Chemical bath deposition of Bi2S3 films by a novel deposition system
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Applied Surface Science 257 (2011) 7529–7533 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc Chemical bath deposition of Bi 2 S 3 films by a novel deposition system Chao Gao, Honglie Shen ∗ , Lei Sun, Zhou Shen College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, PR China article info Article history: Received 9 December 2010 Received in revised form 7 February 2011 Accepted 14 March 2011 Available online 21 March 2011 Keywords: Bi2S3 films Chemical bath deposition Novel deposition system Optical and electrical properties abstract Bismuth sulfide (Bi 2 S 3 ) films were chemically deposited by a novel deposition system in which ammo- nium citrate was used as the chelating reagent. Two sulfur source thioacetamide (TA) and sodium thiosulfate (Na 2 S 2 O 3 ) were used to prepare Bi 2 S 3 films. Both the as-prepared films have amorphous structure. However, annealing can improve the crystallization of the films. The composition of the films prepared by TA and Na 2 S 2 O 3 are all deviate from the stoichiometric ratio of Bi 2 S 3 . The Bi 2 S 3 films are all homogeneous and well adhered to the substrate. The optical properties of the Bi 2 S 3 films are studied. The electrical resistivity of the as-prepared films are all around 7 × 10 3 cm in dark, which decreases to around 1 × 10 3 cm under 100 mW/cm 2 tungsten–halogen illumination. After the annealing, the dark resistivity of the Bi 2 S 3 film prepared by TA decreases by four magnitudes. In contrast, the dark resistivity of the Bi 2 S 3 film prepared by Na 2 S 2 O 3 only decreases slightly. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Bismuth sulfide (Bi 2 S 3 ) is a group V–VI semiconductor that draws much attention for its photo-electrical properties. Bi 2 S 3 are potential candidates for the fabrication of photovoltaic devices, photodiode arrays, photoconductors [1–4], etc. There have been several techniques involving chemical bath deposition [5–13], electro-deposition [14] spray pyrolysis [15] and successive ionic layer adsorption and reaction [16] to prepare Bi 2 S 3 films. Among these methods, chemical bath deposition is particularly important because of its simplicity and simple facility to obtain large area films [17]. In previous reports about the chemical bath deposition of Bi 2 S 3 films, triethanolamine (TEA) and ethylenediaminetetraacetic acid (EDTA) are normally used as the chelating reagent. However, nanostructured or discontinuous Bi 2 S 3 films rather than continu- ous Bi 2 S 3 films are prone to be obtained when they were deposited on glass substrates or TCO substrates. As we know, nanostructured films are not suitable for the application in thin film solar cells because pores in the films may result in the leakage of photo- current. So new deposition system should be developed for the preparation of continuous Bi 2 S 3 films. In this paper, we successfully prepared Bi 2 S 3 films by chemical bath deposition in which ammo- nium citrate was used as the chelating reagent. And two reagents thioacetamide (TA) and sodium thiosulfate (Na 2 S 2 O 3 ) were used as the sulfur source. The structures, morphologies, optical proper- ties and photocurrent response properties of the Bi 2 S 3 films were ∗ Corresponding author. Tel.: +86 25 52112626; fax: +86 25 52112626. E-mail address: [email protected] (H. Shen). studied. According to the test results, continuous Bi 2 S 3 films can be obtained when TA was used as the sulfur source. 2. Experiment details 2.1. Bi 2 S 3 films preparation Bi(NO 3 ) 3 (bismuth nitrate), TA (thioacetamide), Na 2 S 2 O 3 (sodium thiosulfate), AC (ammonium citrate) were used for the preparation of Bi 2 S 3 films. SnCl 2 (tin dichloride) and Na 2 S (sodium sulfide) were used for the preparation of SnS buffer layers, and glass was selected as the substrate. Before the film deposition, the glass substrate was firstly ultra- sonically cleaned by acetone and then ultrasonically cleaned by di-ionized water. Then a thin SnS buffer layer with a thickness of several nanometers was deposited on the substrate to improve the homogeneousness and adhesion of the chemically deposited Bi 2 S 3 films [11]. The SnS buffer layer is prepared by successive ionic layer adsorption and reaction method in which SnCl 2 and Na 2 S solu- tion were used as the precursor solution. The detailed process of the preparation and identification of SnS films can be found in our previous work [18]. For the deposition of Bi 2 S 3 films, Bi(NO 3 ) 3 was firstly dissolved into dilute hydrochloric acid. Then AC solution was added into Bi(NO 3 ) 3 solution, and finally TA solution or Na 2 S 2 O 3 solution was added to the mixed solution. The concentration of the Bi(NO 3 ) 3 , AC and TA (or Na 2 S 2 O 3 ) in the reaction solution were 0.04 M, 0.24 M and 0.06 M, respectively. The pH value of the reaction solution was adjusted to 3 for Na 2 S 2 O 3 and 7 for TA by adding ammonia. After the preparation of reaction solution, glass substrates with SnS buffer layers were inserted into the solution, and each sub- 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.080
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Applied Surface Science 257 (2011) 7529–7533
Contents lists available at ScienceDirect
Applied Surface Science
journa l homepage: www.e lsev ier .com/ locate /apsusc
hemical bath deposition of Bi2S3 films by a novel deposition system
hao Gao, Honglie Shen ∗, Lei Sun, Zhou Shenollege of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, PR China
r t i c l e i n f o
rticle history:eceived 9 December 2010eceived in revised form 7 February 2011ccepted 14 March 2011
a b s t r a c t
Bismuth sulfide (Bi2S3) films were chemically deposited by a novel deposition system in which ammo-nium citrate was used as the chelating reagent. Two sulfur source thioacetamide (TA) and sodiumthiosulfate (Na2S2O3) were used to prepare Bi2S3 films. Both the as-prepared films have amorphous
vailable online 21 March 2011
eywords:i2S3 filmshemical bath depositionovel deposition system
structure. However, annealing can improve the crystallization of the films. The composition of the filmsprepared by TA and Na2S2O3 are all deviate from the stoichiometric ratio of Bi2S3. The Bi2S3 films are allhomogeneous and well adhered to the substrate. The optical properties of the Bi2S3 films are studied.The electrical resistivity of the as-prepared films are all around 7 × 103 � cm in dark, which decreases toaround 1 × 103 � cm under 100 mW/cm2 tungsten–halogen illumination. After the annealing, the darkresistivity of the Bi2S3 film prepared by TA decreases by four magnitudes. In contrast, the dark resistivity
by N
ptical and electrical properties of the Bi2S3 film prepared. Introduction
Bismuth sulfide (Bi2S3) is a group V–VI semiconductor thatraws much attention for its photo-electrical properties. Bi2S3 areotential candidates for the fabrication of photovoltaic devices,hotodiode arrays, photoconductors [1–4], etc. There have beeneveral techniques involving chemical bath deposition [5–13],lectro-deposition [14] spray pyrolysis [15] and successive ionicayer adsorption and reaction [16] to prepare Bi2S3 films. Amonghese methods, chemical bath deposition is particularly importantecause of its simplicity and simple facility to obtain large area films17].
In previous reports about the chemical bath deposition of Bi2S3lms, triethanolamine (TEA) and ethylenediaminetetraacetic acidEDTA) are normally used as the chelating reagent. However,anostructured or discontinuous Bi2S3 films rather than continu-us Bi2S3 films are prone to be obtained when they were depositedn glass substrates or TCO substrates. As we know, nanostructuredlms are not suitable for the application in thin film solar cellsecause pores in the films may result in the leakage of photo-urrent. So new deposition system should be developed for thereparation of continuous Bi2S3 films. In this paper, we successfullyrepared Bi2S3 films by chemical bath deposition in which ammo-
ium citrate was used as the chelating reagent. And two reagentshioacetamide (TA) and sodium thiosulfate (Na2S2O3) were useds the sulfur source. The structures, morphologies, optical proper-ies and photocurrent response properties of the Bi2S3 films were∗ Corresponding author. Tel.: +86 25 52112626; fax: +86 25 52112626.E-mail address: [email protected] (H. Shen).
169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.apsusc.2011.03.080
a2S2O3 only decreases slightly.© 2011 Elsevier B.V. All rights reserved.
studied. According to the test results, continuous Bi2S3 films can beobtained when TA was used as the sulfur source.
2. Experiment details
2.1. Bi2S3 films preparation
Bi(NO3)3 (bismuth nitrate), TA (thioacetamide), Na2S2O3(sodium thiosulfate), AC (ammonium citrate) were used for thepreparation of Bi2S3 films. SnCl2 (tin dichloride) and Na2S (sodiumsulfide) were used for the preparation of SnS buffer layers, and glasswas selected as the substrate.
Before the film deposition, the glass substrate was firstly ultra-sonically cleaned by acetone and then ultrasonically cleaned bydi-ionized water. Then a thin SnS buffer layer with a thickness ofseveral nanometers was deposited on the substrate to improve thehomogeneousness and adhesion of the chemically deposited Bi2S3films [11]. The SnS buffer layer is prepared by successive ionic layeradsorption and reaction method in which SnCl2 and Na2S solu-tion were used as the precursor solution. The detailed process ofthe preparation and identification of SnS films can be found in ourprevious work [18].
For the deposition of Bi2S3 films, Bi(NO3)3 was firstly dissolvedinto dilute hydrochloric acid. Then AC solution was added intoBi(NO3)3 solution, and finally TA solution or Na2S2O3 solution wasadded to the mixed solution. The concentration of the Bi(NO3)3, AC
and TA (or Na2S2O3) in the reaction solution were 0.04 M, 0.24 Mand 0.06 M, respectively. The pH value of the reaction solutionwas adjusted to 3 for Na2S2O3 and 7 for TA by adding ammonia.After the preparation of reaction solution, glass substrates withSnS buffer layers were inserted into the solution, and each sub-
7530 C. Gao et al. / Applied Surface Science 257 (2011) 7529–7533
a b
Fig. 1. XRD patterns of the Bi2S3 films prepared by TA (a) and Na2S2O3 (b).
a b
Fig. 2. EDS spectra of the Bi2S3 films prepared by TA (a) and Na2S2O3 (b).
Fig. 3. SEM images of Bi2S3 films prepared by TA (a, after 12 h deposition; b, after 36 h deposition) and Na2S2O3 (c, after 12 h deposition; d, after 36 h deposition).
C. Gao et al. / Applied Surface Science 257 (2011) 7529–7533 7531
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c
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Fig. 4. Optical transmittance and reflectance spectra of the Bi2S3 films prep
trate was placed at 60◦ angle to the horizontal line. All the filmseposited at room temperature (300 K). The deposition time were2 h and 36 h both for the films prepared by TA and Na2S2O3.fter the deposition, the films were rinsed by di-ionized water andollected. For XRD and photo-current response tests, additionallms were prepared and annealed at 250 ◦C for 1 h in nitrogentmosphere.
.2. Characterization
X-ray diffraction (XRD) analysis was carried out using an ARL’TRA diffractometer with Cu K� radiation (� = 0.15406 nm) over
he 2� collection range of 10–70◦. The morphology of the film
a
Fig. 5. Photo-current response curves of the Bi2S3 films prepar
by TA (a); the Bi2S3 films prepared by Na2S2O3 (b); the SnS buffer layer (c).
was characterized using a LEO-1530VP scanning electron micro-scope. The thickness of the film is measured by an AMBIOS XP-2surface profiler. The photo-current response curves of the filmsbefore and after the annealing were recorded by a Keithley2400digital source-meter (under 100 mW/cm2 tungsten–halogen illu-mination). UV–vis–NIR transmittance and reflectance spectra wererecorded in the wavelength range of 200–2000 nm using a VarianCary5000 spectrophotometer.
3. Result and discussion
Fig. 1 shows the XRD patterns of the Bi2S3 films prepared byTA and Na2S2O3 after 36 h deposition (the thicknesses of the films
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ed by TA (a) and Na2S2O3 (b) before and after annealing.
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re all around 200 nm). The standard diffraction pattern of Bi2S3JCPDS no. 17-0320) is also shown in the figure. From the figure wean see that both the diffraction patterns of the as-prepared films byA and Na2S2O3 show no obvious diffraction peaks, indicating thathe crystallization of the films is poor. However, when the films arennealed at 250 ◦C, diffraction peaks appear for both the films pre-ared by TA and Na2S2O3, indicating that the crystallization of thelms is improved by the annealing. For the Bi2S3 films preparedy Na2S2O3, the diffraction pattern matches well with the stan-ard diffraction pattern of the orthorhombic Bi2S3. The diffractioneaks at 2� = 25.1◦, 28.6◦ and 31.8◦ correspond to the (1 3 0), (2 1 1)nd (2 2 1) planes of the orthorhombic Bi2S3, respectively. For thei2S3 films prepared by TA, though the positions of the diffractioneaks in XRD pattern matches with that in the standard patternf Bi2S3, there is a difference in the relative diffraction intensity ofhe standard data and the experimental data. Nevertheless, we canonclude that the predominant phase in the films prepared by TAnd Na2S2O3 are all orthorhombic Bi2S3.
The composition of the Bi2S3 films is estimated from the EDSpectra of the films which are shown in Fig. 2. The influence ofnS buffer layer on the composition analyses of the Bi2S3 films waselieved to be very weak because the quantity of SnS is much lesshan the Bi2S3 films. We even cannot find obvious peaks for Sn ele-
ent (located at around 3.5 keV) from the EDS spectrum. It can beeen that the molar ratios of bismuth to sulfur are 1:1.4 and 1:1.6or the films prepared by TA and Na2S2O3, respectively. These val-es are close to but deviate from the stoichiometric ratio of Bi2S3,howing that the Bi2S3 films prepared by TA is bismuth-rich. How-ver, for the Bi2S3 films prepared by Na2S2O3, the excess S may alsoesult from the thiosulfate because thiosulfate can decompose to Slement when they are in acid medium.
Fig. 3 shows the scanning electron microscope (SEM) images ofhe Bi2S3 films prepared by TA and Na2S2O3 after different depo-ition time. For the Bi2S3 film prepared by Na2S2O3, we can seehat the film is porous, and dendritic layer can be seen from theEM image. The Bi2S3 film prepared by TA is comprised of cylindri-al cluster in the size of hundred nanometers. Though the surfaceorphology of the film is rough, it is continuous and no obvious
ore can be found in the film. It is particularly important for thepplication of Bi2S3 to thin film solar cells because the leakage ofhoto-current can be prevented when a continuous or compactlm is used. According to the literature [2,19], the deposition ofhe Bi2S3 film follows the hydroxide cluster mechanism, which cane expressed as the equations below:
i3+ + 3H2O = Bi(OH)3 + 3H+ (1)
Bi(OH)3 + 3S2− = Bi2S3 + 60H− (2)
i3+ Ions are prone to hydrolyze especially in the solution with aigh pH value. So when the deposition solution has a high pH value,he size of Bi(OH)3 cluster are bigger than that in the solution withlow pH value. So a rough film comprised of cylindrical cluster isrone to form at high pH value.
Fig. 4 shows the optical transmittance and reflectance spectraf the Bi2S3 films prepared by different sulfur source. The opti-al transmittance and reflectance spectra of the SnS buffer layerre also shown in Fig. 4(c). The sharp adsorption edge at around20 nm originates from the glass substrate. We can see that theransmittance of the SnS buffer layer exceed 80% when the wave-ength is beyond 400 nm. So the influence of the buffer layer on
he transmittance of the Bi2S3 film is weak. It can be seen that thebsorption edges are located at around at 880 nm for Bi2S3 filmsrepared by TA and 810 nm for Bi2S3 films prepared by Na2S2O3.o the optical bandgaps are around 1.4 eV and 1.5 eV for Bi2S3 filmsrepared by TA and those prepared by Na2S2O3, respectively. The[[[
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difference in optical bandgaps may result from the difference in thestoichiometric ratio of the Bi2S3 films [6].
The photo-current response curves of the Bi2S3 films arerecorded under 100 mW/cm2 tungsten–halogen illumination. Forthe measurement, a bias voltage of 20 V is applied on the film andstabilized for a moment. After the stabilization period, the currentwas recorded in the following sequence: 30 s in dark; 40 s underillumination; finally, 40 s in dark.
Fig. 5 shows the photo-current response curves of the Bi2S3films before and after annealing at 250◦ in nitrogen atmospherefor 1 h. As we mentioned above, the thickness of the SnS bufferis only several nanometers. So the electrical resistance of thefilms is very large, we have test the photo response curve of thesolo SnS buffer layer, we found that there is almost no currentwhether the films is in dark or under illuminance. So the influ-ence of the SnS buffer layer on the photo-response test of Bi2S3films is very weak. The electrical resistivity of the as-prepared filmsare all around 7 × 103 � cm in dark, which decreases to around1 × 103 � cm under 100 mW/cm2 tungsten–halogen illumination.After the annealing, the dark resistivity of the Bi2S3 films preparedby TA decreases by four magnitudes. In contrast, the dark resistiv-ity of the Bi2S3 films prepared by Na2S2O3 only decreases slightlywhich may result from the amorphous to crystalline transformation[8]. Furthermore, after the annealing, both the films show obvi-ous persistent photoconductivity (PPC) phenomena. According tothe Bi–S phase diagram [20], bismuth and sulfur will precipitate inbismuth-rich and sulfur-rich Bi2S3 films after annealing at the tem-perature below 270 ◦C, respectively. The precipitated bismuth andsulfur may act as defects or dopant in the films. And the decreaseof the resistivity and the PPC phenomena after the annealing maybe resulted from this process [21].
4. Conclusion
Bi2S3 films were successfully prepared by chemical bath deposi-tion in which ammonium citrate was used as the chelating reagent.The crystallization of the prepared films is poor and it can beimproved by annealing in nitrogen atmosphere. The molar ratiosof bismuth to sulfur are slightly deviated from the stoichiomet-ric ratio of the Bi2S3. The electrical resistivity of the as-preparedfilms is all around 7 × 103 � cm in dark, which decreases to around1 × 103 � cm under 100 mW/cm2 tungsten–halogen illumination.After the annealing, the dark resistivity of the Bi2S3 films preparedby TA decreases by four magnitudes, while the dark resistivity ofthe Bi2S3 films prepared by Na2S2O3 only decreases slightly. Thebandgaps of the films are 1.4 eV and 1.5 eV for Bi2S3 films preparedby TA and for those prepared by Na2S2O3, respectively.
Acknowledgement
This research is financial supported by National High-Tech andDevelopment Program of China Grant 2006AA03Z219.
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