Name: Jia Zhu
Advisor: Prof. Yi Cui
Time: April 26 (Monday: 4pm-6pm)
Location: CIS-X Auditorium
Title: Tuning the Shape of Semiconductor Nanowires for Advanced Photovoltaics
Abstract:
Tuning the shape of nanostructures can have a strong effect on photon
management and charge carrier collection for photovoltaics. Here, I
demonstrate two examples of nanowire shape designing: nanocones and
branched nanowires.
Photon management, involving both absorption enhancement and
reflection reduction, is critical to all photovoltaic devices. It can
improve the efficiency by minimizing optical and electrical losses,
and cut cost by reducing material usage, process time and capital
investment. Here I demonstrate a novel solar cell structure with an
efficient photon management design. The centerpiece of the design is a
novel nanocone structure, which is fabricated by a scalable low
temperature process. With this design, devices with very thin active
layer can achieve near perfect absorption because of both efficient
antireflection and absorption enhancement over a broad spectral range
and a wide range of angles of incidence. More strikingly, the design
and process is not in principle limited to any specific material
system, hence it opens up exciting opportunities for all classes of
photovoltaic devices. I have used amorphous silicon and dye sensitized
solar cells as two examples to demonstrate the concept. The device
efficiencies of this design are significantly better compared to
conventional devices. Moreover, I also have explored absorption
enhancement on a sub-wavelength scale, compared to "classical" light
trapping limits.
PbSe nanocrystals have shown a greatly enhanced multi exciton
generation (MEG) effect, one important step toward third generation
solar cells. However, it is difficult to extract generated carriers
from nanocrystals without good transport pathways. Three dimensional
branched nanowire or nanotube networks, with strong quantum
confinement within two dimensions, and the connected third dimension
as an efficient charge carrier pathway, could be ideal for enhancing
the MEG effect, light absorption, and carrier collection. I
successfully demonstrate a large area growth of PbSe hyperbranced and
chiral branched nanowires on a variety of substrates. More excitingly,
Chiral branched nanowires reveal a new nanowire growth mechanism,
dislocation driven growth, which can be applied to a variety of
materials.
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