Metal-halide perovskites: the next evolution in photovoltaics

D r. C o l i n B a i l i e

Po st d o c , S t a n fo rd U n i ve rs i t y

Fo u n d e r, I r i s P V

* D a t a i n t h i s p r e s e n t a t i o n f r o m t h e l a b a t S t a n f o r d *

Global Climate and Energy Project

Fastest-improving PV technology in history

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‘Perovskite’ describes a crystal structure class

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Generic formula: ABX3

Methylammonium-lead-iodide

CH3NH3PbI3

CH3NH3+ Pb2+ I-

A B X

Perovskite solar cells have versatility in their architecture

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Energy diagram:

Perovskite processing is simple and fast

5 VIDEO CREDIT: JOEL TROUGHTON, YOUTUBE

The perovskite is a strongly-absorbing direct band gap semiconductor

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The perovskite is already an efficient solar cell technology

MaterialBandgap

(eV)q·Voc(eV)

Energy loss (eV)

GaAs 1.43 1.122 0.31

Perovskite(MAPbBr3)0.15(FaPbI3)0.85

1.55 1.19 0.36

Silicon 1.12 0.74 0.38

CIGS ~1.15 0.76 0.39

CdTe 1.49 0.88 0.61

a-Silicon 1.55 0.90 0.65

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M. GREEN ET AL. SOLAR CELL EFFICIENCY TABLES (VERSION 46) 2015

J. P. C. BAENA, A. HAGFELDT, ET AL., ENERGY & ENVIRON. SCI. 2015

W. S. YANG, S. I. SEOK, ET AL., SCIENCE 2015

Time resolved photoluminescence (TRPL): Carrier Carrier lifetime can be long

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τ= 261ns

τ= 4ns

Defects in perovskites are shallow

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Yanfa Yan et al. Adv. Materials, 2014.

Tuning the composition adjusts the band gap

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(MA)Pb(BrxI1-x)3

CH3NH3PbBr3

Eg=2.3 eVCH3NH3PbI3

Eg=1.6 eV

CH(NH2)2Pb(BrxI1-x)3

CH(NH2)2PbBr3

Eg=2.2 eVCH(NH2)2PbI3

Eg=1.48 eV

Snaith et al. Energy Environ. Sci., 7, 982–988 (2014)

Advantages of perovskites

• Tunable band gap

• Highly absorbing

• Long carrier lifetimes (slow bulk recombination)

• Low surface recombination (slow surface recombination)

• They can be printed, even on plastic!

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Use double junction tandems to reach >30% efficiency

12SHOCKLEY AND QUEISSER (1961), DE VOS (1980). NOTE: INPUT SPECTRUM IS 6000 K BLACKBODY; STANDARD AM1.5G SOLAR SPECTRUM

YIELDS SLIGHTLY DIFFERENT VALUES.

Single-Junction Theoretical Efficiency Double-Junction Theoretical Efficiency

Tandems overcome single-junction efficiency limits

Perovskite/silicon tandem

practical efficiency limit: 30-35%

Silicon

practical efficiency limit: 25%

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CH3NH3PbBr3

[Eg=2.3 eV]CH3NH3PbI3

[Eg=1.6 eV]

Perovskite bandgap is tunable over the ideal range for the top cell in a tandem

14DE VOS. J. PHYS. D: APPL PHYS (1980)

CH3NH3Pb(BrxI1-x)3

Two potential scalable tandem architectures

15COLIN D. BAILIE, MICHAEL D. MCGEHEE, MRS BULLETIN (2015)

mechanically stacked monolithically integrated

Mechanically-stacked tandem on silicon using ITO as the rear electrode

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mechanically stacked

17.0%

Separate

12.3% 12.3%

Tandem

5.7%

18.0%

+

K. A. Bush, C. D. Bailie, Y. Chen, T. Leijtens, A. R. Bowring, F. Moghadam, M. D. McGehee, Adv. Materials (2016)Silicon image from Yu et al. J. Micro/Nanolith. MEMS MOEMs (2009)

2-terminals coming out of the junction box of a mechanically-stacked tandem

17C.D. BAILIE, M. G. CHRISTOFORO, J.P. MAILOA, M. D. MCGEHEE, ET AL., ENERGY ENVIRON. SCI., 2015, 8 956

Flexibility to match voltage or current of the top and bottom strings

Current World Record Mechanically Stacked Perovskite on Si Tandem

18J. Werner, C. Ballif et al, ACS Energy Letters 1 (2016) p. 474.

First Monolithic Perovskite/Silicon Tandem – 13.7% with 11.5mA/cm2

• Significant parasitic absorption in the hole transport material – Spiro-OMeTAD

Mailoa, J. P. and Bailie, C. D., et al. (2015). Applied Physics Letters, 106, 121105.

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Bandgap tuning with alloyed materials

Can change the bandgap by controlling the halide composition

Bandgap can be tuned from 1.6–2.3 eV for CH3NH3Pb(BrxI1-x)3

Hoke, E. T. et al. Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem. Sci. 6, 613–617 (2014).

MAPbI, Br

Top cell Eg range

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Phase segregation in mixed halides limits the Voc

Hoke, E. T. et al. Reversible photo-induced trap formation in mixed-halide hybrid perovskites for photovoltaics. Chem. Sci. 6, 613–617 (2014).

Arrows show increasing time

Phase segregation for all CH3NH3Pb(BrxI1-x)3 with x>0.2

x=0.4

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PL spectra very similar over composition range of (MA)Pb(BrxI1-x)3 after light soaking

Mixed halide PL spectra similar to what would be expected for (MA)Pb(Br0.15I0.85)3, (x~0.15)

1.6 1.8 2.0 2.2 2.4 2.60.0

0.5

1.0

P

L (

arb

.un

its)

Energy (eV)

x=0

x=0.1

x=0.2

x=0.3

x=0.4

x=0.5

x=0.6

x=0.7

x=0.8

x=0.9

x=1

800 750 700 650 600 550 500

Wavelength (nm)

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Selecting the high band gap semiconductor

Eg Material(s) Device efficiency

Stable to phase segregation

Group

1.5eV FAPbI3

(FAPbI3)0.85(MAPbBr3)0.15

20.2%17.3%

Yes?

SeokSeok

1.6eV MAPbI3

FA0.9Cs0.1PbI3

Triple cation*

19.7%16.5%21.1%

YesYes

?

ParkGrätzelGrätzel

1.7eV FA0.83Cs0.17(I0.6Br0.4)3

MAPbBr0.8I2.2

17%14.9%

PossiblyNo

SnaithHuang

1.8eV MAPbBr0.9I2.1 12.7% No Zou

1.9eV CsPbBrI2 6.5% Yes McGeheeSnaith

2.3eV MAPbBr3

CsPbBr3

8.7%6.5%

YesYes

GreenCahen

*Triple cation formula: Cs0.05(MA0.17FA0.83)0.95Pb(I0.83Br0.17)3

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Our best monolithic 2-terminal tandem Cs0.17FA0.83Pb(Br0.17I0.83)3 perovskite on heterojunction silicon from Zach Holman’s team at ASU

1cm2 23.6 % efficient very stable

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

0 0.5 1 1.5

Cu

rre

nt

De

nsi

ty (

mA

/cm

2)

Voltage (V)

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Perovskite/HIT Tandem EQE and 1-R

0

10

20

30

40

50

60

70

80

90

100

300 400 500 600 700 800 900 1000 1100 1200

EQ

E a

nd

1-R

(%

)

Wavelength (nm)

EQE Sum IR HIT2 - Perovskite EQE - 18.57mA IR HIT2 - Silicon EQE - 18.26mA IR HIT 2 - 1-R

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Main limitation for perovskite is demonstration of 25-year field lifetime

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Light breaks bonds

Water reacts chemicallyHeat vaporizes organics

Halides corrode metals

Review Article: Tomas Leijtens et al. Stability of Metal Halide Perovskite Solar Cells,” Advanced Energy Materials, 2015.

Several studies demonstrate that non-metal electrodes work better than metal ones

Carbon electrodes enable stable devicesMei, A. et al. A hole-conductor–free, fully printable mesoscopic perovskite solar cell with high stability. Sci. 345, 295–298. (2014).

EPFL: gold diffuses in solar cellsDomanski, K. et al. Not all that glitters is gold: metal-migration-induced degradation in perovskite solar cells. ACS Nano 10, 6306–6314(2016).

Halogens react with metalBack, H. et al. Achieving long-term stable perovskite solar cells via ion neutralization. Energy Environ. Sci. 9, 1258–1263. (2016).

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Stability remains a major barrier to perovskite solar cells

28K. A. BUSH, C. D. BAILIE, Y. CHEN, T. LEIJTENS, A. R. BOWRING, F. MOGHADAM, M. D. MCGEHEE, ADV. MATERIALS (SUBMITTED)

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Aluminum Doped Zinc Oxide (AZO) Enables Sputtering of ITO as the Top Electrode

• Hole blocking layer

• Sputtering buffer layer

Glass

ITO

PEDOT:PSS

Perovskite

PC60BM

Al:ZnO nps

ITO

MAPbI3

PC60BMPEDOT

ITO ITO

-3.9eV

-4.2eV

-4.8eV

-4.8eV

-5.2eV

-6.0eV

-5.4eV

-4.4eV

-7.6eV

ZnO

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Progress in Sputtering ITO as the Top Electrode

Glass

ITO (150nm)

PEDOT:PSS (40nm)

Perovskite (~275nm)

PC60BM (40nm)Al:ZnO nps (50nm)

ITO (500nm)

MgF2 (150nm)

200nm

SunpremeYe Chen, Wei Wang, Wen Ma, FarhadMoghadam

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Improved thermal and environmental stability with sputtered ITO electrode

31K.A. Bush, C.Bailie, M. McGehee et al., Adv. Mat, 28 (2016) 3937.

ITO-sealed perovskite on hot plate at 150°C

32K. A. BUSH, C. D. BAILIE, Y. CHEN, T. LEIJTENS, A. R. BOWRING, F. MOGHADAM, M. D. MCGEHEE, ADV. MATERIALS (SUBMITTED)

Packaging devices

Side View Top View

Solar Glass

Perovskite

Bus Bars

Encapsulant

Edge Seal

Space-filling glass

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• Solar glass (3.2mm, Pilkington)• Edge seal (Butyl, Quanex)• Bus bars (Cu + Sn/Ag/Cu coating, Ulbrich)

• Conductive adhesive (Sn/Bi, Hitachi) • Encapsulants (EVA, PO) • Encapsulant (Surlyn)

Testing of Fully Encapsulated Devices in 85°C/85% RH Damp Heat

6 weeks = 1000 hours

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0.1

1

10

100

1,000

Fra

ctu

re E

ner

gy,

Gc

(J/m

2 )

Dense SiO2

TEOS SiO2 ULK

dielectrics

Gc ~ 5 J/m2

Gc ~ 10 J/m2

OPV

CIGS

CuInxGa(1-x)Se2

Mo

CdSAl doped ZnO

Al foil

Fracture Properties of Device Materials

Silicon PV

Polymers forPackaging

Encapsulation

Structural Materials

Protective Coatings

Perovskites

Ag

P3HT

ZnO

CH3NH3PbI3

ITO-PET

Outlook on stability

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• Using the more stable perovskites, impermeable and unreactive electrodes and proper packaging has improved stability enormously.

• We have passed a temperature cycling test and the damp heat test.

• Long-term testing under light is underway. 1000 hour tests are encouraging.

Outlook

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• Single junction efficiencies approaching 25 % seem possible.

• Band gaps for single and multijunction tandems are available.

• Breaking 25 % efficiency with tandems is inevitable and 30 % look achievable.

• Stability is rapidly improving.

What are the implications of Pb being toxic?

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Babayigit et al. Nature Materials, 15 (2016) 247.

Amount of lead

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• The panels will have about 1 g of lead in the perovskite.

• Silicon panels typically have 16 g of lead in the solder.

• Lead would not easily escape a packaged module.

Perovskite companies

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Company Location Approach

Oxford PV England Perovskite on Si monolithic 2T tandem

Iris PV Silicon Valley Perovskite mechanically stacked on Si Tandem

Hunt Energy Dallas, Texas Single junction perovskites

Saule Poland Flexible perovskite cells

Weihua Solar China Printed single junction panels

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Acknowledgments

• Michael McGehee• Rachel Beal, Kevin Bush, Andrea Bowring, Rongrong Cheacharoen, Eric Hoke,

Tomas Lietjens, Axel Palmstrom, Dan Slotcavage• Duncan Hargrave at D2 solar• Homer Antoniadis at DuPont• Jonathan Mailoa, Robert Hoye, Tomas Leijtens, Sarah Sofia, Tonio Buonassisi

at MIT• Zhengshan J. Yu, Mathieu Boccard, Zach Holman at ASU• Ye Chen, Wei Wang, Wen Ma, Farhad Moghadam at Sunpreme

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