Copyright 2009 by the American Chemical Society
VOLUME 113, NUMBER 35, SEPTEMBER 3, 2009
EDITORIAL Nanotechnology for Next Generation Solar Cells This editorial marks the introduction of a new feature for The Journal of Physical Chemistry: JPC Virtual Issues. Each Virtual Issue will consist of a selection of 40 or fewer articles, including Reviews and Feature Articles, published in JPC over the past two years that report important advances in different subdisciplines of physical chemistry and will be accessible through the JPC A/B/C home pages. For reference, the introduction describing each issue, complete with a list of highlighted articles, will be published as editorials in the journal. It is our hope that readers of JPC will find these topical collections of articles to be a valuable resource in learning about the state of the art in a subdiscipline of physical chemistry. This inaugural JPC Virtual Issue (http://pubs.acs.org/page/jpccck/vi/1) focuses on the important area of solar cell research. The research papers selected from various laboratories around the world provide valuable fundamental information as well as insights into the mechanisms of energy conversion processes, kinetic and thermodynamic limitations, and methodologies to improve energy conversion efficiency. The emergence of new strategies in the design of energy conversion and storage systems during the past couple of years has resulted in many fascinating and important research developments. Of particular interest are efforts to design new nanostructured architectures and molecular assemblies for the next generation of solar cells. Three different types of solar cells based on the advances in nanotechnology have emerged: (i) dye sensitized solar cells (DSSC), (ii) hybrid organic solar cells, and (iii) quantum dot solar cells. The capture and conversion of light energy in these solar cells is facilitated by modifying a nanostructured semiconductor interface with a dye, conjugate polymer, or semiconductor nanocrystals, respectively. Improving the efficiency of photoinduced charge separation and transport of charge carriers across these nanoassemblies remains a challenge. The basic concepts involved in the development of nanoassemblies for light energy harvesting applications are featured in ref 1. The reviews of Peter, Hodes, and Kamat2-4 on the dyesensitized nanocrystalline solar cells (DSSC) and quantum dot solar cells highlight recent progress including the processes that dictate the photoconversion efficiency. The thermodynamic and kinetic criteria for successful cell design are outlined in these articles. Imahori has presented strategies for utilizing photoinduced charge separation in donor-acceptor molecules to fabricate nanostructured semiconductor based solar cells.5 The photosensitization of nanostructured TiO2 films with visible light absorbing dyes has led to the development of DSSC with efficiencies greater than 10%. Although there have been significant successes, certain challenges remain in DSSC research. The focus of recent research has been on maximizing photoconversion efficiency by molecular design, developing new nanostructure architectures, and establishing the fundamental processes in light harvesting assemblies.6-16 The use of ionic liquids as a replacement for common solvents has shown promise in the development of solid state DSSC.17-19 10.1021/jp905378n CCC: $40.75 2009 American Chemical Society Published on Web 08/04/2009
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Editorial
The morphology and optoelectronic properties of polythiophene- and polyphenylenevinylenebased conjugated polymers and oligomers continue to be evaluated for aiding the development of organic hybrid solar cells.20-23 Insight into the origin of disorder in the system, the degree of carrier localization, and the role of chain interactions has been attempted by classical molecular dynamics simulations24 and by monitoring the ultrafast decay component in the polarization anisotropy.25 Time-resolved microwave conductivity experiments have shown that the phase separation of the polymer and PCBM ([6,6]-phenyl C-61-butyric acid methyl ester) achieved through controlled annealing retards the recombination of charge carriers and thus facilitates charge collection in a solar cell.26 Conductive atomic force microscopy (c-AFM) has also been recently employed to map the electronic properties of conducting polymers.27 New strategies are required to improve the performance of these cells by extending the absorption into the red and overcoming the limitations induced by photodegradation. Research emphasis in the area of quantum dot solar cells has been aimed at utilizing the unique optical and electronic properties of semiconductor nanocrystals for capture and conversion of light energy.28-32 The size-dependent properties of CdSe, CdS, and other semiconductor nanocrystals make them suitable for tuning the photoresponse of solar cells. However, the efficiencies of quantum dot sensitized solar cells have remained rather low (1-2%) compared to DSSC and organic hybrid cells. The semiconductor/electrolyte interface plays a crucial role in dictating hole transfer and anodic corrosion of the semiconductor. More concerted efforts are needed to design functionalized or hybrid nanostructures in order to improve the efficiency of these solar cells and minimize the photocorrosion processes.33,34 As the quest for energy solutions continues, we can expect many new exciting discoveries to aid in the capture and conversion of light energy economically and efficiently. Needless to say physical chemistry will continue to play an essential role in providing a fundamental understanding of light induced processes and charge transfer events.
Prashant V. Kamat ExecutiVe Editor
George C. Schatz Editor-in-Chief References and Notes (1) Kamat, P. V. Meeting the clean energy demand: Nanostructure architectures for solar energy conversion. J. Phys. Chem. C 2007, 111, 2834–2860. http://dx.doi.org/10.1021/jp066952u. (2) Peter, L. M. Characterization and modeling of dye-sensitized solar cells. J. Phys. Chem. C 2007, 111, 6601–6612. http://dx.doi.org/10.1021/jp069058b. (3) Hodes, G. Comparison of Dye- and Semiconductor-Sensitized Porous Nanocrystalline Liquid Junction Solar Cells. J. Phys. Chem. C 2008, 112, 17778–17787. http://dx.doi.org/10.1021/jp803310s. (4) Kamat, P. V. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters. J. Phys. Chem. C 2008, 112, 18737–18753. http://dx.doi.org/10.1021/jp806791s. (5) Imahori, H.; Umeyama, T. Donor-Acceptor Nanoarchitecture on Semiconducting Electrodes for Solar Energy Conversion. J. Phys. Chem. C 2009, 113, 9029–9039. http://dx.doi.org/10.1021/jp9007448. (6) Liang, M.; Xu, W.; Cai, F. S.; Chen, P. Q.; Peng, B.; Chen, J.; Li, Z. M. New triphenylamine-based organic dyes for efficient dye-sensitized solar cells. J. Phys. Chem. C 2007, 111, 4465–4472. http://dx.doi.org/ 10.1021/jp067930a. (7) Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh, P. J.; Gordon, K. C.; Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.; Gratzel, M.; Officer, D. L. Highly efficient porphyrin sensitizers for dyesensitized solar cells. J. Phys. Chem. C 2007, 111, 11760–11762. http://dx.doi.org/10.1021/jp0750598. (8) Wang, Z. S.; Cui, Y.; Dan-Oh, Y.; Kasada, C.; Shinpo, A.; Hara, K. Thiophene-functionalized coumarin dye for efficient dye-sensitized solar cells: Electron lifetime improved by coadsorption of deoxycholic acid. J. Phys. Chem. C 2007, 111, 7224–7230. http://dx.doi.org/10.1021/jp067872t. (9) Xu, W.; Peng, B.; Chen, J.; Liang, M.; Cai, F. New triphenylamine-based dyes for dye-sensitized solar cells. J. Phys. Chem. C 2008, 112, 874–880. http://dx.doi.org/10.1021/jp076992d. (10) Fabregat-Santiago, F.; Bisquert, J.; Palomares, E.; Otero, L.; Kuang, D. B.; Zakeeruddin, S. M.; Gratzel, M. Correlation between photovoltaic performance and impedance spectroscopy of dye-sensitized solar cells based on ionic liquids. J. Phys. Chem. C 2007, 111, 6550–6560. http://dx.doi.org/10.1021/jp066178a. (11) Qin, P.; Yang, X. C.; Chen, R. K.; Sun, L. C.; Marinado, T.; Edvinsson, T.; Boschloo, G.; Hagfeldt, A. Influence of pi-conjugation units in organic dyes for dye-sensitized solar cells. J. Phys. Chem. C 2007, 111, 1853–1860. http://dx.doi.org/10.1021/jp065550j. (12) Eu, S.; Hayashi, S.; Urneyama, T.; Matano, Y.; Araki, Y.; Imahori, H. Quinoxaline-fused porphyrins for dye-sensitized solar cells. J. Phys. Chem. C 2008, 112, 4396–4405. http://dx.doi.org/10.1021/jp710400p. (13) Morandeira, A.; Fortage, J.; Edvinsson, T.; Le Pleux, L.; Blart, E.; Boschloo, G.; Hagfeldt, A.; Hanmiarstrom, L.; Dobel, F. Improved photon-to-current conversion efficiency with a nanoporous p-type NiO electrode by the use of a sensitizer-acceptor dyad. J. Phys. Chem. C 2008, 112, 1721–1728. http://dx.doi.org/ 10.1021/jp077446n. (14) Cao, Y. M.; Bai, Y.; Yu, Q. J.; Cheng, Y. M.; Liu, S.; Shi, D.; Gao, F. F.; Wang, P. Dye-Sensitized Solar Cells with a High Absorptivity Ruthenium Sensitizer Featuring a 2-(Hexylthio)thiophene Conjugated Bipyridine. J. Phys. Chem. C 2009, 113, 6290–6297. http://dx.doi.org/10.1021/jp9006872. (15) Colodrero, S.; Mihi, A.; Anta, J. A.; Ocana, M.; Miguez, H. Experimental Demonstration of the Mechanism of Light Harvesting Enhancement in Photonic-Crystal-Based Dye-Sensitized Solar cells. J. Phys. Chem. C 2009, 113, 1150–1154. http://dx.doi.org/10.1021/jp809789s.
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J. Phys. Chem. C, Vol. 113, No. 35, 2009 15475 (16) Barnes, P. R. F.; Anderson, A. Y.; Koops, S. E.; Durrant, J. R.; O’Regan, B. C. Electron Injection Efficiency and Diffusion Length in Dye-Sensitized Solar Cells Derived from Incident Photon Conversion Efficiency Measurements. J. Phys. Chem. C 2009, 113, 1126–1136. http://dx.doi.org/10.1021/jp809046j. (17) Yamanaka, N.; Kawano, R.; Kubo, W.; Masaki, N.; Kitamura, T.; Wada, Y.; Watanabe, M.; Yanagida, S. Dye-sensitized TiO2 solar cells using imidazolium-type ionic liquid crystal systems as effective electrolytes. J. Phys. Chem. B 2007, 111, 4763–4769. http://dx.doi.org/10.1021/jp0671446. (18) Kang, S. H.; Kim, J. Y.; Kim, Y.; Kim, H. S.; Sung, Y. E. Surface modification of stretched TiO2 nanotubes for solid-state dye-sensitized solar cells. J. Phys. Chem. C 2007, 111, 9614–9623. http://dx.doi.org/ 10.1021/jp071504n. (19) Shi, D.; Cao, Y. M.; Pootrakulchote, N.; Yi, Z. H.; Xu, M. F.; Zakeeruddin, S. M.; Graetzel, M.; Wang, P. New Organic Sensitizer for Stable Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes. J. Phys. Chem. C 2008, 112, 17478–17485. http://dx.doi.org/10.1021/jp807191w. (20) Ramakrishna, G.; Bhaskar, A.; Bauerle, P.; Goodson, T. Oligothiophene dendrimers as new building blocks for optical applications. J. Phys. Chem. A 2008, 112, 2018–2026. http://dx.doi.org/10.1021/jp076048h. (21) Greene, L. E.; Law, M.; Yuhas, B. D.; Yang, P. D. ZnO-TiO2 core-shell nanorod/P3HT solar cells. J. Phys. Chem. C 2007, 111, 18451–18456. http://dx.doi.org/10.1021/jp077593l. (22) Tsai, M. S.; Hsu, Y. C.; Lin, J. T.; Chen, H. C.; Hsu, C. P. Organic dyes containing 1H-phenanthro[9,10d]imidazole conjugation for solar cells. J. Phys. Chem. C 2007, 111, 18785–18793. http://dx.doi.org/10.1021/ jp075653h. (23) Mikroyannidis, J. A.; Stylianakis, M. M.; Sharma, G. D.; Bahraju, P.; Roy, M. S. A Novel Alternating Phenylenevinylene Copolymer with Perylene Bisimide Units: Synthesis, Photophysical, Electrochemical, and Photovoltaic Properties. J. Phys. Chem. C 2009, 113, 7904–7912. http://dx.doi.org/10.1021/jp901651z. (24) Vukmirovic, N.; Wang, L.-W. Electronic structure of disordered conjugated polymers: Polythiophenes. J. Phys. Chem. C 2009, 113, 409–415. http://dx.doi.org/doi:10.1021/jp808360y. (25) Dykstra, T. E.; Hennebicq, E.; Beljonne, D.; Gierschner, J.; Claudio, G.; Bittner, E. R.; Knoester, J.; Scholes, G. D. Conformational Disorder and Ultrafast Exciton Relaxation in PPV-family Conjugated Polymers. J. Phys. Chem. C 2009, 113, 656–667. http://dx.doi.org/10.1021/jp807249b. (26) Grzegorczyk, W. J.; Savenije, T. J.; Heeney, M.; Tierney, S.; McCulloch, I.; van Bavel, S.; Siebbeles, L. D. A. Relationship between Film Morphology, Optical, and Conductive Properties of Poly(thienothiophene): [6,6]-Phenyl C-61-Butyric Acid Methyl Ester Bulk Heterojunctions. J. Phys. Chem. C 2008, 112, 15973–15979. http://dx.doi.org/10.1021/jp8044548. (27) Pingree, L. S. C.; MacLeod, B. A.; Ginger, D. S. The changing face of PEDOT:PSS films: Substrate, bias, and processing effects on vertical charge transport. J. Phys. Chem. C 2008, 112, 7922–7927. http://dx.doi.org/ doi:10.1021/jp711838h. (28) Mora-Sero, I.; Bisquert, J.; Dittrich, T.; Belaidi, A.; Susha, A. S.; Rogach, A. L. Photosensitization of TiO2 layers with CdSe quantum dots: Correlation between light absorption and photoinjection. J. Phys. Chem. C 2007, 111, 14889–14892. http://dx.doi.org/10.1021/jp074907w. (29) Lee, H. J.; Yum, J. H.; Leventis, H. C.; Zakeeruddin, S. M.; Haque, S. A.; Chen, P.; Seok, S. I.; Grazel, M.; Nazeeruddin, M. K. CdSe quantum dot-sensitized solar cells exceeding efficiency 1% at full-sun intensity. J. Phys. Chem. C 2008, 112, 11600–11608. http://dx.doi.org/10.1021/jp802572b. (30) Tachibana, Y.; Umekita, K.; Otsuka, Y.; Kuwabata, S. Charge Recombination Kinetics at an in Situ Chemical Bath-Deposited CdS/Nanocrystalline TiO2 Interface. J. Phys. Chem. C 2009, 113, 6852–6858. http:// dx.doi.org/10.1021/jp809042z. (31) Shalom, M.; Dor, S.; Ruhle, S.; Grinis, L.; Zaban, A. Core/CdS Quantum Dot/Shell Mesoporous Solar Cells with Improved Stability and Efficiency Using an Amorphous TiO2 Coating. J. Phys. Chem. C 2009, 113, 3895–3898. http://dx.doi.org/10.1021/jp8108682. (32) Guijarro, N.; Lana-Villarreal, T.; Mora-Sero, I.; Bisquert, J.; Gomez, R. CdSe Quantum Dot-Sensitized TiO2 Electrodes: Effect of Quantum Dot Coverage and Mode of Attachment. J. Phys. Chem. C 2009, 113, 4208–4214. http://dx.doi.org/doi:10.1021/jp808091d. (33) Cui, S.-C.; Tachikawa, T.; Fujitsuka, M.; Majima, T. Interfacial Electron Transfer Dynamics in a Single CdTe Quantum Dot-Pyromellitimide Conjugate. J. Phys. Chem. C 2008, 112, 19625–19634. http://dx.doi.org/ 10.1021/jp807591d. (34) Tvrdy, K.; Kamat, P. V. Substrate Driven Photochemistry of CdSe Quantum Dot Films: Charge Injection and Irreversible Transformations on Oxide Surfaces. J. Phys. Chem. A 2009, 113, 3765–3772. http://dx.doi.org/ doi:10.1021/jp808562x.
