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Silicon-Based Tandem Photovoltaic Cells

Zachary Holman

Arizona State University

2018 SETO Portfolio Review


SETO Mantra: Efficiency, Reliability, Cost

3


10 15 20 25 30 35 400.00

0.10

0.20

0.30

0.40

0.50

2.0%, 10 yr

0.75%, 30 yr0.2%, 30 yr

Module

pri

ce (

$/W

)

Module efficiency (%)

0.2%, 50 yrAnnual degradation, lifetime

10 15 20 25 30 35 400.00

0.10

0.20

0.30

0.40

0.50

2.0%, 10 yr

0.75%, 30 yr0.2%, 30 yr

Module

pri

ce (

$/W

)


0.2%, 50 yrAnnual degradation, lifetime

10 15 20 25 30 35 400

25

50

75

100

125

150

175

2.0%, 10 yr

0.75%

, 30 y

r0.2%

, 30

yr

Module

pri

ce (

$/m

2)


0.2%

, 50

yr

Annual degradation, lifetime

Doubling efficiency more than doubles module selling price

SETO Mantra: Efficiency, Reliability, Cost

4


What Will the Silicon Bottom Cell Look Like?

Silicon heterojunction (SHJ) cells have highest Vocs

Blue parasitic absorption is non-issue in tandems

Front TCO is a natural recombination junction

Expensive low-temperature silver paste not needed for two-terminal tandems

Benefits from a high-lifetime (n-type wafer)

TCOs and rear metallization can be pricey

5


Silicon Heterojunction Bottom Cell: Solar-Grade p-Type Wafers Gettering (G), hydrogenation (H), and advanced hydrogenation (AHP) improves p-type

SHJ lifetime from 30 µs to 300 µs

Certified Voc of 707 mV on p-type CZ, 702 mV on p-type multi

CZ monocrystalline

6


What Will the Top Cell Look Like?

Limiting-efficiency calculations including Auger recombination give 43%

1.72 eV top cell is best for two-terminal tandem; 1.6–1.9 eV for four-terminal

But we don’t make sub-cells that operate at their limiting efficiency!

7


Yu et al, Nature Energy 2016

with

Efficiency resolved by wavelength

Allows direct evaluation of cell pairs—only bother considering a tandem if top cell is considerably better than silicon for λ < 700 nm

Spectral Efficiency Shows the Way

300 400 500 600 700 800 900 1000 1100 12000

10

20

30

40

50

60

70

80Record cells

Wavelength (nm)

Mono-Si

GaAs

a-Si:H

Spectr

al effic

iency (

%)

GaInP

CdTe

Perovskite

8

𝜂 𝜆 =𝐽𝑆𝐶(𝜆) ∙ 𝑉𝑂𝐶 ∙ 𝐹𝐹

𝐼(𝜆)𝐽𝑆𝐶 𝜆 = 𝑞

𝜆

ℎ𝑐𝐸𝑄𝐸(𝜆) ∙ 𝐼(𝜆)




Predicts the maximum possible tandem efficiency for any two sub-cells

300 600 900 1200

0

10

20

30

40

50

60

70

80

90

Spectr

al effic

iency (

%)

Wavelength (nm)

1.9 eV1.4 eV

1.12 eV (Si)

17 1921

23

25

27

15

29

31

13

50 55 60 65 70 75 80 85 90 95 100

1.40

1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

25%-efficient silicon bottom cell

Top cell fraction of detailed-balance efficiency (%)

20

22

24

26

28

30

32

34

36

38

40

42

Top-c

ell

bandgap (

eV

)

Tandem

effic

iency (

%)

26

30

34 40

17 1921

23

25

27

15

29

31

13

50 55 60 65 70 75 80 85 90 95 100

1.40

1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00



20

22

24

26

28

30

32

34

36

38

40

42

Top-c

ell

bandgap (

eV

)

Tandem

effic

iency (

%)

9


Predicts the maximum possible tandem efficiency for any two sub-cells

Of existing cells, GaAs gives highest tandem efficiency despite wrong bandgap!


300 600 900 1200

0

10

20

30

40

50

60

70

80

90

Spectr

al effic

iency (

%)

Wavelength (nm)

1.9 eV1.4 eV

1.12 eV (Si)

26

30

34 40

17 1921

23

25

27

15

29

31

13

50 55 60 65 70 75 80 85 90 95 100

1.40

1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80

1.85

1.90

1.95

2.00

GaAs



20

22

24

26

28

30

32

34

36

38

40

42

Top-c

ell

bandgap (

eV

)

Tandem

effic

iency (

%)

GaInPPerovskite

Perovskite

CdTe


10


Perovskite Top Cells

Bush et al, Nature Energy 2017

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

5

10

15

20

0 10 20 300

10

20

30

23.6 1.65 18.1 79.0

Voc Jsc FF

(%) (V) (mA/cm2) (%)

Cu

rre

nt

de

nsity (

mA

/cm

2)

Voltage (V)

Eff

icie

ncy (

%)

Time (min)

300 600 900 12000

10

20

30

40

50

60

70

80

90

100

18.5 mA/cm2

1.2 mA/cm2

Blue parasitic

Silicon

1-R

and E

QE

(%

)

Wavelength (nm)

18.9 mA/cm2

Perovskite

Reflection

4.8 mA/cm2

NIR parasitic

3.3 mA/cm2

Losses:

+ Amazing lifetime and Voc for polycrystalline material, potentially inexpensive

− Stability, scaling, depositing on textured surfaces

11


0 200 400 600 800 10000

2

4

6

8

10

12

14

16

18

20

0 200 400 600 800 10000

2

4

6

8

10

12

14

16

18

20(a)

Eff

icie

ncy (

%)

Jm

pp (

mA

/cm

2)

Time (hours)

Eff

icie

ncy (

%)

Jm

pp (

mA

/cm

2)

Time (hours)

EVA 1

EVA 2

(b)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Vm

pp (

V)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Vm

pp (

V)

Perovskite Top Cells

Bush et al, Nature Energy 2017

Thermally stable absorber: Cs0.17FA0.83Pb(Br0.17I0.83)3 (CsFA) perovskite

Thermally evaporated PCBM (electron contact)

ALD-deposited SnO2/ZTO buffer layer prevents sputter damage

12


CdMgTe and CdZnTe Top Cells

Zhao et al, Nature Energy 2016; Becker et al, IEEE JPV 2017

+ CdTe cells at 22%, Mg or Zn to get to 1.7 eV, GW-scale manufacturing in place

− Ternary alloys are complicated, Voc of II-VI cells low due in part to back contact

13


CdMgTe and CdZnTe Top Cells

Swanson et al, submitted; Becker et al, IEEE JPV 2018

Best 1.7 eV monocrystalline CdMgTe cell at 15.2%; reached Voc = 1.18 V and FF = 82% separately

Polycrystalline CdMgTe and CdZnTe lose Mg and Zn upon CdCl2 treatment

Best 1.65 eV polycrystalline CdMgTe cell at 10.2%; lifetime of 7 ns with double heterostructure

14


III-V Top Cells

Yu et al, in preparation

+ Monocrystalline cells are very efficient, stable

− Cost, integration with silicon

300 600 900 1200

0

10

20

30

40

50

60

70

80

90

100

GaAs

Silicon

Reflectance

Transmittance

R, T

and s

pect

ral e

ffic

iency

(%

)

Wavelength (nm)

15


Acknowledgements

ANU (Macdonald)

ASU (Bertoni, Bowden, Honsberg, Mani, Smith, Zhang)

CSU (Sampath)

First Solar (Lee, Malik)

Natcore (Levy)

NREL (Bosco, Metzger, Silverman, Stradins, Woodhouse)

Stanford (McGehee)

UNC (Huang)

UNSW (Hallam)

UA (Angel)

UIUC (Lee)

16


What’s next?

Long-lifetime, cheap, stable polycrystalline absorbers

o Perovskite: Degradation mechanisms? Conformal deposition on 156 mm textured wafers?

o II-VI: Maintain alloy stoichiometry during Cl treatment? Surface passivation?

o III-V: Any plausible way to get these to < $100/m2? Passivation of grain boundaries in polycrystalline III-V materials?

o Other absorbers that we’re not yet exploring?

Heterojunction contacts with high or low work function

o Measurement of implied voltage, comparison to extracted voltage?

o New, transparent high- and low-work-function materials?

o Deposition methods that don’t damage absorbers?

Integration with silicon

o Process compatibility (temperature, chemistry)?

o Two, three, or four terminals? Realistic energy yield calculations?

o Metallization when combining a thin-film solar cell with a wafer-based solar cell?

17

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