Solar Cells Based on Nanostructured Materials
Qifeng Zhang, Xiaoyuan Zhou, Christopher S. Dandeneau, Kwangsuk Park, Supan Yodyingyong, Guozhong Cao*
Materials Science and Engineering, University of Washington, Seattle, WA 98195
1.3 Material Fabrication
24
"Somewhat optimized"
Aggregated TiO2 nanotubes 9.9%
Nanostructured materials possess advantage in electric and optical properties, and when
these carefully designed and fabricated nanostructures serve as a photoelectrode film of
solar cells, they can effectively improve the energy conversion efficiency by offering a
large specific surface area, providing direct pathways for electron transport, or generating
light scattering so as to extend the traveling distance of photons within the photoelectrode
film. Our study focus on the synthesis of nanostructured oxides and the application of these
nanostructures on (1) dye-sensitized solar cells, and (2) hybrid solid-state solar cells that
combine inorganic nanostructures with organic polymer materials.
Current Density /mA/cm
2
Abstract
– Chemical solution method for ZnO nanoparticle aggregates and hydrothermal
growth for TiO2 nanotube aggregates
Example:
20
16
12
Aggregated TiO2 nanotubes 7.4%
8
4
0
0
100
200
300
400
500
600
700
800
Voltage /mV

VOC
(mV)
ISC
(mA/cm2)
Vmax
(mV)
Imax
(mA/cm2)
FF
(%)
(%)
*
730
15.9
450
12.1
47
5.4
500 °C *
720
18.7
485
16.6
60
8.0
600 °C *
710
12.8
480
9.7
51
4.7
17.5
449
15.1
57
6.8
20.8
518
19.1
66
9.9
18.8
459
16.4
57
7.5
Sample
400 °C
6  m #, ¶
670
11  m #, ¶
730
15  m #, ¶
700
* Annealing temperature
#
Film thickness
¶
Annealed at 500 °C
1. Dye-sensitized solar cells
2. Hybrid Solar Cells
1.1 Introduction
Dye-sensitized solar cells (DSCs) are a type of photovoltaic device based on an
electrochemical system that can convert the solar energy to electricity. A DSC
includes a photoanode that consists of conductive glass substrate coated with
porous oxide film, a cathode usually formed by platinum-coated silicon wafer, and
liquid electrolyte that is filled into the space between the photoanode. Dye
molecules that adsorb on the surface of crystal oxide play a role of antenna to
capture the incident photons. The photoexcited electrons inject from the dye
molecules to the oxide semiconductor, and then pass through external circuit
forming photocurrent.
Nanocrystalline TiO2 film
Synthesis of ZnO nanoparticle aggregates and formation of hierarchically structured aggregate
films.[3]
Hierarchical structure provides the photoelectrode with both a large specific surface
area for dye adsorption and the light scattering centers for an enhancement in the
light harvesting efficiency. The aggregates are designed to be submicron size that
is comparable with the wavelengths of the incident light so as to cause light
scattering effectively.
Hybrid solar cells combine advantages of both organic and inorganic
semiconductors. The organic materials consist of conjugated polymers that absorb
light as the donor and transport holes, while the inorganic materials in the structure
are used as the acceptor and electron transporter. The use of nanostructured
inorganic materials in hybrid solar cells may significantly boost the interface area of
p-n junction and meanwhile provides direct pathway for electron transport so as to
reduce the electron trapping.
P3HT:PCBM
Ag electrode
ZnO
PEDOT:PSS
1.4 Results
FTO
– Hierarchical structure creates a dramatic improvement in the solar cell conversion
efficiency
Construction and operational principle of dye-sensitized solar cell
device.[1]
1.2 Motivation
► The DSCs with ZnO aggregate films typically generate a conversion efficiency
about 5.4%, which is almost a double of the 2.4% obtained for ZnO nanoparticles.
Our study also demonstrates that a poly-sized distribution of ZnO aggregates is
advantageous to the generation of light scattering. The maximal conversion
efficiency has been achieved on the film that comprises the aggregates with both a
broad size distribution and an average size close to the wavelengths of incident
light.[2, 4-6]
Glass
3. Other Nanostructures with Potential Application in Solar Cells
TiO2 Nanoparticle Aggregates
20
TiO2 Nanorods
15
=2.4%
5
0
100
200
300
400
500
600
700
60 °C
Voltage /mV
Nanotube
95 °C
Nanowall
95 °C
ZnO Nanotubes
A schematic drawing presented to
illustrate the propagation and
multiple scattering of light in highly
disordered films consisting of
spherical aggregates. Multiple
scattering results in the increase of
light traveling distance and the
formation of optical confinement
when the light scattering is trapped
in closed loops.
100nm
► A high conversion efficiency of 9.9% has been achieved on a TiO2 film that
consists of nanotube aggregates synthesized by a hydrothermal method. The
efficiency is closely related to both the annealing temperature and the film
thickness.[7] (This work is being carried out in collaboration with Dr. Jun Liu at Pacific Northwest National
Laboratory.)
* Corresponding author: gzcao@u.washington.edu
http://depts.washington.edu/solgel/
ZnO Nanorod Array
TiO2-B Nanorods
Acknowledgements
10
0
TiO2 Nanotube Array
=5.4%
2
A comparison of the conversion
efficiencies between the ZnO
nanoparticulate film and the
aggregate film. The conversion
efficiency of the aggregate films
shows a dependence on both the
average size and the size
dispersivity of aggregates.
Current density /mA/cm
Light Scattering Effect – A way employed in our dye-sensitized solar cells to increase
the light harvesting efficiency of DSCs. Due to the light scattering generated by
oxide aggregates, the traveling distance of photons within the photoelectrode film
can be increased significantly. This would result in an improvement in the light
harvesting efficiency of photoelectrode and thus contribute to the solar-to-electricity
conversion efficiency of the cells. The aggregates ensure the photoelectrode film
having a large surface area in virtue of the porous structure of individual aggregate
assembled by nano-sized crystallites.[1, 2]
This work is supported in part by US Department of
Energy (DE-FG02-07ER46467), Air Force Office of
Scientific Research (AFOSR-MURI, FA9550-06-10326), and National Science Foundation (DMI-0455994
and DMR-0605159). This research is also supported by
Washington
Research
Foundation,
Washington
Technology Center, National Center for Nanomaterials
Technology (Korea), Intel Corporation, and EnerG2.
References
[1] Zhang, Q. F.; Dandeneau, C. S.; Zhou, X. Y.; Cao, G. Z., ZnO nanostructures for dye-sensitized solar cells. Advanced Materials
2009, 21, 1-22.
[2] Zhang, Q. F.; Chou, T. R.; Russo, B.; Jenekhe, S. A.; Cao, G. Z., Aggregation of ZnO nanocrystallites for high conversion efficiency
in dye-sensitized solar cells. Angewandte Chemie-International Edition 2008, 47, (13), 2402-2406.
[3] Chou, T. P.; Zhang, Q. F.; Fryxell, G. E.; Cao, G. Z., Hierarchically structured ZnO film for dye-sensitized solar cells with enhanced
energy conversion efficiency. Advanced Materials 2007, 19, (18), 2588-+.
[4] Zhang, Q. F.; Chou, T. P.; Russo, B.; Jenekhe, S. A.; Cao, G. Z., Polydisperse aggregates of ZnO nanocrystallites: A method for
energy-conversion-efficiency enhancement in dye-sensitized solar cells. Advanced Functional Materials 2008, 18, (11), 1654-1660.
[5] Cao, G. Z., Popcorn-style solar cells. Photonics Spectra 2008, 42, (7), 60-61.
[6] Popcorn-ball design doubles efficiency of dye-sensitized solar cells. UNIVERSITY OF WASHINGTON NEWS
(http://uwnews.org/article.asp?articleid=40714).
[7] Nie, Z. M.; Zhou, X. Y.; Zhang, Q. F.; Cao, G. Z.; Liu, J., Aggregated TiO2 nanotubes for high efficiency dye sensitized solar cells.
Unpublished.
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