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

8

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

14

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

21

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)

25

)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

26

104 105 106 107 108 109

Dislocation Density (cm-2)

Epitaxial Si on epi Ge on IBAD templates on p p pflexible metal substrate

27

• 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 : selva@uh.edu

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• Web site : http://www.egr.uh.edu/me/faculty/selva