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Final Report Summary - HJSC (Hierarchical Junction Solar Cells: Theory guides Experiements)

HJSC has made significant advances in several areas related to light harvesting systems:

1) Structure and electronic properties of hybrid nanostructures: We have focused on the development of atomistic approaches to understand the structural and electronic properties of nano-hetero-structures, such as core-shell and seeded nanorods:
a. For core-shell nanocrystals, we have shown with molecular dynamics computer simulation that in a model of CdSe/ZnS core/shell nanocrystals the core high pressure structure can be made metastable under ambient conditions by tuning the thickness of the shell. In nanocrystals with thick shells, we furthermore observed a wurtzite to NiAs transformation, which does not occur in the pure bulk materials. These phenomena were linked to a fundamental change in the atomistic transformation mechanism from heterogenous nucleation at the surface to homogenous nucleation in the crystal core. Our results suggest a new route towards expanding the range of available nanoscale materials.
b. For seeded nanorods, we have studied the electronic structure of CdSe/CdS core/shell seeded nanorods of experimentally relevant size is studied using a combination of molecular dynamics and semiempirical pseudopotential techniques, with the aim to address the transition from type-I to a quasi-type-II band alignment. We found that the overlap of the electron density with the core decreases with decreasing size, consistent with a transition to quasi-type II and attributed this transition to interplay of electron-hole interactions and volume effects, and not to band alignment. Our results provide insight to both scanning tunneling and optical measurements.

2) Transport in nano-junctions: We have developed two alternative theoretical frameworks to study transport in hetero-structures:
a. An equation of motion (EOM) nonequilibrium Green function (NEGF) approach: We showed that the EOM approach to NEGF may violate certain symmetry relations and that broken symmetries can lead to unphysical behavior, such as finite current at zero bias. To circumvent this pathological shortcoming we provided a scheme to restore basic symmetry relations. Illustrations were given for the Anderson and double Anderson impurity models. In addition, we have developed new closures to describe the transport in nano-junctions within the EOM NEGF approach and proposed a scheme to assess the accuracy of the closures by comparing the poles of the Green functions to the exact many-particle energy differences for the isolate system. Our analysis provides means to extend the equation-of-motion technique to more elaborate models of large bridge systems with strong electronic interactions.
b. Semiclassical approach to transport: We have developed and applied semiclassical approaches to study the transport in nano-junctions. The many-electron Hamiltonian in second quantization was mapped onto a classical model (two mapping procedures were developed) that preserves the fermionic character of electrons. Comparisons with exact results generated for the resonant level model and for the Anderson impurity model reveals that the semiclassical treatment provides a quantitative description of the dynamics and steady-state currents at all relevant timescales for a wide range of bias and gate potentials, and for different temperatures in the Coulomb blockade regime. The approach opens the door to study more complex transport problems away from equilibrium.

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