CARBON NANOTUBE BASED
ORGANIC SOLAR CELLS
Arun Tej M.
PhD Student
EE Dept. and SCDT
Outline
Carbon Nanotubes
Properties Useful for Solar Cells
Efficiency Limiting Factors
Nanotubes in Organic Solar Cells
Results and Future Challenges
Arun Tej M, REACH - 2008
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Carbon Nanotubes
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
Arun Tej M, REACH - 2008
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www.applied-nanotech.com
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Nanotube Field Emission Display
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W.B. Choi, Samsung, APL, 1999
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Nanotube Random Access Memory
Most Important
Feature
Applications
DRAM
High Density
Computer
Operating Memory
SRAM
Flash Memory
High Speed
Non-volatility
Cell Phones,
Computer Caches
PDAs, Cameras
MRAM
High Density
High Speed
Non-volatility
All Uses
NRAM
High Density
High Speed
Non-volatility
All Uses
Thomas Rueckes, Nantero, 2000
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Type of
Memory
Nanotube Liquid Flow Sensor
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A.K.Sood, IISc Bangalore, Science, 2003
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Nanotube Integrated Circuit
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5 Stage Ring Oscillator on one SWNT
Z.Chen, IBM, 2006
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Nanotube Based Inorganic Solar Cell
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W.J.Ready, Georgia Tech, JOM, 2007
Nanotube Properties Useful for Solar Cells
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
Arun Tej M, REACH - 2008
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Arun Tej M, REACH - 2008
Split-Gate device, Energy band diagram and I-V characteristics
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Efficiency Improvement with SWNTs
SWNTs
Improve mobility
• Low Carrier Mobilities
(~10-5 cm2 V-1 s-1)
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• Low Exciton Diffusion
SWNTs provide
Lengths (5-15 nm)
Large interfacial area
• Large Exciton Binding
SWNTs have
Energies (up to 1.5 eV)
Suitable energy levels
SWNTs have
Low energy gaps
• Large Energy Gaps
(2-3 eV)
Combine the advantages of Organics and SWNTs
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Nanotubes in Organic Solar Cells
Exciton dissociation sites
As electron acceptors in bulk heterojunction solar cells
Carrier transport
Thin transparent films of m-SWNTs as electrodes
Wu et al, Science, 2004
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Chhowalla et al, APL, 2005
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Results
(1)
Arun Tej M, REACH - 2008
Photoluminescence Quenching
Higher Efficiency
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Arun Tej M, S.S.K.Iyer, and B.Mazhari, IEEE INEC, 2008, Shanghai
Results
(2)
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0.01
1E-3
Slope: 4.2
Slope: 4.0
1E-4
Slope: 1.0
1E-5
1E-6
1E-7
1E-8
Slope: 8.7
SWNT wt%
0.1
1
Forward Voltage (Log Scale)
Trap filling behaviour
Current Density (mA/cm2)
Forward Current Density
(Log Scale)
0.1
40
JP3HT
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JSWNT (1wt%)
30
25
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Negative resistance
region showing
tunneling behavior
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10
5
0
0
1
2
3
4
5
6
7
Forward Voltage
Tunneling behaviour
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Arun Tej M, S.S.K.Iyer, and B.Mazhari, IEEE PVSC, 2008, San Diego
Arun Tej M, REACH - 2008
0.0 wt%
0.1 wt%
0.5 wt%
1.0 wt%
2.0 wt%
Results
(3)
1.2
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Open Circuit Voltage (v)
High Voc of 1.15V at 1 Sun
1.0
0.8
0.6
0.4
0.2
P3HT+SWNT (1wt%)
P3OT+SWNT (1wt%)
0
20
40
60
80
100
-2
Light Intensity (mW cm )
High Open Circuit Voltages with
Bulk Heterojunction Devices
Our Work
To be published
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Future REACH
(1)
e-
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• Synthesis of stable organic compounds
• Separate semiconducting and metallic SWCNTs
• Aligned CNTs inside the semiconducting polymers
give improved charge transport
eh+
e-
e-
h+
h+
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Future REACH
(2)
• Add nanoparticles, quantum dots, fullerenes etc to
the side walls of SWNTs
e-
e-
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e-
e-
h+
eh+
h+
h+
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Future REACH
(3)
New device structures
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“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
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Acknowledgements
• Faculty, Staff and Students, SCDT
• Prof. Ashutosh Sharma, Chemical Engineering
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Organic Solar Cell
Schematic and energy diagram of a typical
polymer solar cell and its operation
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Exciton formation
Exciton
diffusion
e
Exciton
Carrier
dissociation
transport
Charge
collection
h+
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Anode
Donor
Acceptor
Cathode
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H.Hoppe and N.S. Sariciftci, 2004
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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
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METALLIC SWNTS
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 Conductance
length.
is independent of the channel
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Landauer Formula:
2e 2
G
T
h
With N parallel 1D channels
(subbands):
2e2
G ( EF ) 
Tn ( EF )

h n
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.
2e 2
4e 2
G
*2 
 155S
h
h
R ~ 6.5k
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
Conductance through a barrier with
transmission probability T.