Softening temperature on sputtered ZnO interfacial barrier layer for an efficient charge transfer P3HT/ZnO and better interfacial stability in plastic organic photovoltaic devices

Softening temperature on sputtered ZnO interfacial barrier layer for an ef fi cient charge transfer P3HT/ZnO and better interfacial stability in plastic organic photovoltaic devices

A. Jouane ^a , R. Moubah ^a , H. Lassri ^a , D. Saifaoui ^a , G. Schmerber ^b , H. Jaouani ^c , H. Ennamiri ^c , Y.-A. Chapuis ^d , Y. Jouane ^{e , *}

a

LPMMAT, Facult e des Sciences Ain Chock, HASSAN II University, Casablanca, Morocco

b

IPCMS, UMR 7504 CNRS-UDS, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France

c

ESITH, Higher School of Textile Industries and Clothing, Casablanca, Morocco

d

University of Strasbourg, ICube, UMR 7357 CNRS-UDS, 23 rue du Loess, BP 20CR, 67037 Strasbourg Cedex, France

e

Kochi University of Technology, 185 Miyanoguchi Tosayamada Kochi, 782-8502 Japan

a r t i c l e i n f o

Article history:

Received 13 June 2016 Received in revised form 8 September 2016 Accepted 25 September 2016

Keywords:

Organic thin film solar cells Air-stable solar cells Magnetron sputtering Photoluminescence spectroscopy Charge transfer

Plastic photovoltaics

a b s t r a c t

We demonstrate the usefulness of RF magnetron sputtering ZnO thin film at softening temperature, as interfacial barrier layer in air stable flexible inverted organic photovoltaic devices. We investigate the in fl uence of annealing on the ZnO crystallinity, on the ITO substrate morphology and charge transport at the ZnO/active layer interface. The photo-physical and structural characteristics of P3HT beside ZnO interfacial layer and the photovoltaic device performances were also studied using UVevis spectroscopy, photoluminescence (PL) and J-V characteristic. Finally, we study the interfacial stability of devices with and without ZnO interfacial layer in both normal and inverted structure OPVs. We show that under optimized sputtering conditions, higher order and orientation structure of P3HT, the ZnO thermally annealed beside active layer offers better ef fi ciency of contact between the active layer and interfacial layer. We also show that ZnO annealed at a softening temperature of 180

C is functional for both photovoltaic devices (rigid and plastic substrates), leading to improved performance and stability of plastic solar cell devices.

© 2016 Elsevier B.V. All rights reserved.

1. Introduction

Recent advances in energy ef fi ciency, reliability of organic photovoltaic cells (OSCs) and perovskite solar cells (PSC) make them very attractive solutions to manufacture [1,2]. By improving their stability, the inverted OSCs have attracted a growing attention since their initial development [3]. However, in order to achieve high power conversion ef fi ciencies, the inverse structure should include interfacial buffer layers (IL) that lead to appropriate ef fi - cient charge collection [3]. Recently, zinc oxide (ZnO) has been explored as a promising interfacial material for photovoltaic con- version mainly due to its good optical properties (near-UV emission and transparency) and high electric conductivity [3]. The incorpo- ration of ZnO IL between a poly(3- hexylthiophene 2,5-diyl):[6,6]-

phenyl-C61-butyric acid-methyl-ester (P3HT:PCBM) photoactive blend and ITO electrode can place the active layer in a more favorable region of the internal electric fi eld. ZnO IL can be considered as a multifunctional buffer layer for several reasons [4,5]: (i) hole blocking layer due to its large optical band gap Eg ¼ 3.37 eV [6]; (ii) optical spacer [7]; and (iii) it forms an ohmic contact with P3HT:PCBM in inverted structures [8]. In recent years, many methods have been employed to deposit ZnO such as: radio frequency (RF) magnetron sputtering [9,10], pulsed-laser deposi- tion (PLD) [11], sol-gel deposition [12] and chemical vapour depo- sition (CVD) [13]. To prepare high quality of ZnO thin fi lms on large- area and fl exible substrates, sputter deposition is a very attractive technique [14,15]. It can operate at low-temperature and develop dense and reproducible fi lms [14]. Sputtering is also a compatible technology with a roll-to-roll coating process [12,16]. Moreover, sputtered ZnO fi lms can be combined with other materials, leading to an IL with improved properties, and could further substitute the more commonly used but expensive ITO electrode. In our study, it is

* Corresponding author.

E-mail address: [email protected] (Y. Jouane).

Contents lists available at ScienceDirect

Organic Electronics

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 / o r g e l

http://dx.doi.org/10.1016/j.orgel.2016.09.024

1566-1199/© 2016 Elsevier B.V. All rights reserved.

seen that ZnO has a band structure well suited for (P3HT:PCBM) bulk hetero-junction (BHJ) solar cells and presents an ohmic con- tact with PCBM. RF-magnetron sputtering is employed to integrate ZnO IL fi lms between glass/ITO cathode and organic active layer in an inverted OSCs. Different annealing treatments were carried out on the ITO-coated ZnO fi lms and their in fl uence on the photovoltaic performances of fi nal devices is investigated. We also present the structural properties and photo-physical of P3HT using PL spec- troscopy to investigate generation of photo-induced charges in P3HT with its fullerene mixtures and the relationship with the annealed ZnO IL in the effective transfer charge of P3HT:PCBM and ZnO. Using different annealing temperatures, the photovoltaic re- sponses of the OSCs devices were very similar with those inte- grating higher quality (i.e. higher annealed temperature) ZnO fi lms.

Our results show that sputtering is readily compatible with manufacturing solar cells on plastic substrates. Moreover, they give an overview of the interface role in enhancing electron transport in OCSs and better device stability. For comparison, the reference sample without ZnO layer (glass/ITO/P3HT:PCBM/PEDOT:PSS) (poly(3,4- ethylene-dioxylenethiophene):(poly-styrene sulfonate acid)) was as well investigated.

2. Experimental details

The photovoltaic cells developed here follow an inverted structure where the (P3HT:PCBM) BHJ layer is positioned between a glass/ITO/ZnO cathode and PEDOT:PSS/Ag anode. The ZnO layer was deposited by RF sputtering and inserted between the BHJ fi lm and the cathode (ITO), as shown in Fig. 1. ITO-coated glass sub- strates (CEC20S, 20 U ^{/sq., 20} 20 mm

) from Prazisions Glass and Optik GmbH were cleaned during 30 min in an ultrasonic bath with subsequently, detergent deionized water (DI water), acetone, iso- propyl alcohol and UV ozone. The ZnO target used for sputtering was purchased from Neyco Co and had a purity of 99.999%. The target diameter was 50.8 mm and the thickness was 3 mm. Sput- tering was performed using Ar plasma. Before deposition, a pre- sputtering process was operated during 5 min in order to clean the ZnO target surface. Once the sputtering conditions optimized, the ZnO target was sputtered at a constant RF power of 100 W. The ZnO layer thickness was 54 nm. The pressure during deposition was fi xed at 8 $ 10

Torr and the substrate temperature was kept below roughly 40

C. Thermal annealing were carried out of the resulting structure at 180

C and 500

C in air outside the glove-box for 1 h.

The photo-active layer was deposited by spin-coating from a P3HT:PCBM solution (1:1 wt ratio in orthodichlorobenzene (ODCB)) to form a 120 nm thick layer. The P3HT (98.5% regioregular from Sigma Aldrich Co.) and the PCBM (from Nano-C.) were used as received without further puri fi cation and the samples were annealed at 140

C for 15 min in the glove-box. After that, PEDOT:PSS CPP 105 DM (H.C. Starck, Newton, MA) was spin-coated on top of the active layer at 1550 rpm for 180 s in air and annealed at 120

C for 30 min in glove-box. A 120-nm-thick Ag top electrode was thermally evaporated onto PEDOT:PSS at a pressure of about 2 $ 10

Torr. Finally, the whole devices were annealed at 140

C for 15 min under nitrogen atmosphere. The device active area was about 9 mm

.

X-ray diffraction (XRD) analyses were recorded using a Bruker D8 Advance diffractometer (40 mA, 40 kV) equipped with a Sol-X detector and using monochromatic CuK a

radiation ( l ¼ 0.154056 nm). Atomic force microscopy (AFM) images were obtained in tapping mode on a Nanoscope Dimension 3100 from Veeco Instruments. Absorption spectroscopy was measured using a conventional UV e Vis spectrophotometer. Photoluminescence (PL) measurements were performed in order to have insight on the electronic level structure of P3HT and the energy transfer from

P3HT:PCBM to ZnO. The excitation was provided by a 532 nm line of a Nd-YAG laser. The current-voltage (J-V) characteristics of organic photovoltaic devices were measured with a HP Agilent source measurement unit under darkness and light exposure. For the latter, a solar simulator under AM 1.5G conditions (Oriel Xenon 150 W) was used. The light intensity was calibrated with a standard silicon solar cell using a light intensity of 100 mW cm

. The device performances were measured under nitrogen atmosphere for each processing condition.

3. Results and discussions

3.1. Structural characteristics of ZnO and P3HT fi lms

Fig. 2 shows XRD patterns of glass/ITO/ZnO/P3HT:PCBM/

PEDOT:PSS heterostructure, with ZnO annealed at different tem- peratures: as prepared, annealed at 180 and 500

C. All patterns show diffraction peaks corresponding to ITO, ZnO and P3HT, sug- gesting a de fi ned crystalline structure for the main layers of our device. Four ITO diffraction peaks were identi fi ed and attributed to the (211), (222), (400) and (411) re fl ections, suggesting a poly- crystalline character of this layer. Two P3HT diffraction peaks (100) and (200) were observed at 10.8

and 25.4

, respectively for both samples (ZnO as prepared and annealed). These peaks suggest a lattice spacing of 1.64 nm, associated with interdigitated alkyl chains [17,18] which shows a good crystallinity of P3HT on ZnO and ITO substrate. Furthermore, the fi rst diffraction peaks of ZnO can be clearly observed at 34.36

for sample annealed at 180

C and 500

C. For the as-deposited layer this peak is hardly visible while its intensity increases with annealing temperature. The fact that only one peak is visible indicates a preferential orientation of the ZnO structure with the c-axis perpendicular to the substrate [10,19].

Using Scherrer ’ s equation, we calculated the average grain size of the ZnO crystallites and it was 13.5 nm for ZnO annealed at 180

C and 20 nm for sample annealed at 500

C. This should have con- sequences on the growth of P3HT on ZnO and induce a modi fi cation of the molecular organization of P3HT at the ZnO/P3HT interface, which can lead to signi fi cant changes in the J-V characteristics of the OSCs [20].

3.2. Optical characterization

The UV e vis optical transmittance spectra of annealed ZnO fi lms

(Fig. A.9 in Supplementary material) were highly transparent in the

UV e vis region with a small sharp fall in transmittance at 370 nm

due to band gap absorption of ZnO, all samples as prepared and

annealed ZnO present a high transmittance in the absorption range

of photo-active layer P3HT:PCBM, which is bene fi cial for device

photovoltaic ef fi ciency. However, the change in optical gap (Eg) of

ZnO by annealing can in fl uence the charge transport at the inter-

face P3HT:PCBM/ZnO, and thus can disrupt the interfacial molec-

ular order of P3HT at the interface with ZnO. Using optical

transmission measurements, we have determined Eg of ZnO in a

glass/ITO/ZnO stack, using the Beer e Lambert relationship (Fig. A.9

in Supplementary material). We obtained a slight change in Eg

(around 3.22 and 3.25 eV) for the samples: without and with

annealing at 500

C, respectively which can be understood by stress

change in the interatomic space of ZnO, which should affect the

energy de fi cit [21]. Therefore, we can conclude that the effect of Eg

of ZnO is very minimal and ZnO annealing will mostly affect its

structural quality (Fig. 2) and its surface roughness (Fig. 5). These

structural defects should mainly affect the interfacial molecular

order of P3HT at the interface with ZnO and thus the loss of elec-

trons caused by charge recombination at defect sites. Fig. 3 shows

the optical absorption spectra of glass/ITO/ZnO/P3HT: PCBM/

PEDOT:PSS stacks integrating ZnO fi lms as-prepared and annealed at 180 and 500

C. All samples show three vibronic absorption peaks characteristic of semi-crystalline P3HT at 505, 550, and 600 nm [10]. The last fi rst two bands (at 505 and 550 nm) can be attributed to the p e p * transition, whereas the peak shoulder at around 600 nm is due to the inter-plane interactions. [22,23]. Be- sides, in case of non-annealed ZnO fi lm, the absorption is strongly improved, thus we can expect a consistently enhanced photovoltaic response. A typical optical signature of the PCBM was visible in the reference sample over the wavelength range of 320 e 400 nm. Fig. 3 shows a typical signature of ZnO layers at band gap of around 375 nm (3.3 eV) and it ’ s more accented at 500

C. It indicates a higher optical quality of these materials and are in agreement with the XRD data for both P3HT and ZnO (Fig. 2).

3.3. Photophysical characterization of P3HT:PCBM/ZnO interfaces

In order to investigate the effects of ZnO annealing on the transport properties of our devices. We have performed (J-V) cur- rent density-voltage measurements under illumination (Fig. 4).

Table 1 displays all photovoltaic parameters extracted during these

tests. For comparison, the results of device conducted without ZnO were added as well as reference. These results initially demonstrate the importance of cathode IL ZnO in the inverted photovoltaic de- vice performance. Indeed, without ZnO, the cells do not reach a suf fi cient high open-circuit voltage (Voc ¼ 0.27 V) as well as its ef fi ciency ( h ¼ 0.7%), which stagnates at a very insuf fi cient level.

Secondly, we note that ZnO annealing is essential to increase the performance of this device. The photovoltaic parameters identi fi ed in the case of device without annealing is not yet satisfactory ( h ¼ 2.2%). A marked improvement in device performance appears after annealing ZnO at 180 and 500

C. The device prepared in these conditions shows an increase of Voc (0.58 e 0.6 V). By-against, the short-circuit current density (Jsc), which respectively reached 9.9 and 10 mA/cm

for 180 and 500

C, respectively which remains stable regardless annealing, since it depends on the morphology of the active layer (P3HT:PCBM) and the mobility of charge carrier through the interface between ZnO and P3HT:PCBM [10,24]. The fi ll factor (FF) of this device sharply increases from 33% (without ZnO annealing) to 44 and 43.7% (for ZnO annealed at 180 and 500

C), respectively. Moreover, the device performances are heavily modi fi ed reaching values of ef fi ciency equal to 2.7 and 2.6%

respectively for 180 and 500

C. This fi rst stage of growth ef fi ciency Fig. 1. Inverted organic photovoltaic structure (a) Device structure. (b) Energy-level diagram of ITO, ZnO, PCBM, P3HT, PEDOT:PSS and Ag.

Fig. 2. XRD patterns of glass/ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag structures. The ZnO layer was deposited by RF sputtering: non annealed and annealed at 180

C and 500

C.

Fig. 3. Absorption spectrum of the structure glass/ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag

at various annealings of ZnO.

was accompanied by a decrease in series resistance (Rs) from 32 to 10 U ^/cm

for non-annealed and annealed at 180

C, respectively (Table 1). This decrease can be explained by the improvement in contacts and/or charge transfers between the ZnO (IL) and P3HT:PCBM fi lm leading to an increase in photovoltaic parameters Jsc, FF and ef fi ciency. It is rather interesting to observe such prog- ress in the cells by the recovery of the J-V characteristics in direct current quadrant, where we can see a S-shape for the device without annealing (Fig. 4). This phenomenon of ” against-diode ” could be induced by phase separation effects of P3HT and PCBM (or also called vertical segregation phenomenon) which would not occur in the same manner depending on the used substrates. To promote an increase of the PCBM phases at the interface with ZnO, it is important to grow P3HT chains in an orderly manner at the interface with the ZnO layer and thus help the diffusion of mole- cules of PCBM through the rich-P3HT phases. By annealing ZnO, these effects may induce a more appropriate phase separation between P3HT and PCBM, which facilitates exciton dissociation and charge transport at the cathode. This latter can explain the decrease of Rs for sample annealed at 180

C that has almost reached its fl oor value (10 U ^/cm

) and its higher fi ll factor (FF) around 44%, compared to samples without annealing. Fig. 4 shows that annealing at 180

C is quite suf fi cient for a better ef fi ciency of the cell. To have further insight on the mechanism driving this improvement in device performance by ZnO annealing. First, we analyze the device without ZnO, we note that it has a very low Voc which shows the important contribution of ZnO IL in the energy level alignment and polarity of the device (see Fig. 1b) [24,25]. The device with ZnO (IL) does not need a higher temperature to be functional and the photovoltaic performance is almost the same at 180

C and 500

C. The crystallization and the optical quality of ZnO layer annealed at 180

C is suf fi cient to improve the ef fi ciency ( h ⁾ relative to annealing at 500

C and it is noted that the electron mobility increases with increasing annealing temperatures.

To explain the difference in series resistance Rs in the solar cell at 500

C compared to that at 180

C and those despite of the high fi ll factor value (43%), we have investigated the surface morphology of ZnO using an atomic force microscope (AFM) in tapping mode for thermal annealing temperatures (180

C and 500

C). The corre- sponding AFM images are shown in Fig. 5. It is noted that the ZnO fi lm annealed at 180

C is suf fi ciently dense and continuous with a

slight surface roughness (Fig. 5a), However, for the sample annealed at 500

C, the roughness is much larger with a strongly granular aspect (Fig. 5b). This latter is con fi rmed by the root-mean- square (rms) measurements ranging from 3 nm for 180

C to almost twice about 7 nm for 500

C. Indeed, we can safely argue that the high surface roughness for the sample annealed at 500

C causes the increase of Rs (38 U ^/cm

) in the photovoltaic device compared to that annealed at 180

C where Rs is 10 U ^/cm

. The surface roughness should in fl uence the interface quality between the P3HT: PCBM and ZnO, which can promote poor charge transport and high value of the series resistance (Rs) of photovoltaic devices.

Under the effect of thermal annealing, the energy transmitted to the ZnO particles allows them to reorganize the surface to fi nd a stable state and a homogeneous structure, which helps spreading ITO and ZnO particles leading to an increase of the crystallite size (20 nm) as calculated in Section 3.1. When P3HT is deposited on the ZnO layer, its growth is on an atomic layer having crystallites of larger sizes, which also results in an increase of the crystallite size of P3HT, this should prevent the diffusion of PCBM at the ZnO interfacial layer, which may explain the lowering of the photovol- taic performance at 500

C.

PL technique has been widely used to provide basic information about the properties of energy levels within the band gap as well as the interface between layers. The PL spectra of P3HT, P3HT beside ZnO and P3HT mixture with fullerene C60 (PCBM) were measured Fig. 4. J-V characteristics of the glass/ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag structure

with various annealings of ZnO under illumination.

Fig. 5. AFM images of ZnO films grown by sputtering on ITO-coated glass substrates

and annealed at (a) 180

C and (b) 500

C.

to check if there is an interaction between different components in the excited state. Fig. 6 shows the PL spectra of P3HT, the mixture of P3HT:PCBM, PCBM, and P3HT/ZnO. The laser excitation wavelength is near the maximum absorption peak of P3HT (550 nm), whereas it is not absorbed at all by the ZnO (IL). As it is known, ZnO material has been always consider as electron transporting layer and hole blocking layer. Here, we examine the role of ZnO (IL) in the charge transport between ZnO and P3HT, and explore the energy levels responsible for the charge transport within each layer. Fig. 6 dis- plays the PL data recorded by scanning various locations of the sample, a signi fi cant decrease in PL intensity can be observed in regions where ZnO is present. Although the P3HT:PCBM BHJ fi lms present a complete quenching, the latter is very important in case of ZnO/P3HT:PCBM, which allows to conclude that the P3HT chains located near the ZnO interface can preferentially transfer electrons from the active layer (P3HT: PCBM) to the ZnO layer thus producing a photo-current and reducing the series resistance [26] for the device with and without ZnO (IL). In contrast, we observe the appearance of a slight reduction of the luminescence for the sample ZnO/P3HT (Fig. 6a). The PL intensity of P3HT was totally quenched in P3HT:PCBM thin fi lms indicating a highly ef fi cient charge transfer between P3HT and PCBM [27]. Furthermore, PL intensity was not totally quenched in the ZnO/P3HT double layer thin fi lms indicating negligible charge transfer from P3HT to ZnO. Indeed, the difference of the exciton dissociation ef fi ciency at different in- terfaces can also be explained by the different exciton lifetime obtained by PL lifetime. Most of the photo generated excitons will be separated at the interface between P3HT and PCBM and only negligible excitons will be separated at the interface between P3HT and ZnO which explains the lower quenching in PL intensity observed in ZnO/P3HT. Note, that the PL lifetime of P3HT on glass and ZnO/P3HT was studied by Lin et al [28] who found that t

ZnO

(643 ps) for ZnO/P3HT is shorter than that of the pure P3HT thin fi lm t

(735 ps). This indicates that the non-radiative process for the photogenerated excitons occurred quickly at the interface be- tween ZnO and P3HT compared to that of pure P3HT. Therefore the observed quenching in PL intensity in (ZnO/P3HT:PCBM) should be stronger than that in ZnO/P3HT, due to the role played by ZnO during the separation and transport of charge at the interface be- tween ZnO and P3HT. This con fi rms our analysis of photophysical characterization of P3HT:PCBM/ZnO by PL experiments. However, Fig. 6 shows that the PL P3HT is completely quenched and the electronic states in photo-excited P3HT are characterized by three emission peaks which can be fi tted using Gaussian function, that matches the resolved vibronic structures at 648 nm, 713 nm and 780 nm. The PL emission peak at about 648 nm (1.91 eV) was assigned to the pure electronic transition of P3HT and the peaks at around 710 nm (1.74 eV) to the fi rst vibronic band [29,30]. The PL emission in the highest wavelength region at 780 nm (1.5 eV) in- dicates an ordering in the lamellar structure of P3HT [31]. The importance of ZnO IL in energy level alignment can be seen by taking into account the following points: (i) the order and orien- tation in lamellar structure of P3HT thin fi lms, (ii) the decrease of PL intensity for device ZnO/P3HT:PCBM compared to that of ZnO/

P3HT, (iii) the vertical segregation phase which is known to occur in the P3HT/PCBM blend. The alignment of ZnO conduction band at 4.2 eV [32] with that of the lowest unoccupied molecular orbital (LUMO) energy level of acceptor PCBM molecules ( 3.7 eV [31]) during the charge transport and therefore an effective charge transfer from P3HT:PCBM active layer to ZnO IL. These results support the key role played by ZnO as electron transporting layer and hole blocking layer. Many authors reported a vertical-phase segregation of the P3HT:PCBM composite fi lm in which the elec- tron acceptor material (PCBM) moves towards the cathode ZnO/

ITO, while the electron donor material (P3HT) diffuses towards the Table 1

Summary of the photovoltaic parameters of inverted OPVs integrating ZnO cathode IL no annealed and annealed at different temperatures (180

C and 500

C).

Substrate Sample Voc (V) Jsc (mA/cm

) FF (%) h (%) R

( U cm

)

Rigid substrate no annealing 0.58 11.8 33 2.29 32

180

C 0.62 9.9 44 2.71 10

500

C 0.59 10.0 43 2.60 38

No ZnO 0.27 7.4 31 0.71 21

Plastic substrate no annealing 0.34 7.9 38 1.07 76

180

C 0.55 8.1 48.7 2.16 22

Fig. 6. a) PL spectra of P3HT, P3HT:PCBM, P3HT/ZnO, P3HT:PCBM/ZnO and PCBM films,

PL spectra of ZnO and P3HT. b) Gaussian band fits of P3HT/ZnO. The measurements

were performed using an excitation wavelength of 532 nm in the range from 500 to

900 nm.

anode (Ag/PEDOT:PSS) [10,24]. Recently Vaynzof. et al. [24] have shown that the formation of the rich-PCBM 1 e 2 nm in ZnO/

P3HT:PCBM interface, while the same amount of P3HT-rich layer was found at the air interface. This segregation, which has a vertical concentration gradient, allows i) an ef fi cient exciton dissociation and ii) an ef fi cient charge transport associated with a strong elec- trode selectivity. Thus, the annealing of ZnO improves the contact between P3HT:PCBM and ZnO, and the molecular ordering of P3HT:PCBM near the interface with ZnO. This allows to generate exciton which are created in the PL process and have a limited life which should diffuse through a photo-induced charge transfer process and fi nally dissociate into free electrons and holes. This latter is a part of an ef fi cient photovoltaic conversion process and remains valid in a better morphology of P3HT and PCBM blend.

3.4. Stability of OPVs

Now, we describe the stability of our device. The reverse the OPVs device allows to gain stability through the device geometry towards both oxygen and water, since the electron collection is dif fi cult because it implies motion of electron carriers to high en- ergy levels (LUMO). The hole carriers do not exhibit the same problems since, silver is used as an electrode. At fi rst, we are going to compare the normal and inverted devices, as shown in Fig. 7a.

The following structures were, normal: Substrate/ITO/PEDOT:PSS/

P3HT: PCBM/ZnO/Al, and inverted: Substrate/ITO/ZnO/P3HT:

PCBM/PEDOT:PSS/Ag. Both architectures give quite comparable performance. However, for the normal device, we have a pro- nounced S-shape of the J-V, with a smaller slope at high voltage with respect the inverted structure. The series resistance of normal structure is higher (45 U ^/cm

) than that of the inverse structure (10 U ^/cm

). Kuwabara et al. [33] have found that the S-shape strength characteristic depends on the crystallinity of the ZnO IL, and it vanishs when the conductivity of ZnO IL is increased. Thus, we can argue that the ohmic contact between the active layer and ZnO IL is better in case of inverted structure, since ZnO IL can prevent the generation of leakage current at the interface P3HT:PCBM/ITO. The inverted OPVs with ZnO ILs have been found to greatly improve the photovoltaic performance compared to those without ZnO IL and those with ZnO IL on the normal device.

Therefore, inverted structures should present a better performance with best stability. In this part of this work a special con fi guration in air stability measurements in a lifetime experience is presented for all devices and it was tested on the roof of our laboratory in weather condition of the city of Strasbourg in FRANCE (average temperature was 17.7

C and humidity of 64%) in the period January, the results are shown in Fig. 7b. Since the degradation of polymer solar cells strongly depends on the exposure to oxygen, water and tempera- ture, in our experience, we simply evaluate the lifetime of our de- vice and check the contribution of ZnO as IL. All the cells were set to air in dark to avoid degradation by light, enabling us to expose the cells to the same degradation conditions. It is found that the photovoltaic ef fi ciency is more stable after annealing (180

C and 500

C), by against device with non-annealed ZnO IL, where a signi fi cant decrease in photovoltaic ef fi ciency within 48 h of experience, due to a decrease of Jsc. This can be explained by mo- lecular oxygen, which can diffuse mainly through microscopic pinholes between the ZnO (which is fl at, see AFM surfaces of ZnO in Fig. 5 and active layer), which causes a circular oriented incorpo- ration and degradation (inhomogeneous).

Finally, these results suggest that even though the ZnO fi lms have better structural and optical properties due to the high tem- perature annealing processes (500

C), the photovoltaic perfor- mances are mainly depending on the interface quality between ZnO and the active layer. Thus, the use of softening temperature

(180

C), which is plastic-compatible, can lead to encouraging photovoltaic responses because of the fl at and low-resistance Fig. 7. a) J-V characteristic for normal and inverted structures. b) Lifetime curves of efficiency stability for devices set to air in the dark condition.

Fig. 8. J-V characteristics of the plastic substrate/ITO/ZnO/P3HT:PCBM/PEDOT:PSS/Ag

structure without and with annealing ZnO at 180

C, under illumination.

character of the ZnO/P3HT:PCBM interface. Thus, we can conclude that similar performances should be obtained when the cells are deposited not on rigid glass but on fl exible polymer substrates such as PEN (Fig. 8 and Table 1). We fi nally managed to manufacture a plastic cell under the same ZnO condition deposition and annealing upstream of the manufacture of the OPVs cell. However, it has the same photovoltaic performance than previously prepared on glass substrate (Fig. 6). Furthermore, we fi nd the same J-V characteristics as before, for both non-annealing and after limit annealing for plastic substrate at 180

C. We have also S-shape on the J-V curves for the device with non-annealed ZnO, which is re fl ected by the series resistors respectively (76 and 22 U ^/cm

), for non-annealed and annealed at 180

C in plastic device. These J-V characteristics recorded under illuminated conditions suggest the crucial role of metal oxide electron-selective layers in the operating mechanisms of bulk-heterojunction polymer-fullerene plastic solar cells. In inverted structures containing ZnO fi lms at the cathode contact, the morphology of the ZnO/P3HT:PCBM interface has to be carefully controlled to enhance photovoltaic performances of the OSCs devices.

4. Conclusion

In conclusion, we have reported the role of ZnO fi lms deposited by RF magnetron sputtering on the photovoltaic properties of inverted OSCs. Sputtered ZnO was shown to be a promising inter- facial material, also compatible with plastic substrate processes.

The in fl uence of thermal annealing on the glass/ITO/ZnO electrode of OSCs devices and on the interface quality between ZnO and the active layer was studied. This treatment has an important impact on the transport, recombination and photovoltaic properties the cells. It has been demonstrated, that although the crystalline structure of the ZnO layer was improved continuously up to annealing temperatures of 500

C. The softening temperature (less than 180

C) on both rigid and fl exible substrates leads to similar photovoltaic responses of OPVs cells with higher lifetime stability.

Acknowledgement

This work was supported by the French Ministry of Research and Education. The authors are very grateful to T. Heiser, A. Dinia and S.

Colis for fruitful discussions and N. Zimmermann and J. Bartringer for technical assistance.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://

dx.doi.org/10.1016/j.orgel.2016.09.024.

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