Nanophotonic light trapping for high efficiency solar cells

Kylie CatchpoleCentre for Sustainable Energy Systems, Research School of EngineeringAustralian National University, Canberra.

Kylie.catchpole@anu.edu.au

Why nanophotonics?

Thin film eg. c-Si, a-Si

mc-Si

Quantum dot Organic

c-SiApplications of nanophotonics

?

New types of solar cells are hard to texture or very thin

Nanophotonics for solar cells

Localized surface plasmons

≈ (wavelength scale)

<< (sub-wavelength): effective medium

>> : geometrical optics

10m

Scattering back reflectors

Diffraction gratings

Catchpole et al. (2011) MRS Bulletin 2011; 36(6) : 461-467

Nanophotonics for light trapping

Mokkapati and Catchpole, Journal of Applied Physics - Focused Review 112, 101101 (2012)

Progress for crystalline silicon

Mokkapati and Catchpole, Journal of Applied Physics - Focused Review 112, 101101 (2012)

Open symbols – theory

Closed symbols - experiment

Nano-cones (Wang et al. Nano Lett. 2012)

Skewed pyramids (Chong et al. J. Opt. 2012)

Snow Globe Coating

A. Basch, F.J. Beck, T. Söderström, S. Varlamov, K.R. Catchpole, Progress in Photovoltaics, 2012

0

0.2

0.4

0.6

0.8

1

Wavelength [nm]

R

SG coating

paint2

paint1

Silver particles and Snow Globe Coating

Snow Globe Coating combined with Plasmonic Nanoparticles

Plain 4.0mA/cm2

Snow Globe 8.0mA/cm2

coated with plamonic particles 100% increase in Jsc

300 600 900 12000

0.1

0.2

0.3

plainSG-coated SP

Wavelength [nm]

EQE

A. Basch, F.J. Beck, T. Söderström, S. Varlamov, K.R. Catchpole, Appl. Phys. Lett. 2012

TiO2 diffraction gratings

0 200 400 600100

300

500

700

Time (s)

eff ( s

)

Light off

Light on

Light induced passivation gives lifetimes of 700µs Barbé et al. Progress in Photovoltaics, 20(2), 143 (2011)

Wang et al. Progress in Photovoltaics (2012), DOI: 10.1002/pip.2294

PassivationLight trapping

Plasmonic enhancementFar-field (Scattering):

Near-field:

• Strong local field enhancement - very thin absorbers• Increased optical local density of states• Parasitic absorption

• Scattering/absorption cross-sections• Diffraction efficiency• Mode coupling/light trapping

Catchpole & Polman, Opt. Express 2008, Atwater & Polman, Nature Mat. 2010.

Metallic perfect absorbers I

C. M. Watts et al., Advanced Mater. 24, OP98-OP120 (2012).

Metallic perfect absorbers II

a) Metamaterial (t, d << l)• Effective medium (e,m)• Impedance matched to free space

b) Resonant cavity (t~ /4)l

c) Plasmonic grating (d~l)• Coupling to SPPs

t

d

dielectricmetal

metal

C. M. Watts et al., Advanced Mater. 24, OP98-OP120 (2012).

Extremely thin absorber cells

Conventional ETA SC• Large absorption volume• Short carrier path length• Transparent transport layers • Solution processed• Uniformity/infiltration issues

Extremely thin absorber cells

Conventional ETA SC• Large absorption volume• Short carrier path length• Transparent transport layers • Solution processed• Uniformity/infiltration issues

Planar ETA SC• Local field-enhancement• Reduced surface area

(recombination?)• Physical layer deposition

(sputtering/evaporation)

Ultrathin absorber geometry

AIR

SUPERSTRATE (n = 3.6)

Silver stripe, 100nm wide, 25nm high

F.J. Beck et al., Opt. Express 19, A146-A156 (2011).

Results

Absorbing layer:• n = 3.6• a = 3.4 x 104 cm-1

~1.8%

5nm

ETM

Numerical simulations: COMSOL (FEM)TM polarization

Results

~4%

5nm

Superstraten = 3.5

~1.8%

Single passOn superstrate

Results

~16%

5nm

Superstraten = 3.5

120nm

25nm Ag

~4%~1.8%

Single passOn superstrate

Grating

Results

Single passOn superstrate

GratingGrating+ mirror

5nm

Superstraten = 3.5

120nm

25nm Ag

~4%~1.8%

~16%

90%

Wang, White & Catchpole, IEEE Photonics Journal 2013.

Results

98%

5nm

Superstraten = 3.5

120nm

25nm

90%

~4%~1.8%

~16%Single pass

On superstrateGratingGrating+ mirror

Total absorption

Wang, White & Catchpole, IEEE Photonics Journal 2013.

Results

5nm

Superstraten = 3.5

120nm

25nm

Single passOn superstrate

GratingGrating+ mirror

Total absorption

Wang, White & Catchpole, IEEE Photonics Journal 2013.

53x increase inabsorption

Results: angular dependence

Angle and polarization averaged path length enhancement = 28Compared to 2D Lambertian limit n 11

Wang, White & Catchpole, IEEE Photonics Journal 2013.

Crystalline-Si tandems

High efficiency c-Si:• UNSW PERL Cell (1998): h = 25% (4cm2)• Sunpower (2010): 24.2% (155cm2)• Panasonic (2014): 25.6% (143cm2)

Low-cost thin film• Bandgap ~1.7eV• Cheap • Earth-abundantEfficiencies 25-30%?

Crystalline-Si Tandems

Nanotechnology 19, 245201 (2008).

Janz et al., EU PVSEC (2013).

Sunshot projects (Next Generation PV II): • III-V Nanowires on c-Si• CdSe on c-Si

• Organic/c-Si tandem: Energy Environ. Sci., 5, 9173 (2012).

• Nature, 501, 395 (2013): “perovskite cells have now achieved a performance that is sufficient to increase the absolute efficiency of high-efficiency crystalline silicon cells”

Tandem solar cells

• How good does the top cell need to be?• Material requirements

• Bandgap• Diffusion length• Luminescence efficiency

• Optical requirements• Low parasitic α• Minimal transparent conductor loss• Wavelength selective light trapping

How good does a top cell need to be?

1.5 1.6 1.7 1.8 1.9Top cell bandgap (eV)

0

50

100

150

2.0Bot

tom

cel

l pow

er(W

/m2 )

T

l

AM1.5G (1000W/m2)

h = 25% c-Si PERL Cell

Jsc = 42.7mA/cm2

Voc = 706mVFF=0.828

How good does a top cell need to be?

1.5 1.6 1.7 1.8 1.9 2.00

5

10

15

20

25

Top cell bandgap (eV)

Req

uire

d to

p ce

ll ef

ficie

ncy

(%)

h = 27.5%

h = 30%

1.5 1.6 1.7 1.8 1.9Top cell bandgap (eV)

0

50

100

150

2.0Bot

tom

cel

l pow

er(W

/m2 )

h = 25% (breakeven efficiency)

T

l

AM1.5G (1000W/m2)

h = 25% c-Si PERL Cell

Jsc = 42.7mA/cm2

Voc = 706mVFF=0.828

How good does a top cell need to be?

1.5 1.6 1.7 1.8 1.9Top cell bandgap (eV)

0

50

100

150

2.0Bot

tom

cel

l pow

er(W

/m2 )

T

l

AM1.5G (1000W/m2)

h = 25% c-Si PERL Cell

Jsc = 42.7mA/cm2

Voc = 706mVFF=0.828

Four-terminal tandem model

Bottom cell: c-Si PERL (25%)

J0 ~ 49 fA/cm2

FF = 82.8%

Top cell (p-i-n):

Bandgap Eg (direct)

Absorption

Diffusion length Ld => carrier collection [1]

Luminescence efficiency => F Voc [2]

FF = 0.8

Strong absorbers (a0~104cm-1)Short diffusion lengths ~200nm

[1] Taretto, Appl. Phys. A 77, 865 (2003)[2] Smestad, Solar Energy Mat. Solar Cells 25 51 (1992)

White, Lal & Catchpole, IEEE J. Photovolt. (2013), DOI: 10.1109/JPHOTOV.2013.2283342

Results: Eg and F dependence

Ld = 100 nma0 = 104 cm-1

External luminescence efficiency:[Green, Prog. Photovolt: Res. Appl. 20, 472 (2012)]

GaAs: > 0.2 c-Si: ~6 x 10-3 CIGS: 10-3 a-Si: 10-7~10-5

F = 10-5

White. Lal & Catchpole, IEEE J. Photovolt. (2013), DOI: 10.1109/JPHOTOV.2013.2283342

• Light trapping can increase efficiency by 3% absolute.• Optimum bandgap increases as decreases to offset Voc loss.

Results: Ld and a0 dependence

Eg = 1.95eVa0 = 104cm-1

F = 10-8 (qVoc = 0.6Eg)Ld = 35 nm

F = 10-6

Ld = 100 nm

F = 10-5

Optical losses

TCO

TCO

Zeng et al. Adv. Mater. 22, 4484-4488 (2010).

Req

uire

d t

op c

ell e

ffici

ency

(%

)

1.5 1.6 1.7 1.8 1.9 2.00

5

10

15

20

25

Top cell bandgap (eV)

htandem = 25%

htandem = 30%

10% parasitic loss in top cell20% parasitic loss in top cell

Parasitic absorption >20% makes reaching 30% practically impossible

Sub-bandgap absorption

400 600 800 1000 120010

1

102

103

104

105

106

Wavelength (nm)

a (c

m-1

)

a-Si:H

CIS

Sb2S3

CZTS

10 100 10000

2

4

6

8

10

12

14

16

18

Top cell thickness (nm)C

urr

en

t lo

st f

rom

bo

tto

m c

ell

(mA

/cm

2)

Sb2S3 (Eg=1.73eV)

CZTS (Eg=1.5eV)

CIS (Eg=1.5eV)

a-Si:H (Eg=1.7eV)

Cell thickness is limited by sub-bandgap loss

CZTS

Sb2S3

a-Si:H CIS

Parasitic absorption of perovskite is very low (similar to a-Si:H) – much more promising than CZTS.

Light trapping

Any light trapping must be wavelength selective

0.220.240.250.260.270.28

0.29

0.3

0.3

0.31

0.31

c (nm)

Rb

to

c

Tandem

efficiency

400 600 800 10000

0.2

0.4

0.6

0.8

1

0.2

0.22

0.24

0.26

0.28

0.3

0.320.318

selective trapping broadband trapping

no light trapping

Broadband trapping for the top cell is detrimental to total efficiency

After Green (2002) Prog Photovolt. Res. Appl. (10) 252, to include transparency in the rear reflector

Lal, White & Catchpole, submitted

Tandem cells on Si • Gaining 5% efficiency on c-Si requires very good top cells

• How to get there:– III-V or perovskites on Si– Identify new materials: Eg, Ld, F

– High bandgap (Eg~1.95eV @ F =10-5)

– Minimize sub-bandgap absorption– Minimize TCO absorption– Wavelength-selective light trapping

Top cell bandgap htandem = 25% htandem = 30%

1.5eV htop > 17% htop > 22%

1.7eV htop > 12% htop >17%

2eV htop > 9% htop > 14%

White, Lal & Catchpole, IEEE J. Photovolt. (2013), DOI: 10.1109/JPHOTOV.2013.2283342

Conventional absorption measurement :

36

Electron-hole pairs

Quantifying light trapping

900 950 1000 1050 1100 1150 12000

20

40

60

80

100

Abs

orpt

ion

(%)

wavelength

900 950 1000 1050 1100 1150 12000

20

40

60

80

100

AFC(ћω) Free carrier absorption& Parasitic absorption

ABB(ћω) for band to band transition

Absorptance = 100% - Reflectance - Transmission

Conventional absorption measurement makes it hard to identify best light trapping structures.

“A good solar cell makes a good LED and a great LED makes a great solar cell”

37

EmissionAbsorptionhigher energy state

lower energy state

ΔEhν hν

Quantifying light trapping

Luminescence

900 1000 1100 1200 1300 1400

PL

Inte

nsity

(A

.U.)

Wavelength(nm)

PL Planar PL with Light-trapping

Characterization

Photoluminescence spectra of cell structure with and without light trapping

hν

hν

39

)(exp))((exp)( 3..

d

kTA

kTCdj VFCF

e

Constant for certain materials at fixed T

Wavelength dependent

T. Trupke et al., Sol. Energ. Mat. Sol. Cells vol 53, (1998)

900 1000 1100 1200 1300 1400

PL

Inte

nsity

(A

.U.)

Wavelength(nm)

PL Planar PL with Light-trapping

Characterization

900 950 1000 1050 1100 1150 12000

10

20

30

40

50

60

70

80

90

100

Abs

orpt

ivity

Wavelength(nm)

Planar with Light-trapping

• Ag nanoparticles & diffused white coating

Structure 1

Structure design

Structure 2

c-Si

diffused white reflector

• Ag nanoparticles with metal reflector

Ag nanoparticles

c-Si

metal reflector

41

Plasmonics & DWC on c-Si cell

Structure 1

c-Si

diffused white coating

Dielectric Environment:• Passivation layers PECVD α-Si:HPECVD Si3N4

ALD Al2O3

Nanoparticle size • Ag film thickness:

15nm 21nm 27nm 33nm

Plasmonics in back contact cell

Structure 2

• 27nm Ag (D=~200nm)

• Capping layer thickness: 60~150nm PECVD Si3N4

c-Si

metal reflector27 nm

1 μm

950 1000 1050 1100 1150 12000

10

20

30

40

50

60

70

80

90

100

Jlambertian

-JP

Quantifying light-trapping

Absolute Absorption

Spectrum %A

Maximum Possible Photon

Current Jsc

950 1000 1050 1100 1150 12000

10

20

30

40

50

60

70

80

90

100

Lambertian

Light trapping Planar

Abs

orpt

ance

(%

)

Wavelength (nm)

950 1000 1050 1100 1150 12000

10

20

30

40

50

60

70

80

90

100

JAg+BSR

-JP

Fraction of Lambertian Enhancement (FLE)

Experimental results

60 90 120 150 DWC40

45

50

55

60

42

44

53

49

57

AgNP/Si3N

4

AgNP/DWC

Fra

ctio

n o

f L

am

be

rtia

n E

nh

an

cem

en

t (%

)

Capping Layer Thickness (nm)

C. Barugkin et al., IEEE Journal of Photovoltaics 2013

67% for Inverted Pyramid Texture with PLE=16 (T. Trupke et al., Sol. Energ. Mat. Sol. Cells, 1998)

c-Si

diffused white coating

c-Si

metal reflector

1 2

Light trapping similar to inverted pyramids but applicable to any cell

Summary

• Metal particles and scattering back reflectors - can give 100% Jsc enhancement.

• Near-field absorption for planar ETA– 90% absorption in 5nm layer

• Defined optical and electrical requirements for high efficiency tandems on Si

• Photoluminescence for quantifying light trapping - 62% of Lambertian increase demonstrated

PTO for conference slide

Metallic perfect absorbers III

K. Aydin et al., Nature Comms. 2, 517 (2011).

• Can be broadband, angle- and polarization-independent• TPV applications (tunable emissivity)

X. Liu et al., Phys. Rev. Lett. 107, 045901 (2011).

Absorber layer thicknessM

axim

um

ab

sorp

tan

ce

Absorber thickness (nm)

Predicted external quantum efficiency

Modelled Jsc: 1000~1200nm

collection η 95%

2.2mA/cm2 2.6mA/cm2

18% enhancement

c-Si

Metal reflector

c-Si

Metal reflector

Need for light trapping

AM 1.5 solar spectrum and solar radiation absorbed in 2 μm c-Si thin film, assuming single pass

t

optical thickness >> physical thickness ‘t’

glass

Thin semiconductor (few µm)

Thin solar cells an alternative for low cost PV