Ph.D. Oral Defense (Vignesh Gowrishankar): Mon, June 11th; 10 am; Packard 101
vigneshg at stanford.edu
Fri Jun 1 15:23:01 PDT 2007
Dear friends, colleagues and professors,
I will be defending my Ph.D. thesis on June 11th.
Please feel free to come. Details are below:
Ph.D Dissertation Oral Examination
Department of Materials Science & Engineering, Stanford University
Title: Nanostructured Inorganic / Polymer Solar Cells
Ph.D Candidate: Vignesh Gowrishankar
Advisor: Prof. Michael D. McGehee
Date: Monday, June 11th, 2007
Time: 10:00 am (refreshments served at 9:45 am)
Venue: Packard 101
Solar power is a clean, renewable source of
energy, but photovoltaic devices (solar cells)
that convert solar power to electricity are
presently too expensive for widespread use. Large
economies of scale of production, the utilization
of cheaper materials and the use of flexible
substrates will help to significantly bring down
the cost of solar cells. Organic materials, for
example conjugated, semiconducting polymers, are
cheap, easily-processable materials than can be
suitably incorporated into inexpensive, highly efficient solar cells.
Typical hybrid inorganic/organic solar cells
comprise an inorganic semiconductor, with a high
electron affinity, in contact with a
semiconducting polymer of lower electron
affinity. Following light absorption by the
semiconducting polymer, bound electron-hole pairs
(excitons) are created in the polymer that must
diffuse to the interface between the polymer and
the electron-accepting semiconductor, for
dissociation into free carriers that can be
extracted as useful photocurrent. Although
polymers have very strong absorption coefficients
(>100,000 /cm) that facilitates the use of
sub-micron-thick layers to absorb almost all
incident light, they suffer from small exciton
diffusion lengths (2 10 nm) and low charge
carrier mobilities (1E-1 1E-7 cm2/Vs).
Consequently, only those excitons generated
within an exciton-diffusion-length of the
semiconductor interface contribute to the photocurrent.
One solution to this problem is the fabrication
of a nanostructure with interpenetrating regions
of polymer and electron-acceptor that are
intimately mixed at a nanometer lengthscale. A
desirable nanostructure would be of sufficient
thickness to absorb most of the incident light
(300 500 nm), with all polymer regions within
an exciton-diffusion-length of an interface.
Additionally, straight regions of polymer and
electron-acceptor would provide for the shortest,
unimpeded paths for the charge carriers to the
electrodes, while also possibly allowing for the
alignment of polymer chains in such a way so as
to increase hole-mobility of the polymer.
In this work, we have fabricated nanostructures,
which are large-area arrays of vertical 15 30
nm diameter nanopillars and nanoridges separated
by 15 30 nm, in silicon and amorphous silicon
using nanopatterning techniques such as block
copolymer lithography and nanosphere lithography.
Nanostructured amorphous silicon / polymer
(poly(3-hexylthiophene) or P3HT) solar cells were
then fabricated, which were found to exhibit
larger photocurrents than non-nanostructured
(bilayer) devices, and consequently higher
power-conversion efficiencies. The exciton
harvesting, charge transfer and charge transport
processes within these devices were also studied.
Similar nanostructures with vertical pillars were
also fabricated in titania via a nanoimprint
lithography technique employing the use of a
Teflon-like polymeric mold. Nanostructured
titania / P3HT solar cells also exhibited
significant efficiency improvements over bilayer devices.
The use of nanostructures is thus shown to be a
promising method for increasing efficiencies of
organic-based solar cells. Hopefully in the near
future, such techniques will enable the
manufacture of highly-efficient, low-cost
photovoltaic devices that will render the
utilization of solar power more affordable and widely prevalent.
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the labmembers