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Journal of Alloys and Compounds 650 (2015) 641e646

Contents lists avai

Journal of Alloys and Compounds

journal homepage: http: / /www.elsevier .com/locate/ ja lcom

Comparison of Cu2ZnSnS4 thin films and solar cell performance usingZn target with ZnS target

Kwang-Soo Lim a, Seong-Man Yu a, Arun R. Khalkar a, Tea-Sik Oh c, Junggyu Nam d,Dong-Wook Shin a, **, Ji-Beom Yoo a, b, *

a SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon, 440-746, Republic of Koreab School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon, 440-746, Republic of Koreac School of Mechanical and ICT Convergence Engineering, Sun Moon University, Asan, 336-708, Republic of Koread PV Development Team, SAMSUNG SDI, Cheonan, 331-710, Republic of Korea

a r t i c l e i n f o

Article history:Received 29 June 2015Received in revised form7 August 2015Accepted 8 August 2015Available online 10 August 2015

Keywords:Cu2ZnSnS4 solar cellStacked precursorSputtering processZn or ZnS sputter target

* Corresponding author. SKKU Advanced InstituteSungkyunkwan UniversitySuwon440-746Republic of** Corresponding author.

E-mail addresses: shindong37@skku.edu (D.-(J.-B. Yoo).

http://dx.doi.org/10.1016/j.jallcom.2015.08.0560925-8388/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t

Cu2ZnSnS4 (CZTS) thin films are considered as ideal absorption materials for next generation thin filmsolar cells due to their excellent optical and electrical properties as well as low cost. Two types of CZTSthin films were prepared via sputtering with Zn or ZnS targets (Cu/SnS/Zn and Cu/SnS/ZnS). After sul-furization, the microstructure and distribution of the elements and the electrical properties of the thinfilms obtained using the Zn and ZnS targets were compared. The CZTS thin films obtained from the Zntarget exhibit a large grain, smooth surface and less ZnS accumulation between the CZTS and Mo,resulting in improved junction characteristics and electrical properties. The CZTS solar cells that werefabricated with the Zn target exhibit an improved conversion efficiency of 5.06% relative to that of CZTSsolar cells using the ZnS target.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

Cu2ZnSnS4 (CZTS) thin film solar cells are considered to be apromising alternative to those made from CIGS and CdTe [1]. CZTS,which has a direct band gap of ~1.5 eV and a high absorption co-efficient of ~104 cm�1, is an excellent p-type absorber, and its ma-terials are abundant [2,3]. The Shockley-Queisser limit indicatesthat CZTS solar cells can achieve an efficiency of 28% [4]. Thehighest power conversion efficiency that has been achieved to datefor CZTS is 9.2% [5], and a CZTSSe cell with a band gap of 1.13 eV hasa 12.6% power conversion efficiency [6]. CZTS has general beenprepared using vacuum technologies, such as thermal evaporation[7], flash evaporation [8], sputtering [9] and pulsed laser deposition[10], and non-vacuum processes, such as electrodeposition [11] andsolution process [6]. Of these, sputtering exhibits outstanding ad-vantages (such as well-adhered film compositions that are close tothat of the source material, uniform deposition with various

of Nanotechnology (SAINT)Korea

W. Shin), jbyoo@skku.edu

materials, and superior step coverage) and has been suggested foruse to manufacture CZTS thin film solar cells at a high volume.

In general, metallic or sulfide precursors of Sn and Zn have beenused in the deposition of CZTS thin films. A Sn and Zn precursorallows for the morphology and defect distribution of the CZTS thinfilm to be changed, which directly affects the electrical properties ofCZTS. The Katagiri group reported that a CZTS thin film solar cellusing a Cu/SnS/ZnS precursor instead of Sn and Zn (metallic pre-cursor) achieved an improved power conversion efficiency byreducing the voids and defects in the film [12]. The effect of thestacking order of the precursor on the properties of the synthesizedCZTS films was studied, and the Cu/SnS/ZnS stacking order wasobserved with no secondary phase, a uniform surface and animproved device performance when compared to that of otherstacking orders [13]. However, the ZnS precursor remained as solidstate material during sulfurization due to its relatively high meltingtemperature, which retarded the transformation to CZTS phase andresulted in a secondary phase that was present in the CZTS film[14]. Variations in the stacking order of each precursor (e.g., Cu/Sn/Zn, Cu/Sn/ZnS and Cu/SnS/ZnS) were investigated in order toconfirm the differences between the metallic and sulfide pre-cursors [15]. The effects of the target materials on the thin filmproperties and the cell performance is still not clearly understood.

Fig. 1. XRD and Raman spectra of the CZTS film obtained with Zn or ZnS. (a) XRD spectra of both CZTS films. The crystallinity of both CZTS films was similar to that of kesterite CZTS.(b) The Raman spectra of both CZTS films at an excitation wavelength of 532 nm. Only the CZTS phase in both films was observed. (c) Raman spectra of the front and back side ofboth CZTS films at an excitation wavelength of 325 nm. A secondary ZnS phase was observed in both CZTS films. (d) Rescaled Raman spectra of the surface of CZTS films with Zn andZnS. CZTS, CTS, and ZnS phases were observed.

K.-S. Lim et al. / Journal of Alloys and Compounds 650 (2015) 641e646642

To this end, we present a comparison of the characteristics CZTSthin films deposited using Zn and ZnS targets. The stackingsequence for the Cu/SnS/Zn or ZnS prepared via magnetron sput-tering was considered in order to prevent Sn and Zn evaporationduring sulfurization [16,17]. The precursors were annealed in anH2S atmosphere in a tubular furnace (Sulfurization process), andthe formation of CZTS and the secondary phase was investigated byconducting a phase analysis, and the microstructure of the CZTSfilms was observed to evaluate their grain size, surface morphologyand cross sectional grain according to the distribution of elementsthrough the depth profile. The CZTS device that was fabricated wascharacterized in terms of the current densityevoltage (JeV) andquantum efficiency. We then conducted a comparison of theproperties of the CZTS obtained using the Zn target with those ofCZTS obtained using the ZnS target.

2. Experimental details

2.1. Preparation of the CZTS thin film

Mo-coated glass was cleaned with acetone, methanol and DIwater before the deposition of the CZTS absorber layer. The stackedprecursor layer (Cu/SnS/Zn or ZnS) for the CZTS absorber wasdeposited on an Mo-coated glass substrate using three two-inchsputter targets, Cu (99.9%), SnS (99.9%), and Zn (99.9%) or ZnS(99.99%), connected to DC, RF1 and RF2 power supplies, respec-tively. The details of the magnetron sputtering system are givenelsewhere [18]. Target powers of 60, 80 and 60 W (or only 110 W)were used for the Cu, SnS and Zn (or ZnS) targets and weredeposited for 13, 15 and 8 min (or 30 min), respectively. The film

thicknesses of the Cu, SnS, Zn and ZnS were 130, 260, 140 and250 nm, respectively, based on the composition reported by Wanget al. [6]. The film was deposited with a working pressure of~1 mTorr at room temperature, and this was followed by annealingthe CZTS layer in a N2/H2S (95/5%) mixed gas environment at 580 �Cfor 1 h in a furnace with a quartz tube. Prior to sulfurization at580 �C, soft annealing (S.A.) was carried out at 300 �C for 30 min inthe same environment at 780 Torr to prevent the evaporation orloss of elements, and after sulfurization, the sample was naturallycooled down to room temperature in the same atmosphere.

2.2. Characterization of the CZTS thin film and fabrication of thesolar cells

The morphology and thickness of the CZTS thin films weremeasured via scanning electron microscopy (SEM, JSM7401F, JEOL).The depth profile of the CZTS thin films was carried out using AugerElectron Spectroscopy (AES, PHI-700Xi, ULVAC-PHI) to assess theelemental distribution throughout the film. The formation andcrystallinity of the CZTS phase was confirmed via X-ray diffraction(XRD, D8 Discover, Bruker) and Raman spectroscopy (532 nm(Witec) and 325 nm (Horriba Jovin Yvon)). The elemental compo-sition of the thin film was characterized via X-ray fluorescencespectrometry (XRF, Primus 2, Rigaku). In order to fabricate the CZTSthin film solar cells, CdSwas deposited as a buffer layer via chemicalbath deposition (CBD), and a transparent conductive oxide (TCO)layer (i-ZnO/Al:ZnO) was deposited via RF magnetron sputtering.The device was then completed with the deposition of Ni/Al upperelectrode grids using a thermal evaporation system.

Fig. 2. SEM images of the CZTS films obtained with Zn (a) and ZnS (b). AFM images of the CZTS film obtained with Zn (c) and ZnS (d). The insets in (a) and (b) are cross-sectionalSEM images and show the dot and layer structure of the ZnS that accumulated near the Mo back contact for both CZTS films (red boxes). AES depth profiles of the CZTS film obtainedfrom Zn (e) and ZnS (f). A high atomic concentration of Zn and S elements was observed at the interface, and their phase was confirmed to be ZnS from the Raman spectra of backside of the CZTS film. In the case of ZnS, a higher atomic concentration of Zn was observed at the interface. (For interpretation of the references to colour in this figure legend, thereader is referred to the web version of this article.)

K.-S. Lim et al. / Journal of Alloys and Compounds 650 (2015) 641e646 643

2.3. Electrical properties of the CZTS solar cells

The CZTS solar cell device without an anti-reflective coating(ARC) was characterized under AM 1.5 G one-sun illumination(100 mW/cm2) by using a solar simulator (Oriel Sol 3A class AAA).The light intensity was calibrated with a Si solar cell in NRELequipped with a KG-2 filter. The external quantum efficiency (EQE)was measured according to the incident photon conversion effi-ciency (IPCE, PV Measurement Inc., a 75W xenon lamp, Ushio).

3. Results and discussion

The formation and crystallinity of the CZTS thin film obtainedwith a Zn or a ZnS target were characterized using X-ray diffraction(XRD) and Raman spectroscopy. The two CZTS thin films (Zn and

ZnS) have an XRD spectra similar to that of kesterite CZTS with amain peak of (112) at 28.5� and other peaks of (101), (200), (220),(224) and (332) at 18.2, 33.0, 47.3, 56.1 and 76.4�, respectively, asshown in Fig. 1(a) (JCPDS 26-0575). The XRD peak at 40.4� is fromtheMo back contact. Any secondary phases related to Cu and Sn arenot observed in the XRD spectra of CZTS thin films. It is difficult todistinguish the diffraction peaks of ZnS and Cu2SnS3 from thediffraction peaks of CZTS due to overlap [19]. In order to confirm theexistence of secondary phases, such as Cu2S, ZnS and Cu2SnS3 (CTS)in the CZTS thin film, the CZTS thin films obtained with the Zn orZnS target were evaluated using Raman spectroscopy with excita-tion wavelengths of 532 and 325 nm. With a 532 nm excitationlaser (Fig. 1(b)), 166, 286, 337 and 370 cm�1 peaks from kesteriteCZTS were observed in both CZTS films, and there were no signs ofsecondary phases [20e22]. The peaks at 286 and 337 cm�1 are

Fig. 3. Illustration of the CZTS films during sulfurization. Zn is in the liquid state duringsulfurization due to the relatively lower melting temperature than that of the sulfu-rization temperature whereas ZnS is in a solid state, resulting in more of the ZnSsecondary phase after sulfurization.

K.-S. Lim et al. / Journal of Alloys and Compounds 650 (2015) 641e646644

identified as the main vibrational A1 symmetry modes from theCZTS thin film, and the peak at 166 cm�1 is considered as the Bsymmetry mode. At the 325 nm excitation wavelength, both CZTSthin films were investigated in order to observe the ZnS secondaryphase in the CZTS thin film because this excitation wavelength isclose to the band gap of the ZnS compound (~3.84 eV), aiming atresonant excitation conditions [23]. A ZnS secondary phase couldbe observed at the front (surface) and back side of each CZTS film(Fig. 1(c)). The back side of the two CZTS films exhibits strong ZnSpeaks at 348, 700, 1045 cm�1, indicating the formation of the ZnSsecondary phase at the interface between CZTS and the Mo backcontact [24]. The intensity of the ZnS peaks of the CZTS film with aZn precursor is weaker than that of CZTS with a ZnS precursor,which may indicate fewer ZnS accumulation in the back side of theCZTS filmwhen using a Zn target. During sulfurization at 580 �C, thediffusion and reactionwith a vigorous liquid phase of Zn, which hasamelting temperature of 419 �C (cf. ZnS: 1185 �C) enabled the dense

Fig. 4. (a) Current density and voltage curve under 1 sun illumination with a solar simulatextrapolation using a ½Elnð1� EQEÞ�2 vs E curve, like in the inset.

formation of CZTSwith fewer ZnS secondary phase [14]. This will bediscussed in detail in the AES depth profile of the CZTS films. Theintensity at 348 cm�1, which indicates ZnS on the surface of theCZTS films, was much lower than that on the back side of the CZTS.Peaks at 263, 297, 337 and 347 cm�1 were observed, as shown inFig. 1(d). The peaks at 263 and 297 cm�1 are attributed to the CTSphase [25]. The XRD and Raman spectra indicate almost the sameCZTS crystallinity and ZnS secondary phase in both kesterite CZTSfilms, irrespective of the target. In addition, in order to probe theirdifferences, we investigate the morphology, distribution of ele-ments, and electrical properties of both CZTS thin films.

Fig. 2 shows the microstructure and AES depth profile of theCZTS thin films obtained with Zn or ZnS. The CZTS film obtainedwith the Zn target showed a larger grain size (1.21 mm) than that ofthe CZTS film obtained with ZnS (0.54 mm), as in Fig. 2(a) and (b).During the sulfurization, the melted Zn in the CZTS thin film mayimprove the grain growth of CZTS. The thickness of both CZTS filmswas measured from the cross-sectional SEM images as 1.01 mm forZn and 0.91 mm for ZnS, as shown in the inset of Fig. 2(a) and (b). Inaddition, the CZTS film from the Zn target had a smooth surfacemorphology with fewer voids than that from the ZnS target, whichwas observed in the AFM images for both CZTS films (Fig. 2(c) and(d)). The root mean squares (RMS) in the AFM image of the CZTSfilms from Zn and ZnS target were 87.79 and 103.4 nm, respectively.The less grain boundary and the smooth surface of CZTS film ob-tained with the Zn target resulted in an improved interface be-tween the CZTS and CdS buffer layer that was deposited viachemical bath deposition (CBD) to fabricate the devices [26]. Thetwo CZTS films that were obtained had a composition ratio of Cu/(Zn þ Sn) ¼ 0.80 and Zn/Sn ¼ 1.20, approximately, as measuredwith the XRF and the AES depth profiles of both CZTS films thatwere obtained to determine the component distributions of bothfilms (Fig. 2(e) and (f)). The Zn concentration of both CZTS thin filmsnear the Mo back contact was relatively high as a result of the Zn-rich composition in these stacked layers that could be used toprevent the formation of secondary phases with a low band gap,such as Cu2S, SnS and Cu2SnS3 that are responsible for reducing Voc[27]. The CZTS film obtained with the ZnS target exhibited a higherZn composition ratio (Cu/(Zn þ Sn) ¼ 0.31, Zn/Sn ¼ 4.33) than thatobtained with the Zn target (Cu/(Zn þ Sn) ¼ 0.47, Zn/Sn ¼ 2.46) atthe accumulation layer (the yellow line in Fig. 2(e) and (f)). Theformation of small grains at the bottom of both CZTS films could beobserved in the cross-sectional SEM images. In the case of the CZTSfilm using the Zn target, the dot shapes could be observed whilelayer shape was observed in the CZTS film obtained using a ZnStarget (the inset in Fig. 2(a) and (b)). The Raman spectra of the back

or, and (b) EQE spectra of a CZTS device with the band gap energy estimated through

K.-S. Lim et al. / Journal of Alloys and Compounds 650 (2015) 641e646 645

side of both CZTS films confirms these to be ZnS secondary phases,and the estimated cause can be explained as shown in Fig. 3.

Fig. 3 show schematic of the formation process of the CZTS filmby Zn and ZnS precursor during sulfurization. These reactionmechanisms of CZTS in sulfur atmosphere have been explained byScragg et al. and Yoo et al. [28,29]. The CuS and SnS are diffusedfrom deposited precursors and transformed into binary phase suchas Cu2S and SnS2 with increasing temperature. These Cu2S and SnS2phases are reacted to form a ternary phase of Cu2SnS3. Finally, theCZTS (Cu2ZnSnS4) quaternary phase is synthesized by reaction ofCu2SnS3 and ZnS phases. In the synthesis process of CZTS with Znprecursor process, the Zn is reacted in liquid state due to lowmelting temperature, and it results in grain growth and form lesssecondary phase of ZnS. Meanwhile, the ZnS is reacted in solid statedue to high melting temperature, and it has occurred secondaryphase of ZnS enough to affect.

To probe the dominant loss factor in the device performance ofthe CZTS cell using the Zn and ZnS target, the electrical properties ofthe solar cell using two types of films were characterized. Fig. 4(a)shows the current densityevoltage (JeV) characteristics of the CZTSsolar cell. The solar cell obtained with a CZTS absorber with a Zntarget achieved a conversion efficiency of 5.06% with a Jsc of18.9 mA/cm2, Voc of 0.59 V, FF of 45.4% and series resistance (Rs) of10.5 Ucm2. The solar cell with the CZTS absorber obtained with theZnS target showed an efficiency of 1.04% with Jsc, Voc, FF and Rs of6.7 mA/cm2, 0.45 eV, 34.4% and 43.2 Ucm2 respectively. The devicefabricated with the CZTS thin film obtained with the Zn targetexhibited better performance than that obtained with the ZnStarget. The interface recombination between the absorber layer andthe buffer layer (CdS) is considered to be a dominant factor inreducing Voc [30]. As mentioned above (the SEM images in the insetof Fig. 2(a) and (b)), the CZTS thin film obtained with the Zn targethas a large grain size, smooth surface, and fewer voids whencompared to the CZTS film obtained with the ZnS target. The lowdensity of the grain boundary and the voids cause less carrierrecombination at the interface of CZTS and CdS, contributing to theimprovement of Jsc and Voc [26]. In addition, devices fabricatedwiththe CZTS film obtained with the ZnS target had a relatively highconcentration of ZnS phase and a small grain layer near theMo backcontact (Figs. 1(c) and 2(f)). A ZnS secondary phase, accumulated atthe interfaces between the CZTS andMo back contact, was reportedto block the current flow, indicating an increase in the seriesresistance [31], and this is directly affected by the degradation inthe electrical properties of the CZTS films obtained from ZnS. Thelower FF in the device obtained from ZnS is attributed to the largeseries resistance (43.2 Ucm2) of the device when compared to thatfrom ZnS (10.5 Ucm2), due to the high concentration of ZnS sec-ondary phase at the interface.

Fig. 4(b) shows the external quantum efficiency (EQE) spectra ofboth cells in wavelengths ranging from 350 nm to 1000 nm. Thedevice from the Zn target showed a higher EQE of ~80% than theCZTS device from the ZnS target for the wavelength range from520 nm to 1100 nm. EQE strongly depends on the quality of theCZTS film, and the CZTS film with the Zn target collects more car-riers for EQE due to the improvement in film quality with largergrains, fewer voids, and less ZnS accumulation. From 520 nm to700 nm the EQE of the cells with CZTS from the Zn target is almostconstant and decreases sharplywhile the EQE of the cells with CZTSfrom the ZnS target decreases gradually, which indicates a higherrecombination loss in the rear area of the absorber layer or a shorterminority carrier diffusion length [32]. The EQE in the short wave-length from 350 nm to 520 nm is related to ZnO (Eg ¼ 3.5 eV) andCdS (Eg ¼ 2.4 eV). The EQE curves of the cells made from CZTS filmsfrom the Zn and ZnS targets have the same shape but differentvalues in this short wavelength region. The CdS layers of the two

CZTS films have the same characterizations. Therefore, the differ-ence in the EQE values may be a result of carrier collection loss bythe ZnS secondary phase and the grain boundary in the CZTSabsorber layer. As a result, the small grain layer and the high con-centration of the ZnS in CZTS film obtained with the ZnS targetdegrade the carrier collection efficiency. The energy band gap (Eg)was estimated through extrapolation using a ½Elnð1� EQEÞ�2 vs Ecurve, as shown in the inset of Fig. 4(b). The band gap of the CZTSfilm using the Zn and ZnS targets exhibited 1.50 and 1.49 eV,similarly. These calculated band gapsmatched the reported value of1.50 eV [2].

4. Conclusion

The CZTS thin film was synthesized via sputtering deposition(Cu/SnS/Zn or ZnS stacked layer) and sulfurization. A large grainsize (1.21 mm), smooth surface morphology, and fewer voids fromthe microstructure of the CZTS film by Zn were observed. A rela-tively low Zn content in the bottom of the CZTS film obtain with aZn target resulted in the dot structure with small grains and lessZnS accumulation. The Zn precursor, which had a lower meltingtemperature than the process temperature, exhibited larger graingrowth, smoother surface and less phase transformation of the ZnSsecondary phase in the CZTS film. The grain boundary density andthe surface morphology of the CZTS film affects the pen junctionformation and interface recombination, and the ZnS accumulationcaused current blocking with an increase in the series resistance. Asa consequence, the CZTS solar cell with Zn (Less grain boundarydensity, smooth surface and less ZnS accumulation) exhibitedimproved electrical performance in terms of the conversion effi-ciency (5.06%) when compared to that of the CZTS solar cell ob-tained with ZnS (1.04% efficiency).

Acknowledgement

This work was supported by Human Resources Program in En-ergy Technology of the Korea Institute of Energy Technology Eval-uation and Planning (KETEP), granted financial resource from theMinistry of Trade, Industry & Energy, Republic of Korea (No.20154030200870). This work was also supported by the NationalResearch Foundation of Korea (NRF) funded by the Ministry ofEducation, Science and Technology (NRF-2012M3A7B4049986).

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