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Synthesis of polymer–PbSnanocomposite by solar irradiation-induced thermolysis process and itsphotovoltaic applicationsDulen Saikia a , P. K Gogoi b , P. K Saikia c & Sidananda Sarma da Material Science Laboratory, Department of Physics , SibsagarCollege , Joysagar, Sivasagar 785665 , Assam , Indiab Department of Chemistry , Centre for Nanoscience andComposite Materials, Dibrugarh University , Dibrugarh 786004 ,Assam , Indiac Department of Physics , Dibrugarh University , Dibrugarh786004 , Assam , Indiad Department of Physics , IIT Guwahati , Guwahati 781039 ,Assam , IndiaPublished online: 08 Feb 2013.

To cite this article: Dulen Saikia , P. K Gogoi , P. K Saikia & Sidananda Sarma (2013)Synthesis of polymer–PbS nanocomposite by solar irradiation-induced thermolysis processand its photovoltaic applications, Journal of Experimental Nanoscience, 8:3, 403-411, DOI:10.1080/17458080.2012.740639

To link to this article: http://dx.doi.org/10.1080/17458080.2012.740639

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Acknowledgements

The authors awe Dr R.S. Ningthoujam, Scientific Officer (E), Chemistry Division, BARC, Mumbai forproviding all the experimental facilities and suggestions for the improvement of this article. L.R. Singhthanks DST, New Delhi, for providing financial assistance under Fast Tract Young Scientist Scheme(SR/FTP/PS-128/2011) during execution of this study.

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Synthesis of polymer–PbS nanocomposite by solar irradiation-inducedthermolysis process and its photovoltaic applications

Dulen Saikiaa*, P.K. Gogoib, P.K. Saikiac and Sidananda Sarmad

aMaterial Science Laboratory, Department of Physics, Sibsagar College, Joysagar, Sivasagar 785665,Assam, India; bDepartment of Chemistry, Centre for Nanoscience and Composite Materials,Dibrugarh University, Dibrugarh 786004, Assam, India; cDepartment of Physics, Dibrugarh University,Dibrugarh 786004, Assam, India; dDepartment of Physics, IIT Guwahati, Guwahati 781039, Assam, India

(Received 7 November 2011; final version received 13 October 2012)

In this study, we report the development of a new route for the synthesis of polymer–PbSnanocomposite thin film by a novel solar irradiation-induced reaction which is completely freefrom any complexing agents and toxic chemicals. The as synthesised polymer–PbS thin films werecharacterised by X-ray diffraction (XRD), scanning electron microscopy (SEM) and UV–Visabsorption spectroscopy. The optical studies showed a direct allowed band gap of PbS lying inthe range 1.78–2.2 eV. The XRD pattern shows the cubic structure of PbS with a lattice parameterof 5.927 A. The SEM micrograph of the as-synthesised PbS thin films shows clusters in arelatively loose compact structure in the poly(vinyl alcohol) matrix and separated by voids, whichare clearly observed in the surface. However, after annealing, most of the voids disappear in thePbS surface and it is likely that the clusters coalesce giving rise to an homogeneous and compactfilm. The PbS thin film after annealing is almost pinhole free and as such suitable for applicationin solar cell. A proto-type thin film solar cell of CdS/PbS was fabricated (1� 1 cm2) on glasssubstrates using this deposition technique for PbS. The CdS layer was deposited by heat-inducedthermolysis technique. The current–voltage (I–V) characteristics of the cells were measured with ahigh-impedance (�1014) ECIL electrometer amplifier (model EA812). The efficiency of the solarcell was found as 2.54%.

Keywords: PbS–polymer nanocomposite; thin film; solar cell; solar irradiation

1. Introduction

Semiconductor nanocrystals embedded in polymeric matrices are of wide interest due tofundamental scientific aspects as well as their scope for technological applications. In recentyears, significant progress has been achieved in the synthesis of various types of polymer-basednanocomposites and studies on various parameters which determine their optical, thermal,electrical and magnetic properties. Nanocomposite-based devices, such as light emitting devices,photodiodes, photovoltaic solar cells and gas sensors, have been developed, often usingchemically oriented synthetic methods such as soft lithography, lamination, spin-coating orsolution casting and dip-coating techniques. Lead sulphide (PbS) is one such important narrowband-gap semiconductor from IV–VI groups with a band gap of 0.4 eV at 300K and has arelatively large excitation Bohr radius (EBR) of 18 nm [1]. Due to its larger EBR, the quantum

*Corresponding author. Email: dulen.s@rediffmail.com

© 2013 Taylor & Francis

Journal of Experimental Nanoscience, 2013Vol. 8, No. 3, 403–411, http://dx.doi.org/10.1080/17458080.2012.740639

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confinement effect could be observed to a much higher particle size and band-gap energy of PbS

can be tuned from 0.41 to 2.3 eV by changing the mean nanocluster size [2]. These properties

make PbS a very suitable candidate for infrared detection application [3], photography [4], Pb2þ

ion-selective sensors [5], display devices [6] and solar absorption [7]. In addition, PbS has been

utilised as a photoresistance, diode lasers, humidity and temperature sensors, decorative and

solar control coatings [8,9]. Recently, thin-film solar cells based on PbS nanocrystalline thin

films are also receiving attention [10,11].Many methods have been developed to synthesise PbS thin films, including vacuum

deposition [12], electrochemical deposition [13], chemical bath deposition (CBD) [14,15], pulsed

laser deposition [16], sonochemical [17], spray pyrolysis [18] and successive ionic layer

adsorption and reaction method [19]. PbS nanowires and nanorods dispersed in mesoporous

silica [20] and polymer films [21] have been prepared through templates and surfactants.

However, one of the simple and energy-efficient methods to synthesise PbS thin film for

application in thin-film solar cell is the CBD technique. This process does not require any special

set-up or any sophisticated instrument. One can also use high-purity starting materials. Further,

solution-processed electronic and optoelectronic devices have a clear superiority over the

conventional crystalline semiconductor devices in terms of easy synthesis, large device area,

physical flexibility, and most importantly, low cost [22]. In comparison with other II–VI

semiconductors like CdS or ZnS, there are not many papers on nanocrystalline PbS or nano

PbS–polymer composite.The chemical deposition of PbS thin film by generally accepted CBD techniques at large

scale could raise environmental problems as this process usually utilises some complexing agents

such as ethylenediamine tetra acetic acid and ammonia, which are not environmentally benign.

Recently, the synthesis of PbS thin film by microwave irradiation, ultrasonic irradiation and

� irradiation techniques [22–24] has received much attention. Parashar et al. [22] reported a

novel strategy for the synthesis of PbS nanocrystals via the ultrasonic irradiation technique

underlining the effect of acoustic cavitation phenomenon on crystallinity and particle sizes in the

presence of thiourea. The photoluminescence property of nanocrystalline PbS thin film prepared

by the � irradiation technique in an ethanol system was reported by Qiao et al. [23]. The

synthesis of PbS nanocrystals with controllable morphologies and sizes under the microwave

irradiation technique was reported by Qiao et al. [24].The purpose of this article is two fold: (1) to present a simple and environmentally benign

method for the synthesis of PbS nanocrystals in a stable polymeric matrix via the solar

irradiation-induced thermolysis technique by reacting Pbþ dispersed poly(vinyl alcohol) (PVA)

with thiourea and (2) to fabricate a thin-film solar cell of CdS/PbS by utilising this deposition

technique for PbS and to measure the photovoltaic parameters of the cell.

2. Experimental

2.1. Materials and characterising techniques

Lead acetate [Pb(CH3COO)2 � 2H2O], thiourea [CS(NH2)2] and PVA were purchased from Merk

(India) Ltd. and used directly as received. Deionised water was used throughout the

experiments. The crystal structures of the films were analysed using an X-ray diffractometer

(make: Seifert, Model 003T/T) with Cu-Ka radiations operated at 40 kV and 30mA. For optical

studies, absorption and transmission spectra were recorded with a Scinco (S 3100) PD UV–Vis

spectrophotometer. Surface morphology of the films was examined by LEO 1430VP scanning

electron microscope (SEM).

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confinement effect could be observed to a much higher particle size and band-gap energy of PbS

can be tuned from 0.41 to 2.3 eV by changing the mean nanocluster size [2]. These properties

make PbS a very suitable candidate for infrared detection application [3], photography [4], Pb2þ

ion-selective sensors [5], display devices [6] and solar absorption [7]. In addition, PbS has been

utilised as a photoresistance, diode lasers, humidity and temperature sensors, decorative and

solar control coatings [8,9]. Recently, thin-film solar cells based on PbS nanocrystalline thin

films are also receiving attention [10,11].Many methods have been developed to synthesise PbS thin films, including vacuum

deposition [12], electrochemical deposition [13], chemical bath deposition (CBD) [14,15], pulsed

laser deposition [16], sonochemical [17], spray pyrolysis [18] and successive ionic layer

adsorption and reaction method [19]. PbS nanowires and nanorods dispersed in mesoporous

silica [20] and polymer films [21] have been prepared through templates and surfactants.

However, one of the simple and energy-efficient methods to synthesise PbS thin film for

application in thin-film solar cell is the CBD technique. This process does not require any special

set-up or any sophisticated instrument. One can also use high-purity starting materials. Further,

solution-processed electronic and optoelectronic devices have a clear superiority over the

conventional crystalline semiconductor devices in terms of easy synthesis, large device area,

physical flexibility, and most importantly, low cost [22]. In comparison with other II–VI

semiconductors like CdS or ZnS, there are not many papers on nanocrystalline PbS or nano

PbS–polymer composite.The chemical deposition of PbS thin film by generally accepted CBD techniques at large

scale could raise environmental problems as this process usually utilises some complexing agents

such as ethylenediamine tetra acetic acid and ammonia, which are not environmentally benign.

Recently, the synthesis of PbS thin film by microwave irradiation, ultrasonic irradiation and

� irradiation techniques [22–24] has received much attention. Parashar et al. [22] reported a

novel strategy for the synthesis of PbS nanocrystals via the ultrasonic irradiation technique

underlining the effect of acoustic cavitation phenomenon on crystallinity and particle sizes in the

presence of thiourea. The photoluminescence property of nanocrystalline PbS thin film prepared

by the � irradiation technique in an ethanol system was reported by Qiao et al. [23]. The

synthesis of PbS nanocrystals with controllable morphologies and sizes under the microwave

irradiation technique was reported by Qiao et al. [24].The purpose of this article is two fold: (1) to present a simple and environmentally benign

method for the synthesis of PbS nanocrystals in a stable polymeric matrix via the solar

irradiation-induced thermolysis technique by reacting Pbþ dispersed poly(vinyl alcohol) (PVA)

with thiourea and (2) to fabricate a thin-film solar cell of CdS/PbS by utilising this deposition

technique for PbS and to measure the photovoltaic parameters of the cell.

2. Experimental

2.1. Materials and characterising techniques

Lead acetate [Pb(CH3COO)2 � 2H2O], thiourea [CS(NH2)2] and PVA were purchased from Merk

(India) Ltd. and used directly as received. Deionised water was used throughout the

experiments. The crystal structures of the films were analysed using an X-ray diffractometer

(make: Seifert, Model 003T/T) with Cu-Ka radiations operated at 40 kV and 30mA. For optical

studies, absorption and transmission spectra were recorded with a Scinco (S 3100) PD UV–Vis

spectrophotometer. Surface morphology of the films was examined by LEO 1430VP scanning

electron microscope (SEM).

2.2. Synthesis of PbS/PVA nanocomposite

PbS/PVA nanocomposite thin films were prepared by the solar irradiation-induced themolysistechnique by reacting Pbþ dispersed PVA with thiourea. The technique reported here is amodification of our earlier work for the deposition of CdS/PVA nanocomposite thin film [25] inwhich the thermolysis was induced by the heat treatment method. Lead acetate, thiourea andPVA were used as the raw materials for the preparation of PbS–polymer nanocomposite thinfilm. Composition ratios of Pb:S were 3:5. In a typical reaction, a matrix solution was preparedby adding 0.6M lead acetate into a 5% (w/v) aqueous solution of PVA and stirred continuouslyfor 90min at 70�C. The solution was left for 24 h to get transparent liquid, indicating completedissolution of cadmium acetate. Equal volume of 1M thiourea was slowly added in to thismatrix solution and the reactants were stirred continuously for another 30min. Then, theresulting precursors/polymer solutions containing the Pb2þ and S�2 ions in the polymeric matrixwere coated on to a chemically clean glass substrate by the dip-coating technique using a singledip coater (model no. SDC 2007C, Apex Instruments Co.). Then, the coated glass was irradiatedunder bright sunlight condition to release the Pb2þ and S�2 ions from the PVA-bound state forthe formation of PbS nanoparticles within the pores of the PVA matrix. A set of films wereprepared by exposing the solution-coated glass substrates into solar radiations of intensity98,000 lux for different intervals of time (0.5–3.5 h) so as to see the impact of irradiation on theoptical and structural properties of the thin film. PbS nanoparticles were formed under solarirradiation and colour of the film changes from transparent to reddish brown within few minutesand gradually turns into blackish brown upon left itself for drying at room temperature for 24 h.This indicates the formation of PbS nanocrystals in the PVA matrix. Further, the films wereannealed at 60�C for 2 h to increase the grain size and reduce the number of grain boundaries.

3. Results and discussion

3.1. Optical studies

In this study, we studied the effect of solar irradiation on the optical properties of chemicallydeposited PbS films for different exposure times. Figure 1(a) shows the UV–Vis absorptionspectra of PbS/PVA nanocomposite thin films prepared by exposing it into solar irradiations for

Re

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u.)

Wavelength(nm)

1 2 3 4 5

1------ 0.5 h2------ 1.5 h 3------ 2.0 h 4------ 3.0 h 5------ 3.5 h

(a) (b)

300 400 500 600 700 800 900 1000

Figure 1. (a) UV–Vis absorption spectra of PbS/PVA nanocomposite thin films prepared at differentexposure times of 0.5, 1.5 and 2 h, 3 and 3.5 h and (b) band-gap calculation of PbS/PVA nanocompositethin film prepared at different exposure times 0.5, 1.5 and 2 h, 3 and 3.5 h.

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different interval of times from 0.5 to 3.5 h. From the spectra, it is observed that absorbanceedges of all the films are blue shifted relative to the bulk PbS band edge (620 nm), indicating thequantum confinement effect in nanoparticles. The observed blue shift could be attributed due tothe decrease in crystallite sizes of the films in comparison to the bulk PbS. The sharp increase inabsorbance near the fundamental absorption edge for the PbS/PVA film is an indication of goodcrystalline nature of the films [26].

The optical band gaps of the films were obtained using the following equation [27] for asemiconductor:

A ¼K h�� Eg

� �m=2

h�ð1Þ

where A is the absorbance, K a constant and m equal to 1 for direct transition and 2 forindirect transition. Linearity of the plots of (Ah�)2 versus photon energy h� for the PbS/PVAfilms indicates that the material is of direct band-gap nature (Figure 1b). The extrapolation ofthe straight line to the (Ah�)2¼ 0 axis (Figure 1b) gives the energy band gap of the film material.The band gap of the films were found to vary from 1.72 to 2.24 eV as the exposure time wasincreased from 0.5 to 3.5 h and are presented in Table 1.

3.2. Structural and surface morphology

The XRD of PbS/PVA nanocomposite thin films prepared by the irradiation of sunlightunder different exposure times from 0.5 to 3.5 h are shown in Figure 2. It shows severaldiffraction peaks at 2� values of 25.9, 29.95, 43.1, 51.25 and 54. These were assigned to thediffraction lines produced by (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of the face-centredcubic structure of PbS. The average size of the prepared PbS samples was determined to be�10 nm. The lattice constant calculated from the above Miller indices was found to bea¼ 5.928 A, which is almost in agreement with the standard data from JCPDS card no. 78-1901(a¼ 5.931 A).

The reflection planes (3 1 1) and (2 2 2) were absent in the case of the films prepared at 0.5 hof exposure time. PVA is a crystalline polymer and the diffraction peaks at 2�¼ 11.4 and 19.55correspond to that of PVA monoclinic crystalline phase [28,29]. Crystalline structure of PVA isdue to the interaction between the PVA chains through intermolecular hydrogen bonding[28,29]. Upon loading PbS, there is a decrease in the intensity of the (1 0 1) reflection peaks ataround 2�¼ 19.55. However, the intensity of the PVA peak at around 2�¼ 11.4 increases withthe increase of exposure time. The XRD pattern of the PbS particles in the polymer matrixshows broadening of the peaks in comparison to bulk PbS, which indicates that as-synthesisedPbS thin film is nanocrystalline in nature in the polymer PVA matrix. The SEM images of the

Table 1. Band gap, shift in band gap calculated from absorption spectra.

SampleExposuretime (h)

Band gap fromUV–Vis (eV)

Shift inband gap (eV)

PbS/PVA 0.5 2.24 1.831.5 2.12 1.712.0 1.96 1.553.0 1.84 1.433.5 1.72 1.31

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different interval of times from 0.5 to 3.5 h. From the spectra, it is observed that absorbanceedges of all the films are blue shifted relative to the bulk PbS band edge (620 nm), indicating thequantum confinement effect in nanoparticles. The observed blue shift could be attributed due tothe decrease in crystallite sizes of the films in comparison to the bulk PbS. The sharp increase inabsorbance near the fundamental absorption edge for the PbS/PVA film is an indication of goodcrystalline nature of the films [26].

The optical band gaps of the films were obtained using the following equation [27] for asemiconductor:

A ¼K h�� Eg

� �m=2

h�ð1Þ

where A is the absorbance, K a constant and m equal to 1 for direct transition and 2 forindirect transition. Linearity of the plots of (Ah�)2 versus photon energy h� for the PbS/PVAfilms indicates that the material is of direct band-gap nature (Figure 1b). The extrapolation ofthe straight line to the (Ah�)2¼ 0 axis (Figure 1b) gives the energy band gap of the film material.The band gap of the films were found to vary from 1.72 to 2.24 eV as the exposure time wasincreased from 0.5 to 3.5 h and are presented in Table 1.

3.2. Structural and surface morphology

The XRD of PbS/PVA nanocomposite thin films prepared by the irradiation of sunlightunder different exposure times from 0.5 to 3.5 h are shown in Figure 2. It shows severaldiffraction peaks at 2� values of 25.9, 29.95, 43.1, 51.25 and 54. These were assigned to thediffraction lines produced by (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of the face-centredcubic structure of PbS. The average size of the prepared PbS samples was determined to be�10 nm. The lattice constant calculated from the above Miller indices was found to bea¼ 5.928 A, which is almost in agreement with the standard data from JCPDS card no. 78-1901(a¼ 5.931 A).

The reflection planes (3 1 1) and (2 2 2) were absent in the case of the films prepared at 0.5 hof exposure time. PVA is a crystalline polymer and the diffraction peaks at 2�¼ 11.4 and 19.55correspond to that of PVA monoclinic crystalline phase [28,29]. Crystalline structure of PVA isdue to the interaction between the PVA chains through intermolecular hydrogen bonding[28,29]. Upon loading PbS, there is a decrease in the intensity of the (1 0 1) reflection peaks ataround 2�¼ 19.55. However, the intensity of the PVA peak at around 2�¼ 11.4 increases withthe increase of exposure time. The XRD pattern of the PbS particles in the polymer matrixshows broadening of the peaks in comparison to bulk PbS, which indicates that as-synthesisedPbS thin film is nanocrystalline in nature in the polymer PVA matrix. The SEM images of the

Table 1. Band gap, shift in band gap calculated from absorption spectra.

SampleExposuretime (h)

Band gap fromUV–Vis (eV)

Shift inband gap (eV)

PbS/PVA 0.5 2.24 1.831.5 2.12 1.712.0 1.96 1.553.0 1.84 1.433.5 1.72 1.31

PbS/PVA nanocomposite thin film prepared by thermolysis under the solar irradiationtechnique for four different exposure times (0.5–3.0 h) are shown in Figure 3(a)–(d). The surfaceof the as-deposited PbS layer shows clusters in a relatively loose compact structure [30]. Theclusters are constituted by nanocrystalline grains and separated in some cases by voids clearlyobserved in the surface. However, after annealing, most of the voids disappear in the PbSsurface and it is likely that the clusters coalesce because less boundaries are observed in theseimages and is shown in Figure 3(e)–(h).

Some nanocrystalline grains emerging from the clusters are observed and distributedthroughout the surface. Thus, thermal annealing has a strong influence on the layermicrostructure, reducing voids and tightening clusters and grains in a much more compactfilm and may be suitable for application in solar cells. Owing to good crystallinity and bettersurface on annealing, the film prepared at 1.5 h of exposure time was found to be suitable forapplication as an active layer in the CdS/PbS solar cell.

4. Device fabrication and characterisation

A thin-film solar cell with the structure SnO2/CdS/PbS/CdTe was fabricated by utilising thisdeposition technique for PbS and is shown in Figure 4. First, the glass substrate was covered by�230 nm thick layer of transparent conducting oxide (TCO), which in our case is tin oxide(SnO2). The CdS/PVA thin film was then deposited on the top of TCO to a thickness ofapproximately 100 nm by the method as described in our earlier work [25]. Before deposition ofPbS thin film, the CdS film was annealed for 1 h at 60�C. A layer of PbS of thickness �670 nmwas then deposited on the top by the technique developed in this study and then annealed at60�C for 1 h. Finally, a layer of HgTe was deposited on the top as a back contact by the thermalevaporation method at a pressure 510–6mbar. The device had an area of 1� 1 cm2 and wasfound stable for several days.

4.1. Characterisation of the cell

The current–voltage (I–V) characteristics of the fabricated cell were measured with a highimpedance (�1014) ECIL electrometer amplifier (model EA812) and are shown in Figure 4(b).

10 15 20 25 30 35 40 45 50 55 60 65 70

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(a.u

.)

Diffraction angle (2θ)

(PV

A)

(111

)

(200

)

(220

)

(311

) (2

22)

0.5 h

2 h

3 h

3.5 h

1.5 h

Figure 2. XRD of PbS/PVA thin film prepared at different exposure times of 0.5–3.5 h.

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Figure 3. SEM images of PbS/PVA nanocomposite thin film prepared by thermolysis under solarirradiation for different exposure times of (a) 0.5 h; (b) 1.5 h; (c) 2.0 h; and (d) 3.0 h; (e) to (f) represents theSEM images of the annealed films prepared by thermolysis under solar irradiation for different exposuretimes of 0.5–3.0 h.

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Figure 3. SEM images of PbS/PVA nanocomposite thin film prepared by thermolysis under solarirradiation for different exposure times of (a) 0.5 h; (b) 1.5 h; (c) 2.0 h; and (d) 3.0 h; (e) to (f) represents theSEM images of the annealed films prepared by thermolysis under solar irradiation for different exposuretimes of 0.5–3.0 h.

The intensity of illumination was measured with a Lutron LX-101 lux meter. The solar cellparameters were measured under illumination with a 100mWcm�2 (one SUN) tungsten lamp.The observed photovoltaic parameters of the cells are tabulated in Table 2. A conversionefficiency of 2.54 % (Voc¼ 460mV, Jsc¼ 67 mAcm�2, FF¼ 0.63) has been achieved for the cell.

5. Conclusions

Nanostructured PbS thin films were prepared by a simple CBD technique followed bythermolysis under solar irradiation. The results of XRD and SEM show that the deposited PbSfilm consists of nano-sized grains. XRD study shows a nanocrystalline structure with the cubicphase. The optical absorption study reveals that PbS thin films have allowed direct transitions.The optical band-gap energy of PbS thin films varies from 1.72 to 2.24 eV with the change ofexposure time. A prototype thin-film solar cell was fabricated using this deposition technique forPbS and its efficiency was found to be 2.57%.

Acknowledgements

The authors are thankful to Physics Department and Central Instrumentation Facility, IIT, Guwahati, forXRD data and SEM images. One of the authors, D. Saikia, acknowledges DIT, New Delhi, for thefinancial support.

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Figure 4. (a) Experimental set-up for measuring the current–voltage characteristics and (b) I–Vcharacteristics of CdS/PbS thin-film solar cell.

Table 2. Current–voltage parameters for the CdS/PbS thin-film solar cell under 100mWcm�2

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Solar cell I (mWcm�2) Voc (mV) Jsc (mAcm�2) FF (%) � (%)

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pp. 16–19.

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