The Solar Industry Since 2000, Perovskite Solar Cells and Thoughts

on Startups

1

Mike McGeheeStanford University

2000

2

• I started at Stanford.

• My PhD advisor, Alan Heeger, won the Nobel Prize.

• Nanotechnology was exploding.

• I created the nanotechnology course at Stanford.

What is the most exciting opportunity for nanotech??

2001

3

Providing clean energy to people throughout the world to give everyone a high standard of living and prevent climate change is the biggest responsibility smart people have in this century.

Billions of people are counting on us.

I decided to make solar cells, not more powerful circuits or better TVs.

Ordered bulk heterojunctions

~ 2 absorption lengths(200 - 300 nm)

< 2 exciton diffusion lengths (~10-20 nm)

Transparentelectrode(e.g. ITO)

Reflectingelectrode

1st semiconductor(e.g. P3HT)

Electron accepting semiconductor(e.g. titania)

• Almost all excitons can be split• No deadends• Polymer chains can be aligned

• Easy to model• Semiconductors can be changed without changing the geometry.

2002

5

• Roscheisen and Sager founded Nanosolar.

• I became an advisor.

• I met venture capitalists and learned the business side of solar.

• Modules were $4/W and the total cost of an installed system was $10/W, which is 10 X too expensive.

www.nanosolar.com

Nanosolar’s Roll-to-Roll Coating

Nanosolar

Nanosolar’s Cell and Module Design

2005

10

• The Department of Energy held a meeting with > 200 scientists, wrote a ~ 275-page report, and released a lot of funding.

• Venture capitalists started funding cleantech startups. They put $1.5 billion into Nanosolar, Solyndra and Miasole alone.

• > 250 solar cell startups were formed.

• With a 5-year head start, my group was at the right place at the right time.

Installed PV cost breakdown

Tracking the Sun VIII (LBNL report 2015)

China provided $50 billion in low-interest loans around 2008. The price of Chinese silicon panels dropped rapidly.

The post-mortem for Silicon Valley startups

12

• Many of the founders did not know much about photovoltaics. The engineers were distributed between > 200 companies. There was no resemblance to Intel in its early days.

• Most startups took on 5-6 highly innovative, but risky, challenges.

• The Chinese companies simply scaled up what they knew could work.

• USA funds basic research. China supports factories. Why are we surprised when China commercializes American inventions?

• Some companies targeted very cheap, but inefficient panels, ignoring the cost of packaging and installation.

• Long-term stability was not addressed until factories were built, promises were made and modules were in the customers hands.

First Solar and SunPower made it

13

• They had started in the 1990’s.

• They started building utility scale plants, giving them an edge over foreign companies.

Is the PV technology that will play a huge role it revolutionizing our

energy infrastructure already developed?

14

Use Double Junction Tandems to Reach >30% Efficiency

De Vos (1980)

Hybrid Tandems Are a High-Performance Low-Cost Option

Efficiency

CostOrganic

12% efficient$30/m2

Epitaxial Crystalline45 % efficient$40,000/m2

Hybrid30% efficient

$100/m2

Established Technology:Silicon or CIGS

Eg ~ 1.1 eV

Low Cost Defect-Tolerant Technology:

Perovskite, Organic, Nanowires or II-VI

Eg ~ 1.9 eV

Perovskites in Polycrystalline Tandems

Colin Bailie, Greyson Christoforo, Jonathan Mailoa, Andrea Bowring, Eva Unger, William Nguyen, Erik Hoke, Julian Burschka, Norman

Pellet, Jungwoo Lee, Alberto Salleo, Rommel Noufi, Michael Grätzel, Tonio Buonassisi, Michael McGehee

Perovskite Solar Cells are Soaring

1970 1980 1990 2000 2010 20200

10

20

30

Re

co

rd e

ffic

ien

cy (

%)

Year

GaAs c-Si CIGS CdTe Perovskite

Achilleas Savva and Stelios A. Choulis 2

Mesoscopic or planar device structures

19

Nature 501, (2013) 395–398.

“Dye sensitized solar cell” with dye replaced with perovskite

Perovskite

or Silveror Silver

Nature 499, (2013) 316–319

Both have achieved efficiencies of 15% or higher!

Energy diagram:

PerovskitesGeneric formula: ABX3 , where X = oxygen or halide

A cation 12-fold, B-cation 6-fold co-ordinated with X anion

CH3NH3PbI3

Methylammonium-lead-iodide

20

CH3NH3+ Pb2+ I-

4

Perovskites are strongly absorbing

21

Demonstrate tunable band structure

22

(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

• Low surface recombination

• Rapid and uniform deposition

• High device efficiency

23

Hybrid Tandem Architectures

Monolithically-integrated• Fewer layers that parasitically

absorb• Module fabrication easier

Mechanically-stacked• Easier prototyping• No current matching required• No tunnel junction or

recombination layer requiredC. D. Bailie, M. G. Christoforo, M. D. McGehee, et al., Energy Environ. Sci., 2015, 8 956-963.C. D. Bailie, M. D. McGehee MRS Bulletin, 40 (2015) 681-5.

Panel design for mechanically stacked tandems

C. D. Bailie, M. G. Christoforo, M. M. McGehee, et al., Energy Environ. Sci., 2015, 8 956-963

Monolithic integration of the perovskite cell onto a silicon wafer

J.P. Mailoa, C. D. Bailie, M. M. McGehee, T. Buonassisi, et al., Appl. Phys. Lett., 2015, 106 121105

1 cm2 prototype performance shows high Voc and FF

J.P. Mailoa, C. D. Bailie, M. M. McGehee, T. Buonassisi, et al., Appl. Phys. Lett., 2015, 106 121105

We now have 22.2 % 2-terminal tandems, but I can’t reveal the

details yet.

28

Three Main Degradation Pathways

29

1. Moisture ingress – structural degradation

2. Corrosion of metal electrodes upon reaction with

halides

3. Methylammonium evolution – accelerated with heat

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

Indium Tin Oxide (ITO) Presents a Holistic Solution to the Main Degradation Pathways

30

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

• Hole blocking layer

• Sputtering buffer layer

31

Glass

ITO

PEDOT:PSS

Perovskite

PC60BM

Al:ZnO nps

ITO

MAPbI3

PC60BMPEDOT

ITOITO ITOITO

-3.9eV

-4.2eV

-4.8eV

-4.8eV

-5.2eV

-6.0eV

-5.4eV

-4.4eV

-7.6eV

ZnO

Progress in Sputtering ITO as the Top Electrode

32

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

Semi-Transparent MAPbI3 Solar Cells

33

0

2

4

6

8

10

12

14

0 2 4 6 8 10

Eff

icie

nc

y (

%)

Time (min)

Opaque Electrode - 13.5%

Semi-Transparent - 12.3%

-20

-16

-12

-8

-4

0

0.0 0.2 0.4 0.6 0.8 1.0

Cu

rre

nt

(mA

/cm

2)

Voltage (V)

Opaque Electrode

Semi-Transparent

JSC [mA/cm2] VOC [mV] FF [-] η [%]

Opaque 18.8 938 0.77 13.5

Semi-Transparent 16.5 952 0.77 12.3

The high fill factor implies that the electrode has low sheet resistance.

18% Efficient, 4-Terminal Perovskite-Silicon Tandems

34

JSC

[mA/cm2]

VOC [mV] FF [-] η [%]

Silicon 38.3 587 0.754 17.0%

Perovskite 16.5 952 0.774 12.3%

Filtered Silicon 13.3 562 0.762 5.7%

Mechanically-Stacked Tandem 18.0%

Improved thermal and environmental stability with sputtered ITO electrode

Glass

Plan to package devices to improve stability

ITO

PEDOT:PSS

Perovskite

PC60BMAl:ZnO nps

ITO Dust

Hole in ITO

No edge seal

ITO capping layer improves stability but holes in ITO due to particulates and lack of edge seal allow slow, but eventual exchange of gases with environment

Glass

Glass Edge sealEncapsulant

Full packaging of cells within glass/device/encapsulant/glass stack with an edge seal will reduce exchange of gases with environment

• Perovskite discoloration (from brown to yellow) as a result of thermal degradation allows for visual inspection as initial test.

• We evaluated a full device stack – Glass/ITO/PEDOT/MAPbI3/PCBM/AZO-nps/ITO

• Tested standard polymer encapsulants EVA and polyolefin (PO)• EVA process is 135°C for 20min

• PO process is 160°C for 20min

• No discoloration observed during lamination• ITO seal stabilizes the perovskite

Damp Heat Testing

MAPbI3 device after encapsulation with EVA –no color change observed during lamination

EVA-encapsulated films after 500 hours at 85°C/85% RH AND 500 hours at 110°C/0% RH. Minimal degradation near large dust particles observed.

Perovskite composition Efficiency Authors Notes Journal

CsPbBr2I 4.7Qingshan Ma, Martin Green, Anita Ho-Bailie

Co-evaporated films with large grains on the order order of 500 nm – 10 um.

Adv. Energy Mater.DOI:

10.1002/aenm.201502202

CsPbBrI2

9.84(5.6

stabilized)

Rebecca Sutton, Henry Snaith

Solution-depositionwith high temperature anneal (350˚C).

Adv. Energy Mater.DOI:

10.1002/aenm.201502458

CsPbBrI2 6.5Rachel Beal, Michael McGehee

Solution-deposition with lower temperature anneal (135˚C).

J. Phys. Chem. Lett.DOI:

10.1021/acs.jpclett.6b00002

Fully Inorganic Absorbers

Perovskite Composition Efficiency Authors Notes Journal

Cs0.1FA0.9PbI3 16.5Jin-Wook Lee, Nam-Gyu Park

First report of mixed Cs/FA cation material

Adv. Energy Mater.DOI: 110.1002/aenm.201501310

Cs0.2FA0.8PbI2.84

Br0.1617.35

Chenyi Yi, Ursula Röthlisberger, Michael Grätzel

Propose that entropicgain from cation mixing stabilizes the perovskite phase of FAPbI3. Br substitution tunes the bandgap.

Energy Environ. Sci.DOI: 10.1039/c5ee03255e

Cs0.15FA0.85PbI3 16.1 Zhen Li, Kai Zhu

Demonstrate that Cs-substitution stabilizes the material both to phase transitions and decomposition to PbI2.

Chem. Mater.DOI: 10.1021/acs.chemmater.5b04107

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

17.1David McMeekin, Henry Snaith

1.74 eV bandgap material is ideal for tandem applications with crystalline Si.

ScienceDOI: 10.1126/science.aad5845

Cs0.05

(MA0.17FA0.83)0.95

Pb(I0.83Br0.17)3

21.1Michael Saliba, Michael Grätzel

Highest achievedefficiency

Energy Environ. Sci.DOI: 10.1039/C5EE03874J

Mixed-cation Absorbers

Cost to upgrade to a tandem

Today’s Si + Perovskite upgrade Perovskite/Si Tandem

$0.51/WDC $0.34/WDC

16% efficient 27.8% efficient

$82/m2 +$13/m2 $95/m2

What is my secret for getting students to start companies?

43

Tell them that they are too young and don’t have the experience needed to start a company

that will have to build a factory.

This never fails to generate a company.

44

A whole course could be taught on how to start a company. Here are a few key points.

45

• Make sure you are an outstanding researcher before trying to start a company.

• Don’t get caught up in what academia finds trendy.

• Don’t get too attached to what your lab does.

• Be brutally honest when you assess whether or not what you are doing is game changing.

• Make sure your material does something valuable, won’t degrade, really is cost-effective and is safe. 3 out of 4 is not good enough.

Companies founded by students of the McGehee group

46

• Thin Silicon• Allion (printing solar cells and using robotics to install them)• One Earth Capital (venture capital firm)• PLANT PV (tandem solar cells and inks for electrodes)• Plotly (scientific plotting)• Sinovia (silver nanowire transparent electrodes)• Dragonfly (electronics for making systems of solar modules more efficient)• NextTint (smart windows)• WellDone Technology (monitoring of water pumps in remote Africa)• CelLink (improved method for assembling solar modules)• Iris PV (perovskite-silicon tandem solar cells)• #12 is in stealth mode