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