3238
J. Phys. Chem. 1984,88, 3238-3243 Materials Sciences Division of the US.Department of Energy under Contract No. DE-AC03-76SF00098. We acknowledge the San Francisco Laser Center for providing the laser system under N S F Grant CHE79-16250. M. Asscher thanks the Chaim Weizmann Post Doctoral Fellowship for providing a Postdoctoral Fellowship.
tributions (e.g., vibrational and rotational states of the product) were found to be characterized by temperatures lower than the crystal temperature.
Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences,
The Energetics of pin Photoelectrolysis Cells L. Fornarini: A. J. Nozik,* and B. A. Parkinson* Photoconversion Research Branch, Solar Energy Research Institute, Golden, Colorado 80401 (Received: May 31, 1983)
The energetics of p/n photoelectrochemical cells, containing simultaneously illuminated p-type photocathodes and n-type photoanodes, have been investigated by using appropriate combinations of n-WSez, n-MoSe2, n-WS2, n-Ti02, p-InP, p-Gap, and p-Si semiconductor electrodes. The open-circuit photovoltages (Eocv) of the p/n cells were measured as a function of the redox reactions in the cell and as function of light intensity. For many semiconductor electrode combinations, the sum of Eocvplus the standard cell voltage for the net cell reaction ( U owas ) found to be constant for a given pair of nand p-type electrodes at a given light intensity. This constancy was shown to be equivalent to the constancy of the sum of the band bending in the semiconductor depletion layers and the sum of the electrode overvoltages. These results are explained by movement of the semiconductor band edges with changes in the redox reactions that occur at the semiconductor electrode. If special care is taken to produce semiconductor electrodes that do not show surface charging effects, Eocv was found to be independent of the redox reaction. This is the expected behavior for pinned band edges. The dependence of Eocv on light intensity for p-InP/n-MoSez cells was either about 60 mV or about 130 mV per decade increase in light intensity. The latter value occurred when both the p-InP and n-MoSez band edges were pinned, while the former value occurred when only the p-InP band edge was pinned. These conditions for band-edge pinning depended upon light intensity and the nature of the electrolyte.
Introduction A photoelectrolysis device has the advantage over a purely photovoltaic device of providing all or some fraction of its output as storable energy. A photoelectrolysis device which employs a single semiconductor photoelectrode (connected to a metal counterelectrode) to provide the necessary photopotential to drive the photoelectrolysis of high-energy reactions must utilize either a large band gap semiconductor or an external bias. A large band gap material reduces the amount of solar radiation absorbed by the photoelectrode, and an external bias requires the input of expensive electrical energy, both of which reduce the practical solar conversion efficiency achievable in the device. One method for augmenting the effective voltage of a photoelectrolysis cell is to simultaneously illuminate ohmically connected p-type and n-type photoelectrodes which are immersed in an electrolyte.'^* Upon illumination the minority carriers are available at the conduction band edge of the p-type electrode and at the valence band edge of the n-type electrode, while the majority carriers recombine at the ohmic contact. The usable photopotential of this type of p/n cell is increased such that smaller band gap semiconductors can be used to drive more energetic electrode reactions, but at the expense of halving the maximum quantum yield for current flow due to the annihilation of the majority carriers at the ohmic contact. Such a p/n cell requires two photons per separated electron-hole pair and is roughly analogous to photosynthesis in green plants, where both the oxidation of water and the reduction of C 0 2 are also photodriven at two physically distinct photosystem sites.3 The ultimate utility of such a device for solar energy conversion would be governed by the band gaps of the repsective p- and n-type semiconductors, as well as by the position of the energy bands in the semiconductor with respect to the redox levels in the electrolyte. Small band gap materials with the conduction band t Current address: Institute of Chemical Physics, University of Rome, Rome, Italy.
0022-3654/84/2088-3238$01.50/0
of the p-type material at a rather negative potential and coupled to an n-type material with a valence band at a rather positive potential would be the most advantageous. Such a system for the efficient photoelectrolysis of HBr and H I has been described previ~usly.~ The prediction of which materials would be suitable for a given photoelectrolysis reaction would be straightforward if the energy position of the bands of the semiconductor would remain fixed (with respect to the solution redox levels) and independent of the redox species in the solution, the existence of surface states at the semiconductor/electrolyte interface, or whether the semiconductor was illuminated or not. Recent work on one-photoelectrode systems has shown that the band positions a t the interface can be changed significantly by any or all of these The work reported herein attempts to analyze the effect of the semiconductor, redox electrolyte, and level of illumination on the open-circuit voltage of a p/n photoelectrolysis cell.
Approach We chose to study semiconductors which were stable in aqueous redox electrolytes and had relatively small band gaps (with the exceptions of TiOz and Gap). Stability is important because any (1) A. J. Nozik, Appl. Phys. Lett., 29, 150 (1976). (2) H. Yoneyama, H. Sakamoto, and H. Tamura, Electrochim. Acta, 20, 341 (1975). (3) A. J. Nozik, Phil. Trans. R. SOC.London, Ser. A , 295, 453 (1980). (4) C. Levy-Clement, A. Heller, W. A. Bonner, and B. A. Parkinson, J . Electrochem. SOC.,129, 1701 (1982). (5) A. J. Bard, A. B. Bocarsly, F. R. F. Fan, E. G. Walton, and M. S. Wrighton, J . A m . Chem. SOC.,102, 3671 (1980). (6) A. J. Bard, F. Fan, A. S. Gioda, G. Nagasubramanian, and H. S. White, Faraday Discuss. Chem. SOC.,70, 19 (1980); A. B. Bocarsly, D. C. Bookbinder, R. N. Dominey, N. S . Lewis, and M. S . Wrighton, J . Am. Chem. SOC.,102, 3683 (1980); J. Gobrecht, H. Gerischer, and H. Tributsch, Ber. Bunsenges. Phys. Chem., 82, 1331 (1978). (7) J. A. Turner, J. Manassen, and A. J. Nozik, Appl. Phys. Lett., 37, 488 (1980).
0 1984 American Chemical Society
Energetics of p/n Photoelectrolysis Cells corrosion reactions which occurred would influence the open-circuit voltage of the device. The illuminated open-circuit voltage (Ewv) of the cell was measured as the primary variable. This is because the sign and magnitude of Eocv reflects the relative energy level positions of the n- and p-type semiconductor electrodes; the sign of Eocv also determines whether a given photoelectrolysis reaction will occur spontaneously under illumination or whether an external bias voltage is required. In our convention, the voltage of the p-type electrode is measured with respect to the n-type electrode. In this convention a positive open-circuit voltage will indicate a spontaneous process under illumination, whereas a negative EOCV will indicate that an additional external bias is required to drive the photooxidation at the n-type electrode and the simultaneous photoreduction at the p-type electrode. It should be noted that Eocv is measured (externally) between the p-type electrode and the n-type electrode, and that Eocv is not the photopotential that is generated within the cell to drive the chemical reaction in the electrolyte. The latter potential reflects the energetics of the minority carriers, while Ewv reflects the energetics of the majority carriers. This is an important distinction that is frequently not appreciated. It is, of course, possible to have Eocv = 0 (short-circuit conditions), but with the chemical reaction in the cell still proceeding under the influence of sufficient effective photopotential generated by the minority carriers. One model, assuming separate equilibrium among minority and majority carriers in their respective energy bands, relates this internal photopotential to the difference in quasi-Fermi levels between the n- and p-type electrodes under illumination.8 Another model, assuming no equilibrium among minority carriers, relates the driving force for the chemical reaction to the difference between the minority-band-edge positions in the two electrode^.^ The efficiency of the device is also not determined by Eocv. The efficiency will be governed by the quantum yield for current flow (in the two-electrode cell, the current will be limited by the value at the electrode with the smallest photocurrent) and the fill factor of the device (which is strongly linked with the electrontransfer kinetics and the quality of the junction at the semiconductor-electrolyte interface). The open-circuit photovoltage is, to the first approximation, independent of the electrode kinetics at the interface but may be influenced by specific interactions of the redox species with interface states which can control the surface recombination velocity.1° Surface recombination is the process which most influences the open circuit photovoltage in a photoelectrochemical device constructed with semiconductors with good bulk properties.
Experimental Section Semiconductors. p-InP crystals were obtained from Varian. Before use they were etched in 1:l methanol/HCl or, in the case of the samples which exhibited fixed energy band positions, in 2:2:1 HNO,/H,O/IICl. (Ohmic contacts were made by electrically arcing Zn to the back of the sample.) When hydrogen was to be evolved from the p-InP, islands of platinum were plated on the surface from a 2 m M solution of chloroplatinic acid by the method described by Heller et al.," or by using the speckle interference pattern produced by the expanded beam of a He:Ne laser. p-GaP was obtained from Metals Research Ltd, ohmically contacted with Zn by the same method used for InP, and etched in H 2 S 0 4 / H 2 0 2 / H 2 0 ,1:1:3, before use. n-Ti02 was obtained from Commercial Crystal Labs and doped by reducing in a Hz-N2 mixture for 20 min at 500 "C. The TiOz sample was etched in concentrated H2S04at 155 "C before use. p-Si was obtained from General Diode and was etched in hydrofluoric acid or 3:1:2 HF/HNO,/CH,COOH. Islands of platinum were deposited on the silicon by the previously described technique or the technique -described by Wrighton.I2 Mott-Schottky data were obtained (8) H. Gerischer in "Photovoltaic and PhotoelectrochemicalSolar Energy Conversion", F. Cardon, W. P. Gomes, and W. Dekeyser, Ed., NATO Advanced Study Institutes, Plenum, New York, 1981, pp 199-262. (9) A. J. Nozik, ref 8, pp 263-312. (10) R. H. Wilson, J . Appl. Phys., 48, 4292 (1977). (11) A. Heller and R. G. Vadimsky, Phys. Reu. Lett., 46, 1153 (1981).
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3239 TABLE I: AEO Combinations
+ Eocv for a Variety of Electrode/Redox Couple ox
Red Ht/HZa
H+/HZa
v3+/vz+ v-'+/v2+ c
Ht/H2"
Ht/H2"
1-p2 Br-/Br, Br-/Br2c
'
psi
p-InP
p-GaP
AE"
n-WSe, 1.12
1.39
1.07 1.39 1.39
0.535 1.06 1.32 0.52 1.35 0.69
1.27 1.30 1.66 1.33 1.21
0.98 1.29
0.535 1.06 1.32 1.35 0.69
1.28 1.54 1.69
1.05 1.39
0.535 1.06 1.32
1.05 1.24
V'+/V4+ C1-IC1,b Fe(CN)t-/ Fe(CN):-
1.41 1.66 1.19 1.40 1.31
n-MoSe2
HC/H2" Ht/HZa
1-112 Br-/Br2
v3+/vz+ c
Br-/Br2'
H+/H2
C1-/CI2b Fe(CN)>-/ Fe(CN)?-
Ht/HZa
0.96 0.96 1.11
n-WS2
H'/H2" Ht/H2"
1-11,
V'+/VZ+
Br-/Brc
Br-/Br,
0.95 1.08 1.23
When Ht is reduced, microscopic islands of platinum are deposited on the InP, p-Gap, or p-Si surface. 15 M LiCl + 1 M HC1 for stability purposes.20 Solutions in both compartments are different, Le., no VSt in the oxidizing side.
with a computer-automated system, using the lock-in amplifier technique to determine capacitance. HBr and H I solutions were prepared by mixing deoxygenated solutions of HC104 and NaI or NaBr such that the final solution was 2 M in HX and 2 M in NaC104. V3+ solutions were prepared from anhydrous VCl, obtained from Strem Chemical and added to degassed solutions of the proper anion. V02+ was obtained from oxidation of the acidic V3+solutions. Eu3+solutions were prepared by dissolving Eu2O3 obtained from Cerac in the appropriate amount of concentrated acid and then diluting to the required concentration. Electrochemical measurements were made with PAR potentiostats and function generators. Illumination of the semiconductor samples was provided from either tungsten halogen lamps, the 632.8-nm line of a Hughes Model 3235 H-PC, 10-mW He:Ne laser, or the 514.5-nm line from a Spectra Physics Model 262A air-cooled argon ion laser. A variable attenuator, Model K1174 from Karl Lambrecht, was used to adjust the laser illumination intensity such that the photocurrent generated in each semiconductor of any given p/n pair was equal under short-circuit conditions. The experiments with the p/n systems were conducted with separate electrode compartment in an H-shaped cell. The compartments were separated by a glass frit that prevented mixing of the electrolyte in the anode and cathode compartments. Depending upon the specific overall cell reaction being investigated, the electrolyte in the two compartments was different or identical. The specific redox reactions studied are indicated in Table I. The open-circuit voltages were measured after photocurrent was generated in the cell by sweeping the cell potential over an appropriate range. This ensured that sufficient concentrations of oxidized and reduced forms of the redox couple were present in each compartment to define A,!?" within 100 mV. Open-circuit voltages determined from the illuminated current-voltage curves were equivalent to those obtained from the simple open-circuit voltage measurements described above. Typical photocurrentvoltage curves for a n-WSez, p-InP cell are presented in Figure 1 for H I (curve A), HBr (curve B), and HC1+ 15 M LiCl (curve C) solutions. A curve for a cell with V3+ in the photocathode compartment and HBr in the photoanode compartment is also shown (curve D). They show little hysteresis dependent on scan direction and indicate that Ewv is a strong function of the solution redox species. (12) R. N. Dominey, N. S. Lewis, J. A. Bruce, D. C. Bookbinder, and M. S. Wrighton, J . Am. Chem. SOC.,104, 467 (1982).
3240 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 I
Fornarini et al.
- 1
E
HI
E (VOLTS)
Figure 1. Normalized current-voltage curves for a cell consisting of a
n-WSe2photoanode and a p-InP photocathode. The photocathode is plated with Pt metal islands in curves A-C but not in curve D. The solutions in both compartments are as follows: A, 2 M HI; B, 2 M HBr; C, 1 M HCI; 15 M LEI; D, the photoanode compartment contains 1 M V"'Br3 + 1 M HBr and the photoanode compartment contains 2 M HBr. The dark current is negligible on this scale. The voltage E is the bias voltage between the p- and n-type electrodes.
HBr
1
Figure 2. Summary of flat-band positions in the dark for semiconductors tested in this study in 2 M HBr solutions.
Results and Discussion Figures 2 and 3 show the positions of the energy levels for the semiconductors and the redox levels for the redox species used in this study. The energy levels for the semiconductors were determined by standard Mott-Schottky techniques.' Figure 2 is for 2 M HBr and Figure 3 is for 2 M HI. The flat-band potentials measured for the various semiconductors in these two electrolytes are quite similar except in the case of the transition-metal dichalcogenide electrodes where the band positions are shifted when even a trace of I< is present.I3 The presence of V3+ or Eu3+was found to not influence the flat-band potential of the p-type electrodes in the dark nor did the presence of Pt islands on the electrode surface influence the flat-band potential. The simplest model for the two-photoelectrode system would predict that Eocv would be proportional to the difference in flat-band potentials between the two electrodes (BE,,) in the dark.'J4 In Figure 4, Eocv is plotted vs. AEFBunder conditions that the laser illumination intensity is divided between the two electrodes such that the current flow through each was equal. In most cases, the light intensity on each electrode was nearly equal, (13) J. A. Turner and B. A. Parkinson, J . Electroanul, Chew., 150, 61 1 (1983). (14) A. J. Nozik in "Semiconductor Liquid-Junction Solar Cells", A. Heller, Ed., Electrochemical Society, Princeton, 1977,pp 277-8.
Figure 3. Summary of flat-band positions in the dark for semiconductors tested in this study in 2 M HI solutions.
Figure 4. Plot of the open-circuit photovoltage (Ew-) measured between two semiconductors in the various redox solutions (see key, upper left) against the difference in the flat-band potentials for the two electrodes in the dark (AEFB).The dashed line has a slope of 1. (No correlation is observed.)
but substantially more monochromatic intensity was needed to generate the same current in the p-GaP electrodes compared to the transition-metal dichalcogenide electrodes. Figure 4 shows no correlation between Eocvand AEFB; instead, the Eocv measured depends on the redox electrolyte present in the system. Table I is a compilation of numbers obtained by adding the Eocv measured in the cell to the difference between the standard redox potentials for the oxidation and reduction reactions ( A E O ) being driven in the cell. It is estimated that the actual redox potentials in the electrolyte are within 100 mV of AEO under conditions where Eocv is measured. The near constancy of the numbers in Table I suggests that larger open-circuit photovoltages are measured when the difference in redox potentials is small (Le., H+ reduction, I- oxidation) and smaller EOCV's are measured when this difference is large (Le., V3+ reduction and Br- oxidation). An understanding of the relationship between Eocvand AEO can be developed with algebraic manipulation of the energy band positions. The open-circuit photovoltage is the difference between the Fermi levels in the two semiconductors; when the earlier described convention is used (positive Eocvsignifies a spontaneous process under illumination) The Fermi levels with respect to the band edges in the bulk of
1
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3241
Energetics of p/n Photoelectrolysis Cells the semiconductors are given by
where subscripts VB and CB represent the respective valence bands and conduction bands and superscript B refers to the bulk semiconductor. N represents the density of states in the band, niis the intrinsic carrier concentration, and n and p are the dopant concentrations of n- and p-type dopants, respectively. The position of the conduction band at the surface of the n-type material (E:,(n)) can be related to the bulk band position by the amount of band bending in the space charge layer of the semiconductor (VB(4)
= + vB(n) and similarly for the p-type material
(4)
= '%B(P) - vB(P)
(5)
The band positions at the surface can be related to the standard potentials of the respective redox reactions ( E O , and Eoc)by the overpotentials of the oxidation reaction (ta)and the reduction reaction ( q c ) . The overpotential for a given reaction is defined as the energy difference between the band edge at the surface and the solution redox level ta
= EtB(n) - En,
1
0 I
A
B
C
Figure 5. Shift in band positions from the dark initial condition (a) to the dark equilibrium position with no band movement (b) to the light open-circuit condition after band movement (c). Arrows indicate direction of band-edge movement.
I
A
B
l
(6)
Substituting eq 2 and 3 into eq 1, we obtain
Further substitution of eq 4 and 5 into eq 8 results in a relation which includes the band positions a t the surface
Eocv =
Large AEo
\
I
(9) The position of the conduction and valence bands at the surface of the p-type and n-type materials are related via the respective band gaps
Small A E o
V, + rl = Constant
Figure 6. Movement of the semiconductor bands with response to the redox levels in the electrolyte with the condition that V, 9 is constant.
+
I
A
B
Rearrangement and combining terms gives us + AE' =
Enrv
where the first parenthetical term on the right-hand side is constant for any given set of semiconductors. Since the left-hand sum in eq 13 (Eocv + AEo) was found to be (approximately) constant (Table I), the second parenthetical term ( C V B xq)in eq 13 must also be constant. That is, for the n- and p-type electrode combinations indicated in Table I, the sum of the overpotentials and band bending at each electrode remains constant as the redox reaction is varied. Such a relationship could only occur if the
+
Figure 7. Possible band-edge positions for an n-type semiconductor electrode in response to a change in the redox potential from E o l to E o 2 with the constraint that 9 + V, remains constant. In B the band-edge positions can be within the shaded region.
band-edge positions of each electrode were free to move as the redox couples varied. This situation is illustrated by the band diagrams in Figures 5-7. Figure 5 shows the bands shifting from the dark initial condition at flat band (A) to the dark equilibrium position with no band movement (B) to the light open-circuit condition after band edge movement in the direction of the arrows (C).
3242 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984
Fornarini et al.
TABLE 11: p-InP and n-Ti02 (Bands Fixed) Red H+ H+
Ox
Eocv, mV 470 538 561 516
1Br-
H+
c1-
H+
H2O
Red Eu3+ Eu’+ Eu3+
Eu3+
2M
Ox BrH2O I-
c1-
HBr
EOCV,mV 70 64 78 15
Figure 6 shows how the bands move in response to a change in the AEo for the redox couples. In Figure 6A with a large A E O , EWvbecomes small. If hEo becomes small and the restraint that V, + 9 remains constant is imposed the result, as is shown in Figure 6B, is that Eocvbecomes larger through movement of the band edges. Figure 7 illustrates that the band movement with response to a change in Eo can divide itself between band bending and overpotential such that the final position of the band at the surface may fall anywhere in the shaded area of Figure 7B. It has been demonstrated that, by careful preparation and treatment of the semiconductor surfaces, semiconductor electodes can be prepared which show no band movement when illuminated with low levels of supraband gap photons.15 A n-TiO, photoanode was prepared which showed no band-edge movement as evidenced by the constancy of the Mott-Schottky plot in the dark and under illumination. This n-Ti02 photoanode was connected to a p-InP photocathode whose band edges could move (because of the specific etching procedure used in preparing the electrode), and Eocv was determined for a variety of redox couples (see Table 11). It should be noted that whether band edges move or not in p-InP (or other) electrodes in the presence of redox couples depends upon the details of the electrode surface preparation, potential, and light i n t e n ~ i t y . ’ ~ . Heller ’~ et a1.I6 have shown that they can prepare p-InP electrodes whose band edges do not move with change in redox potential of a vanadium solution over the potential range +lo0 to -300 mV (vs. SCE). Cooper et al.” show that, at low light intensity, band movement is absent with electrode potentials ranging from +lo00 to 0 mV (vs. SCE) in acetonitrile electrolytes. The results reported here are not inconsistent with these previous studies. The data in Table I1 show that Eocv did not change with the oxidation reaction at the photoanode but did depend upon the reduction reaction at the photocathode. This behavior is consistent with the absence of band-edge movement as a function of the oxidation reaction at the n-TiO, photoanode and the occurrence of band-edge movement with changes in the reduction reaction at the p-InP photocathode. The Eocv values measured in the various Eu3+ salt solutions (see Table 11) were less than the corresponding Eocv values measured in H+ salt solutions by an amount (0.40-0.49 V) that was about equal to the difference in Eo between the two reduction half-reactions (0.45 V). This relationship is predicted by the requirement (deduced and discussed above) that (Emv A P ) is constant for electrodes whose band edges can move. The dependence of the open-circuit photovoltage on light intensity was determined for p-InP and n-MoSe2 electrodes in both HBr (see Figure 8) and HI (see Figure 9). The measurements were made with the individual semiconductor electrodes coupled to metal counterelectrodes and also with the n- and p-type electrodes coupled together in a p/n cell. In Figure 8, the individual p I n P and n-MoSe, electrodes in 2 M HBr electrolyte both showed a Eocv dependence of about 6 5 mV per decade change in illumination intensity; this is comparable to the value reported earlier for p-InP.’* With the two semiconductor electrodes coupled together, Eocv showed a change of about 130 mV per decade change in illumination intensity-the sum of the slopes for individual electrodes. This behavior was generally observed for all electrodes whether or not they exhibited slopes close to the the-
+
(15) J. A. Turner and A. J. Nozik, Appl. Phys. Lett., 41, 101 (1982). (16) A. Heller, B. Miller, and F. A. Thiel, Appl. Phys. Lett., 38, 282 (1981). (17) G. Cooper, J. A. Turner, B. A. Parkinson, and A. J. Nozik, J . Appl. Phys., 54, 6463 (1983). (18) A. Heller, E. Aharon-Shalom, W. A. Bonner, and B. Miller, J . Am. Chem. SOC.,104, 6942 (1982).
-1001 10-2
I
10-1
I
I
100
10’
Light Intensity, mW
Figure 8. Dependence of the open-circuit photovoltage on light intensity for p-InP, n-MoSe,, and for the coupled electrodes in 2 M HBr. The light intensities given in the figure are total values incident upon the sample; intensities in mW/cm2 are obtained by multiplying the total intensity values by 11.O. The open-circuit photovoltages for the individual semiconductor photoelectrodes are measured with respect to SCE.
600
-
2M
HI
>
E a- 400m
2
2
200-
3
?
5 c
n-MoSe?
0-
0.
0
-200
-
10-2
10-1
100
10‘
Light Intensity, rnW
Figure 9. Dependence of the open-circuit photovoltage on light intensity for p-InP, n-MoSe2, and for the coupled electrodes in 2 M HI. The light intensities given in the figure are total values incident upon the sample; intensities in mW/cm2 are obtained by multiplying the total intensity values by 11.0. The open-circuit photovoltages for the individual semiconductor photoelectrodes are measured with respect to SCE.
oretical value. MoSe,, WS,, and WSe, electrodes with less than perfect van der Waals surfaces always exhibited slopes of greater than 60 mV/decade. When H I was used as the electrolyte (see Figure 9), the p-InP electrode individually showed about a 5 5 mV change in Eocv per decade change in illumination intensity over the full light intensity range (5.5 X lo-’ to 220 mW/cmz), while the n-MoSe, electrode showed an Eocv dependence at low light intensities (5.5 X lo-’ to 22 mW/cmz) of about 80 mV per decade change in intensity. At intensities above 22 mW/cm2, Eocv saturated at a constant value for n-MoSe2. The Eocv of the coupled p/n configuration showed voltage changes with decade changes in light intensity of about 130 mV mV at low intensities (5.5 X lo-‘ to 22 mW/cm2) and about 60 mV at higher intensities (22 to 220 mW/cm2). For ideal photovoltaic devices, the dependence of the open circuit photovoltage on light intensity is given by
where T i s temperature, k is Boltzmann’s constant, e is electronic charge, I is illumination intensity, and Io is a constant for a given e1e~trode.l~This expression defines the movement of the Fermi (19) J. I. Pankove, “Optical Processes in Semiconductors”,Prentice-Hall, Englewood Cliffs, NJ, 197 1.
Energetics of p/n Photoelectrolysis Cells level in the semiconductor bulk with increasing light intensity. For ideal semiconductor-liquid junctions with fixed band edges, Eocv changes by 59 mV per decade change in illumination intensity. For an ideal p/n photoelectrolysis cell, the movement of the Fermi level in each semiconductor electrode follows eq 14, and because the movement is in opposite directions the dependence of Eocv on illumination intensity should be 2 X 59 mV/decade or 118 mV per decade change in light intensity. For p-InP and n-MoSe, in 2 M HBr (Figure 8), the ideal behavior described above is approximately observed experimentally if the semiconductor surfaces are carefully prepared. For p-InP and n-MoSe, in 2 M HI (Figure 9), ideal behavior is only observed at low light intensities (5.5 X lo-' to 22 mW/cm2). Above 22 mW/cm2, the p/n cell shows an EWvdependence on light intensity equal to that of the p-InP electrode alone (about 60 mV/decade intensity). These results suggest that at the higher intensity, only the p-InP electrode band edges are fixed, while the band edges of the n-MoSez electrode shift toward positive potentials. This shift cancels the photovoltage produced by band flattening (shift of Fermi level toward negative potentials) of the n-MoSe, electrode. The movement of the band edges of n-type electrodes toward positive potentials is expected if the surface is positively charged. Such net positive charge buildup on the surface would wcur as adsorbed I- is oxidized to I< by positive holes. Such large effects on the band positions of layered chalcogenides, depending upon the 1-/13-ratio, have been previously 0b~erved.l~ The fact that band-edge movement can be observed in a system of two semiconductors simply connected together suggests that these effects might be used to advantage in such a photoelectrochemical device. If, as a result of surface state charging, the bands of the p-type material are moved more negative and the bands of the n-type material are moved more positive, it would follow that redox processes with a larger AEo value may be driven by the device. The fact that the simultaneous oxidation of bromide and reduction of vanadium(II1) yielded an open-circuit potential which indicated that the process was spontaneous for several electrode combinations (see Figure 4) is an example of this effect. However, in practice these apparent benefits may not be realizable in a device. The surface states which are responsible for the band-edge movement will also act as recombination centers. The dominance of recombination near the open-circuit potential will result in low fill factors and thus a lower efficiency for the device. This effect was empirically observed in this study. Fill factors for cells which were driving redox reactions with large AEo tended to be lower than those observed for a pair of couples with a smaller AEO (see Figure 1).
Conclusion The simple model for p/n photoelectrolysis cells, which predicts that the open-circuit photovoltage (Eocv) would be proportional (20) L. Schneemeyer, A. Stacy, and M. S. Wrighton, J . Am. Chem. Soc., 102, 6898 (1980).
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3243 to the difference in flat-band potentials of the n- and p-type electrodes determined in the dark, has been shown to be invalid for many electrode combinations. For these systems, it was observed that the sum of the difference between the standard redox potentials for the oxidation and reduction reaction being driven in the cell ( A E O ) and Em- was approximately constant for a given pair of n- and p-type electrodes at a given light intensity. The constancy of (AEO Eocv) was shown to be equivalent to the constancy of the quantity (CV, Cq),where X V , is the sum of the band bending in the semiconductor depletion layers and xq is the sum of the overvoltages at each electrode; q is defined as the difference between the minority carrier band edge and the standard redox potential of the reaction being driven by the minority carrier at the semiconductor electrode. These results can be explained by the movement of the semiconductor band edges with changes in the redox reaction that occurs at the semiconductor electrode. This band-edge movement is attributed to charging effects at the semiconductor surface produced by the redox species in the electrolyte. If special care is taken to produce semiconductor surfaces that do not show charging effects dependent upon the redox couple in the electrolyte, then the band edges do not move and are fixed independent of the redox reaction. This situation can be determined from Mott-Schottky data. In such cases, where the band edges are fixed, the open-circuit. photovoltage was found not to be dependent upon the redox reaction at the fixed-band-edge electrode. The simple model for p/n cells is valid if both n- and p-type electrodes have fixed band edges. For a given electrolyte composition and pair of semiconductor electrodes, the effect of light intensity on band-edge movement was found to be dependent upon the redox reaction and the electrode material. For p-InP in the presence of HI or HBr, EWv increased approximately 60 mV per decade increase in light intensity over the range 5.5 X lo-' to 220 mW/cm2. For n-MoSez, the same behavior was observed in the presence of HBr. However, in HI electrolyte, the Emv dependence on light intensity was about 80 mV/decade at light intensities less than 22 mw/cm2, and Emv was independent of light intensity above 22 mW/cm2. This behavior is attributed to changes in surface charge related to oxidation of adsorbed I- on n-MoSe,, resulting in band-edge movement. For p/n cells with p-InP and n-MoSe2, these effects were manifested by an Emv dependence on light intensity of about 130 mV/decade at intensities less than 22 mw/cm2, and about 60 mV/decade at intensities above 22 mW/cm2.
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Acknowledgment. This work was supported by the U S . Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under Contract EG-77-(2-4042-01. The authors thank John Turner for discussions and Jerry Cooper for experimental assistance and the preparation of several electrode samples. Registry No. WSe2, 12067-46-8; WS2, 12138-09-9; MoSe2, 1205818-3; TiO,, 13463-67-7; Si, 7440-21-3; InP, 22398-80-7; Gap, 1206398-8.
