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Energy Procedia 10 (2011) 177 – 181
European Materials Research Society Conference Symp. Advanced Inorganic Materials and Concepts for Photovoltaics
Precursor Concentration Effect on the Properties of ZnIn2Se4 Layers Grown by Chemical Bath Deposition P. Babua, K.T. Ramakrishna Reddya and R.W. Miles* a
Thin Film Laboratory, Department of Physics, Sri Venkateswara University, Tirupati – 517 502, India. *School of Engineering, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK
Abstract ZnIn2Se4 (ZIS) films were grown by chemical bath method and the effects of precursor concentration (PC) on the chemical and physical properties were investigated for the first time. The layers were grown on Corning 7059 glass substrates using equimolar aqueous solutions of zinc chloride, indium tri-chloride and sodium selenite as precursors. The deposition was carried out at different PC values that vary in the range, 0.01 - 0.075M at a constant bath temperature of 80oC and deposition time of 90 min. The structural studies showed nanocrystalline nature of the films with the (112) preferred orientation. The crystallite size varied in the range, 54 – 125Å. The composition analysis indicated O and Cl in addition to Zn, In and Se in the films. The optical transmittance changed with PC and layers with PC=0.05M had the transmittance, >75 % in the visible range. The energy band gap of the films varied in the range, 2.91 - 3.09 eV with change of PC. FTIR studies were undertaken to identify the adsorbed functional groups on the surface of the films and photoluminescence spectra revealed the presence of defect states in the films. A detailed analysis of these properties are reported and discussed.
© by Elsevier ElsevierLtd. Ltd.Selection Selectionand/or and/orpeer-review peer-reviewunder underresponsibility responsibility Organizers oforganizers: European © 2011 2010 Published Published by of ofthe symposium Materials Research Society (EMRS) Conference: Symposium on Advanced Inorganic Materials and Concepts G. Conibeer; Yongxiang Li; J. Poortmans; M. Kondo; A. Slaoui; M. Tao; M. Topic for Photovoltaics. Keywords: ZnIn2Se4 films, Chemical bath deposition, Physical properties
1. Introduction ZnIn2Se4 (ZIS) is a ternary semiconductor that belongs to AIIB2IIIX4VI chalcopyrite family. It attracts the attention of many researchers due to its potential application in various fields, including photoelectronics and photovoltaics [1-2]. Photovoltaic devices with the highest conversion efficiency of 15.3% has been reported for Cu(In,Ga)Se2-based solar cells using ZIS as the buffer layer [3]. Investigations on structural, optical and electrical properties of ZIS films grown by various methods such as chemical
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1876-6102© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Organizers of European Materials Research Society (EMRS) Conference: Symposium on Advanced Inorganic Materials and Concepts for Photovoltaics. doi:10.1016/j.egypro.2011.10.173
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transport reaction, chemical vapor deposition with iodine as the transport agent, spray pyrolysis and thermal evaporation have been reported [4-7]. However, to our knowledge, no attempt has been made to prepare ZIS films by chemical bath deposition (CBD) method. CBD is a simple and low cost technique capable of depositing homogeneous, uniform and smooth layers [8]. Generally, the properties of thin films are highly dependent on the technique used and deposition conditions employed to grow the layers. In the previous work, we studied the effect of pH on the physical properties of CBD grown ZIS films [9]. In this paper, we report, for the first time, on the effect of precursor concentration on the structure, composition, optical properties and photoluminescence of ZIS films prepared by CBD technique. 2. Experimental ZIS films were deposited on Corning 7059 glass substrates by chemical bath deposition. Aqueous solutions of zinc chloride (ZnCl2), indium tri-chloride (InCl3) and sodium selenite (Na2SeO3) are used as the precursor materials for Zn, In and Se respectively. The concentration of all the three solutions was maintained constant for a particular deposition and is termed as ‘precursor concentration (PC)’. The chemical bath is made by mixing 10 ml of ZnCl2, 20 ml of InCl3 and 40 ml of Na2SeO3 solutions complexed with 2 ml of (80%) hydrazine hydrate in a separate beaker. Few drops of 25% ammonia (NH3) are also added to the mixture in order to adjust the pH of the solution to a value, 10. Ultrasonically cleaned glass substrates were dipped into the solution bath vertically while stirring of the solution continues. The deposition was carried out at different precursor concentrations that vary in the range, 0.01 - 0.075M at a constant bath temperature of 80 oC and fixed deposition time of 90 min. The crystallinity of ZIS films was measured using a Siefert X-ray diffractometer with Cu-Kα radiation source (λ=1.5402Å). The composition and morphological properties were analyzed using X-ray energy analyzer (Inca Penta FETx3) attached to Zeiss scanning electron microscope (SEM). Optical transmittance studies were performed using Perkin Elmer UV-Vis-NIR spectrophotometer, Fourier transform infrared (FTIR) and photoluminescence (PL) studies made using Thermo Electronic Corporation (IR200 FTIR) and Florolog-3fluorescence spectrometer respectively. 3. Results and discussion The deposited layers were strongly adherent to the substrate surface and appear brick-red in colour. The thickness of the films, determined by a weighing method was approximately 0.75μm. The chemical composition of the films was evaluated from energy dispersive analysis of X-rays (EDAX) measurement. The EDAX spectra indicated the prominent peaks corresponding to Zn, In and Se in addition to O, C and Cl. The evaluated stoichiometry of the grown films deviated from the 1:2:4 atomic ratio of ZnIn2Se4, which is mainly due to the incorporation of oxygen, carbon and chlorine into the films during the growth process. In the EDAX spectra, the peaks of ‘In’ are more pronounced when compared to the other elements. This might be due to the presence of insoluble In(OH)3 in minute quantity on the film surface. The appearance of oxygen in the EDAX spectra might be due to the presence of oxygen in the aqueous solutions of precursors used for the film deposition [10] and /or incomplete reaction of the bath mixtures. The X-ray diffraction (XRD) spectra of all ZIS films grown at various PC values showed a strong (112) orientation, exhibiting the tetragonal crystal structure. The spectra also showed four major peaks related to the (110), (200) and (211) planes in addition to low intensity (103), (213), (204) and (310) planes that correspond to the orientations of ZnIn2Se4. No other peaks related to binary phases of Zn-Se and In-Se were observed. These results are in agreement with the JCPDS and other reported data [11, 12]. Fig. 1(a) shows the typical XRD spectrum of ZIS film formed at PC=0.05M. All the films also showed
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board peaks with a high fringe width at half maximum (FWHM), indicating nanocrystalline nature of the layers. The evaluated lattice parameters, a and c were approximately in agreement with the reported values [12]. The crystallite size, D of the films was evaluated using the Debye Scherer formula [13] D
0.94O E CosT
(1)
where ‘β’ is the FWHM of (112) peak of ZIS, ‘λ’ is the wavelength of X-rays and ‘θ’ is the corresponding diffraction angle. The evaluated crystallite size varied in the range, 54 – 125 Å in the PC range investigated. The SEM surface topography revealed that the layers formed at lower PC values showed number of regions separated by micro-cracks in which unevenly distributed small spherically shaped grains are distributed. Fig. 1(b) shows the SEM image of a typical ZIS film prepared at PC=0.05M.
Fig. 1: (a) X-ray diffraction spectrum (b) corresponding SEM micrograph of ZIS film.
The optical transmittance studies of as-deposited ZIS films were carried out in the wavelength range, 350 - 2500 nm and the corresponding transmission spectra are shown in Fig. 2. A sharp fall in the optical
Fig .2. (a) Optical transmittance versus wavelength spectra of ZIS films and (b) plot of (αhν)2 verses hν.
transmittance revealed that the deposited films had direct optical transition between the parabolic bands and also indicated better crystallinity of the films. The optical transmittance of ZIS films increased with increase of precursor concentration and the films deposited at a PC = 0.05M had an optical transmittance > 75%. However, it is slightly decreased at PC>0.075M. The lower transmittance observed at lower PC values might be due to higher surface scattering since the surface of these layers looked rough, observed from the SEM analysis. Further, the absorption edge shifted towards the shorter wavelength side as with
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the increase of precursor concentration. This might be due the decrease in the grain size of the films. The optical energy band gap ‘Eg’ is determined using the following relation [14] (αhυ) = A (hυ – Eg)1/2
(2)
where α is the absorption coefficient, hυ is the photon energy. The dependence of ( DhQ)2 on hQ for a typical ZIS film grown at PC=0.05M is shown in Fig. 2(b). The extrapolation of the linear portion of the plot on to hQ axis gives the band gap of the film that varied in the range, 2.91-3.09 eV in the investigated Pc range. The optical band gap obtained in this work is in good agreement with the value reported [7]. The FTIR studies were undertaken in order to identify the functional groups present in the films. Fig. 3 shows the FTIR spectrum of ZIS film prepared at PC=0.05M, which showed different bands. The band at 748 cm-1 and 847 cm-1 are due to the incomplete conversion of Zn(OH)2 in the grown films [15], which is also evident from the EDAX spectrum indicating the presence of oxygen in the film. The low intensity band observed at 1640 cm-1 was assigned to the O-H stretching/bending vibration modes. The presence of an intense broad band at approximately 3400 cm-1 was attributed to the symmetric or asymmetric –OH stretching mode and the adsorption of oxygen on the film surface [16].
Fig. 3: FTIR spectrum of ZIS film grown at P C=0.05M.
Fig .4: Photoluminescence spectra of ZIS films.
The photoluminescence spectra of ZIS films are reported for the first time and are shown in Fig. 4 that showed a main peak and shoulders on either side. The main peak corresponds to the energy band gap of the material that shifted towards lower wavelength side with the increase of precursor concentration. The corresponding energy band gap varied in the range, 2.86 – 2.98 eV with the increase of PC, which are slightly lower than the value determined from the optical measurements. The decrease of overall PL intensity with increase in PC could be due to the increased number of trap states in the mid-band gap region for non-radiative transition. Gaussian curve fitting to deconvolute the shoulders in PL spectra revealed two broad peaks centred at 440-455 nm and 460-490 nm, respectively. The peak located at 440455 nm might be associated with zinc interstitials (Zni)/zinc vacancies (VZn) [17], while the other at 460490 nm is due to the donor-acceptor pair and/or oxygen residual impurity (OSe) [18]. A detailed study of PL measurements with temperature is essential to understand the defect structure in ZIS films. 4. Conclusions ZnIn2Se4 films were successfully grown on Corning 7059 glass substrates using a low cost method, chemical bath deposition at different precursor concentrations for the first time. The layers prepared at PC=0.05M showed better crystallinity with a strong (112) orientation with higher grain size compared to
P. Babu et al. / Energy Procedia 10 (2011) 177 – 181
the layers grown at other precursor concentrations. These films showed an average optical transmittance > 75% in the visible region with an energy band gap of 3.09 eV. The FTIR measurements revealed that ZIS layers consisted of adsorbed -OH group because of the presence of Zn(OH)2 and In(OH)3 in the solution, which is commonly observed in the films formed by CBD method. The PL studies revealed defect states corresponding to vacancies/interstitials in association with the related donor-acceptor states. Acknowledgements One of the authors, KTR Reddy, Sri Venkateswara University and Northumbria University wish to thank the European Commission for financial support to carry out this work. References [1] Luengo J, Joshi NV. Composition dependence of the energy gap of Zn xMn1−xIn2Se4 and optical absorption spectroscopy. Material Letters 1996;26:47-50. [2] Fortin E, Raga F. Low temperature photoconductivity of ZnIn2Se4 and CdIn2Se4. Solid State Commum 1974;14:847-850. [3] Delahoy AE, Akhtar M, Cambridge J, Chen L, Govindarajan R, Guo S, Romero MJ. CIGS devices with ZIS, In2S3, and CdS buffer layers. 29th IEEE Photovoltaic Specialists Conference, New Orleans, USA, May 19 - 24, 2002, p. 640. [4] Abdullayev AG, Kerimova TG, Kyazumov MG, Khidirov ASH. An electron diffraction study of the ZnIn2Se4 crystal structure: a novel phase. Thin Solid Films 1990;190:309-315. [5] Paorici C, Zanotti L, Zuccalli G. A temperature variation method for the growth of chalcopyrite crystals by iodine vapour transport. J Cryst Growth 1980;43:705-710. [6] Yadav SP, Shinde PS, Rajpure KY, Bhosale CH. Preparation and properties of spray-deposited ZnIn2Se4 nanocrystalline thin films. J Phys Chem Solids 2008;69:1747-1752. [7] Soliman LI, Hendia TA. Influence of γ-irradiation on the optical and electrical properties of ZnIn2Se4 films. Radiat. Phys Chem 1997;50:175-177. [8] Zhou L, Xue Y, Li J. Study on ZnS thin films prepared by chemical bath deposition. J Environ Sci 2009;21:S76-S79. [9] Babu P, Reddy MV, Revathi N, Reddy KTR. Effect of pH on the physical properties of ZnIn2Se4 thin films grown by chemical bath deposition. J. Nano- Electron. Phys 2011;3:85-91. [10] Gode F, Gumuş C, Zor M. Investigations on the physical properties of the polycrystalline ZnS thin films deposited by the chemical bath deposition method. J Crys Growth 2007;299:136-141. [11] 2002 JCPDS Card No.75-0741 v. 2.3. [12] Zeyada HM, Aziz MS, Behairy AS. Structure formation and mechanisms of DC conduction in thermally evaporated nanocrystallite structure ZnIn2Se4 thin films. Physica B 2009;404:3957-3963. [13] Cullity BD, Stock SR Elements of X-ray Diffraction. Prentice Hall: Pearson; 2001. [14] Tanusevski A. Optical and photoelectric properties of SnS thin films prepared by chemical bath deposition. Semicond Sci Technol 2003;18:501-505. [15] Alberti G, Torracca E, Conte A. Stoicheiometry of ion exchange materials containing zirconium and phosphate. J Inorg Nucl Chem 1966;28:607-613. [16] Kuantama E, Han DW, Sung YM, Song JE, Han CH. Structure and thermal properties of transparent conductive nanoporous. F:SnO2 films Thin Solid Films 2009;517:4211-4214. [17] Becker WG, Bard AJ. Photoluminescence and photoinduced oxygen adsorption of colloidal zinc sulfide dispersions. J Phys Chem 1983;87:4888-4849. [18] Vaksman YF, Nitsuk YA, Purtov YN, Shapkin PV. Native and impurity defects in ZnSe:In single crystals prepared by free growth. Semiconductors 2001;35:883-889.
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