Solution Deposition of Amorphous IZO Films

By Ultrasonic Spray Pyrolysis

Innovation for Our Energy Future

National Renewable Energy Laboratory Colorado School of Mines

By Ultrasonic Spray Pyrolysis

Robert Pasquarelli1, Maikel van Hest2, Alexander Miedaner2, Calvin Curtis2, John Perkins2, Joseph Berry2, Ryan O'Hayre1, and David Ginley2

1) Colorado School of Mines, Golden, Colorado 80401 2) National Renewable Energy Laboratory, Golden, Colorado 80401

H O - based 2

Poor Quality

Formate Precursor Synthesis

Post Processing

20 μm

Post 

Processing

As 

Deposited

Film Quality and Optical Properties

Zn Formate

Zn hydroxy carbonate + formic acid at 800C 1 week, 

white crystals

In Formate

In oxide + formic acid at 800C 2 weeks, white with 

C

O O

H -1

F tI

Transparent Conducting Oxides (TCO)

Dual Function in Solar Cell

• Transparent - allows light to 

enter solar cell to reach the 

absorber

• Conducting - electrical contact

CIS Absorber Layer

Molybdenum Contact

TCO

Solution Deposition Methods

Spin Coating / Spray 

Pyrolysis / Inkjet Printing

• Inexpensive

• Atmospheric

• Scalable

Why Solution Processing ?

Current Deposition Methods

Sputtering / Pulsed Laser 

Deposition

• Expensive

• High vacuum

• Difficult to scale $

Thermal Gravimetric 

Analysis (TGA)

• Formates decompose 

at temperature similar 

to acetates

H2O loss of hydrate

Main loss 

event

• MeOH prevents cracking

• 150-200 nm thick

• SEM suggests porous film 

or small grains

Formate Characterization

MeOH - based 

Good Quality

SEM cross-section 100 nm

20 μm

,

yellow tint powder

In(1-x)-Zn(x) Formate (IZF) Zn 

hydroxy carbonate + In oxide + formic acid at 800C 2 weeks, white 

powder

Formate Ion

M.A. Contreras, et. al, 24th PVSC IEEE December 5-9 (1994)

CIS Solar Cell Cross-Section

• Conducting - electrical contact

for collecting charge carriers

Molybdenum Contact

• AFM RMS roughness 18 nm

• Sprayed films 5-40X rougher than sputtered IZO film

• Simple $ to acetates

• Single / close 

decomposition 

temperatures 

preferred to avoid 

separate phase 

formation

(TGA in air, stacked spectra)

Our Approach

Combinatorially 

sputtered Indium 

Zinc Oxide (IZO) 

shows highest 

conductivity and 

smoothness in 

Indium-rich region

Ink Film

IZF + H2O + 

HCOOH Gray

(In0)

Ink Additives

Ink Formulation

Indium rich region

Current TCOs by solution routes have only focused on 

crystalline, Zinc-rich films (3-5% In)

Our Goals:

•Amorphous IZO film

•Indium-rich composition

•Overcome solubility problems

with conventional Indium

metal-organic compounds

(reducing acid) (In0)

IZF + H2O + 

HNO3

(oxidizing acid)

Yellow

(In2O3)

M.P. Taylor et al, Adv. Funct. Mater. 18 (2008) 3169-3178

Processing Details

• IZF inherently reducing

• Acid addition required for 

IZF solubility

• Acid choice affects redox 

ink properties

+

Zn 

Precursor

In 

In-Zn 

Precursor

Conventional 

Mixed Precursor 

Approach

Single 

Precursor 

Approach

versus

Conductivity

Ink Formulation

Precursors + Additives (surfactants, oxidizing agents, pH 

modifiers) + Solvent

Deposit

Spray film on heated substrate (temperature, 

flow rate, coats)

Ink Formulation

IZF + HNO3 + H2O IZF + HNO3 + MeOH (0.1M) 

Deposition

Sprayed films on glass substrates at 140-2100C

Ar-RTP Processing

Rapid Thermal Processing under Ar for 20min at 3400C 

with a 200C/sec ramp film side up then down

Ar-H2 Annealing

Annealed in tube furnace under Ar-4%H2 at 300-4000C 

for 20min with a 3hr ramp

metal organic compounds

•Develop new In-Zn precursors

Solution Processing Scheme

IZO Film

Precursor

P d FilC iti

• Desired carrier concentration 

can be achieved with 40 and 

65 at% In

• XRD suggests 

40 at% In films 

annealed at 300-

Film Morphology

• Need to boost mobility by 

~1 order of magnitude to be 

in range of sputtered IZO

Composition of IZF precursor (line) 

and annealed films (points) when 

ink aged 4 hrs versus 2 days:

• Measured by XRF

• Confirmed by ICP-AES

• Longer ink aging results in 

precipitation and reduction of In 

content (at%) relative to Zn

)

N2 Flow

Ink 

Mist

Substrate on Hotplate

Ink

X-Y Stage

NREL 

Ultrasonic 

Spray 

System

Conclusions Future Work

Precursor and Film Composition 400oC are 

amorphous

• Higher 

temperature 

processing and 

films with 65 at% 

In show In2O3

crystallization / 

phase separation

• No ZnO observed

Thermal Processing

Convert ink film to TCO (temperature, ramp rate, time, 

annealing gases)

Analyze

Conductivity Transparency

Morphology Film Quality

Conclusions

• Successfully synthesized and 

characterized In, Zn, and In-Zn formates

• In-Zn formate (IZF) can serve as a 

precursor for In-Zn oxide films

– 70:30 In:Zn atomic ratio

– Decomposes near 260oC

– Ink composition dependent on aging

• Spray deposition with an IZF+HNO3+MeOH 

ink and Ar-H2 anneal results in IZO films

– Amorphous, In2O3 forms at higher In 

content and with annealing near 500oC

– Thin, crack-free

– Good optical transmittance (>75%)

– Conductive (~60 S/cm)

• Explore processing to produce 

denser films and increase mobility

• Narrow processing temperature and 

time windows

• Factorial design to determine critical 

processing parameters

• Explore other precursors

Future Work

This work was supported by the U.S. Department of Energy under Contract No. DOE-AC36-08GO28308 with the National Renewable Energy Laboratory.

The information contained in this poster is subject to a government license.

2009 34th IEEE Photovoltaic Specialists Conference • Philadelphia, PA • June 7-12, 2009 • NREL/PO-520-46072

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