The effects of heat-treatments in vacuum on the electrical and

photoconductive properties of screen printed and sintered layers

of cadmium sulphide have been studied. The dark conductivity of

the sintered layers is found to increase by severat orders of magnitude.

These layers are useful for making solar cell

Chockalingam, M.J.

Mahalingam, R.

Nagarajarao, K.

Radhakrishna, I.

Rangarajan, N.

Suryanarayana, C.V.

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Bulletrn of Electrochemrstry \ ( I )  Jan-Feb 1985, 70-71 STUDIES ON VACUUM ANNEALED CADMIUM SULPHlDE SINTERED PHOTO- CON DUCTlVE LAYERS MRS MARY JULIANA Cl-DCKALINGAM, R MAHALINGAM, K NAGARAJA RAO, I RADHAKRISHNA, N RANGARAJAN A N D  C V SURYANARAYANA Central Electrochemical Research Institute, Karaikudi 623 006 ABSTRACT The effects of heat-treatments in vacuum on the electrical and photoconductive properties of screen printed and sintered layers of cadmium sulphide have been studied. The dark co.nductivity of the sintered layers is found to increase by severat orders of magnitude. These layers are useful for making solar cells. INTRODUCTION Thin film phctovoltaic cells h s e d  on heterojunctions have been considered for many years as interesting alternatives to single crystal solar cells. Thin film technology is highly suitable for making cheap and viable photovoltaic cells useful for solar energy conversion. The most promising thin film photovoltaic generator which has been wldely investlgoted and where much progress in research and technology hus been obtalned during the lost fkw years is the d S / C u l ~  solar cell. It is expected that this type o f  solar cell can reach the price goal which has been defined for economical operation competitive to conventional pcwer plants. Thin film solar cells requlre a minimum of material and energy input. This is an ~mportont advantage in view of the availability of materials and pay-back time of solar cells. CdS films required for solar cell fabrication may be obta~ned by ( 1  ) vacuum 'vapour ,deposit~on (2) sputter~ng (3) spray deposition and (4) sintering. Certain advantages are nof~ced in the sinterinq technique : a ]  large area cells can easily be made b) incorporation of impurity is easy c) good bonding to substrate is obtained and d)  the method IS less expenslve. The only disadvantage wlth the method is that the dark res~stiv~ty of the CdS layers sintered in air IS very high of the order of 108 ohrn im.  In this paper, the sintered layers have been annealed in vacuum to  obtain the desired resistivity of 1 o h m i m  to 100 ohmim.  The effects of vacuum a n n e a l ~ n ~  studies on CdS have been studied prevcously / I ,  21. Recently an efficiency of 12.8% has been reported on screen printed and sintered CdS/CdTe solar cells [3]. EXPERIMENTAL High purity CdS powder is obtalned by the rnteraction of cadmium qcetate with thiourea under optimum pH condit~ons. A slurry is prepared using the powder, a fluxing material and a nonuqueous solvent, and then painted on ceramic substrates by the screen print~ng techn~que. The layers ore sintered in on atmosphere of alr in the temperature range between 500°C and 600DC for about 30 minutes. Ohmic contacts of Indium with an inter- electrode spacing of 1 rnm ore applied on the CdS layers by employing Hind Hivac Vacuum coating unit Model 7 2A4. The resistance values have been measured using Keithley Solid State Electrometer. Model 61K.  The th~ckness values of the stntered CdS layers as measured with the TESA Digital micrometer hnwn hnnn frrllnrl tn he 7 5  mirrfinq Fnr vnrllrjm onnccllino I,"",, &,,I , V V l l Y  "" ^-. , ,,. -.-...-. - -- studies, the sintered layers have been introduced Into the Vacutherm Microfurnace. Vacuum heat-treatments have been carried out at various temperatures for a pericd of 30 minutes in a vacuum of 10-5 torr. RESULTS AND DISCUSSION From the present investigation, the values of resistance of the CdS cells and the photosensitivity obtained by dividing dark resistance by light res~stance ore glven in Table I. The sintered cell before vacuum annealing possesses dark resistance value of 500 MA and light resistance value of 50 KA at 100 lux. The photosensitivity before vacuum annealing is equal to 104. On vacuum annealing, the dark res~stance of these cells changes consderably and so also the light resistance. Table I : Relationship between vacuum annealing temperature and resistance Temperature Dark reststance Res~stance at Photo- m OC ~n ohms 100 lux in sensitivity ( X  103) ohms ( x  103) It IS observed that the dark res~stance decreases from 500 M- (0s srntered In o ~ r j t o  250 K A  when heat-treated In vacuum at  250°C reach~ng a mlnlmum of 4 x 103 ohms at 350°C. Thls reduct~on In res~stance IS due to the desorption of oxygen states Above 350°C, agaln the res~stance values Increase as the temperature IS Increased upto 700°C Thus we nof~ce that the dark resistance value decreases by 5 orders of magn~tude at 350°C. Photo- sens~tlv~ty also IS the least at 350°C. Oxygen centres act as sensttlnng centres In CdS. They ~mmobhze holes. When all the oxygen states are desorbed, holes are made free wh~ch recombme w ~ t h  ~hotoexc~ted lectrons thereby destroylnq photosens~trvity - .  The effects of heat-treatments in vacuum and in air at relatively low temperatures (120°C) have been reported 141. In those experiments vccuum heat-treatments have not altered the conductivity markedly while alr treatments decrease it by two . . . .  . . . -  orders ot rnagnltude. However, a decrease In res~stwty by tour Mary JUI;~?W e t  01. - Vacuum annealed CdS sintered photo conducfrve layers orders of magnitude is reported when samples were subjected to annealing ;~t 260° +- 20°C at a high vacuum [5].  This is attributable to oxygen desorption. The present experiments also behave in a similar fashion. The sintered cadmium sulphide layers possess very h~gh  resistance values. Th~s is due to the compensat~on,caused by the acceptor- like oxygen states. Oxygen chemisorption phenomenon on cadmium sulphide crystals both as to its kinetics and energetics has been studied [6]. It has been established that oxygen forms deep acceptor-like states in cadmium sulphide with a level at 0.91 eV below the conduction band. The present investigation confirms this behaviour. The gdsorbed oxygen acts like an acceptor impurity. On the one hand, this causes a decrease in electron density and on the othei hand acts as a trap for the carriers that increase the potential barrier at the grain boundaries. This in turn affects the mobility according to the relation [7, 81. whereprepresent; t6e nhbility, Pathe mobility at absolute zero, q reprksents the number of charge carriers, and f represents the potential barrier between the grains. The reduction in resistivity observed when samples were annealed in vacuum upto 350°C indicates the loss of oxygen and chlorine from grain boundaries with a consequent reduction in 6. The increase in dark resistance that we have observed when the temperature of heat-treatment is above 350°C might be due to the secondary recrystatlization occuring at this temperature as a change in preferential orientation. A recrystallization ternperature at 350°C is low compared with the value of 600°C reported earlier [91 for pure CdS deposited in vocuum. However, the presence of impurities other than oxygen, which are known as inhibitors of recrystallization, may produce this effect. During recrystallization, the gram boundary movement due to crystallite growth would incorporate the impurities into the boundaries. These impurities seem to act like sensitizing centres wrth the result that photosensrtivity increases above 350°C. At 700°C, some of the acceptor-like mpurities wilt be removed. Hence, photosensitwity decreases. It has been noticed earlier that the minimum resistance occurs at 500' C vaccum onnealing jl j. In this case, ?he starting material was cadmium chloride instead of cadmium acetate. When cadmium chloride is the starting material, it is likely that more chloride ions might have entered the lattice during sintering. This has been confirmed and the dark resistance value is found to be 2 Mn for these samples 11 j. When vacuum annealing was carried out w ~ t h  this sample upto the temperature of 500°C. oxygen and chlorine might have come out of the lattice giving rise to increased donor concentrotions. In the present case, since cadmium acetate is the starting material oxygen alone will disappear and this happens at a lower ternperature of 350°C. From the dark resistance value, the dark sensitivity of the CdS layer after vacuum annealmg at 350°C works out to be 100 ohm-cm. Such layers are useful for CdS heterojunction solar cells. Acknowledgement : The authors thank Prof. K.I. Vasu, Director, Central Electrochemical Research Institute, Karaikudi, for his kind encouragement. REFERENCES 1. Mary Juliana C K Nagaraja Roo, N Ransaralan and C Y Suryanarayana. Trans. SAEST. 18 (1 983) 241. 2. L Exosura, E Garcia-Camerero, F Arjona and F Rueda, Solar Cells, 1 1 (1 984) 21 1 . 3. H Matsumoto. K Kur~boyashi. H. Uda, Y Kamatsu, A Nakono and S Ikegami. Solar Cells, 1 1 (1 984) 367. 4. C Wu and R H Bube, J.  Appl. Phys 45 (1974) 648. 5. S Martinuzzi, F Cabane-Brouty, J Qualid, J Gervars. A Mostavan and J L Grainier, in A Strub (Ed), Proc. 1 st Commission of the European Coinmunities Conf. of Photovoltaic Solar Energy, Luxembourg, September 27-30 (1977) Reidel, Dordrecht (1 978). p. 581. 6. P Mark, J Phys. Chem. Solids. 25 (1961) 91 1. 7. L L Kazmerski, W B Berry and C W Allen, J. Appl. Phys. 43, (1 972) 351 5. 8. R L Petritz, Phys. Rev. 104 (1 956) 1 508. 9. W Kahle and H Berger, Phys. Status Solid A. 2, (1970) 717. rJ Bulletin of Nectmchemistry 1 (1) Jan. -Feb. 1985 

Studies on vacuum annealed cadmium sulphide sintered photoconductive layers

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