J . Phys. Chem. 1985,89, 11 16-1 121
1116
are untenable on thermochemical grounds since reaction 33 is endothermic by about 39 kcal m0l-l.4~ Searching for alternatives we propose here a branching reaction in the decomposition of C2H,+
also be postulated for the production of C2H2from C2H4,where H2 elimination has also been shown to 0ccur.4~ In fact, as noted earlier, hydrogen formation in the presence of hydrocarbon additives was observed in the present system, Table 11.
Reaction 35 is analogous to the vacuum UV photolysis of ethane, where molecular H2 elimination has been shown to be. the principal reaction channel.44 Processes similar to reactions 26 and 35 may
Acknowledgment. The authors express their gratitude to Professor K. Kuratani for making available the experimental facility and his kind support throughout this work. E.T.R. thanks the Natural Sciences and Engineering Research Council of Canada for financial support. Registry No. CF2C1CHzCl,1649-08-7; CFzCHC1, 359-10-4; C2H6, 74-84-0; C2H4, 74-85-1; Hz, 1333-74-0; DZ,7782-39-0.
~~
(42) K. J. Laidler, “Chemical Kinetics”, McGraw-Hill, New York, 1965. (43) S. W. Benson, ‘Thermochemical Kinetics”, 2nd ed,Wiley, New York, 1976. (44) H. Okabe and J. R. McNesby, J . Chem. Phys., 34, 668 (1961).
(45) H. Okabe and J. R. McNesby, J . Chem. Phys., 36, 601 (1962).
Evidence for Charge Trapping at the Gold/Chlorogalllum Phthalocyanine Interface Using Photocurrent Spectroscopy with One or Two Illumination Sources William J. Buttner, 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)
Photocurrent vs. wavelength spectra were reported for thin films of chlorogallium phthalocyanine (GaPc-C1) on optically transparent gold substrates in contact with aqueous hydroquinone solutions. Charge carriers were produced which, depending upon the absorptivity of the film and direction of illumination, were localized near either the Au interface or the solution interface. Under positive bias (hole harvesting at the solution interface), with illumination of the Au/Pc interface first, photons near the absorbance maximum in the visible range (600-750 nm) produced charge which was trapped at the Au/Pc interface. This trapping arose because of the formation of a potential well brought about by a mismatch in the Fermi levels of bare gold and the Pc film. The Fermi level of the Pc layer before contact with either phase is ca. 0.5 eV above the valence band edges, thus allowing the formation of space charge layers at both interfaces. The potential well at the Au/Pc interface extends ca. 250 nm into the 1-pm-thicknessfilms for bias potentials 200-300-mV positive of the flat-band potential (e.g., the equilibrium potential for H,Q/BQ couple). Carriers generated by the addition of a second CW light source attenueted the effects of the potential well at the Au/Pc interface and thus enhanced the photocurrent yields from the primary, modulated light source.
Introduction Thin films of chlorogallium phthallocyanine ( G a P A l ) vacuum deposited onto optically transparent metallized polymer gold electrodes (MPOTE) have been shown to be highly active photoconductors capable of positive or negative photopotentials with facile charge-transfer kinetics when contacting various aqueous redox couples.’ The absorbed light photocurrent quantum efficiency for these photoconductors was 2-lo%, indicating considerable trapping and recombination of charge occurred within the Pc film. We have shown that the photocurrent action spectrum for the Au/GaPc-C1 electrodes in excess of 0.3 pm thickness displayed different photocurrent spectral behavior when illuminated through the backside (BS, i.e., illumination at the metal/dye interface first) relative to frontside (FS, Le., illumination at the dye/electrolyte interface first).2 For GaPc-C1 films of sufficient thickness (ca. 1 pm), frontside illumination resulted in an action spectrum possessing a broad featureless current response from about 550 to 850 nm which closely resembles the absorption spectrum. Backside illumination raulted in a diminished response in the same wavelength region and displayed two maxima in the photocurrent response curve. Weakly absorbed light associated with t.he wings of the absorption spectra at ca. 550 and 820 nm was most efficient in generating photocurrent with BS illumination. These results have been documented previously for other Pc systems as well and were rationalized in terms of holes being the ‘Present address: Los Alamos National Laboratories.
0022-3654/85/2089-1116$01.50/0
most mobile photogenerated charge carrier.*q3 In order for the holes to be harvested by the redox active species in solution, they must be generated within the effective collection length ( I ) of the front surface. For frontside illumination, strongly absorbed light was most efficient in generating harvestable holes within a distance 1 of the front surface. In contrast, for backside illumination, only light which had a penetration depth on the order of the film thickness generated harvestable carriers. If a uniform electric field is assumed across the electrode, a quantitative expression can be derived which demonstrated that the maximal photocurrent for backside illumination should occur when the penetration depth of the light (1 /& where @ is the absorption coefficient) was equal to the thickness of the film (d), less one collection length (1/8 = d - I ) . Results shown here and in the following paper indicate that a uniform electric field cannot always be presumed in these Pc films. Recently we reported on a series of experiments in which the photocurrent yield from a modulated light source (probe) was significantly increased by using an auxillary, constant intensity illumination source (pump): an experimental technique first (1) Rieke, P. C.; Armstrong, N. R. J. Am. Chem. SOC.1984, 106, 47. (2) Rieke, P. C.; Linkous, C. L.; Armstrong, N. R. J . Phys. Chem. 1984, 88, 13.51. (3) Faulkner, L.; Tachikawa, H.; Fan, F.-R.; Fischer, S. G. In ‘Interfacial Photoprocesses”; Wrighton, M. S.,Ed.;American Chemical Society: Washington, DC, 1980; Adv. Chem. Ser. No. 184, pp 113-138. (4) Buttner, W.; Rieke, P. C.; Armstrong, N. R. J . Elecrrochem. SOC. 1984, 131, 691.
0 1985 American Chemical Society
Charge Trapping at the Au/GaPc-C1 Interface described by Heller and co-workers.sp6 In their reports and subsequent applications to inorganic semiconductor materials7?* the auxillary light source was found to create charge carriers which fill predominately interband trap sites and thus enhanced photocurrent yields for illumination by light greater than the bandgap energy. In contrast, we observed a pump-induced enhancement of photocurrent yields for GaPc-Cl for all probe wavelengths. The magnitude of the enhancement was, however, probe wavelength dependent. Photocurrents induced by strongly absorbed probe photons (600-750 nm) were enhanced preferentially by the added pump over photocurrents induced by probe light in the weakly absorbing regions of the absorbance spectrum. These results appeared to be consistent with the filling of trap sites in the bulk Pc film by the pump-produced charge carriers. Further experiments which are discussed below, however, have demonstrated the existence of an additional potential barrier for charge transport at the Au/Pc interface. One of the effects of the secondary light source, was to generate charges which lower that barrier. The existence of such a potential barrier can be expected whenever a semiconductor is brought into contact with two substrates such that both interfaces form electron-blocking junctions or both interfaces form hole-blocking junctions. When the photoconductor film is only lightly doped (i.e., the Fermi level is near the center of the bandgap) space charge layers a t both interfaces can extend appreciable distances into the Pc lqyer. Photocurrent spectra can be useful in identifying the presence of such a potential inversion layer at the metal/photoconductor interface, provided that it is possible to illuminate that interface first. Increasing the steady-state charge carrier population with an auxillary light source is also a useful way of quantitating the removal of the inverted layer through the addition of a large excess of charge in that region. Pulsed laser coulostatic experiments are presented in the following paper that provide a quantitative description of the band picture for the Au/GaPc-C1 system in contact with an aqueous redox couple, regardless of the direction of illumination or the initial bias potential.
Experimental Section The synthesis and purification of GaPc-C1 as well as the protocol for vapor deposition onto optically transparent gold electrods have been previously described.2 All photopotentials and photocurrents reported were qualitatively the same for all electrodes explored. The absolute magnitude of the photopotential and photocurrents varied slightly from one family of electrodes to another (generally seven were produced and characterized at one time). In the experiments described below, the electrolyte consisted of 5 mM hydroquinone (HzQ) and 0.2 M potassium hydrogen phthalate (KHP), pH 4, with all measurements performed under oxidizing potentials. These electrodes were mounted in a typical spectroelectrochemical cell which allowed illumination from two directions at once. The effective area of each electrode was approximately 0.3 cm2. The specific applied potential is given in either the figure caption or text. Photocurrent action spectra were recorded by using phase-sensitivedetection methods utilizing a lock-in amplifier (LIA, Ortec, Model 9503). The probe light source consisted of a mechanically chopped 450-W xenon arc lamp (Oriel) passing through a monochromator (Jobin-Yvon, Model H-20). All experiments were performed under potentimtat control (ECO, Model 551). Details of these measurements have been previously presented.2 In addition to phase-sensitive detection, the modulated photocurrent profile was monitored directly on a storage oscil(5) Heller, A,; Chang, K. C.; Miller, R. J. J. Electrochem. Soc. 1977, 124, 691. (6) 684. (7) 21 12. (8) 21 12. (9)
Heller, A.; Chang, K. C.; Miller, B. J. J . Am. Chem. SOC.1978, 110,
The Journal of Physical Chemistry, Vol. 89, No. 7 , 1985
PHOTOCURRENT 632nm
r
20nA
A
e
E=+SOOmv C
1117
PROF I LES
540nm I
50nA
En ' L D/-7
/L.=o
E = + 2OOmV
I
LIGHT ON LIGHT OFF
* 65ms
Figure 1. Photocurrent profiles induced by backside illumination of a GaPc-Cl/Au electrode with 540- (B and D) and 632-nm (A and C) light, modulated at 14 Hz, electrodes biased to the indicated potentials (relative to Ag/AgCl). The electrolyte consisted of 5 m M H2Q and 0.2 M KHP (pH 4). 4 is the dark current (10 nA at +200 mV and -50 nA at +500 mV). The square wave represents the illumination duty cycle.
liscope (Tektronix, Model 5441). Such direct detection of the photocurrent was feasible only with sufficiently light-active, nonporous electrodes which show nearly negligible dark current at the applied potential. In the experiments described below, the auxillary light source, generally a C W He-Ne laser, illuminated the photoconductor from the opposite direction of the probe source.
Results and Discussion Photocurrent Profiles and Action Spectra Using One Light Source. Photocurrent action spectra of photoconductors and semiconductors have usually been measured by directly monitoring dc currents vs. wavelength (e.g., see ref 10) or by r nitoring the ac photocurrent with a phase-sensitive detector m. hod such as a lock-in-amplifier (LIA) (e.g., see ref 2-4). Both of these techniques may incorrectly diagnose effects due to trapping of photogenerated charge at one of the interfaces. Honda and coworkers have shown that, at low light intensities, these effects can be significant." We therefore decided to investigate in detail the wavelength dependence on the photocurrent response. Figure 1 shows the photocurrent-time profiles for the GaPc-Cl electrode at two different potentials with BS illumination at two different wavelengths. At these bias potentials the expected current response was from the photoassisted oxidation of H,Q to BQ. For a bias of +200 mV, the only possible Faradaic response was the photoinduced oxidation of H2Q at the Pc/solution interface since there was no BQ present in solution. At either bias potential, with 540-nm illumination (Figure 1, B and D), the photocurrent responded in the expected anodic direction for a photoinduced oxidation to the on/off nature of the modulated light. For BS illumination, at 632 nm (near the absorbance maximum, Figure 1, A and C) the direction of the current response when the light was on was opposite of that expected, and that seen at 540 nm. At +0.2 V vs. Ag/AgCl which is slightly negative of the dark equilibrium potential but positive of the flat-band potential (0.OV Ag/AgCl), the photocurrent response was cathodic when the light was on and decayed away with an apparent time constant of ca. 20 ms. Upon termination of illumination, an anodic current was
McCann, J. F.; Skyllas-Kazacos, M. J . Electrochem. SOC.1983, 130, Gerischer, H.; Ltibke, M.; Bressel, B. J . Electrochem. SOC.1983, 130,
(10) Leempoel, P.; Fan, F. R. F.; Bard, A. J. J . Phys. Chem. 1983,87, 2948.
Gerischer, H. In "Physical Chemistry; An Advanced Treatise"; Eyring, H., Hendenon, D., Jost, W., Eds.; Academic Press: New York, 1970; p 463.
( 1 1) Minami, N.; Wdtonabe, J.; Fujishima, A.; Honda, K. Ber. Buweges. Phys. Chem. 1979,83,476.
Buttner et al.
1118 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
postulated a similar “Double barrier” for chlorophyl films on metal
substrate^.^^
B
Figure 2. Proposed energy level and band structure diagram for the Au/GaPc-CI/H2Q system for the isolated components (A) and at equilibrium (B). In C is a schematic representation of the excess charge density in Au (region a), Pc (regions b and c), and solution (region d) in the dark and during illumination ( I ) .
generated which was also transient and had a similar time constant for decay as that observed for the light-induced cathodic current. These current excursions were reminiscent of the charging and discharging of a capacitor. As the bias potential was increased to +0.5 V (Figure 1A) it can be seen that these transient photocurrent excursions were superimposed upon the expected Faradaic anodic photocurrent and dark decay. When illumination occurred from the frontside first, charge carriers were generated predominantly a t the Pc/electrolyte interface, and the expected rise and fall of the photocurrent with the on/off transition of the light source was observed a t any wavelength. The absorbance spectra for these PCfilms consisted of a broad peak between 550 and 850 nm. 632-nm light lies very near the absorbance maxima while 540-nm light lies on the shoulder of the absorbance spectrum. The penetration depths (1/,3) of 632and 540-nm light were calculated to be ca. 200 and ca. 900 nm, respectively. The film thickness was about loo0 nm. BS, 632-nm light localized carrier creation nearer the Au/Pc interface. Since the non-Faradaic or capacitive current occurred only for strongly absorbed, backside illumination, the Au/Pc interface must be responsible for the observed behavior. As the bias potential was increased to +0.5 V, a Faradaic current was added to the capacitive current. The effect of changing bias must selectively modify the potential drop at the Au/Pc interface, in addition to the net potential drop across the Pc film. These results may be explained if the substrate/photoconductor/electrolyte system is modeled as possessing an equilibrium band structure as indicated in Figure 2. Owing to an intrinsic mismatch in the Fermi level of the gold substrate (-4.8 eV) and phthalocyanine dye (-4.7 eV) a space charge region has been formed at the metal/dye interface. Voltammetric studies, with a variety of redox couples, have been used to determine the relative position of the Fermi level for gold in contact with the GaPc-C1 film (Le., the flat-band potential for this system).’ Redox couples P both positive and negative of that for the H,Q/BQ couple with J show photopotentials which are constant with a Fermi level of the GaPc-Cl film which is at least 0.5 to 0.7 eV above the valence band edge, prior to contact with the Au and electrolyte phases. This is an expected condition for all photoconductor materials which are only slightly p-type and therefore have an intrinsic Fermi level well within the bandgap region. The exact assignment of the Fermi level of the phthalocyanine film (as discussed below) was obtained from the results presented in the following paper.’* The aqueous redox couple H,Q/BQ also formed a space charge region with the Pc film and the result was a potential well within the film, near the Au/Pc interface. Precedence for this phenomenon can be found in the work of Tang and Albrecht who (12) Rieke, P. C.; Armstrong, N. R. J . Phys. Chem., following paper in this issue.
The charge density vs. distance across the Au/Pc/R,Ox assembly would arrange itself in the following regions (shown schematically in Figure 2C): (a) a compact layer of excess negative charge at the Au interface, (b) a diffuse layer of excess positive charge in the Pc film extending to the base of the potential well, (c) a region of excess positive charge extending to the Pc/solution interface, and (d) a region of net excess negative charge extending into solution. Upon BS illumination with strongly absorbed 632-nm light, contraction of the space charge region at the Au/Pc interface occurs. Holes created near the Au/Pc interface (region b) are driven toward that interface, requiring an increase in negative excess charge density in region a (shaded area). The photogenerated electrons are expected to migrate toward the bottom of the potential well and would also be able to participate in recombination events.I4J5 The net effect is to increase charge density nearer the Au/Pc interface compared to the dark condition. The photocurrent (Figure 1A) is largely capactive in nature and can be related to this movement of charge carriers. Whether charge exchange occurs across the Au/Pc interface under these conditions cannot be determined. Charge movement followed by high recombination rates reestablished a steady-state condition of no net current at these photon fluxes within ca. 20 ms. Since no net charge transfer had occurred under illumination, in order to maintain charge neutrality within the film in the dark cycle, the excess charges a t the Au/Pc interface relaxed back to the initial state, creating an oxidative current transient. In contrast, moderately absorbed light (540 nm) was not significantly attenuated in the Pc Film by BS illumination. A more uniform charge distribution was generated throughout the film (both regions b and c), and those carriers which were generated within one collection length ( I ) of the front surface were harvested, resulting in the expected anodic photocurrent.2 A fraction of the carriers which were generated within the back surface space charge region (region b) by 540-nm light were still trapped and dissipated. These carriers would be a smaller fraction of the total charge generated than those generated with 632-nm BS illumination. The observed photocurrent response at 540 nm and all other wavelengths was therefore the sum of a capacitive current for those carriers generated within the back surface space charge layer, and an anodic Faradic current for those carriers generated within one collection length of the front surface. Even for 632-nm, BS illumination, some of the expected Faradaic current could be obtained by increasing the bias potential (Figure 1A). Increasing the bias potential will increase the electric field strength at every point within the Pc film. This would have the effect of narrowing the space charge region at the Au/Pc interface, providing that no change in charge density of the Pc film occurs in the dark (dark injection of charge carriers). The dark current/voltage behavior of these films shows them to be quite insulating.’ We therefore believe that a t higher bias potentials, a smaller fraction of charge is trapped at the Au/Pc interface and is instead harvested at the Pc/solution interface. Because the transport time of charge carriers goes down with increased electric field strength within the film, the Faradaic photocurrent yield is also expected to increase. In previous studies we have hypothesized that the initial Fermi levels for the Au electrode Pc film and solutions containing H,Q/BQ or Fe(CN),*?” were arranged in descending order, such that the Pc Fermi level lay between the Au and redox couple Fermi levels.’ If such were the case, then the potential drop within the Pc film at +0.2 V vs. Ag/AgCl would be expected to be uniform and positive in direction at every location within the Pc film. Previous action spectra reported have been explained in terms of this model.Z3 The two maxima in the wings of the BS photoaction spectra have been ascribed solely to bulk recombination and C. W.; Albrecht, A. C. J . Chem. Phys. 1975,62,2139. (14) Pope, M.J . Chem. Phys. 1962, 36, 2810. (1 5) Rose, A. “Concepts in Photoconductivity and Allied Phenomena”; Krieggs: Huntington, NY,1978. (13) Tang,
The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
Charge Trapping at the Au/GaPc-C1 Interface
1119
PHOTOCURRENT PROFILES
PHOTOCURRENT RESPONSE
B
A c
C
O m W PHOTOPUMP 012mW PHOTOPUMP 12mW PHOTOPUMP
r 50nA
I20nA
1200nA
n A I
0.0 LIGHTON LIGHTOFF
i
.
400
:
.
500
.
600
:
.
700
:
.
800
:
I
c
900
WAVELENGTH [ n m l Figure 3. Backside photocurrent action spectra measured as the output
of a LIA (14-Hz modulation). The phase angle was adjusted to obtain optimal photocurrent response at 540 nm. The bias potential was +200 (A) or +600 mV (B) (relative to Ag/AgCI). The electrolyte consisted of 5 mM H2Q and 0.2 M KHP (pH 4). Note that trace B is attenuated by a factor of 5. trapping of the charge carriers produced from strongly absorbed photons.2 From the experiments described here we now understand that this is only one of the trapping mechanisms which limits conversion efficiency for charge created near the Au interface. Original conceptions of the collection length (I) using weak, modulated probe light must be modified to reflect that this parameter is actually a combination of the effects of bulk trapping and trapping of charge at the Au/Pc interface (see below).2 The effect of increasing bias potential on the corrected photocurrent action spectra is shown in Figure 3, A and B. In these experiments the photocurrent was recorded as the output of the LIA in which the phase angle was adjusted to the photocurrent observed for 540-nm illumination (net photocurrent at a maxima). At moderate potentials (+0.2 V of Em vs. Ag/AgCl, Figure 3A), the photocurrent action spectrum was qualitatively similar to those previously r e p ~ r t e dbut ~ , ~now showed both anodic and cathodic photocurrents. By referencing the phase adjustment of the LIA to the photocurrent response at 540 rim, the observed BS photocurrent in the region, ca. 600 to 800 nm, was of opposite sign to that observed at other wavelengths. This was caused by the dominance of the capacitive photocurrent in that region. As the applied potential was made increasingly positive (+0.6 V, Figure 3B), the Faradaic photocurrent response increased at all wavelengths. The photocurrent action spectrum obtained at bias potentials near +0.2 V provided an estimate of the size of the space charge region at the Au/Pc interface. It was found that a wavelength which generated zero net photocurrent (Le. the Faradaic and capacitive currents cancel) was near 610 nm. At this wavelength, the number of harvestable carriers must be approximately equal to the number captured at the Au/Pc interface. The penetration depth of 610-nm light ( l o a = 250 nm) should therefore be approximately equal to the length of that space charge layer. This value was estimated from absorption spectra of thin films of GaPc-CI, where the absorptivity at 610 nm was ca. 4 X lo4 cm-'. As the bias potential was increased, the wavelengths where this transition occurs moved closer toward the absorption maximum of thin films of GaPc-C1, where the 1 values are smaller and where the width of the potential well at the back interace was narrowed. Photocurrent Profiles and Action Spectra Using Two Light Sources. These charge trapping effects could be modified or removed by using an unmodulated light source, the pump, which illuminated the film in addition to, and from an opposite direction to, the modulated light source. The effect on the photocurrent/time profiles using BS, strongly absorbed, modulated light
/a
I --L
=o
65ins Figure 4. Photocurrent profiles induced by backside illumination of a
GaPc-Cl electrode with low-intensity 632-nm light (ca. 10 pW) modulated at 14 Hz. The applied potential was +200 mV (relative to Ag/ AgCI): Curve A, no additional light source; curve B, additional 0.12-mW constant frontside illumination within 632.8-nm HeNe laser; curve C, 1.2" frontside incident with 632.8-nm HeNe laser. In all cases, the horizontal line indicated as id represents the level of dark current (,- 10 nA). In curves B and C the horizontal line below the modulated photocurrent is the dc photocurrent level induced by the auxillary light source. Note the differing scales for curves A, B, and C. The square wave represents the illumination duty cycle.
PHOTOCURRENT RESPONSE ?PHOTOPUMP (4mW. 632.8nm)
3
-i c z
LLI
U
a 0.0 3 0
2
8 Q
0.0
*1
- - - -- - - - - - - - 400
500
600
----
700
800
900
WRVELENGTH [ n m l Figure 5. Photocurrent action spectra as measured by the output of a LIA (14-Hz modulation) with the phase angle adjusted at 540 nm illumination for backside (A and B) and frontside (C and D) illumination in the presence (B and D) and absence (A and C) of constant intensity illumination by a 4-mW HeNe laser (632.8 nm). The auxillary light source was incident on the photoelectrode in the opposite direction of the probe. The applied potential was +300 mV (Ag/AgCl). Note that trace B is attenuated by a factor of 2.
was pronounced, as shown in Figure 4. As the intensity of the auxillary FS light source was increased from 0 to 0.12 mW to 1.2 mW, the relative contribution of the capacitive charging/ discharging nature of the BS modulated photocurrent induced by 632-nm light was decreased and the photocurrent became predominantly Faradaic in nature. There was also a measurable dc photocurrent produced by the pump FS illumination, but in
1120 The Journal of Physical Chemistry, Vof.89, No. 7, 1985 P -IOTOC UR R E Y T PROF I L E S 1 0 mW
15mK
0.06m W
015m W
0015mW
015mW
Figure 6. Photocurrent profiles induced by BS modulated 632-nm probe light source at powers of 1.0 (A), 1.5 (B), 0.06 (C), 0.15 (D and F), and 0.015 mW (E) at bias potentials of +200 (A and B), +400 (C and D), and +600 mV (E and F) measured relative to Ag/AgCl. The horizontal lines represents the zero current level (indicated i = 0) and the dark current level id.
utilizing the LIA, the current induced by the modulated probe light source was completely distinguished from that current. Figure 5 shows further the effect of addition of the constant light source on the intensity action spectra by varying intensities of the 632-nm pump. The photocurrent responses were explored for both BS illumination, with and without the photopump (A and B), and FS illumination, with and without photopump (C and D). As with Figure 3, the phase angle of the LIA was adjusted for maximal response at 540 nm. Figure SA shows the same photocurrent response as before (Figure 3A). Upon application of the photopump (Figure SB), the magnitude of the photocurrent was enhanced at all wavelengths, with maximal enhancement at wavelengths having the shortest penetration depth (600-800 nm). This was consistent with the ability of the photoprcduced charge from the pump light source to flatten (saturate) the potential well at the Au/Pc interface, thus allowing some of the otherwise captured holes to be collected at the Pc/electrolyte interface. Note that this effect was so pronounced for the pump intensity used (4 mW) that the current scale for Figure 5B has been attentuated by a factor of 2. When the probe illumination was incident from FS and the pump was from the BS, the action spectra of Figure 5 , C and D, result. Enhancement of the modulated photocurrent was seen at all wavelengths, but the effect produced only about a 25% increase in overall photocurrent yield. Capacitance of the AulPc Interface. The effect on carrier migration due to the inversion layer at the Au/Pc interface is further documented in Figure 6. At bias potentials of +0.2 V vs. Ag/AgCl the cathodic charging currents represented a decreasing fraction of the photocharge p r o d u d , as the power from a BS,modulated probe (632.8 nm, He-Ne laser) was increased from 1 to 1.5 mW. At a bias potential of +0.4 V a lower incident laser power was needed (0.15 mW) to virtually remove the capacitive current from the photocurrent/time transient. At +0.6 V the photocurrent profile assumes nearly normal appearance with less than +0.15 m W of pump power. With BS modulated illumination only, the size of the capacitance for this space charge layer a t the Au/Pc interface was estimated. At a bias potential of +0.2 V (Ag/AgCl) integration of the current time responses during the charging event showed C/cm2 which corresponds to ca. that the charge was 8 X 4 X lo9 holes/cm2. This measured charge density was independent of incident light intensity providing that the incident photon flux was of sufficient intensity to saturate the exam charge at the back surface during the illumination period. This interfacial charge
Buttner et ai. density was seen to decrease at bias potentials of +0.4 and +0.6 V to 7 X lo8 and 4 X lo8 holes/cm2, respectively. This decrease is consistent with the decreased size of this region as the positive bias to the whole electrode assembly was increased. It is clear that the size of this interfacial capacitance is small, and perhaps insignificant, in terms of energy-loss pathways that might be present during high-intensity, constant illumination, in energy conversion applications. In applications of these types of materials where the light intensities are lower and transient however (such as electrophotographic processes), these effects could be significant in determining the efficiency of charge separation. These photocurrent measurements provide a convenient means of estimating the quantum efficiency of photocurrent generation (4). At a low incident light intensity (- 15 gW) the backside depletion layer does not charge completely during illumination. Integration of the light-induced cathodic charging current was found to correspond to 3.5 X lo9 C or 2 X lo9 holes. The total number of incident, BS 632.8-nm photons for the 30-ms illumination duty cycle was 1.4 X 10". Dividing these two numbers the incident light quantum efficiency would be ca. 1.0%. Since there was some attentuation of light intensity as it passes through the Au-MPOTE substrate, 1.0% represents a lower limit for 4, which may be larger by factors of 2-4. It is interesting to note that this efficiency is consistent with absorbed light quantum efficiencies for photoelectrochemical processes reported earlier, which were measured for steady-state currents under much higher illumination intensities, where capacitive effects were less pronounced.1-2 Photocurrent Yeilds us. Light Intensity. The photoelectrochemical activity of similar Au/GaPc-Cl rotating disk electrodes has previously been reported using polychromatic illumination' and has been reexamined in these studies. The photocurrent at high light intensities (ca. 100 mW/cmz polychromatic) was found to be proortional to the intensity raised to the 0.7 power (iPhm P.7)for frontside illumination. A power dependence of photocurrent on light intensity lying between 0.5 and 1.0 has been explained by Rose1Sto be due to an exponential distribution of bandgap states, where the density of states increases as the energy levels approach the band edges. An energy level, the steady-state Fermi level (SSFL), is defined for each carrier, in which the probabilities for thermal exitation (detrapping) and deexitation (recombination) are equal. States which lie between the SSFL for both holes and electrons are recombination centers and those states which lie between the band edges and the SSFL's closer to the band edges. If an exponential distribution of states within the bandgap is present, the density of recombination sites will increase faster than the density of free charge carriers. The result will be an increasing probability of recombination with intensity and as a consequence a power dependence less than one. The enhancement of the modulated photocurrent by the constant intensity photopump as seen for the very low light intensities used here suggests that a power dependence on light intensity of greater than one should be observed. As the intensity of light increases, the potential well at the Au/Pc interface is minimized. Carriers captures in this well have a larger probability of undergoing recombination, and thus, as the well depth diminishes with increasing photopump intensity, the probability that a carrier will transit the film increases. The net result is a photocurrent vs. light intensity which has an exponent of greater than one. Further experiments were conducted to explore the difference between sublinear and supralinear photocurrent vs. light intensities. The difference between the two previously observed behaviors appears to result only from differences in illumination intensities and illumination directions. Using BS illumination (632.8 nm) over a range from 15 pW to 15 mW, at stationary electrodes, we have been able to observe steady-state photocurrents with both a supralinear (slope = 1.2) light intensity dependence (below ca. 0.15 mW), and a sublinear light intensity dependence (slope = 0.7) at higher photon fluxes. At low light intensities, as for most of the experiments reported here, trapping of charge at the Au/Pc interface dominates the energy loss pathways. At higher light intensities, this effect becomes insignificant and the sublinear light
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
