Omar Meglali, Nadhir Attaf, Assia Bouraiou, Mohamed Salah Aida
Sciences and Technologies Faculty, Ziane Achour University, Djelfa, Algeria.
Sciences Faculty, Mohamed Boudiaf University, M’sila, Algeria.
Thin Films and Interfaces Laboratory, Department of Physics, Mentouri University, Constantine, Algeria.
DOI: 10.4236/msa.2013.411089   PDF    HTML   XML   4,422 Downloads   6,775 Views   Citations

Abstract

Cu(In, Al)Se₂ thin films were prepared by electrodeposition from the aqueous solution consisting of CuCl₂, InCl₃, AlCl₃ and SeO₂ onto ITO coated glass substrates. The as-deposited films were annealed under vacuum for 30 min at temperature ranging between 200°C and 400°C. The structural, composition, morphology, optical band gap and electrical resistivity of elaborated thin films were studied, respectively using x-ray diffraction, energy dispersive analysis of x-ray, scanning electron microscopy, UV spectrophotometer and four-point probe method. The lattice constant and structural parameters viz. crystallite size, dislocation density and strain of the films were also calculated. After vacuum annealing, x-ray diffraction results revealed that all films were polycrystalline in nature and exhibit chalcopyrite structure with (112) as preferred orientation. The film annealed at 350°C showed the coexistence of CIASe and InSe phases. The average crystallite size increases linearly with annealing temperature, reaching a maximum value for 350°C. The films show a direct allowed band gap which increases from 1.59 to 1.78 eV with annealing temperature. We have also found that the electrical resistivity of films is controlled by the carrier concentration rather than by their mobility.

Keywords

Electrodeposition; Cu(In, Al)Se₂; Thin Film

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1. Introduction

One of the main challenges in photovoltaic research is the development of lower cost and higher conversion efficiency devices. This primary objective can be achieved by using tandem solar cell structures. For the thin film tandem cell, the top cell requires a wide band gap absorber layer with optical band gap in the range 1.7 - 1.9 eV whereas the bottom cell requires an absorber layer with a band gap around 1.1 eV [1]. Fabrication of thin film tandem cell gets relatively simplified if the desired band gap semiconductor absorber layers for the top and bottom cells can be obtained from a single ternary or quaternary alloy system by varying the alloy composition suitably [1].

Cu(In, Al)Se₂, abbreviated as CIASe, is considered as an quaternary interesting absorber material for the wide band gap cell in the thin film tandem structures, because it requires smaller relative alloy concentration than gallium (Cu(In, Ga)Se₂) or sulphur (CuIn(S, Se)₂) alloys to achieve comparable band gap. Furthermore, the aluminum is a much cheaper and abundant material [2]. The optical band gap of Cu(In_1−xAl_x)Se₂ semiconductor can be controlled from 1 eV (for x = 0) to 2.7 eV (for x = 1) by the partial replacement of indium by aluminum [3,4]. The conversion efficiency has already exceeded 16.9% using CIASe material as absorber layer in solar cell [5]. These progresses open up new perspectives.

The CIASe thin films have been prepared by various methods such as chemical vapor transport [6], evaporation [1,7], selenization of evaporated precursors [8] chemical bath deposition [1], pulsed laser deposition [9], electrodeposition [10], etc. Amongst them, electrodeposition is an appealing technique that offers low-cost equipment, high deposition speed and possibility of large-area polycrystalline films deposition. In the present work, Cu(In, Al)Se₂ films have been deposited on the indium tin oxide (ITO) coated glass substrate by one step electrodeposition process using two electrodes system. After vacuum annealing of the as-deposited samples, they have been characterized by using XRD, SEM, optical absorption and electrical resistivity. The relationship between the properties of the Cu(In, Al)Se₂ films and the annealing temperature is also studied.

2. Experimental

In the present work, electrochemical experiments were carried out in a simple two-electrode cell configuration with a indium tin oxide (ITO) coated glass substrate as a working electrode (area 5 mm × 15 mm), and a palatine sheet as the counter electrode. The electrode position bath containing 10 mM CuCl₂, 20 mM InCl₃, 20 mM AlCl₃ and 20 mM SeO₂ dissolved in de-ionized water. The pH of the solution was adjusted to 2.6. Deposition was carried out at a room temperature, without stirring the solution. The as-deposited films were annealed under vacuum for 30 min at temperature ranging between 200˚C and 400˚C.

The crystalline structure of the resulting films was analyzed by means of x-ray diffractometer using CuK_α radiation of wavelength λ = 1.5418 Å. Since no standard files are available for CIASe material, standard file of CuInSe₂ was used to identify Cu(In, Al)Se₂ elaborated films. The crystallite size C_s, was estimated from the full width at half maximum (FWHM) of the diffraction peak by using Scherr’s formula [11].

The surface morphology of the films was investigated using a scanning electron microscope (SEM). The optical transmittance of the films was recorded using UV-VisNIR spectrophotometer (Shimadzu UV-3101) at room temperature in the wavelength range of 350 - 1500 nm in steps of 2 nm. An identical ITO coated glass substrate was used as reference for recording the optical absorption spectra.

The thicknesses of the films were measured by means of a DEKTAK 3030 surface profile system. Electrical resistivity of the films was determined using four point probe method.

3. Results and Discussion

The x-ray diffraction pattern of the as-deposited film, before annealing, is shown in Figure 1. The XRD spectra appear the peaks correspond to the most intense peaks of ITO phase [12]. It is noted that before annealing we have observed only the XRD patterns related to the ITO phase. This indicates that the as deposited films are amorphous in nature or are composed with micro crystallite that cannot be detected.

The EDAX pattern for the as deposited films is given in Figure 2. The spectrum shows the typical emission lines of copper (CuL), indium (InL), aluminum (AlK) and selenium (SeL) elements in the investigated energy range. On the other hand, the appearance of silicon (SiK), oxygen (OK) and tin (SnL) peaks comes mainly from the

ITO coated glass substrate.

The x-ray diffraction patterns of the elaborated thin films, after annealing at different temperature are shown in Figures 3(a)-(d). All specimens are composed of polycrystalline CIASe and ITO phases, and no distinct peaks particular to CuInSe₂ and CuAlSe₂ are detected. Thereafter, the annealing results in changing CIASe films from amorphous to polycrystalline structure.

The XRD spectra appear the peaks located at 2θ ≈ 26.78˚ and 45.19˚. The present XRD pattern is most satisfactorily indexed on the basis of the chalcopyrite structure of CuInSe₂ phase. The last peaks are the three most intense peaks for CIASe phase in its chalcopyrite structure [13], they correspond respectively to the (112) and (204)/(220) planes. The presences of these most intense reflections confirm the chalcopyrite structure of CIASe material.

Figure 4 shows the XRD patterns of the CIASe films near (112) diffraction peak. As can be seen from this

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	Y. B. K. Reddy and V. S. Raja, “Preparation and Characterization of CuIn0.3Al0.7Se2 Thin Films for Tandem Solar Cells,” Solar Energy Material and Solar Cells, Vol. 90, No. 11, 2006, pp. 1656-1665. doi:10.1016/j.solmat.2005.09.002
[2]	B. Kavitha and M. Dhanam, “In and Al Composition in Na-no-Cu(InAl)Se2 Thin Films from XRD and Transmittance Spectra,” Materials Science and Engineering B, Vol. 140, No. 1-2, 2007, pp. 59-63. doi:10.1016/j.mseb.2007.03.011
[3]	T. Hayashi, T. Minemoto, G. Zoppi, I. Forbes, K. Tanaka, S. Yamada, T. Araki and H. Takakura, “Effect of Composition Gradient in Cu(In,Al)Se2 Solar Cells,” Solar Energy Materials and Solar Cells, Vol. 93, No. 6-7, 2009, pp. 922-925. doi:10.1016/j.solmat.2008.11.007
[4]	D. C. Perng, J. W. Chen and C. J. Wu, “Formation of CuInAlSe2with Double Graded Band Gap Using Mo(Al) Back Contact,” Solar Energy Materials and Solar Cells, Vol. 95, No. 1, 2011, pp. 257-260. doi:10.1016/j.solmat.2010.04.046
[5]	S. Marsillac, P. D. Paulson, M. W. Haimbodi, R. W. Birkmire and W. N. Shafarman, “High-Efficiency Solar Cells Based on Cu(InAl)Se2 Thin Films,” Applied Physics Letters, Vol. 81, No. 7, 2002, pp. 1350-1352. doi:10.1063/1.1499990
[6]	N. Kuroishi, K. Mochizuki and K. Kimoto, “Surface Oxidation of CVT-Grown CuAlSe2,” Materials Letters, Vol. 57, No. 13-14, 2003, pp. 1949-1954. doi:10.1016/S0167-577X(02)01110-2
[7]	Dhananjay, J. Nagaraju and S. B. Krupanidhi, “Structural and Optical Properties of CuIn1-xAlxSe2 Thin Films Prepared by Four-Source Elemental Evaporation,” Solid State Communications Vol. 127, No. 3, 2003, pp. 243-246. doi:10.1016/S0038-1098(03)00389-2
[8]	S. Martin and C. Guillen, “Characterization of Chalcopyrite Cu(In,Al)Se2 Thin Films Grown by Selenization of Evaporated Precursors,” Energy Procedia, Vol. 10, No. 1, 2011, pp. 182-186. doi:10.1016/j.egypro.2011.10.174
[9]	K. H. Kim and Fianti, “Growth of Single-Phase CuInAlSe2 Thin Films by Using Pulsed Laser Deposition and Selenization,” Journal of the Korean Physical Society, Vol. 60, No. 12, 2012, pp. 2001-2006. doi:10.3938/jkps.60.2001
[10]	D. Prasher, R. Sharma, A. Sharma and P. Rajaram, “CuIn1-XAlXSe2 Thin Films Grown By Electrodeposition,” Proceedings of the 55th dae Solid State Physics Symposium, Manipal, 26-30 December 2010, pp. 597-598. doi: 10.1063/1.3605999
[11]	L. I. Malssel and R. Glang, “Handbook of Thin Film Technology,” McGraw Hill Book Company, New York, 1970.
[12]	International Center for Diffraction Data, ICDD, PDF2 Database, File Number 76-0152 for In2O3.
[13]	International Center for Diffraction Data, ICDD, PDF2 Database, File Number 40-1487 for CuInSe2.
[14]	International Center for Diffraction Data, ICDD, PDF2 Database, File Number 73-0609 for InSe.
[15]	J. Muller, J. Nowoczin and H. Schmitt, “Composition, Structure and Optical Properties of Sputtered Thin Films of CuInSe2,” Thin Solid Films, Vol. 496, No. 2, 2006, pp. 364-370. doi:10.1016/j.tsf.2005.09.077
[16]	A. Bouraiou, M. S. Aida, E. Tomasella and N. Attaf, “ITO Substrat Resistivity Effect on the Properties of CuInSe2 Deposited Using Two-Electrode System,” Journal of Material Science, Vol. 44, No. 5, 2009, pp. 1241-1244. doi:10.1007/s10853-009-3252-y
[17]	M. Dhanam, P. K. Manoj and R. R. Prabhu, “High-Temperature Conductivity in Chemical Bath Deposited Copper Selenide Thin Films,” Journal of Cristal Growth, Vol. 280, No. 3-4, 2005, pp. 425-435. doi:10.1016/j.jcrysgro.2005.01.111
[18]	M. Sugiyama, A. Umezawa, T. Yasuniwa , A. Miyama, H. Nakanishi and S. F. Chichibu, “Growth of Single-Phase Cu(In,Al)Se2 Photoabsorbing Films by Selenization Using Diethylselenide,” Thin Solid Films, Vol. 517, No. 7, 2009, pp. 2175-2177. doi: 10.1016/j.tsf.2008.10.083
[19]	E. Halgand, J. C. Bernède, S. Marsillac and J. Kessler, “Physico-Chemical Characterisation of Cu(In,Al)Se2 Thin Film for Solar Cells Obtained by a Selenisation Process,” Proceedings of Symposium O on Thin Film Chalcogenide Photovoltaic Materials, EMRS 2004 Conference, Strasbourg, 24-28 May 2004, pp. 443-446. doi: 10.1016/j.tsf.2004.11.039
[20]	J. Lopez-Garcia, C. Maffiotte and C. Guillen, “WideBandgap CuInx-1AlxSe2 Thin Films Deposited on Transparente Conducting Oxides,” Solar Energy Materials and Solar Cells, Vol. 94, No. 7, 2010, pp. 1263-1269. doi:10.1016/j.solmat.2010.03.022
[21]	F. Urbach, “The Long-Wavelength Edge of Photographic Sensitivity and of the Electronic Absorption of Solids,” Physical Review, Vol. 92, No. 5, 1953, p. 1324. doi:10.1103/PhysRev.92.1324
[22]	H. Moualkia, S. Hariech, M. S. Aida, N. Attaf and L. Laifa, “Growth and Physical Properties of CdS Thin Films Prepared by Chemical Bath Deposition,” Journal of physics D: Applied Physics, Vol. 42, No. 13, 2009, Article ID: 135404. doi:10.1088/0022-3727/42/13/135404
[23]	B. Pejova, “The Urbach—Martienssen Absorption Tails in the Optical Spectra of Semiconducting Variable-Sized Zinc Selenide and Cadmium Selenide Quantum Dots in Thin Film Form,” Materials Chemistry and Physics, Vol. 119, No. 3, 2010, pp. 367-376. doi: 10.1016/j.matchemphys.2009.08.064
[24]	C. de Blasi, G. Micocci, S. Mongelli and A. Tepore, “Large InSe Single Crystals Grown From Stoichiometric and Non-Stoichiometric Melts,” Journal of Cristal Growth, Vol. 57, No. 3, 1982, pp. 482-486. doi:10.1016/0022-0248(82)90062-8
[25]	Y. B. K. Reddy and V. S. Raja, “Preparation and Characterization of CuIn0.75Al0.25Se2 Thin Films by Co-Evaporation,” Physica B, Vol. 381, No. 1-2, 2006, pp. 76-81. doi:10.1016/j.physb.2005.12.256