PHOTOVOLTAIC CELLS CONTAINING DYE SOLGTIONS BH. S. V. RAGHAVA RAO Department of Chemistry, Andhra University, Waltair, South India Received November 9, 1988
It has long been known (7) that when two electrodes of platinum or any other noble metal are introduced into a solution of a fluorescent dye and one of them is illuminated, a difference of potential is set up between the two electrodes, the magnitude and sign of the P.D. depending among other things on the dye, the solvent, the concentration of the dye, and the nature and intensity of the incident light. Various theories have been advanced from time to time to explain the mechanism of this interesting phenomenon, but with apparently little success. From the similarity of the current-potential curves, obtained when an auxiliary potential is impressed on the electrodes, to analogous curves for photoelectric emission, Goldmann (4) was led to the conclusion that photovoltaic effects owed their origin to photoelectric phenomena. Subsequently van Dijck (11) obtained with a copper oxide cell evidence which is singularly a t variance with such conceptions. The extensive work of Grumbach ( 5 ) has fairly well established the fact that the -seat of E.M.F. is in the illuminated electrolyte and that the electrode is practically inactive. Russell (9) pictured the process as a photochemical activation of the dye molecules and the building up of the E.M.F. by the impact of the activated molecules with the electrode surface. He traced the time lag of the photopotential curves to the formation of the active molecules a t some distance from the electrode, which would take a measurable time to diffuse to the electrode. Ghosh (3) developed this concept further, assumed a mutual deactivation of the molecules by collision, and by applying the Nernst equation for concentration cells derived an expression for the time rate of the rise of potential on illumination, which was verified with the experimental data of Rule (8) on fluorescein. He also showed that the maximum photopotential developed would be proportional to the square root of the intensity of the incident radiation, a prediction also proved by the data of Rule. The concept of a photochemical origin for the E.M.F. suggests at once a close relationship between photochemical absorption and photovoltaic potential. The complete lack of data of this type has excluded a crucial examination of the hypothesis of Ghosh. The following work was started 693
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to provide some of the required data. A point of further interest was the question of a possible parallelism between photovoltaic effects and the phenomenon of optical sensitization, so extensively employed in chromatic photography. Accordingly, erythrosin, a fluorescent dye very much used in the preparation of orthochromatic plates and a non-fluorescent dye, chrysoidine, were selected for investigation. EXPERIMENTAL
The dyes employed were of the highest purity obtainable and were further purified by recrystallization from alcohol. A concentrated solution of the dye was first prepared by dissolving in water a weighed amount of the dye and making up the solution to a definite volume; a check on the concentration was obtained by weighing the residue on evaporating a measured volume of the solution to dryness over a water bath. The solution was preserved in the dark in stoppered bottles of resistance glass previously steamed. Under such conditions the stock solution showed no signs of deterioration and a solution of erythrosin aged in this way for four months gave results differing from a freshly made solution of the same concentration by less than 2 per cent. Solutions of desired strength were made by diluting the stock solut,ion in the right proportion. The dye solution was contained in a glass cell with plane parallel walls and about 4 cm. wide. A coating of non-reflecting black on the outer faces with a small rectangular patch 1.5 by 1.0 cm. on one face to admit the light served to shut off effectively stray light from the cell. The cell was mounted on a thick paraffin block. The electrodes were rectangular pieces of platinum foil of the same size as the opening in the glass cell joined by a short length of platinum wire to sealed mercury cups. One face of the electrode and the platinum lead wire were coated with a thin film of paraffin wax, which greatly diminished local cell formation. The illuminated electrode was at a constant distance of 0.5 mm. from the glass wall and directly opposite the unpainted patch. The dark electrode almost touched the opposite wall of the cell. A light trap of a suitable shape immediately behind the illuminated electrode helped to screen efficiently the major portion of the electrolyte from the activating light. The source of light was a transparent silica mercury burner (K.B.B. atmospheric-horizontal type) working a t 85 volts and 2.0 amperes. The current in the burner was maintained constant by means of an adjustable rheostat in series. During the early experiments the entire radiation from the burner after passing through a glass cell containing cold water to filter the heat radiation was utilized for exciting the photopotentials, while, later, narrow spectral regions were isolated by interposing suitable Wratten filters of gelatin.
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PHOTOVOLTAIC CELLS CONTAINING DYE SOLUTIONS
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The cell showed a pronounced tendency to polarize; it was necessary to leave the cell a whole afternoon undisturbed when once it was polarized. The use of a potentiometer for measuring the photopotentials was thus excluded, for, during balancing, a small current is invariably drawn from the cell. A vacuum tube voltmeter was therefore considered to offer a distinct advantage; this instrument could further be made to give a continuous record of the growth of the photopotential. The principle of the valve-voltmeter is too well-known to need a full description here. It is however of interest to mention that a straight circuit utilizing a single valve operating well below its normal rated anode and filament potentials proved highly satisfactory. The arrangement was extremely stable, albeit a slight reduction in its sensitivity resulted thereby. A Philips B405 valve (normal rating,-anode 120 volts and filament 4.0 volts) was made use of with only 50 volts on the anode and 2.8 volts on the filament; the mutual conductance of the valve under these conditions was 1.0 milliampere per volt. This in conjunction with a Cambridge mirror galvanometer of sensitivity 0.33 X lo-* ampere provided a means of measuring potentials of the order of 0.33 X 10-6 volt. The photopotentials were of the magnitude of a few millivolts. It was therefore necessary to shunt the galvanometer suitably in order to bring the potentials to be measured within the range of the instrument. Potentials of 10 millivolts and above were measured on a Cambridge microammeter with a central zero, while lower potentials could be easily measured on the mirror instrument shunted so as to give a deflection of one scale division for a grid swing of 3.3 x volts. It is necessary here to remark on the great stability of the arrangement. During an experiment which usually lasted some hours the creep of the zero of the instrument was ordinarily one scale division, but in inclement weather the creep at times amounted to even three divisions on the mirror galvanometer. Even this large creep meant only a change in the grid potential of a tenth of a millivolt. The cell was connected directly between the grid and the negative end of the filament. The readings on the microammeter and the mirror galvanometer gave not only the magnitude of the photopotentials but also their sign directly. RESULTS
The photopotentials were negative in sign in the case of erythrosin solutions, but positive with chrysoidine. It is yet eaily to attempt an explanation for this difference in behavior,of the two dyes; possibly, the chemical constitution of the dye in addition to the solvent plays a part in determining the sign of the photo-E.M.F. When freshly formed, the cell generally showed no P.D. between its electrodes; even in cases when the cell exhibited a minute P . D . initially, this
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was never more than a fraction of a millivolt and the change of P.D. on illumination was always the same. This minute dark E.M.F.has possibly its origin in the presence of traces of impurities on the electrode surfaces. Figure 1 illustrates the time rate of the rise of the photopotential a t various concentrations of the dye; curves a, b, c, and d refer to 0.035 mg., 0.00017 mg., 0.275 mg., and 0.825 mg., respectively, of erythrosin per cubic centimeter of solution, while curves e and f refer to 0.0364 mg. and 0.0018 mg., respectively, of chrysoidine per cubic centimeter of solution. It will be noticed that the rise of E.M.F. is rapid a t first, then becomes comparatively slow, and finally reaches a saturation value. This saturation P.D. is determined by the concentration of the dye and the color and intensity of the exciting light. The gradual rise of E.M.F.on illumination is in accord with the concept of diffusion of activated molecules t o the electrode, 20 v1
5 15
-3 E
E 12
9 58
. I +
.3
a 4-
E4 0
20
10
60
80
100
120
770
Time in minutes
FIG.1
a process obeying a law similar to the curves of figure 1. Again, the disappearance of the photopotential on cutting off the'illumination (cf. curve c) is a much slower process than the building up. Both erythrosin and chrysoidine exhibit Lhis behavior in all concentrations. (Only one curve has been shown for convenience.) Another interesting feature of the curves of figure 1 is that the maximum potential is reached sooner in dilute solutions. Such a behavior is also explained easily on the diffusion theory, since with a decrease in the concentration of the dye more and more of the active radiation is transmitted t o the molecules contiguous to the electrode, thus reducing the diffusion path. Secondly, decreased viscosity effects a t smaller concentrations of the dye may also tend to accelerate the diffusion of the activated molecules. Further evidence in support of this hypothesis was obtained in this way. By slightly altering the position of the electrode the attainment of the maximum potential could be either retarded or accelerated.
PHOTOVOLTAIC CELLS CONTAINING DYE SOLUTIONS
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PHOTOPOTENTIAL AND CONCEXTRATION O F DYE
The variation of the maximum potential with the concentration of the dye in solution is shown in curves E, and CR of figure 2 for erythrosin and chrysoidine, respectively, in the composite light of the mercury arc lamp. For convenience the concentration of the dye is plotted on a logarithmic scale. Curves Ez and EB,giving the maximum potential developed by erythrosin solutions in the light of the mercury arc after passing through a green and yellow filter, respectively, are of some interest. While in more concentrated solutions green light is more active, in dilute solutions yellow light develops higher potentials. This difference probably arises from a difference in the extinction coefficients of the dye for the two kinds of light.
FIG.2
Another feature of the curves E (1, 2, and 3) is that the maximum in the potential occurs at the same concentration of the dye solution irrespective of the nature of the exciting light. Still another characteristic of the curves of figure 2 (excepting EAwhich will be discussed later) is that after the maximum potential has been reached, the fall of potential thereafter proceeds directly as the logarithm of the concentration of the dye solution. The same cannot be said however of the potential rise. The fall of E.M.F. with increasing concentration of the dye is most probably due to increasing absorption of the active radiation in layers immediately after the glass wall of the cell. Lowry (6) has shown by eliminating the presence of a thick absorption layer in front of the electrode that the potential-concentration
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curve exhibits no maximum, but that the potential merely tends to a saturation value. Of interest in this connection are the results of some experiments in which the exciting light traversed a column 4 cm. long of the dye solution of various concentrations in a separate absorption cell before being incident on the photo-cell. The photo-cell liquid contained 0.035 mg. of the dye erythrosin per cubic centimeter of solution corresponding to the optimum concentration. Curve EA of figure 2 depicts these results, in which the maximum photopotential developed by the photo-cell is plotted against the concentration of the absorbing liquid. EAand the descending part of El are parallel to each other, pointing to a parallelism between the two mechanisms. PHOTOPOTENTIAL AND INTENSITY O F LIGHT
The dependence of the maximum photopotential on the intensity of the incident light was next investigated in a cell containing the optimum con-
centration of the dye solution. Variations in intensity of the light were produced by interposing in its path neutral tint screens of known transmission. The results are shown in figure 3. Curve a refers to erythrosine and curve b to chrysoidine. It will be noticed that with both dyes the photopotential reaches a saturation value a t higher light intensities. In curves a’ and b’ the photo-E.M.F.’S are plotted as functions of the square root of the intensity. The linear course of these curves is in agreement with the deductions of Ghosh (3). PHOTOPOTENTIAL AND WAVELENGTH O F LIGHT
The next point of interest was the location of the spectral region most active in producing the P.D. The qualitative experiments in yellow and green light recorded previously are of no use, since the transmission powers
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of the filters are not known. Therefore the light from the arc was filtered through Wratten light filters; Nos. 26, 22, 12, and 35 appeared most suitable. Of these No. 26 cuts off sharply a t 600pp and transmits about 70 to 75 per cent of the very feeble orange lines of the mercury arc. No. 22 transmits 70 per cent of the yellow lines at 579 pp, while No. 12 allows to pass through 73 per cent of the green line and 75 per cent of the yellow lines. No. 35 cuts off all the ultra-violet up to 340 pp and the visible beyond 460 pp. The transmission of this filter is about 27.5 per cent of the line at 365, 56 per cent of that a t 405, and again only 27.5 per cent a t 436 pp. This is indeed a very wide band, but sufficiently narrow to show the comparative activity of the shorter wavelengths. The results are shown in table 1. Since the distribution of energy in the mercury arc is very different for the different wavelengths, in order to compare the activities of the various
PHOTOPOTENTIAL FILTER
Erythrosin
No. 26. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . No. 2 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . No. 1 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . No. 3 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Composite light. . . . . . . .:. . . . . . . . . . . . . . . . . . . . . . . . .
Chrysoidine
mv.
mn.
1.5 9.0 11.4 6.0 18.3
0 1.4 5.0 2.0 7.8
spectral bands it is necessary to know this energy distribution. Bensley (2) has measured this in an arc of the same type and working under similar conditions of current and voltage. He gives the following values for the energy of the different lines in the mercury arc spectrum. I n addition there is present the luminous background of the arc; since this is very feeble, appreciable errors will not be introduced by neglecting it. Energy distribution in the K.B.B. mercury arc-horizontal type, current 2 amperes, voltage 85. Wane
length
577-9 546
404-8 366
Energy in arbitrary units
13 19 2.5 9
By taking into account this unequal distribution and variation of photopotential with intensity illustrated in figure 3, the transmission factors of
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the Wratten filters employed, the values given in table 2 are calculated for the activity of the different regions of the spectrum in producing the photopotentials. Before discussing these values further it is advantageous to consider the absorption of the dyes. Erythrosin in water shows very slight absorption a t 360pp followed by strong absorption between 455 and 562pp with a maximum at 518pp and is transparent from 562 to 630pp, while chrysoidine is transparent from 330 to 360pp and from 540 to 630pp with a pair of unseparated bands in the region 360 to 540pp (10). It is clear from a comparison of the data for photovoltaic activity with the absorption of the dye, that the maximum potential is induced by regions of the spectrum immediately following the long wavelength limit of the absorption band, both in the case of erythrosin and chrysoidine. Further, erythrosin is known t o sensitize the photographic plate to wavelengths up to 600pp with a pronounced maximum in the neighborhood of 580pp TABLE 2 Calculated activity of different regions of the spectrum in producing photopotentials CALCULATED PHOTOPOTENTIAL WAVELEHGTH
6 1 5 ~ ~ 577-9M.U 5 4 6 ~ ~
