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Antimony sulfide thin films in chemically deposited thin film photovoltaic cells
Thin Solid Films 515 (2007) 5777 – 5782 www.elsevier.com/locate/tsf
Antimony sulfide thin films in chemically deposited thin film photovoltaic cells Sarah Messina ⁎, M.T.S. Nair, P.K. Nair Department of Solar Energy Materials, Centro de Investigación en Energía, Universidad Nacional Autónoma de México, Temixco, Morelos 62580, Mexico Available online 16 December 2006
Abstract Antimony sulfide thin films of thickness ≈500 nm have been deposited on glass slides from chemical baths constituted with SbCl3 and sodium thiosulfate. Smooth specularly reflective thin films are obtained at deposition temperatures from −3 to 10 °C. The differences in the film thickness and improvement in the crystallinity and photoconductivity upon annealing the film in nitrogen are presented. These films can be partially converted into a solid solution of the type Sb2SxSe3 − x, detected in X-ray diffraction, through heating them in contact with a chemically deposited selenium thin film. This would decrease the optical band gap of the film from ≈ 1.7 eV (Sb2S3) to ≈1.3 eV for the films heated at 300 °C. Similarly, heating at 300 °C of sequentially deposited thin film layers of Sb2S3–Ag2Se, the latter also from a chemical bath at 10 °C results in the formation of AgSb(S/Se)2 with an optical gap of ≈ 1.2 eV. All these thin films have been integrated into photovoltaic structures using a CdS window layer deposited on 3 mm glass sheets with a SnO2:F coating (TEC-15, Pilkington). Characteristics obtained in these cells under an illumination of 850 W/m2 (tungsten halogen) are as follows: SnO2:F–CdS–Sb2S3–Ag(paint) with open circuit voltage (Voc) 470 mV and short circuit current density (Jsc) 0.02 mA/cm2; SnO2:F–CdS–Sb2S3–CuS–Ag(paint), Voc ≈ 460 mV and Jsc ≈ 0.4 mA/cm2; SnO2:F–CdS–Sb2Sx Se3 − x–Ag(paint), Voc ≈ 670 mV and Jsc ≈ 0.05 mA/cm2; SnO2:F–CdS–Sb2S3–AgSb(S/Se)2–Ag(paint), Voc ≈ 450 mV and Jsc ≈ 1.4 mA/cm2. We consider that the materials and the deposition techniques reported here are promising toward developing ‘all-chemically deposited solar cell technologies.’ © 2007 Elsevier B.V. All rights reserved. Keywords: Chemical deposition; Antimony sulfide; Thin films; Photovoltaic structures
1. Introduction Chemical deposition of thin films of antimony sulfide has been reported by many authors since early 1990s [1–3]. When heated at 300 °C, these films show well defined crystal structure identical with that of the mineral stibnite (Sb2S3), of the orthorhombic system [2]. The optical band gap of this material is 1.88 eV (direct gap) [4]. Heating a chemically deposited Sb2S3–CuS layer at 350–400 °C (nitrogen, ≈ 40 Pa) results in the formation of CuSbS2 thin film, which occurs in nature as the mineral chalcostibite. An optical band gap of ≈ 1.5 eV and ptype conductivity of 0.03 (Ω cm)− 1 have been reported for this material [5]. Photovoltaic structures, SnO2:F–CdS–Sb2S3– CuSbS2 [6], have shown an open circuit voltage, Voc, of ≈ 350 mV and a short circuit current density, Jsc, ≈ 0.2 mA/cm2.
⁎ Corresponding author. E-mail address: [email protected] (S. Messina). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.12.155
Chemically deposited antimony sulfide thin films have also featured in Schottky barrier and heterojunction solar cells [7,8]. Preparation of AgSbSe2 thin films is possible through the reaction of Sb2S3–Ag layer at 200–300 °C in selenium vapor evolved from a chemically deposited Se thin film [9]. Photovoltaic structures of SnO 2:F:CdS–Sb2S 3–AgSbSe 2 [10,11] showed Voc N 500 mV and Jsc ≈ 2 mA/cm2. Heating an Sb2S3 film at 200–400 °C in contact with a chemically deposited Se thin film produces solid solutions of Sb2SxSe3 − x with the optical band gap decreasing to 1.2 eV [12], which would be an attractive feature for solar cells. In this paper we report a new methodology to produce antimony sulfide thin films of thickness N500 nm from a single bath. The formation of Sb2SxSe3 − x, CuSbS2 and AgSbSe2 through the reactions of Sb2S3/Se, Sb2S3–CuS and Sb2S3– Ag2Se/Se, respectively are illustrated. The application of these films as absorbers in chemically deposited photovoltaic structures is presented subsequently. The results indicate that antimony chalcogenides and related ternary or quaternary
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materials offer prospects as absorber materials in chemically deposited solar cells. 2. Experimental details 2.1. Thin film deposition 2.1.1. Sb2S3 thin films Thin films of Sb2S3 were obtained from a chemical bath prepared by dissolving 650 mg of SbCl3 (Aldrich) initially in 2.5 ml of acetone and subsequently adding 25 ml of 1 M Na2S2O3 (Baker Analyzed) solution and sufficient water to make the volume to 100 ml. For the deposition of thin films of Sb2S3 reported by this method [2], clean microscope glass slides were placed in the bath, supported vertically on the wall of the beaker. Final film thickness of 200 nm was achieved in 4 h deposition inside a refrigerator at 10 °C. In the present work the glass slides were placed in the bath horizontally in order to avoid temperature and concentration gradient in the bath. The bath with the substrates was placed in a temperature controlled circulation bath at − 3, 1, 5 or 10 °C. In all cases the temperature was initially set at 10 °C. Subsequently, the temperature was decreased in steps of 2 °C every 15 min to reach a final deposition temperature of − 3 to 5 °C. Without this gradual decrease in temperature, the film would detach from the glass substrates during rinsing for depositions made at temperatures b5 °C. At 1 °C, a film of ≈ 550 nm in thickness was deposited on glass with a transparent conductive oxide (TCO) of SnO2:F or SnO2:F–CdS substrates in 6 h. This condition of deposition was followed for most studies presented here. 2.1.2. Se thin films These thin films required as source of Se vapor in the processing of the thin films reported here were deposited on microscope glass slides coated with a thin film of ZnS, following the procedure described in [9]. The chemical bath for Se-deposition was prepared using 10 ml of a 0.1 M solution of Na2SeSO3, 70 ml of water, and 2 ml of ∼ 4.4 M acetic acid. A Se thin film of ≈ 300 nm in thickness was deposited from this solution in 8 h at 10 °C in a refrigerator. In order to control the evolution of the Se vapor in the heat treatments of Sb2S3 and Sb2S3–Ag2Se layer, a ZnS thin film was further deposited on the Se film, as suggested in a recent report [13]. 2.1.3. Ag2Se thin films The deposition mixture was prepared by dissolving 100 mg of AgNO3 (Baker Analyzed) in 10 ml of water. Then 4 ml of 7.5 M of ammonia (aq.), 81 ml of deionized water and 5 ml of a 0.1 M solution of Na2SeSO3 were added [14]. Silver selenide thin film was deposited at 10 °C in a refrigerator on a thin film of Sb2S3. Under this condition, a thin film of 150 nm in thickness was deposited in 45 min. 2.1.4. CuS thin film This film was deposited on Sb2S3 thin films using a chemical bath containing 10 ml of 0.5 M CuCl2, 8 ml of triethanolamine
Fig. 1. Growth curve of antimony sulfide thin films deposited at − 3, 1, 5 and 10 °C.
(TEA) 50%, 8 ml of 15 M ammonia (aq.), 10 ml of 1 M NaOH, 6 ml of 1 M thiourea, and water for 100 ml bath. In 1 h at 30 °C, a CuS thin film of estimated thickness 120 nm was deposited on the Sb2S3 film. The latter film was deposited at 1 °C and heated in air at 200 °C to prevent peeling in the CuS deposition bath. 2.2. Heat treatments Individual thin films as well as multilayer films and device structures were heated in air, or in nitrogen (40 Pa) in a vacuum oven. For the formation of Sb2SxSe3 − x solid solution or the ternary compound AgSbSe2, a ZnS–Se–ZnS layer is held facing the Sb2S3 film or Sb2S3–Ag2Se layer and the assembly kept intact using Teflon sealing tape. Here, ZnS remains stable at temperatures up to 350 °C and does not incorporate into the adjacent film — it merely controls the evolution rate of Se vapor for the reaction [13]. 2.3. Characterization X-ray diffraction (XRD) patterns of the films were recorded on a Rigaku D-Max 2000 diffractometer using Cu-Kα (λ = 1.5406 Å) radiation in standard mode (θ–2θ) and grazing incidence angles of θ = 0.5°, 1.5° and 3°. X-ray fluorescence (XRF) spectra were obtained on Philips Magi-X PRO using a pentaerythrite (002) analyzer crystal for Sb-Lβ and S-Kα and a PX1 layered analyzer crystal (2d = 5.1 nm) for Se-Lα fluorescence emission. The optical transmittance and specular reflectance spectra of the films were obtained using a Shimadzu UV 3100 PC spectrophotometer in the wavelength range 250– 2500 nm. Air or front aluminized mirror, respectively, was used in the reference beams. For the electrical characterization, we printed silver paint electrodes (5 mm length at 5 mm separation) on the film surface. A tungsten halogen lamp was used for the illumination. It produced an intensity of illumination 850 W/m2 at the sample plane. The samples were placed in the measurement chamber and maintained in the dark to stabilize the current before a voltage was applied across the electrodes
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Fig. 2. XRF peak intensities in thin films of Sb2S3: as prepared, heated at 300 °C in air or N2 (40 Pa) and in Sb2S3 thin films heated at 300 °C in contact with a Se thin film in air or N2 (40 Pa) as shown in the inset.
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during 30 min in air or N2 atmosphere. During the heating there is a loss of ≈ 6–9% S and Sb, but the ratio of S/Sb intensity remains the same. Since the XRD results show that the material of the film is Sb2S3, this ratio is 1.5. Hence the XRF intensities (raw data) have been multiplied by a sensitivity factor to result in the values shown. When the Sb2S3 film is heated at 300 °C during 30 min in the presence of Se vapor evolved from a ZnS–Se–ZnS placed in contact with the Sb2S3 film (as shown in the inset), the Sb-XRF intensity falls by about 9–10%, but the S-XRF intensity falls by a greater extent. We note that there is an incorporation of Se in the film compensating the relative loss of S. Grazing angle XRD pattern would show that the surface of the Sb2S3 thin film has been converted into a solid solution, Sb2SxSe3 − x during such heating.
using a Keithley 230 programmable voltage source. The current was measured using a Keithley 619 multimeter interfaced with a computer. Thickness of the films was measured using Alpha Step 100 (Tencor, CA). 3. Results and discussion 3.1. Film growth Specularly reflective Sb2S3 thin films devoid of adhering precipitate were obtained at the different temperatures. The following conclusions may be drawn from the growth curve for the films presented in Fig. 1. The terminal (final) thickness of the films increases with reduction in the deposition temperature: at 10 °C, a final thickness of ≈ 470 nm is reached in 5–6 h, whereas at − 3 °C a final thickness of 650 nm is obtained in about 7 h. At 1 °C, a film thickness of 550 nm is obtained in 6 h. For most of the studies, this condition has been chosen for the film deposition. Note that the temperature was reduced in steps of 2 °C from 10 °C to the final deposition temperature. Thus, during an initial period of 1–2 h, the temperature of the bath has not been constant, and hence conclusion on the rate of deposition with temperature may not be drawn from the present set of data. The initial (1–4 h) deposition rate is greater than that in the later period, when the bath becomes depleted of the constituent ions. This is a feature well known in chemically deposited semiconductor thin films. The reason for initiating the deposition at 10 °C is to improve the adhesion of the films to glass substrates during the nucleation phase of the deposition. The films deposited at a constant temperature of − 3 or 1 °C, would otherwise peel-off from the glass substrate when rinsed in water. However, this problem is inhibited when the film is deposited on a CdS thin film, as required for forming the photovoltaic structure. 3.2. X-ray fluorescence analysis Fig. 2 shows the intensity of XRF peaks of Sb-Lβ, and S-Kα for Sb2S3 thin films (≈ 550 nm, as deposited) heated at 300 °C
Fig. 3. XRD patterns of: A — a) Sb2S3 thin films heated at 300 °C in N2; b) Sb2S3 thin films heated in contact with a Se thin film and recorded at θ = 0.5° and 1.5°. B — c) Sb2S3–CuS thin film heated at 400 °C in N2; and d) Sb2S3– Ag2Se thin film heated at 300 °C in nitrogen in contact with a Se thin film.
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AgSbSe2, (PDF # 12-0379). The reaction appears to be complete in this case. 3.4. Optical properties
Fig. 4. Optical transmittance of different crystalline thin films formed from Sb2S3 thin films; the values in the parentheses are the optical band gap obtained from (αhν)n versus hν plots.
Optical transmittance spectra of Sb2S3 thin films and of various other thin films produced from these, as described in Fig. 3, are given in Fig. 4. The following heat treatments have been applied: Sb2S3, heated at 300 °C in N2; Sb2SxSe3 − x produced by heating in nitrogen at 300 °C a Sb2S3 film in contact with ZnS–Se–ZnS thin film (Fig. 2 inset); CuSbS2 produced by heating in nitrogen at 400 °C a Sb2S3–CuS layer; and AgSbSe2 produced by heating in air at 300 °C a Sb2S3– Ag2Se layer in contact with a ZnS–Se–ZnS film. The optical absorption coefficient (α) for photon energies n Þ (hν) was calculated using the equation: a ¼ d1 ln ð1−R 2T þ 2 1=2 ð1−RÞ þR2 g. Here d is the film thickness and T and R are 2T fractional values of transmittance and reflectance. The values of the optical band gap have been calculated from the best straight line fits in (αhν)n versus hν plots. In general we found that for n = ⅔ the correlation factor is the best. Thus, the optical gap is direct but involves forbidden transitions [15]. 2
2
3.3. Crystalline structure XRD peaks are not observed in the case of as-deposited Sb2S3 thin films [1,2]. When heated in air or N2 at temperatures above 250 °C these films produce XRD patterns matching that of the mineral stibnite (PDF # 42-1393, as shown in Fig. 3A a). The XRD patterns for a thin film of Sb2S3 recorded after heating in contact with a Se thin film in N2 (or air) at grazing incidence angles of 0.5° and 1.5°, are shown in Fig. 3A b) along with the standard pattern for Sb2Se3 (PDF # 15-0681). The formation of a solid solution is expected because both materials crystallize in orthorhombic structure. It is seen in Fig. 3A b) that the top layer of the Sb2S3 has been converted into a solid solution, with the XRD peaks recorded at grazing incidence 0.5° staying midway between that of Sb2S3 and Sb2Se3 in 2θ positions. The value of x for the solid solution Sb2SxSe3 − x layer may be estimated using Vegard's law for solid solutions, as illustrated for this material [12]. In the present case x is b1.5 for the top layer. In the pattern recorded at θ = 1.5°, the X-rays penetrate deeper into the layer, and broader XRD peaks arise due to a continuous increase in x toward the interior of the film. We have reported in [12] that heating the layer at a higher temperature or for a longer duration would lead to the formation of a solid solution Sb2SxSe3 − x with a constant value of x throughout the film thickness. However, for photovoltaic application, a film with graded composition and hence of graded optical band gap may be more desirable. The XRD pattern given in Fig. 3B c) is of a CuSbS2 thin film formed by heating a Sb2S3–CuS layer in N2 at 400 °C during 1 h, as suggested in [5]. The XRD pattern for the film matches that of the mineral chalcostibite, (PDF # 44-1417) of composition CuSbS2. The conversion of the layer to the ternary compound predominates, but the presence of Sb2S3 (200) and (120) peaks suggests that the reaction is not complete. The reaction between Sb2S3 and Ag2Se in contact with Se thin film at 300 °C in nitrogen results in the formation of polycrystalline AgSbSe2 thin film, as shown in Fig. 3B d). The XRD peaks observed here match the standard pattern of
Fig. 5. Photoconductivity response curves of: a) antimony sulfide thin films heated in air and N2 with and without Se vapor; b) photoconductivity response of CuSbS2 and AgSbSe2 thin films formed by heating Sb2S3–CuS and Sb2S3– Ag2Se layers.
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3.5. Electrical properties Antimony sulfide thin films heated with and without Se in N2 at 300 °C shows the same order of photocurrent response. However in the film heated in air in contact with a ZnS–Se–ZnS film at 200 °C for 30 min, the photocurrent response improves considerably, compared with an Sb2S3 film subjected to the same condition of heating. These results are shown in Fig. 5. Both CuSbS2 and AgSbSe2 are also photoconductive. The dark conductivity of these materials is ≈ 10− 3 (Ω cm)− 1, well suited to serve as p-absorbers. 4. Photovoltaic structure Current (I)–voltage (V) characteristics of the photovoltaic structures formed using the materials developed above are shown in Fig. 6. The heterojunction SnO2:F–CdS–Sb2S3 was heated in air at 250 °C during 30 min; it shows Voc = 470 mV and Jsc = 0.02 mA/cm2 (Fig. 6a). The presence of a Se thin film placed in contact with the structure during heating at 225 °C for 30 min in air improves the characteristics: Voc = 670 mV and
Fig. 7. I–V characteristics of the structures TCO–CdS–Sb2S3–CuS–Ag(print) and TCO–CdS–Sb2S3–AgSbSe2–Ag(print) in the dark and under illumination.
Fig. 6. I–V characteristics of the structures: a) TCO(SnO2:F)–CdS–Sb2S3; b) TCO–CdS–Sb2S3–Sb2SxSe3 − x; c) TCO–CdS–Sb2S3–CuS; and d) TCO–CdS– Sb2S3–AgSbSe2. In all the cases the illumination is from the TCO side and a silver print contact of about 1 mm2 area painted over the top layer served as pside contact.
Jsc = 0.05 mA/cm2 (Fig. 6b). We found in this case that the silver print electrode reacts with the top layer, probably forming a Ag2Se layer, leading to a cell degradation. But on freshly painted silver print electrodes the same characteristics are obtained. This issue did not come up in cell structures with CuS or AgSbSe2 as the top layer. For the photovoltaic structures consisting of Sb2S3–CuS (Fig. 6c), we used the first layer of Sb2S3 (300 nm) heated in nitrogen at 300 °C, followed by the deposition of the second Sb2S3 layer (300 nm) and subsequently the deposition of a CuS thin film of 50–100 nm and heated in air at 200 °C. This cell presents Voc = 460 mV and Jsc = 0.4 mA/cm2. For the structure with AgSbSe2 (Fig. 6d) the Sb2S3 layer was heated in air at 225 °C during 30 min in contact with a ZnS–Se–ZnS film and another Sb2S3 film was deposited over this. The Ag2Se film was deposited on this and the structure was heated in nitrogen at 200 °C for 90 min in contact with a Se thin film to form AgSbSe2. I–V characteristics for this structure show: Voc = 450 mV and Jsc = 1.4 mA/cm 2 . Fig. 7 shows both I–V curves in the dark and under illumination for these two cell structures. The presence of a high series resistance as well as comparable parallel resistance is obvious in these cells. The deposition procedure described here avoided film peeloff during sequential deposition. This process has to be optimized in further work to improve the cell characteristics.
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5. Conclusions In this paper we presented a methodology to deposit Sb2S3 thin films of 500–600 nm in thickness, adequate for use in photovoltaic structures from a single bath. This is an improvement over the methods reported previously, in which multiple depositions have been used to build up the thickness to this value. These films have been used to prepare solid solutions of the type Sb2SxSe3 − x with graded composition and optical band gap (≈ 1.3–1.7 eV) along the thickness, desired in photovoltaic structures, by heating them in contact with a chemically deposited Se thin film. Photovoltaic absorber films of ternary compositions, CuSbS2 (band gap, 1.54 eV), and AgSbSe2 (1.2 eV) may be produced from the Sb2S3–CuS and Sb2S3–Ag2Se, which are photoconductive and possess dark conductivity 10− 3 (Ω cm)− 1, suitable to serve as p-absorbers. The use of these films in photovoltaic structures has been illustrated. We consider that further work on the optimization of the film thickness, thermal processing and cell configurations would lead toward new solar cell technologies using chemical bath deposition as the principal thin film technique. Acknowledgements The authors are grateful to María Luisa Ramón and Patricia Altuzar Coello, for recording the XRD and XRF data; Oscar Gomez-Daza for the assistance in the optical characterization and José Campos for the electrical charac-
terization. We acknowledge the financial support received from DGAPA-UNAM (IN110906) and CONACYT (Project 40515-F and graduate fellowship for SM). References [1] O. Savadogo, K.C. Mandal, Sol. Energy Mater. 26 (1991) 117. [2] M.T.S. Nair, Y. Peña, J. Campos, V.M. García, P.K. Nair, J. Electrochem. Soc. 145 (1998) 2113. [3] I. Grozdanov, Semicond. Sci. Technol. 9 (1994) 1234. [4] O. Madelung, Semiconductors Other than Group IV Elements and III–V Compounds, Data in Science and Technology, Springer-Verlag, Berlin Heidelberg, 1992. [5] Y. Rodríguez-Lazcano, M.T.S. Nair, P.K. Nair, J. Cryst. Growth 223 (2001) 399. [6] Y. Rodríguez-Lazcano, M.T.S. Nair, P.K. Nair, J. Electrochem. Soc. 152 (2005) G635. [7] O. Savadogo, K.C. Mandal, J. Phys., D, Appl. Phys. 27 (1994) 1070. [8] O. Savadogo, K.C. Mandal, J. Electrochem. Soc. 141 (1994) 2781. [9] K. Bindu, P.K. Nair, Semicond. Sci. Technol. 19 (2005) 1348. [10] K. Bindu, José Campos, M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 20 (2005) 496. [11] K. Bindu, M.T.S. Nair, T.K. Das Roy, P.K. Nair, J. Electrochem. Solid State Lett. 9 (2006) G-195. [12] D.Y. Suarez-Sandoval, M.T.S. Nair, P.K. Nair, J. Electrochem. Soc. 153 (2006) C91. [13] K. Bindu, M.T.S. Nair, P.K. Nair, J. Electrochem. Soc. 153 (2006) C526. [14] B. Pejova, M. Najdoski, I. Grozdanov, S.K. Dey, Mater. Lett. 43 (2000) 269. [15] R.A. Smith, Semiconductors, Second. Ed, Cambridge University Press, Cambridge, 1978, p. 314.
Antimony sulfide thin films in chemically deposited thin film photovoltaic cells Sarah Messina ⁎, M.T.S. Nair, P.K. Nair Department of Solar Energy Materials, Centro de Investigación en Energía, Universidad Nacional Autónoma de México, Temixco, Morelos 62580, Mexico Available online 16 December 2006
Abstract Antimony sulfide thin films of thickness ≈500 nm have been deposited on glass slides from chemical baths constituted with SbCl3 and sodium thiosulfate. Smooth specularly reflective thin films are obtained at deposition temperatures from −3 to 10 °C. The differences in the film thickness and improvement in the crystallinity and photoconductivity upon annealing the film in nitrogen are presented. These films can be partially converted into a solid solution of the type Sb2SxSe3 − x, detected in X-ray diffraction, through heating them in contact with a chemically deposited selenium thin film. This would decrease the optical band gap of the film from ≈ 1.7 eV (Sb2S3) to ≈1.3 eV for the films heated at 300 °C. Similarly, heating at 300 °C of sequentially deposited thin film layers of Sb2S3–Ag2Se, the latter also from a chemical bath at 10 °C results in the formation of AgSb(S/Se)2 with an optical gap of ≈ 1.2 eV. All these thin films have been integrated into photovoltaic structures using a CdS window layer deposited on 3 mm glass sheets with a SnO2:F coating (TEC-15, Pilkington). Characteristics obtained in these cells under an illumination of 850 W/m2 (tungsten halogen) are as follows: SnO2:F–CdS–Sb2S3–Ag(paint) with open circuit voltage (Voc) 470 mV and short circuit current density (Jsc) 0.02 mA/cm2; SnO2:F–CdS–Sb2S3–CuS–Ag(paint), Voc ≈ 460 mV and Jsc ≈ 0.4 mA/cm2; SnO2:F–CdS–Sb2Sx Se3 − x–Ag(paint), Voc ≈ 670 mV and Jsc ≈ 0.05 mA/cm2; SnO2:F–CdS–Sb2S3–AgSb(S/Se)2–Ag(paint), Voc ≈ 450 mV and Jsc ≈ 1.4 mA/cm2. We consider that the materials and the deposition techniques reported here are promising toward developing ‘all-chemically deposited solar cell technologies.’ © 2007 Elsevier B.V. All rights reserved. Keywords: Chemical deposition; Antimony sulfide; Thin films; Photovoltaic structures
1. Introduction Chemical deposition of thin films of antimony sulfide has been reported by many authors since early 1990s [1–3]. When heated at 300 °C, these films show well defined crystal structure identical with that of the mineral stibnite (Sb2S3), of the orthorhombic system [2]. The optical band gap of this material is 1.88 eV (direct gap) [4]. Heating a chemically deposited Sb2S3–CuS layer at 350–400 °C (nitrogen, ≈ 40 Pa) results in the formation of CuSbS2 thin film, which occurs in nature as the mineral chalcostibite. An optical band gap of ≈ 1.5 eV and ptype conductivity of 0.03 (Ω cm)− 1 have been reported for this material [5]. Photovoltaic structures, SnO2:F–CdS–Sb2S3– CuSbS2 [6], have shown an open circuit voltage, Voc, of ≈ 350 mV and a short circuit current density, Jsc, ≈ 0.2 mA/cm2.
⁎ Corresponding author. E-mail address: [email protected] (S. Messina). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.12.155
Chemically deposited antimony sulfide thin films have also featured in Schottky barrier and heterojunction solar cells [7,8]. Preparation of AgSbSe2 thin films is possible through the reaction of Sb2S3–Ag layer at 200–300 °C in selenium vapor evolved from a chemically deposited Se thin film [9]. Photovoltaic structures of SnO 2:F:CdS–Sb2S 3–AgSbSe 2 [10,11] showed Voc N 500 mV and Jsc ≈ 2 mA/cm2. Heating an Sb2S3 film at 200–400 °C in contact with a chemically deposited Se thin film produces solid solutions of Sb2SxSe3 − x with the optical band gap decreasing to 1.2 eV [12], which would be an attractive feature for solar cells. In this paper we report a new methodology to produce antimony sulfide thin films of thickness N500 nm from a single bath. The formation of Sb2SxSe3 − x, CuSbS2 and AgSbSe2 through the reactions of Sb2S3/Se, Sb2S3–CuS and Sb2S3– Ag2Se/Se, respectively are illustrated. The application of these films as absorbers in chemically deposited photovoltaic structures is presented subsequently. The results indicate that antimony chalcogenides and related ternary or quaternary
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materials offer prospects as absorber materials in chemically deposited solar cells. 2. Experimental details 2.1. Thin film deposition 2.1.1. Sb2S3 thin films Thin films of Sb2S3 were obtained from a chemical bath prepared by dissolving 650 mg of SbCl3 (Aldrich) initially in 2.5 ml of acetone and subsequently adding 25 ml of 1 M Na2S2O3 (Baker Analyzed) solution and sufficient water to make the volume to 100 ml. For the deposition of thin films of Sb2S3 reported by this method [2], clean microscope glass slides were placed in the bath, supported vertically on the wall of the beaker. Final film thickness of 200 nm was achieved in 4 h deposition inside a refrigerator at 10 °C. In the present work the glass slides were placed in the bath horizontally in order to avoid temperature and concentration gradient in the bath. The bath with the substrates was placed in a temperature controlled circulation bath at − 3, 1, 5 or 10 °C. In all cases the temperature was initially set at 10 °C. Subsequently, the temperature was decreased in steps of 2 °C every 15 min to reach a final deposition temperature of − 3 to 5 °C. Without this gradual decrease in temperature, the film would detach from the glass substrates during rinsing for depositions made at temperatures b5 °C. At 1 °C, a film of ≈ 550 nm in thickness was deposited on glass with a transparent conductive oxide (TCO) of SnO2:F or SnO2:F–CdS substrates in 6 h. This condition of deposition was followed for most studies presented here. 2.1.2. Se thin films These thin films required as source of Se vapor in the processing of the thin films reported here were deposited on microscope glass slides coated with a thin film of ZnS, following the procedure described in [9]. The chemical bath for Se-deposition was prepared using 10 ml of a 0.1 M solution of Na2SeSO3, 70 ml of water, and 2 ml of ∼ 4.4 M acetic acid. A Se thin film of ≈ 300 nm in thickness was deposited from this solution in 8 h at 10 °C in a refrigerator. In order to control the evolution of the Se vapor in the heat treatments of Sb2S3 and Sb2S3–Ag2Se layer, a ZnS thin film was further deposited on the Se film, as suggested in a recent report [13]. 2.1.3. Ag2Se thin films The deposition mixture was prepared by dissolving 100 mg of AgNO3 (Baker Analyzed) in 10 ml of water. Then 4 ml of 7.5 M of ammonia (aq.), 81 ml of deionized water and 5 ml of a 0.1 M solution of Na2SeSO3 were added [14]. Silver selenide thin film was deposited at 10 °C in a refrigerator on a thin film of Sb2S3. Under this condition, a thin film of 150 nm in thickness was deposited in 45 min. 2.1.4. CuS thin film This film was deposited on Sb2S3 thin films using a chemical bath containing 10 ml of 0.5 M CuCl2, 8 ml of triethanolamine
Fig. 1. Growth curve of antimony sulfide thin films deposited at − 3, 1, 5 and 10 °C.
(TEA) 50%, 8 ml of 15 M ammonia (aq.), 10 ml of 1 M NaOH, 6 ml of 1 M thiourea, and water for 100 ml bath. In 1 h at 30 °C, a CuS thin film of estimated thickness 120 nm was deposited on the Sb2S3 film. The latter film was deposited at 1 °C and heated in air at 200 °C to prevent peeling in the CuS deposition bath. 2.2. Heat treatments Individual thin films as well as multilayer films and device structures were heated in air, or in nitrogen (40 Pa) in a vacuum oven. For the formation of Sb2SxSe3 − x solid solution or the ternary compound AgSbSe2, a ZnS–Se–ZnS layer is held facing the Sb2S3 film or Sb2S3–Ag2Se layer and the assembly kept intact using Teflon sealing tape. Here, ZnS remains stable at temperatures up to 350 °C and does not incorporate into the adjacent film — it merely controls the evolution rate of Se vapor for the reaction [13]. 2.3. Characterization X-ray diffraction (XRD) patterns of the films were recorded on a Rigaku D-Max 2000 diffractometer using Cu-Kα (λ = 1.5406 Å) radiation in standard mode (θ–2θ) and grazing incidence angles of θ = 0.5°, 1.5° and 3°. X-ray fluorescence (XRF) spectra were obtained on Philips Magi-X PRO using a pentaerythrite (002) analyzer crystal for Sb-Lβ and S-Kα and a PX1 layered analyzer crystal (2d = 5.1 nm) for Se-Lα fluorescence emission. The optical transmittance and specular reflectance spectra of the films were obtained using a Shimadzu UV 3100 PC spectrophotometer in the wavelength range 250– 2500 nm. Air or front aluminized mirror, respectively, was used in the reference beams. For the electrical characterization, we printed silver paint electrodes (5 mm length at 5 mm separation) on the film surface. A tungsten halogen lamp was used for the illumination. It produced an intensity of illumination 850 W/m2 at the sample plane. The samples were placed in the measurement chamber and maintained in the dark to stabilize the current before a voltage was applied across the electrodes
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Fig. 2. XRF peak intensities in thin films of Sb2S3: as prepared, heated at 300 °C in air or N2 (40 Pa) and in Sb2S3 thin films heated at 300 °C in contact with a Se thin film in air or N2 (40 Pa) as shown in the inset.
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during 30 min in air or N2 atmosphere. During the heating there is a loss of ≈ 6–9% S and Sb, but the ratio of S/Sb intensity remains the same. Since the XRD results show that the material of the film is Sb2S3, this ratio is 1.5. Hence the XRF intensities (raw data) have been multiplied by a sensitivity factor to result in the values shown. When the Sb2S3 film is heated at 300 °C during 30 min in the presence of Se vapor evolved from a ZnS–Se–ZnS placed in contact with the Sb2S3 film (as shown in the inset), the Sb-XRF intensity falls by about 9–10%, but the S-XRF intensity falls by a greater extent. We note that there is an incorporation of Se in the film compensating the relative loss of S. Grazing angle XRD pattern would show that the surface of the Sb2S3 thin film has been converted into a solid solution, Sb2SxSe3 − x during such heating.
using a Keithley 230 programmable voltage source. The current was measured using a Keithley 619 multimeter interfaced with a computer. Thickness of the films was measured using Alpha Step 100 (Tencor, CA). 3. Results and discussion 3.1. Film growth Specularly reflective Sb2S3 thin films devoid of adhering precipitate were obtained at the different temperatures. The following conclusions may be drawn from the growth curve for the films presented in Fig. 1. The terminal (final) thickness of the films increases with reduction in the deposition temperature: at 10 °C, a final thickness of ≈ 470 nm is reached in 5–6 h, whereas at − 3 °C a final thickness of 650 nm is obtained in about 7 h. At 1 °C, a film thickness of 550 nm is obtained in 6 h. For most of the studies, this condition has been chosen for the film deposition. Note that the temperature was reduced in steps of 2 °C from 10 °C to the final deposition temperature. Thus, during an initial period of 1–2 h, the temperature of the bath has not been constant, and hence conclusion on the rate of deposition with temperature may not be drawn from the present set of data. The initial (1–4 h) deposition rate is greater than that in the later period, when the bath becomes depleted of the constituent ions. This is a feature well known in chemically deposited semiconductor thin films. The reason for initiating the deposition at 10 °C is to improve the adhesion of the films to glass substrates during the nucleation phase of the deposition. The films deposited at a constant temperature of − 3 or 1 °C, would otherwise peel-off from the glass substrate when rinsed in water. However, this problem is inhibited when the film is deposited on a CdS thin film, as required for forming the photovoltaic structure. 3.2. X-ray fluorescence analysis Fig. 2 shows the intensity of XRF peaks of Sb-Lβ, and S-Kα for Sb2S3 thin films (≈ 550 nm, as deposited) heated at 300 °C
Fig. 3. XRD patterns of: A — a) Sb2S3 thin films heated at 300 °C in N2; b) Sb2S3 thin films heated in contact with a Se thin film and recorded at θ = 0.5° and 1.5°. B — c) Sb2S3–CuS thin film heated at 400 °C in N2; and d) Sb2S3– Ag2Se thin film heated at 300 °C in nitrogen in contact with a Se thin film.
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AgSbSe2, (PDF # 12-0379). The reaction appears to be complete in this case. 3.4. Optical properties
Fig. 4. Optical transmittance of different crystalline thin films formed from Sb2S3 thin films; the values in the parentheses are the optical band gap obtained from (αhν)n versus hν plots.
Optical transmittance spectra of Sb2S3 thin films and of various other thin films produced from these, as described in Fig. 3, are given in Fig. 4. The following heat treatments have been applied: Sb2S3, heated at 300 °C in N2; Sb2SxSe3 − x produced by heating in nitrogen at 300 °C a Sb2S3 film in contact with ZnS–Se–ZnS thin film (Fig. 2 inset); CuSbS2 produced by heating in nitrogen at 400 °C a Sb2S3–CuS layer; and AgSbSe2 produced by heating in air at 300 °C a Sb2S3– Ag2Se layer in contact with a ZnS–Se–ZnS film. The optical absorption coefficient (α) for photon energies n Þ (hν) was calculated using the equation: a ¼ d1 ln ð1−R 2T þ 2 1=2 ð1−RÞ þR2 g. Here d is the film thickness and T and R are 2T fractional values of transmittance and reflectance. The values of the optical band gap have been calculated from the best straight line fits in (αhν)n versus hν plots. In general we found that for n = ⅔ the correlation factor is the best. Thus, the optical gap is direct but involves forbidden transitions [15]. 2
2
3.3. Crystalline structure XRD peaks are not observed in the case of as-deposited Sb2S3 thin films [1,2]. When heated in air or N2 at temperatures above 250 °C these films produce XRD patterns matching that of the mineral stibnite (PDF # 42-1393, as shown in Fig. 3A a). The XRD patterns for a thin film of Sb2S3 recorded after heating in contact with a Se thin film in N2 (or air) at grazing incidence angles of 0.5° and 1.5°, are shown in Fig. 3A b) along with the standard pattern for Sb2Se3 (PDF # 15-0681). The formation of a solid solution is expected because both materials crystallize in orthorhombic structure. It is seen in Fig. 3A b) that the top layer of the Sb2S3 has been converted into a solid solution, with the XRD peaks recorded at grazing incidence 0.5° staying midway between that of Sb2S3 and Sb2Se3 in 2θ positions. The value of x for the solid solution Sb2SxSe3 − x layer may be estimated using Vegard's law for solid solutions, as illustrated for this material [12]. In the present case x is b1.5 for the top layer. In the pattern recorded at θ = 1.5°, the X-rays penetrate deeper into the layer, and broader XRD peaks arise due to a continuous increase in x toward the interior of the film. We have reported in [12] that heating the layer at a higher temperature or for a longer duration would lead to the formation of a solid solution Sb2SxSe3 − x with a constant value of x throughout the film thickness. However, for photovoltaic application, a film with graded composition and hence of graded optical band gap may be more desirable. The XRD pattern given in Fig. 3B c) is of a CuSbS2 thin film formed by heating a Sb2S3–CuS layer in N2 at 400 °C during 1 h, as suggested in [5]. The XRD pattern for the film matches that of the mineral chalcostibite, (PDF # 44-1417) of composition CuSbS2. The conversion of the layer to the ternary compound predominates, but the presence of Sb2S3 (200) and (120) peaks suggests that the reaction is not complete. The reaction between Sb2S3 and Ag2Se in contact with Se thin film at 300 °C in nitrogen results in the formation of polycrystalline AgSbSe2 thin film, as shown in Fig. 3B d). The XRD peaks observed here match the standard pattern of
Fig. 5. Photoconductivity response curves of: a) antimony sulfide thin films heated in air and N2 with and without Se vapor; b) photoconductivity response of CuSbS2 and AgSbSe2 thin films formed by heating Sb2S3–CuS and Sb2S3– Ag2Se layers.
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3.5. Electrical properties Antimony sulfide thin films heated with and without Se in N2 at 300 °C shows the same order of photocurrent response. However in the film heated in air in contact with a ZnS–Se–ZnS film at 200 °C for 30 min, the photocurrent response improves considerably, compared with an Sb2S3 film subjected to the same condition of heating. These results are shown in Fig. 5. Both CuSbS2 and AgSbSe2 are also photoconductive. The dark conductivity of these materials is ≈ 10− 3 (Ω cm)− 1, well suited to serve as p-absorbers. 4. Photovoltaic structure Current (I)–voltage (V) characteristics of the photovoltaic structures formed using the materials developed above are shown in Fig. 6. The heterojunction SnO2:F–CdS–Sb2S3 was heated in air at 250 °C during 30 min; it shows Voc = 470 mV and Jsc = 0.02 mA/cm2 (Fig. 6a). The presence of a Se thin film placed in contact with the structure during heating at 225 °C for 30 min in air improves the characteristics: Voc = 670 mV and
Fig. 7. I–V characteristics of the structures TCO–CdS–Sb2S3–CuS–Ag(print) and TCO–CdS–Sb2S3–AgSbSe2–Ag(print) in the dark and under illumination.
Fig. 6. I–V characteristics of the structures: a) TCO(SnO2:F)–CdS–Sb2S3; b) TCO–CdS–Sb2S3–Sb2SxSe3 − x; c) TCO–CdS–Sb2S3–CuS; and d) TCO–CdS– Sb2S3–AgSbSe2. In all the cases the illumination is from the TCO side and a silver print contact of about 1 mm2 area painted over the top layer served as pside contact.
Jsc = 0.05 mA/cm2 (Fig. 6b). We found in this case that the silver print electrode reacts with the top layer, probably forming a Ag2Se layer, leading to a cell degradation. But on freshly painted silver print electrodes the same characteristics are obtained. This issue did not come up in cell structures with CuS or AgSbSe2 as the top layer. For the photovoltaic structures consisting of Sb2S3–CuS (Fig. 6c), we used the first layer of Sb2S3 (300 nm) heated in nitrogen at 300 °C, followed by the deposition of the second Sb2S3 layer (300 nm) and subsequently the deposition of a CuS thin film of 50–100 nm and heated in air at 200 °C. This cell presents Voc = 460 mV and Jsc = 0.4 mA/cm2. For the structure with AgSbSe2 (Fig. 6d) the Sb2S3 layer was heated in air at 225 °C during 30 min in contact with a ZnS–Se–ZnS film and another Sb2S3 film was deposited over this. The Ag2Se film was deposited on this and the structure was heated in nitrogen at 200 °C for 90 min in contact with a Se thin film to form AgSbSe2. I–V characteristics for this structure show: Voc = 450 mV and Jsc = 1.4 mA/cm 2 . Fig. 7 shows both I–V curves in the dark and under illumination for these two cell structures. The presence of a high series resistance as well as comparable parallel resistance is obvious in these cells. The deposition procedure described here avoided film peeloff during sequential deposition. This process has to be optimized in further work to improve the cell characteristics.
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5. Conclusions In this paper we presented a methodology to deposit Sb2S3 thin films of 500–600 nm in thickness, adequate for use in photovoltaic structures from a single bath. This is an improvement over the methods reported previously, in which multiple depositions have been used to build up the thickness to this value. These films have been used to prepare solid solutions of the type Sb2SxSe3 − x with graded composition and optical band gap (≈ 1.3–1.7 eV) along the thickness, desired in photovoltaic structures, by heating them in contact with a chemically deposited Se thin film. Photovoltaic absorber films of ternary compositions, CuSbS2 (band gap, 1.54 eV), and AgSbSe2 (1.2 eV) may be produced from the Sb2S3–CuS and Sb2S3–Ag2Se, which are photoconductive and possess dark conductivity 10− 3 (Ω cm)− 1, suitable to serve as p-absorbers. The use of these films in photovoltaic structures has been illustrated. We consider that further work on the optimization of the film thickness, thermal processing and cell configurations would lead toward new solar cell technologies using chemical bath deposition as the principal thin film technique. Acknowledgements The authors are grateful to María Luisa Ramón and Patricia Altuzar Coello, for recording the XRD and XRF data; Oscar Gomez-Daza for the assistance in the optical characterization and José Campos for the electrical charac-
terization. We acknowledge the financial support received from DGAPA-UNAM (IN110906) and CONACYT (Project 40515-F and graduate fellowship for SM). References [1] O. Savadogo, K.C. Mandal, Sol. Energy Mater. 26 (1991) 117. [2] M.T.S. Nair, Y. Peña, J. Campos, V.M. García, P.K. Nair, J. Electrochem. Soc. 145 (1998) 2113. [3] I. Grozdanov, Semicond. Sci. Technol. 9 (1994) 1234. [4] O. Madelung, Semiconductors Other than Group IV Elements and III–V Compounds, Data in Science and Technology, Springer-Verlag, Berlin Heidelberg, 1992. [5] Y. Rodríguez-Lazcano, M.T.S. Nair, P.K. Nair, J. Cryst. Growth 223 (2001) 399. [6] Y. Rodríguez-Lazcano, M.T.S. Nair, P.K. Nair, J. Electrochem. Soc. 152 (2005) G635. [7] O. Savadogo, K.C. Mandal, J. Phys., D, Appl. Phys. 27 (1994) 1070. [8] O. Savadogo, K.C. Mandal, J. Electrochem. Soc. 141 (1994) 2781. [9] K. Bindu, P.K. Nair, Semicond. Sci. Technol. 19 (2005) 1348. [10] K. Bindu, José Campos, M.T.S. Nair, P.K. Nair, Semicond. Sci. Technol. 20 (2005) 496. [11] K. Bindu, M.T.S. Nair, T.K. Das Roy, P.K. Nair, J. Electrochem. Solid State Lett. 9 (2006) G-195. [12] D.Y. Suarez-Sandoval, M.T.S. Nair, P.K. Nair, J. Electrochem. Soc. 153 (2006) C91. [13] K. Bindu, M.T.S. Nair, P.K. Nair, J. Electrochem. Soc. 153 (2006) C526. [14] B. Pejova, M. Najdoski, I. Grozdanov, S.K. Dey, Mater. Lett. 43 (2000) 269. [15] R.A. Smith, Semiconductors, Second. Ed, Cambridge University Press, Cambridge, 1978, p. 314.