(34c) Nanocrystal Building Blocks in Photofunctional Superlattice Structures
Recent advances in synthesis, characterization, and the emerging understanding of their size-dependent properties have created exciting opportunities for semiconductor nanomaterials to contribute to the development of next-generation energy conversion technologies. Semiconductor nanocrystal quantum dots are particularly attractive material candidates for the efficient capture of solar emission in inexpensive, thin film photovoltaic devices due to their large absorption cross sections, low-cost solution-phase processing and size-tunable energy gaps. Despite the immense promise, the development of cost- and performance-competitive nanocrystal-based solar cells has, to date, fallen short of expectations. This gap is primarily due to the current lack of answers to the challenge of how to efficiently extract photogenerated charges from quantum confined systems and transport them to external electrodes. In addition to the thermodynamic challenge of integrating the nanocrystal into a structure in which separation of photogenerated electron and hole can be separated and transported, there are also important kinetic challenges. Specifically, the rates of the sub-processes of separation and transport across the various interfaces must surpass competing recombination and trapping dynamics. We review our recent work investigating the exciton dissociation and charge transport between quantum confined nanocrystals as a function of the interparticle spacing. Beyond controlling the interdot spacing, directing the nanocrystals into ordered superstructures with predefined translational and orientational order is a critical parameter for controlling the electronic coupling in three dimensional superlattices. We will present recent experiments relating superlattice symmetry and optical properties of the ensemble.