Langmuir 2007, 23, 12445-12449
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Self-Assembled Hybrid Polymer-TiO2 Nanotube Array Heterojunction Solar Cells Karthik Shankar, Gopal K. Mor, Haripriya E. Prakasam, Oomman K. Varghese, and Craig A. Grimes* Department of Electrical Engineering and Department of Materials Science and Engineering, The PennsylVania State UniVersity, UniVersity Park, PennsylVania 16802 ReceiVed July 9, 2007. In Final Form: September 17, 2007 Films comprised of 4 µm long titanium dioxide nanotube arrays were fabricated by anodizing Ti foils in an ethylene glycol based electrolyte. A carboxylated polythiophene derivative was self-assembled onto the TiO2 nanotube arrays by immersing them in a solution of the polymer. The binding sites of the carboxylate moiety along the polymer chain provide multiple anchoring sites to the substrate, making for a stable rugged film. Backside illuminated liquid junction solar cells based on TiO2 nanotube films sensitized by the self-assembled polymeric layer showed a short-circuit current density of 5.5 mA cm-2, a 0.7 V open circuit potential, and a 0.55 fill factor yielding power conversion efficiencies of 2.1% under AM 1.5 sun. A backside illuminated single heterojunction solid state solar cell using the same self-assembled polymer was demonstrated and yielded a photocurrent density as high as 2.0 mA cm-2. When a double heterojunction was formed by infiltrating a blend of poly(3-hexylthiophene) (P3HT) and C60-methanofullerene into the self-assembled polymer coated nanotube arrays, a photocurrent as high as 6.5 mA cm-2 was obtained under AM 1.5 sun with a corresponding efficiency of 1%. The photocurrent action spectra showed a maximum incident photon-to-electron conversion efficiency (IPCE) of 53% for the liquid junction cells and 25% for the single heterojunction solid state solar cells.
1. Introduction Among low dimensional systems and architectures, titania nanotubes (NTs) continue to be rigorously investigated for their promising application in various fields such as hydrogen gas sensing,1 photocatalysis,2 photoelectrochemical water-splitting,3-5 biomedical implants,6 carriers for drug elution,7 and photovoltaics;8,9 recent review papers on the material architecture are available.10,11 We have previously reported on the performance of liquid junction solar cells where ruthenium based dyes were used to sensitize TiO2 nanotube arrays. This report describes an application of vertically oriented highly ordered TiO2 nanotube arrays as the working electrode in polymer-sensitized liquid (1) (a) Varghese, O. K.; Paulose, M.; Ong, K. G.; Dickey, E. C.; Grimes, C. A. AdV. Mater. 2003, 15, 624-627. (b) Mor, G. K.; Varghese, O. K.; Paulose, M.; Grimes, C. A. Sens. Lett. 2003, 1 (1), 42-46. (c) Paulose, M.; Varghese, O. K.; Mor, G. K.; Grimes, C. A.; Ong, K. G. Nanotechnology 2006, 17 (2), 398402. (d) Varghese, O. K.; Yang, X.; Kendig, J.; Paulose, M.; Zeng, K.; Palmer, C.; Ong, K. G.; Grimes, C. A. Sens. Lett. 2006, 4, 120-128. (2) (a) Macak, J. M.; Zlamal, M.; Krysa, J.; Schmuki, P. Small 2007, 3 (2), 300-304. (b) Yang, L.; He, D.; Cai, Q. Y.; Grimes, C. A. J. Phys. Chem. C 2007, 111, 8214-8217. (3) Mor, G. K.; Shankar, K.; Varghese, O. K.; Grimes, C. A. J. Mater. Res. 2004, 19, 2989-2996. (4) (a) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2005, 5 (1), 191-195. (b) Shankar, K.; Paulose, M.; Mor, G. K.; Varghese, O. K.; Grimes, C. A. J. Phys. D: Appl. Phys. 2005, 38 (18), 35433549. (c) Varghese, O. K.; Paulose, M.; Shankar, K.; Mor, G. K.; Grimes, C. A. J. Nanosci. Nanotechnol. 2005, 5, 1158-1165. (5) (a) Shankar, K.; Mor, G. K.; Prakasam, H. E.; Yoriya, S.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nanotechnology 2007, 18 (6), 065707. (b) Mor, G. K.; Prakasam, H. E.; Varghese, O. K.; Shankar, K.; Grimes, C. A. Nano Lett. 2007, 7, 2356-2364. (6) Popat, K. C.; Leoni, L.; Grimes, C. A.; Desai, T. A. Biomaterials 2007, 28 (21), 3188-3197. (7) Popat, K.; Eltgroth, M.; LaTempa, T.; Grimes, C. A.; Desai, T. A. Biomaterials 2007, 28, 4880-4888. (8) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6 (2), 215-218. (9) Paulose, M.; Shankar, K.; Varghese, O. K.; Mor, G. K.; Hardin, B.; Grimes, C. A. Nanotechnology 2006, 17 (5), 1446-1448. (10) Grimes, C. A. J. Mater. Chem. 2007, 17, 1451-1457. (11) Mor, G. K.; Varghese, O. K.; Paulose, M.; Shankar, K.; Grimes, C. A. Sol. Energy Mater. Sol. Cells 2006, 90, 2011-2075.
junction solar cells and single heterojunction as well as double heterojunction solid state solar cells. Poly(3-hexylthiophene) (P3HT) is a conjugated polymer that is strongly absorbing in the wavelength region 450-600 nm and possesses good hole mobility of 0.1 cm2/V s in its regioregular form.12 As a result, it is one of the more heavily investigated materials for use in polymeric photovoltaic cells. P3HT has side chains that make it soluble in a variety of common organic solvents. Solution processing of the polymer is attractive because of its applicability in scalable large area devices. Accordingly, most of the reports on solid state solar cells using conjugated polymers have employed wet processing techniques such as spincoating,13 dip coating,14 drop-casting,15 doctor-blading,16 ink-jet printing,17 and screen printing18 to cast the polymer film from solution. In this report, we employ a carboxylated P3HT derivative that self-assembles on the surface of the nanotubular electron accepting TiO2 substrate upon overnight immersion of the substrate in a solution of the polymer. The formation of the polymeric semiconductor films by selfassembly offers distinct advantages such as cost, uniformity, and scalability over other solution processing techniques. Spincoating has a disadvantage in material cost, since the majority of the dispensed material is splashed from the wafer during the process. Other problems with spin-coating include dependence of the film thickness on the density of the pattern (nanotubes in (12) Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 1998, 280 (5370), 1741-1744. (13) Coakley, K. M.; Srinivasan, B. S.; Ziebarth, J. M.; Goh, C.; Liu, Y. X.; McGehee, M. D. AdV. Funct. Mater. 2005, 15 (12), 1927-1932. (14) Wang, G. M.; Hirasa, T.; Moses, D.; Heeger, A. J. Synth. Met. 2004, 146 (2), 127-132. (15) Park, J.; Lee, S.; Lee, H. H. Org. Electron. 2006, 7 (5), 256-260. (16) Schilinsky, P.; Waldauf, C.; Brabec, C. J. AdV. Funct. Mater. 2006, 16 (13), 1669-1672. (17) Sirringhaus, H.; Kawase, T.; Friend, R. H.; Shimoda, T.; Inbasekaran, M.; Wu, W.; Woo, E. P. Science 2000, 290 (5499), 2123-2126. (18) Shaheen, S. E.; Radspinner, R.; Peyghambarian, N.; Jabbour, G. E., Appl. Phys. Lett. 2001, 79 (18), 2996-2998.
10.1021/la7020403 CCC: $37.00 © 2007 American Chemical Society Published on Web 10/24/2007
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our case) and thickness nonuniformity effects at the edges of the samples. The process of self-assembly by chemisorption is dependent only on the availability of active sites on the surface and is independent of the topographical features or total area of the substrate. The advantages of using conjugated polymers as sensitizers are that they are relatively inexpensive compared to the more commonly used ruthenium based dyes and have much higher absorption coefficients.19 In films sensitized by molecular dyes, the thickness of the nanostructured TiO2 film needs to be at least 10 µm to harvest the maximal amount of incident photons. For a polymer such as P3HT with a high absorption coefficient, a film several hundred nanometers in thickness is sufficient to optimally harvest incident sunlight. Furthermore, in a selfassembled monolayer of dye molecules, an individual molecule may desorb quite easily. On the other hand, a single polymer chain is bound to the TiO2 surface at multiple sites containing carboxylic groups along the length of the chain, and even if an individual TiO2-carboxylate linkage fails, the polymer will not desorb from the surface. Desorption in this case would require failure of multiple linkages between the TiO2 and the polymer. Thus, a polymeric sensitizer is inherently more rugged compared to a monomolecular sensitizer. One disadvantage of the particular polymeric sensitizer used, namely, carboxylated P3HT, is the much smaller overlap of its absorption spectrum with the solar spectrum. Another disadvantage is the possibility of the polymer chains clogging the pores of the nanotubular electrode and limiting the effective surface area available to the redox electrolyte. Yanagida et al. obtained impressive photocurrents of up to 9.75 mA cm-2 by using the carboxylated P3HT derivative poly(3-thiophene acetic acid) in conjunction with 7 µm thick nanocrystalline TiO2 films.20 However, the values of the open circuit voltages obtained were low and close to 0.4 V. We used the carboxylated P3HT derivative regioregular poly[3-(5carboxypentyl)thiophene-2,5-diyl] with 4 µm long TiO2 nanotube arrays. We obtained consistently higher open circuit potentials of 0.7-0.76 V upon AM 1.5 1 sun illumination with our single hetrojunction devices. Mwaura et al.21 used the same carboxylated P3HT derivative and obtained open circuit potentials of 0.54 V when employed with 4 µm thick nanocrystalline TiO2 films. Non-polymeric all-organic dyes such as indoline, when used to sensitize TiO2 nanoparticulate films, have been successful in producing solid state solar cells with efficiencies upward of 4%.22 Bulk heterojunction devices using P3HT-PCBM blends have been used to produce solar cells with efficiencies greater than 5%.23 In comparison to the reports cited above, a significant novelty introduced in this paper is the construction of a double heterojunction device where a 1:1 blend of P3HT and [6,6]phenyl C[6][1] butyric acid methyl ester (PCBM) is infiltrated into the self-assembled polymer coated TiO2 nanotubes. In addition to the P3HT-TiO2 interface present in the single heterojunction device, a second interface between P3HT and PCBM is available for charge separation in the double heterojunction device.24 (19) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16 (23), 45334542. (20) Yanagida, S.; Senadeera, G. K. R.; Nakamura, K.; Kitamura, T.; Wada, Y. J. Photochem. Photobiol., A 2004, 166 (1-3), 75-80. (21) Mwaura, J. K.; Zhao, X. Y.; Jiang, H.; Schanze, K. S.; Reynolds, J. R. Chem. Mater. 2006, 18 (26), 6109-6111. (22) Schmidt-Mende, L.; Bach, U.; Humphry-Baker, R.; Horiuchi, T.; Miura, H.; Ito, S.; Uchida, S.; Gra¨tzel, M. AdV. Mater. 2005, 17 (7), 813-815. (23) Ma, W.; Yang, C.; Gong, X.; Lee, K.; Heeger, A. J. AdV. Funct. Mater. 2005, 15, 1617-1622. (24) Hansel, H.; Zettl, H.; Krausch, G.; Kisselev, R.; Thelakkat, M.; Schmidt, H.-W. AdV. Mater. 2003, 15, 2056-2060.
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Figure 1. Field emission scanning electron microscope (FESEM) image of a mechanically fractured 4 µm long TiO2 nanotube array sample.
2. Experimental Section Three kinds of devices were made with the polymer self-assembled on the TiO2 nanotube array electrode. A liquid junction solar cell was prepared by infiltrating the polymer coated TiO2 electrode with I3-/I2 redox electrolyte where the polymer functioned primarily as a photosensitizer. A single heterojunction solid state solar cell using the self-assembled carboxylated P3HT derivative was fabricated and tested as well as a third device configuration, namely, a double heterojunction solid state solar cell consisting of a P3HT-PCBM blend infiltrated into the pores of self-assembled P3HT derivative coated TiO2 nanotube arrays. 2.1. Fabrication of TiO2 Nanotube Arrays. Pure titanium foil (250 µm thick) was purchased from Sigma Aldrich, and before anodization it was cleaned with acetone followed by an isopropyl alcohol rinse. Nanotube arrays were formed by anodizing the Ti foil for 30 min in an electrolyte mixture containing 0.3 wt % NH4F and 2% deionized H2O dissolved in ethylene glycol as reported elsewhere.25 The anodization was performed in a two-electrode configuration with titanium foil as the working electrode and platinum foil as the counter electrode, under constant potential at room temperature (approximately 22 °C). The as-anodized samples were ultrasonically cleaned in deionized water to remove surface debris and subsequently crystallized by annealing in an oxygen ambient at 580 °C for 4 h with heating and cooling rates of 1 °C min-1. The morphology of the anodized samples was studied with the use of a JEOL JSM-6300 field emission scanning electron microscope (FESEM). The resulting nanotube arrays shown in Figure 1 had an average inner diameter of 70 nm, a wall thickness of 22.5 nm, and a length of 4 ( 0.2 µm. 2.2. Fabrication of Liquid Junction Solar Cells. Crystallized nanotube arrays were immersed overnight in a 7 mg mL-1 solution of regioregular poly[3-(5-carboxypentyl)thiophene-2,5-diyl] (see Figure 2a) in N,N-dimethylformamide (DMF) to form the polymer film. For the liquid junction solar cells, the redox electrolyte contained 0.5 M LiI, 0.02 M I2, 0.6 M N-methylbenzimidazole, 0.10 M guanidinium thiocyanate, and 0.5 M tert-butylpyridine in methoxypropionitrile (MPN). A conductive glass slide sputter coated with 1 nm of Pt was used as the counter electrode while the polymer coated nanotube array functioned as the working electrode in the fabricated cells. Both the electrodes were spaced by Parafilm 120 µm in thickness and pressed by clamps. Electrolyte was introduced into the clamped electrodes by capillary action. A schematic of the liquid junction device geometry is shown in Figure 2b. 2.3. Fabrication of Solid State Solar Cells. For the single heterojunction solid state solar cells, a layer of conducting polymer poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT/ PSS) was formed on the carboxylated P3HT coated nanotube array (25) Prakasam, H. E.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Phys. Chem. C 2007, 111 (20), 7235-7241.
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Figure 3. Current-voltage characteristics of backside illuminated liquid junction solar cells under 1 sun AM 1.5 illumination. electrodes by spin-coating from an aqueous suspension of PEDOT/ PSS (Aldrich) at 5000 rpm. The device was subsequently placed on a hot plate for 5 min at 160 °C to eliminate residual water and annealed at 200 °C in a nitrogen ambient for 1 min to cause the PEDOT/PSS film to penetrate into the pores of the nanotubular structure. A glass slide coated with indium tin oxide (ITO) was spin-coated with PEDOT/PSS from an aqueous suspension at 5000 rpm, baked at 160 °C, and pressed onto the PEDOT/PSS coated polymer sensitized nanotube array electrode and formed the hole injection contact for the solid state solar cell. The solar cell was illuminated through the ITO coated glass slide. A schematic of the solid state device geometry is shown in Figure 2c. To fabricate the double heterojunction devices, we first prepared chlorobenzene solutions of P3HT (10 mg mL-1) and (C71-)PCBM (8 mg mL-1). The solution that resulted from blending the pristine as-prepared solutions of P3HT and PCBM in a 1:1 ratio was spin-coated at 700 rpm onto the carboxylated P3HT coated TiO2 nanotube array substrates. The subsequent steps were identical to those for the single heterojunction cell. This device geometry is shown in Figure 2d. 2.4. Characterization Methodology. The morphologies of the titania nanotubes were studied using a JEOL JSM-6300 field emission scanning electron microscope (FESEM). The UV-vis spectra were recorded with a Varian Cary 100 spectrophotometer. Electrical measurements were performed using a Keithley 2400 source meter and a CHI 600B potentiostat. The photocurrent (I) and photovoltage (V) of the cell were measured with an active area of 0.13 cm2 using simulated sunlight at AM 1.5 produced by a 500 W Oriel solar simulator. The illumination of the fully fabricated devices for the EQE measurement was performed using a 300 W Newport Oriel solar simulator equipped with AM 1.5 filters. The light outputs of the solar simulators were calibrated using an NREL calibrated standard silicon solar cell obtained from Burdick Technologies, Inc. A UV filter was placed at the output of the solar simulators to block high-energy photons and protect the polymers in the solar cells from ultraviolet induced degradation of their electronic properties. Optical power measurements were performed using a Newport Oriel optical power meter.
3. Results and Discussion
Figure 2. (a) Molecular structure of the carboxylated P3HT derivative; (b) schematic of a backside illuminated liquid junction solar cell; (c) schematic diagram of a backside illuminated single heterojunction solid state solar cell constructed from carboxylated P3HT self-assembled onto TiO2 nanotube arrays; and (d) schematic diagram of a backside illuminated double heterojunction solid state solar cell constructed from the P3HT-PCBM blend infiltrated into TiO2 nanotube arrays coated with self-assembled carboxylated P3HT.
The current-voltage (I-V) characteristics of these devices are shown in Figure 3. The polymer sensitized nanotube array based devices exhibit a Jsc of 5.5 mA cm-2, a Voc value of 0.7 V, and a fill factor (ff) of 0.55 to produce an overall conversion efficiency of 2.1%. Backside illumination is not optimal in these solar cells, since incident photons experience absorption losses through the ITO layer, from absorption by iodine in the electrolyte and the carboxylated P3HT on the surface of the nanotubes even before reaching the TiO2-polymer heterojunction. Similarly, the solid state cells experience losses through the ITO layer and the PEDOT/PSS conducting polymer layer.
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In a liquid junction dye sensitized solar cell, the electron injection from the excited-state of the dye into the conduction band of the n-type semiconductor (TiO2) in our case raises the quasi-Fermi level of the TiO2 electrode and brings it closer to the conduction band of the semiconductor. In Gratzel cells, the open circuit voltage is determined by the difference between the quasi-Fermi level of the TiO2 (under illumination) and the potential of the redox couple.26 The theoretical limit for the maximum achievable Voc is of the order of the difference between the positions of the TiO2 conduction band (CB) and the potential of the redox couple. The photovoltage that develops is the result of the difference in the Fermi levels of the TiO2 and the electrolyte redox couple and is largely independent of the dye molecule for the same level of electron injection into the semiconductor. With this perspective, we note that the values of the open circuit photovoltage obtained by us using the carboxylated P3HT as the sensitizer (0.7-0.76 V) are similar to those obtained by us using ruthenium based dyes as the sensitizer (0.76-0.84 V). Despite the photonic absorption losses experienced due to the backside illumination geometry, the values of the open circuit potential obtained in this study (0.7-0.76 V) upon illumination of the liquid junction dye sensitized solar cells are significantly higher than those reported using similar carboxylated P3HT derivatives. Using the monocarboxylated P3HT derivative poly(3-thiopheneacetic acid) and the dicarboxylated derivative poly(3-thiophenemalonic acid), Senadeera and colleagues were able to obtain Voc values of 0.3-0.4 V,20,27 which were attributed to a shift in the position of the surface conduction band of TiO2 due to protonation of the surface by the carboxylic groups. Also, a high density of carboxylic groups anchored to the surface of the n-type semiconductor was found to decrease the open circuit photovoltage due to the creation of interface dipoles at the TiO2polymer interface.28 In any DSSC, the dark currents can significantly reduce the maximum cell voltage obtainable. The authors are of the opinion that when the carboxylic groups are in close proximity to the site of charge generation in the molecule or monomeric unit of the polymer, the reduction of the open circuit potential due either to high dark currents or to the shift in band-offsets is more acute. Thus, when Mwaura et al.21 used the same carboxylated P3HT derivative used in this study, which consists of an alkyl chain of five CH2- groups separating the π-conjugated structure from the carboxylate moiety, they observed relatively higher photovoltages of up to 0.54 V. The carboxylated P3HT derivatives have been reported to function poorly as hole transport materials.29 Hence, an alternative is needed to transport the holes to the collection contact. Instead of using a hole transporting material (HTM) to deliver the holes to the contact, we bring the hole collection contact close to the point of generation of the holes by infiltrating the conducting polymer PEDOT/PSS into the nanotubes. Figure 4 shows the I-V characteristics of the backside illuminated single heterojunction solid state solar cells. The solid state devices show a poor fill factor in their I-V characteristics which we attribute to poor electrical contact between the ITO coated glass slide which is merely pressed onto the polymer coated devices. This contact is less intimate than a contact formed by evaporation or sputtering of metal on top of the polymer. When the devices are (26) Kumara, G.; Tennakone, K.; Perera, V. P. S.; Konno, A.; Kaneko, S.; Okuya, M. J. Phys. D: Appl. Phys. 2001, 34 (6), 868-873. (27) Senadeera, G. K. R.; Kitamura, T.; Wada, Y.; Yanagida, S. Sol. Energy Mater. Sol. Cells 2005, 88 (3), 315-322. (28) Liu, Y. X.; Scully, S. R.; McGehee, M. D.; Liu, J. S.; Luscombe, C. K.; Frechet, J. M. J.; Shaheen, S. E.; Ginley, D. S. J. Phys. Chem. B 2006, 110 (7), 3257-3261. (29) Senadeera, G. K. R.; Fukuri, N.; Saito, Y.; Kitamura, T.; Wada, Y.; Yanagida, S. Chem. Commun. 2005, (17), 2259-2261.
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Figure 4. Current-voltage characteristics of a backside illuminated single heterojunction solid state solar cell under 1 sun AM 1.5 illumination showing the dark current density (×, green), photocurrent density before anneal (0, blue), and photocurrent density after a 1 min anneal at 200 °C ([, red) as a function of potential.
Figure 5. Current-voltage characteristics of backside illuminated double heterojunction solid state solar cell under 1 sun AM 1.5 illumination showing the dark current density (green line), photocurrent density (red line), and power density (blue line) as a function of potential.
subjected to a 1 min anneal at 200 °C in a nitrogen ambient, the photocurrent increases sharply from 1.37 to 2 mA cm-2, which we attribute to improved penetration of the PEDOT/PSS layer into the nanotubes enabling the collection of more charge carriers from the P3HT chains bound to the surface of the nanotubes. Increasing the thickness of the PEDOT/PSS conducting polymer film opens up the possibility of further improved pore penetration upon annealing into the nanotubular structure, but our initial results indicated that any such improvement is obtained at the expense of a substantial drop in the open circuit potential of the fabricated solar cells. Figure 5 shows the I-V characteristics of double heterojunction devices. The double heterojunction devices show a photocurrent density of 6.5 mA cm-2, which is higher than that obtained with liquid junction cells using the same electrode and sensitizer. However, the efficiency of these devices is limited to 1% due to the lower open circuit potential and fill factor. Polymeric sensitizers are attractive because of their low cost and their large extinction coefficients relative to molecular dyes, which enable significant light harvesting using a much smaller photoelectrode thickness. However, a self-assembled polymeric sensitizer has a less than optimal surface coverage resulting in gaps on the surface where the hole transport material and the n-type TiO2 are in direct contact, allowing for the recombination of photogenerated holes in the hole transport material with photogenerated electrons in the TiO2 subsequent to charge separation.30 The (30) Senadeera, G. K. R.; Pathirathne, W. M. T. C. Curr. Sci. 2004, 87 (3), 339-342.
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4. Conclusions
Figure 6. IPCE spectra of backside illuminated liquid (9) and solid state (O) solar cells.
performance of solid state solar cells employing the all-organic non-polymeric sensitizer indoline is superior with reported efficiencies higher than 4%,22 probably due to the higher surface coverage of the dye and the use of spiro-OMETAD as the hole transport material, which shows excellent pore filling characteristics. A second sensitization step by small molecule dyes to coat areas where no polymeric sensitizer is present may ameliorate the recombination problem. Also, treatment of the devices with an ionic liquid containing imidazolium salts may improve device performance due to the improvement of the electronic properties of the π-conjugated polymer.29 Figure 6 shows the action spectra of the liquid junction and single heterojunction solid state solar cells. Due to light absorption by iodine in the redox electrolyte, the IPCE spectrum of the liquid junction cell does not closely resemble that of the solid state cell. A maximum IPCE of 53% is obtained with the liquid junction solar cells, and 25% is obtained with the single heterojunction solid state solar cell.
We have successfully demonstrated the use of the selfassembled polymeric sensitizer poly[3-(5-carboxypentyl)thiophene-2,5-diyl] in liquid junction dye sensitized solar cells as well as solid state solar cells. To the best of our knowledge, this is the first report of the use of TiO2 nanotube arrays used in conjunction with a polymeric sensitizer. We have also demonstrated the viability of a new single heterojunction solid state device configuration wherein the hole collection contact is infiltrated into the pores of the nanotubular structure to collect the holes generated in the surface bound sensitizer layer. This is a new device concept where the conducting polymer PEDOT/ PSS is used to directly form a contact with the surface bound sensitizer layer avoiding the use of a specialized hole transporting semiconductor. We have also successfully fabricated double heterojunction devices with AM 1.5 photocurrents as high as 6.5 mA cm-2. In the backside illuminated liquid junction solar cells, photocurrents up to 5.6 mA cm-2 are obtained along with relatively high values of the open circuit potential ranging from 0.7 to 0.76 V. Maximum IPCEs of 53% for the liquid junction cell and 25% for the solid state solar cell were obtained. Optimization of the PEDOT/PSS layer used in the solid state solar cells and improvement of the surface coverage of the polymeric sensitizer are expected to improve the performance of these devices. Acknowledgment. Support of this work by DARPA under grant DE-FG02-06ER15772 is gratefully acknowledged. LA7020403
