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"ECS Transactions - Boston, MA" Volume 16, "Photovoltaics for the 21st Century 7"

to be published in September, 2011

Fabrication of Cu(In,Ga)Se2 Thin Film by Selenization of Stacked Elemental Layer

with Solid Selenium

Zhao-Hui Lia, Eou-Sik Choa, Sang Jik Kwona,*, and Mario Dagenaisb

a

Department of Electronics Engineering, Kyungwon University, Seongnam, Koreab Department of Electrical and Computer Engineering,University of Maryland, CollegePark, MD20742, USA

* Corresponding author, Email: [email protected](S.J.Kwon)

Cu(In,Ga)Se2 thin films were prepared using the classical two-stepprocess approach. In the first step, the metallic precursors weredeposited on the soda-lime glass substrates by electron-beamevaporation following the Cu/In/Ga sequence. The selenization,called the second step, was produced in a rapid thermal processor.This process performed under nitrogen atmosphere of 200 Torr at200C for 5 minutes followed by 500C for 3 minutes. The devicequality of CIGS film, with the atomic ratios of Cu/(In+Ga) of 0.90and Ga/(In+Ga) of 0.26, was obtained when the Cu/In/Ga metalliclayer had thinness ratio of 900 /600 /200 . The CIGS filmwith homogeneous and dense surface morphology with large grainsize (~ 2 m) was formed.

Introduction

Chalcopyrite Cu(In,Ga)Se2 (CIGS) is a potential absorber material for high-efficiencythin film solar cells due to its favorable band gap (1.04~1.68 eV) and high-absorptioncoefficient (>104 cm-1) for solar radiation (1). Until now, CIGS-based thin film solarcells with the efficiency over 20% have been achieved over the past few years (2).

The two-step approach is one of the promising methods for the manufacture of CIGSsolar cells due to its lower cost, excellent compositional uniformity over large areas, andhigh throughput (3). The metallic precursor is usually deposited by a variety of methods,such as sputtering and evaporation, from element sources or alloy sources. Among them,the stacked elemental layer (SEL) approach is regarded as the most promising technologybecause it is an efficient low cost method for the large area commercial mass production.The Cu-In-Ga precursor is always deposited as Cu/In/Ga/multilayer, which can resultin smoother surfaces and better crystallinity (4). In the present work we proposed that theCu-In-Ga multilayer was deposited using electron-beam (E-beam) evaporation methodand a high quality CIGS film was prepared after selenized the metallic precursor withsolid Se in a rapid thermal processer.

Experimental Procedure

The elemental layer of Cu, Ga, and In were deposited on the Molybdenum-coatedsoda-lime glass (Mo/SLG) Mo/SLG using E-beam evaporation method at roomtemperature from 99.99% pure source. Figure 1(a) shows the schematic of the E-beamevaporation system used in this experiment. The individual elemental layer was once

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deposited by E-beam evaporation without breaking vacuum from a rotatable crucible.The stacked sequence of the elemental layer was Cu/In/Ga/Cu/In/Ga. The thickness ofeach metallic layer was controlled by in-situ quartz crystal monitoring. The unit ofstacked layer was repeated 5~6 times and the total thickness was kept at 0.8~1.3 m. Thethickness ratio of Cu/In/Ga layer was varied to control the finial chemical composition of

the CIGS layer. In order to optimize the chemical composition of CIGS, a pre-experimentwas produced with varying the thickness ratio of In/Ga layer and the Cu layer wasprovisionally kept at 250 . Subsequently, the Cu layer was changed from 600 to 1100 .After evaporation of stacked elemental layer, a Se layer with thickness of ~2 m wasdeposited using thermal evaporation.

Figure 1. Schematic of (a) the E-beam evaporation for deposition of metallic precursorand (b) the rapid thermal processor.

The Se-containing precursor film was subsequently moved into the quartz reaction

tube of a rapid thermal processor, which was provided with 15 tungsten halogen lampsboth on top and at the bottom as shown in Fig. 1(b). The first step of annealing processwas maintained at 200C for 5 minutes to form copper selenide and indium selenide.Then, to from chalcopyrite phase and achieve the recrystallization and grain process, theslenezation temperature was promoted 550C for 3 minutes.

The chemical composition of CIGS thin films was analyzed by an X-ray fluorescence(XRF). The morphology was observed using a scanning electron microscope (SEM) andan optical microscope respectively.

TABLE I. The chemical composition of as-deposited metallic precursor with different thickness ratios ofIn/Ga layers.

Metallic precursor layer(Cu/In/Ga) cycle No.

XRF results (at%) Atomic ratio

Cu In Ga Cu/(In+Ga) Ga/(In+Ga)

(250 /500 /300 ) 6 24.92 46.11 28.97 0.33 0.39

(250 /600 /200 ) 6 24.85 54.77 20.38 0.33 0.27

(250 /700 /100 ) 6 23.19 66.39 10.43 0.30 0.14

Results and Discussion

(b)(a)

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The stochematicy of CIGS includes two main parameters affected the properties ofCIGS absorber layer and the performance of solar cells, the atomic ratio of theCu/(In+Ga) and Ga/(In+Ga). In order to optimize the atomic ratio of Ga/(In+Ga) firstly,we deposited the precursors with different thickness ratios of In/Ga layers while thecopper layer was fixed at 250 . Table I lists the chemical compositions of the metallic

precursors. The atomic ratio of Ga/(In+Ga) varied from 0.39 to 0.14 with the changingthe thickness ratios of In/Ga layers from 500 /300 to 700 /100 . Some researchresults manifested the fact that the optimum composition of a CIGS absorber with aGa/(In+Ga) ratio of about 0.3 (5). From above results, we can found that the In/Ga layerswith thickness ratio of 600 /200 was suited for the formation of high efficiency CIGSabsorber.

Figure 2. Surface morphology of as-deposited metallic precursor with thickness of Culayer: (a) 600 , (b) 750 , (c) 900 , and (d) 1100 . The In and Ga layers were kept at600 and 200 respectively.

TABLE II . The chemical composition of as-deposited metallic precursor with different thicknesses of theCu layers.



Cu In Ga Cu/(In+Ga) Ga/(In+Ga)

(600 /600 /200 ) 6 38.51 45.84 15.65 0.63 0.25

(750 /600 /200 ) 6 44.23 41.84 13.93 0.79 0.25

(900 /600 /200 ) 5 46.55 39.71 13.74 0.87 0.26

(1100 /600 /200 ) 5 51.92 35.56 12.58 1.08 0.26

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Subsequently the atomic ratio Cu/(In+Ga) was changed by changing the thickness ofCu layer from 600 to 1100 . The surface morphology of metallic precursor is shown inFig. 2. The island-like grain covered on the flat surface was distinctly found when the Culayer was about 600 as shown in Fig. 2(a). With increasing of the thickness of Cu layer,this island-like grain became smaller gradually, which is shown in Fig. 2(b) and (c), and

it disappeared completely when the thickness of Cu layer increased up to 1100 asshown in Fig. 2(d). These island-like grains were detected as the In-rich Cu-In alloys.With increasing of the thickness of Cu layer, more copper element reacted with theindium element or/and the Cu-In alloy. So that the In concentration of Cu-In alloy alsobecame less and then the grain size also become smaller. Finally, this large grain sizedisappeared completely.

Figure 3. Surface morphology of the CIGS after selenization process with thicknesses ofCu layer: (a) 600 , (b) 750 , (c) 900 , and (d) 1100 . The In and Ga layers werekept at 600 and 200 respectively.

TABLE III. The chemical composition of selenized CIGS films with different thicknesses of Cu layers.



Cu In Ga Se Cu/(In+Ga) Ga/(In+Ga)

(600 /600 /200 ) 6 26.91 29.05 10.75 33.28 0.68 0.27

(750 /600 /200 ) 6 30.81 26.04 9.48 33.68 0.87 0.27

(900 /600 /200 ) 5 31.96 26.17 9.35 32.51 0.90 0.26

(1100 /600 /200 ) 5 35.27 23.55 8.38 32.80 1.10 0.26

The CIGS surface morphology was analyzed using SEM and it results was shown inFig. 3. The CIGS films has a large, compactly packed, faceted grain. And the grain size

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slightly became smaller and inhomogeneous when the Cu layer thickness in the metallicprecursor was increased. However, for all CIGS films, the grain size was about 1~2 m.Such large grains mean that the CIGS film has a good crystalline. The XRD results alsoconfirmed this point.

Figure 4. Corporation of both Cu/(In+Ga) ratio and Ga/(In+Ga) ratios of CIGS filmsbefore and after selenization process.

The chemical compositions of the films were measured using XRD before and afterselenization process and their results were shown in Table II and Table III respectively.

The Ga/(In+Ga) atomic ratio was kept at a constant because the thicknesses In and Galayers were not changed. As we expected, on the other hand, the Cu/(In+Ga) atomic ratioincreased with increasing of the thickness of Cu layer. For high-efficiency solar cells, it iswell known that the overall composition of CIGS absorber film should be slightly Cu-deficient, with a thin, even more Cu-deficient surface layer. The composition of the thissurface layer corresponds to the stable ordered vacancy compound (OVC) Cu(In,Ga) 3Se5.The formation of the OVC layer occurs automatically on the top surfaces of slightly Cu-deficient CIGS thin films at high temperature when the Cu/(In+Ga) ratio is about 0.9. TheOVC surface layer is weakly n-type, and because the bulk the absorber is p-type, theyform a buriedp-n junction. Thus the inverted surface can minimizes the recombination at

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the CIGS/CdS interface. Otherwise, the wide band gap of the OVC surface layer(1.23~1.3 eV) can increase further the barrier for the recombination at the CIGS/CdSinterface. Therefore, the formation the Cu-deficient bulk or surface layer is a key to high-efficiency solar cells. In this experiment, the weakly Cu-deficient CIGS film wasobtained when the Cu/In/Ga layer was about 900 / 600 / 200 /.

Moreover, the changing of the Cu/(In+Ga) and the Ga/(In+Ga) ratios before and afterselenization process were also invested as shown in Fig. 4. It was clear that bothCu/(In+Ga) and Ga/(In+Ga) ratios of selenized CIGS films were slightly increasedcompared with that of precursors. It may be caused by the loss of In and/or Ga duringselenization process at high temperature. We think that the In and Ga losses duringselenization process in nitrogen atmosphere occurs possibly by evaporation of In2Se/InSeand Ga2Se binaries (6).

Conclusions

CIGS thin films were prepared using the classical two-stage process approach. In thefirst stage, the metallic precursors were deposited on the soda-lime glass substrates by E-beam evaporation following the Cu/In/Ga sequence. The selenization was produced ina rapid thermal processor. This process was realized at 200C for 5 minutes followed550C for 3 minutes. The device quality of CIGS film, with the atomic ratios ofCu/(In+Ga) of 0.90 and Ga/(In+Ga) of 0.26, was obtained when the Cu/In/Ga metalliclayer had thinness ratio of 900 /600 /200 . The homogeneous and dense CIGS had alarger grain size of about 2 m after selenization process using RTP. Otherwise the lossof In and Ga was also found and it might be possibly caused by the evaporation ofIn2Se/InSe and Ga2Se losses during selenization process in nitrogen atmosphere.

Acknowledgments

This research was supported by Basic Science Research Program through theNational Research Foundation of Korea(NRF) funded by the Ministry of Education,Science and Technology(2010-0009454). This research was supported by the KyungwonUniversity Research Fund in 2011.

References

1. H. Neumann, Sol. Cells, 16, 317 (1986).2. P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W.

Wischmann, and M. Powalla, Prog. Photovolt. Res. Appl., 15, (2011).3. K. Knapp, and T. Jester, Sol. Energy, 71, 165 (2001).4. J. Bekker, V. Alberts, A.W. R. Leitch, and J. R. Botha, Thin Solid Films, 431-432,

116 (2003).5. U. Rau, M. Schmidt, A. Jasenek, G. Hanna, and H. W. Schock, Sol. Energy Mater.

Sol. Cells, 67, 137 (2001).6. A. Parretta, M. L. Addonizio, S. Loreti, L. Quercia and M. K. Jayaraj, J. Cryst.

Growth, 183, 196 (1998).

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