Cu(In,Ga)(S,Se)2 solar cells and modules by electrodeposition

Thin Solid Films 480–481 (2005) 526 – 531 www.elsevier.com/locate/tsf

Cu(In,Ga)(S,Se)2 solar cells and modules by electrodeposition S. Tauniera, J. Sicx-Kurdia, P.P. Granda, A. Chomonta, O. Ramdania, L. Parissia, P. Panheleuxa, N. Naghavia, C. Huberta, M. Ben-Faraha, J.P. Fauvarquea, J. Connollya, O. Roussela, P. Mogensenb, E. Mahe´b, J.F. Guillemolesa, D. Lincota, O. Kerreca,* a

Laboratoire Commun EDF-CNRS/ENSCP, Plateau CISEL, 6 Quai Watier, 78401 Chatou cedex, France b Saint-Gobain Recherche, 39, Quai Lucien Lefranc, 93303 Aubervilliers Cedex, France Available online 22 January 2005

Abstract The CIS by electrodeposition (CISEL) project between Electricite´ de France (EDF), Centre National de la Recherche Scientifique (CNRS)/Ecole Nationale Supe´rieure de Chimie de Paris (ENSCP) and Saint-Gobain Recherche (SGR) aims at developing a low-cost electrodeposition process for Cu(In,Ga)(S,Se)2 (CIGS) solar cells. The process is characterized by two main steps: (i) deposition of the precursor film and (ii) thermal annealing. This process enables the preparation of a large range of sulfur containing absorbers, with S/(S+Se) atomic ratio from 0% to more than 90%. The films are single phase over the whole composition range. The influence of Sulfur content on the microstructure has been shown with grain sizes decreasing with increasing sulfur content. Efficient solar cells can be obtained from all the different precursor compositions, with efficiencies of over 10% on lab cells on sulfur-rich absorbers, and 6–7% on 30
30 cm2 devices. The homogeneity of 15
15 cm2 substrates is also discussed. D 2004 Elsevier B.V. All rights reserved. Keywords: Cu(In,Ga)(S,Se)2 solar cells; Electrodeposition; Sulfur

1. Introduction For the PV industry to reach a significant fraction of the electricity market, price per produced kWh must fall and the size of the industry must expand significantly. One of the most promising strategies for lowering PV costs is the use of thin-film technologies in which thin films of photoactive materials (typically b5 Am in thickness) are deposited on inexpensive large-area substrates like for example window glass [1,2]. Chalcopyrite compounds Cu(In,Ga)(Se,S)2 (CIGS) have led to the highest laboratory efficiencies for thin film solar cells (N19%) [3] and CIGS modules have been successfully produced on an industrial scale [13–15]. Although CIGS compounds are comparable in performance and stability to existing crystalline silicon devices, their market share is still very small (b1%) [4]. At present, * Corresponding author. Tel.: +33 1 3087 7135; fax: +33 1 3087 8565. E-mail address: o`livier.kerrec´@edf.fr (O. Kerrec). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.11.200

high cost vacuum based techniques such as coevaporation and sputtering, are used to deposit the CIGS absorbers which is reducing the competitiveness of this technology. There is thus great interest in developing alternative, more cost effective, techniques based on low-temperature, nonvacuum techniques deposition processes such as electrodeposition. Electrodeposition is well suited for large-scale industrial processes, having a low energy consumption and low capital investment [2,5]. Electrodeposition has already been shown to be a promising approach for the production of efficient, low-cost CIGS solar cells. Efficiencies in the range of 6–7% had been reported for the so-called one-step electrodeposition route [6]. The CIS by electrodeposition (CISEL) project was launched in December 2000. The partners involved are Electricite´ de France (EDF), the Centre National de la Recherche Scientifique (CNRS), Ecole Nationale Supe´rieure de Chimie de Paris (ENSCP) and Saint-Gobain Recherche (SGR), with financial support from the Agence de l’Environnement et de la Maıˆtrise de l’Energie (ADEME)

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performed with a LEO 440 microscope equipped with a tungsten filament gun. The efficiencies of the devices were measured on a continuous, class A, 10
10 cm2 simulator. The efficiency values of the laboratory cells were corrected for the spectral mismatch of the CIGS with respect to the c-Si, which served to calibrate the light source, using spectral response measurements.

3. Process The electrolytic step is carried out on 5
5 cm 2 substrates. The precursor elements are first dissolved in an aqueous acidic bath, and then co-deposited at the glass/Mo electrode, according to the following cathodic reaction, exemplified for CuInSe2 [8]: Fig. 1. SEM cross-section of a CuInSe2 precursor layer after electrodeposition. The precursor film is smooth and dense.

[4,7]. In January 2003, a joint EDF and CNRS/ENSCP Thin-Film Solar Cells Laboratory currently employing about 25 people was set up at Chatou near Paris. One of the principal objectives of the CISEL project is the transfer of the process from the laboratory scale cells to the intermediate 30
30 cm2 modules. In this paper, we present the first results obtained within the joint laboratory, in terms of laboratory cells and discuss critical aspects of the up-scaling of the process, such a the control of film composition, quality and homogeneity.

Cu2þ þ In3þ þ 2H2 SeO3 þ 13e þ 8Hþ YCuInSe2 þ 6H2 O

2. Experimental

This is an ideal situation but in practice deviations from the stoichiometric composition are observed. It is thus essential to carefully control the composition and homogeneity of the precursor layer. The morphology is also important: the layer has to be dense, and smooth, as shown in the SEM cross-section (Fig. 1). Although deposited at room temperature and in aqueous medium, this precursor layer is microcrystalline, with grain sizes ranging from 5 to 50 nm. This is illustrated in the diffraction patterns obtained by TEM, which clearly show discrete diffraction spots, instead of the diffuse rings expected for more poorly crystallized materials (Fig. 2).

The CIGS solar cells were deposited on Mo-coated soda-lime glass. The Mo coated substrates were prepared by sputtering by Saint Gobain Recherche, either on 0.09 m2 or 20 m2 substrates. The CISEL process differs from the more classical vacuum based processes mainly by the way the absorber is prepared. This is the reason why the results presented in this paper focus essentially on absorber preparation, i.e. electrodeposition of the precursor layer followed by thermal annealing. The subsequent relatively standard process steps were performed in the joint laboratory, that is the deposition of a CdS buffer layer by chemical bath deposition, and deposition of the ZnO window layer. These techniques are routinely used on substrates sizes from 2.5 to 900 cm2. The thickness and composition measurements were performed by X-ray fluorescence spectroscopy (XRF) using a Fischer XAN Spectrometer (50 kV, 2 mm diameter collimator). The X-ray diffraction (XRD) h–2h diagrams were recorded on a Rigaku-Geigerflex diffractometer, using a Co anode. The TEM studies were performed on a Philips Tecnai F20 STEM microscope, and the SEM images were

Fig. 2. Typical TEM micro-diffraction pattern obtained with our CuInSe2 precursor layers. The actual grain size ranges from 5 to 50 nm.

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Fig. 3. SEM cross-section of CIS film after thermal annealing. Illustration of a bsuitableQ microstructure, with large, densely packed grains.

In order to achieve the desired semi-conducting properties, the precursor films need to be thermally annealed. Under well-controlled conditions, much larger grains are obtained, as illustrated on the SEM cross-section (Fig. 3). This indicates strong recrystallization has taken place during the annealing step. However, we observe that the layers are not always as dense as shown in Fig. 3. In some cases, the layer can become porous after annealing (Fig. 4), which can be detrimental to the cell performance. Efficiencies higher than 8% have nevertheless been measured with such films. Our process has been extended to prepare quaternary as well penternary alloys, with various amounts of Ga and S in the film.

Fig. 5 shows for instance the preparation of sulfur containing absorbers over a large range, by just controlling the sulfur content in the precursor film. The atomic ratio S/(S+Se) after annealing can be varied from 0% to more than 90%. This S content has a clear impact on the microstructure as shown in Fig. 6. The closed triangles represent the evolution of the (112) planes spacing, as a function of the S content. The almost linear decay indicates that we have a true solid solution existing throughout the compositional range, for the quaternary alloy CuIn(Se,S)2. We have also tried to evaluate the crystallite sizes from the diffraction patterns and lines broadening. The estimated values are plotted as open diamonds. The interesting result

Fig. 4. SEM cross-section of CIS film after thermal annealing. Illustration of a bpoorQ microstructure, with many voids leading to a porous layer.

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Table 1 2 examples of bbest laboratory cellsQ (0.1 cm2) obtained so far in the joint laboratory Voc (mV) J sc (mA/cm2) FF Eff. (%)

Fig. 5. Sulfur content of the absorber layer measured by XRF, after the annealing step, as a function of the sulfur content in the precursor (before annealing).

is that the average crystallite size tends to decrease with the increase in S content. The average size is close to 800 nm at low S content and it ends-up at 100 nm at the S-rich side.

4. Devices The best performances obtained so far in the joint laboratory in Chatou have been efficiencies around 10%, on 0.1 cm2 cells. Two examples of good cells are shown in Table 1, at respectively 10.5% and 9.9%. Previously, efficiencies in the range of 6–7% have been reported with the so-called one-step electrodeposited route [6,9]. The NREL group has also demonstrated an efficiency of 15.4%. However, the authors used a hybrid approach consisting of the badjustmentQ of the electrodeposited precursor by evaporating 300 nm Ga and 700 nm In [5]. More recently, 900

3.4

d (112) (A)

Cell b 692 21 68 9.9

the complementary layer of In evaporated was reduced to 200 nm, leading to 9.4% cells [10]. Other groups also reached efficiencies around 9% on metallic substrates by sequential electrodeposition of the Cu, In and Se elements followed by annealing [11,12]. The band gap of the absorbers, as determined from spectral response measurements, is approximately 1.47 eV. The Voc’s obtained compare well with those of the best chalcopyrite thin-film cells with the same gap value [7]. The rather low value of the current is associated with losses in the red, as shown by the spectral response measurements (Fig. 7). Additional studies are under way in order to address the loss mechanisms in these devices, to determine for instance the main recombination mechanisms. In parallel to this bResearchQ activity on laboratory cells, important work has been done on up-scaling the process, the main objective being the production of a 30
30 cm2 prototype module. Currently, all the process steps have been implemented on 30
30 cm2, except for the modularization part (interconnection and encapsulation). The required processing equipment have when necessary been developed in house, and the quality control of the process has also been improved at each step as shown by the statistical studies that we have performed on the different production steps. As an illustration, Fig. 8 shows an histogram of the thickness measured by XRF on 150 electroplated precursor, with 30
30 cm2 area. The standard deviation is around 10% relative to the average, which is quite a good result for this

800

Crystallite size (nm)

3.35

700

500 3.25 400 300

3.2

200 3.15 100 3.1 0

10

20

30

40

50

60

70

80

90

0 100

1

0.8

EQE (%)

600

3.3

Crystallites (nm)

d (112) (Angstrom)

Cell a 718 22.3 66 10.6

0.6

0.4

CuIn(Se,S)2 (9.9%) ZnO CuIn(Se,S)2 (10.5%)

0.2

% S/(S+Se) 0

Fig. 6. ( , left axis) d spacing of the (112) planes as determined by XRD, as a function of the final Sulfur content in the absorber; ( R , right axis) average crystallite size as determined by the XRD (112) line full width at half maximum (FWHM), as a function of the sulfur content in the annealed absorber.

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Wavelength (µm) Fig. 7. External quantum efficiency of the brecordQ laboratory cells. The transmission spectrum of the ZnO window layer is also shown.

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S. Taunier et al. / Thin Solid Films 480–481 (2005) 526–531 60%

40%

(a) (b) (c)

Frequency (%)

Frequency (%)

50%

30%

20%

10%

40% 30% 20% 10%

0% 0

0.5

1

1.5

2

2.5

3

3.5

0%

Thickness (µm)

0

1

Fig. 8. Thickness distribution of one hundred fifty 30
30 cm electroplated CIS precursors measured by XRF. The average value is hti=2.0 Am and the standard deviation is r=0.20 Am.

parameter. The morphology was also very close to that obtained on smaller substrates. The lateral variations on the precursor composition are less than 5% (relative) on a single plate. This is illustrated in Fig. 9 showing the Cu content distribution measured by XRF on a 30
30 cm2 precursor. In this particular case, the standard deviation amounts to only 1.3% of the average value, which is evidence of the good precursor homogeneity. Up-to-now, the best efficiencies obtained on 30
30 cm2 and 15
15 cm2 plates are in the range 6–7%. However, these are average efficiencies measured on 0.4 cm2 cells at different locations on a single large area circuit. Since we are currently working on the implementation of the interconnection steps. The efficiency distributions of three different 8
15 cm2 devices are shown in Fig. 10. These efficiencies were measured on 0.4 cm2 individual cells, with a class A simulator, without any correction for the spectral mismatch. The homogeneity of the device increases from

2

3

4

5

6

7

8

Efficiencies (%)

2

Fig. 10. Histogram of efficiencies on three different 8
15 cm2 devices. The efficiencies are measured on a class A simulator, without any correction for the spectral mismatch effect, on 0.4 cm2 cells. The homogeneity of the device decreases from (a) to (c). The detailed data are given in Table 2.

(c) to (a), mainly due to improvements in the electrodeposition process. Typically, between 20 and 60, single 0.4 cm2 cells were tested on each 15
15 cm2 or larger plate. Table 2 summarizes the main statistics of the three devices of Fig. 10.

5. Conclusion The CISEL project has now entered a new development phase, with the building of a joint laboratory between EDF and the CNRS/ENSCP. The objective is to demonstrate high efficiencies on laboratory cells, as well as a prototype 30
30 cm2 module using the CISEL process, namely the preparation of a precursor film based on electroplating, followed by a thermal annealing step. This process enables us to prepare a large range of sulfur containing absorbers, with %S/(S+Se) atomic ratios from 0% to more than 90%. Efficiencies over 10% have been obtained on lab cells, and 6–7% efficiencies on 30
30 cm2 cells. The performance of 15
15 cm2 and larger devices was assessed from the measurement of many 0.4 cm2 individual cells. In this article, we also showed a few statistics on the efficiency of 8
15 cm2 devices. Table 2 Statistics for three larger-area devices showing the maximum values efficiency, Voc, J sc and FF, as well as the average values 2

Fig. 9. Cu content distribution in a 30
30 cm2 electrodeposited precursor, as measured by XRF (normalized values). The standard deviation is only 1.3% relative to the average Cu content, which illustrates the good homogeneity of the precursor film.

Size (cm ) Number of cells tested Maximum efficiency (%) Average efficiency (%) Standard deviation (%) Average Voc (mV) Average J sc (mA/cm2) Average FF (%)

(A)

(B)

(C)

8
15 28 6.4 5.9 0.3 574 17.6 58

8
15 28 6.6 5.7 0.6 592 19.2 50

15
15 56 5.7 4.5 1.5 528 16.7 49

The size of the actual cell measured was 0.4 cm2.

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In the future, our main focus will be on improving the understanding of the process and device properties, and also to increase the efforts towards industrialization, especially in terms of reliability and yield. Aknowledgments This work was performed with the financial support of ADEME. The authors would like to thank the Materials and Mechanics of Components (MMC) Department of EDF for its contribution to the characterization of the materials: L. Legras and M. Lamirand for TEM studies, P. Todeschini and P. Le Bec for XRD measurements, and M. Mahe´ for SEM studies. References [1] K.L. Chopra, P.D. Paulson, V. Dutta, Prog. Photovolt. Res. Appl. 12 (2004) 69. [2] K. Zweibel, Sol. Energy Mater. Sol. Cells 59 (1999) 1. [3] K. Ramanathan, M.A. Contreras, C.L. Perkins, S. Asher, F.S. Hasoon, J. Keane, D. Young, M. Romero, W. Metzger, R. Noufi, J. Ward, A. Duda, Prog. Photovolt. Res. Appl. 11 (2003) 225.

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