MRS_Oct_7_2010_Workshop

2010 MRS Workshop Thin Film PV

HelioVolt Confidential and Proprietary

CIGS Synthesis by Reactive Transfer Processing of Compound PrecursorsB.J. Stanbery

Chief Scientist, Founder, and Chairman


Outline

• Thermochemistry of Cu–In–Ga–Se material system

• Motivation for alternative CIGS processing approach

• Reactive Transfer Processing and variants for Rigid vs. Flexible substrates

• Current status

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THERMOCHEMISTRY OF CU–IN–GA–SE MATERIAL SYSTEM

2010 MRS Workshop on Thin Film Photovoltaics

7 October 2010; Denver, CO

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2010 MRS Workshop Thin Film PV 4

Cu–(In,Ga)–Se Ternary AlloysMolecularity (M) and Stoichiometry (S)• M= [Cu]/([In]+[Ga])

• S = 2[Se]/[Cu]+3([In]+[Ga])

• ∆M= M-1; ∆S= S-1• ALL high-efficiency

CIGS devices have ∆M<0 and ∆S>0

• Formation reaction:

y Cu2Se + (1-y) (In,Ga)2Se3

+ ∆Se →(Cuy(In,Ga)1-y)2Se3-2y+∆Se

112

Cu In, Ga

Se

247

135

M-axis

∆M<0

247

112 = CuInSe2247 = Cu2In4Se7135 = CuIn3Se5

∆S>0

CuSe.Cu2Se.

Cu2Se3. .(In,Ga)2Se3

.(In,Ga)4Se3

Intermetallic Plethora

.(In,Ga)Se


CIGS Complex Non-Stoichiometric Thermochemical Phase Structure

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• All of the stable thermodynamic phases in the CIGS material system are crystalline but can vary in composition

Metal sub-latticeOrder-DisorderTransition

high quality device domain (2-phase )

Ga–In alloymaximum efficiencyzone


• Peculiar semiconductor behavior: CIGS PV devices insensitive to % atomic composition variations & extended defects>19% efficiencies recently reported† over range:

• 0.69 ≤ [Cu]/([In]+[Ga]) ≤ 0.98 (Group I/III ratio)• 0.21 ≤ [Ga]/([Ga]+[In]) ≤ 0.38 (Group III alloy ratio: Eg)

• Empirical Observations– CIGS PV devices are always copper deficient

compared to α-CuInSe2

– Compositions lie in the equilibrium α+β 2-phase domain

†Jackson et al., Prog. PV, Wiley & Sons, 2007.

CIGS Non-Stoichiometry and Atypical Device Behavior


Role of Nanostructuring in CIGS PV Device Physics• Intra-Absorber Junction (IAJ) model

– Device-quality CIGS is a two-phase mixture of p-type α-CIGS and n-type β-CIGS phases, forming ananoscale bulk heterojunction

– These internal junctions form an interpenetrating percolation network, allowing positive and negative charges to travel to the contacts in physically separated paths, reducing recombination.


MOTIVATION FOR ALTERNATIVE CIGS PROCESSING APPROACH



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Characteristics of an Ideal CIGS Manufacturing Method• High device-quality material

– Ability to create intrinsic defect structures limiting recombination; role of the order-disorder transition?

– Ability to control Group III and VI composition gradients– Control of extrinsic doping (e.g.: sodium)

• High processing rate– Reduces capital cost for targeted throughput

• Low thermal budget– Reduces operating cost and energy payback time

• High materials utilization– Reduced materials consumption and recycling expenses

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Synopsis of Prior Art for CIGS Synthesis:Co-evaporation

• First method to achieve 10% efficiency and research approach used to make all record cells since 1989

• Simultaneous evaporation of the constituent elements onto a high-temperature (450-700°C) substrate to directly synthesize CIGS in a single stage process

• Competition between adsorption and desorption kinetics reduces (1) selenium utilization and (2) indium incorporation at temperatures near/above the order-disorder transition

• Extended dwell at high temperatures generates high thermal budget and equipment costs

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Synopsis of Prior Art for CIGS Synthesis:Metal Precursor Selenization

• Most well-developed, widely used approach for commercial manufacture of CIGS modules, providing good large-area uniformity

• Deposition of multilayer metal films by PVD, plating, or particle suspensions followed by second-stage high-temperature annealing in Se or H2Se/H2S

• Complex intermetallic alloying reactions and differential diffusion during selenization cause uncontrolled segregation

• Selenium/Sulfur diffusion limits reaction rate and resulting extended dwell at high temperature generates high thermal budget; first stage deposition method determines materials utilization efficiency and capital intensity

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Synopsis of Prior Art for CIGS Synthesis:Oxide Precursor Selenization

• High-speed printing of copper indium gallium oxide nanoparticle ink onto a metal foil substrate, subsequently annealed at high temperature in H2Se/H2S to convert the oxide into sulfo-selenide– Enables excellent materials utilization

• Reduced diffusion lengths of chalcogens in nanoparticles speeds displacement reaction

• Difficult recrystallization kinetics limit film densification and large grain growth

• Composition gradient control challenging

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Synopsis of Prior Art for CIGS Synthesis:Stacked Elemental Layers (SEL)

• Differs from the metal selenization approaches by incorporating layers of selenium, as well as the metals, into the precursor film itself– Circumvent the need to diffuse selenium through the

entire thickness of the precursor stack– Enables intervention in intermetallic formation by

stacking sequence control– Multi-step reaction kinetics shown to generate

compound intermediates prior to CIGS formation• Rapid thermal processing used in second stage to

minimize thermal budget and parasitic reactions

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REACTIVE TRANSFER PROCESSING



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Reactive Transfer Processing of Compound Precursors

• Two-stage process– Low-temperature

deposition of multilayer compound precursor films

– RTP reaction of compound precursorsto form CIGS

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112

Cu In, Ga

Se, S

247247

112 = Cu(In,Ga)(Se,S)2247 = Cu2(In,Ga)4(Se,S)7

CuSe.Cu2Se.

Cu2Se3. .(In,Ga)2(Se,S)3

.(In,Ga)4(Se,S)3

Intermetallic Plethora

.(In,Ga) (Se,S)


Reactive Transfer Processing Compound Precursor Deposition• Two methods have been developed for

deposition of compound precursors– Low-temperature Co-evaporation

• Equipment requirements similar to conventional single-stage co-evaporation but lower temperatures lead to higher throughput and reduced thermal budget

– Liquid Metal-Organic molecular solutions• Proprietary inks developed under NREL CRADA• Decomposition of inks leads to formation of inorganic

compound precursor films nearly indistinguishable from co-evaporated films (for some compounds)

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Reactive Transfer ProcessingContact Transfer Synthesis (FASST®)

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Substrate

Print Plate

Metal Contact Layer

Release LayerPrecursor 2

Precursor 1

Print PlateRelease LayerPrecursor 2

SubstrateMetal Contact Layer

Precursor 1

ElectrostaticChuck

Rapid Thermal Processor

Flash Heating

DeviceProcessing


CIGSemitter

Completed Device

Recoat Print Plate


CIGS

Print PlateRelease Layer


Field-Assisted Simultaneous Synthesis and Transfer (FASST®)• Combines features of

– Rapid Thermal Processing and,– Anodic Wafer Bonding

• Advantages– Rapid processing

• Eliminates pre-reaction• Independent pre-heating of precursors

– Confinement of volatile selenium– High electrostatic field provides

intimate precursor film contact• Substrate compliance critical for uniform large-area

contact so FASST® process variant most suitable for flexible substrate processing.

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Recrystallization of Nanoscale Precursor Films Forming Large Grain CIGS

Precursor Film FASST® CIGS cross-section

© 2009 HelioVolt Corporation


CIGSMo

Chalcopyrite CIGS (& Mo) (220/204) preferred orientation

achieved

SIMS Depth Profile Uniform elemental distribution ⇒complete reaction of the two precursors

CIGS Film by FASST® in 6 minutes with Vacuum-based Precursors

XRD


Metal-Organic Decomposition (MOD) Precursor Film Deposition• Inorganic compound reaction CIGS synthesis provides

pathway for evolutionary adoption of MOD precursors• Key drivers

– Low capital equipment cost– Low thermal budget– High throughput

• Flexibility– Good compositional control by chemical synthesis– Variety of Cu-, In- and Ga-containing inks can be synthesized

and densified to form multinary sulfo-selenide precursors• Efficient use of materials

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Cross Section Cross Section

Co-evaporatedCIGS Precursor

Film

Spray Deposited

CIGS Precursor Film

Top View Top View

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MOD Comparison with Vacuum Precursor Deposition Method


SEM

NREL CRADA – Hybrid CIGS by FASST®

Chalcopyrite CIGS (& Mo) (220/204) preferred orientation

achieved Exceptionally large grains Columnar structure

XRD


Reactive Transfer ProcessingNon-Contact Transfer Synthesis (NCT™)

• More suitable for rigid substrates

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Source Plate

SubstrateCIGS Layer

Heat

Source Plate with Transfer FilmPressure

Substrate

Cu, In, Ga, Se

Process Step

• Independent deposition of distinct compound precursor layers on substrate and source plate

• Rapid non-contact reaction– Turns stack into CIGS with high efficiency grains– Combines benefits of sequential selenization

with Close-Spaced Vapor Transport (CSVT) for junction optimization

• CIGS adheres to the substrate and the source plate is reused

Technology

MRS_Oct_7_2010_Workshop