Eray S. Aydil
Chemical Engineering and Materials Science Department
Acknowledgements: National Science Foundation NIRT ProgramMinnesota Initiative for Renewable Energy and the Environment (IREE)
ZnOZnO nanowire based solar cellsnanowire based solar cells
Prof. Uwe Kortshagen
Prof. David Norris
Kurtis LeschkiesJanice Boercker
Prof. C. Barry Carter
Dr. Joysurya Basu
Emil Enache-Pommer
Dr. DivakarRamachandran
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.
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
Dye-sensitized nanowire solar cell
Wide band gap semiconductor nanowires with adsorbed dye monolayer
Light
Liquid electrolyte or hole conductor e-e-
Nanowires provide a direct path to the substrate for efficient charge collection.
Device optical absorption spectrum can be tuned through selection of QD material and size.
400 450 500 550 600 650Wavelength (nm)
Abs
orba
nce
(A.U
.)
Increasing size of QD
ħωEgQD
Electron states
Hole states
Colloidal QDs can generate multiple electron-hole pairs per absorbed photon.
(1) Baxter et al., Nanotechnology, 2006. (2) Law et al., Nature Mater., 2005. (3) Nozik, Physica E, 2003. (4) Schaller et al., Phys. Rev. Lett., 2005. (5) Ellingson et al., Nano Lett., 2005.
Emerging alternatives to Emerging alternatives to DSSCsDSSCs
How can we attach colloidal QDs onto the surface of the nanowires?
Can the QDs harvest light while attached to the nanowires?
Will the QDs transfer photogenerated electrons to the nanowires?
What factors limit the overall performance of these devices?
e-e-
Quantum dots attached to nanowires
Liquid electrolyteor hole conductor
Quantum dots attached to nanowires
Liquid electrolyteor hole conductor
CdSe quantum dots attached to ZnO nanowires
Light
0.05 M I2, 0.05 M LiI, 0.5 M tert-butylpyridinein short chain hydrocarbons
CdSe quantum dots attached to ZnO nanowires
0.05 M I2, 0.05 M LiI, 0.5 M tert-butylpyridinein short chain hydrocarbons
Light
Electrochemical Scale (V vs. NHE)
8.0 +3.5
3.0
4.0
5.0
6.0
7.0
-1.5
-0.5
+0.5
+1.5
+2.5
Solid State Scale (eV)
ZnO5 nm
CdSe QDs Redox
I- / I3-
-
-
)
-
-
)
ZnO
-3--
3-
QuantumQuantum--dotdot--sensitized solar cellssensitized solar cells
Low temperature Low temperature ZnOZnO nanowire growth nanowire growth from zinc nitrate and from zinc nitrate and MethenamineMethenamine solutionsolution
2 µm
0.025 M zinc nitrate, Zn(NO3)2
0.025 M methenamine
95 ºC
30 32 34 36 38 400
400
800
1200
1600
2000
Inte
nsity
2θ (degrees)
(000
2)
(101
0)
(101
1)
200 nm
XRD
Baxter, Walker, van Ommering, and Aydil, Nanotechnology 17, S304 (2006).
100 nm
[0001][1120]
Control of Control of ZnOZnO nanowire dimensionsnanowire dimensions4 cycles
1 µm
1 cycle
1 µm 2 µm
8 cycles
100 nm
0 2 4 6 80
2
4
6
8 684528 Aspect Ratio
Number of Growth Cycles
Leng
th (µ
m)
13
0
100
200
300
400
500
Diam
eter (nm)
CdSeCdSe quantum dotsquantum dots“One-pot synthesis” from CdO and tri-n-octylphosphine selenide (TOPSe) in hexadecylamine(HDA) with tri-n-octylphosphine oxide (TOPO),
QDs are single-crystals with diameters of 3–4 nm and surface-coordinated with TOPO and HDA.
4000 3500 3000 2500 2000 1500 10000
0.02
0.04
0.06
0.08
0.10
0.12
stretching
P=O stretchingN-H
x2
x10
Wavenumber (cm-1)
Abs
orba
nce
O=PTOPO:
H2NHDA:
CdSeCdSe quantum dot size controlled by reaction timequantum dot size controlled by reaction time
350 400 450 500 550 600 650
Wavelength (nm)
Abs
orba
nce
(AU
)
1.9 nm2.3 nm2.6 nm3.7 nm4.0 nm5.1 nm
350 400 450 500 550 600 650Wavelength (nm)
Abs
orba
nce
(AU
)
'As-synthesized' with TOPO ligands
Post surface exchange with MPA
Replace Alkyl ligands on Replace Alkyl ligands on CdSeCdSe with Xwith X--RR--YY
SCH2CH2COOHSCH 2CH 2C
OOH
SCH 2C
H 2CO
OH
SCH
2CH
2CO
OH
SCH2CH
2COOH
SCH2CH2COOH
SCH2CH2COOH SCH2CH2COOH
SCH
2CH
2CO
OH
SCH
2 CH
2 CO
OH
SCH2 CH
2 COOH
SCH2CH
2COOH
Hexane phase
Methanol phase
CH3 stretching from [N(CH3)4]+
3500 3000 2500 2000 1500 1000
0
0.002
0.004
0.006
0.008
0.010
COO–
asymmetric
CH3deformation
from [N(CH3)4]+COO– symmetric
x0.5 x0.25
Wavenumber (cm-1)
Abs
orba
nce
(A.U
.)
C=O from COOH
CdSeCdSe quantum dots adsorbed on quantum dots adsorbed on ZnOZnO NanowiresNanowires
Attaching quantum dots to Attaching quantum dots to ZnOZnO nanowiresnanowires
CdSeCdSe quantum dots on quantum dots on ZnOZnO nanowiresnanowires
0.14 nm
0.14 nm
Optical absorption of Optical absorption of CdSeCdSe QDsQDs on on ZnOZnO nanowiresnanowires
OOC-CH2CH2S-
OOC-CH2CH2S-
OOC-CH2CH2S-
QD optical absorption was preserved when QDs were attached to the nanowires.
400 500 6000
0.05
0.10
0.15
0.20
ColloidalCdSe QDs
CdSe QDson ZnO
ZnO
Wavelength (nm)
Film
Abs
orba
nce
(A.U
.)
CdSeCdSe quantum dot sensitized solar cellsquantum dot sensitized solar cells
Photovoltaic effect observed
0
40
80
120
160
0
0.5
1.0
1.5
350 400 450 500 550 6000
0.5
1.0
1.5
2.0
(b)
ZnO
CdSe QDs on ZnO
Phot
ocur
rent
(µA/
cm2 )
Abso
rban
ce
(a)
(c)
Wavelength (nm)
Irrad
ianc
e (W
m-2 n
m-1)
0
40
80
120
160
0
0.5
1.0
1.5
350 400 450 500 550 6000
0.5
1.0
1.5
2.0
(b)
ZnO
CdSe QDs on ZnO
Phot
ocur
rent
(µA/
cm2 )
Abso
rban
ce
(a)
(c)
Wavelength (nm)
Irrad
ianc
e (W
m-2 n
m-1)
0 0.2 0.4 0.6 0.8-3
-2-1
0
12
3
0 0.1 0.2 0.3 0.4
-0.10
-0.05
0
0.05
0.10
no O2 plasmawith O2 plasma
Potential (V)
Curr
ent D
ensi
ty (m
A/cm
2 )
ZnO
Electron transferred from QD to nanowire
CdSeCdSe quantum dot sensitized solar cellsquantum dot sensitized solar cells
43421IQE
LHEIPCE collinj ϕϕ ××=
injϕ
LHE
collϕ
450 500 550 6000
15
30
45
60
IPCE
LHE (Light harvesting efficiency)
Wavelength (nm)LH
E or
IPC
E (%
)
Internal quantum efficiency (IQE) as high as 45–58% between 500–600 nm.
Power conversion efficiency limited by the LHE and available nanowire surface area.
SummarySummary
CdSe quantum dots and ZnO nanowires form a new type of QDSSC.
Photogenerated electrons from CdSe transferred into ZnO.
Isc ≈ 2 mA/cm2, Voc ≈ 0.6 V, FF ~ 0.3 for typical QDSSCs. IQE as high as 58%.
Nanowire QDSSCs may be a promising solar cell design.
Replace liquid electrolyte.
Increase roughness factor of nanowires.