6670
J. Phys. Chem. 1993,97,6670-6678
Photovoltaic and Electrical Properties of Al/Langmuir-Blodgett Films/Ag Sandwich Cells Incorporating Either Chlorophyll a, Chlorophyll b, or Zinc Porphyrin Derivative A. Desarmeaux, J. J. Max, and R. M. Leblanc' Centre de recherche en photobiophysique, Universitb du Qubbec h Trois-Riuikres, C.P. 500, Trois- Rivikres, Qub., Canada G9A 5H7 Received: October I, 1992;In Final Form: February 19,I993
The photovoltaic properties of sandwich cells of Langmuir-Blodgett films of chlorophyll a, chlorophyll b, and zinc porphyrin derivative have been measured. The rectifying devices were prepared by interposing the pigment multilayers between two semitransparent metal electrodes, Le., aluminum and silver with work functions (4) such that 4 ~ C 1 4pigment C 4 ~ All ~ sandwich . cells exhibit small short-circuit photocurrents and large opencircuit photovoltages (as high as 1.1 V). The low-power conversion efficiency of all three pigments (0.0010.030%) is related to the large internal resistance of the photovoltaic cells (30 to more than lo00 Ma). This suggests that the poor photovoltaic properties of the sandwich cells are extrinsic to the molecular structure of pigments and should rather be related to the presence of the insulating layers of aluminum and chrome oxide and of the monolayer of cadmium arachidate. In addition, a theoretical treatment of the action spectra is presented. The multiple internal reflections of light between the semitransparent metal electrodes strongly modify the photovoltaic properties of the sandwich cells. The characterization of the metal/semiconductor junction was carried out with the low-frequency oscillographic method. Large variations in the relations btween 1/cZ and Vwere observed as a function of the light intensity, the frequency, and the amplitude of the applied bias. These variations have been related to the presence of the insulating layer which modifies the voltage applied to the junction.
Introduction The primary photochemical event in photosynthesisis the lightinduced charge separation.' Considering the high-energy conversion efficiency in higher plants,2 one can suppose that the reaction centers possess the right constituents in a suitable organization for the energy transduction mechanisms. The photovoltaic properties of chlorophyll a (chl a ) have beem widely studied in relation to the primary steps of photo~ynthesis.~-"In these studies, the pigment was disposed under different forms (e.g., amorphous, microcrystalline,Langmuir-Blodgett films, ...) between two semitransparent metal electrodes, and generally, the photovoltaic cells show relatively low energy conversion efficiency (0.02-0.208). In order to improve the energy transduction mechanisms of such systems, the chl a molecules were mixed withvariousphotosynthetic pigments to reproduce partially the conditions encountered in v ~ v o . ~Nevertheless, ~,'~ the energy conversion efficiencyof these mixed photosyntheticpigments was similar to that of the photovoltaic cells of pure chl a. The use of chlorophyll pigments and its derivatives in photovoltaic devices shows generally large internal resistance which can be related to the low energy conversion efficiency. The presence of an insulating layer at the metal/pigment interface can be the origin of the high internal resistance observed in the photovoltaic cells. This layer, generally formed by a metal oxide layer and in some cases by a monolayer of fatty acid acting as a hydrophobic base for the deposition of the Langmuir-Blodgett films, can strongly alter the ability of the photovoltaic devices to separatechargecarriers. Lawrenceet a]." haveshown that better rectification and higher efficiencies were obtained by stopping the corrosion of the aluminum electrode with a dichromate treatment. On the other hand, the presence of long hydrocarbon chains on the molecular structure of the pigment can also bring an additional resistance to the photovoltaic cell to decrease the charge transduction mechanisms. Consequently, molecules with shorter hydrocarbon chains on their structure would improve the process of charge transfer within the cell. The use of synthetic porphyrins for such studies allows two main advantages: (i) one can obtain molecules with various molecular structures and (ii)
synthetic porphyrins are generally less sensitive to photodegradation than chlorophyll pigments. However, to build multilayer arrays of such pigments, a limitation lies in the fact that many pure porphyrin films having short hydrocarbon chains on their structures are unstable at the air-water interface.'"l9 This is explained by the fact that the presence of long hydrocarbon chains is often necessary to provide an orientation and a structural organization of the molecules at the air-water interface. The influence of molecular structure on the performance of merocyanine dyes as solar energy converters has been recently studied.20 In this study, the techniques of thermal evaporation and film casting from solutionswere used to sandwich the pigments between aluminum and silver electrodes. It has been shown that small changes in the molecular structure can strongly affect the quantum efficiency of charge generation of the Bolar cell devices. Effectively, the substitution of a NO2 group by a hydrogen atom on the molecular structure of the merocyanine dye leads to a variation of the quantum efficiency of charge generation from 0.01% to 3.808. The poor performance of the NOpsubstituted dye has been associated with the presence of sites for internal conversion through torsion or photodissociation. Moreover, just by inverting the position of two groups on the molecule, a variation of more than 1 order of magnitude was observed in the quantum efficiency of charge generation. It is thus interesting to study the photovoltaic properties of chlorophyll b (chl b), which differs from chl a only by the presence of an aldehyde group instead of a methyl group on the conjugated porphyrin ring. This small molecular difference causes appreciable variations in the absorption spectrum of the pigment. Furthermore, molecules of chl b are known to be important accessorypigmentsfor the transfer of light energy to the reaction centers in green plants.' This paper reports the photovoltaic and electrical properties of sandwich cells of Langmuir-Blodgett films of chl a, chl b, and zinc porphyrin derivative. The Langmuir-Blodgett films constitute an important model system for such studies since they produce a well-ordered environment which may be suitable for the energy transduction mechanisms. The chl a and b molecules were used because of their relation with the photosynthetic events
0022-3654/93/2097-6670%04.00/0 0 1993 American Chemical Society
Al/Langmuir-Blodgett Films/Ag Sandwich Cells
-R
= CH3
Zn
a -A
= CHO
ietromethyl
Chlorophyll 2 Chlorophyll
-
corboethoxy
-
-12
3.8.13.18
- trihaxyl - 2 , 7.17- paphlne(2nP)
Figure 1. Molecular structure of chlorophyll a, chlorophyll b, and zinc porphyrin derivative.
and their small difference in molecular structure, while the zinc porphyrin derivative was used because it contains short hydrocarbon chains linked to the conjugated porphyrin ring. In addition, a theoretical treatment of the action spectra is presented. The presence of internal reflections of light within the pigment, due to the semitransparent metal electrodes, strongly modifies the photovoltaic properties of sandwich cells.
Experiment.1 Section The molecular structures of the pigments studied are shown in Figure 1. Chl u was extracted from spinach leaves according to the method of Omata and Murata.?' Zinc 12-carboethoxy2,7,17-trihexyl-3,8,13,18-tetramethylporphine (ZnP) was synthesized in Prof. Ringuet's 1aborat0ry.l~The purity of pigments was checked by thin-layer chromatography. Chl b and arachidic acid were respectively purchased from Sigma (St. Louis) and Applied SciencesLaboratories (State College, PA) and were used without further purification. The Langmuir-Blodgett techniquewas used to build multilayer arrays of pigmentZ2between two dissimilar semitransparent metal electrodes, Le., aluminum and silver. Both metal electrodeswere evaporated under vacuum according to the method previously described.'' The freshly evaporated aluminum electrodes were treated with a dichromate solution (1od M)to stop the corrosion of aluminum.11 The transparency range of the aluminum and silver electrodes at 680 nm was respectively 13-38% and 1-38%. The photovoltaic properties of 6-1 0 sandwich cells of each organic pigment have been analyzed. For the multilayer deposition, a M, pH 8.0) was used for all Tris-HC1 buffer solution pigments. The spreading solvent for chl (Iand chl b was benzene, whereas chloroform was used for ZnP. Both solvents were distilled prior to the experiment. Water for the subphase was distilled from a quartz apparatus before use. All experiments were performed at 19 f 1 OC under dim green light. Before the deposition of pigments, the top aluminum electrode was covered with one monolayer of cadmium arachidate to act as hydrophobic base. This cadmium arachidatelayer was transferred at a surface pressure of 30 mN m-1 using as a subphase a Tris-HC1 buffer solution M, pH 8.0) in which approximately 10-4 M of CdC12 was added. The chl o and chl b cells contained 44 monolayers of pigment transferred on top of the arachidate layer at a surface pressure of 20 mN m-l. This pigment thickness has been reported to give the maximum power conversion efficiency for the chl u sandwich cells.lZ For the ZnP cells, 40 monolayers were transferred at a surface pressure of 12 mN m-I. We have noted that the first five monolayers of the porphyrin derivative showed a constant decrease of the deposition ratio on the upward movement of the slide. Afterward, the last monolayers were transferredin anX-typedeposition (i.e., the monomolecular films were transferred only on the downward movement of the slide). The photovoltaic cells to be analyzed were irradiated in a stainless steel Faraday cage with light from a 50-W tungsten-
The Journal of Physicol Chemistry, Vol. 97, No. 25,1993 6671 halogen lamp passing through a monochromator (Jobin-Yvon). Light energy emitted by the source (ranging from 0.1 to 24.0 pW cm-2) was measured with a United Detector Technology 21A power meter. Currents and voltages developed by the cells were measured with a Keithley 616 electrometer. The open-circuit photovoltages are the voltages directly read on the electrometer upon illumination of the cells and are thus the sum total of the voltages produced in dark and in light. The power conversion efficiencies, 1,of the cells were determined as follows. An external load resistance is introduced in the circuit, and its value is varied from 0 to (loo0 Mil in the present case). A plot of the photocurrent against correspondingphotovoltageis then obtained, and the maximum power, Pmax,generated by the cell is the maximum product of photocurrent and photovoltage. Knowing the light power, Pine, incident on the cell, 1 is obtained from the ratio of P- to Pint. The active area of the cell is 0.45 cm2. The capacitance measurements were carried out with the lowfrequency oscillographic method as described by Twarowskiand AlbrechtSz3The measurements have been made in the dark and under illumination in the range 1P2-102 Hz. The current generated by the cells was measured with the use of a currentto-voltage converter using a conversion factor from 5 X 103 to 2 X lo8 V/A. An average of 5-100 I-Vcurves has been carried out for each photovoltaic cell in order to increase the signal/noise ratio. A Perkin-Elmer double-beam spectrophotometer was used to measure the absorption spectrumof the Langmuir-Blodgett films. Due to the presence of the reflecting metal electrodes in the photovoltaic cells, the absorption spectrum of the LangmuirBlodgett films was measured on metal-free glass slides which were prepared at the same time as the multilayer deposition on the metal electrodes. We have thus considered the absorption spectra of the pigments on these metal-free glass slides to be the same as the spectra of the pigment multilayer sandwich in the photovoltaic cells.
ResulCe and Discussion
Jhrk cbrmcterlstics. All photovoltaic cells of chl u, chl b, and ZnP exhibit small dark short-circuit currents between 0.04 and 0.60 nA cm-2 and relatively large dark open-circuit voltages rangingfrom0.03 tomore than0.50 V. Thesedarkcharacterietics have the same polarity as the photocurrents and photovoltages. Dark open-circuit voltages as high as 0.76 V have been observed for some photovoltaic cells of ZnP. The dark photovoltaic characteristics have often been related to an electrochemical reaction of the interfacial oxide layer at the aluminum/pigment However, the use of a too small thickness of pigment in the photovoltaic cells can impede the depletion layer from expanding sufficiently to reach equilibrium,thus generating dark currents and dark voltages. The two last explanations are not contradictory with each other since the expansion of the depletion layer is related to thequalityof the rectifyingaluminum/ pigment contact. Consequently, the large variations observed in the dark characteristics from one cell to another can be related to some variations in the rectification propertiesof the aluminum/ pigment junction. The increase of temperature in the photovoltaic devices m a y also contribute to the generation of dark currents and dark voltages.26 Effectively, preliminary studies on chl b cells have shown large variations of the dark characteristics according to the temperature of the experiments. We have noted an increase of the short-circuit current (0.0154.600 nA cm-2) and of the open-circuitvoltage (0.024.35 V) by increasingthe temperature from -25 to 45 OC. These results are often related to the thermal activation of trapped charge Pbotovolfaic churcteristies. The photovoltaic cells exhibit short-circuit photocurrents between 1.0 and 18.0 nA and open-circuit photovoltages as high as 1.1 V when the pigment is
Dhormeaux et al.
6672 The Journal of Physical Chemistry. Vol. 97, No. 25, 1993
TABLE I: Dark and Photovoltaic Cbrncteristics of the Sandwich Cells of Chlorophyll a, Chlorophyll b, and Zinc Porphyrin Derivative
chl a dark currents (nAcm-2) dark voltages (V) short-circuit photocurrents (nAcm-') open-circuit photovoltages (V) internal resistance (MQ) cell power (nW cm-2) power conversion efficiency (%) 0.5 PHOTOVOLTAGE [ V )
1.0
F g v e 2. Photocurrent versus photovoltage of a sandwich cell of chl a at 678 nm (-), chl b at 656 nm (- -), and ZnP at 600 nm
-
(-.e).
illuminated at the maximum absorption peak in the red region (light intensity = 4-16 pW cm-2 according to the transparency of the electrodes). We have noted that the time to reach a steadystate value for photovoltage is relatively long (>20 min). Figure 2 shows typical photocurrent vs photovoltage curves for the photovoltaic cells of each pigment illuminated at their corresponding maximum absorption in the red region. These curves wereobtained by varying the load resistance (0-lo00 Ma)in the external circuit. One can observe that the curves are far from the almost rectangular shape usually obtained with inorganic solar cells.2' The curves are rather typical of photovoltaic cells having high series resistance.28 In fact, the internal resistance of the cells, measured by the method of impedance matching, varies from 30 to more than 1000 Ma for all three pigments according to the light intensity. Such high resistance values can decrease the efficiency of charge-transfer mechanisms within the cell and can therefore explain the long time required to reach a steady-state value in photovoltage measurements. The large variations of the internal resistance from one cell to another may result from various factors. Since the growth of the oxide layer is time-dependent, the elapsed time between the evaporation of the aluminum film, the deposition of the pigment multilayers, and the experimental measurements of the photovoltaic characteristics can play a major rolein dictating the internal resistance value of the photovoltaic cells. Furthermore, any variation of the experimental conditions in one of these steps of preparation may affect the insulator thickness and its resistance value. On the other hand, a difference in the morphology of the pigment in the multilayer arrays can affect the energy transduction mechanisms within the pigment and, thus, the overall resistance of the cell. The use of ZnP molecules, which possess short hydrocarbon chains, does not tend to decrease the internal resistance of the overall cell. As pointed out before, our cell devices present an insulating layer at the aluminum/pigment junction formed by the aluminum and chrome oxide layers and by the monolayer of cadmium arachidate. The chrome oxide layer is formed from the dichromate treatment of the electrode. The resistance of this insulating layer (100-1000 MQ) is comparable to the series resistance observed in our sandwich cells (vide infra). Consequently, it seems that the molecular structure of the pigments used in this study is not the limiting factor which characterizes the high internal resistance observed in our devices but should rather be related to the aluminum/pigment interface. By illuminating the pigments at their corresponding maximum absorption in the red region with a light intensity ranging from 5.0 to 15.0 pW cm-2, a maximum power output ranging from 0.2 to 2.2 nW cm-2 was usually obtained for all photovoltaic cells. The power conversion efficiency of the photovoltaic devicesvaries from 0.001% to 0.030%for all three pigments. Table I summarizes the photovoltaic characteristics of the sandwich cells of the three organic pigments. One can easily observe that the molecular
chl b
ZnP 0.04-0.60
0.08-0.14 0.01-0.50 1.1-18.0
0.04-0.40 0.08-0.58 2.7-14.5
0.4-1.1
0.8-1.1
30-1000 0.2-2.4 0.001-0.025
55-1000 30-1000 0.2-2.2 0.1-2.0 0.001-0.030 0.001-0.025
0.03-0.76 0.9-11.1 0.41.1
structure of the pigment does not influence the photovoltaic properties of the cells. Despite the large difference between the molecular structure of ZnP and that of the chlorophyll pigments, all photovoltaic characteristics are very similar. The low values of power conversion efficiency can be related to the large internal resistance of the cells which make the short-circuit photocurrent less than the actual photogenerated current. However, the poor semiconducting properties of the pigment multilayers are also responsible for the low power conversion efficiencies of the photovoltaic cells. Light Intemity DependenceoftbePbotoeurrent. Thesandwich cells of all three pigments, illuminated at their corresponding maximum absorption in the red region, exhibit a nonlinear light intensitydependence of the short-circuitphotocurrent that follows the relation
,z
= Kzi";
where ZW is the short-circuit photocurrent, K a proportionality constant, Zinc the incident light intensity, and T the light exponent. The light exponent values vary generally between 0.71 and 0.87 for all photovoltaic cells when the pigments are illuminated at their corresponding maximum absorption in the red region with a light intensity up to =18 FW cm-2. For most photovoltaic studies involving organic pigments, values of 7 ranging from 0.3 to 1.0 are generally r e p ~ r t e d . ' ~ J ~ . ~The ' ~ ~values of light exponents vary with the wavelength of illumination of pigments. Values of T of 1.00f 0.04have been observed for all photovoltaic cells of chl a and chl b when the pigments were illuminated at their corresponding maximum absorption in the blue region with alightintensityrangingfrom 1.5 to6.0MWcm-2. Thisconstitutes a difference in the light exponent values of about 0.20 higher in comparison to the red region values. On the other hand, the photovoltaic cells of ZnP have never shown a linear intensity dependence on the short-circuit photocurrent by illumination of pigment between 400 and 700 nm. Values of T of 0.90 0.04 have been observed for the cells illuminated at the maximum absorption in the blue region. For all photovoltaic cells, a decrease of the light exponent valuea has been observed at each considered wavelength by increasing the light intensity by a factor of -3. This can be related to the presence of the insulating interfacial layer at the aluminum/ pigment junction. The values of light exponents lower than unity have often been interpreted in the literature by the presence of exponentially distributed traps that control carrier recombination.26 However, Fan and Faulkner3' have shown that the presence of an interfacial insulating film in cell devices can greatly affect the light intensity dependence of the short-circuit photocurrent. They have reported a linear or nearly linear relation between the short-circuit photocurrent and light intensity for a metal-free phthalocyanine sandwiched between indium and gold electrodes (In/HzPc/Au). On theother hand,incelldevicesusingalu" contacts (Al/HzPc/Au), the short-circuit photocurrent rises as the cube root of the intensity, and the power conversion efficiency is lower by 2-3 orders of magnitude. These characteristics have been attributed to the poor quality of the aluminum/pigment
*
The Journal of Physical Chemistry, Vol. 97, No.25, 1993 6673
Ai/Langmuir-Blodgett Films/Ag Sandwich Cells
B
-0.48
w
u -0.36 -0.24
a
0.0
-0. I2
: ‘-1
2 a
$ m a
I
\,
,....
.-
,,/0.13
QO 400
000
500
600
700
I
1
500 600 WAVELENGTH (nm)
I
700
800
Figure 4. Uncorrected (-) and corrected (- - -) action spectra of a sandwich cell of chl b illuminated through the aluminum electrode. The absorption spectrum of chl b (- -) and the light intensity profile of the are presented. source (a-)
0.39
0.2
400
0.00
10.52
-1
0.0
800
WAVELENGTH (nm)
Figure 3. Absorption spectrum (-) and corrected action spectra of a sandwich cell of (A) chl u, (B)chl b, and (C) ZnP illuminated through
the aluminum (- - -) and silver (-) electrodes.
junction due to an A1203 interfacial layer. The large variations observed in the light exponent values from one cell to another can thus be attributed tovariations in the aluminum/pigment contact. Action Spectra. The action spectra of the photovoltaic cells are obtained by plotting the short-circuit photocurrent as a function of wavelength of illumination. The action spectra have been corrected for the same number of photons reaching the pigment at each wavelength to better see the relation with the absorption spectrum. This correction has been made by attributing a light exponent value for three or four regions of the visible spectrum. However, as we will see later, this correction can induce some modifications in the profile of the action spectra of pigments. Figure 3 shows typical action spectra of the photovoltaic cells of all three pigments illuminated through each metal electrode and the corresponding absorption spectrum of the pigments. One can observe that the action spectra of the sandwich cells illuminated through the aluminum electrode closely follow the absorption spectrum of pigments in terms of position of the maxima. This suggests that the singlet excitons are the precursors of the charge generation mechanisms. Many studies have reported an inverse relationship between the action spectrum and the absorption spectrum when the cells are illuminated through the back electrode.4+30.35v36 This is explained by the fact that the light absorbed at the front surface is not effective in creating free charge carriers. This front layer is then acting as an inner filter which screens the blocking contact from the incident light. One can observe that the illumination of the cells through the silver electrode does not lead to the inverse absorption spectrum of pigments. In fact, the filter effect is observed only to a small extent in the blue region of the action spectra of the chl a and chl b cells. This can be explained by the low thickness of pigments used in our cell devices. Assuming we found that the thickness of one monolayer of chl a is 14
that the entire pigment thickness of the chl (I cells is 616 A. In this case, the depletion layer covers a large part, or wen the totality of the thickness, so the illumination of either side of the cell will lead to similar results in terms of position of the maxima. Furthermore, as we will see in the theoretical treatment of the action spectra, the filter effect is attenuated by the presence of multiple internal reflections within the cell due to the presence of the reflecting metal electrodes. It has to be mentioned that a lower photocurrent is invariably observed for the illumination through the silver electrode. In this case, the light intensity is mainly absorbed near the pigment/silver interface. Since the electrical field decreases with distance from the rectifying aluminum/pigment junction, one may expect a lower quantum yield of charge generation near the pigment/silver interface, leading to a decrease of the photocurrent. Numerous results published in the literature on photovoltaic action spectra of organic cells have been corrected for a constant photon flux at each wavelength. However, as seen previously, the light intensity dependence of the short-circuit photocurrent varies with the wavelength of illumination and the light intensity. Since the exact relation between the photocurrent and light intensity is not known for each wavelength, such corrections may alter the profile of the action spectra. To illustrate this effect, Figure 4 shows both corrected and uncorrected action spectra of a sandwich cell of chl b illuminated through the aluminum electrode. Thecorrespondingabsorptionspectrumof the pigment and the incident light intensity profile of the source (through aluminum electrode) are also presented. The corrected action spectrum was determined by using the light exponent values 0.98, 0.83,0.82,and 0.81 at 468, 560,610, and 656 nm, respectively. A much lower blue/red band ratio can be observed in the uncorrected spectrum due to the lower light energy emitted by the sourcein the blue region of the visible spectrum. Furthermore, the Soret band in the uncorrected spectrum is shifted to wavelengths of lower energy. Finally, an important difference between both spectra is the presence of the shoulder at 510 nm in the uncorrected spectrum, which was not observed when we corrected the action spectrum for the same number of photons at each wavelength. The presence of this shoulder is related to the large increase of the light intensity emitted by the source in this region. So, in order to avoid any inaccuracies in the profile of the action spectra, we will next consider the exact values of the short-circuit photocurrent by using the uncorrected spectra. The profile of the action spectrum of the photovoltaic cells can be understood by using a model similar to the one developed by Ghoshet al?5-38In this model, weconsideredthat the photocurrent is proportional to the generation of excitons created by the light absorption. These excitons can be separated by the electric field
Dbormeaux et al.
6674 The Journal of Physical Chemistry, Vol. 97, No. 25, 1993 in the depletion layer. It has to be mentioned that this model can also be applied to the processes of generation of charge carriers. So, for the illuminationof the cell through the aluminumelectrode, the density of carriers photogenerated in the depletion layer per unit area at a distance x from the surface is given by the following expression:
where l b is the barrier width, 4 the quantum yield of charge generation, l o the number of incident photons/unit area, and CY the optical absorption coefficient of the pigment at the considered wavelength. In the same way, for the illumination of the cell through the silver electrode, the density of carriers photogenerated in the barrier width per unit area is given by the equation:
where I represents the thickness of the pigment multilayer. Let us consider now the carriers generated at a distance x out of the depletion layer. In thiscase,the photocurrent is proportional to the number of carriers generated by the light absorption times the probability that the carrier will diffuse to the junction and be separated by the internal field. The diffusion of carriers can be treated with the standard diffusion equation for a permanent regime:37 (4)
WAVELENGTH (nm)
-
Experimental (-) and calculated (- -) action spcctra of a sandwich cell of chl 6 illuminated through the (A) aluminum and (B) silver electrodesfor only one passage of light. The correspondingincident is presented. light intensity profile of the source F i e 5.
n~ n 0 = -- -+ #azoexp[-cY(I - x)] ~
6
6x2
70
(5)
(0.0)
where D and 7 represent the diffusion coefficient and the mean lifetime of the carriers. The second and third term of each equation correspond respectively to the recombination and generation of the carriers. In combining the solution of the diffusion equation with eqs 2 or 3, respectively, the total photocurrent of the photovoltaiccells for the illuminationthrough the aluminum and silver electrodes (J = -D 6n/&x)/b+ nAI(2) or nAB(3))is respectively
a4z&e*/b[ 1 - ,-@([-/b) [2e*(’-/b) JAl
=
(p- CY2)
[ 1 - ,-28(/-M
1
,-b(’-/b)]
I+
4ZO[l- B2e4Ib/(o2- a2)] (6)
withLD corresponding to thediffusion where B= 1/LD = 1/(d) length of the carriers. An important point before using such a theoretical treatment for the action spectra is the fact that our cells possess a low pigment thickness. Therefore, only a fraction of the incident light intensity is absorbed by the pigment. As an example, about 70% of the light intensity is absorbed by the 44 monolayers of chl b at the maximum absorption in the blue region, and this value reaches only 15% at 560 nm. Since the pigment is sandwichedbetween two semitransparent metal electrodesof high reflection coefficient,a fraction of the transmitted light is reflected, leading to multipleinternal reflections of light within the pigment. Knowing the transparency value of both metal electrodes at each wavelength and the thickness of the pigment multilayers, one can easily measure the light absorption by the pigment during each reflected wave. So to take into account the multiple internal
reflections of light in the theoretical treatment of the action spectra, the total current is obtained by adding the current values calculated from eqs 6 and 7 using as ZO the total number of photons (Le., incident and reflected photons) comingfrom each metal electrode. To illustrate the theoretical results, Figures 5 and 6 show the experimental and calculated action spectra of a sandwich cell of chl b illuminated through each metal electrode, for one passage of light and for multiple internal reflections, respectively. The calculated action spectra are presented for a barrier width of 250 A, a diffusion length of 200 A, and a quantum yield of charge generation (4 = 0.005) independent of wavelength and constant with the distance in the cell. These values are in good agreement with thevalues~bservedforchlamultilayerarrays.~~J~ However, it has to be mentioned that the variation of the diffusion length and barrier width values affect only to a small extent the profile of the theoretical action spectra. One can observe that the spectrum of the current calculated for only one passage of light (Le., without the internal reflection process) shows a red shift of the &ret band compared to the absorption spectrum for both sides of illumination (Figure 5). This is attributed to the large increase of the incident light intensity in this region. The larger shift observed for the calculated spectrum illuminated through the silver electrode comes from the additional contribution of the filter effect. The calculated spectrum for the longer wavelengths follows reasonably the profile of the experimental spectrum but varies in intensity, especially for the illumination through the silver electrode. The quasi-nonvariation of the light intensity profile between 520 and 750 nm explains the absence of the band shift in comparison with the absorption spectrum of the pigment multilayer. Let us consider now the theoretical action spectra, taking into consideration the multiple internal reflections within the cell. One can notice that the calculated action spectrum illuminated through the aluminum electrode exhibits a shoulder at 510 nm
Al/Langmuir-Blodgett Films/Ag Sandwich Cells
The Journal of Physical Chemistry, Vol. 97, No. 25, 1993 6675
__
I
400
500
700
600
000
WAVELENGTH (nm)
Figure 7. Profile of the quantum yield of charge generation for the illumination of a sandwich cell of chl b through the aluminum (-) and silver (- - -) electrodes and the absorption spectrum of chl b (e-).
400
500
600
700
8
WAVELENGTH (nm)
-
Figure 6. Experimental (-) and calculated (- -) action spectra of a sandwich cell of chl b illuminated through the (A) aluminum and (B) silver electrodes for multiple internal reflections of light. The corresponding total light intensity profile of the source (--) is presented.
which corresponds to the one observed in the experimental action spectrum (Figure 6). The presence of this shoulder is attributed to the combined effect of the large increase of the light intensity profile and the decrease of the extinctioncoefficient of the Rigment in this region. Effectively,a decrease of absorption has the effect of increasing the number of reflections within the pigment that may lead to a higher increase of the current in this region. For the region of lower absorption, one can observe a large increase of the current in comparison with the theoretical spectrum for only one passage of light. This is due to the larger number of internal reflectionsthat highly increase the lightexcitation of the pigment and, consequently, the generation of charge carriers. The theoretical action spectrum for the illumination through the silver electrode also shows a red shift of the Soret band to coincide with the position of the band in the experimental action spectrum. At longer wavelengths, the spectrum is similar to the profile of the experimental spectrum but varies in intensity, particularly for the blue/red band ratio. We noticed that the use ofa counter electrode with a low transparency highly increases the intensity of the current due to a higher light reflection in the cell. The differences in the intensity of the calculated current for some regions, with respect to the experimental action spectrum, can be explained in terms of quantum yield of charge generation. Effectively, as pointed out before, we used a quantum yield of charge generation independent of wavelength and constant in the whole thickness of the pigment in the cell. Figure 7 shows typical apparent profiles of the quantum yield of charge generation for the illumination of a sandwich cell of chl b through each metal electrode. These profiles were obtained by dividing the action spectra of the cells and the total number of photons absorbed by the whole pigment thickness at each wavelength. As can be seen, a large variation of the quantum yield of charge generation is observed with wavelength. Thequantum yield for an illumination through the aluminum electrode decreases from 400 to 500 nm, followed by a quasi-plateau up to 660 nm. For the illumination
0.0
\.. I
I
400
500
I
I
600
700
I
- -.
eoc
WAVELENGTH (nm) and calculated (- -) action
-
Fi’iguo 8. Experimental (-) spectra of a sandwich cell of chl b illuminated through the (A) aluminum and (B)
silver electrodes for multiple internalref’lectionsusing thequantum yield values 0.6095, 0.3595, and 0.1095 for the wavelength rangca 400-460, 500458, and 698-790 nm, respectively.
through the silver electrode, a steeper decrease of the quantum yield is observed in the blue region followed by a slight decrease from 500 to 700 nm. In both cases, an increase of the quantum yield is observed in the far-red region. This can be attributed to some artifacts in the absorption value of the pigment coming from the supporting glass slide. Since the absorption spectrum of the pigment has been corrected for a zero value of absorption at 800 nm,any deviation of the absorbance value in this region leads to large variations in the quantum yield values. The variation of the quantum yield of charge generation with wavelength of illumination has been already observed in amorphous selenium.39 In this study, the dependence of the photogeneration efficiency on wavelength of illumination of these inorganic solar cells was explained in terms of initial separation
Dcaormeaux et al.
6676 The Journal of Physical Chemistry, Vol. 97. No. 25, 1993 7.5
I
I
I
c
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-
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Figure 9. (A) Current-voltage curvca and (B) l/C?-Yrelations for a chl b (- -), and ZnP (-) in the dark at 100 sandwich cell of chl a (e-),
Hz.
1
1
-
between thermalized electron-hole pairs. It was shown that this initial separation varies from 70 A at W - n m excitation to 8.4 A at 620-nm excitation. Such a mechanism can well explain the wavelength dependence of the quantum yield observed in our organic photovoltaic devices. Effectively, the absorption by the pigment of short-wavelengthexcitation will generate excitons in which the Coulomb attraction between the undissociated electronhole pairs will be small compared to that of the excitons generated from long-wavelengthexcitation. This way, the charge separation of the electron-hole pairs will be much more efficient for shortwavelength excitation, resulting in higher values of quantum yield of charge generation. The behavior of the quantum yield with respect to wavelength of illumination is very similar for the sandwich cells of all three pigments. The quantum yield profiles obtained for these organic pigments can be divided in three wavelength regions having different correspondingquantum yield values. To illustrate this effect, Figure 8 shows the theoretical action spectra of the sandwich cell of chl b obtained by using respectively the quantum yield values 0.60%. 0.3596, and 0,1096 for the wavelength ranges 400-460,500658, and 698-790 nm. These values have been taken from the quantum yield profile for the illumination of the chl b cell through the aluminum electrode presented in Figure 7. The connection between each wavelength range has bcen made by considering a linear decrease of the quantum yield on a 40-nm extent. One can observe an excellent agreement between the experimental and theoretical action spectra for the illumination through the aluminum electrode. The small difference between spcctra is explained by the fact that the quantum yield value is not really constant in each wavelength range. With regard to the experimental and theoretical action spcctra for the illumination through the silver electrode, one can observe large variations between spectra due to the difference in the quantum yield profile for illumination through this side. This may come from the fact that the quantum yield values do not represent the effective ones since we have considered the number of photons absorbed in the entire thickness. Moreover, as pointed out before, the magnitude
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0.0 I 3 APPLIED BIAS ( V I Flgnre 10. (A) Current-voltage curvca and (B) l/&Vrelations for a sandwich cell of chl a (-), chl b (- -),and ZIP (-) in the dark at 0.01 Hz. -1.8
-
of the electricalfield, resulting from the metal/pigment junction, varies with distance in the pigment thickness. Therefore, this charactsrieticmay beresponsiblefor thevariationof thequantum yield profile with the side of illumination. Finally, since the presence of multiple internal reflections of light within the cell has been put in evidence, the disagreement between the spectra can be explained by some interferencas between the electromagnetic waves. According to the side of illumination, the position of the ~ n s t r u c t i and/or ~e destructiveinterference fringes in the pigment would greatly affect the quantum yield values. Since the variation of the refractive index asa function of the Wavelength is not well established, the theoretical treatment of such interference phenomena is still not solved. Qpacbam M e a " & The capacitance measurements have been carried out with the low-frequency oscillographic m~thod.2~ This method consists of applying an extemal bias to the cell simultaneouslymeasuring the current as a function of the applied bias. Considering the cell as a simple parallel combmtion of a resistance (R) and of a capacitance (C),the expression of R and C for a triangular bias is given by
where Voandfare, respectively, the amplitude and the frequency of the bias, whilefC andj- refer to the current for increasing and dcmasing bias, respectively. The width of the depletion layer ( W)varies with the extemal bias according to the expression
(9) where Vb is the Schottky barrier voltage, N is the density of charge carriers, and G, and e, are the permittivity of free space and the dielectric constant of the semiconductor, respectively. Rearranging the expressions, one can obtain the followingrelation:
Al/Langmuir-Blodgett Films/Ag Sandwich Cells
-I .4 APPLIED
BIAS ( V I
The Journal of Physical Chemistry, Vol. 97, No. 25, 1993 66?l
I .4
-I A
APPLIED BIAS ( V I
I ,4
prrOr 11. (A) Current-voltage, (e) resistance-voltage, (C) capacitancbvoltage. and (D)l/C?-voltage curvca for a sandwich cell of chl bin the dark (-) and under illumination (*-) at 0.01 Hz.
1
+ ",
-I
c2
v A~N
(10)
where K'= 2 / e s s represents a constant value. Therefore, if the capacitance is attributed to a Schottky junction, a straight line should be obtained in the 1/c2 vs V plots. Such a simple calculation is the major advantageof this method. Consequently, this method has been widely used in the characterization of the electrical properties of organic photovoltaic cells.11~13940-43 One must, however, underline that this simple calculation is due to the simple model employed, Le., that of a capacitance and a resistance in parallel. Figure 9 shows typical current-voltage (I-y) curves and their corresponding l/C? vs Vrelations for the sandwich cells of chl a, chl b, and ZnP in the dark at 100 Hz. As can be seen, the capacitance is voltage independent for all three cases. The nonvariation of the capacitance with the applied bias has often b a n associated to the low mobility of deep trappedcharge carriers which cannot respond rapidly to the polarization cycle.' 1~13.23This way, one can measure the geometric capacitance between the two metal electrodes. Twarowski and Albrtcht have observed the passage of a geometric capacitance to a voltage dependence capacitance upon heating the photovoltaic cells of tetracene or phthalocyanine.40 These results were attributed to the mobilization of trapped charges upon heating the pigment. On the other hand, some authors attributed the nonvariation of the capacitance with the applied bias at high frequencies to the fact that all the pigment thickness is depleted of free charge carriers.Although we agree with the fact that the depletion layer covers the major part of the pigment thickness, it appears that the nonvariation of the capacitance is not solely attributed to the low pigment thickness since the capacitance is still voltage independent for sandwich cells with a much greater thickness of chl u pigment (results not shown). Figure 10 shows the I-Vcurves and their corresponding 1 / 0 vs Vrelations for the sandwich cells of chl a, chl b, and ZnP in
the dark at 0.01 Hz. One can observe that, at this frequency, the capacitance begins to show a voltage dependence, but the ideal linearrelationbetweenl/CLand Visnotobserved. Similarresults have b a n observed for the Al/tetracene/Ncsatron cell and were attributed to a large number of charge carriers which can only marginally respond at this freq~ency.2~Furthermore, the nonlinear relation has also b a n attributed to a nonuniform distribution of impurities in the space charge la~er.134~The presence of the insulating layer in the photovoltaiccells has ban rarely considered in the interpretation of the electrical characteristics of the overall system. However, if the series resistance of the system is negligible, a discontinuity should manifest itself at each inversion of the sign of dY/dt. Such discontinuities have not ban o k e d for all threcpigment cells. These results indicate the prciicnce of a resistance in series in the sandwich celh g i w rise to a certain time constant. From capacitance measurements made on unpigmented cells (i.e., Al/Al203/cadmium arachidate monolayer/Ag), we found that the resistance of this interfacial layer (1W-lOOO MQ)is comparable to the series resistance observed with the pigmented cells. Therefore, we believe that the nonlinear relation observed between l/G and V in the sandwich cells of each pigment should be attributed, in part, to the presence of the insulating layer. Figure 11 shows the effect of the illumination of the pigment on the electrical properties of a chl b cell at 0.01 Hz.The results are presented for the illumination of the cell through the aluminum electrode at maximum absorption of chl b in the red region with a light intensity of 7.4 rW cm-2. As can be seen, a large increase of the capacitance is observed upon illumination of the pigment. This can be explainedby the fact that the photogeneratedcarriers decrease the built-in potential of the cell, thus reducing the width of the depletion layer. On the other hand, the photogenerated current decreases considerably the resistance of the system. Some studies have notcd that the illumination of organic photovoltaic cells allows one to obtain a linear relation between l/eand V.11-23Although the linear relation observed was not
Dcsormeaux et al.
6678 The Journal of Physical Chemistry, Vol. 97. No. 25, 1993 truly ideai, these results were explained by the mobilization of trapped charge carriers upon illumination of the cell. However, this linear relation has not been observed in most of the measurements made on the sandwich cells of chl a, chl 6, and ZnP. Indeed, large variations in the l/C-Vrelation have been observed as a function of the light intensity, the frequency, and the amplitude of the applied bias. In addition, some 1 / C - V relations have shown two or more linear segments on which are associated various values of slope. Similar results were observed on photovoltaic cells of p h t h a l ~ y a n i n e s . Consequently, ~~ the determination of parameters such as the built-in potential (vb), the width of the depletion layer ( W),and the density of charge carriers (N) from the l/G-V relations may lead to erroneous values. It must be pointed out that the electrical properties of silicon solar cells do not show these variations. However, by simulating the presence of an unpigmented cell in series with the silicon solar cell, we have observed the same type of variations obtained with the organic photovoltaic cells.49 The origin of these variations in the I/G-Yrelations can therefore be related to a polarization effect of the insulating layer at the aluminum/ pigment interface that modifies thevoltageapplied to thejunction. This way, only a part of the applied bias is across the junction, the remaining being across the insulator. These results show the nonnegligible role of the interfacial insulating layer on the electrical properties of the sandwich cells.
Conclusion All sandwich cells of chl a, chl 6, and ZnP exhibit similar photovoltaic propertias. It is suggeeted that the poor photovoltaic performances of the cells are extrinsic to the molecular structure of the pigment and should be related to the presence of the insulating layers of aluminum and chrome oxide and of the monolayer of cadmium arachidate. Consequently, with a view to maximizing the efficiency of these organic cells, efforts should be made to decrease the high resistanceinduced by these insulating layers. The correction of the action apectra of the photovoltaic cells for the same number of photons induces some modifications in the profile of the action spectra due to the variation of the light intensity depsndence of the short-circuit photocurrent upon wavelength and light intensity. It is thus preferable to use the uncorrected action spectra ta avoid any inaccuracies in the values of the current. The theoretical treatment of the action spectra of the sandwich cells of the thin organic films has demonstrated the presence of multiple internal reflections of light within the pigment. The band shift as well as the intensity of the cumnt can be explained by the internal light reflections. With regard to the capacitance measurements, the nonlinear relation betwetn 1/cL and the applied bias can be associated with the insulating layer at the aluminum/pigment interface which modifies the voltage applied to the junction. These results put in evidence the nonnegligible role of this interfacial layer on the photovoltaic and electrical properties of the cells.
AcLwwkdgnmt. This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada and the Fonds pour la formation de chercheurs et l’aide
la recherche. We thank Prof. Michel Ringuet for the synthesis of the porphyrin derivativeand Dr. Surat Hotchandani for useful discussions.
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