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Ab initio Study of Hot Carriers in Semiconductors: First Picosecond after Sunlight Absorption in Silicon

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Hot carriers in silicon. Top – relaxation time vs. energy. Bottom – mean free path vs. energy along different directions.

Scientific Achievement
First ab initio calculations of hot carriers’ (HC) properties and dynamics in semiconductors with many-body perturbation theory without using experimentally derived parameters; elucidated mechanisms for energy loss and explained experiments.

Significance and Impact
Hot carrier thermalization is the main source of energy loss in devices such as solar cells, and their dynamics is central to many energy conversion processes. Due to fast time scale and complex physics involved, characterization of hot carriers has long been a challenge. The work opens the avenue to use ab initio means to interpret/guide new experiments to harness the energy from HCs.

Summary
Hot carriers (HCs) are charge carriers (electrons or holes) that have surplus energy compared to those in thermal equilibrium. They may be generated by injection or optical excitations. They lose energy to lattice or other carriers rapidly (< 1 ps) and are found in many electronic/optoelectronic/energy conversion devices. Hot carrier thermalization is the main source of energy loss in solar cells, and their dynamics is central to many energy conversion processes. However, due to the sub-picosecond time scale and complex physics involved, characterization of HCs has long been a challenge even for the simplest materials.

The work by Steve Louie and Jeff Neaton provides the first ab initio calculations of HC properties and dynamics in semiconductors using many-body perturbation theory without any experimentally derived parameters. Using their excited-state methods and codes, they computed from first principles the properties and dynamics of the HCs in Silicon and tracked their behaviors as a function of time within the first picosecond after sunlight absorption. Their results elucidated the mechanisms for energy loss of HCs and explained previous experiments. This work opens the avenue to use ab initio calculations to interpret and guide new experiments to harness the energy from HCs in semiconductors and other systems such as organic-inorganic junctions and nanostructures.

M. Bernardi, D. Vigil-Fowler, J. Lischner, J.B. Neaton and S.G. Louie, Phys. Rev. Lett. 112, 257402 (2014)