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Computational Challenges for Nanostructure Solar Cells

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Key Challenges: Current nanostructure solar cells often have energy efficiencies well below that of traditional solar cells. To understand why, one must understand the complete photoelectron dynamics in a nanostructure - the photon absorption, exciton generation, exciton dissociation, carrier transport and carrier collection.  However, the large number of surface states, the strong exciton binding energies, the nano-interfaces, the lack of doping, and the possibility of unintended internal electric fields make this a daunting task that requires a suite of techniques and computer codes offering different electronic structure methods and varying levels of approximation.

Why it matters: Solar cells made of inorganic nanostructures, or mixtures of nanostructures and organic polymers, offer important advantages for constructing photovoltaic and photoelectrochemical solar cells in which sunlight is either directly converted into electricity or is used to produce a carbon-neutral chemical fuel to replace petroleum.

Accomplishments: A new linearly-scaling 3-D fragment method to self-consistently solve the charge density of a ten- to hundred-thousand atom system with density functional theory has been developed and implemented on NERSC's Cray XT4 (Franklin) system. It can be used to study nanocrystals (e.g., a 5,000 atom CdSe/CdS core/shell structure) in just a few hours.  Such nanostructures have been synthesized but their electronic structure, especially their wave function localizations, cannot be measured directly.  The calculations showed that the the electron is outside the CdSe core but the electron "hole" formed from the sunlight-induced exciton breakdown, stays inside the core.

Investigators: Zhengji Zhao, Lin-Wang Wang, Lawrence Berkeley National Laboratory

More Information: See Condensed Matter 2008, 20, 294203 and Energy & Environmental Science, Volume 2, Number 9, September 2009, Pages 897–1004.