Heat treatments in chemically deposited SnS thin films and theirinfluence in CdS/SnS photovoltaic structures

D. Avellaneda • B. Krishnan • A. C. Rodriguez •

T. K. Das Roy • S. Shaji

Received: 30 June 2013 / Accepted: 28 August 2014

� Springer Science+Business Media New York 2014

Abstract Chemical deposition technique is used to produce

good quality tin sulphide (SnS) thin films of 450 nm in

thickness, from a chemical bath containing tin chloride

(SnCl2�2H2O) and thioacetamide. The SnS thin films were

annealed at 300, 350 and 400 �C for 1 h in vacuum, nitrogen

and argon atmospheres, resulting in the modification of their

optical and electrical properties. The optical band gap showed

a decrease from 1.2 to 1 eV whereas the electrical conduc-

tivity increased by more than two orders in the heat treated

films at 400 �C in comparison with those of pristine samples.

The morphology analysis of the as prepared and heat treated

SnS thin films was done using scanning electron microscopy;

X-ray diffraction patterns showed orthorhombic SnS phase

and the chemical states of the elements were confirmed by

X-ray photoelectron spectroscopy. The heat treated SnS thin

films (1.8 lm thickness) were incorporated to photovoltaic

structures of the type: Glass/TCO/CdS/SnS/C/Ag, showing

open circuit voltages (Voc) from 270 to 330 mV and a short

circuit current density (Jsc) up to 6 mA/cm2.

1 Introduction

One of the most important challenges in solar cell tech-

nology is the development of thin film solar cells based on

inexpensive processes using materials of high abundance.

Many attempts have been made for searching these types of

abundant, non-toxic and cost-effective new materials

which are suitable for that purpose. One of the emerging

materials is CZTS (copper-zinc-tin-sulphide) that has

already been incorporated as absorber material in solar

cells, with conversion efficiencies close to 10 % [1].

Another material that has been received more attention

recently is the IV–VI compound semiconductor tin sul-

phide (SnS), due to the abundance of Sn in the earth’s crust

([2 ppm), and the facility to produce it in the thin film

form by various techniques such as electrochemical depo-

sition [2, 3], spray pyrolysis [4], thermal evaporation [5],

chemical bath deposition [6], etc. Thin films of SnS exhibit

p-type conductivity, optical band gap from 1–1.7 eV, and

orthorhombic or zinc blende crystalline structure, depend-

ing on the preparation method [6–9].

Thin films of tin sulphide have been incorporated to

photovoltaic structures giving open circuit voltages (Voc) in

the range of 120–370 mV and short circuit current densities

(Jsc) between 1 and up to 9 mA/cm2 [10–13]. Wang et al.

reported the fabrication of photovoltaic structures using

nanocrystals of SnS giving Voc = 471 mV, Jsc = 0.3 mA/

cm2, FF = 0.71 and conversion efficiency of 0.1 % [14].

Recently, Bashkirov et al. reported the formation of hetero-

junctions of the type: Mo/SnS/CdS/ZnO:Al with SnS

obtained by hot wall deposition method, CdS by chemical

bath and ZnO by magnetron sputtering. The parameters

reported were: Voc = 132 mV, Jsc = 3.6 mA/cm2, FF =

0.29 and conversion efficiency up to 0.5 % [15]. Among all

photovoltaic parameters reported, the best results were

Voc = 260 mV, Jsc = 9.6 mA/cm2 and FF = 0.53 to

achieve conversion efficiencies of 1.3 % [16], using SnS thin

films of 600 nm in thickness with a band gap close to

1.32 eV, prepared by spray pyrolysis.

D. Avellaneda (&) � B. Krishnan � A. C. Rodriguez �T. K. Das Roy � S. Shaji

Facultad de Ingenierıa Mecanica y Electrica, Universidad

Autonoma de Nuevo Leon, C.P. 66450 San Nicolas de los Garza,

Nuevo Leon, Mexico

e-mail: david.avellanedavl@uanl.edu.mx

B. Krishnan � S. Shaji

CIIDIT- Universidad Autonoma de Nuevo Leon, Apodaca,

Nuevo Leon, Mexico

123

J Mater Sci: Mater Electron

DOI 10.1007/s10854-014-2295-2

In the present work, we report modifications in the

structural, morphological, optical and electrical character-

istics of chemical bath deposited SnS thin films due to post

deposition treatments (annealing) in vacuum (V), nitrogen

(N2) and argon (Ar) atmospheres at temperatures of 300, 350

and 400 �C. Such films were incorporated as absorber layer

in photovoltaic structures using chemical bath deposited CdS

as a window material. Photovoltaic characterizations of the

cells in dark and under illumination are also presented.

2 Experimental

2.1 Chemical deposition

(a) SnS thin films: SnS thin films were deposited on

microscope (Corning) glass substrates (25 9 75 9 1 mm)

cleaned using distilled water and neutral soap, and finally

dried with air. The SnS bath was prepared following the

procedure reported previously [6, 10, 18]: in a 100 ml

beaker, 1 g of tin chloride (SnCl2�2H2O) was dissolved in

5 ml of acetone, followed by the sequential addition of

12 ml of 3.7 M triethanolamine (TEA), 65 ml of distilled

water, 8 ml of 1 M thioacetamide (TA) and 10 ml of 4 M

ammonia, NH3(aq). The substrates were placed vertically

into the beaker with the bath composition, and placed in a

controlled temperature of 40 �C and 17 h to obtain thick-

ness of SnS thin films close to 450 nm, for the character-

ization and annealing of the films.

(b) CdS thin films: The bath composition used to obtain

CdS thin films was reported previously [17]: to 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 M

ammonia (aq), 8 ml of 1 M thiourea, and finally distilled

water. The bath temperature was kept at 70 �C for 3 h,

which resulted in good quality CdS thin films of thickness

close to 200 nm. The thickness of the samples was mea-

sured using an Alpha Step 100 thickness measurement unit

(Tencore, CA).

2.2 Preparation of photovoltaic structures

To develop the PV structures, commercial (TEC-15 from

Pilkington, Toledo) transparent conductive oxide, TCO,

(SnO2:F) coated glass substrates were used to deposit CdS

thin films. These substrates of TCO coated with a layer of

CdS, were placed in four consecutive fresh baths of SnS:

the first one at 40 �C for 17 h, and the next three at 60 �C

for 6 h each, to obtain final thickness close to 1.8 lm. The

photovoltaic structures were completed with a 50 nm of

evaporated Ag (99.999 %) onto colloidal carbon suspen-

sion as electrodes. Thermal evaporation was done in a

high vacuum system (INTERCOVAMEX-TE12P), and

thickness of Ag layer was measured using a quartz crystal

thickness monitor incorporated in the evaporation system.

2.3 Annealing the thin films

The samples of tin sulphide (SnS) deposited on glass

substrates and the photovoltaic structures deposited on

TCO were heated at 300, 350 and 400 �C for 1 h in vac-

uum, nitrogen (300 mTorr) and argon (300 mTorr). The

annealing was done in a vacuum oven (T-M High Vacuum

Products). To heat in nitrogen atmosphere, the samples

were introduced into the oven and the chamber was purged

twice with N2 at a pressure of 100 mTorr, and after that the

nitrogen was introduced at the work pressure (300 mTorr)

the heating of the samples began. Similar procedure was

done in the case of annealing in argon.

2.4 Characterization

X-ray diffraction patterns (XRD) were recorded using an

Empyrean diffractometer with CuKa radiation (k = 1.5406

A), operated at 45 kV and 40 mA; the scans were per-

formed in the 2h range from 20� to 60� in a step of 0.016�and 80 s per step in a continuous mode. Optical transmit-

tance (T) and near-normal specular reflectance (R) were

measured using a UV–Vis spectrophotometer (Shimadzu

UV-1800) in the 300–1,100 nm wavelength range; the

films on one side of the substrates were removed with

cotton swabs moistened in dilute HCl, cleaned with water,

and dried. The spectrometer was calibrated (base line set-

ting process) using air as sample as well as reference for

the transmittance measurements. Similarly, for the reflec-

tance measurements, a front aluminized mirror supplied by

the manufacturer was used. The morphological analysis of

the films was done using a scanning electron microscope

(FEI Nova Nano, SEM), and an energy dispersive X-ray

analyzer (EDX) associated with the SEM was used for the

elemental detection. The X-ray photoelectron spectra were

recorded on K-Alpha (Thermo Scientific) X-ray photo-

electron spectrometer, using Al Ka radiation (1,486.6 eV)

as the exciting source. The electrical characterization was

done using a Keithley Picoammeter/Voltage Source 6487

connected with a computer through RS-232 interface. Pairs

of silver paint electrodes of 5 mm in length separated by

5 mm were printed on the surface of the films for the

photocurrent measurements. The samples were allowed to

stabilize in the dark inside the measurement chamber prior

to applying the voltage. Photocurrent response of the films

was measured under an illumination intensity of 85 mW/

cm2 from a tungsten halogen lamp.

For the characterization of the photovoltaic structures,

carbon electrodes were painted in the top absorber layer

(SnS), and Ag was evaporated on the carbon electrodes after

J Mater Sci: Mater Electron

123

the annealing of the structure. TCO was taken directly as the

negative contact to measure the I–V characteristics in the

dark and under illumination, using an Oriel solar simulator

(model 96000) under an AM1.5 radiation of 100 mW/cm2.

3 Results and discussion

3.1 XRD

Figure 1a–c shows the XRD patterns for SnS thin films of

0.45 lm in thickness, annealed in vacuum, nitrogen and

argon atmospheres, at 300, 350 and 400 �C, respectively. At

different conditions, the recorded patterns are similar: well

defined peaks are present at 2h = 22�, 26�, 30.4�, 31.5�, 39�and 49�. All these peaks match the standard pattern for

mineral herzenbergite (PDF# 39-0354) of chemical formula

SnS. The observed peaks correspond to the crystallographic

planes: (110), (120), (101), (111), (131) and (211), respec-

tively. The shape of the peaks in the range of 2h = 42–46�,

shows overlap of the weak planes: (210), (141) and (002)

located at 2h = 42.5�, 44.7� and 45.5�, as marked in the

figure. Thus, the thin films annealed at 300–400 �C show

orthorhombic SnS as reported previously [10]. There were

no peaks corresponding to other phases such as SnS2, Sn2S3

or SnO2 due to the heat treatment.

3.2 SEM with EDAX

Figure 2a–d shows the scanning electron micrographs for

the as prepared SnS thin films and heat treated films at

350 �C. In each case, the micrograph captured under

magnification 20,000 is given. The micrographs show

folded sheet like structure of an estimated thickness of the

individual sheet less than 100 nm. Also, flower like

assemblies were formed on the surface of all the samples.

This type of morphology was observed in SnS thin films

prepared by chemical bath deposition and other techniques

[19–22]. Similar growth was shown for SnS samples

annealed at 300 and 400 �C irrespective of the annealing

atmosphere. Semi-quantitative analysis for all the samples

using EDAX gave atomic ratios (at%) of Sn/S close to 1.1

for the as prepared, vacuum and nitrogen annealed sam-

ples, and 0.95 for the sample annealed in argon.

3.3 XPS analysis

Figure 3a shows a wide scan X-ray photoelectron spectra

(XPS) of SnS thin films annealed at 350 �C in argon

recorded after etching by 3 keV Ar? ions for 180 s.

Binding energies (BE) of all the peaks were corrected using

C 1 s energy at 284.6 eV corresponding to adventitious

carbon in addition to the charge compensation by the flood

gun associated with the spectrometer. The pattern of the

survey spectrum shows the presence of Sn, S, C and O. The

peaks corresponding to C 1s and O 1s were detected only

on the surface probably due to contamination in the air; no

other impurities were present in the film. The high reso-

lution spectra of S 2p and Sn 3d peaks are given in Fig. 3b

and c, respectively. The peak at 160.9 eV corresponds to

the energies of S 2p3/2 for S in SnS (reduction state -2) and

the peaks at 485.7 and 494.1 eV coincide with Sn 3d5/2 and

Sn 3d3/2 binding energies of Sn in the oxidation state ?2

[23]. The gap between Sn 3d5/2 and Sn 3d3/2 level is

8.4 eV, separated by spin orbit coupling. From the spectra,

the Sn/S ration was evaluated using the relation between

the integral area for S 2p and Sn 3d peaks and the value

was 1.02:1. This result is consistent with that obtained from

EDAX (Sect. 3.2). The BE values and the spectral char-

acteristics are in agreement with previous reports of similar

studies on SnS thin films [24, 25]. Comparable results were

obtained in the case of as prepared, vacuum and nitrogen

annealed samples.

3.4 Optical properties

Figure 4 shows the reflectance (R) and transmittance (T)

spectra of the as prepared and annealed SnS thin films.

From these data we can observe clearly that the transmit-

tance spectra of the annealed samples are shifted to longer

wavelengths, and this is more evident for the samples

annealed at 400 �C. The reflectance spectra are also shifted

in the same way. From this figure, we can observe that for

the visible region, almost all the radiation is absorbed in all

Fig. 1 X-ray diffraction patterns of SnS thin film samples annealed

in vacuum, nitrogen and argon at different temperatures: a 300 �C,

b 350 �C and c 400 �C. Principal planes that match the standard

pattern of the mineral herzenbergite (PDF#39-0354) are indicated

J Mater Sci: Mater Electron

123

the annealed films. From these transmittance and reflec-

tance data, the optical absorption coefficients (a) of the

films at different wavelengths were calculated using the

equation [26]:

a ¼ 1

dln

1� Rð Þ2þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1� Rð Þ4þ 2RTð Þ2q

2T

2

4

3

5;

where d is the thickness of the film. In semiconductors,avaries with hm according to the empirical relation [27]:

a ¼ Aðhm� EgÞm

hm

� �

;

where A is a constant, m = 1/2 or 3/2, for direct allowed or

forbidden transitions and m = 2, for indirect allowed

transitions respectively, across a band gap (Eg). To calcu-

late the value of Eg, we considered the Tauc plot of (ahm)1/m

versus (hm-Eg) from which the straight line region was

extrapolated to the hm axis. In our experiments the best

fitting was obtained for m = 2 corresponding to indirect

transitions, i.e. plots of (ahm)1/2 versus hm given in Fig. 5.

The plots show the values of Eg close to 1.25 eV for the ‘as

prepared’ SnS thin film, whereas for the samples annealed

at 300 �C the values slowly decreases to a minimum of

1.1 eV for the vacuum annealed film (V). For the 350 �C

samples the values of Eg remain *1.1 eV irrespective of

the annealing atmosphere. In the case of the annealed

samples at 400 �C, a similar behavior of slight reduction in

Eg is observed, with values close to 1.1 eV (N2, Ar) and

1 eV for the V annealed samples. The reduction in Eg value

from 1.25 to 1.1 eV in all the samples annealed at different

conditions can be due to an improvement in the crystal-

linity after annealing. Transmittance and reflectance mea-

surements were repeated for various times on the same

sample as well as on different sample prepared at same

conditions. The virtually identical results in spectra of T%

and R% indicated that the analyzed samples were homo-

geneous. The spectrophotometer used in our experiments

can detect 1 nm increase in wavelength. Considering the

propagation of errors for the calculation of absorption

spectra and the optical band gap, the estimated energy gap

is presented with an experimental error of ±0.01 eV.

3.5 Electrical properties

The photocurrent response curves of as prepared and

annealed SnS thin films are shown in Fig. 6. An applied

bias of 100 V was used in all cases, and the measurements

were carried out by collecting the current for 10 s in the

dark, 20 s under illumination and finally 10 s in the dark

again. The dark conductivity (rD) of the as prepared SnS

sample shown in the figure is 2.5 9 10-5 X-1 cm-1, and it

increases to 6.5 9 10-5 X-1 cm-1 under illumination (rL).

The values of rD and rL for the SnS thin films annealed in

vacuum (V), nitrogen (N2) and argon (Ar) atmospheres, at

300, 350 and 400 �C, are given in Table 1. From the table

Fig. 2 SEM images of thin films of SnS as prepared (a) and after annealing at 350 �C in different atmospheres: b vacuum, c nitrogen and

d argon

J Mater Sci: Mater Electron

123

for all annealed samples, conductivity is increased com-

pared to that of as prepared films. However for 300 and

400 �C samples, the vacuum annealed films show the

maximum value, while for 350 �C samples nitrogen

annealed films show the maximum value. All the samples

(as prepared and annealed) show photoconductivity: when

light was turned on (during 20 s) number of charge carriers

increased instantly and hence the photocurrent, as seen

from Fig. 6. However during illumination, there was a

slight increase in the current due to thermal effect of the

light source (tungsten halogen lamp). When light was

turned off, the thermal effect delayed the decay process of

the photocurrent. In general, the samples annealed above

300 �C showed better optical absorption and crystallinity;

hence the increase in conductivity during illumination.

The equipment used for the measurement of electrical

properties (Keithley) was calibrated and certified to operate

in the established ranges. The experimental error in the

electrical properties was mainly originated from the mea-

surement error of the printed electrodes (±1 mm) and the

film thickness (±5 nm) used which was measured with a

profiler. Taking these data into account, the conductivity

value was estimated with an error ±10 %.

3.6 Photovoltaic structures

For the development of photovoltaic structures, we selected

vacuum and argon annealed SnS thin films at 350 �C on the

basis of their electrical conductivity. Even though the

Fig. 3 XPS analysis of the SnS thin film annealed at 350 �C in argon

atmosphere, recorded after etching by 3 keV Ar? ions for 180 s:

a survey scan, b S region and c Sn region

Fig. 4 Transmittance (T %)

and reflectance (R %) spectra of

SnS thin films as-prepared and

annealed at 300, 350 and

400 �C, 1 h in vacuum, nitrogen

and argon (300 mTorr). SnS

thin film of four sequential

deposits (4d-1.8 lm) annealed

at 350 �C in argon atmosphere

is also included

J Mater Sci: Mater Electron

123

400 �C annealed samples were more conductive, the pho-

tovoltaic structures were short circuited probably due to the

diffusion of Sn into CdS layer at that temperature. The

photovoltaic structures were of the type: glass/FTO/CdS/

SnS/C/Ag. The J–V characterization of the structures was

made in the dark and under illumination (AM1.5, 100 mW/

cm2), and the cell parameters (short circuit current density,

Jsc, open circuit voltage, Voc, fill-factor FF and energy

conversion efficiency) were determined using the standard

procedures.

Figure 7a depicts the J–V curve under illumination of

the photovoltaic structure using as prepared SnS. The curve

under dark condition showing good rectification properties

is given in the inset. The principal parameters of the

structure were: Voc = 310 mV, Jsc = 0.43 mA/cm2 and

FF = 0.34, having low conversion efficiencies (0.04 %).

Figure 7b shows the J–V characteristics under dark and

illumination for photovoltaic structures annealed at 350 �C

in vacuum, which presents the parameters: Voc = 323 mV,

Jsc = 3.9 mA/cm2 and FF = 0.44, for conversion effi-

ciencies close to 0.55 %. The curves under dark and light

for the photovoltaic structures annealed at 350 �C in argon,

are presented in Fig. 7c giving the best results obtained

in this work: Voc = 270 mV, Jsc = 6 mA/cm2 and FF =

0.44, conversion efficiencies up to 0.7 %. It is worth to

note that these results are the best results reported so far for

SnS thin films as absorber layer, prepared by chemical bath

deposition technique. A comparative analysis of the present

results revealed that for the PV structures formed by

annealing the SnS thin films Jsc values were higher than

that of using as prepared SnS films. This may be due to

Fig. 5 Band gap energy (Eg) of

SnS samples at the different

annealed temperatures. As

prepared SnS is given for

comparison. Also Eg of SnS

(1.8 lm) annealed at 350 �C in

argon is shown

Fig. 6 Photocurrent response of SnS thin films as prepared and

annealed at 300, 350 and 400 �C in vacuum, nitrogen and argon. Bias

voltage of 100 V was applied in all cases. Light ON and OFF periods

were of 20 and 10 s respectively

Table 1 Electrical conductivity of annealed SnS thin films

Temperature Condition rD (X cm)-1

(9 10-3)

rL (X cm)-1

(9 10-3)

As prepared 0.025 0.065

300 �C Vacuum (V) 0.48 1.4

Nitrogen (N2) 0.04 0.15

Argon (Ar) 0.2 0.8

350 �C Vacuum (V) 2.6 4.9

Nitrogen (N2) 3.0 6.0

Argon (Ar) 0.95 3.0

400 �C Vacuum (V) 4.2 6.6

Nitrogen (N2) 2.7 5.6

Argon (Ar) 3.0 5.6

J Mater Sci: Mater Electron

123

increase in conductivity by the annealing process. The best

results for argon annealed structures, Jsc was increased by

more than ten times.

4 Conclusions

Heat treatments in SnS thin films prepared by chemical

bath deposition were done at 300, 350 and 400 �C for 1 h

in vacuum, nitrogen and argon atmospheres. The XRD

analysis showed that SnS thin films were of orthorhombic

crystal structure and XPS results confirmed the presence of

sulphur and tin with chemical states corresponding to that

in SnS. Heat treatments resulted in the modification of

optical and electrical properties of the thin films. The

optical characterization of these films showed a decrease in

the value of the band gap energy from 1.2 eV for the as

prepared film to a value close to 1 eV for the sample

annealed in vacuum at 400 �C. The maximum increase in

the values of electrical conductivities were observed in the

vacuum annealed thin films at 400 �C corresponding to 4.2

and 6.6 9 10-3 X-1 cm-1, for rD and rL, respectively.

The best photovoltaic results obtained in this work, were an

open circuit voltage, Voc = 270 mV, short circuit current

density, Jsc = 6 mA/cm2, fill factor of 0.44 and efficiency

of 0.7 %. Values of Jsc are smaller than that expected

theoretically from the band gap of absorber layer; this

could be mainly due to insufficient diffusion length in tin

sulphide. Further research on the control and character-

ization of the interface properties of photovoltaic structures

are needed to improve the efficiency of the photovoltaic

structures.

Acknowledgments The authors are thankful to PROMEP, Mexico,

CEMIE-SOL (CONACyT-SENER-UNAM) and PAICYT-UANL for

the financial assistance, CIIDIT-UANL for SEM and Dr. Josue

Amilcar Aguilar from CIMAV-Monterrey for XRD.

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