Reminder: Ph.D. Oral Defense (Vignesh Gowrishankar): Mon, June 11th; 10 am; Packard 101

[Vignesh Gowrishankar]

Dear friends, colleagues and professors,

I will be defending my Ph.D. thesis on Monday, 
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 (near Bytes Cafe)


Abstract:

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.
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