Photoelectrochemical effects on p-silicon of ruthenium dioxide thin films

Solar Energy Materials 10 (1984) 309-316 North-Holland, Amsterdam

309

P H O T O E L E C T R O C H E M I C A L EFFECTS ON p-SILICON OF R U T H E N I U M DIOXIDE T H I N FILMS W . GISSLER and A.J. McEVOY *

Commission of the European Communities, Joint Research Centre, 21020 lspra (Va), Italy Received in revised form 6 April 1984 The catalytic effect on the photoelectrochemical behaviour of p-type silicon surfaces, due to thin ruthenium dioxide films, was investigated. Film deposition was by reactive rf sputtering. There was a drastic anodic displacement of the onset potential for the hydrogen evolution reaction in 2 M HCi, and an analogous displacement in the photoresponse onset potential in an electrolyte containing the ferrous/ferric redox couple. There was also a stabilising effect due to the ruthenium dioxide coating. The results are interpreted on a MIS model. A solar cell conversion efficiency of 5.1% was obtained in the V 2 + / V 3+ redox electrolyte.

1. I n t r o d u c t i o n

Although silicon is the established material for solid state photovoltaic devices, it has proved intractable for semiconductor-liquid junction solar energy conversion [1,2]. Both n- and p-type silicon surfaces in contact with aqueous media degrade through the formation of an oxide layer. Methods for the stabilisation of the electrode characteristics of silicon are therefore of current interest [3-8]. In particular, p-type silicon, if used as a hydrogen-evQlving photocathode, has its behaviour significantly improved by a thin platinum coating [4,5]. It has also been shown that the V 2÷ / V 3÷ redox couple stabilises the p-type silicon electrode, making possible a solar cell with efficiency up to 6.1% [7]. The present work demonstrates another method for the amelioration of p-type silicon electrodes, by surface coating with thin films of ruthenium dioxide. This method should be distinguished from earlier experiments [21,22] with a "hybrid" system composed of a solid state Schottky barrier structure in contact with an electrolyte and where only the kinetics of the charge transfer process of this contact was improved by the use of ruthenium dioxide. The beneficial effect of traces of ruthenium dioxide in association with semiconductors for photoelectrochemical energy conversion was established [9,10] in microheterogeneous systems (colloidal suspensions in aqueous media) where it is presumed to act as an oxygen evolution catalyst. Further investigation has established, however, that the effect of ruthenium dioxide on extended semiconductor surfaces is more evident for photoreduction reactions at p-type electrodes, than for

* Present address: C A M Ltd, Newcastle Road, Galway, Ireland.

0165-1633/84/$03.00 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

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oxidation at n-type surfaces [11]. This reductive catalytic property of ruthenium dioxide has also been recognised elsewhere in electrochemistry and a working mechanism has been proposed [12].

2. Experimental The p-type silicon single crystals used in this investigation were boron-doped, with a resistivity of 0.8 f~ cm. The area of the crystals varied from sample to sample and was in the range 5 × 10-2-15 x 10 _2 cm 2. Each wafer was first etched in a CP-4 solution (5 : 3 : 3, H N O 3 : C H 3 C O O H : HF, + 1% bromine). Ohmic contacts were formed by fusing indium on the rear surface, and each semiconductor chip was then sealed in epoxy resin, leaving only the (111) surface exposed. Immediately prior to deposition of the ruthenium dioxide, or in the case of bare p-silicon electrodes, prior to immersion in the electrolyte, each chip was given a further brief etch, first for 20 s in CP-4, then in HF, to remove the oxide grown on the surface by exposure to the atmosphere. For the application of the ruthenium dioxide coatings, an rf sputtering procedure was employed [13]. The surface was first prepared by a brief sputter-etch, the duration of which was significant for the subsequent photoelectrochemical behaviour. The deposition of the thin film was initiated by sputtering from a ruthenium dioxide powder target in a pure argon plasma: this provided a higher sticking coefficient and avoided the exposure of the silicon surface to an oxidising environment. After 1 min oxygen was admitted to the sputtering chamber, so that further sputtering in a 50% O 2 / A r plasma could maintain film stoichiometry [14]. In electrolyte preparation, analar grade materials were used; water was demineralized and distilled. Photoelectrochemical experiments were carried out in a single compartment cell, adapted for flushing the electrolyte with nitrogen and maintaining an oxygen-free atmosphere in contact with it; this precaution was particularly important when using the vanadium-based redox system. In the preparation of this redox electrolyte, V205 was mixed, dry, with a slight excess of the metallic zinc required to reduce the vanadium species to its + 2 oxidation state. Distilled water was added, and then, under a nitrogen atmosphere, sufficient concentrated HC1 was admitted, dropwise from a burette, to yield a 4 M concentration on completion of the vanadium reduction. Meanwhile, the redox potential of the solution was continuously monitored. This procedure minimized the side reaction of hydrogen evolution by the zinc/acid processes, so that the most cathodic of the three one-electron vanadium couples, the V 2 + / V 3+, is approached [6]. Only in the presence of excess metallic zinc was the redox potential of - 4 9 0 mV (SCE) attained; most of the experiments in this system were carried out at a redox potential of - 4 5 0 mV (SCE). The use of a carbon counter electrode was an essential precaution in this electrolyte, which is unstable in the presence of platinum, which catalyses the reaction: V 2+ + H + --, V 3+ + ½H 2 [15]. In the preparation of the ferrous/ferric redox electrolyte, the sulphate salts were used, due to the problems of hygroscopy and hydrolysis presented by ferric chloride. All potentials were determined with respect to standard calomel reference elec-

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trodes, and controlled by a conventional programmer and potentiostat. Capacitance measurements were made by an ac modulation method, using a phase sensitive detector to measure the reactive component of impedance. The light source was a 450 W dc xenon discharge lamp, for which a monochromator was available.

3. Results

The effect of a 40 ,~ ruthenium dioxide coating on the photoelectrochemical behaviour of p-type silicon is demonstrated in fig. 1 by continuous lines: a) in 2 M HC1; b) in a 0.1 M solution of F e E+ and Fe 3+ ions in 0.1 M H 2 S O 4 ; and c) in a V2+/V 3+ redox electrolyte, with a total vanadium content of 0.35 M, the proportionate concentration of Z n 2 ÷ and 4 M HC1. For comparison, the characteristics of bare p-type silicon electrodes in the same electrolytes are represented by broken lines. In the 2 M HCI electrolyte, without an added redox couple, the photoresponse onset potential was displaced by 0.4 V, so that it lies some 0.35 V anodic of the

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reversible hydrogen potential. This displacement of photoresponse onset potential is comparable with that reported at platinised p-silicon electrodes [4]. However, like platinum, the ruthenium dioxide coating did not fully stabilise the semiconductor surface in this electrolyte: there was a slow regression of the photoresponse onset potential from the value initially attained. The dashed curve in fig. la was obtained with an uncoated p-type silicon surface, freshly etched, first in CP-4, then in HF; some of these electrodes, on cycling, were surprisingly stable. At both coated and bare p-silicon electrodes, evolution of hydrogen was associated with the photocurrent, and the formation of gas bubbles was evident. In the ferrous/ferric redox electrolyte, an even greater anodic displacement of photoresponse onset potential was noted, by 0.6 V, on the ruthenium dioxide coated surface, compared with that of a bare p-silicon electrode. However, this effect was accompanied by the appearance of a relatively strong dark current as seen in fig. 1 b. The photoresponse onset potential at the bare p-silicon surface is also more anodic than that for the same surface in the HC1 electrolyte. The coated electrode was fully stabilized for periods of the order of an hour. The vanadium based redox system in acid solution has been recognized as a suitable stabilizing agent for the operation of regenerative solar cells with p-type semiconductor photocathodes [6,7,15]. The behaviour of the ruthenium dioxide covered p-type silicon electrode was therefore investigated in this medium also. Comparative studies with uncoated electrodes optimised by the procedure of Lewerenz et al. [7] were made, and the i/V characteristics are shown in fig. lc. Typically, those p-type silicon electrodes newly coated with ruthenium dioxide exhibited an anodic dark current at potentials somewhat positive of the electrolyte redox potential. However, this behaviour could be suppressed by passivation procedures (to be described later in the context of solar cell function) without detrimental effect on the saturation current available at the electrode. While results comparable with those at the optimized but uncoated p-silicon electrode were then obtained, the distinct advantage of the ruthenium dioxide coated material observed in the other electrolytes, does not extend to the V 2 ÷ / V 3+ redox electrolyte system. To investigate in more detail the effect of ruthenium dioxide thin films on the photoelectrochemical behaviour of p-type silicon, the space charge capacitance (C) dependence on potential (V) was determined. The results are displayed in the form of Mott-Schottky ( I / C 2 vs. V) plots in fig. 2. The dashed lines were obtained with uncoated p-silicon surfaces, and the continuous lines with surfaces modified by the application of ruthenium dioxide films of a thickness of 40 A. Curves 1 and 2 represent the results in the HCI electrolyte, 3 and 4 in the ferrous/ferric redox electrolyte, and 5 and 6 in the V 2 + / V 3÷ electrolyte. All plots are qualitatively similar, quantitatively differing in that physical and geometric areas may differ. They are comparable with that presented by Lewerenz et al. [7], for the p-type silicon electrode. This observation is surprising, in view of the significant dependence of the photoresponse onset potential on electrode coating, and on the presence and nature of redox couples in the electrolyte. The photoresponse onset potential in the ferrous/ferric redox electrolyte, for example, at 0.6 V (SCE) is incompatible with a value of - 0 . 1 V (SCE) for the flat band potential as deduced from the intercept on

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the potential axis of the abrupt edge of the Mott-Schottky plot [7]. It is noted, however, that experiments in several electrolytes [16,17] have observed an anomalous increment in capacitance at potentials near the reversible hydrogen potential. As this feature may give rise to the abrupt Mott-Schottky plot edge, a more anodic

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flat-band potential is derivable from the extrapolation of the cathodic linear segment of the plot. We favour the value of 0.3 V (SCE) given by Madou et al. [16] as the flat-band potential of p-type silicon in acid media at p i l l . Ruthenium dioxide coated p-silicon electrodes were also investigated as cathodes in regenerative solar cell configurations, using the vanadium-based redox electrolyte and carbon counter electrodes. The precautions already described to attain the V2+,/V -~+ redox couple were carefully maintained, and the redox potential was constantly monitored. In fig. 3 the characteristics of cells with differently treated photocathodes are shown. In curve (a), the result obtained with a newly-sputtered electrode, the open-circuit voltage is limited by the same loss mechanism responsible for the anodic dark current displayed by the same electrode in fig. lc, referenced to SCE. Suppression of this loss mechanism could be achieved by two procedures: i) the ruthenium dioxide coated p-type electrode was biased to a cell potential of + 1.0 V, whereupon the reverse current gradually decayed, and the characteristic of the cell was thereafter that of curve b; ii) the coated semiconductor surface may be submitted to a further brief etch, for some seconds, whereupon the characteristic of curve c was recorded. It should be noted, in the context of method (i), that for an uncoated p-type silicon electrode in this type of cell, operation near open-circuit voltage leads to rapid and irreversible degradation of the characteristic, due to growth of a continuous insulating oxide film [6]. Under the same conditions the photocurrent at a ruthenium dioxide coated silicon electrode was maintained for a time of the order of minutes. With a cell of the type shown in curve c, a conversion efficiency of 5.1% was measured, under illumination by white light from the xenon arc source, at an intensity of 16.5 m W / c m 2, after allowing for absorption of light in the deeply-coloured electrolyte. A long term stabilization of the electrode, however, could not be reached. After several minutes of operation under short-circuit conditions the photocurrent decreased slowly.

4. Discussion

Noble metal overlayers on semiconductor surfaces are known to inhibit photocorrosion, and in special cases to catalyse photoelectrochemical reactions [4,5,18]. Heller [18] developed a model for hydrogen-evolution catalysts like Pt, Rh and Ru which is essentially based on the assumption that the semiconductor/catalyst junction forms a Schottky barrier. As a metallic conductor, and on work function considerations * * Note on work function It has recently been established from a determination of the potential of zero charge of ruthenium dioxide single crystals in aqueous electrolytes, that the work function in such media is not higher than 4.9 eV [19]. This figure is in accord with electron spectroscopy results, and it m a y be even lower for polycrystalline and thin-film material. From the literature value of 4.06 eV for the electron affinity of silicon [20] and the band gap of 1.12 eV, a work function close to 5.1 eV would be expected for the p-type material; hence the expectation of a barrier height of some 200-300 mV. It is also noted that platinum, whose clean-surface vacuum work function of 5.7 eV is reduced on hydrogen evolution in an electrolyte to some 4.9 eV [18] also makes a Schonky contact to p-type silicon [5].

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ruthenium dioxide should form a Schottky barrier of some 300 mV on p-type silicon: this has been confirmed by the behaviour of a solid-state device in which a sputtered ruthenium dioxide layer of 300 ~, thickness contacts a p-type silicon substrate. Experimentally, our results do not allow this model to be applied without qualification: The observed photoresponse onset potentials, and also the dark currents in redox electrolytes, are compatible with a low barrier of some 200-300 mV, but they do not exceed the flat band potential of the uncovered electrodes. The effect of the ruthenium dioxide coating is to reduce the interval between the flat band potential and the onset of photocurrent which is generally observed at p-type semiconductor electrodes. Since the flat band potential itself is unchanged, the Mott-Schottky plots are little influenced by the ruthenium dioxide coating or by the presence and nature of redox couples in the electrolyte. Also, the cell open-circuit voltage exceeds that expected from the Schottky barrier model. It is noted, however, that such a thin coating (40 A) as that applied on the silicon surfaces, cannot be assumed continuous and impermeable. Therefore, a direct action of the electrolyte on the semiconductor must be considered, developing an oxide interlayer, so that the structure approximates to a MIS system. It is known that MIS barriers are higher and more stable than those at intimate metal-semiconductor contacts, thereby explaining the passivation process which leads to the cell characteristics of fig. 3 (curves b and c). It is concluded that the anodic oxidation procedure promotes the formation of this stable interlayer. For these reasons we think that the benefit from ruthenium dioxide coatings on p-type silicon arises from a favoured kinetics of the charge transfer process of certain interfacial reactions, but without influencing its energetics. From fig. 1 it is evident that the kinetics of hydrogen evolution and that of the Fe 3÷ reduction process are favoured, but that for the V 3÷ reduction there is only little influence. The electrode characteristic for hydrogen evolution compares favourably with that obtained with platinized p-type silicon surfaces [15]. The oxide chemistry, the availability of several oxidation states of the ruthenium species, and the possibility of formation of hydroxyl bonds underlie the catalysis mechanism in ruthenium dioxide [12]. This chemistry is maintained with stability during the electrochemical processes: in the acid electrolyte, with hydrogen evolution, the charge passed was three orders of magnitude greater than that required for reduction to the element of all the ruthenium present in the dioxide film.

5. Conclusion

In the present work, surface stabilisation, and catalysis of cathodic photoelectrochemical reactions, have been demonstrated, due to a thin film of ruthenium dioxide on a semiconductor of technological importance, p-type silicon. The lower overpotential for hydrogen evolution in an acid electrolyte is noted. So also is the stabilisation under open-circuit conditions of the p-silicon electrode in a regenerative solar cell using the vanadium redox system, though in this case no overall enhancement of cell efficiency is achieved.

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Acknowledgements T h e a u t h o r s t h a n k D r . R. M e m m i n g

for helpful discussions and Dr. G. Restelli

f o r p r o v i d i n g t h e s i l i c o n c r y s t a l s . T h e t e c h n i c a l c o o p e r a t i o n o f M e s s r s . A. H o f f m a n a n d R. S c h u b e r t is r e c o g n i z e d .

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