University of Pittsburgh Ordered TiO2 Nanotube Arrays on TCO for Dye-sensitized Solar Cells CHENGKUN XU AND DI GAO* * firstname.lastname@example.org DEPARTMENT OF CHEMICAL AND PETROLEUM ENGINEERING UNIVERSITY OF PITTSBURGH12/2/2010 1
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
Vertically Ordered Nanostructures for DSCA 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 .
Challenges of FabricatingVertically Ordered Nanostructures TiO2 nanotube arrays on titanium substrate an illumination on the platinum deposited counter-electrode side has to be used. Ti SubstrateM. 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)
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.
Solution-based synthesis of TiO2 nanotube arraysdirectly 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 arraysGrowth of ZnO nanowire arrays on TCO:or Zn2+ + 2OH- → ZnO + H 2ODeposition of TiO2 on ZnO nanowires: TiF62− + 2H 2O = TiO 2 + 6F − + 4H +Dissolution of ZnO nanowires: ZnO + 2H + = Zn 2+ + H 2OEtching of the top ends of the tubes: TiO 2 + 6F − + 4H + = TiF62− + 2H 2O
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
DSC performance 8 60 7Current 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
Growth of ZnO nanowire arraysConventional 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 consumingIt is challenging to grow ZnO nanowire arrays longer than 10 µm. Chengkun Xu and Di Gao, J. Phys. Chem. C 2010, 114, 125–129
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
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
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)
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
Formation of Potential Barrier within the tube wall n-type E EF,n C Eg EV Solution 20 nm 50-100 nmImplication: allow the use of kinetically fast hole transporting materials and thus make it possible to fabricate efficient solid state DSC.
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.