007) 5771–5776www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Structural and chemical transformations in SnS thin films usedin chemically deposited photovoltaic cells

David Avellaneda ⁎, Guadalupe Delgado, M.T.S. Nair, P.K. Nair

Department of Solar Energy Materials, Centro de Investigación en Energía, Universidad Nacional Autónoma de México, Temixco, Morelos-62580, Mexico

Available online 31 January 2007

Abstract

Chemically deposited SnS thin films possess p-type electrical conductivity. We report a photovoltaic structure: SnO2:F–CdS–SnS–(CuS)–silver print, with VocN300 mVand Jsc up to 5 mA/cm2 under 850 W/m2 tungsten halogen illumination. Here, SnO2:F is a commercial spray-CVD(Pilkington TEC-8) coating, and the rest deposited from different chemical baths: CdS (80 nm) at 333 K, SnS (450 nm) and CuS (80 nm) at 293–303 K. The structure may be heated in nitrogen at 573 K, before applying the silver print. The photovoltaic behavior of the structure varies withheating: Voc ≈ 400 mV and Jscb1 mA/cm2, when heated at 423 K in air, but Voc decreases and Jsc increases when heated at higher temperatures.These photovoltaic structures have been found to be stable over a period extending over one year by now. The overall cost of materials, simplicityof the deposition process, and possibility of easily varying the parameters to improve the cell characteristics inspire further work. Here we reporttwo different baths for the deposition of SnS thin films of about 500 nm by chemical deposition. There is a considerable difference in the nature ofgrowth, crystalline structure and chemical stability of these films under air-heating at 623–823 K or while heating SnS–CuS layers, evidenced inXRF and grazing incidence angle XRD studies. Heating of SnS–CuS films results in the formation of SnS–CuxSnSy. ‘All-chemically depositedphotovoltaic structures’ involving these materials are presented.© 2007 Elsevier B.V. All rights reserved.

Keywords: Chemical deposition; Tin sulfide; Thin films; Photovoltaic structures

1. Introduction

The physico-chemical processes underlying the chemicaldeposition method of semiconductor thin films have been de-scribed in a recent book [1]. Thin films of SnS deposited by thistechnique was first reported in 1987 [2,3], and ever sinceinterest in these thin films has grown [4–6]. Thin films of SnScan be obtained by many other techniques as well: vacuumevaporation [7], electron beam deposition [8], chemical vaportransport [9], and spray pyrolysis [10]. The optical band gapsfor the films vary from 1.0 to 1.5 eV depending on the depo-sition technique and/or measurement methodology employed,as listed in [11].

Chemical bath deposition of SnS [5], its conversion to SnO2

by air annealing [12], the use of SnS–CuS thin films as a

⁎ Corresponding author. Tel.: +52 55 56229731.E-mail addresses: daa@cie.unam.mx (D. Avellaneda), mtsn@cie.unam.mx

(M.T.S. Nair).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.12.078

spectrally selective solar radiation control or absorber layer[13], and the formation of photoconductive p-type Cu4SnS4[14] absorber films, etc., have been reported from our laboratorysince 1991. The use of vacuum deposited tin sulfide as anabsorber layer in heterojunction photovoltaic structures of thetype: n-CdS/p-SnS/Ag, has been demonstrated in 1994 byanother group [15]. An open circuit voltage Voc=120 mV, and ashort circuit current density Jsc=7 mA/cm2 with a conversionefficiency of 0.29% were reported in that structure. This wasfollowed by a report [16] in 2003 of a photovoltaic structurewith brush plated SnS with Voc=370 mV, short circuit current of780 mA, and efficiency 0.63%.

In the present work we show the principal characteristics ofchemically deposited tin sulfide thin films obtained from twodifferent chemical baths. These films have been integrated intophotovoltaic structures in different configurations and withdifferent heat treatments, and the cell characteristics are pre-sented. Our basic intention is to illustrate the many possibilitiesthat exist in developing an all-chemically deposited solar celltechnology in which SnS thin film would constitute the

5772 D. Avellaneda et al. / Thin Solid Films 515 (2007) 5771–5776

principal p-type absorber layer and a CdS film or anothersuitable material as the window layer.

2. Experimental details

2.1. Deposition of thin films leading to photovoltaic structures

2.1.1. Chemical bath for SnS deposition — Bath ATo prepare a 0.1 M solution of Sn(II), 15 ml of glacial acetic

acid was added to 1.13 g tin(II) chloride, SnCl2·2H2O, (BakerAnalyzed Reagent) taken in a beaker and heated at∼363 K withstirring for 6–8 min on a hot plate, followed by the addition of1 ml of conc. HCl to completely dissolve the salt to result in aclear transparent solution, and the volume was taken to 50 mlwith the addition of distilled water in a standard volumetricflask. The deposition bath was prepared by transferring 10 ml ofthis solution to a 100 ml beaker, followed by the sequentialaddition of 30 ml of 3.7 M triethanolamine (as supplied dilutedto 50%), 16 ml of 15 M ammonia, NH3(aq), 10 ml of 0.1 Mthioacetamide (TA), and 34 ml of distilled water with con-tinuous stirring. The substrates, 1 mm thick Corning microscopeglass slides as well as commercial (TEC-8 from Pilkington,Toledo) transparent conductive oxide (SnO2:F) coated glasssubstrates on which a thin layer of CdS with thickness of ≈100 nm was deposited for photovoltaic structure, were im-mersed in the bath. At 293–298 K (room temperature), weobtained a terminal thickness of 100 nm for the tin sulfide filmsin about 5–6 h. Up to five consecutive deposits, each in afreshly prepared bath, were made to achieve a thickness ofabout 500 nm for the SnS film integrated in the photovoltaicstructures reported here. Our effort to deposit thicker films usinga concentrated bath was not successful.

2.1.2. Chemical bath for SnS deposition — Bath BThis bath was prepared following the procedure reported

previously [5]: 1 g of tin chloride (SnCl2·2H2O) was dissolvedin 5 ml of acetone in a 100 ml beaker, followed by the sequentialaddition of 12 ml of 3.7 M triethanolamine (TEA), 65 ml ofdistilled water, 8 ml of 1 M thioacetamide (TA) and 10 ml of4 M ammonia, NH3(aq), to complete a volume of 100 ml. Thedeposition was made on Corning glass slide or on the con-ducting glass substrates with a CdS coating for photovoltaicstructures. At 308 K, a thin film with a thickness of 0.50 μm isdeposited in about 20 h. This film would appear deep red intransmitted daylight.

2.1.3. Deposition of CDS thin filmsTo prepare 100 ml of the deposition bath, 30 ml of 0.1 M

cadmium acetate, 10 ml of 1 M sodium citrate, 10 ml of 1.5 Mammonia (aq), 8 ml of 1 M thiourea, and finally distilled waterwere mixed with stirring in a 100 ml beaker [17]. Thin films of100 nm thickness were deposited at 343 K in 3 h, on SnO2:F-coated conducting glass substrates.

2.1.4. Deposition of CuS thin filmsIn order to complete two of the photovoltaic structures

presented in this work, a layer of CuS was deposited. The bath

composition was the same as that reported in [18]: 5 ml of a 1 Msolution of copper chloride (CuCl2·2H2O), 8 ml of 3.7 Mtriethanolamine (TEA), 8 ml of 15 M ammonia (NH3 aq), 10 mlof 1 M sodium hydroxide (NaOH), and 6 ml of 1 M thiourea,were added in a 100 ml beaker and the volume taken to 100 mlby the addition of water. Two different durations, 30 min and1 h, of depositions at 303 K were used to obtain CuS on SnSthin films.

2.2. Heat treatment

The thin films of SnS, SnS–CuS, and the photovoltaicstructures incorporating the films were heated in nitrogen or inair at different temperatures for modifying their compositionsand structural, electrical and optical properties. Heating in ni-trogen was done in a vacuum oven (T-M High VacuumProducts) and that in air, in a furnace (Sola Basic, Lindberg).For annealing in nitrogen, the samples were first introduced inthe oven and the chamber was evacuated to about 20 mTorr.Subsequently nitrogen was introduced into the chamber to therequired pressure, and the furnace was heated. All through theheating the flow of nitrogen was maintained at the chosenpressure. Samples of SnS (from Baths A and B) were annealedat 573 K in 300 mTorr nitrogen or at 398–823 K in air fordifferent durations. In the case of SnS (Bath A)–CuS samples,the heating was done at 623 K in nitrogen, to convert it toCu2SnS3. Photovoltaic structures, SnO2:F–CdS–SnS–CuS,prepared with SnS thin films from Bath A and CuS filmsdeposited for 1 h were heated at 588 K in nitrogen to convert thetop layer into Cu2SnS3, while that with CuS films of 30 mindeposition were heated at 473 K in air for 10 min in order toconvert the CuS film into a more conductive (p-type) layer, asdescribed in [18]. Another structure, SnO2:F–CdS–SnS, withSnS from Bath B was heated at 398–448 K in air.

2.3. Characterization

Structural characterization of the films was done by X-raydiffraction (XRD), recorded on a Rigaku D/MAX-2000 X-raydiffractometer in the standard mode, using Cu-Kα radiation. Theoptical transmittance (T) and specular reflectance (R) of the filmswere recorded using a Shimadzu 3100 PC spectrophotometer inthe 250–2500 nm wavelength range. For the transmittance, thereference was air, whereas for the reflectance, it was a frontaluminized mirror. For this, the film on one side of the substratewas removed by scrubbing carefully with cotton swabs moist-ened in dilute HCl and subsequently with water. For the elec-trical characterization, we printed two electrodes of 5 mm lengthat 5 mm separation with silver paint on the surface of the films.With the samples placed in the chamber, enough time was givento stabilize the current in the dark before applying a voltageacross the electrodes for recording the photocurrent response ateach 0.5 s: during the first 20 s in the dark, next 20 s under light,and the last 20 s in the dark to record the decay of thephotocurrent. We used a Keithley 230 programmable voltagesource and a Keithley 619 electrometer for recording these data.For the illumination, a tungsten halogen lamp was used.

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3. Results and discussion

3.1. Crystalline structure of the films

Fig. 1(a) shows the XRD pattern of a thin film of SnS (BathB) of 500 nm thickness deposited on Corning substrate. Welldefined peaks at 2Θ=31.5°, 30.4°, and 26° match the standardpattern for the mineral herzenbergite (PDF #39–0354) [19] withSnS composition and correspond to crystallographic planes:(111), (101) and (120), respectively. Fig. 1(b) shows the XRDpattern of a thin film of SnS of thickness about 500 nm obtainedby multiple depositions from Bath A on Corning glass substrate.Irrespective of the presence of well defined XRD peaks inthis pattern, there is no satisfactory match with either theherzenbergite pattern or with any other compound in the SnxSysystem. It is possible that presence of some mixed phases andpreferential orientation have made the pattern not easilydiscernible. However, in Fig. 1(c) it is seen that annealing thisfilm in air at 823 K transforms the film into SnO2, showing welldefined peaks at 2Θ=26.6°, 33.8°, 51.7°, matching well thestandard pattern for the mineral cassiterite (PDF #41–1445)[19] of composition SnO2. Similar XRD patterns with matchingpeak positions corresponding to the crystallographic planes:(110), (101), and (211) of SnO2 are also observed in the case ofthe SnS film from Bath B after they are heated in air at 673 K. InX-ray fluorescence scans of the films, we obtained the S/Snfluorescence peak height ratio of nearly the same for the filmsdeposited from both baths, showing that both have very nearlythe same chemical composition, but might be distinctstructurally. This distinction would lead to distinct opticalband gaps in the samples and to different conversion rates tooxide phase, as can be seen below.

Fig. 1. XRD patterns of thin film samples: a) of SnS from Bath B; b) of SnS from Bd) obtained after heating SnS (Bath A)–CuS layer at 623 K in N2 300 mTorr, alongCu2SnS3.

Fig. 1(d) shows XRD peaks at 2Θ=28.4°, 32.9°, 47.2° and56°, matching closely the standard pattern of the mineral mohite(PDF #27–0198) [19] with composition Cu2SnS3 for a samplewith a layer of CuS on SnS (Bath A) annealed in N2 at 623 K.The observed peaks correspond to the crystallographic planes:(2̄ 1̄1), (2̄06), (2̄010) and (3̄ 2̄10), respectively, of the cubicdisordered structure of the mineral.

3.2. Optical and electrical properties

Fig. 2 shows the optical characteristics, (transmittance, T,and reflectance, R, spectra), of the tin sulfide thin films fromBath A (300 nm) and Bath B (500 nm). These data wereemployed to evaluate the optical absorption coefficient (α) ofthe films plotted in Fig. 2b) and to analyze the optical band gap,Fig. 2c) and d), using the equations [20]:

T ¼ 1−Rð Þ2exp −adð Þ1−R2exp −2adð Þ ; and

a ¼ 1dln

1−Rð Þ2þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1−Rð Þ4þ 2RTð Þ2

q

2T

24

35

We find that the best fit (with a correlation factor for straightline fit of 0.9999) for the film deposited from Bath A is obtainedconsidering forbidden optical transition across a direct gap of1.7 eV, in accordance with [21]:

ahm ¼ C hm−Eg

� �3=2:

In the case of the films deposited from Bath B, an indirectgap of 1.12 eV best fits the data. The difference in the band gaps

ath A; c) obtained after heating in air at 823 K (Bath A) or at 673 K (Bath B);with the standard patterns of herzenbergite, SnS, cassiterite, SnO2, and mohite,

Fig. 2. Optical characteristics of SnS thin films deposited from Baths A and B: a) transmittance (T%), and reflectance (R%) spectra; b) optical absorption coefficient, α;and band gap analyses, c) for film A; d) for film B.

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of the films deposited from the two baths is apparent from thedifference in the wavelengths corresponding to the onset of theoptical absorption in the transmittance curves of Fig. 2a) as wellas in the optical absorption coefficients in Fig. 2b).

From the photocurrent response curves in Fig. 3, recordedwith an applied bias of 100 V, of the films from Bath A (300 nmthickness), before and after annealing in nitrogen at 573 K, wesee that the dark conductivity is 6×10−8 Ω−1 cm−1(Fig. 3a),and that the conductivity under illumination increases by twoorders of magnitude. The conductivity in the dark for the films(500 nm thickness) obtained from Bath B is 1.4×10−6 Ω−1

cm−1, which increases to 2×10−4 Ω−1 cm−1 under illumina-tion. Annealing this sample at 573 K in N2 increases the

Fig. 3. Photocurrent response in SnS thin films: a) from Bath A; b) from Bath B;temperature.

conductivity: 2×10−3 Ω−1 cm−1 in the dark; and 6×10−3 Ω−1

cm−1 under illumination. A bias of 10 V was applied forrecording the photocurrent response in these samples. Hot probemeasurements showed a p-type conductivity in the films afterannealing. We note that the magnitude of this p-type con-ductivity falls in the range desired for a p-absorber in solar cellstructures.

3.3. I–V characteristics of photovoltaic structures

Thin films of both the baths (A and B), were integrated intophotovoltaic structures, by depositing them over thin films ofCdS on conductive glass (SnO2:F) substrates, thus forming an

the data for the films annealed at 573 K in N2 are indicated by the annealing

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absorber layer on a CdS window. The characteristics of thesestructures are presented in Figs. 4 and 5, illustrating the cellstructure in each case. The heat treatment employed, in eachcase is also described in the figure caption.

In the case of the cell structures in Fig. 4, a tin sulfide film of400–500 nm in thickness deposited from Bath Awas used. Thesimplest of the cell structure, SnO2:F–CdS–SnS–silver print,without any heating, shown in Fig. 4c), has a Voc=380 mV, andJsc=0.05 mA/cm2. We consider that the low conductivity of theSnS absorber layer is responsible for the low Jsc. The high Vocof 380 mV suggests that SnS (Bath A) is p-type, since CdS thinfilms obtained by chemical deposition is well established to bean n-type semiconductor. The photovoltaic structure of Fig. 4b),additionally has a layer of CuS thin film of 50–100 nmdeposited for 30 min at 303 K onto SnS (Bath A). This structurewas heated at 473 K in air for 10 min to reduce the CuS thin filmresistance. The silver paint electrode was applied after this, overa 1 mm2 area. This structure shows a very conductive CuS layer,with a sheet resistance of 200 Ω. We isolated an area of 1 cm2

around the silver print electrode, which was consideredin the estimation of Jsc. The values for this cell structure are:Voc=310 mV, and Jsc=1 mA/cm2. For the structure described inFig. 4a), a CuS thin film of approximately 100 nm was

Fig. 4. I–V characteristic of three different photovoltaic structures with SnSfrom Bath A: a) structure heated at 588 K for 1 h in nitrogen; b) structure heatedin air for 10 min at 473 K; c) without any heat treatment.

Fig. 5. I–V characteristic of the photovoltaic structure SnO2:F–CdS–SnS (BathB) before and after heating in air for 15 min at 398–448 K: a) in the dark; and b)under illumination.

deposited on the SnS thin film, and the cell was annealed in anitrogen atmosphere, at 588 K, for 1 h. Grazing angle X-raydiffraction at incidence 0.5° has shown that the formation of aternary compound semiconductor Cu2SnS3, as illustrated inFig. 1d), occurs at the surface through the reaction of SnS(fraction of the thickness) and CuS thin films. The structure wascompleted with silver paint electrodes and baked in air at 353 Kfor 15 min. In the present report, this is the cell showing the bestcharacteristics: Voc=340 mV, and Jsc=6 mA/cm2.

The I–V characteristics in Fig. 5, correspond to thephotovoltaic structure: SnO2:F–CdS–SnS, with a 500 nm ofSnS thin film deposited for 20 h at 308 K (from Bath B). Silverpaint electrodes of approximately 2 mm2 area were printed onthe film surface. The measurements were made on the asprepared structure, and after annealing the structure in air, at398–448 K for 15 min each to assess the stability. Arepresentative curve for the structure annealed at 398 K is given.

The structure, as prepared, shows Voc =380 mV, andJsc = 0.17 mA/cm2. After annealing, Voc decreased to360 mV, and Jsc increased to 0.31 mA/cm2. This increasecan be explained on the basis of the transformations thatoccur in the SnS (B) samples due to the annealing, whichimproves the film conductivity, as shown in Fig. 3b).

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The stability of the I–V characteristics of all the cells pre-sented here are satisfactory. Over a period of one year, the cellcharacteristics remained stable during periodic measurements.

4. Conclusions

In this work, we described the characteristics of the tinsulfide thin films prepared by chemical deposition using twochemical formulations with distinct structural, optical and elec-trical properties, but both useful as p-absorber films in all-chemically deposited photovoltaic structures. Samples of BathA involving the use of acid to dissolve tin(II) chloride showsdirect forbidden band gap Eg=1.7 eV, and electrical conduc-tivities of 6×10−8 Ω−1 cm−1 in the dark and ≈6×10−6 Ω−1

cm−1 under illumination. These values remain approximatelythe same after annealing in nitrogen. When heated in air at 773–823 K, these films convert to SnO2.

Samples of SnS (Bath B), have an indirect band gapEg=1.12 eV, and electrical conductivities of 1.4×10−6 Ω−1

cm−1 in the dark and 2×10−4 Ω−1 cm−1 under the illumi-nation. When this film is annealed in nitrogen at 573 K, theconductivity in the dark increases to 2×10−3 Ω−1 cm−1, whilethat under illumination changes to 6×10−3 Ω−1 cm−1. ThisSnS film is converted to SnO2, by heating in air at 673 K.

Both the thin films were integrated into distinct photovoltaicstructures. The best value we are reporting here is: Voc=340 mVand Jsc=6 mA/cm2 in the cell structure, glass-SnO2:F–CdS–SnS/Cu2SnS3–Ag (print). All the thin films, except the com-mercially available conducting glass, were prepared bychemical bath deposition technique. We consider that the re-sults presented in this report offer various possibilities forlooking for improved characteristics through optimization of thefilm thicknesses and process temperatures. The large-areacapability of the chemical deposition technique is well accepted,and hence a solar cell technology based on this approach canemerge through further work.

Acknowledgments

We are grateful to Maria Luisa for recording the XRDpatterns, Jose Campos for assistance in electrical measurements,Oscar GomezDaza for general assistance in the chemical

laboratory, and Patricia Altuzar for XRF measurements. We alsoacknowledge the financial support provided by CONACyT–México (Project, 40515F) and DGAPA-UNAM (Project,IN110906). David Avellaneda is thankful to CONACyT,México for the graduate student fellowship.

References

[1] G. Hodes, Chemical Solution Deposition of Semiconductor Films, MercelDekker Inc., NY, 2003.

[2] R.D. Engelken, H.E. McCloud, C. Lee, M. Slayton, A. Ghoreshi,J. Electrochem. Soc. 134 (1987) 2696.

[3] P. Pramanik, P.K. Basu, S. Biswas, Thin Solid Films 150 (1987) 269.[4] A. Tanusevsky, Semicond. Sci. Technol. 18 (2003) 501.[5] M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 6 (1991) 132.[6] J.B. Johnson, H. Jones, B.S. Latham, J.D. Parker, R.D. Engelken, C.

Barber, Semicond. Sci. Technol. 14 (1999) 501.[7] M.M. El-Nahass, H.M. Zeyada, M.S. Aziz, N.A. El-Ghamaz, Opt. Mater.

20 (2002) 159.[8] A. Tanusevski, D. Poelman, Sol. Energy Mater. Sol. Cells 80 (2003) 297.[9] A. Ortiz, J.C. Alonso, M. Garcia, J. Toriz, Semicond. Sci. Technol. 11

(1996) 243.[10] K.T. Ramakrishna Reddy, P. Purandar Reddy, R.W. Miles, P.K. Datta, Opt.

Mater. 17 (2001) 295.[11] B. Thangaraju, P. Kaliannan, J. Phys., D, Appl. Phys. 33 (2000) 1054.[12] P.K. Nair, M.T.S. Nair, R.A. Zingaro, E.A. Meyers, Thin Solid Films 239

(1994) 85.[13] P.K. Nair, M.T.S. Nair, V.M. García, O.L. Arenas, Y. Peña, A. Castillo, I.T.

Ayala, O. GomezDaza, A. Sánchez, J. Campos, H. Hu, R. Suárez, M.E.Rincón, Sol. Energy Mater. Sol. Cells 52 (1998) 313.

[14] M.T.S. Nair, C. Lopéz-Mata, O. GomezDaza, P.K. Nair, Semicond. Sci.Technol. 18 (2003) 755.

[15] Hidenori Noguchi, Agus Setitadi, Hiromasa Tanamura, Takao Nagatomo,Osamu Omoto, Sol. Energy Mater. Sol. Cells 35 (1994) 325.

[16] B. Subramanian, C. Sanjeeviraja, M. Jayachandran, Sol. Energy Mater.Sol. Cells 79 (2003) 57.

[17] M.T.S. Nair, P.K. Nair, R.A. Zingaro, E.A. Meyers, J. Appl. Phys. 75(1994) 1557.

[18] P.K. Nair, V.M. García, A.M. Fernández, H.S. Ruiz, M.T.S. Nair, J. Phys.,D 24 (1991) 441.

[19] Powder Diffraction File, Joint Committee on Powder DiffractionStandards, International Center for Diffraction Data, Swarthmore, PA,Cards 39–0354, 41–1445 and 27–0198.

[20] J.I. Pankove, Optical Processes in Semiconductors, Dover Publications,Inc., NY, 1971.

[21] R.H. Misho, W.A. Murad, Sol. Energy Mater. Sol. Cells 27 (1992) 335.