Improving the efficiency of subphthalocyanine based planar organic solar cells through the use of MoO3/CuI double anode buffer layer

(1)

Improving the ef ﬁ ciency of subphthalocyanine based planar organic solar cells through the use of MoO

3

/CuI double anode buffer layer

Z. El Jouad

^a,d

, M. Morsli

^b

, G. Louarn

^c

, L. Cattin

^c

, M. Addou

^d

, J.C. Bernède

^a,n

aUniversité de Nantes, MOLTECH-Anjou, UMR 6200, 2 rue de la Houssinière, 44322 Nantes Cedex, France

bUniversité de Nantes, Département de Physique, 2 rue de la Houssinière, 44322 Nantes Cedex, France

cUniversité de Nantes, Institut des Matériaux Jean Rouxel (IMN), UMR 6502, 2 rue de la Houssinière, BP 32229, 44322 Nantes Cedex 3, France

dLOPCM, CNRST-URAC-14, Université Ibn Tofail, BP 133, Kenitra 14000, Morocco

a r t i c l e i n f o

Article history:

Received 5 March 2015 Received in revised form 6 June 2015

Accepted 9 June 2015

Keywords:

Organic photovoltaic cells Planar heterojunctions Phthalocyanine derivatives Thinﬁlm morphology Hole mobility

a b s t r a c t

Planar organic photovoltaic cells (OPV) based on the heterojunction subphthalocyanine/fullerene (SubPc/C60) were fabricated by varying the nature of hole transporting layer (HTL) and the SubPc thickness. The performances of the efficient heterojunction SubPc/C60are improved through the use of MoO3/CuI double HTL. In comparison with OPV using MoO3alone as HTL, the insertion of CuI leads to a significant increase in the short circuit current due to improved hole mobility. With the MoO3/CuI HTL, the power conversion efficiency was maximized to nearly 5% at a SubPc thickness of 20 nm. The atomic force microscopy study shows that the morphology of the SubPcfilms depends on the HTL. With CuI, the SubPc films are more homogeneous, with a smoother surface. These morphology differences induce modifications of the electrical properties of the SubPc.J–Vcharacteristics of hole only devices, i.e. devices with SubPc inserted between two high work function electrodes, show that the hole mobility in SubPc deposited onto CuI is higher than that infilms deposited onto MoO3.

1. Introduction

Organic photovoltaic cells (OPV) are extensively studied due to their lightness,ﬂexibility and the continuous improvement of their performances, due to the use of new materials and to improved OPV architectures[1,2]. These cells are based on a heterojunction electron donor/electron acceptor (ED/EA). Two OPV families are encountered, using a polymer as an electron donor and small molecules. In poly- meric systems, power conversion efﬁciency (

η

) of more than 9% was achieved through the optimization of materials used in bulk heterojunction (BHJ) conﬁguration[3,4]. Recently, an efﬁciency of 8.5%

was achieved using planar heterojunction (PHJ) with an activefilm which consists of three small molecule organic layers [5]. An advantage of small molecules is that they exhibit inherent mono- dispersity. Usually such PHJ using small molecules are fabricated through vacuum process which allows easy achievement of high purity and reproducibility. Vacuum deposition enables extremely thin yet homogeneous layers. This well-controlled, ultra-thin film process allows deposition of a large number of layers on top of each other. However, these structures suffer from a low short circuit current (Jsc) because of limited exciton dissociation due to insufficient

ED/EA interface area and low sunlight absorption[6]. Therefore, it is necessary to look for organic materials with very high absorption coefficient. The relevant parameters, other thanJsc, derived from the current density–voltage (J–V) are the open circuit voltage,V_oc, and the fill factor, FF. To improve OPV efficiency it is necessary to improve, at least, one of these relevant parameters.

It is now well accepted thatVocdepends linearly on the energy difference between the HOMO energy of the electron donor and the LUMO energy of the electron acceptor [7]. Therefore, it is desirable to increase the energy difference between HOMOEDand LUMOEAin order to obtain a higherVocvalue.

Due to their semiconductor properties, small band gap value and stability, metal-phthalocyanines (M-Pc) such as copper- phthalocyanine (CuPc), and zinc-phthalocyanine (ZnPc) are often used as ED in OPV using the PHJ conﬁguration. However, as the EA usually used is C60, whose LUMO value is 4.5 eV; the HOMO energy value of these M-Pcs, around 5.1–5.2 eV, limits the maximum theoreticalVocvalue 0.6–0.7 eV.

In order to increase theVoc, other molecules belonging to the family of phthalocyanine chromophores, but with higher HOMO energy value, were studied. Subphthalocyanine (SubPc) (Fig. 1), with HOMO energy of 5.6 eV, is one of them; high absorption coefﬁcient

α

¹¼53.4 nm is added to the high value of its HOMO energy[8].

Contents lists available atScienceDirect

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

Solar Energy Materials & Solar Cells

nCorresponding author.

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

(2)

As said above, if it is interesting to use new materials, it is also necessary to improve OPV architectures. The interface electrodes/

organic materials are crucial in determining the OPV performances.

Efﬁcient carrier extraction interfaces are required to collect the pho- togenerated current. Usually, buffer layers are inserted between the electrodes and the organic materials. Among the hole transporting layer (HTL), the insertion of gold[9]or transition metal oxides has shown their effectiveness in improving the OPV performances[10,11].

However, the effectiveness of the gold layer is limited by the value of its work functions[12]. Among the transition metal oxides, MoO3is highly desirable, not only due to its recognized efﬁciency, but also due to its low sublimation temperature, which made it easy to deposit [11]. While MoO3 allows achieving of good band matching at the interface anode/ED, the performances of OPV using phthalocyanine chromophores also depend on their molecular packing and orientation. Actually, they exhibit change in the molecular packing and orientation and even energy level with regard to possible HTL tem- plating effect, resulting in different optical and electrical properties.

For instance, in the case of CuPc, it was shown that a CuI HTL changes the CuPc molecules orientation from perpendicular to parallel to the substrate, which allows a significant improvement of the OPV performances[13,14]. SubPc is a widely used phthalocyanine chromo- phore either as ED[8–16]or EA[5–17]. However, there are not studies based on SubPc as ED with the use of CuI HTL. Following the pre- ceding works, we probe different HTL configurations such as MoO3, CuI, and MoO3/CuI and we proceed to some more characterizations for interpreting the specific effect of CuI on the ED properties. We have made various physicochemical and electrical characterizations.

The AFM study shows that the SubPc thinﬁlm morphology varied according to the HTL underneath it, while we have investigated theJ– V characteristics of the hole-only devices with MoO3, CuI and MoO3/CuI HTL. In the case of CuI HTL the mobility is μ₀

¼1.6910⁵cm²/(V s), while it is μ0_¼1.4110⁶cm²/(V s), with the MoO₃HTL. It means that the hole mobility in SubPc is improved by one order of magnitude when it is deposited on CuI. All this allows achieving of OPV with 4.66% efﬁciency.

2. Experimental

OPV were fabricated by deposition under vacuum (10⁴Pa) on to pre-cleaned ITO covered glass substrate, without breaking the

vacuum. The standard substrate dimensions were 25 mm by 25 mm. Since ITO covered the entire glass substrate, some ITO must be removed to shape the under electrode. After masking a broad band of 25 mm by 20 mm, the ITO was etched using ZnþHCl as the etchant[18]. ITO substrates with a sheet resistance of 20Ω/sq were scrubbed with soap, rinsed with distilled water, dried and then placed in the vacuum chamber. OPV structure consists of ITO/HTL /SubPc /C60/Alq3/Al, the HTL being MoO3, CuI or MoO3/CuI. The three HTL conﬁgurations were probed in order to check the best one. For completing the planar heterojunction, the fullerene was used as an EA. The electron transporting layer (ETL) was a layer of aluminum tris(8-hydroxyquinoline) (Alq3)[18]

and the cathode was an aluminumﬁlm. Alq3was chosen as the ETL because it has been shown that it allows growing of solar cells with higher lifetime [19]. The thickness of the thin ﬁlms and deposition rates were estimated in situ using a quartz monitor. We used a 3 nm thick MoO3layer as one of the HTLs because MoO3

with this thickness is a very efficient HTL in optoelectronic organic devices [20]. In the case of CuI, it has been shown that the deposition rate has a strong influence on the morphology of the films. When deposited too fast, it is highly inhomogeneous, which induces current leakage. Such current leakage leads to a decrease of Voc [13]. Therefore we use a very slow deposition rate i.e 0.005 nm/s, while its thickness was 1.5 nm[14].

According to a previous study [18], the thickness of the C60

layer was 40 nm and that of the Alq3ETL was 9 nm. The cathode was an aluminum ﬁlm of 100 nm thickness, deposited by evaporation. The effective area of each cell was 0.16 cm². Theﬁnal cell architecture was glass/ITO (100 nm)/HTL/SubPc/C60(40 nm)/Alq3

(9 nm)/Al (100 nm), HTL¼MoO3, CuI and MoO3/CuI (Fig. 2a).

The main focus of this work was to study the inﬂuence of the nature of the HTL, MoO3, CuI or MoO3/CuI, on the OPV performances when SubPc is the ED. In order to optimize the OPV performances, the SubPc thickness was varied from 14 nm to 22 nm.

It should be noted that at least six diodes are realized by cycle of deposit to average the OPV performances.

SubPc/ C₆₀ 3.6 eV

4.5 eV

5.6 eV

6.3 eV 1.1 eV

Fig. 1.Chemical structure of electron donor and energy level diagram of the planar heterojunction SubPc/C60.

-200 0 200 400 600 800 1000 1200 -10

-8 -6 -4 -2 0 2 4

J (mA/cm2)

V (mV)

HTL EA ED EBL

Anode Cathode Alq₃

ITO SubPc C₆₀

Al

MoOCuI₃

Glass

Fig. 2.OPV diagram (a) andJ–Vcharacteristics of OPV using SubPc (▼) as electron donor, in the dark (full symbols) and under AM1.5 irradiation (open symbols), cells with MoO3/CuI as HTL (b).

(3)

In order to understand the effect of the different HTL on the OPV cells performances, different characterization techniques have been used for studying the structures ITO/HTL/SubPc.

These thinﬁlms structures were analyzed by X-ray diffraction (XRD) by a Siemens D5000 diffractometer using K

α

radiation from Cu (

λ

Kα¼0.15,406 nm).

The ﬁlm absorbance was measured at wavelengths of 1.2– 0.30

μ

m. The optical measurements were carried out at room temperature using a Carry spectrometer.

Atomic force microscope (AFM) images on different sites of the ﬁlms were taken ex-situ at atmospheric pressure and room temperature. All measurements have been performed in tapping mode (JPK instruments). 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.

Electrical characterizations were performed with an automated I–Vtester, in the dark and under sun global AM 1.5 simulated solar illumination. Performances of photovoltaic cells were measured using a calibrated solar simulator (Oriel 300 W) at 100 mW/cm² light intensity adjusted with a PV reference cell (0.5 cm²CIGS solar cell, calibrated at NREL, USA). Measurements were performed in an ambient atmosphere. All devices were illuminated through ITO electrodes.

In order to determine the effect of the HTL on the electrical properties of the SubPcﬁlms, we have investigated theJ–Vchar- acteristics of hole-only devices with MoO3, CuI and MoO3/CuI HTL.

These devices were grown using high work-function electrode buffer layers. The hole only devices were fabricated by replacing the C60and the Alq3ETL by a high work functionﬁlm. The work function of Au being not more than 5.1 eV, it is expected to lie within the band gap of SubPc, thereby inducing barrier to injection (or collection) of holes. In order to avoid such barrier, a thin MoO3

buffer layer was introduced between the SubPcfilm and the top electrode. It was shown that MoO3acts as efficient hole injecting- collecting layer into most conjugated organic material[21]. Hole only devices have been made using the same ITO covered glass substrate than those used to grow OPV cells. After deposition of the HTL, an organic film thick of 120 nm was deposited. The organicfilm was covered with a MoO3film thick of 7 nm and then a thin goldfilm was deposited. Finally aluminum was used as the top electrode.

3. Experimental results and discussion

Firstly we checked the optimum thickness of the SubPc layer, using MoO3/CuI as bilayer HTL, which gave the best results in the case of other electron donors such as thienylenevinylene–triphenylamine with peripheral dicyanovinylene groups (TDCV–TPA) [14] or CuPc [11]. While the optimum SubPc layer thickness is usually between 10 and 15 nm[15,16], with our MoO₃(3 nm)/CuI (1.5 nm) HTL, the optimum thickness is 20 nm (Table 1). This thickness is higher than that usually encountered. However, another study shows that a thickness of 40 nm was also used successfully[8].

As shown inFig. 2b, our champion OPV had a power conversion efﬁciency (

η

) of 4.97%, with Voc¼1.06 V, Jsc¼9.0 mA/cm² and FF¼52.5%.

The values inTable 1are the average values obtained for series of 6 cells.Vocincreases up to 18 nm of SubPc and then it tends to saturate.Jscand FF increase upto 20 nm and then they decrease.

Then we checked that, here also, the bilayer HTL, MoO3(3 nm)/

CuI (1.5 nm), allows achievement of the best results. This is con- ﬁrmed by the results inTable 2. When a simple HTL is used, MoO₃ permits obtainment of a good Vocvalue, but FF is quite small.

Conversely, with CuI alone,Vocis smaller,Jscis greater, while FF is far higher. Moreover, it can be seen inTable 1that in the case of a bilayer HTL when the CuI thickness is increased there is a decrease ofVoc, even when in parallel, the thickness of the SubPc layer is increased.

At last, in order to check the effect of the anode work function we have also probed ITO alone. The efﬁciency obtained is very low.

Regarding Voc, the experimental value obtained with SubPc is close to the theoretical estimate, since the difference is only 0.07 V (Figs. 1and2). TheVocvalue is inﬂuenced not only by the energy level difference between the LUMO of the EA and the HOMO of the ED but also by the molecular packing and the charge carrier recombination at the interface[7]. In this respect the nearly opti- mumVocvalue obtained with SubPc is a consequence of reduced recombination at ED/EA interface, which results in a reduced dark current. It is known that Voc increases when the dark current is reduced[22]; in the case of OPV with SubPc as ED, it can be seen in Table 2thatVocdepends on the HTL. Actually, inFig. 3one can see that, with CuI alone as HTL, the dark current is higher by one order of magnitude than that of the OPV with MoO3in the HTL. Thus, the Vocis consequently enhanced in the OPV with MoO3. In the same way, when in the bilayer HTL the CuI thickness overpasses the Table 1

Averaged (6 OPV) photovoltaic performance data of OPV using subPc as electron donor, with different thickness, under AM1.5 conditions.

CuI thickness (nm)

SubPc thickness (nm)

Voc(V) Jsc(mA/cm²) FF (%) η(%)

1.5 14 0.8570.02 5.1070.25 3572 1.4870.35

1.5 16 0.9770.01 5.8070.20 5072 2.8770.30

1.5 18 1.0270.01 6.0670.10 53.571 3.2870.25

1.5 20 1.0370.01 8.3170.10 54.571 4.6670.25

3.0 20 0.9570.02 6.6870.25 5072 3.1770.30

3.0 22 0.9670.02 6.4370.20 4973 3.0270.35

1.5 22 1.0370.01 7.0270.10 52.571 3.8070.25

Table 2

Averaged (6 OPV) photovoltaic performance data of OPV using SubPc as electron donor, with different HTL, under AM1.5 conditions (voltage scan speed: 80 ms per point of measure).

HTL (thickness) Voc(V) Jsc(mA/cm²) FF (%) η(%)

MoO3(3 nm) 0.9870.01 6.0570.10 4471 2.6070.2

CuI (1.5 nm) 0.8570.02 6.3870.15 5672 3.0770.3

MoO3/CuI (3/1.5 nm) 1.0370.01 8.3170.10 54.571.0 4.6670.25

No HTL 0.2770.02 3.2570.10 4472 0.3770.3

-200 -100 0 100 200 300 400

-0,01 0,00 0,01 0,02 0,03 0,04

J (mA/cm2)

V (mV) MoO₃/CuI CuI MoO₃

HTL:

Fig. 3. J–Vcharacteristics in the dark of OPV using CuI (○), MoO3(▲) and MoO3/CuI (■) HTL.

(4)

optimum value of 1.5 nm, there is a decrease in the cells performances (Table 1).

Without HTL, there is not a good band matching at the interface (ITO work function around 4.7 eV) and there is a barrier at the interface ITO/SubPc and the OPV performance is very bad (Table 2).

The other relevant parameter which varies signiﬁcantly through the use of the different HTL isJsc.Jscdepends strongly on the light absorption, on the carrier mobility and on the efﬁciency of the carrier collection.

The absorption spectra, inFig. 4, exhibit two absorption bands, a Q-band in the visible range between 500 nm and 650 nm and a Soret band in the UV range, both assumed to correspond to

π

–

π

^*

transitions of the C–N backbone[23]. The maximum of the Q-band is situated at 599 nm with a shoulder at 555 nm and a smaller one at around 500 nm [24]. The spectra of the SubPc layers do not depend signiﬁcantly on the HTL.

In order to check the effect of the HTL on the morphology of the SubPc layers, we have examined the surfaces of the SubPc layers using AFM. The surfaces of SubPc layers deposited on MoO3and on CuI are presented inFig. 5. All images were taken with the same area of 5

μ

^m5

μ

m. It can be seen that the layers exhibit different structures. In the case of SubPc ﬁlms deposited onto MoO3, spherical grains are clearly visible inFig. 5a. Some of these grains percolate between them giving birth to little elongated clusters, with a length of the order of 0.5

μ

m. The root mean square (rms) of these layers is 10.070.2 nm. When deposited onto CuI, the SubPc layer surface is covered with elongated and branched clusters (Fig. 5b). These clusters are continuous all along the AFM images. In that case, the rms is 5.070.2 nm, which corresponds nearly to half of that of layers deposited onto MoO3. These differences in rms are illustrated by the proﬁles (Supporting Infor- mation S1). Similar results are obtained when MoO3/CuI HTL is used instead of CuI. Therefore it can be concluded that these AFM studies suggest that the SubPc thin ﬁlm morphology varied according to the HTL underneath it.

Whatever the HTL is, no differences are visible in the XRD diagrams of SubPc (Supporting Information, S2). Regardless of the HTL, the only peak visible corresponds to the ITO bottom layer, which means that the SubPcﬁlms are disordered without strong interaction between the molecules. While neither the absorbance, nor the XRD studies allow explaination of the different behaviors of the OPV with the different HTL; it is not the same with regard to AFM study. As said above, the carrier mobility is crucial to theJsc

value. The conductivity of the SubPc layers is quite poor [24], which limits Jsc and FF[8]. Therefore, any improvement in the charge mobility in SubPc will signiﬁcantly increaseJscand therefore the performance of the OPV. It is known that charge carrier mobility can be estimated from space-charge limited current–

voltage technique (SCLC). Such SCLC regime can be achieved using hole only (electron only) devices. In a hole only structure, one electrode, here the modiﬁed electrode ITO/HTL, must efﬁciently inject hole into the electron donor, while the other electrode must block electron injection which is achieved through the introduction of a MoO₃ layer below the top electrode (insetFig. 6)[20].

Moreover, in these sandwich-type structures the organic layer must be thick enough to prevent that interface phenomena Fig. 4.Absorbance spectra of SubPc deposited onto MoO3/CuI (^____) and MoO3

(- - - -) and CuI(-–-)HTL.

Fig. 5.AFM topography images (55μm²) of (a) ITO/MoO3/SubPc, and (b) ITO/CuI/

SubPc.

1 1

, 0 1E-5 1E-4 1E-3 0,01 0,1

n = 0.95 n = 1.9 CuI

J (mA/cm2 )

V (V) CuI

MoO₃

ITO SubPc

Al

HTL

Glass MoO₃

Fig. 6.Current density–voltage curves log–log plot of the hole-only devices: ITO/.

HTL/SubPc/MoO3/Al, with as HTL MoO3 (○) or CuI (■).

InsetFig. 6: Current density–voltage curves log–log plot of the hole-only devices:

ITO/CuI/SubPc TDCV–TPA/MoO3/Al (inset: Diagram of the hole only devices).

(5)

dominate those of bulk, therefore we used SubPc layers thick of 100 nm and not the 20 nm used in the OPV.

The results obtained with the hole only devices, are shown in Fig. 6. The curves obtained with CuI and MoO₃/CuI were similar, that is why, for clarity only the curves with single HTL are shown inFig. 6. The devices with the CuI HTL show the largest current density at the same driving voltages compared to the devices with MoO₃ HTL. Both devices display nearly ohmic transport at low voltage as shown in the inset ofFig. 6in the case of CuI.

At higher current density the slope increases, which corresponds to a transition toward Space Charge Limited Current (SCLC) [25]. At higher voltage, the slope is nearly equal to 2, which corresponds to SCLC behavior in the absence of traps or withﬁlled traps.

Assuming uniform charge-carrier mobility, when the SCLC regime is achieved, theJ–Vcurve follows the Mott–Gurney square law[26]:

J=9/8ε ε μ0 r hV L2/ 3 ( )1

whereε0is the vacuum permittivity,

ε

ris the dielectric constant,

μ

his the hole mobility,Vis the voltage, andLis the thickness of the organic layer.

However, the mobility varies with the electricﬁeld. In order to take this variation into account, Murgatroyd assumed that the mobility is[27]as follows:

F

exp 2

h 0

μ =μ (γ ) ( )

whereμ0represents the mobility whenVtends be to zero andγ is a parameter that describes theﬁeld dependence effect, andFthe electricﬁeld.

The ﬁeld dependence arises from some disorder or shallow trapping.

From formulae (1) and (2) Murgatroyd approximated to the following:

J=9/8ε ε^{0 r}V L²/ ³μ0exp 0.89( γ V L/ ) ( )3 As shown in the inset ofFig. 6, at low voltages theJ–Vcurve can be described by an ohmic law, with a slopenE1, which corresponds to the following:

J=qn V Lμ / ( )4

where q is the elementary charge, and n the charge carrier density.

At higher voltage, we have in the inset ofFig. 6,nE2 which means that the SCLC regime is achieved. We proceeded to the fitting of the J–V curves with Eq. (3) by varying γ and μ₀. One should be very careful with the values obtained. Indeed the validity of the calculation assumes that many conditions are ful- filled such as the charge injection is efficient, the series resistance does not dominate, and devices are really unipolar[21]. While the result only gives an order of magnitude of the absolute value of the mobilities, the relative values of a sample to the other are more significant (Table 3). It must be noted that the

γ

value is of the same order of magnitude than that already encountered in the literature[21,28].

In the case of CuI HTL the mobility estimated is μ₀

¼1.6910⁵cm²/(V s), while it is onlyμ₀_¼1.4110⁶cm²/(V s),

with the MoO3HTL. It means that the hole mobility in SubPc is signiﬁcantly improved by the CuI HTL (one order of magnitude).

On the other hand, the values estimated, are in the range of values measured by others groups. For instance, Liu et al.[22]measure, using the same technique, mobilities from 5.7210⁵ to 1.0910⁶cm²/(V s) following the purity of the SubPc ﬁlms.

Therefore this calculated value is credible. Even if it is only indi- cative it can be used for comparisons between similar materials.

Such variation of the value of the mobility with the HTL used can be related to the changes of the morphology of the SubPcﬁlms, changes which were put in evidence by the AFM study. In the presence of CuI, as shown by the rms values, the SubPcﬁlms are more homogeneous which decrease the grain boundary effects and increases the measured mobility. The measured improvement of the hole mobility through the use of MoO3/CuI HTL canjustify the fact that the optimum SubPc thickness estimated in this study is higher than that generally used.

Some authors showed that increasing the deposition rate allows to reduce SubPc crystallization, which induces higher HOMO value and results in higherV_ocand efﬁciency values[15].

We did not see similar effect probably because all our SubPcﬁlms are amorphous whatever deposition rate we used.

As a conclusion of this discussion, we have already shown that CuI HTL tends to increase the leakage current due to some surface in homogeneity of the CuI HTL[13,14]. Therefore the best result is achieved with the double HTL MoO₃/CuI. We assume that the reason for this may be the dual function of MoO3 and CuI. CuI improves the conductivity of the SubPcﬁlms through an increase in the hole mobility and MoO3prevents the OPV cells from leakage path formation and allows achieving of an optimum band matching between the anode and the HTL.

Finally we present the time response of our devices. The devices were encapsulated with an amorphous selenium layer for the lifetime test. To improve the accuracy of the study of aging of the OPV we performeda “semien capsulation” thereof. Without protecting layer the performances of the OPV are known to decline rapidly[29]. In order to limit this instability an encapsulating layer of amorphous selenium (Se-a) was thermally evaporated before exposing the devices to atmospheric conditions. A selenium pro- tective coating (PSe) has been proved to efﬁciently protect the devices against oxygen and water[30], for at least a few hours in air, depending on its thickness [18]. The longer lifetime of the devices thus obtained allows a more precise analysis of the effects causing degradation. Following the protocol proposed in Ref.[31], the procedure used to study the ageing process of our OPV corresponds to the intermediate level labeled ‘‘Level 2’’. The operational lifetimes have been measured under AM1.5, in air and at room temperature. Two processes were used. For theﬁrst one, between every measure samples were stored in air and in the light of day, the cells being in open circuit conditions this process is called“intermittent illumination”. For the second process the OPV were subjected to continuous illumination, while the substrate temperature was stabilized to 35°C.

Typical degradation curves for devices exposed to atmospheric conditions under continuous illumination are presented in Figs. 7and8in the case of a MoO3/CuI DHTL with a 50 nm thick Se layer.

It can be seen inFig. 7thatVocremains nearly constant during the time-scale of experiment, whileJscand

η

follow more or less the same trend. Similar curves are obtained whatever the processes used, under continuous or intermittent light. The operational lifetime t_η0/2 is deﬁned as the time at which the initial performances have been divided by two[29]. It can be seen in Fig. 8 thatt_η0/2 is around 50 h. The lifetime is the same under permanent or intermittent light means that there is no strong degradation under light of the organic molecules present in the Table 3

Parameters (μ₀andγ) deduced from theﬁtting of theJ–Vcurves with Eq.(3).

HTL μ₀(cm²/(V s)) γ((cm/V)^1/2)

CuI 1.6910⁵ 0.00033

MoO3 1.4110⁶ 0.00167

(6)

OPV. It must be noted that similar curves were obtained with CuPc as ED, which suggests that the lifetime of these OPVs depends mainly on the EA.

This stability shows that the lifetime of the OPV depends essentially on the EA, regardless of whether the device is illuminated or not. In an earlier study it was shown that under long- term exposure to light there was a signiﬁcant photobleaching of SubPc; photobleaching attributed to the progressive crystallization of SubPc[32]. We do not observe such behavior with our devices, actually we have shown that our SubPc layer stays amorphous, which can be due to the deposition conditions and due to the thermalization of the substrate temperature during the long-term exposure to light. Therefore, it is assumed that the conductivity decrease of C60ﬁlm due to water and oxygen permeating easily through thermally deposited aluminum cathode is the reason for the degradation of performance of OPV cells (Figs. 7,8)[33].

4. Conclusion

We showed that the performances of the OPV based on the SubPc/C60heterojunction can be improved by use of a double HTL, MoO3/CuI. A high short circuit current was observed for a double HTL, MoO3/CuI. The SEM and AFM studies show that the morphology of the SubPc films depends on the nature of the HTL, MoO3or CuI, thefilms being more homogeneous with CuI under layer. The electrical study of hole only devices show that the morphology induced by the CuI HTL is more favorable to the hole mobility in thefilm, which justifies the higherJsc. In the experimental conditions of the present study, the operational lifetimes of

the different OPV depend mainly on the air contamination of the C60layer.

Acknowledgments

This work wasﬁnancially supported by the Academy Hassan II PPR/2015/9 (Maroco).

Appendix A. Supplementary material

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.solmat.2015.06.

017.

References

[1]F.C. Krebs, N. Espinosa, M. Hösel, R.R. Sondergaard, M. Jorgensen, 25th Anni- versary article: rise to power-OPV-based solar parks, Adv. Mater. 6 (2014) 29–39.

[2]Ning Li, Derya Baran, George D. Spyropoulos, Hong Zhang, Stephane Berny, Mathieu Turbiez, Tayebeh Ameri, Frederik C. Krebs, Christoph J. Brabec, Environmentally printing efficient organic tandem solar cells with highfill factors: a guideline towards 20% power conversion efficiency, Adv. Energy Mater. 4 (11) (2014) 1400084.

[3]Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Enhanced power-conversion efﬁciency in polymer solar cells using an inverted device structure, Nat.

Photon. 6 (2012) 591.

[4]J. You, L. Dou, K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, Chun- Chao Chen, Jing Gao, Gang Li, Yang Yang, A polymer tandem solar cell with 10.6% power conversion efﬁciency, Nat. Commun. 4 (2013) 1446.

[5]K. Cnops, B.P. Rand, D. Cheyns, B. Veeet, M.A. Empl, P. Heremans, 8.4% Efﬁcient fullerene-free organic solar cells exploiting long-range exciton energy transfer, Nat. Commun. 5 (2014) 3406.

[6]J.T. Chen, C.S. Hsu, Polymer conjugated polymer nanostructures for organic solar cell applications, Chemistry 2 (2011) 2707.

[7]K. Cnops, B.P. Rand, D. Cheyns, P. Heremans, Enhanced photocurrent and open- circuit voltage in a 3-layer cascade organic solar cell, Appl. Phys. Lett. 101 (2012) 143301.

[8]J.H. Beck, B. Ray, R.R. Grote, R.M. Osgood, C.T. Black, M.A. Alam, I. Kymissis, Nanostructured electrodes improve theﬁll factor of organic photovoltaics, IEEE J. Photovolt. 4 (2014) 1100–1106.

[9]L. Cattin, S. Tougaard, N. Stephant, S. Morsli, J.C. Bernède, On the ultra-thin goldﬁlm used as buffer layer at the transparent conductive anode/organic electron donor interface, Gold Bull. 44 (2011) 199.

[10]J. Mayer, S. Hamwi, M. Kröger, W. Kowalski, T. Riedl, A. Kahn, Transition metal oxides for organic electronics: energetics, device physics and applications, Adv. Mater. 24 (2012) 5408.

[11]J.C. Bernède, L. Cattin, M. Morsli, About MoO3as buffer layer in organic optoelectronic devices, Technol. Lett. 1 (2) (2014) 5.

[12]J.C. Bernède, L. Cattin, S. Ouro Djobo, M. Morsli, S.R.B. Kanth, S. Patil, P. Leriche, J. Roncali, A. Godoy, F.R. Diaz, M.A. del Valle, Inﬂuence of the highest occupied molecular orbital energy level of the donor material on the effectiveness of the anode buffer layer in organic solar cells, Phys. Status Solidi A 208 (2011) 1989.

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

P. Zamora, F.R. Diaz, M.A. del Valle, Improved performance of organic solar cells by growth optimization of CuI/MoO3double anode buffer, Phys. Status Solidi A 210 (2013) 802.

[14]J.C. Bernède, L. Cattin, M. Makha, V. Jeux, P. Leriche, J. Roncali, V. Froger, M. Morsli, M. Addou, MoO3/CuI hybrid buffer layer for the optimization of organic solar cells based on a donor–acceptor triphenylamine, Sol. Energy Mater. Sol. Cells 110 (2013) 107.

[15]C.-F. Lin, V.M. Nichols, Y.-C. Cheng, C.J. Bardeen, M.-K. Wei, S.-W. Liu, C.-C. Lee, W.-C. Su, T.-L. Chiu, H.-C. Han, L.-C. Chen, C.-T. Chen, J.-H. Lee, Choloroboron subphthalocyanine/C60planar heterojunction organic solar cell with N,N- dicarbazolyl-3,5-benzene blocking layer, Sol. Energy Mater. Sol. Cells 122 (2014) 264.

[16]X. Tong, B.E. Lassiter, S.R. Forrest, Inverted organic photovoltaic cells with high open-circuit voltage, Org. Electron. 11 (2010) 705.

[17]N. Beaumont, S.W. Cho, P. Sullivan, D. Newby, K.E. Smith, T.S. Jones, Boron subphthalocyanine chloride as an electron acceptor for high-voltage fullerene organic photovoltaic, Adv. Funct. Mater. 22 (2012) 561.

[18]Y. Berredjem, N. Karst, A. Boulmokh, A.H. Gheid, A. Drici, J.C. Bernède, Opti- misation of the interface“organic material/aluminium”of CuPc/C60based photovoltaic cells, Eur. Phys. J.: Appl. Phys. 40 (2007) 163.

[19]Q.L. Song, F.Y. Li, H. Yang, H.R. Wu, X.Z. Wang, W. Zhou, J.M. Zhao, X.M. Ding, C.

H. Huang, X.Y. Hou, Small-molecule organic solar cells with improved stability, Chem. Phys. Lett. 416 (2005) 42.

0 20 40 60 80 100 120 140 160 0.0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Normalized values

t (h) Jsc/Jsc₀

FF/FF₀ Voc/Voc₀

η/η

Fig. 7.Evolution with time of the different normalized parameters of an OPV under continuous illumination with SubPc as electron donor. And MoO3/CuI as HTL.

0 20 40 60 80 100 120 140 160 0,0

0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0

/0

t (h)

SubPc under constante illumination SubPc under intermitante illumination

ηη

Fig. 8.Variation with time of the normalized efﬁciency of OPV cells with SubPc (under continuous illumination■ and intermittent illumination ●) as electron donor (cells with MoO3/CuI as HTL).

(7)

[20]L. Cattin, F. Dahou, Y. Lare, M. Morsli, R. Tricot, S. Houari, A. Mokrani, K. Jondo, A. Khelil, K. Napo, J.C. Bernède, MoO3surface passivation of the transparent anode in organic solar cells using ultra-thinﬁlms, J. Appl. Phys. 105 (2009) 034507.

[21]J.C. Blakesley, F.A. Castro, W. Kylberg, G.F.A. Dibb, C. Arentes, R. Valaski, M. Cremona, J.S. Kim, J.-S. Kim, Towards reliable charge-mobility benchmark measurements for organic semiconductors, Org. Electron. 15 (2014) 1263.

[22]S.-W. Liu, W.-C. Su, C.-C. Lee, C.-C. Chou, C.-W. Cheng, Efﬁcient organic photovoltaic device using a sublimated subphthalocyanine as an electron donor, ECS Solid State Lett. 1 (2012) 70.

[23]H. Gommans, D. Cheyns, T. Aernouts, C. Girotto, J. Poortmans, Electro-optical study of subphthalocyanine in a bilayer organic solar cell, Adv. Funct. Mater. 17 (2007) 2653.

[24]P. Heremans, D. Cheyns, B.P. Rand, Strategies for increasing the efﬁciency of heterojunction organic solar cells: material selection and device architecture, Acc. Chem. Res. 42 (2009) 1740.

[25]D. Chirvase, Z. Chiguvare, M. Knipper, J. Parisi, V. Diakonov, J.C. Hummelen, Temperature dependent characteristics of poly(3 hexylthiophene)-fullerene based heterojunction organic solar cells, J. Appl. Phys. 93 (2003) 3376.

[26]P. Qin, G. Fang, W. Zeng, X. Fan, Q. Zheng, F. Cheng, J. Wan, X. Zhao, Inﬂuence of thermal radiation during aluminium thermal evaporation on organic solar cells, Sol. Energy Mater. Sol. Cells 95 (2011) 3311.

[27]P.N. Murgatroyd, Theory of space-charge-limited current enhanced by Frenkel effect, J. Appl. Phys. D3 (1970) 151.

[28] 〈http://www.npl.co.uk/science-technology/electrochemistry/research/

organic-elecronics/mobility-protocol〉, 2014.

[29] G. Dennler, C. Lungenschmied, H. Neugebauer, N.S. Sariciftci, M. Latrèche, G. Czeremuszkin, M.R. Wertheimer, A new encapsulation solution forﬂexible organic solar cells, Thin Solid Films 511–512 (2006) 349.

[30] A. Latef, J.C. Bernède, Study of the thinﬁlm interface aluminium–tellurium, Phys. Status Solidi A 124 (1991) 243.

[31]M.O. Reese, S.A. Gevorgyan, M. Jørgensen, E. Bundgaard, S.R. Kurtz, D.S. Ginley, D.C. Olson, M.T. Lloyd, P. Morvillo, E.A. Katz, A. Elschner, O. Haillant, T.

R. Currier, Vi Shrotriya, M. Hermenau, M. Riede, K.R. Kirov, G. Trimmel, T. Rath, O. Ingan̈as, F. Zhang, M. Andersson, K. Tvingstedt, M. Lira-Cantu, D. Laird, C. McGuiness, S.J. Gowrisanker, M. Pannone, M. Xiao, J. Hauch, R. Steim, D.

M. DeLongchamp, R. R̈osch, H. Hoppe, N. Espinosa, A. Urbina, G. Yaman- Uzunoglu, J.-B. Bonekamp, A.J.J.M. van Breemen, C. Girotto, E. Voroshazi, F.

C. Krebs, Consensus stability testing protocols for organic photovoltaic materials and devices, Sol. Energy Mater. Sol. Cells 95 (2011) 1253.

[32] N. Wang, X. Tong, Q. Burlingame, J. Yu, S.R. Forrest, Photodegradation of small- molecule organic photovoltaics, Sol. Energy Mater. Sol. Cells 125 (2014) 170–175.

[33] A. Hamed, Y.Y. Sun, Y.K. Tao, R.L. Meng, P.H. Hor, Effects of oxygen and illumination on the in situ conductivity of C60thinﬁlms, Phys. Rev. B 47 (1993) 10873.