1. University of Pittsburgh
Ordered TiO2 Nanotube Arrays
on TCO for Dye-sensitized Solar Cells
CHENGKUN XU AND DI GAO*
* gaod@pitt.edu
DEPARTMENT OF CHEMICAL AND PETROLEUM ENGINEERING
UNIVERSITY OF PITTSBURGH
12/2/2010 1

2. Current Dye-sensitized Solar Cells (DSC)
Current DSC anode structure
e mesoporous film
a random TiO2 nanoparticle network
a disordered pore structure
I- I- Consequences:
TCO − − 1. Slow electron transport;
I3 I3
2. Poor electrolyte filling,
hampering further improvement in
Dye coated TiO2 DSC performance.
nanoparticles

3. Vertically Ordered Nanostructures for DSC
A promising solution: replace the nanoparticle film with vertically
aligned nanowire or nanotube arrays.
Advantages:
e
Vertically ordered nanostructures provide:
1. A direct pathway for electron transport, reducing
the probability of electron recombination.
2. A straight channel for electrolyte filling, increasing
the pore filling of solid state DSC with electrolytes .

4. Challenges of Fabricating
Vertically Ordered Nanostructures
TiO2 nanotube arrays on titanium substrate
an illumination on the platinum deposited
counter-electrode side has to be used.
Ti Substrate
M. Paulose et al. Nanotechnol.
17 (2006) 1446–1448
about 20% loss of incident photons
Ordered nanostructure must be fabricated
directly on transparent conducting oxides (TCO)

5. Current Ordered Nanostructures on TCO
η:6.95%
X. Feng et al. Nano Lett. B. Liu et al. JACS O. Varghese et al. Nat.
2008, 8, 3782 2009, 131, 3985 Nanotechnol. 2009, 4, 592.
• The efficiencies: 3 to 7%, still low compared with
nanoparticle-base DSC.
• The cause: low surface area and thus low light absorption.
• Must fabricate high surface area vertically ordered
nanostructures on TCO at a low cost.

6. Solution-based synthesis of TiO2 nanotube arrays
directly on TCO
a b c d
ZnO seed crystal
TiO2 underlayer
1 2 3
ITO substrate ZnO nanowire arrays Top-end closed Top-end opened
TiO2 nanotube arrays TiO2 nanotube arrays
Growth of ZnO nanowire arrays on TCO:or Zn2+ + 2OH- → ZnO + H 2O
Deposition of TiO2 on ZnO nanowires: TiF62− + 2H 2O = TiO 2 + 6F − + 4H +
Dissolution of ZnO nanowires: ZnO + 2H + = Zn 2+ + H 2O
Etching of the top ends of the tubes: TiO 2 + 6F − + 4H + = TiF62− + 2H 2O

7. Morphology Characterization and DSC performance
5 µm
10-11 µm long 5 µm
300-500 nm wide 5 µm 1 µm
1.6×109 wires/cm2
×
Chengkun Xu and Di Gao, Chem. Mater. 2010, 22, 143–148

8. DSC performance
8
60
7
Current Density (mA/cm )
2
6 50
IPCE (%)
5 40
4 VOC: 0.810 V
2
30
3 ISC: 6.8 mA/cm
FF: 0.655 20
2
η : 3.6% 10
1
0 0
0.0 0.2 0.4 0.6 0.8 1.0 400 500 600 700
Voltage (V) Wavelength (nm)
• Demonstrating the success of our fabricating approach.
• Higher surface area arrays required.
Chengkun Xu and Di Gao, Chem. Mater. 2010, 22, 143–148

9. Growth of ZnO nanowire arrays
Conventional solution based method:
ZnO forms simultaneously on seeded substrates
and in the bulk solution.
Rapid depletion of reactants
Slow growth rate of wires on substrates
Time consuming
It is challenging to grow ZnO nanowire arrays longer than 10 µm.
Chengkun Xu and Di Gao, J. Phys. Chem. C 2010, 114, 125–129

10. Our approach to growing ZnO nanowire arrays:
Added NH4OH reducing solution supersaturation.
Added polyethyleneimine (PEI) adsorption
In the bulk solution:
ZnO homogeneous nucleation completely suppressed.
On seeded substrates
Nanowires can normally grow.
Chengkun Xu and Di Gao, J. Phys. Chem. C 2010, 114, 125–129

11. 10 times faster growth rate using our approach
16
14
Wire Length (µm)
12
Our approach
10
8
6
Solution bath refreshed
4
2
0 Conventional process
0 1 2 3 4 5 6 7 8 9 10
Growth Time (h)
Chengkun Xu and Di Gao, J. Phys. Chem. C 2010, 114, 125–129

12. Growth of long TiO2 nanotube arrays
• Reduced reactant concentration
• Increased solution volume to substrate area ratio
• Radial growth of the wires is mass transport limited
VOC: 0.820 V
14
Current Density (mA/cm )
2
2
ISC: 12.2 mA/cm
12 FF: 0.613
η: 6.1%
10
80
70
8 60
IPCE(%)
50
6 40
30
4 20
10
2 0
400 500 600 700
Wavelength (nm)
0
0.0 0.2 0.4 0.6 0.8
Voltage (V)

13. Electron recombination kinetics
a
0.9 Light on (100 mW/cm )
2
Open circuit voltage (V)
Dye 0.8
CB 0.7 Light off TiO2 tube cell
T iO2 particle cell
0.6
0.5
0.4
Voc
0.3
0.2
I3 I−
−
0.1
Pt/TCO
TiO2 0.0
0 50 100 150 200 250 300
Time (s)
Electron lifetime τ n = −( k BT e)( dVoc dt ) −1
Nanotube DSC Nanoparticle DSC
τn(s) ~1 ~0.1-0.2
Chengkun Xu and Di Gao, Chem. Mater. 2010, 22, 143–148

14. Formation of Potential Barrier within the tube wall
n-type
E
EF,n
C
Eg
EV Solution 20 nm
50-100 nm
Implication: allow the use of kinetically fast hole transporting
materials and thus make it possible to fabricate
efficient solid state DSC.

15. Summary
• We have developed a strategy for synthesizing vertically
ordered and long TiO2 nanotube arrays directly on TCO.
• The processes involve solution-based growth of ZnO nanowire
arrays on TCO and subsequent aqueous conversion into TiO2
nanotube arrays.
• All the synthesis steps utilize wet chemical processes, which
feature low cost, low temperature, and ease in scale-up.
• The approach has a potential of being further improved and opens
up opportunities of fabricating efficient and economically viable
dye-sensitized solar cells.