PhD Oral Examination: Ching-Mei Hsu (Thursday, May 5th, 10:00am)

Ching-Mei Hsu chingmei at
Fri Apr 22 09:12:59 PDT 2011

*Photon Management for a-Si:H Solar Cells Using Periodic Nanostructures
Ching-Mei Hsu*

* *

Department of Materials Science and Engineering

Advisor: Prof. Yi Cui

Thursday May 5th 2011, 10:00 am (Refreshments at 9:45 am)

Location: Paul G. Allen Auditorium (CIS-X 101)

 Solar technology is a leading candidate for clean energy production.
Silicon is an excellent material for photovoltaic (PV) applications due to
its low toxicity, abundance, long term stability, and well developed
processing technologies. Crystalline Si solar cells currently dominate the
photovoltaic market despite requiring more material and more
energy-intensive manufacturing processes than their thin-film counterparts.
Thin-film silicon, e.g. amorphous silicon (a-Si:H), provides the advantage
of decreasing material costs over crystalline silicon. Because the material
is amorphous, there are many defects, which results in a small minority
carrier diffusion length. Thus, a thinner absorber is required. However,
thinner absorber layers do not absorb light effectively, resulting in poor
cell performance. If the active material could be made to absorb all of the
light in a film with a thickness approximately equal to the minority carrier
diffusion length, the open-circuit voltage (Voc), short-circuit current
(Jsc), and fill factor (FF) of the device would be greater than those of a
thicker cell.

My research is comprised of three parts: (1) developing a nanostructure
fabrication process, (2) designing device geometries for alternative light
trapping strategies in both substrate and superstrate configurations, and
(3) investigating the effects of nanostructures’ morphologies on the optical
and electrical properties of devices. In contrast to the use of randomized
surface texturing to improve the coupling of light into the active material,
we employed periodic nanostructures to couple incident light into guided
modes that propagate in the plane of the absorber. This approach can
significantly increase the optical path length inside a thin absorber layer.
To achieve this goal, I first developed a nanostructure fabrication process
by combining self-assembly and reactive ion etching. We then employ these
as-made nanostructures in a-Si:H solar cells. The periodic-nanostructure
devices show an enhanced absorption and photocurrent generation in
comparison with planar cells. We used FTDT studies to confirm that the
increased photocurrent was indeed caused by enhanced absorption.  We also
systematically studied the effects of morphological parameters on
light-trapping efficiency and electrical characteristics of the device. With
my optical and electrical findings, we have achieved efficiencies up to 9.7%
for devices with substrate configurations and 10.2 % for devices with
superstrate configurations.

Ching-Mei Hsu
PhD Candidate
Stanford University
Materials Science and Engineering
476 Lomita Mall, McCullough Bld. Rm 217
Stanford, CA 94305
email: chingmei at ;
          chingmei1219 at
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