metal electrodes

Volume 56, number 3

CHEMICAL. PHYSICS LETTERS

MECHANISM OF PHOTOCURRENT

15 June 1978

GENERATION

AT ZINC-TETRAPHENYLPORPHINE/ME+AL

ELECTRODES

Tomoji KAWAI, Katsumi TANIMURA and Tadayoshi SAKATA Institute for Molecular Science. Myodaui, Okazaki 444. Japan

Received 4 January 1978 Revised manuscript received 17 llIarch 1978

T%e energy structure of zinc-tetraphenylporphine(ZnTPP)/metal photoelectrodes and the dynamic process of photocurrent generation have been studied. A mechanism is presented, in which the direction of the photocurrent is determined by the Schottky barrier formed at the ZnTPP/metal interface, whereas the charge carriers are produced at the ZnTPP/solution interface by electron transfer from excited ZnTPP to the acceptor in solution_ An electron transfer process through the lowest triplet state of ZnTPP is suggested.

1. Introduction Solar energy conversion is one of the most important current problems, and from this point of view various types of photoelectrochemical cells have been investigated [I -51. Porphyrin compounds are one of the substances which could be used as a photoelectrode, for they absorb light in the visible region and are stable in electrolytes. Furthermore, the porphyrin skeleton plays an important role in photosynthetic activity in the chlorophyll molecule. Photoelectrochemical cells consisting of metal-tetraphenylporphine (MeTPP) have been studied by several workers [6-g]. In the case of a zinc-tetraphenylporphine(ZnTPP)/Al electrode [7], a photovoltage of 1.1 V is reported. However, it is not been well understood even why the electrical current flows in a given direction, and the energy structure of the electrode interface or the dynamic process of photocurrent generation are still unsolved problems. In this study, in order to elucidate the mechanism of the photocurrent and photovoltage generation, we made the photoelectrode of a thin film of ZnTPP on a metal or semicbnductor surface, and analyzed its absorption, emission, and ultraviolet photoelectron spectra, together with the action spectra of the photocurrent, the current-voltage curve and the photoresponse of this electrode system.

2. Exper.mental The pllotoelectrochemical celI consists of the photoelectrode(ZnTPP on metal or semiconductor), a counter electrode (Pt), the Ag/AgCl reference eIectrode, a gas inlet valve and aE electrometer (Keithey 160B) or a potentiostat (Hokutodenko HZ2) for the detection of photovoltage and photocurrent. Methylviologen or oxygen was added to the solution as an electron acceptor_ The electrode was illuminated throilgh a quartz window by a 500 W xenon lamp (U&o UISOlC) or by a 1.5 @ xenon flash lamp (Xenon Co.). ZnTPP(Strem Co. Ltd.) was evaporated in vacua onto the metal electrode surfaces, the backside of which was connected to a copper wire by Ag paste and sealed by. epoxy in order to eltiinate contact of the bare metal with the Jectrolyte. The thickness of ZnTPP was monitored by a Sloan thickness meter (DTMZOO). Metals

such as platinum (99.9%), Pd, Au, MO, Cu, Zn, Al and a SnO, transparent conductor (95% SnO, + 5% SbZ04) were used as the substrates. 40 &/cm2 of short circuit current, 530 mV of open circuit voltage was obtained for this photocell as the maximum value. The quantum efficiency for incident light of 435 nm, defined as the number of electrons produced from a given number of

absorbed photons, was 2%.

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CHEMICAL

Volume 56, number 3 3_

PHYSICS LETI’ERS

spectra coincide and the spectra are similar in shape in the region above 380 nm. This indicates that the photocurrent is generated via the excited states of the

Results and discussion

3.2. Energy

d&mm

15 June 1978

of the electrode

ZnTPP film. The adsorbed dye on the metal surface is The cathodic

photocurrent

wzs generated

by illumi-

nation on the ZnTPP/Pt elecrrode, that is, the pro-

duced electrons flow from the photoelectrode into the solution. Fig. 1 shows the dependence of the photocurrent upon the wavelength h of iliuminated light (action spectrum) and the absorption spectrum of ZnTPP deposited on a quartz plate (Soret band: 435 run, Q band: 550 um). The peak positions of both

Optical

A-ion

Zn TPPf Quartz

Cl Band

Fluorescence

n coo WAVELENGTH

Excitation

Photocurrent Action znTPP/pt

500

600

(nml

F& I_ The optical absorption spectrum of ZnTPP (200 A)/ qawrtz. and the emission and the photocurrent action spectra of ZnTPP (200.4)/Pt electrode. The emission and the photocurrent action spectra were corrected for the intensity distriburion of the xenon lamp and the characteristics of the monochromator.

542

considered to be ineffective for photocurrent generation in contrast to the clear photosensitization effect on the semiconductor surface, because an electron transfer from the excited molecule into the metal is rapidly compensated for by back injection of an electron from the metal into the hole produced in the dye. Here, the thin film of ZnTPP (50-2000 A) is regarded as au organic semiconductor rather than as a layer of adsorbed dye molecules. The ultraviolet photoelectron spectrum of ZnTPP revealed that this compound is a p-type semiconductor with a work function of 4.3 eV and the top of the valence band at 5.0 eV from the vacuum level. It is well known that, when a metal and a semiconductor of different work fslnction are brought into contact, a Schottky barrier is produced [lo]. By taking into consideration this barrier formation, the energy diagram of the photoelectrode with a Pt substrate is presented in fig. 2. Because the work function of ZnTPP is smaller than that of Pt(6 ev), electrons flow from TPP to platinum until the chemical potentials of the two materials become equal, and consequently, energy level bending of ZnTPP near the interface is produced, i.e. the accumulation layer of holes. When charge carriers are produced in ZnTPP layers by photoexcitation, electrons will move along the bending of the level in the bulk of ZnTPP, and holes in a counter direction. For the ZnTPP/Pt system, electrons move to the solution and holes to the metal and recombine with the electrons in the metal at the interface. For the ZnTPP/Al system, on the other hand, this phenomenon will be reduced because of the small amount of band bending (Al : 4.25 ev). This mechanism was confumed as follows: Since the extent of the level or the band bendkg depends upon the difference between the work function of the metal and the TPP, a large level bending results in the case of a noble metal electrode because of its large work function, while the bending is small in the case of a base metal electrode where the work function is comparable to that of ZnTPP. The photocurrent (ip) and photovoltage (VP) for various metals are shown in table 1. There is a tendency that the metal electrode having a larger work function gives a larger value of ip and VP.

CHEMICAL

Volume 56, number 3

PHYSICS LETTERS

lSJune1978

ZnTPP

Zn TPP

Solution

voc

T-

VC!c

Al (t 25)

I Au(L 761 pt

1 (6 0) @V

%?%8vs EF

MVSV? hJ ---_-_-_-__E

F

Fig- 2. Electron energy diagram for the formation of Schottky barrier at the interface between ZnTPP and Pt. The work functions of Pt, Al and Au are presented on the lefthand side. V.B.: vaJence band, EF: Fermi level, Q: Q band, Vat.: vacuum Ievei, MV 2+/ MV+: the redox potential of methylviologen (-0.44 V versus NHE).

Table 1 Photocurrent (fp) and photovoltage (Yp) of ZnTPP/metal photoelectrode (hv = 43.5 nm, electrolyte: O.LN WI). The counter electrode is platinum. JUF represents the work function of the metals

Pt Pd Au MO

Cu Zn AI

WFWI

ip(nA)

Fp(mV)

6.0 4.97 4.76 4.29 4.29 4.25 4.25

300 250 13.5 40 25 <5 <5

165 150 160 15 6
The photocurrent dependence on the electrode potential was examined. When the electron energy level of platinum is being lowered by applying positive bias voltage, the band becomes flat and fmally bent downward. Accordingly, the photocurrent is expected to decrease and to change sign with increasing positive bias potential_ In fact, we observed the change of cathodic into anodic current at an electrode potential of 500 mV versus Ag/AgCl. 3.2. Interface for the effective charge separation In order to produce photocurrent, charge carriers be produced from a neutral exciton state at the

m-ust

first stage. The place where the effective charge separation occurs was determined by the following experiment. The photoelectrode has two interfaces, i.e. the interface between Pt and ZnTPP (interface I) and that between ZnTPP and electrolyte (interface II). The action spectrum of ZnTFP with a thickness of 800 A deposited on Pt was very similar to that with 200 Athickness shown iu fig. 1. The shape of the spectra did not depend on the thickness of ZnTPP in the thickness region of 80 to 1200 A. Becarrse of the large light absorbance at 435 run (Soret band), 75% of the incident light is absorbed by ZnTPP of 200 A thickness. If the charge carriers are generated at the ZnTPP/metal interface, a dip in intensity in the action spectrum at 435 run should be observed, because at the wave number of 435 nm almost all the light is absorbed in the 800 A ZnTPP film. This dip was observed in the case of the ZnTPF/Sn02 electrode system. The ZnTFF electrode on transparent SnO, was illuminated from two different directions. The action spectrum is similar to the absorption spectrum of ZnTFF when it is illuminated from the backside of the ZnTPP film, whereas it showed the dip in intensity at the Soret band when the electrode is illuminated from the ZnTPP side through the solution_ This indicates that the initial charge carrier generation occurs at the ZnTPP/SnO, interface, whereas it occurs at rhe interface between electrolyte and the ZnTFF film for the Pt electrode. In fact, the photocurrent was enhanced remarkably when an elec543

CHEMICAL

Volume 56, number 3 tron acceptor

such as methylviologen

or oxygen

PHYSICS LFXTERS

was

(ZnTPP/Pt) is similar to that reported bjr Soma [S] for the charge separation on the Hz-tetraphenylporphine crystal electrode on gold coated aluminum. The low quantum efficiency for the charge carrier generation at the metal/ZnTPP interface was examined. The quantum yield of the photocurrent generation for a dry sandwitch cell, Au/ZnTPP/Au, under a bias voltage of lo5 V/cm, was about 10s3 times smaller than that of a wet system. Furthermore, the photocurrent response is relatively slow (e.g. takes a few minutes for attaining equilibrium), whereas a rapid photoresponse in the lOA s region was observed in the corresponding wet cell by the flash photolysis technique. The low efficiency for the charge generation is partly because the exciton of ZnTPP is quenched by the metal substrate added into the solution.

This latter case

by the energy transfer process. This explanation is rationalized by the fact that the quantum efficiency of the fluorescence from ZnTPP (200 A) on Pt was half of that on quartz whose band gap is large enough to inhibit efficient energy transfer from ZnTPP. Consequently, the initial charge carrier generation occurs due to electron transfer from the excited ZnTPP to the acceptor in the solution at the ZnTPP/solution interface. Then the hole thus produced moves towards the metal surface because of the bending of the level. 3.3. Dynamic process of the photocurrent generation The next question is what kind of excited state of ZnTPP contributes to the electron transfer from ZnTPP to the acceptors in solution_ The electron transfer to the acceptor, such as met&ylviologen or oxygen, can occur at the ZnTPP/electrolyte interface through (1) the conduction band (charge carrier produced directly). (2) singlet state (Soret state(Sa) or Q state&)), or (3) the triplet state or ZnTPP. The fluorescence excitation spectrum of the electrode during the operation ofthecellisahowninfig. l.Thisisverysimilartothe photocurrent action spectrum in the same figure. This indicates that the excited ZnTPP decays to the lowest singlet state where it emits fluorescence_ It is known that ZnTPP relaxes from the lowest singlet state to the triplet state with a high effxciency [I l] _ From these results, the lowest singlet state or the triplet state is the probable precursor of the electron transfer to the solution. An electron ejection process via the triplet

544

state would

15 June 1978

be most probable, because (1) TPP’s which _

have a long triplet life time, such as Zn- or Mg-TPP,

were superior to Tl?P% with a short triplet life time in production of the photocurrent and photovoltage, aud (2) the photocurrent generated via the triplet state was estimated to be larger by a factor of 4-5 than that via the singlet state from a simple kinetic consideration. A detailed discussion will be published elsewhere.

4. Conclusion The energy

diagram

of the electrode

and the dynam-

summarized in fig. 3. The incident light excites a ZnTPP molecule to the Soret band or Q band state, to form an exciton. It decays to the lowest singlet state rapidly, which emits fluorescence, decays radiationlessly or crosses to the triplet state. The triplet exciton near the interface ejects an electron into an acceptor in the electrolyte such as methylviologen or oxygen. The remaining hole in ZnTPP moves along the bent band of ZnTPP produced by the contact of metal and ZnTPP (p-type semlconductor), and recombines with an electron in the metal. This mechanism explains the cathodic photocurrent and the other experimental observations. Beside this mechanism, the incident light of wavelength less than 380 nm would excite electrons to the conduction band directly. The photocurrent observed ic processes

are

ZnTPP

Solution

?

Fg. 3. The energy diagram and the dynamical process of photocurrent generation in ZnTPP/Pt photoelectrode in methylviologen sohltion.

Volume 56, number 3

CHEMICAL PHYSICS LElTEBS

in the action spectra below 380 nm would be due to

this mechanism. The behaviour of the excited ZnTPP is thus important for making au efficient wet solar cell, so that we are now attempting to elucidate these mechanisms by the laser flash photolysis technique.

Acknowledgement The authors wish to thank Dr. M. Soma, Dr. S. Tsukada and Dr. T. Kondow for many helpful discussions and Professor Y. Fujita for supplying the ZnTPP. We are also indebted to Mr. N. Sato, Dr. K. Seki and Professor Inokuchi for measuring the UPS spectrum of ZnTPP. We are grateful to Professor K. Yosbihara for continuous encouragement.

15 June 1978

References [l] A. FuJishima and K. Honda, Nature 238 (1972) 371. [ 21 H. Gcrischer, J. Electroanal. Interfacial Electrochem. 58 (1975) 263. 131 J. Manassen, D. Cahen, G. Fades and A. Sofer, Nature 263 (1976) 97. f41 J-M. Bolts, A.B. Ellis, K-D. Legg and M.S. Wrighton, J. Am. Chem. Sot. 99 (1977) 4826. I51 A.J. Nozik, Appl. Phys. Letters 30 (1977) 567. P-51M. Calvin, J. Thcor. Biol. 1 (1961) 258. t71 J-H. Wang, Proc. Natl. Acad. Sci. US 62 (1969) 653. 181 M. Soma, Chem. Phys. Letters 50 (1977) 93. PI M_Shiozawa, H. Y amamoto and Y. Fijita, Bull. Chem. Sot. Japan 50 (1977) 2177. WI W. Schottky, Z. Physik 118 (1942) 539. IllI P.G. Seyhold and M. Goutennan, J. Mol. Spectry. 31 (1969) 1.

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