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Citation: Zoppi, Guillaume, Forbes, Ian, Miles, Robert, Dale, Phillip, Scragg, Jonathan and Peter, Laurence (2009) Quaternary Cu2ZnSnSe4 thin films for solar cells applications. In: 5th Photovoltaic Science Applications and Technology (PVSAT-5), 1-3 April 2009, Glyndŵr University, Wrexham, UK.

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Quaternary Cu2ZnSnSe4 Thin Films for Solar Cells Applications

G. Zoppi1 *, I. Forbes1, R. W. Miles1, P. J. Dale2 †, J. J. Scragg and L. M. Peter21Northumbria Photovoltaics Applications Centre, Northumbria University, Ellison Building,

Newcastle upon Tyne, NE1 8ST, UK2Department of Chemistry, University of Bath, Bath, BA2 7AY, UK

† Now at Laboratoire Photovoltaïque, Université du Luxembourg, c/o CRP - GabrielLippmann, 41, rue du Brill, L-4422 Belvaux, Luxembourg

* Corresponding Author: guillaume.zoppi@northumbria.ac.uk

Abstract

Polycrystalline thin films of Cu2ZnSnSe4(CZTSe) were produced by selenisation ofCu(Zn,Sn) magnetron sputtered metallicprecursors for solar cell applications. Thep-type CZTSe absorber films were foundto crystallize in the stannite structure (a =5.684 Å and c = 11.353 Å) with anelectronic bandgap of 0.9 eV. Solar cellswith the structure were fabricated withdevice efficiencies up to 3.2%.

1. Introduction

Thin film of Cu2ZnSnS4 (CZTS) andCu2ZnSnSe4 (CZTSe) have attractedsignificant interest lately as alternativeabsorber layers in Cu(In,Ga)(S,Se)2(CIGS) thin film solar cells. Both CZTSand CZTSe are direct bandgapsemiconductors with high absorptioncoefficient (10-4 cm-1) [1]. This type ofabsorber derives from the CuInSe2chalcopyrite structure by substituting halfof the indium atoms with zinc and theother half with tin. The resulting energybandgap of this material has beenreported to range from 0.8 eV for theselenide to 1.5 eV for the sulfide [2, 3] anddevices based on CZTS have been madewith efficiencies up to 6.7% [4]. Onlylimited studies have been done on CZTSeand devices have been reported on onlytwo occasions [2, 5].In this work, CZTSe films were producedby sequential deposition of high puritysputtered Cu, Zn and Sn from elementaltargets followed by selenisation at hightemperature. We report on some of theabsorber properties and device results.

2. Experimental

CZTSe thin films were produced by a twostage process by means of selenisation ofmagnetron sputtered metallic Cu(Zn,Sn)(CZT) precursor layers. The Cu, Zn andSn layers were sequentially sputter

deposited using high purity (5N) targetsonto unheated Mo coated soda-lime glasssubstrates on a rotating substrate table.The selenisation process took place in amixed argon and elemental seleniumatmosphere at temperatures of 500°C for30 min. The thickness of the precursorwas adjusted so that the final thickness ofthe CZTSe film was ~2 m. Devices weremade by the deposition of a 70 nm CdSwindow layer using the chemical bathdeposition method, followed by thedeposition of 50 nm i-ZnO and 400 nmindium tin oxide (ITO) using rf sputtering.The cells were finished using sputteredNi/Al contacts.The structural quality of the absorberlayers was examined using X-raydiffraction (XRD) carried out with aSiemens D-5000 diffractometer using aCuKradiation source (= 1.5406 Å). Thefilm morphology and composition weredetermined using a FEI Quanta 200scanning electron microscope (SEM)equipped with an Oxford Instrumentsenergy dispersive X-ray analyzer (EDS).Depth profiling and investigations of lateraluniformity of the layers were investigatedusing a bench-top Millbrook MiniSIMS(secondary ion mass spectroscopy)system with a Ga+ primary ion of 6 keV.Photocurrent spectra of the devices wererecorded using a double gratingmonochromator with chopped illuminationnormalized against calibrated silicon andgermanium photodiodes. Current - voltagemeasurements were recorded at 25°Cunder AM1.5 (100 mW.cm-2) illumination.Reflectance measurements were recordedusing a Shimadzu SolidSpec-3700.

3. Results & Discussion

The XRD pattern of a CZT precursor layeron Mo-coated glass is presented in figure1, the corresponding composition data isshown in Table 1. The film wasintentionally deposited Cu poor(Cu/(Zn+Sn) = 0.85), with approximately

equal amounts of Zn and Sn. Table 1indicates that the film was slightly Sn poor(Zn/Sn = 1.08). XRD analysis of theprecursor film revealed the presence ofelemental Zn and Sn, but not Cu. Cu wasfound to be present in the form of binarieswith Zn (Cu5Zn8) and Sn (-Cu6Sn5) butthere was no evidence of any ZnxSny sub-phase formation during the deposition ofthe precursor. This is in agreement withCu-Sn-Zn solder alloy studies which haveshown that Zn and Sn react preferentiallywith Cu [6, 7]. After selenisation at 500°Cfor 30 min, EDS measurements indicatedthat the film conserves its overall Cu-poorand slightly Sn-poor composition (Table1).

CZTprecursor

CZTSeabsorber

Cu/(Zn+Sn) 0.85 0.83Zn/Sn 1.08 1.15

Se/(Cu+Zn+Sn) - 1.02

Table 1. Elemental composition of theCZT metallic precursor and subsequentselenised films at 500°C for 30 min.

2(°)

30 40 50 60 70 80

Inte

nsity

(a.u

.)

-C

u 6S

n 5S

n

*

Sn

*

Zn, Cu5Zn8-Cu6Sn5

*Sn,

Cu 5

Zn 8

-C

u6S

n 5,

Cu 5

Zn 8

,Sn,

-C

u6S

n 5,C

u 5Zn

8

Figure 1. XRD pattern of a Cu(Zn,Sn)precursor film with Cu/(Zn+Sn)=0.85 andZn/Sn=1.08 deposited on Mo-coatedglass. Peaks marked (*) arise from thesubstrate.

Figure 2 shows the corresponding XRDpattern of a CZTSe film selenised at500°C for 30 min. All the peaks can beattributed to Cu2ZnSnSe4. The calculatedlattice parameters are a = 5.684 Å and c =11.353 Å, in very good agreement with thepublished data from Matsushita et al. [8]and Olekseyuk et al. [9]. The doubletpeaks (312/116) and (400/008) areobserved and together with the previouscited references this indicates that theselenised film adopts the stannite [I4-2M]structure. Figure 3 shows the morphology

of the 2 m thick CZTSe film processed at500°C. The film consists of closely packedgrains which are ~2 m wide.

2(°)

20 30 40 50 60 70 80

Inte

nsity

(a.u

.)

(112

)(1

03)

(211

)

(213

)(2

20/2

04)

(400

/008

)

(332

/316

)

(312

/116

)

*

*

(110

)

(202

)

*

Figure 2. XRD pattern of a Cu2ZnSnSe4(CZTSe) film selenised at 500°C for 30min. Peaks marked (*) arise from thesubstrate, and all other peaks belong tothe CZTSe structure.

(a)

(b)

Figure 3. Scanning electron micrograph ofa CZTSe film on Mo-coated glass. (a)Surface image and (b) cross-sectionalimage.

The elemental distribution of the films as afunction of depth was probed by SIMS,and a depth profile of the four elements isshown in figure 4 for a sample selenised at500°C. This qualitative assessmentindicates a uniform distribution of the Seand Sn metallic elements throughout thefilm thickness. The Cu signal is alsouniform for the majority of the film.However, the increase in the Cu signaltowards the interface with the substrate isattributed to excess Cu deposited duringthe precursor preparation, to improve

adhesion to the substrate. Finally, the Znprofile appears to show a dip in the middleof the film. However, due to the lowsensitivity for the Zn ions in the CZTSematrix, further analysis is required todetermine whether this is a real feature orthis is due to the morphology of the film.

Depth (nm)

0 400 800 1200 1600

Inte

nsity

(cou

nts

per

seco

nd)

101

102

103

Zn64

Cu63

Sn120Se80

Figure 4. MiniSIMS depth profiles of aCZTSe film deposited on glass. Theprecursor was selenised at 500°C for 30min.

Figure 5 shows the current density -voltage (J-V) curves for two selecteddevices under AM1.5 standard illuminationand in the dark. The corresponding cellparameters are shown in Table 2. Cell Arepresents the device with the best fillfactor (FF) of 0.48 while cell B is the solarcell with the best open circuit voltage (Voc)of 0.359 V. This yields an efficiency of3.0% and 3.2% for cell A and B,respectively for a total area of 0.229 cm2.

Voltage (V)

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4

Cur

rent

Den

sity

(mA

.cm

-2)

-20

-10

0

10

20

30

40

Cell A

Cell B

Light

Dark

Figure 5. Dark and light current density –voltage curves of the best performingCZTSe solar cells recorded under AM1.5(100 mW.cm -2, 25°C) illumination and totaldevice area of 0.229 cm2.

The efficiency is limited partly by the fillfactor indicated by the high shuntconductance (Gsh) (8.0 and 9.1 mS.cm

-2

for cell A and B, respectively) and highseries resistance (Rs) (2.2 and 3.9 .cm

2

for cell A and B, respectively). The J-Vmeasurements also showed a crossoverof the dark and light current voltage curve(figure 5, cell A) that is characteristic oflarge series resistance. Due to a non-optimum grid design, an estimated 20% ofthe total area is covered by the grid and asa comparison, this would yield efficienciesof 3.6% and 4.0% for cells A and B,respectively for an active area of 0.184cm2. This constitutes a significantimprovement when compared toFrieldmeier et al’s 0.8% device [2] andAltosaar et al’s 1.8% monograin structure[5].

Voc(V)

Jsc(mA.cm -2) FF

(%)

Cell A 0.304 20.6 0.48 3.0Cell B 0.359 20.7 0.43 3.2

Table 2. Solar cells parameters. Thedefinition of symbols is given in the text.

Wavelength (nm)

400 600 800 1000 1200 1400

Ex

tern

alQ

uant

umE

ffic

ien

cy(%

)

0

10

20

30

40

50

60

70

Re

flect

ance

(%)

5

10

15

20

25

30

35

40

Figure 6. External quantum efficiency(EQE) for a CZTSe solar cell (Cell B)(solid line) and reflectance data for aCZTSe film (dashed line).

Figure 6 shows the external quantumefficiency (EQE) for cell B. The maximumquantum efficiency of 65% is obtained fora photon wavelength of 520 nm andexhibits the characteristic shortwavelength cut-off from the CdS bufferlayer. The CZTSe devices arecharacterized by a long end tail curvesloping gradually from 520 nm which maybe attributed to high doping densities andsmall electron diffusion length. The energybandgap of the CZTSe film wasextrapolated and found to be 0.94±0.05eV. This is in agreement with ourreflectance data, also plotted in figure 6,which shows the main optical transition tobe at 0.88±0.04 eV. Friedlmeier et al.estimated that their evaporated CZTSe

absorber thin films to have an energybandgap of about 0.8 eV [2]. The spreadobserved in bandgap values could be dueto differences in stoichiometry but also tothe presence of additional phases.The EQE spectrum measured under zerovoltage bias, figure 6, was also used tocalculate the expected short circuit currentdensity (Jsc). The value of Jsc calculated byintegrating the EQE response with theAM1.5 solar spectrum is 19.7 mA.cm -2

which is in close agreement with themeasured value from the J-V curve (20.7mA.cm -2). The difference arises due tomissing EQE data at long wavelength.

4. Conclusion

It has been shown that good qualityCu2ZnSnSe4 (CZTSe) material can beproduced for solar cell applications using a2-stage sputtering-selenisation process.The slightly Cu-poor and Zn-richabsorbers showed good optical andelectronic properties and weresuccessfully made into solar cellsachieving efficiency of 3.2%. To theauthors’ best knowledge this is the highestreported efficiency for a device based on aCu2ZnSnSe4 absorber. These devices arecurrently limited by the current generationand collection as indicated by spectralresponse and current-voltagemeasurements.

Acknowledgments

This work was funded by the EPSRCSUPERGEN programme “Photovoltaicsfor the 21st Century”.

References

[1] K. Ito and T. Nakazawa, JapaneseJournal of Applied Physics 27 (1988)2094-2097[2] T. M. Friedlmeier, N. Wieser, T. Walter,H. Dittrich, and H. W. Schock,Proceedings of the 14th EuropeanPhotovotlaic Specialists Conference,Barcelona (1997) 1242-1245[3] H. Katagiri, Thin Solid Films 480-481(2005) 426-432[4] H. Katagiri, K. Jimbo, S. Yamada, T.Kamimura, W. S. Maw, T. Fukano, T. Ito,and T. Motohiro, Applied Physics Express1 (2008) 041201[5] M. Altosaar, J. Raudoja, K. Timmo, M.Danilson, M. Grossberg, M. Krunks, T.Varema, and E. Mellikov, Proceedings of

the 4th IEEE World Conference onPhotovoltaic Energy Conversion, (2006)468-470[6] M.-C. Wang, S.-P. Yu, T.-C. Chang,and M.-H. Hon, Journal of Alloys andCompounds 381 (2004) 162-167[7] T. Ichitsubo, E. Matsubara, K. Fujiwara,M. Yamaguchi, H. Irie, S. Kumamoto, andT. Anada, Journal of Alloys andCompounds 392 (2005) 200-205[8] H. Matsushita, T. Maeda, A. Katsui,and T. Takizawa, Journal of CrystalGrowth 208 (2000) 416-422[9] I. D. Olekseyuk, L. D. Gulay, I. V.Dydchak, L. V. Piskach, O. V. Parasyuk,and O. V. Marchuk, Journal of Alloys andCompounds 340 (2002) 141-145