Hybrid Organic-Nanocrystal Solar Cells
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Hybrid Organic–
Nanocrystal Solar Cells
Delia J. Milliron, Ilan Gur, and A. Paul Alivisatos Abstract Recent results have demonstrated that hybrid photovoltaic cells based on a blend of inorganic nanocrystals and polymers possess significant potential for low-cost, scalable solar power conversion. Colloidal semiconductor nanocrystals, like polymers, are solution processable and chemically synthesized, but possess the advantageous properties of inorganic semiconductors such as a broad spectral absorption range and high carrier mobilities. Significant advances in hybrid solar cells have followed the development of elongated nanocrystal rods and branched nanocrystals, which enable more effective charge transport. The incorporation of these larger nanostructures into polymers has required optimization of blend morphology using solvent mixtures. Future advances will rely on new nanocrystals, such as cadmium telluride tetrapods, that have the potential to enhance light absorption and further improve charge transport. Gains can also be made by incorporating application-specific organic components, including electroactive surfactants which control the physical and electronic interactions between nanocrystals and polymer. Keywords: nanocrystals, organics, photovoltaics, semiconductors, solar cells.
Introduction Solar cells incorporating both organic semiconductors and inorganic semiconductor nanocrystals aim to integrate the benefits of both classes of materials. Inorganic semiconductors are ideal for fabricating highly efficient solar cells, as they absorb a broad range of light and transport charge effectively. However, bulk inorganics require expensive processing, historically limiting the cost-effectiveness of conventional inorganic solar cells. This has motivated extensive research on organic photovoltaic cells, since small-molecule and especially polymer semiconductors have the potential to be inexpensively processed into flexible, thin-film photovoltaics. Semiconductor nanocrystals combine many advantageous characteristics of bulk inorganic materials with the solution processability and low-temperature chemical synthesis of polymers. Additionally, the optical gap of inorganic nanocrystals can be tuned by material selection and quantum confinement, and advances in synthesis techniques allow control over nanocrystal size and shape to optimize photovoltaic MRS BULLETIN • VOLUME 30 • JANUARY 2005
performance. Hybrid solar cells blending organic semiconductors and inorganic semiconductor nanocrystals offer the potential to deliver efficient energy conversion with low-cost fabrication. Not only does the nanocrystal component serve as an active absorbing material in such devices, the absorption properties can be tuned by means of quantum confinement, allowing for further optimization. Materials selection for hybrid photovoltaic cells must reflect the requirements of each step in the energy conversion process. The –* absorption in semiconducting polymers can be rather strong, but occurs over a discrete energy range cove
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