Carbon-Based Solar Cells

Chabot College Guest Lecture

Michael VosgueritchianPhD Candidate

Prof. Zhenan Bao’s Group2-19-2013

1

Research Overview Carbon and Organic Electronics

2

Substrate

Sc-CNT

Anode

Cathode

IR light

ExcitonElectronhole

C60

• Silver or• PDMS / unsorted

CNT

CNT

P3DDT

• ITO / PEDOT or• Graphene

-3.8

-5.1

-4

-6.2

Sc-CNT C60

-4.7

e-

AgITO

-4.25-5

PEDOT

Active layerAnode Cathode

HOMO

LUMO

Graphene

-5.3

P3DDT

-3.5

-5.3

CNT

-4

Current Energy World demand is 15 TW (15 trillion Watts)

Enough power for 15 billion 100W light bulbs US 26% (even though 5% of population)

3

Source: cleantech.org

Sustainable Energy

Wind Energy Solar Energy Ocean Energy Geothermal Energy Biofuel

In ~1 hr we get enough solar power to power the earth for a year!

4

Source: Sandia National Lab

Solar Radiation and Market Enough <1% of landmass enough to provide energy

demand

5

Solar Cells Technologies

Crystalline Si – 27.6% Thin-Film

• CIGS – 20.4%• CdTE – 18.3%• α- Si - 13.4%

OPVs – 11.1% Nanotechnology

• Quantum Dots – 7.0% • Carbon based PVs (CPVs) – 1.2%* (~0.5%)

Other: GaAs, dye-sensitized, etc.

6

NREL.com

GEKonerka

Best Cell Efficiencies

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Solar Cell Uses and Considerations Applications

Industrial Commercial Home Portable

Considerations Cost/efficiency Materials Lifetime Niche applications

8

NREL.com

Portable Solar Cells

Uses Power portable

electronic devices Lighting Transportation

Lighting Africa Project Main failure due to

cracks in the solar cells

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Krebs et al. Energy Environ. Sci., 2010,3, 512-525

Transparent Electrodes (TEs) Materials that offer high conductivity and

high transparency, usually in thin film form

10

Displays

Sony.com

Solar Cells

• LEDs• Touch Screens• Energy Storage• Sensors• Transistors

Konarka.com

Why do we Need New Alternative Electrodes? Replace ITO

Enable flexible (stretchable) organic electronics

Images from

Google 11

Carbon PVs (CPVs) New class of solar cells

First demonstration of all-C solar Cell Stability

Chemical/Environmental: water/O2, heat, etc. Physical: strains, flexible/stretchable devices

Potential for cheap solar cells Solution processable Roll-to-roll fabrication Lightweight

Near-infrared absorption Tandem cells

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Carbon Nanomaterials

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Carbon Nanotubes (CNTs) – 1D• Discovered in 1991 • Single and multi-walled• Semiconducting or Metallic

Fullerenes – 0D• Discovered in 1985 (C60)• C60, C70, C84 • Films – n-type semiconducting

Graphene – 2D • Discovered in 2004• 2010 Nobel Prize• Metallic/transparent

Solar Cell Operation

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Short Circuit Current (Jsc) High absorption Low recombination

Open circuit voltage (Voc) Optimum band gap

in

ocsc

PFFVJPCE

Fill factor (FF) Reduce parasitic

resistances

CPV Structure Design of first demonstration of all-Carbon

solar cell Bilayer active layer: P3DDT sorted CNTs, C60 Electrodes

• Anode: ITO/PEDOT reduced graphene oxide (rGO)

• Cathode: Ag n-doped CNTs

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Substrate

Sc-CNT

Anode

Cathode

IR light

ExcitonElectronhole

C60

• Silver or• PDMS / unsorted

CNT

CNT

P3DDT

• ITO / PEDOT or• Graphene

-3.8

-5.1

-4

-6.2

Sc-CNT C60

-4.7

e-

AgITO

-4.25-5

PEDOT

Active layerAnode Cathode

HOMO

LUMO

Graphene

-5.3

P3DDT

-3.5

-5.3

CNT

-4

M. Vosgueritchian et al. ACS Nano, 2012, 6 (11), pp 10384–10395

Film Fabrication

16

Spray-CoatingSpin-Coating

Roll-to-roll Coater

Konerka.com

Sorting of SC-SWNTs

Lee, H. W. et al. Nature Communication 2011, 2, 541 17

Solution based method to selective sort SWNTs Semiconducting

selectivity by P3DDT

Can be solution deposited: spin-coating, spray coating, etc.

Absorbs in the infrared (IR)

Active Layer Bilayer of sorted SWNTs and C60

SWNT spin coated from solution C60 evaporated in vacuum

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1 2 3 4

50

100

150

200

250

300 Batch 1 Batch 2

Number of Layers

Shee

t Res

istan

ce

sq)

70

75

80

85

90

95

100

% Transm

ittance (at 550nm)

a) b)

Quartz Substrate

Sc-CNT

Reduced GO

n-doped SWNT

IR light

ExcitonElectronhole

C60

c)

400 600 800 1000 1200 1400 16000

5

10

15

20

25

30

35

40

Tran

smiss

ion (%

)

Wavelength (nm)

Drop casting, thin area Spin coating 5X Spin coating 3X Spin coating 1X

M. Vosgueritchian et al. ACS Nano, 2012, 6 (11), pp 10384–10395

Absorption Spectrum

Anode – Graphene Can make large area electrodes

Smooth (2D) structure

Can be made highly conductive (30 ohms/sq at 90%)

Bae et al., Nature Nanotechnology 5, 574–578 (2010)  19

1 2 3 4

50

100

150

200

250

300 Batch 1 Batch 2

Number of Layers

Shee

t Res

istan

ce

sq)

70

75

80

85

90

95

100

% Transm

ittance (at 550nm)

a) b)

Quartz Substrate

Sc-CNT

Reduced GO

n-doped SWNT

IR light

ExcitonElectronhole

C60

c)

Reduced Graphene Oxide

20

1 2 3 4

50

100

150

200

250

300 Batch 1 Batch 2

Number of Layers

Shee

t Res

istan

ce

sq)

70

75

80

85

90

95

100

% Transm

ittance (at 550nm)

a) b)

Quartz Substrate

Sc-CNT

Reduced GO

n-doped SWNT

IR light

ExcitonElectronhole

C60

c)

Oxidation

Reduced Graphene Oxide (rGO)

thermal

reduction

Deposit on Surface by spin-coating

rGO– 2D• Solution Processable• 102-103 Ω/□ at ~80% T • Cheap H. Becerril et al. ACS Nano, 2008, 2 (3), pp 463–470

Cathode – n-doped SWNT TE

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Use stretchable SWNT films on PDMS as the cathode for all-carbon solar cells instead of metal Need n-doping: DMBI organic dopant Previously used as electrodes in pressure an strain

sensors Spray-coated from solution

Biaxially stretched

As-deposited

1 μm1 μm

N

N

o-MeO-DMBI

OH

1 2 3 4

50

100

150

200

250

300 Batch 1 Batch 2

Number of Layers

Shee

t Res

istan

ce

sq)

70

75

80

85

90

95

100

% Transm

ittance (at 550nm)

a) b)

Quartz Substrate

Sc-CNT

Reduced GO

n-doped SWNT

IR light

ExcitonElectronhole

C60

c)

M. Vosgueritchian et al. Nature Nanotech, 2008, 2 , pp 788-792

Device Performance

With traditional electrodes• ~0.5% Efficiency for full spectrum• ~0.2% Efficiency in the IR

With carbon electrodes• ~0.01% Efficiency full and IR

Improving Performance Theoretical Efficiency of ~9-

10% Morphological Issues

Smoothen films: roughness/aggregates can cause leakage/shorting

Contact Issues Better contact between films:

better charge transport, decrease recombination

Active Layer Materials Use variety of SWNTs: increase

absorption Heterojunctions Thicker films

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Heterojunction

Electrodes Improve conductivity

Long Term Introduce flexibility Test stability All solution-processable

SWNTs absorb mostly in the infrared Film thickness only about 5 nm Different deposition process

Absorption Issues

24800 1000 1200 1400 1600 1800

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.00

0.05

0.10

0.15

0.20

0.25

Abs

orba

nce

(a.u

.)

Opt

ical

pow

er in

tens

ity (m

W/c

m2 )

Wavelength (nm)

Light intensity with filter Absorbance semiconducting SWNT

Summary First demonstration of all-carbon Solar Cell

Sorted-SWNTs used as light absorber C60 used to separate excitons Carbon electrodes replace traditional ITO/metal

electrodes Lots of work needs to be done! Acknowledgments

Prof. Zhenan Bao Dr. Marc Ramuz Dr. Ghada Koleilat Evan Wang Ben Naab

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QUESTIONS?

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