Light absorption in the near-infrared and photocurrent synergism observed with vacuum-sublimed films of zinctetraphenylporphyrin

t.h,2n Thin Solid Films 307 (1997) 208-214

ELSEVIER

Light absorption in the near-infrared and photocurrent synergism observed with vacuum-sublimed films of zinctetraphenylporphyrin Yutaka Harima ~, Takuji Kodaka, Paul Price, Tsukasa Eguchi, Kazuo Yamashita Faculty of Imegrated Arts and Sciences, Hiroshima Unicersity, 1-7-1 Kagam~'ama. Higashi-Hiroshima 739, Japan Received 5 March 1997; accepted 23 April 1997

Abstract Photocurrent peaks are found in the near-infrared (NIR) as well as in the visible for zinc(H) tetraphenylporphyrin (ZnTPP) films contacting redox solutions, in contrast to no appreciable photocurrent in the NtR for a sandwich-type celt of A1/ZnTPP/Au. The spectral responses of photocurrents in the NIR for the wet cells resemble that of photocurrent synergism reported earlier for the sandwich-type cell, although the ZnTPP solid is almost transparent in this wavelength region. The observed electronic transition in the NIR is ascribed not to a singlet-to-triplet excitation which is commonly observed with phthalocyanine solids, but to the absorption of the NIR light due to a trace of a ZnTPP photoproduct, most probably an isoporphyrin. The photoproduct, which diminishes a generation efficiency of photocurrents by acting as a recombination center, is responsible for the increase in photocurrents due to Soret band excitation by simultaneous illumination with the NIR light. The difference in the NIR photoresponse between the wet cells and the dry cell is explained on the basis of an energy-level consideration. © 1997 Elsevier Science S,A. Keywords: Zinctetraphenyl porphyrin; Photocurrents; Light absorption: Near-infrared

1. Introduction Molecular solids have attracted increasing interest because of their possible applications in electronic and optoelectronic devices. In order to improve their efficiencies, however, a detailed knowledge of gap states, i.e., energy levels lying within the energy gaps of the materials, is of immense importance since these intermediate states should greatly affect electronic processes such as photogeneration of charge carriers, carrier transport and so on. Recently, we have found a synergistic effect of photocurrent for porphyrin solids sandwiched between A1 and Au [1]. A photocurrent due to Soret band excitation at 430 nm (I~) was enhanced by the simultaneous illumination with monochromatic near-infrared (NIR) light which is hardly absorbed by the porphyrin solids themselves. The extent of the photocurrent synergism ('O) was def'med as follows [1]: r/(%) = 100 × [ I ~ + 2 - (I~ + I p ) ] / I ~

(1)

" Corresponding author. Tel.: +81 824 24 6534; fan: +81 824 24 0757; e-mail: [email protected],jp. 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0040-6090(97)00252- 6

where ]pi + 2 is a photocurrent due to concomitant illumination with the 430 nm light and monochromatic NIR light, and Iv- represents a photocurrent due to the NIR light alone. In most experiments, I~ was small enough to be omitted in the evaluation of "q. A dependence of r/ on the wavelength of the NIR light was termed as a synergism spectrum or an "q spectrum. The synergism spectrum for zinc(II) tetraphenylporphyrin (ZnTPP) solids exhibited two broad peaks as shown by curve a in Fig. 1 [1]. The observed spectral response of 77 indicates clearly that electronic transitions in the NIR are detectable with a high sensitivity when the proposed technique is applied. Indeed, such NIR electronic transitions have neither been found in an absorption spectra of the ZnTPP film nor in a spectral response of photocurrents due to a single illumination. An absorption tail extending to 1000 nm, shown by curve b in Fig. 1, is attributable to a light scattering due to the ZnTPP film surface, because the absorbance in the wavelength region beyond 700 nm varies as the fourth power of the photon energy [2]. Thin films of phthalocyanine (Pc) analogous to porphyrin, on the other hand, are "known to absorb light in the wavelength range of 800-1200 nm as well as in the visible (VIS) range. Absorptions of the NIR light evidenced by a

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Y. Hari~m e~ a I . / T h i n Solid Fibns 307 (1997) 2 0 8 - 2 ! 4

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Wavelengthdnm Fig. 1. The synergism spectrum of (a) the photovoltaic cell AI/ZnTPP (2000 A)/Au and (b) absorption spectrum of the aublimed ZnTPP (500 ,1) film, photoconductivity or photocun'ent measurement [3-8], inelastic electron tunnelling spectroscopy [9,10] and ordinary absorption spectroscopy [t 1] are ascribed to a singlet-totriplet (S-T) transition [3-6,8-10] or a ligand-to-metal charge-transfer [11], although the assignments are arbitrary in some Pc's. A more recent study by us on photocurrent synergism using vanadyl(IV) tetraphenylporphyrin revealed that, first, the gap states in the porphyrin semiconductor are responsible for the NIR light absorption and second, that the electronic transitions to or from the gap states diminishes the rate of recombination of electron-hole pairs via these intermediate states, leading to the enhancement of the 430 nm-photocurrents [12]. Here, the gap states are speculated to function as recombination centers although we currently do not "know if the gap states behave as hole traps or electron traps. The main aim of the present study is to determine the origin of these gap states and to discuss the mechanism of the synergistic effect.

drained from Matsuzaki Shinku. The ITO-coated glasses used for preparation of dry cells were prepared in this laboratory by means of a DC sputtering apparatus (Tokuda CFS-SEP). Fig. 2 shows schematic illustrations of a wet cell and a dry celt. The former consists of an I y K I or an H Q / B Q redox solution in contact with an ITO/ZnTPP electrode and a bare ITO counter electrode. The I2/KI and H Q / B Q redox solutions were, respectively, adjusted to pH 4.6 by an acetate buffer and pH 2 by hydrochloric acid. Accordin&to our previous studies [14_,15], the two redox couples, consisting of a two-electron transfer step, are highly irreversible on ITO, different from other metal electrodes. The finding may be-explained in terms of the ITO being a degenerate n-type semiconductor with a wide band gap ( > 3.5 eV). Other redox couples and a substrate metal replacing ITO were also tested. However, reliable photocurrent action spectra for wet cells were obtained only in the combination of the above redox couples and ITO, which gave small and non-drifting currents in the dark. The ITO/ZnTPP electrodes were illuminated through the redox solutions. These solutions were almost transparent in the 500-1100 nm wavelength range. Photocurrents of the wet- ceEs were stable for many days. The d r y M / Z n T P P / A u cells, where M denotes A1, Pb, In, Bi or ITO, were prepared as described elsewhere [t6]. When the

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2. Experimental

~I,TOZnTFP The ZnTPP, purchased from Strem Chemicals, was purified by ftrst oxidizing the trace contaminant, zinc tetraphenylchlorin, by 2,3-dicyano-7,8-dichlorobenzoquinone (DDQ) in chloroform [13]. The resulting solution was next chromatographed with a neutral aluminum (mesh size 200-300) column with chloroform as the eluting solvent. Finally, it was sublimed in a stream of argon at reduced pressure. Free-base tetraphenylporphyrin (H,_TPP) used for comparison was obtained from Tokyo Kasei and purified by chromatography. Porphyrin films were prepared by a physical vapor deposition at 10 . 4 Pa soon after purification. Film thicknesses were 2000 A unless otherwise stated, p-Benzoquinone (BQ), from Katayama Chemicals, was purified by sublimation before use. p-Hydroquinone (HQ), also from Katayama, was used as received. All other chemicals were of reagent grade and used without further purification. Indium-tin-oxide (ITO) substrates of 10 1-1/[] used for the construction of the wet cells Were

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Pb, In and Bi films were used as blocking electrodes, a 100-A thick A1 film was first vapor-deposited on the pre-cleaned glasses prior to the evaporation of the respective metals on it because these metal films were almost insulating without the A1 underolayer. The thicknesses of the metal films were about 200 A. Illumination was carried out through the M / Z n T P P interface which forms a blocking contact. Photocurrents of the dry cells increased with time for 2 weeks (ca. 10 times for ZnTPP cells and 20 times for HzTPP cells). The anomalous photocurrent increase was ascribed to an increase in a hole drift mobility due to the crystallization of porphyfin films [17]. The photocurrent action spectra were measured by one of two set-ups, depending on the wavelength range: one for the range 500-700 nm and another for 600-1100 nm. In the latter set-up, which can provide about 10 times more intense monochromatic light than the former, a color filter (Toshiba R-60) was used to ensure the removal of light of wavelengths shorter than 600 nm. The photocurrent action spectra measured with the two set-ups were normalized at constant intensities of light after correction for losses due to electrode absorption. They were combined to yield a photocurrent spectrum exceeding the 500-1100 nm wavelength range by comparing photocurrents at overlapped wavelengths. Measurements of these action spectra were made soon after the preparation of the dry cells although their shapes did not change considerably with time in contrast to the increase in the magnitudes of photocurrents. Measurements of absorption spectra were carried out on a UV-VIS-NIR spectrophotometer (Shimadzu UV-3101PC). All measurements were made at room temperature and under air.

3. Results and discussion Open-circuit photovoltages of the dry and wet cells were 1.0 and 0.1 V, respectively. The directions of photocurrents are consistent with a p-type conductance of the ZnTPP solid [18]: upon illumination, electrons flowed from the ZnTPP to the A1 in the dry cell and in the wet cells, electrons were injected from the ZnTPP solid into the electrolyte solutions. It is well-known that the p-type conductance of molecular semiconductors arises from doping of O 2 from air [19-21]. Fig. 3 compares action spectra of short-circuit photocurrents for the A 1 / Z n T P P / A u celt and the I T O / Z n T P P electrode in the redox solutions of I - / I ~ and H Q / B Q . Photocurrents are normalized at 550 nm and those resulting from excitation at wavelengths longer than 620 nm are 10 times magnified for clarification i. The lack of a pho-

I Photocurrents at 20 /zW cm -2 of 550 nm-light incident on the junctions were about 1,5, 0.7, and 0.3 nA cm -2 for the dry cell and the wet cells with I - / I ~ and H Q / B Q , respectively.

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Fig. 3. The photocurrent action spectra for (a) the A1/ZnTPP (2000 A ) / A u cell and the ITO/ZnTPP (2000 A) electrodes in 0.2M Na2SO 4 solutions containing (b) 5raM HQ and 5mM BQ, and (e) 5raM 12 and 0.5M KI. Illumination was made through the interfaces forming a blocking contact, i.e., A1/ZnTPP and solution/ZnTPP interfaces.

tocurrent tail in the NIR supports the view that the absorption tail of the ZnTPP film (curve b of Fig. 1) arises from a light scattering. A salient feature is the difference in the NIR photocurrent between the wet cells and the dry cell in spite of the fact that ZnTPP films in both types of cells were prepared together. As may be expected from the very low absorptivity of the ZnTPP solid in the NIR, no appreciable photocurrents at wavelengths above 700 nm are seen with the dry cell. On the other hand, two photocurrent peaks at 770 and 860 nm can be seen in the action spectra of both of the wet cells even though they have different types of the redox solutions. The ratio of the photocurrent at 860 nm to that at 550 nm amounts to ca. 10%. This value remained almost unchanged even when the ZnTPP thin film electrode in the I - / I 7 solution was illuminated with an intense white light for several hours. Note that the wavelengths of these wet cell peaks correspond fairly well with the synergism spectrum of the dry cell shown in Fig. 1, although the latter is slightly broader. This could indicate a common light absorption process for the photocurrent synergism in the dry cell and the NIR photocurrents in the wet cell. Further possible support for the common process is that the lack of synergism in H2TPP dr3, cells may correspond to the lack of NIR photocurrent in the H,TPP wet celt as well as in the dry cell (Fig. 4) ~Incidentally, Loutfy has investigated the photoresponse of an x-phase metal-free phthalocyanine (x-HePc) solid contacting ZnO, CdS or a redox solution [7]. A photocurrent peak at 980 nm is observed for all these junctions and is assigned to the direct excitation of an electron from the x-H_,Pc valence band to surface states of unknown origin. In contrast to the x-H2Pc case, clear NIR photocurrent peaks are seen only for the wet cells in this study with ZnTPP as a molecular semiconductor.

2 Photocurrents at 15 # W cm -2 of 520 nm-light incident on the junctions were about 0.2 and 0.4 nA cm -2 for the dr3, and wet cells, respectively.

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3. l. Origin of the NIR light absorption The involvement of the redox species such as I - / I 3 or H Q / B Q in the NIR absorption may be ruled out since, being transparent in this wavelength range, effects such as dye sensitization at a semiconductor electrode [22] are unlikely. Furthermore, light absorption due to chargetransfer complexes formed between ZnTPP and redox species is also unlikely because both of the redox couples give photocurrent peaks at the same wavelengths. The involvement of ZnTPP triplet states can also be ruled out. Fig. 5 sunLmarizes the spectroscopic properties of ZnTPP taken from the literature. An emission study of ZnTPP in a methylcyclohexane-isopentane glass at 77 K shows that two phosphorescent bands appear in the NIR [23,24]. One of the bands (781 nm) is assigned t o a 0 - O band of a triplet-to-singlet transition and the other (875 nm) to a 0 - 1 band. Although these band positions are close to the photocurrent peaks (770 nm and 860 nm) in the action spectra of the wet cells, the coincidence of the peak positions does not necessarily imply that the observed photocurrent peaks are assignable to S - T transition. If this

211

were the case, a mirror symmetry relation between the photocurrent and emission spectra should hold [25]. Namely, photocurrent peaks should be shifted to shorter wavelengths by ca. 100 nm if the above assignment is reasonable. The environment of the ZnTPP molecules and the temperature used in their luminescence measurements are different from the conditions in our own study and this is likely to cause shift in the peak wavelengths. Actually, the above argument may not hold in view of the fact that fluorescence peaks of ZnTPP dispersed in the glass at 77 K [23] coincide with those for a sublimed film of ZnTPP at room temperature within a few nanometers. Even so, a rough calculation using a radiative lifetime of the triplet state of ZnTPP, 0.48 s-1 [24], yields a molar extinction coefficient ( e ) of t h e order 10 -4 M - r c m - 1 for S - T absorption in the ZnTPP [26]. Similarly, for H2TPP with a radiative triplet lifetime of 5.6 X 10 .3 s -1 [24], the e value thus calculated is 10 .6 M -1 cm - I . These extremely small e values ex21ain n o observation of photocurrents due to S - T transition at the ZnTPP and H , T P P photoelectrodes. For the same reason S - T transition is unlikely to be responsible for the photocurrent synergism. It has been reported also that a triplet state of ZnTPP in toluene absorbs light at 745 and 845 nm [27]. However, the triplet-to-triplet absorption is not accountable for the NIR photoresponse observed with the wet cells since ZnTPP molecules are in their ground state and so the excited state population is negligible. The remaining and most probable candidate for the NIR absorption is impurity. Indeed, spectroscopic examinations revealed that ZnTPP in solution is sensitive to light, and several photoproducts, which absorb light in the NIR, were generated during the chromatographic purification of ZnTPP in the presence of light [28]. The curve a in Fig. 6 is an absorption spectrum of ZnTPP in chloroform prepared in L'eO' dim light by using ZnTPP purified in the light. The absence of an absorption peak or shoulder at 620 nm indicates successful removal of chlorine, the most common impurity of porphyrins [29]. Spectrum a shows a I

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Fig. 6. The absorption spectra of ZnTPP in chloroform. The solutions were prepared from ZnTPP purified (a) in the light and (b) in vet? dim light. The absorption spectrum of cur,~e b in the VIS is reduced to one third in the figure.

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I7.Harima et aL / Thin Solid Films 307 (1997) 208-214

broad absorption band in the NIR, although the NIR peak absorbance is only 0.4% of the Q-band absorbance in comparison with the N I R / Q - b a n d photocurrent ratio of 10%. When the chromatographic purification of ZnTPP and the solution preparation were both carried out in uerv dim light, the broad absorption band almost disappears, as shown by spectrum b. The subsequent illumination of the pure ZnTPP solution with an intense white light resulted in a substantial change with time in the absorption spectra (Fig. 7). The evolved absorption band has a peak at 810 nm and a shoulder at ca. 755 nm. When similar experiments were made with weaker intensity of UV light, the observed spectrum had a shoulder at ca. 810 nm and a peak at 755 rim. It can be inferred from the difference in shape between these absorption spectra that at least two chemical species are formed upon the illumination of the ZnTPP solution in the presence of O z, one having an absorption peak at 755 nm and the other at 810 nm, although experimental identification of these species was not attempted. An absorption spectrum of ZnTPP recovered from a sublimed film (ZnTPP used being purified in very dim fight) exhibited a similar absorption band with a magnitude slightly greater than spectrum b in Fig. 6, suggesting that the species of our interest cannot be removed by sublimation or, on the contrary, decomposition may be enhanced by heating in vacuo. Since sublimation is the process used for making the ZnTPP films, it can be expected that the impurity was present in the samples used in the photocurrent measurements. However, it should be noted that the absorption spectra of Fig. 7 differ in shape and position from the photocurrent action spectra of the wet cells. Yet further impurity was found on spectroscopic examination of a chloroform solution of ZnTPP containing DDQ; as stated earlier, DDQ is a prerequisite for the removal of chlorine. The DDQ-containing ZnTPP solution was irradiated for 10 rain with an intense white light and chromatographed in dim light with a neutral column and chloroform as the eluting solvent. A green portion ca. 5 cm above the main ZnTPP band was collected and the fraction was dried

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Fig. 8. The absorption spectra of (a) ZnTPP photoproduct and (b) isoporphyrin of ZnTPP in dich[oromethane, and (c) photocurrent action spectrum of the wet ceil using a HQ/BQ redox solution. The two absorption peaks in spectrum a (550 and 600 nm) are from ZnTPP slightly involved in the impurity fraction. Spectrum b is a reproductionof figure 1 in Ref. [30], representing the absorption spectrum of isoporphyrin whose chemical structure is shown in the inset. with a rotary evaporator. The isolated impurity was dissolved in dichloromethane for a spectroscopic examination. The impurity's spectrum in the NIR (curve a of Fig. 8) matches curve b, the absorption spectrum of an isoporphyrin formed by two-electron oxidation of ZnTPP [30]. tn addition, these two spectra well resemble the action spectrum of the wet cell (curve e). Further evidence that photoproduct impurity is involved is that the photocurrent ratio (at 550 nm to that at 860 nm) for the wet cell prepared using ZnTPP purified in the dark was only 1.4% compared with 10% obtained previously. The impuritycontaining ZnTPP films were prepared by co-sublimation of ZnTPP and the isolated impurity in order to see whether the impurity-doping enhances the photocurrent response of the wet cells in the NIR or not. However, reliable action spectra were not obtainable because of a drifting of background currents. On the other hand, the photochemical formation of the isoporphyrins at the surface of a ZnTPP electrode immersed in a redox solution can be ruled out because prolonged illumination of the ZnTPP electrode for several hours did not affect the 860 nm:550 nm photocurrent ratio as stated earlier. On these bases, we currently presume that the NIR absorption in the sublimed ZnTPP solid is attributable to a trace amount of the isoporphyrin, which is formed by a photochemical reaction of ZnTPP with DDQ and cannot be completely removed by chromatography and sublimation. A species with an absorption spectrum similar to that of Fig. 8 has been found as an intermediate during illumination of a 2-propanol solution of ZnTPP in the presence of CdS clusters [3 l]. Finally, that H , T P P wet cells generated no photocurrent in the NIR is supported by H_,TPP showing no absorption in the NIR (Fig. 9).

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Wavelength/nm

Fig. 7. The time-evolution of absorption spectra of ZnTPP in chloroform during illumination of the solution with a 650 W-tungsten halogen lamp for (a) 0, (b) 8, (c) 20, and (d) 60 s, where ZnTPP purified in the dark was used.

3.2. Lack o f NIR photoresponse in the dr 3, cell and photo current synergism

Recent studies on the photovoltaic cell by capacitance and work function measurements show that a well-behaved

Y. Harbna et al. / Thin Sohd Films 307 (1997) 2 0 8 - 2 2 4

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depletion layer is formed in the ZnTPP solid at the A1/ZnTPP junction [32] and a diffusion potential (q5d) at the Schottky junction is 1.0 V [16,32]. In contrast, a e d value at the solution/ZnTPP junction is 0.1 V, when judged from photovoltages of the wet cells. Using these figures, energy level diagrams of the A1/ZnTPP and solution/ZnTPP junctions may be depicted as in Fig. 10. Here, it is assumed that electronic transitions in the NIR (transition b) take place from the valance band to energy levels lying below the conduction band edge of the ZnTPP semiconductor, where the unoccupied energy levels represent LUMO levels for the isoporphyrin. It is further assumed that LUMO levels of other photoproducts, not shown in the figure, are far below that of the isoporphyrin. In the proposed energy schemes, the LUMO levels lie below the Fermi level (E F) at the A I / Z n T P P interface so that electron injection from the ZnTPP solid into A1 will not be induced by the NIR illumination. On the other hand, A1

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electron injection into the redox solutions is feasible upon illumination of the wet cells with the NIR light (transition b) as well as with the VIS (transition a). Evidently, photoproducts other than the isoporphyrin cannot contribute to NIR photocurrent generation in the dry and wet cells because their LUMO levels at the junctions are below E F. On these bases, one can explain the photocurrent synergism observed with the dry cell of A 1 / Z n T P P / A u in the following way: the unfilled energy levels above E F can trap photoexcited electrons (process c) and the trapped electrons recombine nonradiatively with holes in the valence band (process d). Eventually, the impurity suppresses generation of photocurrents by acting as an electron trap. It is reasonable to assume that photocurrents due to the VIS light illumination will be enhanced when the LUMO levels become filled by illumination with the NIR light. Another possibility of recombination responsible for the_photocun'ent synergism involves the filled LUMO levels of the impurity close to the A1/ZnTPP junction. They can trap holes in the valence band (process e), followed by recombination of the trapped holes at Eg~ with electrons in the conduction band (process f). Obviously, the recombination process can also be quenched by the NIR light illumination. The broad ~7 spectrum such as that ~hown in Fig. 1 might be explainable in terms of contribution from other photoproducts to the photocurrent synergism if the LUMO levels of these impurities are located at adequate positions. In order to provide further confirmation for the energy scheme speculated above, Schottky-type cells of ZnTPP with different metal junctions were prepared. Fig. 1 t shows photocurrent action spectra of five photovoltaic cells featuring ITO, Bi, In. Pb and A1 as blocking electrodes, with their photocurrents normalized at 550 nm. Here, the ZnTPP sample chromatographed in the light was employed. The &a values at the junctions of ZnTPP with ITO, Bi, In, Pb and A1 have been evaluated as 0.1, 0.3, 0.4, 0.6 and 1.0 V, respectively, corresponding well to the differences in work function between ZnTPP and the respective metals [16]. In addition, 550 nm-photocurrents at the various junctions increased from 0.02 nA c m - : for ITO to 2 n A c m - : for

Solution/ZnTPP

Fig. i0. The energy level diagrams for the A1/ZnTPP and solution/ZnTPP junctions, where Ec, Ev and EF are the conduction band, valence band and Fermi level energies of ZnTPP, respectively,and Egs the LUMO Ievel energy of the isoporphyrin. Transition a represents bulk excitation in the ZnTPP solid and b represents excitation from the HOMO level to the LUMO level of the isoporphyrin. Processes c and d denote recombination via unfilled gap states whiie processes e and f via filled gap states close the AI/ZnTPP junction. For simplicity, the gap states responsible for the two photocurrent peaks are represented by one line.

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E Harima et al /Thin Solid Films 307 (1997) 208-214

A1 in the above order. A photocurrent shoulder can be clearly seen at ca. 860 nm for the I T O / Z n T P P junction with the smallest q5d value whereas photocurrents for the other metal junctions are small in the NIR and their spectral responses are almost featureless. This finding supports our speculation that the position of the LUMO levels of the impurity relative to E F of the ZnTPP solid at the metal/ZnTPP or solution/ZnTPP interface determine the magnitude of the NIR photocurrent relative to the 550 nm-photocurrent. Photocurrent synergism was measured with these dry cells. However, no systematic change of with 4,d was observed although the r/ value for the I T O / Z n T P P / A u ceU was the smallest among those for the dry cells investigated. A further test for the proposed energy scheme was attempted with wet cells consisting of redox couples whose potentials were more negative than ca. 0.2 V for I - / I 3 and H Q / B Q . In these wet cells, the LUMO levels of the impurity at the solution/ZnTPP interface lie below the Fermi level of the redox solution and thus illumination of the contact with the NIR light should not induce photocurrents. However, the wet cell experiments were unsuccessful because background currents in the dark were large compared with NIR photocurrents of the order 10 -~° A or much below and, more inconveniently, they drifted gradually.

4. Summary It is found that the gap states in the ZnTPP thin films originate from isoporphyrins formed photochemically during the purification of ZnTPP. The gap states are responsible for the NIR photoresponse of the ZnTPP wet cells and, in part, for the photocurrent synergism of the ZnTPP dry cell by acting as recombination centers for photogenerated electron-hole pairs. A change of VIS:NIR photocurrent ratio with different Schottky junctions is reasonably explained in terms of the proposed energy level scheme.

Acknowledgements One of the authors (Y.H.) acknowledges Grant-in-Aid for Scientific Research (No. 06650947) from the Ministry of Education, Science, Sports and Culture.

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