Capacitance measurements of photovoltaic cells based on mixed monolayers of chlorophyll a and sulfoquinovosyldiacylglycerol

Solar E n e ~ Materials and Solar Cells

ELSEVIER

Solar Energy Materials and Solar Cells 45 (1997) 211-225

Capacitance measurements of photovoltaic cells based on mixed monolayers of chlorophyll a and sulfoquinovosyldiacylglycerol Salvator Nsengiyumva a, Chouhaid Nasr a, Surat Hotchandani Roger M. Leblanc b

a,*

a Groupe de Recherche en Energie et Information Biomol~culaires, Universit~ du Quebec ~ Trois-Rivibres, C.P.500, Trois-Rivi~res, Qua., Canada GgA 5H7 b Department ofChemistD,. UniL~ersi~' of Miami, P.O. Box 249118, Coral Gable, FL, USA 33124-0431

Received 28 November 1995

Abstract The capacitance measurements of A1/Chlorophyll a / A g and Al/Chlorophyll aSulfoquinovosyldiacylglycerol/Ag sandwich cells have been carried out at different frequencies in dark and under illumination. The results show that while a voltage-dependent capacitance and a linear 1 / C 2 versus V~ plot is obtained for chlorophyll a (Chl a) at low frequencies, the addition of sulfoquinovosyldiacylglycerol (SQDG) in proportions ~ 0.25 results in totally voltage-invariant capacitance. The capacitance characteristic of Chl a-SQDG (0.25) cells resemble more those for an insulator than those for a Schottky barrier. These results are explained in terms of negatively charged SQDG playing a role of traps which immobilize the holes. Alternatively, the interaction of negative polar head group of SQDG with Mg 2÷ of Chl a can also be invoked as a possible reason for the observed results. The bound Chl a- SQDG species possibly does not form a Schottky barrier or forms a weak barrier with either A1 or Ag electrodes, resulting in insulator-like capacitance characteristics of A1/Chl a-SQDG(0.25)/Ag cells. Keywords: Photovoltaics; Mixed monolayers; Capacitance measurements

* Corresponding author. 0927-0248//97/$17.00 Copyright © 1997 Published by Elsevier Science B.V. PII S0927-0248(96)00031- 1

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1. Introduction

The thylakoid membrane of the chloroplasts, which is the site of primary events of photosynthetic process, contains chlorophyll a (Chl a) molecules coupled to membranebound polypeptides which are arranged into specific complexes required for the process of light energy transduction [1]. These complexes are embedded within a lipid bilayer which also includes all the accessory pigments, the quinones, and the specialized proteins responsible for the electron transport between the two photosystems [2-4]. The membrane lipids, thus, play an important role in primary processes of photosynthesis through specific interactions with proteins and pigments [5]. Among various lipids present are the sulfolipids such as sulfoquinovosyldiacylglycerol (SQDG) which was discovered by Benson et al. in 1959 [6]. Even if. as yet, little is known about the exact role of this sulfolipid in the thylakoid membrane, there are some indications that SQDG is a negatively charged lipid and is closely associated with intrinsic complexes of thylakoid membrane [7]. A direct role for SQDG in photosynthesis has been proposed by Haines in 1973 [8], and, further, Weier and Benson [9] have suggested that the plant sulfolipid may help orient chlorophyll within the choroplast membrane. From fluorescence lifetime measurements of chlorophyll a in lipid vesicles of digalactosyldiacylgycerol (DGDG) and SQDG, Van Gurp et al. [10] have discussed the influence that the polarity of lipid head group exerts on energy transfer in self-assembling pigment-lipid systems. They suggested that the negative head group of SQDG binds to Mg 2+ ion in Chl a and thus keeps Chl a molecules apart from aggregation. They further concluded that by their binding to Chl a, the negatively charged sulfolipid may be involved in stabilizing the pigment-protein complexes in the thylakoid membrane. In view of its negative charge, we became interested in the use of SQDG with Chl a (structures shown in Scheme 1) to obtain efficient solar cells modelled on photosynthesis. A considerable study with regard to the photoelectric properties of Chl a sandwiched between two metals has appeared from our laboratory and elsewhere [11-18]. The two metals employed are such that Chl a forms a rectifying contact with one of them, i.e., A1 or Cr, and an ohmic contact with the other, i.e., Ag or Hg. The photovoltaic studies of these cells have shown that the cells possess low power conversion efficiencies ~ 0.05%- ~ 0.20%, depending upon the ambient conditions. In an attempt to improve the efficiency of the cells. Chl a was mixed with SQDG and the sandwich cells of the type Al/monolayers of Chl a - S Q D G / A g were prepared and their photovoltaic study was carried out [19]. The idea underlying the use of SQDG was to exploit the beneficial role of charged substances in the process of charge separation [20]. The enhancement in charge separation is considered to be due to the capture of a hole from the photogenerated electron-hole pair (in Chl a) by negatively charged SQDG, leaving the electron to be swept away by the Schottky barrier field, thus reducing the charge recombination and resulting in high efficiency of the cells. However, contrary to what was expected, the incorporation of SQDG in ratios higher than 0.025 proved to have deleterious effect on the photovoltaic performance of the cell. Among the possible causes of this drastic effect, we had invoked that the presence of SQDG in Chl a monolayers, in immediate contact with Al, can destroy the rectifying character of

S. Nsengiyumva et al. / Solar Energy Materials and Solar Cells 45 (1997) 211-225

213

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SQDG (ref. 7)

Chl a (ref. 2)

Scheme 1. Molecular structures of Chl a and SQDG. A1/Chl a interface. This leads to a weaker Schottky barrier and therefore results in lower values of various photovoltaic parameters. Since capacitance measurements are usually best suited to examine various characteristics of a Schottky barrier, we have carried out the capacitance measurements of A1/Chl a - S Q D G / A g cells, and the results are presented in this paper.

2. Experimental 2.1. Materials

Chl a was extracted from barley according to the method described by Omata and Murata [21]. Its purity was checked by thin-layer chromatography and by the surface pressure-area isotherm of the pigment at the air-water interface. SQDG and arachidic acid were, respectively, purchased from lipid products, Nutfield, Surrey (UK) and Applied Sciences Laboratories, State College, PA (USA), and were used without further purification. Solvents n-hexane and ethanol were obtained from Fisher Scientific Co., Montreal, Canada, and were distilled prior to their use. Aluminum (99.999%) and silver (99.999%) were obtained from Johnson Mathey, Brampton, Ontario, Canada. 2.2. Methods 2.2.1. Monolayer deposition

The Langmuir-Blodgett technique was used to deposit monolayers of cadmium arachidate (Cd-Ar), Chl a and the mixture of Chl a-SQDG on clean glass iamellae. The

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s. Nsengiymm a et al. / Solar L)wr~,,y Material~ and Solar Cells 45 (1997) 211 225

deposition surface pressure lk)r monolayers of Chl a and its mixture with SQDG was 20 r a N / m , while that lk~r C d - A r monolayers was 30 m N / m . For further details refer to our previous work [21]. 2.2.2. D e z ' i c e . [ i t b r i c a t i o ,

Aluminum base electrode, 80-100,~ thick and 2()-25c} transparency, was first deposited by evaporation under vacuum ( ~ 10 '~ torr). In order to render AI electrode hydrophobic and thus facilitate the multilayer deposition of Chl a and its mixture with SQDG, a single monolayer of cadmium arachidate was deposited by Langmuir-Blodgett technique. 44 monolayers of Chl a or its mixture with SQDG were then deposited at 20 m n / m using a phosphate buffer (10 ~ m, pH 8) as the subphase. This pigment thickness has been reported to give the maximum power conversion efficiency for the Chl a sandwich cells [22]. The fabrication of the cells was completed by the evaporation, under vacuum (10 6 Tow), of a silver collecting electrode ( ~ 25% transparency) on top of Chl a or Chl a-SQDG multilayers. The active area of the cell is 0.45 cm 2. The cells with only Chl a monolayers will be referred to as Chl a cells, while those containing SQDG will be referred to as Chl a-SQDG cells with the SQDG proportion indicated in parentheses. 2.3. C a p a c i t a n c e measurernenl.s

The capacitance measurements have been performed using a set-up similar to the described by Twarowski and AIbreacht [23]. A periodic triangular voltage was applied to the cells, and the current flowing through the cells was recorded with respect to this applied voltage. The capacitance, C, of the cells was then calculated using the following equation [23]: J ~ - ./ C"

(J)

8 V,#"

where ,1. and J refer to the two values of the current at the applied voltage, V,~ is the amplitude, and f is the frequency of the applied voltage.

3. Results 3. I. SuJ;~i¢ce p r e s s u r e - a r e a

isotherms:

The surface pressure-area (~--A) isotherms of the pure Chl a, SQDG and the mixed monolayers are given in Fig. I. The isotherms for the mixtures of different mole fractions have been recorded but, because of the overlap of the isotherms, only two representative examples (i.e., molar fraction of Chl a equal to 0.95 and 0.25) are shown. Isotherms a and b are li)r pure Chl a and pure SQDG with collapse pressure at about 25 and 49 m N / m , respectively. The collapse pressure corresponds to the surface pressure where a sudden change in slope in the ~--A isotherms takes place. For mixtures with a

S. Nsengiyumva et al. / Solar Energy Materials and Solar Cells 45 (1997) 211-225

215

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i

0

40

80

120

160

200

MOLECULAR AREA (~2/molecule) Fig. 1. Surface pressure-area isotherms of pure and mixed Chl a-SQDG monolayers at the nitrogen-water interface. Isotherms a and b are for pure Chl a and SQDG, respectively, while 1 and 2 represent the mixed monolayers with mole fraction of Chl a equal to 0.95 and 0.25, respectively.

low content in Chl a (isotherm 2), two transition points are seen, one at 49 m N / m and the other at 37 m N / m . These transition points correspond to the true and the apparent collapse, respectively. For the mole fraction of Chl a equal to 0.95, only the apparent collapse pressure at 26 m N / m is seen (isotherm 1). This is due to the fact that for this molar fraction of Chl a, the monolayer becomes rather unstable at surface pressures greater than 30 m N / m so that one can not obtain reproducible results for the portion of the isotherm above this surface pressure. The existence of apparent collapse pressure is associated with the rejection, partial or total, of one of the components from the monolayers [24]. Below the apparent collapse pressure, the two components are completely miscible. In view of this, the surface pressure of 20 m N / m , was chosen for the deposition of mixed monolayers. The detailed monolayer study of Chl a-SQDG will be published elsewhere.

3.2. Capacitance The capacitance measurements of the cells have been carried out at various frequencies. Fig. 2, Fig. 3, and Fig. 4 present the current-voltage oscillograms of Chl a, Chl a-SQDG(0.025) and Chl a-SQDG(0.25) cells in dark at 100, 0.1 and 0.01 Hz, respectively. Chl a cells serve as a reference in this study. As seen from the figures, the curves for all the cells at 100 Hz are symmetric with respect to the applied bias. The evaluation of the capacitance with the help of Eq. (1) indicates that the capacitance is independent of the voltage at 100 Hz. However, as the frequency of the applied bias decreases, asymmetry in the current-voltage oscillograms sets in and the capacitance becomes voltage dependent. This is seen in Fig. 5 where capacitance is plotted against applied bias for different frequencies. While the capacitances of Chl a and Chl a-SQDG (0.025)

216

S. Nsengiytmu'a et al. / Solar Energy Materials and Solar Cells 45 (1997) 21l 225

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APPLIED BIAS (V) Fig. 2. C u r r e n t - v o l t a g e o s c i t l o g r a m s o f Chl a cell in dark. T r i a n g u l a r bias a p p l i e d to AI e l e c t r o d e at ( A ) 100

Hz, (B) 0.1 Hz, and (C) 0.01 Hz. Negative voltages represent forward bias while the positive voltages the reverse bias. cells are voltage dependent at low frequency (curves c), the situation with Chl a-SQDG (0.25) is very much different in that the capacitance is practically independent of the voltage even at the lowest of the frequencies employed, i.e., 0.01 Hz.

S. Nsengiyumva et al. / Solar Energy Materials and Solar Cells 45 (1997) 211-225

217

10 (A) v

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APPLIED BIAS (V) Fig. 3. Current-voltage oscillograms of Chl a-SQDG (0.025) cell at (A) 100 Hz, (B) 0.1 Hz, and (C) 0.01 Hz.

In general, when the depletion layer is probed capacitively, the charges responsible for the depletion layer respond to the variations in the oscillating applied bias, and enable the depletion layer to expand or contract. The width, W, of the depletion layer,

218

S. Nsengiyunu,~l et al. / Solar Energy MateriaLs cmd Solar Cells 45 (I 997) 211-225

50 (A)

<

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A P P L I E D BIAS (V) Fig. 4. Current-vohage ~,)scillograms of Chl a-SQDG (0.25) cell at (A) 100 Hz, (B) 0.1 Hz. and (C) 0.01 Hz.

thus, varies with the applied bias, and the two are related as shown in the following expression [23]: i

W=

eN~

(2)

S. Nsengiyumva et al. / Solar Energy Materials and Solar Cells 45 (1997) 211-225

100

219

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A P P L I E D BIAS (V) Fig. 5. Capacitance-voltage curves of (A) Chl a, (B) Chl a-SQDG(0.025), and (C) Chl a-SQDG(0.25) cells in dark at different frequencies, (a) 100 nz, (b) 0.1 Hz, and (c) 0.01 Hz. where e, e0, G, and Na are, respectively, the electronic charge, the permittivity of free space, the dielectric constant of pigment, and charge density of the barrier; Vb and Va are the built-in potential and applied bias, respectively. The negative Va refers to the forward bias. The capacitance of the cell is that of the high resistance depletion layer and is given by expression (3) [23]: eo G A C= - - , W

(3)

220

S. Nsengiyurm,a et al. / S o k w Energy Materials and Solar Cells 45 (1997) 211-225

where A is the active area of the cell. Combining Eq. (2) with Eq. (3), one obtains the following relation [23]: C2 -

eA2et,~, N~

(4)

Thus, a plot of I / C : versus V~ would be linear if capacitance is that of a Schottky barrier. Since width, W, of the depletion layer is voltage-dependent, so will be the capacitance of the Schottky barrier. Our results, however, show that the capacitance is voltage-dependent only at low frequencies, i.e., 0.01 Hz. The voltage dependent capacitance only at low frequencies has also been reported for tetracene [23], magnesium phthalocyanine [25], and other evaporated organic films [26,27]. This was attributed to the fact that in sublimed films the trapped charges are far more numerous than the free charges and that the depletion layer consists mainly of trapped charges. It should be mentioned that in the fabrication of Chl a and Chl a-SQDG cells, Chl a and SQDG have been deposited in form of anhydrous and amorphous monolayers. It is thus possible that similar to the case of evaporated films of organic pigments, there are present a large number of structural imperfections, irregularities and inhomogeneities in monolayers of Chl a and SQDG. Further, SQDG itself may function as a source of defects and imperfections in the structural make-up of Chl a monolayers. These structural defects can give rise to traps and, depending upon the trap depth, can immobilize the charge carriers temporarily or permanently. Thus, in the cells of Chl a and Chl a-SQDG one can assume that the trapped charges outnumber the free charges. It is, therefore, the trapped charge density that will control the width and other properties of the depletion layer. The effect of traps, usually, is to diminish the mobility and increase the response time of the charge carriers. As a result, the charge carriers are not able to respond to the variations in applied voltage at frequencies of even 100 Hz. The depletion layer width, W, remains virtually unaffected, and what one measures is the voltage-independent geometric capacitance, with Chl a or Chl a-SQDG behaving as simple dielectrics. However, as the frequency of the applied bias decreases and falls within the response time of the trapped charges, W expands and contracts, and a voltage-dependent capacitance can be seen for Chl a and Chl a-SQDG(0.025) cells at 0.01 Hz (Fig. 5, curves c). Although, the capacitance does vary with voltage at this frequency for Chl a and Chl a-SQDG (0.025) cells, an ideal linear relationship between I / C 2 versus V~ is observed only for Chl a cells (Fig. 6A). From the analysis of the linear I / C 2 versus V.d plot with respect to Eq. 4. various parameters of the depletion present at Al/Chl a interface have been evaluated. The values of 295A, 0.7 V, and 3.4 × 1023/m ~ obtained for W, Vb and N~, respectively, are in good agreement with our earlier results [28]. The capacitance of the cells of Chl a-SQDG (0.25) is, however, completely invariant of the voltage. This suggests that perhaps, there are still a large number of trapped carriers in Chl a-SQDG cells that marginally respond to the 0.01 Hz frequency of applied bias. To improve the response of the charges, the frequency of the oscillating probe voltage was further lowered to 0.005 Hz: however, due to a lot of noise in the measurements, meaningful results could not be obtained. Besides low frequencies, light can also bring the electrical response time of the trapped charge carriers within the time variation of the applied voltage [25]. Although

S. Nsengiyumva et al. /Solar Energy Materials and Solar Cells 45 (1997) 211-225

12

221

(A)

£8

~

4

0

I

I

I

20

(B)

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0

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3

(C) 2 5

i

.qet*l

¢,q

0

-0.8

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I

I

-0.4

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APPLIED BIAS (V) Fig. 6. 1 / C 2 versus Va plots for (A) Chl a, (B) Chl a-SQDG(0.025) and (C) Chl a-SQDG(0.25) in dark at 0.01 Hz.

the exact mechanism by which mobilization of trapped charges by light takes place is not well understood, it is possible that light supplies the necessary activation energy and frees the trapped charges. The cells were thus illuminated and capacitance measurements

222

S. Nsengiyuml'a et al. / Solar Energy Materials and Solar Cells 45 (1997) 211-225

(A) .-.

6

4

,-

J

2

I

I

J I

(B)

4 3 "-"

, ,','l "

2

0

I

,

I

I

1

I

30 (c)

20 w

~

10

0

-0.8

I

-0.4 0 0.4 A P P L I E D B I A S (V)

0.8

Fig. 7. I / C 2 versus V~ plots for (A) Chl a-SQDG(0.025) and (B) Chl a-SQDG(0.25) under illumination (~t,~ = 676 nm), and (C) 23 monolayers of cadmium arachidate. The frequency of the applied bias is 0.01 Hz.

were made. The wavelength of excitation was 676 nm which is strongly absorbed by Chl a. The plots of I / C : versus ~, for illuminated cells of Chl a-SQDG at 0.01 Hz are shown in Fig. 7. One can note that while there is a tendency towards linearity for Chl

S. Nsengiyumva et al. / Solar Energy Materials and Solar Cells 45 (1997) 211-225

223

a-SQDG(0.025) cells upon illumination (Fig. 7A), there seems to be virtually no effect, whatsoever, of illumination on Chl a-SQDG(0.25) cells, and capacitance shows no signs of voltage dependence. The 1 / C 2 versus Va plots of Chl a and Chl a-SQDG(0.25) at 0.01 Hz are distinctly different from each other, both in dark and under illumination. The plot of Chl a-SQDG(0.25) is more like that for an insulator, for example, cadmium arachidate (shown in Fig. 7C). It seems as if the incorporation of SQDG in Chl a, in proportions larger than 0.025, has only further aided in trapping and immobilization of the charge carriers to a great extent, thus imparting a, sort of, dielectric or insulator like character to Chl a. It should, however, be pointed out that the insulating behaviour of Chl a-SQDG may also be due to the inability of Chl a to form a Schottky barrier with AI in presence of large proportion of SQDG. In absence of the barrier (or a weak barrier), the capacitance observed will correspond to the voltage-independent geometric capacitance.

4. Discussion

As mentioned in Section 1, our prime objective to use Chl a in conjunction with SQDG was to exploit the beneficial role of charged substances (SQDG) in enhancing the charge separation in Chl a. The rationale behind this was that SQDG being negatively charged would capture the photogenerated holes (in Chl a) and help photogenerated electrons escape recombination, thereby improving the charge separation efficiency. The capacitance measurements show that the role of negatively charged SQDG is those of the traps which immobilize the holes. Although the immobilization of the holes by SQDG, as mentioned, should help photogenerated electrons (in Chl a) diffuse away from the holes and should, in theory, increase the charge separation yield and improve the efficiency of the cells; the photovoltaic results, however, show otherwise. The photovoltaic parameters such as short-circuit photocurrent, open-circuit photovoltage, fill factor, quantum yield of charge generation, all decrease as the proportion of SQDG is increased [19]. Similar photovoltaic behaviour, i.e., the decrease in quantum yield of charge generation, has also been reported for mixed monolayers of Chl a and N,N-distearoyl-1,4-diaminoanthraquinone, SAQ (an electron acceptor) with an increasing proportion of SAQ [22]. The authors explained their results in terms of the stabilization of the photogenerated electron (in Chl a) on the acceptor SAQ. They argued that if the lifetime of this metastable ion was longer than the diffusion time of the hole to move away, there would be an increase in quantum yield of charge generation. However, contrary will result if the metastable ion was short-lived, as the acceptor will then provide extra or supplementary paths for charge recombination. The same thing might be happening in Chl a-SQDG system, but with respect to the stabilization of the hole on negatively charged SQDG. In other words, if the metastable species, hole-SQDG, is short-lived relative to the diffusion time of the electron to move away, charge recombination will dominate and charge generation yield will decrease, leading to poor photovoltaic characteristics of the cells. However, in view of the totally voltage-independent capacitance for Chl a-SQDG (0.25) even at 0.01 Hz and under illumination, one can safely presume that the hole had been

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immobilized for sufficiently long time so that the electron has diffused away escaping recombination. This would thus rule out the short-lived nature of the hole-SQDG species as being responsible for the poor photovoltaic performance of Chl a-SQDG cells. Alternatively, one can explain both the photovoltaic and capacitance results in terms of the poor rectifying character of A1/Chl a contact in presence of SQDG or a poor rectifying contact of Chl a-SQDG species with either Al or Ag electrodes. This will lead to a weaker Schottky barrier that will result in low values of various photovoltaic parameters and the capacitance characteristics which will resemble more like an insulator than those of a barrier. The weakening of the barrier would be consistent with our earlier photovoltaic results with Chl a-canthaxanthin mixture [I 2], where canthaxanthin is neither an electron acceptor nor a negatively charged substance (a hole acceptor). How the barrier is weakened is not quite clear to us, but the intervening effect of SQDG might be a factor. Further, the negatively charged head group of SQDG can also bind with Mg 2+ of Chl a, and, as a result, as discussed by Van Gurp et al. [10], can keep Chl a molecules apart and prevent them from forming aggregates. This bound Chl a-SQDG entity possibly makes a poor rectifying contact with either A1 or Ag electrode and results in a weak barrier leading to poor photovoltaic and insulator like response of the Chl a-SQDG cells. Further experiments with Chl a-SQDG monolayers with SQDG deposited farther away from AI electrode and with varying number of SQDG monolayers are, however, necessary to better understand the influence of SQDG on the capacitance and other photoelectric properties of Chl a.

5. Conclusion The capacitance measurements show that the role of SQDG in the mixture of Chl a and SQDG is that of the traps which immobilize the holes. When the proportion of SQDG increases, it immobilizes the holes to a great extent, and strongly intervenes in the expansion and contraction of the depletion layer. In extreme cases, it may render the depletion layer totally insensitive to applied bias, resulting in a voltage-independent geometric capacitance even at frequencies as low as 0.01 Hz. The I / C 2 versus V,~ plots of Chl a and Chl a-SQDG (0.25) at 0.0l Hz are distinctly different from each other both in dark and under illumination. The plot of Chl a-SQDG(0.25) is more like of an insulator, suggesting the complete immobilization of holes by SQDG. Alternatively, the capacitance characteristics can be explained if one considers the interaction of negatively charged SQDG with Mg 2+ of Chl a. This may hinder in the ability of Cbl a to form a Schottky barrier with either A1 or Ag electrode. In the absence or in the presence of a weak barrier, the capacitance would be more like that of an insulator than that of a Schottky barrier and would be voltage-independent as has been presently observed. The photovoltaic parameters will also decrease as the charge separation ability of a weak barrier would be feeble.

References [1] LP. Thornber, J. Markwell and S. Reinman, Photochem. Photohiol. 29 (1979) 1205. [2] R+P.F. Gregory, Biochemistry of Photosynthesis. 2nd ed. (Wiley, NY, 1977).

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