Photoelectrochemical behaviour of tin monosulphide

Solar Cells, 25 (1988) 97 - 107

97

PHOTOELECTROCHEMICAL BEHAVIOUR OF TIN MONOSULPHIDE MAHESHWAR SHARON and KRISHNAN BASAVASWARAN

Department of Chemistry, Indian Institute of Technology, Bombay 400 076 (India) (Received January 27, 1987; accepted in revised form February 29, 1988)

Summary Polycrystalline samples of tin monosulphide were prepared by passing H2S through an acidic solution of stannous chloride. The structure and the composition of the samples were confirmed by X-ray diffraction and energy dispersive analysis of X-rays. Hall measurements showed the samples to be n type and the mobility of the majority carriers was found to be 11.4 cm 2 V -~ s-1. Reflectance spectra of the powder samples indicated a band gap of 1.29 eV. The photoelectrochemical behaviour of n-SnS was studied in Fe3+/Fe 2÷, Ce4+/Ce 3+ and I2/I- redox couples. Capacitance measurements yielded a donor density equal to 1.26 × 10 ~6 cm -3, which is in agreement with the value 2.28 × 10 ~6 cm -3 obtained from Hall measurements. Cyclic voltammetric studies on platinum as well as n-SnS were carried out with the best suitable Ce4+/Ce 3+ redox couple and the onset potential (+0.46 V v s . a saturated calomel electrode (SCE)) was found to be in good agreement with the value of the flat-band potential obtained from impedance measurements. The semiconductor was found to be stable against photocorrosion in the Ce4+/Ce 3+ redox couple over a period of 60 h. A photoconversion efficiency of 0.63% was obtained with the cell n-SnS/Ce 4÷, Ce3+/Pt. The photoelectrochemical studies were carried out with a tungsten-halogen lamp and with solar radiation. Both illuminations gave similar results.

1. Introduction The current emphasis in the search for alternative, renewable energy sources has been in the development of wet photovoltaic cells for the conversion of solar energy into electricity or chemical species [1- 8]. The development of such a cell depends on the discovery of inexpensive and abundant semiconductor materials for use in wet photovoltaic cells with stable operation for a long duration. Of the variety of semiconductors that have been evaluated as candidates for wet photovoltaic cells, the tin chalcogenides are those which have been considered by our group recently [9, 10]. Tin monosulphide is an attractive candidate for photoelectrochemical (PEC) cells because it (1) has a band gap 0379-6787/88/$3.50

© Elsevier Sequoia/Printed in The Netherlands

98 of 1.29 eV (2) is chemically stable in acidic media and (3) is economically competitive compared with other semiconductor materials which have shown high optical conversion efficiencies. Tin monosulphide belongs to the class of IV-VI type semiconductors, and its electrical and optical properties have appeared in the literature [11 14]. However, no report is available on the photoelectrochemical properties of tin monosulphide. We present here the photoelectrochemical properties of tin monosulphide.

2. Experimental details Polycrystalline samples of SnS were precipitated by passing hydrogen sulphide through an acidic solution (pH 1) of stannous chloride. The precipitate was filtered and dried. The X-ray diffraction patterns of SnS powder samples were recorded by a Philips model P W - l l 4 0 X-ray diffractometer. The energy dispersive analysis of X-rays of SnS powder samples was carried out with a Siemen Etch Autoscan instrument. The reflectance spectra of SnS powder samples were recorded in the range 700 - 2100 nm by a Hitachi 330 spectrophotometer. The powder samples were made into pellets (area 1 cm 2) and ohmic contact was induced with conducting silver paste and was experimentally confirmed by the I - V characteristics. The electrodes were fabricated according to the procedure reported in ref. 10. Deionized water was used to make solutions with analytical grade chemicals {without further purification}. A conventional, three-electrode, single-compartment cell was used for all the experiments. The cell consisted of a platinum foil and semiconducting working electrode'and was fitted with an optical window to illuminate the semiconductor electrode. A saturated calomel electrode (SCE) was the reference. The light source was a tungstenhalogen lamp with a water filter. All the electrochemical measurements were made with a PAR model 173 potentiostat/galvanostat, PAR model 175 Universal programmer, Iwatsu model 6430 digital Memoryscope and a Houston 2000 recorder. The light intensity was measured with an Eppley thermophile. The differential capacitance measurements were carried out with a digital LCR meter (Vasavi Electronics, India). Unless otherwise specified, all the potentials are with respect to a saturated calomel electrode.

3. Results and discussion

3.1. X-ray diffraction studies Figure 1 shows the X-ray diffraction patterns of SnS powder samples. The d values and the corresponding intensities are shown in Table 1, which confirmed the formation of tin monosulphide.

99

32 ° SnS

R

39"3~ 51.3°

55

SnS

50

z.5

z,0

I

'~

35

30

25

X-RAY ANGLE(degrees) ( 2g,Cu-K~. )

Fig. 1. X-ray diffraction patterns of tin rnonosulpbide.

TABLE 1 Comparison between d values of SnS and A S T M data

Observed values

ASTM data

d (A)

Intensity (I/Io)

d (A)

Intensity (I/Io)

2.79 1.77 2.82

100 70 25

2.79 1.76 2.83

100 70 25

3.2. Energy-dispersive analysis o f X-rays studies Energy dispersive analysis of X-rays of the SnS samples was carried out to determine the stoichiometry of the compound. The stoichiometry was found to be close to 1:1, which confirmed the formation of SnS. 3.3. Chemical stability The SnS powder samples were kept under different acidic pH solutions over a period of one month to study the chemical stability. The solution was analysed by atomic absorption analysis after filtering out the SnS powder. No traces of decomposition products (Sn 2÷, Sn 4+) were detected, confirming the chemical stability of the semiconductor in the dark. 3.4. Band gap Figure 2 shows the reflectance spectrum of SnS powder samples. A sharp absorption is noted in the spectrum. Figure 3 shows a plot of dif-

100

I t

...........

_

i

I

/ /

2 o

o

5'~

960

~bo

~

1~o

~ioo

goo

(nm)

Fig. 2. Reflectance spectrum of tin monosulphide powder samples.

~( . / ~ " - ' ~

Eg=1'28eV

,{ 6

6

o

700

900

1100

13'00

15'00

1700

(nm)

Fig. 3. A plot of differential transmittance

vs.

wavelength, (dT/d~,)

vs.

)~, for SnS.

ferential transmittance vs. the wavelength (dT/dX vs. X); this yielded a peak at 1.29 eV which corresponds to the band gap of the semiconductor. This value is comparable with the value 1.15 eV reported by Albers et al. [12]. 3.5. A n n e a l i n g e f f e c t s and Hall m e a s u r e m e n t s

The SnS pellets were sintered in an argon atmosphere at 200 °C for 24 h. Before sintering, the pellets showed p-type conductivity (measured by the hot probe technique), but after sintering the conductivity was found to

101

be n type. This could be due to the following processes. During the sintering process at 200 °C for 24 h in an argon atmosphere, small traces of sulphur might have been removed, owing to thermal decomposition of SnS, leaving behind sulphur-deficient SnS. This is supported by the observation that when SnS was annealed at 500 °C, small traces of sulphur (yellow coloured surface) were found to be on the surface of the pellet. At 200 °C, the process of removal of sulphur might be very slow and detection difficult compared with the process at 500 °C. However, removal of sulphur may be sufficient to alter the stoichiometry of the c o m p o u n d to show an n-type behaviour. Hall measurements on the sintered pellets (9 mm × 1.6 mm) were carried o u t at room temperature and the donor concentration and the mobility of the majority carriers were found to be 2.28 × 1016 cm -3 and 11.4 cm 2 V -1 s-1 respectively. The Hall mobility is rather low compared with the mobility of the carriers o f the semiconductors yielding high conversion efficiencies (e.g. for n-Si, the electron mobility is 1350 cm 2 V -1 s-~ and the hole mobility is 480 cm 2 V -1 s-1 [15]). 3.6. P h o t o c u r r e n t m e a s u r e m e n t s

The band edge positions of SnS calculated by the Butler-Ginley method are reported elsewhere [9] and suitable electrolytes were selected in comparison with the band edge positions of SnS to fabricate a PEC cell. Table 2 shows the photocurrent observed with different electrolytes studied. Among the electrolytes studied, a high photocurrent was observed with Ce4+/Ce 3÷ electrolyte. This is due to the fact that, among the electrolytes, SnS can form maximum band bending with the Ce4+/Ce 3+ redox system. 3.7. Capacitance m e a s u r e m e n t s

The impedance measurements were carried o u t at a frequency of 1 kHz by a method described elsewhere [7]. The differential capacitance plot ( 1 / C 2 vs. the electrode potential) showed linear behaviour (see Fig. 4). The intercept at 1/C 2 = 0 on the potential axis after correcting for k T / e came to +0.46 V vs. SCE, which is the flat-band potential of the semiconductor. The donor concentration was also calculated from the M o t t - S c h o t t k y equation TABLE 2 Photocurrent observed with different electrolytes for n-SnS Electrolyte

P h o t o c u r r e n t (pA cm -2)

Eredo x (V vs. NHE)*

Ce4+/Ce 3+ (0.1 M in 0.5 M H2SO4) Fe3+/Fe 2+ (0.1 M in 0.5 M H2SO4) I2/I(0.1 M aqueous)

312.0

+1.42

143.8

+0.71

60.7

+0.31

*Value determined from

cyclic voltammetry.

]02

E

tJ_

x

%

0

*0.2

+O,h 0.~ ÷0-8 I0 I ELECTRODE POTENTIAL(V vs S C E ) ~

Fig. 4. Mott-Schottky plot at 1 kHz for an SnS electrode in 0.1 M Ce4+/Ce3+ redox couple in an acidic pH solution.

1/C2 = (V__ Vfb

kT)__ 2 e eeoeND

(1)

and the value came to 1.25 × 1016 cm -3, which is comparable with the value obtained f r o m Hall measurement data. The dielectric constant [12] was taken as 19 for this calculation. The width of the space charge layer was calculated f r o m the equation

\ e-----N-DD !

(2)

and the value came to 1993 A. This value is rather low to provide a large area for the absorption of p h o t o n s in the space charge region.

3.8. Cyclic voltammetric studies The cyclic v o l t a m m e t r y of a platinum electrode in a 0.1 M Ce4+/Ce 3÷ redox couple at different scan rates is shown in Fig. 5. A set of quasireversible anodic and cathodic peaks were observed at different scan rates. In the Ce4+/Ce 3+ r ed o x couple there appears to be m ore than one chemical reaction, either preceeding or following the actual charge transfer processes, which is evident f r om the fact t ha t the peak current ratio between anodic and cathodic is not close to uni t y [16]. However, classical linear behaviour o f Ip vs. X/v (where Ip is the peak current and v is the scan rate) was observed as shown in Fig. 5 (inset), confirming the diffusion-limited charge transfer process occurring at the s e m i c o n d u c t o r - e l e c t r o l y t e interface. Figure 5 also illustrates the I - V behaviour of the Ce4+/Ce 3+ redox couple on the n-SnS electrode with a scan rate of 100 mV s-1. Scans are shown b o th in the dark and under illumination. These data reveal the expected light d e p e n d e n t anodic current. If the n-SnS had f o r m e d a good

103

a~

T

!11

anod~

Fig. 5. Cyclic v o l t a m m e t r i c curves. Curves a, f o r t h e s y s t e m P t / C e 4+, C J + (0.1 M in 0.5 M H2SO4)/Pt in t h e dark: curve al, scan rate 1 0 0 m V s - l ; curve a2, scan r a t e 50 m V s - l ; curve a3, scan r a t e 20 m V s - l ; curve aa, scan rate 10 m V s -1. Curves b, for t h e s y s t e m n - S n S / C e 4+, Ce 3+ (0.1 M in 0.5 M H2SO4)/Pt: curve bl, in t h e dark, scan rate o f 10 m V s - l ; curve b2, u n d e r i l l u m i n a t i o n , scan r a t e o f 10 m V s -1.

junction barrier, no cathodic current would have been obtained. However, when the reorientation energy of the redox couple is larger than the band gap of the semiconductor, the oxidized or reduced level of the redox electrolyte may overlap with both band edges as shown in Fig. 6. Under these conditions, it is possible to observe electron transfer in both the anodic and cathodic modes. The cea+/ce 3+ redox couple is expected to give a reorientation energy of 1.2 eV [17] and the band gap of SnS is also close to this value. In addition, the cathodic current could be due to the photoreduction of the cation impurities present in the solution or due to the reaction of strongly adsorbed ions on the semiconductor surface with the charge carriers via the surface states [18]. The cathodic current observed in the dark as well as under illumination could also be due to the coexistence of n- and p-type domains on the surface of the semiconductor, as observed by Menezes et al. [19, 20] for WSe2 crystals. If SnS is also to exhibit such behaviour, then one would expect a greater probability of n- and p-character distribution on the surface of a pellet than on the surface of the single crystal. However, distributions on the surface would be such that the overall nature may either be n type or p type (as observed by the hot probe method). When electron transfer reactions are

104

E(Vvs SCE)

E

V

~

Fig. 6. Redox potential of Ce4+/Ce 3+ relative to the band edges of n-SnS.

studied under a particular bias (i.e. in the anodic or cathodic mode), the individual nature of the surface would contribute towards the total current passing through the system. If this is to be true, then it is expected that a cathodic current would be observed in the dark as well as under illumination with the SnS semiconductor electrode. However, to confirm this a collimated beam is required to study the characteristic of each point of the surface of the pellet. The presence of mixed n-type and p-type, if it existed at the surface, would also reduce the photocurrent efficiency of the electrode. The low efficiency observed with the pellet may also be partly due to this behaviour. The observed anodic peak is not as intense as that obtained with platinum. Moreover, the anodic peak is shifted towards more cathodic potentials. The decrease in the anodic peak intensity could be due to the back reduction or oxidation of the photogenerated species induced by the step-sites and grain boundaries [21]. The shift in the anodic peak towards more cathodic potentials may be due to additional intrinsic energy supplied to the electrolyte by the semiconductor due to the formation of the space charge layer at the interface, which is n o t available with a platinum electrode. The onset potential at an anodic sweep mode under illuminated conditions corresponds to a condition when bands are flat (flat-band potential). The onset potential with SnS was found to be +0.46 V vs. SCE, which agrees with the value (+0.46 V vs. SCE) determined by the capacitance method. 3.9. P h o t o c o r r o s i o n s t u d i e s

A major technical difficulty which must be overcome before any PEC cell can be constructed is photocorrosion of the semiconductor leading to the destruction of the electrode. Extensive research has been carried out to understand the mechanism of the photocorrosion process of semiconductors [22- 24]. These studies

105

are normally made using the rotating ring disc electrode (RRDE) technique [25] and atomic absorption analysis [26]. In the absence of the RRDE technique, we have carried out atomic absorption analysis to study the photocorrosion of tin monosulphide. A PEC cell was fabricated with the Ce4+/Ce 3÷ redox couple and a platinum counterelectrode and the photoanode was subjected to prolonged illumination for 10 h with a light intensity of 12 mW cm -2. The resulting solution was analysed by atomic absorption analysis to find traces of photodecomposition products. No detectable amounts of ions of the decomposition products were observed, confirming the stability of the electrode against photodecomposition. A cell of similar type was also irradiated with solar light for 6 days and the solution resulting at the end of the experiment was analysed by atomic absorption analysis to confirm photocorrosion of the semiconductor in sunlight. No traces of tin or sulphur could be recorded by the atomic absorption analysis, suggesting the material to be stable for 6 days of constant illumination by solar radiation. Figure 7 shows the photocurrent-time characteristics in sunlight which shows an almost constant current flow supporting the above experiment. 3.10. Pho toconversion efficiency The power output characteristics of the cell n~SnS/Ce 4÷, Ce3+/pt at a light intensity of 12 mW cm -2 is shown in Fig. 8. The fill factor and the efficiency were calculated to be 0.44 and 0.63% respectively. The low conversion efficiency of n-SnS could be due to the following reasons. The low value of the space charge layer width would lower the probability of effective charge separation of photogenerated charge carriers produced due to absorption of photons by the material. The presence of surface states, grain boundaries and step-sites may also be responsible for the low efficiency. The low mobility of the majority carriers would also contri60 50 C40

E

o

g_10

5'6 Time (hrs)

Fig. 7. Photocurrent

vs.

t i m e characteristics o f the cell n-SnS/Ce 4+, Ce3+/Pt in sunlight.

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u

~: 0"6%

~8 2~

~8 c~

s'o

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Vphoto(rnV) Fig. 8. O u t p u t p o w e r curve o f t h e n-SnS e l e c t r o d e , the i n c i d e n t light i n t e n s i t y is 12 m W cm-2.

bute towards the low efficiency. Above all, pellets are always considered to give low conversion efficiencies and there is a need to carry out these measurements with thin film or single-crystal samples of SnS before any axing is made on this material. Related work is in progress.

4. Conclusions Considering the band gap of SnS (Eg = 1.29 eV), which is in the optimum region required for the conversion of solar energy to electricity, its chemical stability and stability against photocorrosion, there is scope for using this material to develop an economic PEC cell. The flat-band potential obtained by the capacitance measurements corresponds with the values obtained by the photo-onset method. The majority carrier concentration calculated by Hall measurements also corresponds with the values obtained by impedance data. However, the efficiency was found to be low which could be due to reasons explained earlier. Related work is in progress and the results will be published in our subsequent publications.

Acknowledgments One of us (K. B.) is thankful to the Indian Institute of Technology, Bombay, and the Department of Non-Conventional Energy Sources, Government of India, for providing him with a fellowship. We are thankful to the Regional Sophisticated Instrumentation Centre, Bombay, for providing facilities such as XRD, EDAX and other instruments. We are grateful to Prof. Agnihotri, Department of Physics, Indian Institute of Technology, Delhi, for extending his help to us for taking the optical measurements.

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15 16 17 18 19 20 21 22 23 24 25 26

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