Spray Pyrolized Copper Indium Gallium Sulfide Abosrober Layers for Thin Film Solar Cells Nurdan Demirci Sankır
Erkan Aydın, Esma Uğur
Department of Materials Science and Nanotechnology Engineering TOBB University of Economics and Technology Ankara, Turkey
[email protected]
Micro and Nano Technology, Graduate Program TOBB University of Economics and Technology Ankara, Turkey
[email protected],
[email protected]
Abstract—In this study, copper indium gallium sulfide (CuIn1-xGaxS2 ) films was deposited on glass substrates using ultrasonic spray pyrolysis technique (USP). The Ga/(In+Ga) molar ratio in the precursor solutions has been tailored to obtain the stoichiometric films. Chalcopyrite structure of the films was confirmed by XRD analysis. High absorption coefficient values have been obtained for all samples. It has been observed that the optimum Ga/(In+Ga) ratio in precursor was 0.5 for the bestperformed CuInGaS2 thin films. Keywords-copper indium gallium sulfide, ultrasonic spray pyrolysis, chalcopyrite film, thin film solar cells, impact nozzle
I.
INTRODUCTION
Thin film solar cells based on chalcopyrite absorbers have been predicted as one of the most promising candidates for next generation photovoltaic technologies [1]. There are two subclasses of the chalcopyrite compound family, which are based on the use of elements from group I, III and VI or II, IV and V of the periodic table. Among this family, CuIn1-xGaxS2 type chalcopyrite absorbers offer unique advantages such as band gap control, high efficiency and stability [2-4]. Moreover, reduced toxicity compare to the selenium containing absorbers and variety of manufacturing techniques made this material very attractive for large area applications. Numerous researchers have been focused on cost effective manufacturing techniques of thin film chalcopyrite solar cells such as electrodeposition and spray pyrolysis [5,6]. However, there is very limited information in literature about spray pyrolysis of CuIn1-xGaxS2 absorbers. In this study, USP method has employed to obtain CIGS2 absorbers. USP is one of the atmospheric manufacturing techniques, which is very simple, and therefore, very cost effective. It is possible to tailor the optical and electrical properties of the thin films by chaining the molarity of the precursor solution. With this motivation effects of the gallium replacement with indium on the physical properties of the CuIn1-xGaxS2 films have been investigated in this study. II. EXPERIMENTAL CuIn1-xGaxS2 thin films have been deposited on glass using Sono-Tek FlexiCoat USP System. Aqueous precursor solutions were prepared using copper (II) chloride-dehydrate (CuCl2,
Sigma-Aldrich), indium (III) chloride (InCl3, Acros Organics), gallium (III) chloride (GaCl3, Sigma-Aldrich) and thiourea (NH2CSNH2, Acros Organics) as a copper, indium, gallium and sulfur source, respectively. Ga/(In+Ga) ratio in films was controlled by varying Ga/(In+Ga) ratio of precursors from 0.3 to 0.7 while keeping the Cu and S molarities constant at 4.5 mM and 13.5 mM, respectively. The crystal structure of the films was confirmed using Rigaku Miniflex X-ray diffractometer (XRD) (CuKα, λ= 1.5405 Å). The surface morphology of the films was investigated by FEI, Quanta 200 FEG scanning electron microscopy (SEM). Elemental composition of the films was determined by Energy Dispersive X-Ray Analysis (EDX) at 15 kV accelerating voltage. The optical transmittance was recorded in wavelength range of 2002000 nm. Sheet resistivity of the films was measured by Lucas Lab 4-point probe system equipped with Keithley 2400 I-V source measure system. III.
RESULTS AND DISCUSSIONS
A. Structural Properties SEM analysis has been used to investigate the surface morphology of the CuIn1-xGaxS2 films. As can be seen Fig1 all films were pin-hole and crack free. However, some agglomerated shapes in micron size were observed on the surface of the films. Cross sectional SEM studies showed that, thickness of the CuIn1-xGaxS2 films increased by increasing the Ga content in solution (Table 2).
Fig1. SEM micrographs of ultrasonically sprayed CuIn1-xGaxS2 films.
EDX has been used to determine the chemical structure of the CuInGaS2 thin films (Table 1). All sprayed films except x=0.5 one were sulfur rich. Cl contaminations up to 4.4% have been detected for all sprayed films. Presence of chlorine is related with the use of chloride-based precursors for deposition. Also oxygen contamination up to 42% has been detected. This could be attributed to the secondary phase formation across the profile of the films. Targeted Ga/(In+Ga) stoichiometry for CIGS2 films has been obtained for equal amount In and Ga containing precursor solutions. This sample also showed lowest O contamination compare to the other samples. It has been also observed that Cu concentration has been increasing on agglomerated areas. Most probably Cu atoms diffused through to surface of the films resulted the Cu rich secondary phases on the surface. TABLE I ELEMENTAL COMPOSITION FROM EDX OF AS-DEPOSITED CUIN1-XGAXS2 FILMS. x 0.3 0.4 0.5 0.6 0.7
Cu (At%) 13.7 13.7 19.3 14.5 11.2
In (At%) 15.0 14.1 11.0 10.0 6.7
Ga (At%) 4.4 5.5 10.8 8.3 8.5
S (At%) 36.7 38.6 36.5 33.3 27.5
O (At%) 26.6 23.9 19.1 29.5 42.1
Cl (At%) 2.8 3.3 3.3 4.4 4.1
Fig2 shows the XRD patterns of the ultrasonically sprayed CuIn1-xGaxS2 films. Typical X-ray diffraction peaks of chalcopyrite structure have been obtained regardless of chosen x values. The observed X-ray diffraction peaks at 2θ=28.1°, 35.4°, 46.6°, and 55.3° could be indexed to the (112), (004)/(200), (204)/(200), and (116)/(312)/(215) reflections of the CuInGaS2 crystal structure [7]. XRD analysis also revealed that the center of (112) peak had a tendency to increase as Ga incorporation increased up to targeted x=0.5 value.
Fig2. XRD of ultrasonically sprayed CuIn1-xGaxS2 thin films
Well known Debye-Scherrer formula was used to calculate the crystallite size of the films using peaks belong to (112) plane; (1)
where d is the crystallite size; λ is the X-ray wavelength used; β is the angular line width of half maximum intensity; and θ is the Bragg’s angle. As maintained by these calculations all sprayed films have nanocrystalline nature (Table2). TABLE II SOME STRUCTURAL, OPTıCAL AND ELECTRICAL PROPERTIES OF CUIN1-XGAXS2 FILMS. Film Thick. (μm)
Eg (eV)
0.3
1.60
1.49
5.73
6.40 x10
4
0.4
1.95
1.49
5.16
5.76 x10
4
0.5
1.99
1.50
4.14
1.00 x10
4
0.6
2.09
1.49
4.15
1.51 x10
3
0.7
2.14
1.54
6.07
2.06 x10
3
x
Mean Cryst. Size (nm)
ρs (Ω/☐ )
B. Optical and Electrical Properties Optical transmission data was used to determine absorption coefficient (α) and band gap (Eg) values for sprayed CuInGaS2 films. Extrapolation of the linear region of (αhν)2 versus hν graph obtained the Eg of the films (Fig3). Eg of the films were around 1.50 eV. These results are very close to targeted band gap value of CuInGaS2 absorbers. α vs. hν plots of sprayed films revealed that when Ga content in solution increased α of the films has a tendency to decrease.
Fig3. (αhν)2 vs (hν) plots of CuIn1-xGaxS2 thin films (inset; variation of absorption coefficient as a function of photon energy)
Sheet resistivity (ρs) measurements of the films were done via 4-point probe technique. As can be seen in Fig4 when the Ga/(In+Ga) ratio in solution increased ρs of the film increased. Increasing on ρs values has a linear correlation with increasing Ga/(In+Ga) content in films.
ACKNOWLEDGEMENT This study was supported by The Scientific and Technological Research Council of Turkey under the research Grant TBAG-110T326. REFERENCES [1] A. Jager-Waldau, “Progress in chalcopyrite compound semiconductor research for photovoltaic applications and transfer of results into actual solar cell production”, Solar Energy Materials & Solar Cells (2011), 95(6), 1509-1517,
Fig4. Variation of Ga/(In+Ga) ratio in films and sheet resistivity of CuInGaS2 films as a function of Ga/(In+Ga) ratio in solution
IV. CONCLUSIONS CuInGaS2 thin films have been successfully deposited on soda lime glass substrates using very low amount of spraying solution. Structural, optical and electrical properties were studied using XRD, SEM, EDX, UVVIS-NIR, 2-point and 4point measurements. For sprayed films, targeted ratio of Ga/(In+Ga)=0.5 was found for equal amount In and Ga containing solutions.
[2] P. S. Vasekar, Anant H. Jahagirdar, Neelkanth G. Dhere, “Photovoltaic characterization of Copper–Indium–Gallium Sulfide (CIGS2) solar cells for lower absorber thicknesses,” Thin Solid Films vol. 518, pp.1788–1790, 2010. [3] R. Kaigawa, A. Neisser, R. Klenk, M. C. Lux-Steiner, “Improved performance of thin film solar cells based on Cu(In,Ga)S2,” Thin Solid Films, vol. 415, pp. 266-271, 2002. [4] R. Kaigawa, D. M. Souza, Y. Satake, R. Klenk, “Dependence of the Properties of Cu(In,Ga)S2/Mo Films Prepared by Two-Stage Evaporation Method on Degree of Vacuum during Deposition”, Japanese Journal of Applied Physics 51 (2012) 10NC17. [5] F. Long, W. Wang, J. Du, Z. Zou, “CIS(CIGS) thin films prepared for solar cells by one-step electrodeposition in alcohol solution”, Journal of Physics: Conference Series 152 (2009) 012074 [6] T. Ryo, D-C Nguyen, M. Nakagiri, N. Toyoda, H. Matsuyoshi, S. Ito, “Characterization of superstrate type CuInS2 solar cells deposited by spray pyrolysis method”, Thin Solid Films 519 (2011) 7184–7188 [7] S. Y. Kim, J.H. Kim, “Fabrication of CIGS Thin Films by Using Spray Pyrolysis and Post-selenization,” Journal of the Korean Physical Society, vol. 60, no. 12, pp. 2018-2024, 2012.
