of CdTe by electrodeposition
Ann. Chim. Sci. Mat, 1998,23,
OF CdTe BY ELECTRODEPOSITION THERMODYNAMIC ASPECT
M. RAMIl, E. BENAMARI,
F. CHRAIBI1, A. ENNAOUI’
1 Laboratoite de Physique des Mat&iaux, Universite Mohammed V, Faculte des Sciences, B.P. 1014, Rabat, Maroc 2 L.P.M.C., Universit6 Ibn Tofail, Faculd des Sciences, B.P. 133, 14000 Ktnitra, Maroc Summary : The cathodic electrodeposition of cadmium telluride on conducting transparent indium tin oxide (ITO) coated glass plates from an acidic aqueous bath containing CdSO, and Te02 is described The study of the thermodynamic aspect of electrodeposited CdTe has allowed us to predict the deposition potentials range. The influece of deposition potential and bath temperature on the structure and cristallinity were investigated. X-ray difliaction analysis showed that the films deposited at room temperature are not cristallized whereas those deposited at higher temperatures have a cubic structure. A decreaseof the potential improves the cristalline quality. Photoelectrochemical characterization of the deposit in aqueous polysulfide as an electrolyte was used to determine the conduction type ad the effect of deposition potential on the photocurrent. Resume. Electrodtnosition des couches minces de CdTe et asnect thermodvnamiaue: Des couches minces de tellurure deum ont ettcCltsctrodepos&esa partir d’une solution acide de CdSO, et de TeOz sur des substrats conducteurs en verre recouvert d’oxyde d’indium dope a l’etain (ITO). L’&ude de l’aspect thermodynamique de la codeposition cathodique de CdTe nous a permis de prtvoir la gamme de potentiel ou la codtposition peut avoir lieu. L’influence de la temperature du bain et du potentiel de deposition sur la structure et la cristallinid a Cte ttudit?e. L’analyse par diffraction des rayons X redle que les couches obtenues a temperature ambiante sont quasiment amorphes et celles depo&s i des temperatures blev&es sont de structure cubique. Le dtplacement des potentiels de deposition vers des valeurs negatives ameliore la qualitt cristalline des d&p&. La caracterisation photoel&rochimique des depots dam une solution de sulfate de sodium a Cte effectuee pour determiner le type de conduction et l’effet du potentiel de deposition sur le photocourant.
1. INTRODUCTION The properties of cadmium telluride (CdTe) films are of considerable interest because of their potential use for solar cells and others optoelectronic devices. The material has a direct band gap of 1.45 eV (1) which is an almost ideal match to the space solar spectrum and the reason why is used as an absorber for heterojunction solar cells. It has a high absorption coefficient (10’cm~r) (2). It is known that CdTe films can be prepared by several techniques such as closed space sublimation (3), evaporation (4), metalo-organic chemical vapor deposition(5), spray(6) and electrodeposition (7). This latter process has attracted considerable attention due to its simplicity, efficient material utilization and large scale low cost fabrication potential. In fact, cells with over 14.2% efficiency have been reported (8) and 900 cm’ modules have been fabricated on a production basis using the electrodeposition technology (9). The thermodynamic study of the electrodeposition of binary compounds specially cadmium chalcogenides has allowed to understand the formation mechanisms of these materials and the potential range where the codeposition can occur (10). In this paper we will study on the one hand the thermodynamic aspect of the cathodic codeposited of CdTe and, on the other hand, the photoelectrochemical properties and the influence of the deposition parameters (deposition potential and bath temperature) on cristallinity and morphology.
Renrints: M. RAMI, Laboratoire de Physique des Mamriaux, Faculte des Sciences, BP. 1014 RABAT-MAROC
M. Rami et al.
2. L The cathodic deposition of CdTe is related to the simultaneous electrodeposition of two ion species cadmium (Cd) and tellurium (Te). The elementary reactions and the corresponding equilibrium potentials for Cd and Te are : Cd2+ + 2e‘ H Cd(S) 0
‘Cd2+ - U(--d
I ENH - O.O295log-
+ 3H’ + 4e- e Te(S) + 2H20
a = 0.551V I ENH + O.O1481og- meo:
T; - -pH
The cathodic deposition of binary compounds was classified under two classes I and II (lo), depending on whether the difference in equilibrium potentials of the pure components is larger or smaller than the shift in potentials due to variation of the component activity in the deposit resulting from compound formation. The activities of Cd and Te in CdTe depend on the stoichiometric composition of the compound, the two being related through the reaction. Cd(S) + Te(S) + CdTe (S) 131 *%dTe = - 25.5 kcal mol-’ - acdre -- exp(- g$ acdare or for tire = 1
[ 41 acdaT?xp(
For temperatures at which deposition can occur, CdTe has an existance range stretching from CdTe in equilibrium with Cd to CdTe in equlibrium with Te.Thus the Cd activity varies from ~=l at the CdTeKd boundary to ~xp(AG/BT) at the CdTeJ’Te boundary where aT,=l , with an opposite variation of arc. These variation of aor and ar, cause a shift in the Cd and Te potentials. For reactions [l] and  hE = I$& - Etd= 0.954 The free enthalpy of formation of CdTe from Cd and Te is AG= - 25.5 kcal mol.‘. Compound formation causes a shift of the Cd potential from the standard value (-0,403VIENI-I) at the CdTeJCd phase boundary to -0.403 + AG/2F=O.l43V at the CdT&e phase boundary. The Te potential shifts simulaneously from 0.551V at the CdTelTe phase boundary to 0.551 +AG/4F =0.824V at CdTe/Cd boundary [lo]. It is seen that AE > AG/2F. Therfore CdTe has to be classified under classe I. with Cd as the potential determing species under all conditions (10). In order to be able to deposit Cd and Te in almost equal quantities it is necessary to use an electrolyte with a high concentration of the less noble component (Cd) and a low concentation of the noble component (Te). 3.m The cathodic deposition of CdTe films on commercially available conducting indium tin oxide coated glass substrates (ITO) was carried out in an aqueous solution of sulphuric acid containning 0,2 M CdS04 and 1mM HTeO*+. The pH value and the corresponding deposition potential have been chosen in such a way that they provide a common immunity region for simultaneous codeposition of cadmium and tellure. A platinium sheet, saturated calome.1 electrode (SCE) and iridium tin oxide (ITO) coated glass plates were used as the counter electrode, reference electrode and substrates, respectively. A scanning potenial ( Pine instrument model AFRDES Bi-potentiostat) was used for potentiostatic deposition of the CdTe films. Curves of current v.s deposition voltage were recorded with an X-Y recorder. A constant temperature was maintained by immersing the electrochemical cell containing the electrodes, substrate and electrolyte into a thermoregulated bath. The structure of the deposited products was analysed by X-ray diffraction ( XBD), their morphology was studied by using scanning electron microscope (SEM). 4. RESULTS AND DISCUSSION A plot of current density v.s cathode voltage ( measured relative to a saturated calomel electrode SCE ) for deposition of CdTe at different temperatures is shown in fieurel. The current increases rapidly at -0.24 v.s SCE when the interface activity of HTeOr+ is gradually reduced. A plateau is reached between -0.35 and -0.64V when the interface value of the activity of HTeOz+ is reduced to zero. The current density increases again at -O.&W when deposition of pure cadmium becomes possible. The results of preliminary experiments lead us to select the potential range for deposition of CdTe on IT0 substrates, which is ranging from -0.35 to -O&W.
of CdTe by electrodeposition
,’ 0 0
FIG. 1 - Current density vs cathode voltage for deposition of CdTe from 0.2M CdSO4, 1mM TeGz , pH=2 sweep rate lOmV/s at 80°C and inset at 18°C.
29( [email protected]
FlG. 2 - XRD spectra of CdTe films deposited on ITO substrate from 0.2M CdS.04, ImM TeOr, at 80°C; deposition potential: (a) -0.6V. (b) -0.58V , (c) - 0.56 vs SCE.
4.1. Effect of the deposition potential -. The films deposited at potentials ranging from -0.35 to -0.63V vs SCE showed the characteristic X-ray diffraction peaks of cubic CdTe. At more negative potentials the formation of Cd was predominant. Figure 2 shows the XRD of CdTe films electrodeposited at different potentials. It is obvious, first, that there is a preferential reflection for the two samples which corresponds to the (111) direction of the cubic CdTe. The lattice constant corresponding to the main diffraction peak is found to be qx, = 0.640nm which is in good agreement with the theoretical value ( arr,=0.649 nm). Secondly, with decreasing electrodeposition potentials the peak height increases and the full width of the peak at half maximum decreases. This indicates an increasing crystallite size with decreasing eletrodeposition potential. 4.2. Effect
of bath temnerature
Films deposited at an electrolyte temperature of 18°C were not cristallized as seen from XRD, higher deposition temperatures lead to the formation of well cristallized cubic CdTe. The current densities were higher at higher temperature (finure 1). The influence of current densities on the properties of the deposited CdTe films can be
0 1 q*‘-
I.. .,. ,*. I.. .. . I... Wavelength
FIG. 4 - (ahv)* versus hv plot for CdTe electrodeposited at -0.6V vs SCE. The inset is the optical absorption spectrum.
M. Rami et al. summarised as follows: higher current densities lead to films with a rough surfaces and could be easily removed mecanically, whereas films deposited at low current densities have a smooth surfaces and are well adhering. In general, the layers are brown and well adhering to the substrates if they are grown slowly and not too thick. A typical SEM picture of an electrodeposited film is presented in fiaure 3. They reveal the presence of an homogeneous deposit with globular grains which show no apparent sharp edges. The average grain size is about 0.3 pm. Fiaure 4 shows a plot obtained from (ahv)*=A(hv-Eg) equation in which a is the absorption coefficient of light and considered to be proportional to InlO*, A is the absorption and Eg is the band gap. The value of the latter is obtained by extrapolating the linear region plot of (tiv)’ versus energy to zero. It is equal to 1.42 eV. The current-voltage curves in the dark and under illumination for films deposited at different potentials in contact with an electrolyte containning OSM NazS and 0.5 M NaOH are shown in figure 5. The films deposited at low potentials showed the highest photocurrent. This is may be attributed to an improvement of the cristalline quality with decreasing of the deposition potential. It is found that the films deposited in the potential range from -0.50 to -0.64V are n-type.
. _ ---
Under In the
FIG. 5 - Current density-potential curves in the dark and under illumination under two potentials: (a) -0.56 V, (b) -0.6V vs SCE.
, Under In the
for CdTe in Na#NaOH/O.S:OS
CONCLUSION We have studied the electrodeposition of CdTe on conducting substrates (ITO) in an acidic bath from CdSOa and TeOz. The thermodynamic study has allowed us to understand the potential range where the electrodeposition of this material can occur. We found that the CdTe films deposited in a potential range between -0.350 and -0.64V vs SCE have a cubic structure. The films deposited at low temperatures are not well cristallized, the increase in temperature and the decrease in deposition potential improve the crystalline qualify with [ Ill] oriented crysallites. Scanning electron micrographs show an homogeneous deposit with globular grains of about 0.3 pm. The band gap of CdTe derived from absorption measurements is about 1.42 eV. The deposited films exibit a photoresponse when are illuminated in a polysulfide redox solution. They behave as an n-type semiconductor when they are deposited at potentials ranging from 5.
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