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The photoelectrochemical behaviour of polycrystalline AgIn5Se8
0013-4636/96 63 .00+0.00 Pergamoa Journal, Lldd
Nuoo9MVaa Aem, You . 31, No. 10, pp . 1293-1298 . 1986. Britain. Pins in
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THE PHOTOELECTROCHEMICAL BEHAVIOUR OF POLYCRYSTALLINE AgIn 5 Se8 G. RAzztm and L. PERALDO BICELLI Department of Applied Physical Chemistry of the Milan Polytechnic, Research Centre on Electrode Processes of the C.N .R., Milan, Italy and M. ARFELLI and B. ScRosAn Department of Chemistry, University of Rome "La Sapienza", Rome, Italy (Received 2 December 1985; in revised facet 4 February 1986)
Abstraet-Polycrystalline samples of n-type AgIn,Se e were prepared from the constituent element and employed as electrodes in photoeleetrochemical cells . These semiconductor electrodes were characterized by spectral response, Mott-Schottky plots, current-voltage measurements and complex impedance analyses . The polycrystalline n-AgIn,Se s electrodes show a very encouraging photoeleetroehemical behaviour, since capable of offering solar to electrical conversion efficiencies of the order of 3 % . However, stability tests run both under illumination and in the dark, revealed that this interesting photoelectrochemical performance worsened upon time of illumination . This was interpreted in terms of photocorrosion which leads to the formation of a selenium film on the surface of the semiconductor electrode. INTRODUCTION The progress in the development of photovoltaics systems with a semiconductor-redox electrolyte junction has prompted the investigation of new semiconductor electrodes, mainly absorbing in the visible region of the solar spectrum. In this respect, ternary semiconductors, analogues of cadmium dichalcogeaides and obtained by cross-substitution of the electropositive constituents, have lately been the object of growing scientific interest. According to the Grimm-Sommerfeld rule[l], binary and ternary tetrahedrally coordinated semiconductors can be found when the cation-to-anion ratio is equal to 1 and the valence electron-to-atom ratio is 4. Their structure will result to be either a zincblend type or a related one . Thus, in the case of A(I)B(III)C(VI) semiconductors, where A - Ag, Cu, B = Ga, In and C = S, Se, Te, a chalcopyrite structure (strictly related to the zinc-blend structure) is observed . However, in the A(I)B(IIT)C(VI) system, in addition to the already considered one, another phase, having the A(I)Bs(III)C5(VI) formula, is possible . The compounds of the latter type should not have a completely tetrahedral structure, since the cation-toanion ratio is 0.75 and the valence electron-to-anion ratio is around 4.57. Effectively, typical examples of this class, such as AgIn5 Ss and CuIn,S5 , have a spinet structure, this predicting the presence of cation sites with tetrahedral and octahedral coordination in a 1 : 2 ratio. An interesting exception is AgIn,S 5 , for which a diamond-like structure has been reported[2] . This structure was explained assuming that two sites remain vacant in the cationic sublattice[2] . Hence, this compound is currently represented by the D 3 AgIn,Se 8 formula, where D is a cation vacancy[3] . A generalized cation-deficient formula, such as the above one, can
easily be explained following the Grimm-Sommerfeld rule. In fact, when the vacant sites are also counted, the predicted values for the cation-to-anion and for the valence electron-to-atom ratios, are obtained . The remarkable aspect of AgIn,S 8 and similar compounds (ey CdIn2 Se4 ) is in the incomplete lattice of electropositive ions . This is sometimes referred to as "defect" lattice with an uncorrect terminology since no defects as such are implied the structure[4] . AgIn5 Ss was characterized by Palatnik and Rogacheva[2, 5, 6] who investigated by thermographic, microstructural and X-ray methods the phase and structure composition relations in the 3(AgInSe2)-2(In 2 Se,) system. These authors found a relatively narrow region of homogeneity around the composition corresponding to Agin5 Ses . All the lines in the X-ray powder photograph of this ternary phase could be indexed assuming a tetragonal cell and a ratio c/a of about 2 .0, with a about 5 .80 A and c about 11 .59 A[5] . The equilibrium diagram of the Ag2 Se-In2Se, binary system was also examined and the optimum composition for the stoichiometric compound resulted to be 0 .5(Ag 2 Se . Sln 2 Se,)[6] . Further research on the crystalline structure of AgIn 5 Se s was carried out by various authors[3, 7, S] . The lattice parameters most recently determined, are a = 5.7934 A and c = 11 .6223 A[3] . However, the space group still remains uncertain:
by
Dza or S;(2), S,(7) and D' (3) . The interesting properties above illustrated, have prompted us to investigate the photoclectrochemical behaviour of AgInSe s . This study has been carried out within a systematic research program on photoelectrochemical cells based on ternary semiconductors having different structure[9] .
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Polycrystalline samples of n-AgIn,Ses have been kindly provided by Dr J. L . Shay of AT & T Bell Laboratories . They were synthesized from the constituent elements . The conductivity of the material is about 10 -4 (ohm cm) - ' . The Agln 5 Sc a samples were polished to 1 pm finish diamond paste. They were then etched in a I :1 H,SO4 : H 2 O 2 solution up to one minute and rinsed with deionized doubly distilled water. Similarly to the preceding case of CuIn,S s[9], this was the etching solution giving the best results in terms of photoelectrochemical behaviour . A conventional three-electrode glass cell was used for the photoelectrochemical tests . The counter electrode and the reference electrode consisted of a large surface platinum foil (10 em 2 ) and a saturated calomel electrode (see), respectively . The preparation of the semiconductor electrodes and the procedure for assembling the cell were described in detail elsewhere[1O] . The area of the semiconductor electrode exposed to the electrolyte ranged from 0 .05 cm 2 to 0.2 cm' . The aqueous electrolyte used was either 1 M Na2 SO 4 or 2 M KT, 20mM I 2 , the latter hereinafter called "polyiodide" solution . The two solutions, prepared using deionized doubly distilled water and analytical grade reagents, had a pH between 6 and 7 . The instrumentation included a standard photoelectrochemical setup, with an Oriel 150 Xe-arc lamp or a halogen lamp, a UDT 61 radiometer, an AMEL 553 potentiostat-galvanostat, an Applied Photophysics monochromator Model 7000 having 1200 groves mm - ', a Wayne Kerr B 331 Mk I t autobalance precision bridge at a frequency of 1 .6 kHz and a Solartron frequency response analyser, Mod . 1250. A set of neutral density filters was used for changing the light intensity[ 10] . Net photocurrents under monochromatic light excitation were measured with a phase sensitive technique which included a Brookdeal 9479 variable frequency chopper and a PAR lock-in amplifier Model 128A . Computer-aided acquisition and analysis of the experimental data was performed with an Apple HE microcomputer.
RESULTS AND DISCUSSION Spectral response The spectral dependence of the short-circuit photocurrent at constant flux of n-AgIn,S s in the polyiodide solution is illustrated in Fig. 1 . The threshold photoresponse at a wavelength of approximately 975 nm corresponds to a hand gap of 1 .27 eV, which is in close agreement with the value (E, - 1-25 eV) previously found by Benoit et al.[8], who, however, did not report the nature of the transition (eg direct or indirect). According to the general Gartner's theoretical relationship[l1-14] between the photocurrent under reverse bias condition when the semiconductor surface is depleted (le„) and the photon flux (hv > E a ) at the semiconductor surface (0), corrected for reflection losses and solution absorption, we have : Ir„=e'[1-exp(-aW)/(l+aL,)]
( 1)
o
750
850
9sc
Wavelength, nm Fig . 1 . Short-circuit photocurrent vs wavelength of n-Agin,Se a in a 2 M KI, 20 mM I, aqueous solution . The inset shows (I,hv) 1 2 and (Ip hv) 2 vs by and the band gap determination of the indirect (1 .27 eV) and of the direct (1 .47 eV) transition. ~
where a, Wand L p are the optical absorption coefficient of the radiation, the depletion layer width and the minority carrier diffusion length, respectively, and a is the electron charge . This expression only considers minority charge carriers migration in the barrier and their diffusion in the bulk material. Thus, electron-hole recombination effects both in the space charge region and in the surface through surface states and the thermal generation inside the junction are neglected, as well as charge-transfer process at the semiconductorelectrolyte interface, assuming that the electrochemical reaction is not rate-determining[15-18] . By operating at wavelengths close to the semiconductor absorption edge, where both L c and Ware much smaller than the optical penetration depth a - ', equation (1) may be simplified[13] in :
leh = eqbaW
(2)
On the other hand, at the same conditions, the optical absorption coefficient a depends on the photon energy hv, according to the equation : a = A (hv Ear a (3) by where A is a constant, E, the hand-gap energy and n is equal to 1 or 4 for a direct or an indirect transition, respectively . Therefore, the linear x axis extrapolation of (Iv„ - hv) 2 jw as a function of by yields the band gap value . This plot is reported for the case of AgIn 5 Se s in the inset of Fig. 1 for the two n values . The observed linear dependences indicate that there are two energy bands, the lowest at 1 .27 eV, due to the first indirect allowed transition and the other at 1 .47 eV, due to a direct mode. It is worth pointing out that the indirect gap width is close to the optimum value for terrestrial photovoltaics . Mott-Schottky plots Figure 2 presents a typical Mott-Schottky plot for a n-AgIn,Se a /electrolyte (1 M Na 2 SO 4) electrochemical junction in the dark at . 1 6 kHz . The curve is a straight line, though in a narrow potential range . From the
The photoelectrochemical behaviour of polycrystalline Agln,Se,
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12-
inn mACms
a
-a5 v, V ve . SCE v,V n .SCE
Fig. 2 . Mott-Schottky plot of n-AgIn,Se, in a 1 M Na 2 SO4 aqueous solution, at 1 .6 kHz intercept of this curve with the potential axis, a value of -0.52 V vs see is obtained for the flat-band potential, Vf,, of the semiconductor electrode. This experimental value is in agreement with that (-0 .6 V vs sce) calculated from Pauling electronegativities of the semiconductor atoms[19] . The method and the approximations used for this calculation are described in detail in a previous work[9] . Here we would only drive the attention to the fact that the best results are achieved when Pauling instead of Mulliken electronegativities are used and this evidences the empirical basis and, in a way, the weakness of the method . From the slope of the I/C2 vs V plot of Fig. 2, the donor concentration of the semiconductor electrode can be calculated, once the relative static dielectric constant a of the compound is known . To our knowledge, there are no reported values for this quantity . Nevertheless, a may be estimated indirectly from the reflectance at the semiconductor-air interface through the refractive index R and the Maxwell equation . Obviously, this is a very coarse approach, which, however, can allow us to approximate the optical dielectric constant, le to obtain the lower limit of the value of a and thus to calculate many helpful quantities . In the present case, adopting a mean value of R c 0.3[20], a results to be greater than 12 and the carrier density around 10' 6 cttt - a
Photocurrent-voltage curves Figure 3 shows the photocurrent-voltage curves of polycrystalline n-AgIn,Sc s in the 1 M Na 2 SO4 solution (curve b) and in the polyiodide solution (curve a) . In the first case the photocurrent reaches a maximum and the electrode exhibits a passivation behaviour of the same type as that typically observed when photoelectrodes are progressively covered by a layer of chalcogen atoms . When a polyiodide solution is used, the photocurrent onset is shifted cathodically. Moreover, also in this case, the AgIn,Se e electrode decomposes during prolonged anodic polarization with white light irradiation (see curve a of Fig . 3) . As experimentally observed, this photocorrosion results in the formation of a reddish layer on the electrode surface . Microprobe analysis proved that this layer is essentially formed by elemental selenium,
Fig. 3 . Photocurent density us applied voltage characteristics of n-Agln,Sea in a 2 M KI, 20 mM 1 2 (curve a) and in a I M Na 2SO4 (curve b) aqueous solution. Illumination: 100 mW cm -2, white light . Scan rata 10 mV s -1 .
even if small quantities of silver, indium and iodine were detected. On the basis of this result, one may postulate the following tentative scheme for the photocorrosion of AgIn,Se 8 in the polyiodide electrolyte: Agln,Se e +16H'-Ag'+5In3+ +8Sea ,,,t.
(4)
Scanning electron microscope (SEM) observations showed that the hereinafter-called - selenium layer" is highly porous and can be easily detached mechanically from the semiconductor surface (see Fig . 4). Consequently, this layer is not able to protect the underlying specimen surface and thus to entirely prevent the photocorrosion of the AgIn,Se e semiconductor electrode . This quasi-protecting effect exerted by the polyiodide solution is quite interesting and worth further consideration . Therefore, it has appeared to us of importance to integrate the voltammetric results with complex impedance analysis of the semiconductor/electrolyte interface .
Complex impedance analysis Figure 5 shows the complex impedance plot for an AgIn,Se 8 electrode illuminated in a polyiodide electrolyte . Curve a is the response of a freshly prepared electrode after an initial current-voltage sweep, curve b the response after 2 h of illumination and curve c after 3 h of illumination . Two semicircles are distinguishable. The intercepts with the real axis, passing from high to low frequency, may be associated to the resistance of the electrolyte, R„ to the charge-transfer resistance, Rn , and to the resistance of the selenium layer, R f . This interpretation seems to be confirmed by the complex impedance response of the AgIn,Ses electrode after progressive periods of illumination (curves b and c). It may be noticed that while the value of Re remains approximately constant, that of R f increases, as expected by the fact that, as the time of the illumination increases, the selenium layer becomes thicker and its resistance increases accordingly . Furthermore, from the maximum of the low frequency semicircle, a capacitance associabee to a thin layer, can be obtained . Therefore, the frequency response analysis sub-
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Fig. 4 . SEM micrograph of the n-Agln 5 Ses electrode surface after 15 h at short-circuit conditions in the 2 M KI, 20 mM I r aqueous solution under 1110 mW em 2 white light illumination.
nation (compare curve b of Fig. 3). Therefore, also in the dark, there is a passivation of the electrode, again with the possible formation of a resistive layer on its surface. Effectively, if the sweep is repeated, the maximum current decreases, as indicated in curve c of Fig . 6. However, the morphology of the film formed in the dark differs from that obtained under illumination, since a golden layer, instead of the usual reddish selenium one, was observed on the electrode surface . The nature of this film and its mechanism of formation are still under investigation . Various difficulties have been encountered on the identification of the film, mainly due to its very small thickness. This still unidentified golden film is highly protective ; polarization at 1 V vs see for one night of the
Fig. 5. Complex impedance plots for the Agln,Se s electrode illuminated in a polyiodide electrolyte. Curve a: after a current-voltage sweep, curve b : after 2 h illumination, curve c: after 3 h illumination .
stantiates the voltammetric results, confirming that a resistive layer grows on the semiconductor surface upon illumination in the polyiodide electrolyte .
t
.6 .4 .2
Dark current-voltage curves Figure 6 shows the dark voltammetric behaviour of the Agln,Se s electrodes in the polyiodide (curve a) and in the sulphate (curves b and c) electrolytes. The curves in the 1 M Na,SO, electrolyte present a maximum with a trend similar to that observed under illumi-
Fig. 6 . Dark current density vs applied voltage characteristics of a n-Agln,Se, in 2 M KI, 20 mM 1 r (curve a) and in I M Na 2SO, (curve b, first run ; curve c, second run) solutions. Scan rate : 10 mV s - ' .
The photoelectrochemical behaviour of polycrystalline Agln,Se, AgIn,Sc5 electrode in the dark, is sufficient to entirely passivate its surface- In fact, after this treatment, no maxima were observed in the voltage-current curve of the electrode. Disappointingly, the photocurrent exhibited by these coated AgInsSes electrodes are very small and thus the passivating film does not improve the performance of the semiconductor but rather depress it. The fact that the mechanism of passivation in the dark is different than that in the light is not surprising, since the latter involves majority charge carriers, while holes are not available for the dark anodic process . As discussed by Gerischer and coworkers in the case of the MoSe, electrode[21], the photocorrosion is induced by holes homogeneously generated by light absorption underneath the surface . These holes are driven toward s the electrode/electrolyte interface, reach the surface more or less homogeneously and induce there corrosion reactions by bond weakening . On the contrary, corrosion in the dark is more localized since the initial step is electron injection into the conduction band from surface states via tunnelling through the energy barrier of the depletion layer . The fact that in the case here examined, the dark current starts increasing when the band bending (ie the difference between the anodic bias and the flat-band potential) is still small than the band-gap energy (see Fig . 6) is an indication that such states are indeed present on the surface . Under these experimental conditions, electrons cannot tunnel to the conduction band from occupied energy states in the valence band. Therefore, one must consider electronic surface states above the valence band edge at energy levels higher than the conduction band in the bulk. Consequently, dark corrosion of the anodically polarized electrode is expected to he restricted to limited surface areas . The process will then result in a very high local current density and the corrosion product will primarily he formed at the more active surface defects which are thus rapidly passivated . The quasi-lone pair electron states of the anions around and directed towards the pseudovacancies in AgIn,Se, can be considered of similar characteristics as those arising from dangling bonds . Such bonds on the surface of a II-VI compound (eg CdSe) are known to give rise to states within the forbidden gap[4] . Therefore, near the top of the valence band, "vacant" sites are expected to introduce a high density of states which may be involved in the above discussed dark corrosion process . Current-voltage characteristics under monochromaticlight The Gartner equation (1) allows to determine the hole diffusion length L from current vs applied voltage measurements undeer monochromatic light illumination of the semiconductor electrode . To obtain this important parameter in the case here under study, we have examined the current-voltage curves of the AgIn,Se, electrode in the 1 M Na,SO, electrolyte under 700 nm monochromatic light . In the depletion layer approximation and for a homogeneous donor distribution across the spacecharge layer, W, can be expressed as[13] : W-Wo(V-V,5)1'r (5)
1 297
where Wo is the depletion layer width for a 1 V potential drop across it . The introduction of equation (5) in (1) and the manipulation[22] of the Gartner equation, result in the relation: -In(1 -q) > • aW (V-Vr,)112+ln(I+aL,) (6) where q = I,,/e* is the photocurrent efficiency . Since the dielectric constant a and the donor centre density ND have already been calculated, W can be determined from the Shockley-Read approximation: Wo=(2eco/eND)1j2 (7) where vo is the permittivity of the free space . We obtain Wo = 2.9 x 10-5 cm V-112 Efficiencies, as well as other measurements in the Na,SO4 solution, suffer from various uncertainties owing to the formation of the continuously changing surface film . Nevertheless, as predicted by equation (6) alineardependencebetweenln(1-q)and(V-v p 11/2 was experimentally found, even if limited in the ighpotential region[23] . Thus, once again, it was verified that at high anodic values, charge carrier recombination is negligible and the transfer reaction does not represent the limiting step . From the y axis intercept and the slope of the best least square fit to the experimental points, the following values have been calculated: Lr = 0.6 x 10-17 cm and ajz_MOnml - 1 .2 x 104 cm-1 . Owing to the very small value of the hole diffusion length, the flux of holes generated in the bulk that reach by diffusion the depletion region is practically negligible in our electrode specimens, as usually found in polycrystalline materials[24] . Output power characteristics and stability tests The polyiodide electrolyte was selected for the photoclectrochemical tests . Figure 7 shows the output power characteristics of a photoelectrochemical cell using a AgIn,Sea electrode, etched with a 1 :1 : H,O, solution. The values of the maximum H2SO4 power efficiency (3 .06 %) and of the fill factor (0 .49) are
v,v e Fig . 7. Output power characteristics of the polycrystalline n-AgIn,Se,/2 M ICI, 20 mM I,/Pt cell under 85 MW em-2 white light illumination .
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G . RAzzINI el al. (iii) Preliminary data on the behaviour of Agln s Se s in photoelectrochemical cells with the polyiodide electrolyte are encouraging in terms of power conversion efficiency (around 3 %) and fill factor (around 0 .5) . These values, which partly depend on the good match of the semiconducting electrode band gap with the solar spectrum, support us in attempting further approaches for the stability problem .
Iph, .A .Ire
I
00
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Acknowledgements-This work has been carried out with the support of Consiglio Nazionale delle Ricerche (CNR), Progetto Finalizzato Energetica 2, Sotroprogetto carbone e
rdrogeno . Fig. 8 . Short-circuit current density vs time of the unetched n-Agln s Ses /2M KI, 20mM 1 y /Pt cell under continuous 100mW cm - ' white light illumination . encouraging, especially considering that polycrystalline electrode has been used and that the light was not corrected for optical reflection losses and for solution absorption . Figure S illustrates a stability test of the AgIn 5 Se e electrode, determined as photocurrent versus time of continuous illumination . The steady decay in photocurrent indicates the progressive worsening of the electrode performance due to the deterioration of its surface, as also verified by SEM measurements . The limited stability observed in Fig . 9 points out that the pseudovacancies in the crystal structure which could cause rapid diffusion and drift of ionic species leading to fast deactivation of the electrode material by photooxidation[4], have a very detrimental effect in the case of the Agln s Ses photoelectrode . Up to now, attempts to improve material stability through electrode,/electrolyte junction stabilization by surface treatments and modification of the solution composition have not been successful .
CONCLUSIONS On the basis of the above described results, one can derive the following conclusions . (i) Using a Schottky barrier model to describe the AgIn 5 Se s n-type semiconductor in contact with a liquid electrolyte, it has been possible to determine some meaningful physical quantities, like the band gap, the flatband potential, the donor centre density, the barrier width, the minority carrier diffusion length and the transition mode . (ii) The material both as part of a regenerative photoelectrochemical cell and in rest conditions, turned out to be unstable, both under irradiation and in the dark, owing to corrosion phenomena . This behaviour has tentatively been related to the Agln s Ses cation-deficient structure, whose remarkable aspect is the incomplete lattice of electropositive ions .
REFERENCES 1 . H. Hahn, G . Frank, W . Klinger, A . Meyer and G. Storger, Z . Anorg . Ally- Chem. 271, 153 (1953) R. Nitsche, J. Physique 9C, 3 (1975). 2 . L. S . Palatnik and E . I . Rogacheva, Soc . Phys. Cryst llagr. 11, 189 (1966). 3 . P. Benoit, P . Charpin and C . Djega-Mariadassou, Mater. Res. Bull. 18, 1047 (1983) . 4 . R. Tenne, Y . Mirovsky, Y . Greenstein and D . Cahen, J. electrochem . Soc . 129, 1506 (1982) . 5 . L . S . Palatnik and E. I . Rogacheva,1zv. Akad. Nauk SSSR, Neorgan . Marerialy 2, 478 (1966) ; C.A . 65, 6426c (1966). 6. L . S. Palatnik and E . 1 . Rogacheva, Dokl . Akad . Nook SSSR 174, 80 (1967) ; C .A . 67, 77337f (1967) . 7 . M . Robbins and M . A. Miksovsky, Mater. Res. Bull. 6, 359 (1971) . 8 . P . Benoit, C. Djega-Mariadassou, R . Lesueur and J . H . Albany, Phys. Letters 73A, 55 (1979) . 9 . B . Scrosati, L . Fornarini, G- Razzini and L . Peraldo Bicelli, J . electrochem. Soc. 132, 593 (1985) .
10. G . Razzini, M . 1 srzari, L. Peraldo Bicelli, F . Levy, L. De Angelis, F . Galluzzi, E . Scale, L . Fornarini and B . Scrosati, J . Power Sources 6, 371 (1981) . l1 . W . Gartner, Phys. Rev. 116, 84 (1959) . l2 . F. Stern, Solid State Phys 15, 299 (1963). 13 . M . A. Butler, J . appl . Phys 48, 1914 (1977). 14 . R . H . Wilson, J . appl. Phys. 48, 4292 (1977). 15 . J . Rerchmann, Appl. Phys . Lett . 36, 574 (1980) . 16 . F. El Gvibaly and K . Colbow, J. appl . Phys. 53, 1737 (1982). 17 . R . U . E . `t Lam and D_ R . Franceschetti, Mater. Res . Bull. 17, 1081 (1982). 18 . S. K. Kovach and A . T. Vas ko, Elecorokhimiya 19, 252 (1983) . 19. M. A . Butler and D . S. Ginley, J . electrbchem . Soc . 125, 228 (1978). 20. J . L. Shay, B . Tell, H . M. Kasper and L . M . Schiavone, Phys. Rev . B7, 44R5 (1973). 21 . H . Gerisher, D. Ross and M . Lubke, Z . phys. Chem . Neue Folge 139, 1 (1984) . 22. J . H. Kennedy and K. W . 1, rese, J . electrochem . Soc . 123, 1683 (1976) . 23 . P . Salvador, C . Gutierrez, G . Campet and P . Hagenmuller, J . electrechem . Sac. 131, 550 (1984) . 24. S .-E . Lindquist, A . Lindgren and Z . Yan-Ning, J . electrochem . Soc. 132, 623 (1985) .
Nuoo9MVaa Aem, You . 31, No. 10, pp . 1293-1298 . 1986. Britain. Pins in
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THE PHOTOELECTROCHEMICAL BEHAVIOUR OF POLYCRYSTALLINE AgIn 5 Se8 G. RAzztm and L. PERALDO BICELLI Department of Applied Physical Chemistry of the Milan Polytechnic, Research Centre on Electrode Processes of the C.N .R., Milan, Italy and M. ARFELLI and B. ScRosAn Department of Chemistry, University of Rome "La Sapienza", Rome, Italy (Received 2 December 1985; in revised facet 4 February 1986)
Abstraet-Polycrystalline samples of n-type AgIn,Se e were prepared from the constituent element and employed as electrodes in photoeleetrochemical cells . These semiconductor electrodes were characterized by spectral response, Mott-Schottky plots, current-voltage measurements and complex impedance analyses . The polycrystalline n-AgIn,Se s electrodes show a very encouraging photoeleetroehemical behaviour, since capable of offering solar to electrical conversion efficiencies of the order of 3 % . However, stability tests run both under illumination and in the dark, revealed that this interesting photoelectrochemical performance worsened upon time of illumination . This was interpreted in terms of photocorrosion which leads to the formation of a selenium film on the surface of the semiconductor electrode. INTRODUCTION The progress in the development of photovoltaics systems with a semiconductor-redox electrolyte junction has prompted the investigation of new semiconductor electrodes, mainly absorbing in the visible region of the solar spectrum. In this respect, ternary semiconductors, analogues of cadmium dichalcogeaides and obtained by cross-substitution of the electropositive constituents, have lately been the object of growing scientific interest. According to the Grimm-Sommerfeld rule[l], binary and ternary tetrahedrally coordinated semiconductors can be found when the cation-to-anion ratio is equal to 1 and the valence electron-to-atom ratio is 4. Their structure will result to be either a zincblend type or a related one . Thus, in the case of A(I)B(III)C(VI) semiconductors, where A - Ag, Cu, B = Ga, In and C = S, Se, Te, a chalcopyrite structure (strictly related to the zinc-blend structure) is observed . However, in the A(I)B(IIT)C(VI) system, in addition to the already considered one, another phase, having the A(I)Bs(III)C5(VI) formula, is possible . The compounds of the latter type should not have a completely tetrahedral structure, since the cation-toanion ratio is 0.75 and the valence electron-to-anion ratio is around 4.57. Effectively, typical examples of this class, such as AgIn5 Ss and CuIn,S5 , have a spinet structure, this predicting the presence of cation sites with tetrahedral and octahedral coordination in a 1 : 2 ratio. An interesting exception is AgIn,S 5 , for which a diamond-like structure has been reported[2] . This structure was explained assuming that two sites remain vacant in the cationic sublattice[2] . Hence, this compound is currently represented by the D 3 AgIn,Se 8 formula, where D is a cation vacancy[3] . A generalized cation-deficient formula, such as the above one, can
easily be explained following the Grimm-Sommerfeld rule. In fact, when the vacant sites are also counted, the predicted values for the cation-to-anion and for the valence electron-to-atom ratios, are obtained . The remarkable aspect of AgIn,S 8 and similar compounds (ey CdIn2 Se4 ) is in the incomplete lattice of electropositive ions . This is sometimes referred to as "defect" lattice with an uncorrect terminology since no defects as such are implied the structure[4] . AgIn5 Ss was characterized by Palatnik and Rogacheva[2, 5, 6] who investigated by thermographic, microstructural and X-ray methods the phase and structure composition relations in the 3(AgInSe2)-2(In 2 Se,) system. These authors found a relatively narrow region of homogeneity around the composition corresponding to Agin5 Ses . All the lines in the X-ray powder photograph of this ternary phase could be indexed assuming a tetragonal cell and a ratio c/a of about 2 .0, with a about 5 .80 A and c about 11 .59 A[5] . The equilibrium diagram of the Ag2 Se-In2Se, binary system was also examined and the optimum composition for the stoichiometric compound resulted to be 0 .5(Ag 2 Se . Sln 2 Se,)[6] . Further research on the crystalline structure of AgIn 5 Se s was carried out by various authors[3, 7, S] . The lattice parameters most recently determined, are a = 5.7934 A and c = 11 .6223 A[3] . However, the space group still remains uncertain:
by
Dza or S;(2), S,(7) and D' (3) . The interesting properties above illustrated, have prompted us to investigate the photoclectrochemical behaviour of AgInSe s . This study has been carried out within a systematic research program on photoelectrochemical cells based on ternary semiconductors having different structure[9] .
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G . RAZZINI et a!. EXPERIMENTAL
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Polycrystalline samples of n-AgIn,Ses have been kindly provided by Dr J. L . Shay of AT & T Bell Laboratories . They were synthesized from the constituent elements . The conductivity of the material is about 10 -4 (ohm cm) - ' . The Agln 5 Sc a samples were polished to 1 pm finish diamond paste. They were then etched in a I :1 H,SO4 : H 2 O 2 solution up to one minute and rinsed with deionized doubly distilled water. Similarly to the preceding case of CuIn,S s[9], this was the etching solution giving the best results in terms of photoelectrochemical behaviour . A conventional three-electrode glass cell was used for the photoelectrochemical tests . The counter electrode and the reference electrode consisted of a large surface platinum foil (10 em 2 ) and a saturated calomel electrode (see), respectively . The preparation of the semiconductor electrodes and the procedure for assembling the cell were described in detail elsewhere[1O] . The area of the semiconductor electrode exposed to the electrolyte ranged from 0 .05 cm 2 to 0.2 cm' . The aqueous electrolyte used was either 1 M Na2 SO 4 or 2 M KT, 20mM I 2 , the latter hereinafter called "polyiodide" solution . The two solutions, prepared using deionized doubly distilled water and analytical grade reagents, had a pH between 6 and 7 . The instrumentation included a standard photoelectrochemical setup, with an Oriel 150 Xe-arc lamp or a halogen lamp, a UDT 61 radiometer, an AMEL 553 potentiostat-galvanostat, an Applied Photophysics monochromator Model 7000 having 1200 groves mm - ', a Wayne Kerr B 331 Mk I t autobalance precision bridge at a frequency of 1 .6 kHz and a Solartron frequency response analyser, Mod . 1250. A set of neutral density filters was used for changing the light intensity[ 10] . Net photocurrents under monochromatic light excitation were measured with a phase sensitive technique which included a Brookdeal 9479 variable frequency chopper and a PAR lock-in amplifier Model 128A . Computer-aided acquisition and analysis of the experimental data was performed with an Apple HE microcomputer.
RESULTS AND DISCUSSION Spectral response The spectral dependence of the short-circuit photocurrent at constant flux of n-AgIn,S s in the polyiodide solution is illustrated in Fig. 1 . The threshold photoresponse at a wavelength of approximately 975 nm corresponds to a hand gap of 1 .27 eV, which is in close agreement with the value (E, - 1-25 eV) previously found by Benoit et al.[8], who, however, did not report the nature of the transition (eg direct or indirect). According to the general Gartner's theoretical relationship[l1-14] between the photocurrent under reverse bias condition when the semiconductor surface is depleted (le„) and the photon flux (hv > E a ) at the semiconductor surface (0), corrected for reflection losses and solution absorption, we have : Ir„=e'[1-exp(-aW)/(l+aL,)]
( 1)
o
750
850
9sc
Wavelength, nm Fig . 1 . Short-circuit photocurrent vs wavelength of n-Agin,Se a in a 2 M KI, 20 mM I, aqueous solution . The inset shows (I,hv) 1 2 and (Ip hv) 2 vs by and the band gap determination of the indirect (1 .27 eV) and of the direct (1 .47 eV) transition. ~
where a, Wand L p are the optical absorption coefficient of the radiation, the depletion layer width and the minority carrier diffusion length, respectively, and a is the electron charge . This expression only considers minority charge carriers migration in the barrier and their diffusion in the bulk material. Thus, electron-hole recombination effects both in the space charge region and in the surface through surface states and the thermal generation inside the junction are neglected, as well as charge-transfer process at the semiconductorelectrolyte interface, assuming that the electrochemical reaction is not rate-determining[15-18] . By operating at wavelengths close to the semiconductor absorption edge, where both L c and Ware much smaller than the optical penetration depth a - ', equation (1) may be simplified[13] in :
leh = eqbaW
(2)
On the other hand, at the same conditions, the optical absorption coefficient a depends on the photon energy hv, according to the equation : a = A (hv Ear a (3) by where A is a constant, E, the hand-gap energy and n is equal to 1 or 4 for a direct or an indirect transition, respectively . Therefore, the linear x axis extrapolation of (Iv„ - hv) 2 jw as a function of by yields the band gap value . This plot is reported for the case of AgIn 5 Se s in the inset of Fig. 1 for the two n values . The observed linear dependences indicate that there are two energy bands, the lowest at 1 .27 eV, due to the first indirect allowed transition and the other at 1 .47 eV, due to a direct mode. It is worth pointing out that the indirect gap width is close to the optimum value for terrestrial photovoltaics . Mott-Schottky plots Figure 2 presents a typical Mott-Schottky plot for a n-AgIn,Se a /electrolyte (1 M Na 2 SO 4) electrochemical junction in the dark at . 1 6 kHz . The curve is a straight line, though in a narrow potential range . From the
The photoelectrochemical behaviour of polycrystalline Agln,Se,
1 295
12-
inn mACms
a
-a5 v, V ve . SCE v,V n .SCE
Fig. 2 . Mott-Schottky plot of n-AgIn,Se, in a 1 M Na 2 SO4 aqueous solution, at 1 .6 kHz intercept of this curve with the potential axis, a value of -0.52 V vs see is obtained for the flat-band potential, Vf,, of the semiconductor electrode. This experimental value is in agreement with that (-0 .6 V vs sce) calculated from Pauling electronegativities of the semiconductor atoms[19] . The method and the approximations used for this calculation are described in detail in a previous work[9] . Here we would only drive the attention to the fact that the best results are achieved when Pauling instead of Mulliken electronegativities are used and this evidences the empirical basis and, in a way, the weakness of the method . From the slope of the I/C2 vs V plot of Fig. 2, the donor concentration of the semiconductor electrode can be calculated, once the relative static dielectric constant a of the compound is known . To our knowledge, there are no reported values for this quantity . Nevertheless, a may be estimated indirectly from the reflectance at the semiconductor-air interface through the refractive index R and the Maxwell equation . Obviously, this is a very coarse approach, which, however, can allow us to approximate the optical dielectric constant, le to obtain the lower limit of the value of a and thus to calculate many helpful quantities . In the present case, adopting a mean value of R c 0.3[20], a results to be greater than 12 and the carrier density around 10' 6 cttt - a
Photocurrent-voltage curves Figure 3 shows the photocurrent-voltage curves of polycrystalline n-AgIn,Sc s in the 1 M Na 2 SO4 solution (curve b) and in the polyiodide solution (curve a) . In the first case the photocurrent reaches a maximum and the electrode exhibits a passivation behaviour of the same type as that typically observed when photoelectrodes are progressively covered by a layer of chalcogen atoms . When a polyiodide solution is used, the photocurrent onset is shifted cathodically. Moreover, also in this case, the AgIn,Se e electrode decomposes during prolonged anodic polarization with white light irradiation (see curve a of Fig . 3) . As experimentally observed, this photocorrosion results in the formation of a reddish layer on the electrode surface . Microprobe analysis proved that this layer is essentially formed by elemental selenium,
Fig. 3 . Photocurent density us applied voltage characteristics of n-Agln,Sea in a 2 M KI, 20 mM 1 2 (curve a) and in a I M Na 2SO4 (curve b) aqueous solution. Illumination: 100 mW cm -2, white light . Scan rata 10 mV s -1 .
even if small quantities of silver, indium and iodine were detected. On the basis of this result, one may postulate the following tentative scheme for the photocorrosion of AgIn,Se 8 in the polyiodide electrolyte: Agln,Se e +16H'-Ag'+5In3+ +8Sea ,,,t.
(4)
Scanning electron microscope (SEM) observations showed that the hereinafter-called - selenium layer" is highly porous and can be easily detached mechanically from the semiconductor surface (see Fig . 4). Consequently, this layer is not able to protect the underlying specimen surface and thus to entirely prevent the photocorrosion of the AgIn,Se e semiconductor electrode . This quasi-protecting effect exerted by the polyiodide solution is quite interesting and worth further consideration . Therefore, it has appeared to us of importance to integrate the voltammetric results with complex impedance analysis of the semiconductor/electrolyte interface .
Complex impedance analysis Figure 5 shows the complex impedance plot for an AgIn,Se 8 electrode illuminated in a polyiodide electrolyte . Curve a is the response of a freshly prepared electrode after an initial current-voltage sweep, curve b the response after 2 h of illumination and curve c after 3 h of illumination . Two semicircles are distinguishable. The intercepts with the real axis, passing from high to low frequency, may be associated to the resistance of the electrolyte, R„ to the charge-transfer resistance, Rn , and to the resistance of the selenium layer, R f . This interpretation seems to be confirmed by the complex impedance response of the AgIn,Ses electrode after progressive periods of illumination (curves b and c). It may be noticed that while the value of Re remains approximately constant, that of R f increases, as expected by the fact that, as the time of the illumination increases, the selenium layer becomes thicker and its resistance increases accordingly . Furthermore, from the maximum of the low frequency semicircle, a capacitance associabee to a thin layer, can be obtained . Therefore, the frequency response analysis sub-
G . RAVSNt et al.
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Fig. 4 . SEM micrograph of the n-Agln 5 Ses electrode surface after 15 h at short-circuit conditions in the 2 M KI, 20 mM I r aqueous solution under 1110 mW em 2 white light illumination.
nation (compare curve b of Fig. 3). Therefore, also in the dark, there is a passivation of the electrode, again with the possible formation of a resistive layer on its surface. Effectively, if the sweep is repeated, the maximum current decreases, as indicated in curve c of Fig . 6. However, the morphology of the film formed in the dark differs from that obtained under illumination, since a golden layer, instead of the usual reddish selenium one, was observed on the electrode surface . The nature of this film and its mechanism of formation are still under investigation . Various difficulties have been encountered on the identification of the film, mainly due to its very small thickness. This still unidentified golden film is highly protective ; polarization at 1 V vs see for one night of the
Fig. 5. Complex impedance plots for the Agln,Se s electrode illuminated in a polyiodide electrolyte. Curve a: after a current-voltage sweep, curve b : after 2 h illumination, curve c: after 3 h illumination .
stantiates the voltammetric results, confirming that a resistive layer grows on the semiconductor surface upon illumination in the polyiodide electrolyte .
t
.6 .4 .2
Dark current-voltage curves Figure 6 shows the dark voltammetric behaviour of the Agln,Se s electrodes in the polyiodide (curve a) and in the sulphate (curves b and c) electrolytes. The curves in the 1 M Na,SO, electrolyte present a maximum with a trend similar to that observed under illumi-
Fig. 6 . Dark current density vs applied voltage characteristics of a n-Agln,Se, in 2 M KI, 20 mM 1 r (curve a) and in I M Na 2SO, (curve b, first run ; curve c, second run) solutions. Scan rate : 10 mV s - ' .
The photoelectrochemical behaviour of polycrystalline Agln,Se, AgIn,Sc5 electrode in the dark, is sufficient to entirely passivate its surface- In fact, after this treatment, no maxima were observed in the voltage-current curve of the electrode. Disappointingly, the photocurrent exhibited by these coated AgInsSes electrodes are very small and thus the passivating film does not improve the performance of the semiconductor but rather depress it. The fact that the mechanism of passivation in the dark is different than that in the light is not surprising, since the latter involves majority charge carriers, while holes are not available for the dark anodic process . As discussed by Gerischer and coworkers in the case of the MoSe, electrode[21], the photocorrosion is induced by holes homogeneously generated by light absorption underneath the surface . These holes are driven toward s the electrode/electrolyte interface, reach the surface more or less homogeneously and induce there corrosion reactions by bond weakening . On the contrary, corrosion in the dark is more localized since the initial step is electron injection into the conduction band from surface states via tunnelling through the energy barrier of the depletion layer . The fact that in the case here examined, the dark current starts increasing when the band bending (ie the difference between the anodic bias and the flat-band potential) is still small than the band-gap energy (see Fig . 6) is an indication that such states are indeed present on the surface . Under these experimental conditions, electrons cannot tunnel to the conduction band from occupied energy states in the valence band. Therefore, one must consider electronic surface states above the valence band edge at energy levels higher than the conduction band in the bulk. Consequently, dark corrosion of the anodically polarized electrode is expected to he restricted to limited surface areas . The process will then result in a very high local current density and the corrosion product will primarily he formed at the more active surface defects which are thus rapidly passivated . The quasi-lone pair electron states of the anions around and directed towards the pseudovacancies in AgIn,Se, can be considered of similar characteristics as those arising from dangling bonds . Such bonds on the surface of a II-VI compound (eg CdSe) are known to give rise to states within the forbidden gap[4] . Therefore, near the top of the valence band, "vacant" sites are expected to introduce a high density of states which may be involved in the above discussed dark corrosion process . Current-voltage characteristics under monochromaticlight The Gartner equation (1) allows to determine the hole diffusion length L from current vs applied voltage measurements undeer monochromatic light illumination of the semiconductor electrode . To obtain this important parameter in the case here under study, we have examined the current-voltage curves of the AgIn,Se, electrode in the 1 M Na,SO, electrolyte under 700 nm monochromatic light . In the depletion layer approximation and for a homogeneous donor distribution across the spacecharge layer, W, can be expressed as[13] : W-Wo(V-V,5)1'r (5)
1 297
where Wo is the depletion layer width for a 1 V potential drop across it . The introduction of equation (5) in (1) and the manipulation[22] of the Gartner equation, result in the relation: -In(1 -q) > • aW (V-Vr,)112+ln(I+aL,) (6) where q = I,,/e* is the photocurrent efficiency . Since the dielectric constant a and the donor centre density ND have already been calculated, W can be determined from the Shockley-Read approximation: Wo=(2eco/eND)1j2 (7) where vo is the permittivity of the free space . We obtain Wo = 2.9 x 10-5 cm V-112 Efficiencies, as well as other measurements in the Na,SO4 solution, suffer from various uncertainties owing to the formation of the continuously changing surface film . Nevertheless, as predicted by equation (6) alineardependencebetweenln(1-q)and(V-v p 11/2 was experimentally found, even if limited in the ighpotential region[23] . Thus, once again, it was verified that at high anodic values, charge carrier recombination is negligible and the transfer reaction does not represent the limiting step . From the y axis intercept and the slope of the best least square fit to the experimental points, the following values have been calculated: Lr = 0.6 x 10-17 cm and ajz_MOnml - 1 .2 x 104 cm-1 . Owing to the very small value of the hole diffusion length, the flux of holes generated in the bulk that reach by diffusion the depletion region is practically negligible in our electrode specimens, as usually found in polycrystalline materials[24] . Output power characteristics and stability tests The polyiodide electrolyte was selected for the photoclectrochemical tests . Figure 7 shows the output power characteristics of a photoelectrochemical cell using a AgIn,Sea electrode, etched with a 1 :1 : H,O, solution. The values of the maximum H2SO4 power efficiency (3 .06 %) and of the fill factor (0 .49) are
v,v e Fig . 7. Output power characteristics of the polycrystalline n-AgIn,Se,/2 M ICI, 20 mM I,/Pt cell under 85 MW em-2 white light illumination .
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G . RAzzINI el al. (iii) Preliminary data on the behaviour of Agln s Se s in photoelectrochemical cells with the polyiodide electrolyte are encouraging in terms of power conversion efficiency (around 3 %) and fill factor (around 0 .5) . These values, which partly depend on the good match of the semiconducting electrode band gap with the solar spectrum, support us in attempting further approaches for the stability problem .
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Acknowledgements-This work has been carried out with the support of Consiglio Nazionale delle Ricerche (CNR), Progetto Finalizzato Energetica 2, Sotroprogetto carbone e
rdrogeno . Fig. 8 . Short-circuit current density vs time of the unetched n-Agln s Ses /2M KI, 20mM 1 y /Pt cell under continuous 100mW cm - ' white light illumination . encouraging, especially considering that polycrystalline electrode has been used and that the light was not corrected for optical reflection losses and for solution absorption . Figure S illustrates a stability test of the AgIn 5 Se e electrode, determined as photocurrent versus time of continuous illumination . The steady decay in photocurrent indicates the progressive worsening of the electrode performance due to the deterioration of its surface, as also verified by SEM measurements . The limited stability observed in Fig . 9 points out that the pseudovacancies in the crystal structure which could cause rapid diffusion and drift of ionic species leading to fast deactivation of the electrode material by photooxidation[4], have a very detrimental effect in the case of the Agln s Ses photoelectrode . Up to now, attempts to improve material stability through electrode,/electrolyte junction stabilization by surface treatments and modification of the solution composition have not been successful .
CONCLUSIONS On the basis of the above described results, one can derive the following conclusions . (i) Using a Schottky barrier model to describe the AgIn 5 Se s n-type semiconductor in contact with a liquid electrolyte, it has been possible to determine some meaningful physical quantities, like the band gap, the flatband potential, the donor centre density, the barrier width, the minority carrier diffusion length and the transition mode . (ii) The material both as part of a regenerative photoelectrochemical cell and in rest conditions, turned out to be unstable, both under irradiation and in the dark, owing to corrosion phenomena . This behaviour has tentatively been related to the Agln s Ses cation-deficient structure, whose remarkable aspect is the incomplete lattice of electropositive ions .
REFERENCES 1 . H. Hahn, G . Frank, W . Klinger, A . Meyer and G. Storger, Z . Anorg . Ally- Chem. 271, 153 (1953) R. Nitsche, J. Physique 9C, 3 (1975). 2 . L. S . Palatnik and E . I . Rogacheva, Soc . Phys. Cryst llagr. 11, 189 (1966). 3 . P. Benoit, P . Charpin and C . Djega-Mariadassou, Mater. Res. Bull. 18, 1047 (1983) . 4 . R. Tenne, Y . Mirovsky, Y . Greenstein and D . Cahen, J. electrochem . Soc . 129, 1506 (1982) . 5 . L . S . Palatnik and E. I . Rogacheva,1zv. Akad. Nauk SSSR, Neorgan . Marerialy 2, 478 (1966) ; C.A . 65, 6426c (1966). 6. L . S. Palatnik and E . 1 . Rogacheva, Dokl . Akad . Nook SSSR 174, 80 (1967) ; C .A . 67, 77337f (1967) . 7 . M . Robbins and M . A. Miksovsky, Mater. Res. Bull. 6, 359 (1971) . 8 . P . Benoit, C. Djega-Mariadassou, R . Lesueur and J . H . Albany, Phys. Letters 73A, 55 (1979) . 9 . B . Scrosati, L . Fornarini, G- Razzini and L . Peraldo Bicelli, J . electrochem. Soc. 132, 593 (1985) .
10. G . Razzini, M . 1 srzari, L. Peraldo Bicelli, F . Levy, L. De Angelis, F . Galluzzi, E . Scale, L . Fornarini and B . Scrosati, J . Power Sources 6, 371 (1981) . l1 . W . Gartner, Phys. Rev. 116, 84 (1959) . l2 . F. Stern, Solid State Phys 15, 299 (1963). 13 . M . A. Butler, J . appl . Phys 48, 1914 (1977). 14 . R . H . Wilson, J . appl. Phys. 48, 4292 (1977). 15 . J . Rerchmann, Appl. Phys . Lett . 36, 574 (1980) . 16 . F. El Gvibaly and K . Colbow, J. appl . Phys. 53, 1737 (1982). 17 . R . U . E . `t Lam and D_ R . Franceschetti, Mater. Res . Bull. 17, 1081 (1982). 18 . S. K. Kovach and A . T. Vas ko, Elecorokhimiya 19, 252 (1983) . 19. M. A . Butler and D . S. Ginley, J . electrbchem . Soc . 125, 228 (1978). 20. J . L. Shay, B . Tell, H . M. Kasper and L . M . Schiavone, Phys. Rev . B7, 44R5 (1973). 21 . H . Gerisher, D. Ross and M . Lubke, Z . phys. Chem . Neue Folge 139, 1 (1984) . 22. J . H. Kennedy and K. W . 1, rese, J . electrochem . Soc . 123, 1683 (1976) . 23 . P . Salvador, C . Gutierrez, G . Campet and P . Hagenmuller, J . electrechem . Sac. 131, 550 (1984) . 24. S .-E . Lindquist, A . Lindgren and Z . Yan-Ning, J . electrochem . Soc. 132, 623 (1985) .