Chemically Deposited Silver Antimony Selenide Thin Films for Photovoltaic Applications

Transcription

1 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society 1165-M08-25 Chemically Deposited Silver Antimony Selenide Thin Films for Photovoltaic Applications J.G. Garza 1, S. Shaji 1,2, A.M. Arato 1,2, E. Perez-Tijerina 2,3, A. C. Rodriguez 1, T. K. Das Roy 1 and B. Krishnan 1,2 Facultad de Ingenieria Mecanica y Electrica, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Nuevo León, Mexico. 2 CIIDIT- Universidad Autónoma de Nuevo León, Apodaca, Nuevo León, Mexico. 3 Facultad de Ciencias Físico Matemáticas, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Nuevo León, Mexico. ABSTRACT Silver antimony selenide (AgSbSe 2 ) thin films were prepared by heating sequentially deposited antimony sulphide (Sb 2 S 3 ), silver selenide (Ag 2 Se) and Ag thin films in close contact with a selenium thin film. Sb 2 S 3 thin film was prepared from chemical bath containing SbCl 3 and Na 2 S 2 O 3, Ag 2 Se from the bath containing AgNO 3 and Na 2 SeSO 3 and Se thin films from an acidified solution of Na 2 SeSO 3, at room temperature on cleaned glass substrates. Ag thin film was deposited by vacuum thermal evaporation. The annealing temperature was varied from o C in vacuum ( 10-3 Torr) for 1 h. X-ray diffraction analysis showed the films formed at 350 o C was polycrystalline AgSb(S,Se) 2 or AgSbSe 2 depending on selenium thin film thickness. Morphology of these films was analyzed using Atomic Force Microscopy and Scanning Electron Microscopy. The elemental analysis was done using Energy Dispersive X-ray technique. Optical characterization of the thin films was done by optical transmittance spectra. The electrical characterizations were done using Hall effect and photocurrent measurements. A photovoltaic structure: Glass/ITO/CdS/AgSbSe 2 /Ag was formed, in which CdS was deposited by chemical bath deposition. J-V characteristics of this PV structure showed V oc =370 mv and J sc =0.5 ma/cm 2 under illumination using a tungsten halogen lamp. INTRODUCTION Silver antimony selenide (AgSbSe 2 ) belongs to I-V-VI 2 group compound semiconductors having optical absorption coefficient of 10 4 cm -1 with a band gap of ~1 ev [1], and hole mobility of 1500 cm 2 /V-s[1], offering its applications as photovoltaic absorber material. Various methods were reported for the preparation of this material [2-5]. A photovoltaic structure using this material as absorber layer, Glass/SnO 2 :F-CdS-Sb 2 S 3 -AgSbSe 2 -Ag showed V oc = 530 mv and J sc = 1.7 ma/cm 2 [5]. In this case, AgSbSe 2 thin film was formed by annealing of Sb 2 S 3 -Ag layers in contact with chemical bath deposited selenium thin film under nitrogen atmosphere. Preparation of CdS/AgSbSe 2 PV structure by all-chemical bath deposition process using sequentially deposited CdS/Sb 2 S 3 /Ag 2 Se thin films on SnO 2 :F coated glass substrate in contact with selenium thin film [6] was reported. In the present work, we report the formation of AgSbSe 2 thin films by heating a multilayer of Sb 2 S 3 /Ag 2 Se/Ag in contact with a selenium thin film in vacuum. Further, by varying the selenium thickness, AgSb(S,Se) 2 phase was formed. Structure and morphology of these thin films were analyzed using X-ray diffraction, atomic force

2 microscopy (AFM) and scanning electron microscopy (SEM). Elemental analysis was done by energy dispersive X-ray detection employed with the SEM. Optical and electrical characterizations were also done. A photovoltaic structure: Glass/ITO/CdS/AgSbSe 2 /Ag was prepared. The J-V characteristics of this structure showed V oc =370 mv and J sc =0.5 ma/cm 2 under illumination using tungsten halogen lamp. It is to be emphasized that the development of AgSb(S,Se) 2 and AgSbSe 2 thin films using chemical bath deposited Sb 2 S 3, Ag 2 Se and Se precursor thin films, and vacuum evaporated Ag thin films offers a simple, controlled, non-toxic and cost effective process for the PV technology. EXPERIMENT (a) Deposition of Sb 2 S 3 /Ag 2 Se/Ag multilayers: Sb 2 S 3 and Ag 2 Se thin films were deposited sequentially on glass substrates as follows [6]: Sb 2 S 3 thin films: 0.65 g of SbCl 3 was dissolved in 2.5 ml acetone in a 100 ml beaker to which 25 ml of 1 M pre-cooled (10 o C) Na 2 S 2 O 3 solution was added followed by 72.5 ml of deionized water. Well-cleaned microscopic glass substrates were placed horizontally in a Petri dish and the above solution was added to it and maintained at room temperature (25 ºC). After 5 h of deposition, uniform orange color Sb 2 S 3 thin films of 500 nm in thickness were obtained. The thickness was estimated using gravimetric method. Ag 2 Se thin films: In this case, the chemical bath was prepared in a Petri dish by dissolving 100 mg of AgNO 3 in 10 ml of water to which 4 ml ammonia (aq.) solution, 5 ml of 0.1 M Na 2 SeSO 3 and 81 ml of water were added sequentially. Sb 2 S 3 coated glass substrates were placed horizontally in the bath. The duration of deposition was 1h at 25 ºC. Ag layer: % pure silver wire was evaporated onto Ag 2 Se/Sb 2 S 3 /Glass. Various samples with Ag thin films in thickness of 100 Å, 200 Å and 300 Å were prepared. Ag layer thickness was measured using quartz crystal thickness monitor employed in the vacuum deposition system. Selenium thin films: For this, 80 ml of 0.01 M Na 2 SeSO 3 was acidified to a ph of 4.5 by adding 2.2 ml of diluted acetic acid [5]. The selenium thin films formed after 1 h and 3 h of deposition were selected for the heating process. During annealing, the selenium thin film vaporizes to react with Ag/Ag 2 Se/Sb 2 S 3 /Glass multilayers. (b) Heating of Glass /Sb 2 S 3 /Ag 2 Se/Ag Multilayers of Glass/Sb 2 S 3 /Ag 2 Se/Ag were kept in contact with Se thin films and heated in vacuum (4x10-3 Torr) at different temperatures in the range o C for 1 h. Good samples were obtained at 350 o C and these films were selected for further analysis. (c) Preparation of the PV structure: For this, first a CdS thin film (50 nm) was deposited from a solution containing CdCl 2, sodium citrate, NH 4 OH and thiourea [5] on ITO coated glass substrates supplied by Vinkarola Instruments. Subsequently, Sb 2 S 3 /Ag 2 Se/Ag multilayers were deposited on the CdS thin film as explained above. This multilayer structure: Glass/ITO/CdS/Sb 2 S 3 /Ag 2 Se/Ag was thermally annealed in vacuum at 350 o C for 1 h in contact with a selenium thin film. (d) Characterization: X-ray diffraction patterns (XRD) of the thin films were recorded using Siemens D5000 diffractometer. The scan range was (2θ) using Cu Kα 1 radiation (λ= Å). Morphological studies were done using Atomic force microscopy (and scanning electron microscopy (JEOL JSM 6510 LV), elemental analysis using an energy dispersive X-ray detector associated with the SEM. The optical transmittance of the films was measured using Perkin-

3 Elmer UV-VIS- spectrophotometer. Electrical measurements were carried using Keithley 6487 picoammeter/voltage source interfaced with a computer. The contacts used were two planar electrodes of 5 mm in length and 3 mm in separation using silver paint for the dc conductivity measurement. The charge carrier concentration was estimated using a Hall effect system (ECOPIA) in a four probe electrode configuration. The J-V characteristics of the heterostructure under illuminated condition was done on a 2 mm diameter spot using silver paint contact. The light source used was tungsten halogen lamp and the intensity was 870 W/m 2. RESULTS AND DISCUSSION The thin films formed by heating precursor layers of Sb 2 S 3 /Ag 2 Se/Ag in contact with Se thin films at 350 o C in vacuum are denoted as A, B, C and D respectively on the basis of thickness of the evaporated Ag layer and the selenium thin film. A, B and C are with Ag layer in thickness 100 Å, 200 Å and 300 Å, and Se thin film deposited for 1h. Sample D is with Ag layer thickness 100 Å, and selenium layer deposited for 3 h. All these samples are formed with same thickness of Sb 2 S 3 and Ag 2 Se precursor layers. XRD (204) Sb 2 S 3 (200) Sb 2 S 3 Counts (a.u) (002) (400) (512) C B (111) (220) (222) D (002) (31-2) (112) (400) (204) (512) A AgSb(S,Se) 2 monoclinic PDF AgSbSe 2 cubic PDF (111) (200) (220) (222) 311) θ (degree) 2 θ (degree) Figure 1: XRD patterns for samples A, B, C and D. Standard patterns corresponds to AgSb(S,Se) 2 and AgSbSe 2, the major phases present in the thin films. Figure 1 shows XRD patterns recorded for samples A, B, C and D as marked in the figure. The pattern for sample A contains reflections from (002), (400) and (204) planes of monoclinic AgSb(S,Se) 2 (PDF: ) phase present in the film. Also, peaks from (110), (020), (120) planes of orthorhombic Sb 2 S 3 (PDF: ) are present in the film as indicated by shaded circles. For sample B, the peaks present are that of AgSb(S,Se) 2 phase. In this case, (204) peak intensity is relatively higher than (400) as in the standard pattern. Sample C shows peaks corresponding to (002), (400), (204) and (512) reflections from AgSb(S,Se) 2 phase and relative

4 peak intensities are in agreement with the standard pattern. Sample D shows peaks corresponding to cubic AgSbSe 2 phase as marked by the planes of (111), (200), (220) and (222) planes which follow nearly the same intensity pattern as the standard as depicted in the figure. Very weak reflections from un- reacted Sb 2 S 3 phase is also present in the pattern. SEM and AFM (a) Figure 2: (a)se micrographs of samples A, B, C and D with magnification x 15000, the scale shown are 1 µm (b) AFM of sample D A C (b) B D. The morphology of samples A, B, C and D is shown in figure 2(a) by the corresponding SE micrograph. Sample A has nearly spherical particles while samples B and C show nondistinguished particles or grains even though both the sample surfaces are uniform. In the case of selenium rich sample D, spherical grains or particles are observed. Also, this sample shows non-uniformity on the surface with compact grains. AFM image of AgSbSe 2 thin film formed by heating with excess selenium is shown in figure 2(b). Spherical compact grains of average size less than 50 nm are observed in the figure. Grain boundaries are clear in the micrograph. EDX 1E-5 Bias=50 V σ =10-2 (ohm cm) -1 D Current (A) 1E-6 B C σ =5x10-3 (ohm cm) -1 σ =10-4 (ohm cm) -1 Figure 3: EDX spectra for samples A, B, C and D 1E-7 1E-9 A 50 s σ =10-6 (ohm cm) -1 Time (s) Figure 4: photocurrent response for samples A, B, C and D

5 Elemental analysis for a selected area in the SE micrograph is illustrated in figure 3. The presence of Sb, Ag, S and Se are shown in the figure. Additional elements such as Si, O, Na, Mg are from the glass substrates and C is from the graphite conductive coating. As Ag content of the thin films varies from sample A to C, there is a slight increase in the relative intensity of Ag peak showing relatively same Se peak counts. In the case of D, Se peak is relatively high compared to that of other sample confirming the selenium rich AgSbSe 2 as the prominent phase in this film. Electrical properties: Measurements on the d.c conductivity of the thin films under dark and illumination conditions are shown in figure 4. From the figure, it is clear that the dark conductivity increases with Ag layer thickness in the precursor films. For sample A, the value is of the order of 10-6 (Ωcm) -1, for sample B 10-4 (Ωcm) -1 and for sample C 10-3 (Ωcm) -1. All these thin films containing AgSb(S,Se) 2 as major phase posses significant photocurrent response. In the case of selenium rich sample D (AgSbSe 2 phase), conductivity is enhanced by 10 4 times compared to that of selenium deficient sample A without significant change in its photocurrent response. Slight p-type conductivity was shown by samples B and C with carrier concentration in the range of /cm 3. Sample D possessed p-type carriers of the order of /cm 3. The hole mobility for all these films was cm 2 V -1 s -1. Consistent results were not obtained in the case of sample A due to its very high resistance. Optical characterization: Figure 5 shows optical transmittance spectra of samples A, B, C and D in the wavelength range of nm. In sample A, a small hump at 700 nm (1.8 ev) corresponds to the absorption due to Sb 2 S 3 and shows significant transmission in the visible range. Samples B and C, dominant in AgSb(S,Se) 2 phase, have similar transmittance behaviour throughout the specified wavelength range. In these cases, the light absorption begins at nearly 800 nm (1.5 ev) which corresponds to an ideal value for their applications as PV absorber. In the case of sample D, selenium rich AgSbSe 2, shows nearly zero transmittance and hence full absorption of light in the visible range. Also, in this case the light absorption process begins in the near IR region. J (ma/cm 2 ) Voltage (V) -0.4 Figure 5: Transmittance spectra for samples A, B, C and D illumination 870 W/m 2 Figure 6: J-V characteristic of the PV structure: Glass/ITO/CdS/AgSbSe 2 /Ag under illumination

6 Photovoltaic structure: AgSbSe 2 was chosen as the absorber layer for PV structure because of the good electrical and optical properties. Figure 6 shows the J-V characteristics of the PV structure: Glass/ITO/CdS/AgSbSe 2 /Ag. This structure was illuminated from the CdS side using a tungsten halogen lamp. From the J-V curve, V oc = 370 mv and J sc =0.5 ma/cm 2 are obtained. CONCLUSIONS Thin films of AgSb(S,Se) 2 and AgSbSe 2 were formed by heating Sb 2 S 3 /Ag 2 Se/Ag multilayered precursor films in close contact with selenium thin film deposited for 1 h and 3 h respectively. From the studies on electrical and optical properties of these thin films, it is evident that by varying Ag and Se layer thickness in the precursor thin films, AgSbSxSe 2-x thin films of appropriate electrical and optical properties can be obtained which will improve PV characteristics and performance. The advantage of selenization through chemical bath deposited Ag 2 Se or Se thin films ensure a non- toxic and cost effective PV technology using this material. ACKNOWLEDGMENTS The authors are thankful to PAICYT 2007-UANL, N.L, Mexico and SEP- CONACYT - Mexico (project 48609) and PROMEP-Mexico for the financial assistance, Dr.Anabel Alverez, Ciencias Químicas, UANL, N.L, Mexico, for recording XRD. One of the authors, Jorge Oswaldo is grateful to CONACYT-Mexico for providing Fellowship. REFERENCES 1. O. Madelung, Semiconductors other than Group IVElements and III V Compounds (Data in Science and Technology) Berlin-Springer 1992, p H. Soliman, D.Abdel-Hady, and E. Ibrahim, J. Phys.: Condens. Matter., 10, 847 (1998). 3. A. R Patel, and D. Lakshminarayana, Thin Solid Films, 98, 59 (1982). 4. K. Wang, C. Steimer and M. Wuttig, J. Optoelec. Adv. Mat., 9, 2008 (2007). 5. K. Bindu, J. Campos, M. T. S. Nair, A. Sanchez and P. K. Nair, Semicond. Sci. Technol., 20, 496 (2005). 6. K. Bindu, M. T. S. Nair, T. K. Das Roy and P. K. Nair, Electrochem. Solid-State Lett., 9, 195 (2006).