Microsoft Word - 300-308 300 | Physics 2014) عام 3(العدد 27المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (3) 2014 Effect of Thickness on the Electrical Conductivity and Hall Effect Measurements of (CIGS) Films Bushra K.H.Al-Maiyaly Iman.H.Khudayer Ali H. Abd Alrazak Dept. of physics/ College of Education For Pure Science (Ibn Al-Haitham)/ University of Baghdad Received in:27May2014 Accepted in:29September 2014 Abstract The influence of different thickness (500, 1000, 1500, and 2000) nm on the electrical conductivity and Hall effect measurements have been investigated on the films of copper indium gallium selenide CuIn1-xGaxSe2 (CIGS) for x= 0.6.The films were produced using thermal evaporation technique on glass substrates at R.T from (CIGS) alloy. The electrical conductivity (σ), the activation energies (Ea1, Ea2), Hall mobility and the carrier concentration are investigated and calculated as function of thickness. All films contain two types of transport mechanisms of free carriers, and increase films thickness was fond to increase the electrical conductivity whereas the activation energy (Ea) would vary with films thickness. Hall Effect analysis results of CIGS films show all films were (p-type) and both Hall mobility and the carrier concentration increase with the increase of films thickness. Key words: CIGS, Films, Electrical conductivity, Hall Effect, Thermal Evaporation. 301 | Physics 2014) عام 3(العدد 27المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (3) 2014 Introduction Recent environmental and energy resource have an increased interest in renewable energy sources such as photovoltaic devices. CuIn1-xGaxSe2 (CIGS) /CdS heterojunction devices are promising candidates for these applications when produced as polycrystalline materials. [1] In yield solar cells among the various types of thin film based solar cells available in the market, CIGS film solar cells have been considered to be the most promising alternatives to crystalline silicon solar cells because of their high solar to electricity conversion efficiency up to 20.3%., reliability, radiation hardness ,and exhibit excellent stability [2,3,4]. CIGS is used as the absorber layer which is the most important layer in the PV device, reducing the thickness of absorber layer is the another approach for reducing the overall cost of the solar cell, decreasing the thickness of CIGS layer below 1 micron could lead to reduction in the production cost, with no, or only minor, loss in performance. [2] Cu (In,Ga)Se2 is a compound semiconductor exhibiting chalcopyrite crystal structure having direct band gap , large absorption coefficient (α~105 cm-1)[5,6],a moderate surface recombination velocity and radiation resistance[7].These properties give an opportunity for the fabrication of low cost, stable and high efficiency thin film solar cells[7,8]. CIGS is formed by partial substitution of In by group III element Ga. The band-gap can be changed by changing x in CuIn1-xGaxSe2. CIGS is a self-doped (intrinsically doped) material, which means that, when the compound is formed, it automatically becomes either p- or n-type, depending upon the elemental composition present in the bulk and the surface of the film. Absorber p-type CIGS has large concentration of holes, excess electron hole pairs are generated by the light that is absorbed by the CIGS layer. [2, 6] The primary intrinsic defects, which are also called native defects, include copper vacancies (VCu), copper-on-indium of gallium (CuIn/Ga) antisites, which produce acceptor type defects, while indium/gallium-on-copper antisites (In/GaCu), and selenium vacancies (VSe), give rise to donor-type defects. [2] Several methods of deposition techniques have been used to prepare CIGS films, such as co-evaporation [9,10], sputtering techniques [11], sequential evaporation and selenization [12], closed space vapor transport [13] and spray pyrolysis [14]. Even though these methods result in highly efficient CIGS solar cells, they generally require initially high capital investment as well as maintenance capital expense [2]. Many research groups show the effective performance of the CIGS based solar cell, by depositing the CIGS layer using somewhat less expensive deposition methods like flash evaporation[2], thermal evaporation[15,16], screen printing of nano particles [17], electro deposition[8],rapid thermal processor[18,19],resistive heating [5],electron- beam evaporation[20],etc. In this research the electrical conductivity, Hall mobility and the carrier concentration of CuIn1-xGaxSe2 (CIGS) films for x= 0.6 are studied by varying films thickness from 500 nm to 2000 nm, which are prepared by thermal evaporation technique. Experiment CuIn1-xGaxSe2 (CIGS) films for x= 0.6 of different thickness (500, 1000, 1500, and 2000) nm were prepared by the alloy which obtained by fusing the mixture of the appropriate quantities of the elements Cu, In, Ga and Se of high purity (99.999%) in evacuated fused quartz ampoules, heated at (1273 K) for five hour. CIGS films were prepared onto a glass slide substrate by thermal evaporation technique in a high vacuum system of (3x10-6) torr using Edward coating unit model (E 306) from molybdenum boat. The distance from molybdenum boat to substrate was about (15 cm), the deposition rate was about (5 nm/sec) for all the films in room temperature. Al electrodes were used as contact material for making the electrical connections. 302 | Physics 2014) عام 3(العدد 27المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (3) 2014 For D.C. measurement Keithly model 616 has been used to measure the variation of electric resistance (R) with temperature range (298-503) K, then calculated the resistivity (ρ) by the formula [21]:- L tbR   ……………………………………… (1) Where t is film thickness, b is electrodes width; L is distance between two Al electrodes. The conductivity (σ) is related to the resistivity by equation [21]:-   1 c.d ………………………………………… (2) Carrier concentrations and Hall mobility were calculated from resistivity and Hall voltage using equations below. Hall coefficient (RH) is determined by measuring the Hall voltage (VH) that generates the Hall field across the sample of thickness (t) as a function of current (I) at constant magnetic field (0.11) Tesla using Keithly model (616) according to the relation: [22] RH = VH.t / I.B …………………. (3) Where B= magnetic field The carrier concentration is related to the Hall coefficient by equation: [22,23] nH = ±1 / RH.e ………………………… (4) The sign of the Hall coefficient of semiconductor is determined by the sign of the charge carriers. If the conduction is due to one carrier type, we can measure the mobility according to the relation: [23] μH = σ / nH .e = σ . │RH│……………….. (5) Where σ is the conductivity. The thickness of the prepared all films was measured by using the weighing method according to the following relation[24]: t= m / A .ρ …………………. (6) Where: t= film thickness, m = mass of film, ρ = density of film, A= films area. Using a sensitive balance whose sensitivity of the order (10-4) gm. Results and Discussion We can deduce from the variation in the resistivity of CuIn1-xGaxSe2 (CIGS) films for x= 0.6 which grown at R.T as a function of thicknesses (500, 1000, 1500, and 2000) nm, in Fig. (1) that the resistivity values decrease as the thickness increases due to the improvement in the films structure yields more packing density. We believe that the increase in film thickness (t) reduce dangling bonds, defects like vacancy sites, trapping centers of charge carriers and point defect cluster in the films structure, this is, perhaps, because of the decreased grain boundary scattering, therefore the resistivity of the films decrease from (4.87 ×102 Ω .cm to 1.11 Ω .cm) as the thickness increases with the improvement in the electrical conductivity σ (Fig. 2). Figure (2) shows a plot of lnσ of CIGS films versus 103/T for different thickness, the activation energy of the electrical conduction can be determined. It is clear from this figure and Table 1, that the electrical conductivity increases as the film thickness increases because of the increase number of carriers available for transport for the same reasons as we mentioned before, such observations have also seen by ref. [2]. We can notice from figure (2) that all (CIGS) films have two mechanisms for electrical conductivity which means that there is two mechanism of transport of free carriers with two values of activation energy (Ea1, Ea2) each one predominating in a different temperature ranges. The electrical conductivity of these films is affected by the transport of free carriers in extended states beyond the mobility edge at higher temperature range (403-473) K,as well as carriers excited into the localized states at 303 | Physics 2014) عام 3(العدد 27المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (3) 2014 the edge of the band and hopping at other range of temperature (278-393) K, such observations have also seen by ref.[25] . As given in Fig (3) and Table 1, the activation energy varies as the thickness increases and this is because of the improved crystal linty with the increase of the grain size. The Hall coefficient values (RH)were calculated for CIGS films of different thickness from equation(3) which is used to determined carrier density(nH) and Hall mobility (μH ) by using equation (4),(5) respectively. All CIGS films exhibit p-type conductivity, the sign of Hall coefficient for all prepared films is positive, which means that the type of conduction was p- type, i.e. holes are majority charge carriers in the conduction process, this result is in agreement with refs [1,2,6,18].In addition to that the carrier concentration into the order of 1017cm-3 is in good agreement with refs[1,2,6], as well as the value of carrier mobility's in contrast with result obtained by ref.[26] which is found in the range 0.02–0.05 cm2/Vs .All these parameters are shown in Table 2. It is clear from figures (4,5) and Table 2.that both ( nH ) and (μH ) respectively increase with the increase of thickness. This behavior can be attributed to the decrease the trapping centers of charge carriers with the increase of film thickness, this is, perhaps, because of the decreased grain boundary scattering which limits the mobility in thinner films and this is because of the improved film structure, reduce native defect centers and grain boundary defects, therefore the carrier mobility improves. Conclusion In research, we have used thermal evaporation method to study the effect of thickness on the electrical properties of CuIn1-xGaxSe2 (CIGS) films for x= 0.6, through measurements of conductivity and Hall Effect. From the obtained results of the present work, we conclude the following: 1-A thermal evaporation was a good method to prepare (CIGS) film at R.T from alloy. 2-The electrical conductivity and activation energies of CIGS films are seen to be dependent on the film thickness, the electrical conductivity shows as an increase of behavior with an increase of thickness. 3. The behavior of the electrical conductivity of CIGS films as a function of thickness is a result of the community between two mechanisms of transport, hopping charge transport between localized states at the edge of the band  at low temperature  (278-393) K and charge transport to extended state beyond the mobility gap at higher temperature (403-473) K. 4- The resistivity of these films is small; therefore these samples can be used as an absorber layer in the fabrication of solar cell. 5-Hall effect measurements confirmed that holes were predominating in the conduction process. Both the mobility and concentration of the charge carriers increase with the increase of thickness. References 1. David,J. Schroeder;Jose, Luis Hernandez; Gene, D. Berry; and Angus, A. Rockett, (1998),Hole transport and doping states in epitaxial CuIn1-xGaxSe2, J. Appl. Phys., 83,3,1519-1526. 2. Ray,J.R; Desai M.S.; Panchal,C.J; Bharati Rehani, Mehta,P.K., (2013),Effect of Rapid Thermal Annealing of CIGS thin films as an Absorber Layer," Journal Of Nano- and Electronic Physics, 5, 2, 02013(7pp) . 3.AnjunHan,Yi Zhang,Wei;Song;Boyan Li; Wei Liu and Yyn Sun, (2012),Structure ,Morphology and Properties of thinned Cu(In,Ga)Se2 films and solar cell, Semicond.Sci.Technol., 27,035022(8pp). 304 | Physics 2014) عام 3(العدد 27المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (3) 2014 4. Ray,J.R; Panchal,C.J; Desai,M.S;and Trivedi,U.B. (2011),Simulation of CIGS thin film solar cells using amps-1D, J. Nano- Electron. Phys., 3(1): 747-754. 5. Athar JAVED, (2007), Preparation and Study of the Structural, Optical and Electrical Properties of Cu(In,Ga)Se2 thin films, Turk J Phys, 31, 287 – 294. 6. Mahmud Abdul Matin Bhuiyan, Mohammad Shafkat Islam, and Amit Jyoti Datta,(2012),Modeling, Simulation and Optimization of High Performance CIGS Solar Cell," International Journal of Computer Applications,57,16,26-30. 7. Habibe BAYHAN, A. and Sertap KAVASO_GLU, (2003), Admittance and Impedance Spectroscopy on Cu(In,Ga)Se2 solar cell , Turk J Phys", 27, 529 - 535. 8.Fei LONG; Weiming WANG; Jingjing DU,and Zhengguang ZOU, (2009),CIGS thin films prepared for solar cells by one-step electrodeposition in alcohol solution,"Journal of Physics: Conference Series,152 ,012074. 9. Pyuck-Pa Choi; Oana Cojocaru-Mirédin; Roland Wuerz, and Dierk Raabe, (2011), Comparative atom probe study of Cu(In,Ga)Se2 thin-film solar cells deposited on soda-lime glass and mild steel substrates , J. Appl. Phys., 110, 124513 . 10. Jackson,P; Hariskos,D; Lotter,E; Paetel,S; Wuerz,R; Menner,R; Wischmann,W; and Powalla ,M., (2011) , New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%,Prog. Photovolt:Res. Appl., 19, 894. 11. Kihwan Kim; Hanket,G.M.; Huynh,T.; and Shafarman,W.N., (2012),Three-step H2Se/Ar/H2S reaction of Cu-In-Ga precursors for controlled composition and adhesion of Cu(In,Ga)(Se,S)2 thin films, J. Appl. Phys., 111, 083710. 12. Marudachalam,M.; Birkmire,R.W.; Hichri,H.; Schultz,J.M.; Swartzlander,A. M.M. Al- Jassim, (1997), Phases, Morphology and Diffusion in CuInxGa1-xSe2 Thin Films, J. Appl.Phys., 82, 2896. 13. Bouloufa,A.; Djessas,K;and Todorovic, D., (2009), Structural and optical properties of Cu(In,Ga)Se2 grown by close-spaced vapor transport technique, Mat. Sci. Semicon.Proc., 12, 82. 14. Coutts,T.J.and Meakin,J.D. (1990), Current Topics in Photovoltaics: 4, New York: Academic Press:. 15. Mohammad Istiaque Hossain, (2012),Fabrication and Characterization of CIGS solar cells with In2S3 Buffer Layer Deposited by PVD Technique, Chalcogenide Letters ,9,5, 185 – 191, May. 16. Romeo,A.; Gysel,R.; Buzzi,S; Abou-Ras, D; Bätzner,D. L; Rudmann, D; Zogg,H;and Tiwari A. N. ,(2004),Properties of CIGS solar cell developed with evaporated II-VI buffer layers, Technical Digest of the International PVSEC-14, Bangkok, Thailand. 17. Faraj,M.G.; Ibrahim, K.and Salhin,A. (2011), Investigation of CIGS Solar Cells on Polyethylene Terephthalate Substrates, Int. J. Polym. Mater.,60, 817. 18. Bo rner,F.; Gebauer,J.; Eichler,S.; Krause-Rehberg,R.; Dirnstorfer,I.; Meyer,B.K.and Karg,F. (1999),Defects in CuIn(Ga)Se2 solar cell material characterized by positron annihilation: post-growth annealing effects, Physica B, 273-274, 930-933. 19- Laura, E.; Slaymaker; Nathan, M.; Hoffman; Matthew A; Ingersoll, Matthew R; Jensen1; Jiří Olejníček; Christopher L;Exstrom, Scott A; Darveau; Rodney J; Soukup, Natale J; Ianno,Amitabha Sarkar; and Štěpán Kment, (2011),Properties of CuIn1-xGax Se2 films prepared by the rapid thermal annealing of spray deposited CuIn1-xGaxS2 and Se2, Mater. Res. Soc. Symp. Proc., 1324 © Materials Research Society,DOI: 10.1557/opl.2011.1152 20-Zhao-Hui Lia; Eou-Sik Choa; Sang Jik Kwona; and Mario Dagenaisb, (2011),Fabrication of Cu(In,Ga)Se2 Thin Films by Selenization of Stacked Elemental Layer with Solid Selenium, "ECS Transactions - Boston, MA" 16, "Photovoltaics for the 21st Century 7",to be published in September. 305 | Physics 2014) عام 3(العدد 27المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (3) 2014 21. Kasap, S.O. (2002), Principles of Electronic Materials and Devices, 2nd edition, Mc Graw Hill. 22- Kazmerski, L. L. (1980), Polycrystalline and Amorphous Thin Films and Device, Academic Press. 23- William, D. Callister, Jr. (2003), Materials Science and Engineering, An Introduction, 6th edition, John Wiley & Sons, Inc. 24- Donald A.Neamen ,(2003), Semiconductor Physics and Devices, basic principles,3th edition, McGraw-Hill Companies,Inc. 25- Mesa,F.; Calderon,C.; and Gordillo, G. (2010), Study of electrical properties of CIGS thin films prepared by multistage processes, Thin Solid Films,518,1764-1766. 26- Dinca,S.A; Schiff,E. A; Shafarman,W. N; Egaas, B; Noufi,R. and Young D. L., (2012),Electron drift mobility measurements in polycrystalline CuIn1-xGax Se2 solar cell, Applied Physics Letters, 100, 103901. Table No. (1) : The electrical conductivity and activation energies of CIGS films at different thickness Table No. (2): Values of Carrier Concentration and Carrier Mobility for CIGS Films with different thickness Thickness (nm) Carrier Concentration ) 3-( cm Hn Carrier Mobility /v. s.)2(cm Hμ 500 3.9 E+17 0.033 1000 5.4 E+17 0.142 1500 5.8 E+17 0.993 2000 1.8 E+18 3.152 Films thickness (nm) R.T  10-3 (.cm)-1 Ea1 Tem. range Ea2 Tem. range ( eV ) ( K ) ( eV ) ( K ) 500 2.0519 0.013 278-393 0.147 403-473 1000 12.205 0.033 278-393 0.317 403-473 1500 92.165 0.035 278-393 0.32 403-473 2000 898.206 0.0113 278-393 0.058 403-473 306 | Physics 2014) عام 3(العدد 27المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (3) 2014 Figure No.(1): Variation resistivity of CIGS films of different thickness Figure No.(2): Variation lnσ versus 103/T as a function of thickness for CIGS films Figure No.(3): Variation activation energies as a function of thickness for (CIGS) films -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 1.5 2 2.5 3 3.5 4 ln  (o h m .c m )- 1 1000/T (K)-1 t= 500 nm t= 1000 nm t= 1500 nm t=2000 nm 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0 500 1000 1500 2000 2500 A c ti v a ti o n E n e rg y ( e V ) Thickness (nm) Ea2 Ea1 0.00E+00 1.00E+02 2.00E+02 3.00E+02 4.00E+02 5.00E+02 6.00E+02 0 500 1000 1500 2000 2500 R e si st iv it y (  o h m .c m ) Thickness (nm) 307 | Physics 2014) عام 3(العدد 27المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (3) 2014 Figure No.(4): Variation of the charge Carrier’s concentration For CIGS films of different thickness Figure No. (5): Variation Hall mobility as a function of thickness for CIGS films 0 2E+17 4E+17 6E+17 8E+17 1E+18 1.2E+18 1.4E+18 1.6E+18 1.8E+18 2E+18 0 500 1000 1500 2000 2500 n H ( c m ) -3 Thickness (nm) 308 | Physics 2014) عام 3(العدد 27المجلد مجلة إبن الھيثم للعلوم الصرفة و التطبيقية Ibn Al-Haitham Jour. for Pure & Appl. Sci. Vol. 27 (3) 2014 )CIGS( وقياسات تأثير ھول ألغشيةتأثير السمك على التوصيلية الكھربائية بشرى كاظم حسون الميالي ايمان حميد خضير علي حسين عبد الرزاق / جامعة بغداد) بن الھيثم ا الصرفة ( للعلوم كلية التربيةقسم الفيزياء / 2014ايلول 29البحث :قبل 2014ايار27استلم البحث: الخالصة التوصيلية الكھربائية وقياسات تأثير ھول في nm (and 2000 ,1500 ,1000 ,500)حسب تأثير االسماك المختلفة االغشية باستعمال تقنية التبخير ت. حضرx= 0.6 ,إذ 2SexGax-1CuIn (CIGS) ألغشية نحاس انديوم كاليوم سيلينيوم .(CIGS)الحراري على ارضيات من الزجاج عند درجة حرارة الغرفة من سبيكة وتحركية ھول وتركيز الحامالت دالة لتغير السمك. Ea1(Ea ,2( وطاقات التنشيط (σ)التوصيلية الكھربائية تحسب بينما تنوعت ,حامالت الشحنة ولوحظ زيادة التوصيلية الكھربائيةوقد أظھرت كل االغشية آليتين لالنتقال االلكتروني ل كانت من نوع CIGSطاقات التنشيط بزيادة سمك االغشية المحضرة . وبينت نتائج قياسات تأثير ھول ان جميع أغشية (p-type) .كما لوحظ زيادة كل من تركيز وتحركية حامالت الشحنة بزيادة سمك االغشية . تأثير ھول , التبخير الحراري.، التوصيلية الكھربائية ، اغشية، CIGS -: المفتاحيةالكلمات