Journal of the Korean Physical Society, Vol. 59, No. 6, December 2011, pp. 3432∼3435 Brief Reports

Optimum Substrate Temperature in One-stage Co-evaporation ofCu(In,Ga)Se2 Thin Films for High-efficiency Solar Cells

Chan Kim and Ilsu Rhee∗

Department of Physics, Kyungpook National University, Daegu 702-701, Korea

Dae-Kue Hwang and Dae-Hwan Kim†

Green Energy Research Division, Daegu Gyeongbuk Institute of Science and Technology, Daegu 711-873, Korea

(Received 13 October 2011, in final form 18 October 2011)

One-stage co-evaporation of Cu(In,Ga)Se2 (CIGS) thin films has strong potential for wide adop-tion in industry because of its simplicity compared with multiple-stage processes. This studyinvestigated the effects of substrate temperature during CIGS film growth on the efficiency of theresulting ITO/ZnO/CdS/CIGS/Mo structures. CIGS thin films were grown by using one-stageco-evaporation at substrate temperatures ranging from 525 to 550 ◦C. The device with a CIGSfilm grown at 535 ◦C was found to have the highest cell efficiency. In the XRD patterns, filmsgrown at this substrate temperature had the largest texture coefficient for the (220) plane and thesmallest full width at half maximum for both the (112) and the (220) planes. Other cell electricalcharacteristics were also largest for this substrate temperature. Thus, we conclude that the optimalmorphological characteristics of CIGS thin films grown at a temperature of 535 ◦C are responsiblefor the high efficiency.

PACS numbers: 72.40.+w, 73.50.PzKeywords: One-stage co-evaporation, Cu(In,Ga)Se2, Substrate temperature, Texture coefficientDOI: 10.3938/jkps.59.3432

I. INTRODUCTION

Quaternary Cu(In,Ga)Se2 (CIGS) thin films have at-tracted considerable attention owing to their high ab-sorption coefficient, their controllable direct band gapenergy, and thus their possible applications to high-efficiency solar cells [1,2]. Efficiencies as high as 20.3%have been reported [2] for CIGS solar cells fabricatedby using three-stage co-evaporation [3–5]. Furthermore,the characteristics of CIGS thin-film solar cells fabricatedby using this method have been comprehensively stud-ied, including the proportions of constituent elements,the grain size, and the structural properties [6–8]. How-ever, three-stage co-evaporation is not a simple process,but requires real-time control of the composition and thesubstrate temperature. Thus, many commercial manu-facturers are exploring the feasibility of using a simplerprocess such as one-stage co-evaporation [9].

This study investigated the characteristics ofITO/ZnO/CdS/CIGS/Mo solar cells with CIGS filmsfabricated by using one-stage co-evaporation [10]. Inthis process, the four elements can be co-evaporated

∗E-mail: ilrhee@knu.ac.kr†E-mail: monolith@digist.ac.kr

together with inductively controllable evaporationrates. The great advantage of this process is that thesubstrate temperature can be kept constant during theevaporation. This enables the effects of the substratetemperature on properties of the resulting solar cells tobe investigated, and the optimum substrate temperaturefor high-efficiency cells to be found. We grew CIGSfilms at various substrate temperatures ranging from525 to 550 ◦C and characterized their morphologies andcell efficiencies. Note that to systematically study theeffect of substrate temperature on the solar cell, weavoided certain processing steps that are known to affectefficiency, such as post-CdS growth annealing [11] andheating the substrate during window layer deposition[12].

II. EXPERIMENTAL

CIGS thin films were deposited on a 1-µm-thick Molayer on a soda-lime glass (SLG) substrate by using co-evaporation at various substrate temperatures rangingfrom 525 to 550 ◦C. The substrate was heated by us-ing a SiC heater, and its temperature was measuredby using an optical pyrometer. During deposition, the

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Optimum Substrate Temperature in One-stage Co-evaporation · · · – Chan Kim et al. -3433-

Table 1. Composition of the CIGS thin films.

Substrate Cu

In + Ga

In

In + Ga

Ga

In + Ga

Se

In + GaTemperature

550 ◦C 0.91 0.59 0.41 1.99

545 ◦C 0.90 0.58 0.42 1.99

535 ◦C 0.93 0.58 0.42 1.97

525 ◦C 1.01 0.59 0.41 2.04

evaporation rate of each element was controlled by us-ing quartz crystal microbalance (QCM) sensors. TheCdS layer (thickness, 50 nm) was formed on the CIGSfilm by immersing it into a mixed chemical solution ofCdSO4, thiourea (CH4N2S), and NH4OH for 10 min at50 ◦C. A 50-nm-thick ZnO layer was then deposited byrf magnetron sputtering, followed by a transparent elec-trode layer, a 150-nm-thick ITO film by using the samemethod. Note that these two layers were deposited inan atmosphere of Ar at 5 mTorr at a 300 W rf powerwithout heating the substrate to minimize unintendedchanges in the properties of CIGS film. Finally, a pat-terned aluminum layer for efficiency measurements wasdeposited by using a thermal evaporator with a shadowmask. The area of the fabricated CIGS solar cells wasdetermined to be 0.23 cm2 by mechanical scribing.

The atomic composition of the CIGS films was mea-sured by using inductively coupled plasma atomic emis-sion spectroscopy (ICP-AES; Shimadzu, ICPS-8100).The grain size and the morphology of the films wereanalyzed by using field emission scanning electron mi-croscopy (FE-SEM; Hitachi, S-4800). The structuralproperties of the films were analyzed by using X-raydiffraction (XRD; PANalytical, X’pert Pro-MPD). Thecurrent density versus voltage (J-V ) curves of the solarcells were obtained under the AM 1.5 global spectrum(1,000 W/m2). The electrical parameters of solar cellswere determined from these J-V curves.

III. RESULTS AND DISCUSSION

As shown in Table 1, the composition of the CIGSfilms varied with the substrate temperatures. Here,the composition ratio of In to Ga was considered tobe constant. Note from this table that in the formulaCu(InxGa1−x)Se2, the amounts of Cu and Se reach theirrepresentative values of 1 and 2 at 525 ◦C.

As shown in the SEM images in Fig. 1, the grain sizedecreased as the substrate temperature decreased toward535 ◦C, but then increased again at 525 ◦C. However, weobserve in Fig. 2 that the XRD peak for the (220) direc-tion shows the smallest value at 525 ◦C. Furthermore, theFWHM (full width at half maximum) of the XRD peakfor the (112) direction at 525 ◦C is larger than thosefor the peaks at other substrate temperatures. These

Fig. 1. (Color online) SEM images of CIGS films grown atvarious substrate temperatures.

Fig. 2. (Color online) XRD patterns of CIGS films grownat various substrate temperatures.

observations suggest that some of the large grain ob-served at 525 ◦C might be amorphous or in the process

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Table 2. Parameters of the J-V curve for CIGS solar cells. σ represents the standard deviation for eight samples.

Substrate Efficiency [%] Voc [V] Jsc [mA/cm2] Fill factor [%]

Temperature Max Avg. σ Max Avg. σ Max Avg. σ Max Avg. σ

550 ◦C 9.20 7.93 0.86 0.64 0.63 0.01 24.9 23.7 1.5 57.5 52.7 2.9

545 ◦C 10.26 9.87 0.19 0.65 0.64 0.00 24.9 24.4 0.5 63.7 62.7 1.2

533 ◦C 10.38 10.05 0.30 0.66 0.65 0.01 25.5 25.3 0.4 62.1 61.1 1.9

525 ◦C 0.06 0.05 0.01 0.05 0.05 0.01 4.8 4.1 0.9 25.1 25.0 0.2

Fig. 3. (Color online) Texture coefficients and FWHMs of the (112) and the (220) planes as functions of substrate temperature.

of crystallization. In particular, we are interested in the(220) XRD peak because the crystallization along thisdirection is known to be responsible for high efficiency inCIGS solar cells [1,13].

The ratio of the XRD peak of the (220) plane to thatof the (112) plane reached a peak at 535 ◦C. Hereafter,to describe the relative heights of the XRD peaks, we usetexture coefficients (TC) defined as

TC(112) =I(112)

I(112) + I(220),

TC(220) =I(220)

I(112) + I(220), (1)

where I(112) and I(220) are, respectively, the integratedintensities of the (112) and the (220) planes. As seenin Fig. 3(a), the TC of the (220) plane is highest at535 ◦C. The large TC of the (220) plane is known toafford high efficiency in CIGS solar cells [1,13]. Further-more, as shown in Fig. 3(b), the FWHMs for both the(112) and the (220) planes are lowest at 535 ◦C. Theseresults indicate that the XRD peaks become sharpest at535 ◦C, reflecting better crystallization at this tempera-ture.

As seen in Fig. 4 and Table 2, the solar cell with thefilm grown at 535 ◦C had the highest efficiency, open-circuit voltage, and short-circuit current density. By con-trast, the CIGS solar cells with films grown at 525 ◦C didnot show any solar cell characteristics.

Fig. 4. (Color online) Current density-voltage (J-V )curves for solar cells with CIGS films grown at various sub-strate temperatures.

IV. CONCLUSION

We fabricated CIGS solar cells with CIGS films formedby using one-stage co-evaporation. To investigate the ef-fects of substrate temperature on the efficiency of theCIGS solar cells, we grew the CIGS films at various sub-strate temperatures ranging from 525 to 550 ◦C. In theXRD patterns, TC(220) was largest at a substrate tem-perature of 535 ◦C. A large TC(220) is known to be as-sociated with high-efficiency CIGS solar cells. Further-more, the FWHMs of both the (112) and the (220) planeswere smallest at the same substrate temperature. Thus,

Optimum Substrate Temperature in One-stage Co-evaporation · · · – Chan Kim et al. -3435-

the solar cells with CIGS grown at 535 ◦C showed thehighest efficiency, open-circuit voltage, and short-circuitcurrent density. Thus, we can conclude that the opti-mum substrate temperature for the growth of CIGS filmsis approximately 535 ◦C. Moreover, we can note from theXRD patterns that the morphological features of CIGSfilms grown at this temperature are responsible for thehigh efficiency.

ACKNOWLEDGMENTS

This work was supported by the DGIST R&D Pro-gram of the Ministry of Education, Science and Technol-ogy of Korea (11-BD-01), and was also supported by theNational Research Foundation of Korea (2010-0021315).

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