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Optical transmittance enhancement of flexible copper film electrodes with a wetting layer for organic solar cells Guoqing Zhao, Myungkwan Song, Hee-Suk Chung, Soo Min Kim, Sang-Geul Lee, Jong-Seong Bae, Tae Sung Bae, Donghwan Kim, Gun-Hwan Lee, Seung Zeon Han, Hae-Seok Lee, Eun-Ae Choi, and Jungheum Yun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10234 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 20, 2017

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ACS Applied Materials & Interfaces

Optical transmittance enhancement of flexible copper film electrodes with a wetting layer for organic solar cells Guoqing Zhao,† Myungkwan Song,† Hee-Suk Chung,ǁ Soo Min Kim,‡ Sang-Geul Lee,± JongSeong Bae,§ Tae-Sung Bae,ǁ Donghwan Kim,‡ Gun-Hwan Lee,ǀ Seung Zeon Han,ǀ Hae-Seok Lee, *,¶ Eun-Ae Choi,*,† and Jungheum Yun*,† †

Surface Technology Division, Korea Institute of Materials Science, Changwon, Gyeongnam 51508, Republic of Korea ǁ

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

‡

Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea

±

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

§

Busan Center, Korea Basic Science Institute, Busan 46742, Republic of Korea

ǀ

Commercialization Research Division, Korea Institute of Materials Science, Changwon, Gyeongnam, 51508, Republic of Korea ¶

KU-KIST Green School, Graduate School of Energy and Environment, Korea University, Seoul 02841, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT: The development of highly efficient flexible transparent electrodes (FTEs) supported on polymer substrates is of great importance to the realization of portable and bendable photovoltaic devices. Highly conductive, low-cost Cu has attracted attention as a promising alternative for replacing expensive indium tin oxide (ITO) and Ag. However, highly efficient, Cu-based FTEs are currently unavailable because of the absence of an efficient means of attaining an atomically thin, completely continuous Cu film that simultaneously exhibits enhanced optical transmittance and electrical conductivity. Here, it is reported on the strong twodimensional epitaxy of Cu on ZnO by applying an atomically thin (around 1 nm) oxygen-doped Cu wetting layer. Analyses of transmission electron microscopy images and x-ray diffraction patterns, combined with first-principles density functional theory calculations, reveal that the reduction in the surface and interface free energies of the wetting layers with a trace amount (1−2 at%) of oxygen are largely responsible for the two-dimensional epitaxial growth of the Cu on ZnO. The ultrathin 2D Cu layer, embedded between ZnO films, exhibits a highly desirable optical transmittance of over 85% in a wavelength range of 400−800 nm and a sheet resistance of 11 Ω sq−1. The validity of this innovative approach is verified with a Cu-based FTE that contributes to the light-to-electron conversion efficiency of a flexible organic solar cell that incorporates the transparent electrode (7.7%), which far surpasses that of a solar cell with a conventional ITO (6.4%).

KEYWORDS: flexible transparent electrode; organic solar cell; copper; ultrathin film; wetting layer


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1. INTRODUCTION A major degradation in optical transparency due to absorption and reflection is one of the critical issues limiting the widespread use of thin metal films as transparent electrodes in the latest optoelectronic applications such as solar cells, light-emitting diodes, touchscreens, flatpanel displays, and electrochromics. Transparent electrodes are characterized by a high level of electrical conductance and high optical transparency that allows light to pass into and out of an optoelectronic device. Choice of potential candidates is usually limited to coinage-metal-based nanomaterials with the structural geometry of either a continuous film or a grid/nanowire network. Metal grids and nanowires have gained attention due to their simultaneous attainment of highly promising optical transmittance and electrical conductivity. However, for application in optoelectronic devices, their discontinuous, rough geometrical features containing empty nanoor microscale holes lead to several technical issues: (i) serious carrier recombination due to discontinuities in electrical conductance, (ii) device short-circuiting due to extreme surface roughnesses of the protruding 3D structures, and (iii) chemical corrosion due to incomplete anticorrosion sealing of the 3D surfaces of metal grids and nanowires.1−4 In contrast, the 2D geometrical feature of a continuous film structure of metals embedded between thin transparent oxide films can be an excellent alternative for solving these issues arising associated with metal grids and nanowires. To successfully optimize the optoelectrical properties, the morphological transition from discrete granules to a continuous network must occur within a minimum thickness metal film that exhibits excellent dispersion over chemically heterogeneous supporting substrates (predominantly oxides). Ultrathin metal films exhibiting atomically flat and completely continuous morphologies provide the best means of minimizing transmittance losses while attaining a sufficiently high level of conductance.


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However, ideal 2D growth is not obtained for highly conductive coinage metals, such as Ag and Cu. Instead, the 3D (or Volmer-Weber) growth mode of metal nanoclusters, leading to a delay in the development of a completely continuous film, is normally observed with highsurface-free-energy metals on oxide with low surface free energy due to the poor metal-wetting of the oxides.5−7 Efforts to produce optically transparent metal-film electrodes have largely failed to achieve the 2D growth mode of noble metals, because there is currently no means of attaining the complete wetting of chemically heterogeneous oxides with noble metals when the metals are extremely thin.8 The strategies currently available for improving this wetting of oxides with noble metals have centered on applying chemically different transition metals (Al, Cr, Ge, Nb, Ni, Pt, Sn, and Ti) as either an alloy

9−12

or a wetting layer.13−21 However, the prevalent

techniques are not able to attain the desired level of optical transparency because the transition metals readily aggravate the transmittance loss due to their strong photon absorption relative to noble metals. Researchers have yet to identify an efficient means of supporting the fabrication of atomically smooth and continuous noble metal films on oxide substrates while maintaining their integrities and avoiding the use of optically inferior transition metals. Meanwhile, as a result of redirecting our focus to the exploration of the influence of gas additives on metal growth, we obtained solid data indicating the improvement in the wetting of noble metals in the presence of gas additives such as O,22−25 S,25, 26 and N.27, 28 Considering the inherent 3D growth mode of noble metals on chemically heterogeneous substrates resulting from the vigorous coalescence between evolving metal clusters as a means of reducing the high surface energy of the metal clusters,29−31 the abrupt reduction in the surface energy of the metal clusters via substoichiometric oxidation, as reported in the literature,32, 33 provides a particularly effective means of suppressing the coalescence of metal clusters and thus alleviates the 3D


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growth behavior of the noble metals. More recently, researchers have successfully demonstrated that an improvement in the wetting of noble metals as induced by oxygen additives could be achieved without any serious degradation in the electrical conductance or optical transmittance by precisely suppressing the oxygen inclusion to a trace amount of no more than a few atomic percent.34−37 However, the basic working principles and exact mechanisms are still largely unclear because the existing data only provide fragmentary pieces of relevant information. No systematic strategy has, to the best of our knowledge, been proposed to achieve the complete wetting of noble metals on oxide substrates by fully leveraging the advantage of substoichiometric oxidation. The purpose of this study is to achieve the 2D epitaxial growth of pure Cu on a ZnO film, in a way that realizes the complete wetting of Cu on ZnO by controlling the formation and coalescence kinetics of the Cu clusters, which has long been regarded as being an attainable objective using only heterogeneous metal dopants and wetting layers. Here, we report on the 2D growth behavior of pure Cu on a ~1-nm suboxide Cu wetting layer, denoted as Cu-on-Cu(O), completely covering the ZnO surface to considerably enhance the wettability. The experimental and computational results indicated that the Cu(O) layer with a trace oxygen dose (1−2 at%) exhibits a close-packed array of thermodynamically stable Cu(111)-dominant metallic phases with greatly reduced free energies at both its surface and interface. It serves as an ideal platform for the 2D homoepitaxial growth of subsequently deposited Cu with the successful suppression of 3D Cu clustering on its surface. This result is remarkable since the growth process was carried out using a mature sputtering system at room temperature. The potential for application to real optoelectrical devices was explored by fabricating highly efficient ZnO/Cu-on-Cu(O)/ZnO (denoted as ZCOZ), which is a oxide/metal/oxide (OMO) configuration, flexible transparent


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electrodes (FTEs) and organic solar cells (OSCs) based on FTEs with excellent light-to-electron conversion efficiencies far surpassing the performance of solar cells using conventional ZnO/Cu/ZnO (denoted as ZCZ) or ITO electrodes.

2. EXPERIMENTAL METHODS 2.1. Fabrication and characterization of FTEs. The film materials of Cu, Cu(O), ZnO, and ITO, which constituted the FTE structures examined in this study, were synthesized on either 125-µm polyethylene terephthalate (PET, Panac, Co.) substrates or conventional Si wafers by sequential room-temperature deposition processes using a multi-gun magnetron sputtering system (Flexlab system 100, A-Tech System Co., Ltd.).28 Details of sputtering system and operating conditions are provided elsewhere.28,37 The ZnO and ITO films were deposited from ZnO and ITO (Sn = 10 wt%) targets, respectively, at a 200 W radio frequency (RF) power under an Ar plasma condition. The Cu layer was deposited from a Cu target at a 50 W direct current power of 50 W under an Ar plasma condition, while the Cu(O) interlayer was deposited at the same power by using an inlet gas mixture of Ar and O2, while other sputtering conditions were identical to those used for Cu sputtering. The oxygen dose in the Cu(O) interlayer was controlled by changing the inlet O2 gas flow rate when the inlet Ar gas flow rate was fixed to 50 sccm. The optimal Cu(O) interlayer was achieved at a low O2 flow rate of 0.2 sccm. The deposition rates of the Cu and Cu(O) films were 6 and 5 nm min−1, respectively. The ZCZ and ZCOZ FTEs were sequentially prepared by sputtering without vacuum breaking. The morphologies of the ultrathin metal films were observed by ultrahigh-resolution (UHR) field-emission scanning electron spectroscopy (FE-SEM, S-5500, Hitachi, Co.), using an accelerating voltage of 15 kV and a magnification of 500k, at the Jeonju Center of the Korea


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Basic Science Institute (Jeonju, Korea) and tapping-mode atomic-force microscopy (AFM, NX10, Park System). The cross-sectional morphology and crystallography of the ZCZ and ZCOZ configurations were investigated using transmission electron microscopy (ARM200F, JEOL) and annular dark field (ADF) scanning transmission electron microscopy (STEM) analyses, again at the Jeonju Center of the Korea Basic Science Institute. The crystallographic features of the ZCZ and ZCOZ configurations were investigated by X-ray diffraction (XRD, Empyrean, PANalytical) measurements at the Daegu Center of the Korea Basic Science Institute (Daegu, Korea). The preferred orientation was estimated by determining the Cu(111) and Cu(200) intensities. The thicknesses of metal thin films were determined from the X-ray reflectivity, and by performing surface profiler (p-11, KLA-Tencor) measurements. The oxygen doses in Cu films were determined by using X-ray photoelectron spectroscopy (XPS, K-ALPHA+, Thermo Scientific Co.) depth profiling measurements with a 1486.6 eV Al Kα source at the Busan Center of the Korea Basic Science Institute (Busan, Korea). The total (specular + diffusive) transmittance, subtracting the transmittance of the PET substrates, and reflectance spectra of the ZCZ,

ZCOZ,

and

ITO

FTEs

were

measured

by

ultraviolet-visible-near-infrared

spectrophotometry (Cary Series, Agilent Technologies). The sheet resistances of the FTEs were measured using a four-point probe system (MCP-T600, Mitsubishi Chemical Co.) for a sample dimension of 2.5 × 2.5 cm2. The concentration and mobility of electrons were measured by a Hall effect measurement system (8400 series HMS, Lake Shore Cryotronics Inc.) for a sample dimension of 1.0 × 1.0 cm2. A chemical stability test of the ZCZ and ZCOZ FTEs was carried out by exposing the electrodes to accelerated humidity conditions (85 °C, 85% relative humidity). 2.2. Fabrication and characterization of OSCs. The flexible inverted OSCs consisted of a photoactive layer of PTB7 (1-Material Chemscitech):PC71BM (Solemme BV), a hole transport


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layer of PEDOT:PSS (Clevios P VP AI4083), and an evaporated Ag layer as back electrode, fabricated on PET substrates coated with either a ZCZ, ZCOZ, or ITO/ZnO multi-layer as the front transparent electrode. The thickness of the PTB7:PC71BM layer was ~100 nm with a deviation of no more than 5%, and the thickness of PEDOT:PSS layer was 5 nm. The current density−voltage (J−V) characteristics of the OSCs with a defined area (0.38 cm2) were measured using a Keithley 2400 SourceMeter unit under Air Mass 1.5 Global (AM 1.5G) radiation with an intensity of 100 mW cm−2. The external quantum efficiency (EQE) values were measured with a cell response/QE/IPCE measurement system (PV Measurements Inc.). The mechanical stability of OSCs was determined under the influence of the compressive stress produced by the deformation of the PET substrates. Details of fabrication and characterization conditions for OSCs are provided elsewhere.28 2.3. Computational simulations. The first-principle calculations within the density functional theory were performed using the Vienna Ab-initio simulation package code (VASP).38 Ionic pseudopotentials were obtained by the projector augmented wave (PAW) method.39, 40 The Cu and Zn atoms have 3d electrons in the valence states. The wave functions were expanded into plane waves with a cutoff energy of 450eV. The Perdew-Burke-Ernzerhof approximation (PBE) was adopted to determine the exchange and correlation potential.41 The rear of the (4 × 4) (0001ത) ZnO slab system was passivated by pseudo-hydrogen atoms, charged with 1.5 electrons. The lowest three bilayers of the ZnO slab were fixed to hold the crystal structure of the bulk phase of the ZnO, and the following two bilayers were used as buffer layers. Six layers of Cu(111) were built on the oxygen-terminated ZnO(0002) surface. The lattice constant of (4×4) ZnO was approximated to be 22% larger than that of (4×4) Cu and 2% smaller than that of (5×5) Cu. A vacuum region of 10 Å was included to prevent any interaction between adjacent supercells. A


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special gamma k-point was used for the Cu/ZnO slab system. A single oxygen atom was incorporated in the Cu domain consisted of 141 Cu atoms, equivalent to an oxygen does of 0.7 at%, at different sites: interstitial-octahedral, interstitial-tetrahedral, and substitutional sites. All the atomic positions were optimized using the conjugate-gradient method until the residual forces on each atom were less than 0.01 eV/Å. The spreading parameter of pure Cu was set to zero. The optical transmittance and reflectance of the OMO electrodes were calculated by a transfer matrix method using the Silvaco TCAD software package based on the LUMINOUS algorithm of the Atlas module.28 The structural configuration of the multilayer electrodes was designed so that the OMO electrodes with different metal interlayers were formed on a 200-nm quartz substrate. The metal and ZnO layers were assumed to have a completely continuous, flat structural morphology producing only mirror reflection and no diffusive reflection.

3. RESULTS AND DISCUSSION Ultrathin Cu layers (2 to 5 nm) were (i) directly deposited on polycrystalline ZnO films and (ii) deposited on a ~1-nm suboxide Cu interlayer, denoted as Cu(O), through a sequential roomtemperature sputtering process, in which the Cu(O) interlayer was fabricated using a mixture of Ar (50 sccm) and O2 (0.2 sccm) as a sputtering gas for Cu deposition (Figure 1a). The morphologies of the Cu layers were determined by UHR FE-SEM and AFM (Figure1b; Figure S1, Supporting Information). The 3D growth mode, represented by the transition from an array of discrete polygonal clusters to the coalescence of large and irregular particles with large numbers of pinholes and voids, was observed for pure Cu that had been deposited directly onto chemically heterogeneous ZnO. However, unexpected morphologies that cannot be interpreted


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by the 3D growth mode were observed for Cu deposited on the Cu(O) interlayer. The morphological features of the Cu-on-Cu(O) were not coincident with the complete layer-by-layer growth to produce a Cu thickness of 2 nm because boundary grooves were still observed in the Cu-on-Cu(O) layer consisting of small and irregular clusters (Figure S1, Supporting Information). However, the Cu clusters on the Cu(O) interlayer were connected with neck-like bridges but did not coalesce to form the discrete, large particles that were observed for Cu on ZnO. The Cu-on-Cu(O) layer was found to have wet most of the ZnO surface. As a result, a continuous layer with a 2D morphological feature was developed for a 3.5-nm Cu film formed on a Cu(O) interlayer. Cross-sectional STEM images of 3.5-nm Cu films, inserted between ZnO films, clearly demonstrate the 3D to 2D morphological transition as a result of applying a unique Cu(O) interlayer (Figure 1c). This raises a question about how the drastic migration of Cu clusters, mostly driven toward the reduction in the surface free energy of highly volatile, unstable clusters, can be suppressed for ultrathin Cu on a Cu(O) interlayer with a thickness of only 1 nm. This clearly differs from the inherent 3D morphological feature of the Cu directly deposited on ZnO, which continues to exhibit discrete and granular particles. It seems that stagnant Cu(O) clusters coalesced by forming bridges between neighboring clusters rather than through cluster migration on the ZnO. The Cu(O) clusters wet most of the ZnO surface when the thickness was as low as 1 nm (Figure S2, Supporting Information). The surface roughness, estimated to be ~0.3 nm, with no dependency on the Cu thickness, is evidence for the 2D growth of the Cu-on-Cu(O) film (Figure 1d).


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Figure 1. Superwetting of Cu-on-Cu(O) on ZnO. (a) Conceptual diagram representing the 3- to 2-dimensional transition in the growth mode of Cu supported on ZnO films with a Cu(O) wetting layer. (b) FE-SEM (left) and corresponding AFM images (right) of a 3.5-nm Cu film on a 15-nm ZnO film without and with a 1-nm Cu(O) wetting layer (denoted as 3.5-nm Cu or Cu-on-Cu(O)). (c) Cross-sectional STEM images of 3.5-nm Cu, with and without Cu(O) interlayer, embedded between 15-nm ZnO films. (d) AFM results showing the surface-line profiles of Cu and Cu-onCu(O) films corresponding to (b) and root-mean-square surface roughnesses of Cu and Cu-onCu(O) films on the ZnO. The Cu-on-Cu(O) film exhibits a strong (111) orientation, which is aligned in parallel with respect to the underlying polycrystalline ZnO substrate with a preferred orientation of (0002)


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(Figure 2a, b). The cross-sectional high-resolution STEM images (Figure 2c, d) demonstrate unique feature that the (111) preferred orientation develops in the 1-nm Cu(O) layer, even under unfavorable circumstances including lattice mismatch between the Cu and ZnO, and is continuous with the Cu film that is subsequently deposited on the Cu(O) layer. This indicates that the key to the 2D epitaxial growth of the Cu-on-Cu(O) film is the 1-nm Cu(O) interlayer that exhibits good crystallinity, aligned epitaxially with the (111) orientation with strong wetting of the ZnO below. This maintains its 2D structural integrity during subsequent Cu growth. The increase in the intensity of the Cu(111) XRD peak with the use of the Cu(O) interlayer, relative to that of the Cu film without the Cu(O) interlayer, provided evidence of the 2D homoepitaxial growth of Cu on the Cu(O) interlayer (Figure 2a). The exceptionally high degree of the (111) preferred orientation of the Cu-on-Cu(O) film, which was evaluated by the intensity ratio of the Cu(111)/Cu(200) in the XRD patterns of the 2θ−ω scans (Figure 2b), further verified the predominant influence of the Cu(O) layer on the determination of the crystallographic feature of the subsequently grown Cu film during its earliest growth. A noticeable increase in the lattice strain developed during the early growth of the continuous Cu-on-Cu(O) film epitaxially grown on the ZnO with the complete wetting (Figure S3, Supporting Information). The degree of (111) preferred orientation decreased to a level close to that of the Cu film directly deposited on the ZnO as the Cu thickness increased (Figure 2b) because the influence of the Cu(O) interlayer on the Cu growth mode was monotonically reduced as the Cu thickness increased. However, the Cu(111) crystallinity continuously increased with the Cu thickness for both Cu and Cu-on-Cu(O) films.


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Figure 2. Two-dimensional epitaxial growth of Cu-on-Cu(O) on ZnO. (a) X-ray diffraction patterns of 2θ−ω scan for ZCZ (solid line) and ZCOZ (short dot) structures with different thickness of Cu (black, 3nm; red, 8nm; blue, 12nm) deposited on Si substrates. (b) Dependence of the crystallographic orientation of Cu films on their thickness in ZCZ and ZCOZ configurations. The ratio was calculated from the peak intensities of Cu(111) and Cu(200). The inserted schematic images exhibit a distinct difference in the early growth behavior between Cu and Cu-on-Cu(O) clusters. Highly magnified cross-sectional TEM images clearly demonstrating the completely continuous morphology (c) of Cu-on-Cu(O) film embedded between the ZnO films and the preferred Cu(111) orientation (d) well-matched to the ZnO(0002). Note that the primary crystallographic phase in the Cu(O) film, fabricated by reactive


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sputtering utilizing an inlet gas flow rate of Ar:O2 = 50:0.2 sccm, was metallic Cu(111) without a noticeable defined oxide phase, as represented by Cu2O, that may develop at much higher oxidation levels (Figure S4, Supporting Information). The XPS depth profiling of the Cu and Cu(O) films, directly deposited on Si wafers, indicates that the concentration of oxygen in the Cu(O) film is close to the detection limit of XPS (about 1−2 at%), when the oxygen of the native copper oxide at the top Cu(O) surface and native silicon oxide at the Cu(O)/Si interface are excluded (Figure S5, Supporting Information). Given that Cu oxidation to Cu2O is represented by a weak satellite at 945 eV and a Cu2p3/2 peak, which is broader than that of Cu metal, the fact that there is no visible discrepancy in the XPS spectra of the Cu(O) and pure Cu films demonstrates that the trace amount of residual oxygen in the Cu(O) film is insufficient to cause the formation of Cu2O in any measurable level (Figure S6, Supporting Information). Because an XRD signal of Cu2O(111) should be observed at 36.8° [Joint Committee on Powder Diffraction Standard (JCPDS) card No. 78-2076], the intensity of the Cu2O phase in the Cu(O) film was indistinctive relative to that of the metallic copper (Figure S4, Supporting Information). The Cu2O phase was clearly developed in Cu(O) films utilizing an O2 inlet gas flow rate greater than 0.8 sccm. However, such highly oxidized Cu(O) films do not lead to any enhancement of the (111) preferred orientation in Cu-on-Cu(O) films, in which they are used as the Cu(O) interlayer (Figure S7, Supporting Information). Despite the highly metallic state of the Cu(O) with the trace oxygen dose (1−2 at%), the experimental evidence verifies the role of the adsorbed oxygen in improving the wettability of a Cu(O) interlayer. Although the details of the mechanism responsible for the 3D to 2D transition in the growth mode of the Cu film on a ZnO substrate with a Cu(O) interlayer remain difficult to define due to the complexity of the influencing factors, the unique Cu-on-Cu(O) growth mode, which is


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distinct from that of Cu, can be illustrated by a thermodynamic criterion based on the differences in the free energies between the Cu and Cu(O) ultrathin layers adjoining the ZnO substrate. It is well known that complete wetting in accordance with the 2D growth mode is thermodynamically unfavorable when depositing a pure metal with a higher surface energy on a lower-surfaceenergy oxide surface, unless there is extraordinarily strong metal−oxide adhesion.42 The wetting of Cu can be improved by significantly reducing either the surface free energy and/or the interfacial free energy of the Cu because lowering these energies effectively suppresses the cluster migration and 3D coalescence during the early growth stages by generating thermodynamically stable clusters. Here, the working principle of the Cu(O) interlayer as a wetting inducer of Cu on a ZnO substrate was determined by computational studies using firstprinciples density-functional theory. First-principle calculations were performed to determine the stability of an oxygen atom in the Cu(O) interlayer according to the stable geometry of a Cu(111)-on-ZnO(0001ത) slab (Cu/ZnO slab) (Figure 3a; Figure S8, Supporting Information). A geometric configuration of 6 layers of (5 × 5) Cu was expected to have a layer distance of 2.1 Å identical to that of the TEM measurement. However, given the unstable bonding of the first layer of (5 × 5) Cu on (4 × 4) ZnO when compared to other geometric configurations of Cu on ZnO, the geometric configuration consisting of a monolayer of (4 × 4) Cu, followed by multilayers of (5 × 5) Cu on ZnO was chosen for further examination (Figure S9, Supporting Information). An oxygen atom can be incorporated into the Cu(111) by occupying different sites at the Cu and ZnO interface, within the Cu, and at the Cu surface (Figure 3b). The formation energies (∆Ef) of an interstitial and substitutional oxygen atom were determined for the different sites by total energy calculations using


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∆E f = E Cu (O ) / ZnO − slab − ( ECu / ZnO − slab +

1 µO ) , 2 2

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(1)

where E Cu ( O ) / ZnO − slab , E Cu / ZnO − slab and µ O are the total energies of Cu with and without an 2

oxygen atom on the ZnO slab and O2 molecule, respectively (Table S1, Supporting Information). The most stable status of the oxygen atoms in the Cu(111), as represented by a higher negative energy, was as an interstitial oxygen atom occupying a site at the Cu surface. This result indicates that the oxygen atoms involved in the formation of the Cu(O) can accumulate at near surface regions of the Cu(O), thereby promoting the so-called “floating-out” of the oxygen atoms to the metal surfaces.43


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Figure 3. First-principles calculations within the density functional theory. (a) A geometric configuration of an ultrathin Cu film supported on an oxygen-terminated ZnO film used in the computational analyses. A monolayer of (4×4) Cu, followed by five layers of (5×5) Cu, was deposited on a multilayer of (4×4) ZnO. (b) Geometrical configurations of Cu films with an interstitial oxygen atom located at the ZnO-Cu interface, the inside of Cu, and the top surface of Cu. (c) Change in the relative spreading parameters (∆S) of Cu on ZnO films as a function of the position of an oxygen atom incorporated at different sites. (d) Relative spreading parameters (∆S) of Cu supported on ZnO films as a function of the dose of oxygen atoms located at the ZnO-Cu interface, the inside of Cu, and the top surface of Cu. The exact role of the oxygen inclusion in Cu(O) wetting was verified by determining the relative spreading parameter (∆S) as a function of different occupation sites of an interstitial oxygen atom using density functional theory simulations. The spreading parameter (S) of Cu films on ZnO substrates can be obtained as S = γ ZnO − (γ Cu / ZnO + γ Cu ) = E ZnO − ( E Cu / ZnO − nµ Cu ) ,

(2)

where γ ZnO , γ Cu , and γ Cu / ZnO are the surface energies of ZnO and Cu, and the interface energy between the Cu and ZnO, respectively. Also, E ZnO and E Cu / ZnO are the total energies of the ZnO and Cu-on-ZnO slab systems, while n and µ Cu are the number and chemical potential of Cu atoms obtained from Cu bulk, respectively. The relative degree of wetting of the objective metals, Cu and Cu(O), on ZnO substrates can be approximated by the formula ∆S = SCu(O) − SCu = ( γ Cu − γ Cu (O ) ) + ( γ Cu / ZnO − γ Cu ( O ) / ZnO ). The formula suggests that a reduction in the energy terms associated with Cu(O) enhances the relative spreading parameter. An increase in ∆S indicates


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improvement in the Cu(O) wetting relative to that of Cu on ZnO. The computational results show that the interstitial oxygen atom on the Cu(O) surface and at the Cu(O)/ZnO interface increases ∆S by ~0.3 and 0.2 J m−2, respectively (Figure 3c). However, the internal oxygen atom in the Cu(O) bulk has a negligible effect on ∆S, which indicates that it has no effect on Cu(O) wetting. When an interstitial oxygen atom is located at the interface or on the surface, the Cu−O bond can stabilize the Cu atoms by lowering the energy levels of the non-bonding states of the Cu. Furthermore, ∆S increases linearly when the interstitial oxygen atom dose increases to 3 at% (Figure 3d). Without any penetration of oxygen atoms into the Cu bulk or any formation of Cu2O, the introduction of an oxygen dose of 2 at% at the Cu(O) surface led to an increase of ∆S by 0.78 J m−2, corresponding to reduction in the surface energy (~1.8 J m−2)23, 43 of Cu to almost half. Since a slightly less influence was predicted for the introduction of the same oxygen dose at the Cu(O)/ZnO interface, the manipulation of the early growth of Cu(O) clusters due to the existence of oxygen at the interface must not be excluded from the wetting scenario. However, a relatively small increase in ∆S by 0.11 J m−2, predicted for the introduction of the oxygen dose into the Cu(O) bulk, supports the aforementioned floating-out behavior of the oxygen atoms to the surfaces during Cu(O) growth. From the computational results, the critical dynamic features experimentally observed in the Cu-on-Cu(O) growth can be explained as being a result of the unique wetting scenario. When Cu clusters evolve during the early growth stages, the existence of oxygen both at near surface regions of the clusters and at their interfaces with the underlying ZnO leads to a significant reduction in the amount of free energy of the clusters, relative to that of pure Cu as an inevitable consequence of the formation of Cu−O bonds. This is well agree with the previously reporting results that found a significant reduction in the surface energy of the Cu surfaces with oxidation


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states well below those required for the formation of Cu2O,32 together with a greater dispersion of the oxidized Cu nanoclusters on a ZnO substrate.44 Here, a survey of the surface and interface energies of the Cu as a function of oxygen dose indicates that the introduction of an oxygen dose of ~2 at% into the Cu film leads to a significant reduction in the energy to a value close to that of the surface free energy of ZnO (~1 J m−2).45 Therefore, the formation of a 1-nm Cu(O) interlayer leads to the highly effective wetting of the ZnO surface below. This can be attributed to the change in the growth from high-energy Cu on a low-energy ZnO substrate to low-energy Cu(O) on a ZnO substrate, which induces a loss in the driving force for 3D coalescence of the clusters through surface diffusion across the ZnO surface. Notably, despite such drastic reductions in the surface and interfacial free energies, a highly metallic, predominantly Cu-terminated top surface is expected from the Cu(O) interlayer with the trace oxygen dose used. The subsequent growth of pure Cu on the metal-terminated top surface of the Cu(O) interlayer should follow the homoepitaxial growth mode, exhibiting a completely continuous orientation and structure between Cu(O) and Cu despite significant differences in the free energies between the Cu and Cu(O) layers. The 2D epitaxial growth of a Cu-on-Cu(O) layer on a ZnO film promises to provide excellent optoelectrical performance by utilizing a flexible ZCOZ configuration employing sub10 nm Cu layers embedded between 45-nm ZnO films on PET substrates (Figure 4a). The total transmittance of ZCOZ electrodes was superior to that of ZCZ electrodes (Figure 4b). The optimal transmittance was obtained with a ZCOZ electrode utilizing a 6.5-nm Cu layer on a 1nm Cu(O) interlayer. The improvements in the optical and electrical properties of the ZCOZ electrodes can be readily interpreted by the 2D epitaxial growth of a Cu film on the Cu(O) interlayer. The differences in the total transmittances of ZCZ and ZCOZ electrodes at longer


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wavelengths (600−800 nm) could mainly be ascribed to the different plasmonic loss behaviors of those electrodes, when the absorbance was determined by accounting the transmittance and reflectance of incident light in the electrodes (Figure S10-S11, Supporting Information). The formation of completely continuous Cu-on-Cu(O) ultrathin films significantly suppressed the plasmon resonance loss, whereas no such suppression could be obtained with ZCZ electrodes utilizing granular Cu films grown by a typical 3D growth mode. A further increase of oxygen dose in the Cu(O) interlayer from a trace amount (1−2 at%) should affect the transmittance of the ZCOZ electrode by causing uncontrollable photon absorbance in highly oxidized phases, represented by Cu2O, in the interlayer. However, the probability of any negative influence to transmittance being ascribed to the increased oxidation level of Cu(O) was small because the interlayer was strictly limited to an atomically small thickness (~1 nm). The ZCOZ electrodes exhibited highly desirable transmittances without any notable reduction in the transmittance even at relatively large oxygen doses of Cu(O) (Figure S12, Supporting information). To confirm the superiority of the Cu(O) interlayer relative to interlayers based on various other transition metals, the optical properties were computationally estimated for the ZnO/metal/ZnO electrodes using a Cu film on various interlayer materials including Al, Ge, Ni, Pt, Ti, and Cu(O). To simplify the simulation issue by excluding the plasmon resonance interference in transmittance and reflectance, the structure of the interlayers was assumed to have a continuous 2D geometry identical to that of the Cu(O) interlayer. The predicted transmittance spectra of the electrode utilizing a 1-nm Cu(O) interlayer outperformed that of the electrodes utilizing all the other interlayer materials of the same thickness across the entire visible spectrum (Figure 4c). The reliability of the simulation results was proven by comparison with the optical properties that were experimentally observed for the ZCOZ electrode. The large difference in the


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transmittance was attributed to non-negligible light absorption and reflection caused by the interlayers utilizing transition metals other than Cu. The light absorption that adversely affected the optical performances of the electrodes was even more severe once the thickness of the interlayers was increased to 2 nm (Figure S13, Supporting Information).

Figure 4. Performances of Cu-on-Cu(O)-based flexible transparent electrodes (FTEs). (a) Schematic diagram (top) and optical photograph (middle) of a FTE consisting of a ZnO/Cu-onCu(O)/ZnO configuration on PET substrate, and the comparison Optical photographs and sheet resistance values (bottom) of ZCZ (left) and ZCOZ (right) electrodes utilizing a 5.0-nm Cu layer. (b) Total transmittance spectra for the ZCZ and ZCOZ FTEs utilizing two different Cu thicknesses of 6.5 nm and 9.5 nm. (c) Numerically predicted total transmittance spectra for ZnO/6.nm-thick Cu-on-wetting layer/ZnO electrodes utilizing a 1-nm wetting layer of different


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transition metals: Ge, Ni, Pt, Ti, and Al. (d) Change in the sheet resistance of ZCZ and ZCOZ electrodes as a function of Cu thickness. (e) Average total transmittance versus sheet resistance for the ZCZ, ZCOZ, and ITO electrodes. The arrows are guides to the eyes to show the optimized performance. The 2D morphology of Cu-on-Cu(O) films also contributed to an improvement in the electrical properties by dramatically reducing the percolation threshold thickness of Cu films with a Cu(O) interlayer. The electrical conductance of the ZCOZ electrode far surpassed that of the ZCZ electrode with the rapid decrease in the sheet resistance (Figure 4d), which was readily explainable with the rapid increase in the carrier mobility, even with almost identical carrier concentration values (Figure S14, Supporting Information). The dependence of the sheet resistance on the thickness of Cu(O) interlayer demonstrated the full development of the 2D morphology of the Cu(O) interlayer at a thickness of 1 nm, as the reduction in the sheet resistance almost saturated with further increases in the Cu(O) thickness over 1 nm (Figure S15, Supporting Information). The ZCOZ electrode exhibited an average transmittance of 85.8% in the visible spectral range and a sheet resistance of 11.12 Ω sq−1, whereas the ZCZ electrode utilizing a 9.5-nm Cu layer exhibited an optimal average transmittance of 79.5% in the same spectral range at a similar sheet resistance (Figure 4e). More impressive was the fact that the optical and electrical performances of the ZCOZ electrode outperformed those of amorphous ITO single films, which was deposited on a PET substrate. A 240-nm ITO single film showed a sheet resistance of 28.81 Ω sq−1 with an average transmittance of 81.4% (Figure 4e). Although the transmittance was seriously degraded by the change in the thickness of ITO single film (Figure S16a, Supporting Information), the increase in the film thickness did not lead to an effective reduction in the sheet


22

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resistance once the thickness exceeded 150 nm (Figure S16b, Supporting Information). It should be noted that the OMO structural configuration, consisting of a thin metal film embedded between continuous oxide films, provided stable electrical properties against oxidation even under accelerated humidity conditions, whereas a Cu film without any oxide film encapsulation exhibited a complete failure of the electrical conductance within a short time (Figure S17, Supporting Information). The superior optoelectrical performance of the ZCOZ FTE, relative to the conventional ZCZ and ITO FTEs, as manifested by the 2D epitaxial growth of ultrathin Cu films on a 1-nm Cu(O) interlayer, facilitated a photocurrent conversion efficiency (PCE) of 7.72% for flexible OSCs. The PCE value greatly surpassed the efficiencies of 6.72% and 6.38% observed for OSCs utilizing ZCZ and 240-nm ITO FTEs, respectively (Table 1). The flexible OSCs consisted with the following configuration: PET/FTE/ZnO/photoactive layer/hole-transport layer/Ag (Figure 5a). The performance of the ZCOZ FTE as a front electrode was compared with that of ZCZ and ITO FTEs. The photoactive layer consisted of poly[[4,8-bis[(2-ethylhexyl)oxy]-benzo[1,2-b:4,5b′ ] (PTB7)

dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)-carbonyl]thieno[3,4-b]thio-phenediyl]] and

[6,6]-phenyl-C71-butyric

acid

methyl

ester

(PC71BM).

The

poly(3,4-

ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS), was chosen as a hole transport material with its high hole mobility and a work function close to that of PTB7. The schematic energy diagram demonstrates the work function and band gap of each component in the OSCs (Figure 5b). The J−V characteristics (Figure 5c) and EQE spectra (Figure 5d) were determined for the OSCs utilizing ZCOZ, ZCZ, and ITO FTEs. The highest EQE spectrum and Jsc value were achieved by the OSC the ZCOZ with a 6.5-nm Cu layer. Furthermore, the highest fill factor (FF) and lowest series resistance (Rs) were also detected with the same ZCOZ FTE (Table 1).


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These OSC performances, therefore, can mainly be ascribed to the enhanced light absorption associated with the high optical transmittance of the ZCOZ FTE, whilst retaining superior electrical conductivity and structural integrity. The inverted OSC using the ZCOZ FTE exhibited reasonable stability, comparable to that of conventional ZCZ and ITO FTEs, by retaining about 85% of its initial PCE value after its exposure to the ambient environment for 14 days (Figure S18, Supporting Information). The high stability against mechanical bending of the OSC using the ZCOZ FTE was also a distinctive landmark for identifying its advantages relative to ITObased FTEs. When the OSCs are subjected to compressive stress induced by bending (Figure S19, Supporting Information), the OSCs using an OMO configuration, including both ZCOZ and ZCZ, exhibited far superior flexibilities relative to an ITO-based OSC (Figure 5e). Remarkably, the flexible OSC using the ZCOZ FTE with a 6.5-nm Cu layer retained a PCE value well in excess of 90% of its initial value even after bending with a bending radius as little as 0.2 mm. The OSC using the 240-nm ITO film experienced catastrophic degradation (~20% reduction) even after bending with a radius of as much as 8 mm. This difference between the OSCs using the OMO and ITO FTEs may be due to the enhanced mechanical flexibility of the ZnO/6.5-nm Cu-on-Cu(O)/ZnO FTE, compared to that of the 240-nm ITO FTE. The change in electrical conductance of these FTEs under recurring deformation was further determined by performing a cyclic bending test, in which the bending radius was repeatedly changed from ∞ to 8 mm. The ZnO/6.5-nm Cu-on-Cu(O)/ZnO FTE exhibited a relatively small resistance change of 5.9% after 1000 bending cycles, whereas the ITO FTE exhibited the same resistance change after only 12 bending cycles (Figure S20, Supporting Information). The resistance change of the ITO FTE eventually approached a catastrophic loss in conductance after 1000 bending cycles, due to the serious development of cracks.


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Figure 5. Photovoltaic performances of flexible OSCs using different FTEs. Schematic architecture (a) and schematic energy diagram of the electronic structure (b) of flexible OSCs. J−V characteristics (c) and EQE spectra (d) of OSCs using different front electrodes: ZCZ, ZCOZ, and 240-nm ITO single-film electrodes. (e) Change in the power conversion efficiency with bending radius during the irreversible compressive bending of flexible OSCs utilizing different FTEs corresponding to (c). The inserted image shows the photograph of a flexible OSC fabricated in this study.


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Table 1. Photovoltaic performances of flexible OSCs using different FTEs.

Rsheet

Tavg

Jsc

[Ω sq ]

[%]

ITO (240 nm)/ZnO

28.81

ZnO/Cu (9.5 nm)/ZnO

Voc

FF

Rs

[mA cm ]

[V]

[%]

[Ω cm ]

81.43

14.44 ± 0.05

0.71 ± 0.01

62.61 ± 1.04

5.38 ± 0.26

6.38 ± 0.03

9.49

79.47

14.76 ± 0.15

0.71 ± 0.01

64.09 ± 0.22

4.72 ± 0.10

6.72 ± 0.07

ZnO/Cu (6.5 nm)-on11.12 Cu(O)/ZnO

85.84

16.31 ± 0.21

0.72 ± 0.01

65.31 ± 0.49

3.03 ± 0.17

7.72 ± 0.15

ZnO/Cu (9.5 nm)-onCu(O)/ZnO

83.54

15.69 ± 0.07

0.71 ± 0.01

65.03 ± 0.33

3.58 ± 0.19

7.29 ± 0.06

Electrode type

−1

6.26

−2

PCE 2

[%]

Rsheet, sheet resistance; Tavg, average transmittance in the spectral range of 400−800 nm; Jsc, short-circuit current density; Voc, open-circuit voltage; FF, fill factor; Rs, series resistance; PCE, photocurrent conversion efficiency; ITO, indium tin oxide; ZnO, zinc oxide. Each parameter was calculated from the average value for 5 solar cells, fabricated on each electrode type. Error bars represent the standard deviation.

4. CONCLUSION A highly efficient, cost-effective FTE, directly deposited onto a polymer substrate, is crucial to the fabrication of the latest generation of flexible OSCs. However, the material choice for the electrode is still limited to conventional ITO or Ag. We are proposing a unique technique for facilitating a highly transparent and conductive Cu ultrathin film for use in the highly efficient flexible transparent electrodes of organic solar cells fabricated on polymer substrates. A completely continuous, 2D Cu ultrathin film electrode was successfully fabricated on a chemically heterogeneous ZnO film by applying an atomically thin oxygen-doped Cu wetting


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layer. Given the adverse effects of a wetting layer, based on heterogeneous transition metals, on the optoelectrical performance, including relatively poor transmittance and conductivity relative to noble metals, the present Cu-on-Cu(O) geometry offers an ideal platform for a highly 2D morphology, whilst retaining the optoelectrical performance of noble metals. The Cu thin film based electrode with a ZCOZ configuration, consisting of a sub-10-nm Cu-on-Cu(O) layer embedded between ZnO films on a PET substrate, realized promising optoelectrical features (an average transmittance of 85.8% over the visible spectral range and a sheet resistance of 11.12 Ω sq−1) and long-term stability against oxidation. These features are greatly superior to those of transparent electrodes with either an ITO single-film or a ZnO/Cu/ZnO configuration. By employing the ZnO/Cu-on-Cu(O)/ZnO configuration as the front electrode, the resulting flexible OSC exhibited an efficiency of 7.72%, far surpassing that of a solar cell with an ITO single-film and ZnO/Cu/ZnO electrodes.

AUTHOR INFORMATION Corresponding Authors *Address: 797 Changwondaero, Changwon, Gyeongnam, 51508, Republic of Korea. E-mail: [email protected]; [email protected]. 145 Anamro, Seongbukgu, Seoul, 02841, Republic of Korea. E-mail: [email protected].

Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENTS This research was funded by the New & Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning supported by the Ministry of Trade, Industry & Energy of Korea and the Fundamental Research Program of the Korea Institute of Materials Science. H.-S.C. was supported by National Research Foundation of Korea (NRF) grant funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea (No. 2015R1C1A1A01052727).

Supporting Information. FE-SEM images of Cu, Cu(O), and Cu-on-Cu(O) films during the early growth stages. Crystallographic XRD data of Cu, Cu(O) and Cu-on-Cu(O) films embedded between ZnO films. Chemical XPS data of Cu and Cu(O) films. Geometric configurations used in first-principle calculations and computationally predicted formation energies of Cu layers on ZnO films as a function of the number of Cu layers. Optical transmittance, reflectance, and absorbance spectra of OMO and ITO electrodes. Sheet resistances of ZCOZ and ITO electrodes. Carrier mobility and concentration data of ZCZ and ZCOZ electrodes. Change in the sheet resistance of electrodes during the 85 °C and 85% relative humidity test. Long-term stability test results of flexible OSCs using various transparent electrodes. Photograph of the irreversible bending test. Mechanical flexibility test results of the ZCOZ and ITO electrodes. Computationally predicted formation energies of an oxygen atom located at different sites in a multilayer Cu film on ZnO. This material is available free of charge via the Internet at http://pubs.acs.org.


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