EDAXstudy shows that, for lower concentrations of. CdSt& in the .... stoichiometric deposit obtained in the periodic pulse ... Table I EDAXdata for CdTe films.
A COMPARATIVESTUDY OF COMPOSITIONALAND STRUCTURAL PROPERTIES CdTe THINFILMSPREPARED BY DIF’FERENTELECI’ROPLATINGTECHNIQUES P.D.Paulson,
V.Dutta
andkhatar
OF
Singh
Photovoltaic Laboratory, Centre for Energy Studies Indian Institute of Technology, New Delhi-l 10 016, INDIA. ’ Industrial-Tribology Machine Dynamics and Maintenance Engineering Indian Institute of Technology, New Delhi-l 10 016. INDIA.
Centre.
ABSTRACT.
Cd2' The structural and compositional properties of CdTe fdms, deposited from an aqueous solution of CdSO and TeO. by Pulsed Electrodeposition, Potentiostatic and Galvanostatic Electrodeposition. have been studied. Polycrystalline films with a preferred < I1 1> orientation are obtained in all techniques. However films deposited by pulsed technique show the higher intensity for < 111 > peak. EDAXstudy shows that, for lower concentrations of CdSt& in the electrolyte, pulsed films are more stoichiometric compared to films deposited by other two techniques.
(2)
EXPERIMENT CdTe films were deposited on to the ITOlGlass substrates from a deposition bath contains aqueous solution of CdSO. saturated with TeO (= lbfhl) using a conventional three electrode system. The pH of the electrolyte is adjusted to 2.2 using HSO.. The bath temperature is kept constant at 9&J. The deposition technique is described elsewhere[5,7. CdTe films were deposited for different CdSO+ concentrations. The HTeOz concentration is varied by speciating the TeO. first with NaOH then brought in to 2.2 pH by adding H$O, to the solution[81. Deposition is also carried out for different pulse frequencies with 33% duty cycle. The fihns were then characterised using XRD (Rigaku Rotaflex) and EDAX (EDAXPV9900)
Thin film CdTe is one of the most promising materials for terrestrial photovoltaic[l]. The recent reports show that CdTe based solar cells deposited by different techniques like Electra- deposition, Screen printing, PVD, MOCVD[2] and Closed Space Vapor Transport (CSVT)[3] can yield efficiency more than 10% over small area. However the electrodeposition seems to be the ideal technique for the photovoltaic application, as it does not require any costly instrumentation for the vacuum and also can easily be scaled up for large area deposition[4]. CdTe films can be deposited by pulse, Potentiostatic and Galvanostatic Electrodepostion techniques. Since the sohtbility of TeO in HCI acid bath is less compared to sulphuric acid bath, we have used latter for the deposition of CdTe films. CdTe deposition occurs at a potential that is more positive than required for deposition of Cd as a separate phase. This is possible because of the negative free energy of formation of CdTe (-92 W/mole) from Cd and Te. There are two reactions ( eq.(l) and (2)) occurring at this potential [5]
+ 4e--
- Cd
It has been reported that the CdTe films obtained with the pulse technique has reduced porosity and improved conductivity compared with that obtained by the Potentiostatic method[q. In order to compare the compositional and structural properties of pulse plated films with Potentiostatically and Galvanostatically deposited films. we have conducted the EDAXand XRD studies. keeping the deposition conditions nearly identical.
INTRODUCTION.
HTeO,* + 3H'
+ 2e-
RESULT
8 DISCUSStON
At high concentrations of Cd in the electrolyte, the CdTe deposition is independent of the CdSO. concentration. It is only depend on the HTeO. concentrations in the electrolyte. At lower concentrations of CdSq 150 Hz) however, no good film except a black deposit was obtained.
Te + 2H,O -(l)
and 331 CH33654/94/0000-0331
$4.00 0 1994 IEEE
First WCPEC;
Dec. 5-9,
1994; Hawaii
XRD Characterization: Fig.1 shows the XRD pattern of three samples deposited by (a) Galvanostatic (b) Potentiostatic and (c) Pulse electrodepositions from a typical electrodeposition bath. All the three are films essentially cubic < 111 > preferential orientation. However the intensity of < 111 > peak is high for pulse deposited films suggesting a higher degree of preferential orientation. A low value FWHMof the < 111 >peak revealed that Potentiostatically deposited films have the higher grain size. This result is confirmed with the SEM and the grain size measured is = 0.7~. XRDpattem also shows the reflection from CdTe (220), (311) planes. It is also found that Cd (101) Te (112) and Te(lO1) phases are also present in the films.
The variation of Cd and Te content in the Fig.2 film with the concentration of CdSQ+ in the electrolyte
EDAX Analysis Fig.2 shows the variation in the stoichiometry with the concentration of CdSCQAt lower concentration of CdSOY the pulsed films were found to be more stoichiometric. No such difference is observed at higher concentrations and films deposited by either technique is Te rich.
occur on special surface sites designated by s. which is supposed to be constant. The adsorption process of HTeO: and @ions are governed by eq.(3) and (4) respectively.
W
.
A Ah 40 I0 (oqr**)
C.&T
4
fi0 2.0
depoited Fig. 1.Difhaction patterns of different asdeposited CdTe films. (a) Galvanostatic (b) Potentiostatic and (c) Pulse.
6.0
60
I(
lo*vlcn
Fig.3.EDAX Spectra of a typical (a) dc electroand (b) Pulse elecuodeposited films.
HTeO,'(Surf.)+
This behavior can be understood using the two step process proposed by Vedel et.al [9]. A two step process, adsorption of HTeO: ions and the Cd ions followed by the reduction of HTeOiin to CdTe is involved in the CdTe electrodeposition. Adsorption is supposed to
4.0 OC”T O.OIlcEY
Cd2*(Surf.)+
s+
s$
1
HTeO;,s
Cd",s
-(3)
--(4)
Where Land k’ are the rate constants respectively
332
for the adsorption and desorption reactions. The nearstoichiometric deposit obtained in the periodic pulse deposition might be due to the domination of either k’or k. However, a more detailed study will be required in this direction. Table I EDAXdata
I-
static depositions. The structural, and compositional properties of these films have been studied. The Pulse deposition is found to be superior than the other electrodeposition techniques. Good near-stoichiometric films are obtained for low (2O< f> 100)pulse frequencies. The speaciation of TeO with NaOH was found to be advantageous for DC deposition in confirmation with the compositional data. annealing.
for CdTe films
Element
ACKNOWLEDGEMENTS
DC
Cd
43.73
39.84
I--
56.27
60.16
Te
We are indebted to Prof.L.K.Malhotra, Thin Film Lab. IIT, New Delhi for providing the experimental facilities for these studies. The financial support from Council of Scientific and Industrial Research (CSIR), New Delhi is greatfully acknowledged. REFERENCES.
Pulse I
[l] J .Skarp, Y .Koskinen, S.Lindfors. A.Rautianen and T.Suntola, “Development and Evaluation of CdS\CdTe Thin Solar Film PV Cells”, Proceedings of 70 E.C.Photovo/faic Enegy Conference, April 8-12, 1991, Lisbon, Portugal, pp567-569
Cd
11
Te Te
54.33
1 52.65
Table.1 summarises the EDAXresults for effect of NaOH speciation.The introduction of more Te ions using NaOH seems to be advantageous for dc deposition, whereas it does not have any significant effect in Pulse deposited films. Thus a stoichiometric CdTe film can be obtained using DC technique by speciating Te with NaOH. ------..-
[2] H.S.Ullal. J.L.Stone, K.Zweibel, T. Surek, R.L.Mitchel,” Polycrystalline Thin Film Solar Cells and Modules”, P~vceedings of 6 PVSEC, Feb.lO-14, 1992, New Delhi, India, ~~81-87
1oow Lonz 2OHZ oc
[3] J. Britt and C. Ferekides, “Thin Film CdSl CdTe olar Cells With 15.8% Eficiency” Appl.Phy Letf., 62, 1993, pp285 l-2852 [4] S.K.Das and G.C.Morris,“heparation and Properties of CdSlCdTe Thin Film Solar Cells Produced by Periodic Pulse Electrodeposition Technique”. Sdar Energy Materiaials and Solar Cells, 30, 1993. ~107-118 [5] M.P.R.Panicker, M&aster and F.A.Kroger. “Cathodic Electrolyte”, of CdTe from Aqueous Deposition J.Elecfnxhem.Soc., 52, 1978. ~~566-572
0.0
boo
Fig.4.
-
1 600
[6] S.Moorthy Babu, R.Bhaskaran and P. Ramaswami,“Electrodeposition of CdTe by Potentiostatic and Pulse Technique”, Thin Solid films. 202, 1991, ~~67-75 lid0
~ooo
12w WYI WAVELENGTH lnml
Absorption Spectra ofasdeposited
CdTe
films The optical absorption of these films is as shown in the Fig.4. The films deposited by pulsed technique have higher absorption than the films deposited by the direct current technique. CONCLUSION CdTe films were deposited by Pulse and Potentio-
[7] M.W.Verbruggand C.W.Tobbias, “The Periodic Electroof Cadmium Telluride’, AlChE chemical Codeposition Journal, 33, 1987, ~~628-641 [8] R.N.Bhattacharya and K.Rajeshwar, a Electrodeposition of CdTe Thin Films’. J.E/ectmchem. SOC., 131, 1984, ~~2032-2037 [9] Catherine Sella. Pascale Boncorps. and Jacques Vedel. ” The Electrodeposition Mechanism of CdTe from Acidic Sot., 133. 1986, Aqueous Solutions”, J. Electmchem. ~~2043-2047
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