Department of Chemistry. 235 Radiation Laboratory. Notre Dame IN 46556-0579.
USA. Quantum Dot Solar Cells: Semiconductor Nanocrystals As Light ...
1938-2
Workshop on Nanoscience for Solar Energy Conversion 27 - 29 October 2008
Quantum Dot Solar Cells: Semiconductor Nanocrystals As Light Harvesters
Prashant KAMAT University of Notre Dame Department of Chemistry 235 Radiation Laboratory Notre Dame IN 46556-0579 USA
Meeting the Energy Demand
14 TW Challenge
Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters Prashant V. Kamat Dept Of Chemistry and Biochemistry and Radiation Laboratory Dept. of Chemical & Biomolecular Engineering University of Notre Dame, Notre Dame, Indiana 46556-0579 Support: US DOE (BES) http://www.nd.edu/~pkamat
After Oil Beyond Fossil Fuels
Farming Solar
Meeting the Energy Demand
L L I R D “
D Y B A B
” L L RI
www.solarsales.com.au
World Oil Production vs. Discovery Source: Dr. C.J. Campbell
Source:Wikipedia
Can we address the clean energy challenge with Nanotechnology?
Our Research Focus Photoinduced electron transfer in light harvesting systems Donor- Acceptor Semiconductor-Sensitizer Semiconductor-Semiconductor Semiconductor-Metal
e
Molecular linker AcceptorPt
TiO
2
Donor
hν
e hν
e e
e
Ag
h
e h
h
CdSe
h
TiO2 ethanol products
e h
TiO2 e h
hν
e
e
e
C60
C60
C60
Quantum Dot Solar Cells Tunable band edge Offers the possibility to harvest light energy over a wide range of visible-ir light with selectivity
Hot carrier injection from higher excited state (minimizing energy loss during thermalization of excited state)
Multiple carrier generation solar cells. Utilization of high energy photon to multiple electron-hole pairs
M et al IT O
m Aluminu
Glass
m Aluminu
Metal Metal Junction Junction Solar Solar Cell Cell
IT O
T
Semiconductor Nanocrystals
PE DO
Redox or Hole Transport Layer
Glass Active Polymer/ Semiconductor Layer
e
h
CdSe TiO2
Polymer Polymer Solar Solar Cell Cell
e OTE
h
h
hν
Pt
Electrolyte
Quantum Quantum Dot Dot Sensitized Sensitized Solar Solar Cell Cell
1. Semiconductor/metal Interface ECB
e
EF EVB
h
–
CB
–
et
+
ht
+ hν
Red
VB
Ox
Because of smaller dimensions charge separation and transport issues in nanostructure films need to be tackled
Metal/PbSeNC/Metal Sandwich Photovoltaic Cell Aluminum
M et al IT O
PbSe NC film, deposited via layer-by-layer dip coating
Glass
Short-circuit photocurrent (>21 mA cm-2) by way of a Schottky junction EQE of 55-65% in the visible and up to 25% in the infrared region Power conversion efficiency of 2.1%.
Luther et al Nano Lett., Vol 8, 2008, 3488
Redox or Hole Transport Layer
Semiconductor Nanocrystals
IT O
PE DO
m Aluminu
T
2. Polymer –Semiconductor Nanocrystal Hybrids
Glass Active Polymer/ Semiconductor Layer
Adv. Mater, 2004, 16, 1009-1013
O
Hybrid Nanorod-Polymer Solar Cells IT
PE DO T
Aluminum
Glass Active Polymer/ Semiconductor Layer
SCIENCE VOL 295, 2002 2425
3. Quantum Dot Sensitized Solar Cell (QDSSC)
e
hν
CdSe
TiO2
CB
e hν
e
CB
CdSe TiO2 VB VB
e
e
TiO2
e
h
e
e
e
e e
e
e h
e h
e
h
e h
CdSe
h
e e
Red
h
h
Ox
h
hν
Photoelectrochemistry
Spectroscopy detector
e R O
hν pump
–
CB
probe
CB
–
et
ht
+
TiO2 VB
+ VB CdSe
Red Ox
GERISCHER H, LUBKE M A PARTICLE-SIZE EFFECT IN THE SENSITIZATION OF TIO2 ELECTRODES BY A CDS DEPOSIT JOURNAL OF ELECTROANALYTICAL CHEMISTRY 204 (1-2): 225-227 1986
hν
detector
Photoexcitation of CdSe Quantum Dots CB
pump
probe
1P(e)-1P3/2(h)
0.5 1S(e)-1S3/2(h)
-0.04
0
0
440 nm 530 nm Time, ps
440 nm 530 nm
c
3x
d
0.5 1.0 Time, ps
400
a b c d e
450
Δ OD
ΔOD
ΔOD
CdSe
a b
-0.08
Absorbance
VB
0.2 ps 0.3 ps 0.4 ps 0.5 ps 1.1 ps
0.5
e 500
3x
550
Wavelength, nm
600
650
1.0
Charge Separation in TiO2/CdSe Transient Bleaching Recovery of 3 nm CdSe Quantum Dots Ex. 387 nm
B
CdSe-MPA
C
CdSe-MPA-TiO2
0.00
0.00
ΔA
ΔA
-0.04
1 ps 35 ps 400 ps 1500 ps
-0.08
450
500
550
Wavelength, nm
600
-0.04 1 ps 35 ps 400 ps 1500 ps
-0.08
650
450
500
550
Wavelength, nm
600
650
Modulation of the charge injection process by controlling the particle size?
CdSe e Slow Eg=1.9 eV h
TiO2
e CdSe 7.5 nm
CdSe e
Fast
Eg=2.4 eV h
TiO2 e
e TiO2
CdSe 2.4 nm
Photoexcitation of CdSe Quantum Dots CdSe +hν → CdSe (ep +hp) → CdSe (es + hs) (A) Absorbance
400
(B)
Wavelength, nm 500 600
700
CdSe quantum dots of size 2.4 nm to 7,5 nm were excited with 387 nm laser pulse (130 fs)
0.4 0.2 e
0.0
d
c
b
a
As the particle size decreases from 7.5 nm to 2.4 nm, the first (1S3/21Se) excitonic peak shifts from 645 nm (1.92 eV) to 509 nm (2.44 eV).
0.00
ΔA
-0.02
a e
-0.04
d CdSe a 7.5 nm (Eg 1.92 eV) b 4.6 nm (Eg 1.99 eV) c 3.5 nm (Eg 2.18 eV) d 2.7 nm (Eg 2.35 eV) e 2.4 nm (Eg 2.44 eV)
-0.06 400
Transient bleach corresponds to the first excitonic bleach
c b
500 600 Wavelength, nm
700 Robel, Kuno, Kamat, JACS 2007; 129, 4136
Electron transfer between CdSe and TiO2
0.8
a size 7.5 nm λ(1S)=645 nm
b
0.0
CdSe (es) + TiO2 → CdSe + TiO2(e)
Normalized bleach
Analysis of Bleaching Recovery
a
0.8
ΔA(t)=ΔA(0)×exp[-(t/τ)β] - where τ is the peak value of the characteristic lifetime
size 4.6 nm λ(1S)=605 nm
b
0.0 0.8
0.0
a size 3.5 nm λ(1S)=570 nm
b
0.8
a
size 2.7 nm 0.0 λ(1S)=527 nm
b
0.8
a size 2.4 nm λ(1S)=509 nm b
0.0 0 Robel, Kuno, Kamat, JACS 2007; 129(14) pp 4136 - 4137
500
1000
Time (ps)
1500
Size Dependent Quenching Phenomenon
Diameter
Eg
τCdSe
[nm]
[eV]
[ps]
7.5
1.92
2332
0.697
2281
0.755
9.58×106
4.6
1.99
7224
0.474
4961
0.446
6.3×107
3.5
2.18
4420
0.417
1117
0.475
6.7×108
2.7
2.35
6739
0.457
357
0.505
2.65×109
2.3
2.44
23119
0.51
83
0.493
1.2×1010
ΔA(t)=ΔA(0)×exp[-(t/τ)β]
βCdSe
τCdSe-TiO2
βCdSe-TiO2
[s-1]
[ps]
- where τ is the peak value of the characteristic lifetime
ket
1/τ’ – 1/τ = ket
Size Dependent Electron transfer between CdSe and TiO2 ket= 107 s-1
ECB, V vs. NHE -0.9 -1.1 10
e
-1.3
CdSe 7.5 nm
TiO2
CdSe 2.4 nm
2.7 nm
9
10
3.5 nm
-1
ket, s
e
2.4 nm
10
ket= 1.2x1010 s-1
8
10
4.6 nm 7
10
7.5 nm
h 0.4
Eg=2.4 eV
Eg=1.92 eV
6
10
e
e
0.6 -ΔG ,eV
0.8
Robel, Kuno, Kamat, JACS 2007; 129 pp 4136 - 4137
CdSe 7.5 nm
TiO2
h CdSe 2.4 nm
Linking Q-CdSe to TiO2 particles a
e
b c
d
e
CdSe TiO2 hν -0.75
Electrode
nm 0 HOOC
OOC SH
SH
HOOC SH
SH
HOOC
OOC SH
OTE
Bifunctional linker molecule
TiO2
-0.75
OOC SH
2 μm
Chemically Modified TiO2 film
150 nm
OOC OOC
SSH
OOC
SH
OOC
OOC
S-
OOC
SSH
0
CdSe OTE/TiO2/CdSe Electrode
J. Am. Chem. Soc. 2006,128, 2385-2393
0
2 μm
Photoelectrochemical behavior of Q-CdSe-TiO2 films
Potentiostat WE
CE
Voltage vs. SCE, V
RE
TiO2
R O
hν
Photocurrent, μA
0.2
e
0 0.0
-0.2 TiO2
-0.4
-0.6
-0.8
a
100
b 200
TiO2/CdSe
I-V characteristics of (a) OTE/TiO2 and (b) OTE/TiO2/MPA/CdSe films. Electrolyte 0.1 M Na2S. The filtered lights allowed excitation of TiO2 and CdSe films at wavelengths greater than 300 and 400 nm respectively
-1.0
Modification of TiO2 Films with Different Size CdSe Particles
2.6nm
3.7nm 3.7 nm 3.0 nm 2.6 nm 2.3 nm
0.6
Absorbance
2.3nm 3.0nm 3.7nm 2.6nm
3.0nm
0.4
0.2
0.0 300
3.7 nm 3.0 nm 2.6 nm 2.3 nm
0.9
Absorbance
2.3nm
0.6
0.3
0.0
400
500
600
Wavelength, nm
700
450
500
550
600
Wavelength, nm
650
700
Tuning the Photoresponse of Quantum Dot Solar Cells e IPCE or Ext. Quantum Eff. R
hν
= (1240/λ) x (Isc/Iinc) x 100
O
E 50
3.7 nm 3.0 nm 2.6 nm 2.3 nm
(A)
IPCE (%)
40
e
h
h
e
CB
30
h
20
h
RO
10
VB
CdSe
TiO2
0 350
e
e
400
450
500
550
600
650
700
Wavelength (nm) J. Am. Chem. Soc., 130 (12), 4007 -4015, 2008
Photocurrent Response Efficiency of Charge Injection vs. Light absorption
e 2
Solar Flux
O
hν
Current density, mA/cm
R
3.7 nm 3.0 nm 2.6 nm 2.3 nm
(A)
3
2
1
0 0
50
100
Time, sec
150
Can we employ the nanowire/nanorod architecture to improve the performance of quantum dot solar cells? CdSe CdSe
e
e
e
e
TiO2
OTE/TiO2 nanoparticles
Ti/TiO2 nanotubes
(a)
(b)
Recent advances Nanowire dye-sensitized solar cells LAW, GREENE, JOHNSON, SAYKALLY, YANG Nature Materials 4 , 455, 2005
Fast Electron Transport in Metal Organic Vapor Deposition Grown Dye-sensitized ZnO Nanorod Solar Cells Galoppini, Rochford, Chen, Saraf, Lu, Hagfeldt, and Boschloo J. Phys. Chem. B; 2006; 110 16159
Electron transport in solar cells with ZnO-nanorod electrodes was about 2 orders of magnitude faster (30μs) than ZnO-colloid electrodes
Mor, G. K. et al Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells.
Leschkies, K. S et al Photosensitization of ZnO nanowires with CdSe quantum dots for photovoltaic devices.
Nano Lett., 2006. 6, 215-218.
Nano Lett., 2007. 7, 1793-1798.
Martinson, A. B. F. et al., ZnO nanotube based dye-sensitized solar cells ZnO nanotube based dye-sensitized solar cells. Nano Lett., 2007. 7, 2183-2187.
B
A
5μm
200nm C
D
2μm
500nm
(B) (A)
2.3nm 3.0nm 3.7nm 2.6nm
2.3nm
3.0nm 2.6nm
3.7nm
(C) 2.3nm 3.0nm 3.7nm 2.6nm
(B) 3.7 nm CdSe Intensity (a.u.)
10000
Photoemission of CdSe/glass and CdSe/TNP
Intensity (a.u.)
1.2
2.6 nm diameter a. CdSe/glass b. CdSe/TiO2
3.7 nm diameter c. CdSe/glass d. CdSe/TiO2
CdSe/glass
1000 CdSe/TiO2(NP)
100
CdSe/TiO2(NT)
10 prompt
1 0
5
10
15
20
Time, ns
c
a
0.8
, ns CdSe/ glass
0.4 b ×3
0.0 500
550
d ×3
600
650
700
Wavelength, nm
Emission spectra of CdSe QDs (a, c) on glass and (b, d) chemically bound to TiO2 nanoparticle films at 2 different sizes of QDs (2.7 and 3.7 nm). Excitation was at 480 nm.
CdSe/ tnp CdSe/ tnt
ket, sec-1
2.6nm
4.1
3.7nm
7.9
2.6nm
0.4
2.5E+09
3.7nm
1.3
6.3E+08
2.6nm
0.4
2.2E+09
3.7nm
1.5
5.5E+08
25
Quantum Dot Solar Cells – Particle versus Tube Architecture (A) TiO2(NP)
CdSe Diameter 3.7 nm a. 3.0 nm b. 2.6 nm c. 2.3 nm d.
IPCE (%)
40 d
30
50
c
20
(B) TiO2 (NT)
40
IPCE (%)
50
b
CdSe Diameter 3.7 nm a. 3.0 nm b. 2.6 nm c. 2.3 nm d.
d c
30 b
20 a
a
10
10 0
0 350
400
450
500
550
600
650
700
350
400
450
500
550
600
650
700
Wavelength (nm)
Wavelength (nm) 200nm
500nm
ISC mA/cm2
VOC V
Pmax mW/cm2
FF
CdSe-TNP
1.64
0.591
0.25
0.26
CdSe-TNT
1.95
0.582
0.29
0.26
J. Am. Chem. Soc., 130 (12), 4007 -4015, 2008
Power conversion efficiency ~1%
Depositing CdS quantum dots on TiO2 nanotubes
Unsonicated TiO2 NT
0 dip
5 dip
10 dip
20 dip
100kX
500kX
Scale bars: Top 500nm, bottom 50nm
(TiO2)
Cd2+
(TiO2)Cd2+
Wash
S2-
1 dip cycle
(TiO2)CdS
Wash
Photocurrent Response of TiO2 (nanotube)CdS Films IPCE: CdS on Single TiO2 electrode 60
R
50
hν
O Tubes 5 dip
30
10 dip 20 dip
20 10
0 350
400
450
500
550
600
Wavelngth (nm) Reflectence Absorbtion Spectra: CdS/P25/OTE 1 0.9 0.8 0.7 Absorbance
IP C E (% )
40
TiO2
0.6
5dip 0.5
10dip 15dip
0.4
20dip 0.3 0.2 0.1 0 350
400
450
500
Wavelength (nm)
550
600
Carbon nanostructures as conduits to transport charge carriers Advantages ¾ High surface area ¾ Good electronic conductivity, excellent chemical and electrochemical stability ¾ Good mechanical strength
Pt
Goal Effective utilization of carbon nanostructures for improving the performance of energy conversion devices - To develop electrode assembly with CNT supports - Improve the performance of light harvesting assemblies - Facilitate charge collection and transport in nanostructured assemblies
…..towards achieving ordered assemblies on electrode surface
c
SWCNT- TiO2 composite films • Mesoscopic TiO2 films are extensively used in Dye-Sensitized Solar Cells • A carbon nanotube support architecture can disperse the TiO2 particles and facilitate charge collection and charge transport within the film.
e e – hν
et
CB
– +
ht
+
VB
• The first step is to design the SWCNTTiO2 network and test the feasibility of the composite system in solar cells
Kongkanand, A.; Domínguez, R.M.; Kamat, P.V., Single Wall Carbon Nanotube Scaffolds for Photoelectrochemical Solar Cells. Capture and Transport of Photogenerated Electrons. Nano Lett., 2007. 7, 676-680. Vietmeyer, F.; Seger, B.; Kamat, P.V., Anchoring ZnO Particles on Functionalized Single Wall Carbon Nanotubes. Excited State Interactions and Charge Collection. Adv. Mater., 2007, 19: 2935-2940
Electrophoretic Deposition of SWCNT on Electrode Surfaces
Sonication for 1 min SWCNT+TOAB in THF
After Electrodeposition on OTE
SWCNT/TOAB in THF
OTE/SWCNT
0.8 B
0.6
OTE/SnO2/SWCNT
Abs 0.4
b OTE/SWCNT
200 nm
a
0.2 OTE/SnO2
0.0 400
c
600 800 1000 Wavelength, nm J. Am. Chem. Soc., 2004. 126 10757-10762.
A Carbon Fiber Paper (CFE) CFE
50 μm
5 μm
B TiO2 Deposition On CFE CFE/TiO2
50 μm
5 μm
50 μm
1 μm
50 μm
1 μm
C SWCNT –Deposition On CFE CFE/SWCNT
D SWCNT-TiO2 on CFE CFE/SWCNT/TiO2
Photocurrent Generation Photocurrent, μA/cm
2
CFE/TiO2 versus CFE/SWCNT –TiO2 60
a) SWCNT/TiO2 b) TiO2
a
40
b 20
0 0
20
40
60
Time, sec
UV light
The results are indicative of better charge collection and transport provided by the SWCNT -Network
IPCE, %
Higher IPCE (increase of factor ~2) was observed for mesoscopic CFE/SWCNT-TiO2 films
a
15
10
a) SWCNT/TiO2 at 0V b) SWCNT/TiO2 at SC c) TiO2 at 0V d) TiO2 at SC
b c
5
d
0 350
400
450
500
Wavelength, nm
Nano Lett., 2007. 7, 676-680
Dependence of TiO2/SWCNT Ratio on the Photocurrent Generation
2
Photocurrent (μA/cm )
60
a
50 40
b
30
1 μm
20 a) SWCNT/TiO2 b) TiO2
10 0
0
1
2
3
4 2
TiO2 loadings (mg/cm )
Increasing the TiO2 concentration results in enhanced photocurrent as they are dispersed on SWCNT network. At concentrations greater than 2 mg/cm2 the beneficial effect of SWCNT disappears. Under these conditions. TiO2 particles aggregate and the charge recombination dominates Nano Lett., 2007. 7, 676-680
Where do we go from here?
Capping CdSe with an Electron Acceptor Shell
Electrophoretic deposition of Cluster films
CdSe
1 μm
CdSe C60
1.2 μm
0
0 1 μm
0 5
4
IPCE (%)
C60
(a)
nC60
(b) (c)
CdSe CdSe/nC60
c
3
2
b
1 a 0 350
400
450
500
550
600
650
700
Wavelength (nm)
J. Am. Chem. Soc., 2008, 130, 8890–8891
Organized light harvesting assembly using carbon nanostructures hν
e h
1 μm
e
15 nm e h
e h
e
e
0
0 1 μm
0
Graphene-Semiconductor Nanocomposites
ACS Nano, 2008, 2, 1487-1491
hν h TiO TiO ‐GO 2 2
TiO2-GR
Graphene Oxide
hν
0.6 μm
Reduced Graphene
hν
e
Summary
•
Unique properties of quantum dots offer new opportunities to develop low-cost and high efficiency solar cells
•
1-D architectures are useful for designing next generation solar cells.
•
Opportunities exist for carbon nanostructures to facilitate capture and transport of electrons in nanostructure semiconductor based solar cells.
Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion (Review) J. Phys. Chem. C, 2007. 111 2834 - 2860. Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters (Centennial Feature) J. Phys. Chem. C 2008, 112, in press
Researchers/Collaborators Graduate students
Brian Seger (Chem. Eng.) David Baker (Chem. Eng.) Kevin Tvrdy (Chemistry) Clifton Harris (Chemistry) Matt Baker (Physics) Ian Lightcap (Chemistry) Philix Vietmeyer (Chemistry) Yanghai Yu (Chem. Eng.) Istvan Robel (Physics)
Collaborators Dr. K. G. Thomas (India) Prof. Fukuzumi (Osaka U.) Prof Ken Kuno (UND) Prof. K. Vinodgopal (IUN)
Post-Docs/Visiting Scientists
Jin Ho Bang
Undergraduate students
Pat Brown Chris Rodriguez David Riehm Rachel Staran
What will the future hold? Over the last twenty years, the per-kWh price of photovoltaics has dropped from about $500 to nearly $5; think of what the next twenty years will bring.
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