Synthesis of SnS thin films via galvanostatic electrodeposition and fabrication of CdS/SnS heterostructure for photovoltaic applications

Page 1

SC

BA

a

ARRAA

KSEpCH

1

wgwpett

(

0d

Applied Surface Science 256 (2010) 4328–4333

Contents lists available at ScienceDirect

Applied Surface Science

journa l homepage: www.e lsev ier .com/ locate /apsusc

ynthesis of SnS thin films via galvanostatic electrodeposition and fabrication ofdS/SnS heterostructure for photovoltaic applications

iswajit Ghosh, Rajarshi Roy, Sumit Chowdhury, Pushan Banerjee, Subrata Das ∗

dvanced Materials & Solar Photovoltaic Division, School of Energy Studies, Jadavpur University, Raja S.C. Mullick Road, Kolkata 700032, India

r t i c l e i n f o

rticle history:eceived 11 January 2010eceived in revised form 3 February 2010ccepted 4 February 2010vailable online 12 February 2010

a b s t r a c t

In this work SnS thin films were electrochemically deposited on ITO coated glass substrate by galvano-static electrodeposition at different pH of the plating bath. The working electrode used in these studieswas low cost high purity graphite rod. The as-deposited films were found to be smooth, pinhole freeand well adherent to the substrate with no powdery deposition. EDX measurements revealed that allthe SnS films were non-stoichiometric in nature with variation from Sn-excess to S-excess compositions.

eywords:nSlectrochemical depositionHharacterizationeterostructure

XRD pattern showed that all the SnS thin films had orthorhombic polycrystalline structure. The directbandgaps of all the films were found to be in the range 1.54–1.58 eV. ITO/SnS/In structure exhibited lin-ear current–voltage characteristics, establishing the ohmic nature of both ITO/SnS and SnS/In junctions.Furthermore, SnS layer was grown on CdS film using electrodeposition technique. The heterostructureITO/CdS/SnS/In was characterized under dark and illuminated conditions. From forward biased I–V char-acteristics several junction parameters like barrier height, diode ideality factor and series resistance ofthe heterostructure were extracted using Cheung model.

(

. Introduction

Over the past several years many new and advanced materialsere proposed as the potential solar cell material. The research are

oing on to exploit their properties either as individual thin films orhen employed in a device, with an intention to lower the overallrice of the solar cell by augmenting the photovoltaic conversionfficiency. Interestingly, tin sulfide (SnS) has been highlighted ashe most promising material for the fabrication of low cost, non-oxic photovoltaic devices. It fulfills certain basic properties:

a) Several binary tin sulfides (SnS, SnS2, Sn2S3, Sn3S4, Sn4S3) areknown of which SnS and SnS2 are technologically the mostimportant, with bandgaps of 1.3 and 2.07 eV respectively. Tin (II)sulfide, i.e. SnS, has an orthorhombic layered crystal structurewith lattice parameters a = 4.329 Å, b = 11.193 Å and c = 3.98 Åwhich can be fabricated by numerous fabrication methodolo-gies and the optical bandgap reported to be 1.2–1.7 eV. It is to

be noted that single phase SnS is usually accompanied by traceamount of impurity phase SnS2 which is quiet difficult to sep-arate. The wide variation in bandgaps of SnS films reported byearlier researchers is presumably due to the admixture of differ-

∗ Corresponding author. Tel.: +91 33 2414 6823; fax: +91 33 2414 6853.E-mail address: [email protected] (S. Das).

169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2010.02.025

© 2010 Elsevier B.V. All rights reserved.

ent small proportion of SnS2 and other impurity phases present[1,2].

b) It exhibits p-type electrical conductivity with absorption coef-ficient as high as CdTe (104 cm−1).

(c) The constituent elements of SnS are abundant and relativelynon-toxic.

All these characteristics and relatively convenient depositionmethodology, SnS seems to be quite tailor made as an absorberlayer for thin film solar cell [1–4]. Apart from photovoltaic conver-sion, it has potential applications in holographic recording systems[5] and solar control devices [6] as well.

SnS thin films can be fabricated by various methods such as vac-uum evaporation [7], spray pyrolysis [8], chemical deposition [9],electrochemical deposition (ECD) [10–13], R.F. sputtering [14] andSILAR method [15,16]. There are growing interests in electrodepo-sition of various ceramic materials following the pioneering workof Brenner [17].

ECD technique provides some key advantages compared tosome other deposition methods. Apart from very inexpensiveand easily expandable deposition technique occurring at ambient

pressure, it offers excellent control over the properties of semi-conductor films with the variation of process parameters, such ascurrent density, composition of the electrolytes, temperature andpH of the plating bath. Eventually the thickness of the depositedfilms can be directly monitored through the charge consumed dur-

Page 2

ce Sci

iofirrtatuptl

iwtcoZtwgscwotci2moeeuTI

2

2

eiDccbttqw

B. Ghosh et al. / Applied Surfa

ng the deposition process. Previous investigations were focusedn potentiostatic and galvanostatic electrodeposition of SnS thinlms with the ultimate aim of depositing good quality stoichiomet-ic SnS films [9–12]. In galvanostatic mode [12] elimination of theeference electrode makes the process more simple and cost effec-ive. Furthermore, it is a common practice to use platinum sheet asn anode. Apart from the high cost of platinum electrode it tendso corrode in sulfur environment with continuous batch processtilization. So, in our present work comparatively low cost, highlyure graphite rod was employed for galvanostatic electrodeposi-ion. Furthermore, the graphite electrode is very much suitable forarge area deposition.

This paper presents the effort made towards the electrochem-cal growth of SnS on ITO and CdS coated glass substrate along

ith the characterization of CdS/SnS heterostructure. So far elec-rochemical growth of SnS on conducting substrate has been aommon practice. However, electrochemical deposition of SnS onther semiconductors or wide bandgap window materials, i.e. CdS,nO, ZnS is technologically more interesting as it would give riseo a possible heterostructure. In this study initially SnS thin filmsere galvanostatically grown at room temperature on ITO coated

lass substrate corresponding to different pH of the plating batholution. The films were structurally, compositionally and opti-ally characterized. Compositionally the near stoichiometric filmith ordered well distributed grains was found to grow at pH 3.00

f the bath solution. Current–voltage (I–V) measurements showedhat the ITO/SnS/In structure exhibits linear characteristics indi-ating thereby that both ITO/SnS and SnS/In junctions are ohmicn nature. For the fabrication of the said heterostructure, initially00 nm CdS was deposited on ITO coated glass substrate using ther-al evaporation, followed by electrochemical growth of SnS layer

n CdS film at pH 3.00 of the bath solution. Indium was thermallyvaporated onto SnS to form the top contact (2 mm2 size). Thelectrical behavior of the CdS/SnS heterostructure was determinedsing I–V measurements under dark and illuminated conditions.he Junction parameters were extracted from the forward biased–V characteristics using Cheung model.

. Experimental procedure

.1. Fabrication of SnS thin film

The constituents of the electrodeposition bath were adjustedmpirically for the optimal deposition of SnS thin films. All chem-cals were of reagent grade purity and were used as received.eionized water was used as the solvent for the electrolytic pro-ess. Before deposition, ITO coated glass substrates were carefullyleaned with isopropanol in an ultrasonic bath for 10 min, followed

y in deionized water and dried. A conventional two-electrode sys-em was employed for direct current electrodeposition. ITO beinghe cathode, high purity graphite electrode was used as anode. Theuantity of tin sulfide deposited on the ITO coated glass substrateas controlled by the galvanic current passed through the solu-

Fig. 1. Photograph of the electrodeposited SnS film

ence 256 (2010) 4328–4333 4329

tion. The electrodeposition bath consisted of 10 mM/L of SnSO4 and50 mM/L of Na2S2O3. The pH of the bath was adjusted to values like1.50, 2.00, 2.50 and 3.00 by drop-wise addition of dilute H2SO4. Thevaried pH of the deposition bath and its composition were shown tohave direct effect on the stoichiometry and growth rate of the elec-trodeposited films. The electrodeposition was conveniently carriedout at room temperature at a plating current density (through aregulated DC supply) of 1 mA/cm2 for 45 min.

The electrodeposition process involved the formation of SnSthin films, which included simultaneous deposition of tin and sul-fur from the bath by passing current between the anode and thecathode. The basic electrochemical reactions for the simultaneousco-deposition of Sn and S should be characterized by

ESn = ES (1)

where ESn and ES are the equilibrium deposition potential of theelements Sn and S respectively.

The mechanism of SnS formation involves two reactions thathad been taken place simultaneously on cathodic surface.

Sn2+ + 2e− ↔ Sn(s) (2)

S2O32− + 6H+ + 4e− ↔ 2S + 3H2O (3)

According to Nernst equation the equilibrium electrode potentialof the above two reactions can be expressed as

E

(Sn2+

Sn

)= E0

(Sn2+

Sn

)+ RT

2Fln(aSn2+ ) (4)

E

(S2O3

2−

S

)= E0 +

(RT

4F

)ln(aS2O3

2− ) + 3RT

2F pH(5)

In Eqs. (4) and (5), “E” and “a” denote standard electrochemicalpotential and the activities of the relevant ions in the bulk solutionrespectively, R is the universal gas constant, T is the absolute tem-perature, F is the Faraday’s constant. The above equation signifiesthat pH has a direct influence on electrodeposited SnS films.

The resulting electrodeposited SnS thin films appeared uniformand smooth in terms of film quality as shown in the photograph(Fig. 1).

2.2. Fabrication of CdS/SnS heterostructure

200 nm CdS thin films were grown on ITO coated glass substrateby evaporating high purity CdS powder in vacuum evaporationunit at 10−5 Torr base pressure. Thickness of the CdS films wasdetermined in situ with the help of a digital thickness sensor. Theas-deposited films were annealed in air at 400 ◦C for 3–4 min. The

electrodeposition of SnS on CdS was carried out at pH 3.00 of thebath solution. The Sn/S ratio was 1.09 at pH 3, which may be lookedupon as Sn doped SnS. Interestingly, in our earlier work SnS filmwere grown by vacuum evaporation method, the Sn-to-S ratio wasfound to be 1.41 [2].

s grown at different pH of the plating bath.

Page 3

4330 B. Ghosh et al. / Applied Surface Sci

fTttw

2

anbt3Ic

3

ww2tttwtt

cbmsapa(3

ports that for SnS there has been no abrupt change in band structuredespite the wide variation of Sn-to-S ratio. Furthermore, XRD pat-tern of all the SnS films indicates that there has been no significantchange in crystallographic structure as well. All the SnS films werep-type in nature as revealed from thermoelectric probe test.

Table 1Sn-to-S ratio for different pH of the plating bath.

Fig. 2. The X-ray diffraction pattern of SnS films.

SnS was electrodeposited on CdS using galvanostatic techniqueor nearly 2 h at 1 mA/cm2 current density using graphite anode.he growth rate of SnS is expected to be quite slow compared tohat on bare ITO owing to the high sheet resistance of CdS. Thehickness of the SnS layer was found to be of ∼400 nm. p–n junctionas thus formed between n-type CdS and p-type SnS.

.3. Analysis of the film properties

Structural and compositional analyses of the SnS films werechieved by the way of X-ray diffractometry (Bruker D8), scan-ing electron microscopy and EDX (Carl Zeiss EVO-40). Theandgaps were determined by measuring transmission profile ofhe films using a UV–vis spectrophotometer (Perkin Elmer Lambda-5). I–V characteristics of the ITO/SnS/indium structure and theTO/CdS/SnS/In heterostructure were recorded using a Keithley 238urrent source and Interactive Characterization Software (ICS).

. Results and discussion

The thickness of the deposited films, measured gravimetrically,ere found to be in the range of 200–250 nm. Higher growth rateas observed with increase in bath pH. At lower pH values (1.50,

.00) the entire electrolytic bath showed milky white characteris-ics owing to the disproportionation of thiosulfate that gave rise tohe formation of colloidal sulfur. However, the excess sulfur con-ent got diminished with higher pH values and the bath solutionas increasingly become transparent. It may be mentioned here

hat the insulating nature of colloidal sulfur acted as a hindrance tohe ionic mobility.

X-ray diffraction patterns of electrodeposited SnS on ITOorresponding to pH 1.50, 2.00, 2.50 and 3.00 of the platingath are shown in Fig. 2. All SnS peaks in the pattern closelyatched to those of orthorombic SnS (herzenbergite-syn) corre-

ponding to JCPDS card number 39-0354 having lattice constants

= 0.4329 nm, b = 1.1192 nm and c = 0.3984 nm. The diffractioneaks at 2� = 21.15◦, 25.91◦, 26.94◦, 30.09◦, 31.42◦, 38.91◦, 45.09◦

nd 50.41◦ can be indexed to (1 1 0), (1 2 0), (0 2 1), (1 0 1), (1 1 1),1 3 1), (0 0 2) and (1 1 2) planes of SnS respectively. The peaks at0.09◦, 60.06◦ and 35.08◦ may correspond to (2 2 2), (4 0 0) and

ence 256 (2010) 4328–4333

(6 2 2) planes of ITO and the superimposition of the (2 2 2) planeof ITO with (1 0 1) SnS plane might have been taken place becauseof their closeness. Furthermore, it can be noted that because of thenon-stoichiometric nature of the SnS films the stronger diffractionpeak of S (JCPDS 13-0141) corresponding to 2� = 27.08◦ and thestrongest diffraction peak of Sn (JCPDS 04-0673) corresponding to2� = 30.64◦ may also be appeared. In some of the XRD patterns themerger of these two peaks with the nearby SnS peaks might havebeen taken place because of their proximity.

SEM micrographs (Fig. 3a–e) show different morphologies of thesurface grains corresponding to varying pH of the plating bath. Itrevealed that except the films deposited at pH 2.50, all the grainswere compact, well distributed particles of uniform size and shapeand no crack or pinhole were observed. At pH 1.50 (Fig. 3a), wheat-like grains were observed whereas nearly spherical grains wereobserved at pH 2.00 (Fig. 3b). From the SEM images, it was fur-ther observed for the films deposited at pH 2.50 (Fig. 3c) that sometetrahedral shaped grains larger in size and shape were sporadi-cally distributed throughout the surface, which is presumably dueto secondary phase precipitation of segregated sulfur. For the filmdeposited at pH 3.00, well defined compact spherical grains wereobserved (Fig. 3d and e).

Compositional analysis (Sn-to-S ratio) was calculated from EDXanalysis, where the powder scraped from the electrodepositedfilms was used to study the EDX spectra so as to prevent the unde-sirable contribution of indium and tin from ITO itself. The averagevalue of the Sn-to-S ratio for all the SnS films are presented inTable 1. From the table it was found that all the SnS films werenon-stoichiometric in nature, although the film deposited corre-sponding to pH 3.00 was close to the stoichiometric composition.Furthermore, with the increase in pH there was subsequent reduc-tion of the sulfur content in SnS samples leading to the variationfrom S-rich to Sn-rich compositions.

The UV–vis transmission spectra (Fig. 4a) obtained for SnS filmsdeposited on ITO in the region 550–1100 nm, revealed that almostall the films showed good absorption in the visible range. Bandgapswere evaluated from the transmission spectra using the followingrelation:

˛(h�) = A(h� − Eg)n (6)

where A is a constant and n is equal to 1/2 for direct bandgap semi-conductors. The absorption coefficient ˛ can be calculated from therelation:

˛ =(

1t

)ln

(100

T

)(7)

where t is the thickness of the films and T is the percentage trans-mitted.

The plot of (˛h�)2 vs. h� is shown in Fig. 4b. The bandgapsobtained for all the samples were ranging from 1.54 to 1.58 eV, asdepicted in Table 2. Although there has been an appreciable changein Sn-to-S ratio with the variation of bath pH, however, surprisinglynot much variation observed in bandgap. This fact indirectly sup-

Bath pH Average Sn (%) Average S (%) Sn/S ratio

1.50 41.36 58.64 0.702.00 44.39 55.61 0.792.50 45.52 54.48 0.833.00 52.24 47.76 1.09

Page 4

B. Ghosh et al. / Applied Surface Science 256 (2010) 4328–4333 4331

F M mim eposit

fiiS

fiiiS

t

TB

ig. 3. (a) The SEM micrograph (15k× magn.) of SnS deposited at pH 1.50. (b) The SEagn.) of SnS deposited at pH 2.50. (d) The SEM micrograph (10k× magn.) of SnS d

Fig. 5 shows the SEM image of the electrodeposited SnS over CdSlm. It may be noted here that the grains of SnS on CdS were spher-

cal in nature but slightly smaller in size compared to the grains ofnS on bare ITO.

Fig. 6 shows the I–V characteristics of 200 nm as-deposited SnSlms (on ITO) metallized with indium. From the I–V characteristics

t has been noticed that indium formed ohmic contact to p-SnS. This

s in accordance to the observation in our previous studies wherenS thin films were grown using vacuum evaporation method [18].

The dark I–V characteristic of the ITO/CdS/SnS/In heterostruc-ure (Fig. 7) was found to be rectifying with very low reverse

able 2andgaps at different pH of the plating bath.

Bath pH

1.50 2.00 2.50 3.00

Bandgap (eV) 1.54 1.56 1.58 1.57

crograph (20k× magn.) of SnS deposited at pH 2.00. (c) The SEM micrograph (10k×ed at pH 3.00. (e) The SEM micrograph (20k× magn.) of SnS deposited at pH 3.00.

saturation current, which supports heterojunction formation.However, very feeble photovoltaic effect was observed from thisdevice.

The rectifying I–V characteristics of the ITO/CdS/SnS/In het-erostructure could be described by thermionic emission theory. Thecurrent that flows across the junction can be expressed as follows

I = I0

[exp

(qV

nkT

)− 1

](8)

where I0 is the saturation current, k is the Boltzmann constant, nis the ideality factor, T is the absolute temperature, q and V are theelectronic charge and the applied voltage respectively.

According to Cheung’s method [19] the forward current volt-

age characteristics of a device having series resistance RS can beexpressed as

I = I0 exp[

q(V − IRS)nkT

](9)

Page 5

4332 B. Ghosh et al. / Applied Surface Science 256 (2010) 4328–4333

Fh

wo

V

ws

F

To evaluate �b another function was defined

H(J) = V −(

nkT

q

)ln

(J

AefA∗T2

)(13)

ig. 4. (a) Transmission spectra of electrodeposited SnS films. (b) Plot of (˛h�)2 vs.� of SnS films.

here IRs represents the voltage drop across the series resistancef the heterojunction.

Eq. (10) can be rewritten in terms of current density J as,

= RSAefJ + n�b + nkT

qln

(J

A∗T2

)(10)

here Aef is the effective area of the junction and A* is the Richard-on constant.

ig. 5. The SEM micrograph (30k× magn.) of SnS electrodeposited over CdS film.

Fig. 6. I–V characteristics of ITO/SnS/In structure.

Differentiating V with respect to ln J, Eq. (10) reduces to

dV

d(ln J)= JRS + nkT

q(11)

Thus, the plot d(V)/d(ln J) vs. J was found to be linear. RS can beevaluated from the slope of the plot and the intercept would givethe ideality factor of the device. The values of the n and RS werefound to be 5.76 and 1.53 k� respectively.

The barrier height of the heterostructure can be evaluated usingthe relation

J0 = A∗T2 exp(

−qϕb

kT

)(12)

where J0 is the reverse saturation current density and �b is thebarrier height.

A* = 120(m*/m) A cm−2 K2 for SnS the value for m*/m is about0.20 [20].

Fig. 7. I–V characteristics of ITO/CdS/SnS/In heterojunction.

Page 6

ce Sci

H

oawo

4

gtfSwbloerisftpptce

[[

[

[[

[15] B. Ghosh, M. Das, P. Banerjee, S. Das, Semicond. Sci. Technol. 23 (2008) 125013.[16] B. Ghosh, M. Das, P. Banerjee, S. Das, Appl. Surf. Sci. 254 (2008) 6436.[17] A. Brenner, Electrodeposition of Alloys, vols. 1 and 2, Academic, New York, 1963.

B. Ghosh et al. / Applied Surfa

So that Eq. (10) reduced to

(J) = n�b + JRS (14)

Thus, a plot of H(J) vs. J was found to be linear. The barrier heightf the device was evaluated from the intercept using the value of nnd was found to be 0.55 eV. It has been observed that both n and RSas quite higher which is presumably due to high sheet resistance

f CdS and large amount of surface states associated at the junction.

. Conclusion

This work presents the room temperature electrochemicalrowth of SnS films on ITO coated glass substrate using simplifiedwo electrode system. The electrodeposition was carried out at dif-erent pH of the bath solution using high purity graphite electrode.nS films obtained, however, were non-stoichiometric in natureith variation from S-rich to Sn-rich compositions with increase in

ath pH. XRD studies indicated that no abrupt change in the crystal-ographic structure occurred with the variation of Sn-to-S ratio andther impurity phase, i.e. SnS2 was too little to be detected. Inter-stingly the bandgaps of all the SnS films would lie within a shortange despite the wide variation of the non-stoichiomerty, whichndirectly established that there has been little or no change in bandtructure. Furthermore, this paper indicates that it is quite possibleor the electrochemical growth of SnS on CdS coated ITO substrateo simplify the heterostructure fabrication technique. However, the

hotovoltaic effect obtained from this heterostructure was too lowresumably due to (a) the presence of large number of trap states athe interface (which ultimately resulted in lowering the minorityarrier diffusion length) and (b) the high sheet resistively of vacuumvaporated CdS.

[[[

ence 256 (2010) 4328–4333 4333

Acknowledgements

The authors would like to acknowledge to Dr. Subhasis Neogiof School of Energy Studies, J.U. and Mr. Sabyasachai Shome ofGeological Survey of India for extending their help.

References

[1] M. Ristov, G. Sinadinovski, I. Grozdanov, M. Mitreski, Thin Solid Films 173(1989) 53.

[2] B. Ghosh, M. Das, P. Banerjee, S. Das, Sol. Energy Mater. Sol. Cells 92 (2008)1099.

[3] M. Ristov, G. Sinadinovski, M. Mitreski, M. Ristova, Sol. Energy Mater. Sol. Cells69 (2001) 17.

[4] B. Ghosh, M. Das, P. Banerjee, S. Das, Semicond. Sci. Technol. 24 (2009) 025024.[5] M. Radot, Rev. Phys. Appl. 18 (1977) 345.[6] P.K. Nair, M.T.S. Nair, J. Phys. D: Appl. Phys. 24 (1991) 83.[7] H. Noguchi, A. Setiyadi, H. Tanamura, T. Nagatomo, O. Omoto, Sol. Energy Mater.

Sol. Cells 35 (1994) 325.[8] N.K. Reddy, K.T.R. Reddy, Thin Solid Films 325 (1998) 4.[9] M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 6 (1991) 132.10] Z. Zainal, M.Z. Hussein, A. Ghazali, Sol. Energy Mater. Sol. Cells 40 (1996) 347.11] A. Ghazali, Z. Zainal, M.Z. Hussein, A. Kassim, Sol. Energy Mater. Sol. Cells 55

(1998) 237.12] B. Subramanian, C. Sanjeeviraja, M. Jayachandran, Mater. Chem. Phys. 71 (2001)

40.13] S. Cheng, Y. Chen, C. Huang, G. Chen, Thin Solid Films 500 (2006) 96.14] W.G. Pu, Z.Z. Lin, Z.W. Ming, G.X. Hong, C.W. Qun, Proceedings of the 1st World

Conference on Photovoltaic Energy Conversion, Hawaii, 1994, p. 365.

18] B. Ghosh, M. Das, P. Banerjee, S. Das, Solid State Sci. 11 (2009) 461.19] S.K. Cheung, N.W. Cheung, Appl. Phys. Lett. 49 (1986) 85.20] L.I. Berger, Semiconductor Materials, CRC Press, New York, 1997, p. 343.