J . Phys. Ckem. 1987, 91, 150-154
150
+
for the nitroxide moiety to bind to the alumina surface as was demonstrated by adding Tempo (2,2,6,6-tetramethylpiperidino1-oxy) to an alumina suspension. In this case, the ESR spectrum of this probe was clearly that of the probe in aqueous solution. Centrifugation of the mixture further demonstrated this point. Because of the highly anisotropic nature of the spectra in the hemimicelles, it is difficult to determine hyperfine coupling constants for the probes 2 and 3. With I , a relatively high value is obtained for the hyperfine splitting (15.0 G). This supports the model shown in Figure 3 where the nitroxide of 1 resides at the water-SDS interface. ESR spectroscopy with nitroxide spin probes represents a facile method for probing microenvironments. It is possible to determine microviscosities by calibration of the ESR spectra of the probes in solvent mixtures of known viscosities. This method avoids the complications involved in comparing rotational correlation times for different probes, which can vary differently as a function of the solution viscosity, since the response of the probe to solution viscosity is accounted for explicitly. Indeed, the microviscosity can effectively be defined as the homogeneous solution viscosity which results in the same spectrum as that in the microenvironment. With this methodology, while the spin probes 1, 2, and 3 were found to be in similar microenvironments in SDS micelles, they sense different microviscosities in SDS hemimicelles as a function of distance from the alumina surface. The flexibility variations indicated by these results provides a more detailed picture of the structure of hemimicelles than that obtainable by classical bulk property measurements.
I
+I Figure 3. Schematic representation showing the SDS hemimicelle flexibility differences in two probe positions (modeling 1 and 3).
tight so that rotational mobility is severely restricted. Further from the solid surface the alkyl chains can spread out such that the flexibility of the system increases. This is shown pictorially in Figure 3. It should also be noted that there is no tendency
Acknowledgment. We thank NSF and ARO for generous support of this research. K.C.W. acknowledges PHS Grant No. CA07957 (National Cancer Institute, DHHS). Registry No. 1, 53034-38-1; 2, 29545-47-9; 3, 29545-48-0; SDS, 151-21-3.
Cadmium Sumde/Poly(vkrylferrocene)/Gold and Cadmium Sulfide/Polypyrrole/Gold Solid-State Cells Mark P. Hagemeister and Henry S. White* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis. Minnesota 55455 (Received: August 13, 1986)
The fabrication and electrical behavior of solid-state CdS/poly(vinylferrocene)o/+(PVFo/t)/Au and CdS/polypyrroleOl"+(PP/"+)/Au cells are described. Both polymers contact :dS to yield normal Schottky-like (semiconductor/metal) behavior, in the dark and under illumination, when the polymer layers are in the partially oxidized and charged state, PVF"/+ and PPojn+. Photocurrent generation in these cells depends strongly upon oxidation state of the dry polymer film. In the partially oxidized state, CdS/PVF+/O/Au, photocurrents increase linearly with increasing light intensity. When the polymer film is in the fully reduced and neutral state, CdS/PVP/Au, the observed photocurrents are ca. 2 orders of magnitude smaller and nearly independent of incident light intensity. A similar dependence of photocharacteristics on oxidation state is observed for the CdS/PP/Au cell. An analysis of the photovoltaic cell response under redox polymer charge-transport control is presented. I
introduction A difficulty with photoelectrochemical cells for the conversion of solar to electrical or chemical energy is the tendency of the photoactive cathode or anode to passivate or dissolve in These corrosion reactions are especially deleterious in aqueous
solutions where water plays a key role in the solvation of electrode lattice ions or supplies oxygen for forming passive oxide layers. Recently, several reports have described solid-state photovoltaic devices constructed by sandwiching a thin and dry electroactive polymer film between semiconductor and metal electrode^.^-^ In
(1) Gerischer, H. J. Electroanal. Chem. 1977, 82, 133. (2) Bard, A. J.; Wrighton, M. S. J. Electrochem. SOC.1977, 124, 1706. (3) Nozik, A. J. Annu. Rev. Phys. Chem. 1978, 29, 189.
(4) Skotheim, T. A.; Inganas, 0. J. Electrochem. SOC.1985, 132, 2116. (5) Sammells, A. F.; Ang, P. G. P.J . Electrochem. SOC.1984, 131, 617. (6) Sammells, A. F.; Schmidt, S.R. J . Electrochem. SOC.1985, 132, 520. (7) Cook,R. L.; Sammells, A. F.J. Electrochem. SOC.1985, 132, 2429.
0022-3654/87/2091-0150$01.50/00 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. I, 1987 151
CdS/Polymer/Au Solid-state Cells
I Cu wire
n
thin Au laver
I
I -r)
Figure. 1. Schematic representation of CdS/polymer/Au solid-state photovoltaic cell.
principle, the absence of a liquid electrolyte may increase the stability of the resulting device for operation in energy conversion schemes. In addition, the cell geometry and materials naturally lend themselves to the fabrication of thin-film devices, avoiding complex and bulky cell arrangements for containing a fluid phase. In this report, we describe the fabrication and characteristics of two solid-state CdS/polymer/Au photovoltaics (Figure 1) where “polymer” represents either a thin layer of poly(viny1ferrocene) (PVF) or polypyrrole (PP). Previous experimental studies and modeling by Murray and co-workers8 and by others9 of charge transport through redox polymers sandwiched between two metal electrodes immersed in fluid solution and in the solid statesCare especially relevant to the work described herein. The focus of our present investigation is to examine the role of the polymer oxidation state on the performance parameters of thin-film polymer devices for energy conversion. Experimental Section Chemicals. Poly(viny1ferrocene) (Polysciences, Inc.) was used as received. Pyrrole (Aldrich) was distilled under vacuum and stored under refrigeration. Tetraethylammonium tetrafluoroborate (Southwestern Analytical) (TEABF4) was recrystallized twice from methanol/ether. Acetonitrile (CH,CN) (Aldrich, HPLC grade) was stored over molecular sieves. Methylene chloride (CH2C12) (Mallinckrodt, Analytical Reagent) was used as received. Solid-State Cell Fabrication. Solid-state CdS/polymer/Au cells were constructed with single-crystal CdS (Cleveland Crystals). Ohmic contact was made to the rear of the crystal with a In-Ga eutectic. A copper wire was attached to the In-Ga contact with silver paint and the entire assembly was mounted in hard epoxy (Mager Scientific) exposing only the Cd-rich face. The exposed front face was polished with successively finer grades of alumina down to 0.3 pm, etched in 1:1 HC1:H20 for 15 s, rinsed with distilled H 2 0 , and air-dried before each use. The exposed areas of the CdS crystals were 0.05-0.1 cm2. Poly(viny1ferrocene) films were deposited on the CdS surface by evaporation from (8) (a) Pickup, P. G.; Murray, R. W. J. Electrochem. Soc. 1984,131, 833. (b) Pickup, P. G.; Kutner, W.; Leidner, C. R.; Murray, R. W. J. Am. Chem. SOC.1984, 106, 1991. (c) Jerigan, J. C.; Chidsey, C. E. D.; Murray, R. W. J. Am. Chem. SOC.1985, 107, 2824. (d) Chidsey, C. E. D.; Feldman, B. J.; Lundgren, C.; Murray, R. W. Anal. Chem. 1986, 58, 601. (e) Chidsey, C. E. D.; Murray, R. W. Science (Washington, D.C.) 1986, 231, 25. (9) (a) Kittlesen, G. P.; White, H. S.; Wrighton, M. S. J. Am. Chem. SOC. 1985,107,7373. (b) White, H. S.; Kittlesen, G. P.; Wrighton, M. S. J. Am. Chem. SOC.1984, 106, 5375. (c) Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Phys. Chem. 1985, 89, 5133. (d) Wrighton, M. S. Comm. Inorg. Chem. 1985,4,269. (e) Wrighton, M. S. Science (Washington, D.C.) 1986, 231, 32.
CH2C12(1.5 mg PVF/mL).l0 Film thicknesses were estimated from the coulometric charge required to oxidize identically prepared Pt/PVF electrodes in CH3CN/0.1 M TEABF,. All of the polymer was assumed to be electroactive. A reported estimate of 3 M was used as the concentration of electroactive for dry PVF films.’ The CdS/PVF electrodes were cycled under illumination (200 mW/cm2) briefly to establish the electroactivity of the polymer film. PVF was- photooxidized on CdS beginning -at about -0.4 V vs. SSCE (sodium chloride saturated calomel reference electrode). Electrodes were removed from solution in either the oxidized or reduced state after several cycles. For the CdS/PVF“/+ structure, the electrode was held at +0.1 V for ca. 30 s. under illumination in order to photooxidize the PVF layer. The +0.1-V potential was sufficiently positive to cause partial oxidation of the PVF film while not causing extreme photooxidation of the CdS substrate. For the CdS/PVFo structure, the electrode was held at -0.8 V vs. SSCE in the dark to ensure complete reduction of the film. The electrodes were then removed in the partially oxidized or fully reduced state, briefly rinsed with CH3CN to Torr) remove excess electrolyte, and placed under vacuum ( and dried overnight. Polypyrrole was deposited on CdS by electropolymerization under illumination at 1.0 V vs. SSCE in a CH3CN solution containing 0.1 M pyrrole and 0.1 M TEABF4.12 The PP film thickness was controlled by adjusting the time of exposure to the incident light. CdS/PP electrodes were cycled briefly in CH,CN/O. 1 M TEABF, under illumination and removed in either the partially oxidized state at 0.1 V or in the fully reduced state at -0.8 V. The oxidation potential of 0.1 V was typically only 50 mV positive of the polypyrrole oxidation peak on CdS under illumination. Film thicknesses were estimated from integration of charge passed during the reduction of polypyrrole on CdS under illumination. This calculation assumed electroactivity of 25% pyrrole monomer units’, and a polymer density of 1 g/cm3. Quoted film thicknesses probably represent an upper limit to actual values because of the large capacitive charging currents. Electrodes were briefly rinsed with CH3CN and placed under vacuum overnight as described above. Ohmic contact was made by electron-beam evaporation of Au (99.999%, Metron, Inc.) onto the front electrode surface, including the surrounding epoxy shroud. Typically, 100 A of Au was coated at a rate of 4 A/s onto the electrode substrate at ambient temperature and a pressure of lo4 Torr. Au film thickness was monitored by a quartz crystal monitor. Contact was made to the Au surface coated epoxy surrounding the polymer layer with Cu wire and conducting Ag paint. The entire assembly is shown in Figure 1. Solid-state and Electrochemical Measurements. Electrochemical measurements were made in a 5-mL single-compartment cell equipped with a sodium chloride saturated calomel reference electrode (SSCE) and a Pt wire counter electrode. All voltammetric experiments were performed in N2-purged CH&N solutions containing 0.1 M TEABF4. Voltammograms were recorded with a Pine RDE4 potentiostat and a Kipp and Zonen Model BD90 x-y recorder. Current-voltage curves for the solid-state cells were recorded by employing the same instrumentation described above for voltammetric experiments. The working electrode lead from the potentiostat was connected to the Cu wire attached to the thin Au overlayer on the electrode front surface, while the counter and reference electrode leads were shorted together and connected to the Ohmic contact connection on the rear of the CdS crystal. The light source used in these studies was a 500-W projector lamp (General Electric CZA). The total power of the incident beam (10) Chambers, J. Q.; Inzelt, G. Anal. Chem. 1986, 57, 1 117. (11) Nowak, R. J.; Schultz, F. A.; Umana, M.; Lam, R.; Murray, R. W. Anal. Chem. 1980, 52, 315. The value of PVF site concentration was estimated in ref 11 for RF plasma polymerized films to be ca. 3 M. A slightly
smaller value was obtained in ref 9a for films prepared by evaporation. (12) (a) Noufi, R.; Frank, A.; Nozik, A. J. Am. Chem. SOC.1981,103, 1849. (b) Noufi, R.; Trench, D.; Warren, L. J. Electrochem. SOC.1981, 128, 2596. (13) Waltman, R. J.; Bargon, J.; Diaz, A. F. J. Phys. Chem. 1983, 87, 1459.
152 The Journal of Physical Chemistry, Vol. 91, No. 1, 1987 A. CdS/PVF/Au
Hagemeister and White
"Oxidized"
\' -
-
I
0
20
40
60
80
100
Percent Transmittance
Figure 3. Short-circuit current and open-circuit photovoltage for the CdS/PVF"/+/Au cell as a function of incident light intensity. 100% transmission corresponds to 100 mW/cm2 illumination power.
B. CdS / P V F/Au "Reduced"
-
I
I
dark
T
Figure 2. Current-voltage curves in the dark and under illumination for CdS/PVF'/+/Au cells. PVF in the partially oxidized state (60%) (A) and (B) fully reduced state. Illumination: 200 mW/cm2.
was 200 mW/cm2. Neutral density filters (Oriel) were used to vary the incident light intensity.
Results and Discussion Discussion of results are separated into three main sections. First, we present experimental characterization of CdS/ PVF+/O/Au photovoltaic cells. We demonstrate that the electrical behavior of CdS/PVF+/O interface is similar in many respects to that observed for semiconductor/metal interface^.'^ We then present an analysis of the photovoltaic response of an ideal solid-state semiconductor/redox polymer/metal structure. We will present expressions for the photovoltage, photocurrent, fill factor, and output power as functions of the initial polymer oxidation state. This analysis yields quantitative prediction of the behavior obtained for the CdS/PVF+/O/Au cell. Finally, the results of measurements on the CdS/PP"/O/Au cell are presented to compare and contrast solid-state photovoltaic devices fabricated with redox conductive and electronically conductive polymers. CdS/ Poly(vinylferrocene)+/O/AuPhotovoltaic Cell. Typical current-voltage curves obtained in the dark and under illumination for CdS/PVF+/O/Au cells are shown in Figure 2. Measurements were made with the cell arrangement shown in Figure 1 by varying an applied voltage between the Au film and the rear Ohmic contact to CdS and recording the current passed in the external circuit. The curves in Figure 2A correspond to results obtained for a cell where the PVF film ( 1 pm thick) was ca. 60% oxidized (see Experimental Section). The i-V curve in the dark for this cell shows rectifying behavior with a large increase in current obtained under foward bias beginning at an applied voltage of approximately 0.6 V vs. SSCE. Under illumination with white light, a photocurrent is observed beginning at 0.62 V. The shapes of the photocurrent curves are drawn out, indicating a relatively low fill factor, due in part (see below) to slow charge transport through the polymer film. Open-circuit photovoltages of 0.55-0.7 V were obtained for several cells constructed with oxidized PVF films. (14) Sze, S. M. Physics ofSemiconductor Deuices, 2nd ed.; Wiley Interscience New York, 1981.
The dependence of short-circuit photocurrent and open-circuit photovoltage for this cell as a function of light intensity is shown in Figure 3. Open-circuit photovoltages saturated at relatively low light intensity (5 mW/cm2), consistent with a Schottky barrier-like junction at the CdS/PVF-'/+ interface. Photocurrents varied linearly up to an input power of 100 mW/cm2, indicating that photocurrent generation is limited by incident light flux. Entirely different results are obtained when the PVF layer is in the fully reduced state. In the dark, rectifying behavior is again observed, Figure 2b, although the current under foward bias is considerably smaller than that observed for the cell constructed with partially oxidized films. For instance, at an applied voltage of 0.6 V the foward bias current in Figure 2B is approximately 100 times smaller (note scale change between curves A and B) than that observed for the oxidized polymer cell, Figure 2A. Under illumination with white light at the same intensity as above a small steady-state photocurrent is observed at V, larger than 0.4 V and of the opposite sign than that observed h r the oxidized PVF cell. The origin of this photocurrent is not known, but the direction of current is consistent with photoreduction of PVF+ sites at the semiconductor/polymer interface. Photocurrents corresponding to oxidation of PVF sites were suppressed by 100 times and were nearly independent of light intensity. Analysis of Photovoltaic Cell Response under Polymer Charge- Transport Control. To gain a better understanding of solid-state cell characteristics, such as that in the CdS/PVF+/O/Au system, we have modeled the situation of a redox conductive polymer layer sandwiched between metal and semiconductor electrodes of equal cross-sectional areas.I5 Charge transport in redox polymers has been described as a diffusional processt7and is thus dependent upon concentration gradients of the oxidized (PVF') and reduced (PVP) polymer sites. We consider the redox reactions at the two electrodes to be Nernstian, with the concentrations at the electrode surfaces controlled by the photovoltage between the contacting electrodes. In the dark, the concentrations of oxidized and reduced polymer sites are constant throughout the polymer film, their values determined by the potential (vs. a standard reference electrode) at which the polymer layer was removed from solution. If the layer was removed from solution while at the standard potential of the redox polymer, the initial state of the film would be a 1 :1 ratio of oxidized and reduced sites at every point throughout the film. For every oxidized center, an anion (Clod-) from solution phase would be incorporated into the redox layer to maintain electroneutrality. After the layer is removed from solution, a photovoltage can be generated between the two electrodes by illumination of the cell. Since this analysis is for redox films in the solid state, no electrochemical reference (15) The underlying concepts leading to the development of eq 1 and 2 are considered by Reilley (ref 16) in regard to the voltammetric response to a twin-electrode thin-layer cell. (16) Reilley, C. N. Rev. Pure Appl. Chem. 1968, 18, 137. (17) Murray, R. W. Electroanalytical Chemistry; Bard, A. J., Ed.; Wiley: New York, 1984; Vol. 14, and references therein.
The Journal of Physical Chemistry, Vol. 91, No. 1, 1987 153
CdS/Polymer/Au Solid-state Cells electrode can be used. For this reason, the potential of each electrode cannot be controlled independently; only the photogenerated potential difference between the two electrodes can be varied in this arrangement. The concentrations of oxidized and reduced polymer sites at the electrode surfaces will vary with light intensity as reduced sites are converted to oxidized sites at the n-type semiconductor electrode and oxidized sites are converted to reduced sites at the metal electrode. At steady state, a photovoltage will generate linear concentration profiles of the oxidized and reduced species. Since the electroactive centers do not migrate themselves, the sum of the concentrations of oxidized and reduced species at any point in the film is equal to CT,the total concentration of electroactive centers in the redox layer. As the photovoltage is increased with increasing light intensity, more current flows due to the increasing concentration gradients. With continuing larger voltages, a situation develops in which the reaction a t one or both of the electrodes is limited by the amount of reactant at the electrode surface. This occurs when the concentration of oxidized species at the negative metal electrode, or the concentration of reduced species at the positive semiconductor electrode, is depleted to near zero. Under these conditions, the current is limited by the mass transport of charge in the polymer. With the assumption of Nernstian kinetics at each electrode, an expression can be derived for the voltage in terms of concentrations of the limiting redox species initially in the film and also at the electrode surface under ill~mination.'~The limiting species is defined as the species (oxidized or reduced) that has the lower initial concentration in the redox film (e.g., P V P or PVF').
1
k-
0
n
2
a
LL
3
C
\
4
03 (u
5
0
E H a
'
6
v, volts Figure 4. Theoretical current-voltage curves for redox polymer-based solid-state cell as a function of photovoltage between contacting electrodes. Each curve represents the expected response for different fractional concentrations of oxidized polymer redox sites: (1) c, = 0.0 or 1.O; (2) c, = 0.1 or 0.9;(3) ci = 0.2 or 0.8;(4) c, = 0.3 or 0.7; ( 5 ) c, = 0.4 or 0.6;(6)c, = 0.5.
Here, c is the concentration of limiting species a t the electrode surface divided by CT, c, is the initial concentration of limiting species divided by C,, and CT is the total concentration of the electroactive centers. From eq 1 the surface concentration of the limiting reactant, c, reaches its limiting values of 0 and 2ci only when the photogenerated voltage is large. For instance, for an initial condition of 1:l oxidized and reduced polymer site concentrations, the current reaches 99% of its limiting value with an applied voltage of 0.24 V. Values for the current as a function of the concentration profile can be calculated from i = ~ D ~ T ~ F A C-Tq() C /6 (2) where c, ci, and CTare as defined above, A is the cross-sectional area of the polymer layer, 6 is the thickness of the redox layer, and DcT is the diffusion coefficient for charge transport. The limiting current occurs when c = 0 or 2c,. A prediction that can be made from these results is the lack of photoconductivity exhibited by redox layers in a highly reduced or highly oxidized state, Figure 2. This fact arises due to the absence of a redox-active species at the electrode where that species is a reactant. In a highly oxidized or highly reduced redox layer, a very low concentration of redox sites causes the current to reach a diffusion-limited value a t very small photovoltages. Figure 4 shows a family of photocurrent-voltage curves calculated for various values of the fractional concentration of oxidized polymer sites, c,, using eq 1 and 2. The six curves shown correspond to the full range of possible oxidation states, ranging from the fully reduced to the fully oxidized film. As expected from eq 2, the maximum photocurrent is obtained for a half-oxidized polymer film. The power output of a photovoltaic cell is determined by the , open-circuit photovoltage, product of short-circuit photocurrent, Z V,, and fill factor, ff. The fill factor is used to describe the rectangularity of the photocurrent-voltage curve and is defined as ratio of the maximum output power of the device (VZ)to the product of the open-circuit potential, V , ,and short-circuit current, ZSC.
fill factor =
vz
V,4C
0
0.2
0.4
0.6
0.8
10
INITIAL CONCENTRATION, C;
Figure 5. Theoretical output parameters for redox polymer-based solidstate photovoltaic limited by diffusional charge transport in the polymer. (A) Short-circuit photocurrent, I,; (B) fill factor (see text); ( C ) expected output power (V, X I, X ff). The numerical values labeling each curve in (A), (B), and (C) are the open-circuit photovoltages (=IEFB- EordOxl) calculated for a half-oxidized polymer film.
For a solid-state cell, V,, I,,, and ff vary considerably with the oxidation state of the polymer film. Figure 5 shows the limiting current, Zsc, the fill factor, and the relative power efficiency as a function of c, ranging from 0 to 1 for systems operating at different open-circuit voltages. The open-circuit voltage is calculated as the potential difference between the standard polymer redox potential and the flat-band potential of the semiconductor (=IEo - EFbl),both measured on the same undefined reference Z increases linearly with c, ranging from scale. For V, > 0.5 V, , 0 to 0.5 and decreases linearly at larger values, Figure 5A. At lower values of V,, the photocurrent decreases near c, = 0.5, although this effect is not pronounced until V, becomes much less than 0.4 V. The interesting nonlinear behavior of the fill
Hagemeister and White
154 The Journal of Physical Chemistry, Vol. 91, No. I, 1987 A. CdS/PP/Au "Oxidized"
I
L illuminated
B. CdSIPPlAu "Reduced"
T 0 2 pa
1
dark I
3
I
02
generate light-limited photocurrents under the present experimental conditions. CdS/Polypyrrole Au Cell. Results similar to those obtained for the C d S / P V P +/Au system are observed for the CdS/ PP"/*/Au cell. Figure 6 shows typical results for these cells with polypyrrole in either the oxidized or reduced state. Photovoltages up to 0.34 V and a fill factor of 0.56 were obtained for these cells in the oxidized state. The rectifying shape of the i-Vcurve is again consistent with Schottky-like barrier formation at the CdS/PP"Ifl+ interface. Inspection of the curves in Figure 6 shows the dependence of the photocurrent response on the oxidation state of the polymer. For CdS/PP/Au systems employing oxidized PP, large shortcircuit photocurrents are observed with well-developed currentvoltage curves characteristic of Schottky barrier devices. With PP in the reduced state, insignificant photocurrents were obtained (Figure 6b). This dependence of the photocurrent on the oxidation state of films agrees with reported results for polypyrrole cond~ctivity.~~J~ An implication of the dependence of short-circuit current on oxidation state is that the polymer layer is responsible for charge transport and is not shorted out or bypassed by diffusion of Au to the semiconductor surface. However, large short-circuit photocurrents were observed for about 25% of the CdS/PP/Au employing PP in the reduced state. These results are likely due to Au short circuits through the polymer film. An additional evidence minimizing the effect of Au diffusion is the difference in photovoltages obtained from devices using either oxidized PP or PVF films. For P V p / + , V, is consistently greater than 0.55, while for PP"+ V, is consistently less than 0.4 V (typically 0.35 V). This dependence of photovoltage on the polymer overlayer points to the primary role played by the polymer film in these cells and suggests that the effect of the Au overlayer is minimal.
_c_d 01 00
"applied
illuminated
Figure 6. Current-voltage curves in the dark and under illumination for CdS/Pp/"+/Au cells. PP (A) partially oxidized and (B) fully reduced. Illumination: 200 mW/cm2.
factor, Figure 5B, is a consequence of several factors. First, the minimum near c, = 0.5 is due to the larger photovoltage required to reach the limiting plateau current in polymer films prepared in the half-oxidized state (see Figure 4). The tailing at very low or high values of c, is produced by the logarithmic dependence of photovoltage on c,. The power output (=I,V, X fill factor) is shown in Figure 5C. The shape and magnitude of the curve is clearly dominated by the photocurrent and photovoltage, respectively. For any two-state polymer redox system, e.g., PVp/PVF+, the optimum polymer oxidation state for maximum power efficiency corresponds approximately to a 1:l ratio of oxidized and reduced sites. At lower expected photovoltages, i.e., less than 0.4 V, the maximum power point shifts significantly to higher oxidation states in the polymer film. The experimental results for the CdS/PVp/+/Au system are in qualitative agreement with the preceding description of the expected output parameters. Devices constructed from fully reduced polymer layers, i.e., c, = 0, show no significant photocurrent corresponding to PVF oxidation at the semiconductor surface. Devices constructed from partially oxidized films (c, = 0.5-0.7)
Conclusion The electrical properties of solid-state photovoltaic devices constructed with redox cyductive or electronically conductive polymer films depend strongly on the extent of polymer oxidation. W e have demonstrated the ability to control the output characteristics of CdS/PVFo/+/Au and CdS/PP"/*/Au cells by varying the fractional concentration of oxidized sites in the polymer film. Both polymers form rectifying contacts with CdS when partially oxidized. In the fully reduced and neutral state, both PVF and PP behave as thick insulating layers in these solid-state devices.
Acknowledgment. Financial support by the Corrosion Research Center at the University of Minnesota is gratefully acknowledged. Registry No. PVF, 34801-99-5; PP, 30604-81-0; CdS, 1306-23-6; Au, 7440-57-5.
