CARBON NANOTUBE BASED ORGANIC SOLAR CELLS

Arun Tej M.

PhD Student

EE Dept. and SCDT

• Carbon Nanotubes• Properties Useful for Solar Cells• Efficiency Limiting Factors• Nanotubes in Organic Solar Cells• Results and Future Challenges

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Outline

• S. Iijima - MWNT (1990), SWNT (1993)• Rolled graphene sheet with end caps• Large aspect ratios• Unique properties• Finds applications in

• Conductive plastics and adhesives• Energy storage• Efficient heat conduits• Structural composites• Biomedical devices

• Numerous electronic applications www.applied-nanotech.com

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Carbon Nanotubes

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Nanotube Field Emission Display

W.B. Choi, Samsung, APL, 1999

Thomas Rueckes, Nantero, 2000 5

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Nanotube Random Access Memory

Type of Memory

Most Important Feature

Applications

DRAM High Density Computer Operating Memory

SRAMFlash Memory

High SpeedNon-volatility

Cell Phones,Computer CachesPDAs, Cameras

MRAM High DensityHigh SpeedNon-volatility

All Uses

NRAM High DensityHigh SpeedNon-volatility

All Uses

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Nanotube Liquid Flow Sensor

A.K.Sood, IISc Bangalore, Science, 2003

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5 Stage Ring Oscillator on one SWNTZ.Chen, IBM, 2006

Nanotube Integrated Circuit

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Nanotube Based Inorganic Solar Cell

W.J.Ready, Georgia Tech, JOM, 2007

• High carrier mobilities (~1,20,000 cm2 V-1 s-1)

• Large surface areas (~1600 m2 g-1)

• Absorption in the IR range (Eg: 0.48 to 1.37 eV)

• Conductance - Independent of the channel length• Enormous current carrying capability – 109 A cm-2

• Semiconducting CNTs – Ideal solar cells• Mechanical strength & Chemical stability

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Nanotube Properties Useful for Solar Cells

Split-Gate device, Energy band diagram and I-V characteristics

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Combine the advantages of Organics and SWNTs 11

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Efficiency Improvement with SWNTs

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• Exciton dissociation sites• As electron acceptors in bulk heterojunction solar cells • Carrier transport• Thin transparent films of m-SWNTs as electrodes

Chhowalla et al, APL, 2005Wu et al, Science, 2004

Nanotubes in Organic Solar Cells

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Results (1)

Photoluminescence Quenching Higher Efficiency

Arun Tej M, S.S.K.Iyer, and B.Mazhari, IEEE INEC, 2008, Shanghai

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Results (2)

0 1 2 3 4 5 6 70

5

10

15

20

25

30

35

40

JP3HT

JSWNT (1wt%)

Cur

rent

Den

sity

(m

A/c

m2

)Forward Voltage

Negative resistanceregion showing tunneling behavior

Trap filling behaviour Tunneling behaviour

Arun Tej M, S.S.K.Iyer, and B.Mazhari, IEEE PVSC, 2008, San Diego

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0 20 40 60 80 1000.2

0.4

0.6

0.8

1.0

1.2

P3HT+SWNT (1wt%)

P3OT+SWNT (1wt%)

Op

en C

ircu

it V

olt

age

(v)

Light Intensity (mW cm-2)

High Voc of 1.15V at 1 Sun

High Open Circuit Voltages with Bulk Heterojunction Devices

Results (3)

Our WorkTo be published

• Synthesis of stable organic compounds• Separate semiconducting and metallic SWCNTs• Aligned CNTs inside the semiconducting polymers

give improved charge transport

e-

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h+

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Future REACH (1)

• Add nanoparticles, quantum dots, fullerenes etc to the side walls of SWNTs

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e-

h+

h+

e-e-

h+

e-

h+

e-

Future REACH (2)

“A Solar Cell with Improved Light Absorption Capacity”

S. Sundar Kumar Iyer and Arun Tej M.Patent Appln. No. 933/DEL/2006

Dt: 31st March, 2006

New device structures

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Future REACH (3)

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Acknowledgements

• Faculty, Staff and Students, SCDT• Prof. Ashutosh Sharma, Chemical Engineering

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Schematic and energy diagram of a typical polymer solar cell and its operation

e-

h+

Anode Cathode Donor Acceptor

Exciton formation

Exciton diffusionExciton dissociationCarrier transport

Charge collection

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Organic Solar Cell

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Conjugated polymers Conduction due to

sp2– hybridised carbon atoms

and (pz-pz)bonds electrons are

delocalised in nature giving high electronic polarisability

High absorption in the UV-Visible range of the solar spectrum

H.Hoppe and N.S. Sariciftci, 2004

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METALLIC SWNTSMETALLIC SWNTS

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Conductance is independent of the channel length. 25

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Conductance through a barrier with transmission probability T.

Landauer Formula:

With N parallel 1D channels (subbands):

m-SWNTs: Only two subbands cross EF (N=2)

Source of R: Mismatch in the number of conduction channels in the SWNT and the macroscopic metal leads.

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