Cadmium sulphide layers have been prepared on various substrates by the screen printing and sinlering

technique on conducting and nonconducting substrates.These layers have been annealed in vacuum a1

350' C for obtaining enhanced conductivity. Cuprous sulphide layers have been obtained on these layers

by the Clevite process. Post heat-treatment has been carried out in air to form the required p-n

heterojunctions. Solar a 's have been fabricated and the output characteristics have been studied

Chockalingam, M.J.

Jayachandran, M.

Nagarajarao, K.

Radhakrishna, I.

Rangarajan, N.

IR@CECRI

FABRICATION OF Cu,SCdS SOLAR CELL USING SCREEN PRINTED CdS LAYERS M JAYACHANDRAN, (MRS) MARYJULIANA CHOCKALINCAM, K N A C A W A  RAO, I RADHAKRISHNA and N R A N C A W A N  Central Electrochemical Research Institute, Karaikudi -623006 ABSTRACT Cadmium sulphide layers have been prepared on various substrates by the screen printing and sinlering technique on conducting and nonconducting substrates.These layers have been annealed in vacuum a1 350' C for obtaining enhanced conductivity. Cuprous sulphide layers have been obtained on these layers by the Clevite process. Post heat-treatment has been carried out in air to form the required p-n heterojunctions. Solar a 's have been fabricated and the output characteristics have been studied. Key words: Cu,S-CdS solar cell, screen printing, sintering INTRODUCTION C dS layers have been obtained by various methods such as vacuum evaporation [ I ] ,  sputtering 121 (reactive sputtering of Cd in H2S), spray pyrolysis [3,4], screen printing and sintering 151, vapour deposition and electrodeposition 161 to yield solar cells with efficiencies close to that of single crystal values. Although vacuum evaporation of CdS followed by'the conventional Clevite wet process has been the popular method for fabrication of solar :ells, methods like spray pyrolysis and sintering seem to have a promising luture, because of their low cost of productiodunit power. Hence, in this paper the screen printing technique, sintering of CdS and its effect on different substrates are discussed. CdS layers sintered on ceramic substrates were used for making layer type cells by Clevite process. The output of the cells are reported and compared. EXPERIMENTAL Sintered CdS layem made by screen printing technique Photograde CdS powder was prepared by the thioureamethod developed in this laboratory [7] .  This powder was sensitized with cadmium chloride, evaporated to dryness and ground well so as to pass through 400 mesh sieve. A uniform coating was given on stainless steel, molybdenum, and ceramic substrates by silk screen technique. These layers were dried at 120°C in air and sintered between .500-600°C under controlled atmosphere in Gailenkamp Furnace with proportional control. The CdS layers prepared with cadmium chloride alone as the flux material yielded high resistance layers of the order of 100 k.ohms. To bring down the resistance, CdSO, was added along with 10'51 cadmium chloride. Four samples with various concentrations of CdSO, (1 -4 Xt) were prepared and cells were fabricated from each batch. The CdS layers thus obtained and vacuum annealed at 350°C a t  l o - "  tom for45 minutes 181 gave rise toa  layer with dark resistivity of 2.5 ohm-cm. Substrate effect on CdS layer Substrate treatment CdS films were formed on molybdenum sheet etched in 1 :1 H N 0 3 ;  stainless steeI sheet etched in 25 % femc chloride and 5 % HCI and ceramic substrates. Uniform and adherent coatings could be obtained on etched stainIess steel substrates. Preheated Mo and stainless steel substrates gave better adherence perhaps due to the formation of Iow resistance mono- molecular oxide layer. Etching of CdS layer Etching of the CdS layer is a necessary step as it opens up the crystallites for the topotaxial growth of Cu2S and leads to maximum coverage of the CdS layer. Attempts to etch the sintered CdS layer on ceramic substrate with 1 : 1 HCI gave good output. Shorting of the latter was observed in the case of metallic substrates. Hence sintering on metallic substrates not only warranted thick layers unlike in the case of ceramic substrates but also different sintering conditions. Morphology of CdS layer on different ~ b r t r a t e s  Figure 1 (a,b,c) shows the SEM pictures of CdS sintered layers on Mo, stainless steel and oeramic substrate respectively. The degree of conglomeration of grains decreases in the order ceramic, Mo and stainless steel. Less number of pits are seen in the CdS layer on the ceramic substrate. Mo substrate has greater number of pits, but are uniformly distributed. When double coating is given and compacted, it leads to uniform layer. On stainless steel substrate, larger and deeper pits are seen which lead to microcracks and shorting when Cu2S is formed over the layer. The pit formation can be attributed to the differential thermal expansion between the metal substrate and CdS layer. At the higher magnification, the spherical crystalline structure of the CdS layer is well brought out in figure 2 (a,b). The grain size lies in the range 1.5pm to 3.5pm. The crystalline structure is better in Mo (2 a) than in the case of stainless steel (2 b) or ceramic substrate. Heterojunction formation Various processes are available in the literature 1101 for the formation of Cu,S film on CdS layer such as: the Clevite process, solid state reaction (dry process), electrodeposition, vacuum deposition and spraying methods. When attemptsmere made to get thin CdS layers on metallic substrates by sintering technique, the substrate material and the surface preparation conditions were found influencing the characteristics of the CdS layers. The CdS layer was dipped for 10 seconds and Cu2S layer was formed. It was wmhed in triple distilled water to remove the soluble CdCI2 and then in acetone and dried at 1 10°C. As such no rectification was ohserved and apost heat-treatment was given to form the junction. In this work, the cells wereannealed at 2M°C inair for different timings. The optimum time for maximum V, and I,, was found to be 1 1  minutes. fabricated as shown in figute 3. I t  consisted of a sintered polycrystalline CdS layer of about 30 pm on a cerainic substrate. An indium coating covering about I0 W of the Cd? surface was formed and then annealed at about 360°C at a pressure of 1 0  tom for one hour. The Cu2S was formed over the CdS layer leaving some space between In contact and the Cu,S layer. Silver paste was applied LO the Cu2S layer. Leads were connected to the indium and silver contacts to serve aa negative and positive terminals of the cells. Polyatyrene lacquering was done over this and the whole system war encapsulated in an acrylic cup. It was then hermeticaHy sealed with epoxy resin to prevent the entry of atmospheric O2 or water vapour into theaystem. Jayachandran el at- Fabricallon of C u z -  CdS solar cell r -- -- (d Fig. 1 : S~r~lered CdS on diHerenl substrates (X270). (a) Mo (b) Sta~nless steel (c) Ceiam~c Solu cell frbricrtion In order to study the behaviour and contribution of the prepared iow- mistance CdS layer with CulS layer on the same, layer-type solar cells were Fig 2 : Slmered M S  (X2700) on [a) Mo and (bl Sla~oless steel 7 Fig. 3 :  Schemallc d~agram of t t ~ e  cerarnlc subslrale Cu2SICdS solar cells I )  Gmmlc subslrale 2) CdS 3) Cu2S 4)  S~lver conlacl 5 )  lnd~um contacl 5) Lacquer~ng 7) Encapsulanl 0) Eooxy seol T 82 Bulletin of Electrochemistry 2 (2) Mar.-Apr. 1986 RESULTS AND DISCUSSIONS The variation of photosensitivity with time is well brought out in the figure 4. Although various percentages of addition of CdS04 to the sensitized powder were tried, the addition of 2% and 3% of CdS04 gave RD / RL values in the useful range (since the CdS required for photovoltaic work should not only have a lowresistance but RD / RL value tending to 1). As is shown in the figure the sensitivity is maximum for 40 minutes sintering time for2% CdS04 as well as 3% CdS04. During sintering, the cadmium chloride melts at 58S°C and CdS dissolves in the molten cadmium chloride to initiate recrystallisation. Beyond optimized time and temperature, CdCl, volatilizes away.  0.0 10 20 3 0  4 0  5 0  60  ' 70  TIME IN MINUTES Fig. 4 :  Effect ol sinlering on CdS layer resistance: 1 ) 2 % CdS04 and 2) 3% CdSO, Effect of cadmium rulphate on the nrimtance of CdS The addition of CdS04 brings about an increase in cd++/S- -  ratio as revealed by the reactions CdS + CdSO, - 2 ~ d + +   2S02T ' (1 -x) CdS + x ~ d + +  -r CdS, -, CdS reacts with CdS04 to form more ~ d + +  ions and the evolution of SO, from the lattice creates more sulphur vacancy. As a result, ~ d + + /  S-- ratio increases which is reponsible for the lower resistance of the 1ayer:Above 3 %; CdS04 remains as such in the lattice and this bein an insulator does increase the resistance. Below 3% CdS04 sufficient Cdi+/ S-- ratio is not built for want of CdS04. Hence the high resistance. R o b  of crdmium chloride Figure 4 shows a decrease in the valuea of Rd/ RL with time up to 40 minutes and then a rise, which is due to the fact that more and more chloride ions are getting incorporated into the lattice giving rise to more donor levels. Above the sintering time of 40 minutes, more and more of chloride ions begin to get out of the lattice gradually thereby decreasing the donor concentration. Even though Mo subatrate is the best choice with spherical , p i n  structure for maximum efficiency [I 1 ] followed by stainless steel substrate, the cost, short circuiting anqpeeling-off of the layers after CuzS formation are the unfavourable factors, when low cost cells with reasonable conversion efficiency are aimed. The V, and I,, of the final cell with varied parameten are given in Table I. Table I : Properties of Cu,S-CdS Solar cells Cell No. .Deposition Sheet resis- Time of V,, IS, time tance of heat treat- Cu2-,S ment (4 (ohmspersq.) (minutes) (mV) ( m ~  / cm2) Maximum I,, was obtained for the cell No. 62 whose Cu2S sheet resistance is maximum. When the post heat-treatment time was increased from 5 to 14 minutes, the Cu2S sheet resistance also increased from 120 ohms to.200 ohmdsquare. This may be due to the diffusion of cadmium ions into the CuzS lattice. The CuzS being p-type, cadmium ion entering the lattice as donors brings about an increase in resistance [2]. However, the resistance decreases when post heat treatment time exceeds 14 minutes. This may perhaps be attributed to the number of cryper ions entering the CdS lattice exceeding the number of cadmium ions entering the Cu2S lattice. Thus a post heat treatment of 14 minutes tends to form the clialcocite phase of the CuzS layer which absorbs most of the incident solar radiation [13] for current generation. Also a recent work [14] on the physicochemical properties confirms the observation that the resistivity is more when the Cu content in the Cu,S layer increases and is maximum for Cu2S. CONCLUSION The addition of 3% CdS04 is necessary to obtain low-resistance, sintered CdS layen and thereby increasing the cell output. Further investigations are being continued to increase the sheet resistance of the cuprous sulphide layer and optimize <he couditions for chalcocite (near Cu2S) formation for getting maximum output. Acknowledgement: The authors thank Dr VK Venkatesan, Head, Electrochemical Materials Science Division for helpful discussions. The authors also thank Sri R Mahalingam for helping them in gettingsome of the layers vacuum annealed. REFERENCES 1. L C  Burton, Solar CeIk, 1 ( 1  980) 159 2. W Mueller, H Freg, K Radler and K H Schuller, Thin Solid Filmr, 59 (1979) 327 3. R R  Chamberlin and J S  Skarman, J Electrochem Soc, 11 3 (1 966) 86 4. L Escosura, E Garcia-Camarero, F A jona and F Ruedq Solar Celk, 11 (1948) 211 5. H Matsumato, N Nakayamaand S Ikegarni, JpnJApplPhys, 19 (1 980) 129 6. J F'McCann and M Skyllas Kazacas,J Electroad Chem, 11 9 (1 981 )409 7. C V. Suryananyanq N Rangarajan, K Naganja Rao and Mary Juliana ChocMingam, Indian Patent No. 115347 (1 968) 8. Mary Juliana Chockalingam, R Mahalingam, K Nagaraja Rao, I Radhit krishnq N Ranmaian and C V  Suwananyanq Bull Electrochem, 1 " - (1985) 70 9. L C  Burten, International Workshop on 'CdS s o h  ceils and other heterojunctions". April S M a y  2 (1975) p 305 10. M Savelli and J Bougnot, Solar Energy Conversion ~ d :  BO Seraphin, Springer Verlag, N.Y. (1 97 9) 2 13 11. H Saha and S Banerjee, Solar Cells, 7 (1 982) 233 12. FJ Bryant, AK Hani, S Salkalachen and C G  Scott,JPhys D: ApplPhys, 16 (1 983) 1755 13. W Palz and J Besaen, Photovoltaic SpecialirO Con5 May 2-4 (1 975) p 91 14. H Rickert, H D  Wienhafer, T E  Schmidt, R Wagner, Proc Fourth E C Photovoltaic Solar Enngy Confcrenct, (1 982) 827 EN J Bulletin of Electrochemistry 2 (2) Mar.-Apr. 1986 183 

Fabrication of cu,scds solar cell using screen printed cds layers

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