High-efficiency thin film

nano-structured multi-junction solar

cells James S. Harris (PI) (Co-PIs-Mark Brongersma, Yi Cui, Shanhui Fan)

Stanford University

GCEP Research Symposium 2013 Stanford, CA October 9, 2013

2

Single-crystalline

Thin film III-V PV

cSi

Cost per area

Efficiency

40%

30%

20%

10%

Solar Cell Efficiency and Cost

glass

Konarka

(~1 $/W)

(<<1 $/W)

(>>1 $/W)

CPV

High efficiency

Low cost

Higher efficiency reduces entire system cost (BOS)

3

Lowering Cell Cost--Ultra-Thin Films

Substrate 30%

Epitaxial growth

50%

Processing 20%

Estimated cost distribution

for III-V solar cells

Scaled processing (3X)

Substrate recycling (10X)

Ultra-thin films decrease material cost

Increased throughput decreases capital cost (10X)

3% 10%

6%

81% Substrate

Epitaxial growth

Processing

Save cost Cost savings

4

Outline

Photon management in thin film solar cells

• Key to cost reduction and efficiency improvement

• Enhanced optical absorption by nano-structuring

Nanostructured III-V solar cells

• Conventional designs

GaAs nano-junction cell performance and challenges

• New designs for high efficiency nanostructured solar cells

• Dielectric nanostructure window layer solar cell

Ultra-thin film Si cells for tandem junction cells

Conclusions

5

Benefits of Photon Management

-30

-20

-10

0

10

20

0 0.2 0.4 0.6 0.8 1

Cu

rre

nt

de

nsi

ty (

mA

/cm

2)

Voltage (V)

Carrier

confinement

Photon Management

Black: Thin film GaAs cell

Blue: Conventional GaAs cell

Red: Ultra-thin film GaAs cell

with photon management

Photon management is crucial for ultra-thin film solar cells

Y. Kang et al. 39th

PVSC, 2013

6

2Ω/square at 90% transmittance and 10Ω/square at 95% transmittance

H. Wu, D. Kong, S. Fan, Y. Cui: Nature

Nanotechnology 8, 421 (2013)

Metal Nanowire Transparent Conducting Electrodes

7

Meso- and Nano- Wire Transparent Electrodes

P. Hsu, S. Wang, Y. Cui et. al. Nature

Communication 4: 2522, 2013.

Meso-scale metal wires

reduce sheet resistance

by at least one order of

magnitude with the same

transmittance.

8

Enhancing Open Circuit Voltage with Nanophotonic Design

S. Sandhu, Z. Yu, S. Fan, Optics Express 21, 1209 (2013);

44nm GaAs

bulk GaAs

A. Niv et al PRL (2012);

9

Model cell: 1 m thick cSi cell Light absorption vs angle & wavelength

Wave optics regime: Optimizing absorption = Optimizing resonance excitation

1: Local (Mie) resonance 2:Fabry-Perot Resonance 3:Guided mode resonance 4:Diffracted resonance

Z. Yu et al., PNAS 107, 17491, (2010).

Total absorption = Aggregate of contributions of all ‘narrow’ resonances

Nanostructure Light Trapping

10

Outline

Photon management in thin film solar cells

• Key to cost reduction and efficiency improvement

• Enhanced optical absorption by nano-structuring

Nanostructured III-V solar cells

• Conventional designs

GaAs nano-junction cell performance and challenges

• New designs for high efficiency nanostructured solar cells

• Dielectric nanostructure window layer solar cell

Ultra-thin film Si cells for tandem junction cells

Conclusions

11

Prior Nanostructured Solar Cells

Back reflector

Hsu et al. Adv. Energy. Mat. 2012

Light trapping absorbers

Oh et al. Nat. Nanotechnol. 2012

Antireflection

Xi et al. Nat. Photonics, 2007

D. Liang, et.al. Adv. Energy Mat., 2012

Nano-structured, flexible

GaAs thin film

12

Nanostructured p-n Junctions

B. M. Kayes, et al. J. Appl. Phys. 97, 114302 (2005)

Nanostructured p-n junction:

1) Enhance absorption by antireflection and light trapping

2) Decouple the absorption length and carrier transport

13

-0.1 0 0.1 0.2 0.3 0.4 0.5 0.6-5

0

5

10

15

20

25

Voltage (V)

Curr

ent D

ensity (

mA

/cm

2)

Planar

Nano-pyramid

Nano-dome

Challenges for III-V Nano-

structured Solar Cells

200nm

200 nm Cell Jsc (mA/cm2) Voc (V) Fill Factor Eff. (%)

Planar 5.1 0.49 0.57 1.44

Nano-dome 7.5 0.35 0.48 1.21

Nano-pyramid 18.5 0.32 0.28 1.67

Nano-wire * 15.5 0.20 0.27 0.83

Planar and nano-pyramid cells with

200nm thick p-n junction

200nm

Nanostructured GaAs cells enhance Jsc, however degrade Voc and F.F.

New designs required to improve Voc and F.F. * Czaban et al. Nano. Letter 2008

14

Metal Contact Problems and New Nano-Cell Design

Metal Mesa grid design eliminates shunts

Shunts at valleys Add Insulation layer,

still some shunts

Planar junction/mesa

separates contact

from nanostructures

15

Outline

Photon management in thin film solar cells

• Key to cost reduction and efficiency improvement

• Enhanced optical absorption by nano-structuring

Nanostructured III-V solar cells

• Conventional designs

GaAs nano-junction cell performance and challenges

• New designs for high efficiency nanostructured solar cells

• Dielectric nanostructure window layer solar cell

Ultra-thin film Si cells for tandem junction cells

Conclusions

16

Nano-Pyramid Cell Design

Replace nanocone GaAs with nanocone window (AlGaAs, InGaP)

A heterojunction window

in III-V solar cell

• Window layer has wide bandgap --- transparent to most light

• Window layer repels minority carriers back to the junction

• Transport Electron-hole pairs in nanocones to p-n junction

p-GaAs 0.3 um

n-GaAs 2.0 um

Metal

p-AlGaAs 1.0 um

n-AlGaAs 0.1 um

17

Nano-Pyramid Window Solar Cell

Nanocone window

p Al0.8Ga0.2As

p GaAs

n GaAs

Metal mesa

D. Liang et al. Nano Letters 2013

18

Nano-Pyramid Absorption

Reflection < 3% from 450nm to 850nm

Nanocone window

Planar window

500nm Completely black

even under 1-sun

illumination

D. Liang et al. Nano Letters 2013

19

Performance: Measured J-V (1 sun)

Voc (V) Jsc (mA/cm2) Fill Factor Efficiency(%)

Planar window 0.979 21.23 63.1 13.1

Nanocone window 0.982 24.40 71.0 17.0

Improvement 0.3% 15% 13% 30%

0

5

10

15

20

25

0 0.2 0.4 0.6 0.8 1

J (

mA

/cm

2)

Voltage (V)

500nm

Nanocone window

Planar window

D. Liang et al. Nano Letters 2013

20

0

0.5

1

Ba

nd

ga

p O

ffs

et

(eV

) E

g-V

oc

Best Voc in nanostructure-based solar cells

1.12 1.42 1.49 1.75

c-Si GaAs CdTe a-Si

Bandgap Offset (Eg - qVoc)

Mariani et.al. Nano.Lett. 2011

Fan et al. Nat. Mat. 2009 Hsu et al. Adv. Energy. Mat.2012

Zhu et al. Nano. Lett.2010

Czaban et al. Nano.Lett.2008

Oh et al. Nat. Nanotechnol. 2012

Putnam et al. Energy Environ. Sci. 2010

Our work Nov 2012

Our work May 2012

Zhao et al. Appl. Phys. Lett. 1998

Kayes et al. 37th PVSC. 2011

21

Benefits of Nano-Pyramid Window

Enhanced light absorption and carrier collection from nanocone window

---- 15% improvement in Jsc (24.4 mA/cm2)

High-quality and low-area junction minimize J0

----- high Voc (1.003 V)

Metal mesa contact improves shunt and series resistance

----- good FF (71%)

----- 30% enhancement in efficiency (17%)

22

0 20 40 60 800

0.1

0.2

0.3

0.4

Incident Angle (degree)

Re

flecta

nce

(A

.U.)

SLARC

DLARC

Si3N

4 Nano

Nanostructured Dielectric Window

0.4 0.6 0.80

0.5

1

Wavelength ( m)

Re

flection

(A

.U.)

Planar

n=1.9

n=2.0

n=2.1

n=2.2

Reflection < 10%

Transparent, large band gap (5.5eV)

Refractive index ~2 and tunable

Widely used as anti-reflective coating

MgF2

SiO2

Si3N4

ZnO

ZnS TiO2

Al2O3

MgO

3

4

5

6

7

8

9

10

11

1.2 1.4 1.6 1.8 2 2.2 2.4 2.6

Ban

d g

ap

(e

V)

Refractive index

0 20 40 60 800

0.05

0.1

0.15

0.2

Incident Angle (degree)

Re

flecta

nce

(A

.U.)

DLARC

Si3N

4 Nano

AlGaAs Nano

23

0.3 0.4 0.5 0.6 0.7 0.8 0.90

0.2

0.4

0.6

0.8

1

Wavelength ( m)

Absorp

tion (

A.U

.)

Si3N

4 window

Planar

Ultra-Thin Nano-Pyramid Cells

Strong light trapping, 88% absorption enhancement

Guided lateral propagating mode

Jsc = 28.55 mA/cm2

Jsc = 15.18 mA/cm2

Absorption in 200nm GaAs slab

24

Outline

Photon management in thin film solar cells

• Key to cost reduction and efficiency improvement

• Enhanced optical absorption by nano-structuring

Nanostructured III-V solar cells

• Conventional designs

GaAs nano-junction cell performance and challenges

• New designs for high efficiency nanostructured solar cells

• Dielectric nanostructure window layer solar cell

Ultra-thin film Si cells for tandem junction cells

Conclusions

25

Nano-Pyramid Si Cell Structure

Si3N4 window

Si Cell

n-type 0.3um

p-type 1.7um

1 x 1019

p-substrate 1 x 1015

[intrinsic Si] 0.9um

3 x 1018

Metal

contact

Y. Kang et al. to be published, 2013

26

0

5

10

15

20

25

30

0 0.2 0.4 0.6

Cu

rre

nt

den

sit

y

(mA

/cm

^2

)

Voltage (V)

Si3N4 nano window

Si3N4 SL ARC

w/o Si3N4

J-V Characteristics

Voc (V) Jsc (mA/cm^2) Eff. (%) F.F. (%)

Si3N4 Nano window 0.57 28.15 11.44 71.26

Si3N4 SL ARC 0.56 26.11 10.22 69.51

W/O Si3N4 0.54 21.32 8.08 69.32

Si3N4 nano-cone

enhances

Jsc by 32%

Eff. by 43%

Y. Kang et al. to be published, 2013

27

External Quantum Efficiency

0

10

20

30

40

50

60

70

80

90

400 500 600 700 800 900 1000 1100

EQ

E (

%)

Wavelength (nm)

Si3N4 nano window

Si3N4 SL ARC

w/o Si3N4

• In the visible light region, EQE is enhanced by over 30%;

• In the near infrared region, EQE is enhanced by ~15%,

due to the low absorption in thin film. Y. Kang et al. to be published, 2013

28

Summary

Demonstrated Highest Efficiency Nano-structured III-V solar cell

• Nano-scale photon management enables ultra-thin film solar cells

to achieve high efficiency

• The nano-window, planar junction, mesa contact design

simultaneously improves junction quality, light absorption, carrier

collection and eliminates shunting defects

• The AlGaAs nano-cone window solar cell showed and enhanced

efficiency of 17%

• Demonstrated highest transparency nano-metal contact grid

• Demonstrated thin film lift-off and flexible GaAs cell

• The Si3N4 nanostructure window design reduces the optical losses

in window layer and enhances the efficiency by 43%

29

Acknowledgements

Thank You

STUDENTS and POSTDOCS

Brongersma Group Fan Group

Dianmin Lin Sunil Sandhu

Cui Group Harris Group

Hui Wu Yangsen Kang

Desheng Kong Yusi Chen

Po-Chun Hsu Yijie Huo

Shuang Wang

Corporate Collaborators Research Support

Solar Junction

Solexel

OEpic