high efficiency photovoltaics on inexpensive flexible substratesvselvama/projects/cv-_zhang.pdfnovel...
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Novel approach for high efficiency thin film photovoltaics on inexpensive
flexible metal substratesflexible metal substrates
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Multijunction III-Vs yield higher efficiency, but still work only with single crystalline filmswork only with single crystalline films
• Single crystal Ge substrates are available only in small dimensions
• Single-crystalline like thin films with good lattice match with III-V semiconductors on large-area polycrystalline substrates can bepolycrystalline substrates can be game-changing in realizing high-efficiency, lower cost PV.
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Ion Beam Assisted Deposition (IBAD) – A technique to produce near single crystal films on polycrystalline orproduce near single crystal films on polycrystalline or amorphous substrates
• Essentially, any substrate can be used – stainless steel, nickel alloys, glass,Essentially, any substrate can be used stainless steel, nickel alloys, glass, polymer …(room temperature process)
• Biaxial texture achieved in certain conditions of ion bombardment resulting in grain-to-grain misorientation in film plane of about 5 degrees !
• Only 10 nm of IBAD film is needed – very fast process !
spool
substrate
Ion assistDeposition
Biaxial texture
Grains in the IBAD film are arranged in a 3-Deposition source
Grains in the IBAD film are arranged in a 3-dimensional aligned structure with grain-to-grain misorientation in any axis less than 5 degrees –essentially a near-single crystalline structure 3
Epitaxial single crystalline-like films on polycrystalline or amorphous substrates based on IBADamorphous substrates based on IBAD
• A near single crystalline film is achieved by IBAD under specific conditions of ion beam energy, ion/atom ratio and ion beam orientation.
• Once a template is created, this near-single-crystalline structure can be transferred epitaxially to many other films.
3 56.412.022.241.476.9143.0
YBCO
100 nm
YBCO
100 nm100 nm100 nm
1.01.93.5
YBCO(113)Reflection High Energy
LaMnO3
MgO (IBAD + Epi layer)
Y O
LaMnO3
MgO (IBAD + Epi layer)
Y O YBCO(113)g gyElectron Diffraction of growing IBAD film showing biaxial texture development within a few nanometers
X-ray polefigure showing a high degree of biaxial texture in a superconducting YB C O fil
Al2O3
Y2O3
Hastelloy C-276
Al2O3
Y2O3
Hastelloy C-276
YBa2Cu3Ox film grown epitaxially on a IBAD MgO film even though the lattice mismatch is about 8% 4
IBAD on flexible metal substrates have been successfully used in high-performance highsuccessfully used in high-performance high temperature superconductorsUsing IBAD template films on Hastelloy substrate, a multilayer thin film stack is g p y , yepitaxially grown to produce HTS thin film wires with current density > 5 MA/cm2, same as in film on single crystal substrate
LaMnO3 (Lattice match)
< 0.1 mmCu
Homo-epi MgO (texture improvement)IBAD MgO
Y2O3 (nucleation layer)Al2O3 (diffusion barrier)
Cu
Hastelloy substrate
R2R tools in industry for pilot manufacturing of 1,500 m lengths of IBAD-based epi films
MOCVD
Substrate Electropolishing
Hastelloy substrateElectropolishing
IBAD Magnetron Sputtering
R2R tools developed for in-line and off-line characterization of long IBAD-based epi films
R2R XRD for phi-scan measurements
R2R tools developed for in-line and off-line characterization of long IBAD-based epi films
R2R XRD for phi-scan measurements
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es)
6
7
xtur
e (d
egre
e
4
5
In-p
lane
tex
40 200 400 600 800 1000 1200 1400 1600
Tape position (m)
R2R tools developed for in-line and off-line characterization of long IBAD-based epi films
I li lli tIn-line ellipsometry
In-line vision system
5 m
In-line XRD in MOCVD Electrical testing
IBAD-based tapes with epitaxial layers are
Electric Insulation(PPLP + Liquid Nitrogen)
Stainless Steel DoubleCorrugated Cryostat
routinely produced in 1,500 meter lengths
Cu StrandedWire Former
(PPLP + Liquid Nitrogen) g y
135 mm HTS cable
Cu Shield2G HTS wire(2 shield Layers)
2G HTS wire(3 conductor Layers)
IBAD-based superconductors have been successfully inserted in the electric power grid350 m long cable in downtown Albany, NYSupplied electric power to 25,000 households
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IBAD templates provide great opportunities for photovoltaic thin films
n-type epi siliconAntireflection coating
Amorphous layerBiaxially-textured IBAD layer
Epitaxial layerp-type epi silicon
n-type epi silicon
Metal or glass substrate
p y
GaInAsGaInP
Antireflection coating
GaInAsGaInP
Antireflection coating
Metal or Amorphous layer
Biaxially-textured IBAD layer
Lattice match cap layerGeEpitaxial graded layer
Metal or Amorphous layer
Biaxially-textured IBAD layer
Lattice match cap layerGeEpitaxial graded layer
ceramic substrateceramic substrate
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Reel-to-reel deposition of IBAD-based ptemplates for III-V photovoltaicsAll samples fabricated in this work were made by reel-to-reel deposition in two p y pIBAD and two buffer (magnetron sputtering) systems
Ge does not grown epitaxially on MgO g p y g
250030003500
Ge(220)
600°C
Homo-epi MgOIBAD M O
Ge
100015002000
Cou
nts
MgO(200) GeGe
Ge(111)
IBAD MgO
0500
20 30 40 50 60 702 theta (°)
(400)Ge(311)
(111)
4000
Hastelloy substrate
1000
10000MgO(200)
Ge(220)
GeG
Ge(111)
600°C3000
4000 600 650 720
770 820 850
10
100
Cou
nts
Ge(400)
Ge(311)
1000
2000C
ount
s
120 30 40 50 60 70
2 theta (°)
0
64 65 66 67 68 69 702 theta (°) 13
Ge does not grown epitaxially on buffer layers d l d f HTS i IBAD t l tdeveloped for HTS using IBAD templates
LaMnO3Homo-epi MgO
IBAD MgOGe • Lattice constants: MgO – 0.422 nm.
Ge – 0.565 nm.
Hastelloy substrate
IBAD MgO• Even if Ge grows 45° rotated on
MgO, still a 5.2% mismatch.
• LaMnO3 :a = 0 553 nm ( 2 2% mismatch)
6000
Bare LMO 500°C 530°C 550°C 580°C 650°C
LMO (200)
Ge (111)
MgO (200)
Ge (311) Ge (400)
Ni alloy substrate
Hastelloy substrate
a = 0.553 nm (-2.2% mismatch)b = 0.571 nm (1% mismatch)
• No 45° rotation needed. 3000
4000
5000
Coun
ts( )
In spite of the smaller lattice mismatch, Ge does not grow epitaxially on 1000
2000
3000
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LMO surface0
20 30 40 50 60 70
2 theta (°)
Structural match considerations for epi growth of p gGe on IBAD MgO based flexible substrates
Basal plane projectionsp p j
Str ct ral match bet een Ge & MgO and bet een Ge & LMO is poor
Ge – diamond MgO – rock salt CeO2 - fluoriteLaMnO3 – perovskite
• Structural match between Ge & MgO and between Ge & LMO is poor• Very good structural match between Ge & CeO2
• CeO2 – 0.5411 nm (-4.4% mismatch with Ge)
Can Ge grow epitaxially on CeO2 on IBAD MgO based flexible substrates ?
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Strongly textured Ge (400) obtained on CeO2 on g y ( ) 2IBAD MgO tape LaMnO3
Homo-epi MgOIBAD M OGe
CeO2
IBAD MgOGe
7000 CeO2 (200) 10000 CeO2 (200) Ge (400)
Hastelloy substrate
4000
5000
6000
unts
Ge (400)
100
1000
unts
CeO2(400)
MgO (200)
LMO (100)
1000
2000
3000Cou
CeO2(400)
MgO (200)LMO (100)
Ge(111) 10
100C
ou Ge(111)
0
20 30 40 50 60 70
2 theta (°)
(100)1
20 30 40 50 60 702 theta (°)
16V. Selvamanickam et al. J. Crystal Growth 311, 4553 (2009)
Excellent in-plane texture obtained in Ge films on pIBAD templates on metal substrates
ts)
y (a
rb. u
nit
Inte
nsity
0 100 20050 150 250
Phi angle (°)
I l t t f 6 6° hi d i G fil IBAD M O t l t hi h i
Ge (111) polefigure Ge (111) phi scan
Phi angle ( )
• In-plane texture of 6.6° achieved in Ge films on IBAD MgO templates, which is comparable with typical texture of oxide epitaxial films grown on IBAD MgO.
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Excellent in-plane texture obtained in Ge films on pIBAD templates on metal substrates
7
4
5
6
7
extu
re (°
)
1
2
3
In P
lane
Te
00 1 2 3 4 5 6
Ge film thickness (µm)
Ge (111) polefigure Ge (111) phi scan
• In-plane texture improved to 1° in thicker Ge film i.e. all grains aligned with respect to each other within 1° !
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Refraction index and Extinction Coefficient measurements on Ge on IBAD substrates• Data matches very well with that of single crystalline Ge indicating the y g y g
near single crystalline nature of Ge on IBAD substrates
8
5
6
750 nm Ge on CeO/IBAD300K
ctio
n (n
)
Ellipsometry Measurement Reference bulk Ge
2
3
4
5
ex o
f Ref
rac
2 3 4 5 6 70
1
2
Inde
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Energy (eV)
V. Selvamanickam et al. J. Crystal Growth 311, 4553 (2009)
Epitaxial 0.9 μm thick GaAs on Ge on IBAD MgO• GaAs grown by Molecular Beam Epitaxy (MBE) epitaxially on Ge on IBAD
at 550°C
GaAs
Ge
CeO2
LaMnO3
20HastelloyAl2O3
MgO
A. Freundlich et. al. Proc. 35th IEEE Photovoltaic Specialists Conference (PVSC), Honolulu, HI. p 002543-5, (2010)
Strong photoluminiescence obtained in GaAs on g pIBAD MgO-based flexible metal substrate• Narrow (FWHM~20 meV) band-edge excitons measured in this film ( ) g
indicates a good optoelectronic quality of deposited GaAsFreundlich_PV group_April 2009
Band-edge Excitons 1μm MBE grown GaAs
Freundlich_PV group_April 2009
Photoluminescence 1μm MBE grown GaAs G /C2O/LMO/fl ibl t l
EX=1.526 eVΔE~10 meV
units
) 10K PL
on Ge/C2O/LMO/flexible metal
Donor-AcceptorBands
.uni
ts) Ref substrate (10K)
MBE08-025 10K 40K70K
on Ge/C2O/LMO/flexible metal
PL FWHM~ 20 meVen
sity
(arb
.
Defect Bands
GaAstens
ity (a
rb 70K
Bulk GaAsexciton1.515 eV
InteBand-edge
ExcitonInt
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1.50 1.52 1.54 1.56Energy (eV)
1.0 1.1 1.2 1.3 1.4 1.5 1.6Energy (eV)
Defects in Ge layer need to be reduced• Well defined GaAs (004) peaks observed in high resolution XRD• Width of peaks suggest defect/dislocation density ~ 5 ×108 cm-2.• Etch pit measurements on Ge show defect density of 6 ×108 cm-2• Etch pit measurements on Ge show defect density of ~ 6 ×108 cm 2.
GaAs(004)
PV Group_Freundlich_UH
GaAs/Ge/CeO2/LMOGaAs(004)
PV Group_Freundlich_UH
GaAs/Ge/CeO2/LMO
FWHM ~ 1900 arcsecits
) FWHM ~ 1900 arcsecits
)
as grown Ge 1900 arcsec
ensi
ty (a
rb. U
n
Ge/CeO2/LMO
1900 arcsec
ensi
ty (a
rb. U
n
Ge/CeO2/LMORTA @ 800OC -30Sec Ge (004)
FWHM
Int Ge/CeO2/LMO
RTA @ 800OC -30Sec Ge (004)
FWHM
Int Ge/CeO2/LMO
Ge after etch
22-7200 -3600 0 3600 7200
~ 2800 arcsec
Bragg Angle (arcsec)-7200 -3600 0 3600 7200
~ 2800 arcsec
Bragg Angle (arcsec)
Defects concentrated at the CeO2 – Ge interface d d ith i i thi kand decrease with increasing thickness
Ge
Ge
200 nm
CeO2
Hastelloy
Ge
Hastelloy
CeO2
MgO
200 nmHastelloy
23100 nmHastelloy
MgO
Improved GaAs with MBE Ge overlayerp y
MBE GaAs (1μm)/Ge (0.4μm) on IBAD Ge/CeO/LMO
GaAs (400)[110]
- MBE GaAsT~550 C
[110]-
[110]- MBE GaAs
T~550 C
FWHM ~860 arcsec
~x4
T~550 C
~x4
T~550 C
G a A s (0 0 4 )
P V G ro u p _ F re u n d lic h _ U H
G a A s /G e /C e O 2 /L M OG a A s (0 0 4 )
P V G ro u p _ F re u n d lic h _ U H
G a A s /G e /C e O 2 /L M O PV Group_CAM_UHPV Group_CAM_UH
x2F W H M ~ 1 9 0 0 a rc s e c
b. U
nits
) F W H M ~ 1 9 0 0 a rc s e c
b. U
nits
)
R T A @ 8 0 0 O C -3 0 S e c G e (0 0 4 )
Inte
nsity
(arb
G e /C e O 2 /L M OR T A @ 8 0 0 O C -3 0 S e c G e (0 0 4 )
Inte
nsity
(arb
G e /C e O 2 /L M O
24GaAs/Ge MBE growth on Ge IBAD-7 2 0 0 -3 6 0 0 0 3 6 0 0 7 2 0 0
F W H M ~ 2 8 0 0 a rc s e c
B ra g g A n g le (a rc s e c )-7 2 0 0 -3 6 0 0 0 3 6 0 0 7 2 0 0
F W H M ~ 2 8 0 0 a rc s e c
B ra g g A n g le (a rc s e c )
Patterned device structure on flexible substrate
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Modeling solar cell efficiencies on flexible gsubstrates
30GaAs solar cell Conventional cell (n-base= 3 μm)
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)Conventional cell (n base 3 μm)
thin base cell (0.2 μm)
Plasmon-enhanced thin cell
15
20
plasmonicdesignfic
ienc
y (%
10
gdefectreduction
AM
0 E
ff
0
5 Presenttarget
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104 105 106 107 108 109
Dislocation Density (cm-2)
Epitaxial Si on epi Ge on IBAD templates on p p pflexible metal substrate
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• Hall mobility ~ 56 cm2/Vs for 700 nm Si film
Next stepsp• Modify buffer and Ge growth processes to enhance epitaxy and
minimize interfacial defects.• Graded buffer deposition to minimize transition in lattice mismatch
• Minimizing Threading Dislocation Densities (TDD) in MBE/CBE grown GaAs
• Fabrication and testing of photovoltaic properties of GaAs solar cells on flexible substrates
• Set up reel-to-reel MOCVD and PECVD system for scaling up of III-V p y g pand Si deposition respectively on inexpensive, flexible substrates
For more information contact :• Prof. Venkat Selvamanickam (Selva)• E-mail : [email protected]
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• Web site : http://www.egr.uh.edu/me/faculty/selva