The Journal of
Physical Chemistry
0 Copyright, 1986, by the American Chemical Society
VOLUME 90, NUMBER 22 OCTOBER 23, 1986
LETTERS Novel Approach to Efflcient Photoelectrochemical Solar Cells Using Electrolyte/Discontinuous Metal/Semiconductor Junctions Yoshihiro Nakato,* Keiichi Ueda, and Hiroshi Tsubomura* Laboratory for Chemical Conversion of Solar Energy and Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: June 4 , 1986; In Final Form: September 5. 1986) The open-circuit photovoltage (V,) generated at an n-Si electrode coated with a microscopically discontinuous platinum layer in a redox solution is increased by changing the redox potential of the redox couple toward the positive, until it reaches about 0.5 V, much higher than the V, (ca. 0.2 V) for an n-Si electrode coated with a continuous Pt layer. These results verify a theory proposed by us on the discontinuous metal coated semiconductor electrodes and open up a new approach to efficient photoelectrochemical solar cells. Photoelectrochemical reactions on semiconductor electrodes as well as semiconductor powder photocatalysts have been studied extensively in view of solar energy utilization. We reported previously that thin metal coatings not only prevent the semiconductor electrodes from corroding's2 but also catalyze photoelectrode reactions such as hydrogen p h o t o e v ~ l u t i o n . The ~ ~ ~ use of the thin metal coating has, however, a problem in that electrodes coated with continuous metal layers generally lose photovoltages arising from semiconductor/liquid junctions, showing only small photovoltages arising from semiconductor/metal junction^.',^ Recently we have proposed a theory6 on the mechanism of generation of photovoltages at a semiconductor electrode coated with a discontinuous metal layer in redox electrolyte solutions. An important conclusion is that, for such a discontinuous metal coated semiconductor electrode, the surface band energy of the semiconductor at the metal-covered parts is different from that at the naked parts, but this band modulation is rapidly weakened toward the interior of the semiconductor and the effective energy barrier height is nearly equal to that in a naked semiconductor electrode in cases where the metal exists in a form of thinly scattered, minute islands, say, 5 nm wide, separated by more than 20 nm from each other (case 3 of ref 6). Accordingly, for an (1) Nakato, Y.;Ohnishi, T.; Tsubomura, H . Chem. Leu. 1975, 883. (2) Nakato, Y.; Abe, K.; Tsubomura, H . Ber. Bunsen-Ges. Phys. Chem. 1976, 80, 1002. (3) Nakato, Y.; Tonomura, S.; Tsubomura, H. Ber. Bunsen-Ges. Phys. Chem. 1976.80, 1289. (4) Nakato, Y.; Shioji, M.; Tsubomura, H. Chem. Phys. Lett. 1982, 90, 453. ( 5 ) Sze, S . M. Physics of Semiconductor Devices, 2nd ed.; Wiley: New
York, 1981; Chapter 5. ( 6 ) Nakato, Y.;Tsubomura, H . J . Photochem. 1985, 29, 257.
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n-type semiconductor electrode covered with such minute metal islands where photogenerated holes can enter into the solution via the metal islands, the photovoltage increases by changing the redox potential of the redox couple toward the positive, similar to the case of naked electrodes, approaching the equivalent of the band gap. This prediction is quite contrary to that of the conventional potential barrier model for continuous metal/semiconductor contact^.^ In other words, such a microscopically discontinuous metal layer on a semiconductor electrode does not spoil the photovoltage arising from a semiconductorsolution junction and yet retains the catalytic and the stabilizing functions in cases where the naked parts of the semiconductor surface are passivated. A similar conclusion can be derived for p-type semiconductors. The above prediction opens up a new important approach to stable and highly efficient photoelectrochemical solar cells. In the present Letter we will report experimental results verifying this prediction. Discontinuous metal coatings were made as follows: n-Si electrodes, prepared from single crystal n-Si wafers having a resistivity of 0.4-0.8 fl cm and cut perpendicular to the (1 11) axis, were illuminated in a 48% hydrogen fluoride stirred solution under anodic bias for a few minutes, keeping the photocurrent density at 5-10 mA cm-2, followed by immersion in a 10% H F solution in the dark for about Id h. Scanning electron microscopy (Figure 1A) shows that such a photoetching treatment makes the n-Si surface very rough, producing micropores of 10-40 nm in diameter, as reported by other worker^.^^^ Platinum was then deposited on this photoetched n-Si electrode by the electron-beam evaporation method under 5.2 X 10" Torr (1 Torr = 133.3 Pa), with the n-Si surface tilted at an angle of about 15' against the direction of the flow of platinum vapor (cf. Figure 2A). The average thickness of the deposited Pt was 2.0 nm, as measured 0 1986 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 90,.No. 22, 1986
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Figure 2. Schematic illustrations of two different ways to deposit Pt on a porous n-Si surface, showing the formation of a discontinuous or continuous Pt layer.
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Figure 1. Scanning electron micrographs of a photoetched n-Si wafer (A), a photoetched and Pt-coated Pt/n-Si(porous) wafer (B), and a nonphotoetched and Pt-coated Pt/n-Si(flat) water (C). Dark parts in (A) correspond to micropores, and bright parts in (B) correspond to Pt islands. An n-Si wafer not photoetched or coated with Pt showed no structure, as in (C).
with a quartz oscillator whose face was placed perpendicular to the direction of the Pt vapor flow. The Pt-coated n-Si electrodes thus prepared will hereafter be designated as Pt/n-Si(porous), and those prepared from n-Si electrodes not photoetched (namely, only etched in a 10%H F solution in the dark for about 10 s just before the Pt deposition) and hence having flat surfaces will be designated as Pt/n-Si(flat). The scanning electron microscopic inspection showed that the Pt layer of Pt/n-Si(porous) is likely to be discontinuous (Figure lB), consisting of islands 10-40 nm wide and long, though that of Pt/n-Si(flat) looks quite continuous (Figure 1C). The formation of the discontinuous Pt layer in Pt/n-Si(porous) can be understood by the schematic illustration of Figure 2A. The Pt islands in Pt/n-Si(porous) are somewhat larger in size and denser than those assumed in our theory (case 3 of ref 6). The effect will be discussed later. Curve a of Figure 3 shows a photocurrent-potential curve for Pt/n-Si(porous) in a 8.8 M HBr/O.O5 M Br2 aqueous solution (M = mol/dm3) as compared with that for Pt/n-Si(flat) (curve b). The open-circuit photovoltage (V,) for Pt/n-Si(porous) is about 0.5 V, much higher than that for Pt/n-Si(flat), clearly indicating the effect of the discontinuous metal coating. A low V, (0.2-0.3 V) was obtained with an n-Si electrode photoetched and hence having a porous surface when Pt was evaporated on the n-Si surface placed perpendicular to the direction of the Pt vapor flow (Figure 2B), most probably becnuse the Pt layer in this case covers the n-Si surface almost continuously. The Pt(7) Unagami, T. J. Electrochem. SOC.1980, 127, 476. (8) Beale, M. I. J.; Chew, N. G.; Uren, M. J.; Cullis, A. G.; Benjamin, J. D.Appl. Phys. Lett. 1985, 46, 86.
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Figure 4. V, vs. c(R/Ox) for Pt/n-Si(porous) ( 0 )and Pt/n-Si(flat) (0). Redox couples used are (1) 7.6 M HI/0.05 M I,; (2) 6.0 M HI/0.5 M I,; (3) 1.7 M Fe2+/0.1 M Fe3+ (in 7.6 M HCl); (4) 7.6 M HBr/O.5 M Br,; ( 5 ) 7.6 M HBr/l.O M Br,.
coated n-Si electrodes, both porous and flat, were stable in aqueous redox solutions, but the uncoated ones degraded rapidly, showing no steady photocurrents. The stabilization of Pt/n-Si(porous) can be explained by taking into account that Pt-coated parts of the n-Si surface are stabilized by the Pt layer and the noncoated parts are covered with silicon oxide and passivated. In Figure 4 is plotted the V, against the redox potentials c(R/Ox) of the redox couples in solution. The V, for Pt/n-Si(porous) increases linearly with e(R/Ox), whereas V, for Pt/nSi(flat) is nearly constant, irrespective of e(R/Ox). The latter result is the one just expected from the conventional potential barrier model for the metalsemiconductor contacts. The former result indicates the behavior expected from our aforementioned theory for a discontinuous metal coated semiconductor electrode. The maximum V, obtained in the present work is ca. 0.5 V (Figures 3 and 4), somewhat lower than that for p n junction silicon solid solar cells (ca. 0.6 V). Also, the slope in the V, vs. c(R/Ox) relation is much lower than unity (Figure 4). These results can be explained by taking account of the metal layer being too dense (cf. Figure 2B). A much higher V, should be obtained if the metal islands could be made smaller and sparser. It spite of this, we conclude that the present work has given support to our theory and suggests a new approach to efficient photoelectrochemical solar cells. More details of the theory and experimental work will be reported in the near future.
