J. Phys. Chem. 1985,89, 1121-1 126 intensity dependence is then observed. These interpretations are tempered by the fact that strongly absorbed probe and pump illumination sources were used in these experiments which produced highly localized charges in the Pc films. Approximately 2% of the incident frontside pump photons, however, will penetrate the typical Pc f h and be absorbed within the space charge layer at the Au/Pc interface. Since the capacity for charge in this region is small, even the low photon flux from the FS pump should significantly lower the potential barrier for migration of holes generated at the back surface. If both the probe and pump are directed toward the film from the front surface, the modulated photocurrent yield decreases with increasing pump intensity as predicted by the above sublinear photocurrent dependence observed for the rotating disk electrode. Bulk us. Interfacial Effects. It is of interest to reexamine the photocurrent action spectra and the analysis of these spectra according to our earlier model.2 In that case we assumed that maxima occur in the BS action spectra when the collection length of the photogenerated charge was less than the difference between the film thickness and the absorption length of the photon. We have shown here that the parameters which effect the length over which charge can be collected from the Au/Pc interface, could have been either due to bulk trapping or recombination or to trapping of charge at the Au/Pc interface. Since we are dealing here with polycrystalline films, one must also include the possibility that charge trapping can occur between two contacting faces of adjacent Pc microcrystals. In the same manner as at the Au/Pc interface, a mismatch of the Pc surface and bulk composition would lead to mismatch in electron affinities of two contacting crystals, at the interface between them. This may lead to the formaion of a space charge region within both crystals, and could potentially contribute to trapping of charge in that region. This effect is likely to be most pronounced when the electron affinity
1121
of the surface Pc molecules is raised (a hole trap will be formed at the interface). If the orientation of the Pc/Pc interface is parallel to the Au/Pc interface the effect of the charge trap is more likely to be observed than when the Pc/Pc interface is perpendicular to the Au/Pc interface. This last point confirms the need to deposit single layers of microcrystals rather than multilayers. The GaPc-Cl films are predominantly, but not exclusively, single layer deposits with crystallite dimensions of 0.5 to 1.Opm on each side.' It is clear that under conditons of high light intensity, or high electric field strength within the Pc film, trapping of charge is not likely to be observed unless the inversion layer at the back interface is larger in capacitance than seen for this system. The effect of this charging phenomenon at the Au/Pc interface may be alleviated by using metallic substrate materials whose intrinsic Fermi level lies closer to or within the valence band of the photoconductor. This would decrease the size of this inversion layer but may eventually turn the Pc film into a real dark conductor when applied potentials push the solution Fermi energy below the valence band edge. This will remove any of the desirable photoconductive properties of this material. Care must be taken in the choice of such substrates and in the characterization of the metal/photoconductor interface. As shown in the following paper, the size of this potential well and the band diagram for the Au/GaPc-Cl system can be more accurately quantitated through the use of pulsed illumination sources. l 2 Acknowledgment. This research was supported by grants from the National Science Foundation CHE83-17769 and from IBM Corporation. Registry No. Au, 7440-57-5; GaPc-CI, 197 17-79-4; hydroquinone, 123-31-9.
Pulsed Laser Coulostatlc Studles of Phthalocyanlne Photoconductor Electrodes Peter C. Rieke and Neal R. Armstrong* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: May 23, 1984; In Final Form: October 31, 1984)
The Au/chlorogallium phthalocyanine/Fe(CN)6ssh photoconductor system was investigated by pulsed dye laser photocoulostatics. By variation of the direction and intensity of illumination, the Schottky barriers formed between the photoconductor dye and the contacting phases could be studied separately. The potential gradients at each interface were found to oppose one another such that a potential well formed in the dye layer, consistent with previous photocurrent spectroscopy studies. The depth of this well was quantitated and the nonlinear potential gradients across the film defined for any bias potential. This potential well decreases photoelectrochemical efficiencies in the Pc film by the trapping of mobile charges. This effect is most pronounced when there is a large difference between the work function of the conductive substrate and the intrinsic Fermi level of the Pc layer.
Introduction Thin-film photoconductor electrodes have recently been reported by us which consist of a semitransparent gold film (300 A) on a polymer backing (Au-MPOTE) over which a chlorogallium phthalocyanine layer of up to 1 pm thickness is vacuum deposited. These materials possess some unusual features for phthalocyanine thin films, in that the molecular structure of GaPc-Cl promotes a long-range ordering and therefore sizable microcrystallites in each film. Nonporous films have been made which show both positive and negative photopotentials that are typically several hundred millivolts, when placed in contact with aqueous electrolytes.'-4 The GaPc-C1 thin film is representative of organic (1) Linkous, C. L.; Klofta, T.; 1983, 130, 1050.
Armstrong, N. R. J. Electrochem. SOC. 0022-3654/85/2089- 1121$01.50/0
photoconductor thin films which are lightly doped, so that the Fermi level is well above the valence band edge (0.5-0.7 eV) and therefore both positive and negative photopotentials are observable. Highly doped Pc films, where the Fermi level is near the valence band edge, will in general show only positive photopotentials.'*+'' The photoelectrochemical quantum efficiencies of the GaPc-C1 films are in the range of 2-10% for hole harvesting at the solution interface. Similar photopotential excursions, but lower efficiencies, have been previously observed in porphyrin films.5 (2) Rieke, P. C.; Linkous, C. L.; Armstrong, N. R. J . Phys. Chem. 1984, 88, 1351. ( 3 ) Rieke, P. C.; Armstrong, N. R. J. Am. Chem. SOC.1984, 106, 47. (4) Buttner, W.; Rieke, P. C.; Armstrong, N. R.J . Electrochem. SOC. 1984, 131, 225.
( 5 ) Kawai, T.; Tanimura, K.; Sakata, T. Chem. Phys. Let?.. 1978,56,541.
0 1985 American Chemical Society
1122 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
It is of interest to learn where the energy loss pathways exist in these films. In the preceding paper,6 observations of the photocurrent transient response (when holes are being harvested at the Pc/solution interface) showed the existence of a spacecharge region at the Au/Pc interface, in contrast to previous expectations,'-I2 but consistent with chlorophyl/metal interfaces studied by Tang and A1bre~ht.I~This space-charge region was responsible for the trapping of a sizeable fraction of the photogenerated holes a t the Au/Pc interface under low illumination intensities but does not constitute the only mechanism for energy loss. Bulk trapping of holes was also shown to occur even after the effects of the inversion layer at the Au interface were overcome by the addition of a variable-intensity photodopant. Pulsed laser photoelectrochemical experiments have been used to describe interband and intraband surface states and the rates of charge migration within inorganic semiconductor material^.'^'' Delivering the photoexcitation in the form of a short duraction pulse has the advantage of allowing the observation of charge decay rates and, in the cases discussed here, allows the more accurate description of band diagrams for thin film photoconductors. Pulsed laser photocurrent (photocoulometry) measurements have been conducted on other Pc thin-film systems, having the advantage of allowing the quantitation of the photogenerated charge without mass transport control of the photocurrent.'8-20 These photocurrent measurements suffer, however, in that the resolution of the photoelectrochemical process is limited by the R C time constant of the electrochemical cell (typically 20 to 100 ps), as determined by the solution resistance, and the working electrode capacitance in a property designed cell. Perone and co-workersIel6 and others18have shown that this time constant is the major limitation in most transient electrochemical experiments. Coulostatic experiments offer an alternative for following the fastest electrochemical events. In this technique, a semiconductor electrode is initially poised a t some desired potential, the applied potential is removed, and a high impedance differential amplifier is placed across the cell. A laser pulse illuminates the electrode and the potential time transient is recorded. The time constant for the photopotential response is determined by the total impedance of the electrochemical cell and the capacitance of the instrumentation. Since this capacitance is usually orders of magnitude smaller than the working electrode capacitance, very fast potential excursions (down to 10-100 ns) can be reliably recorded.14,1s In the pulsed laser coulostatic experiments a t the semiconductor-redox interface outlined by Perone and co-workers, only a single interface was c~nsidered.'~-'~ In the dark, potentiostatic control charged the capacitance between the space-charge region of the semiconductor and the Helmhotz double layer of solution. Pulsed laser illumination induced charge migration in the space-charge region and across the interface. Four processes have been identified by previous investigators which might occur in the GaPc-C1 film in contact with a ferri/ferrocyanide electrolyte solution, after application of the laser pulse. Two of these processes would result in a potential excursion toward the flat-band potential followed by return to the initial, open-circuit potential in the dark. These processes are (a) elec(6) Buttner, W. J.; Rieke, P. C.; Armstrong, N . R. J . Phys. Chem., preceding paper in this issue. (7) Fan, F. F.; Faulkner, L. R. J . Am. Chem. SOC.1979, 101, 4779. (8) Chamberlain, G. A., Malpas, R. E. Symp. Faraday SOC.# 70, Paper # 15, Oxford, England, 1980. (9) L e t m p l , P.; Fan, F. F.; Bard, A. J. J . Phys. Chem. 1983,87, 2948. (10) Loutfy, R. 0. McIntyre, L. F. Sol. Energy Mater. 1982, 6, 467. (11) Loutfy, R. 0. J . Phys. Chem. 1982,86, 3305. ( I 2) Sussman, A. J . Appl. Phys. 1%7, 38, 2738. (13) Tang, C. W.; Albrecht, A. C. J. Chem. Phys. 1975, 62, 2139. (14) Perone, S. P.; Richardson, J. H.; Deutscher, S. R.; Rosenthal, J.; Ziemer, J. N . J . Electrochem. SOC.1980, 127, 2380. (15) Richardson, J. H.; Perone, S. P.; Deutscher, S.B. J . Phys. Chem. 1981, 85, 341. (16) Deutscher, S. B.; Richardson, J. H.; Perone, S. P.; Rosenthal, J.; Ziemer, J. Symp. Faraday SOC.#70, Paper #2, Oxford, England, 1980. (17) Kamat, D. V.; Fox, M. A. J . Phys. Chem. 1983,87, 59. (18) Chan, K. Ph. D. Dissertation, University of Arizona, 1984. (19) Ropolic, Z. D.; Sharp, J. H. J. Chem. Phys. 1977, 66, 5076. (20) Tang, C. W.; Albrecht, A. C. J . Chem. Phys. 1975, 63, 953.
Rieke and Armstrong tron-hole pair photogeneration and separation in the space-charge region, followed by electron-hole pair recombination, and (b) nonequilibrium expansion and contraction of the space-charge region. No net charge transfer occurs across the interface in either of these cases. While transient potential changes will occur, no net potential change is associated with these effects in the time course of the experiment. The two other processes (c), electron transfer in the dark, and (d), light-induced heterogeneous electron transfer, result in net charge transfer and thus a net potential change. Any electron transfer at the electrode in the dark results in a potential decay toward the equilibrium potential as determined by the redox coupled. Light-induced electron transfer results in potential decay toward the flat-band potential. If the initial open-circuit bias potential in the dark is positive or negative of both the flat-band and equilibrium potentials, both the dark and light electron transfer potential decays will be negative or positive in direction, respectively. If the initial bias potential is between these two potentials the dark decay will be toward the equilibrium potential, the light-induced decay toward the flat-band potential. In the Au/GaPc-C1/Fe(CN)"e aqueous system two interfaces exist. Since the dye is relatively insulating in the dark the system can initially be considered as a simple capacitor with a linear potential drop across the dielectric dye material. Application of a potential charges the capacitor. In order to discharge the capacitor and observe a potential change, charge must migrate across the film. Under illumination, holes (the majority carriers)2*21are created at the illuminated interface. If the potential gradient is such that the illuminated interface is negatively charged, holes may be immediately harvested by negative charge at this interface. Other photoproduced holes then may migrate across the film from the other interface to help neutralize the excess negative charge. The number and distribution of holes which participate in such an event is dependent upon the wavelength of the photons used. The population of photoproduced charges is greatest at the illuminated interface and decays exponentially in accordance with the absorpitivity of the Pc film. The result of this charge neutralization at an interface will be net charge transfer across the film and a net permanent potential change. If the potential gradient is such that the illuminated interface is positiuely charged, the holes created by illumination must migrate across the film. Recombination and trapping will prevent most of these holes from traversing the film (if the film thickness is large compared to one collection length for the photoproduced hole). The result will be little net dissipation of the charge and, therefore, no net potential change. Effects which can be ascribed to inefficient hole transport across a thick Pc film have been observed in photocurrent action spectra by our~elves*~~ and other ~ o r k e r s . ~ ~ ~ ~ ~ - * ~ The previous paper6 illustrated that the potential drop across the Pc film could not be considered linear and that space-charge regions exist at each interface due to the formation of Schottky barriers. Each of the photoeffects observed by Perone and coworkers for single-crystal electrodele16 can occur in these regions. However, in their work, because net charge transfer across the semiconductor was required for a permanent potential change, light-induced charge transfer at one interface was expected to be coupled to dark carrier transport in the rest of the film and at the other interface. In the experiments reported here, observation of the pulsed laser coulostatic response allowed independent estimation of the Schottky barrier heights at each interface. This was made possible by (a) illumination from both directions through the use of a semitransparent metal substrate; (b) the use of laser light, of various intensities, which was very highly absorbed and allowed selective generation of carriers at each interface; and (c) the ability (21) Gutmann, F.; Lyons, L. E. "Organic Semiconductors";Wiley: New York, 1967. (22) Fan. F. F.: Faulkner. L. R. J . Chem. Phvs. 1978. 69. 3341. (23) Ghosh, A.'K.; More1,'D. L.; Feng, T.; Shaw, R. F.'; Rowe, Jr., C. A. J . Appl. Phys. 1974, 45, 230. (24) Loutfy, R. 0.;Sharp, J. H. J . Chem. Phys. 1979, 71, 1211. (25) Linkous, C. L. Ph. D. Dissertation, Michigan State University, 1983.
Coulostatic Studies of Phthalocyanine Electrodes LCLLi
DYE LASER
I
'
S C 6 PE
T I
Figure 1. Experimental setup for the pulsed laser photocoulostatic ex-
periments. to relate the photopotential excursion in the 0-30-ps time domain to the height of the illuminated Schottky barrier. Evidence for two barriers existed only when illumination intensities were moderate and transient conditions were employed. Continuous illumination produces carriers which diffuse across the film such that the photopotential effects at each barrier were not due to carriers created in the interface. Experimental Section GaPc-Cl was synthesized in this laboratory and its preparation is described elsewhere.24 All solutions were prepared from deionized water doubly distilled from permanganate. Reagent grade chemicals were used as obtained. Photoelectrochemical studies were conducted in a vacuum-deoxygenated potassium hydrogen phthalate, p H 4 buffer. The potential scale was referenced to the saturated Ag/AgCl electrode. uType C", GaPc-Cl electrodes were used in these studies. Their preparation, morphology, and electrochemical behavior have been described in In summary, a thin film of GaPc-C1 was vacuum sublimed onto a substrate consisting of a 300-A layer of gold bonded to a clear polyester sheet and referred to as goldmetallized-plastic optically transparent electrode or Au-MPOTE. The GaPc-Cl film consisted of a single layer of crystallites about 1 pm on an edge tightly packed together to form an electrochemically nonporous film, requiring light to be conductive. The coulostatic experiment is block diagramed in Figure 1. Prior to the laser pulse, the photoelectrode of interest (W in Figure 1) was poised at some potential vs. the reference electrode by a conventional three-electrode potentiostat (ECO, Model 555). At some time (-t) the potentiostat was disconnected from the cell by opening the AD7516 switches, leaving the electrode in the open-current condition. A high-input impedance differential amplifier, Tektronix Model 5A22N, with storage oscilloscope was triggered a t this time and monitored the potential difference between the working and reference electrodes. 20-30 ps later, at t = 0, the laser was fired. The potential excursion was recorded on the storage scope and recorded with a scope camera. For time domains less than 200 p, the differential amplifier was ac coupled to the cell. At longer times, the scope was dc coupled (see Discussion section). A timing circuit was used to control the potentiostat, trigger the scope, and fire the laser. A square wave clock based on a Model 555 timer and a series of Model 7490 decade counters was used to control the repetition rate of the experiment. The repetition rate was between 30 and 45 s in order to allow time to change the experimental parameters for each laser pulse and to prevent significant reactant depletion near the photoelectrode. The rising edge of the square wave from the 555/7490 timer was used to set off a 74123 dual retriggerable monostable. One-half of the monostable produced a pulse of 1.0 s in duration. This pulse opened three AD7516 fast CMOS analog switches and closed them after typically 1.O s. This opening and closing of switches connected to the potentiostat determined the maximum duration
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
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of the experiment. The other half of the monostable also fired on the rising edge of the square wave and produced a pulse of variable duration of 0.1 to 1.0ps. A typical value used was 20-30 ps. On the leading edge of this pulse (time = -t), the scope was triggered and the trailing edge (time = 0) was used to fire the laser. The scope could also be triggered on the trailing edge for very short time domains experiments. It was necessary to wait 20-30 ps after opening the AD7516 switches before firing the laser in order to allow a noise spike associated with the switches to die away. The cell was a modified version of a sandwich type spectroelectrochemical cell based on 1 1/2-in.Lucite round stock, which allowed illumination of the electrode from both directions. Backside illumination (BS) is defined by illumination passing through the Au-MPOTE into the Pc film and frontside illumination (FS) as illumination passing through the solution fiit. The counterelectrode consisted of a coiled platinum wire. The reference electrode system consisted of a typical Ag/AgCl electrode in saturated KCl, separated from solution by a glass frit, and placed in parallel with a pseudoreference electrode (PRE). The high impedance of the Ag/AgCl reference electrode prevented accurate measurement of potential changes at the working electrode at short times, while the (PRE) was designed to have a low impedance to facilitate fast measurements. The use of these in parallel allowed accurate potential measurements at both short and long times . The (PRE) consisted of a platinum bilet in series with a 47-pF capacitor. The dimensions of the platinum bilet were 1/4 in. i.d., 3/8 in. 0.d. X ' / 4 in. length. The bilet was placed concentric with the in. diameter hole forming the cell volume in the center of the cell body. The bilet formed the support for a 1/4 in. i.d., 3 / 8 in. 0.d. O-ring against which the working electrode was compressed. Thus the PRE was less than in. (the thickness of the O-ring) from the working electrode. Without the PRE distortion of the potential at times less than 50 ps occurred, but with the PRE, the potential could be accurately measured at times well below 1 1 s . The cell was placed in an radiofrequency (rf) shield constructed from a 3 X 5 X 7 in. aluminum box containing a simple optical rail. Coaxial connections were made directly to the box. The laser pulse entered the box through a in. diameter hole. The rf box and cell were alligned concentric with a HeNe laser used to align the dye laser. The laser was a Phasar DL1100 flash lamp pumped dye laser. The dye used was a fresh M solution of cresyl violet (Exiton) in absolute ethanol with a peak output at 670 nm. The laser was operated at a 13-kV capacitor charge which resulted in an output of 22 mJ through a 350-500-11s pulse. The energy was measured by integrating the current-time response of a Hamamatsu 5780-8BQ photodiode and comparing the results with the response of a CW, 15-mW, HeNe laser. The power of the HeNe laser was previously measured with a Coherent 210 power meter. The photodiode was placed inside the rf box and frosted glass placed over the entrance in order to avoid saturation of the photodiode and to accommodate the different beam diameters. The operation of the coulostat was tested with dummy R C circuits. The observed decay of the potential-time transients matched closely (f25%) the value predicted from the nominal value of the dummy circuit components. Reliable behavior down to 1 ps was observed. Further testing was conducted with a bare Au-MPOTE electrode and an equimolar Fe(CN),'-+ solution. When the cell was open circuited, the expected simple decay from the initial potential to the equilibrium value, as determined by the redox coupled, was observed. The discharge of the laser spark gap created an oscillation less than 5 ps in duration and about 15 mV in magnitude. For the experiments reported here this was an insignificant potential excursion. 14326
(26) Herrmann, C. C.; Derrault, G . G.; Dilla, A. A. Anal. Chem. 1968, 40, 1173.
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1124 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
Figure 2. Cyclic voltammogram (potential referenced to Ag/AgCl) for a 1 mM each ferri/ferrocyanide solution, in the dark and under polychromatic illumination (ca. 100 mW/cm2, 470-900 nm). The zero current potential in the dark is the equilibrium potential, and the zero current potential in the light is the flat-band potential.
Results and Discussion Dark us. Light Current/ Voltage Behavior. Figure 2 shows the current/voltage curve that resulted in the dark when the G a P d l electrode was immersed in an equimolar solution of ferri-ferrocyanide (pH 4). Previous experiments have shown that the dark current on such an electrode was negligible (less than 1%) when compared to the photocurrent response. If the current sensitivity was expanded sufficiently, however, an unusual dark current/ voltage response could be observed. The small pores in the GaPc-C1 layer allowed a fraction of the solution redox coupled to penetrate to the gold substrate and react. The plateau nature of the i/Vcurve p i t i v e of the equilibrium potential was consistent with mass transport of ferri/ferrocyanide through a slightly porous overlayer on the Au electrode, as modeled by Matsuda and coworker~?’.~* From the magnitude of the current at the plateau relative to that observed on the bare gold electrode surface, the surface can be estimated to be a t least 99% covered by the inert GaPc-Cl layer. In subsequent coulostatic experiments it will be seen that these small pores acted as sites for dark decay of the electrode potential. Upon continuous high-intensity illumination, the photocurrent dwarfed the dark current and the zero current potential in the i / V curves shifted to ca. 0.0 V vs. Ag/AgCl, producing a photopotential l i i t e d in size by the difference in work function of the Au substrate and the formal potential of the redox c o ~ p l e d . ~ J ~ ~This * + ~potential, ~ a t zero photocurrent, is the flat-band potential for the Au/GaPc-Cl/ferri, ferrocyanide photoconductor system. PotentiallTime Transients at Times up to 10 ms and High Illumination Power. Figure 3 shows the potential/time transients that were observed in the 0-IO-ms time domain, following illumination with a 22-mJ laser pulse (per electrode area of 0.3 cm2) from the backside, and with initial bias potentials ranging from +400 to -400 mV vs. Ag/AgCl. Also shown are the decays of the electrode potential that resulted if the electrode remained in the dark during that same time period following open circuit. Curves 3a and 3b show that, when the electrode was initially biased positive, the electrode potential, following the laser pulse, moved in a negative direction and then returned to the potential of the open-circuited electrode in the dark. BS illumination at positive bias was unable to fully discharge the capacitance across the Pc film and the initial potential (minus the dark decay) was reestablished. Curves 3c and 3d show that, when the electrode was (27) Gueshi, T.; Tokuda, K.; Matsuda, H. J. EIecfrmnuI.Chem. 1978,89, 247. (28) Gueshi, T.; Tokuda, K.; Matsuda, M. J . Eledroanal. Chem. 1979, 101, 29. (29) Rose, A. “Concepts in Photoconductivity and Allied Problems”; Krieger: Huntington, NY, 1978. (30) Rose, A. Phys. Srafus, Solidi A 1979, 56, 11.
0
msec.
10
Figure 3. Potential-time photocoulostatic transients in the 0-10-ms time domain. Response for four initial bias potentials vs. Ag/AgC1 are shown: (a) +400, (b) +100, (c) -100 mV, (d) -400 mV, with illumination, solid curves, and without illumination, dashed curves. B Frontside
A Backside
o,2E
ul ??
0
>
I
I
I
100
I
‘
0.0
I
I
100
0 psec Figure 4. Potential-time transients in the 0-150-ps time domain, for frontside and backside illumination, with an initial bias of +I80 mV. Numerals refer to the neutral density factor used to attenuate the light; responses over six orders of intensity magnitude are shown. The laser was pulsed at 0.0 ws and the potentials, at a time corresponding to the arrow, were used in Figure 6. O
usec
initially biased negative, the laser pulse was successful in discharging the Pc capacitance. The final potential was positive of the initial potential and was permanently changed within the time scale of the experiment. A negative bias of the Au/Pc/redox system drove holes to the Au/Pc interface, whereas a positive bias drove holes to the solution interface. Because the penetration depth (1/@)of 670-nm photons is much less than the Pc film thickness (I/@ d 250 nm), holes were localized near the Au interface. Holes were harvested efficiently at Au (negative bias) but were unable to move efficiently through the Pc film to the solution interface (positive bias). The size of the permanent potential excursion under negative bias was dependent upon the initial bias potential and the rates of charge transfer a t the two interfaces. We can make a lower estimate of the Pc film capacitance under illumination that leads to the potential excursions of curves 3c and 3d. The charge~ ca. 1 ms and the quantity of charge transfer process O C C U K ~for is related to the solution depletion layer thickness at 1 ms and the quantity of the redox species in that layer. For a 1 mM solution the charge transferred was about 1 X lo-* C/cm2 or 6 X 1013electrons. Since the potential change was about 0.3 V the capacitance is therefore approximately 30 pF/cmZ. The number of photons triking the electrode was 7 X 1016/cm2(the actual number adsorbed maybe a factor of 2-4 lower), leading to an absorbed light quantum efficiency of at least 0.1%. As one would expect from this model, frontside illumination produced a transient voltage excursion at negative bias potentials and produced a more permanent excursion for positive bias potentials, confirming the importance of hole migration to the ca-
Coulostatic Studies of Phthalocyanine Electrodes
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
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N e u t r a l Density D 6 5 4 3 2 1
8001
D 6 5 4 3 2 1
I I I I I I \I
Back
,,q 0 1
Figure 5. Model illustrating the effect of intensity on the photopotential
for backside illumination. The upper left figure shows three hypothetical intensity profiles and the potential drop or band bending, through the film in the dark. The bottom figures show the potential profiles in the film for each intensity. The upper right figure shows the photopotential response plotted vs. light intensity. pacitive discharge process of the Pc film. PotentiallTime Transients at Short Times and Lower Illumination Power Densities. Figure 4 shows the potential transients in the 0-150-ps time scale as a function of decade changes in intensity and illumination direction for an initial bias potential near +200 mV. The curves are numbered with the neutral density (ND) factor used to attenuate the light (diminishing transmittance in powers of ten). Curve 1 corresponded to an intensity of 2.2 mJ. mJ and curve 6 to 2.2 X In this time domain the potential transient was dominated by two regions. At t i m q greater than 30 ws the potentials changed gradually. Here the processes outlined above for the 10-ms domain were just beginning to occur. At times less than 2Cb30 ps an initial potential excursion was followed by decay to a relatively constant value. The initial potential rise occurred in less than 1 1.1s and was determined by the laser pulse width of up to 500 ns. This rapid change of potential at up to 20-30 ps was more prominate at higher light intensities and is characteristic of a rapid nonequilibrium expansion, followed by contraction, of a space-charge region to a near-equilibrium photopotential. This type of rapid expansion and contraction of diffuse layers has been postulated by Feldberg for solutions where the diffusion coefficients of mobile ionic species (ca. 1 X cmz/s) dictate the time frame of the expansion and contraction (ca. s).~' Laser and Bard have predicted a similar effect in intrinsic Ge semiconductor electrodes where the carrier diffusion coefficients are near 100 c ~ * / s . ~For * most phthalocyanines it is assumed that the carrier diffusion coefficient for holes is 1 X lo-* to 1 X cm2/s (calculated from mobilities2'), which would lead to expansion and contraction times for the space charge regions on the order of microseconds, as observed here. At about 30 ps (arrows in Figure 4) the expansion and contraction processes seem to have slowed or ceased. After this time, potential excursions may be dominated by movement of any excess camers which can diffuse to other regions of the Pc film and undergo charge separation. At ND = 3, 2, and 1 for backside and ND = 2 and 1 for frontside illumination the curves sloped toward the flat-band potential at times greater than 30 ps, as a result of this process. In order to get a significant correlation between the position of carrier creation and the region in which the photopotential was developed it was necessary to measure the potential after nonequilibrium effects had decayed but before significant carrier migration recombination or charge transfer had occurred, Le., at about 30 ps after the laser pulse. It can be seen that, in the case of BS illumiantion, at 30 ps the photopotential initially moved in a positive direction and then (31) Feldberg, S . W. J . Phys. Chem. 1970, 74, 87. (32) Laser, D.;Bard, A. J. J . Electrochem. SOC.1976, 123, 1828.
Figure 6. Photopotentials (taken from the transients in Figure 4 at 30 vs. intensity, as determined by the neutral density (ND) factor used to attenuate the light. The potential at ND = D, the dark value, is the ps)
initial bias potential. moved negative toward the flat-band condition as the laser intensity was increased. In contrast, for FS illumination the potential at 30 bs initially moved negative, a s expected, and then overshot slightly the final potential obtained at the highest light intensities. The case for BS illumination, at this initial bias potential, is modeled in Figure 5 for three illumination intensities (three different population distributions of photons within the Pc film). If we assume the potential well model postulated in the previous paper: the band will be inverted at the Au interface and the lowest BS laser intensity (case 1) places charge in the Pc film only sufficient to begin to flatten the bands in that region. Consequently, the initial potential excursion is positiue and becomes negative only when sufficient light intensities are used to fully flatten the band at this back interface and to begin to place charge in the space-charge region at the Pc/solution interface (cases 2 and 3). An opposite process would be expected to occur for frontside illumination. The potential will initially move negative as the intensity increases followed by a small return in the positive direction at the highest intensities. Once an intensity has been reached that places enough charge in the Pc film to saturate one of the potential gradients, further illumination will not increase the equilibrium photopotential developed there. The excess charge carriers can then lead to a large nonequilibrium expansion of the space-charge region at the opposite interface, as observed here. The experiment shown in Figure 4 for a +200-mV bias was repeated for biases ranging from about +800 to -600 mV vs. Ag/AgCl. The transients were very similar except for magnitude. In Figure 6,potential values a t 30 ps after the laser pulse are plotted vs. neutral density factor for each initial bias. Figure 6, therefore, represents a plot of the photopotential as a function of initial bias and intensity. For BS illumination at moderate positive bias potential the positive and then negative potential excursion with increasing light intensity (A in Figure 6 ) has already been attributed to the selective placement of charge at the Au/Pc interface. Increasing the initial bias potential decreased the size of this excursion (15 mV, D in Figure 6). The action of increasing the bias potential was to narrow this space-charge region at the back interface and to make the potential drop there a smaller fraction of the total potential drop across the Pc film. The intensity of light required to just saturate the photopotential mJ. If a carrier generation at the back interface was 22.0 X efficiency (from absorbed photons) of between 0.1 and 1.0% is
1126 The Journal of Physical Chemistry, Vol. 89, No. 7 , 1985
Figure 7. Band diagrams for the Au/GaPc-C1/Fe(CN-)2-se system at four bias potentials; -0.6,O.O (flat-band), +0.25 (equilibrium), and +0.80 V, measured vs. Ag/AgCl. Left-hand axis is the energy of an electron vs. vacuum, with the Fermi level of Au assumed constant at 4 . 8 eV. The top scale is for the conduction band, and the bottom scale is for the Fermi levels and the valence band.
assumed, 0.001-0.0001 pC of charge were created at this laser intensity; and this charge resulted in a 0.1-V photopotential a t 30 ps. From these values the capacitance at the Au/Pc interface can be calculated to be ca. 0.001-0.01 pF/cm2. This value is in good agreement with that calculated in the previous paper6 for the same interface from photocurrent transients. This value is not the same as that capacitance calculated earlier in this paper 30 pF, nor should the two be expected to be similar, since they involved different physical processes. The first capacitance calculated involved complete charge transfer a c r w the illuminated Pc film and the second involved only the placement of charge in the space-charge region formed at the Au/Pc interface in the dark. Moderate negative bias for BS illumination produced an initial positive excursion in potential, but, as the light intensity increased, the potential overshot its final value by as much as 70 mV (B in Figure 5). Even larger initial negative bias of the photoelectrode still produced the overshoot of magnitude 35 mV (C in Figure 6). These potential excursions are indicative of the presence of the other potential well formed near the Pc/solution interface under initial negative bias. As the BS light intensity is increased, the bands near the back interface are flattened and a positive photopotential is developed at the Pc/solution interface. The time duration of the experiment allows this to be observed selectively, and to obtain an indication of the initial size of the potential well within the Pc film. This effect is identical with the effect of a bias potential applied to a single Schottky barrier, in that the barrier height is increased or decreased according to the sign of the bias.33 The difference lies in that two barriers exist within this Pc film, in such a manner that the effect of a bias on one barrier is opposite to the effect of the same bias on the other barrier. The result is a decrease in the depth of the potential well as the bias is moved positive or negative of the equilibrium pote11tia1.l~ These BS potential excursions were used to estimate a band diagram as expected in the Pc film prior to illumination, at several different bias potentials. These band diagrams and the net potential drops within the Pc film dashed lines, potentials (vs. the Ag/AgCl reference) are shown in Figure 7 . Initial bias of the electrode at the equilibrium potential (ca. +0.25 V) and at +0.8 V produced the potential well at the back interface that was quantitated by the potential excursions A and D in Figure 6. The flat-band condition is near 0.0 V, but the bands within the Pc film are not truelyflat as shown by excursion B in Figure 5 . At the (33) Kao, K. C.; Huang, W. “Electrical Transport in Solids”;Pergamon Press: New York, 1981; pp 90.
Rieke and Armstrong apparent flat-band conditions, the band edges have the same relative electrochemical potential at both of the contacting interfaces. Initial negative bias of the photoelectrode produces the potential well at the Pc/solution interface which is documented for -0.6 V and by potential excursion C in Figure 6 . Figure 6 also shows the potential excursions observed for FS illumination. Selective placement of charge near the Pc/solution interfacefirst should produce potential changes in that region first. It is expected that the overshoot of potential for BS illumination should be reflected in a similar overshoot of potential for FS illumination, but at opposite extremes of laser intensity. Excursion A’ in Figure 5 is obtained when the light intensity was sufficient to flatten the bands near the Pc/solution interface and place charge at the Au/Pc interface. Excursions B’, C’, and D’ similarly reflect the excrusions that were seen for BS iluminations. While there was qualitative agreement between the two cases, the absolute magnitudes of the FS excursions was slightly less than for the BS case (A’, 25 mV; B’, 45 mV; C’, 25 mV; and D’, less than 1 mV). We attribute these differences primarily to differences in reflectivity of the Pc film, the Au/MPOTE, the solution, and the windows of the cell for the two different illumination directions. These differences probably resulted in lower photon intensities for FS as opposed to BS illumination. Charge transfer and decay processes of course begin immediately upon the application of the laser pulse. Even at 30 ps, some discharging of these capacitive elements has undoubtedly occurred. Therefore, the band diagrams of Figure 7 have underestimated the sizes of each interfacial behavior. The spacecharge regions could in fact be narrower and the potential wells deeper than actually shown. Conclusion It is apparent from these experiments and those presented in the previous paper6 that the band structure of the Au/GaPcC1/Fe(CN)63-q4-redox system is not as simple as previously imagined. It is also clear that this is likely to be the case with the interfacing of several photoconductor materials with conductive substrates (where the doping level is low, and the materials nearly intrinsic). Rose has pointed out the need to match the conduction band and valence band edges of the photoconductor to the Fermi levels of the contracting phases, to avoid precisely the problem outlined in these two paper^.^^,^^ Our current experiments seek to determine the extent to which this is possible through the use of other metallized polymer substrates and various redox couples. Implicit in this discussion has been the assumption that the photoelectrode consisted of a uniform Pc film rather than a single layer of high surface area microcrystallites. The role that microporous areas play and the effect of film thinning between crystallites are currently being explored. It is expected that pores and regions of thinning in the Pc film could contribute to sizeable differences in potential drop and interfacial capacitance when compared to regions of planar, smooth Pc film. In a pore or thinned area, the capacitance of the contacting phase may be high enough to “short circuit” the photoelectrochemical cell, leading to a significantly reduced, or even zero, photopotential. Consistent with this expectation, attempts to construct metal/GaPc-Cl/metal thin film assemblies have been unsuccessful because of facile electrical communication of the two metals in the small number of pores and thinned areas. In electrochemical systems, however, the extremely low mobility of charge carrying species in solution (as compared to electrons in a metal) allows a dilute solution of redox couple within a pore to become quickly depleted of charge-transfer capacity. This is likely to occur in the dark and most rapidly at the same areas of the Pc film which could act as a short circuit between the metal and solution. Control of the electrical properties is shifted to the flat tops of the crystallites where the redox couple can be replenished or is unaffected by the dark processes. In this respect, the surface roughness of the Pc film may have relatively little effect on the photopotential developed across the film. Registry No. Au, 7440-57-5; GaPc-C1, 19717-79-4.
