Highly flexible, conductive and transparent MoO3/Ag/MoO3 multilayer electrode for organic photovoltaic cells

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Highly ﬂ exible, conductive and transparent MoO

₃

/Ag/MoO

₃

multilayer electrode for organic photovoltaic cells

T. Abachi

^a,1

, L. Cattin

^b

, G. Louarn

^b

, Y. Lare

^a,2

, A. Bou

^c,f

, M. Makha

^a,d

, P. Torchio

^c

, M. Fleury

^b

, M. Morsli

^e

, M. Addou

^d

, J.C. Bernède

^a,

⁎

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

bUniversité de Nantes, Institut des Matériaux Jean Rouxel (IMN), CNRS, UMR 6205, 2 rue de la Houssinière, BP 32229, 44322 Nantes cedex 3, France

cAix-Marseille Université, Institut Matériaux Microélectronique Nanosciences de Provence (IM2NP), CNRS-UMR 7334, Domaine Universitaire de Saint-Jérôme, Marseille, F-13397, France

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

eL'UNAM, Université de Nantes, Faculté des Sciences et des Techniques, 2 rue de la Houssinière, BP 92208, Nantes, F-44322, France

fCrosslux SAS, Avenue Georges Vacher, Immeuble CCE ZI Rousset-Peynier, 13790 France

a b s t r a c t a r t i c l e i n f o

Article history:

Received 27 March 2013

Received in revised form 15 July 2013 Accepted 17 July 2013

Available online 24 July 2013 Keywords:

Flexible substrate ITO-free electrode MoO3/Ag/MoO3electrode Organic solar cell

MoO3/Ag/MoO3(MAM) multilayer structures were deposited by vacuum evaporation on polyethylene terephthalate (PET) substrate. We demonstrate that, as in the case of glass substrate, the sheet resistance of such structures depends significantly on the Agfilm deposition rate. When it is deposited between 0.2 and 0.4 nm/s, an Ag thickness of 11 nm allows achieving sheet resistance of 13Ω/sq and an averaged transmission of 74%. A study of the influence of the PET substrate on the optimum MoO3thicknesses was done. A good qualitative agreement between the theoretical calculations of the variation of the optical transmittance of the MoO3/Ag/MoO3structures is obtained. The optimum MAM structures MoO3(17.5 nm)/Ag (11 nm)/MoO3(35 nm) has a factor of merit FM= 4.21 10⁻³(Ω/sq)⁻¹. Proven by the scotch test the MAM structures exhibit a good adhesion to the PET substrates. The MAM structures were also submitted to bending tests. For outer bending, the samples exhibit no variation of their resistance value, while for inner bending there is a small increase of the resistance of the MAM structures. However this increasing is smaller than that exhibited by Indium Tin Oxide. When the PET/MAM structures are used as anode in organic photovoltaic cells, it is shown that the need to use thicker Agfilms inside the multilayer and to cover the MAM with Au to obtain promising Current density vs Voltage characteristics is due to the heating of the PET substrate during the deposition process.

1. Introduction

Nowadays, the commercial request of transparent conductive electrode (TCE) deposited ontoflexible substrates for optoelectronic devices increases continuously. Usually, indium tin oxide (ITO) is the most often used TCE in optoelectronic devices. However, it presents some disavantages such as the limitation of itsflexibility due to its ceramic structure, which can induces defects when it is too muchflexed [1–4]. Moreover the indium rarity involves high cost and exhaustion possibility[2,4–6]. Thus many works are currently dedicated to In-free TCE. Beside others transparent conductive oxydes (TCOs), conducting polymers, metal grids embedded in polymer, carbon nanotubes, graphene, metal nanowires, semitransparent metal electrodes and multilayer structures have been proposed[7,8].

If, up to now, these alternatives solutions have not all the potential of ITO, some of them demonstrate already the ability to answer the prereq- uisites for specific applications. Among them, a classical and old solution is constituted by the use of semitransparent thin metalfilms. These ultra thin metalfilms are the object of a renewed interest because of their higherflexibility than that of the TCO, what makes them potential candidates for being deposited on plastic substrates[9,10]. Moreover, concept of bilayer thin metalfilms is also investigated due to their higher performances than those of equivalent single layer films [11,12]. The idea of multilayer structures is carried at its peak by the realization and the study of oxide/metal/oxide structures. In these structures, the thin metalfilm is embedded between two metal oxide dielectric layers in order to decrease the high light reflection[13]. For in- stance we showed that MoO3/Ag/MoO3(MAM) multilayer structures can be efficiently employed as alternative transparent conductive electrodes[14–16]. Following the optimisation of these MAM structures on glass substrates, we extend this investigation to the realisation of TCE ontoflexible substrates. After checking the electrical, optical and morphological properties of the MAM multilayer structures deposited ontoflexible substrates, we show that the trilayer electrode is entirely

⁎ Corresponding author. Tel.: +33 251 125 530.

E-mail address:jean-christian.bernede@univ-nantes.fr(J.C. Bernède).

1Permanent address: ENS Kouba, Alger, Algérie.

2Permanent address: Université de Lomé, Faculté Des Sciences, LES, BP 1515 LOME - TOGO.

http://dx.doi.org/10.1016/j.tsf.2013.07.048

Contents lists available atScienceDirect

Thin Solid Films

j o u r n 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 / t s f

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compatible withflexible plastic substrate such as PET (polyethylene terephthalate). It exhibits excellentflexibility due to the ductile nature of the intermediate silver layer. These flexible electrodes are then probed as anode in planar organic photovoltaic cells (OPVCs).

The studied OPVCs were planar heterojunctions composed as follows:

PET/MAM/ABL/CuPc/C₆₀/BCP/Al, where ABL is an anode buffer layer, CuPc the copper pthtalocyanine, C60 the fullerene and BCP the bathocuproine. For comparison, PET/ITO anodes were also probed.

2. Experimental details

2.1. PET/MoO3/Ag/MoO3structures realization and characterization.

The structures were deposited on 50μm-thick polyethylene terephthalate (PET) substrates. The MAM structures were deposited by simple sequential joule effect evaporation without breaking the vacuum (10⁻⁴Pa). Deposition rate andfilm thickness were measured in situ by quartz monitor. We have already showed that, when deposited onto glass substrates, the optimumfilm thicknesses are: MoO3(20 nm)/Ag (10–11 nm)/MoO3 (35 nm) [15]. About the layer deposition rates, when situated between 0.05 nm/s and 0.1 nm/s, that of MoO3was not determining for the structure properties; but we have demonstrated that the properties of the structures are highly sensitive to that of the silver deposition rate[15]. Therefore, wefixed the MoO3films deposition rate at 0.1 nm/s, while we decided to analyze the MAM multilayer structures properties as a function of the silver deposition rate, from 0.15 nm/s to 0.5 nm/s.

The MAM structures deposited ontoﬂexible substrate were charac- terized using different techniques.

A Siemens D-500 X-ray diffractometer using the CuK2radiation was employed to study the structure of thefilms. Theθ-2θconfiguration was used for the measures. The surface topography was observed with a field emission scanning electron microscope (SEM, JEOL F-7600). The SEM operating voltage was 5 kV.

AFM images on different sites of theﬁlms were takenex-situ at at- mospheric pressure and room temperature. All measurements have been performed in tapping mode (Nanoscope IIIa, (Veeco, Inc.). Classical silicon cantilevers were used (NCH, nanosensors). The average force constant and resonance were approximately 40 N/m and 300 kHz, respectively. The cantilever was excited at its resonance frequency.

These structural characterization were performed at the“Centre de microcaractérisation de l'Université de Nantes”.

The optical measurements were carried out at room temperature using a Carry spectrophotometer. The optical transmittance and reﬂec- tance were measured in the 0.3 to 1.2μm spectral range. The four-probe technique was used to measure the electrical conductivity.

Theflexibility of the MAM structures deposited onto PET was studied using a laboratory made bending system. The samples were clamped between two conductive parallel plates. The one wasfixed to a mobile axis moved by the engine in a movement of go and come, 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 by an electrometer. 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 upward. For inner bending, the sample was loaded with the multilayer structure facing downward.

XPS measurements were carried out at room temperature. 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, to obtain data in a reasonable experimental time (energy resolution of 0.48 eV). Concerning the calibration, binding energy for the C1s hydrocarbon peak was set at 284.6 eV. The pressure

in the analysis chamber was maintained lower than 10⁻⁷Pa. The back- ground spectra are considered as Shirley type.

2.2. Organic photovoltaic cells realization.

The OPVCs were realized as already described in previous papers [17]. Shortly, after deposition of the MAM structures, they were trans- ferred from the growth chamber to the OPVCs manufacturing chamber.

The typical OPVCs design was: PET/anode/ABL/CuPc/C₆₀/BCP/Al, with anode =MAM or ITO. PET/ITO anodes were used as reference. The ABL was introduced between the anode and the organic layer in order to improve the holes collection and to prevent the passage of electrons.

It is known that ABL made of MoO3allows improving the band matching at the interface anode/CuPc[18,19]. More recently, it was shown that the CuPc molecules orientation can be managed through the introduction of a thin templatingfilm at the interface between the anode and the organicfilm[20]. As a matter of fact, when the CuPcfilm is deposited onto MoO3, the molecules are perpendicular to the plane of the substrate, while they lie parallel to it when deposited onto CuI (Fig. 2 of Ref. [16]. This last molecule orientation allows improving light absorption (Fig. 3 of Ref.[16]) and OPVCs performances. Therefore, in the present work, a 3 nm-thick CuI layer was deposited onto anodes, with a deposition rate of 0.005 nm/s[21].

The thickness of the CuPcﬁlm was 35 nm, that of C60was 40 nm and that of BCP 8 nm[17]. The ITO and Alﬁlms were 100 nm-thick. The effective area of each cell was 0.25 cm².

The characteristics of the photovoltaic cells were measured using a calibrated solar simulator (Oriel 300 W) at 100 mW/cm²light intensity adjusted with a reference cell (0.5 cm² CIGS solar cell, calibrated at NREL, USA). Measurements were performed at ambient atmosphere.

All devices were illuminated through the transparent electrode.

3. Experimental results

3.1. Characterization of the MAM structures deposited onto PET.

As only the diffraction peaks of PET are visible on the X-ray diffraction diagrams of the PET/MAM structures, it means that the multilayer structures are amorphous. Same results were obtained with commercial PET/ITO structures. The transmittance spectra of PET/ITO and PET/MAM structures are presented inFig. 1. It can be seen that, if the maximum of transmittance of the PET/electrode structures is around 71% (whatever the electrode: PET/ITO or PET/MAM), the transmission domain width of PET/ITO is broader than that of PET/MAM. Conversely, the sheet

400 500 600 700

0 20 40 60 80 100

T (%)

PET/ITO

PET/MoO₃/Ag/MoO₃

λ (nm)

Fig. 1.Spectral transmittance of PET/MAM and PET/ITO structures.

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resistance of the ITO deposited onto PET is 50Ω/sq, while it is around 10–15Ω/sq when the MAM is deposited on PET (Table 1).

The effect of the Ag deposition rate (for thicknesses around 11 nm) is crucial. It is shown inTable 2that the sheet resistance of the structures PET/MoO3(17.5 nm)/Ag (11 nm)/MoO3(35 nm) depends on the deposition of the Agfilm. The silverfilms are resistive when the silver layer is deposited at a deposition rate below 0.2 nm/s, while they are conductive when deposited at a deposition rate ranging between 0.2 and 0.4 nm/s (Table 2). For higher deposition rate, the conductivity of the films decreases. We have already shown that the thickness of the silver film which allows achieving the percolation threshold, i.e. the formation of a closed and continuousfilm, depends on the silver deposition rate.

For low values of deposition rate (below 0.2 nm/s), thickerfilms are needed to reach the percolation threshold thickness[15]. For higher deposition rates (more than 0.5 nm/s), the surface morphologies of the PET/MAM structures show inFig. 2that, the homogeneity of the films decreases and that cracks appears clearly, which justifies the decrease of the conductivity of the structure (Fig. 2d,Table 2). When deposited at a rate ranging between 0.2 nm/s and 0.4 nm/s, thefilms are homogeneous, although deposited on a plastic substrate (Fig. 2a–c).

This result was investigated by AFM study. InFig. 3, an image of a PET substrate covered with a MAM electrode can be seen, the Agﬁlm being deposited at 0.3 nm/s. The average value of the root mean square (rms) roughness deducted from several PET/MAM structures is rms = 2 ± 0.5 nm. It should be noted that the rms roughness of bare PET substrates is about 2.5 ± 0.5 nm. This means that the surface roughness tends to slightly decrease when the PET is covered with a MAM multilayer structure. This could be another beneﬁcial effect of using MAM as electrode in optoelectronic components composed of very thin organic layers.

The scotch tape method[22]was used to estimate the adhesion of the MAM structures to the PET substrate. This test is highly qualitative, but it allows screeningfilms involving poor adhesion from those where adhesion is appreciable. We pressed the tape onto thefilm and then rapidly stripped it. Three possibilities arise: (a) thefilm 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 PET/MAM structures, the trilayer structures are systematically not at all removed, which testifies of their good adhesion to the PET substrates.

In order to check and evaluate theﬂexibility of our trilayer MAM structures onto PET substrates, we fabricated several samples on PET and tested them under outer and inner bending as described above.

The results are compared to commercial 100 nm-thick ITO onto PET.

During the bending tests, the variation of the sheet resistance of the PET/MAM and PET/ITO samples was measured as follows. The change in resistance was expressed as (R-R0)/R0, where R0is the initial resistance and R the measured resistance after bending.Fig. 4shows the change in the resistance of both kinds of samples with increasing bending cycles. It can be seen that the (R-R0)/R0value of the PET/ITO sample increases regularly from theﬁrst cycle. In the case of PET/MAM samples, the answer to the bending test depends on the type of bending. For outer bending, the samples exhibit no variation of their resistance value, indicating a constant resistance of the trilayer structure. Even after 5000 bending cycles the resistance is stable. However in the case of inner bending cycles, during theﬁrst cycles, there is a small increase of the resistance of the PET/MAM structure, while it tends to stabilize after 50 cycles. Although it is there, it must be highlighted that the deg- radation of the conductivity of the PET/MAM structure after the inner bending cycles is far smaller than that of the PET/ITO structures. For in- stance after 10 cycles, the value of (R-R0)/R0reached with ITO is equal to 6 times more than that obtained with MAM.

The good stability of the sheet resistance of the PET/MAM structure is usually attributed to the presence of the ductile Ag metal layer between both MoO3layers[23]. Moreover, the MoO3being amorphous, its failure strain is higher than that of crystallineﬁlms.

We are able to modelize and to calculate the optical properties of multilayer thinfilms. For the current work, a simulation tool based on a 2 × 2 Transfer Matrix Method[24]is used. Based on the Fresnel equa- tions, this method allows us to calculate in particular the Poynting vec- tor, the intrinsic absorption in each layer and the transmittance of the whole structure. Up to now, the thickness values used for the MoO₃ thinfilms were those optimized in the case of glass substrates. In the present work, the substrate is the PET, with a refractive index n slightly higher than glass (Fig. 5) and with a thickness of 50μm (instead of 1000μm for glass). For previous simulations, we did not take into account directly the thickness of glass by considering it as infinite compare to the trilayer and by placing the incident light source in the glass substrate of the numerical design (in order to gain time calculation).

Here, we decide to take into account the PET's thickness of 50μm to improve the accuracy of the results. Such design leads to the presence of interferences in the curves of transmittance due to the multiple reﬂections in the PET. To further compare properly such curves, we deduce the average transmittance value as shown inFig. 6.

Thus we have then studied the influence of the MoO₃films thickness on the optical properties of the MAM structures deposited on PET. Firstly, the thickness of the bottom MoO3film was used as parameter. The experimental transmittance curves of PET/MoO3(x)/Ag (11 nm)/MoO3

(35 nm), with 10 nmbxb20 nm are shown inFig. 7a. The averaged transmittance (300–700 nm) is given in Table 1. It can be seen in Table 1that the highest averaged transmission is obtained when the MoO3thickness is 17.5 nm, even if the transmission data shows a slight decrease for wavelengths superior to 550 nm. We can also notice that there is a slight blue shift of the transmittance curve (Fig. 7a). The theoretical transmittance curves were also calculated (Fig. 7b). The simulated variations and tendencies are similar to those of experimental curves.

However, the differences in transmittance for 10 nmbxb20 nm are rather low, and it appears difﬁcult here to extract an optimal value for x.

Secondly, we investigated the structures PET/MoO3(17.5 nm)/Ag (11 nm)/MoO3(x nm) with 20bxb35 nm (Fig. 8a). If there is no great change of the transmittance, there is on the other hand a consid- erable red shift of the domain of transmission of the PET/MAM structure when x increases. In order to check the presence of this red shift effect we proceeded to theoretical simulation. The air side oxide layer thickness was considered, by setting the thicknesses of the other oxide layer (glass side) to 17.5 nm and of the silver layer to 11 nm. The Table 1

Optimization of the thickness of the MoO3layers, electrical and optical characteristics of PET/MoO3/Ag/MoO3structures.

MoO3

(nm) Ag (nm)

MoO3

(nm) λTM

(nm) TMax

(%)

T (300–700 nm) (%)

Rsq (Ω/sq)

FMx103

(Ω/sq)⁻¹

10 11 35 465 74 63 21.5 0.55

13.5 11 35 468 82 70 22.5 1.45

16 11 35 475 81 72 13 3.26

17.5 11 35 475 84 74 13 4.21

18.5 11 35 483 80 71 14 2.77

20 11 35 485 78 67 20 1.07

17.5 11 20 360 82 64 8 1.43

17.5 11 25 440 84 70 10 2.94

17.5 11 30 473 84 71 10 3.31

17.5 11 35 475 85 74 13 4.22

17.5 15 35 485 66 55.5 3.5 0.78

Table 2

Variation of the sheet resistance of the PET/MoO3(17.5 nm)/Ag (11 nm)/

MoO3(35 nm) structures with the deposition rate of the Agﬁlm.

Ag deposition rate (nm/s) Rsq (Ω/sq)

0.1 320

0.2 13

0.3 16

0.4 14

0.5 80

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calculated transmittance spectra of the structure PET/MoO3(17.5 nm)/

Ag (11 nm)/MoO₃(x nm) are obtained by varying x from 0 to 35 nm.

The frequency shift role of the MoO3top layer (air side) is illustrated inFig. 8b. We clearly observe a red-shift of the transmittance curves according to the increase of the thickness of this oxide layer. These numerical results calculated in the 350–700 nm spectral range are well correlated with the experimental results presented inFig. 8a.

The optimum PET/MoO3 (17.5 nm)/Ag (11 nm)/MoO3 (35 nm) structures have an averaged transmittance of 74%, a sheet resistance of 13Ω/sq and aﬁgure of merit of 4.21.10⁻³(Ω/sq)⁻¹. The factor of merit FMbeing that proposed by Haacke[25]i.e.:

FM= T^q/Rsq, with T = transmittance, Rsq = sheet resistance and q = an exponent that determines which transmittance level is required for a speciﬁc purpose. We used q = 10 which leads to a transmittance of 90 %, as aimed for our application. Finally PET/MoO3(17.5 nm)/Ag (11 nm)/MoO3(35 nm) structures were probed as anode in OPVCs.

3.2. Characterization of the organic photovoltaic cells.

To investigate the possibilities of the PET/MAM structures, asﬂexible transparent conducting electrodes, we fabricated OPVCs using them as anode.

Typical Current density vs Voltage (J-V) characteristics and photovoltaic parameters of the different OPVCs are presented inFig. 9and Table 3. OPVCs onto PET/ITO structures were also realized for comparison. The PET/ITOﬁlm was provided by Aldrich. Its sheet resistance was 60Ω/sq, its transmittance is shown inFig. 1and it is 70%. Firstly, in reference to our previous results obtained with MAM structures deposited on glasses[16], we used MAM structures with an Agﬁlm thick of 11 nm, this anode being covered by CuI (3 nm) before organic layers deposition. It can be seen inFig. 9that the shape of the J-V characteristics of the OPVC using this anode exhibits a kink effect also called S-shaped effect. It is known that this undesirable S-shape behavior can be explained Fig. 2.Surface visualisation of PET/MAM structures, where the Ag layer was deposited at 0.2 nm/s (a), 0.3 nm/s (b), 0.4 nm/s (c) and 0.5 nm/s (d).

a b

Fig. 3.Typical AFM topographic mage (5μm × 5μm) of PET/MAM (a) and PET (b). The vertical scale is 40 nm for the 2 images.

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by imbalanced profiles of charge carrier densities andfield distribution within the layers. This charge accumulation at any interfaces can be due to large injection barrier or resistive contact, with imbalanced mobilities [26,27]. A classical equivalent electrical scheme of OPV cells was used to calculate, the series resistance, Rs, defined by the slope of theJ–Vcurve atJ= 0. As a matter of fact the value of the series resistance Rs is high (Table 2), which can justifies the kink effect. Such S-shaped behavior induces smallfill factor (FF) and open circuit voltage Voc, which decreases the power conversion efficiency (PCE).

There is not such S-shaped behavior when the MAM structure is deposited onto a glass substrate[16], while the physical properties of the MAM structures are similar. The main differences between the MAM structures consist in the thickness and roughness of their substrates. The glass substrate is thick of 1 mm, the PET substrate is thick of only 50μm and while the rms of glass is lower than 0.5 nm, that of PET is 2.5 nm. At the end of the deposition process, the substrate temperature is 80 °C in the case of glass substrate and 110 °C in the case of PET substrate, due to its smaller thickness. Such temperature dif- ference, added to the highest roughness of the PET which can engender more easily paths favoured for silver diffusion, can cause an important Ag migration toward the surface of the MAM structure during the deposit of the organic layers. As a matter of fact this hypothesis is corroborated by XPS study. Here, if we have shown that the MAM structures resist to the scotch test, the organic layers do not. Therefore,

considering this, we can access to the surface of the MAM structure used as anode in the OPV cell by removing the organic layers. We can see inFig. 10that, if before OPV cell deposition the Ag signal is very faint at the surface of the MAM structure, it becomes very important after deposition onto PET. The Ag diffusion, so highlighted, induces a

0 200 400 600 800 1000

0 50 100 150 200 250 300

Number of cycles PET/ITO outer

PET/ITO inner PET/MAM outer PET/MAM inner

Inner Outer

PET

(R-R 0)/R 0

Fig. 4.Resistance evolution after inner (open symbol) and outer (full symbol) bending as a function of the number of bending cycles for PET/MAM (▲) and PET/ITO (■) structures.

Insert: Schematic drawings of the two different bending conﬁgurations (inner vs. outer bending).

Fig. 5.Refractive index n of glass, PET and MoO3[15]in the 350–700 nm spectral range.

Fig. 6.Calculated transmittance spectrum of PET (50μm)/MoO3(17.5 nm)/Ag (11 nm)/

MoO3(35 nm) electrode and its averaged curve.

400 500 600 700 800

0 20 40 60 80 100

T (%)

10.5 nm 13.5 nm 16.0 nm 18.5 nm 20.0 nm 17.5 nm

PET/MoO₃ (x nm)/Ag (11 nm)/ MoO₃ (35 nm)

X=

a

b

λ (nm)

70

60

50

400 500 600 700

λ (nm)

T (%)

Fig. 7.Experimental (a) and theoretical (b) transmittance spectra of PET/MoO3(X nm)/Ag (11 nm)/MoO3(35 nm) structures with X ranging between 10 nm and 20 nm.

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signiﬁcant increase of the sheet resistance of the electrode and the formation of a partially blocking contact at the interface anode/organic material, due to the small work function of Ag (4.3 eV). In order to prevent these effects we have modiﬁed the anode. To preserve the conductivity of the MAM structure we have increased the Ag thickness from 11 nm

to 17 nm. Moreover, as we have shown earlier[28]that an ultra thin goldﬁlm introduced between the anode and the CuPc layer, suppress the kink effect in the J-V characteristics of the planar heterojunction CuPc/C₆₀, by improving the band matching at the interface, we have deposited 0.7 nm of Au onto the MAM structure before depositing the OPV cell. It can be seen inFig. 9that, by using as anode a structure PET/MoO3

(17.5 nm)/Ag (11 nm)/MoO₃ (35 nm)/Au (0.7 nm)/CuI (3 nm), the S-shaped effect has totally disappeared. If we compare this result to those obtained with PET/ITO anodes, we can see that the PCE is higher than that obtained with an ITO film, while it is smaller than that achieved with an ITOfilm covered by CuI. The increase of the Agfilm thickness add to the presence of Au allows to suppress totally the kink effect, through the decrease of the electrode sheet resistance and the suppression of the barrier at the interface anode/organic material. How- ever, if CuI improves the light absorption through its templating effect on the CuPc molecules[16,20,21], the increase of the Ag thickness decreases the transmittance of the anode and, as shown inTable 1and Fig. 9there is a decrease of the short circuit current Jsc, which limits the PCE.

4. Conclusions

MoO₃/Ag/MoO₃structures were deposited onto PET. Performances equivalent to those achieved by MAM structures deposited onto glass substrates are achieved. The high adherence and theﬂexibility of the structures are checked. However, when probed as anode in organic photovoltaic cells, the experiments show that it is necessary to increase the Ag thickness of the MAM structure to preserve its conductivity and to introduce a very thin Au buffer layer to achieve good J-V characteristics.

Therefore, works are in progress to optimize the physical properties of the MAM Structures. Among these works, the introduction of Al diffusion barrier to prevent Ag diffusion[29]. Another possibility, as shown by others authors, is to proceed to planarization of the PET surface [30]to obtain reproducible properties on large surfaces. In order to prevent heating effect during the deposition of the cells, the PET substrate will be cooled by pasting it onto cooling substrate holder and thicker PETﬁlms (150μm) will be probed.

400 500 600 700

40 50 60 70 80 90

PET/MoO₃ (17.5 nm)/Ag (11 nm)/ MoO₃ (x nm)

T (%)

30 nm 25 nm 20 nm 35 nm a

x=

b

λ (nm) 80

70

60

50

40

30

400 500 600 700

λ (nm)

T (%)

Fig. 8.Experimental (a) and theoretical (b) transmittance spectra of PET/MoO3(17.5 nm)/Ag (11 nm)/MoO3(X nm) structures with X ranging between 5 nm and 35 nm.

-200 0 200 400 600

-3 -2 -1 0 1 2 3

V (mV) MA(11 nm)M/CuI MA(17 nm)M/Au/CuI Anode

J (mA/cm2 )

Fig. 9.Typical J–V characteristics of anode/CuPc/C60/Alq3/Al structures, under illumination of AM1.5 solar simulation (100 mW/cm²).

Table 3

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

Anode Voc (V) Jsc (mA/cm2) FF (%) η(%) Rs

MA(11 nm)M/CuI 0.37 2.4 29 0.26 250

MA(17 nm)M/Au/CuI 0.43 2.1 43 0.37 25

ITO 0.41 2.4 25 0.25 60

ITO/CuI 0.425 4.4 36 0.67 13

365.0 367.5 370.0 372.5 375.0 377.5

4000 6000 8000 10000 12000 14000 16000

Intensity (counts/s)

Binding energy (eV)

Fig. 10.XPS spectra of Ag3d before ( ) and after ( ) organic layer deposition.

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Acknowledgements

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

References

[1] J. Lewis, S. Grego, B. Chalamala, E. Vick, D. Temple, Appl. Phys. Lett. 85 (2004) 3450.

[2] Y.-M. Chang, L. Wang, W.-F. Su, Org. Electron. 9 (2008) 968.

[3] C.D. Williams, R.O. Robles, m. Zhang, S. Li, R.H. Baughman, A.A. Zakhidov, Appl. Phys.

Lett. 93 (2008) 183506.

[4] M.-G. Kang, M.-S. Kim, L.J. Guo, Adv. Mater. 20 (2008) 4408.

[5] H.M. Zhang, W.C.H. Choy, Org. Electron. 9 (2008) 964.

[6] Y. Berredjem, J.C. Bernède, S. Ouro Djobo, L. Cattin, M. Morsli, A. Boulmokh, Eur. Phys. J. Appl. Phys. 44 (2008) 223.

[7] K. Ellmer, Nat. Photonics 6 (2012) 809.

[8] C.J.M. Emmott, A. Urbina, J. Nelson, Sol. Energy Mater. Sol. Cells 97 (2012) 14.

[9] S.D. Yambem, A. Haldar, K.-S. Lliao, E.P. Dillon, A.R. Barro, S.A. Curran, Sol. Energy Mater. Sol. Cells 95 (2011) 2424.

[10] S.D. Yambem, K.-S. Lliao, S.A. Curran, Sol. Energy Mater. Sol. Cells 95 (2011) 3060.

[11] D.S. Ghosh, R. Betancur, T.L. Chen, V. Pruneri, J. Martorell, Sol. Energy Mater. Sol. Cells 95 (2011) 1228.

[12] S. Cheylan, D.S. Gosh, D. Krautz, T.L. Chen, V. Pruneri, Org. Electron. 12 (2011) 818.

[13] C. Guillén, J. Herrero, Thin Solid Films 520 (2011) 1.

[14] L. Cattin, M. Morsli, F. Dahou, S. Yapi Abe, A. Khelil, J.C. Bernède, Thin Solid Films 518 (2010) 4560.

[15] D.-T. Nguyen, S. Vedraine, L. Cattin, P. Torchio, M. Morsli, F. Flory, J.C. Bernède, J. Appl. Phys. 112 (2012) 063505.

[16] M. Makha, L. Cattin, Y. Lare, L. Barkat, M. Morsli, M. Addou, A. Khelil, J.C. Bernède, Appl. Phys. Lett. 101 (2012) 2233307.

[17] Y. Berredjem, N. Karst, L. Cattin, A. Lakhdar-Toumi, A. Godoy, G. Soto, F. Diaz, M.A.

del Valle, M. Morsli, A. Drici, A. Boulmokh, A.H. Gheid, A. Khelil, J.C. Bernède, Dyes Pigments 78 (2008) 148.

[18] N. Zhou, X. Guo, R.P. Ortiz, S. Li, S. Zhang, R.P.H. Chang, A. Facchetti, T.J. Marks, Adv.

Mater. 24 (2012) 2242.

[19] X. Guo, N. Zhou, S.J. Lou, J.W. Hennek, R.P. Ortiz, M.R. Butler, P.T. Boudreault, J.W.

Strzalka, P. Morin, M. Leclerc, J.T.L. Navarrete, M.A. Ratner, L.X. Chen, R.P.H. Chang, A. Facchetti, T.J. Marks, J. Am. Chem. Soc. 44 (2012) 18427.

[20] C.H. Cheng, J. Wang, G.T. Du, S.H. Shi, Z.J. Du, Z.Q. Fan, J.M. Bian, M.S. Wang, Appl.

Phys. Lett. 97 (2010) 083305.

[21] L. Cattin, J.C. Bernède, Y. Lare, S. Dabos-Seignon, N. Stephant, M. Morsli, P.P. Zamora, F.R.

Diaz, M.A. del Valle, Phys. Status Solidi A (2013),http://dx.doi.org/10.1002/pssa.

201228665.

[22] K.L. Mittal, Electrocompon. Sci. Technol. 3 (1976) 21.

[23] S.-W. Cho, J.-A. Jeong, J.-H. Bae, J.-M. Moon, K.-H. Choi, S.W. Jeong, N.-J. Park, J.-J. Kim, S.H. Lee, J.-W. Kang, M.-S. Yi, H.-K. Kim, Thin Solid Films 516 (2008) 7881.

[24] S.A. Dyakov, V.A. Tolmachev, E.V. Astrova, S.G. Tikhodeev, V. Yu Timoshenko, T.S.

Perova, International Conference on Micro-and Nano-Electronics 2009, February 26, 2010, p. 75210G-1.

[25] G. Haacke, J. Appl. Phys. 47 (1976) 4086.

[26] C.C. Kulshreshtha, J.W. Choi, J.-K. Kim, W.S. Jeon, M.C. Suh, Y. Park, J.H. Kwon, Appl.

Phys. Lett. 99 (2011) 023308.

[27] J.C. Wang, X.C. Ren, S.Q. Shi, C.W. Leung, P.K.L. Chan, Org. Electron. 12 (2011) 880.

[28] B. Kouskoussa, M. Morsli, K. Benchouk, G. Louarn, L. Cattin, A. Khelil, J.C. Bernède, Phys. Status Solidi A 206 (2009) 311.

[29] I. Pérez Lopéz, L. Cattin, D.-T. Nguyen, M. Morsli, J.C. Bernède, Thin Solid Films 520 (2012) 6419.

[30] H. Cho, C. Yun, J.-W. Park, S.S. Yoo, Org. Electron. 10 (2009) 1163.