EDITORIAL pubs.acs.org/JPCL
Solar Cells by Design A pressing question, “If not now, when will be the right time?” lingers among the scientific community and renewable energy proponents. Fossil fuels, while appearing to be relatively inexpensive, can carry a very large price tag. Our continued consumption of fossil fuels is creating new environmental challenges. Consider any one of a number of recent environmental disasters involving fossil fuels in the U.S., for instance the BP gulf coast oil spill, and the cost of our dependence on fossil fuels becomes clearer. The actual cost of the health, environmental, and economic impacts, however, is inestimable. Recent articles in Science point out difficult strategies that must be adopted in solving the energy problem.1 For instance, the highly renewable solar and wind energies require large installation areas. To deliver power derived from solar cells or wind turbines to a city the size of San Diego, an area bigger than the city itself is needed. Are these energy sources feasible? Alternatively, huge solar power farms could be built in the North African desert and transmit electricity to Europe via subsea cables, providing a new way to utilize the plentiful desert sunshine.2 If implemented, the Saharan desert power farms could contribute 15% of Europe's energy demand by 2050. One way to overcome the large area requirement for PV panels is to exceed the Shockley-Queisser photoconversion efficiency limit of 33%. Recent advances in nanostructure semiconductor materials offer new opportunities to design next-generation solar cells. For example, tapping of hot electrons and multiple carrier generation in nanostructure assemblies can boost the theoretical limit to 66%.3,4 Basic energy research efforts are now on the rise, in part, through the U.S. Department of Energy's Energy Frontier Research Centers (EFRCs), Energy Innovation Hubs, and other initiatives. Physical chemistry provides a fundamental understanding of light-harvesting assemblies and the charge-transfer processes.5 Earlier this year, The Journal of Physical Chemistry Letters Perspectives on organic hybrid solar cells6,7 and solar fuels8,9 highlighted recent advances and merits of nanostructured photocatalyst assemblies. The Perspectives in this issue focus on exploring semiconductor nanostructures for the design of next-generation solar cells. Of particular interest are semiconductor nanocrystals or quantum dots that serve as light absorbers. The size-dependent optical and electronic properties and the ability to capture hot electrons make semiconductor quantum dots such as PbSe and CdSe an ideal choice for solar cells. Bisquert and Mora-Sero10 discuss recent advances in designing semiconductor-sensitized solar cells in their Perspective. The concept of separating photogenerated charge carriers is similar to that of the dye-sensitized solar cells (DSSCs). The efficiencies of these cells (4-5%) are still less than those of DSSCs (∼12%). The authors also draw attention to some recent reports that wrongly execute power conversion efficiency measurements. Any reports of high efficiencies should be scrutinized carefully. Additional research is needed to explore new materials for counter electrodes,
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control of recombination and band alignment by surface treatments, and development of absorbing nanocomposites with enhanced light-harvesting properties. Palomares and coauthors11 present another class of solar cells that uses semiconductor nanocrystals and polymers to achieve charge separation under visible light irradiation. Optimization of capping agents of semiconductor nanocrystals is important in maximizing the interfacial electron transfer between the semiconductor and polymer. The authors discuss the charge recombination kinetics in these hybrid solar cells. Chemical bath-deposited Sb2S3 (antimony sulfide) deposited on nanoporous TiO2 and an organic hole-transporting material has yielded a solar conversion efficiency of 5.2%.12 Sequential deposition of QD and dye sensitization layers has also been used to capture a significant part of the solar spectrum.13 To date, the iodide/triiodide redox couple delivers the highest efficiency for dye-sensitized solar cells. It also plays an important role in improving the charge transport in polymer electrolytes.14 Yet, many researchers pay little attention to its photochemistry and redox chemistry. In their Perspective, Meyer and coauthors discuss the redox chemistry of iodine and its implication in dye-sensitized solar cells.15 Specifically, they focus on the making and breaking of I-I bonds at the TiO2 interface and their role in delivering highefficiency dye-sensitized solar cells. As additional optical and electronic properties of semiconductor nanocrystals are identified and new hybrid architectures emerge, novel ways to design next-generation solar cells will be developed. Steady-state solar cell evaluation and the fundamental understanding of the charge-transfer kinetics will also play key roles in making continued advances in semiconductor nanocrystal based solar cells.
Prashant V. Kamat Deputy Editor University of Notre Dame, Notre Dame, Indiana 46556, United States
REFERENCES (1) (2) (3) (4)
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Cho, A. Energy's Tricky Tradeoffs. Science 2010, 329, 786– 787. Clery, D. Sending African Sunlight to Europe, Special Delivery. Science 2010, 329, 782–783. Pandey, A.; Guyot-Sionnest, P. Hot Electron Extraction from Colloidal Quantum Dots. J. Phys. Chem. Lett. 2010, 1, 45–47. Tisdale, W. A.; Williams, K. J.; Timp, B. A.; Norris, D. J.; Aydil, E. S.; Zhu, X. Y. Hot-Electron Transfer from Semiconductor Nanocrystals. Science 2010, 328, 1543–1547. Kamat, P. V.; Schatz, G. Nanotechnology for Next Generation Solar Cells. J. Phys. Chem. C 2009, 113, 15473–15475.
Received Date: October 4, 2010 Accepted Date: October 4, 2010 Published on Web Date: October 21, 2010
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Kaake, L. G.; Barbara, P. F.; Zhu, X. Y. Intrinsic Charge Trapping in Organic and Polymeric Semiconductors: A Physical Chemistry Perspective. J. Phys. Chem. Lett. 2010, 1, 628– 635. Sagawa, T.; Yoshikawa, S.; Imahori, H. One-Dimensional Nanostructured Semiconducting Materials for Organic Photovoltaics. J. Phys. Chem. Lett. 2010, 1, 1020–1025. Joshi, U. A.; Palasyuk, A.; Arney, D.; Maggard, P. A. Semiconducting Oxides to Facilitate the Conversion of Solar Energy to Chemical Fuels. J. Phys. Chem. Lett. 2010, 1, 2719–2726. Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655–2661. Mora-Sero, I.; Bisquert, J. Breakthroughs in the Development of Semiconductor Sensitized Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3046–3052. Palomares, E.; Martinez-Ferrero, E.; Albero, J. Materials, Nanomorphology and Interfacial Charge Transfer Reactions in Quantum Dot/Polymer Solar Cell Devices. J. Phys. Chem. Lett. 2010, 1, 3039–3045. Moon, S.-J.; Itzhaik, Y.; Yum, J.-H.; Zakeeruddin, S. M.; Hodes, G.; Gratzel, M. Sb2S3-Based Mesoscopic Solar Cell using an Organic Hole Conductor. J. Phys. Chem. Lett. 2010, 1, 1524– 1527. Shalom, M.; Albero, J.; Tachan, Z.; Martinez-Ferrero, E.; Zaban, A.; Palomares, E. Quantum Dot-Dye Bilayer-Sensitized Solar Cells: Breaking the Limits Imposed by the Low Absorbance of Dye Monolayers. J. Phys. Chem. Lett. 2010, 1, 1134–1138. Call, F.; Stolwijk, N. A. Impact of I2 Additions on Iodide Transport in Polymer Electrolytes for Dye-Sensitized Solar Cells: Reduced Pair Formation versus a Grotthuss-Like Mechanism. J. Phys. Chem. Lett. 2010, 1, 2088–2093. Meyer, G. J.; Rowley, J.; Farnum, B.; Ardo, S. Iodide Chemistry in Dye-Sensitized Solar Cells: Making and Breaking I-I Bonds for Solar Energy Conversion. J. Phys. Chem. Lett. 2010, 1, 3132– 3140.
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