Effect of the Ag deposition rate on the properties of conductive transparent MoO3/Ag/MoO3 multilayers

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Effect of the Ag deposition rate on the properties of conductive transparent MoO

3

/Ag/MoO

3

multilayers

L. Cattin

^a,n

, Y. Lare

^b

, M. Makha

^c,f

, M. Fleury

^a

, F. Chandezon

^d

, T. Abachi

^c,1

, M. Morsli

^e

, K. Napo

^b

, M. Addou

^f

, J.C. Bernède

^c

aInstitut des Matériaux Jean Rouxel (IMN), UMR CNRS-Université de Nantes 6502, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France

bUniversité de Lomé, Faculté des Sciences, Laboratoire Energie Solaire, BP 1515 Lomé, Togo

cL’UNAM, Université de Nantes, MOLTECH-Anjou, CNRS, UMR 6200, 2 rue de la Houssinière, BP 92208, Nantes F-44322, France

dUMR SPrAM 5819 (CEA-CNRS-UJF), CEA Grenoble/INAC, 17 Rue des Martyrs, F-38054 Grenoble Cedex 9, France

eL’UNAM, Université de Nantes, Faculté des Sciences et des Techniques, 2 rue de la Houssinière, BP 92208, Nantes, F-44000 France

fLaboratoire Optoélectronique et Physico-chimie des Matériaux, Université Ibn Tofail, Faculté des Sciences, BP 133 Kenitra 14000, Morocco

a r t i c l e i n f o

Article history:

Received 13 February 2013 Received in revised form 3 April 2013

Accepted 12 May 2013

Keywords:

ITO free electrode

Oxide metal oxide structures Evaporation rate

Organic solar cell Transparent electrode

Molybdenum oxide/ silver layer/ﬂexible electrode

a b s t r a c t

The properties of molybdenum trioxide (20 nm)/silver (xnm)/molybdenum trioxide (35 nm) multilayer structures, deposited by simple vacuum evaporation, depend significantly on the deposition rate and on the thickness of the silver layer. If the presence of a commutation from an insulating state to a highly conductive state in these structures is usual, we show that, the thickness of the layer of Ag corresponding to the percolation of the metal paths, decreases from 8 nm to 4 nm when the Ag deposition rate increases from 0.2 nm/s to 0.4 nm/s. The transmission being optimum at 10–11 nm, the calculation of the factor of merit shows that the best structures are obtained for silverfilms approx. 10 nm thick deposited at a rate between 0.3 nm/s and 0.4 nm/s. When the optimal structures MoO3/Ag/MoO3 are used as anode in planar organic solar cells anode/CuI/CuPc/C60/Alq3/Al they allow achieving power conversion efficiency of the same order of magnitude than that achieved by reference cells using ITO as anode.

1. Introduction

Recently the interest in organic optoelectronic devices raised steadily owing to their interesting properties, such as low cost, mechanical flexibility, small weight. Indium tin oxide (ITO) is widely used as transparent anode in these devices. However there are some disadvantages of this transparent conductive oxide (TCO). The limitation of itsflexibility due to its ceramic structure can induce defects if itflexed too much[1–4]. The indium scarcity involves high cost and a real shortage possibility [2,4–6]. The deposition process of ITO needs sputtering and/or heating cycle, which has negative effect on the performances of organic devices, when it should be deposited onto the organic material. For all these reasons, there is a strong need to develop alternative transparent anode for organic optoelectronic devices[7,8].

Oxide/Metal/Oxide structures have been shown to be promising alternatives to ITO. In such structures, in order to decrease the high light reﬂection of the metalﬁlm it is embedded between two

metal oxides dielectric[9]. If transparent conductive oxide (TCO) were often used in these structures, oxides others than the TCO can be also used, the conductivity of the structures being due essentially to the metal layer[10–18]. For instance we showed that MoO3/Ag/MoO3 (OMO) multilayers structures can be used as alternative transparent conductive electrodes [19–21]. In such structures, the metalﬁlm should be thick enough to allow good structure conductivity and thin enough to avoid strong light absorption. It should be noted that such MoO3/Ag/MoO3structures can be easily deposited by simple thermal evaporation onto unheated substrates. We have already optimized the evaporation rate and the thickness of the oxideﬁlms[20].

In the present work we study the influence of the deposition rate of the Agfilm on the properties of the OMO structures. We show that the structures properties are optimum when the Agfilm deposition rate is 0.3–0.4 nm/s. These OMO structures were successfully used as transparent anode in organic photovoltaic cells (OPVs). Optimum power conversion efficiency (PCE) is obtained when a CuI anode buffer layer allowing templating of the organic layer is deposited onto the OMO multilayer structures.

Moreover, this trilayer electrode exhibits excellentﬂexibility due to the ductile nature of the intermediate silver layer.

Contents lists available atSciVerse ScienceDirect

journal homepage:www.elsevier.com/locate/solmat

Solar Energy Materials & Solar Cells

http://dx.doi.org/10.1016/j.solmat.2013.05.026

nCorresponding author. Tel.:+33 251125531.

E-mail address:[email protected] (L. Cattin).

1Permanent address: ENS Kouba, Alger, Algérie.

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2. Experimental

2.1. MoO3/Ag/MoO3structures realization and characterization.

The structures have been deposited either on glass or on PET substrates. After scrubbing with soap, the substrates were rinsed in running deionised water. Then the substrates were dried with an air flow and then loaded into a vacuum chamber (10⁻⁴Pa). OMO structures were deposited by simple sequential joule effect evaporation. The multilayer structures were successively deposited onto substrates at room temperature, using two tungsten crucibles, one loaded with MoO3 powder and the other one with Ag wire. The deposition rate and thefilm thickness were measured in situby a quartz crystal microbalance. Following earlier studies, the deposition rate of the MoO3films was 0.1 nm/s. The thickness of the MoO3films were fixed at 20 nm for the first films deposited onto the glass substrate and 35 nm for the top layer[20]. During the present study, the deposition rate of the Ag film was varied from 0.2 nm/s to 0.5 nm/s, while its thickness was varied from 3 nm to 12 nm.

The OMO structures were characterized using a combination of different techniques. A Siemens D-500 X-ray diffractometer using the CuK2 radiation was employed to study the structure of the films. The surface topography and the cross section of thefilms were observed with afield emission scanning electron microscope (SEM, JEOL F-7600). Atomic force microscope (AFM) images were takenex-situat atmospheric pressure and room temperature on different locations of thefilms. AFM microscopy was performed in contact mode using a Pico SPM microscope (Molecular Imaging).

For each sample, imaging of the surface was carried out in various places with a maximum scan size of 6.26.2mm²and 22mm². The AFM image processing was done using the WSxM program which allows, among others, calculation of the roughness of the surface and proﬁlometry[22]. The surface roughness given in the following has been calculated by averaging the rms roughness obtained from each images for a given sample. Scanning kelvin probe microscopy (SKPM) was performed with a Cervantes AFM- SKPM head and a Dulcinea controller unit (Nanotec Electronica, Madrid, Spain) which allowed obtaining simultaneously an image of the work function of the sample together with the surface topography[23]. The SKPM was operated in air using Si cantilevers with a Cr/Pt coating (resonance frequency 75 kHz) from Budget Sensors. Calibration of the work functionΦof the cantilevers was done using freshly cleaved highly ordered pyrolytic graphite before and after a set of measurements (HOPG SPI-3, SPI supplies, West Chester, Pennsylvania). For each sample, the measurements were performed on several locations and reproduced on different days. The local work function is derived from the SKPM image by averaging the local potential measured and using the work function of HOPG as the reference (Φ¼4.6 eV)[21–24]. The WSxM software was used to drive the Dulcinea controller as well as for the subsequent image analysis. The optical measurements were carried out at room temperature using a Carry spectrometer.

The majority carrier type was checked by the hot probe technique.

A n-type constantan wire was used as the reference sample.

The four probe technique was used to measure the electrical conductivity of the samples.

The ﬂexibility of the OMO structures deposited onto PET was studied using a laboratory made bending set-up. The samples were clamped between two conductive parallel plates. One end of the sample was attached to a mobile axis moved by the engine in a back and forth way, while the other end was attached to a rigid support.

The sample was loaded with the multilayer structure facing upward.

The distance between the two plates in the stretched mode was 30 mm, that of the bent position was 12 mm. The bending radius was approximately 6 mm. During the bending test, the resistance of the sample was measured by an electrometer.

2.2. Organic photovoltaic cells realization and characterization.

After deposition, the OMO structures were transferred from the growth chamber to the OPVs realization chamber. The OPVs probed in the present work were planar organic bilayer heterojunctions. The planar multi-heterojunction solar cells studied were as follow: OMO/ABL/CuPc/C₆₀/BCP/Al. Bathocuproine (BCP) is used as exciton blocking layer (EBL) at the C60/cathode interface. The insertion of an EBL between the electron acceptor (C60) and the Al layer was shown to significantly improve the performances of OPV cells[25]. Similarly it is known that, when a TCO is used as anode, the introduction of an anode buffer layer (ABL) between the anode and the electron donor (ED), here the CuPc, allows obtaining optimum OPVs power conversion efficiency (PCE). MoO3is well known as a very efficient ABL [26–29], therefore the OMO structures can be used without ABL. However, since the OMO structures were transferred from the growth chamber to the OPVs realization chamber, some structures were covered with 3 nm of MoO3 in the OPVs deposition chamber to mask the possible surface contaminations arisen during the transfer. Moreover, if it is known that MoO3allows improving the band matching at the interface anode/ED, it has been shown that the orientation of the CuPc molecules is also very important[30]. For instance, when the CuPcfilm is deposited onto MoO3, the molecules 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. Therefore in some samples, the CuI was used to template the CuPcfilms. The CuI layer thickness was 3 nm[30]and its deposition rate wasfixed at 0.005 nm/s. Also some ITO coated glass substrates were used to serve as references. The thickness of the CuPcfilm was 35 nm, that of C60was 40 nm and that of BCP 8 nm[26]. 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 300 W) at 100 mW/cm² light intensity adjusted with a reference cell (0.5 cm² CIGS solar cell, calibrated at NREL, USA). Measurements were performed at an ambient atmosphere. All devices were illuminated through TCO electrodes.

3. Experimental results

3.1. Effect of the Ag deposition rate on the MoO3/Ag/MoO3structures properties.

First, the DRX diagrams do not exhibit any diffraction peak, which corresponds to amorphousﬁlms, whatever the deposition rate (0.2–0.5 nm/s) and the thickness (4–12 nm) of the Ag layer.

Fig. 1shows that, whatever the Ag deposition rate is, the optical transmission is maximal at around 440–460 nm, for a silver layer thickness of approx. 10 nm. As a matter of fact, when increasing the thickness of the Ag layer thickness, there is a red shift of the transmittance peak from 400 nm with 4nm of Ag to 525 nm with 13 nm of Ag. The maximum value of the transmission is 8673%

with an average value (between 400 and 700 nm) higher than 70%. Such optimum value corresponds to an Ag layer thickness of 10–11 nm (Fig. 2). More precisely, it can be seen inFig. 2that the transmission of the OMO structures increases when the deposition rate of Ag increases from 0.2 to 0.4 nm/s. For higher deposition rate it tends to decrease (for 11 nm of Ag deposited at 0.5 A/s the averaged transmission is 70%). The optimum value of the averaged transmission is 76% (Fig. 2).

The conductivity of theﬁlms varies strongly with the Ag layer thickness and deposition rate (Fig. 3). When the thickness of the Ag layer increases, whatever the deposition rate of Ag, there is

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systematically a sudden increase of the conductivity from around 110⁻⁶to more than 110⁴Ωcm. We attribute such increase to the formation of a threshold percolation path, that is to say the passage from discontinuous to continuous silver layer.Fig. 3shows that the thickness of the layer of silver corresponding to the establishment of percolation decreases when the silver deposition rate increases. When the silver layer is deposited at 0.2 nm/s it is situated at 8 nm, while it is only 4 nm when deposited at 0.4 nm/s.

However the maximum conductivity is obtained at 0.3 nm/s.

Moreover, even if the value of the percolation thickness decreases when the Ag deposition rate increases, it can be seen that the optimum transmittance is obtained for Ag layer thickness of 10–11 nm (Fig. 3). The fairly low transmittance for thin Ag layers was seen due to the absorption of the aggregated Ag islands. An increase in Ag thickness leads to improvement of transmittance because a continuous Ag layer has low absorption. At an Ag thickness of 10–11 nm, it shows the highest transmittance of 89%

at 450 nm wavelength. However, a further increase of Ag thickness resulted in a decrease of transmittance even though it had lowest sheet resistance[31]. Therefore, the optimum Ag thickness in OMO multilayer appears to be approx. 10 nm. The corresponding sheet resistance is about 572Ωsq. Actually, if above the percolation

thickness value, whatever the Ag deposition rate is, the conductivity increases slightly with the silver layer thickness,Fig. 2shows that further increase of Ag thickness results in a decreased transmittance.

Therefore in order to measure the global performance of the trilayer structures we use the classiﬁcation technique proposed by Haacke[32]. The factor of meritΦMproposed by Haacke is ΦM¼T¹⁰=Rsq

withTtransmission andRsq the sheet resistance of the TCE. The factor of meritΦMwas calculated for the averaged transmission (in the spectral range 400–700 nm). It can be seen inFig. 4that the highest factor of merit is achieved for a thickness of the silver layer of 11 nm and a deposition rate of 0.3–0.4 nm/s (around 16–17 10⁻³Ω⁻¹). Therefore the present study of the optimization of the thickness of Ag layer, which is quite crucial, since there is a tradeoff between the sheet resistance and the resistance of the OMO structure, shows that the optimal structure is obtained with 11 nm silver thickness for a deposition rate between 0.3 nm/s and 0.4 nm/s. Beyond 0.4 nm/s deposition rate, the electrical and optical performances of structures are degraded. In addition, the hot probe technique characterization (data not shown) shows that the conductive structures are n-type. When measured as a function of the temperature, the resistance of the conductive trilayer structures increases slightly with the temperature, which means that they have a metallic-like behavior.

In order to check the morphology of the Ag layer, we have imaged the surface of MoO₃/Ag bilayer structures by SEM, the Ag deposition rate being used as parameter, while its thickness is fixed at 11 nm. The SEM micrographs of the MoO3/Ag structures (Fig. 5) show that their morphology depends significantly on the Ag deposition rate. If all the Ag films are continuous, those deposited at 0.4 nm/s are the most uniform which explains why the highest transmittance is achieved for thesefilms, while when thefilms are deposited at 0.2 nm/s the percolation is just achieved.

A representative AFM image of a OMO trilayer structure is displayed inFig. 6a. The surface morphology appears not depend- ing on the Ag deposition rate. The averaged value of the rms roughness as derived from measurements on different samples appears to be approx. 1.5 nm70.5 nm and is independent of the Ag deposition rate. The work function, Φ, corresponding to the surface visualized in Fig. 6b, is 5.270.1 eV as derived from the SKPM measurements. As can be seen The SKPM image is rather homogeneous and the uncertainty accounts for the sample-to- sample and location variations. When the OMO structures is

300 400 500 600 700 800

0 10 20 30 40 50 60 70 80 90

T (%)

σ (nm) 0.4 nm/s 0.3 nm/s 0.2 nm/s

Fig. 1.Spectral transmittance of MoO3/Ag/MoO3structures where the Ag layer was deposited at different rates.

4 5 6 7 8 9 10 11 12

50 55 60 65 70 75 80

T (%)

t_Ag (nm) 0.2 nm/s

0.3 nm/s 0.4 nm/s

Fig. 2.Variation of the averaged transmission (400 nm–700 nm) of MoO3/Ag/MoO3

structures with the Ag deposition rate and the thickness of the Ag layer.

2 4 6 8 10 12

10^-6 10^-5 10^-4 10^-3 10^-2 10^-1 10⁰ 10¹ 10² 10³ 10⁴

σ (scm-1)

t_Ag (nm) 0.2 nm/s

0.3 nm/s 0.4 nm/s

Fig. 3.Variation of the conductivity of MoO3/Ag/MoO3structures with the Ag deposition rate and the thickness of the Ag layer.

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covered with an additional layer of CuI (thickness of 3 nm), there is a signiﬁcant increase of the rms surface roughness of the structure to approx. 9 nm and also of the work function which rises up to 5.470.1 eV.

Regarding the effect of the deposition rate on the properties of the OMO multilayers, the shape of the Ag nanostructures (3 dimensional or 2 dimensional) as obtained during the initial stage offilm formation depends on the mobility of the Ag adatoms and the accumulating shadowing effect[33]. Therefore, the different behaviors of the OMO structures with the deposition rate of silver can be due to the fact that for a slow Ag deposition rate, the deposited Ag atoms have enough time to diffuse and reorganize, which results in granular discontinuousfilms as shown by the SEM study. For higher silver deposition rates, there is a quenching effect with formation of a high density of nucleation centers and the silverfilament paths become continuous for thinnerfilms.

The scotch tape method[34]was used to estimate the adhesion of the OMO structures to the substrate. This test is highly qualitative, but it allows screeningfilms involving poor adhesion from those where adhesion is good. We pressed the tape onto the film and then rapidly stripped it. Three possibilities arise: (a) the film is completely removed from the substrate (b)film is not at all removed, and (c) thefilm is partly removed or removed in patches.

In the case of our OMO structures, systematically, the structures are not at all removed, which testiﬁes of their good adhesion to the glass substrates.

In order to check the ﬂexibility of the OMO structures they were deposited onto PET substrates. To evaluate theﬂexibility of our trilayer structures we fabricated several samples on PET substrates with the best structures geometry of MoO3 (20 nm)/

Ag (11 nm)/MoO3(35 nm) and tested them under tensile bending as described inSection 2. The results are compared to commercial 100 nm thick ITO onto PET. During the bending test, the variation of the resistance of the PET/OMO and PET/ITO samples was measured as follows. The change in resistance was expressed as (R−R0)/R0, whereR0is the initial resistance and Rthe measured resistance after bending.Fig. 7shows the change in the resistance of both kinds of samples with increasing bending cycles. It can be seen that the ((R−R0)/R0) value of the PET/ITO sample increases regularly following the ﬁrst bending cycle. On the contrary, the PET/OMO sample exhibits no variation of its resistance value, indicating a constant resistance of the trilayer structure. Even after 5000 bending cycles the resistance is stable, while, that of

PET/ITO increases rapidly. In the latter case, the relative change ((R–R0)/R0) after 10 bending cycles is 12% and 300% after 1000 ones. The stability of the resistance of the PET/OMO structure is usually attributed to the presence of the ductile Ag metal layer between the MoO3layers[31]. Moreover, MoO3being amorphous its failure strain is higher than that of crystallineﬁlms.

It should be noted that we have carried out studies with other metals such as Au and Cu. In the case of Cu we have shown that there Fig. 5.SEM micrographs of MoO3/Ag bilayer structures, where the Ag layer was deposited with a deposition rate of 0.3 nm/s, 0.4 nm/s and 0.5 nm/s.

8 9 10 11 12

2 4 6 8 10 12 14 16 18

ΦM (ohm−1)

t_Ag (nm)

0.2 nm/s 0.3 nm/s 0.4 nm/s x 10^-3

Fig. 4. Variation of the factor of meritΦMof MoO3/Ag/MoO3structures with the Ag deposition rate and the thickness of the Ag layer.

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is Cu diffusion into the MoO₃layers[35], moreover the diffusion into the bottom layer increases with the deposition rate. On the other hand, in the case of Au the averaged transmission is smaller than that achieved with Ag in the same conditions. For instance, when a metal ﬁlm thick of 10 nm is deposited at 0.2 nm/s the averaged transmission is only 65% with gold, while it is 70% with silver. Therefore we did not pursue the study in the case of gold.

However, it would be interesting to conduct a study based on the nature of the dielectric. ZnS bottom layer, for instance, can be used, since it has been shown to permit the growth of highly homogeneous Cu thinﬁlms[15].

3.2. Organic solar cells

The optimum MoO3/Ag/MoO3 structures have been tested as transparent anode in bilayer heterojunction OPVs. As discussed in the introduction, different anode conﬁgurations were probed directly guided by our previous studies [20,21]. Fig. 8 and

Table 1 show the typical results achieved with different anode conﬁgurations.

Firstly, it can be seen that, as expected, poor results are obtained with bare ITO. As a matter of fact better results are achieved with bare OMO anode, the PCE of the OPVC being 0.68%

in the former case and 1.15% in the latter case. We have shown in a preceding paper that the poor efﬁciency and the kink effect visible in theJ–Vcharacteristics of the cells using bare ITO as anode could be attributed to formation of a parasitic diode at the interface ITO anode/organic donor, such diode effect being due to the large difference between the ITO work function and the highest occu- pied molecular orbital (HOMO)[36]. Since the kink effect is never visible in theJ–Vcharacteristics of the cells using OMO anode, it can be concluded that a good matching is achieved between the work function of the anode and the HOMO of the organic material.

Such result can be expected since the upper layer of the structure used as anode is the MoO₃and it was previously reported that this oxide induces a electronic level matching with the HOMO of CuPc Fig. 6.(a) and (c) Representative AFM and SKPM images of a MoO3/Ag/MoO3structure. (b) and (d) Same for a MoO3/Ag/MoO3/CuI (c) structures. The size of the images is 22mm².

0 200 400 600 800 1000 4800 5000

0 100 200 300 400 500 600 700

(R-R0)/RO (%)

Number of cycles MoO₃/Ag/MoO₃

ITO

Fig. 7.Resistance evolution after bending as a function of bending cycles for PET/

MoO3/Ag/MoO3structure and PET/ITO.

-200 0 200 400 600

-7.5 -5.0 -2.5 0.0 2.5 5.0 7.5

-200 -100 0 100 200 0,00

0,25 0,50

V (mV) OMO OMO/MoO OMO/CuI Anode

J (mA/cm2)

V (mV) ITO

ITO/MoO3

ITO/CuI MAM MAM/MoO3

MAM/CuI Anode

Fig. 8. Typical J–V characteristics of anode/CuPc/C60/Alq3/Al structures, under illumination of AM1.5 solar simulation (100 mW/cm²).

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[20,25,26]. Similar results are obtained when the OMO is covered, or not, by a MoO3ABL thick of 3 nm (Fig. 8,Table 1). It means that the contamination of the OMO structures during their transfer from the growth chamber to the OPVs realization chamber is not too penalizing or that it is reversible [37]. Finally, it must be underlined that if in the case of bare anode, better results are achieved with the MoO3/Ag/MoO3anode, when these anode are covered by a CuI ABL, the best results being obtained with ITO/CuI, even if promising results are achieved with OMO/CuI anodes.

The fact that the best results are obtained with a CuI ABL, whatever the anodes is, must be attributed to the influence of CuI on the orientation of the CuPc molecules. As a matter of fact it was shown that CuI modifies strongly the structural and optical properties of the CuPc layer deposited onto the electrode. Onto CuI the CuPc molecules lie down parallel to the plan of the substrate, while onto MoO3 they stand up perpendicular to the substrate[30]. Such CuPc molecule orientation induces an increase of the absorption of the CuPcfilms and it allows improving the OPVc performances.

It is known that the measure of the External Quantum Efficiency (EQE) allows checking the origin of the improvement of the PCE. It was shown recently that the EQE improvement follows the increase of the absorbance of the absorbance of the organic material[38]. Therefore, we proceeded to the measure of the absorbance of the CuPc films deposited onto PET/OMO and PET/OMO/CuI anodes. InFig. 9 we see that the presence of CuI modifies significantly the optical density curve of the CuPcfilm.

When it is deposited onto MoO3, in the visible region the absorption band corresponding to the Q band (π–πn transition) shows two peaks located at 622 nm and 693 nm [39]. When deposited onto CuI ABL there is an increase of the absorption with a broadening and a red shift of the second peak (703 nm). There- fore, the Q band surface increase will induce an increase of the absorption and therefore of the probability of free carrier formation in OPVs, which explains the increase of theJscand the OPVs performances in the presence of CuI.

The KPFM measurements show that, not only the addition of a CuI layer modifies the CuPc molecules orientation, but also that it increases the surface work function of the anode, from 5.2 eV to 5.4 eV. Such increase can facilitate the hole collection by the anode, which results in an higher short circuit currentJsc. At the same time, the AFM study shows that the presence of CuI increases significantly the surface roughness of the anode. It is known that in planar bilayer OPVs a large value of the surface roughness may decrease the shunt resistance of the cells. So, the Voc and the efficiency are limited by the leakage current induced by the small shunt resistance [29,40,41]. As a matter of fact we have already shown that, when deposited onto ITO anodes, if the CuI modifies the CuPc molecule orientation, it also decreases the reproducibility of the device performances [42]. This phenomenon is in good agreement with the present AFM study. The increase of the surface roughness, induces a tendency to leakage current apparition and a poor reproducibility of the results. As we previously showed[42], it is necessary to use a double anode buffer layer, MoO3/CuI to

improve the reproducibility, because MoO₃ prevents shunt path formation. In the present work, those difﬁculties are avoided by the OMO structure itself, MoO3being systematically present in the anode. In fact, experience shows that the effect of CuI on the value ofVocis twofold. On the one hand it tends to induce the formation of leakage paths of current, which tends to diminish the value of Voc. Secondly by changing the orientation of CuPc molecules it tends to increase the value. As a matter of fact, it is known thatVoc

increases with the difference LUMOEA–HOMOED,where LUMOEAis the lowest unoccupied molecular orbital of the electron acceptor.

It has been shown that the HOMO of organic thinﬁlms is modiﬁed by the molecules orientation. In the case of CuPc it varies from 4.75 eV to 5.15 eV when the molecules orientation changes from perpendicular to the plan of the substrate to parallel to it[43]. In the present study it can be seen that, while the leakage current is higher in the presence of CuI (insetFig. 8), which should lead to a reduction ofV_oc, it is not so. This shows that the effect induced by changing the orientation of the molecules on theVocpremium that due to the effect of leakage currents, probably because these are limited by the presence of MoO3.

Therefore the higher value ofVocobtained with CuI than with MoO3 as ABL can be attributed to the larger value of LUMOEA– HOMOED, the CuPc molecules being parallel to the CuI substrate, while they are perpendicular to the MoO3substrate.

4. Conclusion

MoO3/Ag/MoO3 structure have been characterized and then probed as anode in organic solar cells. The study shows that the properties of the structures depend on the deposition rate of the silver interlayer. It is shown that there is a threshold thickness value where the structures commute from an insulating state to a conductive one. We attribute this commutation to the formation of a percolating silver layer. The value of this percolation thickness decreases when the Ag deposition rate increases. It is shown that the transmittance of theﬁlms, whatever the deposition rate, is optimum when the silver thickness is 10–11 nm, while a further increase results in a lower transmittance. The structures with the best factor of merit are obtained when the Ag thickness is 11 nm and the deposition rate is between 0.3 nm/s and 0.4 nm/s. These optimized anode structures were used as transparent anodes in organic solar cells. We thus showed that OMO structures can be used as efﬁcient anode in organic solar cells when a CuI ABL is Table 1

Photovoltaic performance data under AM1.5 conditions of devices using different anodes.

Anode Voc(V) Jsc(mA/cm2) FF (%) η(%)

ITO 0.437 5.28 30 0.68

ITO/MoO3 0.495 5.26 60 1.56

ITO/CuI 0.530 6.30 54 1.80

MAM 0.430 4.90 52 1.15

MAM/MoO3 0.444 5.12 52 1.19

MAM/CuI 0.490 6.26 52 1.60

400 600 800

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

O. D. (arb. units)

λ (nm)

PET/OMO/CuI/CuPc PET/CuI/CuPc

Fig. 9.UV–vis absorption spectra of CuPc layers deposited on PET/OMO ( ) and PET/OMO/CuI ).

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introduced between the anode and the CuPc. This CuI layer is an efficient template of the CuPcfilms. Even if the device efficiency are presently slightly lower than that achieved with ITO based reference devices, the anodes discussed appear to be a promising alternative to ITO in stacked solar cells taking into account that the structures are deposited by simple evaporation onto substrate at room temperature, thus avoiding any degradation of the organic under layer.

Acknowledgments

This work has beenﬁnancially supported by the OTC-2012– 2013 Project (Nanorgasol Network of Mission Ressources et Compétences Technologiques du CNRS FRANCE) and the France- Maroc contract: PHC Volubilis No. MA/10/228 and the Hassan II Academy of Science and Technology (Morocco).

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