J. MATER. CHEM., 1991, 1(6), 929-933 Characterization and Photoelectrochemical Studies of CulnS, Thin Films Prepared by Sulphurization of Culn Alloy Akhlesh Gupta and A. Surya N. Murthy* Department of Chemistry, Indian Institute of Technology, New Delhi, 110 016, India Thin films of CulnS,, prepared by sulphurization of electroless deposited Culn alloy at different conversion temperatures and time durations, have been characterized by XRD and AES. The films converted at 250 and 350 "C are of mixed phase while those converted in the range 450-550 "C are of single-phase sphalerite with (112) as the preferred orientation. Detailed photoelectrochemical investigations show the best films are those obtained at a conversion temperature at 450 "C. Keywords: Thin Film ; CulnS,; Chalcogenide There is a considerable interest in ternary chalcogenide mater- ials, especially CuInS, and CuInSe, for use in photovoltaic devices.CuInSe, has been found to be promising in solid- state heteroj~nction'-~ and photoelectrochemical (PEC) The band gap of CuInS, (1.55 eV) is improved over that of CuInSe, (1.05 eV) and efficiencies of 9.7% have been reported using CuInS,-based PEC A variety of techniques have been used to prepare CuInS,"-l8 thin films, including a two-step technique involv- ing chalcogenation of CuIn alloy with H2S at high tempera- ture~.'~-,~This technique is useful because of the ease with which the film composition can be controlled, thereby making it possible to optimize the optical and electrical properties.Recently, Gupta et have prepared CuInS, thin films by such a two-step technique, in which electrolytically deposited CuIn alloy films were annealed at only one temperature, uiz. 550 "C, for 30 min in an H2S atmosphere. In this paper we report the effects of conversion temperature and duration of treatment on the structural, compositional and PEC proper- ties of CuInS, films. Experimental The CuIn alloy films were prepared by an electroless technique on etched and cleaned Ti substrate~~~3,~ The sulphurization of CuIn alloy films was carried out in a conventional tubular furnace in flowing 100% H,S gas. Before sulphurization, all films were annealed in an N, atmosphere at 200 "C for 15 min; this was found to ensure a uniform composition in the bulk of material.23 Conversion temperatures in the range 250-650 "C were controlled using a Mesibus temperature controller (DTC-6004). The influence of annealing for different time periods was investigated at a constant temperature of 550 "C.Alloy films made from a solution containing molar ratios of In :Cu =1 (X-type) and 1.5 (Y-type) were employed for conversion. X-Ray diffraction (XRD) patterns, Auger Elec- tion spectroscopic (AES) data and PEC data were obtained as described in ref. 23. X-Ray photoelectron spectra (XPS) were obtained on a Physical Electronics Inc., model PHI- 1800 employing Mg-KE radiation (E = 1253.6 eV). Trans- mission electron diffraction (TED) measurements were carried out using a TEM Model-JEOL JEM 200CX system.The PEC cell was illuminated with a 650 W tungsten- halogen lamp and the intensity of water-filtered light (80 mW ern-,) falling on the glass window was measured by a calibrated thermopile (Kipp and Zonan, CAI-754323, Hol- land). No correction was made for light absorption by electro- lyte and reflection by photoanode. Results and Discussion XRD patterns of CuInS, films obtained from X-type CuIn alloy films (Fig. 1) show that considerable amounts of impurity phases such as Cu,S, Ins, In2S3 or In,03 are present, but the major phase is undoubtedly CuInS,. The amount of impurity phases is larger in films converted at 250 "C than at 350 "C. The occurrence of Cu,S and In2S3 impurity phases decreases with increasing conversion temperature. At higher temperatures, an increasing amount of CuInS, is formed directly from the metal alloy and also indirectly uia inter-diffusion of the binary sulphides.It is interesting that the formation of single-phase material was reported at a conver- sion temperature of 250 "C by Binsma and van der Linden" and at 350 "C by Grindle et aL20 with annealing times of 2 h in each case. Our films prepared at 350 "C (2 h) still contain impurity phases. We observed single-phase CuInS, when the conversion was carried out at 550 "C for 15 min. A single-phase, polycrystalline, sphalerite film with (112) as the pre- ferred orientation could also be obtained when a conversion temperature of 450 "C was applied for 30 min [Fig.l(c)]. The sphalerite structure has been reported for CUI~S,'~*~~ films prepared by the two-step technique. Hodes et al.," however, reported a chalcopyrite structure for CuInS, and conjectured that the high conversion temperature (550 "C) is responsible for such a structure, but our films that were converted at the same temperature and for the same duration are found still to possess the sphalerite structure [Fig. l(d)]. Attempts to convert the sphalerite CuInS, films into the chalcopyrite phase by an additional annealing" at 400 "C for 120 min in N, was unsuccessful. However, the crystalline properties of the film improved, as evidenced by the sharpness of the major diffraction peaks. While the films converted at 550 "C were of single phase [Fig.l(d)], the XRD pattern of the film obtained at 650 "C [Fig. l(e)] show some extra peaks characteristic of Ins impurity phase and of a compound containing Ti and S [Fig. l(f)], along with CuInS, peaks. A comparison of Fig. l(c) and (d) shows that the structure and orientation of the films converted at 450 and 500°C are the same, thus establishing that the crystalline structure is relatively insensi- tive to the conversion temperature in the range 450-550 "C and Cu, In and S form a single phase and stoichiometric CuInS, films. In order to confirm the nature of the phases and to detect any impurity phases, TED measurements were performed. The TED pattern of CuInSz films converted from X- and Y-type CuIn alloy are shown in Fig.2. The d-values, calculated from the diameter of rings in the TED patterns, b I 0 0 0I I.(c)I I I I 0 0 20 40 60 80 20 40 60 80 201° 2el" Fig. 1 XRD patterns of CuInS, films, converted at (a)250, (b)350, (c) 450, (d) 550, (e)650 "C and (f)Ti substrate annealed at 650 "C in H,S. Cu,S; .,0,CuInS,; 0,Ti; x, Ins; 0, (Ti-S); A, In,S,; A,In,O, (a) Fig. 2 TED patterns of CuInS, films from (a)X-type and (b) Y-type,CuIn alloy (see text) are compared with XRD data and standard d-values in Table 1. The data are consistent with the formation of polycrystalline CuInS,. The characteristic d-values for the chalcopyrite. phase and other impurity phases are absent. The TED and XRD patterns23 therefore confirm the forina- Table 1 Comparison of TED and XRD data calculated" dlA hkl XRDb 3.16 112 3.17 3.04 103 - 2.74 200,004 2.75 2.39 21 1 - 2.04 2 13,105 - 1.94 220,204 1.95 1.66 312,116 1.66 1.59 224 1.59 1.37 400,008 1.38 1.26 3 16,332 1.26 1.23 404 - 1.13 228 1.13 "Ref.35. bData from Fig. l(c). 'Data Fig. 2(b). observed, d/!i TED' TEDd 3.16 3.17 - - 2.74 2.74 - - - - 1.94 1.95 1.67 1.67 1.60 1.60 1.39 1.39 1.26 1.28 - - 1.13 1.13 from Fig. 2(a). dData from tion of a single-phase, polycrystalline thin film with the sphalerite structure. AES analysis shows that oxygen, if present, is <5 atom% in all samples, irrespective of the conversion temperature, and the proportion of sulphur on the surface remains uniform. The Cu:In ratio (1.34) at the surface of the film converted at 250 "C progressively decreased towards unity with increasing temperature, the proportion of indium increasing correspond- ingly.This implies that indium loss (probably as In2& seen as the yellow deposition in the cold region of the tube) is greater at low temperatures and decreases with increasing conversion temperature. This, may be due to the fact that at lower temperatures the rate of In loss as In2& is greater than the rate of CuIn alloy conversion to CUI~S~.~' Depth profile J. MATER. CHEM., 1991, VOL. 1 analyses of the film (Fig. 3) show that the films converted at 250 and 350 "C are non-uniform in the bulk in terms of their Cu :In ratio.Increasing the conversion temperature causes the ratio to become more uniform. It was thought that besides annealing temperature annealing duration is also important. Therefore, the films were converted at 550 "C for varying times of 15, 30 and 60 min. The XRD patterns show that all the films are of a single phase with the sphalerite structure. AES analysis of these films at the surface and in the bulk show that conversion duration does not have a marked effect and all the films are almost of the same composition and uniformity in depth. It can be concluded that CuIn alloy can be completely converted into CuInS, using 550 "C conversion for 15 min or conversion at 450 "C for 30 min. XPS of single-phase CuInS, was carried out in order to determine the valence states of Cu and In and their binding energies.The observed binding energies of Cu 2p3 /2, Cu 2p, /2, In 3d,,,, In 3d3,, and S 2p are in good agreement with those reported in the literature (Table 2) for single-phase CuInS, thin films. Since the binding energies for Cu and In in impurity phases (such as Cu2S, In203, In2S3) are very close to those of Cu and In in CuInS,, it is very difficult to establish the absence of impurity phases in CuInS,. However, some peak broadening is expected if impurity phases are present. The full-width at half-maxima (FWHM) calculated for Cu 2p3 /2, In 3d,/, and S 2p for CuInS2 film are in good agreement with literature values (Table 2). The difference in binding energy of Cu 2p3/, and S 2p [A(Cu-S)]; In 3d5,, and S 2p [A(In-S)] are also in agreement with the literature values (Table 2). These results indicate that CuInS, films converted 100 I 60ti 20 .-c 60 a 401 0 O/ 801:,1--------;20 0 0 6 12 18 sputter tirnelrnin Fig.3 AES depth profile of CuInS, film converted at (a)250, (b)350 and (c) 450 "C 93 1 Table2 Binding energies (in eV) for Cu, In and S associated with CuInS, E,IeV thin thin film in film film present thin single (single (impure thin XPS study film" crystal" phase)b phase)b film' Cu 2p,,, 952.3 -951.87 952.5 953.0 -CU 2p3,, 932.3 930.8 931.7 932.7 933.2 932.6 (1.9) (2.5) (1.9) (1.8) (2.3)In 3d,,, 452.1 -452.7 452.3 452.3 451.9 In 3d,,, 444.4 443.4 444.6 444.7 444.6 444.3 (1.4) (2.4) (1.4) (2.1)s 2P 161.7 -161.5 161.8 161.8 -(2.1) (2.4)A(Cu-S) 770.6 -770.2 770.9 771.4 -A(1n-S) 282.7 -283.2 282.9 282.8 -A(Cu-S), Binding energy difference between Cu 2p3,, and S 2p peaks.A(1n-S), Binding energy difference between In 3d,,, and S 2p peaks. Values in parentheses represent the FWHM. "Ref. 26. bRef. 27. 'Ref. 28. at 450°C for 30 min do not have any impurity phases like Cu,S or In,03. Detailed PEC studies on all films have been carried out in polysulphide solution. The stoichiometric CuInS, films (from X-type CuIn alloy) were p-type, showing negligible photo- voltaic activity, while the other films that contained an excess of In were n-type and showed good photovoltaic activity.This is as expected since the n-type CuInS, materials, includ- ing the single crystal29 show more band bending than p-type materials in any redox Therefore, detailed PEC experiments were carried out only for CuInS, films obtained from Y-type CuIn alloy films. The dark and photo I-V characteristics of CuInS, film converted at 450 "C for 30 min are shown in Fig. 4.The short- circuit current (SCC) variation is in the range 8.5-10 mA cm-2 14 f 8 a 6E 2 4 2 ,I I /-.'I I 1 +200 +400 V/mV Fig. 4 Current-voltage characteristics of CuInS, film electrode in polysulphide (3 mol dmP3 NaOH, 3 mol dm-3 Na,S and 4 mol dm-3 S) in the dark and under illumination of 80 mW cm-, 932 J.MATER. CHEM., 1991,VOL. 1 for different batches of samples, and the open-circuit voltage 60 - (V0J remains almost the same (ca. 340 mV). A steep rise in the dark anodic current at a reverse bias of ca. +0.25 V is typical of these films and limits our ability to measure photocurrent where there is strong band bending. From the 50 - photocurrent onset measurements a flat-band potential (Efb) of -0.45 V was obtained. This is larger than Efbfor electro- plated CUI~S,,'~ but it is comparable to that obtained with ~40c .- slurry-painted CUI~S,~' and by sulphurization of electro- C 3 plated CuIn alloy.21 The fill factor is small (0.22),typical of the n-CuInX,/polysulphide ~y~tem,~,~~*~~*~~ and is not neces- n5 30 sarily indicative of the film quality.The spectral response of the cell is shown in Fig. 5. More notable is the almost flat response between short-wavelength absorption of the electro- lyte and the long-wavelength semiconductor absorption edge; all samples gave a similar shape. Some sub-bandgap response was also observed, which may be due to excess of In (excess of In was confirmed by AES analysis but not detected in the XRD pattern).23 The formation of In-S or In-0 compound or CuIn,S8 is a po~sibility'~*~~ and is indicative of the intrinsic defect state in the material, giving rise to optical transitions when the illumination intensity is less than the bandgap energy.33 The extrapolation of the linear portion of (4,h~)~ us. hv plot (Fig. 6) gave the value of the bandgap, 1.44 eV, which is in good agreement with the values of 1.46eV,,' 1.43 V,34 1.45 eVI8 and 1.44 eV', for CuInS, thin films but smaller than that for the single crystal 1.5 eV,35 1.48 eV8 and 601 50.00" g @ \€30-/-g 20 -10 0' I I I I I N h>c:2 20 ' 10. I I 1.3 1.4 1.5 1.6 hvleV Fig. 6 (4,hv)' us. hv for the determination of the bandgap calculated value of 1.55 eV. This difference may be due to sub-bandgap defects. A detailed PEC study of CuInS, films prepared under different conditions has been carried out in polysulphide electrolyte and data are presented in Table 3. The PEC characteristics of the film obtained at 450 "C are better than those for the film obtained by electrodeposition" or screen printing3' and are comparable to the film obtained by sulphurization of electroplated CuIn all~y.'~,~~**~ The PEC characteristics of the film begin to deteriorate as the conver- sion temperature increases above 450 "C, although there was not much deterioration between 450 and 550 "C.When the conversion temperature was 650"C, the PEC response was very poor, possibly due to the presence of impurity phases (Ins and Ti-S). The effect of conversion duration (15, 30 and 60 min) on the PEC properties was also studied, the films converted response curves the the same for all samples shapeof spectral for 30 min gave was best performance. The except that converted at 650 "C. Two values of the bandgap energy were calculated for this sample from its spectral response curve. The stability of the film (Y-type alloy converted at 450 "C for 30 min) has been examined in polysulphide electrolyte at short-circuit current (SCC) and maximum power point (Fig.7).The current initially increases at both points. Since an Table 3 Photoelectrochemical data on CuInSz (from Y-type CuIn alloy) converted under different conditions current conversion temperature/ "C, durationlmin light intensity /mW cm-2 density (J =ISCIA) ImA cm-2 open-circuit voltage, vo,/mv fill factor efficiency("/.I flat-band potential" E,,ImV bandgap energy /eV 450, 30 550,30 80 80 9.87 6.80 335 380 0.22 0.22 0.90 0.73 380 440 1.44 1.45 650, 30 80 1.33 350 0.29 0.17 400 1.41 and 1.33 550,15 550,60 80 80 5.83 4.23 350 340 0.22 0.21 0.50 0.38 370 380 1.44 1.43 "Potential against the solution potential. J.MATER. CHEM., 1991, VOL. 1 c 0 4 8 12 16 tih Fig. 7 Stability of n-CuInS, (obtained from Y-type alloy) converted at 450 "C for 30 min at (a)the short-circuit point and (b)the maximum power point current is observed at the SCC point. Such a fall in current has been observed for thin film~~l,~~and for the single crystaLZ9 However, at the maximum power point, the current is stable [Fig. 7(b)]. Therefore, the operation of a cell at maximum power point is more advantageous than at the SCC point. Conclusion (1) Deposition by the electroless method is as good as elec- trodeposition for the preparation of CuInS, films.(2) XRD, TED and XPS analyses confirm the presence of single-phase, (1 12) oriented, sphalerite structure of CuInS, films obtained at 450 "C. (3) Films converted at low or very high temperature contain impurity phases. (4) The compositional uniformity of films increases with temperature. (5) Films obtained at 450 "C are better suited for PEC cells. (6) Long-term operation is advantageous at the maximum power point rather than at the short-circuit point. The financial support of the Department of Non-conventional Energy Sources (no. 2/5/4/82 and 102/6/86/NT) Government of India, and IIT is gratefully acknowledged. The authors thank Dr. G. K. Padam, National Physical Laboratory, New Delhi, and Dr. S. P. Singh, Department of Physics, IIT Delhi, for helpful discussions.References 1 W. E. Devaney, R. A. Mickelsen and W. S. Chen, in Proc. 10th IEEE Photovoltaic Specialists Conf., Las Vegas, 198.5, New York, 1986, p. 1733. 2 R. Noufi, R. J. Matson, R. C. Powell and C. Herrington, Solar Cells, 1986, 16, 479. 3 R. A. Mickelson, Proc. Polycryst. Thin Film Review Meeting, 1983, Golden, SERI Publication CP211- 1985. 4 G. Razzini, L. P. Bicelli, B. Scrosati and L. Zanotti, J. Electro-chem. SOC., 1986, 133, 351. 5 D. Cahen and Y. W. Chen, Appl. Phys. Lett., 1984,46, 746. 6 D. Haneman and J. Szot, Appl. Phys. Lett., 1985, 46, 778. 7 S. Menezes, Appl. Phys. Lett., 1984, 45, 148. 8 M. A. Russak and C. Carter, J. Electrochem. SOC., 1985, 132, 1741. 9 J.G. Fleming, M. L. Feaheiley and H. J. Lewerenz, J. Electro-chem. SOC., 1989, 136, 1506. 10 H. J. Lewerenz, H. Goslowsky, K. D. Husemann and S. Fiechter, Nature (London), 1986, 321, 687. 11 L. L. Kazmerski, M. S. Ayyagari and G. A. Sanborn, J. Appl. Phys., 1975, 46, 4865. 12 H. L. Hwang, C. C. Tu, J. S. Maa and C. Y. Sun, Solar Energy Mater., 1980, 2, 433. 13 H. L. Hwang, C. L. Cheng, L. M. Liu and C. Y. Sun, Thin Solid Films, 1980, 67, 83. 14 H. L. Hwang, B. H. Tseng, C. Y. Sun and J. J. Loferski, Solar Energy Muter., 1980, 4, 67. 15 P. Rajaram, R. Thangaraj, A. K. Sharma, A. Raza and 0. P. Agnihotri, Thin Solid Films, 1983, 100, 111. 16 G. K. Padam and S. U. M. Rao, Solar Energy Muter., 1986, 13, 297. 17 A. N. Tiwari, D. K.Pandya and K. L. Chopra, Thin Solid Films, 1985, 130, 217. 18 R. N. Bhattacharya, D. Cahen and G. Hodes, Solar Energy Muter., 1984, 10, 41. 19 J. J. M. Binsma and H. A. Van Der Linden, Thin Solid Films, 1982, 97, 237. 20 S. P. Grindle, C. W. Smith and S. D. Mittleman, Appl. Phys. Lett., 1979, 35, 24. 21 G. Hodes, T. Engelhard, D. Cahen, L. L. Kazmerski and C. R. Herrington, Thin Solid Films, 1985, 128, 93. 22 G. Hodes and D. Cahen, Solar Cells, 1986, 16, 245. 23 A. Gupta, A. N. Tiwari and A. S. N. Murthy, Solar Energy Muter., 1988, 18, 1. 24 A. Gupta and A. S. N. Murthy, J. Muter. Sci. Lett., 1989,8, 559. 25 C. D. Lokhande and G. Hodes, Solar Cells, 1987, 21, 215. 26 Y. Mirovsky, R. Tenne, D. Cahen, G. Swatzky and M. Polak, J. Electrochem. SOC., 1985, 132, 1070. 27 A. N. Tiwari, Ph.D. Thesis, Indian Institute of Technology, Delhi, 1986. 28 P. Rajaram, R. Thangaraj, A. K. Sharma and 0. P. Agnihotri, Solar Cells, 1985, 14, 123. 29 Y. Mirovsky, D. Cahen, G. Hodes, R. Tenne and W. Giriat, Solar Energy Muter., 1981, 4, 169. 30 G. Hodes, T. Engelhard, J. A. Turner and D. Cahen, Solar Energy Muter., 1985, 12, 211. 31 Y. Mirovsky and D. Cahen, Appl. Phys. Lett., 1982, 40, 727. 32 F. A. Theil, J. Electrochem. Soc., 1982, 129, 1570. 33 R. Haak, D. Tench and M. A. Russak, J. Electrochem. SOC., 1984, 131, 2709. 34 M. Gorska, R. Beaulieu, J. J. Loferski and B. Roessler, Solar Engery Muter., 1979, 1, 313. 35 A. Venkatarathnam and G. V. Subba Rao, Muter. Chem. Phys., 1987, 16, 145. 36 L. L. Kazmerski, M. A. Ayyagari, G. A. Sanborn, F. R. White and A. J. Merrill, Thin Solid Films, 1976, 37, 323. 37 C. D. Lokhande, J. Power Sources, 1987, 21, 59. Paper 1/O 12641; Received 18th March, 199 1