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Chemist’s Quest for Inexpensive, Efficient, and Stable Photovoltaics
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he need for inexpensive, efficient, and stable photovoltaics continues to intensify. Chemists in the academic community have placed considerable emphasis on “hybrid” solar cells that are ideally fabricated from earth-abundant inorganic and organic components under ambient conditions. Such “hybrid” solar cells are often divided into three broad classifications: dye-sensitized, organic, and quantum dot. It is an unfortunate fact that practical applications of these three classes of solar cells, which represent the vast majority of academic chemistry research, remain far behind the traditional silicon-based photovoltaic materials. If one were to install solar cells today, silicon would be the obvious and correct choice. To ensure that this is not always the case, additional research in hybrid solar cells is clearly needed. There is in fact justified and continued optimism that this situation will soon change as efficiencies and stabilities of hybrid solar cells continue to improve, and some niche applications have been identified. Four interesting perspective articles appear in this issue that articulate fundamental and practical scientific issues relevant to the state-of-the-art organic bulk heterojunction/ polymer blend, dye-sensitized, and quantum dot solar cells that suggest new research directions toward sustainable electrical power through cheap, inexpensive, and stable photovoltaics.1 4 The perspective article authored by Bisquert and Belmonte proposes fundamental device models for the interpretation of current voltage data in organic bulk heterojunction solar cells. The efficiency of such solar cells has recently climbed to about 8%. Fundamental questions exist on how the open circuit photovoltages, short circuit photocurrents, and fill factors abstracted from current voltage data are related to the molecular mechanisms of charge separation and transport. A long-standing issue has been whether electrodes with large differences in work function generate a significant “built-in” electric field that enhances power conversion efficiency. Bisquert and Belmonte conclude that such built-in fields have a negligible role in charge separation and transport. Rather, the kinetics of photoinduced electron transfer in the organic blend was identified as the critically important variable. This finding enabled more flexible boundary conditions at the electrode surfaces that yielded fundamentally new device models that can now be implemented, tested, and further refined by experimentalists working on many different types of bulk heterojunction solar cells. It was particularly noteworthy that the benchmark P3HT C60 blends displayed sufficient stability to enable full characterization by impedance spectroscopy from which many important conclusions were drawn. The insights of this work indicate that highly reactive metal electrodes such as calcium are not necessary for efficient energy conversion in this class of organic solar cells. Two recent perspective articles have focused on quantum dot solar cells.2,3 Oron and Zaban and co-workers discuss the use of Forster resonance energy transfer (FRET) between quantum dots to improve power conversion efficiency in dye-sensitized solar cells based on mesoscopic TiO2 thin films.2 In some aspects, the approaches described are reminiscent of nature’s antennae effect in photosynthetic proteins where quantum dots take the place of chlorophyll pigments. The mechanism for r 2011 American Chemical Society
electrical power generation involves light absorption by the quantum dot followed by FRET to a molecular dye. The excited dye then injects an electron into the TiO2, and is regenerated by a redox mediator present in an external electrolyte. This mechanism differs from the more common approach where the quantum dots simply take the place of the dye molecule. The authors show that with quantum dots as energy transfer donors, enhanced spectral response and absorbance of the solar cell can be achieved. Thus, in principle, energy transfer from quantum dots can enhance photocurrents and enable the use of thinner dye-sensitized electrodes. Furthermore, through physical separation of the energy transfer and electron transfer steps, the quantum dots can be protected from corrosive iodide/tri-iodide redox mediators that should enhance stability as well as efficiency. Rogach and co-workers consider application of quantum dots in three types of photovoltaics that included the quantum dotsensitized materials similar to those described by Oron and Zaban.2,3 A second type was comprised of a quantum dot thin film deposited onto an optically transparent electrode (OTE) over which a gold electrode was evaporated. When the quantum dots were of a sufficient doping level and size, a depletion layer was formed in the quantum dots that facilitated charge separation much like bulk metal-semiconductor interfaces. This type of solar cell is thus referred to as a Schottky cell. Replacement of the OTE with the mesoscopic nanocrystalline TiO2 thin films commonly used in dye-sensitized solar cells yields what have been termed “depleted heterojunction colloidal quantum dot solar cells”, which are intermediate between dye-sensitized and Schottky solar cells. In one embodiment with p-type PbS quantum dots, a 5.1% power conversion efficiency was realized.5 Discussions of how deposition conditions, band edge offsets, multiple exciton generation, and surface passivation could further improve power conversion efficiencies in all three types of quantum dot photovoltaics were presented.3 Peter’s provocative perspective article points out that the exponential growth of publications on dye-sensitized solar cells over the past 10 years has not resulted in significant efficiency increases, which appear to have reached a plateau around 12%. This naturally raised the question of ‘where next?’.4 A description of attempts to improve the key components of the solar cells, i.e., the sensitizers, mesoscopic oxide, and electrolyte, entails some discussion of why such research failed to enhance efficiencies was followed by how this might be overcome in the next generation of dye-sensitized solar cells. A specific example where a multitude of research publications resulted in no practical advances involved efforts to replace the spherical TiO2 nanocrystallites with nanorods, nanowires, nanotubes, and other nanostructures. Peter points out that the efficiency with which injected electrons are collected in the external circuit was already near 100% with spherical nanoparticles. Thus one would have anticipated aux priori that enhanced transport through other nanostructures Received: July 10, 2011 Accepted: July 12, 2011 Published: August 04, 2011 1965
dx.doi.org/10.1021/jz2009314 | J. Phys. Chem. Lett. 2011, 2, 1965–1966
The Journal of Physical Chemistry Letters
GUEST COMMENTARY
would not significantly alter the solar conversion efficiency. This has been found to be the case. It was emphasized, however, that this need not be generally true, and that there may be a benefit for nanowires and other nanostructures in nonconventional dyesensitized solar cells that utilize alternative redox mediators or in solid state embodiments. Peter concludes that progress in the efficiency of dye-sensitized solar cells does not just need more research, it requires better focused research. Indeed, key scientific obstacles must be overcome in order to place any hybrid solar cell on the electrical grid in the short term. Research focused on “champion cells” has been shown to locally optimize efficiencies, while more broadly focused studies may enable more dramatic scientific breakthroughs in efficiency and fundamental knowledge. Many exciting possibilities exist, as hybrid solar cells based on inorganic, organic, and/or quantum dot materials offer the real possibility for low-cost, high-efficiency electrical power generation.1 4,6 10 Gerald J. Meyer Departments of Chemistry and Materials Science and Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland, 21218 United States
’ REFERENCES (1) Bisquert, J.; Garcia-Belmone, G. On Voltage, Photovoltage, and Photocurrent in Bulk Heterojunction Organic Solar Cells. J .Phys. Chem. Lett. 2011, 2, 1950–1964. (2) Buhbut, S.; Itzhakov, S.; Oron, D.; Zaban, A. Quantum Dot Antennas for Photoelectrochemical Solar Cells. J. Phys. Chem. Lett. 2011, 2, 1917–1924. (3) Hetsch, F.; Xu, X.; Wang, H.; Kershaw, S.; Rogach, A. Semiconductor Nanocrystal Quantum Dots as Solar Cell Components and Photosensitizers: Material, Charge Transfer, and Separation Aspects of Some Device Topologies. J .Phys. Chem. Lett. 2011, 2, 1879–1887. (4) Peter, L. The Gratzel Cell: Where Next? J .Phys. Chem. Lett. 2011, 2, 1861–1867. (5) Pattantyus-Abraham, A. G.; Kramer, I. J.; Barkhouse, A. R.; Wang, X.; Konstantatos, G.; Debnath, R.; Levina, L.; Raabe, I.; Nazeeruddin, M. K.; Gr€atzel, M.; Sargent, E. H. Depleted-Heterojunction Colloidal Quantum Dot Solar Cells. ACS Nano 2010, 4, 3374–3380. (6) Mora-Sero, I.; Bisquert, J. Breakthroughs in the Development of Semiconductor Sensitized Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3046–3052. (7) 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. (8) Venkataraman, D.; Yurt, S.; Venkatraman, B. H.; Gavvalapalli, N. Role of Molecular Architecture in Organic Photovoltaic Cells. J. Phys. Chem. Lett. 2010, 1, 947–958. (9) Ratcliff, E. L.; Zacher, B.; Armstrong, N. R. Selective Interlayers and Contacts in Organic Photovoltaic Cells. J. Phys. Chem. Lett. 2011, 2, 1337–1350. (10) Miyasaka, T. Toward Printable Sensitized Mesoscopic Solar Cells: Light-Harvesting Management with Thin TiO2 Films. J. Phys. Chem. Lett. 2011, 2, 262–269.
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