chemical potentials of the metals and oxygen, respectively. The chemical potentials of the Ag and Ge were approximated from the total energy of a face-centered cubic Ag bulk and that of a diamond-structured Ge bulk, respectively. The chemical potential of the oxygen in the plasma was assumed from the formula:

= + 1 eV,

where is the total energy of an oxygen molecule. A negative Ecoh value indicates that the layered geometry of metal atoms, constituting a WL, is energetically stable on the surface of the ZnO substrate. On the other hand, a positive Ecoh value indicates that the layered geometry of the metal atoms is energetically unfavorable, leading to the promotion of 3D clustering of the metal atoms on the surface of the ZnO substrate. A simple geometric configuration exhibiting a monolayer arrangement of metal atoms on the ZnO surface was assumed for the numerical calculations. The preliminary results indicated that Ag and Ge atoms bond with the oxygen atoms of the surface of the ZnO substrate by preferably occupying the energetically favorable sites (Figure S5, Table S2, Supporting Information). The Ecoh calculations clarified the wetting features of Ag, Ge, and Ag(O) incorporated into the tetrahedral configurations on the oxygen-terminated surface of the ZnO substrate (Figure 4). First, the formation of the layered configuration was predicted to be highly favorable for Ge on the ZnO substrate from a thermodynamic aspect because of a large negative Ecoh value of −0.6 eV/atom, whereas the same geometric configuration of Ag was unlikely to form on the ZnO substrate because of a positive Ecoh value of 0.2 eV/atom, indicating the relatively low bonding strength of the Ag to the ZnO substrate, which could lead to the formation of 3D Ag nanoclusters instead of a 2D Ag layer (Figure 4d). Meanwhile, the incorporation of oxygen additives into the Ag(O) was predicted to promote the formation of the layered configuration of the Ag(O) WL, this


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being a consequence of the reduction in the Ecoh of the Ag(O) WL that is predominantly attributed to the oxygen termination of the non-bonding electrons of Ag. The change in the value of Ecoh with the oxygen concentration in the Ag(O) pointed to the strong contribution of the oxygen to the reduction in the Ecoh value of the Ag(O) WL. The Ecoh value was expected to decrease to a negative value as the oxygen concentration in the Ag(O) increased to > 7.5% (Figure 4d). This indicates that the Ag(O) WL becomes energetically more stable as the oxygen concentration increases to 7.5%. Further calculations were performed to determine the wetting stabilities of Ag layers that were subsequently deposited on the layered Ag, Ge, and Ag(O). A strong reduction in the Ecoh value of the Ag layer was predicted with an increase in the oxygen level of the Ag(O) WL, which is strong evidence for the oxygen in the Ag(O) WL contributing to an improvement in the wetting of the Ag layer subsequently deposited on the Ag(O) WL (Figure S6, Supporting Information). When the value of Ecoh was calculated for face-centered cubic Ag(111) monolayers that were deposited and then relaxed on different Ag, Ge, and Ag(O) WLs, the value of Ecoh was predicted to be negative in every case, indicating that the Ag(111) monolayers were energetically stable on each WL. Although the Ag(111) monolayer was predicted to be the most stable on the Ge WL, given that it had the lowest Ecoh value of −0.1 eV atom−1, the continuous reduction in the Ecoh of the Ag(111) monolayer with an increase in the oxygen concentration in the Ag(O) WL was also predicted. The numerical predictions can be readily associated with a thermodynamic view that describes the oxygen-induced change in the wetting of the Ag/Ag(O) layer as being a consequence of the reduction in the driving force for 3D coalescence between nanoclusters. The improvement in the wetting ability of a high-surface-energy noble metal on an oxide substrate can result from


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a reduction in the interface free energy between the metal and the oxide. The reduction in the Ecoh values of the Ag(O) and subsequently deposited Ag layers corresponds with a reduction in the interface free energy of the Ag/Ag(O) layer on the ZnO substrate. This induces the early transition from complete coalescence to incomplete coalescence (Figure S1) and thus the rapid formation of completely continuous layer geometries for the Ag/Ag(O) (Figure 2). 2.3 Optoelectrical performances and photovoltaic applications. The excellent wetting ability of Ag/A(O) on the ZnO substrate leads to the formation of a continuous ultrathin layer, while avoiding the use of optically unfavorable HMWLs. This facilitates an improvement in the optical transmittance relative to a pure Ag single film in an OMO configuration. The Ag/Ag(O) with a ZAOZ configuration, based on bottom and top ZnO films with thicknesses of 5 and 25 nm, respectively, exhibited excellent optoelectrical performance on a flexible PET substrate. The thicknesses of bottom and top ZnO films were determined to optimize the antireflection index matching in the ZAOZ TE at reduced thicknesses that also contribute to suppress the optical absorption of ZnO films (Figure S7, Supporting Information). The ZAOZ TE, optimized with a 6-nm Ag/Ag(O) layer, exhibited a strong transmittance spectrum across the visible range, far surpassing not only that of the ZAZ TE but also those of the ZAGZ and AZCZ TEs (Figure 5a and Figure S8, Supporting Information). When the degree of optical scattering was further evaluated by determining transmittance hazes for different TEs, the optical scattering was found to be negligible for the TEs with very small transmittance hazes clearly less than 1% (Figure S9, Supporting Information). It is expected that it will be possible to maximize the optical transmittance at the minimum continuous thickness of the metal layer by suppressing the serious optical losses that are caused either by the strong reflection of the incident light in the thicker, continuous layers or by the strong scattering in thinner, discontinuous clusters. From this


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viewpoint, the strong reduction in the optimal transmittance spectrum of the ZAZ TE relative to that of the ZAOZ TE could be attributed to the delay in the formation of a continuous Ag layer up to a thickness of ca. 9 nm, accompanied by strong optical reflections (Figure S10, Supporting Information). Meanwhile, the inferior transmittance spectra of the ZAGZ and ZACZ TEs could be attributed to the higher optical absorbance spectra, caused mainly by the HMWLs. The strong absorbance was not detected for the ZAOZ TE at the minimum continuous thickness of the Ag/Ag(O) (Figure S11, Supporting Information). The superior wetting performance of the Ag/Ag(O), without any notable deterioration in the electrical conductivity associated with Ag(O), led to the early realization of good electrical conductivity in the ZAOZ TE fabricated on a flexible PET substrate. When the sheet resistances of different TE types were evaluated as a function of the Ag thickness, the ZAOZ TE exhibited the lowest electrical resistivity among the variations addressed in the present study for Ag thicknesses < 9 nm (Figure 5b). A very narrow distribution of the carrier concentrations, specifically, 3−4 × 1022 cm−3, clearly indicates the metallic characteristics of the conducting layers (Figure S12, Supporting Information). The lower sheet resistance of the ZAOZ TE, relative to those of the ZAZ and ZACZ TEs, was attributed mainly to its higher electron mobility due to the fast development of the electrical paths in the Ag/Ag(O) on the ZnO substrate (Figure 5c). However, although conductive paths were developed in the Ag/Ge, which had the best wetting performance among the candidates, faster than in the Ag/Ag(O), the ZAGZ TE had a sheet resistance that was clearly higher than that of the ZAOZ TE. This result can be explained by the lower electron mobility of the Ag/Ge which would be dominated by the inferior conductivity of the Ge WL. However, it is important to note that the positive influence of the Ag(O) WL on the electrical conductivity of the ZAOZ TE can be maximized under precisely


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optimized conditions, that is, a thickness of 1.5 nm and an standard oxygen level (Ar:O2 flow rate of 45:8 sccm) for the Ag(O). The electrical conductivity of the ZAOZ TE deteriorated considerably with a reduction in either the thickness or oxygen level from the optimal conditions (Figure S13, Supporting Information). Measurement of the sheet resistances of the ZAOZ TEs utilizing non-optimal Ag(O) conditions revealed them to be even higher than that of the ZAZ TE, due to the poor wetting ability of Ag that evolved on the Ag(O) WL. However, due to the use of the optimized Ag(O) as a WL, the simultaneous attainment of strong optical transmittance and electrical conductivity in the ZAOZ TE was confirmed from its figure of merit, which was found to be much better than those of the other OMO candidates (Figure 5d). The mechanical flexibility of the ZAOZ TE was evaluated by performing bending tests, in which that of the ITO TE was also evaluated for comparison (Figure 6). When the mechanical flexibility of both TEs was evaluated as a function of the bending radius of polymer substrates (Figure 6a), the ZAOZ TE showed a negligible increase in the resistance even as the bending radius was reduced to 6 mm (Figure 6b). However, the ITO TE showed a dramatic increase in its resistance as the bending radius was reduced to 9 mm. The reliability of the TEs against their continuous exposures into mechanical deformation geometries was further determined by implementing a cyclic bending test at a minimum bending radius of 9 nm (Figure 6 c). The ITO TE showed a catastrophic increase in its resistance as soon as the bending test was initiated, whereas the ZAOZ TE exhibited a resistance change of less than 3% even after 1100 bending cycles. The excellent stability of the ZAOZ TE in severely deformed geometries is readily attributed to the ductility of the continuous Ag film and the reduced thicknesses of bottom and top ZnO films. The excellent optoelectrical characteristics of the ZAOZ TE, with its high broadband


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transmittance over the entire visible spectral range and low electrical resistivity, provide an opportunity to improve the performance of optoelectrical devices utilizing the ZAOZ TE as their front electrode. The high light harvesting realized by applying high efficient TE is a well-known and frequently reported method to improve solar cell performance.36-40, 42-43 The fabrication of a ZAOZ TE on flexible polymer substrates at room temperature further enhances the potential of using ZAOZ TEs for the preparation of highly efficient flexible optoelectronic devices. In the present study, this was demonstrated using a flexible organic solar cell (OSC). The contribution of the optoelectrical performance of the ZAOZ TE to the improvement of the photocurrent conversion efficiency of the flexible OSC was investigated using a photoactive blend layer. This consisted of an electron donor material, poly[[4,8-bis[(2-ethylhexyl)-carbonyl]-thieno-[3,4-b]thiophenediyl]] (PTB7-Th), and an electron acceptor material, namely, [6,6]-phenyl-C71-butyric acid

methyl

ester

(PC71BM)

in

an

inverted

device

configuration:

PET/TE/PTB7-

Th:PC71BM/poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS)/Ag (Figure 7a). The external quantum efficiency (EQE) spectra (Figure 7b) of the OSCs utilizing different TE types were found to have serious implications with the transmittance spectra (Figure 5a), as measured for the TE types. The exceptionally high broadband transmittance of the ZAOZ TE utilizing a 6-nm Ag/Ag(O) film proved highly advantageous for improving the EQE spectrum and thus the current density across the entire light-harvesting spectral range of the flexible OSCs, relative to other OMO and ITO single-film TEs. When the photovoltaic performance parameters, including the short-circuit current density, open-circuit voltage, fill factor, and the corresponding photocurrent conversion efficiency (PCE) of the flexible OSCs were determined from the current density−voltage (J−V) characteristics (Figure 7c, Table 1), the performance of the OSCs using different TEs was found to be predominantly influenced by the current density, which was


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strongly dependent on the differences in the optical transmittance of the TE types in this study. The highest PCE of the OSC using a ZAOZ TE is thus attributable to the highest transmittance of the ZAOZ TE. However, any notable difference in the FF values of the OSCs using the different TE types was not observed because of the small differences in the sheet resistance of the TE types and the small photoactive size of the OSCs used in the present study (0.1 cm2). For the circumstances, the contribution of the superior optical transmittance of the ZAOZ TE to the improvement in the Jsc was magnified as shown in Table 1. Although the contribution of the electrical conductivity of the TEs to the performance of OSCs should increase with the photoactive size of the OSCs, relative to that used in the present study, the advantages of the ZAOZ TE would be maintained given its superior optical transmittance, even for an identical sheet resistance, relative to the other TE candidates. Since the failure of flexible OSCs in mechanically deformed geometries is attributed to the development of microscopic cracks in the TEs accompanied by their catastrophic electrical failures,36-37,44-45 the excellent mechanical flexibility of the ZAOZ TE ensures superior durability for flexible organic solar cells using the TE as their window electrode when compared with the solar cells using a fragile ITO TE.

3. CONCLUSION In the present study, the use of an ultrathin Ag(O) WL is proposed as a viable alternative for improving the wetting ability of the subsequently deposited Ag while circumventing the opticalloss issue incurred by the use of HMWLs. The chemical and geometrical features of the Ag(O) WL are typified by distinctly separated, irregular island-like nanoclusters of minimally oxidized, but highly metallic, Ag. Our experimental results support the significant contribution of the Ag(O) WL to the rapid development of a completely continuous Ag film with the benefits of the


17


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homogeneous growth of Ag on highly stable Ag(O) nanoclusters grown on a ZnO film. Numerical predictions using first-principles calculations based on density functional theory provided evidence of the oxygen-additive-induced stabilization of an atomically thin Ag/Ag(O) layer on a ZnO substrate, with a reduction in the formation energies of the 2D Ag(O)−ZnO and Ag−Ag(O) interfaces. This supports a significant improvement in the wetting ability of the Ag, thus enabling the formation of a completely continuous, ultrathin Ag film. This minimizes the optical and electrical losses in Ag-based TEs applied to optoelectronic devices. The excellent potential of Ag/Ag(O) for application to state-of-the-art optoelectronic devices was confirmed by a significant improvement in the photon-to-electron conversion efficiency of a flexible photovoltaic device utilizing the Ag/Ag(O) as its front electrode material.

4. EXPERIMENTAL METHODS Deposition and characterization of TEs. The metals (Ag, Ag(O), Cu, and Ge) and oxides (ZnO and ITO) used in various TE types were deposited on either 125-µm PET substrates or Si wafers by conventional multigun magnetron sputtering (Flexlab System 100, A-Tech System Co.) at room temperature. The multilayer configurations of the TEs were fabricated sequentially in a continuous vacuum under the following conditions. Pure metals, including Ag, Cu, and Ge, were deposited by sputtering pure 4-inch Ag, Cu, and Ge targets (Applied Science Co.) under a pure Ar atmosphere, generated with an inlet Ar flow rate of 45 sccm. The Ag(O) was deposited by reactive sputtering the 4-inch Ag target under a mixed atmosphere of Ar and O2, generated with an inlet Ar flow rate of 45 sccm and an inlet O2 flow rate of between 4 and 8 sccm. Direct current (DC) power (50 W) was applied for the sputtering of Ag, Ag(O), and Cu, whereas a relatively low DC power (25 W) was applied to the sputtering of Ge. The ITO and ZnO were


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deposited by sputtering ITO and ZnO targets, respectively, under a pure Ar atmosphere generated by an inlet Ar flow rate of 60 sccm at a radio frequency power of 200 W. The sputtering processes and conditions were as described in the literature.37, 38 Morphological image analyses of the metal nanostructures, formed by the nanoclusters combining into continuous layers, were carried out by UHR FE-SEM (S-5500, Hitachi HighTechnologies Co.) at the Jeonju center of the Korea Basic Science Institute (KBSI Jeonju, Republic of Korea) and AFM (NX10, Park System). The optical characteristics, including the transmittance

and

reflectance,

were

determined

by

ultraviolet/visible/near-infrared

spectrophotometry (Cary Series, Agilent Tech.). The transmittance spectra of the TEs were determined by subtracting that of the PET substrate, while the absorbance spectra of the TEs were determined by ascertaining the aforementioned transmittance and reflectance spectra. The transmittance haze of different TEs was determined from specular and total transmittances using a simple formula, Transmittance haze (%) = (Ttotal − Tspecular)/Ttotal × 100, where Ttotal and Tspecular are the total transmittance and specular transmittance, respectively. The sheet resistances of the TEs were determined by four-point probe (MCP-T600, Mitsubishi Chemical Co.) measurements, and the carrier concentration and mobility were determined by Hall effect (8400 series HMS, Lake Shore Cryotronics Inc.) measurements. The bending tests were performed in a home-made bending test system as a function of the bending radius of PET substrates, in which the TEs were subjected to tensile stresses. The percent change in the resistance of TEs was calculated by ∆R (%) = [(R − R0)/R0] × 100, where R0 and R are the resistance before and after bending, respectively. The surface coverages of different metals on the ZnO substrate were determined using image-processing software (NIS-Elements Basic Research, Nikon). The chemical and crystallographic characteristics of the Ag and Ag(O) layers were determined by XPS (Escalab 200


19


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R, VG Scientific) at the Electronics and Telecommunications Research Institute (Daejeon, Republic of Korea) and by XRD (Empyrean, PANalytical) at KBSI Daegu, respectively. The thicknesses of the metals and oxides were determined from either the X-ray reflectivity (X’pert Pro-MRD, Philips) at KBSI Daegu or surface profiling (Dektak XT, Bruker) measurements. Computational Simulations. Numerical first-principles calculations based on density functional theory were implemented using the Vienna Ab-initio Simulation Package (VASP).46 The ionic potentials were determined by projector-augmented wave potential.47, 48 The kinetic cut-off energy of the plane-wave bases was fixed to 450 eV. The exchange and correlation potentials were determined by Perdew-Burke-Ernzerhof approximation.49 The ZnO [0001] slab configuration with the bottom layer, passivated by pseudo hydrogen atoms with 1.5 electrons, was used for the calculations. The top-most bilayers of the oxygen-terminated surface of ZnO substrates were relaxed when the Ag, Ag(O), and Ge monolayers were deposited on the ZnO substrates. A vacuum region of 18 Å was used on the ZnO slab system to exclude any interaction with the adjacent supercells. A gamma-centered (2 × 2 × 1) Monkhorst-Pack k-point grid was used for the calculations. The residual forces on each atom were reduced to less than 0.01 eV/Å using the conjugate-gradient method. Fabrication and characterization of OSCs. The flexible OSCs were fabricated on PET substrates using different TEs as their front electrodes. The electron donor material, PTB7-Th (1material Chemscitech Inc.), and the electron acceptor material, PC71BM (Solemme BV), were blended at a concentration ratio of 9 mg/ml:13.5 mg/ml in a chlorobenzene solution. The PTB7Th:PC71BM layer with a thickness of around 100 nm was deposited from the solution as a result of the sequential processes including spin-coating and the thermal treatment of the ZnO layers. The PEDOT:PSS layer was subsequently spin-coated on the photoactive layer using a solution of


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PEDOT:PSS and isopropyl alcohol (IPA). A thermal treatment process was conducted to dry the PEDOT:PSS layer by accelerating the evaporation of IPA at a temperature less than 100 °C. The EQE of the OSCs was determined by Newport incident photon conversion efficiency measurements. The J−V characteristics were determined by applying illumination equal to air mass 1.5 global with an intensity of 100 mWcm−2 to ten specimens, each with an area of 0.1 cm2, for each OSC condition.

AUTHOR INFORMATION Corresponding Authors *Address: 797 Changwondaero, Changwon, Gyeongnam, 51508, Republic of Korea. E-mail: [email protected]; [email protected]. Tel: +82 (0)552803515. Fax: +82 (0)552803570. 308 Ningxia Road, Qingdao 266071, P. R. China. E-mail:[email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was funded by the Fundamental Research Programs (PNK5840 and POC2910) of the Korea Institute of Materials Science and by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173030014370). W.S. thanks the Program for Introducing Talents of Discipline to Universities (“111” Plan).

Supporting Information


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Additional Figures (S1-S13) and Tables (S1,S2) are included in the Supporting Information and referring to: Coalescence behaviors Ag(O) nanoclusters, Crystallographic structural and chemical information of Ag(O) films from XRD and XPS analyses, Optimization of Ag(O) acting as WL, surface roughness of Ag films with and without WL, Details on simulation structure, Detail optical and electrical properties of transparent electrodes. REFERENCES (1)

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Table 1 Photovoltaic performances of flexible OSC devices fabricated with TEs optimized according to the conditions shown in Figure 5(a). a

TE type

Rsheet [Ω sq−1]

T [%]

Jsc [mA cm−2]

Voc (V)

FF [%]

PCE [%]

ZAZ

5.83

84.11

15.64 ± 0.14

0.78 ± 0.01

55.30 ± 0.21

6.74 ± 0.12

ZAOZ

11.97

93.75

17.09 ± 0.15

0.78 ± 0.01

55.66 ± 0.41

7.47 ± 0.15

ZAGZ

26.42

86.18

15.90 ± 0.18

0.78 ± 0.01

56.69 ± 0.53

7.02 ± 0.24

ZACZ

6.51

82.17

14.79 ± 0.26

0.78 ± 0.01

56.10 ± 0.14

6.50 ± 0.12

ITO

36.13

85.66

14.43 ± 0.24

0.78 ± 0.01

55.12 ± 0.32

6.20 ± 0.18

Rsheet, sheet resistance; T, total transmittance averaged over visible spectral range (400−800 nm); Jsc, short-circuit current density; Voc, open-circuit voltage; FF, fill factor; PCE, photocurrent conversion efficiency. aEach parameter was determined by averaging the value for ten devices. The error bars represent the standard deviation.


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Figure 1. Influence of WL on optical performance of TEs. (a) Schematic diagrams illustrating OMO TEs utilizing an optically transparent Ag(O) WL (left) and a HMWL (right) on which Ag is subsequently grown. (b) Photographs of proposed ZnO/Ag/Ag(O)/ZnO (left) and conventional ZnO/Ag/Ge/ZnO (right) TEs deposited on highly flexible PET substrates with a 6-nm Ag film. The thicknesses of the Ag(O) and Ge WLs were fixed to 1.5 and 0.7 nm, respectively. The bottom and top ZnO layers were fixed to 5 and 25 nm, respectively. (c) Comparison of total transmittance, averaged over the visible spectral range (400−800 nm), versus sheet resistance of different flexible TEs: Ag, Ag/Ag(O), Ag/HMWL including Ag/Ge and Ag/Cu, and amorphous ITO films. The solid lines are to aid with legibility.


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Figure 2. Influence of WL on morphological evolution of Ag. Highly magnified FE-SEM images of (a) 0.7-nm Ag, Ag(O), Ge, and Cu deposited on ZnO substrates and (b) Ag films evolving on ZnO substrates with and without a WL. The thicknesses of the WLs were optimized at 1.5 nm for Ag(O) and at around 0.7 nm for Ge and Cu. The thickness of the Ag(O) WL, 1.5 nm, was included in the thickness of the Ag that was subsequently grown on it, while the thicknesses of the Ge and Cu were excluded from the thickness of Ag. The thickness of the ZnO was fixed at 5 nm. Scale bar = 50 nm. (c) Change in Ag wetting ability on ZnO substrate as a function of the Ag thickness, where the wettability was determined from the surface area of the Ag evolved with its thickness. The surface area (red) of the Ag on the ZnO was determined by using image processing software with plane FE-SEM images, as demonstrated in (d) with an example of 3.0nm Ag directly grown on the ZnO substrate. Scale bar = 50 nm.


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Figure 3. Improvement of surface roughness of Ag with a WL. (a) Atomic force microscopy images of Ag, Ag/Ag(O), and Ag/Ge films deposited with conditions corresponding to Figure 2. Scale bar = 200 nm. (b) Line profiles of surfaces of Ag, Ag/Ag(O), and Ag/Ge grown at an Ag thickness of 6 nm.


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Figure 4. Numerical predictions of wetting behaviors of different WLs. Probable structural configurations of (a) Ag, (b) Ge, and (c) Ag(O) monolayers deposited on the O-terminated surface of ZnO substrates. (d) Ecoh of Ag(O) monolayer, corresponding to the configuration shown in (c), as a function of the oxygen concentration, which were compared with the cohesive energies of the pure Ag and Ge monolayers corresponding to the configuration of (a) and (b), respectively.


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Figure 5. Optoelectrical performances of flexible TEs fabricated on PET substrates. (a) The best total transmittance spectra of ZnO/metal/ZnO TEs obtained under different metal conditions: Ag (9 nm), Ag/Ag(O) (6 nm) including the Ag(O) thickness of 1.5 nm, Ag (6 nm)/Ge (0.7 nm), and Ag (9 nm)/Cu (0.7 nm), compared to that of a 160-nm amorphous ITO single-film TE. Changes in (b) sheet resistance and (c) carrier mobility of ZnO/metal/ZnO as a function of Ag thickness. (d) Figure of merit (FoM), T10/Rs, where T is the total transmittance, averaged over the visible spectral range (400−800 nm), and Rs is the sheet resistance corresponding to (a) and (b), respectively. The bottom and top ZnO thicknesses were fixed to 5 and 25 nm, respectively. The solid lines in (b−d) are to aid with legibility.


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Figure 6. Mechanical flexibility of TEs fabricated on PET substrates. (a) Photographs of bending tests of ZnO (25 nm)/Ag/Ag(O) (6 nm) /ZnO (5 nm) (ZAOZ) and ZnO (25 nm)/ITO (160 nm) (ITO) TEs with different bending radii. (b) The change in the resistance of the TEs with the bending radius. (c) The change in the resistance of the TEs as a function of bending cycles at a minimum bending radius of 9 mm. Insets show the optical images of the TEs after the cyclic bending test. Scale bar = 100 µm.


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Figure 7. Photovoltaic performances of flexible OSCs fabricated with different TEs. (a) Schematic illustrations of structure and corresponding energy levels of flexible OSC devices fabricated in the present study. Comparison of (b) external quantum efficiency spectra and (c) current density−voltage characteristics of highly flexible OSCs (shown in the inset) utilizing different ZnO/metal/ZnO TEs corresponding to the conditions shown in Figure 5(a).


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