Download Photoelectrochemical characterization of Bi2S3 thin films deposited

Indian Journal of Engineering & Materials Sciences
Vol. 13, April; 2006, pp. 140-144
Photoelectrochemical characterization of Bi2S3 thin films deposited by modified
chemical bath deposition
R R Ahire & R P Sharma*†
Thin Film and Semiconductor Laboratory, Department of Physics, G.T.Patil College, Nandurbar 425 412, India
Received 5 April 2005; accepted 28 December 2005
For the photoelectrochemical (PEC) solar cell, the prime requirement is that photoelectrode/ photoanode should have
bandgap close to the maximum in the visible spectrum. Bismuth sulphide (Bi2S3) is challenging material because of its midway bandgap (Eg = 1.74 eV) and absorption coefficient of the order of 104 cm-1. In the present investigation, bismuth
sulphide (Bi2S3) thin films of thickness about 0.14 μm have been prepared by using modified chemical bath deposition
method onto glass and fluorine doped tin oxide (FTO) coated glass substrate under optimized conditions. The films are
annealed at 200ºC for 2 h in air. It is found that deposited films turn from amorphous to polycrystalline after annealing. The
Bi2S3/NaOH-S-Na2S/C cell has been fabricated by using Bi2S3 annealed films and their photoelectrochemical performance
has been studied. It is found that Bi2S3 films are photoactive. However, conversion efficiency is low (0.056%) due to low
series and high shunts resistance of Bi2S3 films.
IPC Code: H01C17/14
For the last decades, photoelectrochemical (PEC)
processes at semiconductor, electrolyte interface have
found new interest because of their solar and nonsolar applications. Bi2S3 thin films have received great
attention since its bandgap lies close to the range of
theoretically maximum attainable energy conversion
efficiency1. It can also be used in heterojunction, IR
detectors, LUX meters and switching devices.
Photoelectrochemical (PEC) devices based on Bi2S3
thin films have much interest and activity in solar
energy research. Further, such PEC cells have been
applied in rechargeable electrochemical storage
septum cell and redox couple storage devices.
It is known that Bi2S3 films can be prepared by
various methods such as chemical bath in aqueous
and non-aqueous solutions2-4, spray5, solution gas
interface6 and electrodeposition7,8. In these, chemical
bath deposition is a simple, inexpensive and
convenient method for large area deposition. Both
metal ions and chalcogen ions are mixed together in
bath where metallic ions and chalcogen ions are
released. These ions combine together to form metal
chalcogenide films on the substrate a precipitate in the
bulk solution.
In the present work, we report on the preparation of
Bi2S3 films by modified chemical bath deposition
__________
* For correspondence (E-mail: [email protected])
†
Present address: Department of Physics, Dr Babasaheb
Ambedkar Marathwada University, Aurangabad 431 004, India
method on to fluorine doped tin oxide [FTO] coated
glass substrates. Using Bi2S3 films as a photoelectrode
and carbon as a counter electrode formed the PEC cell
and its photoelectrochemical characterization was
carried out.
Experimental Procedure
Film deposition
The Bi2S3 thin films were deposited onto FTO
coated glass substrates of 0.075 × 0.0125 m2. For the
deposition of thin films aqueous 0.003 M bismuth
nitrate solutions were used as cationic precursors.
The pH of this solution was 9. The anionic precursor
was 0.1 M of thioacetamide [CH2 – CS - NH2] with
pH ∼11. The pH of the anionic precursor was raised
with the addition of hydrazine hydrate.
For the deposition of Bi2S3 thin films a wellcleaned glass substrate was immersed in cationic
precursor solution of bismuth nitrate [Bi(NO2)3] for
20 s in which Bi3+ ions are adsorbed on the surface of
the substrate. The substrate was rinsed with ion
exchange water for 40 s to remove unadsorbed ions.
Then substrate was immersed in an anionic precursor
of thioacetamide solution for 20 s in which S2- ions
are reacted with adsorbed Bi3+ ions on glass substrate.
This was followed by rinsing again in ion exchange
water for 40 s to remove unreacted S2- ions. This
completes one deposition cycle for the deposition of
Bi2S3 thin films. By repeating such deposition cycle
AHIRE & SHARMA: PHOTOELECTROCHEMICAL CHARACTERIZATION OF Bi2S3 THIN FILMS
141
for 20 times continuous Bi2S3 film on glass substrate
was obtained. The deposition was carried out at room
temperature (27ºC).
electrical conductivity of Bi2S3 thin films prepared by
using above deposition bath was also confirmed from
thermo-emf measurement of the films11.
Photoelectrochemical (PEC) characterization of Bi2S3 thin
film
Current-voltage (I-V) characteristics in dark and under light
illumination
The photoactivity of film was tested by forming a
PEC cell using Bi2S3 thin film onto FTO coated glass
as working electrode and graphite as counter
electrode. Polysulphide (Na2S-S-NaOH) was used as
an electrolyte. A 200 W tungsten filament lamp was
used as the light source. The current-voltage (I-V)
characterization in dark and under illumination was
obtained at 82 mW/cm2 illumination intensity. The
spectral response of cell was recorded with
monochromator using wavelength between 350 to
1050 nm. Transient photoresponse and capacitancevoltage (C-V) characteristics of cell were used to
calculate open circuit voltage, decay constant and flat
band potential, respectively.
The current-voltage (I-V) characteristics of PEC
cells in dark and under illumination were examined
with photoelectrode of Bi2S3. The nature of the I-V
curves indicated the formation of rectifying junction
(Fig. 1).
The PEC cell was illuminated and the curves were
shifted in the forth quadrant indicating that the cell is
generator of electricity. Increase in the current under
illumination indicates that the Bi2S3 films are
photoactive in nature12. Information of junction
ideality factor can be obtained using diode equation as
Results and Discussion
Bi2S3 film thickness
The thickness of photelectrode has a strong impact
on the performance of photoeletrochemical cell9.
Thickness of Bi2S3 films was measured with the help
of weight difference method employing sensitive
microbalance. Thickness of the film was 0.14 μm and
film was adherent well to the FTO coated glass
substrate.
⎡ eV ⎤
I = I0 ⎢e nkT −1⎥
⎢⎣
⎥⎦
… (2)
where n is junction ideality factor, Io is the reverse
saturation current, V is the forward bias voltage and I
is the forward bias current. The plot of log I versus V
is shown in Fig. 2 (derived from Fig. 1) using forward
bias data only. The values of junction ideality factor
nd for dark and ni for under illumination were
determined from the graph of log I versus V and are
12.4 and 9.5, respectively.
Type of conductivity
The PEC with the configuration Bi2S3/(NaOH-SNa2S)/C was formed. It was seen that PEC cell gives
some dark voltage Vd and dark current Id with Bi2S3
electrode as the negative and graphite electrode as the
positive polarity ends. The origin of this voltage is
attributed to the difference between two half cell
potentials in the PEC cell, which can be written as
E = Egraphite - EBi2 S3
… (1)
where Egraphite and EBi2 S3 are the half cell potentials
when dipped in the polysulphide electrolyte. After
illumination of the cell, the magnitude of this voltage
increases with negative polarity towards the Bi2S3
photoelectrode. Therefore, in the present case,
cathodic behaviour of photovoltage of semiconductor
was observed which indicates that the Bi2S3 thin films
have n-type electrical conductivity10. The n-type
Fig. 1⎯Current-voltage characteristics for Bi2S3 polysulphide
PEC cell in dark and under illumination.
142
INDIAN J. ENG. MATER. SCI., APRIL 2006
Photovoltaic output characteristics
Photovoltaic output characteristics of Bi2S3 PEC
cell under illumination intensity 82 mW/cm2 is shown
in Fig. 3. The calculations lead to the fill factor (ff)
39% and power conversion efficiency (η) 0.056. The
low conversion efficiency may be due to the low
series and high shunt resistance. The series resistance
RS and shunt resistance RSh were calculated from the
slopes of power output characteristic using relations.
1
⎛ dI ⎞
⎜
⎟ =
⎝ dV ⎠I =0 RS
… (3)
1
⎛ dI ⎞
⎜
⎟ =
⎝ dV ⎠V =0 RSh
… (4)
The values of RS and RSh are found to be 53 Ω and
625 Ω, respectively.
Spectral response
Fig. 4 shows the plot of short circuit current (Isc)
against wavelength λ. The decrease of Isc on shorter
wavelength side of the peak may be due to absorption
of light in electrolyte and high surface recombination
of photogenerated carrier by surface states13. The
decrease in Isc on longer wavelength side may be
attributed to the transition between defect levels.
Using peak value band gap energy is 1.78 eV. This
agrees well with the results obtained for optical
absorption of Bi2S3 films14.
Transient photoresponse
Fig. 2⎯Log I versus V plot for Bi2S3 polysulphide PEC cell in
dark and under illumination.
Fig. 3⎯Photovoltaic output characteristics for Bi2S3 polysulphide
PEC cell.
Photovoltaic rise and decay of PEC cell was
studied. Fig. 5 shows rise and decay characteristics of
Bi2S3/polysulphide PEC cells. Theoretically, open
circuit voltage (Voc) decay curve has three distinct
regions15. These three regions correspond to high,
intermediate and low level injection of minority
Fig. 4— Spectral response for Bi2S3 polysulphide PEC cells.
AHIRE & SHARMA: PHOTOELECTROCHEMICAL CHARACTERIZATION OF Bi2S3 THIN FILMS
143
Fig. 7⎯Mott-Schottky plot for Bi2S3 polysulphide PEC cell.
Fig. 5— Rise and decay curve for Bi2S3 polysulphide PEC cell.
to the presence of surface states. The decay constant,
b is calculated to be equal to 1.33.
Capacitance – voltage (C-V) characteristics in dark
Fig. 7 shows Mott-Schottky plot for the PEC cell
formed with Bi2S3 thin film the value of flat band
potential Vfb is obtained using Mott-Schottky
equation.
C2 =
2
(V −V fb − kT / e)
qεεοNd
… (6)
where the symbols have their usual meaning. The
nature of the plot is linear. The flat band potential Vfb
was determined by extrapolating the linear region of
this plot on the voltage axis is –0.55V. The positive
slope of the plot confirmed the n-type electrical
conductivity of Bi2S3 which is in good agreement with
the earlier values reported in literature16.
Fig. 6⎯Log Voc against log t (derived from Fig. 5)
carriers. The open circuit voltage Voc of the PEC cell
was found to persist for some time when the light is
cut off. Fig. 6 shows graph of log Voc versus log t by
using relation.
Voc (t) = Voc (0) e-b
… (5)
where Voc (0) and Voc (t) are the open circuit voltages
at t = 0 and t s respectively and ‘b’ is decay constant
the linearity of log Voc versus log t plot suggest the
kinetic involved in the voltage decay process is of
second order. The slow decay in Voc can be ascribed
Conclusions
A simple and less investigated modified chemical
bath deposition method was used for depositing good
quality thin films of Bi2S3 on FTO coated glass. The
I-V characteristics of the PEC cell formed by using
Bi2S3 thin film with polysulphide electrolyte (Na2S-SNaOH) shows rectifying nature of the junction. Fill
factor and conversion efficiency of the cell are found
to be small due to low series and high shunt resistance
values. The decay constant of Bi2S3 films, which is
found to be 1.33, indicated the second order kinetics
in decay process. Film exhibits n-type electrical
conductivity.
INDIAN J. ENG. MATER. SCI., APRIL 2006
144
Acknowledgements
One of the authors (RRA) is thankful to UGC New
Delhi for the award of the teacher fellowship, and also
thanks to Principal Dr P D Deore, S G Patil College,
Sakri, for the grant of study leave for this work.
Authors are also thankful to the Principal, G T Patil
College, Nandurbar, for the laboratory and computer
facilities.
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