Synthesis of Block Copolymers for the Production of
Titania Nanocomposites for Photovoltaic Applications
Dustin Phillips and Dr. Devon Shipp
Department of Chemistry and Biomolecular Science, Clarkson University
Introduction
Photovoltaic devices or solar devices are used to covert sunlight energy into electrical energy. The
most common type of photovoltaic devices currently in use are based on the semi-conducting material
silicon. However, dye-sensitized solar cells that instead utilize a ruthenium dye attached to TiO2 have the
potential to allow for the production of inexpensive solar cells which can be applied to a variety of flexible
substrates, including clothing.
Current fabrication of dye-sensitized solar cells results in unstructured TiO2 which achieve an efficiency that
is less than 10%. Structuring the solar cell on a nanometer scale will allow for maximized surface area for
the ruthenium dye and less opportunity for electrons to recombine with positively charged molecules, thus
allowing for greater efficiency in energy production. Therefore, one of the ultimate goals of PV device
production is to create a simple, easy method of making nanostructured titania materials in order to improve
photo-generated current efficiency.
Project Research Goals
The primary goal of this research is to synthesize and then use a block copolymer to create
nanometer sized titania nanorods that can be used to structure the dye-sensitized solar cell on a nanometer
scale. The use of a self assembling block copolymer composed of polystyrene
PS block
PVP block
and poly(4-vinylpyridine) [PS-b-P4VP] (see Scheme 1)can be used in structuring
n
m
the dye-sensitized solar cell. Therefore, we describe here our efforts to make the
N
PS-b-P4VP block copolymer and characterize its molecular weight and
composition.
Dustin Phillips
Scheme 1
2008
REU
Chemistry
Dr. Devon Shipp
Results and Discussion
We anticipate that a thin film of the block copolymer will align to form upright rods approximately
10 nanometers in diameter (see Fig. 1), where poly(4-vinylpyridine) forms the cylinder surrounded by
polystyrene. TiO2 nanorods can be created from PS-b-P4VP using similar methods outlined in the literature
by Russell et al. Briefly, this involves the use of selective solvents
to create nanopores that can be backfilled with TiO2 to form the
TiO2 nanorods. We expect that the dye-sensitized solar cell can be
created by attaching the ruthenium dye to the structured titania nanorods.
Fig. 1
The appropriate block copolymer for self assembly of nanorods is composed of a 45,000 molecular
weight polystyrene block with a 20,000 molecular weight poly(4-vinylpyridine) block attached. The use of
reversible addition-fragmentation chain transfer (RAFT) polymerization allows for the production of
polymer chains with these well defined molecular weights and with low polydispersity.
In RAFT polymerization the RAFT agent, typically a thiocarbonylthio compound, undergoes
reversible attachment/reattachment to the polymer chain as it polymerizes, thus slowing and controlling the
process of polymerization. Altering the mole ratio of monomer to RAFT agent alters the resulting molecular
weight of the polymer, therefore allowing for the creation of polymers with specified molecular weights. In
creating the block copolymer, RAFT polymerization is first used to create polystyrene with the desired
molecular weight of approximately 45,000 g/mol. The polystyrene sample is then collected and used in the
creation of the block copolymer. This polystyrene, along with 4-vinylpyridine monomer and the initiator
AIBN, are combined and the RAFT agent that is attached to the polystyrene works in the same way as the
poly(4-vinylpyridine) polymerizes onto the polystyrene. Multiple reactions were done to find the
appropriate mole ratios to yield block copolymer with a poly(4-vinylpyridine) block of molecular weight
20,000 g/mol.
Gel permeation chromatography (GPC) testing of multiple trials showed that a mole ratio of 800:1
styrene monomer to RAFT agent yields the desired polystyrene molecular weight of 45,000 g/mol. (see Fig.
2). Nuclear magnetic resonance (NMR) spectroscopy data showed that the 4-vinylpyridine was successfully
Dustin Phillips
2008
REU
Chemistry
Dr. Devon Shipp
polymerized onto the polystyrene/RAFT (see Fig. 3). Using mole ratio of 250:1 4-vinlypyridine monomer to
polystyrene/RAFT the poly-4-vinylpyrine added a molecular weight of 63,000 g/mol to the polymer. Further
trials will attempt to decrease the poly-4-vinylpyrine weight to achieve the desired additional molecular
weight of 20,000 g/mol.
600
M = 45,000
550
n
M /M = 1.2
RI Signal (mV)
w
n
500
450
400
350
300
10
12
14
16
Retention Volume (mL)
18
20
Figure 2. GPC data of polystyrene.
PS block
PVP block
n
m
N
5
2
2
6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
ppm
8.83
7.0
2.73
7.5
2.54
8.0
1.00
8.5
Figure 3. NMR spectrum of PS-b-PVP block copolymer.
Works Cited
Shipp, D. A. “Living Radical Polymerization: Controlling Molecular Size and Chemical Functionality
in Vinyl Polymers” J. Macromol. Sci. C: Polym. Rev. 2005, 45, 171-194.
Park, S.; Wang, J.-Y.; Kim, B.; Xu, J.; Russell, T. P. “A Simple Route to Highly Orientated and
Ordered Nanoporous Block Copolymer Templates” ACSNano 2008, 2, 766-772.
Dustin Phillips
2008
REU
Chemistry
Dr. Devon Shipp