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Photoelectrochemical behaviour of n-type CdIn2Se4 semiconductor electrodes in polysulphide electrolytes
Solar Cells, 11 (1984) 389 - 400
389
PHOTOELECTROCHEMICAL BEHAVIOUR OF n-TYPE CdIn2Se4 SEMICONDUCTOR ELECTRODES IN POLYSULPHIDE ELECTROLYTES
L. FORNARINI, F. STIRPE, E. CARDARELLI and B. SCROSATI
Dipartimento di Chimica, University of Rome, Rome (Italy) (Received August 2, 1983;accepted November 4, 1983)
Summary The photoelec~rochemical behaviour of n-type CdIn:Se4 single-crystal electrodes in a typical photoelectrochemical cell having a polysulphide electrolyte and a platinum counterelectrode was examined. The output photocharacteristics of this cell are greatly influenced by the surface treatment of the ternary chalcogenide semiconductor. The stability of the cell is controlled by the S2--Se 2- exchange reaction which takes place on the semiconductor surface on illumination in the polysulphide electrolyte.
1. Introduction
Photoelectrochemical solar cells based on a semiconductor-liquid electrolyte junction have stimulated much interest as promising devices for the conversion of solar energy. Various semiconductor-electrolyte systems have been investigated to define the performance and the characteristics of this particular type of solar cell and to evaluate finally their effective relevance to practical utilizations. It is now well recognized that, while photoelectrochemical solar cells benefit from a very easy assembly procedure and from a reasonably high energy conversion efficiency, they may suffer poor stability. This limitation prevents their use for long-term applications and thus a breakthrough in the field can only be achieved if stable semiconductor-electrolyte systems are found. Recently, persistent research has been devoted to this topic and various approaches have been proposed. One of these concerns the use of transition metal dichalcogenides (e.g. MoSs, MoSes and WSe2) as photoanodes. These layered semiconductors have optical transitions which do not involve bonding orbitals [1 ] and, in principle, this should prevent photocorrosion. Effectively, layered semiconductors are very stable in conjunction with an I3--I- redox couple, as experimentally verified by various researchers [ 2 - 4 ] . However, 0379-6787/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
390 the photoelectrochemical behaviour of layered semiconductors is largely influenced by the morphological state of the surface [5, 6 ]. Surface defects, such as steps between the layers, act as recombination centres for the photogenerated carriers, greatly reducing the fill factor and efficiency values. Specific surface treatments, such as selective electropolymerizations [7], may partly block the defects and thus overcome the problem. The long-term effectiveness of these treatments, however, is still under study and possibly a subject of further refinements [8]. An alternative route in the research of stable photoelectrode materials may be found in the use of ternary semiconductors (e.g. CuInS2, CuIn2Se4 and CdInzSe4) obtained by cross-substitution of the electropositive constituent of the binary chalcogenide analogue (i.e. CdS and CdSe). According to our knowledge, the use of one of these ternary semiconductors (CuInS2) in a photoelectrochemical solar cell was originally proposed by the Bell Laboratories group, U.S.A. [9]; since then, other researchers [10] and in particular a group at the Weizmann Institute of Science, Israel [11, 12], have been and currently are investigating the properties of the entire family. Tenne et al. [12] described the ternary semiconductors as having high density non-bonding electronic states at the top of the valence band, which allow photoelectrochemical processes without exerting a direct influence on the chemical bonds. Therefore, ternary chalcogenides are expected to be much more stable than their binary analogues. Indeed, good stability has been inferred for CuInS2 [9, 11], CdIn2S4 [10] and CdInzSe4 [12], all in conjunction with polysulphide electrolytes. However, the stability of these ternary chalcogenide photoelectrochemical cells has not yet been evaluated under prolonged illumination and/or diversified working conditions. Furthermore the role of the S2--Se 2exchange reaction on the semiconductor surface on illumination, which has been shown to be critical to the stability of the CdSe/polysulphide cells [13 - 15] has not been thoroughly examined in the CdIn2Se4/polysulphide cell. It therefore seemed to us of interest to investigate further the photoelectrochemical behaviour o f the n-CdIn2Se4/(Sx2-,S 2-) photoelectrochemical cell, with particular emphasis on the relation between the surface morphology and stability of the semiconductor photoanode. 2. Experimental details
The n-type CdIn2Se4 single crystals were kindly provided by Dr. F. Ldvy of the Polytechnic o f Lausanne. These crystals were grown from the vapour phase by heating the elements in a stoichiometric ratio to react in an evacuated crucible. Iodine was used as the transport agent. The chemical transport reaction was run for 4 days at 600 °C [16]. The basic characteristics of the n-type CdIn:Se4 crystals so obtained have been described elsewhere in detail by Ldvy and coworkers [16].
391 The semiconductor crystals were first polished and then etched in a 1:12 dilute aqua regia solution. Ohmic contacts were made by soldering the back surface of the crystal with pure indium onto a copper support. Insulating epoxy was then used to seal the electrode so as to expose only the front surface to the electrolyte. The exposed surfaces were of the order of a few tenths o f a square centimetre (0.1 - 0.3 cm: ). The polysulphide solutions were prepared by dissolving reagent-grade chemicals in doubly distilled water. Various solutions were considered. Typical concentrations of the c o m p o n e n t reagents of the solution used for the photoelectrochemical studies were 3 M Na:S, 3 M KOH and 4 M S. More dilute polysulphide solutions were used for the preliminary tests and for the spectral responses. Finally a " n e u t r a l " electrolyte (0.5 M Na2SO4 in H20) was used for comparison. The procedure described by Tenne e t al. [12] was used for the photoetching processe,;. These were performed potentiostatically under a reverse bias of 1 or 2 V with respect to a platinum electrode in a 1:12 aqua regia solution and with an illumination intensity of 80 mW cm -2. Typical photoetching intervals ranged around 30 s. Standard optical and electrochemical equipment was used for the photoelectrochemical measurements. A halogen-filtered lamp was used as the light source. The illumination intensity at the electrode was 80 mW cm -2 as determined by a solarimeter. The cadmium ion concentration in the solutions of the cell exposed to prolonged illumination was determined using atomic absorption spectroscopy. For this analysis, 2 cm 3 of the polysulphide solution were acidified with concentrated HNOa, heated to boiling for few minutes, filtered to eliminate the elemental sulphur which had formed and finally diluted to 10 cm a with deionized water. The cadmium concentration was determined with an International Laboratory atomic absorption spectrometer equipped with a flameless system and using a wavelength o f 228.8 nm. All the determinations were carried o u t with the standard addition m e t h o d to minimize the matrix effect. The sensitivity o f the instrument was 0.2 mg 1-1 of cadmium. Blanks were also prepared and tested in the same way, in order to evaluate the cadmium impurities contained in the reagents (i.e. Na2S, KOH and sulphur) used for the electrolyte solution. The scanning electron microscope observations and the energy-dispersive X-ray microanalysis were carried out with a Stereoscan 100 Cambridge microscope.
3. Results and discussion
The wavelength dependence o f the q u a n t u m yield for the n-type CdIn2Sea semiconductor crystals in a sulphide-rich electrolyte was similar to that observed by Tenne e t al. [12]. The etched semiconductor crystals show an indirect band gap at 1.58 eV and a direct band gap at 1.75 eV. Similar
392 i 6 ~-
j (mA cn~2)
v
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--r F i g 1 Onset of photocun'ent density in a Cdln2Se4 pho~electrode and its dependence
on the Na2S concentration in the electrolyte: curve a, neutral solution, Yonse t = --160 mV; curve b, 10 -a M Na2S, Yonse t = --313 mV; curve c, 10 -2 M Na2S, Vor~et = --400 mV; curve d, 10 -1 M Na2S, Yonse t = - - 5 2 5 mY; curve e, 1 M Na2S, Yonset = --600 mY. band gap values have also been r e p o r t e d by L~vy and coworkers [16]. At wavelengths shorter than 600 nm, the spectral response showed a decay corresponding to the absorption of the o r a n g e - r e d polysulphide solution which acts as a c u t - o f f filter in this region. No evidence o f subband states, previously n o t e d by T enne e t al. [12], was observed for the s e m i c o n d u c t o r crystals used in this work. The onset o f a p h o t o c u r r e n t density at the CdIn2Se4 p h o t o e l e c t r o d e in a solution containing various NaES concent rat i ons was examined (Fig. 1). As in the binary chalcogenide analogue CdSe [17], by increasing the conc e n t ratio n o f the Na2S t he onset of the p h o t o c u r r e n t density is shifted towards mo r e negative values. The increase in the sulphide c o n c e n t r a t i o n t h e n also seems to favour in this case the absorption of S 2- ions on the s e m i c o n d u c t o r surface with a consequent negative shift o f the flat-band potential. Th e evidence o f this process is shown in Fig. 1 , where the value of Vo,,et is given f o r various Na2 S concentrations; as the c o n c e n t r a t i o n increases f r o m 10 -3 M to 1 M, the onset shifts f r o m - - 1 60 mV t o - - 6 0 0 mV respectively with respect to a standard calomel electrode (SCE). It is generally possible to regard the Vo,_,et trend as an a p p r o x i m a t e indication of the behaviour of the flat-band potential. A m o r e precise d e t e r m i n a t i o n would obviously require the d e t e r m i n a t i o n of M o t t - S c h o t t k y plots in these media. The measurements o f t h e M o t t - S c h o t t k y plots f or CdIn2 Sea, as well as for o t h e r ternary chalcogenide p h o t o a n o d e s , in polysulphide electrolytes are in progress in our laboratory. In o u r work, a t t e n t i o n has been mostly focused on the operational characteristics o f t he n-type CdIn2Sea p h o t o a n o d e . In Fig. 2 are shown typical cu r r en t d e n s i t y - v o l t a g e curves for an etched CdIn:Sea crystal in a neutral Na2SO4 electrolyte and in a polysulphide electrolyte of composition
393 J
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. . . . . .
-600
-400
-200
-r
200
400
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600
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Fig. 2. Current density-voltage curves in the dark and under illumination for a Cdln2Se4
photoelectrode in a neutral electrolyte (0.5 M Na2SO4) (curve b) and in a polysulphide electrolyte (3 M Na2S, 3 M KOH and 4 M S) (curve a}.
3 M Na2S, 3 M KOH plus 4 M S. The values o f photocurrent density are rather modest and such a p o o r performance indicates the presence of a large concentration o f semiconductor surface defects which act as recombination centres for the photogenerated carriers. Indeed, such a highly defective surface condition is typical of ternary chalcogenide semiconductors of the CdIn2Se4 t y p e [12]. The surface treatment is therefore of crucial importance to the improvement in the photoelectrochemical response o f these semiconductors. In the case examined here, various types of surface treatment were evaluated. The procedure usually adopted was a chemical etching of the semiconductor crystal, followed by immersion of the crystal in solutions of various metal ions in the hope that the absorption of such ions on the surface would favourably affect the electrode performance. While no appreciable effects were noticed after treatment with RuC13 solution, slight improvements in the performance of etched crystals were observed after they had been immersed in a ZnC12 solution. Energy-dispersive X-ray microanalysis revealed zinc absorption on the surface of the treated CdIn2Se4 crystal. Probably this absorption beneficially removed some surface states, b u t still in such a modest fashion as to maintain the performance o f the p h o t o a n o d e well below any value of significance. It has been reported by various researchers that selective photochemical etching may produce dramatic improvements in the photocharacteristics of binary chalcogenides, such as CdSe [15], and in those of their ternary analogues, such as CdIn2Se4 [12]. This is confirmed in this work, since large increases in photocurrent density were observed for p h o t o e t c h e d CdIn2Se4 crystals, as typically shown by the current density-voltage curve in Fig. 3. The effect of the photoetching treatment is also revealed in Fig. 4, where the o u t p u t characteristics o f a CdIn2Se4/(Sx 2-, S 2-) photoelectrochemical cell after consecutive treatments of the semiconductor crystal are
394
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10-
V(mV
-800
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Fig. 3. Current d e n s i t y - v o l t a g e curves in the dark and under illumination o f a CdIn2Se4 p h o t o e t c h e d e l e c t r o d e in a p o l y s u l p h i d e e l e c t r o l y t e (3 M Na2S, 3 M K O H and 4 M S).
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.......
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...............
........
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Fig. 4. O u t p u t characteristics o f a C d I n 2 S e 4 / ( 3 M Na2S, 3 M K O H , 4 M S)/Pt p h o t o e l e c t r o c h e m i c a l cell ( i l l u m i n a t i o n , 8 0 mW c m - 2 ) : curve a, a s - m o u n t e d s e m i c o n d u c t o r crystal ( F F = 0 . 2 9 ; 7"/= 0.15%); curve b, c h e m i c a l l y e t c h e d s e m i c o n d u c t o r crystal ( F F = 0 . 3 0 ; 17 = 0.21%); curve c, after t h e crystal had been p h o t o e t c h e d at a bias o f 1 V ( F F = 0 . 3 2 ; 17 = 0 . 4 4 % ) ; curve d, after the crystal had been p h o t o e t c h e d at a bias o f 2 V ( F F = 0 . 3 4 ; 17 = 2.5%).
395 displayed. Curve a was obtained for a cell employing an as-mounted crystal, curve b for a cell with a chemically etched (in 1:12 aqua regia solution) crystal, curve c after the crystal had been p h o t o e t c h e d at a bias of 1 V and curve d after the crystal had been p h o t o e t c h e d at a bias of 2 V. An outstanding improvement was obtained by the photoetching treatment; the values of the short-circuit current density Jsc, the fill factor FF and the efficiency 77 respectively changed from 1.90 mA cm -2, 0.29 and 0.3% for the untreated crystal (curve a) to 19.6 mA cm -2, 0.34 and 2.5% for the p h o t o e t c h e d crystal {curve d). Photoetching is then a very effective treatment. As pointed o u t by Tenne e t al. [ 12], photoetching acts as "deliberate photocorrosion", insomuch as it removes selectively the surface defects, thus favouring large improvements in photoelectrochemical behaviour such as that illustrated in Fig. 4. A comparison of the scanning electron micrographs of a chemically etched CdIn2Se4 crystal (Fig. 5(a)) and of the same crystal after photoetching (Fig. 5(b)) shows that the photoetching treatment produced an increase in surface roughness. This morphological change may also contribute to improve the o u t p u t characteristics further, since it increases the effective surface area and decreases the surface reflectivity of the semiconductor p h o t o a n o d e [15, 18]. The results described indicate that, with an appropriate surface treatment of the semiconductor, the CdIn2Se4/(Sx 2-, S 2- ) photoelectrochemical cell may eventually reach efficiency values of interest. In fact, even with the crude laboratory cell used in this work (for which no attempts were made to optimize the working conditions) an energy conversion of 2.5% was obtained at an illumination o f 80 mW cm -2. This efficiency value compares well with that offered by other similar photoelectrochemical cells, when the values of the direct band gap (1.75 eV) and o f the indirect band gap (1.58 eV) of the CdIn2Se4 semiconductor p h o t o a n o d e are considered with respect to those of other ternary chalcogenides. The main factor which limits the conversion
(a) (b) Fig. 5. Scanning electron micrographs of a Cdln2Se4 crystal (a) before and (b) after photoetching.
396
efficiency in the Cdln2Se4/(S~ 2 , S 2-) cell studied here is the low opencircuit voltage (about 0.3 V), since short-circuit current densities as high as 20 mA cm -2 can easily be obtained at 80 mW cm -2. This low open-circuit voltage is a typical feature of the majority o f ternary chalcogenide photoelectrochemical cells. However, as o f t e n remarked, a long operational stability rather than a high efficiency is the key factor in the d e v e l o p m e n t of practical photoelectrochemical cells. Therefore, in the cell studied here, particular at t ent i on was also addressed to the study o f the cell's behaviour under continuous illumination. Such a stability test was p e r f o r m e d by examining the trend of t h e o u t p u t cu r r en t density o f the CdIn2Se4/(Sx 2-, S 2-) cell as a funct i on of the total charge delivered. In Fig. 6 are shown the results obtained for two cells working under a load o f 10 ~2 (curve a) and a load of 100 [2 (curve b), with an unstirred electrolyte, at r o o m t e m p e r a t u r e and at a continuous illumination o f 80 mW cm -2. When the cell is run under a load of 10 [2 (close to short-circuit conditions), a stable o u t p u t behaviour is obtained for only a limited a m o u n t o f time, after which progressive decay takes place (see curve a). However, when the o u t p u t current is reduced, the cell shows a much more stable behaviour, since only a slight deterioration is observed after the passage o f a charge which is f o u r times larger (see curve b). A similar crucial dependence o f the stability on the values of the initial c u r r en t density has been observed f or cells based on the binary chalcogenide CdSe [15, 19]. In such a case it has been proposed t hat the o u t p u t stability
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I 200
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Fig. 6. Current density o u t p u t as a f u n c t i o n o f t h e total charge passed for a Cdln2Se4/ (Sx 2--, S 2 - ) p h o t o e l e c t r o c h e m i c a l cell operating under an external load o f 10 ~ (curve a) and o f 100 ~'1 (curve b) with an unstirred electrolyte and at r o o m temperature.
397
could be affected by the formation of a blocking layer on the semiconductor surface as the result o f an S2--Se 2- exchange reaction [19]. To ascertain whether in our study the same effect also predominates over the more c o m m o n photocorrosion phenomenon, the solutions of the CdIn2Se4/(Sx 2-, S 2- ) cell were analysed for cadmium ions using atomic absorption spectroscopy at the end of the prolonged illumination tests illustrated in Fig. 6. A concentration of cadmium ions less than 1 - 2 ppb was detected in both cases. A comparison of this value with t h a t calculated on the basis o f a corrosion reaction of the t y p e CdIn2Se4 + 8h ÷ = Cd 2÷ + 2In 3+ + 4Se(surface)
(1)
according to the total charge passed ( 1 2 5 0 0 0 ppb Cd2+), excludes the possibility o f any consistent dissolution of the p h o t o a n o d e on illumination. Figure 7 is a scanning electron micrograph of the surface of the CdIn2 Se4 p h o t o a n o d e exposed to the continuous illumination test. The observation under the microscope and, in particular, the energy-dispersive X-ray analysis, markedly indicated the formation o f a sulphur-rich layer on the semiconductor surface. As observed for the binary chalcogenide analogues [13, 19], the contact of the ternary selenide semiconductor with the polysulphide electrolyte induces a surface S2--Se 2- exchange reaction, here tentatively indicated as CdIn2Se4 + xS 2- = CdIn2Se4_ xSx + xSe 2-
(2)
which may eventually proceed to completeness as CdIn2Se4_ xS~ + (4 -- x)S 2- = CdIn2S4 + (4 -- x)Se 2-
(3)
or even to a higher degradation state, such as CdIn2Se4 + S 2- = CdS + In2Se3 + Se 2-
(4)
The evaluation of the effects of the exchange reaction on the stability of CdSe photoanodes is still uncertain. Heller e t al. [ 19] postulated a blocking
Fig. 7. Scanning electron micrograph o f a C d I n 2 S e 4 crystal a f t e r t h e passage o f 2 0 0 0 C c m - 2 d u r i n g continuous illumination in conjunction with a p o l y s u l p h i d e e l e c t r o l y t e .
398 action of the sulphur-rich layer which occurred for two concurrent reasons: the formation of an energy barrier for the transfer of holes to the solution (due to the different band gap values of the CdSe and the CdS) and the degradation of the semiconductor crystallinity (due to the lattice mismatch of the two chalcogenides). Accordingly, the same workers have shown that the barrier may be lowered and the original structure less perturbed if selenium is added to the solution in order to promote the formation of selenium-rich films of the type CdSe~ xSx. Such a composite layer would not block the current if x remains low [19]. Also Manassen and coworkers [13, 15] have shown that the stabilization of the CdSe photoanodes is strongly related to the S2--Se 2- exchange reaction. According to these workers, if the surface layer is maintained below a critical thickness (indicated as about 100 A), the o u t p u t behaviour remains stable while, for thicker layers, deterioration takes place on illumination. The reason for this deterioration was again associated with an increase in the transfer resistance and with a degradation of surface crystallinity of the semiconductor [13, 15]. The relationship between the binary CdSe and the ternary chalcogenide analogue CdIn2Se4 is obviously very close. Even if in the present state of knowledge it is difficult to speculate on the exact nature of the exchange reactions, nevertheless it seems reasonable to assume that in the CdIn2Se4/ (Sx 2-, S 2-) electrolyte system the c o m p o u n d (or compounds) of the external layer also has a (have) different band gap value(s) from that (those) of the bulk ternary chalcogenide semiconductor. Therefore, in this system it also appears possible to infer the formation of a barrier at the interface which hinders the transfer of the minority carriers to the solution. Furthermore, from the structural changes revealed in the scanning electron micrograph in Fig. 7, it may reasonably be assumed that the exchange reaction is also accompanied by a degradation of the crystallinity of the semiconductor surface. It is then possible to conclude that, by analogy with the parent binary c o m p o u n d , the stability of the CdIn2Se4 photoanode in polysulphide electrolytes is controlled by the growth of a blocking film which results from an S2--Se 2- exchange reaction. The thickness and the crystallinity of this film depend on various factors, such as the initial morphology of the semiconductor crystal and, in particular, on the operating photocurrent density. At high current densities the film grows very quickly, leading to a rapid deactivation of the semiconductor surface. At low current densities the film growth process is moderate, probably as a result of an initial S2--Se 2exchange, followed by an $2--S 2- exchange with a consequent stabilization of the o u t p u t performance. It should be pointed o u t that, if the formation of the blocking film is continuous, a progressive decay in the performance of the semiconductor p h o t o a n o d e would be expected. Instead, the experimentally observed trend of the o u t p u t photocurrent density shown in Fig. 6, curve a, indicates that the process is catastrophic and produces an abrupt fall in the cell performance.
399
Such a behaviour, which appears to be typical of binary chalcogenide photoanodes (e.g. CdSe [15]), has so far never been observed for ternary chalcogenides. On the contrary, a very high stability has been reported for photoanodes such as CuInSe2 in a polysulphide electrolyte photoelectrochemical cell, where the photocurrent density output remains stable even after the passage of 20 000 C cm -2 under short-circuit conditions [20]. Furthermore, for this ternary copper selenide, no evidence of surface S2--Se 2exchange reaction was found [20]. Such a difference in the behaviour of the two chalcogenides can be explained on the basis of the related structures. In fact, even if both CdIn2 Se4 and CuInSe2 are ternary analogues of the II-VI binary compounds, the former belongs to the II-(III)2-(VIh family (defective zinc blende) and the latter belongs to the I-III-(VI)2 family (chalcopyrite). The difference between the two structures and particularly the incomplete lattice of the defective zinc blende CdIn2Se4 may account for the poorer stability of the cadmium salt. As result o f this, CdIn2Se4 does not appear to be a good candidate for the development of a stable ternary chalcogenide-based photoelectrochemical cell, unless specific treatments for the stabilization of the semiconductor surface can be profitably adopted. Possible approaches for achieving this goal appear to be mainly the following. (i) Selenium should be added to the electrolyte solution, in order to favour the formation of selenium-rich films. This should reduce both the barrier height and the lattice mismatch. (ii) Chemical etching and other similar treatments should be carried out on the semiconductor crystals, in order to induce porous morphology. Such a condition would reduce the stresses between the bulk and the externally growing film. (iii) Polycrystalline rather than single-crystal photoanodes should be used. (iv) Cells should be operated at high temperatures.
Acknowledgments The authors wish to thank Dr. F. Ldvy, Polytechnic of Lausanne, for having kindly provided the single-crystal samples and Dr. G. Razzini, Polytechnic of Milan, for helpful discussions. Financial support from the University of Rome, Progetto di Ateneo, is also acknowledged. References 1 2 3 4
H. Tributsch,J. Electrochem. Soc., 125 (1978) 1086. H. Tributsch, Sol. Energy Mater., 1 (1979) 257. W. Kautek and H. Gerischer, Bet. Bunsenges. Phys. Chem., 84 (1980) 645. L. Fornarini, F. Stirpe, B. Scrosati and G. Razzini, Sol. Energy Mater., 5 (1981) 107.
400 5 H. J. Lowerenz, A. Heller and F. Di Salvo, J. Am. Chem. Soc., 102 (1980) 1877. 6 L. De Angelis, E. Scaf~, P. Galluzi, L. Fornarini and B. Scrosati, J. Electrochem. Soc., 129 (1982) 1237. 7 L. Fornarini and B. Scrosati, Electrochim. Acta, 28 (1983) 667. 8 L. Fornarini, F. Stirpe and B. Scrosati, J. Electrochem. Soc., 130 (1983) 2184. 9 M. Robbins, K. J. Bauchmann, V. G. Lambrecht, F. A. Thiel, J. Thomson, R. G. Vodimisky, S. Menzes, A. Heller and B. Miller, J. Electrochem. Soc., 125 (1978) 831. 10 G. F. Epps and R. S. Becker, J. Electrochem. Soc., 129 (1982) 2628. 11 Y. Mirovsky, D. Cahen, G. Hodes, R. Tenne and W. Giriat, Sol. Energy Mater., 4 (1981) 169. 12 R. Tenne, Y. Mirovsky, Y. Greenstein and D.Cahen, J. Electrochem. Soc., 129 (1982) 1506. 13 D. Cahen, G. Hodes and J. Manasseh, J. Electrochem. Soc., 125 (1978) 1623. 14 R. N. Noufi, P. A. Kohl, J. W. Rogers, Jr., J. M. White and A. J. Bard, J. Electrochem. Soc., 126 (1979) 949. 15 G. Hodes, J. Manassen and D. Cahen, J. Electrochem. Soc., 128 (1981) 2325. 16 G. Margaritondo, A. D. Katnani and F. L6vy,Phys. Status SolidiB, 103 (1981) 725. 17 R. H. Wilson, J. Electrochem. Soc., 126 (1979) 1187. 18 R. Tenne, N. Muller, Y. Mirovski and D. Lando, J. Electrochem. Soc., 130 (1983) 852. 19 A. Heller, G. P. Schwartz, R. G. Vadimsky, S. Menzes and B. Miller, J. Electrochem. Soc., 125 (1978) 1156. 20 Y. Mirovsky and D. Cahen, Appl. Phys. Lett., 40 (1982) 727.
389
PHOTOELECTROCHEMICAL BEHAVIOUR OF n-TYPE CdIn2Se4 SEMICONDUCTOR ELECTRODES IN POLYSULPHIDE ELECTROLYTES
L. FORNARINI, F. STIRPE, E. CARDARELLI and B. SCROSATI
Dipartimento di Chimica, University of Rome, Rome (Italy) (Received August 2, 1983;accepted November 4, 1983)
Summary The photoelec~rochemical behaviour of n-type CdIn:Se4 single-crystal electrodes in a typical photoelectrochemical cell having a polysulphide electrolyte and a platinum counterelectrode was examined. The output photocharacteristics of this cell are greatly influenced by the surface treatment of the ternary chalcogenide semiconductor. The stability of the cell is controlled by the S2--Se 2- exchange reaction which takes place on the semiconductor surface on illumination in the polysulphide electrolyte.
1. Introduction
Photoelectrochemical solar cells based on a semiconductor-liquid electrolyte junction have stimulated much interest as promising devices for the conversion of solar energy. Various semiconductor-electrolyte systems have been investigated to define the performance and the characteristics of this particular type of solar cell and to evaluate finally their effective relevance to practical utilizations. It is now well recognized that, while photoelectrochemical solar cells benefit from a very easy assembly procedure and from a reasonably high energy conversion efficiency, they may suffer poor stability. This limitation prevents their use for long-term applications and thus a breakthrough in the field can only be achieved if stable semiconductor-electrolyte systems are found. Recently, persistent research has been devoted to this topic and various approaches have been proposed. One of these concerns the use of transition metal dichalcogenides (e.g. MoSs, MoSes and WSe2) as photoanodes. These layered semiconductors have optical transitions which do not involve bonding orbitals [1 ] and, in principle, this should prevent photocorrosion. Effectively, layered semiconductors are very stable in conjunction with an I3--I- redox couple, as experimentally verified by various researchers [ 2 - 4 ] . However, 0379-6787/84/$3.00
© Elsevier Sequoia/Printed in The Netherlands
390 the photoelectrochemical behaviour of layered semiconductors is largely influenced by the morphological state of the surface [5, 6 ]. Surface defects, such as steps between the layers, act as recombination centres for the photogenerated carriers, greatly reducing the fill factor and efficiency values. Specific surface treatments, such as selective electropolymerizations [7], may partly block the defects and thus overcome the problem. The long-term effectiveness of these treatments, however, is still under study and possibly a subject of further refinements [8]. An alternative route in the research of stable photoelectrode materials may be found in the use of ternary semiconductors (e.g. CuInS2, CuIn2Se4 and CdInzSe4) obtained by cross-substitution of the electropositive constituent of the binary chalcogenide analogue (i.e. CdS and CdSe). According to our knowledge, the use of one of these ternary semiconductors (CuInS2) in a photoelectrochemical solar cell was originally proposed by the Bell Laboratories group, U.S.A. [9]; since then, other researchers [10] and in particular a group at the Weizmann Institute of Science, Israel [11, 12], have been and currently are investigating the properties of the entire family. Tenne et al. [12] described the ternary semiconductors as having high density non-bonding electronic states at the top of the valence band, which allow photoelectrochemical processes without exerting a direct influence on the chemical bonds. Therefore, ternary chalcogenides are expected to be much more stable than their binary analogues. Indeed, good stability has been inferred for CuInS2 [9, 11], CdIn2S4 [10] and CdInzSe4 [12], all in conjunction with polysulphide electrolytes. However, the stability of these ternary chalcogenide photoelectrochemical cells has not yet been evaluated under prolonged illumination and/or diversified working conditions. Furthermore the role of the S2--Se 2exchange reaction on the semiconductor surface on illumination, which has been shown to be critical to the stability of the CdSe/polysulphide cells [13 - 15] has not been thoroughly examined in the CdIn2Se4/polysulphide cell. It therefore seemed to us of interest to investigate further the photoelectrochemical behaviour o f the n-CdIn2Se4/(Sx2-,S 2-) photoelectrochemical cell, with particular emphasis on the relation between the surface morphology and stability of the semiconductor photoanode. 2. Experimental details
The n-type CdIn2Se4 single crystals were kindly provided by Dr. F. Ldvy of the Polytechnic o f Lausanne. These crystals were grown from the vapour phase by heating the elements in a stoichiometric ratio to react in an evacuated crucible. Iodine was used as the transport agent. The chemical transport reaction was run for 4 days at 600 °C [16]. The basic characteristics of the n-type CdIn:Se4 crystals so obtained have been described elsewhere in detail by Ldvy and coworkers [16].
391 The semiconductor crystals were first polished and then etched in a 1:12 dilute aqua regia solution. Ohmic contacts were made by soldering the back surface of the crystal with pure indium onto a copper support. Insulating epoxy was then used to seal the electrode so as to expose only the front surface to the electrolyte. The exposed surfaces were of the order of a few tenths o f a square centimetre (0.1 - 0.3 cm: ). The polysulphide solutions were prepared by dissolving reagent-grade chemicals in doubly distilled water. Various solutions were considered. Typical concentrations of the c o m p o n e n t reagents of the solution used for the photoelectrochemical studies were 3 M Na:S, 3 M KOH and 4 M S. More dilute polysulphide solutions were used for the preliminary tests and for the spectral responses. Finally a " n e u t r a l " electrolyte (0.5 M Na2SO4 in H20) was used for comparison. The procedure described by Tenne e t al. [12] was used for the photoetching processe,;. These were performed potentiostatically under a reverse bias of 1 or 2 V with respect to a platinum electrode in a 1:12 aqua regia solution and with an illumination intensity of 80 mW cm -2. Typical photoetching intervals ranged around 30 s. Standard optical and electrochemical equipment was used for the photoelectrochemical measurements. A halogen-filtered lamp was used as the light source. The illumination intensity at the electrode was 80 mW cm -2 as determined by a solarimeter. The cadmium ion concentration in the solutions of the cell exposed to prolonged illumination was determined using atomic absorption spectroscopy. For this analysis, 2 cm 3 of the polysulphide solution were acidified with concentrated HNOa, heated to boiling for few minutes, filtered to eliminate the elemental sulphur which had formed and finally diluted to 10 cm a with deionized water. The cadmium concentration was determined with an International Laboratory atomic absorption spectrometer equipped with a flameless system and using a wavelength o f 228.8 nm. All the determinations were carried o u t with the standard addition m e t h o d to minimize the matrix effect. The sensitivity o f the instrument was 0.2 mg 1-1 of cadmium. Blanks were also prepared and tested in the same way, in order to evaluate the cadmium impurities contained in the reagents (i.e. Na2S, KOH and sulphur) used for the electrolyte solution. The scanning electron microscope observations and the energy-dispersive X-ray microanalysis were carried out with a Stereoscan 100 Cambridge microscope.
3. Results and discussion
The wavelength dependence o f the q u a n t u m yield for the n-type CdIn2Sea semiconductor crystals in a sulphide-rich electrolyte was similar to that observed by Tenne e t al. [12]. The etched semiconductor crystals show an indirect band gap at 1.58 eV and a direct band gap at 1.75 eV. Similar
392 i 6 ~-
j (mA cn~2)
v
( SCt'; ))
--r F i g 1 Onset of photocun'ent density in a Cdln2Se4 pho~electrode and its dependence
on the Na2S concentration in the electrolyte: curve a, neutral solution, Yonse t = --160 mV; curve b, 10 -a M Na2S, Yonse t = --313 mV; curve c, 10 -2 M Na2S, Vor~et = --400 mV; curve d, 10 -1 M Na2S, Yonse t = - - 5 2 5 mY; curve e, 1 M Na2S, Yonset = --600 mY. band gap values have also been r e p o r t e d by L~vy and coworkers [16]. At wavelengths shorter than 600 nm, the spectral response showed a decay corresponding to the absorption of the o r a n g e - r e d polysulphide solution which acts as a c u t - o f f filter in this region. No evidence o f subband states, previously n o t e d by T enne e t al. [12], was observed for the s e m i c o n d u c t o r crystals used in this work. The onset o f a p h o t o c u r r e n t density at the CdIn2Se4 p h o t o e l e c t r o d e in a solution containing various NaES concent rat i ons was examined (Fig. 1). As in the binary chalcogenide analogue CdSe [17], by increasing the conc e n t ratio n o f the Na2S t he onset of the p h o t o c u r r e n t density is shifted towards mo r e negative values. The increase in the sulphide c o n c e n t r a t i o n t h e n also seems to favour in this case the absorption of S 2- ions on the s e m i c o n d u c t o r surface with a consequent negative shift o f the flat-band potential. Th e evidence o f this process is shown in Fig. 1 , where the value of Vo,,et is given f o r various Na2 S concentrations; as the c o n c e n t r a t i o n increases f r o m 10 -3 M to 1 M, the onset shifts f r o m - - 1 60 mV t o - - 6 0 0 mV respectively with respect to a standard calomel electrode (SCE). It is generally possible to regard the Vo,_,et trend as an a p p r o x i m a t e indication of the behaviour of the flat-band potential. A m o r e precise d e t e r m i n a t i o n would obviously require the d e t e r m i n a t i o n of M o t t - S c h o t t k y plots in these media. The measurements o f t h e M o t t - S c h o t t k y plots f or CdIn2 Sea, as well as for o t h e r ternary chalcogenide p h o t o a n o d e s , in polysulphide electrolytes are in progress in our laboratory. In o u r work, a t t e n t i o n has been mostly focused on the operational characteristics o f t he n-type CdIn2Sea p h o t o a n o d e . In Fig. 2 are shown typical cu r r en t d e n s i t y - v o l t a g e curves for an etched CdIn:Sea crystal in a neutral Na2SO4 electrolyte and in a polysulphide electrolyte of composition
393 J
/" // I I s / I
I
-800
. . . . . .
-600
-400
-200
-r
200
400
V(mV t
I
600
800
(SCE)) I
Fig. 2. Current density-voltage curves in the dark and under illumination for a Cdln2Se4
photoelectrode in a neutral electrolyte (0.5 M Na2SO4) (curve b) and in a polysulphide electrolyte (3 M Na2S, 3 M KOH and 4 M S) (curve a}.
3 M Na2S, 3 M KOH plus 4 M S. The values o f photocurrent density are rather modest and such a p o o r performance indicates the presence of a large concentration o f semiconductor surface defects which act as recombination centres for the photogenerated carriers. Indeed, such a highly defective surface condition is typical of ternary chalcogenide semiconductors of the CdIn2Se4 t y p e [12]. The surface treatment is therefore of crucial importance to the improvement in the photoelectrochemical response o f these semiconductors. In the case examined here, various types of surface treatment were evaluated. The procedure usually adopted was a chemical etching of the semiconductor crystal, followed by immersion of the crystal in solutions of various metal ions in the hope that the absorption of such ions on the surface would favourably affect the electrode performance. While no appreciable effects were noticed after treatment with RuC13 solution, slight improvements in the performance of etched crystals were observed after they had been immersed in a ZnC12 solution. Energy-dispersive X-ray microanalysis revealed zinc absorption on the surface of the treated CdIn2Se4 crystal. Probably this absorption beneficially removed some surface states, b u t still in such a modest fashion as to maintain the performance o f the p h o t o a n o d e well below any value of significance. It has been reported by various researchers that selective photochemical etching may produce dramatic improvements in the photocharacteristics of binary chalcogenides, such as CdSe [15], and in those of their ternary analogues, such as CdIn2Se4 [12]. This is confirmed in this work, since large increases in photocurrent density were observed for p h o t o e t c h e d CdIn2Se4 crystals, as typically shown by the current density-voltage curve in Fig. 3. The effect of the photoetching treatment is also revealed in Fig. 4, where the o u t p u t characteristics o f a CdIn2Se4/(Sx 2-, S 2-) photoelectrochemical cell after consecutive treatments of the semiconductor crystal are
394
4oi
J
3O/-
20-
10-
V(mV
-800
l
I
I
200
400
600
(SCE))
I 800
Fig. 3. Current d e n s i t y - v o l t a g e curves in the dark and under illumination o f a CdIn2Se4 p h o t o e t c h e d e l e c t r o d e in a p o l y s u l p h i d e e l e c t r o l y t e (3 M Na2S, 3 M K O H and 4 M S).
3
',mA cm 2 )
20
15
-, .d\ N
\ \ \ \ ....
.......
~
......
...............
........
\ i
100
200
300
400
V',mV )
Fig. 4. O u t p u t characteristics o f a C d I n 2 S e 4 / ( 3 M Na2S, 3 M K O H , 4 M S)/Pt p h o t o e l e c t r o c h e m i c a l cell ( i l l u m i n a t i o n , 8 0 mW c m - 2 ) : curve a, a s - m o u n t e d s e m i c o n d u c t o r crystal ( F F = 0 . 2 9 ; 7"/= 0.15%); curve b, c h e m i c a l l y e t c h e d s e m i c o n d u c t o r crystal ( F F = 0 . 3 0 ; 17 = 0.21%); curve c, after t h e crystal had been p h o t o e t c h e d at a bias o f 1 V ( F F = 0 . 3 2 ; 17 = 0 . 4 4 % ) ; curve d, after the crystal had been p h o t o e t c h e d at a bias o f 2 V ( F F = 0 . 3 4 ; 17 = 2.5%).
395 displayed. Curve a was obtained for a cell employing an as-mounted crystal, curve b for a cell with a chemically etched (in 1:12 aqua regia solution) crystal, curve c after the crystal had been p h o t o e t c h e d at a bias of 1 V and curve d after the crystal had been p h o t o e t c h e d at a bias of 2 V. An outstanding improvement was obtained by the photoetching treatment; the values of the short-circuit current density Jsc, the fill factor FF and the efficiency 77 respectively changed from 1.90 mA cm -2, 0.29 and 0.3% for the untreated crystal (curve a) to 19.6 mA cm -2, 0.34 and 2.5% for the p h o t o e t c h e d crystal {curve d). Photoetching is then a very effective treatment. As pointed o u t by Tenne e t al. [ 12], photoetching acts as "deliberate photocorrosion", insomuch as it removes selectively the surface defects, thus favouring large improvements in photoelectrochemical behaviour such as that illustrated in Fig. 4. A comparison of the scanning electron micrographs of a chemically etched CdIn2Se4 crystal (Fig. 5(a)) and of the same crystal after photoetching (Fig. 5(b)) shows that the photoetching treatment produced an increase in surface roughness. This morphological change may also contribute to improve the o u t p u t characteristics further, since it increases the effective surface area and decreases the surface reflectivity of the semiconductor p h o t o a n o d e [15, 18]. The results described indicate that, with an appropriate surface treatment of the semiconductor, the CdIn2Se4/(Sx 2-, S 2- ) photoelectrochemical cell may eventually reach efficiency values of interest. In fact, even with the crude laboratory cell used in this work (for which no attempts were made to optimize the working conditions) an energy conversion of 2.5% was obtained at an illumination o f 80 mW cm -2. This efficiency value compares well with that offered by other similar photoelectrochemical cells, when the values of the direct band gap (1.75 eV) and o f the indirect band gap (1.58 eV) of the CdIn2Se4 semiconductor p h o t o a n o d e are considered with respect to those of other ternary chalcogenides. The main factor which limits the conversion
(a) (b) Fig. 5. Scanning electron micrographs of a Cdln2Se4 crystal (a) before and (b) after photoetching.
396
efficiency in the Cdln2Se4/(S~ 2 , S 2-) cell studied here is the low opencircuit voltage (about 0.3 V), since short-circuit current densities as high as 20 mA cm -2 can easily be obtained at 80 mW cm -2. This low open-circuit voltage is a typical feature of the majority o f ternary chalcogenide photoelectrochemical cells. However, as o f t e n remarked, a long operational stability rather than a high efficiency is the key factor in the d e v e l o p m e n t of practical photoelectrochemical cells. Therefore, in the cell studied here, particular at t ent i on was also addressed to the study o f the cell's behaviour under continuous illumination. Such a stability test was p e r f o r m e d by examining the trend of t h e o u t p u t cu r r en t density o f the CdIn2Se4/(Sx 2-, S 2-) cell as a funct i on of the total charge delivered. In Fig. 6 are shown the results obtained for two cells working under a load o f 10 ~2 (curve a) and a load of 100 [2 (curve b), with an unstirred electrolyte, at r o o m t e m p e r a t u r e and at a continuous illumination o f 80 mW cm -2. When the cell is run under a load of 10 [2 (close to short-circuit conditions), a stable o u t p u t behaviour is obtained for only a limited a m o u n t o f time, after which progressive decay takes place (see curve a). However, when the o u t p u t current is reduced, the cell shows a much more stable behaviour, since only a slight deterioration is observed after the passage o f a charge which is f o u r times larger (see curve b). A similar crucial dependence o f the stability on the values of the initial c u r r en t density has been observed f or cells based on the binary chalcogenide CdSe [15, 19]. In such a case it has been proposed t hat the o u t p u t stability
I J
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24
cm 2)
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I I 20
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16
12
..........
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I 200
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J 400
I 600
J 800
10100
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| 1400
I 1600
I 1800
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Fig. 6. Current density o u t p u t as a f u n c t i o n o f t h e total charge passed for a Cdln2Se4/ (Sx 2--, S 2 - ) p h o t o e l e c t r o c h e m i c a l cell operating under an external load o f 10 ~ (curve a) and o f 100 ~'1 (curve b) with an unstirred electrolyte and at r o o m temperature.
397
could be affected by the formation of a blocking layer on the semiconductor surface as the result o f an S2--Se 2- exchange reaction [19]. To ascertain whether in our study the same effect also predominates over the more c o m m o n photocorrosion phenomenon, the solutions of the CdIn2Se4/(Sx 2-, S 2- ) cell were analysed for cadmium ions using atomic absorption spectroscopy at the end of the prolonged illumination tests illustrated in Fig. 6. A concentration of cadmium ions less than 1 - 2 ppb was detected in both cases. A comparison of this value with t h a t calculated on the basis o f a corrosion reaction of the t y p e CdIn2Se4 + 8h ÷ = Cd 2÷ + 2In 3+ + 4Se(surface)
(1)
according to the total charge passed ( 1 2 5 0 0 0 ppb Cd2+), excludes the possibility o f any consistent dissolution of the p h o t o a n o d e on illumination. Figure 7 is a scanning electron micrograph of the surface of the CdIn2 Se4 p h o t o a n o d e exposed to the continuous illumination test. The observation under the microscope and, in particular, the energy-dispersive X-ray analysis, markedly indicated the formation o f a sulphur-rich layer on the semiconductor surface. As observed for the binary chalcogenide analogues [13, 19], the contact of the ternary selenide semiconductor with the polysulphide electrolyte induces a surface S2--Se 2- exchange reaction, here tentatively indicated as CdIn2Se4 + xS 2- = CdIn2Se4_ xSx + xSe 2-
(2)
which may eventually proceed to completeness as CdIn2Se4_ xS~ + (4 -- x)S 2- = CdIn2S4 + (4 -- x)Se 2-
(3)
or even to a higher degradation state, such as CdIn2Se4 + S 2- = CdS + In2Se3 + Se 2-
(4)
The evaluation of the effects of the exchange reaction on the stability of CdSe photoanodes is still uncertain. Heller e t al. [ 19] postulated a blocking
Fig. 7. Scanning electron micrograph o f a C d I n 2 S e 4 crystal a f t e r t h e passage o f 2 0 0 0 C c m - 2 d u r i n g continuous illumination in conjunction with a p o l y s u l p h i d e e l e c t r o l y t e .
398 action of the sulphur-rich layer which occurred for two concurrent reasons: the formation of an energy barrier for the transfer of holes to the solution (due to the different band gap values of the CdSe and the CdS) and the degradation of the semiconductor crystallinity (due to the lattice mismatch of the two chalcogenides). Accordingly, the same workers have shown that the barrier may be lowered and the original structure less perturbed if selenium is added to the solution in order to promote the formation of selenium-rich films of the type CdSe~ xSx. Such a composite layer would not block the current if x remains low [19]. Also Manassen and coworkers [13, 15] have shown that the stabilization of the CdSe photoanodes is strongly related to the S2--Se 2- exchange reaction. According to these workers, if the surface layer is maintained below a critical thickness (indicated as about 100 A), the o u t p u t behaviour remains stable while, for thicker layers, deterioration takes place on illumination. The reason for this deterioration was again associated with an increase in the transfer resistance and with a degradation of surface crystallinity of the semiconductor [13, 15]. The relationship between the binary CdSe and the ternary chalcogenide analogue CdIn2Se4 is obviously very close. Even if in the present state of knowledge it is difficult to speculate on the exact nature of the exchange reactions, nevertheless it seems reasonable to assume that in the CdIn2Se4/ (Sx 2-, S 2-) electrolyte system the c o m p o u n d (or compounds) of the external layer also has a (have) different band gap value(s) from that (those) of the bulk ternary chalcogenide semiconductor. Therefore, in this system it also appears possible to infer the formation of a barrier at the interface which hinders the transfer of the minority carriers to the solution. Furthermore, from the structural changes revealed in the scanning electron micrograph in Fig. 7, it may reasonably be assumed that the exchange reaction is also accompanied by a degradation of the crystallinity of the semiconductor surface. It is then possible to conclude that, by analogy with the parent binary c o m p o u n d , the stability of the CdIn2Se4 photoanode in polysulphide electrolytes is controlled by the growth of a blocking film which results from an S2--Se 2- exchange reaction. The thickness and the crystallinity of this film depend on various factors, such as the initial morphology of the semiconductor crystal and, in particular, on the operating photocurrent density. At high current densities the film grows very quickly, leading to a rapid deactivation of the semiconductor surface. At low current densities the film growth process is moderate, probably as a result of an initial S2--Se 2exchange, followed by an $2--S 2- exchange with a consequent stabilization of the o u t p u t performance. It should be pointed o u t that, if the formation of the blocking film is continuous, a progressive decay in the performance of the semiconductor p h o t o a n o d e would be expected. Instead, the experimentally observed trend of the o u t p u t photocurrent density shown in Fig. 6, curve a, indicates that the process is catastrophic and produces an abrupt fall in the cell performance.
399
Such a behaviour, which appears to be typical of binary chalcogenide photoanodes (e.g. CdSe [15]), has so far never been observed for ternary chalcogenides. On the contrary, a very high stability has been reported for photoanodes such as CuInSe2 in a polysulphide electrolyte photoelectrochemical cell, where the photocurrent density output remains stable even after the passage of 20 000 C cm -2 under short-circuit conditions [20]. Furthermore, for this ternary copper selenide, no evidence of surface S2--Se 2exchange reaction was found [20]. Such a difference in the behaviour of the two chalcogenides can be explained on the basis of the related structures. In fact, even if both CdIn2 Se4 and CuInSe2 are ternary analogues of the II-VI binary compounds, the former belongs to the II-(III)2-(VIh family (defective zinc blende) and the latter belongs to the I-III-(VI)2 family (chalcopyrite). The difference between the two structures and particularly the incomplete lattice of the defective zinc blende CdIn2Se4 may account for the poorer stability of the cadmium salt. As result o f this, CdIn2Se4 does not appear to be a good candidate for the development of a stable ternary chalcogenide-based photoelectrochemical cell, unless specific treatments for the stabilization of the semiconductor surface can be profitably adopted. Possible approaches for achieving this goal appear to be mainly the following. (i) Selenium should be added to the electrolyte solution, in order to favour the formation of selenium-rich films. This should reduce both the barrier height and the lattice mismatch. (ii) Chemical etching and other similar treatments should be carried out on the semiconductor crystals, in order to induce porous morphology. Such a condition would reduce the stresses between the bulk and the externally growing film. (iii) Polycrystalline rather than single-crystal photoanodes should be used. (iv) Cells should be operated at high temperatures.
Acknowledgments The authors wish to thank Dr. F. Ldvy, Polytechnic of Lausanne, for having kindly provided the single-crystal samples and Dr. G. Razzini, Polytechnic of Milan, for helpful discussions. Financial support from the University of Rome, Progetto di Ateneo, is also acknowledged. References 1 2 3 4
H. Tributsch,J. Electrochem. Soc., 125 (1978) 1086. H. Tributsch, Sol. Energy Mater., 1 (1979) 257. W. Kautek and H. Gerischer, Bet. Bunsenges. Phys. Chem., 84 (1980) 645. L. Fornarini, F. Stirpe, B. Scrosati and G. Razzini, Sol. Energy Mater., 5 (1981) 107.
400 5 H. J. Lowerenz, A. Heller and F. Di Salvo, J. Am. Chem. Soc., 102 (1980) 1877. 6 L. De Angelis, E. Scaf~, P. Galluzi, L. Fornarini and B. Scrosati, J. Electrochem. Soc., 129 (1982) 1237. 7 L. Fornarini and B. Scrosati, Electrochim. Acta, 28 (1983) 667. 8 L. Fornarini, F. Stirpe and B. Scrosati, J. Electrochem. Soc., 130 (1983) 2184. 9 M. Robbins, K. J. Bauchmann, V. G. Lambrecht, F. A. Thiel, J. Thomson, R. G. Vodimisky, S. Menzes, A. Heller and B. Miller, J. Electrochem. Soc., 125 (1978) 831. 10 G. F. Epps and R. S. Becker, J. Electrochem. Soc., 129 (1982) 2628. 11 Y. Mirovsky, D. Cahen, G. Hodes, R. Tenne and W. Giriat, Sol. Energy Mater., 4 (1981) 169. 12 R. Tenne, Y. Mirovsky, Y. Greenstein and D.Cahen, J. Electrochem. Soc., 129 (1982) 1506. 13 D. Cahen, G. Hodes and J. Manasseh, J. Electrochem. Soc., 125 (1978) 1623. 14 R. N. Noufi, P. A. Kohl, J. W. Rogers, Jr., J. M. White and A. J. Bard, J. Electrochem. Soc., 126 (1979) 949. 15 G. Hodes, J. Manassen and D. Cahen, J. Electrochem. Soc., 128 (1981) 2325. 16 G. Margaritondo, A. D. Katnani and F. L6vy,Phys. Status SolidiB, 103 (1981) 725. 17 R. H. Wilson, J. Electrochem. Soc., 126 (1979) 1187. 18 R. Tenne, N. Muller, Y. Mirovski and D. Lando, J. Electrochem. Soc., 130 (1983) 852. 19 A. Heller, G. P. Schwartz, R. G. Vadimsky, S. Menzes and B. Miller, J. Electrochem. Soc., 125 (1978) 1156. 20 Y. Mirovsky and D. Cahen, Appl. Phys. Lett., 40 (1982) 727.