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Solar Energy Materials & Solar Cells 93 (2009) 783–788

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

� Corr

E-m

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

Nanoparticle-based approach for the formation of CIS solar cells

Seokhyun Yoon �, Taehun Yoon, Kyoung-Soo Lee, Seokhee Yoon, Jeong Min Ha, Seungbum Choe

LG Chem, Ltd./Research Park, 104-1, Moonji-dong, Yuseong-gu, Daejeon 305-380, Republic of Korea

a r t i c l e i n f o

Article history:

Received 21 December 2007

Received in revised form

22 September 2008

Accepted 27 September 2008Available online 26 November 2008

Keywords:

Core–shell

Nanoparticle

CIS layer

Binary phase

48/$ - see front matter & 2008 Elsevier B.V. A

016/j.solmat.2008.09.061

esponding author. Tel.: +82 42 870 6276; fax:

ail address: shyoond@lgchem.com (S. Yoon).

a b s t r a c t

In this study, nanoparticle-based approach was suggested for the formation of CuInSe2 (CIS) layer.

Nanoparticles with core–shell structure were used as the precursor material, and binary phases were

used as core and shell material in the core–shell structure to maximize the kinetics of CIS formation

reaction. From the investigation of the effect of heating rate on Se-loss, it was concluded that Se-loss

could be minimized by using high heating rate and core–shell structure with a binary compound. By

minimizing Se-loss before and during CIS formation reaction, it was shown that CIS layer could be

formed without Se overpressure. CIS solar cell made in this study showed the highest efficiency of 1.11%,

which showed a CIS layer for the device could be obtained without selenization process.

& 2008 Elsevier B.V. All rights reserved.

1. Introduction

Though CIGS material has strong potential as an absorber layerfor thin-film solar cells, there has not been much progress in massproduction. As is well known, it is mainly due to the vacuum-basedprocess required for the fabrication of CIGS layer with high quality,which leads to the high efficiency of the final device. Actually, high-efficiency solar cells including record cells have been fabricatedusing the so-called ‘‘3-stage process’’, which includes the period ofCu–Se deposition during the deposition process [1]. The effect ofthis Cu–Se deposition period on the growth process was known toinduce liquid-assisted growth and the enhancement of graingrowth resulting from the formation of the CuSe liquid phase at agrowth temperature between 500 and 600 1C. However, the flux ofeach element should be controlled tightly during the entiredeposition process of the 3-stage growth method.

Instead of the 3-stage process, many approaches have beentried to overcome the difficulties in mass production. They includevacuum-based processes such as sputtering and selenization [2],reactive sputtering [3] as well as non-vacuum-based processesincluding chemical bath deposition [4] and coating of nanopar-ticles [5,6]. Among these approaches, coating of nanoparticles hasbeen considered as one of the most viable methods for massproduction due to its easiness of scale-up and high material usage.In their research, metal oxide nanoparticles or metal alloynanoparticles [5,6] were used in the form of aqueous ink andtransformed into the CIGS phase. Efficiency as high as 13.6% was

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+82 42 861 2057.

obtained in this type of approach for small-area cells [5]. However,it requires a long periods of selenization process, which use highlytoxic H2Se gas.

From the Cu–Se binary phase diagram, the CuSe phase showsa-b and b-g phase transition with increasing temperature. It isfinally transformed into Cu2Se with Se-loss at 377 1C as in Fig. 1.Therefore, starting from the CuSe phase, will end up with theCu2Se phase at the reaction temperature for the formation of CISphase (500–600 1C). Hence, Se overpressure is applied during theformation reaction of the CIS phase as in the 3-stage process orrapid thermal annealing (RTA) process using the bilayer structure.Se overpressure is required to make up for the Se-loss andsuppress the phase change of CuSe to Cu2Se during the reactionperiod. However, if Se-loss is minimized or the reaction kineticsfor the CIS formation is maximized, the phase change of CuSe toCu2Se can be suppressed and the CIS phase can be formed throughliquid-assisted growth without Se overpressure.

In this study, nanoparticle with a core–shell structure was usedas a precursor material for the formation of the CIS layer. Byforming a bilayer structure of binary compounds such as Cu–Seand In–Se in nano-scale, the kinetics could be maximized, whichresulted in a short reaction time and in the minimization of Se-loss. The core-shell structure with minimized Se-loss also made itpossible to have liquid-assisted growth with the help of the CuSephase. Se-loss during the time required to reach RTA temperaturecould also be suppressed by using RTA with a high heating rate.The CIS layer formed by this nanoparticle-based approachcombined with the RTA process showed morphology with goodinterparticle adhesion and resulted in the formation of actualdevice, which makes this approach promising one for massproduction.

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1600

1400

1200

1000

800

600

400

200

00 10 20 30 40 50 60 70 80 90 100

SeAtomic Percent SeleniumCu

Tem

pera

ture

°C

0 10 20 30 40 50 60 70 80 90 100

L1

1.8

1084.87°C1100°C

1063°C

1130°C

L3

523°C

377°C

332°C221°C

CuS

e 2

51°C120°C

�CuS

e

�CuSe�CuSe112°C

Cu3Se2� (Cu2-xSe)

123±15°C

(Cu)

� (Cu2-xSe)

L2

~52.5

(Se)

Fig. 1. Cu–Se binary phase diagram.

Fig. 3. Morphology and phase of InSe nanoparticle: (a) Morphology of InSe

nanoparticles and (b) XRD pattern of InSe nanoparticles.

Fig. 2. Approach in this study.

S. Yoon et al. / Solar Energy Materials & Solar Cells 93 (2009) 783–788784

2. Experimental details

Nanoparticles having core-shell structure were synthesizedusing a well-known seed-mediated growth method [7,8]. In thisstudy, the In–Se phase was first synthesized as a core material,and then followed by the formation of Cu–Se phase as a shellmaterial. Both core and shell materials were prepared by apatented process [9]. Core–shell structure of the as-preparednanoparticles was investigated and confirmed by high-resolutiontransmission electron microscope (HRTEM). Effect of heating rateon Se-loss from CuSe phase and on its phase transition wasstudied using high-temperature X-ray diffraction (HT-XRD)equipped with a position sensitive detector.

Precursor ink, or suspension of nanoparticles, was prepared bydispersing nanoparticles in ethanol with a proper binder materialsuch as polyvinylpyrrolidone. Precursor ink was coated on Na-freemolybdenum-coated glass (Na-free Mo glass substrate) using theelectrospray method with an electric field of 6 kV. The Coated CISlayer was dried, and then covered with a thin layer of Se by usingink including Se nanoparticles to compensate for the loss of Seduring the RTA process.

RTA was performed under nitrogen atmosphere using peaktemperature between 400 and 600 1C for 3–5 min. The formationof the CIS phase was confirmed by XRD, and the morphology ofthe layer was investigated by a scanning electron microscope(SEM). For cell fabrication, subsequent layers such as CdS, i-ZnO,n-ZnO and Al-grid were made by well-known processes such aschemical bath deposition, sputtering and e-beam evaporation. The

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S. Yoon et al. / Solar Energy Materials & Solar Cells 93 (2009) 783–788 785

performance of the CIS solar cell was evaluated by measuringcurrent–voltage characteristics under the simulated AM1.5 con-dition.

3. Results and discussion

As explained before in this article, the approach in this study isto form a CuSe/InSe bilayer structure in nanoparticle, and totransform it into CIS phase as shown in Fig. 2. InSe nanoparticlesprepared as core materials were observed by SEM as in Fig. 3.Nanoparticles with size 30–60 nm were identified as InSe phaseby XRD and by a In–Se ratio of 1.0 from the bulk-compositionanalysis. The formation of core-shell structure was tried byforming a CuSe phase as a shell material. The observation ofSEM as shown in Fig. 4 shows that the nanoparticles were amixture of small particles (�100 nm) and large ones (�500 nm).As is shown in Fig. 5a, the core-shell structure of smallnanoparticle was verified from an HRTEM image, which showeda bright InSe core region as well as a dark CuSe shell region. InFig. 5a, it is also observed that the InSe core region is composed ofmany nanocrystalline domains. From the lattice fringe image for alarge nanoparticle as in Fig. 5b, it turned out that large

Fig. 4. Analysis of core-shell nanoparticles: (a) Morphology of core-shell

nanoparticles and (b) XRD pattern of core-shell nanoparticles.

nanoparticles have single-crystalline nature with an interplanarspacing of 0.35 nm, which is very close to the separation between½1 0 1̄ 0� crystal planes for the g-CuSe phase. Therefore, it can beclaimed that product nanoparticles showed a mixed pattern ofcore-shell-type nanoparticles (broad XRD peaks) and an isolatedCuSe phase (sharp XRD peaks). This mixed pattern of nanoparti-cles suggests that core-shell growth was incomplete in thesynthesis. However, by forming a core-shell structure with anInSe phase as the core, the oxidation of InSe phase, which leads tothe formation of a very stable In2O3 phase and is detrimental toCIS formation, could be avoided during the RTA process and theformation of CIS phase could be possible without reducingthe environment as in this study. To suppress the loss of Se andthe CuSe-Cu2Se phase change until CIS formation reactionbegins, high heating rate to the reaction temperature of the RTAprocess was suggested before in this article. The effect of heatingrate on the change of CuSe phase was studied by synthesizing

Fig. 5. HRTEM analysis of nanoparticles: (a) Lattice image of small nanoparticle

and (b) Lattice image of large nanoparticle.

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CuSe-only nanoparticles and by monitoring their phasechange using HT-XRD with different heating rates as shown inFig. 6.

As in Fig. 6, when the heating rate was low, the point of phasechange (�370 1C) was close to that of the equilibrium case.However, the point of Se-loss was actually delayed (396–426 1C)when the heating rate was high and consequently the resolutionof HT-XRD could not be increased due to the required scan time(at least, 15 s). Therefore, it can be speculated that with a higherheating rate, e.g. 1800 1C/min, which is usual in an RTA process,the point of phase change or Se-loss can be delayed until CISformation reaction begins. Nanoparticles with a core-shellstructure were made into precursor ink, and the precursor inkwas coated on Na-free Mo glass substrates. The coated precursorlayer was transformed into CIS phase with RTA conditions such asheating rate of 1800 1C/min (30 1C/s), reaction temperature of500 1C, and reaction time of 5 min. The CIS layer that formed afterthe RTA process showed good grain growth with grain sizes of upto 2mm as shown in Fig. 7. It is obvious from the observation oflarge CIS grains that grain growth was enhanced through liquid-assisted growth by CuSe phase with the suppression of Se-loss.

Fig. 6. Phase change of CuSe-Cu2Se with different heating rate: (a) Phase change

with 2 1C/min and (b) Phase change with 120 1C/min.

However, this CIS layer could not be used as an absorber layer forsolar cell owing to very rough surface, incomplete coverage, andcrack formation.

Other nanoparticles were tested and selected as precursors forthe formation of CIS layer. The CIS layer formed from thesenanoparticles showed a fully covered surface, good interparticleadhesion, and fewer cracks as shown in Fig. 8. From this layer, CISsolar cell with the general structure of Al-grid/n-ZnO/i-ZnO/CdS/CIS/Na-free Mo glass/ was fabricated. The performance of solarcells was evaluated under an AM1.5 simulated condition, and thecurrent–voltage characteristic of CIS solar cells with the highestefficiency of 1.1% was displayed, as shown in Fig. 9. As for cellparameters, low fill factor and low VOC may be due to theroughness and micro-crack formed in the CIS layer, which led tothe increase of leakage current and shunting in the device. It isalso suggested that the low value of JSC might have resulted fromthe increase of carrier recombination due to the surface roughnessas well as the organic residue acting as barrier to carrier transportin the absorber layer. It is also speculated that the absence of Nadue to the Na-free Mo glass substrate used in this study mighthave decreased net carrier density in the CIS layer and might have

Fig. 7. Analysis of CIS phase formed after RTA process: (a) Morphology of CIS layer

after RTA and (b) XRD pattern of CIS phase after RTA.

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Fig. 8. Morphology of CIS layer used for solar cells: (a) Crack in CIS layer, (b) Morphology of CIS layer and (c) cross-section.

Fig. 9. Plot of current density vs. voltage of CIS solar cell.

S. Yoon et al. / Solar Energy Materials & Solar Cells 93 (2009) 783–788 787

increased the activity of grain boundary defects, which led to lowVOC and low JSC, respectively.

4. Conclusions

CIS solar cell with an efficiency of 1.11% was first made withoutreduction or selenization process using the nanoparticle-basedapproach. Efficient reaction for the formation of CIS phase wasmade possible by using binary phases connected in the form ofcore–shell structure nanoparticles. Combined with the RTAprocess, our approach based on core–shell structure couldminimize the loss of Se and could exploit liquid-assisted growthby CuSe phase during the reaction period. Using our nanoparticle-

based approach, it was possible to make CIS phase for the devicewithout reduction or selenization processes as used in previousnon-vacuum-based approaches. The uniformity of nanoparticlesas well as the completeness of core–shell structures should beimproved for the formation of CIS layer with more uniform andlarger grains. Also, the formation of crack and the roughness of CISlayer will be minimized by optimizing the RTA process and inkformulation to increase cell performance in the future.

Acknowledgements

The authors would like to thank Dr. K.H. Yoon at Korea Instituteof Energy Research (KIER) in Korea, for the formation of CdS/i-ZnO/n-ZnO/Al-grid structure in the final device and its perfor-mance evaluation. The authors would also like to acknowledge Dr.Chul-Hee Park for the HT-XRD measurement in the study ofheating rate effect on the change of the CuSe phase.

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