J. Phys. Chem. 1882,86, 3796-3802
3788
fundamental of CH31.13 We display, here, the deconvolved line shape of the neat v2, not just its correlation function. In addition, the original, nondeconvolved data were also clearly Gaussian in nature. The striking difference in the profile of the neat u2 as compared to the dilute u2 and both profiles for 2vz is most intriguing. It is likely that we are looking at a temporal effect vis-&vis the shapes of the 1249-cm-' signal and the 2470-cm-l signal in the neat CH31 in regard to cage formation. However, there is no current physical model which will explain uz being Gaussian and 2vz being Lorentzian in shape (neat-liquid data).23 Apparently, Arndt and YarwoodZ0did not observe this Gaussian shape of the v2 signal in the neat CH31for reasons which are unclear. While the data indicate resonant transfer as the primary dephasing mechanism for u2 and 2u2, the behavior of the fundamental (neat liquid) indicates a great deal more complexity for the situation as noted above. The modulation time appears to be much longer for the neat liquid and localized site effects appear to have a much greater importance than in the case of either the dilute fundamental or the overtone, neat or dilute. This cage effect was noted earlier by Wang.24 The dynamic force constant of the intermolecular interaction was calculated by using Oxtoby's equation19 ~~
~
(23) Oxtoby, D., private communication. (24) Whittenburg, S. L.; Wang, C. H. J. Chem. Phys. 1977, 66, 4255.
where R, is the contribution from resonant transfer or the change in the 2uz line width upon dilution, qs is the upper state level number (2 for an overtone), and p is the g term from the inverse kinetic energy matrix for the Al bending mode of a five-atom symmetrical top molecule.25 The value of the transition dipole interaction constant was found to be (2.63 f 0.38) X lo4 mdyn A. This value is 2-3 orders of magnitude smaller than has been found for intramolecular bending force constants, as would be exp e ~ t e d . ~ ~It?is, ~ 'nevertheless, significant and indicative of a relatively strong transition dipole-transition dipole interaction and further evidence for a resonant transfer dephasing mechanism. Moreover, it offers a clear understanding of at least one dynamic source of the cage effect observed in CHJ which is independent of its permanent dipole moment. Furthermore, the results given here conform to a case I1 system (signal narrowing with dilution) with a dynamic mechanism as opposed to the static mechanism proposed in phenylacetylene.28 (25) Wilson, E. B.; Decius, J. C.; Cross, P. C. 'Molecular Vibrations, The Theorv of Infrared and Raman Vibrational SDectra": McGraw-Hill New York,-1955; p 306. (26) Reference 25, p 174. (27) Decius, J. C. J. Chem. Phys. 1948, 16, 214. (28) Baglin, F. G. J . Raman Spectrosc. 1980, 9, 202.
Comparison of the Action of Some Merocyanine Dyes as Solar Photovoltaic Elements and as Photographic Sensitizers Philemon Yamln,+ Allan P. Piechowakl, George R. Blrd,' LJepartmnt of Chemistry and of Chemical and Biochemical Engineering, Rutgers, The Stete University of New Jersey, New Brunswick, New Jersey 08903
and Don Morel Emon Research and Engineering Corporation, Linden, New Jersey 07036 (Received May 29, 1981; I n Final Form: May 27, 1982)
Experimental photovoltaic (PV) cells have been constructed from a combination of semitransparent aluminum (alumina surface), polycrystalline organic dye, and silver backing electrodes. Working with merocyanine dyes familiar in photographic sensitization, these cells have given open-circuit voltages of 1.1 V and short-circuit current quantum efficiencies of 15-30%. The fill factors, which govern efficiency under load, are on the order of 0.25, and the operating lifetimes are not yet adequate. Here we compare some individual dyes 88 photovoltaic sensitizers and as silver bromide photoconductivity sensitizers. The electronic thresholds for AllA120Bldyeand AgBrldye appear to be approximately equal. Since the dye packing structures are different (being three-dimensional crystals in the PV cells and close-packed monolayers on the AgBr surface), the absorption spectra differ grossly in form. The patterns of wavelength-dependentquantum yields also differ in the two applications. Field-dependent ionization sets an important limitation on many of the dyes in P V cells, as does the familiar drop of AgBr sensitization as monolayer coverage is exceeded. Ultimately one hopes for a synthesis of understanding of these two limitations, and even for a common solution. In the past, organic dyes have been dismissed as impractical for use in solar photovoltaic devices. Recently Morel and others'v2 have reported on simple and potentially inexpensive solar cells having an organic chromophore as the active component between electrodes of Al(Al2O3and Ag. These cells have functioned at shortcircuit current quantum efficiencies of 16% (carrier pairs 'Present Address: Polychrome Corp., Clark, NJ 07066.
per absorbed photon) and have exhibited open-circuit voltages as high as 1.2-1.3 V. Also, in spite of the relatively shorbwavelength absorptions, these cells have given overall sunlight conversion efficiencies as high as 1% . While none of the present materials has either sufficient conversion (1) D. L. Morel et al., Appl. Phys. Lett., 32, 495 (1978). (2) (a) V. Y. Merritt and H. J. Hovel, Appl. Phys. Lett., 29,414 (1976); (b) K. Iriyama et al., Jpn. J. Appl. Phys., 19, Supplement 19-2, 173-7 (1979).
0022-3654/82/2006-3796$0 1.2510 0 1982 American Chemical Society
Merocyanine Dyes as Solar Photovoltaic Elements
efficiency or sufficient stability under irradiation to justify the immediate design of useful devices, these results are more than 1order of magnitude higher than the fondest hopes of 5 years ago and would seem to justify continued efforts in this peculiar field. The long-wavelength “spectral sensitization” of photographic action in silver halide films was discovered more This effect has been enormously than 100 years useful in the development of practical panchromatic, color, and infrared films. The debate on “THE” mechanism of spectral sensitization has continued to this day, though a growing consensus is emerging that the fundamental step is the transfer of electrons from the excited dye to the silver halide ~ubstrate.~ With well-chosen materials the transfer process operates near 100% efficiency, so that body-absorbed short-wavelength photons and long-wavelength dye-absorbed photons are equally effective in producing the complex solid-state reactions lumped together as “latent image”. While this initial long-wavelength step converts a neutral dye into an oxidized cation radical, it does not necessarily follow that the donor molecule is irreversibly destroyed. In fact, in photoconductivity experiments with dyed single crystals of AgBr, the totality of electrons injected is likely to be greater than the number of available dye molecules long before any appreciable degradation of the dye is observed. Two of us once had the frustrating experience of setting up an experiment to monitor the degradation of some efficient photographic sensitizers as a function of environmental variations and then not being able to produce any degradati~n!~ The use of organic dyes in solar photovoltaic devices was suggested, in part, by analogies to photographic sensitization. The most efficient solar dyes are neutral molecules in the ground state, falling in the photographic dye class of merocyanines? whereas most useful photographic sensitization is accomplished with the ionic, mostly cationic, cyanine dyes. Merocyanines are used sparingly for special photographic applications and seem to operate with efficiencies comparable to the cyanines. While the cyanines are well-known for their chemical instability in solutions, the merocyanines are considerably more durable as a class, and both classes of dyes are highly stable in the solid, crystalline state. The fabrication of these cells has been described by Morel et al.’ but will-per request of a helpful referee-be briefly summarized here. The steps in making an experimental cell are as follows: (1)Evaporate semitransparent aluminum (ca. 50% T) on a clear glass or plastic substrate. (2) Break vacuum, and allow the spontaneous oxidation of aluminum to go to its natural limit of about 20-A thickness of oxide. (3) Either (a) restore vacuum and evaporate solid dye onto the AllA1203or (b) cast a dye layer out of solvent onto the AlIA1203surface. Spin-casting, the process used for covering computer chips with photoresist lacquer, gives surprisingly uniform layers. (4) Vacuum evaporate a back layer of silver over the dye. The configuration of a typical cell is shown in Figure 1. Note that all of the above processes, especially the cycle 1,2,3a, 4, are amenable to mass production of large areas of material. This is essential in any photovoltaic system that is to be of largescale practical use in harvesting solar energy. From ~
(3) W. West, Photogr. Sci. Eng., 18,35 (1974). (4) W. West and P. B. Gilman in “The Theory of the Photographic Process”,4th ed.,T. H. James, Ed., Macmillan, New York, 1977,Chapter 10, see especially pp 272-86. (5) A. P. Piechowski, Doctoral Thesis, Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, NJ, May 1976. (6) D. M. Sturmer and D. W. Heseltine, ref 4,Chapter 8.
The Journal of Physical Chemistry, Vol. 86, No. 19, 1982 3797
\--. J
,-
~
IRRADIATION THROUGH BASE
*
I
Figure 1. Figure 1 shows the configuration of a cell made by the procedure described In the text: (a) a clear substrate, glass or plastic; (b) a semitransparent layer of vacuum-evaporated aluminum metal, covering all but the area reserved for the connection to the opposite silver electrode; (b-1) a layer of AI,O, formed by breaking vacuum and allowing spontaneous aerial oxidation; (c) a layer of evaporated dye from 200 to 2000 A thick-alternatively, this layer may be cast from a solvent; (d) a layer of evaporated silver, covering all of the dye layer, but not covering the area reserved for contact to the aluminum (-) electrode. This figure Is drawn vertically on a roportionate scale, so that the dimensions represent 50 A of AI, 20 !of AI,O,, 300 A of dye, and 300 A of Ag. On this scale, the horizontal scale Is compressed by a factor of about lo-’.
this point of view, single-crystal silicon cells are a magnificent and tantalizing failure. The present investigation was initiated to explore the behavior of some of the best photovoltaic merocyanines in their other mode as AgBr sensitizers. To a first approximation the two processes are similar, involving electron injection from the excited dye to the A11A120, electrode in the PV cell and electron injection from the excited dye into AgBr in the photographic process. Of course, the most striking difference is that photovoltaic sensitization is accomplished with solid, polycrystalline or amorphous dye films of thickness from 10 to 200 nm, while photographic sensitization is generally accomplished with a close-packed monolayer of dye on the surfaces of the AgBr crystals. Over-dying silver halide systems beyond a monolayer usually produces a sharp drop in efficiency, although there are notable exceptions to this One of the mechanisms for this drop appears to be an attack on the silver latent image,8 but another more subtle mechanism apparently produces a drop in the efficiency of carrier-pair formation and electron injection as monolayer coverage is exceeded. We have not been able to accomplish AgBr photoconductivity sensitization with thick merocyanine dyes layers in this present investigation.
Experimental Section The AgBr single crystal used in these experiments was a sliced wafer (111)obtained from an ingot prepared by the Harshaw Chemical Co. The crystal orientation was determined by overgrowingNaCl from a saturated solution and observing the orientation of the resulting parallel microcubes. The crystal thickness decreased from 1.10 to 1.00mm during the course of the experiments as a result of repeated cleaning by etching in hypo. The dyes employed in this investigation were all synthesized at Exxon Research and Engineering Co., Linden, NJ, under the direction of Dr. E. L. Stogryn. Since these (7) B. Zuckerman, Photogr. Sci. Eng., 11, 156 (1967). (8) (a) J. Spence and B. H. Carroll, J. Phys. Colloid Chem., 52, 1098 (1948); (b)V. I. Borin, Usp. Nauchn. Fotogr., Acad. Nauk SSSR,Otdel Khim. Nauk, 7, 183 (1960). (9) J. P.Sage, J. Phys. Chem. Solids, 26, 145 (1965).
3798
The Journal of Physical Chemistry, Vol. 86, No. 19, 1982
Yamin et al.
TABLE I: Physicochemical Properties of Dyes Used in This Worka
dye code
chemcial name
absorp- extinction tion wavecoeff, length, (M/L)-' nm cm-'
-
half-wave potentials, V ER(112)
EOX(,,2)
M-8
3-carboxymethyl-5-[p(3-ethyl-2(3H)-benzothia-
518
93200
-1.31
+0.54-0.60
M-43
zolylideneethylidene Irhodanine 3-carboxymethyl-5-[ (3,3-dimethyl-1-ethyl-2(3H)indolylidene )ethylidene Irhodanine
498
82500
-1.34
(+0.87)
M-165
3-carboxymethyl-5-[p(3-ethyl-2(3H)-cr-naphthothiazol- 533
101100
-1.32
(t0.67)
y1idene)ethylidene Irhodanine
2,4-bis[3,3-dimethyl-l-ethyl-2(3H)-indolylidenemethyl I- 624 3 2 7 0 0 0 (-1.10) ndb 1,3-cyclobutadienediylium-l,3-diolate Table I lists the full chemical names and properties of the dyes discussed in this paper. The absorption wavelengths and
M-93 a
aoldh, % 0.37 at 544 nm 0.43 at 535 nm 0.67 at 562 nm ndb
extinction coefficients were obtained in CH,OH solutions and are in the usual units. The half-wave reduction potentials are measured relative t o a saturated calomel electrode (SCE) connected by a KCl/agar salt bridge t o a solution of CH,OH t 0.05 M LiCl. Direct current polarographic techniques were employed with a dropping mercury electrode. The one reduction potential in parentheses was obtained for a related molecule, the 1,l'-dimethyl derivative. The upper oxidation potential was obtained by any of the following: dc polarography (done vs. SCE), differential pulse polarography, or cyclic voltammetry on a CH,OH t 0.05 M LiCl solution, using a spinning Pt measuring electrode, a Pt auxiliary electrode, and an SCE reference electrode. The lower potentials in parentheses were calculated by an empirical equation nearly identical with that developed by S. a h n e , 2. Wiss. Photogr., Photophys. Photochem., 59, 1 1 3 (1966). The fluorescence intensities are peak values measured o n dilute methanol solutions at room temperature and corrected for the fractional absorption of the solution. The valuessre given relative t o rhodamine B chloride (laser grade, Eastman Kodak), which has an overall quantum yield of fluorescence in ethanol of 0.69, as reported by C. A. Parker and W. T. Rees, Analyst (London),85, 587 (1960). Since all of the dyes discussed here have similar fluorescence bandwidths, the tabulated values are roughly proportional t o their relative Not determined, quantum yields of fluorescence,
dyes had been carried to high purity by multiple recrystallizations, no further purifications were attempted before the studies of sensitized electron injection into AgBr. The dyes and their properties are given in Table I. To prepare for dyeing, the AgBr crystal was dipped briefly into sodium thiosulfate (photographicfixer) solution at 50% of saturation concentration and then rinsed repeatedly with deionized water, finally rinsed in MeOH, and dried in a N, gas stream. The dyes were dissolved in methanol-water solutions 1:l by volume, and the crystal was suspended in a silver holder in the solution for 60 min. We refer to this as a "quasi-equilibrium" procedure, since the surface concentration does not depend critically on time or conditions of dyeing, even though times of hundreds of hours may be required for the reorganization of dye aggregates on the surface to be completed.1° The dye solution is of low absolute concentration (ca. 1-2 mg of dye/25 mL of solution) but approaches saturation level so that aggregation and complete surface coverage are favored. The photoconductivity apparatus is essentially identical with that described by SageBand by Z ~ c k e r m a n .The ~ crystal is sandwiched between a Nesa (transparent tin oxide) coated quartz electrode and a Mylar-blocked brass base electrode, the sandwich acting as one arm of a capacitive Wheatstone bridge. With this configuration, the dyed surface does not have to be disturbed by the intrusion of electrodes. Since AgBr is an ionic conductor having a large concentration of mobile interstitial Agi+ ions, a steady dc field cannot be maintained within such a crystal. Accordingly, the bridge is driven by a 100-kHz sine wave of 300-V amplitude, and the constant, small, interstitial conductivity is nulled by the act of balancing the bridge. Then a light pulse of microsecond duration from a xenon flash lamp is impinged on the crystal through a monochromator, the pulse being timed to coincide with the forward electric field peak (positive inward from the front surface) or the reverse peak. Since the present crystals were dyed on both sides, there was only a small difference (10) A. E. Rosenoff, V. K. Walworth, and G. R. Bird, Photogr. Sci. Eng., 14, 328 (1970).
PHOTOCONDUCTANCE iBFm 1 ?--=-,
APPARATUS
Flgure 2. Block diagram of the photoconductivity apparatus used for observations on silver halide single crystals. Key features of this apparatus are as follows: (1) the cancellation of ionic conductivity of interstitial Ag,' ions present in the crystal, (2) capacitive coupling to the dyed crystal, which leaves the dyed surface undisturbed, and (3) pulsed light excitation inphase or out of phase wkh the high-frequency, high-voltage driving signal applied to the capacitance bridge.
in signal between the two directions, favoring the forward field, which draws electrons inward from the more intensely illuminated front surface. A block diagram of the apparatus is shown in Figure 2. The light absorption of the dyed crystal is measured by use of a Cary Model 14 (later 17) spectrophotometer equipped with an integrating sphere. Since the preliminary thiosulfate etch leaves the crystal with a clean but textured surface, the sphere is used to collect all of the light propagated through the crystal, regardless of lateral refraction or scattering from the textured surface. This measurement is by no means simple or easy, as the measured dye absorptions are at most 6-10%. These small absorptions must be separated from the residual (base-line) scattering effects of the textured crystal surface. The separation is quite easy when the dye absorption band is sharp and narrow, but very difficult when the dye absorption is broad and runs into the rising AgBr absorption below 500 nm. The photoconductive response of the dyed crystal to monochromatic light is interpreted relative to the response
Merocyanine Dyes as Solar Photovoltaic Elements
in the strong AgBr absorption region at and below 475 nm, with a correction made for the relative flux of incident photons and the absorption a t each wavelength. One wishes to know the number of charge carriers released but measures instead the product of charge and displacement qw as a voltage generated across the bridge and displayed on the recording CRT. The mean free e- path w is obtained by measuring the photoresponse of the crystal to shortwavelength intrinsic silver bromide absorption with field forward and field reversed: rf/rr = 1 ko
+
where rf, r, = responses (CRT) with field forward and field reversed, w = mean free path (cm), and k(h) = absorption coefficient (cm-l). In this equation, as derived by Van Heyningen and Brown,ll k is the absorption coefficient (cm-’) for crystal absorption on the natural log basis corresponding to the equation I / I o = e-kx here Io = incident monochromatic light intensity, I = intensity at depth x in the crystal, and k(X) = absorption coefficient (cm-l). There is one serious discrepancy introduced by this procedure of determining the mean free path w(cm), and it is that the mean free path is depth dependent within the AgBr crystal. The results of West and Saunders12have established the presence of a layer of excess electron traps (interstitial Agi+ ions) immediately below the crystal surface to a depth of 6 pm. The effect of this trapping layer appears as an apparent decrease in quantum yield for intrinsic carriers produced by decreasing wavelengths from 425 to 275 nm (k = 6.3 X 102-6X lo6 cm-l).I3 We believe this decrease in quantum yield to be apparent only but will follow the past practice of reporting dye quantum yields “as read” relative to intrinsic 475-nm response. It is highly probable that these quantum yields should be increased 3-4X to agree with photogtaphic quantum yields near 100% and to correct for the inapplicability of the Van Heyningen-Brown equation to a medium having nonuniform trapping properties. Optical measurements on the photovoltaic assemblies are somewhat simpler since the dye absorption is large and not subject to ambiguities. The “tail” absorption measurement on both sides of the absorption band is, however, rendered somewhat uncertain by the large correction for the absorption by the semitransparent aluminum layer and the special semitransparent silver layer. To measure quantum efficiency of carrier generation, one irradiates the PV device with monochromatic light of measured intensity (tungsten-quartz iodide lamp Schoeffel0.25-m monochromator, typically 10-nm slits). The current is calculated in electron charges flowing at low impedance (short circuit) and is then divided by the flux of incident photons per second yielding the quantum efficiency for incident photons. This ratioing measurement is accomplished by diverting 30% of the monochromator output with a neutral beam splitter to a spectrally flat (Si) detector. The ratio of short-circuit photocurrent to reference photocurrent is the “normalized photocurrent”. This process of taking a normalized photocurrent is only justified when the photocurrent has been shown to be linear with light intensity. This is usually the case. The normalized photocurrent is proportional to the quantum yield per incident photon and need only be divided by the fractional absorption to give
+
(11) R. Van Heyningen and F.C . Brown, Phys. Rev., 111,462 (1958). ( 1 2 ) W. West and V. 1. Saunders, J. Phys. Chem., 63, 45 (1959). (13) F. Moeer and F. Urbach, Phys. Rev., 102, 1519 (1956).
The Journal of Physical Chemistty, Vol. 86, No. 19, 1982 3799
-
100
v
>
u
2 -
h I \
““i v-L 80
r.
\
U U
w
2 b-
%
CY
\
\
40
‘
* O t / Y
400
1
+ABSORPTION b
I
--+
-
1
600 700 WAVE L E NC TH , I N NANOMETER S
500
Flgum 3. Absorption of l i i (- - -) and shortcircuit quantum efficiency (-) as measured for a favored merocyanine dye M-8. Note that the quantum efficiency is only moderate (-30%) in the main absorption band but that it rises to nearly 100% at shorter wavelengths. This is a typical pattern for merocyanines. (Reprinted with permission from ref 1. Copyright 1978 American Institute of Physics.)
a quantum yield per absorbed photon. When the normalized photocurrent is corrected for the known quantum efficiency of the reference silicon detector (ca. 80%) and is divided by the measured dye absorption (in the cell, corrected for partial blocking by the A1 electrode) at a particular wavelength, we have a true quantum yield, observed as a current. This ratio has been determined at low impedance (short circuit) and has come to be known as .,PC One wishes CPec were 1.00. It is occasionally 0.30, more often 0.15 at best, and the improvement of CPsc for these cells is a major research problem. When the low-impedance meter is replaced by a highimpedance voltmeter (open circuit), V,,, the open-circuit voltage is measured. V, is nearly independent of the light flux. When a cell is observed under a resistive load, to measure the maximum useful work, a power output equal to VJw represents a theoretical ideal. The optimized power VI is compared to this ideal by the fill factor, ff = VI/ ( V J A . Like a,, this is a pure number, generally 0.25 for organic cells, but ranging as high as 0.8 in silicon cells. Contrary to intuition, the low fill factor of organic cells is not simply the result of poor conduction in the body of the organic material. It is instead associated with the phenomenon of field-dependent ionization-a more difficult problem. For the present, we shall be concerned with CPsc. This is the most descriptive single parameter, and it can be appreciated in exactly the same sense as the quantum yield of a photochemical reaction.
Results Figure 3 shows the absorption spectrum and the quantum yield (short-circuit mode) of a photovoltaic cell made with the benzothiazole-substituteddye M-8. (The numbers used here are simply a sequential identification (merocyanine no. 8),used in place of the fullorganic namw (see Table I). This figure is reproduced from ref 1. Figure 4 shows the absorption and quantum yield of charge carriers for this same dye acting as a monolayer on the surface of the AgBr crystal. We note immediately that the absorption bands have different locations and shapes. In particular, the long-wavelength half-maximum absorption falls at 627 nm on AgBr but at 590 nm in the PV cell layer. (In viewing broad solid-state spectra of this sort, it is helpful to report the more precisely determined wavelength of half-maximum absorbance, and not just the peak X of the broad band.)
3000
The Journal of phvsical Chemistry, Vol. 86, No. 79, 1982
Yamin et ai.
n
-a‘
.05-
E
I
Qoz
- .04-
d
0 0
m
N -
dz
L
m
m
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-20
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W
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2.03z 0 z
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m
0
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560‘
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600 WAVELENGTH (nm)
i
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700
Figure 4. Performance of M8, the dye of Figure 3, acting now as a sensitizer of photoconductivity on the surface of a single crystal of AgBr. The absorbance (-) spectrum is quite different here, with dye present as a monolayer on AgBr, from the spectrum of evaporated microcrystayinedye shown in F$re 3. The pattem of reiative quantum yields also shows a rlse to shorter Wavelengths on AgBr. The factors governing thls shor-wavelength rise are considered to be quite different in the two cases.
The trend of quantum efficiencies-rising to shorter wavelengths-seems similar with this dye on AgBr and in the PV cell. However, the PV quantum efficiency falls nearly to zero (ca.5 % ) on the long-wavelength absorption tail and rises to nearly 100% a t the shorter wavelengths. The quantum efficiency on AgBr rises from 10% (X4?)for the absorption at 625 nm, associated with the largest agg r e g a t e ~to , ~20% ~ for absorptions at 535 nm, a wavelength appropriate to monomeric dye adsorbed flat-on to the AgBr surface. As will be seen, the origins of rising quantum yield at shorter wavelengths are quite different for dye on AgBr and in the PV cell. The optical response of a A11Al,031dyelAg PV cell is strongly dependent on the direction of illumination, as shown in Figure 5. Irradiation from the Ag side through a specially prepared semitransparent silver deposit only produces a photocurrent a t those wavelengths for which absorption is weak. Strongly absorbed light, being confined to the solid dye nearest to the silver electrode, is quite ineffective; weakly absorbed light penetrates to the AllA12031dyeinterface and is much more effective. Clearly, charge carrier separation occurs near the aluminum interface with these electron-injecting dyes. The electrochemical potentials and other properties are given in Table I for the dyes discussed here. A general study of dye potentials and an examination of electrochemical irreversibility in solution and its bearing on solid-state sensitization is discussed in separate papers.lSa (14)B. Zuckerman and H. Mingace, J. Chem. Phys., 60,3432(1969). (15)A. P. Piechowski and G. R. Bird, Photogr. Sci. Eng., 22, 306 (1978). (16)G.R. Bird, K. Norland, A. E. Roaenoff, and H. Michaud, Photogr. Sci. Eng., 12, 196 (1968). (17)(a) L.G.S. Brooker et aL,J. Am. Chem. Soc., 73,5332(1951).See ref 6 for related papers in the “Color and Constitution” series. (b) J. R. Platt, J. Chem. Phys., 26, 80 (1956). (18)J. Potenza and B. Toby, Department of Chemistry, Rutgers University, private communication, manuscript in preparation. (19)C. Reich, Photogr. Sei. Eng., 18,335 (1974). (20)P.J. Melz, J. Chem. Phys., 67,1694 (1972). (21)(a) V. Czikkely, H. Forsterling, and H. Kuhn, Chem. Phys. Lett., 6,207 (1970); (b) H.Biicher and H. Kuhn,ibid., 6,183 (1970). (22)J. Potenza and B. Toby, Department of Chemistry, Rutgers University, private communication, manuscript in preparation.
I
400
5 00 WAVELENGTH (nm)
6 00
Figure 5. Pattern of normalized photocurrents for a special photovoltaic cell made with 200 A of dye and a semitransparent siiver back eiectrode as well as the normal milransparent Ai front eiectrode. The solid curve shows the normal current obtained from irradiating the AI side, while the dashed m e shows the resuit of irradiatingthe Ag side. These observations establish that the Agldye interface is inactive for carrier generation. The bng-wavelength back irradiation peak occurs at 567 nm, where dye transmission is l/e. The corresponding short-wavelength peak at 429 nm is missing (larger exciton range) and Is replaced by an artifact at 412 nm associated with a transmission maximum of coiioidai sliver. 1.o -
- .020 I
-I
I
35-
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0
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a
5
-
N v)
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5 00 WAVELENGTH (nm)
600
Figure 6. This figure shows in superposition the photovoitaic current quantum yield and the AgBr photosensitization ad of another favored merocyanine dye, M-43. Here the absorption patterns are entirely different for the microcrystalline solid (-) in the PV cell and the adsorbed monolayer (- - -) on AgBr, the latter resembling a photogaphic “J” aggegate. The pattern of quantum yields is also entirely different, with the PV cell showing a slightly rising %=to short wavelength (X) much as was seen with the dye M-8 in Figure 3. The AgBr photoconductivequantum yield ad (0)is flat and constant across the narrow absorption band, except for scatter of data. Use the left-hand scale as a pure number to read both kinds of quantum yields.
Figure 6 shows the combined data for M-43,the merocarbocyanine dye synthesized with a rhodanine and a pseudoindole group. Here the PV absorption (solid line) and the AgBr absorption (dashed line) are entirely different. In fact, the narrow absorption and action band (567-nm peak) on AgBr is reminiscent of a strongy redshifted photographic “J” aggregate.16 The influence of the nonplanar gem-dimethyl group in controlling aggregate (23)A. P. Piechowski, J. Electroanal. Chem., in press.
Merocyanlne Dyes as Solar Photovoltalc Elements
f ,501
The Journal of phvsical Chemktry, Vol. 86, No. 79, 1982 3801
.03
-
-1.02
6
3-
N
w
0 z
.25
-.01
2 8
v)
v)
U
U
m
m
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5 00 WAVELENGTH (nm)
600
Flguro 7. Figure 7 resembles Figure 6, showing the photovoltaic and photoconductive performance of a related dye M-165. Here, use the right-hand numerlcal scale multlpiled by 10 to read both photovoltaic (0). This dye shows a quantum yleld0, (X) and silver bromide PV quantum yleld which Is fiat or dropping silaMly to shorter A. However, since Its PV quantum yleld is generally lower than the yields of M-8 and M-43, no major new interpretationof thls pattern is warranted.
structure and spectrum will be discussed below. This dye shows a flat spectrum of quantum yields across the narrow aggregate band on AgBr, but a typical rising spectrum of quantum yields with shorter wavelengths in the PV cell. The spectrum of absorption and action of the a-naphthothiazole dye M-165 is shown in Figure 7. This dye sensitizes AgBr at somewhat lower efficiency and does so with a relatively flat spectrum of quantum yields (circles) over the aggregate absorption. The spectrum of photovoltaic responses is also shown in Figure 7. Finally, Figure 8 shows one kind of total exception to combined AgBr and AlIA1203sensitization. This is the squarylium dye M-93. This dye sensitizes PV action with high efficiency but with a relatively short time duration of good performance; by contrast it has not given appreciable sensitization of AgBr photoconductivity in any of our experiments. We assume that this dye would show some sensitization of photographic image formation in an actual photographic emulsion, since this is a much more sensitive test than our AgBr photoconductivity measurementa. The relationship between the centrosymmetric molecular structure and protruding nonplanar groups (C(CH,),)of this dye and ita failure to sensitize AgBr will be discussed below.
Discussion The sensitization of AgBr crystals by cyanine dyes often occurs with a preferential increase of quantum yields toward shorter wavelengths-wavelengths which can be identified with smaller aggregates or even flat-on adsorbed monomeric dye. It is easier to satisfy electrochemical (thermodynamic) criteria for electron injection with monomers and smaller aggregates, and it is also possible that smaller aggregates do not contain exciton-trapping or electron-trapping impurities (sometimes generated in situ from the starting dye). Thus, the rising quantum yield seen in AgBr photoconductivity with the benzothiazole dye M-8 is a familiar photographic effect. For dyes of high stability and large, negative reduction potential, wavelength independence of quantum yield on silver halide is sometimes observed, as with M-43. The absorption bands of polycrystalline or amorphous M-8 and M-43 dyes in the PV cells are characteristic of the solid packing of merocyanines. (Both M-8 and M-43 crystallize from solutions and have well-defined X-ray
2-
w
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0 v)
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4 00
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600 WAVELENGTH (nm)
700
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Flgurr 8. Figure 8 shows the absorption spectra of a 350-A evapoand a 1200-A soiventcast film (ex pytidlne) of the squaMium rated ~h dye M-93. This dye gives no observable photoconductive sensltizatkm of AgBr but does ghre good PV quantum yields in the AI1Al2O3ldyelAg cell, reaching 0, = 16.5% in solvent-cast films and 12.8% in evaporated films. These yields are relatively short-lived, and the dye is performing in an unstable polymorphic crystal structure.
diffraction patterns. Discrete X-ray peaks are not always observable in the thin dye layers appropriate to PV device optimization.) These dyes have large dipole moments17 and generally planar structures. Nearest neighbors tend to pack flat-to-flat at the van der Waals interplanar distance of 3.4-3.6 A, with the dipoles aligned antiparallel.ls Since the generally planar merocyanines have no symmetry element other than the molecular plane, the transition dipoles for the visible absorption band are generally not parallel to each other in nearest-neighbor pairs, and the aggregate or crystallite absorption splits into two subbands. This type of packing and spectral splitting has been analyzed by ReichlPfor the cyanines, and the analysis seems applicable to the merocyanines as well. The low photovoltaic quantum yield of the amorphous or polycrystalline merocyanines has sometimes been described in terms of an internal resistance, suggesting poor carrier transport properties (especially hole transport) in the solid dyes. However, when the light intensity is reduced, the quantum yield of current carriers does not rise as it should if a simple internal resistance were acting. It is more fruitful to interpret this low quantum yield as a consequence of field-dependent ionization,2oi.e., the need for an assisting electrical field to facilitate the production of a separated e-,h+ pair from a molecular exciton of low energy (a molecular exciton is simply an excited singlet molecule capable of rapid molecule-to-molecule migration by energy transfer). At shorter wavelengths, the exciton has energy sufficient to produce the separated e-,h+ pair without assistance-thus, the quantum efficiency is near 100%. Unfortunately, the rise in quantum efficiency seems always to occur just outside the useful limits of the absorption band. Field-dependent ionization is a serious limitation on many (but not all!) organic photoconductors. The internal field of the device seems to be concentrated near the A12031dyeinterface, and excitons trapped outside this region are relatively ineffective in forming carriers. Ghosh and Feng%have discussed the detailed behavior of M-8 in cells of different thicknesses in terms of exciton (24) A. K.Ghosh and T.Feng,J. Appl. Phys., 49, 5982 (1978).
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The Journal of i%ysical Cbmistty, Vol. 86, No. 19, 1982
mobility. They found a high exciton range in the shortwavelength region where the quantum yield is near loo%, but a much lower range for excitons of lower energy. The dye M-165 seems to be an exception to the usual pattern of field-dependent, wavelength-dependent ionization. However, the quantum efficiency is not high. The dye M-93 is not a merocyanine but can be viewed as an extension of the cyanines and merocyanine. It is not ionic (not a cyanine), nor does it have a large dipole (as do the merocyanines). It can be considered as two merocyanines joined head-to-head, or as a cyanine (+) chromophore crossed with oxanol (4. However one may view it, the dye has no particular inhibition against packing with the molecular centers offset, viz. the "slipped deck of cardsw18or even the "brickstoneW2* structures, both of which produce red-shifted absorptions. The failure of this squaric acid dye to sensitize AgBr efficientlycan be explained in purely structural terms. The dye has an appropriate reduction potential (-1.10 V) to be potentially a good silver halide sensitizer. However, the centrosymmetric structure (center of symmetry at the center of the squaric acid cyclobutene ring) which has emerged from an X-ray crystallographic studyz2does inhibit the kind of long edge-on adsorption so familiar with the cyanines. Also, this dye has an excess of nonplanar substituents (two gem-dimethyl groups and two N-ethyl groups) and cannot pack plane-to-molecular plane at the normal van der Waals distance. The most significant feature of the X-ray crystal structure is a nearest-neighbor plane-to-plane separation of 4.1 A. It has been shown that the 3.4-3.5-A packing of ordinary cyanines and merocyanines leads to helpful epitaxial coincidences between monolayer lattice parameters and normal dye packing in monolayer sheets.Is These favorable structural coincidences do not occur with M-93, and the dye is conspicuously weak as a sensitizer for AgBr. Even the flabon type of packing on AgBr seems to be inhibited for this dye. The performance of this same dye in PV cells is not correspondingly inhibited. The dye sensitizes with a high quantum yield, but the device lifetime is shorter than would be expected from the chemical stability. This large molecule can be vacuum sublimed without change, and chemical decomposition in a period of hours following sublimation or solvent deposition seems unlikely. One clue to the short life comes from the crystal study, which revealed that this dye crystallizes in at least two well-characterized polymorphs. In retrospect, we have to consider the hypothesis that vacuum evaporation or solvent coating from pure pyridine may deposit the dye in an unstable (0) form and that upon standing the 0 crystals disorder
Yamin et al.
through local formation of a domains. The 0form is PV active, and the a form might also be, but the disordered alp mixed system is expected to be inactive. When the polarographic reduction potentials of photographic sensitizers are compared with their sensitizing action, the reduction potential of -0.85 V (relative to (Ag(AgCllKC1(saturated) as reference electrode) is found to be a threshold for sensitization of AgBr photographic emulsions prepared at a pAg of 7. The potentials shown in Table I will suggest that the effective conduction band of A11A1203may lie a bit closer to the vacuum level than does the conduction band of AgBr (pAg -7) at -3.3 eV. The two substrate levels are very close together, and interesting comparisons can be made.
Conclusions Good photovoltaic dyes for the cell AlIA1203)dyelAgare likely to be good photographic sensitizers for electron injection into AgBr. Where photovoltaic and photographic results do not run parallel, the explanation can sometimes be found in structural control of crystal or aggregate packing by the nonplanar substituents on the generally planar dye chromophore. In a photovoltaic cell the dye exists as a layer 10-200 nm thick, while on the surface of AgBr crystals the same dye exists and functions best in a close-packed monolayer structure (
