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Title: Extracting hot carriers from photoexcited semiconductor nanocrystals

This research program addresses a fundamental question related to the use of nanomaterials in solar energy -- namely, whether semiconductor nanocrystals (NCs) can help surpass the efficiency limits, the so-called “Shockley-Queisser” limit, in conventional solar cells. In these cells, absorption of photons with energies above the semiconductor bandgap generates “hot” charge carriers that quickly “cool” to the band edges before they can be utilized to do work; this sets the solar cell efficiency at a limit of ~31%. If instead, all of the energy of the hot carriers could be captured, solar-to-electric power conversion efficiencies could be increased, theoretically, to as high as 66%. A potential route to capture this energy is to utilize semiconductor nanocrystals. In these materials, the quasi-continuous conduction and valence bands of the bulk semiconductor become discretized due to confinement of the charge carriers. Consequently, the energy spacing between the electronic levels can be much larger than the highest phonon frequency of the lattice, creating a “phonon bottleneck” wherein hot-carrier relaxation is possible via slower multiphonon emission. For example, hot-electron lifetimes as long as ~1 ns have been observed in NCs grown by molecular beam epitaxy. In colloidal NCs, long lifetimes have been demonstrated through carefulmore » design of the nanocrystal interfaces. Due to their ability to slow electronic relaxation, semiconductor NCs can in principle enable extraction of hot carriers before they cool to the band edges, leading to more efficient solar cells.« less
Publication Date:
OSTI Identifier:
Report Number(s):
DOE Contract Number:
Resource Type:
Technical Report
Research Org:
University of Texas at Austin
Sponsoring Org:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
Country of Publication:
United States
36 MATERIALS SCIENCE; 14 SOLAR ENERGY Nano materials, solar energy conversion