Cu-Ag bi-layer fi lms in dielectric/metal/dielectric transparent electrodes as ITO free electrode in organic photovoltaic devices
M. Hssein
a,d, S. Tuo
b,1, S. Benayoun
c, L. Cattin
a, M. Morsli
e, Y. Mouchaal
b,f, M. Addou
d, A. Khelil
f, J.C. Bern ede
b,*aUniversite de Nantes, Institut des Materiaux Jean Rouxel (IMN), CNRS UMR 6502, 2 rue de la Houssiniere, BP 32229, 44322, France
bUniversite de Nantes, MOLTECH-Anjou, CNRS UMR 6200, 2 rue de la Houssiniere, BP 92208, Nantes F-44000, France
cUniversite de Lyon, Ecole Centrale de Lyon, LTDS, UMR CNRS 5513, 36 Av. Guy de Collongue, F-69134 Ecully Cedex, France
dLMVR, FST, Universite Abdelmalek Essaidi, Tanger, Ancienne Route de l'Aeroport, Km 10, Ziaten, BP 416, Morocco
eUniversite de Nantes, Faculte des Sciences et des Techniques, 2 rue de la Houssiniere, BP 92208, Nantes F-44000, France
fUniversite d'Oran 1dAhmed Ben Bella, LPCMME, BP 1524 ELM Naouer, 31000 Oran, Algeria
a r t i c l e i n f o
Article history:
Received 3 July 2016 Received in revised form 4 November 2016 Accepted 16 December 2016 Available online 19 December 2016
Keywords:
ITO free electrode Multilayer structure Dielectric/metals/dielectric0 Adhesion
Flexibility
Organic photovoltaic cells
a b s t r a c t
Among ITO alternative, dielectric/metal/dielectric multilayer structures are one of the most often studied possible substituent. However, if their optical and electrical properties are systematically investigated it is not the same with regard to their mechanical properties. In the present manuscript we have studied the properties of ZnS/Cu/Ag/ZnS, ZnS/Cu/Ag/MO3(with M¼Mo or W) structures. With a maximum transmission of 90% and a sheet resistance of 5U/sq the optimum structure exhibits afigure of merit of 82 103U1whenl¼600 nm. Beyond these standard measures we proceeded to the study of the mechanical properties of the multilayer structures. The inner and outer bending tests show that the ZnS/
Cu/Ag/ZnS (or MO3) structures are moreflexible than ITO, while their responses to scotch tests show that they exhibit a large adhesion to the substrate, glass or plastic. The scratching adhesion test puts in ev- idence that the adhesion to the substrate of the Ag layer is smaller than that of ZnS/Cu/Ag/ZnS, which is smaller than that of ITO. On the other hand, this test shows that the ZnS/Cu/Ag/ZnS (no cracks for L¼25 N) is less brittle than ITO (cracks L¼15N). Finally, when used as anode in organic solar cells, the structure ZnS/Cu/Ag/WO3allows achieving the best efficiency, similar to that obtained with ITO.
©2016 Elsevier B.V. All rights reserved.
1. Introduction
The need for transparent conductive electrode (TCE) is increasing rapidly due to applications in optoelectronic devices.
Indium tin oxide (ITO) is the most common transparent conductive material used in industrial processes. It has many advantages, high transmission in the visible range, good conductivity. After cleaning by UV-ozone treatment, its high work function allows using it easily as anode in optoelectronic devices. Actually, ITO is the transparent conductive electrode that gives organic optoelectronic devices with optimum performances. Nevertheless, it has also some disadvantages. Indium is scarce, aggressive techniques of
deposits for organic materials and brittleness. About the In limited resources, it must be noted that, at the moment, the main difficulty is not related to In scarcity, but to the fact that it is extracted as a byproduct of Zn mining at very low concentration, which limits In production. Usually ITO thin films are deposited by magnetron sputtering, which is not compatible with organic devices, if you want to use ITO as top electrode. Moreover, in order to achieve quite high conductivity the ITO electricalfilms properties are optimized by annealing at 250 C or higher temperatures, which is not compatible with plastic substrates. At last, ITO is brittle, which made it incompatible withflexible substrates. All that requires the development of alternative TCEs. This new TCE must fulfill, different requirements such as: High transmittance (z80%), good conductivity (Rs<20U/sq), easiness in controlling the surface work function, abundance of the constituents, good stability and envi- ronmental neutrality, soft and low temperature deposition tech- nique (T < 150C),flexibility, adhesion compatible with plastic substrates, low surface roughness (rms z 1 nm), scalability for
*Corresponding author.
E-mail address:jean-christian.bernede@univ-nantes.fr(J.C. Bernede).
1 Permanent address: Laboratoire de physique de la Matiere Condensee et Technologie (LPMCT) 22 BP 582 Abidjan 22, Universite Felix Houphou€et Boigny, Abidjan, C^ote d'Ivoire.
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Organic Electronics
j o u rn a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / o r g e l
http://dx.doi.org/10.1016/j.orgel.2016.12.030 1566-1199/©2016 Elsevier B.V. All rights reserved.
industrial application.
Different possibilities were experimented, all of them have some advantages, but all of them have some weaknesses. The conductive polymers, such as PEDOT:PSS, are easy to deposit in thin films form by spin coating and ink-jet printing[1]. However, their conductivity is limited. Different proposition were presented to increase it, such as the introduction of silver printed grid, but it induces some complexity in the electrode process[2]. Due to its high conductivity, graphene is a promising material[3]. However, up to now it is very difficult to obtain conductive graphenefilms with large area. The utilization of carbon nanotube induces others difficulties, surface roughness of the electrode, adhesion to the substrate[4]. Moreover, it is not so easy to achieve carbon nanotube films, which are simultaneously conductive and transparent. This difficulty is easier to address using silver nanowires[5]. However, the problems of adhesion to the substrate and of surface roughness are still present. During these last years, a new interest was dedi- cated to ultra thin metalfilms, since it was shown that using of a double metal layer allows improving the ultra thin metal films properties[6,7]. However, here also it is difficult to obtainfilms that are conductive and transparent. Achieving conductive films re- quires an increase of the metalfilms thickness which implies an increase of thefilm reflexivity. A possibility consists in inserting this ultra-thin film between two dielectric films with high refractive index, which allows decrease the electrode reflexivity. Dielectric/
metal/dielectric0(D/M/D0) multilayer structures have been exten- sively studied [8,9]. If these structures exhibit small sheet resis- tance, their spectrum of transparency is narrower than that of ITO.
In a recent work we propose a strategy to overcome this limitation.
The experience shows that the transmittance peak wavelength, lMax, depends on the metal, lMax Ag ¼ 490 nm and lMax
Cu¼580 nm, using ZnS as dielectric. Therefore,flat broad trans- mittance spectra can be expected when these two metals, are involved. We showed that the use of a bilayer M12¼Cu-Ag allows widening significantly the value of the averaged (400 nm-700 nm) transmittance of the D/M12/D0structures, while small sheet resis- tance were preserved, which allows to obtain electrodes with high figure of meritFM[10]. Here, ZnS was used in place of transition metal oxide such as MoO3due to the fact that Cu diffuse strongly into MoO3 [11]. On the other hand, if the optical and electrical properties of the new ITO free electrodes are often studied, it is not the case of their adhesion and brittleness[8,9]. In the present work we probe theflexibility of these new structures deposited onto plastic substrates, and their adhesion and brittleness using scratch tests. We show that the D/M12/D0 multilayer structures are less brittle than those of ITO.
After these characterizations, D/M12/D0structures were probed as transparent conductive electrode in organic photovoltaic cells (OPVCs) based on the planar heterojunction: Copper phthalocya- nine/fullerene, CuPc/C60. We show that the best performances are obtained when the structure ZnS/Cu/Ag/WO3is used as anode.
2. Experiments
2.1. Realization and characterization of the dielectric/metal/
dielectric0structures
The structures were deposited under vacuum onto glass or PET substrates. They were loaded in the vacuum chamber (103Pa). The dielectric was ZnS, while two metals, Cu and Ag, were inserted between the two dielectric layers. The experiments described in paragraph 3.2 show that, when these structures are used as ITO free anodes in organic photovoltaic cells (OPVCs), it is necessary to substitute, MoO3or better, WO3to ZnS in order to achieve high hole collection efficiency, therefore we also studied structures with MO3
(M¼Mo, W) as top layer. The ZnS/Cu/Ag/ZnS(MO3) were deposited by simple joule effect sublimation (ZnS, MO3) or evaporation (Cu, Ag). The different layers were successively deposited onto sub- strates at room temperature. The thickness and deposition rate of the differentfilms were measured using a quartz monitor, after calibration through the use of a scanning electron microscope. The cross sections of the structures were observed with afield emission scanning electron microscope (SEM, JEOL F-7600). In order to improve the visualization of the cross section, images in back- scattering (BEI) mode were done. The thin films structures are analyzed by X ray diffraction (XRD) by a Siemens D5000 diffrac- tometer using Karadiation from Cu (lKa¼0.15406 nm).
The optical measurements were carried out at room tempera- ture using a Perkin spectrophotometer. The optical transmission was measured in the 0.3mm-1.2mm spectral range. The four-probe technique was used to measure the electrical conductivity.
Theflexibility of the ZnS/Cu/Ag/ZnS(MO3) structures deposited onto PET was studied using a laboratory made bending system[12].
The samples were clamped between two conductive parallel plates.
The one wasfixed to a mobile axis moved by the engine with a rectilinear reciprocating motion, while the other one wasfixed to a rigid support. The distance between the two plates in the stretched mode was 30 mm, while that of the bent position was 12 mm. The bending radius was approximated to 6 mm. During the bending test, the resistance of the sample was measured using an elec- trometer. The sample was loaded with the multilayer structure either facing upward (outer-bending) or downward (inner- bending).
The scotch tape method[13]was used to estimate the adhesion of the structures to the glass. This test is highly qualitative but it allows screeningfilms involving good adhesion from those where adhesion is poor. We pressed the tape onto the film and then stripped it, in order to check the adhesion of the multilayer struc- tures to their substrates.
Scratching adhesion testing was performed using commercial microscratch tester MST-CSEMEX® equipped with an integrated optical microscope, an acoustic emission detection system and a tangential friction force (Ft) sensor, with a sensitivity detection of approximately 10 mN. During the normal load application with a Rockwell C diamond indenter (cone apex angle 120, tip radius R¼200mm), the specimen has been moved horizontally until some failures occur[14].
2.2. Realization and characterization of the organic photovoltaic cells
After deposition, the ZnS/Cu/Ag/ZnS(MO3) structures were transferred from the growth chamber to the OPVCs realization chamber. The OPVCs probed in the present work were based on the classical planar organic bilayer heterojunctions: CuPc/C60. The organic heterojunction was sandwiched between two electrodes, one of them being transparent. In order to improve the OPVCs ef- ficiency, buffer layers were inserted between the electrodes and the organic materials. Therefore, the planar multi-heterojunction solar cells studied were as follow: ZnS/Cu/Ag/ZnS(MO3)/ABL/CuPc/C60/ Alq3/Al.
Tris-(8-hydroxyquinoline)Aluminium(Alq3) is used as exciton blocking layer (EBL) at the C60/cathode interface. The ABL (anode buffer layer) used consists in a coevaporated MO3:CuI layer. The insertion of an EBL between the electron acceptor (C60) and the Al layer was shown to significantly improve the performances of OPVCs[15,16]. Similarly it is known that, when a TCO is used as anode, the introduction of an anode buffer layer between the anode and the electron donor (ED), here the CuPc, allows obtaining op- timum OPVCs power conversion efficiency (h).
MO3,i.e.MoO3or WO3, are well known as very efficient ABL [16e19]. Nevertheless, if it is known that MO3allows improving the band matching at the interface anode/ED, it has been shown that the orientation of the CuPc molecules is also very important[20].
For instance, when the CuPcfilm is deposited onto MO3, the mol- ecules are perpendicular to the plane of the substrate, while they lie flat on it when deposited onto CuI. The latter case allows to significantly improve light absorption and OPVs performances.
Moreover, it was shown that CuI is a very efficient electron blocking layer[21,22]. If CuI is an efficient hole selective layer, its surface roughness can have negative effect on the J-V characteristics of the OPVC through leakage induced current[23,24]. Therefore in the present work, in order to avoid such roughness negative effects, CuI was coevaporated with MO3, since we have shown that such co- evaporated ABL allows combining the advantages of CuI and of the transition metal oxide[25]. The MO3:CuI layer thickness was 3 nm and its deposition rate wasfixed at 0.01 nm/s. In order to estimate the potential of our new electrodes, some ITO coated glass substrates were used as references. The thickness of the CuPcfilm was 35 nm, that of C60was 40 nm and that of Alq39 nm[16]. The ITO and Al layers were 100 nm thick. The effective area of each cell was 0.16 cm2. The characteristics of the photovoltaic cells were measured using a calibrated solar simulator (Oriel 300W) at 100 mW/cm2light intensity adjusted with a reference cell (0.5 cm2 CIGS solar cell, calibrated at NREL, USA). Measurements were per- formed at an ambient atmosphere. All devices were illuminated through TCO electrodes.
3. Experimental results and discussion
3.1. Adhesion and mechanical properties of the D/M/D0structures We have shown in Ref.[10]that the optimal thicknesses of the different layers constituting the structure were as follow:
ZnS(50 nm)/Cu(3 nm)/Ag (9 nm)/ZnS (45 nm). The transmission maximum of such structure was 90%, while the averaged trans- mission between 400 and 1000 nm was 85%. Its sheet resistance was Rsq¼5U/sq. Knowing that thefigure of meritFMproposed by Haacke is[26]:
FM¼T10/Rsq, (1)
with T transmission and Rsq the sheet resistance of the structure, thefigure of merit calculated for the optimum structure was 82 103U1whenl¼600 nm.
In the continuation of the present article, the structures will be carried out with these layer thicknesses. InFig. 1the transmission of such structure using the double metal layer Cu/Ag is compared with that of structures with only one metalfilm, Cu or Ag. It can be seen that, not only the transmission range of the structure with two metal layers is wider than that of structures with only one metal layer, but also that the transmission spectrum of the ZnS/Cu/Ag/ZnS structures tends towards that of ZnO.
One can see inFig. 2that, in these structures, the metalsfilm is continuous all along the multilayer ZnS/Cu/Ag/ZnS structures. The insert inFig. 2allows seeing that ITO and ZnS/Cu/Ag/ZnSfilms have a quite similar appearance.
A typical XRD pattern is presented in Fig. 3, two diffraction peaks are clearly visible.
The diffraction peak situated at 2q¼2866 is due to ZnS and that at 2q¼3833 corresponds to Ag[27,28]. The grain size D of the differentfilms is estimated by the Scherrer formula, which gives a lower limit of the coherent diffracting domain size:
D¼(0.94l)/bcosq (2)
with, l¼0.1541 nm, wavelength of the X-ray source andbthe broadening at the full width at half maximum (FWHM).bis such that b ¼ (bexp2 bo2)1/2, with bexp experimental FWHM) and bo
FWHM due to the apparatus itself [29]. With 0.30and 1.21 as FWHM for ZnS and Ag respectively, the estimated grain size are 42 nm for ZnS and 9 nm for Ag.
We checked the flexibility of the ZnS/Cu/Ag/ZnS structures deposited onto PET substrates. The results are compared to com- mercial 100 nm-thick ITO onto PET, which sheet resistance is 100U/ sq. The change in resistance was expressed as (RR0)/R0, where R0 is the initial resistance and R the measured resistance after bending. The bending apparatus was designed to perform outer bending and inner bending tests. For outer bending measurements, the sample was loaded with the multilayer structure facing up- ward. For inner bending, the sample was loaded with the multilayer structure facing downward.
Fig. 4shows the change in the resistance of the samples with increasing number of bending cycles. Since, as it will be shown in the next paragraph, the best OPVCs are obtained when WO3is the
Fig. 1.Transmission spectra of ZnS/M/ZnS, with M¼Cu (13 nm), Ag (11 nm) or Cu (3 nm) and Ag (9 nm), of ZnS/Cu/Ag/WO3and of ZnO.
Fig. 2.Cross section of a structure ZnS (50 nm)/Cu (3 nm)/Ag (9 nm)/ZnS (45 nm).
Insert: Photographs of an ITO and a ZnS/Cu/Ag/ZnS structure.
top layer of the new electrode, we present also the performance of the ZnS/Cu/Ag/WO3 structures. For all samples, the (R R0)/R0
value increases quite quickly during thefirst 50 cycles and then it tends to stabilize. However the relative resistance increase is four time higher in the case of ITO. It means that ZnS/Cu/Ag/D0
structures exhibit smaller variation of their resistance value, indi- cating a better flexibility of these multilayer structures. Similar result is obtained for inner and outer bending tests. The good sta- bility of the sheet resistance of the PET/DAD0structures is usually attributed to the presence of the ductile Ag metal layer between both dielectric layers[30]. Nevertheless, it must be noted that the ITO probed here exhibits betterflexibility properties than usually encountered[12]. It may be due to the fact that the PET substrate thickness of ITO is only 50mm, while that of our structure is 125mm, which make that the mechanical stress are different. As a matter of fact, it is known that the minimum allowed radius of curvature scales linearly with the total thickness of the sample[31]. It can also be seen that the structures with WO3as top electrode are slightly more stable than those with ZnS. Actually, the failure strain of amorphousfilm is higher than that of crystallinefilms. In crystal- line films, bending stresses result in periodic cracks that run perpendicular to the bending direction. The amorphous material cannot release strain by dislocation motion and therefore they deforms elastically until they cracks [32]. Since only diffraction peak due to ZnS and Ag are visible in the XRD patterns of the structure, even when WO3is substituted to ZnS as top layer in the structure, it is logical that structures with WO3are moreflexible. It can be noted that the ZnS/Cu/Ag/ZnS(MO3) are less stable under bending tests than MoO3/Ag/MoO3structures, which is probably due to the fact that our MoO3/Ag/MoO3are amorphous[12], while ZnS/Cu/Ag/ZnS(MO3) are not.
The scotch tape method[13]was used to estimate the adhesion of the ZnS/Cu/Ag/ZnS(MO3) structures to the PET substrate. As said above, this test is highly qualitative, but it allows estimating the adhesion to the substrate. After pressing the tape onto thefilm we stripped it. Three possibilities arise:
(a) thefilm is completely removed from the substrate (b) thefilm is not at all removed,
(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. The ZnS/Cu/Ag/ZnS(MO3) multilayer structures are systematically not at all removed, which testifies of their good adhesion to the PET substrates.
In order to test the coating scratching behavior, the samples were moved horizontally under the tip until some failures occur.
The Constant Load Mode (CLM) tests were conducted on three kinds of samples, ZnS/Cu/Ag/ZnS structures, commercial ITO (SOLEMS) and thin metalfilm. The samples were moved with a scratching speed of 10 nm/min whereas the constant load applied, L, varied between 5 and 30 N. The length of each scratch is 5 mm but the friction coefficient (m), defined as the ratio Ft/L, with Ft the friction force, is determined as the mean value ofmon a distance of 4 mm in the middle of the scratch (stationary state of the sliding contact). During a scratchmis very constant and the ratio (mMax- mmin)/maveragenever exceed 8% except for one point (L¼5 N)Fig. 7.
The micrographFig. 7shows the delaminating of the silver coating which induces an erratic phenomenon and a higher variation of the friction coefficient. The critical load, Lc, was defined from the op- tical observations of thefilm damage in the track. By progressively increasing the loading charge, the occurrence of cracking allows determining the brittleness while the delaminating allows deter- mining the sample adhesion. The coating ZnS/Cu/Ag/ZnS structure - substrate is resistant to scratching. Neither cracking nor delami- nating is observed before a critical load Lc¼26N (Fig. 5), thus demonstrating good adhesion of this layer. We notice a sudden change in behavior between L¼25 N and 26 N. For L>26 N, the film is completely delaminated from the contact area (Fig. 5).
In the case of ITO coating (Fig. 6), as soon as the loading charge Fig. 3.XRD patterns of a ZnS/Cu/Ag/ZnS structure.
Fig. 4.Resistance evolution after inner and outer bending as a function of the number of bending cycles for PET/ITO (-),PET/ZnS/Cu/Ag/ZnS ( ) and PET/ZnS/Cu/Ag/WO3 ( ) structures. Insert: Schematic drawings of the two different bending configurations (inner vs. outer bending).
L¼15 N, cracking of the coating is observed, however, the crack does not propagate at the interface with the substrate. The ITO adhesion to the substrate is very good because no delaminating is visible until a loading charge of 30N.
One can see inFig. 7that, in the case of silverfilm, a loading charge of only 5 N is sufficient to cause the delaminating of the Ag film. A part of the film remains in the trace, crushed by the tip, another part is folded by the tangential force. The adhesion of the metal to the substrate is poor.
Between ZnS/Cu/Ag/ZnS and Ag samples, interfacial cohesion loss processes are relatively close and strongly conditioned by the value of the corresponding tangential force, Ftc, allowing a relevant comparison of results from Lc. This point is reinforced by the fact that the coefficients of friction are very close (m~ 0.07) and the
simple product Lc$mc¼Ftc is a representative value of the bond.
The comparison may be completed by the analysis on the ITO layer, which does not delaminate for a load of 30N (maximum load of the unit). Thus, if“A”is the adhesion of the layer to the substrate we have:
A (Ag)<A (ZnS-Ag-ZnS)<A (ITO).
On the other hand, the cracking process being similar, a direct comparison is here also justified and it is shown that the layers of ZnS-Ag-ZnS (no cracks for L¼25 N) are less brittle than those of ITO (cracks L¼15N).
3.2. Organic photovoltaic cells using D/M/D0structures as anode In afirst time we use the structures giving the highestfigure of merit, ZnS(50 nm)/Cu(3 nm)/Ag (9 nm)/ZnS (45 nm), as anode in OPVCs based on CuPc/C60planar heterojunction such as:
ZnS/Cu/Ag/ZnS/(MO3:CuI)/CuPc/C60/Alq3/Al.
If ZnS is a n-type semiconductor [29], its work function is 5.26 eV. Following the example of MoO3[16e19], such high work function allowed thinking that the ZnS thinfilm at the top of the multilayer structure could be an effective holes extracting layer, without any ABL. However, the experimental results were quite disappointing, since the devices are semi-insulating and the effi- ciencies very small (Fig. 8a). ZnS is insulating and, maybe, the Fig. 5.Evolution of the friction coefficient in function of the loading charge applied to
the ZnS/Cu/Ag/ZnS coating, micrographs of the corresponding stripe.
Fig. 6.Evolution of the friction coefficient in function of the loading charge applied to the ITO coating, micrographs of the corresponding stripe.
Fig. 7.Evolution of the friction coefficient in function of the loading charge applied to the Silverfilm coating, micrographs of the corresponding stripe.
Fig. 8. Typical J-V characteristics of anode/MoO3:CuI/CuPc/C60/Alq3/Al, (a) anode: ZnS/
Cu/Ag/ZnS and (b) ZnS/Cu/Ag/MoO3.
thickness of the upper ZnS layer, 45 nm, is too high to easily allow passage of the holes. In order to avoid this negative effect, we probe OPVCs with thinnest upper ZnS layer, unfortunately without suc- cess. Probably, ZnS is not an efficient hole extracting layer. As a matter of fact, it seems more efficient as electron transport material [33].
MoO3being often used in D/M/D0devices and as hole extracting layer in optoelectronic devices, we substituted MoO3to the ZnS top layer. If the electrode ZnS/Cu/Ag/MoO3, MoO3being thick of 15 nm, allows achieving better results (Fig. 8b), the efficiencies achieved are quite small and, moreover, the reproducibility is not good.
Sometimes a quite high leakage current involves very small Voc and FF. At first glance, this disappointing result is contradicts those obtained with electrodes MoO3/Ag/MoO3 [12]. However, in the present study, under the Ag layer, there is a thin Cu layer. The Ag layer being thick of only 9 nm, it is probable that in some small areas, Cu is in direct contact with MoO3. We have already shown that Cu diffuse spontaneously into MoO3[11], which will induce the leakage currents encountered. If MoO3is often used as anode buffer layer do to its high effectiveness, WO3 has also been used with success. Recently it was shown that WO3 allows growing more stable OPVCs than with MoO3[34]. Therefore, in order to improve the reproducibility of the results, we probe the structure ZnS/Cu/
Ag/WO3, WO3 being thick of 15 nm, as transparent electrode in OPVCs. In that case, since the top layer of the structure was WO3, the anode buffer layer was WO3:CuI.
A cross section of this electrode is presented inFig. 9a. It can be seen that the metal layer is continuous all along the structure, while we see clearly that, as expected, the ZnS layer is thicker than that of WO3. The image of the surface visualization is shown inFig. 9b, the surface of the structure is composed of micro grains and it is
homogeneous. The sheet resistance of the ZnS/Cu/Ag/WO3struc- ture was 6U/sq. Apart from the replacement of ZnO by WO3, all things being equal, it is normal that the sheet resistance is similar, since it is it is essentially determined by the metal layer[8,9]. It can be seen inFig. 1that the maximum transmission at 600 nm is 90%
i.e. the same value than that obtained with Zn as top layer. All that make thatFM¼68 103U1whenl¼600 nm. If slightly smaller than the maximum value obtained with ZnS/Cu/Ag/ZnS structures it is still among the highest values ever measured, for instance, in Ref.[35]FM¼44 103U1, but after annealing at 100C or, other example,FM¼78 103U1but with more complex ZnS/Ag/ZnO/
Ag/WO3structures[36]. However, it can be seen inFig. 1that when WO3 is substituted to ZnS, if the maximum transmission value obtained at 600 nm is the same; there is a bleu shift of the trans- mission spectrum. The refractive index n of both dielectrics, WO3
and ZnS being quite similar (>2) this bleu shift effect, for the most, cannot be attributed to the substitution of ZnS by WO3, but to the difference of thickness of thesefilms. Actually it was shown that, in order to obtain the optimal transmission properties, the ZnS thickness must be 45 nm[10]. On the other hand, in order to grow efficient OPVCs, the thickness of the top layer of the D/M/D struc- ture must not overpass 15 nm. We have already shown that the increase of the top dielectricfilms in accompanied by a red shit of the transmission range of the structures[37]. This explains the red shift effect visualised inFig. 1, the thickness of top dielectricfilm being smaller in the case of WO3.
When OPVCs based on the planar heterojunction CuPc/C60are grown on these ZnS/Cu/Ag/WO3 electrodes, reproducible results are obtained. Typical J-V characteristics obtained with OPVC such as ZnS/Cu/Ag/WO3/WO3:CuI/CuPc/C60/Alq3/Al, are shown in Fig. 10.
For comparison the characteristics obtained with a classical ITO anode are also shown. We would like to stress here that we do not intend to achieve the best overall cell performance. We rather show that a substitution of ITO by our new transparent conductive electrode does not penalize the performance of cells. Therefore more information is given by the relative variation of the value of the energy conversion efficiency from one anode configuration to another one, than by the absolute value of the efficiency.
Fig. 10shows that nearly similar performances are obtained by an OPVC with ITO or ZnS/Cu/Ag/WO3as anode, with, in the case of ITO, Open circuit voltage Voc ¼ 0.46 V, Short circuit current Jsc¼6.77 mA/cm2, Fill Factor FF¼56% and efficiencyh¼1.75% and,
Fig. 9.SEM visualization of (a) the cross section and (b) the surface of a structure ZnS (50 nm)/Cu (3 nm)/Ag (9 nm)/WO3(15 nm).
Fig. 10.Typical J-V characteristics of ITO/MO3:CuI/CuPc/C60/Alq3/Al ( ) and ZnS/Cu/Ag/
WO3/WO3;CuI/CuPc/C60/Alq3/Al ( ) (in the dark, full symbols; under light, open symbol). Insert Figure 10: Evolution of the parameters of ZnS/Cu/Ag/WO3/WO3;CuI/
CuPc/C60/Alq3/Al versus number of bending test.
in the case of ZnS/Cu/Ag/WO3, Voc¼0.47 V, Jsc¼7.15 mA/cm2, Fill Factor FF¼52% and efficiencyh¼1.74%. It means that, while the same Voc values are obtained whatever the anode used, the Jsc value is a little better with ZnS/Cu/Ag/WO3, while ITO allows achieving a slightly higher FF value. The higher Jsc value can be attributed to the smaller sheet resistance of ZnS/Cu/Ag/WO3. We estimated the values of the shunt resistance (Rsh) and the series resistance (Rs), in order to assess the effect of the different anodes on the values of Rsh and Rs. The slopes at the short circuit point and at the open circuit voltage are the inverse values of Rsh and Rs of the equivalent circuit scheme of a solar cell respectively. The ob- tained values are Rs¼4U, Rsh¼600Uin the case of ITO and Rs¼4.5Uand Rsh¼425Uin the case of ZnS/Cu/Ag/WO3. It means that the main difference is in the Rsh value. The root mean square (rms) roughness value of the ITO used is only 0.8 nm[16], while that of the new electrode is 1.56 nm[10], which may explain the difference of the Rsh values. Actually, the thickness of the organic layers being small, an increase in the surface roughness of the bottom electrode, results in an increase of the leakage current and therefore in a decrease of thefill factor[38]. Therefore, this differ- ence in the Rsh value may justify the smaller FF value obtained with the roughest anodei.e.ZnS/Cu/Ag/WO3. Nevertheless, the perfor- mances of the cells, whatever the anode is, are nearly similar.
In the Insert ofFig. 10we report the evolution of the parameters of ZnS/Cu/Ag/WO3/WO3;CuI/CuPc/C60/Alq3/Al devices versus number of bending tests with a binding radius of 6 mm. It can be seen that after a quite fast decrease during thefirst cycles, the ef- ficiency of the cells tends to decrease more slowly during the following cycles. The devices maintain their performance relatively well with a small drop observed with the number of bending test.
This evolution can be compared with other obtained from OPVCs using Ag mesh as transparentflexible electrode[39]. In that case, after 100 bending tests the devices still yielded 90% of the initial efficiency, while our case it is 80%. This difference can be attributed, at least partly, to the fact that, in Ref.[39], the tests were performed in a nitrogenfilled glovebox, while they are in room air in our case.
It is well known that air and water contamination deteriorate the OPVCs performances. We have shown that the degradation with time of our planar heterojunctions is mainly due to C60 contami- nation which is present in the OPVCs tested here[40]. Moreover, the shape of the curve,i.e.fast initial and then slower decrease and dominating contribution of Jsc in the degradation, are similar in the ageing curves and in those obtained after binding. It can be concluded that at least for a significant part, the degradation of the performances of the OPVCs submitted to bending test is due to air contamination, which shows that they support pretty well the bending tests. Here, the ductile metal prevents failure formation in the transparent electrode.”
4. Conclusion
To achieve a performing new ITO free transparent flexible electrode it is necessary to not only check the optical, electrical, morphological properties and the flexibility of these new elec- trodes, but also to control their adhesion to the substrate and their mechanical properties, which is rarely made. These properties are nevertheless very important for commercial use of these new electrodes. That is why we study these properties in this manu- script, using original techniques seldom used during the charac- terization of the D/M/D structures. This allows us to show in an original way that our ZnS/Cu/Ag/ZnS(MO3) multilayer structures not only exhibit high transmission and small sheet resistance, but also that they have good adhesion to PET substrates and that they areflexible and less brittle than ITO. Actually, even if their adhesion to the substrate is smaller than that of ITO, it is quite large; this
allows using them as bottom electrode in optoelectronic devices.
Therefore they were introduced in organic solar cells based on the planar heterojunction CuPc/C60. If disappointing results are ob- tained when ZnS is the top layer of the structure, better results when a transition metal oxide is used, mainly in the case of WO3. The best results are obtained when the WO3:CuI hybrid anode buffer layer is used, the efficiency of the cell being 1.74%, which is very near from the 1.75% achieved with ITO. Moreover it is shown that these OPVCs support pretty well the bending tests.
Acknowledgements
The authors acknowledge funding from the European Commu- nity ERANETMED_ENERG-11-196: Project NInFFE and from CNRST (PPR/2015/9) Ministere, Morocco).
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