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Quantum-Dot-Sensitized Solar Cells: Effect of Nanostructured TiO2 Morphologies on Photovoltaic Properties Taro Toyoda*,† and Qing Shen†,‡ †
Department of Engineering Science, The University of Electro-Communications, 1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
‡
ABSTRACT: There is a great deal of interest in dye-sensitized solar cells (DSCs) fabricated with nanostructured TiO2 electrodes. Many different dye molecules have been designed and synthesized to achieve high photovoltaic conversion efficiency. Recently, as an alternative to organic dyes, semiconductor quantum dots (QDs) have been studied for their light-harvesting capability compared with other sensitizers. Accordingly, an attractive configuration to exploit these fascinating properties of semiconductor QDs is the quantum-dot-sensitized solar cell (QDSC) due to their high photoactivity, process realization, and low cost of production. The morphology of TiO2 electrodes included with surface orientation is important for satisfactory assembly of QDSCs in order to improve the efficiency. Breakthroughs allowing an increase in efficiency will advance on two areas of electrode morphology control, namely, (A) TiO2 nanotube electrodes and (B) inverse opal TiO2 electrodes.
T
dipole moments, leading to rapid charge separation.8 The demonstration of multiple exciton generation (MEG) has fostered an interest in colloidal semiconductor QDs.9−11 The formation of more than one exciton upon the absorption of a single photon not only is a process of a great current scientific interest but is potentially important for solar cells. The demonstration of MEG in colloidal semiconductor QDs could push the thermodynamic photovoltaic conversion efficiency up to 44%12 from the current 31% of the Shockley−Queisser detailed balance limit.13 Accordingly, one of the most attractive configurations to exploit these fascinating properties is the quantum-dot-sensitized solar cell (QDSC).14−16 QDs are excited by light to generate electron−hole pairs. The electrons are injected into a nanostructured TiO2 electrode, and they are transported to the transparent conductive oxide electrode. The holes are injected into the liquid or solid electrolyte, which acts as a hole-transporting medium. The resulting holes are transported to the counter electrode, where the oxidized counterpart of the redox system is reduced.7 The use of semiconductor QDs as sensitizers goes back to the 1990s.17−22 During recent years, semiconductor QDSCs have attracted much attention. The photovoltaic conversion efficiencies of QDSCs lag behind those of DSCs, while the use of semiconductor QDs as light absorbers requires the development of new strategies. The poor performance of QDSCs may be ascribed to the difficulty of assembling the semiconductor QDs into a mesoporous TiO2 matrix to obtain a well-covered QD layer on the TiO2 crystalline surface. The other problem that one would encounter is the selection of an efficient electrolyte (including the solid case) in which the
he increasing demand for renewable and low-cost energy has engendered some outstanding research in the field of next-generation solar cells. There is a great deal of interest in the technological applications to dye-sensitized solar cells (DSCs) made with nanostructured TiO2 electrodes because of their high photovoltaic conversion efficiency, which exceeds 10%.1 In DSCs, the applications of organic dye molecules as a photosensitizer, nanostructured TiO2 as an electron transport layer, and an iodine redox couple for hole transport dramatically improve the light-harvesting efficiency. Many different dye molecules have been designed and synthesized to achieve high photovoltaic conversion efficiency. However, further effort is needed to improve DSCs in practical applications. The main undertaking for those developing next-generation solar cells is to improve the photovoltaic conversion efficiency, together with the long time stability. One of the promising approaches is to replace the organic dyes by inorganic substances with high optical absorption characteristics and longer stability over time. Recently, as an alternative to organic dyes, semiconductor quantum dots (QDs) have been studied for their light-harvesting capability compared to other sensitizers.2−7
When semiconductor materials are translated to the nanoscale, new physical and chemical properties are realized for useful applications. Semiconductor QDs exhibit attractive characteristics as sensitizers due to their tunable band gap (or HOMO− LUMO gap) by size control to match the absorption spectrum to the sunlight spectral distribution. Moreover, semiconductor QDs possess higher extinction coefficients and greater intrinsic © 2012 American Chemical Society
Received: April 16, 2012 Accepted: June 28, 2012 Published: June 28, 2012 1885
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photosensitization of TiO2 nanotube array electrodes prepared on Ti foils adsorbed with CdS, CdSe, and CdTe QDs.35−40,48,53,54 An electrodeposited CdS QDs-modified, highly ordered TiO2 nanotube array electrode and its application to photoelectrochemical cells have been reported with a three-electrode system.53 The results showed the formation of thin CdS QDs on the anodically synthesized TiO2 nanotube array electrode. The addition of CdS QDs increased the photocurrent by a factor of 16. Their results showed that an electrochemically synthesized CdS QD could be used to sensitize the TiO2 nanotube array electrode, making it more responsive to the visible spectrum. A comparison between the photoelectrochemical behavior of CdS QDs deposited on a TiO2 nanotube array electrode and on nanoparticles was made.38 The nanotube array electrode exhibited superior performance with a greater photocurrent generation efficiency than that of the nanoparticulate electrode. This higher performance was also shown in the incident photon to current conversion efficiency (IPCE) spectra, which showed that the nanotublar architecture provided more than double the maximum nanoparticulate IPCE. V oc values were not affected by the different architectures, signaling that the nanotubular morphology does not affect the electronic structure of the materials.38 In general, the electrochemical etching of Ti foils in a fluoride medium produces TiO2 nanotube arrays. However, as Ti electrodes are opaque, only one side will be accessible for illumination. Several advantages exist if nanotube arrays can be detached and reassembled on other conductive surfaces. A few efforts have been made to remove TiO2 nanotube arrays as freestanding membrane or bundled arrays.55,56 Removal of nanotubes from the Ti substrate allows one to use the full potential of the material. Additionally, the detached tubes can be conveniently transferred onto transparent electrodes for spectroscopic studies or onto flexible conducting substrates for use on nonconventional surfaces.37 CdS QDs were deposited on the restructured electrodes to compare their performance as QDSCs to aligned nanotube electrodes. The sensitized photoresponse of the photoelectrochemical cell created from reassembled TiO2 nanotubes was very similar to that of aligned TiO2 nanotube arrays. Finally, we present the recent development on sandwichstructured solar cells with TiO2 nanotube array electrodes. Lee et al. reported that they attained an IPCE peak value of 36% for a sandwich-structured solar cell consisting of CdSe QDsensitized TiO2 nanotubes.57 A CdSe QDSC employing TiO2 nanotubes has been proposed by our group.40 One key factor in the QDSC is that a Cu2S film58 with a ring shape, prepared on a brass sheet, was used as a counter electrode. Optical absorption measurements showed that the optical absorption in the visible light region due to the adsorbed CdSe QDs could be clearly observed. The IPCE spectra of the TiO2 nanotube electrodes with different CdSe adsorption times are shown in Figure 1a.40 Red shifts in the IPCE spectra were observed with increasing adsorption time. The IPCE peak value increases with increasing adsorption times, and a maximum of 65% is achieved for 24 h adsorption; it then decreases for longer adsorption times, indicating the presence of CdSe−CdSe boundaries that cause an increasing loss of electrons and holes during the transport from CdSe QDs to TiO2 nanotubes and electrolytes. The photovoltaic characteristics of the sandwich-structured solar cells with different adsorption times are shown in Figure 1b.40 As the adsorption time increases, the short-circuit current
metal chalcogenide can run stably without serious degradation. Even though QDSCs still have low photovoltaic conversion efficiency, they have attracted significant attention among researchers as promising third-generation photovoltaic devices due to a rapid increase of photovoltaic conversion efficiencies around 4−5% at 1 sun illumination (AM 1.5, 100 mW/ cm2).23−25 Also, recent studies show the record photovoltaic conversion efficiencies of 5.4% with Mn-doped QDSC26 and 6.3% with panchromatic photon harvesting in inorganic− organic heterojunction Sb2S3-sensitized solar cells.27 Moreover, the signature of MEG has also been observed in PbSe QDSCs.28
One of the main factors of the photovoltaic performance is the morphology of the TiO2 electrode for satisfactory assembly of QDs and the improvement of the photovoltaic conversion efficiency. The morphology of TiO2 electrodes, including surface orientation, is important for satisfactory assembly of QDSCs for improving the photovoltaic conversion efficiency.29 In nanostructured sensitized solar cells, the nanostructured photoanode should have high surface area to increase the amount of sensitizer loading to enhance light harvesting. However, the recombination process is proportional to the electrode surface area. Because QDs have a higher extinction coefficient than conventional dyes, the surface area in QDSCs may not need as much increase as those in DSCs.30 The opencircuit voltage, Voc, in sensitized solar cells is significantly affected by the recombination process. A balance between recombination and light harvesting is needed to maximize sensitized solar cell performance.30,31 The other limiting factors for the overall photovoltaic performance of QDSCs is the transport of photogenerated electrons through the nanostructured photoanode network. TiO2 is the recipient of injected electrons from optically excited QDs and provides the conductive pathway from the site of electron injection to the transparent back contact. Thus, TiO2 electrodes with a higher degree of order than those conventionally made from a disordered assembly of nanoparticles are desirable for achieving high photovoltaic efficiency of QDSCs through improvements in the electron-transfer rate, as well as good hole electrolyte penetration.3,32−45 From our point of view, the breakthroughs will come from two areas of electrode morphology control, namely, (A) TiO2 nanotube electrodes and (B) inverse opal TiO2 electrodes. A. TiO2 Nanotube Array Electrode. The one-dimensional (1D) tubular structure, size confinement in the radial direction, and a larger surface-to-volume ratio of TiO2 nanotube array electrodes are useful for separating and directing electrons to the collecting electrode surface. Directionality in photoelectrochemical systems increases electron mobility because the travel time in the TiO2 electrode is greatly reduced.46,47 Anodic etching of TO2 nanotubes has been a key player in developing next-generation solar cells. Recent studies have shown that etched nanotubes outperform the traditional nanoparticulate electrodes.34,48−52 There remain several issues that need to be addressed.37 Several studies have been reported concerning the 1886
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Figure 1. (a) IPCE spectra and (b) photocurrent density−photovoltage characteristics of CdSe QD-sensitized TiO2 nanotube solar cells with different adsorption times.40 Parameters in photovoltaic solar cell are listed in the table.
advantages with this fibrous QDSC based on TiO2 nanotubes: (1) the conflict between illumination and opaque electrodes is solved; (2) many types of counter electrodes can be selected, giving the possibility of better photovoltaic performance; and (3) it is flexible and easily weaved and integrated. B. Inverse Popal TiO2 Electrode. The relatively low photovoltaic conversion efficiency obtained in DSCs compared to that for conventional Si solar cells and the theoretical values (∼29%)13 are ascribed to the poor penetration of the materials into thick TiO2 electrodes and the detachment of holetransport layers from TiO2 electrodes, especially in the case of solid-state cells, some specific factors such as the detachment of the hole-transport layer from the TiO2 electrode, and the decrease of the shunt resistance associated with internal leakage.61 To address the penetration of both sensitizers and redox couples, an approach has been proposed using mesoporous inverse opal TiO2 produced from self-organizing systems.62,63 Inverse opal TiO2 has large interconnected pores that lead to a better infiltration, although the BET surface area of inverse opal TiO2 (43 m2/g) is smaller than that of the nanoparticle one in our case (80 m2/g in TiO2 nanoparticles of 15 nm diameter). In addition, it also exhibits a photonic band
density Jsc, Voc, and fill factor (FF) increase, indicating the increase of photovoltaic conversion efficiency up to 1.8% under 1 sun illumination for 24 h of adsorption. It should be noted that the FFs of the CdSe QDSCs have been improved greatly to as high as 0.53 by using the Cu2S counter electrode compared to those using the Pt counter electrodes (∼0.4). Improvement in the FFs was achieved by using Cu2S as a counter electrode due to its good electrocatalytic activity. Cu2S counter electrode performance has been studied in detail.58,59 However, the FF is still poor compared with that obtained in the DSCs (∼0.7). The FF can be improved by reducing the resistance of each interface. Further optimization of the structure of TiO2 nanotubes, the conditions of CdSe QD adsorption, the electrolyte concentration, and, especially, the design of the Cu2S counter electrodes is desirable in future practical applications to QDSCs. An original new type of fibrous QDSC has been designed with CdS and CdSe QD cosensitized TiO2 nanotubes on Ti wire and highly active Cu2S as the counter electrode by Meng’s group.60 By optimizing the CdSe QD adsorption time and the length of nanotubes, a photovoltaic conversion efficiency of 3.18% has been obtained under 1 sun illumination.60 It is revealed that there are several 1887
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Figure 2. (a) IPCE spectra and (b) Photocurrent density−photovoltage characteristics of CdSe QD-sensitized inverse opal TiO2 solar cells with different adsorption times.69 Parameters in the photovoltaic solar cell are listed in the table.
gap (photonic crystal), which depends on the filling fraction of TiO2 in the inverse opal structure. The photonic crystal plays two roles, as a dielectric mirror for wavelengths corresponding to the stop band and as a medium for enhancing light absorption on the long-wavelength side of the stop band. A method of enhancing the light-harvesting efficiency has been proposed by confining photons to the high refractive index layer of dye-sensitized TiO2.64 Several mechanisms for light interaction in these structures, including localization of heavy photons near the edges of a photonic gap, Bragg diffraction in the periodic lattice, and multiple scattering events at disordered regions in the photonic crystals, lead ultimately to enhanced backscattering.65 However, the photovoltaic properties of the DSCs were not investigated in the above references 64 and 65. Dyes molded in the shape of an inverse opal showed a decrease of photovoltaic conversion efficiency for the spectral region in which a photonic stop band opened up. By contrast, when a standard nanocrystalline TiO2 was coupled to an inverse opal, the mirror effect of the photonic crystal at band gap frequencies
increased the light-harvesting efficiency of the cell and showed an increase of IPCE.42 Amplification of light conversion has been demonstrated with the CdS QDSCs using inverse opal TiO2 electrodes at the blue edge of the stop band and in highly randomized media, and the gain was larger than red-edge gains in photocurrent.43 A small amount of photovoltaic conversion efficiency of only 0.6% was reported in DSCs by coupling to an inverse opal TiO2 electrode at an early stage.66 The use of inverse opal TiO2 electrodes in QDSCs with CdSe QDs prepared by the chemical bath deposition (CBD) method has been reported in our laboratory,3,32,33 with ZnS surfaced passivation.67 Inverse opal TiO2 electrodes were prepared by the bottom-up method, which was simple and relatively fast in our laboratory.32 In this method, the voids of artificial opal latex are filled with nanometer-sized TiO2 particles made by adding a drop of TiCl4 into a polystyrene template, hydrolyzing and heating in air to calcine the template and to anneal TiO2. The important point is that it is necessary to a make perfectly ordered template 1888
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Figure 3. Photocurrent density−photovoltage characteristics of CdSe QD-sensitized inverse opal TiO2 and nanoparticulate TiO2 solar cells with the same electrode thickness.69 Parameters in the photovoltaic solar cell are listed in the table.
Figure 4. Time dependence of the transient grating signal intensity of CdSe QD-sensitized inverse opal TiO2 solar cells with different adsorption times.33.
for fabricating thicker inverse opal films. CdSe QDs adsorption was observed not only on the portion near the surface of the inverse opal structure but within the entire inverse opal structure. When the adsorption time increased, the CdSe QDs diameter increased and showed a tendency toward saturation, which depended on the morphology of the TiO2 electrode.68 The IPCE spectra of the inverse opal TiO2 electrodes with different CdSe adsorption times, which have been characterized in our laboratory, are shown in Figure 2a.69 Red shifts in the IPCE spectra are observed with increasing adsorption time. The IPCE peak value increases with increasing adsorption times, and a maximum of 62% was achieved for 8 h adsorption; then, it decreased for longer adsorption times, indicating CdSe− CdSe boundaries that cause an increasing loss of electrons and holes during the transport from CdSe QDs to inverse opal TiO2 and electrolyte. The photovoltaic performances of the sandwich-structured solar cells (counter electrode: Cu2S;
electrolyte: polysulfide) with different adsorption times have been characterized in our laboratory and are shown in Figure 2b.69 As the adsorption time increases, Jsc increases with the increase of adsorption times, showing the increase of photovoltaic conversion efficiency up to 3.1% under 1 sun illumination for 8 h of adsorption, although the amount of CdSe QDs is smaller than that on the nanoparticulate TiO2 electrode due to the smaller surface area in the inverse opal TiO2electrode. Voc and FF are independent of the adsorption times. The photovoltaic characteristics decrease for longer adsorption times. This indicates that the pore size in the inverse opal structure becomes so small that the redox couple cannot easily penetrate through the structure, resulting in poor charge transfer at the CdSe QDs/electrolyte interfaces. Furthermore, the lower FF compared to that of DSCs may be attributed to resistance losses in the interfaces of the cell (TiO2/CdSe, CdSe/CdSe, CdSe/polysulfide electrolyte). In order to study 1889
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Figure 5. Time dependence of the TG signal intensity of CdSe QD-sensitized inverse opal (IO) and nanoparticulate (NP) TiO2 solar cells.33.
corresponds to the photoexcited electron relaxation process. The electron relaxation process is determined by not only the trapping and recombination but also the electron-transfer rate to the TiO2 conduction band. At the initial adsorption where the fraction of CdSe/TiO2 interfaces is larger than that for CdSe/CdSe interfaces, the slow decay time constant, τ2, is mainly attributed to the electron-transfer rate from CdSe QDs to the TiO2 conduction band. As the adsorption time increases, the fraction of CdS/CdSe interfaces become larger, causing the retardation in the electron-transfer process. In order to study the unique properties of the inverse opal structure in the photoexcited carrier dynamics, the TG responses of CdSe QD-sensitized inverse opal TiO2 and nanoparticulate TiO2 electrodes are compared in Figure 5.33 τ1 and τ2 were determined to be slow from 4 to 7 and 80 to 200 ps, respectively, by changing the inverse opal electrode to a nanoparticulate electrode. Because there was no difference in the particle sizes of CdSe QDs on both electrodes, the CdSe/ CdSe interfaces are mainly responsible for the hole relaxation process. The large quantity of CdSe/TiO2 interfaces that resulted from better penetration of CdSe QDs also contributes to the faster electron relaxation rate for the CdSe QDs on the inverse opal TiO2 electrode relative to that for the nanoparticulate TiO2 electrode.
the unique properties of the inverse opal structure in sensitized solar cell applications, the typical photovoltaic performances of CdSe QD-sensitized inverse opal TiO2 and nanoparticulate TiO2 solar cells were compared, which have been characterized in our laboratory, as shown in Figure 3.69 The thickness of both TiO2 films was similar (using a thicker one of ∼9 μm). Although the Jsc of the inverse opal case was similar to that of the nanoparticulate case, the inverse opal case showed a higher photovoltaic conversion efficiency (3.5%) than that of the nanoparticulate case (2.4%) due to the higher Voc, although the amount of CdSe QDs in the inverse opal case might be a half of that in the nanoparticulate case due to the difference of surface area. The higher Voc in the inverse opal case was due to the larger fraction of electron injection to the TiO2 resulting in a higher quasi Fermi level, also indicating the higher recombination resistance (lower recombination rate). The higher Voc observed in the inverse opal TiO2 case is a superior characteristic compared to the nanoparticulate TiO2 ones, while the somewhat lower photocurrent property may be improved by fabricating thicker inverse opal films. It is important to note that sensitization can be achieved if the photoexcited carriers in QDs can sufficiently transfer to the conduction band of the electrode and to the electrolyte (charge separation). Regarding this point, the photoexcited carrier dynamics in the QDs play an important role in improving the photovoltaic conversion efficiency of solar cells. Ultrafast carrier dynamics of CdSe QDSCs have been studied using an improved transient grating (TG) technique.33,70−73 The improved TG method is a type of ultrafast dynamics characterization. The TG responses of an inverse opal TiO2 electrode adsorbed with CdSe QDs for different adsorption times have been reported and are shown in Figure 4.33 The TG signals for the inverse opal TiO2 electrode with adsorbed CdSe QDs were due to the photoexcited carrier dynamics of the CdSe QDs. Because the hole-trapping time is known to be much faster than electron relaxation,8 the fast decay process in the TG responses is assumed to reflect the decrease of photoexcited hole carrier numbers due to the trapping at the CdSe QD surface states (hole relaxation process). As the adsorption time increases, the fast decay time constant, τ1, increases, indicating that the slower hole relaxation rate or the photoexcited holes take more time to be trapped at the CdSe QD surface states. The slow decay process in the TG responses
It is necessary to develop suitable combinations of QDs and nanostructured TiO2 electrodes with different morphologies to enhance photovoltaic conversion efficiencies of QDSCs.
Future Issues and Challenges. The anodically etched TiO2 nanotube on Ti foils has been a key player in developing nextgeneration solar cells. Ti has higher conductivity74 and is much cheaper than transparent electrodes. Recent studies have shown that etched nanotubes outperform the traditional nanoparticulate electrodes. Moreover, the ease of synthesis and reproducibility of TiO2 nanotubes allows for easy scale-up for larger-scale production. There remain several issues that need 1890
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(3) Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T. High Efficiency of CdSe Quantum-Dot-Sensitized TiO2 Inverse Opal Solar Cells. Appl. Phys. Lett. 2007, 91, 023116. (4) Mora-Seró, I.; Bisquert, J. Breakthroughs in the Development of Semiconductor-Sensitized Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3046−3052. (5) Ruhle, S.; Shalom, M.; Zaban, A. Quantum-Dot-Sensitized Solar Cells. Chem. Phys. Chem. 2010, 11, 2290−2304. (6) Emin, S.; Singh, S. P.; Han, L.; Satoh, N.; Islam, A. Colloidal Quantum Dot Solar Cells. Solar Energy 2011, 85, 1264−1282. (7) Hetsch, F.; Xu, X.; Wang, H.; Kershaw, S. V.; Rogach, A. L. Semiconductor Nanocrystal Quantum Dots as Solar Cell Components and Photosensitizers: Material, Charge Transfer, and Separation Aspects of Some Topologies. J. Phys. Chem. Lett. 2011, 2, 1879−1887. (8) Underwood, D. F.; Kippeny, T.; Rosenthal, S. J. Charge Carrier Dynamics in CdSe Nanocrystals: Implications for the Use of Quantum Dots in Novel Photovoltaics. Eur. Phys. J. D 2001, 16, 241−244. (9) Schaller, R. D.; Sykora, M.; Pietryga, J. M.; Klimov, V. I. Seven Excitons at a Cost of One: Redefining the Limits for Conversion Efficiency of Photons into Charge Carriers. Nano Lett. 2006, 6, 424− 429. (10) Trinh, M. T.; Houtepen, A. J.; Schins, J. M.; Hanrath, T.; Piris, J.; Knulst, W.; Goossens, A. P. L. M.; Siebbeles, L. D. A. In Spite of Recent Doubts Carrier Multiplication Does Occur in PbSe Nanocrystals. Nano Lett. 2008, 8, 1713−1718. (11) Nozik, A. J. Multiple Exciton Generation in Semiconductor Quantum Dots. Chem. Phys. Lett. 2008, 457, 3−11. (12) Hanna, M. C.; Nozik, A. J. Solar Conversion Efficiency of Photovoltaic and Photoelectrolysis Cells with Carrier Multiplication Absorbers. J. Appl. Phys. 2006, 100, 074510. (13) Shockley, W.; Queisser, H. J. Detailed Balance Limit of Efficiency of p−n Junction Solar Cells. J. Appl. Phys. 1961, 32, 510− 519. (14) Nozik, A. J. Quantum Dot Solar Cells. Phys. E 2002, 14, 115− 120. (15) Klimov, V. I. Mechanism for Photogeneration and Recombination of Multiexcitons in Semiconductor Nanocrystals: Implications for Lasing and Solar Energy Conversion. J. Phys. Chem. B 2006, 110, 16827−16845. (16) Gur, I.; Fromer, N. A.; Geier, M. L.; Alivisatos, A. P. Air-Stable, All-Inorganic Nanocrystal Solar Cells Processed from Solution. Science 2005, 310, 462−465. (17) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. Photophysical and Photochemical Aspects of Coupled Semiconductors. Charge Transfer Processes in Colloidal CdS−TiO2 and CdS−AgI Systems. J. Phys. Chem. 1990, 94, 6435−6440. (18) Vogel, R.; Pohl, K.; Weller, H. Sensitization of Highly Porous, Polycrystallne TiO2 Electrodes by Quantum Sized CdS. Chem. Phys. Lett. 1990, 174, 241−246. (19) Liu, D.; Kamat, P. V. Photoelectrochemical Behavior of Thin CdSe and Coupled TiO2/CdSe Semiconductor Films. J. Phys. Chem. 1993, 97, 10769−10773. (20) Vogel, R.; Hoyer, P.; Weller, H. Quantum-Sized PbS, Ag2S, Sb2S3, and Bi2S3 Particles as sensitizers for Various Nanoporous WideBandgap Semiconductors. J. Phys. Chem. 1994, 98, 3138−3188. (21) Zaban, A.; Micic, O. I.; Gregg, B. A.; Nozik, A. J. Photosensitization of Nanoporous TiO2 Electrodes with InP Quantum Dots. Langmuir 1998, 14, 3153−3156. (22) Toyoda, T.; Saikusa, K.; Shen, Q. Photoacoustic and Photocurrent Studies of Highly Porous TiO2 Electrodes Sensitized by Quantum-Sized CdS. Jpn. J. Appl. Phys. 1999, 38, 3185−3186. (23) Gónzalez-Pedro, V.; Xu, X.; Mora-Seró, I.; Bisquert, J. Modeling High-Efficiency Quantum Dot Sensitized Solar Cells. ACS Nano 2010, 10, 5783−5790. (24) Chang, J. A.; Rhee, J. H.; Im, S. H.; Lee, Y. H.; Kim, H.-J.; Seok, S. I.; Nazeeruddin, M. K.; Grätzel, M. High Performance Nanostructured Inorganic−Organic Heterojunction Solar Cells. Nano Lett. 2010, 10, 2609−2612.
to be addressed. FF can be improved by reducing the resistance of each interface. To address the penetration of both sensitizers and redox couples, an approach has been proposed using mesoporous inverse opal TiO2 created from self-organizing systems. In addition, this also exhibits a photonic band gap, which depends on the FF of TiO2 in the inverse opal structure. The inverse opal structure plays two roles, as a dielectric mirror for wavelengths corresponding to the stop band and as a medium for enhancing light absorption on the long-wavelength side of the stop band. Their roles are useful for the QDSC application. Although the Jsc of the inverse opal case was somewhat lower than that of nanoparticulate case, the inverse opal case showed a higher photovoltaic conversion efficiency than that of the nanoparticulate case due to the higher Voc. The higher Voc in the inverse opal case is due to the larger fraction of electron injection from QDs to the TiO2 resulting in a higher quasi Fermi level (higher recombination resistance). The higher Voc observed in the inverse opal TiO2 case is a superior characteristic in the QDSC application, while the lower photocurrent property may be increased by fabricating thicker inverse opal films with a narrower band gap and/or combining two QDs. Also, it should be necessary to make a perfectly ordered template for fabricating thicker inverse opal films. Further optimization of the structure of nanotubes and the inverse opal structure, the conditions of QD adsorption, the electrolyte concentration, together with the study of a better counter electrode than Cu2S are all desirable in future applications to QDSCs.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. Biographies Taro Toyoda is a Professor at the Department of Engineering Science at The University of Electro-Communications. His research focuses on basic studies of the synthesis, assembly, and optical spectroscopy of semiconductor quantum dots and their use for photovoltaic solar cell applications. Qing Shen is an Assistant Professor at the Department of Engineering Science at The University of Electro-Communications. Her research involves the basic studies of semiconductor quantum dots, including photoexcited carrier dynamics for photovoltaic solar cell applications.
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ACKNOWLEDGMENTS This work was partially supported by the Strategic Japanese− Spanish Cooperative Program (Japan Science and Technology Agency: JST), the PRESTO Program (JST), and a Grant in Aid for Scientific Research (No. 21310073) from the Ministry of Education, Sports, Science and Technology of the Japanese Government.
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REFERENCES
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NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on June 28, 2012, with a misspelling in the title. “Nanostuctured” was corrected to “Nanostructured”. The corrected version was reposted on July 9, 2012.
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