Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
CHAPTER
THREE
Chemosynthesis and Photoelectrochemical Performance
of PbS Thin Films
3.1 Introduction
Band gap is a fundamental property of the semiconductor. Energy
separation between the filled valence band and empty conduction band is known
as band gap. As the particle size decreases the band gap of semiconductor is
found to increases; this is known as the quantum size effect. This happens
because of the confinement of charge carriers in potential wells of small
dimensions. The recent interests in size quantization effects of nanocrystalline
semiconductors have drained great attention toward metal-chalcogenide based
systems in solar cell. In developing group II-VI semiconductor nanocrystalline
thin films for harvesting light energy of particular interest are CdX and PbX (X =
S, Se, and Te) nanocrystalline thin films, which have quite small band gap and
thus are competent of harvesting photons in the visible and infrared region.
Along with light harvesting abilities the semiconductors have potential
applications in biology, optics, electronics and transport [1 - 4]. Numerous
efforts have been done to synthesize nanocrystalline thin films with various
particle size and shape.
The lead sulphide (PbS) is an important direct narrow-band gap IV-VI
semiconducting material with a bulk band gap of 0.41 eV at 300 K [5]. The
optical properties of nanocrystals are strongly depending on its size and shape
[6]. The exciton Bohr radius for PbS, PbSe and PbTe chalcogenides are 18, 46
and 152 nm respectively making them an interesting material to study the
quantum effects over the wide range of particle size [7]. Quantum size effects are
usually characteristic of nanocrystallites measuring less than 10 nm. Due to this
effect band gap of the material increases as the size of the particle decreases.
This property makes it an excellent candidate for opto-electronic applications in
many fields such as photography, IR detectors, solar absorbers, light emitting
devices and solar cells [8-12]. Various nanostructures such as nantubes,
nanorods, star-shaped, dendrites etc. of PbS have been synthesized using
surfactant-assisted solution growth, Chemical Bath Deposition (CBD),
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
nonhydrolytic colloidal routes, polyol-mediated synthesis and electrochemical
methods [13-16].
Among aforementioned methods, CBD has its own advantages such as
simplicity, reproducibility, nonhazardous, cost effective and well suitability for
large area deposition. Moreover, we can grow various morphologies of
semiconducting materials by adjusting the growth parameters like pH of growth
solution, deposition time, temperature and addition of different additives in bath.
Furthermore, it facilitates better orientation of crystallites with improved grain
structure.
In the present investigation, we have carried out the synthesis of
nanostructured PbS thin films with tunable optical properties by CBD method.
The films deposited at various time intervals in aqueous medium, at room
temperature using low precursor concentration. The deposited PbS films exhibit
highly dense and well-defined grains with compact structure over the entire
substrate. Further, the effect of deposition time on crystallinity, morphology and
optical properties were studied in detail. The possible growth mechanism of
nanosphere and nano cubo-octahedron is discussed. The photoelectrochemical
(PEC) performance such as J-V characteristics in dark and under illumination,
photovoltaic output of prepared films are studied.
Finally, the performance
parameters like short circuit current density (Jsc), open circuit voltage (Voc), fill
factor (FF) and photo-conversion efficiency (η) in dark and under illumination
are discussed.
3.2 Experimental details
All chemical were of AR grade and used without any further purification.
The lead acetate, Pb(CH3COO)2.3H2O was used as source of Pb (II) source and
thiourea, H2N⋅CS⋅NH2 as source of S2- source, tri-sodium citrate, Na3C6H5O7,
c.a.TSC is used as complexing agent and sodium hydroxide, NaOH was used for
pH adjustment. Solution of above precursor chemicals were prepared in doubly
distilled water at room temperature.
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
3.2.1 Substrate cleaning
Cleaning of the substrates for thin film depositions is most important
factor. It affects the adherence, smoothness and uniformity of the deposited thin
film. The techniques to be adopted for cleaning depend on nature of substrate,
degree of cleanliness required and nature of contaminants to be removed. The
common contaminants are grease, adsorbed water, air borne dust and oil
particles. Cleanliness is the process of breaking the bonds between substrates and
contaminants without damaging the substrates. There are various methods to
supply energy for breaking such bonds, such as heating, bombarding by ions
scrubbing etc. The following cleaning procedure was used for glass substrates.
Firstly clean the microslides by applying labolene over it using cotton and
wash it with ordinary water, then by double distilled water. After that,
microslides are put into the newly prepared chromium trioxide solution prepared
in double distilled water upto boiling the solution. After boiling take aside the
beaker and cool it at room temperature for few minute. Then the microslides
deep into a beaker containing distilled water for the minutes. Then, remove slides
from the beaker and put it in ultrasonic bath for 10 minutes for constant
temperature. Now these slides put in rack and then keep it in oven for drying.
Then put few drops of ultrapure methanol into the container. The dried
microslides with rack are put into container close the container and heat it
about 10 minutes. Take container aside and cool it up to room temperature. Now
cleaned microslides are clean by cotton and ready to wrap into the butter paper
for experimental work.
Cleaning of FTO Coated Glass Substrates:
Fluorine Doped Tin Oxide (FTO) coated glass substrates with a sheet
resistance of < 15 Ω / sq cm (Kintec Company, Hung Hom, Kowloon, Hong
Kong) were first cleaned with detergent, ultrasonicated in deionized water,
acetone and isopropyl alcohol for 15 minutes, respectively.
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
3.2.2 Solution preparation and thin film formation
The concentrations of precursors, pH of bath solution, temperature and
rate of substrate rotation were finalized at the initial stages of the thin film
deposition. By obtaining proper conditions good quality and uniform films were
obtained on substrate support. The film growth involves reaction of Pb2+ and S2ions in aqueous medium. At alkaline pH ~ 12 Pb-TSC arrested metal ions slowly
dissociate from complex and reacts with chalcogen ion S2-. Ion by ion
condensation took place which results in thin film deposition on substrate
surface. When ionic product K exceeds the solubility product Ksp condensation
of metal ions and chalcogen ions [17] results into binary PbS thin film formation.
Aqueous solutions of 10 ml 0.1 M lead acetate and 3 ml 1 M TSC was
taken in a beaker. Initially, the solution becomes turbid and milky due to the
complex formation. Then the solution of 1 M sodium hydroxide was added into
above solution to dissolve the complex and to keep pH of solution at ~ 12.
Finally, 10 ml 0.5 M thiourea was added into the above solution with constant
stirring and make final volume of solution 100 ml by adding distilled water into
it. The glass substrates were immersed in the bath at room temperature. The
temperature of the bath was around 270 C. The samples dipped in the bath, were
extracted after a time interval of 20, 30, 40, 50, 60, 70, 80 and 90 min and are
abbreviated as PbS20, PbS30, PbS40, PbS50, PbS60, PbS70, PbS80 and PbS90
respectively. The deposited films were rinsed with double distilled water and
allowed to dry at room temperature. The as-deposited films were found to be
uniform, specularly reflective (mirror-like) to the glass substrates. The schematic
of CBD method for deposition of PbS thin films shown in Fig. 3.1.
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
Figure 3.1: Schematic experimental set up of CBD method for deposition of PbS thin films
3.2.3 Effect of preparative parameters
3.2.3.1 Complexing agent
Growth of PbS thin films is influenced by organic complexing agent. In
absence of complexing agent rate of reaction would be very fast which result
simply precipitation of metal chalcogenides in the reaction bath, thus thin films
of metal chalcogenides cannot be grown on substrate support. Therefore in the
present investigation Pb2+ ions is arrested by using TSC as a complexing agent to
slow down the release of metal ions in the reaction bath. The stability constant
indicates the strong affinity of the complexing agent (TSC) towards the Pb2+ ions
and its tendency to arrest Pb2+ ions in solution even at alkaline pH, where metal
hydroxide precipitate is possible. At optimum concentration of TSC, dissociation
of Pb-TSC complex is took place thus Pb2+ ions is released to react with S2- ions
which result in creation of nucleation centre on the surface of substrate support
followed by ion-by-ion condensation.
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
3.2.3.2 Concentration of precursor
The quality and growth rate of the deposited films was greatly influenced by the
concentration of the reacting species. Therefore we have studied the effect of
concentration of the reacting species for various concentrations. The film
deposited by using low concentration is thin and non uniform. This observation
can be related to the insufficient supply of ionic species at such concentration
levels. On the other hand when concentration of the species was increased, the
quality and uniformity of the films goes on increasing. This is true up to a certain
level of concentration and then saturation in the growth process was observed.
The uniform and good quality thin films of PbS were obtained by using
concentration that is 0.1 M solution of Pb(CH3COO)2.3H2O and by using 0.5
H2N⋅CS⋅NH2 solution. The deposition time were varied in order to obtain various
PbS thin films.
3.2.3.3 pH of reacting mixture
The growth of metal chalcogenide thin films using CBD depends on pH
value of the reacting solution. Increase in the pH value causes increase in the
relative molecular surface area and the solubility of the chalcogen species [18]. It
is common observation that with increasing pH complexation of metal ions by
complexing agent increases with decreasing hydrogen ion concentration. At low
pH value due to unavailability of OH- ions the HS- ions will not be formed
because rate of reaction between Pb2+ and S2- ionic species is very high which
results in to precipitate formation. At higher pH value Pb-TSC complexe slowly
release ions which facilitate the slow rate of reaction. Thus at pH 12 ± 0.2 Pb2+
ions react with S2- ions to form PbS on surface of the material.
3.2.3.4 Deposition temperature
The temperature dependence of growth rate observed in literature survey
is that the rate of deposition increases with bath temperature resulting into
formation of fine granular structure. The effect of bath temperature on the
growth of thin films can be very extreme with respect to the terminal thickness,
morphology, optical and electronic properties. The rate of deposition increases
with the increase in bath temperature. The higher temperature of reaction mixture
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
is responsible for precipitation rather than the film deposition. At lower value of
temperature, it was observed that Pb2+ and S2- ions reacted slowly. Hence the
optimum temperature selected for deposition of PbS thin film is 27 ± 2˚C.
3.2.3.5 Deposition period
The growth rate of thin films was studied at various deposition times. For
deposition time of less than 10 min, films were discontinuous and therefore
deposition time was varied between 10 and 90 min. A brownish-black precipitate
gradually fills the bath. The growth rate slows down after deposition period of 70
min. So deposition after 70 min results in less material formation on the
substrate. On addition of the reactant together reaction start it can be observed by
change in color and ultimately it ends in deposition of film.
3.2.4 Mechanism of thin film deposition
The detail of nucleation and growth depends on the material being
deposited and other parameters such as reaction time, temperature, complexing
agent, pH, and required precursors to maintain a high simultaneous nucleation
rate and good size distribution for deposition of PbS thin films. CBD is suitable
for the deposition of PbS thin films. According to Ostwald ripening law if metal
ions in solution are arrested using complexing agent like TSC, the rate of
reaction between metal ions and chalcogenide ions can be well controlled to get
desired quality of metal chalcogenide thin films. The film growth can takes place
either by ion-by-ion condensation of the material particle or by the adsorption of
the colloidal particles from the solution on to the substrates depends on
deposition conditions. The process of deposition of a material from the solution
onto the substrate depends mainly on the formation of nucleation centre and
subsequent growth to produce a film. The terminal thickness of the film almost
depends on the ionic and solubility products of the reacting species in the
solution. Nucleation process results that the clusters of molecules formed
undergo rapid decomposition and combine to grow up to a certain terminal
thickness beyond which no further building up of thickness occurs. The
nucleation process starts when the ionic product exceeds the solubility product of
reacting species. Growth of these nuclei by the addition of more and more ions
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
from the solution results in to the formation of stable nucleus of the size greater
than the critical size. The stirring of solution increase the rate of arrival of the
ions on the substrate surface and hence speed up the rate of coagulation of the
colloidal particles in the solution.
3.2.5 Characterization of PbS thin films
The thickness of the films was measured by using Ambios XP-1 surface
profiler. The absorption measurement was performed in the wavelength range
over 400-2000 nm by using a Hitachi-330 (Japan) UV-vis-NIR double beam
spectrophotometer. Room temperature Photoluminescence (PL) was studied on
Luminescence Spectrometer (model: F900, resolution: 0.1 nm, Edinburgh
Instruments, UK).
The X-ray Diffraction (XRD) spectrum of the films was recorded using
X-ray diffractometer (Bruker AXS D8 Advance X-ray Diffractometer) with Cu
Kα target having wavelength 1.542Å. Surface morphology of deposited films
was examined using Field Emission Scanning Electron Microscopy (FE-SEM)
(JEOL JSM-6500F). Energy Dispersive X-ray Spectroscopy (EDS) was
performed on a JEOL-JSM-6360A scanning microscope. Transmission Electron
Microscopy (TEM) for PbS thin films was performed using a TECNAI G2 20U
TEM equipped with a field emission gun operating at 20 kV to 120 kV
accelerating voltage in combination of bright-field and dark-field STEM
imaging. The chemical composition and valence states of constituent elements
were analyzed by X-ray Photoelectron Spectroscopy (XPS) Thermo K-Alpha
with multi-channel detector, which can suffer high photonic energies from 0.1 to
3 KeV.
The Fourier Transform Infrared (FT-IR) spectra of samples were collected
using a Spectrum 100 Perkin Elmer FT-IR spectrophotometer in the frequency
range 450-4000 cm-1 using pellets made by mixing the sample with KBr, to
investigate the functional groups of the PbS nanoparticles. The wetting behaviors
of the samples were checked by means of contact angle measurement using
Rame-hart model 500-F1, USA. The J-V characteristics were measured using
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
Semiconductor Characterization System SCS-4200 Keithely, Germany using two
electrode configurations.
3.3 Results and discussion
3.3.1 Reaction and growth mechanism
The precipitation of metal chalcogenides in CBD occurs only when the
ionic product exceeds the solubility product of metal chalcogenides (PbS in our
case) [19]. The film growth takes place via ion-by-ion condensation of the
materials or by adsorption of colloidal particles from the solution onto the
substrate. The complexing agents help to control the reaction rate. PbS thin films
were deposited from an aqueous alkaline medium containing Pb-TSC, complexe
and thiourea solution. The deposition process based on simple ion by ion
mechanism involves three steps i) Formation of complex ii) Formation of S2ions iii) Formation of PbS via ion by ion condensation. PbS forms via the overall
reactions to take place inside the bath:
Pb2+ + OH- + Cit3- → PbOHCit
2-
3.1
NH2 -SC-NH2 + OH- → NC-NH2 + HS- + H2 O
3.2
HS- + OH- → H2 O + S2-
3.3
PbOHCit + S2- → PbS + OH- + Cit3-
3.4
2-
In this reaction process initially lead acetate dissociates as Pb2+ and
(CH3COO-) ions. After addition of TSC it forms complex with Pb2+ as lead
hydroxyl citrate [Pb (OH) Cit2-], then the solution became milky turbid. Further
addition of excess alkaline NaOH causes dissolution of turbidity leading to a
homogeneous solution. Due to excess of NaOH in solution provides OH- ions,
thiourea hydrolyzed into cyanamide (N≡C-NH2), which is unstable in an aqueous
medium and readily converts into urea (O=C-(NH2)2) following base catalyzed
reaction and release S2- ions into solution [20]. The deposition of the films from
the solution involves a nucleation phase in which an initial layer of [Pb (OH)
Cit2-] formed on the glass substrate and is subsequently converted into PbS by the
reaction with S2− ions available in the bath. Finally, the as deposited PbS thin
films are found to be uniform and well adherent to the substrate. As deposited
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
thin film samples are cut into pieces of different size for characterization
purpose.
Growth mechanism
Fig. 3.2 shows the schematic growth model of PbS crystals and its
transformation from granular seeds to cubes and finally, to cubo-octahedra. It is
widely accepted that the formation of crystals is mainly achieved through two
stages: nucleation and growth. In the solution-phase synthesis of nanocrystals,
‘nucleation’ is generally referred to the formation of tiny seeds with a stable
structure and well-defined crystallinity and the shape of seeds is primarily
determined by the minimization of surface energy [21].
Generally, a basic crystal shape is determined by two growth process:
habit formation and branching growth. The former is determined by the relative
order of surface energies of crystallographic planes of a crystal, while the latter is
determined by a diffusion effect. The surface energies associated with different
crystallographic planes are usually different, and the growth rates on different
facets are dominated by the surface energy. In a solution-phase process,
impurities or capping agents are usually adopted to alter the surface free energies
via adsorption or chemical interaction and thus induce new shapes [22].
Figure 3.2: Schematic illustration of the different morphologies of the PbS thin films
deposited at various deposition time
It has also been known that rock-salt phase (fcc) of the PbS crystals
generally nucleates as tetra decahedron seeds, exposing six (100) facets and eight
(111) facets. Subsequent competitive growth on these two different types of
crystalline facets determines the final shape [22].
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
In the present system, initially granular seed formation takes place due to
nucleation. The intrinsic surface energy of (111) facet is higher than that of the
(200) facet for PbS crystals [5, 22], relatively fast growth along eight equivalent
(111) directions from the tetra decahedron seeds under conditions of low
temperature (27 °C) results in the formation of cube-shaped nanocrystals. This is
because the interaction strength of TSC with (111) facets at low temperature is
relatively weak and the effects on the surface energies of PbS crystals can be
ignored. When the reaction time was increased, this interaction strength was
greatly enhanced and it could efficiently lower the surface energies of (111)
facets, which thus would lower the growth on (111) facets and facilitate the
growth on (200) facets. The different growth rates on the (200) and (111) facets,
lead the formation of the cubooctahedra PbS crystals [22].
3.3.2 Thickness measurement
The thickness of the films was found to vary from 712 to 3256 nm with an
increase in deposition time from 20 to 90 min. The values of film thickness of all
films are given in Table 3.1. It was observed that the thickness increases with
deposition time up to PbS70 thereafter, it decreases rapidly. The saturation can be
related to insufficient quantity of reactive species after PbS70.
3.3.3 Optical absorption studies
The optical absorption spectra for the films were recorded in the
wavelength range of 400-2000 nm at room temperature. Fig.3.3 shows variation
in the optical absorption with wavelength.
The absorption coefficient for all the films was found to be of the order of
10-10 cm-1. The optical band gap energy of all films is calculated using a classical
equation:
α0 (hν-Eg ) 2
α=
hν
1
3.5
where, Eg is the separation between the bottom of conduction band and
top of the valence band, hν is the photon energy and n is the constant. For
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
allowed direct transitions n = 1/2 and for indirect transition n = 2. The plots of
(αhν)
2
against hν for all the films are shown in Fig. 3.4 (a) and (b). The
extrapolation of straight line portions to zero absorption coefficient (α = 0), leads
to the estimation of the band gap energy values. It is noted that the band gap
energy varied ranging from 2.06 to 0.99 eV with deposition time. Tabulation of
optical band gap of the PbS thin films deposited at various deposition time 20,
30, 40, 50, 60, 70, 80 and 90 min is given in Table 3.1.
Figure 3.3: Absorption spectra of the PbS thin films deposited at various deposition time
20, 30, 40, 50, 60, 70, 80 and 90 min
The variation in Eg is attributed to the quantum confinement effect.
Because, for a semiconductor crystal, electronic excitation consists of a loosely
bounded electron-hole pair, usually delocalized over a length much longer than
the lattice constant. As the diameter of the semiconductor crystallite approaches
this exciton Bohr diameter, its electronic properties start to change. This
quantum confinement effect can be explained qualitatively by considering a
particle-in-a-box like situation where the energy separation between the level’s
increases as the dimensions of the box is reduced. In addition, quantum
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
confinement leads to a collapse of the continuous energy bands of a bulk
material into discrete, atomic like energy levels. The discrete structure of energy
states leads to a discrete absorption spectrum of QDs. Thus, one observes an
increase in the band gap of the semiconductor with a decrease in the particle size
[23].
Figure 3.4: Tauc plots to estimate the direct band gap of the PbS thin films deposited at
various deposition time (a) 20-60 min (b) 70-90 min
Chapter III
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
Table 3.1: Tabulation of the thickness and optical band gap of the PbS thin films
deposited at various deposition time 20, 30, 40, 50, 60, 70, 80 and 90 min
Sample
Time (min)
Thickness (nm)
Optical band gap (eV)
PbS20
PbS30
PbS40
PbS50
PbS60
PbS70
PbS80
PbS90
20
30
40
50
60
70
80
90
712
1057
2004
2336
2506
3256
2375
2207
2.06
1.67
1.50
1.12
1.10
0.99
1.05
1.07
It is interesting to note that this facile and low cost chemosynthesis
method enables band gap energy tuning from as low as 0.99 eV to 2.06 eV,
capable of harnessing photons over visible and NIR regime. This would help to
prepare an efficient solar absorber with minimum interfacial stress and to absorb
the maximum span of the solar spectrum in PbS-based solar cells.
3.3.4 Photoluminescence (PL) study
PL is an important tool to investigate quality of the thin film, which
depends on size of crystallites, morphology and chemical environment. Our
results set in evidence both blue and UV luminescence of PbS thin films at room
temperature. To optimize the deposition time, we have elaborated the PbS thin
films onto glass substrate at different times, ranging from 20 to 90 min and the
luminescence spectra have been recorded for 416 nm excitation wavelength
using xenon lamp as a light source which is laid out in Fig. 3.5. Twin peaks at
405 nm and 393 nm, along with low intense peak at 460 nm are observed in the
PL spectra. An intense emission band was observed for the sample which
deposition time was 70 min. The peaks positions are almost the same for all
samples. However, there is increase in PL intensity with the deposition time 20
to 70 min observed, after that it decreases.
PL intensity shift towards low wavelength can be related to the
nanoparticles size. Precisely, as in nanocrystals prepared by other methods [24,
25], this shift is attributed to the spatial confinement of electron-hole pairs. Cao
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
et. al. [26] also observed a large blue shift of the absorption edge for their
synthesized PbS nanocubes. Chattopadhyay and co-workers [27] reported a blue
shift for their prepared tetrapod PbS. They shows that the PL peak around 440
nm wavelength is due to the transition of electrons from the conduction band
edge to holes trapped at surface states located within the band gap energy.
Figure 3.5: Room temperature photoluminescence spectra of the PbS thin films deposited
at various deposition time 20, 30, 40, 50, 60, 70, 80 and 90 min
Under PL excitation at 416 nm, the PbS nanoparticles emit blue light as
393 nm. The PL spectrum consists of one strong peak at 405 nm, that can be
ascribed to a high level transition in PbS semiconductor crystallites. Emission
bands at 405 nm are usually related to the transition of electrons from the
conduction band edge to holes, tapped at interstitial Pb2+ sites [28]. There is
another possibility that the observed peaks may correspond to transitions into
high-energy bands rather than excitonic transitions. According to Sagar et al.
[29], the photoluminescence is sensitive to the quality of crystal structure and to
the presence of defects. We think that the intense emission of the PbS thin films
is due to the presence of low defects density introduced in the thin film during
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
the CBD process. The origin of the observed emission peaks in the blue-light
bands is still far from well understood and more detailed investigations are
needed.
3.3.5 X-ray Diffraction (XRD) studies
The XRD patterns of the PbS thin films are shown in Fig. 3.6. The
comparison of observed XRD patterns with the standard JCPDS data (Card no.
78-1058) confirms the formation of a PbS phase with cubic crystal structure. The
lattice parameter ‘a’ of PbS was calculated using the following equation:
1
h2 + k 2 + l 2
=
d2
a2
3.6
The mean value of a = 5.9320 Å is in good agreement with the reported
value in JCPDS data, a = 5.9315. The cell parameters, observed and standard d
values with respect to their hkl planes are given in Table 3.2.
Table 3.2: Structural and morphological characterization of PbS thin films
d-values (Å)
hkl
Standard Observed Planes
3.4245
2.9657
2.0971
1.7884
1.7122
1.4828
1.3607
1.3263
1.2107
1.1415
3.4243
2.9666
2.0971
1.7893
1.7114
1.4827
1.3617
1.3264
1.2110
1.1413
111
200
220
311
222
400
331
420
422
511
Cell parameter
a = b = c (Å)
5.9310
5.9333
5.9317
5.9344
5.9285
5.9308
5.9355
5.9322
5.9327
5.9306
A thinner film PbS20 exhibits XRD peak corresponding to (111) plane,
which ameliorates with the increase in the deposition time up to PbS40,
afterwards preferred orientation changes to (200) plane. Besides this major peak,
eight more peaks corresponding to (220), (311), (222), (400), (331), (420), (422)
and (511) planes are observed. This suggests that, PbS phase is stable and its
formation is independent on the deposition time.
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
Figure 3.6: X-ray diffraction patterns of the PbS thin films deposited at various deposition
time 20, 30, 40, 50, 60, 70, 80 and 90 min. The standard JCPDS data (Card no.78-1058) for
PbS is shown as vertical lines
Further, the average crystallite size was calculated using a well-known
Debye-Scherrer’s formula:
D=
0.9λ
β cos θ
3.7
where, λ is the wavelength of X-rays (1.5406 Å), β the full width at half
maximum (in radian) of the peak and θ is Bragg’s angle of XRD peak and listed
in Table 3.3. The crystallite size is varied from 23 to 34 nm with deposition time
changed from 20 to 90 min. PbS20 films exhibits smaller crystallite size (23 nm)
whereas PbS70 film exhibits higher crystallite size (34 nm). Since smaller crystals
tend to have surfaces with sharper convexity, they gradually disappear by
feeding the larger crystals, as the thickness increases. The net effect is crystal
growth. Hence PbS70 film has larger crystallite size as compared with other
films.
Table 3.3: Tabulation of crystallite size of PbS thin films for different deposition time
Sample
Crystallite
size (nm)
Chapter III
PbS20
23
PbS30
26
PbS40
27
PbS50
29
PbS60
30
PbS70
34
PbS80
30
PbS90
26
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Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
3.3.6 Field Emission Scanning Electron Microscopy (FE-SEM)
Fig. 3.7 (a-h) shows typical FESEM images of all the PbS thin films. The
surface morphology changes from granular to cubic to cubo-octahedron with
deposition time. Densely packed spherical grains are formed on the surface of
PbS20 to PbS40 films. The formation of cubic to cubo-octahedron morphology can
be seen for PbS50 to PbS90 thick films.
The nucleation and growth mechanism is important for thin film
formation in chemical methods. Initially, the small particles get nucleated on the
surface of the substrate, which provide effective growth sites for the thin films.
Large number of these sites comes together to form particles with well defined
facets; during the growth processes and finally, this large number of particles
agglomerates to form a grain. Moreover, the smaller grains with sharper
convexity disappear and feed to relatively large grains to form a matured grain.
Thus, a grain is composed of particles made up of the tiny nucleates.
Consequently, grains are bigger than the crystallites and the particles.
Chapter III
129
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
Figure 3.7: Field emission scanning electron microscopy images of the films deposited at
various deposition time (a) 20, (b) 30, (c) 40, (d) 50, (e) 60, (f) 70, (g) 80 and (h) 90 min. All
the films are recorded at x 25,000 magnification. Inset shows water contact angle
measurement of PbS thin films
The contact angle (CA) of a water drop is influenced by the roughness of
the surface. Inset Fig. 3.7 (a-h) shows water CA measurement of PbS thin films
and are found to be 82o to 47o (±1o) variation for PbS20 to PbS90 respectively. PbS
thin films shows hydrophilic nature because the acetate ions from the PbS
precursor strongly influence to hydrophilic nature. This is beneficial to the better
access of electrolyte into the film structure which enhances the PEC
performance.
3.3.7 Energy Dispersive X-ray Spectroscopy (EDS)
The elements present in the sample qualitatively identified by their
characteristics wavelengths in Energy dispersive X-ray spectroscopy (EDS).
Quantitative estimation is also possible by measuring relative intensities in the
spectra. For compositions greater than or about 1% and elements separated by
Chapter III
130
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
few atomic numbers, EDS analysis is very useful because the intensities are
increased about 100-fold. Fig. 3.8 represents EDS pattern of PbS70 thin film.
This analysis shows that the atomic percentage of Pb and S confirms
stoichiometry of the synthesized compound. The expected and atomic percentage
of Pb and S are as shown in inset of Fig. 3.8.
Figure 3.8: EDS scanning pattern of PbS70 thin film
The EDS data showed close agreements in theoretical and experimental
values of Pb2+ and S2-. So atomic weight percent suggest the chemical formula
PbS of as deposited lead sulphide thin films. The percentage of Pb in the film is
higher than expected this is attributed to the fact that lead is more metallic and its
reactivity towards S2- is higher. This is may be responsible for slightly distortion
from stoichiometry of ternary PbS thin film.
3.3.8 Transmission Electron Microscopy (TEM) study
Here, Transmission Electron Microscopy (TEM) and Selected Area
Electron Diffraction (SAED) were employed further to characterize the
morphology and crystal structure of PbS nanoparticles. TEM for PbS70 is shown
in Fig. 3.9 (a) to (c), were used to examine the PbS growth and quality.
Chapter III
131
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
Figure 3.9: (a) to (c) TEM images of the PbS70 thin film (d) SAED pattern of PbS thin film
From the TEM images, PbS nanoparticles, somehow aggregation are
found. The aggregation may result because of the following reasons. First, for
smaller QDs, there exists a weak, although long range attraction that persists up
to higher concentrations in the solution of PbS [30]. Second, for small
nanoparticles, the absorption of complexing agent occurs on the surface of PbS
[30], and when the film dried, the citrate ions may link the particles together.
Through the measurement the dark particles are PbS nanoparticles. The white
areas without crystal lattice between them may be the citrate ions. In the TEM
images of the dark particles for higher magnification marked red squares clearly
demonstrates the lattice planes for the PbS nanoparticles which is shown in Fig.
3.9 (b) and (c). The SAED pattern in Fig. 3.9 (d) proves that the PbS
nanoparticles are crystalline and lattice planes for PbS are good accordance with
XRD data.
3.3.9 X-ray Photoelectron Spectroscopy (XPS)
Chapter III
132
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
Electronic structures and chemical states of PbS film have been performed
by XPS. The values of the binding energy were calibrated using C 1s peak (284.8
eV) as the internal standard. Fig. 3.10 shows a full scan XPS spectrum of PbS70
film in which the peaks corresponding to the energies of several orbitals of Pb, S,
C and O are easily identified. The C and O peaks, mainly from atmospheric
contamination due to exposure of the sample to air. The area analysis of more
detailed XPS spectra of the individual peaks are shown in Fig. 3.11 (a) and (b).
Figure 3.10: Full scan XPS spectra of PbS70 thin film
Fig. 3.11(a) illustrates the Pb (4f) high resolution XPS spectrum. The finescanned Pb 4f7/2 and 4f5/2 peaks are observed at 138.95 and 141.02 eV
respectively of Pb (4f). Each peak is resolved into two bands by fitting it into the
Vioget function. For Pb 4f7/2 peak, the resolved bands referred as P1 and P2 and
this is due to the absence of residual ligands on the film surface. The band P1 at
138.4 eV and P2 at 139.1 eV are corresponding to the Pb-S and Pb-O bonding
structures respectively. Jang et al. reported PbS nanowires using three different
routes; solvothermal (PS1), chemical vapor transport (PS2) and gas phase
substitution reaction (PS3) [31]. They found that Pb 4f7/2 peak cannot be resolved
into two bands, and this corresponds to the presence of the residual ligands on
Chapter III
133
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
the surface. In addition, Pb 4f7/2 peak of PS2 sample cannot well resolve into two
components. The PS3 having Pb 4f7/2 peaks resolved into two components at
137.3 and 139.0 eV respectively indicates the thinner oxide layer. They showed
the improvement in mobility of holes due to the thinner oxide layer. The peak
corresponding to Pb 4f5/2 peak has been resolved into two components at 140.9
and 141.2 eV, which is referred as P3 and P4 respectively.
Figure 3.11: High resolution XPS peaks of (a) Pb 4f7/2 and Pb 4f5/2, (b) S 2p3/2 and S 2p1/2
The XPS spectrum of Sulfur S2p peak is shown in Fig. 3.11 (b). The
corrected S2p peaks of PbS are deconvoluted into S2p1/2 and S2p3/2 peaks. The
Chapter III
134
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
S2p1/2 and S2p3/2 peaks positions for PbS are 159.2 eV and 159.5 eV respectively.
The approximate composition of the surface can be determined by dividing the
individual peak area, after appropriate background subtraction, by their
respective atomic sensitivity factor (ASF). The obtained ratio is 1:1for PbS70 thin
film deposited by CBD. Table 3.4 shows standard and observed binding energies
for PbS core orbitals.
Table 3.4: Standard and observed binding energies for PbS core orbitals
Element
Core orbital
Pb
4f5/2
4f7/2
2p3/2
2p1/2
S
Binding energy (eV)
Standard
Observed
141.70
141.07
137.80
138.99
159.90
159.57
159.46
159.28
3.3.10 Fourier Transform Infra-Red (FT-IR) spectroscopy
Figure 3.12: FTIR transmission spectra of the PbS thin film was recorded in the wave
number range of 500 - 4000 cm-1
Fig. 3.12 shows the FTIR spectra of PbS nanoparticles. The
antisymmetric and symmetric C-H stretching vibration of the CH2 group around
2921 and 2856 cm−1 respectively, the CH2 deformation vibration at 1465 cm−1
[32]. The peak at 1095 cm−1 corresponds to one of the fundamental frequency of
Chapter III
135
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
C-O bond. The peak near 1608 cm−1 is observed due to (H-O-H) stretching
vibration. Strong bands associated with Si-O stretching and bending vibrations
are apparent to 798 and 1024 cm-1. Because the bond of Pb-S is mainly
electrovalent bond, the FTIR spectra of PbS does not show strong bands
associated with Pb-S stretching and bending vibrations.
3.3.11 Photoelectrochemical (PEC) study
3.3.11.1 Choice and selection of suitable electrolyte
The formal redox potentials of the various electrolytes were studied and
with reference to that the suitability of electrolyte was checked. A suitable
electrode was selected by determining λmax of various electrolytes. e.g.
sulphide/polysulphide
(S-/S2-),
ferricyanide/ferrocyanide
(Fe3+/Fe2+),
iodide/polyiodide (I-/I3-) solution. The λmax of polysulphide solution was found to
be suitable and it shows maximum absorption in visible (~390 nm) range of
spectra. The sulphide/polysulphide couple with formal redox potential -0.70 V vs
Standard Calomel Electrode (SCE) ± 0.005 V was found suitable for
constructing PEC cell of material under investigations. The formal redox
potentials of the various electrolytes are shown in Table 3.5.
Table 3.5: The formal redox potentials of the various electrolytes used for construction of
PEC cell
Sr. No.
1
2
3
Electrolyte for PEC cell
Sulphide/Polysulphide
Potassium ferricyanide /
potassium ferrocynide
Iodide/Polyiodide
Formal redox potential (V)
-0.70 V vs 0.005 V
-356 V vs 0.005 V
-315 V vs 0.005 V
3.3.11.2 Preparation of the electrolyte
Various electrolytes were prepared in double distilled water and used
immediately in PEC cell.
Sulphide / Polysulphide
1 M sodium hydroxide was mixed with 1 M sulfur powder and the
solution was stirred continuously. To this 1 M Na2S was added and the resultant
solution was stored in an air sealed bottle.
Chapter III
136
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
Iodide / Polyiodide
0.0625 M sodium iodite was mixed with iodine crystals and dissolved in
double distilled water.
Ferri / Ferrocyanide
0.5 potassium ferrocyanide and 0.5 M potassium ferricyanide were mixed
together to form a ferri/ferrocynide.
3.3.11.3 Selection of redox couple
The physical and chemical properties of an electrolyte decide the power
conversion efficiency of the electrochemical photovoltaic cells. The electrolyte
properties that govern the conversion efficiency of PEC cell are,
a) pH of the electrolyte
b) Ionic conductivity
c) The absorption of light by electrolyte solution
d) Redox potential of electrolyte
Optical absorption study of the electrolyte system is therefore necessary
as it gives information about part of the visible light that can be utilized in a
semiconductor depletion region. The sulphide/polysuphide redox couple shows
absorption cut off at 450 nm. However in case of ferro/ferricyanide and
iodide/polyiodide, absorption cut off observed between 500-850 nm. This means
that photons with wavelength lower than 500 nm are observed in the electrolyte
itself before reaching to the semiconductor/electrolyte interface [33, 34]. This
indicates that sulphide/polysulphide redox electrolyte system show least optical
absorption in the visible region of the spectrum. Therefore sulphide/polysulphide
electrolyte system is selected for PEC study of ternary and quaternary thin films.
3.3.11.4 Construction of PEC cell
The J-V characteristics of the thin films were recorded by using a
semiconductor characterization system (SCS - 4200 Keithley, Germany) with
two electrode configuration under a halogen lamp (30 mW/cm2). The
semiconductor film (average area 1.0 cm2) was employed as the working and
graphite as counter electrode respectively. The distance between the working and
counter electrode was 0.5 cm. An aqueous 1 M polysulphide (Na2S + S + NaOH)
Chapter III
137
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
solution was used as the redox electrolyte. Measurements for the power output
characteristics and J-V plots were made at fixed intervals after waiting a
sufficient amount of time for the system to reach equilibrium (both in dark and
under illumination). The photo anode area exposed to light was 1 cm2. The area
of semiconducting thin film other than that in contact with electrolyte was
covered by insulating tape. A typical PEC measurement setup is shown in Fig.
3.13.
Figure 3.13: A typical PEC measurement setup
For the photoelectrochemical characterization of the PbS thin film
samples PbS20 to PbS90, all the measurements were performed in an electrolyte of
1 M polysulfide (Na2S-NaOH-S) in a two-electrode arrangement of following
configuration:
Glass/FTO/PbS | 1 M (Na2S-NaOH-S)aq. | Counter electrode
3.8
Transient photoresponse characteristics of the cell were used to calculate
the decay constant.
3.3.11.5 J-V characteristics
Fig. 3.14 shows the J-V characteristics of PbS thin film samples PbS20 to
PbS90. The J-V characteristic in the dark resembles diode-like characteristics for
Chapter III
138
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
the PEC cells fabricated with all the samples. Under light, J-V curves shifts in
the IVth quadrant indicating generation of electricity, typical of solar cell
characterization. The use of nanocrystalline in place of single crystal is desired
from the realistic point, realizing a large area photoelectrode in any PEC cell.
But the problems on utilizing nanocrystalline thin film semiconductor in PEC
cell is the absence of space charge layer at the electrode-electrolyte interface.
Under these circumstances, photogenerated charge carriers can move in both
directions may be recombine readily with the hole or leak out at the electrolyte
interface instead of flowing through the external circuit [35].
Figure 3.14: J-V characteristics of PbS thin film samples PbS20 to PbS90
3.3.11.6 Photovoltaic output characteristics
Photovoltaic output characteristic for PbS thin film samples PbS20 to
PbS90 shown in Fig. 3.15. The open circuit voltage Voc found to be from 60 to
120 mV and short-circuit current Isc are 84 to 205 µA/cm2 respectively for
sample PbS20 to PbS90.
PCE is one of the most import parameter to characterize solar cell
performances. It is defined as the percentage of maximum output of electrical
power to the incident light power. The PCE can be described as,
Chapter III
139
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
PCE (η) =
Pm Jsc Voc =
Pin
Pin
3.9
Where, Pm is maximum power point, Pin is the incident light intensity,
Jsc is the short-circuit current density, Voc is the open-circuit voltage and FF is
the fill factor, which is defined as the ratio of Pm to the product of Jsc and Voc.
The FF is determine from the following relation
FF where, Imax
Pm
Imax ×Vmax
Jsc ×Voc
Jsc ×Voc
3.10
= the current at the maximum power output,
Vmax = the voltage at the maximum power output,
Isc
= the short-circuit current,
Voc
= the open-circuit voltage and
Pm
= maximum power point.
The FF is directly affected by the values of the cells series and shunt
resistance. Increasing the shunt resistance (Rsh) and decreasing the series
resistance (Rs) will lead to higher FF, thus resulting in greater efficiency.
Figure 3.15: Output characteristics of PbS thin film samples PbS20 to PbS90
Chapter III
140
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
The significant improvement in conversion efficiency is observed in PbS70
thin films. This is because the densely packed cubo-octahedra grains of PbS70
thin films can absorb enough light. Furthermore the photo generated electrons
can transport through the compact layers to the conducting substrate with
minimum loss. This greatly reduces the recombination losses of the photo
generated charge carriers due to decrease in grain boundary resistance in charge
transport process. The FF and the light to electricity conversion efficiency (η)
along with the Jsc and Voc are given in Table 3.6.
Table 3.6: Comparative chart for the PbS thin film samples PbS20 to PbS90 indicating Isc,
Voc with efficiency
Sample
Voc
(mV)
Isc
(µA/cm2)
Jsc
(µA/cm2)
Imax
(µA/cm2)
Vmax
(mV)
FF (%)
Efficiency
η (%)
PbS20
PbS30
PbS40
PbS50
PbS60
PbS70
PbS80
PbS90
61
70
73
100
95
108
119
94
84
91
98
101
105
205
139
105
168
183
197
203
210
410
278
210
52
47
50
54
73
94
78
70
29.65
37.04
38.45
47.18
52.72
56.29
58.65
48.33
29.80
26.98
26.47
25.02
38.73
23.91
27.43
34.43
0.0031
0.0035
0.0038
0.0051
0.0077
0.0106
0.0091
0.0068
The current-voltage characteristics are largely dependent on the Rs and Rsh
resistance. A lower Rs means that higher current will flow through the device and
high Rsh corresponds to fewer shorts or leaks in the device. The ideal cell would
have Rs approaching zero and Rsh approaching infinity. The Rs can be estimated
from the inverse slope at a positive voltage where the J-V curves become linear.
The Rsh can be derived by taking the inverse slope of the J-V curves around zero
(0) voltage. The resistances Rs and Rsh were analyzed from the J-V curves of the
PbS thin films using the relations,
dI
1
= dV I 0 Rs
Chapter III
dI
1
= dV V 0
Rsh
3.11
3.12
141
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
A slight reduction of Rsh is observed, that can be due to shorts or leaks in
the device. The Rs can be expressed as the sum of the bulk and interfacial
resistance. The increasing the thickness of PbS thin film, decreases the Rs in the
device and thereby increases the current. The high Rs is due to the barriers at the
interface between the film and electrolyte. Where, the Rs and the Rsh values were
estimated from the slopes of the J-V curve at the vicinity of the intersections of
the J-V curve with the V and J axes.
The ideality factor ‘nd’ of prepared PbS films are determined from diode
equation,
I = I0 e
qV
nd kT
-1
3.13
The ideality factor is determined under forward bias and is normally
found to be in between 1 to 2 depending up on the relation between diffusion
current and recombination current. When diffusion current is more than
recombination current then ideality factor becomes 1 and it becomes 2 in
opposite case. Most solar cells, which are quite large compared to conventional
diodes, well approximate an infinite plane and will usually exhibit near-ideal
behavior under standard test condition (nE1).
Under certain operating conditions; however, device operation may be
dominated by recombination in the space charge region. This is characterized by
a significant increase in I0 as well as an increase in ideality factor to nE2. The
latter tends to increase solar cell output voltage while the former acts to corrode
it. The net effect, therefore, is a combination of the increase in voltage shown for
increasing ‘n’ and the decrease in voltage shown for increasing I0. Typically, I0 is
the more significant factor and the result is a reduction in voltage. The ideality
values of PbS solar cells lies between 2.5 to 1.77. This indicates that current
transport in the grain boundary region of the PbS solar cell is controlled by
recombination at a high density of defect states.
Chapter III
142
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
3.4 Conclusion
Nanocrystalline PbS thin films of different thickness with deposition time
have been successfully deposited by a simple and cost effective CBD method.
The XRD shows the films are polycrystalline consisting of cubic phase. The FESEM micrographs shows that the nanoparticles of PbS, changes from spherical to
cubic to cubo-octahedra in nature. PbS thin films are direct band nature and band
gap energy of PbS films are varied from 2.06 to 0.99 eV, which are higher than
the bulk due to quantum confinement effect in PbS nanocrystallites. Electronic
structures and chemical states of PbS film is analyzed by XPS. Existence of
different peaks of PbS nanoparticles in FTIR revealed the formation of PbS and
presence of different functional groups. A photoluminescence spectrum of PbS
sample shows green and yellow band emission. The maximum of efficiency is
found to be 0.01% for PbS70 sample. The efficiency of PbS thin films is low it
may be due to the anodic corrosion of the films and defects sites present in the
films.
We foresee a simple and effective chemical approach to deposit good
solar absorber that can harness a wide Vis-to-NIR spectral range to increase
solar-cell efficiency.
Chapter III
143
Chemosynthesis and Photoelectrochemical Performance of PbS Thin Films
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