CdS solar cells

Thin Solid Films, 144 (1986) 223-228 PREPARATION AND CHARACTERIZATION

223

REACTIVE S P U T T E R I N G O F L A R G E - A R E A Cu2S/CdS SOLAR CELLS* E. VANHOECKE, M. BURGELMAN AND L. ANAF

Laboratory of Electronics, Ghent State University, Sint-Pietersnieuwstraat 41, B-9000 Ghent (Belgium) (Received August 8, 1984; accepted November 11, 1985)

Reactive sputtering is a promising technique for large-scale solar cell production. We grew thin chalcocite (Cu2S) films by sputtering a pure copper target in an H2S-Ar atmosphere. When sputtered onto glass substrates, the layers showed excellent stoichiometric and crystallographic quality. Sputtering similar films onto evaporated CdS layers gave rise to solar cells with an area of 3.6 cm 2 and efficiencies of up to 4.9~o. With experiments conducted on large-area cells (21 cm 2) we have already obtained an efficiency of 2.4~o. Several cells showed an open-circuit voltage Vocin excess of 550 mV; for one cell we even measured 612 mV, which is the highest value ever reported for a Cu2S/CdS solar cell. All this proves that excellent quality heterojunctions can be obtained by this epitaxial technique. Sputter etching to clean the CdS substrates prior to deposition gives satisfactory results. In addition, we experimented with wet etching (HC1 solution) of the CdS layers. It is known that the more common Clevite process to fabricate the Cu2S layer needs a rather thick (i.e. 25 tam) CdS layer to prevent shunt conductance. We conducted successful experiments to reduce this thickness and hence the production cost of these cells: CdS layers 10 lam thick still allow us to obtain good solar cells.

1. INTRODUCTION

We used the reactive sputtering technique for the production of Cu2S/CdS thin film solar cells. This type of cell has been brought to its state of the art by using two more common methods: the wet dipping process and the dry method. Sputtering now offers an alternative method that avoids all wet steps: it could be carried out in an all-vacuum process. In that way contamination of the constituent layers is not possible, the production process itself is shorter and less complicated and there are no fundamental problems for continuous large-scale production. * Paper presented at the Sixth International Conference on Thin Films, Stockholm, Sweden, August 13-17, 1984. 0040-6090/86/$3.50

© Elsevier Sequoia/Printed in The Netherlands

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E. VANHOECKE, M. BURGELMAN, L. ANAF

In the literature there are reports on solar cells with CuzS sputtered onto single crystal CdS ~, onto evaporated CdS 2 9, onto sputtered CdS 7'a°'~ and onto evaporated (Cd, Zn)S layers 6'7. 2.

EXPERIMENTAL PROCEDURE

We sputter Cu2S layers with a 99.998~o pure copper target (diameter, 10 cm) in an H 2 S - A r atmosphere. We keep the d.c. voltage at the target constant at 1.6 kV and the total pressure at 0.4 Pa; the incident r.f. power is then about 180 W and the deposition rate about 0 . 6 n m s -x. The HzS-to-argon flow ratio is regulated automatically to a preset value that is mostly of the order of 20~o-25~o. The substrates are always sputter cleaned before deposition of the CuzS layer. The thickness of the layers varies from 50 to 300 nm. Four substrates of area 2.5 cm × 2.5 cm are placed on a water-cooled holder 5.5 cm above the target and are coated simultaneously. Two of them are bare C o m i n g 7059 borosilicate glass substrates for characterizing the Cu/S layer itself and the other two are previously coated with evaporated chromium (50 nm), silver (1 ~tm) and CdS (5-30p.m) layers for fabricating solar cells (area, 3.6cmZ) 12. Sometimes only one 5.1 cm × 5.1 cm CdS substrate is used for fabricating a largerarea solar cell (21 cm2). The heterojunctions are provided with a gold contact grid 250 nm thick, evaporated through a molybdenum mask. The finger spacing was 700 tam and the finger width 80 lam. The top contact is completed with a collecting contact in a sandwich structure: Au(40nm)/Cu(1 ~tm)/Au(40 nm). The total grid transmission is about 84~o. All solar cell measurements are performed under Standard Test Conditions 13. We use a short arc xenon lamp to establish illumination of 100 m W cm -2. All efficiencies stated in this paper are referred to the total cell area. 3.

C u 2 S FILMS ON GLASS

We investigated our CuxS films sputtered on glass by means of X-ray diffraction, by analysing the optical reflection and transmission spectra R(2) and T(2) and by electrical measurements such as determining the resistivity. During previous work 8'9 we have determined a wide range for the crucial process parameter, i.e. the H2S-to-argon flow ratio, in which we obtain pure chalcocite films with a resistivity of about 10- 2 fl cm. These films are suited for solar cell fabrication, also with respect to the crystal orientation. Indeed, owing to the sputter etching of the glass substrates, we observe only three peaks occurring at 20 = 26 ° 24', 54 ° 24' and 86 ° 30' (Fig. 1). They correspond to the 004, 008 and (by extrapolating the two former d/n values) 00 12 directions of Cu2S according to the pseudo-orthorhombic indexation of Potter and Evans 14 (the Powder Diffraction File, Card 33-49015, based on ref. 14 has a monoclinic indexation). They also correspond exactly to the 002, 004 and 006 direction of Cu2S (high form) according to the hexagonal indexation of the Powder Diffraction File, Card 24-57A x5. Hence we conclude in any case that only the preferable 001 directions of CUES are detected in the X-ray diagram.

SPUTTERING OF C u 2 S / C d S SOLAR CELLS

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Sputtering Cu2Slayers in the way described above onto evaporated CdS layers gave rise to solar cells with efficiencies of up to 4.9~o (area A = 3.6 cm2; open-circuit voltage Voc = 585 mV; short-circuit current density Jsc = 13.3 mA c m - 2 ; fill factor F F = 63~o). The high values of Voc that we regularly measure on these cells (on one cell even 612mV!) prove that good quality heterojunctions are formed by this technique. We conducted experiments on large-area solar cells (21cm 2) and obtained an efficiency of 2.4~. The current-voltage plot of the best cell is given in Fig. 2. The characteristics are as follows: 15 ~tm evaporated CdS; 120 nm sputtered Cu2S; Voc = 500 mV; Js~ = 10.5 mA cm 2; F F = 47~. Sputtering the Cu2S allows the use of thin CdS layers. We fabricated good solar cells with a 10 ~tm CdS layer whereas the more c o m m o n Clevite process requires rather thick CdS films (about 25 lam) to avoid shunt conductance. Since the CdS layer need not be thick, it could be sputtered as well, giving rise to an all-sputtered heterojunction as prepared by Thornton and Anderson ~~. We sputtered CdS layers about 10 ~tm thick from a CdS target; the resulting solar cells had rather poor efficiencies of the order of 1 ~ owing to the high resistivity of the CdS (about 30kf~cm). For the best cell we found an efficiency q =1.1~o, Vo¢ = 4 7 0 m V , J,¢ = 6.4mA cm -a and F F = 37~o. A typical feature of all our cells is that they show no photovoltaic effect in their as-deposited state. This is in disagreement with the results reported in refs. 7 and 11. Only by annealing the cell is the diode behaviour formed, as shown in Fig. 3. The best results were obtained by heating in air at 160 °C for at least 60 min. Longer heating up to 2-3 h does not improve the efficiency but does not seem to harm the cell, from which we conclude that the duration is not critical. An alternative annealing with good results is heating in vacuum for 10-30 min at 140 °C.

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E. VANHOECKE~ M. BURGELMAN, L. ANAF

All the CdS layers were sputter etched prior to the deposition of the CUES. This improves the crystallographic orientation of the CuzS; however, in this case the 106 direction (and a small 115 peak) remain present (Fig. 1) owing to the imperfect orientation of the CdS films 1z. Some CdS layers were chemically etched in an HCI solution (1:3 H C I : H 2 0 , 60°C, 5-50s). Such an etch causes the CdS to have a textured surface and hence a lower reflectance, as pointed out in ref. 16. Generally we observed a rise in Isc of 10%-30% owing to the increased light trapping but also a drop in Vo~ and in FF, both to various extents. This behaviour is still under investigation. As a result of this wet etch the efficiency improves slightly (by about 0.35% absolutely) although the best smaller cell (4.9%) was not etched chemically. 5

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Finally, some typical characteristics of the~e cells (the smaller cells) were measured. Measurement of Voc and lsc at various light intensities and at various temperatures revealed a diode factor of about unity (illuminated cell), independent of temperature. An Arrhenius plot revealed a barrier height of about 0.9 eV and a recombination velocity $1 at the interface of 10 6 c m s - 1. The values of $1 measured on Cu2S/CdS solar cells fabricated by the dry and the wet method, both topotaxial methods, are in the same range. This is a good indication that we obtain an epitaxial growth with this sputtering technique 1. The diode factor in the dark ranged from 2 to 3, but depended on temperature. Capacitance-voltage measurements revealed for the CdS a doping concentration N ~ 3 x 1017 c m 3, and from Hall measurements on cross-shaped Cu2S samples we derived p ,~ 2 x 102° c m - t,/~ ~ 2 - 4 cm 2 V - 1 s - 1 a n d p ~ 10-2 f~cm.

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5. CONCLUSIONS W e have d e m o n s t r a t e d the possibility of f a b r i c a t i n g C u 2 S / C d S solar cells by s p u t t e r i n g the Cu2S layer o n t o e v a p o r a t e d C d S s u b s t r a t e s in an a l l - d r y process. H i g h q u a l i t y h e t e r o j u n c t i o n s can be formed, as i n d i c a t e d b y the high Voc values (more t h a n 550 mV) which we m e a s u r e o n several cells a n d b y the p r o d u c t i o n of a 3.6 cm 2 cell with 4.9~o efficiency. So far we have o b t a i n e d an efficiency of 2 . 4 ~ on l a r g e r - a r e a cells (21 cm2). W h e n s p u t t e r i n g the Cu2S layer, o n l y r a t h e r thin ( a b o u t l 0 ~tm) C d S layers are required, so t h a t s p u t t e r i n g of these C d S layers b e c o m e s of interest. W e realized an a l l - s p u t t e r e d C u E S / C d S h e t e r o j u n c t i o n t h a t resulted in a solar cell efficiency of 1.1~. ACKNOWLEDGMENTS This text presents research results of the Belgian N a t i o n a l E n e r g y Research a n d D e v e l o p m e n t P r o g r a m ( P r i m e M i n i s t e r ' s Office, Science P o l i c y P r o g r a m m i n g ) . T h e scientific r e s p o n s i b i l i t y is a s s u m e d b y its authors. M.B. t h a n k s the N a t i o n a l Science F o u n d a t i o n for a fellowship. REFERENCES 1 G. Armantrout, J. Yee, E. Fischer-Colbrie, J. Leong, D. Miller, E. Hsieh, K. Vindelov and T. Brown, Photovoltaic Properties of reactively sputtered Cu~S films and reactively sputtered CuxS-CdS heterojunctions, Proc. 13th IEEE Photovoltaic Specialists" Conf., Washington, DC, 1978, IEEE, New York, 1978, pp. 383-391. 2 L. Partain, G. Armantrout and D. Okubo, Nondestructive SEM measurements of minority-carrier transport parameters of Cux S/CdS solar cells as a function of heat treatment, IEEE Trans. Electron Devices, 27(1980) 2127-2133. 3 E. Elizalde, M. Leon, F. Rueda and F. Arjona, Thin film CuzS/CdS junctions produced by evaporation and sputtering: effect of thermal treatments in vacuum, Proc. 4th Commission of the Eur. Communities" Photovoltaic Solar Energy Conf., Stresa, 1982, Reidel, Dordrecht, 1982, pp. 809-817. 4 W. Anderson, A. Jonath and J. Thornton, Magnetron reactive sputtering deposition of Cu2S/CdS solar cells, Proc. 2nd Commission of the Eur. Communities" Photovoltaic Solar Energy Conf., West Berlin, 1979, Reidel, Dordrecht, 1978, pp. 890-897. 5 A. Jonath, W. Anderson, J. Thornton and D. Cornog, Copper sulfide films deposited by cylindrical magnetron reactive sputtering, J. Vac. Sci. Technol., 16 (1979) 200-203. 6 J. Thornton, D. Cornog, R. Hall and L. Dinetta, Apparatus surface conditioning effects in coppersulfide reactive sputtering for photovoltaic applications, J. Vac. Sci. Technol., 20 (1982) 296-299. 7 J. Thornton, Copper sulfide/cadmium sulfide heterojunction cell research by sputter deposition, Proc. 16th IEEE Photovoltaic Specialists" Conf., San Diego, CA, 1982, IEEE, New York, 1982, pp. 737-742. 8 E. Vanhoecke and M. Burgelman, Reactive sputtering of thin Cu2S films for application in solar cells, Thin Solid Films, 112 (1984) 97-106. 9 E. Vanhoecke, M. Burgelman and L. Anaf, Reactively sputtered Cu2S films and Cu2S-CdS solar cells, Proc. 17th IEEE Photovoltaic Specialists" Conf., Orlando FL, 1984, IEEE, New York, 1984, pp. 890-895. 10 W. Muller, H. Frey, K. Radler and K.-H. Schuler, CdS-Cu:,S heterojunctions produced by r.f. sputtering, Thin Solid Films, 59 (1979) 327-336.

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11 J. Thornton and W. Anderson, High performance all-sputter deposited CuzS/CdS junctions, Appl. Phys. Lett., 40 (1982) 622-624. 12 A. de Vos, K. Stevens, L. Vandendriessche and M. Burgelman, X-ray diffraction study of the dry formation of Cu2S-CdS solar cells, Sol. Cells, 8 (1983) 33-45. 13 Standard Procedure For Terrestrial Photovoltaic Performance Measurements, Specification 101, Commisssion of the European Communities, Brussels, 1979. 14 R. Potter and H. Evans, Definite X-Ray Powder Data for Covellite, Anilite, Djurleite and Chalcocite, J. Res. U.S. Geol. Survey, 4 (1976) 205-212. 15 Powder Diffraction File, Joint Committee on Powder Diffraction Standards, Swarthmore, PA, Card 33-490; 1974, Card 24-57A. 16 J. Bragagnolo, Photon loss analysis and design of thin-film planar junction CuzS/CdS devices, Proc. 2nd Commission of the Eur. Communities' Photovoltaic Solar Energy Conf.. West Berlin. 1979, Reidel, Dordrecht, 1979, pp. 882-889.