Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #1
Current Status of Solar Photovoltaic Technology Platforms, Manufacturing
Issues and ResearchSteve Hegedus
Institute of Energy ConversionUniversity of Delaware
With assistance from IEC staff: Brian McCandless (CdTe), Bill Shafarman (CIGS)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #2
Outline Introduction to IEC at U of Delaware
PV trends, growth in scale, contribution to energy production
Crystalline Si PV status, baseline technology, near term focus: New methods emitter formation, passivation and device
architecture
Thin Film PV status, baseline technology and near term focus: Cu(InGa)Se2 : wide gap alloys, improved 2-step selenization CdTe: higher deposition T , substrate a-Si/nc-Si (briefly) : multijunction, collapse of the a-Si industry
Two common PV myths
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #3
Institute of Energy Conversion at U of Delaware Founded in 1972 to perform thin-film PV research
World’s oldest continuously operating
solar research facility
First 10% efficient thin film solar cell (1980)
Dept of Energy University Center of Excellence
for Photovoltaic Research and Education (1992)
Soft funded - government and industry contracts
2012 staff: 11 professional, 3 tech, 2 admin, 5 post doc, >20 grad students (4 depts)
Recently rec’d $8.4M from DOE (3 year grants)
First flexible10% cell
4x4 inchminimodule
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IEC Research Program Goals Expand the fundamental science and engineering base for
thin film and c-Si photovoltaics to improve performance
Transfer these technologies to large-scale manufacturing IEC has been responsible for growth of several PV start-ups
through technology transfer and validation
Provide workforce with PV scientists and engineers
>40 graduates since 1992 (PV Center of Excellence)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #5
IEC Technology Thrust Areas Thin film polycrystalline CuInGaSe2-based (CIGS) solar cells
Thin film polycrystalline CdTe solar cells
Silicon-based solar cells
Front and back contact heterojunction (a-Si/c-Si)
Thin film tandem a-Si and nc-Si
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IEC Facilities: complete capability for fabrication and characterization of thin film and c-Si solar cells
Over 20 thin-film deposition systems: PECVD (vhf/rf/dc), HWCVD, PVD, Vapor Transport, sputtering, H2S/H2Se reaction, chemical bath
Materials characterization: XRD, GIXRD, VASE, EDS, SEM, AFM, AAS, XPS, FTIR, Raman, optical trans+refl, Hall effect
Device fabrication: complete capability for high efficiency solar cells:
c-Si (front heterojunction and IBC), CdTe, Cu(InGa)Se2 , a-Si
Laser and mechanical scribing for monolithic module fabrication
Device characterization: J-V, J-V-T, QE, C-V, OBIC, accelerated life stress (damp-heat, ambient, light)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #7
The Big Picture: PV applications and achievements
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #8
PV can be installed anywhere, 10’s Watts to 100’s Megawatts
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Recent worldwide achievements Installation: 17 GW in 2010 (100% growth), 30 GW in 2011 (70% growth)
Average annual growth >50% p/y for decade
EU: PV providing 2-4% of annual electricity in Spain, Germany, Italy May 2012 Germany received >10% from PV On one day >40% (22 Gigawatts peak supply out of 27 GW installed)
US: 5.7 GW installed, 2 GW in CA Over 70% of 2011 installations are ‘utility scale’ or > 100 kW Worlds largest PV power plant 250 MW Aqua Caliente Project (CA, thin film)
Creating hundreds of thousands of jobs 400,000 in Germany; 100,000 in US R&D, manufacturing, supply chain (materials), system design, installation
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #10
Trend in PV applications: 1990-2009 1995: PV demand driven by off-grid applications
After 1995: Innovative policy in Japan, Germany stimulated market for grid-connected residential and commercial
>2008: Asian Si modules drove down prices, increased installations
>2010: Significant growth in utility scale > 1 MW projects
2012: First year of flat or negative growth in decades,
projected to recover 2013
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #11
Industry in turmoil: ‘roller coaster ride’ Significant consolidation, bankruptcy, closures in past 2 years
Top companies for years suddenly quit PV or bankrupt
Worldwide capacity ~ twice demand yet demand still growing
Huge excess inventory
Shrinking profits - many companies selling at loss to compete
c-Si done much better in price and efficiency than many expected, squeezing thin film start ups
Renewed emphasis on improving performance since costs so low
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #12
Brief Overview of PV Basics
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What is a PV device?Direct converter of light into electricity:photons in, electrical current (DC) out
Three critical processes:Light Absorption + Carrier Generation + Carrier Collection
(current flow) (deliver P to load)
e -h+
e -
h+
load
charge separation
V+
V-
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Cell efficiencies vs. bandgap EG Record performance single junction cells vs. theoretical limit
Expect maximum performancewith EG ≈ 1.5 eV
But theor eff >25% possible EG ≈ 1 – 1.8 eV
Many thin film, III-V options a-Si/nc-Si 2J
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Commercial Scale PV Devices Single crystal or multicrystalline Si wafers
Dominate market: 85-90% of sales Solar grade Si, lower qual than IC Module efficiency: 14-20% Low cost Asian Si driving prices down
Thin films (1-3 µm polycrystalline or amor) Ultimately lower cost than Silicon wafers (??) On glass, metal or plastic foils Diverse materials, techniques Lower quality, imperfect crystallization, more defects Module efficiency: 8-14% Unique advantages in building integrated products 10-15% of market, #2 PV company is TF CdTe (First Solar)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #16
One common PV challenge: reducing gap between champion cell and module efficiency
Wolden et al, JVST-A 29 (2011) 030801
Multi c-Siand TF CIGSboth ~20%cell efficiency.But mc-Si has more mature module technology.
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #17
Why efficiency matters – fixed BOS costs
Wang et al Renewable and Sustainable Energy Rev (2011)
Levelized cost of Energy:• Lifecycle costs/energy • LCOE costs include Balance of System whichscale with # modules, area• Lower eff = higher BOS$• More rack, wiring, install $• y-intercept is system price without module• Si modules ‘selling’ at $0.9/WOr $120/m2
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #18
Crystalline Si (c-Si) Technology and Advanced Concepts
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #19
Standard commercial Si PV cell processstart
finish
900°C
400°C
900°C
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Commercial Si Solar Cell, Eff ~ 15-17%
Front contact (Screen printed Ag fired through SiN)
(~ 0.3 µm)
RandomtexturedF and R
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #21
World record Si solar cell: PERL
Cell Voc (V) Isc (mA/cm2) FF(%) Eff(%)PERL 0.70 42 81 24.7
• PERL cell: Passivated Emitter, Rear contact Locally diffused• 2-step emitter (thin n between contact and thick n+ under contact)• UNSW, AU, 1998; very complex design, not manufacturable
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #22
Conflicting emitter properties: pn junction vs resistance vs absorbing ‘dead layer’
property Advantage DisadvantageIncrease thickness •Reduce lateral R for
current flow to Ag contact•Prevent melting Ag SP metal penetrate to base
•Increase absorption in highly defect layer (photons not converted to e-h pair), lower blue QE and Jsc
Increase doping •Reduce lateral R•Reduce contact R with Ag or other metal grid
•Increase defects and recombination (Io) so decrease Voc
Increase bandgap •Decrease absorp loss•Increase band bending, reduce recomb (Io)
•Increase lateral resistance significantly, req second conductive layer ($$)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #23
Why not ‘tune’ the emitter to have properties it needs only where it needs them?
Why not a 2 step emitter – spatially specific? different thickness and doping where needed? acknowledge that current flow is 2D not 1D
Why not replace with wider bandgap material?
Why not get rid of the emitter all together*
* On the front of the device
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #24
Industrially proven high efficiency Si solar cell concepts
Three commercial proven enhancements (full size wafer results) PERL/SE: passivated+selective emitter, rear localized contact HIT: ‘HJ intrinsic thin’ a-Si/c-Si heterojunction IBC: ‘interdigitated back contact’ rear emitter and base contact
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #25
Hi Eff concept #1: the 2-step ‘selective emitter’Conventional 1 stepthick emitter
2 step emitter: thinner n+ everywhere except under metal n++
AppliedMaterialswebsite
Increased blue response with thinner n+
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #26
SE option 1: laser doping + plating metal
http://www.photonics.com/Article.aspx?AID=40098
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #27
SE option 2: deposit thicker n++ then etch
back
• requires alignment of front metalto thicker n++ mesa
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Thinner vs selective emitter: Voc, Isc, FF, Eff
Gauthier “Industrial Approaches of Selective Emitter on Multicrystalline Silicon Solar Cells”24th Eu-PVSEC (2009) 2-CV.5.46
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #29
Hi Eff #2 Heterojunction Solar Cell: deposited a-Si passivation layers reduce surface recombination
Device: n-type c-Si wafer and 5-10 nm PECVD a-Si layers (EG=1.7-1.8 eV)
(i) a-Si:H surface passivation layers (both sides)(p)a-Si:H emitter (front)(n)a-Si:H back contact (rear)All a-Si and contacts deposited <200°C (low $, less defects, no warping thin wafers)
High efficiency and VOC (Sanyo/Panasonic): η = 23% champion cells, 19% modulesVOC = 740mV
10nm (i)a-Si:H30nm (n)a-Si:H
10nm (p)a-Si:H
60nm ITO
300μm n-c-Si wafer
Rear Contact (Al)
Front Contacts (Ag)
Hole Current
Electron Current
EV
EFEC
hυ
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #30
Two modes of c-Si Surface Passivation by a-Si:H
c-Si Substrate a-Si:H film
EFEG,c-Si EG,a-Si:H
EC
EV
PECVD a-Si:H provides best passivation and processing < 300 °C, high VOC
Si surface cleaning critical to good passivation
Well-established, slow deposition rate, easily control thickness ~ 5-10 nm
Defect neutralization by H atoms:Reduce c-Si surface dangling bonds
Reduce recombination (IO), increase VOC
(n)c-Si Substrate (i)a-Si:H film
EF
EG,c-SiEG,a-Si:H
EC
EV
Field effect passivation:Increased band bending at junction
Repel/separate majority or minority carrierReduce Io, increase VOC
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #31
Hi Eff Si #3: Interdigited back contact (IBC) cell
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Standard front junction vs all back contact
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IBC spectral response higher in short (blue) and long (IR) wavelengths
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #34
Sunpower IBC: highest efficiency module
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #35
Integration of both device concepts: SHJ-IBC Cell
Interdigitated back contact (IBC) solar cell.
n-type c-Si
intrinsic a-Sip-type a-Si
TCO
intrinsic a-Sin-type a-Si
TCOSilicon heterojunction (SHJ) solar cell.
IBC-SHJ solar cell (IEC structure)
First publishedresults on SHJ-IBCBy IEC (APL 2007)
•intrinsic a-Si buffer•separate a-Si pand n regions
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #36
IEC Multichamber PECVD for HIT and IBC-SHJ
4 chambers plus 2 load lock, DC/RF/VHF plasmaMultiple substrate sizes (1x1 up to 12x12 inch)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #37
Thin Film PV Common Features
Monolithic Integration via laser patterning: enabling technology
Lower efficiency: TF PV best suited for BIPV, large power plants
Status and Critical Issues Cu(InGa)Se2
CdTe A-Si/nc-Si (briefly)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #38
Monolithic Series Interconnection : laser scribe Allows structuring of large area uniform thin film layers into series
connected junction diodes; critical technology for TF PV
Three laser scribing steps (patterning steps P1, P2, P3) P1) bottom conductor; P2) semiconductor junction; P3) top conductor
Width of cells determines module current (ISC), # in series determines VOC
Chapter 12, Handbook of Photovoltaic Science and Eng (Luque, Hegedus), Wiley 2011
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TFPV applications: BIPV Appearance preferred for building-integrated PV
85kW Shell Solar Cu(InGa)Se2 in Wales
Semitransparent a-SiArchitectural skylight
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Flexible PV for roll-out rooftop installation (USSC triple junction/SS)
Flexible a-Si on SS: BIPV (flex laminate)
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4 MW of CdTe installed by Tucson Electric
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15 MW of 3Sun Tandem Thin Si in Altomonte, Calabria Italy
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #43
Help from Bill Shafarman
Institute of Energy ConversionUniversity of Delaware
Cu(InGa)Se2Thin Film Solar Cells
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #44
Why thin film CuInSe2 alloys for PV? Direct bandgap chalcopyrite materials with high absorption coefficient
Extraordinary compositional tolerance
Alloy with Ga, Al, Ag, S to engineer bandgap improve performance, TF tandem
Can be deposited on glass or light flexible substrates: polymer, foils
Highest device and module efficiency of any TF PV technology
Multiple deposition technologies with promise of scalability
Attracted considerable private investor funding
Outdoor module stability demonstrated
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #45
Cu(InGa)Se2 Thin Film PV Performance
Highest cell efficiency = 20.3% (ZSW 2010) Efficiency ≥ 18% from several laboratories Sub-module eff. = 17.8% with
area > 800 cm2 (Solar Frontier 2012) 12–14% module efficiency from
companies worldwide
Manufacturing Many companies with various approaches1. Reaction of metal precursors (2-step)
Low cost deposition of metals Batch process: “selenization”
2. Multi-source evaporation (1-step) In-line process, high temp
Substrate
Mo
Cu(InGa)Se2
Grid
CdSZnO:Al
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #46
Cu(InGa)Se2 Optical Absorption High optical absorption of sunlight
Direct bandgap Complete absorption in ~ 1 µm thickness (CdTe very similar) Reduces requirements for minority carrier transport
0
0.2
0.4
0.6
0.8
1
0.001 0.01 0.1 1 10 100 1000
Rel
ative
Abs
orpt
ion
thickness (µm)
Si
CIGS
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #47
Cu(InGa)Se2 Grain Structure Films are polycrystalline with rough surface
Grain size ~ 0.1 – 1 µm depends on deposition conditionse.g. substrate temperature during evaporation
But device performance is remarkably insensitive to grain size, morphology
1µm
Cu(InGa)Se2
Modeposited at 400°C deposited at 550°C
Wilson, Birkmire, Shafarman Proc. 33rd IEEE PVSC (2008)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #48
CuInSe2 Alloys with wider EG: Al, Ga, S CuInSe2 has EG=1.0 eV, limits eff., major focus 20 yrs is to raise EG
Wide range of ternary, quaternary alloy options Recent focus on alloys with Ga/(Ga+In), S/(S+Se), Ag/(Ag+Cu)
Alloying changes EG also lattice constants (no epi!), band alignment
x = alloy fraction:Al/(In+Al)Ga/(In+Ga)S/(Se+S)
Ga/(In+Ga)a x=0b x=0.24c x=0.61
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #49
Increasing efficiency and VOC at higher EG Increasing EG with alloy (Ga, Ag, S) to push efficiency at EG >1.3 eV
Failure to capture benefit of larger EG due to VOC/ EG<1 is critical issue
Contreras et al, 37th IEEEE PVSC, Seattle 2011
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #50
Process 1: Elemental Co-evaporation Simultaneous delivery of elemental vapors
to hot substrate Makes higher efficiency devices than 2-step Independent control of each element
Ga/(In+Ga) gradient bandgap gradient
To vacuum pumpPbase ≈ 1x10-6 TorrPrun ≈ 2x10-5 Torr
Substrate HeaterSubstrate at 400-600°C
Film Growth MonitorThermal Evaporation
Sources for Cu, In, Ga, Se, (S)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #51
IEC Cu(InGa)(SeS)2 Co-evaporation Five source system for Cu-In-Ga-Se-S
Boron nitride Knudsen cells Typical temperatures
T(Cu) = 1350°C T(In) = 1000°C T(Ga) = 1100°C T(Se) = 300°C
Source design and T control are critical features
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #52
Process 2: 2-step Precursor Reaction Advantages: lower cost, higher uniformity + materials utilization Many precursor deposition options with Cu, In, Ga –
sputtering – commercially available electrodeposition – high utilization, non-vacuum, batch ink printing – high utilization, non-vacuum, continuous
Reaction in hydride gases (H2Se, H2S) or elemental vapors (Se,S) Multi-step reaction pathway to form Cu(InGa)Se2
Mo/Cu/Ga/In Mo/Cu(InGa)Se2ReactionH2Se/H2S
or Se/S
400 - 600°C
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Cu(InGa)Se2 Precursor Reaction IEC H2Se / H2S reactor
reaction in quartz tube of glass/Mo/Cu-In-Ga precursor layers atmospheric pressure with flowing H2Se / H2S / Ar / O2
Temperature-time cycle critical to uniformity, manufacturibility
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #54
CIGS Module Process Flow
CleanSubstrate
Mo dcsputtering
P1 laserscribe
CIGSdeposition
P3 mech.Scribe
EVA/glasslamination
CdS bathdeposition
HR ZnOdeposition
P2 mech.scribe
ZnO:Aldeposition
Modulecompletion
Attachbuss bars
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #55
Manu-facturer
DepositionTechnology
Champion product % apa
Current nameplate capacty MW/a
Deposition Process Pro´s
Deposition Process Con´s
comments
Manz (Würth)Solibro
1-stage co-evaporation
15.1
14.4
30 120
Simpler, more advanced process
Sacrifices efficiency Glass-glassCd- buffer
Global Solar Energy
3-stage co-evaporation
15 (cell)13 (mod)
Highest known TF module efficiency
Complex process SS substrate. glass/polymer
MiaSolé Reactive sputter 15.7 >40? good efficiency potential, rel. small capex exp.
Complex process SS substrate, cut + stitch, glass-glassCdS-dry
Solar Frontier,Stion/TSMC
2-step: Sputter+ H2Se/S-Selenization
SF: 17.8SF: 14.1 (manu)Stion: 14.5TSMC 15.1
980
5 + 135+300
advanced process, potentially higher CapEx
Sacrifices efficiencyHigher OpEx than evap.
glass-glassCdSglass-glassCd-free
Avancis, Hyundai
2-step: Sputter+Se-evap.+ RTP-cryst./H2S
30 + 100+100
Glass-glassCdS (Cd-free)
Solo Power 2-step electroplate-Selenization
15.1% cell13.5% mod
30 + 400 Good metal utilization, rel. low CapEx
Sacrifices efficiency SS substratePolymerCdS
Source: Markus Beck / Photon San Francisco‚ Feb. 2012 plus adds from the author
CIGS thin film manufacturing status: > 30MW real capacity
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #56
Two Current research issues: Cu(InGa)Se2
Fundamental understanding of relation between wide EG alloys and device performance with multisource evaporation
Control uniformity and reduce process time with Se/S reaction
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #57
1. New wide gap alloy (AgCu)(InGa)Se2 Ag addition to Cu(InGa)Se2 lowers melting temperature, better surface
mobility, potential for improved structural hence electronic quality
Ag increases bandgap by up to 0.25 eV,
Single phase over entire range of Ag–Cu and Ga–In alloying
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #58
Notable wide EG cell results with (AgCu) Cells: SLG/Mo/(AgCu)(InGa)Se2/CdS/ZnO/ITO/grid/MgF2
High Eff = 17.6% with Eg ≈ 1.3 eV High VOC = 890 mV with Eg ≈ 1.6 eV
Hanket, et al., Proc. 34th IEEE PVSC
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2. H2Se/H2S reaction of Cu-Ga-In Precursors:Ga segregation at rear limits EG and Voc
0
10
20
30
40
50
60
0 20 40 60 80
com
positio
n (%
)
sputter time (min)
Se
Cu
In
Ga
Mo
S0
10
20
30
40
50
60
0 20 40 60 80co
mpo
sitio
n (%
)
sputter time (min)
Se
Cu
In
Ga
Mo
S
• 30’ H2Se@450°C, 15’ H2S@450°C• Complete H2Se reaction prior to H2S• Ga segregated at back, none at front• Low EG at front junction, low Voc
• 15’ H2Se@450°C, 15’ H2S@450°C• Partial H2Se reaction prior to H2S• Ga uniform, higher at front• Higher EG at front junction, higher Voc
Hankett et al, Proc 4th World PVSEC, 2006
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #60
Single step H2Se process vs. Three-step H2Se/H2S process
550
Time, min
Tem
p,
°C
20 50 10 10
1st step :H2Se
20 10
400
Time, min
Tem
p,
°C
20
Single step:H2Se
450
2nd step :Ar
3rd step :H2S
60 ~ 90 Mo
Mo
Ga accumulation
Ga homogenization
Process optimization Ga distributed uniformly with 3 step H2Se/H2S Major improvement in VOC and Eff.
Single step process
Three-step process
-0.2 0.0 0.2 0.4 0.6 0.8
-40
-20
0
20
J (m
A/c
m2 )
V (V)
Eff Voc Jsc FF Single-step : 8.3% 0.383 V 37.0 mA/cm2 58.7% 3-step : 14.2% 0.599 V 32.2 mA/cm2 73.5%
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CdTe Thin Film Solar Cells
Help from Brian McCandless
Institute of Energy ConversionUniversity of Delaware
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #62
Why CdTe TF solar cells? Chemically stable, simple phase diagram, easy surface passivation Film deposition by a variety of scalable techniques Optimized cells require post-dep halide (Cl) exposure at ~ 400C Easily adapted to monolithic integration Low cost: thin absorber, low cap ex equipment costs, high dep rate Best laboratory cell efficiency >17%, best module 14% (First Solar) Presently lowest price PV available (First Solar)
• First Solar <$0.75/W manufacturing cost• 2-3 hours from glass to module with 13% efficiency• Responsible for US lead in module manufacturing• Thin film success story!
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #63
Superstrate CdTe/CdS cell configuration
Glass superstrate (SLG or advanced)
SnO2 (comm or more complex TCO)
HR layer (intrinsic TCO, ~ 20 nm)
CdS (n-type, 30-50 nm)
CdTe (p-type, 2-4µm) + CdCl2 step
Contact (contains Cu as dopant)
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CdTe/CdS heterojunction cell structure:grain boundaries, S-Te interdiffusion, QE
Contact(Cu+other)
p-CdTe
n-CdS
Zn2SnO4Cd2SnO4
glass 400 500 600 700 800 900
1.0
0.8
0.6
0.4
0.2
0
Wavelength (nm)Q
uant
um e
ffic
ienc
y
CdCl2 HTas-deposited
CdS depletionCdTe1‐xSx alloy
Bandgap shift
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CdTe/CdS module processing
Clean glass/SnO2Substrate
DepositCdS, CdTe
FirstScribe
Post DepCdCl2 Treat
BackContact
SecondScribeThird
Scribe
Tabbing, Junct box
EncapsulateTest
CdTe Surface Etch
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #66
CdTe deposition technology
AtmosphericHigh Cl
O2
VacuumNo Cl
Low O2
T > 500°C T < 400°C
Spray ScreenPrint
CSSVT
PVDED
rf Sp
Golden PhotonMatsushita
Abound, USFFirst Solar, Calyxo
PrimeStar/GE, NREL, IEC
Canrom
BP Solar
XunLight 26
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #67
IEC CdTe Research using Vapor Transport Dep
CdTeSourceVapor Transport System CdCl2 Reactor
Develop process for Eff > 15% on moving glass substrate ( 3 cm/min) to provide basis for in-line manufacturing
Optimize: CdTe deposition, substrate, contacts, CdCl2 annealing Surface chemistry, interdiffusion, impurities, grain growth
Currently IEC CdTe work is proprietary; evaluating alternative glass, TCO, buffer layers
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #68
CdTe film growth: TSS, thickness, rateAFM 20 x 20 mSubstrate temp
Film thickness
Growth rate
500ºC5 m, 8 m/min
570ºC5 m, 8 m/min
550ºC1 m, 8 m/min
550ºC6 m, 8 m/min
550ºC23 m, 8 m/min
550ºC6 m, 81 m/min
Faceted morphology, grain size increases with substrate temperature
Grain size increases proportional with CdTe thickness
Grain size inversely proportional to CdTe growth rate
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #69
Fundamental CdTe issues Compensating defects → doping and defect control difficult
Difficult to achieve NA>5E14 cm3
High hole affinity → non-ohmic back contact (needs p+ Cu2Te) Unable to increase Voc>0.85V which is only 60% of EG
Highest efficiencies with non-commercial substrate Replace SLG with BSG glass ($) : more transparent, higher T deposition
Replace SnO2:F (FTO) with Cd2SnO4/Zn2SnO4 (CTO/i-ZnO)
Higher CdTe dep T allows improved grain structure (higher Voc, FF)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #70
New substrates enable higher CdTe dep T higher Eff
[McCandless et al, to be submitted (2012)]
Commercial Tec10 SLG/SnO2:F/i-ZTO vs R&D GL/Cd2SnO4/i-ZTO
Record CdTe FF>81% With new substrates
13.5%
16.5%
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #71
Manu-facturer
DepositTech.
Champion product % apa
Current nameplate capacity MW/a
Depo Process Pro´s
Depo ProcessCon´s
module
First Solar Low Press Vapor Transport
14.4 modChamp lab cell: 17.3%
2,700closing several lines
Simple and matured process, high thruput
Global player,Quality ??
Glass-glass
Primestar/GE
“Thermal evap.“ 12.8 mod 30, started construct 400 MW, on hold
? Techn. Status ?
Glass-glass
Abound Solar(CSU)
Low PressThermal evap.
15.7 cell Started construct,Closed 2012
Glass in, module out.
Lower efficiency?
Glass-glass
Calyxo(Q-cell)
Atmos. press. thermal evap.
13.4%Champion lab cell: 16.2%
80 Potential low cost, high rate
Quality? Techn. Status ?
Glass-glass
Source: Schock / SNEC Shanghai‚ May 2012 plus adds from Dimmler 38th IEEE PVSC
CdTe thin film manufacturing status
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #72
Brief discussion of a-Si based multijunction PV
Steven Hegedus
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #73
Why amorphous Si (a-Si:H) for thin film PV? Unique among thin film PV technologies
Easily vary doping (p or n), bandgap, crystallinity (a-nc), thickness, Well-established commercially viable multijunction process (>20 yrs)
Minimal deposition steps: PECVD plus sputter back contact (<200°C)
Highest cell efficiency (3-junction a-Si/a-SiGe/nc-Si) : 14% (stabilized)
Commercial module stabilized efficiency (2J a-Si/nc-Si) : 9-10%
Least difference between best cell and typical module efficiency: Oerlikon, Sharp, AMAT have tandem 12% cell and 10% module
Challenge: native and light induced defectslow mobility+lifetime limit efficiency; even as lowest cost PV in market, cannot compete
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #74
Multijunction multibandgap a-Si solar cells
Chapter 12, Handbook of Photovoltaic Science and Eng (Luque, Hegedus), Wiley 2011
‘micromorph’a-Si/nc-Si tandemoptimum trade-offbetween cost and efficiency
Eff=6-7% Eff=9-11% Eff=12-13-% Eff=12-13%
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #75
Current a-Si PV commercial status: deathwatch ~ 10-15 companies bought Oerlikon or AMAT turn-key fab lines 2007-
2009 during Si feedstock shortage and PV price increase Low efficiency, high cap ex, new a-Si PV at disadvantage Most now closed (bankrupt), few operating in Asia Subsequent c-Si overcapacity and PV module price collapse squeezed a-
Si from the cost side (its strength) now multi-Si comparable cost Appl Matl closed their a-Si fab line 2010, Oerlikon Solar closed 2012
United Solar (USSC/ECD) flex roofing product: closed 2012 after 30 yr
Sharp (?), Panasonic, Kaneka, NexPower, Astraenergy-Chint; 3Sun (Italian JV Sharp-Enel) are leading players, ~9-10% efficient 2J 58 MW Sharp micromorph tandem installed in CA in 2012
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #76
Dispell Two PV Myths
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #77
I heard it takes more energy to make a solar module than they can ever produce?
NO!!!! How many years of operation before reach energy break-even point?
Energy payback is <1.5 years for today’s crystalline Si wafer modules, even less for next generation thin film modules
With 25 year warrantees, today’s PV modules will be net producers of clean, CO2-free electricity for at least 23 years!
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #78
I heard you would have to cover the country with solar modules to make any ‘real’ energy?
Total energy:200x200 miles(50% coverage)
Electricity only:100x100 miles(50% coverage)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #79
Questions? Then buy this book!
2nd Edition (2011) of the mostcomprehensive PV book-1100 pages-6 new chapters-8 different cell technologies-TCO’s-performance characterization-batteries, inverters, system-policy, economics
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #80
Back-up slides
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #81
2012 TF PV industry expected production
0 200 400 600 800 1000 1200 1400 1600
First Solar - CdTe
Solar Frontier - CIGS
Sharp - thin Si
Astroenergy - thin Si
NexPower - thin Si
Trony - thin Si
Miasole - CIGS
T-Solar - thin Si
3Sun - thin Si
Solibro - CIGS
2102 Top Thin Film PV manufacturers
2012 MW produced
20
30
35
40
80
90
120
180
620
1530
Very fluid, obtained from Renewable Energy World on-line 6/4/2012
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #82
Double vs Triple Junction: United Solar/ECD
Guha, SPIE Photovoltaics 2009
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #83
Cadmium toxicity: perception problem Widely studied by Brookhaven Natl Lab and NREL Cd: lung, kidney, bone carcinogen CdTe: less soluble, more stable, less toxic Cd is by-product of Zn mining Choices: react Cd with Te and encapsulate behind glass in controlled
environment and generate clean energy; or leave it in exposed ore tailings
Not released to environment during roof-top residential fire EU: granted CdTe PV an exemption; politically vulnerable US: CdTe PV not classified as toxic waste (EPA) Japan, Korea: banned CdTe PV and closed research First Solar recycling/insurance program, “behind the fence” www.nrel.gov/cdte