2316
J . Phys. Chem. 1988, 92, 2316-2324
Effect of Microscopic Discontinuity of Metal Overlayers on the Photovoltages in Metal-Coated Semiconductor-Liquid Junction Photoelectrochemical Cells for Efficient Solar Energy Conversion Yoshihiro Nakato,* Keiichi Ueda, Hiroyuki Yano, and Hiroshi Tsubomura* Laboratory f o r Chemical Conversion of Solar Energy and Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan (Received: August 4, 1987; I n Final Form: October 9, 1987)
The open-circuit photovoltage V, of a photoelectrochemicalcell equipped with a platinum-coated n-Si semiconductor electrode remarkably increased when the Pt layer was made microscopically discontinuous. A strikingly high V, of 0.685 V has been obtained for a Pt-coated and alkali-etched n-Si electrode in which Pt existed in the form of islands 5-20 nm wide. The V, is much higher than those for normal pn-junction Si solid solar cells (about 0.59 V). This is an encouraging result, which possibly makes a novel approach to highly efficient solar cells. Theoretical considerations on the effect of the discontinuity of the metal overlayers in metal-coated electrodes have shown that high photovoltages can be generated in cases where the metal exists in the form of extremely small, sparsely scattered islands, say about 5 nm wide, separated by about 20 nm from each other, even though no increase in the intrinsic barrier height at the metal-island/semiconductorinterface is assumed to occur. The effective barrier height at the semiconductor-liquid junction can increase to the equivalent of the band gap by choosing appropriate redox couples due to the rapid decay of the potential modulation near the metal-island/semiconductor interface. In silicon electrodes covered with minute metal islands, the naked parts of the silicon surface are covered with insulating thin oxide layers, and the metal-covered parts serve as gates for the carrier transport. Accordingly, the saturation current can become quite low because most of the silicon surface is covered with the oxide layer, producing only low densities of intragap states, and the surface recombination at these parts becomes negligibly slow.
Introduction Photoelectrochemical (PEC) cells based on semiconductorliquid junctions have attracted much attention from the viewpoint of solar energy This method has a merit in that photovoltages arise only by immersing a semiconductor electrode in a liquid, without forming a p n junction, thus enabling us to utilize low-cost semiconductor materials such as polycrystalline or amorphous films easily. This method has another merit in that solar energy can be converted directly into storable chemical energy. The main difficulty in this method lies in that nearly all known semiconductors having band gaps suitable for the solar energy conversion, such as Si, GaAs, and CdSe, are corrosive in aqueous electrolyte solutions. Effort has been made either to stabilize these corrosive semiconductors by choosing appropriate redox couples or solvent^^*^ or to find new stable semiconductor material^.^.' The most successful result for the prevention of corrosion of silicon and other materials in aqueous solutions has probably been the one obtained by the present authors by coating the semiconductor surface with a very thin layer of platinum or other metals.*q9 Many years ago we reportedlOJ1 that thin noble metal layers deposited by vacuum evaporation effectively protected the semiconductor electrodes such as n-Si and n-GaP from corrosion. In the initial stage of our studies, we found that some of the goldcoated n-GaP electrodes generated very high photovoltages that could be explained only by assuming that the height of the potential barrier at the metal/semiconductor contact was increased ( 1 ) Gerischer, H. Top. Appl. Phys. 1979, 31, 115. (2) Parkinson, 9.A. J. Chem. Educ. 1983, 60, 338. (3) Hodes, G.;Fonash, S. J.; Heller, A,; Miller, B. Adu. Electrochem. Electrochem. Eng. 1984, 13, 113. (4) Gibbons, J. F.; Cogan, G. W.; Gronet, C. M.; Lewis, N.S. Appl. Phys. Lett. 1984, 45, 1095.
( 5 ) Licht, S.; Tenne, R.; Dagan, G.;Hodes, G.; Manassen, J.; Cahen, D. Appl. Phys. Lett. 1985, 46, 608. (6) Tributsch, H . J . Photochem. 1985, 29, 89. (7) Ennaoui, A,; Fiechter, S.; Jaegermann, W.; Tributsch, H. J . Electrochem. SOC.1986, 133, 97. (8) Nakato, Y.; Hiramoto, M.; Iwakabe, Y.; Tsubomura, H . J. Electrochem. Soc. 1985, 132, 330. (9) Nakato, Y.;Iwakabe, Y . ;Hiramoto, M.; Tsubomura, H . J . Ejectrochem. SOC.1986. 133. 900. (10) Nakato[’Y.;~Ohnishi,T.; Tsubomura, H . Chem. Lett. 1975, 883. (1 1) Nakato, Y . ;Abe, K . ; Tsubomura. H. Ber. Bunsen-Ges. Phys. Chem. 1976,80, 1002.
0022-3654/88/2092-2316$01.50/0
by illumination,” in striking contrast to the conventional theory.I2J3 We tentatively explained such an anomalous photovoltaic effect as being due to the discontinuity of the metal layer.]’ We found also that p-Si and p-GaP electrodes coated with photoelectrochemically deposited noble metal layers efficiently generated hydrogen-evolving photocurrents starting favorably at potentials more positive than those for naked electrode^.'^ On the contrary, the p-type electrodes coated with noble metals by vacuum deposition gave nearly ohmic contacts and no photoc~rrent.’~ Later, it was reported by other workersi6 that the abovementioned experimental result for the Au/n-GaP electrode was not reproducible. This was probably because it was difficult to completely protect the n-GaP electrode from photoanodic dissolution by a porous gold layer. We later confirmed,17J8however, by using stable n-Ti02 electrodes and by directly measuring the potential of the metal overlayer, that the above-mentioned increase in the barrier height by illumination really existed. After further studies, both experimental and theoretical, on the photovoltages for metal-coated semiconductor electrodes, we recently made it clearI9 that, in cases where the metal layer consisted of minute islands, a variety of photovoltaic characteristics arose on changing the size and the density of the metal islands, which could be explained without assuming any change in the intrinsic barrier height at the metal-island/semiconductor interface. Most interesting is the prediction that very high photovoltages could be generated in electrodes coated with extremely small, sparsely scattered metal islands, say about 5 nm wide, separated by 20 nm or more from each other. To prove the above-mentioned prediction, we tried various methods to deposit very small and sparsely scattered metal islands (12) Sze, S. M . Physics ofSemiconductor Deuices, 2nd ed.; Wiley: New York, 1981; Chapter 5. ( 13) Metal-Semiconductor Schottky Barrier Junctions and Their Applications; Sharma, B. L., Ed.; Plenum: New York, 1984. (14) Nakato, Y.; Tonomura, S.; Tsubomura, H. Ber. Bunsen-Ges. Phys. Chem. 1976, 80, 1289. ( 1 5 ) Nakato, Y. Yano, H.; Nishiura, S.; Ueda, T.; Tsubomura, H. J . Electroanal. Chem. Interfacial Electrochem. 1987, 228, 97. (16) Wilson, R. H.; Harris, L. A,; Gertsner, M. E. J. Electrochem. Soc. 1977, 124, 1511. (17) Nakato, Y.; Tsubomura, H. Isr. J . Chem. 1982, 22, 180. ( 1 8) Nakato, Y.; Shioji, M.; Tsubomura, H. Chem. Phys. Lett. 1982, 90, 453. (19) Nakato, Y.; Tsubomura, H . J. Photochem. 1985, 29, 257.
0 1988 American Chemical Society
Microscopic Discontinuity of Metal Overlayers
m /
metal islands
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988 2317
r-----l 30
1 ight t -
-0.8
-0,6
-0,4
-0.2
0
U vs, Pt-counterelectrode / V
'redox electrolyte
semi conductor
(e,g,, n-Si)
insulating epoxy
Figure 2. Photocurrent density (j)-potential (U) curves for a Pt-coated n-Si electrode under simulated solar AM1 (100 mW cm-2) irradiation: (a) after and (b) before alkali etching.
(A)
monitored with a quartz crystal oscillator. The structure of the Pt layers was investigated by using an Akashi Seisakusho DS-130 scanning electron microscope (SEM) having a resolution of about 6 nm. The photoelectrochemical (PEC) solar cell was composed of a Pt-coated n-Si electrode, a Pt-plate counter electrode, and an aqueous electrolyte solution of a redox couple (Figure 1A). The solutions were prepared by using special-grade chemicals and deionized water without further purification. Photocurrent density (j)-potential (v> curves were measured with a potentiostat and a potential sweeper, U being measured vs the counter electrode. A saturated calomel electrode (SCE) or a silver/silver chloride (Ag/AgCl) electrode was sometimes used as the reference electrode. A Wacom solar simulator that emits a simulated solar AM1 (100 mW/cmz) spectrumz4 was used as the light source. spacer
glass
(B) Figure 1. Schematicdiagrams for the photoelectrochemical (PEC) solar cells (A) used in the present experiments and (B) suitable for practical
application. on n- and p-Si electrodes and found that remarkably high photovoltages are really generated. Parts of the experimental results for Pt-coated n-Si electrodes were already reported.z+z3 The results of studies on Pt- or Au-coated p-Si electrodes were also re~0rted.l~ In the present paper we report the experimental results for the Pt-coated n-Si electrodes in more detail, together with some new results for the Pt-coated p s i electrodes. We also give details of our theory since our previous paper19 was too brief to be well understood, especially for the most interesting case of coating with sparsely scattered, minute metal islands.
Experimental Section Single-crystal n- and p-Si (CZ) wafers were obtained from Shin-Etsu Handotai Co. These wafers were cut to pieces, about 8 mm X 8 mm, ultrasonically washed in acetone and water, and etched in CPD-2 (a mixture of hydrofluoric acid, nitric acid, and a small amount of bromine) or in a 12% hydrogen fluoride solution. Ohmic contacts were made with an indium-gallium alloy. Electrodes of n- and p-Si were prepared in the usual way using silver paste, copper wire, and epoxy adhesive. Platinum was deposited by the electron-beam evaporation method, in most cases under (1-4) X lo4 Pa and in some cases under an argon atmosphere of ca. 0.13 Pa. Just before the Pt deposition, the Si electrodes (or chips) were etched in 10% HF for about 10 s. The average thickness of the deposited Pt was (20) (21) (22) (23) 405.
Nakato, Y.; Yano, H.; Tsubomura, H. Chem. Lett. 1986, 987. Nakato, Y . ;Ueda, K.; Tsubomura, H. J. Phys. Chem. 1986,90,5495. Tsubomura, H.; Nakato, Y. Nouu.J . Chim. 1987, 11, 167. Nakato, Y.; Tsubomura, H. Ber. Bunsen-Ges. Phys. Chem. 1987.91,
Results Pt-Coated and Alkali-Etched n-Si electrode^.^^^^^^^^ Figure 2b shows a j-U curve for an n-Si (0.4-0.8 Q cm, (1 11) face) electrode coated with a 3-nm-thick Pt layer vacuum-evaporated under 0.13-Pa argon atmosphere, measured in a stirred 8.6 M HBr/O.l M Br2 solution ( M = mol/dm3). The open-circuit photovoltage (V,) was only 0.28 V. Thej-Ucurve was strikingly improved after the electrode was treated in a 4 M NaOH solution at 90 OC for about 3 min and then in a 0.25 M N a O H solution containing 0.6 M 2-propanol at 90 O C for about 3 min. The highest V, so far obtained was 0.685 V, as is shown by curve a in Figure 2, which yields a short-circuit current (,j) of 25.0 mA/cm2, a fill factor of 0.666, and a conversion efficiency of 11.4% under simulated solar AM1 (100 mW/cm2) irradiation. Similar results were obtained for n-Si (1 11) and (100) electrodes coated with Pt vacuum-evaporated under (1-4) X Pa (i.e., in the absence of the above 0.13 Pa of argon). The j-U curves for the Pt-coated and alkali-etched n-Si electrodes in the HBr/Br2 solution were stable during cyclic scans for at least 10 min. The j-U curves were much more stable (for 1 h or more) in solutions of ot4er redox couples, for instance, Fez+/Fe3+. Gradual dissolution of the deposited platinum seems to take place in the HBr/Br2 solution.2s Naked n-Si electrodes, on the contrary, degraded seriously in the first forward (negative to positive) scan in any redox solutions. Figure 3 shows V, versus IUh - c(R/Ox)l, where U, is the flat-band potential for naked n-Si, determined from the onset potential of the anodic photocurrent in a redox-free electrolyte soluti0n,2~and c(R/Ox) is the redox potential of the redox couple used. V, for the Pt-coated and alkali-etched n-Si (open circle) increased linearly with IU, - e(R/Ox)] and then became constant in a range where lU, - e(R/Ox)l exceeded the equivalent of the band gap of n-Si (about 1.1 V). On the contrary, V, for the Pt-coated but not alkali-etched n-Si (closed circle) stayed nearly (24) Nakato, Y.; Egi, Y.; Hiramoto, M.; Tsubomura, H. J . Phys. Chem. 1984, 88, 4218. ( 2 5 ) Nakato, Y . ;Ueda, T.; Egi, T.; Tsubomura, H. J . Electrochem. SOC. 1987, 134, 353.
2318
Nakato et al.
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988
I 4
I
6
I
/
>
I
4 1.5
2
6
X
I 1
J
0' 0,3
1,2
0.9
0.6
I Ufb
-
E( R/OX)
1
1,5
/ V
Figure 3. V, versus IV, - c(R/Ox)) for Pt-coated and alkali-etched n-Si (0) and for Pt-coated n-Si (a). Redox couples used are as follows: 1, Fe(C204)3'/Fe(C204)33- (pH 6 . 2 ) ; 2, I-/I,- (7.6 M H '); 3 , Fe2+/Fe3+ (pH 0.5);4, Br-/Br2 (8.6 M H+); 5 , Fe(CN)6'/Fe(CN),)- (pH 7 . 6 ) ;6 , Br-/Br2 (pH 2.9).
constant, irrespective of t(R/Ox). The SEM inspection showed that the Pt layer vacuum-deposited on n-Si was quite continuous (Figure 4A), whereas after the alkali etching it existed in the form of minute islands 5-20 nm wide (Figure 4B). It is noted here that many (not all) of the Pt-coated and alkali-etched n-Si electrodes had a number of tiny Pt flakes 1-5 pm X 1-5 pm on the surface, though the micrograph of Figure 4B was taken for the electrode having no such Pt flakes. The j-U curves of the electrodes with such flakes were the same as those without the flakes. We obtained j-U curves similar to curve b in Figure 2 even for the cases of very thin pt coating, 0.5-0.1 nm average thickness. The V, for such thinly coated n-Si electrodes was increased by the alkali etching up to 0.6 V or more, like the case of the 3nm-thick coatings (Figure 2). However, the electrodes were not so stable, and in most cases the j-U curve shifted toward the positive potential by say 0.03 V in each cyclic scan. Photoetched and Pt-Coated n- and p-Si Electrodes.2',22 The similar n-Si electrodes coated with discontinuous Pt layers were also prepared by the following method: n-Si (0.4-0.8 Q cm,(1 1 1) face) electrodes were illuminated in a 48% HF 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% HF solution in the dark for about 10 h. The SEM investigation showed that such a photoetching treatment makes the n-Si surface very rough, producing micropores of 10-40 nm in diameter (Figure 5A), as reported by other worker^.^^^^^ Platinum was then vacuum-deposited on the photoetched n-Si electrode under 6.4 X Pa, with the n-Si surface tilted at an angle (0) of about 20° against the direction of the flow of Pt vapor (cf. Figure 6A). The average thickness of the deposited Pt, as measured with a quartz oscillator, was 0.7 nm. The Pt-coated n-Si electrodes thus prepared will hereafter be designed as Pt/n-Si (porous, 0 = 20'). According to the SEM inspection, the Pt layer for Pt/n-Si (porous, 0 = 20°) seems to be discontinuous (Figure SB), consisting of islands 10-40 nm wide and long. The formation of such a discontinuous Pt layer in Pt/n-Si (porous, 0 = 20') can be understood by the schematic illustration of Figure 6A. The Pt islands for Pt/n-Si (porous, 0 = Z O O ) were larger and denser than those of the aforementioned Pt-coated and alkali-etched electrodes (Figure
4B). Figure 7 shows a j-U curve for Pt/n-Si (porous, 0 = 20') in a 8.8 M HBr/0.05 M Br, aqueous solution (curve a) and for Pt/n-Si (porous, 0 = 90°) (curbe b). The V, values for Pt/n-Si (porous, 0 = 20') were about 0.51 V, much higher than those for Pt/n-Si(porous, 0 = 90°), 0.2-0.25 V, which were nearly equal to those for Pt/n-Si(flat surface) (closed circles in Figure 3). The (26) Unagami, T. J . Electrochem. Soc. 1980, 127, 476. (27) Beale, M. I. J.; Chew, N. G.; Uren, M. J.; Cullis, A. G.; Benjamin, J. D.Appl. Phys. L e f t . 1985, 46, 86.
Pt/n-Si(porous, 0 = 20') electrodes were stable in aqueous redox solutions. Similar results were obtained with n-Si( 100) electrodes. The V , for Pt/n-Si(porous, 0 = 20°) increased linearly with c(R/Ox) (open circles in Figure 8), and the V, for Pt/n-Si(porous, 0 = 90°) stayed nearly constant (closed circles). The slope in the V, - e(R/Ox) relation for Pt/n-Si(porous, 0 = 20') was considerably lower than unity, contrary to the case of the Pt-coated and alkali-etched n-Si (Figure 3). Similar studies have been made for p-Si. The p-Si electrodes having porous surfaces were prepared by electrochemical etching in 48% HF, by passing a dark anodic current of 10 mA cm-2 for about 1 min, and then Pt was deposited at a tilt angle (0) of 20° (cf. Figure 6). The Pt-coated p-Si electrodes thus prepared, designated as Pt/gSi(porous, 0 = 20°), showed efficient hydrogen evolution photocurrent (curve a in Figure 9) in 0.5 M HzS04, contrary to Pt/p-Si(porous, 0 = go'), which showed a dark j-U curve (curve c) similar to a Pt metal electrode. No photoeffect was detected in the latter case as was reported for Pt/p-Si(flat surface) in our previous paper.15 The generation of the photocurrent for Pt/p-Si(porous, 0 = Z O O ) clearly indicates an increase in the effective barrier height at the Pt/p-Si interface by the discontinuous Pt coating. Theory Charge and Potential Distributions and Effective Barrier Heights in Semiconductor Electrodes Coated with Discontinuous Metal Layers. We will discuss here only n-type semiconductors. (Similar conclusions can be obtained for p-type semiconductors.) The concept of charge and potential distributions for a naked semiconductor electrode' and a semiconductor electrode coated with a continuous metal layer12 is fairly well established as schematically illustrated in Figure 10, where interfacial charges arising from surface states and adsorbed ions as well as interfacial bond dipoles are neglected for simplicity. For PEC cells in their working states, the Fermi level EF(SC)of the semiconductor is higher than that of the redox couple EF(redox) in the electrolyte solution by qU, where q is the elementary charge and U the photovoltage when series resistance is negligible. First, let us consider the naked surface of the semiconductor. The barrier height q&, at the semiconductor/electrolyte interface is given by the difference between the conduction-band edge Fc(E) at the interface and EF(redox) (Figure 10A) qt#& = Ef(E) - EF(redox)
(1)
where e and (E) both designate that the quantity is for the semiconductor surface facing the electrolyte. EF(redox) is related with the redox potential e(R/Ox) by EF(redox) = -qc(R/Ox)
+ Eo
(2)
where Eois a constant estimated to be about -4.5 eV for the case of a normal hydrogen electrode (NHE) as the reference electrode.' Ec(E) is related with the flat-band potential Ubversus N H E by the equation
(3) where qU, is the energy difference between the conduction-band edge and EF(SC)in a neutral region (Figure 10). E*,(E) is determined by the electron affinity x of the semiconductor, the potential drop due to bond dipoles at the semiconductor surface, the potential drop due to the Helmholtz double layer caused by ion adsorption equilibria, etc.,' and is in general independent of EF(redox) unless high-density and chemically stable surface states are p r e ~ e n t . ~ ~ Accordingly, .*~ &, increases linearly with the downward shift of EF(redox). Second, we consider the barrier height q& at the semiconductor/metal interface. This is given by the difference between (28) Bard, A. J.; Bocarsly, A. B.; Fan, F.-R. F.; Walton, E. G.; Wrighton, m. Chem. SOC.1980, 102
Microscopic Discontinuity of Metal Overlayers
The Journal of Physical Chemistry. Vol. 92, No. 8. 1988 2319
Figure 4. Scanning electron micrographs for ( A ) Pl(3 nml-coated "-Si and (R) Pt(3 nm)-coated and alkali-etched "-Si. White pa* in B mrmpand to Pt islands.
theconduction-band edge Fc(M) at the metaliwered surface and the Fermi level E,(M) of the metal layer
@F = K ( M ) - EF(M) (4) where m and (M) designate that the quantity is for the metalcovered part of the semiconductor surface. It is empirically known that, for ionic-type semiconductors such as ZnO and S n 0 2 , 46;; is given by the difference between x and the work function of the metal, whereas, for covalent-type semiconductors such as Si and GaAs. q& is nearly independent of the metal work function and is mainly determined by the difference between x and the Fermi level of the interfacial states that are preexisting or formed by metalsemiconductor reaction."^" In either case, 4;; is normally regarded as being constant for a given semiconductor-metal combination. irrespective of whether the metal-semiconductor contact is immersed in a solution or not and whether it is illuminated or not. The €&redox) deviates from €&M) by 41.where rt is the overpotential depending on the current density. For reversible or nearly reversible redox couples such as Br-/Br2, 1 is small and EF(M) is nearly equal to €,(redox). If the donor density in the space charge layer of the semiconductor is assumed to be uniform, the potential +,(x) in this layer is expressed by a function of x. the distance from the surface to the interior (cf. Figure IO). as follows: rb&)
= ( ~ N D / ~ % ) ( ~-WX2) ,J 0
, 0 u
>
0.2 D
-
S P O ChorQe ~
0
0.2 0.4 0.6 E(R/Oxl vs. Ag/AgCl I V Figure 8. V, versus c(R/Ox) far Pt/n-Si(porous, E = 20°) (0)and for FY/n-Si(porous, E = 90") (0). Redox couples used are as follows: I , 7.6 M HI/O.OS M I?; 2, 6.0 M HI/0.5 M I,; 3, 1.7 M Fe*+/O.l M Fe3+in 7.6 M HCI; 4, 7.6 M HBr/O.O5 M Br,; 5, 7.6 M HBr1l.O M Br,. I
U
VS.
,
l o w
redox'
Schematic diagram of charge distributions far ductor electrode coated with minute metal islands.
Fiyre 11.
SCE I V
eleCtrOlYfe
a
semican-
conduct Ion band
____----
Figure 12 Schematic energy-band diagram for a semiconductor elec-
trode coated with minute metal islands.
Figure 9. Hydrogen evolution current density (,?-potential (v) curves for Pt/pSi(porous, E = 20°) in 0.5 M H2SOI (a) under simulated solar AMI (100 mW cm-l) illuminationand (b) in the dark. Curve c is for h/pdi(porous, E = 90")and is independent of whether the electrode is illuminated or not
,
,I/
cm,
Figure 13. Schematic two-dimensional energy-band diagrams for semiconductor electrodes mtcd with metal islands when F,(M) is lower than
