C ARBON N ANOTUBE B ASED O RGANIC S OLAR C ELLS 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 2 Arun Tej M, REACH 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 3 Arun Tej M, REACH Carbon Nanotubes

Arun Tej M, REACH Nanotube Field Emission Display W.B. Choi, Samsung, APL, 1999

Thomas Rueckes, Nantero, Arun Tej M, REACH Nanotube Random Access Memory Type of Memory Most Important Feature Applications DRAMHigh DensityComputer Operating Memory SRAM Flash Memory High Speed Non-volatility Cell Phones, Computer Caches PDAs, Cameras MRAMHigh Density High Speed Non-volatility All Uses NRAMHigh Density High Speed Non-volatility All Uses

Arun Tej M, REACH Nanotube Liquid Flow Sensor A.K.Sood, IISc Bangalore, Science, 2003

7 Arun Tej M, REACH Stage Ring Oscillator on one SWNT Z.Chen, IBM, 2006 Nanotube Integrated Circuit

Arun Tej M, REACH Nanotube Based Inorganic Solar Cell W.J.Ready, Georgia Tech, JOM, 2007

High carrier mobilities (~1,20,000 cm 2 V -1 s -1 ) Large surface areas (~1600 m 2 g -1 ) Absorption in the IR range (E g : 0.48 to 1.37 eV) Conductance - Independent of the channel length Enormous current carrying capability – 10 9 A cm -2 Semiconducting CNTs – Ideal solar cells Mechanical strength & Chemical stability 9 Arun Tej M, REACH Nanotube Properties Useful for Solar Cells

Split-Gate device, Energy band diagram and I-V characteristics 10 Arun Tej M, REACH

Low Carrier Mobilities (~10 -5 cm 2 V -1 s -1 ) Low Exciton Diffusion Lengths (5-15 nm) Large Exciton Binding Energies (up to 1.5 eV) Large Energy Gaps (2-3 eV) Combine the advantages of Organics and SWNTs SWNTs Improve mobility SWNTs provide Large interfacial area SWNTs have Suitable energy levels SWNTs have Low energy gaps 11 Arun Tej M, REACH Efficiency Improvement with SWNTs

Arun Tej M, REACH 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, 2005 Wu et al, Science, 2004 Nanotubes in Organic Solar Cells

Arun Tej M, REACH Results (1) Photoluminescence Quenching Higher Efficiency Arun Tej M, S.S.K.Iyer, and B.Mazhari, IEEE INEC, 2008, Shanghai

Arun Tej M, REACH Results (2) Trap filling behaviourTunneling behaviour Arun Tej M, S.S.K.Iyer, and B.Mazhari, IEEE PVSC, 2008, San Diego

Arun Tej M, REACH High Open Circuit Voltages with Bulk Heterojunction Devices Results (3) Our Work To be published

Synthesis of stable organic compounds Separate semiconducting and metallic SWCNTs Aligned CNTs inside the semiconducting polymers give improved charge transport e-e- e-e- e-e- h+h+ h+h+ 16 Arun Tej M, REACH e-e- h+h+ Future REACH (1)

Add nanoparticles, quantum dots, fullerenes etc to the side walls of SWNTs 17 Arun Tej M, REACH e-e- h+h+ h+h+ e-e- e-e- h+h+ e-e- h+h+ e-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: 31 st March, 2006 New device structures 18 Arun Tej M, REACH Future REACH (3)

Arun Tej M, REACH 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-e- h+h+ Anode Cathode Donor Acceptor Exciton formation Exciton diffusion Exciton dissociation Carrier transport Charge collection 21 Arun Tej M, REACH Organic Solar Cell

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 Conjugated polymers  Conduction due to sp 2 – hybridised carbon atoms   and  (p z -p z )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, Arun Tej M, REACH

M ETALLIC SWNT S 24 Arun Tej M, REACH

Conductance is independent of the channel length. 25 Arun Tej M, REACH

Conductance through a barrier with transmission probability T. Landauer Formula: With N parallel 1D channels (subbands): m-SWNTs: Only two subbands cross E F (N=2) Source of R: Mismatch in the number of conduction channels in the SWNT and the macroscopic metal leads. 26 Arun Tej M, REACH