MoO3/Ag/MoO3 anode in organic photovoltaic cells: Influence of the presence of a CuI buffer layer between the anode and the electron donor

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MoO3/Ag/MoO3 anode in organic photovoltaic cells: Influence of the presence of a CuI buffer layer between the anode and the electron donor

M. Makha, L. Cattin, Y. Lare, L. Barkat, M. Morsli et al.

Citation: Appl. Phys. Lett. 101, 233307 (2012); doi: 10.1063/1.4769808 View online: http://dx.doi.org/10.1063/1.4769808

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MoO

₃

/Ag/MoO

₃

anode in organic photovoltaic cells: Influence of the presence of a CuI buffer layer between the anode

and the electron donor

M. Makha,^1,2L. Cattin,³Y. Lare,^1,a)L. Barkat,⁴M. Morsli,⁵M. Addou,²A. Khelil,⁴ and J. C. Berne`de^1,b)

1Universite de Nantes, MOLTECH-Anjou, CNRS, UMR 6200, 2 rue de la Houssinie`re, BP 92208, Nantes F-44000, France

2Laboratoire Optoelectronique et Physico-chimie des Materiaux, Universite Ibn Tofail, Faculte des Sciences, BP 133 Kenitra 14000, Morocco

3Universite de Nantes, Institut des Materiaux Jean Rouxel (IMN), CNRS, 2 rue de la Houssinie`re, BP 32229, 44322 Nantes cedex 3, France

4Universite d’Oran, LPCM2E, Oran, Algerie

5Universite de Nantes, Faculte des Sciences et des Techniques, 2 rue de la Houssinie`re, BP 92208, Nantes F-44000, France

(Received 8 November 2012; accepted 19 November 2012; published online 7 December 2012) MoO₃/Ag/MoO₃ (MAM) multilayer structures (layers thickness 20 nm/10 nm/35 nm) are used as anode in CuPc/C₆₀/Alq₃/Al organic photovoltaic cells. The averaged transmittance (400 nm-800 nm) of these MoO₃/Ag/MoO₃ multilayer structures is 70%62% and their sheet resistance is 3.5 61.0X/sq. When these multilayer structures are used as anode, the power conversion efficiency of the MoO₃/Ag/MoO₃/CuPc/C₆₀/Alq₃/Al cells is around 1%, this efficiency is increased of 50% when a thin CuI film (3 nm) is introduced at the interface between the anode and the organic film. This improvement is attributed to the templating effect of CuI on the CuPc molecules.V^C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4769808]

The significant and continuous progresses of the power conversion efficiency (PCE) of the organic photovoltaic cells (OPVCs) performances demonstrate that OPV cells are a potential avenue to low cost and flexible next generation solar cells.¹Some years ago, Peumans and Forrest shown that high efficiency organic photovoltaic cells can be grown by vacuum deposition using double-heterostructure copper phthalocyanine/C60.²The fabrication of OPV cells by vacuum deposition of small molecules presents the advantage of simple fabrication of multilayer devices and straightforward control of the layers thickness. Afterward, it was shown that the introduc- tion of a thin MoO3 anode buffer layer (ABL) between the anode, usually indium tin oxide (ITO), and the electron donor (ED) allows improving significantly the PCE and the stability of the OPVCs, through an improvement of the band matching at the interface ITO/ED.^3,4More recently, the PCE of OPVCs was improved by templating of the organic layer. For instance, CuI was shown as a very efficient template for CuPc, by changing CuPc molecules orientation from perpendicular to the plan of the substrate to parallel to it.⁵

On the other hand, the need of transparent conductive coatings is increasing continually due to numerous applica- tions of these coatings in many optoelectronic devices. ITO is the current choice. However, indium being scarce makes ITO expensive. Moreover, an increasing interest concerns the use of flexible substrates for optoelectronic devices and ITO films are well known to perform quite poorly under repeated bending.⁶ As a consequence, there is a need for

alternative transparent conductive electrode. Among the possible alternatives, oxide/metal/oxide multilayer structures were shown as promising. In order to achieve good conductivity and transmittance, a very thin silver film sandwiched between two oxide films offers a reasonable solution.⁷ In a recent publication, after optimization of the thicknesses of the various layers constituting the structures, we showed that it is possible to obtain structures MoO₃/Ag/MoO₃ (MAM) applicable as transparent electrode in optoelectronic devices.⁸ In the present work, we show that it is possible to cumulate the advantages of the MoO3and the CuI ABLs by recovering the MAM structure with a CuI ABL. Therefore, the anode of the multi-heterojunction OPVCs studied in the present work was MAM, MAM/CuI, and ITO for control. The averaged transmittance and sheet resistance of these ITO anodes were 90% and 20X/sq., respectively.

Before thin films deposition, the substrate was scrubbed with soap, rinsed with distilled water, dried and next placed in the vacuum chamber (10⁴Pa). The different films were deposited sequentially onto the substrate by sublimation or evap- oration. The MAM structures were grown following the process described in Ref.8, that is to say the structures were as follow: substrate/MoO₃ (20 nm)/Ag (10 nm)/MoO₃ (35 nm).

According to the works of Chen andet al.,⁵we chose to work with a CuI layer thick of 3 nm, while the thickness of the CuPc was 35 nm, that of C₆₀was 40 nm and that of bathocuproine (BCP) 9 nm³. The effective area of each cell was 0.16 cm². The thin films thicknesses were estimatedin situusing a quartz monitor. Finally, the cell arrangement was glass/anode/

CuPc(35 nm)/C₆₀(40 nm)/BCP(9 nm)/Al(100 nm).

The characteristics of the photovoltaic cells were measured using a calibrated solar simulator (Oriel 300 W) at

a)Permanent address: Universite de Lome, Faculte Des Sciences, Laboratoire sur l’energie solaire, BP 1515 Lome, Togo.

b)Author to whom correspondence should be addressed. E-mail: jean-christian.

[email protected].

0003-6951/2012/101(23)/233307/3/$30.00 101, 233307-1 VC2012 American Institute of Physics APPLIED PHYSICS LETTERS101, 233307 (2012)

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100 mW/cm²light intensity. Measurements were performed at an ambient atmosphere. All devices were illuminated through transparent conducting oxide (TCO) electrodes.

Also, the transmission spectra and the sheet resistance of the transparent electrodes were measured following techniques described previously.⁸

Typical J-V curves are shown in Figure 1 for OPVCs using MAM, MAM/CuI, and ITO as anode. The average values obtained with 5 OPVCs are given in the TableI. It can be seen that for some runs, as shown in Figure1, the CuI ABL allows achieving performances nearly equals, whatever the transparent electrode is. However, the averaged values obtained from a wider sampling show that MAM/CuI anodes allow to approach performances obtained with ITO/CuI, without being however able to equal them. In the case of bare MAM anode, the performances are significantly smaller (TableI).

The influence of the properties of the anode was checked through the use of MAM structures with different conductivity and transmittance. As a matter of fact, the properties of the MAM structures depend strongly on the Ag film thickness.⁸ When the Ag film is thick of 10–11 nm, the transmittance of the electrodes is maximal while they are conductive, which is to say the average transmittance (350 nm to 800 nm) is 70%62% and its sheet resistance is 3.561.0X/sq. For an Ag film thick of only 8 nm that is to say just at the percolation threshold of the metal film, the average transmittance is 69%62%, while its sheet resistance is 1062.0X/sq. These differences involve different performances for the OPVCs using these electrodes. The PCE of the devices using the MAM structures with the thinner Ag film are only half that of optimal MAM structure. The degradation of the performances is mainly due to a decrease of Jsc and fill factor (FF).

In order to investigate the effects of CuI on the performances of the OPVCs, the ITO/CuI/CuPc, MAM/CuI/CuPc, and MAM/CuPc structures have been characterized by UV-Vis spectroscopy and X-ray diffraction. As shown in Figures 2 and3, the presence, or not, of CuI onto the MAM structure modifies strongly the structural and optical properties of the CuPc layer deposited onto the electrode.

As expected, the X-ray diagram of CuPc depends strongly on the presence, or not, of CuI. A peak centred in 2h¼6.8is visible in the case of MAM anodes, while a peak centred in 2h¼27.8 is visible in the case of MAM/CuI, which situated at 6.8 having completely disappeared. It should be noted that two features corresponding to the very thin CuI film are visible. The peak situated at 2h¼6.8 can be attributed to the (200) direction of thea-CuPc, which situated at 27.8 corresponds to an interlayer separation of 0.32 nm. Therefore, as already shown in the case of ITO substrate, these results indicate that onto CuI the CuPc molecules lie down parallel to the plan of the substrate, while onto MoO₃, they stand up perpendicular to the substrate.⁵

In a similar way, it can be seen in Figure3that the optical density of the CuPc film depends strongly on the anode configuration. It is increased, mainly in the region of the peak situated at around 695 nm. When deposited onto CuI

FIG. 1. Typical I-V characteristics, under AM 1.5, of OPVCs deposited at the same time, but with different anodes.

TABLE I. Typical parameters of organic solar cells with different anode buffer configurations.

Anode Voc (V) Jsc (mA/cm²) FF (%) g(%)

MAM 0.40560.010 4.9560.2 52.562.0 1.0760.15 MAM/CuI 0.49560.010 6.0060.2 52.562.0 1.5660.15 ITO/CuI 0.50560.010 6.9560.2 48.3362.0 1.7360.15

FIG. 2. X-ray diffraction diagrams of CuPc thin films deposited onto ITO/MoO3(—) and ITO/CuI (----).

FIG. 3. Optical density of CuPc thin films deposited onto ITO/MoO3(—) and ITO/CuI (---).

233307-2 Makhaet al. Appl. Phys. Lett.101, 233307 (2012)

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ABL, there is a broadening of 30 nm and a red shift of 5 nm of the second peak. As a matter of fact, these results confirm those obtained with ITO anode. Indeed, the results presented in the TableIare completely coherent with the specific properties of the diverse structures that we have just studied. So the highest value of Voc achieved when the CuI ABL recov- ers the anode can be explained by the CuPc molecule orientation induced by CuI. It is known that Voc increases with the difference LUMO_EA-HOMO_ED, where LUMO_EA is the lowest unoccupied molecular orbital of the electron acceptor and HOMO_ED is the highest occupied molecular orbital of the electron donor. It has been shown that the HOMO of organic thin films is modified by the molecules orientation.

In the case of CuPc, it varies from 4.75 eV to 5.15 eV when the molecules orientation changes from perpendicular to the plan of the substrate to parallel to it.⁹Therefore, the higher value of Voc obtained with CuI than with MoO₃as ABL can be attributed to the larger value of LUMO_EA-HOMO_ED, the CuPc molecules being parallel to the CuI substrate and perpendicular to the MoO₃substrate. In the same way, in the case of the MAM anode, the increase of Jsc measured when the anode is covered with CuI can be attributed to the increase of the absorption of the MPc films deposited onto CuI, this increase being itself due to the modification of the orientation of the MPc molecules. In the case of the anode covered by CuI, Jsc is higher with the ITO anode because of its higher transmittance, while the higher FF value obtained with MAM anode can be attributed the high conductivity of these anodes. About the different performances achieved with cells using MAM/CuI anodes with different Ag thickness, it is clear that the differences are mainly due to conductivity difference of the MAM structures. The series resistance Rs of the diode is directly related to the conductivity of the electrode. Here, Rs change from 5X/sq. to more than 30X/sq. when the Ag thickness decreases from 10 nm to 8 nm.

As a conclusion, it can be said that if, actually, MoO₃/Ag/

MoO₃ multilayer structures, when optimized, appear to be

possible substituent to ITO in OPVCs, we show that it is desir- able to introduce a thin buffer layer between the anode and the electron donor to achieve power conversion efficiency of the same order as those obtained with ITO anode. The CuI ABL deposited on these structures acts in the same way as when it is used on ITO. It modifies the orientation of the CuPc molecules from perpendicular to the plan of the substrate when deposited onto MoO₃to parallel at this plan when deposited onto CuI. The small difference of PCE compared with the ITO anode is due to the fact that the transmission range of the MoO₃/Ag/MoO₃ multilayer structures is less wide than that of ITO. So, we are working at present on the realization of structures with a transmission range widened.

This work has been financially supported by the OTC- 2012-2013 Project (Nanorgasol network of Mission Ressour- ces et Competences Technologi-ques du CNRS FRANCE”) and the France-Maroc contract: PHC Volubilis No. MA/10/

228 and the Hassan II Academy of Science and Technology (Morocco).

1F. C. Krebs, “Fabrication and processing of polymer solar cells: A review of printing and coating techniques”,Sol. Energy Mater. Sol. Cells93, 394 (2009).

2P. Peumans and S. R. Forrest, “Very-high-efficiency double-heterostructure copper phthalocyanine/C60photovoltaic cells,”Appl. Phys. Lett. 79, 126 (2001).

3L. Cattin, F. Dahou, Y. Lare, M. Morsli, R. Tricot, S. Houari, A. Mokrani, K. Jondo, A. Khelil, K. Napo, and J. C. Berne`de,J. Appl. Phys. 105, 034507 (2009).

4N. Zhou, X. Guo, R. P. Ortiz, S. Li, S. Zhang, R. P. H. Chang, A. Facchetti, and T. J. Marks,Adv. Mater.24, 2242 (2012).

5Y.-C. Chen, P.-C. Kao, Y.-C. Fang, H.-H. Huang, and S.-Y. Chu,Appl.

Phys. Lett.98, 263301 (2011).

6C. D. Willams, R. O. Robles, M. Zhang, S. Li, R. H. Baughman, and A. A.

Zakhidov,Appl. Phys. Lett.93, 183506 (2008).

7C. Tao, G. Xie, C. Liu, X. Zhang, W. Dong, F. Meng, X. Kong, L. Shen, S. Ruan, and W. Chen,Appl. Phys. Lett.95, 053303 (2009).

8D.-T. Nguyen, S. Vedraine, L. Cattin, P. Torchio, M. Morsli, F. Flory, and J. C. Berne`de,J. Appl. Phys.112, 063505 (2012).

9W. Chen, H. Huang, S. Chen, Y. L. Huang, X. Y. Gao, and A. T. S. Wee, Chem. Mater.20, 7017 (2008).

233307-3 Makhaet al. Appl. Phys. Lett.101, 233307 (2012)