J. Phys. Chem. 1996, 100, 5481-5484
5481
Photoelectrochemical Properties of Poly(3-alkylthiophene) Films in Aqueous Solution Ola A. El-Rashiedy and Steven Holdcroft* Department of Chemistry, Simon Fraser UniVersity, Burnaby, BC V5A 1S6, Canada ReceiVed: October 30, 1995; In Final Form: January 1, 1996X
The photoelectrochemical properties of π-conjugated poly(3-hexylthiophene) films in aqueous solution are presented. Application of potential biases more negative than +0.3 V (SCE) to the polymer film immersed in an aqueous electrolyte gives rise to a depletion layer at the polymer/solution interface. Photoexcitation of the polymer leads to photoreduction of water. The photovoltage observed is linearly related to pH of the solution in accordance with the variation in redox potentials of the H+/H2 couple, indicating the absence of Fermi level pinning. Quantum efficiencies are dependent on the pH of the solution, irradiation power, excitation wavelength, and film thickness. The capture cross section of irradiation was varied by controlling the film thickness. With an irradiation wavelength of 435 nm, >90% of the incident light was absorbed within the first 280 nm. Using this wavelength of incident light, quantum efficiencies reached a maximum for 220 nm thick films. Quantum efficiencies decreased with increasing incident power.
Introduction An intense study of photoelectrochemical (PEC) solar energy conversion occurred during the past two decades.1 This was largely inspired by the perception that the semiconductor/ electrolyte interface would exhibit fewer problems regarding interface states than their solid state analogs and the promise of large-scale PEC devices using less expensive photoelectrodes. Indeed, it has been found that PEC cells can operate with polycrystalline and amorphous photoelectrodes, although high efficiencies (>15%) are typically reserved for highly crystalline electrodes.1e However, a number of difficulties are associated with photoelectrochemical cell design including problems in sealing liquid systems; semiconductor band gaps that are too large or too small to harness solar energy; semiconductor interface and surface states which lead to Fermi level pinning and loss of photoenergy; photocorrosion of photoelectrodes; and high costs of processing large area electrodes for commercial PEC cells. As an alternative source for semiconductor electrodes for photoelectrochemical cells, we have investigated thin films of the processable π-conjugated polymer poly(3-hexylthiophene). π-Conjugated polymers are intriguing candidates for photoelectrodes for the following reasons: (i) They exhibit semiconducting properties, as evidenced from a number of reports on solid state devices such as Schottky barrier devices,2 electroluminescent displays,3 and field effect transistors.4 (ii) Evidence exists for the formation of oxidized polymer upon irradiation, due to electron injection into solution. In our laboratory we have observed the photolytic formation of radical cation species of poly(ω-(3-(thienyl)alkanesulfonates) in aqueous solution.5 (iii) Although susceptible to photodegradation reactions, in the presence of moisture poly(3-alkylthiophene) films are remarkably stable.5 (iv) They can be readily processed in the form of thin films of variable thickness. (v) Hole mobilities are as high as 10-3 cm2 V-1 s-1.4 (vi) The conduction band electrons have sufficient energy to reduce protons in aqueous solution. A number of reports exist concerning the photoelectrochemistry of π-conjugated polymers.6 Polyacetylene was the first to be reported. Thin films were shown to develop a small * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-5481$12.00/0
Figure 1. Photoelectrochemical reaction scheme.
photovoltage (40-100 mV) upon illumination in aqueous solutions.7 Similarly, polypyrrole was reported to form a liquid junction which led to photoelectrochemical doping of the films.8 Photoelectrochemistry of polyaniline has been studied in aqueous and nonaqueous solution. In both cases, a photocurrent was observed which was affected by the oxidation state of the polymer.9 Poly(3-methylthiophene) shows a significant stable photocurrent when irradiated in acetonitrile solutions, although the electrochemical reaction leading to photocurrent was not reported.10 These early reports used conjugated polymer films which were generally intractable and difficult to characterize. The majority of studies required in situ preparation of polymer films via electrochemical polymerization. Since this time, synthetic routes have been developed which allow the preparation of soluble and processible materials. Thus, a great deal more is known regarding the chemical and physical attributes of these polymers by virtue of the fact that they can be fully characterized. The photophysics and photochemistry of poly(3-alkylthiophenes), for example, has been extensively documented.11,12 Furthermore, by controlling the nature of the functional group and the degree of stereoregularity, the electrical and physical properties can be readily tuned to enhance or diminish a particular property.13,14 With this knowledge, we have initiated a photoelectrochemical study which utilizes the positive attributes and electronic tunability of poly(3-alkylthiophenes). In this report, we describe photoelectrochemical hydrogen evolution at thin films of poly(3-hexylthiophene). A three-electrode system was employed so that the polymeric photocathode can be studied independently of the counter electrode. A schematic diagram illustrating the PEC process is shown in Figure 1. © 1996 American Chemical Society
5482 J. Phys. Chem., Vol. 100, No. 13, 1996
Figure 2. Absorption spectra of a poly(3-hexylthiophene) thin film, 0.4 µm.
El-Rashiedy and Holdcroft
Figure 4. Current-voltage curve of P3HT films irradiated with monochromatic light (435 nm); pH 4.0; film thickness, 0.8 µm; I0, 4.05 mW/cm2.
Figure 3. Thermodynamic potentials of H+/H2 and O2/ H2O redox couples in relation to the band energies of P3HT.
Experimental Section Polymerization was achieved by mixing a solution of 3-hexylthiophene in chloroform with a chloroform solution of anhydrous ferric chloride in a molar ratio 1:4.15 The mixture was stirred for 24 h. The solid product was filtered and washed successively with NH4OH (28%), water, and acidified methanol. The crude polymer was dissolved in hot dichloromethane, and insoluble products were removed by filtration. The polymer was purified by Soxhlet extraction and extensive washing with ammonia. The precipitate was dried under reduced pressure at 50 °C. Absorption spectra of polymer films were obtained using a Hewlett Packard 8452A diode array spectrophotometer. Measurements were conducted at 25 °C. Thin polymer films were prepared by solvent evaporation of polymer solutions onto a glassy carbon electrode. The illumination source was a 200 W Hg-Xe lamp (Illumination Industrial Ltd.). Irradiation was carried out using a 300 nm cutoff filter. The wavelength of irradiation was selected using an appropriate 10 nm bandwidth interference filter. The intensity of the incident light was measured by radiometer/photometer (International Light, Model IL 1350). Current density versus voltage (I-E) curves were obtained using a PAR 173 potentiostat and a Model 175 universal programmer and recorded on a HP 7046A X-Y recorder. The reference electrode was a saturated calomel electrode (SCE), and the auxiliary electrode was a Pt foil. Results and Discussion A UV/vis absorption spectrum of poly(3-hexylthiophene) (P3HT) is shown in Figure 2. The spectrum lacks a sharp absorption edge observed for conventional semiconductor electrodes, but the pseudo-bandgap can be estimated from the low-energy band edge of the spectrum.10,16 The bandgap energy (Eg) was estimated to be 2.0 ( 0.1 eV. Figure 3 shows the redox potentials of the H+/H2 and O2/H2O redox couples with respect to the energy bands of the polymer. The flat band potential was taken as +0.35 V (SCE), as previously reported.10 The position of the valence band (VB) for P3HT has been previously determined electrochemically from the onset of the
Figure 5. Effect of pH on the photocurrent of P3HT films irradiated with monochromatic light (435 nm); film thickness, 0.8 µm; pH 4.0; I0, 4.05 mW/cm2.
oxidation process.17A value +0.56 V (SCE) was previously reported for films in organic media.10 It is likely that the onset potential is different in aqueous solutions due to differences in solvation energy. Unfortunately, the polymer is electrochemically inactive in water. The presence of a small fraction of CH3CN (5% vol/vol) was necessary to swell the polymer and provide electrochemical activity in aqueous solutions. The valence band was determined to be situated +0.52 V in relation to the reference electrode. The conduction band (CB) was thus situated -1.48 V versus the SCE. The data indicate that conduction band electrons generated via excitation of the polymer have sufficient energy to reduce protons in solution. The effect of light on the observed cathodic current for P3HT is shown in Figure 4 for a film 0.8 µm thick in a solution of pH 4.0. At this pH the thermodynamic redox potential associated with hydrogen evolution is -0.48 V (SCE). The current-voltage curve indicates that the polymer film passivates the electrode to hydrogen evolution in the absence of light. Upon irradiation with 435 nm light, a photocurrent is observed with an applied potential more negative than +0.3 V (SCE). The photocurrent does not reach a limiting value but increases with applied potential. The onset of the photocurrent correlates with the flat band potential. At potentials more negative than this, the energy bands of the polymer are bent downward toward the polymer/electrolyte interface, consistent with a p-type semiconductor electrode. The effect of pH on the photocurrent is shown in Figure 5. Notably, the onset potential remains constant at +0.3 V (SCE), i.e., the flat band potential, which indicates that the energy bands do not experience Fermi level pinning due to the buildup of a surface charge. This is consistent with the fact that the semiconductor is an organic material largely devoid of subbandgap interfacial states between the film and the electrolyte. The redox potential of the H+/H2 couple shifts negatively with
Poly(3-alkylthiophene) Films in Aqueous Solution
Figure 6. Photovoltage (at iph f 0) versus pH. Data extracted from Figure 5.
Figure 7. Dependence of quantum efficiency on film thickness of P3HT films irradiated with monochromatic light (435 nm); E, -0.48 V (zero photovoltage); pH 4.0; I0, 4.05 mW/cm2.
an increase in pH of the solution. Since the onset potential of the I-E curves remains constant, this means that the photovoltage (obtained by extrapolation to zero current) increases with increasing pH. A plot of photovoltage (Vph at Iph f 0) versus pH is shown in Figure 6, together with the change in redox potential of the H+/H2 couple. The linear relationship of Vph versus pH confirms the absence of Fermi level pinning. Although the hydrogen ion concentration was varied by a factor of 1010, from pH 2 to 12, the photocurrent decreased only by a factor of 2. This implies that the photocurrent is not limited by an electrochemical kinetic event, otherwise the iph would be more strongly dependent on the concentration of H+. The quantum efficiencies of iph were found to be relatively low. For a solution of pH 4.0, a film thickness of 0.22 µm, and an applied potential equivalent to -0.48 V (SCE), i.e. zero photovoltage, the quantum efficiency was only 0.05%. Possible reasons for the origin of low quantum efficiencies include nonoptimized film thickness, low hole mobility, low electron mobility, or rapid electron-hole recombination. The quantum efficiency was dependent on the film thickness, as shown in Figure 7. An optimum film thickness was found under these conditions. The depth of penetration of 435 nm light was calculated from Beer’s law and knowledge of the extinction coefficient of the polymer ( ) 5480 mol-1 L cm-1 at 435 nm)18 and is shown in Figure 8. The plot indicates that >90% of incident light is absorbed in the first 280 nm. Films much thicker than this require photogenerated holes to travel unnecessarily large distances through the film to reach the current collector. The reason for the significant decrease in quantum efficiencies with very thin polymer films is under current investigation. The dominant factor leading to low quantum yields appears to be geminate electron-hole recombination; thus, the increase in photocurrent observed with more negative applied potentials is the result of an increase in efficiency of electron-hole pair separation with a larger electric field. In this regard, the P3HT-based photoelectrochemical
J. Phys. Chem., Vol. 100, No. 13, 1996 5483
Figure 8. Depth of penetration of 435 nm light through a P3HT polymer film.
Figure 9. Dependence of quantum efficiency and photocurrent on the light intensity, -0.48 V (zero photovoltage); λ, 435 nm.
Figure 10. Photochemical stability of P3HT films irradiated with monochromatic light (435 nm); E, -0.28 V; pH 4.0; I0, 4.05 mW/cm2, film thickness, 1.73 µm.
system is not unlike solid state photoconduction devices.19 Quantum yields of Al/poly(3-dodecylthiophene/Al sandwich structures, for example vary from 10-5 up to ∼1 when the electric field is increased from 100 to 5 × 105 V/cm. Figure 9 shows that the cathodic photocurrent increases with increasing light intensity. This can be explained by an increase in the electron-hole concentration resulting from an increase in incident photons. Electron-hole recombination becomes more probable with increasing concentration, and a lower quantum efficiency results. This is consistent with previous studies conducted on Al/polymer/Au photovoltaic sandwich cells which show a linear correlation of photocurrent with incident irradiation power and quantum yields close to unity at low levels of illumination.20 Figure 10 shows the variation of iph with the irradiation time. Poly(3-hexylthiophene) is known to degrade when irradiated in the form of thin films or dissolved in nonaqueous solvents containing dissolved molecular oxygen.21 Degradation takes the form of both reduced π-conjugation, chain scission, and cross-linking. The former manifests itself as photobleaching and is largely the product of photosensitization and reaction of singlet oxygen. Chain scission and cross-linking have been shown to occur via the classical route of photooxidation. The photochemistry of aqueous solutions of water-soluble derivatives
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El-Rashiedy and Holdcroft lead to major improvements in quantum efficiency. Only one polymer has been examined in this study, whereas it is well documented that this class of polymer can be synthesized in such a way as to control the energy bandgap, the degree of crystallinity, the flat band potential, and carrier mobility.13,14 Thienyl oligomers, known to self-assemble and to yield molecular layers with often superior electronic properties, might also prove to be interesting candidates for photoelectrochemical semiconductor electrodes.24 Acknowledgment. Financial support of this work by the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged. References and Notes
Figure 11. Current-voltage curves of P3HT and P3HT/C60(1 wt %) films irradiated with monochromatic light (435 nm), pH 4.0; film thickness, 0.87 µm; I0, 4.05 mW/cm2.
of poly(3-hexylthiophene) containing alkanesulfonate substituents has also been investigated, and they are substantially more photostable because the lifetime of the reactive intermediate, singlet oxygen, is much lower in aqueous solution.5 The photostability of the P3HT-photocathode is consistent with the observation that, in the presence of water, poly(3-alkylthiophenes) are quite stable to photochemical degradation. In an attempt to further investigate whether inefficient electron-hole separation was responsible for low quantum efficiencies, we have incorporated an electron acceptor, buckminsterfullerene (C60), into the polymer films. It has been previously demonstrated that the incorporation of small quantities of C60 enhances photoconductivity in π-conjugated polymers. In this context C60 serves as an electron acceptor. Back electron transfer to the polymer is found to be retarded, and thereby electron-hole separation is enhanced.22,23 Figure 11 shows the I-E curves of pristine and C60-containing polymer films irradiated with 435 nm light in an aqueous solution of pH 4.0. The onset of the photocurrent is not affected by the presence of C60, indicating that the energy bands of the polymer, with respect to the reference electrode, are not affected by the addition of C60. Moreover, an increase in photocurrent, up to 5-fold, is observed for the C60-containing polymer. This is consistent with the hypothesis that separation of the electronhole pair is enhanced by the electron acceptor. The light dependence on the poly(3-hexylthiophene)/C60 composite films was similar to polymer films in the absence of C60; that is, the photocurrent increased with decreasing light intensity, but preliminary studies indicate that the poly(3-hexylthiophene)/ C60 films were less stable under sustained irradiation. Conclusion A PEC photocathode employing a conjugated poly(3-alkylthiophene) film has been demonstrated in aqueous solutions. Under conditions of pH 4.0, I0 ) 0.05 mW/cm2, and film thickness 220 nm, the quantum efficiency of the P3HT photoelectrochemical cell was 3.9% at zero photovoltage. While the quantum efficiencies are low compared to conventional inorganic semiconductors, the use of polymer films is attractive because of their versatile and film-forming nature. Quantum efficiencies appear to be limited by electron-hole recombination. Incorporation of electron acceptors into the films may
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