950
J . Phys. Chem. 1984,84950-955
the squarylium dyes, the short time stability in water buffers was not good and any pKa results listed are only very approximate. Further details concerning the pKa determinations can be found in the discussion. Registry No. M-1, 3568-36-3; M-4, 25962-05-4; M-8, 25962-03-2; M-9, 23350-56-3; M-10, 14207-70-6; M-13, 29453-90-5; M-26, 1341614-3; M-28, 74833-88-8; M-40, 88454-38-0; M-41, 3783-14-0; M-43, 80530-93-4; M-47, 42906-02-5; M-54, 64285-51-4; M-55, 3443 1-79-3; M-57, 88454-39-1; M-58, 83846-68-8; M-65, 20339-33-7; M-69, 88454-40-4; M-72, 30457-59-1; M-73, 88454-41-5; M-74, 88454-42-6; M-75, 88454-43-7; M-78, 32682-09-0; M-85, 12243-46-8; M-93,
88475-75-6; M-102, 13056-24-1; M-106, 88454-34-6; M-107, 8845444-8; M-108, 88454-45-9; M-109, 43134-05-0; M-114, 88454-46-0; M-119, 4731-29-7; M-123, 7693-03-0; M-124, 88454-37-9; M-126, 16368-56-2; M-127, 88454-47-1; M-133, 88454-48-2; M-134, 8845449-3; M-136, 61413-28-3; M-139, 88454-50-6; M-141, 88454-51-7; M-143, 88454-52-8; M-144, 88454-53-9; M-146, 88454-54-0; M-148, 10150-12-6; M-150, 88454-55-1; M-153, 43134-04-9; M-157, 2323618-2; M-160, 88454-56-2; M-165, 82497-78-7; M-166, 88454-57-3; M-167, 88475-76-7; M-169, 76971-10-3; M-170, 88454-58-4; M-171, 88454-59-5; M-175, 88475-77-8; M-178, 88475-78-9; M-185, 8847579-0; M-197, 68842-66-0; M-198, 88475-80-3; M-199, 88475-81-4; A1203, 1344-28-1.
Improvement of Ai/AI,O,/Multilayer Array of Chlorophyll a/Ag Photovoltaic Cell Characteristics by Treatment of the Aluminum Electrode M. F. Lawrence, J. P. Dodelet,* and L. H. Dao INRS-Energie, C.P. 1020, Varennes, Qudbec, Canada JOL 2PO (Received: February 14, 1983; In Final Form: May 23, 1983)
The characteristics and behavior of the photovoltaic cell A1/Al2O3/44layers of chlorophyll a/Ag have been examined. It was found that when the freshly evaporated A1 electrode is exposed to atmospheric air having a relative humidity 260% at -23 OC, a large detrimental corrosion current is present and very little photocurrent is observed. The existence of corrosion spots of -0.1 pm in diameter has been demonstrated by electronic microscopy. The dark current at zero bias is assigned to the corrosion current of the AI electrode. This corrosion current disappears when the freshly evaporated A1 electrode is dipped into a dichromate solution where [Cr2072-]1 10” M. Efficienciesof the photovoltaic cells with dichromate treated AI electrodes are improved mainly because of.the large increase in the open circuit voltage V,. J-V characteristics and C2-Vmeasurements have been used to draw a detailed energy band diagram of the cell. The large V, values are explained in terms of an additional space charge produced by the trapping of some of the photogenerated charges when the cell is illuminated.
Introduction Electrical, photoelectrical, and photoelectrochemical properties of chlorophyll a (Chla) Langmuir-Blodgett films have been investigated in relation with the primary processes of photosynthesis.’-8 In these studies, the photoconductor was a film of one monolayer or a stack of many monolayers (up to 100) consisting of pure Chla or a mixture of Chla with other molecules of interest in photosynthesis (like quinones, carotenoids, lecithins, etc.). For photovoltaic purposes, the monolayers have always been deposited on semitransparent aluminum electrodes. The choice of this metal resulted from its relatively low work function, the mechanical characteristics of the semitransparent film, and the very hydrophilic behavior of the freshly vacuum-evaporated A1 film. The low work function (4.17 eV9) is necessary to produce the rectifying contact with Chla, a p-type semiconductor,I0 and ~~~
(1) Simpson, W. H.; Reucroft, P. J. Thin Solid Films 1970, 6, 167. (2) Reucroft, P. J.; Simpson, W. H. Discuss. Faraday Soc. 1971,51,202. (3) Janzen, A. F.; Bolton, J. R. J . A m . Chem. 1979, 101, 6342. (4) Jones, R.; Tredgold, R. H.; O’Mullane, J. E. Photochem. Photobiol. 1980, 32, 223. (5) Lawrence, M. F.; Dodelet, J. P.; Ringuet, M. Photochem. Photobiol. 1981, 34, 393. (6) Miyasaka, T.; Watanabe, T.; Fujishima, A.; Honda, K. J . Am. Chem. SOC.1978, 100, 6657. (7) Miyasaka, T.; Watanabe, T.; Fujishima, A.; Honda, K. Nature (tondon) 1979, 277,638. (8) Miyasaka, T.; Honda, K. Surf. Sci. 1980, 101, 541. (9) Rivitre, J. C. “Solid State Surface Science”; Green, M., Ed.; Marcel Dekker: New York, 1969; Vol. 1, p 244. (10) Putseiko, T. E.; Akimov, I. Discuss. Farodoy SOC. 1959, 27, 83.
0022-3654/84/2088-0950$01.50/0
the hydrophilic surface of the A1 electrode allows the transfer of the pigment monolayer from the Langmuir-Blodgett trough to the electrode with a deposition ratio of 1 0.02. Aluminum electrodes were also extensively used in organic photovoltaic cells where the pigment is not deposited by the Langmuir-Blodgett technique as, for example, in ref 11-1 8. In a previous paper,5 we have shown that photovoltaic cells of the type A1/A1203/multilayer array of Chla/Ag have a low conversion efficiency and a low quantum yield, even when monochromatic light is used. These cells are evidently not suitable for energy conversion where high efficiencies are required under solar illumination. They are, on the other hand, particularly useful for photoconduction studies where Chla, associated with other molecules of photosynthetic interest, is concerned. The Langmuir-Blodgett technique permits the organization of pigments at the molecular level and therefore allows us to know the composition of the photoconductor when different pigments are mixed. AI/A1203/multilayer array of Chla/Ag cells always show a dark current which seems to be related to the corrosion of the A1 (11) Ghosh, A. K.; Morel, D. L.; Feng, T.; Shaw, R. F.; Rowe, Jr. C. A. J . Appl. Phys. 1979, 45, 230. (12) Tang, C. W.; Albrecht, A. C. J . Chem. Phys. 1975, 62, 2139. (13) Kampas, F. J.; Gouterman, M. J . Phys. Chem. 1977, 81, 690. (14) Fan, F. R.; Faulkntr, L. R. J . Chem. Phys. 1978, 69, 3334, 3341. (15) Loutfy, R. 0.;Sharp, J. H. J . Chem. Phys. 1979, 71, 1211. (16) Martin, ,M.;Andrt, J. J.; Simon, J. Nouv. J . Chim. 1981, 5 , 485. (17) Yamashita, K.; Kihara, N.; Shimidzu, H.; Suzuki, H. Photochem. Photobiol. 1982, 35, 1. (18) Dodelet, J. P.; Pommier, H. P.; Ringuet, M. J . Appl. Phys. 1982, 53, 4270.
0 1984 American Chemical Society
A1/AIzO,/Chla Multilayer Electrodes
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 951
electrode. When this dark current is large, the rectifying properties of the cells are ruined. We found that dipping the AI electrode in a dichromate solution immediately after vacuum evaporation practically stops the corrosion of the metal and drastically improves the behavior of the cells when compared to cells made with untreated electrodes. This paper reports the changes in the characteristics of A1/AlZ0,/44 layers of Chla/Ag cells after dichromate treatment of the AI electrode.
Experimental Section Chlorophyll a was purchased from Sigma and used without purification. The vacuum system used for metal depositions is a Varian VK12B cryogenic pump. Aluminum shots (99.5% pure) from Alfa Chemicals are placed in a BN crucible and heated with a W filament. The microscope slides used as substrate for the AI electrode are 40 cm from the crucible. They are not cooled during the evaporation. The cleaning procedure for the microscope slides has already been d e s ~ r i b e d .The ~ pressure at evaporation was always below 5 X lo-’ torr. After the evaporation was complete, the bell jar is brought back to atmospheric pressure with dry air and the electrodes to be treated are immediately plunged into the appropriate dichromate solution. The treatment consists of dipping for 5 min the freshly evaporated AI electrode in a stirred dichromate solution of concentration ranging from lo-* to lo-’ M. The relative humidity in atmospheric air is measured by the dry and wet bulb thermometric method. The dry temperature throughout the experiments varied between 22 and 25 “C. Before the deposition of Chla, all AI electrodes are covered with one monolayer of cadmium arachidate. This yields a smooth hydrophobic electrode on which 44 layers of Chla are deposited. The monolayer depositions are made at temperatures between 18 and 20 “C in air for cadmium arachidate and under Nzfor Chla. All cells contained 44 monolayers of Chla because it was found previously5 that this provided the optimum thickness as far as quantum yield and efficiency are concerned. The LangmuirBlodgett technique used for the pigment deposition has already been reported.s The cell is completed by the evaporation under vacuum of a collecting and guard electrode with either Ag or Au (99.9% pure from Alfa). The cells are irradiated, in a Faraday cage, with light from a 650-W tungsten lamp passing through a Kratos monochromator. The light energy is measured with a United Detector Technology 21A power meter. After correction for the light absorption by the AI electrode, the pigment is irradiated at 680 nm with an Currents and voltages energy ranging from 0.5 to 60 MW produced by the cells are measured with a 616 Keithley electrometer. Capacitance measurements are made with a setup similar to the one described by Twarowski and A1bre~th.I~The source of alternative voltage is a PAR universal programmer. The current vs. voltage curves are recorded with an XY EGG recorder. All measurements were performed in atmospheric air between 22 and 25 “C. The electronic scanning microscope used in the investigation of the AI electrodes is a JEOL JSM-35CF, and the transmission microscope is a Philips EM300. The thickness of the evaporated AI was measured with a Model 980-4020 Varian interferometer.
-0.3 + Y-
-
15-
5.6 pW ~ r n - ~
LL
Figure 1. (A) Effect of [Cr20?-] on the efficiency improvement factor IF(7) and on the quantum yield factor F(+)for A1/AI20,/44 layers of Chla/Ag cells irradiated at different intensities with monochromatic light (680 nm). (B) Variation with [Cr20,”] of the opten circuit photovoltage V, (0)and fill factor ff ( X ) for an Al/Al20,/44 layers of Chla/Ag cell under monochromatic light (680 nm) at 27 pW cm-2.
Cells having high JD values develop little photovoltage (1100 mV) when they are irradiated at 680 nm (the maximum of the action spectrum in the red) with energies up to 27 WWcm-z on the pigment. Since these large values of JD seemed to be associated with corrosion of the A1 electrode, we used potassium dichromate solutionsz0to inhibit the attack on the metal. At [Cr207z-]= and M, JD is hardly affected by the treatment but JD drops drastically to -0.4 nA when [CrZ0,*-] 1 10” M and also becomes independent of the relative humidity to which the freshly evaporated AI electrode is exposed. The effect of dichromate treatment on the cell efficiencies is shown in Figure 1A. The three full curves represent the efficiency improvement factor IF(q) vs. dichromate concentration at different light intensities W T ) = v(tae)/q(uae) (1) where q(tae) and q(uae) are the efficiencies of the cells made with a dichromate treated aluminum electrode (tae) and untreated aluminum electrode (uea), respectively. The efficiencies are calculated from the following equation: 1OOJ,, V,ff
v
=
Results Photovoltaic cells of the type A1/A120,/cadmium arachidate/44 monolayers of Chla/Ag always generate a dark current, JD, which is a function of the relative humidity (rh) to which the freshly evaporated AI electrode is exposed when the bell jar is opened. at 40% rh to 12 nA Dark currents ranging from -2 nA at 65% rh were measured directly after completion of the cells. JD slowly decreases with time (after 3 to 4 days JD is 50% of its initial value) when the cells are kept in the dark at -10 OC to avoid degradation of Chla.
where J , is the short circuit photocurrent density and V, the open circuit photovoltage when the cell is irradiated at 680 nm. The cell fill factor is ff and W is the monochromatic light power incident on the pigment. q(uae) was measured for nine cells. The mean values obtained 1.9 X IO-,%, and 3.0 X are 1.2 X at 5.6, 10.7, and 27 MWmr2. The increase of cell efficiency with the rise in light intensity is due to an abnormally low V, at low light intensity for all the (uae) cells measured. The maximum improvement in power conversion efficiency is obtained when [Cr2072-]= M . At that concentration, tae cells have a mean efficiency of 2.2 X which is hardly affected by variation of light intensity within the range used in our experiments. Between two and five cells were used for each dichromate concentration and the efficiencies measured where always found to be *30% of the mean value reported. The same is also true for uae cells. It would be of interest to see if the positive effect of dichromate treatment on the device efficiency demonstrated at very low light
(19) Twarowski, A. J.; Albrecht, A. C. J . Chem. Phys. 1979, 70, 2355.
(20) Vermilyea, D. A,; Vedder, W. Trans. Faraday SOC.1970,66, 2644.
952
Lawrence et ai.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984
- 80
forward I
I.o
I
I
reverse
I
0 APPLIED
I
I
I
1.0 BIAS ( V I
2.0
Figure 3. A1/AI20,/44 layers of Chla/Ag cell. Forward and reverse bias dark current, JD,for a dichromate treated A1 electrode (broken line) and an untreated AI electrode (full line).
Figure 2. (A) Scanning electron microscope photograph of a 2000-Athick untreated AI electrode at a magnification of 4.8 X lo4. (B) Transmission electron microscope photograph of a 2000-A-thick untreated A1 electrode at a magnification of 1.14 X lo5.
levels persists at much higher intensities (100 mW cm-2). Unfortunately, irradiation of Chla with high light intensities causes a rapid degradation of this biological pigment. The factor concerning the quantum yield for charge production, F(+), calculated like for IF(TJ),is found to be 1 within experimental error. F(+) is shown in Figure 1A (broken curve) for 27 pW cm-2 of monochromatic light (680 nm) on the pigment and the results is the same for the other intensities used. Quantum yields, 4, for charge production are calculated with the following equation: 100J,hv +(%) = (3) qw where hu is the monochromatic (680 nm) photon energy and q is the electron charge. The mean quantum yields found for both kinds of cells are 0.17%, 0.17%, and 0.14% at 5.6, 10.7, and 27 pw cm-2, respectively. If the dichromate treatment has very little effect on the quantum yield, it is obvious from Figure 1B that the change in efficiency with [Cr2072-]results from the combined effect of a large variation in the open circuit photovoltage V, and the fill factor ff. In Figure lB, these variations are given for 27 pW cm-2 but are similar for other incident powers. Figure 2A shows a scanning electron microscope photograph of a 2000-A-thick untreated A1 electrode at a magnification of 4.8 X lo4. The picture has a dark background with many white surface spots having a diameter of about 0.08 pm. They are interpreted as corrosion points on the A1 electrode and are present at about the same density on all pictures of untreated and dichromate treated A1 electrodes. This implies that their formation must occur when the freshly evaporated A1 comes into contact with the moisture in air. These corrosion spots cover roughly 7%
of the surface. It was impossible to get a good resolution for the I electrodes used in our photovoltaic 300-&thick semitransparent A cells but we assume that the 300- and 2000-A-thick electrodes behave in a similar manner. Figure 2B is a picture of an untreated 2OOO-A A1 electrode taken with a transmission electron microscope at a magnification of 1.14 X los. The white surface spots in Figure 2A are now seen more clearly as black spots having diameters ranging between ~ 0 . 0 2 and 0.2 pm. In this photograph, owing to a much better resolution than that obtained with the scanning electron microscope, we estimate that 17% of the surface is covered with corrosion points by measuring the fraction of the picture surface covered with dark areas believed to be the corroded aluminum. When an external voltage (0.8 V) is applied, in the dark, to cells having an A1 electrode treated with a dichromate solution of concentration L 10” M, J D is small and remains constant when both forward and reverse biases are applied. For the untreated cells (and cells treated with [Cr20,2-] Ilod M) however, J D is large and remains constant if the cells are forward biased (negative potential on the A1 electrode), but the dark current decreases when they are reverse biased. This decrease results in the electrochemical formation, at the corrosion point level, of an A1203layer through which diffusion of water and oxygen would be restricted. Figure 3 shows the forward and reverse bias dark current for two cells, one with an untreated A1 electrode after stabilization of the dark current following the electrochemical formation of the A120, layer (full line), the otherr with a M dichromate treated A1 electrode (broken line). The rectification ratio at 1.35 V is 21 for the former and 645 for the latter. Figure 4 shows capacitance measurements for untreated (full line) and dichromate treated (broken line) aluminum electrode cells in the dark (4A) and under illumination (4B) (- 10 pW cm-2 at 680 nm), respectively. These measurements were performed by using a triangular applied voltage of amplitude V, = 1.4 V centered at zero bias and a frequency f = 0.1 Hz. The capacitance C is given” by C = (J+ - J-)/8Vaf (4) where J+ and J- refer to the two values of the current at the applied voltage V. The capacitance changes with the applied voltage. This is consistent with the existence of a Schottky depletion layer W varying with the bias21a w = [2t,(Vbi - v - k T / q ) / q “ / 2 (5) (21) Sze, S. M. “Physics of Semiconductor Devices” Wiley: New York, 1969; (a) pp 371 and 372; (b) p 369.
The Journal of Physical Chemistry, Vol. 88, No. 5, 1984 953
A1/A1203/Chla Multilayer Electrodes
TABLE I: Parameters of the Schottky Depletion Layer for an AI/A1,0,/44 Monolavers of ChlaiAn Cell illumination
AI
electrode
N , 1017 carriers cm-’
Vbi, V
W,A
no no
untreated treated
0.48 0.68
2.5 1
1.14
281 3 89
Yes
untreated
0.40
yes
treated
4.47 2.60
26 1
0.46
196
i
I .o
0 V
-1.0
Figure 4. Al/Al2O3/44layers of Chla/Ag cell. J vs. Vcurve recorded in the dark (A) and under illumination (B) for a dichromate treated AI electrode (broken line) and an untreated AI electrode (full line). Triangular applied bias to the AI electrode at 0.1 Hz.
rn
0
I
-1.0
I
I
0
I
1.0
V Figure 5. AI/AI2O3/44layers of Chla/Ag cell. C2vs. V plot for a dichromate treated AI electrode (A) and an untreated AI electrode (B) in the dark ( 0 )from J-V readings of Figure 4A and under illumination (0)from J-V readings of Figure 4B.
where cs is the permittivity of the semiconductor, Vbi is the built in potential, and N is the carrier density of the semiconductor. The other symbols have their usual meaning, and Cz= 2(Vbi
- v - kT/q)/qc,N
I
0.8
I
I
1.2
V
i
/d
I
0.4
Figure 6. AI/AI20,/44 layers of Chla/Ag cell. Curve calculated by using a modified Schottky equation for the forward J-V characteristics of a dichromate treated AI electrode. J-V readings are the same as in Figure 3.
2.0
1
I
0
(6)
Figure 5 shows the C2vs. Vrelation for tae cells (A) and uae cells (B). Since the dielectric constant for Chla in an array of monolayers is not known, a value of 3.5 was used in the calculations. In contrast to what would be expected if Chla behaved like a standard semiconductor, a straight line for eq 6 in the dark was never obtained. A similar behavior has already been noticed19 for tetracene and was rationalized in terms of deeply trapped
charge carriers with very low mobility. Under illumination straight lines are obtained for C2vs. V plots, which is explained by the mobilization of charge carriers with the use of light.19 The average parameters of the Schottky depletion layer obtained for treated and untreated cells under illumination (- 10 MW and in the dark are given in Table I. The results appearing for untreated A1 electrodes also include those obtained for cells made of an A1 electrode treated with a dichromate concentration
