PHOTOVOLTAIC EFFECTS I N GRIGNARD SOLUTIONS. I11
NEW OBSERVATIONS.A POSSIBLE THEORY R. T. DUFFORD Department of Physics, The University of Missouri, Columbia, Missouri Received November 7, 1918
This article1 records certain new observations on photovoltaic effects in Grignard solutions and in other substances, and outlines briefly certain theoretical considerations which may apply to the effect. The work is a continuation of that described in previous papers by the writer (1). NEW OBSERVATIONS
The results here reported are summarized from observations on about eighty cells, some of which, because of changes of electrodes or other experimental conditions, are equivalent to several single cells each. About sixty of the cells contained Grignard solutions, and about twenty were cells of other types which it was desired to study for purposes of comparison. In all, results are now available from more than three hundred cells, on which the total number of readings would probably exceed two hundred thousand. The tests with the device for circulating the solutions, described in the previous paper (l),have been continued; so far, they seem to confirm the conclusions stated there. The writer wishes to make further experiments before stating final conclusions; the question whether two effects exist is still open, though the conclusion seems very probable. For work in this field, it would be very convenient if some electrode could be found which would be entirely free from any response to light, when immersed in Grignard solutions. So far, no substance has been found which does not give some small photovoltaic effect in these solutions. Mercury covered with carefully dried calomel, in ethylmagnesium bromide solution, gives only a few millivolts; probably the calomel protects the mercury surface from the light, or from the products formed by the light. But even this small response is thirty to forty times the error of measurement. And with phenylmagnesium bromide, the solution becomes discolored. Hence this type of electrode, while perhaps the most nearly inactive so far found, is 1 Abstracted from a thesis presented to the Graduate School of the University of Missouri in partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1931. 709
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far from satisfactory. It is therefore necessary in studying the responses of single electrodes to use H-shaped cells in which the electrode in one arm can be kept completely dark. While these cells give satisfactory results, they require the use of larger volumes of solution than would otherwise be necessary. The use of standard reference electrodes, such as the calomel half-cell, has not been attempted, because of the diffiulty in bridging over from aqueous solutions to ether solutions which must remain absolutely anhydrous, and because of the uncertainty concerning the electromotive forces that might be introduced at the liquid junctions. A really inactive electrode would be very convenient. The failure of phenylmagnesium bromide solutions to transport any considerable amount of effect of illumination with it in the circulation experiments seems to indicate that the effect in this case may be due to some kind of sensitive film formed on the electrode surface. The capacitance measurements made by Hammond (2) seem to make it clear that thin films always exist on the electrode surfaces in these cells. Tests reported in the preaeding article (1)indicated that the response with platinum electrodes in Grignard solutions is not due to adsorbed oxygen; electrodes held at 800°C. in a hydrogen atmosphere for some hours, and cooled in hydrogen, behaved the same as electrodes flamed and cooled in air, or electrodes kept between slices of sodium in ether. These precautions seem sufficient to exclude the possibility of effects due to oxygen, which might have been expected toproduce some effect if present in or on the electrodes, where it could react under the stimulation of light. Electrodes cooled in nitrogen behave similarly. It seems probable, therefore, that the regularly observed effects are not due to films of gas on the electrodes. However, it seems very clear that other kinds of film on the surface of a platinum electrode can affect its response to light. For example, a film of the oxidation product formed by the momentary exposure of an electrode wet with Grignard solution to the air, or the film formed by dipping a moist electrode into a Grignard solution, will change the behavior of a platinum electrode, as shown in figure 1. These films raise the dark voltage to unusually high values, sometimes more than one volt, and they increase the resistance of the cells enormously. As the figure shows, the response to light may be even greater than without the film. Exact values of the thickness of these films cannot be given, but the order of magnitude is about 0.1 to 0.01 mm. Preliminary results on several other metals in Grignard solutions are available. With lead electrodes, the presence of a surface layer of oxide seems to make no important difference in the response. With aluminum, the response is slightly larger if the layer of oxide is left on the surface, as the sheet comes from stock, than if the sheet is freshly polished just before inserting it in the solution. Clean copper surfaces show a good response.
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Surface layers of oxide give rise to differences in the response which are undoubtedly due to the light-sensitiveness of the cuprous oxide itself. The response of cuprous oxide in the solutions mentioned is less than in some aqueous solutions that have been studied. The curve in figure 2 was obtained from a type of cell obtainable commercially, which contains a sensitive electrode of cuprous oxide on copper, an
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FIG.1. CELL No. 101, Pt 1 CaHSMgBrI Pt, WITH ONE PLATINUM ELECTRODE ILLUMINATED Curve A is for a clean bright platinum electrode; curves B and C are for another similar electrode, but taken on different days, Curve D is for the electrode of curve A after it was coated with the oxidation products from the Grignard reagent; curves E and F are for other coated electrodes coated with hydrolysis products of the Grignard reagents. The upward-pointing arrows indicate the time a t which illumination begins; the downward-pointing arrows indicate the end of the illumination.
inert electrode of lead, and a solution of lead acetate or nitrate. The curve was obtained with potentiometric equipment, so that the cell supplied no current except the minute amounts used in obtaining balance. Used with a milliammeter, t.he response and recovery of the cell are even more rapidtoo rapid for an instrument to follow. The current changes from zero in t8hedark to several milliamperes in intense light. The tendency to reach a
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maximum response quickly, and then to show a decreasing response with continued illumination, seems to be usual in cuprous oxide. Cells with sodium and potassium electrodes in Grignard solutions have been given some study. The metals were cut and handled under anhy-
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drous ether, except that they were kept in a nitrogen atmosphere while they were transferred to the solutions. The surfaces, bright when first cut, become bluish gray on standing. This change of color probably indicates formation of some kind of surface film on the electrodes; but it is doubtful if the films contain oxygen, or if they do, whether these metals would release
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PHOTOVOLTAIC EFFECTS I N GRIGNARD SOLUTIONS. I11
it under illumination. The sodium electrodes gave small but remarkably consistent responses both to light and t o x-rays. Typical curves are shown in figure 3. The potassium gave smaller and more erratic responses; on standing a few days, the metal dissolved completely, and was replaced by a dark fibrous mass, probably magnesium deposited electrolytically by local action. This reaction deserves further study.
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FIG.3. CELLNo. 105, Na 1 CzHrMgBrI Na, ONEELECTRODE ILLUMINATED Curve A shows the response to white light, the results being unusually reproducible. Curve B shows the voltage response of this cell to x-rays; curve C the currentresponse to x-rays; and curve D shows another voltage response curve taken j u s t after the current curves were obtained, showing that polarization by the current did not affect the reproducibility of the voltage responses.
Somewhat similar cases of formation of sensitive films in aqueous solutions are known. Grube and Baumeister (3) and others have studied the light-sensitivity of anodically polarized platinum electrodes. The response is of the type that shows a reversal on continued illumination. On the other hand, the writer found t,hat anodically polarized aluminum in aqueous solutions is relatively insensitive both to visible light and to x-rays. Ulhraviolet light gives a small but consistent effect.
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The general conclusion from these observations seems to be that while surface layers do not necessarily cause photovoltaic effects to appear, they do modify the effect frequently, and may in some cases be responsible for the appearance of the effect. The experiments described above seem to indicate that the effect is not due to oxygen in all cases. The study of such surface effects seems to the writer to be well worth continuing, since it seems probable that the results can be of great help in deciding the nature of photovoltaic action. The study of the effects of depolarizers, described briefly in the preceding article, seems equally promising, and is being continued. A number of cells have been made in which Grignard solutions were hermetically sealed in glass. These cells have shown a good response to light after three years; under proper conditions, therefore, their useful life is considerable. In collaboration with Dr. H. E. Hammond, the writer has studied the response of a number of Grignard cells to x-rays. In general, the responses obtained have been large compared with those from visible light; a typical case is shown in figure 4. They seem to obey the same logarithmic law as applies t o other cases of the photovoltaic effect, though the law is only a veryrough approximation. These observations were made under conditions in which the shielding of the circuits was not entirely satisfactory, so that there is some possibility that electromagnetic induction, together with rectifying properties of the cells, could give rise to a spurious effect. While auxiliary experiments made at the time seemed to prove that such spurious effects were negligible, there were discrepancies in the behavior of certain cells which did not seem explainable otherwise. The results are therefore regarded as preliminary, and are communicated with due reservation. More satisfactory apparatus has been prepared, and additional work is to be carried out with it. The effect gives some promise as a method of measuring x-ray intensities. It is interesting to compare with the preceding results the curves obtained from photovoltaic cells which contain no electrolyte. The light-sensitive substance was either cuprous oxide or selenium, though other substances are known which give the effect, Such cells can be made by forming a layer of cuprous oxide on copper by heating (such plates are used commercially in the cuprous oxide type of rectifiers which have come on the market recently), and then pressing a layer of wire cloth against the oxide, or plating or sputtering a metallayer on it. When illuminated through the wire cloth, current tends to flow, the wire cloth becoming positive by several hundredths of a volt. While some investigators have regarded this phenomenon as a photoelectric effect, the writer has included it with the photovoltaic effects, since it is essentially the production of an electromotive force by light, and since the response not only follows the same roughly logarith-
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mic law as other photovoltaic effects, but shows the same tendency to diminish and even to reverse its direction under prolonged intense illumination. Typical curves are shown in figure 5. Copper and nickel wire cloths have been used, the nickel giving a somewhat larger response. The response and recovery are very rapid in these cells, so that they can follow fluctuations in the illumination up to frequencies of 1000 per second, beyond which VOLTS
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the response becomes small. Their dark voltage is usually zero. The reversal of the response is shown in the last two illuminations in figure 5. The recovery from such a reversed response is direct; positive values are not retraced. Subsequent responses are reversed. It is not known how long such a negative response would continue; the cell which gave the curve reproduced here had completely recovered its normal charac-
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teristics by the following morning. The reversal seems to occur equally quickly whether the cell delivers maximum current or none. Substances which behave in this respect like cuprous oxide, generally show a reduction in electrical resistance on illumination. They also show a certain amount
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FIQ.5 Curve A shows the voltage response t o white light of different intensities, of Cell No. 400, Cu I CUZOI Cu, a "dry" cell with the outer coating of copper electroplated on the CUZO. The reversal of the voltage response on intense illumination is noteworthy. Curves B and C compare the voltage and current responses of Cell No. 405, a dry Cu I Cut0 I Cu cell in which the outer electrode was a copper wire cloth pressed against the cuprous oxide, with the similar responses in curves D and E for Cell No. 404, a Ni I Cut0 I Cu cell with the copper wire cloth replaced by one of nickel.
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of unilateral conductivity, which permits their use in rectifiers. Some writers have claimed that the rectifying ability and the photovoltaic responses are directly proportional to each other, but this statement is disputed by others. The striking similarity of the behavior of these dry cells to that of electrolytic cells raises the question whether the effect in the dry cells can be due to traces of moisture. Apparently the answer to this question is negative. One such cell was kept heated to above 100°C. for some time, then was cooled in a desiccator in which it was kept for several weeks. Its characteristics were not changed a t all. THEORETICAL CONSIDERATIONS
It seems worth while now to examine the experimental evidence so far obtained, in search of indications as to the probable nature of the processes that underlie the photovoltaic effect. In discussing photovoltaic effects, it is usual to divide the phenomena into two classes: (a) the group in which the electrode is the light-sensitive part of the cell (this is the type discovered by Becquerel) ; and (b) the type in which the electrolyte is the sensitive part. But the Grignard cells containing solutions of substances like phenylmagnesium bromide, which transport little or no effect with the electrolyte but instead seem to form a sensitive layer on the surface of an otherwise inert electrode, seem to belong in a third class, or a t least in a very special subdivision of the second. The dry cells containing substances like cuprous oxide or selenium, but no electrolyte, seem to belong in a still different class, which quite possibly should include some of the cases usually placed in the first group. While many authorities regard the last group as photoelectric rather than photovoltaic in nature, they are placed in the latter class here because of the very great resemblance of their behavior to that of other kinds of photovoltaic cell. The points of resemblance may be stated, for emphasis: (a) the effect observed is a change in electromotive force; relatively large currents can be obtained, but the currents are not always proportional to the illumination; (b) the responses occur with wavelengths too long to excite a surface photoelectric effect in the substances concerned, so that it must be explained by a special internal photoelectric effect if it is due to such an effect; (e) the maximum electromotive force developed is roughly proportional to the logarithm of the illumination, and not a linear function of it, nor is it a linear function of the frequency; (d) the maximum current obtainable is likewise not always proportional to the illumination, a t least for high intensities of illumination, but is more nearly proportional to the logarithm of the illumination; (e) the E.M.F. decreases and sometimes actually reverses on long-continued illumination; (f) the response shows a time lag so great that such cells cannot follow light variations of frequency very far above 1000 per second. It is of course true
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that dry cells show differences in behavior from that of the other classes: they respond and recover more rapidly; the response and recovery curves are somewhat different in shape, possibly indicating a different mechanism for the response; and they show relatively large decreases in resistance when illuminated. The statements which follow are intended to apply primarily to the effects observed in Grignard solutions, although they apply to a certain extent in a great many other cases. It is not yet possible to give an entirely satisfactory theory of all photovoltaic effects. The writer regrets sincerely that the severe limitations of available space at present necessary have compelled omission of adequate review or reference to the many theories presented by other workers. A picture is here suggested tentatively as being a probable one; it necessarily contains many elements suggested by other workers, but contains a few points that are apparently new. There are strong reasons for doubting that the effect can be due simply to the emission of electrons under the influence of light. One such reason is the existence of “Minchin reversals,’’ in which the photovoltaic E. M. F. reverses its direction if the illumination is continued. This may not be a conclusive argument, since it has been shown earlier in this article that the same type of reversal can be obtained from the “dry” type of photovoltaic cell, in which the effect is usually explained as being due to the emission of photoelectrons at a barrier layer. However, it may be that this commonly accepted explanation of the action in dry cells is incorrect. Another reason for doubting the simple electronic explanation, in the present case, is the fact that some of the solutions carry the effect with them, when circulated, so that a previously illuminated solution can affect an electrode in the dark, just as if the electrode itself had been illuminated. Not all of the solutions seem to be able to do this; but the fact that some of them do suggests that the effect depends in some way on the formation of either ions or excited molecules by the light. Other reasons for doubting the electronic theory are suggested in the second paragraph above; the theory is quantitatively unsatisfactory in several respects. The suggestion that light actually ionizes the solutions seems to be definitely contradicted by the experimental evidence. If a Grignard cell be regarded as a concentration cell, for the purpose of computing the change in the concentration of the ions controlling the electrode potential that would be necessary to explain the observed changes in E. M. F., it is found that the concentrations would need to change in a ratio which often would exceed a millionfold. From the conductivity, the ionization is already considerable, and any such change as demanded by the theory would almost certainly give rise to a measurable change in the conductivityof the solutions. This point has been tested thoroughly. Hammond (2), in a number of measurements of the resistance of such cells, found that the changes pro-
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duced by light were very small. In one case a 2 per cent change was observed, but all the others were considerably smaller. Harty (4), in a large number of measurements, found that the changes rarely exceeded one-half of 1 per cent. In the majority of observations, the change was an increase rather than a decrease in resistance, as if the light had suppressed rather than increased the ionization. In view of these facts, it seems more probable that light produces some type of excited molecules, rather than ions. It is very likely that the electrodes in the Grignard solutions are covered by layers of adsorbed molecules, which probably are not in quite the same state as the molecules in the solution. There are two chief reasons for such a statement. First, the direct-current resistance of the cells often exceeds the electrolyte resistance, as determined by alternating-current methods, by as much as a thousandfold. Apparently the D. c. resistance is due largely to polarization. It can be decreased by the addition of suitable depolarizing substances. Further, the capacitance of these cells, which can be determined simultaneously with the A. c. resistance, proves to be very large, so that the most reasonable interpretation of the observations seems to be that the capacitance is that between electrodes and electrolyte. Since values of as high as seven microfarads per square centimeter have been observed, it appears that the relatively non-conducting film that covers the electrodes is often only a few molecules thick, possibly a t times only one. The sensitiveness of the voltage of some of these cells to shaking may indicate that in some cases these surface layers are very loosely held. The variability of the dark voltage suggests that the films may be continually changing. The photovoltaic effect is then visualized as being due to the formation of excited molecules by the light, either throughout the liquid or, in some cases, only in the layer of electrostatically strained molecules making up the surface film on the electrode; the surface-layer molecules are then more or less completely replaced by excited molecules which are formed in the layer or which drift in to it, or else the excited molecules form an additional layer on the electrode surface. There is evidence in the capacitance measurements of Hammond and of Harty that both of these possibilities actually occur. These adsorbed layers of molecules introduce adsorption potentials which must modify the potentials of thc cells. The action of depolarizers in reducing the D. c. resistance of the cells seems to be due to the substitution in the surface films on the electrodes of other molecules which offer less hindrance to the processes which occur at the electrode surfaces in conduction. The depolarizing substances are not good conductors by themselves, and it is found that at the same time that they reduce the D. c. resistance of the cells, they increase the electrolyte resistance, as would be expected from diluting the electrolyte with a nonconducting liquid. The depolarizing action occurs in systems in which there seems to be no possibility of chemical reaction, and no other explanation than the one just suggested has been put forward for such cases.
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While the subject is by no means settled, it is probable that no photochemical reaction occurs in the Grignard solutions which is able to explain the observed photovoltaic effects. It is the opinion of the writer, based on a considerable amount of evidence collected from the literature as well as from observation, that even in those cases in which photochemical reactions are known to occur, in most cases a photovoltaic effect exists independently of and in addition to the changes in E,M. F. due to the chemical reaction. I n view of the facts, now well established (new evidence on this point will be forthcoming from the writer’s laboratory later), that a photovoltaic effect can exist independently of fluorescence of or any selective absorption of light by the electrolyte, it seems certain that any idea of a necessary associat,ion of these phenomena must be abandoned. A suggestion as to the nature of the excited molecules postulated above may add a clarifying detail to the picture presented. Most writers have suggested simply an electronic displacement for the excitation process. However, such excitation is usually associated with selective absorption, and, judging from the duration of the associated fluorescence, the life of excited molecules of this type is entirely too short to explain the long-continued photovoltaic effects. It seems that for the present purpose something further is needed. Either more stable types of electronic displacement than are known from other phenomena must be assumed, or else other types of excitation must be sought. Now, another type of excitation is known, from its occurrence in connection with the Raman effect. I n this type, the atoms which make up the molecules are separated somewhat by the light, and are set in vibration, the frequency of the internal vibrations being related to the characteristic infra-red frequencies of the molecules. Whether excitation of this type is sufficiently long-lived to explain photovoltaic effects is not known. The writer believes that either this type, or a similar type in which the vibration is missing and the life is longer, will be found able to supply the necessary mechanism for explaining photovoltaic effects. For a given molecule, there are as many types of excitation of this sort as there are types of interatomic linkages. It seems necessary, and not unreasonable, to suppose that at least two types of excited molecules are formed in the cases in which reversals occur; if the type which forms more slowly, or which diffuses to the electrodes more slowly, is also the more stable type, so that its effect ultimately outweighs that of the less stable variety, a possible basis is found for at least a qualitative explanation of the reversal phenomena. The picture suggested has been described very sketchily and with many details omitted, since it is still in the formative stage. It has, however, already justified itself from the point of view that it has suggested many new experiments which have yielded valuable information regarding the photovoltaic effect.
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The writer is grateful to irofessors 0. M. Stewart and H. M. Reese, of the Department of Physics, the University of Missouri, for continued encouragement and support of this work; and to the gradually increasing group of graduate students who by working out important details are making possible more rapid progress on the problem. Thanks are due the Research Laboratory, Incandescent Lamp Dept., General Electric Co., Cleveland, Ohio, and to Mr. Samuel Wein, of New York, for certain materials used in this work. REFERENCES (1) (2) (3) (4)
DUFFORD, R. T . : J. Phys. Chem. 34: 1544 (1930); 36: 988 (1931). HAMMOND, H. E. : Thesis, University of Missouri, 1929; Phys. Rev. 36: 998 (1930). GRUBEAND BAUMEISTER: Z. Elektrochem. 30: 322 (1924). HARTY,JOHNHENRY:Thesis, University of Missouri, 1932.
