CARBON NANOTUBE BASED ORGANIC SOLAR CELLS Arun Tej

CARBON NANOTUBE BASED ORGANIC SOLAR CELLS Arun Tej M. Ph. D 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 • • • 2

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 • • • www. applied-nanotech. com 3

Nanotube Field Emission Display Arun Tej M, REACH - 2008 W. B. Choi, Samsung, APL, 1999 4

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 5 Arun Tej M, REACH - 2008 Type of Memory

Nanotube Liquid Flow Sensor Arun Tej M, REACH - 2008 A. K. Sood, IISc Bangalore, Science, 2003 6

Nanotube Integrated Circuit Arun Tej M, REACH - 2008 5 Stage Ring Oscillator on one SWNT Z. Chen, IBM, 2006 7

Nanotube Based Inorganic Solar Cell Arun Tej M, REACH - 2008 W. J. Ready, Georgia Tech, JOM, 2007 8

Nanotube Properties Useful for Solar Cells 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 (Eg: 0. 48 to 1. 37 e. V) 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 • • 9

Arun Tej M, REACH - 2008 Split-Gate device, Energy band diagram and I-V characteristics 10

Efficiency Improvement with SWNTs Improve mobility • Low Carrier Mobilities (~10 -5 cm 2 V-1 s-1) Arun Tej M, REACH - 2008 • Low Exciton Diffusion SWNTs provide Lengths (5 -15 nm) Large interfacial area • Large Exciton Binding SWNTs have Energies (up to 1. 5 e. V) Suitable energy levels SWNTs have Low energy gaps • Large Energy Gaps -3 e. V) (2 Combine the advantages of Organics and SWNTs 11

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 Arun Tej M, REACH - 2008 • • Chhowalla et al, APL, 2005 12

Results (1) Arun Tej M, REACH - 2008 Photoluminescence Quenching Higher Efficiency Arun Tej M, S. S. K. Iyer, and B. Mazhari, IEEE INEC, 2008, Shanghai 13

Results (2) Arun Tej M, REACH - 2008 Trap filling behaviour Tunneling behaviour 14 Arun Tej M, S. S. K. Iyer, and B. Mazhari, IEEE PVSC, 2008, San Diego

Results (3) Arun Tej M, REACH - 2008 High Open Circuit Voltages with Bulk Heterojunction Devices Our Work To be published 15

Future REACH (1) e- Arun Tej M, REACH - 2008 • 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+ 16

Future REACH (2) • Add nanoparticles, quantum dots, fullerenes etc to the side walls of SWNTs e- e- Arun Tej M, REACH - 2008 e- e- h+ eh+ h+ h+ 17

Future REACH (3) New device structures Arun Tej M, REACH - 2008 “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 18

Acknowledgements • Faculty, Staff and Students, SCDT • Prof. Ashutosh Sharma, Chemical Engineering Arun Tej M, REACH - 2008 19

Arun Tej M, REACH - 2008 Thank You 20

Organic Solar Cell Schematic and energy diagram of a typical polymer solar cell and its operation Arun Tej M, REACH - 2008 Exciton formation Exciton diffusion e Exciton Carrier dissociation transport Charge collection h+ 21 Anode Donor Acceptor Cathode

Arun Tej M, REACH - 2008 22

§ § H. Hoppe and N. S. Sariciftci, 2004 Arun Tej M, REACH - 2008 § Conjugated polymers Conduction due to sp 2– 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 23

METALLIC SWNTS Arun Tej M, REACH - 2008 24

Arun Tej M, REACH - 2008 Conductance is independent of the channel length. 25

Conductance through a barrier with transmission probability T. Landauer Formula: With N parallel 1 D 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. Arun Tej M, REACH - 2008 26