This study investigates atmospheric-pressure solution deposition routes as an alternative to these traditional high-vacuum techniques

Berry, J.

Curtis, C.

Ginley, D.

Miedaner, A.

O'Hayre, R.

Pasquarelli, R.

Perkins, J.

van Hest, M.

UNT Digital Library

Solution Deposition of Amorphous IZO FilmsBy Ultrasonic Spray PyrolysisInnovation for Our Energy FutureNational Renewable Energy Laboratory Colorado School of Mines   Robert Pasquarelli1, Maikel van Hest2, Alexander Miedaner2, Calvin Curtis2, John Perkins2, Joseph Berry2, Ryan O'Hayre1, and David Ginley21) Colorado School of Mines, Golden, Colorado 80401   2) National Renewable Energy Laboratory, Golden, Colorado 80401H2O -based Poor QualityFormate Precursor SynthesisPost Processing20 µmPost ProcessingAs DepositedFilm Quality and Optical PropertiesZn FormateZn hydroxy carbonate + formic acid at 800C 1 week, white crystalsIn FormateIn oxide + formic acid at 800C 2 weeks, white with COOH -1F t ITransparent Conducting Oxides (TCO)Dual Function in Solar Cell• Transparent - allows light to enter solar cell to reach the absorber• Conducting - electrical contactCIS Absorber LayerMolybdenum ContactTCOSolution Deposition MethodsSpin Coating / Spray Pyrolysis / Inkjet Printing• Inexpensive• Atmospheric• ScalableWhy Solution Processing ?Current Deposition MethodsSputtering / Pulsed Laser Deposition• Expensive• High vacuum• Difficult to scale $Thermal Gravimetric Analysis (TGA)• Formates decompose at temperature similar to acetatesH2O loss of hydrateMain loss event• MeOH prevents cracking• 150-200 nm thick• SEM suggests porous film or small grainsFormate CharacterizationMeOH -based Good QualitySEM cross-section      100 nm20 µmyellow tint powderIn(1-x)-Zn(x) Formate (IZF)                                                                 Zn hydroxy carbonate + In oxide + formic acid at 800C 2 weeks, white powderorma e onM.A. Contreras, et. al, 24th PVSC IEEE December 5-9 (1994)CIS Solar Cell Cross-Section   for collecting charge carriers • AFM RMS roughness 18 nm• Sprayed films 5-40X rougher than sputtered IZO film• Simple • Single / close decomposition temperatures preferred to avoid separate phase formation(TGA in air, stacked spectra)Our ApproachCombinatorially sputtered Indium Zinc Oxide (IZO) shows highest conductivity and smoothness in Indium-rich regionInk FilmIZF + H2O + HCOOH Gray(In0)Ink AdditivesInk Formulation 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 problemswith conventional Indiummetal-organic compounds(reducing acid)IZF + H2O + HNO3(oxidizing acid)Yellow(In2O3)M.P. Taylor et al, Adv. Funct. Mater. 18 (2008) 3169-3178Processing Details• IZF inherently reducing• Acid addition required for IZF solubility• Acid choice affects redox ink properties+Zn PrecursorIn  In-Zn PrecursorConventional Mixed Precursor ApproachSingle Precursor ApproachversusConductivityInk FormulationPrecursors + Additives (surfactants, oxidizing agents, pH modifiers) + SolventDepositSpray film on heated substrate (temperature, flow rate, coats) IZF + HNO3 + H2O IZF + HNO3 + MeOH        (0.1M) DepositionSprayed films on glass substrates at 140-2100CAr-RTP ProcessingRapid Thermal Processing under Ar for 20min at 3400C          with a 200C/sec ramp film side up then downAr-H2 AnnealingAnnealed in tube furnace under Ar-4%H2 at 300-4000C              for 20min with a 3hr ramp •Develop new In-Zn precursorsSolution Processing SchemeIZO FilmPrecursorP d Fil C 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 IZOComposition 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 ZnN2 FlowInk MistSubstrate on HotplateInkX-Y StageNREL Ultrasonic Spray SystemConclusions Future Workrecursor an  m ompos on 400ºC are amorphous• Higher temperature processing and films with 65 at% In show In2O3crystallization / phase separation• No ZnO observedThermal ProcessingConvert ink film to TCO (temperature, ramp rate, time, annealing gases)AnalyzeConductivity    TransparencyMorphology    Film Quality• 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 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-520-46072NREL is a national laboratory of the U.S. Department of Energy    Office of Energy Efficiency and Renewable Energy            Operated by the Alliance for Sustainable Energy, LLC

Solution Deposition of Amorphous IZO Films By Ultrasonic Spray Pyrolysis (Poster)

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Solution Deposition of Amorphous IZO Films By Ultrasonic Spray Pyrolysis (Poster)

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