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The influence of surface oxide films on the stabilization of n-Si photoelectrode
75
Surface Science 109 (1981) 75-81 North-Holland Publishing Company
THE INFLUENCE OF SURFACE OXIDE FILMS ON THE STABILIZATION
OF
n-Si PHOTOELECTRODE B.H. LOO, K.W. FRESE, Jr. and S. Roy MORRISON Materials Research Laboratory,
SRI International,
Menlo Park, California 940.25, USA
Received 13 January 1981
We have provided direct evidence of the enhanced effectiveness of stabilizing agents due to thin surface oxide films, ca. 15-25 A, on n-Si photoelectrode. Rotating ring disc electrode and ellipsometric experiments are combined to show the stabilization efficiency of potassium ferrocyanide improves with oxide thickness. A band model describing the observed effect is given.
1. Introduction In an earlier study [ 1J, using voltammetry, we have investigated the effectiveness of various reducing or stabilizing agents in the corrosion suppression of n-Si photoelectrode. In the absence of fluoride ions or high concentration of hydroxide ions in solution, corrosion leads to the formation of an SiOZ film on the surface of the silicon semiconductor electrode. The voltammetry measurements indicated the oxide film could substanti~y increase corrosion resistance. It was suggested that the oxide film could aupport a field which raises the energy level of the stabilizing agent with respect to the valence band edge of the Si electrode to a position favorable for hole capture from the valence band. Holes that come to the surface of the Si photoelectrode can oxidize the stab~~g agent and/or the silicon electrode. The effectiveness of the stabilizing agent in hole capture reflects the degree of corrosion suppression of the silicon electrode by the stabilizing agent. Thus shifting the energy level of the stabilizing agent to a position more favorable for hole capture means that the corrosion is suppressed. Potassium ferrocyanide and N, N, N’, N’-tetramethyl-p-phenylene-diamine (TMPD) were shown to be quite effective in the hole capture [l-3]. It was suggested that after a thin oxide fti was grown on the electrode surface, these ions suppressed further corrosion, so the film grew very slowly. In this paper, we report rotating ring disc electrode (RRDE) studies on n_Si photoelectrode using potassium ferrocyanide and TMPD, and provide direct evidence on the enhanced stabilization of r&i photoelectrode by thin surface oxide films. The thickness of the surface oxide fti that gives the maximum stab~ation 0039 -6028/8 1/OOOO-0000/$02.50 0 North-Holland
16
B.H. Loo et al. / Surface oxide films and stabilization of n-Si photoelectrode
efficiency is estimated by ellipsometric given to explain this observed effect.
measurements.
Finally,
a band model is
2. Experimental The RRDE apparatus was similar to that described before [4]. Single crystal n-type Si(l1 l), obtained from Silicon Materials (Mountain View, CA), was used as the disc electrode, of area 0.2 cm2. Ohmic contacts were made using an In-Ga alloy and silver epoxy. The oxidation products of the stabilizing agents K,Fe(CN), and TMPD were detected on the gold ring. The collection efficiency of the RRDE assembly was determined to be 0.22. A platinum electrode was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. All potentials are reported with respect to SCE. The potentials of the ring and disc electrodes were controlled independently by means of a Pine Instrument RDE 3 bipotentiostat (Grove City, PA). The silicon disc electrode was etched each time prior to RRDE measurements with 20% aqueous HF solution and followed by an absolute methanol rinse. A 150 W xenon lamp was used to illuminate the Si disc electrode. The absolute photocurrents at the ring and disc electrodes were determined with chopped light at a frequency of 1 Hz. The ellipsometric measurements were made with a Gaertner L119 ellipsometer (Chicago, IL) at an incidence angle of 70” and a wavelength of 546.1 mn. The thickness of the surface oxide film was calculated using n(Si) = 4.050 - 0.028i [5-71, and the film refractive index of 1.50 [S] .
3. Results As we mentioned earlier, photogenerated holes that come to the surface of the Si photoelectrode can oxidize the stabilizing agent and/or the silicon electrode. The stabilizing agent normally captures less than 100% of the holes, and the remainder oxidize silicon to silicon dioxide. As the Si02 increases in thickness, the photocurrent at constant light intensity gradually decreases over time. In the RRDE measurements, one obtains two measures of the effectiveness of a stabilizing agent: first, the quantity of oxidized stabilizing agent as monitored on the ring electrode; second, the decrease in photocurrent. The stabilizing efficiency of a stabilizing agent is defined as s = (IJNIn)
x lOO%,
(1)
where In is the ring photocurrent, In is the disc photocurrent and N is the collection efficiency of the ring disc electrode assembly. Obvious, S is 100% when the holes are entirely captured by the stabilizing agent. Fig. 1 shows a typical RRDE study of the decay of the photocurrents at the ring
B.H. Loo et al. /Surface 150
oxide films and stabilizationof n-S photoelectrode
I
I
I
I
5
10
77
\
a ,z !z 2 IX z
100
50
43 O_
0
TIME OF OXIDATION
1 15 (mid
Fig. 1. Decay of photocurrents at the ring (ZR) and disc (ID) electrodes as a function of the time of oxidation. Electrolyte: O.lM K4Fe(CN)6 + 0.5M KCI; disk voltage: 1.0 V; ring voltage: -0.5 V (versus SCE); pH = 8; rotation speed: 1000 rpm. Note the fast decay ofzD near t = 0, indicating most of the photogenerated holes are consumed initially in the formation of the surface oxide film.
and the disc electrodes as a function of the time of oxidation or the oxide thickness. The stabilizing agent was potassium ferrocyanide (O.lM and pH = S), and its stabilization efficiency was less than 100%. The decrease in 1, is attributed to the resulting growth of surface oxide fii. The ring photocurrent, monitoring the oxidized product of the stabilizing agent, decreases less rapidly, on a percent basis, than the disc photocurrent, especially in the first few minutes of oxidation. Thus S in eq. (1) increases. Fig. 2 shows the stabilization efficiency as a function of time of oxidation in aqueous solution of O.lM K.,Fe(CN), at pH = 3.5,5 and 8, and saturated TMPD at pH = 8. An interesting feature shown in fig. 2 is the direct observation of the initial rise in the stabilization as predicted in our earlier voltammetry work [l] . In the case of ferrocyanide, the stabilization increases as the pH is lowered. At pH = 3.5, the stabilization efficiency is comparable to that of TMRD, which is known to be a better stabilizing agent for n-Si photoelectrode [2,3]. We also performed RRDE measurements at the same photocurrent level with a lower concentration of stabilizing agent, 0.05M I(4Fe(CN)a, in 0.5M KC1 aqueous solution. The result shows S decays faster after the initial rise. The faster decay in stabilization was due to the rapid growth of a thick surface oxide. A larger propor-
78
B.H. Loo et al. / Surface oxide films and stabilizationof n-S photoelectrode
6 A l 0.1 M K4 Fe&N),
+ 0.5 M KC1
o TMPD + 0.5 M KCI (pH = 8) 0’
0
I
I
I
5
10
15
TIME OF OXIDATION
(mid
Fig. 2. Stabiiation of n-silicon as a function of time of oxidation (oxide thickness) in aqueous solutions of O.lM K4Fe(CN)6 (pH as indicated) and saturated TMPD (pH = 8). Note the initial rise in the stabilization in each case.
tion of the photogenerated holes were consumed in the oxidation of the Si electrode with the concentration of stabilizing agent lowered. To permit direct interpretation of these measurements, we have performed ellipsometric measurements of the oxide thickness as a function of time of oxidation. Ellipsometric measurements of the electrode surface were made after the electrode was oxidized under identical conditions, removed from the solution, rinsed several times with absolute alcohol and blown dry, Fig. 3 shows the resulting curve of the stab~ation efficiency with ferrocyanide as a function of the oxide thickness. Five different oxide thicknesses were determined and the best curve was drawn through
5 --I 50 F I8 3 m 2 v) 0-A 0
10
20 OXIDE
30
40
THICKNESS,
50
60
70
(A)
Fig. 3. Effect of oxide thickness on stab~ation
of n-silicon by ferrocymice
(0.1 mold.
B.H. Loo et al. /Surface
oxide films and stabilization of n-Si photoelectrode
19
these points. Note that the error bars do not represent the experimental errors but rather they represent our estimated experimental uncertainties involved with the ex situ measurements. The data points are averages of several thickness measurements at different spots of the oxidized surface. The measurements agreed with each other within 10% indicating the surface oxide is quite homogeneous. We found that after the HF rinses, the oxide films are lo-15 A thick, this is consistent with the observation made by Archer [5]. In the presence of 1% HF, the stabilization efficiency was very low, about 10%. No ellipsometric measurements were made in this case, we assume the oxide thickness is about O-10 A. Fig. 3 clearly shows maximum stability is achieved after the surface oxide grows to a certain thickness. This is fairly convincing evidence that a thin oxide present on the surface of a semiconductor can greatly enhance the efficiency of a stabilizing agent.
4. Discussion The band model for the observed effect is illustrated in fig. 4. Fig. 4a shows the band model of Si/SiOJelectrolyte in the dark, when no charges are present on the interfacial states at the Si/SiOZ interface. Under such circumstance, no appreciable voltage would develop across the oxide layer, and the relative positions of the valence band and the energy levels in solution are the same as they were in the case where there was no surface oxide present. The valence band edge of silicon at pH = 3 is 0.35 eV below the Fermi energy of the calomel electrode [9]. The redox potential of ferrocyanide indicates that the characteristic energy level EFedox of ferrocyanide is 0.12 eV below the Fermi energy of the calomel electrode [IO]. Thus, the characteristic energy level of ferrocyanide is close to the valence band edge of silicon. The density of the energy levels of the ions in solution as a function of energy is maximum at an energy A less that cedox, namely at Ered, as shown. h is the reorganization energy and, for ferrocyanide, it is assumed to be 0.7 eV. The density of energy levels forms a Gaussian distribution centered at Ered with the standard deviation of (2MT)‘/2 [ 11,121. To optimize the effectiveness of the stabilizing agent, it is necessary to have a high density of states at or above the valence band edge. For optimal hole capture, it is therefore desirable to decrease the energy of the valence band of silicon to a point closer to f&j. This can be accomplished if there is an oxide present on the electrode surface. Under illumination, the silicon electrode produces holes that can be trapped at the Si/SiOZ interface, and as a result, the interface states become positively charged, and a large double layer appears, as indicated in fig. 4b. This lowers the valence band, and E,, can then approach the energy Er,+ This illustration shows why a thin surface oxide enhances the stabilization of n-Si photoelectrode. As Em is lowered toward the energy Ered, S increases (at constant oxide thickness). S reaches a maximum when Ew = Ered as indicated in fig. 3. With increasing oxide thickness, hole capture will increase because Em approaches Ered, but will
80
B.H.Loo et al. /Surface
oxide films and stabilization of n-S photoelectrode
Si
I
SiO,
I
Electrolyte
Positive
Pin $--t Eg = 1.1 eV
Calomel Zero
States
Approx. 2000A
Fig. 4. Band model
Negative Ions Solution
158
of Si/SiOz/electrolyte
to show field in oxide when interface
states become
charged. decrease because the holes must tunnel through a or just before E, reaches Ered, a maximum in S is thickness increases beyond this point, this simple dict whether S will maintain its high value or through the oxide has to be taken into account.
thicker oxide. When E,, = Erd expected. However, as the oxide theoretical model does not predecrease. Tunneling of charges
B.H. Loo et al. / Surface oxide jilms and stabilization of n-Si photoelectrode
More quantitatively,
the rate of hole capture by a stabilizing
81
agent is given by
[41 k3 = B exp(-yXO)
exp {- [E,-=%,d
- 4v,,]
*/4MT)
2
(2)
with B, 7 and h constants, where the first exponential factor represents a tunneling probability across an oxide thickness X0, the second represents iso-energetic hole transfer as developed in the fluctuating energy level model of electrode reactions [ 11,121. To determine S we must compare such a process with the oxidation of the silicon as a function of Xc. Because the tunneling probability decreases exponentially with X0, whereas the oxidation rate by field-aided diffusion is more likely to vary inversely with X,,, one might expect S to decreases as the oxide becomes considerably thicker, which was indeed observed as in fig. 3. The presence of a thin oxide on a photoelectrode decreases the total current that can be passed and thus could adversely affect its solar cell performance. A compromise may therefore be necessary in certain cases. In conclusion, we have shown quantitatively that the presence of a thin surface oxide may substantially improve the stabilization of solar cells. This result adds to our knowledge and our models of corrosion-stabilization, and alerts us to the possibility that a high gross stabilization efficiency may unfortunately indicate that a film is present on the surface that may eventually detract from the conversion efficiency of solar cells.
Acknowledgement This work was supported by the Solar Energy Research Institute under Contract Xp-g-8002-6. Technical assistance of Dr. Kenneth Sancier in the ellipsometric measurements is gratefully acknowledged.
References [l] M.J. Madou, K.W. Frese, Jr and S.R. Morrison, J. Phys. Chem. 84 (1980) 3423. [2] D. Laser and A.J. Bard, J. Phys. Chem. 80 (1976) 463. [3] A.B. Bocarsly, E.G. Walton, M.G. Bradley and MS. Wrighton, J. Electroanl. Chem. 100 (1979) 283. [4] K.W. Frese, Jr., M.J. Madou and S.R. Morrison, J. Phys. Chem. 84 (1980) 3172. [5] R.J. Archer, J. Opt. Sot. Am. 52 (1962) 970. [6] K. Vedam and W. Knausenbaerger, J. Opt. Sot. An. 59 (1969) 64. [7] S.S. So and K. Vedam, J. Opt. Sot. Am. 62 (1972) 596. [8] A.C. Adams, T.E. Smith and C.C. Chang, J. Electrochem. Sot. 127 (1980) 1787. [9] M.J. Madou, B.H. Loo, K.W. Frese, Jr. and S.R. Morrison, Surface Sci. 108 (1981) 135. [lo] W.M. Latimer, Oxidation Potentials (Prentice-Hall, New York, 1953). [ 111 H. Gerischer, Z. Phys. Chem. 26 (1960) 233; 27 (1961) 40. [ 121 H. Gerischer, Surface Sci. 18 (1969) 97.
Surface Science 109 (1981) 75-81 North-Holland Publishing Company
THE INFLUENCE OF SURFACE OXIDE FILMS ON THE STABILIZATION
OF
n-Si PHOTOELECTRODE B.H. LOO, K.W. FRESE, Jr. and S. Roy MORRISON Materials Research Laboratory,
SRI International,
Menlo Park, California 940.25, USA
Received 13 January 1981
We have provided direct evidence of the enhanced effectiveness of stabilizing agents due to thin surface oxide films, ca. 15-25 A, on n-Si photoelectrode. Rotating ring disc electrode and ellipsometric experiments are combined to show the stabilization efficiency of potassium ferrocyanide improves with oxide thickness. A band model describing the observed effect is given.
1. Introduction In an earlier study [ 1J, using voltammetry, we have investigated the effectiveness of various reducing or stabilizing agents in the corrosion suppression of n-Si photoelectrode. In the absence of fluoride ions or high concentration of hydroxide ions in solution, corrosion leads to the formation of an SiOZ film on the surface of the silicon semiconductor electrode. The voltammetry measurements indicated the oxide film could substanti~y increase corrosion resistance. It was suggested that the oxide film could aupport a field which raises the energy level of the stabilizing agent with respect to the valence band edge of the Si electrode to a position favorable for hole capture from the valence band. Holes that come to the surface of the Si photoelectrode can oxidize the stab~~g agent and/or the silicon electrode. The effectiveness of the stabilizing agent in hole capture reflects the degree of corrosion suppression of the silicon electrode by the stabilizing agent. Thus shifting the energy level of the stabilizing agent to a position more favorable for hole capture means that the corrosion is suppressed. Potassium ferrocyanide and N, N, N’, N’-tetramethyl-p-phenylene-diamine (TMPD) were shown to be quite effective in the hole capture [l-3]. It was suggested that after a thin oxide fti was grown on the electrode surface, these ions suppressed further corrosion, so the film grew very slowly. In this paper, we report rotating ring disc electrode (RRDE) studies on n_Si photoelectrode using potassium ferrocyanide and TMPD, and provide direct evidence on the enhanced stabilization of r&i photoelectrode by thin surface oxide films. The thickness of the surface oxide fti that gives the maximum stab~ation 0039 -6028/8 1/OOOO-0000/$02.50 0 North-Holland
16
B.H. Loo et al. / Surface oxide films and stabilization of n-Si photoelectrode
efficiency is estimated by ellipsometric given to explain this observed effect.
measurements.
Finally,
a band model is
2. Experimental The RRDE apparatus was similar to that described before [4]. Single crystal n-type Si(l1 l), obtained from Silicon Materials (Mountain View, CA), was used as the disc electrode, of area 0.2 cm2. Ohmic contacts were made using an In-Ga alloy and silver epoxy. The oxidation products of the stabilizing agents K,Fe(CN), and TMPD were detected on the gold ring. The collection efficiency of the RRDE assembly was determined to be 0.22. A platinum electrode was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. All potentials are reported with respect to SCE. The potentials of the ring and disc electrodes were controlled independently by means of a Pine Instrument RDE 3 bipotentiostat (Grove City, PA). The silicon disc electrode was etched each time prior to RRDE measurements with 20% aqueous HF solution and followed by an absolute methanol rinse. A 150 W xenon lamp was used to illuminate the Si disc electrode. The absolute photocurrents at the ring and disc electrodes were determined with chopped light at a frequency of 1 Hz. The ellipsometric measurements were made with a Gaertner L119 ellipsometer (Chicago, IL) at an incidence angle of 70” and a wavelength of 546.1 mn. The thickness of the surface oxide film was calculated using n(Si) = 4.050 - 0.028i [5-71, and the film refractive index of 1.50 [S] .
3. Results As we mentioned earlier, photogenerated holes that come to the surface of the Si photoelectrode can oxidize the stabilizing agent and/or the silicon electrode. The stabilizing agent normally captures less than 100% of the holes, and the remainder oxidize silicon to silicon dioxide. As the Si02 increases in thickness, the photocurrent at constant light intensity gradually decreases over time. In the RRDE measurements, one obtains two measures of the effectiveness of a stabilizing agent: first, the quantity of oxidized stabilizing agent as monitored on the ring electrode; second, the decrease in photocurrent. The stabilizing efficiency of a stabilizing agent is defined as s = (IJNIn)
x lOO%,
(1)
where In is the ring photocurrent, In is the disc photocurrent and N is the collection efficiency of the ring disc electrode assembly. Obvious, S is 100% when the holes are entirely captured by the stabilizing agent. Fig. 1 shows a typical RRDE study of the decay of the photocurrents at the ring
B.H. Loo et al. /Surface 150
oxide films and stabilizationof n-S photoelectrode
I
I
I
I
5
10
77
\
a ,z !z 2 IX z
100
50
43 O_
0
TIME OF OXIDATION
1 15 (mid
Fig. 1. Decay of photocurrents at the ring (ZR) and disc (ID) electrodes as a function of the time of oxidation. Electrolyte: O.lM K4Fe(CN)6 + 0.5M KCI; disk voltage: 1.0 V; ring voltage: -0.5 V (versus SCE); pH = 8; rotation speed: 1000 rpm. Note the fast decay ofzD near t = 0, indicating most of the photogenerated holes are consumed initially in the formation of the surface oxide film.
and the disc electrodes as a function of the time of oxidation or the oxide thickness. The stabilizing agent was potassium ferrocyanide (O.lM and pH = S), and its stabilization efficiency was less than 100%. The decrease in 1, is attributed to the resulting growth of surface oxide fii. The ring photocurrent, monitoring the oxidized product of the stabilizing agent, decreases less rapidly, on a percent basis, than the disc photocurrent, especially in the first few minutes of oxidation. Thus S in eq. (1) increases. Fig. 2 shows the stabilization efficiency as a function of time of oxidation in aqueous solution of O.lM K.,Fe(CN), at pH = 3.5,5 and 8, and saturated TMPD at pH = 8. An interesting feature shown in fig. 2 is the direct observation of the initial rise in the stabilization as predicted in our earlier voltammetry work [l] . In the case of ferrocyanide, the stabilization increases as the pH is lowered. At pH = 3.5, the stabilization efficiency is comparable to that of TMRD, which is known to be a better stabilizing agent for n-Si photoelectrode [2,3]. We also performed RRDE measurements at the same photocurrent level with a lower concentration of stabilizing agent, 0.05M I(4Fe(CN)a, in 0.5M KC1 aqueous solution. The result shows S decays faster after the initial rise. The faster decay in stabilization was due to the rapid growth of a thick surface oxide. A larger propor-
78
B.H. Loo et al. / Surface oxide films and stabilizationof n-S photoelectrode
6 A l 0.1 M K4 Fe&N),
+ 0.5 M KC1
o TMPD + 0.5 M KCI (pH = 8) 0’
0
I
I
I
5
10
15
TIME OF OXIDATION
(mid
Fig. 2. Stabiiation of n-silicon as a function of time of oxidation (oxide thickness) in aqueous solutions of O.lM K4Fe(CN)6 (pH as indicated) and saturated TMPD (pH = 8). Note the initial rise in the stabilization in each case.
tion of the photogenerated holes were consumed in the oxidation of the Si electrode with the concentration of stabilizing agent lowered. To permit direct interpretation of these measurements, we have performed ellipsometric measurements of the oxide thickness as a function of time of oxidation. Ellipsometric measurements of the electrode surface were made after the electrode was oxidized under identical conditions, removed from the solution, rinsed several times with absolute alcohol and blown dry, Fig. 3 shows the resulting curve of the stab~ation efficiency with ferrocyanide as a function of the oxide thickness. Five different oxide thicknesses were determined and the best curve was drawn through
5 --I 50 F I8 3 m 2 v) 0-A 0
10
20 OXIDE
30
40
THICKNESS,
50
60
70
(A)
Fig. 3. Effect of oxide thickness on stab~ation
of n-silicon by ferrocymice
(0.1 mold.
B.H. Loo et al. /Surface
oxide films and stabilization of n-Si photoelectrode
19
these points. Note that the error bars do not represent the experimental errors but rather they represent our estimated experimental uncertainties involved with the ex situ measurements. The data points are averages of several thickness measurements at different spots of the oxidized surface. The measurements agreed with each other within 10% indicating the surface oxide is quite homogeneous. We found that after the HF rinses, the oxide films are lo-15 A thick, this is consistent with the observation made by Archer [5]. In the presence of 1% HF, the stabilization efficiency was very low, about 10%. No ellipsometric measurements were made in this case, we assume the oxide thickness is about O-10 A. Fig. 3 clearly shows maximum stability is achieved after the surface oxide grows to a certain thickness. This is fairly convincing evidence that a thin oxide present on the surface of a semiconductor can greatly enhance the efficiency of a stabilizing agent.
4. Discussion The band model for the observed effect is illustrated in fig. 4. Fig. 4a shows the band model of Si/SiOJelectrolyte in the dark, when no charges are present on the interfacial states at the Si/SiOZ interface. Under such circumstance, no appreciable voltage would develop across the oxide layer, and the relative positions of the valence band and the energy levels in solution are the same as they were in the case where there was no surface oxide present. The valence band edge of silicon at pH = 3 is 0.35 eV below the Fermi energy of the calomel electrode [9]. The redox potential of ferrocyanide indicates that the characteristic energy level EFedox of ferrocyanide is 0.12 eV below the Fermi energy of the calomel electrode [IO]. Thus, the characteristic energy level of ferrocyanide is close to the valence band edge of silicon. The density of the energy levels of the ions in solution as a function of energy is maximum at an energy A less that cedox, namely at Ered, as shown. h is the reorganization energy and, for ferrocyanide, it is assumed to be 0.7 eV. The density of energy levels forms a Gaussian distribution centered at Ered with the standard deviation of (2MT)‘/2 [ 11,121. To optimize the effectiveness of the stabilizing agent, it is necessary to have a high density of states at or above the valence band edge. For optimal hole capture, it is therefore desirable to decrease the energy of the valence band of silicon to a point closer to f&j. This can be accomplished if there is an oxide present on the electrode surface. Under illumination, the silicon electrode produces holes that can be trapped at the Si/SiOZ interface, and as a result, the interface states become positively charged, and a large double layer appears, as indicated in fig. 4b. This lowers the valence band, and E,, can then approach the energy Er,+ This illustration shows why a thin surface oxide enhances the stabilization of n-Si photoelectrode. As Em is lowered toward the energy Ered, S increases (at constant oxide thickness). S reaches a maximum when Ew = Ered as indicated in fig. 3. With increasing oxide thickness, hole capture will increase because Em approaches Ered, but will
80
B.H.Loo et al. /Surface
oxide films and stabilization of n-S photoelectrode
Si
I
SiO,
I
Electrolyte
Positive
Pin $--t Eg = 1.1 eV
Calomel Zero
States
Approx. 2000A
Fig. 4. Band model
Negative Ions Solution
158
of Si/SiOz/electrolyte
to show field in oxide when interface
states become
charged. decrease because the holes must tunnel through a or just before E, reaches Ered, a maximum in S is thickness increases beyond this point, this simple dict whether S will maintain its high value or through the oxide has to be taken into account.
thicker oxide. When E,, = Erd expected. However, as the oxide theoretical model does not predecrease. Tunneling of charges
B.H. Loo et al. / Surface oxide jilms and stabilization of n-Si photoelectrode
More quantitatively,
the rate of hole capture by a stabilizing
81
agent is given by
[41 k3 = B exp(-yXO)
exp {- [E,-=%,d
- 4v,,]
*/4MT)
2
(2)
with B, 7 and h constants, where the first exponential factor represents a tunneling probability across an oxide thickness X0, the second represents iso-energetic hole transfer as developed in the fluctuating energy level model of electrode reactions [ 11,121. To determine S we must compare such a process with the oxidation of the silicon as a function of Xc. Because the tunneling probability decreases exponentially with X0, whereas the oxidation rate by field-aided diffusion is more likely to vary inversely with X,,, one might expect S to decreases as the oxide becomes considerably thicker, which was indeed observed as in fig. 3. The presence of a thin oxide on a photoelectrode decreases the total current that can be passed and thus could adversely affect its solar cell performance. A compromise may therefore be necessary in certain cases. In conclusion, we have shown quantitatively that the presence of a thin surface oxide may substantially improve the stabilization of solar cells. This result adds to our knowledge and our models of corrosion-stabilization, and alerts us to the possibility that a high gross stabilization efficiency may unfortunately indicate that a film is present on the surface that may eventually detract from the conversion efficiency of solar cells.
Acknowledgement This work was supported by the Solar Energy Research Institute under Contract Xp-g-8002-6. Technical assistance of Dr. Kenneth Sancier in the ellipsometric measurements is gratefully acknowledged.
References [l] M.J. Madou, K.W. Frese, Jr and S.R. Morrison, J. Phys. Chem. 84 (1980) 3423. [2] D. Laser and A.J. Bard, J. Phys. Chem. 80 (1976) 463. [3] A.B. Bocarsly, E.G. Walton, M.G. Bradley and MS. Wrighton, J. Electroanl. Chem. 100 (1979) 283. [4] K.W. Frese, Jr., M.J. Madou and S.R. Morrison, J. Phys. Chem. 84 (1980) 3172. [5] R.J. Archer, J. Opt. Sot. Am. 52 (1962) 970. [6] K. Vedam and W. Knausenbaerger, J. Opt. Sot. An. 59 (1969) 64. [7] S.S. So and K. Vedam, J. Opt. Sot. Am. 62 (1972) 596. [8] A.C. Adams, T.E. Smith and C.C. Chang, J. Electrochem. Sot. 127 (1980) 1787. [9] M.J. Madou, B.H. Loo, K.W. Frese, Jr. and S.R. Morrison, Surface Sci. 108 (1981) 135. [lo] W.M. Latimer, Oxidation Potentials (Prentice-Hall, New York, 1953). [ 111 H. Gerischer, Z. Phys. Chem. 26 (1960) 233; 27 (1961) 40. [ 121 H. Gerischer, Surface Sci. 18 (1969) 97.