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Combined CBD-CVD Technique for Preparation of II-VI Semiconductor Films for Solar Cells
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 57 (2014) 24 – 31
2013 ISES Solar World Congress
Combined CBD-CVD Technique for Preparation of II-VI Semiconductor Films for Solar Cells I.R. Chávez Urbiola*a, J.A. Bernal Martínezb, J. Hernández Borjaa, C.E. Pérez Garcíaa, R. Ramírez Bona, Y.V. Vorobieva. a CINVESTAV-IPN, Unidad Querétaro, Libramiento Norponiente 2000, 76230 Querétaro, México. CIMAV Chihuahua/Monterrey, Av. Miguel de Cervantes 120, 31109 Chihuahua, Mexico.
b
Abstract II-VI and IV-VI semiconductor films for solar cell applications, namely, CdTe, CdSe, PbSe and PbTe, can be prepared in two-stage deposition process. In this work we illustrate the two-stage process to obtain PbTe films from precursor plumbonacrite films deposited by chemical bath deposition (CBD). In the first stage, plumbonacrite Pb10(CO3)6O(OH)6) films were deposited onto glass substrate by (CBD), using ammonia-free low-temperature process in alkaline aqueous solution with corresponding ion sources. Then, in the second stage, the obtained film was placed in a Chemical Vapor Deposition (CVD) Hot Wall reactor with gas transportation, where it acted as substrate in a reaction of substitution of non-metallic film component by Te, thus forming PbTe films. The nitrogen flux of 0.25 lt/min was used as transporting gas. The source temperature was adjusted between boiling (Tb) point and melting point (Tm) with the aim to control the flux gas of the source(Tellurium source, Tm=449.51°C,Tb=988°C). The substrate temperature was adjusted to improve the quality of the film. Besides, a two-stage CBD process was also studied, where first a higher band gap semiconductor layer was deposited (CdS) and then a smaller gap material (PbS) was deposited above it. The structural and optical investigation of the films obtained proved their high quality that determines the possibility of utilization them as elements of solar cells, in particular, multi-junction cells. As an example, solar cell of CdS/PbS type was constructed showing quantum efficiency above 25%. The technique as a whole is versatile and relatively cheap, it could be used for preparation of a large variety of semiconductor materials. © 2014 Authors. Published by Elsevier This is Ltd. an open access article under the CC BY-NC-ND license © 2013The The Authors. Published by Ltd. Elsevier (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES Selection and/or peer-review under responsibility of ISES.
Key words: Semiconductors; Chemical vapor deposition; Thin film; Thick film; Crystal structure
_________ * Corresponding author. Tel.: (+52442)2119938; fax: (+52442)2119938.
E-mail address: [email protected].
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES. doi:10.1016/j.egypro.2014.10.004
I.R. Chávez Urbiola et al. / Energy Procedia 57 (2014) 24 – 31
1. Introduction A typical thin film solar cell is composed of 4 layers, namely: transparent conductive oxide (TCO) which acts as the front electrical contact, window layer, absorber layer and finally the back electrical contact on top of the absorber layer. Among the semiconductor materials with adequate properties for window or absorber layers in thin film solar cells are the cadmium and lead chalcogenides (CdS, CdSe, CdTe, PbS, PbSe and PbTe). Heterostructure solar cells based on these chalcogenide semiconductor materials have been reported for: CdS/CdTe [1, 2], CdS/PbS [3], CdS/CdSe [4], CdSe/PbSe [5], etc. These semiconductor materials have also been applied in other types of devices such as visible and infrared detectors [6-8]. Several methods can be used for the film deposition of these materials, including chemical or physical routes. Each deposition technique has its own advantages and disadvantage, therefore, depending on the sought properties, for each material it is necessary to investigate several techniques and analyze the film properties to find the most convenient one for specific applications. For example, it is well known that CdS films obtained by chemical bath deposition (CBD) perform very well as window layers in CdS/CdTe solar cells. On the other hand, CBD technique is not appropriate for the deposition of absorber materials because it is difficult to attain the large film thickness of the absorber layer required for solar cell applications. An alternative to deposit thick absorber layers, taking advantage of the characteristics of the CBD technique, is the ion exchange of appropriate precursor films previously deposited by this technique. Based on this scheme, in this work we report the setup of a chemical vapor deposition (CVD) system with a single chalcogen source to obtain cadmium or lead chalcogenides by conversion of cadmium or lead oxides or hydroxides previously deposited by CBD. The conversion to chalcogenide films is attained by thermal ion exchange by exposing the precursor metal oxide film to the chalcogen gas flux at high temperature. To show the feasibility of this two-stage combination process, in this work we applied it to the deposition of PbTe films from a lead-based compound, plumbonacrite (Pb10(CO3)6O(OH)6), deposited by CBD. The chalcogen source in this CVD process was Te. 2. Material and methods The plumbonacrite films were deposited on glass substrates by means of the CBD technique. The CBD recipe to deposit the plumboncrite films will be published elsewhere. The plumbonacrite films were converted to PbTe through a thermal substitution reaction in a CVD reactor. The thermal substitution reaction consist in the exchange of O, OH and CO3 groups in the plumbonacrite molecule by Te to obtain PbTe molecules. The CVD process was performed as follows; the precursor plumbonacrite film and the Te source were introduced into a quartz growth chamber separated by 120 mm. This separation allows different temperatures for the Te source and the plumbonacrite film. At the beginning of the process the growth chamber is filled with nitrogen to produce an inert atmosphere in order to avoid the oxidation of the Te source, then source and substrate are heated until temperatures attain 500°C and 300°C, respectively. After this heating, the nitrogen is made to flow as carrier gas for transporting Te gas from the source to the substrate. The Te gas flux over the surface of the precursor film is sustained for defined time, which is considered the deposition time. We found that after 20 min the PbTe film is formed and after that a tellurium film is deposited above the PbTe film. For the second part of this work, the deposition of cadmium sulfide films by CBD was made-up in a solution prepared in a beaker by the sequential addition of 0.05 M CdCl 2, 0.5 M C6H5O7Na3O7-2H2O (sodium citrate), 0.5 M KOH diluted, pH 10 borate buơer, 0.5 M CS(NH2)2 (thiourea) and deionized water. Glass slides substrates were placed in the reaction beaker and it was immersed in a water bath at 70°C during one hour after what the substrates were removed from the reaction solution [3]. On the other hand, lead sulfide films deposition by CBD was performed in a water bath at 70°C during one hour in a reaction solution with a mixture of: lead acetate 0.5M, sodium hydroxide (NaOH) 2M, thiourea 1M, triethanolamine 2M (C6H15NO3) and deionized water.
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2.1. Material characterization The precursor and deposited films were characterized by X-ray diffraction using the equipment RIGAKU, Dmax2100, Cu-Kα radiation, 40kV and 20 mA, λ=1.54056Å Kα1. The surface morphology of the films was analyzed by SEM, using Scanning Electron Microscope SEM (JSM-5800LV JOEL). The optical constants of the chemical deposited CdS and PbS films were obtained at room temperature under a normal incidence in the spectral range of 240-840 nm (5.16-1.75 eV) using a Film Tek_3000 (SCI, Inc.) spectrometer. 3. Results and discussions 3.1. Structural analysis
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PbTe Film PDF 65-2953 PbO PDF 05-0561 * (101)* (110) *
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PbTe Film PDF 65-2953 Te PDF 65-3370 *
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Figure 1 illustrates the X-ray diffraction patterns of the precursor plumbonacrite film and two converted samples in different stages of the CVD process with Te gas flux (20 and 30 min of deposition time). As can be seen at the bottom, the precursor film pattern displays the diffraction peaks corresponding to lead oxide carbonate hydroxide (plumbonacrite PDF 19-0680). The pattern of the sample exposed by 20 min to Te flux corresponds to PbTe with some traces of PbO as shown by the weak diffraction signals at 33 and 37°. The high intensity of the diffraction peaks in this pattern show that PbTe film has good crystallinity. Similar XRD patterns of PbTe films with PbO traces were reported in references [9, 10]. The formation of PbO can be due to the existence of oxygen inside the growth chamber according to reference [11]. However, in our case it is due to the incomplete transformation of plumbonacrite to PbTe, which can be concluded from the analysis of the XRD pattern of the sample deposited for 30 min. In this pattern, at top of Fig. 1, the diffraction signals of PbO have disappeared and instead Te diffraction peaks are observed in addition to those of PbTe. It is an evidence that conversion to PbTe continues after 20 min of exposition of plumbonacrite to Te gas flux and after it is completed, sometime between 20 and 30 min, a crystalline layer of Te is formed on top of PbTe film.
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Fig. 1 XRD patterns of Plumbonacrite and PbTe films.
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3.2. SEM (Scanning electron microscopy) The evolution of the surface morphology from the plumbonacrite film to PbTe films is shown in the SEM images in Fig. 2. It is observed that the plumbonacrite surface, Fig. 2 a), is composed by round leaves, like wafers, with size of several microns and very thin thickness. In some regions, the thin wafers are arranged forming flower-like shapes. The surface morphology of this precursor film exposes large surface area for a better interaction with Te gas, which is favorable for the thermal exchange process. The surface morphology of the PbTe film deposited for 20 min, shown in Fig. 2 b), displays the wafer-like shapes, lying horizontally on the substrate. The wafers in this case form a much more dense film and smaller formations are observed on their surface. The shape of these formations is diverse including round grains, prisms and even a hexagonal microtube can be observed. The unusual surface morphology of this PbTe is clearly influenced by the morphology of the precursor film. The common surface morphology of conventional PbTe films consists in grains with not well-defined shape, as it has been reported in references [11-15]. The image in fig. 2 c) shows the surface morphology of the film deposited for 30 min. when is covered by tellurium. The observed morphology with smaller grains covering the wafer shapes correspond to the Te layer on top of the PbTe film.
Fig. 2 Top view images of a) Pb10(CO3)6O(OH)6) film, b) PbTe film and c) PbTe-Te film.
These results show that thermal ion exchange process was effective to substitute O, OH and CO 3 groups by Te in the precursor plumbonacrite films to obtain PbTe films. The control of the deposition time is crucial to obtain PbTe crystalline films without PbO due to an incomplete substitution or Te due to an excess of exposure to Te gas.
Fig. 3 a) XRD patterns of PbS films b) XRD patterns of CdS films.
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3.3. Analysis of the CdS and PbS films deposited by CBD. In fig. 3 are shown the XRD patterns the PbS and CdS films deposited by CBD. In fig 3 a) the pattern display diffraction peaks at 26, 30, 43 and 61°, which correspond to the diffraction lines produced by the (111), (200), (220) and (311) crystalline planes of the PbS cubic phase (galena), respectively. It is observed a preferential growth along the (200) direction. In fig 3 b) the pattern displays an intense diffraction peak at about 26°, which correspond to the diffraction line produced by the (002) crystalline planes of the CdS hexagonal phase. The surface morphology of PbS and CdS films is shown in the SEM images of Fig. 4. In fig. 4 a) it can be observed that the surface of the PbS film is very compact and homogenous. In fig. 4 b), for the case of CdS film, it is observed that the surface is compact, showing a granular structure with very small grains and well-defined grain boundaries. There are also large aggregates dispersed on the film surface. The formation of aggregates on the surface is typical of the CBD technique [16], and it has also been observed in other types of materials deposited by CBD [17].
Fig.4 Top view SEM images of a) PbS film and b) CdS film.
The measured optical transmission (T) and optical reflection (R) spectra of the PbS and CdS films are plotted as solid lines in fig. 5. The absorption edge in transmission spectra of the films is observed for CdS at approximately 530 nm. The transmission and reflection spectra of CdS films were fit using a model with two layers, representing the CdS layer on the glass substrate [18]. The transmission spectrum of PbS is approximately zero in the visible region due to the large thickness of the films.
Fig.5 Optical transmission and reflectance spectra of PbS and CdS films.
I.R. Chávez Urbiola et al. / Energy Procedia 57 (2014) 24 – 31
3.4. Heterostructured CdS-PbS solar cell. Some of the application for these materials is in solar thin film cells. We have built a PbS/CdS solar cell with the structure: glass/ITO/CdS/PbS/gold, which is shown in Fig 6 (see [3] for details).
Fig. 6 Photo and structure of the CdS/PbS solar cell.
In previous works we made solar cell glass/ITO/CdS/PbS/Graphite and studied the heterojunction CdS/PbS and we obtain a efficiency of K 1.63% with a Fill Factor FF=0.36. The area of the solar cells was 0.16 cm2. The J–V characteristics were obtained in darkness and under 90 mW/cm2 illumination using a tungsten-halogen lamp, with a mask inside in a black box, the parameters of a semiconductor are measured using a Semiconductor Parameter Analyzer 4155C Agilent in the voltage range between 0.4 and 0.7 V in steps of 0.01 V. The J–V characteristics presented in Fig. 7 have rectifying character in the dark, showing sharp increase of current at voltages around 0.3–0.4 V (it indicates that the barrier is of order of 0.3–0.4 eV). Being illuminated, the structures present characteristics typical for solar cells: the values of open-circuit voltage Voc= 290 mV, max voltage Vmax=0.160 V, max current J max=9.2mW/cm2, short circuit current Jsc=14 mA/cm2 , with the corresponding efficiency. (see details in [3])
Fig. 7 Current–voltage characteristics in the dark and under illumination of 90 mW/cm2 of CdS/PbS solar cell.
Fig 8 presents spectrum of external quantum efficiency (EQE) obtained for the cell PbS/CdS (see [3] for details) from spectral measurements of short circuit current. The EQE values in the range 0.6–3.5 eV are almost constant, varying slightly around 20% in accordance with the spectral dependence of reflectance from CdS film (see [3] for details).
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Fig. 8. Spectrum of external quantum efficiency (EQE) of PbS/CdS cell.
Taking into account that the total photon flux in the corresponding part of solar spectrum is F=3.7x1017 photons/cm2 s [19], and the Jsc value for this cell is 14 mA/cm2 , we calculate an average quantum efficiency to be Jsc/Fe=24% (‘‘e’’ is an electron charge), which agrees well with Fig. 7. 4. Conclusions In this work we have shown that the combination of two processes, CBD and CVD, can be successfully used for the preparation of PbTe films from precursor plumbonacrite films deposited by CBD. The properties of CdS and PBS films deposited by CBD are suitable for their application as window and absorber layers, respectively, in CdS-PbS solar cells. Acknowledgements I.R. Chávez-Urbiola and C.E. Pérez García wish to thank CONACYT for their scholarship. We acknowledge the technical assistance of: C.A. Avila Herrera, M. A. Hernández Landaverde, J.E. Urbina Alvárez and A. Jiménez Nieto. References 1. 2. 3. 4. 5. 6. 7.
Morales-Acevedo, A., Can we improve the record efficiency of CdS/CdTe solar cells? Sol. Energy Mater. Sol. Cells, 2006. 90: p. 2213-2220. Junfeng Hana, C.S., G. Haindl , Ganhua Fu, V. Krishnakumar , J. Schaffner, Chunjie Fan, Kui Zhaoa, A. Kleinb, W. Jaegermann, Optimized chemical bath deposited CdS layers for the improvement of CdTe solar cells. Sol. Energy Mater. Sol. Cells, 2011. 95: p. 816-820. J. Hernández-Borja, Y.V.V., R. Ramírez-Bon, Thin film solar cells of CdS/PbS chemically deposited by an ammonia-free process. Sol. Energy Mater. Sol. Cells, 2011. 95: p. 1882-1888. Yu-LongXie, Enhanced photovoltaic performance of hybrid solar cell using highly oriented CdS/CdSe-modified TiO2 nanorods. Electrochim. Acta, 2013. 105: p. 137-141. V. Sholin, A.J.B., I.E. Anderson Y. Sahoo, D. Reddy, S.A. Carter, All-inorganic CdSe/PbSe nanoparticle solar cells. Sol. Energy Mater. Sol. Cells, 2008. 92: p. 1706-1711. Dalven, R., A review of the semiconductor properties of PbTe, PbSe, PbS and PbO. Infrared Phys. Technol., 1969. 9: p. 141-181. S.K.J. Al-Ani, H.H.M., E.M.N. Al-Fwade, The optoelectronic properties of CdSe:Cu photoconductive detector. Renew. Energy, 2002. 25: p. 585–590.
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8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.
Sushil Kumar, e.a., Studies on thin films of lead chalcogenides. Curr. Appl. Phys., 2005. 5: p. 561-566. A.A. Yadav, M.A.B., E.U. Masumdar, Studies on cadmium selenide (CdSe) thin films deposited by spray pyrolysis. Mater. Chem. Phys., 2010. 121: p. 53-57. L. Kungumadevi, R.S., A. Subbarayan, AC conductivity and dielectric properties of thermally evaporated PbTe thin films. Solid-State Electron., 2010. 54: p. 58-62. Z. Dashevsky, E.S., V.Kasiyan, E.Flitsiyan, L.Chernyak, Influence ofoxygentreatmentontransportproperties of PbTe:Inpolycrystallinefilms. Physica B, 2010. 405: p. 2380–2384. S. Kertoatmodjo, N., F. Guastavino, Formation reactions of PbTe thin film deposition by closespaced vapor transport (CSVT) technique. Thin Solid Films, 1998. 324: p. 25–29. Xiaohong Li, I.S.N., Direct electrodeposition of PbTe thin films on n-type silicon. Electrochem. Commun., 2008. 10: p. 363–366. Heini Saloniemi, T.K., Mikko Ritala, Markku Leskela, Electrodeposition of PbTe thin films. Thin Solid Films, 1998. 326: p. 78–82. Y.Y.Wang, K.F.C., X.Yao, Facile synthesis of PbTe nanoparticles and thin films in alkaline aqueous solution at room temperature. J. Solid State Chem., 2009. 182: p. 3383–3386. D. Lincot, R.O.-B., Mechanism of Chemical Bath Deposition of Cadmium Sulfide Thin Films in the AmmoniaϋThiourea System. J. Electrochem. Soc., 1992. 139: p. 1880-1886. R. Bayón, J.H., Structure and morphology of the indium hydroxy sulphide thin films. Appl. Surf. Sci., 2000. 158: p. 49-57. M.B. Ortuño-López, M.S.-L., A. Mendoza-Galván, R. Ramírez-Bon, Optical band gap tuning and study of train in CdS thin films. Vacuum, 2004. 76: p. 181-184. Henry, C.H., Limiting efficiencies of ideal single and multiple energy gap terrestrial solar cells. J. Appl. Phys., 1980. 51: p. 4494–4499.
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ScienceDirect Energy Procedia 57 (2014) 24 – 31
2013 ISES Solar World Congress
Combined CBD-CVD Technique for Preparation of II-VI Semiconductor Films for Solar Cells I.R. Chávez Urbiola*a, J.A. Bernal Martínezb, J. Hernández Borjaa, C.E. Pérez Garcíaa, R. Ramírez Bona, Y.V. Vorobieva. a CINVESTAV-IPN, Unidad Querétaro, Libramiento Norponiente 2000, 76230 Querétaro, México. CIMAV Chihuahua/Monterrey, Av. Miguel de Cervantes 120, 31109 Chihuahua, Mexico.
b
Abstract II-VI and IV-VI semiconductor films for solar cell applications, namely, CdTe, CdSe, PbSe and PbTe, can be prepared in two-stage deposition process. In this work we illustrate the two-stage process to obtain PbTe films from precursor plumbonacrite films deposited by chemical bath deposition (CBD). In the first stage, plumbonacrite Pb10(CO3)6O(OH)6) films were deposited onto glass substrate by (CBD), using ammonia-free low-temperature process in alkaline aqueous solution with corresponding ion sources. Then, in the second stage, the obtained film was placed in a Chemical Vapor Deposition (CVD) Hot Wall reactor with gas transportation, where it acted as substrate in a reaction of substitution of non-metallic film component by Te, thus forming PbTe films. The nitrogen flux of 0.25 lt/min was used as transporting gas. The source temperature was adjusted between boiling (Tb) point and melting point (Tm) with the aim to control the flux gas of the source(Tellurium source, Tm=449.51°C,Tb=988°C). The substrate temperature was adjusted to improve the quality of the film. Besides, a two-stage CBD process was also studied, where first a higher band gap semiconductor layer was deposited (CdS) and then a smaller gap material (PbS) was deposited above it. The structural and optical investigation of the films obtained proved their high quality that determines the possibility of utilization them as elements of solar cells, in particular, multi-junction cells. As an example, solar cell of CdS/PbS type was constructed showing quantum efficiency above 25%. The technique as a whole is versatile and relatively cheap, it could be used for preparation of a large variety of semiconductor materials. © 2014 Authors. Published by Elsevier This is Ltd. an open access article under the CC BY-NC-ND license © 2013The The Authors. Published by Ltd. Elsevier (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES Selection and/or peer-review under responsibility of ISES.
Key words: Semiconductors; Chemical vapor deposition; Thin film; Thick film; Crystal structure
_________ * Corresponding author. Tel.: (+52442)2119938; fax: (+52442)2119938.
E-mail address: [email protected].
1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and/or peer-review under responsibility of ISES. doi:10.1016/j.egypro.2014.10.004
I.R. Chávez Urbiola et al. / Energy Procedia 57 (2014) 24 – 31
1. Introduction A typical thin film solar cell is composed of 4 layers, namely: transparent conductive oxide (TCO) which acts as the front electrical contact, window layer, absorber layer and finally the back electrical contact on top of the absorber layer. Among the semiconductor materials with adequate properties for window or absorber layers in thin film solar cells are the cadmium and lead chalcogenides (CdS, CdSe, CdTe, PbS, PbSe and PbTe). Heterostructure solar cells based on these chalcogenide semiconductor materials have been reported for: CdS/CdTe [1, 2], CdS/PbS [3], CdS/CdSe [4], CdSe/PbSe [5], etc. These semiconductor materials have also been applied in other types of devices such as visible and infrared detectors [6-8]. Several methods can be used for the film deposition of these materials, including chemical or physical routes. Each deposition technique has its own advantages and disadvantage, therefore, depending on the sought properties, for each material it is necessary to investigate several techniques and analyze the film properties to find the most convenient one for specific applications. For example, it is well known that CdS films obtained by chemical bath deposition (CBD) perform very well as window layers in CdS/CdTe solar cells. On the other hand, CBD technique is not appropriate for the deposition of absorber materials because it is difficult to attain the large film thickness of the absorber layer required for solar cell applications. An alternative to deposit thick absorber layers, taking advantage of the characteristics of the CBD technique, is the ion exchange of appropriate precursor films previously deposited by this technique. Based on this scheme, in this work we report the setup of a chemical vapor deposition (CVD) system with a single chalcogen source to obtain cadmium or lead chalcogenides by conversion of cadmium or lead oxides or hydroxides previously deposited by CBD. The conversion to chalcogenide films is attained by thermal ion exchange by exposing the precursor metal oxide film to the chalcogen gas flux at high temperature. To show the feasibility of this two-stage combination process, in this work we applied it to the deposition of PbTe films from a lead-based compound, plumbonacrite (Pb10(CO3)6O(OH)6), deposited by CBD. The chalcogen source in this CVD process was Te. 2. Material and methods The plumbonacrite films were deposited on glass substrates by means of the CBD technique. The CBD recipe to deposit the plumboncrite films will be published elsewhere. The plumbonacrite films were converted to PbTe through a thermal substitution reaction in a CVD reactor. The thermal substitution reaction consist in the exchange of O, OH and CO3 groups in the plumbonacrite molecule by Te to obtain PbTe molecules. The CVD process was performed as follows; the precursor plumbonacrite film and the Te source were introduced into a quartz growth chamber separated by 120 mm. This separation allows different temperatures for the Te source and the plumbonacrite film. At the beginning of the process the growth chamber is filled with nitrogen to produce an inert atmosphere in order to avoid the oxidation of the Te source, then source and substrate are heated until temperatures attain 500°C and 300°C, respectively. After this heating, the nitrogen is made to flow as carrier gas for transporting Te gas from the source to the substrate. The Te gas flux over the surface of the precursor film is sustained for defined time, which is considered the deposition time. We found that after 20 min the PbTe film is formed and after that a tellurium film is deposited above the PbTe film. For the second part of this work, the deposition of cadmium sulfide films by CBD was made-up in a solution prepared in a beaker by the sequential addition of 0.05 M CdCl 2, 0.5 M C6H5O7Na3O7-2H2O (sodium citrate), 0.5 M KOH diluted, pH 10 borate buơer, 0.5 M CS(NH2)2 (thiourea) and deionized water. Glass slides substrates were placed in the reaction beaker and it was immersed in a water bath at 70°C during one hour after what the substrates were removed from the reaction solution [3]. On the other hand, lead sulfide films deposition by CBD was performed in a water bath at 70°C during one hour in a reaction solution with a mixture of: lead acetate 0.5M, sodium hydroxide (NaOH) 2M, thiourea 1M, triethanolamine 2M (C6H15NO3) and deionized water.
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2.1. Material characterization The precursor and deposited films were characterized by X-ray diffraction using the equipment RIGAKU, Dmax2100, Cu-Kα radiation, 40kV and 20 mA, λ=1.54056Å Kα1. The surface morphology of the films was analyzed by SEM, using Scanning Electron Microscope SEM (JSM-5800LV JOEL). The optical constants of the chemical deposited CdS and PbS films were obtained at room temperature under a normal incidence in the spectral range of 240-840 nm (5.16-1.75 eV) using a Film Tek_3000 (SCI, Inc.) spectrometer. 3. Results and discussions 3.1. Structural analysis
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PbTe Film PDF 65-2953 PbO PDF 05-0561 * (101)* (110) *
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30 min.
PbTe Film PDF 65-2953 Te PDF 65-3370 *
(012)* (220) (110)* (111)* (003)* (021)* (222)
(200)(001)*
Figure 1 illustrates the X-ray diffraction patterns of the precursor plumbonacrite film and two converted samples in different stages of the CVD process with Te gas flux (20 and 30 min of deposition time). As can be seen at the bottom, the precursor film pattern displays the diffraction peaks corresponding to lead oxide carbonate hydroxide (plumbonacrite PDF 19-0680). The pattern of the sample exposed by 20 min to Te flux corresponds to PbTe with some traces of PbO as shown by the weak diffraction signals at 33 and 37°. The high intensity of the diffraction peaks in this pattern show that PbTe film has good crystallinity. Similar XRD patterns of PbTe films with PbO traces were reported in references [9, 10]. The formation of PbO can be due to the existence of oxygen inside the growth chamber according to reference [11]. However, in our case it is due to the incomplete transformation of plumbonacrite to PbTe, which can be concluded from the analysis of the XRD pattern of the sample deposited for 30 min. In this pattern, at top of Fig. 1, the diffraction signals of PbO have disappeared and instead Te diffraction peaks are observed in addition to those of PbTe. It is an evidence that conversion to PbTe continues after 20 min of exposition of plumbonacrite to Te gas flux and after it is completed, sometime between 20 and 30 min, a crystalline layer of Te is formed on top of PbTe film.
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Fig. 1 XRD patterns of Plumbonacrite and PbTe films.
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3.2. SEM (Scanning electron microscopy) The evolution of the surface morphology from the plumbonacrite film to PbTe films is shown in the SEM images in Fig. 2. It is observed that the plumbonacrite surface, Fig. 2 a), is composed by round leaves, like wafers, with size of several microns and very thin thickness. In some regions, the thin wafers are arranged forming flower-like shapes. The surface morphology of this precursor film exposes large surface area for a better interaction with Te gas, which is favorable for the thermal exchange process. The surface morphology of the PbTe film deposited for 20 min, shown in Fig. 2 b), displays the wafer-like shapes, lying horizontally on the substrate. The wafers in this case form a much more dense film and smaller formations are observed on their surface. The shape of these formations is diverse including round grains, prisms and even a hexagonal microtube can be observed. The unusual surface morphology of this PbTe is clearly influenced by the morphology of the precursor film. The common surface morphology of conventional PbTe films consists in grains with not well-defined shape, as it has been reported in references [11-15]. The image in fig. 2 c) shows the surface morphology of the film deposited for 30 min. when is covered by tellurium. The observed morphology with smaller grains covering the wafer shapes correspond to the Te layer on top of the PbTe film.
Fig. 2 Top view images of a) Pb10(CO3)6O(OH)6) film, b) PbTe film and c) PbTe-Te film.
These results show that thermal ion exchange process was effective to substitute O, OH and CO 3 groups by Te in the precursor plumbonacrite films to obtain PbTe films. The control of the deposition time is crucial to obtain PbTe crystalline films without PbO due to an incomplete substitution or Te due to an excess of exposure to Te gas.
Fig. 3 a) XRD patterns of PbS films b) XRD patterns of CdS films.
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3.3. Analysis of the CdS and PbS films deposited by CBD. In fig. 3 are shown the XRD patterns the PbS and CdS films deposited by CBD. In fig 3 a) the pattern display diffraction peaks at 26, 30, 43 and 61°, which correspond to the diffraction lines produced by the (111), (200), (220) and (311) crystalline planes of the PbS cubic phase (galena), respectively. It is observed a preferential growth along the (200) direction. In fig 3 b) the pattern displays an intense diffraction peak at about 26°, which correspond to the diffraction line produced by the (002) crystalline planes of the CdS hexagonal phase. The surface morphology of PbS and CdS films is shown in the SEM images of Fig. 4. In fig. 4 a) it can be observed that the surface of the PbS film is very compact and homogenous. In fig. 4 b), for the case of CdS film, it is observed that the surface is compact, showing a granular structure with very small grains and well-defined grain boundaries. There are also large aggregates dispersed on the film surface. The formation of aggregates on the surface is typical of the CBD technique [16], and it has also been observed in other types of materials deposited by CBD [17].
Fig.4 Top view SEM images of a) PbS film and b) CdS film.
The measured optical transmission (T) and optical reflection (R) spectra of the PbS and CdS films are plotted as solid lines in fig. 5. The absorption edge in transmission spectra of the films is observed for CdS at approximately 530 nm. The transmission and reflection spectra of CdS films were fit using a model with two layers, representing the CdS layer on the glass substrate [18]. The transmission spectrum of PbS is approximately zero in the visible region due to the large thickness of the films.
Fig.5 Optical transmission and reflectance spectra of PbS and CdS films.
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3.4. Heterostructured CdS-PbS solar cell. Some of the application for these materials is in solar thin film cells. We have built a PbS/CdS solar cell with the structure: glass/ITO/CdS/PbS/gold, which is shown in Fig 6 (see [3] for details).
Fig. 6 Photo and structure of the CdS/PbS solar cell.
In previous works we made solar cell glass/ITO/CdS/PbS/Graphite and studied the heterojunction CdS/PbS and we obtain a efficiency of K 1.63% with a Fill Factor FF=0.36. The area of the solar cells was 0.16 cm2. The J–V characteristics were obtained in darkness and under 90 mW/cm2 illumination using a tungsten-halogen lamp, with a mask inside in a black box, the parameters of a semiconductor are measured using a Semiconductor Parameter Analyzer 4155C Agilent in the voltage range between 0.4 and 0.7 V in steps of 0.01 V. The J–V characteristics presented in Fig. 7 have rectifying character in the dark, showing sharp increase of current at voltages around 0.3–0.4 V (it indicates that the barrier is of order of 0.3–0.4 eV). Being illuminated, the structures present characteristics typical for solar cells: the values of open-circuit voltage Voc= 290 mV, max voltage Vmax=0.160 V, max current J max=9.2mW/cm2, short circuit current Jsc=14 mA/cm2 , with the corresponding efficiency. (see details in [3])
Fig. 7 Current–voltage characteristics in the dark and under illumination of 90 mW/cm2 of CdS/PbS solar cell.
Fig 8 presents spectrum of external quantum efficiency (EQE) obtained for the cell PbS/CdS (see [3] for details) from spectral measurements of short circuit current. The EQE values in the range 0.6–3.5 eV are almost constant, varying slightly around 20% in accordance with the spectral dependence of reflectance from CdS film (see [3] for details).
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Fig. 8. Spectrum of external quantum efficiency (EQE) of PbS/CdS cell.
Taking into account that the total photon flux in the corresponding part of solar spectrum is F=3.7x1017 photons/cm2 s [19], and the Jsc value for this cell is 14 mA/cm2 , we calculate an average quantum efficiency to be Jsc/Fe=24% (‘‘e’’ is an electron charge), which agrees well with Fig. 7. 4. Conclusions In this work we have shown that the combination of two processes, CBD and CVD, can be successfully used for the preparation of PbTe films from precursor plumbonacrite films deposited by CBD. The properties of CdS and PBS films deposited by CBD are suitable for their application as window and absorber layers, respectively, in CdS-PbS solar cells. Acknowledgements I.R. Chávez-Urbiola and C.E. Pérez García wish to thank CONACYT for their scholarship. We acknowledge the technical assistance of: C.A. Avila Herrera, M. A. Hernández Landaverde, J.E. Urbina Alvárez and A. Jiménez Nieto. References 1. 2. 3. 4. 5. 6. 7.
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