Designed for Charge Transfer: Complexes of CdSe Nanocrystals and Oligothiophenes
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Designed for Charge Transfer: Complexes of CdSe Nanocrystals and Oligothiophenes Delia J. Milliron, Claire Pitois, Carine Edder, Jean M.J. Fréchet, and A. Paul Alivisatos Department of Chemistry, University of California, Berkeley, CA 94720 and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Hybrid organic-inorganic solar cells promise to utilize the advantages of each class of materials and their complementary nature to produce power efficiently with inexpensive processing. Combining the efficient charge transport typical of inorganic semiconductors with the solution processibility of semiconducting polymers, nanocrystal-polymer photovoltaics hold tremendous potential[1]. Electronic energy levels of inorganic and organic semiconductors tend to be significantly staggered, creating a large energetic driving force for charge transfer. However, the hybrid approach requires control over the interface between dissimilar materials in order to mix them on the nanoscale. This mixing creates the very high area interface responsible for charge creation. Molecular level control of the interface, absent in current designs, could optimize charge separation. Semiconductor nanocrystals useful for solar cells, such as CdSe, absorb strongly across the visible spectrum, enabling efficient collection of sunlight. Due to quantum confinement, they also exhibit a size-tunable bandgap. Different cells using nanocrystals of different size can be used in series to maximize electrical power production and minimize heat. CdSe nanocrystals have also been prepared in a variety of shapes[2], including long nanorods, which can be used to transport electrons efficiently out of the cell. Finally, unlike conventional inorganic materials, nanocrystals are prepared at low temperatures (around 300°C) and yet contain very few defects [3]. Poly- and oligothiophenes appropriately complement CdSe nanocrystals in solar cells. Among organic semiconductors, they have high hole mobilities, only exceeded by single crystals. Thus, efficient pathways exist for transporting charges out of the device: electrons through the inorganic and holes through the organic material. Polythiophenes also absorb in the visible, contributing to light collection. When the organic-inorganic interface is controlled, these materials efficiently create and transport charge under illumination. To improve solar cell performance, the nanocrystals used continue to evolve to larger and longer shapes [4] and new materials. These changes challenge the delicate control over the inorganic-organic interface that had been achieved with smaller nanocrystals of CdSe. And, even in the case of these smaller nanocrystals, molecular level control over the interface is lacking. Such control could be used to reduce recombination, which limits device efficiency at high light intensities found in full sunlight. We propose to introduce a third component into nanocrystal-polymer solar cells to optimize and generalize interface control. An electroactive surfactant, bound
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