Challenges in Solar-to-Electric Energy Conversion: an Introduction · cells would produce enough...
Transcript of Challenges in Solar-to-Electric Energy Conversion: an Introduction · cells would produce enough...
Eray S. Aydil
Chemical Engineering and Materials Science Department
Acknowledgements: National Science FoundationMinnesota Initiative for Renewable Energy and the Environment (IREE)
Challenges in Challenges in SolarSolar--toto--Electric Energy Conversion:Electric Energy Conversion:
an Introductionan Introduction
United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005).
www.er.doe.gov/bes/reports/files/SEU_rpt.pdf
Ener
gy C
onsu
mpt
ion
(TW
)
0
1
2
3
4
5
6
7
5.3
4.2
3.5
0.9 0.9
0.07 0.03 0.01 0.004
oil
coal
gas
hydr
oele
ctric
nucl
ear
wind
etha
nol
geot
herm
alph
otov
olta
ic
Find ways to provide clean energy to ~ 10 billion people.
The Energy ChallengeThe Energy Challenge
Solar powerSolar power
World demand ~ 15 TWSun ~ 120,000 TW
Covering 0.125 % of earth’s surface with 10% efficient solar cells would produce enough energy to supply the annual global demand.
Global solar PV productionGlobal solar PV production
~30-50% growth
At 35% growth rate we will reach 1 TW in ~ 20 Years
StateState--ofof--thethe--art in solar cellsart in solar cells
United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005) and from J. Crystal Growth 275, 292 (2005) by T. Surek.
Module efficiency ~ 0.5 -
0.8 × lab efficiencies
c-Si(89.6%)
CdTe(4.7%)
thin film Si(5.2%) CIGS
(0.5%)
2007 market share for various technologies2007 market share for various technologies
0
5
10
15
20
25
30
Coal Gas Oil Wind Nuclear Solar
Cos
t (¢
/kW
-hr)
1-4 ¢ 2-5 ¢ 6-8 ¢ 5-7 ¢ 6-7 ¢
20-40 ¢
~ ~ ××
5 more expensive than other sources5 more expensive than other sources
Residential
~ 40 ¢Commercial ~ 30 ¢Industrial
~ 22 ¢
M. Green Third Generation Photovoltaics” Advanced Solar Energy Conversion, Springer Verlag, Berlin (2004).
0 200 400 6000
20
40
60
80
100
Effic
ienc
y (%
)
Cost $/m2
I$ 4/W
$ 1/W
$ 0.5/W$ 0.2/W
II
Shockley-
QuessierLimit
III
Three generations of solar cellsThree generations of solar cellsI.
Crystalline Si solar cells ($ 8/W ~ 40 ¢/kWh)II.
Thin film solar cellsIII.
Advanced future structures
Installed PV System Cost = Module Cost + Balance of System (BOS)
Future projections for existing technologiesFuture projections for existing technologies
United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005) and from J. Crystal Growth 275, 292 (2005) by T. Surek.
What do you have to do to convert photons to current?What do you have to do to convert photons to current?
Separate photogenerated
+ve
and –ve
chargesMinimize recombinationMaterial and interfacial properties control rates
A BC
e-
h+or
A Be-
h+
DoE/NREL, www.pv.unsw.edu.au
pp--nn
junction solar celljunction solar cell
Challenge is in cost reduction
New ways of making c-Si, thinner, cheaper, “solar grade”
www.semiconductor-sanyo.com
Thin film amorphous silicon solar cellThin film amorphous silicon solar cell
CuInCuIn11--xx
GaGaxx
SeSe22
(CIGS) solar cells(CIGS) solar cells
Record 19.9% efficiency achieved through empirically derived deposition
Interfaces not well understood
Relation between microstructure, composition and performance not well understood
Large area production difficult
In is scarce
Organic solar cellsOrganic solar cells
max
sc oc
PFFVI
=×
V
I
Voc
Isc
Pmax
Power
V
Pmax
max sc oc
S S
VP FF II I
η ××= =
Fill Factor
Overall Efficiency
AM1.5 2ssolar
0
I I ( ) d 1000 W / mA
∞
= λ λ ≈∫
Solar cell figures of meritSolar cell figures of merit
M. Green Third Generation Photovoltaics” Advanced Solar Energy Conversion, Springer Verlag, Berlin (2004).
0 200 400 6000
20
40
60
80
100
Ef
ficie
ncy
(%)
Cost $/m2
I$ 4/W
$ 1/W
$ 0.5/W$ 0.2/W
II
Shockley-
QuessierLimit
III
ShockleyShockley--QuessierQuessier
LimitLimit
ShockleyShockley--QuessierQuessier
LimitLimit
IncreasingEnergy
conduction band
valence band
e
h
bandgap,Eg
light
e
h
e
(1)(2)
(3)
energy lost to heat
Shockley-Queisser
limit ~ 33%
Surpassing ShockleySurpassing Shockley--QuessierQuessier
limit with limit with multijunctionmultijunction
solar cellssolar cells
www1.eere.energy.gov
Novel methods for concentratingNovel methods for concentrating
Baldo
et al. Nature 403, 750 (2000)
Design and synthesis of dyes or inorganic particles that can absorb diffuse light and reemit anisotropicaly
and efficiently
http://web.mit.edu/newsoffice/2008/solarcells-0710.html
What do you have to do to convert photons to current?What do you have to do to convert photons to current?
Separate photogenerated
+ve
and –ve
chargesMinimize recombinationMaterial and interfacial properties control rates
A BC
e-
h+or
A Be-
h+
NanostructuredNanostructured
materials are emerging as materials are emerging as potential solar cell architecturespotential solar cell architectures
Large surface and interfacial areas found in
nanostructured
materials present significant
advantages both for light absorption and for charge separation, the two critical steps in solar-to-electric energy conversion.
A
B
C
ChallengeChallenge
Find A, B and C with appropriate electronic and optical propertiesProvide means to separate charge at the A-B-C interfaceMaximize optical absorptionAssemble into high interfacial area nanostructured film Minimize premature charge recombinationCost → 0
A
B
C
HeterojunctionsHeterojunctions
between between nanostructurednanostructured
materialsmaterials
A
B
C
B
A
Some architectures that have emerged so far
Nanoparticle based dye sensitized solar cells (Gratzel, 1991)
Bulk heterojunction solar cells (Heeger, 1995 and Alivisatos, 2002)
Nanowire based dye sensitized solar cells (Baxter, 2005; Law 2005)
Nanoparticle Quantum dot sensitized solar cells (Vogel, 1990; Nozik
2003)
Nanowire quantum dot sensitized solar cells (Leschkies, 2007)
Quantum dot solar cells (Nozik, 2007, 2008)
Dye Sensitized Solar CellsDye Sensitized Solar Cells
O’Regan
& Grätzel, Nature 353, 737 (1991).Grätzel, Nature 414, 338 (2001).
Nanocrystalline, mesoporous TiO2 photoelectrode on TCO.
TiO2 is photosensitized with a monolayer of dye.
Efficient light harvesting with large dyed surface area: ~ 1000 × flat film
Heeger. Science 270, 1995.
Fulerenes blended (80 wt%) with conjugated polymer host Polymer absorbs light donates electronFullerenes are electron acceptorsBicontinuous, interpenetrating D-A heterojunction
MEH-PPV: poly[(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene]
Bulk heterojunction solar cellsBulk heterojunction solar cells
•
CdSe
nanorods replace fullerenes–
semiconductor also absorbs light –
increased aspect ratio gives better transportP3HT: poly-(3-hexylthiphene) ; PEDOT: Polyethylenedioxythiophene
Tran
spar
ent e
lect
rode
met
al
Hybrid Hybrid NanorodNanorod--Polymer Solar CellPolymer Solar CellAlivisatos. Science 295, (2002).
ChallengesChallenges
•
Can we design appropriate donors and acceptors from first principles?–
molecular structure to maximize charge transport–
molecular features to phase separate at right length scales–
appropriate energy level alignments for exciton dissociation
•
Understand and determine energy level alignments at the D-A interface
•
What is the ideal interface structure that minimizes charge recombination?
Multiple exciton generation in quantum dotsMultiple exciton generation in quantum dots
conduction band
valence band
e
hhν
= 2Eg
e e
h h
ħωEgQD
Electron states
Hole states
Colloidal QDs can generate multiple electron-hole pairs per absorbed photon.
Nozik (2003) - multiple exciton generation (MEG) in quantum dots may occur with high probability
Klimov (2005) - first demonstration of MEG in PbSe quantum dots
Challenges in making Challenges in making nanostructurednanostructured solar cellssolar cells
Challenges
create high surface area nanostructured materials that enable efficient light absorption, charge separation and charge transport
establish the fundamental scientific principles that will enable novel solar cell architectures based on nanostructured materials
Synthesis Solar cellassembly
Understanding the materials & the device physics
Design