J . Phys. Chem. 1987,91, 5651-5659 favor the formation of extremely small crystallites, with a high surface-to-volume ratio, it is reasonable to expect that these may be so highly doped as to be nearly degenerate semiconductors. The properties of the TiOPc and VOPc films explored so far have given rise to good rectifying behavior in the dark, sizable photopotentials for a variety of redox couples, and some of the highest photoelectrochemical efficiencies reported for Pc thin films. It is clear, however, that further improvements will require more careful control of the thin-film composition and morphology, as can be achieved by deposition in UHV environments. Recent microcircuit photoconductivity experiments in UHV show that postannealing of the TiOPc films can cause a transition in visible absorbance and photocurrent spectra from that of Figure l a to that of Figure lb.” Similar transitions have been previously noted for VOPc films3 Preliminary studies indicate a sizable increase in photoconductivity accompanying this transition. The molecular nature of the chemisorbed oxygen in these tetravalent metal Pc films is still not clear. Added oxygen forms appear with binding energies suggesting bonding to the central metal and additional forms of oxygen in less electronegative environments. The other higher binding energy forms may represent oxygen as found in OH-like environments. Some caution must be used in the interpretation of these XPS data. The sensitivity of this surface spectroscopy is generally considered to be at the part-per-thousand level for elements such as oxygen. XPS-detectable forms of oxygen reported in these experiments are above that level. These new forms of oxygen are found, however, after rather extreme treatment procedures in VOPc, or at low TiOPc coverages on the Au-MPOTE, and may be representative of secondary species which arise from the reaction of O2 with the (24) (a) Fan, F.-R. F.;Faulkner, L. R. J. Chem. Phys. 1978,69,3341. (b) Fan, F.-R.F.; Faulkner, L. R. J. Am. Chem. SOC.1979, 101,4719. (25) Larsson, R.; Folkesson, B.; Shon, G. Chem. Scr. 1973, 3, 88.
5651
Pc film, but which may not directly affect the conductivity or photoconductivity. The fact that the UHV-deposited Pc/AuMPOTE films give nearly identical photoresponses to those shown in Figure 2 lends support to that hypothesis. Dopants at partper-million concentration levels can affect the electronic properties of semiconductors and may not be detected by many spectroscopic probes. Hydrogen-doping experiments described in the following paper appear to support this hypothesis.’ For 02-doped trivalent-metal Pc films, treatment in atmospheric pressure hydrogen at 140 OC for up to 48 h causes a transition in photoelectrochemical behavior from p-type to a behavior more consistent with a lightly doped semiconductor thin film. Similar treatments of VOPc and TiOPc thin films, which exhibited p-type character as above, showed no change after this same hydrogen treatment. The dopant incorporated during vacuum deposition is not removed from these materials by reductants such as hydrogen. One possibility currently being explored is that the oxovanadium or oxotitanium cation, left as a result of demetalation during deposition, may act as an irreversible dopant. Studies detailed in the subsequent paper show that when the oxygen affinity of the Pc is lower than is apparent for VOPc and TiOPc, oxygen levels can be controlled, which cause significant changes in the photoelectrochemical response. Simultaneous microcircuit conductivity, microgravimetry, and surface spectroscopic experiments are under way to further elucidate the molecular nature of dopant species that affect electronic properties of extremely thin (1-50 monolayers) Pc films. Acknowledgment. Support from the National Science Foundation and from the Materials Characterization Program, State of Arizona, is gratefully acknowledged. Registry No. TiOPc, 26201-32-1; VOPc, 13930-88-6; Fe(o-ph),2+, 14708-99-7; K3Fe(CN)6, 13746-66-2; Au, 7440-57-5; 02,7782-44-7; anthraquinonesulfonate, 30637-95-7.
Spectroscopic and Photoelectrochemical Studies of Trivalent Phthalocyanine Thin Films. The Role of Gaseous Dopants (0, and H,) in Determining Photoelectrochemical Response T. J. Klofta, T. D. Sims, J. W. Pankow, J. Danziger, K. W. Nebesny, and N. R. Armstrong* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: February 19, 1987)
Thin films of vacuum-deposited chlorogallium phthalocyanine (GaPc-C1) and other trivalent-metal Pc’s show widely variable photoelectrochemical properties depending upon vacuum deposition conditions and postdeposition doping with either O2or H1 Combinationof scanning electron microscopy (SEM), infrared spectroscopy (ATR-RIR), visible absorbance spectroscopies, and X-ray diffraction indicates that typical GaPc-C1 films consist of a mixture of at least two different phases, with different proportions of each depending upon growth conditions. High-temperature O2doping of GaPc-C1 can turn a film that behaves like a lightly doped semiconductor into a p-type material; H2 doping can reverse the effect. ESR experiments indicate the presence of high concentrations of radical species (ca. 10’’ cm-,) whose populations can be increased or decreased with O2 or H2doping, respectively. Photocurrent yield spectra of lightly doped or p-type GaPc-C1 films confirm that photocurrents are limited by hole transport in the first instance and electron transport in the second instant.
Introduction Most previous studies of phthalocyanines (pc) for photovoltaic and photoelectrochemical use have generally concluded that Pc thin films are highly doped, p-type semiconductors.’-2 Our results with tetravalent-metal PC’S such as vanadyl phthalocyanine
* Author to whom correspondence should be addressed.
(VOPc) and titanyl phthalocyanine (TiOPc) are consistent with those findings*3 As shown in the preceding paper, these (1) (a) Simon, J.; Andre, J.-J. Molecular Semiconductors; SpringerVerlag: New York, 1985; pp 101, 102, 112-121, and references cited therein. (b) Martin, M.; Andre, J.-J.; Simon, J. J . Appl. Phys. 1983, 54, 2792. (c) Gutman, F.; Lyons, L. E. Organic Semiconductors; Wiley: New York, 1967. (d) Sussman, A. J . Appl. Phys. 1967, 38, 2738, 2748.
0022-3654/87/2091-5651$01.50/00 1987 American Chemical Society
5652 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
films demonstrate good rectification properties and high contrast between photocurrent and dark currents. Certain trivalent-metal Pc's, however, such as GaPc-C1, can show photoelectrochemical behavior more consistent with a lightly doped semiconductor thin film.435 Positive and negative photovoltages can be obtained across the lightly doped versions of these thin films, controlled by the sign and magnitude of the difference in work function of the two contacting phases. The electrical contact between the lightly doped trivalent-metal Pc layer and a gold thin-film substrate is not necessarily ohmic, in contrast to conclusions from previous studies of Pc films on Recently our attention has been drawn to the differences in photoelectrochemical properties that manifest themselves with variations in vacuum deposition conditions for GaPc-C1 and InPc-C1 films.4~~ GaPc-C1 films, grown at rates of deposition of less than 1 equivalent monolayer/min, can be produced in a state that produces behavior consistent with low concentrations of dopant, with thicknesses ranging from ca. 0.8 to 1.3 pm. We have noticed, however, the possibility for formation of GaPc-C1 films that behave more like highly doped ptype semiconductors, through changes in growth rate and substrate ~ o n d i t i o n .In ~ this paper we summarize several recent spectroscopic, electrochemical, and photoelectrochemical studies that can be correlated to show how surface morphology, surface composition, and small molecule dopants may control the electronic properties of the trivalent-metal Pc's. The transformation of the lightly doped, trivalent-metal Pc thin films into ptype semiconductors, with apparent ohmic contact to gold, can be reversible through O2 and H2 doping.
Experimental Section The experimental details of this work have been explained in previous publication^.^" Synthesis and purification of GaPc-C1 was carried out with GaC1, and o-phthalonitrile as the starting materials. The solid product (formed in a benzene/nitrobenzene solvent) was isolated, washed with ethanol, and repeatedly vacuum sublimed to achieve acceptable purity. The sublimed powder still contained measurable free radical states as discussed below, which may have an influence on the electronic and photoelectrochemical properties. Vacuum deposition of the GaPc-C1 film was carried out in either of two chambers. The first system was a diffusion-pumped (10" Torr base pressure) glass chamber where the substrate electrodes were placed on an aluminum mask directly above a small charge of GaPc-C1. For large crystallite films the Pc charge was heated to ca. 200 O C to cause a slow deposition, the substrate temperature rose to ca. 100 OC, (up to eight electrodes were prepared simultaneously by deposition of the Pc through a mask), and the deposition rates were such that it took 48-96 h to produce a Pc film of 1.O-pm thickness. Smaller crystallite GaPc-C1 films were produced with faster growth rates (ca. 4 h to produce a 0.5-pm film) and lower substrate temperatures (ca. 65 "C), as for VOPc and TiOPc films.3 The electrode substrates for the Pc films were primarily thin gold films (ca. 300 A) on a smooth polyester substrate (Au-MPOTE), which allowed for illumination from two different directions. Electrolytes and redox couples were recrystallized to achieve acceptable purity or were used as reag~~~~~
( 2 ) (a) Giraudeau, A,; Fan, F.-R. F.; Bard, A. J. J. Am. Chem. SOC.1980, 102, 5137. (b) Jaeger, C . D.; Fan, F.-R. F.; Bard, A. J. J . Am. Chem. SOC. 1980,102,2592. (c) Tachikawa, H.; Faulkner, L. R. J . Am. Chem. SIX.1978, ZOO, 4379. (d) Fan, F.-R. F.; Faulkner, L. R. J. Chem. Phys. 1978,69, 3341. ( e ) Fan, F.-R. F.; Faulkner, L. R. J . Am. Chem. SOC.1979, 101, 4779. (f) Leempoel, P.; Fan, F.-R. F.; Bard, A. J. J . Phys. Chem. 1983, 87, 2948. ( 3 ) (a) Klofta, T. J.; Linkous, C. L.; Armstrong, N. R. J . Electroanal. Chem. Interfacial Chem. 1985, 185, 73. (b) Klofta, T. J.; Danziger, J.; Lee, P.; Pankow, J.; Nebesny, K. W.; Armstrong, N. R. J . Phys. Chem., accompanying paper in this issue. ( 4 ) Klofta, T. J.; Buttner, W. F.; Armstrong, N. R. J. Electrochem. SOC. 1986, 133, 1 5 3 1 . ( 5 ) Klofta, T. J. Ph.D. Dissertation, University of Arizona, 1986. (6) (a) Rieke, P. C.; Armstrong, N. R. J . Phys. Chem. 1985,89, 1121. (b) Buttner, W. F.; Rieke, P. C.; Armstrong, N. R. J. Phys. Chem. 1985,89, 1 1 16. (c) Rieke, P. C.; Linkous, C. L.; Armstrong, N. R. J . Phys. Chem. 1984,88, 1351.
Klofta et al. ent-grade materials where possible. Water used in the electrochemical studies was deionized and doubly distilled from permanganate. Photoelectrochemical studies were conducted as described elsewhere.',* All Pc electrodes were mounted in a spectroelectrochemical cell that provides for illumination of the electrode/electrolyte interface first (FSillumination) or the metal/Pc interface first (BS illumination). Electrolytes were deoxygenated before all electrochemical studies. Photocurrent action spectra were obtained by monochromatizing the output of the xenon arc lamp used for all studies (450 W). The lamp was modulated at low frequencies (13 Hz) and the current-to-voltage output of the potentiostat demodulated with a lock-in amplifier (LIA). Currents are reported in absolute values obtained from calibration of the LIA response. Photovoltammetric studies were conducted by using the filtered output of the arc lamp (water filter plus IR glass filter plus long-pass filters, to give a 470-900-nm response. Some of the Pc films for surface analysis studies were grown in an ultra-high-vacuum (UHV) vessel with a base pressure of ca. Torr. The freshly purified Pc was loaded into a specially designed effusion cell, the entire system brought to the base pressure by means of pumping and bakeout cycles, and the substrate (Au foils or Au-MPOTE electrodes, see below) positioned within 2-3 cm of the tip of the effusion cell. The substrate temperature rose to ca. 35-40 O C during the deposition process, and the growth rates were ca. 5 monolayers/min. The samples could be transferred at any time from this vessel under UHV, into the spectrometer, for surface analysis. Low-pressure exposure of the samples to O2was conducted in another sample chamber coupled to the surface spectrometer. Samples were occasionally removed from UHV and exposed to pure O2or atmosphere in the sample entry airlock (atmospheric pressure exposures). Some samples for surface analysis were prepared in the diffusion-pumped vacuum system described above (exactly as for those studied photoelectrochemically) and transferred in atmosphere or under argon to the UHV surface spectrometer. XPS/UPS studies were conducted by using a VG ESCALAB MKII spectrometer with an analyzer base pressure of ca. 5 X Torr. XPS spectra were obtained with primarily A1 K a radiation in the constant analyzer energy mode (CAE) with a pass energy hE of 20 V. The data was subsequently corrected for analyzer transmission function and secondary electron background and was curve fit to resolve overlapping peaks by using software developed in this laboratory (see ref 10 in the preceding paper). For UPS spectra of low-coverage Pc films the data were corrected for Au valence-band spectral contributions by normalizing the bare Au UPS spectrum to the intensity of the Au Fermi edge in the Pccoated Au spectrum and subtracting the normalized Au spectrum from the UPS spectrum of the Au/Pc sample. This does not provide complete removal of Au contributions to the Pc UPS spectrum, but adequately defined spectral features do result. Results and Discussion Infrared and Visible Spectroscopic Studies-Correlation with Variations in Morphology As Seen by SEM. Figures la-c and 2 show the ATR-IR spectra of the morphological extremes of the GaPc-Cl films we have explored to date. The IR spectra correlate to the SEM's shown in Figure Id-f. Figure Id shows a GaPc-C1 thin-film electrode like those whose photoelectrochemical properties we have documented previously.' This type of electrode consists of primarily blocklike crystals as seen by SEM, is nonporous, and yields sizable photopotentials and photocurrents. The size of the average crystallite varies depending upon deposition rate and duration of deposition. Electrodes with widely variable photoelectrochemical properties can be p r o d ~ c e d(see ~ , ~ below). By extending the growth time of films like those in Figure Id and lowering the sublimation temperature slightly, a GaPc-C1 phase characterized by platelets begins to form in addition to the (7) (a) Rieke, P. C.; Armstrong, N. R. J . Am. Chem. SOC.1984, 106,47. (b) Buttner, W. F.; Rieke, P. C.; Armstrong, N. R. J . Am. Chem. SOC.1985, 107, 3738.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5653
Trivalent Phthalocyanine Thin Films CHLOROGALLIUM PHTHALOCYANINE
Y.AVENUNBERS
CM-
(a> CHLOROGALLIW PHTHALOCYMINE
c
sM3
760
780
740
720 ~
YAVENUMBERS
800
780
760
CM-
7UO
YAVENUMBERS
720
7
CII-
Figure 1. ATR-FTIR spectra of the C-H out-of-planeband region and S E M s of three representative types of GaPc-Cl/Au-MPOTE film electrode. (a and d), (b and e), and (c and f) correspond to the same film. (See text for explanation.)
associated with the CpFlle-Nai bonds.8 Thsassignment for the region around 754 cm-' has not been made but from chlorinesubstitution experiments is known not to be associated with the ( Y ~ modes8 - ~ The spectrum of Figure l a and the SEM of Figure Id show that there is one crystallite phase that is dominant, with a C Y C - ~at 732 cm-'. As the plateletlike crystallites form, up to two new bands are clearly defined as 725 and 729 cm-'. These same three f h were also examined in the spectral region between 300 and 500 cm-' (Figure 2). In addition to some metaldependent modes associated with the isoindole ring (340-355 cm-'), we were able to assign a peak at 431 cm-' as the Ga-Cl stretch, by inference to infrared data on other halogenated, trivalent-metal phtha1ocyanines?*loa For the blocklike crystallites of Figure Id a single band at 431 cm-' was observed, whereas for the mixed block/platelet film of Figure lc,e, bands at 431 and 438 cm-' were observed. As discussed elsewhere,1° visible absorbance spectra were recorded for these same films even though the thickness was such that absorbance values in excess of 5.0 were common in the wavelength region from 600 to 800 nm. Through the use of a CCD-camera detector, coupled with a double monochromator, single beam absorbance spectra were recorded in the wavelength region from 400 to 900 nm.lob For the GaPc-C1 films like those in Figure la, the absorbance spectrum showed a dominant band at 605 nm with an extended shoulder from 605 to 900 nm. For the film in Figure lb,c, the absorbance spectrum was broadened to form two bands at 665 and 760 nm and had the spectral shape most often associated with thinner GaPc-C1 films examined in this group in the p a ~ t . ~The * ~films of Figure l a were typical in that they contained few of the platelet forms seen in Figure lb,c. Most vacuum-deposited GaP-C1 films are composed of combinations of the two morphologies, with the blocklike crystallites being dominant. Resolution of these two phases by photocurrent action spectroscopies has not been possible, as has been the case for TiOPc and VOPc films.3 X-ray crystallographic structures have been determined for GaPc-Cl single crystals as well as several other halogenated trivalent-metal Pc's and the tetravalent metal Pc's.' These studies, in addition to the results presented here, have pointed to the possibility for at least two types of structures to coexist in these thin films. One possible structure is a head-to-tail coupling of the adjacent Pc's through electrostatic interactions along the metal-counterion bond. The other structure involves at least one interleaved arrangement of the Pc's with a head-to-head tail-to-tail . arrangement of the Pc's, slipped horizontally with respect to each other. Where there is UV-visible spectroscopy to correlate with these structures, it has been proposed that one or more of the slipped-stack arrangements is responsible for the red-shifted visible absorbance of these materials.'lb Further details of these spectroscopic studies will be presented elsewhere; however, the following summary can be made: (a) We assume that a head-to-tail linear stack will give rise ~ and lower energy Ga-Cl stretching to higher energy C Y C -modes modes than for any of the slipped-stack arrangements. (b) From IR, SEM, and X-ray diffraction analyses of many of the GaPc-C1 films studied and reported to date, the blocklike crystallite phase (which shows the highest energy C Y C - ~and lowest energy Ga-Cl stretching vibrations) appears to be dominant under most vacuum deposition conditions employed so far, even though the percentage of the other phase may be appreciable (10-20%). (c) The visible absorbance spectra suggest that the blocklike crystallites result (8) (a) Kendall, D. N. Anal. Chem. 1953,25,382. (b) Kotlyar, I. P. Opt. Spektrosk. 1961, ZZ, 92. (c) Ogorodnik, K. Z. Opt. Spektrosk. 1975,39,223. (d) Debe, M. K. J. Appl. Phys. 1984,55,3354. (9) Linsky, J. P.; Paul, T. R.; Nohr, R. S.; Kenney, M. E. Znorg. Chem.
blocklike crystallites (Figure le,f). For the most advanced stage 1980,19,3131. of this platelet growth the photoelectrochemical activity is dete(10) Sims, T. D. M. S. Thesis,Universityof Arizona, 1987: (b) Epperson, riorated from that previously reported' and that shown below. P.; Denton, M. B.; manuscript in preparation. (1 1) (a) Wynne, K. J. Znorg. Chem. 1984,23,4658. (b) Griffiths, G. H.; In the IR spectra the region around 725 cm-' has been preM. S.; Goldstein, P. Mol. Cryst. Liq. Cryst. 1976,33, 149. (c) Ziolo, viously assigned to the C-H out-of-plane bending mode ( ( Y ~ - ~ ) Walker, R. F.; Griffiths, C. H.; Troup., J. M. J. Chem. SOC., Dalton Tram. 1980,2300. and is a sensitive indicator of the crystallographic phase for (d) Hiller, W.; StrSihle, J.; Kohl, W.; Hanack, M. 2. Kristallogr. 1982, 159, phthalocyanines.8 The spectral region around 780 cm-' has been 173. ~
5654 The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
Klofta et al.
-
& % . ,C ~ 6 r P c BLOCK PHASE
PUTELET
AND BLOCK I
1 438
I
450 400 350 YAVENURBERS (CR-1
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450
350
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YAVENURBERS
500
#I
450
(CR')
350
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Figure 2. ATR-FTIR spectra of the Ga-C1 stretching frequency region for the same GaPc-C1 films in Figure 1.
from a cofacially stacked GaPc-C1 phase and that the slipped-stack conformations result in the plateletlike phase seen in the SEM. (d) The gas and heat treatments discussed below have little detectable effect on changing the gross morphology of the GaPc-C1 films we have studied, as indicated by S E M and IR data. (e) It still has not been ascertained for GaPc-C1 which of the phases posses higher photoconductivity. Attempts to address this last question are under way. Recent experiments to photoelectrochemically decorate each type of GaPc-C1 crystallite with silver nuclei have confirmed that the two phases can be differentiated when light of ca. 640 or 800 nm is used. These experiments will be discussed in detail elsewhere, but preliminary results indicate that the phase which strongly absorbs light at 800-820 nm is the more photoelectrochemically active. Photoelectrochemical Behavior. Figure 3b shows the dark and illuminated behavior of a GaPc-Cl/Au photoelectrode like that shown in Figure Id, in millimolar ferri/ferrocyanide electrolyte, vs. the electrochemical behavior of a bare gold electrode (Figure 3a). The electrochemical behavior in Figure 3b is consistent with a situation where the Fermi potential of the Pc before contact to Au or the electrolyte is negative (on the electrochemical scale) of the Eo' of the redox couple and the EF of the Au substrate. After contact and equilibration, illumination results in a negative photopotential, which is the result of, but not as large as, the potential difference between the substrate metal and the redox electrolyte. The dark currents on this kind of photoelectrode are negligible in the potential region from +0.8 to -0.5 V vs. Ag/AgCI. The i/V behavior of this type of photoelectrode is consistent with that expected of a semiconductor thin film with a low doping density and a Fermi level nearer to the middle of the band gap than the p-type TiOPc and VOPc films discussed previ~usly.~ Figure 3c represents the photovoltammetric behavior of a GaPc-C1 film grown under vacuum conditions that result in faster deposition rates (by a factor of 10) than those used for the film type in Figures 3b and Id. The resulting film (as viewed by SEM) consists of crystallites like those seen in Figure Id but which are least afactor of 10 smaller in the lateral dimension. They remain tightly packed and produce a nonporous layer over the Au substrate. The film thickness for optimum photoresponses was generally 0.3-0.5 Irm. Au-polymer substrates used to produce the GaPc-C1 films in Figure 3b were normally pretreated by ethanol/water rinses and vacuum- or air-drying procedure^.^ If this pretreatment step was eliminated, low levels of contamination (Cl-, N-, and 0-containing species) were left on the surface which
3 k .l
r VO'? \jYhU )
Lj yvO,ca
I
7 1 TVOC(3) 1
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I
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Figure 3. Dark and illuminated electrochemical behavior (vs. a bare Au
electrode) of GaPc-C1films that exhibit behavior suggesting either lightly doped or p-type semiconductor properties. (a) Cyclic voltammogram of an equimolar (1 mM) solution of K,Fe(CN), and K3Fe(CN), in a pH 4 electrolyte at bare Au. (b) The same voltammogram for an illuminated, large crystallite GaPc-Cl film like that shown in Figure Id. The dark current/voltage trace is for this film. (c) The same voltammogram for an illuminated GaPc-CI film that had been grown in such a way as to show p-type semiconductor properties (see text). act as nucleation sites for the deposited GaPc-CI crystallites. Films grown at the same growth rate on treated and untreated AuMPOTE substrates would then produce much different crystallite geometries. Clearly, both growth rate and substrate composition could have a large affect on Pc-film morphology and photoelectrochemical behavior. Reflectance IR studies, however, showed that the films of Figure 3c produced basically the same aC+, spectrum as those in Figure l a ; only the crystallite dimensions were different (see discussion below of spectroscopic results). The electrochemical response of the smaller crystallite films showed negligible dark currents for a variety of redox couples at potentials negative of +0.8 V vs. Ag/AgCl, but a facile oxidation-reduction process at potentials positive of that value, exactly as for the p-type TiOPc and VOPc films. The photovoltammetric behavior for the ferri/ferrocyanide electrolyte produced a positive photopotential vs. bare gold. The data shown in Figure 3 demonstrate the feasibility of producing a photovoltaic device based on GaPc-C1 photoanodes and photocathodes arising from these
Trivalent Phthalocyanine Thin Films
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5655 Au/GcIPc-CI/F~CN$-'~.
n
K
t
After 1 week
e
E(vs AgIAgCI) r 10 months
t-08 Figure 4. Plots of open-circuitphotopotential (V,) vs. formal potential of the redox couple employed for (a) GaPc-CI films like those of Figures Id and 3b, (b) GaPc-CI films like those of Figure 3c, and ( e ) TiOPc or VOPc films like those described in ref 3. different growth conditions and then illuminated sim~ltaneously.~ The open-circuit photovoltage of such a cell is ca. 0.65 V, in 10 mM ferri/ferrocyanide. The power conversion efficiency is ca. 0.05% with a fill factor of 0.5. Higher electrolyte concentrations resulted in improved efficiencies but decreased fill factors. The photocurrents remain limited by the redox electrolyte concentration and the photoconductivity of each GaPc-C1 film. Plots of V , vs. Eo' of the redox electrolyte were made for GaPc-C1 films of the type shown in Figure 3b,c, where Eo' values were varied over 1.6 V (Figures 4a,b). Also shown for comparison is a plot produced for TiOPc and VOPc photoelectrodes (Figure 4c). The slope of these plots should ideally be 1.0; however, values typically range from 0.4 to 0.75.' The two types of GaPc-C1 films clearly produce widely varying responses for each redox couple, but the fraction of photopotential recovered per unit change in Eo' of the redox couple remains approximately constant. The similarity between the plots made for the small crystallite GaPc-C1 films and those made for the TiOPc and VOPc electrodes should also be noted. The TiOPc and VOPc electrodes have been grown in optimum form with small-dimension crystallite^.^ It is clear that the small crystallite GaPc-C1 electrodes like those of Figure 4b behave with p-type semiconductor properties. MottSchottky plots of these p-type electrodes in inert electrolytes led to calculated values of donor density of ca. (2-5) X 10l8 per cm3, as for other ptype Pc electrodes. V, vs. Eo' plots that ran parallel to and in between those in Figure 4a,b could be obtained for GaPc-C1 films produced with crystallite dimensions and/or doping densities intermediate between the two extremes described so far. Effects of O2and H2 on the Photoelectrochemical Response. The role of weak oxidant dopants on conductivity and photoconductivity in various Pc's has been addressed by previous workers and suggests that these effects can be profound.'-13 Laboratory atmosphere aging of GaPc-C1 thin films was investigated by producing a large batch of photoelectrodes simultaneously, storing in the laboratory atmosphere at 22 f 2 OC, and periodically characterizing (1-month intervals) for a period of up to 1 year.5 Figure 5a shows the photovoltammetric behavior of a large crystallite GaPc-Cl film 1 week after vacuum deposition showing (12) (a) Sussman, A. J . Appl. Phys. 1967,38, 2738,2748. (b) Harrison, S.; Ludewig, K. H. J . Chem. Phys. 1966, 45, 343. (c) van Ewyk, R. L.; Chadwick, A. V.; Wright, J. D. J. Chem. Soc., Faraday Trans. I 1980, 76, 2194. (d) Wright, J. D.; Chadwick, A. V.; Meadows, B.; Miasik, J. L. Mol. Cryst. Liq. Cryst. 1983,93, 315. (e) Wohltjen, H.; Barger, W. E.: Snow, A. W.; Jarvis, N. L. IEEE Trans. Electron Devices 1985, ED-32, 1170. (13) (a) Marks, T. J.; Kalinain, D. W. Extended Linear Chain Compounds; Miller, J . S., Ed.; Plenum: New York, 1982; Vol. 1, pp 197-331 and references cited therein. (b) Inabe, T.; Gaudiello, J. G.; Moguel, M. K.; Lyding, J. W.; Burton, R. L.: McCarthy, W. J.; Kannewurf, C. R.; Marks, T. J. J . Am. Chem. SOC.1986, 108, 7595.
b' Figure 5. Dark and illuminated voltammograms for the type GaPc-CI electrode in Figure 3b, after a 1-week exposure to laboratory atmosphere and after a 10-month exposure. little change in photopotential from an electrode examined immediately after production. After 1 month, however, the photovoltammogram for the ferri/ferrocyanide couple (and the apparent EFB) was shifted positive with respect to the fresh film. This shift was progressive with time and resulted in the photovoltammogram in Figure 5b for an electrode aged for 10 months. Apparent flat-band potentials measured by plots of 1/c" vs. E, flat-band potentials measured from the square of the photocurrent vs. E (at moderately absorbed illumination wavelengths), or EFB measured by minima in modulated photocurrent vs. potential curves (measured by-low frequency modulation) showed a systematic shift to more positive values as aging in atmosphere progressed. These changes were accelerated by annealing freshly deposited GaPc-C1 films in atmospheric pressure, pure O2 at 140 OC, for periods of up to 48 h. For a film such as that shown in Figure 5a, with an apparent flat-band potential of ca.0.0 V vs. Ag/AgCl, treatment in high-pressure O2 resulted in a positive shift of the entire photovoltammogram to ca. +0.4-+0.5 V. For the film in Figure 5a the dark currents were negligible in the potential range between 0.8 and -0.4 V. After O2 treatment, the dark currents in the range of 0.8 V (in contact with redox couples like o-tolidine) became appreciable. When a variety of redox couples were examined, the V, vs. E l I 2behavior was shifted from that of Figure 4a for the fresh film to near that of Figure 4b for the 02-treated film. Shorter times of oxygen treatment produced responses intermediate between those two extremes5 The original photovoltammetric behavior could be restored by taking the 02-treatedfilms (even those that had seen electrolyte) and annealing them in atmospheric pressure H 2 at 140 "C for periods of up to 48 h. Intermediate H2 treatment times resulted in incomplete restoration of the original activity. After several 02/H2treatment cycles, the photoelectrochemical response of these electrodes deteriorated, apparently because of loss of adherence of the Pc film to the Au/polymer substrate. Annealing small crystallite GaPc-C1 films, like those of Figure 3c and 4b, in H2 at 140 OC for periods of up to 48 h showed the converse effect. Before H2treatment these photoelectrodes gave dark current responses at potentials positive of 0.8 V and only positive photopotentials for the redox couples in Figure 4b. After H2 treatment the photopotentials were made smaller as the photovoltammograms shifted negatively. Subsequent treatment with atmospheric pressure O2 at the same temperature restored the original photovoltammetric activity.
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It should be noted that 0, treatments shifted the photovoltammetric responses of the electrodes in Figure 3b toward those of Figure 3c, but never caused a complete shift of the photopotential, regardless of the duration of O2 treatment beyond that point. Similarly, H 2 treatment caused the voltammograms of Figure 3c to shift toward those of Figure 3b, but the transformation was never complete. N2treatment at the same temperature and duration as for O2 and H2 treatments was unsuccessful in causing measurable changes in photoelectrochemical response for either type of GaPc-C1 film. Similar experiments on p-type InPc-C1 electrodes, however, did show some EFBshifts after N2 treatment that were similar to H2 treatment of the 0,-doped GaPc-C1 films.5 0, treatments and H 2 treatments of an identical nature were also applied to the p-type VOPc and TiOPc thin films, resulting in no appreciable change in their photoelectrochemical properties. To the extent that we have studied these systems, the trivalentmetal Pc’s appear to be amenable to reversible 0, doping, whereas the tetravelent systems are not as easily affected. Photocurrent Action Spectra. Figure 6a,b shows the photocurrent yield spectra for a large crystallite GaPc-CI film (Figure la,d) which gave photoelectrochemical activity like those of Figures 3b and 4a. Hydroquinone and benzoquinone were used to harvest the charge at the solution interface depending upon the potential applied to the Pc electrode. Photocurrent yield spectra like that of Figure 6a have been shown previously.6 When the bias potential is such that holes are harvested at the solution interface, the photocurrent yield spectrum for illumination of the solution interface first (FS) shows a response similar to the absorbance spectrum expected of a thin film of GaPc-C1. For illumination of the Au/Pc interface first (BS), the photocurrent spectrum is greatly attenuated in the region of strongly absorbed photons, 620-740 nm. Charge is created near the Au/Pc interface because of the high optical density and is not successful in traversing the full thickness of the Pc film. When the bias potential is held just positive of the flat-band potential, the photocurrent in the region between 620 and 740 nm may even be cathodic, owing to charging currents resulting from the nonohmic contact between the metal substrate and the Pc film.6 The experiments reported here were conducted at higher bias voltages so that these effects were not so pronounced. We have shown earlier that approximations can be made (provided a uniform composition and potential gradient are assumed) which indicate that the maxima in the BS photocurrent spectra for these lightly doped GaPc-CI films occur at wavelengths where the absorption length of the photon (1 / p ) is equal to the difference between the film thickness (d) and the average length that the charge carrier will diffuse before annihilation (e), which for these films is ca. 250-300 nm.6 At high enough bias potentials the BS spectrum of Figure 6a grows in intensity, the minimum in the region between 620 and 740 nm becomes less pronounced, and the photocurrent maxima begin to move toward the center of the spectrum. This is consistent with the increased likelihood that charges created at the Au/Pc interface will survive to be harvested at the solution interface. As the bias potential was adjusted to be negative of the flat-band potential, the BS spectrum of Figure 6a grew continuously in magnitude and the FS spectrum decreased in the region between 620 and 740 nm until the spectrum of Figure 6b resulted. Under these conditions electrons are now harvested at the solution interface and the BS spectrum takes on the appearance of the absorbance spectrum. The BS spectrum of Figure 6b is skewed slightly toward another maximum at ca. 840 nm, but the arc lamp source has several maxima in this region (for which we corrected), which nevertheless lower the precision of the photocurrent data. The data in Figure 6a,b indicate that the transport and/or production of holes in the Pc film is limiting photocurrent in the large crystallite GaPc-CI films. For either bias condition, when the harvesting of holes occurred at the interface that was illuminated, the photocurrents tracked the absorbance spectrum. When hole harvesting occurred away from the illuminated interface, the photocurrent spectrum was attenuated at wavelengths where photon penetration lengths were small.
Klofta et al.
t
Llghty doped GaPc-C1
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i
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.
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.
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Figure 6. Photocurrent action spectra for GaPc-C1 films where illumination of the electrolyte/Pc interface occurred first (FS) or illumination of the Au-MPOTE/Pc interface occurred first (BS). (a) A GaPc-C1 electrode like that in Figure 3b biased positive of EFB. (b) The same electrode biased negative of EFB.(c) A p-type GaPc-C1electrode biased negative of EFB.
This behavior is in contrast to that observed in Figure 6c for the small crystallite GaPc-C1 films like those of Figures 3c and 4b. In this case only the photocurrent spectrum for bias potentials negative of the flat-band potential is shown. For bias potentials positive of the flat-band potential (ca. +0.8 V), dark currents were appreciable and the photocurrent spectra were not well-defined. The spectrum of Figure 6c should be directly compared with the spectrum of Figure 6b, since for both electrodes the bias potential was held negative of the flat-band potential by the same amount. For the small crystallite ptype and GaPc-C1 films the FS spectrum mimicked the absorbance spectrum for electron harvesting at the
Trivalent Phthalocyanine Thin Films solution interface and the BS spectrum is attenuated-exactly the opposite to that seen for the BS and FS spectra of Figure 6b. The effects of O2and H2treatments on the photocurrent action spectra were studied next. When the large crystallite film of Figure 6b was subjected to the O2treatment described above, the BS spectrum showed a distinct minimum appearing in the wavelength region between 620 and 740 nm, while the FS spectrum was significantly increased in that same wavelength region. Both of the spectra doubled in absolute current density at every wavelength. 0, treatment caused a partial inversion of the BS and FS spectra, but this transformation was not complete. When the small crystallite GaPc-C1 film of Figure 6c was treated in H2 as described above, complete inversion of the FS and BS epectra did not appear, but the BS spectrum grew significantly in intensity in the region between 620 and 740 nm and the FS spectrum decreased in the same wavelength region as H2 treatment progressed. ESR Studies. Powdered versions of GaPc-C1 and VOPc that had been extensively purified by sublimation were subjected to ESR analysis. For the purified GaPc-C1 powder a strong resonance centered at g = 2.0023 was observed which, when integrated, gave an approximate spin density of 1 X 10’’ per cm3. Subjecting this powder to the 0, treatment described above caused the ESR signal to increase slightly; however, treating this powder in H2 as described above, caused the signal intensity to decrease by a factor of 50%. Longer treatment times in H2were unsuccessful in lowering the signal intensity beyond the original observation. Retreatment in O2 caused the ESR signal to return its original intensity. Half of the radical species responsible for the g = 2.0023 signal (vs. a DPPH reference) can be removed by the hydrogen treatment, but not all of this population is affected. This is consistent with the photoelectrochemical observations, where the small crystallite films could be only partially affected by H2 treatment. For VOPc there is an unpaired electron in the d,2+ orbital of the VOPc molecule which could provide a free spin even without adventitious dopants. The powder ESR signal showed a broad spectrum with 0, unresolved hyperfine splitting. 0, and H2 treatments of VOPc powders had little discernible effect on these ESR spectra. The spin density of 0,-induced radical species was not high enough to be easily detected by this spectroscopic scheme. Previous workers have also noted the presence of free spins in purified Pc’s and have attributed them in part to the presence of oxygen radical anions and sublimation decomposition products.’ Surface Mediation of the GaPc-CI Photocurrents. Figure l a shows voltammograms of an oxygen-annealed ptype GaPc-C1 film in contact with millimolar anthraquinone and is typical of the response for all of the p-type Pc films we have studied. The forward sweep under illumination produces the expected photoassisted reduction currents, which result in a mass transport limited peak current. Upon scan reversal the oxidation peak current is clearly observable. If the Pc film were truly rectifying, it is expected that no oxidative currents would be observed until the potential was swept positive of the flat-band p0tentia1.l~ This oxidative peak is observed for all photovoltammograms produced with bias potentials swept initially negative but stopped and returned before reaching -0.5 v. When the potential was swept negatiue of -0.5 volts, or held at that potential for several seconds, the voltammogram of Figure 7b resulted. The oxidative peak current is completely absent at its original location, and any oxidative current observed is spread out over the potential region positive of +0.4 V. This same effect could be obtained by simply increasing the concentration of AQ to greater than 5 X IO-, M.S The peak currents for reduction increased with increasing concentration as expected, until the currents were limited by the PC film resistance. The reverse oxidative currents were attenuated with respect to the reduction currents. After the electrode was held at -0.5 V and then poised at positive potentials for several minutes, removal of the electrode 0 4 ) Gerischer, H. In Solar Energy Conversion; Seraphin, B. O., Ed.; Springer-Verlag: New York, 1979; pp 115-132.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5657 f-
AwGaPc-CIIAO Initial
v
\
\dI
E(V vs AgAgCl)
Hold d -0.5V for 30s and mturn
I
E(V vs Ag/AgCI) Figure 7. Dark and illuminated voltammograms for a p-type GaPc-C1 film in millimolar anthraquinone, pH 4 electrolyte. (a) Normal dark and illuminated behavior. (b) Behavior after poising electrode at -0.5 V for several seconds.
and storage in laboratory atmosphere and/or replenishment of the solution did not return the original photovoltammetric activity. When the electrode was held at +0.8 V (positive of the apparent flat-band potential) and illuminated for periods of at least 5 min, the original voltammetric activity of Figure 7a was completely restored. Apparently the oxidative activity seen in Figure 7a and previous figures is mediated by the presence of an electroactive group on the Pc film surface. Sufficiently negative potentials are capable of attenuating its activity, through a redwtion process. A well-defined voltammetric wave for this reduction was not observed for these electrodes in inert electrolytes, either in the dark or in the light. The rate of mediated oxidation and/or surface coverage of the mediator is low enough that high electrolyte concentrations attenuate its effect. This mediation process can be fully recovered in low electrolyte concentrations by photoassisted oxidation. It is interesting, however, that the O2and H2 treatments described earlier had little effect on the rectifying properties of these thin films. After the electrode was poised at -0.5 V, the absence of the surface mediation process improved the open-circuit photopotential and photocurrents by only a small amount, suggesting that this site of back-reaction on the untreated Pc films is less important than other bulk recombination processes. XPS/UPS Studies. XPS and UPS spectra were recorded for all of the types of Pc films thus far produced in the normal diffusion-pumped vacuum system (base pressure ca. Torr) and for Pc films produced in the special UHV preparation chamber coupled directly to the surface spectrometer. For the GaPc-C1 films Ga(3d and 2p3/2), C(ls), N(ls), C1(2p), and O(1s) spectra were recorded for the as-prepared films from the diffusion-pumped system and as a function of film thickness for films grown under UHV on bulk gold and the Au/polymer substrates. For all but the lowest coverage GaPc-C1 films the Ga, C, N, and C1 atomic percentages were within 1-3% of that expected for the pure material, when the spectral intensities were corrected for photoemission probability, escape depth, and analyzer transmission f ~ n c t i o n .GaPc-C1 ~ films grown at 1-10-monolayer coverages on the Au-MPOTE and on bulk gold showed some deficiency in chlorine, which is discussed further below. Low-coverage spectra have also shown some unusual splitting of the N ( 1s) line shapes, which is the subject of further study.
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The Journal of Physical Chemistry, Vol. 91, No. 22, 1987
t
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Oxygen(1s)
Small Grains
Klofta et al.
r
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t
Large Grains
559
534 5 21 BINDING ENERGY QV
546
508
Figure 8. O(1s) XPS spectra for GaPc-C1 films like those in (a) Figure 3c (p-type) and (b) Figure 3b.
The C(1s) and N(1s) line shapes and binding energy peak positions were exactly those predicted for metalated phthaloc y a n i n e ~(Le., ~ ~ no oxidized forms of C or N were observable). Figure 8 shows the O(1s) spectra for GaPc-C1 films grown in the diffusion-pumped vacuum system to produce the large and small crystallite films discussed in Figures 3 and 4. Some exposure to laboratory atmosphere occurred during transfer of the samples to the surface spectrometer. A difference in O( 1s) intensity of a factor of 5-1OX was seen when the small vs. large crystallite films were compared. The O( 1s) signal is a single peak centered at 533.0 eV. It is tempting to speculate that the difference in photoelectrochemical properties between the two types of films is due to the difference in oxygen levels as evidenced by the data in Figure 8. XPS samples only the near-surface region of these materials, however, and if O2doping did occur in the diffusionpumped vacuum system, the doped state may be destroyed during the transfer, resulting in the spectra shown in Figure 8. Oxygen radical anions present in the XPS sampling region would be expected to give a spin-orbit splitting of the O(1s) line shape,I6 which was not observed. There is a difference in surface roughness between the small and large crystallite films which would certainly enhance the intensity of an adventious dopant on the small crystallite film. This difference in surface-to-volume ratio also enhances the likelihood for doping during the vacuum preparation steps (see discussion below). GaPc-C1 films prepared under UHV, at any coverage, showed no detectable O(1s) signal. Several films, with GaPc-C1 coverages from a few monolayers to ca. 1000 A, were exposed to several thousand langmuirs of pure O2at room temperature in a UHV dosing cell at pressures ranging from 10” Torr up to atmospheric pressure, repositioned in the analysis chamber at ca. 5 X Torr, and reexamined immediately by XPS. Once again no detectable oxygen signal was seen. The work of Dahlberg and co-workers has shown that the interaction of O2with certain Pc films is weak and reversible within this pressure range.” Removal of the UHV-prepared GaPc-C1 film from the vacuum and subjection to the same high-temperature O2treatment as described earlier, followed by reexamination with XPS, produced an O( 1s) signal of the type and magnitude shown in Figure 8a. Once again, some unavoidable atmosphere exposure was incurred during sample transfer. UHV-prepared films exposed to atmosphericpressure O2at room temperature showed only a barely detectable O( 1s) signal. We can infer from these studies that the diffusion-pumped system normally produces an 02-doped GaPc-C1 film, although the exact chemical nature of the product species cannot be inferred (15) (a) Zeller, M. V.; Hayes, R. G . J . Am. Chem. SOC.1973,95, 3855. (b) Niwa, Y.;Kobayashi, H.; Tsuchiya, T. J . Chem. Phys. 1980, 73, 642. (1 6) Orchard, A. F. In Handbook of X-ray and Ultrauiolet Photoelectron Spectroscopy; Briggs, D., Ed.; Heyden: Philadelphia, 1977; pp 15-19 and references cited therein. (17) (a) Dahlberg, S. C. Appl. Surf. Sci. 1982, 14, 47. (b) Dahlberg, S. C.; Musser. M. E. Surf. Sci. 1979, 90, 1.
I
-13.0 -12.0 -110 -100 -90 -80 -7.0 -60 INITIAL ENERGY (eV) E= ,O
-5C
Figure 9. UPS spectra of GaPc-CI. Upper trace: gas-phase UPS spectrum of GaPc-C1 (He I radiation, energy referenced to E F in spectrometer). Lower trace: UPS spectrum for a thin film of GaPc-CI after correction for contribution from Au valence band region (He I radiation, energy referenced to EF of Au substrate).
from the surface analysis data alone. We also note that if each O2 species was responsible for the creation of one charge carrier, then the XPS detection limit for oxygen (parts per thousand) would represent an extraordinarily high dopant concentration, which is not consistent with the photoelectrochemical data. It can be reasoned that the added oxygen forms operate with low efficiency to produce charge carriers which affect photoconductivity. UPS (He I) spectra were also recorded for various Pc films as a function of surface coverage on both bulk Au and the AuMPOTE substrate. Figure 9a shows the gas-phase spectrum for GaPc-C1, and Figure 9b shows the solid H e I spectrum for a coverage of ca. 39 A on either type of Au substrate. The energies associated with each of the peaks in the gas-phase spectrum are referenced to the vacuum level, while those of the solid are referenced to the Fermi edge for the Au substrate. Solid-state relaxation effects alone can account for the difference in energies of each of the peaks.]* These solid-state spectra are quite similar to UPS spectra obtained for several other Pc’s using either He I, He 11, or 75-eV synchrotron radiation s o ~ r c e s . ~Koch J ~ and co-workers have shown that the highest lying orbitals in d10metal Pc’s are likely to be. C(2pz,a;),’’ labeled peak A in Figure 9. After correction for the work function of the Au substrate (obtained from the width of the UPS spectrum on the clean substrate20), the energy noted for peak A in our UPS data is within 0.1 eV of those values reported by Koch and co-w~rkers.’~ Peak B is predominantly N(2pZ)in character and also corresponds well with the assignments made by Koch. Fenske-Hall calculations conducted on the basic framework of the GaPc-C1 molecule show good agreement between the predicted assignments of these peaks and (18) Williams, P. M.In Handbook of X-ray Ultraviolet Photoelectron Spectroscopy; Briggs, D., Ed.; Heyden: Philadelphia, 1977; pp 313-330 and references cited therein. (19) Iwam, M.; Eberhardt, W.; Kalkoffen, G.;Koch, E. E.; Kunz, C. Chem. Phys. Left. 1976,62, 344. (b) Koch, E. E.; Grobman, W. D. J . Chem. Phys. 1977,67, 837. (c) Koch, E. E.; Iwan, M.; Hermann, K.; Bagus, P. S. Chem. Phys. 1981.59, 249. (d) Tesler, E.; Iwan, M.; Koch, E. E. J. Electron Specfrosc. Relat. Phenom. 1981, 22, 297. (e) Iwan, M.; Koch, E. E.; Chiang, T. C.; Eastman, D. E.; Himpsel, F. J. Solid State Commun. 1980, 34, 57. (20) Kellog, G.Ph.D. Dissertation, University of Arizona, 1986.
Trivalent Phthalocyanine Thin Films their acutal location^.^*^^ Peak C is predicted from these calculations to originate from C1 p-orbital mixing with N p-orbitals and is missing from UPS spectra of demetalated (H,Pc) and zinc phthal~cyanine.’~Interestingly, this peak was attenuated with respect to the others when the coverage of GaPc-C1 was below 1.0 nm. All of the rest of the bands were broadened at even the lowest coverages, indicating that solid-state broadening effects were prevalent at even these thicknesses. This is consistent with the notion that nucleation occurs immediately during the vacuum deposition of the Pc films and that isolated crystallites are formed which eventually coalesce into a film. Sacrificial chemical analysis of a much thicker Pc film, conducted after the XPS/UPS experiments;showed that even when the thickness was ca. 100 nm, the Au(4f) signal from the substrate could still be seen in the XPS spectrum. This further supports the idea that microislands are forming. Previous electron micrographs have shown that growth of 100-1 000-nm-thick films occurs by this process.6
Conclusions These studies serve to underscore the importance to preparation condition, growth rates, the presence of oxidizing and reducing dopants, and the nature of the transition metal complexes in the Pc ring in determining the photoelectrochemical properties of phthalocyanine thin films. Previous studies have indicated that thin films of GaPc-C1, VOPc, and TiOPc might have multiple crystallite phases (as revealed primarily by photocurrent action spectra). In the VOPc and TiOPc cases it seemed that these phases were spatially separated within the film (i.e., layered), resulting in assymetry in the FS and BS photovoltammetric and action spectral r e s p o n ~ e s .In ~ the case for the large crystallite GaPc-C1 films, the presence of two or more phases, segregated or otherwise, was not as clear. The IR data presented here and elsewhere make a strong case for the presence of two or more phases in all of the Pc thin films.l0 This appears to be a second-order effect in determining the photoelectrochemical properties of these materials, relative to some of the other processes detailed above. Studies are under way to spectroscopically probe the difference in Pc films that have been O2doped to different extents, but at the present time the effects appear to be small and difficult to quantitate by I R or UV-visible spectroscopy. XPS studies indicate that for GaPc-C1 films grown in pressure ranges of ca. 10“ Torr (diffusion-pumped vacuum system) the surface concentration of oxygenous species is less than that seen for VOPc or TiOPc films. Large crystallite GaPc-C1 films grown slowly and a t high substrate temperatures show bound oxygen concentrations that are just above detection limits (ca. monolayer levels). VOPc and TiOPc films contain (in addition to the oxygen bound to the metal) up to 2-3 times as much oxygen as expected, apparently uniformly distributed throughout the near-surface r e g i ~ n .The ~ origin of this extra oxygen is clearly due in part to the takeup of 0,in the vacuum deposition at 10” Torr, since UHV-prepared films had no extra oxygen forms and showed a low affinity for O2 at room temperature. High pressures of 0, and high temperatures are needed to incorporate oxygenous forms into the already formed Pc film. Recent studies also suggest that oxygen-containing molecules can be brought up from the MPOTE substrate during deposition to dope the first Pc layers deposited.2’ The shift in photovoltammograms for the GaPc-C1 films with O2and H2treatments should be viewed in the context of solid-state studies of Pc’s where Fermi energies were shifted with the same dopants.’ The molecular nature and depth distribution of oxygen as a dopant is still not understood and must be further explored in order to correctly interpret these shifts in potential. The lightly (21) Lee, P.; Pankow, J.; Danziger, J.; Armstrong, N. R. manuscript in preparation.
The Journal of Physical Chemistry, Vol. 91, No. 22, 1987 5659 doped large crystallite GaPc-C1 films behave as though the Fermi level is near the middle of the band gap and that ohmic contact to the Au substrate is not made. UPS studies showed that the relative positions of the valence-band edge in GaPc-C1 and EF in the bare Au or Pc-covered gold would require charge transfer mediated by interfacial states above the VB edge in order to form an ohmic contact. These states are apparently present only at low concentrations in the lightly doped GaPc-C1. 02-treated GaPc-C1 films or GaPc-C1 films grown in such a way as to incorporate more oxygen behave as though ohmic contact is made to the Au substrate and that the effective donor density is above lo’* ~ m - as ~ for , the VOPc and TiOPc films. This suggests that the act of 0,doping increases the hole-carrier population and (probably by the same mechanism) increases the Au/Pc interfacial state density to facilitate charge transfer. The photocurrent yield spectra confirm that O2treatments lead to enhanced hole transport vs. electron transport in the GaPc-C1 films. Photocurrent spectra like those in Figure 6 can have different interpretations depending upon (a) the film thickness vs. the photon penetration depth and (b) the homogeneity of Pc film When minima are seen in the photocurrent spectra at wavelengths of strongly absorbed light, it can be argued that the light is being absorbed at an “inactive” interface or that light is absorbed at an interface that is as “active” as the rest of the Pc film, but that the distance that carrier must travel is greater than the average diffusion length and they are therefore annihilated before being harvested. For the lightly doped large crystallite GaPc-C1 films we have argued that the latter case holds in their action spectra.6 For the faster grown, smaller crystallite VOPc and TiOPc films, photocurrent action spectra show clearly the presence of spatially separated phases with different photocurrent or H2-treated yieldse3 The photocurrent spectra for the 0,GaPc-C1 films presented here suggest that doping with these gases is not uniformly effective and that spatially separated regions are formed. The dark conductivity of Pc’s is known to increase with O2 doping,’ and that is confirmed in these studies. From the photocurrent yield and photovoltammetric studies alone we postulate that the act of O2doping creates a film that is spatially segregated in terms of differing regions of dark conductivity and photoconductivity and that nonuniform potential drops across the film give rise (at least in part) to the flat-band-potential shifts observed. Conclusions drawn from these studies must be tempered by the interpretation of the ESR results. The purified GaPc-C1 powder from which vacuum deposition begins still contains a high concentration of free spins which are enhanced by O2doping and which are attenuated (but only by 50%) by H2 doping. This result argues for the presence of two or more radical forms in the Pc. One is accessible to H2 (probably a radical generated by interaction and another (uniformly distributed) is inaccessible to with 0,) H2or unreactive. It has been suggested this is a radical fragment (Pc) resulting from Pc decomposition during purification.] It should be noted that the concentration of these radical forms need only be at the 1-100-ppm level (barely or not detectable by normal spectroscopic means) in order to dominate the eletronic properties of these molecular semiconductors. Tailoring the photoconductive properties of these Pc thin films will obviously require UHV deposition conditions with complete control of dopant levels and careful control of substrate order and microcrystallinity of the organic layer. Acknowledgment. Funding from the National Science Foundation, and from the Materials Characterization Program, State of Arizona, is gratefully acknowledged. Registry No. GaPc-CI, 19717-79-4; VOPc, 13930-88-6; TiOPc, 26201-32-1; 02,7782-44-7; H2, 1333-74-0; K,Fe(CN),, 13943-58-3; K3Fe(CN),, 13746-66-2; anthraquinone, 84-65- 1; hydroquinone, 12331-9; benzoquinone, 106-51-4; gold, 7440-57-5.
