5436
J. Phys. Chem. B 2000, 104, 5436-5447
Photoelectrochemical Behavior of n-GaAs and n-AlxGa1-xAs in CH3CN Louis G. Casagrande, Agnes Juang, and Nathan S. Lewis* DiVision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 ReceiVed: NoVember 18, 1999; In Final Form: February 24, 2000
Current density vs potential, open-circuit voltage vs temperature, and differential capacitance vs potential measurements have been used to show that n-GaAs and n-AlxGa1-xAs electrodes exhibit partial Fermi level pinning in contact with CH3CN over a wide range of redox potentials. Despite a change of over 1.2 V in redox potential of the solution, the open-circuit voltage only changed by ∼300 mV. The slope of the opencircuit voltage vs redox potential of the solution was typically 0.33-0.44. Differential capacitance vs potential data also yielded a barrier height change of less than 300 mV for over 1.2 V change in the redox potential of the solution. The dependence of the current density vs potential behavior of n-GaAs/CH3CN-ferriceniumferrocene+/0 on variables such as the illumination intensity, dopant density of the semiconductor, concentration of redox acceptor in the solution, crystal face, electrolyte, and cell temperature was evaluated. The resultant kinetic data indicate that surface-state recombination is the dominant recombination mechanism at these interfaces, which are capable of producing an open-circuit voltage of 0.83 V at a short-circuit current density of 20 mA cm-2, as well as energy conversion efficiencies of > 10%. X-ray photoelectron spectroscopy investigation of n-GaAs confirmed surface changes were induced by electrochemical operation of n-GaAs electrodes in CH3CN-cobaltocenium-cobaltocene+/0 electrolyte. The presence of Fermi level pinning and the existence of changes in n-GaAs and n-AlxGa1-xAs electrode surfaces when these electrodes are in contact with CH3CN-cobaltocenium-cobaltocene+/0 electrolyte complicates the extraction of ket values from the steadystate current density vs potential behavior of n-GaAs or n-AlxGa1-xAs/CH3CN contacts.
I. Introduction The photoelectrochemical behavior of GaAs/liquid contacts has attracted much attention recently in attempts to compare theoretical predictions with the experimental behavior of semiconductor electrodes.1-16 Steady-state current density (J) vs potential (E) data obtained on n-GaAs/CH3CN-cobaltocenium-cobaltocene (CoCp2)+/0 contacts in the limit where the observed rate does not exhibit the expected second-order rate law (first order in the surface electron concentration and first order in the redox acceptor concentration) have recently been interpreted to indicate that ket values at semiconductor/liquid contacts can be as high as 10-10 cm4 s-1.6 Subsequent reports on n-GaAs/CH3CN contacts have relied upon solvent-suspended quartz crystal microbalance measurements to indicate that both cobaltocene and cobaltocenium adsorb onto the GaAs surface.7 Assumptions about the effective electron-transfer distance to the adsorbed species, the distance dependence of the orbital coupling overlap, and the surface coverage of adsorbed electroactive species in this system have recently yielded a 10 000fold reduction in the inferred ket value to ket ≈10-14 cm4 s-1.6,7 It is therefore of great interest to examine in detail the J-E properties of the n-GaAs/CH3CN interface. The behavior of n-GaAs in contact with CH3CN has been studied previously by a number of workers. Wrighton and Bard used cyclic voltammetry techniques to conclude that Fermi level pinning was present at n-GaAs/CH3CN contacts over a wide range of redox potentials.17 Nagasubramanian, Wheeler, and Bard subsequently reached identical conclusions based on impedance measurements of n-GaAs/CH3CN contacts.18 Other studies indicating nonideal junction behavior of n-GaAs electrodes have been performed in propylene carbonate, tetrahydrofuran, dimethylformamide,
and aluminum chloride-butylpyridinium chloride molten salt electrolyte systems.19-27 Reduction of the influence of surface states has, in certain cases, resulted in efficient GaAs-based photoelectrochemical cells, specifically for n-GaAs/CH3CNferricenium-ferrocene(Fc)+/0 contacts.28,29 In this work, we report additional details of investigations into the photoelectrochemical J-E behavior of n-GaAs/CH3CN-Fc+/0 contacts.28-30 The focus of this work is to evaluate the dependence of the J-E data of this contact on variables such as the illumination intensity, dopant density of the semiconductor, concentration of redox acceptor in the solution, crystal face, electrolyte, and cell temperature to elucidate the recombination mechanisms that control the interfacial chargetransfer flux at the n-GaAs/CH3CN-Fc+/0 contact. Because the Fc+/0 couple is apparently a well-behaved outer-sphere redox system, information on ket in this system can be related through the use of electron-transfer theories to the behavior expected for the n-GaAs/CH3CN-CoCp2+/0 contact.4,6,13,31-33 In addition, we report the J-E behavior of n-GaAs/CH3CN contacts with a series of metallocenes for which the redox potential, E(A/A-), has been varied over a wide (> 1 V) range of potentials. These data allow evaluation of the degree of Fermi level pinning exhibited by n-GaAs electrodes in an CH3CN-based electrolyte. These data are valuable in assessing the assumption required in the assignment of ket ) 10-10 to 10-14 cm4 s-1 values6,7 that the band edges do not move in response to variation in the redox potential of the solution. We also report J-E data on the photoelectrochemical behavior of n-AlxGa1-xAs/CH3CN contacts, to evaluate the generality of our observations on the n-GaAs/CH3CN system with respect to closely related III-V ternary alloy semiconducting electrodes. Finally, we report X-ray
10.1021/jp9941155 CCC: $19.00 © 2000 American Chemical Society Published on Web 05/19/2000
n-GaAs and n-AlxGa1-xAs in CH3CN photoelectron spectroscopic (XPS), J-E, and cyclic voltammetric studies designed to probe for the presence of irreversible chemical and/or electrochemical changes on n-GaAs electrodes immersed in CH3CN-CoCp2+/0 solutions. II. Experimental Section A. Electrodes and Materials. Most samples were grown by organometallic vapor-phase epitaxy on n+-GaAs substrates oriented 2° off (100) and doped with Se to produce a donor density, Nd, > 5 × 1018 cm-3. The photoactive epilayers were 10-15 µm thick, Nd ) (1-2) × 1017 cm-3 Se-doped GaAs or 5 µm thick, Nd ) 1 × 1017 cm-3 Se-doped n-AlxGa1-xAs (x ) 0.09, 0.16, 0.24, or 0.31). The composition of the alloys was confirmed by electron microprobe measurements, and the band gap energy for each sample was verified by either photoluminescence or spectral response measurements (vide infra). Samples used for XPS studies and barrier height measurements were (100)-oriented n-GaAs doped with Si to Nd ) (0.9-1.9) × 1017 cm-3 (Laser Diode Co.). Ohmic contacts were prepared by either electron beam deposition of a 1000-Å layer of In followed by annealing in forming gas for 15 min, or by thermal evaporation of a 1000-Å layer of 2% Ge-98% Au alloy or of 12% Ge-88% Au followed by annealing in forming gas at 475 °C for 5 min. Electrodes were fabricated as described previously,30 with typical exposed electrode areas of 0.05-0.15 cm2 for electrical measurements and 0.15-0.25 cm2 for barrier height measurements. All areas were determined either from enlargements of photographs of the electrodes or by digitizing magnified scanned images of the electrodes. The electrode surfaces were etched in a solution of 0.05% (v/v) Br2 in CH3OH followed by immersion in 1.0 M KOH(aq). The KOH solution was replaced with a 4.0 M solution of NH3 in CH3OH for the experiments carried out in a N2-purged glovebox. The etching protocol was identical to that described in prior reports from this laboratory.34,35 n-AlxGa1-xAs samples with x ) 0.24 and 0.31 were pretreated with 1:1 HF(49%): H2O2(30%) immediately before the standard etching procedure in order to improve their photocurrent response. The solvents, CH3OH, C2H5CN, and CH3CN, were obtained from EM Science. The CH3OH was distilled over magnesium methoxide. The C2H5CN was distilled over P2O5 and stored over activated 3 Å sieves. The CH3CN was predried over CaH2, then dried over P2O5 and stored over activated 3 Å sieves. All solvents were distilled, collected, and stored under N2(g). Ferrocene (Fc, Aldrich), 1,1′-dimethylferrocene (Fe(C5H4CH3)2, Aldrich), bis(pentamethylcyclopentadienyl)iron (Fe[C5(CH3)5]2, Strem), and cobaltocene (CoCp2, Strem or Aldrich) were sublimed in vacuo. Ferrocenium hexafluorophosphate (FcPF6) was prepared by chemical oxidation of recrystallized Fc with benzoquinone and HPF6 and dried in vacuo at room temperature. [Fe(C5H4CH3)2][BF4] was oxidized from Fe(C5H4CH3)2 in CH3OH with benzoquinone and HBF4 and dried in vacuo at room temperature. CoCp2PF6 (Strem or Aldrich) was recrystallized from CH3CN and dried in vacuo, or prepared via the method of Sheats and Rausch36 and dried in vacuo at room temperature. Mono- and di-substituted derivatives of CoCp2PF6 ([CoCp(C5H4CO2CH3)][PF6] and [Co(C5H4CO2CH3)2][PF6]) were prepared and purified as described by Sheats and Rausch,36 characterized by IR and NMR spectroscopy, and dried in vacuo at room temperature. The reduced forms of the CoCp2PF6 derivatives were prepared by in-situ electrolysis (vide infra). Lithium perchlorate (LiClO4, Aldrich) was dried by fusing at 240 °C under vacuum. Tetraethylammonium tetrafluoroborate ([(C2H5)4N][BF4], Aldrich), TEABF4, was dried at ∼160 °C in vacuo for 24 h.
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5437 B. Electrical Measurements. A standard three-electrode potentiostatic setup was used for all photoelectrochemical measurements. The reference electrode for J-E experiments was a Pt wire poised at the Nernstian solution potential, and the counter electrode was either a Pt foil or a Pt gauze of area at least 10 times larger than that of the working electrode. All solutions were magnetically stirred, except during the cyclic voltammetry experiment. All current density vs potential experiments were performed using an EG&G Princeton Applied Research (PAR) Model 173 potentiostat/galvanostat, which was interfaced with either a PAR Model 176 current follower or a PAR Model 179 digital coulometer. The potential was controlled by a PAR Model 175 universal programmer. Traces were recorded on a Houston Instruments Omnigraphic 2000 recorder at a scan rate of 50 mV s-1. Light intensities were controlled by the use of a 300 W ELH-type tungsten-halogen bulb in conjunction with neutral density filters. Open-circuit voltages were measured with the sample held at open-circuit, and short-circuit currents were measured with the sample held at 0 V vs the cell potential. All voltages and currents were monitored using a Beckman Tech 310 digital multimeter, a Fluke 27 multimeter, a Keithley 177 microvolt multimeter, or a Keithley 130A digital multimeter. Spectral response data were obtained using a 100 W tungsten-halogen source, powered by a Hewlett-Packard 6024A dc power supply. The beam was passed through a SPEX 1670 Minimate monochromator equipped with a 5000 Å grating, and modulated at 13 Hz using a Frequency Control Products taut band chopper. The signal was filtered using a 330 nm long pass or a 550 nm long pass filter to remove higher order frequencies, and the beam was then split off by a Pyrex slide to a Si cell reference diode that followed fluctuations in light intensity. The optical arrangement was calibrated using another diode of known spectral response that was placed in the light beam precisely where the sample would be. Both detectors were United Detector Technology (UDT) model 260 Si diodes, sensitive to wavelengths ranging from 360 to 1100 nm. The reference detector current was processed through a custom-built current-voltage converter and lock-in amplifier. The sample current was processed by one of the potentiostat arrangements detailed above or by a Bioanalytical Systems, Inc. model CV-1B cyclic voltammograph, in conjunction with a PAR model 5204 lockin amplifier or a PAR model 124A lock-in amplifier equipped with a PAR model 116 differential preamplifier. Both signals were monitored by an IBM PC via a Data Translation DT 2801/ 5716 or DT2801-A A/D converter, and the monochromator and the external input to the potentiostat were controlled from the computer. Photocurrent densities during spectrum acquisition were on the order of 1 nA cm-2. No correction was made for any optical absorption or transmission losses in the cell, so the reported values are true external quantum yields for the system under study. The open-circuit potential, Voc, as a function of temperature was measured by submersing a specially designed photoelectrochemical cell into a strip-silvered dewar containing an ethanol bath through which a copper coil was passed. A narrow strip on the side of the dewar allowed visible light through, and the illumination was attenuated with neutral density filters to obtain the desired light intensity. A current-potential curve was taken at room temperature at each light intensity in order to determine the photocurrent density. Solutions were composed of low concentrations of supporting electrolyte and redox species so that lower temperatures could be achieved without precipitation of the solutes. The temperature of the cell was lowered by
5438 J. Phys. Chem. B, Vol. 104, No. 23, 2000 passing liquid N2 through the copper coil. The cell temperature was measured using an Omega type T, 1/32" ungrounded junction copper-constant thermocouple equipped with an Omega cold junction compensator and an Omega thermocouple amplifier. The cooling rate was 10-30 °C min-1. The open-circuit voltage was measured directly against the cell potential, and both Voc and the temperature were continuously monitored by an IBM PC equipped with a Data Translation DT2801-A A/D converter. C. X-ray Photoelectron Spectroscopy (XPS) Investigation of n-GaAs Surfaces. The chemistry of n-GaAs after exposure to CH3CN-CoCp2+/0 solution was investigated with X-ray photoelectron spectroscopy (XPS). Immediately before each experiment all samples were etched in a N2-purged glovebox using the procedure described above. Four samples without back ohmic contacts were immersed in the following four CH3CN solutions for 10 min. Solution 1 contained 0.7 M LiClO4, solution 2 contained 0.7 M LiClO4 and 10 mM CoCp2, solution 3 contained 0.7 M LiClO4 and 10 mM CoCp2PF6, and solution 4 contained 0.7 M LiClO4 and 10 mM each of CoCp2 and CoCp2PF6. Another n-GaAs electrode was immersed in an electrochemical cell with solution 4, and a potential sweep at 50 mV s-1 between -0.5 and 0.3 V vs E(A/A-) was applied for 10 min. All samples were rinsed with CH3CN and transferred under N2 into a flush box connected to the XPS via a highvacuum load lock. A Varian V-200 turbo pump was used to evacuate the load lock to approximately 10-7 Torr prior to introduction of the samples into the ultrahigh-vacuum (UHV) system. The XPS experiments were conducted in an M-probe surface spectrometer (Surface Science Instrument (Fisons)) pumped by a CTICryogenics 8 cryo pump. The instrument was maintained at a base pressure of < 5 × 10-10 Torr, although the operating pressure was 5 × 10-9 to 2 × 10-8 Torr. Monochromatic Al KR X-rays (hν ) 1486.6 eV) incident at 55° from surface normal were used to excite electrons from the sample, while the emitted electrons were collected by a hemispherical analyzer at a takeoff angle of 35° from the plane of the sample surface. Data collection and analysis were done with the M-probe package software version 3.4. D. Barrier Height Measurements. The barrier height of the n-GaAs/CH3CN-CoCp2+/0 junction was calculated from electrochemical impedance data. Impedance measurements were carried out in a N2-purged glovebox. The cell solution consisted of 0.7 M LiClO4, 10 mM CoCp2, and 10 mM CoCp2PF6. All electrodes were etched immediately before each experiment. Impedance measurements were performed with a Solartron 1260 impedance gain-phase analyzer equipped with a Solartron 1286 electrochemical interface. A 10 mV ac signal with a frequency sweep from 100 Hz to 100 kHz was superimposed upon a dc bias stepped in 50 mV intervals between 0.4 and 1.0 V vs E(A/ A-). The method used for extraction of barrier heights was the same as that described in a prior report from this laboratory.37 III. Results A. Current Density vs Potential Characteristics of n-GaAs/ CH3CN-Fc+/0 Junctions. 1. J-E Properties Under Illumination. Figure 1 depicts the J-E behavior of a mirror-finished n-GaAs electrode at a scan rate of 50 mV s-1 in contact with the CH3CN-0.7 M LiClO4-0.5 mM Fc+-0.09 M Fc electrolyte. Under 88 mW cm-2 of ELH-type tungsten-halogen illumination, this interface produced an open-circuit voltage, Voc, of 822
Casagrande et al.
Figure 1. Current density-potential (J-E) behavior for the n-GaAs/ CH3CN-LiClO4-Fc+/0 system at Γ0 ) 88 mW cm-2 ELH illumination. The reference electrode was a Pt wire in a Luggin-Haber capillary. E(A/A-) was +0.184 V vs SCE. Voc ) 827 mV, Jph ) 19.8 mA cm-2, and η ) 11%.
TABLE 1: Kinetic Parametersa and Open-Circuit Voltages of n-GaAs/CH3CN-Ferrocene+/0 Contacts Voc, V
Jph, mA cm-2
0.693 1.206 d d 0.699 0.084 averages
γb
log(J0)c
Voc,calc, V (Jph ) 1 mA cm-2)
1.61 ( 0.05 1.42 ( 0.10 1.50 ( 0.04 1.51 ( 0.10
-10.2 ( 0.2 -12.0 ( 0.8 -11.0 ( 0.2 -11.2 ( 1.0
0.69 ( 0.03 0.70 ( 0.07 0.71 ( 0.03 0.70 ( 0.01
a Kinetic parameters collected from photoelectrochemical cells with average E(A/A-) ) 0.184 ( 0.011 V vs SCE. b Diode quality factor; see text for discussion. c J0 ) reverse saturation current density (A cm-2). d Voc not measured near Jph ) 1 mA cm-2.
( 4 mV and a photocurrent density, Jph, of 20.2 ( 0.8 mA cm-2. The energy conversion efficiency, η, of this cell was 10.9 ( 0.7%, with no correction for residual uncompensated resistance losses, concentration overpotentials, or optical reflection losses at the various interfaces in the device. The value of the diode quality factor, γ, can be obtained using the equation38
Voc ) (γkT/q) ln(Jph/J0)
(1)
where J0 is the reverse saturation current density of the solid/ liquid contact. Thus, measuring Jph and Voc at a variety of illumination levels of the semiconductor/liquid contact allows extraction of both γ and Jo. A value of γ ) 1.51 ( 0.10 was typically observed for the n-GaAs/CH3CN-Fc+/0 system (Table 1). The Voc of 827 mV at 88 mW cm-2 of illumination is the highest value, to our knowledge, for any n-GaAs electrode under AM1.5 conditions in nonaqueous solvents reported to date. Similarly, the uncorrected 11% energy conversion efficiency is also the highest value reported, to our knowledge, for any stable n-GaAs/nonaqueous liquid contact reported to date under AM1.5 illumination conditions.38-41 2. Spectral Response Data. The spectral response data for the n-GaAs/CH3CN-Fc+/0 contact are displayed in Figure 2. The data displayed a limiting plateau value of the external quantum yield, Φext, of ∼0.80 over most of the visible region of the spectrum. The optical reflection losses at these mirrorfinished GaAs electrode surfaces account for ∼20% of the incident photons at each wavelength. These spectral response data therefore indicate that the internal quantum yield for charge separation and flow through the external circuit, based on photons actually absorbed by the semiconductor, were very close to unity throughout the visible region of the spectrum.
n-GaAs and n-AlxGa1-xAs in CH3CN
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5439 TABLE 3: Electrolyte, Crystal Face, and Acceptor Concentration Dependence of the Current Density-Voltage Properties of n-GaAs/CH3CN-Ferrocene+/0 Contacts n-GaAs(100)/CH3CN-Electrolyte-90 mM Fc-0.5 mM Fc+ electrolyte TEABF4 LiClO4
Jph, mA cm-2
Voc,a V
20.2 20.2
0.856 ( 0.010 0.822 ( 0.004
n-GaAs/CH3CN-0.7 M LiClO4-90 mM Fc-0.5 mM Fc+ crystal face
Jph, mA cm-2
Voc, V
(110) (100)
4 4
0.613 ( 0.010 0.793 ( 0.010
n-GaAs(100)/CH3CN-0.7 M LiClO4-Ferrocene+/0 +
Figure 2. Quantum yield vs wavelength of incident light for the n-GaAs/CH3CN-LiClO4-Fc+/0 system.
TABLE 2: Open-Circuit Voltage vs Temperature Parameters of n-GaAs/CH3CH2CN-Ferrocene+/0 Contacts Jph, mA cm-2
Eact,a eV
slope, mV K-1
20 10 5 4.5 2 1 0.5 0.2 0.1
1.42 ( 0.06 1.41 ( 0.08 1.41 ( 0.05 1.39 ( 0.05 1.38 ( 0.04 1.36 ( 0.03 1.37 ( 0.03 1.33 ( 0.05 1.30 ( 0.02
-2.10 ( 0.18 -2.2 ( 0.3 -2.25 ( 0.02 -2.24 ( 0.02 -2.31 ( 0.16 -2.33 ( 0.1 -2.46 ( 0.13 -2.5 ( 0.2 -2.60 ( 0.04
a Activation energy obtained from extrapolated intercept of V vs T oc data to 0 K.
The spectral response data, which were obtained at relatively low ( 1.2 V, were used in this study. These redox couples produced a variation in the Nernstian potentials of the electrolyte solutions from -1.2 V to +0.2 V vs SCE. In each case, the J-E curves exhibited
5440 J. Phys. Chem. B, Vol. 104, No. 23, 2000
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TABLE 4: Kinetic Parameters for Different Redox Couples of n-GaAs/CH3CN Interfaces Jph mA cm-2
+0.079
0.689
1.009
1.47 ( 0.04
Fe[C5(CH3)5]2
-0.210
0.632
1.190
1.557 ( 0.016
-9.85 ( 0.07
0.626 ( 0.009
[Co(C5H4CO2CH3)2]+/0
-0.527 -0.524 -0.493 -0.548 -0.52 ( 0.02
0.464 0.509 0.51 0.478
0.803 0.902 0.71 0.947
1.26 1.42 1.48 1.23 1.35 ( 0.12
-9.37 -9.14 -9.0 -9.63 -9.3 ( 0.3
0.471 0.513 0.526 0.479 0.479 ( 0.005
-0.813 -0.799 -0.784 -0.799 ( 0.015
0.416 0.424 c
0.840 0.983 c
1.34 1.49 1.65 1.49 ( 0.16
-8.37 -7.93 -7.02 -7.7 ( 0.7
0.422 0.432 0.390 0.415 ( 0.002
-1.075 -1.070 -1.109 -1.09 ( 0.02
0.448 0.545 0.484
1.243 0.800 1.303
1.20 1.36 1.43 1.33 ( 0.12
-9.21 -9.90 -8.61 -9.2 ( 0.6
0.439 0.553 0.475 0.49 ( 0.06
-1.210 -1.094 -1.238 -1.211 -1.19 ( 0.06
0.496 0.486 0.378 0.397
0.988 1.051 1.040 1.150
1.25 1.33 1.52 1.50 1.40 ( 0.13
-9.76 -9.22 -7.23 -7.45 -8.4 ( 1.3
0.495 0.485 0.377 0.392 0.44 ( 0.06
[Fe(C5H4CH3)2]+/0 +/0
averages [CoCp(C5H4CO2CH3)]+/0 averages CoCp2+/0 averages [Co(C5H4CH3)2]+/0
averages
Ecell V vs SCE
Voc,calc, V (Jph ) 1 mA cm-2)
Voc, V
redox couple
γa
log(J0)b -11.0 ( 0.2
0.69 ( 0.03
a Diode quality factor; see text for discussion. b J ) reverse saturation current density (A cm-2). c V not measured near J ) 1 mA cm-2. All 0 oc ph Voc values are generally to within (5 mV; Jph values are measured with 4-digit multimeters.
Figure 3. A plot of the open-circuit photovoltage (calculated) vs equilibrium cell potential for the n-GaAs/CH3CN-LiClO4 system at Jph ) 1.0 mA cm-2. The redox couples were: (A) [Co(C5H4CH3)2]+/0, (B) CoCp2+/0, (C) [CoCp(C5H4CO2CH3)]+/0, (D) [Co(C5H4CO2CH3)2]+/0, (E) Fe[C5(CH3)5]2+/0, (F) [Fe(C5H4CH3)2]+/0, and (G) Fc+/0.
rectification, with a small, limiting anodic current value in the dark and an exponential increase in current in response to negative, forward bias relative to E(A/A-). Each system also exhibited an anodic photocurrent proportional to the light intensity incident on the semiconductor surface. Table 4 summarizes the electrochemical parameters obtained from the J-E data of such systems, while Figure 3 displays the opencircuit photovoltage, Voc, of each system, recorded at a light intensity sufficient to provide a photocurrent density of 1 mA cm-2 at each n-GaAs/CH3CN contact. The striking feature of these data is that the J-E behavior and the Voc values were relatively similar for each system, regardless of the value of E(A/A-) or the nature of the redox couple. For example, the open-circuit voltage changed only by ∼300 mV, despite a change of over 1.2 V in E(A/A-) between the n-GaAs/CH3CN-Co(C5H4CH3)2+/0 and the n-GaAs/CH3CN-Fc+/0 contacts. In the ideal model of semiconductor photoelectrochemistry, the degree of rectification would decrease for an n-type semiconductor as E(A/A-) became more nega-
tive.32,48,49 Similarly, Voc should be linearly dependent on E(A/ A-), with a slope close to unity, for a homologous series of redox species. Instead, as depicted in Figure 3, a much weaker dependence of either the J-E data or Voc was observed as E(A/ A-) was varied. These data clearly indicate that Fermi level pinning is present in this system over the range of potentials investigated. The presence of partial Fermi level pinning was further confirmed by collecting Voc vs T data for the following redox couples, Fc+/0, CoCp2+/0, and CoCp(C5H4CO2CH3)+/0. As summarized in Table 5, CoCp2+/0 and CoCp(C5H4CO2CH3)+/0, exhibited anomalously large Eact values in Voc vs T plots compared to expectations for systems having such negative redox potentials. These data are thus consistent with the trends in Voc vs E(A/A-) discussed above and underscore the nonideal Voc vs E(A/A-) behavior exhibited by these systems. C. X-ray Photoelectron and Cyclic Voltammetry Data of n-GaAs Surfaces in Contact with the CH3CN-CoCp2+/0 Electrolyte. The data of Figure 3 and Tables 4 and 5 signal the presence of a chemical reaction between the n-GaAs and the CH3CN-CoCp2+/0 solution under the conditions explored in this work. This hypothesis was confirmed by XPS studies of the surface of n-GaAs electrodes before and after they had been exposed to the CH3CN-CoCp2+/0 electrolyte. Figure 4a depicts the XPS survey spectrum for an n-GaAs surface before exposure to a CH3CN-CoCp2+/0 solution. Inspection of the As 3p and Ga 3d region in high resolution mode indicated that the As peak displayed negligible contribution from excess elemental As, as has been observed previously for n-GaAs surfaces exposed to the KOH and Br2-CH3OH sequential etching process.35,50,51 This XP spectrum has been shown to remain constant after immersion or electrochemical operation of n-GaAs electrodes in the CH3CN-Fc+/0 electrolyte.35,50,51 In contrast, Figure 4b displays the XPS data that were recorded after immersion of an n-GaAs electrode into a CH3CN-CoCp2+/0 solution. Peaks for C and O were seen but the Ga and As peaks were reduced in intensity. Also, no peaks for Co were observed, and a silvery-white film was often visible on the electrode surface.
n-GaAs and n-AlxGa1-xAs in CH3CN
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5441
TABLE 5: Open-Circuit Voltage vs Temperature Parameters for Different Redox Couples of n-GaAs/CH3CH2CN Contacts redox couple
Ecell V vs SCE
Jph mA cm-2
[CoCp(C5H4CO2CH3)]+/0
-0.758
∼15 1.0 0.23
average CoCp2+/0
-1.026
∼20 ∼10 0.7 0.13
average a
Eact,a eV
slope, mV K-1
0.987 0.996 0.975 0.986 ( 0.010 1.022 1.005 1.008 0.996 1.008 ( 0.011
-1.53 -1.84 -1.91 -1.38 -1.46 -1.55 -1.69
Activation energy obtained from extrapolated intercept of Voc vs T data to 0 K.
Figure 5. Cyclic voltammogram of n-GaAs in CH3CN-0.7 M LiClO4 in the absence of a redox species. A reduction wave at -1.5 V vs SCE confirmed the presence of irreversible surface changes at negative potentials in CH3CN. The scan rate was 50 mV s-1.
Figure 4. XP spectra of n-GaAs (a) immediately following etching process and (b) after 10 min of J-E scans in a CH3CN solution containing LiClO4 (0.7 M), CoCp2PF6 (10 mM), and CoCp2 (10 mM).
The presence of changes in the GaAs surface chemistry at negative potentials in CH3CN was confirmed by investigating the cyclic voltammetry of n-GaAs in CH3CN-0.7 M LiClO4, in the absence of a redox-active species. As displayed in Figure 5, a reduction wave was observed at -1.5 V vs SCE. Because the operating limit of the solvent/electrolyte window is < -2.5 V vs SCE, this reduction wave must be associated with chemical changes in the surface of the GaAs electrode. This observation is in excellent agreement with prior work by Kohl and Bard, who observed similar behavior for n-GaAs electrodes in contact with CH3CN-0.1 M tetra-n-butylammonium perchlorate (TBAP).46 D. Photoelectrochemical Behavior of n-AlxGa1-xAs Surfaces in Contact with CH3CN-Fc+/0. The generality of the J-E behavior observed for n-GaAs surfaces in contact with CH3CN was explored by investigating the J-E behavior of a series of n-AlxGa1-xAs (0.09 e x e 0.31) photoelectrodes, where the Al composition was increased until the direct-to-indirect band gap transition was reached. These semiconductor samples
Figure 6. Current density-potential (J-E) behavior for the n-AlxGa1-xAs/CH3CN-LiClO4-Fc+/0 series at Γ0 ) 88 mW cm-2 ELH illumination. (s) x ) 0.09, Eg ) 1.51 eV; (- - -) x ) 0.16, Eg ) 1.63 eV; (-----) x ) 0.24, Eg ) 1.72 eV; (- - - -) x ) 0.31, Eg ) 1.82 eV.
were high-quality epilayers grown by organometallic vapor phase deposition on n+-GaAs substrates. 1. J-E Properties Under Illumination. Figure 6 displays the J-E data for the series of n-AlxGa1-xAs electrodes in contact with the CH3CN-Fc+/0 electrolyte. The key electrochemical parameters of these semiconductor/liquid junctions are listed in Table 6. These data are not only of interest in establishing the generality of the J-E behavior of n-GaAs surfaces in the III-V ternary alloy mixture, but also in establishing whether increasing the band gap through variation of alloy composition produces increased efficiencies in these photoelectrochemical cells. The Voc values obtained at 88 mW cm-2 of ELH-type illumination exhibited a monotonic increase as the Al content of the electrode (and thus the band gap) increased. However,
average Ecell V vs SCE +0.189 ( 0.005
Voc, V
Jph mA cm-2
0.759 0.780 0.757 e
averages
n-Al0.09Ga0.91As; Eg ) 1.51 eV 0.765 0.841 0.783 0.863 0.745 0.840 0.755 0.838 0.762 ( 0.016 0.846 ( 0.012
12.84 13.22 16.41 13.36 14.0 ( 1.7
6.6 ( 0.6 6.9 ( 0.4 8.83 ( 0.05 4.3 ( 0.3 6.7 ( 1.9
8.0 ( 0.6 8.6 ( 0.4 11.61 ( 0.12 6.3 ( 0.3 9(2
-14.17 -12.967 -13.6 ( 0.9
n-Al0.16Ga0.84As; Eg ) 1.63 eV 0.783 0.859 0.782 0.867 0.783 ( 0.001 0.863 ( 0.006
12.63 12.05 12.3 ( 0.4
6.8 ( 0.4 6.4 ( 0.4 6.6 ( 0.3
8.6 ( 0.4 9.2 ( 0.5 8.9 ( 0.9
1.5 1.53 1.3 1.44 ( 0.13
-12.7 -12.2 -14.0 -13.0 ( 0.9
n-Al0.24Ga0.76As; Eg ) 1.72 eV 0.839 0.928 0.825 0.919 0.838 0.918 0.834 ( 0.008 0.922 ( 0.006
11.27 11.15 11.70 11.4 ( 0.3
5.7 ( 0.5 6.4 ( 0.5 7.4 ( 0.3 6.5 ( 0.9
8.0 ( 0.5 8.8 ( 0.6 9.2 ( 0.3 8.7 ( 0.6
1.318 1.318 1.55 1.32 1.38 ( 0.12
-15.01 -14.70 -12.7 -14.2 -14.0 ( 1.3
n-Al0.31Ga0.69As; Eg ) 1.82 eV 0.925 0.999 0.891 0.959 0.886 0.971 0.870 0.946 0.890 ( 0.02 0.97 ( 0.02
8.12 7.74 8.95 8.26 8.3 ( 0.5
5.5 ( 0.3 4.5 ( 0.3 4.9 ( 0.3 4.8 ( 0.3 4.9 ( 0.4
6.5 ( 0.3 5.8 ( 0.3 7.4 ( 0.3 5.9 ( 0.3 6.4 ( 0.7
1.186 1.26 1.34 1.244 1.26 ( 0.06
-13.97 -13.60 -12.41 -13.300 -13.3 ( 0.7
0.773 e
0.729 e
1.193 1.331 1.26 ( 010
0.834 0.829 e
0.824 1.194 e
0.191 0.907 0.879 e
0.906 1.587 0.871 e
averages +0.188 ( 0.008
ηcorr,d % (Γ0 ) 88 mW cm-2)
0.790 0.825 1.406 e
averages +0.188 ( 0.009
η,c % (Γ0 ) 88 mW cm-2)
log(J0)b
averages +0.191 ( 0.007
Jph, mA cm-2 (Γ0 ) 88 mW cm-2)
γa
Voc,calc, V (Jph ) 1 mA cm-2)
Voc, V (Γ0 ) 88 mW cm-2)
5442 J. Phys. Chem. B, Vol. 104, No. 23, 2000
TABLE 6: Photoelectrochemical Cell Characteristics of n-AlxGa1-xAs/CH3CN-Ferrocene+/0 Contacts
a Diode quality factor; see text for discussion. b J ) reverse saturation current density (A cm-2). c Percent cell efficiency, calculated as the ratio of the cell output at maximum power over the incident 0 illumination intenisty, Γ0 (mW cm-2). d Percent cell efficiency corrected for solution resistance loss. e Voc not measured near Jph ) 1 mA cm-2.
Casagrande et al.
n-GaAs and n-AlxGa1-xAs in CH3CN
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5443
TABLE 7: Open-Circuit Voltage vs Temperature Parameters of n-AlxGa1-xAs/CH3CN-Ferrocene+/0 Contacts Jph, mA cm-2
Eact,a eV
slope, mV K-1
17 15 10 1.1 0.9
n-Al0.09Ga0.91As; Eg ) 1.51 eV 1.53 ( 0.01 1.47 ( 0.01 1.46 1.52 1.47
-2.49 ( 0.02 -2.23 ( 0.10 -2.33 -2.70 -2.48
14 10 0.9
n-Al0.16Ga0.84As; Eg ) 1.63 eV 1.56 ( 0.03 -2.38 ( 0.05 1.46 -2.33 1.54 -2.58
8 0.5
n-Al0.24Ga0.76As; Eg ) 1.72 eV 1.64 ( 0.02 -2.61 ( 0.05 1.61 -2.82
8 0.5
n-Al0.31Ga0.69As; Eg ) 1.82 eV 1.74 ( 0.01 -2.77 ( 0.03 1.71 -2.96
a Activation energy obtained from extrapolated intercept of Voc vs T data to 0 K. Uncertainties are values (1 standard deviation based on multiple measurements.
Figure 7. Effective activation energies as a function of band gap energy for the n-GaAs/C2H5CN-LiClO4-Fc+/0 system and the n-AlxGa1-xAs/C2H5CN-LiClO4-Fc+/0 series. The semiconductor samples were: (A) n-GaAs, (B) n-Al0.09Ga0.91As, (C) n-Al0.16Ga0.84As, (D) n-Al0.24Ga0.76As, and (E) n-Al0.31Ga0.69As.
as expected, the Jsc value decreased as the band gap increased. Unlike the situation for the n-GaAs1-xPx electrode series described previously,34,52 the increase in Voc for the n-AlxGa1-xAs/CH3CN-Fc+/0 contacts is not sufficient to offset the decrease in Jsc observed under 88 mW cm-2 of ELH-type illumination, so the energy conversion efficiency of a photoelectrochemical cell decreases as the Al content increases. 2. ActiVation Energies for the Dominant Recombination Process. Table 7 lists the activation energies at a variety of illumination intensities for the entire series of n-AlxGa1-xAs/ CH3CN-Fc+/0 contacts. In all cases, linear relationships were observed in Voc vs T plots over the temperature range studied. The values of Eact vs Eg are plotted in Figure 7. The activation energies are approximately equal to the band gap energy for each value of x investigated in the n-AlxGa1-xAs series. However, the increases in Voc are larger than the changes in the conduction band edge position calculated from the reported conduction band discontinuity of n-AlxGa1-xAs relative to GaAs. This behavior clearly rules out thermionic emission-type processes as the dominant recombination mechanism at these solid/liquid contacts, because thermionic emission currents would have an activation energy equal to the difference between the energy of the bottom of the conduction band, Ecb,
and qE(A/A-). The observed behavior is consistent with the interpretation that the dominant recombination current of n-AlxGa1-xAs/CH3CN-Fc+/0 contacts, like that of n-GaAs/Fc+/0 contacts, results from surface-state recombination that behaves in accord with classical Shockley-Reed-Hall statistics for carrier recombination processes.42,53,54 E. Dependence of the J-E Behavior of the Series of n-AlxGa1-xAs/CH3CN Contacts on the Redox Potential of the Solution. The J-E data were obtained for all of the n-AlxGa1-x As samples in contact with CH3CN-LiClO4 electrolytes containing the same collection of ferrocene and cobaltocene derivatives used in the investigation of the Voc vs E(A/A-) properties of n-GaAs/CH3CN junctions. Stable J-E curves, characterized by large fill factors and semiconductor-limited anodic currents, were observed for all samples. Table 8 lists the key electrochemical properties obtained from this series of experiments. Figure 8 displays Voc as a function of E(A/A-). These data were obtained for all of the n-AlxGa1-xAs electrodes at light intensities sufficient to provide the same photocurrent density, 1 mA cm-2. The Voc vs E(A/A-) behavior of the ternary alloy samples is similar to that of the parent n-GaAs sample with the exception that Voc increased monotonically for each redox couple as Eg increased. As found for n-GaAs (Figure 3), the largest photovoltages for the n-AlxGa1-x As materials were observed for electrodes in contact with CH3CN-Fc+/0 solution, with the Voc values decreasing as the value of E(A/A-) was made more negative. The slope of Voc vs E(A/A-) is typically 0.33-0.40 for the range of E(A/A-) under investigation. Thus, nonideal behavior, consistent with partial Fermi level pinning, is present for all of these electrode materials in contact with the CH3CN electrolyte. Also, as observed for n-GaAs, all of the n-AlxGa1-xAs electrode materials exhibited anomalously high Voc values in contact with CoCp2+/0 and Co(MeCp)2+/0 electrolytes. The indications of surface changes in contact with these electrolytes for n-GaAs thus seem extendable to the entire family of n-AlxGa1-xAs compounds explored in this study. IV. Discussion A. Electrochemical Properties of n-GaAs and n-AlxGa1-xAs/ CH3CN Contacts. It is clear that n-GaAs and n-AlxGa1-xAs electrodes exhibit partial Fermi level pinning in contact with CH3CN over a wide range of redox potentials. In this work, the Fermi level pinning behavior has been probed through use of steady-state J-E and Voc vs T data. The band edge shifts documented herein are often difficult to resolve when a small potential range is explored, but such behavior is clearly seen with a large variation in E(A/A-). These conclusions are also consistent with differential capacitance measurements of the band edge positions of n-GaAs/ CH3CN contacts. Prior studies of the n-GaAs/CH3CN-Fc+/0 contact have yielded a barrier height of 1.1 V for this system,8 in which the Nernstian solution potential, E(A/A-), was +0.32 V vs SCE. For comparison, Mott-Schottky measurements of the n-GaAs/CH3CN-CoCp2+/0 contact indicate a barrier height of 0.9 V, even though the redox potential of these solutions is -0.9 V vs SCE. Thus, the barrier height changed by less than 300 mV for >1.2 V change in the redox potential of the solution, clearly indicating the presence of Fermi level pinning for these contacts. Similar conclusions regarding Fermi level pinning of GaAs electrodes in contact with nonaqueous solvents have been drawn previously from cyclic voltammetric and/or impedance studies of GaAs/CH3CN contacts.18-20,24,25,46 The cyclic voltammetric
5444 J. Phys. Chem. B, Vol. 104, No. 23, 2000
Casagrande et al.
TABLE 8: Kinetic Parameters for Different Redox Couples of n-AlxGa1-xAs/CH3CN Contacts redox couple Fe[C5(CH3)5]2+/0 [Co(C5H4CO2CH3)2]+/0 average [CoCp(C5H4CO2CH3)]+/0 average CoCp2+/0 [Co(C5H4CH3)2]+/0
average Fe[C5(CH3)5]2+/0 average [Co(C5H4CO2CH3)2]+/0 [CoCp(C5H4CO2CH3)]+/0 average CoCp2+/0 [Co(C5H4CH3)2]+/0 average Fe[C5(CH3)5]2+/0 [Co(C5H4CO2CH3)2]+/0 [CoCp(C5H4CO2CH3)]+/0 average CoCp2+/0 average [Co(C5H4CH3)2]+/0 Fe[C5(CH3)5]2+/0 average [Co(C5H4CO2CH3)2]+/0 average [CoCp(C5H4CO2CH3)]+/0 average CoCp2+/0 average [Co(C5H4CH3)2]+/0 a
Ecell,V vs SCE
log(J0)b
Voc,calc, V (Jph)1 mA cm-2)
-12.03 ( 0.10 -9.32 -11.59 -10.5 ( 1.6 -8.93 -11.79 -10 ( 2
0.710 ( 0.012 0.591 0.575 0.583 ( 0.011 0.479 0.516 0.50 ( 0.03
1.307 ( 0.010 1.207 1.24 1.285 1.374 1.28 ( 0.07
-11.11 ( 0.05 -11.50 -11.28 -11.88 -11.54 -11.6 ( 0.3
0.627 ( 0.006 0.603 0.604 0.627 0.689 0.63 ( 0.04
-0.212 -0.203 -0.208 ( 0.006 -0.552 -0.755 -0.777 -0.766 ( 0.016 -1.090 -1.181 -1.220 -1.20 ( 0.03
n-Al0.16Ga0.84As; Eg ) 1.63 eV 0.680 0.707 1.241 0.732 0.933 1.253 1.247 ( 0.016 0.636 1.360 1.118 ( 0.015 0.519 0.755 0.994 0.515 0.761 1.033 1.014 ( 0.028 0.734 1.573 1.251 ( 0.013 0.668 0.976 1.544 0.745 1.544 1.340 1.44 ( 0.14
-12.47 -12.96 -12.7 ( 0.3 -12.47 ( 0.11 -12.08 -11.65 -11.9 ( 0.3 -12.73 ( 0.08 -10.37 -12.27 -11.3 ( 0.3
0.690 0.735 0.71 ( 0.03 0.622 ( 0.011 0.526 0.522 0.524 ( 0.003 0.720 ( 0.010 0.668 0.729 0.70 ( 0.04
-0.190 -0.534 -0.749 -0.771 -0.760 ( 0.016 -1.072 -1.079 -1.076 ( 0.005 -1.194
n-Al0.24Ga0.76As; Eg ) 1.72 eV 0.816 1.303 1.241 0.711 0.785 1.118 ( 0.015 0.565 1.025 0.994 0.574 1.117 1.033 1.014 ( 0.028 0.684 0.819 1.40 0.747 1.647 1.117 1.29 ( 0.16 0.821 1.530 1.298 ( 0.011
-12.47 -12.47 ( 0.11 -12.08 -11.65 -11.9 ( 0.3 -11.39 -13.61 -12.5 ( 1.6 -13.57 ( 0.08
0.81 ( 0.03 0.720 ( 0.013 0.572 0.580 0.576 ( 0.006 0.639 0.735 0.687 ( 0.068 0.808 ( 0.009
-0.216 -0.196 -0.206 ( 0.014 -0.518 -0.515 -0.517 ( 0.002 -0.776 -0.759 -0.768 ( 0.012 -1.099 -1.078 -1.089 ( 0.015 -1.218
n-Al0.31Ga0.69As; Eg ) 1.82 eV 0.771 0.926 1.29 0.899 0.903 1.266 1.287 ( 0.017 0.655 1.045 1.08 0.732 0.788 1.49 1.3 ( 0.3 0.639 0.706 1.23 0.562 0.832 0.980 1.11 ( 0.18 0.755 0.826 0.938 0.850 1.103 1.157 1.05 ( 0.16 0.995 1.699 1.515 ( 0.013
-13.18 -15.04 -14.1 ( 1.3 -13.68 -11.6 -12.6 ( 1.5 -12.05 -12.80 -12.4 ( 0.5 -16.77 -15.46 -16.1 ( 0.9 -13.95 ( 0.08
0.773 0.894 0.83 ( 0.09 0.674 0.754 0.71 ( 0.06 0.651 0.564 0.61 ( 0.06 0.760 0.849 0.81 ( 0.06 0.976 ( 0.011
-0.198 -0.510 -0.556 -0.53 ( 0.03 -0.801 -0.783 -0.792 ( 0.013 -1.100 -1.167 -1.150 -1.166 -1.226 -1.18 ( 0.03
Voc, V
Jph mA cm-2
γa
n-Al0.09Ga0.91As; Eg ) 1.51 eV 0.709 0.984 1.34 ( 0.02 0.579 1.047 1.59 0.569 0.803 1.14 1.37 ( 0.32 0.473 0.833 1.377 0.511 0.741 1.01 1.2 ( 0.3 0.626 0.598 0.601 0.675 0.690
0.912 0.922 0.833 1.120 1.052
Diode quality factor; see text for discussion. b J0 ) reverse saturation current density (A cm-2).
studies included an even wider potential range than was investigated in the J-E experiments of the present study, spanning the potential range from -2.5 V to +0.8 V vs SCE. Kohl and Bard46 reported the photoelectrochemical behavior of n-GaAs in CH3CN solutions with a range of redox couples including photovoltages for some redox couples having formal potentials negative of the flat-band potential. The ability to generate a photovoltage at the most negative cell potentials was attributed to pinning of the Fermi level by surface states. The current-voltage properties of the n-GaAs/tetrahydrofuran system were characterized using cyclic voltammetry by DiQuarto and Bard.19 Saturation of the photovoltage at cell potentials negative of the valence band for n-GaAs and positive of the conduction band for p-GaAs was interpreted as indicating Fermi level pinning due to either a high density of surface states or
carrier inversion at the interface. Yeh and Hackerman20 investigated the behavior of the n-GaAs/N,N-dimethylformamide interface. Using cyclic voltammetry in solutions of various redox couples, they observed negative shifts in the oxidation/reduction wave of benzoquinone at n-GaAs under illumination relative to the values of these peaks in the dark, and they ascribed this behavior to nonideality in the GaAs/liquid junction. Although our work has used a KOH(aq) etch followed by a Br2-CH3OH etch, which has been reported to yield the most optically abrupt GaAs/air interface, similar Fermi level pinning behavior has been observed for n-GaAs electrodes etched with HCl.46 The Fermi level pinning behavior observed herein is also consistent with a variety of prior studies of GaAs surface chemistry in other ambients. For example, nonideal energetic behavior has been observed for n-GaAs surfaces exposed to a
n-GaAs and n-AlxGa1-xAs in CH3CN
Figure 8. A plot of the open-circuit photovoltage (calculated) vs equilibrium cell potential for the n-AlxGa1-xAs/CH3CN-LiClO4-Fc+/0 series at Jph ) 1.0 mA cm-2. (-b-) x ) 0.09, Eg ) 1.51 eV; (- -O- -) x ) 0.16, Eg ) 1.63 eV; (-[-) x ) 0.24, Eg ) 1.72 eV; (- -9- -) x ) 0.31, Eg ) 1.82 eV. Error bars not shown are within the size of the symbols. The redox couples were: (A) [Co(C5H4CH3)2]+/0, (B) CoCp2+/0, (C) [CoCp(C5H4CO2CH3)]+/0, (D) [Co(C5H4CO2CH3)2]+/0, (E) Fe[C5(CH3)5]2+/0, (F) [Fe(C5H4CH3)2]+/0, and (G) Fc+/0.
variety of impurities in UHV, with formation of As defects or excess elemental As thought to be the source of the persistent Fermi level pinning of n-GaAs surfaces.55-57 In addition, formation of metal contacts on n-GaAs is well-documented to result in Fermi level pinning.53,58,59 In fact, we are not aware of any series of n-GaAs contacts reported to date in which Fermi level pinning has not been observed when the electrochemical potential of the contacting phase has been varied over a significant range. B. Photoelectrochemical Behavior of n-GaAs/CH3CNFc+/0 Contacts. Despite the partial Fermi level pinning behavior of n-GaAs/CH3CN contacts, it is still apparently possible to construct relatively efficient photoelectrochemical energy conversion devices from such systems. This evidently occurs because the equilibrium Fermi level position of the GaAs electrode is sufficiently close to the valence band edge energy that large photovoltages can be obtained through the use of redox couples with very positive values of E(A/A-). Evidence supporting this hypothesis is obtained from the differential capacitance measurements for the n-GaAs/CH3CN-Fc+/0 system, which indicate that the Fermi level is 1.1 eV positive of the conduction band edge under equilibrium conditions.8 Thus, if thermionic emission is suppressed and if the direct electrontransfer process is sufficiently slow, it will be possible to observe photovoltages limited only by surface-state-induced recombination at the solid/liquid contact as described below. The kinetic data support the notion that surface-state recombination, and not direct electron transfer, dominates interfacial current flow at n-GaAs/CH3CN-Fc+/0 contacts.29 A bulk recombination/diffusion-limited current density should depend on the dopant density of the solid,53 but no such dependence of Voc in response to changes in Nd is observed experimentally. Furthermore, the bulk recombination/diffusion-limited Voc is calculated to be ∼1.00 V for the semiconductor/liquid contacts used in this work at a light intensity sufficient to provide Jph ) 20 mA cm-2,60 yet the observed Voc values are much lower than this upper limit. If direct electron transfer to the acceptor species in the electrolyte were dominant, these contacts should exhibit a first-order dependence on the concentration of acceptor species in solution;31,38,48 this also is not observed for the n-GaAs/CH3CN-Fc+/0 contact. Additionally, the activation energy in a Voc vs T plot for an electron transfer limited process
J. Phys. Chem. B, Vol. 104, No. 23, 2000 5445 should be approximately qE(A/A-) - Ecb,29,44 but the experimentally determined Eact value obtained from Voc vs T plots was essentially equal to the band gap energy of the GaAs. The kinetic data therefore imply that surface-state recombination is the dominant recombination mechanism at the n-GaAs/CH3CN-Fc+/0 contacts studied in the present work. Despite this residual surface-state recombination activity, the Voc values of the n-GaAs/CH3CN-Fc+/0 system are, to our knowledge, the highest reported to date for any nonaqueous n-GaAs/liquid junction.38,40,61,62 The optical-to-electrical conversion efficiency of these devices is ∼10% without correction for optical reflection losses or residual uncompensated resistance losses. The optical reflection losses alone account for a loss of approximately 5 mA cm-2 of photocurrent at this light intensity, based on measurements of the increase in photocurrent observed upon use of matte-etched n-GaAs surfaces in other systems.63 Thus, photoelectrode energy conversion efficiencies of 12-13% should be readily obtainable from these systems. It should be possible, in principle, to obtain even higher photovoltages from these electrodes provided that the bulk recombination/diffusion limit is not reached. Thus, further reductions in surface recombination could produce additional, significant gains in efficiency above the high levels observed in the present work. The behavior of the n-AlxGa1-xAs electrode series is similar, in many respects, to that of the parent n-GaAs. The same dominant charge-transfer mechanism is indicated by the J-E and Voc vs T data, and all of the n-AlxGa1-xAs electrodes display high energy conversion efficiencies in contact with the CH3CN-Fc+/0 electrolyte. Unlike the n-GaAs 1-xPx electrode series, however, the increase in Voc as x increases in the n-AlxGa1-xAs/CH3CN-Fc+/0 contacts is not sufficient to compensate for the decrease in Jph arising from the larger band gaps of such samples. These data are also consistent with the hypothesis that surface recombination dominates the recombination processes at n-AlxGa1-xAs/CH3CN-Fc+/0 contacts. C. Electrochemical Properties of n-GaAs and n-AlxGa1-xAs/ CH3CN-CoCp2+/0 Contacts. Operation of n-GaAs electrodes in contact with the CH3CN-CoCp2+/0 electrolyte clearly induced irreversible changes in the surface chemistry of the GaAs electrode. Similar changes have been observed previously in the electrochemistry of n-GaAs/KOH-Se-/2-(aq) contacts in response to exposure of n-GaAs to CH3CN-CoCp2+/0 solutions.35 Such changes were not, however, observed in KOHSe-/2-(aq) solutions after exposure of n-GaAs surfaces to CH3CN-methyl viologen2+/+ solutions.64 These observations indicate that the CH3CN-CoCp2+/0 contact is the source of the electrical changes in the n-GaAs surface, and by extension, of changes in the n-AlxGa1-xAs surfaces as well. Such changes are persistent and affect the electrochemistry of the nonaqueous systems studied in this work. These surface changes were confirmed by the XPS analysis of the GaAs system, which shows the formation of a carbonaceous overlayer on the GaAs surface subsequent to electrochemical measurements in the CH3CN-CoCp2+/0 solution. D. Interfacial Rate Constant Determinations Using nGaAs and n-AlxGa1-xAs/CH3CN Contacts. Several conditions must be met in order to extract robust ket values from the steadystate J-E data of semiconducting electrodes.2,3,8,9 First, the expected rate law must be validated, i.e., the interfacial flux must be shown to be first-order in the concentration of electrons at the semiconductor surface and first-order in the concentration of electron acceptors in the solution.4,48 Second, the band edges must not move with variations in the concentration of the redox acceptor species. Otherwise, both the electron concentration and
5446 J. Phys. Chem. B, Vol. 104, No. 23, 2000 the redox acceptor concentration are changing simultaneously, and validation of the rate law is not readily possible. Third, the surface must be defect-free enough so that capture of the electrons by surface states, which may arise either from states that are intrinsic to the surface or from adsorption of the redox ions used in the ket measurement, does not compete kinetically with the direct electron capture process from the semiconductor to the redox acceptor species.4,31,41,48,65,66 Otherwise, adsorption will preclude measurement of the desired rate of interfacial charge transfer. Evaluation of the validity of the second and third conditions, which are critical to establish a valid experimental determination of ket from steady state J-E data for n-GaAs/CH3CN and n-AlxGa1-xAs/CH3CN contacts, can be obtained from the data described in this work. The evidence for irreversible surface changes upon immersion of n-GaAs or n-AlxGa1-xAs electrodes into CH3CN-CoCp2+/0 solutions, as probed by the XPS data presented herein or by the experiments involving the KOH-Se-/2-(aq) electrolyte described elsewhere,35 implies that a significant portion of the interfacial current in the n-GaAs/CH3CN-CoCp2+/0 contact flows through pathways that do not involve the direct transfer of electrons from the conduction band of the semiconductor into dissolved, nonadsorbed redox acceptor species. The XPS data clearly indicate that a chemical reaction, which produces a carbonaceous surface film, dominates the interfacial chemistry of the n-GaAs/CH3CN-CoCp2+/0 contact at potentials sufficiently negative to produce cathodic current flow through the interfaces. This film might introduce surface states and thereby raise the overall flux, act as an insulator and block the interfacial current from flowing, or both. The presence of such a film also implies that the concentration of adsorbed electroactive CoCp2+/0based sites cannot be directly deduced under these experimental conditions from mass-change measurements of the electrode. Of note is that the n-GaAs electrodes investigated in this work produced higher open-circuit voltages, and therefore must have lower values of ket, than have been reported for n-GaAs electrodes exposed to less optimal etching procedures.6 Since the rate constant ket describes the direct transfer of electrons from the conduction band of the semiconductor to nonadsorbing electron acceptor ions in the solution phase, ket cannot depend on the surface condition or etching procedure. The photovoltage value observed at Jph ) 1 mA cm-2 in Figure 3 can therefore be used to set an upper bound on ket for the n-GaAs/CH3CNCoCp2+/0 system, because the rate of the direct electron-transfer process is equal to, or less than, the observed interfacial chargetransfer flux. This produces an upper bound on ket for the n-GaAs/CH3CN-CoCp2+/0 contact of
