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Thin Solid Films
journal homepage:www.elsevier.com/locate/tsf
Stabilisation of Cu fi lms in WO
3/Ag/Cu:Al/WO
3structures through their doping by Al and Ag
D.-E. Rabia
a, M. Blais
a, H. Essaidi
b,d, N. Stephant
a, G. Louarn
a, M. Morsli
c, S. Touihri
d, J.C. Bernède
b, L. Cattin
a,⁎aInstitut des Matériaux Jean Rouxel (IMN), Université de Nantes, CNRS, UMR 6502, 2 rue de la Houssinière, BP 32229, 44322 Nantes, Cedex 3, France
bMOLTECH-Anjou, CNRS, UMR 6200, Université de Nantes, 2 rue de la Houssinière, BP 92208, Nantes F-44322, France
cFaculté des Sciences et des Techniques, 2 rue de la Houssinière, BP 32229, 44322 Nantes, Cedex 3, France
dUnité de Physique des Dispositifs à Semi-conducteurs, Université El Manar, Faculté des Sciences de Tunis, Campus Universitaire, 2092 Tunis, Tunisia
A R T I C L E I N F O
Keywords:
Transparent electrode Indium tin oxide-free Copper aluminum alloy Flexible electrode Tungsten oxide Multilayer structures
A B S T R A C T
Indium tin oxide (ITO) is the most common transparent conductive material used in industrial processes. It has many advantages, but also some disadvantages: Indium is scarce and ITO deposition techniques are aggressive for organic materials, making it difficult to use it as top electrode in organic devices. Moreover its ceramic structure limits its application inflexible devices. Among the possible new In free transparent conductive electrode, dielectric/metal/dielectric multilayer structures such as WO3/M/WO3 appear very promising.
However, silver, which is the metal the more often used is expensive. Therefore it would be very profitable if copper, which is abundant on earth, could be substituted for silver. However the stability with time of the structure using Cu is questionable due to the high Cu diffusivity. In the present manuscript we improve sig- nificantly the lifetime of the structures using the alloy Cu:Al when a thin silver layer (2 nm) is introduced between the WO3bottom layer and the Cu:Al. It is shown that the Cu atom mobility is significantly decreased by the presence of Al of the alloy and of Ag which appears to diffuse into the metal layer forming an eutectic with Cu.
1. Introduction
Today the need inflexible optoelectronic devices is increasing ra- pidly. In optoelectronic devices it is necessary that at least one of the electrodes is transparent and conductive, so that it can pass photons to the active layers of the component and it allows the charge carrier to move freely. Up to date, Indium Tin Oxide (ITO) is the transparent electrode the most widely used for its remarkable optical and electrical properties. However, the natural scarcity of In and ITO brittleness have induced developments of alternative materials. Different substituents were proposed. Carbon based materials such as carbon nanotubes, polymer (PEDOT:PSS) and graphene were often proposed and inter- esting results were obtained for specific utilisations [1,2]. In the same way, metallic nanowires, mainly Ag, sometimes Cu, were probed with some success [3]. Cheap wet techniques were used for the realisation of these new transparent conductive electrodes. This choice may be in- teresting for some applications. Nevertheless, for other uses it is inter- esting to deposit using a vacuum technique. This allows good control of the conditions of deposits and thus high reproducibility of the
properties of the electrodes obtained [4]. Among them, dielectric/
metal/dielectric (D/M/D) structures have obtained significant interest due to their good optical and electrical properties [4–6]. High con- ductivity and optical transparency are mutually exclusive and si- multaneous optimisation of these both properties presents a stimulating challenge. The metal layer allows achieving a high conductivity, but at the expense of transmitting the light that is reflected by the thin me- tallic layer. However, the insertion of this thin metal layer between two dielectric layers, the D/M/D structures can suppress the reflexion from the metal in the visible due to the high refractive index of dielectrics. As metal, Ag exhibits the smallest resistivity:ρAg= 1.60 × 10−6Ωcm. It was used in many different D/M/D structures: ZnO/Ag/ZnO [7–9], MoOx/Ag/MoOx [10,11,13] and different others oxide/Ag/oxide (O/
Ag/O) [1–3,14]. These multilayer structures can be used as efficient transparent electrodes in different optoelectronic devices.
Because of its high price, it is desirable to substitute silver for a less expensive metal. From this point of view, Cu is a very good candidate because its conductivity,ρ = 1.67 × 10−6Ωcm, is very near from that of Ag and its natural reserves are far larger, which made that it is far
https://doi.org/10.1016/j.tsf.2018.11.059
Received 17 July 2018; Received in revised form 27 November 2018; Accepted 28 November 2018
⁎Corresponding author.
E-mail address:linda.cattin-guenadez@univ-nantes.fr(L. Cattin).
Available online 29 November 2018
0040-6090/ © 2018 Published by Elsevier B.V.
T
cheaper [15]. So different works were dedicated to D/Cu/D structures and performing electrodes were grown [15–18]. But the problem with Cu is that it migrates easily, so the D/Cu/D structures are relatively unstable [16]. Therefore, the stability of Cu based multi layer structures needs significant improvements. Some studies have already been dedicated to the improvement of the stability of these structures. For instance, using the alloy Cu:Cr allows reducing the oxidation-reduction reaction between the Cu electrode and the electrolyte in dye-sensitized solar cells [19]. Therefore the use of a Cu alloys can improve the electrode stability. Recently, we have shown that using Cu:Ni alloy allows improving the lifetime of the WO3/Cu/WO3 electrodes [20].
However, the melting point temperature of Ni being far higher than that of Cu, it is quite difficult to vary the atomic percentage of Ni introduced in the alloy by using simple thermal joule effect. Another possibility to prevent Cu diffusion consists in introducing diffusion barriers. If this technique has been used with success in the case of Ag [21], the success is only partial in the case of MoO3/Al/Cu/Al/MoO3[16]. Moreover, the addition of two ultra thin layers in the structures complicates the de- position process and it seems simpler to attempt to stabilize the struc- tures by using a metal alloy. Therefore, the melting point of Al being smaller than that of Ni, in the present work we have studied the properties of WO3/Cu:Al/WO3multilayer structures. It must be noted that the choice of WO3as dielectric is justified by the fact that it was shown that WO3is more stable than MoO3in organic optoelectronic devices [22] and that it allows to obtain D/Cu:Ni/D structures far more stable than when MoO3is the dielectric. We attributed this better sta- bility to the fact that the WO3thinfilm density is higher than that of MoO3[20].
2. Experimental techniques
Two kinds of substrates were used during this study, soda-lime glass and PET (polyethylene terephthalate). The PET substrates were thick of 150μm. Commercial PET needs a specific surface treatment before use as electrode substrate. After cleaning by a detergent, Micro Son (Ref 4890D) provided by Fisher Scientific, the PET substrates were rinsed in running distilled water. Then they were treated in 0.5 M NaOH at 90 °C for half an hour, rinsed again with distilled water andfinally dried in an oven and introduced into the deposition apparatus. In the case of glass substrates, after scrubbing with soap, substrates were rinsed in running deionised water. Then, they were dried with an argonflow, heated for 10 min at 100 °C, andfinally loaded into a vacuum chamber (10−4Pa).
The size of the samples was 2.5 cm × 2.5 (or 1.6) cm. Multilayer structures, WO3/Cu:Al/WO3, were deposited by using a simple joule effect evaporation/sublimation system. The three layers of the struc- tures were successively deposited onto the substrates without breaking the vacuum. Tungsten crucibles were used for metal and oxide de- position. The thickness and deposition rate of the differentfilms were measured using a quartz monitor. The thicknesses measured by the quartz monitor were checked by cross-section visualization using a scanning electron microscope (SEM). The optimal thicknesses and de- position rate of WO3layers have been determined in a previous work [24]. The bottom WO3layer thickness was 20 nm and the top WO3layer was and 35 nm, while the deposition rate was 0.05 nm/s. The deposi- tion rate of the Cu:Al alloy and its composition, were used as para- meters. Two alloy composition were investigated, Cu:Al 6 wt% and Cu:Al 3 wt%, while the deposition rate varies from 0.1 nm/s to 1 nm/s.
The surfaces of the structures were observed with afield emission scanning electron microscope (JEOL JSM 7600F). Secondary electron images (SEI) and backscattered electron images (BEI) have been done.
The composition of thefilms has been studied by energy dispersive spectrometry (EDS) with a - SAMx silicon drift detector (SDD) mounted to - another SEM (JEOL JSM 5800LV).
Atomic force microscopy (JPK instruments, NanoWizard, Berlin,Germany) was used for topography imaging and roughness measurements of the surfaces. The images were taken in air. The
intermittent contact mode was used for AFM imaging. Classical silicon cantilevers were used (NanoWorld NCHR probe). The average force constant and resonance were approximately 42 N/m and 320 kHz, re- spectively. Topographic images were recorded on different areas of the surface. The SPM Image Processing software (v.2–47) from Gwyddion was used to calculate the surface roughness (analyzed areas were:
5 × 5μm2and 10 × 10μm2).
X ray diffraction (XRD) scans were carried out using a Siemens D5000 diffractometer using Cu Kαradiation (λKα= 0.1542 nm), using a standard Bragg-Brentanoθ/2θgeometry.
The optical measurements were carried out at room temperature using a Perkin spectrophotometer (PERKIN ELMER Lambda 1050). The optical transmission was measured in the 0.3μm–1.0μm spectral range.
The conductivity at room temperature has been measured by the Van der Pauw four probes technique at room temperature. The linearity of I (V) characteristics was systematically checked and possible ther- moelectric effects were cancelled by inverting the polarity current. All measurements were computer controlled.
The optical and electrical properties of the transparent conductive electrodes being decisive when it comes to the quality of the electrode, it is customary to use afigure of merits (ΦTC) which makes it possible to weight these two properties. One of the most often usedfigure of merits ΦTCis that defined by Haacke as [25]:
=
ΦTC T /R10 sh (1)
with T transmission and Rsh the sheet resistance of the electrode.
Concerning XPS measurements, an Axis Nova instrument from Kratos Analytical spectrometer with Al Kαline (1486.6 eV) as excitation source has been used. The core level spectra were acquired with an energy step of 0.1 eV and using a constant pass energy mode of 20 eV, (energy resolution of 0.48 eV). Concerning the calibration, binding energy for the C1s hydrocarbon peak was set at 284.8 eV. Then data were analyzed with the CasaXPS software.
The depth profile of the structures was studied by recording suc- cessive XPS spectra obtained after argon ion etching for short periods.
Sputtering was accomplished at pressures of less than2 × 10−6Pa, 47μA/cm2current density and a 4 kV beam energy using an ion gun.
With these experimental conditions, all the analyzed surface was sputtered (raster size: 3 × 3 mm2).
The scotch tape method [26] was used to estimate the adhesion of the structures to the substrate. This test is only qualitative but it allows screeningfilms involving poor adhesion from those where adhesion is appreciable. We pressed the tape onto the film and then rapidly stripped it. Three possibilities arise: (a) thefilm is completely removed from the substrate (b) thefilm is not at all removed, and (c) thefilm is partly removed or removed in patches. In the present work, the results of the scotch tape tests are evaluated through this classification.
Theflexibility of the W/A/W structures was analyzed by a labora- tory made bending test system. The samples were clamped between two parallel plates. One plate was mounted to the shaft of the motor, the other wasfixed to afixed support. The distance between the plates in the stretched mode was 25 mm. The bending radius was around 6 mm and the bending frequency was 1 Hz. During the bending test, the re- sistance of the samples was measured by a multimeter. The actual sheet resistance of the ITO samples used for comparison with the WO3/Ag/
Cu:Al/WO3stack was 100Ω/sq.
3. Experimental results
Firstly, we have tried to optimize the Cu:Al layer thickness in the WO3/Cu:Al/WO3multilayer structures in order to achieve transparent and conductive structures. As shown inFig. 1, even with a Cu:Al layer thickness as high as 17 nm, transparent structures, withT= 78% ± 4 etλ= 600 nm, were obtained, the black lines drawn on the paper being clearly visible through the multilayer structure.
Unfortunately, even with this high metal layer thickness, the
conductivity of these structures was quite small, and moreover the re- sults were not reproducible. If the sheet resistance of the sample of Fig. 1a is around 100Ω/sq., that ofFig. 1b is far higher, outside the measuring range of the apparatus used. It can be seen inFig. 1that the conductivefilms are pink while the insulating are grey.
Moreover, it must be noted that the Al atomic concentration in the Cu:Al layers is more than 2 at.% in the case of conductive structures and only 1 at.% in the case of insulating structures. These color and Al atomic concentration differences mean that, probably, there is some Cu diffusion into the WO3layers, in the case of grey samples. This hy- pothesis is corroborated by the fact that when pure Cu is used in the WO3/Cu/WO3 structures they are systematically grey and insulating (Fig. 1c).
This preliminary study showed two important results. The con- centration of aluminum is decisive on the quality of the electrode.
Without, or with a too low concentration of Al, there is diffusion of the Cu into WO3, which makes the electrode insulating. Moreover, the sheet resistance of the conductive pink samples is relatively high and needs to be reduced.
It can be concluded from thisfirst series of characterizations that the properties of the WO3/Cu:Al/WO3depend significantly of the Al at.%
present in the Cu layer. If it is positive to show that Al has a real in- fluence on the Cu atoms diffusion, the fact that 1 at.% of Al does not stabilizes the Cu layer, indicates some probable limit in the Al efficiency in stopping Cu diffusion.
During thesefirst experiments, the deposition rate of the alloy was between 0.1 and 0.6 nm/s, which can justifies the variation of the Al atomic concentration from one Cu:Al layer to another one.
Therefore we decided, to study the influence of the deposition rate of the metal alloy on the composition of the deposited layer. We checked the homogeneity of the composition of the layers by taking measurements at several points of the samples. Averaged values are reported inTable 1, it can be seen that there is a correlation between the increase in deposition rate and the aluminum concentration. The increase in Al content in the depositedfilm at higher deposition rate is because of the lower melting point of Al compared to Cu. There is also a good correlation with the composition of the starting bulk alloy. The first results above show that it is desirable to make layers whose Al
concentration is between 2 and 3 at.%. Actually, if the concentration is lower, as shown above, the aluminum will not play its role and the copper diffusion in WO3will not be prevented. On the other hand, too high concentration will reduce transmission and conductivity. There- fore the Cu:Al layers in the electrodes were deposited at a deposition rate of 0.75 nm/s, using Cu:Al 6 wt% as starting source alloy. It must be noted that, in the case of pure Cu, it diffuses into WO3, whatever is its deposition rate (0.1 and 0.6 nm/s).
However, some scatter of the measured conductivity is still present and the sheet resistance is not in the range of the results obtained with similar structures using Ag as metal layer. Therefore, in addition to choosing an optimized deposition rate of the alloy, it was decided, in order to improve the reproducibility of the results and especially the conductivity of the structures, to insert a thin layer of silver between the WO3and the alloy, the diffusion of silver in the transition metal oxides being smaller than that of Cu [16].
Therefore, we used a double metal layer Ag/Cu:Al with a total thickness from 14 nm to 16 nm. When the thickness of the Ag layer was decreased that of Cu:Al was increased. We started with an Ag layer thick of 5 nm and progressively decreased it to 1 nm, while that of Cu:Al was increased. The results obtained are reported inTable 2. Even for Ag films thick of only 1 nm, the structures are pink, which testifies of the efficiency of Ag to prevent Cu diffusion in WO3. However, when the Ag layer was thick of only 1 nm, the value of the sheet resistance could significantly change from one sample to another. In addition some structures lost their pink color and saw their conductivity strongly di- minish in a few days.
Therefore, for the continuation of our study we decided to use a layer of Ag thick of 2 nm and we tried to optimize thefigure of merits of the structures by modifying the thickness of the Cu:Al layer. The results are summarized inFig. 2andTable 3.
Here, while the thickness of Ag was at 2 nm, that of Cu:Al ranged from 11 to 17 nm. When the thickness of the Cu:Al layer is 110 nm, the structure is nearly insulating andΦTCwas not calculated. The best re- sults are obtained for 14 nm–15 nm of Cu:Al, with Tmax = 82%. For 14 and 15 nm, the small difference in average transmission between the structures is compensated by the difference in conductivity, so that their performances are nearly equivalent. It means that there is per- colation of the metal layer. For thinner metalfilms, not only the sheet resistance increases rapidly, but also, the heterogeneities due to the discontinuity of the metal layer induce some light diffusion, which decreases the light transmission. For thickerfilms, the reflectivity of the metal layer increases and the transmission decreases. Therefore for the more specific characterization the Cu:Al thickness was 14.5 nm.
The thickness of the silver layer, which permits to stabilize the Cu:Al layer when deposited onto WO3, is only 2 nm, the thickness being measured by a quartz monitor. In that range of thickness, the value measured is averaged, the realfilms being discontinuous. In the present study, we used SEM for estimating the rate of covering of the WO3film by the silver. Images in secondary and backscattering mode have been
a b
1.6 cm
Fig. 1.Photography of WO3/Cu:Al/WO3structures with 3 at.% (a), 1 at.% (b) and 0 at.% (c) of Al in the Cu:Al layer.
Table 1
Variation of the Al atomic concentration in the Cu/Al alloy with the deposition rate of the alloy and the concentration of the alloy source.
Al (W.% bulk) Deposition rate (nm/s) Al W.% Al at.%
Cu:Al (6w.%) 0.10 0.44 1.2
0.30 0.55 1.5
0.60 0.72 2
0.75 1.10 2.6
1.00 2.20 5.5
Cu:Al (3w.%) 0.30 0.46 1.2
0.75 0.56 1.5
1.00 0.62 1.6
Table 2
Variation of the main parameters of the WO3(20 nm)/Ag/Cu:Al/WO3(35 nm) multilayer structures with the thickness of the Ag and Cu:Al layers.
Thickness (nm) Tmoy
(400 nm–700 nm)
ρ(Ω.cm) Rs (Ω/sq) Figure of merit T10/Rs Ag Cu: Al
1 13 53 9.42 × 10−3 1338 –
1 14 56 2.57 × 10−4 36.5 7.84 × 10−5
1 15 54 2.45 × 10−4 34.8 6.06 × 10−5
2 13 71 2.33 × 10−4 33.3 9.77 × 10−4
2 13 73 2.31 × 10−4 33.1 1.30 × 10−3
3 12 69 1.66 × 10−4 24.2 1.01 × 10−3
4 11 70.5 2.96 × 10−4 42.2 7.2 × 10−4
4 12 68.5 1.63 × 10−4 23.3 9.8 × 10−4
5 10 71 2.36 × 10−4 33.5 9.7 × 10−4
done. Because the average atomic number of silver is far from that of WO3 atoms, the backscattering mode allowed us to obtain well-con- trasted images for further treatment. The images have been exploited using the open-source software ImageJ (http://rsb.info.nih.gov/ij/).
Such software allows improving the exploitation of the images them- selves. It was already used to estimate the rate coverage of the TCO films by the silver nanodots [27] and gold nanodots [28]. SEM back- scattered images werefirstfiltered to reduce the noise and then con- verted to binary image by threshold operation to see, in black, the area covered by silver particles, while the substrate is white. Then, ImageJ can measure this covering [28]. An example is shown inFig. 3. A WO3
layer covered by 2 nm of silver is shown in backscattering mode in Fig. 3a. Lighter grains are visible on the entire surface of the layer. From Fig. 3, thefirst objective was to evaluate the ratio of covering of the thin WO3 film by the silver. After the ImageJ treatment of the image (Fig. 3b), the results of the calculations give a WO3surface coverage by silver of 35%. This means that a coverage of only 35% by Ag of WO3is enough to be active to prevent Cu diffusion in WO3/(Ag)Cu:Al/WO3
structures.
InFig. 3c, it can be seen that the surface of the structure WO3(/Ag (2 nm)/Cu:Al (14.5 nm) is homogeneous, without any grain visible at the surface.
The X-ray diffraction diagram of WO3/Ag/Cu:Al/WO3is presented inFig. 4, only diffraction peaks due to Cu are visible. While Cu layers are polycrystalline, WO3layers are amorphous. The diffraction peaks (111), (200) and (220) of Cu are clearly visible inFig. 4. As shown by the inset ofFig. 4, the position of the (111) peak is 2θ= 43.26° cor- responds to the value expected for Cu (JCPDS N° 04–0836).
The samples were submitted to the tape test in order to roughly estimate its adhesion to the substrate. When the tape was pressed onto
thefilm just after deposition and then rapidly stripped it, thefilm was partly removed, which means that its adhesion to the substrate, glass or PET, was poor. Fortunately one day after deposition, the WO3/Ag/
Cu:Al/WO3structures pass the tape test with success, thefilms are not at all removed. They exhibit good adhesion to the substrates, whatever the substrate is, glass or PET.
Then we proceeded to the study of theflexibility of the structures.
During the bending tests, the variations of the sheet resistance of the PET/WO3/Ag/Cu:Al/WO3 and PET/ITO samples were measured as follows. The change in resistance was expressed as (R-R0)/R0, where R0 is the initial resistance and R the measured resistance after bending.
Fig. 5shows the change in the resistance of both kinds of samples with increasing bending cycles. Values reported in this Figure correspond to averaged measures obtained with three different samples. It can be seen 400 500 600 700 800 900 1000
0 20 40 60 80 100
T (%)
λ (nm)
14 nm (PET) 14 nm (Glass) 15 nm (Glass) 13 nm (Glass) 16 nm (Glass)
Fig. 2.Variation of the light transmission of the WO3(20 nm)/Ag/Cu:Al/WO3 (35 nm) multilayer structures with different thicknesses of the Cu:Al layer.
Table 3
Variation of the main parameters of the WO3/Ag/Cu:Al/WO3 multilayer structures with the thickness of the Cu:Al layer, the Ag layer thickness being fixed at 2 nm.
Thickness (nm) Rsh
Ω/
sq T (%) average⁎
ΦTC
average⁎ (×10−3)
T max % ΦTCmax (×10−3) WO3 Ag Cu: Al WO3
20 2 11 35 – 65 – 70 –
20 2 12 35 43 72 0.87 79 2.20
20 2 13 35 33 73 1.30 79 2.86
20 2 14 35 24 78 3.47 82 5.70
20⁎⁎ 2⁎⁎ 14⁎⁎ 35⁎⁎ 27 73 1.60 79 3.50
20 2 15 35 21 76 3.06 82 6.54
20 2 16 35 20 68 1.05 74 2.46
20 2 17 35 20 60 0.30 67 0.91
⁎ Average value between 400 nm and 700 nm.
⁎⁎ PET substrate.
Fig. 3.SEM study of WO3(20 nm)/Ag (2 nm), (a) Image in the backscattering mode, (b) Image treated with ImageJ. (c) Surface image of WO3(20 nm)/Ag (2 nm)/Cu:Al (14.5 nm).
that the (R-R0)/R0 value of the PET/ITO sample increases with the number of bending cycles, this increase being higher in the case of outer bending. In the case of PET/ WO3/(Ag)Cu:Al/WO3samples, the answer to the bending test does not depend on the type of bending. For outer and inner bending the samples exhibit nearly no variation of their re- sistance value, indicating a constant resistance of the trilayer structure.
Whatever the possible small modifications of the sheet resistance of the
PET/ WO3/(Ag)Cu:Al/WO3samples after outer or inner bending cycles they are far smaller than those of the PET/ITO structures as it can be seen inFig. 5.
This good stability of the PET/ WO3/(Ag)Cu:Al/WO3structure can be attributed to the ductile metal layer between the both WO3 layers [29]. Moreover, the WO3being amorphous, its failure strain is higher than that of crystallinefilms.
A surface visualization of a Glass/ WO3/(Ag)Cu:Al/WO3structure is shown in Fig. 6. The surface of the structure is smooth and highly homogeneous, its surface roughness was checked by atomic force mi- croscopy (InsetFig. 6). The Root Mean Squared (RMS) Roughness de- duced is 1.94 nm. Such small surface roughness is favourable to the use of these structures for the production of components using low active layer thicknesses which is the case of cells.
To track the evolution with time of the electrode properties, three typical types of structures were studied: WO3/Cu (16 nm) /WO3, WO3/ Ag (2 nm)/Cu:Al 1 at.% (13 nm)/WO3and WO3/Ag (2 nm)/Cu:Al 3 at.
% (13 nm)/WO3the thickness of the WO3layers being 20 nm for the bottom layer and 35 nm for the top layer. Between each measurement, the samples were stored in ambient air in transparent boxes. These boxes were stored in the experiment room. The statistics of recent years show that during the period of tracking the structures, the relative humidity of the room was 40%–60%, while its temperature was si- tuated between 18 °C and 25 °C. While the optical properties of the structures were stable during the duration of the study, the evolution during these three months of the sheet resistance of the different samples is shown inFig. 7. First of all it must be noted that the re- sistance of the WO3/Cu (16 nm)/WO3multilayer structure is out of the measuring range of the apparatus used.
It can be seen inFig. 7that when a thin Ag layer is introduced between a WO3layer and Cu:Al, the degradation of the conductivity is slowed down. But the conductivity of a structure using Cu:Al 1 at.%
becomes quite rapidly poorly conductive. In the case of the WO3/Ag/
Cu:Al 3 at.%/WO3 structure, after an initial increase similar to that encountered in the preceding structures, the sheet resistance tends du saturate, which allows preserving an acceptable conductivity. Standard deviations are indicated inFig. 7. It must be noted that they increase with time. It is less than 1Ω/sq. during thefirst days and ± 20Ω/sq.
three months later for the most resistive sample, while it is smaller in the case of the more conductive sample which sheet resistance tends to saturate. These quantitative results confirm the qualitative study de- scribed above. The Cu:Al alloy must contain at least 2 at. % of Al to be
30 40 50 60 70 80
200 400 600 800 1000
40 41 42 43 44 45 2 (θ)
2 = 43.266
Cu (200)
46 Cu
(220) 20
).u.a(I
2 (°) CuAl 1A/s
CuAl 6A/s CuAl 10A/s
Cu (111) 100
Fig. 4.XRD diagrams of WO3 (20 nm)/Ag (2 nm)/Cu:Al (14.5 nm)/WO3
(35 nm) multilayer structures, the Cu:Al source (6 at.%) being deposited at different deposition rates.
0 1000 2000 3000 4000 5000
0 10 20 30 40 50 60 70
(R-R0)/R0 (%)
Number of cycles ITO
WO3/Ag/Cu:Al/WO3 (a): Outer bending
0 500 1000 1500 2000
0 5 10 15 20 25
(R-R0)/R0 (%)
Number of cycles W03/Ag/Cu:Al/WO3 ITO
(b): Inner-bending
Fig. 5.Resistance evolution after outer bending (a) and inner bending (b) as a function of the number of bending cycles for PET/WO3(20 nm)/Ag (2 nm)/
Cu:Al (14.5 nm)/WO3(35 nm) and PET/ITO structures.
Fig. 6.Surface visualization of a WO3(20 nm)/(Ag)Cu:Al/WO3(35 nm) mul- tilayer structure. Inset: AFM image of a WO3(20 nm)/(Ag)Cu:Al/WO3(35 nm) multilayer structure.
efficient, while 2 nm of Ag allows limiting the diffusion of Cu. It can be concluded that, inserting 2 nm of Ag on the bottom WO3and introdu- cing 3 at.% of Al in Cu allows improving significantly the stability of the WO3/M /WO3structures using Cu as metal.
In order to check the influence of these different parameters on the Cu diffusion we proceeded to XPS measurements. Typical profiles ob- tained on different samples are presented inFig. 8. The different pro- files were obtained on samples 2 weeks old. The profile ofFig. 8a cor- responds to the structure WO3/Cu/WO3, it confirms the high diffusion of Cu in WO3 since there is only small increase of Cu atomic con- centration in the center of the profile, so that the layers of the stack are not clearly distinguishable from each other. Moreover, there is Cu ac- cumulation at the surface of the multilayer structure. In, the case of WO3/Ag/Cu:Al 3 at.%/WO3 structures theFig. 8b shows that the se- quence oxide/metal/oxide is clearly visible, which testifies that the stability of these structures is far better than that of structures using Cu alone as metal. Nevertheless, the profile shows also that the metal layer consists more in a Cu:Al:Ag monolayer than in a Ag/Cu:Al bilayer.
Therefore it seems that Ag does not behave as a barrier diffusion layer but as a stabilizer of the Cu:Al layer though the formation of a Cu:Al:Ag alloy. As shown by the inset ofFig. 8b, the Ag profile is not symmetric, the Ag concentration increases faster on bottom side of the metal layer, which is in good agreement with the fact that Ag is introduced between the WO3bottom layer and the Cu:Al layer. If the diffusion of Ag was due to the etching, its concentration should increase with the etching time which is not the case here. This shows that, if some Ag diffusion may occur during the etching process, it is not the main contribution to Ag diffusion.
About Cu:Ag alloys, it must be noted that Ag an Cu have large miscibility and a relative atomic size difference of 12%, which allows this large miscibility and eutectic possible formation [29]. The max- imum solubility of Ag in Cu is 7.47 at.%, which may possible the Ag diffusion in the present structures, the maximum of Ag in the profile being 4 at.%. On the other hand, it was already shown, in the case of TiOx/Ag/TiOx structures, that the presence of a thin layer of Cu in- terlayer between TiOx and Ag, thick of 2 nm allows enhancing the stability of the structure by a factor 10 [30]. When deposited on both sides of the Ag layer, Cu can improve the stability by a factor exceeding 100. In the same way, it is possible to improve the stability of TiOx/Ag/
TiOx structures by using as metal interlayer an alloy Ag/ Cu 8 at. %.
Such Cu:Ag solubility and stability improvement of D/M/D structures using Cu:Ag alloys confirms that, as shown during the present work, the presence of Ag in the structures WO3/Cu:Al/WO3 induces an
improvement of their stability. The hypothesis of Ag diffusion is re- inforced by the fact that, as the SEM study has shown, the Ag layer is discontinuous, which makes it unlikely that it can act as an effective diffusion barrier. About the Al effect, it is probable that it is present in the structure in the form of Al2O3. These Al2O3particles may acts as pinning at the grain boundary, which limits the Cu atomic motion and therefore enhances the structure stability [31].
4. Conclusion
The present study shows that the combined effect of Al and Ag makes it possible to improve the stability of the structuresWO3/Cu/
WO3. Actually, if using Cu:Al alloy, with 2–3 at. % of Al, allows de- creasing significantly the high Cu diffusion, the stability and reprodu- cibility of the properties of the structures remains insufficient. The in- sertion of a thinfilm of Ag (2 nm) at the bottom WO3/Cu:Al interface permits to stabilize in an acceptable way the multilayer structures. The depth profile of the structures studied by recording successive XPS spectra have shown that, the presence of Al and Ag decreases strongly the Cu diffusion into the oxide, while Ag appears to be present in the whole thickness of the metal layer. Following these results, a study on 0 10 20 30 40 50 60 70 80 90 100
50 100 150 200
Rs (Ohm/sq)
Ag (2 nm)/Cu:Al 1at.% (13 nm)
Ag (2 nm)/Cu:Al 3at.% (13 nm)
Time (d)
Fig. 7.Evolution with time of the sheet resistance of different typical samples:
WO3/Cu (16 nm) /WO3, WO3/Ag (2 nm)/Cu:Al 1 at.% (13 nm)/WO3and WO3/ Ag (2 nm)/Cu:Al 3 at.% (13 nm)/WO3, the thickness of the WO3layers being 20 nm for the bottom layer and 35 nm for the top layer. The values obtained with WO3/Cu (16 nm) /WO3, are not shown due to its too high resistivity.
0 200 400 600 800 1000 1200 1400 1600 0
20 40 60 80 100
)%.ta(noitartnecnoC
Etching time (s) (a)
WO3
Cu
WO3-Top Cu:Al Ag WO3- Bottom
Substrate
(c)
0 200 400 600 800 1000 1200 1400 1600 0
20 40 60 80 100
(b)
400 600 800 10001200 0
1 2 3 4
)%.ta(noitartnecnoC
Etching time (s) Cu WO3
Ag
Fig. 8.XPS profiles of (a) WO3(20 nm)/Cu:Al 3 at.% (16 nm)/WO3(35 nm) and (b) WO3(20 nm)/Ag (2 nm)/Cu:Al 3 at.% (14.5 nm)/WO3(35 nm) structure, (c) typical structural diagram of a structure.
the use of Cu:Ag as a metal alloy is being carried out.
Acknowledgement
The authors acknowledge funding from the European Community ERANETMED_ENERG-11-196: Project NInFFE.
References
[1] L. Zhang, A. Aboagye, A. Kelbar, C. Lui, H. Fong, A review: carbon nanofibers from electrospun polycrytonitrile and their applications, J. Mater. Sci. 49 (2014) 463–480.
[2] R. Garg, S. Elmas, T. Nann, M.R. Andersson, Deposition methods of graphene as electrode material for organic solar cells, Adv. Energy Mater. 7 (10) (2016) 1601393.
[3] D. Langley, G. Glusti, C. Mayousse, C. Celle, D. Bllet, J.-P. Simonato, Flexible transparent conductive materials based on silver nanowire networks: a review, Nanotechnology 24 (2013) 452001.
[4] C. Guillén, J. Herrero, TCO/metal/TCO structures for energy andflexible electro- nics, Thin Solid Films 520 (2011) 1–17.
[5] L. Cattin, J.C. Bernède, M. Morsli, Toward indium-free optoelectronic devices:
Dielectric/Metal/Dielectric alternative conductive transparent electrode in organic photovoltaic cells, Phys. Status Solidi A 210 (2013) 1047–1061.
[6] W. Cao, J. Li, H. Chen, J. Xue, Transparent electrodes for organic optoelectronic devices: a review, J. Photonics Energy 4 (2014) 040990.
[7] Y. Peng, L. Zhan, N. Cheng, T.L. Andrew, ITO-free transparent organic solar cell with distributed bragg reflector for solar harvesting windows, Energies 10 (2017) 707.
[8] J.H. Kim, J.H. Lee, S.-W. Kim, Y.-Z. Yoo, T.-Y. Seong, Highlyflexible ZnO/Ag/ZnO conducting electrode for organic photonic devices, Ceram. Int. 41 ( (2015) 7146–7150.
[9] S. Vedraine, A. El Hajj, P. Torchio, B. Lucas, Optimized ITO-free tri-layer electrode for organic solar cells, Org. Electron. 14 (2013) 1122–1129.
[10] L. Cattin, Y. Lare, M. Makha, M. Fleury, F. Chandezon, T. Abachi, M. Morsli, K. Napo, M. Addou, J.C. Bernède, Effect of the Ag deposition rate on the properties of conductive transparent MoO3/Ag/MoO3multilayers, Sol. Energy Mater. Sol.
Cells 117 (2013) 103–109.
[11] M. Makha, L. Cattin, Y. Lare, L. Barkat, M. Morsli, M. Addou, A. Khelil, J.C. Bernède, MoO3/Ag/MoO3anode in organic photovoltaic cells: influence of the presence of a CuI buffer layer between the anode and the electron donor, Appl. Phys. Lett. 101 (2012) 233307.
[13] J.C. Bernède, L. Cattin, M. Morsli, T. Abachi, Elargissement du domaine de trans- mission des structures multicouches diélectrique/métal/diélectrique via l'utilisation d'une double couche de métal cuivre /argent, Technol. Lett. 1 (5) (2014) 5–12 (in French).
[14] A. Bou, Ph. Torchio, S. Vedraine, D. Barakel, B. Lucas, J.C. Bernède, P.Y. Thoulon, M. Ricci, Numerical optimization of multilayer electrodes without indium for use in organic solar cells, Sol. Energy Mater. Sol. Cells 125 (2014) 310–317.
[15] S. Lim, D. Han, H. Kim, S. Lee, S. Yoo, Cu-based multilayer transparent electrodes: a low-cost alternative to ITO electrodes in organic solar cells, Sol. Energy Mater. Sol.
Cells 101 (2012) 170–175.
[16] I. Pérez Lopéz, L. Cattin, D.-T. Nguyen, M. Morsli, J.C. Bernède, Dielectric/Metal/
Dielectric structures using copper as metal and MoO3as dielectric for use as transparent electrode, Thin Solid Films 520 (2012) 6419–6423.
[17] X.-Y. Liu, Y.-A. Li, S. Liu, H.-L. Wu, H.-N. Cui, ZnO/Cu/ZnO multilayerfilms:
Structure optimization and investigation on photoelectric properties, Thin Solid Films 520 (2012) 5372–5377.
[18] D.R. Sahu, J.-L. Huang, The properties of ZnO/Cu/ZnO multilayerfilms before and after annealing in the different atmosphere, Thin Solid Films 516 (2007) 208–211.
[19] T.-C. Lin, W.-C. Huang, F.-C. Tsai, The structural and electro-optical characteristics of AZO/Cr:Cu/AZO transparent conductivefilm, Thin Solid Films 589 (2015) 446–450.
[20] L. Tuo, H. Cattin, L. Essaidi, G. Peres, Z. Louarn, M. El Jouad, S. Hssein, S. Touihri, P. Yapi Abbe, M. Torchio, J.C. Bernède Addou, Stabilisation of the electrical and optical properties of dielectric/Cu/dielectric structures through the use of efficient dielectric and Cu:Ni alloy, J. Alloys Compd. 729 (2017) 109–116.
[21] L. Zhou, X. Chen, F. Zhu, X.X. Sun, Z. Sun, Improving temperature-stable AZO-Ag- AZO multilayer transparent electrodes using thin Al layer modification, J. Phys. D.
Appl. Phys. 45 (2012) 505103.
[22] W. Greenbank, L. Hirsch, G. Wantz, S. Chambon, Interfacial thermal degradation in inverted organic solar cells, Appl. Phys. Lett. 107 (2015) 263301.
[24] H. Essaidi, L. Cattin, Z. El Jouad, S. Touihri, M. Blais, E. Ortega, G. Louarn, M. Morsli, T. Abachi, T. Manoubi, M. Addou, M.A. del Valle, F. Diaz, J.C. Bernède, Indium free electrode, highlyflexible, transparent and conductive for optoelec- tronic devices, Vaccum 153 (2018) 225–231.
[25] G. Haacke, Newfigure of merit for transparent conductors, J. Appl. Phys. 47 (2012) 4086.
[26] M. Hssein, S. Tuo, S. Benayoun, L. Cattin, M. Morsli, Y. Mouchaal, M. Addou, A. Khelil, J.C. Bernède, Cu-Ag bi-layerfilms in dielectric/metal/dielectric trans- parent electrodes as ITO free electrode in organic photovoltaic devices, Org.
Electron. 42 (2017) 173–180.
[27] F. Papadopulos, M. Spinelli, S. Valente, L. Foroni, C. Orrico, F. Alviano, G. Pasquinelli, Ultrastruct. Pathol. 31 (2007) 401–407.
[28] L. Cattin, S. Tougaard, N. Stephant, S. Morsli, J.C. Bernède, On the ultrathin gold film used as buffer layer at the transparent conductive anode/organic electron donor interface, Gold Bull. 44 (2011) 199–205.
[29] H. Chen, J.-M. Zuo, Structure separation of Ag-Cu alloy thinfilms, Acta Mater. 55 (2007) 1617–1628.
[30] K. Chiba, K. Suzuki, Effects of heterogeneous metal atoms on the stability of a silver layer of a heat mirror coating, Sol. Energy Mater. Sol. Cells 25 (1992) 113–123.
[31] H. Han, T.L. Alford, Texture and surface morphology evolution of Ag(Cu) layers on indium tin oxide thinfilms, J. Phys. D. Appl. Phys. 41 (2008) 155306.
