Reminder: EE PhD Oral Defense - Jia Zhu, Monday, April 26, 2010; 4-6pm, CIS-X Auditorium

Jia Zhu jiazhu at stanford.edu
Fri Apr 23 10:14:33 PDT 2010


Stanford University Ph.D. Dissertation Defense

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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