ORI GIN AL PA PER

Synthesis and Characterization PbS and Bi2S3

Nanostructures via Microwave Approachand Investigation of Their Behaviors in Solar Cell

Mohammad Yousefi • Mohammad Sabet •

Masoud Salavati-Niasari • Hamid Emadi

Received: 6 February 2012 / Published online: 25 April 2012

� Springer Science+Business Media, LLC 2012

Abstract PbS and Bi2S3 nanostructures were synthesized successfully via a

microwave approach. For synthesis of PbS nanoparticles, a new precursor,

[bis(salisylate) lead (II)]; [Pb(Hsal)2] was used. The products were characterized by

X-ray diffraction, scanning electron microscopy, and photoluminescence spectros-

copy. Thin film of Bi2S3 was prepared by doctor’s blade technique and solar cell

made from ITO/Bi2S3/PbS/Pt layers. I–V characterization was investigated for this

cell and fill factor, open circuit voltage and short circuit current values were

obtained.

Keywords PbS � Bi2S3 � Nanostructures � Microwave � Solar cells

Introduction

In recent years, semiconductor nanomaterials have received considerable attention

due to their wide applications in the fabrication of optical and electronic devices

[1–3]. Bi2S3 is a kind of semiconductors with an Eg of 1.3 eV [4]. Bi2S3 has potential

applications including photovoltaics [5], IR spectroscopy [6] and thermoelectrics

[7, 8]. Many techniques including hydrothermal [9, 10] solvothermal [11–13]

biomolecule assisted method [14] thermal decomposition, ionic liquid-assisted

M. Yousefi

Islamic Azad University, Shahre Rey Branch, P.O. Box 18155-144, Tehran, Islamic Republic of Iran

M. Sabet � M. Salavati-Niasari (&)

Institute of Nano Science and Nano Technology, University of Kashan, P.O. Box 87317-51167,

Kashan, Islamic Republic of Iran

e-mail: salavati@kashanu.ac.ir

H. Emadi

School of Chemistry, College of Science, University of Tehran, Tehran, Islamic Republic of Iran

123

J Clust Sci (2012) 23:511–525

DOI 10.1007/s10876-012-0463-1

templating route [15], photochemical synthesis method [16], sonochemical [17],

template-synthesis methods [18] and microwave [19] have been applied to prepare

Bi2S3 nanoparticles or superstructures. Bi2S3 nanostructures with different morphol-

ogies, such as nanorods [20] nanotubes [21], flower-like structures [22], nanoribbions

[23] and nanowire [24], have been synthesized.

As an important IV–VI group semiconductor, lead sulfide (PbS) has attracted

considerable attention owing to its especially small direct band gap (0.41 eV) and

larger excitation Bohr radius (18 nm) [25–27], and has been widely used in many

fields such as Pb2? ion-selective sensor [28], photography [29], IR detector [30] and

solar absorber. Moreover, an exceptional third-order nonlinear optical property of

PbS nanoparticles has been found, which makes PbS nanocrystals a promising

candidate for photonic and optical switching device applications [31]. The

absorption edge of PbS exhibits a large blue shift [32] when one shrinks the

crystallite size to the nanometer size regime. Recently, various methods have been

developed to synthesize PbS submicro-/nano-crystals, including solvothermal

method [33], chemical vapor deposition (CVD) method [34], ultrasonic method

[35], and electrodeposited route [36]. Microwave-assisted synthesis [37] is a very

attractive process for producing a variety of materials. In this work two types of

nanostructured semiconductors with different features were synthesized via facile

microwave approach. The effects of several parameter on product size and

morphology such as sulfur sources, sulfur concentration, microwave irradiation time

and power were investigated. The optical properties were studied by PL

spectroscopy. Bi2S3 Thin film was fabricated by Doctor’s blade technique.

Subsequently PbS thin film was covered on Bi2S3 film via Doctor’s blade method

and their features were obtained from I–V curve.

Experimental

Materials

All the chemical reagents used in experiments were of analytical grade and used as

received without further purification. [Pb(Hsal)2] precursor was synthesized as

follow: Pb(NO3)2 was dissolved in 50 mL of distilled water to form a homogeneous

solution. The stoichiometric amount of sodium salicylate dissolved in an equal

volume of distilled water, and then was added dropwise into the above solution

under magnetic stirring. The solution was stirred for about 30 min and refluxed for

3 h. The obtained precipitate was isolated, washed several times with distilled water

and ethanol and dried at 60 �C for 2 h. The achieved product was characterized by

X-ray diffraction (XRD). Anal. calcd for C14H10O6Pb: C, 34.93; H, 2.09; Pb, 43.04.

Found: C, 34.76; H, 1.97; Pb, 42.11 %. The molecular formulae of the complex has

been assigned on the basis of the results of their elemental analyses. The lead(II)

complexes are extremely stable in the solid state and in solution and are relatively

stable against ligand dissociation even in highly acidic solutions. The molar

conductance values of the complexes measured in DMF corresponds to

nonelectrolytes.

512 M. Yousefi et al.

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Characterization

X-ray diffraction patterns were recorded by a Rigaku D-max C III, X-ray

diffractometer using Ni-filtered Cu Ka radiation. Scanning electron microscopy

(SEM) images were obtained on LEO-1455VP equipped with an energy dispersive

X-ray spectroscopy. FE-SEM images were obtained on HITACHI S-4160. Room

temperature photoluminescence (PL) was studied on an F-4500 fluorescence

spectrophotometer. Elemental analysis was obtained from Carlo ERBA Model EA

1108 analyzer. The metal contents of the samples were measured by atomic

absorption spectrophotometer (AAS-Perkin-Elmer 4100–1319) using a flame

approach.

Preparation of PbS and Bi2S3 Nanostructures

In a typical experimental process, the stochiometric amount of Pb(Hsal)2 and

different sulfur sources were dissolved in 30 ml ethylene glycol separately and

stirred for 20 min. Then these solutions mixed together and stirred for 30 min.

Finally solution was transferred to 100 ml beaker and the contents were exposed to

microwave irradiation in a domestic microwave oven, operating at 2,450 MHz, for

different output power and time. The obtained black powder was washed several

times with distilled water and ethanol to remove any unreacted reactants and

impurity and dried in vacuum. Experimental conditions for preparation of PbS are

shown at Table 1.

Table 1 Experimental condition for the preparation of PbS nanoparticles

Sample

no.

Figure

no.

Sulfur source Microwave

time (min)

Microwave

power (W)

Pb:S mole

ratio

1 2a Thioacetamide 2 750 1:1

2 2b Thiourea 2 750 1:1

3 2c L-cysteine 2 750 1:1

4 2d Carbon disulfide 2 750 1:1

5 2e Thiosemicarbazide 2 750 1:1

6 2f Ammonium sulfide 2 750 1:1

7 2g Sodium sulfite 2 750 1:1

8 2h Thiosemicarbazide ?

Thioacetamide

2 750 1:1

9 3a Thioacetamide 2 750 1:2

10 (optimized

sample)

3b Thioacetamide 2 750 1:3

11 4a Thioacetamide 2 600 1:3

12 4b Thioacetamide 2 900 1:3

13 5a Thioacetamide 3 750 1:3

14 5b Thioacetamide 5 750 1:3

Synthesis and Characterization PbS and Bi2S3 Nanostructures 513

123

The reaction for preparation of Bi2S3 was similar to PbS synthesis. In a typical

synthesis 1.00 g (0.002 mol) of Bi(NO3)3�5H2O powder and 0.14 ml (0.002 mol)

thioglycolic acid (TGA) as sulfur source were dissolved in 30 ml ethylene glycol,

separately. Then the solutions were mixed together and stirred for 30 min. Finally

solution was exposed to microwave irradiation for different time and powers. The

obtained black powders were washed several times with water and ethanol and dried

at 60 �C for 2 h. Experimental conditions for preparation of Bi2S3 are shown at

Table 2.

Manufacturing

Bi2S3 film as absorber layer was prepared by doctor’s blade coating using slurry of

0.05 g Bi2S3 powder dispersed in 5 ml ethanol, 0.05 g ethyl cellulose and 0.05 ml

Triton X-100 mixture on Indium Tin Oxide (ITO)-coated glass substrate. Then, the

glass substrate was maintained at 200 �C for 30 min to remove the organic

compounds. Subsequently, other similar paste from PbS nanoparticles was prepared

and technique was deposited on Bi2S3 layer by doctor’s blade. In current

investigation, PbS was selected as n-layer. The counter-electrode was prepared by

chemical deposition of a platinum solution above an ITO electrode. This electrode

was placed over the PbS/Bi2S3 electrode. The redox electrolyte consisted of 0.05 M

LiI, 0.05 M I2 and 0.5 M 4-tert-butylpyridine at acetonitrile as a solvent. The

photocurrent–voltage (I–V) curves were used to calculate short-circuit current (Isc),

open-circuit voltage (Voc) and fill factor (FF). I–V measurement was made under

simulated Air Mass 1.5 global illumination. Also, the active cell area was 1 cm2.

The image of the fabricated cell process is depicted at Scheme 1.

Table 2 Experimental condition for the preparation of Bi2S3 nanostructures

Sample no. Figure

no.

Microwave

time (min)

Microwave

power (W)

Bi:S mole

ratio

Morphology

15 6a 2 750 3:1 Aggregated nanoparticles

16 6b 2 750 2:1 Aggregated nanoparticles

17 6c 2 750 1:1 Petal-like structures

18 6d 2 750 1:2 Flower-like nanostructures

19 6e 2 750 1:3 Nanorods

20 6f 2 750 1:4 Aggregated nanorods

21 (optimized

sample)

6g 2 750 1:5 Urchin-like nanostructures

22 7a 2 600 1:5 Aggregated nanorods

23 7b 2 900 1:5 Aggregated nanorods

24 8a 5 750 1:5 Aggregated

nanorods ? irregular

masses

25 8b 7 750 1:5 Aggregated nanorods

514 M. Yousefi et al.

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Results and Discussion

X-ray Diffraction Pattern

Figure 1 shows the XRD pattern of Pb precursor (Fig. 1a), PbS (Fig. 1b) and Bi2S3

(Fig. 1c) nanostructures. As shown in Fig. 1b, the main diffraction peaks were

observed at 26.134�, 30.238�, 43.234�, 51.133�, 53.590�, 62.69�, 69.043�, 71.041�,

and 79.049� in the XRD pattern of the PbS which confirm the formation of PbS with

tetragonal structure. (JCPDS No. 78-1897, a = 5.9143A). The crystal size of the

PbS nanoparticles was estimated by Debye–Scherrer equation [38] (d = 0.9 k/bcosh) and it was about 29.2 nm. In Fig. 1c all the peaks reflected from (3 1 0),

(2 1 1), (2 2 1), (3 0 1), (3 1 1), (4 2 0), (4 3 0), (2 5 0), (4 4 0), (5 0 1), (3 1 2),

(2 4 2), (1 7 1), (7 2 1), (6 5 1) and (8 1 1) planes matched well with the standard

JCPDS-17-0320 pattern of Orthorhombic Bi2S3. The crystal size of the Bi2S3

nanostructures was calculated about 14.6 nm. No impurity peaks was observed for

PbS and Bi2S3 which indicate the products have high purity.

SEM Images

Figure 2a–h shows the SEM images of PbS nanostructures that prepared from

several sulfur sources. Based on modern complex theory, the sulfide source

molecules, some of which dissociate and release S2-, also behave as potential

ligands [39]. Therefore by using different sulfur sources different morphologies

were obtained (Fig. 2). Table 3 described the morphology of PbS nanostructure

prepared with eight different sulfur sources. Figure 4 exhibits the effect of sulfur

concentration on morphology of PbS nanostructures. By increasing the mole ratio of

S:Pb to 2:1 (Fig. 4a), the size of particles becomes smaller and further increase (3:1)

will cause to the formation of small and separated particles (Fig. 4b).

Figure 5 shows the effect of microwave power on shape of the PbS

nanostructures. At 600 W microwave irradiation due to little energy of microwave,

the product was mainly composed from agglomerated particles (Fig. 5a). By

increasing the power to 750 W the sufficient energy for preparation of PbS

nanoparticles has been created and hence PbS nanoparticles formed which were

Scheme 1 Schematic diagram illustrating the image of solar cell fabrication process

Synthesis and Characterization PbS and Bi2S3 Nanostructures 515

123

consisted of separated particles with smaller diameter (Fig. 4b). By further increase

of microwave power, due to high energy of microwave irradiation, the active

surface of nanoparticles [40] adhere together and bigger particles have been

achieved (Fig. 5b). In fact, due to diffusion process of the reactants to the surface of

the growing crystallite led to growth proceeds through the new nucleations and

increasing the number of particles [41]. Therefore, the optimum power for PbS

nanoparticles formation is 750 W.

Fig. 1 X-ray diffraction pattern of a Pb precursor, b sample no. 10 and c sample no. 21

516 M. Yousefi et al.

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Figure 6 shows the SEM images of PbS nanostructures that prepared at 3

(Fig. 6a) and 5 (Fig. 6b) min. The influence of microwave time on morphology is

Fig. 2 Scanning electron microscopy images of a–h sample no. 1–8 respectively

Synthesis and Characterization PbS and Bi2S3 Nanostructures 517

123

similar to the power effect. By increasing time the energy of microwave has been

increased and aggregated particles were obtained.

Figure 7 shows the SEM image of Bi2S3 nanostructured synthesized from

Bi(NO3)3�5H2O and TGA at different mole ratio of Bi:S. When the Bi:S mole ratio

was 3:1, irregular masses were obtained (Fig. 7a). By increasing the sulfur source to

2:1 (Fig. 7b) the product shape was similar to 3:1 mol ratio. The increase of sulfur

concentration to 1:1 (Fig. 7c), leads to significant difference from particles to flower

Fig. 3 Scanning electron microscopy images of a sample no. 9 and b sample no. 10

Table 3 The morphology of PbS nanostructure prepared with eight different sulfur sources

Figure no. Sulfur source Morphology

2a Thioacetamide Nanoparticles

2b Thiourea Nanocubics

2c L-cysteine Nanocubics ? nanoparticles

2d Carbon disulfide Nanocubics ? nanoparticles

2e Thiosemicarbazide Star-like nanostructures

2f Ammonium sulfide Aggregated nanocubics

2g Sodium sulfite Sphere-like structures

2h Thiosemicarbazide ? Thioacetamide Aggregated nanoparticles

Fig. 4 Scanning electron microscopy images of a sample no. 11 and b sample no. 12

518 M. Yousefi et al.

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like morphology. As shown in this figure concentricity plates were created petal-like

structure. By further increase in sulfur concentration (1:2), the flower structure

becomes perfect (Fig. 7d). When the Bi:S mole ratio was changed to 1:3 besides the

flower-like structure, rod-like structure obtained (Fig. 7e). When the Bi:S mole ratio

was changed to 1:4 (Fig. 7f) another significant variation in morphology was

observed. In other words, in this concentration, the flower-like structure replaced

with the aggregated rods. Finally, at 1:5 mol ratio the product was mainly composed

of urchin-like and rod-like structures (Fig. 7g). Hence the best mole ratio for

preparation of Bi2S3 nanostructures was 1:5. Actually, the trick in microwave

synthesis of urchin-like Bi2S3 nanocrystal presented here is the usage of TGA as a

sulfur source and stability agents, which was previously used as the stability agent

to prevent the chalcogenide nanocrystals from aggregating [42].

Figures 8 and 9 show the effect of microwave power and time on Bi2S3

morphology. The best power for Bi2S3 nanostructured formation was 750 W

(Fig. 7g) which is similar to the PbS results. At 600 W (Fig. 8a) and 900 W

(Fig. 8b) the products haven’t specified morphology. Also the best time for creation

proper morphology was 2 min (Fig. 7g) and in the other time such as 3 (Fig. 9a) and

5 min (Fig. 9b) the obtained products do not have good structures.

Figure 10 shows the SEM of Bi2S3 thin film. The fabricated thin film has non-

uniform surface and several lumps are located at thin film surface that may be a

strong factor for electron trap and weak operation of solar cell.

Photoluminescence spectrum

Figures 11 and 12 show the room temperature PL spectroscopy of PbS and Bi2S3

nanostructures, separately. As shown in this figures, The calculated band gaps from

PL were 2.9 and 2.24 eV for PbS and Bi2S3 respectively that show a large blue shift

in comparison with their bulk samples with 0.41 eV (PbS) [43] and 1.3 eV (Bi2S3)

[4]. These different values are due to quantum size effect. In other words, by

decreasing the particle size, the PL band shifts to lower wavelength. Therefore the

bandgap becomes higher.

Fig. 5 Scanning electron microscopy images of a sample no. 13 and b sample no. 14

Synthesis and Characterization PbS and Bi2S3 Nanostructures 519

123

Fig. 6 Scanning electron microscopy images of a–g sample no. 15–21 respectively

520 M. Yousefi et al.

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I–V Characterization

The current–voltage measurement for PbS/Bi2S3 solar cell is shown in Scheme 1. It

was found that the fabricated device has solar cell behavior. The FF, Voc and Isc

Fig. 7 Scanning electron microscopy images of a sample no. 22 and b sample no. 23

Fig. 8 Scanning electron microscopy images of a sample no. 24 and b sample no. 25

Fig. 9 Scanning electronmicroscopy image of Bi2S3 thinfilm

Synthesis and Characterization PbS and Bi2S3 Nanostructures 521

123

were 25 %, 40 and 0.004 mA respectively. The main reasons for these low values

are mentioned below:

1. Due to high bandgap of Bi2S3 the generation of electron–hole becomes low and

hence the Voc value decreased.

2. The urchin-like morphology of Bi2S3 leads to the caption of the generated

electrons in Bi2S3 structure and therefore they can’t transfer to n-layer and so

the Isc becomes low.

3. Another factor that restricts the Isc is presence of many small particles in PbS

nanostructures. By decreasing the particle size the grain boundaries that located

Fig. 10 Room temperaturephotoluminescence of sampleno. 10

Fig. 11 Room temperaturephotoluminescence of sampleno. 21

522 M. Yousefi et al.

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between the particles have been increased and subsequently, electron trap states

become high.

4. The Bi2S3 and PbS have orthorhombic and tetragonal structures, respectively.

These phase difference is an important factor that restrict the cell operation and

therefore FF has been decreased.

5. The presence of several lumps at thin film surface and non-uniform structure of

film are two strong parameters that restrict the solar cell operation.

A characterization comparison for solar cell with other similar works is shown in

Table 4.

Conclusion

In this study, two nanostructured semiconductors were synthesized via microwave

approach. Several effects on product morphology and size such as sulfur

Fig. 12 I–V characterization of solar cell

Table 4 The characterization comparison for solar cell with other similar works

Structure Voc (mV) Jsc(mA/cm2) FF Ref.

Glass/ITO/CdS/PbS/conductive graphite 290 14 0.36 [44]

TCO/window (CdS-ZnS)/Bi2S3/PbS/graphite

electrode/Ag

130–310 0.5–5 0.25–0.42 [45]

ITO/ZnO/PbS/CdS/Pt 281.3 1.48 57 [46]

ITO/ZnO/CdS/PbS/Pt 311.4 2.20 55 [46]

ITO/ZnO/CdS/Pt 292.2 1.69 59 [46]

ITO/ZnO/PbS/Pt 272.9 1.27 55 [46]

TCO/Bi2S3/PbS/graphite electrode/Ag 220–280 1.1–2.1 0.26–0.34 [47]

TCO/CdS/Sb2S3/PbS/graphite electrode/Ag 620–660 2.95–3.73 0.29–0.031 [48]

Synthesis and Characterization PbS and Bi2S3 Nanostructures 523

123

concentration, microwave power and time and sulfur sources have been investi-

gated. Best structures were selected for studying their behavior in solar cell.

Because of the Bi2S3 had smaller bandgap from PbS nanoparticles, it used to as

p-layer. Optical properties of products were studied by PL spectroscopy and it was

found that due to quantum size effect, the bandgap of two semiconductors were

shifted to lower wavelength. Thin film of Bi2S3 and PbS were prepared by doctor’s

blade technique and solar cell was made from ITO/Bi2S3/PbS/Pt-ITO layers. I–V

characterization was obtained from this cell. Calculated FF, Voc and Isc were

obtained from I–V curve showing very low values. Several parameters such as

morphology of products, high band gap of p-layer, non-uniform thin film and

difference of products structure are responsible to weak operation of solar cell.

Acknowledgments Authors are grateful to Council of Islamic Azad University, Shahr-e Rey Branch,

Tehran and University of Kashan for providing financial support to undertake this work.

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