Synthesis of ZnO Nanowires and TiO2 Nanowires for Dye-Sensitized Solar Cells
Janice Boercker (CEMS): advisor: Eray Aydil (CEMS)
The burning of fossil fuels releases almost 7 Gt of CO2 into the atmosphere each year.1 CO2 is a greenhouse gas and is the main pollutant responsible for global warming. In order to combat the catastrophic consequences of global warming, carbon-free alternative energy sources will need to be implemented. A potential carbon-free energy source is solar cells because the energy from the sun is very large, ~9x1016 watts. The sun provides almost 10,000 times more energy than the current global demand of 10TW; if just 0.1% of the earth was covered by 10% efficient solar cells all of the current global energy need could be satisfied using solar power. The main barrier to wide use of solar cells is the high cost. Inexpensive solar cells need to be developed in order for solar energy to become a viable economical alternative to burning fossil fuels.
Figure
1: Schematic of a nanowire
dye-sensitized solar cell. 3
The nanowire
dye-sensitized solar cell (DSSC), shown in figure 1, was developed recently,3,4,5 and is
a modification to the nanoparticle DSSC develop by Gratzel and O’Regan.6 The nanowire
DSSC is composed of three main components: (1) a ~10 micron thick ZnO nanowire array, (2) a monolayer of dye adsorbed onto
these nanowires, and (3) an electrolyte interpenetrating the nanowire
array. Transparent conducting oxide
(TCO) substrates contacting the semiconductor film and the electrolyte complete
the cell.
When solar radiation is incident on the DSSC electrons are excited in the dye and are injected into the semiconductor nanowires. The electrons travel through the semiconductor to the photoanode and through the load to the cathode where they reduce oxidant in the electrolyte (typically I3- of the I-/I3- couple). The reductant (e.g. I-) then completes the circuit by reducing the photooxidized dye. The advantage of the nanowires in the DSSC is that they allow for efficient electron transport. The nanowires act as a “highway” for the electrons. The current efficiency of a nanowire DSSC is about 2.4%4. This relatively low efficiency is mainly due to a lack of surface area for dye absorption. To improve the efficiency I am looking into ways to grow longer, thinner, denser ZnO nanowires by examining the nanowire nucleation and growth.
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(2) Petit, J. R.; Jouzel, J.; Raynaud, D.; Barkov, N. I.; Barnola, J.M.; Basile, I.; Bender, M.; Chappellaz, J.; Davis, M.; Delaygue, G.; Delmotte, M.; Kotlyakov, V. M.; Legrand, M.; Lipenkov, V. Y.; Lorius, C.; et al Nature 1999, 399, 429-436.
(3) Baxter, J.; Walker, A.; Ommering, K.; Aydil, E. Nanotechnology 2006, 17, S1-S9
(4) Guo, M.; Diao, P.; Wang, X.; Cai, S. Journal of
(5) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nature Materials 2005, 4, 455-459.
(6) O'Regan, B.; Gratzel, M. Nature 1991, 353, 737-739.