Nanotechnology in
Solar Energy Conversion
David F. Kelley
University of California, Merced
Energy production and use in the United States
How much energy do we use?
How do we use it? Where do we get it?
Total energy usage
(the situation isn’t getting any better)
World energy consumption
10
9
8
7
6
5
4
3
all
oil
coal
gas
biomass
nuclear
Hydro
geothermy
sun; wind & other
2
1
0
1850
1875
1900
1925
year
1950
1975
2000
Economics of solar energy
cost of production, ¢ per kW-hr (U.S. in 2002)
25
20
15
Cost
10
5
0
Coal
Gas
Oil
Wind
Nuclear
Solar
Efficiency of Photovoltaic Devices
25
Efficiency (%)
20
15
10
5
1950
crystalline Si
amorphous Si
nano TiO2
CIS/CIGS
CdTe
1960
1970 Year 1980
1990
2000
Bulk semiconductor photovoltaics
„
„
„
Existing photovoltaics are simple, relatively efficient, but are expensive.
Charge separation occurs in the bulk (not at an interface) and is driven by
the local electric potential gradient.
Efficient charge collection requires very pure materials. (Avoid
recombination centers.)
Dye-sensitized solar cells
„
„
„
Light is absorbed by surface adsorbed dye (or nanoparticle) .
Charge separation occurs at an interface.
Charge migration occurs in a chemical potential (concentration) gradient.
-O
Electron
injection
3IO
TiO2
MLCT
2e-
N
2e-
Ru2+
cathode
N
O
-O
I3-
Bulk heterojunction polymer solar cells
„
„
„
„
Light is absorbed primarily by the polymer
Charge separation at the polymer/fullerene interface
Hole transport through the polymer
Electron transport through the fullerene
Nanotechnology in solar energy conversion:
Nanoparticle based solar cells
Application in “dye” sensitized or bulk heterojunction
photovoltaics
Advantages:
„ More photostable than dyes
„ Tunable through quantum size effects (quantum
confinement)
„ Not sensitive to impurities
„ Possible multiple exciton generation from excitation
in the blue regions of the solar spectrum (reverse
Auger process). Potentially very efficient.
Passive luminescent solar concentrators
„
„
„
„
Total internal reflection
directs light to small, highly
efficient PV. Two big
technical problems:
1) luminescence quantum
yield
2) self absorption
Possible solution: two sizes of
core/shell nanorods.
Most photon absorption is by
smaller (and more numerous)
CdSe or CdTe nanorods.
Energy transfer from smaller
(blue absorbing) to larger
(red emitting) nanorods. The
spectral difference minimizes
self-absorption.
Total
internal
reflection
Energy
transfer
Photovoltaic
Polymer or glass film with aligned nanorods
Larger bandgap shell passivates the core semiconductor.
CdTe/CdS/ZnS lattice matching results in highly
luminescent core/shell semiconductor nanorods
ZnS
CdTe
CdTe
CdS