The document provides an overview of current status and research focus areas for various solar photovoltaic technology platforms, including crystalline silicon PV and thin film technologies. For crystalline silicon PV, the baseline technology is discussed along with near term research focus on new methods for emitter formation, passivation and device architecture to improve performance. For thin film technologies, the status and research focus for CIGS, CdTe, and a-Si/nc-Si technologies is summarized. The document also discusses the Institute of Energy Conversion at the University of Delaware and its research program goals and facilities for thin film and crystalline silicon photovoltaic research.
Current Status of Solar Photovoltaic Technology Platforms, Manufacturing Issues and Research
1. Current Status of Solar Photovoltaic
Technology Platforms, Manufacturing
Issues and Research
Steve Hegedus
Institute of Energy Conversion
University of Delaware
With assistance from IEC staff:
Brian McCandless (CdTe), Bill Shafarman (CIGS)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #1
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 #2
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) First flexible
Soft funded - government and industry contracts 10% cell
2012 staff: 11 professional, 3 tech, 2 admin, 5 post
doc, >20 grad students (4 depts)
4x4 inch
Recently rec’d $8.4M from DOE (3 year grants) minimodule
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #3
4. 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 #4
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
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #5
6. 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)
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7. The Big Picture:
PV applications and achievements
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8. PV can be installed anywhere, 10’s Watts to 100’s Megawatts
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #8
9. 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 #9
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
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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
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12. Brief Overview of PV Basics
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #12
13. 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- V+
e-
h+ load
h+ V-
charge separation
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #13
14. Cell efficiencies vs. bandgap EG
Record performance single junction cells vs. theoretical limit
Expect maximum
performance
with EG ≈ 1.5 eV
But theor eff >25%
possible EG ≈ 1 – 1.8 eV
a-Si/nc-Si 2J
Many thin film, III-V options
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #14
15. 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 #15
16. One common PV challenge: reducing gap
between champion cell and module efficiency
Multi c-Si
and TF CIGS
both ~20%
cell efficiency.
But mc-Si
has more
mature module
technology.
Wolden et al, JVST-A 29 (2011) 030801
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17. Why efficiency matters – fixed BOS costs
Levelized cost of Energy:
• Lifecycle costs/energy
• LCOE costs include
Balance of System which
scale with # modules, area
• Lower eff = higher BOS$
• More rack, wiring, install $
• y-intercept is system price
without module
• Si modules ‘selling’ at $0.9/W
Or $120/m2
Wang et al Renewable and Sustainable Energy Rev (2011)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #17
18. Crystalline Si (c-Si) Technology
and Advanced Concepts
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #18
19. Standard commercial Si PV cell process
start
900°C
900°C
400°C finish
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #19
20. Commercial Si Solar Cell, Eff ~ 15-17%
Front contact (Screen printed Ag fired through SiN)
Random
textured
F and R
(~ 0.3 µm)
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21. World record Si solar cell: PERL
• 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
Cell Voc (V) Isc (mA/cm2) FF(%) Eff(%)
PERL 0.70 42 81 24.7
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22. Conflicting emitter properties: pn junction vs
resistance vs absorbing ‘dead layer’
property Advantage Disadvantage
Increase thickness •Reduce lateral R for •Increase absorption in
current flow to Ag highly defect layer
contact (photons not converted
•Prevent melting Ag SP to e-h pair), lower blue
metal penetrate to base QE and Jsc
Increase doping •Reduce lateral R •Increase defects and
•Reduce contact R with recombination (Io) so
Ag or other metal grid decrease Voc
Increase bandgap •Decrease absorp loss •Increase lateral
•Increase band bending, resistance significantly,
reduce recomb (Io) req second conductive
layer ($$)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #22
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 #23
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 #24
25. Hi Eff concept #1: the 2-step ‘selective emitter’
Conventional 1 step 2 step emitter: thinner n+ everywhere
thick emitter except under metal n++
Increased blue Applied
response with Materials
thinner n+ website
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #25
26. SE option 1: laser doping + plating metal
http://www.photonics.com/Article.aspx?AID=40098
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27. SE option 2: deposit
thicker n++ then etch
back
• requires alignment of front metal
to thicker n++ mesa
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #27
28. 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 #28
29. Hi Eff #2 Heterojunction Solar Cell: deposited a-Si
passivation layers reduce surface recombination
10nm (p)a-Si:H 10nm (i)a-Si:H
30nm (n)a-Si:H
Rear
Device: n-type c-Si wafer and 5-10 nm
60nm
ITO Contact PECVD a-Si layers (EG=1.7-1.8 eV)
(Al)
hυ (i) a-Si:H surface passivation layers (both sides)
(p)a-Si:H emitter (front)
Front
Contacts
(n)a-Si:H back contact (rear)
(Ag)
300μm n-c-Si wafer All a-Si and contacts deposited <200°C (low $,
Electron Current less defects, no warping thin wafers)
High efficiency and VOC (Sanyo/Panasonic):
EF EC
η = 23% champion cells, 19% modules
VOC = 740mV
EV
Hole Current
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #29
30. Two modes of c-Si Surface Passivation by a-Si:H
Field effect passivation:
Defect neutralization by H atoms: Increased band bending at junction
Reduce c-Si surface dangling bonds Repel/separate majority or minority carrier
Reduce recombination (IO), increase VOC Reduce Io, increase VOC
EC
EC
EF EF
EG,c-Si EG,a-Si:H EG,a-Si:H
EG,c-Si
EV
EV
c-Si Substrate a-Si:H film (n)c-Si Substrate (i)a-Si:H film
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
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #30
31. Hi Eff Si #3: Interdigited back contact (IBC) cell
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32. Standard front junction vs all back contact
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #32
33. IBC spectral response higher in
short (blue) and long (IR) wavelengths
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #33
35. Integration of both device concepts: SHJ-IBC Cell
TCO
p-type a-Si
intrinsic a-Si
n-type c-Si
intrinsic a-Si
n-type a-Si
TCO
Silicon heterojunction (SHJ) solar cell. Interdigitated back contact (IBC) solar cell.
First published •intrinsic a-Si buffer
results on SHJ-IBC •separate a-Si p
By IEC (APL 2007) and n regions
IBC-SHJ solar cell (IEC structure)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #35
36. IEC Multichamber PECVD for HIT and IBC-SHJ
4 chambers plus 2 load lock, DC/RF/VHF plasma
Multiple substrate sizes (1x1 up to 12x12 inch)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #36
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 #37
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
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #38
39. TFPV applications: BIPV
Appearance preferred for building-integrated PV
Semitransparent a-Si
Architectural skylight
85kW Shell Solar
Cu(InGa)Se2 in Wales
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #39
40. Flexible a-Si on SS: BIPV (flex laminate)
Flexible PV for roll-out rooftop installation (USSC triple
junction/SS)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #40
41. 4 MW of CdTe installed by Tucson Electric
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #41
42. 15 MW of 3Sun Tandem Thin Si in
Altomonte, Calabria Italy
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #42
43. Cu(InGa)Se2
Thin Film Solar Cells
Help from
Bill Shafarman
Institute of Energy Conversion
University of Delaware
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #43
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 #44
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 Grid
ZnO:Al
Manufacturing CdS
Many companies with various approaches
Cu(InGa)Se2
1. Reaction of metal precursors (2-step)
Low cost deposition of metals Mo
Batch process: “selenization”
Substrate
2. Multi-source evaporation (1-step)
In-line process, high temp
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #45
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
1
bsorption
0.8
CIGS
0.6
Si
elative A
0.4
0.2
R
0
0.001 0.01 0.1 1 10 100 1000
thickness (µm)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #46
47. Cu(InGa)Se2 Grain Structure
Films are polycrystalline with rough surface
Grain size ~ 0.1 – 1 µm depends on deposition conditions
e.g. substrate temperature during evaporation
But device performance is remarkably insensitive to grain size,
morphology
Cu(InGa)Se2
1µm
Mo
deposited 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 #47
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)
Ga/(In+Ga)
S/(Se+S) a x=0
b x=0.24
c x=0.61
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #48
49. Increasing efficiency and VOC at higher EG
Increasing EG with alloy (Ga, Ag, S) Failure to capture benefit of larger EG
to push efficiency at EG >1.3 eV 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 #49
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
Substrate Heater
Substrate at 400-600°C
Film Growth Monitor
To vacuum pump
Thermal Evaporation Pbase ≈ 1x10-6 Torr
Sources for Cu, In, Ga, Se, (S) Prun ≈ 2x10-5 Torr
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #50
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 #51
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 Mo/Cu(InGa)Se2
Mo/Cu/Ga/In H2Se/H2S
or Se/S
400 - 600°C
Reaction in hydride gases (H2Se, H2S) or elemental vapors (Se,S)
Multi-step reaction pathway to form Cu(InGa)Se2
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #52
53. 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 #53
55. CIGS thin film manufacturing status: > 30MW real capacity
Manu- Deposition Champion Current Deposition Deposition Process comments
facturer Technology product % nameplate Process Con´s
apa capacty MW/a Pro´s
Manz 1-stage co- 15.1 30 Simpler, more Sacrifices efficiency Glass-glass
(Würth) evaporation 120 advanced process Cd- buffer
Solibro 14.4
Global Solar 3-stage co- 15 (cell) Highest known TF Complex process SS substrate.
Energy evaporation 13 (mod) module efficiency glass/polymer
MiaSolé Reactive sputter 15.7 >40? good efficiency Complex process SS substrate, cut +
potential, rel. stitch, glass-glass
small capex exp. CdS-dry
Solar 2-step: Sputter+ SF: 17.8 980 advanced Sacrifices efficiency glass-glass
Frontier, H2Se/S- SF: 14.1 process, Higher OpEx than CdS
Stion/ Selenization (manu) 5 + 135 potentially higher evap. glass-glass
TSMC Stion: 14.5 +300 CapEx Cd-free
TSMC 15.1
Avancis, 2-step: 30 + 100 Glass-glass
Hyundai Sputter+Se- +100 CdS (Cd-free)
evap.+ RTP-
cryst./H2S
Solo Power 2-step 15.1% cell 30 + 400 Good metal Sacrifices efficiency SS substrate
electroplate- 13.5% mod utilization, rel. low Polymer
Selenization CapEx CdS
Source: MarkusSpectra/ Webinar San Francisco‚ Feb. 2012 plus adds from09/27/12 #55
Photonic Beck Photon “PV manufacturing and research” Hegedus the author
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 #56
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 #57
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
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #58
59. 2. H2Se/H2S reaction of Cu-Ga-In Precursors:
Ga segregation at rear limits EG and Voc
• 30’ H2Se@450°C, 15’ H2S@450°C • 15’ H2Se@450°C, 15’ H2S@450°C
• Complete H2Se reaction prior to H2S • Partial H2Se reaction prior to H2S
• Ga segregated at back, none at front • Ga uniform, higher at front
• Low EG at front junction, low Voc • Higher EG at front junction, higher Voc
60 60
50 Se 50
)
)
com osition (%
Se
positio (%
40 40
n
30 Cu Mo 30
Cu
Mo
20 In
p
20
In
com
10 10
S Ga S Ga
0 0
0 20 40 60 80 0 20 40 60 80
sputter time (min) sputter time (min)
Hankett et al, Proc 4th World PVSEC, 2006
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #59
60. Single step H2Se process vs. Three-step H2Se/H2S process
20
Single step process Eff Voc Jsc FF
Temp,
2
Single-step : 8.3% 0.383 V 37.0 mA/cm 58.7%
°C
450 2
3-step : 14.2% 0.599 V 32.2 mA/cm 73.5%
Single step: 0
J (mA/cm )
H2Se
2
20 60 ~ 90 Time, min Mo
Three-step process -20 Ga accumulation
550
Temp,
Mo
°C
400 -40 Ga homogenization
1st step : 2nd step : 3rd step :
H2Se Ar H2S
-0.2 0.0 0.2 0.4 0.6 0.8
20 50 10 10 20 10 V (V)
Time, min
Process optimization Ga distributed uniformly with 3 step H2Se/H2S
Major improvement in VOC and Eff.
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #60
61. CdTe Thin Film Solar Cells
Help from
Brian McCandless
Institute of Energy Conversion
University of Delaware
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #61
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 <$0.75/W manufacturing cost
• 2-3 hours from glass to module with 13% efficiency
• Responsible for US lead in module manufacturing
• Thin film success story!
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #62
63. Superstrate CdTe/CdS cell
configuration
Glass superstrate (SLG or advanced)
SnO2 (comm or more complex TCO)
HR layer (intrinsic TCO, ~ 20 nm)
CdS (n-type, 30-50 nm)
CdTe (p-type, 2-4µm) + CdCl2 step
Contact (contains Cu as dopant)
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #63
65. CdTe/CdS module processing
Clean
glass/SnO2 First Deposit Post Dep
Substrate Scribe CdS, CdTe CdCl2 Treat
CdTe Surface Second
Third Back
Etch Scribe
Scribe Contact
Tabbing,
Junct box Test
Encapsulate
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #65
66. CdTe deposition technology
T > 500°C T < 400°C
Atmospheric Screen
Spray
High Cl Golden Photon Print
O2 Matsushita
Vacuum Abound, USF PVD Canrom
CSS
No Cl First Solar, Calyxo
ED
BP Solar
VT PrimeStar/GE,
Low O2 NREL, IEC
XunLight 26
rf Sp
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #66
67. IEC CdTe Research using Vapor Transport Dep
Develop process for Eff > 15% on moving glass substrate ( 3 cm/min) to
provide basis for in-line manufacturing
Optimize: CdTe deposition, substrate, contacts, CdCl2 annealing
Surface chemistry, interdiffusion, impurities, grain growth
Currently IEC CdTe work is proprietary; evaluating alternative glass, TCO, buffer
layers
CdTe
Vapor Transport System Source CdCl2 Reactor
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #67
69. Fundamental CdTe issues
Compensating defects → doping and defect control difficult
Difficult to achieve NA>5E14 cm3
High hole affinity → non-ohmic back contact (needs p+ Cu2Te)
Unable to increase Voc>0.85V which is only 60% of EG
Highest efficiencies with non-commercial substrate
Replace SLG with BSG glass ($) : 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 #69
70. New substrates enable higher CdTe dep T higher Eff
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%
[McCandless et al, to be submitted (2012)]
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #70
71. CdTe thin film manufacturing status
Manu- Deposit Champion Current Depo Depo Process module
facturer Tech. product % apa nameplate Process Con´s
capacity MW/a Pro´s
First Solar Low Press 14.4 mod 2,700 Simple and Global player, Glass-
Vapor Champ lab cell: closing matured Quality ?? glass
17.3% process, high
Transport several lines
thruput
Primestar/ “Thermal evap.“ 12.8 mod 30, started ? Techn. Status Glass-
GE construct ? glass
400 MW, on
hold
Abound Low Press 15.7 cell Started Glass in, Lower Glass-
Solar Thermal evap. construct, module out. efficiency? glass
(CSU) Closed 2012
Calyxo Atmos. press. 13.4% 80 Potential low Quality? Glass-
(Q-cell) thermal evap. Champion lab cost, high rate Techn. Status glass
cell: 16.2% ?
Source: Schock / SNEC Shanghai‚ May 2012 plus adds from Dimmler 38th IEEE PVSC
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #71
72. Brief discussion of a-Si based multijunction PV
Steven Hegedus
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #72
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 defects low mobility+lifetime
limit efficiency; even as lowest cost PV in market, cannot compete
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #73
74. Multijunction multibandgap a-Si solar cells
‘micromorph’
a-Si/nc-Si tandem
optimum trade-off
between cost and
efficiency
Eff=6-7% Eff=9-11% Eff=12-13-% Eff=12-13%
Chapter 12, Handbook of Photovoltaic Science and Eng (Luque, Hegedus), Wiley 2011
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #74
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 #75
76. Dispell Two PV Myths
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #76
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 #77
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 #78
79. Questions? Then buy this book!
2nd Edition (2011) of the most
comprehensive 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 #79
81. 2012 TF PV industry expected production
Very fluid, obtained from Renewable Energy World on-line 6/4/2012
2102 Top Thin Film PV manufacturers
Solibro - CIGS 20
3Sun - thin Si 30
T-Solar - thin Si 35
Miasole - CIGS 40
Trony - thin Si 80
NexPower - thin Si 90
Astroenergy - thin Si 120
Sharp - thin Si 180
Solar Frontier - CIGS 620
First Solar - CdTe 1530
0 200 400 600 800 1000 1200 1400 1600
2012 MW produced
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #81
82. Double vs Triple Junction: United Solar/ECD
Guha, SPIE Photovoltaics 2009
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #82
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
Photonic Spectra Webinar “PV manufacturing and research” Hegedus 09/27/12 #83