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Depositing HfO2 Thin Films by ALD
for Photovolatics
Luping Li
Department of Chemical Engineering
University of Florida
Footprints…
2
Outline
• Background
• Atomic layer deposition
• HfO2 thin films/characterizations
• Applications in dye-sensitized solar cells • Nanoparticle DSSCs
• Nanowire DSSCs
3
Introduction & Motivation
Dye sensitized solar cells (DSSCs)
• Photoanode: FTO glass with a TiO2-
nanoparticle thin-film loaded with dye
molecules
• Dye inject e- to TiO2 to FTO to external
circuit
Electron recombination greatly reduces
device efficiency
• Back flow of e- reduces current density
• Major pathways – FTO to electrolyte,
TiO2 to electrolyte/dye
High bandgap thin films can suppress
electron recombination
4
Introduction to ALD
• ALD is an alternating, self-
limiting CVD thin-film process
with sub-nanometer precision
• Thermal ALD
• Plasma ALD
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Pros Cons
Lower deposition T More complicated chamber
Broader range of chemistry possible More complicated reactions
Denser film Potentially poorer conformality
Higher purity
Higher throughput
Potential damage to films
Plasma ALD Characteristics
Liu et al., J. Electrochem. Soc., 152 (2005) G213-G219.
ALD Growth Characteristics
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Kim et al. Thin Solid Films 519 (2011) 6639–6644.
Intermediate T: Sufficient
thermal energy - process
window
Low T: Slow growth
High T: Fast growth
Red: Plasma ALD
Different Plasma Systems
Direct plasma ALD
Simple chamber
Plasma damage
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Remote plasma ALD
Independent plasma
Versatility
Radical enhanced ALD
Controlled plasma density
Sensitive substrate
Kim et al., J. Appl. Phys. 98 (2005) 094504.
Home-Made ALD System
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Characteristics
• Direct plasma ALD
• Thermal/plasma dual mode
• SS 316L construction
• Up to 450 ⁰C
• MFC up to 1000 sccm
• LabView controlled
Cross-Sectional TEM
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Deposition conditions • Thermal mode
• T=200 ⁰C
• H2O pulse 0.06 s
• Purge 60 s
• Hf pulse 0.1 s
• Purge 30 s
• Cycles = 140
TEM: JEOL 2010F
Results • Thickness = 13 nm
• Rate: 0.9 Å/cycle
HfO2 thin film has been successfully deposited!
Thermal vs Plasma: XPS
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Hf 4f
• A Perkin Elmer 5100 XPS system was used
• Thermal and plasma mode deposited HfO2 films with the same chemistry
Thermal vs Plasma: Growth Rate
• A Horriba UVISEL 2
ellipsometer was used
• Low T has high rate:
Condensation
• High T has high rate:
Decomposition
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Plasma mode
Wider process window
Higher growth rate
Reaction Mechanism
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2nd reaction
Thermal mode
Plasma mode
1st reaction
Same
Plasma ALD: AFM
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• Used a Digital Instruments Dimension 3100 AFM
• Lower plasma power resulted in smoother surface
Plasma ALD: Resistivity
• Used a 4-point probe
technique
• 20 nm thick HfO2 film
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Resistivity decreased with longer plasma time
Are they helpful in DSSCs…
Cell Efficiency: HfO2 on ITO
• Control has an efficiency of 6.45%
• Voc and Jsc increase with the
number of ALD cycles for up to 2
cycles
• Highest efficiency is 7.09% (10%
increase)
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Voc [V] Jsc [mA·cm-2] FF [%] η [%]
Control 0.66±0.02 15.01±0.06 65.0±0.6 6.45±0.03
1 Cycle on ITO 0.69±0.02 15.91±0.07 63.0±0.7 6.88±0.04
2 Cycle on ITO 0.68±0.02 16.38±0.08 63.5±0.7 7.09±0.03
4 Cycle on ITO 0.70±0.02 16.09±0.05 60.8±0.5 6.83±0.05
6 Cycle on ITO 0.66±0.01 15.73±0.05 62.9±0.8 6.51±0.05
HfO2 on TiO2
Voc [V] Jsc [mA·cm-2] FF [%] η [%]
Control 0.66±0.02 15.01±0.06 65.0±0.6 6.45±0.03
1 Cycle on TiO2 0.67±0.02 16.36±0.07 61.0±0.6 6.65±0.03
2 Cycle on TiO2 0.70±0.01 16.91±0.08 61.6±0.7 7.25±0.04
4 Cycle on TiO2 0.66±0.01 16.35±0.5 56.4±0.7 6.10±0.05
6 Cycle on TiO2 0.63±0.02 16.60±0.07 46.8±0.5 4.95±0.03
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• Voc and Jsc increase with
the number of ALD cycles
for up to 2 cycles
• Highest efficiency is 7.25%
HfO2 on TiO2 is similar to HfO2 on ITO…Differences?
Differences: ITO vs TiO2
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ITO Blocking
Different mechanisms
On ITO: Suppressed ITO to electrolyte
On TiO2: Suppressed TiO2 to dye/electrolyte
Limitations in Nanoparticle
• Random walk
• Limited diffusion distance (10 µm)
• Grain boundaries
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Better structures are desirable….
ITO Nanowire for DSSCs
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• ITO-TiO2 core-shell
• Highly conductive core
• Decoupling of injection and transport
• Less recombination
• Longer nanowires
Nanowire Characterizations
• Vertically-aligned nanowires
• Good crystallinity
• Single crystalline
• ITO-TiO2 core-shell nanowires
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The fabricated nanowires are promising…
Depositing Thin Films
Ready for cell assembly & testing…
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No ALD
HfO2 ALD
TiO2 ALD
Porous TiO2 shell
Cell Performance
Voc (V) Jsc (mA·cm-2) FF (%) η (%) Dye loading
(x10-8 mol·cm-2)
No ALD 0.52±0.03 9.96±0.07 54.2±0.9 2.82±0.08 1.51
HfO2 ALD 0.71±0.03 12.17±0.06 55.8±0.6 4.83±0.09 1.61
TiO2 ALD 0.63±0.04 16.80±0.05 50.7±0.7 5.38±0.05 1.57
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• Both HfO2 & TiO2 increased
efficiency dramatically
• HfO2: Higher Voc
• TiO2: Higher Jsc
TiO2 is more effective: 90% efficiency increase
Thin Film Bandgaps…
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A: Injection
B: Recombination
TiO2: No blocking of e- injection so higher Jsc
HfO2: Higher CB edge so higher Voc
How is nanowire compared with nanoparticle…
Nanoparticle vs Nanowire
• HfO2 in nanowire DSSCs showed higher percentage of
efficiency increase
• Further increase in nanowire DSSCs: More surface areas
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Summary
• A thermal/plasma dual mode ALD system was used
to deposit HfO2/TiO2 thin films
• Plasma ALD has wider process window and faster
deposition rate
• Lower plasma power resulted in smoother surface
and longer plasma time resulted in lower resistivity
• Thin films deposited in nanowire-DSSCs showed
higher percentage of efficiency increase
• Efficiency can be further improved with higher
surface area
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Acknowledgement
Advisor: Dr. Kirk Ziegler
Group members:
Jung Kim
Cheng Xu
Yang Zhao
Justin Clar
Justin Wong
Akshita Dutta
Financial support from NSF is acknowledged
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Depositing HfO2 Thin Films by ALD
for Photovolatics
Luping Li
Department of Chemical Engineering
University of Florida