Dr. Stanbery's presentation at the MRS 2010 Workshop

1. CIGS Synthesis by Reactive Transfer
Processing of Compound Precursors
B.J. Stanbery
Chief Scientist, Founder, and Chairman
HelioVolt Confidential
2010 MRS Workshop
and Proprietary
Thin Film PV

2. 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
2010 MRS Workshop
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3. 2010 MRS Workshop on Thin Film Photovoltaics
7 October 2010; Denver, CO
THERMOCHEMISTRY OF
CU–IN–GA–SE MATERIAL SYSTEM
2010 MRS Workshop
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4. Cu–(In,Ga)–Se Ternary Alloys
Molecularity (M) and Stoichiometry (S)
• M= [Cu]/([In]+[Ga]) M-axis 112 = CuInSe2
Se 247 = Cu2In4Se7
• S = 2[Se]/[Cu]+3([In]+[Ga]) 135 = CuIn3Se5
• ∆M= M-1; ∆S= S-1
• ALL high-efficiency ∆S>0
CIGS devices have Cu2Se3. 135 .(In,Ga)2Se3
∆M<0 and ∆S>0 CuSe. 112 .(In,Ga)Se
247
247
Cu2Se. .(In,Ga)4Se3
• Formation reaction: ∆M<0
y Cu2Se + (1-y) (In,Ga)2Se3
+ ∆Se → Cu In, Ga
(Cuy(In,Ga)1-y)2Se3-2y+∆Se Intermetallic Plethora
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5. CIGS Complex Non-Stoichiometric
Thermochemical Phase Structure
high quality Metal sub-lattice Ga–In alloy
device domain Order-Disorder maximum
(2-phase ) Transition efficiency
zone
• All of the stable thermodynamic phases in the CIGS material
system are crystalline but can vary in composition
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Thin Film PV
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6. CIGS Non-Stoichiometry and
Atypical Device Behavior
• 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.
2010 MRS Workshop
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7. 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 a
nanoscale 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.
2010 MRS Workshop
Thin Film PV

8. 2010 MRS Workshop on Thin Film Photovoltaics
7 October 2010; Denver, CO
MOTIVATION FOR ALTERNATIVE
CIGS PROCESSING APPROACH
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9. 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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10. 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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11. 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
2010 MRS Workshop
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12. 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
2010 MRS Workshop
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13. 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
2010 MRS Workshop
Thin Film PV
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14. 2010 MRS Workshop on Thin Film Photovoltaics
7 October 2010; Denver, CO
REACTIVE TRANSFER PROCESSING
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15. Reactive Transfer Processing of
Compound Precursors
• Two-stage process Se, S
112 = Cu(In,Ga)(Se,S)2
– Low-temperature 247 = Cu2(In,Ga)4(Se,S)7
deposition of multilayer
compound precursor
Cu2Se3. .(In,Ga)2(Se,S)3
films CuSe. 112 .(In,Ga) (Se,S)
247
247
– RTP reaction of Cu2Se. .(In,Ga)4(Se,S)3
compound precursors
to form CIGS
Cu In, Ga
Intermetallic Plethora
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16. 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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17. Reactive Transfer Processing
Contact Transfer Synthesis (FASST®)
Rapid Thermal Processor
Electrostatic
Chuck
Print Plate
Release Layer
Precursor 2
Print Plate
Release Layer
Precursor 2
Precursor 1
Precursor 1 Metal Contact Layer
Metal Contact Layer Substrate
Substrate
Flash Heating
Print Plate
Recoat Print Plate Release Layer
emitter
Device
CIGS Processing CIGS
Metal Contact Layer Metal Contact Layer
Substrate Substrate
Completed Device
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18. 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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19. Recrystallization of Nanoscale Precursor
Films Forming Large Grain CIGS
Precursor Film FASST® CIGS cross-section
2010 MRS Workshop
Thin Film PV © 2009 HelioVolt Corporation

20. CIGS Film by FASST® in 6 minutes
with Vacuum-based Precursors
XRD
CIGS
Mo
SIMS Depth Profile  Chalcopyrite CIGS (& Mo)
 (220/204) preferred orientation
 Uniform elemental distribution ⇒
complete reaction of the two precursors achieved
2010 MRS Workshop
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21. 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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22. MOD Comparison with Vacuum
Precursor Deposition Method
Co-evaporated Top View Top View Spray
CIGS Precursor Deposited
Film CIGS Precursor
Film
Cross Section Cross Section
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23. NREL CRADA – Hybrid CIGS by FASST®
XRD
SEM
 Chalcopyrite CIGS (& Mo)
 (220/204) preferred orientation
 Exceptionally large grains achieved
 Columnar structure
2010 MRS Workshop
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24. Reactive Transfer Processing
Non-Contact Transfer Synthesis (NCT™)
Process Step
Cu, In,
Ga, Se • Independent deposition of distinct
compound precursor layers on
Substrate
substrate and source plate
Source Plate with Transfer Film • Rapid non-contact reaction
Pressure – Turns stack into CIGS with high efficiency grains
Heat – Combines benefits of sequential selenization
with Close-Spaced Vapor Transport (CSVT) for
junction optimization
Source Plate
• CIGS adheres to the substrate and
the source plate is reused
Substrate
CIGS Layer
• More suitable for rigid substrates
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Thin Film PV
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