- Email: [email protected]
Studies on properties of spray deposited Bi2S3 films and electrochemical photovoltaic cells formed with Bi2S3 films
Mutcrials
C’lrrmistrj,
md Hl?,sics,
I1 (1984)
401
401-412
ST!JDIES ON PIIOPERTIES OF SPRAY DEPOSITED Bi2S3 FILMS AND -_ ELECTRCGHEMICAL PHOTDVOLTAIC CELLS FORMED XTH -
S.H. PAWAR, Miss S.P.T~~~~
Bi.&
FILMS
and C.D. LOKHANDE
Energy Conversion Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004 (India)
Received 7 March 1984; accepted 5 April
19%
ABSTRACT Bismuth sulfide films are deposited by a spray pyrolysis technique on conducting and amorphous glass substrates at optimised preparative parameters and their electrical and optical properties are studied. The elec,trochemical photovoltaic (ECPV) cells with a configuration Bi S3/NaOH - Na2S - S / C, are formed and the results on I-V characz eristics, spectral response and C-V measurements are reported and discussed. INTRODUCTION Seniconductor-liquid
junction solar cells have attracted
attention in the last few years, due to growing interest in the solar energy conversion [l-E]. These cells are simple in construction and have the advantages that they can be used for both photovoltaic and chemical energy conversion. However, the problem is to find semiconductor electrode materials which are suitable and inexpensive for an efficient ECPV cell. The semiconductors which possess bandgaps in the range of 1.4 to 1.7 eV are suitable in photovoltaic solar energy conversion devices [9], In this respect Bi2S3 seems to be a promising material, since it shows a strong absorption of light in the span of wavelengths shorter than 900 nm [lO'J. Many methods have been employed for the preparation of Bi2S3 films. Bhattacharya and Pramanik deposited BlaSg films by the chemical bath deposition technique [lo] and Miller and Heller deposited Bi2S3 films on bismuth substrate anodically [ll].
@ Elsevier Se~]u~i~/P~nt~d
in The Netherlands
402
Palyakav et al.formed a layer of Bi2S3 on bismuth film by reactive vacuum diffusian in sulfur vapor [12j.
Pawar et al. deposited
3i2S3 films using a solution gas interface technique [13], while Bhosale prepared tion [14j.
Bi2S3 films by dipping Bi2C3 films in Na,S solu-
These films have been employed in electrochemical
photovoltaic cells and their properties have been reported by many workers [10,11,143.
However, as far as the authors
dre
aivare,
no
data are available on spray-deposited Bi2S3 films. In this paper, we report the deposition of Bi2S3 films by the spray pyrolysis technique on conducting and amorphous glass substrates at optimized preparative parameters.
The natLlre of the
contact between the conducting glass and the Bi2S3 film is determined.
Electrical and optical properties of Bi,S, films
reported.
are The electrochemical photovoltaic c~1l.s with a configu-
ration of Bi2S3/1M NaOH - 1M Na2S - 0.2M S/C are formed and the results on the I-V characteristics, photoresponse, spectral response, C-V measurements etc. are reported and discussed.
Bi2S3 films were deposited by the spray pyrolysis technique described elsewhere [15]. thiourea
The solutions of bismuth nitrate 3rd
were mixed in the appropriate volume so as to obtain a
Bi:S ratio of 2:3; substrate temperature was 3CQ0G and spray rate was 3 cc/mir,. The concentration of the solution was O.lM and the total quantity of mixed solution was 2cX, CC.
Air was used to
atomize the spray. Fluorine-doped conducting glasses (F-doped SnO*) with a resistance of 30-hjcm2 and aasrpho?ls glasses were used as the substrates for Bi2S3 films. Conducting glasses with 85-S@ transparency and 30-60b/cm2 resistance were prepared by the spray pyrolysis technique [16]. Electrical and optical properties of the Bi2S3 films were measured by the following techniques. A two-probe method was used for studying electrical resistivity of the films. A DC power supply was used for passing the current through the film sample. Aplab FET Nanommeter TFM 13 and PLA digital voltmeter,
DPM 10 were used for measuring current and voltage, respectively. A 65 watt strip heater sandwiched between two brass plates was used to study the variation of film resistivkty with temperature.
403
l(mA)
-0.6 -0.4 -0.2 0.2
0.4
0.6
V(volts)
-401
Fig. 1. Current-voltage characteristics for a Bi2S3 film junction.
CsndLfCting
glass/
Thermoelectric power measurements were carried out on a specially designed brass block which keeps a Linear t'7crma.lgradie -nt along its length. A Chromel-Alumel therrocouple was used for measuring the temperatures.
The mean temperature was measured
with a PLA digital millivoltmeter DPM 10, while the differential thermal gradient and thermoelectric voltages were measured with a oioital microvoltmeter, VMV 15. Optical absorption of the film at different wavelengths was studied with a monochromator (Carl absorption
was
Zeiss
Zena,
Germany).
Optical
recorded from t.he longer to shorter wavelength
side. The electrcchemical photovoltaic (ECPV) cell of the configuration Bi2S3/1M N?OH - 1M NJ~S - 0.2M S/C "ias formed.
The area of the Bi2S3 electrode was defined w,\..th the application cf parafin wax by coating tha unexposed area of the film.
Silver paste ~'35
applied to the conducting glass substrate and a copper contact was m;de to it. AR grade chemicals were used for preparing the electrolyte. Appropriate amounts of NaOH and Na2S were dissolved in double distilled water at room temperature. powder was added.
To this solution sulfur
The color of the solution was yellowish pink.
A graphite rod of 6 x 1 cm was used as a counter electrode. The rod was dipped in a cobalt sulfide solution before use in the ECPV cell [17].
A
coming
tube was modified by fixing it inside
a copper calorimeter having a window 2 x 0.5 cm for illumination of the Bi2S3 photoelectrode. A rubber bung was used to make the
404
cell air tight.
A standard Calomel electrode (sCE) was used as
the reference electrode.
The electrolyte was stirred during the
experiment. Electrical and optical properties of the cell were studied with an Aplab Nanoamemeter TFM 13 and PLA digital voltmeter DPM 10. A tungsten filament lamp (500 W.) was employed for illuminating the cell. The intensity was measured with the help of 9 A water filter was interposed between the
Suryamapi luxmeter.
lamp and the ECPV cell in order to avoid heating the cell. The spectral response of the cell was studied with the monochromater (Carl Zeiss Jena) by noting the variation of short circuit photocurrent, Isc, with wavelength.' The capacitance voltage (C-V) measurement was carried out with a digital capacitance meter Type VCM 13 A. RESULTS AND DISCUSSION The optical and electrical properties of spray-deposited Bi2S3 films are discussed in Section I.
Electrochemical photo-
voltaic cell properties are discussed in Section II. Section I Properties of spray deposited Bi2S3 _- films Bi2S3 films deposited at 300°C were found to be uniform and adhesive to the glass substrate. by the weight-difference 8.0
The film thickness was measured
technique.
It was observed that the
1
0
/@
/
7.0. l
cc: m
/
/ 0
"06.0. /
5.0
/ . 1.7 ,,$(: lo$3K-12:6
i.9
Fig, 2. Arrhenium plot (log iiversus l/T) for a typicalBi2S3
film.
405
Mean
temp Oc
Fig. 3, Variation of thermoelectric power (TEP) with temperature for a Bi2S3 film.
thickness of Bi2S3 films deposited on the conducting glass is always smaller than the amorphous glass substrates. Similar observations have been reported by Uplane and Pawar in the case of Cdl_$nxS
films [lo].
The contact between the substrate and the film can be ideal if it is of an ohmic nature.
It was reported that silver paste
gives ohmic contact to Bi2S3 [lo] films; therefore, in order to study the nature of the contact between the conducting glass substrate and Bi2S3, silver paste was applied to both to make the contact, To study the I-V characteristics, Fig.1 shows the nature of the I-V characteristic for the Bi2S3/Sn02 junction.
In the
low-voltage region, the conducting glass gives an ohmic contact to Bi2S3 films. The resistivity of the film was studied by using a two-point D.C. probe method in the temperature range 300 to 500 K. The area of the film on the glass substrate was defined and the silver paste was applied for making the contacts to Bi2S3 films.
The
dark resistivity q is of the order of 104 ohm-cm, while the resistance of Bi2S3 films prepared by the reactive vacuum diffusion in sulfur vapor is reported to be of the order of KA
1123. Higher
resistivity of the film may be attributed to the grain boundary discontinuity and thickness of the film [lo]. Variation of the resistance with temperatureshows th?t Ri2S3 films are semiconducting.
An Arrhenius plot for a typical film is
406
shown in Fig.?.
The activation energ\/,Ea, is calc!Jlated by using the relation R = Roe-Ea/KT
(1)
whore all terms have their usual meanings.
The activation energy
was 0.68 eV in the temperature range 350 - 450 K.
Results availa-
ble in the literature show that the activation energy varies from 0.72 to 1.3 eV for polycrystalline Bi2S3 films [10,19,20]. The type of conductivity was determined from the thermoelectric power (TEP) measurement. The polarity of the thermally generated voltage at the hot end was positive, indicating that the films are of the n-type [13,13,20].
TEP is of the order of pVv/'C.
Fig.3 shows the variation of the TEP with temperature, it increases with increasing temperature.
The optical absorption cf the films
was studied in the wavelength range 403 to 300 nm. The variation of absorption coefficient, d, with wavelength, is shojwn in Fi.cJ.4. is of the order of 104 cm- 1 for Bi2S3 films, showing that Bi2S3 is a direct-band-gap material . The band gap is determined from 0 ( aChv)l versus hv plots, to be 1.67 eV. This is in good agreement with the results reported by others [13,20].
A (nm)
Fi 4. Variation of absorption coefficient,4 A 9hrn) for a Bi2S3 film.
, with wavelength
Section II Properties of ECPV cells formed with Bi.,-& films It is seen that a voltage (called the dark voltage, V,, and a dark current (called ID) were developed in the dark for all films. Polarity of the dark voltage is negative towards Bi2S3 and posi-
407
tive towards the carbon electrode.
The origin of the dark voltage
is attributed to the difference between the two half-cell potentials in the EGPV cell and can be written as : E = E Bi2s3 - E EBi
(2)
carbon
are the half-cell potentials developed and E carbon 33 when Bi2S3 and carbon electrodes are introduced in an electrolyte.
where
s
The presence of the dark current in the cell suggests tliat there is some deterioration of the photoanode in the dark. -0.3
-9.1
\i appii ed 0. 1
0.3
0.4-
I -0. ,5 0i, 55 -0.75 vs SCE Fig. 5. Current-voltage characteristics in the dark and under illumination for a ECPV cell formed with Bi$S3/1M NaOH - i?.2M S/C. The intensity of illumination is 100 mW/cm, -0.6
.
To study the nature of the junction in the ECPV cell formed with Bi S films and an electrolyte, dynamic current-voltage (I-V) 23 characteristics were studied. Fig.5 shows the I-V characteristics for the Bi2S3 films. In the electrode-electrolyte system the nature of the charge-transfer reaction is given by Butler-Volmer equation as [21] I
= Io [e(1-B)VF/RT _ e- BVF/RT I
where
(3)
I, is the equilibrium exchange current density, p is the
symmetry factor, V is the over voltage, R is the Iuniversal gas constant and F is the Faraday constant. For voltages > 100 mV,
eqn (3) can be written as : (4)
408
iWhen i;= 3.5
the jlJnction shoves rectifying properties.
The
magnitude of b was calculated by plotting the lsg I _-versus V. The magnitude of fiwas 0.9, which shows that the junction is rectifying in nature.
0
40 80 Intensity(mW/cm2)
120
6. Variation of short circuit photocurrent, ISC, and open circuit voltage, Voc, with light intensity for a cell formed with a Bi2S3 film as a photoanode. However, the current in reverse bias does not saturate. In the ECPV cells this behaviour is due to the follcwing reasons[22]: (i) the effective barrier height decreases because of the interPC t ron pairs are thermally generated facial layer; (ii) .ho1 e- e 1_ in the depletion layer under the conditions of large reverse bias and (iii) the current increases due to the on-set of the electron injection from an electrolyte, because the barrier height becomes thin enough for tunneling to take place. To estimate the energy conversion efficiency of the ECPV cell, I-V curves under illumination were recorded by using a variable D.C. power supply in series with the external circuit. Fig.5 also shows the I-V characteristic of the ECPV cell under 100 mW/cm illumination intensity. Energy conversion efficiency,q was calculated as 0.01% and fill factor, ff, as 47.8%. The ECPV cells were formed with various Bi2S8 films deposited in the same rUn and reproducibility of the cells was checked by noting the Isc and the Voc. The magnitudes of the ISC and the
409
Voc differ from film to film.
Observations of five films from the
same run showed a lQ.,deviation from t5-emean values of the Isc and the Voc. The deviation is attribute d to the different positicns dar;ng spraying the solution and iifferent resistances of t&
conducting 11ass substrates.
1200.
300
-
0
I
0.
480
a40
-
600X(270 Fig. 7. Rela.tive spectral response of an &PV 3i?L3 film as a photoanode.
cell formed with a
In the present case, the cllrrent magnitudes obtained by the sprayed Bi2S3 films dre comparable with the magnitudes reported by Miller and Heller [ll] for anodically deposited Bi2S3 films and by Bhattacharya and Pramanik [lOI for chemically deoosited Bi2S3 films.
However, the voltages for sprayed 9i2S3 films are
low in comparison with the above methods.
This might be due to
the pinning of the Fermi level by the surface states during illumination.
This aspect invites more attention.
The photoresponse of the cell was measured by noting the Isc and the Voc as a function of the light intensity, fL. The equivalent circllit diagram of the ECPV cell implies that Isc varies linearly with the light intensity, i.e. Isc
=
CfL
(5)
where C is a constant. Fig.6 shows the variation of the Isc and the Voc with the light intensity. The Isc shows some deviation from linearity.
In the case of solid junctions, this deviation
is attributed to the series resistance of the cell.
In the
present case, it may be attributed to resistance of the film and the substrate.
410
The relJtisn
between
the Voc and the light intensity for
equilibrium distribution cf the charge carriers in the space charge region is given by [?3] Vcc = const3nt + y
In
fL -
y
In (Voc KyT )
(6)
From Fig.6, it is seen that the Voc satiJrJt@s at the intensity, which is in good agreement twit.!-! theory.
higher
The spectral respcns of the EGPV cell was studied by determining variation of the Isc with wavelrngth, A.
Fig.7 represents
a typical relative spectral response of the cell., The ISC shows a peak at 730 nm which (Eg = 1.7 eV).
corresponds
to
the band-gap
the optical absorption method (Eg = 1.67 lower phd.cc:Jrrent the
absorption
surface
oil
of
the
att.ributed
to the
[1,24].
shorter
the
the light
recombination
Similarly, used
lower
of
on the
absorption
case
since the depletion layer, and the Gouy diffuse layer
of
Bi2S3.
to the depletion
flat
band potential,
in Section I, side
minority longer
(C-V)
wavelength
layer. is
2 oE:Eo%
side
negligible
were made The capaci-
semiconductor
due to the
high
and to the large magni.tude On the above assumption, Vfb,
calculated
(V-Ufh-
using Y)
the
is
photoanode
due to the Helmholtz
cJpJci.tJnces are
amount of
carriers,
measi!renents
to the
The
may be due to
and large
tha ?_ight at the
corresponds
in an electrolyte
red
=
the
f:>rmedwSth Bi2S3 film photoanodes.
tance observed in this
l/C2
wavelength
the photogenerated
photocurrent
lower
ev)
in an electrolyte
The capacitance-voltage
on ?h+ ECPV cells
concentration
of
This agrees well with the band-gap determined frcm
relation
[26]
layer
ionic compathe : (7)
where E; is the semiconductor die1ectrj.c constant., E, is the permittivity of vaculum, i$ is the carrier density and V is the applied voltage. The plot of in Fig.8. slopes,
The
which
plot
are
l/C” verstis V for a typical Bi2S3 cell is shown shows tiwo straight portions with different attributed
to the
states present on the Bi2S3 films, 0.3 eV for Bi,S3/S/S-2 redox couple.
nature and the surface defect The Vfb is calculated as Since the Vfb is a measure
of potential which must be applied to the semiconductor such that the bands remain flat at the interface, the Vfb determines the amount of band bending.
411
-850
-7.75 V vs
-700
SCE (mV)
Fig. 8. l/C' versus voltage plot for sn ECPV cell formed with a Ri2S3 film as a photoanode.
ACKNOWLEDGEMENTS Two of the autliors (.>HPand SPT) are yrateful to the Department of Non-conventional Energy
Sources,
New Delhi, for
financial support, and one of the authors (CDL) is indebted to the Council of Scientific and Industrial Research, New Delhi, for the award of a Senior ,iesearch Fellowship. REFERENCES A.Heller, K.C.Chany and B.MilLer, .T. Electrochem. Sot., 124 (1977) 697. H.Minoura, M.Tsui!ti and T.Oki, Ber. Busenges, Phys. Chem.,81 (1977) 588. A.B.Ellis, S.W.Kaiser and M.S.Wriyhton, J. Ame,r. Chem.--Soca9 (1977) 2839. K.W.Frese, J. Appl. -P&s. 40 (1982) 275. -__ Lett., _A.Heller, B.Miller, S.S.Chu and Y.T.Lee, -J. Amer. Chem. Sot., 101 (1979) 7633. R.L.Vdn Mirhaeghe, F.Cardan dnd W.P.Gomes. 3er. Bunsen*, Phys. _~ Chem., 83 (1979) 236.
412
7
F.R.F.Fan and A.J.Bard, J. Amer. Chem. Sot., 102 (1980) 3677.
8
T.S.Jayadevaish, Appl. Phys. Lett. 25 (1979) 399.
9
M.Prince, J. kppl. Phys. 26 (1955) 534,
10
H.N.Bhattacharya and P.Pramanik, J. Electrochem. SOC.
129
(1982) 332. 11
B.Miller and A.Heller, Nature, 262 (1976) 680.
12
S.M.Polyakav, E.N.Laverko, I.S.Lisker, A.L.Fukshanskii and M.Yagodking, Sov. Phys. Solid State, 17 (1975) 386
13
S.H.Pawar, P.N.Bhosale, M.D.Uplane and Shobha Tamhankar, Thin Solid Films, 110 (1983) 165.
14
C.H.Bhosale, M.Phil. Dissertation, Shivaji University, Kolhapur, India, 1983.
15
S.H.Pawar, Miss S.P.Tamhankar and C.D.Lokhande, Proc. Nuclear Physics and Solid State Physics Symposium, Mysore, India,1983. ___._-.__. -
16
E.Shanthi, A,Banerjee, V.Dutta and K.L.Chopra, Thin Solid Film, 71 (1980) 237.
17
J.Menessen,G.Ho&s
g3.Cahen
in A.Heller (ed.) 'Semiconductor
Liquid Junction Solar Cells' Electrochem.Soc,Inc., Princeton, 1977, p.34. 18
M.D.Uplane and S.H.Pawar, Mat. Res. Bull. (in press).
19
J.Black, E.M.Conwell, L.Seigle and C.W.Soencer, J. Phys. Chem. Solids, 2 (1957) 240.
20
P.Pramanik and R.N,Bhattac~~arya, J. Electrochem. Sot., 127 (1980) 2087.
21
J.O.M.Bockris and A.K.N.Reddy, Modern Electrochem. Vo1.2, Plenum Rosetta Edi.tion, (1973) p.862.
22
K.Rajeshwar, L.Thompson, P.Singh, R.C.Kainthia and K.L.Chopra, J. Electrochem. Sot., 128 (1981) 1744.
23
H.Gerischer, J. Electroanalyt. Chem., 58 (1975) 263.
24
A.Heller, K.C.Chang and B.Miller, in A.Heller (ed.) Semiconductor Liquid Junction Solar Cells, Electrochem. Sot. Inc., Princeton N.J., 1977, p.54,
25
J.R.iiJilsonand S.M.Park, J.Electrochem. Sot., 128 (1980) 338.
C’lrrmistrj,
md Hl?,sics,
I1 (1984)
401
401-412
ST!JDIES ON PIIOPERTIES OF SPRAY DEPOSITED Bi2S3 FILMS AND -_ ELECTRCGHEMICAL PHOTDVOLTAIC CELLS FORMED XTH -
S.H. PAWAR, Miss S.P.T~~~~
Bi.&
FILMS
and C.D. LOKHANDE
Energy Conversion Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004 (India)
Received 7 March 1984; accepted 5 April
19%
ABSTRACT Bismuth sulfide films are deposited by a spray pyrolysis technique on conducting and amorphous glass substrates at optimised preparative parameters and their electrical and optical properties are studied. The elec,trochemical photovoltaic (ECPV) cells with a configuration Bi S3/NaOH - Na2S - S / C, are formed and the results on I-V characz eristics, spectral response and C-V measurements are reported and discussed. INTRODUCTION Seniconductor-liquid
junction solar cells have attracted
attention in the last few years, due to growing interest in the solar energy conversion [l-E]. These cells are simple in construction and have the advantages that they can be used for both photovoltaic and chemical energy conversion. However, the problem is to find semiconductor electrode materials which are suitable and inexpensive for an efficient ECPV cell. The semiconductors which possess bandgaps in the range of 1.4 to 1.7 eV are suitable in photovoltaic solar energy conversion devices [9], In this respect Bi2S3 seems to be a promising material, since it shows a strong absorption of light in the span of wavelengths shorter than 900 nm [lO'J. Many methods have been employed for the preparation of Bi2S3 films. Bhattacharya and Pramanik deposited BlaSg films by the chemical bath deposition technique [lo] and Miller and Heller deposited Bi2S3 films on bismuth substrate anodically [ll].
@ Elsevier Se~]u~i~/P~nt~d
in The Netherlands
402
Palyakav et al.formed a layer of Bi2S3 on bismuth film by reactive vacuum diffusian in sulfur vapor [12j.
Pawar et al. deposited
3i2S3 films using a solution gas interface technique [13], while Bhosale prepared tion [14j.
Bi2S3 films by dipping Bi2C3 films in Na,S solu-
These films have been employed in electrochemical
photovoltaic cells and their properties have been reported by many workers [10,11,143.
However, as far as the authors
dre
aivare,
no
data are available on spray-deposited Bi2S3 films. In this paper, we report the deposition of Bi2S3 films by the spray pyrolysis technique on conducting and amorphous glass substrates at optimized preparative parameters.
The natLlre of the
contact between the conducting glass and the Bi2S3 film is determined.
Electrical and optical properties of Bi,S, films
reported.
are The electrochemical photovoltaic c~1l.s with a configu-
ration of Bi2S3/1M NaOH - 1M Na2S - 0.2M S/C are formed and the results on the I-V characteristics, photoresponse, spectral response, C-V measurements etc. are reported and discussed.
Bi2S3 films were deposited by the spray pyrolysis technique described elsewhere [15]. thiourea
The solutions of bismuth nitrate 3rd
were mixed in the appropriate volume so as to obtain a
Bi:S ratio of 2:3; substrate temperature was 3CQ0G and spray rate was 3 cc/mir,. The concentration of the solution was O.lM and the total quantity of mixed solution was 2cX, CC.
Air was used to
atomize the spray. Fluorine-doped conducting glasses (F-doped SnO*) with a resistance of 30-hjcm2 and aasrpho?ls glasses were used as the substrates for Bi2S3 films. Conducting glasses with 85-S@ transparency and 30-60b/cm2 resistance were prepared by the spray pyrolysis technique [16]. Electrical and optical properties of the Bi2S3 films were measured by the following techniques. A two-probe method was used for studying electrical resistivity of the films. A DC power supply was used for passing the current through the film sample. Aplab FET Nanommeter TFM 13 and PLA digital voltmeter,
DPM 10 were used for measuring current and voltage, respectively. A 65 watt strip heater sandwiched between two brass plates was used to study the variation of film resistivkty with temperature.
403
l(mA)
-0.6 -0.4 -0.2 0.2
0.4
0.6
V(volts)
-401
Fig. 1. Current-voltage characteristics for a Bi2S3 film junction.
CsndLfCting
glass/
Thermoelectric power measurements were carried out on a specially designed brass block which keeps a Linear t'7crma.lgradie -nt along its length. A Chromel-Alumel therrocouple was used for measuring the temperatures.
The mean temperature was measured
with a PLA digital millivoltmeter DPM 10, while the differential thermal gradient and thermoelectric voltages were measured with a oioital microvoltmeter, VMV 15. Optical absorption of the film at different wavelengths was studied with a monochromator (Carl absorption
was
Zeiss
Zena,
Germany).
Optical
recorded from t.he longer to shorter wavelength
side. The electrcchemical photovoltaic (ECPV) cell of the configuration Bi2S3/1M N?OH - 1M NJ~S - 0.2M S/C "ias formed.
The area of the Bi2S3 electrode was defined w,\..th the application cf parafin wax by coating tha unexposed area of the film.
Silver paste ~'35
applied to the conducting glass substrate and a copper contact was m;de to it. AR grade chemicals were used for preparing the electrolyte. Appropriate amounts of NaOH and Na2S were dissolved in double distilled water at room temperature. powder was added.
To this solution sulfur
The color of the solution was yellowish pink.
A graphite rod of 6 x 1 cm was used as a counter electrode. The rod was dipped in a cobalt sulfide solution before use in the ECPV cell [17].
A
coming
tube was modified by fixing it inside
a copper calorimeter having a window 2 x 0.5 cm for illumination of the Bi2S3 photoelectrode. A rubber bung was used to make the
404
cell air tight.
A standard Calomel electrode (sCE) was used as
the reference electrode.
The electrolyte was stirred during the
experiment. Electrical and optical properties of the cell were studied with an Aplab Nanoamemeter TFM 13 and PLA digital voltmeter DPM 10. A tungsten filament lamp (500 W.) was employed for illuminating the cell. The intensity was measured with the help of 9 A water filter was interposed between the
Suryamapi luxmeter.
lamp and the ECPV cell in order to avoid heating the cell. The spectral response of the cell was studied with the monochromater (Carl Zeiss Jena) by noting the variation of short circuit photocurrent, Isc, with wavelength.' The capacitance voltage (C-V) measurement was carried out with a digital capacitance meter Type VCM 13 A. RESULTS AND DISCUSSION The optical and electrical properties of spray-deposited Bi2S3 films are discussed in Section I.
Electrochemical photo-
voltaic cell properties are discussed in Section II. Section I Properties of spray deposited Bi2S3 _- films Bi2S3 films deposited at 300°C were found to be uniform and adhesive to the glass substrate. by the weight-difference 8.0
The film thickness was measured
technique.
It was observed that the
1
0
/@
/
7.0. l
cc: m
/
/ 0
"06.0. /
5.0
/ . 1.7 ,,$(: lo$3K-12:6
i.9
Fig, 2. Arrhenium plot (log iiversus l/T) for a typicalBi2S3
film.
405
Mean
temp Oc
Fig. 3, Variation of thermoelectric power (TEP) with temperature for a Bi2S3 film.
thickness of Bi2S3 films deposited on the conducting glass is always smaller than the amorphous glass substrates. Similar observations have been reported by Uplane and Pawar in the case of Cdl_$nxS
films [lo].
The contact between the substrate and the film can be ideal if it is of an ohmic nature.
It was reported that silver paste
gives ohmic contact to Bi2S3 [lo] films; therefore, in order to study the nature of the contact between the conducting glass substrate and Bi2S3, silver paste was applied to both to make the contact, To study the I-V characteristics, Fig.1 shows the nature of the I-V characteristic for the Bi2S3/Sn02 junction.
In the
low-voltage region, the conducting glass gives an ohmic contact to Bi2S3 films. The resistivity of the film was studied by using a two-point D.C. probe method in the temperature range 300 to 500 K. The area of the film on the glass substrate was defined and the silver paste was applied for making the contacts to Bi2S3 films.
The
dark resistivity q is of the order of 104 ohm-cm, while the resistance of Bi2S3 films prepared by the reactive vacuum diffusion in sulfur vapor is reported to be of the order of KA
1123. Higher
resistivity of the film may be attributed to the grain boundary discontinuity and thickness of the film [lo]. Variation of the resistance with temperatureshows th?t Ri2S3 films are semiconducting.
An Arrhenius plot for a typical film is
406
shown in Fig.?.
The activation energ\/,Ea, is calc!Jlated by using the relation R = Roe-Ea/KT
(1)
whore all terms have their usual meanings.
The activation energy
was 0.68 eV in the temperature range 350 - 450 K.
Results availa-
ble in the literature show that the activation energy varies from 0.72 to 1.3 eV for polycrystalline Bi2S3 films [10,19,20]. The type of conductivity was determined from the thermoelectric power (TEP) measurement. The polarity of the thermally generated voltage at the hot end was positive, indicating that the films are of the n-type [13,13,20].
TEP is of the order of pVv/'C.
Fig.3 shows the variation of the TEP with temperature, it increases with increasing temperature.
The optical absorption cf the films
was studied in the wavelength range 403 to 300 nm. The variation of absorption coefficient, d, with wavelength, is shojwn in Fi.cJ.4. is of the order of 104 cm- 1 for Bi2S3 films, showing that Bi2S3 is a direct-band-gap material . The band gap is determined from 0 ( aChv)l versus hv plots, to be 1.67 eV. This is in good agreement with the results reported by others [13,20].
A (nm)
Fi 4. Variation of absorption coefficient,4 A 9hrn) for a Bi2S3 film.
, with wavelength
Section II Properties of ECPV cells formed with Bi.,-& films It is seen that a voltage (called the dark voltage, V,, and a dark current (called ID) were developed in the dark for all films. Polarity of the dark voltage is negative towards Bi2S3 and posi-
407
tive towards the carbon electrode.
The origin of the dark voltage
is attributed to the difference between the two half-cell potentials in the EGPV cell and can be written as : E = E Bi2s3 - E EBi
(2)
carbon
are the half-cell potentials developed and E carbon 33 when Bi2S3 and carbon electrodes are introduced in an electrolyte.
where
s
The presence of the dark current in the cell suggests tliat there is some deterioration of the photoanode in the dark. -0.3
-9.1
\i appii ed 0. 1
0.3
0.4-
I -0. ,5 0i, 55 -0.75 vs SCE Fig. 5. Current-voltage characteristics in the dark and under illumination for a ECPV cell formed with Bi$S3/1M NaOH - i?.2M S/C. The intensity of illumination is 100 mW/cm, -0.6
.
To study the nature of the junction in the ECPV cell formed with Bi S films and an electrolyte, dynamic current-voltage (I-V) 23 characteristics were studied. Fig.5 shows the I-V characteristics for the Bi2S3 films. In the electrode-electrolyte system the nature of the charge-transfer reaction is given by Butler-Volmer equation as [21] I
= Io [e(1-B)VF/RT _ e- BVF/RT I
where
(3)
I, is the equilibrium exchange current density, p is the
symmetry factor, V is the over voltage, R is the Iuniversal gas constant and F is the Faraday constant. For voltages > 100 mV,
eqn (3) can be written as : (4)
408
iWhen i;= 3.5
the jlJnction shoves rectifying properties.
The
magnitude of b was calculated by plotting the lsg I _-versus V. The magnitude of fiwas 0.9, which shows that the junction is rectifying in nature.
0
40 80 Intensity(mW/cm2)
120
6. Variation of short circuit photocurrent, ISC, and open circuit voltage, Voc, with light intensity for a cell formed with a Bi2S3 film as a photoanode. However, the current in reverse bias does not saturate. In the ECPV cells this behaviour is due to the follcwing reasons[22]: (i) the effective barrier height decreases because of the interPC t ron pairs are thermally generated facial layer; (ii) .ho1 e- e 1_ in the depletion layer under the conditions of large reverse bias and (iii) the current increases due to the on-set of the electron injection from an electrolyte, because the barrier height becomes thin enough for tunneling to take place. To estimate the energy conversion efficiency of the ECPV cell, I-V curves under illumination were recorded by using a variable D.C. power supply in series with the external circuit. Fig.5 also shows the I-V characteristic of the ECPV cell under 100 mW/cm illumination intensity. Energy conversion efficiency,q was calculated as 0.01% and fill factor, ff, as 47.8%. The ECPV cells were formed with various Bi2S8 films deposited in the same rUn and reproducibility of the cells was checked by noting the Isc and the Voc. The magnitudes of the ISC and the
409
Voc differ from film to film.
Observations of five films from the
same run showed a lQ.,deviation from t5-emean values of the Isc and the Voc. The deviation is attribute d to the different positicns dar;ng spraying the solution and iifferent resistances of t&
conducting 11ass substrates.
1200.
300
-
0
I
0.
480
a40
-
600X(270 Fig. 7. Rela.tive spectral response of an &PV 3i?L3 film as a photoanode.
cell formed with a
In the present case, the cllrrent magnitudes obtained by the sprayed Bi2S3 films dre comparable with the magnitudes reported by Miller and Heller [ll] for anodically deposited Bi2S3 films and by Bhattacharya and Pramanik [lOI for chemically deoosited Bi2S3 films.
However, the voltages for sprayed 9i2S3 films are
low in comparison with the above methods.
This might be due to
the pinning of the Fermi level by the surface states during illumination.
This aspect invites more attention.
The photoresponse of the cell was measured by noting the Isc and the Voc as a function of the light intensity, fL. The equivalent circllit diagram of the ECPV cell implies that Isc varies linearly with the light intensity, i.e. Isc
=
CfL
(5)
where C is a constant. Fig.6 shows the variation of the Isc and the Voc with the light intensity. The Isc shows some deviation from linearity.
In the case of solid junctions, this deviation
is attributed to the series resistance of the cell.
In the
present case, it may be attributed to resistance of the film and the substrate.
410
The relJtisn
between
the Voc and the light intensity for
equilibrium distribution cf the charge carriers in the space charge region is given by [?3] Vcc = const3nt + y
In
fL -
y
In (Voc KyT )
(6)
From Fig.6, it is seen that the Voc satiJrJt@s at the intensity, which is in good agreement twit.!-! theory.
higher
The spectral respcns of the EGPV cell was studied by determining variation of the Isc with wavelrngth, A.
Fig.7 represents
a typical relative spectral response of the cell., The ISC shows a peak at 730 nm which (Eg = 1.7 eV).
corresponds
to
the band-gap
the optical absorption method (Eg = 1.67 lower phd.cc:Jrrent the
absorption
surface
oil
of
the
att.ributed
to the
[1,24].
shorter
the
the light
recombination
Similarly, used
lower
of
on the
absorption
case
since the depletion layer, and the Gouy diffuse layer
of
Bi2S3.
to the depletion
flat
band potential,
in Section I, side
minority longer
(C-V)
wavelength
layer. is
2 oE:Eo%
side
negligible
were made The capaci-
semiconductor
due to the
high
and to the large magni.tude On the above assumption, Vfb,
calculated
(V-Ufh-
using Y)
the
is
photoanode
due to the Helmholtz
cJpJci.tJnces are
amount of
carriers,
measi!renents
to the
The
may be due to
and large
tha ?_ight at the
corresponds
in an electrolyte
red
=
the
f:>rmedwSth Bi2S3 film photoanodes.
tance observed in this
l/C2
wavelength
the photogenerated
photocurrent
lower
ev)
in an electrolyte
The capacitance-voltage
on ?h+ ECPV cells
concentration
of
This agrees well with the band-gap determined frcm
relation
[26]
layer
ionic compathe : (7)
where E; is the semiconductor die1ectrj.c constant., E, is the permittivity of vaculum, i$ is the carrier density and V is the applied voltage. The plot of in Fig.8. slopes,
The
which
plot
are
l/C” verstis V for a typical Bi2S3 cell is shown shows tiwo straight portions with different attributed
to the
states present on the Bi2S3 films, 0.3 eV for Bi,S3/S/S-2 redox couple.
nature and the surface defect The Vfb is calculated as Since the Vfb is a measure
of potential which must be applied to the semiconductor such that the bands remain flat at the interface, the Vfb determines the amount of band bending.
411
-850
-7.75 V vs
-700
SCE (mV)
Fig. 8. l/C' versus voltage plot for sn ECPV cell formed with a Ri2S3 film as a photoanode.
ACKNOWLEDGEMENTS Two of the autliors (.>HPand SPT) are yrateful to the Department of Non-conventional Energy
Sources,
New Delhi, for
financial support, and one of the authors (CDL) is indebted to the Council of Scientific and Industrial Research, New Delhi, for the award of a Senior ,iesearch Fellowship. REFERENCES A.Heller, K.C.Chany and B.MilLer, .T. Electrochem. Sot., 124 (1977) 697. H.Minoura, M.Tsui!ti and T.Oki, Ber. Busenges, Phys. Chem.,81 (1977) 588. A.B.Ellis, S.W.Kaiser and M.S.Wriyhton, J. Ame,r. Chem.--Soca9 (1977) 2839. K.W.Frese, J. Appl. -P&s. 40 (1982) 275. -__ Lett., _A.Heller, B.Miller, S.S.Chu and Y.T.Lee, -J. Amer. Chem. Sot., 101 (1979) 7633. R.L.Vdn Mirhaeghe, F.Cardan dnd W.P.Gomes. 3er. Bunsen*, Phys. _~ Chem., 83 (1979) 236.
412
7
F.R.F.Fan and A.J.Bard, J. Amer. Chem. Sot., 102 (1980) 3677.
8
T.S.Jayadevaish, Appl. Phys. Lett. 25 (1979) 399.
9
M.Prince, J. kppl. Phys. 26 (1955) 534,
10
H.N.Bhattacharya and P.Pramanik, J. Electrochem. SOC.
129
(1982) 332. 11
B.Miller and A.Heller, Nature, 262 (1976) 680.
12
S.M.Polyakav, E.N.Laverko, I.S.Lisker, A.L.Fukshanskii and M.Yagodking, Sov. Phys. Solid State, 17 (1975) 386
13
S.H.Pawar, P.N.Bhosale, M.D.Uplane and Shobha Tamhankar, Thin Solid Films, 110 (1983) 165.
14
C.H.Bhosale, M.Phil. Dissertation, Shivaji University, Kolhapur, India, 1983.
15
S.H.Pawar, Miss S.P.Tamhankar and C.D.Lokhande, Proc. Nuclear Physics and Solid State Physics Symposium, Mysore, India,1983. ___._-.__. -
16
E.Shanthi, A,Banerjee, V.Dutta and K.L.Chopra, Thin Solid Film, 71 (1980) 237.
17
J.Menessen,G.Ho&s
g3.Cahen
in A.Heller (ed.) 'Semiconductor
Liquid Junction Solar Cells' Electrochem.Soc,Inc., Princeton, 1977, p.34. 18
M.D.Uplane and S.H.Pawar, Mat. Res. Bull. (in press).
19
J.Black, E.M.Conwell, L.Seigle and C.W.Soencer, J. Phys. Chem. Solids, 2 (1957) 240.
20
P.Pramanik and R.N,Bhattac~~arya, J. Electrochem. Sot., 127 (1980) 2087.
21
J.O.M.Bockris and A.K.N.Reddy, Modern Electrochem. Vo1.2, Plenum Rosetta Edi.tion, (1973) p.862.
22
K.Rajeshwar, L.Thompson, P.Singh, R.C.Kainthia and K.L.Chopra, J. Electrochem. Sot., 128 (1981) 1744.
23
H.Gerischer, J. Electroanalyt. Chem., 58 (1975) 263.
24
A.Heller, K.C.Chang and B.Miller, in A.Heller (ed.) Semiconductor Liquid Junction Solar Cells, Electrochem. Sot. Inc., Princeton N.J., 1977, p.54,
25
J.R.iiJilsonand S.M.Park, J.Electrochem. Sot., 128 (1980) 338.