A STUDY O F PHOTOVOLTAIC CELLS BY CARL W. TUCKER
Introduction If a photosensitive substance is placed upon two metal electrodes which dip into some solution, the potential difference between these two electrodes, in the dark, may be brought to zero. But if one of these electrodes is illuminated while the other is darkened, the photochemical changes which take place on the illuminated surface, may be expected to produce a potential difference between the exposed and darkened electrodes. What the magnitude of this voltage will be and what will be the sign of the charge on the illuminated electrode, may be expected to vary with the nature of the photosensitive substance and its light-reaction. Becquerel’ was the first to prepare a light-sensitive cell of this type. The effect produced upon proper illumination of such a cell has since been termed the “Becquerel effect.” The cell arrangement, as described is designated as a “photovoltaic cell.” Becquerel adopted silver chloride, bromide, and iodide as photosensitive substances, and these were placed upon electrodes of silver or platinum. The cell solution used was a dilute sulfuric acid. He observed, from the beginning, that the sign of the charge on the illuminated electrode varied with the thickness of the photosensitive material on the metal surface. He also pointed out that the observed voltage was inconstant during the illumination and that its value vaned with the intensity of the illumination. From Becquerel’s experiments, we may draw the following conclusions: (Throughout this paper, that electrode from which current flows in the cell solution, is considered as being negatively charged, and is called an anode, i. e . as zinc in a copper-zinc cell. An arrow included in the cell diagram designates the direction of current flow in the cell solution.) I. The photosensitive silver halides, placed on platinum sheet and made up into a photovoltaic cell with dilute sulfuric acid, form an electrode which functions as a cathode in the cell under proper illumination. 2. The photosensitive silver halides, placed on silver sheet and made up into a photovoltaic cell with a solution containing a small amount of sulfuric acid, all form photosensitive electrodes which vary in their electrical behaviour, as the silver halide thickness or uniformity on the electrode surfaces is varied. With a thin layer of the halides on the silver electrode, it becomes a cathode upon illumination and with a thicker or more uniform layer, it functions as an anode upon illumination. In 1891, Minchin2 recorded his observations on photovoltaic cells made up from several different electrode metals and using varied cell solutions. Min1
2
La Lumiere, 11, 1 2 1 . Phil. Mag., (5) 31, 207 (1891).
‘358
C.4RL W. T U C K E R
chin’s results again impress one with the indefiniteness of the electrical effects observed. I n some cells the illuminated electrode was found to be an anode. I n others the reverse effect was observed. I n still other cells both effects were found. Indeed, in regard to this latter point, I wish to quote &Tinchin exactly: “In nearly every cell that I used with tin plates-whose surfaces, as stated above, had not been treated in any way-the exposed plate was positive to the unexposed; but after a time varying from a few minutes to a few hours, it was found that this positive current died out and was replaced by an apparently stronger current, in which t’heexposed electrode was negative. Thus there was a change in the sign of the E. 121. F. produced by the continuous action of light. This again reminds us of 31. Becquerel’s observation about the thickness of sensitive layers. There seemed to be almost no exception to the rule that the exposed electrode begins by being positive and ends by being negative, the negative regime lasting for many days of prolonged exposure to light.” I should point out that Llinchin called the positive plate or electrode, the plate from which current flowed in the cell. That is, his nomenclature is the reverse of the one that I have adopted. Among the first photosensitive materials that hlinchin used, again, were the three silver halides placed on metallic silver. Alinchin prepared these sensitive electrodes by making a collodion emulsion of the silver halide which was then allowed to harden as a film on the silver plate. His cell solutions were different from those of Becquerel; sodium chloride, sodium bromide, and silver nitrat,e solutions being used with the silver chloride, silver bromide, and silver iodide coated electrodes, respectively. The results which Minchin found upon individual illumination of these three photovoltaic cells, are different from those recorded by Becquerel. With the cell arranged as follows: Illuminated Ag 1 AgCl NaCl AgClI Ag t
the exposed electrode was found to be a cathode. I n the cell, Illuminated Ag [ hgBr KBr AgBr 1 Ag t
the illuminated electrode was again found to be a cathode. But in the cell, Illuminated Ag i AgI AgSOs AgI Ag ~
+ the exposed electrode \vas found to be a n anode. Minchin devotes the remainder of his paper to a record of results observed with photovoltaic cells made up of different metals in various solutions. Since many of his conditions are not sufficiently described in detail, it is difficult to interpret all of his results. We may at least note in his work the following points:
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STUDIES I N PHOTOVOLTAIC CELLS
Illumination of a metal surface which has some photosensitive subI. stance present, either as a film or in densermasses, will produceaphotovoltaic effect. 2. The nature of the photochemical changes taking place at an electrode surface is recorded indirect:). by the direction of flow of the current produced in the photovoltaic cell. 3. This direction of current flow is found to vary with the nature of the cell solution, the photosensitive substance, the electrode metal, and the thickness of the photosensitive substance as an electrode surface layer. Hence we conclude that the nature of the photochemical actions must vary with each of these variables. 4. Minchin's record of the fact that in many cells he found an initial effect in which the current flowed in one direction in the cell, and then reversed as illumination continued, indicates that a change in the various possible photochemical or electrochemical actions at the illuminated electrode face, must occur, during the period that the cell is being illuminated. Wildermann' studied very carefully some photovoltaic cells of the simpler type. By a photographic method, he succeeded in constructing an accurate representation of the relation between galvanometer deflections and time of illumination. He did not study any of those cells types in which the photosensitive substance is present as a thick or uniform layer on the electrode surfaces. Wildermann found that the illuminated electrode was a cathode in all of the following systems: O . I X NaCl AgCl rig Illuminated .4g AgCl J, (Thin layer) t (Thin layer) ~
h g ' XgBr (Thin layer)
0.1sS a B r
Ag ' AgBr (Thin layer)
0.1sKBr
,,
Xg 1 AgBr (Thin layer)
O . I N LiBr
,, ,,
rig I &I (Thin layer)
0.1sK I
Hg HgrSOI (Thin layer)
0.1sKZSO,
c u I CUO (Thin layer)
11'40N NaOH
I, t,
t) JI
J,
,,
~
Jt
,,
t
t
t
t
t
t
'TVildermann: Z. physik. Chem.. 59, 553, 703 (1907).
XgBr I Xg (Thin layer) AgBr I Ag (Thin layer) XgBr I Ag (Thin layer) AgI Ag (Thin layer)
HgzSOa Hg (Thin layer)
'
CUO c u (Thin lager)
1360
CARL W . TUCKER
From Wildermann’s time-volt curves we can deduce that the photovoltage slowly approaches a maximum and falls quite rapidly to the original potential difference between the unilluminated electrodes, as the source of light is removed. Case’ produced a photovoltaic cell with electrodes consisting of a uniform layer of cuprous oxide on metallic copper. The illuminated electrode in this cell was an anode when the cell solution contained about 2 . ~ 5 7copper ~ formate and 0.47~formic acid. The new observation which Case made, was the behavior of this particular cell when illuminated with the external circuit closed. It was found that, if one electrode was illuminated and the other kept dark, a current was produced which flowed in the cell solution away from the illuminated plate and developed initially an E.PI1.F. of 0.11 volts. Upon continued illumination with the external circuit closed through a current measuring instrument, the current finally hopped to zero; but if the cell was now rotated, illuminating the previously darkened plate, the E.M.F. reappeared, current again flowing away from the exposed electrode. This action could be continued indefinitely. Case further stated that if these same electrodes with their photovoltaic coating, were transferred to a dilute sodium chloride solution and one electrode illuminated, the exposed electrode became a cathode. We may draw these conclusions from this paper by Case: A uniform layer of cuprous oxide on copper in a proper solution, beI. haves electrically in the same manner as did Becquerel’s uniform layer of silver chloride on silver in a dilute sulfuric acid solution. A uniform layer of cuprous oxide on copper, illuminated in a sodium 2. chloride solution as a cell electrode, behaves in the same manner as did Minchin’s uniform layer of silver on silver chloride in a dilute sodium chloride solution. 3. Upon continued illumination of Case’s photovoltaic cell with the external circuit closed, the photosensitive material upon the copper electrode surface seems to be destroyed. Simultaneously, it is reformed on the darkened electrode. Cuprous oxide-copper photovoltaic c ~ l electrodes l were carefully studied for the first time as regards density and uniformity of the cuprous oxide film, and the effect of a variation in the cell solutions used, by Garrison.* He devised a method of placing a layer of cuprous oxide on copper by which the density of the layer could be readily varied, as desired. His method consisted in dipping a polished copper sheet into a cupric formate solution and hydrol>-zingthe cuprous formate formed on the copper surface. Each time this procedure was repeated, the density and uniformity of the oxide layer was increased. Garrison records a point which is of interest as viewed in the light of the preceding researches. He found that many cuprous oxide-copper electrodes gave an initial anode tendency when illuminated, but upon continuous Case: Trans. Am. Electrochem. Sac., 31, 351 (1917). *Garrison: J. Phys. Chem., 27, 601 (1923).
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STUDIES I N PHOTOVOLTAIC CELLS
illumination this effect reversed. The initial anode effect was found to increase and the reverse cathode effect to decrease, as the density and uniformity of the cuprous oxide layer on the copper electrode surfaces was increased. Garrison in this respect substantiated the reversal rule first pointed out by hfinchin. This paper by Garrison may be summarized as follows: In the cell type, Illuminated Cu 1 CuaO (Thin layer)
K?SO4 t
CUSO I c u (Thin layer)
the illuminated electrode is a cathode. If this cell is illuminated with a closed external circuit, a current reversal takes place when the light-source is removed. If a potentiometer is used to observe the photovoltage, no current being allowed to pass through the cell as measurements are being made, this reversal tendency does not exist. I n the cell type, Illuminated Cu 1 Cu?O (Dense layer) the illuminated electrode is an anode. In a cell of the type, Illuminated Cu 1 CueO (Medium density)
XIS04
+
K~SOI
CU?O c u (Dense layer) CU?O 1 c u (lledium density
in which current tends to flow from the illuminated electrode in the cell, addition of sulfuric acid to the cell solution reverses the anode tendency. Addition of potassium hydroxide increases the original anode tendency. When solid copper sulfate is added to the potassium sulfate cell solution the original tendency of current flow from the illuminated electrode is diminished. On the other hand, addition of copper sulfate to a cell of the type, Illuminated Cu 1 CunO (Thin layer)
&SOn t
c u i 0 ' Cu (Thin layer)
in which the illuminated electrode is originally cathode, causes a reversal and an anode effect becomes apparent. I n all of the above types in which current reversal tends to take place as the nature of the cell solutions are varied, the Minchin reversal phenomena always becomes prominent at the time the change in direction of current flow takes place. Csing the same method of attack, Garrison' made a study of the photovoltaic cell, Illuminated Ag I AgI l - 1 . Phys. Chem., 28, 333 (1924).
Solution
.%I 1 4 2
1362
CARL W . TUCKER
in which he varied again the density of the halide layer on the silver surface, as well as the cell solution. I t was again found that the illuminated electrode was cathode in Illuminated Ag I AgI (Thin layer)
K2SOJ
+
&I I x g (Thin layer)
By coating the silver with the silver iodide through an electrochemical method, Garrison found that the illuminated electrode became an anode when the density and uniformity of the silver halide layer was increased. Illuminated Ag j AgI (Dense layer)
K2S04
+
.w
I Ag (Dense layer)
He also again found, that with continued illumination of the above cell, the original anode tendency of the exposed electrode was reversed. He found that addition of silver nitrate to the potassium sulfate solution increased the original anode tendency, while additon of potassium iodide increased the cathode tendency of the illuminated electrode. We may conclude from Garrison's papers, that the diverse observations recorded in the extant literature, regarding photovoltaic cells in general, may be eventually explained by a proper study of the density and uniformity of the photosensitive layer and the effect of the cell solution, since even slight variations in these two factors change the behaviour of the cells under illumination. EXPERIMENTAL STUDY Introduction This paper attempts to explain the behaviour of illuminated photovoltaic cells, through a discussion and experimental study of the cuprous oxide and silver halide photovoltaic systems. The viewpoints which will be introduced may be summarized as follows: I. Both the nature of the cell solution and the nature of the metal with which the photosensitive material is in contact will be factors in determining the photochemical reactions which take place under illumination. Local cells may form on the surfaces of the illuminated electrode. 2. These can have a marked effect upon the nature of the potential difference found to exist between the two massive electrodes of the photovoltaic cell proper. 3 . I n the case of a thick or uniform layer of the light-sensitive substance on an illuminated metal sheet, these local cells will be a t a minimum. They will be at a maximum in the case of thin or non-uniform layers of the photosensitive substance on an illuminated metal sheet. 4. The solutions used in the photovoltaic cell may be varied in such a way as to throw these local cells into the reversible or irreversible types of cells. The behaviour of the photovoltaic cell under illumination will accordingly be found to vary as the cell solution is varied.
STUDIES I S PHOTOVOLTAIC CELLS
I363
Apparatus I n order to simplify the cell behavior as much as possible, the photovoltage obtained upon illumination was always determined by a potentiometric method. Since practically no current passes through the cell when this method is used, the occurrence of a secondary electrode polarization is prevented. The cell electrodes used were of two types, the sheet electrode and the wire gauze electrode. The sheet electrodes were made from a thin rolled sheet, 8 X 3 cm. A copper wire connection was attached by soldering, in the case of the copper electrodes and by wrapping the wire into the thin sheet, in the case of the silver electrodes. In use, about a 6 cm. length of the electrode was immersed in the cell solution. Except in a few cases, the electrodes were not coated with an insulator of any kind on their back and unilluminated surface. The gauze electrodes were slightly larger than the sheet, having dimensions of about 6 X 6 cm. and a gauze wire diameter of . 5 mm. with a gauze size of 40 mesh /inch. The photovoltaic cell vessel consisted of an ordinary, glass, flat-walled, absorption cell, 4 X I O X I O cm. The exposed electrode was shielded from the illuminated electrode by a piece of black sheet rubber dropped down between the two electrodes. A Leeds and Northrup student potentiometer in connection with a Leeds and Northrup reflecting galvanometer sensitive to 82.2 megohms, was used for measuring the photovoltages. The source of light was a zoo-watt bulb on a I I O A.C. line, placed at a distance of I O cm. from the electrode face, with no reflector. In practice, electrodes were always made in duplicate, and by a system of two zoo-watt bulbs, each cell electrode could be individually illuminated and observed, the one serving as a check on the behaviour of the other. Since in many cases, important reversals in the photovoltage occur during the period of illumination, the ordinary tap-key was not considered sufficiently rapid, as a galvanometer connection. For this reason, a switch was arranged so that the galvanometer could be permanently connected to the cell during the I O to I j second illumination periods. The photovoltaic current was at the same time accurately balanced by changing the potentiometer slide-wire adjustment in such a way that the galvanometer showed zero deflection at all times. By this method, variations in the directional tendency of the cell current were accurately registered in the varying settings of the potentiometer. The Cuprous Oxide-Platinum Photovoltaic Cells Bancroft' has pointed out that the nature of a photochemical action can be determined by the chemical environment of the illuminated system. A photosensitive substance may be expected to be photochernically oxidized when illuminated in an oxidizing environment. A photosensitive substance may be expected to be photochemically reduced when illuminated in a reducing environment. Cuprous oxide, as a photosensitive substance, would be expected to possess the possibility of undergoing either photochemical oxidation J Phys C h e m , 12,
2 q
(1908).
1364
CARL W. TUCKER
to cupric oxide or a photochemical reduction to copper. If we place cuprous oxide on platinum, as an inert metal, and make it an electrode in a photovoltaic cell in which both oxidizing and reducing solutions are used, we should be able to obtain a confirmation of these idcas, as indicated by a variation in the cell behaviour as various solutions are used. Consider the photovoltaic cell, Illuminated P t I Cu2O
Reducing Solution
CunO 1 P t
in which one electrode is illuminated. Photochemical reduction of the cuprous oxide will tend to take place at the surface of the exposed electrode. X potentiometer applied to the cell terminals should show that the two electrodes are not in equilibrium, and that the exposed electrode is tending to function as an anode, in order to restore the equilibrium through an oxidation of reduced cuprous oxide and a reduction of cuprous oxide at the darkening electrode. Similarly, if we have the cell, Illuminated P t 1 CuzO
Oxidizing solution
CUQO1 P t
illumination of one electrode will produce a photochemical oxidation of the cuprous oxide on the platinum. The cell now being out of equilibrium, a potentiometer will show that a current is tendlng to flow in a way that will make the illuminated electrode a cathode. Equilibrium is tending to be restored through a reduction of oxidized cuprous oxide at the illuminated surface and an oxidation of cuprous oxide at the unexposed electrode surface. In such a cell with a neutral solution, the presence of dissolved atmospheric oxygen in the solution may possibly supply enough of an oxidation potential to cause the illuminated electrode to appear as a cathode. These various cell types were set up and their behaviour observed. I t is difficult to place cuprous oxide on a platinum surface in such a way that it will adhere when the electrode is dipped into a solution. This problem was solved by making precipitated and washed cuprous oxide into a thick paste with a little water, and rubbing it upon the face of a platinum gauze electrode. The cuprous oxide penetrated into the interstices of the gauze, and was found to remain satisfactorily in that location, even when the electrode wasdipped into a cell solution. All of these platinum-cuprous oxide electrodes were prepared in this fashion. The cuprous oxide was made by precipitation from hot Fehling’s solution with glucose and washing the precipitate well with hot water. The results obtained with these electrodes illuminated in various solutions, are indicated in the curves shown in Fig. I . In these curves the ordinates represent anode or cathode behaviour of the illuminated electrode and total voltage difference between the two cell electrodes. The abscissas correspond very roughly to time of illumination in seconds. These curves are not to be considered as a quantitative representation of behaviour, but rather, as a convenient method of illustrating qualitative results. They have all been drawn free-hand, after the cell had been set up and illuminated.
STUDIES IS PHOTOVOLTAIC CELLS
1365
I n none of these cells was a maximum voltage obtained in the time of illumination. The cell was only illuminated for a period of I O t o 1 5 seconds, since the thermal effects began to become appreciable after that time. The cell solutions used were 0.1?; except in the case of the acidified potassium sulfate and the potassium hydroxide and potassium sulfate mixtures. In these the acid or alkali were added in solution and dropwise until a definite indication of acidity or alkalinity was shown by litmus paper. In order to ensure that these observed effects were produced by the photosensitive cuprous oxide alone, the platinum gauze, free from any oxide, was illuminated in all of the solutions used. Either no effect or negligible effects 008 O'O
I
"t
010
FIG.I Behaviour of platinum-cuprous oxide electrodes illuminated in oxidizing and reducing solutions.
were found. The photovoltages observed with cuprous oxide on platinum were small but definite. They indicate that in all reducing solutions, photochemical reduction of cuprous oxide tends to take place; and in all oxidizing solutions, photochemical oxidation of cuprous oxide tends to take place.
The Cuprous Oxide-Copper Photovoltaic Cells In order t o determine whether or not the electrode metal has its own specific effect on the action of the photovoltaic cell, it would be desirable to repeat the above experiments, with the one substitution of a copper gauze electrode. Any variations in behaviour can then be attributed at once t,o the difference between the metals, copper and platinum. Before making the experiments, we can point out another possible variation in the cell behaviour, which may be expected. If we consider again, the platinum gauze electrode containing cuprous oxide in its surface, and if we agree that the photovoltaic behaviour of this electrode is produced by the illuminated cuprous oxide, a complicating factor enters.
1366
CARL W. TUCKER
I n electrodes of the gauze type, the back of the illuminated electrode is darkened but is intimately connected electrically, by the metal and the conducting solution, with the front and illuminated surface. Hence, we should expect a series of local cells to be set up on the face of the illuminated electrode, in which current will flow between illuminated and unilluminated points of cuprous oxide on platinum, or even between points of illuminated cuprous oxide on platinum and illuminated platinum. As pointed out, the possibility of this current flow in local cells is greatly increased by the peculiar construction of the gauze electrode, as compared with a sheet electrode. It follows also, that the potential difference between the illuminated and unilluminated mas-
co7L
FIG.2 Behaviour of copper gauze-cuprous oxide electrodes illuminated in oxidizing and reducing solutions.
sive electrodes of the photovoltaic cell proper, is determined only as a secondary thing and after the equilibrium adjustment is attained among the local cells on the illuminated gauze electrode surface. In the case of cuprous oxide on platinum, the effect of these local cells will be negligible, since the inert platinum cathode and anode in the local cell tends to polarize at once if any current passes through the system. If, on the other hand, the platinum gauze is replaced by a copper galizel this polarization may not occur. Since copper electrodes are not inert, Lhe metal anodes of these local cells will tend to pass into solution. These ions passing into solution about the illuminated electrode surface, at once increase the possibility of many new variations in electrode behaviour. But in every case, the photovoltage which we can determine between the illuminated and unilluminated massive electrodes will be determined only after the local cells have reached an equilibrium or attempt to reach equilibrium. The results of experiments using copper gauze electrodes, pasted with cuprous oxide, in various cell solutions, are shown in Fig. 2 . In this case, the
1367
STUDIES I N PHOTOVOLTAIC CELLS
clean copper gauze free of oxide was previously illuminated in each of these solutions and found not to be photosensitive. The results obtained can be attributed to the original photosensitivity of cuprous oxide. We may summarize these results as follows: I. The presence of copper metal in the electrode has a specific effect on the behaviour of the copper-cuprous oxide cell. 2. These cells with copper gauze electrodes show the so-called “Minchin effect.” The illuminated electrode is anode at first, then becomes cathode upon continued illumination.
ISeconds \O
1
I5
FIG.3 Behaviour of copper sheet electrodes, uniformly coated with cuprous oxide and illuminated in oxidizing and reducing solutions.
3 . The initial anode effect is more permanent when an oxidizing solution is in the cell. It is least permanent when a reducing solution is present. 4. I n every case, removal of the light source after the illuminated electrode has become a cathode, shows an increase in the cathode tendency. The potential difference between the two cell electrodes then slowly returns to zero. This effect is indicated by the dotted portion of the curves. Minchin also mentioned this same effect in his report. Before entering into the theoretical explanation of the behaviour of these cells, I shall present results obtained with electrodes in which local cell formation is minimized but not entirely removed. Such an electrode can be prepared by coating the copper sheet electrodes previously described, with a uniform and dense layer of cuprous oxide, but not insulating the back and unilluminated surfaces from the solution. The cuprous oxide was formed on copper sheet by the copper formate method as described by Garrison. The curves shown in Fig. 3 show the results which were obtained with these sheet copper electrodes. The solutions iised were 0.1K as before.
1368
CARL W. TUCKER
These results may be summarized as follows: I. The anode effect is magnified as the local cell formation is decreased. 2 . The voltage niaxiniuni indicates that the local cell action is still appreciable and that even these elcctrodcs would show a reversal in sign, if illuminated over a long period of time. 3 . The anode effect is greatest in reducing solutions and least in oxidizing solutions. 4. The general shape of the time-volt curve for cells with sheet coppercuprous oxide electrodes, is independent of the solution used. I wish to propose the following theoretical explanation for the behaviour of the copper gauze-cuprous oxide, and copper sheet-cuprous oxide electrodes. I n all of these experiments, the behaviour of the cell under illumination was markedly different from the behaviour of illuminated cuprous oxide on platinum. Also, the general behaviour of these latter cells was independent of the solution used. K e must, therefore, conclude that the change to a metallic copper electrode niust provide the basis for all the altered behaviour. Either the copper itself influences the progress of the photochemical action or the local cell action on the face of the illuminated electrode has become a predominating factor. Consider first the possible photochemical behaviour of cuprous oxide in contact with copper. If we could set up an electrocheniical cell of the type: CU (electrode)
1
Solution
j
Cu10 (electrode)
it would seem logical to predict that, the current would tend to flow in this cell solution from the copper to the cuprous oxide electrode. I n other words metallic copper tends to reduce cuprous oxide and copper itself tends to be oxidized by cuprous oxide. If now, these two cell electrodes are brought together until they are in contact, the same tendency must still exist. The system produced by bringing into contact these two cell electrodes is exactly that system which we have in a dense and uniform layer of cuprous oxide placed on copper sheet. I t follows that as far as photochemical tendencies are concerned, the cuprous oxide is in a reducing environment, exactly as if it, were in contact with a reducing solution. I therefore conclude that illuminated cuprous oxide on copper tends to be photochemically reduced. Consider now an ideal cuprous oxide-copper phot'ovoltaic cell electrode, consisting of a molecularly uniform copper sheet, perfectly insulated at its back or unilluminated side, and uniformly covered upon its front surface by a continuous layer of cuprous oxide one molecule deep. Let two such electrodes be built up into a photovoltaic cell, and one electrode be uniformly illuminated : Illuminated Cu , Cu?O Solution CUlO 1 c u 50 local cells can exist 011 the illuniinated surface. The illuminated cuprous oxide in contact with copper is tending tc be photochemically reduced. The illuminated cell is out of equilibrium and a potentiometer will show that the
1369
STUDIES I N PHOTOVOLTAIC CELLS
illuminated electrode is tending to function as a n anode, in order to bring the cell back into equilibrium. A constant photovoltage would be obtained which would be a direct measure of the photochemical reduction tendency of the illuminated cuprous oxide on copper. With a reducing solution in this cell the reduction tendency and the corresponding photovoltage will be increased; and with an oxidizing solution in this cell, the reduction tendency will be decreased and the corresponding photovoltage decreased. The cuprous oxide-copper sheet electrodes, whose behaviour upon illumination is illustrated in Fig. 3, are approaching this ideal electrode just described. Their anodic photovoltage is weakened and inconstant through the formation of local cells. The anode tendency is greatest in reducing solutions and least in oxidizing solutions. The thick and thin electrode film types first described by Becquere1,“une couche mince d’iodide et une couche 6paisse d’iodide,”-were really varying degrees of uniformity of layer of photosensitive substance on the metal electrode surface. Local cells on the face of the illuminated electrode can possibly exist in the following forms: c u I CUSO (light)
Solution
CunO 1 c u (dark)
c u I CUSO (light)
Solution
cu (light)
(2)
c u I CUSO (light)
Solution
Icu (dark)
(3)
c u ,CUP0 (very uniform and dense. Light)
Solution
cu20 I c u (uniform and thin. Light)
(5)
~
(1)
On the sheet copper electrodes coated uniformly with cuprous oxide, the local cells formation similar to ( I ) would be expected to occur most commonly. On the gauze electrodes, pasted with cuprous oxide, any of these local cells might exist. Further, in the case of the copper sheet electrode, the local cell action may be considered as being comparatively small. There is a large internal resistance in each local cell, in this case, since local cell currents flowing between the front and back surfaces of the illuminated electrode must traverse an appreciable length of the cell solution. A question at once arises in regard to the direction of flow of current in these local cells. In considering the behaviour of the photovoltaic cell with ideal electrodes free from local cells, it was found that the illuminated electrode possessed a negative charge as compared with the darkened electrode and current tended to flow in that direction which would equalize this difference of charges between the two electrodes. In these local cells now being examined, the two electrodes of the local cell are one and the same; they are identical with the massive illuminated electrode. There can be no difference
CARL W. TUCKER
I370
of charges between the two local cell electrodes. Since the whole existencz of the local cell must depend upon a localized photochemical tendency of illuminated cuprous oxide to reduce, current will flow in the local cells to apoint of illuminated cuprous oxide on copper from any other dissimilar point. The local cells are thus always tending to accommodate themselves to the photochemical reduction of cuprous oxide by copper. If the various cell solutionsusedupto the present time are reviewed, it will be observed, that each of them in the local cells, Illuminated Cu 1 CunO
Solution t
CueO i Cu (dark)
or
,,
c u CUZO ~
Solution t-
~
cu (light)
produces an irreversible electrode system. Since cuprous oxide is by postulation, always present in excess at a local cell cathode, it will be found that the local cells will only polarize anodically, with an increasing cuprous or cupric ion concentration at each local cell anode. It follows that, in all such cell solutions, the massive illuminated cuprous oxide-copper electrode, will ultimatelycbtain a surface polarized anodically, if local cell action issufficiently large and time of illumination is sufficiently great. This explains the behaviour of the cuprous oxide-copper gauze electrodes, which initially showed a small anode tendency which was immediately overbalanced by an increasing anodic polarization of the local cells. This explanation also produces a complete solution of Minchin’s reversal rule, previously cited. Even in the case of the gauze electrode systems, the initial anode effect though small, was appreciable and tended always to decrease the cathode effect produced by the anodically polarized local cells. As the light source was removed, the anodic polarization manifested in the cathodic effect in the massive illuminated electrode tended to apparently increase, as the initial arode effect of the illuminated massive electrode was at that moment completely removed. This explains the behaviour of the gauze dectrode systems indicated in the dotted extension of the curves in Fig. 2 . It will be noted that the entire theory of this local cell action upon an illuminated electrode surface, has been built up with the idea that the solution in the local cell converted it into an irreversible cell system. It should be possible t o select cell solutions that will throw these local cells into the class of completely reversible cells. If this can be done, the entire behaviour of the photovoltaic cell proper, may be expected to be again changed. The best approximation to Such solutions will be dilute neutral sodium chloride or sodium formate. A copper anode in such solutions tends t o pass into solution as cuprous oxide, through an immediate hydrolysis of any cuprous chloride or cuprous formate formed.
STUDIES IN PHOTOVOLTAIC CELLS
1371
The local cells,
c u 1 cuz0
NaCl
cu20 I c u
t
c u I CunO
NaCO2H
cu20 1 c u
t
may be considered as being completely reversible, or nearly 80.
010
-
ole L
FIG.4 Behaviour of copper gauze-cuprous oxide electrodes illuminated in solutions containing sodium chloride or sodium formate.
The results of illuminating both copper gauze and copper sheet electrodes coated with cuprous oxid?, in these special solutions, and with these solutions combined with oxidizing or reducing agents, are shown in Figs. 4 and 5 . With a copper gauze electrode coated with cuprous oxide and illuminated in a photovoltaic cell, all local cell action on the exposed electrode surface will tend to be increased in sodium chloride or sodium formate cell solutions. KO polarization occurs. The initial anode effect upon illumination will become negligible. If the local cells were completely reversible, no voltage difference whatever would be produced between the two massive cell electrodes. Cnder proper conditions the local cell, c u I CUlO
KaC1 c
Icu
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CARL W. TUCKER
would later produce cuprous oxide on all the exposed copper surfaces of the massive electrode and the maesive electrode would be converted into a copper electrode uniformly coated with cuprous oxide, and behave upon illuminations as such. If this case is realized, the illuminated electrode will became anode in these special cell solutions. I n reducing solutions, this effect will be most possible, since the efficiency of cuprous oxide formation may then be expected to be increased at the copper anode of the local cell. I n an oxidizing solution the reverse thing will occur, the efficiency of cuprous
FIG.5 Behaviour of copper sheet electrodes, uniformly coated with cuprous oxide and illuminated in solutions containing sodium chloride and sodium formate.
oxide formation a t a local cell anode will decrease, and some cupric ions will pass into solution producing an anodic polarization, with a resulting cathodic tendency appearing in the illuminated massive electrode. Since dissolved atmospheric oxygen ordinarily will produce an oxidation tendency in a neutral solution, just as was postulated in considering the behaviour of cuprous oxide on platinum electrodes illuminated in neutral potassium sulfate solution; the ideal completely reversible local cell may not be attained in these neutral special solutions The illuminated massive electrode will behave therefore, as a cathode in the neutral special solutions. All of the above discussion dealt with the gauze electrodes, and an inspection of the corresponding experimental curves shown in Fig. 4 will indicate that the theory of the action of these special photovoltaic cells, agrees in general with the experimental results. In the sheet copper electrodes uniformly coated with cuprous oxide, the local cell effect ordinarily, is of much less importance, since even when the local cells do exist on such an illuminated surface, they polarize themselves i n most cell solutions. In these special cell solutions, however, the action of the local cells on the sheet electrode is greatly increased, and as a direct re-
I3 73
STUDIES I N PHOTOVOLTAIC CELLS
sult, the original anode tendency of the illuminated massive electrode is correspondingly decreased. This fact is illustrated by the various experimental curves shown in Fig j The Cuprous Oxide-Copper Photovoltaic Cell with no Local Cell Formation Finally, in order t o bring out in the most convincing manner, the effect of local cells in decreasing the anode tendency of a n illuminated copper electrode uniformly coated with cuprous oxide, the following trial was made. h copper sheet uniformly coated with cuprous oxide by the copper formate method was made the illuminated electrode in a photovoltaic cell, and the cell solutions were varied. The same electrode was then coated on its back or unilluminated surface with paraffine, and the photovoltages again measured in the same solutions. These results were obtained: Solution
Insulated electrode
Uninsulated electrode
Sodium oxalate Potassium sulfate Potassium bromate
.133 volts (anode)
.230 volts (anode)
.1r8
”
”
,215
”
”
.IO5
”
”
,210
”
”
These varying values show at once that the photovoltages produced by an illuminated copper sheet-cuprous oxide electrode, may be widely varied as local cell formation on the illuminated surface is prevented. The Silver Halide Photovoltaic Cells The silver chloride, silver bromide, and silver iodide photovoltaic cells may be studied in the same manner as were the cuprous oxide cells. 0 . I N solutions were used in all cases, as before, unless otherwise specified. Since silver gauze was not available as an electrode material, all measurements of silver halide on silver and platinum electrodes, were made on sheet electrodes, of 3 X 8 cm. dimensions, with a copper wire folded into the silver sheet to permit ready attachment to the potentiometer. The method of observing the photovoltages, the cell arrangement, and light sources, were the same as in the cuprous oxide series. The Silver Halide-Platinum Photovoltaic Cells Silver chloride, bromide, and iodide, was made by adding silver nitrate in excess to the potassium halide salt solution, and washing the precipitate carefully with hot water by decantation. It was found that this finely divided silver halide precipitate could be rubbed on a platinum surface as a moist paste and then allowed to dry in the air. An electrode prepared in this fashion possessed a layer of the halide upon its surface which adhered when the platinum was submerged in the cell solution.
CARL Tv. TUCKER
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As before, illumination of silver halide in the presence of an oxidizing solution, may be expected to result in a photochemical oxidation, with silver ion and the free halogen as the products of the light reaction. The silver halideplatinum electrodes, when illuminated, may be expected to behave in such a
010 Oo6
t
FIG.6 Behaviour of platinum-silver iodide electrodes illuminated in different solutions. 010
-
008 -
5
s 008Oo8 010
t
FIG.7 Behaviour of platinum-silver bromide electrodes illuminated in different solutions.
way as will bring the illuminated and darkened portions of the cell into equilibrium. Illumination of this photovoltaic cell, with an oxidizing solution, may be expected then, t o produce a reducing or cathode tendency in the illuminated electrode.
STCDIES Izi PHOTOVOLTAIC CELLS
'375
Conversely, silver halide illuminated in reducing solution, may be expected to undergo a photochemical reduction to metallic silver and halide ions. The illumination of this photovoltaic cell with a reducing solution, may be expected then, to produce an anode or oxidizing tendency in the illuminated electrode. The photo-current is tending to pass in that direction which will bring the two cell electrodes back into equilibrium. noor 008 OW-
0 0
t
004-
002 -
9
002 -
B
004
: J"
-
006-
008 OOlOL
FIG.8 Behaviour of platinum silver chloride electrodes illuminated in dfferent solutions
Figs. 6, 7 , and 8, show the behaviour of the various silver halide-platinum electrodes, in different types of solutions. The abscissas and ordinates in these diagrams have the same significance as before. The behaviour of these cells experimentally, was in accord with the theoretical requirements. I n general the photovoltage of the silver chloride is greater than that of the silver bromide coated electrodes. The silver bromide cell photovoltage is in turn greater than that shown by the silver iodide cell. The observed differences were very small, however, and could not be accurately recorded in this method of electrode preparation and observation. The Silver Halide-Silver Photovoltaic Cells Since these preliminary experiments on the behaviour of illuminated silver halide on platinum indicate that the halides behave in the same general manner as did the cuprous oxide on platinum, we may expect the same general theories to apply when considering the silver halide on silver electrodes. Silver halide illuminated while in contact with a silver sheet, should behave photochemically as though it were in a reducing environment; just as did cuprous oxide illuminated while in contact with copper. If the silver halide layer on the silver electrode surface is uniform and possible local cell formation is thereby minimized, the illuminated electrode tends to photochemically reduce and the two cell electrodes are not in equilibrium. The
CARL W. TUCKER
1376
tendency which the cell shows to return to an equilibrium is shown by an anodic behaviour of the illuminated cell electrode. As the possibility of local cell formation on the illuminated electrode is increased, this initial anode behaviour is decreased and may even be over-balanced by a cathode effect
5
004
006
FIG.9 Behaviour of silver sheet electrodes, uniformly coated with silver iodide, silver bromide, or silver chloride and illuminated in different solutions.
i-
3 ,004 a 'Oo6
,002
t
OlOL
FIG.I O Behaviour of silver sheet electrodes, not uniformly coated with silver iodide, silver bromide, or silver chloride and illuminated in different solutions.
produced through the anodic polarization of the irreversible local cells. If the corresponding sodium halide salt is used in the cell solutions, the local cells will become non-polarizable. Local cell action is correspondingly increased. In such a special cell solution, the initial anode effect of the illuminated massive electrode will be at a minimum, and if the silver halide formation were perfectly efficient a t the local cell anodes, no photo-effect whatever would be observed at the initial moment of illumination. Actually,
STUDIES IT PHOTOVOLTAIC CELLS
I3ii
this ideal case would be difficult to realize, since the reducing or oxidizing tendencies of the cell solutions will have a direct influence on the efficiency of the local cell actions. The curves shown in Figs. 9, I O , and 11 illustrate the behaviour of these photovoltaic cells, and confirm this theoretical discussion. With thick or uniform lay-ers of the silver halide on silver, illuminated, the exposed electrode is initially anode in all ordinary solutions, tending to revert to a cathode as continued illumination breaks down the uniformity of the photosensitive layer. 010
Oo8 010
t
FIG.1 1
Behaviour of silver sheet electrodes, uniformly coated with silver chloride and illuminated in solutions containing chlorides.
With thin or uniform layers of the silver halides on silver, illuminated, the exposed electrode is initially very slightly anodic but rapidly becomes cathodic. With uniform or non-uniform layers of the silver halide on silver and illuminated in a halide solution, the local cells become reversible and the exposed massive electrode is always a cathode. The cathode effect becomes apparent, even if very small amounts aof halide salt are present in the solution, decreasing the initial anode effect produced in thick or dense layered electrodes in a reducing solution, and overbalancing the anode effect of a similar electrode illuminated in an oxidizing solution. The sensitivity of the local cells upon the illuminated surface, to halide ions, is shown by the abnormal behaviour of a dense or uniform layered silver chloride electrode illuminated in a 0.I N potassium sulfate solution saturated with silver chloride. Upon illumination an initial cathode effect is shown, because of the appreciable concentration of chloride ion present. As this chloride concentration is decreased by a constant precipitation of silver chloride a t the local cell anodes, the illuminated electrode regains its normal behaviour and becomes an anode. The experimental curve obtained is shown in Fig. 1 1 .
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CARL W. TUCKER
Summary From this theoretical discussion and study of the behaviour of the cuprous oxide and silver halide photovoltaic cells, the following general rules can be set down, as a basis for explaining and predicting the behaviour of a given photovoltaic cell: I. In a photovoltaic cell with two electrodes consisting of a photosensitive substance on platinum and dipping into an oxidizing solution, illumination will cause the exposed electrode to behave as a cathode, if the photosensitive substance present is tending to be photochemically oxidized when illuminated in contact with the same oxidizing solution. If this same electrode system is dipped into a reducing solution, the illuminated electrode will behave as an anode, if the photosensitive substance is tending to be photochemically reduced when illuminated in contact with the reducing solution. 2. I n a photovoltaic cell with electrodes consisting of the same metal from which the metallic constituent of the photosensitive material is derived, illumination tends to produce a photochemical reduction of the photosensitive substance. 3 . If the photosensitive substance is on the illuminated electrode as a uniform layer, local cell formation on the illuminated surface will be at a minimum, and the exposed electrode will be an anode. 4. If the photosensitive substance is on the illuminated electrode as a non-uniform layer, local cells will be set up in the illuminated electrode. If these local cells tend to polarize, the illuminated electrode will eventually become a cathode after continued illumination. If these local cells do not polarize but are completely reversible, the electrical behaviour of the illuminated electrode will be determined by the oxidizing or reducing nature of the cell solution. In an oxidizing solution or a neutral solution the illuminated electrode will be a cathode while in a reducing solution, the illuminated electrode will be an anode. If these local cells approach but do not attain complete reversibility, the illuminated electrode will be cathode in oxidizing or neutral solutions, and may or may not be cathode in reducing solutions.
Discussion of Recorded Photovoltaic Cells The first photovoltaic cells made by Becquerel consisted of electrodes with each of the three silver halides; silver chloride, bromide and iodide, on platinum. The cell solution was in every case a dilute sulfuric acid. Since an acid solution may be considered as an oxidizing solution, the illuminated electrode in these cells should have been in every case a cathode. Becquerel obtained this result experimentally. Becquerel next found that uniformly coated silver halide-silver electrodes were always anode in this same acid solution, while non-uniformly coated electrodes were always cathode, upon illumination. I n the first case, no local cells or a minimum of local cell action occurred upon the illuminated surface. The illuminated electrode was anode as a result of the cell reaction to a tendency toward a photochemical reduction of silver
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STUDIES I N PHOTOVOLTAIC CELLS
halide, I n the second case the local cell action was a t a maximum on the illuminated electrode, and since these local cells in a sulfuric acid solution were anodically polarized, the illuminated electrode was a cathode. Minchin next prepared silver halide-silver electrodes by drying an emulsion of the silver halide on the silver surface. This can be reasonably interpreted to mean that the silver halide was present upon the electrode surface in such a large amount, that the silver was completely coated with the photosensitive substance. I n other words Minchin had made a uniformly layered electrode in which local cells upon the illuminated surface would be a t a minimum. He found, however, that his illuminated silver chloride in a sodium chloride solution was on a cathode. His illuminated silver bromide on silver, in potassium bromide, also became cathode. This is as it should be, since the chloride and the bromide solutions increased any local cell action by converting these cells into non-polarizable types. The cathode effects produced by the local cells overbalanced the initial anode tendency of the illuminated electrode. I n silver nitrate solution, his illuminated silver iodidesilver electrode, became anode. I n this case the local cells polarized and had less influence upon the cell behaviour. The illuminated electrode was anode as a result of the cell reaction toward the photochemical reduction of the silver iodide. The behaviour of the photovoltaic cells studied by Wildermann, is theoretically satisfactory. All of his photosensitive layers were of the non-uniform type. The illuminated electrode in all of his cells was a cathode. The peculiar behaviour of the cuprous oxide-copper cell described by Case, should be readily explained. This cell was constructed as follows: Illuminated
Cu 1 CulO
C U ( C O ~ H ) ~ ~ H C O ~ HCuzO 1 Cu
+
Upon illumination of the photovoltaic cell, he found a current flowing in the solution, away from the illuminated electrode. Upon continued illumination with a closed circuit, the photovoltage slowly fell to zero. From Case’s description of his electrodes, they consisted of a uniform layer of cuprous oxide on copper. Any local cells which may have been formed on the illuminated surface would polarize: Cu 1 CUZO
CU(CO~H)~,HCO~H
CUZO I Cu
Current passage through such a cell would result in the formation of cuprous oxide a t the cell cathode, but the formic acid present would tend to prevent the formation of cuprous oxide at the cell anode. The resulting polarization of the local cell would allow the initial anode tendency of the illuminated massive electrode to predominate. However, with such a cell solution present, cuprous oxide would tend to be electrochemically precipitated on the darkened massive electrode, or cathode, as the current produced by illumination passed through the cell. At the same time cuprous oxide would be oxidized to copper formate at the anode, or illuminated massive electrode,
1380
CARL W . TUCKER
and through this removal of the photosensitive cuprous oxide, the photosensitivity of the cell will be decreased and eventually the photovoltage will drop to zero. If the electrodes are now reversed and the formerly darkened electrode with its new coating of deposited cuprous oxide is illuminated, the cell photosensitivity will be regained and the entire process can be again repeated if the external circuit is kept closed. Case noted that his cell electrodes behaved in a reverse manner in sodium chloride solution; the illuminated electrode was a cathode. I t can be seen a t once that a sodium chloride solution in the local cell, c u I CuaO NaC1 CuaO 1 c u will convert it into a reversible cell. Cuprous oxide is formed at the cell anode and destroyed at the cell cathode as current passes. This increased activity of the local cells upon the illuminated surface caused the original anode tendency to fall to zero and a cathode tendency to become apparent as a result of some slight anodic polarization in the local cells. Garrison studied the effect of variation of density of cuprous oxide layer on copper. His results agree with those previously recorded in the experimental part of this paper. Garrison also recorded the influence of hydrogen, hydroxyl, and cupric ions when added to the cell solution. He points out, however, that his study of the effect of varying the concentration of these particular ions, was made always on electrodes with an intermediate density or uniformity of layer,that is, a layer which we now know may either produce an anode or cathode effect in the illuminated massive electrode, depending upon the character of the local cells in the illuminated surface. The general rules which he laid down for the behaviour of the copper-cuprous oxide photovoltaic cell as hydrogen, hydroxyl, and cuprous ion concentrations are varied, will not hold for all the possible densities or uniformities of cuprous oxide layer on copper. Garrison confirmed by quantitative measurements, the effect of varying the density of the silver iodide layer on the silver electrode. He also accurately checked the tehaviour of the cell as known amounts of potassium iodide or silver nitrate were added to the solution. Addition of silver nitrate was found to increase the anode tendency of an illuminated silver iodide-silver electrode ; addition of potassium iodide to the cell solution increased the cathode tendency upon illumination. This is the correct behaviour of the cell in such solutions since variation of the solution is again varying the behaviour of the local cells in the illuminated cell electrode.
Acknowledgment To Professor Wilder D. Bancroft, who by helpful suggestion, inspirational criticism, and personal direction, has made this study possible, I am sincerely grateful. Cornell Universaly
